Semiconductor device

Abstract

To provide a semiconductor device capable of high output and low voltage operation by eliminating potential spikes without increasing optical loss.SOLUTION: A semiconductor device of the present invention includes: a substrate; two or more electronically conductive layers each including group III nitride semiconductors and having different compositions, which are laminated in order on the substrate; and a compositionally graded layer that is arranged between at least part of adjacent electronically conductive layers in which the composition changes relatively from that of one electronically conductive layer to that of the other electronically conductive layer from the one electronically conductive layer to the other electronically conductive layer. The compositionally graded layer further includes doped n-type impurities.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a semiconductor device, and more particularly, to a semiconductor device used in a GaN-based semiconductor laser device.

Background Art

[0002] Semiconductor lasers have excellent characteristics such as being small, inexpensive, and having high output. Therefore, they are used in a wide range of technical fields such as communication, IT technologies such as optical disks, medical treatment, and some lighting applications. In recent years, in particular, high-power semiconductor lasers have been used for processing light source applications for various materials such as metals, resins, and composite carbon materials (CFRP). Especially, by means of a photosynthesis technology that bundles a plurality of laser emission points to increase the output power, a processing machine system having a large optical output of several hundred watts to several kilowatts has been developed and put into practical use.

[0003] Here, conventionally, for semiconductor laser light sources for processing, GaAs-based near-infrared lasers using GaAs as a semiconductor substrate with a wavelength of about 1 μm have been mainly used. This depends on the fact that the formation technology of GaAs-based semiconductors has reached a high level of maturity. However, such near-infrared wavelength lasers have a problem that it is difficult to process copper materials frequently used in EV motors and the like because the materials reflect the laser light and the energy does not enter the heat. For this reason, a processing light source (also referred to as a "semiconductor device" or "semiconductor light-emitting device") having a wavelength of 405 nm to 540 nm using a GaN-based semiconductor laser that emits light in a blue wavelength region where the absorption rate of copper materials is large (reflectivity is low) has attracted attention. Against this background, it is required to operate a GaN-based semiconductor laser at high power.

[0004] During such high-power operation, the power consumption becomes larger compared to low-power operation. However, in order to reduce the environmental impact, reducing power consumption (improving energy efficiency) becomes extremely important. In order to reduce the power consumption and increase the output power of a semiconductor laser, it is necessary to efficiently convert the input power into light. Therefore, it is crucial to increase the power conversion efficiency, which indicates the ratio of the input energy converted into laser light. By converting the input power into light with high efficiency, not only can the optical output be increased, but it is also possible to prevent the excess energy from being converted into heat, thereby reducing the decrease in optical output caused by heat generation and the adverse effects on long-term reliability characteristics.

[0005] On the other hand, as a method for realizing high output power, in a nitride semiconductor device having an n-side nitride semiconductor layer, an active layer, and a p-side nitride semiconductor layer, a technique of doping the n-side nitride semiconductor layer with n-type impurities at a high concentration (about 10 19 cm -3 is known (Patent Document 1).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, according to the inventors' studies, in a GaN-based semiconductor laser, potential spikes that increase the operating voltage by hindering electron injection occur at the interfaces of the respective layers constituting the n-side layer, specifically, between the n-side AlGaN cladding layer and the n-side GaN guide layer, and between the n-side GaN guide layer and the n-side InGaN guide layer. Such potential spikes, as in Patent Document 1 above, occur when the n-side layer is doped at a high concentration (10 19 cm -3It is considered that it can be reduced by doping to a certain degree). However, according to the inventors' study, when such high-concentration doping is performed on a laser element, it becomes a source of optical loss due to the interaction between light and carriers, reducing the optical output. Thus, in order to reduce the operating voltage, high-concentration doping is necessary, but there is a trade-off problem that when high-concentration doping is performed, the optical output decreases.

[0008] The present invention solves the above problems, and an object thereof is to provide a semiconductor laser element that eliminates potential spikes and operates at high output and low voltage without increasing optical loss.

Means for Solving the Problems

[0009] To achieve the above object, one aspect of the present invention is A substrate, two or more electron-conductive layers including a group III nitride semiconductor, which are sequentially stacked on the substrate and have different compositions from each other, and disposed between at least some adjacent ones of the electron-conductive layers, from one side of the electron-conductive layer to the other side of the electron-conductive layer, having a composition gradient layer whose composition changes so as to approach from the composition of the one electron-conductive layer to the composition of the other electron-conductive layer, and the composition gradient layer includes a doped n-type impurity, relating to a semiconductor element.

Effects of the Invention

[0010] According to the present invention, it is possible to realize a semiconductor laser element that eliminates potential spikes and operates at high output and low voltage without increasing optical loss.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be specifically described.

[0013] (Embodiment 1) 1. Semiconductor element Hereinafter, a semiconductor element 100 according to an embodiment of the present invention will be described. In the present embodiment, as an example of the semiconductor element 100, a blue (wavelength 445 nm) semiconductor laser using a hexagonal group III nitride semiconductor will be used for explanation. Hereinafter, the explanation will be made with reference to the drawings.

[0014] FIG. 1 and FIG. 2 are diagrams of a semiconductor element 100 according to an embodiment of the present invention. FIG. 1 is a plan view of the semiconductor element 100 as viewed from the upper surface direction. FIG. 2 is a cross-sectional view of the semiconductor element 100 cut along the paper surface perpendicular to the direction along FIG. 1A - B including the optical waveguide 20. The semiconductor element 100 according to the present embodiment has a structure that reduces the potential spike on the conduction band side at the interface of each layer with different compositions by simultaneously performing a composition gradient and n-type doping in the n-side layer in a pn junction semiconductor element, particularly in a semiconductor laser.

[0015] First, a simple configuration of the semiconductor element 100 will be described. In FIGS. 1 and 2, semiconductor device 100 is, for example, sequentially formed on semiconductor substrate 1 which is an n-type hexagonal GaN substrate having a (0001) plane, an n-type cladding layer 2 containing, for example, n-AlGaN, an n-side first composition gradient layer 2-2 which is laminated such that the composition of, for example, Al gradually decreases and is doped with an n-type impurity, an n-side first optical guide layer 3 containing, for example, n-GaN, an n-side second composition gradient layer 3-2 which is laminated such that the composition of, for example, In gradually increases and is doped with an n-type impurity, an n-side second optical guide layer 4 containing, for example, n-InGaN, an active layer 5 containing, for example, an InGaN layer, a p-side optical guide layer 6 containing, for example, i-GaN, an electron blocking layer 7 containing, for example, p-Al 0.35 GaN, and a p-type cladding layer 8 containing, for example, a p-AlGaN / GaN superlattice and a p-type contact layer 9 containing, for example, p-GaN are laminated.

[0016] The optical waveguide of semiconductor device 100 is insulated on both sides by an insulating film 10 which is, for example, SiO2, and on the uppermost surface of the optical waveguide, a p electrode 11 which is, for example, Pd / Pt, a wiring electrode 12 which is, for example, Ti / Pt / Au, and a pad electrode 13 which is, for example, Ti / Au are formed in a predetermined pattern. Before and after the optical waveguide 20 of semiconductor device 100, a rear coat film 15 composed of, for example, a dielectric multilayer film for reflecting light in the optical waveguide 20 and a front coat film 14 composed of, for example, a dielectric multilayer film for emitting light are formed. On the surface opposite to semiconductor substrate 1, an n electrode 16 which is, for example, Ti / Au is formed.

[0017] In the above semiconductor device 100, the n-type cladding layer 2 (first layer), the n-side first optical guide layer 3 (second layer), and the n-side second optical guide layer 4 (third layer) are electron conductive layers having a function of conducting electrons in the stacking direction. On the other hand, the p-side optical guide layer 6, the electron blocking layer 7, and the p-type cladding layer 8 are hole conductive layers having a function of conducting holes. Then, electrons are injected from n electrode 16 and holes are injected from p electrode 11, conducted through the above respective layers, recombined in active layer 5, and converted into light.

[0018] The n-type cladding layer 2, the n-side first optical guide layer 3, and the n-side second optical guide layer 4, which are two or more electron conductive layers, contain a group III nitride semiconductor and have different compositions from each other. That the compositions are different from each other means, for example, that the compositions of group III (Al, Ga, In, B) are different from each other and / or the n-type impurity doping concentrations are different, and preferably means that the compositions of group III (Al, Ga, In, B) are different from each other. The composition of group III means, for example, the ratio (atomic ratio) of each element constituting group III. Examples of different compositions of group III include that the difference in the Al or In composition between adjacent electron conductive layers is 0.5% or more, and in a more effective configuration, 3%. And a first composition gradient layer 2-2 doped with an n-type impurity is disposed between the n-type cladding layer 2 and the n-side first optical guide layer 3; a second composition gradient layer 3-2 doped with an n-type impurity is disposed between the n-side first optical guide layer 3 and the n-side second optical guide layer 4.

[0019] Regarding the first composition gradient layer 2-2: The first composition gradient layer 2-2 is disposed between the n-type cladding layer 2 and the n-side first optical guide layer 3, and the composition changes so as to relatively approach the composition of the n-side first optical guide layer 3 from the composition of the n-type cladding layer 2 from the n-type cladding layer 2 side toward the n-side first optical guide layer 3 side.

[0020] Here, "the composition changes so as to relatively approach the composition of the n-side first optical guide layer 3 from the composition of the n-type cladding layer 2" means that in the first composition gradient layer 2-2, the composition gradually changes from a composition close to the composition of the n-type cladding layer 2 to a composition close to the composition of the n-side first optical guide layer 3. The composition change may be any of positive, negative, and zero in the second derivative value of the composition change amount with respect to the film thickness, but it is preferably monotonically decreasing. By monotonically decreasing the composition, the fixed charges caused by the tensile strain of AlGaN, which are the origin of spikes, can be more easily spatially distributed. In the present embodiment, the Al composition of the first composition gradient layer 2-2 decreases from the n-type cladding layer 2 side toward the n-side first optical guide layer 3 side. In the present specification, the Al composition (Al composition) indicates the ratio of the number of Al atoms occupied by group III elements.

[0021] The composition near the boundary between the first compositionally graded layer 2-2 and the adjacent layer may be the same as (continuous) or different from (discontinuous) the composition of the adjacent layer. For example, the Al composition near the boundary between the first compositionally graded layer 2-2 and the n-type cladding layer 2 may be the same as, more than, or less than the Al composition of the n-type cladding layer 2, and it is preferably the same as or less than the Al composition of the n-type cladding layer 2. The Al composition near the boundary between the first compositionally graded layer 2-2 and the n-side first optical guide layer 3 may be the same as, more than, or less than the Al composition of the n-side first optical guide layer 3, and it is preferably the same as or less than the Al composition of the n-side first optical guide layer 3.

[0022] Examples of the n-type impurities doped into the first compositionally graded layer 2-2 include Group IVB and VIB elements such as Si, Ge, Se, S, and O. Among them, Si, Ge, and O are preferred, and Si is more preferred. The doping concentration of the n-type impurities in the n-side first compositionally graded layer 2-2 depends on the thickness of the first compositionally graded layer 2-2, but is preferably 2×10 18 cm -3 or less, for example, about 1×10 18 cm -3 is good.

[0023] The thickness of the first compositionally graded layer 2-2 depends on the doping concentration of the n-type impurities, but is preferably 15 nm or more, more preferably 50 nm or more. The upper limit value of the thickness of the first compositionally graded layer 2-2 is not particularly limited, for example, it is 100 nm.

[0024] Regarding the second compositionally graded layer 3-2: The second compositionally graded layer 3-2 is disposed between the n-side first optical guide layer 3 and the n-side second optical guide layer 4, and the composition changes such that the composition relatively approaches the composition of the n-side second optical guide layer 4 from the composition of the n-side first optical guide layer 3 as it goes from the n-side first optical guide layer 3 side to the n-side second optical guide layer 4 side.

[0025] Here, "the composition changes so as to approach the composition of the n-side second optical guide layer 4 from the composition of the n-side first optical guide layer 3 relatively" means that in the second composition gradient layer 3-2, the composition gradually changes from a composition close to that of the n-side first optical guide layer 3 to a composition close to that of the n-side second optical guide layer 4. The composition change may be such that the second derivative value of the amount of composition change with respect to thickness (the amount of change in the In composition in this embodiment) is positive, negative, or zero. However, in this embodiment, it is preferably negative, that is, it monotonically increases. By monotonically increasing the composition, the fixed charges caused by the compressive strain of InGaN, which are the origin of spikes, can be spatially distributed. In this embodiment, the In composition of the second composition gradient layer 3-2 increases from the n-side first optical guide layer 3 side toward the n-side second optical guide layer 4 side. In this specification, the In composition (In content) indicates the ratio of the number of In atoms among the group III elements.

[0026] The composition near the boundary of the second composition gradient layer 3-2 with the adjacent layer may be the same as or different from the composition of the adjacent layer. For example, the In composition near the boundary of the second composition gradient layer 3-2 with the n-side first optical guide layer 3 may be the same as, more than, or less than the In composition of the n-side first optical guide layer 3; it is preferably the same as or less than the In composition of the n-side first optical guide layer 3. The In composition near the boundary of the second composition gradient layer 3-2 with the n-side second optical guide layer 4 may be the same as, more than, or less than the In composition of the n-side second optical guide layer 4; it is preferably the same as or less than the In composition of the n-side second optical guide layer 4.

[0027] The doping concentration and thickness of the second composition gradient layer 3-2 can be the same as or similar to those of the first composition gradient layer 2-2.

[0028] When there is no such compositional gradient region and the composition changes abruptly, positive and negative fixed charges due to polarization occur at the opposing interfaces. Since positive and negative charges occur on the opposing surfaces of the thin interface, a huge electric field strength is generated. Such a huge electric field generates a band spike. From the perspective of electrons passing through the interface, such an electric field functions as a static force, so it cannot pass through without applying an additional voltage to accelerate. This additional voltage component causes the operating voltage to increase. Also, even if a compositional gradient region is provided, when the n-type impurity is not doped, the generation of potential spikes cannot be sufficiently suppressed. On the other hand, when a compositional gradient region as described above is provided and the composition is inclined, it becomes possible to make the fixed charges approach a spatially uniform distribution state, and the electric field strength becomes smaller. Furthermore, by n-type doping the compositional gradient region, it becomes possible to assist electron conduction and effectively reduce the electric field strength. That is, by providing a compositional gradient layer doped with n-type impurities, the optical output can be increased without increasing the internal optical loss of the semiconductor element 100. Note that although the compositional change is defined as the composition increasing or decreasing monotonically, as long as the composition is substantially increasing or decreasing monotonically as a whole, it may have a region where the composition is constant or slightly increasing inside.

[0029] Regarding other layers: Other layers other than the above will be described in the manufacturing method of the semiconductor element 100 described later.

[0030] Hereinafter, the specific configurations of the first compositional gradient layer 2-2 and the second compositional gradient layer 3-2 (hereinafter, these are also collectively referred to as "compositional gradient layers") in the present embodiment will be described while comparing with comparative examples.

[0031] In FIG. 3, in the embodiment shown in FIG. 2, a structure without the composition gradient layers shown in 2-2 and 3-2 is shown as a comparative example. Note that the top view of the semiconductor device of the comparative example is the same as that of FIG. 1, so it is omitted. Also, in the comparative example of FIG. 3, since it is the same as the embodiment of FIG. 1 except for the presence or absence of the doped composition gradient layer, the description of the layer structure and the like is omitted. Also, in the present application, the Al and In compositions indicate the ratio (atomic %) of the number of atoms occupied by Al and In atoms in the group III elements of each layer. FIG. 4 shows a band structure diagram on the conduction band side between C and D in FIG. 3 when a bias voltage of 4 V is applied to the pn junction in the comparative example shown in FIG. 3. Labels such as n-AlGaN in FIG. 4 represent the stacked structure. n-AlGaN, n-GaN, and n-InGaN correspond to the n-type cladding layer 2, the n-side first optical guide layer 3, and the n-side second optical guide layer 4 in FIG. 3. Note that in the figure, the Al composition in the group III of the AlGaN layer is 2.6%, the In composition in the group III of the InGaN layer is 3%, and the Si doping concentrations of the AlGaN layer and the GaN layer are 1×10 18 cm -3 This is the calculation result when it is set as. Also, the upper side of FIG. 4 shows the location dependence of the Al, In compositions, and Si doping concentration.

[0032] In FIG. 4, two positive potential jumps (spikes) occur in the conduction band side band at the interfaces of n-AlGaN and n-GaN, and the interface of n-GaN and n-InGaN. Here, the spike at the n-AlGaN / n-GaN interface is denoted as spike A, and the n-GaN / n-InGaN interface is denoted as spike B. In FIG. 3, electrons are injected from the n electrode 16 and holes are injected from the p electrode 11, and they recombine in the active layer 5 after passing through each layer in the middle and are converted into light. Here, between C and D in FIG. 3, in order for electrons to conduct on the conduction band towards the active layer 5, due to the existence of spike A and spike B, it is necessary to apply a potential (voltage) to overcome each spike. In other words, the driving voltage required to obtain the same electron current increases, which can be said to be equivalent to an increase in the device resistance. Such an increase in device resistance leads to an increase in the driving voltage required to obtain the same light output, that is, an increase in driving power, resulting in an increase in power consumption, a decrease in light output and a breakdown associated with an increase in device temperature, which is not preferable.

[0033] As a structure capable of eliminating such potential spikes, it is effective to increase the doping concentrations of the n-type cladding layer 2 which is n-AlGaN and the n-side first optical guide layer 3 which is n-GaN.

[0034] FIG. 5A shows the results calculated by the inventors of the values of potential spikes A and B when the Si doping concentrations of the n-type cladding layer 2 which is n-AlGaN and the n-side first optical guide layer 3 which is n-GaN in FIG. 4 are changed to 1×10 18 cm -3 ~1×10 19 cm -3 It can be seen that the potential spike decreases with the doping concentration, and with a doping of 5×10 18 cm -3 or more, the spike is sufficiently low at about 0.05 eV. However, there are also problems with such high-concentration doping.

[0035] Fig. 5B shows the calculated values of the internal optical loss of the laser element when the doping concentrations of the n-type clad layer 2 made of n-AlGaN and the n-side first optical guide layer 3 made of n-GaN are changed in the comparative example shown in Fig. 3. Here, the internal optical loss was obtained by measuring the refractive index and the optical attenuation coefficient of each layer by spectroscopic ellipsometry and applying this to the scalar wave equation. The transfer matrix method is used to simplify the solution of the wave equation. As shown in Fig. 5B, when the Si doping concentration exceeds 2×10 18 cm -3 the internal optical loss increases significantly, and at 1×10 19 cm -3 it increases by about twice compared to 1×10 18 cm -3 . On the other hand, since the optical output of the laser decreases due to the internal optical loss, it is preferably less than about 3 cm -1 . From this, the doping concentration is preferably 5×10 18 cm -3 or less, more preferably about 2×10 18 cm -3 or less. However, there is a trade-off relationship in that when the doping concentration is decreased, the potential spike shown in Fig. 5A increases.

[0036] Fig. 6 shows the calculation results of the band structure and potential spike in the example of this embodiment. The upper part of Fig. 6 shows the location dependence of the Al, In composition and Si doping concentration. The solid line is the composition and concentration profile in the case without the composition gradient layer (comparative example), and the dotted line is the case with the composition gradient layer with doping (example). The example is the calculation result when the thickness of the composition gradient layer is 50 nm and the Si doping concentration is 1×10 18 cm -3 , and other conditions (bias voltage, etc.) are the same as in the comparative example.

[0037] As shown in Fig. 6, potential spikes are reduced by the composition gradient and Si doping of the composition gradient layer, and a reduction in the operating voltage is expected. Specifically, spike A is reduced by the first composition gradient layer 2-2, and spike B is reduced by the second composition gradient layer 3-2.

[0038] Fig. 7 summarizes the results of calculating potential spikes A and B when the thickness and Si doping concentration of the composition gradient layer described in Fig. 6 are varied in the examples.

[0039] As shown in Fig. 7, the potential spike is reduced when the doping concentration of the composition gradient layer is increased and the thickness is increased. In a laser element, it is preferable to set the potential spike to 0.2 eV or less (an increase of approximately 0.2 V in the operating voltage). From this, when the doping concentration is 7.5×10 17 cm -3 , the thickness of the composition gradient layer is 40 nm or more, when the doping concentration is 1.0×10 18 cm -3 it is 20 nm or more, and when it is 2.0×10 18 cm -3 it is desirably 10 nm or more. From these facts, when the doping concentration is 7.5×10 17 cm -3 or less, the thickness of the composition gradient layer is preferably 50 nm or more. In this example as well, from the doping concentration dependence of the internal optical loss of the laser element shown in Fig. 5B, the doping concentration is desirably 2×10 18 cm -3 or less. The lower limit of the doping concentration depends on the group III composition, but can be, for example, about 5×10 17 cm -3 or so.

[0040] Figures 8A and 8B redraw the potential spike amounts in FIG. 7 as contour maps, separated into potential spikes A (FIG. 8A) and B (FIG. 8B), showing the dependence on the thickness and doping concentration of the compositional gradient layer. The two dotted lines in each figure correspond to the lines of spike amounts 0.1 eV and 0.2 eV, respectively.

[0041] To reduce the operating voltage of the device, the spikes are preferably 0.2 eV or less, more preferably 0.1 eV or less. From FIGS. 8A and 8B, a combination of doping concentration and thickness of the compositional gradient layer that can reduce the potential spike can be visually confirmed. The upper right region in each figure is the region where the spike is small. When the doping concentration is high and the thickness of the compositional gradient layer is large, the spike decreases.

[0042] In the potential spike A of FIG. 8A, the spike becomes 0.2 eV or less when the thickness d and doping concentration c of the first compositional gradient layer 2-2 are set in the upper right region of the figure (the region where d and c take values larger than the following linear equations), surrounded by the following two straight lines. Equation (1): d ≧ a1·c + b1 (5.9×10 17 cm -3 ≦ c ≦ 6.2×10 17 cm -3 ), a1 = -1.73×10 -15 , b = 1122.7 Equation (2): d ≧ a2·c + b2 (6.2×10 17 cm -3 ≦ c ≦ 2.03×10 18 cm -3 ), a1 = -3.4×10 -17 , b = 69.1

[0043] Similarly, in the potential spike A of FIG. 8A, the spike becomes 0.1 eV or less when the thickness d and doping concentration c of the first compositional gradient layer 2-2 are set in the upper right region of the figure (the region where d and c take values larger than the following linear equations), surrounded by the following three straight lines. Equation (3): d ≧ a1·c + b1 (7.25×10 17 cm-3 ≤ c ≤ 7.5×10 17 cm -3 )、a1 = -2.08×10 -15 、b = 1608.7 Equation (4): d ≥ a2·c + b2(7.5×10 17 cm -3 ≤ c ≤ 1.03×10 18 cm -3 )、a1 = -1.51×10 -16 、b = 161.2 Equation (5): d ≥ a2·c + b2(1.03×10 18 cm -3 ≤ c ≤ 1.65×10 18 cm -3 )、a1 = -1.04×10 -18 、b = 17.2

[0044] From these, from the viewpoint of making it easier to reduce spike A, it is preferable to set the thickness d and the doping concentration c of the first composition gradient layer 2-2 so as to satisfy Equation (1) or (2), and it is more preferable to set them so as to satisfy any one of Equations (3) to (5).

[0045] In the potential spike B of FIG. 8B, the spike becomes 0.2 eV or less when the thickness d and the doping concentration c of the second composition gradient layer 3-2 are set in the upper right region of the figure (the region where d and c take values larger than the following linear equations), which is surrounded by the following four straight lines. Equation (6): d ≥ a1·c + b1(6.0×10 17 cm -3 ≤ c ≤ 7.27×10 17 cm -3 )、a1 = -4.8×10 -16 、b = 388.2 Equation (7): d ≥ a2·c + b2(7.27×10 17 cm -3 ≤ c ≤ 8.86×10 18 cm -3 )、a1 = -1.23×10 -16 、b = 128.2 Equation (8): d ≥ a2·c + b2(8.86×10 17 cm -3≤ c ≤ 1×10 18 cm -3 )、a1 = -1.58×10 -16 、b = 159.4 Equation (9): d ≥ a2·c + b2(1×10 18 cm -3 ≤ c ≤ 1.25×10 18 cm -3 )、a1 = -6×10 -18 、b = 7.5

[0046] Similarly, in the potential spike B of FIG. 8B, the spike becomes 0.1 eV or less in the upper right region of the figure (the region where d and c take values larger than the following linear equations) surrounded by the following three straight lines when the thickness d and the doping concentration c of the second composition gradient layer are set. Equation (10): d ≥ a1·c + b1(7.25×10 17 cm -3 ≤ c ≤ 9.38×10 17 cm -3 )、a1 = -3.78×10 -16 、b = 374 Equation (11): d ≥ a2·c + b2(9.38×10 17 cm -3 ≤ c ≤ 1×10 18 cm -3 )、a1 = -1.21×10 -16 、b = 133 Equation (12): d ≥ a2·c + b2(1×10 18 cm -3 ≤ c ≤ 2×10 18 cm -3 )、a1 = -6×10 -18 、b = 18

[0047] From these, from the viewpoint of making spike B easier to reduce, it is preferable to set the thickness d and the doping concentration c of the second composition gradient layer 3-2 so as to satisfy any one of Equations (6) to (9), and it is more preferable to set them so as to satisfy any one of Equations (10) to (12).

[0048] 2. Function As described above, wurtzite GaN-based materials (such as AlGaN, GaN, InGaN, etc.) that make up a GaN-based semiconductor laser element are characterized by large polarization charges defined by the crystal symmetry. For example, when AlGaN is laminated on GaN, on the C(0001) plane, negative fixed charges are formed on the surface side and positive fixed charges are formed on the back side, respectively. Polarization is the sum of spontaneous polarization determined by the film composition and piezoelectric polarization determined by the strain in the film. Since AlGaN is formed on GaN with strain, the fixed charges are larger in AlGaN than in GaN. Therefore, at the AlGaN / GaN heterointerface, since there are more positive fixed charges than negative fixed charges, a large number of free electrons / holes are formed at that interface to balance the charges. These free electrons deform the band potential and become the origin of the potential spike that inhibits electron injection.

[0049] On the other hand, the semiconductor element 100 according to the present embodiment is disposed between at least some adjacent electron-conductive layers (between the n-type cladding layer 2 and the n-side first optical guide layer 3, and between the n-side first optical guide layer 3 and the n-side second optical guide layer 4), and has composition gradient layers (the first composition gradient layer 2-2 and the second composition gradient layer 3-2) containing doped n-type impurities. According to the configuration according to the above embodiment, for example, by providing a region with a composition gradient between the n-type cladding layer 2 containing n-AlGaN and the n-side first optical guide layer 3 containing n-GaN, the spatial distribution of the fixed charges can be dispersed, and the distribution of free electrons / holes can be dispersed. Furthermore, by appropriately doping the gradient region with n-type impurities, the band potential can be effectively reduced or burned out. That is, by providing a composition gradient layer doped with n-type impurities, the potential spike can be reduced and the operating voltage can be lowered without performing high-concentration doping to the extent that laser light absorption occurs, thereby improving the optical output.

[0050] 3. Manufacturing Method of Semiconductor Element Subsequently, the manufacturing method of the semiconductor element 100 will be described in conjunction with the detailed configuration.

[0051] First, on an n-type hexagonal GaN substrate 1 with a main plane being the (0001) plane, for example, using the Metalorganic Chemical Vapor Deposition (MOCVD method), from the n-type cladding layer 2 to the p-type contact layer 9 are continuously formed. First, an n-type cladding layer 2 containing Al 0.03 GaN is laminated, for example, in a thickness of 0.5 to 5 μm, preferably about 3 μm.

[0052] Here, as gas raw materials for film formation, for example, trimethylgallium (TMG), trimethylindium (TMI), trimethylaluminum (TMA) for group III raw materials, silane for n-type impurities, ammonia for group V raw materials, etc. may be used. The Si doping concentration of the n-type cladding layer 2 containing n-AlGaN is, for example, 1×10 18 cm -3 and it is good to be about this value.

[0053] Next, while reducing the composition of Al from 3% to 0% over about 50 nm, a first composition gradient layer 2-2 containing n-AlGaN is laminated. As a method for changing the composition, the supply amount of the Al raw material (TMA) may be gradually reduced. Alternatively, with the supply amount of TMA being constant, the supply amount of the Ga raw material (TMG) may be gradually increased. In this case, in order to keep the Si doping concentration at a substantially constant value, it is good to increase the supply amount of the SiH4 raw material in accordance with the increase in the supply amount of TMG. Further, instead of gradually changing the supply amounts of the Al and Ga raw materials, the Al raw material may be supplied pulsedly and the DUTY may be gradually reduced to substantially continuously reduce the composition. Here, the composition change, the composition of Al near the boundary with the adjacent layer, and the doping concentration of the n-type impurities are as described above.

[0054] Next, n-GaN constituting the n-side first optical guide layer 3 is grown, for example, in a thickness of 0.05 to 0.5 μm, preferably about 0.25 μm. Here, the Si doping concentration of the n-side first optical guide layer 3 containing n-GaN is preferably 1×10 18 cm -3 and it is good to be about this value.

[0055] Furthermore, the n-InGaN that constitutes the second compositionally graded layer 3-2 is laminated while gradually increasing the In composition from the n-GaN layer side of the n-side first optical guide layer 3. As a method for increasing the composition, the supply amount of the In raw material (TMI) may be gradually increased. Alternatively, the supply amount of TMIn may be kept constant, and the supply amount of the Ga raw material (TMG) may be gradually decreased. In this case, in order to keep the Si doping concentration substantially constant, it is advisable to increase the supply amount of the SiH4 raw material in accordance with the increase in the supply amount of TMG. Furthermore, instead of gradually changing the supply amounts of the In and Ga raw materials, the In raw material may be supplied pulsedly, and the duty may be gradually increased to substantially continuously increase the composition. Alternatively, since the amount of In incorporated increases when the temperature is lowered, the In composition may be increased by gradually lowering the growth temperature while keeping the supply amounts of TMI and TMG constant. In the temperature range of 800°C, with a low temperature of about 20°C, here, the compositional change, the In composition near the boundary with the adjacent layer, and the doping concentration of the n-type impurity are as described above.

[0056] An n-side second optical guide layer 4 containing n-InGaN is formed on the second compositionally graded layer 3-2. The thickness of the n-side second optical guide layer 4 is, for example, 10 to 500 nm, preferably about 200 nm.

[0057] The method for laminating the active layer 5 will be described below. In 0.03 GaN barrier layer and In 0.18 A quantum well active layer composed of two periods of In 0.03 GaN barrier layer and In 0.18 GaN quantum well layer is grown. First, a first In 0.08 GaN barrier layer is grown to a thickness of 20 nm. Next, a first In 0.03 GaN quantum well layer is grown to a thickness of about 3.0 nm. Subsequently, after growing a second In

[0058] Here, if the thickness of the InGaN barrier layer is large, the internal quantum efficiency will decrease due to radiative recombination in the barrier layer. Therefore, it is preferable to limit the thickness of each barrier layer to about 40 nm at most. Also, in the above example, the thickness was made constant at 10 nm, but from the viewpoint of the uniformity of carrier injection, etc., the thicknesses of the first to third barrier layers may be different.

[0059] Next, a layer composed of an i-InGaN layer, i-GaN or i-AlGaN, or a combination of these layers that constitutes the p-side optical guide layer 6 is laminated to about 0.2 μm. Here, from the viewpoint of light confinement inside the element, it is preferable to adopt a configuration in which the refractive index decreases from the active layer 5 side to the n-type cladding layer 2 side. That is, by forming the guide layer in the order of InGaN, GaN, and AlGaN from the side close to the active layer 5, light confinement can be increased.

[0060] Next, p-Al 0.35 GaN (Mg concentration 5×10 19 cm -3 ) is laminated to 5 nm. The p-AlGaN layer may be, for example, made to have a Mg concentration of 5×10 19 cm -3 using cyclopentadienylmagnesium (Cp2Mg).

[0061] Next, a p-type cladding layer 8 made of a 600-nm p-Al 0.03 GaN layer is laminated, for example, with a Mg concentration of 1×10 19 cm -3 . Further, a p-contact layer 9 containing 10-nm-thick p-GaN is laminated with a Mg concentration of 1×10 20 cm -3 .

[0062] Next, the grown wafer is processed into a ridge stripe type laser. First, for example, by using the thermal CVD method, an SiO2 insulating film (not shown) made of SiO2 with a film thickness of 0.3 μm is formed on the p-type contact layer 9. Further, by using a photolithography method and an etching method using hydrofluoric acid, the SiO2 insulating film is left in a stripe shape with a width of 16 μm, and other regions are etched. At this time, considering forming the end face of the laser by using the natural cleavage plane (m-plane) of the hexagonal nitride semiconductor, the direction of the stripe is made parallel to the m-axis direction of hexagonal GaN.

[0063] Next, by using the inductively coupled plasma (ICP) etching method, the upper part of the stacked structure is etched to a depth of 1.0 μm using the SiO2 insulating film, and a ridge stripe part constituting the optical waveguide 20 is formed from the upper parts of the p-type contact layer 9 and the p-type clad layer 8. Then, the second mask film is removed using hydrofluoric acid, and again, by using the thermal CVD method, an insulating film 10 made of SiO2 with a film thickness of 200 nm is formed again over the entire surface including the ridge stripe part on the exposed p-type clad layer 8.

[0064] Next, by using a lithography method, a resist pattern (not shown) having an opening with a width of 15.5 μm along the ridge stripe part is formed on the upper surface of the ridge stripe part (optical waveguide 20) in the insulating film 10. Subsequently, for example, by using reactive ion etching (RIE) using trifluoromethane (CHF3) gas, the SiO2 insulating film is etched using the resist pattern as a mask, and the p-type contact layer 9 is exposed from the upper surface of the ridge stripe part.

[0065] Next, for example, by using an electron beam (EB) evaporation method, a metal laminated film constituting a p-electrode 11 made of, for example, palladium (Pd) with a thickness of 40 nm and platinum (Pt) with a thickness of 35 nm is formed on at least the p-type contact layer 9 exposed from the upper surface of the ridge stripe part. Then, by using a lift-off method for removing the resist pattern, the metal laminated film in the region other than the upper part of the ridge stripe is removed to form the p-electrode 11.

[0066] Next, as shown in FIG. 1, by means of a lithography method and a lift-off method, a wiring electrode 12 made of, for example, Ti / Pt / Au is selectively formed so as to cover the p electrode 11 on the upper part of the ridge stripe portion on the insulating film 10. Here, the wiring electrode 12 has a planar dimension in a direction parallel to the ridge stripe portion of 750 μm and a planar dimension in a direction perpendicular to the ridge stripe portion of 150 μm. Here, the wiring electrode 12 is formed of a metal laminated film of titanium (Ti) / platinum (Pt) / gold (Au) having thicknesses of 50 nm, 200 nm, and 100 nm, respectively. Generally, a plurality of laser devices are formed in a matrix on the main surface of a wafer. Therefore, when the wiring electrode 12 is cut when dividing the substrate in the wafer state into individual laser chips, the p electrode 11 in close contact with the wiring electrode 12 may be peeled off from the p-type contact layer 9. Therefore, as shown in FIG. 1, it is desirable that the wiring electrodes 12 are not connected between adjacent chips. Subsequently, an Au layer having a thickness of, for example, 10 μm is formed on the upper part of the wiring electrode 12 by an electrolytic plating method to form a pad electrode 13. By doing so, it becomes possible to mount the laser chip by wire bonding, and since the heat generated in the active layer 5 can be effectively dissipated, the reliability of the semiconductor element 100 can be improved.

[0067] Next, the back surface of the semiconductor element 100 in the wafer state formed up to the Au pad electrode is polished with a diamond slurry to thin it until the thickness of the semiconductor substrate 1 becomes about 100 μm. Thereafter, for example, by an EB evaporation method, a metal laminated film made of, for example, Ti having a thickness of 5 nm, platinum having a thickness of 10 nm, and Au having a thickness of 1000 nm is formed on the back surface of the semiconductor substrate 1 (the surface opposite to the surface on which the optical waveguide 20 is formed) to form an n electrode 16.

[0068] Next, the semiconductor element 100 in the wafer state is cleaved (primary cleavage) along the m-plane so that the length in the m-axis direction is, for example, 1200 μm. Subsequently, for example, using an electron cyclotron resonance (ECR) sputtering method, a front coat film 14 is formed on the cleavage plane from which the laser light is emitted, and a rear coat film 15 is formed on the opposite cleavage plane. Here, as the material of the front coat film 14, a dielectric film such as a SiO2 single-layer film is used. Also, as the material of the rear coat film 15, a dielectric film such as a ZrO2 / SiO2 laminated film is used. By setting the reflectance on the front side (light emission side) of the semiconductor element 100 to, for example, 6% and the rear side (opposite side to the light emission side) to, for example, 95%, a highly efficient semiconductor element 100 can be configured.

[0069] Subsequently, the laser chip is completed by cleaving (secondary cleavage) the semiconductor element 100 that has been primarily cleaved along the a-plane between the optical waveguides 20 formed at a pitch of, for example, 200 μm in the a-axis direction.

[0070] In the above-described embodiment, the semiconductor element 100 has the layer structure shown in FIG. 2, but it is not limited thereto. For example, in this embodiment, the number of electron conductive layers is two, but it may be one or three or more. For example, the semiconductor element 100 may have at least a substrate 1, an n-type clad layer 2, a doped first composition gradient layer 2-2, an n-side first optical guide layer 3, an active layer 5, a p-side optical guide layer 6, and a p-type clad layer 8.

[0071] Also, in this embodiment, the number of composition gradient layers is two, but it may be set according to the layer structure of the semiconductor element 100 and may be one or three or more. For example, in this embodiment, when there is no n-side second optical guide layer 4, the second composition gradient layer 3-2 may not be provided.

[0072] In addition, in this embodiment, the case where the n-type cladding layer 2 containing AlGaN is in contact with the n-side first optical guide layer 3 containing GaN, and the n-side first optical guide layer 3 containing GaN is in contact with the n-side second optical guide layer 4 containing InGaN has been described. However, for example, at the interface where a layer containing AlInGaN is in contact with a layer containing GaN, the composition of In or the composition of Al may be decreased toward the interface of GaN.

[0073] In addition, in this embodiment, GaN-based materials have been described. However, other material systems that generate polarization by laminating layers with different compositions, such as AlGaAs / InGaAs on GaAs, InGaAsP, AlGaAsP on InP, and GaInSb on GaSb, etc., may also be used. That is, the type of group V of the group III-V semiconductor, the type of group III, their combinations, and those with their ratios changed may be used.

[0074] In addition, in this embodiment, the case of doping with n-type impurities and compositional gradient in the n-side layer has been described. However, the p-side layer may be similarly doped with p-type impurities and compositional gradient.

[0075] In addition, in this embodiment, the chip is divided into a width of 200 μm. However, an array element in which a plurality of light-emitting elements are continuously formed may also be used.

[0076] In addition, in this embodiment, a pn junction semiconductor laser has been described. However, as long as it has a structure that conducts electrons in the stacking direction and has a potential in layers with different compositions, a pn junction semiconductor LED or a unipolar electron-conducting type quantum cascade laser may also be used.

[0077] In addition, in the above embodiment, in the manufacturing method of the semiconductor element 100, the case where the MOCVD method is used for the crystal growth method when forming the stacked structure has been described. However, for example, a growth method capable of growing a GaN-based blue-violet semiconductor laser structure, such as the molecular beam epitaxy (MBE) method or the chemical beam epitaxy (CBE) method, etc., may also be used.

Description of Symbols

[0078] 1 Semiconductor substrate 2 n-type cladding layer 2-2 First composition gradient layer 3 n-side first optical guide layer 3-2 Second composition gradient layer 4 n-side second optical guide layer 5 Active layer 6 p-side optical guide layer 7 Electron barrier layer 8 p-type cladding layer 9 p-type contact layer 10 Insulating film 11 p-electrode 12 Wiring electrode 13 Pad electrode 14 Front coat film 15 Rear coat film 16 n-electrode

Claims

1. A substrate, Two or more electron conductive layers which are sequentially stacked on the substrate, contain group III nitride semiconductors, and have different compositions from each other, A composition gradient layer which is disposed between at least some adjacent ones of the electron conductive layers and whose composition changes so as to approach the composition of the other electron conductive layer from the composition of the one electron conductive layer toward the other electron conductive layer, and the composition gradient layer contains doped n-type impurities, A semiconductor device.

2. The semiconductor device according to Claim 1, wherein as the two or more electron conductive layers, a first layer containing AlGaN, a second layer containing GaN, and a third layer containing InGaN are included, and as the composition gradient layer, a first composition gradient layer which is disposed between the first layer and the second layer, contains the doped n-type impurities, and has a decreasing Al composition from the first layer toward the second layer, a second composition gradient layer which is disposed between the second layer and the third layer, contains the doped n-type impurities, and has an increasing In composition from the second layer toward the third layer, and includes at least one of them, A semiconductor device.

3. The semiconductor device according to Claim 2, wherein The doping concentration of the n-type impurity in at least one of the first composition inclined layer and the second composition inclined layer is 2×10 １８ cm －３ or less. A semiconductor device.

4. The semiconductor device according to Claim 2, wherein The thickness of at least one of the first composition inclined layer and the second composition inclined layer is 50 nm or more, and the doping concentration of the n-type impurity is 7.5×10 １７ cm －３ or less. A semiconductor device.

5. The semiconductor device according to Claim 3, wherein when the thickness of the first composition gradient layer is d and the doping concentration of the n-type impurities is c, d and c satisfy the following formula (1) or (2), A semiconductor device.

6. Formula (1): d ≥ a1·c + b1 (5.9×10 １７ cm －３ ≤ c ≤ 6.2×10 １７ cm －３ ), a1 = -1.73×10 －１５ , b = 1122.7 Equation (2): d ≥ a2·c + b2 (6.2×10 １７ cm －３ ≤ c ≤ 2.03×10 １８ cm －３ ), a1 = -3.4×10 －１７ , b = 69.1 The semiconductor device according to Claim 3, wherein when the thickness of the first composition gradient layer is d and the doping concentration of the n-type impurities is c, d and c satisfy any one of the following formulas (3) to (5), A semiconductor device.

7. The semiconductor device according to Claim 3, wherein when the thickness of the second composition gradient layer is d and the doping concentration of the n-type impurities is c, d and c satisfy any one of the following formulas (6) to (9), Equation (3): d ≥ a1·c + b1 (7.25×10 １７ cm －３ ≤ c ≤ 7.5×10 １７ cm －３ ), a1 = -2.08×10 －１５ , b = 1608.7 Equation (4): d ≥ a2·c + b2 (7.5×10 １７ cm －３ ≤ c ≤ 1.03×10 １８ cm －３ ), a1 = -1.51×10 －１６ , b = 161.2 Equation (5): d ≥ a2·c + b2 (1.03×10 １８ cm －３ ≤ c ≤ 1.65×10 １８ cm －３ ), a1 = -1.04×10 －１８ , b = 17.2 A semiconductor device.

8. The semiconductor device according to Claim 3, wherein when the thickness of the second composition gradient layer is d and the doping concentration of the n-type impurities is c, d and c satisfy any one of the following formulas (10) to (12), A semiconductor device. Equation (6): d ≥ a1·c + b1 (6.0×10 １７ cm －３ ≤ c ≤ 7.27×10 １７ cm －３ ), a1 = -4.8×10 －１６ , b = 388.2 Equation (7): d ≥ a2·c + b2 (7.27×10 １７ cm －３ ≤ c ≤ 8.86×10 １８ cm －３ ), a1 = -1.23×10 －１６ , b = 128.2 Equation (8): d ≥ a2·c + b2 (8.86×10 １７ cm －３ ≤ c ≤ 1×10 １８ cm －３ ), a1 = -1.58×10 －１６ , b = 159.4 Equation (9): d ≥ a2 · c + b2 (1 × 10 １８ cm －３ ≤ c ≤ 1.25 × 10 １８ cm －３ ), a1 = -6 × 10 －１８ , b = 7.5

9. The semiconductor device according to any one of Claims 1 to 8, wherein it is a pn junction type semiconductor light emitting device, A semiconductor device. Equation (10): d ≥ a1·c + b1 (7.25×10 １７ cm －３ ≤ c ≤ 9.38×10 １７ cm －３ ), a1 = -3.78×10 －１６ , b = 374 Equation (11): d ≥ a2·c + b2 (9.38×10 １７ cm －３ ≤ c ≤ 1×10 １８ cm －３ ), a1 = -1.21×10 －１６ , b = 133 Equation (12): d ≥ a2·c + b2 (1×10 １８ cm －３ ≤ c ≤ 2×10 １８ cm －３ ), a1 = -6×10 －１８ , b = 18

10. The semiconductor device according to any one of Claims 1 to 8, wherein it is a semiconductor laser device, A semiconductor device.

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