H loss mechanism during anneal of silicon nitride: chemical dissociation

Journal ArticleRemote plasma enhanced chemical vapor deposited silicon nitride (SixNyHz), produced at high ammonia to silane flow rates (ammonia rich) shows a reduction of hydrogen during rapid thermal anneal at temperatures that exceed the deposition temperature. This H release could be either due to a ‘‘slow'' atomic diffusion of the covalent bonded atoms between bonding sites, or to a ‘‘fast'' molecular diffusion of hydrogen containing molecules (e.g., H2, NH3, SiH4), which dissociate from the network before they diffuse. In order to determine the dominant mechanism, layers of deuterated and hydrogenated silicon nitride on top of a crystalline Si substrate were annealed and the development of the NH- and ND-area densities were measured with Fourier transform infrared spectroscopy. Comparison of theoretical models with the measurements showed that chemical dissociation and subsequent rapid diffusion are the dominant processes. These results were confirmed by secondary ion mass spectroscopy. The experiments indicate that the H reduction in silicon nitride antireflection coatings of solar cells is mostly due to H migration out of the system and not into the Si area and make the hypothesis that postdeposition annealing of solar cell antireflection coatings can cause H-related bulk passivation of the underlying c-Si therefore questionable

Diffusion of hydrogen and deuterium in stack systems of SixNyHz/SixNyDz and crystalline Si

Journal ArticleH/D-, N-H/D- and Si-H/D-bond density changes were investigated in stacks consisting of a Cz-Si substrate, a thin layer of SiC>2, amorphous deuterated silicon nitride as well as amorphous hydrogenated silicon nitride in order to see if the post deposition anneal of a-SixNyHz layers on crystalline silicon wafers can actually lead to a migration of H atoms into the Si-bulk, which is an important question in regard to emitter passivation of Si-solar cells. The stacks were grown with remote plasma enhanced chemical vapor deposition (RPECVD). A low temperature (=200°C) process of down stream injected ammonia (NH3) and silane (SiH4) activated by an upstream injected He-plasma, produced through RF-radiation (13.65MHz) was used. Thermal treatment was executed by ex situ rapid thermal anneal in Ar ambient. For the measurements of H and D bond densities, FTIR was employed while SIMS determined atomic densities of H, D and O in the c-Si/nitride interface region. The experiments showed that H transport in silicon nitride is determined by several mechanisms including diffusion and dissociation processes and that silicon nitride deposited with high ammonia to silane ratios can produce molecular species like ammonia and H2. The study of the reaction dynamics showed that the production of molecular hydrogen is the most dominant process as long as Si-H-bonds are present in the system. After their exhaustion, an ammonia producing reaction prevails that leads with increasing temperatures to lower densities in the nitride films

A First-Principles Approach to Insulators in Finite Electric Fields

We describe a method for computing the response of an insulator to a static, homogeneous electric field. It consists of iteratively minimizing an electric enthalpy functional expressed in terms of occupied Bloch-like states on a uniform grid of k points. The functional has equivalent local minima below a critical field E_c that depends inversely on the density of k points; the disappearance of the minima at E_c signals the onset of Zener breakdown. We illustrate the procedure by computing the piezoelectric and nonlinear dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm

Pressure dependence of the Boson peak in glassy As2S3 studied by Raman Scattering

A detailed pressure-dependence study of the low-energy excitations of glassy As2S3 is reported over a wide pressure range, up to 10 GPa. The spectral features of Boson peak are analysed as a function of pressure. Pressure effects on the Boson peak are manifested as an appreciable shift of its frequency to higher values, a suppression of its intensity, as well as a noticeable change of its asymmetry leading to a more symmetric shape at high pressures. The pressure-induced Boson peak frequency shift agrees very well with the predictions of the soft potential model over the whole pressure range studied. As regards the pressure dependence of the Boson peak intensity, the situation is more complicated. It is proposed that in order to reach proper conclusions the corresponding dependence of the Debye density of states must also be considered. Employing a comparison of the low energy modes of the crystalline counterpart of As2S3 as well as the experimental data concerning the pressure dependencies of the Boson peak frequency and intensity, structural or glass-to-glass transition seems to occur at the pressure ~4 GPa related to a change of local structure. Finally, the pressure-induced shape changes of the Boson peak can be traced back to the very details of the excess (over the Debye contribution) vibrational density of states.Comment: To appear in J. Non-Cryst. Solids (Proceedings of the 5th IDMRCS, Lille, July 2005

Normal and anti Meyer-Neldel rule in conductivity of highly crystallized undoped microcrystalline silicon films

We have studied the electrical conductivity behavior of highly crystallized undoped hydrogenated microcrystalline silicon films having different microstructures. The dark conductivity is seen to follow Meyer Neldel rule (MNR) in some films and anti MNR in others, which has been explained on the basis of variation in the film microstructure and the corresponding changes in the effective density of states distributions. A band tail transport and statistical shift of Fermi level are used to explain the origin of MNR as well as anti-MNR in our samples. The observation of MNR and anti MNR in electrical transport behavior of microcrystalline silicon is discussed in terms of the basic underlying physics of their origin and the significance of these relationships.Comment: 5 pages, 1 figur

Intermediate Phases, structural variance and network demixing in chalcogenides: the unusual case of group V sulfides

We review Intermediate Phases (IPs) in chalcogenide glasses and provide a structural interpretation of these phases. In binary group IV selenides, IPs reside in the 2.40 < r < 2.54 range, and in binary group V selenides they shift to a lower r, in the 2.29< r < 2.40 range. Here r represents the mean coordination number of glasses. In ternary alloys containing equal proportions of group IV and V selenides, IPs are wider and encompass ranges of respective binary glasses. These data suggest that the local structural variance contributing to IP widths largely derives from four isostatic local structures of varying connectivity r; two include group V based quasi-tetrahedral (r = 2.29) and pyramidal (r = 2.40) units, and the other two are group IV based corner-sharing (r = 2.40) and edge-sharing (r = 2.67) tetrahedral units. Remarkably, binary group V (P, As) sulfides exhibit IPs that are shifted to even a lower r than their selenide counterparts; a result that we trace to excess Sn chains either partially (As-S) or completely (P-S) demixing from network backbone, in contrast to excess Sen chains forming part of the backbone in corresponding selenide glasses. In ternary chalcogenides of Ge with the group V elements (As, P), IPs of the sulfides are similar to their selenide counterparts, suggesting that presence of Ge serves to reign in the excess Sn chain fragments back in the backbone as in their selenide counterparts

Ab Initio Phonon Dispersions for PbTe

We report first principles calculations of the phonon dispersions of PbTe both for its observed structure and under compression. At the experimental lattice parameter we find a near instability of the optic branch at the zone center, in accord with experimental observations.This hardens quickly towards the zone boundary. There is also a very strong volume dependence of this mode, which is rapidly driven away from an instability by compression. These results are discussed inrelation to the thermal conductivity of the material.Comment: 3 figures; typos corrected. Figure 1 replaced to correct labe

Enhancement of piezoelectricity in a mixed ferroelectric

We use first-principles density-functional total energy and polarization calculations to calculate the piezoelectric tensor at zero temperature for both cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest piezoelectric coefficient for the tetragonal configuration is enhanced by a factor of about three with respect to that of the cubic configuration. This can be attributed to both the larger strain-induced motion of cations relative to anions and higher Born effective charges in the tetragonal case. A normal mode decomposition shows that both cation ordering and local relaxation weaken the ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure