Much excitement has surrounded the accelerating development of β-Ga2O3 for electronics due to its ultrawide band gap, high breakdown voltage, compatibility with many dopants, and comparative ease of producing large substrates via meltgrowth techniques. Our research has focused on growth and characterization of Czochralski (CZ) and vertical gradient freeze (VGF) single crystals of β-Ga2O3 with various dopants, including donors (Zr, Hf, Cr), acceptors (Mg, Zn, Fe, Ni, Cu), and alloying elements (Al). We find in general that doping in CZ and VGF materials can be different and sometimes non-uniform due to the interaction with crucible material (Ir), selective evaporation, and thermal profile. We have also explored the creation and identification of gallium vacancies (VGa) through annealing, by using positron annihilation spectroscopy (PAS), hydrogenated Fourier Transform Infrared (FTIR) spectroscopy, and electrical measurements. Different analysis techniques probe different spatial and depth averages, and thus careful consideration must be given to correctly interpret results and significance of defect concentrations determined. Insights from our work to date are offered, in terms of their applicability to devices.
Non-stoichiometry related extended defects in CdTe/CZT, such as tellurium inclusions and precipitates are known to be detrimental bulk defects in detector grade cadmium zinc telluride. In our attempt to minimize the size of tellurium inclusions we have employed accelerated crucible rotation technique in modified vertical Bridgman growth configuration. Acceleration and deceleration rate as high as 900 rpm2 was successfully applied during superheated melt mixing and growth. By comparing growths with and without ACRT under otherwise identical growth conditions, it was observed that the average inclusion size reduced by more than 50 percent due to ACRT. Additionally, we will discuss the effect of forced melt convection on the axial zinc and dopant segregation profile. Electrical characterization, spectrometric performance and purity analysis of the grown crystals will be presented.
Different rotation profiles were applied to the ampoule during Vertical Bridgman (VB) growth of Cd0.9Zn0.1Te (CZT) for
enhancing compositional homogeneity, minimizing radial and axial segregation and stabilizing growth interface. The
radial Zinc (Zn) segregation was significantly minimized. Improvements in the axial Zn segregation were also
noticeable. The modifications in interface shapes during the growth were prominent from the photoluminescence (PL)
studies. Improved grain structure was observed with growths containing lower (0.5 wt. %) initial excess Te; however
with higher amount of initial excess Te (7.5 wt. %) high resistivity ingots with improved carrier properties were grown
consistently but with lower single crystal yield. With more intense rotations, resistivity values within the ingot varied by
an order of magnitude showing non uniformity in distribution of dopants.
Cadmium Zinc Telluride (CdZnTe/CZT) crystals were grown using a modified vertical Bridgman growth technique with
10 % Zn concentration at Washington State University (WSU). Analyses of the effects of volume (vol.) %, number
density (cm-3), mean diameter (μm) of secondary phases (SPs) and thickness (mm) of the CZT crystals on single crystal
properties such as carrier mobility lifetime (μτe) and resistivity (ρ) were performed. Some correlations were observed
between μτe values of different CZT crystals and vol. %, number density, mean diameter of SPs and thickness of the
crystals. High μτe and lower SP vol. % values were obtained for the ingots grown with rapid cool down times and with
no intentional amounts of excess Te/Cd. For the selected samples, the effects of the SPs on the μτe values were
established for the SPs whose mean diameters were ≤4 μm and >4 μm. These studies indicate vol. % and mean diameter
of SPs are the important parameters for CZT crystal performance as a radiation detector.
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