TECHNIQUE FOR THE GROWTH OF EPITAXIAL COMPOUND SEMICONDUCTOR FILMS

Abstract: A technique for the deposition of group III-V compound semiconductor films in epitaxial form wherein the group V source material employed is in solid elemental or compound form. The prime advantage of such technique resides in the elimination of the need for the highly toxic arsine gas for this purpose while permitting the preparation of a product essentially free of contamination.

Description: TECHNIQUE FOR THE GROWTH OF EPITAXIAL COMPOUND SEMICONDUCTOR FILMS This invention relates to the growth of epitaxial films comprising group III-V semiconductor compounds. More particularly, the present invention relates to a method for the deposition of epitaxial layers of group III-V semiconductor compounds by means of an organometallic chemical vapor deposition process. During the past decade, there has been a burst of interest in the generation of techniques for the epitaxial growth of compound semiconductors. Among the more popular of such techniques is that which is commonly termed the organometallic chemical vapor deposition process (OMCVD) . In the implementation of such technique, it has been common to utilize hydrides such as arsine, which is arsenic trihydride (AsH_) and phosphine which is phosphorous trihydride (PH 3 ) as the source material of the group V element, or, alternatively, a combination of such hydrides with an organometallic compound. Unfortunately, these techniques have not proven to be entirely satisfactory in that the presence of oxygen and water vapor in the AsH_ and PH, tend to be deleterious. Furthermore, and of greater significance is the fact that the use of these hydrides frequently leads to toxicity problems. In accordance with our invention these prior art limitations are obviated by a process which involves deposition of the group III-V semiconductor compound in a conventional cold wall reactor using as a source material a solid source of the group V element in combination with an organometallic compound of the group III element. Heretofore, workers in the art' would not have contemplated the use of solid sources for this purpose since it was widely accepted that vapors of the group V elements could not be transported in a compatible fashion with vapors of the group III organometallic compounds, the latter decomposing at vaporization temperatures of the group V element. We have now discovered that this limitation may be overcome by initially maintaining the vapor streams in different conduits and upon their confluence diluting the group V element vapors with hydrogen to prevent condensation thereof as the temperature is lowered to a temperature which is compatible with the vaporized organometallic compound. Our invention will be more readily understood by reference to the accompanying drawing the single Figure of which is a front elevational view, in cross-section, of an apparatus suitable for use in the practice of the present invention. With reference now more particularly to the drawing, there is shown a conventional cold wall reactor 11 having disposed therein a quartz block 12, a susceptor 13 which serves as a support for a substrate wafer 31, and a quartz support member 30 for susceptor 13. Reactor 11 is adapted with an inlet conduit 14, an exhaust conduit 15 and a cap member 16 which serves as a seal at the exit end of reactor 11. Rf coil 17 is provided around the midsection of reactor 11 for the purpose of heating the susceptor during the reaction. A heating tape 18 is also provided around the inlet end of the reactor for the purpose of obviating the likelihood of the group V element condensing during the course of the reaction but it does not heat this region sufficiently to cause parasitic reactions between the group III organometallic compound and the vapors of the group V element. The temperature in this region is within the range of 200-300°C. Also included as part of the apparatus used in the practice of the invention is a vaporizing furnace 19 for vaporization of the group V source material, furnace 19 having disposed therein boat 20 for containing the source material. boat 20 being coupled with quartz tube 21 having a soft iron bar 22 sealed therein. A hook means 23 connected to tube 21 permits the magnetic movement of the boat during the processing sequence. Furnace 19 also includes inlet conduit 24, a cap 25 to seal the inlet end and exit conduit 26 which is connected to inlet conduit 14 of cold wall reactor 11. Heating of the source material exiting from furnace 19 is effected by means of a heating tape 27 wrapped around exit conduit 26. Also shown connected to conduits 26 and 14 is conduit 28 for introduction of organometallic compounds and hydrogen to reactor 11. A brief description of the procedure followed in the practice of our invention will now be given. Initially, a substrate member is selected upon which deposition of the desired epitaxial films will be effected. Substrates suitable for this purpose may be conductive or insulating in nature, ultimate selection being dependent upon the type of device contemplated. Typical materials found suitable for this purpose are gallium arsenide, indium phosphide and the like. These substrates may conveniently be obtained from commercial sources. Prior to insertion of the substrate wafer 31 upon the surface of susceptor 13, the substrate is subjected to a conventional degreasing and etching sequence, thereby assuring the presence of a clean surface for deposition. A typical cleansing sequence would involve degreasing with trichloroethylene, acetone and methyl alcohol followed by etching with a 3:1:1 mixture of sulfuric acid, hydrogen peroxide and water. Etching is continued for a time period sufficient to remove approximately 2 microns from the surface of the substrate. The degreased, etched substrate 31 is then placed upon susceptor 13 in cold wall reactor 11. In accordance with an aspect of our invention, a group V source material in solid, elemental or compound form of high purity is selected. For this purpose, it is desirable to employ the material in a form of the highest purity available, typically 99.9999+% purity. Arsenic, gallium arsenide polycrystals, indium arsenide polycrystals, indium phosphide polycrystals, and phosphorous are the materials most commonly employed for this purpose. The source material so selected is then placed in boat 20 in vaporizing furnace 19. The weight of the source material is not considered critical, the only requirement being that sufficient material be present to obtain the desired III-V compound. The source selected may be obtained in solid form from known commercial sources or it may be generated in situ by cracking halides such as arsenic trichloride or phosphorous trichloride and condensing the arsenic or phosphorous vapors prior to a deposition run. In this latter case, the furnace 19 is made movable and the movable boat eliminated. In the operation of the process, the source material in boat 20 is heated with hydrogen flowing into the furnace via inlet 24. Heating is effected at a * temperature sufficient to vaporize the solid source, the hydrogen serving as a means for transporting the vaporized source to reactor 11. The amount of vapor which is transported determines the carrier -type and concentration level, the morphology of the deposited layer and the growth rate. These parameters must be determined experimentally for each system, such being dependent upon considerations relating to the intended use of the deposited film. Heating tape 27 is heated to a temperature equal to that of furnace 19, thereby assuring that source vapors leaving furnace 19 will not condense in the conduit leading to cold walled reactor 11. Simultaneous with heating of the source material, hydrogen is bubbled through a liquid organometallic compound of a group III element at a temperature a few degrees lower than room temperature or lower depending on the vapor pressure of the group III organometallic compound, so resulting in vapors of the organometallic compound being transported via conduit 28 past the region 33 of confluence with the vapor from solid source 20 and through conduit 14 into the growth region in reactor 11. Transport of the organometallic compound may be regulated by controlling the rate at which hydrogen is bubbled through the organometallic compound. The hydrogen introduced through conduit 28 serves to dilute the group V vapors in the region 33. It will be understood by those skilled in the art that during the growth sequence it is feasible to alter the source of the group III compound and/or add suitable dopants to the system such as hydrogen selenide, silane, diethylzinc and the like. The substrate material 31 contained in the cold wall reactor 11 is heated to a temperature sufficient to permit growth of the desired compound semi-conductor. In general this temperature may range from 475-750°C with a general preference , being found for a range of 600-750°C, such range being dictated by considerations relating to layer quality. Epitaxial films prepared in accordance with the described procedure may be used in a wide variety of device applications .which will be readily appreciated by those skilled in the art. Typical of such devices are field effect transistors, light emitting diodes, lasers, etc. An example of the practice of the present invention is set forth below. It will be understood that this example is solely for purposes of exposition and is not to be construed as limiting. This example describes the growth of a gallium arsenide epitaxial film utilizing an apparatus of the type shown in the Figure. The substrate employed was a semi- insulating chromium doped gallium arsenide wafer oriented six (6) degrees off the (100) crystalline surface toward the (lll)A surface. The source material chosen was 99.9999% purity elemental arsenic, obtained from commercial sources. The arsenic source material was placed in the boat of the vaporizing furnace and heated to a temperature of approximately 450°C. The susceptor in the cold wall reactor was then heated using radio frequency induction to a temperature of 650°C, the susceptor being heated without substantially heating the quartz reactor tube above the wafer of gallium arsenide. High purity hydrogen was then passed through the vaporizing furnace at a flow rate of 6 liters per minute, so resulting in a flow velocity of about 25 cm./sec. at the leading edge of the susceptor. The amount of arsenic transported to the deposition region ranged from 0.025 grams/min to 0.125 gra s/min corresponding to an arsenic source temperature of 425- 470°C. Hydrogen was then bubbled through trimethyl gallium and trimethyl aluminum and the resultant vapors transported to the cold wall reactor. At the deposition site, a trimethyl gallium partial pressure of 1x10 -4 was established to commence growth. Growth was initiated at a rate of approximately 0.125 μm/min. The resultant grown layer was p-type and had a carrier concentration of approximately 2x10 15 cm-3. Studies of the reaction described in that foregoing example revealed that deposition was dominated by the decomposition reaction of the As. molecule produced by heating solid arsenic. This was found to result in deposition characteristics markedly different from those observed with the commonly used arsine. This conclusion is supported by the fact that a (100) facet was observed on an epitaxial layer deposited on a 6° off (100) toward (111)A substrate, the layer being of p-type when using a solid arsenic source. However, when arsine is used, no (100) facet is observed and the resultant layers are under similar growth conditions generally of n-type. As indicated above, the prime advantage of the described process resides in the elimination of the use of the highly toxic arsine gas. However, ancillary benefits include the elimination of parasitic reactions between the group V source and t-he organometallic compounds, such reactions being of particular concern with indium containing organometallics. Lastly, it will be appreciated that optimization of the deposition parameters permits deposition of any of the group III-V compounds or their alloys.

Claims: CLAIMS 1. A method for the growth of group III-V epitaxial semiconductor compounds upon a substrate which comprises reacting vapors of a group V source material with a group III compound at compound semiconductor growth temperatures characterized in that the group V source material is a group V material in elemental or compound soU- d form. 2. A method as in claim 1, characterized in thaϋ said group III compound is an organometallic compound. 3. A method as in claim 1, characterized in that said group V source material is arsenic or gallium arsenide?• 4. A method as in claim 3 , characterized in that- said group III compound is trimethyl gallium. 5. A method as in claim 3, characterized in that said arsenic is heated to a temperature of approximately 450°C. 6. A method as in claim 3, characterized in that- said substrate is heated to a temperature within the range of 475-750°C. 7. A method as in claim 6, characterized in that said group III compound is trimethyl gallium. 8. A method as in claim 1, characterized in that the vapors of a froup V source material comprise a first vapor stream of the group V material in elemental or compound sol d form, the group III compound is in the form of a second vapor stream and, upon confluence of said first and second vapor streams, said first vapor stream is diluted with hydrogen to prevent condensation thereof as the temperature is lowered to a temperature compatible with said second vapor stream. 9. A method as in claim 8, characterized in that said " group III compound is an organometallic compound. 10. A method as in claim 9 , characterized in that said substrate is a gallium arsenide wafer, said group V material is elemental arsenic, and said group III compound is trimethyl gallium. -Si ¬ ll. A method as in claim 8, characterized in that said second vapor stream contains said hydrogen to dilute said first vapor stream upon confluence therewith. 12. A method as in claim 11, characterized by expos said substrate to said confluent vapor streams, thereby causing deposition from said vapor streams onto said substrate.

Inventors: BHAT RAJARAM, COX HERBERT MICHAEL, BHAT, RAJARAM, COX, HERBERT, MICHAEL
Applicants: BELL COMMUNICATIONS RES
Application year: 1985
Main classification: H01L21
IPC codes: C30B25, H01L21