Materials Science and Engineering B89 (2002) 263 – 268 Crystalline to amorphous phase transition in very low temperature molecular beam epitaxy M. Bauer *, M. Oehme, E. Kasper Institut fu ¨r Halbleitertechnik, Uniersita ¨t Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany Abstract For the first time, we use in situ time resolved reflectivity (TRR) measurement to determine the thickness for crystalline – amor- phous transition. The use of TRR measurements to study the breakdown of the single crystalline growth and the following deposition of amorphous silicon ( -Si) is based on the higher refraction index of  -Si at the wavelengths used (950 and 670 nm). An increasing  -film thickness causes cycles of constructive and destructive interference and leads to a time-dependent reflectivity. Final amorphous film thicknesses have been confirmed by ex situ spectroscopic ellipsometry. Samples were prepared by the deposition of electron-beam evaporated Si in an MBE equipment. The temperature, as a function of heater power, was carefully calibrated for highly doped substrates, with a thermocouple mounted into a test substrate. Substrates were kept at constant growth temperatures between 100 and 250 °C. From an Arrhenius-type expression, we can deduce an activation energy of 0.36 0.05 eV. The growth rate dependence was investigated between 0.013 and 0.7 nm s -1 at a constant temperature of 200 °C. Rate dependent data can be fitted by a power law. A variation of the deposition rate by a factor of 100 causes a variation in epi thickness by a factor 5. Similar experiments were also performed on low doped substrates and with non-linear temperature ramp. The obtained substrate temperature depends strongly on doping because of free carrier absorption (differences up to 50 °C). © 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystalline to amorphous phase transition; Molecular beam epitaxy (MBE); Amorphous silicon; Low temperature epitaxy (LTE); In-situ monitoring; Time resolved reflectivity (TRR) www.elsevier.com/locate/mseb 1. Introduction Breakdown in homoepitaxy occurs during growth on Si(100) at very low temperatures (LT). Si layers de- posited at low temperatures initially develop single crystalline material, but then turn to amorphous ( ) at a critical thickness h epi . This epitaxial thickness h epi depends strongly on substrate temperature and also on the presence of impurities (doping atoms and residual gas atoms) and substrate crystal orientation. Besides the fundamental question of the low temper- ature limit of silicon molecular beam epitaxy (MBE), there are two reasons to investigate this regime. In view of the doping problem in silicon, the epitaxial tempera- ture is particularly significant for this system. If the epitaxial temperature is low enough to suppress all segregation effects while growth is still crystalline, then there is a wide range of practical applications for Si MBE, involving atomically sharp junctions [1 – 4]. The second reason is the usefulness of supersaturation of point defects, which are injected/created during very low temperature Si and SiGe epitaxy, that allows the production of very thin strained relaxed buffer (SRB) layers, called virtual substrates, where point defects and their interactions with dislocations play a key role [5]. Measurement of growth characteristic in very LT Si-MBE have been performed by several groups [6–13] using various techniques, such as cross-section trans- mission electron microscopy (TEM), reflection high en- ergy electron diffraction (RHEED) [14 – 16], low energy electron diffraction (LEED) [17 – 19] and Rutherford backscattering (RBS) spectrometry and channeling [8,20,21]. The various applied techniques have different sensi- tivity to disorder and one would indeed expect some variation in epitaxial temperatures for this reason * Corresponding author. Tel.: +49-711-6858-011; fax: +49-711- 6858-044. E-mail address: bauer@iht.uni-stuttgart.de (M. Bauer). 0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0921-5107(01)00777-2