High-energy x-ray diffraction study of pure amorphous silicon

Abstract

Medium and high-energy x-ray diffraction has been used to study the atomic structure of pure amorphous Si prepared by MeV Si implantation into crystalline silicon. Both as-implanted and annealed samples were studied. The inelastically scattered x rays were removed by fitting the energy spectrum for the scattered x rays. The atomic scattering factor of silicon, previously known reliably up to 20 Å Ϫ1 , has been extended to 55 Å Ϫ1. The radial distribution function of amorphous Si, before and after annealing, has been determined through an unbiased Fourier transformation of the normalized scattering data. Gaussian fits to the first neighbor peak in these functions shows that scattering data out to at least 40 Å Ϫ1 is required to reliably determine the radial distribution function. The first-shell coordination number increases from 3.79 to 3.88 upon thermal annealing at 600°C, whereas that of crystalline Si determined from similar measurements on a Si powder analyzed using the same technique is 4.0. Amorphous Si is therefore under coordinated relative to crystalline Si. Noise in the distribution function, caused by statistical variations in the scattering data at high-momentum transfer, has been reduced without affecting the experimental resolution through filtering of the interference function after subtracting the contribution of the first-neighbor peak. The difference induced by thermal annealing in the remainder of the radial distribution functions, thus revealed, is much smaller than previously believed. ͓S0163-1829͑99͒00943-1͔ I. INTRODUCTION

Thermal conductivity accumulation in amorphous silica and amorphous silicon

“…Also shown in Fig. 1(b) is an experimental measurement [57] which compares well with our current sample, but with a slight broadening in the first peak.…”

Disentangling Neighbors and Extended Range Density Oscillations in Monatomic Amorphous Semiconductors

“…This deviation shows that the dihedral angle distribution is not uniform and must include a preferred dihedral angle, not observed in as-implanted a-Ge, between 94° and 135°. Comparing the dots with the dashed line, an increase in dihedral ordering well outside the statistical noise can be seen upon thermal annealing; a similar ordering was suspected to occur in a-Si [5]. Dihedral order peaked at 120 degrees is required for, or evidence of, 6-membered rings.…”

“…The short range order in a-Si [5] and a-Ge [2,6,7] is believed to approximate a fully fourfoldcoordinated random network. In spite of the close resemblance between short range order in the crystal and amorphous phases, the intermediate range order (2 nd and 3 rd neighbours) is so different that in the radial distribution function (RDF) no clear peak is found at a position corresponding to the 3 rd neighbour distance of the crystalline phase.…”

“…Thermal annealing provides atomic mobility as required for defect removal which is accompanied by relaxation of the compressed tetrahedrons towards nearly ideal volumes and a reduction in average bond angle distortion. It is a testament to the power of RDF type analysis of high quality data that many elements of this scenario (bond length, average density, coordination number, average bond angle distortion, average tetrahedron size and volume, and even defect density [5]) can be deduced directly from the diffraction data. [5] and for a theoretical model [15].…”

“…It is a testament to the power of RDF type analysis of high quality data that many elements of this scenario (bond length, average density, coordination number, average bond angle distortion, average tetrahedron size and volume, and even defect density [5]) can be deduced directly from the diffraction data. [5] and for a theoretical model [15]. Curves have been vertically offset for clarity.…”