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.
We measured the resonator insertion losses, slope efficiencies and output power of six Nd doped Y3Al5O12 (YAG)laser crystals with and without chemical etching. It was found that chemical etching lowered the insertion losses and increased the slope efficiency. Point defects were investigated by positron annihilation spectroscopy (PAS). PAS depth profiles showed that the etching efficiently removed near surface atomic-scale defects. The results suggested that chemical etching is an effective means to improve lasing properties of Nd:YAG crystals, reduce the thermal loading and increase the strength.
The effects of two intrinsic deep levels on electrical compensation and effects of dislocations on carrier mobility in semi-insulating CdTe and CdZnTe radiation detector crystals are reported here. These levels were found in samples grown by conventional Bridgman and high-pressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of Es1 equals 0.27 +/- 0.07 eV, Ed1 equals Ev + 0.735 +/- 0.005 eV and Ed2 equals Ev + 0.743 +/- 0.005 eV. The shallow level (Es1) is associated with dislocations. The first deep level (Ed1) is associated with the doubly ionized Cd vacancy acceptor and the second deep level (Ed2) is associated with the Te-antisite (TeCd). The second deep level (TeCd) was found to electrically compensate the material to produce high resistivity CdTe and CdZnTe, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing. The dislocations were found to affect the mobility of the carriers in the CdTe and CdZnTe crystals.
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