JP2010027924A - Group iii nitride light-emitting diode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a group III light-emitting diode having a quantum well structure capable of emitting a polarized light. <P>SOLUTION: In a light-emitting diode 11, a main surface 13a of a substrate 13 is tilted from c-plane in a-axis direction or m-axis direction of a group III nitride at an angle of not less than 10°and not more than 60°. The main surface 13a is semi-polar plane. A lattice constant D<SB>B</SB>of a barrier layer 23 can be made smaller than a lattice constant D<SB>W</SB>of an InAlGaN well layer 21 because the barrier layer 23 consists of GaN, etc. A large compression strain can be applied to the InAlGaN well layer 21. The compression strain is effective in enhancing polarization degree. A quantum containment can be reduced by adjusting a thickness of the InAlGaN well layer in a multiple quantum well structure of an active layer. Strong quantum containment is not exerted due to a thick well because the thickness of the InAlGaN well layer is more than 5 nm. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a group III nitride light emitting diode.

Non-Patent Document 1 shows a theoretical calculation for the degree of polarization. Non-Patent Documents 2 to 4 describe light-emitting diodes manufactured on a semipolar surface.
Japanese Journal of Applied Physics, vol. 46, No.33, 2007, pp.L789 Applied Physics Letters, vol. 89, 2006, pp.211907 Applied Physics Letters, vol. 87, 2005, pp.231110 Electronics Letters 31st January 2008 Vol. 44 No.3

  Non-Patent Document 1 relates to polarization in GaN strained quantum well structures on semipolar and nonpolar substrates. In a gallium nitride-based strained quantum well structure provided on a GaN substrate with an off angle of 70 ° or less, the negative polarization degree increases as the quantum confinement increases. Further, when the strain of the quantum well structure increases, the degree of positive polarization increases. In a gallium nitride-based strained quantum well structure on an off-substrate with an off angle of 70 ° or more and 90 ° or less, the positive degree of polarization increases when the quantum confinement in the quantum well structure increases, and the strain becomes negative when the strain in the quantum well structure increases. The degree of polarization increases.

  There is a need in the field of display devices for light emitting diodes that generate light of a large degree of polarization. Currently available GaN-based light emitting diodes are fabricated on c-plane GaN substrates, and light from this GaN-based light emitting diode exhibits random polarization. On the other hand, a GaN-based light emitting diode can also be manufactured using a semipolar plane GaN substrate and a nonpolar GaN substrate. These light emitting diodes can generate polarized light rather than randomly polarized light.

  Non-Patent Document 1 does not show a specific structure related to a light emitting diode. On the other hand, in Non-Patent Documents 2 to 4, InGaN / GaN-based light-emitting diodes are fabricated on a semipolar plane GaN substrate, and Non-Patent Documents 2 and 3 describe measured values of polarization.

  The degree of polarization is related to the quantum confinement property and the strain in the quantum well structure, the quantum confinement property is related to the band gap difference between the well layer and the barrier layer, and the strain is related to the lattice constant difference between the well layer and the barrier layer. is doing. Since quantum confinement is related to quantum efficiency, quantum well structures are configured to enhance quantum confinement, and well layer strain is related to reliability, so quantum well structures are formed to avoid increasing strain. Is done. For this reason, in the InGaN / GaN quantum well structure, adjusting the band gap difference and the lattice constant difference for the degree of polarization and satisfying both the fabrication of the quantum well structure to obtain the desired light emission characteristics It's not easy.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a group III nitride light-emitting diode having a quantum well structure capable of generating polarized light.

  According to one aspect of the present invention, a group III nitride light-emitting diode is a group III nitride light-emitting diode that generates polarized light having a predetermined peak wavelength, and comprises (a) a hexagonal group III nitride. And (b) an n-type gallium nitride based semiconductor layer provided on the substrate, (c) the substrate having a principal surface inclined from the c-plane at an angle of 10 degrees to 60 degrees in a predetermined direction. A p-type gallium nitride semiconductor layer provided on the substrate; and (d) an active layer provided between the n-type gallium nitride semiconductor layer and the p-type gallium nitride semiconductor layer. The active layer includes a well layer made of a gallium nitride based semiconductor mixed crystal containing Al element, Ga element and In element as a group III constituent element, and a barrier layer made of a gallium nitride based semiconductor, and has the predetermined peak wavelength. The barrier layer has a lattice constant smaller than that of the well layer and includes at least one of a GaN layer, an InGaN layer, and an AlGaN layer.

  According to this group III nitride light-emitting diode, the well layer is composed of a gallium nitride semiconductor mixed crystal containing Al element, Ga element and In element as group III constituent elements, and the gallium nitride semiconductor mixed crystal has a predetermined peak. It is a quaternary mixed crystal that can generate light of a wavelength. The lattice constant of this quaternary mixed crystal is larger than the lattice constant of InGaN in a light emitting device that generates light having a predetermined peak wavelength using an InGaN well layer. Therefore, the lattice constant difference between the gallium nitride semiconductor of the barrier layer and the quaternary mixed crystal can be made larger than the lattice constant difference between the InGaN and the gallium nitride semiconductor of the barrier layer. The lattice constant of the barrier layer is smaller than that of the well layer. Therefore, the quaternary mixed crystal well layer contains a relatively large compressive strain. Compressive strain is effective for increasing the degree of polarization. On the other hand, a well layer made of a quaternary mixed crystal and a well layer made of InGaN can generate light having the same predetermined peak wavelength, and therefore exhibit substantially the same quantum confinement property at the same well layer thickness. . That is, by using a quaternary mixed crystal instead of InGaN, a large compressive strain can be realized without increasing the quantum confinement property of the well layer. Therefore, the group III nitride light-emitting diode generates polarized light by a quantum well structure having a quaternary mixed crystal well layer and a barrier layer made of GaN or ternary mixed crystal.

  In the group III nitride light emitting diode according to the present invention, the thickness of the well layer is preferably larger than 5 nm. The quantum confinement property can be weakened by adjusting the thickness of the InAlGaN well layer in the multiple quantum well structure of the active layer. Since the thickness of the InAlGaN well layer exceeds 5 nm, the quantum confinement property is not strong. Therefore, the group III nitride light-emitting diode generates polarized light by a quantum well structure having a weak quantum confinement property and a compressive strain well layer.

  In the group III nitride light emitting diode according to the present invention, the thickness of the well layer may be 10 nm or less. According to this group III nitride light-emitting diode, when the well layer is thicker than 10 nm, the light emission efficiency decreases.

  In the group III nitride light emitting diode according to the present invention, it is preferable that the band gap of the barrier layer is equal to or smaller than the band gap of gallium nitride and larger than the band gap of the well layer.

  According to this group III nitride light-emitting diode, since the band gap of the barrier layer is less than or equal to that of gallium nitride, the band gap energy difference between the well layer and the barrier layer in the multiple quantum well structure of the active layer can be adjusted. Can weaken the quantum confinement property.

  In the group III nitride light-emitting diode, the well layer is made of InAlGaN. Therefore, compressive strain is applied to the well layer, and the accompanying increase in the band gap energy difference between the well layer and the barrier layer can be avoided. it can. Therefore, a quantum well structure having an InAlGaN well layer and a barrier layer is effective for increasing the degree of polarization. In the group III nitride light emitting diode according to the present invention, the barrier layer may be made of GaN. Alternatively, the barrier layer can be made of InGaN. The change of the band gap with respect to the change of the lattice constant is small.

  In the group III nitride light emitting diode according to the present invention, it is preferable that the well layer and the barrier layer are provided so that a peak wavelength of light from the active layer is in a wavelength range of 400 nm to 600 nm. The above-mentioned wavelength range of this group III nitride light emitting diode is suitable for a light emitting diode for a display device.

  In the group III nitride light-emitting diode according to the present invention, the predetermined direction may be the direction of the a-axis or m-axis of the group III nitride.

  In the group III nitride light emitting diode according to the present invention, the substrate is preferably made of a gallium nitride based semiconductor. According to this group III nitride light-emitting diode, a substrate material having an appropriate lattice constant according to the emission wavelength is provided. In the group III nitride light emitting diode according to the present invention, the substrate may be made of a conductive GaN semiconductor. According to this group III nitride light-emitting diode, the GaN substrate is suitable for growing an active layer having a low dislocation. Accordingly, a strain-encapsulated well layer is provided without causing strain relaxation, and the strain amount of the well layer can be easily controlled.

In the group III nitride light emitting diode according to the present invention, the n-type gallium nitride based semiconductor layer, the active layer, and the p-type gallium nitride based semiconductor in an orthogonal coordinate system composed of mutually orthogonal X1, X2, and X3 axes. The layers are arranged in the direction of the X3 axis, the X2 axis indicates the inclination direction of the c-axis of the group III nitride, and the lattice constant of the barrier layer is the hexagonal III of the substrate. A lattice constant less than or equal to the lattice constant of the group nitride, and at least one of a band gap energy difference between the well layer and the barrier layer and a lattice constant difference between the well layer and the barrier layer in the multiple quantum well structure of the active layer However, the degree of polarization P in the light emission of the light emitting diode is 0.6 or more, and the degree of polarization P is equal to the electric field component I1 in the X1 direction of the light in the light emitting diode and the X2 By using the electric field component I2 countercurrent,
P = (I1-I2) / (I1 + I2)
Can be defined by

  According to the group III nitride light-emitting diode, the quantum well structure is configured such that the polarization degree P in light emission of the light-emitting diode becomes 0.6 or more by adjusting the band gap energy difference in the multiple quantum well structure of the active layer. Is formed.

  The above and other objects, features, and advantages of the present invention will become more readily apparent from the following detailed description of preferred embodiments of the present invention, which proceeds with reference to the accompanying drawings.

  As described above, according to the present invention, a group III nitride light emitting diode having a quantum well structure capable of emitting polarized light is provided.

  The knowledge of the present invention can be easily understood by considering the following detailed description with reference to the accompanying drawings shown as examples. Subsequently, an embodiment according to the group III nitride light-emitting diode of the present invention will be described with reference to the accompanying drawings. Where possible, the same parts are denoted by the same reference numerals.

FIG. 1 is a drawing showing the structure of a group III nitride light emitting diode according to the present embodiment. The group III nitride light-emitting diode 11 generates polarized light having a predetermined peak wavelength. The group III nitride light-emitting diode 11 includes a substrate 13, an n-type gallium nitride semiconductor region 15, a p-type gallium nitride semiconductor region 17, and an active layer 19. The substrate 13 is made of, for example, a hexagonal group III nitride. The main surface 13a of the substrate 13 is inclined from the c-plane at an angle θ of 10 degrees or more and 60 degrees or less in the a-axis and / or m-axis direction of the group III nitride. The inclination angle θ is defined by (θ _A ² + θ _M ² ) ^1/2 . The inclination angles θ _A and θ _M indicate inclination angles in the a-axis direction and the m-axis direction, respectively. Further, the inclination of the c-axis can be, for example, the direction of the a-axis or the m-axis. When the tilt angle is 10 degrees or more, the degree of polarization varies greatly depending on the magnitude of lattice distortion and the magnitude of quantum confinement, and the degree of polarization can be controlled by lattice distortion and quantum confinement. When intending to increase the emission wavelength, it is advantageous to increase the width of the well layer, in which case the quantum confinement effect is reduced. On the other hand, since it is necessary to increase the In composition of the well layer, the lattice constant difference between the well layer and the barrier layer increases, and the strain of the well layer increases. When the tilt angle is 60 degrees or less, the increase in distortion gives a positive degree of polarization, and the influence of the quantum effect on the degree of polarization is that the degree of polarization is close to zero or positive when the well width is wide. As a positive degree of polarization. On the other hand, when the tilt angle is 60 degrees or more, an increase in distortion gives a negative degree of polarization, and the influence of the quantum effect on the degree of polarization is such that a relatively large positive degree of polarization is given even when the well width is wide. work. As a result, they cancel each other and the total degree of polarization becomes small. Therefore, in order to obtain a particularly long emission wavelength, it is desirable to use a substrate having an inclination angle of 60 degrees or less. Vector _{V C} indicates the c-axis direction, the vector _{V N} indicates the normal direction of the principal surface 13a of the substrate 13. The c-plane of the substrate 13 is orthogonal to the vector V _C , and a typical c-plane is drawn with a broken line in FIG. The main surface 13a of the substrate 13 is a semipolar surface. The n-type gallium nitride based semiconductor region 15 and the p-type gallium nitride based semiconductor region 17 are provided on the main surface 13 a of the substrate 13. The active layer 19 is provided between the n-type gallium nitride semiconductor region 15 and the p-type gallium nitride semiconductor region 17, and the active layer 19 has a multiple quantum well structure and has a predetermined peak wavelength. Configured to generate.

  Referring to FIG. 1, an orthogonal coordinate system S for showing a geometric structure is shown. In the present embodiment, a pair of side surfaces of the light emitting diode 13 extends in the X-axis direction, and another pair of side surfaces extends in the Y-axis direction. The n-type gallium nitride semiconductor region 15, the active layer 19, and the p-type gallium nitride semiconductor region 17 are arranged in the Z-axis direction. The active layer 19 includes a well layer 21 made of a gallium nitride based semiconductor mixed crystal containing an Al element, a Ga element, and an In element as a group III element, and a barrier layer 23 made of a gallium nitride based semiconductor. The well layers 21 and the barrier layers 23 are alternately arranged in the Z-axis direction. The well layer 21 is sandwiched between the barrier layers 23, and the well layer 21 receives stress according to the difference between the lattice constant of the well layer 21 and the lattice constant of the barrier layer 23.

  In the light emitting diode 11, the well layer 21 is made of a mixed crystal containing an aluminum (Al) element, an indium (In) element, and a gallium (Ga) element as a group III constituent element and a nitrogen (N) element as a group V constituent element. As this mixed crystal, there is InAlGaN. The well layer 21 is made of InAlGaN. Since the barrier layer 23 includes at least an Al element as a group III element, it is suitable for applying compressive strain to the well layer 21. The barrier layer 23 includes at least one of a GaN layer, an InGaN layer, and an AlGaN layer.

FIG. 2 is a diagram showing a band lineup of gallium nitride based semiconductors. According to this group III nitride light-emitting diode 11, since the well layer 21 includes Al element, Ga element and In element as group III constituent elements, the gallium nitride based semiconductor mixed crystal generates light of a predetermined peak wavelength. Possible quaternary mixed crystals (for example, points W1 and W2 in FIG. 2). Referring to FIG. 2, the quaternary mixed crystal (for example, FIG. 2) is compared with the lattice constant d _{InGaN of the} InGaN in a light emitting device that generates light having a predetermined peak wavelength using an InGaN well layer (band gap E _W ). The lattice constant d _{InAlGaN at} point W1) in FIG. Thus, as compared with the difference in lattice constant between the lattice constant _{d B} of lattice constant _{d InGaN} and barrier layers of gallium nitride-based semiconductor of the InGaN (e.g. GaN), gallium nitride in the lattice constant _{d InAlGaN} and the barrier layer of the quaternary mixed crystal systems difference in lattice constant between the lattice constant d _B of the semiconductor can be increased. The lattice constant _{d InAlGaN} smaller lattice constant _{d B} is the well layer of the barrier layer. Therefore, the quaternary mixed crystal well layer 21 contains a relatively large compressive strain. This compression strain is effective for increasing the degree of polarization. On the other hand, since both the well layer 21 made of quaternary mixed crystal and the well layer made of InGaN can generate light having the same predetermined peak wavelength, substantially the same quantum confinement property is obtained at the same well layer thickness. Show. That is, by using a quaternary mixed crystal instead of InGaN, an increase in compressive strain can be realized without increasing the quantum confinement property of the well layer 21. This therefore does not reduce the degree of polarization. Therefore, the group III nitride light-emitting diode can generate polarized light by a quantum well structure having a well layer having a large compressive strain without increasing the quantum confinement property.

FIG. 3 is a view showing an active layer structure in the light emitting diode according to the present embodiment. As already described, the barrier layer 23 may include at least one of a GaN layer, an InGaN layer, and an AlGaN layer, and FIG. 3 shows the GaN layer as a barrier layer. Lattice constant D _B of the barrier layer 23 made of gallium nitride semiconductor is less than the lattice constant D _W of the well layer 21. The lattice constant _DW of the InAlGaN well layer 21 is smaller than the lattice constant of the hexagonal group III nitride of the substrate 13. For this reason, a relatively large compressive strain is applied to the well layer 21. This compression strain is effective for increasing the degree of polarization. Figure 3 (b), the lattice constant _{D InGaN} of the InGaN is shown by a chain line, the InGaN has the same band gap as the band gap _{E W} of the InAlGaN well layer 21 shown in FIG. 3 (c) Have That is, the InAlGaN well layer 21 can increase the lattice constant difference between the well layer 21 and the barrier layer 23 without changing the band gap.

  Further, the quantum confinement property can be weakened by adjusting the thickness of the InAlGaN well layer in the multiple quantum well structure of the active layer. In the light emitting diode 11, it is preferable that the thickness of the well layer 21 made of InAlGaN exceeds 5 nm. Since the thickness of the InAlGaN well layer exceeds 5 nm, strong quantum confinement is not exhibited due to this well thickness. This is effective for increasing the degree of polarization. The thickness of the InAlGaN well layer 21 can be 10 nm or less. If the InAlGaN well layer 21 is thicker than 10 nm, the light emission efficiency may be reduced.

  Accordingly, the group III nitride light-emitting diode generates polarized light by a quantum well structure that increases the strain to the well layer and does not enhance (or weaken if necessary) the quantum confinement property.

  Referring again to FIG. 1, an orthogonal coordinate system SP for defining the degree of polarization is shown. The orthogonal coordinate system SP includes an X1 axis, an X2 axis, and an X3 axis that are orthogonal to each other. In this orthogonal coordinate system SP, the X2 axis indicates the inclination direction of the c-axis of the group III nitride. In the direction of the X3 axis, an n-type gallium nitride based semiconductor region 15, an active layer 19, and a p-type gallium nitride based semiconductor region 17 are arranged. As shown in FIG. The layer 23 and the well layer 21 are arranged.

In the following description, the bandgap energy difference between the well layer 21 and barrier layer 23 are code △ E - is referred to as _{(= △ E E △ E H} ), difference in lattice constant between the well layers 21 and barrier layers 23 Is referred to as a symbol Δd (= D _W −D _B ). In the active layer 19, at least one of the band gap energy difference ΔE and the lattice constant difference Δd between the well layer 21 and the barrier layer 32 has a polarization degree P in the light emission of the light emitting diode 13 of 0.6 or more. It is provided as follows. The degree of polarization P is obtained by using the electric field component I1 in the X1 direction and the electric field component I2 in the X2 direction of the light L emitted from the light emitting diode 11.
P = (I1-I2) / (I1 + I2)
It is prescribed by.

According to the light emitting diode 11, as shown in FIG. 3 (b), the lattice constant D _W of the well layer 21 is greater than the lattice constant D _B of the barrier layer 23, also the group III nitride of the hexagonal substrate 13 Greater than the lattice constant of the object. For this reason, a relatively large compressive strain is applied to the well layer 21. The compressive strain is effective for increasing the polarization degree P. Further, by changing the band gap energy difference ΔE (for example, the band gap of the barrier layer 23) in the multiple quantum well structure of the active layer 19, the quantum confinement property can be adjusted. Therefore, the band gap energy difference ΔE is provided so that the polarization degree P in the light emission L of the light emitting diode 11 is 0.6 or more. Further, the lattice constant difference Δd is provided so that the polarization degree P is 0.6 or more. Therefore, the light emitting diode 11 generates light having a desired polarization by a quantum well structure having a weak quantum confinement property and a large distortion to the well layer 21.

  In the light emitting diode 11, the substrate 13 is preferably made of a gallium nitride based semiconductor. A substrate material having an appropriate lattice constant according to the emission wavelength is provided. As the material of the substrate, for example, GaN, AlGaN or the like can be used. In the light emitting diode 11, the substrate 13 can be made of a conductive GaN semiconductor. A GaN substrate is suitable for growing a low dislocation active layer. Accordingly, a strain-encapsulated well layer is provided without causing strain relaxation, and the strain amount of the well layer can be easily controlled.

  The n-type gallium nitride based semiconductor region 15 can include, for example, one or a plurality of n-type gallium nitride based semiconductor layers. As the n-type gallium nitride semiconductor layer, for example, a material such as GaN or AlGaN can be used. The p-type gallium nitride based semiconductor region 17 can include, for example, an electron block layer 25 and a p-type contact layer 27. As the electron block layer 25, for example, a material such as AlGaN or InAlGaN can be used. For the p-type contact layer 27, for example, a material such as p-type GaN or p-type AlGaN can be used. On the main surface 13 a of the substrate 13, a gallium nitride based semiconductor stack 29 is mounted. The gallium nitride based semiconductor laminate 29 inherits the semipolar nature of the main surface 13 a of the substrate 13. Similar to the substrate 13, the c-axis of the gallium nitride based semiconductor stack 29 is inclined with respect to the main surface 13 a of the substrate 13, and the c-axis in the crystal of the active layer 19 is a stack of the well layer 21 and the barrier layer 23. It is inclined with respect to the direction (Z-axis direction). A first electrode (for example, an anode) 31 is formed on the gallium nitride based semiconductor stack 29. The first electrode 31 can be formed on the entire surface of the gallium nitride based semiconductor stack 29 so that a current can be supplied over the entire active layer 19. The light L from the active layer 19 is emitted through the first electrode 31. Most of the light L from the active layer 19 is transmitted through the first electrode 31. A pad electrode 35 is provided on the first electrode 31. A second electrode 33 is provided on the back surface 13 b of the substrate 13. The second electrode 33 can be formed on the entire back surface 13 b of the substrate 13. Polarized light is generated in the active layer 19 by the current flowing through the first electrode 31 and the second electrode 33, and this light L is emitted from the emission surface to the outside in the direction of the axis Ax.

  In the light emitting diode 11, the well layer 21 and the barrier layer 23 are preferably provided so that the peak wavelength of light from the active layer 19 is in a wavelength range of 400 nm to 600 nm. The wavelength range is substantially the same as the wavelength range of visible light. Therefore, the light emitting diode 11 is suitable as a light emitting diode for a display device.

Preferably the band gap E _B of the barrier layer 23 is less than the band gap gallium nitride (GaN), and larger than the band gap E _W of the well layer 21. The well layer 21 receives compressive stress from the barrier layer 23.

FIG. 4 is a view showing the structure of the active layer of the light emitting diode according to the present embodiment. In this embodiment, as shown in FIG. 4A, the light emitting diode 11a includes an active layer 19a. The barrier layer 23a is made of GaN. As shown in FIG. 4B, the lattice constant of the barrier layer 23a is indicated by the reference symbol D _GaN , and the lattice constant D _InAlGaN of the InAlGaN layer 21a is larger than the lattice constant D _{GaN of} gallium nitride. For this reason, this barrier layer 23a applies compressive strain to the InAlGaN well layer 21a. The magnitude of the compressive strain is adjusted by the Al composition of the well layer 21a. FIG. 4 (c), are shown conduction band _{G C} and the valence band _{G V} is. In the present example, these are the same as the conduction band DC _GaN and valence band DV _{GaN of GaN} . Code _{E B} represents the band gap of the barrier layer, if code _{E W} is showing a band gap of the well layer, the band gap difference _{△ E GaN (= E B -E} W) , together with an increase in the Al composition of the well layer 21a In addition, the In composition of the well layer 21a is adjusted in order to adjust the band gap of the well layer 21a to a predetermined wavelength. In order to obtain a suitable quantum confinement with respect to the degree of polarization, the band gap difference ΔE _GaN can be, for example, 1 eV or less. Further, in order to obtain an improvement in light emission efficiency due to the quantum well structure, the band gap difference ΔE _GaN can be, for example, 0.1 eV or more.

FIG. 5 is a view showing an active layer structure in the light emitting diode according to the present embodiment. In this embodiment, as shown in FIG. 5A, the light emitting diode 11b includes an active layer 19b. The barrier layer 23b is preferably made of InGaN. The InGaN barrier layer 23b can apply compressive strain to the InAlGaN well layer 21b, and can avoid an increase in the band gap energy difference. Therefore, compressive strain is effective for increasing the degree of polarization in the quantum well structure having the InAlGaN well layer 21b and the InGaN barrier layer 23b. As such, the lattice constant _{D InAlGaN} is larger than the lattice constant _{D GaN} gallium nitride lattice constants _{D InGaN} and InAlGaN layer 21b of InGaN layer 23b shown in Figure 5 (b). This InGaN layer 23b applies compressive strain to the InAlGaN well layer 21b. The magnitude of the compressive strain increases as the Al composition of the well layer 21b increases and increases as the In composition of the barrier layer 23b decreases. On the other hand, the magnitude of the compressive strain is that the well layer 21b is a quaternary mixed crystal. Therefore, the lattice constant and the band gap can be independently changed by adjusting the composition of the group III element. In FIG. 5 (c), it is shown conduction band _{G C} and the valence band _{G V} is. For comparison, the GaN conduction band DC _GaN and valence band DV _GaN are also shown. Code _{E B} represents the band gap of the barrier layer, when the sign _{E W} is showing a band gap of the well layer, the band gap difference _{△ E InGaN (= E B -E} W) is, Al composition of the well layer 21b is increased _However , the band gap difference ΔE _InGaN can be set to a desired value within a certain range by adjusting the In composition. Therefore, the strain due to the difference in lattice constant between the InAlGaN well layer and the InGaN barrier layer can be set separately from the band gap difference ΔE _InGaN quantum confinement between the InAlGaN well layer and the InGaN barrier layer. In order to obtain a suitable quantum confinement property with respect to the degree of polarization, the band gap difference ΔE _InGaN can be, for example, 1 eV or less. Further, in order to obtain an improvement in light emission efficiency due to the quantum well structure, the band gap difference ΔE _InAlGaN can be, for example, 0.1 eV or more.

  Note that AlGaN can be used instead of GaN of the barrier layer 23a. At this time, the magnitude of the compressive strain is adjusted by the Al composition of the barrier layer 23a. The material of the well layer and the barrier layer can be InAlGaN and GaN, respectively. This combination is suitable for a light emitting diode having an emission wavelength range A (400 nm to 520 nm). At a wavelength of 520 nm or more, the band gap energy difference between the barrier layer GaN and the well layer InAlGaN becomes too large, the quantum confinement property increases, and the polarization degree decreases. Also, the material of the well layer and the barrier layer can be InAlGaN and InGaN, respectively. This combination is suitable for a light emitting diode having an emission wavelength range B (480 nm to 600 nm). In particular, at a long wavelength, when the barrier layer is made of InGaN, the band gap of the barrier layer can be reduced, and the decrease in the degree of polarization due to the enhanced quantum confinement property can be suppressed. At long wavelengths, the lattice constant of the well layer can be increased relatively easily. Even if the lattice constant of the barrier layer increases by using InGaN for the barrier layer, sufficient strain can be applied to the well layer. it can. Conversely, in the short wavelength region of 480 nm or less, there is a merit that the band gap difference can be reduced, but it becomes difficult to sufficiently strain the well. Further, the material of the well layer and the barrier layer can be InAlGaN and AlGaN, respectively. This combination is suitable for a light emitting diode having an emission wavelength range C (400 nm to 450 nm). By using AlGaN for the barrier layer, the well layer can be sufficiently distorted even in a relatively short wavelength region, and the degree of polarization can be increased. In addition, if it is a short wave, the band gap energy difference can be reduced relatively easily, and the degree of polarization can be increased even if the band gap energy difference increases by using AlGaN as the barrier layer.

  When a light-emitting diode manufactured on a c-plane substrate is used in a display device such as a liquid crystal device, backlight light is provided through a polarizing plate in order to generate polarizing light. Since the light from the light-emitting diode fabricated on the c-plane substrate is non-polarized light, when the non-polarized light passes through the polarizing plate, the light component orthogonal to the desired polarization direction is lost, and the polarizing plate The light output is greatly reduced by the passage of.

  The light from the light emitting diodes produced on the semipolar and nonpolar substrates has a polarization property, but the active layer is not configured so that the degree of polarization is sufficiently large. When the light from the light emitting diode passes through the polarizing plate of the display device, the light output is greatly reduced by the passage of the polarizing plate.

  On the other hand, as described above, in the light emitting diodes 11, 11a, and 11b according to the present embodiment, the active layer is provided with the structure of the high strain well layer or the structure of low quantum confinement so as to increase the degree of polarization. The degree of polarization of light from a light emitting diode fabricated on a semipolar surface can be increased.

(Example)
According to the process flow shown in FIG. 6, a light emitting diode structure was fabricated by metal organic vapor phase epitaxy. Trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH ₃ ), monomethylsilane (MMSi), biscyclopentadienylmagnesium (CP ₂ Mg) are used as raw materials for epitaxial growth It was.

In step S101, a GaN wafer showing semipolarity was prepared. This GaN wafer has a surface inclined at an off angle of 18 degrees from the c-axis to the a-axis direction. After installing the GaN wafer in the reactor, the surface of the GaN wafer was heat-treated in step S102. Using a heat treatment temperature of 1100 degrees Celsius and an in-furnace pressure of 27 kPa as heat treatment conditions, heat treatment was performed for 10 minutes while flowing ammonia and hydrogen through the reactor. Thereafter, in step S103, for example, a Si-added GaN layer was grown. This growth temperature is, for example, 1100 degrees Celsius. The thickness of the GaN layer is 5 μm, for example. Subsequently, an active layer is formed in step S104. The substrate temperature was changed to 840 degrees Celsius, and an InGaN barrier layer was grown in Step S105. For this purpose, TMG, TMI and NH ₃ were fed into the reactor. The thickness of the barrier layer is, for example, 15 nm. Thereafter, the substrate temperature was changed to 700 degrees Celsius, and an InAlGaN well layer was grown in Step S106. The thickness of this well layer is 10 nm. The band gap of the InAlGaN well layer is equal to the same band gap as In _0.20 Ga _0.80 N. In step S107, the barrier layer-well layer structure was grown for three cycles, and then an InGaN layer (15 nm) having the same composition was grown thereon to form an active layer. After forming the active layer, a p-type gallium nitride based semiconductor region was grown in step S108. First, the supply of TMG, TMI and TMA (Group III raw material) was stopped, and the substrate temperature was raised to 1000 degrees Celsius. At this substrate temperature, TMG, TMA, NH ₃ and CP ₂ Mg were introduced into the reactor, and an electron block layer was grown in step S109. This electron block layer is made of Mg-added p-type AlGaN and has a thickness of 20 nm. After growing the p-type AlGaN, the supply of TMA was stopped, and a p-type contact layer was grown in step S110. The thickness of the p-type contact layer is 50 nm. Thereafter, the temperature was lowered to room temperature, and the epitaxial wafer was taken out of the reactor. Note that a GaN barrier layer may be formed instead of the InGaN barrier layer (InGaN having a small In composition). The GaN barrier layer is, for example, 870 degrees Celsius.

The laminated structure obtained by the above epitaxial growth is as follows.
GaN wafer: n-type GaN,
n-type GaN layer: Si added, thickness 5 μm,
Barrier layer: undoped In _0.02 Ga _0.98 N, 15 nm
Well layer: undoped In _0.30 Al _0.05 Ga _0.65 N, 10 nm
Electron blocking layer: Mg-added p-type Al _0.12 Ga _0.88 N layer, thickness 20 nm,
Contact layer: Mg-doped p-type GaN layer, thickness 50 nm.

  The wafer grown by this method was taken out of the reactor, and the polarization property of the photoluminescence light was measured. A light-emitting element having a linear polarization with a degree of polarization as high as 0.8 could be produced.

In this embodiment, it is used GaN or InGaN as a barrier layer, it is set to 10nm in spreading width of the well layer, _In the quaternary mixed crystal as a well layer _{U Al V Ga 1-U-} V N (0 <U <1, 0 <V <1) was used. For this reason, in a GaN wafer in which the c-axis is inclined at an off angle of 70 degrees or less, the degree of polarization increases as the quantum confinement is weakened and the well layer strain is increased. For the former purpose, the well width was set to a wide 10 nm. A well width that is too wide eliminates the quantum confinement effect and lowers the internal quantum efficiency of light emission. Therefore, the well width preferably does not exceed 10 nm. On the other hand, quaternary mixed crystal InAlGaN was used for the well layer so that the lattice constant difference between the barrier layer and the well layer was increased to increase the strain. By using a quaternary mixed crystal, the composition of In and Al can be adjusted to keep the band offset from the well layer substantially constant, while the lattice constant difference can be made larger than that of a well layer made of InGaN. As described above, in the quantum well structure including the InAlGaN well layer and the InGaN (or GaN) barrier layer, the strain can be increased and the barrier height can be maintained or reduced.

  The combination of an InGaN well layer and an InAlGaN barrier layer cannot achieve both an increase in compressive strain and a reduction in quantum confinement. Therefore, the InAlGaN barrier layer cannot improve the degree of polarization. On the other hand, by using InAlGaN for the well layer, in order to provide the same emission wavelength as that of the light-emitting element using InGaN and to increase the degree of polarization, the strain between the barrier layer and the well layer is reduced while increasing the band offset. Can be big. Therefore, the loss due to the polarizing plate can be reduced by using a light-emitting diode having a high degree of polarization for a device that requires polarization.

  The well layer has a larger lattice constant than the barrier layer, and is subjected to compressive strain in the interface direction at the two interfaces between the well layer and the barrier layer. In addition, since the well layer has a smaller band gap than the barrier layer, the potential barrier of the barrier layer acts on the well layer from both sides thereof. Due to the compressive strain and quantum confinement, the conduction band that determines the electronic state involved in light emission and the valence band that determines the hole state are changed. In particular, the rearrangement of quantum levels in the valence band is greatly related to the degree of polarization. As shown in FIG. 1, an axis X1 is defined in the c-axis off direction within the well layer plane, an axis X2 is defined in a direction perpendicular to the off direction, and an axis X3 is defined in the well layer stacking direction. By changing the quantum level, light having polarization in each of these three directions is generated by recombination of holes existing in the quantum levels of energy E1, E2, and E3 and electrons in the conduction band. At this time, since the level energy E3 is always lower than the level energies E1 and E2, holes do not exist at the level of the energy E3 during recombination light emission. Therefore, the light from the LED has no polarization component in the X3 direction.

  Further, the off-angle is zero or substantially zero, that is, on the c-plane GaN substrate, the energies E1 and E2 are always equal, and the polarization components in the X1 and X2 directions are equal, so the degree of polarization is zero.

  When the off angle is 70 degrees or less, as the strain of the well layer increases, the energy E2 becomes higher than the energy E1 due to the elastic constant of the lattice and the anisotropy of the deformation potential. Therefore, the degree of polarization in the X2 direction is increased. On the other hand, when the quantum confinement of the well layer increases, the energy E1 becomes higher than the energy E2 due to the anisotropy of the polarization vector, and the degree of polarization in the X1 direction increases.

  Thus, the effect of strain and quantum confinement works in the opposite direction with respect to the magnitude relationship between energy E1 and energy E2, and in order to increase the degree of polarization in the X2 direction in particular, a quantum well having large strain and weak quantum confinement. It is effective to form a structure.

  As described above, it is understood that the degree of polarization can be improved by weak confinement and large distortion in the semipolarity in the off-angle range in the present embodiment.

  When the polarization degree of light emission of the light emitting element is 0.6 or more, the component I1 is four times larger than the component I2. Ideally, about 80% of the light passes through the polarizing plate. A light-emitting element that satisfies the lower limit (0.6) of the degree of polarization has a great advantage over a light-emitting element that does not have a degree of polarization (that is, light transmission of about 50%).

  In this embodiment, a quantum well structure is used that increases the degree of polarization in the X2 direction by large strain and weak confinement. On the contrary, a quantum well structure that increases the degree of polarization in the X1 direction by a small strain and strong confinement can be used. However, in order to strengthen the quantum confinement, it is necessary to use a thin well layer and increase the band offset. In particular, reducing the width of the well layer is effective for enhancing the quantum confinement effect. However, in order to obtain light having a long wavelength of 500 nm or more, the width of the well layer cannot be reduced so as to increase the wave length, and it is not a good measure to sufficiently increase the quantum confinement. At such a wavelength, a quantum well structure having a sufficient degree of polarization cannot be produced.

  On the other hand, the measure to increase the degree of polarization in the X2 direction by large strain in the well layer and weak quantum confinement is effective for increasing the degree of polarization by increasing the width of the well layer. This corresponds to the production of a light emitting device having a wavelength. This increases the positive degree of polarization.

  The material of the well layer and the barrier layer can be InAlGaN and AlGaN, respectively. This combination is suitable for a light emitting diode having an emission wavelength range of 400 nm to 450 nm. In this wavelength range, the lattice constant of the active layer is relatively small, and when the barrier layer is GaN and InGaN, the lattice constant difference between the well layer and the barrier layer is small, and the well layer cannot be sufficiently distorted. On the other hand, when AlGaN is used for the barrier layer, a sufficient difference in lattice constant is produced even in this wavelength range, and the well layer can be sufficiently distorted to increase the degree of polarization. In addition, within the above wavelength range, the difference in band gap energy between the well layer and the barrier layer is originally small, and even if the band gap energy difference is increased by using AlGaN as the barrier layer, the effect of reducing the degree of polarization is small.

  While the principles of the invention have been illustrated and described in the preferred embodiments, it will be appreciated by those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. The present invention is not limited to the specific configuration disclosed in the present embodiment. We therefore claim all modifications and changes that come within the scope and spirit of the following claims.

FIG. 1 is a drawing showing the structure of a group III nitride light emitting diode according to the present embodiment. FIG. 2 is a diagram showing a band lineup of a well layer and a barrier layer of the group III nitride light emitting diode according to the present embodiment. FIG. 3 is a view showing an active layer structure in the light emitting diode according to the present embodiment. FIG. 4 is a drawing showing an example of an active layer structure in the light emitting diode according to the present embodiment. FIG. 5 is a drawing showing another active layer structure in the light emitting diode according to the present embodiment. FIG. 6 is a drawing showing a main process flow in a method for producing a light emitting diode structure by metal organic vapor phase epitaxy.

Explanation of symbols

13 ... substrate, 15 ... n-type gallium nitride based semiconductor region, 17 ... p-type gallium nitride based semiconductor region, 19 ... active layer, a vector showing the V C _... c-axis direction, the normal direction of the V N _... substrate main surface 21, 21a, 21b ... well layer, 23, 23a, 23b ... barrier layer, ΔE ... band gap energy difference, Δd ... lattice constant difference, P ... polarization degree, 25 ... electron blocking layer, 27 ... p Type contact layer, 31, 33 ... electrode, 35 ... pad electrode

Claims (11)

A group III nitride light emitting diode that generates polarized light of a predetermined peak wavelength,
A substrate comprising a hexagonal group III nitride and having a principal surface inclined from the c-plane at an angle of 10 degrees to 60 degrees in a predetermined direction;
An n-type gallium nitride based semiconductor layer provided on the substrate;
A p-type gallium nitride based semiconductor layer provided on the substrate;
An active layer provided between the n-type gallium nitride semiconductor layer and the p-type gallium nitride semiconductor layer;
The active layer includes a well layer made of a gallium nitride semiconductor mixed crystal containing Al element, Ga element, and In element as a group III element and a barrier layer made of a gallium nitride semiconductor, and generates the predetermined peak wavelength. Configured to
The lattice constant of the barrier layer is smaller than the lattice constant of the well layer,
The group III nitride light-emitting diode, wherein the barrier layer includes at least one of a GaN layer, an InGaN layer, and an AlGaN layer.   The group III nitride light-emitting diode according to claim 1, wherein the well layer has a thickness greater than 5 nm.   3. The group III nitride light-emitting diode according to claim 1, wherein the well layer has a thickness of 10 nm or less.   The band gap of the said barrier layer is below the band gap of gallium nitride, and is larger than the band gap of the said well layer, The III described in any one of Claims 1-3 characterized by the above-mentioned. Group nitride light-emitting diodes.   The said well layer and the said barrier layer are provided so that the peak wavelength of the light from the said active layer may exist in the wavelength range of 400 nm or more and 600 nm or less, The any one of Claims 1-4 characterized by the above-mentioned. A group III nitride light-emitting diode according to item 1.   The group III nitride light-emitting diode according to any one of claims 1 to 5, wherein the predetermined direction is an a-axis or m-axis direction of the group III nitride.   The group III nitride light emitting diode according to any one of claims 1 to 6, wherein the substrate is made of a conductive GaN semiconductor. In an orthogonal coordinate system composed of an X1 axis, an X2 axis, and an X3 axis orthogonal to each other, the n-type gallium nitride semiconductor layer, the active layer, and the p-type gallium nitride semiconductor layer are arranged in the X3 axis direction. The X2 axis indicates the direction of inclination of the c-axis of the group III nitride,
The lattice constant of the barrier layer is less than or equal to the lattice constant of the hexagonal group III nitride of the substrate;
At least one of a band gap energy difference between the well layer and the barrier layer and a lattice constant difference between the well layer and the barrier layer in the multiple quantum well structure of the active layer is a degree of polarization in light emission of the light emitting diode. P is set to be 0.6 or more,
The degree of polarization P uses the electric field component I1 in the X1 direction and the electric field component I2 in the X2 direction of light in the light emitting diode,
P = (I1-I2) / (I1 + I2)
The group III nitride light-emitting diode according to any one of claims 1 to 7, characterized by:   The group III nitride light emitting diode according to any one of claims 1 to 8, wherein the barrier layer is made of GaN.   The group III nitride light emitting diode according to any one of claims 1 to 8, wherein the barrier layer is made of InGaN. The group III nitride light-emitting diode according to claim 1, wherein the barrier layer is made of AlGaN.

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