Growth of InAs nanocrystals embedded in SiO2 films by radio-frequency magnetron cosputtering
Introduction
A wide gap optically transparent insulating matrix, such as SiO2, provides an ideal host environment for nanocrystals. The optical properties of semiconductor nanocrystals with size as small as a few nanometers (usually called quantum dots) embedded in SiO2 have been extensively studied 1, 2, 3because they are expected to exhibit a very large optical nonlinearity and offer a possibility of fabricating novel photoelectric devices. Semiconductor-doped SiO2 thin films are often fabricated using traditional radio-frequency magnetron cosputtering technique, which has many advantages as described by Tsunetomo et al. [4]. Until now most of the experimental work based on linear and nonlinear optical spectroscopy have been made in order to clarify the size-quantized electronic states in these dots and explore new optical phenomena related to the size quantization (quantum size effects) 5, 6, 7. However, to the best of our knowledge, no systematic investigation on the growth behavior of semiconductor nanocrystals embedded in SiO2, as a function of the RF cosputtering parameters has been reported. Knowledge of the growth behavior is indispensable for fabrication of desired samples with a good size distribution.
InAs is one of the most important semiconductors with a band gap of 0.35 eV which falls into the infrared spectral region. During the last two years, considerable efforts have been focused upon InAs self-assembled quantum dots prepared by molecular beam epitaxy 8, 9, 10. It has been revealed that InAs epitaxially grows in coherent island Stranski–Krastanow (S–K) mode. Very recently, InAs nanocrystals, ranging from 4 to 14 nm in size embedded in SiO2 matrices by applying a RF cosputtering technique, have been successfully prepared. The results of room temperature photoluminescence (PL) measurements have been reported in Ref. [11], which demonstrate that the PL from InAs nanocrystals originates from the radiative recombination of the quantum-confined electron-heavy hole excitons and electron-splitoff hole excitons. This paper is an extension of the previous work and reports the results of transmission electron microscope (TEM) observation and optical absorption measurements. We will focus our attention on the studies of the growth behavior of InAs nanocrystals in composite films, as a function of the RF cosputtering parameters.
Section snippets
Sample preparation
Two steps have been usually employed to synthesize semiconductor nanocrystals by RF magnetron cosputtering 12, 13: deposition of an amorphous InAs/SiO2 composite film, and subsequent post-annealing at a temperature of 700–1000°C for several or tens of hours to form nanocrystals. Because of this long high-temperature annealing step, a poor InAs size distribution of nanocrystals occurs, making it difficult to obtain a narrow-linewidth luminescence and a sharp optical absorption edge. The samples
Substrate temperature
During deposition, substrate heating was caused by two independent cumulative processes, i.e., inherent heating from the plasma and conduction heating from the sample holder. It was observed that the substrate temperature was raised to ∼65°C solely by the hot plasma. The TEM images for the films deposited at different substrate temperatures are presented in Fig. 1. For the 65°C sample, no InAs nanocrystals were found (Fig. 1a), indicating that InAs nanocrystals are not yet formed at such a low
Conclusions
The growth behavior of InAs nanocrystals embedded in SiO2 matrices has been studied systematically. The fractal phenomenon of InAs in InAs–SiO2 composite film is first observed. With increasing substrate temperature, InAs undergoes transitions from a dispersed phase, to a fractal distribution, particle formation, and finally to grain growth. The average sizes of InAs nanocrystals are well described as a linear function of substrate temperature ranging from 250 to 400°C. Above 400°C, the average
Acknowledgements
This work was supported by the Chinese Academy of Sciences. We would like to thank Professor J.P. Shui for fruitful discussion and Dr. X.Y. Qin for help in language.
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