Structural and electrical contact properties of LPE grown GaAs doped with indium

We have studied the effects of In doping on the structural and electrical properties of a liquid phase epitaxially (LPE) grown GaAs. The results of surface morphology studies show that macroscopically, a terrace-free area in certain regions can be seen on the surface of a GaAs layer doped with In of 2.4 × 10¹⁹ cm³. The full widths at half-maximum (FWHM) of x-ray double crystal rocking curves show that a GaAs epi-layer of good crystalline quality can be obtained by doping In to a concentration up to 4.3 × 10¹⁹ cm^-3, beyond which a sharp increase in the FWHM is observed. Etch pit density (EPD) study also shows that the dislocation density is reduced by doping the epi-layer with In. At an optimum In concentration, 2.4 × 10¹⁹ cm^-3, the EPD was reduced by a factor of 20 when measured at the surface of a 9μm thick epi-layer. The I-V characteristics of Au-GaAs Schottky diodes show, for the layer with an optimal In concentration, an ideality factor close to 1.04 over more than seven decades of current. For the same layer, the reverse I-V characteristics are close to an ideal Schottky diode and could be fitted by a theoretical curve, combining the thermionic field emission and thermionic emission. For doping levels higher than 6 × 10¹⁹ cm^-3, the epitaxial layer quality deteriorated. We report the results obtained from the Nomarski optical microscope, double crystal x-ray rocking curves, etch pit density, forward and reverse I-V characteristics, and the theoretical current transport models.     

GaAs planar gunn devices with sulfur-ion implanted n layers

Planar type Gunn effect devices have been fabricated by sulfur-ion implantation into the Cr doped semi-insulating GaAs substrates. The high doping efficiency as 90% was obtained as a result of long heat treatment. The mobility of the sulfur-ion implanted n layers with average carrier concentration of 4 × 10¹⁶cm^?3 was 5200 cm²/Vsec at room temperature and 12,000 cm²/Vsec at 77 K. The minimum gate trigger voltage of the Gunn effect digital devices was 100 mV. Sulfur-ion implanted Gunn effect devices have shown superior current drop ratio dependence on doping-depth product, compared to the devices prepared from the epitaxial layer.     

Green emitting Zn-diffused LPE GaP diodes

The growth of liquid phase epitaxial (LPE) layers of n-type GaP on GaP substrates was investigated using a multi-wafer growth system constructed of fused quartz which had provisions for gas phase saturation and doping of the gallium melt. The effect of growth temperature, substrate orientation, doping level, and repeated use of the same melt on the properties of zinc diffused electroluminescent diodes fabricated from these epitaxial layers were investigated. After an initial increase, the carrier concentration remained relatively constant (1.7 × 10¹⁷/cc) throughout a series of eighteen runs from the same melt. Using conventional commercial techniques, zinc diffused diodes with efficiencies of 0.05% at 30A/cm² and a brightness of 1200 fL at 10A/cm² were produced. These diodes had a limited area n and p-type contact and had an epoxy dome. Layers grown on the 〈 111 〉P orientation had the best surface quality whereas those grown on the 〈 100 〉 plane incorporated less background impurities. The use of relatively low growth starting temperatures (∼ 920°C) was found to minimize the background impurity of the layers and the substrate surface deterioration due to the reaction with ammonia. This work was, in part, sponsored by the Air Force Materials Laboratory under the direction of Mrs. E. H. Tarrants, contract number F33615-71-C-1621     

Current controlled liquid phase epitaxial (CCLPE) growth of InGaAs on (100) InP

High quality epitaxial layers of In_xGa_1−xAs (x = 0.53) were grown on semi-insulating (Fe-doped) (100) InP sub-strates. The layers were grown at a constant furnace temperature of 640°C by passing a direct electric current (0–10A/cm²) from the substrate to the melt. In order to minimize the out-diffusion of Fe atoms from the bulk of the substrate during the melt saturation, the substrate was kept at a cold temperature region (340°C) within the growth chamber and remotely loaded in the graphite boat just prior to the initiation of the growth cycle. In addition to pre-venting the out-diffusion of Fe atoms, this procedure sub-stantially reduced the thermal degradation of the InP sub-strate surface. The above technique produced high quality layers having uniform thickness and good surface morphology. A study of the dependence of growth rate on the applied current density yielded an average growth rate of 0.06μm/ A-min. Room temperature Hall measurements on layers grown by CCLPE resulted in Hall mobilities μ₃₀₀ = 8900cm²/V-sec at a carrier concentration of 6.2 × l0¹⁶cm⁻³. The improve-ment in the mobility achieved by the CCLPE technique is attributed to a reduced out-diffusion of scattering centers from the substrate into the growth layer, as well as to the higher quality of epitaxial layers normally achieved by CCLPE.     

Growth condition of iodine-doped n+-CdTe layers in metal-organic vapor phase epitaxy

Iodine doping of CdTe layers grown on (100) GaAs by metal-organic vapor phase epitaxy (MOVPE) was studied using diethyltelluride (DETe) and diisopropyltelluride (DiPTe) as tellurium precursors and ethyliodine (EI) as a dopant. Electron densities of doped layers increased gradually with decreasing the growth temperature from 425°C to 325°C. Doped layers grown with DETe had higher electron densities than those grown with DiPTe. When the hot-wall temperature was increased from 200°C to 250°C at the growth temperature of 325°C, doped layers grown with DETe showed an increase of the electron density from 3.7×10¹⁶ cm⁻³ to 2.6×10¹⁸ cm⁻³. On the other hand, such an increase of the electron density was not observed for layers grown with DiPTe. The mechanisms for different doping properties for DETe and DiPTe were studied on the basis of the growth characteristics for these precursors. Higher thermal stability of DETe than that of DiPTe was considered to cause the difference of doping properties. With increasing the hot-wall temperature from 200°C to 250°C, the effective ratio of Cd to Te species on the growth surface became larger for layers grown with DETe than those grown with DiPTe. This was considered to decrease the compensation of doped iodine and to increase the electron density of layers grown with DETe. The effective ratio of Cd to Te species on the growth surface also increased with decreasing growth temperature. This was considered to increase the electron density with decreasing growth temperature.     

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 10¹⁸ cm⁻³ in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N₂ dopant flow. Due to the high n-type background carrier concentration of the order of 10¹⁷ cm⁻³ in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 10¹⁸ cm⁻³. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H₂ was used as carrier gas.     

Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

Growth condition dependence for GaAs selective epitaxial growth by molecular beam epitaxy

GaAs selective epitaxial growth by conventional molecular beam epitaxy (MBE) was studied while varying its growth conditions, such as substrate temperature. As pressure, growth rate, and Si or Be doping. Selectivity is improved with the increase in substrate temperature, and with the decrease in As pressure or growth rate. Si and Be were doped up to 3 x 10¹⁸ and 3 × 10¹⁹ cm⁻³, respectively. While no Si doping influence was observed, Be doping degraded the surface morphology. Selective epitaxial growth by conventional MBE with appropriate growth conditions will be applicable to device fabrication.     

AlGaSb Buffer Layers for Sb-Based Transistors

InAs quantum wells can serve as the channel for high-electron-mobility transistors. Structures are typically grown on semi-insulating GaAs substrates with 1.5 μm to 3.0 μm buffer layers of AlSb and AlGaSb accommodating the lattice mismatch. We demonstrate that high electron mobility in the InAs (>20,000 cm²/V s at 300 K) and smooth surfaces can be achieved with Al_0.8Ga_0.2Sb buffer layers as thin as 600 nm, grown at rates of 1.5 monolayers/s to 2.0 monolayers/s. The use of thinner buffer layers reduces molecular beam epitaxial growth time and source consumption. The buffer layers also exhibit higher resistivity, which should reduce excess gate leakage current and improve device isolation.     

Transport properties and defects in semi-insulating and Si-implanted InP

The transport properties and defect levels in Si-implanted semi-insulating and liquid phase epitaxial InP have been studied by Hall and photoconductivity measurements. Wide variations in the conductivity and Hall coefficient have been measured in semi-insulating InP:Fe and the results have been analyzed and interpreted by appropriate charge neutrality models. The energy position of the Fe and Cr acceptor levels have been determined to be 0.68 and 0.40 eV, respectively, below the conduction band minimum. The implantation studies indicate that the electrical properties of the layers are very sensitive to implant dose and energy, the type and thickness of encapsulant and the anneal temperature. High-resistivity or p-type conductivity was observed in layers implanted with 6.0 × 10¹¹ to 4.0 × 10¹² cm⁻² Si⁺. In general, better results were obtained with sputtered Si₃N₄ encapsulation. Varying amounts of Fe and Cr outdiffused to the active layer during annealing and a dominant defect, 0.56 eV below the conduction band, was observed in the photoconductivity spectra.     

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, x_Bi, lower than 0.2 at.%, an excess deep-donor level (activation energy E_d≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 10¹⁷ cm⁻³. Inductive coupled plasma (ICP) emission analysis shows that, for x_Bi=1 at.%, Bi concentration in the epitaxial layer is as high as N_Bi=2.3–2.7 × 10¹⁹ cm⁻³. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10⁻⁵ torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.     

Vapor growth of InP for MESFET’S

Epitaxial layers of InP have been grown by the conventional In/PCl₃/H{ion2} technique. With the aim of fabricating FET’s structures, we have studied the growth of low doped buffer layers and the doping by H₂S. It has been shown, that the purity of the layers increases from experiment to experiment and that low doped layers, in the 10¹³ – 10¹⁴ cm^-3 range, are obtained after growth of about 10 layers. Evidence for the purity of these layers have been obtained from Hall, photoluminescence and SIMS measurements. Cr and Fe outdiffusion from the substrate has been studied by SIMS. Fe is found to diffuse from the substrates, even in the case of substrates which are not intentionally doped with Fe. Some FET’s have been fabricated on epitaxial structures with and without buffer layers: the static characteristics of the transistors are encouraging (I_Dss = 24 mA, g_m = 19 mS for a gate of 2 μm and 200 μm in length and width, respectively); the pinch-off is better in devices fabricated from structures with buffer layers.     

Improvements of the SiC homoepitaxy process in a horizontal cold-wall CVD reactor

In this research effort, we investigate the influence of the cold-wall reactor geometry on the chemical vapor deposition (CVD) growth process of 4H-SiC and the quality of lightly doped epitaxial layers. Stable growth conditions with respect to growth rate and C/Si ratio of the gas-phase can be achieved by the appropriate choice of the distance between susceptor and walls of the inner quartz tube. A background doping concentration in the range of 10¹⁴ cm⁻³ is realized by employing a high temperature stable and hydrogen etch resistant coating of the graphite susceptor. Doping and thickness homogeneity of epitaxial layers on 35 mm diam. 4H-SiC substrates, expressed by σ/mean, are as low as 6.9 and 7.7%, respectively. From deep level transient spectroscopy measurements, the concentration of the frequently reported intrinsic Z₁-center in 4H-SiC is determined to be below the detection limit of 10¹² cm⁻³.     

The properties of Si/Si1-xGex films grown on Si substrates by chemical vapor deposition

A growth parameter study was made to determine the proper of a SiGe superlattice-type configuration grown on Si substrates by chemical vapor deposition (CVD). The study included such variables as growth temperature, layer composition, layer thickness, total film thickness, doping concentrations, and film orientation. Si and SiGe layers were grown using SiH₄ as the Si source and GeH₄ as the Ge source. When intentional doping was desired, diluted diborane for p-type films and phosphine for n-type films were used. The study led to films grown at ∼1000°C with mobilities from ∼20 to 40 percent higher than that of epitaxial Si layers and ∼100 percent higher than that of epitaxial SiGe layers grown on (100) Si in the same deposition system for net carrier concentrations of ∼8x10¹⁵ cm^-3 to ∼2x10¹⁷ cm^-3. Enhanced mobilities were found in multilayer (100)-oriented Si/Si_1-xGe_x films for layer thicknesses ≥400A, for film thicknesses >2μm, and for layers with x = 0.15. No enhanced mobility was found for (111)-oriented films and for B-doped multilayered (100)-orlented films. Supported in part by NASA-Langley Research Center, Hampton, VA, Contract NAS1-16102 (R. Stermer & A. Fripp, Contr. Mon.)     

Investigations on low temperature mo-cvd growth of GaAs

The growth conditions for the deposition at low temperatures of epitaxial layers of GaAs on (100) GaAs crystals using TMG and arsine are studied in detail. The films are grown at atmospheric pressure in a vertical reactor in which the arsine is fed in through the rf heated susceptor for precracking. The growth temperature was varied between 680°C and 450°C. In the whole temperature range epitaxial growth was obtained. The growth rate at temperatures below 600°C depends on the AsH₃ flow, suggesting that the availibility of As vapor species, not AsH₃ limits epitaxial growth in this temperature range. For a constant AsH₃ /TMG ratio of 8 the growth rate decreases by exp (-E/kT) with an activation energy of E = 1.5 eV. Growth rates as low as 0.5 um/h have been achieved. Unintentionally doped layers show semi-insulating behaviour at growth temperatures below 500° C, similar to the behaviour seen from MBE layers. However, n-type layers with reasonable mobilities can be grown in the low temperature range (450 ° C) using H₂ Se as the doping gas.     

Doping of LPE layers of CdTe grown from te solutions

Good quality epitaxial layers of CdTe have been grown by LPE from Te-rich solutions at ˜ 500‡C onto (111) CdTe substrates. The layers have been characterised by a wide variety of techniques including capacitance-voltage profiling, photoluminescence at 4K and secondary-ion mass spectrometry (SIMS) . Undoped layers had good electrical properties (p ˜ 1 × 10¹⁶ cm⁻³, L_e ˜ 3 μm) and SIMS showed the layers to be of high purity. Those doped with In and Al however, had low n-type carrier concentrations and very short diffusion lengths, while the photoluminescence spectra showed a strong peak at ˜ 1.4 eV commonly seen in n-type bulk CdTe. The most heavily doped layers showed marked decreases in lattice parameter.     

Molecular beam epitaxy growth of high-quality arsenic-doped HgCdTe

We have initiated a joint effort to better elucidate the fundamental mechanisms underlying As-doping in molecular beam epitaxy (MBE)-grown HgCdTe. We have greatly increased the As incorporation rate by using an As cracker cell. With a cracker temperature of 700°C, As incorporation as high as 4×10²⁰ cm⁻³ has been achieved by using an As-reservoir temperature of only 175°C. This allows the growth of highly doped layers with high quality as measured by low dislocation density. Annealing experiments show higher As-activation efficiency with higher anneal temperatures for longer time and higher Hg overpressures. Data are presented for layers with a wide range of doping levels and for layer composition from 0.2 to 0.6.     

Liquid phase epitaxial growth and assessment of Pb1−xSn x Te alloys

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3.     

Slider LPE of Hg1-xCdxTe using mercury pressure controlled growth solutions

Homogeneous, nearly perfect single crystals of Hg_1-xCd_xTe are extremely difficult to prepare due primarily to the high vapor pressure of mercury. However, epitaxially grown Hg_1-xCd_xTe layers have a high potential for yielding material of a substantially higher quality. Using a new, open-tube, horizontal slider-type liquid phase epitaxial (LPE) growth technique, in which mercury pressure controlled growth solutions are used, a high degree of growth solution compositional control has been demonstrated. LPE layers of Hg_1-xCd_xTe have been grown on CdTe substrates and their high quality has been confirmed by optical, transport and electron microprobe measurements. Layer thicknesses are uniform and have been varied from 5 to 40 μ by changing the degree of supercooling or the growth time. An electron carrier concentration as low as 8.6 × 10¹⁵/cm³ and electron Hall mobilities up to 2.8 × 105 cm²/V-sec at 77K have been measured on in situ annealed samples. This work was sponsored by the Department of the Air Force and the U.S. Army Research Office.     

Errata

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3. The online version of the original article can be found at