JP5409210B2 - Semiconductor light emitting device - Google Patents

Description

  The present invention relates to a semiconductor light emitting device. In particular, the present invention relates to a semiconductor light emitting device that emits deep ultraviolet light.

  Since the realization of high-intensity blue light-emitting diodes (LEDs) by Shuji Nakamura, nitride semiconductors have received much attention as materials for light-emitting devices. So far, blue, green, white and other light emitting diodes have been put into practical use as traffic signals, various indicators, illumination, liquid crystal backlights and the like. Furthermore, development of semiconductor lasers using this material has progressed, and blue-violet and pure blue lasers have been developed so far, which have been put to practical use in next-generation DVD (Blu-ray) light sources and full-color displays, respectively. It is being done. As described above, the development of light-emitting devices using nitride semiconductors is steadily progressing, but the band gap of nitride semiconductor materials is 210 nm (deep ultraviolet) to 1.8 um (infrared) in terms of the wavelength of light. The potential of the material is that it is possible to realize LEDs and lasers of various wavelengths (colors) of deep ultraviolet, ultraviolet, visible light, and infrared. In particular, deep ultraviolet LEDs are required to be put to practical use for sterilization, disinfection, purification, dermatological medical treatment, and the like, and deep ultraviolet lasers are used for high density optical disk recording. Currently, there are mercury lamps and gas lasers as ultraviolet light sources, but the dangers and environmental problems of the gas used for mercury and gas lasers, the size of the equipment, power consumption, etc. are major problems, which are environmentally friendly. In addition, realization of a semiconductor light emitting device with a small size, energy saving, and long life is desired.

JP 2009-158955 A JP 2009-123969 A JP 2009-1111012 A

In order to realize a deep ultraviolet semiconductor light emitting device, the In _x Ga _1-x N currently used in the light emitting layer of the white LED is changed to Al, and the Al composition (x) is further increased. Is the simplest approach. However, when the Al composition of the AlGaN light emitting layer is increased, the band crossing of the valence band occurs, and light is emitted with the polarization of the electric field parallel to the c axis. However, there is a problem that almost no light is emitted from the element surface, and the characteristics of the light emitting element are greatly deteriorated. In addition, this valence band crossing causes oscillation with TM polarized light in a semiconductor laser, which is different from polarized light (TE polarized light) oscillated by a normal semiconductor laser.

  As a technique for solving the above problem, there is an idea of using a nonpolar substrate (a substrate having a plane orientation perpendicular to the c-axis). On a nonpolar substrate, even if the polarization from the active layer becomes an electric field vector parallel to the c-axis, light can be emitted to the surface of the element, and even a semiconductor laser can oscillate with TE polarization, convenient. However, it is very difficult to grow an AlGaN crystal on a nonpolar surface, and it is difficult to produce a high-quality active layer because it is impossible to suppress mixing such as stacking faults and dislocations. For this reason, there is a demand for a method for solving the above problem with a light emitting element on a c-plane or a plane orientation substrate close to crystal growth that is easy to grow.

  The present invention has been made in view of these problems, and an object thereof is to provide a semiconductor light emitting device having excellent light emission characteristics in the deep ultraviolet region.

  One embodiment of the present invention is a semiconductor light emitting device. The semiconductor light emitting device includes a slightly inclined substrate whose c-axis is inclined by 5 ° to 15 ° with respect to a direction perpendicular to the substrate surface, or a half whose c-axis is inclined from 40 ° to 70 ° with respect to a direction perpendicular to the substrate surface. An AlN substrate, which is a polar substrate, and a light emitting layer formed on the AlN substrate and having a quantum well structure provided between the n-type cladding layer and the p-type cladding layer, It includes a quantum well layer made of an AlGaN layer (Al (x) Ga (1-x) N: 0.75 <x ≦ 1), and the light emitting layer emits deep ultraviolet light.

  According to this aspect, a semiconductor light emitting device having excellent light emission characteristics in the deep ultraviolet region can be obtained.

  In the semiconductor light emitting device of the above aspect, the quantum well layer may have a thickness of 2 nm or less. The semiconductor light emitting device of the above aspect may be a light emitting diode.

  The semiconductor light emitting device of the above aspect may be a semiconductor laser in which polarized light in a direction in which the c-axis of the AlN substrate is projected onto the substrate surface is emitted from the light emitting end surface. In this case, the cleavage plane of the light emitting layer may be a resonator mirror.

  The semiconductor light emitting device of the above aspect further includes a pair of reflective layers sandwiching the n-type clad layer, the light-emitting layer, and the p-type clad layer, and polarized light in a direction in which the c-axis of the AlN substrate is projected onto the substrate surface. It may be a vertical cavity surface emitting laser that is emitted in a direction perpendicular to the surface.

  ADVANTAGE OF THE INVENTION According to this invention, the semiconductor light-emitting device excellent in the light emission characteristic of the deep ultraviolet region is provided.

1 is a schematic diagram illustrating a configuration of a semiconductor light emitting element according to a first embodiment. It is a schematic diagram showing how to take the coordinate axis in the (light emitting layer) quantum well on an arbitrary plane orientation substrate. It is a graph which shows the Al composition dependence of the optical matrix element of the AlGaN quantum well (well width 1.5nm) formed on the AlN substrate whose substrate surface is c plane. It is a graph which shows the Al composition dependence of the optical matrix element of the AlGaN quantum well (well width 1.5nm) formed on the AlN board | substrate with which c-axis inclined 10 degrees with respect to the direction perpendicular | vertical to a substrate surface. It is the graph which plotted the emitted light intensity from the element surface at the time of using the AlN board | substrate which changed the inclination angle of the c-axis with respect to the direction perpendicular | vertical to a substrate surface at 0 degree, 5 degrees, and 10 degrees as a function of Al composition. It is a graph which shows the polarization characteristic of the AlGaN quantum well coherently grown on the AlN board | substrate with which the c-axis inclined 58 degrees with respect to the direction perpendicular | vertical to a substrate surface. FIG. 3 is a schematic diagram illustrating a configuration of a semiconductor light emitting element 10 according to a second embodiment. FIG. 6 is a schematic diagram illustrating a configuration of a semiconductor light emitting element according to a third embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same reference numerals are given to the same components, and the description will be omitted as appropriate.

(Embodiment 1)
FIG. 1 is a schematic diagram illustrating a configuration of a semiconductor light emitting element 10 according to the first embodiment. The semiconductor light emitting device 10 of the first embodiment is a light emitting diode. The semiconductor light emitting device 10 according to the first embodiment includes a substrate 20, an n-type semiconductor layer 30, a light emitting layer 40, a p-type semiconductor layer 50, and an n-type semiconductor layer 30 that are sequentially formed on the substrate 20 by crystal growth. And the p-side electrode 70 formed on the p-type semiconductor layer 50.

  The substrate 20 is an AlN substrate, and the c-axis is inclined slightly by 5 ° to 15 ° with respect to the direction perpendicular to the substrate surface, or the c-axis is inclined from 40 ° to 70 ° with respect to the direction perpendicular to the substrate surface. Semipolar substrate. The substrate 20 can be formed using, for example, the MOCVD method. The thickness of the substrate 20 is, for example, 100 μm.

The n-type semiconductor layer 30 is an n-AlGaN cladding layer, and more specifically is formed by doping Al _0.87 Ga _0.13 N with, for example, Si as an n-type impurity. The n-type semiconductor layer 30 has a thickness of 1 μm, for example.

  On the other hand, the p-type semiconductor layer 50 is provided on the light emitting layer 40, and includes a p-GaN layer 52, a p-AlGaN cladding layer 54, and a p-AlGaN block layer 58 in this order from the p-side electrode 70 side.

  The p-GaN layer 52 is a low resistance layer for making ohmic contact with the p-side electrode 70. The p-GaN layer 52 is formed by doping, for example, Mg as a p-type impurity in GaN. The thickness of the p-GaN layer 52 is, for example, 0.1 μm.

Specifically, the p-AlGaN cladding layer 54 is formed by doping Al _0.87 Ga _0.13 N with, for example, Mg as a p-type impurity. The thickness of the p-AlGaN cladding layer 54 is, for example, 0.5 μm.

The p-AlGaN block layer 58 has a role of suppressing the outflow of electrons from the light emitting layer 40 and increasing the efficiency of recombination of electrons and holes. Specifically, the p-AlGaN block layer 58 is formed by doping, for example, Mg into Al _0.98 Ga _0.02 N as a p-type impurity. The thickness of the p-AlGaN block layer 58 is, for example, 0.05 μm.

  The light emitting layer 40 has a multiple quantum well structure (MQW) in which three AlGaN layers (quantum well layers) doped with Si and three AlGaN layers (barrier layers) are alternately stacked. Emit light by recombining.

The composition of the AlGaN layer (quantum well _{layer), (Al (x) Ga (} 1-x) N: 0.75 <x ≦ 1) is represented by, _typically, Al 0.8 _{Ga 0.2} N It is. The thickness of the AlGaN layer (quantum well layer) is preferably 2 nm or less, and typically 1.5 nm. When the thickness of the AlGaN layer is greater than 2 nm, the polarization characteristics are greatly changed, and the intensity of light emitted from the element surface may be extremely reduced.

More specifically, the AlGaN layer (barrier layer) is Al _0.9 Ga _0.1 N. The thickness of the AlGaN layer (barrier layer) is, for example, 5 nm.

  The light emitted from the light emitting layer 40 is deep ultraviolet light, and the emission wavelength is, for example, 220 nm.

  The n-side electrode 60 is provided on the n-type semiconductor layer 30 and is made of, for example, Ni / Au.

  The p-side electrode 70 is provided on the p-type semiconductor layer 50. The p-side electrode 70 is made of, for example, Ni / Au. The n-side electrode 60 and the p-side electrode 70 can be formed by a vapor deposition method.

  The light emission characteristics of the semiconductor light emitting element 10 (light emitting diode) having the layer structure described above will be described.

  In the following description, X1-polarized light, X2-polarized light, and X3-polarized light are defined as follows. That is, as shown in FIG. 2, X1 polarized light is polarized light perpendicular to the c-axis in the substrate plane. X2 polarized light is polarized light in a direction in which the c-axis is projected onto the substrate surface. X3 polarized light is polarized light in a direction perpendicular to the substrate. The “polarization direction” refers to the “direction of the electric field vector of light”.

  FIG. 3 is a graph showing the polarization characteristics of an Al (x) Ga (1-x) N quantum well having a width of 1.5 nm coherently grown on an AlN substrate having a c-plane substrate surface (hereinafter referred to as a c-plane substrate). It is. In FIG. 3, the horizontal axis represents the Al composition (x), and the vertical axis represents the optical matrix elements for the X1-polarized light, the X2-polarized light, and the X3-polarized light.

  As shown in FIG. 3, it can be seen that in the AlGaN quantum well formed on the c-plane substrate, the polarization characteristics change greatly when the Al composition is around 0.77. This is due to band crossing of the valence band, and it is known that this crossing occurs near x = 0.25 in bulk AlGaN. However, in this quantum well, x = 0. The polarization characteristic is maintained without crossing the valence band up to 77. However, when x is larger than 0.77, the valence band crossing occurs, so that the X1 polarized light and the X2 polarized light decrease rapidly.

  FIG. 4 shows a 1.5 nm width Al (coherently grown on an AlN substrate (an example of a slightly tilted substrate: hereinafter referred to as a 10 ° off-substrate) whose c-axis is tilted by 10 ° with respect to a direction perpendicular to the substrate surface. x) is a graph showing the polarization characteristics of Ga (1-x) N quantum wells. In this case, as in the case of the c-plane substrate, band crossing occurs, but it can be seen that the polarization characteristics themselves change gradually. This is due to the effect of band mixing caused by breaking the 6-fold symmetry in the substrate plane. Because of this effect, light emission with an Al composition of about 0.8 to 0.9 is emitted from the light emitting layer (active layer) on the 10 ° off substrate to some extent from the device surface, and a c-plane substrate is used. Compared with the conventional semiconductor light emitting device, a great merit can be obtained. In other words, in the semiconductor light emitting device of the first embodiment, the Al composition of the quantum well layer can be made higher, that is, the wavelength of light emitted from the light emitting layer can be made shorter. Such an effect becomes remarkable in a slightly inclined substrate in which the c-axis is inclined by 5 ° to 15 ° with respect to a direction perpendicular to the substrate surface.

  FIG. 5 is a graph plotting the light emission intensity from the element surface as a function of the Al composition when using an AlN substrate in which the inclination angle of the c-axis with respect to the direction perpendicular to the substrate surface is changed to 0 °, 5 °, and 10 °. It is. As shown in FIG. 5, it can be seen that the emission intensity when the Al composition is 0.9 is increased by about 1 digit by turning off the substrate by 10 °, which shows that a very slight substrate tilt is very effective. Yes.

  FIG. 6 shows a 1.5 nm wide Al layer (coherently grown on an AlN substrate (an example of a semipolar substrate: hereinafter referred to as a 58-degree off substrate) whose c-axis is inclined by 58 ° with respect to a direction perpendicular to the substrate surface. x) is a graph showing the polarization characteristics of Ga (1-x) N quantum wells. As shown in FIG. 6, when the light emitting layer (active layer) is formed on the 58-degree off substrate, the emission intensity of X2 polarized light gradually decreases even when the Al composition is larger than 0.77, and the element surface Sufficient luminescence is emitted from. Such an effect becomes remarkable in a semipolar substrate in which the c-axis is inclined by 40 ° to 70 ° with respect to a direction perpendicular to the substrate surface.

  The manufacturing method of the semiconductor light emitting device 10 of the first embodiment is not particularly limited, and the semiconductor light emitting device 10 can be manufactured by a well-known light emitting diode manufacturing method.

(Embodiment 2)
FIG. 7 is a schematic diagram showing the configuration of the semiconductor light emitting device 10 according to the second embodiment. The semiconductor light emitting device 10 of the second embodiment is a semiconductor laser. The semiconductor light emitting device 10 according to the second embodiment is provided on the back surface of the substrate 20, the n-type semiconductor layer 30, the light emitting layer 40, the p-type semiconductor layer 50, and the substrate 20 that are sequentially formed on the substrate 20 by crystal growth. In addition, a p-side electrode 70 formed on the n-side electrode 60 and the p-type semiconductor layer 50 is provided.

  Regarding the semiconductor light emitting device 10 of the second embodiment, the description of the same configuration as that of the semiconductor light emitting device 10 of the first embodiment will be omitted as appropriate, and the description will focus on the configuration different from the semiconductor light emitting device 10 of the first embodiment.

  The substrate 20 is a slightly inclined substrate or a semipolar substrate made of AlN as in the first embodiment.

  The n-type semiconductor layer 30 includes an n-AlN cladding layer 32 and an n-AlGaN guide layer 36 in order from the substrate 20 side.

  The n-AlN cladding layer 32 has a heterojunction with the n-AlGaN guide layer 36. Thereby, the n-AlN cladding layer 32 has a role of confining the light emitted from the light emitting layer 40 in the light emitting layer 40. Specifically, the n-AlN clad layer 32 is formed by doping AlN as an n-type impurity, for example, Si. The thickness of the n-AlN cladding layer 32 is, for example, 1 μm.

  The n-AlGaN guide layer 36 has a role of confining light from the light emitting layer 40. Specifically, the n-AlGaN guide layer 36 is formed by doping, for example, Si as an n-type impurity in AlGaN. The thickness of the n-AlGaN guide layer 36 is, for example, 0.1 μm. The n-AlGaN guide layer 36 only needs to be able to confine light from the light emitting layer 40 and does not need to contain an n-type impurity.

  On the other hand, the p-type semiconductor layer 50 is provided on the light emitting layer 40, and includes a p-GaN layer 52, a p-AlN cladding layer 54 ', and a p-AlGaN guide layer 56 in this order from the p-side electrode 70 side.

  The p-GaN layer 52 is a low resistance layer for making ohmic contact with the p-side electrode 70. The p-GaN layer 52 is formed by doping, for example, Mg as a p-type impurity in GaN. The thickness of the p-GaN layer 52 is, for example, 0.1 μm.

  The p-AlN cladding layer 54 ′ has a refractive index smaller than that of the p-AlGaN guide layer 56 and has a heterojunction with the p-AlGaN guide layer 56. Thereby, the p-AlN clad layer 54 ′ has a role of confining light emitted from the light emitting layer 40 in the light emitting layer 40. Specifically, the p-AlN cladding layer 54 ′ is formed by doping, for example, Mg as a p-type impurity in AlN. The thickness of the p-AlN cladding layer 54 ′ is 0.5 μm, for example.

The p-AlGaN guide layer 56 has a role of confining light from the light emitting layer 40. Specifically, the p-AlGaN guide layer 56 is formed by doping Al _0.9 Ga _0.1 N with, for example, Mg as a p-type impurity. The thickness of the p-AlGaN guide layer 56 is, for example, 0.1 μm. The p-AlGaN guide layer 56 only needs to be able to confine light from the light emitting layer 40 and does not need to contain a p-type impurity.

  The light emitting layer 40 has a quantum well structure similar to that of the first embodiment. Light (X2 polarized light, wavelength 220 nm) from the light emitting layer 40 is radiated in a direction orthogonal to the end surface A from the end surface A corresponding to a cross section orthogonal to the longitudinal direction of the p-side electrode 70. In the present embodiment, the end face A that becomes the resonator mirror is a cleavage plane.

  The n-side electrode 60 is electrically connected to the back surface of the substrate 20. The n-side electrode 60 is made of Al, for example.

  The p-side electrode 70 is provided on the p-type semiconductor layer 50 in a stripe shape in a direction perpendicular to the c-axis. The p-side electrode 70 is made of, for example, Pd / Au.

  In the semiconductor light emitting device 10 (semiconductor laser) having the layer structure described above, since the substrate 20 is a slightly inclined substrate or a semipolar substrate, the surface that can be cleaved crystallographically is limited, and the cleavage surface is a resonator mirror. Therefore, the stripe direction needs to be the X1 direction. In order to oscillate with TE polarized light in such a laser structure, the optical matrix element of X2 polarized light must be large, but the c-axis is inclined 5 ° to 15 ° with respect to the direction perpendicular to the substrate surface. By using a slightly tilted substrate or a semipolar substrate whose c-axis is inclined by 40 ° to 70 ° with respect to the direction perpendicular to the substrate surface, the value of the optical matrix element of X2 polarization is greatly increased. (See FIG. 4). Therefore, the semiconductor light emitting device 10 (semiconductor laser) according to the present embodiment is expected to exhibit excellent device characteristics in the deep ultraviolet region.

  The method for manufacturing the semiconductor light emitting device 10 of the second embodiment is not particularly limited, and the semiconductor light emitting device 10 can be manufactured by a known semiconductor laser manufacturing method.

(Embodiment 3)
FIG. 8 is a schematic diagram showing the configuration of the semiconductor light emitting device 10 according to the third embodiment. The semiconductor light emitting device 10 of the third embodiment is a vertical cavity surface emitting laser. The semiconductor light emitting device 10 according to the third embodiment includes a substrate 20, an n-type DBR (distributed Bragg reflector) layer 80 formed in order on the substrate 20 by crystal growth, an n-type semiconductor layer 30, a light emitting layer 40, p. An n-side electrode 60 provided on the n-type DBR, a p-side electrode 70 formed on the p-type semiconductor layer 50, and a DBR layer 90 formed on the p-side electrode 70. .

  Regarding the semiconductor light emitting device 10 of the third embodiment, description of the same configuration as that of the semiconductor light emitting device 10 of the first embodiment will be omitted as appropriate, and a description will be given focusing on a configuration different from the semiconductor light emitting device 10 of the first embodiment.

  The substrate 20 is a slightly inclined substrate or a semipolar substrate made of AlN as in the first embodiment.

The n-type DBR layer 80 has a high reflectance with respect to light emitted from the light emitting layer 40 (wavelength 220 nm), and is paired with the DBR layer 90 to form a resonator surface. In the present embodiment, the n-type DBR layer 80 has a structure in which n-AlN (thickness 25 nm) and n-Al _0.9 Ga _0.1 N (thickness 25 nm) are alternately stacked for 30 periods.

  The n-type semiconductor layer 30 is an n-AlN cladding layer, and more specifically, is formed by doping, for example, Si as an n-type impurity in AlN. The n-type semiconductor layer 30 has a thickness of 1 μm, for example.

  The light emitting layer 40 has a quantum well structure similar to that of the first embodiment. The light (X2 polarized light) from the light emitting layer 40 is radiated in a direction perpendicular to the substrate surface after resonating between the n-type DBR layer 80 and the DBR layer 90.

  The p-type semiconductor layer 50 is a p-AlN cladding layer, and is formed by doping AlN with, for example, Mg as a p-type impurity. The thickness of the p-type semiconductor layer 50 is, for example, 0.5 μm.

  The n-side electrode 60 is provided on the n-type DBR layer 80 and is made of, for example, Al.

  The p-side electrode 70 is provided on the p-type semiconductor layer 50. The p-side electrode 70 is formed of, for example, an ITO transparent conductive film.

The DBR layer 90 is provided on the p-side electrode 70. In the present embodiment, the DBR layer 90 has a 10-period stacked structure in which SiO ₂ (thickness 35 nm) and ZrO ₂ (thickness 35 nm) are alternately arranged.

  In the semiconductor light emitting device 10 (vertical cavity surface emitting laser) having the layer structure described above, the substrate 20 is a slightly inclined substrate whose c-axis is inclined from 5 ° to 15 ° with respect to the direction perpendicular to the substrate surface, or the substrate surface By using a semipolar substrate whose c-axis is tilted from 40 ° to 70 ° with respect to the direction perpendicular to the vertical axis, the value of the optical matrix element of the X2 polarized light increases dramatically, and the vertical cavity surface emission that emits deep ultraviolet light It is expected to exhibit excellent device characteristics as a laser.

  The method for manufacturing the semiconductor light emitting device 10 of the third embodiment is not particularly limited, and the semiconductor light emitting device 10 can be manufactured by a well-known method for manufacturing a vertical cavity surface emitting laser.

  The semiconductor light emitting device 10 only needs to have the substrate 20, the n-type cladding layer, the light emitting layer 40, and the p-type cladding layer described above, and can be applied to applications other than light emitting diodes, semiconductor lasers, and vertical cavity surface emitting lasers. It is.

DESCRIPTION OF SYMBOLS 10 Semiconductor light emitting element, 20 board | substrate, 30 n type semiconductor layer, 40 light emitting layer, 50 p type semiconductor layer, 60 n side electrode, 70 p side electrode, 80 n type DBR layer, 90 DBR layer

Claims (5)

An AlN substrate that is a slightly inclined substrate in which the c-axis is inclined by 5 ° to 15 ° with respect to a direction perpendicular to the substrate surface;
A light emitting layer formed on the AlN substrate and having a quantum well structure provided between an n-type cladding layer and a p-type cladding layer;
With
The quantum well structure includes an AlGaN layer (Al (x) Ga (1-x) N: 0.75 <x ≦ 1), and includes a quantum well layer having a thickness of 2 nm or less ,
The light emitting layer emits deep ultraviolet light, and is a semiconductor light emitting element. The semiconductor light emitting device according to claim 1, wherein the semiconductor light emitting device is a light emitting diode. The semiconductor light emitting element according to claim 1, wherein the semiconductor light emitting element is a semiconductor laser in which polarized light in a direction in which the c-axis of the AlN substrate is projected onto the substrate surface is emitted from the light emitting end face. 4. The semiconductor light emitting device according to claim 3 , wherein the cleavage plane of the light emitting layer is a resonator mirror. A pair of reflective layers sandwiching the n-type cladding layer, the light emitting layer, and the p-type cladding layer;
2. The semiconductor light emitting element according to claim 1 , which is a vertical cavity surface emitting laser in which polarized light in a direction in which the c-axis of the AlN substrate is projected onto the substrate surface is emitted in a direction perpendicular to the substrate surface.

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