Growth process studies by reflectance anisotropy spectroscopy on MOVPE ZnSe

The growth of ZnSe on GaAs by metal organic vapour phase epitaxy (MOVPE) has been studied using reflectance anisotropy spectroscopy (RAS). The RA spectra of ZnSe are significantly different for growth on initially Se- or Zn-exposed GaAs surfaces. The Se-terminated GaAs (001) RA spectrum has Se-dimer-related features at 3.3 and 5.1 eV, and the large, high energy peak dominates during ZnSe growth on this surface. Transmission electron microscopy (TEM) analysis has been used to show that these large RA signals arise from anisotropic surface corrugation of the growing ZnSe epilayer. Under initially Zn-stabilised growth conditions, the ZnSe epilayer RA spectrum is largely featureless, showing only a weak peak at 4.7 eV and a dip at 5.1 eV. The corresponding surface anisotropy is greatly reduced in comparison with growth from the initially Se-terminated surface. These observations reveal RAS to the an important technique for ensuring the desired initial GaAs surface since the grown ZnSe surface morphology is critically dependent on the pre-growth substrate surface treatment. However, as the characteristic ZnSe RA spectra are relatively insensitive to changes in substrate temperature and VI–II ratio, RAS is of more limited use as an in-growth surface probe for MOVPE-grown ZnSe.     

Über die Züchtung von ZnS-, ZnSe- und ZnTe-Einkristallen unter Anwendung der chemischen Transportreaktion

In connection with investigations on the synthesis of ZnS, ZnSe and ZnTe single crystals by chemical transport using iodine as transport agent in sealed tubes the mechanism of mass-transport and the relationship between material-transport and growth process were studied. In this way the optimum conditions for growing single crystals were found. The dependence of the transport rate on the undercooling ΔT, on the diameter of the ampoule and on the pressure in the system is described. Relative large crystals in good quality could be propared by influencing the nucleation and the growth process by help of well defined transport rates. In all cases crystals with sphalerite structure only were obtained. The habit with the most common faces (110), (111) and (211) was found to be predominant. The dominant habit is responsive to variations in the experimental parameters. The crystal perfection have been determined by etch patterns and by X-ray topography.     

Studies on the vapour growth of ZnS,ZnSe and ZnTe single crystals

Large, highly perfect single crystals (up to 10 × 10 × 5 mm³) of ZnS, ZnSe and ZnTe have been grown from the vapour phase by dissociative sublimation and chemical transport with iodine. Good quality crystals were obtained when the growth rate was limited by diffusion of the vapours rather than by thermal convection or by reaction rates at the charge and or the growing surface. The presence of an inert gas and defined iodine concentrations increase the growth stability, especially for charges with slight deviations from stoichiometry. Electronmicrographs, X-ray topographs and etching experiments revealed low dislocation densities and relatively large inclusion-free regions.     

Growth and annealing effect of high-quality ZnSe:N/ZnSe by MOCVD

The ZnSe : N epitaxial layers were grown on (1 1 0) ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas, and using ammonia as a dopant source. In order to obtain highly doped ZnSe : N epitaxial layers, the optimum growth and doping conditions were determined by studying the photoluminescence (PL) spectra from the ZnSe epitaxial layers grown at different ammonia flux and VI/II flux ratio. Furthermore, in order to enhance the concentration of active nitrogen in ZnSe epitaxial layer, a rapid thermal anneal technique was used for post-heat-treating. The results show that the annealing temperature of over 1023 K is necessary. Beside, a novel treatment method to obtain a smooth substrate surface for growing high quality ZnSe epitaxial layers is also described.     

Studies in the growth of ZnSe on GaAs(001)

This paper reports a study of the molecular beam epitaxial (MBE) growth of ZnSe on GaAs substrates using elemental sources. Growth rates of ZnSe as a function of Se:Zn flux ratio for constant Zn flux were determined over a wider range of values than previously reported. Careful measurements of atomic fluxes and sample thickness lead to a determination of the sticking coefficients of Zn and Se which are at variance with many previously reported values. The temperature dependence of the sticking coefficients of Zn and Se has been measured carefully and provides evidence for a greater desorption of Zn from the growing surface than previously thought, an effect which persists at low growth temperatures. Measurements at high flux ratios support the use of a precursor model to describe MBE growth of ZnSe on GaAs substrates.     

Metalorganic chemical vapor epitaxy and doping of ZnMgSSe heterostructures for blue emitting devices

Blue-green semiconductor laser diodes operating at room temperature are still the domain of wide bandgap II–VI compound semiconductors. CW operation at room temperature and hours of lifetime were reported. However, the conductivity control, defect generation and the ohmic contacts still need improvement. Therefore we focused our work on the MOVPE growth and the optimization of ZnMgSSe/ZnSSe/ZnSe heterostructures as well as on nitrogen doping of ZnSe. To verify the layer quality characterization was carried out by X-ray diffraction, electron probe microanalysis, electrical measurements and photoluminescence. ZnMgSSe/ZnSSe/ZnSe and ZnSSe/ZnSe quantum wells and superlattices were grown to investigate structural as well as interface properties. Electron beam and optical pumping was used to clarify the laser mechanism and to clarify the suitability of a MOVPE process to grow laser quality material. The electrical compensation of ZnSe doped with nitrogen is still controversially discussed whereas high n-type doping with chlorine was reproducible achieved. ZnSe: N doped at different growth conditions (II/VI ratio, growth temperature, nitrogen supply) using N₂ excited in a plasma source or by the use of nitrogen containing precursors was investigated to study the compensation mechanisms.     

Growth of ZnSe single crystal by CVT method with self-moving convection shield

The self-moving convection shield was used in the growth of ZnSe single crystal by chemical vapor transport method using iodine as a transport agent. The reduction of the convection enables the growth of a 1-in diameter ZnSe single crystal. The incorporation efficiency of iodine on (1 1 1)B facet was proved to be larger than that on (1 0 0) facet. Impurity-hardening effect of incorporated iodine in the grown ZnSe crystal is also suggested.     

Atomic‐scale study of vapour growth morphology of crystalline urea

The role of surface relaxation on habit controlling energetics and growth morphology are investigated within the framework of Burton‐Cabrera‐Frank (BCF) and Hartman‐Perdok (HP) models. The habit controlling energetics has been calculated using first principles method. The growth morphology obtained using BCF theory shows that the structural relaxation has considerable effect on growth morphology. The relaxed growth morphology obtained using BCF model match with the experimental result from vapour phase. On the other hand the shape obtained using HP model does not correspond with the experimental shape. Observed polar growth morphology of urea crystal has been discussed particularly in the context of different atomic environments of (111) and (111) faces. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ reflectance difference spectroscopy of ZnSe-based semiconductor surfaces

We will give a brief report on applications of reflectance difference spectroscopy (RDS) for in situ growth monitoring during MOVPE. The experiments were made on GaAs(001) substrates using dimethylzinc-triethylamine, ditbutylselenide and as metalorganic precursors in a laminar flow reactor. We analyzed the influence of the in situ GaAs-substrate preparation on the optical response of the surface. The growth of ZnSe was investigated and compared to data obtained in an MBE process. Spectra at various stages of growth and time-dependent kinetic RDS records were measured during deposition and fitted with a three-layer growth model. Furthermore we utilized the surface sensitivity of the RDS technique to demonstrate surface kinetic processes during p-doping with tbutylamine.     

ZnSe薄膜的化学反应辅助热壁外延法生长及特性研究

本文通过将新型化学气相反应促进剂Zn(NH4)3Cl5引入到热壁外延系统中,以二元素单质Zn和Se为原料,直接在Si(111)衬底上生长了高质量的ZnSe晶体薄膜,薄膜成分接近理想化学计量比。研究了主要工艺参数对薄膜生长形貌和性能的影响。采用SEM、AFM、EDS和PL谱技术研究了生长的ZnSe薄膜的形貌、成分和发光特性。研究结果表明,热壁温度和生长时间是影响ZnSe薄膜形貌的主要因素;气相反应促进剂在薄膜生长和调节成分方面扮演了关键角色,Zn(NH4)3Cl5的存在使得Zn(g)和Se2(g)合成ZnSe晶体的反应转变为气固非一致反应,从而更容易获得近乎理想化学计量比的ZnSe薄膜。ZnSe薄膜在氦镉激光激发下,室温下PL谱由近带边发射和(VZn-ClSe)组合的SA发光组成,而在飞秒激光激发下,仅在481nm处显示出强烈的双光子发射峰。     

Structural Perfection of ZnSe Crystals Grown by Travelling Heater Method

ZnSe crystals have been grown by the travelling heater method (THM) with PbCl₂ as solvent. Our investigations have shown that ZnSe crystals could be obtained with high crystalline perfection referred to a full width at the half maximum (FWHM) down to 20 arcsec of double crystal rocking curves measured at chemically polished (100)-oriented surfaces. Solvent growth techniques like THM should have a remarkable potential for ZnSe bulk growth if the capture of solute as inclusion and precipitation can be avoided.     

Synthesis,spectral, thermal and NLO properties of N,N ‐Dimethyl anilinium picrate

The crystalline substance N,N ‐dimethyl anilinium picrate(DMAP) was synthesized and the single crystals of the title compound were grown by slow evaporation solution growth technique at room temperature. The crystalline nature of the material has been confirmed by powder X‐ray diffraction method. The elemental analysis data confirm the stoichiometry and hence the molecular formula of the synthesized material. The formation of the charge‐transfer complex has been confirmed by UV‐visible spectral data. The infrared and ¹H NMR spectroscopic data indicate a charge transfer activity associated with a proton transfer from the acceptor to the donor followed by intermolecular hydrogen bonding. Thermal behavior and stability of crystal were studied using thermo gravimetric (TG) and differential thermal analyses (DTA) techniques. The second harmonic generation (SHG) of the material was confirmed using Nd: YAG laser. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the thermal and optical properties of ZnSe and doped ZnSe crystals grown by PVT

Single crystals of ZnSe, phosphorus-doped ZnSe and phosphorus with Gallium-doped ZnSe have been grown by physical vapour transport technique. The thermal properties like thermal diffusivity, effusivity and conductivity are studied using photoacoustic spectroscopy and differential scanning calorimetry, for the use of substrate. Their optical properties are also studied using Raman, photoluminescence and photocurrent for the study of the site symmetry of the phosphorus. Our measurements reveal the possibility of phosphorus going to deep level thus reducing to C_3v level.     

Influence of the crucible bottom shape on the heat transport and fluid flow during the seeding process of oxide Czochralski crystal growth

For the seeding process of oxide Czochralski crystal growth, influence of the crucible bottom shape on the heat generation, temperature and flow field of the system and the seed‐melt interface shape have been studied numerically using the finite element method. The configuration usually used in a real Czochralski crystal growth process consists of a crucible, active afterheater, induction coil with two parts, insulation, melt, gas and seed crystal. At first, the volumetric distribution of heat inside the metal crucible and afterheater inducted by the RF‐coil was calculated. Using this heat generation in the crucible wall as a source the fluid flow and temperature field of the entire system as well as the seed‐melt interface shape were determined. We have considered two cases, flat and rounded crucible bottom shape. It was observed that using a crucible with a rounded bottom has several advantages such as: (i) The position of the heat generation maximum at the crucible side wall moves upwards, compared to the flat bottom shape. (ii) The location of the temperature maximum at the crucible side wall rises and as a result the temperature gradient along the melt surface increases. (iii) The streamlines of the melt flow are parallel to the crucible bottom and have a curved shape which is similar to the rounded bottom shape. These important features lead to increasing thermal convection in the system and influence the velocity field in the melt and gas domain which help preventing some serious growth problems such as spiral growth. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Initial growth behavior of GaAs on ZnSe in MOVPE

We have used atomic force microscopy to investigate the initial stages of the growth of GaAs on ZnSe by metalorganic vapor phase epitaxy. Underlying ZnSe with an atomically flat surface is achieved by growth at 450°C and post-growth annealing at the same temperature. The growth modes of GaAs on the ZnSe surface strongly depend on growth temperatures. The growth carried out at 450°C is 2-dimensional, while that at 550°C is highly 3-dimensional (3D), where the 3D islands are elongated in the [110] direction. The growth behavior, unlike homoepitaxy, is well interpreted in terms of low sticking coefficient and anisotropic lateral growth rate in the heterovalent heteroepitaxy.     

Specific features of the phase formation,synthesis, and growth of ZnMoO4 crystals

The ZnO-MoO₃ phase diagram in the range of ZnO compositions from 0.95 to 1.05 mol % is refined by differential scanning calorimetry. Data on crystal stoichiometry are obtained using the weighting method by reducing ZnMoO₄ in a hydrogen atmosphere. The specific features of solid-phase synthesis of ZnMoO₄ are studied, and its heat of fusion is measured. The modes of solid-phase synthesis and growth of ZnMoO₄ crystals are optimized. Some experimental data on the ZnMoO₄ crystal growth in the [001] direction by the low-thermal gradient Czochralski method are presented. Crystals with a cross section of ～50 × 50 mm², a length of 160 mm, and a weight up to 1 kg have been obtained.     

Preparation and investigation of (CuInSe2)x(2ZnSe)1‐x and (CuInTe2)x(2ZnTe)1‐x solid solution crystals

The (CuInSe₂)_x(2ZnSe)_1‐x and (CuInTe₂)_x(2ZnTe)_1‐x solid solution crystals prepared by Bridgman method and chemical vapor transport have been studied. The nature of the crystalline phases, the local structure homogeneity and composition of these materials have been investigated by X‐ray diffraction (XRD) and Electron Probe Microanalysis (EPMA) methods. The analysis revealed the presence of chalcopyrite‐sphalerite phase transition between 0.6 ≤ X ≤ 0.7. Lattice constants, value of σ position parameter and bond length between atoms were also calculated. It was found that the lattice parameters exhibit a linear dependence versus composition. The transmission spectra of solid solution crystals in the region of the main absorption edge were studied. It was established that the optical band gap of these materials changes non‐linearly with the X composition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal conductivity of ZnSe by molecular dynamics simulation

Molecular dynamics simulation using the new fit of the empirical Tersoff Potential is applied to study the thermal properties of ZnSe at and below the room temperature. The resulting diffusivity and thermal conductivity are compared with the photoacoustic measurements on the grown ZnSe crystal. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The growth of ZnSe by photo-assisted metalorganic chemical vapor deposition (MOCVD)

ZnSe epitaxy layers were grown on (1 0 0)GaAs substrates by photo-assisted MOCVD using DMZn and DMSe as group II and VI sources, respectively. Irradiation can improve the growth rate efficiently, but the irradiation intensity influences the growth rate and the crystalline quality negligibly in a large range. Due to an oxidation reaction on the surface of ZnSe, the growth rate and the flow ratio of group II and VI sources influence the crystalline quality.