Grain Boundaries In Polycrystalline Semiconductors Research Articles

Role of Grain Boundaries in Exceptionally H 2 Sensitive Highly Oriented Laser Ablated Thin Films of SnO2

A comparative study on growth, electrical conductivity, and sensor characteristics of highly oriented (transparent) and randomly oriented thin films of grown by pulsed laser (KrF; ablation technique have been carried out. Sensors made of randomly oriented polycrystalline films (deposited at 725°C on alumina) exhibited a sensitivity of about 90% for 50 ppm of at sensor operating temperatures above 240°C with a good response (∼30 s) and retracing times (180 s). Sensors made of a axis orientated films [deposited on at 525°C] exhibited an exceptionally high sensitivity of 30 to 40% even for 1 ppm of at 310°C with a shorter response time of about 15 s. However, the retrace time was very long (about 20 min). Sensors made of predominantly (101) orientated films [grown at 525°C on sapphire (1102)] exhibited an exceptional sensitivity of 90% even for 5 ppm at 300°C also had remarkably short response times of a axis oriented films as well as the quick retracing times of polycrystalline films. Thin films, which exhibited exceptionally high sensitivity showed large changes in electrical conductivity and activation energy as function of oxygen partial pressure. Atomic force microscopy investigation reveals that the films are highly granular with average size of about 150-200 nm which is ten times larger than the critical size of 8 nm as being 8 nm for Analysis of results based on the model for carrier transport across the grain boundaries in polycrystalline semiconductors reveals that the surface barrier height of the grain boundaries is responsible for the large variation in activation energy and sensitivity. © 2001 The Electrochemical Society. All rights reserved.

Charge-transfer theory in polycrystalline semiconductors with deep impurity centers

This paper discusses the static and dynamic properties of charge transfer through electrically active grain boundaries in polycrystalline semiconductors with deep impurity centers in the bulk of the grains. The admittance of the grain boundary is computed as a function of frequency and applied dc bias Ub. When Ub≠0, the admittance is mainly controlled by charge-exchange processes of the intergrain boundary states and to an insignificant extent by the charge exchange of deep traps in the bulk of the semiconductor. The application of this theory to the spectroscopy of intergrain boundary states is considered.

Structure of Grain Boundaries in Polycrystalline Semiconductors

Structure of Grain Boundaries in Polycrystalline Semiconductors

Electrical measurement of the dopant segregation profile at the grain boundary in silicon bicrystals

The paper presents a new method of determining the dopant concentration profile at the grain boundaries in polycrystalline semiconductors, based on high-frequency capacitance and dc conductance measurements. The method takes advantage of the electrostatic potential barrier associated with charged grain boundaries, and resembles, in that respect, the well-known junction capacitance technique of dopant profiling in semiconducting single crystals. As an example of application, the doping profile has been determined for the Σ=25 twin boundary in n-type (phosphorus-doped) and p-type (boron-doped) silicon bicrystals. The dopant distribution has been found, in both types of samples, very nearly homogeneous on a range of a few microns about the grain-boundary plane. This has been verified, in the case of the boron-doped bicrystals, by a secondary ion mass spectroscopy measurement of the boron concentration profile at the boundary. Annealing treatments on the n-doped bicrystals induce a slight segregation of phosphorus at the boundary. The form of the doping profile, as given by the electrical measurements, is discussed on the basis of McLean’s model of impurity segregation. The binding energy of phosphorus at the boundary is estimated on the order of 0.6 eV, comparable in order of magnitude to the values obtained by microanalytical studies of the dopant segregation at the boundaries in microcrystalline silicon.

Transient response of electrically active grain boundaries in polycrystalline semiconductors

In many polycrystalline semiconductors, the current transport is controlled by potential barriers formed at the grain boundaries. We investigate the response of ZnO grain boundaries to high-voltage pulses with risetime of less than 1 μs. The experimental results, in particular the voltage overshoot effect, cannot be explained by the dynamics of electrons (majority carriers) alone. A model which includes holes (generated by hot electrons in the depletion regions) yields an appreciable overshoot when the steady state is close to a current-voltage bistability. In this case, the approach to equilibrium is altered by the delayed response of the barriers to the presence of holes. Further implications of the model are also discussed.

Recombination processes at grain boundaries in polycrystalline semiconductors

A model for a constant distribution of localized interface states in polycrystalline semiconductors is proposed. The grain boundary diffusion potential under equilibrium conditions calculated analytically for n-type silicon is in a good agreement with experimental measurements. The non-equilibrium analyses show the carrier concentration at the grain interface for large densities of interface states and small doping concentrations of the donors. The recombination velocity and the effective density of interface states essentially depends on the height of the diffusion potential under illumination. Le modèle de la distribution constante des états localisées dans les interfaces des semiconducteurs polycrystallines est proposé. Le potentiel diffuse du limitè des grains sous les conditions d'équilibre calculé analytiquement pour n-type silicon est en bon coïncidence avec les mesuragess expérimentales. Les analyses déséquilibrées montrent la concentration des porteurs dans la limite du grain pour grandes densités des états d'interface et pour petites concentrations des doneurs. La vitesse de la recombination et la densité effective des états d'interface suffissament dépendent du niveau de potentiel diffusé sous l'illumination.

Lowering of grain-boundary barrier heights by grain curvature

A model is presented for the potential barrier height at grain boundaries in polycrystalline semiconductors, which takes into account a curved grain boundary under equilibrium and nonequilibrium conditions, assuming flat quasi-Fermi levels. An analytical method to calculate the reduction of the potential barrier due to the grain curvature, without using the depletion approximation, is developed and applied to compute the barrier lowering as a function of the bulk doping concentration, interface state density, and the grain radius in silicon. The barrier height can vary along a grain boundary, due to different local curvatures, being lowest at corners of grains.

The dc voltage dependence of semiconductor grain-boundary resistance

A model is developed to describe the potential barriers which often occur at grain boundaries in polycrystalline semiconductors. The resistance of such materials is determined by thermionic emission over these barriers. The dc grain-boundary current density as a function of applied voltage is calculated using several forms for the density of defect states within the boundary region. In all cases, the currents are Ohmic at low voltages; they can attain a quasisaturated level at intermediate voltages, and they display a sharp bias dependence at high voltages. The details of the intermediate and high-voltage characteristics are found to depend strongly on the grain-doping density and on the density and energy distribution of defect states at the grain boundary. Contrary to previous assertions, we find that the large current-voltage nonlinearities found in real materials are most likely associated with defect-state densities that decrease above the zero-bias Fermi level. The results of the model are compared with previous experimental data on Si and Ge bicrystals and on polycrystalline ZnO varistors. Finally, a detailed method for determining the energy density of grain-boundary defect states from current-voltage data is developed.

Effect of grain boundaries on the performance of polycrystalline tunnel MIS solar cells

Tunnel MIS solar cells utilizing polycrystalline semiconductors are an attractive alternative to conventional p–n junction devices for fabricating large-scale solar energy conversion arrays. The main effect of the grain boundaries in polycrystalline semiconductors is to reduce the lifetime of the minority carriers. Experimental work indicates that the potential barrier near the grain boundary has a very limited effect on the barrier height which, in turn, controls open-circuit voltage of the device. The collection efficiency depends upon the grain size of the crystallites, in addition to other device parameters. A simple description of the performance of polycrystalline MIS solar cells will be presented, based on the above mentioned electrical characteristics.

Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> , grain size <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</tex> and interface state density N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">is</inf> at the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> adjacent to the boundaries, and ranges from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\simeq 10^{2}</tex> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm . s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> depending on N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">is</inf> and N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> . Under illumination, the population of the interface states is altered considerably from its dark level and as a result, V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> decreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> . Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\simeq 10^{11}</tex> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> eV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , τ decreases from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> s as the grain size is reduced from 1000 to 0.1 µm (for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N_{d} = 10^{16}</tex> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ). For a constant grain size, τ decreases with increasing N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> . The open-circuit voltage of p-n junction solar cells decreases for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau \leq 10^{-7}</tex> s, whereas that for Schottky barrier cells remains at its maximum value until <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau \lsim 10^{-8}</tex> s.