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Low-temperature-grown (LTG) GaAs is a unique material that has been used in a variety of device applications to achieve record performance. LTG GaAs used as a buffer layer eliminates sidegating and backgating and in GaAs integrated circuits. Record output power density (1.57 W/mm) and superior microwave-switch performance were demonstrated when LTG GaAs was used at a gate insulator in a metal-insulator-semiconductor field-effect transistor. High-speed (0.5 ps) and high-voltage (1 kV) LTG GaAs photoconductive switches have also been demonstrated. Using the same material, researchers have demonstrated highresponsivity (0.1 A/W), wide-bandwidth (∼ 375 GHz) LTG GaAs photodetectors. Devices incorporating LTG GaAs are currently being optimized for systems applications. LTG GaAs technology can enhance system performance and enable new systems for military and commercial applications in the areas of radar, communications, instrumentation, and highspeed computing.
Since its initial report by the IBM/Purdue University group in 1990, GaAs with As precipitates (GaAs:As) has been shown by this group to exhibit unusual and useful electrical and optical properties. In this paper we review our progress in understanding the fundamental properties of this material. We have shown that both the electrical and optical properties of GaAs:As arc explained by assuming that the GaAs is of good crystalline quality and that the As precipitates act as buried Schottky barriers. This model accounts for its semi-insulating stability against both n- and p-type doping, its high-speed photoconductive behavior, and its ability to detect 1.3 micron light when it forms the “I” layer of a PIN photodiode via the internal photoemission process. Using modulation spectroscopy we clarify the fundamental differences between GaAs:As and unannealed GaAs grown at 200 C. We also show that GaAs:As used as a 1.3 micron detector in the metal-semiconductor-metal device structure format, has a photoconductive bandwidth in excess of 50 GHz.
The localized vibrational mode (LVM) of silicon donor (SiGa) in molecular beam epitaxial GaAs layers grown at various temperatures is studied using the infrared absorption technique. It is found that the total integrated absorption of this LVM is decreased as the growth temperature decreases. This finding suggests a nonsubstitutional incorporation of Si in GaAs layers grown at ∼200 °C. On the other hand, an almost complete substitutional incorporation is obtained in GaAs layers grown at temperatures higher that 350 °C. Thermal annealing does not cause any recovery of the SiGa LVMs in present GaAs layers grown at ∼200°C.
The compensation mechanism and transport properties of annealed GaAs grown by molecular beam epitaxy at low substrate temperature (LT-GaAs) and Cu diffused InP are analyzed by using a deep donor band model and a precipitate model. It was found that the compensation in highly resistive LT GaAs can not be explained by the precipitate model alone, and therefore a high donor density had to be considered. In Cu diffused InP, the precipitate model gives a consistent explanation for the observed carrier compensation and mobility data. For both semi-insulating LT-GaAs and fully-compensated, lightly-doped InP:Cu, the neutral impurity scattering was found to be a major carrier scattering mechanism.
The electrical characteristics of an N(LT)N structure are studied through implementation of numerical simulation techniques for the case of donor traps 0.83 ev below the conduction band and acceptor traps 0.3 ev above the valence band. The results show characteristics sensitive to the relative densities of the traps. In particular, high acceptor trap / low donor trap concentrations generally result in low breakdown voltages, whereas high acceptor / high donor concentrations result in higher breakdown voltages.
LT GaAs(220°C) was grown on an n+ substrate and capped with n+ GaAs grown at 600°C (n-i-n). Complete IV and CV measurements were performed. The IV characteristics exhibit ohmic, trap-filling and space-charge-limited regimes. We have developed a model based upon the compensation of background shallow acceptors by deep donor traps, large concentrations of which have been shown to exist in LT GaAs. Computer simulation of the IV curve is compared with experimental results. The “breakdown” is attributed to trapfilling under electron injection. It is also found that when the voltage across the structure is changed, the current takes several seconds to reach steady state. This is consistent with our model, which assumes slow trapping and detrapping in the LT GaAs. High frequency CV measurements show the capacitance to be fairly constant for voltages below “breakdown”.
The effect of growth conditions on the properties of GaAs grown by molecular beam epitaxy at low substrate temperatures has been studied. It has been found that the response time to 100 fsec 830nm light pulses is a function of substrate temperature and arsenic flux. The reason for variation of optical response with growth conditions is related to the nature of the incorporation of excess arsenic. A recent model proposed by Warren and others is invoked to explain the change in optical response with growth conditions. Further substantiation of this model comes from experiments on the annealing of low substrate temperature GaAs which has been doped with silicon.
A zero-bias thermally stimulated current (TSC) spectroscopy under both optical (1.96eV) and electrical excitation using samples with a Schottky contact on the top was applied to annealed LTMBE GaAs grown at different temperatures, and bulk SI GaAs with different stoichiometries. The results show that: 1) the new TSC technique is capable of revealing the traps at 235K<T<380K and is effective in indicating the crystal stoichiometry of bulk SI GaAs; 2) the driving force for the currents under a zero-bias comes from the builtin surface field as well as a thermal gradient; 3) the trap species in the annealed LTMBE GaAs samples and the annealed control SI GaAs sample are similar, especially for TA(0.79eV), Ta (0.33eV), T4 (0.29eV), and T6*(0.16eV); and 4) the trap densities in LTMBE samples are higher than those in the control samples and are dependent on the MBE growth temperature.
We have developed a simpler and more reliable method of thermoelectric effect spectroscopy (TEES) by eliminating the second heater in the technique. We have applied this method to the deep level studies in semi-insulating(SI) GaAs epitaxial layers grown at a low temperature by molecular beam epitaxy (LT-GaAs) and SI-undoped GaAs, Cr-doped GaAs. We have found that the electrical contacts on front and back surfaces of the sample are more reliable for the TEES measurement than both contacts made on the same surface. In this contact arrangement, the temperature difference of about 1–2K between the back and front surfaces was enough to produce a clear and reliable TEES data, without the need for a second heater. The results obtained by TEES are consistent with the results obtained by photo-induced current transient spectroscopy (PICTS) and by thermally stimulated current (TSC) measurements. The TEES results clearly distinguish between the electron traps and the hole traps. We will discuss the results on the various semi-insulating GaAs samples and the advantages and limitations of the TEES technique.
The native defects in LTMBE III-V layers have been studied by the electron paramagnetic resonance (EPR) technique for three different systems: GaAs on GaAs, GaAs on Si and InP on InP. The GaAs layers are characterised by high concentrations of ionized arsenic antisite defects(1019 cm −3), with properties similar to those of the native AsGa in amorphous GaAs. Their variation with the growth temperature, layer thickness and thermal annealings has been assessed.The results are independant on the nature of the substrate, GaAs or Si. Inspite of a 1% phosphorous excess no phosphorous antisites could be detected in the as-grown, undoped or Be doped InP layers.
Observation of Arsenic antisites (AsGa) in GaAs layers grown by molecular beam epitaxy (MBE) at low substrate temperatures (∼ 200°C) is reported, using electron paramagnetic resonance (EPR), magnetic circular dichroism in absorption (MCDA), and MCDA tagged by optically detected magnetic resonance (MCDA-ODMR). This experiment confirms that there is a MCD absorption band directly associated with AsGa in the GaAs layers. The AsGa concentration in the GaAs layers is found to decrease by about one order of magnitude after annealing at 600°C for two minutes.
We report a cathodoluminescence (CL) and photoluminescence (PL) study of molecular beam epitaxy grown GaAs at low substrate temperatures (LT GaAs), and semi-insulating LEC GaAs. The as grown LT GaAs material shows intense deep level emissions which can be associated with an excess concentration of Arsenic. These emissions subside with annealing for a few minutes at temperatures above 450 ° C. CL measurements clearly show an extremelly reduced concentration of traps in the post-growth 600 ° C annealed material. These results account for a diminished role of electronic point defects in controlling the insulative behavior of LT GaAs and strongly support the “buried” Schottky barrier model.
It has recently been shown that a 1000Å cap layer of molecular beam epitaxial (MBE) GaAs grown at 200°C passivates the surface of a GaAs active layer (n≃2×1017cm−3) in the sense of reducing the free–carrier depletion which arises from surface acceptor states. The same phenomenon holds for active-layer concentrations up to 7×1018cm−3, for caps as thin as 14Å, and for either As2 or As4 anion species. In an attempt to understand these effects, we have applied photoreflectance (PR) and x–ray photoelectron spectroscopy (XPS). In general, the PR shows contributions from the surface, cap/active–layer interface, and active–layer/buffer–layer interface, because each of these regions can have a different electric field. In fact the various field strengths can be determined from Franz–Keldysh oscillations (FKO), and good agreement with Hall–effect measurements is usually found. However, for 200°C material, no PR is seen, suggesting that there is no surface charge (no surface acceptor states below the Fermi level) or at least no surface–charge modulation by the light. The XPS data, which arise only from the near–surface (∼30Å) region, show that the binding energies in the capped samples are increased (i.e., surface Fermi pinning energy decreased) by 0.2 eV with respect to those in the uncapped samples. These data are discussed in relation to a passivation model.
We have studied the annealing characteristics of acceptor doped GaAs:Be grown at Low substrate Temperatures (300°C) by Molecular Beam Epitaxy (LTMBE). The Be was introduced in a range of concentrations from 1016 –1019 cm−3. As-grown material was found to be n-type even up to the highest Be concentration of 1019 cm−3 although Raman spectroscopy of the Be local vibrational mode indicates that the majority of the Be impurities are substitutional. We propose that Be acceptors are rendered inactive by the high concentration of AsGa-related native donor defects present. Results of slow positron annihilation studies indicate an excess concentration of VGa in LTMBE layers over bulk grown crystals. A distinct annealing stage at 500°C, similar to irradiation damaged and plastically deformed GaAs, marks a rapid decrease in the AsGa defect concentration. A second annealing stage at 800°C corresponds to the activation of Be acceptors. Analysis of isothermal annealing kinetics for the removal of AsGa-related defects gives an activation energy of 1.7 ±0.3 eV. We model the defect removal mechanism with the VGa assisted diffusion of ASGa to As precipitates.
The structural quality of GaAs layers grown at low temperatures by solid-source and gassource MBE at different growth conditions is described. Dependence on the growth temperature and concentration of As [expressed as As/Ga beam equivalent pressure (BEP)] used for the growth is discussed. A higher growth temperature is required to obtain the same monocrystalline layer thickness with increased BEP The annealing of these layers is associated with the formation of As precipitates. Semicoherent precipitates with lowest formation energies arc formed in the monocrystalline parts of the layers grown with the lowest BEP. Precipitates with higher bormation energies are formed when higher BEP is applied; they are also formed in the vicinity of structural defects. Formation of As precipitates releases strain in the layers. Arsenic precipitates are not formed in annealed ternary (InAlAs) layers despite their semi-insulating properties. The role of As precipitates in semi-insulating properties and the short lifetime of minority carriers in these layers is discussed.
Excess arsenic can be incorporated in GaAs and AIGaAs epilayers by growing at low substrate temperatures (LT-GaAs and LT-AIGaAs) by molecular beam epitaxy (MBE). Upon annealing these epilayers, the excess As precipitates forming GaAs:As and AIGaAs:As. Using transmission electron microscopy (TEM), we have measured the densities and sizes of the As precipitates and thereby determined the amount of excess As incorporated in these epilayers. The volume fraction of excess As as a function of inverse substrate growth temperature follows an Arrhenius-type behavior with an activation energy of 0.87 eV. The sizes of the As precipitates increase and the densities decrease with increase anneal temperatures; for Si-doped GaAs:As this results in n-type material when the densities become small enough that the depletion regions around the As precipitates no longer overlap. Also investigated is the formation of As precipitates at GaAs/AIGaAs heterojunctions and superlattices, and our attempts to tailor the As precipitate distribution.
GaAs layers grown by molecular beam epitaxy (MBE) at low substrate temperatures (LT GaAs) were studied in a novel purpose designed X-ray experiment. It combines X-ray double crystal rocking curve measurements with some elements usually found in optical setups like light illumination at liquid nitrogen temperatures applied to transfer EL2 type defects into metastable state. Ability to record such transfers with the X-ray experiment as well as large lattice relaxation accompanying this process is presented.
For the first time, surface acoustic waves (SAWs) were used to study the lattice relaxation of metastable defects. A persistent increase of as much as 0.4% of the SAW velocity at low temperatures was observed after illumination of LT-GaAs; this increase could be quenched by annealing at 120–130°K. This behaviour is caused by the metastable transition of EL2-like AsGa defects and constitutes the direct experimental proof of the illumination induced large lattice relaxation of this defect.
Low-temperature GaAs layers (LTGaAs) grown by molecular beam epitaxy on GaAs substrates have been characterized by x-ray diffraction techniques. X-ray rocking curve measurements on more than 200 anneal conditions show that through appropriate choice of growth condition, layers with different states of strain are obtained. Three distinct ranges of low temperature growth are defined, labelled as “low-range,” “mid-range,” and “high-range,” corresponding to growth temperatures less than 260 °C, between 260 and 450 °C, and more than 450 °C, respectively. 0.5μm thick films grown in the low-range are amorphous, whereas those in the mid-range are fully strained and lattice-matched to the substrate, and those grown above 450 °C are indistinguishable from ordinary GaAs. Notable properties of mid-range layers are the random behavior of the as-grown strain, and the expansion and contraction of the lattice parameter with thermal anneals up to 900 °C. A growth model for LTGaAs based on arsenic antisite defects is proposed.
We have measured the excess As atoms present in GaAs layers grown by molecular beam epitaxy at low substrate temperatures using particle induced x-ray emission technique. The amount of excess As atoms in layers grown by MBE at 2000C were found to be ∼4×1020 cm−2. Subsequent annealing of the layers under As overpressure at 600'C did not result in any substantial As loss. However, transmission electron microscopy revealed that As precipitates (2- 5nm in diameter) were present in the annealed layers. The lattice location of the excess As atoms in the as grown layers was investigated by ion channeling methods. Angular scans were performed in the <110> axis of the crystal. Our resutls strongly suggest that a, arge fraction of these excess As atoms are located in an interstitial position close to an As row. These As “intersitials” are located at a site slightly displaced from the tetrahedral site in a diamond cubic lattice. No interstitial As signal is observed in the annealed layers.