Gas-Source Molecular Beam Epitaxy Growth and Characterization of GaNP/GaP Structures
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279 Mat. Res. Soc. Symp. Proc. Vol. 618 0 2000 Materials Research Society
EXPERIMENT The GaNP bulk layers (7500 A thick) and GaNP/GaP MQWs samples were grown on (100) GaP substrates by GSMBE. 7N elemental Ga and thermally cracked PH 3 at 980'C were used. High-purity N2 was injected through a N radical beam source (Oxford Applied Research Model MPD21) operated at 13.56 MHz to generate active N species. For the GaP buffer layers, the growth temperature was 640'C. For GaNP bulk layers and GaNP/GaP MQWs, the growth temperature was decreased to 520'C to incorporate N. The N composition was determined by high-resolution XRC measurements and theoretical dynamical simulations. XRC measurements were performed using a Phillip Xray diffractometer. Low-temperature PL measurements were carried out by mounting the samples in a liquid He cryostat and using the 514.5 nm line of an Ar÷ laser as the excitation source. A GaAs cathode photomultiplier tube was used to detect the signal at the exit of a 50-cm monochromator through an amplifier. Optical absorption measurements were performed using a broad band halogen lamp. The signal was detected at the exit of a 600 lines/mm monochromator by a Si photodiode. RESULTS AND DISCUSSIONS Fig. I shows the (400) XRCs of GaNxP,-x bulk layers (7500 A thick) with different N concentrations. With increasing N concentration, the X-ray peak of GaNP shifts away the substrate peak, indicating increased tensile strain. Due to the much larger thickness than the critical layer thickness calculated from the Matthews and Blakeslee's model (for example, 500 A for 2.3% N concentration)[7], the GaNxP1 .x sample was partially relaxed and the X-ray peak becomes broader with increasing x. Low-temperature PL measurements show that for a GaNxP1 .x sample with very low N concentration (x = 0.05%, corresponding to 1019 cm- 3), there are a series of sharp emission lines from different NNi (i • 5) pairs. With increasing N concentration up to 0.43%, the sharp emissions from NNi pairs disappear and a broad PL peak with strong intensity from the GaNP alloy appears. The PL peak red-shifts, and the intensity also increases with increasing N concentration. Fig. 2 shows room-temperature (RT) PL spectra for GaNxP1 -x (x > 0.7%) bulk layers. The PL intensity is very strong. A red-spot can be seen even with naked eyes. To our knowledge, this is the first report of red emission from GaNP alloys at RT. The PL intensity of GaNP bulk layers increases with increasing N concentration, up to 1.3%, contrary to Baillargeon et al.'s report[6]. The PL intensity increases with higher N concentration mainly due to the increased matrix element for the transition from the conduction band to the valence band[8]. With a N concentration more than 1.3%, PL intensity decreases due to decreased sample quality partly as a result of increased strain. Our results qualitatively agree with the theoretical calculation of Bellaiche et al., where they found a transition point from indirect to direct bandgap at a N concentration of 3% for GaNP[8]. Yaguchi
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