Effective Separation of Photogenerated Electron-Hole Pairs by Radial Field Facilitates Ultrahigh Photoresponse in Single Semiconductor Nanowire Photodetectors

We report an investigation on the observation of ultrahigh photoresponse (photogain, G(Pc) > 10(6)) in single nanowire photodetectors of diameter <100 nm. The investigation, which is a combination of experimental observations and theoretical analysis of the ultrahigh optical response of semiconductor nanowires, has been carried out with an emphasis on Ge nanowires. Semiconductor nanowire photodetectors show a signature of photogating where G(Pc) rolls off with increasing illumination intensity. We show that surface band bending due to depleted surface layers in nanowires induces a strong radial field (similar to 10(8 )V/m at the nanowire surface) that causes physical separation of photogenerated electron-hole pairs. This was established quantitatively through a self-consistent theoretical model based on coupled Schrodinger and Poisson equations. It shows that carrier separation slows down the surface recombination velocity to a low value (<1 cm/s), thus reducing the carrier recombination rate and extending the recombination lifetime by a few orders of magnitude. An important outcome of the model is the prediction of G(Pc) similar to 10(6) in a single Ge nanowire (with diameter 60 nm), which matches well with our experimental observation. The model also shows an inverse dependence of G(Pc) on the diameter that has been observed experimentally. Though carried out in the context of Ge nanowires, the physical model developed has general applicability in other semiconductor nanowires as well

Temperature-dependent Thermal Conductivity of a Single Germanium Nanowire Measured by Optothermal Raman Spectroscopy

We investigate temperature-dependent thermal conductivity kappa (T) in a single Ge nanowire (NW) using optothermal Raman spectroscopy, which utilizes the temperature dependence of Raman lines as a local probe for temperature. The experiment is carried out from 300 K to above 700 K, a temperature range in which thermal conductivity of single NWs has been rarely explored. The thermal conductivity of Ge NWs (grown by vapor-liquid-solid mechanism) at around room temperature is observed to lie in the range 1.8-4.2 W/m K for diameters between 50 and 110 nm. The thermal conductivity at a given temperature is found to follow a linear dependence on NW diameter, suggesting that the low magnitude of kappa (T) is determined by diffused scattering of phonons from the surface of NWs that reduces it severely from its bulk value. kappa (T) shows approximately 1/T behavior which arises from the Umklapp processes. The quantitative estimation of errors arising from the optothermal measurement and methods to mitigate them is discussed. We also suggest a quick way to estimate approximately the thermal conductivity of Ge and Si NWs using the above observations

Si microline array based highly responsive broadband photodetector fabricated on silicon-on-insulator wafers

We report a high responsivity broad band photodetector working in the wavelength range of 400-1100 nm made from a horizontal array of Si microlines (line width similar to 1 mu m) fabricated on a silicon-on-insulator (SOI) wafer. The array was made using a combination of plasma etching, wet etching and electron beam lithography. It forms a partially suspended (nearly free) Silicon microstructure on SOI. The array detector under full illumination of the device shows a peak Responsivity of 28 A W-1 at 750 nm, at a bias of 1 V which is more than an order of magnitude of the responsivity in a typical commercial Si detector (<= 1 A W-1). In a broad band of 400-1000 nm the responsivity of the detector is in excess of 10 A W-1. We could isolate the contributions of different parts of the microline to the photocurrent by using focused illumination. It was established through simulation that the partial suspension of the microlines in the array is necessary to obtain such high responsivity. The partial suspension isolates the microlines from the bulk of the wafer and inhibits carrier recombination by the underlying oxide layer leading to enhanced photoresponse which has been validated through simulation

Restoration of perovskite phase in the top layer of thin BTO film by plasma treatment and annealing

We report a simple method to restore the perovskite phase in the top surface/sub-surface region of a thin film (similar to 100 nm) of barium titanate (BTO) fabricated by pulsed laser deposition on a platinized silicon surface and thus enhance its dielectric and ferroelectric properties. Phase evolution, surface morphology with local chemical composition of as-grown BTO films have been studied as a function of laser fluence. Investigations using x-ray diffraction, grazing-angle incidence x-ray diffraction and depth resolved x-ray photoelectron spectroscopy show that even after achieving a good phase formation there can be a presence of non-perovskite TiO2 phase at the surface and subsurface in such films that degrades its dielectric and ferroelectric response. The restoration of the degraded top layer was done by a combination of low energy Ar plasma treatment followed by an annealing process that enhances Ba content

Phonons and Thermal Properties of Ge Nanowires: A Raman Spectroscopy Investigation and Phonon Simulations

We have investigated phonon an harmonicity related thermal properties e.g., coefficient of thermal expansion (alpha), Gruneisen parameter (gamma), and phonon mean free path as limited by Umklapp scattering (lambda(mfp))] for Ge nanowires (NWs) using temperature-dependent Raman spectroscopy as well as phonon dynamics simulations. The experiments were carried out in two types of NW ensembles. One type of NWs has only the native oxide layer on Ge, and the other type has relatively thicker GeO2 on the surface forming a core-shell structure. The temperature-dependent shift of the LO/TO Raman line of Ge (300 cm(-1)) was used to determine the alpha gamma product in the temperature range of 80-800 K. The alpha gamma product is enhanced compared to that observed in the bulk crystalline Ge over the whole temperature range. The experimental work was complimented by phonon simulations with quasi-harmonic approximation using density functional perturbation theory. The simulation allowed us to determine the thermodynamic parameters like bulk modulus, specific heat capacity (C-v), alpha, and gamma. We have determined the anharmonicity coefficients and phonon lifetimes in Ge NWs and also estimated the lambda(mfp) arising from phonon-phonon scattering (Umklapp process). Comparison of the computed thermal parameters with the experimental data allowed us to place a confidence limit on the calculated parameters, which was used to separate out the two parameters alpha and gamma for the NWs from the observed alpha gamma product. The enhancement of alpha, in particular, in the Ge NWs has been explained as arising from significant softening of theta(D) in the NWs as observed from the low temperature C-v calculated from the phonon simulations. Comparison of the computed phonon density of states shows appearance of excess weights in the phonon spectrum, which contributes to enhancement of heat capacity in NWs compared to that in the bulk