Ge based nanostructures for electronic and photonic devices S.K. Ray * , R.K. Singha, S. Das, S. Manna, A. Dhar Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India article info Article history: Received 4 December 2009 Received in revised form 7 January 2010 Available online 26 February 2010 abstract Self-assembled Ge x Si 1x islands were grown on Si(0 0 1) substrates by solid source molecular beam epi- taxy. Two different morphological shapes with different sizes were evolved by tuning the growth time at a constant deposition temperature. Micro-Raman analysis was carried out to investigate the composition, intermixing and strain of resultant islands. The observed broad infra-red photoluminescence signal from grown samples was associated with radiative recombination of holes confined in the Ge islands and elec- trons localized in the Si buffer layer. The PL peak position and intensity were found to be influenced by the islands size and intermixing of Si and Ge. The electrical properties of the islands were studied through photoexcited I–V characteristics and current imaging using conducting mode atomic force microscopy. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Self-assembled Ge nanoislands formed by molecular beam epi- taxy (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 comple- mentary metal–oxide semiconductor compatible light emitters operating in the 1.3–1.6 lm spectral range [1,2]. Photolumines- cence (PL) properties of these nanostructures were extensively studied. Compared to PL in planar Si/SiGe superlattices, the quan- tum 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-inter- faces 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 molecu- lar beam epitaxially (MBE) grown Ge/Si islands along with their correlation with interband photocurrent characteristics. The man- ifestation of electrical charging of Ge quantum dots has been investigated by conducting atomic force microscopy (C-AFM). 2. 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 X 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 heated at elevated temperature 800 °C for 10 min and the deposi- tion of Si buffer layer started on fresh Si surface. Then gradually the substrate temperature was reduced to 550 °C and maintained dur- ing the growth of Ge. The Ge source was heated up to 1150 °C. Ge was deposited for 5 min (sample A) and 2 min (sample B) for the present set of samples. In Stranski–Krastanov growth mechanism, the arrangement of deposited Ge atoms begins with the formation of a strained planar layer called the wetting layer (WL), until a crit- ical thickness is reached. A further increase of the deposited thick- ness leads to the nucleation of three-dimensional islands on the wetting layer. The growth was monitored using in situ reflection high energy electron beam diffraction (RHEED) pattern. The shape and size of the grown islands were determined using Veeco, Nanoscope-IV atomic force microscope. The composition and in-plane strain of the samples were investigated through micro-Raman experiment in backscattering geometry using a REN- ISHAW 1000 system equipped with a Leica microscope. Measure- ments were performed at room temperature with 514.5 nm line 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.049 * Corresponding author. E-mail address: physkr@phy.iitkgp.ernet.in (S.K. Ray). Microelectronics Reliability 50 (2010) 674–678 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel