Raman investigation of GaP–Si interfaces grown by molecular beam epitaxy A. Bondi a , C. Cornet a , S. Boyer a , T. Nguyen Thanh a , A. Létoublon a , L. Pedesseau a , O. Durand a , A. Moreac b , A. Ponchet c , A. Le Corre a , J. Even a, ⁎ a Université Européenne de Bretagne, INSA, FOTON, UMR CNRS 6082, 20 Avenue des Buttes de Coësmes, F-35708 Rennes, France b Institut de Physique de Rennes, UMR-CNRS n°6251, Université Rennes1, Campus de Beaulieu — 35042 Rennes cedex, France c CEMES, UPR CNRS 8011, F-31055 Toulouse, France abstract article info Available online 16 December 2012 Keywords: Raman spectroscopy Strain Defects Raman spectroscopy was used to investigate the residual strain in thin GaP layers deposited on Si substrates by molecular beam epitaxy. Different growth conditions were used to obtain a clean GaP–Si interface, includ- ing migration enhanced epitaxy. The strain induced Raman shifts of the longitudinal and the transverse optical GaP lattice modes were analyzed. The effects of crystalline defects are discussed, supported by high resolution transmission electron microscopy and X-ray scattering studies. Finally, Raman Spectroscopy reveals the presence of disorder (or surface)-activated optical phonons. This result is discussed in the light of surface morphology analyses. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Today, an important challenge in the development of Opto-Electronic Integrated Circuits is to realize efficient light emitters on Si substrate, in order to combine and integrate III–V and Si technologies [1]. However, the lattice mismatch between most of the III-V materials and Si leads to the creation of a large number of dislocations [2]. This difficulty is by-passed by the pseudomorphic growth of a thin GaP layer below the critical thickness (~0.4% lattice mismatch) [3]. This route bypasses expensive production steps encountered by wafer bonding and die- bonding. The Si lattice-matched GaP(N) diluted-nitride alloy (2.2% N assuming Vegard's law), known to promise a high luminescence effi- ciency, can then be grown, opening the way for subsequent defect-free device elaboration [4]. In this approach, the reduction of defects gener- ated at the GaP–Si hetero-interface and propagating in the structure is a key issue for the upcoming realization of optical emitters, especially lasers, on Si substrate. Various epitaxial growth modes have been studied using mainly Molecular Beam Epitaxy (MBE) or Metal Organic Vapor Phase Epitaxy to control the nucleation of GaP on Si. The best results were obtained using special growth modes such as Migration Enhanced Epitaxy (MEE) [3,5], or Flow-rate Modulated Epitaxy [6] instead of conven- tional continuous epitaxy. These techniques result in higher diffusivity of the atoms during the nucleation allowing to maintain 2D growth mode and thus low surface roughness. The key issue of the GaP nucle- ation on Si, is to avoid the formation of crystalline defects: antiphase domains (APD), stacking faults (SF), and microtwins (MT) [3,7,8] orig- inating from bad stacking of atoms, resulting in lattice symmetry breaking. To this end, High Resolution Transmission Electron Microscopy (HRTEM) is used to characterize these defects at a local scale [9], X-ray diffraction (XRD) to study the structural properties throughout the film in its entirety [10–12] and Atomic Force Microscopy (AFM) to analyze surface morphology [11,12]. In this paper, Raman spectroscopy is used to characterize GaP layers grown on Si substrate. The analysis of longitudinal optical (LO) and transverse optical (TO) phonons lines (frequencies, intensities, shapes) is expected to yield useful information on the strain state, porosity and crystalline quality of the GaP samples [13–15]. The exper- imental observations are supported by empirical and ab initio simula- tions of the atomic vibrations. A comparison with TEM analyses and previous AFM and XRD results is then performed [11,12]. 2. Experimental details Four samples were grown in a Riber Compact 21 Solid Source Molecular Beam Epitaxy (MBE) system, on a (001) Si substrate misoriented of 4° in the [110] direction to favor the formation of dou- ble Si steps and, hence, to reduce the APD density [11,12]. These sam- ples are labeled 90-MBE, 20-MBE, 20-MEE and 20-a-MEE, depending on their growth conditions and thicknesses, as described in the following. Si substrates were cleaned using a standard RCA procedure in order to eliminate surface contaminants. The 90-MBE and 20-MBE samples have been grown by conventional MBE growth mode under a phosphorus excess during the growth sequence. The 20-MEE and 20-a-MEE samples have been grown using MEE [3] in order to enhance the smoothing of the growth front and therefore favor a pseudo-2D growth. The 90 nm-thick 90-MBE sample has been grown at 580 °C whereas for the three 20 nm-thick GaP layers (20-MBE, 20-MEE and 20-a-MEE) were grown at 450 °C. For the 20-a-MEE sample, an Thin Solid Films 541 (2013) 72–75 ⁎ Corresponding author. E-mail address: jacky.even@insa.rennes.fr (J. Even). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.132 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf