Structural And Opticai Investigation On InGaN/GaN Multiple Quantum Wells

Recently, great achievements have been made in the field of semiconductor materials with the progress of science and technology, especially for the III-V nitride semiconductor materials, such as AlN, GaN, InN and their alloys, which have got rapid development. As representative of III-V compound semiconductor materials, GaN has been the focus of intense research due to the excellent physical and chemical properties and broad application prospects. GaN and their alloys have direct energy gaps, covering a wavelength range from the infrared (InN,-0.7eV) to the ultraviolet (AlN,～6.0eV). Moreover, GaN can work under those conditions such as strong acid, alkali and radialization due to its high degree of hardness and stable chemical properties. As a result, it is an important material for optoelectronic devices. In the field of optoelectronic devices, InGaN-based ultra-high brightness blue and green LEDs have been commoditized, and blue LED has become the key development project of the major companies and research institutions around the world. In the field of microelectronic devices, great development has been made for AlGaN/GaN high electron mobility transistor (HEMT) and field effect transistor (HFET) due to the large electron mobility of the two dimensional electron gas (2DEG) generated in the AlGaN/GaN heterojunction interface. In the field of detector devices, deep research has been made for the GaN-based ultraviolet detector and p-n junction photovoltaic detector, which make the performance of the devices up to a high level.The development of optoelectronic devices has come to the era of blue light. InGaN alloy has been widely used for the fabrication of blue LED and LD devices. Thus, the study of InGaN multiple quantum well structure emitting mechanism is essential for further development of such optoelectronic devices. The most accepted point of view so far is that the inhomogeneous distribution of In component leads to the In-rich localized states, which prevent the transfer of the carriers to the nonradiative recombination centers in the material. However, high density of dislocations usually appears in the InGaN epitaxial layer due to the large lattice mismatch between GaN and sapphire substrate. Together with the natural defect in the GaN epitaxial layer and the unintentional dopant during the growth process, nonradiative recombination centers are usually formed in the materials, reducing the recombination efficiency. Meanwhile, the strong polarization field induced by large lattice mismatch in the epitaxial layer and the spontaneous polarization in GaN will also reduce the recombination efficiency. In addition, the so-called efficiency droop usually appears in the electroluminescence, which resulted from the large build-in electric field and applied bias, the non-uniform distribution of the carriers among the quantum wells and the self-heating of the devices under high current. Even so, the InGaN/GaN multiple quantum well LEDs usually exhibit extremely high quantum efficiency, which resulted from the effect of In-rich localized states. The luminescence of the devices is governed by the competition of the above mentioned factors at different conditions. Thus, the study of InGaN multiple quantum well structure emitting mechanism is important for both the luminescence theory and the development of such devices. In this paper, we study the optical behavior of the InGaN MQWs LED in detail by measuring the spectrum of the sample at different temperatures, excitation powers and current densities, to seek the carrier dynamic mechanics in the devices. The main conclusions of the dissertation are listed below.(1) We have studied the influence of Stark effect resulting from the built-in electric field on the photoluminescence (PL). It can be inferred from the excitation power dependences of PL peak energy and full width at half maximum (FWHM) that with increasing the excitation power at6K, the luminescence process undergoes a mechanism converting from the screening of Stark effect to the high energetic band filling of the localized states. While at300K, nonradiative recombination centers are thermally activated, and defect-related nonradiative recombination dominates the process in the low excitation power range.(2) We have studied the anomalous temperature behavior of the peak energy and linewidth at different excitation powers. At low excitation power, the S-and W-shaped temperature dependences of the emission energy and linewidth reflect the conversion of the carrier transferring mechanisms from nonthermalized to thermalized distribution of localized carriers, and finally to the regular thermalization of the carriers.The disappearance of the S-and W-shaped temperature dependences with increasing excitation power, is attributed to the reduced localization effect. The initial decreasing in the W-shaped temperature dependent linewidth strengthens first and then weakens with increasing excitation power, which we attribute to the exponentially increased density of states with energy in the band tail.(3) We fitted the temperature-indueced blueshit of the peak energy by the modified Varshni empirical law, in consideration of the influence of the localized states. The results indicate that the increasing excitation power can reduce the localization effect.(4) We have studied the excitation power dependence of the internal quantum efficiency. The excitation power dependence of the emission intensity shows that the emission process of the MQWs is dominated by the radiative recombination at low temperature, and by nonradiative recombination at room temperature within low excitation range. The conclusion is that the internal quantum efficiency is determined by nonradiative recombination, radiative recombination, Stark effect and band filling of localized states.(5) We have studied the carrier transfer and recombination mechanism in the InGaN/GaN MQWs in detail. Two InGaN-related emission peaks observed in the full PL spectrum at moderate temperatures and low excitation powers are assigned to the QDs and the InGaN matrix, due to a strong phase separation confirmed by HRTEM. With increasing temperature, the intensity behavior of the two peaks is attributed to that with increasing temperature the transfer of the photo-generated carriers from the InGaN matrix into the QDs first is enhanced below-100K due to the increased mobility in the low temperature range, and then is gradually suppressed up to300K due to the reduced nonradiative lifetime. In addition, the S-shaped temperature behavior of the InGaN matrix-related emission peak energy reflects a slight composition fluctuation in the InGaN matrix, while the monotonic decreasing of the QDs-related peak energy with increasing temperature is ascribed to the relaxation process via hopping of the carriers inside the QDs.(6) We have studied the carrier transfer and recombination mechanism in the InGaN/GaN MQWs by time-resolved photoluminescence (TRPL). All the curves for both the peaks show multiple-exponential decay, including a relatively faster decay (a few nanoseconds) in the early stage and a slower decay (several tens nanoseconds) in the extended range, which are due to the carrier escape into either higher or lower levels and the effective decay times, including the mechanisms of radiative recombination, nonradiative recombination, and possibly extended carrier transport. We make the conclusion that the carrier lifetime is determined by escape rate, capture rate, radiative recombination lifetime, nonradiative recombination lifetime and transition probability. The TRPL measurement results explain the carrier transfer and recombination mechanism in the InGaN/GaN MQWs more clearly.(7) We have measured the current-voltage (I-V) characteristics and electroluminescence (EL) spectra of the InGaN/GaN MQWs LED. From the I-V curves, we deduced that the major carrier transfer mechanism is tunneling current and the forward bias to get a certain current is significantly increased when the current is increased, especially at lower temperatures.The anomalous behavior of the EL spectra at low temperatures is attributed to the electron leakage resulting from the large forward bias since the electron leakage results in the failure of Coulomb screening effect and the relative enhancement of the low-energetic localized state filling. The current dependences of the integrated EL intensity and EQE further show that the electron leakage occurs in the whole current range at low temperatures, whereas only in high current range at high temperatures. The temperature dependences of the EL intensity at different current densities indicate that the EL efficiency is determined by not only carrier transfer, Stark effect, nonradiative recombination within the quantum well, but also carrier transport and distribution among the quantum wells.Thus, to achieve a high-quantum efficiency of InGaN-based LED, it is essential to overcome the nonradiative recombination, weaken the built-in electric field, increase the depth of the localized states and balance the distribution of electrons and holes in the MQWs region to reduce the electron leakage.