Introduction to photoconductivity in nipi structures

Nipi structures (doping superlattices) consist of a series of alternating n-type and p-type doped layers. This leads to space charge regions between the layers and therefore to a modulation of the conduction and valence bandedges in growth direction. The amplitude of the modulation depends on the doping levels, the layer thicknesses and the bulk properties of the host crystal. For studying the optoelectronic pecularities in most cases it is sufficient to examine the properties of a single period of a nipi-structure, and for experiments, it will thus be sufficient to use e.g. a p-n-p- structure.

Due to the built-in bandedge modulation (potential modulation) optically generated e-h-pairs are separated in space and their lifetime is enhanced as compared to bulk material (figure ). In general there are two types of mechanisms governing the recombination in nipi structures: tunnelling through the barriers and thermally activated recombination across the barriers both depending exponentially on the potential modulation V. On the other hand, the value of the potential modulation V depends also on the excess carrier concentrations. The photogenerated and spatially separated e-h-pairs screen the space charge regions between the p- and n-layers resulting in a decreasing potential modulation. Since the shape of the potential modulation affects the carrier distribution and the excess carriers theirselves influence the shape of the potential modulation this feed back mechanism on the potential modulation V in general has to be described in a self consistent manner.

Because of this feedback on the potential modulation V also the recombination rate r is strongly dependent on the excess carrier concentration and thus the dependence of the excess carrier density on the illumination is extremely nonlinear. In special cases an inverse proportional dependence of the lifetime on the optical generation rate occurs.

For lateral transport measurements on p-n-p-structures selective contacts to the embedded n-layers are used. An external lateral constant current, supplied between two selective contacts at the ends of a linear shaped sample causes a lateral voltage drop in the n-layer and thus locally effects the magnitude of the potential modulation V (figure ): Starting from the negative contact the supplied current produces a potential gradient dVn/dx of the bottom of the conduction bandedge Vn. Due to this potential gradient the magnitude of the potential modulation V is increasing, which leads to a local increase of the extension of the space charge regions and at the same time to a locally decreasing width of the conducting n-channel. Going forward, nearer to the positive contact, the effect on the channel width escalates until, at higher currents, the channel vanishes at the positive contact (pinch off effect). On the other hand, an increasing potential modulation near the positive contact will make the recombination process much less likely there and the area where recombination is possible shrinks to a region near the negative contact. Therefore the recombination near the negative contact has to become more effective in order to keep the total balance of the generation and recombination, which means that the potential modulation has to become smaller as compared to the conditions without any current.

For an understanding of the effects concerning the spatial separation of excess carriers (and their feedback on the potential modulation V and on the lifetime) and the lateral external current (which also influences the local channel width and local potential modulation) one has to consider the simultaneous combination of their influences on each other locally.