Enhanced Performance of Infrared Detectors and Solar Cells by Sige Nanostructures

摘要

Short-period silicon/germanium (Si_mGe_n ) superlattice and SiGe quantum well (QW) structures have been grown by molecular beam epitaxy (MBE) on < 100 > Si substrates for near (1.3μ) and mid-infrared (3μ — 5μ; 8μ — 12μ) detection. For the near IR detection-suitable for fiber optical communication within an Si IC chip-SiGe QW with 2 to 4 monolayers thin, p-doped Ge quantum well layers and short-period Si_mGe_n superlattices with period lengths of several atomic monolayers (e.g. m = n = 5ML; 1ML ～ 1. 4 A ) have been grown. For Si_mGe_n superlattices the zone folding effect in growth direction has been predicted to produce a direct bandgap and strong optical interband transitions near 0. 8eV (≈ 1.3μ), in the SiGe QW's sharp Si/Ge hetero-interfaces break the k-selection rules and lead to strong spatial localization of electron and hole wave function within the wells which results in a strong interband excitonic transition at 1.3μ. An integrated waveguide-/ photodetector deposited on a SIMOX (Si substrate with separation by implantation of oxygen) substrate has been fabricated, an external quantum efficiency of 11 % at 2 GHz modulation frequency has been observed. For the mid IR range (3μ—5μ) highly p-doped Si/SiGe quantum well detectors have been deposited on undoped, double-sided polished Si substrates based on hetero-internal photoemission (HIP) over the Si/SiGe barrier. The absorption and photocurrent spectra have been measured from mesa detectors at 77K. The photoresponse spectrum of the HIP detectors has been shown to be widely tunable in the technological important wavelength range between 3μ — 5μ by choice of Ge-content, well thickness and doping level, the spectrum is considerably broader than that of Pt: Si. Quantum efficiencies of ～ 0. 6 % at 4 (and 77K have been achieved from SiGe HIP structures, dark currents as low as 10~(-8)A/ cm~2 and detectivity values of D~* 9 10~(11) cm (Hz)~(1/2)/W have been obtained in SiGe HIP detectors. In addition, growth of self-organized Ge-islands on Si-substrates deposited in the Stranski-Krastanow growth mode by Si MBE has been performed to increase absorption and quantum efficiency in thin film solar cells for space applications. Photoluminescence measurements, atomic force and transmission electron microscopy exhibit optimized three dimensional growth at temperatures around 700℃ and 8 monolayers of Ge coverage. By use of Sb surfactant during growth highly ordered Ge-islands with lateral densities up to 1 X 10~(11) cm~(-2) have been deposited on Si and periodically stacked on top of each other.