CA1319587C - Metalorganic chemical vapor depositing growth of group ii-vi semiconductor materials having improved compositional uniformity - Google Patents

Abstract

Abstract of the Disclosure A method for growing a Group II-VI epitaxial layer over a substrate is described. The method includes the steps of directing a plurality of vapor flows towards the substrate, including a Group II organic vapor, a Group VI organic vapor, and a Group II elemental mercury vapor. At least one of the Group II organic vapor and Group VI organic vapor has organic groups which sterically repulse the second one of the Group II and Group VI organic vapors or which provide electron transfer to the Group II atom or electron withdrawal from the Group VI atom. With the particular arrangements described, it is believed that substantially independent pyrolsis of the Group II organic vapor is provided over the growth region of the substrate, and accordingly, Group II depletions such as cadmium depletion in the epitaxial films provided over the substrate is substantially reduced.

Description

1~'5 ~ 19~8'7 ~

METALORGANIC CHEMICAL VAPOR D~POSITION GROWT~ OF
GROUP II-VI SEMICONDUCTO~ MATERIALS HAVING IMPROVED
COMPOSITIONAL UNIFORMITY
Back~round of the Invention This invention relates generally to epitaxial growth techniques~ and more particularly ts growth of Group II-VI
- semiconductor crystalline materials~
As is known in the art, Group II-VI semiconductor epitaxial mater;als such as cadmium telluride and mercury cadmium telluride have important applications as photodetector elements for detection of electromagnetic energy in the spectral range from approximately 0.8~ m to 30~ m. By adjust-ing an alloy composition of cadmium and mercury, photodetector elements are provided which are sensitive to different wave-length ranges withi~ the 0.8 ~ m to 30 ~m wavelength band.
Several different techni~ues have been suggested for providing cadmium telluride and mercury cadmium telluride suitable for use in photodetector applications. One method suygested is metalorganic vapor phase expitaxy (MOVPE), also referred to metalorganic chemical vapor deposition (MOCVD).
As it is known, the MOCVD technique for growing mercury cadmium telluride involves directing vapors of mercury, dimethylcadmium, and diethyltelluride into a reactor vessel and chemically reacting the directed vapors to provide the epitaxial material.

~ 3 ~ 7 Several problems are enco~ntered in the art of growing mercury cadmium telluride epitaxial layers by the MOCVD
technique! One problem of particular importance is the compositional uniformity of the deposited epitaxial layers provided by the MOCVD technique.
Generally, the composition of these layers varies from the downstream portion of the substrate to the upstream co~ ~ Os ~-h`o~ a/
, ` portivn. This ~osit-on-a~- variation generally involves a progressively increasing depletion of Cd towards the downstream or back portion of the substrate whereas the upstream portion or front portion of the substrate is generally excessively rich in Cd. Variations in the lateral and side to side compositional uniformity Cd are ~enerally also present.
The prevalent view regarding the chemical reaction mechanisms which occur in Group II-VI materials yrown by MOCVD is set forth in an article entitled "Organometallic Growth of II-YI Compounds~ by J.B. Mullin et al~ Journal of Crystal Growth, Volume 55r 1981~ pp~ 92-106~ In this article, the authors suggest that the directed alkyls of tellurium and cadmium may not pyrolyse independently. Rather, the authors suggest that adducts or complexes of these compounds are produced because D~Cd (dimethylcadmium) and DETe (diethyltelluridP) are attracted to one another in the vapor phase forming a weak bond~ In the authors' v;ew, the decompo-sition of these adducts leads to the formation of the Group II ~I materials and o~her productsO

~3~L9~8~

Aecording to the presen~ invention there is provided a method of providing a layer comprising a Group II-VI semiconductor material over a substrate, comprises the step of:
directing a flow comprising a Group II metalorganic yroup, and a Group VI organic comprising a Group VI moie-ty attached to at least one organic group towards the substrate wherein the said organic group attached to the Group VI moiety is a large and bulky organic group, characterized in tha~ the said organic group attached to the &roup II moiety is a sufficiently large and bulky organic group to sterically repulse the large bulky organic group of the Group VI organic, such that the Group II metalorganic and the Group VI organic pyrolyse substantially independently.
The invention further provides a method of providing a layer comprising a Group II-VI material which comprises the steps of:
directing a flow comprisiny a Group VI organic incorporating a Group VI moiety towards a substrate;
directing a flow comprising a Group II organic towards the substrate, with said Group II organic comprising a Group II moiety with sufficiently large and bulky organic groups attached thereto to sterically repulse the Group VI organic, such that the Group II
organic and the Group VI organic pyrolyse substantially independently; and reacting the Group VI and Group II moieties thereby released from the said organics ~o form the Group II-VI material over the substrate.

- 2a -, :`, :.:

1 3 1 ~ ~ g 7 62901-710 The invention additionally provides a method ~or growing a layer comprising Group II-VI material over a subs~rate which comprises the steps of:
directing a first flow comprising a Group VI oryanic towards the substrate;
directing a Group II organic towards the substrate, with saicl Group II organic having a Group II moiety with organic groups bonded directly to the Group II moiety which provicle electron transfer to the electroposi~ive Group II moiety, such that the Group II organic and the Group VI organic pyrolyse substantially independently.

- 2b -~, . . .

8 7 6290l-7l0 Ideal MOCVD Growth of HgCdTe, for example, may be viewed as an irreversible pyrolyRis in which the primary alkyl ~1 order) of the tellurium and the 0 order alkyl of the cadmium independently decompose into elemental telluriu~ and elemental cadmi~m. The ~lemental tellurium react~ with the elemental cadmium and elemental mercury provided in the vapor stream to form mercury telluride ~nd cadmium telluride. The mercury telluride and cadmium telluride are then deposited over the Rubstrate to form the mercury cadmium tellur~de as ~how in Reactions 1-5 below:

DETe - ~ Te ~ H.C. ~Reaction 1) Te ~ H~ HgTe tReaction 2) DMCd - ~ Cd ~ H.C. (Reaction 3) Cd ~ Te - ~ CdTe (Reaction 4) HgTe ~ CdTe ~ Hgl_xc~xT~ ~Reaction 5) where ~.CO stands for hydrocarbons Although there i~ evldenc2 that mercury tellurlde i~
grown by the independent pyroly~i~ oF the primary allcyl of tellurium and the rcaction between mercury and elemental tellurium ~Reaction~ 1 and 2~, cadmium telluride i~ now be1ieved to be grown by ~ different proces~ which occur.~ at elevated temperatures. ThQ simpllfled overa11 reaction i~ as shown in ~eaction 6, DMCd ~ DETe ~ CdTe ~ ~C ~Reaction 6) :.. ..

~ ~9~

In this reaction, the electropositive cadmium atom in dimethylcadmium (DMCd) is attracted to the electronegative tellurium atom in diethyltelluride. At low temperatures, this attraction is not significant enough to form a bond creating a stable adduct. How~ver, at the ele~ated tempera tures over the growth region in the reactor vessel, this attraction leads to a chemical reaction in which the alkyls of cadmium and tellurium react to form cadmium telluride plus other hydrocarbons. This chemical reaction leads to a rapid depletion of cadmium in thP downstream portions of the HgCdTe layers formed over the substrate. Accordingly, the nonpyrolytic behavior of the cadmium alkyl is seen as a first cause of compositional nonuniformity in H~CdTe films formed by metal-organic ch mical vapor deposition. In general, therefore, the nonpyrolytic behavior of the Group II alkyl, caused by attraction between the electropositive Group II atom and the electronegative Group VI atom, is seen as the first cause of compositional nonuniformity in MOCVD growth of Group II-VI
materials.
A second cause of comDositional nonuniformity in Group II-VI materials , particularity materials such as HgCdTe which involve two Group II elements, is believed to involve a A reversible, exchange reaction in the vapor phase between the primary alkyl of the ~roup IX atom and elemental Group II
atom. In particular in ~gCdTe, the primary alkyl of Cd `8 ~

and the elemental mercury are involved in the following reaction:

DMCd + Hg --3 DMHg + Cd ~Reaction 7) Typically, with the MOCVD technique, the reactor walls and the mercury source are heated to a temperature of about 220C
to prevent condensation of the mercury from the vapor stream.
At these wall temperatures, Reaction 7 has an equilibrium constant in which the reaction is primarily driven towards the right, that is in a direction sn which dimethylcadmium decomposes. At these temperatures, however, the rate of the reaction is relatively low, and accordingly the reaction i5 not a significant cause of Cd depletion. However, when the vapor stream arrives at the substrate, there is a sudden increase in temperature which causes the phase composition o the vapor stream to change since Reaction 7 is now driven-very strongly to the right. Consequently, there is a strong variation in Cd concentration in which elemental cadmium is produced by the reaction. The elemental Cd is even more attracted to the diethyltelluride than DMCd~ As shown in Reaction (8), it reacts with diethyltellurid~ to form cadmium telluride over the upstream portions of the susceptor. That - is, CdTe may be deposited out of the vapor stream prior to reaching the substrate. This arran~ement again lead to a cadmium depletion in downstream portions of he layers grown over the substrate.

1 319~87 Cd + DETe ~ CdTe + H.C. (Reaction 8) Accordingly, the nonpyrolytic behavior of the cadmium alkyl is seen as a major cause of compositional nonuniformity in Group II-VI materials such as ElgCdTe. This compositional nonuniformity is also believed present with other Group II-VI
materials. For example, mercury zinc telluride has been proposed as a replacement material for mercury cadmium tellu-ride. Zinc is a significantly more electropositive atom than cadmium. Accordingly, alkyl zinc compounds should be signifi-cantly more reactive towards electronegative species such as ~ellurium than alXyl cadmium compounds. As a consequence of this attraction, the first cause of compositional nonuniformity, i.e. the chemical reaction between the Group II alkyl and the Group VI alkyl, maybe even more significant for a material such as mercury zinc telluride. This mechanism is also believed present in mercury manganese telluride, a second potential replacement or mercury cadmium telluride. Manganese is also significantly more electropositive than cadmium, and consequently~ a manganese alkyl would be significantly more reactive towards the Group VI alkyl than the cadmium alkylO
With mercury zinc telluride, since zinc is significantly more electropositive than Cd, there results a negligible exchange reaction with mercury, and accordingly, Reaction 9 occurs readily with substantial no reverse reaction present.

- ~3~9~8~

Zn ~ DMHg ~ DMZn ~ ~g (~eaction 9) In accordance with the present invention, a Group II-VI
layer is provided over a substrate. A first flow comprising a selected Group II organic is directed towards the substrate and a second flow comprising an organic of the Group VI
el~ment is also directed towards the substrate. At least one of the Group II and Group VI organic compounds have at least one organic group that sterically repulses the organic groups of the second one of the Group II and Group VI organic compounds.
Preferably, the Group II organic has large, bulky organic groups which surround the Group II atom and sterically repulse the organic groups of the selected Group VI organic, thus reducing or substantially eli~inating reactions between the Group II organic and the Group VI organic in the vapor stream.
Preferably still, the selected Group II organic includes a ,., ,~e~.lsi~r1 branched organic group which increases the steric r-s~
of the Group VI organic compound. With this particular - arrangement, by providing a Group II or Group VI organic source, having large organic groups surrounding the ~roup II
element or Group VI element, steric hindrance is provided between the Group II organic, and the Group VI organic source.
That i5, by surroundin~ the Group II and/or Group VI element with large, bulky groups which sterically repulse one another, the Group II and/or Group VI atoms are prevented from getting 5 close to each other, and ~hus the reactions involving the L 9 ~ 8 7 Group II organic, and the Grou~ VI organic described above, are substantially reduced. The selected Group II and Group VI
organic sources are thus substantially less reactive towards each other in the vapor stream when compared to conventional S Group II organics and Group VI organic sources. Accordingly, the Group II organic and Group VI organic pyrolyse or decompose substantially independently of one another over the growth region above the su~strate. Since the Group II and Group VI
organics decompose substantially independently over the substrate, the concentration of Cd varies significantly less in the vapor stream, and hence the deposited layer has a substanti~lly more uniform composition.
In accordance with a still further aspect of the present invention, the Group II organic sterically repulses the Group VI organic source which includes an organic group having a relatively low activation energy compared to the activation Pnergy of a primary alkyl of the Group VI element for the formation of a free radical during pyrolysis of the Group VI
organic compound. A flow comprising an elemental source of a Group II metal is also directed towards the substrate~ The selected Group II organic having the large bulky organic groups is sterically hindered from reactin~ with the elemental Group II metal. Further, the sterically hindered Group II
organic is selected to have a thermal stability comparably to the thermal stability o~ the selected Group VI organic. With ~ 3 ~

this particular ar~angementr low temperature growth of compositionally uniform Group II-VI materials is provided.
The low growth temperatures should kinetically limit the rate of the exchange reaction involviny the selected Group II
organic and elemental Group II metal, and the steric replusion - effects should further kinetically limit this exchange reaction~
Thus, the second additional cause of compositional nonun~formity in MOCVD growth of Group II-VI materials is also substantially reduced, since the rate constant at low temperatures for the exchange reactio~, is relatively small in the direction with provides the exchange of Group II metals. Side to side compositional uniformity is also believed improved, since with prior approaches, any cause of side to side nonuniformity, such as, variations in flow patterns and temperature gradients were amplified due to the uncontrolled and non-independent character of the chemical reactions which were occuring~
In accordance with a still further aspect of the present invention, à mercury ca~mium telluride crystalline layer is grown over a crystalline substrate by directing a plurality of vapor ~lows ~owards the substrate. A first vapor flow comprises a source of mercury, a second vapor flow comprises an organic source of cadmium selected from the group consisting of diethylcadmium (DECd), di-N-propylcadmium (DPCd~, di-iso-butylcadmium (DIBCd) r and di-neopentylcadmium ~DnPCd). The .
_ g _ 1319~8~

tellurium organic includes at least one organic group selected from the group consisting of a secondary alkyl, a tertiary alkyl, an allyl, a benzyl, and a cycloallyl group bonded to the tellurium atom. The selected Cd organic, Te organic, and ~g are reacted at a temperature at which the j exchange reaction involving the Cd organic and Hg is kineti-cally limited. With this arrangement, by providing a cadmium organi~ having organic groups surrounding the cadmium atom :j .
which-~terically hinder the tellurium atom from reacting with the cadmium atom, a substantially reactive-free transport of these vapors through the reactor tube towards the elevated temperature region over the substrate is provided. Further, since the temperature over the growth region is such that the exchange reaction is kinetically limited ~i,e. the rate of the reaction is low) substantially more uniform growth of HgCdTe is provided since there is less elemental Cd available to react with the Te organic prior to the substrate. At the growth temperatures over the substrate, the eadmium organic and tellurium-organic pyrolyse substantially independently, providing ~ree tellurium and cadmium. The free tellurium reacts with the elemental mercury and free ~admium to form mercury cadmium telluride. Since cadmium is not lost in the :j vapor stream prior to pyrolysis of the cadmium organic over the su~strate, the front to back compo~itional uniformity of the deposited layers will be substantially more uniform. Due ~ 31~8'~

to the controlled reactions which are occuring, i.e. substan~
tially independent pyrolysis of the cadmium and tellurium alkyls, improvement is also anticipated in the side to side compositional uniformityO
In accordance with a further aspect of the present invention, a Group II-VI layer is provided over a substrate, A first flow comprising a selected Group II organic is directed towards the substrate and a second flow comprising an or~anic of the Group VI element is also directed towards the substrate.
At least one of the Group II and Group VI organic compounds have organic groups which provide either electron transfer to the electropositive Group II element or electron transfer from the electronegative Group VI element. Preferably, the Group II organic has an electron releasing organic group bonded directly to the Group II element. The electron releasing group should preferably be a stable group where the potential for incorporation of unintended dopants could be a problem.
An example of an electron releasing group is the phenyl group (C6Hs). With this particular arrangement, by providing an organic Group II source having electron releasing groups bonded to the Group II element, the electropositivity-of the Group II atom is reduced by the transfer of electronic charge from the selected electron releasing group. In particular, since the electropositivity of the Group II element is reduced ~5 by the presence of the phenyl groups, this will concomitantly ~ 31~58~

reduce the attractive force between Group II atoms and Group VI atoms while the Group II organic and the Group VI organic are in thP vapor stream. The electronegative nature of ~he phenyl group should also reduce the interaction between an organic Group VI compound and the organic Group II compound having the phenyl group attached.
In accordance with a still further aspect of the present invention, the Group II compound is selected having an organic group bonded directly to the Group II atom which provides electron transfer to the electropositive Group II atom and provides steric hinderance between the Group II atom and Group VI atom in the Group VI organic sourceO Preferably, the Group II element has a pair of phenyl groups bonded directly to it with at least one hydrogen atom of one of the phenyl groups replaced by an organic group. Preferably, electrophilic substitution at a hydrogen position on the phenyl ~roup is from a species belonging to the class of groups which are generally classified as activating, ortho, para directors. Examples of organics classified as ~eakly activating groups include phenyls and alkyls (methyl, ethyl e t~ ro~Ds etc.). ~e~e~o~ that is groups containing atoms other than hydrogen and carbon and classified as moderately and strongly activating groups include alkoxides, -OCH3, -OC2Hs etcO; -NHCOCH3; -OH and -NH2 ~-NHR, NR) where R is a radical, may also be used, keeping in mind the potential for in~roduction ~3 ~ ~87 of oxygen and nitrogen. For example, a methyl group may be used to replace a hydrogen at one of the ortho positions or the para position of each one of the phenyl groups. The presence of the large organic groups will sterically hinder the Group II atom from reacting with the Group VI atom by the presence of out-of-plan~ hydrogen atoms associated with the methyl group. That is, these hydrogen atoms will partially shield the Group II atom. Furth~rmore, with this shielding of the Group II atom by the out-of-plane hydrogen atoms the vapor pressure of the Group II organic will also increase, since the inten~olecular attraction between a cadmium atom of one molecule and a phenyl group of a similar molecule is reduced. Further, the use of ~he activating ortho and para directors will increase the electron transfer and, hence, further reduce the electropositivity of the Group II atom.
In accordance with a further aspect of the present invention, the hydrogens at both ortho positions of each phenyl group are replaced by a large organic group such as a phenyl group, or an alkyl group such as a methyl, ethyl etcu Substitution of a methyl group, for example, in each ortho position of each phenyl group will sterically repulse the methyl groups of the other phenyl groups and, accordingly, the methyl groups will be rotated 90 from each other resulting in a nonplanar molecule. With this particular arrangement, in addition to the steric hinderance provided by the presence 9 ~ ~ 7 ~

of the substituted groups, and the electron releasing of the phenyl groups, an additional feature of a molecule having both ortho positions of each phenyl group substituted is tha~
the central Group II atom is enclosed by a cage fonmed by the rotated methyl groups. This structure results in a molecule whicht although heavier, is believed to have a higher vapor pressure thanr a nonsubstituted Group II phenyl, or the single ortho position substituted &roup II phenyl, since the cage of methyl groups around the Group II atom should signifi-cantly reduce intermolecular attractions between the Group II
atom of one molecule and a phenyl group of a second molecule.
Moreove., the cage of methyl groups surrounding the Group II
atom should make this atom particularly unreactive towards Group VI alkyls.
In accordance with a still further aspect o~ the present invention, a mercury cadmium telluride crystalline layer is grown over ~ substrate by directing a plurality of vapor flows towards the substrate. The first vapor flow comprises a source of mercury, the second vapor flow comprises an organic source o- cadmium selected from the group consisting of di-phenylcadmium ~DPCd), di-orthotolylcadmi-~ (DOTCd), di-(2,6 xylyl)cadmium (DXCd), and di mesitylcadmlum (DMSCd).
The Group VI organic includes organic groups selected from the group consisting of a primary alkyl, a secondary alkyl, a tertiary alkyl, an allyl, a benzyl r and a cycloallyl group ~3~8~

bonded to the Grou~ VI element. With this particular arrange-ment, by providing a cadmium organic having organic groups which sterically hinder reactions with the tellurium organic, and provides for electron transfer to the electropositive cadmium atom, the attraction between ~he cadmium organic and the tellurium organic is concominantly reduced. The steric hinderance will also reduce the exchange reaction between the cadmium organic and mercury. Accordin~ly, substantially independent pyrolysis of each organic source over the substrate is provided, thereby, providing a mercury cadmium telluride layer having improved compositional unifor~ity.

. 15 , 13~93~ ~
~ .
Brief Descr_ tion of ~
The foregoing features of this invention, as well as the invention itself, may be more fully understood from the detailed description of the drawings, in which;
FIG. 1 is a plan view of a photodetector element, here a photoconductive element including cryst~l layers comprising Group II-~I semiconductor materials;

FIG~ 2 is a cros~-sectional view taken along line 2-2 of FIG. l;

FIG. 3 is a view showing the relationship between FIGS.
3A and 3B;

FIGS. 3A, 3~ are schematic diagrams oE a growth apparatus for use in growing the epitaxial la~er shown in FIG. l; and .~5 FIG. 4 is a schematic diagram of an alternate reactor vessel having a reservoir for those Group II organic sources having a high melting temperature.

a 8 7 Descri~tion of the_Preferred Embodiments Referring now to FIGSo 1 and-2, a typical photoconductive element 10, suitable for use in a photoconductive array (not shown) is shown to include a substrate 11, here comprising cadmi~m telluride (CdTe) or gallium arsenide ~GaAs), indium - antimonide (InSb1 or other suitable Group II-VI or Group III-V
; ~ubstrate materials or sapphire (A12O3)~ Disposed over and here on a substrate 11 is a Group II-VI epitaxial buffer layer 12a here comprising cadmium t~lluride ICdTe), and a second epitaxial layer 12b of cadmium telluride (CdTe) or mercury cadmium telluride tH9cdTe)~ or other suitable Group II-VI material such as HgZnTe, or a material such as HgMnTe.
Disposed on portions of the epitaxial layer 12b are a pair of electrical ohmic-type contacts 13 each provided from a patterned composite layer comprising sequentially deposited layers 13a~
: 13b, and 13c respectively, of indium ~In) 10,000 A thick, chromium (Cr) 500 A, and gold ~Au) 5,000 A thicko Pads 14 comprising gold each 1.5 m thick are disposed over the contacts 13 to provide a bonding point for external components.
Disposed in a channel region 15 between the ohmic contacts 13 is a passivation layer 16a, here of an in situ anodic oxide formed from a portion of the HdCdTe layer 12b as is known, 800 A thick and an anti reflection coating layer 16b. Layer 16a, 16b are used to protect the channel region 15 and to provide a composite layer window 16 which is trans - }7 -3 1 ~

parent to incident electromagnetic energy 17 generally in the wavelength range o approximately 0.8~ m to 30~ m which is directed towards the window 16. In response to such incidence radiation 17, the conductivity of the epitaxial layer 12b changes, thus permitting the photoconductive element to det~ct the presence of the incident electromagnetic radiation 17. Further, the ratio x of Cd to Te may be adjusted, as is known, to sèlectively cover different ranges of wavelengths within the band of approximately 0.8~ m to 30~ m.
Referring now to FIGS. 3, 3A, and 3B, a schematic representation of a vapor phase epitaxial apparatus 20 ~FIG.
3) used for growing epitaxial layers 12a, 12b of cadmium telluride or mercury cadmium telluride, as described in conjunction with FIGS. 1 and 2 above, includes a vapor apparatus 2~a (FIG. 3A) having a manifold 26 with mass flow controllers 26a-26~, and bubbler apparatus 39 and 55, as shown. During operation, hydrogen is fed, via H~ purifier 22 and valve 24, to manifold 26, whereas, helium is fed through apparatus 20 when the apparatu~ 20 is inoperative and exposed to air. The vapor phase apparatus 20 also includes a vapor phase epitaxial reactor 20b ( FIGo 3B), here including an open quartz reaction tube 60, as shown. Suffice it here to say that a graphite susceptor 63 i5 disposed in the quartz reaction tube 60 and the susceptor is inductively heated by an r.f. coil 620 R. f . coil 62 is disposed around the periphery of quartæ

13~9~7 :~
;

reactor tube 60 and is activated to raise the temperature of the susceptor 63, the substrate 11 disposed on the susceptor 63, and the immediate region 61 around the substrate 11 to a ~- predetermined temperature. The temperature of the substrate 11 S is monitored, via a thermocouple ~not shown), embedded in the susceptor 63. Prior to the susceptor 63 and the substrate 11 being heated, however, the system is purged of atmospheric gasses by introducing helium, then hydrogen into the interior of the furnace tube 60 and vapor apparatus 20a. Then vapors , from lines 27e-27g, 31c and 47c are fed in~co rhe tube where th~y decompose and react to provide the epitaxial layers 12a, 12b. Quartz reaction tube 60 also includes a cap 72 at an opposite end from lines 27e, 279, 31c and 47 which is coupled to a quartz exhaust line 74 used to exhaust gasses from tube 60.
Referring now particularly to FIG. 3A, the vapor apparatus 20a provides tubes 31c, 47c and 27e-27g which feed vapors to the quartz reaction tube 60 (FIG. 3B), as shown.
Tube 31c the Group II organic source ~H2 tube is fed from a junction member 32. Junction member 32 is u ed to mix flows from two gas sources delivered to a pair of ports thereof, and direct said mixed gas flow to third port thereof, which is coupled to the quartz tube 31c. The first port of junction 32 is fed from the bubbler apparatus 39~ Bubbler apparatus 39 includes a pair of solenoid control valves 28, ~ 19 --~3~9~7 `

30. A first one of.the solenoid contxol valves, 28, 30, hore solenoid control valve 28, has a first port coupled to a first mass flow controller 26a, via tubes 27a and has a second port coupled to a bubbler 36, via tube ~9a. Bubbler 36 here has disposed therein the selected Group II organic compound, as will be described hereina~ter. The bubbler 36 is provided ln recirculating temperature control bath 40 which provides a constant flow of a liquid around the bubbler to maintain the organic Group II compound 36 at a predetermined temperature to provide a sufficient vapor pressure. This range of temperature may extend but not necesarily be limited to the range of -20C to 100C. A second tube 29c is disposed into bubbler 36, above surface of the organic Group II source and is coupled to a port of solenoid control valve 30. A
15 third tube 2~b is coupled between the remaining ports Qf solenoid control valves 28 and 30~
The normally deactivated state of solenoid control valves 28 and 30 enables hydrogen gas to pass from the hydrogen source, here the mass flow controller ~6at via tube 27a, to tube 29b and on through to tube 31c to purge ~he reactor vessel of atmospheric gasses, as described above~
During epitaxial growth of cadmium telluride or mercury cadmium telluride, for example, solenoid control valves 28 and 30 are placed in their activated state enabling hydrogen gas to pass through tube 29a into bubbler 36 which contains a :~19~87 selected organic cadmium source 37. The hydrogen gas bubbles through the organic cadmium source 37 and picks up molecules of the organic cadmium source 37. Therefore, a mixture oE
the organic cadmium source and hydrogen (Cd-organic ~H2) emerges from bubbler 36, via line 29c, and is routed by solenoid control valve 30 to line 31a. A second mass flow controller 26b is activated to provide a predetermined flow of a carrier gas, here hydrogen, through a valve 36, via line 31b, to junction member 32. Therefore, emerging from line 31c is a diluted vapor flow of the Cd organic with respect to the carrier gas, here hydrogen.
Tube 47c, the "Group VI-organic tube," is fed from a junction member 48. Junction member 48 is used to mix flows from two gas sources and delivers said mixed gas flow to a third port coupled to tube 47c. The first port of junction member 48 is fed from the bubbler apparatus 55. Bubbler apparatus 55 includes a pair of solenoid control valves 44, 46.
first one of said solenoid control valves, here solenoid control valve 44, has a irst port coupled to a third mass flow controller 26c, via tube 27c, and has a second port coupled to a bubbler 52, via tube 45a. Bubbler 52 here has disposed therein a Group VI organic 53 as will be described hereinafter, Suffice it to say here that the Group VI organic, ~ay be a primary alkyl of the Group VI element or alternatively is selected to have an activation energy for the formation of 1319~8 a free radical during dissociation o the the Group VI-organ;c that is lower than the activation energy during disassociation of a primary alkyl of the Group VI element. The bubbler 52 is provided in a recirculating temperature control bath 56 which provides a constant flow of a liquid around the bubbler 52 to maintain the tellurium organic 53 in bubbler 52 at a predetermined temperature sufficient to provide ade~uate vapor pressureO This range may extend to but is not necessar-ily limited to the range of -20C to +100C. A second tube 45c is disposed into bubbler 52, above the surface of the Group VI organic, and is coupled to a port of solenoid control valve 46. A third tube 45b is coupled between remaining ports of solenoid control valves 44 and 46.
The normally deactivated state of solenoid control valves 44 and 46 enables hydro~en gas to pass from the hydrogen source, here the mass flow controller 26c, via tube 27c, to tube 45b, and on throu~h tube 47c to purge the reactor ve~sel of atmos-pheric gasses, as described above. Duxing epitaxial growth of cadmium telluride or mercury cadmium telluride over substrate 11, valves 44 and 46 are placed in their activated state, enabling hydrogen gas to pass through tube 45a into bubbl~r 52 which contains the Group VI organic 53. The hydrogen gas bubbles through the Group VI organic 53 and picks up molecules of the Group VI organic 53. Therefore, a mixture of the Group VI organic and hydrogen (Group VI-organic ~ H2) emerges ~- ~ 319~8~1 ~

from the Group VI organic 53, via line 45c, and is routed by solenoid control valve 46 to line 47a. A fourth mass flow controller 26d is activated to provide a predetermined flow of a carrier gas, here hydrogen, through a YalVe 50 and via S line 47b to junction member 48. Therefore, emerging from , line 47c is a diluted vapor flow of the Group VI organic with respect to the concentration of the carrier gas, here hydrogen.
Tube 27e is fed from a fifth mass flow controller 26e to a quartz reservoir 66 (FIG. 3B) containing a liquid source of a Group II element such as mercury. Hydrogen gas is directed over the surface of the liquid mercury, and vapor ; molecules of mercury over the liquid mercury surface are picked up by thç hydrogen gas flow, providing a vapor flow of ~ mercury and hydrogen (Hg+ H2). The vapor flow is fed to a quartz junction element 70 (FIG. 3B). A second input port of quartz junction element 70 is fed via a quartz tube 71a which is coupled to a sixth mass flow controller 26f, via a valve 72 and tube 27f. Emerging from junction element 70 via tube 71b and into tube 60 is, therefore, a diluted flow of mercury vapor and hydrogen.
Referring particularly now to FIG. 3B~ as previously mentioned, the susceptor 63 is heated by an r.fO coil disposed -~ around the quartz reaction tube 60.
A quartz reservoir 66 containing the liquid elemental mercury and the region adjacent thereof is resistively heated !
- 23 ;

1 3 ~ 7 by a resistance heat source 68, as shown, to a temperature of at least 100C, but generally less than 250C preferable within the range of 150C to 180C. The zone immediately after the reservoir 66 and past the substrate 11 is then heated by banks of infrared lamps 64 to a temperature in the range of lOO~C to 250C with 150C to 180C being the preferred range~
Heating of the walls prevents premature condensation of ~ mercury from the vapor stream.
; The outwardly exposed surface of the substrate 11 is degreased and cleaned using appropriate solvents and then polished us;ng an appropriate material which will etch the material of the substrate. For example, a bromine methanol solution is used to chemically polish CdTe or GaAs before growth of the various epitaxial layers. The substrate 11 is lS then placed on the susceptor 63 which is then disposed in the quartz reaction tube 600 In operation, furnace tube 60 is purged of atmospheric gasses by introduction of helium and then hydrogen gas as described abovc. The susceptor 63 is then inductively heated by the r.f. coil 62, the reservoir 66 by the resistive heating element 68, and reaction tube 60 by the infrared lamps 64.
Each is then allowed to reach the growth temperaturesO When the apparatus 20b has reached the growth temperatures, valves 28, 30, 34, 44, 46, 50, and 72 are activated enabling diluted mix~ures of hydrogen gas + Group II organic, hydrogen gas +

3~ 9~87 the Group VI organic, and hydrogen gas + mercury to emerge from tubes 31c and 47c and 71b, respectively, upstream from the substrate 11.
The hydrogen r mercury, and organic vapors are at the desired operating temperature provided by the uniform heating of the substrate 11 and the region 61 around the substrate 11. It is believed that the directed, selected organic source will pyrolyse substantially independent of one another and produce mercury cadmium telluride in accordance with chemical Reactions lA-5A below:

Te organic -~ Te + ~.C. (Reaction lA) Te ~ ~g ~ HgTe ~Rèaction 2A) Cd organic -~ Cd ~ H.C. (Reaction 3A) Cd + Te ~ CdTe ~Reaction 4A) HgTe ~ CdTe ~ Hgl_xCdxTe ~Reaction 5~-) where H.C. stands for hydrocarbons The composition x is controlled by regulating the flow of H2 into the Hg reservoir, the temperatu~e of the Hg reservoir and the concentration of cadmium organic and the tellurium organic.
The mole fraction (i.e., concentration of Cd-organic, Te organic and Hg) is given by:

MF(Cd organic)= H2 t~.ru bubbler 36 x Cd or~anic Va~or Pressure~Torr) Total H2 Flow in Tube 60 760 (Torr) MF(Te organic)- ~ thru bubbler 52 x Total H2 Flow in Tube 60 Te orqanic Va~or Pressure_(Torr) ~ ~3~8~ f MF(Hg) = H2 oveE reservoir 66 x Hg Vapor Pressure (Torr) Total H2 Flow in Tube 60 760 ~Torr) Only a portion of the organic vapors which are directed over the substrate 11 is actually reacted. Unreacted organic vapors are exhausted from the reactor tube 60, via the exhaust line 74, and are directed towards an exhaust cracking furnace ~not shown) which cracks the remaining organic gasses into the elements and provide a gas stream which comprises substan-tially hydrogen and various hydrocarbons.
In accordance with one aspect of the invention, the cadmium source is an organic source having organic groups which are selected to sterically hinder the cadmium atom from reacting with a tellurium atom provided in the organic tellurium sourc~. Preferably, the selected organic group is not bonded directly to the cadmium atom since bonding of the organic group directly to the cadmium atom will increase the reactivity of the cadmium organic.
The selected Cd organic has a gen~ral chemical structure as:
Rl - Cd -R2 where Rl, R2 may or may not be the same, and at least one of Rl, R2 has the general chemical structure as set forth below:

~ ~3~9~

Y --C--l2 where Xl~ X~ may or may not be the same and preferably are ~elected from the group of hydrogen, a halogen, or an organic~ Y has the general chemical structure as set forth below:
Yl where Yl, Y2, Y3 may or may not be the same and are preferably hydrogen, a halogen, or an organicO
As shown below, the Cd organic has an organic group which incorporates the carbon atom at the Q position of the chain of the Cd organic.
Yl Y2 C - C - -Cd - C - C - Y2 With this particular arrangement, the large bulky groups at the ends of the chain will sterically hinder the cadmium atom from reacting with the tellurium atom in the tellurium organicO
One preferred example of a Group II organo having a large bulky group at the ~ position carbon in the organic groupr thereof, is the chemical di-neopentylcadmium (~CH3)3CCH2)2Cd.

31~87 Di-neopentylcadmium has a general chemical structure as set forth below:

cl3 H ~ C~3 C~3 - ~ _ C --Cd - C ~ - CH3 The molecule contains two tertiary butyl groups which are separated from the cadmium atom by a ~ position carbon atom, here a CH2 group. Since the tertiary ~utyl groups are not bonded directly to the cadmium atom, they do not signiicantly destabilize the di-neopentylcadmium molecule. Accordin~ly, di-neopentylcadmium ~DNPCd) should have a thermal stability comparable to diethylcadmium (DECd). D~PCd has several advantages. DNPCd is ~elieved to reduce reactions between itself and the selected tellurium organic due to steric repul sion provided by the tertiary butyl groups in the DNPCd molecule. The presence of these tertiary butyl group~ makes it difficult for the two molecules and, in particular, for the two atoms of the two molecules to come within a close enough distance to react. Furthermore, by selecting an appropriate tellurium source, low temperature ~rowth of mercury cadmium telluride will be provided. Accordingly, Reaction 7, the exchange reaction between the Group II
organic and mercury should be kinetically limited and, therefore, not be a major c~use of cadmium depletion~ The ~319a~7 neopentyl groups ((~H3)3CCH~) due to their weight and size should also reduce the rate of free radical chain reactions and, therefore, provide a molecule which is substantially less reactive in the vapor stream than dimethylcadmiumO
A second preferred example is the chemical diisobutyl-cadmium ((CH3)2CHCH2)2Cd which has an isopropyl group separated from the Cd atom by a c~ position carbon atom, here a CH2 group. Di-isobutylcadmium has a general chemical structure as set forth below:

CH3 H H ~ CH3 CH - C - Cd - C - -CH
t I ~
CH3 . H H CH3 Other examples include di-N propylcadmium and diethyl-cadmium, each has the respective general chemical structure set forth below:
CH3 -CH2- CH2- Cd - CH2~ ~H2- CH3 - di N-propylcadmium CH3- CH~- Cd- CH2 - CH3 - diethylcadmium Alternatively, the cadmium organic may have organic groups bonded directly to the cadmium atom which transfers electron charge to the electroposi~ive cadmium atom. By reducing the electropositivity of the cadmium atom with the electron releasing organic ~roup, the cadmium organic will be less reactive towards the organic tellurium molecule than prior known dimethylcadmium An example of such a compound ~ 2~ -` ~3~87 is diphenylcadmium having the general chemical formula set forth below:
H H H H

H--~-- Cd --~--H

H H H H
As shown, diphenylcadmium contains two phenyl groups which are sources of electrons because of their ~ level electron clouds. The central cadmium atom is electropositive.
Consequently, the electropositivity of the cadmium atom should be reduced by the presence of the phenyl groupsO This will concomitantly reduce the attractive force between the cadmium organic and tellurium organic. The negative charge nature of the phenyl groups should further reduce the inter-action between a selected tellurium organic and diphenylcadmium~
since the phenyl groups should repel the electronegative tellurium atom. Another feature of using an aromatic cadmium compound such as diphenylcadmium is that typically aromatic cadmium compounds are relatively s~able. Accordingly, it can - be stored for long periods of time without decomposition.
Furthermore, the phenyl groups themselves are also stable entities, and it is believed that the rings will not be broken during pyrolysis. Accordingly, it is also believed that MOCVD growth using diphenylcadmium should result in little carbon incorporation into the mercury cadmium telluride - ~ 3 ~

films. Although diphenylcadmium has a relatively low vapor pressure and is a solid having a malting point of 174C, it is nevertheless believed that such a source may ba used.
- A heated reservoir arrangement such as shown in FIG. 4 may be used to provide a suitable vapor flow of diphenylcadmium, in a similar manner as reservoir 66 pro~ides the Hg vapor flow. That is the line 27a may be directed to a reservoir 87 containing the Cd organic 86~ Hydrogen gas is passed over the reservoir and picks up molecules of the Cd organic 86 and directs this vapor stream into the reactor vessel via tube 31c after predetermined dilution with H2 as described above. The reservoir is disposed within a heated furnace at a predetermined temperature, as shown. The furnace may be a multizone furnace to heat the Cd organic reservoir and Hg reservoir to selected temperatures.
Alternative oadmium sources having electron releasing phenyl groups rings include di-orthotolylcadmium having the gener21 chemical structure set forth below:
H~ C,H3 H~ H

H- ~ - Cd - ~ -H

Di~orthotolylca~mium is similar to diphenylcadmium except that one ortho position hydrogen on each benzene ring is 5 replaced by a methyl groupJ The methyl groups also increase ~ ~19~7 the trans~er of electron charge from the phenyl groups to the cadmium atoms, consequently, reducing the positive charge of the Cd atom. Although this molecule is heavier than diphenylcadmium, it is believed nevertheless, di-orthotolyl-cadmium (DOTCd) will have a higher vapor pressure, because by attaching the methyl group at one of the ortho positions, the planar symmetry of the molecule is altered by the ou~-of-plane methyl hydrogens. These hydrogens atoms partially shield the central cadmium atom, and as a consequence reduce the inter-molecular attraction between a cadmium ato~ of one molecule with a benzene ring of another molecule. Di-orthotolylcadmium has a melting point of 115C which may indicae that DOTCd will have a higher vapor pressure than DPCd. Furthermore, it is believed that the partial shielding of the cadmium atom by the out of-plane hydrogen atoms will result in reduced attraction between the cadmium atom and a tellurium atom in the tellurium organic.
Further alternate examples of cadmium compounds having increased electron transfer and increased steric hinderance are di-(2,6 xylyl~ cadmium (DXCd) and di-mesitylcadmium (DMSCd). These molecules have the gen0ral chemical structure as set forth below:

~5 ~ 3~8~

H--~-- Cd ~--H
DXCd . H c~3 ~H3 H

DPlSCd~ \ ~
CH3--~-- Cd ~ CH3 H ~H3 ~ CH3 These compounds are the same as di-orthotolylcadmium except that DXCd has methyl groups at both ortho positions on each benzene ring and DMSCd has methyl groups at both ortho posi-tions and the para position of each benzene ring. With two ortho groups attached to each benzene ring, the electron charge transferred to the cadmium atom is 4urther increased.
Further, the ortho mPthyl groups attached to the benzene ring should sterically repulse each other resulting in a nonplanar molecule~ A further important feature of this structure is that because of the steric hinderance, the cadmium atom will be enclosed by a cage of ~our methyl groups. This increased steric hinderance should concominantly increase the vapor pressure of DXCd. Furthermore, the cage o~ methyl groups around the cadmium atom should reduce the attraction between the tellurium organic and DXCd or DMSCd and should prevent or ~ 33 -~ 8 ~

substantially limit the exchange reaction between cadmium and mercury.
Accordingly, the selected Cd organic having electron releasing groups has the general chemical structure as set forth below:
R3 Cd R4 where R3, R4 may or may not be the same and at least one of Rl, R2 has the general chemical structure as set ~orth below:

3 ~

, 15 where hydrogen (H~ is generally, but not necessarily, provided at the me~a positions and where Yl, Y2 are at the ortho positions and Y3 i5 at the para position and are each selected from the group of hydrogen and an organic group.
Preferably, the organics are activating groups such as phenyls and alkyls (C6Hs, CH3, C~s etc~) or heteroatoms such as the alkoxides, -OCH3, -OC2Hs etc; -NHCOCH3; -OH; and h~te r~at~f~
~ NH2 (N~R, NR) where R is a radical. The he~o~ may be ~ ,, .
used where the potential for O or N incorporation into the 2S deposited films is not a problem~

~.31 9~87 Preferably, in order to increase steric h~nderanc~
between the sterically hindered cadmium organic and the tellurium organic, the tellurium organic i5 selected to include large bulky organic groups which will likewise ste~i-cally hlnder the tellurlum organic molecule from reacting with the c~dmium organic. A tellurium source naving a relatively low activation energy for ~orma-tion of a free radical during pyrolysis whe~ compared to the activa-tion energy for diethyl-telluride is th~ tertia~y alkyl ditertiarybutyltelluride. Diter-tiarybutyltelluride has a general cnemical structure given below: .

cl3 Cl3 CH3 ~ C Te - C ~ CH3 CH3 ~H3 Ditertiarybutyltelluride lncludes two tertiary butyl groups bonded directly to the tellurium atom. Accordingly, the presence of the tertiary butyl groups ln th~ tellurium organo ditertiarybutyltelluride, destabilize the tellurium organic and st~rieally hinders tha tellurium atom. Selection of ditertiarybutyltellurld~ as the tellurium organie ~ource And selection o one of the ~terically h~ndered cadmium ~ource mention~d abov~ will provid0 lncreas~d ~teric hlnderanc~ and, 13~9~87 there~ore, increase reactive ~r~a transport o~ tha cadmlum and tellurium or~anic~ thru the reactor vess21.
Other sources o t~llurium (Group VI alement) ~nclud~
dllsopropyltellur1de tDIpTe) having the general chemical structure given below-CH3 ~ / CH3 CH -- T~ -- CH
CH3 . CH3 Di~.~opropyltellurld~ has a lo~er stability andr hence, enhanced cracking efficiency when compared to the cracking efEiciency of DETe. DIPTe is a preferred example o~ a secondary tellurium alkyl. The bulky isopropyl als.o sterically hinder the tellurium atom from reacting with the Cd organic.

Greater delocatization and consequently ].owor activation ene.rgies are provided using the overlap of the p orbital of the unpaired electron with double bonds instead of single bonds. The ... ~.

1319~7 ` . 62901-710 allyl radical, the benzyl radical, and cycloallyl radical each delocal~ze the fr~e ~lectron chargQ over the entire carbon chain. Pr~ferr~d examples o~ allyls, benzyl-~, and cycloallys of the Group VI element are shown ln the Table.
Therefore, tellurium organic sources such as diter-tlarybutyltelluride or the a~orementioned secondary alkyls, allyls, cycloallyls, or benzyls, each provide lower activation energy for formation of a free radical and, c~nsequently, reduced growth temperatures. ~y selecting the tellurium organic ln ~unction w~th the ~elactlon of the cadmium organlc compound, growth of Group II-VI matsrials such a~ mercury cadmium telluride will occur at lowar ~rowth t~mpQrature~, the ~elected organic~ will st2rlcally hinder each other, and the exchange react~on b~tween tha Group II or~anic 80urc3 and mercury will be k~netically limited.

Growth of Gr~up II-VI ~em~conductor materlals usln~ dltertiary~utyltellurld~
as the tellur~um organic can occur at temperatures as low a~
about 230C. It is believed that at thi3 temperature~ the exchange reactlon (Reaction 7) between mercury and the cadmium organic, a~ previously mentlone.d, will be substantially kinetically limited. Thu~, the exchange re~ction will not be a significant source of cadmium depletion a~ in prior tQchn~que~. Accordlngly, wlth the abova described arran~ement, ; 37 3 t'~ 8 a .~
h ~ ~ a hal I I Cl~
O
0 ~i_' I I h o -V
0 r~
E~ a~ I o ~ ~ a) ~ ~a ~1 Z --I ~ O -- V
I ~ l ~ I
N rl ~ _I V --I
.c ~ s Q JJ ~ ~ ~
~ E~ ~ E3 ~a ~D i ~: ~ C~
E~ O
U~ U
~ o ~o) 3:~
c ~ UE~ U ~) h t~ U~_) V ~ ~
. ~ ~D ~ ~ I I
V
U C~

Q) ~ _ c.) c~
_~

_, E u~ u v U ~ u O
~D ~ 3 U ~ U ~r U ~
-- U ~ U

8 ~ ~ -substantially reactive free transport of the cadmium organic and the tellurium organic will be provided, and the cadmium organic and the tellurium organic will undergo substantially independent pyrolsis over the elevated temperature region of the substrate, thereby, providing`mercury cadmium telluride films having improved front to back compositional uniformity.
It is also believed that the side to side compositional uniformity of the deposited mercury cadmium telluride films will also be improved.
To further reduce the attraction between the electrv-positive Cd atom (Group II atom) and electronegative Te atom ~Group VI atom), electron withdrawal from the Te (Group VI) atom may be accomplished by selecting the Te (Group VI) organic source to have electron withdrawal groups, as shown below for Te:

X~ T~ Xl where Xl and X2 are ~enerally hydrogen and Y is a meta position deactivating group selected from the group -NO2;
-N(CH3)3+; -CN; -COOH ~-COOR); SO3H -CHO (-CRO)where R is an ~ \ .

-~ ~ 3~9~87 organic group, keeping in mind the potential for N, o, S
etc. incorporation into the Group II-VI layers.
Alternatively, the para, ortho position hydrogens ``~ halacien .~ (Xl, X2 positions) may be replaced by a ~a~n ~-F, -Cl, -Br, I) and the meta positions groups are hydrogen.
Having described preferred embodiments of the invention, it will now be apparent to one of skill in the art that other embodiments incorporating their conce.pts may be used. It is felt, thereforet that these embodiments should not be limited to the disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.

Claims (36)

1. A method of providing a layer comprising a Group II-VI semiconductor material over a substrate, comprises the step of:
directing a flow comprising a Group II metalorganic group, and a Group VI organic comprising a Group VI moiety attached to at least one organic group towards the substrate wherein the said organic group attached to the Group VI
moiety is a large and bulky organic group, characterized in that the said organic group attached to the Group II moiety is a sufficiently large and bulky organic group to sterically repulse the large bulky organic group of the Group VI organic, such that the Group II metalorganic and the Group VI organic pyrolyse substantially independently.

2. A method according to claim 1, wherein the said large bulky organic groups are branched organic groups.

3. A method according to claim 1, wherein the at least one large and bulky organic group attached to the Group II moiety includes a secondary or tertiary alkyl group bonded to a beta position carbon atom in the said one group or is a phenyl group bonded directly to the Group II moiety and the said at least one large and bulky organic group of the Group VI organic is bonded directed to the Group VI

moiety and is a secondary alkyl, a tertiary alkyl, an allyl, a benzyl, or a cycloallyl group.

4. A method of providing a layer comprising a Group II-VI material which comprises the steps of:
directing a flow comprising a Group VI organic incorporating a Group VI moiety towards a substrate;
directing a flow comprising a Group II organic towards the substrate, with said Group II organic comprising a Group II moiety with sufficiently large and bulky organic groups attached thereto to sterically repulse the Group VI organic, such that the Group II organic and the Group VI organic pyrolyse substantially independently; and reacting the Group VI and Group II moieties thereby released from the said organics to form the Group II-VI
material over the substrate.

5. A method according to claim 4, wherein the Group II organic has an organic group which incorporates a beta position carbon atom.

6. A method according to claim 5, wherein the said beta position carbon atom has an isopropyl or a tertiary butyl group bonded thereto.

7. A method according to claim 6, wherein the Group VI organic has at least one organic group consisting of a secondary alkyl, a tertiary alkyl, an allyl, a benzyl, or a cycloallyl group.

8. A method according to claim 7, wherein the said at least one organic group of the group VI organic is bonded directly to the Group VI moiety.

9, A method according to claim 4, wherein the Group II organic has the general chemical structure as set forth below:
where A is the Group II moiety, and where R1, R2 may or may not be the same and where at least one of the R1, R2 has the general chemical structure as set forth below:

where X1, X2 may or may not be the same and are selected from hydrogen, a halogen, and an organic; and where Y has the general chemical structure set forth below:

where Y1, Y2, and Y1 may or may not be the same, at least two are organic groups and the remaining one is selected from hydrogen, a halogen, and an organic group.

10. A method according to claim 9, wherein the Group II organic is dineopentylcadmium.

11. The method as recited in claim 9, wherein the organic has at least one organic group selected from a secondary alkyl, a tertiary alkyl, an allyl, a benzyl, a cycloallyl group.

12. A method according to claim 11, where the said selected organic group is bonded directly to the Group VI
moiety in the Group VI organic.

13. A method according to claim 4, further comprising the step of:
directing a flow of an elemental source of a Group II
element towards the substrate; and wherein the reacting step includes reacting said Group II element with the Group II

moiety of the Group II organic and Group VI moiety of the Group VI organic to provide the layer.

14. A method according to claim 13, wherein the reaction step to provide the Group II-VI material occurs at a temperature at which an exchange reaction involving the Group II element and Group II organic is substantially kinetically limited.

15. A method according to claim 14, wherein the reaction step occurs at a temperature of less than 320°C,

16. A method according to claim 15, wherein the Group II organic has at least one organic group which includes a beta position carbon atom in said organic group and the Group VI organic has at least one organic group selected from a secondary alkyl, a tertiary alkyl, an allyl, a cycloallyl, and a benzyl group.

17. A method according to claim 14, wherein the reaction step occurs at a temperature of less than 280°C.

18. A method according to Claim 17, wherein the Group II organic has at least one organic group which includes a beta position carbon atom in said organic group; and the Group VI organic has at least one organic group selected from a tertiary alkyl, an allyl, a cycloallyl, and a benzyl group.

19. A method of providing a layer comprising mercury cadmium telluride over a substrate, comprises the steps of:
directing a flow of a source of mercury towards the substrate;
directing a flow of a source of tellurium towards the substrate; and directing a flow comprising a cadmium organic towards the substrate, with said organic having the general chemical formula:
R1_Cd_R1 where R1, R2 may or may not be the same at least one of R1, R2 has the general chemical formula:

wherein X1 and X2 may or may not be the same and are selected from hydrogen, a halogen and an organic and Y1, Y2 and Y3 may or may not be the same and are selected from hydrogen, halogen and an organic.

20. A method according to claim 19, wherein the source of mercury in an elemental source of mercury and the source of tellurium is an organic source of tellurium having at least one organic group selected from a primary alkyl, a secondary alkyl, a tertiary alkyl, an allyl, a benzyl, and a cyaloallyl group bonded directed to the tellurium atom.

21. A method according to claim 20, wherein the tellurium metalorganic is selected from diethyltelluride, di-isopropyltelluride, ditertiarybutyltelluride, dibenzyltelluride, di-(2-propen-1-yl) telluride, di-(2-cyclopropen-1-yl) telluride, X-ethyltelluride, X-isopropyltelluride, X-tertiarybutyltelluride, X-benzyltelluride, X-(2-propen-1-yl) telluride, and X-(2-cyclopropen-1-yl) telluride;
where X is selected from a halogen, hydrogen, and an organic group.

22. A method for growing a layer comprising Group II-Vi material over a substrate which comprises the steps of:
directing a first flow comprising a Group VI organic towards the substrate;
directing a Group II organic towards the substrate, with said Group II organic having a Group II moiety with organic groups bonded directly to the Group II moiety which provide electron transfer to the electropositive Group II
moiety, such that the Group II organic and the Group VI
organic pyrolyse substantially independently.

23. A method according to claim 22, wherein at least one of the said organic groups is a phenyl group.

24. A method according to claim 23, wherein the electron releasing phenyl group further comprises at least one group substituted for a hydrogen atom at a first one of the para and one of the pair of ortho positions of the phenyl group.

25. A method according to claim 24, wherein the substituting group is an electrophilic activating group.

26. A method according to claim 25, wherein the substituting group is an alkoxide, or NH2, or -NHR, or -NRR, or -OH, or -NHCOCH3, where R is a radical.

27. A method according to claim 25, wherein the substituting group is a phenyl or an alkyl group.

28. A method according to claim 24, wherein the group substituted for the hydrogen at the first one of the para and one of the ortho positions of the phenyl group is an organic group and sterically repulses the Group VI organic as the Group VI organic and the Group II organic are directed toward the substrate.

29. A method according to claim 28, wherein the substituted organic group bonded to one of the ortho positions of the phenyl group is a methyl group.

30. A method according to claim 20, wherein the phenyl group has each ortho position hydrogen atoms substituted with a methyl group.

31. A method according to claim 28, wherein the para position hydrogen atom and both ortho position hydrogen atoms are substituted by methyl groups.

32. A method of growing a layer comprising mercury cadmium telluride over a substrate comprises the steps of:
directing a first flow comprising a tellurium organic towards the substrate;
directing a flow comprising a source of mercury towards the substrate; and directing a flow comprising a cadmium organic towards the substrate, wherein the cadmium organic is diphenylcadmium, or di-orthotolylcadmium, or di-(2,6 xylyl) cadmium, or di-mesitylcadmium.

33. A method according to claim 32, wherein the tellurium organic has at least one organic group which is a primary alkyl, or a secondary alkyl, or a tertiary alkyl, or an allyl, or a benzyl, or a cycloallyl group bonded to the tellurium moiety.

34. A method of growing a Group II-VI layer comprising the steps of:
directing over a substrate n flow comprising a Group II
organic;
directing over the substrate a flow comprising a Group VI organic having at least one sufficiently large and bulky organic group attached to a Group VI moiety to sterically repulse the Group II organic, the said organic group including at least one phenyl group so substituted as to withdraw electrons from the Group VI moiety, such that the Group II organic and the Group VI organic pyrolyse substantially independently over the substrate.

35. A method according to claim 34, wherein the organic group is a phenyl group bonded directly to the Group VI moiety having deactivating meta directors chosen from -NO2; -N(CH3)3+; -CN; -COOH; -SO3H; -CHO; -COOR; and -COR where R is chosen from any suitable organic group, or any suitable element.

36. A method according to claim 35, wherein the phenyl group bonded directly to the Group VI moiety has deactivating para and ortho directors substituting for hydrogen at the para and ortho positions and consisting of halogens.

Case No. 34429