Miscibility gap in the GaAsy Sb1−y system

Measurements have been made to determine accurately the solidus curve of the pseudobinary section GaAs_ySb_1−y, by annealing samples to equilibrium and determining compositions by x-ray powder photography. It is found that the equilibriui diagram shows a peritectic form with a peritectic temperature of 745 ± 1‡C and a miscibility gap at that temperature extending from y = 0.38 to y = 0.68. It is also shown that as the temperature is lowered the miscibility gap widens rapidly, being from y = 0.30 to y = 0.95 at 700‡C. The form of these phase boundaries is important when growth of GaAs_ySb_1−y alloys by liquid phase epitaxy or similar techniques is considered.     

Metalorganic vapor phase epitaxial growth of GaInAsP/GaAs

Ga_xAs_yP_1−y lattice matched to GaAs has been grown by low pressure metalorganic phase vapor epitaxy over the entire compositional range. At T_G = 670°C broad peaks of low intensity are observed in the 10K photoluminescence for y = 0.2–0.4 due to the predicted miscibility gap in this compositional region. An increase in growth temperature leads to a smaller miscibility gap. The band gap as well as the morphology show a strong dependence on substrate misorientation. The smoothest GalnAsP surfaces are obtained on exact oriented substrates. For the ternary GalnP the surface roughness is correlated to the degree of ordering in the temperature range of 600 to 750°C. The smallest band gap together with the smoothest surface is obtained on (100) 2° off to (111)B. Ordering effects are also observed in the quaternary GalnAsP. Broad-area lasers processed from the grown layers show high slope efficiency (0.9 W/A) and low internal losses (<3 cm⁻¹).     

Structural properties of ZnSy Se1−yZnSe/GaAs (001) heterostructures grown by photoassisted metalorganic vapor phase epitaxy

ZnS_ySe_1−yZnSe/GaAs (001) heterostructures have been grown by photoassisted metalorganic vapor phase epitaxy, using the sources dimethylzinc, dimethylselenium, diethylsulfur, and irradiation by a Hg arc lamp. The solid phase composition vs gas phase composition characteristics have been determined for ZnSy_ySe_1−y grown with different mole fractions of dimethylselenium and different temperatures. Although the growth is not mass-transport controlled with respect to the column VI precursors, the solid phase composition vs gas phase composition characteristics are sufficiently gradual so that good compositional control and lattice matching to GaAs substrates can be readily achieved by photoassisted growth in the temperature range 360°C ≤ T ≤ 400°C. ZnSe/GaAs (001) single heterostructures were grown by a two-step process with ZnSe thicknesses in the range from 54 nm to 776 nm. Based on 004 x-ray rocking curve full width at half maximums (FWHMs), we have determined that the critical layer thickness is h_c ≤200 nm. Using the classical method involving strain, lattice relaxation is undetectable in layers thinner than 270 nm for the growth conditions used here. Therefore, the rocking curve FWHM is a more sensitive indicator of lattice relaxation than the residual strain. For ZnS_ySe_1−y layers grown on ZnSe buffers at 400°C, the measured dislocation density-thickness product Dh increases monotonically with the room temperature mismatch. Lower values of the Dh product are obtained for epitaxy on 135 nm buffers compared to the case of 270 nm buffers. This difference is due to the fact that the 135 nm ZnSe buffers are pseudomorphic as deposited. For ZnS_ySe_1−y layers grown on 135 nm ZnSe buffers at 360°C, the minimum dislocation density corresponds approximately to room-temperature lattice matching (y ∼ 5.9%), rather than growth temperature lattice matching (y ∼ 7.6%). Epitaxial layers with lower dislocation densities demonstrated superior optical quality, as judged by the near-band edge/deep level emission peak intensity ratio and the near band edge absolute peak intensity from 300K photoluminescence measurements.     

Growth and characterization of lead-tin-ytterbium-telluride for diode lasers

PbSnYbTe is a new and potentially useful material for fabricating double hetero junction PbSnTe diode lasers by molecular b<sam epitaxy. These lasers should have improved carrier and photon confinement, resulting in improved external quantum efficiency and higher device operating temperature. It is found from room temperature optical transmission studies that the band gap of Pb_1−x Yb_x Te increases approximately as dEg/dx =; 3.3 eV for x < 0.04, and that the index of refraction decreases with increasing x. Doping studies of (Pb_1−y Sn_y )_0.97 Yb_0.03Te indicate that> it can be doped heavily p-type at low temperatures for y ∼ 0.10. The lattice constant of (Pb_0.85 Sn_0.15)_1−x Yb_x Te is independent of x up to x ∼ 0.10. these characteristics make (Pb_1−y Sn_y )_1−x Yb_x Te well-suited for the fabrication of lattice-matched double heterojunction lasers with y∼ 0.10.     

Vacuum deposited epitaxial layers of PbSl−xSex for laser devices

This paper describes an epitaxial vacuum deposition system used to grow heterostructure PbS_l−xSe_x diode lasers that operated cw at 12K with threshold current densities as low as 60A/cm₃. The relatively low temperature (300°C) growth process, which simulates closed tube vapor phase growth, minimizes substrate-epilayer strain and vacancy interdiffusion. Laser devices were fabricated by sequential evaporation of n-type and p-type PbS_l−xSe_x layers onto PbS_l−ySe_y substrates. n-type grown layers were sometimes found to have Pb-rich droplets on their surfaces, and a correlation has been made between the presence of these droplets and the starting source material.     

The organometallic VPE growth of GaAs1−ySby using trimethylantimony and Ga1−xInxAs using trimethylarsenic

The organometallic vapor phase epitaxial growth of Ga_1−xIn_xAs and GaAs_1−ySb_y using trimethylarsenic and trimethylantimony as the Group V sources is reported and the relevant chemistry is discussed. Growth rate and composition variations as a function of temperature are given for GaAs_1−ySb_y. A result of particular importance is that no detrimental room temperature gas phase reaction is observed between triethylindium and trimethylarsenic. Consequently, the growths are performed at one atmosphere pressure without the need for any complicated injection schemes in the reactor design. This work was supported by the U.S. Dept. of Energy. First, the concept of III-V growth using column V trialkyls was verified using TMSb for the growth of GaAsSb under Contract No. EY-76-C-03-1250 from the Div. of Materials Sciences Branch of the Office of Basic Energy Sciences. After successful verification, TMAs was tried for the growth of InGaAs, which is supported by the Solar Energy Research Institute under Contract No. XP-9-8081-1.     

Liquidus isotherms,solidus lines and LPE growth in the Te-rich corner of the Hg-Cd-Te system

Liquidus isotherms for the Hg_1−xCd_xTe primary phase field in the Te-rich corner of the Hg-Cd-Te ternary system have been determined for temperatures from 425 to 600‡C by a modified direct observational technique. These isotherms were used to help establish conditions for the open-tube liquid phase epitaxial growth of Hg_1−xCd_xTe layers on CdTe_1−ySe_y substrates. Layers with x ranging from 0.1 to 0.8 have been grown from Te-rich HgCdTe solutions under flowing H₂ by means of a horizontal slider technique that prevents loss of Hg from the solutions by evaporation. Growth temperatures and times of 450–550‡C and 0.25–10 min, respectively, have been used. The growth solution equilibration time is typically 1 h at 550‡C. Source wafers, supercooled solutions, and (111)-oriented substrates were employed in growing the highest quality layers, which were between 3 and 15 Μm thick. Electron microprobe analysis was used to determine x for the epitaxial layers, and the resulting data, along with the liquidus isotherms, were used to obtain solidus lines. In addition to EMP data, optical transmission results are given. This work was sponsored by the Department of the Air Force and the U. S. Army Research Office.     

Comparison of In1−xTlx Sb and Hg1−x Cdx Te as long wavelength infrared materials

Cohesive energies, elastic constants, band structures, and phase diagram are calculated to evaluate the In_1−xTl_xSb alloy (ITA) as a long-wavelength infrared (LWIR) material compared to Hg_1−xCd_xTe (MCT). To obtain a 0.1 eV gap at zero temperature, the x value for ITA is estimated to be x=0.083 as compared to x=0.222 for MCT. At this gap, ITA is more robust than MCT because the cohesive energies order as InSb>TlSb>CdTe>HgTe, and ITA has the stronger bonding InSb as the majority component. Although TlSb is found to favor the CsCl structure, ITA is a stable alloy in the zincblende structure for low x values. However, our phase diagram indicates that it is difficult to grow the 0.1 eV gap ITA from the melt, because above the eutectic the liquidus curve is flat, and the solidus drops rapidly. Moreover, the width of the stable concentration range of the zincblende solid phase shrinks at low temperatures due to the presence of the CsCl structure.     

Phase equilibria of the Si-Ge-Ti system relevant to the reactions between SiGe alloys and Ti

The semiconductor-rich region of the Si-Ge-Ti ternary isotherm at 900°C was determined by metallography, x-ray diffraction, and electron microprobe analysis. The sample alloys were prepared by arc-melting. These alloys were brought to equilibrium by annealing at 900°C for 400 h. It was confirmed that at 900°C, TiSi₂ and TiGe₂ form a continuous solid solution Ti(Si_1−yGe_y)₂ with the C54 crystal structure. It was also shown that, other than Ti(Si_1−yGe_y)₂ and Si_1−xGe_x, there is not any binary or ternary phase within the Si-Ge-TiGe₂-TiSi₂ trapezoid region. Between the Ti(Si_1−yGe_y)₂ and Si_1−xGe_x single-phase fields is the Ti(Si_1−yGe_y)₂-Si_1−xGe_x two-phase region. The tie-lines for this two-phase region were determined. The tie-lines tilt slightly toward the TiSi₂ and Ge corners. In other words, at equilibrium, the silicon to germanium atomic ratio is larger in Ti(Si_1−yGe_y)₂ than in Si_1−xGe_x (x>y). This tendency for tie-lines to tilt toward the TiSi₂ and Ge corners had been proposed in the literature as the reason for the interesting microstructure evolution during the reactions between SiGe alloys and Ti. In addition, the possible diffusion paths for the reactions between SiGe alloys and Ti were discussed based on the obtained isotherm. Recognizing Si and Ge have higher mobilities in Ti(Si_1−yGe_y)₂, it is predicted that for SiGe the extent of concentration change is large but occurs over a shorter distance, and for TiSi₂ the extent of concentration change is small but occurs over a longer distance.     

Growth and characterization of lattice-matched epitaxial films of GaxIn1−xAs/InP by liquid-phase epitaxy

We determined the conditions for successful lattice-matched growth by liquid-phase epitaxy near T = 620‡ C of Ga_XIn_1−XAs on [111B] InP substrates. We have used the results of the growth of both lattice-matched and intentionally lattice-mismatched epitaxial layers, (0.4 ≤ X ≤ 0.7) to calculate a phase diagram which gives the correct liquidus temperature, (T_L ± 1‡ C), and the correct solid composition, (± 5 % of the nominal composition), for the entire range of growth solutions considered for this important ternary semi-conductor system. The parameters appropriate to this calculation are significantly different from those used to describe the growth of Ga_XIn_1−XAs on GaAs. The results of this calculation play an important part in the better understanding of the quaternary alloy Ga_XIn_1−XAs_yP_1−y. Our measurements show that the ternary alloy lattice-matched to InP is Ga_0.47In_0.53As, semiconductor with a direct band gap about 0.75 eV at room temperature. We have grown p-n junction homostructures and double-heterostructures on InP substrates. These wafers have been used to make detectors in the 1.0 – 1.7/um range of the optical spectrum.     

On-line growth monitoring of InP-based device structures by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) has been used to study the metalorganic vapor phase epitaxy growth process for Ga_xIn_1−xAs_yP_1−y/InP light emitting diodes. The sensitivity of RAS to morphology changes is demonstrated by InP growth on different InP:Fe substrates. RAS reveals not only development of dull surfaces but also detects initial temporary roughness of mirror-like layers. Based on the RAS results the substrate preparation was optimized. RAS spectra measured on n- and p-type InP and p-type GaInAsP during light emitting diodes production are suitable for finger-printing of the growth process. Spectra from InP:Si and InP:Zn layers show characteristic features near 4.3 eV which allow for assessment of doping level at growth temperature (640°C). Correlation of RAS spectra and transients during growth with the quaternary composition was achieved. A change in composition of only Δx=0.01, Δy=0.03 corresponding to a shift of photoluminescence-peak position by 16 nm was detectable in RAS spectra. The results demonstrate the high sensitivity and thus the suitability of RAS for on-line control during growth of device structures.     

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of In_xGa_1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk In_xGa_1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.     

Application of urbach rule optical absorption to composition measurement of Cd1−yZnyTe

The optical absorption coefficient of Cd_1−yZn_y Te near the fundamental band edge was measured at room temperature using transmission spectroscopy. Like in other II–VI semiconductors, it was found that the absorption coefficient exhibits an exponential dependence on incident photon energy according to Urbach’s rule. It was also found that the exponential parameters depend on composition, y, of Cd_1−y Zn_yTe. A technique is described for determining the composition of Cd_1−y Zn_yTe from optical transmission spectroscopy. This technique has been implemented in the manufacturing of Cd_1−yZn_yTe substrates for lattice matched epitaxial growth of HgCdTe.     

Engineering phase formation thermo-chemistry for crystal growth of homogeneous ternary and quaternary III–V compound semiconductors from melts

Based on intrinsic alloy phase formation chemistry and thermodynamics, a novel and unique way of producing compositionally homogeneous multi-component (binary, ternary, quaternary) semiconductor materials is presented. A free energy minimization computer program licensed from AEA Technology Engineering Software, Inc., has been employed to study the composition of the solidifying phases from Ga-In-As-Sb melts at different temperatures and with various liquid compositions. The solid phases have been identified (theoretically and experimentally) to be either ternary compounds of Ga_1−xIn_xAs_ySb_1−y depending on the melt temperature and composition. By engineering the thermochemistry of preferential phase formation in the Ga-In-As-Sb melt, compositionally uniform, single phase, crack free, large polycrystalline Ga_1−xIn_xSb and Ga_1−xIn_xAs have been grown.     

Spinodal decomposition and clustering in III/V alloys

The criteria for clustering and spinodal decomposition in III/V pseudobinary and quaternary solid alloys are examined. A chemical driving force for clustering and phase separation exists in some ternary and most quaternary alloys. However, single crystalline alloys are shown to be stabilized by the coherency strain energy inherent in any clustering or spinodal decomposition in alloys where lattice parameter is a function of composition. Analytical expressions are derived for T_s, the temperature above which no clustering or phase separation should occur. Most III/V pseudobinary and quaternary alloys are stable at all temperatures. Numerical techniques are used to calculate spinodal isotherms. Results are presented for the systems Ga_xIn_1−x As P_1−y, A1_xGa_1−xAs_ySb_1−y, and Ga_xIn_1−x As P_1−y. This work was supported by the Department of Energy, contract No. DE-AT 03-81 ER 10934.     

Vapor phase equilibria in the Cd1−xZnxTe alloy system

Cd_1−xZn_xTe compounds of different compositions have been prepared at temperatures ranging from 400 to 1000°C by annealing elemental Te in sealed quartz ampoules, in an atmosphere comprising vapors of Cd and Zn whose partial pressures were varied by varying the composition of the binary Cd_1−yZn_y alloys which provided the Cd and Zn vapors in these annealing experiments. The chemical compositions of the resulting Cd_1−xZn_xTe compounds have been analyzed using electron probe microanalytical techniques. Results indicate that presence of a 0.5%Zn along with Cd in a closed or semi-closed system may prove to be beneficial in preventing decomposition and/or formation of a metal/non metal phase during annealing of Cd_0.96Zn_0.04 Te substrates. Using the thermodynamic data in the literature for the binary Cd_1−yZn_y alloys and with the assumption that the activities of the Cd and Zn components are weakly dependent on temperature, the partial pressures of Cd and Zn in equilibrium with the Cd_1−xZn_xTe compounds at various temperatures have been evaluated.     

Step structure of GaInAsSb grown by organometallic vapor phase epitaxy

The surface step structure of Ga_1−xIn_xAs_ySb_1−y grown by organometallic vapor phase epitaxy on GaSb substrates has been studied by atomic force microscopy. Epilayers were grown at 525°C and 575°C on (001) GaSb substrates misoriented 2° toward (101) or 6° toward (1 1)B. For Ga_0.88In_0.12As_0.1Sb_0.9 grown at 575°C, the surface exhibits step-bunching on both types of substrates. When the composition is increased to Ga_0.86In_0.14As_0.12Sb_0.88, the periodic step structure breaks down and the surface becomes irregular. The deterioration of the step structure is a consequence of phase separation at the surface of the metastable GaInAsSb epilayer, which leads to the formation of GaAs- and InSb-rich regions. The photoluminescence (PL) of such layers show significant broadening due to carrier recombination in the lower energy gap InSb-rich quaternary regions. On the other hand, the surface of GaInAsSb epilayers grown at a lower temperature of 525°C is vicinal with steps heights of one to two monolayers. The PL FWHM values are considerably smaller for these layers. This improvement in material quality is related to smaller adatom lifetimes at the lower growth temperature. The importance of surface kinetics as it influences the step structure and thermodynamically driven phase separation is discussed.     

Liquid phase epitaxial growth and characterization of InAsxSb1−;x AND In1−yGaySb ON (111)B InSb substrates

Liquid phase epitaxial growth of InAs_xSb_1−x, for 0<_x<0.27 and In_1−yGa_ySb, for 0<y<0.37, has been successfully accomplished on (111)B InSb substrates between the temperatures of 450 and 520°C. The phase diagrams and the growth conditions for high-quality planar epitaxial layers have been determined. For growth of InAs_xSb_1−x for high values of x, the strong tendency of the ternary melt to dissolve the substrate, even when the liquid is a few degrees below its melting point, was negated by using large supercooling. Small supercooling of zero to 5.6°C were required over the whole range of composition examined for (In.Ga)Sb, whereas, for example, supercooling greater than 30°C was required to grow InAs_o.26Sb_o.74 to avoid substrate dissolution. Lattice mismatch to the substrate was relieved by compositional grading. Etch pit studies in both materials yielded dislocation densities ranging from 5.8 × 10² to 2×10⁶ cm⁻² with most materials in the low 10⁴ range. Hall and resistivity measurements performed at 300K and 77K on most samples showed an impurity contamination of the epitaxial layers. Some samples were n-type (carrier concentration approximately 10¹⁷/cm³), with varying degrees of acceptor compensation and others were n-type (carrier concentration approximately l0¹⁷/cm³) at room temperature due to intrinsic conduction, but exhibited p-type conduction (carrier concentration approximately 5×l0^l6/cm³) at 77K. Hall measurements performed on one of the latter samples ofvery low As content from 77K to 4.2K to examine hole freeze-out yielded an acceptor level ionization energy of 0.0126eV which is close to the effective mass acceptor level ionization energy in InSb. The electron-to-hole mobility ratio was also found to be 65.9. Electron microprobe analysis showed silicon to be the dominant impurity.     

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.     

The phase transformation of Bi-Pb-Sr-Ca-Cu-O bulk materials are rapidly melted and solidified by a CO2 laser

The phase transformation of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O_y bulk materials rapidly melted and solidified by a CO₂ laser with the scanning speed of 40 mm/s were investigated. Results of x-ray diffraction pattern, scanning electron microscopy and energy-dispersive x-ray analysis showed the decomposition of the high-T_c phase in the laser irradiated region. Nonsuperconducting phases such as CaO and (Sr_1−xCa_x)CuO_y were found to be in the melting zone. On the other hand, (Sr_1−xCa_x)CuO_y and 2212 phase were also found in the heat-affected zone. When the irradiated samples were treated with 835‡C for 72 h in air, the laser treated region changed into the high-T_c as a major phase, in addition to the low-T_c phase and nonsuperconducting phase. However, the high-T_c phases are piled up randomly. The transport critical-current density of the laser treated samples after annealing is lower than that of the original sintered one, i.e. at 77K and zero magnetic field.