Thermodynamic stability of bulk and epitaxial CdHgTe,ZnHgTe, and MnHgTe alloys

The thermodynamic stability of Cd_1?xHg_xTe, Mn_xHg_1?xTe, and Zn_xHg_1?xTe alloys is studied. Calculations performed in the context of the δ lattice-parameter model indicate that CdHgTe and ZnHgTe alloys are stable over the entire range of compositions at typical growth temperatures. At the same time, a miscibility gap is found in Mn_xHg_1?xTe at 0.33 < x < 1 at T = 950 K, which is consistent with the known experimental data. It is shown that the biaxial strains observed in Mn_xHg_1?xTe/CdTe and Mn_xHg_1?xTe/Cd_0.96Zn_0.04Te thin epitaxial films lead to a narrowing of the miscibility gap and to insignificant lowering of critical temperatures.     

Thermodynamic stability of bulk and epitaxial Ge1−x Snx semiconductor alloys

The total energy for an unconstrained and biaxially confined model of Ge_1?x Sn_x alloys was calculated using molecular-dynamics simulation. This made it possible to study the thermodynamic stability of both disordered and ordered phases of the semiconductor alloys. A remarkable suppression of the phase separation in Ge_1?x Sn_x due to biaxial strain was found.     

Step structure during organometallic vapor phase epitaxial growth of ordered GaInP

The nature of the steps on the nominally (00l)-oriented surface of Ga_0.5In_0.5P lattice matched to GaAs has been studied using high resolution atomic force microscopy. The layers were grown by organometallic vapor phase epitaxy (OMVPE) at a temperature of 620°C on substrates misoriented by angles,?_m, from 0 to 9° toward the $[\bar 110]$ direction in the lattice. An array of bunched steps from 25 to 50Å in height, depending on the substrate misorientation angle, is observed on the surface. An unusual feature of these bunched or super-steps, as compared to those seen for GaAs surfaces, is that they have relatively short lengths of a few thousand Angstroms. In addition, not all of the steps congregate into the surface steps. Thus, the surface consists of three “phases”: (001) flats, (11n) facets, and misoriented areas covered by an array of monolayer steps. The fraction of steps contained in the supersteps decreases monotonically as ?_m increases from 3 to 9°. Again, this differs from reports of the nature of GaAs surfaces grown under similar conditions where essentially all of the steps congregate into supersteps. The value of n for the (lln) facets also varies with misorientation angle: The angle between the (001) and the (lln) facets increases from approximately 11-12° for ?_m = 3° to nearly 30? for ?_m = 9°. An attempt was made to correlate the surface structure with ordering, which is observed to vary significantly with misorientation angle. The degree of order is found to increase monotonically with the fraction of steps forming supersteps.     

InSb,GaSb, and GaInSb grown using trisdimethylaminoantimony

GalnSb alloys as well as the constituent binaries InSb and GaSb have been grown by organometallic vapor phase epitaxy using the new antimony precursor trisdimethylaminoantimony (TDMASb) combined with conventional group III precursors trimethylindium (TMIn) and trimethylgallium (TMGa). InSb layers were grown at temperatures between 275 and 425°C. The low values of V/III ratio required to obtain good morphologies at the lowest temperatures indicate that the pyrolysis temperature is low for TDMASb. In fact, at the lowest temperatures, the InSb growth efficiency is higher than for other antimony precursors, indicating the TDMASb pyrolysis products assist with TMIn pyrolysis. A similar, but less pronounced trend is observed for GaSb growth at temperatures of less than 500°C. No excess carbon contamination is observed for either the InSb or GaSb layers. Ga_1-xIn_xSb layers with excellent morphologies with values of x between 0 and 0.5 were grown on GaSb substrates without the use of graded layers. The growth temperature was 525°C and the values of V/III ratio, optimized for each value of x, ranged between 1.25 and 1.38. Strong photoluminescence (PL) was observed for values of x of less than 0.3, with values of halfwidth ranging from 13 to 16 meV, somewhat smaller than previous reports for layers grown using conventional precursors without the use of graded layers at the interface. The PL intensity was observed to decrease significantly for higher values of x. The PL peak energies were found to track the band gap energy; thus, the luminescence is due to band edge processes. The layers were all p-type with carrier concentrations of approximately 10¹⁷ cm³. Transmission electron diffraction studies indicate that the Ga_0.5In_0.5 Sb layers are ordered. Two variants of the Cu-Pt structure are observed with nearly the same diffracted intensities. This is the first report of ordering in GalnSb alloys.     

On the growth,structure, and surface morphology of epitaxial CdTe films

The structure and surface morphology of epitaxial CdTe films grown on glassy substrates with and without compensation with an additional Te vapor source during growth are studied. The optimal conditions of the production of structurally perfect epitaxial films with a pure smooth surface with no inclusions of another phase (T _so = 1000–1100 K, T _su = 570–670 K) are determined. It is established that, on glassy substrates, the epitaxial films grow via the (111) plane of the face-centered cubic (fcc) lattice with the parameter a = 6.481 Å. By varying the temperature of the main and compensating sources, CdTe films with n- and p-type conductivity are produced.     

Surface morphology of silicon carbide epitaxial films

Silicon carbide (SiC) semiconductor technology has been advancing rapidly, but there are numerous crystal growth problems that need to be solved before SiC can reach its full potential. Among these problems is a need for an improvement in the surface morphology of epitaxial films that are grown to produce device structures. Because of advantageous electrical properties, SiC development is shifting from the 6H to the 4H polytype. In this study of both 6H and 4H-SiC epilayers, atomic force microscopy and other techniques were used to characterize SiC epilayer surface morphology. Observed features included isolated growth pits a few micrometers in size in both polytypes and triangles (in 4H only) approximately 50 um in size for epilayers 3 um in thickness. Also observed in some epilayers were large steps with heights greater than 20 nm. We found that there are significant differences between the morphology of 6H and 4H epilayers grown under identical conditions. We were able to improve surface morphology by avoiding conditions that lead to excess silicon during the initial startup of the growth process. However, the observed morphological defect density in both 6H and 4H epilayers was still the order of 10⁴ cm^-2 and varied widely from run to run. As expected, we found that morphological defects in the SiC substrates play a role in the formation of some epilayer surface features.     

Characterization of vapor phase epitaxial ZnSe films

Properties of ZnSe films doped with donor impurities were investigated. The ZnSe films were grown at 350°C by using metallic zinc and selenium as the source materials; their vapors were transported separately by H₂ gas under atmospheric pressure. Iodine-doped ZnSe films were grown using CH₃I (1000 ppm, diluted in helium) as a dopant source. However, it was necessary to stop this dopant flow during the film growth to obtain epitaxial films. HC1 gas etching and evacuation of the reaction apparatus before the film growth began were employed to obtain epitaxial films and to avoid redistribution of impurities without heat-treatment at higher temperature. Secondary ion mass spectroscopy analysis indicated that both chlorine and gallium were included in the layers, as well as iodine, because of residual HC1 gas. Optically high-quality and rather highly conductive n-type ZnSe films were obtained. Maximum electron concentration was 3.3 × 10¹⁷ cm⁻³.     

The growth, structure, and magnetic properties of thin epitaxial films of GdMnO3 multiferroics

Thin epitaxial gadolinium manganite (GdMnO₃) films were manufactured on single-crystal substrates of neodymium gallate (001) (NdGaO₃) and strontium titanate (001) (SrTiO₃) by the high-frequency magnetron sputtering method. The RBS analysis demonstrated that the thickness of the obtained films was ??100 nm and the chemical composition corresponded to the stated stoichiometry. It was established by the X-ray diffraction analysis of the structure and phase composition of the obtained films that all films were single-phase but, depending on the temperature of the substrate during sputtering they have one or several types of orientations relative to the substrate. The X-ray diffraction analysis and high-resolution electron microscopy data on gadolinium manganite films on neodymium gallate substrates were verified. Features pointing to phase transitions in GdMnO₃ that were earlier discovered on bulk single-crystal samples were found in the temperature dependence of the magnetic susceptibility.     

GaInSb单结热光伏电池的设计与优化

使用Silvaco/Atlas软件设计、模拟并优化了GaInSb/GaSb单结热光伏电池,研究了器件材料厚度和掺杂浓度对电池性能的影响。主要从最大输出功率(Pm)、开路电压(Voc)和短路电流(Isc)这三个参数表征并分析电池器件的优劣。器件厚度主要通过对入射光的吸收率和光生载流子的收集效率影响热光伏电池的性能。而掺杂浓度对于热光伏电池特性的影响主要是从复合机制以及少子迁移率等方面体现。最后得到了优化后的器件结构。     

Growth and characterization of germanium and boron doped silicon epitaxial films

The epitaxial growth and characterization of in-situ germanium and boron (Ge/B) doped Si epitaxial films is described. As indicated by secondary ion mass spectroscopy and spreading resistance measurements, the total and electrically activated B concentrations are essentially identical and independent of Ge incorporation. The B and Ge concentrations are uniformly distributed in these Ge/B doped films. A slight enhancement of Hall mobility is obtained, possibly due to the stress relief induced by Ge counterdoping. Carrier conduction in these films is due to the activated B with an activation energy of 0.04 eV as revealed by conductivity versus temperature measurements. Ge atoms appear to be isoelectronic with Si atoms in these films. A slight degradation of minority carrier diffusion length is observed. Electrical characterization of PN diodes on these Ge/B doped films do not reveal any anomaly. SiO₂ on these Ge/B doped films has similar oxide fixed charge density, interface state density and dielectric breakdown strength compared to silicon dioxide on boron doped epitaxial films. Electron injection reveals a different transport mechanism of the SiO₂ grown on these Ge/B doped films.     

Electrical properties of Pb1−xCdxS epitaxial films

We have measured the resistivity ρ and Hall coefficient R_H at 300, 77, and 4.2 K of p-type Pb_1−XCd_XS epitaxial films as a function of substrate temperature T_s, film thickness d, and composition x. The films were vapor deposited on cleaved (111) BaF₂ (111) SrF₂ , and (001) NaCl and polished (001) BaF₂ substrates. The Hall mobility μ_H at 77 K of p-type PbS films increased approximately linearly from 1 × 10⁴ to 2 × 10⁴ cm² V⁻¹ sec⁻¹ as T_s was varied from 400 to 500°C, respectively. Both μ_H and R_H increased with d due to the presence of a strong p-type surface layer on the exposed surface. The x of the films was controlled by the x of the source material and T_s. The mole fraction of CdS could be varied between 0.002 < x < 0.06 by varying T between 513 and 410°C, respectively, and using source material with x = 0.06. The electrical properties of samples grown on freshly cleaved (111) BaF₂ and (111) SrF₂ were essentially identical even though the lattice constant of SrF₂ is a better match to Pb_1−XCd_XS than BaF₂. The R_H and μ_H at 77 K were independent of thickness for low substrate temperatures and were observed to increase with increasing thickness for high substrate temperatures. The μ_H increased with decreasing temperature and became temperature independent below about 30 K, which is similar to the behavior observed in other lead salt compounds. However, the magnitude of μ_H was considerable lower throughout the 300 to 4.2 K temperature range than for PbS films. The R_H showed little temperature variation, which is typical lead salt behavior. Supported by Naval Surface Weapons Center Independent Research Funds.     

外延BiFeO_3薄膜的温度特性

基于Landau-Devonshire唯象理论,采用等效衬底晶格常数的方法,并拟合了磁刚度系数的温度函数,对多铁性外延BiFeO3薄膜的铁电性和磁性进行了研究。结果表明,70nm厚薄膜的磁化强度在10～731℃先增加后减小,在371℃时有最大值64569A/m,随膜厚的增加磁化强度的极值减小;自发极化、c轴晶格常数随膜厚的增加而减小,同一厚度薄膜随温度的升高自发极化强度减小;压电常数、相对介电常数随温度升高而增大,随膜厚增加而增加。     

Investigation of short-term current gain stability of GaInP/GaAs-HBTs grown by MOVPE

Initial current gain variation in GaInP/GaAs-HBTs stressed at high-current density was investigated in dependence on base–dopant concentration and hydrogen passivation. It is shown that current-induced acceptor activation results in short-term current gain reduction, but does not affect emitter–base heterojunction properties. The incorporation of hydrogen during growth and the resulting passivation of carbon acceptors in the base layer depends mainly on carbon doping level and the precursors present during growth and cool-down. Under the growth conditions used, in-diffusion of hydrogen within AsH₃-rich growth steps turned out to be the major source of base–dopant passivation. Utilizing the blocking effect of n-type layers for H⁺ in-diffusion we were able to reduce the base passivation ratio [H]/[C] to 0.1 leading to significant improvements in gain stability. It is shown that current gain instabilities can be completely suppressed by using a high-current density pre-stress treatment.     

Thermodynamic stability and redistribution of charges in ternary AlGaN,InGaN, and InAlN alloys

A model of the delta lattice parameter is used to study the thermodynamics of AlGaN, InGaN, and InAlN alloys. The phase diagrams obtained indicate that AlxGa_1?xN is stable in the entire range of x, whereas the miscibility gap corresponds to 0.2 < x < 0.69 for InxGa_1?xN and to 0.16 < x < 0.7 for In_xAl_1?xN at 1000 K. Biaxial stresses lower the critical temperature and narrow the miscibility gap. The charge-density distribution is analyzed using the pseudopotential method to obtain an approximation of 32-atom supercells. The results of the analysis show that the stability of these alloys is controlled by the competition between the destabilizing contribution of strains related to the mismatch between the lattice constants and a stabilizing charge exchange between various chemical bonds. Biaxial stress reduces the charge redistribution caused by strains and thus increases the stability of an alloy.     

Investigation of the thermal stability of metastable GeSn epitaxial layers

A stack of five elastically strained metastable GeSn layers with a thickness of 200 nm each separated by Ge spacer layers with a thickness of 20 nm is grown on a (001) Si/Ge virtual substrate. The molar fraction of Sn in the GeSn layers is 0.005, 0.034, 0.047, 0.072, and 0.10. After growth the structure is subjected to thermal annealing for 2 min at a temperature of 400°C. It is demonstrated that during the course of annealing the GeSn alloy, along with plastic relaxation, undergoes phase separation; this phase separation begins before the end of plastic relaxation. The structural degradation of the GeSn layers increases with increasing concentration of Sn accumulated on the structure surface in the form of an amorphous layer.     

Different characteristics of high-temperature superconducting infrared detectors with granular and epitaxial films

We describe the measurements of infrared detection in granular and epitaxial films of YBa₂Cu₃O_7?δ . The experimental frequency response measurements stated that the response voltage of detector for epitaxial film decreased much more rapidly than that for granular film. Different relationship of infrared response and bias current for detectors from two types of films has been demonstrated. We attribute the descrapency to the characters of detectors associated with different types of film structures.     

Morphological dependence of passive epitaxial oxide films on nanoparticles of iron

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Direct bonding of GaAs films on silicon circuits by epitaxial liftoff

Epitaxial liftoff has emerged as a viable technique to integrate GaAs with silicon. The technique relies on the separation of a thin epi-GaAs film from its substrate followed by direct bonding of the thin film to a silicon substrate. The silicon substrate has to meet certain planarity and smoothness conditions in order to obtain high quality bonding. Unfortunately, processed silicon IC chips do not satisfy these conditions. In this paper, we report on the results of two different planarization techniques, plasma etch back and chemical-mechanical polishing, to integrate GaAs LEDs with silicon circuits using epitaxial liftoff. 4 by 8 arrays of GaAs LEDs have been integrated with silicon driver circuits using plasma etch back. We also have lifted off areas as large as 500 mm² and bonded them on 5″ device wafers by chemical-mechanical polishing. This can be essential for mass production of optoelectronic devices based on epitaxial liftoff.     

Cathodoluminescence and Raman scattering in Ga1−x AlxP epitaxial films

Low-temperature cathodoluminescence and Raman scattering of Ga_1?x Al_xP epitaxial layers (0≤x≤0.8) grown by liquid phase epitaxy on the GaP(100) substrate are studied. The obtained cathodoluminescence spectra indicate that the dependence of the indirect energy gap on the composition parameter x is nonlinear. This nonlinearity can be described by the parabolic function with the inflection parameter b=0.13. Raman scattering studies show that the phonon spectrum of Ga_1?x Al_xP consists of one (Al-P)-like vibrational mode and three (Ga-P)-like modes.