Drift-diffusion of highly mobile dopants in CdTe

The diffusion of Ag in CdTe exhibits anomalous concentration profiles, which essentially reflect the profile of the deviation from stoichiometry. At a diffusion temperature of about 800 K, the Ag dopant atoms are present as charged interstitials. The deviation from stoichiometry at diffusion temperature substantially changes upon an external source of Cd atoms. Such an external source can be represented either by the vapor pressure from metallic Cd or by a Cd layer arising at the interface to an evaporated layer of Cu or Au. Also, the Co diffusion in CdZnTe is shown to be strongly affected by the presence of an external vapor pressure of Cd, but in a substantially different way compared to the Ag diffusion in CdTe

Characterization of CZT crystals grown by the Boron Oxide Encapsulated Vertical Bridgman technique for the preparation of X-ray imaging detectors"

CdZnTe crystals are employed for the preparation of X- and Gamma- ray detectors. However, the large-scale production of these detectors is limited by the low yield of material with the required material properties, i. e. high resistivity, high carrier mobility lifetime product, and low inclusion density. In this frame, a new technique for the growth of CZT crystals has been developed based on a modification of the vertical Bridgman technique consisting in the encapsulation of the molten charge by a layer of boron oxide [1]. The authors have found that due to a chemical interaction with the quartz ampoule [2], a boron oxide layer fully encapsulates the crystal during growth preventing the direct contact of the crystal and the crucible walls. As a consequence, 2-inches CZT crystals characterized by large single grains and extremely low dislocations density have been grown. More recently, x-ray detectors with spectroscopic characteristics and good mobility lifetime product have been obtained using these crystals [3]. In this work we present an extensive characterization of these crystals with different techniques, ranging from energy dispersion x-ray analysis, photoluminescence mapping, contactless resistivity mapping, and infrared transmission. As a result, it is determined the interface shape and the zinc concentration distribution, that is related to the fluido-dynamic properties of the melt during growth. Moreover, the good homogeneity of resistivity is demonstrated. Finally, the typical inclusion density of these crystals is studied, hence the possibility to use them for the preparation of imaging and high flux x-ray detectors

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

The interaction of free carriers with defects and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here, a multi-method approach is used to quantify and characterize defects in single crystal MAPbI(3), giving a cross-checked overview of their properties. Time of flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo-Hall and thermoelectric effect spectroscopy assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation allows for quantifying the lifetime, mobility, and diffusion length of holes and electrons separately. Here, it is demonstrated that the dominant part of defects releases free carriers after trapping; this happens without non-radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease drift mobility sensibly. The results are the key for the optimization of the charge transport properties and defects in MHP and contribute to the research aiming to improve perovskite stability. This study paves the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes

Oxidation - inhibited electroless preparation of (CdZn)Te - Au contacts

V článku jsou studovány elektrické a mechanické vlastnosti zlatých elektrických kontaktů nanesených na sloučeninu (CdZn)Te z vodného a metanolového roztoku bez použití elektrolýzy. Zatímco kontakty nanesené na vertikální plochy materiálu vykazovaly silnou injekci nosičů náboje při napětích větších než 100 V, kontakty nanesená na plochy horizontální injekci nevykazovaly. Tento jev jsme vysvětlili rozdílnou krystalovou orientací kontaktovaných povrchů. Po aplikaci dodatečných povrchových úprav a doplnění ochr anné elektrody („guard ring“) jsme získali materiál vykazující vysoký specifický odpor 1011 Ωcm. Kontakty nanesené z roztoku na bázi alkoholu vykazovaly oproti kontaktům z roztoku vodného jak lepší mechanické vlastnosti (adheze a homogenita), tak i mírně l epší elektrické vlastnosti (volt - ampérové charakteristiky).Gold contacts electrolessly deposited on (CdZn)Te surface from methanol and water - based solutions were studied. We have observed strong injection at voltages higher than 100 V that disa ppeared when using contacts on lateral sides instead of bottom ones. In our opinion this phenomenon depends on crystallographic orientation of contact surfaces. After additional surface treatment and applying guard ring we have obtained sample with bulk sp ecific resistivity of 1011 Ωcm. Alcohol - based contacts showed better mechanical (adhesion and uniformity) and very similar electrical (I - V curves) properties in compliance with water - based ones

Automatic Control System for the High Pressure CdTe Crystal Growth Furnace

CdTe and (CdZn)Te bulk single crystals have been widely used as substrates for MBE and LPE epitaxy of infrared (HgCd)Te as well as gamma- and X-ray detectors. The Cd1-xZnxTe (x = 0.04-0.1) single crystals with diameter up to 100 mm and height at most 40 mm were prepared in our laboratory in a vertical arrangement by gradual cooling of the melt (the Vertical Gradient Freezing method). Achievement of excellent crystal quality required full control of Cd pressure during the growth process and application of high Cd pressures (up to 4 bar) at growth temperature. An electronic control system was designed to control both temperature and internal pressure of two zones CZT crystal growth furnace by using two high performance PID controllers/setpoint programmers. Two wire current loop serial communication bus was used for the data exchange and computer control of the furnace electronics setup. Control software was written to supervise the crystal growth process and to collect all important data and parameters