Growth, modulation and photoresponse characteristics of vertically aligned ZnO nanowires

Background

ZnO nanowires are studied for sensors and optoelectronic devices because of the wide bandgap (3.4 eV) and large exciton binding energy (60 meV) of ZnO, and device performance depends on the quality, aspect ratio, alignment and density of the nanowires. The substrates with the smallest lattice misfit for ZnO, ScAlMgO4 and sapphire, are expensive and incompatible with Si-based device processing, whereas the large lattice misfit on Si degrades nanowire quality. Patterning nanowires grown by MOCVD is also difficult, because conventional sacrificial layers have low temperature stability or require hazardous etchants.

Design and approach

Three substrate modifications were compared under the same MOCVD growth conditions. A 500 nm aluminum nitride (AlN) film deposited by RF reactive magnetron sputtering served as an intermediate layer. A thermally evaporated barium fluoride (BaF2) film, chosen for its high temperature stability and solubility in deionized (DI) water, was patterned by lift-off to a thickness of 850 nm; nanowires were grown over it at 630 °C, and dissolving the BaF2 in DI water removed the unwanted nanowires with it. In the third route, V-shaped microcavities were etched into Si with potassium hydroxide (KOH) at 80 °C through a 200 nm silicon nitride mask, so that nanowires inside and outside a cavity were obtained in a single growth without any interfacial or sacrificial layer.

How it was evaluated

Films and nanowires were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). UV detectors with sputtered Au/Ti electrodes (inter-electrode spacing 1 mm, photosensitive area 0.3 mm²) were fabricated on nanowires grown outside and inside the microcavity, and their time-resolved current was measured at 3 V under 352 nm UV exposure.

Key results

The sputtered AlN film showed an XRD peak around 36.1°, indicating (0002) orientation, an FTIR absorption peak at 682 cm⁻¹ from Al–N bonds, an average grain size of about 50 nm and a root-mean-square (RMS) roughness of 2 nm. Nanowires grown on AlN/Si were 3–4 μm long and 80 nm wide and had a higher aspect ratio than nanowires grown under the same conditions on bare Si, which the authors attribute to the lower lattice mismatch and the compressive stress in the AlN film.

The BaF2 film was polycrystalline with an RMS roughness of 21 nm and dissolved in DI water at about 50 nm/min without evolving bubbles. Removing the patterned BaF2 after growth left circular patterns 3 μm in diameter and square patterns in the nanowire arrays.

KOH etching produced V-shaped trenches with sidewalls at 54.7° to the surface. Outside the cavity, the nanowires had an aspect ratio of about 10 and a number density of about 10⁷ per mm²; on the inclined surfaces inside the cavity, the aspect ratio was 20 and the density 5 × 10⁷ per mm². The nanowires inside the cavity were narrower, which the authors relate to stagnant gaseous species and the lack of fresh precursor inside the cavity. XRD and HRTEM indicated single-crystalline growth along the c-axis, with an interplanar spacing of 0.52 nm and no noticeable defects.

The photo/dark current ratio was 1.8 for detectors outside the microcavity and 6 for detectors inside it, and the decay time after the UV light was turned off was about 100 s and about 5 s, respectively; similar behavior was observed over several on–off cycles. The authors attribute the higher ratio to the higher surface-to-volume ratio of the nanowires in the cavity and the faster decay to rapid recombination associated with oxygen adsorption on the nanowire surfaces, which forms a surface depletion layer.

Open questions

The authors note that the AlN and BaF2 routes require more complicated processing than the microcavity route, and suggest that the photoresponse could be improved further by functionalizing the nanowire surface and increasing its surface-to-volume ratio.

How it connects to other research

An earlier paper by J.P. Kar, J.M. Myoung and co-workers fabricated ZnO nanowire UV detectors using a silicon microchannel and, according to its abstract, already compared nanowires on the top surface and towards the bottom of the cavity, with the same aspect ratios (about 10 and 20) and number densities (about 10⁷ and 5 × 10⁷ per mm²) reported here and a better UV response from the bottom nanowires; the present study reports this microcavity comparison together with AlN and BaF2 layers as separate controls of nanowire growth and patterning.