Excitonic ultraviolet laser emission at room temperature from naturally made cavity in ZnO nanocrytal thin films
Introduction
Metal oxides have recently shown such exotic properties as high temperature superconductivity and colossal magnetoresistance, tempting us to expect the birth of a new field of microelectronics based on hetero-epitaxial oxide structures [1]. As a process especially suitable for atomic scale control of the epitaxy of metal oxides, we have developed laser molecular beam epitaxy (MBE) [2]and demonstrated that it can be used for manufacturing quantum structures and exploring novel properties of oxides [3]. As a novel function of metal oxides, we have recently reported an excitonic laser action at room temperature in ZnO thin films composed of hexagonally shaped nanocrystal thin films 4, 5.
Very recently, a laser diode based on III-nitrides has achieved continuous-wave blue lasing at room temperature [6]. Some critical issues, however, have still been remaining to be solved for practical use. In widegap semiconductors, high carrier concentration is needed to achieve a gain enough high to give laser action in an electron-hole plasma (EHP) process [7], which is a laser operation mode in conventional laser diodes. This fact unfortunately results in a high threshold for lasing [8], unless much more efficient lasing process is taken into account. Moreover, it is difficult to fabricate p-type materials having low resistivity in widegap semiconductors. In III-nitride laser diode, most of the applied power is indeed consumed in the p-type layer and the interface with contact metal, resulting in Joule heating.
In order to decrease threshold for lasing, current trend in compound semiconductor laser is concentrated on the fabrication of such low-dimensional structures as quantum well and dot [9]. This is because the quantum effect modifies the profile of density of states so that the transfer integral at the band-edge becomes much larger than that of bulk semiconductor, facilitating efficient stimulated emission. The use of excitonic recombination is another approach to enable intrinsically large matrix element because of its bosonic nature. Therefore, the threshold of excitonic laser action was predicted [10]and verified [11]to be much lower than conventional EHP lasers.
For achieving efficient excitonic laser action at room temperature, exciton binding energy (Ebex) has to be much lager than the thermal energy at room temperature (26 meV). In this viewpoint, ZnO is a suitable material for ultraviolet light emission. ZnO has room temperature band gap of 3.37 eV and has a much larger Ebex (60 meV) than those of ZnSe (22 meV), ZnS (40 meV), and GaN (25 meV). However, stimulated emission from ZnO bulk crystals has been observed only at cryogenic temperatures and it quenched rapidly at higher temperatures probably due to defects [12]. It should be possible to overcome these problems by fabricating high-quality ZnO thin films with using modern thin film technology.
In this paper, we overview our current status towards ZnO lasers. Thin film fabrication, optical properties, and laser operation are described
Section snippets
Experimental
ZnO films were grown by laser MBE (compact laser MBE system; Pascal Co., Ltd.) equipped with reflection high energy electron diffraction (RHEED). The background pressure was better than 1×10−9 Torr. Ceramic ZnO targets (99.9999%) were placed in the chamber and ablated with KrF excimer laser (254 nm, 20 ps, 10 Hz) pulses focused into a spot of 0.5×2 mm with a fluence of 0.6 J cm−2. Sapphire(0001) substrates (14 mmφ×0.4 mmt), polished on the both sides, were mounted on a holder placed 50 mm away
Crystal structure of ZnO films
RHEED patterns of the ZnO films had sharp streaks from the beginning of the deposition, indicating epitaxial growth. X-ray diffraction pattern of the films showed only ZnO(0001) peaks together with the sapphire(0001) peaks. A 2 μm-thick film obtained under optimal growth condition had full width at half maximum (FWHM) of 50 and 210 arcsec for the ZnO(0002) Bragg peak and rocking curve, respectively, indicating very high crystallinity. In-plane mosaicness was evaluated to be 470 arcsec by φ-scan
Conclusions
We have reviewed our progress on ZnO nanocrystal ultraviolet laser research. The structure and formation mechanism of hexagonally shaped nanocrystal were described. Excitonic stimulated emission and laser cavity formation were discussed in relation with the nanocrystal structure.
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2019, Journal of LuminescenceCitation Excerpt :At RT the amplification mechanism is difficult to recognize because the newly occurred band at excitation density increase is located in the area of energies where there may be bands of different nature. The proposed mechanisms are inelastic exciton-exciton scattering (Ex-Ex) and EHP, the identification being based on the band energy position or/and electron-hole density estimation [e.g. [5–7]]. While the mechanism of EHP may be affirmed through a band red-side shift at excitation rising according a band renormalization dependence, Ex-Ex band may be confused with recombination of inelastic scattering of an electron or LO-phonons on exciton or electron-hole (e-h) correlated pair [4].
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Also a member of CREST, Japan Science & Technology Corporation.