Abstract:
A backflow liner in an epitaxial growth system is provided in order to control gas flow and protect the surface of substrates throughout an epitaxial growth cycle. The backflow liner provides critical protection during the warming time prior to substrate pre-treatment, while the growth environment reaches steady state condition between the pre-treatment and the growth process, during pauses between the layer depositions in case of multilayer structure growth, and during the cooling process. The direction of the gas flow through the backflow liner is counter to the deposition gas flows directed from the source end of the growth system. The backflow liner is therefore designed to shape the flow of gases to prevent formation of the vortex-type streams in the growth system that may negatively affect the growth process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/660,586 filed Jun. 15, 2012. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to epitaxial growth systems for production of semiconductor materials and devices, in particular. More specifically, the invention relates to the design of hydride vapor phase epitaxy (HVPE) growth systems and reactors, the design of internal components of HVPE growth systems and reactors, and HVPE-based processes for growth of group III-nitride materials and devices that can be used in optoelectronics as well as in high-power high-frequency electronics. 
     2. Prior Art 
     The development of GaN-based optoelectronics and power electronics has led to widespread research into the growth and applications of compounds of aluminum, gallium, indium, boron, and nitrogen (collectively, the “III-nitrides,” “group III-nitrides,” or “Al x In y Ga 1-x-y N” in which 0≦x+y≦1). Group III-nitride templates, junctions, heterojunctions, multi-layer structures, thick layers/films, and bulk materials are commonly grown epitaxially via chemical vapor deposition methods including, but not limited to, hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD, MOVPE, or OMVPE). 
     During these deposition processes, a group III-nitride is grown upon a substrate or template consisting of, but not limited to, sapphire, silicon, silicon carbide, magnesium aluminate spinel, gallium nitride, aluminum nitride, aluminum-gallium nitride alloys, indium nitride, and/or lithium aluminate. A template shall be understood to be a substrate of one of the preceding materials coated with a layer of group III-nitride material. For the purposes of this invention, the terms “substrate” and “template” will be used interchangeably, though one skilled in the art will recognize that slightly different growth chemistries are required to optimize a group III-nitride deposition process for each. The differences in required chemistries are independent, however, of the implementation of the invention as described below. 
     The choice of substrate material, the crystallographic orientation of the substrate, and the deposition method/chemistry strongly influence the crystalline and morphological quality of the group III-nitride grown upon the substrate/template. 
     During active growth of a III-nitride (the process by which group III-nitride material is added to the surface of a substrate), it is common for a substrate or template to be exposed to a “growth atmosphere” that contains both one or more group III precursors (including but not limited to gallium chloride, aluminum chloride, indium chloride, trimethyl gallium, triethylgallium, trimethyl aluminum, gallium hydride, and gallium metal) and an active nitrogen precursor (typically but not limited to ammonia, hydrazine, or dihydrazine). For the purposes of the present invention, the atmosphere or ambient conditions within a group III-nitride epitaxy chamber will be referred to as a “non-growth atmosphere” if either or both a group III precursor or a nitrogen precursor are absent from the gas phase chemistry in the vicinity of the substrates or templates on which the group III-nitride is grown. 
     Studies of the thermal and chemical stability of GaN and other III-nitride epilayers in various ambient gases have been undertaken that have demonstrated thermal instability of some of the III-nitrides in common growth ambient environments (see M. A. Mastro, O. M. Kryliouk, M. D. Reed, T. J. Anderson, A. Davydov, and A. Shapiro, Thermal Stability of MOCVD and HVPE GaN Layers in H 2 , HCl, NH 3  and N 2 , Phys. Stat. Sol. (a) 188 (2001) 467-471 and M. A. Mastro, O. M. Kryliouk, T. J. Anderson, A. Davydov, A. Shapiro, Influence of polarity on GaN thermal stability, Journal of Crystal Growth 274 (2005) 38-46.). For example, on heating in N 2 , H 2 , NH 3 , and HCl, gallium nitride (GaN) can undergo dissociative sublimation or thermal decomposition accompanied in some instances by gallium droplet formation. In both cases the flatness and smoothness of the surface of the epilayer will be adversely affected, making it unusable for further device epitaxy. 
     It has been found that group III-nitride surfaces are generally more stable in non-growth atmospheres containing predominantly N 2  and NH 3  than in those containing principally H 2 , HCl, or Ar. The protective properties of nitrogen and ammonia are considered to be very useful when for some reason growth interruption is required and III-nitride surfaces are left exposed to the non-growth atmosphere (an ambient in which the III-nitride is not being actively deposited or grown). One skilled in the art of III-nitride epitaxy/crystal growth will recognize that III-nitride films and crystals are frequently exposed to such non-growth atmospheres during typical deposition/growth processes, such as during a waiting period for gas mixture homogenization at the beginning of an epitaxial run or during a slow cooling process at the end of the run. Growth interruptions also occur in the middle of the deposition cycles or runs for annealing to improve crystalline quality of the epilayer. Protection of the III-nitride surfaces is specifically important during the interruptions of an epitaxial process in which one of the components of a gas mixture may adversely influence surface morphology. An example of such an interruption can be found in the HVPE III-nitride deposition process employing HCl flow for the in-situ GaCl formation. The unreacted portion of the HCl flow is capable of etching unprotected surfaces of the substrate and epilayer. Indeed, such growth interruptions occur too frequently during a typical growth cycle for simple relocation of the epiwafers or crystals away from the growth zone of the reactor to adjacent so-called dwell zones to be sufficient or practical for ultimate preservation of the episurface. It is clear that there is a need for an effective means to protect heated III-nitride films, episurfaces, and crystals from decomposition. More preferably, there is a need for a means of providing protective gases through the dwell zone of the reactor to the III-nitride materials therein. 
     It was emphasized during the study of the first steps of the HVPE GaN growth on sapphire substrates that the results of the growth can be significantly influenced by the initial nucleation and nitridation conditions (see S. Gu, R. Zhang, Y. Shi, Y. Zheng, L. Zhang, F. Dwikusuma, T. F. Kuech, The impact of initial growth and substrate nitridation on thick GaN growth on sapphire by hydride vapor phase epitaxy, Journal of Crystal Growth 231 (2001) 342-351). To preserve the surface of the substrates or templates up to the growth temperature of 1100° C., an additional region, called a backflow tube, was introduced into a vertical HVPE reactor. The gas ambient within the backflow tube was chosen to be either pure N 2  or N 2 +NH 3  mixture depending on the choice of pregrowth treatment. The study confirmed that in the reactor with the backflow tube, improved initiation of the growth can be achieved. Apart from the improvement in the crystalline structure of GaN epilayers, their surface was generally smoother and had reduced density of surface pits, which was a common morphological feature of the grown epilayers. However, there was no specific consideration given to the geometry and position of the backflow tube inside the reactor that would help to prevent potential eddy backflows in the growth zone that could adversely affect III-nitride uniformity and quality. From the point of view of the present invention, when the position of the end of the backflow tube is too close to the growth zone or shape of this end coincides with the shape of the growth zone, gases flowing through the backflow tube will effectively block outflow of the reactive gasses from the growth zone. This blocking will negatively influence epitaxy in the growth zone and reduce all benefits of the backflow use to a minimum by unfavorably modifying the gas phase chemistry in the vicinity of the substrates. 
     In reference F. Dwikusuma and T. F. Kuech, X-ray photoelectron spectroscopic study on sapphire nitridation for GaN growth by hydride vapor phase epitaxy: Nitridation mechanism, Journal of Applied Physics 94 (2003) 5656-5664 the study of the sapphire nitridation in the backflow region of a vertical HVPE system under a NH 3  and N 2  ambient was described. It was mentioned that the backflow region allowed the sample to be heated to the temperature of 1100° C. under a countercurrent gas flow, protecting the sample from a gallium precursor stream. Nitridation was carried out by exposing the sapphire to a mixture of NH 3  and N 2  at a total flow rate of 2 slpm and a total pressure of 1 atm. The nominal reactor diameter near the sample that corresponded to the diameter of backflow region was 6 cm. The strict correspondence of the diameters and shapes of the backflow and growth regions inevitably leads to the generation of the gas vortexes in the growth zone resulting in irreproducible epitaxial condition. Indeed, the prior art has focused on the use of radially symmetrical backflow liners in vertical configurations that are of no use in horizontally configured flow paths and fail to address deleterious eddy current formation. 
     In a series of patents, the use of a backflow of ammonia for protection of the grown epilayers is claimed. For example, in U.S. Pat. No. 7,727,333 at a final step of the HVPE deposition of indium gallium nitride epilayer, the backflow of ammonia is provided into the reactor to prevent thermal decomposition of the grown epilayer. In the backflow, the substrate with the epilayer is allowed to cool down to the temperature at which decomposition is negligibly small even without ammonia. 
     In U.S. Pat. Nos. 6,656,272 and 7,670,435 to achieve sharp interlayer interfaces in multilayer structures the backflow gas sources and substrate movement within the growth zone are proposed. Once the growth of one sublayer is completed, the substrate is moved into the growth interruption zone where the backflow of an inert gas insures the interruption of the growth. While the substrate is in the growth interruption zone, the growth zone can be purged with the inert gas and active gas mixture including ammonia is reintroduced. After the growth mixture is uniformly distributed, the substrate is moved back into the growth zone. The Patents description does not include any specifications for optimal geometry of the growth interruption zone. 
     In U.S. Pat. No. 6,890,809 the backflow direction of argon and/or ammonia gases in the HVPE reactor is proposed to prevent undesirable growth during cooling the substrate with already grown epitaxial structure. In the preferred embodiment, the epitaxial structure comprises a GaN—AlGaN p-n heterojunction and p-type GaN capping layer helping to avoid surface oxidation of p-type AlGaN. 
     Along with a number of advantages that substantially increase flexibility of the HVPE process, the use of the backflow streams as disclosed in the prior art suffers a major drawback: it is a cause for induced eddy currents that can destroy the laminar pattern of the main gas flow and compromise stability and reproducibility of the group III-nitride growth process (see E. Richter, Ch. Hennig, M. Weyers, F. Habel, J. D. Tsay, W. Y. Liu, P. Brückner, F. Scholz, Yu. Makarov, A. Segal, J. Kaeppeler, Reactor and growth process optimization for growth of thick GaN layers on sapphire substrates by HVPE, Journal of Crystal Growth 277 (2005) 6-12). 
     The main objective of the present invention is the introduction of a backflow liner that decouples the active/main and counter/backflow gas streams in the growth reactor enriched with the gas counter-flow functionality. A further objective is to provide a backflow liner design that avoids deleterious eddy current formation as has been observed in the prior art and enables laminar gas flow in the vicinity of the growing group III-nitride films. 
     These objectives are achieved in the epitaxial growth reactor geometry provided herein comprising a main reactor element with an inserted backflow liner axially aligned to the adjacent growth liner and separated from it by the shaped opening that directed gas flow from the growth and backflow liners toward the reactor exhaust. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of an example HVPE growth system with the backflow liner including elements: ( 101 ) Growth reactor; ( 102 ) Heater; ( 103 ) Source zone; ( 104 ) Growth liner; ( 105 ) Substrate holder; ( 106 ) Backflow liner; ( 107 ) Dwell zone; ( 108 ) Reactor exhaust; ( 109 ) gas supply tubes, and ( 110 ) backflow gas supply injector, Block ( 111 ) depicts the direction of source gas flow, block ( 112 ) depicts the gas flow direction within the backflow liner, and block ( 113 ) illustrates the gas flow exiting the oblique angled end of the backflow liner. 
         FIG. 2  is an isometric view of the quartz backflow liner including elements: ( 201 ) skids at the shaped oblique end of the liner made to align its position with the growth liner; ( 202 ) the rectangular body of the liner; ( 203 ) and ( 204 ) thermal shields; ( 205 ) thermal insulation ring at the flange end of the liner; ( 206 ) an engaging hole; and ( 207 ) a thermal shield. 
         FIG. 3  is a schematic illustration of the backflow liner aligned with the growth liner position of the backflow liner according to the present invention. 
         FIG. 4  shows a simulation pattern of the gas flow distribution between the growth liner (lower part) and backflow liner (upper part of the figure). It is shown that due to the pushing effect of the gases from the growth liner, vortices are displaced out of the growth zone to the backflow liner. 
         FIG. 5  is a representation of a corresponding vertical-flow, hot-wall Hydride Vapor Phase Epitaxy reactor incorporating the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention addresses the needs of the prior art by presenting a backflow liner component of a III-nitride epitaxy system that ensures the back flow of protective gases to the dwell zone of the reactor. In particular, the present invention provides an epitaxial growth chamber element identified as a backflow liner that provides superior control of gas ambient environments during group III-nitride deposition processes in both growth and non-growth atmospheres. Key details of the invention can be summarized as follows. First, it is found absolutely necessary that the backflow liner shape gas flow so as to protect a subtle epi-ready surface of the substrate during the warming-up time prior to the growth and during growth interruptions in an environment that differs from that intended for treatment and growth. 
     Second, the protection is fulfilled inside the backflow liner via the protective gas flow entering the reactor counter to the main gas mixture direction. Such a counter flow provides for protection of the substrate while the growth environment reaches steady state condition between the treatment and growth processes. The substrate can be retracted from the growth region into the backflow liner preventing decomposition of the substrate and/or group III-nitride surfaces in the inhomogeneous transient gas flow when the active gases are just enabled. 
     Third, the use of the backflow liner provides for the avoidance of decomposition of the group III-nitride film grown on the substrate following the growth process. For example, unless protected by an ammonia ambient environment, a GaN film will decompose at typical crystal growth temperatures on the order of 1000 degrees Celsius. After the completion of the growth process, the substrate is retracted into the backflow liner where the ambient is ammonia or nitrogen, where it is protected from such thermal decomposition. 
     More particularly, the present invention provides for the implementation of a chemical vapor deposition internal component configuration that allows laminar flow delivery of III-nitride protecting gases to substrates and templates without creation of deleterious eddy currents. For the purposes of this specification, reference will be made to the internal component designs of a HVPE crystal growth machine. However, one skilled in the art will recognize that the geometries and designs illustrated herein may be readily implemented in analogous form in MOCVD tools. Further, while the embodiments described herein utilize a horizontal geometry, the invention can be readily implemented in a vertical configuration. Such a vertical reactor is shown in  FIG. 5 , with the various parts thereof labeled as per the embodiments to be described, which but for the vertical orientation, are of corresponding design and function. 
     An axial cross-sectional illustration of one embodiment is provided in  FIG. 1 . The growth system  100  includes growth chamber/reactor  101  represented by a horizontally configured containment tube surrounded by an external heater  102 . While the external heater is illustrated as being external to the containment tube, implementing a “hot-walled” growth chamber design, the invention may be executed utilizing an internal heater in close proximity to the substrates (a “cold-walled” design). The embodiment also envisions a resistive tubular heater. However, other heating methods including but not limited to lamp and radio-frequency heating are compatible with the invention. The heater illustrated in  FIG. 1  may consist of a single temperature zone, but more commonly will be divided into two to twenty independently controlled heating zones to provide for shaping of the thermal profile in the growth chamber. 
     A variety of components are commonly included inside of the containment tube as shown in  FIG. 1 . Block  103  illustrates the source zone of the chamber from which precursor and carrier gases are injected into the chamber through one or more delivery tubes. Block  104  represents the growth liner that surrounds the substrate holder  105 . The backflow liner  106  is positioned downstream of the growth liner, within which a dwell zone  107  exists to which substrates can be retracted and held in a favorable non-growth atmosphere. Protective gases are injected into the backflow liner via one or more injectors  110 . The reactor exhaust  108  is downstream of the dwell zone. The general flow direction of the source gases is depicted by block  111 , the counter flow of gases injected into the backflow liner is depicted by block  112 , the combined source and backflow gases exit the growth region via flow lines generally depicted by block  113 . 
     The backflow liner  106  in  FIG. 1  is illustrated in detail in  FIG. 2  as block  200 . This liner can be generically characterized as a gas flow directing element consists of an assembly of welded plates having a generally rectangular cross-section over the majority of its length. The backflow liner consists of rectangular cross-section region  202  in which protective gases may be injected counter to the flows from the source end of the growth chamber. The backflow liner includes an oblique angle  208  at the end of the element that lies in proximity to the growth liner  104  as illustrated in  FIG. 1 . The oblique angle  208  may vary from approximately 1 degree to approximately 179 degrees depending on the design of adjacent components in the growth chamber  101 . Most preferably, the oblique angle should be approximately 45 degrees. Optional features of the backflow liner that are illustrated in  FIG. 2  include alignment skids  201  that position the growth liner relative to the backflow liner, thermal shields and insulating rings  203 ,  204 ,  205  and  207  to block transmission of infrared radiation into cold regions of the growth chamber, and an alignment hole  206  for positioning the growth liner relative to other chamber components. 
     In the preferred embodiment, the backflow liner  106  is fabricated of fused silica (silicon dioxide), but other materials including but not limited to sapphire, alumina, silicon carbide, boron nitride or a combination thereof are suitable for its fabrication as well. The backflow liner  106  is designed to transport protective ammonia gas at a flow rate ranging from &gt;1 to &lt;50 slpm flow rate. More preferably, the liner illustrated in  FIG. 2  transports approximately 5 slpm of ammonia counter to the principal gas flow direction within in the growth chamber. Alternately, the backflow liner may transport a mixture of gases including but not limited to ammonia, nitrogen, argon, and/or hydrogen at flow rates ranging from &gt;1 to &lt;50 slpm in the direction opposite to the main flow in the reactor. The flow in the backflow liner may be continuous, or only when needed, or may be adjusted between two or more levels when used and when simply being purged to be ready for use. 
     In the preferred embodiment, the backflow liner  106  is inserted in the main reactor tube from the growth chamber&#39;s substrate loading end. The rectangular cross-sectional shape of the backflow liner  106  coincides with the cross sectional shape of the majority of the opposing growth liner  104 . The two liner elements are axially aligned and preferably separated by a gap leaving the shaped oblique opening for the gas stream from the liners to be directed towards the reactor exhaust ( FIG. 3 ). Preferably the size of the exit of the backflow liner  106  will be less than the size of the exit of the growth liner  104 , whether the exit of the growth liner  104  has a configuration as shown in  FIG. 1 , as shown in  FIG. 3  or some other shape. The value of the oblique angle  208  has been chosen to be 45 degrees in the preferred embodiment. This angle is important for proper ratio of back and direct gas flow rates that defines vortex-free condition in the growth reactor. The backflow gas may be, by way of example, NH 3 , N 2 , Ar, or a mixture thereof as supplied by a source connected to the injector  110 . 
     The opposing flows from the growth and backflow liners meet at the gap between their ends. The shaped oblique end of the backflow liner  106  promotes further direction of the gas flow mixture towards the exhaust from the main reactor tube. Due to the low disturbance of the gas stream at the end of the growth liner  104 , vortex-free gas flow inside the growth liner can be obtained. 
     Achievement of vortex-free gas flow within the growth liner depends both on the design of the backflow liner as provided herein and optimization of the ratio of gas injected from the source end of the chamber to that injected through the backflow liner. This source-to-backflow gas ratio is generally preferred to range from 1 to 10, and more preferably from 3 to 6. For the particular geometry utilized in this embodiment, vortex-free conditions in the growth region of the reactor are achieved when 10 slpm of NH 3  and 23 slpm of Ar are directed through the growth liner, while 5 SLPM of NH 3  and 5 SLPM Ar was directed through the backflow liner. Such vortex-free conditions have been demonstrated both experimentally in HVPE group III-nitride growth and confirmed using numerical simulation of the growth reactor as illustrated in  FIG. 4 . Deviation of the source-to-backflow ratio from the range prescribed above leads to the parasitic deposition of group III-nitrides inside the backflow liner and could result in surface etching of the epilayers since gas pressure at the front of backflow liner is insufficient to force gases from the growth liner to the reactor exhaust. 
     Implementation of the preferred embodiment in a group III-nitride HVPE growth system yielded superior uniformity of deposition across multiple substrates placed on the substrate holder  105 . In practice, the invention provided for achievement of less than 5% thickness variation across individual 2-inch diameter substrates and less than 10% thickness variation within a batch of 12 co-loaded substrates in the growth zone on the substrate holder. The invention provides a further advantage over the prior art in that at optimal source-to-back flow gas flow ratio a parasitic deposition in the backflow liner and in the main reactor tube is reduced, in many cases to zero parasitic deposition. 
     As an another advantage of using optimal source-to-backflow ratio in conjunction with the backflow liner, simultaneous etch cleaning of the reactor with hydrogen chloride during cooling of the templates in the backflow liner can be achieved without fear of damaging their smooth epitaxial surface. Performing such etching/cleaning processes without the need to cool the chamber to unload the group III-nitride materials that have been previously grown reduces process cycle time and increases throughput compared to the prior art. 
     An added advantage of the invention is the reduction of the time required for purging of the growth chamber after insertion of the epitaxial substrates into it, either before or after group III-nitride deposition. Such time reductions are firstly due to uninterruptible purging of the backflow liner with protective gases and secondly due to reduced volume of the backflow liner compared to the full reactor. Gas flowing through the backflow liner constantly purges it. While the substrate is still cooling down within the liner, the protective environment inside the liner makes simultaneous post-growth etching of the reactor possible. 
     The backflow liner described in the preferred embodiment can be used not only during the growth of a single epitaxial layer but repeatedly for the multiple epitaxial layers. A structure that includes multiple epitaxial layers may have different constituents, like GaN and AlGaN, different compositions of the constituents, like Al x Ga 1-x N and Al y Ga 1-y N, or different sequences of those compositions. The main purpose for using the backflow liner remains unchanged: to protect the substrates or grown epilayers from the harsh, unsteady environment outside the backflow liner. It is necessary while the growth environment reaches the steady state condition between the treatment and the growth process; while the wafers are at a high temperature close to decomposition temperature after the growth or between the following growth interruptions. Unless being protected by ammonia ambient in the backflow environment, GaN or its III-N alloys are prone to decompose with time. Every time when decomposition is probable wafers are retracted into the backflow liner filled with the ammonia-rich protective atmosphere. 
     One skilled in the art will recognize that many variations of the invention may be implemented that are wholly or partially equivalent to those described in the present application, and it is here intended to cover all said equivalent measures and approaches falling in the scope of the present invention and defined by the following claims. For example, but without limitation, it may be desirable for the backflow liner to possess an oblong cross-section either wholly or in part as opposed to the rectangular cross-section described in the preferred embodiment. Similarly, the optional features illustrated in  FIG. 2  may be omitted entirely, or may be designed as separate, interlocking components that mate with the backflow liner or adjacent components. Alternately, the backflow liner could be vertically oriented rather than horizontally oriented. The dimensions of such a vertically oriented backflow liner may be modified to account for convective flow effects, or the desirable source-to-backflow flow ratios may be adjusted to account for convection. Such examples of variations of the backflow liner design are consistent with the intent of the invention. 
     Thus the present invention has a number of aspects, which aspects may be practiced alone or in various combinations or sub-combinations, as desired. While a preferred embodiment of the present invention has been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the full breadth of the following claims.