Ge based nanostructures for electronic and photonic devices
Introduction
Self-assembled Ge nanoislands formed by molecular beam epitaxy (MBE) on Si have been intensively investigated in recent years. These nanoislands have sizes allowing the manifestation of quantum effects, making possible to study the exciting quantum phenomenon for spatially confined phonons and charge carriers. In addition, by changing the size, shape, density and composition of nanoislands, it is possible to design structures with tunable properties for optoelectronic and nanoelectronic applications. These three-dimensional SiGe nanoislands grown in the Stranski–Krastanov (SK) growth mode have attracted significant attention in optoelectronics due to their potential applications in complementary metal–oxide semiconductor compatible light emitters operating in the 1.3–1.6 μm spectral range [1], [2]. Photoluminescence (PL) properties of these nanostructures were extensively studied. Compared to PL in planar Si/SiGe superlattices, the quantum efficiency at elevated temperatures is found to be enhanced for the dots, most likely due to 3D carrier localization within the SiGe clusters and large energy barriers formed at the hetero-interfaces between the SiGe clusters and the surrounding Si matrix [2], [3]. These three-dimensional carrier confinement and hot carrier dynamics in the quantum dots open up the possibility of normal-incidence operation, which is forbidden in the Ge/Si quantum well superlattice [4], [5]. It has also been proposed that quantum dot infrared photodetectors are superior to superlattice due to lower dark current, and higher operating temperatures.
This paper reports the efficient photoluminescence of molecular beam epitaxially (MBE) grown Ge/Si islands along with their correlation with interband photocurrent characteristics. The manifestation of electrical charging of Ge quantum dots has been investigated by conducting atomic force microscopy (C-AFM).
Section snippets
Experimental
The samples investigated in this work were grown by solid source molecular beam epitaxy (MBE, Riber Supra 32) on p-Si (0 0 1) substrates with a resistivity of 7–14 Ω cm. Prior to growth the chamber was first evacuated to 5 × 10−10 Torr with both ion and cryo-pumps. A typical growth started with the deposition of a Si buffer layer of 50 nm using electron gun followed strained Ge layer using a heated Knudsen cell. To remove the thin native oxygen and other impurities, the Si substrate was first rapidly
Structural analysis
The typical AFM topographic images of the as-grown samples viz., samples A and B are presented in Fig. 1a and b, respectively. From AFM topographic images, the variation of island shape, sizes and density is clearly visible. By eliminating the low frequency noise signal from the topographic images and fixing the respective threshold heights of images, the density of islands in these two samples has been evaluated. The density of islands in these two samples are 4.74 μm−2 (sample A) and 378 μm−2
Conclusions
In conclusions, we have demonstrated the time evolution of Ge islands on Si during growth. Two different characteristic sizes and shapes are evolved by the termination of growth in two different times. As revealed by AFM and micro-Raman analysis, the sizes of the short time grown islands are smaller compared to longer time one with the composition of the former being Ge dominated. The size and composition dependent photoluminescence emission have been investigated. It has been observed that the
Acknowledgement
The work was supported by FIR project grant sponsored by DRDO, Government of India.
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Structural and electrical investigations of MBE-grown SiGe nanoislands
2018, Applied Physics A: Materials Science and Processing