Herstrøm, Berit

Jensen, Flemming

Lin, Li

Ou, Haiyan

Transparent conductive aluminum-doped zinc oxide (AZO) layer was deposited on GaN-based near-ultraviolet (NUV) light emitting epitaxial wafers as current spreading layer by a sputtering process. Efforts were made to improve the electrical properties of AZO in order to produce ohmic contact

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Downloaded from orbit.dtu.dk on: Dec 16, 2017Current Spreading Layer with High Transparency and Conductivity for near-ultravioletlight emitting diodesLin, Li; Jensen, Flemming; Herstrøm, Berit; Ou, HaiyanPublication date:2017Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Lin, L., Jensen, F., Herstrøm, B., & Ou, H. (2017). Current Spreading Layer with High Transparency andConductivity for near-ultraviolet light emitting diodes. Paper presented at 5th International workshop on LEDsand solar application, Kgs. Lyngby, Denmark.  5th International workshop on LEDs and solar applications Current Spreading Layer with High Transparency and Conductivity for near-ultraviolet light emitting diodes Li Lin 1,*, Flemming Jensen 2, Berit Herstrøm 2, Haiyan Ou 1 1 Department of Fotonik Engineering, Technical University of Denmark, Ørsteds Plads 343, 2800 Kgs. Lyngby, Denmark 2 DTU Danchip, Technical University of Denmark, Ørsteds Plads 347, 2800 Kgs. Lyngby, Denmark llin@fotonik.dtu.dk  Abstract— Transparent conductive aluminum-doped zinc oxide (AZO) layer was deposited on GaN-based near-ultraviolet (NUV) light emitting epitaxial wafers as current spreading layer by a sputtering process. Efforts were made to improve the electrical properties of AZO in order to produce ohmic contact. Keywords— near ultraviolet light emitting diodes; transparent conductive current spreading layer; aluminum-doped zinc oxide I. INTRODUCTION Transparent conductive current spreading layer (CSL) is more advantageous in improving light extraction efficiency for NUV LEDs than conventional Ni/Au and indium tin oxide (ITO) is a widely used transparent conductive CSL for LEDs. However, ITO is expensive because of its In-content [1] while AZO is a well-known alternative candidate to ITO since it has similar electrical and optical properties but is low-cost, nontoxic and more stable at high temperatures [2]-[3].  There has been very few research reported on using AZO-based reflective CSL for GaN-based NUV LEDs with flip-chip configuration. Hence, this work focuses on modifying electrical behavior of AZO on p-GaN to produce ohmic contact. In the future, AZO-based CSL will be used together with Al reflector to form AZO-based reflective CSL in flip-chip NUV LED application. II. EXPERIMENTS AND RESULTS  Fig. 1. EL of LEDs with different CSLs Six types of AZO-based CSLs including as-deposit 110nm AZO, annealed 110nm AZO, annealed 5nm Ni/ as-deposit 110nm AZO, annealed 5nm Ni/ annealed 110nm AZO, annealed 5nm Ni/as-deposit 250nm AZO and annealed 5nm Ni/annealed 250nm AZO were fabricated on the mesas of NUV LED wafers. Ni and AZO were deposited by e-beam evaporation and DC sputtering respectively. After deposition, Ni was annealed in air at 550 ℃ for 5 min. 110nm AZO and 250nm AZO were annealed at 550℃ for 5min in N2 and at 800℃ for 1min in N2 respectively. I-V measurement was carried out on the 6 types of CSLs. In the end, annealed 5nm Ni/annealed 250nm AZO gives the best I-V properties since it almost presents ohmic behavior. One NUV LED device with annealed 5nm Ni/annealed 250nm AZO as its CSL was fabricated and its electroluminescence (EL) graph together with that of an LED using conventional Ni/Au CSL were displayed in Figure 1. The EL intensity of the LED with Ni/AZO is weaker than that of the LED with Ni/Au due to the high specific contact resistivity of Ni/AZO. III. SUMMARY I-V behavior of CSLs with different compositions, AZO thicknesses and annealing conditions were tested and compared. The annealed 5nm Ni/annealed 250nm AZO presents the best electrical properties. Afterwards, an NUV LED with transparent conductive Ni/AZO was fabricated. Although EL can be observed, its intensity is still lower than that of the LED with Ni/Au. This is because the contact between Ni/AZO and p-GaN is not perfect ohmic indicating much larger specific contact resistivity than that of Ni/Au. Although Ni/AZO possesses higher transparency than that of Ni/Au, its electrical behavior still needs further modification. ACKNOWLEDGMENT This work was supported by Innovation Fund Denmark (project No 4106-00018B). REFERENCES [1] S.M. Pan, R.C. Tu, Y.M. Fan, R.C. Yeh, and J.T. Hsu, “Improvement of InGaN-GaN light-emitting diodes with surface-textured indium-tin-oxide transparent ohmic contacts,” IEEE Photonics Technology Letters, vol. 15(5), pp. 649-651,  2003. [2] C.H. Kuo, C.L. Yeh, P.H. Chen, W.C. Lai, C.J. Tun, J.K. Sheu, and G.C. Chi, “Low operation voltage of nitride-based LEDs with Al-doped ZnO transparent contact layer,” Electrochemical and Solid-State Letters, vol. 11(9), pp. H269-H271, 2008. [3] B.H. Kong, H.K. Cho, M.Y. Kim, R.J. Choi, and B.K. Kim, “InGaN/GaN blue light emitting diodes using Al-doped ZnO grown by atomic layer deposition as a current spreading layer,” Journal of Crystal Growth, vol. 326(1), pp. 147-151, 2011. 

Current Spreading Layer with High Transparency and Conductivity for near-ultraviolet light emitting diodes

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Current Spreading Layer with High Transparency and Conductivity for near-ultraviolet light emitting diodes

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