The microstructural, electrical and optical properties of GaN/InGaN light emitting diodes (LEDs) with various material quality grown on sapphire have been studied. Burger's vector analyses showed that edge and mixed dislocations were the most common dislocations in these samples. In defective devices, a large number of surface pits and V-defects were present, which were found to be largely associated with mixed or screw dislocations. Tunneling behavior dominated throughout all injection regimes in these devices. The I-V characteristics at the moderate forward biases can be described by I = I0 exp (eV/E), where the energy parameter E has a temperature-independent value in the range of 70 -110 meV. Deep level states-associated emission has been observed, which is direct evidence of carrier tunneling to these states. Light output was found to be approximately current-squared dependent even at high currents, indicating nonradiative recombination through deep-lying states in the space-charge region. In contrast, dislocation bending was observed in a high quality device, which substantially reduced the density of the mixed and screw dislocations reaching the active layer. The defect-assisted tunneling was substantially suppressed in this LED device. Both forward and reverse I-V characteristics showed high temperature sensitivity, and current transport was diffusion-recombination limited. Light output of the LED became linear with the forward current at a current density as low as 1.4x10-2 A/cm2, where the nonradiative recombination centers in the InGaN active region were essentially saturated. This low saturation level suggests optical inactivity of the edge dislocations in this LED.
Uniform current spreading is desirable for both light emitting diodes (LEDs) performance and reliability. It enhances optical efficiency because the joule losses due to current crowding in some parts of the die would be eliminated. The LED design for optimal light extraction and uniform current spreading is therefore a necessity. In this paper we report on preliminary current spreading results obtained from circuit simulation, using Pspice and Aimspice, for LED designs with and without an n-metal ring as well as the epi-up and flip chip LEDs. For the epi-up, both the lateral and vertical resistances of the transparent metals were taken into account. Whereas in the flip chip, the lateral resistance was negligibly small thus only the vertical component contributed to the total p-lump resistance. The n-lateral resistance in the active mesa was critical to uniform current spreading. It was found that the lower the n-lateral resistance, the more uniform the current spreads and flows through the active region. In both the epi-up and flip-chip structures, the contact resistance of the p-metal (including the thin Ni/Au transparent metal) dominated the total p-lump resistance. The larger this value, with fixed n-layer lateral resistance, the more uniform the current spreads in the device. However, high p-contact resistance is not desirable as it reduces the overall efficiency of the device due to excessive heating and increased leakage current. Therefore, for uniform current spreading, the n-lateral resistance should be made small while maintaining an optimum p-lump resistance to achieve a high efficiency.
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