United States Patent (19) 11 Patent Number: 4,833,103 Agostinelli et al. (45) Date of Patent: May 23, 1989

54 PROCESS FOR DEPOSITING A III-V Crystal Growth of III-V Using Coor COMPOUND LAYER ON A SUBSTRATE dination Compounds as Starting Material in the 75) Inventors: John A. Agostinelli; Henry J. MOCVD Process', Journal of Crystal Growth, vol. 55, Gysling, both of Rochester, N.Y. 1981, pp. 135-144. Maury et al., "Raman Spectroscopy Characterization of 73 Assignee: Eastman Kodak Company, Polycrystalline GaP Thin Films Grown by MO-CVD Rochester, N.Y. Process Using Et2Oa-PEt23 as Only Source', Journal 21 Appl. No.: 62,670 de Physique, Colloque C1, Suppl. No. 10, vol. 43, Oct. 22 Filed: Jun, 16, 1987 1982, pp. C1-347 to C1-252. 51) Int. Cl." ...... B05D 5/12 Primary Examiner-Janyce Bell 52 U.S. Cl...... 437/231; 427/108; Attorney, Agent, or Firm-Carl O. Thomas 427/110; 427/123; 427/226; 427/229; 437/234; 437/245 58 Field of Search ...... 427/87, 229, 226, 88, 57 ABSTRACT 427/08, 123, 110,383.7, 383.5, 372.2; 437/104, A process comprising applying to a substrate a thin film 231, 82,234, 245 comprised of a liquid carrier and a precursor selected (56) References Cited from among compounds in which one or more pairs of U.S. PATENT DOCUMENTS group III and V elements are each joined by a thermally stable bond and the group III and V elements are each 3,802,967 4/1974 Ladany et al...... 14.8/171 substituted with two thermally volatilizable ligands. 3,877,982 4/1975 Coldren ...... 437/234 4,250,205 2/1981 Constant et al. ... 427/87 The precursor is heated to a temperature in excess of 4,427,714 l/1984 Davey ...... 427/87 200 C. to remove its volatilizable ligands while leaving 4,510,182 4/1985 Cornils ...... 427/226 a ligand free III-V compound as a monophasic layer on 4,594,264 6/1986 Jensen ...... 427/53.1 the substrate. OTHER PUBLICATIONS Zaouk et al., "Various Chemical Mechanisms for the 10 Claims, No Drawings 4,833,103 1. 2 MOCVD Process', Journal of Crystal Growth, Vol. 55, PROCESS FOR DEPOSTING AII-V COMPOUND 1981, pp. 135-144 discloses a variation on the process of LAYER ON A SUBSTRATE Constant et all wherein elimination of one ligand from each of the III and V elements of the precursor is recog FIELD OF THE INVENTION nized to occur during heating. Maury et al., “Raman The present invention is directed to a processof pro Spectroscopy Characterization of Polycrystalline Gap ducing a layer of a III-V compound. Thin Films Grown by MO-CVD Process. Using Et2. Ga-PEt3 As Only Source', Journal de Physique, Col BACKGROUND OF THE INVENTION loque C1, suppl. no. 10, vol. 43, Oct. 1982, pp. C1-347 References to Group III, IV, and V elements follow O to C1-252, is essentially cumulative with Constant et al art established designations of elements found in groups and Zaouk et al, except for employing polymeric pre 13, 14, and 15, respectively, of the Periodic Table of cursors as starting materials. Zaouk et al and Maury et elements as adopted by the American Chemical Soci all share the disadvantages of Constant et al. ety. Davey U.S. Pat. No. 4,427,714 describes forming Following the discovery of the , semicon 15 III-V compound layers by spraying. For example, gal ductor application interest focused on group IV ele lium arsenide layers are disclosed to be formed by pro ments, first primarily on germanium and then on silicon. cesses including It was later recognized that useful and, for many appli (1) spraying a solution of arsenide or a pre cations, superior properties are pro cursor thereof with an inert gas propellant in a reducing vided by III-V compounds-that is, compounds con gaseous atmosphere; sisting of group III and group V elements. This has led (2) spraying a solution of gallium/arsenic complex to intensive investigations of processes for preparing (each of the gallium and arsenic atoms having three layers of III-V compounds, particularly processes of substituent legands) with an inert gas propellant in an fering the stringent control of III-V compound layer inert or reducing atmosphere; stoichiometry, purity, uniformity, and thickness re 25 (3) creating a stable aerosol of trimethyl gallium dis quired for successful semiconductor applications. The most commonly employed approach for prepar persed in arsine, which is sprayed on a hot substrate; ing III-V compound layers is chemical vapor deposi and tion (CVD), which includes both vapor phase epitaxy (4) spraying a polymeric complex formed between (VPE) and metalorganic chemical vapor deposition 30 trimethyl gallium and methyl/phenyl arsine. All of the (MOCVD). A gaseous compound of a group III ele spraying processes are unattractive, since considerable ment and a gaseous compound of a group V element are unwanted deposition occurs on spray confining walls. introduced into a vacuum chamber and thermally de Thus precursor waste and burdensome cleaning of composed in the presence of a substrate. Although ex equipment is encountered. tensively used, this process exhibits a number of disad 35 It has been recognized that III-V compound layers vantages. First, there is the safety hazard of working can be produced by supplying liquids to substrate sur with toxic gases. Second, each of the group III element faces. Ladany et al U.S. Pat. No. 3,802,967 discloses and group V element compounds are pyrophoric, react first forming a thin III-V compound layer by CVD ing spontaneously with oxygen. Third, with the group techniques and then increasing the thickness of this III and group V elements being introduced as separate layer by conventional liquid phase epitaxy. For in gases, the potential for layers which are stoichiometri stance, in Example 1 a liquid consisting of 97 percent cally unbalanced in either the concentration of the gallium, 2.99 percent , and 0.01 percent group III or group V element is always present, and tellurium is flowed over a 10 micrometer CVD GaAs precise gas metering is required for balanced stoichion layer on a spinel substrate by tipping a graphite boat etry. Fourth, working with high vacuum equipment is 45 containing the liquid and substrate. The temperature of time consuming, cumbersome, and operationally limit the liquid is maintained at 700° C. The Ladany et al 1ng. process, since it begins with CVD, incurs all of the Constant et al U.S. Pat. No. 4,250,205 discloses a disadvantages of that process and in addition is unat variation on the CVD process described above. Instead tractive in requiring very high temperatures for liquid of employing a gaseous compound of a group III ele SO phase epitaxy. ment and a gaseous compound of a group V element as Jensen U.S. Pat. No. 4,594,264 discloses a process for separate precursors for III-V compound deposition, a preparing gallium arsenide layers on monocrystalline, single gaseous precursor is employed which is a coor gallium arsenide or silicon substrates. A gallium-arsenic dinatin compound of one group III element substituted complex is employed of the formula with three volatilizable ligands and one group V ele 55 ment substituted with three volatilizable ligands. Such (I) X3GaAsR3 coordination compounds are also referred to in the art as III-V donor acceptor complexes and as III-V Lewis where acid and Lewis base adducts. Constant et all teaches X is chlorine, bromine, iodine, phenyl, benzyl, avoiding ligand elimination leading to polymeric com methyl, or trifluoromethyl, and pounds. Although the coordination compound ap R is hydrogen, phenyl, benzyl, methyl, or trifluoro proach offers better replication of ratios of III and V methyl. elements and to some extent ameliorates problems of The complex is dissolved in a hydrocarbon or chlori toxicity and oxygen sensitivity, the limitations of using nated hydrocarbon solvent which is free of oxygen, high vacuum equipment for coating remain unabated. 65 sulfur, and nitrogen. The resulting solution is coated as Zaouk et al, "Various Chemical Mechanisms for the a film on the substrate in an amount sufficient to form a Crystal Growth of III-V Semiconductors. Using Coor gallium arsenide layer of from 1 to a few micrometers dination Compounds as Starting Material in the (um) in thickness. The film is then heated to a tempera 4,833,103 3 4. ture of less than 200 C. to volatilize the solvent while referred to simply as III-V compounds. These com avoiding decomposition of the gallium-arsenic complex. pounds are formed of group III (boron, aluminum, gal The next step of the process is to convert the complex lium, and indium) and group V (nitrogen, phosphorus, coating remaining to gallium arsenide by exposing the arsenic, , and bismuth) elements. The III-V coating to ultraviolet (UV) radiation, such as the UV compound layer can contain one or a combination of radiation from a laser. The presence of moisture and III-V compounds. For example, layers of binary III-V oxygen is avoided. All reactions were carried out under compounds, such as aluminum nitride, aluminum phos an inert, dry atmosphere (typically less than 1 ppm phide, aluminum arsenide, aluminum antimonide, gal oxygen content) using purified, dry, oxygen-free sol lium nitride, , gallium arsenide, gal vents. Analysis of a layer produced from a complex of 10 lium antimonide, indium nitride, indium phosphide, C13GaAsOC6H5)3 revealed that it had lost only 70 per indium arsenide, , boron nitride, cent of the carbon and 54 percent of the chlorine of its boron phosphide, boron arsenide, and boron antimon parent coating as measured prior to UV exposure. ide, are specifically contemplated. These residual carbon and chlorine levels are, of course, It is also contemplated that the III-V compound unacceptably high for the majority of semiconductor 15 layer can be formed with a mixture of III-V com SCS. pounds, if desired. For example, in light guides and lasers it is conventional practice to employ ternary SUMMARY OF THE INVENTION III-V compounds, such as gallium aluminum arsenide, In one aspect, this invention is directed to a process with the proportion of aluminum being varied as needed comprising applying to a substrate a precursor of a 20 to modify the layer for its intended light guiding func III-V compound, the precursor consisting of one or tion. Other combinations of ternary III-V compounds more pairs of ligand substituted group III and group V specifically recognized to be useful are gallium arsenide elements and thermally decomposing the precursor. phosphide, indium gallium arsenide, indium gallium The process is characterized in that (i) the precursor is phosphide, indium arsenide phosphide, aluminum gal selected fromamong compounds in which the group III 25 lium phosphide, and aluminum gallium antimonide, as and V elements of each pair are joined by a thermally well as quaternary compounds, such as indium gallium stable bond and the group III and V elements are each arsenide phoshpide. substituted with two thermally volatilizable ligands, (ii) For semiconductor applications the monophasic a thin film comprised of a liquid carrier and the precur layer usually contains minor amounts of one or more sor is applied to the substrate, and (iii) the liquid carrier 30 dopants intended to impart N or P type conductivity. N and ligands are removed from the film, this step includ type conductivity can be achieved by substituting for ing heating the film to a temperature in excess of 200 C. some of the group III elements in the III-V compound to remove the volatilizable ligands while leaving a li an element such as silicon or tin, or by by substituting gand free III-V compound as a monophasic layer on from some of the group V elements sulfur, selenium, or the substrate. 35 tellurium. P type conductivity is achieved by substitut The process of the present invention offers significant ing for some of the group III elements in the III-V advantages over prior art approaches to forming mono compound an element such as zinc, cadmium, beryl phasic layers of III-V compounds. The process of the lium, or magnesium, or by substituting for some of the present invention in employing precursors containing group V elements germanium. Dopant levels are gener pairs of III and V elements is inherently biased toward ally limited to amounts sufficient to impart semiconduc the desired stoichiometric balance of these elements. tive properties. The exact proportions of dopant will The process requires no particular control of ambient vary with the application intended, with semiconductor pressure during any step. Thus, the high vacuum equip dopant levels of from 1015 to 1018 ions per cc being ment required by CVD can be entirely eliminated. At common. For some applications, such as those in which the same time, since spraying is not required to bring the 45 higher than semiconductive levels of conductivity are precursor into contact with the substrate to be coated, required, higher, degenerate dopant levels can be in indiscriminate loss of the precursor and fouling of evidence. It is a distinct advantage of the process of this equipment is avoided. No spray confining walls are invention that all dopant concentration levels conven required, and equipment is readily maintained free of tionally found in III-V compound layers, particularly unwanted depositions. The process of this invention 50 those employed in the fabrication of solid state-e.g., allows thin, uniform layers of ligand free III-V com semiconductor-elements, can be reproducibly pounds to be formed on a variety of substrates. Precise achieved. control of III-V compound layer thickness is readily III-V compound layers of from about 20 to 2000 achieved with the process of this invention. The process Angstroms in thickness can be deposited in a single of this invention allows removal of precursor ligands 55 iteration of the process of the present invention, with (including all of their constituent elements) to leave single iteration layer thicknesses preferably being in the III-V compound layers of purity levels satisfactory for range of from about 100 to 1000 Angstroms. Reiteration semiconductor applications, including those requiring of the process can be undertaken to build up still thicker coatings of highly controlled purity levels. However, layers, if desired. Additionally, where the composition the process of the present invention does not require of the III-V compound layer deposited according to exceptionally high temperatures-i.e., temperatures in this process matches that of the substrate on which it is excess of about 650 C. deposited, the III-V compound layer can form an epi DESCRIPTION OF PREFERRED taxial extension of the initially present substrate. Thus, EMBODIMENTS depending upon the substrate chosen, a single iteration 65 of the process of the present invention can result in a The present invention is directed to processes of III-V compound layer of any desired thickness. forming layers of ligand-free III-V compounds, herein The III-V compound layer is monophasic, meaning after, in accordance with art established usage, also that the layer consists of a single phase of material. The 4,833,103 5 6 layer can be monocrystalline, polycrystalline, or amor R, R2, R3, and R' are independently chosen volatiliz phous, depending upon the manner of its formation and, able ligands; and particularly, the substrate on which it is formed. n is an integer of from 1 to 5. The coating process of the present invention begins Preferred group III elements are boron, aluminum, with the selection of a substrate for coating. Any con 5 gallium, and indium. Preferred group V elements are ventional substrate for a III-V compound layer can be nitrogen, phosphorus, arsenic, and antimony. Specifi selected. While the III-V compound layer can be cally preferred group III and V element combinations formed on any refractory-e.g., tungsten, silicon car are gallium and arsenic, aluminum and arsenic, gallium bide, or the like, it is preferred for semiconductor appli and phosphorus, indium and phosphorus, boron and cations that the III-V compound layer be formed on an 10 nitrogen, and aluminum and nitrogen. insulative or, particularly, a semiconductive substrate. Preferred volatilizable ligands are hydrogen, halogen Exemplary of useful insulative substrates are silicon (e.g., fluoride, chloride, bromide, and iodide), and hy nitride, aluminum oxide (particularly monocrystalline drocarbon and substituted hydrocarbon ligands. The aluminum oxide-i.e., sapphire), and silicon dioxide hydrocarbon ligands can be chosen from among ali (including amorphous, monocrystalline, and glass 15 phatic and aromatic hydrocarbons generally. Among forms). Glasses containing elements in addition to sili useful hydrocarbon ligands are those consisting of sim con and oxygen are contemplated, particularly glasses ple hydrocarbon moieties, such as alkyl, alkenyl, alky having a thermal coefficient of expansion which ap nyl, and aryl groups, as well as hydrocarbon ligands proximates that of the III-V compound layer to be which are composites of simple hydrocarbon moieties, formed-e.g., borosilicate glass and borophosphosili such as alkaryl, aralkyl, aralkaryl, and alkaralkyl cate glass. groups. Hydrocarbon ligand substituents can be se Semiconductive elements, particularly monocrystal lected from among a wide variety of known organic line semiconductive wafers, are highly useful substrates substituents and functional groups, including, but not for the practice of the invention. For example, any of limited to, halogen, hydroxy, mercapto, ether, thioe the monocrystalline group IV or III-V compound wa 25 ther, alkanoyl, ester, thioester, dithioester, amino, fers conventionally employed in the manufacture of amido, carbamoyl, sulfonamido, sulfamoyl, ureido, thi semiconductor elements can be employed as substrates oureido, carboxy, sulfonyl, sulfato, sillyl, and similar for the deposition of a III-V compound layer according groups. It is specifically contemplated that one or more to the present invention. For example, a major face heteroatoms, such as oxygen, sulfur, selenium, tellu lying in a 111}, {110}, or 100 crystal plane of a 30 rium, or nitrogen atoms can with the hydrocarbon sub silicon or III-V compound (e.g., gallium arsenide, gal stituent form a heterocyclic ring. lium phosphide, or indium phosphide) wafer can serve Where it is desired that the ligand be entirely volatil as an ideal substrate for a III-V compound layer. Sub ized and none of its constituent elements employed to strate surfaces which match or at least approximate the modify the III-V compound layer resulting, it is pre crystal habit of the III-V compound forming the layer 35 ferred to limit the ligands to those containing only one to be deposited are particularly advantageous in allow or more of hydrogen, halogen, carbon, and oxygen. ing III-V compound layers to be deposited which are Because of the recognized response of semiconductive monocrystalline and form an epitaxial extension of the layers to trace impurity concentrations, ligands contain original substrate. ing elements which are themselves group III or V ele An important aspect of the process of the present ments or which are known to be useful N or P conduc invention lies in the selection, from among known com tivity type impurity dopants of III-V compounds are pounds, of the precursor of the III-V compound. For generally avoided, except where the ligands are inten the practice of the present invention a precursor is tionally chosen to modify the properties of the III-V chosen consisting of at least one pair of ligand substi compound layer to be formed. tuted group III and group V elements. The group III 45 The primary function of the ligands is to satisfy the and V elements of each pair are joined by a thermally two valence bonding positions of each group III and V stable bond, and the group III and V elements are each element not satisfied by the thermally stable bond be substituted with two thermally volatilizable ligands. tween pairs of group III and V elements. In addition the These precursors differ markedly in their properties ligands are preferably chosen to facilitate coating the 50 III-V compound precursor in a liquid carrier on the from the III-V coordination compounds (III-V donor substrate. By choosing one or more of the ligands to be acceptor complexes and III-V Lewis acid and Lewis a hydrocarbon or a substituted hydrocarbon the vapor base adducts) of the prior art in which each group III pressure of the III-V compound precursor can be re and group V element is substituted with three volatiliz duced, thereby avoiding loss of III-V compound pre able ligands so that only a weak coordination bond 55 cursor molecules from the coating by evaporation. In exists between the group III and group V atoms. addition, the increased bulk provided to the III-V com In one aspect III-V compound precursors herein pound precursor by hydrocarbon ligand inclusion can employed are those represented by formula II: be usefully employed to raise the viscosity of the coat ing composition to optimum levels. On the other hand, (II) 60 if hydrocarbon ligands of excessive bulk are employed, R1 R3 N / the choice of liquid carriers capable of acting as sol M(III)-M'(V) vents can be reduced, and coating composition viscosi ties can be raised beyond optimum levels. t Substituents of the hydrocarbon ligands can be 65 chosen to facilitate dispersion of the III-V compound where precursor in the carrier liquid and to modify the viscos M(III) is a group III element; ity of the coating composition. For example, polar sub M'(V) is a group V element; stituents of the hydrocarbon ligands can facilitate dis 4,833,103 7 8 solving the III-V compound precursor in polar sol vents. In many instances the inclusion of substituents -continued permit inclusion of higher numbers of carbon atoms in the hydrocarbon ligands than would otherwise be at tractive. While individual hydrocarbon and substituted hydro carbon ligands can have up to 20 or more carbon atoms, it is generally preferred to employ hydrocarbon and substituted hydrocarbon ligands containing from 1 to 10 carbon atoms. Specifically preferred hydrocarbon li 10 PC-3 gands are those in which aliphatic moieties consist of from 1 to 6 carbon atoms and aromatic moieties consist of from 6 to 10 carbon atoms. In their simplest from the III-V compound precur sors of this invention contain a single pair of group III 5 and group V elements. For example, in formula II, n is 1. In this instance it can be seen that each group III and PC-4 group V element exhibits only three bonding sites, and there is no weak fourth coordination bond, such as is in evidence in III-V coordination compounds. More par 20 ticularly, the bond between the group III and group V elements is highly thermally stable. The group III and group V bond is, in fact, significantly more thermally stable than that linking the group III and group V ele ments with their ligands. 25 PC-5 When the III-V compound precursor contains more than one pair of group III and group V elements--e.g., when n in formula II is greater than 1, it is generally accepted that the group III and group V elements are conjugated in a cyclic chain or ring. That is, each group 30 III element is bonded to two ligands and to two adja cent group V elements, and each group V element is PC-6 similarly bonded to two ligands and two adjacent group III elements. On heating these polymeric III-V com GameP pound precursors are believed to revert to correspond 35 / N ing monomer (e.g., n = 1 in formula II). C2H5 C2H5 3 In one specifically preferred form III-V compound precursors contemplated for use in the practice of the PC-7 invention are those satisfying formula III:

R R (III) N / Rl M'(V) Rl N / MID M(III) 45 Rl M'(V) Rl PC-8 / N R R where M(III) is a group III element, most preferably gal 50 lium, aluminu, or indium; M'(V) is a group V element, most preferably nitro gen, phosphorus, or arsenic; and PC-9 R and R' are independently hydrogen, halogen, or a hydrocarbon of from 1 to 10 carbon atoms, most prefer 55 ably a hydrocarbon of from 1 to 6 carbon atoms. Specific illustrations of III-V compound precursors contemplated for use in the practice of this invention are the following: Techniques for preparing III-V compound precur sors for use in the practice of the present invention will CH3 As N be readily apparent from the teachings of A.M. Arif, Ga B.L. Benac, A.H. Cowley, R. Geerts, R.A. Jones, K.B. / 65 Kidd, J.M. Power, and S.T. Schwab, "Mono- and Di CH3 As nuclear Phosphido and Arsenido Complexes of Gal lium; Ga(EButt)3, Ga(PH(2,4,6-BulC6H2)3 and Ga(u- EBU2)R22, E = P, As; R =Me, Bun', J. Chem. Soc., 4,833,103 9 10 Chem. Commun., 1986, pp. 1543-1545; A. Zaouk, E. specific carrier liquids are hydrocarbons, including Salvetat, J. Sakaya, F. Maury, and G. Constant, "Vari substituted hydrocarbons, such as halogenated hydro ous Chemical Mechanisms for the Crystal Growth of carbons, which are liquid under ambient conditions-- III-V Semiconductors. Using Coordination Compounds e.g., benzene, toluene, octane, decane, terpenes, and as Starting Material in the MOCVD Process', Journal 5 similar hydrocarbons. Other liquid organic solvents are of Crystal Growth, Vol. 55, 1981, pp. 135-144; F. Maury, contemplated, particularly those that contain only car H. Combes, R. Caries, and J. B. Renucci, "Raman Spec bon, hydrogen, halogen, or oxygen atoms. Alcohols, troscopy Characterization of Polycrystalline GaP Thin such as methanol, ethanol, isopropyl alcohol, and the Films Grown by MO-CVD Process. Using EtGa like are contemplated liquid carriers. Ethers, such as PEt2)3. As Only Source', Journal de Physique, Colloque 10 dimethyl ether and diethyl ether, constitute another C1, suppl. No. 10, vol. 43, Oct. 1982, pp. C1-347 to advantageous class of liquid carriers. Polar solvents, C1-252; and C. G. Pitt, A. P. Purdy, K. T. Higa, and R. such a formamides (e.g., dimethylformamide), tetrahy L. Wells, “Synthesis of Some Arsinogallanes and the drofuran, and water, can be used with precursors polar Novel Rearrangement of a Dimeric Bis(arsino), or polar substituent containing ligands (e.g., polar moi Bis(bis(bis(tri-methylsilyl)methyl)arsinolarsino)- 5 ety substituted hydrocarbons). chlorogallane)", Organometallics, 1986, Vol. 5, p. 1266. The primary function of the carrier liquid is to pro To apply the III-V compound precursor to the sub vide a liquid phase for dispersing the III-V compound strate a liquid coating composition is formed. The coat precursor. The carrier liquid is also chosen for its ability ing composition can be formed merely dissolving the to cover the substrate uniformly. Thus, an optimum precursor into a carrier liquid. To achieve the highest 20 liquid carrier selection is in part determined by the possible level of dispersion of the precursor in the car substrate chosen. Generally more desirable film forming rier liquid, it is preferred to employ carrier liquids properties are observed with more viscous carrier liq which are solvents for the precursor. Generally useful uids and those which more readily wet the substrate coating compositions can be formed when the precur alone, or with an incorporated volatilizable film form sor forms greater than 1 percent by weight of the coat- 25 ing agent. In one form the film forming agent can take ing composition, preferably at least about 5 percent by the form of a wetting agent, such as a volatilizable sur weight of the coating composition, based on total factant. Depending upon the composition of the atmo weight. The precursor can be introduced into the car sphere chosen for carrier liquid volatilization, other film rier liquid up to its solubility limit, if desired. A gener forming agents can be employed. For example, organic ally optimal precursor concentration range in the liquid 30 compounds capable of being volatilized in a reducing carrier is from about 10 to 15 percent, based on total (e.g., hydrogen) atmosphere can be employed. weight. After the III-V compound precursor coating has The next step is to apply the coating composition been formed on the substrate and preferably after the uniformly to the substrate. Any one of a variety of liquid carrier has spontaneously evaporated or been conventional coating techniques known to be capable of 35 removed by mild heating, the coating is heated to vola laying down thin, uniform coatings can be chosen for tilize the ligands. Although precursor decomposition this purpose. For example, immersion or dip coating can actually begin at ambient temperatures or tempera can be employed. A preferred technique for forming a tures only slightly above ambient, to eliminate the li thin, uniform coating on the substrate is by spin coating. gands in their entirety (apart from selected elements This technique, which is widely used in forming thin, 40 intentionally retained to modify the III-V compound) it uniform photoresist composition coatings on semicon is necessary to heat the substrate and coating to a tem ductors, involves applying a small amount of the coat perature in excess of 200 C. ing composition to the substrate and then rotating the The success of the process of this invention in remov substrate. The centrifugal forces exerted by the spinning ing hydrogen, halogen, and organic ligands is attributa motion distribute coating composition uniformly on the 45 ble to the much higher temperatures required to cleave substrate and result in excess coating material being the thermally stable group III and V element bonds expelled from the substrate surface along its edges. By than the ligand bonds. A temperature range is available controlling the rate of spinning and the viscosity of the from about 200 C. to about 450° to 650 C., depending coating composition, the thickness of the stable coating upon the III-V compound selected for preparation, in remaining of the substrate can be controlled. When the 50 which ligand volatilization can be effected without concentration of the III-V compound precursor in the cleaving bonds between group III and V elements. It is coating composition is further taken into account, the usually preferred to heat the III-V compound precur coating required to produce a III-V compound layer of sor coating to a temperature in the range of from about a desired thickness can be routinely ascertained. 250 to 450° C., optimally from about 300 to 400 C. Preferred carrier liquids are those which are solvents 55 With more thermally resistant III-V compounds this for the III-V compound precursor. In addition, the range can be increased and with more readily volatil carrier liquid is preferably chosen to avoid reaction ized. ligands the lower temperature can approach 200 with the precursor. That is, a relatively nonreactive or C. as a lower limit. inert carrier liquid is preferably chosen. It is also advan As is well understood in the art, III-V compounds tageous to choose a carrier liquid that is readily volatil when heated well above ambient temperature are sus ized after coating. For example, carrier liquids which ceptible to oxidation. It is therefore contemplated that evaporate on standing under ambient conditions are the heating step to volatilize the ligands will be under advantageous, although carrier liquids that exhibit sig taken the presence of an inert or reducing atmosphere, nificant evaporative loss prior to coating are generally such as nitrogen, argon, hydrogen, carbon monoxide, or avoided. An alternative is to choose carrier liquids 65 a mixture of these or similar gases. which are volatilized on heating prior to or along with Following heating the III-V compound layer and its the ligands in the course of converting the precursor substrate can be brought back to ambient temperature into the desired III-V compound layer. Illustrations of by any convenient method. To minimize thermal stress 4,833,103 11 12 on the product article it is generally preferred to return the precursor is selected from among compounds in to ambient temperatures gradually. Generally the mag which the group III and V elements of each pair nitude of thermal stress increases with the area of the are joined by a thermally stable bond and the group layer being prepared. Thus, in working with larger III and V elements are each substituted with two diameter wafers annealing and slow rates of cooling are thermally volatilizable ligands, desirable. It is also desirable to maintain the layer in a film comprised of the precursor and a liquid carrier, contact with the inert or reducing atmosphere until which is a solvent for the precursor and which can temperatures below about 100° C. are reached. be entirely volatilized, is applied to the substrate Upon cooling the III-V compound layer can be put and spread to form a uniform coating by rotating to use without further processing, if desired. For exam O the substrate, and ple, the III-V compound layer can be used as formed as the liquid carrier and ligands are removed from the a light guide. In most instances further steps are under film, this step including heating the film to a tem taken to produce a desired end product, such as a semi perature in excess of 200 C. to remove the volatil conductor device. These steps can be carried out in any izable ligands while leaving a ligand free III-V convenient conventional manner for acting on conven 15 compound as a monophasic layer on the substrate. tional III-V compound coatings and therefore require 2. A process according to claim 1 further character no detailed description. ized in that the Group III elements are chosen from the EXAMPLES group consisting of boron, aluminum, gallium and in dium. The invention can be better appreciated by reference 20 3. A process according to claim 1 further character to the following specific examples: ized in that the Group V elements are chosen from the group consisting of nitrogen, phosphorus, arsenic, and EXAMPLE 1. antimony. The utility of the organometallic compound PC-4 as 4. A process according to claim 1 further character a thermal precursor for Gap was initially evaluated by 25 ized in that the ligands are chosen from the class consist heating a solid sample of PC-4 for 3 hours at 500 C. in ing of hydrogen, halogen, hydrocarbons, and substi a flow of argon. tuted hydrocarbons. The resulting black solid was identified as Gap by its 5. A process according to claim 1 further character powder X-ray diffraction pattern (i.e., by comparison ized in that the film is heated to a temperature in the with authentic GaP pattern JCPDS #12-191) and its 30 Raman spectrum. The Raman specturm contained range of from 200 to 650 C. strong absorptions at 356 and 389 cm and no absorp 6. A process according to claim 1 further character tions in the spectral regions where residual C-H or ized in tht the precursor satisfies the formula amorphous carbon would be expected to give absorp tions-e.g., see S. Hayashi, Solid State Commun., 56, 35 Rl R 375 (1985). N / EXAMPLE 2 M(III)-M'(V) Gallium phosphide was formed on a glass substrate R2 R4 by coating an 8% by weight solution of PC-4 in toluene on glass substrate and heating the coated layer for 1 where hour at 425 C. in a flow of argon. Examination of the M(III) is a group III element; resulting layer by Raman spectroscopy showed the M'(V) is a group V element; presence of GaP. R, R2, R3, and Rare independently chosen volatiliz 45 able ligands; and EXAMPLE 3 n is an integer of from 1 to 5. This examples illustrates the thermal decomposition 7. A process according to claim 6 further character of PC-1 to give gallium arsenide. ized in that the film is heated to a temperature in the The utility of PC-1 as a thermally decomposable range of from 250 to 450° C. precursor for GaAs was demonstrated by heating a 50 8. A process according to claim 6 further character sample of PC-1 in an argon atmosphere for 1 hour at ized in that the substrate is a glass or monocrystalline 500 C. substrate. The resulting black solid was identified as monopha 9. A process comprising sic GaAs by X-ray diffraction and comparison of results applying to a substrate a precursor of a III-V com with authentic GaAs pattern JCPDS #32-389. 55 pound, the precursor consisting of one or more The invention has been described in detail with par pairs of ligand substituted group III and group V ticular reference to preferred embodiments thereof, but elements and it will be understood that variations and modifications thermally decomposing the precursor, can be effected within the spirit and scope of the inven characterized in that tion. a film comprised of the precursor and a liquid carrier, What is claimed is: which is a solvent for the precursor and which can 1. A process comprising be entirely volatilized, is applied to the substrate applying to a substrate a precursor of a III-V com and then spread to form a uniform coating, pound, the precursor consisting of one or more the liquid carrier and ligands are removed from the pairs of ligand substituted group III and group V 65 film, this step including heating the film to a tem elements and perture in excess of 200 C. to remove the volatiliz thermally decomposing the precursor, able ligands while leaving a ligand free III-V com characterized in that pound as a monophasic layer on the substrate, and 4,833,103 13 14 the precursor satisfies the formula: -continued t-C4H9 t-C4H9 N / R R Rl Y6 R1 N / N / M(III) M(III) R^ Yevy Rl / N R R O N Cl where N / M(III) is a group III element chosen from the class Ga / N consisting of gallium, aluminu, or indium; Cl M'(V) is a group V element chosen from the class / consisting of nitrogen, phosphorus, or arsenic; and 15 R and R' are independently hydrogen, halogen, or a hydrocarbon of from 1 to 10 carbon atoms. 10. A process according to claim 9 further character GameP ized in that the precursor is chosen from the class con 20 / N sisting of

CH3 As 25 N Ga / CH3 As

30

As

As 35

CH3 H3

45

50

55

60

65