Patent Publication Number: US-2007111372-A1

Title: Methods of forming a p-type group ii-vi semiconductor crystal layer on a substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This patent application is a continuation of U.S. Divisional application Ser. No. 10/896,826, filed Jul. 20, 2004, which claims priority under Title 35, United States Code §119(e) to U.S. provisional application No. 60/391,507 filed Jun. 24, 2002 and nonprovisional application Ser. No. 10/422,568 filed Apr. 23, 2003 naming as inventors Jeffrey E. Nause, Joseph Owen Maciejewski, and Vincente Munne as inventors, which applications are incorporated herein by reference. Both the subject continuation application, divisional application, nonprovisional patent application, and its provisional application have been or are under obligation to be assigned to the same entity. 
    
    
     STATEMENT OF GOVERNMENT RIGHTS IN THE INVENTION  
      This invention was made pursuant to a Small Business Innovative Research project funded by the U.S. Government as represented by the Office of Naval Research under Contract No. N00014-00-C-0362. The U.S. Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to forming compound semiconductor layers using techniques such as metal-oxide chemical vapor deposition (MOCVD) or metal-oxide vapor phase epitaxy (MOVPE). More particularly, this invention relates to methods for forming magnesium, cadmium, and/or zinc oxide crystalline semiconductor layers useful for making electrical and electro-optical devices such as light emitting diodes (LEDs), laser diodes (LDs), field effect transistors (FETs), and photodetectors.  
      2. Description of the Related Art  
      For some time there has been interest in producing II-VI compound wide band gap semiconductors to produce green/blue LEDs, LDs and other electrical devices. Historically, attempts to produce these devices have centered around zinc selenide (ZnSe) or gallium nitride (GaN) based technologies. However, these approaches have not been entirely satisfactory due to the short lifetime of light emission that results from defects, and defect migration in these devices.  
      Recently, because ZnO has a wide direct bandgap of 3.3 electron-Volts (eV) at room temperature and provides a strong emission source of ultraviolet light, ZnO thin films on suitable supporting substrates have been proposed as new materials for LEDs and LDs.  
      Undoped, as well as doped, ZnO films generally show n-type conduction. Impurities such as aluminum and gallium in ZnO films have been studied by Hiramatsu et al. who report activity as n-type donors (Transparent Conduction Zinc Oxide Thin Films Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci. Technol. A 16(2), March/April 1998). Although n-type ZnO films have been available for some time, the growth of p-type ZnO films necessary to build many electrical devices requiring p-n junctions has been much slower in developing.  
      Minegishi et al. (Growth of P-Type ZnO Films by Chemical Vapor Deposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recently reported on the growth of nitrogen doped ZnO films by chemical vapor deposition and on the p-type conduction of ZnO films at room temperature. Minegishi et al. disclose the growth of p-type ZnO films on a sapphire substrate by the simultaneous addition of NH 3  in carrier hydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10 mol % was used, secondary ion mass spectrometry (SIMS) confirmed the incorporation of nitrogen into the ZnO film, although the nitrogen concentration was not precisely confirmed. Although the films prepared by Minegishi et al. using a Zn/ZnO ratio of 10 mol % appear to incorporate a small amount of nitrogen into the ZnO film and convert the conduction to p-type, the resistivity of these films is too high for application in devices such as LEDs or LDs. Also, Minegishi et al. report that the carrier density for the holes is 1.5.×10 16  holes/cm 3 , which is considered to be too low for use in commercial light emitting diodes or laser diodes.  
      Park et al. in U.S. Pat. No. 5,574,296 disclose a method of producing thin films on substrates by doping IIB-VIA semiconductors with group VA free radicals for use in electromagnetic radiation transducers. Specifically, Park et al. describe ZnSe epitaxial thin films doped with nitrogen or oxygen wherein ZnSe thin layers are grown on a GaAs substrate by molecular beam epitaxy. The doping of nitrogen or oxygen is accomplished through the use of free radical source which is incorporated into the molecular beam epitaxy system. Using nitrogen as the p-type dopant, net acceptor densities up to 4.9×10 17  acceptors/cm 3  and resistivities less than 15 ohm-cm were measured in the ZeSe film.  
      However, the net acceptor density is too low and the resistivity is too high for use in commercial devices such as LEDs, LDs, and FETs.  
      White et al in U.S. Pat. No. 6,291,085 disclose a method for producing ZnO films containing p-type dopants, in which the p-type dopant is arsenic and the substrate is gallium arsenide (GaAs). The method of preparation of the film is laser ablation. However, the crystal quality of the films prepared by such a process is inferior and not suitable for device applications.  
      Although some progress has recently been made in the fabrication of p-type doped ZnO films which can be utilized in the formation of p-n junctions, a need still exists in the industry for ZnO films which contain higher net acceptor concentrations and possess lower resistivity values.  
     SUMMARY OF THE INVENTION  
      The invented method described herein overcomes the disadvantages noted above with respect to previous techniques for making p-type zinc oxide layers. This method can be used to make relatively high-quality light emitting diodes (LEDs), laser diodes (LDs), field effect transistors (FETs), and photodetectors, and other electrical , electro-optic, or opto-electrical devices.  
      In one embodiment, a method in accordance with the invention comprises forming a p-type II-VI Group compound semiconductor layer represented by the formula Mg 1-x-y Cd x Zn y  O, in which 0&lt;x&lt;1, 0&lt;y&lt;1, and x+y=0.1 to 1, on a zinc oxide (ZnO) substrate having an (002) crystallographic orientation. The variables x and y can be such that y=1 and x=0, so that the formed layer is composed of ZnO. The forming may comprise heating the substrate; and supplying reaction gases comprising a first gas containing a zinc compound, a second gas containing oxygen, a third p-type dopant gas with at least one element from one of Groups IA, IB, VA and VB of the periodic table of the elements, and a fourth inert carrier gas used to carry the reaction gases to the surface of the heated ZnO substrate to grow the p-type II-VI Group compound semiconductor crystal layer on the ZnO substrate. At least one of the gases may be directed in a flow transverse to the substrate to carry the reaction gases to the substrate to form the layer. The heating and supplying of reaction gases may be performed in a chamber. The heating can be performed so that the temperature of the substrate is maintained at a temperature in a range from two-hundred-fifty (250) to six-hundred-fifty (650) degrees Celsius during growth of the layer. The zinc compound in the first gas comprises diethylzinc, dimethylzinc, or a mixture thereof. The first gaseous material may further comprise magnesium metalorganic, cadmium metalorganic or a mixture thereof. The oxygen in the second gas can be provided as oxygen (O 2 ) or nitrous oxide (N 2 O). The third p-type dopant gas can comprise at least one of nitrogen (N), copper (Cu), arsenic (As), and phosphorus (P) as the dopant species. The method can further comprise a step of rotating the substrate at a rate in a range from one-hundred (100) to one-thousand (1,000) revolutions per minute (rpm) during growth of the layer. The heating can be performed by a heater comprising an electrically-resistive element. The substrate can be maintained at a target temperature by a temperature controller and temperature sensor. The third p-type dopant gas can contain arsenic (As) or phosphorus (P), or both, as dopant. The ZnO substrate can be produced by containing liquid-phase ZnO in a solid-phase ZnO “skull” during growth of the crystal from which the ZnO substrate is formed. The method can further comprise rotating the substrate by a drive unit connected to a shaft that turns a susceptor and carrier plate upon which the substrate is situated during growth of the layer.  
      A method in accordance with another embodiment of the invention comprises the steps of forming a zinc oxide (ZnO) crystal by containing liquid-phase ZnO in a solid-phase ZnO “skull” during growth of the ZnO crystal; forming a ZnO crystal by cutting and polishing the ZnO crystal; and forming a p-type II-VI Group compound semiconductor represented by the formula Mg 1-x-y Cd x Zn y O, in which 0&lt;x&lt;1, 0&lt;y&lt;1, and x+y=0.1 to 1, on the zinc oxide (ZnO) substrate. The forming of the p-type II-VI Group compound semiconductor comprises heating the ZnO substrate; and supplying reaction gases comprising a first gas containing a zinc compound, a second gas containing oxygen, a third p-type dopant gas with at least one element from one of Groups IA, IB, VA and VB of the periodic table of the elements, and a fourth inert carrier gas used to carry the reaction gases to the surface of the heated ZnO substrate to grow the p-type II-VI Group compound semiconductor crystal layer on the ZnO substrate. The forming steps are carried out so that the p-type II-VI Group compound semiconductor crystal layer produced by the method has an acceptor concentration of at least 10 17  atoms per cubic centimeter and a resistivity of at least one-tenth (0.1) Ohm-centimeter. This makes the resulting p-type II-VI Group compound semiconductor crystal layer suitable for producing high-quality light emitting diodes (LEDs), laser diodes (LDs), field effect transistors (FETs), photodetectors, and other devices. The inert carrier gas can comprise argon gas. The gas flow rate for each of the first, second and third gases can be maintained at from ten (10) to five-thousand (5000) standard cubic centimeters per minute (sccm) during growth of the layer. The gas flow rates can be maintained by mass flow controllers. The gases can be maintained under a pressure in a range from five (5.0) to fifty (50.0) torrs during growth of the layer. The gases can be maintained under pressure by an exhaust pump which controls the rate of exit of the gases from a chamber in which the substrate is situated during growth of the layer. The pressure of the gases can be further maintained by pressure flow meters.  
      The p-type ZnO-based layer produced by the disclosed methods can be used in LEDs, LDs, FETs, and photodetectors, in which both n-type and p-type materials are required, as a substrate material for lattice matching to other materials in such devices, and/or as a layer for attaching electrical leads, among other possible uses.  
      Additional objects and advantages of the invention are set forth in the description which follows. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  shows an apparatus of the invention, which can be suitably used for the practice of the method of the present invention.  
       FIG. 2  is a flowchart of a method for producing a M 1-x-y Cd x Zn y O II-VI Group compound semiconductor crystal layer on a substrate in accordance with the invention. 
    
    
      The invention is now described with reference to the accompanying drawings which constitute a part of this disclosure. In the drawings, like numerals are used to refer to like elements throughout the several views.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention is directed to a method of depositing a ZnO-based II-VI Group compound semiconductor crystal layer on a substrate by the metalorganic chemical vapor deposition or metalorganic vapor phase epitaxy technique. ZnO-based II-VI Group compound semiconductors include zinc oxide (ZnO), magnesium zinc oxide (MgZnO), cadmium zinc oxide (CdZnO), and magnesium cadmium zinc oxide (MgCdZnO). These semiconductors may be represented by the formula:
 
Mg 1-x-y Cd x Zn y O, in which 0&lt; x&lt; 1, 0&lt; y≦ 1, and  x+y= 0.1 to 1.
 
      In the present invention, a reaction gas comprising a first organometallic gas containing zinc, and optionally also magnesium and/or cadmium, and a second gas containing oxygen, are supplied to the surface of the heated substrate surface. The substrate can comprise a bulk ZnO crystal chemically matched to the ZnO layer to be formed. A third dopant gas is also supplied to the heated surface to produce p-type conductivity by introducing dopant atoms into the ZnO layer as it grows. The ZnO-based II-VI Group compound semiconductor crystal grows on the heated substrate surface through the reaction of the first and second gas, and the p-type dopant is uniformly incorporated into the lattice of the crystal as it grows.  
      The first gas can be at least an organozinc compound such as diethylzinc, dimethylzinc or a mixture thereof. The first gas can further contain an organic compound of a Group III element, other than organogallium compounds. Examples of such an organic compound includes an organomagnesium compound such as bis(cyclopentadienyl)magnesium, bis(methylcyclopentadienyl)magnesium or a mixture thereof, or an organocadmium compound such as dimethylcadmium.  
      The second gas is oxygen which can react with the first gas to produce a layer of Mg 1-x-y Cd x Zny y O compound on the substrate.  
      The third gas contains a gaseous p-type dopant source, such as metalorganic or other precursors from Groups IA, IB, VA or VB from the period table of the elements. In the preferred embodiment, p-type dopant sources include bis(tetramethylheptanedianol)copper, arsine, phosphine, or tertiarybutylphosphine to introduce p-type dopant atoms copper, arsenic, and phosphorus, respectively, into the Mg 1-x-y Cd x Zn y O layer.  
       FIG. 1  schematically shows an apparatus  20  that can be used to perform the method of the present invention. As shown in  FIG. 1 , the apparatus  1  has a reaction chamber  2  made of, for example, stainless steel. A carrier plate  3  and susceptor  4  are arranged in the chamber  2  to place a substrate  5  thereon substantially horizontally. The substrate(s)  5  is loaded/unloaded through a load/unload port  6  arranged in the chamber  2 , as is well known in the art. The susceptor  4  is a round column having a diameter of, for example, thirty (30) to one-hundred-fifty (150) millimeters (mm) and a height of, for example, ten (10) to thirty (30) mm. The carrier plate  3  and susceptor  4  are made of a high heat-resistant material which does not contaminate the gases in the chamber  2  upon heating. Such a material includes carbon surface-coated with silicon carbide.  
      A shaft  7  is fixed to the center of the lower surface of the susceptor  4 , and air-tightly extends outside the chamber  2 . The shaft  7  can be connected to and rotated by a drive unit  8 , such as an electric motor or servo motor, to rotate the susceptor  4 , and hence the carrier plate  3  and substrate  5 , during the growth of an M 1-x-y Cd x Zn y O layer  9  on the respective substrate  5 . The drive unit  8  is connected to the rotation speed controller  9  that regulates the rotation speed imparted by the drive unit  8  to the shaft  7  to maintain a target rotation speed. The target rotation speed can be constant, or alternatively, can be time-varying according to a predetermined rotation-speed-versus-time profile. In some implementations of the method, it can be desirable, for example, to set the rotation speed at a relatively slow rate at the beginning of the process of forming the M 1-x-y Cd x Zn y O layer(s)  9 , speeding up to be relatively fast toward the end of the process of forming the layer. The controller  10  can be a computer that senses the rotation speed of the shaft  7 .  
      For example, the rotation speed can be sensed by a tachometer internal to the drive unit  8 , which generates an electric signal indicating the rotation speed on conductive line  11 . The rotation speed controller  10  senses the rotation speed, and subtracts it from the target rotation speed internally to the controller  10 , to generate an error signal. As previously mentioned, the target rotation speed can be constant in which case it is time-invariant over the entire process. Alternatively, the target rotation speed corresponding to the time elapsed from the start of the timed process can be read from a target rotation speed profile, in which case the target speed for the elapsed time from commencement of the process is read by the controller  10  from its memory, and subtracted from the rotation speed signal, to generate the error signal. The rotation speed controller  10  uses the error signal to adjust the magnitude of the drive current to either speed, slow, or maintain the rotation speed, based on the error signal. The controller  10  supplies the adjusted drive current to the conductive line  12  to the drive unit  8  to drive the shaft  7  to rotate, thereby also rotating the susceptor  4  and carrier plate  3 , to rotate the substrate(s)  5 .  
      A heater  13  is arranged to heat the susceptor  4 , and hence the carrier plate  3  and substrate  5 , to a temperature suitable to grow the desired ZnO-based II-VI Group compound semiconductor crystal layer on the substrate, e.g., about 400 degrees C. or more. In  FIG. 1 , such the heater  13  is provided close to, but away from, the lower surface of the susceptor  4 . The heater  13  can be an electrically-resistive element that is controlled to heat the susceptor  4  to the required temperature by a temperature controller  14  and a temperature sensor  15 . The temperature sensor  15  can be incorporated in the susceptor  4 . The temperature sensor  15  can be a thermistor, for example. The temperature sensor  15  generates an electric signal proportional to the temperature it senses. The sensor  15  is connected via electrically-conductive lines  16 ,  17  such as metal wires, and slip ring  18 . Because the shaft  7  rotates relative to the temperature controller  14 , slip ring  18  is connected between the wire  16  from temperature sensor  15  and a conductive wire  17  to temperature controller  14 . Alternatively, an optical temperature sensor  19  can be arranged to view the substrate(s)  5  through transparent window  20  air-tightly sealed in the wall of chamber  2 . The optical temperature sensor  15  generates the sensed temperature signal on line  21  that is supplied to the temperature controller  14 . The temperature controller  14  can include an on/off control or supply current control, for example, that generates electric current based on the signal from the temperature sensor  15  or  17 . The temperature controller  14  is connected to supply the electric current it generates to the heating element  13  via electrically-conductive line  22  which extends through bushing  23  in the wall of chamber  2 . A user can set a target temperature with the temperature controller  14 . Alternatively, if a user desires to vary the temperature of the substrate(s)  5  during the layer growth process, the user can set a target-temperature-versus-time profile to control the temperature of the substrate as a function of time during the layer growth process. If the temperature sensed by the sensor  15  or  17  is below the target temperature, the temperature controller  14  supplies electric current on the line  22  to the heating element  13  to increase the temperature of substrate(s)  5 . Conversely, if the sensed temperature is at or above the target temperature, the temperature controller  14  does not generate electric current to permit the substrate(s)  5  to cool to the target temperature. The temperature sensor  15  or  17  and controller  14  thus function to maintain the temperature of the substrate(s)  5  and the respective layer(s)  9  growing thereon, at the target temperature during the growth process.  
      Injection tubes  24 ,  25 ,  26  extend through a wall of chamber  2 , and are air-tightly sealed thereto. The injection tubes  24 ,  25 ,  26  can be arranged perpendicularly or transversely, to the substrate(s)  5  positioned on the carrier plate  5 . Zinc-, cadmium-, and magnesium-containing reaction gases are blown through respective injection tubes  24 ,  25 ,  26  perpendicularly to the substrate surface, together with a carrier gas such as argon gas. More specifically, the injection tubes  24 ,  25 ,  26  can be connected to respective pressure flow controllers  27 ,  28 ,  29 . The pressure flow controllers  27 ,  28 ,  29  are in turn connected to bubblers  30 ,  31 ,  32  containing respective liquid zinc-, cadmium-, and magnesium-organic compounds  33 ,  34 ,  35 . The bubblers  30 ,  31 ,  32  are connected to respective mass flow controllers  36 ,  37 ,  38 . The mass flow controllers  36 ,  37 ,  38  are connected through respective conduits to the tank  39  of carrier gas such as argon. The pressure and mass flow controllers  27 ,  28 ,  29 ,  36 ,  37 ,  38  can be electrically connected via lines  40  to the flow controller  41 . The flow controller  41  can be implemented as a computer with an input device such as dials or keys permitting a user to set the pressure flow rates for controllers  27 ,  28 ,  29  and the mass flow rates for controllers  36 ,  37 ,  38 . More specifically, the flow controller  41  controls the mass flow controllers  36 ,  37 ,  38  to regulate the amount of carrier gas permitted to pass into respective liquid zinc-, cadmium-, and magnesium-organic compounds  33 ,  34 ,  35  in the bubblers  27 ,  28 ,  29 . Through bubbling in liquids  33 ,  34 ,  35 , the carrier gas flows pick up vaporous zinc-, cadmium-, and magnesium-organic compounds for transport to respective pressure flow controllers  27 ,  28 ,  29 . The flow controller  41  controls the pressure flow controllers  27 ,  28 ,  29  via lines  40  to regulate the pressures at which the respective zinc-, cadmium-, and magnesium-containing gases are introduced into the chamber  2 .  
      To provide oxygen to form the p-type M 1-x-y Cd x Zn y O layer(s)  9  on respective substrate(s)  5 , injection tube  48  is air-tightly sealed to the wall of chamber  2 . The injection tube  42  is connected to a mass flow controller  43  which in turn is connected to a tank  44  of oxygen gas. The injection tube  42  can also be connected to a mass flow controller  45  which is in turn connected to tank  46  of carrier gas such as argon. The mass flow controllers  45  and  46  can be manually-set, or alternatively, can be electronically-controlled with the flow controller  41  via lines  40 . The mass flow controllers  43 ,  45  can be used to generate a flow of carrier gas from tank  44 , along with oxygen containing gas from tank  46 , enters the chamber  2  through injection tube  42  and is uniformly distributed in a vertical flow towards the substrate  5  via a distribution plate  47 . In  FIG. 1 , the distribution plate  47  extends horizontally across the chamber and partitions the upper portion of the chamber from its lower portion. The distribution plate  47  is positioned above the points of entry of other reaction gases into the chamber  2  to promote chemical reaction of gases at the surface of the substrate(s)  5  rather than in the upper part of the chamber away from the substrate(s). The distribution plate  47  has spaced holes defined therein to disperse the carrier and oxygen containing gases, and is positioned relative to the substrate(s)  5  so that the flow of carrier and oxygen-containing gas is directed vertically and thus transversely to the surface(s) of the substrate(s)  5  to promote the reaction forming the M 1-x-y Cd x Zn y O layer(s)  9  on the respective substrate(s). This oxygen-containing gas flow thus presses the reaction gases against the substrate(s)  5  where the reaction occurs.  
      An injection tube  48  is provided, and has an outer surface air-tightly sealed to the wall of chamber  2 . The injection tube  48  can be used for separate injection of dopant or other reaction gases. The injection tube  48  extends through the dispersion plate  47  and can have a diffusion head  49  defining spaced holes to diffuse carrier and dopant gas received through tube  48 . The diffusion head  43  diffuses and directs the carrier and dopant gas against the surfaces of the substrate(s)  5  to form the M 1-x-y Cd x Zn y O layer(s)  9  on the respective substrate(s)  5 . If the dopant is gaseous, such as a nitrogen- or phosphorus-containing gas, the opposite end of the injection tube  42  can be connected to a mass flow controller  50  that is in turn connected to tank  51  which contains the dopant gas.  
      Alternatively, if the dopant is in liquid form, such as a cadmium- or magnesium-containing volatile liquid, then the injection tube  48  can be connected to a pressure flow controller  52  which is in turn connected to a bubbler  53  containing the dopant-organic compound liquid  54 . The bubbler  53  is connected to mass flow controller  55  which is in turn connected to tank  56  of carrier gas such as argon. The controllers  50 ,  52 ,  55  can be manually-set, or can be electronically-controller by the flow controller  41  via lines  40 . In the case of use of a gaseous dopant, the flow controller  41  regulates flow through the mass flow controller  50  to a regulated level as set by a user. In the case of liquid dopant, the flow controller  41  controls the mass flow controller  55  to permit a regulated amount of carrier gas to pass through controller  55  to bubble in the liquid dopant compound  54  to generate dopant gas that passes at a pressure regulated by controller  41  via pressure flow controller  52  at a desired pressure through injection tube  48  and diffusion head  49  into the chamber  2 .  
      It should be noted that the flow controller  41  can be such as to control the mass flow and pressure flow controllers  27 ,  28 ,  29 ,  36 ,  37 ,  38 ,  43 ,  45 ,  50 ,  52 ,  55  to produce constant mass and pressure flows, or alternatively, can vary one or more of such flows during the process of growing the p-type Mg 1-x-y Cd x Zn y O layer(s)  9  on the substrate according to a flow-versus-time profile stored in the memory of the controller  41  by a user.  
      At the lower portion of the chamber  2 , an exhaust tube  57  is air-tightly sealed to the chamber  2 . Gases inside the chamber  2  can be exhausted using an exhaust pump  58 . The exhaust pump  58  can be manually-set to maintain a specified rate of exhausting of gases from the chamber  2 . Alternatively, the exhaust pump  58  can be electronically-controlled via a pump controller  59 . More specifically, the pump controller  59  receives a pressure signal on line  61  from butterfly valve  60  positioned in the exhaust tube  57 . The pressure signal on line  61  indicates the pressure of the gases in the chamber  2 . If the pressure of the chamber gases is at or below the target pressure set by a user in the pump controller  59 , the pump controller  59  generates the pump signal on line  62  to control the pump  58  to slow the rate of exhausting of gases from the chamber. Conversely, if the sensed pressure is above the target pressure set in the pump controller  59 , the pump controller  59  generates the pump control signal to cause the pump  58  to speed the rate of exhausting of gases form the chamber  2  to lower the gas pressure inside the chamber  2 . The gas pressure within chamber  2  can thus be maintained at a regulated target pressure.  
      In a mass production environment, it can be desirable to provide the apparatus  1  with a main controller  61 . The main controller  61  is connected to the rotation speed controller  10 , the temperature controller  14 , the flow controller  41 , and the pump controller  59 , and can be used to automatically activate or deactivate the such units to execute the process of forming the M 1-x-y Cd x Zn y O layer(s)  9  on respective substrate(s)  5 .  
      A substrate  5  can be formed, for example, from a stoichiometric powder of elements comprising the target crystal composition with a gas overpressure using an apparatus and method such as that described in U.S. Pat. No. 5,900,060 issued May 4, 1999 to Jeffrey E. Nause et als., which is incorporated herein by reference. This patent is commonly assigned to the owner of this application, Cermet, Inc., Atlanta, Ga. For example, a ZnO powder can be used in this apparatus and method to produce a ZnO crystal with few or virtually no impurities or defects. The nature of this apparatus and method is such as to produce the crystal from a liquid phase that is contained by a cooler outer solid phase of the same material. After formation, the substrate is cut, polished, and cleaned with a suitable etchant or other chemical agent to produce a flat, defect- and contaminant-free surface on which the M 1-x-y Cd x Zn y O layer(s)  9  can be formed.  
      It is generally preferred that the substrate(s)  5  has crystalline structure with a lattice spacing closely matched to that of the Mg 1-x-y Cd x Zn y O layer(s)  9  to be formed thereon. This helps to lower the number of defects in the layer(s)  9  that would otherwise be caused by lattice mismatch. Thus, ZnO is most preferred compound for use as the substrate(s)  5  because of its close or exact match with the lattice structure of the M 1-x-y Cd x Zn y O layer(s)  9  to be formed thereon. However, this does not exclude the possibility of using other substrate compositions, such as sapphire (Al 2 O 3 ), silicon carbide (SiC), silicon (Si), gallium arsenide (GaAs), and gallium nitride (GaN).  
      In operation of the apparatus  1 , a substrate(s)  5  such as a zinc oxide (ZnO) substrate(s) is placed on the carrier plate  3 , which is subsequently placed on the susceptor  4  through port  6 . The susceptor  4  is heated with the heater  13  to a temperature of, for example, 250 degrees C. to 1050 degrees C. to heat the substrate  5  to that temperature. The heater  13  can be controlled by temperature controller  14  using a sensed temperature signal from either of temperature sensors  15 ,  19 . The susceptor  4 , and hence the carrier plate  3  and substrate  5 , is rotated by driving the shaft  7  with drive unit  8 . The drive unit  8  controller to rotate the substrate(s)  5  via shaft  7 , susceptor  4 , and plate  3  at a constant speed or according to a rotation-speed-versus-time profile that varies over time. The dopant, reaction, and carrier gases are supplied in the chamber  2  through respective injection tubes  24 ,  25 ,  26 ,  42 , and  48  in a direction perpendicular, or at least transverse, to the substrate  5 . The mass and pressure flows of the reaction gases can be set either manually or by using flow controller  41  to generate signals supplied to the controllers  27 ,  28 ,  29 ,  36 ,  37 ,  38 ,  43 ,  45 ,  50 ,  52 ,  55 , to regulate the flows of dopant, reaction, and carrier gases into the chamber  2 . Exhaust pump  58  controls the pressure of chamber gases to a target pressure level. This can be done by manual setting of the exhaust pump  58 . Alternatively, the chamber pressure can be sensed using the butterfly valve  60  to generate a sensed pressure signal which the pump controller  59  uses to generate the pump control signal to control the pump  58  to maintain the chamber gas pressure to the target pressure level. The chamber pressure can be controlled via the exhaust pump  58  to maintain a pressure of five (5) to fifty (50) torrs of pressure during the layer growth operation. The exhaust pump  58  can control the chamber gas pressure to a constant target pressure throughout the layer growth operation, or alternatively, can vary the gas pressure in the chamber  2  according to a pressure-versus-time profile stored in the memory of the pump controller  59 . Spinning of the susceptor  4  and the attached carrier plate  3  and substrate  5  converts the reaction gas flow to one essentially parallel with the surface(s) of substrate(s)  5 . The reactants of the reaction gas react with each other to grow the desired ZnO-based II-VI Group compound semiconductor crystal layer on the entire substrate surface.  
      In  FIG. 2 a  method for forming a p-type zinc-oxide-based crystalline semiconductor layer on a substrate(s)  5  begins in step S 1  in which the substrate is placed in the chamber  2  through port  6 . In step S 2  the chamber  2  is sealed by closing port  6 . In step S 3  the substrate(s)  5  is rotated at speed from one-hundred (100) to one-thousand (1,000) rpm. This can be done by the drive unit  8  under control of the rotation speed controller  10 . In step S 4  the substrate(s)  5  is heated to a temperature in a range from two-hundred-fifty (250) to six-hundred-fifty (650) degrees C. This can be done with the heater  13 . The substrate temperature can be sensed by the temperature sensor  15  or  19  for use by the temperature controller  14  to control the heater  13  to regulate the substrate temperature. In step S 5  the zinc-containing reaction gas, optionally with magnesium- and cadmium-containing reaction gases, oxygen-containing gas, dopant gas, and any carrier gas used to transport the reaction and dopant gases, are supplied to the substrate(s)  5  to form the Mg 1-x-y Cd x Zn y O layer(s)  9  thereon. As previously described, the flows of gases can be produced and controlled by some or all of the injection tubes  24 ,  25 ,  26 ,  42 ,  48 , dissipation plate  47 , dissipation head  49 , pressure flow controllers  27 ,  28 ,  29 ,  52 , bubblers  30 ,  31 ,  32 ,  53 , containing respective liquids  33 ,  34 ,  35 ,  54 , mass flow controllers  36 ,  37 ,  38 ,  43 ,  45 ,  52 ,  55 , and tanks  46 ,  39 ,  51 ,  56  containing respective gases. In step S 6 , during the process of growing the M 1-x-y Cd x Zn y O layer(s)  9 , the gases are exhausted from the chamber  2  at a rate to maintain a target pressure of gases in the chamber  2 . The target pressure can be set by a user via the exhaust pump  58  directly, or via a pump controller  59  which controls the exhaust pump  58  based on the pressure sensed by butterfly valve  60 . In step S 7  a determination is made to establish whether the growth of the M 1-x-y Cd x Zn y O layer(s)  9  is complete. This determination can be made using a timer (not shown) internal to the flow controller  41  or monitored by a user to establish whether a predetermined period of time has elapsed from start of growth of the layer(s)  9 . By knowing the layer growth rate under the process parameters (e.g., reactant species, reactant mass flow and concentration, chamber pressure, substrate temperature, substrate spin rate, and reaction time) and the desired thickness of the substrate, the user can determine the amount of time necessary for the reaction to continue in order to grow the layer(s)  9  with the desired thickness. The present thickness standard for homogeneous p-type layer(s)  9  is at present one-hundred (100) microns for integration of electronic devices, but the layer thickness standard tends to be reduced over time as integrated device features become smaller due to improving integration technology. Alternatively, for electro-optic and opto-electric devices, because ZnO is inherently an n-type conductivity material, it is possible to make alternating p-type and n-type layers of a few nanometers to several nanometers thickness to produce an active layer stack for use as a LD or optical sensor, for example. This can be done merely by modifying the flow of p-type dopant gas to a level sufficiently high to produce a p-type layer of desired conductivity, and sufficiently low to produce a n-type layer of desired conductivity. If the determination in step S 7  is negative, e.g., insufficient time has elapsed for the gases to form the layer(s)  9  to the desired thickness, then the process continues by repeat of steps S 3  through and subsequent steps. The repeat of steps S 3  through S 8  can be performed with process parameters that remain static throughout the process. Alternatively, steps S 3  through S 8  can be performed using updated process parameters corresponding to the elapsed time from start of the process. Returning to consideration of step S 7 , if it is determined that growth of layer(s)  9  on substrate(s)  5  is complete, e.g., enough time has elapsed for the layer(s) to grow the desired thickness given the growth rate, in step S 8  the flow of zinc-containing reaction gas, and optionally also magnesium- and cadmium-containing reaction gases, as well as the dopant gas, is halted. In step S 9 , the substrate(s)  5  and layer(s)  9  are permitted to cool to room temperature by cutting off electric current to the heater  13  using the temperature controller  14 . In step S 10  the flow of oxygen and carrier gases is stopped. This can be done after the substrate(s)  5  and layer(s)  9  have cooled to room temperature. In step S 11  the rotation of the subtrate is stopped. In step S 12  the chamber is evacuated using the exhaust pump  58 . In step S 13  the substrate(s)  5  and the Mg 1-x-y Cd x Zn y O layer(s)  9  thereon are extracted from the chamber  2 . The Mg 1-x-y Cd x Zn y O layer(s)  9  of the substrate(s)  5  can then be used to form electrical, electro-optic, or opto-electric devices. One or more of steps S 1  through S 13  can be performed under control of the main controller  61  by activating and deactivating the controllers  10 ,  14 ,  41 ,  59  in accordance with the process parameters set for layer growth. The present invention will be described below by way of Examples which follow. In the following Examples, the apparatus of  FIG. 1  was used to grow a ZnO-based II-VI Group compound semiconductor crystal layer on a zinc oxide substrate.  
     EXAMPLE 1  
      A p-type ZnO layer was grown on a ZnO substrate by the following steps. 
          1. Chemically cleaned, n-type ZnO single crystal substrates  5  of (002) crystallographic orientation were placed on the substrate carrier plate  3 . This was then loaded onto the reactor susceptor  4  through port  6  and the chamber  2  sealed.     2. The chamber  2  was evacuated using the exhaust pump  58  to less than one-tenth (0.1) torr pressure.     3. A flow of three-thousand (3000) sccm argon and three-hundred (300) sccm oxygen was introduced into the chamber  2  through injection tube  42  via controllers  41 ,  43 ,  45 , and tanks  44 ,  46  and the chamber pressure regulated to ten (10) torr with pump  58 .     4. Meanwhile, the substrate susceptor temperature was increased to four-hundred (400) degree C. and rotation rate increased to six-hundred (600) revolutions per minute (rpm) in a period of sixty (60) minutes.     5. This state was maintained a period of time until the temperature of the susceptor stabilized at four-hundred (400) degrees C.     6. Diethylzinc vapor was introduced into the chamber at a rate of 1.4×10 −4  moles per minute (mol/min) entrained in an argon carrier flow of three-hundred-fifty (350) sccm using controllers  27 ,  36 ,  41 , bubbler  30 , and zinc-containing liquid  33 .     7. Meanwhile, tertiarybutylphosphine  54  was introduced into the chamber at a rate of 1.5×10 −5  mol/min entrained in an argon flow of three-hundred-fifty (350) sccm using controllers  41 ,  52 ,  55 , bubbler  53 , and tank  56  of argon gas.     8. Meanwhile, the previous three-thousand (3000) sccm argon and three-hundred (300) sccm oxygen flows mentioned were maintained, and the chamber pressure was regulated at ten (10) torr.     9. This state was maintained for one (1) hour during growth of the films  9  on substrates  5 .     10. Subsequently, all gas flows were halted except for one-thousand (1000) sccm argon carrier gas from tank  46  via controllers  41 ,  45 , and three-hundred (300) sccm oxygen via controllers  41 ,  43  and tank  44 . The chamber pressure was maintained at ten (10) torr.     11. The state of the system was maintained for sixty (60) minutes after deactivating the heater  13  with the temperature controller  14  while the substrates  5  cooled to near room temperature.     12. All gas flows were halted, and the chamber  2  was evacuated using the exhaust pump  58  to less than one-tenth (1.0) torr.     13. The chamber was then vented with atmosphere, and the substrates were removed.        

     EXAMPLE 2  
      MgZnO alloy films were grown on ZnO substrates by the following steps. 
          1. Chemically cleaned, n-type ZnO single crystal substrates  5  of (002) crystallographic orientation were placed on the substrate carrier plate  3 . This was then loaded onto the reactor susceptor  4  and the chamber  2  sealed.     2. The chamber  2  was evacuated using the exhaust pump  58  to less than one-tenth (0.1)torr pressure  3 . A flow of three-thousand (3000) sccm argon and three-hundred (300) sccm oxygen was introduced into the chamber  2  through injection tube  42  via controllers  41 ,  43 ,  45 , and tanks  44 ,  46  and the chamber pressure regulated to ten (10) torr with pump  58 . 4 . Meanwhile, the substrate susceptor temperature was increased to four-hundred (400) degree C. using heater  13 , temperature sensor  15  or  19 , and temperature controller  14 , and rotation rate increased to six-hundred (600) rpm in a period of sixty (60) minutes. This state was maintained a period of time until the temperature of the susceptor  4  stabilized at four-hundred (400) degree C.     3. Diethylzinc vapor was introduced into the chamber at a rate of 2.9×10 −5  mol/min entrained in an argon carrier flow of three-hundred-fifty (350) sccm using controllers  27 ,  36 ,  41 , bubbler  30 , and zinc-containing liquid  33 .     4. Meanwhile, bis(methylcyclopentadienyl)magnesium vapor was introduced into the chamber  2  via controllers  29 ,  38 ,  41 , bubbler  32 , magnesium-containing liquid  35 , and tank  39  of argon carrier gas  41 , at a rate of 5.8×10 −6  mol/min entrained in an argon carrier flow of three-hundred-fifty (350) sccm.     5. Meanwhile, the previous three-thousand (3000) sccm argon and three-hundred (300) sccm oxygen flows mentioned were maintained with units  42 - 47 , and the chamber pressure was regulated at ten (10) torr with exhaust pump  58 .     6. This state was maintained for ninety (90) minutes during growth of the films.     7. Subsequently, all gas flows were halted except for one-thousand (1000) sccm argon carrier gas from tank  46  via controllers  41 ,  45  and three-hundred (300) sccm oxygen via controllers  41 ,  43  and tank  44 . The chamber pressure was maintained at ten (10) torr by exhaust pump  58 .     8. The state of the system was maintained for sixty (60) minutes after deactivating the heater  13  using the temperature controller  14  while the substrates cooled to near room temperature.     9. All gas flows were halted, and the chamber  2  was evacuated using the exhaust pump  58  to less than one-tenth (1.0) torr.     10. The chamber  2  was then vented with atmosphere, and the substrates  5  were removed through port  6 . Due to the fact that the substrate and M 1-x-y Cd x Zn y O layer grown thereon are relatively pure and are closely matched in their crystalline lattice spacing, as well as the activation of a large number of dopant atoms in the layer, net acceptor concentration within the layer is relatively high and resistivity is relatively low as compared to layers produced with previous methods. For example, it is possible to produce layers with net acceptor concentrations of at least 10 17 /cm 3  and resistivities of at least one-tenth (0.1) Ohm-cm. Hence, the methods disclosed herein are useful for generating relatively high-performance, commercially-viable devices such as LEDs, LDs, and FETs.        

      Although the methods of the invention have been described herein with reference to specific embodiments and examples, it is not necessarily intended to limit the scope of the invention to the specific embodiments and examples disclosed. Thus, in addition to claiming the subject matter literally defined in the appended claims, all modifications, alterations, and equivalents to which the applicant is entitled by law, are herein expressly reserved by the following claims.