GB2142045A

GB2142045A - Growth of semiconductors

Info

Publication number: GB2142045A
Application number: GB08414018A
Authority: GB
Inventors: Rodney Hollands Moss
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1983-06-15
Filing date: 1984-06-01
Publication date: 1985-01-09
Also published as: GB2142045B; GB8414018D0

Abstract

Semiconductor material is grown on a substrate in an apparatus comprising a tube 2, 3 and, within and along the tube, an elongate member 4 having means 6 for the location of the substrate on its side surface 5. Suitable means 14, 15, 16, 17, 18 are used to establish two or more gas flows along the tube and impinging on the side surface of the elongate member at differing circumferential positions. By rotating the elongate member relative to the tube the substrate can be moved from one gas flow to another sufficiently rapidly to permit the deposition of quantum well layers. <IMAGE>

Description

SPECIFICATION Growth of semiconductors The present invention relates to the growth of semiconductors.

Various vapour growth techniques are known for semiconductors wherein an appropriate gas composition is passed over a substrate on which the growth occurs.

It is frequently necessary to employ two or more differing gas compositions sequentially to a substrate. This may be necessary even in the growing of a single semiconductor layer on the substrate (eg for protection or etching of surfaces). It will be necessary also when two or more distinct layers are to be grown.

It is possible to achieve the sequential exposure of the substrate to differing gas compositions simply by changing the gas flows through a reactor zone in which the substrate is situated. However, this changeover technique does not offer very good control. In particular, the durations of exposure to each of the compositions or to (undefined) mixtures of the two compositions during the changeover period may be uncertain. This lack of control is especially serious when one or more of the layers to be grown is very thin (ie when the corresponding growth time is very short).

Very thin layers are required, for example, in "quantum well" devices, in particular in quantum well lasers. Typically, the layer thicknesses involved in quantum well lasers are envisaged to be from 50 A (5nm) to 200 A (20 nm) - see for example M G Burt, Electronics Letters, volume 19, pages 210 to 211 (1983).

Of course, more rapid changeover of gas compositions in a reactor may be achieved by the use of higher gas flow rates. The use of higher flow rates is believed to affect the crystal quality adversely, so that the advantage of the improved control of growth times is offset.

An alternative technique is to physically move the substrate from one gas flow to another, but we are not aware of any previous particularly convenient method of doing this.

The methods of which we are aware tend to be complicated and/or offer a high risk of cross-contamination of the gas flows, either directly or by carry-over with the moving substrate.

Previous descriptions of apparatus for the growth of semiconductor materials by passing gas compositions over substrates include R H Moss and J S Evans, J Crystal Growth, volume 55, pages 129-134 (1981); A K Chatterjee, M M Faktor, R H Moss, and E A D White, Journal de Physique, Colloque C5, supplement to volume 43, nos 12, pages C5 491-503(1982); and G H Olsen and T J Zamerowski, Progress in Crystal Growth and Characterisation, volume 2, pages 309-375 (1979).

The present invention provides apparatus for the growth of a semiconductor material on a substrate which comprises a tube; an elongated member within and along the tube and having means on the side surface thereof for the location of a substrate; means for establishing two or more gas flows along the tube and impinging on the side surface of the elongate member at differing circumferential positions; and means for relative rotation of the elongate member and the tube permitting the movement of a substrate located on the said side surface from one gas flow to another.

Preferably, the said side surface is substantially conical. The term "conical" herein is said to be understood to include "frustoconical". Preferably, the means for the location of the substrate on the side surface include a depression, especially a longitudinal groove.

The present invention further provides a method of growth of semiconductor material on a substrate by use of the apparatus which comprises steps of (i) locating the substrate in the side surface of the elongate member by the relevant means; (ii) establishing two or more gas flows by the relevant means; and (iii) exposing the substrate sequentially to at least two such gas flows by relative rotation of the elongate member and the tube by the relevant means.

The apparatus provided by the present invention will now be more specifically described, by way of example, with reference to the accompanying Figures, of which Figure 1 is an associated plan and section of an apparatus according to the invention; and Figure 2 is an elevation of the elongate member in Fig. 1 carrying three substrates, this elevation being in the direction of arrow B in Fig. 1.

In the Figures, 1 represents a water jacket having inner and outer walls 2 and 3 respectively which serves as the tube in accordance with the invention. The elongate member 4 includes a frustoconical side surface 5. The elongate member and the tube have a common vertical axis. Typically, the dimensions of the frustoconical surface 5 would be 44 mm at the narrow end and 60 mm at the broad end, the overall length being 90 mm.

Substrates 7, 8, and 9 are located on surface 5 by means of the shallow longitudinal groove 6 and appropriate protusions or other formations (not shown) on the bottom or sides of the groove.

The apparatus is designed for the establishment of up to four distinct gas flows, each of these within quadrants 10, 11, 1 2, and 1 3 and directed downwards towards the broader end of the conical surface 5. The means for establishing these flows are not fully shown but include four similar fins 14, 1 5, 16, and 1 7 in the upper extension 1 8 of tube 1.

The substrates can be moved between the gas flows by rotation of the elongate member 4 about axle 19.

The groove 6 is of constant depth so that substrate 9 is at a greater distance from the axis of the elongate member than substrate 8, which is in turn at a greater distance than substrate 7. The substrates lie flat in the groove so that the normal to their faces is inclined to be slightly above the horizontal.

This arrangement serves to reduce the effect of depletion of the gas flow by previous exposure to other parts of an individual substrate or to other substrates. The placing of the substrates in a groove makes the establishment of non-turbulent flow over the substrates easier. The vertical orientation of the axes of the tube and elongate member reduce effects due to convection.

The apparatus as described above may be used conveniently for the deposition of semiconductor material by the metal organic chemical vapour deposition method (MOCVD) which has been described elsewhere, eg in the papers by Moss el al and Chatterjee et al already referred to. In this method, the metallic elements of a desired semiconductor material are provided in the form of volatile organometallic compounds included in the gas flow. For example, indium may be provided in the form of (CH3)3 lnP(C2H5)3, and the use of a mixture of (CH3)3 lnP(C2H5)3 and PH3 in H2 is a convenient way of growing InP on a heated substrate.

Thus, in the case where one wishes to grow two layers of material on substrate, the first of a quaternary material Gax Inq xAsyP1 and the second of InP (both with dopants in general), one might proceed as follows: At room temperature the substrate is loaded into position 7, 8, or 9 and the elongate member is rotated to the position for which the substrate lies centrally in quadrant 10 (as, in fact, shown in Fig. 1).

Then the following non-turbulent gas flows are established, at flow rates of about 1 metre/minute: in quadrant 10, a stream of phosphine in hydrogen; in quadrant 11, a stream of hydrogen chloride in hydrogen; in quadrant 12, a stream of hydrogen carrying Ga, In, P, and As in the manner known for MOCVD; and in quadrant 13, as stream of hydrogen containing In and P and a dopant in the manner known for MOCVD.

The substrate is now heated in quadrant 10 (ie in a protective atmosphere) by means of a heater on the elongate element and then moved to quadrant 11 so as to etch the surface clean. Then the substrate is moved successively to quadrants 1 2 and 1 3 for the two depositions to occur, and it is finally moved back to the protective atmosphere of 10 for cool down. The approximate circular symmetry of the tube/elongate member arrangement permits the sample to be moved rapidly without excessive disruption of the gas flows or cross-contamination of the gas flows.

The use of the water jacket in the above procedure is as known for MOCVD.

The procedure described above can be extended to the sequential exposure of a substrate to more than four different gas compositions. This is possible because at any given time one may change the gas composition in any quadrant other than that in which the substrate is then situated. Likewise, the sequential exposure of a substrate to four different gas compositions could be achieved in accordance with the present invention by the use of apparatus permitting the simultaneous establishement of only two or three gas flows.

However, we believe that an apparatus as shown in the Figures permitting the simultaneous establishment of just four gas flows represents a particularly good combination of flexibility and ease of operation.

If it is particularly important to avoid crosscontamination of two particular gas flows, it is possible to use the apparatus shown in the Figures to advantage by putting the two flows in question in opposite quadrants (say, 10 and 12) and putting two buffer flows, eg of pure hydrogen, in the intermediate quadrants 11 and 13.

Claims

1. Apparatus for the growth of a semiconductor material on a substrate which comprises a tube; an elongate member within and along the tube and having means for the location of a substrate on the side surface thereof; means for establishing two or more gas flows along the tube and impinging on the side surface of the elongate member at differing circumferential positions; and means for relative rotation of the elongate member and the tube permitting the movement of a substrate located on the said side surface from one gas flow to another.

2. Apparatus according to claim 1 wherein the said side surface is substantially conical and wherein the gas flows that are capable of being established by the relevant means are in the direction of the broader end of the conical side surface.

3. Apparatus according to any preceding claim wherein the elongate member and the tube are substantially vertical.

4. Apparatus according to any preceding claim wherein the number of gas flows that are capable of being established by the rele vant means is at least four.

5. Apparatus according to any claim 4 wherein the said number is four.

6. Apparatus according to any preceding claim wherein the means for the location of the substrate include a depression in the said side surface.

7. Apparatus according to claim 6, wherein the said depression is a longitudinal groove.

8. Apparatus according to any preceding claim wherein means are provided for the location of two or more substrates at the same circumferential location on the said side surface but at different longitudinal locations.

9. Apparatus for the growth of a semiconductor material on a substrate substantially as described herein with reference to the accompanying Figures.

10. A method of growth of a semiconductor material on a substrate by use of an apparatus according to any preceding claim which comprises the steps of (i) locating the substrate on the side surface of the elongate member specified in claim 1 by the relevant specified means; (ii) establishing two or more gas flows as specified in claim 1 by the relevant specified means; and (iii) exposing the substrate sequentially to at least two such gas flows by relative rotation of the elongate member and the tube specified in claim 1 by the relevant specified means.

11. A method as claimed in claim 10, wherein at least one of the gas flows comprises an organometallic compound.

1 2. A method for the growth of a semiconductor material substantially as described herein with reference to the accompanying Figures.