Visible Light-Emitting Diodes Grown by Plasma Assisted Molecular Beam Epitaxy on Hydride Vapor-Phase Epitaxy GaN Templat
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Visible Light-Emitting Diodes Grown by Plasma Assisted Molecular Beam Epitaxy on Hydride Vapor-Phase Epitaxy GaN Templates and the Development of Dichromatic (Phosphorless) White LEDs Jasper S. Cabalu, Adrian D. Williams, Tai-Chou P. Chen, Ryan France, and Theodore D. Moustakas Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215 ABSTRACT Much of the work on III-Nitride-based LEDs that has been published and applied commercially has been done using metal-organic chemical vapor deposition (MOCVD) as a method of film growth. We report on the growth and fabrication of visible light emitting diodes, by combining hydride vapor-phase epitaxy (HVPE) and rf plasma-assisted MBE (PAMBE) methods. Thick (~7 µm to 10 µm) HVPE n+-GaN smooth and textured templates, were used as substrates for the growth of LED structures by rf-PAMBE. The active regions of the LED structures, which consist of InGaN/GaN MQWs, were grown using the pulsed nitrogen plasma technique leading to abrupt well and barrier interfaces as confirmed by x-ray diffraction (XRD) measurements. Using this method, we obtained InGaN/GaN MQWs whose room temperature photoluminescence (PL) spectra have a full width at half maximum (FWHM) of 12 nm (105 meV). Visible LEDs on smooth GaN templates emitting in the blue to green were produced with EL spectrum FWHM as narrow as 27 nm. On the other hand, white LEDs without the use of phosphor have been produced utilizing textured MQWs as the active region, a phenomenon we initially attribute to different incorporation of In on the different QW-planes. The growth and fabrication of these devices was preceded by detailed growth and doping studies of the various layers of the LED structure. These include detailed nucleation studies on (0001) sapphire substrates as well as identification of kinetic factors which lead to good crystalline-quality InGaN alloys and InGaN/GaN MQWs. INTRODUCTION LEDs, based on the III-Nitrides family of materials, have seen intense research over the past decade. LEDs emitting in the visible to the deep-ultraviolet regions of the electromagnetic spectrum have been successfully made by different growth methods, owing to the direct bandgap characteristics of GaN and its alloys with In and Al. With the successful commercialization of visible LEDs using MOCVD as the method of growth, much of the III-Nitride research community have focused on the vapor-phase growth of these devices. Although there were early efforts to produce LED structures by MBE [1], little research has been done to further optimize MBE growth methods to produce LEDs with improved optical and electrical characteristics. Recently, the III-Nitrides community has seen progress in the MBE growth of these devices with the demonstration of room temperature, pulsed operation of a violet and a blue-violet laser diode grown by ammonia-MBE [2] and rf-plama-assisted MBE (PAMBE) [3], respectively, and a submilliwatt power blue LED grown by PAMBE [4]. In this paper, we present our efforts to produce visibl
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