JPH1051073A - Semiconductor light emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the threshold current density of a semiconductor light emitting device for emitting a short wavelength light, by providing a GaN active layer on a low temperature grown buffer layer formed on a ZnO substrate whereinto a tensile strain originating from the difference between thermal expansion coefficients of the GaN active layer and the ZnO substrate is introduced. SOLUTION: On a ZnO substrate 1, a GaN buffer layer 2 is made to grow. On this GaN buffer layer 2 subjected to a low-temperature growth, a GaN buffer layer 3, an Si-doped n-(Al0.1 Ga0.9 )N barrier layer 4, an undoped GaN active layer 5 and an Mg-doped p-(Al0.1 Ga0.9 )N barrier layer 6 are made to grow in succession. Since the thermal expansion coefficients of the ZnO substrate 1 and the GaN active layer 5 are respectively 2.9×10<-6> /K and 5.59×10<-6> /K, a tensile strain of about 0.3% can be introduced in between them. Therefore, the edge of the valence band of the active layer 5 is occupied only by CH band to make the threshold current density of a semiconductor light emitting device reducible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a semiconductor light emitting device using a GaN-based material and introducing tensile strain.

In recent years, short-wavelength semiconductor lasers that generate light having a wavelength in the blue to near-ultraviolet range have been actively developed.

[0003] In general, materials for blue-emitting semiconductor lasers include ZnSe-based materials of Group II / VI and GaN of Group III / V.
Although a system is known, the ZnSe system has been considered to be advantageous up to now because it is lattice-matched to GaAs which has been used as a high-quality substrate.

[0004] As such, most of the researchers in the world are engaged in research on the ZnSe system, and that research has preceded other researches, and room temperature continuous oscillation has already been reported. However, since ZnSe is a material that is inherently easily deteriorated, there is a problem in reliability, and it has not yet been put to practical use.

[0005] The present invention can provide one means for realizing a short-wavelength semiconductor laser that is sufficiently commercially available.

[0006]

2. Description of the Related Art Two years ago, high brightness L made of GaN was used.
Research results were announced on ED (Light Emitting Diode) (in short, "Magazine" Nikkei Microdevice "published April 1, 1994, Nikkei BP, April 1994, pp. 99-103). reference).

[0007] After this announcement, GaN having excellent environmental resistance has been reviewed instead of the ZnSe material whose reliability has been a bottleneck, and research has been expanding.

As for the oscillation of the GaN-based current injection type semiconductor light emitting device, although pulse oscillation at room temperature has just been reported, it has been pointed out that the threshold current density is large. Similarly, it is predicted that the threshold current density will be very large because of the large mass.

According to theoretical calculations, when a GaN-based material is used, only the CH band forms a band edge due to the introduction of tensile strain, so that tensile strain reduces the threshold current density. It is expected to be effective.

FIG. 2 is a diagram showing the strain dependence of the valence band structure. The ordinate shows the valence band energy level, and the abscissa shows the strain.

In the figure, since the vertical axis is taken positively for the electrons, the energy of the hole increases as it goes down the vertical axis. Therefore, the band end is the top band.

From the figure, it can be seen that in the negative direction of the compressive strain, that is, the amount of distortion, HH and LH are present close to the band edge. It is very large, and it is estimated that the Fermi level is hard to rise.

On the other hand, in the tensile strain, that is, when the strain amount is in the positive direction, it is observed that the band edge is only the CH band. From this, the density of states at the band edge is reduced, and the compression is reduced. It is expected that the Fermi level will increase more easily than in a strained or unstrained state.

[0014]

SUMMARY OF THE INVENTION As described above, Ga
When an N-based material is used, it is apparent that the threshold current density can be reduced by introducing tensile strain,
It is very difficult to achieve.

Usually, to introduce strain into a semiconductor crystal,
It is common to use the shift of the lattice constant.
This is because there is no material having a large lattice constant in an appropriate amount as compared with N, that is, within a range of 0.5%.

According to the present invention, by applying extremely simple means, it is possible to introduce tensile strain into a GaN-based material and to reduce the threshold current density of a semiconductor light emitting device that generates short wavelength light. And

[0017]

The present inventors have studied GaN-based crystals formed on a sapphire substrate. However, in a crystal layer grown on a low-temperature buffer layer, the amount of distortion is small. It was confirmed that the determination was made only by the difference in thermal expansion coefficient with the substrate, without depending on the lattice constant difference.

By utilizing this phenomenon, it is possible to introduce tensile strain into the GaN-based crystal by utilizing the difference in thermal expansion coefficient.

Incidentally, the ZnO substrate has an a-axis lattice constant of 3.252 [Å], and when GaN is directly grown thereon, a tensile strain of about 3.2 [%] is introduced.
Since the amount of strain is too large, a proper crystal cannot be obtained.

However, when a low-temperature growth buffer layer is interposed between the ZnO substrate and the GaN layer, the amount of strain in the crystal on the low-temperature growth buffer layer substantially depends only on the difference in thermal expansion coefficient.

The thermal expansion coefficient of the ZnO substrate is 2.9 × 10 ^-6
/ K, and the coefficient of thermal expansion of the GaN layer is 5.59 ×
Since it is 10 ^-6 / K, about 0.3 [%]
Tensile strain can be introduced.

From the above, in the semiconductor light emitting device according to the present invention, (1) the semiconductor light emitting device is grown on a low-temperature growth buffer layer (for example, 500 ° C.) formed on a ZnO substrate (for example, ZnO substrate 1). GaN buffer layer 2) and a GaN active layer formed on the low-temperature growth buffer layer and having a tensile strain derived from the difference in thermal expansion coefficient between the ZnO substrate and the GaN active layer (for example, an undoped GaN active layer having a tensile strain introduced) 5) is provided, or

(2) a low-temperature growth buffer layer formed on a ZnO substrate;
or a GaInN active layer into which a tensile strain derived from a difference in thermal expansion coefficient from the nO substrate has been introduced, or

(3) In the above (1), the GaN active layer into which the tensile strain has been introduced is made of AlGaN or AlIn.
A double hetero structure formed between barrier layers made of a material selected from N or AlGaInN, or

(4) In the above (2), the GaInN active layer in which the tensile strain has been introduced is made of AlGaN or AlGaN.
It is characterized by having a double hetero structure formed between barrier layers made of a material selected from InN or AlGaInN.

By adopting the above-mentioned means, it becomes possible to easily introduce tensile strain into the GaN-based active layer, and only the CH band is formed at the band edge, and the state density is reduced. The generated semiconductor light emitting device can be oscillated at a low threshold current density.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a cutaway side view of a main part of a semiconductor laser for generating short-wavelength light for explaining an embodiment of the present invention.

In the figure, 1 is a ZnO substrate, 2 is a GaN buffer layer grown at a low temperature, 3 is a GaN buffer layer, and 4 is an n-
(Al _0.1 Ga _0.9 ) N barrier layer, 5 is undoped GaN
The active layer, 6 indicates a p- (Al _0.1 Ga _0.9 ) N barrier layer, 7 indicates an n-side electrode, and 8 indicates a p-side electrode.

The process of manufacturing this semiconductor laser is extremely simple, and an example will be described below.

(1) MOCVD (metalorga)
nic chemical vapor depos
The GaN buffer layer 2 having a thickness of, for example, 500 [Å] is grown on the ZnO substrate 1 at a growth temperature of, for example, 500 [° C.] by applying the method.

(2) Subsequently, MOCVD is applied,
The temperature required to grow each material coating (~ 1
100 [° C.]), the low-temperature-grown GaN buffer layer 2
A GaN buffer layer 3 having a thickness of, for example, 0.5 [μm] and a Si-doped n- (Al
_0.1 Ga _0.9 ) N barrier layer 4 having a thickness of, for example, 0.1 [μ
m] undoped GaN active layer 5 having a thickness of, for example, 1 μm
m] Mg-doped p- (Al _0.1 Ga _0.9 ) N barrier layer 6
Grow in order.

(3) By applying a resist process in the lithography technique and a dry etching method using CHF ₃ or C ₂ H ₆ as an etching gas, a portion where an n-side electrode is to be formed is formed. And n from the surface
The etching reaching the (Al _0.1 Ga _0.9 ) N barrier layer 4 is performed to form a notch 4A.

(4) A part of the cutout 4A is exposed by applying a resist process, a vacuum deposition method, a lift-off method for dissolving the resist, or the like in the lithography technique. N- (Al _0.1
Ga _0.9 ) N barrier layer 4 has a thickness of, for example, 1000 [Å].
The n-side electrode 7 made of Ti is formed, and p- (Al
_0.1 Ga _0.9 ) The thickness on the N barrier layer 6 is, for example, 1000
[Å] The p-side electrode 8 made of Ni is formed.

In the semiconductor laser fabricated as described above, a tensile strain of about 0.3% is introduced into the undoped GaN active layer 5, the band edge becomes only the CH band, and the threshold current density is reduced. Could be lowered.

[0035]

In the semiconductor light-emitting device according to the present invention, the low-temperature growth buffer layer formed on the ZnO substrate and the difference in thermal expansion coefficient between the low-temperature growth buffer layer and the ZnO substrate formed on the low-temperature growth buffer layer. A GaN active layer into which tensile strain has been introduced.

By employing the above configuration, it is possible to easily introduce tensile strain into the GaN-based active layer, and only the CH band is formed at the band edge, and the state density is reduced. The generated semiconductor light emitting device can be oscillated at a low threshold current density.

[Brief description of the drawings]

FIG. 1 is a cutaway side view of a main part of a semiconductor laser for generating short-wavelength light for explaining an embodiment of the present invention.

FIG. 2 is a diagram showing the strain dependence of the valence band structure.

[Explanation of symbols]

Reference Signs List 1 ZnO substrate 2 GaN buffer layer at low temperature growth 3 GaN buffer layer 4 n- (Al _0.1 Ga _0.9 ) N barrier layer 5 undoped GaN active layer 6 p- (Al _0.1 Ga _0.9 ) N barrier layer 7 n-side electrode 8 p-side electrode

Claims (4)

[Claims] 1. A low-temperature growth buffer layer formed on a ZnO substrate, and GaN formed on the low-temperature growth buffer layer and having tensile strain induced by a difference in thermal expansion coefficient between the ZnO substrate and the GaN.
A semiconductor light emitting device comprising an active layer. 2. A low-temperature growth buffer layer formed on a ZnO substrate, and a GaI formed on the low-temperature growth buffer layer and having tensile strain induced by a difference in thermal expansion coefficient between the ZnO substrate and the low-temperature growth buffer layer.
A semiconductor light emitting device comprising an nN active layer. 3. A double hetero structure comprising a GaN active layer in which a tensile strain has been introduced and sandwiched by a barrier layer made of a material selected from AlGaN, AlInN, or AlGaInN. 1
14. The semiconductor light emitting device according to claim 1. 4. The method according to claim 1, wherein the GaInN active layer in which the tensile strain is introduced is made of AlGaN, AlInN, or AlGaInN.
3. The semiconductor light emitting device according to claim 2, comprising a double hetero structure formed between barrier layers made of a material selected from the group consisting of: