WO1997015082A1

WO1997015082A1 - Semiconductor devices

Info

Publication number: WO1997015082A1
Application number: PCT/GB1996/002564
Authority: WO
Inventors: John Emyr Macdonald; David Ian Westwood
Original assignee: University College Cardiff Consultants Limited
Priority date: 1995-10-18
Filing date: 1996-10-17
Publication date: 1997-04-24
Also published as: GB9521460D0; AU7314196A

Abstract

A semiconductor device arranged to emit blue light comprises a substrate (10) and an active layer (12) formed of semiconductor materials, the lattice constant X of the active layer material being related to the lattice constant Y of the substrate material by the square root of 2 (∑2), and the [100] crystallographic axis of the second layer being aligned at 45° to the [100] crystallographic axis of the first layer. With this arrangement the crystal lattices of the two layers (10, 12) are aligned uniformly, so that lattice mismatch defects do not occur between layers having quite different lattice constants.

Description

Semiconductor Devices

This invention relates to semiconductor devices and is particularly directed to achieving a lattice match between adjacent layers in semiconductor devices, particularly light emitting diodes, laser diodes and transistors. The invention has particular but not sole reference to the growth of nitride layers.

Nitrides, particularly GaN, are amongst group III-V semiconductor compounds in which there is considerable present- day interest, particularly with a view to producing diodes emitting light in the blue part of the visible spectrum. Blue LED*s have been produced, but so far it has not been possible to produce blue laser diodes because of excessive numbers of defects in the crystal growth: generally these defects derive from a lattice mismatch between the nitride and the substrate on which it is grown. Thus, the problem arises from the fact that the lattice constants of nitrides are quite different from the lattice constants of suitable substrates (including sapphire, silicon and GaAs) .

We have now devised a semiconductor device structure in which problems of lattice mismatch are substantially alleviated despite a large difference between the lattice constants of adjacent layers.

In accordance with this invention there is provided a semiconductor device which comprises first and second layers of materials the lattice constants of which are related to each other by substantially the square root of 2 and the [100] crystallographic axis of the second layer is aligned at 45° to the [100] crystallographic axis of the first layer.

This invention is based on the realisation that suitable combinations of materials are available, whose lattice constants are related by substantially V2 . These include nitrides (such as inN, GaN and AIN) on the one hand and substrate materials InSb, CdTe and InAs on the other hand. Thus, the lattice constants of InN, GaN and AIN are respectively 4.98, 4.52 and 4.38 A, V2 times these lattice constants giving 7.04, 6.39 and 6.19 A, which values are close to the lattice constants of InSb (6.48A) and InAs (6.C6 A) . It is therefore possible, with different compounds based on different combinations and different proportions of elements, to achieve a close match between the two layers which have their [100] axes at 45° to each other. For example In_(l)4Ga₀₈₆N would then match exactly onto InSb.

Preferably, in the device according to the invention, the first layer is a substrate and the second layer is grown on the substrate. Typically the device comprises a plurality of further layers, including one or more layers above and/or below the second layer, preferably with their crystallographic axes aligned with that of the second layer.

Preferably the second layer comprises a nitride material. Preferably the further layers comprise nitride materials. Preferably the semiconductor device is a light emitting diode or a laser diode, preferably emitting blue light.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which:

FIGURE 1 is a schematic cross-section through a typical laser diode in accordance with this invention;

FIGURE 2 is a schematic plan view to show the relative alignments of the lattices of the substrate and layers formed on the substrate; and

FIGURE 3 is a cross-section similar to Figure 1, through a more detailed semiconductor laser diode structure.

Referring to Figure 1, there is shown a laser diode comprising a substrate 10 of e.g. InSb or InAs, an active layer 12 of e.g. GaN, and cladding layers 11,13 above and below the active layer, e.g. of Al GaN. The laser diode in practice comprises additional layers which need not be described in detail for the purpose of this specification. In accordance with the invention, the substrate lattice constant Y is equal to V2 times the lattice constant X of the layers grown on the substrate 10. Figure 2 shows diagrammatically the lattices of layers 11,12,13 with their [100] crystallographic axes aligned with the [110] axis of the substrate: the lattice of the substrate is shown by the solid lines and the lattice of the layers 11, 12, 13 is shown by the dotted lines. It will be appreciated that the [100] axis of the layers 11,12,13 is at 45° to the [100] axis of the substrate layer 10.

It is envisaged that the layers 11,12,13 will grow naturally at the required orientation on the substrate.

A more detailed semiconductor laser diode structure is shown, for the sake of example, in figure 3: the reference numerals used in Figure 3 correspond with those used in Figure 1.

Claims

1) A semiconductor device which comprises first and second layers of materials the lattice constants of which are related to each other by substantially the square-root of 2 and the [100] crystallographic axis of the second layer is aligned at 45° to the [100] crystallographic axis of the first layer.

2) A semiconductor device as claimed in claim 1, in which the first layer comprises a substrate, the second layer being grown on the substrate.

3) A semiconductor device as claimed in claims 1 or 2 , comprising a third layer disposed above or below the second layer.

4) A semiconductor device as claimed in claim 3, in which the crystallographic axes of the second and third layers are aligned.

5) A semiconductor device as claimed in any preceding claim, in which the second layer comprises a nitride material.

6) A semiconductor device as claimed in claim 5 as appended to claim 3, in which the third layer comprises a nitride material.

7) A semiconductor device as claimed in any preceding claim, in which the semiconductor device is a light emitting device.

8) A semiconductor device as claimed in claim 7, which is arranged to emit blue light.

9) A semiconductor device as claimed in any preceding claim, in which the [100] crystallographic axis of the second layer is aligned with the [110] axis of the first layer.