Reabsorption Effects of Direct Band Emission of Ge

摘要

Ge is an indirect bandgap material for research interest with possible photonic applications. In previous work, only direct bandgap emission was observed in the epitaxial Ge on Si, but both direct and indirect emissions were observed in the bulk Ge substrates. This discrepancy between epitaxial Ge and Ge substrate may be due to reabsorption of direct emission. In this work, the reabsorption is systematically studied by thinning the Ge wafer from 500 μm to 1 μm. The lifetime of bulk Ge substrate can also be extracted using thickness dependent photoluminescence (PL). A (100) single-sided polished, Sb-doped n-Ge wafer with doping concentration of 4E17 cm-3 measured by Hall measurement. The Ge wafers were bonded on Si wafers by epoxy, and were etched by HNO3, CH3COOH and HF solution. The thickness of Ge wafer bonded on Si is reduced from 500 μm to 1 μm by etching. The power density of PL was 360 mW / mm² using a 671 nm laser source. Fig. 1 shows the normalized PL spectra of the n-Ge bonded on Si with different Ge thicknesses with respect to the peak of indirect bandgap emission at 693 meV at room temperature. The relative intensity of the direct emission increases dramatically with decreasing Ge thicknesses due to the reabsorption of direct band emission. The PL spectra of 3 μm n-Ge bonded on Si as well as the 2.8 μm epitaxial Ge on Si in Ref. 2 are shown in Fig. 2. The low PL intensity of epitaxial Ge on Si were probably due to the larger density of point defects and dislocations (~ 3.7E6 cm-2) in epitaxial Ge on Si [1]. Moreover, the electron-hole plasma (EHP) recombination model and the direct bandgap recombination model [2] were used to fit the spectra of indirect and direct bandgap emission (Fig. 3). The integrated intensity ratio of the direct to indirect bandgap emission for 3 μm thick n-Ge bonded on Si is 0.43. Since the reabsorption depends on the depth of the photon emission in Ge, the carrier concentra- ion profiles are essential to calculate the reabsorption effect. The surface recombination velocities (SRVs) of 532 cm/s (obtained by quasi-steady-state photoconductance on double polished wafers) and 1E4 cm/s at the front surface and rear surface, respectively, are used to determine the carrier profiles using lifetime as a fitting parameter. The large rear SRV is due to the rough rear surface of Ge. The carrier distribution is affected by rear SRV seriously (Fig. 4). Fig. 5 shows the intensity ratio of the direct emission at 782 meV to the indirect emission at 693 meV for Ge bonded on Si with different thickness. For the best fit, a lifetime of 2.5 μs is obtained from the ratio versus thickness curve. The sample temperature of 3 μm Ge bonded on Si, obtained by EHP model, was about 20 °C higher than room temperature since the epoxy between Ge and Si is not a good thermal conductor and heat dissipation is not effective. The higher temperature excited more electrons to Γ-valley and enhanced the direct bandgap emission and the direct to indirect emission ratio should be ever lower at room temperature. Fig. 6 shows the integrated intensity ratios of direct to indirect bandgap emission for Ge bonded on Si as well as the epitaxial Ge on Si in Ref. 1. The integrated intensity ratio of direct to indirect bandgap emission for n-Ge bonded on Si samples varies from 0.11 to 0.48 from 500 to 1 μm. However, this ratio of epitaxial Ge on Si is 2.5, which is much larger than n-Ge on Si probably due to the defects in epitaxial Ge on Si can reduce the indirect emission by nonradiative Shockley-Read-Hall (SRH) recombination. References [1] S. -R. Jan, C. -Y. Chen, C. -H. Lee, S. -T. Chan, K. -L. Peng, C. W. Liu, Y. Yamamoto, and B. Tillack,