CdTe Epilayers Research Articles

Donor-acceptor pair luminescence involving the iodine A center in CdTe

Photoluminescence studies from 5 to 296 K have been performed on a series of iodine-doped CdTe epilayers grown by molecular beam epitaxy. The samples exhibit excess electron concentrations in the range from 8×1016 to 3×1018 cm−3. Bright edge emission is observed at 296 K from all samples. A deep-level band centered near 1.45 eV is observed at T<210 K and increases in intensity with doping level. A correlation of growth parameters with photoluminescence data fits a model of the deep-level band being predominantly donor-acceptor pair recombination involving the shallow iodine donor (ITe) and the iodine A-center acceptor complex (VCd-ITe). Zero-phonon emission related to this pair recombination occurs at 1.470 eV at 5 K. Thermal quenching of the integrated intensity of this donor-acceptor band is described by activation energies of 15 and 125 meV corresponding to thermalization of electrons from shallow ITe donors to the conduction band and complete thermalization from the valence band to iodine A centers, respectively.

The effect of a ZnTe buffer layer on the structural and optical properties of the CdTe/ZnTe/GaAs strained heterostructures grown by temperature-gradient vapor-transport deposition

Lattice-mismatched CdTe epilayers on GaAs (100) substrates with and without ZnTe buffer layers were grown by the simple method of double-well temperature gradient vapor-transport deposition. X-ray diffraction measurements were performed to investigate the structural properties of the epitaxial layers. Photoluminescence and transmission electron microscopy measurements showed that the crystallinity of the CdTe epilayers grown on the GaAs substrates was remarkably improved using the ZnTe buffer. The strain of the CdTe layer was determined from photoreflectance measurements. These results indicated that the CdTe epitaxial films grown on GaAs substrates with the ZnTe buffer can be used for applications as buffer layers for the growth of HgxCd1−xTe and CdxZn1−xTe.

Electrical and structural properties of epitaxial CdTe/HgCdTe interfaces

In this study, CdTe epilayers were grown by metalorganic chemical vapor deposition on epitaxial HgCdTe with the purpose of developing suitable passivation for HgCdTe photodiodes. Two types of CdTe layers were investigated. One was grown directly,in situ, immediately following the growth of HgCdTe. The second type of CdTe was grown indirectly, on top of previously grown epitaxial HgCdTe samples. In this case, the surface of the HgCdTe was exposed to ambient atmosphere, and a surface cleaning procedure was applied. The material and structural properties of the CdTe/HgCdTe interfaces were investigated using secondary ion mass spectroscopy, Auger electron spectroscopy, Rutherford back scattering, and x-ray double crystal diffractometry techniques. Electrical properties of the CdTe/HgCdTe heterostructure were determined by capacitance-voltage (C-V) characterization of Schottky barrier devices and metal insulator semiconductor devices. Also, a preliminary current-voltage characterization of n+ p photodiodes was performed. A theoretical model suitable for analysis of graded heterojunction devices was used for interpretation of C-V measurements.

The growth and characterization of (211) and (133) Oriented (Hg,Cd)Te epilayers (211)B GaAs by organometallic vapor phase epitaxy

The growth of CdTe buffer layers on (211)B GaAs substrates by organometallic vapor phase epitaxy (OMVPE) was studied, and it was found that, depending on the growth conditions, either the (211) or (133) epitaxial orientation could be formed. In some cases, an epilayer showing a mixed (211) and (133) orientation was also observed. The influence of several growth parameters on the orientation of the CdTe layer was investigated, and it was found that the Te/Cd ratio, together with the growth temperature, have the most significant effect in determining the epilayer orientation. From these results, it was then possible to select nominally optimized growth conditions for CdTe buffer layers of both orientations. (Hg,Cd)Te layers of the same orientations could then be grown and characterized. Although double crystal x-ray diffraction measurements indicated a somewhat better crystalline perfection in the (133) (Hg,Cd)Te layers, these layers showed a poor surface morphology compared to the (211) orientation. Measurement of etch pit densities also indicated defect densities to be typically half an order of magnitude higher in the (133) orientation. Diodes were formed by ion implantation in both orientations and significantly better results were obtained on the (211) (Hg,Cd)Te layers.

Photoluminescence and raman studies of high quality CdTe:I Epilayers

Low-temperature photoluminescence (PL) studies of iodine-doped CdTe epilayers have been performed. A compensating acceptor center which gives rise to deep-level PL emission at 1.491 eV is identified. From selective excitation PL studies, we assign this 1.491 eV line to the recombination of an associate donor-acceptor close pair, consisting of nearest neighbor substitutional sodium and iodine atoms (NaCd-ITe). This neutral defect complex has a localized mode of 36.5 meV, which is much larger than the bulk CdTe lattice mode of 21.3 meV. The electronic energy level associated with this defect is 115 meV below the conduction band. Also, we use a combination of selective excitation PL and Raman spectroscopies to determine the ionization energy of the isolated shallow iodine donor (ITe) in CdTe. We find that the donor binding energy of this anion-site hydrogenic donor is 15.0 (±0.2) meV.

Effect of growth conditions on crystalline quality of metalorganic chemical vapor deposition (111)B CdTe epilayers characterized by x-ray diffraction

A study of the effect of growth parameters on the resulting crystalline quality of CdTe epilayers grown by metalorganic chemical vapor deposition (MOCVD) technique is reported. The crystalline quality of the MOCVD CdTe (111)B epilayers is investigated by the double crystal rocking curve (DCRC) and Laue x-ray diffraction techniques. The rocking curve full width at half-maximum (FWHM) is used as a quantitative measure of structural perfection of the epilayers. The results indicate that a major parameter that affects the crystalline quality is the growth temperature. Epilayers grown at 480 °C exhibit high crystalline quality that is hardly affected by the partial vapor pressure of the metalorganic sources or the growth rate, whereas these growth parameters strongly affect the crystalline quality of epilayers grown at 430 °C. This phenomenon is attributed to the different mechanisms governing the growth process at these growth temperatures, namely mass transfer and surface kinetics. The crystalline quality of the MOCVD CdTe layers grown on the (111)B face of CdZnTe (4% Zn) or CdTe substrates can consistently match that of MOCVD (001) layers previously considered to have the best crystalline quality, with FWHM less than 90 arcsec for 6–7 μm thick layers. Linewidths as narrow as 56 arcsec are observed for epilayers thicker than about 11 μm. Microtwins, identified by the sixfold symmetry of Laue and double crystal 360° φ-scan diffraction patterns, are present even in epilayers with the narrowest FWHM. No straightforward correlation between visual morphology and crystalline quality was found. The growth of CdTe or CdZnTe epilayers is proposed by the MOCVD technique as buffer layers to obtain super substrates (‘‘superstrates’’) for the subsequent liquid phase epitaxy (LPE) growth of HgCdTe layers and additional heterostructures.

Metalorganic chemical vapor deposition CdTe passivation of HgCdTe

CdTe epilayers are grown by metalorganic chemical vapor deposition (MOCVD) on bulk HgCdTe crystals with x ~ 0.22 grown by the traveling heater method (THM). The THM HgCdTe substrates are (111) oriented and the CdTe is grown on the Te face. The metalorganic sources are DMCd and DETe, and the growth is performed at subatmospheric pressure. Ultraviolet (UV) photon-assisted hydrogen radicals pretreatment plays a dominant role in the electrical properties of the resulting heterostructures. The requirements of a good passivation for HgCdTe photodiodes vis-a-vis the passivation features of CdTe/HgCdTe heterostructures are discussed. The effect of valence band offset and interface charges on the band diagrams of p-isotype CdTe/HgCdTe heterostructures, for typical doping levels of the bulk HgCdTe substrates and the MOCVD grown CdTe, is presented. Electrical properties of the CdTe/HgCdTe passivation are determined by capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor test devices, where the MOCVD CdTe is the insulator. It is found that the HgCdTe surface is strongly inverted and the interface charge density is of the order of 1012cm2 when the CdTe epilayer is grown without the UV pretreatment. With the in-situ UV photon-assisted hydrogen radicals pretreatment, the HgCdTe surface is accumulated and the interface charge density is -4. 1011 cm-2.

Selective excitation of an associate donor-acceptor pair complex in iodine-doped CdTe

Luminescence at 1.491 eV was detected at liquid-helium temperatures from CdTe epilayers heavily doped with iodine. The relatively sharp emission band (FWHM=4.4 meV) was observed under both above-band-gap and below-band-gap excitation. The intensity of the 1.491 eV emission was highly dependent on sample temperature and could only be detected below 45 K. Using the tunable output from a titanium:sapphire laser, selective excitation of this emission was performed to identify the recombination center. A localized mode of 36.5 meV, much larger than the 21.3 meV bulk CdTe mode, is associated with the defect center. We identify the defect as an associate donor-acceptor pair complex resulting from nearest neighbor cation (NaCd) and anion (ITe) point defects. The energy level associated with the (Na−Cd-I+Te) neutral associate pair is approximately 0.115 eV below the CdTe conduction band.

Molecular-beam epitaxial growth of CdTe(112) on Si(112) substrates

High crystalline quality epitaxial CdTe(112)B/ZnTe films were deposited by molecular-beam epitaxy directly onto vicinal Si(112) substrates, without use of GaAs interfacial layers. The films were characterized with x-ray diffraction, optical microscopy, and wet chemical defect etching. Single crystal, twin-free CdTe(112)B films exhibit structural quality exceeding that previously reported for CdTe(112)B heteroepitaxy on GaAs/Si(112) or GaAs(112)B substrates. X-ray rocking curve full width at half-maximum of 72 arcsec for CdTe(224) reflection and near-surface etch pit densities (EPD) of 2×106 cm−2 have been observed for 8-μm-thick CdTe films. EPD depth profiles indicate that the threading dislocation density decreases with increasing II–VI epilayer thickness up to approximately 5 μm thickness and saturates at 2×106 cm−2 for thickness exceeding 5 μm. The CdTe epilayer orientation was observed to tilt 2° away from the Si(112) substrate orientation toward the [001] direction.

Structural and optical properties of a strained [formula omitted] heterostructure grown by temperature-gradient vapor transport deposition at low temperature

A lattice-mismatched CdTe epilayer on a GaAs(100) substrate was grown by the simple method of temperature-gradient vapor deposition in the growth temperature range between 200 and 350 °C. From X-ray diffraction analysis, the layers grown were found to be CdTe epitaxial films. Double-crystal X-ray diffraction was used to evaluate the crystallization and the crystal quality of the CdTe films. The stoichiometry of the CdTe films was investigated by Auger electron spectroscopy. Transmission electron microscopy measurements showed that there was a lattice mismatch between the CdTe epitaxial layer and the GaAs substrate. Photoluminescence measurements at 15 K showed that a CdTe film grown on GaAs(100) in the temperature range 200–350 °C appeared to have an optimum crystal perfection at a substrate temperature of about 255 °C. Electrolyte electroreflectance spectroscopy measurements were carried out to determine the energy gap and strain in the CdTe thin films on the GaAs(100).

N and p Type Doping Control of CdTe and CdZnTe Epilayers and CdTe/CdZnTe Quantum Structures

n and p Type Doping Control of CdTe and CdZnTe Epilayers and CdTe/CdZnTe Quantum Structures

Twinning in epilayers of CdTe on <211> Si: Influence of ZnTe buffer layer and substrate misorientation

ABSTRACTX-ray diffraction spectra of CdTe epilayers grown with and without ZnTe buffer layers on <211> Si substrates by molecular beam epitaxy consist of 422 and 331 reflections. We interpret these as evidence for the existence of twins within the volume of a <211> oriented epilayer and show that twin volume is dependent on the ZnTe buffer layer and substrate misorientation.

Characteristics of MOCVD-Grown High-Quality CdTe Layers on GaAs Substrates

CdTe epitaxial layers are grown successfully on a (100)-GaAs substrate by metalorganic chemical vapor deposition (MOCVD) using dimethylcadrnium (DMCd) and diethyltelluride (DETe) as alkyl sources. The CdTe epilayers grown between 365°C and 380°C possess the best surface morphology. DETe is used as the controlling species of this growth system. Typical growth rates are varied from 2.51µm/hr to 5.31µm/hr. Low-temperature (12K) photoluminscence (PL) measurements reveal that 380°C is the best growth temperature and the full width at half maximum (FWHM) of the dominated peak is about 1.583eV by the bound-exciton emission of 9.38meV. The double crystal X-ray rocking curves (DCRC) indicate that the FWHM decreases while increasing the epilayer thickness and approaches a stable value about 80 arc sec under the growth rate of 5.2µm/hr, the growth temperature of 380°C and the DETe/DMCd concentration ratio of 1.7. The value of 80 arc sec in FWHM is the smallest one ever reported to date.

STRUCTURAL AND ELECTRICAL STABILITY OF METAL CONTACTS TO MBE GROWN CdTe LAYERS

The properties of ohmic and rectifying contacts to CdTe epilayers have been studied with a view to establishing a contacting technology with sufficient reliability and stability for use in the fabrication of a variety of CdTe-based devices involving single and multilayer structures grown by MBE. In the case of n-type CdTe layers, evaporated gold has been found to yield Schottky barriers with initial height in excess of 0.9 eV but re-examination after a period of a few weeks at room temperature (or a much shorter time at elevated temperatures) has revealed a significant decline in the barrier height. Associated changes in the electrical properties of the semiconductor layers in the vicinity of the contact interface have been explored using C-V depth profiling measurements and these have been correlated with changes in the chemical structure observed by means of SIMS imaging techniques. In particular, a significant reduction in the uncompensated shallow donor density just below the metal-semiconductor interface has been found to be accompanied by an out diffusion of Te and Cd to the surface of the metal contact.

CdTe/GaAs(100) heterojunctions: Growth modification by thin silicon interface layers

The growth of CdTe epilayers on GaAs has been explored as a potential buffer layer for the fabrication of CMT/CdTe/GaAs structures on silicon substrates. CdTe layers were deposited by molecular beam epitaxy onto GaAs(100) specimens transferred into the II–VI reactor protected by an amorphous arsenic (a-As) cap. Reflection high energy electron diffraction and double- crystal-x-ray diffraction studies show that CdTe{111} layers grow epitaxially on both As- and Ga-rich GaAs(100) surfaces. The relatively narrow x-ray linewidths associated with the layers demonstrate that the layers are of high structural quality. The detailed nature of the interface between CdTe and GaAs has been studied by x-ray photoemission spectroscopy and low energy electron diffraction. It is shown that the interface is highly reactive, leading to the formation of gallium telluride, which influences the growth orientation of the CdTe layer. In an attempt to prevent such interface chemical reactions, we have explored the use of ultrathin silicon interface layers. Thus, GaAs(100) substrates were covered with 1 ML of Si before being capped with a-As. The samples were transferred to a surface science spectrometer, where the a-As cap was removed, producing an ordered Si-(3×1)/GaAs(100) surface. The subsequent growth of CdTe on the Si/GaAs(100) samples yielded well-ordered epilayers, always displaying the (100) orientation. Possible reasons for the change in epilayer crystallographic orientation due to the presence of the Si interlayer are considered.

Photoluminescence from heteroepitaxial (211)B CdTe grown on (211)B GaAs by molecular beam epitaxy

Low temperature (∼5 K) photoluminescence spectroscopy was performed on undoped CdTe epilayers grown by molecular beam epitaxy on (211)B oriented bulk GaAs substrates at substrate temperatures ranging from 230 to 275 °C. The emission spectra from all samples studied contained evidence of the diffusion of gallium and arsenic atoms from the substrate. A broad, low amplitude emission band observed at 1.594 eV was related to the GaCd donor level in CdTe. Donor-acceptor pair recombination observed at 1.51 eV was due to the substitutional GaCd donor and AsTe acceptor. The level of compensation in the CdTe layers was determined from the energy shift of the donor-acceptor emission peak with excitation power, with the lowest degree of compensation observed in a sample grown at 230 °C. In addition, a bright emission peak was observed at 1.47 eV. This peak, which had been observed previously in homoepitaxial and heteroepitaxial growth of CdTe, was related to electron-hole recombination of a structural defect in the CdTe/GaAs epilayers with an electronic binding energy of ∼130 meV.

Direct growth of Hg 1- xCd xTe alloys in a cold-wall, annular reactant inlet inverted-vertical organometallic vapor phase epitaxy reactor at 300°C

An annular reactant inlet inverted-vertical (ARIIV) organometallic vapor phase epitaxy (OMVPE) reactor, designed specifically for direct growth of Hg 1- x Cd x Te (MCT), is presented. The reactor was characterized in terms of the measured properties of CdTe, HgTe and MCT epilayers grown in it. The growth rates were correlated with substrate temperature, reactant flow rates (dimethylcadmium — DMCD, methylallyltelluride — MATe, and Hg) and pedestal position relative to the annular reactant inlet. The composition and growth rate of MCT were determined as a function of Cd-to-Hg ratio, at constant Te flow rate, at 300°C. The thickness and compositional uniformities of 16 cm 2 MCT epitaxial layers were better than ±2% and 0.602±0.002 in x, respectively.

Donor doping of (211) CdTe epilayers and CdTe/CdZnTe piezoelectric heterostructures by molecular beam epitaxy

Shallow indium donors have been incorporated in (211) CdTe and CdZnTe epilayers by molecular beam epitaxy. For this orientation, no Cd overpressure is necessary to incorporate the indium donors. Essentially 100% activation of donors was obtained in the 1016–7×1017 cm−3 concentration range. Electron mobility was 7300 cm2 V−1 s−1 at 40 K for 3×1015 donors cm−3. Transmission electron microscopy shows that the doped (211) CdTe layers have high structural quality. Optical spectra of modulation-doped (211) CdTe/CdZnTe quantum wells show that an electron gas present in a piezoelectric CdTe well produces strong screening of the internal electric field.

Pressure tuning of strain in CdTe/InSb epilayer: A photoluminescence and photomodulated reflectivity study

The heavy-hole and light-hole excitons of a CdTe epilayer, pseudomorphically grown on an InSb epilayer by molecular beam epitaxy, are studied with a diamond anvil cell as a function of applied hydrostatic pressure via photoluminescence (PL) and photomodulated reflectivity (PR) spectroscopies. They are compared with the excitonic features in the simultaneously measured PL spectra of a sample of bulk CdTe. Under applied pressure, the lattice mismatch-induced splitting between the light-hole and heavy-hole related transitions increases in a continuous and reversible manner because of the additional pressure-induced compression due to the difference in the compressibilities of CdTe and InSb. The unusually large strain sustained by the CdTe epilayer under pressure is discussed in the light of various models. The PR signal vanishes after the InSb epilayer goes through a structural phase transition at approximately 20 kbar, while the PL signal persists until it is irreversibly quenched by the CdTe epilayer undergoing a structural phase transition at approximately 30 kbar. For pressures between 20 and 30 kbar, the behavior of the CdTe epilayer is similar to that of the bulk sample; the strain appears to have been relaxed due to the structural phase transition which has taken place in InSb. Values of the first- and second-order pressure coefficients for bulk CdTe and for the CdTe epilayer as well as values of the hydrostatic and shear deformation potentials are obtained at 14 and 80 K and compared with previously quoted values.

UV photon assisted control of interface charge between CdTe substrates and metalorganic chemical vapor deposition CdTe epilayers

The technology to control the interface charge density between CdTe substrates and CdTe epilayers grown by metalorganic chemical vapor deposition is studied. The interface charge is determined by the modified built-in potential derived from capacitance-voltage characteristics of Schottky contacts formed on the epilayers. Novel ultraviolet (UV) photon assisted and photo thermal surface pretreatments that control the interface between p-type CdTe substrates and CdTe epilayers are reported. The substrates are exposed to UV radiation provided by a high pressure Hg lamp operating at 600 W with a wide emission spectrum between 190 and 300 nm. The UV photon assisted surface pretreatment with hydrogen is compared with additional surface pretreatments: thermal pretreatment with hydrogen (without UV photons) and UV photo thermal pretreatment with hydrogen. The UV photon assisted and the UV photo thermal surface pretreatments with hydrogen reduce the interface charge density to a practically negligible value. In addition, the p-type doping level of the substrate is reduced considerably in a layer of few microns adjacent to the interface.