Epitaxial CdTe Research Articles

Control of microtwins in MOCVD of CdTe on sapphire

Microtwins in epitaxial (111)Te-face CdTe films grown by MOCVD on (0001) sapphire substrates can be detected by optical reflection from the as-grown surface, which is microscopically facetted, as well as by X-ray diffraction. Microtwin content can be made very small, much less than 1% volume fraction, by proper choice of MOCVD reactor operating conditions. Annealing after completion of the CdTe deposition can remove the microtwins.

Structural defects in bulk and epitaxial CdTe

The advantages of growing bulk CdTe from the vapour are summarized and the structural defects present in material grown using three different vapour methods compared critically. Kawano's growth rate explanation of twin distribution is developed and the effect of growth conditions in anomalous cases is speculated upon. Experimentally observed tilts and rotations in CdTe/GaAs(001) are explained using a dislocation model which indicates a degree of cooperative alignment of certain interfacial dislocation components. The equilibrium distribution of threading dislocations and the prospects for filtering using soft, tensed layers, e.g. (HgMn)Te, are discussed.

Effects of growth conditions on the surface structure of CdTe studied by a scanning tunneling microscope

CdTe buffer layers are widely used as substrate materials for the molecular beam epitaxial (MBE) growth of Hg 1-x Cd x Te because of the small lattice mismatch. With the help of a scanning tunneling microscope (STM) and a scanning electron microscope (SEM) the surface topographies of epitaxial CdTe layers grown under different conditions were studied quantitatively. It was found that photoassistance during growth enhances the formation of pyramidal hills with a typical height of 1000 A. The use of an excess of Cd completely changes the surface morphology towards 2-dimensional growth and the remaining layer roughness was less than 50 A. These initial results suggest that the Cd rich conditions provide a promising method for the growth of large, flat layers

Complete characterization of epitaxial CdTe on GaAs from the lattice geometrical point of view

The complete characterization of two monocrystalline samples of epitaxial CdTe(111) on GaAs(001) rotated 2° around [1 1 0] using a new description tool and X-ray techniques is given. In addition, the structural quality of the deposits was correlated to the angles of epitaxial misorientation.

Heteroepitaxial CdTe layers on Si-substrates for radiation sensor arrays with integrated Si-electronics

Abstract Heteroepitaxial CdTe sensor arrays on silicon substrates have the possibility to monolithically combine the most suited materials for low noise radiation detection (e.g. soft X-rays) with integrated signal processing electronics in the Si-substrate. This opens the way to a starring imaging sensor array for low energy X- and gamma-ray detection which require high signal to noise ratios without cooling. We describe the key process towards this goal, growth and doping of epitaxial CdTe layers of sufficient quality on Si(100) substrates by molecular beam epitaxy (MBE). In order to obtain high quality epitaxy despite the up to 20% lattice mismatch, we use stacked CaF2/BaF2 as a buffer layer p-type doping is achieved with Sb or As under Cd-rich flux conditions and laser induced activation.

Substrate heating effect on the growth of a CdTe film on an InSb substrate by vacuum evaporation

Epitaxial CdTe thin films were grown on the (111) oriented InSb substrate by vacuum evaporation, with the substrate kept at 190–225°C during the film deposition. The chamber pressure during film deposition was at 3.5 × 10−6 mbar. X-ray diffraction was used to determine the film structure, while the full width at half maximum (FWHM) of the X-ray diffraction peak was used to examine the crystallinity of the as-deposited films. The film morphology was observed by the scanning electron microscope (SEM), and the film composition was determined by electron probe microanalysis (EPMA). The film quality was examined by infrared transmission spectroscopy. Results indicate that the quality of the grown CdTe films was improved with the higher substrate temperature during the film deposition.

Substrate heating effect on the growth of a CdTe film on an InSb substrate by vacuum evaporation

Epitaxial CdTe thin films were grown on the (111) oriented InSb substrate by vacuum evaporation, with the substrate kept at 190–225°C during the film deposition. The chamber pressure during film deposition was at 3.5 × 10−6 mbar. X-ray diffraction was used to determine the film structure, while the full width at half maximum (FWHM) of the X-ray diffraction peak was used to examine the crystallinity of the as-deposited films. The film morphology was observed by the scanning electron microscope (SEM), and the film composition was determined by electron probe microanalysis (EPMA). The film quality was examined by infrared transmission spectroscopy. Results indicate that the quality of the grown CdTe films was improved with the higher substrate temperature during the film deposition.

Growth of CdTe films on GaAs by ionized cluster beam epitaxy

Abstract Stoichiometric epitaxial films of CdTe were grown on (100)GaAs substrates by ionized cluster beam (ICB) epitaxy. Streaky RHEED patterns indicated good crystallinity and surface flatness of the epitaxial CdTe films. CdTe(100) orientation was obtained when the substrate preheating temperature was 480°C, while CdTe growth inboth (100) and (111) orientations occured when the substrate preheating temperature was above 550°C. The characteristics of the ICB growth process were investigated and the cluster-involving growth behavior has been evidenced. When sufficient clusters were generated in the deposition beam under adequate source vapor pressures, the crystalline quality of the resulting CdTe epilayers improved significantly with the increase of kinetic energy of the CdTe clusters. The best CdTe epilayer obtained exhibited a CdTe(400) double crystal rocking curve (DCRC) having a FWHM of 630 arc sec.

Heteroepitaxy of CdTe(100) on Si(100) using BaF 2−CaF 2(100) buffer layers

Epitaxial CdTe(100) layers have been grown on Si(100) by conventional molecular beam epitaxy (MBE) and photo-assisted MBE (PAMBE) using stacked BaF 2−CaF 2 as a buffer layer. Two-dimensional (2D) growth of BaF 2(100) is obtained using a two-temperature growth method. In conventional MBE, CdTe grows 2D for substrate temperatures above 270°C with a 2 × 1 surface reconstruction indicating a Te-stabilized surface. In PAMBE, 2D growth of CdTe is obtained at lower substrate temperatures (≈200°C) with 1 × 1 RHEED patterns indicating that Ar laser illumination induces Te desorption. PAMBE grown layers have superior structural quality than layers grown by conventional MBE. Sb doped CdTe layers grown by PAMBE exhibit a 2 × 2 surface reconstruction, their 10 K photoluminescence spectra show a single dominant (A 0, X) peak at 1.588 eV, and resistivities of 10 3 Ω cm are measured.

Electrical characterization of CdTe–InSb heterojunctions

Planar and vertical transport studies were carried out on samples of epitaxial (001) CdTe grown by pulsed laser evaporation and epitaxy on n- and p-type (001) InSb substrates. Characterization was performed in the temperature range of 4.2–300 K, including Hall effect, current–voltage, and capacitance–voltage measurements. Ohmic contacts to intrinsic CdTe were obtained by application of a AuCl3, aqueous solution. Similar contacts on grown layers showed no Schottky-barrier characteristics. Layers on n-type substrates did not exhibit rectification. Hall effect and resistivity measurements using contacts to the epilayer are in agreement with bulk n-InSb for the range 120–300 K. Below 120 K current dependence in the measured resistivity is observed with a saturation value at low currents, indicating confinement of the transport to the epilayer. CdTe–p-InSb junctions exhibit rectification characteristics below 80 K with C–V measurements yielding a barrier height of 0.17 V. Hall measurements with contacts to the epilayer showed erratic Hall-voltage measurements, but substrate contacts indicate conduction through InSb implying a depleted layer. Estimated electron concentration and mobility in the CdTe layer are n > 8 × 1016 cm−3, and u ~ 104 cm2 V−1 s−1.This indicates that n-type doping of the CdTe layers, possibly due to the incorporation of In, may take place even at the lowest growth temperature.

Molecular beam epitaxial growth of (100) oriented CdTe on Si (100) using BaF2-CaF2 as a buffer

Epitaxial CdTe (100) has been grown on (100) oriented Si by molecular beam epitaxy using BaF2-CaF2 as a buffer. Two-dimensional (2-D) growth of BaF2(100) is obtained using low-temperature thermal cycles during growth. CdTe growth is also 2-D above 270 °C substrate temperature and a 2×1 surface reconstruction indicating a Te-stabilized surface is obtained. The growth is 3-D at lower substrate temperatures. Good structural quality films showing sharp electron channeling patterns and pronounced photoluminescence at 77 K are obtained. The full width at half maximum of the band-edge peak is 12 meV at 77 K.

Electrophysical characteristics of epitaxial cadmium telluride films

Equilibrium electrophysical characteristics of epitaxial CdTe films grown by the thermal screen method and the quasiclosed volume method are investigated. It is established that the films in both cases are semiconductors with inhomogeneous potential relief of the band to whose formation the main contribution is from barriers on the boundaries of the growth patterns. It is shown that jump conductivity with a variable jump length due to inhomogeneity of the potential relief of the bands and the high states density at the Fermi level is observed in CdTe films synthesized by the quasiclosed volume method for relatively high temperatures.

Laser-induced selected area epitaxy of CdTe and HgTe

The enhancement of growth rate of epitaxial CdTe on GaAs(100)2°→(110) substrates is investigated. Photolytic decomposition of the precursors dimethylcadmium with either dimethyl telluride or diethyl telluride is brought about by absorption of 257 nm radiation from an intra cavity doubled argon ion laser. Growth rate was studied as a function of ultraviolet (UV) intensity by the time resolved reflectivity method to monitor CdTe film thickness. Evidence for enhancement of growth rate from surface absorption of uv radiation is discussed together with the factors that can affect pattern definition. The photoenhancement factor depends primarily on the ratio of photolytic to pyrolytic growth rates, the more stable dimethyl telluride yielding higher photoenhancement factors than diethyl telluride. Results on photopatterned bar and mesa structures are presented with feature sizes down to 50 μm. Preliminary experiments on HgTe photopatterning indicate similar kinetic mechanisms to those for CdTe but slow desorption of reaction products might be responsible for lower growth rates. Possible surface reaction mechanisms are discussed and the prospects for selected area epitaxy of mercury cadmium telluride (MCT) considered.

Growth and microstructural properties of cadmium telluride thin films

For many applications, such as IR detectors and high speed devices, we need high quality cadmium telluride (CdTe) films. To fabricate CdTe epitaxial films we are using a home-built closed hot wall epitaxy system. In this paper, we shall report the complex nature of the CdTe film's microstructure as grown on InSb substrates. Films were grown under various preparation conditions. We observe profound effects of substrate temperature and source temperature on the microstructure of CdTe films. This study together with other properties will allow us to correlate various effects and to design an optimum preparation condition for epitaxial CdTe thin films. An epitaxial CdTe film on different substrates is a solution to the presentshortage of large area CdTe substrates.

Metal-semiconductor-metal (MSM) photodetectors fabricated on MOCVD grown Hg1−xCdxTe

The fabrication of the first metal-semiconductor-metal photodetectors on Hg1−xCdxTe is reported using MOCVD grown layers on GaAs substrates. An epitaxial CdTe overlayer has been incorporated in the device structure for the enhancement of Schottky barrier characteristics. The interdigitated devices (2.3 μm electrode width, 3.3 μm spacing) exhibited a breakdown voltage of −60 V and responsivities of more than 1.0 A/W at a wavelength of 1.3 μm and bias voltage of 40 V. Over the range of bias voltage examined, the dark leakage current of the detectors was dependent on the choice of contact metal, with minimum values of 10 nA at <1 V for Pt/CdTe/Hg1−xCdxTe.

Microstructural, optical and electrical studies of sputtered epitaxial CdTe films

Growth of thick (5 μm or more) epitaxial CdTe(100) films on KBr(100) substrates by planar r.f. magnetron sputtering is discussed. The films are characterized by means of X-ray diffraction, scanning and transmission electron microscopy, UV reflectance and visible and near-IR transmittance measurements.

Molecular beam epitaxy and metalorganic chemical vapor deposition growth of epitaxial CdTe on (100) GaAs/Si and (111) GaAs/Si substrates

Epitaxial CdTe has been grown on both (100)GaAs/Si and (111)GaAs/Si substrates. A combination of molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) has been employed for the first time to achieve this growth: the GaAs layers are grown on Si substrates by MBE and the CdTe film is subsequently deposited on GaAs/Si by MOCVD. The grown layers have been characterized by x-ray diffraction, scanning electron microscopy, and photoluminescence.

Surface nuvleation kinetics of molecular beam epitaxial doped (001) and (111) CdTe

Reflection high-energy electron diffraction desorption studies have been used to determine the reaction kinetics of (001) and (111) CdTe surfaces. The addition of Sb was found to impede epitaxy on the (111) surface, but to be compatible with epitaxy on the (001) surface. The desorption of Cd, Te and Sb from (001) CdTe were observed to follow an Arrhenius relationship with activation energies of 7.70, 1.95 and 3.69 eV, respectively. Reflection high-energy electron diffraction intensity oscillations were observed for the growth of (001) CdTe and used to study interface smoothness. The effect that photo-assisted molecular beam epitaxy has on the reaction of Sb doping of (001) CdTe was determined.

TEM studies of epitaxial CdTe and (Hg, Cd)Te grown by MOVPE on GaAs and CdTe substrates

Epitaxial layers of {111}-oriented CdTe have been grown on {111}A CdTe, {111}B GaAs and {100} GaAs by MOVPE. Cross-sectional transmission electron microscope (TEM) studies have revealed the presence of many lamella twins lying parallel to the substrate-layer interface in all of these layers. In contrast, {100} CdTe deposited onto {100} GaAs exhibits a large number of misfit dislocations at the interface. A structural comparison of epitaxial layers of (Hg, Cd)Te deposited onto CdTe/{111}B GaAs hybrid substrates and onto bulk {111}A CdTe substrates is also reported.

Properties of graded group IIa fluoride buffer layers for semiconductor heteroepitaxy

Composition graded epitaxial CaF 2-SrF 2-BaF 2 stacks with up to 14% lattice mismatch have been grown onto Si(111) by molecular beam epitaxy (MBE). Despite large lattice- and thermal expansion mismatch, the fluoride layers exhibit specular surfaces, are untwinned, and the minimum ion channeling yield is nearly as low as in bulk material. A unique feature is their ability to relax mechanical stress due to lattice- and thermal expansion mismatch even near room temperature in a nonthermal manner. They were applied as buffer layers to grow device-quality epitaxial lead chalcogenides and CdTe on Si, and may find further applications as epitaxial buffers for still other combinations of semiconductor materials with lattice mismatches of up to 20%.