Te Epitaxial Layers Research Articles

Electrophysical Properties of p-Type Undoped and Arsenic-Doped Hg1 – xCd x Te Epitaxial Layers with x ≈ 0.4 Grown by the MOCVD Method

The temperature dependences of the charge-carrier concentration and lifetime of minority carriers in undoped and arsenic-doped p-type Hg1 – xCd x Te epitaxial layers with x ≈ 0.4 grown by the MOCVD-IMP (metalorganic chemical vapor deposition–interdiffusion multilayer process) method are studied. It is shown that the temperature dependences of the charge-carrier concentration can be described by a model assuming the presence of one acceptor and one donor level. The ionization energies of acceptors in the undoped and arsenic-doped materials are 14 and 3.6 meV, respectively. It is established that the dominant recombination mechanism in the undoped layers is Shockley–Read–Hall recombination, and after low-temperature equilibrium annealing in mercury vapors (230°C, 24 h), the dominant mechanism is radiative recombination. The fundamental limitation of the lifetime in the arsenic-doped material is caused by the Auger-7 process. Activation annealing (360°C, 2 h) of the doped layers makes it possible to attain the 100% activation of arsenic.

Электрофизические свойства нелегированных и легированных мышьяком эпитаксиальных слоев Hg-=SUB=-1-x-=/SUB=-Cd-=SUB=-x-=/SUB=-Te p-типа проводимости с x~0.4, выращенных методом MOCVD

AbstractThe temperature dependences of the charge-carrier concentration and lifetime of minority carriers in undoped and arsenic-doped p -type Hg_1 – x Cd_ x Te epitaxial layers with x ≈ 0.4 grown by the MOCVD-IMP (metalorganic chemical vapor deposition–interdiffusion multilayer process) method are studied. It is shown that the temperature dependences of the charge-carrier concentration can be described by a model assuming the presence of one acceptor and one donor level. The ionization energies of acceptors in the undoped and arsenic-doped materials are 14 and 3.6 meV, respectively. It is established that the dominant recombination mechanism in the undoped layers is Shockley–Read–Hall recombination, and after low-temperature equilibrium annealing in mercury vapors (230°C, 24 h), the dominant mechanism is radiative recombination. The fundamental limitation of the lifetime in the arsenic-doped material is caused by the Auger-7 process. Activation annealing (360°C, 2 h) of the doped layers makes it possible to attain the 100% activation of arsenic.

Changes of PbSnTe thin film thickness due to the oxidation by different methods

Nanostructures based on PbTe seem promising for production of thermoelectric converters. However surface oxidation of nanoparticles that occurs during the manufacturing processes can significantly change the nanocrystals' dimensions and properties. Three oxidation methods, i.e. thermal, electrochemical and chemical, were compared with the objective to minimize the loss of semiconductor material during the oxidation. Correlation between thickness of an oxide layer and a semiconductor material layer used to grow the oxide were determined from Auger Electron Spectroscopy profiling through PbTe and Pb0.8Sn0.2Te epitaxial layers down to the BaF2 substrate. The thermal oxidation was found to ensure minimum semiconductor material loss. In addition, material loss in electrochemical processes can be reduced through appropriate choice of solvents.

Model representation of the transmission of high-power pulsed laser IR radiation in the fundamental absorption region in Cd x Hg1–x Te epitaxial layers

A model which describes the changes in the transmission of high-power laser pulses by Cd x Hg1–x Te in view of the variations of the optical parameters of the material with temperature is presented. The thermal fields, absorptance profiles, and transmittance of the semiconductor epitaxial structure have been calculated and compared with experimental data obtained for the wavelength of a carbon dioxide laser.

Raman spectroscopic study of HgCdTe epilayers for infrared detector array fabrication

Micro-Raman spectroscopy was used to evaluate the surface quality of Hg 1− x Cd x Te ( x = 0.3) epitaxial layers polished with chemo-mechanical and free chemical etching processes. Raman technique has been employed to monitor the HgCdTe detector array fabrication process to ensure epilayer surface devoid of impurities and minimal damage. Chemical etching with bromine–methanol solution is seen to leave residual damage or non-stoichiometric HgCdTe surface layer indicated by an asymmetrical broadening and down shift in all Raman peaks. A peak at 112 cm −1 as defect mode or due to disordering on the epilayer surface has been noticed.

Reconstruction of lattice structure of ion-implanted near-surface regions of Hg 1 − XCd XTe epitaxial layers

The results of X-ray, SIMS, SEM and AFM studies of near-surface regions of Hg 1 − X Cd X Te (111) and (110) structures have been presented. These structures were obtained during annealing in the vapour of main components of ISOVPE Hg 1 − X Cd X Te epitaxial layers with the surface implanted with arsenic ions. Berg-Barrett topography method and two-crystal spectrometry technique were exploited. For calculation of depth dependences of deformations and concentrations of dominating defects in the layer's near-surface regions, the Takagi-Taupin equation and generalized dynamic theory of X-ray scattering were used. From experimental SIMS analysis, the distributions of both impurity and main components of Hg 1 − X Cd X Te solid solution were determined providing more accurately the deformation profiles in near-surface regions of the structures. SIMS and SEM analyses have given evidence of abrupt gradients of content in the Hg 1 − X Cd X Te solid solution, due to small changes of thermodynamical equilibrium conditions of high-temperature annealing in the near-surface regions. These are the cause of formation of a graded-gap structure with strains in the crystal lattice. AFM studies have demonstrated that the morphology of Hg 1 − X Cd X Te structures is subjected to the influence of CdTe substrate orientation.

Effect of source composition on the vapor phase epitaxy of Cd1−xZnxTe large-area layers

Abstract Cadmium zinc telluride (Cd 1− x Zn x Te) epitaxial layers were grown by vapor phase epitaxy (VPE) to obtain material suitable for applications in photonics and radiation detectors. The VPE experiments were carried out in the modified Pfeiffer Classic 500 coating system, using Si 6-in substrates and Cd 1− x Zn x Te ternary sources grown by the Bridgman method. To study the effect of source composition, the Cd 1− x Zn x Te sources and as-grown layers were characterized by a scanning electronic microscopy (SEM), composition analysis (EDAX) and X-ray diffractometry (XRD). It was shown that the prolonged VPE growth is followed by the continuous deterioration of source composition that results in an inhomogeneous enrichment of Cd 1− x Zn x Te layers with Zn. Nevertheless, the fast VPE runs using the sources in the form of thin polycrystalline wafers provide the uniform Cd 1− x Zn x Te layers that could be considered for applications.

Structural changes in arsenic ion‐implanted Hg1–xCdxTe epitaxial layers

AbstractResults of X‐ray investigations of Hg1–x Cdx Te (x = 0.25) epitaxial layers after single and two‐stage arsenic ion implantation with the energy E = 100 keV are represented. It has been established that for doses D1 = 2 × 1014 ions/cm2 and D2 = (2 × 1014 + 1015) ions/cm2 the Frenkel pair (anion vacancy + interstitial mercury atom) generation is the most probable process. The model of a possible system of structural defects in the near‐surface layers of ion‐implanted structures Hg1–x Cdx Te is proposed. The model allows for the presence of certain dimensions and concentrations of both growth and new defects – the Frenkel pairs, cluster formations and dislocation loops formed under ion radiation. The calculated deformation profiles are complex in shape with characteristic peaks at the depth of maximum nuclear power ion loss. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth characteristics of ZnMgTe layer on ZnTe substrate by metalorganic vapor phase epitaxy

The ternary Zn 1− x Mg x Te epitaxial layers have been successfully grown on (1 0 0) ZnTe substrates by atmospheric pressure metalorganic vapor phase epitaxy using dimethylzinc, bis-methylcyclopentadienyl-magnesium ((MeCp) 2Mg) and diethyltelluride (DETe) as source materials. The effects of source transport rates and total gas flow rate upon the Mg composition in the Zn 1− x Mg x Te layers have been investigated. It was found that the Mg composition drastically increase with decreasing DETe transport rate or with increasing the total gas flow rate, which may be associated with the reduction of the premature reaction between DETe and (MeCp) 2Mg.

Control of Zn composition (0 < x < 1) in Cd 1− xZn xTe epitaxial layers on GaAs substrates grown by MOVPE

Cd 1− x Zn x Te epitaxial layers over the entire composition range ( x from 0 to 1) were grown on (1 0 0) GaAs substrates by atmospheric pressure metalorganic vapor phase epitaxy. A growth temperature of 560 °C, and the group VI/II precursor flow rate ratio of 2 or larger enabled us to control the Zn composition strictly on the grown epilayers. Epitaxial layers with high crystal quality were obtained in a wide range of Zn composition. The double crystal rocking curves (DCRC) full-width at half maximum (FWHM) values were between 260 and 670 arcsec at the end points of alloy range. The low-temperature PL measurements showed distinct bound-exciton emissions and weak deep-level emissions.

Electrical Characterization of p-Type Pb1−xEuxTe

The transport properties of p-type Pb 1-x Eu x Te epitaxial layers were studied as a function of Eu content, temperature, and magnetic field. The low-temperature hole mobility is drastically reduced when the Eu concentration is increased from 0 to 6%, while the hole concentration remains almost constant. A metal-insulator transition was observed for x ≃ 0.04, which is probably induced by the disorder caused by the introduction of Eu. For temperatures down to 10K, only positive magnetoresistance has been observed at low magnetic fields. An anomalous behavior of the resistivity as a function of temperature has been detected for a Eu content about 5%, which is attributed to the resonance between the localized Eu 4f level and the valence band maximum.

Type conductivity conversion in As-, Sb-doped p-Cd xHg 1− xTe under ion beam milling

The mechanism of p-to-n type conductivity conversion under ion beam milling (IBM) in extrinsically (As or Sb)-doped p-Cd x Hg 1− x Te with x≈0.2 both experimentally and theoretically was studied. It has been experimentally revealed that the character of changes in the As- or Sb-doped samples after IBM is similar to those in the vacancy-doped p-Cd x Hg 1− x Te and is determined by super-fast mercury interstitial atoms diffusion. The basis of theoretical analyze is the model of super-fast Hg interstitial atoms diffusion that has permitted to explain the similar conversion occurred in Hg vacancy-doped p-type Cd x Hg 1− x Te. In this case the conversion involves only the simple intrinsic point defects. In an acceptor doped material a donor is generated due to the formation of a complex—interstitial Hg atom—As or Sb atom located in the Te site. This model provides reasonably good fits with the experimental results obtained for As- and Sb-doped Cd x Hg 1− x Te epitaxial layers where the electron concentration in the converted n-layer corresponds to the concentration of the p-type dopants. Different efficiency of the conductivity conversion observed for As- and Sb-doped samples may be explained by different enthalpy of complex formation calculated for (As Te–Hg I) and (Sb Te–Hg I) pairs.

Efficient and Long-Lived Green Light-Emitting Diodes Based on ZnSSe:Te Active Layer

Detailed optical characteristics of excitonic green emission/absorption in ZnSSe:Te epitaxial layers, grown by molecular beam epitaxy, were studied by photoluminescence (PL) and PL excitation measurements. Based on these optical properties, we have developed bright and long-lived green (∼500 nm) light-emitting diodes (LEDs) using ZnS0.11Se0.85:Te0.04 epilayers as active layers. The ZnSSe:Te-based LEDs exhibit a fairly long device lifetime (>2000 h) when operated at 3 A/cm2 under CW condition at room temperature. The green LEDs show only slow-mode degradation, and the degradation mode is quite different from that of II–VI-based laser diodes (LDs) and LEDs employing the ZnCdSe–ZnSe system. It is confirmed that the Te-doping-induced “alloy-hardening effect” plays an important role in both efficient emission and strong suppression of the device degradation.

Controlled arsenic diffusion in epitaxial Cd xHg 1− xTe layers in the evaporation–condensation–diffusion process

The results of controlled doping of Cd x Hg 1− x Te epitaxial layers are presented. The investigated layers were obtained by the evaporation–condensation–diffusion method in the process of isothermal growth. The process of auto diffusion from the solid phase was investigated. This process consists in the diffusion of As dopants from CdTe substrate into the grown epitaxial layer. Two types of CdTe substrates, uniformly doped in the process of synthesis and unalloyed with ion implanted surface layer, have been used as sources of As diffusant. Comparative analysis of galvanomagnetic measurements and SIMS spectra was carried out. The results of investigations indicate the very high, almost 100%, electrical activity of As dopants in the Cd x Hg 1− x Te epitaxial grown layer.

Defect Structure Rebuilding by Ion Beam Milling of As and Sb Doped p-Hg1-xCdxTe

This study focuses on developing an understanding of the mechanisms of ion beam milling induced p-to-n conversion in extrinsically (As or Sb) doped p-Hg 1-x Cd x Te with x 0.2. The basis of modeling is the quasichemical approach and the model of superfast Hg interstitial atoms diffusion that has permitted to explain the similar conversion occurred in Hg vacancy-doped p-type Hg 1-x Cd x Te. In an acceptor doped material a donor is generated due to the formation of a complex (of interstitial Hg atom and an As or Sb atom located in the Te site). This model provides reasonably good fits with the experimental results obtained for As and Sb doped Hg 1-x Cd x Te epitaxial layers where the electron concentration in the converted n-layer corresponds to the concentration of the p-type dopants. Different efficiency of the conductivity conversion observed for As and Sb doped samples may be explained by different enthalpy of complex formation calculated for As Te -Hg 1 and Sb Te -Hg 1 pairs.

Long-range order in Cd xZn 1− xTe epilayers grown on GaAs substrates

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the ordered structure in Cd x Zn 1− x Te epitaxial layers grown on (0 0 1)GaAs substrates. The SADP showed two sets of superstructure reflections with symmetrical intensity, and the high-resolution TEM (HRTEM) image showed doublet periodicity in the contrast of the (1 1 1) lattice planes. The results of the SADP and HRTEM measurements showed that CuPt-type ordered structures with two different variants were observed in the Cd x Zn 1− x Te epitaxial layers. The formation of a CuPt-type ordered structure in the Cd x Zn 1− x Te epitaxial layer might originate from the minimization of strain relaxation energy in the reconstructed GaAs(0 0 1) surface. A possible atomic crystal structure for the Cd x Zn 1− x Te epitaxial layer is presented based on the HRTEM results. These results provide important information on the microstructural properties for enhancing device efficiencies of operating at the blue–green region of the spectrum.

Effects of passivation treatment on the surface stability in indium-doped Hg 0.8Cd 0.2Te epilayers grown on p-Cd 0.96Zn 0.04Te substrates

Passivation treatment on indium-doped Hg 0.8Cd 0.2Te epitaxial layers grown on p-Cd 0.96Zn 0.04Te substrates by molecular beam epitaxy has been performed in order to improve the surface stability of the Hg 0.8Cd 0.2Te layers. Room-temperature capacitance–voltage measurements clearly revealed metal-insulator–semiconductor (MIS) behavior for the Al/ZnS/passivated Hg 0.8Cd 0.2Te layer/Cd 0.96Zn 0.04Te diodes. The fast state density and the fixed charge density of the Al/ZnS/passivated Hg 0.8Cd 0.2Te/Cd 0.96Zn 0.04Te diode with a sulfur-treated Hg 0.8Cd 0.2Te layer were smaller than those with a chemically oxidized Hg 0.8Cd 0.2Te layer. The interface state density at the ZnS/sulfur-treated Hg 0.8Cd 0.2Te interface were low at 10 11 eV −1 cm −2 at the middle of the Hg 0.8Cd 0.2Te energy gap. These results indicate that the Hg 0.8Cd 0.2Te epilayer is significantly passivated by sulfur treatment and that the passivated Hg 0.8Cd 0.2Te layers can be used for Hg 1− x Cd x Te-based MIS diodes and MIS field-effect transistors.

Improvement of the surface stability of indium-doped Hg0.8Cd0.2Te epitaxial layers grown on p-Cd0.96Zn0.04Te substrates by photochemical passivation

Improvement of the surface stability of indium-doped Hg0.8Cd0.2Te epitaxial layers grown on p-Cd0.96Zn0.04Te substrates by photochemical passivation

Shear deformation and strain relaxation in HgCdTe on (211) CdZnTe

Shear strain is present in Hg0.68Cd0.32Te epitaxial layers grown by molecular beam epitaxy on (211)-oriented Cd1−yZnyTe substrates. Differences in the substrate zinc composition led to lattice mismatch between the epitaxial layer and the substrate. The shear strain induced by the mismatch was measured using reciprocal space maps in the symmetric (422) and asymmetric (511) and (333) reflections. In addition, strain relaxation through the formation of misfit dislocations was confirmed using double crystal x-ray topography. Both the shear strain and the misfit dislocation density increased with increasing mismatch between the epitaxial layer and the substrate. Lattice-matched layers were free of misfit dislocations and exhibited triple axis diffraction rocking curve widths of approximately 6 arcsec. The combination of a thick epitaxial layer, a low index substrate, and the potential for lattice mismatch indicates that both shear strains and misfit dislocations must be considered in the structural analysis of HgCdTe/CdZnTe heterostructures.

Structural, electrical, and optical property studies of indium-doped Hg0.8Cd0.2Te/Cd0.96Zn0.04Te heterostructures

Transmission electron microsopy (TEM), Hall effect, and Fourier transform infrared (FTIR) transmission measurements were performed to investigate the structural, electrical, and optical properties of indium-doped Hg0.8Cd0.2Te epitaxial layers grown on Cd0.96Zn0.04Te (211) B substrates by molecular-beam epitaxy. The TEM measurements showed that high-quality Hg0.8Cd0.2Te epitaxial layers with interfacial abruptnesses were grown on the Cd0.96Zn0.04Te substrates. The Van der Pauw Hall effect measurements on typical indium-doped Hg0.8Cd0.2Te/Cd0.96Zn0.04Te heterostructures with a doping concentration of 6 × 1016 cm−3 at 10 K in a magnetic field of 0.5 T yielded a carrier density and a mobility of 2.2 × 1016 cm−3 and 40,000 cm2/V s, respectively. The FTIR spectra showed that the absorption edges of the indium-doped Hg0.8Cd0.2Te/Cd0.96Zn0.04Te heterostructures shifted to a shorter wavelength range than those of the undoped samples, which was caused by the Burstein–Moss effect. The FTIR spectra also showed that the transmittance intensities of the indium-doped Hg0.8Cd0.2Te/Cd0.96Zn0.04Te heterostructures increased compared with those of the undoped heterostructures, which is due to the compensation of the Hg vacancy defects by the indium atoms. These results indicate that the indium-doped Hg0.8Cd0.2Te epitaxial layers were high-quality n-type layers and that p-HgxCd1−xTe epilayers can be grown on indium-doped Hg0.8Cd0.2Te/Cd0.96Zn0.04Te heterostructures for the fabrication of HgxCd1−xTe photoconductors and photodiodes.