F. Marzo

A MOVPE technology for fabrication of CdTe-based homoepitaxial p-i-n diode structures as nuclear radiation detectors

The homoepitaxy of n-CdTe:l layers is reported as a technological step towards the fabrication of CdTe-based p-i-n diode nuclear radiation detectors. CdTe:I layers were grown at 330degC on detector-grade (111)-oriented CdTe crystals by metalorganic vapour phase epitaxy. To ensure CdTe homoepitaxy, as-received substrates were treated before growth by etching in Br2-methanol and in-situ H2 heat-cleaning at 350degC. (111)-oriented layers with fairly good surface morphology were obtained on as-prepared substrates by growing under Cd-rich vapour conditions. I-doped samples turned out to be n-type with resistivity values around a few Omega.cm and electron concentration ~1016 cm-3, but substantial electrical compensation of I donors occurs in the material, likely due to the formation of unintentional VCd-lTe acceptor centres. Homoepitaxial n-CdTe:I/i-CdTe samples were used to fabricate a preliminary Pt/i-CdTe/n-CdTe:I/AI device structure. Improvement of electrical insulation between back and front electrodes in this M-i-n device was achieved by reactive ion etching of the CdTe:I layer around the Al electrode. This treatment turned out to be effective in reducing the current flowing through the device under reverse bias conditions by more than one order of magnitude.

Development of a CdTe/CZT epitaxial technology for fabrication of large area RT 1-100 keV X-ray photon detectors

This paper reports on the principles and application of H/sub 2/-transport vapor phase epitaxy (H/sub 2/T-VPE), an innovative growth technology for the fabrication of thick (i.e., up to several hundreds micron) CdTe and CdZnTe single crystal epitaxial layers for fabrication of X-ray detector arrays. Details and advantages of the method are illustrated along with properties of materials grown using a simple proof-of-concept H/sub 2/T-VPE reactor. These growth experiments, besides showing the potentials of the method, allowed also to identify critical parameters of the H/sub 2/T-VPE process and its control requirements. Such knowledge has been used to design, build and test an innovative H/sub 2/T-VPE reactor prototype, whose characteristics and operations are here described.

Au-Catalyst Assisted MOVPE Growth of CdTe Nanowires for Photovoltaic Applications

Vertically-aligned CdTe nanowire (NWs) were grown for the first time by metalorganic vapor phase epitaxy, using diisopropyl-telluride and dimethylcadmium as precursors, and Au nanoparticles as metal catalysts. The NWs were grown between 485 and 515 °C on (111)B-GaAs substrates, the latter overgrown with a 2-μm thick CdTe epilayer. To favor the Au-catalyst assisted process against planar deposition of CdTe, an alternate precursors flow process was adopted during NW self-assembly. Field emission electron microscopy observations and X-ray energy dispersive analyses of CdTe NWs revealed the presence of Au-rich droplets at their tips, the contact-angle between Au-droplets and NWs being ~130°. The NW height increases exponentially with the growth temperature, indicating that the Au-catalyzed process is kinetics-limited (activation energy: ~57 kcal/mol), but no tapering is observed. Low temperature cathodoluminescence spectra recorded from single NWs evidenced a band-edge emission typical of zincblend CdTe, and a dominant (defects-related) emission band at 1.539 eV.

On the MOVPE growth and luminescence properties of GaAs-AlGaAs core-multishell nanowire quantum structures

We report on the growth of GaAs-AlGaAs core-multishell nanowire quantum heterostructures by metalorganic vapor phase epitaxy, and their photoluminescence (PL) properties. Dense arrays of vertically-aligned GaAs nanowires were fabricated onto (111)B-GaAs wafers by Au-catalyzed self-assembly, and radially overgrown by two AlGaAs shells between which a few-nm thin GaAs shell was introduced to form a quantum well tube (QWT). Besides the GaAs nanowire core emission band peaked at around 1.503 eV, 7K PL spectra showed an additional broad peak in the 1.556- 1.583 eV energy interval, ascribed to the transition between electron and hole confined states within the QWT. The emission blue-shifts with the shrinkage of as-grown GaAs well tubes, as the nanowire local (on the substrate) density and height change.

On Absorption Properties of GaAs/AlGaAs Nanowire Arrays

Arrays of well-aligned GaAs/AlGaAs core-shell nanowires have strong potential for photovoltaic applications. We analyze simulated reflection, transmission and absorption spectra of periodic arrays of these nanostructures, showing high absorption that compares favorably with experimental data.

On the Luminescence of VLS-grown GaAs-AlGaAs Core-Shell Nanowires and its Dependence on MOVPE Growth Conditions

We report on the photoluminescence (PL) of GaAs-Al 0.32 Ga 0.68 As core-shell nanowires grown by MOVPE, and their dependence on the precursors V:III molar ratio utilised in the vapor during growth. It is shown that the PL emission of the GaAs nanowire core red-shifts with decreasing the V:III ratio from 30:1 to 4:1, an effect tentatively ascribed to the build-up of a space-charge region at the core-shell hetero-interface, the latter associated to the unintentional incorporation of impurities, namely C in GaAs and Si in AlGaAs.

A novel CdTe/CZT epitaxial technology for large area RT 1-100 keV X-ray photon detectors

We report on an innovative vapor phase epitaxial (VPE) technology for the growth of thick (i.e., up to several hundreds micron) CdTe and CdZnTe single crystal layers. The aim of this technology is to achieve the fabrication of large area X-ray detector arrays for operation in the 1-100 keV photon energy range. The new VPE method is a modification of the H/sub 2/-transport VPE (H/sub 2/T-VPE) technique previously developed in the authors laboratory for CdTe. These early growth experiments, besides showing the potentials of the method, allowed to identify critical parameters of the H/sub 2/T-VPE process, and its limitations. The new VPE method proposed here is based on the combination of metalorganic-VPE and H/sub 2/ reduction of solid CdTe to obtain the chemical transport of CdTe/CZT constituent elements into a hot-wall VPE reactor. Controlled and reproducible addition of dopant elements to the vapor can be similarly achieved for the in-situ (i.e. during growth) intentional doping/compensation of the epitaxial layers. Such technology is currently under testing using an ad-hoc designed and built reactor prototype, whose main characteristics are also described.