Surface preparation effect of GaAs(110) substrates on the ZnSe epitaxial layer grown by molecular beam epitaxy

The effect of the surface preparation of the GaAs(110) substrate on the ZnSe epitaxial layer grown by molecular beam epitaxy (MBE) was investigated by means of etch-pit density (EPD) measurements, surface morphology observation, and reflection high-energy electron diffraction (RHEED) analysis. The ZnSe epitaxial layer grown on a GaAs(110) surface prepared by cleaving the (001)-oriented wafer in ultrahigh vacuum (UHV) showed about 5×10⁴ cm^-2 of EPD. This value is much lower than that observed from both the samples grown on the mechanically polished surface with and without a GaAs buffer layer. Due to the non-stoichiometric surface after thermal evaporation of the surface oxide, three-dimensional growth can easily occur on the mechanically polished GaAs(110) substrate. These results suggest that the stoichiometric and atomically flat substrate surface is essential for the growth of low-defect ZnSe epitaxial layers on the GaAs(110) non-polar surface. Received: 21 August 1998 / Accepted: 19 October 1998 / Published online: 28 April 1999     

Estimation of quality of GaAs substrates used for constructing semiconductor power devices

X-ray topography and high-resolution diffractometry methods are used for testing GaAs wafers from different manufacturers, which are used as substrates for epitaxial growth in construction of power semiconductor devices. Typical features of such wafers are a distorted surface layer, bent, and growth dislocations with two types of distribution with a density of (1–2) × 10⁴ cm^?2. The best and worse substrates are determined from the finishing of the working surface, and the optimal combination of X-ray methods for estimating the quality of finishing of the working surface of the crystals with a high level of X-ray absorption is established.     

Optical and electrical properties of quantum wells with electrically tunable two-dimensional electron density by selective contacts

We report on photoluminescence and absorption measurements in type-I hetero n-i-p-i structures. The electron density in the pseudomorphic InGaAs/GaAs quantum wells is tunable between zero and more then 5 · 10¹² cm^-2. This electrical tuning of the subband filling is achieved by a variable voltage applied between selective n-and p-contacts fabricated by epitaxial shadow mask MBE. One of the advantages of having selective contacts to the n- and p-layers is to get reliable information about the electron density, independent of measured absorption and luminescence spectra. This allows a more rigorous analysis of the data on bandgap renormalization, bandfilling and k_∥-conservation. Moreover, the experiments can be performed at low optical power and low carrier temperatures. In the present investigation a bandgap renormalization of -20 meV and a bandfilling induced shift of the absorption edge as large as +50 meV was observed for a sheet electron density of 5 · 10₁₂ cm_-2.     

Determination of deep electron traps in GaAs by time-resolved capacitance measurement

A useful technique of determining the energy levels and the spatial density distributions of multiple electron traps in semi-conductor has been developed using the time-resolved measurement of the Schottky barrier junction capacitance, and this technique has been applied to characterize the electron traps inn-GaAs. In the present technique, the energy levels are determined from single scan of temperature, and the density distributions are calculated from a set of capacitance-voltage relationships. Four traps which lay at 0.39, 0.73, 0.79, and 0.58 eV below the conduction band edge were observed in boat grown or vapor phase epitaxially grown crystals. Many layers which were obtained by a vapor phase epitaxial growth system with N₂ carrier gas were measured and it was found that almost all of them include the 0.73 eV and the 0.79 eV trap with the density between 1×10¹³ and 2×10¹⁵ cm⁻³.     

Defect-free growth of epitaxial silicon at low temperatures (500–800 °C) by atmospheric pressure plasma chemical vapor deposition

Epitaxial Si growth at low temperatures (500–800 °C) by atmospheric pressure plasma chemical vapor deposition has been investigated. Silicon films are deposited on (001) Si wafers using gas mixtures containing He, H₂, and SiH₄. The effects of deposition parameters (composition of reactive gases, very high frequency (VHF) power, and substrate temperature) on film properties are investigated by reflection high-energy electron diffraction, atomic force microscopy, cross-sectional transmission electron microscopy, and plasma emission spectroscopy. It is found that epitaxial temperature can be reduced by increasing VHF power, and that an optimum range of VHF power exists for Si epitaxy, depending on the substrate temperature and the composition of the reactive gases. The result of the H₂ concentration dependence of H_α emission intensity, shows that hydrogen atoms generated in the atmospheric pressure plasma play an important role in Si epitaxial growth. Under the optimized growth conditions, defect-free epitaxial Si films (as observed by transmission electron microscopy) with excellent surface flatness are grown at 500 °C with an average growth rate of approximately 0.25 μm/min. PACS 81.05.Cy; 81.15.Gh; 68.55.Jk     

AlGaN/GaN heterostructure grown on 1 -tilt sapphire substrate by MOCVD

AlGaN/GaN epitaxial layers were grown on 0^°-tilt and 1^°-tilt sapphire substrates by metalorganic chemical vapor deposition (MOCVD). With exactly the same growth conditions, it was found that dislocation density was smaller and crystal quality was better for the AlGaN/GaN epitaxial layers prepared on 1^°-tilt sapphire substrate. We also found that AlGaN/GaN epitaxial layers on 1^°-tilt sapphire substrate were grown with step growth mode while those on 0^°-tilt substrate were grown with two-dimensional island growth. From the temperature-dependent mobility, it was found that crystal quality of the AlGaN/GaN epitaxial layer prepared on 1^°-tilt sapphire substrate is better.     

TEM study of the structure of gallium nitride epitaxial films grown on substrates with different interface morphologies

The structure of gallium nitride epitaxial layers grown on patterned substrates was studied by transmission electron microscopy. The engraving of a substrate surface with pits or pyramids undoubtedly leads to structural improvement. The best results are obtained at a pattern density of about 10⁷ cm^?2.     

Transport characteristics of a single-layer graphene field-effect transistor grown on 4H-silicon carbide

We have investigated transport characteristics of epitaxial graphene grown on semi-insulating silicon-face 4H-silicon carbide (SiC) substrate by thermal decomposition method in relatively high N₂ pressure atmosphere. We have succeeded in forming 1–2 layers of graphene on SiC in controlled manner. The surface morphology of formed graphene was analyzed by atomic force microscopy (AFM), low-energy electron diffraction (LEED) and low-energy electron microscope (LEEM). We have confirmed single-layer graphene growth in average by this method. Top-gated, single-layer graphene field-effect transistors (FETs) were fabricated on epitaxial graphene grown on 4H-SiC. Increased on/off ratio of nearly 100 at low temperature and extremely small minimum conductance (0.018–0.3 in 4 e²/h) in gated Hall-bar samples suggest possible band-gap opening of single-layer epitaxial graphene grown on Si-face SiC.     

Quantum dots-templated growth of strain-relaxed GaN on a c-plane sapphire by radio-frequency molecular beam epitaxy

We investigated the quantum dots-templated growth of a(0001) GaN film on a c-plane sapphire substrate.The growth was carried out in a radio-frequency molecular beam epitaxy system.The enlargement and coalescence of grains on the GaN quantum dots template was observed in the atom force microscopy images,as well as the more ideal surface morphology of the GaN epitaxial film on the quantum dots template compared with the one on the AlN buffer.The Ga polarity was confirmed by the reflected high energy electron diffraction patterns and the Raman spectra.The significant strain relaxation in the quantum dots-templated GaN film was calculated based on the Raman spectra and the X-ray rocking curves.Meanwhile,the threading dislocation density in the quantum dots-templated film was estimated to be 7.1×107cm-2,which was significantly suppressed compared with that of the AlN-buffered GaN film.The roomtemperature Hall measurement showed an electron mobility of up to 1860cm2 /V·s in the two-dimensional electron gas at the interface of the Al 0.25Ga0.75 N/GaN heterojunction.     

Formation of p-n junctions in tellurium-doped AlxGa1−xSb(As) (x = 0.15–0.20) solid solution layers

We study the variation of electron density n in depth of tellurium-doped epitaxial Al_xGa_1–xSb(As) (x = 0.15–0.20) layers. It is established that n decreases in proportion to the growth of the layer and, under definite conditions, the formation of p-n junctions in layers grown from a single melt or the growth of layers having an electron density below 10¹⁶ cm^–3 is possible. It is shown that the cause of such a decrease of electron density is the variation of the composition of the solid solution in depth of the layer and the accompanying increase in the concentration of residual acceptor defects in the material.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 82–86, November, 1984.In conclusion the authors wish to express their gratitude to senior scientist M. D. Vilisovaya, junior scientist G. K. Arbuzovaya, Z. V. Korotchenko, and V. A. Pozolotin for assistance in carrying out the experiments.     

Ion-beam-assisted MBE growth of semi-spherical SiGe/Si nano-structures

Semi-spherical SiGe/Si nano-structures of a new type are presented. Epitaxial islands of 30–40 nm in base diameter and 11 nm in height and having a density of about 6×10¹⁰ cm^-2 were produced on (001) Si by molecular beam epitaxial growth of Si/Si_0.5Ge_0.5 layers with in situ implantation of 1-keV As⁺ ions. It was found by cross-section transmission electron microscopy that the islands have a complicated inner structure and consist of a micro-twin nucleus and semi-spherical nano-layers of various SiGe compositions. The nature of the surface patterning is interpreted by stress relaxation through implantation-induced defects. Received: 12 July 2001 / Accepted: 4 September 2001 / Published online: 2 October 2001     

Electron microscopy study of the structural defect formation in ZnS under irradiation by electrons with energy of 400 keV

ZnS crystals grown form the vapor phase and ZnS/(001)GaAs epitaxial structures grown by metalorganic vapor phase epitaxy are studied by transmission electron microscopy after in situ irradiation in an electron microscope at an electron energy of 400 keV and intensity of (1–4) × 10¹⁹ electrons/cm² s. It is shown that irradiation leads to the formation of small (2.5–45 nm) dislocation loops with a density of 1.4 × 10¹¹ cm^–2, as well as voids and new phase inclusions ≤10 nm in size. Using the moire fringe contrast analysis, these inclusions were identified as ZnO and ZnO₂.     

Effect of surface morphology on the electron mobility of epitaxial graphene grown on 0° and 8° Si-terminated 4H-SiC substrates

Graphene with different surface morphologies were fabricated on 8° -off-axis and on-axis 4H-SiC(0001) substrates by high-temperature thermal decompositions. Graphene grown on Si-terminated 8° -off-axis 4H-SiC(0001) shows lower Hall mobility than the counterpart of on-axis SiC substrates. The terrace width is not responsible for the different electron mobility of graphene grown on different substrates, as the terrace width is much larger than the mean free path of the electrons. The electron mobility of graphene remains unchanged with an increasing terrace width on Siterminated on-axis SiC. Interface scattering and short-range scattering are the main factors affecting the mobility of epitaxial graphene. After the optimization of the growth process, the Hall mobility of the graphene reaches 1770 cm 2 /V·s at a carrier density of 9.8.×10 12 cm 2 . Wafer-size graphene was successfully achieved with an excellent double-layer thickness uniformity of 89.7% on a 3-inch SiC substrate.     

Transport properties of epitaxial graphene formed on the surface of a metal

The transport properties of epitaxial graphene formed on the surface of a metal substrate have been considered within the approach based on the model Anderson-Newns Hamiltonian. An analytical expression for the density of states of epitaxial graphene has been obtained and the renormalization of the Fermi velocity in doped epitaxial graphene has been investigated. The real part of the dynamic conductance of epitaxial graphene has been examined and the limiting values of conductance have been analyzed. When there is no interaction between the graphene and the substrate, the static conductance of epitaxial graphene takes on the universal value 2e ²/π² ?. The fundamental problems considered in this study are of crucial importance in the study of optical, magneto-optical, thermoelectric, and thermomagnetic properties of epitaxial graphene. The obtained results are of great interest for practical use of epitaxial graphene as a promising material for microwave technology.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Spontaneous formation of a system of highly ordered germanium nanoislands upon epitaxial deposition on profiled (111) silicon substrates under electromigration conditions

Atomic-force microscopy was used to study the surface topography of SiGe structures grown by epitaxial deposition of Ge on profiled Si(111) substrates under electromigration conditions. Systems of highly ordered germanium nanosized islands with dimensions of 10–20 nm and a density of 6×10¹⁰ cm^?2 were obtained. It is shown that the geometrical parameters of self-organizing nanoislands can be controlled by a proper choice of the growth and postgrowth annealing conditions for these structures.     

Electronic structure of Ag adsorbed on Si(111); experiment and theory

Experimental results (low energy electron loss spectroscopy) and band structure calculations relating to the early stages of Ag growth on a Si(111) surface are presented. Crystallography and thermal desorption kinetics studies of this interface, previously published, gave rise to the following conclusions. At room temperature and below 200°C, two-dimensional (2D) (111) epitaxial layers develop on top of a first ordered layer (√3 × √3), while at higher temperatures three-dimensional (3D) clusters develop over this first layer. Low energy electron loss experiments were performed at various surface coverages θ. They display different evolutions according to the growth modes. For the 2D epitaxial growth, one observes the disappearance of the peaks characteristic of a Si surface and the onset of Ag induced peaks located at 7.1 and 4.6 eV at completion of the √3 layer. These peaks narrow and shift to the bulk Ag excitation energies at 7.5 and 4 eV when a second Ag layer is deposited. In order to explain these results, we present a theoretical calculation of the electronic density of states of the interface using a tight binding approximation. This calculation accounts for the development of the Ag d band from the √3 coverage range to the (111) epitaxial Ag planes. The evolution of the spectra when θ is increased is discussed in view of these results.     

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C-SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar⁺ ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2-4 keV energy and fluence ranging from 3 × 10¹³ ions/cm² to 1 × 10¹⁸ ions/cm². Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N₂⁺ ions and 7.2 × 10¹⁷ ions/cm² are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar⁺ ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.     

Growth and properties of KLu(WO4)2, and novel ytterbium and thulium lasers based on this monoclinic crystalline host

High‐quality crystals of monoclinic KLu(WO₄)₂, shortly KLuW, were grown with sizes sufficient for its characterization and substantial progress was achieved in the field of spectroscopy and laser operation with Yb³⁺‐ and Tm³⁺‐doping. We review the growth methodology for bulk KLuW and epitaxial layers, its structural, thermo‐mechanical, and optical properties, the Yb³⁺ and Tm³⁺ spectroscopy, and present laser results obtained in several operational regimes both with Ti:sapphire and direct diode laser pumping using InGaAs and AlGaAs diodes near 980 and 800 nm, respectively. The slope efficiencies with respect to the absorbed pump power achieved with continuous‐wave (CW) bulk and epitaxial Yb:KLuW lasers under Ti:sapphire laser pumping were ≈ 57 and ≈ 66%, respectively. Output powers as high as 3.28 W were obtained with diode pumping in a simple two‐mirror cavity where the slope efficiency with respect to the incident pump power reached ≈ 78%. Passively Q‐switched laser operation of bulk Yb:KLuW was realized with a Cr:YAG saturable absorber resulting in oscillation at ≈ 1031 nm with a repetition rate of 28 kHz and simultaneous Raman conversion to ≈ 1138 nm with maximum energies of 32.4 and 14.4 μJ, respectively. The corresponding pulse durations were 1.41 and 0.71 ns. Passive mode‐locking by a semiconductor saturable absorber mirror (SESAM) produced bandwidth‐limited pulses with duration of 81 fs (1046 nm, 95 MHz) and 114 fs (1030 nm, 101 MHz) for bulk and epitaxial Yb:KLuW lasers, respectively. Slope efficiency as high as 69% with respect to the absorbed power and an output power of 4 W at 1950 nm were achieved with a diode‐pumped Tm:KLuW laser. The slope efficiency reached with an epitaxial Tm:KLuW laser under Ti:sapphire laser pumping was 64 %. The tunability achieved with bulk and epitaxial Tm:KLuW lasers extended from 1800 to 1987 nm and from 1894 to 2039 nm, respectively.     

Large area SiC substrates and epitaxial layers for high power semiconductor devices — An industrial perspective

We review the progress in the industrial production of SiC substrates and epitaxial layers for high power semiconductor devices. Optimization of SiC bulk growth by the sublimation method has resulted in the commercial release of 100 mm n-type 4H-SiC wafers and the demonstration of micropipe densities as low as 0.7 cm⁻² over a full 100 mm diameter. Modelling results link the formation of basal plane dislocations in SiC crystals to thermoelastic stress during growth. A warm-wall planetary SiC-VPE reactor has been optimized up to a 8×100 mm configuration for the growth of uniform 0.01–80-micron thick, specular, device-quality SiC epitaxial layers with low background doping concentrations of <1×10¹⁴ cm⁻³, and intentional p- and n-type doping from 1×10¹⁵ to >1×10¹⁹ cm⁻³. We address the observed degradation of the forward characteristics of bipolar SiC PiN diodes [H. Lendenmann, F. Dahlquist, J.P. Bergmann, H. Bleichner, C. Hallin, Mater. Sci. Forum 389–393 (2002) 1259], and discuss the underlying mechanism due to stacking fault formation in the epitaxial layers. A process for the growth of the epitaxial layers with a basal plane dislocation density <10 cm⁻² is demonstrated to eliminate the formation of these stacking faults during device operation [J.J. Sumakeris, M. Das, H.McD. Hobgood, S.G. Müller, M.J. Paisley, S. Ha, M. Skowronski, J.W. Palmour, C.H. Carter Jr., Mater. Sci. Forum 457–460 (2004) 1113].