Jin Jang

Temperature dependence of photoconductivity and recombination in hydrogenated amorphous silicon

The recombination processes in undoped hydrogenated amorphous silicon under steady‐state illumination have been studied in order to explain the temperature dependences of photoconductivity. The recombinations of free carriers through the exponential tail states and through the Gaussian distributed dangling bond states were calculated in terms of the Shockley–Read theory and the occupation statistics of correlated defects. An additional recombination of trapped electrons and holes in the tail states through the dangling bonds by tunneling should be included in order to explain the experimental data of the activated photoconductivity at low temperatures.

EFFECT OF DISCHARGE CONDITIONS ON CHARACTERISTICS OF HYDROGENATED AMORPHOUS-SILICON DEPOSITED BY DC GLOW-DISCHARGE DECOMPOSITION

Abstract Electrical, optical characterization of hydrogenated amorphous silicon films deposited by plasma decomposition in the various dc discharge conditions reveals substantial diffences in a number of properties. A general trend of decreasing defect density with proximity to the cathode electrode is observed. Of specific interest to material properties is the observation of differences of characteristics between samples prepared longitudinal and transverse to the discharge direction.

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Abstract Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below Ec, the mobility edge at the conduction band.

Thermal equilibrium process in hydrogenated amorphous silicon npnp doping-modulated superlattices

Abstract Thermal equilibrium process in hydrogenated amorphous silicon(a-Si:H) npnp doping-modulated superlattices has been studied. Thermal quenching from above 130°C, equilibrium temperature of n-type a-Si:H, gives rise to the increase of defect density of p-layers and raises the Fermi-level of n-layers in a-Si:H npnp superlattices. The resultsv suggest that both dangling bonds and dopants are involved in thermal equilibration of doped a-Si:H and the dangling bonds of a p-type a-Si:H increase upon thermal quenching.

Photo and bias effects in coplanar conductance of doping-modulated amorphous silicon superlattices☆

Abstract The effects of high temperature annealing on persistent photoconductivity (PPC) and of bias on the coplanar conductance in doping-modulated super-lattices and compensated films of hydrogenated amorphous silicon (a-Si : H) have been investigated. All the multilayers showing PPC have nonlinear current-voltage characteristics, which are not observed in single-layered a-Si : H, even in compensated films showing a large PPC. At low bias voltage (E ≤ 10 V/cm) the coplanar conductance is constant with time. At higher field the conductance decays slowly until it reaches a saturation value. The bias effect as well as the PPC is drastically reduced after annealing at 300 °C for 30 min. Experimental results can be explained by assuming H complex centers in the region with excess hydrogen at the n-p interfaces.

Light-induced effects in hydrogenated amorphous silicon at low temperatures☆

Abstract Photoluminescence (PL) intensity and luminescence fatigue with illumination time and annealing in a-Si:H have been measured at various temperatures. The results show that the luminescence fatigue is minimum at 150K and there exist at least two kinds of defect or trap at low temperatures, giving rise to opposite light-soaking effect each other. The results are interpreted in terms of the change in band bending caused by optically induced defect or surface states at the native oxide layer of a-Si:H.

Effect of preparation condition on conductivity activation energy and downward conductivity kink of undoped hydrogenated amorphous silicon

Abstract Temperature dependence of conductivity has been measured for a series of undoped a-Si:H films prepared by glow discharge decomposition of silane. Conductivity activation energy and the existence of downward conductivity kinks around 430 K were observed to depend on preparation conditions such as substrate temperature, annealing temperature and discharge configuration. The undoped a-Si:H films whose Fermi levels are well-above the midgap, always show downward conductivity kinks. Variation of conductivity activation energies among undoped a-Si:H films can be interpreted by the transfer of electrons between islands and tissue.

Persistent photoconductivity decay mechanism in compensated amorphous silicon

We studied the effect of band tail carriers on the annealing of metastable persistent photoconductivity(PPC). We created persistent photoconductivity by light illumination on fully compensated amorphous silicon at several temperatures and measured the decay of PPC. The decay behavior fits quite well calculated values introducing a band tail carrier dependent hydrogen diffusion coefficient. It is concluded that the annealing of the PPC is accelerated with the increase in density of band tail carriers.

Bias-induced conductance change in NPNP doping-modulated amorphous silicon superlattices

Abstract Bias-induced metastabilities in the coplanar and transverse conductances have been studied for amorphous silicon npnp doping-modulated superlattices. The metastabilities, arised from the transverse electric field, are probably due to the creation of dangling bonds and the trapping of charges in p- and n-layers.

Low hydrogen content a-Si:H produced by layer by layer deposition technique

We applied the layer by layer deposition technique to make low band gap a-Si:H by varying preparation parameters of substrate temperature and hydrogen plasma exposure (HPE) time. With the increase of HPE time up to 24s, the dark conductivity decreases, and then increases rapidly, and the change in photoconductivity is relatively small. Above 50s of HPE time, the microcrystalline Si is formed, confirmed from the Raman scattering and conductivity. We obtained a-Si:H film by HPE technique with optical gap of 1.67eV, photosensitivity of > 106, conductivity activation energy of 0.95eV, and hydrogen content of