Impact of the H 2 anneal on the structural and optical properties of thin and thick Ge layers on Si; Low temperature surface passivation of Ge by Si J.M. Hartmann a,n , A. Abbadie a , J.P. Barnes a , J.M. Fe´de´ li a , T. Billon a , L. Vivien b a CEA-LETI, Minatec – 17, Avenue des Martyrs, 38054 Grenoble Cedex 9, France b IEF Bˆ at. 220, University Paris 11, 91405 Orsay, France article info Article history: Received 2 July 2009 Received in revised form 12 November 2009 Accepted 27 November 2009 Communicated by D.W. Shaw Available online 5 December 2009 Keywords: A1. Surface structure A1. Volume defects A1. Diffusion A1. Stresses A3. Chemical vapor deposition processes B2. Semiconducting germanium abstract Using a low temperature/high temperature strategy, we have grown thin (0.27 mm) and thick (2.45 mm) Ge layers on Si(0 0 1) substrates that we have submitted to various constant temperature (750 1C) or cyclic (750 1C/890 1C) H 2 anneals, the objective being to identify those yielding the smoothest surfaces, the lowest threading dislocations densities (TDDs) and the highest near infra-red optical absorptions. The best trade-off for thin layers was 750 1C, 60 min H 2 anneals. Using longer duration 750 1C anneals and especially 750 1C/890 1C cyclic anneals indeed yielded rougher surfaces and vastly degraded optical absorption (deleterious formation of GeSi alloys). By contrast, short 750 1C/890 1C thermal cyclings yielded the best metrics in thick Ge layers (while being at the same time the best in terms of throughput): root mean square surface roughness around 0.8 nm, TDD around 10 7 cm 2 , slightly tensily-strained layers (which a plus for optical absorption as the absorption edge is shifted to higher wavelengths), a limited penetration of Si into Ge (and thus absorption coefficients at 1.3 and 1.55 mm almost equal to those of as-grown layers), etc. We have also described the low temperature (450 1C/525 1C) process that we have developed to passivate Ge surfaces thanks to SiH 4 prior to gate stack deposition. Si layer thickness should be below 20 ˚ A in order to have conformal deposition. A transition of the growth front to 3 dimensions has indeed been evidenced for 20 ˚ A and higher. & 2009 Elsevier B.V. All rights reserved. 1. Introduction The demand for low-cost and efficient photo-detectors operating in the low loss windows (1.3–1.6 mm) of silica fibers is growing rapidly in the fields of telecommunications and optical interconnections [1]. Pure Ge grown directly onto Si(1 0 0) is the best candidate for such photo-detectors, due to its bandgap of 0.77 eV at room temperature (slight tensile strain) [2–4]. Ge(0 0 1) layers can also be used in conjunction with advanced gate dielectrics such as HfO 2 for the formation of bulk Ge [5,6] or Ge-On-Insulator (GeOI) [7–9] – based Metal Oxide Semiconductor Field Effect transistors (MOSFETs) with superior hole mobilities. Finally, due to their small lattice mismatch with GaAs (a Ge = 5.65785 ˚ A3a GaAs = 5.6533 ˚ A) and similar thermal expansion coefficients, Ge(0 0 1) layers can be used as templates for the growth of GaAs-based heterostructures such as diodes and solar cells [10], laser diodes [11], high electron mobility and hetero- junction bipolar transistors [12,13], etc. However, due to the large lattice mismatch with Si (4.2%), it is not easy to obtain on Si(1 0 0) Ge films with suitable character- istics. Rather thick ( 41 mm), flat Ge layers with a defect density as low as possible are indeed mandatory (i) for the fabrication through layer transfer of high quality GeOI or SOLES (which stands for silicon on lattice-engineered substrate [13]) substrates and (ii) in order to obtain high speed electronics devices [5,6]. In contrast, rather thin (a few hundreds of nm) Ge layers with a defect density as low as possible are normally used as the active media of low dark current, near Infra-Red (IR) photo-detectors at the end of SiO 2 -clad Si waveguides (butt or evanescent coupling). Several routes have been explored the last 12 years or so to obtain such films. SiGe virtual substrates graded all the way up to pure Ge yield low threading dislocations densities (  10 6 cm 2 ), fully relaxed but rough (i.e. cross-hatched) Ge layers on top [14–16]. Chemical mechanical polishing is then mandatory to recover smooth surfaces. Another route, initially proposed by Colace et al. [17] (followed quite closely by Hernandez et al. [18]), relies on the low-temperature growth of a thin Ge ‘‘seed’’ layer, followed by the high temperature deposition of a thick Ge layer [2,17–23]. The Low Temperature (LT: 330–450 1C) adopted for the first Ge layer plastically relaxes the strain in the Ge film without any 3D ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.11.056 n Corresponding author. Tel.: + 33 04 38 78 95 24; fax: + 33 04 38 78 30 34. E-mail address: HartmannJM@chartreuse.cea.fr (J.M. Hartmann). Journal of Crystal Growth 312 (2010) 532–541