NANO LETTERS
Ultraviolet Electroluminescence from ZnO/Polymer Heterojunction Light-Emitting Diodes
2005 Vol. 5, No. 10 2005-2008
R. Ko1 nenkamp,* R. C. Word, and M. Godinez Physics Department, Portland State UniVersity, 1719 Southwest 10th AVenue, Portland, Oregon 97201 Received August 1, 2005; Revised Manuscript Received September 1, 2005
ABSTRACT We report ultraviolet electroluminescence at 390 nm from diode structures consisting of electrodeposited ZnO nanorods sandwiched between a transparent SnO2 film and a p-type conducting polymer. The nanorods are embedded in an insulating polystyrene layer. ZnO deposition occurs at 90 °C and produces vertically oriented nanorods with very high uniformity over areas of ∼20 cm2. Electron diffraction shows the nanorods to be single crystalline wurtzite ZnO. As-grown films show a broad electroluminescence band over the visible spectrum. Annealing at moderate temperatures (T ) 300 °C) increases the emission and strongly raises the excitonic contribution. Optimally processed films show a narrow ultraviolet electroluminescence line at ∼390 nm.
With its large exciton binding energy of ∼60 meV, ZnO is a promising candidate for high-stability, room-temperature luminescent and lasing devices. Recent research has demonstrated strong ultraviolet and visible photoluminescence1-4 as well as optically pumped lasing4 in ZnO nanocrystals. So far, however, p-type doping of ZnO has remained a challenge, and as a consequence, only a few LED structures could be fabrictated successfully.5 Until now, only electroluminescence in the visible region has been observed from homo-5 and heterojunction6 nanorod structures, but ultraviolet (UV) electroluminescence from nanostructures has remained out of reach. In most of the ongoing work, ZnO nanostructures are prepared in a variety of gas-phase methods. Typical deposition temperatures are 300-500 °C for CVD- and MOCVD-based techniques,7 600-900 °C for vapor-transport techniques,8 and 900 °C for thermal evaporation techniques.9 Vapor-liquid-solid2 techniques at 900 °C have also been applied. A much lower deposition temperature regime is accessible from solutions. Lincot et al.10,11 have reported epitaxial thin film and nanorod growth on sapphire and GaN grown in electrodeposition at temperatures as low as 80 °C. Nonepitaxial nanorod growth has been demonstrated by us in the same technique on a variety of polycrystalline metal and metal-oxide films.12 Here we report ultraviolet electroluminescence from ZnO nanowire heterojunctions prepared in this low-temperature regime. We show that high-quality single-crystalline nanowire crystals with surface roughnesses of ∼3 nm can be obtained at temperatures of 70-90 °C. In an LED config* Corresponding author. E-mail:
[email protected]. 10.1021/nl051501r CCC: $30.25 Published on Web 09/17/2005
© 2005 American Chemical Society
uration, the as-grown material exhibits broad-band electroluminescence across the spectral region from 350 to 850 nm.6 Processing and annealing at temperatures of 300 °C in air improves the sample quality, reduces the surface roughness, increases the electroluminescence intensity, and shifts the emission maximum to the ultraviolet region. A narrow UVelectroluminescence line is observed at 393 nm in the annealed material. The ZnO nanorods were grown at 90 °C on 5 × 5 cm2 fluorine-doped SnO2 films on glass substrates using a standard three-electrode electrochemical cell with a Pt counter electrode and a Ag/AgCl reference electrode. Before deposition, the substrates were cleaned in an ultrasonic bath of acetone and methanol with subsequent rinsing in HNO3 and distilled H2O. The deposition used aqueous solutions of 0.1 M KCl and 0.1 mM ZnCl2 saturated with O2 gas. The uniformity of the deposition over the complete substrate area was excellent. More details on the electrodeposition are given in refs 6 and 12. Figure 1 a and b shows an electron-diffraction micrograph and a high-resolution STEM obtained from a single as-grown nanorod. The images demonstrate clearly that the as-grown material is single-crystalline. From the orientation of the nanorod in the microscope, it is concluded that the growth direction and the nanorod axis coincide with the c axis of the wurtzite unit cell. Previous studies12 showed that the carrier mobilities in the as-grown material are as high as 20 cm2/Vs. Annealing took place in dry air at a temperature of 300 °C for 1 h at atmospheric pressure and was followed by
Figure 1. (a) Electron diffractogram of a single as-grown ZnO nanorod electrodeposited at T ) 90 °C from an aqueous solution; (b) high-resolution transmission electron micrograph of an as-grown nanorod; (c) high-resolution transmission electron micrograph obtained after annealing at 300 °C.
Figure 2. Scanning electron micrograph of a ZnO nanorod embedded in a polystyrene layer that was subsequently plasmaetched to reduce the coverage of the nanorod tip.
cooling in ambient air. Figure 1c shows markedly improved crystallinity at the nanorod surface. This observation has been confirmed in a large number of micrographs. It is difficult to assert that the higher surface crystallinity is the cause for the electronic improvement of the material. But the finding is at least consistent with the notion that the processing temperature in the low-temperature regime has considerable influence on the O-vacancy density and that the prevalent defect configuration is a deficiency in O at the surface. Because the electrodeposition process involves the transfer of electrons from the substrate to ZnO, the electronic contact between the substrate and the nanorods is necessarily conductive; we also find it to be mostly ohmic at room temperature. Diode-like structures can be created by bringing the electrodeposited nanorods into contact with a p-type material, such as thin-film inorganic or organic semiconductors. To fabricate light-emitting diodes, we have grown the ZnO nanorods on fluorine-doped SnO2/glass substrates and then applied an insulating polystyrene film in order to 2006
Figure 3. (a) Room-temperature photoluminescence obtained on ZnO nanorods showing excitonic and defect-related transitions; (b) electroluminescence obtained from as-grown nanorods showing a broad emission band from defect-related transitions and a small ultraviolet shoulder; (c) electroluminescence of nanorods annealed at T ) 300 °C for 2 h in air. Current values refer to the driving current for a device area of 0.3 cm2.
insulate the uncovered SnO2 areas. The polystyrene deposition typically involves a multilayer process using high molecular weight solutions (MW ≈ 1 800 000) of polystyrene in toluene. The deposition is carried out in such a way that the space between nanorods is homogeneously filled with the insulator, while there is only very thin coverage of insulator material on the tip of the nanorods. Subsequently, a strongly p-doped layer of PEDOT/PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) is deposited on top of the polystyrene layer serving as a hole-injecting contact. The layer thickness of the PEDOT/PSS is typically 0.5 µm. The top contact is made by vacuum-evaporation of a ∼200nm Au layer. This contact configuration to ZnO is strongly rectifying. With a tip coverage of less than 50 nm, carrier Nano Lett., Vol. 5, No. 10, 2005
Figure 4. (a) Current-voltage characteristics of the nanorod-LED structure. The arrow indicates the emission threshold. (b) Energy diagram for the nanorod LED with work function values for the contact and transport layers, and conduction and valence band edges for ZnO.
injection is found to be sufficient to produce visible electroluminescence at room temperature.6 To further reduce the insulator coverage, we can thin the polystyrene at the ZnO tips by mild plasma-etching in oxygen. Typically we used 3 min, 100 W O2 rf-plasma exposure at a pressure of 30 Pa to obtain nearly bare ZnO tips as shown in Figure 2. Figure 3 shows the electroluminescence results for asgrown and annealed ZnO. Although visible broad-band luminescence over the 360-850 nm region is observed in the as-grown ZnO nanorods, improved intensities and an additional narrow UV emission line is observed in LEDs made of material annealed at 300 °C. The UV-emission is centered on a wavelength of 393 nm and has a full width of 24 nm at half-maximum. The band gap energy of ZnO is 3.37 eV, corresponding to 360 nm. In polycrystalline films and nanostructures, room-temperature photoluminescence and lasing has been observed in the region around 390 nm13 and has been ascribed to bound-exciton and vacancy-related transitions.14,15 A comparison between the photoluminescence and the electroluminescence spectra of the annealed ZnO shows nearly identical features in the two spectra, indicating that the injection and transport processes in the annealed ZnO are very efficient. For the UV transitions to have intensities similar to those of the defect transitions, the electronic transport for electrons and holes must occur with only a very small reduction in carrier energy over the transport path length, which corresponds to the nanorod height, that is, ∼2 µm. The unannealed material shows a weak shoulder in the UV region, indicative of less efficient transport and the presence of other nonradiative recombination channels, bypassing the excitonic transitions. Figure 4 shows the current-voltage characteristics of the SnO2/ZnO/PEDOT/ Au heterostructure and an energy level diagram based on published data for the involved materials. The I-V characteristics show excellent rectification; the luminescence onsetvoltage is typically about 5-7 V. A comparison to the energy level diagram indicates that a horizontal band alignment, which is necessary for carrier injection, requires a forward potential of at least 3.1 V: there is a 2.4 eV barrier for hole injection from the PEDOT Nano Lett., Vol. 5, No. 10, 2005
transport level to the ZnO valence band edge and an additional 0.7 eV barrier between the SnO2 Fermi energy and the ZnO conduction band edge. It can be expected that a more elaborate choice of contact materials will bring about substantial reduction of these barriers or that a sequence of small energy barriers can be constructed in multilayer contacts. These refinements may eventually lead to further improvement of the luminescence onset voltage and the current densities in this type of heterojunction device. Overall, this device presents an interesting alternative to organic LEDs. Because the optically active material in this hybrid structure is inorganic, stability issues may be less critical than in all-organic LEDs. The device also satisfies all of the requirements for flexible-substrate and large-area applications. Processing temperatures and procedures principally allow the use of polymeric substrates such as Kapton, and the nanorod/polystyrene composite appears sufficiently robust to absorb large mechanical strain. To conclude, we have reported narrow-line ultraviolet electroluminescence in ZnO nanorods, grown at 90 °C in electrodeposition and annealed at 300 °C. The LED consisted of SnO2/ZnO/PEDOT structures with an internal polystyrene insulator layer. The complete processing procedure is compatible with large-area fabrication on flexible substrates. Acknowledgment. We gratefully acknowledge Dr. Chunfei Li for help with the microscopy work. References (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (2) Yatsui, T.; Lim, J.; Ohtsu, M.; An, S. J.; Yi, G.-C. Appl. Phys. Lett. 2004, 85, 727. (3) Park, W. I.; Jun, Y. H.; Jung, S. W.; Yi, G. C. Appl. Phys. Lett. 2003, 82, 964. (4) Han, X.; Wang, G.; Wang, Q.; Cao, L.; Liu, R.; Zou, B.; Hou, J. G. Appl. Phys. Lett. 2005, 86. (5) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M., Makino T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Nat. Mater. 2005, 4, 42. (6) Ko¨nenkamp, R.; Word, R.; Schlegel, C. Appl. Phys. Lett. 2004, 24, 6004. 2007
(7) Park, W. I.; Kim, D. H.; Jung, S.-W.; Yi, G. C. Appl. Phys. Lett. 2002, 80, 4232. (8) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. AdV. Mater. 2001, 13, 113. (9) Pan, Z. W.; Dai, Z. R.; Wang Z. L. Science 2001, 291, 1947. (10) Peulon, S.; Lincot, D. J. Electrochem. Soc. 1998, 145, 864. (11) Pauporte, T.; Lincot, D. Appl. Phys. Lett. 1999, 75, 3817. (12) Ko¨nenkamp, R.; Boedecker, K.; Lux-Steiner, M. Ch.; Poschenrieder, M.; Zenia, F.; Levy-Clement, C. Appl. Phys. Lett. 2000, 77, 2575.
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(13) Cho, S.; Ma, J.; Kim, Y.; Sun, Y.; Wong, G. K. L.; Ketterson, J. B. Appl. Phys. Lett. 1999, 75, 2761. (14) Teke, A.; O ¨ zgu¨r, U ¨ .; Dogan, S.; Gu, X.; Morkoc¸ , H.; Nemeth; Nause, B.; J.; Everitt, H. O. Phys. ReV. B 2004, 70, 195207. (15) Zhang, B. P.; Binh, N. T.; Wakatsuki1, K.; Segawa, Y.; Kashiwaba, Y.; Haga K. Nanotechnology 2004, 15, S382.
NL051501R
Nano Lett., Vol. 5, No. 10, 2005
