The phenomenology is reviewed for several enhanced diffusion mechanisms: the normal ionization-enhanced diffusion mechanism, the Bourgoin mechanism, the energy-release mechanism and some recoil mechanisms. Application of these mechanisms are discussed for crystalline and amorphous semiconductors, super-ionic materials and insulators in radiation damage, impurity and self-diffusion, ion-implantation, and dislocation-motion-experiments.

Enhanced diffusion mechanisms

In this chapter we will focus on the physics of the defect creation process in semiconductors. In its broadest interpretation this topic can encompass all of the physics of radiation effects in semiconductors; other chapters in this treatise cover many of these other aspects, and consequently we will adopt a narrower view of the topic, considering the question, “How are defects created?,” reserving for ourselves the right to draw on other areas where necessary.

Defect Creation in Semiconductors

The first electron bombarded semiconductor (EBS) devices have recently appeared on the market. These devices have already demonstrated that EBS has considerable promise as an important new electron device for power amplification and control. EBS devices are described with particular emphasis on power devices. The basic EBS principle, some of the analysis used in device design, general considerations in designing the various elements of the device, overall device design, semiconductor processing, and reliability considerations are discussed. Predictions of general directions for future work are made. Some historical information is also presented as well as a brief comparison with other competing power devices.

Electron bombarded semiconductor devices

Negative dilatation (shrinkage) induced by 10‐keV electron bombardment of SiO2 films thermally grown on Si has been studied. Measured deflection changes in continuous SiO2 bridges etched following irradiation showed a maximum shrinkage of [inverted lazy s] 0.8%, which occurred at 0.2 C/cm2, followed by a slight expansion, and subsequent saturation at higher exposures. Exposures up to 3 C/cm2 were utilized. Isochronal annealing studies show the activation energy for recovery to be approximately 1 eV with 2% recovery occurring in 1 h at 400°C.

Electron irradiation dilatation in SiO2

Our work indicates that the behavior of injected majority and minority carriers in representative Si/spinel samples is dominated by deep-level defects that are agglomerated into defect clusters rather than being uniformly distributed in the volume of the Si layer. Since these defect clusters will be surrounded by space-charge regions, the resulting trap occupancy-dependent potential barriers result in slow, nonexponential majority carrier trapping effects but subnanosecond minority carrier lifetimes. Accurate numerical calculations of the electrical properties of a sheet of clustered defects are shown to agree with detailed measurements at 90 and 300°K of the transient conductance decay in P - and N -type films following pulsed carrier injection from electron bombardment. A significant conclusion of the theoretical calculations is that the electrical behavior of a defect cluster is insensitive to any of the model parameters chosen as long as there are a sufficient number of defects to completely deplete the material in the vicinity of the defect plane. Our results show that the observed short minority carrier lifetimes, long-term majority carrier trapping effects and mobility temperature dependences can all be explained by the same cluster defect model and cannot be explained by uniformly distributed point defects. Our calculations also imply that minority carrier lifetime in these films should be strongly injection level dependent. Thus, lifetime measured by the MOS transient capacitance technique will not be the lifetime needed to evaluate the material for bipolar device applications.

Electrical effects of clustered defects in heteroepitaxial Si films

This paper investigates the defect production in Si reported to occur at electron irradiation energies far below the theoretically predicted 170‐keV bulk‐damage threshold. In contrast to published results, our experiments show no permanent resistance changes in bulk samples at fluences as high as 2×1018 20‐keV e/cm2 (78 K). Furthermore, fundamental damage studies using EPR techniques and optical spectroscopy confirm that fewer than 10−4−10−5 atomic displacements occur per 20‐keV electron. These results disagree with previous studies of irradiation‐induced resistance changes in bulk samples and epitaxial films from which it was indirectly determined that subthreshold atomic displacements appear following ionization of a Si K‐shell electron. However, we do find large resistance decreases in p‐type Si/spinel films at fluences ≳1015 20‐keV e/cm2 (300 K) that appear similar to anomalous results in the literature. But the energy and fluence dependences of the resistance change in our experiments are correctly predicted by theory for an irradiation‐induced charge accumulation layer localized within <500 A of the irradiated Si surface. The results do not indicate the presence of a bulk‐damage mechanism. The K‐shell hypothesis is clearly ruled out since the resistance effect remains at energies well below the K‐shell ionization potential. We do not find a corresponding irradiation effect in n‐type Si/spinel films which is consistent with the fact that nearly all the previously reported subthreshold irradiation effects have appeared in p‐type Si. Our results clarify some puzzling aspects of previously reported low‐energy electron irradiation effects in Si and also indicate that the unambiguous determination of defect introduction or existing defect modification by subthreshold irradiation of semiconductors requires fundamental techniques that can be rendered immune to surface phenomena.

C. B. Norris

Papers

Investigation of static electron irradiation effects in bulk Si and thin Si films at energies far below threshold