Effect Of Localized States On The Optical Properties Of InGaN/GaN Multiple-Quantum-Well Structures

The III-V nitride materials A1N, GaN, InN and their alloys have seen rapid development in recent years. They can work under those conditions such as high temperature, acid, alkali and radialization due to their excellent photoelectric performance and stable chemical properties. Moreover, the direct energy gaps of these materials cover a wavelength range from the infrared (InN,-0.7eV) to the ultraviolet (A1N,～6.0eV). As a result, they have a wide range of applications in blue、green and ultraviolet optoelectronic device. As an important Ⅲ-Ⅴ nitride material, the study and development of InGaN will establish a base for actualization of semiconductor illumination of the world.In this paper, we studied the optical properties of InGaN/GaN multiple quantum wells (MQWs) by use of the method of photoluminescence (PL) and electroluminescence (EL). The main research work and results are as follows:1. Excitation power dependence of the PL spectra was studied in InGaN/GaN MQWs at a temperature of30K. The excitation power dependences of the PL peak energy and linewidth indicate that the emission process of the MQWs is dominated first by the screening effect of the quantum confined Stark effect (QCSE) and then by the band-filling effect.2. The dependence of the PL spectra on various temperatures was also reported. We observed an S-shaped (decrease-increase-decrease) temperature dependence of the peak energy for InGaN-related PL with increasing temperature. This anomalous emission behavior is attributed to a change in the carrier dynamics with increasing temperature due to inhomogeneity and carrier localization in the InGaN/GaN MQWs.3. Two InGaN/GaN MQWs samples with different growth rates were grown for the comparison of localization states. By analyzing different temperature dependences of the PL spectra of the two samples, we deduced that the sample with a slower growth rate has a relatively uniform distribution of localization states.4. In the PL spectra, there is a broad deep level emission at lower energy under the conditions of low excitation powers and medium temperatures. This can be attributed to the emission from deep localization states which contain higher indium contents.5. The forward current-voltage (I-V) characteristics of the InGaN/GaN MQWs LED were measured at different temperatures. It is found that when decreasing temperature, the trend of I-V characteristics persists down from300K to180K duo to the carrier transport dominated by tunneling. But below180K, the forward bias starts to significantly increase to obtain a necessary current level due to the electrical characteristic dominated by a high series resistance at lower temperatures.6. The temperature dependence of the EL spectral intensity has been investigated in detail between T=10and300K at various injection current levels. When the injection current level is low at0.001mA, the leading EL band exhibits the highest intensity at10K, and the EL peak intensity decreases as the temperature is increased. This decrease of the EL intensity with increasing the temperature is ascribed to an enhancement of nonradiative recombination processes, i.e., a reduction of the radiative recombination efficiency. When the current level is increased, however, the temperature dependence of the EL intensity is drastically changed. The highest intensity of EL spectra is no longer appearing at the lowest temperature of10K. At50mA, for example, the EL intensity increases with increasing temperature from10K to130K. After reaching the maximum EL intensity around130K, the EL peak intensity decreases as the temperature is further increased. It can be concluded that the EL efficiency is reduced at lower temperatures and higher injection levels. There are two reasons for this phenomenon. First, applying higher forward biases, the carriers are transferred to nonradiative recombination centers as a result of escape from the MQW region, thus reducing the EL efficiency. Another point is that the higher field existing in the well under the higher forward bias decreases the radiative recombination rate due to the QCSE, which also causes the reduced EL intensity.