It is well known that impurities and defects in semiconductors are associated with energy levels in the forbidden gap. Similar states occur at the surface of a semiconductor where the crystal lattice and the symmetry are strongly disturbed. These states are called surface states. Owing to the two-dimensional nature of the surface, their density is measured per unit area, in contrast to bulk states, which are measured per unit volume. A third type of states, similar to surface states, occurs at the interface between two adjacent materials. These states are called interface states. Very often they are also simply called surface states.

Interface states on semiconductor/insulator surfaces

The electrical effects of included silicon carbide (SiC) particles in edge‐defined film‐fed grown silicon ribbons have been examined. By employing a scanning electron microscope operated in the electron‐beam‐induced current mode, as well as observing the effects of SiC particles on solar‐cell characteristics, the electrical activity of particles and particle‐generated defects were studied. The influence of SiC particles largely appears to be a result of impurity accumulation around them rather than a direct effect of the inclusions.

Electrical effects of SiC inclusions in EFG silicon ribbon solar cells

Capacitance‐voltage measurements were used to analyze buried oxide layers formed by the implantation of oxygen into silicon substrates. The analysis is based on the extension of conventional metal/oxide/semiconductor capacitor theory to the two buried oxide interfaces of the epilayer/oxide/substrate structure. The processing parameters include oxygen doses of 1.8 or 2.0×1018 cm−2, implant temperature of 500 °C, and anneal temperature of 1250 °C for 2 or 16 h. The results are fixed oxide charge densities of 2–3×1011 and 1–2×1010 cm−2 for the epi/oxide and substrate/oxide interfaces, respectively; interface trap densities are 0.5 to 2×1011 cm−2 eV−1 for the epi/oxide interface, and not measurable for the substrate/oxide interface. The epilayer/oxide interface was found to be more sensitive to processing variations than the substrate/oxide interface.

Determination of the fixed oxide charge and interface trap densities for buried oxide layers formed by oxygen implantation

The effect of emitter-edge dislocations on the low-frequency noise of bipollar transistors was studied via phosporus surface concentration. A threshold in both noise factor and dislocation density was observed for surface concentration greater than 4.3 × 10 20 cm −3 . It has been found that the dislocations affect the noise considerably when their density is in excess of 10 6 cm −2 .

Noise phenomena associated with dislocations in bipolar transistors

vii effective diffusion length on dislocation density has been developed. For this purpose the model published by Donolato [J. Appl. Phys. 84, 2656 (1998)] has been extended regarding the following features: (i) The use of the »quantum efficiency effective diffusion length«. This quantity is compatible with the usual diffusion length extraction from quantum efficiency data and is more adequate for the typical operation conditions of a solar cell than the quantity introduced in the original work. (ii) The appropriate description of thin devices with a finite thickness and a finite back surface recombination velocity. Since the associated boundary value problem does not allow a straightforward analytical solution, an approximate expression has been derived, which was validated by numerical simulations. The model has been applied to the characterization of thin-film solar cell devices using the techniques of etch pit density (EPD) mapping and spectrally resolved light beam induced current (SR-LBIC). For EPD measurements a special setup has been developed, which is based on an automated microscope and digital image analysis. This system is capable of producing EPD maps with tens of thousands of data points. To correlate EPD and SR-LBIC diffusion length data, the Gaussian profile of the LBIC beam has to be considered. Further, charge carriers can diffuse from the point of generation into neighboring areas (which might have different dislocation densities). Both effects were taken into account by convolving the EPD map with a filter kernel that contains information on LBIC profile and diffusion parameters. The result is an effective dislocation density (EDD) map. At the investigated structures, these EDD maps correlated well with SR-LBIC diffusion length maps. For a quantitative analysis, the extended version of Donolato’s model was locally fitted to EDD and effective diffusion length data. The determined normalized recombination strength values are in the range of 0.006 to 0.016, and are comparable to values measured on conventional multicrystalline silicon solar cells. In addition, the model has been applied to open circuit voltage data measured on samples with different dislocation densities. For these devices a strong decrease of open circuit voltage with increasing dislocation density was found. With the consideration of space charge region recombination, the experimental data could be modeled very well. However, in order to obtain satisfactory fitting results, it had to be assumed that the region of high recombination is much wider

Zone-Melting Recrystallization for Crystalline Silicon Thin-Film Solar Cells

The effects of dislocations on the SiO2/Si interface have been studied at dislocation levels of from 104 to 109/cm2. At dislocation levels of 106/cm2 or less the MOS properties are unaffected; at levels from 107 to 109/cm2 the flat‐band voltage increases with dislocation density to the point where it is not measurable at 109 dislocations/cm2, being greater than the SiO2 breakdown voltage. No effects on the interface state density are seen.

D. V. McCaughan

Papers

Effects of dislocations on the properties of metal SiO2‐silicon capacitors