Effects of in situ thermal annealing on the structural, optical, and electrical properties in Hg0.7Cd0.3Te epilayers grown on CdTe buffer layers

Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) transmission, and Hall effect measurements were performed to investigate the structural, optical, and electrical properties of as-grown and in situ-annealed Hg_0.7Cd_0.3Te epilayers grown on CdTe buffer layers by using molecular beam epitaxy. After the Hg_0.7Cd_0.3Te epilayers had been annealed in a Hg-cell flux atmosphere, the SEM images showed that the surface morphologies of the Hg_0.7Cd_0.3Te thin films were mirror-like with no indication of pinholes or defects, and the FTIR spectra showed that the transmission intensities had increased in comparison to that of the as-grown Hg_0.7Cd_0.3Te epilayer. Hall-effect measurements showed that n-Hg_0.7Cd_0.3Te epilayers were converted to p-Hg_0.7Cd_0.3Te epilayers. These results indicate that the surface, optical, and electrical properties of the Hg_1 − xCd_xTe epilayers are improved by annealing and that as-grown n-Hg_1 − xCd_xTe epilayers can be converted to p-Hg_1 − xCd_xTe epilayers by in situ annealing.     

Modification of Cd1−xMnxTe crystal surface layers by nanopulsed laser irradiation

The surface modification of Cd_1−xMn_xTe (x = 0-0.3) crystal wafers under pulsed laser irradiation has been studied. The samples were irradiated by a Q-switched ruby laser with pulse duration of 80 ns. Optical diagnostics of laser-induced thermal processes were carried out by means of time-resolved reflectivity measurements at wavelengths 0.53 and 1.06 μm. Laser irradiation energy density, E varied in the range of 0.1-0.6 J/cm². Morphology of irradiated surface was studied using scanning electron microscopy. The energy density whereby the sample surface starts to melt, depends on Mn content and is equal to 0.12-0.14 J/cm² for x ≤ 0.2, in the case of x = 0.3 this value is about 0.35 J/cm². The higher Mn content leads to higher melt duration. The morphology of laser irradiated surface changes from a weakly modified surface to a single crystal strained one, with an increase in E. Under irradiation with E in the range of 0.21-0.25 J/cm², the oriented filamentary crystallization is observed. The Te inclusions on the surface are revealed after the irradiation of samples with small content of Mn.     

NiP:Mn as a potential magnetic contacting material to Cd1−xMnxTe

Injection of spin-polarized current into spintronic devices is a challenge to the semiconductor physicists and technologists. II-VI compound semiconductors can act as the spin aligner on the top of GaAs light emitting diode. However, II-VI compound semiconductor like Cd_1−xMn_xTe is still suffering from contacting problem. Application of electroless deposited magnetic NiP:Mn contact would enhance efficient current injection into Cd_1−xMn_xTe than the standard gold contact. A technique for electroless deposition of NiP:Mn on Cd_1−xMn_xTe have been described here. The electronic and magnetic properties of the contact material NiP:Mn and the contact performance of NiP:Mn relative to evaporated gold have been evaluated. The contact fulfills the requirements of resistivity and ferromagnetism for application to Cd_1−xMn_xTe.     

Resonant photoemission study of Eu1−xGdxTe layers

Resonant photoemission study of electronic structure of molecular beam epitaxy grown Eu_1−xGd_xTe layers without and with cover protected layer of Te were performed using synchrotron radiation. The analysis of the valence band and shallow core levels spectra of the clean surface of Eu_1−xGd_xTe obtained in situ under UHV conditions showed the existence of Eu²⁺ and Eu³⁺ ions in the layers. The trivalent europium ions mostly are located at the surface and its amount strongly depends on sample surface preparation conditions. The prolonged annealing of Eu_1−xGd_xTe layers covered with protected layer of Te leads to formation of clean surface of the sample not changing the stoichiometry of it and without the accumulation of Eu³⁺ ions at the surface region.     

Crystal structures of two variants for CuPtB-type ordering in strained CdxZn1−xTe epilayers

Two variants of CuPt_B-type orderings in strained Cd_xZn_1−xTe epilayers were investigated by using transmission electron microscopy (TEM) and selected area diffraction pattern (SADP) measurements. The TEM images on the Cd_0.15Zn_0.85Te epilayers depicted strong contrast modulations along the [110] direction, and the SADP images showed superstructure reflection spots corresponding to a CuPt_B-type ordering. Possible crystal structures for the two variants of CuPt_B-type ordering in the Cd_xZn_1−xTe epilayers, which were determined from the SADP images, are presented.     

Structural,electronic and optical properties of CdxZn1 − xS alloys from first-principles calculations

Structural, electronic and optical properties as well as structural phase transitions of ternary alloy Cd_xZn_1 − xS have been investigated using the first-principles calculations based on the density functional theory. We found that the crystal structure of Cd_xZn_1 − xS alloys transforms from wurtzite to zinc blende as Cd content of x=0.83$x=0.83$. Effect of Cd content on electronic structures of Cd_xZn_1 − xS alloys has been studied. The bandgaps of Cd_xZn_1 − xS alloys with wurtzite and zinc blende structures decrease with the increase of Cd content. Furthermore, dielectric constant and absorption coefficient also have been discussed in detail.     

Structural, optical and electrical properties of Zn1−xCdxO thin films prepared by PLD

Ternary polycrystalline Zn_1−xCd_xO semiconductor films with cadmium content x ranging from 0 to 0.23 were obtained on quartz substrate by pulse laser deposited (PLD) technique. X-ray diffraction measurement revealed that all the films were single phase of wurtzite structure grown on c-axis orientation with its c-axis lattice constant increasing as the Cd content x increasing. Atomic force microscopy observation revealed that the grain size of Zn_1−xCd_xO films decreases continuously as the Cd content x increases. Both photoluminescence and optical measurements showed that the band gap decreases from 3.27 to 2.78 eV with increasing the Cd content x. The increase in Cd content x also leads to the broadening of the emission peak. The resistivity of Zn_1−xCd_xO films decreases evidently for higher values of Cd content x. The shift of PL emission to visible light as well as the decrease of resistivity makes the Zn_1−xCd_xO films potential candidate for optoelectronic device.     

Ab-initio calculations of Co-based diluted magnetic semiconductors Cd1−xCoxX (X=S, Se, Te)

Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic semiconductors composed of II-VI compounds Cd_1−xCo_xX (X=S, Se, Te) at x=0.25. From the calculated results of band structure and density of states, the half-metallic character and stability of ferromagnetic state for Cd_1−xCo_xS, Cd_1−xCo_xSe and Cd_1−xCo_xTe alloys are determined. It is found that the tetrahedral crystal field gives rise to triple degeneracy t_2g and double degeneracy e_g. Furthermore, we predict the values of spin-exchange splitting energies Δ_x(d) and Δ_x(p−d) and exchange constants N₀α and N₀β produced by the Co 3d states. Calculated total magnetic moments and the robustness of half-metallicity of Cd_1−xCo_xX (X=S, Se, Te) with respect to the variation in lattice parameters are also discussed. We also extend our calculations to x=0.50, 0.75 for S compounds in order to observe the change due to increase in Co.     

X-ray photoelectron study of Sn1−xMnxTe semimagnetic semiconductors

X-ray photoelectron (XPS) studies of core-levels in Sn_1−xMn_xTe (x < 0.1) semimagnetic semiconductors have been performed. The spectra were acquired under UHV conditions from the clean (as-cleaved or in-situ scraped) crystal surface. The single-phase NaCl structure of the alloys studied was verified by X-ray diffraction (XRD). The structure of Sn 3d and Te 3d core-levels in SnMnTe was found fully consistent with that of SnTe. Remarkable qualitative similarity of the Mn 2p spectrum of Sn_1−xMn_xTe (x = 0.09) with the case of zinc-blende MnTe [R.J. Iwanowski, M.H. Heinonen, E. Janik, Chem. Phys. Lett. 387 (2004) 110] has been shown: (1) the same binding energies (BEs) of the main contributions to the Mn 2p_3/2 line, related to Mn²⁺ state of the bulk MnTe bond; (2) occurrence of low BE component in the Mn 2p spectrum, indicative of clean-surface species containing reduced-valence Mn ions (i.e. Mn^q+, where 0 < q < 2); (3) strong satellites of the 2p_3/2 (Mn²⁺ related) parent lines. In SnMnTe, the highest intensity ratio of the satellite to main peak (ever reported for Mn 2p photoelectron spectrum) was revealed; this was interpreted in terms of the so-called charge-transfer model.     

Spin effects on oscillatory magnetoresistances in CdxHg1−xTe

A study is made of spin effects on the Shubnikov-de Haas oscillations in Cd_xHg_1?xTe alloys. The difference between the longitudinal and transverse magnetoresistances can be interpreted in terms of the spin-orbit couplings.     

High-temperature electrical properties of magnesiowustite Mg1 − xFexO and spinel Fe3 − x − yMgxCryO4 ceramics

Phase relationships, thermal expansion and electrical properties of Mg_1 − xFe_xO (x = 0.1-0.45) cubic solid solutions and Fe_{3 − x − y}Mg_xCr_yO_4 ± δ (x = 0.7-0.95; y = 0 or 0.5) spinels were studied at 300-1770 K in the oxygen partial pressure range from 10 Pa to 21 kPa. Increasing iron content enlarges the spinel phase stability domain at reduced oxygen pressures and elevated temperatures. The total conductivity of the spinel ceramics is predominantly n-type electronic and is essentially p(O₂)-independent within the stability domain. The computer simulations using molecular dynamics technique confirmed that overall level of ion diffusion remains low even at high temperatures close to the melting point. Temperature dependencies of the total conductivity in air exhibit a complex behavior associated with changing the dominant defect-chemistry mechanism from prevailing formation of the interstitial cations above 1370-1470 K to the generation of cation vacancies at lower temperatures, and with kinetically frozen cation redistribution in spinel lattice below 700-800 K. The average thermal expansion coefficients of the spinel ceramics calculated from dilatometric data in air vary in the range (9.6-10.0) × 10^− 6 K^− 1 at 300-500 K and (13.2-16.1) × 10^− 6 K^− 1 at 1050-1370 K. Mg_1 − xFe_xO solid solutions undergo partial decomposition on heating under oxidizing and mildly reducing conditions, resulting in the segregation of spinel phase and conductivity decrease.     

Spin effects on oscillatory magnetoresistances in n-Pb1−xSnxTe

Shubnikov-de Haas oscillations in n-Pb_1?xSn_xTe have been measured in the magnetic field parallel to the [100] crystal direction at 1.5 K. In the longitudinal magnetoresistances, the one-side peaks of spin-splitting pair series are completely missing. This anomaly is well explained by the selection rules, theoretically derived in the study on a similar effect in Hg_1?xCd_xTe. Landau sublevel-crossings are also discussed.     

Cyclotron resonance in Hg1−xCdxTe at 337 micrometre in the range 77 K – 150 K

Cyclotron resonance experiments have been performed in transmission at 337 μm on Hg_1?xCd_xTe single crystals with x = 0.20 and 0.215, at temperatures between 77 and 150 K.     

Evidence for the spin-dependent scattering of conduction electrons on Mn2+ ions in Hg1−xMnxTe and Cd1−xMnxSe mixed crystals

Changes in the resistivity of Hg_1?xMn_xTe and Cd_1?xMn_xSe mixed crystals associated with paramagnetic resonance of the Mn²⁺ ions have been observed at liquid helium temperature in a strong magnetic field. The effect was recorded by monitoring the submillimeter radiation induced photoconductivity in a swept magnetic field. An increase in the resistivity associated with EPR of the Mn²⁺ ions is interpreted in terms of the spin- dependent scattering of electrons on magnetic impurities, the spins of which are selectively depolarised by means of paramagnetic resonance. Some additional effects influencing the experiments are also discussed.     

Structural and surface properties of Si1−xGex thin films obtained by reduced pressure CVD

This paper investigates the structure and surface characteristics, and electrical properties of the polycrystalline silicon-germanium (poly-Si_1−xGe_x) alloy thin films, deposited by vertical reduced pressure CVD (RPCVD) in the temperature range between 500 and 750 °C and a total pressure of 5 or 10 Torr. The samples exhibited a very uniform good quality films formation, with smooth surface with rms roughness as low as 7 nm for all temperature range, Ge mole fraction up to 32% (at 600 °C), textures of 〈2 2 0〉 preferred orientation at lower temperatures and strong 〈1 1 1〉 at 750 °C, for both 5 and 10 Torr deposition pressures. The ³¹P⁺ and ¹¹B⁺ doped poly-Si_1−xGe_x films exhibited always lower electrical resistivity values in comparison to similar poly-Si films, regardless of the employed anneal temperature or implantat dose. The results indicated also that poly-Si_1−xGe_x films require much lower temperature and ion implant dose than poly-Si to achieve the same film resistivity. These characteristics indicate a high quality of obtained poly-Si_1−xGe_x films, suitable as a gate electrode material for submicron CMOS devices.     

Facile synthesis of thiol-stabilized CdSexTe1−x nanocrystals

CdSe_xTe_1−x nanocrystals (x=0.25, 0.40, 0.50, 0.60 and 0.75) were synthesized using thioglycerol as a stabilizing agent. The composition of the CdSe_xTe_1−x nanocrystals was precisely controlled by tuning the precursor (Se/Te) ratio. The structural, morphological and optical properties of the nanocrystals were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), diffused reflectance spectroscopy (DRS) and photoluminescence (PL) measurements. It is found that the Se/Te ratio significantly affects the properties of the resultant CdSe_xTe_1−x nanocrystals. XRD pattern of the CdSe_xTe_1−x nanocrystals revealed cubic, hexagonal and mixed phases depending on the ratio of Se:Te. Surface morphology of the CdSe_xTe_1−x nanocrystals showed nanoclusters of sizes ∼50 nm, with the adjacent cluster interlinking each other. DRS revealed the size dependence band gap energy prevailing in the CdSe_xTe_1−x nanocrystals from 1.52 to 2.66 eV due to the optical bowing effect. PL measurements exhibited band edge emission in the visible spectral region, and are red shifted with increase in Se concentration. The facile route employed in the present work to synthesis the CdSe_xTe_1−x nanocrystals in an aqueous medium is simple and controllable, and the strategy presented will be handy in preparing diverse semiconducting nanocrystals.     

Observation of short-order clustering effect in Cd1−xHgxTe

The infrared lattice-vibration reflection spectra of Cd_1-xHg_xTe (0?x?0.8) solid solution were measured. For the composition x from 0.1 to 0.6 on the top of the CdTe-like band one can observe considerable fine structure. To account for its nature the short-range clustering of the cations (Cd, Hg) around the anions (Te) characterized by parameter β was supposed. A good fit of the calculated and measured reflectively spectra was obtained provided the clustering parameter β = 0.6.     

Ab-initio investigations of structural, electronic, magnetic and optical properties of ferromagnetic Cd1-xMnxTe

We report ab-initio calculations of the structural, electronic, magnetic and optical properties of the alloy Cd_1-xMn_xTe as a function of the Mn concentration ‘x’. Ab-initio calculations are based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice constants of the Cd_1-xMn_xTe alloys exhibit Vegard's law downward bowing parameter. For the minority spin channel the Fermi level shifts toward higher energy with the value of ‘x’ in Cd_1-xMn_xTe. The spin-exchange splitting energy Δ_x(d) increases with increasing ‘x’ in Cd_1-xMn_xTe and the values of p-d exchange splitting energy Δ_x(pd) of Cd_1-xMn_xTe show that the effective potential for the minority spin is more attractive than that for the majority spin. The values of exchange constants N₀α and N₀β obtained for Cd_1-xMn_xTe are in agreement with the reported data. The magnetic moment per Mn atom reduces from its free space charge value of 5μ_B to around 4μ_B due to p-d hybridization and this results into an appearance of small local magnetic moments on the non-magnetic Cd and Te sites. The absorption threshold shifts toward higher energy and the static refractive index decreases with the increasing value of ‘x’ in Cd_1-xMn_xTe.     

Influence of quantum dot density on excitonic transport and recombination in CdZnTe/ZnTe QD structures

We report on dynamics of excitons in Cd_xZn_1−xTe/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping (r_HB=2700 ns⁻¹) is two orders of magnitude more efficient than in the low QD-density samples (r_HB=33 ns⁻¹). Radiative recombination rates are similar in both types of samples, r_R=1-1.3 ns⁻¹. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs.     

Growth and characterization of Cd1−xZnxTe thin films prepared from elemental multilayer deposition

Cd_1−xZn_xTe is a key material for fabrication of high-energy radiation detectors and optical devices. Conventionally it is fabricated using single crystal growth techniques. The method adopted here is the deposition of elemental multilayer followed by thermal annealing in vacuum. The multilayer structure was annealed at different temperatures using one to five repetitions of Cd-Zn-Te sequence. X-ray diffraction pattern for the multilayer with five repetitions revealed that annealing at 475 °C yielded single-phase material compared to other annealing conditions. EDX spectroscopy was carried out to study the corresponding compositions. Photoluminescence properties and change of resistance of the multilayer under illumination were also studied. The resistivity of the best sample was found to be a few hundreds of Ω cm.