JP2004006957A - Optical semiconductor equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize short wavelength configuration with optical properties of high emission efficiency and an optical absorption factor maintained in optical semiconductor equipment with quantum well texture using a nitride III-V group compound semiconductor. <P>SOLUTION: In an LED with a light emitting layer of quantum well texture consisting of AlGaN, a difference in an AL composition between a barrier layer forming the quantum well texture and the quantum well texture layer is within 15%, and the AL composition of the quantum well texture layer is 2% or more, and the thickness of the quantum well texture layer is 4 nm or less. <P>COPYRIGHT: (C)2004,JPO

Description

<< The present invention relates to an optical semiconductor device having a quantum well structure using a nitride semiconductor, for example, a nitride III-V compound semiconductor.

For example, in a conventional optical semiconductor device having a quantum well structure using a nitride III-V compound semiconductor, in order to form a light emitting layer, an InGaN mixed crystal thin film is used as a quantum well layer, and InGaN or GaN is mainly used as a barrier layer. Has been used.
With such a structure, a light emitting diode (LED) and a laser diode (LD) having a relatively short emission wavelength of about 400 nm have been realized.
In order to obtain a light source with high optical resolution and high chemical activity in the LED and LD, when a shorter wavelength range is to be realized, (1) the quantum well layer should be about 1 to 2 molecular layers. It has been studied to make the thickness extremely thin and (2) to use a material having a high Al composition as the barrier layer.
In the LED or LD, a carrier supply layer that supplies n-type and p-type carriers sandwiching a light-emitting layer includes an AlGaN mixed crystal having a larger band gap energy than the emission wavelength (Non-Patent Document 1 below), A GaN / AlGaN superlattice structure (Non-Patent Document 2 below) has been used. In addition, the applicant has shown that an LED having a light-emitting layer (optically active layer) containing Al can achieve a shorter wavelength than an LED having a conventional GaN / AlGaN quantum well structure as a light-emitting layer ( The following non-patent document 3).

Jpn. J. Appl. Phys., 35 (1996) L74 Appl. Phys. Lett., 72 (1998) 211 Phys. Stat. Sol. (A) 176, 45 (1999)

In the case (1), there is a problem that it is difficult to control the thickness of the quantum well layer.
In the case of the above (2), the piezoelectric field effect is remarkable in the nitride quantum well structure, so that the luminous efficiency and the optical characteristics of light absorption are poor, and it has been impossible to realize an effective optical semiconductor device.
FIG. 6 is a conceptual diagram illustrating the principle of the present invention and the related art, and is a diagram illustrating a change in light emission intensity due to a piezo effect. (A) shows the case where the piezo effect in the present invention is small, and (b) shows the case where the piezo effect in the conventional art is large.
In order to achieve a shorter wavelength range for the purpose of a light source having high optical resolution and high chemical activity, an optically active light-emitting layer containing Al is required. The n-type and p-type carrier supply layers sandwiched between them require a layer having a larger band gap energy and a smaller electric resistance than the emission wavelength. However, when the band gap energy is increased by increasing the Al composition, the AlGaN mixed crystal has a problem that the carrier concentration and the mobility are reduced and the electric resistance is increased. Further, when the GaN / AlGaN superlattice structure is used, an electric barrier is formed for the current flow in the film thickness direction, and the barrier becomes effective. There were problems such as difficulty in increasing the gap energy.
In addition, studies on nitride semiconductors have been conducted in the long wavelength region, which is the emission wavelength of the InGaN emission layer, which is considered to have relatively high luminous efficiency. Regarding the carrier supply layer of a light emitting element having a short wavelength, deterioration of crystal quality due to an increase in the Al composition is overestimated, and no systematic study has been performed. However, we have eliminated such prejudices by improving the growth technology to enable a superlattice structure that does not contain GaN to be manufactured satisfactorily enough to realize, for example, a p-type conductive layer. Concluded
In a conventional quantum well structure, since the quantum well layer is made of GaN or InGaN, the distortion of both layers is large due to the difference in lattice constant between the barrier layer and the quantum well layer. As shown in b), the piezo effect is large and the piezo electric field increases.
For this reason, the separation of the existing positions of electrons and holes in the vertical direction increases, and as a result, the optical transition probability decreases, the luminous efficiency and the light absorption coefficient decrease, and a good optical semiconductor device is realized. It becomes difficult.
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical semiconductor device having a quantum well structure using a nitride III-V compound semiconductor, having a high luminous efficiency and a high light absorption coefficient. An object of the present invention is to realize a shorter wavelength while maintaining optical characteristics.
Another object of the present invention is to realize a semiconductor device using a nitride III-V compound semiconductor, which has a low resistance while maintaining optical characteristics of light transmission efficiency.

In order to achieve the above object, the present invention provides an optical semiconductor device having a quantum well structure using a nitride III-V compound semiconductor, wherein Al is used as a quantum well layer as a quantum well layer as a quantum well structure, That is, the barrier layer and the quantum well layer both contain Al, and the Al composition of the barrier layer is larger than the Al composition of the quantum well layer, thereby realizing a shorter wavelength. Even if the Al composition of the barrier layer is increased, by incorporating Al into the quantum well layer, the difference in lattice constant can be reduced and the piezoelectric field can be suppressed.
In particular, light having a quantum well structure in which the difference between the Al composition of the barrier layer and the quantum well layer is 15% or less and the Al composition of the quantum well layer is 2% or more (the quantum well layer contains 2% or more of Al). It is preferable to configure a semiconductor device. Accordingly, the wavelength can be effectively shortened, and the difference in lattice constant between the quantum well layer and the barrier layer becomes 0.5% or less, and the piezoelectric field induced and formed in the quantum well layer is reduced to 1MV / cm or less. It can be suppressed to a degree.
For this reason, for example, when a quantum well layer of about 2 nm is formed, as shown in FIG. 6A, the piezo electric field is kept small, and the separation of the existing positions of electrons and holes in the vertical direction is reduced. The optical transition probability increases, and an optical semiconductor device having high luminous efficiency and high light absorption coefficient can be realized.
Furthermore, the present invention provides an optical semiconductor device having a light emitting layer made of a nitride semiconductor containing Al, wherein the difference in Al composition between the barrier layer and the well layer is 2% or more and 15% or less, and the average of the barrier layer and the well layer is It is desirable that the AlGaN superlattice structure having an Al composition of 7% or more and the well layer having a thickness of 4 nm or less be used as the carrier supply layer.
With this structure, in a semiconductor device using a nitride III-V compound semiconductor, a reduction in resistance can be realized while maintaining optical characteristics of light transmission efficiency.
That is, the present invention relates to an optical semiconductor device having a light emitting layer having a quantum well structure made of AlGaN, wherein the difference between the Al composition of the barrier layer and the quantum well layer constituting the quantum well structure is within 15%, and The well layer has an Al composition of 2% or more, and the quantum well layer has a thickness of 4 nm or less.
Further, the quantum well layer contains less than 1% of In.
It is needless to say that the effect of suppressing the piezo electric field of the present invention shown here can be applied not only to a light emitting device such as an LED or an LD, but also to a light receiving device such as a photodiode (PD) or an optical modulator.

According to the present invention, as the quantum well structure, Al is used as a component of the quantum well layer, and the Al composition of the barrier layer is made larger than the Al composition of the quantum well layer. It has become possible to improve the operation efficiency of an optical semiconductor element using a group III compound, for example, the luminous efficiency in the case of an LED. By using this structure, the operating wavelength of the nitride optical semiconductor device can be greatly shortened from about 370 nm in the related art. An AlGaN superlattice structure in which the difference between the Al composition of the barrier layer and the well layer is 2% or more and 15% or less, the average Al composition of the barrier layer and the well layer is 7% or more, and the thickness of the well layer is 4 nm or less. By using as a carrier supply layer, in a semiconductor device using a nitride III-V compound semiconductor, a reduction in resistance can be realized while maintaining optical characteristics of light transmission efficiency.
Such effects are not limited to light-emitting devices such as LEDs and LDs, but can be widely used to improve characteristics such as optical spatial resolution in nitride optical semiconductor devices such as light-receiving devices such as PDs and optical modulators. It is also very effective in improving the sensitivity in the case where a chemical substance is photoexcited or detected using an element.

Embodiment 1
FIG. 1 is a schematic sectional view showing the structure of an LED element according to one embodiment of the present invention.
In FIG. 1, 1 is a negative electrode (Au), 2 is a negative electrode (Ti), 3 is an n-type SiC substrate, 4 is an n-type AlGaN layer, 5 is an AlGaN multiple quantum well light emitting layer (MQW (Multi Quantum Well) layer). ) And 6 are p-type AlGaN layers, 7 is a p-type GaN contact layer, 8 is a positive electrode (Ni), and 9 is a positive electrode (Au).
That is, as the substrate on which the LED of the present embodiment is formed, an n-type 6H-SiC substrate with a carrier concentration of 10 ¹⁸ cm ⁻³ and a carrier concentration of 10 ¹⁸ cm ⁻³ oriented in a (0001) Si plane with a plane orientation accuracy of ± 0.2 °. 3 was used.
A vertical MOVPE (Metal Organic Vapor Phase Epitaxy) furnace was used for crystal growth, and the growth pressure was 39997 Pa (300 Torr). Growth was performed using ammonia (NH ₃ ), silane (SiH ₄ ), and cyclopentadienyl magnesium (Cp ₂ Mg) with a group V / III ratio of about 10,000.
In the LED shown in FIG. 1, an n-type Al _0.15 Ga _0.85 N layer 4 having a thickness of 400 nm is grown on the n-type SiC substrate 3 by using TMG, TMA, NH ₃ , and SiH _4, and using an AlGaN multiplex. TMG, TMA, TEG, and NH ₃ are used for growing the quantum well light emitting layer 5, and TMG, TMA, NH ₃ , and Cp ₂ are used for growing the p-type Al _0.15 Ga _0.85 N layer 6 having a thickness of 400 nm. Mg is used, and TMG, NH ₃ , and Cp ₂ Mg are used for GaN growth of the p-type GaN contact layer 7 having a thickness of 15 nm. The AlGaN multiple quantum well light emitting layer 5 which is a quantum well portion has five sets of quantum well layers having a thickness of 2 nm and an Al composition of 5% (0.05), and a set of 5 nm and an aluminum composition of 15% (0.15). Consists of a barrier layer. That is, in the present embodiment, the difference in Al composition between the barrier layer and the quantum well layer is within 10%, and the quantum well layer contains 5% Al. The growth temperature is 1030 ° C. An Ni layer and an Au layer were laminated as the positive electrodes (anodes) 8 and 9, and a Ti layer and an Au layer were deposited on the back surface of the substrate 3 as the negative electrodes (cathodes) 2 and 1 to form an LED element.

FIG. 2 is a diagram showing an emission spectrum at room temperature of the LED manufactured as described above. The horizontal axis represents the wavelength λ (nm), and the vertical axis represents the emission intensity.
As is clear from FIG. 2, in the LED of the present embodiment, an emission peak wavelength of about 345 nm was obtained. This is because the emission wavelength of an LD or LED (360 nm or more) which has been generally reported up to now, or a recently reported GaN / AlGaN-based LED having a thin quantum well layer (for example, Appl. Phys. Lett., Vol. 73) , P. 1668, 1998), the emission wavelength is the shortest, and the effect of the present invention is clear. That is, in an LED having a quantum well structure using a nitride III-V compound semiconductor, it was possible to shorten the wavelength while maintaining high luminous efficiency and optical characteristics of a light absorption coefficient.

Embodiment 2
With an element structure substantially similar to that of the first embodiment shown in FIG. 1, when less than 1% of In is added to the quantum well layer 5, the peak emission wavelength is 350 nm, which is slightly smaller than that of 345 nm in the first embodiment. Although the wavelength was increased, the emission intensity was about 10 times higher than that of the first embodiment.

Embodiment 3
FIG. 3A is a cross-sectional view of a nitride semiconductor light emitting diode (LED) having a semiconductor heterostructure according to another embodiment of the present invention, and FIG. 3B is a nitride semiconductor light emitting diode shown in FIG. 3 is a graph showing the Al composition with respect to the total amount of Ga and Al in each layer.
As shown in the figure, on an n-type SiC substrate 11 made of 6H—SiC and oriented on the (0001) Si plane and having a carrier concentration of 10 ¹⁸ cm ⁻³ , the composition is constant at a thickness of 300 nm and an Al composition of 15%. An n-type AlGaN layer 12 composed of a region is formed.
On the n-type AlGaN layer 12, a 75-period n-type superlattice structure (carrier supply layer) 13 made of AlGaN having two kinds of Al compositions is formed. The Al composition of the well layer constituting the n-type superlattice structure 13 is 14% (the band gap wavelength of the bulk is 340 nm or less), and the Al composition of the barrier layer is 18%. The thickness of each well layer and each barrier layer is 2 nm, and the average Al composition of the barrier layer and the well layer is 16%.
On the n-type superlattice structure 13, a multiple quantum well light emitting layer 14 made of AlGaN is formed. The multiple quantum well light emitting layer 14 includes five pairs of a quantum well layer having a thickness of 2 nm and an Al composition of 10%, and a barrier layer having a thickness of 2 nm and an Al composition of 14%.
On the multiple quantum well light emitting layer 14, a 75-period p-type superlattice structure (carrier supply layer) 15 made of AlGaN having two kinds of Al compositions is formed. The Al composition of the well layer constituting the p-type superlattice structure 15 is 14%, and the Al composition of the barrier layer is 18%. The thickness of each well layer and each barrier layer is 2 nm, and the average Al composition of the barrier layer and the well layer is 16%.
A p-type contact layer 16 made of GaN is formed on the p-type superlattice structure 15, and a positive electrode 17 made of Ni and a positive electrode 18 made of Au are laminated on the contact layer 16.
On the back surface of the n-type SiC substrate 11, a negative electrode 19 made of Ti and a negative electrode 20 made of Au are stacked.
Here, before the negative electrodes 19 and 20 were formed on the n-type SiC substrate 11, the photoluminescence spectrum of the light-emitting layer was measured. As a result, blue light emission specific to the conductive p-type nitride was shown.

FIG. 4 is a diagram showing a current density-voltage (JV) curve of the AlGaN multiple quantum well LED manufactured as described above.
At a bias voltage of 4 V, a current density of 21 A / cm ² and a differential resistance per electrode unit area of 22 mΩ / cm ² were obtained.
This result was compared with an InGaN-based LED having an average Al composition of the carrier supply layer as low as 7% and an emission wavelength as long as 400 nm of an LED element manufactured using the same apparatus. The current-carrying characteristics of the multiple quantum well LED were comparable to those of the InGaN-based LED.

FIG. 5 is a diagram showing an emission spectrum of the LED.
The light emission main peak was 343 nm, and the shortest wavelength was possible among the conventional nitride LEDs.
The energy band edge of the light emitting layer becomes the main peak of the emission spectrum, and the nitride LED has the shortest wavelength, but shows low resistance comparable to that of the InGaN-based LED. is there. To obtain such an effect, it is desirable to satisfy the following conditions (1) to (3). (1) Use a well layer of 4 nm or less which is less affected by an internal electric field due to polarization. (2) A small composition difference of 15% or less in which polarization is hardly generated. (3) A composition difference of 2% or more at which activation of impurities is effective. (4) The average Al composition of the barrier layer and the well layer is 7% or more.
That is, in the present embodiment, the structure of the carrier supply layer is such that the difference in Al composition between the barrier layer and the well layer is 2% or more and 15% or less, the average Al composition is 7% or more, and the thickness of the well layer is 4 nm or less. By doing so, it has become possible to improve the operation efficiency of the short-wavelength nitride optical semiconductor device containing Al in the light-emitting layer, and in this case, to reduce the device resistance in the case of an LED. By using such a structure, the operating wavelength of the nitride III-V semiconductor can be greatly shortened from about 360 nm in the related art.
It is needless to say that the present invention can be variously modified without departing from the gist described in the claims. That is, the configuration of the above-described embodiment shown in FIG. 1 is merely an example, and it goes without saying that the detailed configuration can take various forms. Further, in the LED element shown in FIG. 1, the difference in Al composition between the barrier layer and the quantum well layer is within 15%, and the quantum well layer preferably contains 2% or more of Al. It is needless to say that the present invention is not limited to this. Further, in the LED element shown in FIG. 3, the difference between the Al composition of the barrier layer and the well layer constituting the AlGaN superlattice structure as the carrier supply layer is 2% or more and 15% or less, and the average Al composition of the barrier layer and the well layer. Is 7% or more and the thickness of the well layer is 4 nm or less.

FIG. 1 is a schematic cross-sectional view illustrating a structure of an LED element according to an embodiment of the present invention. FIG. 2 is a diagram showing an emission spectrum of the LED element shown in FIG. 1. (A) is a cross-sectional view showing a nitride semiconductor light emitting diode (LED) having a semiconductor heterostructure according to another embodiment of the present invention. It is a graph which shows Al composition with respect to the total amount of Al. FIG. 5 is a diagram illustrating a current density-voltage (JV) curve of the AlGaN multiple quantum well LED of FIG. 3. FIG. 4 is a diagram showing an emission spectrum of the LED of FIG. 3. (A) is a conceptual diagram for explaining the principle of the present invention, and (b) is a conceptual diagram for explaining a change in emission intensity due to a piezo effect.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 ... Negative electrode (Au), 2 ... Negative electrode (Ti), 3 ... n-type SiC substrate, 4 ... n-type AlGaN layer, 5 ... AlGaN multiple quantum well light emitting layer, 6 ... p-type AlGaN layer, 7 ... p-type GaN contact layer, 8: positive electrode (Ni), 9: positive electrode (Au).

Claims (2)

In an optical semiconductor device having a light emitting layer having a quantum well structure made of AlGaN, a difference between an Al composition of a barrier layer and a quantum well layer constituting the quantum well structure is within 15%, and an Al composition of the quantum well layer is less than 15%. An optical semiconductor device, wherein the thickness is 2% or more and the thickness of the quantum well layer is 4 nm or less. 2. The optical semiconductor device according to claim 1, wherein the quantum well layer contains less than 1% of In.

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