S.M. Cho

Hydrogenated amorphous silicon-nitrogen alloys, a-Si,N:H: A candidate alloy for the wide band gap photo-active material in tandem photovoltaic (PV) devices

Abstract a-Si,N:H alloys were prepared by remote plasma-enhanced chemical-vapor deposition (PECVD), and evaluated for applications as wide band gap, photo-active materials for PV devices. These alloys have been deposited using SiH 4 as the silicon-atom source gas, and either N 2 or NH 3 as the nitrogen-atom source gas to yield films with E 04 band gaps up to ∼2.2 eV. We have characterized the microstructure, and have studied selected optical and electrical, and transport properties, comparing films prepared from N 2 and NH 3 N-atom source gases.

Hydrogenated Amorphous Silicon-Nitrogen, a-Si,N:H ALLOYS: An Alternative to A-SI,C:H for the Wide Band Gap Photo-Active Material in Tandem PV Cells

We have investigated a-Si,N:H alloys as an alternative wide band-gap, photo-active material. The entire alloy range between a-Si:H and a-Si 3 N 4 :H can be formed by a remote plasma-enhanced chemical-vapor deposition (PECVD) process. Other studies have demonstrated that a-Si,N:H alloys could be doped to form window materials for p-i-n devices. This paper focuses on alloy materials with E 04 bandgaps to about 2.2 eV. We have prepared these a-Si,N:H alloys, characterized their microstructure, and studied their photoconductivity, sensitivity to light-soaking and transport properties. For example, with increased alloying we show that i) the white-light photoconductivity and ii) the kinetics and magnitude of the decay of photoconducitivity under intense illumination (the Staebler-Wronski effect), are about the same as for PV-grade a-Si:H.

Deposition of microcrystalline hydrogenated silicon,germanium alloy (μc-SixGe1 − x:H) films by reactive magnetron sputtering (RMS)

Abstract Hydrogenated silicon,germanium alloys, which span the transition from amorphous to polycrystalline microstructures, have been prepared by reactive magnetron sputtering from pure crystalline Si and Ge targets in a hydrogen ambient using argon as the sputtering gas. X-ray diffraction, Raman scattering, Fourier transform infrared spectroscopy, and high-resolution transmission electron microscopy have been used for microstructural characterization of films with different Ge concentrations. Microstructural changes produced by rapid thermal annealing in an Ar environment were also studied.

Nitrogen: Not a Dopant in Crystalline Si (C-Si), But an N-Type Dopant in A-Si:H, Why?

We have incorporated N-atoms into hydrogenated amorphous silicon in the Si-rich alloy regime to ∼12 at.% N, and have observed a transition from n-type doping to alloying as the concentration of N-atoms is increased above about 5 at.%. By analogy with the local bonding arrangements of P-donors in n-doped a-Si:H, we attribute the doping to four-fold coordinated N-atoms with second neighbor H-atoms as in N + -Si-H linkages. The occurrence of these arrangements is supported by (i) IR studies which indicate a non-statistical association of N and H-atoms bonded to the same Si-atom, and (ii) a chemical bonding model in which the large effective electronegativies of four-fold coordinated N + atoms and neutral O-atoms promote similar bonding properties with respect to their nearest-neighbor arrangements with Si and H atoms such as N + (O) -Si-H linkages

Energy Differences Between the Si and the Ge Dangling Bond Defects in a-Si 1-x Ge x Alloys

We have investigated the difference in the electronic energies of neutral Si and Ge dangling bond states in undoped a-Si 1-x Ge x alloys as a function of the alloy composition, x, and local bond-angle distortions. The local density of states, LDOS, in a-Si 1-x Ge x alloys has been calculated using nearest-neighbor interactions, and employing the Cluster Bethe Lattice method. We conclude that for ideal, tetrahedrally bonded amorphous semiconductors alloys, the Ge dangling bond energy is lower than that of Si dangling bonds by ∼ 0.13 eV, independent of the specific nearest neighbors to the dangling bond (3 Si-atoms, 2 Si-atoms and 1 Ge-atom, etc.), but that the spread in dangling bond energies associated bond-angle variations of the order of 6–8 degrees can be larger than this energy difference (∼0.3 eV or greater). This means that structural disorder, rather than chemical disorder causes Si and Ge-atom dangling bond states to overlap in their energy distributions.

Localized states in amorphous Si and Si,Ge alloys

This paper discusses applications of the tight‐binding method to: i) localized anti‐bonding states of Si‐H groups in a‐Si:H, and ii) dangling bond defect states in a‐Si,Ge:H alloys. The a‐Si:H calculations demonstrate that anti‐bonding states of Si‐H groups with bond‐angle distortions of ∼16°–20° or more can be localized below the conduction band edge, and display the trapping properties of floating‐bond defect states. The calculations for the a‐Si,Ge:H alloys show that the average‐alloy bond‐angle disorder, ∼6–8°, can induce a spread in the energies of Si and Ge dangling bond states of ∼0.3 eV, that is larger than that due to chemically‐induced splittings, 0.11–0.15 eV, which include the effects of different distributions of nearest‐neighbor Si and/or Ge‐atoms. The bond‐angle disorder induced dispersion leads to a spectral overlap of the Ge and Si dangling bonds, so that both types of defects can be observed in alloy samples.

Minority Carrier Diffusion Lengths And Photoconductivity In a-Si,N:H Deposited By Remote Pecvd

We have deposited films of a-Si,N:H by remote PECVD from N 2 and SiH 4 for N-concentrations, [N], to about 12 atomic percent (at. %). Bonded-H concentrations were ∼7–10 at. %, Mostly in Si-H groups. The films with [N] = 9–12 at. % have eθ4 bandgaps of ∼2.0 to 2.2 eV, which makes them potentially useful as wide bandgap photo-active materials in tandem PV cells. Several properties are of special interest for PV applications. First, like many other a-Si:H-based alloys, the photoconductivity relative to a-Si:H is degraded by alloying, but less than for a-Si,C:H alloys with the same bandgaps. Second, the ambipolar diffusion lengths (Ld) obtained with the Steady State Photocarrier Grating (SSPG) technique for films with [N] = 10 at. % and eθ4 = 2.1eV, are comparable to those of a-Si:H. For lightly-nitrided films to [N] ∼5 at. %, Ld first decreases with respect to a-Si:H and then increases as [N] increases from ∼7 at.% to 10–12 at. %. These trends follow the dark conductivity activation energy, E a , which initially drops due to doping, and then increases into an alloy regime for [N] > 5 at. %. Films with [N1=10 at. % have dark conductivities and E a s comparable to those of undoped a-Si:H. Third the magnitude of the Staebler-Wronski effect, as monitored by the photo- to dark conductivity ratio after a 1000 Minute lightsoak, was about the same as in a-Si:H. Finally, we contrast the properties of these films prepared from N 2 with a-Si,N:H alloys with the same [N] and E 04 , but prepared from an ammonia N-atom source gas and attribute differences in their photoelectronic behavior such as a significantly enhanced Staebler-Wronski effect.to the presence of Si-NH bonding arrangements in the films grown from NH 3 .