Surface Space Charge Layer Research Articles

Determination of surface properties by means of large signal photovoltage pulses and the influence of trapping

Large signal photovoltage pulses, measured on real surfaces of high-resistivity p-type silicon as a function of excitation intensity and induced charge, show characteristic features, especially smooth minima in the negative pulses. It is shown how the results can be used for a determination of surface potentials (band bendings) and surface- or interface-state distributions. Comparisons between theoretically and experimentally determined photovoltages show that the exchange of charge carriers between surface states and space charge layer during electron-hole nonequilibrium was not negligible and has to be taken into account for an accurate determination of surface potentials. The influence of such trapping processes is analysed graphically and analytically for continuously distributed surface states and the modifications due to trapping are determined. It is shown that the bands become asymptotically flat for the limit of large excitation even when strong trapping in a system of continuously distributed surface states with arbitrarily large concentrations prevails.

High frequency space charge layer capacitance of strongly inverted semiconductor surfaces

The finite spatial extension of the inversion layer minority carriers shunts the dielectric capacitance of the inversion layer and increases the high frequency semiconductor surface space charge layer capacitance in the strong inversion range by about 5 per cent. This distributed minority carrier distribution also gives rise to a small (about 1 per cent) high frequency capacitance minimum near the onset of strong surface inversion. A simple two-lump model is developed which is accurate to within 0·4 per cent of the numerical solution obtained from the exact transmission line model. Applied gate voltages at the capacitance minimum are presented graphically as a function of oxide thickness with the substrate impurity concentration as a parameter. Surface quantization effect is not taken into account.

A model for surface space charge in thin films of AgBr

A model for the surface space charge layer in ionic crystals has been developed for thin films where the film thickness is less than or comparable to the Debye length. A potential difference between the surface and the bulk is present which depends on the film thickness and the impurity concentration. The implications of this model have been examined with respect to thin films of AgBr at room temperature.

The influence of the electrical properties of the solid phase on impurity incorparation during crystal growth

Control of impurity incorporation during crystal growth requires consideration of the relationships between the concentration of the impurity in the liquid phase and its concentration in the growing solid phase. At the solid-liquid interface, the surface space-charge layer in the solid has been found to dominate the incorporation of slowly diffusing impurities. For extrinsic conditions, the amount of slowly diffusing impurity incorporated in the solid varies linearly with the amount in the liquid. Detailed analysis in terms of the surface-space charge layer is given for GaAs liquid-phase epitaxial layers doped with radioactive Te129m. Not only is this linear dependence observed for all the group VI donors S, Se, and Te, but also for the slowly diffusing acceptor Ge. For Te in GaAs, the distribution coefficient goes from greater than unity at low temperatures to less than unity at high temperatures. Rapidly diffusing impurities are not influenced by the surface space-charge layer, and for singly ionized substitutional acceptors or donors the amount of impurity incorporated into the extrinsic solid is expected to vary as the square root of the amount in the liquid. This square-root behavior has been observed for the acceptor Zn which diffuses rapidly in GaAs.

The electrical effect of atomic hydrogen on lead telluride

Epitaxial films of PbTe were treated with molecular hydrogen in the presence of a hot filament. Large changes occurred in the conductivity and Hall coefficient, denoting a transfer of electrons (up to 10 14 cm −2) to the semiconductor. This is believed to involve the creation of a surface space-charge layer, followed by a certain amount of in-diffusion. The effect was reversible by exposure to oxygen.

A model for the AgBr (111) surface, based on the symmetry of nucleation sites for evaporated metal

Metallic nuclei vacuum-deposited on the (111) faces of epitaxial silver bromide films are highly ordered, compared with those formed on inactive substrates. Low-angle electron diffraction reveals a pattern from this deposit consisting of twelve spots, in the directions of the 642 silver bromide reflections, and at radial distances corresponding to a 70 Å spacing. The nuclei are assumed to form on preferential sites of some unidentified type. Their symmetry presumably reflects the symmetry of the surface layer of ions on the silver bromide film. By analysis of the optical Fourier transform of an electron microscope image, it is shown that the deposit is a mosaic of two types of domain in crystallographically equivalent rotational orientations, each having hexagonal symmetry. A model for the surface ionic layer is proposed, having the same symmetry. This layer contains somewhat less than the number of ions which would be required for electrical neutrality, namely, half the number in a (111) lattice plane. The charge discrepancy is similar to the charge density of the diffuse surface space charge layer, as determined from measurements of ionic conductivity.

Bulk trapping effect on carrier diffusion length as determined by the surface photovoltage method: Theory

The theory underlying a method for measuring carrier diffusion length based on the surface photovoltage is extended to include bulk trapping effects at low injection levels. This is done through recognizing that even in the presence of trapping when their lifetimes may differ, excess electrons and holes both diffuse in the bulk with the same characteristic length. The theory has been examined in terms of quasi-equilibrium conditions across the surface space charge layer; however, the method is expected to work under less restrictive conditions. As with no trapping, the method should yield the minority carrier diffusion length in extrinsic material for majority carrier trapping in general, but in the case of minority carrier trapping, only for moderate trapping. When strong minority carrier trapping occurs in slightly extrinsic material, the effective diffusion length, owing to trap-modified ambipolar diffusion in a Dember field, may be greater than the usual minority carrier diffusion length which obtains in the absence of trapping. A treatment of recombination statistics is included to illustrate how trapping effects may arise from the presence of deep-lying impurities such as gold in solicon. It also provides a basis for a detailed examination of the low injection-level assumptions used in simplifying the problem of trap-modified carrier diffusion in the bulk. It is shown, for example, that in extrinsic semiconductor, the usual assumptions that the excess carrier densities be much smaller than their equilibrium values need apply to majority carriers only. For minority carriers, the limitations are far less severe in most cases of practical interest, and depend on the characteristics of the trapping centers involved.

Transverse Magnetoresistance in Thin Epitaxial Films of PbS: Evidence for a Layered Distribution of Charge Carriers

Room-temperature transverse magnetoresistance measurements on n-type PbS films are reported. The results are different for magnetic fields perpendicular and parallel to the film faces. This anisotropy is sensitive to ambient and is understandable in terms of a model consisting of an n-type bulk region and p-type surface space-charge layer.

Approximations for accumulation and inversion space-charge layers in semiconductors

The solution of Poisson's equation for heavily accumulated or inverted space-charge regions in semiconductors is discussed. Closed form solutions are obtained for such regions. These solutions make it possible to obtain good approximations to potential and carrier density variations with distance in semiconductor surface space-charge layers. An example is also given of the use of the approximations in the analysis of n n + junctions.

Surface properties of n-type gallium arsenide

This paper describes an investigation of GaAs surfaces by means of field effect, surface photovoltage and photoconductance measurements. A wide variety of different etch treatments and also the effect of depositing SiO 2 on the surface have been investigated. It is found that the surface barrier always bends towards depletion or inversion. On some etched surfaces the total effective surface trap density is at least 10 12 cm −2 but some treatments were found to reduce this to about 3 × 10 11 cm −2. The effective surface trap density of SiO 2 covered surfaces was 7 × 10 11 cm −2 and was not sensitive to the etch treatment of the underlying surface. Under ac fields the mean surface barrier height becomes biased in the direction of further depletion due to an increase in the mean occupancy of the traps. The ac field effect mobility is of the order of 2000 cm 2 V −1 sec −1 despite the large surface trap densities. It is also deduced from the ac field effect that fast bulk traps with densities of about 10 15 cm −3 are active in the surface space charge layer. A model of the surface is given which explains the observed phenomena.

Ambipolar Carrier Transport and Surface Recombination Velocity in Semiconductor Surface Layers

In discussing charge-carrier recombination at a semiconductor surface it is natural to characterize the recombination properties of the surface itself by a true surface-reflection coefficient which expresses the non-recombination probability for an excess carrier in a single collision with the surface. It is also possible to arrive at an effective surface-reflection coefficient from experimental data by ignoring any effects which might be associated with the presence of a surface space-charge layer. The latter quantity is simply an effective value which involves effects contributed not only by the actual surface, but also by the space-charge layer which is present in the neighborhood of the surface. For a sample with a given photoconductive response, by equating expressions involving the reflectance calculated for a sample having the actual space-charge layer and for one in which space-charge effects are ignored, one may arrive at a relation between the true and effective surface-reflection coefficients. By this procedure one can define in an operational way an effective surface recombination velocity. The combined effects of a space-charge layer and an actual physical surface can then be studied as a function of the space-charge parameters and sample thickness. Results relating true and effective recombination velocities are presented, using an ambipolar-transport formulation, for both depletion-inversion and accumulation layers. Previously developed methods for relating true and effective surface recombination velocities are reviewed and compared to the operational method used herein. Certain advantages connected with the definition adopted here are pointed out. The calculations are based upon known diffusion and mobility parameters for $p$-type silicon.

Conditions for quasi-equilibrium in a semiconductor surface space-charge layer

The transport equations in a surface space-charge layer are solved approximately. The results show that the quasi-equilibrium approximation is generally invalid for the bulk minority carriers in an accumulation layer or in an inversion layer under extraction, and for the bulk majority carriers in an inversion layer under injection. The possibility of complete depletion of carriers of one or the other type at the surface imposes a finite upper limit on the effective surface recombination velocity.

Electron Mobility Studies in Surface Space-Charge Layers in Vapor-Deposited CdS Films

Hall and field-effect surface mobility has been measured in polycrystalline films of CdS of thickness 1000–4000 Å. At low surface potential, the mobility increases with surface potential. The Hall mobility was observed to increase exponentially with temperature from −80° to 120°C. These effects can be explained using a polycrystalline film model. The effects of diffuse surface scattering were not observed at low surface potential due to the small mean free path of electrons. However, when the channel thickness was comparable to the electron mean free path a decrease in mobility was observed.

Properties of Surface Imperfections Produced on Germanium by Cleaning Treatments

Surface conductivity and Hall coefficient of germanium cleaned by ion-bombardment and subsequent annealing have been measured over the temperature range 1.5–300°K. The results indicate that donor levels are produced near the surface by ion-bombardment and are eliminated with subsequent annealing at 500°C in high vacuum making the surface space charge layer p-type. The density of the excess holes near the surface has a magnitude of 8×10 11 cm -2 for an n-type of 8\(\varOmega\) cm and 8.5×10 10 cm -2 for a p-type of 20\(\varOmega\) cm. Ion-bombardment gives rise to a small Hall maximum at low temperatures around 5°K. This maximum shifts to higher temperatures as annealing treatments are repeated and disappears if air is introduced. This maximum is attributed to holes of the order of 10 7 cm -2 in the space charge layer.

Anti-parallel ferroelectric domains in surface space-charge layers of BaTiO3

It is shown on simplified models of a surface space-charge layer of a single crystal of BaTiO3that in the surface region, in which the direction of the electric field is opposite to the direction of spontaneous polarization inside the crystal, anti-parallel domains can exist. The dimensions of these domains are determined. The use of the results obtained is proposed for interpreting the microrelief of the etch figures and for explaining the different phases of the polarization reversal process of a single-domain single crystal of BaTiO3.

Surface Space-Charge Layers in Barium Titanate

Above the Curie temperature, pyroelectric currents can be produced in single crystals of barium titanate even though there is no electric field applied. The polarization that remains at these temperatures is ascribed to space-charge fields in the crystal. From studies of the wave forms of the pyroelectric current signals it is concluded, tentatively, that space-charge layers of up to ${10}^{\ensuremath{-}5}$ cm in thickness reside at the crystal surface and that these space charges also produce a field through the interior of the crystal. Further evidence for space-charge fields is provided by the occurrence of an associated photovoltaic effect and by asymmetrical hysteresis loops. The space-charge fields vary considerably in magnitude from crystal to crystal. In general, they can be modified by suitable heat treatment but return to their original condition when fields are applied above the Curie point.The space charge fields apparently influence the direction in which the domains polarize when the crystal is cooled through the Curie point. They will also affect capacity measurements above the transition and will influence the actual temperature of the transition. It is quite possible that the fields play an important role in the process of domain nucleation.

Effective Carrier Mobility in Surface-Space Charge Layers

Carriers held to a region near the surface by the potential well of a space charge layer may have their mobility reduced by surface scattering, if the width of the well is of the order of a mean free path. An effective mobility, which may differ from the bulk mobility by as much as a factor of ten, has been obtained from a solution of the Boltzmann equation. Solutions have been carried out for two types of potential functions: (a) a linear potential corresponding to a constant space-charge field, and (b) a solution of Poisson's equation including an external bias applied normal to the surface. The results have been used to calculate changes in surface conductance of germanium with changes in surface potential and predict the "field effect" and "channel effect" mobilities.