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

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 ∼1 μm) 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.

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We have investigated phonon an harmonicity related thermal properties [e.g., coefficient of thermal expansion (α), Gruneisen parameter (γ), and phonon mean free path as limited by Umklapp scattering (Λ 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 GeO 2 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 αγ product in the temperature range of 80−800 K. The αγ 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 ), α, and γ. We have determined the anharmonicity coefficients and phonon lifetimes in Ge NWs and also estimated the Λ 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 αandγ for the NWs from the observed αγ product. The enhancement of α, in particular, in the Ge NWs has been explained as arising from significant softening of θ 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.