Synthesis and epitaxial growth of CdTe films by neutral and ionized beams

The growth of CdTe thin films has been studied by epitaxial processes on the cleavage surface of rock salt in vacuum, using electron microscopic and electron diffraction techniques. The crystallinity and structure of the films depend largely upon the intensities and species of the incident beams. The use of two beams effused separately from the Knudsen cells resulted in the growth of films of good crystallinity when the intensity ratio N_Cd : N_Te was 10 : 1. The epitaxial relationships were studied over the range of substrate temperatures between room temperature and 350°C. The co-existence of α-hexagonal and β-cubic modifications of CdTe and their proportions in the film were revealed as a function of the growth processes and substrate temperature. If two beams were ionized by electron bombardment inside the cells and were incident upon the substrate by applying a dc voltage between source and substrate, the epitaxial temperature can be lowered to near room temperature, giving good epitaxy. The epitaxial relationships in the CdTe/NaCl system have been studied.     

Initial stage and kinetics of epitaxial film growth of A3B5 compounds

Thermodynamic and kinetic characteristics of GaAs film decomposition, mechanisms of film formation and growth, main features of the film growth under epitaxy in chemical transport were considered. The stages of epitaxial film growth and the relief smoothing of film surface were discussed applying data of electron microscopy investigations and theoretical considerations. The period of continuous film formation and degrees of the surface covering of the film growth by adsorbed substance were estimated.     

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.     

Vapour phase growth of epitaxial silicon carbide layers

After a brief overview of different epitaxial layer growth techniques, the homoepitaxial chemical vapour deposition (CVD) of SiC with a focus on hot-wall CVD is reviewed. Step-controlled epitaxy and site competition epitaxy have been utilized to grow polytype stable layers more than 50 μm in thickness and of high purity and crystalline perfection for power devices. The influence of growth parameters including gas flow, C/Si ratio, growth temperature and pressure on growth rate and layer uniformity in thickness and doping are discussed. Background doping levels as low as 10¹⁴ cm⁻³ have been achieved as well as layers doped over a wide n-type (nitrogen) and p-type (aluminium) range.

Furthermore the status of numerical process simulation is mentioned and SiC substrate preparation is described. In order to get flat and damage free epi-ready surfaces, they are prepared by different methods and characterised by atomic force microscopy and by scanning electron microscope using channelling patterns. For the investigation of defects in SiC high purity CVD layers are grown. The improvement of the quality of bulk crystal substrates by micropipe healing and so-called dislocation stop layers can further decrease the defect density and thus increase the yield and performance of devices. Due to its high growth rate functionality and scope for the use of multi-wafer equipment hot-wall CVD has become a well-established method in SiC-technology and has therefore great industrial potential.     

Optical and structural properties of epitaxial GaN films grown by pulsed laser deposition

Epitaxial GaN films have been grown on c-cut sapphire substrates by pulsed laser deposition (PLD) using a KrF laser. The properties of GaN films were improved by increasing the growth temperature to 800°C and the nitrogen pressure during growth to 10 mTorr. Room-temperature photoluminescence exhibits a strong band-edge emission at 3.4 eV. From transmission electron microscopy (TEM), the predominant defects in PLD-GaN observed are stacking faults parallel to the interface and screw dislocations along c-axis, the latter differing from the previously published results where most of the threading dislocation in GaN grown by other techniques are of edge type with Burgers vector of .     

On the substructure of epitaxial (Pb,Sn)Te films grown on KCl cleavage faces

The X-ray diffraction rocking curve analysis has been employed for the measurement of crystallite sizes of (Pb, Sn)Te films for different thickness, epitaxially grown on KCl cleavage faces by the hot-wall technique. The size of the coherently scattering regions parallel to the reflecting planes did not depend on film thickness and agreed with the mean distance of nuclei just before coalescence, investigated by means of replica electron microscopy. The results are consistent with a model of stalk-like growth of crystallites in film growth direction.     

Molecular beam epitaxial growth and thermodynamic analysis of InGaAs and InAlAs lattice matched to InP

Reflection high-energy electron diffraction (RHEED) intensity oscillations have been used to detect the onset of metal droplet formation on the surface of InAs, InGaAs and InAlAs during molecular beam epitaxy. The growth conditions which produce such droplets are shown to be in good agreement with predictions based on thermodynamic arguments.     

Multilayered structures of epitaxial indium phosphide

Oscillators with efficiencies over 20% at 15 GHz that have small temperature sensitivity, and transferred electron amplifiers that have less than 9 dB noise figures at 15 GHz have been made from epitaxial multilayers of indium phosphide. The techniques involved in growing and characterising these epitaxial layers are presented.     

Numerical design of Metal‐Organic Vapour Phase Epitaxy process for gallium nitride epitaxial growth

The paper presents the results of numerical simulations and experimental measurements of the epitaxial growth of gallium nitride in Metal Organic Vapor Phase Epitaxy within a AIX‐200/4RF‐S reactor. The aim was to develop optimal process conditions for obtaining the most homogeneous crystal layer. Since there are many factors influencing the chemical reactions on the crystal growth area such as: temperature, pressure, gas composition or reactor geometry, it is difficult to design an optimal process. In this study various process pressures and hydrogen volumetric flow rates have been considered. Due to the fact that it is not economically viable to test every combination of possible process conditions experimentally, detailed 3D modeling has been used to get an overview of the influence of process parameters. Numerical simulations increased the understanding of the epitaxial process by calculating the heat and mass transfer distribution during the growth of gallium nitride. Appropriate chemical reactions were included in the numerical model which allowed for the calculation of the growth rate of the substrate. The results obtained have been applied to optimize homogeneity of GaN film thickness and its growth rate.     

RHEED monitoring of AlN epitaxial growth by plasma-assisted molecular beam epitaxy

We have studied the epitaxial growth of AlN layers by plasma-assisted molecular beam epitaxy (MBE) on 6H–SiC substrate. Reflection high-energy electron diffraction (RHEED) was used to monitor the growth by the observation of the 2D–3D growth transition, respectively, in Al- and N-rich conditions. Special attention was given to the elimination of the Al droplets which often form in Al-rich conditions. Different growth procedures are proposed to avoid the appearance of these droplets while keeping a 2D growth. Each of the procedure gives AlN epilayers with identical crystalline quality and low surface roughness as measured, respectively, by X-ray diffraction and atomic force microscopy.     

Bulk and epitaxial growth of silicon carbide

Silicon carbide (SiC) is a wide bandgap semiconductor having high critical electric field strength, making it especially attractive for high-power and high-temperature devices. Recent development of SiC devices relies on rapid progress in bulk and epitaxial growth technology of high-quality SiC crystals. At present, the standard technique for SiC bulk growth is the seeded sublimation method. In spite of difficulties in the growth at very high temperature above 2300 °C, 150-mm-diameter SiC wafers are currently produced. Through extensive growth simulation studies and minimizing thermal stress during sublimation growth, the dislocation density of SiC wafers has been reduced to 3000–5000 cm⁻² or lower. Homoepitaxial growth of SiC by chemical vapor deposition has shown remarkable progress, with polytype replication and wide range control of doping densities (10¹⁴–10¹⁹ cm⁻³) in both n- and p-type materials, which was achieved using step-flow growth and controlling the C/Si ratio, respectively. Types and structures of major extended and point defects in SiC epitaxial layers have been investigated, and basic phenomena of defect generation and reduction during SiC epitaxy have been clarified. In this paper, the fundamental aspects and technological developments involved in SiC bulk and homoepitaxial growth are reviewed.     

Influence of cooling rate on the liquid-phase epitaxial growth of Zn3P2

We report the liquid-phase epitaxial growth of Zn₃P₂ on InP (1 0 0) substrates by conventional horizontal sliding boat system using 100% In solvent. Different cooling rates of 0.2–1.0 °C/min have been adopted and the influence of supercooling on the properties of the grown epilayers is analyzed. The crystal structure and quality of the grown epilayers have been studied by X-ray diffraction and high-resolution X-ray rocking measurements, which revealed a good lattice matching between the epilayers and the substrate. The supercooling-induced morphologies and composition of the epilayers were studied by scanning electron microscopy and energy dispersive X-ray analysis. The growth rate has been calculated and found that there exists a linear dependence between the growth rate and the cooling rate. Hall measurements showed that the grown layers are unintentionally doped p-type with a carrier mobility as high as 450 cm²/V s and a carrier concentration of 2.81×10¹⁸ cm⁻³ for the layers grown from 6 °C supercooled melt from the cooling rate of 0.4 °C/min.     

Optimized growth conditions of GaN epitaxial layers

Optimized growth conditions of the epitaxial GaN layers on the (0001) oriented sapphire substrates in the Ga/HCl/NH₃/H₂ system have been proposed. The corresponding growth rate varied about the value 0.5 μm · min⁻¹. The study of surface morphology and X-ray diffraction have confirmed the single crystalline character of the layers even though the surface shows a faceted structure. The c/a ratio calculated from our measured data of the lattice parameters was found 1.624 which is relatively close to the ideal close-packed wurtzite structure value. The cathodoluminescent spectra of the layers with a sufficient thickness were characterized by a peak at 3.35 eV having a halfwidth of about 0.2 eV.     

The growth of epitaxial aluminium on As containing compound semiconductors

The growth of epitaxial aluminium on different (1 0 0) oriented compound semiconductors grown using the molecular beam epitaxy technique have been studied. After deposition of the first complete adlayer between the aluminium and the GaAs surface as evidenced by in situ reflection high electron energy diffraction (RHEED), ex situ atomic force microscopy (AFM) images agree that subsequent aluminium deposition is via a 3D nucleated growth mode. RHEED observations during continued deposition of epitaxial aluminium indicate a 2D growth mode dominated by the (1 0 0) orientation. AFM images of the surface of the aluminium reveal that the surface morphology consists of a plateau–valley structure, while transmission electron microscopy characterisation reveals that the aluminium is a (1 0 0) oriented single crystal. For growth of epitaxial aluminium on different (1 0 0) compound semiconductors the resultant hillock–valley morphology of the aluminium is remarkably similar regardless of the underlying semiconductor. There is no apparent difference between the aluminium growth on GaAs and Al_0.6Ga_0.4As indicating that the aluminium content of the semiconductor is having no effect on the growth of the aluminium, whereas there can be a difference in the hillock widths for aluminium grown on In_0.53Al_0.47As and In_0.55Ga_0.45As. The dominant orientation that the aluminium recrystallises to, appears to be determined by the strain between the aluminium 3D nucleates and the underlying semiconductor with (1 0 0) oriented aluminium for tensile strain (growth on GaAs and AlGaAs) and (1 1 0) oriented aluminium for compressive strain (growth on InAlAs and InGaAs).     

Growth and electrical properties of liquid phase epitaxial layers of GaxIn1-xP lattice-matched to GaAs substrates

From the measurement of the evaporation rate of phosphorus the favourable conditions for the layer growth were determined. Further, we have studied the influence of zinc and silicon on the growth and the morphology. The mobilities, the electron and hole concentrations in the temperature range from 77 to 300 K are presented. In the Si-doped samples we could not find a correlation between the silicon concentration in the melt and the donor concentrations in the layer. An estimation of the acceptor ionization energy in Zn-doped samples yields E_A = (15 ± 4) meV.     

Characterization of the carrot defect in 4H-SiC epitaxial layers

Characterization of the epitaxial defect known as the carrot defect was performed in thick 4H-SiC epilayers. A large number of carrot defects have been studied using different experimental techniques such as Nomarski optical microscopy, KOH etching, cathodoluminescence and synchrotron white beam X-ray topography. This has revealed that carrot defects appear in many different shapes and structures in the epilayers. Our results support the previous assignment of the carrot defect as related to a prismatic stacking fault. However, we have observed carrot defects with and without a visible threading dislocation related etch pit in the head region, after KOH etching. Polishing of epilayers in a few μm steps in combination with etching in molten KOH and imaging using Nomarski optical microscope has been used to find the geometry and origin of the carrot defects in different epilayers. The defects were found to originate both at the epi-substrate interface and during the epitaxial growth. Different sources of the carrot defect have been observed at the epi-substrate interface, which result in different structures and surfaces appearance of the defect in the epilayer. Furthermore, termination of the carrot defect inside the epilayer and the influence of substrate surface damage and growth conditions on the density of carrot defects are studied.     

Solid phase epitaxial growth of Si through Al film

Uniform epitaxial growth has been obtained by dissolution and transport of an evaporated Si film through an evaporated Al film at temperatures below 500°C. By analyzing the samples made in different ways we show that the presence of a cap on the metal layer, which inhibits the diffusion of the metal through the evaporated Si, plays a fundamental role. The cap consists of a thin oxide layer grown on top of the metal. The cap is made by leaving the sample in vacuum for two days or by heating the sample in vacuum before the Si deposition. The study of the initial growth rate on 〈100〉 and 〈111〉Si substrates reveals that the growth starts as islands which grow until they coalesce to form a continuous layer. Different growth rates have been obtained by using 〈100〉 and 〈111〉Si substrates. Typical growth rates are 50 Å/min at 330°C on 〈100〉 and 100 Å/min at 392°C on 〈111〉. The activation energy of the process is 1.2 eV.     

Electron microscopy studies of epitaxial MgB2 superconducting thin films grown by in situ reactive evaporation

The morphology and chemistry of epitaxial MgB₂ thin films grown using reactive Mg evaporation on different substrates have been characterized by transmission electron microscopy methods. For polycrystalline alumina and sapphire substrates with different surface planes, an MgO transition layer was found at the interface region. No such layer was present for films grown on MgO and 4-H SiC substrates, and none of the MgB₂ films had any detectable oxygen incorporation nor MgO inclusions. High-resolution electron microscopy revealed that the growth orientation of the MgB₂ thin films was closely related to the substrate orientation and the nature of the intermediary layer. Electrical measurements showed that very low resistivities (several μΩ cm at 300 K) and high superconducting transition temperatures (38 to 40 K) could be achieved. The correlation of electrical properties with film microstructure is briefly discussed.     

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.     

Properties of (Ga0.47In0.53)As epitaxial layers grown by metalorganic vapor phase epitaxy (MOVPE) using alternative arsenic precursors

In this study, the use of novel, liquid, organic arsenic precursors as substitutes for the highly toxic hydride gas arsine (AsH₃) in low pressure metalorganic vapor phase epitaxy (LP-MOVPE) of (GaIn)As lattice matched on InP has been investigated. The model precursors out of the classes of (alkyl)3-nAsH_n (n = 0,1,2) are tertiarybutyl arsine (TBAs), ditertiarybutyl arsine (DitBAsH) and diethyltertiarybutyl arsine (DEtBAs). The MOVPE growth has been investigated in the temperature range of 570–650°C using V/III ratios from 2 to 20. The obtained epitaxial layer quality as examined by means of optical and scanning electron microscopy (SEM), high resolution double crystal X-ray diffraction, temperature-dependent van der Pauw-Hall, as well as photoluminescence (PL) measurements, will be compared for the different source molecules. Under optimized conditions almost uncompensated n-type (GaIn)As layers with carrier concentrations below 1 × 10¹⁵ cm⁻³ and corresponding mobilities above 80 000 cm²/V · s have been realized. For TBAs and DitBAsH in combination with the corresponding P sources TBP and DitBuPH, respectively, we have worked out a process parameter area for the growth of layers with device quality, as proven by the realization of a pin-detector structure.