A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH 4 /͑SiH 4 ϩH 2 ͒ varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy ͑TEM͒ and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a ͑220͒ crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherrer's equation illustrates the known limitations of the latter method for grain size determination in complex microstructures.

The amorphous and microcrystalline silicon films have been prepared by hydrogen dilution from pure silane to silane concentrations G 1.25%. At silane concentrations of less than 10%, a transition from the amorphous phase to the microcrystalline phase can be observed. X-ray diffraction spectroscopy indicates a preferential growth of the crystallites in w x the 220 direction. Additionally, the transition into the microcrystalline regime is accompanied by a shrinking of the optical gap, a reduction in hydrogen content and by a modified trend of the deposition rate. The observed changes in the infrared absorption modes indicate modifications in the hydrogen bonding and can be correlated with results known from monocrystalline silicon. Close to the transition zone, but still in the amorphous regime, the hydrogen content is increased, whereas the microstructure parameter reaches its smallest value. Precisely these films have a 0.06 eV higher optical gap and a reduced defect density by a factor of 4 as compared to a-Si:H layers prepared from pure silane.

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An optically pumped semiconductor disk laser using submonolayer quantum dots (SML QDs) as gain medium is demonstrated. High-power operation is achieved with stacked InAs∕GaAs SML QDs grown by metal-organic vapor-phase epitaxy. Each SML-QD layer is formed from tenfold alternate depositions of nominally 0.5 ML InAs and 2.3 ML GaAs. Resonant periodic gain from a 13-fold nonuniform stack design of SML QDs allows to produce 1.4W cw at 1034nm. The disk laser demonstrates the promising potential of SML-QD structures combining properties of QD and quantum-well gain media for high-power applications.

31P NMR spectroscopy was used to study the products of the decomposition of the antitumor drug ifosfamide (IF, 1d) and its N-dechloroethylated metabolites, namely, 2,3-didechloroethylIF (1a) and 2- (1b) and 3-dechloroethylIF (1c), in buffered solutions at acidic pH. The first stage of acid hydrolysis of these four oxazaphosphorines is a P-N bond cleavage of the six-membered ring leading to the phosphoramidic acid monoesters (2a-d) of type R'HN(CH(2))(3)OP(O)(OH)NHR, with R and/or R' = H or (CH(2))(2)Cl. The electron-withdrawing chloroethyl group at the endocyclic and/or exocyclic nitrogens counteracts the endocyclic P-N bond hydrolysis. This effect is even more marked when the N-chloroethyl group is in the exocyclic position since the order of stability is 1d > 1c > 1b > 1a. In the second stage of hydrolysis, the remaining P-N bond is cleaved together with an intramolecular attack at the phosphorus atom by the non-P-linked nitrogen of the compounds 2a-d. This leads to the formation of a 2-hydroxyoxazaphosphorine ring with R = H (3a coming from compounds 2a,c) or (CH(2))(2)Cl (3b coming from compounds 2b,d) and to the release of ammonia or chloroethylamine. The third step is the P-N ring opening of the oxazaphosphorines 3a,b leading to the phosphoric acid monoesters, H(2)N(CH(2))(3)OP(O)(OH)(2) (4a) and Cl(CH(2))(2)HN(CH(2))(3)OP(O)(OH)(2) (4b-1), respectively. For the latter compound, the chloroethyl group is partially (at pH 5.5) or totally (at pH 7.0) cyclized into aziridine (4b-2), which is then progressively hydrolyzed into an N-hydroxyethyl group (4b-3). Compounds 3a,b are transient intermediates, which in strongly acidic medium are not observed with (31)P NMR. In this case, cleavage of the P-N bond of the type 2 phosphoramidic acid monoesters leads directly to the type 4 phosphoric acid monoesters. The phosphate anion, derived from P-O bond cleavage of these latter compounds, is only observed at low levels after a long period of hydrolysis. Compounds 1a-c and some of their hydrolytic degradation products (4b-1, 4b-2, diphosphoric diester [Cl(CH(2))(2)NH(CH(2))(3)OP(O)(OH)](2)O (5), and chloroethylamine) did not exhibit, as expected, any antitumor efficacy in vivo against P388 leukemia. (31)P NMR determination of the N-dechloroethylated metabolites of IF or its structural isomer, cyclophosphamide (CP), and their degradation compounds could provide an indirect and accurate estimation of chloroacetaldehyde amounts formed from CP or IF.