Optical device

Abstract

A p-type structure (107) is configured of a first structure (107a) and a second structure (107b) formed on an active layer (105). The first structure (107a) is configured by stacking one to six first compositional gradient layers comprising AlxGa1−xN. The main surfaces of the first compositional gradient layers are polar surfaces or semipolar surfaces. Furthermore, in each of the first compositional gradient layers, the composition x of Al decreases in the positive c-axis direction. The second structure (107b) is configured of one second compositional gradient layer comprising p-type AlyGa1−yN. The main surface of the second compositional gradient layer is a polar surface or a semipolar surface. In the second compositional gradient layer, the composition y of Al decreases in the positive c-axis direction.

Description

Optical Devices

[0001] The present invention relates to optical devices.

[0002] Nitride semiconductors, such as AlN, GaN, InN, and their alloys, are characterized by their large band gap Eg and direct transition type semiconductor materials. For this reason, these nitride semiconductors have attracted attention as materials for semiconductor light-emitting devices such as LEDs (light-emitting diodes) and LDs (laser diodes), which can emit light over a wide wavelength range from ultraviolet to red.

[0003] In recent years, various research institutes have been vigorously researching and developing semiconductor light-emitting devices that use nitride semiconductors and emit light in the deep ultraviolet region (wavelength: 200 nm to 350 nm) by shortening the emission wavelength.Light-emitting devices that emit light in the deep ultraviolet region are expected to be applied in a very wide range of fields, including virus inactivation, sterilization, water purification, microfabrication, various medical fields, and high-speed decomposition treatment of pollutants.

[0004] Recently, a technology for oscillating a current injection type LD in the ultraviolet region using a nitride semiconductor has been disclosed (Non-Patent Document 1). The LD in Non-Patent Document 1 has an AlN substrate on which Si is doped as an n-type layer. 0.7 Ga 0.3 An N layer (350 nm thick) was used, and a p-type layer was formed by varying the Al composition x from 1 to 0.7. x Ga 1-x An N layer (thickness: 320 nm) is used.

[0005] Since p-type nitride semiconductors have a deep acceptor level, it is difficult to fabricate a low-resistance p-type semiconductor layer with a high hole concentration at room temperature. For this reason, it is necessary to fabricate a p-type nitride semiconductor with a low resistance, particularly a p-type nitride semiconductor with a compositionally graded Al layer having a large band gap. x Ga 1-x The realization of a low-resistance p-type layer in N will greatly contribute to the development of ultraviolet light-emitting devices. x Ga 1-x In N-type LDs, the p-type layer is designed to be thicker than that of a typical ultraviolet LED, with a thickness of 200 nm or more, in order to improve light confinement.

[0006] However, as shown in Non-Patent Document 2, the p-type layer has a composition gradient of Al. x Ga 1-x When an N layer is applied, the hole concentration is x Ga 1-x Since the hole concentration is inversely proportional to the thickness of the N-type layer, the hole concentration decreases as the thickness of the p-type layer increases, resulting in a decrease in hole injection efficiency and an increase in the laser oscillation threshold current density.

[0007] Z. Zhang. et al., "A 271.8 nm deep-ultraviolet laser diode for room temperature operation", Applied Physics Express, vol. 12, 124003, 2019. J. Simon et al., "Polarization-Induced Hole Doping in Wide-Band-Gap Uniaxial Semiconductor Heterostructures", Science, vol. 327, no. 5961, pp. 60-64, 2010.

[0008] As mentioned above, conventionally, compositionally graded Al x Ga 1-x When the N layer is applied to the p-type layer of an ultraviolet LD, the composition gradient Al x Ga 1-x As the N layer becomes thicker, the efficiency of hole injection decreases, and the threshold current density of laser oscillation increases.

[0009] The present invention has been made to solve the above problems, and provides a compositionally graded Al x Ga 1-x The purpose is to suppress the decrease in hole injection efficiency that accompanies thickening of the N layer.

[0010] The optical device according to the present invention comprises an n-type structure made of an n-type nitride semiconductor and formed on a substrate, an active layer made of a nitride semiconductor and formed on the n-type structure, and a p-type structure formed on the active layer, the p-type structure having a first structure and a second structure formed on the active layer, the first structure being Al x Ga1-x The first composition gradient layer is made of N and has a polar or semi-polar main surface. The first composition gradient layer is made of 1 to 6 layers stacked together. The second structure is made of p-type Al y Ga 1-y The first composition gradient layer has an Al composition x that decreases toward the +c-axis direction, and the second composition gradient layer has an Al composition y that decreases toward the +c-axis direction.

[0011] As described above, according to the present invention, the p-type structure is x Ga 1-x A first structure in which 1 to 6 first composition gradient layers made of N are stacked, and a p-type Al y Ga 1-y The second structure is made up of a single second compositionally graded layer made of N. x Ga 1-x This can suppress the decrease in hole injection efficiency that occurs when the N layer is made thicker.

[0012] 1 is a cross-sectional view showing the structure of an optical device according to an embodiment of the present invention, and FIG. 2 is an explanatory diagram illustrating the formation of negative polarization charges in a composition-graded AlGaN layer.

[0013] An optical device according to an embodiment of the present invention will be described below with reference to FIG. 1 . This optical device can be, for example, a light-emitting diode (LED). Furthermore, by combining a resonator, it can be made into a laser diode (LD). This optical device includes a substrate 101, a buffer layer 102, an n-type structure 103, a first guide layer 104, an active layer 105, a second guide layer 106, a p-type structure 107, a first contact layer 108, and a second contact layer 109.

[0014] For example, a buffer layer 102 made of AlN is formed on a substrate 101 made of AlN, and an n-type structure 103, a first guide layer 104, an active layer 105, a second guide layer 106, a p-type structure 107, and a contact layer 108 are sequentially stacked on top of this. Each of these layers can be formed by crystal growth in the c-axis direction on the substrate 101 whose main surface is the (0001) plane (C-plane). The substrate 101 can be, for example, a SiC substrate, a sapphire substrate, a GaN substrate, a MgAlO substrate, a SiO substrate, a MgO substrate, a ZnO substrate, a NdGaO substrate, a ScAlMgO substrate, a ZnS substrate, a GaAs substrate, or a Si substrate.

[0015] The n-type structure 103 is made of an n-type nitride semiconductor, and may include, for example, an AlN layer doped with Si and having a thickness of 1.3 nm, and an AlN layer doped with Si and having a thickness of 3.0 nm. 0.71 Ga 0.29 The periodic structure (number of periods: 349) is composed of N layers as a unit. The Si doping amount is 1.5×10 19 cm -3 In an LD that emits light in the ultraviolet region, it is preferable to thicken the n-type structure 103 to reduce the device resistance and achieve oscillation characteristics with a lower threshold current density in order to prevent deterioration of device performance due to heat. By using the periodic structure described above, the n-type structure 103 can be thickened without causing lattice relaxation.

[0016] The first guide layer 104 and the second guide layer 106 are made of undoped Al. 0.52 Ga 0.48 The active layer 105 may be made of nitride semiconductor such as AlGaN and have a thickness of 50 nm. For example, the active layer 105 may be made of AlGaN with a predetermined composition ratio. Alternatively, the active layer 105 may have a multiple quantum well structure (2MQWs) including two well layers made of AlGaN. The active layer 105 is sandwiched between a first guide layer 104 and a second guide layer 106.

[0017] The p-type structure 107 is composed of a first structure 107a and a second structure 107b formed on the active layer 105. In this example, when viewed from the active layer 105 side, the second structure 107b is formed on the first structure 107a.

[0018] The first structure 107a is Al x Ga 1-x The semiconductor device is configured by stacking one to six first composition gradient layers made of N. The first composition gradient layers have a polar or semi-polar main surface. Furthermore, in each of the first composition gradient layers, the Al composition x decreases along the +c-axis direction. The first composition gradient layers may have an Al composition x varying in the range of 0.4 to 1. Furthermore, the first composition gradient layers may have an Al composition x varying in the range of 0.7 to 1. Furthermore, each of the first composition gradient layers may have a thickness of 4 to 62 nm. In the first composition gradient layers, holes are induced by the formation of negative polarization charges, resulting in a p-type conductivity without doping.

[0019] The second structure 107b is p-type Al y Ga 1-y The second composition gradient layer is composed of one layer of N. The second composition gradient layer has a major surface that is a polar or semi-polar plane. The second composition gradient layer has an Al composition y that decreases in the +c-axis direction. For example, the second composition gradient layer may have an Al composition y that varies in the range of 0.4 to 1. Furthermore, the second composition gradient layer may have an Al composition y that varies in the range of 0.7 to 1. The second composition gradient layer may have a thickness of 120 to 670 nm.

[0020] For example, the first contact layer 108 may be a compositionally graded Al doped with Mg. x Ga 1-x For example, the first contact layer 108 has an Al composition x that linearly changes (graded) from 0.7 to 0 in a direction approaching the second contact layer 109. For example, the second contact layer 109 can be made of GaN doped with Mg.

[0021] Furthermore, this optical device is formed into a columnar element shape from the middle of the n-type structure 103 in the thickness direction, and an n-type electrode 111 is formed on the upper surface of the n-type structure 103 on the side of this element shape. The n-type electrode 111 may be, for example, a V-based electrode. Furthermore, a p-type electrode 112 is formed on the second contact layer 109. The p-type electrode 112 may be, for example, made of Ni / Pt / Au.

[0022] According to the optical device of the above-described embodiment, the p-type structure 107 has the first structure 107a, which makes it possible to improve the hole injection efficiency in the p-type structure 107 and reduce the laser oscillation threshold current density of the ultraviolet LD.

[0023] Here, the formation of negative polarization charge in a compositionally graded AlGaN layer will be described with reference to FIG. 2. Below, we calculated the polarization charge of a compositionally graded AlGaN layer formed on an AlN substrate with a (0001) plane as the main surface orientation, with an AlN buffer layer interposed between them. In this calculation, the compositionally graded AlGaN layer was set to a thickness of 200 nm or more, and the Al composition decreased with increasing distance from the buffer layer (in the +c-axis direction). Furthermore, n unit cells were set in the thickness direction.

[0024] Figure 1(a) shows the state of the i-th and (i+1)-th unit cells. Positive and negative polarization charges are generated in the unit cells. The effective polarization charge at the interface between the i-th unit cell and the (i+1)-th unit cell can be expressed by the following equation:

[0025]

[0026] In the above formula, P sp is the spontaneous polarization, P pe denotes the piezoelectric polarization, and q denotes the elementary charge.

[0027] At the interface between the i-th unit cell and the (i+1)-th unit cell, the effective polarization charge becomes negative. Therefore, negative polarization charge occurs in the compositionally graded AlGaN layer as shown in Figure 1(c). As shown in Non-Patent Document 2, the negative polarization charge density of a compositionally graded AlGaN layer decreases in inverse proportion to the layer thickness. Therefore, in a compositionally graded AlGaN layer, the negative polarization charge density decreases as the layer thickness increases, and the hole concentration induced in the compositionally graded AlGaN layer decreases.

[0028] On the other hand, in the embodiment, Al having a layer thickness of 4 to 62 nm x Ga 1-x A first structure 107a is formed by stacking 1 to 6 first composition gradient layers N made of N, and a p-type Al y Ga 1-y The p-type structure 107 is formed by the second structure 107b, which is a single second composition gradient layer made of N. By providing the first structure 107a in the p-type structure 107, the aforementioned composition gradient Al x Ga 1-x Compared to the case of an N layer, the negative polarization charge density can be increased, and the hole concentration in the p-type structure 107 can be increased.

[0029] When the optical device is an ultraviolet LD, the p-type structure 107 needs to have a thickness of 200 nm or more from the viewpoint of light confinement in the active layer 105. However, according to the embodiment, the p-type structure 107 can increase the hole injection efficiency even in a thick film of 200 nm or more, and can reduce the laser oscillation threshold current density of the ultraviolet LD.

[0030] The first structure 107a can increase the hole concentration without doping with an acceptor impurity, but can be doped with an acceptor impurity. The acceptor impurity is generally Mg, but is not limited to this. Acceptor impurities such as Zn, Be, Ca, Sr, and Ba can also be doped. The acceptor impurity concentration is 2×10 20 cm -3 The concentration of the acceptor impurity is preferably 2×10 or less. 20 cm -3If the concentration of the acceptor impurities exceeds 100%, the hole concentration may be significantly reduced due to a self-compensation effect, resulting in high resistance. The concentration of the acceptor impurities can be confirmed by secondary ion mass spectrometry (SIMS).

[0031] In the first structure 107a, positive polarization charges are generated at the stacking interface of the stacked first composition gradient layer, which may cause discontinuity in the hole distribution at this interface, resulting in potential depletion. The delta-doping of the aforementioned acceptor impurities can be considered as a method for compensating for the depletion at the stacking interface. By delta-doping the acceptor impurity, for example, Mg, at the stacking interface, the valence band barrier can be lowered.

[0032] The p-type structure 107 is preferably a single crystal structure throughout, but may have polycrystalline or amorphous regions distributed in parts to the extent that the various properties of a crystalline nitride semiconductor are not lost.

[0033] The thickness of the first composition gradient layer constituting the first structure 107a is preferably 4 to 62 nm. If the thickness of the first composition gradient layer is less than 4 nm, the three-dimensional hole effect characteristic of the composition gradient layer may not be fully obtained, and the shape may approach two-dimensional holes, which may result in a decrease in hole injection efficiency. Furthermore, if the thickness of the first composition gradient layer exceeds 62 nm, the hole concentration of the first composition gradient layer may decrease. The thickness of the first composition gradient layer is more preferably 7 to 56 nm, and even more preferably 12 to 51 nm.

[0034] Furthermore, the total thickness of the p-type structure 107 is preferably 200 nm or more and 780 nm or less. If the total thickness of the p-type structure 107 is less than 200 nm, the optical confinement coefficient of the active layer 105 in the p-type structure 107 decreases, and the laser oscillation threshold current density may increase. If the total thickness of the p-type structure 107 exceeds 780 nm, the resistance of the p-type structure 107 increases, and when the optical device is used as an ultraviolet LD, the element may be destroyed by heat generation. The total thickness of the p-type structure 107 is more preferably 230 nm or more and 670 nm or less, and even more preferably 270 nm or more and 510 nm or less.

[0035] In p-type structure 107, the number of layers of the first compositionally graded layer in first structure 107a is preferably 1 to 6. If the number of layers of first structure 107a is 0, p-type structure 107 is composed only of second structure 107b, resulting in a single compositionally graded layer. Therefore, if the p-type structure is made thick, the layer thickness of second structure 107b increases, which may result in a decrease in hole concentration.

[0036] If the number of layers of the first composition gradient layer in the first structure 107a exceeds six, the number of divisions of the first structure 107a increases, which increases the number of potential barriers in each first composition gradient layer, and this may result in a decrease in the hole injection efficiency in the optical device. The number of layers of the first composition gradient layer in the first structure 107a is more preferably 1 to 5 layers, and even more preferably 1 to 3 layers.

[0037] To turn this optical device into an ultraviolet LD with a laser oscillation wavelength of 300 nm or less, the first compositionally graded layer constituting the first structure 107a and the second compositionally graded layer constituting the second structure 107b each can have an Al composition gradient (change) in the range of x = 1 to 0.40. Note that if x is less than 0.40, the optical confinement coefficient in the active layer 105 decreases, and the laser oscillation threshold current density may increase. A more preferable Al composition gradient can be in the range of x = 1 to 0.62, and an even more preferable Al composition gradient can be in the range of x = 1 to 0.7.

[0038] The cross-sectional structure of the p-type structure 107 can be confirmed using a transmission electron microscope (TEM). A TEM allows for the observation of the sample's microstructure by irradiating a thinly processed sample with an electron beam and imaging the transmitted or diffracted electron beam. High-magnification observations allow for the observation of the arrangement of atoms in the crystal (lattice fringes) (lattice image). Polycrystals and crystal defects can also be observed. Furthermore, electron diffraction patterns can be used to identify materials and analyze their crystalline state. The elemental distribution can be determined by elemental analysis using energy dispersive X-ray spectroscopy (EDS). EDS measurements are performed using an EDS measurement device attached to the STEM device. SIMS is another elemental analysis method.

[0039] The hole concentration in the p-type structure 107 can be measured by a Hall effect measurement method. Details will be described later. When the hole concentration in the p-type structure 107 is 4.0×10 16 cm -3 As described above, when the optical device according to the embodiment is used as an ultraviolet LD, the oscillation threshold current density of the laser can be reduced.

[0040] Each layer of the p-type structure 107 can be formed by epitaxial growth. For example, MOVPE (metal organic vapor phase epitaxy) is preferably used as the epitaxial growth method. The epitaxial growth method is not limited to MOVPE, and other methods such as HVPE (hydride vapor phase epitaxy), MBE (molecular beam epitaxy), and sputtering may also be used.

[0041] In fabricating the p-type structure 107, the first compositionally graded layer constituting the first structure 107a and the second compositionally graded layer constituting the second structure 107b can be stacked by epitaxial growth. When an MOVPE apparatus is used as the epitaxial growth apparatus, trimethylaluminum (TMAl) can be used as the source gas for Al. Trimethylgallium (TMGa) can be used as the source gas for Ga. NH can be used as the source gas for N. Biscyclopentadienylmagnesium (CpMg) can be used as the source gas for Mg, an impurity contributing to p-type conductivity. Examples of carrier gases that can be used include H gas, N gas, and a mixture of H gas and N gas. The source gases are not particularly limited, and examples include triethylgallium (TEGa) as the source gas for Ga and a hydrazine derivative as the source gas for N.

[0042] Annealing can be performed to activate the p-type impurity. As an annealing device for performing the annealing, for example, a lamp annealing device, an electric furnace annealing device, etc. can be used. The p-type impurity means an acceptor impurity, such as Mg.

[0043] The first compositionally graded layer constituting the first structure 107a and the second compositionally graded layer constituting the second structure 107b can have a major surface that is the (0001) plane, the (000-1) plane, or the (1-102) plane, respectively. For example, when growing with the major surface being the (000-1) plane, increasing the Al composition x in the crystal growth direction can induce negative polarization charges in the first compositionally graded layer, generating holes, for example.

[0044] In the above description, the p-type structure 107 is made of AlGaN. x Ga 1-x N and In x Al 1-x N and In x Al y Ga 1-x-y N or a combination thereof.

[0045] According to the embodiment, the p-type structure 107 has the first structure 107a, and high hole injection efficiency can be obtained, so that it can be suitably used in ultraviolet LDs with an emission wavelength of 300 nm or less.

[0046] The results of the devices actually fabricated will be described below.

[0047] First, an AlN buffer layer was grown on a semi-insulating 4H-SiC (0001) substrate by MOVPE, and then a 20-nm-thick undoped compositionally graded AlN layer was grown on top of the AlN buffer layer as a p-type structure. x Ga 1-x The first structure is a two-layer structure of N (Al composition x is linearly graded from 1 to 0.7 in the +c-axis direction) grown on a 280 nm thick undoped compositionally graded Al x Ga 1-x A second structure was then stacked on the p-type structure, in which one layer of GaN (Al composition x is linearly graded from 1 to 0.7 in the +c-axis direction) was grown. Furthermore, a Mg-doped GaN (Mg concentration 3.0×10) was grown on the p-type structure as a contact layer. 19 cm -3 ) was grown to 10 nm.

[0048] After fabrication, the sample was annealed at 750°C for 10 minutes in a lamp annealing system with N2 flow. The sample was diced into a shape of approximately 5 mm x 5 mm, and electrodes made of Pd / Au laminates were formed at the four corners of the sample by electron beam evaporation. The contact layer other than the electrodes was removed using an inductively coupled plasma (ICP) dry etching system to form a Hall element.

[0049] The hole concentration of the fabricated Hall element (sample) was measured using the van der Pauw method and found to be 1.3 × 10 17 cm -3 The Hall effect measuring device and its measurement conditions are shown below.

[0050] Hall effect measuring device: "Resi Test 8300" manufactured by Toyo Corporation. Measurement conditions: room temperature (approximately 25°C), approximately 0.25 [T], approximately 10 -4 ~10 -9 [A] AC magnetic field Hall measurement. Sample size: approximately 5 mm x 5 mm.

[0051] Next, the results of actually fabricating the ultraviolet LD described with reference to Figure 1 will be described. [Examples 1 to 8] The substrate 101 was an AlN substrate with a (0001) main surface. The buffer layer 102 was made of AlN. The n-type structure 103 was made of a 1.3 nm thick Si-doped AlN layer and a 3.0 nm thick Si-doped AlN layer. 0.71 Ga 0.29 The periodic structure (number of periods: 349) was composed of N layers as units. The Si doping amount was 1.5 × 10 19 cm -3 It was decided.

[0052] The first guide layer 104 and the second guide layer 106 are made of undoped Al. 0.52 Ga 0.48 The active layer 105 was made of N and had a thickness of 50 nm. The active layer 105 sandwiched between the first guide layer 104 and the second guide layer 106 had a multiple quantum well structure (2MQWs) including two well layers made of AlGaN.

[0053] Al of the first structure 107a x Ga 1-xThe first composition gradient layer made of N has an Al composition x that is linearly graded from 0.7 to 0 in the +c-axis direction. y Ga 1-y In the second composition gradient layer made of N, the Al composition x was linearly graded from 0.7 to 0 in the +c-axis direction.

[0054] The first contact layer 108 is a compositionally graded Al doped with Mg. x Ga 1-x The Al composition x was linearly graded from 0.7 to 0 with respect to the +c-axis direction. The second contact layer 109 was composed of Mg-doped GaN.

[0055] From X-ray reciprocal lattice mapping in the asymmetric plane (-1-124), it was confirmed that the periodic structure of the n-type structure 103 grew pseudomorphically relative to the buffer layer 102 .

[0056] A mask was formed on the second contact layer 109 and dry etching was performed to form an element shape, exposing a part of the n-type structure 103. A V-based electrode was used for the n-type electrode 111 on the n-type structure 103, and Ni / Pt / Au was used for the p-type electrode 112 on the second contact layer 109.

[0057] In Example 1, the active layer 105 is made of Al as a multiple quantum well. 0.4 Ga 0.6 N (3 nm) / Al 0.52 Ga 0.48 The p-type structure 107 was made of an undoped compositionally graded AlN (6 nm) layer with a thickness of 20 nm, which had the same structure as the Hall element (sample) fabricated for measuring the Hall effect. x Ga 1-x The first structure 107a is a two-layer structure of N (Al composition x is linearly graded from 1 to 0.7 in the +c-axis direction) grown on the substrate, and the second structure 107b is a 280 nm thick undoped compositionally graded Al x Ga 1-x The second structure 107b, in which one layer of N (Al composition x is linearly graded from 1 to 0.7 in the +c-axis direction) is grown, is stacked in order, and the first structure 107a is placed near the active layer 105.

[0058] The substrate 101 was divided into stripes by cleaving in the <11-20> direction, forming individual LDs with m-plane mirror end faces. The contact area of ​​each electrode was 4 μm × 520 μm, and the current density was calculated by dividing the current by the contact area between the p-type electrode 112 and the second contact layer 109. The electrical characteristics of the LD were investigated using room-temperature pulse driving (current pulse width 50 ns, period 500 μs). Furthermore, the emission spectrum from one mirror surface was measured using a spectrometer.

[0059] Injection current density 13.2kA / cm 2 A sharp increase in the emission intensity was observed at this current density, and a steep spectrum characteristic of laser oscillation was confirmed from the spontaneous emission light. The laser oscillation peak wavelength was 282.7 nm, the peak width (FWHM) was 0.43 nm, and the oscillation threshold current density was 13.2 kA / cm. 2 The element resistance was 11.9 Ω.

[0060] In Examples 2 to 8, LDs were fabricated and evaluated by changing the MQWs structure of the active layer 105, the structure of the p-type structure 107, and the contact area between the p-type electrode 112 and the second contact layer 109 as shown in Tables 1 to 4. The other structures were the same as in Example 1. As in Example 1, the first structure 107a was not doped with acceptor impurities. When the LDs were evaluated using room temperature pulse driving (current pulse width 50 ns, period 500 μs), a steep spectrum characteristic of laser oscillation was observed. Tables 1 to 4 show the results of the laser oscillation peak wavelength, peak width (FWHM), oscillation threshold current density, and device resistance.

[0061]

[0062]

[0063]

[0064]

[0065] Next, comparative examples will be described. In comparative example 1, undoped Al was used as a p-type structure. x Ga 1-xA structure similar to that of Example 1 was grown by MOVPE, except that one N-based compositionally graded layer (Al composition x linearly graded from 1 to 0.7 in the +c-axis direction) was used. A mask was formed on the second contact layer, and dry etching was performed to form the device shape, exposing a portion of the n-type structure. As in Example 1, a V-based electrode was used as the n-type electrode connected to the n-type structure, and Ni / Pt / Au was used as the p-type electrode connected to the second contact layer.

[0066] The substrate was divided into stripes by cleaving in the <11-20> direction, forming individual LDs with m-plane mirror end faces. The contact area of ​​the p-type electrode with the second contact layer was 4 μm × 520 μm. The electrical characteristics of the LDs were investigated using room-temperature pulse drive (current pulse width 50 ns, period 500 μs), and the emission spectrum from one mirror surface was measured using a spectrometer.

[0067] Injection current density 25.1kA / cm 2 A sharp increase in the emission intensity was observed at this current density, and a steep spectrum characteristic of laser oscillation was confirmed from the spontaneous emission light. The laser oscillation peak wavelength was 283.1 nm, the peak width (FWHM) was 0.45 nm, and the oscillation threshold current density was 25.1 kA / cm. 2 The element resistance was 16.7 Ω, and it was confirmed that the oscillation threshold current density was significantly increased compared to Example 1. Furthermore, the Hall effect of the p-type structure of Comparative Example 1 was measured in the same manner as the Hall element (sample) described above, and the hole concentration was 1.1 × 10 16 cm -3 It was confirmed that the hole concentration was lower than that of the p-type structure 107 having the first structure 107a and the second structure 107b.

[0068] In Comparative Examples 2 and 3, ultraviolet LDs were fabricated by changing the MQWs structure constituting the active layer, the structure of the p-type layer, and the contact area of ​​the p-type electrode connected to the second contact layer from Comparative Example 1 as shown in Table 5. The other structures were the same as in Comparative Example 1. As in Comparative Example 1, no acceptor impurities were doped into the p-type structure. As shown in Table 5, the oscillation threshold current density was 25 kA / cm 2 As described above, the oscillation threshold current density was significantly increased compared to the example.

[0069]

[0070] As described above, according to the embodiment of the present invention, the p-type structure is x Ga 1-x A first structure in which 1 to 6 first composition gradient layers made of N are stacked, and a p-type Al y Ga 1-y The second structure is made up of a single second compositionally graded layer made of N. x Ga 1-x This makes it possible to suppress the decrease in hole injection efficiency that occurs when the N layer is made thicker.

[0071] According to the embodiment of the present invention, the threshold current density of laser oscillation can be reduced even if the p-type structure is thickened, and therefore the present invention can be suitably used for ultraviolet LDs.

[0072] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0073] 101...substrate, 102...buffer layer, 103...n-type structure, 104...first guide layer, 105...active layer, 106...second guide layer, 107...p-type structure, 107a...first structure, 107b...second structure, 108...first contact layer, 109...second contact layer, 111...n-type electrode, 112...p-type electrode.

Claims

1. An n-type structure made of an n-type nitride semiconductor and formed on a substrate; an active layer made of a nitride semiconductor and formed on the n-type structure; and a p-type structure formed on the active layer, wherein the p-type structure has a first structure and a second structure formed on the active layer, and the first structure is Al x Ga 1-x The first composition gradient layer is made of N and has a polar or semi-polar main surface, and is configured by stacking 1 to 6 layers, and the second structure is made of p-type Al y Ga 1-y An optical device comprising a single second composition gradient layer made of N and having a main surface that is a polar or semi-polar plane, wherein the Al composition x of the first composition gradient layer decreases toward the +c-axis direction, and the Al composition y of the second composition gradient layer decreases toward the +c-axis direction.

2. An optical device according to claim 1, wherein the Al composition x of said first composition gradient layer varies in the range of 0.4 to 1, and the Al composition y of said second composition gradient layer varies in the range of 0.4 to 1.

3. An optical device according to claim 2, wherein the Al composition x of said first composition gradient layer varies in the range of 0.7 to 1, and the Al composition y of said second composition gradient layer varies in the range of 0.7 to 1.

4. An optical device according to claim 1, wherein said first composition gradient layer has a thickness of 4 to 62 nm, and said second composition gradient layer has a thickness of 120 to 670 nm.

5. An optical device according to claim 1, wherein said second structure is formed on said first structure.

6. An optical device according to any one of claims 1 to 5, comprising a first guide layer and a second guide layer made of an undoped nitride semiconductor and sandwiching the active layer therebetween.

7. An optical device according to claim 6, wherein the first guide layer and the second guide layer are made of AlGaN.

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