We describe elements of a thermodynamical ensemble theory of electronic states in a variety of disordered systems. Equilibrium energies and kinetics of phase space exploration combine to determiine the ensembles describing disordered systems. Electronic properties are then related to structural energies. This relationship serves to determine the distribution of electronic states present in real materials. Thus we obtain directly electronic properties without a need for detailed microscopic information about the diverse systems. Applications range from the Urbach edge to defect properties providing a unified understanding.

This paper reviews the recent evidence for thermal equilibrium effects in the electronic behavior of hydrogenated amorphous silicon, and relates the thermally induced changes to the motion of bonded hydrogen. The electronic properties are studied through measurements of d.c conductivity and-.weep out, and the role of hydrogen is explored through its diffusion. The magnitude and doping dependence of the diffusion coefficient DH matches the data on the equilibration of the electronic states. Furthermore both the diffusibn and the relaxation can be described by the same dispersive time dependence. It is argued that the diffusion of hydrogen is the rate limiting step in the the equilibration mechanisms and determines the kinetics.

In the last twenty years, the dominant paramagnetic defect in a-Si (the D center) was believed to be threefold-coordinated Si (dangling bond) which is not mobile. Many observations, however, have not been explained in a satisfactory way. This paper summarizes recent contributions of the present author which offer a novel and systematic description of the properties of a-Si. It is shown that overcoordinazion is an essential concept and that fivefold- and threefold-coordinated Si are the primitive conjugate intrinsic defects. Overcoordination is the key to defect migration and thus to the realization of reactions between intrinsic defects, hydrogen, and impurities. Most properties of a-Si are a consequence of a small set of reactions. The available data favor the identification of the D center as fivefold-coordinated Si (floating bond). Dangling bonds are nonmagnetic and their signature is a photoluminescence peak at ∼0.9 eV.

We have carried out a detailed study of the competition between optical and thermal transitions from mobility gap states in hydrogenated amorphous silicon films. By employing transient photocapacitance and capacitance DLTS techniques on n-type doped films, we have observed the dangling bond optical transitions:D− → D0 + e and D0 → D+ + e, and the thermal transitions: D− → D0 + e and D0 → D0 + h. From the transition energies we have determined that the degree of lattice relaxation for dangling bonds does not exceed 0.1 eV and that the value Ueff of the D−/D0 splitting is 0.3 ± 0.1 eV. Voltage pulse photocapacitance studies on undoped films revealed the existence of the optical transition D−/D0 + e. In addition, in all films we have observed a photocapacitance signal due to optical transitions from the valence band tail states (over the energy range 1.4 to 1.9 eV) at temperatures as high as 440 K, in spite of the expectation that such shallow photoexcited holes would immediately be thermally emitted yielding no photocapacitance signal. We discuss possible explanations for this result by considering two different trapping models.

Doped films of amorphous silicon nitride have been prepared by the glow discharge technique over a wide range of ammonia/silane mixtures. P- and B-doped specimens were produced by admixing phosphine or diborane during deposition. Interstitial doping was attempted by Na-ion implantation, and by in-diffusion of evaporated Li. For values of the ammonia/silane volume ratio R < 10−2, the optical gap remains unchanged and all the above dopants raise the room temperature conductivity to σRT ≅ 10−2(Ω cm)−1. With increasing R the efficiency of P, B and Na doping decreases rapidly. However, incorporation of Li continues to significantly increase σRT at higher R where the optical gap approaches 5eV. Near-stoichiometric, transparent specimens have been prepared with σRT increased by up to 14 orders of magnitude by Li doping. A discussion of the doping results is given.

The steady state photomodulation spectrum, its temperature and excitation intensity dependences have been studied in phosphorous doped and undoped a-Si:H. The spectra are analyzed in terms of photocarriers trapped in band-tail states and dangling bonds (DB) defects in undoped samples and impurities and charged DB states introduced by doping in P-doped samples. In undoped a-Si:H we found that the two DB energy levels D± are symmetrically located about midgap with an effective correlation energy Ueff of 0.6±0.2eV. In P-doped a-Si:H D− is pushed toward midgap by about 0.25 eV and Ueff = 0.4±0. 2 eV.

The self-consistent pseudopotential method has been used to calculate the electronic structure of a periodic model of hydrogenated amorphous silicon containing, independently, P and B impurities. We have investigated both impurities in threefold- and fourfold-coordinated sites. Both types of sites studied also include the realistic distortion inherent in disordered systems. In fourfold coordination, both P and B introduce gap states, the energy and spatial extent of which depend upon the local geometrical distortion. In threefold coordination, the lone pair state of P is found below the valence band edge, while the B non-bonding state is found above the conduction band edge. The effects of local geometrical distortion on both of these states is also discussed. Comparisons with other calculations and experimental results are made.

Charge carrier dynamics in doped and undoped a-Si:H after subbandgap excitation at 1064 nm was followed by transient photoconductivity experiments. The kind of optical transitions induced by 1064 nm light is strongly dependent on the position of the Fermi-level. This allows the seperation of excess electron and excess hole kinetics as in n-doped samples mainly mobile electrons and in moderately p-doped samples mainly free holes are generated by 1064 nm light. From comparison of the photoconductivity after 532 nm and 1064 nm excitation it is inferred that the rate controlling step for the electron decay is the availability of free holes.

The effect of the implantation of oxygen ions on the density of states in the mobility gap of n type hydrogenated amorphous silicon (a-Si:H) is studied using drive-level capacitance profiling, deep-level transient spectroscopy( DLTS), and transient voltage-pulse photocapacitance. After implantation, we observe a broadening of both the conduction and valence band tails and an increase in the dangling bond density near midgap, as well as a shift of the Fermi level toward midgap by approximately 100 meV in two out of the throe films studied. We also observed the addition of a distinct defect band at approximately 0.7 eV below the conduction nd mobility edge in films implanted with oxygen doses greater than 1012 cm−2.

The extension of a recently reported application of a dielectric spectroscopic technique on c-GaAs will be shown in the case of a-Si:H. This technique is based upon the application of a small signal a.c. voltage at several fixed frequencies using a Frequency Response Analyzer.

The technique is particularly useful for applications to material study where more traditional methods (i.e. DLTS) fail to apply. Thus we can probe the nature of bulk transport processes in the most resistive of materials and also study the interface between the contact and the substrate. It therefore becomes relatively simple to distinguish Schottky or blocking contacts from Ohmic or quasi-Ohmic contacts.

Typical capacitance and dielectric loss data will be presented as a function of frequency. Arrhenius plots obtained from the temperature dependence of the above parameters will show that activated processes have been observed and may be related to specific defect centres.

Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are briefly reviewed. The small but systematic changes of subgap absorption (α) with the applied external stress have been found. When light-soaking is done under the applied stress the strong increase of α is observed. The μτ (mobility-lifetime) versus α duality is demonstrated. The increase of the dangling bond density, combined with the change of its charge state, is used for the explanation of duality. The light induced creation of dangling bond-impurity intimate pairs is speculated to be the driving force for the Fermi level shift.

A new method for the determination of the deep states density in amorphous semiconductors is presented. The method is based on the carriersemission- time dependence of the capacitance-voltage characteristics of Schottky barriers. The applicability of the method for the study of hydrogenated amorphous silicon materials is demonstrated. The pitfalls associated with trying to deduce the density of states from a single capacitance-voltage characteristic are also discussed.

The drift mobility μd and the mobility-lifetime product μτ in undoped a-Si:H have been studied up to 130°C. The electron μde is temperature-activated with Eae = 0.13 to 0.16 eV. The electron (μτ)e increases with temperature T. For hole transport, we observe the transition from dispersive to non-dispersive transport with increasing T. The hole μdh is ∼ 1/100 of μde, and is activated with Eah = 0.34 to 0.48eV. The hole (μτ)h does not change much with T. A computer simulation demonstrates the high sensitivity of μd to the band tail width.

Drift mobility measurements under high level double injection in p/i/n and single injection in M/i/n devices have been made as a function of d.c. voltage and temperature using a small step voltage technique. For comparable ratio's of V/L2 the transit time of injected electrons was of order one decade shorter under double injection than under single injection. The room temperature drift mobility for single injection was approximately 1cm2−1s−1. We attribute the difference in drift transit time to a higher extended state mobility with double injection than with single injection due to the neutralization of charged defects.

We analyze the transient response of i/n/i and i/p/i semiconductor structures in terms of space-charge-limited and emission-limited currents. In this ‘sweep-out’ experiment, an initial space-charge-limited current governed by the i-layer characteristics gives way at later times to an emission-limited current from the doped layer. The emission-limited current response can be used as a new spectroscopy for determining the electronic density of states near the mobility edge of doped semiconductors. We summarize the published experimental sweep-out data and draw conclusions about the density of electronic states of both n-type and p-type hydrogenated amorphous silicon.

Experimental results on the temporal response of i/n/i hydrogenated amorphous silicon structures are presented as a function of voltage. The current is initially space charge limited followed by a transition to an emission limited current. The data generally exhibits four subsequent time dependence regimes e.g. :(1) an exponential decay, (2) followed by a t−2 decay, (3) then a t−1 decay and (4) lastly a t(α−1) decay where α<1. The current versus time allows one to derive a density of states. The deduced density of states in the arsenic doped n layers displays a peak at shallow energies followed by an exponential with an energy decay constant of approximately 300°K followed by a flat region and finally followed by a slowly rising density of states. The initial Fermi level is estimated to be within the peak located less than 0.3eV from the mobility edge.

Transient photoconductivity measurements in a-Si:H at different n- and p-doping level point to decay kinetics which strongly depend on the doping level and excitation densities. In undoped a-Si:H excess charge carriers decay mainly by a transition between electrons and holes at their respective mobility edges. The generation of hole traps by n-doping quenches this decay channel and transitions between localized states in the bandgap play an important role in charge carrier kinetics. In p-doped material the availability of a large density of mobile holes and positively charged recombination centers increases the electron decay drastically.

Based on modifications of the standard multiple trapping approach we present a theory of photoconductivity decay from the steady state in which explicit account is taken of the minority carrier behaviour. We use this to develop a physical understanding of the decay process. We obtain the intensity dependence of the decay time and compare it with experimental results for hydrogenated amorphous silicon.

Heavily phosphorus and boron doped hydrogenated amorphous silicon films were deposited in the temperature range 50 to 300 °C. Concentrations of P, B and H, IR spectra and room temperature conductivity have been measured. When the deposition temperature is raised from 50 to 300 °C the concentration of P increases, while the concentration of B decreases. The dark conductivity of both P and B doped films decreases dramatically when the deposition temperature is lowered. We interpret these results on the basis of assumptions concerning the microstructure of the deposited films, and especially the variation of this structure with deposition temperature.

Effect of low level doping of boron and phosphorus on the properties of amorphous silicon films (a-Si:H) was studied. Doping level of both boron and phosphorus was in the range of 1017 atoms/cm3. Apparent improvement in the stability of dark and photoconductivity of a-Si:H films upon low level doping does not result from the elimination of light-induced defects. The stability of the dark and photoconductivity upon doping is an indication of pinning of the Fermi level.