Luminescence of (GaAl)As layers grown by temperature gradient liquid phase epitaxy

In this paper the growth of Al_xGa_1–xAs layers with nearly constant composition along their thickness is described. By means of electroluminescence and cathodoluminescence measurements recombination mechanisms connected with the presence of Si are discussed. The aging behaviour of the light emitting diodes is supposed to depend on the AlAs content of the layers and to be connected with the diffusion of acceptors in the p-n junction region.     

Electro-optical properties of InGaAs layers grown by hydride vapour phase epitaxy

A series of epitaxial layers of the InGaAs alloy were deposited on (001) oriented InP substrates by using hydride VPE technique. The layers were characterized by Double Crystal Diffractometry (DCD), Photoluminescence (PL), Hall effect and Capacitance-Voltage (C-V) measurements. The growth parameters and the quality of the grown layers are discussed on the basis of electrical and structural data analysis.     

A structural study of phase transitions within GaN layers grown by low-temperature molecular beam epitaxy

The epitaxial growth of GaN layers on sapphire substrates by molecular beam epitaxy at low temperatures (500°C) has been investigated. Samples exhibited a transition from hexagonal to mixed hexagonal/cubic phase under conditions of increasing Ga flux as determined using a TEM-RHEED technique with complementary SEM and PL observations. Embedded cubic grains adopted two domain variants with additional evidence for twinning.     

Annealing-induced relief of compressive misfit strain in liquid phase epitaxial yttrium iron garnet films

Films of yttrium iron garnet were grown on (111) gadolinium gallium garnet by liquid phase epitaxy from a PbO-B₂O₃ flux. Incorporation of Pb as a substitutional impurity produced an increase in film lattice parameter which resulted in initial compressive misfit strains in the films. The initial strains were relieved by annealing in O₂. The relief process was studied by X-ray double-crystal diffractometry and topography supplemented by optical and scanning electron microscopy. Strain relief was found to be associated with the occurrence of defects which were imaged in both film and substrate topographs. When the initial compressive misfit was sufficiently large, annealing eventually resulted in a tensile strain and the development of cracks which propagated through the films and into the substrates.     

Isothermal liquid phase epitaxy

The epitaxial growth of semiconductor solid solutions from a liquid phase appears to be possible under stringent isothermal conditions. The driving force of such epitaxy can be liquid phase supersaturation caused by isothermal mixing of liquidus solutions or supersaturation in the solid phase resulting from thermodynamic nonequilibrium of the substrate and the solution. The feature of the method is that film growth requires no control temperature and time of crystallization while the film properties are set by the initial process conditions. Isothermal liquid phase epitaxy (ILPE) makes it possible to form hyperfine films whose component distribution can be controlled.ILPE inevitably starts on the initial stage of a common liquid phase epitaxy, therefore this phenomenon must be taken into consideration. The above mentioned factors account for practical importance of ILPE technique for modern electronics.The present paper reviews the mechanism as well as the main regularities of ILPE for A³B⁵ ternary alloys.     

Properties of gallium- and aluminum-doped bulk ZnO obtained from single-crystals grown by liquid phase epitaxy

Bulk properties of gallium (Ga)- and aluminum (Al)-doped zinc oxide (ZnO) were studied using bulky single-crystalline thick films grown by liquid phase epitaxy (LPE). The highest possible dopant concentration was 1×10¹⁹ cm^–3 for LPE growth at around 800 °C. The electron concentration was nearly same to the Ga and Al concentrations. The donor binding energy decreased to nearly zero with an increase in dopant concentration, and electron mobility of the sample with relatively high dopant concentration (1×10¹⁹ cm^–3) was more than 60 cm² V^–1 s^–1 at room temperature. The LPE technique is a potential solution for the production of ZnO for optical applications because the well-defined excitonic luminescence could be seen from the LPE-grown-doped single-crystals.     

Efficiency optimization of p-type doping in GaN:Mg layers grown by molecular-beam epitaxy

The Mg-doping efficiency in GaN layers grown by molecular-beam epitaxy has been studied as a function of the growth temperature, the growth rate, and the Mg beam flux. The Mg cell temperature window for efficient p-type doping is rather narrow, being limited by the GaN n-type background doping density (lower limit) and by the Mg surface coverage that, beyond a threshold, induces a layer polarity inversion (N-polarity), leading to a reduction of the Mg incorporation (upper limit). An increase of the growth temperature avoids this polarity inversion, but the Mg flux must be increased to compensate the strong desorption rate. Thus, a trade-off between both temperatures has to be reached. A reduction of the growth rate has a strong effect on the p-type doping level, yielding up to 7×10¹⁷ holes/cm³ for a total Mg concentration of 1×10¹⁹ cm⁻³. This high Mg concentration does not seem to generate Mg-related defects or deep traps.     

A steady-state procedure of liquid phase epitaxy

A turning-shifting technique is reported permitting the deposition of epitaxial layers at constant temperatures and with small melting volumes. Basic parameters of GaAs are studied and the development of crystal growth is followed up in dependence on temperature.     

Effect of sapphire substrate orientation on the surface morphology and structural quality of thick GaN layers grown by hydride vapor phase epitaxy

The effect of substrate orientation on the surface orientation of thick GaN layers grown by hydride vapor phase epitaxy (HVPE) has been established. Layers oriented along the (0001), (11$ bar 2 $ bar 2 0), and (10$ bar 1 $ bar 1 3) planes have been obtained on, respectively, c- and a-, r-, and m-oriented substrates. Depending on the orientation of the GaN layer surface, surface defects (terraces and growth pits) are faceted by different planes whose intersections with the growth surface are perpendicular to the direction of growth pit faces. It is found that the sapphire substrate surface orientation has an effect on the layer structural quality (which increases with an increase in the layer thickness, regardless of the layer orientation). The directions of crack propagation in the GaN layer also depend on the surface orientation of the layer and are mainly determined by the intersections of the {1$ bar 1 $ bar 1 00} planes of the layer with the surface.     

Deep hole trap level of nitrogen-doped ZnSe grown by metalorganic vapor phase epitaxy

We investigated the hole trap level of nitrogen-doped ZnSe grown by a metalorganic vapor phase epitaxy method. The deep level transient spectroscopy (DLTS) signal and the C-V profile were measured to obtain the trap level, the capture cross section and the trap concentration. A deep hole trap level of about 1 eV from a valence band (labeled TLIS) was extracted from the tail of the DLTS peak. The distributions of the capture cross section and the trap level of the samples for different growth conditions were drawn in a figure to investigate the validity of the resolution of the DLTS signal. The origin of TLIS is thought to be in relation to the ionized acceptor or the charged acceptor-like localized defects.     

Strain-reduced GaN thick-film grown by hydride vapor phase epitaxy utilizing dot air-bridged structure

A 300 μm GaN thick-film, in diameter 1.5 in, was demonstrated without any crack by hydride vapor phase epitaxy (HVPE) growth. The technique used in relaxing the residual stress caused by differences of thermal expansion coefficients (TEC) and lattice constants between GaN and sapphire substrate to prevent GaN film from crack is called a dot air-bridged structure. After the laser lift-off process, 300-μm-thick freestanding GaN wafer, in diameter 1.5 in, could be fabricated. The compressive stress in the dot air-bridged structure was measured by micro-Raman spectroscopy with the E₂(high) phonon mode. The compressive stress could be reduced to as small as 0.04 GPa, which could prevent the crack during the epitaxial process for GaN growth by HVPE. It is important to obtain a large-area crack-free GaN thick-film, which can be used for fabricating freestanding GaN wafer.     

Coexistence properties of phase separation and CuPt-ordering in InGaAsP grown on GaAs substrates by organometallic vapor phase epitaxy

CuPt-ordering and phase separation were directly investigated in In_1-xGa_xAs_yP_1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.     

Growth and characterisation of Zn:LiNbO3/Mg:LiNbO3 multilayer thin films grown by liquid phase epitaxy

1, 3 and 5 mol% ZnO doped LiNbO₃ film and 2 mol% MgO doped LiNbO₃ multilayer films were grown on the LiNbO₃ (001) substrate by liquid phase epitaxy (LPE) method with a Li₂O‐V₂O₅ system. We examined the optical transmission spectra of the Zn:LiNbO₃ by Fourier Transform‐Infrared Spectrophotometer (FT‐IR). The crystallinity and the lattice mismatch between the Zn:LiNbO₃ film and Mg:LiNbO₃ film was confirmed by x‐ray rocking curve (XRC) and observed the ZnO and MgO distribution in the cross‐section of the multilayer thin films by electron probe micro analyzer (EPMA). Furthermore, the surface morphology of the films was observed using atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermally induced stress in GaN layers with regard to film coalescence

Thermally induced plane stress in GaN layers of different thicknesses, grown by metalorganic vapour phase epitaxy on sapphire, is investigated. Thin layers, characterized by isolated grains, are found to be stress-free. With increasing layer thickness, however, grains start to coalesce and stress can build up when the samples are cooled down following growth. As soon as the coalescence process is completed and a compact film has been formed, a maximum stress level is reached which does not further increase for still thicker layers. Therefore, it is proposed that grain edges enable non-compact films to elastically relieve in-plane stress.     

Studies about wafer bow of freestanding GaN substrates grown by hydride vapor phase epitaxy

Free-standing GaN wafers grown by hydride vapor phase epitaxy are typically concavely bowed. In situ and ex situ curvature measurements indicate that some strain developing at the very beginning of the epitaxial process or even in the template grown by metalorganic vapor phase epitaxy may be the origin of this bow. It can be only partly released by etching the defective back-side of the samples indicating that the strong dislocation density gradient is not the only reason for strain in free-standing GaN.     

High-quality InP grown by chemical beam epitaxy

InP films were grown by chemical beam epitaxy using trimethylindium (TMI) and pure phosphine (PH₃) in a flow control mode with hydrogen as the carrier gas, with the TMI flow rate fixed at 3 SCCM. Substrate temperatures were varied between 505 and 580°C and V/III ratios from 3 to 9. InP layers with high optical quality (intense and narrow excitonic transition lines) and high crystalline quality (narrow and symmetric X-ray diffraction peaks) could be grown only within a narrow parameter window around a substrate temperature of 545°C (δT_s ≤ 25°C) and a V/III ratio of 5.5 (δ(V/III) ≤ 2). Carrier densities of 8 × 10¹⁴ cm^-3 with mobilities of 70000 cm²/V.s measured at 77 K were obtained for growth conditions close to the edge of this parameter window towards low V/III ratios. The growth rate of inP was also clearly at its maximum in the given parameter window. Leaving the window, by changing either the growth temperature or the V/III ratio, significantly decreased the growth rate. This reduced growth rate was accompanied by a degradation in the crystalline quality. We also demonstrate that for higher TMI flow the parameter window shifts to higher growth temperatures. The InP could be doped effectively with Si in the range from 9 × 10¹⁵ to 3 × 10¹⁸ cm^-3.