[Truncated] Gallium nitride (GaN)-based electronic devices such as light emitting diodes and n-p-n heterojunction bipolar transistors require a good quality p-type GaN layer for high performance and efficiency. Therefore, assessment of material quality for p-type GaN is of critical importance in the advancement of GaN-based technology. One way of quantifying material quality is to measure the minority carrier diffusion length. In this work, the photocurrent technique is applied to determine this parameter. To be confident that the results obtained using this technique are accurate for GaN, which exhibits diffusion length ranging from tens of nanometers to several micrometers, the validity of the standard mathematical model with respect to GaN is investigated.
The validity of the model is assessed by examining four of the assumptions made in the derivation of this model, namely, that there is no excess minority carrier density at the depletion edge plane, the absorption coefficient is uniform throughout the material, including in the depletion region, there is no recombination in the depletion region and the induced photocurrent consists of the minority carrier current only. The validity of these four assumptions, their effects on the accuracy of the diffusion length values obtained and the overall applicability of the photocurrent technique are investigated.
The investigations found that all four assumptions are not always valid for GaN and therefore the standard mathematical model is also not always valid. The invalidity of the assumption of no recombination in the depletion region was found to have the most significant negative impact on the accuracy of the diffusion length value while the invalidity of the remaining three assumptions do not have a significant detrimental effect. Overall, precaution is recommended when applying the standard mathematical model to GaN samples, particularly if the samples have been subjected to energetic post-growth processes, which can give rise to recombination in the depletion region.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2014|