Polaron transport of amorphous semiconductors with embedded crystallites

Near-room-temperature (narrow-band) polaron transport of an amorphous semiconductor with embedded annealing-induced semiconducting crystallites is treated within an effective-medium approach. Carrier mobilities in the crystallites are assumed much larger than those of the amorphous phase. Nonetheless, crystallites act as macroscopic traps when their carriers’ energies lie below those in the amorphous phase. Then the mixture’s dc conductivity falls below that of the amorphous phase at low enough carrier concentrations. However, with increasing carrier concentration the shifting chemical potential diminishes this trapping effect, enabling crystallites’ larger mobilities to drive the mixtures’ electrical conductivity above that of the amorphous phase. Meanwhile the Seebeck coefficient remains insensitive to the annealing-induced introduction and growth of embedded crystallites. These features are qualitatively similar to those reported for an amorphous organic polymer FET with annealing-induced embedded crystallites.     

Anderson-Mott transition driven by spin disorder: spin glass transition and magnetotransport in amorphous GdSi

A zero temperature Anderson-Mott transition driven by spin disorder can be "tuned" by an applied magnetic field to achieve colossal magnetoconductance. Usually this is not possible since spin disorder by itself cannot localize a high density electron system. However, the presence of strong structural disorder can realize this situation, self-consistently generating a disordered magnetic ground state. We explore such a model, constructed to understand amorphous GdSi, and highlight the emergence of a spin glass phase, Anderson-Mott signatures in transport and tunneling spectra, and unusual magneto-optical conductivity. We solve a disordered strong coupling fermion-spin-lattice problem essentially exactly on finite systems and account for all the qualitative features observed in magnetism, transport, and the optical spectra in this system.     

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.     

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.     

Magnetic and electrical transport properties of delta-doped amorphous Ge:Mn magnetic semiconductors

We report on the growth and characterization of delta-doped amorphous Ge:Mn diluted magnetic semiconductor thin films on GaAs (0 0 1) substrates. The fabricated samples exhibit different magnetic behaviors, depending on the Mn doping concentration. The Curie temperature was found to be dependent on both the Mn doping concentration and spacing between the doping layers. A sharp drop in magnetization and rise in resistivity are observed at low temperature in samples with high Mn doping concentrations, which is also accompanied by a negative thermal remanent magnetization (TRM) in the higher temperature range. The temperature at which the magnetization starts to drop and the negative TRM appears show a correlation with the Mn doping concentration. The experimental results are discussed based on the formation of ferromagnetic regions at high temperature and antiferromagnetic coupling between these regions at low temperature.     

Phonon conductivity of insulators and semiconductors

Phonon conductivity calculations for materials from groups IV, III–V, II–VI and I–VII are presented. The method uses an effective relaxation time for a phonon taking into account departure from equilibrium of all other phonons which participate in both three-phonon N- and U-processes. It is shown that an important role is played by the off-diagonal part of the three-phonon collision operator above the boundary regime. In particular it is found that the off-diagonal U-processes operator gives a negative contribution to the conductivity at high temperatures. The role of optical phonons in heat transport and in acoustic-optical phonon scattering is also studied. For materials with the mass ratio m₁/m₂ ? 1 we have used the Debye dispersion law in the ‘reduced zone’ scheme, while for materials with m₁/m₂ > 1 but <4 we have used the Debye dispersion law for the acoustic phonons and the Einstein dispersion law or the optical phonons. Inclusion of the optical phonons does not modify the temperature dependence of the conductivity but does change the magnitude significantly.     

Doped amorphous semiconductors

The article reviews the advances that have been made during the past two years in the new field of substitutionally doped a-semiconductors, particularly a-Si. After introducing some of the basic concepts used in the treatment of a-materials, the preparation and doping of a-semiconductors from the gas phase and the role of hydrogen are discussed in some detail. Recent results on doping by ion implantation have also been included. The following sections are concerned with the effect of n and p-type impurities on the electronic transport properties of a-Si and a-Ge. The discussion is based on results from conductivity, drift mobility, Hall effect and thermoelectric power measurements and leads to the main conclusion that the donors introduce a new hopping path through the system which begins to dominate over tail-state hopping when their density exceeds about 10¹⁸ cm^-3. Recent photo-conductivity and luminescence results on doped a-Si are then discussed; they give information on recombination and also demonstrate the pronounced effect of doping on these properties. In the final section amorphous p-n junctions are considered as well as their possible applications in photovoltaic solar energy conversion.     

Time-resolved luminescence in amorphous semiconductors

Time-resolved luminescence spectra of some amorphous semiconductors are reported. In a-Si:O:H, the Coulomb interaction of the recombining electron and hole is explicitly observed in the time dependence of the luminescence peak position, at delay times ranging from 200 nsec to 30 μsec. Peak shifts at longer times are analyzed in terms of a distribution of recombination energy due to band tailing. It is deduced that the addition of oxygen to a-Si:H broadens the tails. In As₂S₃ we find no evidence of the Coulomb interaction, and this result supports the charged defect model.     

Diamagnetic enhancement in amorphous semiconductors

We analyse the diamagnetic susceptibility χ of a model two-dimensional semiconductor both in crystalline and amorphous phases using a linear combination of hybrids model. We show that the large diamagnetic enhancement in amorphous Si and Ge is due to the reduction of the Van Vleck type paramagnetic term.     

Frequency-dependent loss in amorphous semiconductors

Recent developments in the theoretical analysis and experimental study of frequency-dependent loss by relaxation in amorphous semiconductors are reviewed. A unified theoretical treatment of the complex a.c. conductivity is given, within the pair approximation, for single electron tunnelling and hopping in both uncorrelated and strongly correlated cases, and the discussion is then extended to pair processes and to atomic relaxation. The problems associated with measuring the frequency dependent conductivity of amorphous samples are considered, and relevant measurements reported for the different classes of amorphous semiconductors, tetrahedral and group V materials and chalcogenides are reviewed in the light of the available theoretical models. The similarity in the magnitudes and frequency, temperature and electric field dependences of the losses observed in many different systems at liquid helium temperatures is noted, and the possible physical reasons for this are examined.     

Hopping conduction in amorphous semiconductors

It is suggested that conduction in localized states at the band edge of an amorphous semiconductor occurs by variable-range hopping. The hopping length is then typically ten times larger than the average interatomic separation and increases with decreasing temperature. The hopping energy is several times smaller than Arrhenius plots of the mobility would indicate.     

Band-gap fluctuations in amorphous semiconductors

The exponential absorption tails of amorphous semiconductors are described by integrating the band-to-band absorption over an exponential distribution of band-gap. This requires that the tail states are derived from the band states, and are therefore not highly localised. We show that the fluctuations required by the absorption predict the principal features of the intrinsic 1.4eV luminescence of a-Si:H. The origin of the fluctuations is attributed to ‘frozen’ phonons which constitute the amorphous short-range disorder.     

Photoconductivity of amorphous semiconductors

An analysis of photoconductivity is presented for a material with a carrier mobility which decreases as a power law with time. We calculate the steady state and transient response, and show that the model applies to hydrogenated amorphous silicon and other amorphous semiconductors. In a-Si:H the effective mobility is obtained from 10^?6 to 1 sec. The recombination is of bimolecular type and the carrier density is almost independent of excitation intensity.     

Theory of amorphous semiconductors

The basic concepts underlying the present theory of amorphous semiconductors are reviewed. Emphasis is put on simple band models.     

Phonon spectroscopy measurements at amorphous films

Phonon spectroscopy measurements were used to examine the scattering of high frequency phonons (300 GHz-1 THz) in amorphous materials. The experiments were done with the use of time and frequency resolved measurements of the phonon transmission behaviour through amorphous single films of different thicknesses. The typical film thicknesses were of the order of 10 nm. In contrast to the pure amorphous semiconductors Si and Ge our experiments show inelastic phonon scattering processes in the case of SiO₂ and SiH. This inelastic phonon scattering also occurs when the pure semiconductors Si and Ge are prepared in an O₂ or H₂ atmosphere, but is missing when the preparation process is done in an N₂ atmosphere. In films of the pure semiconductors a-Si and a-Ge we only found evidence to elastic scattering processes. In further experiments at heated a-SiH samples we could examine the atomical bonded hydrogen to be the center of the inelastic phonon scattering.The measurements and investigations described in this work were done in time of preparing a thesis at: Physikalisches Institut Teil 1, Universität Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany     

Glass transition in charge carrier transport phenomena of amorphous photoconductive polymers

The critical temperature T_o in Gill's empirical equation for the charge carrier transport was found to have aclose connection with the glass transition temperature T_g for poly-N-vinylcarbazole containing model compound systems. Interpretations of T₀ and μ₀, i.e., the drift mobility at T₀, were presented.     

Phonon driven nonlinear electrical behavior in molecular devices

Electronic transport in a model molecular device coupled to local phonon modes is theoretically analyzed. The method allows for obtaining an accurate approximation of the system's quantum state irrespective of the electron and phonon energy scales. Nonlinear electrical features emerge from the calculated current-voltage characteristics. The quantum corrections with respect to the adiabatic limit characterize the transport scenario, and the polaronic reduction of the effective device-lead coupling plays a fundamental role in the unusual electrical features.