Showing papers by "Warren B. Jackson published in 1985"

Light-induced metastable defects in hydrogenated amorphous silicon: A systematic study.

Abstract: We study the magnitude of metastable light-induced changes in undoped hydrogenated amorphous silicon (the Staebler-Wronski effect) with electron-spin-resonance and photoconductivity measurements. The influence of the following parameters is investigated in a systematic way: sample thickness, impurity content, illumination time, light intensity, photon energy, and illumination and annealing temperatures. The experimental results can be explained quantitatively by a model based on the nonradiative recombination of photoexcited carriers as the defect-creating step. In the framework of this model, the Staebler-Wronski effect is an intrinsic, self-limiting bulk process, characterized by a strongly sublinear dependence on the total light exposure of a sample. The experimental results suggest that the metastable changes are caused by recombination-induced breaking of weak Si--Si bonds, rather than by trapping of excess carriers in already existing defects. Hydrogen could be involved in the microscopic mechanism as a stabilizing element. The main metastable defect created by prolonged illumination is the silicon dangling bond. An analysis of the annealing behavior shows that a broad distribution of metastable dangling bonds exists, characterized by a variation of the energy barrier separating the metastable state from the stable ground state between 0.9 and 1.3 eV.

Light-induced metastable defects in hydrogenated amorphous silicon

Abstract: Recent developments in the investigation of light-induced metastable defects in a-Si:H are discussed. A kinetic model that quantitatively describes the creation of metastable dangling bonds in UHV deposited a-Si:H can be extended for a qualitative understanding of defect creation in SiGe alloys, samples containing common impurities (C,N,O), and in compensated a-Si:H. It is also shown that mechanical stress enhances the defect creation efficiency. Detailed information about the energy barrier distribution of the metastable dangling bonds can be obtained by ESR annealing studies. Finally, new results showing a light-induced, reversible quenching of low energy tunneling modes are presented.

Effects of dopants and defects on light-induced metastable states in a -Si:H

Abstract: Using photothermal deflection spectroscopy we measure the gap-state optical absorption of light-induced metastable defects in undoped, singly-doped, and compensated a-Si:H. We observe an enhancement in the gap-state absorption after illumination which is shown to be due to the creation of new silicon dangling-bond defects and not to a shift in the Fermi level. For singly-doped material, the number of light-induced defects scales with dopant concentration, while full compensation (counter doping) drastically minimizes the density of these defects. The results provide evidence that the doping level influences the light-induced defect formation mechanism, and imply that simply breaking Si--Si bonds may not be the primary mechanism.

Dopant and defect states in a-Si:H

Abstract: The changes in the density-of-states distribution upon doping of hydrogenated amorphous silicon (a-Si:H) are discussed A previous doping model is extended to explain the presence of occupied band-tail states, and their predicted doping dependence is shown to be consistent with experimental observations The energy levels of dangling bonds are discussed, and we describe a complete set of optical and thermal emission data which place the upper dangling bond level at around 0·8 eV below the conduction band The results also indicate that the defect has little atomic relaxation It is argued that the defect-dopant pair does not have a negative correlation energy, contrary to a recent suggestion

Implications of recent density-of-states measurements for optical and transport properties of a-Si:H

Abstract: In this paper, recent developments in the determination of the density-of-states for hydrogenated amorphous silicon are reviewed, and the implications of these measurements for optical and transport measurements are discussed. The DOS derived from electron spectroscopies is remarkably consistent with DOS obtained from optical absorption, photoconductivity, and capacitance measurements. Both optical and transport measurements indicate that the phase coherence of the conduction band states is less than one lattice spacing. Some remaining questions regarding the DOS are briefly presented.

Optical spectroscopy of the trivalent silicon defect at the Si-SiO2 interface.

Abstract: Optical absorption by the trivalent silicon defect at the Si-${\mathrm{SiO}}_{2}$ interface has been measured by electroabsorption spectroscopy. On metal-oxide\char21{}silicon (MOS) capacitors, electric field modulation of the occupancy of the silicon dangling orbitals is directly detected as a synchronous modulation of the absorption of subband gap light; in passivated capacitors, a residual absorption is detectable because of free carriers in the accumulation layer of the MOS structure. Both photon energy and surface-potential controlled trap occupancy provide probes of the interface-state distribution. In addition, the results provide an estimate of ${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$ (at 1 eV) for the effective optical cross section of the 0\ensuremath{\rightarrow}1 electron transition of the interfacial defect.

Energy placement of the D− dangling-bond transition in a-Si:H from photocapacitance and photocurrent spectroscopies

Abstract: The characteristic gap — state distribution in a-Si:H was measured by photocapacitance transient spectroscopy and steady-state photoconductivity on p-n junction diodes. The results yield a peak placement of 0.8–0.9 eV for the D− transition of the silicon dangling-bond defect and a value of ∼30 A 2 for the square of the average optical matrix element for this transition.

Light-Induced Metastable Defects in a-Si:H: Towards an Understanding

Abstract: The dependence of the creation and the annealing of metastable dangling bonds in hydrogenated amorphous silicon on various material parameters will be discussed in the context of a recently proposed model. After a brief review of the kinetic behaviour governing defect creation and annealing in undoped a-Si:H, a number of special cases will be analyzed: the influence of alloying with O, N, C, and Ge, changes introduced by doping and compensation, and the role of mechanical stress. Finally, possibilities to increase the stability of a-Si:H based devices will be examined.

Comparison of the Optical Cross Section for the Si Dangling Bond in a — Si:H and at the c — Si/SiO2 Interface

Abstract: The optical properties of the trivalent silicon dangling bond defect in hydrogenated amorphous silicon and at the Si/SiO2 interface are compared. While both defects give rise to ambipolar deep levels within the gap, significant differences in the optical properties between the two systems are found. The absorption in a-Si:H is significantly stronger and is dominated by a transition from the defect to the conduction band while the absorption at the interface is dominated by hole emission. The average dipole matrix element squared is roughly independent of energy in both systems with a magnitude of ∼30A2. Implications of these results for optical measurements in other silicon systems are discussed.

Energy Dependence of the Optical Matrix Element for Hydrogenated Amorphous Silicon

Abstract: The valence band and conduction band density of states of hydrogenated amorphous silicon are determined using x-ray photoemission and bremstrahlung isochromat spectroscopy, respectively. From these densities of states and measurements of the imaginary part of the dielectric function using ellipsometry, the energy dependence of the average transition matrix element squared has been determined. The matrix element exhibits a maximum near the direct edge of crystalline silicon (~3.35 eV) and is very similar to the energy dependence calculated for a complex crystalline form of silicon with short range disorder and long range order.