Grain-boundary-controlled transport in GaN layers

Abstract

An exponential dependence of the photoconductivity on the surface photovoltage at GaN layers is predicted theoretically and confirmed experimentally. The prediction is based on the assumption that the material is mainly an ordered polycrystal, consisting of columnar grains. Accordingly, transport is expected to be limited by potential barriers at the grain boundaries, arising from the charge trapped at grain-boundary defects. The observed exponential dependence provides evidence that strongly supports the model by establishing a direct link between the bulk conductivity and the surface potential barrier. The same model is shown to successfully explain several other defect-related findings as well. In recent years, technological breakthroughs in GaN growth, doping, and contacting technologies have resulted in numerous devices, notably the blue GaN-based laser. 1 Progress, however, is still challenged by a high density of defects. 2 A typical finding is a columnar grain structure often observed in cross-sectional micrographs. 2 The effect of grain boundaries on transport in other semiconductors, especially polycrystalline Si, has been extensively studied. 3 It is widely accepted that trapped charge at grain-boundary interface states leads to the formation of potential barriers and depletion regions, as shown schematically in Fig. 1. These potential barriers resist intergrain transport, a hypothesis successfully used for explaining Hall 4 and ion-implantation 5 experiments in GaN films. If this mechanism is in effect, it can inclusively explain several intriguing transport-related phenomena observed at GaN layers, such as: ~i! a persistent and nonexponentially decaying photoconductivity ~PC!, 6,7 ~ii! a huge ultraviolet gain of GaN-based photodetectors, 8,9 ~iii! in ion-implanted films, a highly superlinear decrease of film conductivity with increasing implanted ion dose, 5 and ~iv! an increasing mobility with increasing carrier concentration, rather than the usual decrease. 4 In this paper, we present more direct evidence for grain-boundary limited transport in GaN, using surface photovoltage and photoconductivity measurements. We then rationalize the above-mentioned transport phenomena. Assuming a thermionic emission over a barrier and a small applied bias ~i.e., an effective bias much smaller than the thermal voltage across each grain!, barrier-limited transport is characterized by an exponential dependence of the conductivity, s, on the average grain-boundary barrier height f gb 3,10