Te Epilayers Research Articles

Improvement of the crystallinity of a Cd xZn 1− xTe epitaxial film grown on a GaAs substrate using a ZnTe buffer layer

The effects of a ZnTe buffer layer on the structural and the optical properties of the Cd x Zn 1− x Te films were investigated using Cd x Zn 1− x Te epilayers on GaAs (100) substrates with and without ZnTe buffer layers. X-ray diffraction measurements showed that the grown layers were Cd x Zn 1− x Te epitaxial films. Photoluminescence and transmission electron microscopy measurements showed that the crystallinity of the Cd x Zn 1− x Te epilayers grown on the GaAs substrates was remarkably improved using the ZnTe buffer. These results suggest that Cd x Zn 1− x Te epitaxial films grown on GaAs substrates with ZnTe buffers by a simple technique can be used for applications as buffer layers for high-quality Hg x Cd 1− x Te layers.

Influence of Magnetic Impurities upon Universal conductance Fluctuations

Mesoscopic phenomena in quantum structures which incorporate magnetic impurities with localized spins may exhibit a number of novel features driven by spin-disorder scattering, exchange spin-splitting of electron bands, and the formation of bound magnetic polarons. After brief information on these effects, their influence on universal conductance fluctuations as well as on low frequency noise and quantum localization is presented. Millikelvin investigations of diffusive charge transport, which have been carried out for submicron wires of n+-Cdi_ xMnx Te epilayers, are reviewed in some details. These studies have provided information on the significance of spin-disorder scattering in semiconductors and put into the evidence a new driving mechanism of the magnetoconductance fluctuations — the redistribution of the electrons between energy levels of the system, induced by the giant s-d exchange spin-splitting. Important implications of these findings for previous interpretations of spin effects in semiconductor and metal nanostructures are discussed. PACS numbers: 72.15.Rn, 73.20.Fz, 73.61.Ga, 75.50.Pp

The behavior of the surface charge density in Hg xCd 1− xTe epilayers due to hydrogenation and annealing

Electrolyte electroreflectance (EER), Fourier transform infrared (FTIR) transmission, and Hall effect measurements have been carried out to investigate the behavior of the surface charge density in hydrogenated and annealed Hg x Cd 1− x Te epilayers grown on nominally undoped p-CdTe (211) B-orientation substrates by molecular beam epitaxy. The transition energy and the broadening parameter were obtained from the fitting in the third-derivative functional form of the EER spectra. The results of the EER measurements showed the decrease in the surface charge density as a result of the passivation of the internal impurities after hydrogenation. The EER results after hydrogenation and after annealing showed an increase in the surface electric field resulting from a decrease in the number of acceptors. The behavior of the surface charge density in hydrogenated and annealed Hg x Cd 1− x Te epilayers obtained from the EER measurements is in good agreement with that measured from the Hall effect measurements.

Growth and characterisation of Hg 1 − xCd xTe (0.21 < x < 0.36) epilayers grown from Te-rich solution by the dipping technique

Hg 1 − x Cd x Te (0.21 < x < 0.36) epilayers were grown on 〈111〉 CdTe substrates by liquid phase epitaxy from a supersaturated Te-rich solution by the dipping technique at 440 and 470°C. Mercury vapour loss was accounted for by adding the estimated loss of Hg in the form of HgTe in the Te-rich solution. The physical parameters such as surface morphology, composition, thickness, full width at half maximum (FWHM) of double crystal X-ray diffraction (DCXRD) rocking curve and broadening parameters from electrolyte electroreflectance (EER) are given. The effect of growth rate on the crystalline quality is investigated. It is shown by X-ray diffraction and electrolyte electroreflectance measurements that the quality of an epilayer deteriorates when the epilayers are grown at a cooling rate in excess of 2°C h − 1.

Growth and characterization of HgCdTe heterostructures by metalorganic molecular beam epitaxy

HgCdTe continues to be the most important semiconductor material for infrared detector applications. To date issues relating to spatial uniformity and repeatability of alloy composition have hampered the development of this technology. In this paper, we describe the growth and properties of Hg0.72Cd0.28Te epilayers and Hg0.72Cd0.28TeCdTe superlattices grown by metalorganic molecular beam epitaxy. The HgCdTe epilayers exhibited a high degree of lateral and vertical alloy composition homogeneity and also very high run-to-run uniformity. HgCdTeCdTe superlattices were also grown with a variable barrier and constant well thickness. The measured periodicity from X-ray diffraction rocking curves closely agreed with the desired values. Infrared Photoluminescence studies confirmed the high crystallinity and suggested that the luminescence was strongly excitonic with a bound exciton linewidth of 8 meV at 6 K.

New Method of the Determination of HgCdTe/CdZnTe Composition by Infrared Transmission

A new method for compositional determination in Hg1-x Cdx Te/CdZnTe has been studied. In case of Hg1-x Cdx Te (MCT) epilayer grown onto Cd(Zn)Te substrates by liquid phase epitaxy (LPE), compositional graded region exists between the substrate and epilayer. This compositional graded region affects the infrared (IR) transmission curve. In this study, the IR transmission curves as a function of compositional profile and epilayer thickness were simulated based on the theoretical IR transmission model. From these results, a new and simple equation for calculating the composition of an epilayer MCT was obtained. This equation is related to the wavenumber (Z1/2) of the 50% T max transmission point, where T max is the maximum transmission, the wavenumber (Z500) of the 500 cm-1 absorption coefficient point, and the wavenumber (Z1000) of the 1000 cm-1 absorption coefficient point. This equation is as follows: x=a+b×Z+(c+d×Z)×ln (10/T), where Z is the specific wavenumber in cm-1, T is the epilayer thickness in µ m, and a, b, c, and d are fitting parameters depending on the specific wavenumber. The compositions can be calculated easily using the new equation with an accuracy of ±0.002 in the range of 0.17≤x≤0.30. The new method can be also applied successfully to the samples with nonuniformity into depth and radial directions.

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.

Mercury cadmium tellurium photodetectors fabricated by liquid phase epitaxy and double boron implantation for fiber optic communication

The p-type Hg 0.3Cd 0.7Te epilayers were grown on CdTe substrates by open tube liquid phase epitaxy technique. The formation of n + - p junction was made by double boron ion-implantation and post-thermal annealing. The quantum efficiency was 0.43 and the detectivity was 2.1 × 10 11 cm Hz 1 2 W -1 for the Hg 0.3Cd 0.7Te photodetector.

Measurement of composition in Hg1−xCdxTe epilayers

A new model has been presented for evaluating the composition distribution in both the transverse (perpendicular to the direction along the depth) and longitudinal (along the depth) directions in HgCdTe epitaxy materials by fitting room temperature transmission spectroscopy data.

Localized exciton magnetic polarons in Cd1-xMnxTe.

Using the method of selective excitation of the exciton luminescence in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te epilayers we have measured energies of localized magnetic polarons (LMP's) for alloys with manganese mole fractions x\ensuremath{\le}0.34. The suppression of the LMP energy has been studied in external magnetic fields and with temperature increase. Polaron formation times and exciton lifetimes have been measured by time-resolved photoluminescence. We have found that in alloys with x0.17 the polaron formation process is interrupted by exciton recombination and, as a result, the LMP does not reach its equilibrium energy. This dynamical effect on the polaron energy together with the strong sensitivity of the LMP formation to the conditions of primary exciton localization causes the absence of the LMP formation in layers with x0.1. Antiferromagnetic clustering of Mn ions, which leads to the spin-glass phase formation at low temperatures, affects the polaron energy and results in the increasing stability of LMP's against suppression by temperature increase and magnetic fields. In ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te with xg0.20 a considerable part of the polaron energy is controlled by the input of clusters of antiferromagnetically coupled Mn spins located in the nonuniform molecular field of localized excitons. The comparison of the exciton Zeeman splitting and the LMP magnetic-field suppression provides insight into the internal structure of LMP's.

Hg 0.8Cd 0.2Te grown by liquid phase epitaxy using Cd 0.94Zn 0.06Te buffer layer

Buffered structures of Hg 0.8Cd 0.2Te/Cd 0.94Zn 0.06Te/Cd 0.97Zn 0.03Te were grown by a slider LPE technique and their material properties were compared with those of Hg 0.8Cd 0.2Te/Cd 0.97Zn 0.03Te structures which have no buffer layer. Photoluminescence spectroscopy (PL) was used to determine the compositions of the buffer layers and the substrates and analyse the impurities in the buffer layers. Auger electron spectroscopy (AES) was used to analyse impurities. Either sodium or lithium related donor-acceptor pair (DAP) peaks were observed in the substrate, but not in the Cd 0.94Zn 0.06Te buffer layer. AES analysis revealed that the substrate contained some impurities such as Si, Al, O, K, Fe, and Mg. In the Cd 0.94Zn 0.06Te buffer layer, the above impurities were not detected. The distribution of dislocations in the epi-layers were analysed by defect etching as well as double crystal X-ray diffraction. For the buffered structure, most misfit etch-pits were located just above the interface between the Hg 0.8Cd 0.2Te and Cd 0.94Zn 0.06Te buffer, and their density was much lower than that in the unbuffered structure, in which etch-pits were distributed rather broadly in the epi-layer. Electrical properties of the Hg 0.8Cd 0.2Te epi-layers, which were Hg-annealed at 250°C, were analysed by Hall measurement at 77 K. The buffered structure showed the carrier concentrations of 7 × 10 13 to 1 × 10 14 cm -3 and Hall mobilities of 2 to 5 × 10 5 cm 2/V · s.

Raman spectroscopy of the paramagnetic spin flip in Cd1-xMnxTe, the role of band-gap excitons as intermediate states, and optically detected electron-nuclear double resonance.

We have studied the paramagnetic resonance signal (PRS, \ensuremath{\Delta}m=+1) of the 3${\mathit{d}}^{5}$ spin of ${\mathrm{Mn}}^{2+}$ in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te epilayers and, less extensively, in a quantum-well structure by means of Raman spectroscopy in an external magnetic field of 6 T at low temperatures. The exciting laser or the scattered light were in resonance with electronic transitions (free exciton transitions) between the valence and the conduction band. The resonances have been observed by continuously tuning the wavelengths of the exciting dye laser in Voigt and Faraday backscattering geometries. For the off-resonance case the PRS is only expected in Voigt geometry with crossed polarizations of incident and scattered light (single resonance case, either ingoing or outcoming). Contrary to that, in resonance, the transition appears with large intensities as double resonances in the ``forbidden'' parallel polarizations and also in Faraday configuration. These results can be interpreted by considering a spin flip of a nucleus with I\ensuremath{\ge}1/2 ${(}^{111}$Cd${,}^{113}$Cd${,}^{125}$Te${,}^{55}$Mn) simultaneously with the reorientation of the electronic spin of the 3d shell.

Substrate photoluminescence: An aid in the evaluation of pulsed laser evaporation and epitaxial growth of Cd1−xMnxTe epilayers on (111)GaAs

Photoluminescence (PL) emitted from (111)Cd1−xMnxTe/(111)GaAs:Si heterojunctions produced using pulsed laser evaporation and epitaxy (PLEE) has been studied at 11.5 K. The heterojunctions show PL from the substrate E0 and E0+Δ0 gap regions as well as from the epilayer. The substrate E0+Δ0 signal is particularly sensitive to the epilayer growth temperature and is strongest for samples grown on substrates held at Tg=290 °C. The heterojunction PL also includes a component produced by Si dopant atoms in the substrate that undergo clustering changes as the growth temperature is raised. Through comparison with annealing studies of the substrate, it is argued that the PLEE ablated material possesses sufficient kinetic energy to increase the effective temperature above Tg at the growth surface.

Controlled p-type Sb doping in LPE-grown Hg1−x Cdx Te epilayers

A Te-rich liquid-phase-epitaxial growth process is reported whereby reproducible Sb-doped layers are prepared with hole concentrations and hole mobilities ranging from 1.8×1016 to 1.3×1019 cm−3 and 280 to 29 cm2/V s, respectively, at 77K for x-values ranging from 0.23 to 0.29. An effective electronic distribution coefficient for Sb of 0.01 is calculated from the hole concentration at 77K and the concentration of Sb in the growth solution. The process for group Va doping of low-x Hg1−x Cdx Te from Te-rich solutions and the procedure for the growth of a CdZnTe buffer layer on a CdTeSe substrate are described. For Te-rich Cd−Zn−Te growth solutions the distribution coefficient of Zn was found to be 18. The growth of a structure consisting of an Sb-doped HgCdTe epilayer on a CdZnTe buffer layer lattice matched (Δa/a<10−4) to a CdSeTe substrate has been demonstrated.

Excitons as intermediate states in spin-flip Raman scattering of electrons bound to donors in Cd1-xMnxTe epilayers.

The resonance behavior of the spin-flip Raman scattering in semimagnetic Cd 1-x Mn x Te epilayers has been used to investigate the scattering process itself as well as to identify the state of the carrier that changes its spin. The characteristic variation of the Stokes shift with magnetic field, a magnetic polaron effect for vanishing magnetic field, and the independence of the scattering efficiency of laser power indicate that a donor-bound electron changes its spin orientation. According to magnetic dipole selection rules of electrons for the spin-flip scattering, the Raman signal shows resonances with excitonic transitions involving the m j =±1/2 hole states as they can be characterised by luminescence and photoluminescence excitation spectroscopy

Growth of MgTe and Cd 1− xMg xTe thin films by molecular beam epitaxy

We report on the growth of the compound semiconductor MgTe as well as the ternary alloy Cd 1− x Mg x Te by molecular beam epitaxy. This is to our knowledge the first time that this material has been grown by any epitaxial technique. Bulk MgTe, which is hygroscopic, has a band gap of 3.0 eV and crystallizes usually in the wurtzite structure. Pseudomorphic films were grown on zincblende CdTe substrates for a MgTe thickness below a critical layer thickness of approximately 500 nm. In addition, Cd 1- x Mg x Te epilayers were grown with a Mg concentration between 0 and 68%, which corresponds to a band gap between 1.5 and 2.5 eV at room temperature. The crystalline quality of the layers is comparable to CdTe thin films as long as they are fully strained. The lattice constant of zincblende MgTe is slightly smaller than that of CdTe, and the lattice mismatch is as low as 0.7%. In addition highly n-type CdMgTe layers were fabricated by bromine doping. The tunability of the band gap as well as the rather good lattice match with CdTe makes the material interesting for optoelectronic device applications for the entire visible range.

Thermomigration of Te precipitates and improvement of (Cd,Zn)Te substrate characteristics for the fabrication of LWIR (Hg, Cd)Te photodiodes

(Cd,Zn)Te wafers containing Te precipitates have been annealed under well defined thermodynamic conditions at temperatures below and above the melting of Te. Results of the examination of the wafers with infrared microscopy before and after the anneals indicate a substantial reduction of the Te precipitates in wafers annealed at temperatures in excess of the melting point of Te compared with those annealed at temperatures below the melting point of Te. These results confirm the thermomigration of liquid Te precipitates to be the principally operative mechanism during annealing in the elimination of these precipitates in (Cd,Zn)Te wafers. The occurrence of Te precipitates in (Hg,Cd)Te epitaxial layers grown on (Cd,Zn)Te substrates containing Te precipitates is also explained on the basis of thermomigration of these precipitates during LPE growth from the substrates to the epilayers. Absence of occurrence of Te precipitates in (Hg,Cd)Te epilayers grown on annealed (Cd,Zn)Te substrates with negligible Te precipitates is also confirmed. Usefulness of annealing (Cd,Zn)Te substrates—to eliminate Te precipitates—prior to epilayer growth is confirmed via demonstration of improved long wavelength infrared (Hg,Cd)Te device array performance uniformity in epitaxial layers grown on (Cd,Zn)Te substrates with negligible Te precipitates after annealing.

The growth of various buffer layer structures and their influence on the quality of (CdHg)Te epilayers

The suitability of various buffer layer structures on (100) GaAs for (CdHg)Te growth by organometallic vapour phase epitaxy (OMVPE) was investigated. The preferred epitaxial orientation of (100)GaAs (111)CdTe was found to be unsuitable due to the formation of electrically active defects in the material. An intermediate ZnTe layer was used to select the (100) orientation and (100) CdTe layers were when deposited on this ZnTe layer. The quality of the resultant CdTe buffer was found to critically depend on the thickness of this intermediate ZnTe buffer, with a ZnTe thickness of approximately 500 Å producing the best CdTe buffer. (CdHg)Te epilayers grown on these ZnTe CdTe buffers had improved electrical properties, but still suffered from a poor surface morphology. This surface morphology could be improved by using a lattice matched Cd 0.96Zn 0.04Te alloy as the final buffer layer, but the surface pyramids typical of the (100) orientation could never be completely eliminated.

A simple reactor for growth of HgCdTe films using LPE technique

A reactor for growth of Hg1−x Cd x Te epilayers by liquid phase epitaxy has been designed and developed. The layers have been successfully grown using this liquid phase epitaxy (LPE) reactor.

The growth of diffusion doped ZnSe: Te epilayers by atmospheric pressure metalorganic chemical vapour deposition

We have used atmospheric pressure metalorganic chemical vapour deposition to deposit tellurium (Te) doped ZnSe: Te epilayers on (100) GaAs substrates. A diffusion doping method was used to achieve controllable doping levels in the 10 18 to 10 19 cm -3 range to overcome the difficulties encountered when using conventional bubbler cell growth methods. The relative increase in Te related emission in the photoluminescence spectrum with increasing concentration was consistent with the opinion that only Te ion doublets or large Te ion clusters lead to exciton trapping in ZnSe. In addition, a band edge emission line (I T 1) showing a peak shift to lower energies with increasing growth temperature was observed. This may be attributed to a complex involving Te ions and ZnSe I D 1 centres.