Experimental realization of purely excitonic lasing in ZnO microcrystals at room temperature: transition from exciton-exciton to exciton-electron scattering

Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion , . We show that ZnO microcrystals with a size of a few micrometers exhibit purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission. We then present the distinct experimental evidence that the room-temperature 2 excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. As the temperature is lowered below ~150 K, the lasing mechanism is shifted from the exciton-electron scattering to the exciton-exciton scattering. We also argue that the ease of carrier diffusion plays a significant role in showing room-temperature excitonic lasing. Among other lasing materials, ZnO is one of the most well-studied optical semiconductors because of its relatively large exciton binding energy Eb of about 60 meV [1,2]. Previously, there have been a number of studies on room-temperature laser emission from ZnO nanostructures, such as thin films [37], nanowires [811], nanodisks [1214] and nanoparticles [1518]. Two main mechanisms are generally invoked as responsible for optical gain: excitonic and electron-hole plasma (EHP) recombinations [1,2]. An EHP state is formed when the density of electron-hole pairs exceeds the Mott density nM, where screening reduces the Coulomb interaction sufficiently that no bound excitonic states are present. Room-temperature EHP emission is observed commonly in most ZnO nanostructures under sufficiently high optical excitation [47,11,17,18]. On the other hand, 3 room-temperature excitonic lasing is achieved almost exclusively in high-quality ZnO thin films [47] despite of the fact that the exciton biding energy (Eb = ~60 meV) is larger than the thermal energy at room temperature (~25 meV). Note also that the exciton dominated regime generally overlaps with the EHP dominated regime [47]. This is most likely because rather high excitation density, which will result in the electron-hole density close to nM, is required for excitonic laser action to compensate the heavy optical loss inherent to these nanostructures. Thus, realization of pure excitonic lasing at room temperature would be difficult for ZnO nanostructured materials, as has been proved in the case of ZnO nanowires [11]. To make matters more challenging, the origin of the excitonic gain in ZnO at room temperature is still controversial. Although the room-temperature excitonic lasing was originally attributed to the exciton-exciton (ex-ex) scattering process [37], Klingshirn and his coworkers [19,20] have recently argued that other mechanisms similar to those based on the exciton-LO phonon (ex-LO) or exciton-electron (ex-el) scattering processes are more likely to be responsible for the optical gain at room temperature. Thus, it is still an open question whether the excitonic process, especially the ex-ex scattering process, indeed accounts for the room-temperature lasing in ZnO or not [9,21]. In order to shed new light on the excitonic lasing process in ZnO, we here employ 4 well-annealed, or well-crystallized, micrometer-sized ZnO crystals. Although lasing characteristics of the ZnO microcrystals have hardly been paid attention to previously [2224], we show that these microcrystals have some advantages over the well-studied nanostructured materials to identify the origin of the excitonic optical gain. First, purely excitonic lasing is achieved in the ZnO microcrystals because of their high crystallinity and low optical loss. Second, individual ZnO microcrystals can serve as effective resonators to show random laser action because their size is larger than the emission wavelength [25]. Although the resulting random feedback is incoherent, the lasing line will represent the maximum of the net gain of the medium [25,26]. That is, the emission spectrum narrows continuously towards the center of the amplification line with increasing pumping intensity. As for the coherent random laser based on ZnO nanostructures [1517], it is not straightforward to determine the position of the gain maximum on account of the presence of multiple resonance modes. From a detailed analysis of the temperature dependent lasing spectra of the ZnO microcrystals, we present the convincing experimental evidence that the excitonic lasing in ZnO at room temperature is not induced by the ex-ex scattering but by the ex-el scattering. We also demonstrated that the transition from ex-ex to ex-el process