3D Numerical Carrier Transport Study by Considering Nano-Scale Structures in LEDs

• Main Authors: Chen-Kuo Wu, 吳鎮國
• Other Authors: 吳育任
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/04091736941980802442

碩士 === 國立臺灣大學 === 光電工程學研究所 === 104 === The experimental results show that there are nano-scale composition fluctuations existing in the ternary alloy of InGaN quantum wells (QWs) and AlGaN electron blocking layer (EBL). The scales of fluctuations are ranging from the units nanometer scale (random alloy fluctuations), tens nanometer scale (imperfect QWs), or hundreds nanometer scale (V-pits). The existence of nano-scale fluctuations will affect the carrier transport and radiative recombination strongly. Therefore, we need to develop a suitable model to analyze these effects. In this thesis, we applied our inhouse 3D FEM Poisson and drift-diffusion solver to analyze these problems. In the beginning, to understand how the piezoelectric barrier influence the carrier injection in GaN device system, we took the n-i-n InGaN system, n-i-n AlGaN quantum barrier (QB) and light emitting diodes (LEDs) with different EBLs to analyze the conduction band potential distribution, I-V performance and internal quantum efficiency (IQE) by considering the random alloy fluctuation. The results show a better fit in I-V curve and reveal that the random alloy fluctuation will affect the carrier confinement and transport significantly, epecially in a thinner epi-layer case. Besides, the imperfect QWs which commonly exist in the green emission LEDs are modeled by our 2D and 3D simulation programs. According to the calculated results, we can more approach the experimental IV performance by considering imperfect QW structures. With properly modeling the electric property, this model could provide a basis for further modeling other physical properties in green LEDs. In the last part, a V-pit embedded inside the blue InGaN LED was studied. A 3D strain-stress sovler and carrier transport model were employed to study the current path, where the quantum efficiency and turn-on voltage will be discussed. Our calculated results show that the shallow sidewall QWs will provide extra hole current flow paths, and make the carrier distribution more uniform along lateral QWs than traditional planar MQWs, which have high piezoelectric barriers make carriers hard to flow through. In addition, the random alloy fluctuation model is applied in the V-pit structure to compare the turn-on voltage and quantum efficiency with planar structure LEDs. The sidewall structure will provide more percolation paths for carriers and improve the carrier injection so that the V-pit LEDs perform smaller turn-on voltage and higher simulated IQE value than planar MQW LEDs. Moreover, the simulated turn-on voltage of the V-pit LED with the random alloy fluctuation model can be pushed earlier to appropriately explain the experimental data. In the last part of this section, the carrier transport by considering the size effect is studied. The variation of the internal quantum efficiency (IQE) for different V-pit sizes is due to the trap-assisted nonradiative recombination and QW areas. The V-pit structure would not only enhance the hole percolation length but act as a potential barrier to prevent carriers from nonradiatively recombining in threading dislocations (TDs). Keywords: blue-green light emitting diode, alloy fluctuation, imperfect quantum well structure, V-shaped pit, GaN, InGaN, AlGaN