C. Boschetti

Molecular beam epitaxial growth of PbTe and PbSnTe on Si(100) substrates for heterojunction infrared detectors

Abstract We have investigated the molecular beam epitaxial (MBE) growth of the IV–VI compounds PbTe and PbSnTe directly on Si(1xa00xa00) substrates, by in situ reflection high energy electron diffraction characterization. PbTe/Si(1xa00xa00) heterostructures were previously obtained only by hot wall epitaxy, and were shown to work as room temperature heterojunction photovoltaic detectors for the mid-infrared band ( 3–5 μ m). Present work has shown that IV–VI single crystal layers, crack-free and as thick as 0.5 μm, can be obtained by MBE on thermally deoxidized Si(1xa00xa00) substrates. The layers were found to be rotated by 45° relative to the substrate azimuthal orientation, and their quality depended on the use of an additional Te flux.

Molecular beam epitaxial growth of high quality Pb1-xSnxTe layers with 0 ≤ x ≤ 1

Abstract Lead tin telluride epitaxial layers with tin concentrations covering the whole compositional range have been grown by molecular beam epitaxy on cleaved BaF 2 (1xa01xa01) substrates, using PbTe and SnTe solid sources. The full width at half maximum of the (2xa02xa02) rocking curve of these samples, measured by a four crystal high resolution X-ray diffraction apparatus, was about 400xa0arcsec for the ternary layers, and 150xa0arcsec for the binary samples. Low temperature Hall mobilities decreased from 5×10 5 xa0Vxa0cm 2 /s, for PbTe, to 2×10 3 xa0Vxa0cm 2 /s for SnTe, although the crystalline quality is approximately the same. This decrease in mobility is a consequence of the increase in hole concentration from 2×10 17 xa0cm −3 to 2×10 20 xa0cm −3 , when the Sn concentration changes from 0 to 100%. This high carrier concentration in the samples with high Sn content showed a large Burstein–Moss shift in the infrared transmission spectra measured between 77 and 300xa0K.

Growth of narrow gap epilayers and p-n junctions on silicon for infrared detectors arrays

Abstract Lead telluride epilayers and p-n junctions were grown on silicon substrates by hot wall epitaxy in order to construct monolithic infrared (IR) detectors arrays, using II-a fluorides as buffer layers. Schottky barriers were also made in the same epilayers to compare the figures of merit. The p-n junctions had performances comparable with the metal barriers and both devices had noise sources in addition to the expected thermal and background ones.

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.

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.

Band crossing evidence in PbSnTe observed by optical transmission measurements

Using high quality epitaxial layers, we have obtained direct evidence of the band inversion in the Pb1-xSnxTe system .The samples, covering the whole composition range, were grown by molecular beam epitaxy on (111)BaF2 substrates. A minimumin the resistivity as a function of temperature was observed for all samples with Sn composition 0.35 £ x £ 0.70. In the same samples and at the same temperature, temperature dependent optical transmission measurements have revealed a change in signal of the energy gap temperature derivative, a direct evidence of the band inversion. However, the temperature for which the inversion occurs is not the one expected by the band inversion model. This discrepancy is supposed to be due to the Burstein-Moss shift caused by the relatively high hole concentration observed in these samples.

Influence of interdiffusion on N-PbTe⧸P-Pb0.80Sn0.20Te heterojunction diodes

Abstract A study of the spectral response, detectivity and I vs V characteristics was made on N -PbTe/ P -Pb 0.80 Sn 0.20 Te heterojunctions HJ grown on BaF 2 substrates by the HWE technique. The results confirm that a potential barrier that appears at the HJ interface depends on the gradation width and on the relation between the N and P concentration. The PbSnTe HJ behavior indicates that the gradation model proposed for the HgCdTe HJ gives a better fit to our data than the junction migration model.

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.