Growth of large-diameter ZnTe single crystals by liquid-encapsulated melt growth methods

The 80-mm-diameter ZnTe single crystals were successfully obtained by the liquid-encapsulated Kyropoulos (LEK) method. Both 〈100〉- and 〈110〉-oriented single crystals were reproducibly grown by using ZnTe seed crystals. Furthermore, 80-mm-diameter, 〈100〉 and 〈110〉 ZnTe single crystals were obtained by the pulling method. The etch pit densities (EPDs) of the grown crystals by the LEK and pulling methods were lower than 10,000 cm⁻². The strain in the grown crystals by the pulling method was lower than that of LEK crystals.     

Lateral epitaxial overgrowth of GaN films on SiO2 areas via metalorganic vapor phase epitaxy

Lateral epitaxial growth and coalescence of GaN regions over SiO₂ masks previously deposited on GaN/AlN/6H-SiC(0001) substrates and containing 3 μm wide rectangular windows spaced 7 μm apart have been achieved. The extent and microstructural characteristics of these regions of lateral overgrowth were a complex function of stripe orientation, growth temperature, and triethylgallium (TEG) flow rate. The most successful growths were obtained from stripes oriented along 〈1 00〉 at 1100°C and a TEG flow rate of 26 μmol/min. A density of ∼10⁹ cm⁻² threading dislocations, originating from the underlying GaN/AlN interface, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The surfaces of the coalesced layers had a terrace structure and an average root mean square roughness of 0.26 nm.     

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.)     

The growth of low dislocation densityp-lnP single crystals

High qualityp-type InP is critical for devices ranging from high power injection lasers to space-based solar cells. The growth of 50 mm diameter, low defect density,p-type, Zn:InP substrates has been achieved for the first time at doping levels below 10¹⁸ cm⁻³. The 600 gram 〈111〉 B-seeded crystals were grown by the vertical dynamic gradient freeze technique. Dislocation densities are more than an order of magnitude below those achieved in comparable LEC-growth material. These range from 300 cm⁻² at the seed end to 1200 cm⁻² in the 50 mm diameter portion of the crystal. Single crystals were grown with carrier concentrations ranging from 1–5 × 10¹⁷ cm⁻³ as determined by Hall measurements. Hole mobilities as high as 100 cm² volt⁻¹ sec⁻¹ were achieved. The in-corporation of the zinc dopant follows normal freezing and a distribution coefficient of 0.67 ± .09 was determined. Infrared transmission imaging shows a lower level of stria-tion contrast relative to that observed for sulfur doped InP.     

Substrate orientation and processing effects on GaAs/Si misorientation in GaAs-on-Si grown by MBE

The angular misorientation of GaAs epitaxial layers grown on silicon substrates by molecular beam epitaxy has been measured using x-ray diffraction. A significant misorientation, or tilt, between the epitaxial layer and the substrate has been observed. The magnitude of the tilt depends on the initial substrate orientation, the silicon substrate type (float zone or Czochralski), postgrowth annealing, and epitaxial layer thickness. In almost all cases, the sense of the tilt is such that the GaAs 〈001〉 lies between the surface normal and the silicon 〈001〉. While the presence of interfacial dislocations with Burgers vectors that are approximately parallel to the heterointerface does predict a tilt between the substrate and the epitaxial layer, the sense of the tilt that arises from these dislocations is opposite to that observed experimentally. A model, based on the relief of misfit by dislocations inclined approximately 45 degrees to the interface, is proposed that correctly describes the observed tilt.     

The growth of Magnesium-doped GaAs by the Om-Vpe process

Magnesium-doped GaAs has been grown by organometallic vapor phase epitaxy (OM-VPE). Bis (cyclopentadienyl) mag-nesium (Cp₂Mg) is used as the organometallic precursor to Mg. The epitaxial layers have been characterized by resis-tivity and Hall measurements, photoluminescence spectro-scopy and optical microscopy. The material is of high electrical and optical quality; controllable doping over the range 10¹⁵ to 10¹⁹cm^-3 is reproducibly attained. The ionization energy of the Mg acceptor is determined to be 30 ± 2.5 meV at 77K. Negligible compensation is observed, consistent with clean thermolysis of the Cp₂Mg under growth conditions. GaAs diodes have been fabricated using Mg as the p-dopant and either Se, Si, or Sn as the n-dopant. The diodes show very low leakage currents under reverse bias, even at relatively high doping levels. Degenerately-doped junctions for interconnecting monolithic cascade concentrator solar cells have also been successfully grown, displaying forward conductivities as high as 19 amps V⁻¹ cm^-2 at 0.05V forward bias.     

Cathodoluminescence assessment of GaAs1−x Px for light emitting diodes

The room temperature cathodoluminescence (CL) properties of selenium doped epitaxial layers of GaAs_1−xP_x, in the composition range 0·35 < x < 0.45, have been examined as a function of the Hall electron concentration. Material selected for this investigation had less than 2 per cent of the total CL emission in the i.r. For a fixed alloy composition the CL intensity is shown to increase with increasing electron concentration, while for a fixed electron concentration the intensity decreases with increasing GaP content. These results have been correlated with the electroluminescent efficiencies of zinc diffused diodes fabricated from the same material. It is shown that CL provides a rapid and reliable means of assessing the composition and emitting efficiency of epitaxial layers for use in the fabrication of light emitting diodes.     

Growth and properties of VPE GaP for green LEDs

The PH₃-HCl-Ga-H₂ technique for VPE growth of GaP is described. The influence of various growth parameters, including substrate temperature, orientation, and PH₃ flow rate on morphology and growth rate are described. For both VPE and LPE nitrogen doping is known to be a major factor in obtaining high green luminescence efficiency. The major emphasis of this paper is an examination of the effect of nitrogen concentration in the range less than 10¹⁹ cm⁻³ (using the Lightowlers correction factor) on the growth process and materials properties, such as defect structure, photoluminescence spectra (at 300 and 77K) and photoluminescence intensity and lifetime. The LED device performance (B/J and efficiency) is used as the final test of material quality. Nitrogen is found to be incorporated far in excess of the solubility limit, and the solid gas distribution coefficient for nitrogen is found to increase rapidly with decreasing temperature below 840°C . The optimum nitrogen concentration for high diode efficacy, photoluminescence intensity, and lifetime is found to be approximately 5 × 10¹⁸ cm⁻³, where diodes fabricated by Zn diffusion into the VPE GaP have efficiencies at a current density of 10 A/cm² of 0.1%, comparable to the state-of-the-art in the more widely used grown p-n junctions using LPE.     

Structural and electrical properties of unintentionally doped 4H-SiC epitaxial layers—Grown by hot-wall CVD

This paper focuses on growth of 4H−SiC epitaxial layers using the hot-wall CVD technique. The relation between the growth regime like total flow, system pressure, C/Si ratio and growth temperature and the characteristics of nominally undoped epilayers, such as thickness uniformity and background doping concentration have been investigated. The epitaxial layers were investigated by optical microscopy, capacitance-voltage measurements, x-ray rocking curve maps, electron channelling patterns and secondary ion mass spectroscopy. Layers up to 40 μm in thickness with a variation of about ±4% and with residual n-type doping levels in the low 10¹⁴ cm⁻³ ranges have been obtained on Si faces wafers. SIMS measurements have shown that the impurity concentration of acceptors like B and Al is below 2×10¹⁴ cm⁻³.     

Electric current controlled liquid phase epitaxy of GaAs on N+ and semi-insulating substrates

Electric current controlled liquid phase epitaxy (LPE) of GaAs has been performed on both n⁺ and semi-insulating substrates. Growth is induced by current flow across the substrate-melt interface. The furnace temperature is held constant during growth so that direct electrical control of the growth process is achieved. The dependence of the growth rate on both the electric current density across the substrate-melt interface and the ambient furnace temperature was determined. Current densities from 5 to 20 A/cm² were employed and furnace temperatures ranging from 680 to 800°C were used. Sustained steady state growth rates as small as 0.022μm/min and as large as 1.4μm/min were obtained. For a given furnace temperature and current density, the measured growth rates on semi-insulating substrates range from 48% to 77% of the rates obtained on n⁺ n substrates. The surface morphology of the epitaxial layers is observed to depend on the electric current density employed during growth. Electric current controlled doping modulation was studied in epitaxial layers grown from unintentionally doped melts. The degree of doping modulation achieved is approximately proportional to the change in applied current density. Approximately a 40% increase in the net electron concentration is obtained by changing the current density from 10 to 30 A/cm² during growth. Preliminary experiments with tin doped epitaxial layers indicate that similar changes in the amount of tin incorporation can be achieved.     

Effects of substrate orientation and surface reconstruction on patterned substrate OMVPE of GaAs

Organometallic vapor phase epitaxial growth of GaAs on 320 nm high mesas was used to study the dependence of lateral growth upon the substrate misorientation from (100) and the mesa wall orientation on the substrate. GaAs (100) substrates were misoriented by 3° toward eight major crystallographic directions, consisting of the four nearest [111] and [110] directions. The mesa sidewalls were oriented either parallel to the 〈011〉 and 〈01 〉 directions or rotated by 45° to be parallel to the 〈001〉 and 〈010〉 directions. GaAs films were grown with TMGa and TBA at T=575°C. The lateral growth rates were up to 25 times higher than the vertical growth rate of 1.3 μm/hour. Optical microscopy and atomic force microscopy (AFM) showed that under the given growth conditions lateral growth off mesa sidewalls is most rapid in the 〈011〉 and/or 〈0 〉 directions and less in the perpendicular 〈01 〉 and 〈0 1〉 directions (lateral growth anisotropy). By raising the temperature to 625°C lateral growth in the 〈01 〉 -〈0 1〉 directions increased while it remained almost constant in the 〈011〉 -〈0 〉 directions. Published results show that the partial pressure of As also affects lateral growth. Differences in the lateral growth rates in the 〈011〉 and its opposite 〈0 〉 directions result from substrate misorientation but not from the orientation of the mesa walls on the substrate. Anisotropic lateral growth rates in different crystallographic directions appear to be caused by both, (1) 1-dimensional Ga diffusion defined by surface reconstruction, and (2) a relatively low energy barrier to atoms flowing over high-to-low terrace steps. A lateral growth model is proposed that describes anisotropic lateral growth at mesa sidewalls in terms of growth conditions and substrate misorientations. The model also explains the difference in the preferential lateral growth directions between MBE and OMVPE.     

Electrical characterisation of Cu-Diffusedn-GaAs epitaxial layers using optical deep level transient spectroscopy

Undopedn ^--GaAs epitaxial layers were grown by OMVPE onn ⁺ (1 × 10¹⁸ cm³) and semi-insulating (SI) GaAs substrates. The as-grown epitaxial layers grown onn ⁺ substrates contained several deep level defects whereas those grown on SI substrates were, apart from the EL2, virtually “defect free”. Upon Cu diffusion, deep levels which may reduce hole and electron diffusion lengths and lifetimes, were formed. Optical deep level transient spectrocopy (ODLTS) has been used to identify such levels atE _v + 0,41 eV andE _c-0,31 eV respectively. The EO1 (EL2) trap concentration reduced after Cu had been diffused into the epitaxial layers. The magnitude of this reduction was approximately equal to the concentration of the trap found atE _c - 0,31 eV which suggests that the two may be related. Activation energies and capture cross-section values are presented for the deep levels detected in these epitaxial layers.     

Dislocations in vapor phase epitaxial GaP

Dislocations in VPE GaP grown on (100) oriented LEC GaP substrates have been characterized, and their origins and effects on LED performance have been investigated. In non-nitrogen doped epilayers, the dislocations are found to originate in the substrate and propagate through the epilayers in straight lines in [100] and <211> directions. The dislocation density of the epilayer is found to be nearly equal to that of the substrate. Introduction of nitrogen during growth of the epilayer has been observed to bend these so-called “inclined≓ dislocations propagating through the layer into [0−1 1] directions in the (100) plane and thus produces segments of [0 −1 1] dislocations to relieve the lattice parameter mismatch due to N. The mismatch dislocation density is observed to be proportional to the N doping level. At very high N doping levels, > 10¹⁹ cm^-3, a large number of new inclined dislocations are observed, which may be in part due to GaN precipitation. The effects of dislocations on LED properties were investigated by measuring dislocation densities in the individual diodes using the electron beam induced current mode of the SEM and comparing this with the spot brightness and luminous flux. The dislocations were observed to produce dark spots in the EL emission in many cases. For a series of runs where all growth and processing parameters were fixed, a good correlation between B/J and dislocation density was observed with B/J decreasing with increasing dislocation density in the range < 1 × 10⁴ cm⁻² to 1 × 10⁶ cm⁻².     

Physico-chemical properties of Ga and In dopants during liquid phase epitaxy of CdxHg1−xTe

This work deals with the study by means of radioactive tracers and autoradiography, as well as measuring of galvanomagnetic properties, of Ga and In doping of epitaxial Cd_xHg_1−xTe layers during their crystallization from a Te-rich melt. Ga and In were introduced in the form of Ga⁷² and In¹¹⁴ master alloys with Te. The effective distribution coefficients of Ga and In during the crystallization of the Cd_xHg_1−xTe solid solutions with x=0.20 to 0.23 were determined by cooling the Te-base melt to 515–470°C. Depending on the concentration of the dopants and the time-temperature conditions of Cd_xHg_1−xTe growth, these ratios for Ga and In were 1.5–2.0 and 1.0–1.5, respectively. The electrical activity of Ga and In was determined after annealing of the Cd_xHg_1−xTe layers in saturated Hg vapor at 270–300°C. In doping of the epitaxial layers to (3–8)×10¹⁴ cm⁻³ with subsequent annealing in saturated Hg vapor at ∼270°C increases the carrier lifetime approximately by a factor of two as compared with the undoped material annealed under the same conditions.     

Effect of growth temperature on oxygen incorporation in GaP

The temperature dependence of oxygen incorporation in GaP has been studied over the range 900–1000°C using liquid phase epitaxy. The growth solutions were oxygen saturated to provide the maximum oxygen concentration in the solid. The concentration of substitutional (donor) oxygen was determined by electrical compensation measurements in p-type material. It has been found that whereas the oxygen concentration in the liquid increases with increasing growth temperature, the distribu-tion coefficient decreases. This behavior results in a maximum oxygen donor concentration of ∼3×10¹⁷ atoms/cm³ which is realized in the temperature range 970–1060°C. For T > 1100°C, the oxygen donor concen-tration falls below 1×10¹⁷ atoms/cm³, in agreement with prior solution growth results. These results thus suggest an optimum temperature range for oxygen doping and fix an upper limit to the number of Zn-O radiative centers (hence light output) for GaP red emitting diodes .     

A study of chemical beam epitaxy of GaAs using tris-dimethylaminoarsenic

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 10¹⁵ cm⁻³ with an electron mobility of 5700 cm²/V·s at 300K and 40000 cm²/V·s at 77K.     

Comparison of the homoepitaxial growth of AlN crystals on Al and N surfaces

A special experimental technique capable of excluding the influence of differences between sources, substrate crystals, and growth process parameters has been employed to correctly compare the quality of homoepitaxial AlN layers grown by sublimation on Al and N surfaces of the substrate crystal with 〈0001〉 orientation. It has been found that, in most cases, the quality of layers grown on the N surface is somewhat higher; however, no differences have been observed in separate cases. Hence the conclusion follows that the range of growth process parameters is substantially wider for the N side compared with the Al side.     

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.     

Efficient Outdiffusion of Hydrogen from Mg-Doped Nitrides by NF<Subscript>3</Subscript> Annealing

High concentration (more than 1 × 10¹⁸ cm⁻³) of hydrogen atoms remaining in Mg-doped GaN epitaxial layers grown by metalorganic chemical vapor deposition even after conventional annealing in N₂ ambient could induce degradation in GaN-based devices containing Mg-doped layers. In this study, by annealing Mg-doped nitrides in NF₃ ambient, we successfully reduced residual hydrogen below mid-10¹⁷ cm⁻³, which is much smaller than by N₂ annealing. NF₃ annealing enhances outdiffusion of hydrogen from the bulk, which is possibly because the nitrogen and fluorine radicals decomposed from NF₃ accelerate desorption of hydrogen adatoms from the surface. The proposed method for Mg activation would improve the reliability of GaN-based light-emitting diodes and laser diodes.     

Molecular beam epitaxy grown long wavelength infrared HgCdTe on Si detector performance

The use of silicon as a substrate alternative to bulk CdZnTe for epitaxial growth of HgCdTe for infrared (IR) detector applications is attractive because of potential cost savings as a result of the large available sizes and the relatively low cost of silicon substrates. However, the potential benefits of silicon as a substrate have been difficult to realize because of the technical challenges of growing low defect density HgCdTe on silicon where the lattice mismatch is ∼19%. This is especially true for LWIR HgCdTe detectors where the performance can be limited by the high (∼5×10⁶ cm⁻²) dislocation density typically found in HgCdTe grown on silicon. We have fabricated a series of long wavelength infrared (LWIR) HgCdTe diodes and several LWIR focal plane arrays (FPAs) with HgCdTe grown on silicon substrates using MBE grown CdTe and CdSeTe buffer layers. The detector arrays were fabricated using Rockwell Scientific’s planar diode architecture. The diode and FPA and results at 78 K will be discussed in terms of the high dislocation density (∼5×10⁶ cm²) typically measured when HgCdTe is grown on silicon substrates.