H. Closs

Electrical properties of Pb1-xSnxTe layers with 0⩽x⩽1 grown by molecular beam epitaxy

In this work, the electrical properties of Pb1−xSnxTe epitaxial layers with Sn composition covering the whole range were investigated. The samples were grown on (111)BaF2 substrates in a molecular beam epitaxy system using PbTe, SnTe, and Te solid sources. As the alloy composition varies from PbTe to SnTe, the hole concentration increases exponentially from 1017 to 1020 cm−3 for Te-rich sources and from 1017 to 1019 cm−3 for stoichiometric ones. The resistivity of the samples, which depends mainly on their hole concentrations, shows an exponential dependence on the temperature with a slope which decreases as x goes from 0 to 1. For all Pb1−xSnxTe samples with x in the range of 0.35–0.7, the resistivity curve shows a very well defined minimum at low temperatures. This anomalous behavior is supposed to be related to the band crossing, where the energy gap temperature coefficient changes sign. The temperatures where the minimum in the resistivity occurs only agree with the ones predicted by the band inversio...

Reciprocal space maps of PbTe/SnTe superlattices

PbTe/SnTe superlattices were grown on (111) BaF2 substrates by molecular beam epitaxy using PbTe as buffer layers. The individual layer thickness and number of repetitions were chosen in order to change the strain profile in the superlattices from completely pseudomorphic to partially relaxed. The superlattices structural properties were investigated by making reciprocal space maps around the asymmetric (224) Bragg diffraction points and ω/2Θ scans for the (222) diffraction with a high resolution diffractometer in the triple axis configuration. With the strain information obtained from the maps, the (222) ω/2Θ scan was simulated by dynamical diffraction theory. The simulated spectra of the pseudomorphic superlattices, in which the in-plane lattice constant is assumed to be the same as the PbTe buffer throughout the superlattice, fitted in a remarkably good agreement with the measured data, indicating that almost structurally perfect samples were obtained. For the thicker superlattices, the (224) reciproca...

IV–VI Compound heterostructures grown by molecular beam epitaxy

Abstract Structural and optical characterization of some IV–VI superlattices (SL) and multi-quantum wells (MQW) grown by molecular beam epitaxy (MBE) on BaF 2 (111) substrates are shown. Three different types of systems were investigated, namely, PbTe/PbSnTe, PbTe/SnTe and PbTe/PbEuTe. High-resolution X-ray diffraction analysis was performed to determine the strain in the structures. The analysis revealed sharp interfaces and good thickness control. The transition energies between the confined levels in the wells were obtained from the absorption steps observed in infrared transmission measurements. Preliminary results on PbTe/Si heterojunction grown by MBE are also presented.

Electrical properties of PbTe doped with BaF2

We study here the p-type doping of PbTe with BaF2. For the investigation, PbTe layers were grown on (111) BaF2 substrates by molecular beam epitaxy. The beam flux ratio between BaF2 and PbTe, defined as the nominal doping level, was varied from 0.02% to 1%. The hole density increases from 5×1017 to 1×1019 cm−3 as the doping level rises from 0.02% to 0.4% and saturates at p∼1019 cm−3 for higher levels. The saturation effect was attributed to self-compensation. The carrier concentration of all samples remained almost constant as the temperature was varied from 10 to 350 K, indicating that no thermal activation is present in the whole doping range. It suggests that the impurity level in PbTe doped with BaF2 remains resonant with the valence band, similar to the native defects behavior. The low-temperature mobility showed a pronounced reduction from 50 000 to 2 000 cm2/V s as the doping level rises from 0.02% to 1%, mainly due to the substantial increase in the hole concentration. For temperatures higher than...

Experimental observation of band inversion in the PbSnTe system

The band inversion in the PbSnTe compound has been observed by optical and electrical measurements. The samples, high quality Pb1−xSnxTe epitaxial layers, have been grown by molecular-beam epitaxy on (111)BaF2 substrates. Optical transmission measurements have revealed a change in signal of the energy gap temperature derivative for samples with 0.35<x<0.70. In the same samples and at the same temperature, a minimum in the resistivity has been observed, showing a interrelation between the optical and electrical measurements. However, the temperature, for which the inversion occurs, is not that predicted by the band inversion model. This discrepancy is supposed to be due to the relatively high hole concentration of these samples.

Investigation of the surface properties of CaF2 layers on (1 1 1) Si as a function of growth temperature

This work reports on the study of surface properties of CaF 2 films (30 and 10 nm thick) grown on (1 1 1) Si by molecular beam epitaxy at substrate temperatures from 400 to 700 °C. Reflection high-energy electron diffraction (RHEED) analysis indicated that CaF 2 films with smooth surfaces were obtained in temperature ranges 500―550 °C and 620-700 °C, while at temperatures from 400 to 500 °C and in the vicinity of 600 °C the films showed grains randomly oriented on top of the surface. Atomic force microscopy (AFM) investigation corroborated with the RHEED results and confirmed the presence of grains on the film surface, with an evident transition near 600 °C. The dependence of grain density on the growth temperature followed the expectation from the RHEED analysis. The arithmetical average roughness of the CaF 2 surface obtained from the AFM images remained below 1 nm for the best quality films. The x-ray reflectivity curves of all samples exhibited well-defined interference fringes, whose oscillation damping behaviour agreed with the RHEED and AFM results. The CaF 2 layer thickness and roughness were accurately determined by a best-fit procedure applied to the x-ray reflectivity data. By combining all results, the temperature range between 525 and 550 °C was found to be the most suitable to grow CaF 2 layers on (1 1 1) Si. For growth temperatures above 650 °C, pinholes and cracks started to reduce the CaF 2 surface quality.