JPH04328877A - Semiconductor device provided with chirp light reflection layer - Google Patents

Abstract

PURPOSE:To prevent a chirp light reflection layer from deteriorating in reflectivity to light of shorter wavelengths due to the absorption of light by the semiconductor which forms the reflection layer concerned. CONSTITUTION:Unit semiconductors formed of two types of semiconductors different from each other in composition are repeatedly laminated to form a light reflection layer 14a widened in wavelength of reflection light by varying the unit semiconductors in thickness, where the unit semiconductors which reflect light of shorter wavelengths are provided to the upper part of the layer 14a or the light incident side of the layer 14a.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a semiconductor device equipped with a chirped light reflecting layer.

0002

2. Description of the Related Art Light emitting diodes are widely used in optical communications, displays, sensors, and the like. Such light emitting diodes have an active layer that emits light formed on a semiconductor substrate by an epitaxial growth method such as liquid phase growth or vapor phase growth. There is a surface-emitting type that extracts light from a light extraction surface formed approximately parallel to the active layer.

By the way, the light output of a light emitting diode is determined by the internal quantum efficiency when converting electrical energy into light energy and the external quantum efficiency when extracting the generated light to the outside. In this case, for example, a light reflecting layer that reflects light by light wave interference, known as Bragg reflection, is provided on the opposite side of the light extraction surface with the active layer in between, and the light traveling to the opposite side of the light extraction surface is reflected. It is known that the light output is improved by reflecting and increasing the external quantum efficiency. The light reflecting layer has a multilayer structure in which unit semiconductors are repeatedly laminated with multiple types of semiconductors having different compositions, and reflects light of a specific wavelength based on the difference in their refractive index. For example, in the case of an infrared or red light emitting diode made of AlX Ga1-X As, a light reflecting layer is formed by epitaxially growing AlAs and GaAs to a predetermined thickness alternately. Such A
The thicknesses TA and TG of lAs and GaAs are AlX
When the emission wavelength of Ga1-X As, that is, the wavelength of the light to be reflected is λB, the refractive index of AlAs is nA, and the refractive index of GaAs is nG, the following equations (1) and (2) are obtained, respectively.
The thickness T of the unit semiconductor obtained by overlapping them is (TA + TG).

TA=λB/4nA
・・・
・(1) TG = λB /4nG

...(2)

However, the light that can be reflected by such a light reflecting layer is only light of a specific wavelength that satisfies the conditions for light wave interference, and the reflected wavelength width is relatively narrow, and the wavelength is expressed by the above equation (1), ( 2) As shown in the formula, it depends on the thickness and refractive index of the unit semiconductor. Therefore, even if there is even a slight change in the thickness or composition of the semiconductor that makes up the light-reflecting layer, the reflected wavelength range will shift from the wavelength range of light emitted from the active layer, resulting in a decrease in optical output, making manufacturing extremely difficult. There was a problem with that. Incidentally, in the case of a GaAs infrared light emitting diode, its emission wavelength has a spread of approximately ±35 nm around 880 nm, and extremely accurate film thickness control technology is required to completely cover this emission wavelength range. Furthermore, when performing epitaxial growth on a large substrate, it is difficult to grow a film with a strictly uniform thickness on the substrate surface, and uneven film thickness causes in-plane non-uniformity in reflected wavelengths, which reduces yield. It also included

On the other hand, it has been considered to expand the reflection wavelength range by changing the film thickness of the unit semiconductor. In other words, the wavelength of light reflected by the light reflecting layer is determined by the film thickness of the unit semiconductor as described above, so for example, the film thickness of the unit semiconductor may be changed stepwise for each predetermined number of layers, or if the layer is continuously stacked. In other words, the film thickness of each unit semiconductor is continuously changed. With such a light-reflecting layer, even if the thickness or composition of each semiconductor changes due to slight control errors during manufacturing, the wavelength range of light emitted from the active layer will deviate from the wavelength range reflected by the light-reflecting layer. This can be effectively prevented, and the light output improvement effect of the light reflection layer can be sufficiently obtained, and a surface-emitting light emitting diode equipped with such a light reflection layer can be manufactured easily. Further, even when epitaxially growing on a large substrate, a decrease in yield due to in-plane non-uniformity of reflection wavelength due to non-uniform film thickness on the substrate surface can be avoided. In this specification, a light reflecting layer in which the film thickness of the unit semiconductor is changed stepwise or continuously in this way is conveniently referred to as a chirped light reflecting layer.

[0007]

[Problems to be Solved by the Invention] However, after repeated research on the light reflection characteristics of such a chirped light reflection layer, it was found that the effect of improving the reflectance of light on the short wavelength side is not necessarily sufficient. This is thought to be because the semiconductor that makes up the light reflective layer absorbs light, and specifically, the wavelength λ at the absorption edge energy of the semiconductor
Since it does not absorb light with a longer wavelength than C but absorbs light with a shorter wavelength, the reflectance of light on the shorter wavelength side decreases by that amount. In the case of the light reflecting layer made of GaAs and AlAs, the absorption edge wavelength λC of GaAs is 880
nm, the half of the light on the short wavelength side of the emission wavelength range of 880±35 nm of the GaAs infrared light emitting diode is absorbed by the reflective layer and the reflectance of the light decreases. As a countermeasure for this, the absorption edge wavelength λC is shorter than the shortest wavelength 845 nm
Although it is possible to use l0.2 Ga0.8 As in place of GaAs, it is difficult to control the mixed crystal ratio and the refractive index varies, and the difference in refractive index with AlAs becomes small, which is not preferable in terms of light wave interference.

The present invention has been made against the background of the above-mentioned circumstances, and its object is to prevent the decrease in reflectance on the short wavelength side due to light absorption by a simple method.

[0009]

[Means for Solving the Problems] In order to achieve the above object, the present invention has a unit semiconductor in which a plurality of types of semiconductors having different compositions are stacked on top of each other, and the unit semiconductor is repeatedly stacked to reflect incident light by light wave interference. In a semiconductor device including a chirped light reflecting layer whose reflection wavelength range is expanded by changing the thickness of the unit semiconductor, the thinner the unit semiconductor is, the closer to the light incident side the unit semiconductor is provided. It is characterized by

0010

[Operation and Effects of the Invention] That is, as is clear from equations (1) and (2) above, since a thin unit semiconductor reflects light on the short wavelength side, if it is provided on the light incident side, , the light on the short wavelength side is reflected without entering deep into the light reflection layer, preventing absorption by the light reflection layer and improving reflectance. On the other hand, light on the long wavelength side enters deep into the light reflective layer and is reflected, but this light on the long wavelength side is not absorbed by the light reflective layer, so this does not reduce the reflectance. . Therefore, the reflectance of light on the short wavelength side can be improved without impairing the reflectance of light on the long wavelength side, and light in a wide wavelength range can be reflected well over the entire range. .

Furthermore, in the present invention, since it is only necessary to provide a thin unit semiconductor on the light incident side, the light reflecting layer is constructed using a semiconductor whose absorption edge wavelength λC is shorter than the wavelength range of the incident light. It is possible to stably and easily form a chirped light-reflecting layer having predetermined light-reflecting characteristics without problems such as finely controlling the semiconductor composition or reducing the difference in refractive index compared to when Can be done.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a diagram illustrating the structure of a surface-emitting light emitting diode 10 which is an embodiment of the present invention.
On the s-substrate 12 are an n-AlAs/n-GaAs light reflection layer 14 and an n-Al0.45Ga0.55As cladding layer 1.
6, p-GaAs active layer 18, p-Al0.45Ga0
．． A 55As cladding layer 20 and a p-GaAs cap layer 22 are sequentially laminated, and a double heterostructure is formed by the cladding layer 16, the active layer 18, and the cladding layer 20. A + electrode 26 and a - electrode 28 are provided on a part of the upper surface 24 of the cap layer 22 and on the lower surface of the substrate 12, respectively, and by applying a forward voltage between them, the activation of the double heterostructure is activated. Light is emitted from layer 18 and extracted from top surface 24 of cap layer 22 . The upper surface 24 corresponds to a light extraction surface. Further, the light reflecting layer 14 reflects the light traveling toward the substrate 12 by light wave interference, thereby improving the light output.

Each semiconductor of the surface-emitting type light emitting diode 10 is epitaxially grown using an MOCVD (metal organic chemical vapor deposition) device, and the thickness of the cladding layer 16 is approximately 2 μm, and the thickness of the active layer 18 is approximately 2 μm. is about 0.1 μm, the thickness of the cladding layer 20 is about 2 μm, and the thickness of the cap layer 22 is about 0.1 μm. Further, the light reflecting layer 14 is formed by laminating a large number of unit semiconductors 30 consisting of two types of n-AlAs semiconductors and n-GaAs semiconductors, as shown in FIG.
- The film thickness of the GaAs semiconductor is determined based on the wavelength range of light emitted from the active layer 18, that is, 880±35 nm. The composition of the semiconductor is controlled by the type and flow rate of the raw material gas introduced into the reactor of the MOCVD apparatus, and the film thickness is controlled by the flow rate and introduction time of the raw material gas. Note that the film thicknesses of each semiconductor in FIGS. 1 and 2 are not necessarily shown in accurate proportions.

Here, the light reflecting layer 14 has a chirp shape in which the thickness of the unit semiconductor 30 changes as shown in FIG. 3 or 4, and the thickness of the unit semiconductor 30 is as follows.
Based on the thickness (TA + TG), which is the sum of the thickness TA of the n-AlAs semiconductor and the thickness TG of the n-GaAs semiconductor, which are determined according to equations (1) and (2) above, assuming that the wavelength to be reflected is λB = 880 nm. The thickness is set as T. In addition, n-A in each unit semiconductor 30
The thicknesses of the lAs semiconductor and the n-GaAs semiconductor are determined such that the ratio is maintained at a constant value of TA:TG.

In the light reflecting layer 14a of FIG. 3, the film thicknesses of all the unit semiconductors 30 change continuously and linearly, and the thickness of the unit semiconductors 30 first formed on the bottom layer, that is, the substrate 12, The thickest film thickness is T(1+DD),
It decreases linearly toward the top, that is, toward the light incident side, and reaches T(1-DD) in the uppermost layer. DD is the thickness change ratio with respect to the reference thickness T, and the amount of change in the total film thickness is 2T·DD. Further, the reflection wavelength at the bottom layer unit semiconductor 30, which is the thickest, is the product λB of the reflection wavelength λB at the reference thickness T and (1+DD).
D), and the reflection wavelength at the top layer unit semiconductor 30 with the thinnest film thickness is the reflection wavelength λB at the reference thickness T.
The product of λB and (1-DD) is λB·(1-DD).

The number N of stacked unit semiconductors 30 in such a light reflection layer 14a is 30, the thickness change ratio DD is 0.05, that is, the reflection wavelengths of the unit semiconductors 30 in the bottom layer and the top layer are 880+44 nm, respectively. 88
The results of a simulation study of the light reflection characteristics in the case of 0-44 nm are shown in FIG. 5(a). For comparison, the light reflection characteristics of the light reflection layer 40a where the film thickness of the unit semiconductor 30 becomes thicker toward the top as shown in FIG. 9 was investigated under the same conditions as above, and the results are shown in FIG. (
Shown in b). The conditions for the simulation are that the light reflecting layer 14
The light is incident perpendicularly to a, 40a, and n-GaA
This takes into consideration the absorption of light by the s-semiconductor. Also,
The medium on the incident side is Al0.45, which is the same as the cladding layer 16.
Ga0.55As was used, and the medium on the opposite side was n-GaAs, which is the same as the substrate 12. From these simulation results, it has been found that the light reflection layer 14a, which has a thinner film thickness on the light incident side of the unit semiconductor 30, has improved reflectance particularly for light on the short wavelength side compared to the light reflection layer 40a on the opposite side. area 880
It can be seen that light of ±35 nm can be well reflected over the entire area.

Further, the number N of stacked layers of the unit semiconductor 30 is 30, the thickness change ratio DD is 0.1, that is, the reflection wavelength of the unit semiconductor 30 in the bottom layer and the top layer is 880+, respectively.
The light reflection characteristics in the case of 88 nm and 880-88 nm were investigated by simulation under the same conditions as above, and the results are shown in FIG. 6(a). FIG. 6(b) shows the light reflecting layer 4 of FIG.
This is the case where N=30 and DD=0.1 at 0a, and it can be seen that in this case as well, the light reflective layer 14a has improved reflectance for light on the short wavelength side compared to the light reflective layer 40a. .

On the other hand, in the light reflecting layer 14b shown in FIG. 4, the film thickness of the unit semiconductor 30 is changed in three steps for each predetermined number n of stacked layers, and the film thickness Tb on the lower layer side is larger than the reference thickness T. The film thickness Ta on the upper layer side is made thinner than the reference thickness T. In this case, the film thickness Tb is set to a value that reflects 950 nm light, that is, T·(950/880), and the film thickness Ta is set to a value that reflects 800 nm light, that is, T·(880).
00/880), the light reflection characteristics when n=5 are:
The results of a simulation under the same conditions as above are shown in FIG. 7(a). FIG. 7(b) shows a light reflecting layer 40b in which the lower layer is thinner and the upper layer is thicker as shown in FIG. This is the light reflection characteristic when set, and it can be seen that in this case as well, the light reflection layer 14b has improved reflectance for light on the short wavelength side compared to the light reflection layer 40b.

As described above, in the surface-emitting type light emitting diode 10 of this embodiment, the film thickness of the unit semiconductor 30 constituting the chirp-shaped light reflection layer 14 is made thinner toward the light incident side, so that light on the short wavelength side Since the light is reflected by the incident side portion of the light reflection layer 14, absorption by the light reflection layer 14 is prevented and the reflectance is improved. On the other hand, the light on the long wavelength side enters deep into the light reflection layer 14 and is reflected.
Since this light on the longer wavelength side is not absorbed by the light reflecting layer 14, the reflectance does not decrease as a result. Therefore, the reflectance of light on the short wavelength side is improved without impairing the reflectance of light on the long wavelength side, making it possible to reflect light in a wide wavelength range well over the entire wavelength range, making it possible to improve the reflectance of light in a surface-emitting type. The light output of the diode 10 is improved.

Furthermore, since it is sufficient to simply provide the thin unit semiconductor 30 on the light incident side, it is possible to use a semiconductor whose absorption edge wavelength λC is shorter than the emission wavelength range from the active layer 18, specifically, n-Al0. 2 Ga0.8 As is used instead of n-GaAs to form the light reflecting layer 14, it is difficult to control the mixed crystal ratio and the refractive index varies, and n-AlAs
A light reflecting layer 1 having predetermined light reflecting properties without impairing light wave interference due to a small difference in refractive index between the
4 can be formed stably and easily.

Although one embodiment of the present invention has been described above in detail based on the drawings, the present invention can also be implemented in other embodiments.

For example, in the light reflection layer 14a of the above embodiment, the film thickness of all the unit semiconductors 30 changes continuously, and in the light reflection layer 14b, the film thickness changes in stages, but as shown in FIG. As shown in FIG. 2, there is a uniform thickness part 32 in which the film thickness of the unit semiconductor 30 is a constant reference thickness T, and a uniform thickness part 32 that is provided on the upper layer side of the uniform thickness part 32 and whose film thickness increases continuously and linearly toward the top. A light reflection consisting of a first variable thickness section 34 that becomes thinner and a second variable thickness section 36 that is provided on the lower layer side of the equal thickness section 32 and whose film thickness becomes continuously and linearly thicker toward the bottom. layer 14c
can also be adopted.

The surface-emitting type light emitting diode 10 of the above embodiment has a double heterostructure having a p-GaAs active layer 18, but GaP, InP, InGaAsP
The present invention can be similarly applied to surface-emitting light-emitting diodes with a double heterostructure or single heterostructure, or surface-emitting light-emitting diodes with a homostructure, made of other compound semiconductors such as. The present invention can also be applied to other semiconductor devices including a light reflective layer such as a semiconductor laser.

Further, in the above embodiment, n-GaAs/n-
Although the AlAs light reflection layer 14 is provided, the type, composition, and film thickness of the semiconductor crystal constituting the light reflection layer are appropriately set based on the refractive index of the semiconductor crystal, the emission wavelength of the light emitting diode, and the like.

Furthermore, although the surface-emitting type light emitting diode 10 has a light extraction surface 24 formed on the opposite side of the substrate 12, the present invention can also be applied to a surface-emitting type light-emitting diode that extracts light from the substrate 12 side. .

Further, in the light reflecting layer 14a of the above embodiment, the film thickness of the unit semiconductor 30 is varied linearly, but it can also be varied along a smooth curve.

[0028] The light reflecting layer 14c also includes a unit semiconductor 3.
Although the equal thickness portion 32 has a thickness of 0 and the reference thickness T, it is also possible to provide a plurality of equal thickness portions with different thicknesses as necessary, such as when having a plurality of active layers with different emission wavelengths. It is possible. The reference thickness T does not necessarily have to correspond strictly to the emission wavelength of the active layer 18, and may be set so as to reflect light having a wavelength near the emission wavelength.

Furthermore, the light reflecting layers 14a, 14 of the above embodiments
c, the film thickness varies by ±T・DD around the reference thickness T, but it is necessary if there is a deviation in the direction of the film thickness error during manufacturing, in other words, in the direction of deviation in the reflected wavelength range. The film thickness may also be changed asymmetrically depending on. To specifically explain the light reflecting layer 14c as an example, the change rate D of the film thickness in the pair of variable thickness portions 34 and 36 is as follows.
There is no problem even if D and the number of laminated layers are set to different values, and in extreme cases, either of the variable thickness parts 34 or 3
6 can also be omitted.

Further, in the light reflecting layer 14b of the above embodiment, the film thickness of the unit semiconductor 30 is changed in three steps;
It can also be changed in stages or in four or more stages. The number of laminated layers n for each film thickness does not necessarily have to be the same,
The number n of laminated layers can also be changed as appropriate. Note that the number n of laminated layers is preferably 4 or more.

Further, in the above embodiment, the case where the surface emitting type light emitting diode 10 was manufactured using an MOCVD apparatus was explained, but it is also possible to manufacture it using other epitaxial growth techniques such as molecular beam epitaxy.

Although no other examples are given, the present invention can be implemented with various modifications and improvements based on the knowledge of those skilled in the art.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the structure of a surface-emitting light emitting diode equipped with a chirped light-reflecting layer, which is an embodiment of the present invention.

FIG. 2 is a diagram illustrating a light reflecting layer in the surface-emitting light emitting diode of FIG. 1;

FIG. 3 is a diagram illustrating a change in the film thickness of a unit semiconductor in the light reflecting layer of FIG. 2;

FIG. 4 is a diagram illustrating another aspect of change in film thickness of a unit semiconductor in the light reflecting layer of FIG. 2;

5 is a diagram showing the light reflection characteristics of the light reflection layer of FIG. 3 when the number of laminated layers N=30 and the thickness change ratio DD=0.05 in comparison with that of the light reflection layer of FIG. 9. FIG.

6 is a diagram showing the light reflection characteristics of the light reflection layer of FIG. 3 when the number of laminated layers N=30 and the thickness change ratio DD=0.1 in comparison with that of the light reflection layer of FIG. 9. FIG.

7 is a diagram showing the light reflection characteristics of the light reflection layer of FIG. 4 in comparison with that of the light reflection layer of FIG. 10. FIG.

8 is a diagram illustrating still another aspect of the change in the film thickness of the unit semiconductor in the light reflecting layer of FIG. 2. FIG.

FIG. 9 is a diagram illustrating a case where the film thickness change of a unit semiconductor is opposite to that in FIG. 3;

FIG. 10 is a diagram illustrating a case where the change in film thickness of a unit semiconductor is opposite to that in FIG. 4;

[Explanation of symbols]

10: Surface emitting type light emitting diode (semiconductor device) 14,1
4a, 14b, 14c: chirped light reflecting layer 30: unit semiconductor

Claims (1)

[Claims] Claim 1: A unit semiconductor in which multiple types of semiconductors with different compositions are stacked is repeatedly stacked to reflect incident light through light wave interference, and the thickness of the unit semiconductor is changed to expand the reflected wavelength range. What is claimed is: 1. A semiconductor device having a chirp-like light-reflecting layer, characterized in that the thinner the unit semiconductor is, the closer the unit semiconductor is provided to the light incident side.