Large and Stable Field-Emission Current from Heavily Si-Doped AlN Grown by MOVPE
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Large and Stable Field-Emission Current from Heavily Si-Doped AlN Grown by MOVPE Makoto Kasu and Naoki Kobayashi NTT Basic Research Laboratories 3-1 Morinosato-Wakamiya, Atsugi, 243-0198, Japan ABSTRACT We investigated electron field emission (FE) from heavily Si-doped AlN grown by metalorganic vapor phase epitaxy. We found that, as the Si-dopant density increases, the threshold electric field decreases, which indicates that electrons are supplied to the surface effectively as a result of Si doping. We show that heavily Si-doped AlN has a maximum FE current of 347 µA (the maximum current density of 11 mA/cm2), stable FE current (fluctuation: 3%), and a threshold electric field of 34 V/µm. We observed visible light emission (luminance: about 1200 cd/m2) from phosphors excited by the field-emitted electrons. INTRODUCTION The interest in electron field emitters using wide-bandgap semiconductors such as AlN has been increasing because the electron affinity of these materials is expected to be very small [1-3]. In materials with small electron affinity, electrons can be easily extracted from the surface to the vacuum. The consequent large electron field emission (FE) will lead to very thin flat panel displays and microwave power amplifier tubes. There have been few studies on AlxGa1-xN (x=0.1 in ref. 4 and 0.05 ≤ x ≤ 0.9 in ref. 5) and AlN [5]. However, the reported FE current from AlxGa1-xN and AlN was as low as 0.01 to 0.12 µA [4, 5]. This is probably because the AlxGa1-xN and AlN samples in those studies were not intentionally doped, and, as a result, few electrons were supplied from the substrate to the surface. Therefore, we investigated the effect of Si doping on the electron field emission from AlN. We found that, as the Si-dopant density in AlN increases, the threshold electric field decreases. We show that heavily Si-doped AlN has a larger maximum FE current than that previously reported for AlN [5], a lower threshold electric field, and an extremely stable emission current. We observed visible light emission from phosphors excited by the field-emitted electrons. RESULTS AND DISCUSSION Heavily Si-doped 0.2-µm-thick AlN was grown at 1100oC by low-pressure (300 Torr) R5.12.1
MOVPE. We chose an n-type (1 x 1018 cm-3) 6H-SiC (0001) substrate because it has high electric conductivity and because SiC and AlN have almost the same lattice constants (mismatch: 1%) and thermal expansions along the a axis. The sources were trimethylaluminum (TMA), trimethylgallium (TMG), and ammonia (NH3). The Si-dopant gas was silane (SiH4), the gas most widely used in GaN MOVPE growth. The Si dopant density was measured using secondary ion mass spectrography. In GaN, Si atoms form a shallow donor level. It has been reported that, in Si-doped AlxGa1-xN (0 ≤ x ≤ 0.33), the resistivity increases exponentially as the Al content x increases [6]. The resistivity of the AlN with the highest Si dopant density (2.5x1020 cm-3) was so high that the free electron density could not be measured at room temperature (RT) by Hall measurement. The full w
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