Drift Mobility Measurements Research Articles

Transport in Si and Ge polymers above and below T g-comparison with a-Se and other glasses

Drift mobility measurement on prototypical sigma bonded Si and Ge backbone polymers carried out over a temperature range broad enough in each case to encompass the glass transition are described. Below T g the hole drift mobility exhibits a convoluted field and temperature dependence similar in detail to that observed in diverse glassy media, including chalcogenides and molecularly doped polymers. It is demonstrated that drift mobility in Si and Ge polymers can be directly modified by doping. A relatively abrupt and reversible change in transport activation occurs when various Si and Ge polymers are thermally cycled through their respective glass transitions. A similar effect on mobility has been observed in other organic and inorganic molecular glasses, including a-Se. However, it is only in Se and Se based glasses where transport is controlled by thermodynamic defects, that pronounced mobility hyteresis associated with glassification and relaxation, is simultaneously observed.

Transient photoconductivity measurements on halogenated a-Se1−x Tex photoconductors

Time-of-flight (TOF) transient photoconductivity measurements, as functions of temperature and applied field, were carried out on chlorinated a-Se1−x Tex (x < 0.1) chalcogenide semiconductor films to study the nature of charge transport in these commercially important xerographic alloys. The films were prepared by conventional vacuum-deposition techniques and were typically 60–70 μm thick. The TOF hole drift mobility was found to follow a power-law field dependence of the manner μh ~ Fn with the index n increasing with Te and Cl additions as well with decreasing temperature. At the highest fields the drift mobility increased sharply with the applied field. The strong field dependence became more pronounced as the temperature was decreased. TOF drift mobility measurements, as a function of temperature at various applied fields, indicated a thermally activated hole-transport process with a well-defined but field-dependent mobility activation energy, which exhibited an approximately linear field dependence, similar to the mobility activation energy reported for doped a-As2Se3. The zero-field extrapolated activation energy, Eμ0, was about ~ 0.43 eV for the whole range of compositions investigated from x ≈ 0.02 to 0.09, and for the Cl additions in the ~ 100 at. ppm range. Two possible interpretations are discussed. The first involves Te-or Cl-introduced shallow defects that result in a distinct feature on the density of localized states peaking at around ~ 0.43 eV above Ev. It is assumed that the the conductivity mobility in this model remains to represent microscopic transport via extended states as in pure a-Se, i.e., μ0 ~ T−n with n ≈ 1. The other possibility is that increased disorder, as a result of alloying with Te or Cl addition, causes the basic microscopic mobility to become thermally activated owing to hopping in the band-tail states with a hop activation energy of ~ 0.14 eV. It is assumed that the energy location of the shallow-hole traps at ~ 0.29 eV above Ev remains reasonably unaffected, but the energy width and the concentration of native defects controlling the mobility are modified. The latter model is essentially a trap-controlled hopping transport. The present TOF measurements could not conclusively infer one model over the other. There was no observable electron transport in chlorinated a-Se1−x Tex films, whereas in unhalogenated a-Se1−x Tex films the electron-drift mobility decreased with the Te content and exhibited a stronger power-law field dependence. The TOF electron photocurrents were highly dispersive and the drift mobility – temperature data indicated a thermally activated charge-transport process with a well-defined but field-dependent mobility activation energy. The zero-field activation energy was ~ 0.49 eV, which is ~ 0.14 eV higher than that for electron transport in pure a-Se. The two models discussed for hole transport may also be applied to electron transport. It is suggested that the Te addition modifies the already present defect population leaving their energy location below Ec unchanged so that the increase in the mobility activation energy of ~ 0.14 eV corresponds to the hop activation energy of electrons in the band-tail localized states.

Picosecond electron drift mobility measurements in hydrogenated amorphous silicon

The electron photocarrier drift mobility in undoped, hydrogenated amorphous silicon was measured in the picosecond domain using optically detected time-of-flight techniques based on the electroabsorption effect. The electron drift mobility at room temperature was approximately 3 cm2 /V s between 50 and 500 ps at electric fields of (1–3)×105 V/cm, essentially the same as the mobility measured in the nanosecond domain. The picosecond measurement constrains transport models postulating larger electron mobilities shortly after photogeneration; the mobility lifetime product for a short-time process must be less than 3×10−11 cm2 /V. A slightly superlinear dependence of drift velocity upon an electric field was also found which could not be explained by dispersion effects.

Spatial Damage Distribution in Electron-Beam Processed Gaas-Algaas Heterostructures, Experiment and Theory

ABSTRACTThe effect of electron beam irradiation on the transport properties of HEMT structures has been studied. The data is found to agree with a model containing no free parameters which predicts the spatial extent of the damage. As grown HEMT material was fabricated into Hall bars. The material was then characterized by measurement of 2D EG number density and drift mobility. The material was then irradiated at constant dose with electron energies between 2.5 and 20 keV. Damage was assessed by changes in the number density and mobility. The number density was essentially unchanged at all energies. No change in mobility occurred for 2.5, 15, and 20 keV, however, a decrease in the mobility occurred for energies from 5.0 to 12.5 keV. These results agree with our model of the electrons energy loss as a function of its path length. The defects are shown to be charged, and possible sources of the defects are discussed.

Adiabatic and non-adiabatic small polaron hopping in molecularly doped polymers

New hole drift mobility measurements for the molecularly doped polymer DEH ( p-diethylaminobenzaldehyde-diphenyl hydrazone) in polycarbonate and literature data for TPD (N,N′-diphenyl-N,N′-bis (3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine) in polycarbonate are analyzed by a new procedure which separates the functional dependence of the mobility on temperature and molecular spacing ϱ. Our analysis indicates that the dependence of the mobility on ϱ arises from different sources. In DEH: polycarbonate it is due to an overlap integral, as normally expected. In TPD: polycarbonate it is due entirely to the dependence of the activation energy on p, a significant clue to the underlying microscopic hole hopping mechanism. These data are consistent with the small polaron hopping theory in two regimes, non-adiabatic hopping in DEH:polycarbonate and adiabatic hopping in TPD: polycarbonate.

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.

Hole carrier drift-mobility measurements in a-Si: H, and the shape of the valence-band tail

Abstract A detailed study has been performed concerning the temperature and electric-field dependence of hole carrier time-of-flight pulses in amorphous silicon. Various aspects of the experimental behaviour are employed in the estimation of the energy distribution and other characteristics of localized states in the films. Over the depth range 0·2 to 0·45eV, the trap concentration is found to vary more rapidly than the exponential form assumed in earlier studies, and a Gaussian tail is in better agreement with the data. Localized-state capture cross-sections are calculated as 1–3 × 10−16 cm2, and are essentially independent of depth over the range studied. Taking the present data in conjunction with the energy distribution of states as determined by other techniques allows an estimation of the mobility of free holes. The value of about 10cm2V−1s−1 thus obtained is an order of magnitude larger than the previous estimate from time-of-flight measurements, but is comparable to the figures suggested by other transport data.

Deep Levels and Drift Mobility Measurements in Hydrogenated Amorphous Silicon

Deep Levels and Drift Mobility Measurements in Hydrogenated Amorphous Silicon

Dual-beam-modulated photoconductivity in hydrogenated amorphous silicon response-time and drift-mobility measurements

Abstract Dual-beam-modulated photoconductivity in a-Si:H is studied at room temperature. The complex modulation signal is analysed in terms of three independent relaxing components. It is shown that the fast-relaxing term is due to electron detrapping from states in the conduction-band tail and that its relaxation time is the response time τR of the photoconductivity. The product μτ is measured by steady-state photoconductivity, and we proceed by investigating the dependence of the ratio μτ/τR = μ/(1 +n t/n) on the photogeneration rate f where n t and n are the trapped and free electron densities respectively. The f dependence of the ratio n t/n is discussed. A model based on the existence of an exponential distribution of tail states allows us to show that n t/n∝ f 1−2γ (σp∝fγ), in agreement with the experimental data. This implies that the response time of the photoconductivity follows a τR∝ fγ dependence. The link between μτ/τR and the drift mobility μD measured by time of flight is discussed. It is found that μ/τR is, at the low photogeneration rates studied in this paper, lower than μD and increases with f The reason is that, whereas time of flight probes shallowly trapped electrons, and measures a ratio (n t/n)*, secondary steady-state photoconductivity probes deeply trapped electrons and measures a ratio xn t/n>(n t/n)*.

The investigation of transport properties in a-si by fast transient techniques

The paper reviews the growing range of information on the electronic properties of glow discharge amorphous Si that has been obtained from transient experiments. The interpretation of recent drift mobility and carrier lifetime measurements will be discussed.

Time-of-flight drift mobility measurements on chlorine-doped amorphous selenium films

Time-of-flight drift mobility experiments have been carried out on pure and chlorine-doped amorphous selenium films. The temperature and field dependence of the drift mobility of both types of carriers have been investigated and the charge transport mechanism has been critically examined in terms of various shallow-trap-controlled transport processes, including hopping for the microscopic mobility. Addition of 30 and 40 wt ppm Cl to a-Se has been found to increase the hole drift mobility activation energy by approximately 0.16 eV. Two interpretations have been shown to be plausible: (i) Cl doping diminishes the existing charged centres (possibly Se1-) acting as shallow hole traps and/or changes the mode of microscopic transport, (ii) Cl introduces a new set of charged hole traps (probably Cl-) at approximately 0.45 eV above Ev. Furthermore, it is shown that at low temperatures the hole drift mobility may be described by an empirical expression similar to that due to Gill (1972, 1976).

Electron drift mobility in a-Si:H; Comparison of two measuring techniques

Time-of-light measurements of the electron drift mobility in hydrogenated amorphous silicon are reported and compared with mobility values obtained by the travelling wave technique. The results obtained by the two methods on samples prepared under the same conditions agree within experimental error.

Drift mobility measurements of small-polaron transport in SrTiO3

Transient-charge-technique transport measurements have been made on pure strontium titanate. The trapping times at low temperature and the drift mobility for temperatures between 100 and 400K have been measured. It is found that the drift mobility is activated at high temperature and that, for one sample, it increases exponentially at low temperature. This behaviour is interpreted as evidence for small-polaron transport, both hopping- and band-type mobility being seen on the same sample with a transition temperature of about 180K. At low temperature the behaviour also depends on the impurity content. The relatively small (0.1 cm2 V-1 s-1) activated mobility, as compared with the larger nonactivated mobilities in doped or reduced samples, is described in terms of the screening of the electron-phonon interaction.

Measurement of drift mobility and carrier density profiles in In0.53Ga0.47As long-gate JFETs

Measurements of the drift mobility profile and the carrier density profile in LPE-In0.53Ga0.47As channel of InGaAs/InP:Fe JFETs with a gate length of 100 µm are reported. Nearly flat profiles of drift mobility on channel depth with maximum values of 7500 cm2/Vs at a net carrier density of 1.5×1016 cm¯3 were realised.

The study of transport and related properties of amorphous silicon by transient experiments

Transient experiments, particulary drift mobility measurements, have made valuable contributions to the understanding of electronic transport in amorphous (a-) semiconductors. The paper begins with a review of recent work on carrier mobilities in a-Si and a discussion of dispersive behaviour in relation to the tail state distributions. We then turn to the present controversy over the interpretation of transient signals and suggest that the conclusions of Silver et al cannot be upheld. The final part deals with information from transient experiments on carrier lifetimes and μτ-products in a-Si and a possible explanation for their thickness dependence is given. Further evidence is presented that neutral dangling bonds (D 0) are the predominant sites for deep trapping.

Drift mobility measurements on single crystals of orthorhombic benzophenone using the electron bombardment technique

Drift mobility measurements on single crystals of orthorhombic benzophenone using the electron bombardment technique

Urbach edge and the density of states in hydrogenated amorphous silicon

Photoconductivity measurements of the optical absorption edge and time-of-flight measurements of the hole drift mobility, on the same amorphous silicon film, enable us to identify the origin of the Urbach edge in amorphous silicon with the density of states in the valence band tail.

The effect of Te alloying on the electronic gap of a-Se

The effect on the electronic density of gap states in a-Se produced by the isoelectronic additive Te has been studied by a combination of drift mobility and xerographic measurements. Drift mobility experiments probe the shallow gap states which control photoinjected carrier transport. Addition of Te to Se in the nominal range 0–18 wt.% progressively decreases both electron and hole drift mobility while simultaneously increasing dispersion. The number and distribution of deep gap states which control injected carrier range have been determined in the same films from analysis of xerographic residual potentials. Taken together these measurements show that progressive addition of Te to a-Se increases the integrated number of deep traps and broadens the distribution of both deep and shallow transport interactive electronic gap states.

Hydrogenated amorphous silicon solar cells

A simple description of the operation of the hydrogenated amorphous silicon (a-SiH x ) pin solar cell is given and general guidelines for increasing the efficiency are established. The use of heterostructures in which the n and p layers have larger band gaps than the intrinsic (i) layer helps to reduce losses in efficiency due to optical absorption in the doped layers and back diffusion of carriers across the n-i and p-i interfaces. The density of gap states of the intrinsic layer is inferred from measurements of optical absorption, photoconductivity, drift mobility, collection efficiency and capacitance. Based on the density of gap states of the best intrinsic material available, an upper theoretical limit of 17% is estimated for the solar cell efficiency. Materials' improvements required to achieve experimentally a 10% efficient a-SiH x solar cell are discussed.

Transport properties of a-Si: H alloys prepared by r.f. sputtering

Abstract Conductivity and thermopower measurements as a function of temperature are reported for a series of r.f. sputtered hydrogenated a-Si samples doped with P. The results are analysed, along with others in the literature, in terms of several models for transport. It is concluded that the best overall fit to the data involves conduction in an intrinsic band above about 400 K and conduction in a P-related impurity band between about 200–400 K. The proposed transport in the latter temperature regime disagrees with earlier conclusions of Spear and Le Comber, and it also implies a smaller shift of the Fermi level with doping than does their model. The implications of this interpretation for current measurement of Hall effect and drift mobility are briefly outlined, as are difficulties in the detailed interpretation and complications related to heterogeneity in structure.