WO1991009995A2

WO1991009995A2 - Process for the deposition of thin films on solids

Info

Publication number: WO1991009995A2
Application number: PCT/GB1990/001967
Authority: WO
Inventors: John Stuart Foord; Dermot O'hare
Original assignee: Isis Innovation Limited
Priority date: 1989-12-21
Filing date: 1990-12-17
Publication date: 1991-07-11
Also published as: JPH05501480A; WO1991009995A3; GB8928824D0

Abstract

A method of forming a group III metal film, or a III - V semiconductor film, on a solid substrate comprises applying to the substrate a gaseous group III precursor having the formula MH3L or MH3L2 where M is Al, In or Ga and L is a Lewis donor. Preferred Lewis donors are based on As, P or N, for example NMe3.

Description

PROCESS FOR THE DEPOSITION OF THIN FILMS ON SOLIDS

Background

The invention relates to processes for depositing on the surfaces of solids films of the metals Ga and In and of III - V semiconductors such as for example the binary, ternary and quarternary compounds of one or more of Ga, Al, In with one or more of As, P, Sb.

The microelectronic and optoelectronic industries have a need to be able to deposit the compounds mentioned above on solids to form materials to be processed into icroelectronic and optoelectronic devices. A variety of methods have been proposed to carry out such processes. These include metal organic vapour phase epitaxy or chemical vapour deposition (MOVPE or MOCVD), low pressure metal organic vapour phase epitaxy or chemical vapour deposition (LPMOVPE or LPMOCVD) , metal organic molecular beam epitaxy (MOMBE) and chemical beam epitaxy (CBE). Although these techniques differ in their exact details the common factor which underlies them is that they all involve the reaction of chemical compounds either in the gas phase in contact with the solid and/or within the adsorbed phase at the surface of the solid resulting in the deposition of the material required on the solid surface. They have also in common the fact that they use metal-organic (compounds with chemical bonds between metal and carbon atoms) compounds as the source of the Group III elements. Such compounds are generally referred to as being the Group III precursors for the process in question. The problem with these approaches is that carbon bonded to the metal atoms in these precursors also is incorporated to a significant extent in the final materials formed. This in many cases can be very disadvantageous since the carbon can degrade the properties of the compounds deposited with reference to their performance in microelectronic and optoelectronic devices. Thus although the development of the vapour phase growth processes MOVPE (or MOCVD), LPMOVPE (or LPMOCVD) , MOMBE, CBE has only been possible because of the availability and use of metal- organic compounds this very same feature has also been disadvantageous as regards the preparation of pure materials.

This patent describes new processes, hydride source vapour phase epitaxy (HSVPE), hydride source chemical vapour deposition (HSCVD), hydride source molecular beam epitaxy (HSMBE), hydride source beam deposition (HSBD) for depositing the compounds listed in the first paragraph and the metals gallium and indium. The key new feature is to use volatile Group III precursors which contain no metal-carbon bonds and which consequently do not result in the incorporation of significant quantities of carbon in the material deposited. The use of these compounds also does not yield significant contamination of the grown materials by other species in contrast to the situation arising in the past when precursors other than organometallic (e.g. Ga, In, Al halides) have been used. Thus the invention overcomes a major problem associated with the growth of the films of semiconductors and metals noted above.

The compound A1H₃.N(CH₃)₃ is known (USP

3375129) and has been used to deposit Al films on substrates by CVD (J. Va. Sci. Technol. A., Vol. ^'7, No. 5, 3117-8, 1989).

The compounds GaH-L are also known (Greenwood N. N., Storr A. and Wallbridge M. G. H., Inorg. Chem 2 (1963) 1036) but their use to deposit metal films on substrates has not been disclosed. The compounds InH₃L can be made similarly. 5 The use of these substituted hydrides as sources of Group III metals for III - V semiconductors has not been described. The Invention

In one aspect the invention provides a 0 method of forming a III - V semiconductor film on a solid substrate by applying a gaseous Group III precursor and a gaseous Group V precursor at the substrate, characterised in that the Group III precursor has the formula MH₃L or H₃L₂ where M is a 5 Group III metal and L is a Lewis donor.

The Group III metal is preferably Ga, Al or In. Semiconductors which comprise Ga and/or In, optionally together with Al are particularly preferred. According to another aspect of the 0 invention, when M is Ga or In, the Group V precursor may be omitted in order to form a film of metallic Ga or In.

The Group V precursors which produce the cleanest films are the hydrides of these elements with 5 the general formula XH₃ (X = As, P, Sb) which should be precracked in the gas admission system. However if it is desired to use precursors with lower volatilities then one or more of the hydrogen moieties in the precursor may be substituted by C1 - C12 _Q hydrocarbon (e.g. methyl, ethyl, tertiarybutyl , triisobutyl, phenyl, substituted phenyl) groups although this risks carbon contamination in the final material. Elemental Group V sources can be used instead of using the Group V reagents outlined above. Alternatively Group V elemental beams can be derived from evaporation from GaAs (arsenic) or InP (phosphorus) or similar materials.

The preferred Group III precursors used in this invention are the compounds MH.-.L or MH₃L₂ where M = Ga, Al, In and L is a Lewis donor. Lewis donors which can be used span the range NH₂R, NHR₂, NR₃, PH₃, PH₂R, PHR₂, R₃, P(0R)₃, PF₃, AsH_3> AsH₂R, AsHR₂, AsR₃, AsF₃, As(OR)₃, R₂0, R₂S (R = C1 - C12 hydrocarbons e.g. alkyl, phenyl or substituted phenyl), C₅H₅N. The general structure of these precursors are illustrated below.

L

MH₃L MH₃L₂

Bidentate ligands L of the Lewis donor kind can also be used. These may be of the form R₂Q(CH₂)_nYR₂ (Q, Y = N, P, As); RZ(CH₂)_nWR (Z, W = 0, S) n is a positive integer and R is an organic group.

Preferably the precursors are chosen such that the semiconductor film has the formula

^My "l-y ^xz ^x1-z

where M is Ga, Al or In, 2 is Ga or In,

X¹ is As, P or Sb,

X² is As, P or Sb, y is from 0 to below 1 , z is from 0 to 1.

The nature of the solid substrate and the condition under which the films are formed on it are not critical to the invention and may be conventional. (J. Crystal Growth, 95 (1989) 121-131). Two sets of conditions are described below, by way of example, one employing molecular beams at low pressure and the other employing chemical vapour deposition using a carrier gas.

Hydride Source MBE (HSMBE) and Hydride Source Beam ₅ Deposition (HSBD)

In this description we use the term HSMBE to refer to the process below which results in the epitaxial growth of material and HSBD where the process conditions are chosen to produce non- 0 crystalline deposits or crystal growth with a low degree of crystal lographic orientation with respect to the underlying substrate.

In these processes the sample on which deposition is to be carried out is placed in a vacuum 5 chamber on a stage which enables the sample to be heated. In order to obtain the purest films it is helpful if the vacuum chamber is designed for ultra- high vacuum capabilities using standard design and is equipped with liquid nitrogen cooled cryopanels and _Q large capacity diffusion or turbomolecular pumps to provide large pumping speeds for the gases to be used in the process. Means are provided to heat the sample uniformly, accurately and reproducibly to a given temperature ideally up to 1000° in particular instances (e.g. for deposition on silicon). The temperature measurement can be done for example by thermocouples or pyrometry with calibration by for example using the melting point of known materials. It is also helpful if the chamber is equipped with reflection high energy electron diffraction (RHEED) which can be used to follow the extent of deposition in many instances using intensity oscillations in the observed RHEED pattern. The chamber is also equipped with gas admission facilities appropriate to the number of elements required in the grown material. For uniform growth the inlets should be designed to achieve a uniform gas flux across the sample. this can be done for example by ensuring that the final passage through which the gas passes before entering the bulk of the vacuum chamber consists of a straight 5 section of tube with low length: radius ratio. The gas admission facilities should also enable reproducible fluxes to be obtained to the precision appropriate to that required in the stoichiometry of the material to be grown through the use for example of fast acting _Q mass flow controllers (e.g. as manufactured by M S). Gas admission can be improved if the inlet equipment for the aluminium, gallium and indium precursors can be heated to avoid unwanted condensation of the precursor in the admission lines. For the precursors 5 for As, P and Sb it is desirable to flow the gases through an inlet nozzle heated to temperatures in excess of 1000°C and packed with crushed tantalum foil since this brings about their partial cracking resulting in enhanced growth rates. _Q In a typical experiment the desired surface of the solid on which deposition is to be carried out is heated to temperatures in the range 100°C - 800°C while gaseous Group V precursors are beamed at the surface. The temperature required depends on the 5 material to be deposited and the substrate used but temperatures in the range quoted are found to yield high deposition rates.

The gaseous Group III precursors are then switched on. The precursors decompose on the heated 0 surface of the solid to deposit the Group III metals (Al, Ga, In) and volatile reaction products which desorb from the surface and are pumped out of the system. The Group V atoms (P, As, Sb) from the Group V precursors also adsorb on the surface to produce the 5 desired compounds listed above. The reaction probability of the Group III precursors with the surface is high hence growth rates of typically

1μm hr can be obtained with surface fluxes of

19 -2 -1 approximately 10 molecules s . In order to avoid forming material depleted in Group V species it is helpful if effective fluxes of the Group V species to the surface considerably exceeds this figure.

Typical chamber pressures used in the growth process

-4 are up to 10 bar.

The Group III stoichiometry achieved in the grown material is roughly proportional to the incident fluxes of the Group III precursors used in an experiment and can for example be accurately controlled by calibration obtained by measuring the stoichiometry of grown materials. Likewise the Group V stoichiometries can also be accurately controlled by such calibration procedures.

In the case where growth is carried out on silicon or 11-V semiconductors epitaxial single crystal material can be grown if appropriate surface preparation procedures are used as is current practice in MBE to prepare atomically ordered surfaces and where there is little lattice mismatch between the compound to be grown and the underlying substrate. In other cases disordered growth can occur. Differing phases can be grown sequentially on the underlying substrate by controlling the time dependence of the incident flux accordingly. Ultimately the phase can be grown 1 ayer-by- 1 ayer as is the case with conventional atomic layer epitaxy. If the Group III reagents defined above are used alone in the absence of the Group V reagents then the growth of the metallic elements Ga or In is achieved. Hydride Source Vapour Phase Epitaxy (HSVPE) and Hydride Source Chemical Vapour Deposition (HSCVD) Again we use the term HSVPE for the process where crystal lographical ly oriented growth results and HSCVD where growth without a high degree of orientation occurs.

In these processes the gaseous precursors specified above are employed but the reaction conditions are changed to operate at pressures up to around normal atmospheric pressure. A typical experiment involves placing the substrate on which deposition is required in a reactor consisting of a gas tight container through which gases can be flowed and which can be heated/cooled, with facilities for controlling sample temperatures in the range up to 1000°C. Standard reactor designs are used in conventional atmospheric MOVPE are suitable. The Group III precursors noted above are placed inside bubblers in thermostatically controlled environments and carrier gas is flowed through the bubblers and into the reactor. Carrier gases which can be used include hydrogen, inert gases or the Lewis donors used in forming the Group III precursors from their trihydrides above and flow control can be achieved with mass flow controller such as manufactured by MKS. Carrier gases can again be employed if the Group V precursor used is of low volatility. A high excess of the Group V precursor over the Group III precursor should be employed. The sample is heated in the reactor and deposition occurs on the sample surface as in conventional MOVPE. The key feature here however is that the invention using the hydride precursors of the Group III element leads to very low levels of carbon in the deposits. Again epitaxial growth of the III-V semiconductors can be achieved on lattice matched substrates provided cleaning procedures are used as in conventional MOVPE. If the Group III precursors of Ga and In are used but the Group V precursors are omitted then the films deposited are of Ga or In. Low pressure variants of the above procedures can be achieved as in conventional low pressure MOVPE using a reactor which is pumped

-5 preferably down to less than 10 mbar before gas is admitted. Under these circumstances carrier gases for the Group III precursors do not need to be used for low pressure deposition experiments at total pressures less than 1mbar.

Example 1 Hydride Source Beam Deposition (HSBD) of Al on GaAs

A (100) oriented GaAs single crystal wafer was introduced into the ultra-high vacuum system and cleaned by a combination of argon ion bombardment and annealing such that the surface displayed a sharp 4 x 1 LEED pattern and was ato ically clean as monitored by X-ray photoelectron spectroscopy (XPS). The surface was then heated and a molecular beam of AlH₃(NMe₃)₂ was directed at the surface at an effective pressure of 10^" mbar. When heated to temperatures in excess of 430K Al was deposited on the surface and continued to be deposited subsequently even when the surface temperature was reduced to 400K. The reaction probability of the Al precursor was found tc be high (>0.5). Subsequent analysis of the surface of the GaAs sample in situ by XPS showed that a film of Al had been deposited on the surface and no impurities could be detected by XPS. Detailed surface spectroscopic measurements were performed to show that the NMe₃ ligands do not dissociate to any significant extent hence the films can be extremely pure. Films of Ga and In can be grown similarly using GaH₃(NMe₃)₂ and InH₃(NMe₃)₂ respectively. Example 2 Hydride Source Beam Deposition of AlAs

The second series of benchmark experiments were carried out similarly and involved the preparation of a clean GaAs (100) sample in ultra-high vacuum as described above. The sample was then heated to 700K in a molecular beam of arsenic (mainly As₂) derived from the evaporation of GaAs. The molecular beam of AlH₃(NMe₃)₂ was then turned on. The effective pressure of the arsenic beam was approximately 10^" mbar and the pressure of the Al precursor was roughly an order of magnitude lower. The reaction was carried out for 10 minutes during which time molecular beam measurements by quadrupole mass spectrometry showed the reaction rate was high. The molecular beams were then turned off and the sample analysed by XPS. XPS showed deposition of a high purity AlAs layer. Example 3 GaAs Growth by Hydride Source Beam Deposition

The third series of experiments were a repeat of the second series except using the Group III precursor GaH₃(NMe₃)₂ instead of the Al precursor noted above. After growth of the film XPS analysis in situ demonstrated the deposition of a high purity GaAs fi lm.

There was good scientific evidence that the NMe₃ ligands did not dissociate in these experiments to a significant extent. Having carried out these experiments described above there is every reason to believe that all of the processes mentioned in the patent, which essentially generalise the experiments mentioned in 2 and 3 above are technically straightforward.

Claims

CLA IMS

1. A method of forming a III - V semiconductor film on a solid substrate by applying a gaseous Group III precursor and a gaseous Group V precursor at the substrate, characterised in that the Group III precursor has the formula H₃L or H₃L₂ where M is a Group III metal and L is a Lewis donor.

2. A method as claimed in Claim 1, wherein M is Al, Ga or In, and L is selected from NH₂R, NHR₂, NR₃, PH₃, PH₂R, PHR₂, PR₃, P(0R)₃, PF₃, AsH₃» AsH₂R, AsHR₂, AsR₃, AsF₃, As(0R)₃, R₂0, R₂S, C^N, R₂Q(CH₂)_nYR₂ and RZ(CH₂) WR where Q and Y are As, P or N, and where W and Z are 0 or S, and where n is a positive integer and where R is C1 - C12 hydrocarbon.

3. A method as claimed in Claim 1 or Claim 2, wherein the Group V precursor is selected from XH₃, XH₂R, XHR₂ and XR₃ where X is As, P or Sb and R is C1 - C12 hydrocarbon.

4. A method as claimed in any one of Claims 1 to 3, wherein the precursors are chosen such that the

1 2 1 2 semiconducto Λr film has the formula y .i -y Xz' Xi -z where M is Ga, Al or In, 2 is Ga or In,

X¹ is As, P or Sb,

X² is As, P or Sb, y is from 0 to below 1 , z is from 0 to 1.

5. A method as claimed in any one of Claims 1 to 4, wherein the substrate and conditions are chosen to produce epitaxial growth of the film.

6. A method as claimed in any one of Claims 1 to 4, wherein the substrate and conditions are chosen o produce polycrystal 1 ine growth of the film.

7. A method of depositing an Indium film on a solid substrate by applying a gaseous precursor to the substrate, characterised in that the precursor has the formula InH₃L or InH₃L₂ where L is a Lewis donor.

8. A method of depositing a Gallium film on a solid substrate by applying a gaseous precursor to the substrate, characterised in that the precursor has the formula GaH₃L or GaH₃L₂ where L is a Lewis donor.

9. A method as claimed in any one of Claims 1 to 8, wherein growth of the film is effected by a molecular beam technique.

10. A method as claimed in any one of Claims 1 to 8, wherein growth of the film i s effected by a chemical vapour deposition technique.