National Cheng Kung University Zn Obased thin film

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

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

National Cheng Kung University Introduction The applications of the UV light： l Anti-counterfeiting detectors l UV treatment l Air and water purification The advantages of the UV LEDs： l l Portable Safety Long lifetime Environmental protection －no mercury (Hg) pollution Jun. 2012 P. 3 Institute of microelectronics OEIC Lab.

National Cheng Kung University The advantages of the Zn. O semiconductor： l l Wide direct band gap (3. 37 e. V) Large exciton binding energy (60 me. V) Low cost, high stability, non-toxic The energy bandgap of the Mg. Zn. O film could be modulated from 3. 37 e. V to 7. 7 e. V The common deposition methods of the Mg. Zn. O film： ü ü MOCVD [1] MBE [2] PLD [3] Sputtering [4] High-temperature process Ref：[1] J. Phys. D: Appl. Phys. , 42, 235101 (2009). [3] IEEE J. Sel. Top. Quantum Electron. , 14, 1048 (2008). [2] Appl. Phys. Lett. , 86, 192911, (2005). Jun. 2012 [4] IEEE Photon. Technol. Lett. , 20, 2108 (2008). P. 4 Institute of microelectronics OEIC Lab.

National Cheng Kung Characteristics of the Zn. O and the Mg. O materials： [1] University materials Zn. O Mg. O energy bandgap 3. 37 e. V 7. 7 e. V ionic radius Zn 2+ = 0. 6 Å Mg 2+ = 0. 57Å lattice constant a=3. 249Å c=5. 207Å a=4. 213 Å hexagonal wurtzite cubic rock-salt (Na. Cl) structure lattice structure Ref：[1] IEEE Photon. Technol. Lett. , 20, 2108 (2008). Jun. 2012 P. 5 Institute of microelectronics OEIC Lab.

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

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

National Cheng Kung Universityprocess of the Zn. O-based thin film double Fabrication heterostructured-ultraviolet LEDs) light-emitting diodes (UV u ULEDs was patterned by the conventional photolithography and lift-off process. u The electrodes were deposited by the evaporator. electron-beam u The ohmic contacts of the Ni/Au metals and p-Al. Ga. N 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-Zn. O: In was performed at 200 o. C in a N 2 ambient for 3 min. u The p-Al. Ga. N/i-Mg. Zn. O/i-Mg. Zn. O/n-Zn. O: In UV LEDs and the conventional p-Al. Ga. N/i-Zn. O/n-Zn. O: In UV LEDs were fabricated. Jun. 2012 P. 8 Institute of microelectronics OEIC Lab.

National Cheng Kung The schematic diagram of the p-Al. Ga. N/i-Mg. Zn. O/i-Zn. O/i. University Mg. Zn. O/n-Zn. O: In UV LEDs Mg. Zn. O CB VB A energy level schematic diagram of the Mg. Zn. O/Mg. Zn. O double heterostructure u Carrier confinement u Enhance the radiative recombination rate Jun. 2012 P. 9 Institute of microelectronics OEIC Lab.

National Cheng Kung University The influence factor of the light-extraction efficiency of the LEDs：[1] Ø Critical angle loss (internal total reflection) Snell`s law n 1 sin c = n 2 sin 90 Þ c =sin-1(n 2 / n 1) n 2 n 1 ØFresnel loss T+R = 1 R = (n 2 - n 1)2 / (n 2 + n 1)2 T = 1 -R = 4 n 2 n 1/ (n 22 +2 n 2 n 1 + n 12) Reduce the light extraction loss Jun. 2012 P. 10 n 2 n 1 > n 2 c T R Ref：[1] S. M. Sze, Semiconductor Devices: Physics and Technology. New York: Wiley, 2002. Institute of microelectronics OEIC Lab.

National Cheng Kung The schematic cross sectional view of the UV LEDs University u Deposited the transparent oxide films of the Si. O 2 and Ti. O 2 , respectively, on the top and sidewall of the UV LEDs by using a RF sputtering system. u The contribution of the oxide passivation layer ： Refractive index： n(air) = 1 n(Si. O 2) = 1. 45 n(Zn. O) = 2 n(Ti. O 2) = 2. 3 Jun. 2012 P. 11 p reduce the light extraction loss p reduce the leakage current Institute of microelectronics OEIC Lab.

National Cheng Kung Results and discussion University The properties of the p-type Al. Ga. N p The energy band gap of the p-type Al 0. 18 Ga 0. 82 N layer was about 3. 71 e. V. p Activation： 750 °C in N 2 ambient for 30 min p Hole concentration = 3. 0 × 1017 cm-3 , Hole mobility = 3. 86 cm 2/V-s Hall measurement results of the films deposited by the vapor cooling condensation system Jun. 2012 Thin film Carrier concentration (cm-3) Mobility (cm 2/V-s) i-type Zn. O 3. 12× 1015 6. 2 n-type Zn. O: In Mg. Zn. O 3. 62× 1019 × 3. 7 × P. 12 Institute of microelectronics OEIC Lab.

National Cheng Kung University 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 Thin film Mg. Zn. O i-Zn. O n-Zn. O: In Optical energy bandgap (Eg) 4. 01 e. V 3. 27 e. V 3. 25 e. V Ref：[1] Phys. Stat. Sol. , 15, 627 (1966). Jun. 2012 P. 13 Institute of microelectronics OEIC Lab.

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

National Cheng Kung Photoluminescence spectra University Near-band edge (NBE) emission u The photoluminescence spectra was excited by a He–Cd laser with a wavelength of 325 nm. u The NBE emission peak of the itype Zn. O film at 380 nm was observed. u Defect emission at the visible region was small enough. Jun. 2012 P. 15 Institute of microelectronics OEIC Lab.

National Cheng Kung Why the films deposited at low-temperature have lower University defect concentration? Room temperature photoluminescence spectra of the high temperature (HT)-Zn. O films and the low temperature (LT)-Zn. O 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-Zn. O thin films deposition at low temperature by vapor cooling condensation technique, ” J. Appl. Phys. , 108, 073119 (2010). Jun. 2012 P. 16 Institute of microelectronics OEIC Lab.

National Cheng Kung University Current-Voltage measurement u A typical rectifying behavior was clearly observed by the semiconductor parameter analyzer. . u The forward turn-on voltage and the reverse breakdown voltage were about 3. 25 V and -9. 4 V, respectively. Jun. 2012 P. 17 Institute of microelectronics OEIC Lab.

National Cheng Kung Electroluminescence spectra University u The emission peaks were at 380 nm u EL peak intensity and total emission power of double heterostructured. UV LEDs were much higher, about 3. 08 and 1. 82 times. visible region Jun. 2012 u Only a pure UV emission was observed, without defect emission at the visible region. P. 18 Institute of microelectronics OEIC Lab.

National Cheng Kung Conclusions University u High quality Zn. O and Mg. Zn. O film with low defect concentration were successfully deposited by the vapor cooling condensation system. u The UV LEDs with a pure UV emission and without defect emission at the visible region was achieved. u Double heterostructure was contributed to the carrier confinement and the enhancement of the radiative recombination rate in the active i-Zn. O layer. u The EL emission peak intensity and the total emission power of the double heterostructured-UV LEDs were much higher than that conventional UV LEDs. Jun. 2012 P. 19 Institute of microelectronics OEIC Lab.

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