Solid-State Electronics 82 (2013) 99–102
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Optical and electrical properties of hydrothermally grown Al-doped ZnO nanorods on graphene/Ni/Si substrate L.L. Wang a,b, B.Z. Lin a,b, M.P. Hung b, L. Zhou a,b, G.N. Panin b,c, T.W. Kang b, D.J. Fu a,⇑ a
School of Physics and Technology and Key Laboratory of Artiﬁcial Nano- and Micro-materials of Ministry of Education, Wuhan University, 430072 Wuhan, China QSRC, Dongguk University, 3-26 Pildong, Junggu, 100-715 Seoul, Republic of Korea c Institute for Microelectronics Technology, 142432 Chernogolovka, Russia b
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 9 October 2012 Accepted 9 January 2013 The review of this paper was arranged by Dr. Y. Kuk Keywords: Zinc oxide Al Resistivity Photoluminescence
a b s t r a c t We present a simple way to prepare low-resistance ZnO nanorods by hydrothermal self-assembled growth at 95 °C and in situ doped with Al. The NRs were grown on graphene/Ni/Si and annealed at 400 °C. Few layer graphene was used to assist aligned growth of the NRs and acted as an electrode during electric measurement. The measurement showed resistance of the Al-doped ZnO NRs 100 times lower than that of undoped ZnO NRs. Photoluminescence measurement showed enhanced deep level emission for the Al-doped NRs and low temperature photoluminescence study showed coexistence of acceptor bound-exciton (3.353 eV) and donor bound-exciton (3.362 eV). Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide as a II–VI compound semiconductor has attracted considerable attention due to its application in ﬁeld emission displays [1,2], piezoelectric transducers , gas sensing devices , as transparent conductive oxide in solar cells and short wavelength optoelectronic devices [5,6], owing to its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature. However, there are three main disadvantages of ZnO nanorods (NRs) for ﬁeld emission displays: (1) high resistivity of the NRs, (2) high contact resistance between the NRs and substrates, and (3) screen effect of NRs at high densities. This work concentrated on reducing of the resistivity of the ZnO NRs which were grown by hydrothermal method. Doping is usually used to increase conductance of the nanorods; sometimes, annealing is also effective because it can change the density of zinc vacancy and oxygen vacancy of ZnO NRs. Nowadays, efforts have been made to improve the properties of ZnO nanostructures by doping with various elements such as Ga, In, Sn, Mn, Mg, Bi, and Al into ZnO. Among them, Al-doped ZnO nanowires are capable of reaching very high conductivity without deterioration in optical transmission and crystallinity , and thus have been regarded as a poten-
⇑ Corresponding author. Fax: +86 27 6875 3587. E-mail address: [email protected]
(D.J. Fu). 0038-1101/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2013.01.012
tial alternative to the currently used transparent conductive material . The Al-doped ZnO was studied by many groups. Zhang et al.  found that hydrothermal process temperature has a signiﬁcant effect on the growth rate and photoluminescence of ZnO nanoarrays. Wang et al.  reported that the heating temperature plays a key role on morphologies of the Al–ZnO hierarchical nanostructures through temperature dependent migration of Al atoms. Yun et al.  shown that the morphology, density, and surface composition of ZnO nanorod arrays are sensitive to the concentration and variety of zinc and aluminum precursors used in sample preparation. ZnO nanorods have already been synthesized by many methods such as chemical vapor deposition and hydrate vapor deposition which usually need high temperature for growth (500–800 °C). Hydrothermal growth used in this work is of low cost and works at low temperatures (60–120 °C), so some very cheap and ﬂexible substrates such as glass and polymer can be used .There is no oxidation of the substrate during the hydrothermal growth, unlike at high temperature CVD growth. In addition, doping is possible and easy by directly adding the required impurity into the reaction solution. Aluminum salt Al(NO3)39H2O, AlCl36H2O or Al2(SO4)3 xH2O were used as the doping precursor by many groups [11– 16]. In this work, we have prepared Al-doped ZnO nanorods by hydrothermal method using aluminum diacetate hydroxide as the doping precursor. Photoluminescence and current–voltage characteristics are studied.
L.L. Wang et al. / Solid-State Electronics 82 (2013) 99–102
2. Experimental Aligned ZnO nanorod arrays were grown using a simple twostep process: synthesis of nanoparticle (NP) seedlayers, followed by synthesis of ZnO NRs using hydrothermal growth. Silicon and graphene/Ni/Si substrates were cleaned with the following procedure: methanol, ethanol and deionized water for 5 min each at 60 °C, and drying with nitrogen gas. The seedlayer solution was obtained by dissolving 35 mmol zinc acetate dehydrate (Zn(CH3COO)22H2O) in 10 mL deionized water, stirring 4 h at 60 °C and then putting the solution in the oven statically for 20 h at 60 °C. ZnO seed NPs were synthesized using spin coating, in which one drop of the above solution was taken onto the cleaned substrate at a low speed 700 rpm during 10 s and then changed to high speed 3000 rpm for 30 s. When stopped, the substrates were put on a heater to ensure good particle adhesion to the substrates for 10 min at 170 °C. This coating step was repeated three times to obtain uniform ﬁlm of zinc acetate crystallites. Finally the substrates were heated to 350 °C in air for 20 min to yield layers of ZnO islands with their (0 0 0 1) planes parallel to the substrate surface [17–19]. The ZnO nanorods were synthesized in a hydrothermal reactor which contained 50 mM of zinc nitrate hexahydrate (Zn(NO3)26H2O), 50 mM of Hexamethylenetetramine ((CH2)6N4)  by keeping the temperature at 95 °C for 4 h. The formation process of the hexagonal ZnO structure using the hydrothermal method can be expressed as follows :
ðCH2 Þ6 N4 þ 6H2 O ! 6HCHO þ 4NH3
NH3 þ H2 O ! OH þ NH4þ
2OH þ Zn2þ ! ZnO þ H2 O:
The Al doped ZnO nanorods were synthesized by putting 10 mM of the impurity aluminum diacetate hydroxide  into the above solution. The reactor was quickly cooled to room temperature by cooling water and then the samples were washed several times by deionized water when ﬁnished. The few layer graphene on Ni/ Si substrate was used to ensure the NRs grow vertically with high density and as electrode during electric measurement. In Al–ZnO NRs, Zn2+ is substituted by Al3+. There are three electrons in the outermost shell of an Al atom, two of them form bond with O, and the third one is free. The lower resistivity of the Al–ZnO NRs compared with the as-grown ZnO NRs is duo to this free electron of the Al atom. The morphology of the Al-doped ZnO NRs are analyzed by a ﬁeld emission scanning electron microscope (FE-SEM) using a Phillips XL-30 with Mono CL system. The crystal quality and defect structure are analyzed by cathodoluminescence (CL) and photoluminescence (PL). The current vs voltage measurement using a Keithley 4200-SCS analyzer of the NRs gave the resistivity of the samples. 3. Results and discussion Fig. 1 shows a Raman spectrum of the few layer graphene grown on Ni/Si substrate. Distinct G and 2D peaks are observed at 1600 cm1 and 2700 cm1, respectively, with large peak intensity ratio of 2D to G, I2D/IG, which indicates the presence of monolayer or bilayer graphene. The small D-peak at 1330 cm1 indicates that the graphene used here contains small amount of defects . The few layer graphene was obtained from spin coating followed by annealing at 950 °C in an ambient of N2 and H2, the details were described elsewhere . A layer of Ni deposited by electron beam evaporation on Si was used as catalyst for graphene growth.
Raman shift (cm ) Fig. 1. Raman spectrum of graphene on Ni/Si substrate.
Fig. 2 shows the SEM images of the grown ZnO NRs on different substrates. The SEM image of the as grown ZnO NRs on Si substrate is shown in Fig. 2a, which displays a low density and randomly arranged NRs. Vertically aligned ZnO NRs image with high density grown on graphene/Ni/Si substrate is shown in Fig. 2b. This is due to the graphene layer have the same hexagonal structure with the ZnO NRs, so the ZnO nanoparticles were easily attached to the graphene uniformly. Therefore, the Al-doped ZnO NRs were grown on graphene/Ni/Si substrate. The Al–ZnO NRs are 2 lm in length and 100 nm in diameter obtained from SEM images (not shown here). Fig. 2c shows the SEM image of Al-doped ZnO NRs before annealing. Many wrinkles on the surfaces of the NRs are clearly revealed. This indicates that Al doping changes the hexagonal structure and degrades the morphology of ZnO NRs. Fig. 2d shows the SEM image of the Al-doped ZnO NRs after annealing for 30 min in vacuum at 400 °C. The surface of the NRs has been improved after annealing and some caps on the top of the NRs were formed, as a result of Al+3 out-diffusion from Al-doped ZnO NRs during annealing. CL spectra of Al-doped ZnO NRs before and after annealing are shown in Fig. 3. The strong UV peak intensity of Al-doped ZnO NRs at the wavelength of 380 nm attributed to band edge emission after annealing compared with the peak before annealing indicates improved structure of ZnO NRs, and this is conformable with the SEM images shown in Fig. 2c and d. This is because some atoms may not occupy their proper lattice sites during hydrothermal growth process at temperatures below 95 °C, and they move to the lattice position after annealing at 400 °C. The ratio of UV/yellow intensity decreases from 9.87 (before annealing) to 5.03 (after annealing), which indicates that many deep level defects are introduced by annealing. It is well known that a visible emission of ZnO can be green or yellow, depending on the oxygen states. The green emission, referred to as a deep-level emission, originates from the recombination of the holes with electrons occupying the singly ionized O vacancy. The yellow emission results from the interstitial oxygen ions . The interstitial oxygen ions can ﬁnd their lattice positions, and some lattice oxygen can also move to interstitial sites. The enhanced broad visible emission after annealing is attributed to combination of two oxygen states. The monochromatic CL image acquired at k = 380 nm of Al doped ZnO NRs after annealing is shown in Fig. 4a. Some gray parts and dark parts are displayed. The gray parts correspond to the NR bodies and the dark parts correspond to the Al cap on the tops of the NRs. Fig. 4b shows the position dependent CL spectra. The ratio (3.3) of UV/yellow on the top part is much smaller than the ratio (5.5) on the center part, reﬂecting clearer Al doping effect on the top of the Al-doped ZnO NRs after annealing.
L.L. Wang et al. / Solid-State Electronics 82 (2013) 99–102
Fig. 2. SEM images of ZnO NRs: (a) Si substrate, (b) graphene/Ni/Si substrate, (c and d) Al-doped ZnO NRs on graphene/Ni/Si before and after 400 °C.
CL intensity (arb.unit)
64000 56000 Before annealing After annealing
48000 40000 32000 24000 16000 8000 350
Wavelength (nm) Fig. 3. CL spectra of Al doped ZnO NRs before and after annealing.
Intensity (arb. unit)
The PL spectra of ZnO and Al-doped ZnO NRs at 10 K are shown in Fig. 5. Only one peak locates at 3.371 eV of the as grown ZnO NRs appeared which can be contributed to the donor bound-exciton [24,25] as seen at 10 K, and its narrow and sharp peak indicates good crystal quality. Acceptor bound-exciton and donor boundexciton peaks [24–26] of Al-doped ZnO NRs coexist which locate at 3.353 and 3.362 eV respectively, indicating that acceptors have been introduced during Al doping. The peaks located at 3.229 and 3.072 eV are contributed, respectively, to donor–accepterpairs (DAP) and the second longitudinal–optical (2LO) of the DAP in the Al-doped ZnO NRs . The above phenomena indicate that Al has incorporated into ZnO NRs. The I–V curves of Al-doped ZnO NRs are shown in Fig. 6. The resistance calculated from the curves is 0.98 104 X, 1.25 105 X, and 8.33 105 X for the annealed Al–ZnO NRs, asgrown Al-doped ZnO NRs, and as-grown ZnO NRs respectively. The resistance of Al-doped ZnO NRs decreases about 10 times after annealing compared with before annealing NRs and 100 times compared with the as grown ZnO NRs. The schematic diagram of
large area center of the selected nanorod top of the nanorod
(b) Fig. 4. CL image (a) and spatial resolved CL spectra (b) of Al-doped ZnO NRs.
ZnO NRs resistivity measurements is shown in the insert. The graphene ﬁlm on Si substrate is used as one electrode, while in-
L.L. Wang et al. / Solid-State Electronics 82 (2013) 99–102
D X (3.362 eV)
3.353 eV 0 AX
DX (3.371 eV)
enhanced deep level emission from the Al-doped ZnO NRs after annealing at 400 °C was observed. The lower resistance is most likely caused by Al doping, through the effect of self-compensation of donors and acceptors, which was conﬁrmed by observation of donor bound-exciton and acceptor bound-exciton on the low-temperature PL spectrum.
DAP 3.229 eV
This work was supported by the International Cooperation Program of Ministry of Science and Technology of China under 2011DFR50580 and Leading Foreign Research Institute Recruitment Program through National Research Foundation of Korea funded by Ministry of Education, Science and Technology (MEST) (No. 2011-00125).
2LO (DAP) 3.072 eV
Photon energy (eV) Fig. 5. PL spectra of the ZnO NRs and Al-doped ZnO NRs at 10 K.
Al-doped NRs before annealing
Al-doped NRs after annealing
Voltage (V) Fig. 6. I–V curves of Al doped ZnO NRs before and after annealing. The inset is a schematic diagram of measurement conﬁguration.
dium on top of the NRs was used as another electrode. The voltage mainly drop on the ZnO NRs, because the resistivity of graphene and indium is far less than that of ZnO NRs (RG+In << RZnO). 4. Conclusion Al-doped ZnO nanorods were prepared by using the simple hydrothermal method and by directly adding the impurity material aluminum salt into the reaction solution at a low temperature of 95 °C. The lower resistance of the annealed Al-doped ZnO NRs is 100 times lower than that of the ZnO NRs without Al doping, and
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