Injection spectroscopy of localized states in thin insulating layers on semiconductor surfaces

Abstract

An overview of charge injection techniques for studies of localized electron states in insulators is presented. These states control most important electrical properties of the materials, especially their thin layers and the multilayered structures containing them. Electronic properties of thin insulating layers are usually different from those of bulk material because in vicinity of an interface the thin film is subjected to action of structural strain, spatial confinement, extra energy release during growth or deposition, e.c. All these factors substantially affect the spectrum of localized electron states (intrinsic defects, dopants, impurities) and influence their spatial distribution. The interfaces often exhibit an unusual behaviour relative the defect generation and an impurity transport and segregation comparing with a bulk of insulator. The characterization of these states in thin-layered systems suffers from low sensitivity of both electron and optical spectroscopies to shallow electron states those have to be detected on the background of the signal from substrate. Nevertheless, the problem may be resolved by means of charge carrier injection into the insulating layer followed by monitoring of the charge becomes trapped. The localized states are detected by this way as a potential wells for the injected a mobile charge carriers. In thin layers the mean path of injected electron (hole) with respect to interaction with such a trap may greatly exceed thickness of the layer. In offers possibility to use linear modes for the trapping and detrapping analysis. We will show that under these conditions the charge-injection methods provide the most important phenomenological parameters of a localized state: the density per unit area, in-depth location, cross section for capture of carriers or recombination, energy position of the trapped carrier level in the insulator bandgap, e.c. On the base of these possibilities a non-destructive methods for characterization of extrinsic and intrinsic defects states are developed. Various charge injection and charge detection methods are compared. It shows unambiguously the advantages of using a semiconducting substrate for the insulator trapping studies. The limits of the linear models applicability are discussed in detail and the effects arising from the non-linear correlation phenomena are analyzed.