JPH10242511A - Strained multiple quantum well structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize a strained multiple quantum well structure by preventing the occurrence of composition separation even when an InGaAsP barrier layer is grown on a well layer having a large strain, by forming the InGaAsP barrier layer by specifying the As content of the layer within a specific range. SOLUTION: A strained multiple quantum well structure is formed on an InP substrate 1. The barrier layer 6 constituting the multi-strain quantum well structure has an In1-x Gax Asy P1-y composition (where, x and y respectively represent the Ga and As compositions which fall within the range of 0 to 1) and the As composition (y) is set at <=0.5. When the As composition (y) is <=0.5, strong PL light emission is obtained, but, when the composition (y) is >0.5, the intensity of the PL light decreases.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a strained multiple quantum well structure used for an active layer of an optical semiconductor device.

[0002]

2. Description of the Related Art A multiple quantum well structure (MQW) in which a quantum well layer having a thickness of several tens of millimeters (hereinafter simply referred to as a well layer) is sandwiched by barrier layers having a larger band gap and stacked in multiple layers. Is widely used for the active layer of the current optical semiconductor device. In recent years, a laser using a strained multiple quantum well structure (strained MQW) in which a compressive strain is introduced into a well layer as an active layer has been developed.
Many research organizations have reported that the device characteristics (threshold current, optical output, etc.) are improved as compared to MQW having a well layer lattice-matched to a conventional substrate. The element characteristics improve as the strain amount of the well layer increases. However, a well layer having a desired strain amount is sandwiched between barrier layers lattice-matched to a substrate, and these layers are alternately stacked in multiple layers to form a strain MQW.
When the thickness of the entire strained MQW exceeds a certain critical value (a so-called critical film thickness), misfit dislocations occur at the interface between the substrate and the strained MQW. The larger the strain of the well layer, the smaller the critical film thickness. This is because the stress due to the strain in the well layer is accumulated in the strain MQW as the number of well layers increases, and this stress causes misfit dislocation. To improve the device characteristics, a large strain needs to be applied, but the thickness of the strain MQW is limited by the critical film thickness. In order to avoid this, a distortion compensation type distortion MQW has been proposed. In this strain compensation type strain MQW, the stress that causes misfit dislocations is offset by combining a well layer having compressive strain with a barrier layer having tensile strain, thereby suppressing the occurrence of misfit dislocations. is there. As a guideline, when fabricating a strain-compensated strain MQW, it has been proposed that the effective strain ε defined by the following equation (1) is made substantially zero [BI Miller et al.
I., Appl. Phys. Lett., 58, (1991), p. 1952]. ε = (ε _w h + ε _b H) / (h + H) (1) where h and H in the above equation (1) are the thickness of the well layer and the barrier layer, respectively, and ε _w and ε _b Represents the strain of the well layer and the barrier layer (the tensile strain uses a sign of − and the compressive strain uses a sign of +). That is, selecting the thickness and strain of the well layer and the barrier layer so as to minimize the effective strain ε is a condition for growing the strain-compensated strain MQW without misfit dislocation. In the strain-compensated strain MQW, as described above, the well layer of the compressive strain and the barrier layer of the tensile strain (or
The combination of the tensile strain well layer and the compressive strain well layer) causes a large strain discrepancy at the interface between the well layer and the barrier layer. Therefore, it is known that when growing a barrier layer on a well layer, the barrier layer does not grow in a layered manner but grows in a three-dimensional island shape. Therefore, when islands are united, defects (defects other than misfit dislocations) are formed, and the strain MQ
W. The optical properties of W were degraded [AG Culliset al.,
Journal of Crystal Growth, 158, (1996), pp. 15-2
7]. Similar defects are introduced when a well layer is grown on the barrier layer. Such defect introduction becomes more remarkable as the difference in strain between the well layer and the barrier layer becomes larger, and becomes more remarkable as the growth temperature becomes higher. Therefore, the distortion compensation type distortion MQ
Even if the effective strain is completely zero in W, there is a problem that optical characteristics are deteriorated due to defects other than misfit dislocations. Applying tensile strain on the InP substrate causes the composition of InGaAsP, which is a barrier layer, to approach or enter the immiscible region. Therefore, it is considered that the InGaAsP layer becomes thermodynamically unstable and causes composition separation, which causes the above-described island-like growth.

[0003]

As described above, in the conventional strain-compensated strain MQW, a large difference in strain occurs at the interface between the well layer and the barrier layer. Therefore, when the barrier layer is grown on the well layer, the barrier layer does not grow in a layered form but grows in a three-dimensional island form, and defects (misfit dislocations) are formed when the grown layers are united. Defects other than the above) and deteriorated the optical characteristics of the strain MQW. Further, when a well layer is grown on the barrier layer, the same defects as described above occur. Therefore, in the strain compensation type MQW, even if the effective strain is almost zero (zero), optical defects are caused by defects other than misfit dislocations. There is a problem that characteristics are deteriorated. Also, InP
Applying a tensile strain on the substrate is caused by the barrier layer I
It is considered that the composition of nGaAsP approaches or enters the immiscible region, so that the InGaAsP layer becomes thermodynamically unstable and causes compositional separation, which causes the island growth.

An object of the present invention is to solve the above-mentioned problems in the prior art and to form InGaAs on a well layer having a large strain.
No composition separation occurs even when a barrier layer made of P is grown.
That is, the barrier layer is prevented from growing in a three-dimensional island shape, and the generation of defects other than misfit dislocations is suppressed,
An object of the present invention is to provide a strained multiple quantum well structure having a stable MQW structure and excellent optical characteristics.

[0005]

Means for Solving the Problems In order to achieve the object of the present invention, the present invention is configured as described in the claims. That is, according to the present invention, as described in claim 1, a strained multiple quantum well structure formed on an InP substrate, wherein the barrier layer constituting the strained multiple quantum well structure is formed of In _1-x Ga _x As _y P _1-y (where x is Ga composition, y
Represents an As composition, each of which represents a range of 0 to 1. ) And a strained multiple quantum well structure with an As composition y of 0.5 or less. According to a second aspect of the present invention, in the first aspect, the well layer is made of InGaAs and has a strained multiple quantum well structure in which the emission wavelength of the strained multiple quantum well structure is in the 2 μm band. Further, the present invention provides the first aspect as described in the third aspect.
Wherein the well layer is made of InAsP and has a strained multiple quantum well structure in which the emission wavelength of the strained multiple quantum well structure is in the 1.55 μm band. According to a fourth aspect of the present invention, there is provided a strained multiple quantum well structure according to any one of the first to third aspects, wherein the number of well layers is four or more. The strained multiple quantum well structure of the present invention comprises:
The barrier layer in the strained multiple quantum well structure formed on the InP substrate may be made of In.
_{1-x Ga x As y P} 1-y consisting of a composition, and the barrier layer A
By setting the s composition y to 0.5 or less, it is possible to realize a strained multiple quantum well structure having a stable structure and excellent optical characteristics with no composition separation even when a barrier layer is grown on a well layer having a large strain. effective.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION

<Embodiment 1> FIG. 1 is a schematic diagram showing a configuration of a strained multiple quantum well structure exemplified in this embodiment. In the figure, an In-type InP substrate 1 having a (001) plane is
P buffer layer 2, InGaAsP guide layer 3 having a thickness of 1000 °, four InGaAs well layers 5, five In layers
Strained multiple quantum well structure 4 comprising GaAsP barrier layer 6, InGaAsP guide layer 7 having a thickness of 1000 °, thickness 800
The InP cap layer 8 is laminated. InGaAs
The P guide layer 3 is lattice-matched to the n-type InP substrate 1, and its composition can be changed according to the emission wavelength of the strained multiple quantum well structure. Further, the compositions of the InGaAs well layer 5 and the InGaAsP barrier layer 6 can be changed according to the emission wavelength. In order to obtain strained MQW having an emission wavelength of 2 μm, which is one application of the present invention, it is necessary to form a well layer of In _0.76 Ga _0.24 As with a thickness of 120 °.
(Strain: 1.65%), lattice matched to the substrate as a barrier layer, and InGaAsP (thickness: 200 μm) having an emission wavelength of 1.2 μm.
Å), the same emission wavelength as the barrier layer as the guide layer: 1.2 μm
Of InGaAsP is used. Since a laser in the 2.05 μm band has an oscillation wavelength corresponding to an absorption band of gas molecules such as carbon dioxide (CO ₂ ), it is expected to be applied to gas sensors and medical equipment (for example, JP-A-5-142146). ). The growth of thin films is based on metalorganic molecular beam epitaxy (MO
MBE) method [H. Sugiura, M. Mitsuhara, H. Oohashi,
T. Hirono, and K. Nakashima, J. Crystal Growth, 1
47, (1995), 1.]. Growth by the MOMBE method
A V-400 CBE device manufactured by VG was used. Group III materials used were trimethylindium (TMI) and triethylgallium (TEG), and group V materials used were phosphine (PH ₃ ) and arsine (AsH ₃ ). The substrate temperature during growth was 520 ° C. The substrate temperature was measured using a pyrometer calibrated with the melting point of InSb. In FIG. 2, FIG.
3 shows the composition dependence of the PL intensity of the strained multiple quantum well structure of FIG. The barrier layer was lattice-matched to the InP substrate. Also,
In order to obtain an emission wavelength in the 2 μm band, the thickness of the well layer is set to 120 ° and the composition is set to In _0.76 Ga _0.24 As (strain: 1.65).
%). The composition of the guide layer was the same as that of the barrier layer. The composition of the barrier layer is such that the emission wavelength is 1.1 μm, 1.2 μm,
1.3 μm and 1.5 μm InGaAsP (1.1 μm each)
Q, 1.2Q, 1.3Q, 1.5Q) and InG
aAs was used. 1.1Q, 1.2Q, 1.3Q, 1.5Q
And InGaAs have an As composition y of 0.3, respectively.
3, 0.48, 0.61, 0.85, and 1.0. Also,
The thickness of the barrier layers was all 200 °. When the As composition is less than 0.5, strong PL emission is obtained, but when the As composition exceeds 0.5, the PL emission intensity decreases. And A
When barrier layers with s composition y of 0.85 and 1.0 are used, PL
No luminescence was obtained. FIG. 3 schematically shows a result of observing a cross section of a strained MQW composed of InGaAs (well layer) / InGaAsP (barrier layer) with a transmission electron microscope (TEM). When the As composition y of the InGaAsP barrier layer 16 is 0.61 [FIG. 3B], the interface between the barrier layer and the InGaAs well layer 15 grown thereon fluctuates. This fluctuation increases as the barrier layers are stacked. Then, in the third barrier layer, the fluctuation of the interface becomes upwardly convex (1).
11) Defects along the plane have occurred. It is considered that PL emission could not be obtained due to the generation of this defect. Considering the state where the interface between the barrier layer and the well layer grown thereon fluctuates, when the barrier layer grows, irregularities are generated on the surface of the barrier layer, and the interface with the well layer fluctuates. It is considered something. Then, the InGaAsP barrier layer 16
When the As composition y is 0.48 (FIG. 3A), the interface between the barrier layer and the InGaAs well layer 15 grown on the barrier layer is flat, and generation of defects along the (111) plane is observed. Did not. Since no irregularities were generated on the surface during the growth of the barrier layer, it is considered that the interface between the barrier layer and the well layer grown thereon became flat. That is, it is considered that when the As composition y of the barrier layer is reduced from 0.61 to 0.48, the growth state of the barrier layer is changed and a flat growth surface is obtained. Next, the cause of unevenness on the surface of the barrier layer will be considered. FIG.
3 shows the relationship between the composition of nGaAsP and the immiscible region. A straight line extending obliquely from the lower left to the upper right represents a composition lattice-matched to InP. The inside of the ellipse shown in FIG. 4 represents an immiscible region at a temperature of 520 ° C. InGaAsP having a composition in this region has a temperature of 52%.
At 0 ° C., a phase having a composition close to GaP
There is a tendency to separate into two phases with compositions close to s.
In a composition lattice-matched to InP, the range of As composition y from 0.45 to 0.98 is 520 close to the growth temperature.
Enter the immiscible region at ° C. The range of the As composition y from 0.45 to 0.98 substantially corresponds to the range of the As composition y of the barrier layer when the PL emission intensity decreases in FIG. From this, it is considered that the composition separation in the barrier layer may be the cause of the occurrence of unevenness on the barrier layer surface during growth. The fact that the compositional separation in the barrier layer causes unevenness of the barrier layer surface during growth and, consequently, the occurrence of defects, was achieved by growing strained MQW by a metal organic vapor phase epitaxy method which is a growth method different from the present invention. Has also been reported to occur (Elimination of wavy layer gr
owth phenomena in strain-compensated GaInAsP / GaI
nAsP multiple quantum well stacks.RLGlew, K.Scarr
ott, ATRBriggs, ADSmith, VAWilkinson.Journal o
f Crystal Growth, vol. 145, published in 1994, pp. 764-77
0). Further, InGaAs having an emission wavelength of 1.3 μm and 1.55 μm, which has been used as an active layer of an optical element, has been used.
InGaAsP having a composition in an immiscible region including sP
It is evident from the observation with a transmission electron microscope that composition separation occurs (Spinodal-like decomposition
of InGaAs (P) / (100) InP grown by gas source mole
cular beam epitaxy, RRLaPierre, T.Okada, BJRobins
on, DAThompson, GCWeatherly, Journal of Crystal G
rowth, vol.155, published in 1995, pp.1-15). FIG. 5 is a sectional view of a semiconductor laser having a strained multiple quantum well structure of the present invention in an active layer. N-type In having (001) plane as substrate
A P-substrate 21 (electron concentration: 2 × 10 ¹⁸ cm ³ ) was used, and a 0.5 μm thick Sn-doped InP buffer layer 22 (electron concentration: 5 × 10 ¹⁷ cm ³ ) and a thickness 1000
Ｉｎ InGaAsP guide layer 23, four layers of InGaAs
A well layer (thickness: 120 °, 1.65% compressive strain) and five InGaAsP barrier layers (thickness: 200 °, As composition: 0.4)
8, strained multiple quantum well structure 2)
4, Be-doped InP cladding layer 28 of InGaAsP guide layer 27, the thickness of 0.3μm thickness 1000 Å (hole ^{concentration: 5 × 10 17 cm ~ 3} ), a thickness of 1.2 [mu] m Zn-doped InP cladding layer 29 ( Hole concentration: 2 × 10 ¹⁸ cm
~ ^3), Zn-doped InGaAs contact layer 30 (hole concentration of thickness 0.3 [mu] m: are stacked ^{5 × 10 18 cm ~ 3)} . After the growth from the Sn-doped InP buffer layer 22 to the Be-doped InP clad layer 28 was performed by the MOMBE method, the wafer was once taken out into the atmosphere, and then the Zn-doped InP clad layer 29 was formed thereon by the metal organic vapor phase epitaxy method. And Zn-doped InGaAs contact layer 30
Grew. The Zn-doped InGaAs contact layer 30 is lattice-matched to the InP substrate. An n-electrode 31 is placed on the n-type InP substrate 21 side of the grown wafer,
forming a p electrode 32 on the nGaAs contact layer 30 side;
A light emitting device having a strained multiple quantum well structure 24 as shown in FIG. 5 was completed. The wafer having the laser structure shown in FIG. 5 was mesa-processed on the side of the p-electrode 32 to form a broad contact laser, and the threshold current density was evaluated. as a result,
When the cavity length was 900 μm, the threshold current density was 2.0 kA / cm ² . The oscillation wavelength is 2.05μ
m.

[Embodiment 2] FIG. 6 is a structural view of a second embodiment of the present invention. N-type InP with (001) plane
An InP buffer layer 42 having a thickness of 1000
歪 InGaAsP guide layer 43, four InAsP well layers 45, and five InGaAsP barrier layers 46, strained multiple quantum well structure 44, 1000 Å thick InGaAs
sP guide layer 47, 800 nm thick InP cap layer 4
8 are stacked. The InGaAsP guide layer 43 is lattice-matched to the n-type InP substrate 41, and its composition is changed according to the emission wavelength of the strained multiple quantum well structure 44. Also, InA
The compositions of the sP well layer 45 and the InGaAsP barrier layer 46 are also changed according to the emission wavelength. In order to obtain strained MQW having an emission wavelength of 1.55 μm, which is one application of the present invention, InAs _0.7 P _0.3 (strain: 2.1%) having a thickness of 50 ° is used as a well layer.
And lattice-matched to the substrate as a barrier layer to obtain an emission wavelength of 1.1.
InGaAsP having a thickness of 100 .mu.m and InGaAsP having a light emission wavelength of 1.1 .mu.m, which is the same as that of the barrier layer, are used as the guide layer. The strain MQW using InAsP as a well layer is InA
Since the conduction band discontinuity at the interface between sP and InGaAsP is large, there is theoretical prediction that a laser with a small change in the oscillation wavelength due to temperature and a small change in the threshold current can be obtained. T. Hirono, S. Seki,
H. Sugiura, J. Nakano, M. Yamamoto, Y. Tohmori, an
d K. Yokoyama, J. Appl. Phys., 77, (1995), p. 411
9.]. FIG. 7 shows the dependency of the PL emission intensity on the As composition of the barrier layer in the strained multiple quantum well structure shown in FIG. The barrier layer was lattice-matched to InP. The thickness and composition of the well layer are adjusted to 50
Å and InAs _0.7 P _0.3 (strain: 2.1%). Also,
The composition of the guide layer was the same as that of the barrier layer. The composition of the barrier layer is such that the emission wavelengths are 1.1 μm and 1.3 μm InGaAsP.
(Abbreviated as 1.1Q and 1.3Q, respectively). 1.1
As compositions of Q and 1.3Q are 0.33 and 0.61, respectively.
It is. The thicknesses of the barrier layers were all 100 °. As
When the composition was 0.33, strong PL emission was obtained, but when the As composition was 0.61, no emission was observed. When the As composition of the barrier layer was 0.33, no compositional separation occurred in the barrier layer, and thus a good strain-free MQW without defects was obtained. Therefore, strong PL emission was obtained. However, when the As composition of the barrier layer was 0.61, composition separation occurred in the barrier layer, and PL was not emitted due to generation of defects.

[0008]

According to the strained multiple quantum well structure of the present invention,
InGaAsP having an As composition y of 0.5 or less as a barrier layer
Is used, no compositional separation occurs even when grown on a well layer having a large strain. Therefore, there is a remarkable effect that a strained multiple quantum well structure having a stable structure and excellent optical characteristics can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of a strained multiple quantum well structure exemplified in Embodiment 1 of the present invention.

FIG. 2 is a diagram showing a relationship between an As composition of a barrier layer of a strained multiple quantum well structure and a PL emission intensity exemplified in the first embodiment of the present invention.

FIG. 3 shows the results of observation of strain MQW by a transmission electron microscope (TEM) when the As composition of the barrier layer of the strained multiple quantum well structure exemplified in the first embodiment of the present invention is 0.48 and 0.61. FIG.

FIG. 4 is an explanatory diagram showing a relationship between an immiscible region and an As composition of a barrier layer in the strained multiple quantum well structure exemplified in the first embodiment of the present invention.

FIG. 5 is a schematic diagram showing a configuration of a semiconductor laser having a strained multiple quantum well structure exemplified in Embodiment 1 of the present invention in an active layer.

FIG. 6 is a schematic diagram showing a configuration of a strained multiple quantum well structure exemplified in Embodiment 2 of the present invention.

FIG. 7 is a diagram showing a relationship between an As composition of a barrier layer of a strained multiple quantum well structure and a PL emission intensity exemplified in the second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... n-type InP board 2 ... InP buffer layer 3 ... InGaAsP guide layer 4 ... Strained multiple quantum well structure 5 ... InGaAs well layer 6 ... InGaAsP barrier layer 7 ... InGaAsP guide layer 8 ... InP cap layer 13 ... InGaAsP guide layer 15 ... InGaAs well layer 16 ... InGaAsP barrier layer 17 ... InGaAsP guide layer 21 ... n-type InP substrate 22 ... Sn-doped InP buffer layer 23 ... InGaAsP guide layer 24 ... Strained multiple quantum well structure 27 ... InGaAsP guide layer 28 ... Be-doped InP clad layer 29 Zn-doped InP cladding layer 30 Zn-doped InP cladding layer 31 n-electrode 32 p-electrode 41 n-type InP substrate 42 InP buffer layer 43 InGaAsP guide layer 44 strained multiple quantum well structure 45 InA P-well layer 46 ... InGaAsP barrier layer 47 ... InGaAsP guide layer 48 ... InP cap layer

Claims (4)

[Claims] 1. A strained multiple quantum well structure formed on an InP substrate, a barrier layer constituting a strained multiple quantum well structure, in _{In 1-x Ga x As y} P 1-y ( wherein, x is a Ga composition,
y is an As composition, and each represents a range of 0 to 1. ), And wherein the As composition y is 0.5 or less. 2. The method according to claim 1, wherein the well layer is made of InGaAs.
Wherein the emission wavelength of the strained multiple quantum well structure is in the 2 μm band. 3. The structure of claim 1, wherein the well layer is made of InAsP, and the emission wavelength of the strained multiple quantum well structure is 1.55 μm.
A strained multiple quantum well structure characterized by being a band. 4. The strained multiple quantum well structure according to claim 1, wherein the number of well layers is four or more.