Growth by CVT and characterization of Hg1-xCdxTe epitaxial layers

Single crystal epitaxial layers of Hg_1-x Cd _x Te were grown on CdTe substrates employing the chemical vapor transport technique. Different growth temperatures, substrate orientations, and various pressures of Hgl₂ as a transport agent were used while the source materials had a fixed composition ofx = 0.2. The epilayers are of nearly uniform composition to a depth of about one-half of the layer thickness. Chemical etching of the as-grown epilayers revealed low etch pit densities in the range of 10³–10⁴ cm⁻². Rectangle-shaped etch pits are observed for the first time on the (100) oriented epilayers of this material. The growth temperature and Hgl₂ pressure used for the growth experiments have significant effects on the layer morphology and composition.     

Electrical properties and compositional distributions of CVT and PVT grown Hg1−xCdxTe epilayers

Epitaxial layers of Hg_1−xCd_x Te were grown on CdTe substrates by the chemical vapor transport technique using Hgl₂ as a transport agent. The epilayers were of nearly uniform composition both laterally and to a depth of about one-half of the layer thickness. By comparison, the composition varied continuously throughout the depth of the layer for epilayers grown by the physical vapor transport technique. Layers were grown both p- and n-type with carrier concentrations on the order of 10¹⁷ cm⁻³. Low-temperature annealing was used to convert the p-type layers into n-type. The room-temperature carrier mobilities of as-grown and converted n-type layers ranged from 10³ to 10⁴ cm²/V-s depending on the composition and are comparable to previous literature values for undoped Hg_1−xCd_xTe crystals.     

The growth of GaAs,AIGaAs, InP and InGaAs by chemical beam epitaxy using group III and V alkyls

The growth kinetics of chemical beam epitaxy (CBE) were investigated with the growth of GaAs, AIGaAs, InP, and InGaAs. Results obtained with epilayers grown by using trimethylarsine (TMAs) and triethylphosphine (TEP) instead of arsine (AsH3) and phosphine (PH₃) were reviewed with some additional results. The CBE grown epilayers have similar optical quality to those grown by molecular beam epitaxy (MBE). Superlattices of GaAs/AlGaAs with abrupt interfaces have been prepared. Since trimethylindium (TMIn) and triethylgallium (TEGa) used in the growth of InGaAs emerged as a single mixed beam, spatial composition uniformity was automatically achieved without the need of substrate rotation in the InGaAs epilayers grown. Lattice-mismatch Δα/α< 1 x 10^-3 have been reproducibly obtained. For epilayers grown with high purity TMAs source, room-temperature electron mobility as high as 9000 cm²/V sec and concentrations of ˜7 x 10¹⁵ cm^-3 were produced. In general, the electron mobilities were as good as those obtained from low-pressure metalorganic chemical vapor deposition. (MO-CVD). Unlike MBE, since the In and Ga were derived by the pyrolysis of TMIn and TEGa molecules at the heated substrate surface, respectively, oval defects observed in MBE grown epilayers due to Ga splitting from Ga melt were not present in CBE grown epilayers. This is important for integrated circuit applications. Unlike MO-CVD, the beam nature of CBE allows for selective area growth of epilayers with well-defined smooth edges using mask shadowing techniques. Typically, growth rates of 2-5μm/h for InP, 2-6μm/h for GaAs and AIGaAs, and 2-5μm/h for InGaAs were used.     

Effect of the post-As+-lmplantation thermal treatment on MBE HgCdTe optical properties

HgCdTe epilayers were grown by molecular beam epitaxy. A series of As⁺-implanted CdTe and HgCdTe epilayers annealed under different temperatures were investigated by photoluminescence spectroscopy. More As⁺ ions can occupy the Te sublattice after the samples were annealed at 450°C, and the acceptor level of As⁺ on the Te sublattice for HgCdTe material (x ≈ 0.39) is 31.5 meV. The Raman spectrum study indicates a recovery of the crystalline perfection after the post-As⁺-implantation thermal treatment.     

Electrical properties of indium-doped LPE layers of Pb1−xSnx Te

Liquid-phase epitaxial (LPE) layers of Pb_1−xSn_xTe with an alloy composition 0≤×≤0.25 were doped n-type by adding from 0.002 to 10 at.% indium to the growth solution. Doping characteristics of indium and electrical properties of the epilayers at 77 and 4.2K were studied by Hall and resistivity measurements made directly on the grown layers. Electron concentration and mobility at 77 and 4.2K are presented as a function of indium doping for various x values. Doping coefficients of ~0.05 and ~0.03 are found for PbTe and Pb_0.8Sn_0.2Te, respectively, grown at ~450°C. For medium to high indium doping, the electron concentration saturates to a constant value independent of doping and LPE growth temperature. The saturation values decrease substantially with increasing x and increase with a decrease in sample temperature. Bulklike mobilities practically independent of doping are recorded up to an indium concentration N_ln~0.3 at.%, above which the mobility decreases with increasing indium concentration. The data shows that indium is a suitable donor in liquid-phase epitaxial layers of Pb_l-XSn_xTe.     

Low carrier concentration (PbSn)Te by molecular beam epitaxy

pb_0.77.77SN_0.23Te was grown by evaporation of the pure elements or of the binary compounds from Knudsen cell sources at l0^-10 torr. The growth chamber was provided with ion bombardment, LEED, Auger spectroscopy and quartz crystal deposition moni-toring. A Sn-wetted heating block was used for close control of substrate temperature. On chemomechanically-polished BaF₂ (100) substrates, monocrystalline films smooth to tens of Angstroms were obtained at 1 micrometer/hour growth rate over a substrate temperature range from 300 to 420°C; above A20°C, re-evaporation occurred. In growth from the elements, excess Pb remains as droplets on the film surface, but most excess Te re-evaporates. The remaining Te produces Pb vacancies which generate p-carriers; conversely, excess Pb produces n-carriers. A quartz crystal deposition rate monitor operat-ing at the growth temperature was used to tune impinging (Te/ Pb+Sn) ratio by observing the discontinuity in rate vs. (Te/ Pb+Sn) which occurred at the stoichiometric ratio. This technique yielded films with as-grown carrier concentrations in the high l0¹⁷ p/cm³ range at 77K. Closer stoichiometry control was obtained by growing from PbTe and SnTe sources with the use of a separate Pb source for reduction of inher-ent Te excess. For Pb_0.77Sn_0.23Te, The metal-rich single-phase boundary was found to cross the stoichiometric composition at 395 + 5°, so that only p-type material was obtainable above this temperature. By using 0.014 atomic fraction excess Pb, it was possible to constrain the film composition to this boundary and thus obtain close control over carrier concen-tration by appropriate adjustment of substrate temperature. Carrier concentrations of 2×l0¹⁷/cc at 77K were obtained in this manner for both n- and p-type material. Excess Pb coalesced into droplets spaced sparsely enough on the sur-face so as not to interfere with device fabrication. Hall mobilities were about 1×10⁴ cm² /v⋅sec at 77K. Attempts to reduce carrier concentration below 2×10¹⁷ cm^-3 resulted in anomalously low Hall voltages, suggesting existence of p and n domains in the films. The origins of this inhomogeneity are not clear at present.     

Electrical properties of HgCdTe epilayers doped with silver using an AgNO3 solution

We employed AgNO₃ solutions for doping Ag in liquid phase epitaxy (LPE) grown Hg_0.78Cd_0.22Te epilayers and found that the minority carrier lifetimes became longer so that the diode properties improved. After annealing LPE grown Hg(1-x)Cd(x)Te layers (x=0.22) in Hg atmosphere, the epilayers were immersed in an AgNO₃ solution at room temperature. The typical carrier concentrations of holes was 3 × 10¹⁶ cm⁻³ at 77K. These values were almost the same as for the nondoped wafers. Also, its acceptor level was 3 to 4 meV. This shows that the Ag was activated. The doped crystals have lifetimes several times longer than those of the nondoped crystals. Numerical fitting showed the lifetime was limited mostly by the Auger 7 process. The Shockley-Read-Hall recombination process was not effective. To examine the Ag-doped wafer, we fabricated photodiodes using standard planar technology. The diodes have an average zero-bias resistance of several MΩ and a shunt resistance of about 1 GΩ for a 10 μm cutoff wavelength at 78K. These values are about four times higher than those of nondoped diodes. The photo current is also two times higher at the same pixel size. This shows that the quantum efficiency is increased. The extension of the lifetime contributes to the high resistance and the high quantum efficiency of the photodiode.     

Use of tertiarybutylphosphine for the growth of InP and GaAs1-xPx

A newly-developed phosphorus source, tertiarybutylphosphine (TBP), which is much less toxic than PH₃, has been used to grow InP and GaAs_1-xP_x by atmospheric pressure organometallic vapor phase epitaxy (OMVPE). Excellent morphologies are obtained for the growth of InP between 560 and 630° C for TBP partial pressures larger than 0.5 x 10^-3. For the first time, V/III ratios as low as 3 have been used to grow InP epilayers with featureless morphologies at 600° C. To obtain good morphologies at both lower and higher temperatures, higher TBP partial pressures are necessary. The electron mobility increases and the electron density decreases as the temperature is increased. The highest room temperature mobilities and lowest electron densities, obtained at 630° C, are 3800 cm²/V-sec and 3 x 10¹⁵ cm^-3, respectively. The 10 K photoluminescence spectra of the InP epilayers at higher growth temperatures show no carbon contamination. Bound excition half widths as low as 3.0 meV have been measured. The use of TBP to replace PH₃ in the growth of GaAs_1-xP_x results in a nearly linear relationship between vapor and solid composition at 610° C,i.e., the P distribution coefficient is nearly unity. This contrasts sharply with the very low P distribution coefficient obtained using PH₃ at such low growth temperatures.     

Automated compositional control of Hg1−xCdxTe during MBE, using in situ spectroscopic ellipsometry

The implementation of a feedback control system for maintaining a desired compositional value in Hg_1−xCd_xTe epilayers is reported. An 88-wavelength ellipsometer monitored the Cd content (x) of a Hg_1−xCd_xTe film during molecular beam epitaxy, and deviations from a pre-determined set-point were automatically corrected via adjustments in the CdTe effusion cell temperature. The accuracy of this system (Δx∼0.002) was confirmed by Fourier transform infrared transmission measurements made ex situ on the epilayers.     

Growth and properties of Hg1-x Cdx Te epitaxial layers

The growth of epitaxial layers of mercury-cadmium-telluride (Hg_1-xCd_xTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd_1-yZ_yTe and Hg_1-xCd_xTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·10¹⁷ cm⁻³, hole mobility of about 300 cm²·V⁻¹ sec⁻¹ and excess minority carrier life-time of 3 nsec, at 77 K.     

The compositional grading of MOCVD-grown GaAs1−xPx via substrate temperature changes

The first application of MOCVD compositional grading by means of substrate temperature changes is reported for the case of GaAs−P_x (0 ≤ x ≤ 0.30). The method is based upon the higher pyrolysis efficiency ratio of AsH₃ relative to PH₃ at lower growth temperatures. The growth upon GaAs substrates commences at a temperature at which this PH₃/ASH₃ thermal cracking efficiency ratio is extremely high; raising the growth temperature decreases this ratio, and hence the As/P ratio in the solid. GaAs_1−xP_x epilayers grown on GaAs by this method display excellent materials properties and are virtually optically and electrically indistinguishable from conventionally-graded epilayers in which the gas phase mole fractions of transport agents are varied at constant temperature. The method is applicable to systems where temperature-dependent cracking efficiency differentials exist.     

Hydrogen‐Assisted Growth of Ultrathin Te Flakes with Giant Gate‐Dependent Photoresponse

Tellurium (Te), as an elementary material, has attracted intense attention due to its potentially novel properties. However, it is still a great challenge to realize high‐quality 2D Te due to its helical chain structure. Here, ultrathin Te flakes (5 nm) are synthesized via hydrogen‐assisted chemical vapor deposition method. The density functional theory calculations and experiments confirm the growth mechanism, which can be ascribed to the formation of volatile intermediates increasing vapor pressure of the source and promoting the reaction. Impressively, the Te flake‐based transistor shows high on/off ratio ≈10⁴, ultralow off‐state current ≈8 × 10^?13 A, as well as a negligible hysteresis due to reducing thermally activated defects at 80 K. Moreover, Te‐flake‐based phototransistor demonstrates giant gate‐dependent photoresponse: when gate voltage varies from ?70 to 70 V, I_on/I_off is increased by ≈40‐fold. The hydrogen‐assisted strategy may provide a new approach for synthesizing other high quality 2D elementary materials.     

In-situ control of temperature and alloy composition of Cd1−xZnxTe grown by molecular beam epitaxy

In this work in-situ spectroscopic ellipsometry (SE) has been applied for the simultaneous determination of the growth temperature and alloy composition for the epitaxial Cd_1−xZn_xTe(211)/Si(211) structure. The optical dielectric functions of CdTe and Cd_0.96Zn_0.04Te (CZT) epilayers were studied as a function of temperature both ex-situ and in-situ in the range from 1.6 eV to 4.5 eV. We employed parametric models for the simulation of the optical properties of CZT at and between the critical points (CP) E₀, E₀ + Δ₀, E₁, E₁ + Δ₁, E₂(Σ) and E₂(Σ). Critical point energies and line widths for Cd_1−xZn_xTe were obtained through the fitting process, which included both zero order and higher order derivatives of the SE pseudo dielectric function. The dependence of the different critical points on Zn concentration x is discussed. It has been demonstrated that the energy of the weak E₀ + Δ₀ transition can be used to measure composition, while the E₁ energy can be used as a real-time temperature measure. The model parameters were optimized through the simultaneous analysis of multiple data sets, and the temperature dependent model was developed for in-situ application. Our analysis is estimated to produce uncertainties of only ±0.5°C in measuring the temperature and ±0.5% in measuring the composition of only the zero order dielectric function is being fitted. The effects of a surface overlayer, of reflected beam deflections, and of other experimental problems on the overall accuracy, are discussed as well as ways to improve the in-situ SE data quality.     

Lead-europium-selenide-telluride grown by molecular beam epitaxy

The band gap of lead-europium-telluride (Pb_1-x Eu_xTe) was determined from room temperature optical absorption measurements and increases as dEg/dx = 3.5 eV for x ≤ 0.044. Eu atoms bond strongly to a PbTe surface during MBE growth and have a small diffusion coefficient (<1 x 10⁻¹⁶ cm /sec at 370°C). The lattice constant of Pb_1-x, Eu_x Te is a nonlinear function of composition, and lattice-matched growth of Pb_1-x Eu_x Se_y Te_1-y, on PbTe is demonstrated. Preliminary studies of the electrical properties of Pb_1-x Eu_x Te indicate compensation of n-type (Bi) and p-type (Tl) dopants. These results indicate that Pb_1-x Eu_x Se_y Te_1-y, may be useful for obtaining diode lasers which emit at wavelengths shorter than those available from Pb_1-x, Sn_x Te.     

Minority carrier lifetime in indium-doped HgCdTe(211)B epitaxial layers grown by molecular beam epitaxy

We have studied the minority-carrier lifetime on intentionally indium-doped (211)B molecular beam epitaxially grown Hg_1-xCd_xTe epilayers down to 80K with x ≈ 23.0% ± 2.0%. Measured lifetimes were explained by an Auger-limited band-to-band recombination process in this material even in the extrinsic temperature region. Layers show excellent electron mobilities as high as ≈2 x 10⁵ cm²v^-1s^-1 at low temperatures. When the layers are compensated with Hg vacancies, results show that the Schockley-Read recombination process becomes important in addition to the band-to-band processes. From the values of τ_n0 and τ_p0 of one sample, the obtained defect level is acceptor-like and is somewhat related to the Hg vacancies.     

N-Type Hg1−xCdxTe: Undoped x = 0.3 LPE material for sprite IR detectors

The requirement for two color Sprite detectors, with elements sensitive in the ranges 3-5 μn (MW) and 8-14 μn (LW) at 77K, is met using Hg_1−xCd_xTe elements of composition x = 0.3 and x = 0.2, respectively. The need for low defect levels for increased performance indicates the use of liquid phase epitaxy (LPE). While LW material is fairly well characterized, the growth and conversion to n-type of MW LPE has proved more difficult. Reported work shows limited data and limited success in converting MW LPE to n-type, and this primarily in donor-doped material. This paper describes the growth, annealing to n-type and characterization of Hg_0.7Cd_0.3Te. High n-type conversion yields were obtained, with low donor levels (mid-10¹³ to mid-10¹⁴ cm⁻³), high mobility (>10⁴ cm² (Vs)⁻¹) and long minority carrier lifetime (>10 us).     

Electrical transport in n-type ZnMgSSe grown by molecular beam epitaxy on GaAs

Significant progress in improving the performance of blue-green II-VI semiconductor injection lasers has come about from advances in the epitaxial growth and doping of ZnMgSSe on GaAs substrates. This paper investigates electrical transport and its relation to structural quality in n-type Zn_1-yMg_y S_xSe_1-x epilayers doped with Cl, grown by molecular beam epitaxy. The composition parameters x and y vary from about 0.12-0.18 and 0.08-0.15, respectively. The quaternary epilayers studied are lattice-matched (or nearly so) to the GaAs substrate. Temperature-dependent Hall-effect measurements are performed on seven n-type ZnMgSSe:Cl epilayers, and a technique is presented whereby the resulting mobility-vs-temperature data is compared with data for ZnSe to obtain a structural figure of merit that is useful in characterizing the quaternary epilayer.     

Interrelations between defects in the Hg1−xCdxTe epilayers and their measured lattice parameters and composition

Absolute values of lattice parameters in Hg_1−xCd_xTe epilayers were precisely measured by high-resolution x-ray diffraction (Bond method), and then compared with those calculated using Cd contents, x, which were derived from Fourier transform infrared transmission spectra. A part of the samples revealed significant discrepancies between measured and calculated lattice parameters, the differences being both of positive and negative signs. The obtained results are discussed in the framework of clusters of the point defects that were visualized by high-resolution scanning electron microscopy.     

Metalorganic vapor phase epitaxyin-situ growth of p-on-n and n-on-p Hg1-xCdxTe junction photodiodes using tertiarybutylarsine as the acceptor source

We report arsenic doping of Hg_1-xCd_xTe (0.2 < x < 0.3) grown using metalorganic vapor phase epitaxy (MOVPE) by the direct alloy growth (DAG) technique. Tertiarybutylarsine (TBAs) was used as a precursor for As doping. Several epilayers were grown at different Hg partial pressures and TBAs bubbler temperatures in order to study the doping characteristics. The amount of As incorporated in the layer as well as the acceptor concentration were found to be a strong function of the Hg pressure. Secondary ion mass spectrometric studies on heterostructures showed that the compositional interdiffusion is less than the diffusion of As during growth. P-N junctions were grown using TBAs for the first time and several of these layers were processed to fabricate photodiodes. A p-on-n grown junction photodiode with a cutoff wavelength of 8.2 μm had an R_oA value of 241 ohm-cm² at 80K and is the highest reported value for p-on-n DAG-MOVPE devices. Methods to improve the device R_oA of the grown junctions are also proposed.     

Ten-inch molecular beam epitaxy production system for HgCdTe growth

Growth of Hg_1−xCd_xTe by molecular beam epitaxy (MBE) has been under development since the early 1980s at Rockwell Scientific Company (RSC), formerly the Rockwell Science Center; and we have shown that high-performance and highly reproducible MBE HgCdTe double heterostructure planar p-on-n devices can be produced with high throughput for various single- and multiplecolor infrared applications. In this paper, we present data on Hg_1−xCd_xTe epitaxial layers grown in a ten-inch production MBE system. For growth of HgCdTe, standard effusion cells containing CdTe and Te were used, in addition to a Hg source. The system is equipped with reflection high energy electron diffraction (RHEED) and spectral ellipsometry in addition to other fully automated electrical and optical monitoring systems. The HgCdTe heterostructures grown in our large ten-inch Riber 49 MBE system have outstanding structural characteristics with etch-pit densities (EPDs) in the low 10⁴ cm⁻² range, Hall carrier concentration in low 10¹⁴ cm⁻³, and void density <1000 cm². The epilayers were grown on near lattice-matched (211)B Cd_0.96Zn_0.04Te substrates. High-performance mid wavelength infrared (MWIR) devices were fabricated with R₀A values of 7.2×10⁶ Ω-cm² at 110 K, and the quantum efficiency without an antireflection coating was 71.5% for cutoff wavelength of 5.21 μm at 37 K. For short wavelength infrared (SWIR) devices, an R₀A value of 9.4×10⁵ Ω-cm² at 200 K was obtained and quantum efficiency without an antireflection coating was 64% for cutoff wavelength of 2.61 μm at 37 K. These R₀A values are comparable to our trend line values in this temperature range.