Y. S. Wu

THE EFFECTS OF LASER ILLUMINATION AND HIGH-ENERGY ELECTRONS ON MOLECULAR-BEAM EPITAXIAL-GROWTH OF CDTE

We report the results of a detailed investigation on the Te‐stabilized (2×1) and the Cd‐stabilized c(2×2) surfaces of (100) CdTe substrates. The investigation demonstrates for the first time that both laser illumination and, to a greater extent, high‐energy electron irradiation increase the Te desorption and reduce the Cd desorption from (100) CdTe surfaces. Thus it is possible by choosing the proper growth temperature and photon or electron fluxes to change the surface reconstruction from the normally Te‐stabilized to a Cd‐stabilized phase.

Photoassisted molecular beam epitaxy of wide gap II-VI heterostructures

Abstract Photoassisted molecular beam epitaxy (PAMBE) is a new growth technique that has been developed to enhance the substitutional doping of II–VI compound semiconductors. During the process the substrate is illuminated during the growth sequence. In the present work a modified Riber MBE 2300 system was employed. A modification to the commercial system was made such that the substrate could be illuminated with an argon-ion laser during the deposition. The effect of illumination power density on the PAMBE growth of undoped CdTe has been studied in detail. With increasing power density we see the appearance of a new, very narrow (0.18 meV) and very intense luminescence feature. Polarization studies have been recently completed showing that this line is strongly linearly polarized along one of the crystallographic axis. We have observed for the first time strongly linearly polarized luminescence for undoped (100)CdTe epitaxial layers grown by photoassisted molecular beam epitaxy. The observation of this linear polarization leads us to postulate that the A-line and B-line observed in PAMBE grown CdTe are associated with vacancy-donor complexes that are preferentially oriented during the layer by layer growth process. The effect of illumination power density on the mobility and carrier concentration in undoped and lightly doped CdTe epilayers has been studied in detail. At low power densities we see an increase in both the mobility and carrier concentration.

Investigation of CdTe surfaces by x‐ray photoelectron spectroscopy

The influence of different CdTe substrate preheats prior to II‐VI molecular beam epitaxial growth on surface stoichiometry and oxygen contamination has been studied using x‐ray photoelectron spectroscopy. For 15 min preheats with temperatures ranging from 100 to 450 °C, the cadmium to tellurium ratio and the oxide overlayer thickness of (100) CdTe surfaces was determined. A preheat temperature of 200 °C is found to produce optimum stoichiometry. For lower temperatures the CdTe surface is still tellurium rich, as left after etching with bromine‐methanol. For higher temperatures, cadmium evaporates faster than tellurium, leaving again a tellurium‐rich surface. The oxygen contamination remains nearly unchanged for temperatures below 250 °C. Oxygen starts to blow off for preheat temperatures above 250 °C, with a steep decrease between 250 and 350 °C. For preheat temperatures higher than 350 °C, the oxygen contamination drops below the detection limit.

HIGH-MOBILITY CDTE-FILMS GROWN BY MOLECULAR-BEAM EPITAXY WITH EXCESS CD

We report the fact that n‐type conducting CdTe can be grown by molecular beam epitaxy (MBE) with excess Cd flux and show for the first time details of the CdTe/Cd surface phase reconstruction diagram.The surface phase diagram has been mapped over the substrate temperature range 180–375 °C using Cd/CdTe ratios between 0.0 and 1.0. At low Cd/CdTe ratios and high temperatures a (2×1) reconstruction is observed and at high Cd/CdTe flux ratios we observe a c(2×2) reconstruction. We have also observed a strong influence of substrate illumination on the surface reconstruction.

Oxygen on the (100) CdTe surface

We have investigated oxygen on CdTe substrates by means of x‐ray photoelectron spectroscopy (XPS) and reflection high‐energy electron diffraction (RHEED). A Te oxide layer that was at least 15 A thick was found on the surface of as‐delivered CdTe substrates that were mechanically polished. This oxide is not easily evaporated at temperatures lower than 350 °C. Furthermore, heating in air, which further oxidizes the CdTe layer, should be avoided. Etching with HCl acid (15% HCl) for at least 20 s and then rinsing with de‐ionized water reduces the Te oxide layer on the surface down to 4% of a monoatomic layer. However, according to XPS measurements of the O 1s peak, 20%–30% of a monoatomic layer of oxygen remains on the surface, which can be eliminated by heating at temperatures ranging between 300 and 340 °C. The RHEED patterns for a molecular beam epitaxially (MBE)‐grown CdTe film on a (100) CdTe substrate with approximately one monoatomic layer of oxidized Te on the surface lose the characteristics of the ...

Thermal effects on (100) CdZnTe substrates as studied by x‐ray photoelectron spectroscopy and reflection high energy electron diffraction

The influence of different CdZnTe substrate treatments prior to II‐VI molecular beam epitaxial growth on surface stoichiometry, oxygen, and carbon contamination has been studied using x‐ray photoelectron spectroscopy and reflection high energy electron diffraction. Heating the substrate at 300 °C can eliminate oxygen contamination, but cannot completely remove carbon from the surface. Heating at higher temperatures decreases the carbon contamination only slightly, while increasing the Zn–Cd ratio on the surface considerably. The magnitude of the latter effect is surprising and is crucial when one is using lattice matched CdZnTe (Zn 4%) substrates.

Surface sublimation of zinc blende CdTe

The surface sublimation of Cd and Te atoms from the zinc blende (111)A CdTe surface has been investigated in detail by reflection high energy electron diffraction and x‐ray photoelectron spectroscopy. These experiments verify that Te is much easier to evaporate than Cd. The experimental value for the Te activation energy from a Te stabilized (111)A CdTe surface is 1.41±0.10 eV, which is apparently inconsistent with recent theoretical results.

Molecular beam epitaxial growth and characterization of (100) HgSe on GaAs

Abstract In this paper, we present results on the first MBE growth of HgSe. The influence of the GaAs substrate temperature as well as the Hg and Se fluxes on the growth and the electrical properties has been investigated. It has been found that the growth rate is very low at substrate temperatures above 120°C. At 120°C and at lower temperatures, the growth rate is appreciably higher. The sticking coefficient of Se seems to depend inversely on the Hg/Se flux ratio. Epitaxial growth could be maintained at 70°C with Hg/Se flux ratios between 100 and 150, and at 160°C between 280 and 450. The electron mobilities of these HgSe epilayers at room temperature decrease from a maximum value of 8.2×10 3 cm 2 /V·s with increasing electron concentration. The concentration was found to be between 6×10 17 and 1.6×10 19 cm -3 at room temperature. Rocking curves from X-ray diffraction measurements of the better epilayers have a full width at half maximum of 550 arc sec.

Investigation of molecular‐beam epitaxially grown CdTe on GaAs by x‐ray photoelectron spectroscopy

X‐ray photoelectron spectroscopy studies of CdTe‐GaAs interfaces are reported. The growth start of CdTe on GaAs can be nearly stoichiometric if convenient growth parameters are chosen. The valence‐band offset between these two materials is found to be large (470 meV). Cd‐Te‐metal‐GaAs multilayers have been grown with very thin metal films. The CdTe‐GaAs band offset is not influenced by such intermediary metal layers. The experimentally obtained value for the valence‐band offset is compared with recent theoretical calculations taking into account interface dipoles.

Photo-assisted molecular beam epitaxy of CdTeCdMnTe heterostructures

Abstract Photo-assisted molecular beam epitaxy (PAMBE) is a new growth technique that has been developed to enhance the substitutional doping of II–VI compound semiconductors. To investigate the underlying mechanism of the PAMBE growth process, detailed reflection high energy electron diffraction studies on the reconstruction of CdTe(100) substrates has been completed. These studies have shown that one of the major effects of the PAMBE growth process is to enhance the cadmium concentration at the growth surface. As an application of the PAMBE growth technique, high quality CdMnTeCdTe heterostructures have been grown. The successful MBE growth of CdMnTeCdTe heterostructures and superlattices has demonstrated the feasibility of growing layered structures incorporating dilute magnetic semiconductor (DMS) materials. The reflectance, photoluminescence and excitation photoluminescence of CdTeCdMnTe single- and multiple-quantum-well structures with well widths in the range 30–45 A have been investigated at low temperatures. These show bright, narrow (less than 1 mev) luminescence features. Recently we have observed for the first time free magnetic polarons in DMS quantum wells. Free- and bound-magnetic-polaron energies were determined by the suppression of polaron formation with increasing temperature and in external magnetic fields.