CdTe Epitaxial Layers Research Articles

Orientation of CdTe epitaxial films on GaAs(100) grown by vacuum evaporation

Abstract The growth of (100)- and (111)-oriented CdTe epitaxial layers on (100)-oriented GaAs substrates were investigated. Ar+ plasma bombardment was used to remove the surface oxide layer, while preheating the substrate before evaporation was performed to deplete arsenic on the GaAs substrate surface. Results indicate that the CdTe(100) will grow on GaAs(100) with an oxide layer remaining on the surface. For the GaAs(100) substrate with the oxide layer removed by plasma bombardment, CdTe(100) will grow on the arsenic-depleted GaAs substrate, while CdTe(111) will grow on the GaAs substrate without arsenic depletion. A model is proposed that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through Ga-Te bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. The photoluminescence investigation conforms to our proposed model.

Incorporation of hydrogen in CdTe and HgTe epitaxial layers grown by MOCVD

The incorporation of hydrogen into MOCVD-grown layers of CdTe, HgTe and CdHgTe using H2 as the vector gas has been studied. Concentrations of incorporated H going from 6.5 × 1017 cm-3 to 5 × 1018 cm-3 have been found by SIMS in CdTe layers. This concentration decreases with increasing growth temperature and decreasing bond strength of the host material.

The observation of induced stresses in cadmium telluride films grown on Si by closed hot wall epitaxy system

X-ray diffraction has been used to study the induced stress in cadmium telluride (CdTe) epitaxial layer, grown in a closed hot wall epitaxial system on (100) Silicon (Si) substrates. The difference between the lattice constants and the thermal expansion coefficients of CdTe and Si are about 20% and 50%, respectively. These mismatches could produce tetragonal distortion in CdTe thin films.

High resolution X-ray diffraction studies of CdxHg1- xTe/ CdTe epitaxial layers grown by MOVPE on GaAs substrates

We have applied high resolution X-ray diffractometry and topography techniques to investigate both the lateral uniformity and structural properties of Cd x Hg 1− x Te layers grown by MOVPE onto CdTe buffer layers on GaAs. On samples ∼1–2 cm square, maps of rocking curve width (β) have shown values varying from 60 arc sec (comparable to the best reported) to over 1000 arc sec on the same slice, indicating the superior value of mapping over single point measurements on this material. A good correlation has been observed between rocking curve widths, lattice tilts and the density of pyramid-like surface defects, the last of which are also associated with an increased twin density. However, on rotating the sample about its surface normal, the 400 surface symmetric β-value varies by up to an order of magnitude, indicating that lattice tilts play an important role in broadening the rocking curve. X-ray topography reveals large tilt boundaries in the CMT epilayer which correlate with the dislocation structure in the GaAs substrate.

Photoassisted molecular beam epitaxy of wide gap II–VI heterostructures

Photoassisted molecular beam epitaxy (PAMBE) is a new growth technique that has been developed to enhance the substitutional doping of II–VI compound semiconductors. During the process the substrate is illuminated during the growth sequence. In the present work a modified Riber MBE 2300 system was employed. A modification to the commercial system was made such that the substrate could be illuminated with an argon-ion laser during the deposition. The effect of illumination power density on the PAMBE growth of undoped CdTe has been studied in detail. With increasing power density we see the appearance of a new, very narrow (0.18 meV) and very intense luminescence feature. Polarization studies have been recently completed showing that this line is strongly linearly polarized along one of the crystallographic axis. We have observed for the first time strongly linearly polarized luminescence for undoped (100)CdTe epitaxial layers grown by photoassisted molecular beam epitaxy. The observation of this linear polarization leads us to postulate that the A-line and B-line observed in PAMBE grown CdTe are associated with vacancy-donor complexes that are preferentially oriented during the layer by layer growth process. The effect of illumination power density on the mobility and carrier concentration in undoped and lightly doped CdTe epilayers has been studied in detail. At low power densities we see an increase in both the mobility and carrier concentration.

X-ray double-crystal diffraction studies of CdTe/GaAs heteroepitaxial layers

The full width at half maximum (FWHM) of the rocking curves and its corresponding strains in CdTe epitaxial layers grown on (111) and (100) GaAs substrates with a metalorganic chemical vapor deposition (MOCVD) technique have been investigated by means of X-ray double-crystal diffractometry. The (333)CdTe rocking curves of (111)CdTe/(111)GaAs epilayers showed that the FWHM values decreased from 750 to 432 arc sec as the thickness increased from 0.5 to 1.8 μm. However, when the growth temperature was raised to 390°C, the epilayer growth mechanism changed from 2D to 3D fashion and the FWHM increased sharply to 1080 arc sec. Compressive strains were present in these (111)CdTe layers and were decreased with increasing thickness. The (400)CdTe rocking curves of (100)CdTe/(100)GaAs epilayers exhibited similar behavior to those of (333)CdTe peaks. Nevertheless, these (100) layers exhibited a 3D growth mechanism even at low growth temperatures (340°C). Epilayer morphologies in consistency with the proposed growth mechanism were also observed.

Far-IR spectroscopy of bulk and surface phonon-polaritons on epitaxial layers of CdTe deposited by plasma MOCVD on GaAs substrates

Far-IR measurements by dispersive Fourier transform spectroscopy (DFTS) and attenuated total reflection (ATR) spectroscopy have been made on semiconductor samples consisting of epitaxial layers of CdTe on GaAs substrates. The two techniques have enabled all bulk and surface phonon modes and guided waves propagating in the system to be investigated. Both the CdTe and GaAs reststrahlen bands, which lie in well separated spectral regions, are observed by DFTS. ATR measurements have revealed a p polarized surface polariton in the CdTe restsrahlen band and both s and p polarized guided waves in the GaAs reststrahlen band. In each case the measured results are in good agreement with calculations based on standard multilayer optics techniques, and each technique provides a sensitive probe of the layer thickness.

X-ray Diffraction Investigation of Epitaxial Layers of CdTe on Sapphire

The anomalous scattering of X-rays has been used to determine the polarity of CdTe epitaxial layers on sapphire. The results for two samples are presented, one of (111) orientation ('A face'), the other of (III) orientation (,B face'). The (III) layer is twinned, the two twin species being related by a 180� rotation about the [1111 axis. The twin fraction shows considerable variation for different positions on this sample, and must be taken into account when analysing the integrated X-ray intensities, in order to get meaningful Bijvoet ratios. The polarities of the two twin species are found to be the same.

Epitaxial Layer Misorientation in CdTe on GaAs‡

ABSTRACTX-ray diffraction has been used to characterize the relative misorientation of [001] and [111] CdTe layers grown by hot-wall epitaxy on GaAs substrates. The magnitude of the misorientation of the CdTe epitaxial layer relative to the GaAs substrate depends on the magnitude of the miscut of the substrate; in addition, the [111] oriented CdTe exhibited significantly larger misorientations than did the [001] CdTe films. The azimuthal direction of the tilt of the epitaxial layer depends strongly on the nominal crystallographic orientation of the film. The [111] CdTe exhibited an azimuthal dependence of the tilt that is approximately coincident with the miscut of the substrate, while the azimuthal direction of tilt in the [001] CdTe layers differed from the substrate miscut direction by as much as 116°. These observations of epitaxial layer misorientation are discussed in terms of a dislocation model for layer tilt and azimuthal rotation in lattice-mismatched epitaxial systems.

Linear polarized luminescence from CdTe epilayers

We report for the first time the observation of strongly linearly polarized luminescence from CdTe epitaxial layers grown by photoassisted molecular beam epitaxy. Strong linear polarization was detected in the acceptor-bound exciton spectral region and in the pair band regions (1.54–1.56 eV and 1.46–1.48 eV). The linear polarization is believed to be due to the preferential alignment of complex centers along one of the [110] lattice directions. The cause of the preferential orientation of these complex centers is the inequivalence during the planar growth on the (100) surface of the [110] and [1̄10] directions in the zinc-blende lattice.

Photoluminescence of gallium impurity in cadmium telluride

We have studied low-temperature photoluminescence in CdTe containing Ga impurity in the 1.28–1.61 eV energy region. The impurity was introduced either by implantation of bulk CdTe wafers or by doping CdTe ingots during Bridgman growth. The presence of Ga was discovered and studied in epitaxial layers of CdTe grown on GaAs substrates by pulsed laser evaporation and epitaxy. Extra features caused by the Ga presence were observed in the edge emission and in the excitonic regions. Implantation results in extended mechanical damage, and only a small fraction of the implanted Ga is optically active. Samples doped during growth by the Bridgman technique exhibit changes in the excitonic region. The photoluminescence spectra of CdTe epitaxial films grown on GaAs exhibit features which are similar to those observed in bulk CdTe with Ga impurity.

The film/substrate orientation relationships of CdTe grown on Si and GaAs by low pressure metalorganic chemical vapour deposition

CdTe epitaxial layers have been grown on (100) Si, (100) GaAs and (111) GaAs substrates by low pressure metalorganic chemical vapour deposition method with dimethylcadmium (DMCd) and diethyltelluride (DETe) as source materials. The orientation relationships between epitaxial layers and substrates are determined experimentally by X-ray diffraction, and the results show that (100) and (111) oriented CdTe epitaxial layers with parallel epitaxial relationships can be grown on (100) and (111) GaAs substrates, respectively. However, the epitaxial orientation relationship is (111)CdTe ‖ (100) Si for the Si substrate. To investigate the factors which determine the orientation relationships, plane-view transmission electron microscopy observations of the interface were carried out. The results seemed to indicate the existence of a thin amorphous layer between CdTe epitaxial layer and Si substrate, and a dislocation structure in the CdTe/GaAs interface. On the basis of these observations, a Te-O bond atomic arrangement of the CdTe/Si interface is tentatively suggested and discussed for the (111) CdTe epitaxial layer formed on (100) Si substrate. On the other hand, the dislocation interface probably acts as a stress buffer layer and plays a major role for the parallel growth orientation in the (100) CdTe ‖ (100) GaAs case.

Temperature-independent unassisted pyrolytic MOCVD growth of cadmium telluride at 290°C and mercury telluride at 325°C using dimethylcadmium methylallyltelluride and dimethylmercury

Methylallyltelluride (MATe) was synthesized and tested as part of a joint effort between American Cyanamid Co. and Hughes Research Laboratories to develop a Te alkyl suitable for low-temperature, high-growth-rate epitaxy of Hg1-xCdxTe (MCT) alloys by unassisted pyrolytic metalorganic chemical vapor deposition (MOCVD). Cadmium telluride and HgTe growth studies using MATe were performed at substrate temperatures between 250 and 350°C. Reproducible temperature-dependent CdTe growth rates as high as 30 μm/h were achieved at or above 290°C. The decomposition characteristics of the Hg source, dimethylmercury (DMHg), limited the minimum temperature for temperature-independent growth of HgTe to 325°C. Between 325 and 340°C the maximum HgTe growth rates were 15 μm/h. Room-temperature Hall measurements were performed on CdTe epitaxial layers grown on high-resistivity CdTe substrate at 300°C. These layers were p-type. Their carrier concentrations (NA-ND), resistivities (ρ) and Hall mobilities (μH) were 3.9X1015 ⩽ NA-ND ⩽ 2.7X1016cm-3, 14.2 ⩽ ρ ⩽ 54.3 Ω cm, and 15 ⩽ μH ⩽51 cm2/V h- s, respectively.

HgCdTe photovoltaic detectors on Si substrates

HgCdTe infrared photovoltaic detectors were fabricated on silicon substrates for the first time by using intermediate CdTe and GaAs epitaxial layers. No cracking or degradation was observed after thermal cycling these devices (cutoff wavelength of 5.5 μm and R0A as high as 200 Ω cm2 at 80 K). Secondary ion mass spectrometry and Auger data substantiate that a CdTe buffer layer can prevent Ga diffusion from the intermediate GaAs epitaxial layer from inadvertently converting the p-HgCdTe to n-type at growth temperatures as high as 500 °C.

Static atomic displacements in a CdTe epitaxial layer on a GaAs substrate

A (001)CdTe epitaxial layer on a (001)GaAs substrate was studied by x-ray diffraction between 10 and 360 K. The CdTe growth took place at 380 °C in a vertical gas flow metalorganic chemical vapor deposition reactor. Lattice parameters and integrated intensities of both the substrate and the epitaxial layer using the (00l) and (hhh) Bragg reflections reveal three important features. Firstly, the GaAs substrate does not exhibit severe strain after deposition and it is as perfect as a bulk GaAs. Secondly, the CdTe unit cell distorts tetragonally with a⊥>a∥ below 300 K. The decay of the (00l) reflection intensities as a function of the temperature yields a Debye temperature of 142 K, the same value as for bulk CdTe. Thirdly, a temperature-dependent isotropic static displacement of the Cd and the Te atoms is introduced to account for the anomalous behavior of the (hhh) intensities.

Low temperature epitaxial growth of cadmium telluride from mercury solvent

The concept of “infinite” melt was used to grow liquid phase epitaxial layers of CdTe in the vertical dipping configuration. Approximately 10 kg of Hg were used as the solvent. The layers were grown on CdTe, Cd 0.96Zn 0.04Te and on in-house grown CdTe on sapphire substrates in the temperature range 200–245°C. Techniques such as photoluminescence, X-ray diffraction and optical microscopy were employed to study the purity, crystallinity and the surface morphology of the layers.

Defect structure of epitaxial CdTe layers grown on {100} and {111}B GaAs and on {111}B CdTe by metalorganic chemical vapor deposition

The technique of cross-sectional transmission electron microscopy has been used to investigate the defect content of epitaxial CdTe layers grown on {100} and {111}B GaAs and on {111}B CdTe by metalorganic chemical vapor deposition. Growth on {111} oriented substrates invaribly gave rise to layers of {111} orientation and these contained a large number of thin (100–1000 Å) lamella twins lying parallel to the interface. In contrast, layers grown on {100} GaAs substrates were found to exhibit either {100} or {111} orientation. Epilayers with the former alignment contained arrays of misfit dislocations at the interface, whereas those with the latter orientation exhibited a density and distribution of lamella twins which were comparable with those of layers grown on {111} substrates. The presence of these defects in homoepitaxially grown CdTe, where the effects of lattice mismatch do not arise, clearly indicates that twinning in {111}B CdTe epilayers is a growth phenomenon.

An electroreflectance study of CdTe

Electroreflectance measurements are reported for bulk and epitaxial CdTe layers grown by molecular-beam epitaxy (MBE). Spectra are analyzed to determine the E0, E1, and E1+Δ1 transition energies as well as the associated broadening energies. The broadening energies are found to correlate with the structural quality of the epilayers. Additional spectral features near the band edge (1.45–1.51 eV) and in the UV (3.1–4.3 eV) are discussed. The extremely sharp spectra obtained for the MBE-grown epitaxial layers indicate high-quality material.

Characterization of thin films, epitaxial layers, and superlattices using the precession camera

This article describes a simple method to provide qualitative structural information on thin films, epitaxial layers, and superlattices using the Buerger precession camera. This method is especially useful for epitaxially grown films because it gives a rapid and easy-to-interpret photograph of the reciprocal space of all the components at once: substrate, film or buffer layer, and/or superlattice. A wide variety of examples - Al on Si, Ge on Si, CdTe epitaxial layers on GaAs, CdTe/ZnTe superlattices on GaAs, and Mo/Ni superlattices on mica - are used to illustrate the present technique.

X-ray observation of twin faults in (1,1,1) CdTe epitaxial layers and in (1,1,1) Hg1−x X xTe/CdTe superlattices

This letter describes x-ray precession photographs of (1,1,1) CdTe epitaxial layers on GaAs, CdTe, and Cd0.96Zn0.04Te substrates. The precession method is further applied to some (1,1,1) Hg1−xXxTe/CdTe superlattices where X=Cd or Mn. More than half of the samples used in this study were grown by molecular beam epitaxy, others by metalorganic chemical vapor deposition, and a few by liquid phase epitaxy. The results unambiguously show that the common (1,1,1) face centered cubic twin fault, or stacking disorder, seen in bulk growth methods also exhibits its effects with the mentioned layer deposition growth techniques, except when liquid phase epitaxy is used. It seems that the twinning is intrinsic to the (1,1,1) growth in face centered cubic systems. In addition, devices with twin domains may have lower electronic mobilities. It further means that more experimental work is needed to produce untwinned single-crystal materials deposited by molecular beam techniques.