CdSe-based nanocrystal quantum wells (QWs) were synthesized around CdS nanocrystal quantum dots and were bandgap- and strain-engineered to achieve high-efficiency short-wavelength luminescence. Tuning the CdSe QW width in the range of 1.05 to 1.58 nm has led to blue-green light emission, whose quantum yield was improved up to 48% through strain compensation by an optimized ZnS outer shell. The luminescence spectrum can be modified by adding a ZnS inner barrier layer to block charge and exciton transfer between the QW and CdS core. Strain management by adjusting the well and barrier thickness has proven critical in such a complex multilayer quantum system for obtaining high-quality nanocrystals and light emission.
InGaN-based LEDs suffer from a significant drop in quantum efficiency (QE) under high-current operation. We studied
the Electroluminescence (EL) of InGaN-based multiple-quantum-well green LEDs on both Sapphire and free standing
Bulk GaN, in an attempt to shed light on the underlying mechanism for the efficiency droop problem. The density of
microstructural defects in the LED on GaN was substantially reduced, leading to a significant reduction in defectassisted
tunneling currents and an improved injection efficiency under low bias. The LED on GaN outperformed the
LED on sapphire at low injection currents and exhibited a ~65% peak internal quantum efficiency. However, it suffered
from even more dramatic efficiency roll-off which occurs at a current density as low as 0.3 A/cm<sup>2</sup>. The EQE roll-off is
mitigated when the LEDs were tested at elevated temperatures. These results are explained as the combined result of
efficient current injection and significant carrier overflow in a high-quality LED.
The growth of violet light emitting diodes (LEDs) was optimized using a statistical design of experiment (DOE) approach and several important interaction effects were found. The DOEs studied the effect of several variables on the well layer, the barrier layer, and the pAlGaN cladding layer. These variables included the gallium flow rate, the indium flow rate, the growth temperature, and the growth time for the well layer, the ammonia flow in the active region, the barrier growth time, and the Si doping of the barrier, as well as the growth time, growth temperature and Mg doping of the pAlGaN cladding layer. The LEDs were optimized based on combinations of several responses from photoluminescence and electroluminescence measurements. An overall process desirability was obtained, based on achieving the desired wavelength and maximizing the PL intensity and optical output power. Significant interactions between variables played a major role in the optimization of optical output power as well the emission wavelength. The understanding of these interactions led to the optimization of the LEDs both by improvements in the structure and improvements in the quality of the layers. Several of the interactions will be explained based on kinetic models of GaN growth by MOCVD.
Blue and near-ultraviolet (UV) InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) were grown on GaN and sapphire substrates using metalorganic chemical vapor deposition. The homoepitaxial LEDs exhibited greatly improved microstructural and electrical properties compared to the devices grown on sapphire. As a result of defect reduction, the reverse-bias leakage current was reduced by more than six orders of magnitude. At forward bias, thermally activated current rather than carrier tunneling was dominant in the LEDs on GaN. The improvement of optical characteristics was found to be a strong function of In content in the active region. At low and intermediate injection levels, the internal quantum efficiency of the UV LED on GaN was much higher compared to that on sapphire, whereas the performance of the blue LEDs was found to be comparable. At high injection currents, both the blue and UV LEDs on GaN greatly outperformed their counterparts on sapphire. The homoepitaxial LEDs with a vertical geometry had a much smaller series resistance and were capable of operating at 600 A/cm<sup>2</sup> in cw mode due to uniform current spreading and efficient heat dissipation.
In this paper we present the electro-optical model, using Aimspice in conjunction with a resistor network, for evaluating the LED designs for optimum uniform current spreading and efficient light extraction. Since high brightness is a critical factor for solid-state lighting, the ability for LED designs to be scalable is important, and we use the pinwheel design, which is aimed at increasing the p-contact area to aid in uniform current spreading, to demonstrate our model. The pinwheel LED design does not scale up because the percentage current uniformity decreases with device size and bias current. To validate the model the current voltage characteristics curves for the two LED sizes (X and 3X) are matched and the recombination saturation current densities values extracted as 7.8 x 10<sup>-8</sup> A/cm<sup>2</sup> and 8.6 x 10<sup>-9</sup> A/cm<sup>2</sup>, respectively. The tunneling saturation current densities for smallest LED, X, is two orders of magnitude lower than the larger device (3X). Although the larger the LED the higher the photon generation, only a small fraction of these photons can escape the device. The largest photon density is generated under the p-metal contact, with decreasing generation towards the edge of the mesa. Since the metal is opaque to the photons, there is that tendency for most of the photons in this region to bounce back and forth in the device and finally get absorbed. For the 3X pinwheel LED at 20 mA forward current, the complimentary experimental and calculated results show that only 2.2% of the generated photons can escape the active region and make it to the outside world.
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 = I<sub>0</sub> 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<sup>-2</sup> A/cm<sup>2</sup>, 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.