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Defect-Mediated Carrier Transport Mechanisms in Vertical GaN p-n Diodes

  • Author(s): Robertson, Christian Andrew
  • Advisor(s): Speck, James S
  • et al.

In recent years, GaN power semiconductor devices have been the focus of research and development due to the favorable material properties of the III-N system. III-N semiconductors have been widely used in high-speed transistors, visible and ultraviolet (UV) optoelectronics, and vertical power electronics. Vertical device topologies are useful for reducing the wafer footprint of power electronics by allowing voltage to be held across epitaxially grown interfaces rather than lateral ones. One of the major challenges to the performance of GaN vertical power devices has been the ubiquitous presence of defects – mainly threading dislocations – in GaN substrates and their respective epitaxial layers. Recent work has also revealed that Ca point defects are present in significant concentration in MBE-grown GaN around the world and that these point defects are likely compensating n- and p-type GaN; furthermore, a systematic way of controlling these defects was also discovered.

The effect of Ca point defects on p-n junction diode behavior was studied by comparing the transport behaviors in samples with and without high Ca incorporation. It was determined that Ca does not have a substantial effect on the ideality factor of the diode in the regime where the diode dictates the I-V behavior. However, it was found that Ca has a significant effect on other aspects of the device demonstrated by an increase in p-GaN resistivity (+107%) and decrease in p-GaN contact resitivity (-35%) when Ca was present. A similar study was also performed to observe the effects of threading dislocation density on the transport properties of vertical GaN p-n junctions. As has been reported by other growth technqiues, vertical GaN p-n diodes grown by NH3-MBE demonstrated substantial effects of threading dislocation density on leakage currents. These results were then compared to a simulation modeling leakage mechanisms of threading dislocations in vertical GaN p-n interfaces.

To study threading dislocations in GaN p-n junctions, Silvaco’s Atlas and TonyPlot softwares were used to compare vertical GaN p-n diodes with and without a dislocation. At zero bias, it was observed that the depletion region width (using the Depletion Approximation) and the maximum electric field were reduced drastically near the dislocation line. More significantly, an asymmetric reduction in the diffusion barrier for electrons and holes was observed due to the asymmetric nature of the dislocation band bending related to the doping. The reductions in electric and diffusion barrier properties persisted into forward bias and asymmetric current profiles for electrons and holes were observed. Lastly, this diffusion barrier reduction carrier resulted in an additional leakage mechanism via Shockely-Read-Hall non-radiative recombination mediated by a high np-product and trap state density near the intersection of the dislocation with the junction.

This model was also used to analyze the dislocation mediated mechanisms in reverse bias. It was found that the defects coalesced by the dislocation strain field will mediate electron-hole pair generation by a trap-assisted tunneling mechanism occurring at a peak electric field in the junction near the dislocation. These electron-hole pairs are then swept away from the junction by the strong, reverse bias electric field thereby resulting in a reverse bias leakage current mediated by the dislocation trap states.

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