Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 10⁶ cm⁻² for 8 μm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 10⁶ cm⁻². These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64 × 10⁴ Ωm₂ (0 FOV) for devices with 7.8 μm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.     

EPD measurements for low dislocation density GaAs and InP substrates

The Japan Manufacturers' Society of Compound Semiconductor Materials (JAMS-CS) performed etch pit density (EPD) round robin evaluations of undoped semi-insulating GaAs substrates for three years from 1986 to 1988 [1]. More than ten years have passed since then, and with applications for semiconductor lasers escalating, the market for low dislocation density substrate is increasing rapidly. EPD measurements for low dislocation density substrates are obviously different from that of a conventional semi-insulating GaAs substrate with EPD around 1.0 × 10⁴ cm⁻².     

Control and growth of middle wave infrared (MWIR) Hg<Subscript>(1−x)</Subscript>Cd<Subscript>x</Subscript>Te on Si by molecular beam epitaxy

Middle wave infrared (MWIR) HgCdTe p-on-n double-layer heterojunctions (DLHJs) for infrared detector applications have been grown on 100-mm Si (112) substrates by molecular beam epitaxy (MBE) for large format 2,560×512 focal plane arrays (FPAs). In order to meet the performance requirements needed for these FPAs, cutoff and doping uniformity across the 100-mm wafer are crucial. Reflection high-energy electron diffraction (RHEED), secondary ion mass spectrometry (SIMS), Fourier transform infrared spectrometry (FTIR), x-ray, and etch pit density (EPD) were monitored to assess the reproducibility, uniformity, and quality of detector material grown. Material properties demonstrated include x-ray full width half maximum (FWHM) as low as 64 arc-sec, typical etch pit densities in mid-10⁶ cm⁻², cutoff uniformity below 5% across the full wafer, and typical density of macrodefects <1000 cm⁻². The detector quality was established by using test structure arrays (TSAs), which include miniarray diodes with the similar pitch as the detector array for easy measurement of critical parameters such as diode I-V characteristics and detector quantum efficiency. Typical I-V curves show excellent R₀A products and strong reverse breakdown characteristics. Detector quantum efficiency was measured to be in the 60–70% range without an antireflection coating.     

Current-voltage characteristics of electric contacts on p-type ZnSe

The current-voltage characteristics of electric contacts made of different materials on p-type ZnSe that form Schottky barriers from 0.3 to 1.2 eV are studied theoretically using the formula $$J = \frac{{A^* T}}{k}\int_0^\infty {T(E)[F(E) - F(E - eV)]dE,} $$ where T(E) is the energy-dependent quantum tunneling probability and F(E) is the Fermi distribution function. The contribution to the total current of both the thermionic emission and the tunneling are therefore included. The net doping concentrations under study range from 1.0×10¹⁷ cm^?3 to 1.0×10¹⁹ cm^?3. The reverse bias voltage across the barrier at a current density of 200 A/cm² is used to assess whether the barrier is reduced to an ohmic contact. A barrier of 0.3 eV is already an ohmic contact at doping concentration p=1.0×10¹⁷ cm^?3, while a barrier of 1.2 eV still behaves like a diode event at p=1.0×10¹⁹ cm^?3.     

An electrical method to characterize thermal reactions of Pd/GaAs and Ni/GaAs contacts

Capacitance-voltage (C-V) and current-voltage (I-V) measurements were used to study the thermal reaction of Pd/GaAs contacts and Ni/GaAs contacts. The thickness of GaAs consumed by the metal/GaAs reaction during annealing was calculated from C-V analyses and I-V analyses. For annealing temperatures below 350°C, the Schottky characteristics of the diodes were good but the electrical junction moves into the GaAs after annealing. The amount of junction movement was calculated directly from our measurements. The diffusion coefficients of Pd and Ni in GaAs at 300°C were estimated both to be around 1.2 × 10¹⁴ cm₂/s.     

Enhancement of Si-donor incorporation by Ga adatoms in Si delta-doped GaAs grown by molecular beam epitaxy

We report on the realization of a modified delta doping technique to obtain doping profiles in MBE grown GaAs, measured by capacitance-voltage (C-V) methods with full-widths at half-maximum (FWHM)s of 25 ± 5Å and peak concentrations of up to 1.1 × 10¹⁹ cm^?3. In this modified delta doping technique, both the Ga and Si shutters were opened for 15 sec during the delta doped layer growth while only the Si shutter is opened during conventional delta doping. Comparison of the two techniques under the same dopant flux and shutter-open-time interval shows that higher sheet-carrier concentrations with narrower FWHMs and higher peak concentrations are obtained with the modified delta doping than with the conventional delta doping method. This suggests that Si donor incorporation is enhanced by the Ga adatoms while broadening of the Si donor distribution is still negligible for this short time interval. The effects of the substrate temperature and the shutter-open time on the Si donor distribution have also been investigated.     

Effects of In and Sb doping in LPE growth thermodynamics and in GaAs layer qualities

We have experimentally determined the distribution coefficients of In and Sb in liquid phase epitaxially (LPE) grown GaAs at several different temperatures and found them to be consistent with the values calculated from the pseudobinary phase diagram of the ternary system at a dilute In or Sb concentration. Both In and Sb are found to be effective in reducing the dislocation densities in the LPE grown GaAs epi-layers. Studies of the phase diagram and the surface morphology indicate that Sb is more effective than In. Based on the results from the surface morphology, x-ray broadening and etch pit density (EPD) data, the optimal Sb concentration was 2–3 × 10¹⁹ cm⁻³.     

Growth of (GaAs)1_x(Ge2)x by metalorganic chemical vapor deposition

We report deposition of (GaAs)_{1_x}(Ge₂)_x on GaAs substrates over the entire alloy range. Growth was performed by metalorganic chemical vapor deposition at temperatures of 675 to 750°C, at 50 and 760 Torr, using trimethylgallium, arsine, and germane at rates of 2–10 μ/h. Extrinsic doping was achieved using silane and dimethylzinc in hydrogen. Characterization methods include double-crystal x-ray rocking curve analysis, Auger electron spectroscopy, 5K photoluminescence, optical transmission spectra, Hall-effect, and Polaron profiling. Results achieved include an x-ray rocking curve full-width at half maximum as narrow as 12 arc-s, Auger compositions spanning the alloy range from x = 0.03 to x = 0.94, specular surface morphologies, and 5K photoluminescence to wavelengths as long as 1620 nm. Undoped films are n type, with n ≈ 1 × 10¹⁷ cm⁻³. Extrinsic doping with silane and dimethylzinc have resulted in films which are n type (10¹⁷ to 10¹⁸ cnr⁻³) or p type (5 × 10¹⁸ to 1 × 10²⁰ cm⁻³). Mobilities are generally ≈ 50 cm²/V-s and 500 cm²/V-s, for p and n films, respectively.     

Doping dependence of the properties of GaAsAlxGa1-xAs single heterostructure luminescent diodes

GaAs luminescent diodes with single heterostructure have been fabricated. The dependence of light power output and time behaviour on doping of the active layer has been investigated. An optimal doping of p = 2.5 × 10¹⁸ cm^?3 with respect to light power and 8 × 10¹⁸ cm^?3 with respect to the power bandwidth product has been achieved.     

MeV energy sulfur implantation in GaAs and InP

Room temperature and elevated temperature sulfur implants were performed into semi-insulating GaAs and InP at variable energies and fluences. The implantations were performed in the energy range 1–16 MeV. Range statistics of sulfur in InP and GaAs were calculated from the secondary ion mass spectrometry atomic concentration depth profiles and were compared with TRIM92 values. Slight in-diffusion of sulfur was observed in both InP and GaAs at higher annealing temperatures for room temperature implants. Little or no redistribution of sulfur was observed for elevated temperature implants. Elevated temperature implants showed higher activations and higher mobilities compared to room temperature implants in both GaAs and InP after annealing. Higher peak electron concentrations were observed in sulfur-implanted InP (n ≈ 1 × 10¹⁹ cm⁻³) compared to GaAs (n ≈ 2 × 10¹⁸ cm⁻³). The doping profile for a buried n⁺ layer (n ≈ 3.5 × 10¹⁸ cm⁻³) of a positive-intrinsic-negative diode in GaAs was produced by using Si/S coimplantation.     

Current transport in fluorine implanted GaAs

Effects of fluorine implantation in GaAs have been investigated by electrical characterization. Ion implantation at 100 keV energy was conducted with doses of 10¹¹ and 10¹²/cm². The effect of fluorine implantation on current-voltage (I-V) characteristics of Schottky diodes was significant. Carrier compensation was observed after implantation by the improved I-V characteristics. The lower dose implanted samples showed thermionic emission dominated characteristics in the measurement temperature range of 300 to 100K. The starting wafer and the low dose implanted samples after rapid thermal annealing (RTA) showed similar I-V properties with excess current in the lower temperature range dominated by recombination. The higher dose implanted samples showed increased excess current in the whole temperature range which may result from the severe damage-induced surface recombination. These samples after RTA treatment did not recover from implantation damage as in the low dose implantation case. However, very good I-V characteristics were seen in the higher dose implanted samples after RTA. The influence of the higher dose ion implantation was to produce more thermal stability. The results show the potential application of fluorine implantation in GaAs device fabrication.     

<Emphasis Type="Italic">Ex Situ</Emphasis> Thermal Cycle Annealing of Molecular Beam Epitaxy Grown HgCdTe/Si Layers

We present the results of ex situ thermal cycle annealing (TCA) of molecular beam epitaxy grown mercury cadmium telluride (HgCdTe) on Cd(Se)Te/Si(211) composite substrates. We examined the variation in the etch pit density (EPD) and overall crystalline quality with respect to annealing temperature, number of annealing cycles, total annealing time, pre-annealed EPD/crystal quality, buffer layer quality, and buffer layer lattice constant. Using TCA we observed an order of magnitude reduction in the dislocation density of the HgCdTe layers and a corresponding decrease in x-ray full width at half maximum, when the as-grown layer EPD was on the order of 1 × 10⁷ cm⁻². Among all the parameters studied, the one with the greatest influence on reducing EPD was the number of annealing cycles. We also noticed a saturation point where the HgCdTe/Si EPD did not decrease below ∼1 × 10⁶ cm⁻², regardless of further TCA treatment or the as-grown EPD value.     

Effect of high-temperature annealing on GaInP/GaAs HBT structures grown by LP-MOVPE

We have investigated the effect of high-temperature annealing on device performance of GaInP/GaAs HBTs using a wide range of MOVPE growth parameters for the C-doped base layer. Carbon doping was achieved either via TMG and AsH₃ only or by using an extrinsic carbon source. High-temperature annealing causes degradation of carbon-doped GaAs in terms of minority carrier properties even at doping levels of p=1 × 10¹⁹ cm⁻³. The measured reduction in electron lifetime and luminescence intensity correlates with HBT device results. It is shown that the critical temperature where material degradation starts is both a function of doping method and carbon concentration.     

Potential barrier formation at a metal-semiconductor contact using selective removal of atoms

The possibility of forming a potential profile in a semiconductor by forming a metal film on its surface via selective removal of oxygen atoms from a deposited metal oxide layer was studied. Selective removal of atoms (SRA) was performed using a beam of accelerated protons with an energy of about 1 keV. Epitaxially grown GaAs films with a thickness of ～100 nm and an electron concentration of 2×10¹⁷ cm^?3 were chosen as the semiconductor material, and W obtained from WO₃ was used as the metal. The potential profile appeared due to the formation of a Schottky barrier at the metal-semiconductor interface. It was found that the Schottky barrier formed at W/GaAs contacts made by the SRA method is noticeably higher (～1 eV) than the barrier formed at the contacts made by conventional metal deposition (0.8 eV for W/GaAs). The data presented indicate that there is no damaged layer in the gate region of the structures, which is most strongly affected by the proton irradiation. Specifically, it was shown that the electron mobility in this region equals the mobility in bulk GaAs with the same doping level.     

Growth and structural properties of low defect,sub-grain free CdTe substrates grown by the horizontal bridgman technique

Large area, low defect CdTe substrates are essential for high quality epitaxy of HgCdTe in infrared detector applications. Vertical Bridgman (VB) CdTe normally exhibits higher than desired dislocation density and sub-grain structure. A seeded Horizontal Bridgman (HB) technique has been used to grow CdTe single crystals which exhibit superior crystalline qualities when compared to standard VB substrates. The HB grown CdTe crystals were not intentionally doped and had resistivities in the 10⁷ ohm-cm range. The etch pit density (EPD) near the seed and the tail end sections is 5 × 10⁴ cm⁻²s, while wafers from the middle section of the ingot have EPDs in the 10⁴ cm⁻² range. Furthermore, HB EPD patterns indicate the absence of sub-grain boundaries. X-ray rocking curves are very sharp and exhibit FWHMs as low as 9 arc-sec. By comparison, the best samples from standard VB CdTe ingots exhibit x-ray rocking curves with FWHMs in the >30 arc-sec range. The IR transmission of HB material is as high as 57% in the 2.5 to 20 μm region. Results of electrical and optical characterization are presented.     

The ohmic contact formation mechanism of Au/Pt/Pd ohmic contact to p-ZnTe

The ohmic contact formation mechanism and the role of Pt layer of Au(500Å) Pt(500Å)/Pd(100Å) ohmic contact to p-ZnTe were investigated. The specific contact resistance of Au/Pt/Pd contact depended strongly on the annealing temperature. As the annealing temperature increased, the specific contact resistance decreased and reached a minimum value of 6×10^?6 Θcm² at 200°C. From the Hall measurement, the hole concentration increased with the annealing temperature and reached a maximum value of 2.3×10¹⁹ cm^?3 at 300°C. The Schottky barrier height decreased with the increase of annealing temperature and reached a minimum value of 0.34 eV at 200°C and it was due to the interfacial reaction of Pd and ZnTe. Therefore, the decrease of contact resistance was due to the increase of doping concentration as well as the decrease of Schottky barrier height by the interfacial reaction of Pd ZnTe. The specific contact resistances of Au Pd, Au/Pt/Pd and Au/Mo/Pd as a function of annealing time was investigated to clarify the role of Pt layer.     

Interface state density in Au-nGaAs Schottky diodes

A method for determining the surface state density in Schottky diodes taking into account both I–V and C–V data while considering the presence of a deep donor level is presented. The model assumes that the barrier height is controlled by the energy distribution of surface states in equilibrium with the metal and the applied potential and does not include, explicitly, an interfacial layer. The model was applied to extract interface state densities of Au-nGaAs guarded Schottky diodes fabricated from bulk and VPE (100) GaAs with carrier conentrations between 3 × 10¹⁵ and 8 × 10¹⁶ cm^?3. These diodes exhibited ideality (n) factors of approximately 1.02 and room temperature saturation current densities ～10^?8 A/cm². This model is in substantial agreement with forward bias measurements over the 77–360°K temperature range investigated, in that a temperature-independent energy distribution of interface states was obtained. In reverse bias the interface state model is most valid with the higher carrier concentration material and at high temperature and low bias voltage. Typical interface state densities from 0.07 eV above the zero bias Fermi level to 0.01 eV below the Fermi level were 2 × 10¹³ cm^?2 eV^?1. The validity of the model under reverse bias is restricted by a non-thermionic reverse current, thought to be enhance field emission from traps.     

Characteristics of AuGeNi ohmic contacts to GaAs

We have studied AuGeNi ohmic contacts to n-type MBE grown GaAs epitaxial-layer with doping in the (10¹⁶?10¹⁹) cm^?3 range, and found several new effects: (a) Contact resistivity exhibit a weak dependence on carrier concentration (much weaker than 1/N_D depencence); (b) We find evidence for a high resistivity layer under the contact at least several thousands angstroms deep, which dominate the contact resistance in most cases; (c) We find a peripheral zone around the contact, about 1 μm wide which differs chemically from the GaAs epi-layer; (d) SIMS analysis reveals a deep diffusion into the GaAs of Ni and Ge; (e) Correlation between density of GeNi clusters in the contact and the contact resistivity are found; (f) Temperature measurements justify that tunneling is responsible for the ohmic contact. We discuss also the validity of the transmission line method and the commonly accepted model of the contact.     

Growth and characterization of Si doped vapor phase epitaxial GaAs for mesfet

This paper describes the Si-doping of GaAs that was grown using the AsCl₃:H₂:GaAs, Ga Chemical vapor deposition process. The doping sources were AsCl₃:SiCl₄ liquid solutions which proved to be highly reproducible for Si doping within the range, 1×1O¹⁶ to 2×10¹⁹ cm^?3. Incorporation of Si into the GaAs apparently occurs under near equilibrium conditions. This point is considered in detail and the consequences experimentally utilized to grow n, n+ bilayers using a single AsCl₃:SiCl₄ doping solution. Si impurity profiles based upon differential capacitance and SIMS data are presented. These can be very abrupt for n, n⁺ structures with order of magnitude changes occurring within 500 Å. For the 1×10₁₆ to 8×l0¹⁸ cm^?3 doped samples the mobilities at 78 and 298°K are comparable to the higher values reported for GaAs thin films grown by CVD. Power FET devices made from this material have demonstrated an output density of 0.86 watts/mm at 10 GHz.     

Study and modeling of the transport mechanism in a Schottky diode on the basis of a GaAs semiinsulator

The current through a metal-semiconductor junction is mainly due to the majority carriers. Three distinctly different mechanisms exist in a Schottky diode: diffusion of the semiconductor carriers in metal, thermionic emission-diffusion (TED) of carriers through a Schottky gate, and a mechanical quantum that pierces a tunnel through the gate. The system was solved by using a coupled Poisson-Boltzmann algorithm. Schottky BH is defined as the difference in energy between the Fermi level and the metal band carrier majority of the metal-semiconductor junction to the semiconductor contacts. The insulating layer converts the MS device in an MIS device and has a strong influence on its current-voltage (I-V) and the parameters of a Schottky barrier from 3.7 to 15 eV. There are several possible reasons for the error that causes a deviation of the ideal behaviour of Schottky diodes with and without an interfacial insulator layer. These include the particular distribution of interface states, the series resistance, bias voltage and temperature. The GaAs and its large concentration values of trap centers will participate in an increase in the process of thermionic electrons and holes, which will in turn act on the I-V characteristic of the diode, and an overflow maximum value [NT = 3 × 10²⁰] is obtained. The I-V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.