Growth and Characterization of Optical and Electrical Properties of ZnO Nano- and Microwires
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0957-K06-09
Growth and Characterization of Optical and Electrical Properties of ZnO Nano- and Microwires Marius Grundmann, Andreas Rahm, Thomas Nobis, Holger von Wenckstern, Christian Czekalla, Jörg Lenzner, and Michael Lorenz Institute of Experimental Physics II, University of Leipzig, Linnéstraße 5, Leipzig, 04103, Germany
ABSTRACT ZnO nanowires are readily fabricated with a novel and unique, high-pressure pulsed laser deposition process developed recently by us. It allows the flexible and reproducible control of morphology, diameter, lenght and composition. Furthermore, we report on the electrical characterization as well as stimulated emission from a zinc oxide (ZnO) microcrystals grown by carbothermal evaporation observed by spatially resolved photoluminescence (PL) and high excitation spectroscopy (HES). INTRODUCTION Currently there is an intense interest in developing small sized semiconductor devices like LEDs and lasers. The wurtzite semiconductor ZnO is a promising material for such future optoelectronic devices due to its band gap in the ultra-violet range and its large exciton binding energy of about 60 meV. In contrast to GaN, this allows the observation of excitonic effects at or even above room temperature (RT). ZnO nano- and microsized structures were synthesized by two growth methods- first by pulsed laser deposition (PLD) and second by a carbothermal evaporation process. GROWTH MECHANISM OF NANOWIRES SYNTHESIZED BY PLD Whiskers were synthesized on 10x10 mm2 a-plane sapphire substrates by a special highpressure PLD process for nanostructure growth [1] at a temperature of about 850°C. The targetto-substrate distance was 25 mm and 1,000-16,000 pulses of a KrF excimer laser were used to ablate a target with a laser energy density of about 2 J/cm2. The pulse repetition rate was 10 Hz. In this work, growth from ZnO rotating targets made from pressed and sintered 5N ZnO powder was investigated [2]. Ar was used as a carrier gas at a background pressure of 100 mbar and a constant flow rate of 100 sccm. The substrates were decorated with 10 nm colloidal gold particles (Ted Pella, Inc.) as a growth catalyst prior to growth. Typical as-grown ZnO nanowire structures are shown in the SEM images in Fig. 1 a) and b). Nanowires with hexagonal cross section are clearly resolved mimicking the wurzite crystal structure of ZnO. The prism side faces are oriented parallel to each other indicating that there is a very good in-plane epitaxial relationship. This behavior has been reported earlier for similar ZnO nanocrystals [3]. In order to observe the nanowhisker growth at different stages, a series of 5 samples with different growth
times or, correspondingly, a different number of laser pulses (1,000-16,000). Most of the whiskers are between 20 nm and 70 nm in diameter (Fig. 1c), d)) and the length increases with increasing pulse number. In later growth stages (sample with 16,000 pulses) the length and the diameter increase by similar relative amounts, resulting in a nearly constant aspect ratio. Furthermore, we foun
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