Electronic behaviors of the gap states in amorphous semiconductors

An extension of the Adler-Yoffa's calculation on the localized gap states in amorphous semiconductors has been made with a more realistic model which takes into account an influence of the diffused gap states and impurity states. A considerable large difference in the electron occupation probability of the gap states with sign of the correlation energy U has been examined on the tetrahedrally bonded and chalcogenide glasses. Variations of the Fermi level as a function of doped impurity concentration and gap state density have been studied.     

Polaron effect and the dc phonon-assisted hopping conductivity in amorphous semiconductors

A new small parameterW of off-diagonal electron-phonon matrix elements with respect to diagonal ones is introduced. Limiting ourselves to terms of the second order inW, the dc phonon assisted hopping conductivity is calculated to the infinite order in the electron-phonon coupling constant. Result clearly exhibits the polaron effect which is discussed in detail.     

Electronic transitions in mobility gap in glassy semiconductors

Electronic transitions in mobility gap in glassy semiconductors are considered. Energies characteristics of the transitions, including de-trapping and luminescence energies, are in a simple way related to both the gap width and each other. Basic types of luminescence centres and some types of radiationless centres, including centres of photo-structural changes, are revealed. Correlations of the luminescence and its fatigue with some other photoinduced transitions are discussed. Some features of the luminescence are presented, including those associated with the inverse- Arrhenius law.     

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e²/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.     

On the hopping conduction mechanism of amorphous semiconductors in a strong electric field

The theory of the hopping conduction by the radiative tunnel transitions (RTT) in a strong electric field is applied to amorphous silicon and germanium. On the basis of numerical estimations we suggest that the relative contribution of the RTT mechanism into the total strong electric field hopping conductivity of an amorphous semiconductor could be significant.     

Chemical trends for localized gap states in amorphous semiconductors

The absence or presence of unpaired spin and of variable range hopping due to the localized gap states (dangling bond states) in various amorphous semiconductors is explained based on the chemical nature of constituent atoms and randomness of the glass structure. The randomness is also correlated with optical absorption data.     

Spatial correlations and temperature dependence of the phonon-assisted hopping thermoelectric power in amorphous semiconductors

The assumption that the thermoelectric power of amorphous elemental semiconductors remains constant (in the hopping regime) down to zero temperature (which accords with some experiments on e.g. amorphous Ge performed above 100 K) is shown to be incompatible with the lowest order single-phonon hopping theory until the spatial interlevel correlations are taken into account.     

Comment on two-dimensional variable-range hopping conduction in group IV amorphous elemental semiconductors

The temperature T and thickness t dependence of the d.c. conductivity σ in very thin films of amorphous germanium, silicon and carbon is discussed. It is shown that a simple two-dimensional variable-range hopping model in a homogeneous medium cannot account for the experimental results, although the predicted $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}$ dependence of ?n σ is often observed. The consequences of this result for hopping transport in group IV amorphous semiconductors are pointed out.     

Non-uniqueness of the heat current operator and the hopping thermoelectric and Nernst-Ettingshausen effects in amorphous semiconductors

The Hardy correction in the heat current operator is generalized to the case of non-zero electron-phonon coupling constantg. It is then shown that not only the lowest (minus first) order but also the fist order contribution ing to the thermoelectric power of amorphous semiconductors in the hopping regime disappears. Consequently, since the lowest (zero) order contribution to the thermoelectric effects in the magnetic field yields no Nernst-Ettingshausen effect in these materials, the last effect is of the second order ing, at least.     

Rate equations in the hopping transport III. A completely new interpretation of the dc phonon-assisted hopping in the mobility gap

A new interpretation of the low-frequency phonon-assisted hopping conduction in the mobility gap of disordered semiconductors which is inherently connected with the effect of broadening of electronic levels due to electron-phonon coupling is suggested. This interpretation is fully compatible with our previous reported mathematical theory of the hopping conduction. It is argued that the current understanding of the hopping transport is based on a dubious neglect of the above mentioned broadening which makes the usual theories of the dc phonon-assisted hopping conduction inconsistent.     

The density of states in the mobility gap of halogenated amorphous silicon

Summary The density of states in the mobility gap of halogenated amorphous silicon has been measured by the field effect technique. The results are discussed to shed light on the reality of a peak found in the DOS. A comparison is made with the DOS curves obtained by SCLC in the same material. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Multiphonon hopping in disordered semiconductors

Equations of motion for the nonequilibrium single-frequency density matrix are used to obtain a balance equation describing the kinetics of electron hopping between localized states in disordered semiconductors. Conclusions are drawn about the applicability of the theory of multiphonon processes to systems with a small nonadiabaticity. The decoupling used is a generalization of one used earlier to the multiphonon case. The balance equation found in the Markov approximation for the average number of sites occupied in an electric field contains the field dependence of the multiphonon-transition probability, tending to the well-known weak-field limit.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp.35–42, February, 1976.In conclusion the author is grateful to V. L. Bonch-Bruevich and A. G. Mironov for discussing this work.     

A study of the density of gap states in amorphous semiconductors from thermostimulated conductivity spectra

A theoretical analysis of thermostimulated conductivity spectra _TSC(T) has been applied to determine the density of gap states g(E) in a-Si: H and a-Si: H/a-SiN _x : H multilayer structures. The results for g(E) are consistent with the results deduced from Fritzsche's analytical approach as well as other methods. A comparison has been made between the two different analytical approaches for _TSC(T). We discuss the relationship between the energy of maximum thermostimulated current emission E _m and quasi-Fermi level E _q. We demonstrate that E _q could be a better parameter than E _m in the general theoretical treatment of thermostimulated conductivity.     

Theory of hopping conductivity in disordered semiconductors

The bond percolation method is used to investigate the conditions under which a linear (and not exponential) temperature dependence of hopping conductivity can be observed.Translated from Izvestiya Vysshikh Uchbenykh Zavedenii, Fizika, No.2, pp. 52–62, February, 1976.     

Hydrogen in amorphous semiconductors

Over the past 5 years, there has been growing interest in a class of amorphous semiconductors deposited in thin-film form in the presence of hydrogen. The interest has derived primarily from certain electronic properties, such as the ability to control the Fermi level by substitutional doping, that make the materials potential candidates for solar photovoltaic energy conversion and thin-film device applications. These same properties have also made the materials attractive “test-beds” for basic research into electronic processes and defect states in amorphous semiconductors.     

Luminescence in amorphous semiconductors

A review of the luminescence properties of amorphous semiconductors is presented. The materials covered are chalcogenide glasses, silicon and arsenic. Luminescence spectra, excitation spectra, temperature dependences and lifetimes are described.

The radiative transition in chalcogenides is the recombination of an electron in the conduction band tail and a trapped hole. A strong electron-phonon coupling distorts the lattice near the trapped hole, lowering its energy. This interaction is responsible for the broadness of the luminescence band and its position at about half the band gap energy. The recombination centre is thought to be a charged dangling bond with density 10¹⁷ cm^-3 in arsenic chalcogenides and 10¹⁶ cm^-3 in selenium. The same centre is observed in the hole drift mobility, and thermally stimulated conductivity.

Luminescence in amorphous silicon also originates from recombination between the band tails and deep centres, with three separate transitions identified. In contrast to chalcogenides the electron-phonon coupling is not strong. The shape and intensity of the spectra are very sensitive to sample preparation and treatment, and correlate with other electrical and optical properties of Si.