FR2608637A1 - PROCESS FOR THE GROWTH OF SEMICONDUCTOR MATERIALS BASED ON GROUP II AND VI ELEMENTS BY CHEMICAL DEPOSITION OF ORGANOMETALLIC COMPOUNDS IN VAPOR PHASE, WITH IMPROVED COMPOSITION UNIFORMITY - Google Patents

Abstract

A PROCESS FOR THE GROWTH OF AN EPITAXIAL LAYER OF ELEMENTS OF GROUPS II AND VI ON A SUBSTRATE IS DESCRIBED. THE PROCESS INCLUDES THE STEP OF SENDING MULTIPLE STEAM STREAMS ONTO THE SUBSTRATE, COMPRISING A STEAM OF AN ORGANIC COMPOUND OF A GROUP II ELEMENT, A STEAM OF AN ORGANIC COMPOUND OF A GROUP VI ELEMENT, AND AN ELEMENTARY MERCURY VAPOR OF GROUP II. AT LEAST ONE VAPOR OF THE ORGANIC COMPOUND OF GROUP II ELEMENT AND THE VAPOR OF ORGANIC COMPOUND OF GROUP VI ELEMENT CONTAINS ORGANIC RADICALS THAT STERICALLY REPRESENT THE SECOND VAPOR OF ORGANIC COMPOUNDS FROM GROUP II ELEMENTS AND GROUP VI, OR WHO ARE RESPONSIBLE FOR A TRANSFER OF ELECTRONS TO THE GROUP II ELEMENT OR WITHDRAWAL OF ELECTRONS FROM THE GROUP VI ELEMENT. THANKS TO THE PARTICULAR PROVISIONS DESCRIBED, IT IS EXPECTED TO OBTAIN A PRACTICALLY INDEPENDENT PYROLYSIS OF THE VAPOR OF THE ORGANIC COMPOUND OF THE ELEMENT OF THE GROUP II ABOVE THE REGION OF GROWTH OF THE SUBSTRATE, AND THUS CONSIDERABLY REDUCE THE EXHAUSTS OF ELEMENTS SUCH AS THE EXHAUST OF CADMIUM, IN EPITAXIAL FILMS DEPOSITED ON THE SUBSTRATE.

Description

- i -

  The present invention relates generally to epitaxial growth techniques, and more particularly to the growth of crystalline semiconductor materials based on

elements from groups II and VI.

  As is known in the art, semiconductor epitaxial materials based on elements of groups II and VI, such as cadmium telluride and mercury and cadmium telluride, have important applications as photodetector elements for detection of electromagnetic energy in the spectral range ranging from approximately 0.8 pm to 30 gm. By adjusting an alloy composition of

  cadmium and mercury, photodetect elements are obtained

  teurs which are sensitive to different wavelength ranges in the wavelength band from 0.8 pm to pim. Several different techniques have been proposed for

  obtaining cadmium telluride and sea telluride-

  cure and cadmium, suitable for use in photodetector applications. A proposed method is the epitaxy of organometallic compounds in the vapor phase

  (EOMPV), also known as chemical deposition of organic compounds

  - 2 nometallic vapor phase (DCOMV). As is known, the DCOMV technique for the growth of mercury and cadmium telluride comprises sending mercury, dimethylcadmium and diethyl telluride vapors to a reactor, and chemically reacting the vapors sent, for

  obtaining the epitaxial material.

  Various problems are encountered in the technique of growing epitaxial layers of mercury telluride and

  of cadmium by the DCOMV process. A special problem-

  important is the uniform composition of the layers

  epitaxial deposits, obtained by the DCOMV technique.

  In general, the composition of these layers varies, from the

  part of the substrate which is downstream to the part of the sub-

  strat which is upstream. Variation in composition

  generally implies an exhaustion of the progressively increasing Cd

  towards the downstream part, or the rear part,

  of the substrate, while the upstream part, or front part

  tale, from the substrate, is generally excessively rich in Cd.

  In general, there are also deviations from the lateral uniformity, and from side to side, in the content

in Cd.

  The prevailing view of mechanics

  my chemical reactions that occur in materials

  rials based on elements of groups II and VI, with growth effected by DCOMV, is given in an article entitled Organometallic Growth of II-VI Compounds "(by Organometallic growth of compounds of elements of groups II and VI), by JB Mullin et al., Journal of Crystal Growth, vol. 55, 1981, pp. 92-106. In this article, the authors suggest that the tellurium and cadmium alkyl compounds sent may not undergo pyrolysis independently l These authors rather suggest that adducts or complexes of these compounds are produced, because dimethylcadmium (DMCd) and diethyl telluride (DETe) are attracted to each other in the vapor phase, -3

  by forming a weak bond. According to these authors, the breakdown

  sition of these adducts leads to the formation of materials to

  based on elements of groups II and IV and other products.

  The theoretical growth, for example, of HgCdTe by DCOMV, can be seen as an irreversible pyrolysis, in which the primary (1st order) alkyl compound of tellurium and the alkyl, order 0, cadmium compound, decompose

  independently in elementary tellurium and elementary cadmium.

  Elemental tellurium reacts with elemental cadmium and with elemental mercury brought into the vapor duct,

  to form mercury telluride and cadmium telluride

  mium. The mercury telluride and the cadmium telluride are then deposited on the substrate, to form mercury and cadmium telluride, as indicated in reactions 1-5 below: DETe> Te + HC (reaction 1) Te + Hg - HgTe (reaction 2) DMCd - Cd + HC (reaction 3) Cd + Te - CdTe (reaction 4) HgTe + CdTe HglxCdxTe (reaction 5),

  in which H.C. means "hydrocarbons".

  Although there is evidence for the growth of mercury telluride by independent pyrolysis of the primary alkyl compound of tellurium and by reaction between mercury

  and elemental tellurium (reactions 1 and 2), we now think

  holding that the growth of cadmium telluride takes place by a different process which occurs at high temperatures. The simplified overall reaction is shown in the

reaction 6.

  DMCd + DeTe - CdTe + H.C. (reaction 6).

  In this reaction, the electropositive cadmium atom pre-

  feels in dimethylcadmium (DMCd) is attracted to the atom

  of electronegative tellurium present in the telluride of di-

  ethyl. At low temperatures, this attraction is not

  large enough to form a bond giving rise to

- 4 -

  sance to a stable adduct. However, at high temperatures

  prevailing over the growth region in the reaction

  this attraction leads to a chemical reaction in

  which the alkyl compounds of cadmium and tellurium react

  lie to form cadmium telluride and other hydrocarbons. This chemical reaction leads to a rapid depletion of cadmium in the downstream parts of the HgCdTe layers formed on the substrate. Consequently, the non-pyrolytic behavior of the alkyl cadmium compound is retained.

  as a root cause of the non-uniformity of the composition

  sition in HgCdTe films formed by chemical deposit

  mique of organometallic compounds in vapor phase. In general

  ral, we therefore consider the non-pyroly-

  tick of the alkyl compound of the group II element, caused by the attraction between the electropositive atom of the group II element and the electronegative atom of the group VI element, as the primary cause of non-uniformity of the composition in the growth by DCOMV of materials based

elements from groups II and VI.

  A second cause of non-uniformity of the composition in materials based on elements of groups II and VI, in

  especially in materials such as HgCdTe which com-

  take two elements from group II, is considered to involve a reversible exchange reaction, in the vapor phase, between the primary alkyl compound of the element of group II and the atom of the element of group II. Especially in

  HgCdTe, the primary alkyl compound of Cd and the mercury elec-

  are involved in the following reaction:

  DMCd + Hg - DMHg + Cd (reaction 7).

  In the DCOMV technique, normally the reactor walls and the mercury source are heated to a temperature of around 220 C, to avoid condensation of the mercury outside

  vapor current. At these wall temperatures, the reaction

  tion 7 has an equilibrium constant in which the reaction is mainly directed to the right, that is to say in - 5

  a direction in which dimethylcadmium decomposes.

  However, at these temperatures, the reaction rate is relatively slow, and therefore the reaction is not a major cause of Cd depletion. However, when the vapor stream reaches the substrate, there is a

  sudden rise in temperature, which causes a change

  cation of the composition of the phases of the vapor stream, since reaction 7 is then very strongly directed to the right. Consequently, there is a large variation in the concentration of Cd, in which elementary cadmium is produced by the reaction. Elementary CD is even more

  attracted to diethyl telluride than is DMCd.

  As indicated in reaction 8, it reacts with telluride

  of diethyl, to form cadmium telluride on the par-

  ties of the susceptor (i.e. the support on which the substrate rests) which are located upstream. That means that

  CdTe can settle out of the vapor stream before reaching

  stain the substrate. This state of affairs again leads to an exhaustion of the cadmium in the parts of the layers, formed

  on the substrate, which are downstream.

  Cd + DETe - CdTe + H.C. (reaction 8)

  Consequently, we see in the non pyro-

  lytic of the alkyl cadmium compound, a major cause of nonuniformity of the composition in materials based on elements of groups II and VI, such as HgCdTe. It is believed

  that this non-uniformity of composition is also pre-

  sense with other materials based on elements of groups II and VI. For example, mercury and zinc telluride has been proposed as an alternative material to be used instead of mercury and cadmium telluride. The zinc atom is a

  atom clearly more electropositive than cadmium. Therefore

  As a result, alkyl zinc compounds should be significantly more reactive towards electronegative bodies than are

  alkyl cadmium compounds. Following this attraction-

  tion, the root cause of the non-uniformity of the composition

  -6- tion, namely the chemical reaction between the alkyl compound of an element of group II and the alkyl compound of an element of group VI, can be even more marked for a material such as telluride of mercury and zinc . It is believed that this mechanism is also present in mercury telluride

  and manganese, a second potential alternative material

  tiel to use instead of mercury telluride and cad-

  mium. Manganese is also significantly more electroposi-

  tif than cadmium, and therefore an alkyl manganese compound would be much more reactive towards the alkyl compound of an element of group VI than is the alkyl compound of cadmium. With mercury and zinc telluride, since zinc is much more electropositive than Cd, this results in a negligible exchange reaction with

  mercury, and therefore reaction 9 occurs easily-

  practically in the absence of a reverse reaction.

  Zn + DMHg - DMZn + Hg (reaction 9).

  According to the present invention, a substrate is obtained

  a layer of elements from groups II and VI. A first cou-

  a rant comprising an organic compound chosen from an element from group II is directed towards the substrate, and a second stream comprising an organic compound from an element from group VI is also directed towards the substrate. At least one of the organic compounds of the elements of group II and of group VI comprises at least one organic radical which sterically repels the organic radicals of the second of the organic compounds of

  elements of group II and group VI. Preferably, the com-

  organic pose of the element of group II contains radi-

  voluminous organic cals, which surround the atom of the element

  ment of group II and sterically repel the organic radicals of the organic compound chosen from the element of group VI, thus limiting, or practically eliminating, the reactions between the organic compound of the element of group II and the organic compound of the element of group VI, in the vapor stream. Even better, the organic compound chosen 7 -

  of the element of group II comprises a branched organic radical

  trust which increases the steric repulsion towards the organic compound

  the element of group VI. Thanks to this particular arrangement, consisting in providing an organic source of a group II or group VI element, comprising voluminous organic radicals surrounding the group II element or the group VI element, a steric hindrance is obtained between the organic source of the group II element and the organic source of the group VI element. That is, by surrounding the group II element and / or the group VI element with bulky radicals, which sterically repel each other, the atoms of the group II element and / or of the element of group VI do not come close to each other, and this thus largely limits the possibility of reactions involving the organic compound of the element of group II and the organic compound of the element of group VI, described above. The organic sources chosen from the group II element and the group VI element are therefore much less reactive towards each other.

  the other, in the vapor stream, that the organic sources

  classic classics of elements from group II and group VI. Consequently, the organic compound of the group II element and the organic compound of the group VI element undergo pyrolysis or decomposition practically independently of each other in the growth region above the

  substrate. Since the organic compounds of the ele-

  group II and group VI decomposes practically-

  Independently above the substrate, the concentration of Cd varies much less in the vapor stream, and consequently the layer deposited has a much more uniform composition.

  According to another aspect of the present invention, the com-

  organic pose of the element of group II steric regrowth-

  ment the organic source of the element of group VI, which contains an organic radical having an activation energy

- 8 -

  relatively low, compared to the activation energy of a primary alkyl compound of the group VI element, for

  the formation of a free radical during the pyrolysis of the

  organic pose of the group VI element. Compressed current

  An elementary source of a Group II metal is also directed towards the substrate. The organic compound chosen from the group II element, comprising the bulky organic radicals, is sterically prevented from reacting with the elementary metal from group II. In addition, the organic compound of the sterically hindered group II element is chosen so that its thermal stability is comparable to the thermal stability of the chosen organic compound of the group VI element. Thanks to this particular arrangement, growth is obtained, at low temperature, of materials based on elements of groups II and VI which have a composition

  uniform. Low growing temperatures should limit

  kinetically ter the rate of the exchange reaction involving the organic compound chosen from the group II element and the elementary metal from group II, and the steric repulsion effects must still kinetically limit this exchange reaction. So, we also reduce in a

  largely the second additional cause of non-uniform

  mity of composition, in the growth of materials to

  based on elements of groups II and VI by DCOMV, because the cons-

  aunt of speed of the exchange reaction, at low temperatures

  tures, is relatively weak in the direction leading to the exchange of Group II metals. It is believed that uniformity

  of the composition, from one side of the substrate to the other, is also

  slightly improved, because with the previous methods, any cause of non-uniformity from one side to the other, such as variations in flow diagrams and gradients

  temperature, was amplified due to the inconsistency

  controlled and not independent of the chemical reactions which take place

duis.

- 9 -

  According to yet another aspect of the present invention,

  a crystal layer of sea telluride is grown

  cure and cadmium on a crystalline substrate, directing several streams of vapor towards the substrate. A first vapor stream comprises a source of mercury, a second vapor stream comprises an organic source of cadmium, chosen from diethylcadmium (DECd), di-N-propylcadmium

  (DPCd), di-isobutylcadmium (DIBCd) and di-neopentylcad-

  mium (DnPCD). The organic tellurium compound comprises at least one organic radical chosen from a secondary alkyl radical, a tertiary alkyl radical, an allyl radical, a benzyl radical and a cycloallyl radical, linked to the tellurium atom. The selected organo-Cd, the organo-Te cheisi and Hg, are reacted at a temperature at which the reaction

  exchange, involving organo-Cd and Hg, is limited

  kinetically. Thanks to this arrangement, consisting of

  to provide an organic cadmium compound having radicals

  organic cals, surrounding the cadmium atom, which sterically prevent the reaction of the tellurium atom with the cadmium atom, a virtually reaction-free transport of these vapors is obtained in the reactor tube, towards the

  high temperature region which is above the sub-

  strat. In addition, since the temperature above the growth region is such that the exchange reaction is kinetically limited (i.e., the reaction rate is slow), growth of the HgCdTe significantly more uniform, because there is less elementary Cd

  available to react with organo-Te before the substrate.

  At growth temperatures above the substrate, the organic cadmium compound and the organic tellurium compound undergo pyrolysis almost independently, yielding free tellurium and cadmium. Free tellurium reacts with elemental mercury and with free cadmium, to give mercury and cadmium telluride. Since there is no loss of cadmium in the current

- 10 -

  vapor before the pyrolysis of the organic cadmium compound above the substrate, the uniformity of composition of the deposited layers, from the front part to the rear part of the substrate, is markedly increased. Due to the controlled reactions which take place, i.e. due to a practically independent pyrolysis of the alkyl compounds of

  cadmium and tellurium, an improvement is also expected

  ration of uniformity in the composition of a sub

strat compared to each other.

  According to another aspect of the present invention, it is

  deposits a layer of elements of groups II and VI on a sub-

  strat. A first stream comprising an organic compound chosen from an element of group II is directed towards the substrate, and a second stream comprising an organic compound of

  the element of group VI is also directed towards the substrate.

  At least one of the organic compounds of group II elements

  and of group VI contains organic radicals which provide

  either a transfer of electrons to the electro- element

  positive of group II, i.e. a transfer of electrons coming from the electronegative element of group VI. Preferably the

  organic compound of the element of group II has a radi-

  organic cal electron donor, directly linked to the element

  ment of group II. The electron donor radical should preferably be a stable group, when the possibility of

  the incorporation of unanticipated dopants could pose a risk

  pale. An example of an electron donating radical is the phenyl group (C6H5). Thanks to this particular arrangement consisting in providing an organic source of an element of group II, comprising electron donor radicals, linked to the element of group II, the electropositivity of the atom of the element of group II is reduced by the transfer of the electronic charge from the electron donor radical

  who was chosen. In particular, since the electro-

  positivity of the group II element is decreased by the pre-

  sence of phenyl groups, this must at the same time reduce the

- 11 -

  force of attraction entered the group II element atoms

  and the atoms of the group VI element, while the com-

  organic pose of the element of group II and the organic compound

  The group VI element is found in the vapor stream S. The electronegative nature of the phenyl group should also reduce the interaction between an organic compound

  of an element of group VI and the organic compound of the element

  ment of group II, to which the phenyl group is attached.

  According to yet another aspect of the present invention,

  one chooses as compound of the element of group II, a com-

  posed comprising an organic radical linked directly to the atom of the element of group II, which confers an electron transfer to the electropositive atom of the element of group II, and provides a steric hindrance between the atom of l group II element and the atom of the group VI element present in the organic source of the group VI element. Preferably, two phenyl radicals are linked directly to the element of group II, at least one hydrogen atom of one of the phenyl radicals being replaced by an organic group. Preferably, the electrophilic substitution at a position of a hydrogen atom on the phenyl radical is carried out with a radical belonging to the category of radicals which are generally classified as activating radicals orienting in ortho or para. Examples of organic radicals classified as weakly activating radicals include the phenyl radical and radicals

  alkyl (methyl, ethyl, etc.). The heteroradicals, that is to say

  say radicals containing atoms other than hydrogen and carbon, and classified as moderately or strongly activating radicals, include alcoholates, -OCH3, -OC2H5, etc .; one can also use -NHCOCH3; -OH and -NH2 (-NHR, NR, R being a radical), keeping in mind the possibility of the introduction of oxygen and nitrogen. For example, you can use a methyl group to replace a

  hydrogen atom in one of the ortho positions or in the posi-

- 12 -

  para of each of the phenyl radicals. The presence of bulky organic radicals will sterically prevent the atom of the group II element from reacting with the atom of the group VI element, by the presence of out-of-plane hydrogen atoms associated with the methyl group. This means that these hydrogen atoms will partially protect the atom from the group II element. In addition, due to this protection of the atom of the group II element by the hydrogen atoms out of plane, the vapor pressure of the organic compound of

  the element of group II will also increase, because the attractiveness

  intermolecular tion between a cadmium atom of a mol-

  cule and a phenyl group of a similar molecule is reduced. In addition, the use of activating radicals orienting in ortho and para should increase the transfer

  of electrons, and therefore further decrease the electroposi-

  tivity of the atom of the element of group II.

  According to another aspect of the present invention,

  places the hydrogen atoms in the two ortho positions of each phenyl group, by a bulky organic radical, such as a phenyl radical or an alkyl radical, such as for example the methyl, ethyl radical, etc. Substitution with a methyl radical, for example at each ortho position of

  each phenyl group must sterically repel the radicals

  methyl cals from other phenyl groups, and as a result the methyl radicals will rotate 90 relative to each other, leading to a non-planar molecule. Thanks to this particular arrangement, in addition to the steric hindrance provided by the presence of the substituent groups and by the

  release of electrons by phenyl groups, a characteristic

  The additional aspect of a molecule substituted in the two ortho positions of each phenyl group is that the central atom, constituted by the element of group II, is enclosed in a cage formed by the methyl groups which have carried out a rotation. This structure leads to a molecule which, although heavier, is considered to have a tension of

- 13 -

  higher vapor than a phenyl compound of an unsubstituted group II element, or a phenyl compound

  an element of group II which is substituted only in one posi-

  ortho tion, because the methyl radical cage around the atom of the group II element must clearly reduce the intermolecular attractions between the atom of the group II element of a molecule and a phenyl group of a second molecule. In addition, the cage of methyl radicals surrounding

  the atom of the element of group II must make this atom par-

  particularly non-reactive towards alkyl compounds of ele

Group VI.

  According to yet another aspect of the present invention,

  a crystal layer of sea telluride is grown

  cure and cadmium on a substrate, directing multiple streams of vapor toward the substrate. The first vapor stream includes a source of mercury. the second vapor stream comprises an organic source of cadmium, chosen from diphenylcadmium (DPCd), di-o-tolylcadmium

  (DOTCd), di- (2,6-xylyl) cadmium (DXCd) and di-mesityl-

  cadmium (DMSCd). The organic compound of the element of group VI comprises organic radicals chosen from a primary alkyl radical, a secondary alkyl radical, a tertiary alkyl radical, an allyl radical, a benzyl radical and a cycloallyl radical, linked to the group element VI. Thanks to this particular arrangement, consisting in providing an organic cadmium compound comprising organic radicals which sterically prevent reactions with the organic tellurium compound and are responsible

  of an electron transfer to the electropositive cadmium atom

  tif, we decrease at the same time the attraction between the compound

  organic cadmium and organic tellurium compound.

  steric hindrance should also limit the reaction

  of exchange between the organic cadmium compound and sea-

  priest. As a result, a pyrolysis is obtained practically

  independent of each organic source above the sub-

- 14 -

  strat, which leads to a layer of mercury telluride and

  of cadmium having improved compositional uniformity.

  The invention is explained in more detail with the aid of the attached drawings, in which FIG. 1 is a plan view of a photodetector element, here a photoconductive element containing layers of crystals comprising semiconductor materials based on 'elements of groups II and VI; Figure 2 is a cross-sectional view along line 2-2 of Figure 1; Figure 3 is a view showing the relationship between Figures 3A and 3B; Figures 3A and 3B are schematic diagrams of a growth apparatus to be used for growth of the epitaxial layer shown in Figure 1; and Figure 4 is a schematic diagram of another reactor having a reservoir for organic sources of group II elements having a high melting temperature. Referring to Figures 1 and 2, it can be seen that a characteristic photoconductive element 10, suitable for use in a photoconductive assembly (not

  shown), includes a substrate 11, here containing tellu-

  cadmium rure (CdTe) or gallium arsenide (GaAs), indium antimonide (InSb) or other suitable support materials belonging to groups II and VI, or groups III and V, or good sapphire (A1203). Above the substrate 11, and in the present case, there is an epitaxial buffer layer 12a based on elements of groups II and VI, here comprising cadmium telluride (CdTe), and a second layer epitaxial 12b of cadmium telluride (CdTe) or mercury and cadmium telluride (HgCdTe), or of another suitable material based on an element of groups II and VI, such as HgZnTe, or else of a

  material such as HgMnTe. On parts of the epi- layer

- 15 -

  taxiale 12b are two electrical contacts 13 of the ohmic type, each provided with a configured composite layer,

  comprising layers 13a, 13b and 13c, deposited sequentially-

  also consisting of indium (In) 1 im (10,000 A) thick, chromium (Cr) 50 nm (500 A)

  thick, and gold (Au) 500 nm (5,000 A) thick.

  Tablets 14 containing gold, 1.5 µm thick each, are placed on the contacts 13 to provide a

  connection point for external components.

  In a channel-shaped zone 15, between the contacts

  ohmic 13, there is a passivation layer 16a, cons-

  here constituted of an oxide formed in a known manner by anodization in situ from a portion of the layer of HdCdTe 12b, of nm (800 A) thickness, as well as an anti-reflection coating layer 16b. We use layers 16a and 16b to protect the region of channel 15 and to spare in the layer

  composite a window which is transparent to the electrical energy

  tromagnetic 17, generally in the wavelength range from about 0.8 im to 30 pm, which is directed towards the window 16. In response to this incident radiation 17, the conductivity of the epitaxial layer 12b is modified, which allows the photoconductive element to detect the presence of the incident electromagnetic radiation 17. It is also possible,

  as we know, adjust the ratio x of Cd to Te, to

  selectively experience different wavelength ranges in

  the band ranging from about 0.8 µm to 30 µm.

  Referring now to Figures 3, 3A and 3B, a schematic representation of a vapor phase epitaxy 20 (fig. 3), used for the growth of the epitaxial layers 12a, 12b of cadmium telluride or of

  mercury and cadmium telluride, as described in conjunction

  tion with Figures 1 and 2 above, includes an apparatus

  steam generator 20a (fig. 3A) comprising a collection

  26 with mass flow regulators 26a-26g and bubblers 39 and 55, as shown in the

- 16 -

  figure. During the operation, hydrogen is sent to the col-

  reader 26 via the H2 purifier 22 and

  valve 24, while helium is sent into the apparatus

  reil 20 when the device 20 is not in use and is 3 exposed to air. The vapor phase production apparatus 20 also comprises a vapor phase epitaxy reactor 20b (FIG. 3B), which here comprises, as shown in the figure, an open quartz reactor tube 60. It suffices to say here that a graphite susceptor 63 is disposed in the quartz reactor tube 60, and the susceptor is heated by induction by means of a radiofrequency shock coil (r.f.) 62. The shock coil r. f. 62 is wound at the periphery of the quartz reactor tube 60, and it is activated to raise the temperature of the susceptor 63, of the substrate 11 placed on the susceptor 63, and of the region 61 in the immediate vicinity of the substrate 11, up to a preset value. The temperature of the substrate 11 is controlled by means of a thermocouple (not shown) embedded in the susceptor 63. However, before heating the susceptor 63 and the substrate 11, the gases are removed from the air outside the system by introducing d helium, then hydrogen, inside the tube-furnace 60 and the steam generator 20a. The vapors from conduits 27e-27g, 31c and 47c, are then introduced into the tube, where they decompose and react to give the epitaxial layers 12a and 12b. The quartz reactor tube 60 also includes a cap 72 at an end opposite to the conduits 27e, 27g, 31c and 47, which is connected to an evacuation conduit 74 made of quartz, used for rejection

* gases outside tube 60.

  Referring now in particular to FIG. 3A, the steam generator 20a supplies the conduits 31c, 47c and 27e-27g which send vapors to the quartz reactor tube 60 (FIG. 3B), as shown on the drawing.

- 17 -

  The conduit 31c, that is to say the conduit bringing H2 and the organic source of the element of group II, is supplied

  by a junction element 32. The junction element is used

  tion 32 to mix the streams from two sources of gas, arriving at two ports thereof, and to direct said stream of mixed gas to the third port thereof, which is connected to the quartz tube 31c. The first

  orifice of the junction element 32 is supplied by the apparatus

  bubbling reil 39. The bubbling apparatus 39 comprises two control solenoid valves 28,30. One of the solenoid valves

  28.30, here the control solenoid valve 28,

  carries a first orifice connected to a first regulator of

  flow 26a through the tube 27a, and has a second orifice

  strung to a bubbling device 36 through the conduit 29a. The bubbling device 36 here contains the organic compound chosen from the element of group II, as described below. The

  bubbling device 36 is placed in a thermo-bath

  recycle 40, which provides constant circulation

  liquid around the bubbling device, to maintain

  provide the organic compound of the group II element (37) at a preset temperature to give sufficient vapor pressure. This temperature range can range from -20 to 1000C, but it is not necessarily included within these limits. A second conduit 29c is placed in the device

  bubbling 36, above the surface of the organic source

  group II element, and it is connected to an ori-

  control solenoid valve 30. A third conduit

  29b is connected between the remaining holes of the electro-

control valves 28 and 30.

  Normally disabled state of the solenoid valves

  control 28 and 30 allows the gaseous hydrogen to pass from the hydrogen source, here the flow regulator 26a, through the pipe 27a, in the pipe 29b, and from there, in the pipe 31c, to purge the reactor from air gas therein,

as described above.

- 18 -

  During epitaxial growth, for example, such

  luride of cadmium or telluride of mercury and cadmium, the control solenoid valves 28 and 30 are put in their activated state, which allows the gaseous hydrogen to pass, via the conduit 29a, into the bubbling device 36 which contains a selected organic source of cadmium 37. The gaseous hydrogen bubbles through the organic source of cadmium 37 and

  entails molecules from the organic source of cadmium 37.

  Consequently, a mixture of hydrogen and of organic source of cadmium (organo-Cd + H2) leaves the bubbling device 36 by the conduit 29c, and it is sent to the conduit 31a by

  the control solenoid valve 30. A second regulation is activated.

  mass flow sensor 26b for sending to the rod element

  tion 32 a current of a carrier gas, here hydrogen, at a predetermined flow rate, through a valve 34 and the conduit 31b. In

  Consequently, the current leaving the conduit 31c is a current

  rant of dilute vapor of the organo-Cd, compared to the gas

  in this case, hydrogen.

  Line 47c, i.e. the compound line

  organic element of group VI, is powered by an element

  Junction 48. Junction 48 is used to mix the streams from two gas sources, and

  this element sends said stream of mixed gases to a third

  fifth port connected to conduit 47c. The first opening of

  the junction element 48 is supplied by the bar appliance

  botage 55. The bubbling device 55 comprises two electro-

  control valves 44 and 46. One of these solenoid valves

  control, here the control solenoid valve 44, has a pre-

  mier orifice connected to a third mass flow regulator

  sic 26c, via conduit 27c, and a second orifice connected

  to the bubbling device 52 through the conduit 45a. The disposi-

  bubbling tif 52 here contains an organic compound of an element of group VI (53), as will be described later. Suffice it to say here that the organic compound of the group VI element may be a primary alkyl compound of the element

- 19 -

  of group VI, or can also be chosen so as to have an activation energy, for the formation of a radical

  free during the dissociation of the organic compound from the element

  ment of group VI, which is lower than the activation energy

  tion upon separation of a primary alkyl radical from the element of group VI. The bubbling device 52

  is placed in a thermostatically controlled recycling bath 56, which provides

  nit constant circulation of a liquid around the device

  bubbling tif 52, to maintain the organic tellurium compound 53, contained in the bubbling device 52, at a preset temperature, sufficient to obtain an adequate vapor pressure. This temperature range can range from -20 to +100 C, but it is not necessarily included within these limits. A second conduit 45c is placed in the bubbling device 52, above the surface of the organic source of the element of group VI, and it is connected to an orifice of the control solenoid valve 46. A

  third conduit 45b is connected between the resistor orifices

  of the control solenoid valves 44 and 46.

  Normally disabled state of the solenoid valves

  control 44 and 46 allows the gaseous hydrogen to pass from the hydrogen source, here the flow regulator 26c, through the pipe 27c, in the pipe 45b, and from there, in the pipe 47c, to purge the reactor from air gas therein, as described above. During epitaxial growth of the

  cadmium telluride or mercury telluride and cad-

  mium, on the substrate 11, the valves 44 and 46 are put in their activated state, which allows the gaseous hydrogen to pass, through the conduit 45a, into the bubbling device 52 which

  contains the organic compound of the group VI element (53).

  Hydrogen gas dabbles in the organic compound of the element of group VI (53), and entrains molecules of the

  organic compound of the element of group VI (53). Therefore

  quence, a mixture of hydrogen and the organic compound of the element of group VI (organo-group VI + E2) leaves the

- 20 -

  organic source of the element of group VI (53), by the

  conduit 45c, and it is sent to conduit 47a by the electro-

  control valve 46. A fourth mass flow regulator 26d is activated to send to the junction element 48 a current $ of a carrier gas, here hydrogen, at a flow

  pre-established, by a valve 50 and the conduit 47b. Therefore

  quence, the current leaving the conduit 47c is a diluted vapor current of the organic compound of the group VI element, relative to the concentration of the carrier gas, in the case

present, hydrogen.

  The conduit 27e, supplied by a fifth mass flow regulator 26e, terminates in a quartz reservoir 66 (FIG. 3B) containing a liquid source of an element of group II such as mercury. Hydrogen gas is sent over the surface of liquid mercury, and molecules of mercury vapor, located above the surface of liquid mercury, are entrained by the stream of hydrogen

  gaseous, giving a stream of mercury vapor and hydro-

  gene (Hg + H2). The vapor stream is brought to an element

  of quartz 70 junction (fig. 3B). A second inlet opening

  of the quartz junction element 70 is powered by

  a quartz tube 71a, which is connected to a sixth regulator

  mass flow controller 26f via a valve 72

  and conduit 27f. The current leaving the rod element

  2 tion 70 through the conduit 71b, and arriving in the tube 60, is therefore a dilute stream of mercury vapor and hydrogen. If we now refer in particular to FIG. 3B, as mentioned previously, the susceptor 63 is heated by a coil r.f. placed around the reactor tube in

quartz 60.

  A quartz 66 tank containing the elemental mercury

  liquid shutter, as well as the region adjacent thereto, are heated by means of a heat source 68 constituted by

  resistance, as indicated in the drawing, to a temperature

- 21 -

  ture of at least 100 C, but generally less than 250 C, of

  preferably in the range of 150 to 180 C. The area immediately below

  diat after the reservoir 66 and after the substrate 11, is then heated by rows of infrared lamps 64, to a temperature in the range of 100 to 250 C, the preferred range being 150 to 180 C. The heating of the walls prevents premature condensation of mercury out of the current

steam.

  Using suitable solvents, the surface of the substrate 11 facing outward is degreased and cleaned, then it is polished using an appropriate product which attacks the material of the substrate. For example, a solution of bromine in methanol is used to chemically polish CdTe or GaAs before the growth of the various layers.

  epitaxial. The substrate 11 is then placed on the suscep-

  tor 63, which is then placed in the reactor tube in

quartz 60.

  In operation, the tube oven 60 is purged from the gases in the air

  being there, by introducing helium and then hydro-

  gas gene, as indicated above. The susceptor 63 is then heated by induction, by means of the r.f. coil. 62, the reservoir 66 by the resistance heating element 68, and the reactor tube 60 by means of the infrared lamps 64.

  then let each of these elements reach the temperatures

  growth tures. When the apparatus 20b has reached the growth temperatures, the valves 28, 30, 34, 44, 46, 50 and 72 are activated, which allows dilute hydrogen mixtures

  gaseous + organic compound of group II element, hydro-

  gas gene + organic compound of the group VI element, and

  hydrogen gas + mercury, to exit through the conduits, res-

  pectively, 31c, 47c and 71b, upstream of the substrate 11.

  The vapors of hydrogen, mercury and organic compounds are at the desired operating temperature, provided by the uniform heating of the substrate 11 and of the region 61 around the substrate 11. It is believed that the selected sources of the

- 22 -

  organic compounds which are directed undergo pyrolysis practically independently of each other, and give

  cadmium and mercury telluride according to chemical reactions

  mics 1A-5A below: organo-Te - Te + HC (reaction 1A) Te + Hg M HgTe (reaction 2A) organo-Cd - Cd + HC (reaction 3A) Cd + Te - CdTe (reaction 4A) HgTe + CdTe * HglxCdxTe (reaction 5A),

  in which H.C. means 'hydrocarbons'.

  The composition x is adjusted by adjusting the flow rate of H2 in the Hg reservoir, the temperature of the Hg reservoir and the concentration of the organic cadmium compound thus

  as organic tellurium compound.

  The molar fraction (FM), i.e. the concentration of organo-Cd, organo-Te and Hg, is given by: H2 passing through the available vapor pressure FM (organo-Cd ) = bubbling sifter 36 of the organo-Cd (kPa) Total current of H2 101.3 kPa in the tube 60 H2 passing through the device- Vapor pressure FM (organo-Te) = bubbling sifter 52 of the organo -Te (kPa) Total current of H2 101.3 kPa in tube 60 H2 above Vapor pressure FM (Hg) = reservoir 66 of Hg (kPa) Total current of H2 101.3 kPa in tube 60

  Only part of the organic vapors which are direct

  gées above the substrate 11 actually react.

  The unreacted organic vapors are expelled from the reactor tube 60 through the exhaust duct 74, and are

  sent to an exhaust gas cracking furnace (not shown

  smelled) which breaks down the remaining organic gases into

  elements, and gives a gas stream which essentially comprises-

- 23 -

  hydrogen and various hydrocarbons.

  According to one aspect of the invention, the source of cadmium is an organic source comprising organic radicals

  which are chosen so as to sterically prevent the reaction

  tion of the cadmium atom with a tellurium atom found in the organic tellurium source. Preferably, the organic radical chosen is not linked directly to the cadmium atom, since a direct bond of the organic radical to the cadmium atom would increase the reactivity of the compound

organic cadmium.

  The organo-Cd chosen corresponds to the general chemical formula

  following route: R1 - Cd - R2 in which R1 and R2 may be the same or different,

  and at least one of the radicals R1 and R2 has the general formula

following route: X1

Y - C -

''

  X2 x2 in which X1 and X2 may be the same or different, and are preferably chosen from hydrogen, a halogen and an organic radical. Y corresponds to the following general chemical formula: Y1

Y2 - C

Y3

  in which Y1, Y2 and Y3 may be the same or different

  rents, and are each preferably a hydrogen atom or

  halogen, or an organic radical.

  As shown below, the organo-Cd has a radi-

  organic cal which includes the carbon atom in position

- 24 -

e of the organo-Cd chain.

  Y1 Xl Xl Y i I I I Y2 - C - C - Cd - C - C - Y2 Il I!

Y3 X2 X2 Y3

  3 a to B With this particular arrangement, bulky radicals

  at the ends of the chain sterically prevent reac-

  tion of the cadmium atom with the tellurium atom present

  in the organic tellurium compound.

  A preferred example of an organic compound of an element of group II, comprising a bulky radical on carbon

  in position a in its organic radical, is the di-neo-

  pentylcadmium [(CH3) 3CCH2] 2Cd. Dienopentylcadmium has the following structural formula:

CH3 H H CH3

I I I I

CH3 - C - C - Cd - C - C - CH3

1 1 1 1

CH3 H H CH3

O

  The molecule contains two tert-butyl groups which are separated

  res of the cadmium atom by a carbon atom in posi-

  tion a, here a CH2 group. Since the tertiary groups

  butyl are not directly linked to the cadmium atom they

  do not significantly destabilize the di-

  neopentylcadmium. Consequently, di-neopentylcadmium (DNPCd) should have a thermal stability comparable to that of diethylcadmium (DECd). DNPCd offers several advantages. DNPCd is believed to limit reactions between itself and the selected tellurium organic compound, due to steric repulsion due to the tert-butyl groups present

  in the DNPCd molecule. The presence of these tert-

  butyl makes it difficult to bring the two molecules together, and in particular the two metal atoms of the

- 25 -

  two molecules, to allow a reaction. In addition, by choosing an appropriate source of tellurium, a

  growth of mercury telluride and cadmium at low tem-

  temperature. Consequently, reaction 7, namely the exchange reaction between the organic compound of the group II element and mercury, should be kinetically limited,

  and therefore not become a major cause of exhaustion

  cadmium. Due to their weight and size, the neopentyl radicals [(CH3) 3CCH2] should also decrease the rate of free radical chain reactions, and therefore give a molecule significantly less

  reactive as dimethylcadmium in the vapor stream.

  A second preferred example is di-isobutylcadmium [(CH3) 2CHCH2] 2Cd, which has an isopropyl group separated from the Cd atom by a carbon atom in position a, here a

  CH2 group. Di-isobutylcadmium has the development formula

following sword:

CH3 H H CH

H CH3

I I

CH - C- Cd - C- CH

CH3 H H CH3

  As other examples, mention may be made of di-n-propyl-

  cadmium and diethylcadmium, each corresponding to the following general formulas: CH3-CH2-CH2-Cd-CH2-CHCH2-CH3 di-n-propylcadmium

  CH3 -CH2 -Cd-CH2 -CH3 diethylcadmium.

  It is also possible that the organic cadmium compound has organic radicals linked directly to the cadmium atom, which transfer an electronic charge to

  the electropositive cadmium atom. As a result of the decrease

  tion of the electropositivity of the cadmium atom by the organic electron donating radical, the organic cadmium compound must be less reactive towards the molecule of the organic tellurium compound than is dimethylcadmium

- 26 -

  known. An example of such a compound is diphenylcadmium, corresponding to the following structural formula:

H H H H

\ I !! H --- Cd - -H

/ \ I \

H H H H

  As can be seen, diphenylcadmium contains two phenyl groups which are sources of electrons, due to their electron cloud 71. The central cadmium atom is electropositive. Consequently, the electropositivity of the cadmium atom should be reduced by the presence of the phenyl groups. This must at the same time reduce the force

  of attraction between the organic cadmium compound and the com-

  layered organic tellurium. The electronegative nature of the phenyl groups should further reduce the interaction between a selected organic tellurium compound and diphenylcadmium,

  because the phenyl groups should repel the atom of such

  electronegative lure. Another peculiarity linked to the use

  sation of an aromatic cadmium compound, such as diphenylcadmium, is that normally aromatic cadmium compounds are relatively stable. As a result, this compound can be stored for long periods of time.

  durations without decomposition. In addition, the phenyl groups themselves

  themselves are also stable units, and we consider that

  the cycles are not broken during pyrolysis. Therefore

  quence, it is also estimated that growth by DCOMV uses

  reading diphenylcadmium should lead to a low incorporation of carbon in the films of mercury and cadmium telluride. Although diphenylcadmium has a relatively low vapor pressure and is a solid with a melting point of 174 C, it is nevertheless believed to be

  possible to use such a source.

- 2.7 -

  A heated reservoir system, such as that shown in Figure 4, can be used to provide an appropriate stream of diphenylcadmium vapor, in the same way that reservoir 66 provides the vapor stream of Hg. That is, one can direct the conduit 27a to a reservoir 87 containing

  organo-Cd 86. Hydrogen gas is passed through

  above the tank, and it drives molecules from the organ-

  no-Cd 86 and conducts this vapor current in the reactor, via line 31c, after a predetermined dilution with H2, as indicated above. The tank is placed inside

  an oven heated to a preset temperature, as shown

  felt on the drawing. The oven can be a multi-zone oven.

  multiple, for heating the organo-Cd and the

Hg at selected temperatures.

  Other possible sources of cadmium, including

  electron donor phenyl radicals, include di-o-

  tolylcadmium, with the following structural formula:

H CH3 H H

% I I

H- Cd -H

H H CH3 H

  Di-o-tolylcadmium is similar to diphenylcadmium, except

  except that a hydrogen in ortho position on each nucleus ben-

  zenic is replaced by a methyl group. The groups

  methyl also increase the charge transfer electro-

  nique, phenyl groups with cadmium atoms, consequently decreasing the positive charge of the Cd atom. Although this molecule is heavier than diphenylcadmium, it is nevertheless admitted that di-otolylcadmium (DOTCd) must have a tension of higher vapor, because due to the fact that the methyl group is fixed in one of the ortho positions, the plane symmetry of the molecule is altered by the out of plane hydrogen atoms of the methyl group. These hydrogen atoms protect

- 28 -

  partially the central cadmium atom, and consequently reduce the intermolecular attraction between a cadmium atom of one molecule and a benzene nucleus of another molecule. Di-o-tolylcadmium has a melting point of 115 C, which may be an indication that DOTCd will have a higher vapor pressure than DPCd. Furthermore, it is believed that partial protection of the cadmium atom by out-of-plane hydrogen atoms will lead to a reduced attraction between the cadmium atom and a tellurium atom present in the

organic tellurium compound.

  As other examples of cadmium compounds with increased electron transfer and increased steric hindrance,

  we can cite di- (2,6-xylyl) cadmium (DXCd) and di-

  mesitylcadmium (DMSCd). These compounds have the following developed formulas:

H CH3 CH3 H

H- / rH - Cd -H DXCd \ \

H CH3 CH3 H

H CH3 CH3 H

DMSCd $ CH3- - Cd CH3 \ I

H CH3 CH3 H

  These compounds are similar to di-o-tolylcadmium, except

  that DXCd has methyl groups in both posi-

  ortho of each benzene nucleus, and the DMSCd has methyl groups in the two ortho positions and in the para position of each benzene nucleus. With two groups

- 29 - ortho linked to each benzene nucleus, the electronic charge transferred to

  the cadmium atom is further increased. In addition, the methyl groups linked to the benzene nucleus in the ortho position must sterically repel each other, which leads to a non-planar molecule. Another feature

  important of this structure is that, because of the

  sterically, the cadmium atom is enclosed in a cage of four methyl groups. This increased steric hindrance must at the same time raise the vapor pressure of the DXCd. In addition, the methyl group cage around the cadmium atom should reduce the attraction between the organic compound of

  tellurium and DXCd or DMSCd, and should prevent or nota-

  seriously limit the exchange reaction between cadmium and mercury. Consequently, the organo-Cd chosen, comprising electron donor groups, corresponds to the following general chemical formula: R3 - Cd - R

3 4

* in which R3 and R4 can be identical or different, and at least one of the radicals R1 and R2 has the following general formula: Yi! I

Y3 - -

  H in which the hydrogen atom (H) is generally, but not necessarily, in the meta positions, and in which Y1 and Y2 are in the ortho positions, and Y3 and in the para position, and are chosen each among an atom

  of hydrogen and an organic group.

  Preferably, the organic compounds are radicals

  activating cals such as phenyl and alkyl radicals

  3S (C6H5, CH3, C2H5, etc., or groups comprising hetero-

- 30 -

  atoms, such as alcoholates, -OCH3, -OC2H5, etc; -NHCOCH3; -OH; and NH2 (NHR, NR, R being a radical). Groups containing heteroatoms can be used when

  the possibility of incorporating O or N into the films

  S deposited deposits are no problem.

  Preferably, in order to increase the steric hindrance between the organic cadmium compound, sterically hindered, and the organic tellurium compound, the organic tellurium compound is chosen so that the latter comprises

  bulky organic radicals which, likewise, will prevent ster-

  rically the reaction of the organic tellurium compound with the organic cadmium compound. As described in application US 844 489 of March 26, 1986

  a tellurium source having a relative activation energy

  low for the formation of a free radical during pyrolysis, when compared to the activation energy of the

  diethyl telluride, is the tertiary alkyl compound tellu-

  di-tert-butyl rure. Di-tert-butyl telluride pos-

sedes the following formula:

CH3 CH3

I i CH3 - C - Te - C - CH3

I I

CH3 CH3

  Di-tert-butyl telluride contains two tert-

  butyl linked directly to the tellurium atom. Consequently, the presence of tert-butyl groups in the organic tellurium compound di-tert-butyl telluride destabilizes the compound.

  organic tellurium and sterically blocks the atom of such

  lure. The choice of di-tert-butyl telluride as the source of organic tellurium compound and the choice of one of the sterically hindered cadmium sources, mentioned more

  high, will give increased steric hindrance and, therefore,

  quent, an increased lack of reaction during transport of

  organic cadmium and tellurium compounds in the reaction

- 31 -

tor.

  Other sources of tellurium (element of group VI) include

  take diisopropyl telluride (DIPTe) of formula:

CH3 CH3

"xCH - Te - CH

CH3 \ CH

3 3

  Diisopropyl telluride, as described in in application US 918 697 of October 16, 1986 has a lower stability than DETe, and therefore an improved cracking yield compared to the yield of

  DEET cracking. DIPTe is a preferred example of a

  layered secondary tellurium alkyl. In addition, the bulky isopropyl radical sterically prevents the reaction of

  the tellurium atom with the organo-Cd.

  As also described in ds-red-hej fi is087 637, greater delocalization is obtained, and consequently lower activation energies, by using overlapping by double bonds, instead of single bonds, of the orbital p of the unpaired electron. The allyl radical, the benzyl radical and the cycloallyl radical each delocalize the free electronic charge over the entire carbon chain. Preferred examples of allyl, benzyl or cycloallyl compounds of the group VI element are given

in the table below.

BOARD

  Formul Representation Denomination (C6H5CH2) 2Te C6Hs-CH2-Te-ClI2-C6H5 dibenzyl tellurium C [13 (C6H5CH2) Te CH3-Te-C [12-C6115 methylbenzyl tellurium (CH2CHCH2) 2Te CH2-CH-CH2-Te- C1! 2-CIl-CH2 di (2propene-1-yl) tellurium CH3 (CH2CHCH2) Te CH3-Te-C112-Cli-CH2 methyl- (2propene-1-yl) tellurium (C3H3) 2Te HH di tellurium L (2-cyclopropene-1-y1e) I! IlC-C-Te-C-C-l1

\\ 1 \ 11

CC

I I

11 a.m.

  H I CH3 (C3H3) Te C113-Te-C-C-H methyl tellurium- (2-cyuclopropene-1-yl) C Il II

- 33 -

  Consequently, organic sources of tellurium such as di-tert-butyl telluride or the above-mentioned secondary alkyl, allyl, cycloallyl or benzyl compounds, each cause a lower activation energy for the formation of a free radical, and therefore lower growing temperatures. By the choice of the organic tellurium compound, in conjunction with the choice of the organic cadmium compound, the growth of materials based on elements of groups II and VI, such as mercury and cadmium telluride, will occur at temperatures of

  lower growth, selected organic compounds become

  will sterically fish each other, and the exchange reaction between the organic source of a group II element and the

  mercury will be kinetically limited.

  1z As described in conjunction with US application 844 489 the growth of semiconductor materials based on elements

  groups II and VI, using di-tert-telluride

  butyl as the organic tellurium compound, can be

  produce at temperatures as low as about 230 C.

  As mentioned above, it is believed that at this temperature the

  exchange reaction (reaction 7) between mercury and com-

  organic cadmium placement will be very kinetically limited.

  Thus, the exchange reaction will not be a significant source of cadmium depletion, as it is in the previous techniques. Consequently, thanks to the arrangement described above, it will be possible to transport the organic cadmium compound and the organic tellurium compound practically in the absence of reaction, and the organic cadmium compound and the organic tellurium compound will undergo pyrolysis. of

  practically independently over the high temperature region

  erasing of the substrate, thus giving films of mercury and cadmium telluride having an improved composition uniformity from the front part to the rear part. It is also believed that the uniformity of composition, from side to side of the substrate, of mercury telluride films

- 34 -

  and cadmium thus deposited, will also be improved.

  To further reduce the attraction between the electro-

  positive of Cd (atom of an element of group II) and the electronegative atom of Te (atom of an element of group VI), one can carry out a withdrawal of electrons out of the atom of Te (group VI) by the choice of the organic source of Te (group VI), so that it includes radicals attracting the electrons, as indicated below for Te: Y x2 2 t Xl- Q Te - - --1 XI- / X y

Y XZ2 Y

  X1 and X2 being generally hydrogen atoms, and Y being a deactivating group in the meta position, chosen from the groups -NO2, -N (CH3) 3, CN, -COOH (-COOR), -SO3H, -CHO (-CRO), R being an organic radical, without losing sight of the possibility of incorporating N, O, S, etc. in the

  layers of elements of groups II and VI.

  Another possibility consists in replacing the hydrogen atoms in para and ortho position (positions of X1 and X2) by a halogen (F, Cl, Br, I), the meta positions being

  occupied by hydrogen atoms.

  It should be understood that the foregoing description has not

  been given by way of illustration and not limitation, and that any variant or modification can be made without departing from the general scope of the present invention,

  as defined in the appended claims.

- 35 -

Claims (37)

  1. Method for obtaining a layer comprising a semiconductor material based on elements of groups II and   VI, on a substrate, comprising the step of sending a cover   rant comprising an organometallic compound of an element of group II and an organic compound of an element of group VI, towards the substrate, the two organic compounds of elements of   group II and group VI each comprising at least one radi-   voluminous organic callus sterically repelling the bulky organic radical of the other organic compounds of   elements of group II and group VI.   2. Method according to claim 1, characterized in that   that the voluminous organic radicals chosen are radicals branched organic radicals.   3. Method according to claim 2, characterized in that the voluminous branched organic radical chosen is chosen from secondary or tertiary alkyl radicals and a phenyl radical, and in that the radicals chosen are linked directly to the atom of the element of group VI and the radicals chosen for the atom of the element of group II include a carbon atom in position 0 in the chain organic radical.   4. A method for obtaining a layer comprising a material based on elements of groups II and VI, comprising the steps of sending a current comprising an organic compound of an element of group VI to the substrate; sending a current comprising an organic compound of an element of group II towards the substrate, said organic compound of an element of group II comprising radicals   voluminous organics chosen so as to repel ster-   only the organic compound of an element of group VI; and reacting said organic compounds of group VI and group II elements, to form the material to   base of elements of groups II and VI. - 36 -   5. Method according to claim 4, characterized in that the organic compound of the group II element comprises an organic radical which comprises a carbon atom in position a in the organic compound of the group II element. 6. Method according to claim 5, characterized in that the organic radical comprising the carbon atom in   position a is chosen from isopropyl, tert- butyl and phenyl.   7. Method according to claim 6, characterized in that the organic radical comprising the carbon atom in   position a is chosen from isopropyl and tert-   butyl. 8. Method according to claim 7, characterized in that the organic compound of the element of group VI comprises at least one organic radical chosen from a secondary alkyl group, a tertiary alkyl group, an allyl group, a   benzyl group and a cycloallyl group.   9. Method according to claim 8, characterized in that   that the organic compound chosen is linked directly to the element Group VI ment.   10. Method according to claim 4, characterized in   what the organic compound of the group II element corresponds to   pond with the following general chemical formula: R1 - A - R2   in which A is an atom of the element chosen from group II, and in which R1, R2 may be identical or different, and at least one of the radicals R1 and R2 corresponds to the following general chemical formula: X1   X1 Y- C- X2 in which X1 and X2 can be the same or different, - 37 -   and are preferably chosen from a hydrogen atom, a   halogen and an organic radical; and in which Y corresponds to   pond to the following general chemical formula: Y1 I Y2 - C - Y3   in which Y1, Y2 and Y3 may be the same or different   rents, and are each chosen from a hydrogen atom, a halogen and an organic radical.   11. Method according to claim 10, characterized in that Y1, Y2 and Y3 are chosen from a hydrogen atom   and alkyl-type organic radicals.   / 5 12. Method according to claim 10, characterized in   what the organic compound of group VI element includes   carries at least one organic radical chosen from a secondary alkyl group, a tertiary alkyl group, a group   allyl, a benzyl group and a cycloallyl group.   13. Method according to claim 11, characterized in that the organic radical chosen is linked directly to the element of group VI.   14. The method of claim 4, further comprising the step of sending a current from an elementary source of a group II element to the substrate; and wherein the reacting step comprises reacting said group II element with the organic compound of a group II element and with the organic compound of a group II element   group VI, for obtaining the layer.   15. The method of claim 14, characterized in that the reaction step, for obtaining the material based on elements of groups II and VI, is carried out at a   temperature at which an exchange reaction, making   come the group II metal and the organic compound of the element   ment of group II, is to a large extent limited kinetics only. - 38 -   16. Method according to claim 15, characterized in that the reaction step is carried out at a temperature less than 320 C.   17. Method according to claim 16, characterized in   $ what the organic compound of the group II element contains   carries at least one organic radical which comprises a carbon atom in position a in said organic radical; and in that the organic compound of the element of group VI comprises at least one radical chosen from a secondary alkyl group, a tertiary alkyl group, an allyl group, a group cycloallyl and a benzyl group.   18. The method of claim 15, characterized in that the reaction step is carried out at a temperature less than 280 C.   19. The method of claim 18, characterized in   what the group II element organic compound   carries at least one organic radical which comprises a carbon atom in position a in said organic radical; and the organic compound of the element of group VI comprises at least one radical chosen from a tertiary alkyl group, a group   allyl, a cycloallyl group and a benzyl group.   20. Method for obtaining a layer comprising   mercury and cadmium telluride, on a substrate,   taking the steps of sending a current from a source of mercury to the substrate; sending a current from a tellurium source to the substrate; and sending a current comprising an organic compound of   cadmium, towards the substrate, said corresponding organic compound   dant to the general chemical formula: R1 - Cd - R2 in which R1 and R2 can be identical or different, at least one of the radicals R1, R2 corresponding to the general chemical formula: - 39 - Y2 Xi I i Y1 - C - C- i l y I Y3 X2   in which X1 and X2 may be the same or different, and are chosen from a hydrogen atom, a halogen and an organic radical; and in which Y1, Y2 and Y3 may be the same or different, and at least two of the radicals Y1,   Y2 and Y3 are organic radicals.   21. The method of claim 20, characterized in that the source of mercury is an elementary source of mercury, and the source of tellurium is an organic source of tellurium comprising at least one organic radical chosen from a primary alkyl group, an alkyl group. secondary, a tertiary alkyl group, an allyl group, a benzyl group and a cycloallyl group, directly linked to the tellurium atom. 22. Method according to claim 21, characterized in that the organometallic telluride compound is chosen from diethyl telluride, diisopropyl telluride, di-tert-butyl telluride, dibenzyl telluride,   di- (2-propene-1-yl) telluride, di- (2-   cyclopropene-1-yl), X-ethyl telluride, Xisopropyl telluride, X-tert-butyl telluride, telluride   of X-benzyl, telluride of X- (2-propene-1-yl) and tel-   X- (2-cyclopropene-1-yl) luride, X being chosen from a   hydrogen atom, halogen and an organic radical.   23. A method for growing a layer comprising a material based on elements of groups II and VI, on a substrate, comprising the steps of sending a first stream, comprising an organic compound chosen from an element of the group VI, towards the substrate; sending an organic compound of an element of   group II towards the substrate, said organic compound of the element - 40 -   ment of group II containing bound organic radicals   directly to the atom of the group II element, which provides   create an electron transfer to the electropositive atom of the element of group II.   24. Method according to claim 23, characterized in that the organic radical is an organic donor radical   of electrons and includes a phenyl group.   25. The method of claim 24, characterized in   what the electron donating phenyl radical further comprises   takes at least one group substituting a hydrogen atom   unenptiond = Lsieparmi the para position and one of the two posi- ortho of the phenyl group.   26. Method according to claim 25, characterized in   what the substituent group is an electric activator group trophile.   27. Method according to claim 26, characterized in   that the substituent group is chosen from a phe-   nyle and an alkyl group, an alcoholate, -NH2, -NHR, -NRR, -OH and -NHCOCH3, R being a radical.   28. The method of claim 27, characterized in   that the substituent group is chosen from a phe- nyle and an alkyl group.   29. The method of claim 25, characterized in that the organic radical chosen, substituting hydrogen dam u psiticndxsie among the para position and one of the ortho positions of the phenyl group, sterically repels the chosen organic compound of tellurium, while the organic tellurium compound and the selected organic cadmium compound are   directed to the substrate in the vapor stream.   30. The method of claim 29, characterized in that the substituent organic radical is a methyl radical and the latter is linked in one of the ortho positions of the phenyl group.   31. The method of claim 29, characterized in that each of the hydrogen atoms in the ortho position of the - 41-   phenyl group is replaced by a methyl radical.   32. The method of claim 29, characterized in that the hydrogen atoms of both the ortho positions and the para position of the phenyl groups are replaced by methyl radicals. 33. Method for growing a layer comprising   mercury and cadmium telluride, on a substrate,   nant the steps of sending a first stream comprising an organic compound of an element of group VI to the substrate; sending a current comprising a source of mercury to the substrate; and sending a current comprising an organic cadmium compound to the substrate, said organic cadmium compound being chosen from diphenylcadmium, di-o-tolylcadmium,   di- (2,6-xylyl) cadmium and dimesitylcadmium.   34. Process according to claim 33, characterized in that the chosen organic compound of tellurium comprises at least   at least one organic radical chosen from a primary alkyl group   mayor, a secondary alkyl group, a tertiary alkyl group   tiary, an allyl group, a benzyl group and a group   cycloallyle, linked to the element of group VI.   35. A method of growing a layer of elements of groups II and VI, comprising the steps of sending a current comprising an organic compound of an element of group II; sending a current comprising an organic compound of an element of group VI, comprising at least one organic radical which attracts electrons out of the element of group VI.   36. Method according to claim 35, characterized in that the organic radical is a phenyl radical, directly linked to the element of group VI, comprising deactivating groups orienting in the meta position, chosen from -NO2, -N (CH3) 3 , -CN, -COOH, -SO3H, -CHO, -COOR and -COR, R - 42 -   being chosen from all appropriate groups or elements.   37. The method of claim 36, characterized in that the organic radical is a phenyl radical, directly linked to the element of group VI, comprising deactivating groups orienting in para and ortho positions, replacing   cutting hydrogen into the para and ortho positions, and making both in halogens.