Factors affecting Ge nanocrystal size in co-sputtered Ge+SiO 2 films
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Factors affecting Ge nanocrystal size in co-sputtered Ge+SiO2 films WK Choi, V Ng, YW Ho, TB Chen, V Ho Microelectronics Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576
ABSTRACT The high resolution transmission electron microscopy and Raman spectroscopy results of germanium nanocrystals embedded in SiO2 synthesized by rapid thermal processing (RTA) have been presented. From the results of samples with different Ge concentrations, it was concluded that there is a narrow window in the Ge concentration that can produce nanocrystals. We also showed that it is possible to vary RTA duration or temperature to produce Ge nanocrystals with varying sizes. Our results therefore suggest that it is possible to utilize (i) annealing duration and; (ii) temperature to tune crystal sizes for optoelectronic applications. INTRODUCTION Since the observation of photoluminescence from group IV (Si, Ge) nanocrystals, immense interest has developed in the growth, synthesis and characterization of these materials due to their potential in silicon-based optoelectronics application [1]. In this paper, germanium nanocrystals (nc-Ge) were synthesized by rapid thermal annealing (RTA) cosputtered thin film composing of silicon dioxide and germanium. The synthesis of nc-Ge via this method had been reported, where Raman spectroscopy was used to estimate the size of the nanocrystals synthesized [2]. The size variation of the Ge nanocrystals is of interest as this could assist in supporting any quantum confinement effects of the system. We report here an attempt to change the size of the Ge nanocrystals by (i) changing the Ge concentration during deposition and (ii) varying the annealing profile. We characterized the different films by Raman spectroscopy and high resolution transmission electron microscopy (TEM). EXPERIMENT The samples were prepared by co-sputtering of SiO2 and Ge (99.99%) on to a n-type Si substrate. Different number of Ge pieces (each of size 10 x 10 x 0.3 mm3) were attached to a 4" SiO2 target and co-sputtered in an Anelva sputtering system (SPH-210H) in argon ambient. The sputtering pressure was maintained at 3x10-3 Torr with rf sputtering power set to 100W. The film thickness was kept constant at approximately 3000Å. RTA was carried out in an A.S.T. rapid thermal processor (SHS 10). The annealing profile is shown in figure 1 with annealing temperature Tp varying from 600-1000ºC and annealing time tp varying from 0-300s. The ramp-up and ramp-down rates were fixed at 30ºC s-1. Raman measurements were performed on a Renishaw 2000 Micro-Raman spectroscopy system with a 514.5nm argon laser. High resolution transmission electron microscopy was carried out using model Philips FEG300CM operating at 300kV.
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Temperature (ºC) tp Tp
ramp-up, ramp-down rate fixed at 30ºC/s
Time (s)
Figure 1. Rapid thermal annealing profile of the samples. Tp was varied from 600-1000ºC while tp was varied from 0-300s. RESULTS AND DISCUSSION (i) Ge concentrat
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