Focused Ion Beam Etching of Nanometer-Size GaN/AlGaN Device Structures and their Optical Characterization by Micro-Photo
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ABSTRACT We report on the nano-fabrication of GaN/AlGaN device structures using focused ion beam (FIB) etching, illustrated on a GaN/AlGaN heterostructure field effect transistor (HFET). Pillars as small as 20nm to 300nm in diameter were fabricated from the GaN/AlGaN HFET. Micro-photoluminescence and UV micro-Raman maps were recorded from the FIB-etched pattern to assess its material quality. Photoluminescence was detected from 300nm-size GaN/AlGaN HFET pillars, i.e., from the AlGaN as well as the GaN layers in the device structure, despite the induced etch damage. Properties of the GaN and the AlGaN layers in the FIB-etched areas were mapped using UV Micro-Raman spectroscopy. Damage introduced by FIB-etching was assessed. The fabricated nanometer-size GaN/AlGaN structures were found to be of good quality. The results demonstrate the potential of FIB-etching for the nano-fabrication of III-V nitride devices.
INTRODUCTION Reactive ion etching [1] and wet etching techniques [2] have mostly been employed for the fabrication of III-V nitride devices with their wide spectrum of applications ranging from short-wavelength light emitters, solar-blind detectors to hightemperature devices [3,4]; these, however, are not the preferred techniques for the nanofabrication of III-V nitrides. Focused ion beam (FIB) etching is one of the most promising techniques for the fine patterning of III-V nitrides, however, only basic etching parameters have been investigated so far [5,6,7]. Alternatively, small nitride structures can be fabricated by selective area growth [8]. The direct write facility of FIB-etching – a well-established technique for optical mask repair and for IC failure analysis and repair – allows the nanometer-scale fabrication of III-V nitride devices without the requirement for depositing an etch mask. A focused gallium ion beam of 5-20nm size is used in FIB to ablate the III-V nitride material. In this paper, we report on the nano-fabrication of a GaN/AlGaN heterostructure field effect transistor (HFET) into pillars as small as 20300nm in diameter by FIB etching. The use of micro-photoluminescence and ultraviolet (UV) micro-Raman mapping to assess 300nm-size FIB-etched HFET structures is demonstrated. UV Raman scattering probes the vibrational states of the GaN and the AlGaN in the sample and provides information on the aluminum composition, the strain and the free carrier concentration in the fabricated structures [9-11]. Etch damage can easily be assessed from the photoluminescence and the Raman scattering intensity in the FIB-etched areas.
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EXPERIMENT The GaN/AlGaN heterostructure field effect transistor (HFET) employed in this study was grown by molecular beam epitaxy (MBE) on a sapphire (0001) substrate, and FRQVLVWHGRIDQPWKLFNORZWHPSHUDWXUHJURZQ*D1EXIIHUOD\HUD
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nominally undoped GaN layer, followed by a 3nm-thick nominally undoped Al0.15Ga0.85N, a 22nm-thick mid-1018cm-3 Si-doped Al0.15Ga0.85N, and a 15nm-thick nominally undoped Al0.15Ga0.85N layer. The piezo-electric fie
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