Lihua Bai

Date of Graduation


Document Type



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 μm 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 ⊥ 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-μm 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-μm 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.