Tome Kosteski

Infrared vibration spectra of hydrogenated, deuterated, and tritiated amorphous silicon

This article presents infrared absorption data of amorphous silicon alloys in which the hydrogen isotopes deuterium and tritium have been substituted for hydrogen. Silicon–deuterium and silicon–tritium vibration frequencies are related to silicon–hydrogen vibration frequencies by simple mass relationships. The silicon–deuterium wagging vibration is broadened and blueshifted due to strong coupling to the amorphous silicon network vibrations.

Tritiated amorphous silicon films and devices

The dc saddle-field glow discharge system was used to stably bond tritium in hydrogenated amorphous silicon films. A betavoltaic battery is demonstrated using tritiated-hydrogenated amorphous silicon as the intrinsic layer in a p–i–n diode and betaconductivity was observed in a-Si:H:T films. Although the half-life of tritium is 12.5 years, the decay of tritium appeared to rapidly increase the midgap density of states which decreased the excess carrier lifetime and decreased the power from the betavoltaic battery. The betaconductivity properties of the a-Si:H:T film were also affected.

Effect of dangling-bond density on luminescence in tritiated amorphous silicon

Self-luminescence in tritiated amorphous silicon is observed. The substitution of tritium for hydrogen in a hydrogenated semiconductor provides a powerful tool for the study of dangling bonds. The radioactive decay of tritium leads to an increase in the dangling-bond concentration in the material of several orders of magnitude over a period of a few weeks with little other modification in the structure of the material. We report here results of a study of photoluminescence as a function of time in tritiated amorphous silicon. An analysis of the data in terms of a simple theoretical model in which dangling bonds are treated as nonradiative recombination centers yields a capture radius of 32 A for the D0 defect.

Hydrogen effusion from tritiated amorphous silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a‐Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300°C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritiu...

Tritiation of amorphous and crystalline silicon using T2 gas

Incorporation of tritium in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at 250°C using tritium (T2) gas at pressures of up to 120atm is reported. The tritium is stored in a surface layer which is approximately 150 and 10nm for a-Si:H and c-Si, respectively. The concentration of tritium occluded in planar and textured c-Si is linearly dependent on the total surface area. The tritium is stable and the dominant tritium evolution occurs at temperatures above 300°C. The concentration of tritium locked in a-Si:H and c-Si was 20 and 4at.%, respectively. Self-catalysis appears to be important in the tritiation process.

Influence of Hydrogen Dilution on Properties of Silicon Films Prepared by D.C. Saddle-Field Glow-Discharge: Observation of Microcrystallinity

ABSTRACT In the D.C. saddle field glow discharge deposition the transition from amorphous to microcrystalline silicon thin films occurs when the silane concentration in the gas phase drops below 10%. We report here the results of Raman spectroscopy, SEM, TEM, and HRTEM studies of the film morphology. We estimate the average crystallite size to be in the range of 5 to 7 nm and the crystalline volume fraction of 25 to 35%. INTRODUCTION Amorphous Si is widely used for large area photovoltaic and microelectronic applications [1]. The use of microcrystalline Si is expected to improve stability against light-induced degradation and provide more efficient doping over that offered by amorphous silicon. Recently, we reported on the growth of mixed phase amorphous-microcrystalline silicon using the D.C. saddle field glow discharge deposition method [2]. The films were grown using hydrogen dilution of silane during the deposition. We were able to identify the growth conditions and the types of substrates that promote microcrystallinity. In this work we present the structural properties of saddle field glow discharge deposited microcrystalline Si films as a function of hydrogen dilution. The films were studied using Raman spectroscopy, SEM, TEM and high-resolution TEM.

Metastability in tritiated amorphous silicon

Abstract The photoluminescence of tritiated, hydrogenated amorphous silicon films was studied as a function time. The samples were deposited over two years prior and tritium decay had resulted in more than 10 20 cm −3 dangling bonds being created in the material. Consequently, photoluminescence was not present. Annealing for 1 h at 150 °C resulted in essentially all dangling bonds being pacified and the photoluminescence being restored. The luminescence decayed to a low value in 2 weeks. Subsequent annealing led to the recovery of the luminescence. The annealing process involves hydrogen diffusion and material reconstruction. The activation energy for the annealing process was found to vary with microstructure with increasing presence of clustered monohydrides leading to a lowering of the activation energy.

Density of states in tritiated amorphous silicon obtained with the constant photocurrent method

The constant photocurrent method was implemented to determine the density of states in tritiated hydrogenated amorphous silicon below the Fermi level. The tritiated hydrogenated amorphous silicon samples were grown using a mixture of silane and tritium gases in the dc saddle-field deposition system. Over time the tritium incorporated in the amorphous silicon undergoes beta decay and transmutation into helium. The transmutation leads to an increase in the density of states in the energy gap of the material. These states are linked to silicon dangling-bond defects. Given its well-characterized decay process, tritium can be used as an experimental tool to study the behavior of hydrogenated amorphous silicon thin films as a function of their defect state density. The density-of-states information in this work indicates a dominant peak of doubly occupied dangling bonds (D−) positioned 1.24eV below the conduction band. The defect density was monitored in time showing an increase of more than two orders of magni...

Nuclear Batteries Using Tritium and Thin Film Hydrogenated Amorphous Silicon

p-i-n junction nuclear devices have been made using tritiated amorphous silicon in the intrinsic region. In this unique device, tritium passivates defects and at the same time is an internal source of beta particles. The beta particles traverse the i-layer and through impact ionization, electron-hole pairs are generated. These charges are separated by the built-in field of the p-i-n junction and electrical power is generated. The power from the devices is about 0.2 nW cm-2 in a device of 400 nm thickness. The decay of tritium leads to the formation of dangling bonds and strain related defects in the silicon lattice. These defects lead to a decrease in the effective width of the space charge region and thereby to an increase in the recombination rate of carriers. As a consequence the electric power decreases with time. To overcome this degradation in performance, delta layered devices were made by selectively introducing tritium into the intrinsic region by modulating the tritium gas fraction during film deposition. The electric power from devices with a delta layer have better stability.

Time evolution of the density of states of tritiated hydrogenated amorphous silicon

Abstract The change with time in the electrical conductivity versus temperature of a tritiated hydrogenated amorphous silicon film was studied. The radioactive decay of tritium produces β particles with a mean energy of 5.7keV and 3He atoms. The high-energy β particles create electron-hole pairs in the film. The 3He atoms diffuse away, leaving dangling bonds. At first the conductivity decreases with time. After approximately 10 days the conductivity increases. We propose that, during the first stage, neutral dangling bonds (D0) are responsible for the decrease by acting as recombination centres. We attribute the subsequent increase to the formation of positively charged dangling bonds. The conductivity data obey the Meyer-Neldel rule.