Characterizing and Modeling Transient Photoconductivity in Amorphous In-Ga-Zn-O Thin Films

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Amorphous In-Ga-Zn-O (a-IGZO) and other amorphous oxide semiconductors are attracting increasing attention from the display industry for their high electron mobility, ease of large-area manufacture, and potential for future flexible electronics. However, such amorphous materials often show instability under gate voltage bias, temperature, and illumination stress, with extremely slow relaxation times. Previous research has focused on empirical solutions to the instability problem, such as laser-assisted annealing and passivation to reduce device degradation. But a complete characterization of the degradation process is still lacking, and understanding of the underlying physical mechanism is still limited. This work focuses on the transient photoresponse of a-IGZO thin films. By measuring the basic electrical properties of the thin films under photo-excitation and dark relaxation, it is confirmed that the conductivity photoresponse is mostly due to the creation and trapping of free electrons, while electron mobility remains mostly constant. However, transient photoresponse in a-IGZO does not follow the simple exponential behavior typically observed crystalline materials. It shows a faster transient at short time scales and slower transient at long time scales comparing to a simple exponential response. Proper characterization of such a photoresponse requires improvements to the conventional van der Pauw and Hall measurement methods. To make fast and accurate Hall measurements, a heterodyne Hall method is developed using simple analog signal processing. This method not only enables continuous measurement of the carrier density transient at a single magnetic field, but also extends the lowest mobility that can be measured by Hall effects. For the non-exponential photoresponse in a-IGZO with fast initial transient and extremely slow long-term transient, a modulated time-division multiplexing apparatus is also introduced to measure several samples in parallel while capturing the initial transients in all samples with high time-resolution. While many previous reports analyzed non-exponential transients by assuming the transients to follow certain function forms, this work introduces a distributed time constant analysis that can be applied to any relaxation response. By transferring the transient response as a function of log-scale time, any relaxation response can be represented as the convolution of a time constant distribution. Therefore, the minimum measurement duration to correctly characterize the response is identified as the inflection point on a semi-log plot versus log-scale time. With the visual features on the semi-log plot, a method to estimate the entire distribution spectrum is also introduced. This allows reasonable estimation of the asymptotic response value, which cannot be directly measured in systems involving large time constants. In the a-IGZO system, the transient photoresponse fits best to a stretched exponential function. This work discusses the applications and properties of the stretched exponential function. Two contrasting physical explanations to the stretched exponential behavior, the distributed activation energy model and the continuous-time random walk model, are discussed. While the distributed activation energy model fails to explain why an asymmetric activation energy distribution appears universally in many distinct systems, the continuous-time random walk model explains the stretched exponential behavior as arising from an exponential tail of activation energies, which fits the disordered nature of amorphous materials. Based on the continuous-time random walk model, a microscopic photoresponse mechanism compatible with the observed stretched exponential transient is proposed for the a-IGZO system

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  • 02/13/2018
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