In the current paper we apply catalyst assisted vapour phase growth technique to grow ZnO nanowires (ZnO nws) on p-GaN thin film obtaining EL emission in reverse bias regime. ZnO based LED represents a promising alternative to III-nitride LEDs, as in free devices: the potential is in near-UV emission and visible emission. For ZnO, the use of nanowires ensures good crystallinity of the ZnO, and improved light extraction from the interface when the nanowires are vertically aligned. We prepared ZnO nanowires in a tubular furnace on GaN templates and characterized the p-n ZnO nws/GaN heterojunction for LED applications. SEM microscopy was used to study the growth of nanowires and device preparation. Photoluminescence (PL) and Electroluminescence (EL) spectroscopies were used to characterize the heterojunction, showing that good quality of PL emission is observed from nanowires and visible emission from the junction can be obtained from the region near ZnO contact, starting from onset bias of 6V.
© 2015 Optical Society of America
Among inorganic oxides ZnO is one of the most attractive electroluminescent materials for light source: this is due to its wide band gap (3,37 eV) and large exciton binding (60meV). ZnO attracted interest as thin films and as nanowires coupled to p-type Cu2O for application in solar cells [1,2]. Moreover, ZnO based LED represents a promising alternative to III-nitride LEDs, as In free devices: the potential is in near-UV emission and visible emission.
Possible emission wavelengths of ZnO are 3,28 eV (UV emission) due to excitonic recombination in ZnO and yellow orange emission in the visible that depends on preparation conditions [1,3–7]. Due to difficulties to p-type doping of ZnO, heterojunctions with easily p-dopable materials are preferred to homojunctions. Further, to provide an efficient holes injection in the n-ZnO regions, one has to select materials with lower (or negative) valence band offsets as compared to ZnO: the best suited material is p-type GaN .
It is well known that crystallinity and optical properties of Vapour Solid deposited ZnO nanowires on GaN is much better than thin films due to the easily accommodated lattice mismatch. In addition ZnO nanowires (ZnO nws) are appealing for LED since they show high crystallinity (low density of interface defects). The nanowires used in LED are usually considerably larger than the ZnO Bohr radius (2.34 nm) and quantum confinement effects can be neglected. Indeed nanowires growth produces grain-boundary free and a much less strained and defective interface, which are sources of non-radiative recombination, compared to thin films. In addition, nanowires can act as direct waveguides  and favour directional light extraction without use of lenses and reflectors and ZnO nanorods were employed to enhance the light extraction efficiency of a GaN LEDs .
Heterojunctions LED based on p-GaN thin film and n-ZnO nws were demonstrated in recent years: ZnO nws in LED were grown by different technique like nanoparticle assisted pulsed laser deposition , CVD [12,13] and by low temperature wet chemical techniques [14–16]. On the contrary ZnO nws grown by the vapour liquid phase technique was not much investigated to this purpose , despite their competitive low cost and very good quality of nanocrystal obtained using high-temperature vapour-liquid-solid growth techniques, and no report on the use of vapour grown ZnO nanowires in LED heterojunction operated at reverse bias was reported yet.
We have demonstrated in recent years nontoxic and relatively inexpensive routes for the fabrication of high-quality ZnO nws , and in the current paper we apply catalyst assisted vapour phase growth technique to grow ZnO nanowires on p-GaN thin film. The heterojunction between n-type ZnO nanowires and p-type GaN thin film emits light in reverse bias regime. We investigated PL emission and EL emission, observing visible EL signal, that is a function of applied voltage. The light emission mechanism is also discussed.
2.1 LED device realization
For the fabrication of n-ZnO nws /p-GaN heterojunction devices, n-ZnO nanowires were grown on 100 nm thick p-GaN on 6 micron thick n-GaN on sapphire substrate.
GaN templates were provided by OSRAM Opto Semiconductors. These consist of a magnesium doped p-GaN layer (100 nm, Mg concentration 1020 cm−3) connected to an underlying 6 μm thick n-type GaN:Si layer by a tunnel junction providing an ohmic, low resistance p-type layer . All GaN layers were grown on (0 0 0 1) oriented sapphire substrates by metal organic chemical vapour deposition (MOCVD). Lithography and ion beam etching steps were performed to expose n-GaN region for contacts. A scheme of the device is reported in Fig. 1(a).
We used a vapour phase growth technique in an evacuated tubular furnace to obtain ZnO nanowires; more details on growth technique are already reported elsewhere . A Pt catalyser deposited by RF sputtering was used to obtain dense growth on GaN layer. Since ZnO and GaN have the same wurtzite structure and low lattice mismatch, the growth parameters can be adjusted to obtain aligned nanowires oriented perpendicular to the surface. The GaN templates were kept at a temperature of 630°C during ZnO NWs growing.
After ZnO nanowires growth, Poly Methyl Methacrylate (PMMA) was spin-coated prior to the deposition of the metal contact to work as an insulating layer to prevent short circuit between the top electrode and p-GaN thin film. To remove the PMMA from the tip of the nanowires, plasma etching treatment in Ar was performed in a custom made system (Colibrì – Gambetti) with parallel plate electrodes.
2.2 Characterization of LED
SEM analysis was carried out by field-emission LEO 1525 microscope, equipped with In-Lens detector for secondary-electrons imaging. The SEM was operated at 5 kV accelerating voltage range to prevent the insulating substrate from electrostatic charging, thus allowing observation of uncoated samples.
IV curves were acquired using a Keithley 2410 sourcemeter. Continuous-wave PL and EL characterization were carried out at room temperature in a micro configuration using a He–Cd laser as light source at 325nm. PL spectra were acquired at normal incidence to the surface of the samples. In the micro configuration 40X UV and 50X Long Working Distance objective were used to focus laser and to send signal to the single monochromator and Peltier cooled CCD detector (Horiba). A wedge filter was used to get rid of Rayleigh line.
3. Results and discussion
3.1 SEM characterization of the ZnO nanowires
FE-SEM was used to monitor growth in different conditions. By changing the growth parameters, it is possible to produce nws that are growing almost in upward direction, with a tilted angle with respect to the normal to the surface. Figure 1(b) reports the SEM top view of ZnO nws at 50KX magnification. The growth of nws starts with a basal pedestal of ZnO (dimension in the range 100-200 nm) from which nanowire protrudes: the nanowires are growing mostly upward with a bending angle with respect to the vertical. The diameter of the nanowire at the tip ranges from about 30 to 50 nm. Most of the nws have hexagonal shape. Figure 1(c) shows the nws before the deposition of the Au contact: they were partially covered by PMMA layer and the exposed tips protruding from PMMA appear brighter than the insulating layer. Thus electrical contacts - that were deposited afterwards - make electrical connection only with ZnO and do not short circuit GaN layer underneath.
3.2 IV tests
IV tests on the junction acquired in the range (−10V; + 10V) with 0.1 ms delay time are shown in Fig. 2(a). Poor rectifying behavior observed could be due to states at the interface between GaN and ZnO, due to growth at high temperature. The inset shows the IV characteristic acquired at ZnO-ZnO contacts and GaN/GaN contacts. While an ohmic behavior is observed for contacts on GaN, a non-ohmic behavior can be observed on the ZnO side, ascribable to the onset of space-charge limited current in nws .
3.3 EL and PL spectra
To collect EL spectra, the device was located under the microscope, and the contacts were connected by means of angled metal tips. A 50x long working distance objective was needed to focus on the device, due to angled metal tips.
EL spectra of n-ZnO nanowires/p-GaN heterojunctions were measured at forward bias and no light emission was observed in the measured spectral range up to 15 V. In reverse bias voltages the device showed EL emission starting from 6V bias [Fig. 2(b)] up to 15 V. The EL spectrum at an applied reverse-bias voltage of 6V showed a yellow emission band with emission peaks at 1,7 eV, 1,9 eV and 2,1 eV. At 10 V the spectrum can be fitted with peaks at 2,2 eV, 2,4 eV and 2,9 eV [Fig. 2(c)]. By increasing the voltage up to 15 V the intensity of these peaks increases with no peak shift. Figure 2(d) displays the band diagram of the heterojunction at zero bias. The mechanism has been ascribed to easy tunnelling of electrons - even at small reverse bias - from the GaN valence band to the ZnO surface conducting band [13,22]. By increasing the bias voltage, higher energy peaks appears. Figure 2(e) shows the CIE 1931 color coordinate calculated from the measured EL spectra in Fig. 2(b) for the hetero-junction LED under different reverse bias ((8 V, 10 V, 12 V, and 14 V). The chromaticity coordinates are (0.35, 0.5), (0.3, 0.48), (0.28,0.45), and (0.31, 0.46) for the LED operated at the reverse bias voltage of 8 V, 10 V, 12 V, and 14 V, respectively. It can be seen that the output color of the LED moved from green towards blue region for bias up to 12V and then comes back into the green region for higher bias. As the lighting images illustrated in the inset of Fig. 2(e) for the reverse bias 12 V, the EL emission can be clearly seen by the naked eyes.
The origins of the EL emission peaks were investigated by comparing the EL and PL spectra of n-ZnO/p-GaN nanowires heterostructure.
PL emission was investigated using the same 50x long working distance objective used for EL acquisition and using 40x UV objective for comparison. The height of all peaks in the UV region is altered by the transmission curve of the 50X objective, which is only 25% in this region.
Figure 2(e) reports the PL spectra of ZnO nanowires and GaN p and n regions. PL spectrum of ZnO nanowires (green line) is dominated by NBE emission of ZnO in the UV at 3,28 eV; very small emission from defects in the green region can be observed at 2,4 eV. The origin of green emission in ZnO nws is still debated: the defect involved could be a single ionized oxygen vacancy , an antisite oxygen , donor–acceptor complexes , or interstitial Zn . Defect emission contribution from underlying GaN can be observed at 2,21 eV (yellow emission). For n-GaN (black line) we observed the free exciton recombination emission at 3,39 eV and the yellow luminescence at 2.21 eV. The yellow band is typically assigned to a transition from a silicon donor to a deep acceptor . PL spectrum of p-GaN shows the emission at 3,39 eV in the UV region and an emission in the blue at 2,8 eV, related to Mg doping of GaN .
The comparison with PL data, strongly suggest that the yellow and green emission in the EL spectra originate from the deep defects level in the ZnO and GaN cited before in the PL section. Contribution from blue luminescence from GaN is present starting from 8V. Despite the high crystal quality of the ZnO nws, no emission in the UV energy range is observed up to the maximum tested bias (15 V). This might be due to inter-diffusion effects at the ZnO/GaN interface, due to the high growth temperature of the ZnO nws. Diffusion of ZnO into GaN have been measured by XPS at the interface between ZnO nws and GaN in a different device (gold catalyzed ZnO nws) , and Zn diffusion at the interface between Ga- doped ZnO and GaN has been reported in literature at high temperatures (800°C) .
Light emission only under reverse bias configuration indicates that EL mechanism is different from that of conventional forward – biased p-n junction light emitting diodes reported in literature . The mechanism proposed by Park et Yi  is that in reverse bias carrier transport is explained in terms of band alignment of p-GaN/n-ZnO heterojunction at the interface. In the formed heterojunction the thickness of the tunnelling barrier is very thin. With a small reverse bias voltage the unoccupied conduction band minimum of n-ZnO would be lower than the occupied valence band maximum of p-GaN. Hence carrier transport by tunnelling can occur even at small reverse bias voltages and the probability increases with reverse bias voltage.
In summary, we have realized ZnO nws/GaN heterojunction by directly growing ZnO nanowires on top of GaN templates using an evaporation-condensation technique. We observed that nanowires can be obtained with a tapered shape and growing almost in upward direction. After device preparation, ZnO nws emit an intense PL signal at room temperature composed by NBE emission in the UV and small contribution from defect emission. IV characterization of the heterojunction showed non optimal rectifying behavior, that we can ascribe to defect states at the interface of GaN and ZnO. Further investigation is on-going to investigate them. Despite this, EL emission in the visible range was observed when the junction is polarized in reverse bias, starting from onset voltage of 6V. Green yellow EL can be due to defect emission from ZnO at 2.4 eV and yellow emission can be due to contribution of GaN at 2.21eV. Blue electroluminescence observed starting from 8 V is due to p-GaN emission. The obtained results are promising in view of the realization of a LED ZnO nws/GaN using the evaporation condensation technique to grow nws.
We would like to thank the group of late Prof. B K Meyer from University of Giessen for lithography and ion beam etching and OSRAM Opto Semiconductors for GaN substrates. The research leading to these results has received funding from the European Communities 7th Framework Programme under grant agreement NMP3-LA-2010-246334. The financial support of the European Commission is therefore gratefully acknowledged.
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