Single Quantum Well Heterostructures of MgZnO/ZnO/MgZnO on C-Plane Sapphire
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ZnO/Mg0 2Zn00 .80 superlattice structures have been studied using the photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy [6]. A blue shift of the PL and PLE spectra has been observed indicating a quantum-size effect. However, the superlattice structures are complicated for fitting calculation. SQW structures are desirable for fundamental studies and calculation. Here, we report for the first time, MgZnO/ZnO/MgZnO single quantum well heterostructures grown on c-plane sapphire substrate by pulsed laser deposition (PLD). PL and PLE results showed the evidence of quantum confinement effects. The results are compared with predictions based on the simple square well model. EXPERIMENT A KrF pulsed excimer laser was used for ablation of the ZnO and Mg0 Zn0 90 targets. The heterostructures were deposited onto c-plane (0001) sapphire substrates at 750'C in an oxygen background pressure of I X 10-' Torr [2,3]. The SQW heterostructures consisted of a narrow band gap thin layer of ZnO (3.3 eV) sandwiched epitaxially between two wider band gap MgxZn1 .,O (3.6 eV) layers. The Mg content of the targets used in this study was fixed at 0.1 based on the quality of the films from XRD and RBS/ion channeling results. The Mg content of MgxZnl1 x0 films grown by PLD has been previously reported [7,8] to be 2.5 times larger than that of the targets due to differences in the vapor pressure of ZnO and MgO at high temperature. The thickness of ZnO layer was varied from 10, 20 to 40 nm, where as thickness of the bottom MgZnO layer was 200 nm for all experiments, and the top layer was fixed at 100 nm. The film 359 Mat. Res. Soc. Symp. Proc. Vol. 623 ©2000 Materials Research Society
thickness was controlled by varying the deposition time calibrated from the average growth rate per pulse of the single film layer on sapphire. Photoluminescence (PL) measurements were performed with both CW and pulsed lasers at room temperature and at liquid nitrogen temperature (77 K). A He-Cd laser (325 nm) laser and N-gas (337 nm) were used as the CW and pulsed excitation source, respectively. The output signal was collected from the edge-emitting configuration. RESULTS AND DISCUSSION The 77 K CW PL spectra of the SQW samples with 10, 20 and 40 nim quantum well thickness are shown in Fig 1. For reference sample, PL from 500-nm thick ZnO film on sapphire is also shown in Fig. 1. This reference sample has a peak position at 369 nm corresponding to 3.36 eV. This emission peak is attributed to radiative recombination of bound excitons. The spectrum also shows the two-shoulder peak at a higher wavelength position that is attributed to donor-acceptor pair transitions at 374 nim with phonon replicas at 384 nm. The peak position at around 340 nm represents the emission from MgZnO barrier layer. The variation of the peak position from 339 to 342 nm could be caused by variation of Mg content in the layer due to small interdiffusion during the growth process. The more the Mg content the lower the wavelength emission (higher energy). The weak peaks
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