Optical gain and lasing of ZnO/ZnMgO multiple quantum wells

J. Cui, S. Sadofev, S. Blumstengel, J. Puls, and F. Henneberger

ZnO has attracted more research interests for its unique properties. The wide band gap of 3.36 eV at room temperature make it a possible candidate for the next generation optoelectronic devices. The large exciton binding energy ensures excitonic transitions up to room temperature. The prediction of room temperature ferromagnetism of ZnO based dilute magnetic semiconductor make it important in the rising spintronics.

The samples were fabricated by radical source molecular beam epitaxy[1]. Two multiple quantum well samples were studied by means of gain and lasing measurement. One (sample A) is with additional optical confinement cladding layers, the other one (sample B) without optical confinement.

Fig. 1	Fig. 2

The structure of sample A is shown in Fig. 1 (a). The waveguide mode density is shown in the Fig. 1 (b). The confinement factor is about 6%.

The geometry in which the lasing measurement was performed is shown in Fig. 2. The incident laser light is shaped in a stripe. We can measure the gain by the so-called variable stripe-length method[2]. The stripe-length dependent PL intensity can be described by Equ. (1).

	Fig. 3

In general cases, by using two stripe lengths the gain values can be calculated with Equ. (2). However, the as grown edges of our samples are of less quality. To remove the influence from the rough edge, we calculated the gains by using three stripe-lengths as shown in Fig. 3 according to Equ. (3).

Fig. 4	Fig. 5

The main results are shown in Fig. 4. The surface photoluminescence (PL) with two excitation intensities are shown in Fig. 4 (a) for the two samples. Below the threshold, the PL intensity is proportional to the excitation intensity. Above the lasing threshold, the edge PL shows lasing features. The feedback from the cracks in the samples make it possible to observe the lasing actions. The gain spectra are shown in Fig. 4 (c).

Several lasing mechanisms has been proposed, such as exciton-exciton scattering[3-8], electron-hole plasma[9]. However, for our samples the energy shift between PL and lasing is below the predicted value for the exciton-exciton scattering process at both low temperature and room temperature. The exciton density is also below the Mott density. The biexciton is also not observed.

ZnO is an intrinc n-type semiconductor. The exciton binding energy is very large. Thus we assign the laser action to negatively charged exciton (trion) localized on well width fluctuations and alloy disorder at the well-barrier interface.

The temperature dependent lasing thresholds for the two samples are shown in Fig. 5. The threshold can be fitted by the Equ.(4). The fitting parameters are shown in Table 1. The sample with optical confinement shows lower threshold at low temperatures.