R. W. Collins

The temperature dependence of photoluminescence in a-Si: H alloys

Abstract Photoluminescence intensity observed near 1.3 eV in sputtered a-Si : H has been measured as a function of temperature for several samples prepared under differing conditions. The data are shown to obey an expression derived from a law of the form p nr p r ∝ e T T 0 where pnr and pnr are the probabilities for non-radiative and radiative recombination. We find T0≈23 K independent of sample preparation conditions.

Effects of partial evolution of H from a-Si:H on the infrared vibrational spectra and the photoluminescence

Abstract H-evolution vs. T in a-Si:H often displays two features near 300°C and 600°C. Evolution of H at 390°C and subsequent redetermination of the evolution vs. T characteristic removes the 300°C feature without affecting other features. IR measurements show that the peaks at 2090, 2000, 890 and 850 cm −1 are reduced uniformly. Thus, the assignments of the 2090 cm −1 peak and the 300°C feature to SiH 2 complexes cannot both be correct. IR spectra of one sample annealed at 265 and 310°C show no reduction in peak intensity, implying that the evolved H was IR inactive. The same anneals do affect the energy of the PL peak, indicating that this H does influence the EDOS.

The effect of gap state density on the photoconductivity and photoluminescence of a-Si:H

Abstract This paper discusses experimental evidence that a fixed amount of hydrogen introduced into amorphous silicon at different hydrogen partial pressures during sputtering may have quite different results for the total density and distribution in energy of states in the pseudogap. Whereas the expected effect is the removal of gap states through the formation of strong Si-H bonds, we show evidence that the gap state densities in some films increase by an order of magnitude over other similar hydrogen content samples. Moreover, such material has much increased photoconductivity response and additional photoluminescence features. We speculate briefly on the possible origin of this higher density of gap states in terms of either natural defects of a -Si or states associated with the Si-H complex.

Photoluminescence in sputtered amorphous Si:H alloys

Abstract We present excitation spectra and photoluminescence (PL) spectra as a function of temperature for films of hydrogenated amorphous silicon (a-Si:H) prepared by sputtering in argon with a partial pressure of hydrogen. For samples prepared at 10 −4 –10 −3 Torr of H, a single PL peak near 1.3eV is observed. At higher partial pressures of H an additional peak near 0.95eV appears. Using PL spectra, photoconductivity-derived absorption and detailed balance arguments, we present a model for radiative recombination at 1.3eV, not strongly Stokes shifted, based on a proposed model of the gap density of states (GDOS). For PL(1.3) and PL(0.95) excitation spectra are presented and compared with absorption spectra. Our model of the GDOS is then expanded to include samples exhibiting PL(0.95). PL(1.3) and PL(0.95) are presented as a function of temperature and examined in terms of present models of non-radiative recombination.

Comparison of a‐Si:H produced by rf sputtering and plasma decomposition methods

We investigate the differences between sputter (S) produced and representative plasma decomposition (GD) produced a‐Si:H in electron drift mobility activation energy, photoluminescence temperature quenching and peak energy, optical absorption extended by photoconductivity, and midgap state density from conductance/capacitance measurements. From these property measurements we conclude that for the GD material less modification of the bands occurs for the hydrogenation required to obtain a comparable midgap state density. This density, measured in both materials, is larger than that expected from an extrapolation of the band tails.

Saturation of band tail states in a-Si:H

Abstract The excitation intensity dependence of the intrinsic photoluminescence (PL) vs. temperature for sputtered and glow discharge hydrogenated amorphous silicon is reported. We find that a transition from linearity to sub-linearity in the excitation intensity dependence of the PL intensity is consistent the onset of photocarrier band tail saturation.