TY - JOUR A1 - Becker, Charles R. A1 - Wu, Y. S. A1 - Waag, A. A1 - Kraus, M. M. A1 - Landwehr, G. T1 - The orientation independence of the CdTe-HgTe valence band offset as determined by x-ray photoelectron spectroscopy N2 - No abstract available Y1 - 1991 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-30784 ER - TY - JOUR A1 - Wu, Y.S. A1 - Becker, Charles R. A1 - Waag, A. A1 - Kraus, M. M. A1 - Bicknell-Tassius, R. N. A1 - Landwehr, G. T1 - Correlation of the Cd-to-Te ratio on CdTe surfaces with the surface structure N2 - We report here that reconstruction on (100), (1lIlA, and (1l1lB CdTe surfaces is either C(2X2), (2X2), and (l X I) or (2X I), (l X I), and (l X I) 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 3d\(_{3/ 2}\) core levels is about 20% larger for a Cd-stabilized (1lIlA, (1lIlB, 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 (lll)A and aTe-stabilized (1l1lB 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 (Ill) A, (l1l)B, and (100) surfaces, while the opposite is true for Cd from Cd-stabilized (Ill) A and (Ill)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 2oo·C to 300 ·C. KW - Festkörperphysik Y1 - 1991 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-37789 SN - 0163-1829 ER - TY - JOUR A1 - Wu, Y. S. A1 - Becker, Charles R. A1 - Waag, A. A1 - Bicknell-Tassius, R. N. A1 - Landwehr, G. T1 - The effects of laser illumination and high energy electrons on molecular-beam epitaxial growth of CdTe N2 - We report the results of a detailed investigation on the Te-stabilized (2 x 1) and the Cdstabilized c( 2 X 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. Y1 - 1991 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-30795 ER - TY - JOUR A1 - Wu, Y. S. A1 - Becker, Charles R. A1 - Waag, A. A1 - Bicknell-Tassius, R. N. A1 - Landwehr, G. T1 - Thermal effects on (100) CdZnTe substrates as studied by x-ray photoelectron spectroscopy and reflection high energy electron diffraction N2 - 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. Y1 - 1992 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-37801 SN - 0003-6951 ER - TY - JOUR A1 - Kraus, M. M. A1 - Regnet, M. M. A1 - Becker, Charles R. A1 - Bicknell-Tassius, R. N. A1 - Landwehr, G. T1 - Comparison of band structure calculations and photoluminescence experiments on HgTe/CdTe superlattices grown by molecular beam epitaxy N2 - We have grown HgTe/CdTe superlattices by molecular beam epitaxy; barrier thicknesses were in the range from 15 to 91 Å and the well thickness was maintained at a constant value of 30 Å. The infrared photoluminescence was investigated by means of Fourier transform infrared spectroscopy in the temperature range from 4.2 to 300 K. All superlattices showed pronounced photoluminescence at temperatures up to 300 K. To gain more detailed insight into the band structure of the HgTe/CdTe superlattices, band structure calculations were performed. The concept of the envelope function approximation was followed. Employing the transfer matrix method, the calculations were completed taking into account an eight band k·p model. An important parameter in these calculations is the natural valence band offset (VBO) between the well and barrier materials. As a general trend, the value for the direct gap decreases with increasing VBO. The experimentally determined energies of the band gap are in reasonable agreement with the values obtained by the theoretical calculations. A comparison between theory and experiment shows that the observed transition energies are closer to calculations employing a large offset (350 meV) as opposed to a small VBO (40 meV). Y1 - 1992 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-37878 ER - TY - CHAP A1 - Bicknell-Tassius, R. N. A1 - Scholl, S. A1 - Becker, Charles R. A1 - Landwehr, G. T1 - High magnetic field transport in II-VI heterostructures N2 - In the present work we report the results of magneto-transport measurements on some Hg-based li-VI semiconductor epitaxiallayers grown by molecular beam epitaxy. The transport measurement were carried out at temperatures in the range 0.4 - 4.2 K in magnetic fields up to 10.0 T. Further, we point out the necessity of using multicarrier models for data interpretation and show finally some Shubnikov-de-Haas results on sampies with high mobility carners. Y1 - 1992 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-37797 ER - TY - JOUR A1 - Becker, Charles R. A1 - He, L. A1 - Einfeldt, S. A1 - Wu, Y. S. A1 - Lérondel, G. A1 - Heinke, H. A1 - Oehling, S. A1 - Bicknell-Tassius, R. N. A1 - Landwehr, G. T1 - Molecular beam epitaxial growth and characterization of (100) HgSe on GaAs N2 - 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 lOO and ISO, 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 x 10^3 cm2 /V' s with increasing electron concentration. The concentration was found to be between 6xlO^17 and 1.6x10^19 cm- 3 at room temperature. Rocking curves from X-ray diffraction measurements of the better epilayers have a full width at half maximum of 5S0 arc sec. KW - Physik Y1 - 1993 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-50947 ER - TY - JOUR A1 - Becker, Charles, R. A1 - Latussek, V. A1 - Heinke, H. A1 - Regnet, M. M. A1 - Goschenhofer, F. A1 - Einfeldt, S. A1 - He, L. A1 - Bangert, E. A1 - Kraus, M. M. A1 - Landwehr, G. T1 - Molecular beam epitaxial growth and characterization of (001) Hg\(_{1-x}\) Cd\(_x\) Te-HgTe superlattices N2 - The molecular beam epitaxially growth of (001) Hg\(_{1-x}\) Cd\(_z\) Te-HgTe superlattices has been systematically investigated. The well width as well as the period were determined directly by X-ray diffraction. This was accomphshed for the well width by exploiting the high reflectivity from HgTe and the low reflectivity from CdTe for the (002) Bragg reflection. Knowing the well and barrier thicknesses we have been able to set an upper limit on the aver~ge composition of the barriers, Xl, by annealing the superlattice and then measuring the composition of the. resultmg alloy. Xb was shown to decrease exponentially with decreasing barrier width. Xb is appreciably smaller m. narrow barriers due to the increased significance of interdiffusion in the Hg\(_{1-x}\)Cd\(_x\) Te/HgTe interface in narrow barriers. The experimentally determined optical absorption coefficient for these superlattices is compared WIth theoretical calculations. The absorption coefficient was determined from transmission and reflection spectra at 300, 77 and 5 K. Using the thickness and composition of the barriers and wells, and an interface width due to interdiffusion, the complex refractive index is calculated and compared with the experimental absorption coefficient. The envelope function method based on an 8 x 8 second order k . p band model was used to calculate the superlattice states. These results when inserted into Kubo's formula, yield the dynamic conductivity for interband transitions. The experimental and theoretical values for the absorption coefficient using no adjustable parameters are in good agreement for most of the investigated superlattices. Furthermore the agreement for the higher energetic interband transitions is much worse if values for the barrier composition, which are appreciably different than the experimentally determined values, are used. The infrared photoluminescence was investigated at temperatures from 4.2 to 300 K. Pronounced photoluminescence was observed for all superlattices in this temperature range. KW - Physik Y1 - 1993 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-50959 ER - TY - JOUR A1 - Waag, A. A1 - Heinke, H. A1 - Scholl, S. A1 - Becker, Charles R. A1 - Landwehr, G. T1 - Growth of MgTe and Cd\(_{1-x}\)Mg\(_x\)Te thin films by molecular beam epitaxy N2 - We report on the growth of the compound semiconductor MgTe as weil as the ternary alloy Cd\(_{l-x}\)Mg, Te by molecular hcam cpitaxy. This is to our knowkdgc thc first time that this material has heen grown by any epitaxial technique. Bulk MgTe, which is hygroscopic, has a band gap of 3.0 eV and crystallizcs usually in thc wurtzite structure. Pseudomorphic films were grown on zincblende Cd Te suhstrates for a MgTe thickness helow a critical layer thickncss of approximately 500 nm. In addition, Cd\(_{l_x}\),Mg\(_x\)Te epilayers were grown with a Mg concentration between 0 and 68%, which corresponds to a band gap betwcen 1.5 and 2.5 eV at room temperature. The crystalline quality of thc layers is comparabk to CdTc thin films as long as they are fully strained. The lauice constant of zincblende MgTe is slightly smaller than that of CdTe, and the lattice mismatch is as low as O.7%. In addition highly n-type CdMgTe layers were fabricatcd by hromine doping. The tunability of the band gap as weil as the rather good laUice match with CdTc makes the matcrial interesting for optoelectronic device applications for the entire visible range. Y1 - 1993 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-37917 ER - TY - JOUR A1 - Herrmann, K. H. A1 - Happ, M. A1 - Möllmann, K.-P. A1 - Tomm, J. W. A1 - Becker, Charles R. A1 - Kraus, M. M. A1 - Yuan, S. A1 - Landwehr, G. T1 - A new model for the absorption coefficient of narrow gap (Hg,Cd)Te that simultaneously considers band tails and band filling N2 - A semiempirical model is presented that correlates the broadening of the absorption edge with both transitions below the energy gap and with transitions by the Kane band model. This model correctly fits both the absorption and luminescence spectra of narrow-gap (Hg,Cd)Te samples that have been grown by the traveling heater method as well as by molecular-beam epitaxy. The accuracy of the band-gap determination is enhanced by this model. Y1 - 1993 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-37894 ER -