The fabrication of white light-emitting diodes using the n-ZnO/NiO/p-GaN heterojunction with enhanced luminescence
© Abbasi et al.; licensee Springer. 2013
Received: 10 April 2013
Accepted: 30 June 2013
Published: 13 July 2013
Cheap and efficient white light-emitting diodes (LEDs) are of great interest due to the energy crisis all over the world. Herein, we have developed heterojunction LEDs based on the well-aligned ZnO nanorods and nanotubes on the p-type GaN with the insertion of the NiO buffer layer that showed enhancement in the light emission. Scanning electron microscopy have well demonstrated the arrays of the ZnO nanorods and the proper etching into the nanotubes. X-ray diffraction study describes the wurtzite crystal structure array of ZnO nanorods with the involvement of GaN at the (002) peak. The cathodoluminescence spectra represent strong and broad visible emission peaks compared to the UV emission and a weak peak at 425 nm which is originated from GaN. Electroluminescence study has shown highly improved luminescence response for the LEDs fabricated with NiO buffer layer compared to that without NiO layer. Introducing a sandwich-thin layer of NiO between the n-type ZnO and the p-type GaN will possibly block the injection of electrons from the ZnO to the GaN. Moreover, the presence of NiO buffer layer might create the confinement effect.
KeywordsWhite light-emitting diode ZnO nanorods Nanotubes NiO buffer layer
Zinc oxide (ZnO) is very much popular among the researchers due its wide direct band gap (3.37 eV) and high exciton binding energy (60 meV) at room temperature. The wide band gap and high exciton binding energy provides a solid platform for the ZnO in the fabrication of optoelectronic nanodevices. Specifically, light-emitting diodes (LEDs) and laser diodes based on the applications of the ZnO material explored its usability, thus ZnO-based light-emitting diodes are considered as the next-generation light-emitting diodes due to their cheap fabrication process and enhanced optical properties . Several synthesis routes have been used for the fabrication of ZnO films and nanostructures, and the prepared ZnO material exhibits good crystalline and optical properties [2–4]. Recently, some ZnO p-n homojunction-based light-emitting diodes have been fabricated [5–7]. Due to the absence of a stable and reproducible p-type doped material with desired quality, ZnO-based light-emitting diodes are not considered up to the level of commercialization. Because of the lack of stable p-type ZnO, most ZnO heterojunctions are developed with the other existing p-type materials including p-type GaN [8–10], Si  and SiC (4H) . Gallium nitride (GaN) is used effectively in the fabrication of heterojunction with ZnO for the development of light-emitting diodes because both materials exhibit a similar crystal wurtzite structure and electronic properties and differ by 1.8% lattice mismatch. The ZnO material is accompanied by the deep-level photoluminescence and electroluminescence (EL) in addition to near-band gap UV emission [11–14]. The deep-level emission is a critical issue which is not yet clear, but it is generally accepted that the possible oxygen vacancies or zinc interstitials are responsible for deep-level emissions . The deep-level emission given by ZnO covers the wide range of visible spectrum, and theoretically, white emission can be obtained by hybridizing the deep-level emission of ZnO with the blue emission of GaN.
In order to improve the luminescence of ZnO-based light-emitting diodes, an interlayer of any other suitable material acting as a buffer medium is highly required for the significant improvement of the internal structure because the interlayer provides a stable charge environment during hole and electron injections in the light emitting part of the diode. Since the introduction of interlayers, such as TiO2, Ag, MoO3, WO3 or NiO interlayers, of different materials has improved the performance of polymer LEDs significantly, it has brought the change in the barriers for electrodes and also increases the hole injection which in result lowers the turn on and working voltage [16–20]. It is also reported that when a thin layer of NiO is deposited at the anode of ITO, then it has enhanced the optoelectronic working activity of double-sided emission devices using the thin-film-based heterojunction of p-NiO and n-ZnO materials . ZnO-based white light-emitting diodes have also been fabricated on GaN substrate by our group previously [22, 23].
Herein, we have developed n-ZnO/p-GaN heterojunctions with the presence and absence of a NiO buffer layer. The NiO buffer layer was deposited by the sol-gel method prior to the growth of the ZnO nanorods and nanotubes on GaN substrate. Four devices are prepared with ZnO nanorods and nanotubes on the GaN substrate: two with NiO buffer layer and the other two without. The devices were characterised by the X-ray diffraction (XRD), scanning electron microscopy (SEM), parameter analyser and the cathodoluminescence (CL) and EL techniques.
Commercially available p-type GaN substrate was used in the development of the present p-n heterojunction. Prior to the growth of the n-type ZnO nanorods, a NiO buffer layer was deposited by the following sol-gel method. A sol-gel of nickel acetate was prepared in the 2-methoxyethanol having a concentration of 0.35 M, and di-ethanolamine was added dropwise under vigorous stirring at 60°C for 2 h by keeping the 1:1 molar ratio of nickel acetate and di-ethanolamine constant. After the synthesis of the sol-gel, cleaned GaN substrate was spin coated with the prepared sol-gel three to five times for the deposition of a thin NiO buffer layer; consequently, the substrate was annealed at 180°C for 20 min. After the annealing, the sample was left in the preheated oven for 4 h at 450°C in order to have a pure phase of NiO. After the deposition of the NiO buffer layer, the substrates were spin coated two to three times with a seed layer of zinc acetate for the growth of the ZnO nanorods and likewise annealed at 120°C for 20 min. Then, the annealed substrates containing the NiO buffer layer were dipped vertically in an equimolar 0.075 M precursor's solution of zinc nitrate hexahydrate and hexamethylenetetramine for 4 to 6 h at 90°C. After the growth of the ZnO nanorods, the nanotubes were obtained by chemical etching using 5 M potassium chloride solution at 85°C for 14 to 16 h.
After the growth of the ZnO nanorods and nanotubes with and without a NiO buffer layer, SEM was used to investigate the morphology of the prepared samples. The X-ray diffraction technique was used for the study of crystal quality and elemental composition analysis. The heterojunction analysis was performed using a parameter semiconductor analyser. CL and EL studies were carried out for the investigation of luminescence response of the prepared devices.
For the device fabrication, the bottom contacts are deposited by the evaporation of the 20-nm thickness of nickel and the 40-nm thickness of gold layers, respectively. Insulating layer of Shipley 1805 photoresist (Marlborough, MA, USA) was spin coated for the filling of vacant spaces between the nanorods, nanotubes and the growth-free surface of the GaN substrate. Reactive ion etching was used for exposing the top surface of the ZnO nanorods and nanotubes for the top contact of aluminium.
Results and discussion
In this study, n-type ZnO/p-type GaN- and n-type ZnO/NiO/p-type GaN-based white light-emitting diodes are designed using two known morphologies of ZnO including nanorods and nanotubes. ZnO nanorods were well aligned and perpendicular to the GaN substrate, and some of the samples were almost fully chemically etched into nanotubes. XRD study shows the c-axis-oriented growth of the ZnO crystal structure with the possible involvement of GaN at (002) crystal plane. Both the CL and EL intensities were significantly increased by inserting a thin layer of NiO at the interface between the n-type ZnO and the p-type GaN due to possible blocking of electron injections from the ZnO to the GaN. Using the NiO buffer layer, the confinement is created which helps in the development of efficient LEDs based on n-type ZnO/NiO/p-type GaN heterojunctions.
We are grateful to the University of Sindh, Pakistan, NED University, Pakistan and Linköping University, Sweden for their financial support.
- Chen Y, Bagnall D, Yao T: ZnO as a novel photonic material for the UV region. Mater Sci Eng B 2000, 75: 190–198. 10.1016/S0921-5107(00)00372-XView Article
- Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, Weber E, Russo R, Yang P: Room-temperature ultraviolet nanowire nanolasers. Science 2001, 292: 1897–1899. 10.1126/science.1060367View Article
- Park WI, Jun YH, Jung SW, Yi GC: Excitonic emissions observed in ZnO single crystal nanorods. Appl Phys Lett 2003, 82: 964–966. 10.1063/1.1544437View Article
- Özgür Ü, Alivov YI, Liu C, Teke A, Reshchikov MA, Doan S, Avrutin V, Cho SJ, Morkoç H: A comprehensive review of ZnO materials and devices. J Appl Phys 2005, 98: 041301. 10.1063/1.1992666View Article
- Wang G, Chu S, Zhan N, Lin Y, Chernyak L, Liu J: ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection. Appl Phys Lett 2011, 98: 041107. 10.1063/1.3551628View Article
- Chen MT, Lu MP, Wu YJ, Song J, Lee CY, Lu MY, Chang YC, Chou LJ, Wang ZL, Chen LJ: Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays. Nano Lett 2010, 10: 4387–4393. 10.1021/nl101907hView Article
- Sun XW, Ling B, Zhao JL, Tan ST, Yang Y, Shen YQ, Dong ZL, Li XC: Ultraviolet emission from a ZnO rod homojunction light-emitting diode. Appl Phys Lett 2009, 95: 133124. 10.1063/1.3243453View Article
- Chang SP, Chuang RW, Chang SJ, Chiou YZ, Lu CY: MBE n-ZnO/MOCVD p-GaN heterojunction light-emitting diode. Thin Solid Films 2009, 517: 5054–5056. 10.1016/j.tsf.2009.03.059View Article
- Li S, Ware M, Wu J, Minor P, Wang Z, Wu Z, Jiang Y, Salamo GJ: Polarization induced pn-junction without dopant in graded AlGaN coherently strained on GaN. Appl Phys Lett 2012, 101: 122103. 10.1063/1.4753993View Article
- Li S, Ware ME, Wu J, Kunets VP, Hawkridge M, Minor P, Wang Z, Wu Z, Jiang Y, Salamo GJ: Polarization doping: Reservoir effects of the substrate in AlGaN graded layers. J Appl Phys 2012, 112: 053711. 10.1063/1.4750039View Article
- Wang T, Wu H, Chen C, Liu C: Growth, optical, and electrical properties of nonpolar m -plane ZnO on p -Si substrates with Al2O3 buffer layers. Appl Phys Lett 2012, 100: 011901. 10.1063/1.3673346View Article
- Shih YT, Wu MK, Chen MJ, Cheng YC, Yang JR, Shiojiri M: ZnO-based heterojunction light-emitting diodes on p -SiC(4H) grown by atomic layer deposition. Appl Phys B 2010, 98: 767–772. 10.1007/s00340-009-3809-0View Article
- Lim JH, Kang CK, Kim KK, Park IK, Hwang DK, Park SJ: UV electroluminescence emission from ZnO light-emitting diodes grown by high-temperature radiofrequency sputtering. Adv Mater 2006, 18: 2720–2724. 10.1002/adma.200502633View Article
- Liu W, Gu SL, Ye JD, Zhu SM, Liu SM, Zhou X, Zhang R, Shi Y, Zheng YD, Hang Y, Zhang CL: Blue-yellow ZnO homostructural light-emitting diode realized by metal organic chemical vapor deposition technique. Appl Phys Lett 2006, 88: 092101. 10.1063/1.2169908View Article
- Du GT, Liu WF, Bian JM, Hu LZ, Liang HW, Wang XS, Liu AM, Yang TP: Room temperature defect related electroluminescence from ZnO homojunctions grown by ultrasonic spray pyrolysis. Appl Phys Lett 2006, 89: 052113. 10.1063/1.2245217View Article
- Bian J, Liu W, Sun J, Liang H: Synthesis and defect-related emission of ZnO based light emitting device with homo- and heterostructure. J Mater Process Technol 2007, 184: 451–454. 10.1016/j.jmatprotec.2006.12.011View Article
- Børseth TM, Svensson BG, Kuznetsov AY, Klason P, Zhao QX, Willander M: Identification of oxygen and zinc vacancy optical signals in ZnO. Appl Phys Lett 2006, 89: 262112. 10.1063/1.2424641View Article
- Hou L, Liu P, Li Y, Wu C: Enhanced performance in organic light-emitting diodes by sputtering TiO2 ultra-thin film as the hole buffer layer. Thin Solid Films 2009, 517: 4926–4929. 10.1016/j.tsf.2009.03.017View Article
- Yang LY, Chen XZ, Xu H, Ye DQ, Tian H: Surface modification of indium tin oxide anode with self-assembled monolayer modified Ag film for improved OLED device characteristics. Appl Surf Sci 2008, 254: 5055–5060. 10.1016/j.apsusc.2008.02.012View Article
- Guo TF, Wen TC, Huang YS, Lin MW, Tsou CC, Chung CT: White-emissive tandem-type hybrid organic/polymer diodes with (0.33, 0.33) chromaticity coordinates. Opt Express 2009, 17: 21205–21215. 10.1364/OE.17.021205View Article
- Kim JH, Lee YJ, Jang YS, Jang JN, Kim DH, Song BC, Lee DH, Kwon SN, Hong MP: The effect of Ar plasma bombardment upon physical property of tungsten oxide thin film in inverted top-emitting organic light-emitting diodes. Org Electron 2011, 12: 285–290. 10.1016/j.orgel.2010.10.023View Article
- Chan IM, Hsu TY: Enhanced hole injections in organic light-emitting devices by depositing nickel oxide on indium tin oxide anode. Appl Phys Lett 2002, 81: 1899–1901. 10.1063/1.1505112View Article
- Wang JY, Lee CY, Chen YT, Chen CT, Chen YL: Double side electroluminescence from p -NiO/ n -ZnO nanowire heterojunctions. Appl Phys Lett 2009, 95: 131117. 10.1063/1.3232244View Article
- Alvi NH, Hussain S, Jensen J, Nur O, Willander M: Influence of helium-ion bombardment on the optical properties of ZnO nanorods/p-GaN light-emitting diodes. Nano Res Lett 2011, 6: 628. 10.1186/1556-276X-6-628View Article
- Sadaf JR, Israr MQ, Kishwar S, Nur O, Willander M: White Electroluminescence Using ZnO Nanotubes/GaN Heterostructure Light-Emitting Diode. Nano Res Lett 2010, 5: 957–960. 10.1007/s11671-010-9588-zView Article
- Nalage SR, Chougule MA, Sen S, Joshi PB, Patil VB: Sol–gel synthesis of nickel oxide thin films and their characterization. Thin Solid Films 2012, 520: 4835–4840. 10.1016/j.tsf.2012.02.072View Article
- Aranovich JA, Golmayo DG, Fahrenbruch AL, Bube RH: Photovoltaic properties of ZnO/CdTe heterojunctions prepared by spray pyrolysis. J Appl Phys 1980, 51: 4260–4268. 10.1063/1.328243View Article
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