The influence of natural convection on the mass transfer kinetics in LPE growth

The simplified scheme has been suggested for quantitative estimation of the effect of natural convection on the LPE layers thickness while growing from a finite solution with parallel horizontal and vertical substrates. The convective-diffusive mass transfer model is being developed on the base of boundary layer approximation. In the case of horizontal growth the layer thickness is determined by effective solution thickness which is distinguished for an upper and lower substrate and depends on convection intensity. For the vertical growth system the expression describing the variation of layer thickness along the substrate has been obtained and the vertical temperature gradient necessary for eliminating a wedge shaped growth in this case is calculated. The experimental investigations of LPE growth of GaP layers from finite solution with horizontal and vertical substrates are carried out. The thickness and shape of LPE layers obtained in both cases coincide well with results of calculation.     

A Multipurpose Graphite Boat for LPE Growth of Multilayer Heterostructures

In the paper the results of the authors' work are summarized on the preparation of multilayer A³B⁵ heterostructures with the use of originally designed graphite boat. The main scheme of the boat design is described in detail and it is shown that the growth apparatus combine the favourable features of the known LPE growth techniques: epitaxy from thin solution layer, sliding boat technique and the technique based upon the displacement of one melt by another.     

Calculations of LPE layer thickness

Three basic cases of LPE growth with and without nucleation were examined. The expressions for layer thickness and growth efficiency calculations were developed. Criteria for LPE growth classification were obtained. It was shown the nucleation takes place at critical values of supersaturation of the solution. The values of critical supersaturation may be calculated from growth efficiency experimental data for LPE growth.     

LPE growth rate calculations

The expressions for LPE growth rate calculations are developed on the basis of the steadystate growth model assuming the nucleation at the second interface and in the volume of the solution. The solution layer thickness can be calculated at which the LPE growth rate reaches maximum for any solution cooling rate. The interface and volume nucleation parameters, critical supersaturation and supercooling of the solution have been determined from experimental GaP LPE data within temperature range 800 – 1030°C. Calculated growth rates fit very closely with experimental rates of GaP LPE growth.     

LPE growth and characterization of InP/InGaAsP ridge-waveguide lasers at 1.3 μm-wavelength

This paper presents the fabrication and the lasing characteristics of 1.3 μm-wavelength ridge-waveguide laser. The epitaxial material used in this study was grown applying the step-cooling technique of liquid phase epitaxy (LPE). The growth conditions for InGaAsP layers lattice-matched to the (001) InP-substrate are reported for lattice compositions corresponding to photoluminescence peak wavelengths of 1.07, 1.14, and 1.31 μm. We have used a conventional multiple-bin sliding boat to grow the LPE layers and a second apparatus for achieving batches of melts of uniform compositions. In the LPE apparatus the various batch melts (In–Sn In–Zn; In–Ga–As of different composition) were saturated with phosphorus using the seed dissolution technique. The epitaxial layers were grown by a single phase technique at a constant temperature. This LPE growth technique is useful for the fabrication of double-heterostructure wafers with an uniform alloy composition and a well-defined layer thickness. Using these epitaxial materials, metal-clad ridge-waveguide (MCRW) lasers have been prepared with stripe widths of 3.5 μm. CW threshold currents of 18 mA at room temperature are achieved for 200 μm long cavities. These lasers have T₀ values ranging from 50 to 70 K and well linear L-I-characteristics.     

A new technique for multilayer LPE

A novel technique is presented which allows LPE deposition of a large number of layers without the disadvantages arising from sliding container parts. Solutions of different compositions or containing different dopants are arranged in separate chambers. By rotation of the whole device the solutions move from one chamber to the next one without intermixing, and by controlled cooling, epitaxial layers are deposited onto the substrates fixed at the chamber walls. The sequence of the layers is determined by the sense and the angles of rotation, and the thickness of each layer is determined, among many other factors, by the cooling rate and dipping time. Up to 15 layers of III–V compounds have so far been produced in a double-screw device, with thicknesses between 0.1 to 10 μm and thickness reproductivity of about 10%. In addition, the fabrication of LED's, III–V lasers, and solar cells, and possibly superlattice devices by the MultiLPE technique can be considered.     

Information about the Growth Process from Aluminium Concentration Profiles in Ga1−xAlxAs Liquid Phase Epitaxial Layers

The variation in the composition of Ga_1−xAl_xAs LPE layers with x < 0.37 as a function of layer thickness is examined by electron beam microprobe analysis. From the combination of these results with the experimentally established relation between layer thickness and cooling interval information is obtained about the onset of homogeneous nucleation in the LPE growth.     

Mass transfer during growth of Ga?In?P?As solid solution in the equilibrium cooling growth technique

Kinetical analysis of crystallization of Ga In P As solid solutions grown by equilibrium cooling growth technique was performed. Theoretical data were compared with experimental data on LPE of the material. The dependencies of layer thickness, lattice mismatch variation between the substrate and layer and energy gap in the layers lattice matched to InP were calculated for different growth temperatures.     

Effect of the Temperature Fluctuation of the Melt on β‐BaB2O4 (BBO) Crystal Growth

In this paper the effect of the growth temperature fluctuation, for instance, the transient furnace temperature variation due to a short‐term electric power supply interruption on BBO crystal growth was investigated based on the theory of temperature wave transmitting in melt and the boundary layer theory of melt. It was found that the critical width of the temperature pulse to avoid the temperature wave penetrating through the boundary layer and reaching to the growth interface at a constant rotation speed (9∼4 r/min) is 69∼150 s and the corresponding amplitude of the temperature pulse is high more than 60 °C due to the large thickness of the velocity boundary layer of the melt. This result indicates that a small transient temperature fluctuation has no significant effect on the crystal quality, and therefore implies that not only transport processes but interface growth kinetics, a two‐dimensional nucleation growth mode at the interface may also dominate the crystal growth.     

Secondary ion mass spectrometry of compositional changes in garnet films

Secondary ion mass spectrometry (SIMS) has been used as the analytical method to obtain directly depth concentration profiles of Pb (Ca and Ga) in magnetic garnet films. The films had the general composition (Y,Pb)₃(Fe,Si,Ga)₅O₁₂ and were grown onto substrates of Ca-doped Y₃Fe₅O₁₂ by means of the LPE technique. The film-substrate interface was found to be reasonably sharp. For dipping without rotation, a transient layer of about 0.28 μm thickness, in which the Pb content changed appreciably, was found arising from the initial non-steady state growth regime. In the case of dipping with rotation, this transient layer was of the order of 0.05 μm and displayed much smaller changes in the Pb content. Changes were also found at the surface of the film due to withdrawal of the specimen from the melt.     

Effect of bismuth on liquid-phase epitaxy (LPE) grown GaAs layer using Ga–As–Bi melt

GaAs epitaxial layers of high structural quality have been realised from Ga–As–Bi melt using liquid-phase epitaxy (LPE). LPE grown GaAs epitaxial layer using bismuth solvent on GaAs substrate has been found to be of good structural perfection as compared to layers using gallium solvent. The temperature-dependent PL spectra of GaAs layer, grown from Ga+Bi mixed solvent has shown that the use of bismuth does not change the band energy. ECV depth profile of heavily zinc-doped epitaxial layer shows uniform doping in the GaAs layer grown using gallium solvent as compared to the layer grown using bismuth solvent.     

Growth of Zn3As2 on GaAs by liquid phase epitaxy and their characterization

Zn₃As₂ epitaxial layers were grown on GaAs (1 0 0) substrates by liquid phase epitaxy (LPE) using Ga as the solvent. Zinc mole fraction in the growth melt was varied from 1.07×10^?2 to 6×10^?2. X-ray diffraction spectrum exhibits a sharp peak at 43.3° characteristic of Zn₃As₂ crystalline layer. The peak intensity increases with increase in zinc mole fraction in the growth melt. The compositions of the as-grown Zn₃As₂ layers were confirmed by energy dispersive X-ray (EDX) analysis. Surface morphology was studied using scanning electron microscopy (SEM) and the thickness of the epilayers was also determined. The Hall measurements at 300 K indicate that Zn₃As₂ epilayers are unintentionally p-doped. With an increase of zinc mole fraction in the growth melt, carrier concentration increases and carrier mobility decreases. Infrared optical absorption spectroscopy showed a sharp absorption edge at 1.0 eV corresponding to the reported band gap of Zn₃As₂.     

GaAs LPE growth and its application to FET

Residual impurities and deep levels in the LPE GaAs layers grown by a sliding boat method were studied. Residual impurities were investigated by monitoring oxygen and water vapor contents in the exhaust gas during heat treatment. The results are satisfactorily explained by assuming oxygen as a dominant residual impurity. Electron traps with a density higher than 5 × 10¹² cm^-3 were not observed in the LPE layers, whereas in VPE layers, 0.82 eV electron traps were always observed. LPE double layers (high purity buffer layer and active layer) were fabricated into FET's. GaAs FET's with a 1 μm gate showed no hysteresis loops in the I–V characteristics and had fairly good high-frequency characteristic (f_max = 70 GHz, NF = 2.4 dB at 10.4 GHz).     

Hg-rich liquid-phase epitaxy of Hg1−xCdxTe

Over the past decade, liquid-phase epitaxy (LPE) has become an established technique for the growth of HgCdTe. This article reviews one of the successful LPE technologies developed for HgCdTe, specifically, “infinite-melt” vertical LPE (VLPE) from Hg-rich solutions.

In spite of the relatively low solubility of Cd in Hg-rich solutions and the relatively high Hg pressure at the usual growth temperatures, this approach has been found to offer superior results for growth of HgCdTe suitable for various compositions and layer structures.

An historical perspective and the current status of VLPE technology are presented. Particular emphasis is placed on the important role of the thermodynamic parameters (phase diagram), on control of stoichiometry (defect chemistry) and on impurity doping (distribution coefficient) for growth of HgCdTe layers from Hg solutions. Critical material characteristics, such as transport properties, minority-carrier lifetime, morphology and crystal structure, are also discussed.     

Study of the metastable region in the growth of GaN using the Na flux method

It was revealed that the metastable region, in which liquid phase epitaxy (LPE) of GaN single crystals proceeds without the generation of polycrystals, expands with growth temperature in the Na flux method. The metastable region appears when LPE growth proceeds at temperatures above 1073 K, although generation of polycrystals inevitably occurs on a crucible at temperatures less than 1073 K. The highest growth rate of 14 μm/h in a small experimental setup was achieved at a temperature of 1163 K with a nitrogen pressure of 5.5 MPa due to complete suppression of the growth of polycrystals on a crucible, even though the supersaturation at this condition reached a fairly high level.Also, an LPE crystal with a flat surface could easily be obtained under high-temperature conditions.     

Inversion of conductivity type of III‐V compound layers in experiments with LPE growth from rare‐earth containing melts

Rare‐earth (RE) elements present in the growth melt of the LPE process are known to have a purifying effect on the grown layers of III‐V compounds. The RE atoms exhibit high chemical affinity preferentially to shallow donors, forming insoluble aggregates that remain in the melt and do not, ordinarily, enter the solid phase. The aim of the paper is to simulate the situation, sometimes observed experimentally, where the gradual gettering of donor impurity, consequent upon increasing the RE content in the melt, leads to an inversion of the electrical conductivity type of the grown layer from n to p. Usefulness of the approach is demonstrated by interpreting results of an experimental work. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm² was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.     

A new substrate holder for liquid-phase epitaxy by the dipping method and its application to Hg1-xCdxTe

This paper describes a substrate holder for liquid-phase epitaxy (LPE) by the dipping method and its use on Hg_1-xCd_xTe LPE. The holder consists of a shaft, a pivotable plate, and a paddle that is fed through the shaft. This holder automatically places the substrate in a horizontal position during dipping and in a vertical position during withdrawal ensuring uniform film growth and good melt drainage. The holder has been successfully used on the liquid-phase epitaxy of Hg_1-xCd_xTe and excellent results were obtained.     

A new graphite boat construction for the LPE growth of thin GaAs layers with a new technique

The characteristics of planar devices made of thin epitaxial GaAs layers on semiinsulating substrates, such as Gunn efect devices and MeSFETs, sensitively depend on the quality of the interface between the epitaxial layer and the substrate. The origin of an n-type, low resistivity interface layer between the epitaxial layer and the semiinsulating substrate is the semiinsulating substrate itself. The measured changes in the substrate-surface layer characteristics occur during the pregrowth heat treatment of the substrate in purified hydrogen. One solution of the problem could be shortening of the pregrowth heat treatment, but a low background doping concentration and a layer surface without any morphological defects only can be obtained after a relatively long pregrowth heat treatment of the melt. In this paper the technique of pregrowth melt preparation by heat treatment of the melt and the melt-back of the substrate prior to the growth is described. A new turnable sliding boat is presented which allows the use of as many melts as necessary for the required device (multilayer structures). Results (change of the electron concentration at the interface epitaxial layer-semiinsulating substrate, trap concentration, Hall mobility) and device characteristics (Gunn device under pulse and dc conditions and MeSFET) obtained with the improved technology are presented.