Light-emitting device and range-measuring device

Abstract

The present invention provides a light-emitting device that improves the peak value of the light pulses used for irradiation. [Solution] The light-emitting device has a spacer portion that includes a saturable absorption layer provided between a first electrode, which is a back electrode, and a resonator portion. The spacer portion has a first spacer layer provided on the first side with respect to the saturable absorption layer, and a second spacer layer provided on the second side with respect to the saturable absorption layer. The first spacer layer and the second spacer layer are of a first conductivity type.

Description

[Technical Field]

[0001] This invention relates to a light-emitting device and a distance-measuring device. [Background technology]

[0002] The document states that a VCSEL (Vertical Cavity Surface Emitting Laser) will be used as the light source for Time of Flight (ToF) LiDAR. VCSELs have the advantage of being less wavelength-dependent to temperature.

[0003] Patent Document 1 describes a VCSEL that can increase the peak value of the irradiated light pulse. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-176886 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, the VCSEL described in Patent Document 1 has room for further improvement. The present invention aims to provide a light-emitting device that is improved from that described in Patent Document 1, and a distance measuring device using such a light-emitting device. [Means for solving the problem]

[0006] According to one disclosure of this specification, a means for solving the problem is a light-emitting element comprising: a first reflector; a second reflector; a resonator portion including an active layer provided between the first reflector and the second reflector; a first electrode provided below the first reflector; a second electrode provided above the core base; and a spacer portion including a saturable absorbing layer provided between the first electrode and the resonator portion, wherein the spacer portion comprises a first spacer layer provided on a first side with respect to the saturable absorbing layer and a second spacer layer provided on a second side opposite to the first side with respect to the saturable absorbing layer, wherein the first spacer layer and the second spacer layer are of a first conductivity type, and the second reflector is of a second conductivity type opposite to the first conductivity type. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a light-emitting device that is an improvement over Patent Document 1, and a distance measuring device using such a light-emitting device. [Brief explanation of the drawing]

[0008] [Figure 1] A schematic cross-sectional view showing a light-emitting element of the first embodiment. [Figure 2] A schematic cross-sectional view showing details of the spacer layer of the light-emitting element of the first embodiment. [Figure 3] A schematic cross-sectional view showing details of the spacer layer of the light-emitting element of the second embodiment. [Figure 4] An example of calculating the electron density of the spacer layer of the light-emitting element in the second embodiment. [Figure 5] This figure shows the relationship between the electron density in the quantum well layer of the light-emitting element of the second embodiment and the photoresponse waveform. [Figure 6] A diagram showing the relationship between electron density and peak intensity in the quantum well layer of the light-emitting element of the second embodiment. [Figure 7] A schematic cross-sectional view showing details of the spacer layer of the light-emitting element of the third embodiment. [Figure 8] A block diagram showing the schematic configuration of a distance measuring device according to the fifth embodiment. [Figure 9]A schematic cross-sectional view showing a comparative example light-emitting element. [Figure 10] A diagram showing the relationship between light extraction efficiency and bandgap difference. [Figure 11] An example of calculating the electron density of the spacer layer of a light-emitting element, and a diagram showing the relationship between electron density and doping concentration. [Figure 12] An example of calculating the electron density of the spacer layer of a light-emitting element, a diagram showing the relationship between electron density and barrier layer thickness, and a diagram showing the relationship between current value and barrier layer thickness. [Figure 13] An example of calculating the electron density of the spacer layer of a light-emitting element. [Figure 14] A schematic cross-sectional view showing a light-emitting element of the fourth embodiment. [Figure 15] An example of calculating the electron density of the spacer layer of a light-emitting element. [Modes for carrying out the invention]

[0009] The embodiments shown below are intended to embody the technical concept of the present invention and do not limit it. The size and positional relationships of the components shown in each drawing may be exaggerated for clarity of explanation. In the following description, identical components may be given the same number and their explanation may be omitted.

[0010] Embodiments of the present invention will be described in detail below with reference to the drawings. In the following description, terms indicating specific directions or positions (e.g., "up," "down," "right," "left," and other terms including these terms) will be used as needed. The use of these terms is to facilitate understanding of the embodiments with reference to the drawings, and the technical scope is not limited by the meaning of these terms. For example, in VCSELs, terms such as lower reflector, upper reflector, lower electrode, and upper electrode are used, but these are simply expressions to specify relative positions. Therefore, if the light-emitting element is mounted upside down, the reflector located at the top can be called the "lower reflector," and the reflector located at the bottom can be called the "upper reflector."

[0011] In the following explanation, the first conductivity type semiconductor will be described as an N-type semiconductor, and the second conductivity type semiconductor, which is the opposite conductivity type to the first, will be described as a P-type semiconductor. However, the first conductivity type semiconductor may also be described as a P-type semiconductor, and the second conductivity type semiconductor as an N-type semiconductor. Furthermore, a non-doped or undoped semiconductor can be called an i-type semiconductor, and the i-type can also be called a third conductivity type.

[0012] In this specification, when the term "impurity concentration" is used, it refers to the net impurity concentration after subtracting the amount compensated by reverse-conductivity impurities. In other words, "impurity concentration" refers to the NET doping concentration. The region where the doping concentration of P-type impurities is higher than that of N-type impurities is a P-type semiconductor region. Conversely, the region where the doping concentration of N-type impurities is higher than that of P-type impurities is an N-type semiconductor region. Note that the process of adding impurities to a semiconductor to generate carriers may also be referred to as doping below.

[0013] [Comparative Example] Before describing the first embodiment of the present invention, a comparative example will be described using Figure 9. Figure 9(A) is a schematic cross-sectional view showing the structure of the comparative example.

[0014] The comparative example, light-emitting element 1100, is a vertical-cavity surface-emitting laser (VCSEL) having a distributed Bragg reflector (DBR). As shown in Figure 9(A), the light-emitting element 1100 has, in this order, a semiconductor substrate 10, a lower DBR layer 12 (first reflector), a spacer portion 14, and an upper DBR layer 28 (second reflector). The light-emitting element 1100 also has electrodes 40, 42 and a protective film 44. The lower DBR layer 12 is provided on the semiconductor substrate 10. The spacer portion 14 is provided on the lower DBR layer 12. The upper DBR layer 28 is provided on the resonator portion 18. The layer located between the lower DBR layer 12 and the upper DBR layer 28 (spacer portion 14 and resonator portion 18) is the resonator spacer portion.

[0015] A saturable absorption layer 16 is provided within the spacer portion 14. The resonator portion 18 is composed of an n-type layer 20 provided on the spacer portion 14, a non-doped spacer portion 22 provided on the n-type layer 20, and a p-type layer 26 provided on the non-doped spacer portion 22. An active layer 24, which is a three-layer quantum well layer, is provided within the non-doped spacer portion 22. An oxidative constriction layer 38 is provided within the upper DBR layer 28. The total number of quantum well layers is not limited to three, but may be four or other.

[0016] The undoped spacer portion 22, the p-type layer 26, and the upper DBR layer 28 are processed into a mesa shape. An electrode 40 electrically connected to the n-type layer 20 is provided on the n-type layer 20 that is exposed by processing the undoped spacer portion 22, the p-type layer 26, and the upper DBR layer 28 into a mesa shape. An electrode 42 electrically connected to the upper DBR layer 28 is provided on the upper DBR layer 28. A protective film 44 is provided on the upper surface of the n-type layer 20 and the sides and top surface of the mesa, excluding at least a portion of the surfaces of the electrodes 40 and 42.

[0017] The semiconductor substrate 10 may be made of, for example, a GaAs substrate. The lower DBR layer 12 may be made of, for example, Al with an optical film thickness of 1 / 4λc. 0.1 GaAs layer and Al 0.9 The structure can be constructed by stacking 35 pairs of laminates, with each pair consisting of a GaAs layer and a sub-layer. Here, λc is the central wavelength of the high-reflection band of the lower DBR layer 12. For example, λc is 940 nm.

[0018] The saturable absorption layer 16 in the spacer portion 14 may be composed of a multiple quantum well including, for example, three quantum wells, each consisting of an 8 nm thick InGaAs well layer sandwiched between 10 nm thick AlGaAs barrier layers. The rest of the spacer portion 14 may be composed of an undoped GaAs layer.

[0019] The resonator section 18 is composed of a p-i-n junction including an n-type layer 20, a non-doped spacer section 22, and a p-type layer 26. Each of the three active layers 24 disposed in the non-doped spacer section 22 can be composed of, for example, a multiple quantum well including four quantum wells in which an InGaAs well layer with a thickness of 8 nm is sandwiched by AlGaAs barrier layers with a thickness of 10 nm. In this case, a total of 12 quantum wells are included in the resonator section 18. The n-type layer 20 can be composed of an n-type GaAs layer, the p-type layer 26 can be composed of a p-type GaAs layer, and the other part of the non-doped spacer section 22 can be composed of a non-doped GaAs layer, respectively.

[0020] Thus, the resonator section 18 is composed of a p-i-n junction that also exists in a normal VCSEL, and has a configuration similar to a resonator section including an active layer in the i layer. However, the number of quantum well layers in the resonator section 18 is larger compared to the number of quantum well layers (about three layers) in a normal VCSEL. The effective resonator length in the resonator section 18 is 10 μm.

[0021] The upper DBR layer 28 can be configured, for example, by laminating 20 pairs of a laminate of an Al 0.1 Ga 0.9 As layer and an Al 0.9 Ga 0.1 As layer as one pair. An oxidation constriction layer 38 formed by oxidizing a part of an Al 0.98 Ga 0.02 As layer with a thickness of 30 nm is provided in the upper DBR layer 28. The oxidation constriction layer 38 can be formed, for example, by oxidizing the Al 0.98 Ga 0.02 As layer from the side surface of the mesa with water vapor during manufacturing. The oxidation constriction layer 38 has a non-oxidized part at the central part of the mesa and an oxidized part near the side wall of the mesa. Due to the oxidation constriction layer 38, the current injected into the light-emitting element 1100 flows only through the non-oxidized part, so that the light-emitting element 1100 laser-oscillates only in the part overlapping the central part of the mesa in plan view.

[0022] The laser light generated by the light-emitting element 1100 may be configured to be emitted from the side of the upper DBR layer 28, or from the side of the semiconductor substrate 10.

[0023] In the comparative example light-emitting element 1100, the configuration of a normal VCSEL is used as a base, with the following three elements added. The first of the three elements added to the VCSEL is to substantially increase the volume of the active layer. For example, a normal VCSEL is composed of three quantum wells, but this is increased to about 20 layers in terms of the volume of quantum wells. The second is to introduce a saturable absorption layer 16. The third is to extend the effective resonator length as a VCSEL. The effective resonator length is the resonator length that light perceives within the resonator. More specifically, it is the average distance that light propagates from the active layer in the resonant direction until it is reflected by the two mirrors constituting the resonator and passes through the active layer again. By adding at least one, preferably three of these elements, it is possible to realize a VCSEL capable of generating light pulses with high peak values ​​and short pulse widths.

[0024] In the comparative example light-emitting element 1100, oscillation begins approximately 600 ps after the start of current injection. The reason why the oscillation start is later than in a normal VCSEL is that the effective volume of the active layer 24 is larger, and for a certain period of time after the start of current injection, oscillation is inhibited by the absorption of light in the saturable absorption layer 16. When light is absorbed by the saturable absorption layer 16, the absorbed light is accumulated in the saturable absorption layer 16 as carriers. As light is absorbed, the number of carriers increases, and when the carrier density in the saturable absorption layer 16 reaches the transparent carrier density, the saturable absorption layer 16 stops absorbing light. As a result, the effect of inhibiting laser oscillation disappears, and the light-emitting element starts to oscillate.

[0025] The purpose of inhibiting laser oscillation for a certain period of time by the saturable absorption layer 16 is to accumulate carriers exceeding the threshold carrier density in the active layer 24. Here, the threshold carrier density is the carrier density required to generate the gain necessary for laser oscillation.

[0026] The carrier density in the active layer 24 begins to rise as current injection starts. Before laser oscillation begins, carriers temporarily accumulate above the threshold carrier density. Then, when laser oscillation begins, the carriers are rapidly consumed by stimulated emission and converge to a stable value. This allows for the output of optical pulses with high peak values ​​and short half-widths.

[0027] The reason why carriers exceeding the threshold carrier density can be accumulated in the active layer 24 is that laser oscillation is suppressed for a certain period of time using the saturable absorption layer 16. By achieving such a high carrier density, after oscillation, light pulses with high peak values ​​and short pulse widths can be generated inside the light-emitting element.

[0028] The condition for laser oscillation to continue after the generation of an optical pulse is that the maximum gain obtained by the active layer 24 exceeds the absorption of the entire resonator. Specifically, laser oscillation can continue when the relationship expressed by the following equation (1) holds true. In equation (1), Γa is the optical confinement coefficient of the saturable absorption layer 16, Γs is the optical confinement coefficient of the active layer 24, and gmax(Iop) is the maximum gain obtained in the active layer 24 when the current value is Iop. Also, α2 is the absorption coefficient of the saturable absorption layer 16, αm is the mirror loss, and αi is the optical absorption by semiconductor carriers, etc. Γs×gmax(Iop) > Γa×α2+αm+αi …(1) [Challenges of the comparative example] Figure 9(B) shows the current injection path of the comparative example. Electrons are injected from electrode 40 and are injected into the active layer 24 via the n-type layer 20. On the other hand, holes are injected from electrode 42 and are injected into the active layer 24 via the p-type upper DBR layer 28 and the p-type layer 26.

[0029] Thus, when electrodes 40 are placed adjacent to a mesa structure, the size of the light-emitting element increases as the size of the electrode 40 increases. As a result, when multiple light-emitting elements are arranged in an array, it may become difficult to achieve a narrow pitch between the light-emitting elements. Furthermore, when an array is formed, it may become difficult to route the wiring that electrically connects to the electrode 40.

[0030] [First Embodiment] Figure 1(A) is a schematic cross-sectional view showing the structure of the light-emitting element of the first embodiment. Unlike the comparative example, a back electrode 140 (first electrode) is provided at the bottom of the lower DBR layer 12, which is the back side of the light-emitting element.

[0031] Components similar to the light-emitting element in the comparative example are given the same reference numerals, and their descriptions are either omitted or simplified.

[0032] Figure 1(B) shows the current injection path of the first embodiment. Carriers from electrode 42 (second electrode) and back electrode 140 (first electrode) move vertically through the semiconductor layer and are injected into the active layer. Unlike the comparative example, the light-emitting element of the first embodiment does not require carrier injection in the lateral direction through the n-type layer 20. Therefore, it is not necessary to secure an area for placing electrode 40, and the size of the light-emitting element can be reduced. Although the size of the light-emitting element can be reduced according to this embodiment, the reduced size is not shown in Figures 1(A) and (B).

[0033] Furthermore, this configuration offers the following secondary benefits. Specifically, during the etching process for forming the mesa structure, the thickness of the n-type layer 20 may vary from element to element. In a configuration where carriers are injected laterally through the n-type layer 20, as in the comparative example, variations in the thickness of the n-type layer 20 result in variations in electrical resistance. In particular, when multiple elements are arranged in an array shape, even if the distance between each element and each electrode 40 is made uniform, a distribution of electrical resistance may occur. In contrast, the light-emitting element of the first embodiment makes it possible to suppress variations in electrical resistance more effectively than in the comparative example.

[0034] Figure 2 is a schematic cross-sectional view showing in detail the semiconductor layers constituting the spacer portion 14. The saturable absorption layer 16 is composed of multiple quantum wells, with the quantum well layer 124 sandwiched between barrier layers 114. The saturable absorption layer 16 is sandwiched between a lower spacer layer 134 (first spacer layer) and an upper spacer layer 144 (second spacer layer), thus forming the spacer portion 14. The quantum well layer 124 is, for example, InGaAs, the barrier layer 114 is, for example, GaAs, and the lower spacer layer 134 and upper spacer layer 144 are, for example, AlGaAs.

[0035] The barrier layer 114 may be AlGaAs instead of GaAs. However, by using GaAs for the barrier layer, the band gap of the barrier layer can be made smaller than when AlGaAs is used. As a result, carriers can be accumulated in the GaAs barrier layer, making it easy to increase the amount of carriers required. This allows for a reduction in the number of quantum well layers. A favorable condition for accumulating carriers in the barrier layer is that the energy difference between the photon energy at the oscillation wavelength (which can also be called the band gap of the active layer) and the band gap of the barrier layer is 230 meV or less.

[0036] Here, from the viewpoint of reducing light absorption by the semiconductor constituting the barrier layer, it is desirable that the energy difference with respect to its band gap be above a certain value. In this embodiment, suitable conditions are considered when the barrier layer material is GaAs. The absorption coefficient of GaAs has wavelength dependence, and the light extraction efficiency calculated using this wavelength dependence is shown below.

[0037] Figure 10 shows the relationship between optical extraction efficiency and bandgap difference. The vertical axis represents optical extraction efficiency, and the horizontal axis represents bandgap difference. The bandgap difference is the difference in band gap between the barrier layer and the quantum well layer. The optical extraction efficiency when there is no optical absorption by GaAs is set to 1. The points in the figure show the values ​​when the bandgap difference is 25, 35, 45, 55, 65, 75, 85, 95, 105, and 115 meV. When the bandgap difference is 105 meV, the optical extraction efficiency decreases by 2% compared to when there is no absorption due to the bandgap. Similarly, when the bandgap difference between the barrier layer and the quantum well layer is 60 meV, 48 meV, and 44 meV, the optical extraction efficiency decreases by 3%, 4%, and 5%, respectively, compared to when there is no absorption due to the bandgap. Note that when there is no absorption due to the bandgap, the optical extraction efficiency is 1.

[0038] The difference between a 2% decrease in light extraction efficiency (105 meV) and a 3% decrease (60 meV) is 45 meV. The difference between a 3% decrease (60 meV) and a 4% decrease (48 meV) is 12 meV. The difference between a 4% decrease (48 meV) and a 5% decrease (44 meV) is 4 meV. In other words, the difference decreases as the light extraction efficiency decreases.

[0039] Here, in compound semiconductors with three or more elements, the controllability of the elemental composition during crystal growth is said to be about 1%. The change in band gap when the composition changes by 1% is 12 meV for AlGaAs systems and 14 meV for InGaAs systems. It can be seen that the energy difference when these compositions change by 1% is similar to the energy difference when the light extraction efficiency changes from 3% to 4% as described above. Thus, considering the 1% change in composition during crystal growth, it is preferable that the band gap difference be 60 meV or more, which corresponds to the design value where the decrease in light extraction efficiency is 3% as described above. By designing to such a value, it is possible to maintain the characteristic stability of the device even if manufacturing errors occur during crystal growth.

[0040] Based on the above, we will examine a suitable range for the bandgap difference, considering both the perspective of providing carriers in the barrier layer and the perspective of reducing light absorption in the barrier layer. In order to minimize the decrease in light extraction efficiency to 2% or less, while prioritizing the reduction of the effect of light absorption at the band edge, a bandgap difference in the range of 105 meV to 230 meV is preferable. Within such a range, the light extraction efficiency of the device will improve.

[0041] Furthermore, if a decrease in light extraction efficiency of about 4% due to light absorption at the band edge is acceptable, it is preferable to set the band gap difference to a range of 60 meV to 230 meV, taking into consideration the controllability during crystal growth. Such a range improves the characteristic stability of the device.

[0042] Even if a compound semiconductor material different from that of this embodiment is used, as long as it is a direct bandgap semiconductor material, the wavelength dependence of the absorption coefficient for wavelengths below the band gap does not change significantly, and therefore the above values ​​can be applied.

[0043] In addition to the effects described above, the configuration that actively accumulates carriers in the barrier layer also has secondary effects. The first effect is that the cumulative strain of the semiconductor layer can be reduced. Assuming the oscillation wavelength is set to 940 nm, the active layer is InGaAs and the substrate is GaAs, so strain occurs in the active layer grown on the substrate due to the difference in lattice constants. Furthermore, the cumulative strain increases as the number of quantum well layers increases. Therefore, the design that reduces the number of quantum well layers by reducing the band gap of the barrier layer, as in this embodiment, has the effect of reducing cumulative strain. This is not an effect unique to the case where the oscillation wavelength is 940 nm, but an effect that can be obtained in common when the lattice constant of the constituent material of the substrate and the lattice constant of the constituent material such as the crystal grown on it are different.

[0044] The second effect is that it reduces carrier consumption due to luminous recombination. In all semiconductors, including quantum well layers and barrier layers, when both holes and electrons are present simultaneously, carriers are consumed by luminous recombination (spontaneous emission). In the case of semiconductor lasers, a large amount of carrier consumption due to luminous recombination (spontaneous emission) is undesirable because it raises the laser oscillation threshold and reduces power conversion efficiency. It is known that this luminous recombination is proportional to the square of the carrier density. Therefore, even when accumulating the same amount of carriers, the carrier density, and thus the amount of carriers consumed by luminous recombination, changes depending on the volume of the carrier accumulation region.

[0045] Furthermore, the undoped spacer section 22, which functions as the active layer, can have a similar configuration. Specifically, the quantum well layer is made of InGaAs, the surrounding barrier layer is made of GaAs, and the band gap difference between the two layers is 230 meV or less. The preferred lower limit of the band gap is the same as described above. With such a configuration, at least one of the following multiple effects can be obtained. These multiple effects are: the ability to increase the number of carriers accumulated in the active layer, the ability to reduce the number of quantum wells required to accumulate the necessary number of carriers in the active layer, the ability to reduce the amount of accumulated carriers consumed by luminescence recombination, and the ability to reduce cumulative strain. Specifically, with this configuration, the total number of quantum well layers can be reduced from 12 to about 4 layers. In addition, the peak value of the optical pulse immediately after the start of oscillation can be increased, and the energy of the optical pulse can be increased. Note that when this configuration is used as the active layer, the preferred range of band gap difference between the quantum well layer and the barrier layer is the same as when it is used as the spacer section 14 described above. However, while a configuration in which the non-doped spacer portion 22 and spacer portion 14 have a bandgap difference of 230 meV or less is preferable, it is not an essential requirement for achieving the effects of the present invention. For example, the effects of the present invention can also be achieved even if the non-doped spacer portion 22 is configured as in the comparative example.

[0046] As described above, in the light-emitting element of the first embodiment, carriers are moved vertically by providing a back electrode 140 on the back side. For this reason, the semiconductor substrate 10, the lower DBR layer 12, and the materials constituting the spacer portion 14 are all n-type semiconductors. That is, the lower spacer layer 134, barrier layer 114, quantum well layer 124 (saturable absorption layer 16), and upper spacer layer 144 that constitute the spacer portion 14 are all n-type semiconductors. However, since doping with impurities may introduce defects into the quantum well layer 124, the quantum well layer 124 may be an undoped semiconductor layer.

[0047] In the above example, the resonator length of the laser is extended by using the spacer portion 14 to increase the distance from the lower DBR layer 12 to the upper DBR layer 28. The purpose of this is to widen the pulse width. The optical thickness of the resonator spacer portion should preferably be at least five times the resonance wavelength, and more preferably at least eleven times the resonance wavelength. Optical thickness refers to the value obtained by multiplying the physical thickness by the refractive index of the medium.

[0048] Furthermore, in the above example, the effective resonator length is extended by increasing the thickness of the spacer portion 14 and other layers to widen the physical distance between the lower DBR layer 12 and the upper DBR layer 28. However, the method of extending the effective resonator length is not limited to this. For example, by providing a third reflector between the lower DBR layer 12 and the upper DBR layer 28 to form a coupled resonator, it is possible to effectively extend the resonator length even if the distance between the lower DBR layer 12 and the upper DBR layer 28 is shorter compared to this embodiment.

[0049] Furthermore, the optical thickness of the spacer portion 14 may be set to an integer multiple of λ / 2, for example, and a pair of semiconductor layers with different refractive indices of λ / 4 may be stacked on top of the spacer portion 14. In this case, the spacer portion 14 is part of the lower DBR layer 12, and it can be said that the spacer portion 14 is included in the lower DBR layer 12. Such a configuration is not excluded in this embodiment either.

[0050] Furthermore, matters common to the comparative examples, the first embodiment, and each of the following embodiments are those described in the comparative examples, which also apply to these embodiments.

[0051] [Second Embodiment] A light-emitting element according to the second embodiment will be described with reference to Figure 3. Figure 3 is a schematic cross-sectional view showing the structure of the spacer portion 14 of the light-emitting element according to this embodiment.

[0052] In Figure 2, all semiconductor layers constituting the spacer portion 14 are made of n-type semiconductors. However, in Figure 3(A), the semiconductor layers constituting the saturable absorption layer 16 are made of undoped semiconductor material, while the lower spacer layer 134 and the upper spacer layer 144 are made of n-type semiconductors. In other words, region 120 in Figure 3(A) is an undoped semiconductor region.

[0053] This light-emitting element allows for a smaller size compared to the comparative example. As a secondary benefit, it makes it possible to suppress variations in electrical resistance.

[0054] Doping the saturable absorption layer 16 with impurities can lead to defects and a decrease in luminous efficiency. Therefore, according to the second embodiment, it is possible to suppress the decrease in luminous efficiency compared to the first embodiment.

[0055] As shown in Figure 3(B), an undoped semiconductor layer 154 may be provided between the saturable absorption layer 16 and the lower spacer layer 134. Alternatively, an undoped semiconductor layer 164 may be provided between the saturable absorption layer 16 on the opposite side and the upper spacer layer 144. In this case, region 120 in Figure 3(B) is an undoped semiconductor region. Furthermore, only one of the undoped semiconductor layers 154 or 164 may be provided. The undoped semiconductor layers 154 and 164 are, for example, undoped AlGaAs.

[0056] Figure 4 shows an example of electron density calculation. The quantum well layer 124 is undoped InGaAs, the barrier layer 114 is undoped GaAs, and the lower spacer layer 134 and upper spacer layer 144 are n-type AlGaAs. The thickness of the quantum well layer 124 is 16 nm, the thickness of the barrier layer 114 is 10 nm, and the impurity concentration of the n-type AlGaAs in the lower spacer layer 134 and upper spacer layer 144 is 1 × 10⁻¹⁶. 18 cm -3 Therefore, in this calculation, to simplify the calculation, two quantum well layers are treated as one layer of quantum well layer 124.

[0057] Figure 4 shows a calculation example when a forward bias is applied to the device, and the electron density from the lower spacer layer 134 to the upper spacer layer 144 is above a predetermined value. This confirms that electrons injected from the lower spacer layer 134 are injected into the upper spacer layer 144 via the lower barrier layer 114, the quantum well layer 124, and the upper barrier layer 114.

[0058] The thickness of the undoped region 120 must be such that carriers from the lower spacer layer 134 diffuse sufficiently. The preferred thickness will be discussed later.

[0059] The rate of carrier consumption Rc in the quantum well layer is given by equation (1) below. Rc = B × (n0 + n1) × p1 ···(1) Here, B is a physical constant, n0 is the electron density when the light-emitting element is not operating, n1 is the increased electron density when the light-emitting element is operating, and p1 is the increased hole density when the light-emitting element is operating. If Rc becomes large, carriers will be consumed in the quantum well layer, and the laser oscillation will not continue. Therefore, it is necessary to design the system so that n0 is not a large value.

[0060] Figure 5 shows the relationship between the electron density n0 present in the well layer and the photoresponse waveform. Figure 5 shows the change in light intensity over time. When the electron density n0 is 0 cm -3In this case, as indicated by symbol 511, the waveform shows a peak with a high peak value, after which the laser oscillation continues and converges to a stable value. Next, when the electron density n0 is 5 × 10 18 cm -3 In this case as well, as indicated by symbol 512, the waveform shows a peak with a high peak value, after which the laser oscillation continues and converges to a stable value. The light-emitting element emits light that has a maximum peak value and a profile that converges to a stable value of a predetermined light intensity after the maximum peak value. Next, the electron density n0 is 1 × 10⁻⁶ 19 cm -3 In this case, as indicated by symbol 513, after showing a peak with a high peak value, the laser oscillation pauses, and then shows another peak. From there, such peaks appear periodically. This is called self-pulsation. Next, when the electron density n0 is 2 × 10⁻⁶ 19 cm -3 In this case as well, as indicated by symbol 514, after showing a peak with a high peak value, the laser oscillation pauses, and then another peak is shown. From there, such peaks appear periodically. Self-pulsation that shows multiple peaks during the period when current is flowing is difficult to handle as a light source. Therefore, it is necessary to set the value of the electron density n0 so that self-pulsation does not occur.

[0061] Figure 6 is a graph showing how the light intensity of the first peak and the light intensity of the second peak, as shown in Figure 5, change with respect to the electron density n0. As can be seen in Figure 5, the smaller the ratio of the light intensity of the second peak to the light intensity of the first peak, the more likely self-pulsation is to occur.

[0062] Figure 6(b) shows the ratio of the light intensity of the second peak to the light intensity of the first peak. Electron density n0 is 7 × 10⁻⁶. 18 cm -3 When the electron density n0 is higher than the case shown, the rate of change in the ratio of peak intensity to electron density becomes considerably larger. Therefore, when the electron density n0 is 7 × 10⁻⁶ 18 cm -3 It is best to set it as follows. Also, the electron density n0 is 3 × 10⁻⁶.18 cm -3 When the value is higher than the case shown, the rate of change in the ratio of peak intensity to electron density becomes larger. Therefore, when the electron density n0 is 3 × 10⁻⁶ 18 cm -3 The following settings are preferable.

[0063] By the way, regarding the calculation example shown in Figure 4, let's try calculating under the condition that the quantum well layer 124 and the barrier layer 114 are n-type semiconductor layers, and the quantum well layer 124, the barrier layer 114, the lower spacer layer 134, and the upper spacer layer 144 all have the same impurity concentration. In other words, these conditions are the calculation example of the first embodiment. When calculated, the electron density n0 in the quantum well layer was found to be 7 × 10⁻⁶. 18 cm -3 To achieve the following, the n-type doping concentration in these layers should be 5.6 × 10⁻⁶. 18 cm -3 The following was found. From these results, in the first embodiment, in order to suppress the occurrence of self-pulsation, the doping concentration of the saturable absorption layer 16 should be 5.6 × 10 18 cm -3 The following applies:

[0064] Figure 11(A) is a graph showing the calculation results of the electron density distribution. Figure 11(A) shows the calculation results when the quantum well layer 124, barrier layer 114, lower spacer layer 134, and upper spacer layer 144 are given the same doping concentration. In the graph, the vertical axis represents electron density (cm³). -3 The horizontal axis shows the position (nm). Above the graph is a schematic diagram showing the positions of the quantum well layer 124, the barrier layer 114, the lower spacer layer 134, and the upper spacer layer 144. The width of the lower barrier layer 114 is shown as W1, the width of the quantum well layer 124 as W2, and the width of the upper barrier layer 114 as W3.

[0065] Figure 11(B) shows the electron density n0 and doping concentration (10) of the quantum well layer 124, as read from Figure 11(A). 18 cm -3 Here, the electron density n0 is the average value. From Figure 11(B), it can be seen that the electron density n0 is dependent on the dope concentration.

[0066] As mentioned above, the electron density n0 in the quantum well layer is 7 × 10 18 cm -3 To achieve the following, the n-type doping concentration in these layers should be 5.6 × 10⁻⁶. 18 cm -3 This can be achieved by doing the following: A more desirable electron density n0 within the quantum well layer is 3 × 10⁻¹⁰. 18 cm -3 To achieve this, the n-type dope concentration should be 2.1 × 10⁻⁶. 18 cm -3 This can be achieved by doing the following:

[0067] On the other hand, the lower limit of the preferred dope concentration from a manufacturing standpoint is 1 × 10⁻⁶. 16 cm -3 This is the result. 1 × 10 16 cm -3 The following electron densities are difficult to control. Considering these lower limits can improve characteristic stability.

[0068] The spacer portion 14 in the second embodiment is as follows. The spacer portion 14 in the second embodiment has a lower spacer layer 134 and an upper spacer layer 144. Figure 12 shows the change in electron density n0 of the quantum well layer 124 in the undoped region 120 due to the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144. Figure 12 shows the doping concentrations and electron densities of the spacer layers. The vertical axis represents electron density (cm²). -3 The horizontal axis shows the position (nm). From Figure 12, the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144 are 5.0 × 10⁻⁶. 18 cm -3 Even after this change, the electron density of quantum well layer 124 remains 5 × 10⁻¹⁴. 17 cm -3As shown in Figure 6, this is within the preferred range. Furthermore, even if the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144 change, the electron concentration n0 of the quantum well layer 124 hardly changes. As described above, Figures 5 and 6 show that the electron concentration in the quantum well layer 124 affects the optical pulse characteristics. Therefore, in the spacer portion 14 of the second embodiment, even if the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144 change, the change in the electron concentration n0 of the quantum well layer 124 is small, and thus the change in optical pulse characteristics can be reduced in response to variations in doping concentration during manufacturing.

[0069] Figure 12(A) is a graph showing the calculation results of the electron density n0. The horizontal axis indicates position, similar to Figure 11. Here, the electron concentration n0 in the quantum well layer 124 is shown when the thickness d1 of the barrier layer placed between the doped layer and the quantum well layer 124 in the spacer portion 14 of the second embodiment is changed. It can be seen that the electron concentration in the undoped portion decreases as the thickness d1 of the barrier layer increases. At a barrier layer thickness d1 = 47.5 nm, the lowest electron concentration is approximately 1 × 10⁻¹⁶. 16 cm -3 In general, for compound semiconductors such as GaAs, 1 × 10⁻⁶ 16 cm -3 The following electron densities are difficult to control. If the barrier layer thickness d1 is increased beyond 47.5 nm, the carriers are canceled out by holes and electrons that occur during manufacturing, and in some cases, regions may occur where there are more holes than carriers. In other words, it becomes difficult to control the device characteristics. For this reason, it is preferable that the barrier layer thickness d1 be 47.5 nm or less.

[0070] Figure 12(B) is a graph showing the relationship between the electron density n0 (average value) in the quantum well layer 124 and the barrier layer thickness d1. The vertical axis represents the electron density in the quantum well layer, and the horizontal axis represents the change in the barrier layer thickness d1. Even at d1 = 2.5 nm, a favorable electron density of 7 × 10⁻¹⁰ is obtained from the viewpoint of the optical pulse waveform shown in Figures 5 to 7 above. 18 cm -3 It can be seen that the following applies. Therefore, it is preferable that the thickness d1 of the barrier layer be d1 = 2.5 nm or more.

[0071] Figure 12(C) is a graph showing the relationship between the current flowing through the spacer portion 14 and the thickness d1 of the barrier layer. This graph was calculated by applying a potential difference of 0.1V across the spacer portion 14. It can be confirmed that current flows when the thickness d1 of the barrier layer is 47.5nm or less, confirming the effect of the spacer portion 14 in the second embodiment, which is that current can pass through while the barrier layer is undoped. Furthermore, when the thickness d1 of the barrier layer becomes less than 30nm, the rate of increase of the current (slope of the graph) increases. Therefore, it is more preferable to set the thickness d1 of the barrier layer to 30nm or less, as this allows more current to pass through.

[0072] Figure 15 shows an example of calculating the electron density of the spacer layer of a light-emitting element. Figure 15 shows the electron density distribution when the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144 are changed in the configuration d1=10nm shown in Figure 12. 18 cm -3 , 3 x 10 18 cm -3 , 1 x 10 18 cm -3 This shows the case when the values ​​are changed. From this, it can be seen that the electron density of the well layer does not change even when the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144 are changed. In other words, it can be seen that the electron density of the quantum well layer is determined by the structure of the barrier layer and the quantum well layer.

[0073] Considering the effect of the spacer portion 14 in the second embodiment, the range of the barrier layer thickness d1 is 2.5 nm to 47.5 nm, preferably 2.5 nm to 30 nm. With such a range, it becomes easy to pass current through the barrier layer while keeping it undoped. Furthermore, the barrier layer thickness d1 may differ above and below the quantum well layer. In that case, it is preferable that the thickness d1 of each barrier layer is within the above range.

[0074] The barrier layer, lower spacer layer 134, and upper spacer layer 144 in the first and second embodiments may be made of the same material and have the same dope concentration, or they may be made of different materials and have different dope concentrations. Furthermore, the structure may be made by omitting at least one of the lower spacer layer 134 and the upper spacer layer 144.

[0075] Note that the carrier consumption in equation (1) above only takes into account the phenomenon of luminescent recombination. On the other hand, in actual semiconductors, semiconductor defects can occur due to crystal growth conditions or the addition of dopants. Depending on the type of semiconductor defect, non-luminescent recombination may occur. In such cases, the amount of carrier consumption increases, so self-pulsation tends to occur at a lower n0 than in the calculation example in Figure 4.

[0076] Furthermore, the electron mobility in undoped semiconductors is faster than that in doped semiconductors. For example, in undoped GaAs, the electron mobility is 1 × 10⁻⁶. 18 cm -3 Compared to GaAs doped with , the mobility is twice or more than twice as high. Therefore, in the second embodiment, there is also the advantage that the electrical resistance can be reduced while keeping the electron density n0 low, due to the high mobility.

[0077] As a modification of this embodiment, there is also a configuration in which the undoped region 120 shown in Figure 3 is doped with a sufficiently low concentration. It can be understood from equation (1) and Figure 5 that if the electron density generated by doping is sufficiently low, a similar effect can be achieved. Comparing the reference numerals 511 and 512 in Figure 5, there is no significant difference in the photoresponse waveform, so 1 × 10⁻⁶ 18 cm -3 It is also possible to dope the electron density with impurities to achieve the following electron densities.

[0078] When the undoped region 120 in Figure 3(A) is changed to a low-concentration impurity region, the quantum well layer 124 and the barrier layer 114 become n-type semiconductors with a lower impurity concentration than the lower spacer layer 134 and the upper spacer layer 144.

[0079] If the undoped region 120 in Figure 3(B) is changed to a low-concentration impurity region, the quantum well layer 124 and the barrier layer 114 will be made of an n-type semiconductor with a lower impurity concentration than the lower spacer layer 134 and the upper spacer layer 144. In addition, the semiconductor layer 154 between the saturable absorption layer 16 and the lower spacer layer 134 will be made of an n-type semiconductor with a lower impurity concentration than the lower spacer layer 134 and the upper spacer layer 144. Furthermore, the semiconductor layer 164 between the saturable absorption layer 16 and the upper spacer layer 144 will be made of an n-type semiconductor with a lower impurity concentration than the lower spacer layer 134 and the upper spacer layer 144. Note that either semiconductor layer 154 or semiconductor layer 164 may be provided.

[0080] Next, we will consider the preferred impurity concentration when the undoped region 120 in Figure 3(A) is changed to a low-concentration impurity region. Figure 13 is a graph similar to Figure 11(A), showing the electron density (cm³) under three conditions. -3 This is a graph showing the distribution of ). Of the three conditions, condition 1 is that the total impurity concentration of the spacer part 14 is 3 × 10 18 cm -3 In this case, condition 2 is that the overall impurity concentration of the spacer part 14 is 1 × 10 18 cm -3 This is the case. Condition 3 is that the impurity concentration of the lower spacer layer 134 and the upper spacer layer 144 is 3 × 10⁻⁶. 18 cm -3 The quantum well layer 124 and the barrier layer 114 are set to 1 × 10 18 cm -3 This is the case. Furthermore, Figure 13(A) shows the case where the barrier layer thickness d1 is 10 nm, and Figure 13(B) shows the graph when the barrier layer thickness d1 is 2.5 nm. The cases where W1 and W3 are changed are shown.

[0081] In both graphs in Figure 13(A) and Figure 13(B), the electron density under condition 2 in the quantum well layer 124 overlaps with the electron density under condition 3. In other words, when the undoped region 120 is changed to a low-concentration impurity region, if the thickness d1 of the barrier layer is 2.5 nm or more, the electron density of the quantum well layer 124 is determined by the doping concentration of the quantum well layer 124 and the barrier layer 114, regardless of the film thickness d1. The reason why the electron density of the quantum well layer 124 is not greatly affected by the doping concentration of the lower spacer layer 134 and the upper spacer layer 144 and the thickness of the barrier layer is as follows: Most of the electrons flowing in from the lower spacer layer 134 and the upper spacer layer 144 are located away from the quantum well layer 124, specifically near the interface between the lower spacer layer 134 and the upper spacer layer 144 and the barrier layer.

[0082] As another embodiment, a configuration in which the lower spacer layer 134 and the upper spacer layer 144 are doped more thinly than the quantum well layer 124 and the barrier layer 114 will be described. When the doping concentrations of the quantum well layer 124 and the barrier layer 114 are within the above range, the lower spacer layer 134 and the upper spacer layer 144 may be doped more thinly than the quantum well layer 124 and the barrier layer 114. For example, the doping concentrations of the lower spacer layer 134 and the upper spacer layer 144 may be 5 × 10⁻¹⁶. 17 cm -3 The concentrations of the quantum well layer 124 and the barrier layer 114 are set to 1 × 10⁻¹⁰ 18 cm -3 This is also possible. Although reducing the doping concentration in the spacer portion results in the disadvantage of increased electrical resistance of the current flowing through the spacer portion 14, the effects of the present invention are still achieved.

[0083] In another embodiment, the quantum well layer 124 can be undoped, while the barrier layer 114, lower spacer layer 134, and upper spacer layer 144 are doped. In this case, the electron concentration n0 of the quantum well layer 124 will be lower than when both the quantum well layer 124 and the barrier layer 114 are doped, by the amount of doping in the quantum well layer. Therefore, the doping concentration of the barrier layer 114 can be set to 5.6 × 10⁻⁶. 18 cm -3More preferably, it is 2.1×10 18 cm -3 or less, thereby realizing a preferable electron concentration n0 of the quantum well layer 124. With such a configuration, it is possible to realize a preferable optical pulse waveform.

[0084] When changing the non-doped region 120 to a low-concentration impurity region, it can be understood that the quantum well layer 124 and the barrier layer 114 may have the doping concentrations shown in FIG. 11(B). Specifically, the doping concentrations of the quantum well layer 124 and the barrier layer 114 are 5.6×10 18 cm -3 or less, more preferably 2.1×10 18 cm -3 or less, thereby realizing a preferable optical pulse waveform. Note that from the viewpoint of the optical pulse waveform, there is no lower limit value for the preferable doping concentration. However, from the manufacturing viewpoint, the lower limit value of the preferable doping concentration is 1×10 16 cm -3 and it becomes.

[0085] [Third Embodiment] The light-emitting element according to the third embodiment will be described using FIG. 7. FIG. 7 is a schematic cross-sectional view showing the structure of the light-emitting element according to the present embodiment.

[0086] The same components as those of the light-emitting element according to the above-described embodiment are denoted by the same reference numerals, and the description thereof is omitted or simplified.

[0087] In FIG. 7, the semiconductor layer constituting the saturable absorption layer 16 is a p-type semiconductor, and the lower spacer layer 134 and the upper spacer layer 144 are n-type semiconductors. That is, the region 130 in FIG. 7 is a p-type semiconductor region. For this reason, a depletion layer 310 is formed from the junction surface between the saturable absorption layer 16 and the lower spacer layer 134 and the junction surface between the saturable absorption layer 16 and the upper spacer layer 144. Specifically, in the saturable absorption layer 16, both the quantum well layer 124 and the barrier layer 114 can be p-type semiconductors.

[0088] As shown in Figure 7, in order to form a depletion layer 310 over the entire saturable absorption layer 16, the impurity concentration of the barrier layer 114 constituting the saturable absorption layer 16 is made lower than the impurity concentrations of the lower spacer layer 134 and the upper spacer layer 144. When a forward bias is applied to the light-emitting element, carriers can freely pass through the saturable absorption layer 16.

[0089] This light-emitting element allows for a smaller size compared to the comparative example. Additionally, it makes it possible to suppress variations in electrical resistance.

[0090] Furthermore, among the saturable absorption layers 16, the quantum well layer 124 can be undoped, and the barrier layer 114 can be made of a p-type semiconductor. By making the quantum well layer 124 undoped, a decrease in luminescence efficiency can be suppressed.

[0091] In the first to third embodiments, a reflector made of semiconductor material is used as the reflector, but a dielectric DBR may also be used. However, in that case, even if the upper and lower electrodes are placed on the dielectric DBR, current cannot be injected through the dielectric DBR, so they are arranged to make electrical contact with the semiconductor part without going through the dielectric DBR. For example, if at least a part of the upper DBR layer is made of dielectric DBR, the electrodes are arranged to make electrical contact with the semiconductor layer located below the upper DBR layer. In this case, the upper electrode 42 can be described as being located on the upper part of the resonator, between the resonator and the dielectric DBR.

[0092] [Fourth Embodiment] In the embodiments described above, a configuration having one pin structure within the resonator, a so-called single-junction type, was explained, but the present invention is not limited thereto. In this embodiment, a multi-junction type configuration having multiple pin structures within the resonator 29 will be described. Figure 14 is a schematic cross-sectional view showing the light-emitting element of this embodiment. The light-emitting element of this embodiment is a so-called two-junction type, having two pin junctions within the resonator 29.

[0093] Figure 14 has the same configuration as shown in Figure 1(A), except for the resonator portion 29. The light-emitting element in Figure 14 has, in this order, a back electrode 140, a semiconductor substrate 10, a lower DBR layer 12, a spacer portion 14, a resonator portion 29, an upper DBR layer 28, an electrode 42, and a protective layer 44. The spacer portion 14 includes a saturable absorption layer 16. The upper DBR layer 28 includes an oxide constriction layer 38.

[0094] The resonator section 29 of this embodiment includes an n-type layer 281, an undoped spacer section 292, a p-type layer 293, a tunnel junction layer 294, an n-type layer 291, an undoped spacer section 292, and a p-type layer 283. The n-type layer 281, the undoped spacer section 292, the p-type layer 293, the tunnel junction layer 294, the n-type layer 291, the undoped spacer section 292, and the p-type layer 283 are arranged in this order from the side closest to the saturable absorption layer 16.

[0095] n-type layer 281 contains n-type AlGaAs, p-type layer 293 contains p-type AlGaAs, n-type layer 291 contains n-type AlGaAs, p-type layer 283 contains p-type AlGaAs. p-type layer 293 is 30 nm thick Al 0.98 Ga 0.02 It includes an oxidized constriction layer 293a which is an As layer. However, the p-type layer 293 does not necessarily have to have an oxidized constriction layer 293a.

[0096] Each of the two undoped spacer sections 292 includes a quantum well layer. More specifically, the undoped spacer section 292 includes a spacer 292a, a single-layer active section 292b, and a spacer 292c, arranged in this order. The single-layer active section 292b is positioned at the antinode of a standing wave. The active section 292b includes three quantum wells. The active section 292b can be composed of a multiple quantum well including, for example, three quantum wells, each consisting of an 8 nm thick InGaAs well layer sandwiched between a 10 nm thick GaAs barrier layer. The remaining parts of spacer 292a and spacer 292c that constitute the undoped spacer section 292 are composed of undoped GaAs layers. In this configuration, the band gap difference between the barrier layer and the quantum well layer is 105 meV, and the active layer configuration also accumulates carriers in the aforementioned barrier layer.

[0097] The tunnel junction layer 294 is 10 in order to improve the tunnel probability. 19 cm -3 It may contain p-type and n-type layers (not shown) of a certain degree or higher. The high-concentration p-type layer may be located closer to the p-type layer 293, and the high-concentration n-type layer may be located closer to the n-type layer 291.

[0098] With this configuration, even if the current injected into the element is the same, the number of carriers generated in the resonator section 29 becomes twice that of a single-junction type, making it possible to increase the optical power.

[0099] Although Figure 14 depicts the layers constituting the light-emitting element as being in direct contact, other functional layers may be placed between any of these layers.

[0100] In this embodiment, an example of a two-junction design has been described, but the design is not limited to this, and it is possible to use three or more junctions based on a similar concept. Furthermore, in this embodiment, an example of applying a multi-junction type resonator spacer to the first embodiment has been described, but the design is not limited to this, and it can be appropriately combined with other embodiments.

[0101] Furthermore, even in a configuration where the p-type layer 293 does not have an oxidative constriction portion 293a, it is possible to reduce variations in emission timing within the array by changing the optical loss in the multi-junction type.

[0102] In the first to fourth embodiments, the light-emitting element has a configuration in which a saturable absorption layer, including a quantum well layer that absorbs light, is sandwiched between semiconductors of the same conductivity type. In other words, the light-emitting element is not a pn junction or pin junction, where layers of different conductivity types are sandwiched above and below a layer that absorbs or emits light, such as a quantum well layer. On the other hand, as a VCSEL using a light-absorbing layer, a pin configuration is also known in which both the active layer and the light-absorbing layer are sandwiched between p-type and n-type semiconductor layers. In this VCSEL, in addition to electrodes that electrically contact the p-type and n-type layers that sandwich the active layer for generating laser oscillation gain, the light-absorbing layer is sandwiched between the p-type and n-type layers to form a pin or pn junction. Furthermore, it has electrodes that electrically contact the p-type and n-type layers, respectively. In this configuration, it is possible for any two of the four electrodes to be combined into one electrode. Therefore, the number of electrodes is generally three or four. In this configuration, carriers for light emission are injected into the active layer by applying a voltage in the forward direction of the pin. Furthermore, a voltage is applied to the light-absorbing layer in the forward or reverse direction of the pin, and the oscillation timing is controlled by the conditions and time variation of the voltage applied to the light-absorbing layer. In this configuration, the oscillation timing is controlled by changing the characteristics of the light-absorbing layer, such as the absorption coefficient, by the voltage and current applied to the light-absorbing layer.

[0103] In such a configuration, it is essential to sandwich the light-absorbing layer between a p-type layer and an n-type layer to form a pin junction in order to electrically control it. If the spacer layer containing the saturable absorption layer in the above embodiment is sandwiched between layers of the same conduction type (for example, n-type in the first embodiment), the above-mentioned electrical control becomes impossible in principle. Therefore, the design philosophy differs depending on whether the conduction types of the doped semiconductor layers on both sides of the saturable absorption layer are the same or different.

[0104] Furthermore, the VCSEL configuration of the above-described embodiment differs significantly from a VCSEL with a pin configuration in which the optical absorption layer is sandwiched between p-type and n-type semiconductor layers in terms of the design philosophy for the absorption coefficient of the optical absorption layer. In a VCSEL with a pin configuration in which both the active layer and the optical absorption layer are sandwiched between p-type and n-type semiconductor layers, the design philosophy is as follows: The idea is to make the absorption coefficient of the optical absorption layer sufficiently large so that laser oscillation does not occur even after time has elapsed since the start of current injection into the active layer. On the other hand, in the state in which laser oscillation occurs, the absorption coefficient of the optical absorption layer is designed to be sufficiently small through electrical control. Therefore, if the layers above and below the optical absorption layer are made of the same conductivity type based on this structure, electrical control becomes impossible. Thus, the light-emitting element will either oscillate before time has elapsed since the start of current injection, or it will start oscillating immediately after the start of current injection, similar to a normal VCSEL, and will not produce short pulses with high peak values.

[0105] In the embodiment described above, the problems in the comparative example are solved as described above by passing current in the vertical direction in Figure 1 from the electrode 42 and the electrode 140 on the back of the substrate, as shown in Figure 1. Furthermore, in order to pass current in the vertical direction through the spacer portion 14, at least a part of the spacer portion 14 is doped with impurities. By doping with impurities, it was found that, under certain conditions, the optical pulse waveform may become undesirable, as shown in Figures 5 to 7, compared to the configuration of the comparative example. Specifically, in the comparative example, only one pulse with a high peak value and short duration is generated at the start of laser oscillation, and thereafter, as the laser oscillation continues, a small peak is generated when transitioning to a stable state. This is a desirable characteristic for a light source for distance measurement. The reason is that the high peak value of the optical pulse at the start of oscillation makes it possible to measure distances further, and the short duration improves the distance measurement accuracy. Also, because the second and subsequent peaks are low, the probability of misidentifying them as the first peak and increasing the error in the distance measurement value is reduced. However, doping the spacer portion 14 may result in an undesirable waveform under certain conditions, where the peak value of the light pulse decreases and the intensity of the second and subsequent peaks increases.

[0106] The inventors identified a problem in a configuration where the spacer portion 14 is doped and current is passed in the vertical direction as shown in Figure 1: a change in the shape of the optical pulse caused by doping. To address this problem, they discovered that there are desirable conditions for the spacer portion 14, such as doping concentration and film thickness, that allow for the maintenance of an optical pulse waveform equivalent to that of the comparative example, and they have achieved a balance between these conditions. The conditions under which this balance is possible are explained in the calculation results for each embodiment.

[0107] In the embodiment described above, the spacer portion 14 was positioned between the lower DBR layer 12 and the active layer 24. However, in order to generate short optical pulses, the spacer portion 14 is not limited to this position. Based on the physical principle of optical pulse generation described above, it will be effective to position it in the part that constitutes the optical resonator. More specifically, the spacer portion 14 may be positioned between the lower DBR layer 12 and the upper DBR layer 28. Furthermore, the spacer portion 14 may be positioned between the layers that constitute the lower DBR layer 12.

[0108] [Fifth Embodiment] A distance measuring device according to a fifth embodiment of the present invention will be described with reference to Figure 8. Figure 8 is a block diagram showing the schematic configuration of the distance measuring device according to this embodiment.

[0109] The distance measuring device 200 according to this embodiment is a distance measuring device (LiDAR device) that applies a surface-emitting laser array, in which the light-emitting elements of the above embodiment are arranged in a two-dimensional manner, to the light source.

[0110] The distance measuring device 200 according to this embodiment may consist of a control unit 210, a surface-emitting laser array driver 212, a surface-emitting laser array 214, a light-emitting optical system 218, a light-receiving optical system 220, an image sensor 222, and a distance data processing unit 224.

[0111] The surface-emitting laser array 214 is a package-mounted light-emitting element 100 according to the fourth embodiment. The surface-emitting laser array driver 212 is a drive unit that receives a drive signal from the control unit 210, generates a drive current for oscillating the surface-emitting laser array 214, and outputs it to the surface-emitting laser array 214. Note that the surface-emitting laser array 214 and the surface-emitting laser array driver 212 may be a single light-emitting device.

[0112] The light-emitting optical system 218 is an optical system that emits laser light generated by the surface-emitting laser array 214 toward the range to be measured. The light-receiving optical system 220 is an optical system that guides the laser light reflected by the object to be measured 2000 to the image sensor 222. In Figure 8, the light-emitting optical system 218 and the light-receiving optical system 220 are represented by a single convex lens-shaped component, but these are not composed of only a single convex lens-shaped component, but are composed of a lens group made up of multiple lenses.

[0113] The image sensor 222 is a photoelectric conversion device in which multiple pixels, including a photoelectric conversion unit, are arranged in a two-dimensional array, and is a light receiving device that outputs an electrical signal corresponding to the incident light. The image sensor 222 may be, for example, a CMOS image sensor or a SPAD sensor. The distance data processing unit 224 functions as a distance information acquisition unit that generates and outputs information regarding the distance to the object to be measured 2000 located within the distance measurement target range, based on the signal from the image sensor 222. The distance data processing unit 224 only needs to be electrically connected to the image sensor 222, and may be located in the same package as the image sensor 222, or in a separate package from the image sensor 222.

[0114] The control unit 210 is composed of an information processing device including a microcomputer and logic circuits, and functions as a central processing unit that controls the operation of the distance measuring device 200, including the operation control of each part and various calculation processing.

[0115] As mentioned above, a suitable light-emitting device for a LiDAR system is one that can generate light pulses with a short pulse width and high peak value. Specifically, the ideal light pulse width for a light source suitable for a LiDAR system is, for example, in the range of 50 ps to 1 ns. On the other hand, from the electrical standpoint of the VCSEL and the VCSEL driver, it is not easy to emit light with such a short pulse width. Since the VCSEL emits light in response to the amount of current injected, in order to make the light pulse about 50 ps to 1 ns, the current pulse driving the VCSEL must be of a similar magnitude. In other words, the circuit from the driver to the VCSEL needs to have excellent electrical characteristics in the high-frequency band, such as 1 GHz or 10 GHz, and to handle currents exceeding 1 A. This presents the challenge of higher costs compared to configuring the electrical transmission section to the driver and VCSEL using only electrical circuits that handle lower frequency bands.

[0116] Therefore, in this embodiment, by using the light-emitting element described in the above embodiment, the VCSEL itself is configured to generate short pulses. This avoids increasing the cost of the driver unit and electrical transmission unit while achieving optical pulses of approximately 50 ps to 1 ns, which are desirable for LiDAR systems.

[0117] First, the control unit 210 outputs a drive signal to the surface-emitting laser array driver 212. The surface-emitting laser array driver 212 receives the drive signal from the control unit 210 and injects a current of a predetermined value into the surface-emitting laser array 214. As a result, the surface-emitting laser array 214 oscillates and laser light is output from the surface-emitting laser array 214. At this time, the pulse width of the light emitted from the surface-emitting laser array 214 is narrower than the pulse width of the injected current, as described above.

[0118] The laser light generated by the surface-emitting laser array 214 is emitted by the light-emitting optical system 218 toward the range to be measured. Of the laser light irradiated onto the object to be measured 2000 within the range to be measured, the laser light reflected by the object to be measured 2000 and incident on the light-receiving optical system 220 is guided by the light-receiving optical system 220 to the image sensor 222.

[0119] Each pixel of the image sensor 222 generates an electrical signal pulse corresponding to the timing of the incident laser light. The electrical signal pulses generated by the image sensor 222 are input to the distance data processing unit 224.

[0120] The distance data processing unit 224 generates distance information to the object to be measured 2000 along the direction of light propagation, based on the reception timing of the electrical signal pulses output from the image sensor 222. By calculating distance information based on the electrical signal pulses output from each pixel of the image sensor 222, three-dimensional information of the object to be measured 2000 can be obtained.

[0121] The distance measuring device 200 of this embodiment can be applied, for example, to control devices in the automotive field to prevent collisions with other vehicles, or to control devices for automatic driving that follows other vehicles. Furthermore, the distance measuring device 200 of this embodiment can be applied not only to automobiles, but also to other moving objects (mobile devices) such as ships, aircraft, or industrial robots, as well as to moving object detection systems. The distance measuring device 200 of this embodiment can be widely applied to devices that utilize information about objects recognized three-dimensionally, including distance information. These moving objects may be configured to include the distance measuring device of this embodiment and control means for controlling the moving object based on distance information acquired by the distance measuring device.

[0122] Furthermore, the three-dimensional information, including depth, obtainable by the distance measuring device 200 of this embodiment can also be used in image capture devices, image processing devices, display devices, etc. For example, it is possible to use the three-dimensional information obtained by the distance measuring device 200 of this embodiment to display a virtual object on top of a real-world image without any sense of incongruity. In addition, by saving the three-dimensional information together with the image information, it is possible to correct the blur of the captured image after shooting.

[0123] Thus, according to this embodiment, a high-performance distance measuring device equipped with a light-emitting device capable of generating light pulses with a short light pulse width and high peak value can be realized.

[0124] The present invention is not limited to the embodiments described above and can be modified in various ways. For example, an example in which a part of the configuration of one embodiment is added to another embodiment, or in which a part of the configuration of another embodiment is replaced, is also an embodiment of the present invention. Furthermore, in this specification, some of the matters described in the comparative examples can also be added to or replaced in the above-described embodiments.

[0125] Furthermore, in the above embodiment, GaAs, AlGaAs, and InGaAs were given as examples of semiconductor materials capable of crystal growth when a GaAs substrate is used as the semiconductor substrate 10, but the semiconductor substrate 10 is not limited to a GaAs substrate. For example, an InP substrate can also be used as the semiconductor substrate 10. Examples of semiconductor materials capable of crystal growth when an InP substrate is used as the semiconductor substrate 10 include InP, InGaAs, InGaP, and InGaAsP.

[0126] Furthermore, the DBR layer in the light-emitting element according to the above embodiment does not necessarily have to be made of a semiconductor material, and may be made of a material other than a semiconductor material.

[0127] The embodiments described above are merely examples of how the present invention can be implemented, and the technical scope of the present invention should not be interpreted as being limited by them. In other words, the present invention can be implemented in various ways without departing from its technical concept or its main features.

[0128] The above-disclosed embodiment includes the following configuration.

[0129] (Composition 1) A light-emitting element, The first reflecting mirror, The second reflector, A resonator section including an active layer provided between the first reflector and the second reflector, A first electrode provided at the lower part of the first reflector, A second electrode provided on the upper part of the resonator section, The device comprises a spacer portion including a saturable absorption layer provided between the first electrode and the resonator portion, The spacer portion comprises a first spacer layer provided on the first side with respect to the saturable absorption layer, and a second spacer layer provided on the second side opposite to the first side with respect to the saturable absorption layer. The first spacer layer and the second spacer layer are of the first conductivity type, The light-emitting device is characterized in that the second reflector is of a second conductivity type opposite to the first conductivity type.

[0130] (Configuration 2) The saturable absorption layer comprises a quantum well layer and a barrier layer. The light-emitting device according to configuration 1, characterized in that the barrier layer is of the first conductive type.

[0131] (Composition 3) The light-emitting device according to configuration 2, characterized in that the quantum well layer is of the first conductivity type.

[0132] (Composition 4) The light-emitting device according to Configuration 3, wherein the impurity concentrations of the barrier layer and the quantum well layer are lower than the impurity concentrations of the first spacer layer and the second spacer layer.

[0133] (Configuration 5) The light-emitting device according to Configuration 3, having a semiconductor layer with an impurity concentration lower than the impurity concentrations of the first spacer layer and the second spacer layer between at least one of the saturable absorption layer and the first spacer layer and between the saturable absorption layer and the second spacer layer.

[0134] (Configuration 6) The impurity concentrations of the barrier layer and the quantum well layer are 5.6×10 18 cm -3 or less, and the light-emitting device according to any one of Configurations 2 to 5 is characterized in that.

[0135] (Configuration 7) The impurity concentrations of the barrier layer and the quantum well layer are 2.1×10 18 cm -3 or less, and the light-emitting device according to any one of Claims 2 to 5 is characterized in that.

[0136] (Configuration 8) The saturable absorption layer has a quantum well layer and a barrier layer, and the quantum well layer and the barrier layer are non-doped semiconductor layers, and the light-emitting device according to Configuration 1 is characterized in that.

[0137] (Configuration 9) The light-emitting device according to Configuration 8, wherein a non-doped semiconductor layer is provided between at least one of the saturable absorption layer and the first spacer layer and between the saturable absorption layer and the second spacer layer.

[0138] (Configuration 10) The thickness of the barrier layer is 2.5 nm or more and 47.5 nm or less, and the light-emitting device according to Claim 8 or 9 is characterized in that.

[0139] (Composition 11) The light-emitting device according to claim 8 or 9, characterized in that the thickness of the barrier layer is 2.5 nm or more and 30 nm or less.

[0140] (Composition 12) The saturable absorption layer comprises a quantum well layer and a barrier layer. The light-emitting device according to configuration 1, characterized in that the barrier layer is of the second conductive type.

[0141] (Composition 13) The light-emitting device according to configuration 12, characterized in that the quantum well layer is of the second conductivity type.

[0142] (Composition 14) The light-emitting device according to configuration 13, characterized in that a depletion layer is formed over the entire saturable absorption layer.

[0143] (Composition 15) The light-emitting device according to any one of configurations 1 to 14, characterized in that the spacer portion is included in the first reflector.

[0144] (Composition 16) The light-emitting device according to any one of configurations 1 to 15, characterized in that the light-emitting element emits light having a maximum peak value and a profile that converges to a stable value of a predetermined light intensity after the maximum peak value.

[0145] (Composition 17) Let Γs be the optical confinement coefficient of the active layer, Γa be the optical confinement coefficient of the saturable absorption layer, gmax(Iop) be the maximum gain obtained in the active layer when the injected current is Iop, α2 be the absorption coefficient of the saturable absorption layer, αm be the mirror loss, and αi be the optical absorption by the carriers. Γs×gmax(Iop) > Γa×α2+αm+αi A light-emitting device according to any one of configurations 1 to 16, characterized in that it satisfies the following relationship.

[0146] (Composition 18) The quantum well layer is composed of InGaAs, The barrier layer is made of GaAs. A light-emitting device according to any one of configurations 2 to 13, characterized by the features described above.

[0147] (Composition 19) The light-emitting device according to any one of configurations 2 to 18, wherein the active layer has a quantum well layer and a barrier layer, and the band gap difference between at least one of the quantum well layer and the barrier layer of the active layer, or the quantum well layer and the barrier layer of the saturable absorption layer, is 60 meV or more and 230 meV or less.

[0148] (Composition 20) The light-emitting device according to any one of configurations 2 to 18, wherein the active layer has a quantum well layer and a barrier layer, and the band gap difference between at least one of the quantum well layer and barrier layer of the active layer, or the quantum well layer and barrier layer of the saturable absorption layer, is 105 meV or more and 230 meV or less.

[0149] (Composition 21) The light-emitting device according to any one of configurations 1 to 20, characterized in that the optical thickness of the layer located between the first reflector and the second reflector is at least five times the resonance wavelength.

[0150] (Composition 22) The optical thickness of the layer located between the first mirror and the second mirror is at least 11 times the resonance wavelength. A light-emitting device according to any one of configurations 1 to 21, characterized by the features described above.

[0151] (Composition 23) A light-emitting device as described in any one of configurations 1 to 22, A light receiving device that receives light emitted from the light-emitting device and reflected by the object to be measured, A distance information acquisition unit acquires information regarding the distance to the object to be measured based on the timing at which light is emitted from the light-emitting device and the timing at which the light-receiving device receives the light. A distance measuring device characterized by having the following features.

[0152] (Composition 24) It is a mobile object, The distance measuring device described in configuration 23, A mobile body characterized by having control means for controlling the mobile body based on distance information acquired by the distance measuring device. [Explanation of symbols]

[0153] 10 Semiconductor substrates 12, 30 Lower DBR layer 14 Spacer section 16 Saturable absorption layer 18, 32 Resonator section 28, 36 Upper DBR layer 38 Oxidized constriction layer 40, 42, 140 electrodes 44 Protective film

Claims

1. A light-emitting element, The first reflecting mirror, The second reflecting mirror, A resonator section including an active layer provided between the first reflector and the second reflector, A first electrode provided at the lower part of the first reflector, A second electrode provided on the upper part of the resonator section, The device comprises a spacer portion including a saturable absorption layer provided between the first electrode and the resonator portion, The spacer portion comprises a first spacer layer provided on the first side with respect to the saturable absorption layer, and a second spacer layer provided on the second side opposite to the first side with respect to the saturable absorption layer. The first spacer layer and the second spacer layer are of the first conductivity type, The light-emitting device is characterized in that the second reflector is of a second conductivity type opposite to the first conductivity type.

2. The saturable absorption layer comprises a quantum well layer and a barrier layer. The light-emitting device according to claim 1, characterized in that the barrier layer is of the first conductive type.

3. The light-emitting device according to claim 2, characterized in that the quantum well layer is of the first conductivity type.

4. The light-emitting apparatus according to claim 3, characterized in that the impurity concentrations of the barrier layer and the quantum well layer are lower than the impurity concentrations of the first spacer layer and the second spacer layer.

5. The light-emitting apparatus according to claim 3, characterized in that at least one of the saturable absorption layer and the first spacer layer, and the saturable absorption layer and the second spacer layer, has a semiconductor layer having an impurity concentration lower than that of the first spacer layer and the second spacer layer.

6. The impurity concentrations in the barrier layer and the quantum well layer are 5.6 × 10 １８ cm －３ The light-emitting device according to claim 2, characterized in that it is as follows:

7. The impurity concentrations in the barrier layer and the quantum well layer are 2.1 × 10⁻⁶. １８ cm －３ The light-emitting device according to claim 2, characterized in that it is as follows:

8. The saturable absorption layer comprises a quantum well layer and a barrier layer. The light-emitting apparatus according to claim 1, characterized in that the quantum well layer and the barrier layer are undoped semiconductor layers.

9. The light-emitting apparatus according to claim 8, characterized in that a non-doped semiconductor layer is provided between the saturable absorption layer and the first spacer layer, and between the saturable absorption layer and the second spacer layer.

10. The light-emitting device according to claim 8, characterized in that the thickness of the barrier layer is 2.5 nm or more and 47.5 nm or less.

11. The light-emitting device according to claim 8, characterized in that the thickness of the barrier layer is 2.5 nm or more and 30 nm or less.

12. The saturable absorption layer comprises a quantum well layer and a barrier layer. The light-emitting device according to claim 1, characterized in that the barrier layer is of the second conductivity type.

13. The light-emitting device according to claim 12, characterized in that the quantum well layer is of the second conductivity type.

14. The light-emitting device according to claim 13, characterized in that a depletion layer is formed over the entire saturable absorption layer.

15. The light-emitting device according to claim 1, characterized in that the spacer portion is included in the first reflector.

16. The light-emitting device according to claim 1, characterized in that the light-emitting element emits light having a maximum peak value and a profile that converges to a stable value of a predetermined light intensity after the maximum peak value.

17. Let Γs be the optical confinement coefficient of the active layer, Γa be the optical confinement coefficient of the saturable absorption layer, gmax(Iop) be the maximum gain obtained in the active layer when the injected current is Iop, α2 be the absorption coefficient of the saturable absorption layer, αm be the mirror loss, and αi be the optical absorption by carriers. Γs×gmax(Iop) > Γa×α2+αm+αi The light-emitting device according to claim 1, characterized in that it satisfies the relationship.

18. The aforementioned quantum well layer is composed of InGaAs, The barrier layer is made of GaAs. The light-emitting device according to feature 2.

19. The active layer has a quantum well layer and a barrier layer, The light-emitting apparatus according to claim 2, characterized in that the band gap difference between the quantum well layer and the barrier layer of the active layer, or between the quantum well layer and the barrier layer of the saturable absorption layer, is 60 meV or more and 230 meV or less.

20. The active layer has a quantum well layer and a barrier layer, The light-emitting apparatus according to claim 2, characterized in that the band gap difference between at least one of the quantum well layer and barrier layer of the active layer or the saturable absorption layer is 105 meV or more and 230 meV or less.

21. The light-emitting device according to claim 1, characterized in that the optical thickness of the layer located between the first reflector and the second reflector is at least five times the resonance wavelength.

22. The light-emitting device according to claim 1, characterized in that the optical thickness of the layer located between the first reflector and the second reflector is at least 11 times the resonance wavelength.

23. The light-emitting device according to claim 1, A light receiving device that receives light emitted from the light-emitting device and reflected by the object to be measured, A distance information acquisition unit acquires information regarding the distance to the object to be measured based on the timing at which light is emitted from the light-emitting device and the timing at which the light-receiving device receives the light. A distance measuring device characterized by having the following features.

24. It is a mobile object, The distance measuring device according to claim 23, A mobile body characterized by having control means for controlling the mobile body based on distance information acquired by the distance measuring device.

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