Analysis of the variation in the composition as a function of growth parameters in the MBE growth of indium doped Hg1−xCdxTe

Compositional changes induced in growing Hg_1−xCd_xTe layers as a function of the changes in temperature of the indium source in the MBE chamber have been analyzed in terms of the Hg-In alloy thermodynamics and changes in the activity of indium over the growing films as the Hg flux changes. Compositional changes induced by changes in the Te source temperature have been analyzed in terms of the changes in the activity of Cd over the growing layers. These analysis are helpful in understanding the compositional changes with variation of the growth parameters in the MBE chamber.     

Simulation of two-dimensional tagged MRI

MR tagging is a recent imaging development that, in cardiac applications, makes possible the tracking of points in the myocardium during the cardiac cycle. Researchers have developed semiautomated, computer-based methods for analyzing tagged images, but the images are complex and present a challenge to automated tracking systems. Simulation can provide an inexhaustible supply of images for testing and validation of tag tracking software and preview the effect of parameter changes in acquisition. SIMTAG is an interactive computer program that simulates two-dimensional tagged-MR experiments. The mathematic model used in the simulation and algorithms for simulating image noise and object deformation are described. Examples of the use of simulated images in SPAMM parameter selection, a comparison of tag contrast in signal-averaged SPAMM and CSPAMM, and simulated images as test sets for tag-tracking software are presented.     

Absorption of Narrow-Gap HgCdTe Near the Band Edge Including Nonparabolicity and the Urbach Tail

An analytical model describing the absorption behavior of Hg_1−x Cd _x Te is developed. It simultaneously considers the contributions from non-parabolic conduction/light hole bands and parabolic heavy hole bands obtained from 14-band k·p electronic structure calculations and the Urbach tail. This model smoothly fits experimental absorption coefficients over energies ranging from the Urbach tail region to the intrinsic absorption region up to at least 300 meV above the band gap.     

Selective epitaxy of cadmium telluride on silicon by MBE

CdTe B was grown on As-terminated Si(111) by molecular beam epitaxy (MBE). Nucleation and interface properties were studied by photoelectron spectroscopy, scanning tunneling microscopy, electron diffraction, and energy-dispersive spectroscopy of x-rays. Selective growth on Si(111) was investigated either by using SiO₂ as a mask, or by growing on a patterned CdTe seed layer. The highest temperature where CdTe nucleates on As-terminated Si(111) surfaces is typically in the range of 220–250°C. On a SiO₂ mask, CdTe nucleates at the same temperatures, leading to polycrystalline growth. However, homoepitaxy of CdTe is possible around 300°C. Hence, CdTe can be grown selectively on a patterned CdTe seed layer on Si(111). This is confirmed by scanning electron microscopy and scanning Auger microscopy.     

Measurements of left ventricular dimensions using real-time acquisition in cardiac magnetic resonance imaging: comparison with conventional gradient echo imaging

This study investigates the use of real-time acquisition in cardiac magnetic resonance imaging (MRI) for measurements of left ventricular dimensions in comparison with conventional gradient echo acquisition. Thirty-one subjects with a variety of left ventricular morphologies to represent a typical clinical population were studied. Short-axis data sets of the left ventricle (LV) were acquired using a conventional turbo-gradient echo and an ultrafast hybrid gradient echo/echo planar sequence with acquisition in real-time. End-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF) and left ventricular mass (LV mass) were measured. The agreement between the two acquisitions and interobserver, intraobserver and interstudy variabilities were determined. The bias between the two methods was 5.86 ml for EDV, 0.23 ml for ESV and 0.94% for EF. LV mass measurements were significantly lower with the real-time method (mean bias 14.38 g). This is likely to be the result of lower spatial resolution and chemical shift artefacts with the real-time method. Interobserver, intraobserver and interstudy variabilities were low for all parameters. In conclusion, real time acquisition in MRI can provide accurate and reproducible measurements of LV dimensions in subjects with normal as well as abnormal LV morphologies, but LV mass measurements were lower than with conventional gradient echo imaging. Presented in abstract form at the International Society of Magnetic Resonance in Medicine meeting in Denver, Colorado in April 2000.     

Microstructure of Heteroepitaxial ZnTe Grown by Molecular Beam Epitaxy on Si(211) Substrates

The interface of ZnTe/Si(211) grown by molecular beam epitaxy was investigated by high-resolution transmission electron microscopy. Several types of defects such as misfit dislocations, stacking faults, agglomerations of vacancies, and precipitates were observed and studied by electron microscopy at the ZnTe/Si interface. The distribution of misfit dislocations at the interface was revealed with the assistance of the fast Fourier transformation filtering technique. A stick-and-ball interface model including misfit dislocation geometry is proposed. The possible origins of the stacking faults, vacancies, and precipitates are discussed.     

The Effect of Wet Etching on Surface Properties of HgCdTe

The surface of HgCdTe, grown by molecular-beam epitaxy and liquid-phase epitaxy, was studied by atomic force microscopy and x-ray photoelectron microscopy after etching in different solutions such as Br:methanol and HBr:H₂O₂:H₂O. Minority-carrier lifetime and surface recombination velocity were measured by photoelectron decay spectroscopy. The same measurements were repeated after exposure to air for periods from 2 h to 2 days. Although these surfaces are rather complicated, the main feature is that Br-based etchants produce elemental Te at the surface, which oxidizes rapidly in air. Without elemental Te, there is less Te oxide, even after longer exposure to air. The existence of elemental Te is correlated with higher surface recombination velocity. This can be explained in terms of band bending and band offsets at Te/HgCdTe and TeO₂/HgCdTe interfaces.     

Mode of arsenic incorporation in HgCdTe grown by MBE

The results of arsenic incorporation in HgCdTe layers grown by molecular beam epitaxy (MBE) are reported. Obtained results indicate that arsenic was successfully incorporated as acceptors in MBE-HgCdTe layers after a low temperature anneal. Secondary ion mass spectrometry and Hall effect measurements confirm that arsenic is incorporated with an activation yield of up to 100%. This work confirms that arsenic can be used as an effective dopant of MBE-HgCdTe after a low temperature annealing under Hg-saturated conditions.     

Correlation of CdZnTe(211)B substrate surface morphology and HgCdTe(211)B epilayer defects

We present results on the surface morphology and recombination lifetimes of molecular-beam epitaxy (MBE)-grown HgCdTe (211)B epilayers and correlate them with the roughness of the CdZnTe substrate surfaces. The substrate surface quality was monitored by in-situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction (RHEED). The SE roughness of the substrate was measured after oxide desorption in the growth chamber. The RHEED patterns collected show a strong correlation with the SE roughness. This proves that SE is a valuable CdZnTe prescreening tool. We also found a correlation between the substrate roughness and the epilayer morphologies. They are characterized by a high density of thin elongated defects, “needle defects,” which appear on most samples regardless of growth conditions. The HgCdTe epilayers grown on these substrates were characterized by temperature-dependent, photoconductive decay-lifetime data. Fits to the data indicate the presence of mid-gap recombination centers, which were not removed by 250°C/24-h annealing under a Hg-rich atmosphere. These centers are believed to originate from bulk defects rather than Hg vacancies. We show that Te annealing and CdTe growth on the CdZnTe substrates smooth the surface and lower substantially the density of needle defects. Additionally, a variety of interfacial layers were also introduced to reduce the defect density and improve the overall quality of the epilayer, even in the presence of less than perfect substrates. Both the perfection of the substrate surface and that of its crystalline structure are essential for the growth of high-quality material. Thus, CdZnTe surface polishing procedures and growth techniques are crucial issues.