Vapour-phase epitaxial growth of ZnS on GaP

Single crystal layers of ZnS about 100 μm thick were grown epitaxially on GaP substrates in an open tube system using source ZnS powder and a flowing hydrogen atmosphere. The growth rate for different substrate temperatures increases with increasing hydrogen flow rate, but the growth rate profiles resemble each other in shape. The profile shifts towards the low temperature side as the source temperature is decreased. The (111)B substrate orientation is found to be preferable to the (111)A or the (100) with respect to surface morphology and crystal quality. X-ray diffraction investigations and luminescent properties show that the (111)B grown layers are of high quality. All ZnS layers grown on GaP substrates are craked on cooling, which may be due to the thermal expansion mismatch between the layer and the substrate. Heat-treatment of the grown layer does not reduce the resistivity, but increases the photoluminescence intensity markedly. Selective vapour-phase epitaxial growth has been successfully applied resulting in crack-free ZnS layers on GaP substrates.     

Transmission electron microscopy observation of GaAs/Al0.3Ga0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al_0.3Ga_0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only an the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (T_s) as high as 630°C, under a V/III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.     

Model for defect-free epitaxial lateral overgrowth of Si over SiO2 by liquid phase epitaxy

A growth model for defect-free epitaxial lateral overgrowth by liquid phase epitaxy is presented. Growth in the pure step flow mode and a preferential development of (111) planes in liquid phase epitaxy permit one to quantitatively predict the overgrowth. The shape and size of the Si lamellae which grow over SiO₂ depend on the crystallographic orientations of the substrate growth face and of the seeding windows. Overgrowth experiments with Si on oxidized, (111)- and (100)-oriented Si wafers serve to verify the model. Growth experiments from In and Bi solutions in temperature intervals between 950 and 800°C yield overgrowth widths up to 120 μm and aspect ratios of 40:1 on (111) oriented wafers.     

The epitaxial growth of thick smooth films of ZnS on GaAs

The growth of smooth thick (80 μm) films of highly-ordered ZnS on GaAs substrates [oriented near the (100)] by open tube vapour transport in H₂:HCl gas flow has been investigated. We have found the surface features of the layers to be similar to those associated with GaAs epitaxy, particularly the “sand-dune” type of hillock. To obtain smooth layers, the substrates must be off the (100) plane, and precleaned in HCl acid. The most critical factor during growth was the HCl transport gas concentration (1.2-1.4%). We have investigated the influence of growth parameters on surface quality and Si contamination. The Si concentration in the films was found to be approximately directly proportional to the growth rate of the layers.     

Vapor growth of GaxAl1−xSb

The vapor growth of Ga_xAl_1?xSb was experimentally studied using a closed tube transport technique. In the closed tube process with iodine as a transport agent, Ga_xAl_1?xSb was successfully grown on (111) oriented GaSb substrates at a relatively low temperature. The composition of the grown layer depended primarily on that of the source, and had only a small graded region (<0.5 μm) between the substrate and the epitaxial layer. The growth rate was almost constant irrespective of the growth time, whenever the other growth parameters were unchanged.     

Crystallographic defects in epitaxial layers of cadmium sulphide

Thin epitaxial films of CdS deposited in high vacuum on ionic single crystal substrates have been studied by transmission electron microscopy. The (100), (110) and (111) faces of NaCl and the (111) face of BaF₂ were used as substrate surfaces. Both cubic sphalerite and hexagonal wurtzite structure films have been produced. The orientations of the sphalerite structure films were (100) and (110) and were produced on substrate faces having the same two orientations. The wurtzite structure films were in (0001) orientation and grew on (111) oriented substrate faces. For a fixed rate of deposition both the number and type of defects found in the films appear to be dependent upon the growth temperature and the crystal structure. Annealing the films at a high temperature has been tested as a means for reducing their defect content and the effect is very different for the two crystal structures. A reduction in the defect content of the wurtzite structure films is induced but no change in the crystal structure occurs. In contrast to this, the sphalerite structure films undergo a progressive phase transformation to the wurtzite stucture while at the same time losing a high proportion of their defects.     

Thermodynamic feasibility study for chemical vapour deposition of some ternary crystals using iodine and hydrogen iodide

Thermodynamic feasibility of transition metal dichalcogenide (TMDC) single crystals of Cu Al S₂, Cu Al Se₂, Cu Al Te₂, Cu Ga S₂, Cu Ga Se₂, Cu Ga Te₂, Cu In S₂, Cu In Se₂, Cu In Te₂, Ag Ga S₂, Ag Ga Se₂, Ag Ga Te₂, Ag In Se₂, Ag In Te₂, using iodine and hydrogen iodide as transporting agents has been reported in this paper. Results give range of temperature for the iodide transport and prevention of starting material formation (elemental transport) in the growth zone. From the range of temperature for the growth of crystals, selected source zone and growth zone temperatures with a differential of 100 °C are also listed. Referring the data listed in the tables can grow good quality crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Selective area growth of GaAs using a Ga beam with a step-function lateral intensity profile

Selective area growth of GaAs has been carried out in order to investigate the surface diffusion of Ga atoms using molecular beam epitaxy (MBE) with the aid of a Ga beam with a lateral step-function intensity profile. This step-function profile was obtained using a closely fitted GaAs shadow mask. When the mask edge was parallel to [01 ], a (311)A facet was typically observed near the edge of the Ga beam, while in the case of the mask edge parallel to [011], a (111)B facet was formed. MBE growth simulation based on the diffusion model was carried out in order to understand the mechanism of this selective area growth. The calculated results were in good agreement with the experimental results, and the diffusion lengths of Ga atoms were determined to be 0.10 μm along [011] direction on the (100) GaAs surface, 0.37 μm along [233] direction on the (311)A GaAs surface and 0.17 μm along [21 ] direction on the (111)B GaAs surface during MBE growth. These diffusion lengths seem to be smaller than those previously observed, which is probably due to a large V/III ratio in the region of the substrate close to the mask edge.     

Reflection high-energy electron diffraction intensity oscillations during growth of (Al,Ga)As on GaAs(111)A

We have made a reflection high-energy diffraction (RHEED) intensity oscillation study of the growth of (Al,Ga)As on GaAs and (Al,Ga)As (111)A surfaces. The RHEED intensity oscillations during growth of (Al,Ga)As are dependent on both growth temperature and As₄:Ga flux ratio. In addition, the oscillation period decreases as the Al fraction in the (Al,Ga)As is increased. Changes in the oscillation period during growth of the first few monolayers of both GaAs on (Al,Ga)As and (Al,Ga)As on GaAs may indicate the intermixing of Al and Ga near the heterointerface.     

Surface diffusion lenght during MOMBE and CBE growth measured by μ-RHEED

The growth rates of layers grown on a mesa-etched (001) GaAs surface were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion lenght of the surface adatoms of column III elements was determined from the gradient of the variation of the growth rates in the cases of MBE, MOMBE using trimethylgallium (TMGa) and CBE using TMGa or triethylgallium (TEGa) and arsine (AsH₃). The obtained values of the diffusion lengths were of the order of a micrometer in every case of the source-material combination. In the case of metalorganic materials as Ga source, it was found that the diffusion length was larger than that of Ga atom from metal Ga source. Since the substrate temperature of the present experiment is high enough to decompose TMGa and TEGa on the surface, Ga adatoms are considered to be responsible to the surface diffusion. Therefore, it is considered that the derivatives of the metalorganic molecules such as methyl radicals affect the diffusion of Ga adatoms.     

Influence of phenol on the habit of sodium chloride crystals

The influence of phenol on the habit of sodium chloride crystals was studied quantitatively. At small concentrations the presence of phenol has no influence whatsoever on the growth rate of the (100) face. At greater concentrations this growth rate increases. Spiral growth was observed on the (100) face by surface dissolution. Phenol slows down the growth rate of the (111) face, the energy barrier being about 600 cal/mole. Roughening of this face is observed when crystallization takes place from pure solutions and in the presence of small amounts of phenol. At a phenol concentration of 0.065 mole/l the growth rates of the (100) and (111) faces become equal.     

Surface diffusion length of Ga adatoms in molecular-beam epitaxy on GaAs(100)-(110) facet structures

By the molecular-beam epitaxial (MBE) growth of GaAs on [001]-mesa stripes patterned on GaAs(100) substrates, (110) facets were formed on the mesa edges defining (100)-(110) facet structures. The surface diffusion length of Ga adatoms along the [010] direction on the mesa stripes was obtained for a variety of growth conditions by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). Using these values and the corresponding growth rate on the GaAs(110) facets, the diffusion length on the (110) plane was estimated. We found that the Ga diffusion length on the (110) plane is longer than that on the (100) and (111)B planes. The long diffusion length on the (110) plane is discussed in terms of the particular surface reconstruction on this plane.     

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.     

Surface reconstructions and growth mode transitions of AlAs(100)

The surface reconstructions of AlAs(100) layers grown by molecular beam epitaxy (MBE) on GaAs(100) were mapped as a function of substrate temperature and arsenic flux. Three main reconstructions were observed - a c(4×4) at lower temperatures and higher arsenic fluxes, a (2×4) at middle temperatures, and a (3×2) at higher temperatures and lower arsenic fluxes. Growth of AlAs on AlAs(100) is layer-by-layer for the high temperature and low temperature reconstructions. In the mid-temperature region, AlAs grows rough on (2×4) reconstructed AlAs(100) as indicated by rapidly damped reflection high-energy electron diffraction (RHEED) intensity oscillations and the appearance of three-dimensional (3D) features. The addition of fractional layers of Ga enhances the smooth growth of AlAs. A metastable (5×2) reconstruction was observed when a fraction of a layer of Ga was present on the surface. The results indicate that Ga segregates during the growth of AlAs on GaAs(100) at temperatures at least as low as 500°C, and that annealing at temperatures above 700°C removes most of the Ga from the surface.     

Large area lateral overgrowth of mismatched InGaP on GaAs(111)B substrates

Application of InGaAs/InGaP double‐heterostructure (DH) lasers increases the band offset between the cladding layer and the active layer more than the use of conventional 1.3 µm InGaAsP/InP lasers. As a first step in realizing 1.3 µm InGaP/InGaAs/InGaP DH lasers, we proposed InGaP lattice‐mismatched epitaxial lateral overgrowth (ELO) technique and successfully carried out the InGaP growth on both GaAs (100), (111)B and InP (100) substrates by liquid phase epitaxy. In this work, we grew the InGaP crystal on GaAs (111)B substrate by adjusting Ga and P composition in In solution, to obtain In_0.79Ga_0.21P (λ = 820 nm) virtual substrate for 1.3 µm InGaAs/InGaP DH lasers. To grow the InGaP all over the lateral surface of the substrate, the growth time was extended to 6 hours. The amount of InGaP lateral growth up to 2 hours was gradually increased, but the lateral growth was saturated. The InGaP lateral width was about 250 µm at the growth time of 6 hours. We report the result that optical microscope observation, CL and X‐ray rocking curve measurements and reciprocal lattice space mapping were carried out to evaluate the crystal quality of the grown InGaP layers. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE Growth of Wide Band-Gap II—VI Compounds for Near-UV and Deep-Blue Light Emitting Devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity ^tBuSH, ^tBu₂Se and the adduct Me₂Zn:Et₃N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MQWs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm².     

Twins in GaAs Crystals Grown by the Vertical Gradient Freeze Technique

The growth of facets and the generation of twins on <100> VGF (vertical gradient freeze technique) grown GaAs were investigated using DSL (diluted Sirtl-like etchant with light) photoetching and transmission X-ray topography. Due to the polarity of the (111) plane in GaAs, As facets are larger and more irregular than Ga facets and twins always occur on As facets. Twins are initiated at the change of boundary condition which is affected by temperature gradient and crucible shape. The mechanism of twin generation is explained by considering the edge concavity at the solid-liquid interface and the supercooling required for initial nucleation of a facet. Twins are more often produced in Si-doped crystals than undoped ones due to the constitutional supercooling.     

Investigation of Residual Oxide Layers on GaAs Surfaces

GaAs(100) and (111) oriented samples, treated by means of different chemical procedures, are studied by X-ray photoelectron spectroscopy (XPS). The thickness and composition of the residual oxide layers are estimated using the intensity ration of Ga 3d and As 3d core levels from the substrate and the oxide overlayers, respectively. Residual oxides containing As₂ O₃ and Ga₂ O₃ on GaAs surface are observed. The thickness of the oxide layers is found to range from 3Å to 15 Å. Optimal conditions for pre-epitaxial surface preparation of GaAs substrates are suggested.     

Theoretical analysis of equilibrium adsorption layers in CVD systems (Si-H-Cl,Ga-As-H-Cl)

Adsorption of Cl, GaCl, As, As₂, As₄, Ga and AsGaCl species on GaAs(111) in the temperature range of 973–1223 K, and Cl, H, Si, SiCl and SiCl₂ on Si(111) in the range of 1425–1525 K, has been considered for different gas phase compositions. All the faces are covered mainly with Cl and H atoms. The fraction of vacancies on GaAs(111)Ga varies in the range of 4–13% reaching ?50% on GaAs(111)As. On the Si(111) the typical fraction of the vacancies is ?1%. The main adsorbed molecules containing Ga and As atoms are GaCl and As₄. For example, at and T = 1073 K, the surface concentration of these molecules reaches ?4 and 2% on GaAs(111)Ga and ?26 and 6% on the GaAs(111)As faces, respectively. Under these conditions, AsGaCl complexes occupy only ?10^-4?10^-5 adsorption sites on both faces. The set of other data is presented. Surface diffusion in the dense adsorption layer is seriously hindered by the lack of vacancies. A rough estimate gives a value of the order of 10^-8 cm²/sec for the diffusion coefficient and diffusion length of several tens of distances for AsGaCl or As₄ complexes on the GaAs(111)Ga face. The local electric field caused by adatoms on the interface may seriously influence the reaction rate.     

Study on the Morphological Change of Growing Ag Crystallites by in-situ SEM Observation

The morphology of the initial growth stage of Ag thin films on Mo(110) substrate was investigated in-situ by the scanning electron microscope (SEM). A great majority of the Ag crystallites showed a truncated Wulff's equilibrium shape of the face-centered-cubic (fcc) structure with epitaxial orientation [1 0]Ag//[11 ]Mo, (111)Ag//(110)Mo with the substrate temperature of 400 °C. In-situ SEM observation during the growth revealed two kinds of particular morphological change of Ag crystallites, namely (1) rotation of an Ag crystallite and (2) reformation of the crystal faces of the Ag crystallite. However, a small number of Ag crystal particles showed Wulff-polyhedron truncated by the (100) plane parallel to the substrate surface (oriented to the 〈100〉 axis normal to the substrate surface).