Liquid phase epitaxial process
United States Patent Weyrieh et al.  
 Inventors: (laus Weyrieh, (iauting; (iiinter Winstel. ()ttuhrunn, huth of (iermany Assignee: Siemens Aktiengesellsehalt, llerlin and Munich. (iermany l-&#39;iletl: Sept. 27, I973 Appl, Nu; 4tll,4l4  
 I30] Fnreign Application Priurity Data Sept. 28, W71 (iermany i e v e i i v i 22-17711) [52] [1.8. (l. i. l48/l7l; l4H/l72; Hit/I73; ll7/Zttl; 252N123 (5A [SI I Int. (&#39;l. lltlll 7/38 X| Field of Search l4H/l7l I72 I73; I l7/2tH; 252N113 (IA l5t References (&#39;itetl llNl&#39;ll-ll) S&#39;I&#39;A&#39;IliS lA&#39;ll-IN&#39;I&#39;S l t|il,)( Hi l/ll&#39;lZ Jarvela et I&#39;m/l7} X HIHLUU 0/1072 Dusen et .&#39;|l......l l4H/l7l 1451 Apr. 29, I975 llmrm&#39;t, /l tft&#39;lll, ur Firm llill (iruss, Simpson. Van Sattten Steatlman, (&#39;hiara 8L Simpsnn {57] ABSTRACT The inventiun emnprises a prueess for the prmluetinn ul a layer which is epitaxially gruwn on a surface til at suhstrate in whieh the material which is tu he epitaxially grown is hruught intu contact with the surface til the suhstrate, and in which at a high temperature sume ul the material at the surface of the suhstrate is ilissulvetl in a melt aml in the liquid phase the melt l&#39;urms a surface cuntaet with the surlaee til the substrate, and in whieh the material til the melt is eum pelletl l0 assume the t&#39;urm ul :1 thin layer on the sur l&#39;aee til the suhstrate hy means ul a hmly whuse material at the surface with which the lmtl) enntaets the liquitl phase is inert in relation In the material til the liquitl phase 9 flaims. 6 Drawing Figures LIQUID PHASE EPITAXIAL PROCESS FIELD OF THE INVENTION It is known to epitaxially deposit semiconductor material on a substrate. The substrate consists, in particular, of the same material as that to be deposited or of a material which differs only in respect to its doping. During the deposition process, the material which is to be deposited is first arranged on the surface of the substrate in the form of a melt. The melt is subsequently allowed to slowly cool for the deposition of the material.  
  The melt is, conventionally, poured&#34; onto the surface of the substrate by a tipping process. In the epitaxy of gallium arsenide and/or phosphide layers on a substrate consisting of gallium arsenide and/or phosphide, generally one uses a melt containing gallium in which the material of the layer is dissolved. For the deposition process, the melt is first heated to a high operating temperature which is dependent upon the material and amounts generally to approximately 600 900C for gallium arsenide and 900 1200C for gallium phosphide. Even when it is already saturated with the material of the substrate body to a greater or lesser extent, the melt, when heated to such a temperature, then dissolves the surface of the substrate body and forms an intimate contact with this surface. During the subsequent cooling of the melt, the material which is to be deposited and which is dissolved in the melt is epitaxially grown.  
  Additional details of the prior art may be found, for example, in RCA Review&#34; 24 (1963), at page 603 et seq. German printed Pat. application No. 2 039 172 laid open for public inspection describes a process which is known as shift epitaxy in which semiconductor material to be deposited on a substrate wafer takes the form of a melt. The material to be deposited is already in the melt before the shift processing step is effected, i.e., before this melt is brought into contact with the surface of the substrate wafer. Above the melt is arranged a body which touches the upper surface of the melt so that the melt assumes the shape of a thin zone. No concrete statements are made on the thickness of the zone. The material which is to be deposited on the substrate wafer is not only obtained from the supply in the melt, but further material to be deposited is taken from the body arranged above the melt which serves as a source of this material, and is deposited on the substrate wafer. The deposition takes place under temperature conditions which are uniform in respect to time. For the deposition, a temperature gradient is provided, the body arranged above the melt being maintained at a higher temperature than the substrate wafer, so that the transport of the material to be deposited is in this case effected by thermo-diffusion. [n the event that the body arranged above the melt does not itself serve as source for the semiconductor material, it is provided that this body is porous, and that the semiconductor material provided by way of additional supply is introduced in a gaseous state through the body into the melt.  
 BRIEF SUMMARY OF THE INVENTION The present invention provides a novel epitaxy process which is more advantageous than the prior art.  
  The present invention is characterized by the fact that the material forming the melt is brought into contact with the surface of the substrate at a temperature which is very much lower than that of prior processes, and it is then heated to a high temperature wherein some substrate material dissolves in the melt. Thereafter, by temperature reduction this dissolved material is redeposited on the substrate as an epitaxial layer. The thickness of the layer of the melt is maintained between predetermined upper and lower limits by a cover plate formed of an inert material to the melt. The upper limit of the thickness of the layer from the melt (for instance gallium) which contains the material to be deposited (for instance gallium arsenide) should not be thicker than the length of the diffusion distance which the material to be deposited can cover during the period when the temperature decrease for the epitaxial depositing takes place. This is to be understood that it is assured that the material to be deposited, which at the predetermined high temperature is evenly distributed in the melt, has sufficient time to move from a zone which is most remote from the substrate surface down to this surface, namely, within the period of the temperature decrease.  
  In the invention, the material which is to be epitaxially deposited is first dissolved from the substrate wafer and then, as a result of temperature reduction, is redeposited, from the melt in which it is dissolved, onto the substrate.  
  ln accordance with the invention a measured quantity of the melt which is to be brought into contact with the surface of the substrate body is applied to the surface of the substrate body and the afoorementioned body is placed onto this material as a cover. At the aforementioned high operating temperature on which the cooling is based, for the intiation and execution of the deposition process, the material which is to be deposited is in dissolved form, in the liquid phase. In accordance with this embodiment of the invention, the cover is so contrived in terms of weight and its area which faces the melt that it floats on the melt. In actual fact the cover rests on the liquid phase material, and the cohesion and adhesion forces of this material prevent the cover completely crushing this material by its pressureThe arrangement of this cover results in a uniformly thin layer, in accordance with the invention, of the liquid phase, on the surface of the substrate body. If the quantity of the material of the melt which was applied was too great, this material is pushed away over the edge of the cover and/or over the edge of the substrate wafer by the cover in the liquid phase.  
  In accordance with another embodiment ofa process in accordance with the invention, the cover is arranged on spacers above the surface of the substrate body in such a manner that the given uniform distance is set between cover and substrate body. In this embodiment, the cover possesses such a heavy weight and such a crosssection that the aforementioned cohesion and adhesion forces of the melt are not able to alter the prede termined distance between the cover and the substrate body. When gallium is employed as a component of the melt it is advantageous to introduce a sufficient quantity of this element into the space between substrate body and cover to fill up this space. This measure can be carried out in a particularly determinate fashion by means of a calibrated syringe since gallium liquifies even as low as 29C. The last described measure can also be used in a corresponding fashion for other materials of the melt, taking into account the relevant melting point thereof, when any additional material contained in the melt such as, e.g., dopants, are in a suffi ciently finely distributed form even at the insertion temperature.  
  When the above mentioned thin layer of the melt is used, it is advantageous also to carry out a doping process through the gas chamber. Exploiting the principle of the invention, it is even possible to carry out this in one operation i.e., i is possible to consecutively grow layer sequences of differing conductivity characteristics. Layer sequences of opposite conductivity characteristics are required, for example, for luminescence diodes and laser diodes. The simplicity which results from the application of the process in accordance with the invention consists in the fact that in spite of the fact that the liquid phase epitaxy principle is applied, it is nevertheless possible to change the doping through the gas chamber. These measures may be carried out both in an open system and also in a system in a vessel which is sealed to a greater or lesser extent.  
  Another aspect of the present invention is embodied in a device which serves to carry out a process in accordance with the invention as described above. Further details on the process in accordance with the invention, the embodiments thereof, and a device as mentioned above and the embodiments and further developments thereof are disclosed in the description of Figures of preferred exemplary embodiments.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration of a substrate carried in a recess of a supporting carrier, and upon which substrate the material of a melt has been placed;  
  FIG. 2 is a diagrammatic illustration of a substrate carried in a recess of a supporting carrier with a melt on the substrate and with a floating inert cover on the melt;  
  FIGSv 3 and 4 diagrammatically represent different embodiments wherein the cover is supported on the edge wall of the carrier after forming the melt into a predetermined thickness;  
  FIG. 5 diagrammatically illustrates an arrangement for carrying out the invention of FIG. 4 in a furnace; and  
  FIG 6 is a diagram, not to scale, showing the temperature characteristics in relation to time on the substrate. of a preferred embodiment of the invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, this drawing schematically illustrates how a limited quantity of the material of a melt 1 may be distributed on the surface 2 of a substrate body 3, and as a result of the cohesion forces of the material of the melt in the liquid phase and of the adhesion forces between this material and the material of the substrate body, the melt will form a layer on the substrate. It is conceivable that cases might exist in which the characteristics of the cohesion and adhesion forces will lead to a different result, L6,, to a distribution in the form ofa thin film on the surface 2. This case does not occur in particular when gallium is a basic component of the melt. Gallium generally is being used for epitaxia] layer production of gallium arsenide and gallium phosphide. The quantity of the melt l of the further material as shown in FIG. 1 is the amount which is to be applied to the substrate body 3 in accordance with a process of the invention, as a thin layer. 4 identifies a carrier body which consists of a material which possesses stability of form and which is resistant to any gases or vapors which may be present, even at the high est operating temperatures. This material is also selected to be such that it does not give rise to any disturbances as a result of reactions with the material of the substrate body 3, the material of the melt l or any other materials present in gaseous form.  
  FIGS. 2, 3 and 4 show different specific embodiments and details of devices serving to execute a process in accordance with the invention. FIG. 2 again shows a carrier body 4 on which the substrate body 3 is arranged in a recess 15. The surface 2 of the substrate body is, however, entirely covered with the material of the melt 21 in the exemplary embodiment represented in FIG. 5. The melt 21 consists of the same quantity as the melt l, and this will in fact serve to explain a principle of the invention. As a result of the so to speak floating cover 26 arranged on the melt 21, above the surface 2, the melt 2] assumes the form of a thin layer as is represented. As stated above, the measurements and properties of the cover 26 depend upon the physical properties of the melt. Any excess melt material is pushed away to the side over the edge of the substrate body 3, as represented by the reference numeral 21 l. The illustration in FIG. 2 is to explain the principle of an embodiment of a process in accordance with the invention. Therefore, further details such as means which serve to safeguard the cover 26 from slipping, have been omitted in this illustration.  
  One proceeds in a corresponding fashion to the previously known liquid phase epitaxy process, (ensuring that the represented arrangement and in particular the melt 21 is heated to a sufficiently high degree) to dissolve a portion of the substrate body from the surface 2 by means of the melt. The higher the temperature of the melt, the more mu aerial of the substrate body is dissolved therein. In th-. subsequent cooling of the melt, the previously dissolved material of the substrate body is redeposited in known fashion as an epitaxially grown layer on to the substrate body.  
  The cover 26 or at least the surface 261 thereof, which rests on the melt 21 consists of a material which is inert, vis-a-vis, the melt and the other materials in the reaction chamber, and indeed, even at the highest occuring temperatures.  
  In a specific embodiment where a substrate body of gallium arsenide and/or phosphide and a melt 21 of gallium is employed, it is advisable to employ a material such as graphite, quartz, aluminum oxide, silicon carbide or boron nitride for the carrier body 4 and for the cover 26.  
  A fundamental principle on which the invention is based consists in the fact that as a result of the very thin, (e.g., only I mm thick) layer of the melt 21 on the surface 2 of the substrate body, such as for example, a layer 1 mm thick, only a small quantity of the material of the melt is arranged on the surface which although sufficient to dissolve the immediate surface 2 of the substrate to form a union (which is favorable for the subsequent epitaxial deposition), it is not sufficient to dissolve the entire substrate body 3. In the prior art example shown in FIG. 1, it was possible for the entire substrate body to be completely locally dissolved in the region of the melt l, by the latter, before the high tem perature which forms the basis of the cooling, had been reached by the supply of heat from the exterior. This has been the reason why previously the melt was not tipped onto the substrate wafer until the maximum temperature was reached.  
  In the description of the present invention, thin is to be understood, in particular, as a layer thickness whose upper limit is determined by the path covered by the material which is to be deposited and which is dissolved in the liquid phase, during the period of the growth in the liquid phase, as a result of diffusion. The lower limit is governed in particular by the predetermined thickness of the layer which is to be grown.  
  The advantage associated with the invention consists not only in the fact that it enables the tipping process which is technically difficult to handle and the requisite devices to be avoided, but, in particular, an advantage is also achieved by the fact that in the process of this invention, the entire substrate surface is covered by the melt from the very beginning of the main period of the heating and in the case of gallium, from the melting point thereof, i.e., 29C onwards. Thus, in the process in accordance with the invention. the entire surface of the substrate body is constantly protected from the environmental atmosphere even during the heating period. This is of particular importance when reactive dopants, such as oxygen, are to be introduced into the gaseous atmosphere.  
  By a process in accordance with the invention, it is possible to achieve a particularly uniform thickness of the layer to be grown, by adjusting the distance between substrate and the body or cover which limits the thickness of the layer of the melt.  
  In the embodiments represented in FIGS. 3 and 4, the cover 36 and 46 respectively are provided with rests on the carrier body 34 and 44, respectively, edge to edge.  
  In the example shown in FIG. 3, a compensating volume 37 is provided which is arranged between the cover 36 and the carrier body 34 at one or several points of the edge. When the melt 31 has been applied to the substrate wafer 3, and the cover 36 has been placed in position, excess material 311 of the melt 31 is pushed away by the weight of the cover into the overflow region 37. In accordance with the invention, the melt 31 is arranged in the form of a thin layer above the substrate body 3. In this embodiment, as also in the embodiment shown in FIG. 2 and the embodiment shown in FIG. 4, which remains to be described, the material ofthe melt 31 can also initially take the form of a powder or a granular material, or a paste, which is transformed into liquid form during the course of the heating. If an atmosphere prevails which is likely to impair the substrate body, it must be ensured, by appropriate selection, that the material ofthe thin layer assumes the liquid phase at the latest at the instant in which the atmosphere could have a damaging effect on the material of the substrate body 3.  
  In FIG. 4, the cover 46 contains at least one aperture 461, through which the material of the melt 41 can be introduced into the area between the carrier body 44 with the substrate wafer 3 and the cover 46. In the event that the material of the melt has already been previously introduced into this area, any excess melt material is pushed out through the aperture 461 as a result of the pressure ofthe cover. Thus, an aperture 461 provided in the cover 46 can possess different functions depending upon the individual circumstances. The distance between the under surface of the cover and the recess in the carrier body 44, and between the surface 2 of the substrate 3 is again in accordance with the thickness which is to be provided, for the layer of the melt.  
  In accordance with one form of the invention, the cover 46 contains a large number of apertures. Apart from the significance of these apertures which has already been explained above, a large number of apertures are particularly favorable in order to enable a gaseous dopant to penetrate into the material of the melt 46 and thus to enable the dopant to be incorporated into the epitaxial layer which is to be grown on the surface 2. Where, for example, a gallium phosphide epitaxial layer for luminescence diodes is used, oxygen and nitrogen, and also zinc vapor or a gaseous Zn compound are particularly advantageous. For this purpose, the arrangement described above and illustrated in FIG. 4 is arranged in a chamber which is sealed off to a greater or lesser extent within vessel 48. Materials can be introduced into this vessel in a known manner through the opening 49 in the form of gases, or in vaporizable, liquid or solid form.  
  FIG. 5 shows an arrangement featuring an embodiment which is provided for the carrying out, for exam ple, the invention of FIG. 4, in a furnace 51. Details, which have already been described in connection with the device illustrated in FIG. 4, are given the same reference numerals in FIG. 5. 149 marks a vessel which is sealed off to a greater or lesser extent and which contains the carrier body 44 in a recess in which is arranged the substrate wafer 3. The carrier body 44 is supported by a carrier 52 (FIG. 4) which enables the carrier body to be moved in the longitudinal direction into the furnace and out of the furnace. The carrier 52 extends through an opening 53 out of the vessel 149. Conduits 54 and 55 are provided for the supply and discharge of gases to and from the vessel 149. For example, argon is supplied as shield gas and oxygen as dopant through the conduit 54.  
  An additional opening 56 is, for example, provided through which a further carrier 57 extends into the vessel 149. This further carrier 57 can be axially movable, to and fro, in the region of the furnace SI in the vessel 149. With the aid of this further carrier, material arranged at the tip 157 thereof can be inserted into the region of the furnace 51. The material, which may be displaced with the aid of the carrier 57, can in particular take the form of dopants such as zinc, which, by being introduced into the region of the furnace, can be vaporized at a time which may be accurately determined.  
  FIG. 6 is a diagram showing the temperature characteristic in relation to time (but not to scale) on the substrate, which has proved favorable for the execution of the process in accordance with the invention, for the production of epitaxial layers for red-luminescent GaP diodes. The time is plotted (but not to scale) on the abscissa 61 and the temperature is plotted (but not to scale) on the ordinate 62. The time period up to the mark 63 on the abscissa approximately 10 minutes represents the time in which the carrier body, together with the gallium phosphidc substrate wafer arranged in its recess, and the gallium which is arranged on this substrate wafer and which is doped with Te, are heated to a predetermined high temperature, e,g., l I00C, in an atmosphere which contains oxygen and which additionally dopes the melt. When this temperature has been reached, a cooling phase takes place up to the point 64. This phase lasts for approximately l minutes, and cooling is carried out at approximately lO50C., maintaining the doping atmosphere. During this phase, an epitaxial deposition takes place. during which semiconductor material contained in the melt is deposited. During the section up to the mark 65 the temperature, is for example, maintained constant, and zinc is vaporized in the vessel 149. This section lasts, e.g., for minutes, and during this time, when the epitaxial deposition is practically at a halt, a zinc doping is carried out of the remainder of the material which has not yet been deposited, is still in the liquid phase and which is still arranged on the substrate wafer. During the further cooling which takes place in the section 0 between the marks 65 and 66, the epitaxial deposition continues, now with material doped with zinc which, in the case of gallium phosphide, is pconducting. At the mark 66 a temperature of 600C has been reached. Up to the time mark 67, the temperature is maintained constant for the purpose of tempering the substrate wafer with the epitaxial layer arranged there&#39; upon which is nand p-doped by layers. After the time mark 67, the final cooling takes place, and subsequently the substrate wafer is withdrawn from the arrangement or device which is provided for the execution of one of the processes in accordance with the invention and which are represented in special embodiments in FIGS. 1 to 5.  
  The process in accordance with the invention may also be used for the simultaneous processing of a plurality of substrate wafers. To this end, it is necessary to provide either an appropriate number of substrate bodies and one carrier body with a recess of correspondingly large surface extent, for the accommodation of a plurality of substrate wafers, or else an appropriate number of individual recesses. Also, a furnace 51 is required which must possess a plateau of constant temperature which is of sufficient length for the overall area occupied by the substrate wafers.  
  It is possible to carry out a regeneration of the material of the melt which has been doped in the stated manner in which the relevant dopant is revaporized out of this material after it has been used, in accordance with a further development of the invention.  
  The process in accordance with the invention is particularly suitable for the production of red-luminous gallium phosphide luminescence diodes. Previously, these diodes have been produced by a relatively expensive double-epitaxy process in which the pn-junction has been accomplished by the successive growth of two melt-epitaxial layers arranged in separate apparatus.  
  It will be apparent to those skilled in the art that many modifications and variations may be effected without departing from the spirit and scope ofthe novel concepts of the present invention.  
 We claim as our invention:  
  1. A process for epitaxially growing a layer on the surface of a substrate which includes bringing melt material which includes gallium into contact with the surface of said substrate at a low temperature of approximately 29C gradually raising the temperature of the melt to a high temperature, wherein some substrate material is dissolved in the melt, and thereafter reducing the temperature of the melt, thereby causing the dissolved material to be redeposited on said substrate, the thickness of melt at said high temperature being maintained between predetermined upper and lower limits by a cover of material inert to said melt, said upper limit being not greater than the length of the diffusion distance over which said dissolved material can cover during the period when the temperature decreases for the epitaxial growth.  
  2. A process according to claim 1, in which said cover floats on said melt.  
  3. A process according to claim 1, in which said substrate is galliumphosphite and in which said melt is gallium.  
  4. A process according to claim 1, in which said upper limit above said substrate is fixed by said cover being physically supported above said substrate.  
  5. A process as claimed in claim 1, characterized in that the material of the melt is applied in a measured quantity to the surface of the substrate, and that the cover is placed onto this material, wherein the cover rests on the thin layer of the material of the liquid phase, at least at the high temperature.  
  6. A process as claimed in claim 1, characterized in that at least one dopant which is soluble at the operating temperature in the melt is added to the material of the melt which is to be brought into contact with the surface of the substrate body.  
  7. A process as claimed in claim 1, characterized in that at least one dopant is conducted via the gas phase to the material of the melt, and that this dopant is incorporated into the layer which is to be epitaxially deposited during the growth of said layer.  
  8. A process as claimed in claim 7, characterized in that a plurality of different dopants are simultaneously supplied.  
 9. A process as claimed in claim 7, characterized in that a plurality of dopants are consecutively supplied. i