H.C. Card

Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration N<inf>d</inf>, grain size<tex>d</tex>and interface state density N<inf>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>d</inf>adjacent to the boundaries, and ranges from<tex>\simeq 10^{2}</tex>to 10⁶cm . s^-1depending on N<inf>is</inf>and N<inf>d</inf>. Under illumination, the population of the interface states is altered considerably from its dark level and as a result, V<inf>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>d</inf>. Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities of<tex>\simeq 10^{11}</tex>cm^-2eV^-1, τ decreases from 10^-6to 10^-10s as the grain size is reduced from 1000 to 0.1 µm (for<tex>N_{d} = 10^{16}</tex>cm^-3). For a constant grain size, τ decreases with increasing N<inf>d</inf>. The open-circuit voltage of p-n junction solar cells decreases for<tex>\tau \leq 10^{-7}</tex>s, whereas that for Schottky barrier cells remains at its maximum value until<tex>\tau \lsim 10^{-8}</tex>s.

Aluminum&#8212;Silicon Schottky barriers and ohmic contacts in integrated circuits

Experimental observations (electrical characteristics and in depth Auger analysis) have been made of the interface behavior in aluminum-silicon contacts. The barrier heights of these contacts (φ<inf>bn</inf>for n-type, φ<inf>bp</inf>for p-type silicon) are sensitive to heat treatments (HT) that are a part of normal integrated circuit processing. If oxide layers (≃20 Å) are present in the Al-Si interface, φ<inf>bn</inf>can be as low as 0.45 eV and φ<inf>bp</inf>as high as 0.75 eV. One can obtain reproducible barrier heights φ<inf>bp</inf>≃ 0.7 eV and φ<inf>bp</inf>≃ 0.5 eV by HT at T ≤ 300deg;C. As the temperature of HT is increased (up to ≃ 550deg;C) φ<inf>bn</inf>can reach ≃ 0.9 eV and φ<inf>bp</inf>drop to < 0.35 eV. The HT at higher temperatures are accompanied by changes in the Al and Si profiles across the interface region. Two mechanisms have been found to be responsible for the changes in barrier height: 1) the removal of positive charges from the oxide, and 2) metallurgical reactions between the Al and Si. These two mechanisms have been separated and their individual behaviors qualified.

Internal photoemission mechanisms at interfaces between germanium and thin metal films

The quantum efficiency associated with the internal photoemission of electrons over the Schottky barrier (of height φ B ) at the metal-Ge interface has been studied experimentally for several metals (Au, Cu, Ag, Pb, and Ni). A theoretical description of this mechanism has been developed in which we take into account the front and back optical absorptance, hot electron scattering, and multiple reflections of excited electrons from the surfaces of the thin electrode film. We have found it necessary to impose a modification of the Fowler theory of photoemission when applied to internal photoemission from thin metal films over a Schottky barrier. This modification relates to an enhanced photoexcitation within the metal films which is attributed in the present theory to a density of states which exhibits a peaked distribution in energy rather than the simple parabolic bands assumed by Fowler. It is clear from the present study that the majority of photoelectron excitation occurs from a small region of energy of the order of a fraction of an electron volt near the Fermi energy. The theoretical model presented here defines two important parameters: a hot-electron mean free path (L e ) and an energy (E ef ) given by the difference between the Fermi level and the effective conduction hand minimum associated with the region of energy in the metal near the Fermi level where the electron distribution is strongly peaked. Values of L e for Au is 550 A, Ag is 570 A, Cu is 450 A, and Pb is 55 A. E ef for Au is 0.1 eV, Ag is 0.152 eV, Cu is 0.11 eV, Pb is 0.1 eV. The validity of this model is confirmed by the experimental finding that the parameters L e and E ef are independent of metal thickness.

Grain boundary effects on the electrical behavior of Al-poly-Si Schottky-barrier solar cells

Schottky-barrier diodes using aluminum on p-type polycrystalline silicon have been fabricated. The contrast of the orientation of neighboring grains is observed after chemical etching of the surface. Comparing the surface morphology of the substrate with the electronic behavior of the Schottky diode, we are able to identify the influence of grain boundaries. It is found that the low-angle boundary has little effect on theI-Vcharacteristics since near ideal SchottkyI-Vcurves are obtained. The barrier height is calculated to be 0.83 V which is higher than that of the single-crystal substrate. The ideality factor is 1.17 for a device containing a twin and low-angle boundaries. The high-angle grain boundary, however, significantly alters both theI-Vand low-frequencyC-Vplots. The experimental data indicate that recombination centers and traps are introduced, resulting in an increase in recombination current and a reduction of the effective mobility. The conduction mechanisms for the two types of diodes are clearly distinguishable both in the dark and under illumination. In the photovoltaic operation under a tungsten lamp, we obtain an open-circuit voltage of 0.48 V and a fill factor 0.51. It appears that the chemical etching along with Schottky-barrier fabrication will provide a useful method to study the polycrystalline substrate for low-cost solar cell applications.

Majority carrier current characteristics in large-grain polycrystalline-silicon-Schottky-barrier solar cells

A theoretical analysis is made of the nonexponential current-voltage characteristics observed in Al-poly-Si (Wacker) Schottky-barrier solar cells fabricated in our laboratory. In this model, we consider a grain boundary effectively in parallel with the Schottky junction. Comparison between experimental data and numerical calculation indicates that the grain boundary may be represented by a fixed interface charge, a uniformly distributed interface state density, and a neutral level E0. Diodes fabricated in regions with high-angle grain boundaries behave in a manner which conforms closely with the proposed model.

Transition with grain size from electrode‐limited to bulk‐limited conduction in polycrystalline semiconductors

An experimental study of Au‐polycrystalline GaAs Schottky barriers has been made. The current‐voltage and capacitance‐voltage characteristics of GaAs films of various grain sizes have been measured. Analysis of these data indicate that the transport may be electrode‐limited, bulk‐limited, or a combination of electrode‐limited and bulk‐limited conduction processes, depending upon the (average) grain size.

Transport of majority and minority carriers in 2-&#181;m-diameter Pt-GaAs schottky barriers

An experimental study of small area (2-µm-diameter) Pt-GaAs Schottky barrier diodes has been made, by using a wafer chip with a matrix of these diodes lying within approximately a minority carrier diffusion length of one another. Using one diode as collector and another as emitter, transistor measurements indicated that the dominant contribution to the current is the majority-carrier thermiconic field emission current for large forward-bias voltage (V_{EB} \gsim 0.4V), whereas the smaller forward-bias (V_{EB} \lsim 0.4V) recombination in the space-charge region was most important. The minority carrier injection ratio is measurable only for large forward-bias voltages, decreasing from ≃ 10-2to 10-5as VEBincreases from 0.5 to 1.0 V. The minority carrier diffusion length was measured to beL_{p} \sime 1.3µm. These results are of considerable significance for the understanding and optimization of the performance of these devices as classical detectors and mixers.