Emission at 247 nm from GaN quantum wells grown by MOCVD
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INTRODUCTION Blue light emitting diodes (LEDs) and blue laser diodes (LDs) have been developed recently using InGaN quantum wells (QWs) as the active layers and are being used in full-color display and high-density optical storage [1,2]. GaN QWs are also of great interest for the development of new UV light emitters and for fundamental research into strong quantum confinement systems, since the huge band offsets between GaN and Al(Ga)N are favorable for increasing quantum confinement energy or reducing emission wavelength. Large quantum confinement requires epitaxial growth of AlxGa1-xN barrier layers with high aluminum contents x, on which very thin GaN well layers have to be formed with precise control of layer thickness and surface smoothness. Although photoluminescence (PL) spectra from thin GaN QWs have previously been reported, peak wavelengths were not very much lower than the bulk value. This is because the aluminum contents x in AlxGa1-xN barrier layers were around 10-20 % and, therefore, barrier heights or band offsets were small [3-13]. Recently, Hirayama and Aoyagi have reduced the PL wavelength down to 280 nm by using Al0.11Ga0.89N for QW layers [14,15]. However, to realize stronger quantum confinement and shorter emission wavelengths, further development of the epitaxial growth of nitride semiconductors is very important. In this work, we measured PL spectra at room temperature for GaN QWs with Al0.8Ga0.2N barriers, which were grown by atmospheric-pressure metal organic chemical vapor deposition (MOCVD). The thickness of the GaN QW layers was systematically varied from one monolayer (ML) to four MLs. We have achieved PL emission at a wavelength as short as 247 nm (5.03 eV) in 1ML QWs. The effective confinement energy, or difference between this recombination energy and the band gap of bulk GaN, is as large as 1.63 eV. Such strong confinement was achieved by reducing the well thickness down to 1 ML and increasing the aluminum content x in
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the AlxGa1-xN barrier layers up to 0.8. SAMPLE PREPARATION
Normarized PL intensity (arb. units)
We grew four samples of GaN multiple QWs (MQWs) containing well layers of different thicknesses, viz., 1, 2, 3, and 4 MLs, in an atmospheric-pressure two-flow MOCVD system with a horizontal quartz reactor. After growing a 30-nm-thick GaN nucleation layer at 480 ºC on a (0001)-oriented sapphire substrate, a 1.5-µm-thick GaN buffer layer was deposited at 1071 ºC. During the growth of the GaN buffer layer, the flow rate of TMG was 88 µmol/min with carrier gases of H2 at 4 l/min and N2 at 12 l/min. NH3 was used as the group V source with a flow rate of 4 l/min, which corresponds to a V/III ratio of about 2000. Then, the substrate was heated up to 1092 ºC to grow MQW layers. The thickness of Al0.8Ga0.2N barrier layers was 3 nm, while the thicknesses of GaN well layers were from 1 ML to 4 MLs. The layer
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Figure 1 Photoluminescence spectra for GaN quantum wells, whose thicknesses are from one monola
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