1. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 1 ZnO-based thin film double heterostructured- ultraviolet light-emitting diodes grown by vapor cooling condensation technique Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China. Po-Ching Wu and Ching-Ting Lee

2. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 2 Outline  Introduction  Experiments  Results and discussion  Conclusions

3. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 3 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 Introduction

4. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 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 3.37 eV to 7.7 eV Ref ： [1] J. Phys. D: Appl. Phys., 42, 235101 (2009). [2] Appl. Phys. Lett., 86, 192911, (2005). [3] IEEE J. Sel. Top. Quantum Electron., 14, 1048 (2008). [4] IEEE Photon. Technol. Lett., 20, 2108 (2008). The common deposition methods of the MgZnO film ： MOCVD [1] MBE [2] PLD [3] Sputtering [4] High-temperature process

5. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 5 Characteristics of the ZnO and the MgO materials ： [1] materials ZnOMgO energy bandgap3.37eV7.7eV ionic radiusZn 2+ = 0.6 ÅMg 2+ = 0.57Å lattice constant a=3.249Å c=5.207Å a=4.213 Å lattice structure hexagonal wurtzite structure cubic rock-salt (NaCl) structure Ref ： [1] IEEE Photon. Technol. Lett., 20, 2108 (2008).

6. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 6 Mixed Phase Hexagonal MgZnO Cubic MgZnO Lattice structure of the MgZnO films ： Ref ： [1] J. Appl. Phys., 94, 7336 (2003).  When the Mg content of the MgZnO films is lower than 36%, the lattice structure is still as hexagonal structure. [1]

7. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 7 The vapor cooling condensation system Experiments Deposition layers ： i-type MgZnO film i-type ZnO film n-type ZnO:In film Deposition conditions ： Pressure ： 10  4 torr Deposition Rate ： 1 Å/s Substrate Temperature ： 80K

8. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 8  ULEDs was patterned by the conventional photolithography and lift-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 500 o C 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 200 o C in a N 2 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. Fabrication process of the ZnO-based thin film double heterostructured-ultraviolet light-emitting diodes (UV LEDs)

9. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 9 The schematic diagram of the p-AlGaN/i-MgZnO/i-ZnO/i- MgZnO/n-ZnO:In UV LEDs CB VB MgZnO ZnO  Carrier confinement  Enhance the radiative recombination rate A energy level schematic diagram of the MgZnO/ZnO/MgZnO double heterostructure

10. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 10  Critical angle loss (internal total reflection) Snell`s law n 1 sin  c = n 2 sin90   c =sin -1 (n 2 / n 1 ) n2n2 n1n1 cc n 1 > n 2  Fresnel loss T+R = 1 R = (n 2 - n 1 ) 2 / (n 2 + n 1 ) 2 T = 1-R = 4 n 2 n 1 / (n 2 2 +2n 2 n 1 + n 1 2 ) n2n2 n1n1 T R The influence factor of the light-extraction efficiency of the LEDs ： [1] Reduce the light extraction loss Ref ： [1] S. M. Sze, Semiconductor Devices: Physics and Technology. New York: Wiley, 2002.

11. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 11 The schematic cross sectional view of the UV LEDs Refractive index ： n(air) = 1 n(SiO 2 ) = 1.45 n(ZnO) = 2 n(TiO 2 ) = 2.3  The contribution of the oxide passivation layer ：  reduce the light extraction loss  reduce the leakage current  Deposited the transparent oxide films of the SiO 2 and TiO 2, respectively, on the top and sidewall of the UV LEDs by using a RF sputtering system.

12. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 12 Hall measurement results of the films deposited by the vapor cooling condensation system Thin film Carrier concentration (cm  3 ) Mobility (cm 2 /V-s) i-type ZnO 3.12×10 15 6.2 n-type ZnO:In 3.62×10 19 3.7 MgZnO ××  The energy band gap of the p-type Al 0.18 Ga 0.82 N layer was about 3.71 eV.  Activation ： 750 °C in N 2 ambient for 30 min  Hole concentration = 3.0 × 10 17 cm  3, Hole mobility = 3.86 cm 2 /V-s Results and discussion The properties of the p-type AlGaN

13. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 13 Transmittance and optical energy bandgap d ： Thickness  ： Absorption coefficient T ： Transmittance h ： Planck`s constant ： Photon frequency E g ： Optical energy bandgap Tauc plot [1] Thin filmMgZnOi-ZnOn-ZnO:In Optical energy bandgap (E g )4.01 eV3.27 eV3.25 eV Ref ： [1] Phys. Stat. Sol., 15, 627 (1966). visible region

14. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 14 EDS measurement  The magneisum content of the MgZnO film was about 25%. [1] XRD measurement Ref ： [1] J. Appl. Phys., 101, 033502 (2007). [2] Thin Solid Films, 372, 173 (2000).  The (0 0 2) diffraction peak of the hexagonal structure in the MgZnO film was measured. [2]

15. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 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. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 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. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 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. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 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. visible region 380 nm

19. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 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. National Cheng Kung University Institute of microelectronics OEIC Lab. Jun. 2012 P. 20 Thanks for your attention !