The effects of vacuum annealing on the optical absorption spectra in the visible and infrared ranges, photoluminescence intensity, and concentration of paramagnetic centers in free-standing porous silicon films were investigated in a temperature range of 100-600°C. It was found that heat-induced hydrogen desorption decreased the porous silicon band gap, which suggests that band-gap energy depends on hydrogen coverage of nanoparticles. The annealing also leads to increasing concentration of defects that were identified as silicon dangling bonds. The energy distribution of the dangling-bond states was estimated from the absorption spectrum.
In the earlier report (1) we observed negative and positive charging of the oxidized germanium surfaces induced by illumination.In the present work we attempted to get information on the nature of traps in germanium dioxide, thermally grown on germanium, by means of electrophysical methods and the electron spin resonance (ESR). The charging of slow surface states (SS) during the illumination of n-Ge single crystals (8 = 25 to 30ncm), oxidized in wet oxygen, was investigated by the field effect at a large sinusoidal Signal.The illumination was performed by a grating monochromator with a high-pressure xenon lamp. The ESR investigations were made on oxidized germanium powders a s well as on single crystals. The uniform optical excitation of powders was realized by continuous rotation of the ampoules with the powder during illumination.The illumination of single crystals with photons having energy 1.8 eV < h IJ < 1 < 3.3 eV, led to negative SS charging (AQ,), Fig. 1. This charging is caused by the transitions of excited electrons from the valence band of germanium indelocalized o r shallow localized electron states of the amorphous Ge02 film, and followed by trapping of electrons in deep traps in the insulator, Fig. 2, transition 1. On the oxidized germanium powder we observed a single symmetric ESR signal I1 (gl = 2.0025 2 0.0005, A H = 3 to Fig. 1. a) SS charging during illumination for samples heated in vacuum at the temperatures (1) 470 K , (2) 570 K, (3) 670 K. b) The ESR signals 2 physica (a)
The nature of surface states on clean Ge and Si surfaces is discussed. New experimental data about the interaction of different acceptor molecules with clean Ge and Si surfaces are presented. New paramagnetic adsorption centres are detected by the EPR method. The characteristic feature of the EPR signal detected is a superfine splitting on the aeSi nuclei. This result shows that the unpaired electron in paramagnetic complexes formed by adsorption are largely drawn into the Si atoms. The initial heats of O? adsorption on clean Ge and Si surfaces, measured by calorimetric method, confirm the dissociative character of the first stages of oxidation and double bond formation.Die Art der Oberflachenzustande anf reinen Ge-oder Si-Oberflachen wird diskutiert. Neue experimentelle Werte der Wechselwirkung von verschiedenen Akzeptor-Molekiilen mit reinen Ge-und Si-Oberf kchen werden dargestellt. Neue paramagnetische Adsorptionszentren werden mittels EPR nachgewiesen. Das charakteristische Merkmal des gefundenen EPR-Signals ist seine Hyperfeinaufspaltung an dem eQSi-Kern. Dieses Ergebnis zeigt, da13 das ungepaarte Elektron in dem paramagnetischen Komplex, der durch Adsorption gebildet wird, weit in die Si-Atome hineinreicht. Die Bildungswirme der 0,-Adsorption auf reinen Ge-und Si-Oberflachen, die mittels kalorimetrischer Methode gemessen wird, bestatigt den dissoziativen Charakter der ersten Stadien der Oxidation und Doppelbindungsbildung.
We have previously observed /1, 2/ photoinduced EPR signals (PS EPR) in Si and Ge monocrystals. It was shown, on the basis of the spin-dependent recombination model /3/, that the Q value registered as PS EPR change of the resonator is connected with the resonance change of photoconductivitv. If the injection level is constant the intensity of PS EPR (I) with a single recombination channel does not depend on the concentration of paramagnetic recombination centers /2/. I With several recombination channels being available, either dependent on the spin polarization o r independent, the change of I will be connected with the variation of the relative contribution of the spin-dependent component of recombination to the total recombination rate (6). F o r example, the &crease of I due to active surface treatment can be connected not only with the reduction of the paramagnetic center concentration, but with the increase of the number of non-paramagnetic recombination centers. It is known /4/ that one can distinguish two surface recombination rate components at the real surface of such semiconductors as Si and Ge. The first being strong dependent on the surface potential (bell shape component Sb, and the second weakly dependent on the potential Sc /4/). To explain the problem which component is connected with the spin polarization, we have studied the influence of various active surface treatments on the intensity of PS EPR. The experiments were made with Si monocrystals of p-and n-type ( 9 = 3000 to 1000 Qcm) and Ge monocrystals of p-type ( 8 = 20 to 30 Rcm). A l l the Samples were etched either in C P o r KOH. A s shown in Fit. 1 the intensity of PS EPR (curves 1 , 2 ) decreases when Si -6 monocrystals in C P etched a r e heated in vacuum (P ~. l 0 ence of oxvgen (P * 50 Torr). Signals a r e absent on samples etched in KOH, Torr) o r in the pres-
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