JPH09246671A - Strained multiple quantum well structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inhibit the generation of a misfitted dislocation in a distorted multiple quantum well structure in the case where an effective strain in the structure is zero by a method wherein the inverted mesa sector substrate of a wafer having the orientation (001) is used as the substrate for the structure. SOLUTION: A multiple quantum well structure consisting of well layers (the strain is εw and the thickness is (h).), which have a grating constant different from that of an inverted mesa sector substrate, and barrier layers (The strain is εb and the thickness is (h).), which have a lattice constant different from that of the substrate, have a strain of a sign different the sign of the strain of the well layers and have a band gap wider than that of the well layers, is formed on the inverted mesa sector substrate. The inverted mesa sector substrate indicates a substrate cut off from two inlays in the orientation [110] and the orientation [-1-10] in a wafer having the orientation (001). By using the inverted mesa sector substrate in such a way, the generation of a misfitted dislocation in the structure can be inhibited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a strained multiple quantum well structure used for an active layer of an optical semiconductor device. Especially,
The present invention relates to a strain-compensated multiple quantum well structure having a structure in which strain of opposite sign is introduced into a barrier layer so as to cancel strain of the well layer.

[0002]

2. Description of the Related Art A multi-quantum well structure (MQW) in which a well layer having a thickness of several nm is sandwiched between barrier layers having a bandgap larger than that and a multi-layered structure is widely used as an active layer of a current optical semiconductor device. It's being used. In recent years, a laser using an MQW in which a compressive strain has been introduced into the well layer as an active layer has improved device characteristics (threshold current, optical output) as compared with a conventional MQW using a well layer lattice-matched to a substrate. It has been reported by many research institutions. However,
When an MQW is formed by sandwiching a well layer having a desired amount of strain between barrier layers lattice-matched to a substrate and alternately stacking them in multiple layers, the total MQW thickness has a certain critical value (so-called critical film thickness). ), Misfit dislocations occur at the interface between the substrate and MQW. The critical film thickness decreases as the strain of the well layer increases. This is because the stress due to the strain in the well layer is accumulated every time the number of well layers increases, and this stress causes the occurrence of misfit dislocations. In order to improve the device characteristics, it is necessary to apply a large strain, but the thickness of MQW is limited by the critical film thickness. In order to avoid this, distortion compensation type M
QW was invented. In the strain-compensated MQW, a well layer having tensile strain is combined with a barrier layer having compressive strain to cancel the stress that causes misfit dislocations and suppress the occurrence of misfit dislocations. Therefore, an MQW having a large thickness can be obtained even for a well layer having a large strain.

In producing a strain-compensated MQW, it has been proposed as a guideline that the effective strain ε defined by the following equation be made almost zero (BIMiller et.al., Appl.
Phys. Lett., 58 (1991) 1952). ε = (ε _w h + ε _b H) / (h + H) (1) where h and H are the thicknesses of the well layer and the barrier layer, respectively, and ε _w
And ε _b are strains of the well layer and the barrier layer, respectively (tensile strain is −, compressive strain is +). That is, selecting the thickness and strain of the well layer and the barrier layer so that the effective strain ε is as small as possible is a guideline for growing a multiple quantum well layer without misfit dislocations.

In the strain-compensated MQW, there is a large strain discrepancy at the interface between the well layer and the barrier layer as described above. Therefore, when growing the barrier layer on the well layer,
It is known that the barrier layer does not grow in layers but grows in three-dimensional islands. Therefore, defects (defects other than misfit dislocations) were introduced when the islands were united, and the optical characteristics of the MQW were deteriorated. Similar defect introduction occurs when growing a well layer on a barrier layer. The introduction of such defects is more remarkable as the difference in strain between the well layer and the barrier layer is larger. Therefore, in the strain-compensated MQW, even if the effective strain is set to zero, there is a problem that optical characteristics are not improved due to defects other than misfit dislocations.

The closer the effective strain is to zero, the larger the strain difference between the well layer and the barrier layer. If the effective strain can be stopped at a certain finite value other than zero, the strain discrepancy between the well layer and the barrier layer need not be large, and defects other than misfit dislocations can be suppressed. It is desired to suppress the occurrence of misfit dislocations when the effective strain is not zero.

[0006]

The present invention has been proposed in order to improve the above-mentioned drawbacks, and an object thereof is to suppress the occurrence of misfit dislocations when the effective strain is not zero.

[0007]

In order to achieve the above object, the present invention provides a well layer having a lattice constant different from that of a substrate and a lattice constant different from that of the substrate. In a strained multi-quantum well structure composed of a layer and a barrier layer having a different sign strain and a bandgap wider than that of the well layer, the substrate is an inverted mesa sector of a wafer having a (001) plane orientation. Is characterized by.
Conventionally, a forward mesa direction sector and a reverse mesa direction sector have been used without distinction as a substrate for forming an MQW. In contrast, the present invention is different in that only the reverse mesa sector is used.

A misfit dislocation of MQW is formed by a threading dislocation from the substrate bending at the interface between the MQW and the substrate. When a film having a lattice constant different from that of a substrate having a (001) plane orientation is grown, if the film thickness increases and exceeds a certain value, the [-110] direction and the [1-10] direction ( If misfit dislocations extending in the forward mesa direction are formed and the film thickness is further increased, [11
Misfit dislocations extending in the [0] direction and the [-1-10] direction (reverse mesa direction) are formed. That is, the critical film thickness for misfit dislocations in the reverse mesa direction is larger than that in the forward mesa direction. Since the reverse mesa sector substrate used in the present invention does not have dislocations that form misfit dislocations extending in the forward mesa direction, the critical film thickness becomes larger than in the past, and the formation of misfit dislocations is suppressed.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention has a well layer having a lattice constant different from that of a substrate, a lattice constant different from that of the substrate, and a strain having a sign different from that of the well layer. However, in a strained multiple quantum well structure composed of a barrier layer having a bandgap wider than that of the well layer, the substrate is an inverted mesa sector of a wafer having a (001) plane orientation. Conventionally, a forward mesa direction sector and a reverse mesa direction sector have been used without distinction as a substrate for forming an MQW. In contrast, the present invention is different in that only the reverse mesa sector is used.

[0010]

FIG. 1 schematically shows a strained multiple quantum well structure according to the present invention. A well layer (strain ε _w , thickness h) having a lattice constant different from that of the substrate on the reverse mesa sector substrate
And a barrier layer (strain ε _b , thickness H) having a lattice constant different from that of the substrate, a strain having a sign different from that of the well layer, and a bandgap wider than that of the well layer. MQW consisting of The effective strain ε of MQW is calculated using the equation (1).

The reverse mesa sector substrate is, as shown in FIG.
A substrate cut out from two quadrants in the [110] and [-1-10] directions in a wafer having a (001) plane orientation. Further, the one cut out from the two quadrants in the [-110] and [1-10] directions will be referred to as a forward mesa sector substrate. In the reverse mesa sector substrate, there are many dislocations that become misfoot dislocations that extend in the reverse mesa direction, and there are almost no dislocations that become misfit dislocations that extend in the forward mesa direction. On the other hand, in the forward mesa sector substrate, many dislocations are misfit dislocations extending in the forward mesa direction, and few dislocations are misfit dislocations extending in the reverse mesa direction. This is due to the thermal history of growing a wafer ingot by the liquid sealed Czochralski method (H.Ono, T.Kitano, and
J. Matsui, Slip dislocation in In-doped liquid enca
psulated Czochralski GaAs duringcrystal growth, Ap
pl.Phys.Lett.vol.51, No.4, p.238-240,1987).

FIG. 3 is a diagram showing the effect of the present invention.
On an InP 2-inch wafer with a (001) plane orientation,
It is a diagram schematically showing the occurrence of misfit dislocations when growing MQW. Tensile strain as well layer-
InGaAsP with a thickness of 1.8% and a thickness of 100Å is used as a barrier layer and InGaA with a compression strain of 0.6% and a thickness of 150Å.
sP is used. The effective strain is -0.36%. The growth was performed by a metalorganic molecular beam epitaxy (MOMBE) method. The growth temperature is 520 ° C. When the presence or absence of misfit dislocations was evaluated using the μ-PL method, there were no misfit dislocations in the two quadrants of the [110] and [−1-10] directions, and the [−110] and [1-10] directions were found. The presence of misfit dislocations extending in the forward mesa direction was confirmed in the two quadrants. Despite forming MQW with the same thickness and the same effective strain, misfit dislocations occurred in the forward mesa sector and no misfit dislocations occurred in the reverse mesa sector. Therefore, by using the reverse mesa sector substrate, It can be seen that the occurrence of misfit dislocations is suppressed.

[0013]

EFFECT OF THE INVENTION A well layer having a lattice constant different from that of the substrate, a lattice constant different from that of the substrate, and a strain having a sign different from that of the well layer, In the strained multiple quantum well structure including the barrier layer having a wide band gap, the use of the inverted mesa sector substrate has an effect of suppressing the occurrence of misfit dislocations.

[Brief description of drawings]

FIG. 1 shows a strained multiple quantum well structure according to the present invention.

FIG. 2 is a diagram illustrating an inverted mesa sector substrate.

FIG. 3 is a diagram showing an effect of the present invention.

Claims (1)

[Claims] 1. A well layer having a lattice constant different from that of the substrate, a lattice constant different from that of the substrate, and a strain having a sign different from that of the well layer. A strained multiple quantum well structure comprising a barrier layer having a wide bandgap, wherein the substrate is an inverted mesa sector of a wafer having a (001) plane orientation.