GB2244285A - Phosphide films produced by pulsed plasma deposition - Google Patents

Abstract

Phosphous-rich non-stoichiometric materials are formed by pulsed plasma deposition from gaseous precursors. The materials may be binary, ternary or quaternary compounds. Typically the materials are deposited on a substrate disposed in a hollow cathode electrode arrangement 14. The materials have suitable properties for the construction of infra-red transmitting elements. <IMAGE>

Description

PHOSPHIDE FILMS This invention relates to phosphide materials and in particular to non-stoichiometric phosphorus-rich material having advantageous physical properties. The invention also relates to a process for forming these materials.

Infra-red elements such as windows which are transparent in the 8-12 micron band are currently constructed from zinc sulphide (Zn S). This material has suitable infra-red properties, but its physical properties are significantly less than ideal. In particular it has poor resistance to rain erosion and the thermal shock.

In an attempt to overcome these disadvantages the use of metallic phosphides as infra-red materials has been proposed. Many of these materials have suitable infra-red properties. However, severe difficulties have been experienced in the preparation of these materials.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a generally amorphous, non-stoichiometric, phosphorus-rich compound MP formed by pulse plasma deposition from vapour phase precursors at or below ambient temperature.

According to the invention there is further provided a method of forming on a substrate surface a generally amorphous non-stoichiometric, phosphorus-rich compound MPX, the method including exposing a vapour mixture containing volatile precursors of the compound to a pulsed radio-frequency whereby to deposit the compound on the substrate, and wherein said substrate is maintained at or below ambient temperature.

Advantageously the phosphorus-rich compounds further include sulphur, e.g. in the form of Zn S.

The use of a pulsed plasma allows the phosphide material to be prepared at ambient temperature. This greatly facilitates the preparation process and expands the range of material compositions that can be formed.

A general description of the pulsed plasma process is provided in our specification No. 21 05729B.

One form of electrode structure suitable for preparation of the phosphide materials described below is disclosed in our copending application No ...........

of even date which derives priority from application No.

90 10000.9.

It is believed that phosphorus has the ability to accommodate local structure disorder because it can adopt a threefold coordination with widely varying bond angles. A significant excess of phosphorus is thus thought to minimise or eliminate 'dangling bond' or 'wrong bond' type defects thus eliminating non-intrinsic infra-red absorption bands and preventing the formation of impurity bands.

We have found that the best infra-red performance and highest chemical stability is obtained at higher phosphorus concentrations. E.g. in a binary compound MP , x may exceed 8. However, the optimum mechanical and thermal properties are obtained at or close to the stoichiometric composition. Thus, by providing a trade off between these two extremes, an optimum composition range can be obtained.

The compounds are formed by deposition from a pulsed radio frequency plasma containing precursors of the constituent elements in vapour form, e.g. as hydrides or as alkyls. Thus, phosphorus may be provided as phosphine (PH3) and sulphur as hydrogen sulphide (H2S). Metallic elements such as Zn, Cd, and Al may be provided as alkyls, e.g. Al (CH3)3. Deposition may be effected by exposure of the vapour mixture at a pressure of about 200m torr and a generator power of about 100 to 300 watts per cc. Typically we employ a pulse width of 100 microseconds, a pulse repetition rate of 25 Hz and a generator frequency of about 13.5MHz. It will be appreciated however that these conditions are by no means critical and, in general will be adapted to match a particular deposition process. For example, a microwave plasma may be employed.It is important that the deposition conditions are such as to ensure the absence of hydrogen in the deposited material.

An embodiment of the invention will now be described with reference to the accompanying drawing in which: Fig. 1 is a schematic diagram of a pulsed plasma deposition apparatus and Fig. 2 illustrates the optical properties of a number of binary phosphide materials and Fig. 3 illustrates the relationship between stoichiometry and optical properties from the germanium/phosphorus system.

Referring to Figure 1, the plasma deposition apparatus includes a reactor chamber 11 having an inlet 110 whereby gases are supplied to the chamber, and an outlet 111 whereby the chamber may be evacuated via a vacuum pump 12. A throttle valve 13 controls the evacuation rate and thus controls the gas or vapour pressure within the chamber 11. Typically the reactor chamber comprises a tube 112, e.g. of quartz, sealed by end plates 113 and 114, the inlet and outlet being provided in one end plate (113). Deposition may be effected via an electrically conductive hollow cathode electrode structure 14 within which a substrate 15 is mounted. Typically the hollow cathode structure comprises a cup-like member 141 supported on a conductive pillar 142 and partially closed by an annular conductive member 143 to define a cavity within which the substrate is exposed to the plasma.A grounded plate electrode 16 is mounted adjacent the electrode structure 14. Pulsed radio frequency energy is applied to the electrode structure 14 from a generator 17 via the support pillar 142. Advantageously the hollow cathode is water cooled to prevent excessive heating of the substrate 15. In some applications the substrate may be cooled below ambient temperature. A plasma generated in the reactor chamber extends into the hollow cathode. Typically we employ an energy density of about 100 to 300 watts/cc to achieve substantially complete dissociation of the plasma. In some applications an impedance matching transformer (not shown) may be employed to couple the generator 17 to the electrode structure 14.

Reactant materials are supplied to the chamber in gaseous form, e.g. as hydrides and/or alkyls. The reactants may be supplied as a uniform mixture or in the form of pulses of different composition, the gas pulses being synchronised with the generator pulses.

In use, the electrical bias appears between the grounded plate electrode 16 and the electrode structure 14. The plasma is substantially fully dissociated by each generator pulse and comprises a mixture of electrons and ions. Electrons have a high mobility, and those near to the grounded electrode 16 migrate to that electrode. The result of this charge migration is to provide the electrode 14 with a negative electrical bias, typically of 300 to 1000 volts. This electrical bias of that electrode provides positive ion bombardment of the substrate 15. We have found that control of this bias results in an improvement of the deposition process.

We have found that, in use, application of a radio frequency to the assembly via the pillar generates a discharge in the space between the electrode bodies.

Although the structure confines the discharge to the electrode region, it permits a free flow of gas to allow interchange between reactant gas and spent reaction products.

Advantageously either or both the electrode bodies 23, 24 are provided on their inwardly directed faces with one or more recesses 29. These recesses provide a local concentration of the plasma discharge and thus enhance the efficiency of the plasma process.

The process may be used to prepare binary compounds (MPx) or it may also be employed to form mixed ternary and quaternary phosphides. It will be appreciated that the pulsed plasma process is particularly suitable for these materials as it permits a continuous adjustment of the compositions.

Furthermore, the plasma composition may be varied during deposition to provide either a graded structure or a layered structure.

In some applications the deposited amorphous material may be annealed, e.g. by heating to 200 to 3000C in an inert gas such as nitrogen. We have found that this generally results in an increase of about 0.3eV in the band gap. It is also thought that the annealing process removes any residual hydrogen from the deposited material.

To illustrate the process a number of binary phosphides have been prepared. In each case the optical band gap has been calculated by observation of the energy at which the optical absorption coefficient * reaches 103 and 104, these being typical values of at the band edge predicted by theoretical models. The results are summarised in Figure 2 of the drawings. For comparative purposes the results for elemental phosphorus are included in Figure 2.

The effect of annealing for a particular binary system, i.e. germanium/phosphorus is illustrated in Fig. 3 of the drawings which illustrate the variation of the 5% optical transmission point with composition in amorphous Ge Px Particularly advantageous binary compounds include, but are not limited to, Ge P , SiPX, A1Px, CP x and ZnPx. These materials are discussed below.

Si Px This material, prepared by pulsed plasma deposition from e.g. silane (SiH4) and phosphine (pH3) exhibits intrinsic absorption bands at 20 microns and at 10 microns. We have measured a Vickers hardness of 420 (for Si P2) and an electronic band gap of about 1.9 eV. The intensity of the 10 microns absorption band is significantly reduced by heating the material to a temperature of 5000C. This is thought to be the result of improved bond retention.

Al x This material prepared by pulsed plasma deposition from trimethyl aluminium and phosphine shows a high stability, even in water for values of x above 5. Films of the material show no absorption bands between 8 and 12 microns and are scratch resistant with a measured Vickers hardness of 390.

Cp This material has particularly advantageous optical properties as the C-P vibrational frequencies are significantly spaced from the infra-red frequencies that are currently of interest. It is important that the material should be formed so as to have substantially no double bonds and substantially no hydrogen. We have found that to achieve these requirements in a plasma deposition process from phosphine and methane it is necessary to provide a considerable excess of methane. We have prepared material of the empirical formula CP6 and have found it to be extremely hard and to have a high band gap of about 2eV. The material has a week absorption band centred at 12.8 microns.

Zn P Typically this is prepared from diethyl zinc and phosphine. The material shows no specific absorption peaks below 20 microns apart from a very weak sharp peak at 11 microns which is thought to be extrinsic. The material is very stable and has a measured Vickers hardness of 280.

The above results illustrate the feasibility of these materials for the construction of infra-red optical elements, e.g. lenses and windows, or for the provision of hard impervious coatings on such devices.

The non-stoichiometric phosphides may advantageously incorporate sulphur. In addition, ternary or quaternary compositions such as Zn Ge - Ga P x may be employed. These materials may be homogeneous or may be provided as atomic layered structures by suitable control of the plasma composition and pulse repetition rate.

Claims (9)

1. A generally amorphous, non-stoichiometric, phosphorus-rich compound MP x formed by pulse plasma deposition from vapour phase precursors at or below ambient temperature.

2. A method of forming on a substrate surface a generally amorphous non-stoichiometric, phosphorus-rich compound MPX, the method including exposing a vapour mixture containing volatile precursors of the compound to a pulsed radio-frequency whereby to deposit the compound on the substrate, and wherein said substrate is maintained at or below ambient temperature.

3. A method as claimed in claim 2, wherein M comprises Zn, Cd, B, AL, Ga, Ge, C, Si, or mixtures thereof

4. A method as claimed in claim 2 or 3, wherein the compound is annealed at a temperature of 200 to 3000C in an inert atmosphere.

5. A method as claimed in claim 2, 3 or 4, wherein the radio-frequency plasma has an energy density of 100 to 300 watts per cc.

6. A method as claimed in any one of claims 2 to 5, wherein the plasma is a microwave plasma.

7. A method as claimed in any one of claims 2 to 6, wherein the substrate is exposed to negative ion bombardment.

8. A method of forming an amorphous phosphorus-rich compound substantially as described herein with reference to Figure 1 of the accompanying drawings.

9. An infra-red transmission element incorporating a phosphorus-rich compound prepared by a method as claimed in any one of claims 2 to 8.