光電所所訊

Ordering InGaP epilayer directly grown on Ge substrate

Professor Hao-Hsiung Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所林浩雄教授

Ternary InGaP lattice-matched to GaAs has been well studied for decades and found applications to several important photonic and electronic devices, such as heterojunction bipolar transistors, laser diodes, and so on. Recently, the ternary has been deposited on Ge substrates as one of the subcell of the InGaP/InGaAs/Ge triple-junction tandem solar cells for high concentration photovoltaic. In this application, although the lattice constant of InGaP has been constrained to that of Ge substrate, one still can adjust the energy gap of InGaP by controlling the ordering of the ternary. The growth of the ternary semiconductor under certain conditions has a tendency towards atomic ordering due to the differences of the atomic size and the binding energy, which can spontaneously adopt a CuPt-type ordered structure. Fig. 1(a), indicates a good InGaP/Ge interface. Note that the miscut of the Ge substrate is towards direction. Steps resulting from the miscut are expected to be observed in the lattice image of the plane. However, as shown in Fig. 1(b), we did not observe the expected steps. The TED patterns with and zone axes are shown in Fig. 2(a) and (b), respectively. As can be seen, only superstructure spots … along direction are observed. For growth plane, there are four planes possible for Cu-Pt ordering. In Fig. 3(a), a sharp peak at 354 cm^-1 is observed. The peak has been previous reported in the literature, and was attributed to the trigonal cationic arrangement with LO symmetry. In order to understand the behavior of this mode, the normalized Raman spectra are plotted in Fig. 3(b). According to the C_3v selection rule, the 354 cm^-1 mode is only forbidden in configuration.

Fig. 1 High resolution cross-sectional TEM images of the InGaP/Ge interfaces. (a) on plane and (b) on plane.

Fig. 2 TED patterns of InGaP epilayer. (a) Zone axis is and (b) zone axis is .

Fig. 3 Room-temperature Raman spectra of the ordering InGaP with eight different polarization configurations. The eight configurations are , , , , , , and . (a) Full spectra with the eight configurations. The orientations of the polarizations are also shown in the inset. (b) Normalized spectra showing the peak at 354 cm^-1, which is relevant to the CuPt-B ordering phase.