Excitonic ultraviolet laser emission at room temperature from naturally made cavity in ZnO nanocrytal thin films

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Abstract

Hexagonally shaped ZnO nanocrystal thin films were fabricated on sapphire(0001) substrates by laser molecular beam epitaxy. Nanocrystal structure was investigated by atomic force microscopy and transmission electron microscopy. Epitaxial growth of ZnO nanocrystal thin films on sapphire substrates was found to occur in a spiral and grain growth mode. The grain growth mode was interpreted by taking higher order epitaxial relationship of oxygen sublattice units between ZnO and sapphire into account. Nanocrystal size could be tuned from 50 to 200 nm controlling film thickness, growth conditions and stoichiometry of the target. The films having small nanocrystal size of about 50 nm showed excitonic stimulated emission having peak energy of 3.2 eV at room temperature with a very low threshold (24 kW cm−2). Mode transition from excitonic stimulated emission to electron hole plasma appeared above another threshold (50 kW cm−2). Well defined Fabry–Perot cavity mode was observed in the emission spectra measured from side edge of the film. It was concluded that the grain boundaries between nanocrystals serve not only as potential barriers confining excitons but also as cavity mirrors.

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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