Super-flat interfaces in In0.53Ga0.47As/In0.52Al0.48As quantum wells grown on (411)A InP substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (“(411)A super-flat interfaces”) were successfully achieved in In_0.53Ga_0.47As/In_0.52Al_0.48As quantum wells (QWs) grown on (411)A InP substrates by molecular beam epitaxy (MBE) at a substrate temperature of 570°C and V/III=6. Surface morphology of the In_0.53Ga_0.47As/In_0.52Al_0.48As QWs was smooth and featureless, while a rough surface of those simultaneously grown on a (100) InP substrate was observed. Photoluminescence (PL) linewidths at 4.2 K from the (411)A QWs with well width of 0.6–12 nm were 20–30 % narrower than those grown on a (100) InP substrate and also they are almost as narrow as each of split PL peaks for those of growth-interrupted QWs on a (100) InP substrate. In the case of the (411)A QWs, only one PL peak with very narrow linewidth was observed from each QW over a large distance (7 mm) on a wafer.     

Thermal stability of Al0.48In0.52As/Ga0.47In0.53As/InP heterostructure and its improvement by phosphidization

A drastic decrease in the sheet carrier concentration of modulation-doped Al_0.48In_0.52As/Ga_0.47In_0.53As/InP heterostructures has been observed after O₂ plasma treatment followed by thermal treatment up to 350°C. The decrease in sheet carrier concentration, which is speculated to be caused by both plasma damage and impurities penetrating from the surface of the epilayer, can be suppressed substantially by using PH₃ plasma treatment prior to the O₂ plasma and thermal treatments.     

Characteristics and device applications of erbium doped III-V semiconductors grown by molecular beam epitaxy

We have studied the properties of molecular beam epitaxially (MBE)-grown Erdoped III-V semiconductors for optoelectronic applications. Optically excited Er³⁺ in insulating materials exhibits optical emission chiefly around 1.54 μm, in the range of minimum loss in silica fiber. It was thought, therefore, that an electrically pumped Er-doped semiconductor laser would find great applicability in fiber-optic communication systems. Exhaustive photoluminescence (PL) characterization was conducted on several of As-based III-V semiconductors doped with Er, on bulk as well as quantum-well structures. We did not observe any Errelated PL emission at 1.54 μm for any of the materials/structures studied, a phenomenon which renders impractical the realization of an Er-doped III-V semiconductor laser. Deep level transient spectroscopy studies were performed on GaAs and AlGaAs co-doped with Er and Si to investigate the presence of any Er-related deep levels. The lack of band-edge luminescence in the GaAs:Er films led us to perform carrier-lifetime measurements by electro-optic sampling of photoconductive transients generated in these films. We discovered lifetimes in the picosecond regime, tunable by varying the Er concentration in the films. We also found the films to be highly resistive, the resistivity increasing with increasing Er-concentration. Intensive structural characterization (double-crys-tal x-ray and transmission electron microscopy) performed by us on GaAs:Er epilayers indicates the presence of high-density nanometer-sized ErAs precipitates in MBE-grown GaAs:Er. These metallic nanoprecipitates probably form internal Schottky barriers within the GaAs matrix, which give rise to Shockley-Read-Hall recombination centers, thus accounting for both the high resistivities and the ultrashort carrier lifetimes. Optoelectronic devices fabricated included novel tunable (in terms of speed and responsivity) high-speed metal-semiconductor-metal (MSM) photodiodes made with GaAs:Er. Pseudomorphic AlGaAs/ InGaAs modulation doped field effect transistors (MODFETs) (for high-speed MSM-FET monolithically integrated optical photoreceivers) were also fabricated using a GaAs:Er buffer layer which substantially reduced backgating effects in these devices.     

利用全固态分子束外延方法在Ge(100)衬底上异质外延GaAs薄膜及相关特性表征

利用全固态分子束外延(MBE)方法在Ge(100)衬底上异质外延GaAs薄膜,并通过高能电子衍射(RHEED)、高分辨X射线衍射(XRD),原子力显微镜等手段研究了不同生长参数对外延层的影响.RHEED显示在较高的生长温度或较低的生长速率下,低温GaAs成核层呈现层状生长模式.同时降低生长温度和生长速率会使GaAs薄膜的XRD摇摆曲线半高宽(FWHM)减小,并降低外延层表面的粗糙度,这主要是由于衬底和外延薄膜之间的晶格失配度减小的结果.     

Molecular beam epitaxial growth of MgZnCdSe on (100) InP substrates

Wide-gap II-VI MgZnCdSe quaternary compounds were grown on InP substrates by molecular beam epitaxy, for the first time. Changing the Mg composition (x = 0 to 0.63), various Mg_x(Zn_yCd_{1_y})_{1_x}Se lattice-matched to InP were grown. Mirror-like surface morphologies and streaky reflection high energy electron diffraction patterns of MgZnCdSe were obtained. With increased Mg compositions, the band-edge emissions wavelength in photoluminescence spectra was shifted from 572 nm (2.17 eV) to 398 nm (3.12 eV) at 15K. Furthermore, the absolute PL peak intensity increased drastically with increased band-edge emission, being accompanied by a relative decrement in the deep level emission intensities were also observed.     

Photoluminescence study of GaAs films on Si(100) grown by atomic hydrogen-assisted molecular beam epitaxy

Preliminary experimental results and analysis of photoluminescence (PL) measurements performed on GaAs heteroepitaxial films, which have been grown on Si(100) substrates by atomic hydrogen-assisted low-temperature molecular beam epitaxy technique have been presented and discussed. The results have also been compared with those obtained for GaAs homoepitaxial films. Furthermore, minority carrier lifetimes in n-GaAs on Si have been characterized by the PL decay method and an average lifetime of as high as 8.0 ns has been successfully obtained, which is the highest value ever reported to date.     

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x<0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density n_s from 0.78×10¹²cm⁻² for x=0.15 to 1.80×10¹² cm⁻² for x=0.40 at 300K, and from 0.75×10¹²cm⁻² to 1.67×10¹²cm⁻² at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm²/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm²/Vs for x=0.15 to 33000 cm²/Vs for x=0.40. The ratios (τ_t/τ_q) at 1.6K between the electron relaxation time τ_t and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al_0.35Ga_0.65As/In_0.15Ga_0.85As structures grown on GaAs substrates, and In_0.52Al_0.48As/In_0.53Ga_0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τ_t which, at low temperatures, is limited principally by alloy scattering.     

InAs/GaSb超晶格中波焦平面材料的分子束外延技术

报道了InAs/GaSb超晶格中波材料的分子束外廷生长技术研究.通过改变GaSb衬底上分子束外延InAs/GaSb超晶格材料的衬底温度,以及界面的优化等,改善超晶格材料的表面形貌和晶格失配,获得了晶格失配△a/a=1.5×10-4,原子级平整表面的InAs/GaSb超晶格材料,材料77 K截止波长为4.87 μm.     

Microstructural and optical characterization of CdTe(211)B/ZnTe/Si(211) grown by molecular beam epitaxy

CdTe layers have been grown by molecular beam epitaxy on 3 inch nominal Si(211) under various conditions to study the effect of growth parameters on the structural quality. The microstructure of several samples was investigated by high resolution transmission electron microscopy (HRTEM). The orientation of the CdTe layers was affected strongly by the ZnTe buffer deposition temperature. Both single domain CdTe(133)B and CdTe(211)B were obtained by selective growth of ZnTe buffer layers at different temperatures. We demonstrated that thin ZnTe buffer layers (<2 nm) are sufficient to maintain the (211) orientation. CdTe deposited at ∼300°C grows with its normal lattice parameter from the onset of growth, demonstrating the effective strain accommodation of the buffer layer. The low tilt angle (<1°) between CdTe[211] and Si[211] indicates that high miscut Si(211) substrates are unnecessary. From low temperature photoluminescence, it is shown that Cd-substituted Li is the main residual impurity in the CdTe layer. In addition, deep emission bands are attributed to the presence of As_Te and Ag_Cd acceptors. There is no evidence that copper plays a role in the impurity contamination of the samples.     

Dual-band infrared detectors made on high-quality HgCdTe epilayers grown by molecular beam epitaxy on CdZnTe or CdTe/Ge substrates

In this paper, we present all the successive steps for realizing dual-band infrared detectors operating in the mid-wavelength infrared (MWIR) band. High crystalline quality HgCdTe multilayer stacks have been grown by molecular beam epitaxy (MBE) on CdZnTe and CdTe/Ge substrates. Material characterization in the light of high-resolution x-ray diffraction (HRXRD) results and dislocation density measurements are exposed in detail. These characterizations show some striking differences between structures grown on the two kinds of substrates. Device processing and readout circuit for 128×128 focal-plane array (FPA) fabrication are described. The electro-optical characteristics of the devices show that devices grown on Ge match those grown on CdZnTe substrates in terms of responsivity, noise measurements, and operability.     

Strain effects in CdTe (111) epitaxial layers grown on GaAs (100) substrates by molecular beam epitaxy

Spectroscopic ellipsometry and photoreflectance measurements on CdTe/GaAs strained heterostructures grown by moleculclr beam epitaxy were carried out to investigate the effect of the strain and the dependence of the lattice parameter on the CdTe epitaxial layer thicknesses. Compressive strains exist in CdTe layers thinner than 2 μm. As the strain increases, the value of the critical-point energy shift increases linearly. These results indicate that the strains in the CdTe layers grown on GaAs substrates are strongly dependent on the CdTe layer thickness. Author to whom all correspondence should be addressed.     

Structure of GaN films grown by molecular beam epitaxy on (0001) sapphire

GaN films grown by electron-cyclotron resonance plasma-assisted molecular beam epitaxy were studied by transmission electron microscopy and x-ray diffraction (XRD). Two sets of films were compared that were grown under identical conditions except for the ratio of the Ga to N flux. Films with a 30% higher Ga to N ratio (A films) were found to contain inversion domains (IDs). No IDs were found in films grown with a lower Ga to N ratio (B films), but instead the zinc-blende GaN was found near the film substrate interface. A narrower XRD rocking curve width along the (0002) direction and a broader rocking curve width along the asymmetric (1102) axis were found for A films compared to B films.     

Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Lateral uniformity in HgCdTe layers grown by molecular beam epitaxy

The fabrication of high-quality focal plane arrays from HgCdTe layers grown by molecular beam epitaxy (MBE) requires a high degree of lateral uniformity in material properties such as the alloy composition, doping concentration, and defect density. While it is well known that MBE source flux nonuniformity can lead to radial compositional variation for rotating substrates, we have also found that composition can be affected significantly by lateral variations in substrate temperature during growth. In diagnostic experiments, we systematically varied the substrate temperature during MBE and quantified the dependence of HgCdTe alloy composition on substrate temperature. Based on these results, we developed a methodology to quickly and nondestructively characterize MBE-grown layers using postgrowth spatial mapping of the cutoff wavelength from the Fourier transform infrared (FTIR) transmission at 300 K, and we were able to obtain a quantitative relationship between the measured spatial variations in cutoff and the substrate temperature lateral distribution during growth. We refined this methodology by more directly inferring the substrate temperature distribution from secondary ion mass spectroscopy (SIMS) measurements of the As concentration across a wafer, using the fact that the As incorporation rate in MBE-grown p-type layers is highly sensitive to substrate temperature. Combining this multiple-point SIMS analysis with FTIR spatial mapping, we demonstrate how the relative contributions from flux nonuniformity and temperature variations on the lateral composition uniformity can be separated. This capability to accurately map the lateral variations in the substrate temperature has been valuable in optimizing the mounting and bonding of large substrates for MBE growth, and can also be valuable for other aspects of MBE process development.     

Origin of void defects in Hg1−xCdxTe grown by molecular beam epitaxy

Characterization of defects in Hg_1−xCd_xTe compound semiconductor is essential to reduce intrinsic and the growth-induced extended defects which adversely affect the performance of devices fabricated in this material system. It is shown here that particulates at the substrate surface act as sites where void defects nucleate during Hg_1−xCd_xTe epitaxial growth by molecular beam epitaxy. In this study, we have investigated the effect of substrate surface preparation on formation of void defects and established a one-to-one correlation. A wafer cleaning procedure was developed to reduce the density of such defects to values below 200 cm⁻². Focal plane arrays fabricated on low void density materials grown using this new substrate etching and cleaning procedure were found to have pixel operability above 98.0%.     

Growth and electronic properties of ZnO epilayers by plasma-assisted molecular beam epitaxy

ZnO thin films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The crystalline properties of the layers as measured by x-ray diffraction were found to improve with lower growth temperatures, where the full-width at half-maximum (FWHM) of the x-ray rocking curves was shown to be in the range of 100 to 1,100 arcsec. The electronic properties were found to improve for higher growth temperatures, with n-type carrier concentration and electron mobility in the range of 1×10¹⁷ −5×10¹⁸ cm⁻³ and 80–36 cm²/Vs, respectively. Photoluminescence (PL) measurements indicated that growth at higher temperatures provided superior band edge radiative emission, while growth at lower temperatures resulted in significant deep level radiative emission centered at 2.35 eV. Photoconductive decay measurements exhibit a slow decay indicating the presence of hole traps, where Zn vacancies are believed to be the source of both the slow decay and the deep level emission observed in PL spectra.     

Effect of in situ annealing on highly-mismatched In0.75Ga0.25As on InP grown using molecular beam epitaxy

We demonstrate that the electrical quality of junctions fabricated in lattice-mismatched In_0.75Ga_0.25As on InP grown by molecular beam epitaxy can be improved with the addition of in situ anneals in the buffer layer that separates the substrate from the In_0.75Ga_0.25As device layers. Near infrared photodetectors fabricated using this material had dark current densities of approximately 2.5 mA/cm² at a reverse bias of 1 V, which is more than one order of magnitude smaller than commercially available photodetectors grown using vapor phase epitaxy. Transmission electron microscopy revealed that dislocations due to the lattice mismatch between the substrate and the epitaxial layer are confined primarily to the buffer layer for all samples studied. No significant differences in x-ray diffraction spectra or dislocation distribution were observed on samples with and without in situ annealing. Atomic force microscopy indicated that all samples had a crosshatch pattern, and that the average surface roughness of the sample that contained in situ anneals is a factor of three greater than the sample without in situ anneals.     

Transmission electron microscopy studies of defects in HgCdTe device structures grown by molecular beam epitaxy

Transmission electron microscopy (TEM) was used to evaluate the microstructure of molecular beam epitaxy (MBE) grown (211)B oriented HgCdTe films. TEM analysis of in-situ doped p-on-n and n-p-n device structures will be presented. Under fully optimized growth conditions the substrate-epilayer interface is free of threading dislocations and twins, and a high degree of structural integrity is retained throughout the entire device structure. However, under non-optimal growth conditions that employ high Hg/Te flux ratios, twins can be generated in the p-type layer of p-on-n device structure, resulting in roughness and facetting of the film surface. We propose a mechanism for twin formation that is associated with surface facetting. TEM evaluation of voids, threading dislocations and Te-precipitates in HgCdTe films are also discussed.     

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].     

Use of electron cyclotron resonance plasmas to prepare CdZnTe (211)B substrates for HgCdTe molecular beam epitaxy

Without any additional preparation, Cd_1−yZn_yTe (211)B (y∼3.5%) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H₂ plasma and used as substrates for HgCdTe molecular beam epitaxy. Auger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330–340°C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ∼1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ∼1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2–20 s) and to changes in radio frequency input power (20–100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ∼0.4 nm were measured. Characteristics of HgCdTe epilayers deposited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 10⁵ cm⁻² have been achieved. ECR Ar/H₂ plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.