GB1586083A

GB1586083A - Growth of semiconductor materials

Info

Publication number: GB1586083A
Application number: GB1067377A
Authority: GB
Original assignee: Post Office
Current assignee: Post Office
Priority date: 1978-03-14
Filing date: 1978-03-14
Publication date: 1981-03-18

Description

(54) IMPROVEMENTS IN OR RELATING TO THE GROWTH OF SEMI CONDUCTOR MATERIALS (71) We, the POST OFFICE, a British corporation established by Statute, of 23 Howland Street, London W1P 6HQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the growth of semiconductor materials, particularly group III- V semiconductor compounds, from the vapour phase. The invention has particular application to the growth of gallium arsenide.

When growing semiconductor materials by vapour phase epitaxy, e.g. gallium arsenide epitaxial layers are grown by the Ga/AsCl3/H2 process, it is desirable that the growth of the layers can be monitored.

This invention provides apparatus which enables the growth of the semiconductor materials to be monitored.

According to the present invention there is provided apparatus for growing semiconductor materials by vapour phase epitaxy the apparatus allowing the growth of the semiconductor materials to be monitored, said apparatus comprising a growth enclosure in which the semi-conductor materials are grown, weighing means disposed above said growth enclosure, means linking said growth enclosure with said weighing means, and a holder for carrying a substrate or substrates on which the semiconductor materials are to be grown, said holder being suspended in the growth enclosure from said weighing means.

The growth enclosure may be generally tubular and may have its axis disposed substantially horizontally.

The weighing means may be a microbial ance which is disposed in a chamber above the growth enclosure, said chamber being linked to the interior of the enclosure by a tube which extends upwardly from the enclosure. The upwardly extending tube may have a capillary section. The chamber may have a port which is connected to a source of hydrogen.

The substrate holder may be arranged to support the or each substrate so that the plane of the substrate is substantially perpendicular to the axis of the growth enclosure. The substrate holder may comprise a lid portion which acts as a baffle to deflect gas issuing from said upwardly extending tube away from the or each substrate, and one or more discs which depend from the lid portion so that the plane of each disc is inclined at slightly less than 90" to the axis of the lid portion. The holder may be suspended from said balance by a silica fibre which extends freely through said upwardly extending tube.

By use of the present apparatus it is possible to monitor accurately the weight of a layer of semi-conductor material grown on a substrate. The weight can be related to the thickness of the layer and thus the thickness of the layer being grown can be monitored.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 is a schematic illustration of apparatus in accordance with the present invention; Figure 2 is a side view of a substrate holder used in the apparatus of Figure 1; Figure 3 is an end view of the substrate holder, and Figures 4 to 10 are plots obtained using the present apparatus.

Figure 1 shows part of an apparatus which is used to grow gallium arsenide by vapour phase epitaxy. An example of a complete growth apparatus is described in U.K. Patent Specification No. 2008084A. The apparatus shown in Figure 1 comprises a generally cylindrical silica work tube 10 which is disposed with its axis substantially horizontal within a tubular furnace (not shown). The furnace has two zones the temperatures of which can be controlled independently.

One end of the tube 10 has two inlet conduits 11, 12. The upper conduit 11 extends into the interior of the work tube so that its end is disposed beyond a boat 14 mounted within the tube 10 below the conduit 11. The conduit 12 terminates at the end of the tube 10 adjacent to the boat 14. The conduit 11 is connected to a source of hydrogen and dopant material such as sulphur. The conduit 12 is connected to a source of hydrogen and arsenic trichloride. The boat 14 contains gallium arsenide source material.

The work tube 10 has an upwardly extending limb 18 which communicates with the tube 10 at a position spaced from the end of the conduit 11. The limb 18 is generally tubular and has a capillary portion 20 adjacent its junction with the tube 10. The limb extends through the wall of the furnace, the gap in the wall of the furnace around the limb 18 being filled with an insulating plug (not shown). At its upper end the limb 18 communicates with a chamber 22. The chamber 22 has a first downwardly extending tubular limb 24, the lower end of which has a spherical joint 25 which seats against the upper end of the limb 18 and is sealed therein by means of an O-ring. The chamber 22 has a second downwardly extending tubular limb 27 the lower end of which has a spherical joint 28 which seats in the upper end of a tube 30 and is also sealed therein by means of an O-ring.The chamber 22 has an inlet port 23. The chamber 22 accommodates a microbalance which is shown schematically at 32. A silica fibre 34 is suspended from one arm of the balance 32 and extends downwardly through the limbs 24 and 18 and into the tube 10. The silica fibre 34 carries at its lower end a substrate holder 36. The length of the fibre 34 is carefully selected so that the substrate holder is suspended within the tube 10 so that it does not touch the sides of the tube. The other arm of the microbalance 32 has suspended therefrom another silica fibre 38 on the end of which is supported a counter weight 40. The balance 32 can be microbalance model 2CT5 manufactured by C.I. Limited with the maximum capacity of 5 grams. The output of the balance is connected to a recorder with a full scale deflection of 2mV.

Referring to Figures 2 and 3 the substrate holder 36 comprises a curved silica lid portion 44 from which depend discs 45 also made of silica. The plane of each disc 45 is inclined at an angle slightly less than 90" to the axis of the lid portion 44 so that when the holder is suspended in the furnace the discs are inclined at a small acute angle to the vertical. The discs are connected towards their lowest extremity by silica rods 46. A silica rod 48 extends between the rods 46 in closely spaced relationship with each disc 45.

Each disc 45 forms a support for a substrate 42 which is mounted on an optical flat. The rods 48 also support the substrates and prevent them slipping from a near vertical position.

In use, hydrogen and sulphur dopant material are passed into the tube 10 through the conduit 11, hydrogen and arsenic trichloride are passed into the tube 10 over the gallium arsenide source in the boat 14 through the conduit 12 and hydrogen is passed into the chamber 22 through an inlet 23. The hydrogen which is a high purity grade hydrogen, has been initially purified by allowing it to diffuse through a heated palladium membrane and then piping it through stainless steel and pyrex (Registered Trade Mark) tubing to the growth apparatus. A minimum number of joints and taps are used in the piping arrangement in order to minimise possible leaks. Purified hydrogen is passed into the chamber 22 through the port 23 at 110 ml per minute.

In order to obtain the hydrogen and arsenic trichloride stream, purified hydrogen is bubbled through high purity arsenic trichloride, which is maintained at 20"C, at a rate of 85 ml per minute. The hydrogen, arsenic trichloride stream enters through the conduit 12 and passes over the gallium arsenide source material. The boat 14 containing the gallium arsenide source material is 5 cm long and is divided into five compartments to prevent diffusion and hence the formation of a gallium arsenide crust at the ends of the boat which might be slightly cooler.The zone of the furnace within which the boat 14 is contained is held at about 840"C. As the arsenic trichloride passes over the boat 14 it undergoes a reversible chemical reaction with the gallium arsenide and effectively transports the gallium arsenide to the substrate holder 36 where the original chemical reaction operates in reverse so that gallium arsenide is deposited on the substrate 42 in the substrate holder 36. This is a well known process and will not be described in any more detail here.

In order for the reaction to operate in reverse it is necessary that the zonw of the furnace containing the holder 36 is maintained substantially at a temperature of 750".

The suphur dopant material is introduced into the hydrogen stream from a capsule having a capillary outlet of known dimensions.

The sulphur in the capsule is heated to a temperature of substantially 750" so that the sulphur can diffuse out of the capsule into the hydrogen stream to form hydrogen sulphide.

This method of producing the dopant stream enables an accurate concentration of dopant to be provided in the semiconductor layer grown on the substrates 42. The dopant material is passed through the conduit l l so that it flows through the rectangular holder 36.

The purpose of the capillary 20 is to main tain at a minimum the concentration of corrosive gases which can diffuse from the work tube 10 and into the balance chamber 22.

Such corrosive gases could corrode the balance mechanism. The capillary also keeps to a minimum possible transport into the tube 10 of undesirable products which could affect the electrical properties of the epitaxial layer being grown on the substrates. Using conventional diffusion analysis it is possible to evaluate the optimum size of the capillary 20 taking into account the diffusion coefficients of the gases, the flow rates and the temperature. It should be noted that the length of tubing between the capillary and the chamber 22 also provides resistance to gas diffusion. The lid portion 44 of the holder 36 acts as a baffle to deflect gas issuing through the limb 18 away from the substrates 42.

The apparatus described above enables the weight of the substrate being grown to be monitored continuously by the balance 32.

The weight is related to the thickness and thus the thickness of the layer being grown can be monitored continuously. It is therefore possible to stop growth when the required thickness of semiconductor material has been grown. The technique can be used for layers of less than litm thickness.

The arrangement has been used to investigate several parameters in the growth of gallium arsenide by vapour phase epitaxy. We have found that the growth rate along the length of the substrate holder 36 is more uniform (~5% variation across the substrate) when the substrates are positioned as shown in Figures 2 and 3 than if the substrates are layed horizontally along a holder. It will be noted that in the holder arrangement shown the plane of each substrate is substantially normal to the gas flow. When the substrates are layed horizontally there is a substantial difference in thickness of the grown layer between the front and rear of the substrate.

Other examples of the use of the apparatus described above will now be given.

We have found that when using a solid source of gallium arsenide in the boat 14 the growth rates of gallium arsenide on the substrates are very constant and do not vary even when growth is stopped and restarted after say 20 minutes. When hydrogen is bubbled through arsenic trichloride there is an induction period before steady state growth is reached. This is illustrated in Figure 4 in which the thickness of the layer is plotted against time. It is apparent that this induction period is very small. of the order of 10 seconds. It appears that the initial growth rate is negative but this is because the density of the gas stream increases which reflects in an apparent loss of weight. When bubbling is stopped there is also a short induction period before a zero growth is obtained. The apparent increase in growth rate at this point is again due to change in the gas density.Figure 4 also indicates that the present apparatus can be used for growing epitaxial layers to a tolerance of 0.1,um. However, in computing thicknesses from the weight changes in addition to preventing growth on the back of the substrate by using an optical flat, the area of the substrate must be measured and the growth on the sides of the substrate must be taken into account. For a typical substrate, 20 mm x 20 mm x o.5 mm, the sides of the substrate account for about 10% of the area.

Taking this into account good correlation between the computed and required thicknesses is achieved consistently for epitaxial layers less than 1 um which are required in the manufacture of field effect transistors.

This type of assembly can also be used for growing material for IMPATT diodes where the thickness of the thin top layer is critical.

We have also used the assembly to investigate the variation of carrier concentration with growth rate. The carrier concentration is affected by variations in the temperature of the gallium arsenide source. Hydrogen sul phide dopant at a concentration of 1 x l 10-6 is used and the results of the investigation are illustrated in Figure 5 in which carrier concentration is plotted against growth rate. The decreasing carrier concentration at lower growth rates implies that non-equilibrium concentrations of sulphur are incorporated into the growing epitaxial layer. The concentration of sulphur species on the gallium arsenide surface must therefore be higher than the equilibrium concentration of sulphur for bulk gallium arsenide and these sulphur species become buried by the growing epitaxial layer before they have time to desorb.

The arrangement has also been used to monitor variations of growth rate using a "liquid gallium" source in the boat 14. The weight of the substrate in the substrate holder was monitored as a function of time and growth was stopped for about 15 minutes and then continued. Growth rates, in terms of increase of thickness, were determined from the total thickness of the layer which is obtained from the carrier concentration profile. The results are shown in Figure 6 for a source which has been resaturated with arsenic. This shows that there is a small amount of growth at a high growth rate at the beginning of growth, and after stopping growth this is repeated. After the initial high rate the growth rate falls to a fairly steady rate but is not as constant as that obtained using a solid source of gallium arsenide.The results of the subsequent investigation are shown in Figure 7 where a similar trend is obtained but with the initial high growth rates persisting over a greater period of time and hence a greater thickness. An extreme case of very high initial growth rate over an extended thickness is shown in Figure 8.

Here the source was used after saturation on two previous occasions over a period of 70 minutes. The results demonstrate that on commencing growth the source has not reached a steady state and are consistent with other reported observations. It is apparent that the state of the source changes with time which means that, unless the source is resaturated before every experiment, variations occur from one experiment to another.

We have also investigated variation of carrier concentration using a "liquid gallium" source. Sulphur was introduced as a dopant in an experiment similar to that giving the results of Figure 7. Initially a high concentration was introduced, growth was stopped the sulphur concentration was lowered and growth recommenced. The growth of the lower doped layer was interrupted in order to ascertain the effects on carrier concentration. The result is shown in Figure 9 which shows that after interruption the growth rate begins at about 3 times the steady value while the carrier is about 5 times higher. This shows that an increase in the growth rate increases the carrier concentration slightly. It appears that increasing the gallium to arsenic ratio increases the carrier concentration considerably. A carrier concentration profile corresponding to Figure 8 is shown in Figure 10.This shows very high carrier concentration at high growth rates.

WHAT WE CLAIM IS: 1. Apparatus for growing semiconductor materials by vapour phase epitaxy the apparatus allowing the growth of the semiconductor materials to be monitored, said apparatus comprising a growth enclosure in which the semiconductor materials are grown, weighing means disposed above said growth enclosure, means linking said growth enclosure to said weighing means, and a holder for carrying a substrate or substrates on which the semiconductor materials are to be grown. said holder being suspended in the growth enclosure from said weighing means.

2. Apparatus as claimed in claim 1 wherein the growth enclosure is generally tubular and has its axis disposed substantially horizontally.

3. Apparatus as claimed in claim 1 or claim 2 wherein the weighing means comprises a microbalance which is disposed in a chamber above the growth enclosure. said chamber being linked to the interior of the enclosure by a tube which extends upwardly from the enclosure.

4. Apparatus as claimed in claim 3 wherein the upwardly extending tube has a capillary section.

5. Apparatus as claimed in claim 3 or claim 4 wherein said chamber has a port which is connected to a source of hydrogen.

6. Apparatus as claimed in any preceding claim wherein said substrate holder is arranged to support the or each substrate so that the plane of the substrate is substantially perpendicular to the axis of the growth enclosure.

7. Apparatus as claimed in claim 6 wherein the substrate holder comprises a lid portion which acts as a baffle to deflect gas issuing from said upwardly extending tube away from the or each substrate, and one or more discs which depend from the lid portion so that the plane of each disc is inclined at slightly less than 90" to the axis of the lid portion.

8. Apparatus as claimed in claim 6 or claim 7 wherein the substrate holder is suspended from said balance by a silica fibre which extends freely through said upwardly extending tube.

9. Apparatus as claimed in claim 1 for growing semiconductor materials by vapour phase epitaxy substantially as hereinbefore described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. case of very high initial growth rate over an extended thickness is shown in Figure 8. Here the source was used after saturation on two previous occasions over a period of 70 minutes. The results demonstrate that on commencing growth the source has not reached a steady state and are consistent with other reported observations. It is apparent that the state of the source changes with time which means that, unless the source is resaturated before every experiment, variations occur from one experiment to another. We have also investigated variation of carrier concentration using a "liquid gallium" source. Sulphur was introduced as a dopant in an experiment similar to that giving the results of Figure 7. Initially a high concentration was introduced, growth was stopped the sulphur concentration was lowered and growth recommenced. The growth of the lower doped layer was interrupted in order to ascertain the effects on carrier concentration. The result is shown in Figure 9 which shows that after interruption the growth rate begins at about 3 times the steady value while the carrier is about 5 times higher. This shows that an increase in the growth rate increases the carrier concentration slightly. It appears that increasing the gallium to arsenic ratio increases the carrier concentration considerably. A carrier concentration profile corresponding to Figure 8 is shown in Figure 10.This shows very high carrier concentration at high growth rates. WHAT WE CLAIM IS:

1. Apparatus for growing semiconductor materials by vapour phase epitaxy the apparatus allowing the growth of the semiconductor materials to be monitored, said apparatus comprising a growth enclosure in which the semiconductor materials are grown, weighing means disposed above said growth enclosure, means linking said growth enclosure to said weighing means, and a holder for carrying a substrate or substrates on which the semiconductor materials are to be grown. said holder being suspended in the growth enclosure from said weighing means.