Radiative Versus Nonradiative Decay Processes in Germanium Nanocrystals Probed by Time-resolved Photoluminescence Spectr
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Radiative Versus Nonradiative Decay Processes in Germanium Nanocrystals Probed by Time-resolved Photoluminescence Spectroscopy P. K. Giri1, R. Kesavamoorthy2, B. K. Panigrahi2, and K.G.M. Nair2 1 Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India 2 Materials Science Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India ABSTRACT Ge nanocrystals (NCs) of diameter 4−13 nm are grown embedded in a thermally grown SiO2 layer by Ge ion implantation and subsequent annealing. Steady state and time-resolved photoluminescence (PL) studies are performed on these embedded Ge nanocrystals to understand the origin of the PL emission at room temperature. Steady state PL spectra show a broad peak consisting of a peak at ~2.1 eV originating from Ge NCs and another peak at ~2.3 eV arising from ion-beam induced defects in the Ge/SiO2 interface. Time-resolved PL studies reveal double exponential decay dynamics of the PL emission on the nanoseconds time scale. The faster component of the decay with large amplitude and having a time constant τ1~3.1 ns is attributed to the nonradiative lifetime, since the time constant reduces with increasing defect density. The slower component with time constant τ2~10 ns is attributed to radiative recombination at the Ge NCs. These results are in close agreement with the theoretically predicted radiative lifetime for small Ge NCs. INTRODUCTION Over the last decade, studies on the optical properties of group IV semiconductor nanocrystals (NCs) have attracted much interest because of their potential applications in Sibased optoelectronics, nanophotonics, and electronic/optical memory devices. Si NCs of several nanometers in diameter show indirect band gap nature [1] and this results in a relatively long photoluminescence (PL) lifetime [2], which is one of the main obstacles to realizing Si-based light emitting devices. In contrast, Ge NCs show a stronger confinement effect [3] resulting in a direct gap semiconductor nature [4]. There have been several theoretical predictions on the superior optical properties of Ge NCs [5] as compared to the Si NCs. However, there is a lack of consensus about the origin of intense visible PL from Ge NCs [6,7]. While majority of the reports have indicted that the defects in the host matrix are primarily responsible for broad PL in the visible region [8,9], some studies have attributed the visible and near-infrared PL to the Ge NCs. Size dependent near-infrared PL [10] and fast (
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