Extended State Mobility Research Articles

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals (Si NCs) prepared by annealing of $a\ensuremath{-}\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}:\mathrm{H}$ precursors is studied using temperature-dependent current-voltage measurements supported by electron spin resonance and mass spectrometry data. Transport is Ohmic in all films at all temperatures and the temperature dependence of conductivity shows that the materials behave as disordered semiconductors, exhibiting extended-state transport at high temperature and variable-range hopping transport at low temperature. Grain-boundary-, surface-, and interface-dominated transport is systematically ruled out. Films are $n$ type, and films with Si NCs exhibit up to ${10}^{3}$ times higher conductivity (up to $0.1\phantom{\rule{0.16em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$) after exposure to a hydrogen plasma which passivates dangling bonds, particularly on Si NCs. A forming gas anneal does not have such an effect, indicating that atomic rather than molecular hydrogen is required. The conductivity of SiC films without Si NCs is largely unchanged by passivation and the Fermi level is not raised nearly as closely to the conduction band. This is attributed to a type I band offset between Si NCs and SiC that leads to extended-state conduction in films with Si NCs taking place in a Si network. This is confirmed by the dependence of the extended-state mobility on the volume fraction of excess Si. Variable-range hopping is relatively insensitive to the presence of excess Si and is hence considered to take place via shallow defect states throughout the volume of the films. The high conductivities are found to be a consequence of background doping by oxygen and nitrogen.

Charge carrier transport and collection enhancement of copper indium diselenide photoactive nanoparticle-ink by laser crystallization

There has been increasing needs for cost-effective and high performance thin film deposition techniques for photovoltaics. Among all deposition techniques, roll-to-roll printing of nanomaterials has been a promising method. However, the printed thin film contains many internal imperfections, which reduce the charge-collection performance. Here, direct pulse laser crystallization (DPLC) of photoactive nanoparticles-inks is studied to meet this challenge. In this study, copper indium selenite (CIS) nanoparticle-inks is applied as an example. Enhanced crystallinity, densified structure in the thin film is resulted after DLPC under optimal conditions. It is found that the decreased film internal imperfections after DPLC results in reducing scattering and multi-trapping effects. Both of them contribute to better charge-collection performance of CIS absorber material by increasing extended state mobility and carrier lifetime, when carrier transport and kinetics are coupled. Charge carrier transport was characterized after DPLC, showing mobility increased by 2 orders of magnitude. Photocurrent under AM1.5 illumination was measured and shown 10 times enhancement of integrated power density after DPLC, which may lead to higher efficiency in photo-electric energy conversion.

Transient photoconductivity studies of band tails states in compensated a-Si:H

The density of conduction band tail states has been determined for a series of compensated a-Si:H films using transient photoconductivity measurements. Relative to undoped material, the energy width of shallow tail states lying within 0.45 eV of the conduction band edge are found to be insensitive to the level of compensated doping for doping levels up to 1000 vppm. An observed reduction in photocurrent magnitude as the doping is increased is consistent with a corresponding reduction in the electron extended state mobility. The magnitude of the mobility reduction is found to be quantitatively consistent with potential fluctuations which arise from ionized dopants for compensated doping levels up to about 100 vppm.

Transport in tunneling recombination junctions: A combined computer simulation study

The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (μc-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si∕a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilities should be determined. Apparent discrepancies between simulation results and measurements are explained and eliminated making use of Gill’s law. For application in tandem and triple solar cell structures, tunneling recombination junctions made of (μc-Si) are the most promising of all examined structures.

Influence of the light-induced degradation on the extended state mobility in hydrogenated amorphous silicon

We measured the effects of light soaking on the extended state electron mobility in intrinsic and n-type doped hydrogenated amorphous silicon samples. We obtained the temperature dependence of the mobility in the range 0–80 °C, using a recently proposed method [Dawson et al., Appl. Phys. Lett. 63, 955 (1993)]. We found a decrease of the mobility as the degree of light-induced degradation increased. We suggest that these changes in the extended state transport are caused by an enhancement in the magnitude of the potential fluctuations introduced by the extra created charged defects.

Study of the temperature and field dependence of electron drift mobility in α-Si 1−χC χ:H using the time-of-flight technique

The time-of-flight transient photoconductivity experiment has been used to investigate electronic transport in hydrogenated amorphous silicon carbide. The samples in the present study were deposited in the low power regime with carbon contents below x = 0.1. Results show that electron transport is anomalously dispersive at low temperatures, as also observed in α-Si:H. The extended state mobility for these materials was estimated to be between 1 and 10 cm 2V −1s −1 and mobility activation energies at low fields indicate that significant trapping occurs further from the conduction band edge with increasing carbon concentration. DOS profiles are derived directly from the experimental data.

Determination of Drift, Extended State Mobility and Recombination Lifetime in Compensated a-Si:H by Photomixing

Mobilities of a series of compensated a-Si:H samples, measured earlier by the time-of-flight technique [1], were determined by the technique of photomixing. We have found that both the extended state and the drift mobilities decrease as the compensation increases. Modelling these transport processes in the context of the photomixing technique, it is shown that long-range potential fluctuations can account for the decrease in the extended state mobility in compensated samples.

The Extended State Mobility in Amorphous Silicon Alloys

ABSTRACTThe total scattering time of free carriers injected optically in the extended states of amorphous semiconductors has been measured using the techniques of femtosecond time-resolved spectroscopy. We find that this scattering time is less than 1 femtosecond, independent of alloy composition (a-Si:H, a-Si,Ge:H, a-Si,C:H) and temperature (from 77 K to 400 K). The extended state mobility deduced from the measurement of the scattering time and of the effective mass is approximately 6 cm2/Vs. A model is proposed to explain these results

Steady State Photocarrier Charge Collection in a-Si:H for Electrons and Holes

AbstractCarrier transport in amorphous silicon pin-diodes has been analyzed by steady state photocarrier charge collection applying strongly absorbed light. For low intensities at room temperature electron charge collection is limited by recombination in the generation region. For increasing intensity ø and/or decreasing temperature charge collection becomes nonlinear in ø and shows S-like characteristics versus voltage.We present a model for this behaviour, including space charge limitation which e.g. for holes in a-Si:H limits charge collection even at room temperature due to low extended state mobility of holes and dark Fermi level position above midgap.

Extended state mobility in a-Si:H measured by femtosecond spectroscopy

We have measured the optical properties of free carriers in a-Si:H by pump and probe femtosecond spectroscopy. The Drude model is found to be applicable and the momentum scattering time that also enters in the expression for the mobility of the extended state carriers is of the order 0.6 femtosecond for electrons. The upper bound for the mobility of free electrons approximately 100 meV above the mobility edge is 6 cm 2/V sec.

Space Charge and Recombination Limited Currents-How Different Are They?

Abstract Space charge limits the current under single carrier injection because only approximately CV can be injected. On the other hand, under double injection one requires only that |Q+ - Q−| ≃ CV. In this paper, I discuss the conditions where Q− ≫ CV and the recombination limited current is very large compared with the space charge limited current. It is shown that Q− ≫ CV can be achieved providing that the capture rate constant to neutral recombination centers is very small compared with the capture rate constant to occupied charged centers and the ratio of the extended state mobility μ to the capture rate constant to charged centers is also large.

Characteristics of hydrogenated amorphous silicon thin film transistors fabricated by d.c. magnetron sputtering

Hydrogenated amorphous silicon (a-Si:H) thin film transistors with channel lengths of 30 μm and with channel widths of 500 μm were made using d.c. magnetron sputtering. By optimizing the fabrication process, an “on” current of about 10−5 A and an off current of about 10−12 A were obtained. We studied the influence of the thickness of the intrinsic layer and the influence of the hydrogen contents in the a-Si:H and in the hydrogenated amorphous silicon nitride (a-SiNx:H) films mainly on the on current and on the extended state mobility. The optimal thickness of the insulating layer was determined to be 0.1 μm and the hydrogen contents to be 19% in a-Si:H and 10% in a-SiNx:H. The maximum mobility attained was 0.55 cm2 V−1 s−1.

Electron-transport parameters and tail-state distribution in hydrogenated amorphous silicon obtained from position and tail-state matrix simulation of drift experiments

A new matrix simulation method has been developed in order to predict transient charge transport in amorphous semiconductors. This matrix method takes into account the simultaneous evolution of spatial positions and trap energy levels of carriers by means of a discretization of time, space, and energy of the trapping levels. Diffusion and space-charge effects, either static or due to the moving carriers, are taken into account. It is shown that these effects are noticeable for small drift fields (smaller than 5\ifmmode\times\else\texttimes\fi{}${10}^{3}$ V/cm for a-Si:H). Our calculational method agrees with the thermalization approximation or the Schmidlin approximation in the regions where these approximations are valid; these validity regions are analyzed. The published experimental time-of-flight data on a-Si:H can be fully understood in the framework of multiple-trapping model with the following characteristics: the trap density of states consists of two exponential functions of energy crossing 140 meV below the conduction-band edge, with slopes defined by equivalent temperature ${T}_{1}$=370 K and ${T}_{2}$=200 K. Neither the dominant state, linear distribution, nor the single exponential tail proposed in the literature is able to describe the whole set of data. The capture cross section \ensuremath{\sigma} is independent of trap level energy and is equal to ${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$; the weak electron-photon coupling, which implies an exponential behavior of the capture cross section, is then ruled out. Extended-state mobility (${\ensuremath{\mu}}_{\mathrm{ext}}$) temperature dependence as ${T}^{\mathrm{\ensuremath{-}}1}$ is favored. Room-temperature ${\ensuremath{\mu}}_{\mathrm{ext}}$ is 30 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ if the first exponential extends from conduction band to 140 meV; this mobility increases up to 130 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ if the density of states is flat between the conduction band and 90 meV.

Microscopic transient simulation of semiconductors and insulators

We have developed a general-purpose simulator for transient experiments in semiconductors and insulators based on microscopic transport of carriers and including an arbitrary distribution of localized states in the band gap. As a first application of the simulator we show the results of simulated time-of-flight experiments on amorphous silicon hydride. We demonstrate that due to the large dynamic range of microscopic transit times, different definitions of the mean transit time result in different observed mobilities. We also show that such experiments may underestimate the extended state mobility.

Drift mobility under single and double injection in hydrogenated amorphous silicon

Abstract To test the effect of charged defect centres in a-Si: H we have performed drift-mobility measurements using a small-voltage-step technique in p-i-n and M-i-n samples already under strong forward bias. These measurements have been made as a function of forward bias and temperature. The drift mobility under double injection was of the order of one decade larger than that under single injection. The respective activation energies were approximately 0·13 and 0·17eV while the activation energies for the forward-bias currents were approximately 0·45 and 0·64 eV. We suggest that the differences in drift mobility are a result of an increase in the extended-state mobility arising from neutralization of the charge defects under double injection. which is not possible under single injection.

Extended State Mobility and Tail State Distribution of a-Si1-xGex:H Alloys

Extended State Mobility and Tail State Distribution of a-Si1-xGex:H Alloys

Drift Mobility Measurements Under Single and Double Injection in a-Si:H

AbstractDrift 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.

Extended-state mobility and its relation to the tail-state distribution in a-Si:H.

The differing interpretations of drift-mobility data for electrons in a-Si:H have been examined. On the one hand, there are the analyses which involve the ``thermalization approximation'' (TA). One such analysis, based on the density-of-states profile deduced by Spear from field-effect data, leads to the unexpectedly high value of the extended-state mobility ${\ensuremath{\mu}}_{\mathrm{ext}=500}$ ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. Another analysis, which assumes an exponential distribution of tail states, results in the more commonly accepted value ${\ensuremath{\mu}}_{\mathrm{ext}=13}$ ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. In this work, we show that both models are inconsistent with experimental data when a more thorough examination is made of the field dependence and the activation energy of the drift mobility ${\ensuremath{\mu}}_{d}$ as predicted by those models. On the other hand, we examined Spear's analysis, which does not involve the TA. We show that a thorough analysis of multiple-trapping transport in a tail-state distribution as proposed by Spear leads to more dispersion than he experimentally observed. Our examination is based on a newly developed computational procedure, which analyzes multiple-trapping transport by discretizing the continuous distribution of localized states. Apart from the relevance with respect to electron transport in a-Si:H, this work shows how a relatively simple but accurate analysis of drift mobility and transient photocurrents can be performed with a much wider applicability than the analyses based on the TA.

Recombination processes and optical bias in undoped a-Si:H

A quasi-monomolecular recombination model is shown to account for coplanar electrode transient photocurrent measurements in undoped amorphous, hydrogenated silicon (a-Si:H). Optical bias is found to modify both photocarrier dynamics and recombination, but with a response time which exceeds the recombination response time of the specimen. This model provides a new method to obtain the ratio of the microscopic trapping rate b + and extended-state mobility μ 0. Estimates were obtained using several specimens as a function of optical bias, temperature, and optical exposure history; the values range from 10 −12 to 10 −10 V-cm. These values are substantially lower than the previous estimate (10 −9 V-cm) based on extrapolated time-of-flight transient photocurrents, with possible implications for the magnitude of μ 0.

Tail-state distribution and extended-state mobility in a-Si:H

Analyses of drift mobilities in a-Si:H have been carried out for a number of model density of states distributions by means of an algebraic discretization formalism for multiple trapping transport. The extended state mobility υo is found to be not larger than 50 cm 2 V −1 s −1. None of the distributions proposed so far in the literature give satisfactory agreement with experimental data.