Optical absorption property of glasses coloured with Cd (S, Se, Te)

In the glasses coloured with Cd (S, Se, Te), the dependence of the absorption coefficient () on photon energy (E) and temperature (T) has been determined. Some results that differ from the Urbach rule were obtained.     

Self-selecting vapour growth of bulk crystals – Principles and applicability

A method of self-selecting vapour growth (SSVG) for bulk binary and multernary crystals of semiconducting materials is reviewed comprehensively for the first time. Although it has been developed over three decades, the method is less well known – even though it is physically distinct from the more widely used ‘Piper–Polich’ and ‘Markov–Davydov’ vapour transport bulk growth methods. The means by which growth takes place on a polycrystalline source to form a crystal free from the walls is described. Modelling and empirical observations have been used to establish the characteristics of the almost isothermal temperature fields that drive the transport in SSVG. It is demonstrated that precise control of thermal radiation is a fundamental requirement for tailoring the temperature distribution—a fact that has been used well in the design of horizontal tube furnace growth rigs. Achievements in the growth of useful PbS, PbSe, PbTe, CdTe and ZnTe compound crystals are described. The SSVG method has proved to be particularly well suited to the growth of solid solutions, and the results of growth experiments, and of compositional and structural analysis, are presented for Pb(Se,S), (Pb,Sn)Se, (Pb,Sn)Te, (Pb,Ge)Te, Cd(Te,Se), Cd(Te,S) and (Cd,Zn)Te. The excellent compositional uniformity delivered is attributed to entropy driven mixing in the low thermal gradients present in SSVG.

To date, most SSVG has been done at the <50 g level for research or small scale production use. Prospects for scaling up the growth are considered, there being no barriers identified in principle. However, there is a limitation in that the shape of the grown crystals is not accurately controlled at present. To overcome this, and to offer an alternative method of scaling up, the use of vertical tube systems is explored. A significant additional advantage of the vertical configuration is that it allows for continuous recycling of the source/crystal mass so as to continuously self-refine the increasingly uniform – and crystalline – product. Achievements to date in growing II–VI and IV–VI crystals are described for prototype vertical SSVG systems. Finally, future prospects for the SSVG method in terms of further developments to the method, and the specific materials that will benefit from it are highlighted.     

IR-device Grade (Hg,Cd)Te Crystals Grown by a Modified Bridgman Method

Electrical and photoelectrical properties of IR-devices manufactured on (Hg, Cd)Te-wafers cut from single crystals grown by modified Bridgman method are reported and compared to those of devices made on THM-(Hg, Cd)Te. MIS-structures and photodiodes were used in order to investigate the different materials. The influence of material parameters and device technology respectively is involved in our discussion of device properties. The quality of modified Bridgman-(Hg, Cd)Te was found to be comparable to that of the THM-(Hg, Cd)Te. At T = 80 K and FOV = 60° background limited detectivity of photodiodes with a cut-off wavelength of λ_co = 10.7 μm was achieved.     

Comprehensive characterization of CdTe and (Cd,Zn)Te single crystals by a chemical etchant

Using a known chemical etchant low- and high angle boundaries and lamellar twins can be seen on CdTe and (Cd, Zn)Te crystal ingots as a whole as well as on slices with naked eyes. Also the polarity of {111} samples can be determined in this way. Etch pits are produced on cut and polished surfaces independent of their crystallographic orientation. A new modified etchant was used to study the low angle subgrain structure on (111)Te surfaces.     

Liquid-Phase Epitaxy of (Hg,Cd)Te on CdTe Substrates by a Vertical Dipping Technique

Hg_1−xCd_xTe layers on CdTe substrates were grown from Te-rich melt solutions by a vertical dipping technique using a special quasi-closed system with ground-glass sealing. Results are good reproducibilities of the electrical properties after annealing in Hg-rich atmosphere (p77 ≈ 2 · 10¹⁶ cm⁻³ μ77 ≈ 500 cm² V⁻¹ s⁻¹) and of the x-value, respectively. A horizontal position of the substrate downwards to the melt solution yields, in difference to a vertical one, to homogeneous layer thicknesses. Short meltback steps before growth leads to sharper profiles of composition.     

Hydrogen and nitrogen ambient effects on epitaxial growth of GaN by hydride vapour phase epitaxy

This study presents the influence of the composition of the carrier gas on the growth of GaN by HVPE. Since no hydrogen is introduced in the vapour phase, the deposition is expected to be controlled by Cl desorption in the form of GaCl₃, as has been proposed for GaAs. However, our published model predicts much lower growth rates than those observed. We can account for both the observed parasitic deposition and GaN growth rate if we assume that GaCl₃ is not at its equilibrium pressure in the deposition zone and where nucleation takes place on the walls as well as on the substrate. This yields a high rate of parasitic nucleation even though the nominal supersaturation is vanishing small. Very little growth takes place on the substrate where the equilibrium pressure of GaCl₃ is reached. We describe similar experiments performed with a H₂/N₂ mixture as the carrier gas. In this case, we expect GaN deposition to be controlled by desorption of Cl as HCl, which is known as the H₂ mechanism. It is speculated that the results show the existence of a new growth mechanism.     

Te溶剂-Bridgman法Cd0.9Mn0.1Te晶体生长习性研究

采用Te溶剂-Bridgman法生长了尺寸为φ30 mm× 60 mm的Cd0.9Mn0.1Te:In晶锭,通过淬火得到了生长界面形貌.测试了晶片在近红外波段的透过率和电阻;采用化学腐蚀的方法观察了晶片中位错,Te夹杂和孪晶界;采用光学显微镜和红外成像显微镜观察了生长界面处附近的形貌.测试结果表明,晶锭中部结晶质量较好的晶片红外透过率达到60％,电阻率达到2.828×1011Ω · cm.位错密度在106 cm-2数量级,Te夹杂密度为1.9×104 cm-2,同时孪晶密度明显低于Bridgman法生长的晶锭.生长界面宏观形貌平整,呈现微凹界面.但由于淬火过程的快速生长,界面微观形貌发生变化,呈现不规则界面,并在界面附近形成富Te相的包裹.     

Crystal growth of PbTe and (Pb,Sn)Te by the bridgman method and by THM

Synthesis and growth of PbTe and (Pb, Sn)Te single crystals by the Bridgman method and by the Travelling Heater Method (THM) from Te-rich solutions are described. It is to be seen from comparative investigations that seeded THM growth reproducibly provides oriented single-crystalline ingots free of low-angle grain boundaries and with etch pit densities of 8–12 × 10⁴ cm⁻². All the materials were p-type with carrier concentrations from 1 to 2 × 10¹⁸ cm⁻³.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.     

Growth Mechanism in Atomic Layer Epitaxy (I) Re-evaporation of Cd and Te from CdTe(111) Surfaces Monitored by Auger Electron Spectroscopy

The mechanism of atomic layer epitaxy (ALE) of cadmium telluride has been studied. Auger electron spectroscopy is used to measure the isothermal re-evaporation rates of elemental Cd and Te deposits on the (111)A and (111)B surfaces of CdTe substrates. The results include an observation that the sticking coefficients of Cd and Te are smaller than unity at the growth temperatures typical of CdTe ALE. After desorption the substrates are left partially covered: 35% by a Cd overlayer on the (111)B surface and 72% by Te on the (111)A surface. The re-evaporation rates of Cd and Te experience a drastic change near the substrate-deposit interface. These rates appear two orders of magnitude smaller than those of bulk-like amorphous Cd and Te solids. The activation energies for reevaporation of the near-interface layer region are estimated to be: 1.5 eV for Te on the (111)A face, 1.0 eV for Te on (111)B and 0.5 eV for Cd on (111)B. It has also been shown that AES can be used to identify the polarity of the CdTe(111) surfaces. The relative difference in peak-to-peak intensity ratios of Cd MNN to Te MNN for (111)A and (111)B is (11 ± 2)%.     

Growth mechanism in atomic layer epitaxy (III) reevaporation of Cd and Te from CdTe (111) surfaces and thick elemental deposits monitored by quadrupole-mass spectrometry

Quadrupole-mass spectroscopical (QMS) studies on isothermal reevaporation of Cd and Te species from the CdTe (111) surface have been performed in two extreme cases. The first concerns the reevaporation of thick, bulk-like non-crystalline Cd and Te films deposited in high vacuum at room temperature on the (111) surface, whereas in the second case evaporation of the constituent species from the bare single crystalline (111) surface has been investigated. The fluxes of the species desorbing in high vacuum (10⁻⁶ Pa) have been monitored with QMS and the desorption temperatures have been measured with a thermocouple mounted as near as possible to the sample surface. The following values E_a(Cd)_bulk = 1.13 ± 0.12 eV, E_a(Te)_bulk = 1.64 ± 0.18 eV and E_a(Cd)₍₁₁₁₎ = 1.13 ± 0.06 eV, E_a(Te)₍₁₁₁₎ = 1.92 ± 0.13 eV of the activation energies for these two cases have been determined from the slopes of the Arrhenius plots. Using these experimental values, the numbers of atomic bonds N_Cd and N_Te occurring in the atomic aggregates of quasi-gas molecules forming the near surface quasi-gas transition layers have been estimated. For Cd quasi-gas molecules 2 ≤ N_Cd ≤ 5, whereas for Te molecules 3 ≤ N_Te ≤ 10. However, no prediction concerning the number of atoms creating the quasi-gas molecules could be made on the basis of the QMS investigations. It has also been shown that Cd atoms evaporate from the bare single crystalline CdTe (111) surface with an activation energy that is equal (in the limits of the experimental error) to the activation energy for sublimation of Cd atoms from pure, non-crystalline, bulk Cd pieces. The analogous activation energy measured for Te atoms is about 20% larger than that of the relevant sublimation process. This result confirms the fact that Te atoms are bound much stronger in the CdTe crystal lattice than Cd atoms.     

Improvement of the electrical properties of Se3Te1 thin films by In additions

The electronic and photoconductivity properties of semiconducting chalcogenide glasses have been largely stimulated by attractive micro-electronic device applications. The present paper aims to study the effect of In additions on the steady state and transient photoconductivity of amorphous In_x(Se₃Te₁)_100 ? x (0 ≤ x ≤ 10 at.%) chalcogenide films. It was found that, the Indium additions lead to the decrease of both the activation energies (ΔE_dc in the dark and ΔE_ph for the photoelectrical conduction) and the optical band gap E_g that improved the electrical properties of these films. The photoconductivity increases while photosensitivity changes from 8.73 to 7.18 with the increase of In content. The exponential dependence of photocurrent on the light intensity suggests that, the recombination mechanism in these films is due to bimolecular recombination. The transient photoconductivity measurements stated that, the carrier lifetime decreased by the increase of the light intensity and In content. The obtained results were discussed in terms of the width of localized states (Mott and Davis model) and the chemical-bond approach.     

Theoretical Study of Structural,Electronic, Optical,and Elastic Properties of KLiX (X: S,Se, and Te) under Hydrostatic Pressure: A Pseudo Potential Plane Wave (PP-PW) Contribution

Crystallography Reports - We have studied structural, electronic, optical, and elastic properties of KLiX under pressure using the Density Functional Theory (DFT) within the Generalized Gradient...     

Cd4BiO（BO3）3的生长与透过光谱（英文）

采用顶部籽晶法生长出了40×10×3 mm3和32×10×2 mm3的非线性光学晶体Cd4BiO(BO3)3。用XRD粉末衍射和热重-差示扫描量热仪确定了该晶体为同成分熔融晶体,熔点为897℃,在990℃以上开始分解。测量了晶体300～6500 nm的室温透过光谱,结果表明Cd4BiO(BO3)3晶体在750～2550 nm的透过率约为80%,紫外截止波长为395 nm。     

Study of the ZnO crystal growth by vapour transport methods

The crystal growth of ZnO by vapour transport is classically made with the assistance of additional species that produce a gaseous mixture, the role of which remains often uncertain in the transport and growth process. Initially, in order to study the mass transport process, a numerical simulation is made to analyse which are the requirements to have an effective transport. As the pressure of each gaseous species is generally unknown, the numerical study has been performed for different total pressures. It is found that, if congruent and equilibrium conditions are assumed at the sublimation and crystallisation interfaces, effective growth conditions can only be attained for a narrow range of total pressures. Nevertheless, it is well known that ZnO growth by vapour transport is possible for a wide range of pressures of gaseous species. As a consequence, partial pressures higher than the equilibrium ones must be present in order to justify the experimental results. We suggest that the thermal decomposition of ZnO is given by an activated process. The analysis of different mechanisms that could justify the activated decomposition, in accord with a systematic set of growth experiments, suggests that some additional species in the growth of ZnO by vapour transport promote the generation of an additional Zn pressure. This zinc pressure would act autocatalytically inducing O₂ and Zn partial pressures higher than the equilibrium ones and promoting thermal decomposition. The above-cited set of experimental growth experiences, that include the presence of C, Zn, Fe, Cu and H₂, will be analysed and interpreted according to this approach.     

The growth of single crystals of langbeinites Rb2Cd2(SO4)3, Tl2Cd2(SO4)3, K2Co2(SO4)3, and their phase transitions

Single crystals of dicadmium dirubidium sulphate and dicadmium dithallium sulphate have been grown from non-stoichiometric aqueous solutions by various methods. Optical and thermal properties in the vicinity of their phase transitions have been studied. Polycrystalline samples of dicobalt dipotassium sulphate have been prepared using Bridgman method. Optical and specific heat measurements have shown a new phase transition at −148°C.