ZnO-Based Ultraviolet LEDs with Double Heterostructured Thin Films: Vapor Cooling Condensation Technique

1. ZnO-based thin film double heterostructured-ultraviolet light-emitting diodes grown by vapor cooling condensation technique Po-Ching Wuand Ching-Ting Lee Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China.

2. Outline • Introduction • Experiments • Results and discussion • Conclusions

3. Introduction • The applications of the UV light： • Anti-counterfeiting detectors • UV treatment • Air and water purification • The advantages of the UV LEDs： • Portable • Safety • Long lifetime • Environmental protection －no mercury (Hg) pollution

4. The advantages of the ZnO semiconductor： • Wide direct band gap (3.37 eV) • Large exciton binding energy (60 meV) • Low cost, high stability, non-toxic • The energy bandgap of the MgZnO film could be modulated from 3.37eV to 7.7 eV • The common deposition methods of the MgZnO film： • MOCVD [1] • MBE [2] • PLD [3] • Sputtering [4] High-temperature process [3]IEEE J. Sel. Top. Quantum Electron., 14, 1048 (2008). [4] IEEE Photon. Technol. Lett., 20, 2108 (2008). Ref：[1] J. Phys. D: Appl. Phys., 42, 235101 (2009). [2] Appl. Phys. Lett., 86, 192911, (2005).

5. Characteristics of the ZnO and the MgO materials： [1] Ref：[1] IEEE Photon. Technol. Lett., 20, 2108 (2008).

6. Lattice structure of the MgZnO films： Mixed Phase • When the Mg content of the MgZnO films is lower than 36%,the lattice structure is still as hexagonal structure.[1] Hexagonal MgZnO Cubic MgZnO Ref：[1] J. Appl. Phys., 94, 7336 (2003).

7. Experiments The vapor cooling condensation system Deposition layers： i-type MgZnOfilm i-type ZnO film n-type ZnO:In film Deposition conditions： Pressure ：10-4 torr Deposition Rate ：1 Å/s Substrate Temperature：80K

8. Fabrication process of the ZnO-based thin film double heterostructured-ultraviolet light-emitting diodes (UV LEDs) • ULEDs was patterned by the conventional photolithography andlift-off process. • The electrodes were deposited by the electron-beam evaporator. • The ohmic contacts of the Ni/Au metals and p-AlGaN was processed with sulfide treatment and performed at 500oC in an air ambient for 10 min in the rapid thermal annealing (RTA) system, while the Ti/Au metals and n-ZnO:In:In was performed at 200oC in a N2 ambient for 3 min. • The p-AlGaN/i-MgZnO/i-ZnO/i-MgZnO/n-ZnO:In UV LEDs and the conventional p-AlGaN/i-ZnO/n-ZnO:In UV LEDs were fabricated.

9. The schematic diagram of the p-AlGaN/i-MgZnO/i-ZnO/i-MgZnO/n-ZnO:In UV LEDs MgZnO ZnO MgZnO CB A energy level schematic diagram of the MgZnO/ZnO/MgZnO double heterostructure • Carrier confinement • Enhance the radiative recombination rate VB

10. The influence factor of the light-extraction efficiency of the LEDs：[1] n1 > n2 • Critical angle loss • (internal total reflection) • Snell`s law • n1sinc = n2 sin90 • c =sin-1(n2 / n1) T n2 n2 • Fresnel loss • T+R=1 • R=(n2-n1)2/ (n2+n1)2 • T=1-R=4 n2n1/ (n22+2n2n1 + n12) n1 n1 R c Reduce the light extraction loss Ref：[1] S. M. Sze, Semiconductor Devices: Physics and Technology. New York:Wiley, 2002.

11. The schematic cross sectional view of the UV LEDs • Deposited the transparentoxide films of the SiO2and TiO2, respectively, on the top and sidewall of the UV LEDs by using aRF sputtering system. • The contribution of the oxide passivation layer ： • reduce the light extraction loss • reduce the leakage current Refractive index： n(air) = 1 n(SiO2) = 1.45 n(ZnO) = 2 n(TiO2) = 2.3

12. Results and discussion The properties of the p-type AlGaN • The energy band gap of the p-type Al0.18Ga0.82Nlayer was about 3.71 eV. • Activation：750 °C in N2 ambient for 30 min • Hole concentration = 3.0 × 1017 cm-3 , Hole mobility =3.86 cm2/V-s Hall measurement results of the films deposited by the vapor cooling condensation system

13. Transmittance and optical energy bandgap Tauc plot [1] visible region d：Thickness ：Absorption coefficient T ： Transmittance h：Planck`s constant ：Photon frequency Eg：Optical energy bandgap Ref：[1] Phys. Stat. Sol., 15,627 (1966).

14. EDS measurement XRD measurement • The magneisum content of the MgZnO film was about 25%.[1] • The (0 0 2) diffraction peak of the hexagonal structurein the MgZnO film was measured. [2] Ref：[1] J. Appl. Phys., 101, 033502 (2007). [2] Thin Solid Films, 372, 173 (2000).

15. Photoluminescence spectra Near-band edge (NBE) emission • The photoluminescence spectra was excited by a He–Cd laser with a wavelength of 325 nm. • The NBE emission peak of the i-type ZnO film at 380 nm was observed. • Defect emission at the visible region was small enough.

16. Why the films deposited at low-temperature have lower defect concentration? Room temperature photoluminescence spectra of the high temperature (HT)-ZnO films and the low temperature (LT)-ZnO films excited with a He–Cd laser with a wavelength of 325 nm. [1] Ref：[1] H. Y. Lee, S. D. Xia, W. P. Zhang, L. R. Lou, J. T. Yan, and C. T. Lee, “Mechanisms of high quality i-ZnO thin films deposition at low temperature by vapor cooling condensation technique,” J. Appl. Phys., 108, 073119 (2010).

17. Current-Voltage measurement • A typical rectifying behavior was clearly observed by the semiconductor parameter analyzer. . • The forward turn-on voltage and the reverse breakdown voltage were about 3.25 V and-9.4 V,respectively.

18. Electroluminescence spectra • The emission peaks were at 380 nm. • Only a pure UV emission was observed, without defect emission at the visible region. • EL peak intensity and total emission power of double heterostructured-UV LEDs were much higher, about 3.08 and 1.82 times. 380 nm visible region

19. Conclusions • High quality ZnO and MgZnO film with low defect concentration were successfully deposited by the vapor cooling condensation system. • The UV LEDs with a pure UV emission and without defect emission at the visible region was achieved. • Double heterostructure was contributed to the carrier confinement and the enhancement of the radiative recombination rate in the active i-ZnO layer. • The EL emission peak intensity and the total emission power of the double heterostructured-UV LEDs were much higher than that conventional UV LEDs.

20. Thanks for your attention!