Liquid phase epitaxial growth and assessment of Pb1−xSn x Te alloys

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3.     

Thick crack-free CaF2 epitaxial layer on GaAs (100) substrate by molecular beam epitaxy

The crystallographic orientation of low temperature (LT) grown CaF₂ on GaAs (100) substrates is investigated. LT epitaxial (100) CaF₂ layers are obtained on a thin (100) oriented CaF₂ template at growth temperatures down to room temperature. This makes it possible to grow crack-free CaF₂ (100) using a multiple-temperature-growth scheme at any desired temperature. The resulting CaF₂ layers, with thickness up to 680 nm, can withstand temperature cycling from RT to 650°C without cracking. Based on these results, a four pair Ga_0.5Al_0.5As/CaF₂ quarter-wavelength Bragg reflector was fabricated with center wavelength at 880 nm. The reflector, with a total CaF₂ thickness of 615 nm, shows broadband high reflectivity with a crack-free surface. This crack-free surface can then be overgrown with further device layers.     

Errata

The liquid-phase epitaxial growth of Pb_1−xSn_x Te on PbTe (100) substrates has been investigated over a range of growth temperatures from 600-400°C, and has been found to produce material with good uniformity and reproducibility of carrier concen-tration and alloy composition. The assessment of the epitaxial layers by such techniques as x-ray diffraction, dislocation etching and thermo-electric power measurements is described. Various features of the epitaxial layers such as interface irregularity, dislocation and diffusion effects are discussed, and likely mechanisms for their existence are proposed. The hole concentrations of the epitaxial layers, obtained by thermoelectric power measurements, are shown to have a similar dependence on preparation temperature as for bulk annealed material, suggesting that native defects are the dominant source of carriers above~ 2×10* cm^-3. The online version of the original article can be found at     

Variation of the thickness and composition of lpe InGaAsP,InGaAs, and InP layers grown from a finite melt by the step-cooling technique

Constant composition InGaAsP and InGaAs epitaxial layers can be grown using the step-cooling technique. However, the requirement of a fixed growth temperature limits the maximum thickness that can be obtained. The thickness of InGaAsP (λ_g = 1.15 μm@#@), InGaAs (λ_g = 1.68 μm), and InP liquid phase epitaxial layers grown on (100) InP sub-strates by the step-cooling technique has been measured as a function of growth time. (λ_g is defined as the wave-length corresponding to the band gap of the epitaxial layer). For long growth times, the effect of the finite growth solution becomes important, and beyond a distinct growth time, constant composition growth can no longer be maintained. The maximum constant composition layer thick-ness obtainable is not severely restricted by the fixed growth temperature, and from the experimental results this maximum thickness can be estimated for any melt size.     

Metalorganic vapor phase epitaxy of (100) CdZnTe layers using diisopropylzinc source

Growth characteristics of (100) Cd_1−xZn_xTe (CZT) have been studied using metalorganic vapor phase epitaxy. CZT layers were grown on (100) GaAs substrates using diisopropylzinc (DiPZn), dimethylcadmiun (DMCd), and diethyltelluride (DETe) as precursors. Growths were carried out in the temperature range from 375 to 450°C. Since DiPZn has lower vapor pressure than DMCd, CZT layers with Zn composition below 0.06 were grown with good compositional control. Layers with uniform Zn composition and thickness over an area of 10 × 15 mm² were grown. Enhancement of CZT growth rate was observed when a small amount of DiPZn is introduced under fixed flows of DMCd and DETe. Zn composition increases abruptly for further increase of DiPZn flow rate, where growth rate decreases. Growth mechanisms for the above growth conditions were also discussed.     

Resonance characteristics of Y1 Ba2 Cu3 O7-8 superconductor ceramic coil

This research is concerned with Y₁ Ba₂ Cu₃ O_7-8 a superconductor ceramics. This ceramic pressed into a cylindrical form was cut into a wire coil by manual work and the Q-value as against frequency was measured in parallel resonance of a 5 turn coil. When the coil was cooled below the critical temperature, the electrical resistance of the superconductor vanished and the Q-value increased by about 9 times from 13 to 112 for a parallel capacity of 100 pF. The inductance of the coil was about 0-201μ H for a frequency of 25-2 MHz     

Lattice constant,bandgap, thickness,and surface morphology of InGaAsP-InP layers grown by step-cooling,equilibrium-cooling,supercooling and two-phase-solution growth techniques

Constant composition InGaAsP epitaxial layers can be grown on (100) InP substrates at a constant temperature using the diffusion-limited step-cooling growth technique, and in general, compositionally graded layers result when the diffusion-limited equilibrium-cooling, supercooling, and two-phase-solution growth techniques are used. The lattice constant and energy gap of the epitaxial layers grown using the step-cooling technique are nearly independent of small variations of X _p ^l and the amount of step cooling, but are dependent on growth temperature. The dependence of lattice constant and energy gap of the epitaxial layers on X _Ga ^l and X _As ^l has been determined for the step-cooling and supercool ing techniques.     

Hydrogen transport vapor phase epitaxy of CdTe on hybrid substrates for x-ray detector applications

The growth of thick CdTe epitaxial layers by the hydrogen transport vapor phase epitaxy (H₂T-VPE) method is reported for the first time. The thermodynamics of the H₂ transport method of CdTe is analyzed to determine the equilibrium partial pressures of the molecular species in the vapor and its supersaturation as a function of growth conditions. (100)-oriented CdTe epilayers are successfully grown by H₂T-VPE on hybrid ZnTe/GaAs(100) substrates prepared by metalorganic vapor phase epitaxy. Growth rates up to 10 μm/h are obtained at temperatures ∼760°C and with the CdTe source temperature at 827°C. The achievement of even higher growth rates can be foreseen by using the present method under slightly different conditions; several hundreds micron thick CdTe layers can be thus grown by the H₂T-VPE. CdTe samples have mirror-like, nearly featureless surfaces. Also, CdTe epilayers have shown a medium-to-high resistivity at room temperature, possibly as a result of compensation by donor impurities diffusing from GaAs. Still the growth of highly resistive layers by in-situ chlorine doping during the H₂T-VPE growth is possible. In summary, H₂T-VPE is a potential alternative to traditional melt- and vapor-growth methods for the synthesis of detector-grade CdTe for application to the 1–100 keV x-ray energy range.     

Growth by CVT and characterization of Hg1-xCdxTe epitaxial layers

Single crystal epitaxial layers of Hg_1-x Cd _x Te were grown on CdTe substrates employing the chemical vapor transport technique. Different growth temperatures, substrate orientations, and various pressures of Hgl₂ as a transport agent were used while the source materials had a fixed composition ofx = 0.2. The epilayers are of nearly uniform composition to a depth of about one-half of the layer thickness. Chemical etching of the as-grown epilayers revealed low etch pit densities in the range of 10³–10⁴ cm⁻². Rectangle-shaped etch pits are observed for the first time on the (100) oriented epilayers of this material. The growth temperature and Hgl₂ pressure used for the growth experiments have significant effects on the layer morphology and composition.     

The properties of Si/Si1-xGex films grown on Si substrates by chemical vapor deposition

A growth parameter study was made to determine the proper of a SiGe superlattice-type configuration grown on Si substrates by chemical vapor deposition (CVD). The study included such variables as growth temperature, layer composition, layer thickness, total film thickness, doping concentrations, and film orientation. Si and SiGe layers were grown using SiH₄ as the Si source and GeH₄ as the Ge source. When intentional doping was desired, diluted diborane for p-type films and phosphine for n-type films were used. The study led to films grown at ∼1000°C with mobilities from ∼20 to 40 percent higher than that of epitaxial Si layers and ∼100 percent higher than that of epitaxial SiGe layers grown on (100) Si in the same deposition system for net carrier concentrations of ∼8x10¹⁵ cm^-3 to ∼2x10¹⁷ cm^-3. Enhanced mobilities were found in multilayer (100)-oriented Si/Si_1-xGe_x films for layer thicknesses ≥400A, for film thicknesses >2μm, and for layers with x = 0.15. No enhanced mobility was found for (111)-oriented films and for B-doped multilayered (100)-orlented films. Supported in part by NASA-Langley Research Center, Hampton, VA, Contract NAS1-16102 (R. Stermer & A. Fripp, Contr. Mon.)     

Supercooling studies and LPE growth of Hg1−xCdxTe from Te-Rich solutions

Hg_1-xCd_xTe liquid phase epitaxial (LPE) layers were grown from well-stirred large (100 g) Te-rich Hg-Cd-Te solutions by the dipping method. Supercooling below the liquidus temperature in Te-rich solutions was studied by differential thermal analysis (DTA) and film growth results. Although supercooling of 20 to more than 100° C was routinely measured in small (2 g) sample melts, supercooling in larger melts (>100 g) was erratic and smaller. Factors affecting the degree of supercooling were identified and a Hg-reflux was found to be a major cause of erratic melt behavior. The LPE reactor was modified to correct the Hg-reflux action and a visual technique was developed for in situ determination of the liquidus temperature. A limited amount of supercooling was found in the melt after reactor modification but it was difficult to maintain for extended durations before spontaneous nucleation occurred. Consequently, programmed cooling rather than isothermal LPE was employed to grow many of the films reported here. Hg_1−xCd_xTe epitaxial layers ofx = 0.2 to 0.25 were grown on (111)B oriented CdTe substrates by cooling the melts only 1–2° C below the previously measured crystallization temperature. The small amount of cooling minimized composition variation with film thickness. Excellent surface morphology was obtained when slow cooling rates of 0.02–0.05° C/ min were used. Cooling rates greater than 0.2° C/min created rough, pitted surface. Precise substrate orientation was important in reducing surface terracing. Composition and thickness uniformities of the epitaxial films were excellent as a result of substrate rotation. Run-to-run reproducibility of film composition was ±0.01 inx. Hall measurements showed carrier concentrations in the range 2–20 × 10¹⁴ cm⁻³ with photoconductive lifetimes of 0.5–3.0 dms forx = 0.20 to 0.25.     

Infrared absorption and far-infrared diffuse reflection spectra in Pb-doped Bi−Sr−Ca−Cu−O high-temperature superconductor

In this paper we study the room-temperature infrared transmission spectra (400–1600cm^?1) and far-infrared diffuse reflection spectra (50–450cm^?1) in Pb-doped Bi?Sr?Ca?Cu?O (2223) single phase (Tc=107k, sp1), multiphase (Tc=110k, sp2) and nonsuperconducting samples (sp3). The spectral features in superconductor are totally different from those in nonsuperconductor, which show the different crystal structure. The correlation existing between a factor group analysis of the phonons in (2223) and (2212) compounds affords a tentative assignment of ir-active modes in Pb-doped (2223) single phase by comparison with reported data in (2212) materials. The Cu?O stretching E_u vibration (605cm^?1) of CuO₂ layers is the characteristic vibrational mode related perovskitelike crystal structures. Two phonon coupling effect emerges in the infrared transmission spectra in Pb-doped superconductor. The Ca?O vibration A_2u (254cm^?1) might be related to superconductivity of Bi-based family.     

Silicide formation in co-deposited TiSix layers: The effect of deposition temperature and Mo

The silicide reaction in co-deposited TiSi_x layers on single crystal and pre-amorphized Si has been studied in detail. Both the co-deposition ratio and the co-deposition temperature were found to have a strong effect on the formation of the C54-TiSi₂ phase in these films. An unusual dependence of the sheet resistance on the co-deposition ratio was observed for films deposited at room temperature and those deposited at 400°C: the C54-TiSi₂ phase forms more easily for layers deposited at 400°C in the co-deposition ranges x∼0 and x>1.5, while it forms more easily for layers deposited at room temperature in the co-deposition ratio range of x∼0.2–1.5. These dependencies are explained by the formation of crystalline silicide phase(s) with composition close to the co-deposition ratio. With a Si rich ratio, the C49-TiSi₂ phase forms at 400°C with very small grain size, which facilitates the C54-TiSi₂ phase formation. The initial reaction of Ti-rich layer deposited at 400°C involves the formation of metal rich silicide, which impedes the formation of the C54-TiSi₂ phase. An ultra-thin MoSi_2.0 layer (<0.5 nm) was found to promote the formation of the C54-TiSi₂ phase in layers co-deposited at room temperature, but it showed little effect on layers co-deposited on pre-amorphized substrates at elevated temperature.     

Electrodeposited Gd2Zr2O7 and Gd2O3 Buffer Layers for YBa2Cu3O7-δ Superconductors

Non-vacuum electrodeposition (ED) was used to prepare a biaxially textured Gd₂Zr₂O₇ (GZO) and GZO/Gd₂O₃ (GO) buffer layer on a Ni-W substrate. The YBa₂Cu₃O_7−δ (YBCO) superconductor was deposited by pulsed-laser deposition (PLD) on a simplified ED-GZO and ED-GZO/ED-GO buffer layer. The buffer layers and YBCO superconductor were characterized by X-ray diffraction (including θ/2θ, pole figure, omega scans, and phi scans) and atomic force microscopy. Full-width at half maximum values of the omega (ω) and phi (Φ) scans of the electrodeposited GZO and GZO/GO layer were better than those of the Ni-W base substrate. At 77 K and a self-magnetic field, the critical current density of PLD YBCO on the electrodeposited-based buffer layer was 3.3 × 10⁶ A/cm², using the field criterion of 1 μV/cm.     

Elastic strains in heteroepitaxial ZnSe1−xTex on InGaAs/InP (001)

Here we report on the elastic strains in ZnSe_1−xTe_x (x<0.9) epitaxial layers grown using photo-assisted metalorganic vapor phase epitaxy on In_0.53Ga_0.47As/InP (001) substrates. High-resolution x-ray diffraction was used to determine their composition and strain. At room temperature, we observed an apparent asymmetry in strains for tensile and compressive layers. However, when we accounted for the difference in thermal expansion between the substrate and epitaxial material, the growth temperature strain relaxation appears symmetric with respect to the sign of mismatch. The growth temperature strains are in agreement with the Matthews and Blakeslee (MB) model [J.W. Matthews and A.E. Blakeslee, J. Cryst. Growth 27, 118 (1974)] for both compressive (x>0.6) and tensile (x<0.4) layers. However, for the layers with composition in the range 0.4<x<0.6, the growth temperature strains exceed the values predicted by the MB theory. Apparently, low-mismatch layers experience a kinetic barrier to relaxation. The overall behavior can be fit by the relaxation model of Dodson and Tsao [B.W. Dodson and J.Y. Tsao, Appl. Phys. Lett. 51, 1325 (1987)] using the values Cμ²=80 s⁻¹ and γ₀=10⁻⁹.     

In‐Situ Compositional and Structural Analysis of Plastic Solar Cells

Bulk‐heterojunction photovoltaic cells consisting of a photoactive layer of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) and a C₆₀ derivative, (1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐methanofullerene), (PCBM), sandwiched between an indium tin oxide (ITO) anode covered with poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and an aluminum cathode have been analyzed using transmission electron microscopy (TEM) and cryogenic Rutherford backscattering spectrometry (RBS) to assess the structural and elemental composition of these devices. TEM of cross sections of fully processed photovoltaic cells, prepared using a focused ion beam, provide a clear view of the individual layers and their interfaces. RBS shows that during preparation diffusion of indium into the PEDOT:PSS occurs while the diffusion of aluminum into the polymer layers is negligible. An iodinated C₆₀ derivative (I‐PCBM) was used to determine the concentration profile of this derivative in the vertical direction of a 100 nm active layer.     

High-Vacuum Evaporation of <Emphasis Type="Italic">n</Emphasis>-CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> Photoabsorber Films for Hybrid PV Structures

Thin films of Cu-In-Se (CISe) photoabsorber with an overall composition of CuIn₃Se₅ were deposited onto glass/indium tin oxide (ITO) substrates from a polycrystalline bulk CuIn₃Se₅ source using the high-vacuum evaporation technique. Thermal conditions for the substrates during the evaporation process and the subsequent annealing in vacuum were selected to prepare polycrystalline n-CuIn₃Se₅ photoabsorber layers for use in hybrid photovoltaic structures based on an inorganic photoabsorber and conductive polymer functional layers. The CISe layers were deposited at a substrate temperature of 200°C and were annealed at temperatures from 300°C to 500°C in vacuum. Part of the as-deposited CISe was annealed twice, in argon and in vacuum at 500°C. These layers exhibited high photosensitivity and photoconductivity when illuminated with white light at an intensity of 100 mW/cm². The results showed that the chalcopyrite structure of the prepared CISe photoabsorber films adhered well to the glass/ITO substrate. The average value of charge carrier concentration and the profile of charge carrier concentration in the annealed CISe photoabsorber layer were calculated using impedance spectroscopy.     

The use of metalorganic chemical vapor deposition to prepare device quality Ga(AsP) strained-layer superlattices

Strained-layer superlattices (SLS’s), which consist of thin (<300 Å) alternating layers of Ga(AsP) and GaAs or GaP, have been prepared by metalorganic chemical vapor deposition (MOCVD). Transmission electron microscopy and x-ray diffraction of the SLS’s indicate that the layers are coherently strained and dislocation free. The mismatch between these very thin layers is totally accommodated by strain for misfits of one percent or less. The layer thickness for the binaries and the ternary is controlled by the TMG flow while the solid composition for the ternary is determined by the arsine/phosphine ratio. The solid composition for a fixed arsine/phosphine ratio is a function of temperature and arsine partial pressure. Uniformly doped SLS’s have properties similar to the ternary of the same composition. A photodiode has been prepared from a GaAs_0.2P_0.8/ GaP SLS with a leakage current of 155 × 10^-6 A/cm² at -5V and a quantum efficiency of 40% at 420 nm.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

Fabrication of Bi2Sr2CaCu2O x and Bi2Sr2Ca2Cu3O x Superconductor Thick Films by Using Cu-Free Precursors on Ni Substrates

We studied the formation of Bi₂Sr₂CaCu₂O _x (Bi-2212) and Bi₂Sr₂Ca₂Cu₃O _x (Bi-2223) thick films in a heat treatment process of the Ni-sheathed Bi-Sr-Ca-Cu-O (BSCCO) system. Cu was electrodeposited initially on the Ni substrates (Cu/Ni). Well-oriented Bi-2212 superconductor thick films were formed successfully on Ni tapes by liquid reaction between Cu-free precursors and Cu/Ni tapes. However, only a small amount of Bi-2223 was formed. Thick films were prepared by screen-printing with Bi₂O₃, SrCO₃, and CaCO₃ powders on Cu/Ni tapes and heat treating them. Heat treatment was performed in the temperature range of 750–850°C in a tube furnace for several minutes to hours. The phases and the microstructures of the high temperature superconductor thick films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electrical properties were examined by the standard four-probe method. At the heat treatment temperature, the specimens were in a partially molten state during reaction between the oxidized copper layer and the screen-printed precursors on the Cu/Ni tapes.