No abstract

No abstract

No abstract

Extended x-ray absorption fine structure measurements have been made on amorphous germanium, a-Ge, and amorphous germanium with 4.9 and 6 at. % hydrogen concentration levels as determined by infrared absorption. It is found that the structural disorder of the first Ge coordination shell in all forms of the amorphous Ge is somewhat greater than the crysta11ine form and decreases as hydrogen is added. The disorder of the first shell is reduced by 15% for the 4.9 at. %%u o Han dby 35% for the 6.0 at. % H relative to the unhydrogenated amorphous Ge produced at the same substrate temperature, T,. The decrease of the first-shell coordination number of all forms of the amorphous Ge is small, being within the uncertainty of 1.5% and the change in first-neighbor dis-

Extended x-ray-absorption fine-structure (EXAFS} measurements were made on three samples of germanium films that were sputtered on substrates at temperatures of 175'C, intermediate between 175 and 250'C, and 325'C. Analysis of the second coordination shell of these samples gives clear evidence of incipient heterogeneous crystallization from an amorphous matrix.Although x-ray diffraction and optical-absorption measurements indicated that the low-and intermediate-T, samples were amorphous, EXAFS indicated that the intermediate-T, sample 0 was heterogeneous, 20'/o of which consisted of microcrystallites approximately 10 A in size embedded in an amorphous matrix. The fact that EXAFS can distinguish between microcrystalline and amorphous states gives the most direct experimental evidence to date that the amorphous state is a continuous random network.The transition of a material from an amorphous state to a crystalline one has long been a matter of both theoretical and experimental interest. '~I t has been generally suspected that heterogeneous crystallization is the route followed in transforming the amorphous samples to the crystalline state. Instead of the entire sample becoming more ordered and crystallizing homogeneously, the discontinuous phase transition between the amorphous and crystalline states more likely occurs by heterogeneous crystallization about nucleation sites which grow in size with temperature to constitute the whole sample. Interest in this subject has recently led to new experimental studies.An extended x-ray-absorption fine-structure (EXAFS) study, 'o made on vapor-deposited amorphous Ge as a function of substrate temperature, concluded that a continuous transition occurred from the amorphous to the crystalline phase and that the results suggested a microcrystalline model of amorphous Ge films. That study based its analysis on the size of the peaks in the real-space Fourier transform of the EXAFS data and made the implicit assumption of a homogeneous sample. The results we report here suggest that the sample was not homogeneous, placing in question the conclusions of Ref. 10. In a subsequent study, x-ray diffraction and Raman scattering were employed in addition to EXAFS. It was concluded that above a substrate temperature of 240'C and below about 270'C a heterogeneous region exists where crystallites coexist in an amorphous matrix. The crystallites are detected by x-ray Bragg peaks and a relatively sharp Raman scattering peak. From the sharpness of the Bragg peaks it was estimated that the crystallite sizes are all greater than = 200 A. Although no Bragg peaks are visible below T, of 240'C, the EXAFS do show increasing ordering in the second shell occurring with increasing T,. The limited EXAFS analysis in this paper could not reveal any further details about this interesting regime.In this paper we investigate this regime with EXAFS and, using a more complete analysis, show explicitly the coexistence of two phases, one microcrystalline and one amorphous. These two phases have an interesting t...

No abstract

Nominal Y0.3B&.6Na0.1CuO~6 thin films were prepared with small grains having an orientation of the c axis perpendicular to the MgO substrate. The electrical resistivity and the thermoelectric power in the absence and in the presence of a magnetic field are reported. The magnetic field dependence of the irreversibility line is discussed. The role of the field for disentangling intragrain and intergrain effects in both coefficients is emphasized.

Ultraviolet photoelectron spectra taken with 40.81-eV photons are presented for three series of Au alloys: AuGa, AuIn, and AuCd. The results are discussed with reference to similar work on a series of Ag'alloys previously reported and are also compared with x-ray photoelectron spectroscopy data. The results are consistent with the previous work on Ag alloys in that the Au Sd doublet width and splitting are (i) independent of the secondary or alloying metal, and (ii) strongly dependent on the mean number of nearest Au neighbors. The behavior of the Au 5d-band. width, as a function of Au concentration, as measured here using Heu radiation, difFers significantly from that measured previously using x-ray photoelectron spectroscopy. This difference cannot be attributed wholly to the difference in linewidth of the two-photon sources or to surface enrichment of the samples. The similar variations of the Ga, In, and Cd d bands as a function of Au concentration are discussed with reference to previous work on the alloys of AgIn and AgCd.