Y. S. Wu

Bio: Y. S. Wu is an academic researcher from University of Würzburg. The author has contributed to research in topics: X-ray photoelectron spectroscopy & Molecular beam epitaxy. The author has an hindex of 10, co-authored 15 publications receiving 251 citations.

The effects of laser illumination and high-energy electrons on molecular-beam epitaxial-growth of cdte

Abstract: 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.

Investigation of CdTe surfaces by x‐ray photoelectron spectroscopy

Abstract: 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.

Correlation of the Cd-to-Te ratio on CdTe surfaces with the surface structure

Abstract: We report here that reconstruction on (100), (111)A, and (111)B CdTe surfaces is either c(2\ifmmode\times\else\texttimes\fi{}2), (2\ifmmode\times\else\texttimes\fi{}2), and (1\ifmmode\times\else\texttimes\fi{}1) or (2\ifmmode\times\else\texttimes\fi{}1), (1\ifmmode\times\else\texttimes\fi{}1), and (1\ifmmode\times\else\texttimes\fi{}1) when they are Cd or Te stabilized, respectively. There is a mixed region between Cd and Te stabilization in which the reflected high-energy electron-diffraction (RHEED) patterns contain characteristics of both Cd- and Te-stabilized surfaces. We have also found that the Cd-to-Te ratio of the x-ray photoelectron intensities of their 3${\mathit{d}}_{3/2}$ core levels is about 20% larger for a Cd-stabilized (111)A, (111)B, or (100) CdTe surface than for a Te-stabilized one. According to a simple model calculation, which was normalized by means of the photoelectron intensity ratio of a Cd-stabilized (111)A and a Te-stabilized (111)B CdTe surface, the experimental data for CdTe surfaces can be explained by a linear dependence of the photoelectron-intensity ratio on the fraction of Cd in the uppermost monatomic layer. This surface composition can be correlated with the surface structure, i.e., the corresponding RHEED patterns. This correlation can in turn be employed to determine Te and Cd evaporation rates. The Te reevaporation rate is increasingly slower for the Te-stabilized (111)A, (111)B, and (100) surfaces, while the opposite is true for Cd from Cd-stabilized (111)A and (111)B surfaces. In addition, Te is much more easily evaporated from all the investigated surfaces than is Cd, if the substrate is kept at normal molecular-beam-epitaxy growth temperatures ranging from 200 \ifmmode^\circ\else\textdegree\fi{}C to 300 \ifmmode^\circ\else\textdegree\fi{}C.

Oxygen on the (100) CdTe surface

Abstract: 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 ...

Progress, challenges, and opportunities for HgCdTe infrared materials and detectors

Abstract: This article presents a review on the current status, challenges, and potential future development opportunities for HgCdTe infrared materials and detectortechnology. A brief history of HgCdTe infrared technology is firstly summarized and discussed, leading to the conclusion that HgCdTe-based infrared detectors will continue to be a core infrared technology with expanded capabilities in the future due to a unique combination of its favourable properties. Recent progress and the current status of HgCdTe infrared technology are reviewed, including material growth,device architecture, device processing, surface passivation, and focal plane array applications. The further development of infrared applications requires that future infrared detectors have the features of lower cost, smaller pixel size, larger array format size, higher operating temperature, and multi-band detection, which presents a number of serious challenges to current HgCdTe-based infrared technology. The primary challenges include well controlled p-type doping, lower cost, larger array format size, higher operating temperature, multi-band detection, and advanced plasma dry etching. Various new concepts and technologies are proposed and discussed that have the potential to overcome the existing primary challenges that are inhibiting the development of next generation HgCdTeinfrared detectortechnology.

Measurement of band offsets and interface charges by the C–V matching method

Abstract: The present article describes a novel application of capacitance–voltage measurements to determine simultaneously the band discontinuities (ΔEV, ΔEC) and interface charge density (σ) of heterojunctions. The method, which we refer to as C–V matching, complements the most versatile C–V profiling technique proposed by Kroemer and successfully applied by others. In contrast to the C–V profiling which is limited to isotype heterojunctions, the new method is applicable to p-n heterojunctions as well. The methodology is based on three cardinal equations which are not controversial—the lineup of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi levels (when voltage is applied), the charge neutrality and the expression for the total capacitance of the heterostructure. The three equations are formulated for equilibrium as well as nonequilibrium conditions, using quasi-Fermi levels and the quasi-equilibrium approximation. The three cardinal equations are defined by the two constant (al...

Partial density of Mn 3d states and exchange-splitting changes in Zn1-xMnxY (Y=S,Se,Te).

Abstract: Mn 3d states in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Y (Y=S,Se,Te) have been investigated using resonant synchrotron-radiation photoemission at the Mn 3p-3d threshold. We observe chemical trends due to anion substitution for the main Mn 3d photoemission feature at 3.5, 3.6, and 3.8 eV below the valence-band maximum as well as for the d(\ensuremath{\uparrow})-d(\ensuremath{\downarrow}) exchange splitting for ${\mathrm{Zn}}_{0.81}$${\mathrm{Mn}}_{0.19}$S, ${\mathrm{Zn}}_{0.81}$${\mathrm{Mn}}_{0.19}$Se, and ${\mathrm{Zn}}_{0.68}$${\mathrm{Mn}}_{0.32}$Te, respectively. The size of relaxation effects associated with the resonant-photoemission process is derived. Valence-band offsets of 0.3 eV are found between the semimagnetic Zn compounds and their parent binary alloys.

Indium doping of CdTe and Cd1−xZnxTe by molecular‐beam epitaxy: Uniformly and planar‐doped layers, quantum wells, and superlattices

Abstract: CdTe and Cd1−xZnxTe layers and microstructures were doped with indium donors during their growth at low temperatures (200–220 °C) by molecular‐beam epitaxy under Cd overpressure. Uniform and planar doping of layers and local doping of quantum wells and superlattices are presented. Characterization techniques include secondary‐ion mass spectroscopy (SIMS), capacitance‐voltage and Hall‐effect measurements, optical spectroscopy, x‐ray double diffraction, and x‐ray photoelectron spectroscopy. In the range of indium concentrations 2×1016–1×1018 cm−3, the donor activation efficiency is 100% for uniform doping. A low‐temperature carrier mobility of up to 5300 cm2/V s is obtained. The highest measured carrier concentration is 1.3×1018 cm−3; at a higher doping level, strong compensation occurs, related to dopant migration and cadmium vacancy formation. Planar doping also yields ≊100% activation efficiency for moderate values of sheet density (≊1011 cm−2) but has the same limit of about 1018 cm−3 for total carrier ...