GB2248456A - A method of growing III-V compound semiconductor material on a substrate - Google Patents

Abstract

A substrate is placed in a low-pressure environment and a gallium or indium containing or indium-containing first stream formed from a gaseous source and at least one further stream containing a group V element are directed towards the substrate to cause growth of a III-V compound semiconductor material on the substrate. The method is characterised by forming the first stream a gaseous from a carbon-free source selected from gallium chloride, gallium fluoride, gallium bromide, indium chloride, indium fluoride and indium bromide so as to reduce carbon contamination of the III-V compound semiconductor material being grown on the substrate. The substrate is preferably placed in an environment of such a low pressure that the streams form molecular beams directed towards the substrate. <IMAGE>

Description

DESCRIPTION A METHOD OF GROWING III-V COMPOUND SEMICONDUCTOR MATERIAL ON A SUBSTRATE.

This invention relates to a method of growing III-V compound semiconductor material on a substrate, which method comprises placing the substrate in a low pressure environment and directing a first gallium-containing or indium-containing stream formed from a gaseous source and at least one further stream comprising a group V element towards the substrate to cause growth of a III-V compound semiconductor material on the substrate.

EP-A-202329 (PCT WO 86/03231) describes such a method in which the low pressure environment is an ultra-high vacuum environment so that the streams form so-called molecular beams, that is the mean free path of the molecules, atoms or ions forming the streams is comparable to or greater than the distance from the source within the low pressure environment of the stream to the substrate, so that the number of collisions occuring before the molecules, atoms or ions reach the substrate is negligible.

In the example described in EP-A-202329 the first stream is provided as a molecular beam formed from an organometallic compound of gallium or indium with the gallium or indium typically being provided directly at the substrate by pyrolysis of the metalorganic compound due to the heating of the substrate. The group V element containing molecular beam may, where the desired group V element is phosphorus or arsenic, similarly be formed from an organometallic compound such as triethylphosphine or triethylarsine or may be formed from a hydride such as phosphine or arsine.

The use of such gaseous sources for the group III and group V elements has several advantages over the use of the solid sources used in conventional molecular beam epitaxy (MBE). Thus, in conventional MBE, the solid sources of the elements are heated in Knudsen cells provided within the MBE chamber to produce the desired molecular beam. Although such solid sources may be extremely pure, it is necessary from time to time to recharge the Knudsen cells with fresh solid source. This is not only time consuming but also requires a subsequently lengthy process to re-establish a clean ultra-high vacuum environment.In contrast, where gaseous sources are used, then the supply may be adjusted or renewed without effecting the ultra-high vacuum conditions by appropriate use of valve systems connecting the gaseous supplies to the chamber within which the ultra-high vacuum environment is established.

This means that it is easier and less time-consuming to change or refresh the source of a particular element when using gaseous sources, which should enable costs to be reduced. However, because the gaseous sources commonly used, with the exception of the group V hydrides, are organometallic compounds carbon contamination of the III-V compound semiconductor material is significantly higher than the carbon contamination which occurs during conventional MBE growth using solid sources.Typically the carbon contamination occurring in material grown by a conventional MBE process may be about 2x1014 atoms cm~3 whilst the carbon contamination occurring where organometallic compounds are used as described above may be 1017 atoms cm~3. Carbon contamination of the as grown material may cause significant problems in devices manufactured from the material. Thus, for example, a high carbon contamination may result in difficulties in dopant control in heterojunction bipolar transistors (HJBTs) and may adversely limit or restrict the two dimensional electron gas formed in a high electron mobility transistor (HEMT).

It is an aim of the present invention to provide a method of manufacturing a III-V compound semiconductor material using a carbon-free gaseous source of gallium or indium.

According to the present invention, there is provided a method of growing III-V compound semiconductor material on a substrate, which method comprises placing the substrate in a low-pressure environment and directing a gallium-containing or indium-containing first stream formed from a gaseous source and at least one further stream comprising a group V element towards the substrate to cause growth of a III-V compound semiconductor material on the substrate, characterised by providing the first stream from a gaseous carbon-free source selected from the group consisting of gallium chloride, gallium fluoride, gallium bromide, indium chloride, indium fluoride and indium bromide.

Thus, using a method in accordance with the invention enables gallium or indium containing III-V compound semiconductor materials to be grown using a gaseous source of gallium or indium without causing the significant carbon contamination problems which may arise when an organometallic compound such as triethylgallium, trimethylgallium, triethylindium or trimethylindium is used.

Conveniently, the substrate is placed in an environment of such low pressure that the streams form molecular beams directed towards the substrate. As used herein the term "molecular beam" should be understood to mean a stream of molecules, atoms and/or ions (or other species) which is provided under ultra-high vacuum conditions so that the mean free path of the molecules, atoms and/or ions is comparable to or greater than the distance from the source within the low pressure environment of the stream to the substrate. Also, as used herein the term gaseous source means a source which is in the form of a gas at the temperature and pressure maintained in the low pressure environment. Thus, the source may not be gaseous at standard temperature and pressure.

The use of gallium chloride or indium chloride is particularly advantageous because these chlorides can be produced in an extremely pure state comparable to that of a conventional MBE solid source. Gallium chloride has a vapour pressure at 700C of about 7 Torr (9.3x102Pa) making it particularly suitable for ultra-high vacuum techniques such as gaseous source MBE and also for low pressure chemical vapour deposition.

The one further stream may be provided from a gaseous source selected from the group consisting of arsine, tertiarybutylarsenic and phosphine. The use of arsine or phosphine, although potentially more dangerous than other sources, should further assist in reducing carbon contamination of the grown material.

Tertiarybutylarsenic is also a suitable gaseous source of arsenic because the organic component has a relatively high volatility and is chemically stable and so is incorporated into the material being grown less readily than a methyl or ethyl group. Other group III or V element compounds in which the group III-V element component forms a relatively weak bond with the remainder of the compound, for example as is the case for a Lewis Acid base, and in which the remainder of the compound itself forms a chemically stable entity, may be used.

A further stream comprising another group III element may be provided for enabling a tertiary compound semiconductor material to be grown on the substrate, for example both an indium and a gallium containing stream may be provided. The first stream and the further group III element-containing stream may be controlled to form alternate layers of two different III-V compounds on the substrate.

The further group III element containing material may be provided from an aluminium-containing gaseous source such as alane compounds, for example monotrimethylamine alane (AlH3(NMe3)), or tertiarybutylaluminium. Although such an alane compound dissociates by splitting off the amine group which is of relatively high volatility and is chemically stable and thus should be less readily incorporated into the material being grown, it has been found that significant carbon contamination still arises where an alane compound is used with an organometallic compound of gallium or indium because the carbon provided by the gallium or indium compound reacts with the aluminium species. In contrast where, in accordance with the present invention, a carbon-free gaseous source of gallium or indium is used very little carbon contamination results when an alane compound such as monotrimethylamine alane is used as the aluminium source. Similar considerations apply to tertiarybutylaluminium and so again the organic group will not readily be incorporated into the III-V compound semiconductor material being grown.

Conveniently, the substrate comprises a monocrystalline III-V compound semiconductor material selected from the group consisting of gallium arsenide, gallium phosphide, other suitable III-V compound semiconductor materials and other suitable semiconductor materials such as silicon.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which the single Figure is a very schematic representation of a molecular beam epitaxy machine adapted to use gaseous sources.

The machine shown in the single Figure of the drawing comprises a stainless steel vacuum chamber 1 capable of use with ultra-high internal vacua. An ultra-high vacuum pumping system (for example an ion-pump and possibly also a cryogenic pump, a trapped diffusion pump and/or a turbomolecular pump) 2 is connected via a valve system and stainless steel piping to the chamber to enable a desired ultra-high vacuum to be attained within the chamber 1.

A preparation chamber 4 is connected via a first valve system 4a to the vacuum chamber 1. The preparation chamber 4 is connected to an entry lock system 5. The entry lock system comprises a first valve system 6 connected to an outlet 7 of the preparation chamber 4 and a branched or T-junction stainless steel pipe 8 providing one line 8a leading via a further valve system 9 to an ultra-high vacuum pumping system 10 (for example the same pumping system 2 mentioned above) and a second line 8b leading via a third valve system 11 to an entry lock 12 which is evacuated by a rough pump 13 connected to the sample entry lock by a pipe 12a and a valve system 13a.A transfer mechanism (not shown) is provided for mechanically transferring substrates or samples between the sample lock 12, the preparation chamber 4 and a rotatable support stage 18 within the vacuum chamber via a window 19 through hollow cryogenic panels la provided within the chamber 1. Liquid nitrogen is passed through the cryogenic panels la which serve to provide thermal isolation and to condense gaseous material not impinging on a substrate mounted on the rotatable support stage 18. Although not shown in the Figure, the support stage 18 is provided with a heater for heating a substrate 23 mounted on the support stage 18 to the desired growth temperature.

In place of the conventional solid source Knudsen cells, the machine is provided with a number of gas source devices 30. Three gas source devices 30 are shown in the Figure, although the number of gas source devices provided will of course depend upon the type of material to be grown.

As shown in the Figure, each of the gas source devices 30 comprises first and second gas supply lines 31 and 32 with appropriate valves 33 and a precision electronic flow controller 34. Each first gas supply line 31 provides the gaseous source of a group III or V element whilst the second supply line 32 provides a supply of a carrier gas, usually nitrogen, argon or hydrogen, which is required to enable operation of the flow controllers 34.

Alternative methods of controlling the flow of gas from the outlet orifices 30a of the gas source devices 30, for example control of the pressure at the outlet orifices 30a, may be used which do not require the use of a carrier gas. In such cases, the second supply lines 32 may be omitted. Although not shown in the Figure, further source devices may be provided to provide sources of dopant material such as silicon and beryllium. These may be conventional solid sources or gaseous sources, as appropriate.

Each of the gaseous source devices 30 and any dopant source device may have a mechanical shutter 35 which is movable to switch off and on the flow from the outlet orifice 30a of the associated gaseous source devices. Alternatively, the flow from the outlet orifices 30a may be controlled by the valves 33 in which case the mechanical shutters would be omitted or kept in the open position.

Heaters (not shown) may be associated with one or more of the gaseous source devices 30 to enable thermal decomposition of the gaseous source material prior to entry into the chamber 1. Whether or not this is necessary or desirable will depend upon the particular gaseous material. Thus, for example, where arsenic or phosphorus is to be provided by a gaseous source device 30 providing arsine or phosphorine then the gaseous source device 30 may be provided with a heater to enable thermal dissociation to provide group V atoms or molecules. The group III element gaseous sources will be delivered to the substrate without prior decomposition so that dissociation occurs at the heated substrate as will the Group V element gaseous sources if sources such as tertiarybutylarsenic or other similar compounds are used.

A quadrupole mass spectrometer (not shown) may be provided for residual gas analysis and a reflection high energy electron diffraction (RHEED) arrangement for assessment of surface structure. The RHEED arrangement consists simply of a 5-20 keV (kilo electron volt) electron gun 20 and a fluorescent screen 21, each aligned with a respective window 19a and 19b in the cryogenic panels la with the electron beam emitted by the gun 20 arranged to be at a very shallow angle (1 to 3 degrees) to the substrate surface. Additionally there may be an Auger electron spectrometer to determine surface composition and purity. Some or all of these analytical facilities may be housed in the separate analysis/preparation chamber 4 mounted between the sample insertion interlock system 5 and the chamber 1.

The vacuum chamber 1 is provided with a viewing port 22 to enable a sample to be observed.

An example of a method in accordance with the invention will now be described.

The substrate, in this example a semi-insulating monocrystalline gallium arsenide substrate, is first introduced into the entry lock 12 and after the pressure within the entry lock has been reduced by operation of the rough pump 13, the substrate is then transferred to the preparation chamber 4 by the sample transfer mechanism (not shown). A pressure of about 10~10Torr (1.33 x 1O#8Pa) is maintained in the preparation chamber and the sample is heated to about 400 degrees Celsius in the preparation chamber to cause outgassing of undesired impurities.

The substrate is then transferred by the transfer mechanism (not shown) to the support stage 18 which is then rotated. The Figure illustrates diagrammatically the substrate 23 mounted on the support stage 18. A pressure of about 5 x lO#11Torr (3.3 x 10-8Pa) is maintained within the vacuum chamber 1. To provide a basis for high quality epitaxial layers, a substrate surface free of crystallographic and other defects and clean on an atomic scale (0.01 ML (monolayer) of impurities) should be prepared. This involves free etching of the surface by an oxidative process which removes any carbon and leaves the surface covered with a protective volatile oxide.The protective volatile oxide is removed by heating the substrate 23 (using the heater (not shown) mounted to the substrate stage or possibly using an external radiant heat source (not shown)) with the appropriate valves 33 and mechanical shutter 35 opened to provide a molecular beam formed from the group V element containing gaseous source, in this example arsine (AsH3), to the substrate 23. As mentioned above, the arsine gaseous source device 30 has a heater to enable thermal dissociation at the source to provide arsenic molecules or atoms.

The temperature of the substrate 23 is then raised to the desired growth temperature, typically 6500C, in the molecular beam formed from the group V element containing gaseous source.

The shutters 35 (or valves 33 depending upon the manner in which flow from the outlet orifices 30a is controlled) of the gaseous source device 30 providing the carbon free gallium-containing gaseous source, in this example gallium chloride, and the shutter 35 (or valve 33) of a further gaseous source device 30 providing, in this example, an aluminium-containing gaseous source in the form of an alane compound such as monotrimethylaminealane (AlH3(NMe3)) are then opened to direct molecular beams of gallium chloride and the alane compound towards the substrate 23.

It is preferred in the present example that the flux of the molecular beams be controlled by controlling the pressure behind the outlet orifices 30a in the manner used in conventional molecular beam epitaxy (MBE). However, if the flux of the molecular beams is to be controlled by precision flow meters 34 then each of the gaseous sources will be supplied in an inert carrier gas, for example nitrogen, argon or possibly hydrogen, to enable the flow rates to be controlled. In this example, the fluxes are controlled to provide growth of about IML (monolayer) per second that is with a beam equivalent pressure of 10~6Torr (1.33 x 10-4Pa) and a flux of the order of 6 x 1014 0-2S-1 for each of the molecular beams.

The gallium chloride and alane compound molecular beams are thermally decomposed at the heated substrate (the arsine having been predecomposed by the heater at the arsenic gaseous source device) enabling growth on the gallium arsenide of a layer of aluminium gallium arsenide, AlxGal#xAS with, for the molecular beam fluxes given above, x = 0.5. Of course by adjusting the relative fluxes of the aluminium and gallium providing gaseous sources different compositions of aluminium gallium arsenide may be provided and by switching off the aluminium source, gallium arsenide may also be grown.

The aluminium gallium arsenide layer grown on the gallium arsenide substrate 23 is substantially free of carbon because in this particular example the only deliberately provided source of carbon is the aluminium-containing gaseous source. Thus, the use of gallium chloride to provide a gaseous source of gallium which is free of carbon considerably reduces the possibility of carbon contamination of the aluminium gallium arsenide layer.

Furthermore, because the alane compound dissociates to provide an amine group which is of relatively high volality and is chemically stable, the amine group readily leaves the surface of the material being grown and the possibility of carbon incorporation from the aluminium-containing gaseous source is small. This possibility is also reduced because of the fact that the gallium-containing gaseous source is free of carbon and so does not provide any carbon-containing species which might otherwise react with the aluminium species to produce free carbon or methyl groups which could be incorporated into the material being grown.

It will be appreciated that, by appropriate operation of the mechanical shutters 35 (or valves 33), alternate layers of gallium arsenide and aluminium arsenide to form, for example, a superlattice, may be grown instead of an aluminium gallium arsenide alloy. Also, alternative carbon-free gallium sources may be used.

Thus, for example, gallium bromide or gallium fluoride could be used in place of the gallium chloride. The use of gallium chloride is, however, particularly advantageous because gallium chloride can be produced in an extremely pure state comparable to that of conventional MBE solid gallium sources and has a vapour pressure at 700C of about 7 Torr (9.3 x 102Pa) making it particularly suitable for use as a gaseous source in ultra-high vacuum techniques such as gaseous source MBE.

Although a gallium containing, in particular a gallium chloride, gaseous source is used to form the first stream in the example described above, the gallium-containing gaseous source may be replaced by or supplemented with a carbon-free indium-containing source enabling compounds such as aluminium indium arsenide (AlInAs), indium arsenide (InAs), gallium indium arsenide (GaInAs) and aluminium gallium indium arsenide (AlGaInAs) to be grown. The indium-containing source is selected from the group consisting of indium chloride, indium fluoride and indium bromide. Again for reasons similar to those given for the gallium-containing source discussed above, in particular for reasons of purity and suitability of vapour pressure, indium chloride provides a particularly advantageous carbon-free source of indium.

Alternative aluminium-containing gaseous sources could be used, for example tertiarybutylaluminium which dissociates to provide a relatively highly volatile and chemically stable organic group and so does not result in significant incorporation of carbon into the material being grown. In addition other gaseous sources of arsenic, for example tertiarybutylarsenic which similarly dissociates to provide a chemically stable organic group of relatively high volatity and which is thus not easily incorporated into the growing material could be used in place of arsine. Arsine has advantages in being a very pure source but it is a more hazardous material to use. Other group V element-containing gaseous sources may be provided, for example a phosphorus-containing gaseous source such as phosphine enabling the growth of, for example, gallium aluminium phosphide or tertiary compounds.Also gaseous sources of other group III elements may be provided enabling the growth of epitaxial layers of other gallium or indium containing III-V semiconductor materials. In addition dopant sources such as sources of silicon and beryllium may be provided to enable doped layers to be grown to allow formation of, for example, pn junction laser diodes. The dopant sources may be similarly provided as gaseous sources, for example using silane to provide silicon. However because considerably less dopant material is required, solid sources of the dopant materials could be used as these would need replacing considerably less frequently than the sources of the materials for forming the III-V semiconductor material itself, such solid dopant sources can however be susceptible to contamination by metalorganic compounds.

Although a method in accordance with the invention has been described above with reference to the ultra-high vacuum technique of MBE, it may also be applied to low pressure chemical vapour deposition techniques where the low pressure environment is typically in the region of a few Torr.

From reading the present disclosure, other modifications and variations will be apparent to persons skilled in the art. Such modifications and variations may involve other features which are already known in the semiconductor art and which may be used instead of or in addition to features already described herein.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (9)

CLAIM(S)

1. A method of growing III-V compound semiconductor material on a substrate, which method comprises placing the substrate in a lowpressure environment and directing a gallium-containing or indium-containing first stream formed from a gaseous source and at least one further stream comprising a group V element towards the substrate to cause growth of a III-V compound semiconductor material on the substrate, characterised by providing the first stream from a gaseous carbon-free source selected from the group consisting of gallium chloride, gallium fluoride, gallium bromide, indium chloride, indium fluoride and indium bromide.

2. A method according to Claim 1, further characterised by placing the substrate in an environment of such a low pressure that the streams form molecular beams directed towards the substrate.

3. A method according to Claim 1 or 2, further characterised by providing the at least one further stream from a gaseous source selected from the group consisting of arsine, tertiarybutylarsenic and phosphine.

4. A method according to Claim 1,2 or 3, further characterised by providing a further stream comprising another group III element for enabling a tertiary compound semiconductor material to be grown on the substrate.

5. A method according to Claim 1,2 or 3, further characterised by providing a further stream from a gaseous source comprising another group III element and controlling the first stream and the further group III element-containing streams to form alternate layers of two different III-V compounds on the substrate.

6. A method according to Claim 4 or 5, further characterised by providing the further group III element-containing material from an aluminium-containing gaseous source.

7. A method according to Claim 6, further characterised by providing the further stream from a gaseous source selected from the group consisting of monotrimethylamine alane and tertiarybutylalumium.

8. A method according to any one of the preceding claims, further characterised by providing the substrate of a monocrystalline III-V compound semiconductor material selected from the group consisting of gallium arsenide and gallium phosphide.

9. A method of growing a III-V compound semiconductor material on a substrate, substantially as hereinbefore described with reference to the accompanying drawings.