Interface recombination velocity

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

We note in this context that in Si based MIS (metal-insulator-semiconductor) solar cells one of the roles of the 20-60A thick Si02 layer may well be reduction of the recombination velocity at the Si surface. Chambouleyron and Soucedo noted a decrease in the recombination velocity at the conductive Sn02/Si interface8 relative to that at the Si surface and Michel and Lasnier find a recombination velocity of less than 2xl04 cm/sec at the conductive indium tin oxide/Si interface.9 In both cases heating the metal oxides present on the elemental Si produces an intermediate Si02 layer. 

The presence of arsenic at the interface implies that surface states within the band gap will be introduced (see Fig. 1). We associate the high surface recombination velocity with the presence of arsenic. The formation of elemental As on the GaAs surface explains the difference in behavior of InP and of GaAs. In InP the thermodynamically stable phase that results from oxidation of the surface is colorless InP04 which straddles the band gap. In GaAs it is Ga203 and small band gap As. 

Some metastable centers may be associated with doped layers or with interface states. The light-induced generation of these centers appears to increase the surface recombination velocity in some cells, causing a decrease in the spectral response at short wavelengths. This effect and the others mentioned above are reversible annealing the a-Si H cells at 200°C for several minutes restores the cells to their initial conditions. 

The recombination velocity, which characterizes the recombination process, may vary over a wide range, from 1 to lO cm/s, at room temperature. Surface recombination centers that can be described by the one discrete recombination center model have been found to exist in different sihcon/electrolyte systems. ° The states that can exchange charge carriers with only one of the bands are traps for electrons or holes. Surface states that contribute to the interface capacitance but do not act as the... 

The surface recombination velocity can be introduced by a further boundary condition. At the semiconductor-liquid interface the recombination rate is given by... 

Mercury cadmium teiluride (HgCdTe) is a direct bandgap semiconductor widely used as a material for infrared detectors due to his narrow variable band gap. The achievement of high-performance detectors depends critically on a low surface recombination velocity of the minority carriers. The chemical growth of a passivation oxidized superficial layer in an aqueous Fe(CN)g3- basic solution is studied in this work. The depth profiles of the different elements in the oxidized layer superficial layer and its thickness are studied by X-ray photoelectron spectroscopy. The electrical properties of the interface are evaluated from MIS devices. The conditions of oxidation have been optimized. 

Ellis has reported the surface recombination velocity of H2O2" HF-H2O treated surfaces (94) to be on the order of 10 cm sec l for 1500 A thick films formed in etches with reduced oxidant content. The value of "s" was further reduced by exposure of the film to HF acid fumes for about 10 sec. This resulted in 5-10 cm sec l values which persisted for several hours. That value is reasonably consistent with measurements of Loferski and Rappaport (96) on freshly HF-etched surfaces without initial oxidation films. The low values of s on those films, however, only lasted for tens of seconds once the HF vapor was removed. The large variability of s in these and other experiments (25,27,37,99) indicate that the preoxidation treatment is also an important factor in the final interface condition and that the interface chemistry is still only vaguely understood. 

The results of one such calculation are shown as solid curves in Fig. 5.15 [5.110]. Additional generalized curves and closed-form solutions appear in the references [5.14, 15, 89, 92, 93, 95, 96, 101, 104, 110]. The various solutions all agree on the importance of low recombination velocity at the substrate/active layer growth interface and thus on the importance of low lattice mismatch. They also indicate that near threshold the quantum efficiency can be higher in transmission than in the reflection mode owing to one added optical reflection at the vacuum surface [5.95]. From the various separate models have come several versions of an optimum structure, but all are similar in general dimensions and structure as outlined above. 

Besides the stability issues encountered at the solid-liquid interface, its electronic quality defines the efficiency of the system. This encompasses the control of surface recombination but, also, the control of the energy band alignments between light absorbers, interfacial films, and electrocatalysts. Accordingly, two avenues have to be followed (i) the reduction of the surface recombination velocity (compare Eqs. 10,11,14) and (ii) the use of surface- or interface dipoles for band alignment tuning purposes. 

Partial pressures above the limits in figure 6 lead to sulfur rich phases of Cu S. The CUj S films sputtered on CdS films exhibited epitaxial growth as demonstrated by Armantrout et al. /22/ by electron diffraction and hence showed no higher recombination velocity at the interface than in topotacially grown junction layers. 

Surface recombination rate constant, Kr, at the interface is one of the most important factors that influence the overall rate, and this quantity depends mainly on the density of surface states, Dss E) and can be determined quantum mechanically, using the Anderson Hamiltonian formalism/ In this expression (76), cTc is the recombination cross section. The value of cTc can be obtained using the scattering theory. Sp E) is the velocity of photoexcited electron, and f E) represents the distribution of surface states which one may consider to be of Gaussian type of distribution, centered at the midgap energy. 

The theories of the electronic and ionic currents have some features in common. One may formulate models in which the current is limited by the injection into the film from the contacts of positively or negatively charged carriers, or one may consider an equilibrium state to exist across either or both interfaces. One may postulate space-charge limited currents, trapping, and recombination processes. One of the chief differences between the ionic and the electronic currents is that the average velocity of the ions is approximately exponentially dependent on the field for fields which produce experimentally observable ionic currents, whereas the average velocity of electrons is linearly dependent on the field at low fields with different types of nonlinearity at high fields. 

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