Abstract:
The subject invention comprises a plurality of serially connected small volume photovoltaic devices. A single device includes a first layer of n +  doped InSb, a second layer of doped IDAs 1-x  Sb x  and a third layer of p +  doped InSb. From 2-50 of these devices are grown by either a low-pressure MOCVD or MBE process to a maximum thickness of about 10 μm. Alternatively, the first layer may be n +  doped In y  Ga 1-y , Sb the second layer p +  doped InAs 1-x  Sb x  and the third layer In y  Ga 1-y  Sb, repeated to a maximum thickness of 10 m.

Description:
This invention is made with government support under Grants N00014-93-0931 and N00014-92-3-1951 awarded by the Department of the Navy. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This application relates to Sb-based semiconductor heterostructures for use in photodetectors and more particularly series connected Sb-based semiconductor heterostructures with reduced dark current and increased resistance for use at room temperature. 
     BACKGROUND OF THE INVENTION 
     I. PHOTODETECTORS 
     Unselective thermal detectors such as radiation thermocouples, bolometers and pyroelectric detectors are operated mainly at ambient temperature and can be used in an extremely wide range of electromagnetic spectrum, from x-rays to microwaves. Uncooled thermal detectors are lightweight, rugged, reliable and convenient to use. However, their detectivity is relatively low, especially when operated at high frequencies. They are used in applications which do not require high performance or fast response. The response time of thermal detectors is adequate for some imaging applications, while the wide spectral band and large number of elements can compensate for modest performance. 
     There are numerous applications such as fast imaging, laser receivers, real time spectroscopy and fast pyrometry, which do require high performance and fast response as well. A high sensitivity and fast response detection of infrared radiation can be achieved with the use of semiconductor photodetectors. 
     The major drawback of current IR photodetectors is the need for cooling to suppress thermal processes which compete with the optical ones in the generation of free carriers in a semiconductor. The capability to operate with background-limited sensitivity (BLIP) is considered the ultimate performance goal. Generally, cryogenic cooling is required to achieve this ideal situation. The cooling requirements of intrinsic narrow band gap semiconductor photodetectors are much less stringent than that of extrinsic devices, Schottky barrier and quantum well devices. Typically, to obtain background-limited performance, detectors for the 3-5 μm spectral region are operated at 200 K or less, while those for the 8-14 μm region are typically operated at liquid nitrogen temperature (77 K). 
     The conventional liquid nitrogen cooling technique is inconvenient in industry and field use. One alternative is to use Joule-Thompson microcoolers to change high-pressure nitrogen gas into a liquid directly at the cold finger of the dewar. Unfortunately, this cooling system requires a high-pressure gas tank or compressor. Another alternative is the Stirling-cycle cooler. No liquid or gas cryogen is required, only electrical power. The Stirling-cycle cooler is quite bulky, expensive and still unreliable. 
     The need for cooling is a considerable problem which inhibits the widespread use of IR systems. The cooling requirements add considerably to the cost, bulk, weight, power consumption and inconvenience of infrared systems and it is highly desirable that they be eliminated or reduced. 
     II. PHOTOCONDUCTORS 
     Photoconductivity is still the most common mode of operation of a 3-5 μm and 8-14 μ Hg 1-x  Cd x  Te photodetectors. Recent advances in material preparation and detector technology have led to devices approaching theoretical limits of responsivity over wide ranges of temperatures and background. The simple n-type Hg 1-x  Cd x  Te photoconductor operated at liquid nitrogen temperature is a mature product that has been in production for many years. The performance of near room temperature photodetectors has been further improved by the use of monolithic optical immersion, heterojunction contacts, 3D heterostructure design and non-equilibrium operation. 
     Generally, the present high-temperature photoconductors have relatively poor low-frequency properties, exhibiting a high level of l/f noise with a knee of about 10 kHz or more. The l/f noise is a critical issue. The problem is especially stringent in uncooled and longest wavelength devices, which require a high biasing for best performance. 
     However, the measured detectivity of uncooled 10.6 μm photoconductors at high frequencies is several orders of magnitude higher compared to other ambient temperature 10.6 μm detectors with subnanosecond response time such as photon drag detectors, fast thermocouples, bolometers and pyroelectric ones, approaching the performance of slow thermal detectors. 
     In contrast to photoconductors, photovoltaic detectors can operate without bias which enable good performance at low frequencies. Most of the previous work has been done using Hg 1-x  Cd x  Te. 
     The development of photodiodes was stimulated initially by their application as high-speed detectors, mostly for direct and heterodyne detection of 10.6 μm CO 2  laser radiation, but present efforts have been dominated by applications for large arrays for LWIR and MWIR thermal imaging systems. In contrast to photodetectors, photodiodes with their very low power dissipation can be assembled in two-dimensional arrays containing a very large (≈10 6 ) number of elements. 
     SUMMARY OF THE INVENTION 
     The subject invention comprises a plurality of serially connected small volume photovoltaic devices. A single device includes a first layer of heavily doped n +  InSb, a second layer of lighter p +   doped In 1-x  As x  Sb and a third layer of heavily doped InSb. From 2-50 of these devices are grown by either a low-pressure MOCVD or MBE process to a maximum thickness of about 10 μm. 
     Single-element photodiodes can be fabricated from the heterostructures of the subject invention using wet chemical etching and standard photolithographic techniques. Lactic-nitric acid mixtures provide a controllable etching rate. Metallization is realized using electron-beam evaporated TiAu layers. Annealing is not necessary because of the heavy doping used in the contact layers. The mesa top is fully metallized in order to decrease series resistance and increase quantum efficiency by optical double-pass. In a first stage, cleaved chips are mounted on standard laser heat-sinks with drilled hole for backside illumination. Optical immersion is achieved by using either refractive or Fresnel lenses prepared in the substrate. Refractive lenses can be prepared by mechanical grinding followed by chemical polishing. Fresnel lenses are prepared using multistage photolithography followed by ion-beam etching. 
     For three-dimensional heterostructures, the fabrication sequence is as follows: First, an n +  InSb/π-InAsSb heterostructure as grown on a semi-insulating GaAs substrate. Second, mesas are etched down to the p +  -InSb contact layer. Third, a wider gap lightly doped semiconductor (e.g. GaInSb) is regrown for lateral carrier confinement. Fourth, following removal of the mask, the n +  -InSb top contact layer is grown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the LP-MOCVD reactor for use in the method of the subject invention. 
     FIG. 2 is a cross-section of an InSb/InAsSb/InSb heterostructure for a photodetector operating at room temperature with an epitaxial layer structure on a GaAs substrate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The reactor and associated gas-distribution scheme used herein is shown in FIG. 1. The system includes a cooled quartz reaction tube (diameter 5 cm in the substrate area) pumped by a high-capacity roughing pump (120 hr -1 ) to a vacuum between 10 and 300 Torr. The substrate was mounted on a pyrolytically coated graphite susceptor that was heated by rf induction at 1 MHz. The pressure inside the reactor was measured by a mechanical gauge and the temperature by an infrared pyrometer. A molecular sieve was used to impede oil back-diffusion at the input of the pump. The working pressure was adjusted by varying the flow rate of the pump by using a control gate valve. The gas panel was classical, using 1/4 -inch stainless steel tubes and Swagelock fittings. Flow rates were controlled by mass flowmeters. 
     The reactor was purged with a nitrogen flow of 4 liters min -1 , and the working pressure of 75 Torr was established by opening the gate valve that separated the pump and the reactor. The evacuation line that was used at atmospheric pressure was automatically closed by the opening of the gate valve. The gas flow rates were measured under standard conditions, i.e., 1 atm and 20° C., even when the reactor was at subatmospheric pressure. The pressure in the gas panel was regulated by the needle valve placed between the gas panel and the reactor. The needle valve was adjusted to maintain a constant pressure of 1 atm on the gas panel, thereby ensuring reproducibility of flow-rate measurements. 
     Examples of the gas sources used in this study for the growth of InAsSb and InGaSb and doping sources are listed in the accompanying tabulation. 
     
         ______________________________________Sb(CH.sub.3).sub.3         AsH.sub.3    Sn(CH.sub.3).sub.4In(CH.sub.3).sub.3         Be(CH.sub.3).sub.2                      (C.sub.5 H.sub.5).sub.5 MGGa(C.sub.2 H.sub.5).sub.3         S.sub.2 H.sub.4______________________________________ 
    
     The organometallic group-III species trimethyl indium (TMI) and methyl antimony (TMA) are contained in stainless steel bubblers, which are held in controlled temperature baths at 31° and 0° C., respectively. An accurately metered flow of purified H 2  for TMI and TMA is passed through the appropriate bubbler. To ensure that the source material remains in vapor form, the saturated vapor that emerges from the bottle is immediately diluted by a flow of hydrogen. The mole fraction, and thus the partial pressure, of the source species is lower in the mixture and is prevented from condensing in the stainless steel pipe work. 
     Semi-insulating or n +  substrates are supplied by MCP, Crystacomm, Metals Research, or Sumitomo in the form of ingots or polished wafers. Wafers are sliced 2° off (100) toward (110) and chemically etched for 10 seconds at room temperature in a 15% bromine-methanol solution in order to remove 20 μm from each side. The wafers were then mechanochemically polished in a solution of 1.5% bromine in methanol, removing a further 80-100 μm. The substrates were finally cleaned in methanol and rinsed in isopropyl alcohol. The substrates were etched again, just before use, by dipping in 1% bromine in methanol at room temperature for one minute, rinsing in warm isopropyl alcohol, and drying. An n +  tin-doped substrate and a semi-insulating iron-doped substrate were generally used for each experiment. 
     The epitaxial layer quality is sensitive to the pretreatment of the substrate and the alloy composition. 
     Pretreatment of the substrates prior to epitaxial growth was thus found to be beneficial. One such pretreatment procedure is as follows: 
     1. Dipping in H 2  SO 4  for 3 minutes with ultrasonic agitation; 
     2. Rinsing in Deionized H 2  O; 
     3. Rinsing in hot methanol; 
     4. Dipping in 3% Br in methanol at room temperature for 3 minutes (ultrasonic bath); 
     5. Rinsing in hot methanol; 
     6. Dipping in H 2  SO 4  for 3 minutes; 
     7. Rinsing in deionized H 2  O, and 
     8. Rinsing in hot methanol. 
     After this treatment, it is possible to preserve the substrate for one or two weeks without repeating this treatment prior to growth. 
     Growth takes place by introducing metered amounts of the group-III alkyls and the group-V hydrides into a quartz reaction tube containing a substrate placed on an rf-heated susceptor surface. The hot susceptor has a catalytic effect on the decomposition of the gaseous products; the growth rate is proportional to the flow rate of the group-III species but is independent of temperature between 500° and 600° C. and of the partial pressure of group-V species as well. The gas molecules diffuse across the boundary layer to the substrate surface, where the metal alkyls and hydrides decompose to produce the group-III and group-V elemental species. The elemental species move on the hot surface until they find an available lattice site, where growth then occurs. They are contained in stainless steel bubblers, which are held in controlled temperature baths at 31° and 0° C., respectively. An accurately metered flow of purified H 2  for TMI and TEG is passed through the appropriate bubbler. To ensure that the source material remains in vapor form, the saturated vapor that emerges from the bottle is immediately diluted by a flow of hydrogen. The mole fraction, and thus the partial pressure, of the source species is lower in the mixture and is prevented from condensing in the stainless steel pipe work. 
     Molecular Beam Epitaxy (MBE) and low-pressure Metalorganic Chemical Vapor Deposition (MOCVD) are used to grow the Sb-based heterostructures of the subject invention. The growth is performed using readily available, high quality and low cost substrates, such as GaAs, and GaAs-coated silicon. 
     The practical near room temperature photodetectors with optimum performance require narrow gap π absorber layers and heavily doped wider gap n +  and p +  contact layers (n +  πp +  structures). In addition, the devices can be supplied with optical concentrators which can be prepared on the substrate. These optical concentrators are either refractive lenses or binary (e.g. Fresnel) lenses. The realization of lateral carrier confinement with wider bandgap semiconductors require either selective epitaxy and/or regrowth. The material growth requirements for the successful realization of detector arrays of the subject invention are the ability to grow high quality narrow-bandgap heterostructures with tight composition and doping control on foreign substrates such as GaAs and GaAs-coated Si and the ability to perform selective epitaxy and/or regrowth. 
     More specifically, the doping requirements for n + πp +  Sb-based heterostructures are the following: The doping of the narrow bandgap active layer should be in the range of 10 15  to 3×10 17  cm -3 . Moreover, the bandgap of this absorber material should be optimized for operation at any specific wavelength (8-14 μm) and temperature (200-300 K). The doping of the degenerate n +  contact materials should be in the range of 10 17  to 10 20  cm -3  and the bandgap should be wider than that for the absorber region, and the doping of the wider gap p +  contact material should be in the range of 10 17  to 10 20  cm -3 . 
     P-type doping may be achieved using Zn or Mg with MOCVD. Doping source temperatures, flows or fluxes will be adjusted to cover the doping range required for the contact layers. 
     Another possible candidate material for the contact layer (n +  and p + ) is In 1-y  Ga y  Sb. By alloying InSb with Ga, it is possible to increase the heterojunction barrier heights and improve carrier confinement, while maintaining identical lattice parameters. 
     In the method of the subject invention an InAs 1-x  Sb x  layer will be grown on top of the InSb buffer layer deposited on GaAs or GaAs-coated Si substrates and n +  doped. In 1-y  Ga y  Sb could be used on an InAsSb buffer layer as an alternative, with a lattice parameter matched to that of InAs 1-x  Sb x  (x=y=0.65-1.0). 
     The subject invention includes a multiple stack of n +  πp +  Sb-based heterostructures. The total thickness of such multijunction devices should be kept between about 5 and about 10 μm: In the case of single junction devices, the thickness of the absorber region is determined by the ambipolar diffusion length (a few micrometers). In the case of multijunction devices, the thickness of each individual absorber should be kept well below diffusion length, while the total absorber thickness should be close to the effective absorption length. This means that the proposed MOCVD and MBE growth techniques are the most appropriate for the realization of such structures requiring a large number of relatively thin layers for a total thickness on the order of a few microns. 
     Referring now to FIG. 2, there is shown one iteration of the device of the subject invention having a Gallium Arsenide substrate on which is grown a layer of InSb, with a further layer of InAsSb and a layer of InSb grown on top of that. Generally speaking, multiple stacks of these Sb-based heterostructures will be grown. Each such heterostructure will be approximately 0.1-0.7 μm with approximately 2-50 repetitions, up to a maximum of about 10 μm. The total thickness of the structure should be lower than the inverse of the absorption coefficient of radiation, i.e. ##EQU1## Each heterostructure layer is grown and then doped as follows. The first InSb layer is optimally 0.1 μm and doped at 10-20  atoms cm -3 , n-type. This layer may be between 0.05 and 1.5 μm and doped between 10 18  and 10 20   atoms cm -3  concentration. 
     The next layer, InAsSb, is optimally 0.5 μm and doped 2×10 16 , p-type. The thickness may be anywhere from 1 micron to 5 microns, while the doping concentration may be within the range of 1-5×10 16  atoms cm -3 . 
     The top layer of InSb is optimally 0.1 μm with p-type doping of 10 19  cm -3 . The thickness of this top layer may be between 0.05 to 0.15 μm with a doping level of 10 18  to 10 20  cm -3  p-type. 
     It is important that the lattice matches of the various layers be as close as possible. Thus, InGaSb is an acceptable alternative to InSb, both having ideal lattice matches with InAsSb. 
     The following examples illustrate the subject invention: 
     EXAMPLE 1 
     
         ______________________________________MOCVD Grown Structure   (μm)       (atms)/cm.sup.-3                            DopingLayer   Thickness     Level Doping                            Element______________________________________InSb    0.1           10.sup.19  SnInAsSb  0.5           2 × 10.sup.16                            ZnInSb    0.1           10.sup.20  Zn______________________________________ 
    
     The above structure was repeated 15 times using MOCVD growth. Good structural quality and composition of the ternary layers were observed using high resolution x-ray diffraction. Photovoltaic response was demonstrated up to 12 μm at 200° K and up to 14 μm at room temperature. 
     EXAMPLE 2 
     
         ______________________________________MBE Grown Structure     (μm)     (atms)/cm.sup.-3                            DopingLayer     Thickness   Level Doping                            Element______________________________________InSb      .05         10.sup.19  SiIn.sub.0.8 As.sub..2 Sb     .25         10.sup.16  BeInSb      .05         10.sup.19  Be______________________________________ 
    
     32 layers of the above heterostructure are prepared utilizing MBE procedures, p-type layer being doped to the concentration indicated with Bis(cyclopentadienyl) magnesium. Similar characteristics as set forth above in Example 1 are observed. 
     EXAMPLE 3 
     
         ______________________________________MBE Grown Sructure     μm      (atms/cm.sup.-3)                          DOPINGLAYER     THICKNESS  DOPING    ELEMENT______________________________________1   InAsSb    1.0        ≃10.sup.19                            Si      Buffer                                    Layer2   In.sub..65 Ga.sub..35 Sb         0.2        ≃10.sup.18                            Si3   InAs.sub..35 Sb.sub..65         0.2        2 × 10.sup.16                            Be4   In.sub..65 Ga.sub..35 Sb         0.2        ≃10.sup.18                            Be______________________________________ 
    
     Layers 2, 3 and 4 of the above heterostructure are grown six successive times to achieve characteristics similar to those observed in Example 1. 
     EXAMPLE 4 
     
         ______________________________________MOCVD Grown Structure     μm       (atms/cm.sup.-3)LAYER     THICKNESS   DOPING    DOPING EFFECT______________________________________1   InAsSb    1.0         ≃10.sup.19                             Sn2   In.sub..65 Ga.sub..35 Sb         0.2         ≃10.sup.18                             Sn3   InAs.sub..35 Sb.sub..65         0.2         ≃2 × 10.sup.16                             Mg4   In.sub..65 Ga.sub..35 Sb         0.2         ≃10.sup.18                             Mg______________________________________ 
    
     Layers 2, 3 and 4 of the above heterostructure are grown six successive times to achieve characteristics similar to those observed in Example 1. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments and equivalents falling within the scope of the appended claims. 
     Various features of the invention are set forth in the following claims.