MOVPE Growth of Quaternary (Al, Ga, In)N for UV Optoelectronics
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ABSTRACT We report the growth and characterization of quaternary AlGaInN. A combination of photoluminescence (PL), high-resolution x-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) characterizations enables us to explore the contours of constant- PL peak energy and lattice parameter as functions of the quaternary compositions. The observation of room temperature PL emission at 351nm (with 20% Al and 5% In) renders initial evidence that the quaternary could be used to provide confinement for GaInN (and possibly GaN). AlGaInN/GaInN MQW heterostructures have been grown; both XRD and PL measurements suggest the possibility of incorporating this quaternary into optoelectronic devices. INTRODUCTION Development of nitride-based ultraviolet (UV) emitters has thus far been overshadowed by the intense effort in the InGaN-based visible light emitters.1 The potential usage of a compact UV light source ranges from energy-efficient indoor lighting (as a white-light source), free space satellite communication, compact chemical, environmental, and biological sensing devices, applications in medical diagnosis, treatment, and possibly optical storage. With all the rewarding promises, however, the development of UV emitters faces many challenging issues. Preliminary work in the AlGaN/GaN quantum well (QW) UV emitters2 suggested that the use of a binary GaN active region results in a low optical output power. Furthermore, mismatch-induced tensile stress was observed during growth of AlGaN heterostructures on thick GaN templates for UV devices,3 leading to relaxation through crack generation. Quaternary AlGaInN compound semiconductors are expected to enclose a finite (non-zero) area on the plot of energy gap versus lattice constant. In principle the employment of quaternary compounds should render flexibility in tailoring bandgap profile while maintaining lattice matching and structural integrity. In this paper we will summarize our investigation on the issues of optical efficiency and strain control using quaternary AlGaInN compound. EXPERIMENTAL DETAILS The MOVPE growth is carried out in a vertical rotating-disc reactor. All of the quaternary epilayers and multiple quantum well (MQW) structures were grown on 1-µm GaN epilayers at 1050°C using a standard two-step nucleation procedure (on sapphire) with low-temperature GaN grown at 550 °C.4 The growth temperature of the quaternary *
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AlGaInN was varied between 750 and 820 °C. The reactor pressure was held constant at 200 Torr. The NH3 and N2 flows were set at 6 l/min each. An additional flow of H2 (~400 cm3/min) was employed as a carrier gas. Trimethylgallium, triethylaluminum, and trimethylindium were employed as metalorganic precursors. Given the scarcity of reliable information concerning the quaternary, it is imperative to accurately determine the concentrations of the constituent elements. Concentrations of In and Al in the films were measured using Rutherford backscattering spectrometry (RBS)5 with a 2.5 MeV 4He+
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