Theoretical Studies Of Zn O-based Electroluminescent Semiconductor Materials And Devices

As a novel semiconductor with the direct wide bandgap 3.37 e V and the exciton binding energy as large as 60 me V, Zn O has emerged as a promising candidate for electrically pumped excitonic UV lasing at RT. Due to the great potential of realizing ultraviolet laser, in recent years, many domestic and abroad groups have been studying on this topic. However, more than ten years and a great deal of manpower and material resources have been spended, electrically pumped Zn O UV lasers are still difficult to achieve. The main reason is that it is still an international scientific conundrum to obtain stable and low-resistance p-type Zn O film to realize Zn O-based UV lasers. In order to achieve electrically pumped Zn O-based UV lasers, we carried out phased theoretical research as follows:1. We have searched for new strategies of overcoming the intrinsic compensation and increasing the p-type doping efficiency in Zn O. By using the first-principles method, we simulated the dynamic growth processes of N-doped Zn O by MBE. It is found that the NZn-VO will emerge during the zinc-polar growth processes. As a metastable donor complex, NZn-VO will be free from the spontaneous compensation by the intrinsic donors in Zn O during the growth processes. After activation, NZn-VO will transform to a stable acceptor complex of NO-VZn and the energy barrier of this procedureis about 1.1e V. The calculated ionization potential of NO-VZn which is about0.16 e V indicates that NO-VZn is a shallow acceptor defect. Based on the our calculation results, some suggestions were proposed to improve the p-type doping efficiency in Zn O, including selecting Zn-polar substrates, keeping the balance between the Zn flow and O flow and choosing NO or NO2 as the N source instead of N2. The key problems of Zn O p-type doping have been overcomed.2. We have explored a new way——quantum cascade way—— to realize electrically pumped Zn O-based UV lasers. For the traditional interband optical transition,the p-type doping efficiency of Zn O is difficult to meet the laser emission requirement by far. We attempted to use a new-type optical transition method—the intersubband transition within conduction bands to realize electrically pumped Zn O-based UV lasers, and only n-type Zn O were employed. But limited by the intrinsic property of one-dimensional confined quantum well, it is difficult to realize population inversion with shorter-wavelength photon emission. Thus traditional one-dimensional quantum cascade lasers(QCLs) can only work at mid- and far infrared regions. It was found that by adding another dimensional confinement to the traditional one-dimensional confined QCLs, electron LO-phonon resonance access will be established. The lifetime of electrons in lower states of the optical transition levels will be decreased due to electron LO-phonon scattering, thus the population inversion could be established. Taking the advantage of the large band offset(about 3.59 e V)between Zn O and Mg O, we theoretically designed two-dimensional confined Zn O/Mg O QCLs operating at λ=2.95μm,1.57μm,1.13μm and734 nm. Although the device have not been achieved by now, Our work may pave new ways for fabricating shorter-wavelength QCLs and the realization of Zn O based UV lasers.