Abstract
Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.
Highlights
Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size
The morphology is similar to that of 700 °C annealed film, but the top part zone without SiGe NCs is expanded to 100 nm (Fig. 2a)
The size of the SiGe NCs are similar like in the film annealed at 700 °C but the crystallization of SiGe is better as selected area electron diffraction (SAED) pattern reveals (Fig. 2b)
Summary
Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. The scientific effort is spent to understand the conditions of SiGe NCs formation into dielectric matrices (e.g. SiO2, HfO2, Si3N4, Al2O3 or TiO2) which goes through the usual sequence of problems regarding reduction of the oxides, diffusion of Si and Ge in the oxide matrix, nucleation and growth, frequently followed by coarsening of nanocrystals due to Ostwald ripening[4,14] In these films, the optical and electrical properties can be managed by controlling the NC density and sizes that influences the carrier quantum confinement and in turn the materials properties[15,16,17]. Introducing SiGe NCs into oxides is beneficial for tuning the crystallization temperature, controlling the NCs size (in the case of multilayers) and minimizing the effects given by defects like recombination centers or fast leakage paths in SiGe-based films[4] All these properties are dependent on the matrix material. We obtained films with good photoresponse in VIS-SWIR, the main contribution in SWIR being given by the SiGe NCs
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