Development Of High Power Green Light-emitting Diodes On Silicon Substrate

Further improvement of the green light emitting diodes(LEDs) performance must be made for the widespread application of nitride-based LEDs in fields such as backlighting liquid-crystal-display panels, large-scale full- color displays and solid-state lighting. However, the quantum efficiency of the green LEDs is about half of that of AlInGaN blue LEDs, and lags further behind that of AlGaInP red LEDs, which is known as ―green gap‖. Moreover, the gap gets deeper at high injection current densities, as green LEDs exhibit more remarkable efficiency droop than other visible LEDs. In this thesis, first, the common technologies in GaN LEDs such as high growth rate for GaN and the temperature dependent electroluminescence characteristics of chips were developed.To improve the efficiency of green LEDs and close the ―green gap‖, then we have design and grown a new epitaxial structure on the silicon substrate. The main achievements are as follows:1. The unintentional carbon doping concentration of GaN films grown by lo w pressure metal organic chemical vapor deposition(LP-MOCVD) depends strongly on the growth rate. The carbon concentration increases from 2.9×1017 to 5.7×1018 cm-3 when the growth rate increases from 2.0 to 7.2 μm/h. A correlation between carbon concentration and N vacancies is found. The intensity ratio of yellow and blue luminescence to band edge luminescence in the samples increases linearly with carbon concentration. The present results demonstrate direct and quantitative evidence that the C-related defects are the origin of yellow and blue luminescence. The related results were reported in The Journal of Semiconductors.2. Performance improvements in Ga InN blue light-emitting-diodes(LEDs) including decreased wavelength shift, enhanced quantum efficiency, and reduced operating voltage are obtained by an abruptly and heavily Mg-doped p-AlGaN electron blocking layer. Moreover, the commonly observed efficiency collapse at cryogenic temperatures for conventional LEDs is not found in abrupt doped LEDs. Under high level injection conditions, the conventional LEDs have much higher electric field in the p-type neutral region than the abruptly doped LEDs at cryogenic temperatures. The higher electric field leads to the greater electron leakage, as well as the cause of efficiency collapse. The related results were reported in The Electrochemical Society.3. Many GaInN light-emitting diodes(LEDs) are subjected to a great temperature variation during their serving. In these applications, it is advantageous that GaInN LEDs have a weak temperature dependence of forward voltage. However, the factors determining the exact temperature dependence of the forward voltage characteristics are not fully understood. In this paper, two series of GaInN LEDs are prepared for investigating the correlation between the epitaxial structural and the temperature dependence of the forward voltage characteristics. The forward voltage characteristics of samples are studied in a temperature range from 100 K to 350 K. The curves of forward voltage versus temperature(dV/dT) are compared and analyzed. For the three samples in series I, according to the barrier thickness and emitting wavelength, they are designated as blue multiquantum well(MQW) with thin barrier(sample A), blue MQW with thick barrier(sample B), and green barrier with thick barrier(sample C) respectively. Their structures of active region including the insertion layer between n-GaN and MQW, the MQW, and the emitting wavelength are diff erent from each other. However, the same slopes of d V/d T at room temperature(300 K±50 K) are observed in the samples. Moreover, samples B and C with the same p-type layer design also have the same slopes of dV/dT at cryogenic temperatures. Sample A with a much thinner p-type layer shows a lower slope than samples B and C. Based on the these experimental data, it is deduced that the intrinsic physic properties of active region such as structure and emission wavelength have a little in?uence on the variation of the slope of d V/d T either at room tempe rature or at cryogenic temperatures. Moreover, the Mg concentration of the p-GaN main region determines the slope of dV /d T at cryogenic temperatures. Low doping concentration leads to a high slope of dV /d T. In order to ?nd the decisive factor determining the slope of dV /dT at room temperature, three samples in series II are grown. For sample E, at the MQW-EBL(electron blocking layer) interface, the Mg concentration increases very slowly while an abruptly varying doping pro?le is observed for samples D and F. The slopes of samples D and F are both-1.3 mV·K-1. This is very close to the calculation value of the lower bond for the change in forward voltage(-1.2 mV·K-1). Meanwhile, the slope of sample E is-2.5 mV·K-1, which is much higher than those of samples D and F. Thus, it is suggested that the major factor in?uencing the slope of dV /d T at room temperature is the Mg doping pro?le of the initial growth stage of the p-AlGaN electron blocking layer. These phenomena are mainly attributed to the changes of the activation energy of p-AlGaN and p-GaN, since it relies on the doping concentration and temperature. Our ?ndings clarify the roles of active region, p-AlGaN and p-GaN in the temperature dependence of the forward voltage characteristic. More importa ntly, the results obtained in this study are helpful for optimizing the growth parameters to achieve LED devices with forward voltage that has a low sensitivity to temperature. More importantly, it is helpful in optimizing the growth parameters to achieve LED devices with forward voltage have low temperature dependence sensitivity. The related results were reported in Acta Physica Sinica.4. A newly designed green epitaxial structure on the silicon substrate have been optimized as follows: a) Through the optimization of n-GaN doping level, the current spreading was significantly improved; b) A new insertion layer which contains low-temperature GaN, InGaN/GaN superlattice and blue multi qutum wells(MQW) was introduced between n-GaN and green MQW. The thickness of LT-GaN, the In content in InGaN/GaN SL and well thickness in blue MQW were optimized to form InGaN staircase. The staircase act as electron cooler reduces the electron overflow, leading to the enhancement of quantum efficiency; c)The thickness of green quantum well thickness and barrier design were optimized. The use of the InGaN/AlGaN/InGaN barrier enhances the performances of devices. After all, at 35 A/cm2, 300 K, the internal quantum efficiency of 1mm×1mm chips at 515 nm, 520 nm and 525 nm were 45.2%,42.5% and 41.6%, respectively.5. The effect of p-AlGaN thickness on the leakage current characteristics and efficiency with large V-pits were investigated. According to the structure of V-pits, it is found that the thickness of p-AlGaN EBL in the sidewalls of V-pits is increased while increasing EBL thickness on the(0001) plane. The increased EBL thickness in the sidewalls can provide thicker energy barrier and then screens the defects more effectively. As a result, the leakage current of LEDs with thicke r EBL is significantly reduced. For example, when the EBL thickness increases from to 20 nm to 40 nm, the reverse leakage current is from-2846 nA down to-465 nA. The evolution of EQE of samples at different current density is also discussed. At low curre nt densities, EQE decreases first and increases afterward with the increase of EBL thickness, which should be ascribed to the competition between enhanced radiative recombination rate and reduced hole injection efficiency. At high current density, EQE almo st increases with increasing EBL thickness, which is due to the improved electron confinement in the active region by preventing electrons leakage to the p-type layer. However, as EBL thickness continue to increase, the EQ E is decreased due to the lower ho le concentration in EBL. Finally, EBL thickness was optimized by evaluating the green LED performance. Packaged 1mm×1mm chips with a 40 nm EBL emits up to 260 mW(wavelength: 520nm) at 35 A/cm2, the EQE is 31.2%.