Date of Graduation
CdGeAs2 is a nonlinear optical material that can be used in frequency conversion applications in the infrared. In this work, absorption and photoluminescence (PL) experiments were performed on over 200 CdGeAs 2 bulk crystals from 5 K to room temperature. The data can be explained by a model of three defects, including one shallow donor, one shallow acceptor, and one deep acceptor. The samples were grown by the horizontal gradient freeze technique, and as-grown unintentionally doped samples are p-type. An absorption band near 5.5 Î¼m due to an intervalence-band transition is observed in those samples which have enhanced concentration of the shallow acceptor. Three other absorption bands were separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E || c. A second broad band peaking near 0.52 eV is detected using E âŠ¥ c. These two bands are transitions from the top two valence bands to the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E || c. This band is assigned to transitions from the shallow acceptor to the shallow donor states and/or conduction band states. The absorption edge of CdGeAs2 agrees with models invoking the Urbach tail. Structural disorder and the exciton-phonon interaction are responsible for the broadening of the absorption edge. Two PL bands are generally seen in p-type CdGeAs 2 crystals and both are donor-acceptor pair recombinations. The 0.55-eV PL band is the transition from a 14-meV donor to a 120-meV shallow acceptor with the presence of potential fluctuations. The 0.35-eV PL band is a common feature for the p-type samples, and is the transition from the same donor to a 260-meV deep acceptor. Correlations have been established between the 5.5-Î¼m absorption band and a 0.55-eV PL band, an electron paramagnetic resonance signal, and hole concentration in p-type CdGeAs2 samples. The donor-doped CdGeAs2 n-type samples do not have the 5.5-Î¼m absorption band, and display high free carrier absorption. Samples doped with Te and Se impurities do not have the 0.55-eV PL band, and highly In-doped samples do not have the 0.35-eV PL band. These results support that the shallow acceptor is GeAs and the deep acceptor is the Cd vacancy.
Bai, Lihua, "Optical properties of cadmium germanium arsenide." (2004). Graduate Theses, Dissertations, and Problem Reports. 8433.