Gap States In Amorphous Silicon Research Articles

Investigation of interface and band gap states in amorphous silicon p—i—n solar cells using current deep level transient spectroscopy

Abstract Current deep level transient spectroscopy (DLTS) studies were performed on as-deposited and reverse biased annealed samples of a-Si:H p—i—n solar cells. These studies indicate interface states at p/i and i/n junctions, in addition to the band gap states in the i-layer. The hole and electron trap states are located at 0.8 and 1.12 eV, respectively, above the valence band, whereas the interface states at the p/i junction are located at 0.55 eV and those at the i/n junction at 1.2 eV above the valence band. A decrease in the interface states with reverse biased annealing treatment has been observed.

Improved analysis of the constant photocurrent method

Abstract A numerical model has been developed to simulate constant photocurrent method spectra. It takes into account the position of the Fermi energy and the full set of optical transitions between localized and extended states under subbandgap illumination, capture, emission and recombination processes. The comparison of simulated and measured spectra yields information about the density of localized gap states in amorphous silicon, that is the valence-band tail, the integrated defect density, the defect distribution in energy and the charge state of the defect states. For n- and p-type hydrogenated amorphous silicon (a-Si:H) we achieved good agreement between simulation and experimental data. In the annealed state the defect absorption is dominated by a single defect peak which can be attributed to D − states in n-type material and D + states in p-type a-Si: H. In undoped a-Si: H we observed more charged than neutral defect states, confirming the predictions of the defect-pool model. Furthermore, the simulations reveal that the defect chemical potential depends on the Fermi level as postulated by the defect-pool model. In the light-soaked state, undoped material shows an enhanced defect density. We observe an increase in the density of both neutral and charged defect states. The charged-to-neutral defect ratio does not change upon light soaking.

Gap states in amorphous silicon—dangling and floating bonds

It is shown that the primitive intrinsic defects in amorphous Si (a-Si) are threefold- and fivefold-coordinated Si atoms. A variety of experimental and theoretical information indicates that both these defects are likely to play a role in determining the properties of a-Si. Both have localized states in the energy gap. Electron-spin-resonance (ESR) data show that the dominant paramagnetic defect is more likely to be fivefold-coordinated Si (floating bond). A luminescence peak at ∼0.9 eV, on the other hand, is likely to be due to threefold-coordinated Si (dangling bond). In hydrogenated a-Si (a-Si:H), interstitial H (either at a “bond center” or “channel” site) may also have a state in the gap.

Experimental determination of the density of gap states in amorphous silicon by Schottky barrier admittance

Abstract Experimental admittance measurements are presented for an n-type doped amorphous-silicon Schottky barrier. The measurements are shown to be quite consistent with the theory described by Archibald and Abram (1983, 1986) and, using this theory, an estimate of the density of states in the upper half of the mobility gap is calculated. The average value is ∼ 1017 cm−3 eV−1 and there is a minimum situated approximately 0˙4 eV below the conduction-band mobility edge. This result is in approximate agreement with the density of states deduced by Lang, Cohen and Harbison (1982) using deep-level transient spectroscopy. The test-fit value for the gap-state capture/emission time constant τ0 is found to be 10−12 s.

Thermally stimulated current studies of the density of gap states in amorphous silicon

Careful thermally stimulated current experiments were carried out on undoped amorphous hydrogenated silicon. We observe two peaks in the TSC profile, one at 120°K and the other at 300°K. We construct a theoretical framework based on the retrapping mechanism to understand this phenomena. The agreement between experiment and the theoretical prediction is good. In contrast the non-retrapping, high field formulation provides a poor fit to the experiment. We deduce that the density of states (DOS) in the gap has a maxima at 0.45 eV in broad agreement with the results of field effect measurements. Deep level transient spectroscopy (DLTS) measurements, which predict a minima in the DOS at 0.4 eV, find no support from our calculations.

Doping and gap states in amorphous silicon

The author shows that the decline in doping efficiency in a-Si:H with increased doping level arises because the dopants act as negative U centres and pin Ef during deposition. He also shows that only deep states from dangling bonds and dopants respond to coordination changes so unlike in chalcogenides, Ef is pinned in a finite density of shallow tail states. Defect bonding can now be expressed in a single scheme valid for all amorphous semiconductors.

Amorphous silicon-silicon nitride thin-film transistors

A thin-film field-effect transistor has been fabricated using glow-discharge amorphous silicon as the semiconductor and silicon nitride as the insulator. The transistor operates in the electron (n type) accumulation mode and by changing the gate potential from zero to only 3 V a change in the source-drain conductance of greater than four orders of magnitude is obtained. The results imply upper limits to the density of gap states in amorphous silicon and interface states at the amorphous silicon-silicon nitride interface of 3×1016 cm−3 eV−1 and 5×1011 cm−2 eV−1, respectively.