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A major challenge in the stoichiometric growth of complex oxide compounds is the control of the relative compositions of the constituent materials. A potential avenue for compositional analysis during growth is the use of x-ray fluorescence generated during reflection high energy electron diffraction measurements. Using this technique, relative compositions of Y and Mn in molecular beam epitaxy grown YMnO3 samples were studied. Comparing the results with Rutherford back scattering spectroscopy suggests that the technique has the potential for real-time analysis of elemental fluxes and stoichiometry control during sample growth. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) from undoped and chromium-doped ZnTe epilayers was studied in the spectral range from 0.3 to 2.4 eV over the temperature range from 5 K to room temperature. Films approximately 2 μm thick were grown by molecular-beam epitaxy on GaAs substrates. For undoped ZnTe, spectra are dominated by a sharp acceptor-bound-exciton line related to arsenic, with only weak emission observed due to substitutional oxygen (OTe). An analysis of strain-induced shifts in excitonic energies was performed to clarify transition assignments for two PL bands near 2.375 and 2.379 eV. For ZnTe:Cr epilayers, the infrared Cr2+ emission near 2.5 μm (0.5–0.6 eV) was found to be a function of both doping concentration and Zn/Te beam-equivalent-pressure ratio. Direct excitation of the Cr2+ ions with 1.89-μm light produced emission up to room temperature. Indirect excitation with 514.5-nm light produced emission that could be detected up to 180 K. Luminescence decay curves measured from a ZnTe:Cr film ([Cr]∼1×1018 cm-3) using pulsed excitation at 355 and at 532 nm indicated two lifetimes: τ1=3 (±1) μs and τ2>=10 μs. The shorter lifetime agrees with values reported previously for Cr-doped ZnTe bulk crystals and thin films under 1.9-μm pulsed excitation. [ABSTRACT FROM AUTHOR]

Presents a study that investigated photoluminescence from heteroepitaxial (211)B cadmium telluride grown on (211)B gallium arsenide by molecular beam epitaxy. Methods used in the preparation of the samples; Analysis of the edge photoluminescence emission region; Common method of evaluating layer quality by the photoluminescence technique.

Presents information on a study which discussed electroreflectance (ER) measurements for bulk and epitaxial cadmium tellurium layers grown by molecular beam epitaxy. Use of the ER experimental configuration; Discussion on transverse ER measurements; Analysis of additional spectral features near the band edge.

Focuses on the effects of intentional incorporation of hydrogen and deuterium in nitrogen-doped zinc selenide (ZnSe) grown by conventional molecular beam epitaxy and the local vibration modes for N-D bonding in ZnSe. Details of the growth of nitrogen-doped ZnSe layers; Description of the depth profile for nitrogen and hydrogen concentrations in a nitrogen-doped ZnSe sample; Significance of the effect of isotopic substitution on the observed vibrational frequency.

Using spectroscopic ellipsometry, the temperature-dependence of the dielectric functions of a series of Hg1−xCdxSe thin films deposited on both ZnTe/Si(112) and GaSb(112) substrates were investigated. Initially, for each sample, room-temperature ellipsometric spectra were obtained from 35 meV to 6 eV using two different ellipsometers. Subsequently, ellipsometry spectra were obtained from 10 K to 300 K by incorporating a cryostat to the ellipsometer. Using a standard inversion technique, the spectroscopic ellipsometric data were modeled in order to obtain the temperature-dependent dielectric functions of each of the Hg1−xCdxSe thin films. The results indicate that the E1 critical point blue-shifts as a function of Cd-alloy concentration. The temperature-dependence of E1 was fitted to a Bose-Einstein occupation distribution function, which consequently allowed us to determine the electron-phonon coupling of Hg1−xCdxSe alloys. From the fitting results, we obtain a value of 17 ± 2 meV for the strength of the electron-phonon coupling for Hg1−xCdxSe alloy system, which compares nominally with the binary systems, such as CdSe and CdTe, which have values around 38 meV and 16 meV, respectively. This implies that the addition of Hg into the CdSe binary system does not significantly alter its electron-phonon coupling strength. Raman spectroscopy measurements performed on all the samples show the HgSe-like transverse optic (TO) and longitudinal optic (LO) phonons (∼ 130 cm−1 and ∼ 160 cm−1, respectively) for all the samples. While there is a slight red-shift of the HgSe-like TO peak as a function of the Cd-concentration, HgSe-like LO peak does not significantly change with the alloy concentration. [ABSTRACT FROM AUTHOR]

Iodine-doped CdTe and CdMgTe layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-typ e carrier concentrations of 7.4 × 10 cm for CdTe and 3 × 10 cm for CdMgTe were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for CdMgTe is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 10 cm, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 10 cm. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe. [ABSTRACT FROM AUTHOR]

Iodine-doped CdMgTe/CdSeTe double heterostructures (DHs) have been grown by molecular beam epitaxy and studied using time-resolved photoluminescence (PL), focusing on absorber layer thickness of 2 μm. The n-type free carrier concentration was varied to ∼7 × 10 cm, 8.4 × 10 cm, and 8.4 × 10 cm using iodine as dopant in DHs. Optical injection at 1 × 10 photons/pulse/cm to 3 × 10 photons/pulse/cm, corresponding to initial injection of photocarriers up to ∼8 × 10 cm, was applied to examine the effects of excess carrier concentration on the PL lifetimes. Iodine-doped DHs exhibited an initial rapid decay followed by a slower decay at free carrier concentration of 7 × 10 cm and 8.4 × 10 cm. The optical injection dependence of the carrier lifetimes for DHs was interpreted based on the Shockley-Read-Hall model. The observed decrease in lifetime with increasing n is consistent with growing importance of radiative recombination. [ABSTRACT FROM AUTHOR]

Spectroscopic ellipsometry, ranging from 35meV to 6 eV, was used to determine the dielectric functions of a series of molecular beam epitaxy-grown Hg1-xCdxSe thin films deposited on both ZnTe/Si(112) and GaSb(112) substrates. The fundamental band gap as well as two higher-order electronic transitions blue-shift with increasing Cd composition in Hg1-xCdxSe, as expected. Representing the free carrier absorption with a Drude oscillator, we found that the effective masses of Hg1-xCdxSe (grown on ZnTe/Si) vary between 0.028 and 0.050 times the free electron mass, calculated using the values of carrier concentration and the mobility obtained through Hall measurements. Using these effective masses, we determined the carrier concentrations of Hg1-xCdxSe samples grown on GaSb, which is of significance as films grown on such doped-substrates posit ambiguous results when measured by conventional Hall experiments. These models can serve as a basis for monitoring Cd-composition during sample growth through in-situ spectroscopic ellipsometry. [ABSTRACT FROM AUTHOR]

A method is described for directly measuring the temperature of a substrate in a molecular-beam epitaxy (MBE) growth system. The approach relies on the establishment of the temperature dependence of Raman-active phonons of the substrate material using independently known calibration points across the range of interest. An unknown temperature in this range is then determined based on the Raman peak position with the substrate in situ the MBE chamber. The apparatus relies on conventional optics and Raman components. Shifting and broadening of the Raman spectrum are described based on the effects of thermal expansion and anharmonic decay. The choice of reference temperature is discussed. The method is qualified by examining the substrate temperature dependence, relative to that of a standard thermocouple, during a commonly used ramp procedure. Both temperature difference and time lag are obtained. [ABSTRACT FROM AUTHOR]

Cross-section electron micrographs, cathodoluminescence images, and confocal photoluminescence (cPL) images have been acquired for ZnTe layers deposited to various thicknesses on GaSb substrates with (211)B and (100) orientations. The critical thickness of ZnTe on GaSb is predicted to range between 115 nm and 329 nm, depending on the theoretical approach chosen. For ZnTe layers grown on (211)B GaSb with thickness exceeding 150 nm, dark spots and lines are present in all images. We associate these with dislocations generated at the ZnTe/GaSb interface. The discrepancy between this thickness value and a critical thickness value (350 nm to 375 nm) obtained for the (211)B orientation in a previous study is related to the distinction between the onset of misfit dislocations and the onset of significant plastic deformation. The former requires a direct imaging technique, as strain-related measurements such as x-ray diffraction do not have the resolution to detect the effects of small numbers of dislocations. For ZnTe layers on (100) GaSb, x-ray diffraction measurements indicate an abrupt change characteristic of dislocation multiplication at a thickness value in the range from 250 nm to 275 nm. High-resolution electron micrographs of the ZnTe/GaSb interface indicate that deoxidation using atomic hydrogen produces GaSb surfaces suitable for ZnTe epitaxy. cPL images of a 1.2- μm-thick lattice-matched ZnTeSe layer grown on a 150-nm-thick ZnTe buffer layer on a (211)B GaSb substrate yield a threading dislocation density of ∼7 × 10 cm. [ABSTRACT FROM AUTHOR]

In-situ cathodoluminescence (CL) measurement during growth of gallium nitride (GaN) by molecular beam epitaxy (MBE) was investigated for determining substrate temperature. Cathodoluminescence was easily observed from room temperature up to and beyond typical temperatures used during growth. Determination of the peak energy of the CL spectrum allows unambiguous determination of the substrate temperature. Temperature drift and reproducibility issues during growth of GaN were observed using the in-situ CL measurement. [ABSTRACT FROM AUTHOR]

We have initiated a joint effort to better elucidate the fundamental mechanisms underlying As-doping in molecular beam epitaxy (MBE)-grown HgCdTe. We have greatly increased the As incorporation rate by using an As cracker cell. With a cracker temperature of 700°C, As incorporation as high as 4 x 1020 cm-3 has been achieved by using an As-reservoir temperature of only 175°C. This allows the growth of highly doped layers with high quality as measured by low dislocation density. Annealing experiments show higher As-activation efficiency with higher anneal temperatures for longer time and higher Hg overpressures. Data are presented for layers with a wide range of doping levels and for layer composition from 0.2 to 0.6. [ABSTRACT FROM AUTHOR]

Variable magnetic-field Hall and transient photoconductance-lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy (MBE). Use of quantitative mobility-spectrum analysis (QMSA) combined with multiple carrier-fitting (MCF) techniques indicates that the majority of samples contain an interfacial n-type layer that significantly influences the interpretation of the electrical measurements. This n-type layer completely masks the high-quality electrical properties of undoped or low n-type In-doped HgCdTe, as well as complicating the interpretation of activation in As-doped p-type HgCdTe. Introduction of an intentional n-type background, typically created through doping with In to "recover" high mobility, is actually shown to increase the "bulk" layer conductivity to a level comparable to the interface layer conductivity. Photoconductance-lifetime measurements suggest that In-doping may introduce Shockley-Read-Hall (SRH) recombination centers. Variable-field Hall analysis is shown to be essential for characterizing p-type material. Photoconductance-lifetime measurements suggest that trapping states may be introduced during the incorporation and activation of As. Two distinctly different types of temperature dependencies were observed for the lifetimes of As-doped samples. [ABSTRACT FROM AUTHOR]

We show that variations of the E1 reflectance peak in Hg-based superlattices can be used to probe low-temperature interdiffusion by monitoring the shift of the E1 peak with time over extended periods. Little evidence of interdiffusion was detected for a number of HgTe/CdTe and HgCdTe/CdTe superlattices stored at room temperature for approximately two years. Two HgTe/CdTe superlattices and one HgCdTe/CdTe superlattice were subsequently annealed in a dry nitrogen atmosphere at 100°C for approximately six months, and then at 150°C for 24 days. During these intervals, the superlattices were periodically removed from the anneal for reflectance measurements to assess the extent of the interdiffusion. Comparison of these results with calculations of superlattice bandgaps and interdiffusion profiles has led to an evaluation of the low temperature interdiffusion coefficients. These extend previous results to lower temperatures and confirm that the degradation of Hg-based superlattices devices due to thermal interdiffusion under normal processing, storage, and operating conditions should not be an issue of concern. [ABSTRACT FROM AUTHOR]

Threading dislocations in gallium nitride epilayers intermittently doped with oxygen were examined using transmission electron microscopy. Dislocations of all types were observed to incline away from the [0001] growth direction at the first introduction of oxygen, remaining inclined through subsequently doped layers. The type and the sign of the Burgers vector of inclined dislocations were characterized by large angle convergent beam electron diffraction, which revealed that the direction of inclination was not primarily driven by misfit stresses. In contrast, dislocations were observed to incline towards pronounced surface pits. It is concluded that the inclination is driven by surface roughening induced by oxygen doping, in contrast to previously published results. [ABSTRACT FROM AUTHOR]

Variable magnetic field Hall effect, photoluminescence, and capacitance-voltage (CV) analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. Early indications suggest the Mg acceptor level in InN may lie near 110 meV above the valence band maximum. The development of p-type doping techniques offers great promise for future InN based devices. [ABSTRACT FROM AUTHOR]

Epitaxial YMnO3 films were grown on (0001) GaN-on-sapphire templates using molecular beam epitaxy. The YMnO3 maintained the (0001) orientation with an in-plane YMnO3/GaN epitaxial relationship of (0001)∥(0001); [1100]∥[1120]. The YMnO3 was ferroelectric at room temperature with a remanent polarization of ∼3.2 μC/cm² and a saturation polarization of ∼12 μC/cm². This heterostructure is a promising candidate for multifunctional structures that integrate ferroelectrics with GaN-based high-power and short-wavelength light-emitting devices. [ABSTRACT FROM AUTHOR]

Positron annihilation spectroscopy is used to study vacancy defects in GaN grown by molecular-beam epitaxy due to different polar directions and varying stoichiometry conditions during oxygen doping. We show that Ga-polar material is free of compensating Ga vacancies up to [O]=1018 cm-3 in Ga stable growth, but high concentrations of VGa are formed in N-stable conditions. We also show that vacancy clusters are formed in N-polar material grown in Ga stable conditions, which may be related to the higher reactivity of the N-polar surface. These clusters have no apparent influence on the electrical properties of the material. We thus infer that their charge state is neutral. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High-quality (0001) and (0001¯)-GaN films were grown by plasma-assisted molecular-beam epitaxy to study the dependence of oxygen incorporation on polarity and oxygen partial pressure. Oxygen incorporates at a rate ten times faster on nitrogen-polar GaN than on the Ga polarity. Oxygen doping is controllable, reproducible, and produces low compensation material up to concentrations of at least 10[sup 18] cm[sup -3] with higher levels showing significant compensation. Layers containing oxygen at levels above 10[sup 22] cm[sup -3] exhibit severe cracking while oxygen concentrations less than 10[sup 21] cm[sup -3] do not introduce significant strain. The oxygen incorporation rate has a weak dependence on Ga overpressure during Ga-stable growth but dramatically increases for conditions approaching N-stable growth. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A pronounced dependence of Mg incorporation on surface polarity was observed in a series of Mg step-doped epitaxial GaN layers grown by rf-plasma-assisted molecular-beam epitaxy. Incorporation was studied for both (0001), or Ga-polarity and (0001¯) or N-polarity orientations. Up to a factor of 30 times more Mg was incorporated in Ga-polarity layers under certain conditions, as determined by secondary ion mass spectrometry. Measurements indicate surface accumulation of Mg occurs during growth, with stable accumulations of close to a monolayer of Mg on the Ga-polarity surface. The presence of atomic hydrogen during growth significantly increased incorporation of Mg without also incorporating potentially compensating hydrogen. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

GaN was grown by molecular beam epitaxy using an rf plasma source. Growth under gallium-rich conditions at 730 °C was required to produce high quality layers as indicated by photoluminescence, Hall effect, atomic force microscopy, and x-ray diffraction measurements. Atomic hydrogen has a significant effect for Ga-rich growth, increasing growth rates by as much as a factor of 2. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Atomic hydrogen is demonstrated to effectively clean GaAs substrates for subsequent growth of ZnSe by molecular beam epitaxy. Optical fluorescence microscopy is shown to be a useful technique to image nonradiative defects related to stacking faults. While the density of stacking faults in ZnSe films grown using conventional thermal cleaning is greater than 107 cm-2, stacking fault densities lower than 104 cm-2 are obtained using atomic hydrogen cleaning. Low-temperature photoluminescence spectra of undoped ZnSe are dominated by excitonic transitions for the low defect density samples in contrast to the high level of defect-related emission from high defect density samples. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the successful p-type doping of CdTe films with arsenic using the photoassisted molecular beam epitaxy growth technique. These doped epilayers were grown at substrate temperatures as low as 180 °C. The room-temperature hole concentrations in the CdTe:As layers ranged from 7×1015 to 6.2×1018 cm-3 as determined by van der Pauw–Hall effect measurements. We propose a doping mechanism responsible for the high p-type doping levels observed in the films. The arsenic acceptor ionization energy was found to ∼58–60 meV using low-temperature photoluminescence measurements. [ABSTRACT FROM AUTHOR]

Variable magnetic field Hall and transient photoconductance lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy and metalorganic chemical vapor deposition. Temperature variation and, in the case of Hall, magnetic-field variation are needed to give a more complete picture of the mechanisms that control lifetimes in HgCdTe samples. Recent predictions of recombination lifetimes from full band structure calculations were compared to experimental lifetimes at various doping levels at long-wave infrared (LWIR) and mid-wave infrared (MWIR) compositions. For n-type material, lifetimes from low doping levels fall well below the predictions, implying that Shockley-Read-Hall (SRH) recombination is still dominant. MWIR samples have a lifetime that increases somewhat with carrier concentration, suggesting that In doping passivates the SRH defects for that composition. Lifetimes in p-type MWIR material appear to be well-explained by recent theoretical calculations. In p-type material, trapping states may be introduced during the incorporation and activation of As, since some samples with unusually long lifetimes had a distinctly different type of temperature dependence. [ABSTRACT FROM AUTHOR]

Homoepitaxial GaN films, doped with Mg, were grown by rf-plasma molecular-beam epitaxy on Ga-polarity (0001) templates. Convergent-beam electron diffraction analysis establishes that the film polarity changes from [0001] to [0001_] when the Mg flux during growth is approximately 1 ML/s. Secondary ion mass spectrometry indicates a doping concentration of ∼10[sup 20] cm[sup -3] in the film where the inversion occurs, and a reduced Mg incorporation in the [0001_] material. Transmission electron microscopy shows that the inversion domain boundary is faceted predominantly along the {0001} and {h,h,-2h,l} planes, with l/h approximately equal to 3. Using first-principles total energy calculations, we show that the {h,h,-2h,l} segments of the boundary are stabilized by the incorporation of Mg in threefold coordinated lattice sites. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]