IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 19 (2008) 145703 (5pp) doi:10.1088/0957-4484/19/14/145703 Band offsets and photocurrent spectroscopy of Si/Ge heterostructures with quantum dots S V Kondratenko 1 , A S Nikolenko 1 , O V Vakulenko 1 , M Ya Valakh 2 , V O Yukhymchuk 2 , A V Dvurechenskii 3 and A I Nikiforov 3 1 Kyiv National Taras Shevchenko University, Physics Department, 2 Academician Glushkov Avenue, Kyiv 03022, Ukraine 2 Lashkaryov’s Institute of Semiconductor Physics, NAS of Ukraine, 45 Prospekt Nauky, Kyiv 03028, Ukraine 3 Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Prospekt Lavrent’eva 13, 630090 Novosibirsk, Russia E-mail: kondr@univ.kiev.ua Received 10 October 2007, in final form 23 January 2008 Published 5 March 2008 Online at stacks.iop.org/Nano/19/145703 Abstract Raman and lateral photoconductivity spectra of self-assembled SiGe nanoislands were studied with a height of ∼2 nm and a base of ∼20 nm formed at a temperature of 500 ◦ C. It was estimated that the value of elastic deformation (ε xx ) was −0.022 (ε zz = 0.017), while the germanium content in the islands (x ) was 0.66. The obtained values of x and ε were used to calculate band offsets at the interfaces and the energy of interband transitions of structures under study. It was shown that the minimal energy of photocurrent observation is 0.52 eV, which is below the bandgap of the QDs under study. The first photocurrent component which began to contribute at 0.52 eV and had a peak at 0.68 eV is explained by optical transitions of electrons from the QD HH localized states of the valence band to the conduction band  2 valley of the surrounding silicon matrix in which tensile strains are present. The second component with limiting energy of 0.73 eV can be caused by interband electron transitions from the HH valence band of the QDs to the  4 valley of the QD conduction band. 1. Introduction Great interest in Si/Ge heterostructures formed on the basis of Ge quantum dots (QDs) embedded in an Si matrix is caused by the possibility of their use for the formation of active elements of microchips and photodetectors with improved characteristics as compared with 3D structures and the creation of principally new electronic and optoelectronic devices [1–3]. Quantum dots formed by the Stranski–Krastanow mechanism due to the difference in Ge and Si lattice constants (∼4%) are stressed and have nonuniform composition due to the processes of Si–Ge intermixing at 300–700 ◦ C[4–6]. A specific feature of these structures is the presence of nonuniform stresses both in the QDs and in the surrounding Si matrix. The energy spectrum of such structures is modified due to the quantum confinement effect and mechanical strains. Besides, the latter causes the QD component intermixing, which depends on the growth conditions, and affects the energy spectrum. Knowledge of the specific features of the energy spectrum and band alignment of the QDs embedded in the Si matrix is necessary for the development of electronic devices on the basis of such structures. Spatially direct and indirect interband transitions in SiGe heterostructures with QDs were studied by luminescence spectroscopy [7, 8], photocurrent spectroscopy [9–11] and optical absorption spectroscopy [12]. The intraband absorption due to transitions between confined holes and band-to- continuum electron transitions allows one to use such structures as infrared quantum dot photodetectors. The information about band offsets of the Si/SiGe interface is important from the practical point of view. This work is devoted to the analysis of lateral photoconductivity spectra and the calculation of the heterojunction band diagram using the values of strain and composition obtained from Raman scattering (RS) measurements. 0957-4484/08/145703+05$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1