Light emitting diodes based on ZnO nanowires

Abstract

The fabrication and characterization of light emitting diodes (LEDs) based on n-ZnO nanowires/p-GaN thin film heterojunction were studied. Closely packed and vertically aligned n-ZnO nanowire array was first grown on Si substrates by a hydrothermal method and then optimized on the p-GaN substrates. The average diameter and length of the n-ZnO nanowires were around 95 nm and 520 nm, respectively. The perfect wurtzite structure of n-ZnO nanowires was verified by the dominant E2 high mode (439 cm-1) in the Raman spectrum of the n-ZnO nanowires. The sharp (002) diffraction peak with a small full width at half maximum (FWHM) of 0.3° in X-ray Diffraction pattern shows the good crystallinity and [001] preferred growing direction of the n-ZnO nanowires. LEDs based on n-ZnO nanowires/p-GaN thin film heterojunctions were successfully fabricated by a hydrothermal method. Arrays of n-ZnO nanowires were grown on two commercially available p-GaN substrates (S1 and S2). According to the results of Four-point Hall Measurement, the carrier density of substrate S1 (2.00 × 1017 cm-3) is double of that of S2. Substrate S2 on the other hand has higher carrier mobility (176 cm2/Vs) than S1 (33 cm2/Vs). Both assembled LEDs exhibit diode-like rectifying behavior in current-voltage (I-V) measurements. Blue emission (440 nm) is dominated in the assembled LED based on substrate S1, while weak UV (370 nm) and green (550 nm) emissions are also detected. The blue emission is visible even at a forward bias as small as 7 V. Interference fringes found in the Electroluminescence emission of the assembled LED based on substrate S2 indicate that those well-faceted n-ZnO nanowires act as waveguiding cavity in the photon emission.