Clean, As- and P-terminated Si(100) surfaces were prepared with H-2 carrier gas and AsH3 and PH3 as precursors in an MOCVD chamber. Reflectance difference spectra (RDS) were taken in situ. Samples were then transferred under vacuum to an ultrahigh-vacuum (UHV) chamber for analysis with AES, LEED, and STM. Clean, As-terminated Si(100) surfaces were achieved by annealing the sample at T < 900 degreesC supplying either AsH3 or hydrogen (plus background As-x). Various preparation procedures were applied and benchmarked in UHV. Extended annealing under AsH3 led to strongly faceted surfaces whereas AsH3 flow of moderate concentration, temperature, and time led to flat, two-domain, (2 x 1)/(1 x 2) reconstructed surfaces. RD spectra almost identical to RD spectra of As/Si(100) surfaces prepared in UHV with MBE were obtained by annealing the samples under AsH3. Annealing under PH3 supply bore a new, two-domain (6 x 3)/(3 x 6) surface reconstruction at low flows, and a SiP compound at high flows. (C) 2004 Elsevier B.V. All rights reserved.