Incipient amorphous-to-crystalline transition in Ge

Abstract

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...

Low-temperature growth of polycrystalline Ge thin film on glass by in situ deposition and ex situ solid-phase crystallization for photovoltaic applications

“…Although a shift in peak position can also be due to stress in the film, these results clearly indicate a trend that the peaks shift toward 300 cm À1 with increasing T s and the transition between amorphous and poly-Ge occurs at T s = 255-280 8C. The beginning of this transition temperature is similar to that reported in the literature [17].…”

Surface structure of cadmium selenide nanocrystallites

“…Amorphous or partially crystallized diamond and zinc-blende materials have been measured many times with EXAFS, and it is always found that the second-shell Debye-Waller factor, which can increase through bond bending, is much larger than in the corresponding bulk crystal. 21 In contrast, the first-shell Debye-Waller factor can only increase through energetically expensive bond stretching, and remains very close to the bulk value, as we find for the CdSe NC's.…”

“…As has been observed before, for zincblende materials, the first-shell Debye Waller factor is reduced by the stiffness of the bond lengths compared to the relatively low energy required to induce bond bending. 21 The absence of an observable second shell, even at 80 K, in the NC samples indicates that there is a significant static disorder in the tetrahedral bond angle in the interior of the NC's as well as at the surface.…”

In-situ extended energy-loss fine structure analysis of the solid and liquid phases in sub-micron Al particles

“…A similar study was made in amorphous Ge using EXAFS (Ref. 35) and the mean-square displacement at room temperature is an order of magnitude higher compared to crystalline Ge, suggesting that the main contribution to the broadening of the RDF peaks in amorphous Ge is from the structural disorder.…”