Core-shell Ge nanowires and heterojunctions for memory and photodetector devices

Germanium nanowires are attractive due to their higher carrier mobilities and lower effective masses and compatibility with Si-based CMOS devices. Semiconductor heterojunctions grown on one dimensional nanowire templates have been demonstrated to be useful for photoelectrochemical cell, light-emitting diodes, electrochromic devices, and sensor systems. This core-shell nanowire heterojunction possesses a unique advantage of high surface area-to-volume ratio with carrier separation taking place in the junction in radial direction within one diffusion length of minority carriers. These nanowires sandwiched/embedded in polymers are also attractive for applications in organic light emitting diodes, photovoltaic cells, flat panel displays and in organic field effect transistors. In addition, the memory devices using semiconductor/organic heterostructures make them useful for the realization of flexible and low cost polymeric electronic and optoelectronic devices. We shall present the growth and characteristics of Ge-CdS core-shell nanowire radial heterojunctions and memory characteristics of Ge/GeO₂ core-shell nanowires sandwiched between a tunnel silicon oxide and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) polymer. The growth of core-shell nanowires was based on vapour-liquid-solid (VLS) mechanism using a Ge ingot, under controlled conditions with the assistance of a gold catalyst. Before the deposition of Ge, a thin gold film was evaporated on oxidized p-type silicon substrates. The growth of nanowires was carried out at temperatures ranging from 750-950?C for different times in nitrogen ambient at a flow rate of 100 seem. Transmission electron micrographs revealed the formation of Ge-GeO core-shell nanowires, having core diameter of 20-50 nm² and shell diameter of 10-15 nm. The selected area electron diffraction pattern clearly indicated the formation of single crystalline Ge in the core region. The core-shell Ge/CdS nano- wires heterojunction was fabricated using a simple chemical process. Raman spectrum revealed the formation of phonon confined nanocrystalline CdS shell on Ge nanowires surfaces. Room temperature and low temperature photoluminescence characteristics of the nanowires as well as core-shell Ge/CdS nanowire heterostructure showed dominant defect related emission from the GeO shell region whereas a strong band edge emission from CdS was recorded. A blue shift of the energy gap of CdS with decreasing temperature was observed from the PL spectra. The photocurrent characteristics of Ge/CdS core-shell nanowires junction under reverse bias were studied in detail. An enhanced photoresponse with broad spectral bandwidth was observed in core-shell nanowire devices in the wavelength range of 450 - 1000 nm. The observed photoresponse can be essentially useful for fabricating broad band photovoltaic and photodetection devices based on nanowire heterojunctions. The hybrid polymer heterostructure was prepared by spin coating of MEH-PPV solution on Ge/GeO core-shell nanowires. The capacitance-voltage characteristic´s (C-V) of core-shell nanowires/conducting polymer device exhibited excellent metal-insulator-semiconductor behaviour with hysteresis due to charge storage at 300 K. The C-V characteristics at different operating frequencies revealed that the hysteresis was mainly due to the trapping of charge carriers in Ge nanowires rather than in the interfacial states. A simplified model based on tunneling of carries into the valence subbands of Ge was used to explain the charge storage behaviour. The study indicates that the core-shell nanowire/conducting polymer heterostructure holds promise for potential applications in next generation nonvolatile memory for polymer electronic and optoelectronic devices.