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,Outline,I
2、ntroductionExperimentsResults and discussionConclusions,The applications of the UV light：Anti-counterfeiting detectorsUV treatmentAir and water purificationThe advantages of the UV LEDs： PortableSafetyLong lifetimeEnvironmental protection no mercury (Hg) pollution,Introduction,The advantages of the
3、ZnO semiconductor：Wide direct band gap (3.37 eV)Large exciton binding energy (60 meV)Low cost, high stability, non-toxicThe 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.
4、Sel. Top. Quantum Electron., 14, 1048 (2008).4 IEEE Photon. Technol. Lett., 20, 2108 (2008).,Characteristics of the ZnO and the MgO materials： 1,Ref：1 IEEE Photon. Technol. Lett., 20, 2108 (2008).,Lattice structure of the MgZnO films：,Ref：1 J. Appl. Phys., 94, 7336 (2003).,When the Mg content of the
5、 MgZnO films is lower than 36%, the lattice structure is still as hexagonal structure.1,The vapor cooling condensation system,Experiments,Deposition layers：i-type MgZnO filmi-type ZnO filmn-type ZnO:In filmDeposition conditions：Pressure ：10-4 torrDeposition Rate ：1 /sSubstrate Temperature：80K,ULEDs
6、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 500oC in an air ambient for 10 min in the rapid thermal anne
7、aling (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.,Fabrication process of the ZnO-based thin film double heterostructured-
8、ultraviolet light-emitting diodes (UV LEDs),The schematic diagram of the p-AlGaN/i-MgZnO/i-ZnO/i-MgZnO/n-ZnO:In UV LEDs,Carrier confinement Enhance the radiative recombination rate,A energy level schematic diagram of the MgZnO/ZnO/MgZnO double heterostructure,Critical angle loss (internal total refl
9、ection)Snells lawn1sinc = n2 sin90c =sin-1(n2 / n1),Fresnel lossT+R = 1R = (n2 - n1)2 / (n2 + n1)2 T = 1-R = 4 n2 n1/ (n22 +2n2n1 + n12),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 Technolo
10、gy. New York: Wiley, 2002.,The schematic cross sectional view of the UV LEDs,Refractive index：n(air) = 1 n(SiO2) = 1.45 n(ZnO) = 2 n(TiO2) = 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 S
11、iO2 and TiO2 , respectively, on the top and sidewall of the UV LEDs by using a RF sputtering system.,Hall measurement results of the films deposited by the vapor cooling condensation system,The energy band gap of the p-type Al0.18Ga0.82N layer was about 3.71 eV.Activation：750 C in N2 ambient for 30
12、min Hole concentration = 3.0 1017 cm-3 , Hole mobility = 3.86 cm2/V-s,Results and discussion,The properties of the p-type AlGaN,Transmittance and optical energy bandgap,d：Thickness：Absorption coefficientT ： Transmittanceh：Plancks constant：Photon frequency Eg：Optical energy bandgap,Tauc plot 1,Ref：1
13、Phys. Stat. Sol., 15, 627 (1966).,visible region,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 measu
14、red. 2,Photoluminescence spectra,Near-band edge (NBE) emission,The photoluminescence spectra was excited by a HeCd 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.,Why the films deposited at
15、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 HeCd 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.
16、Lee, “Mechanisms of high quality i-ZnO thin films deposition at low temperature by vapor cooling condensation technique,” J. Appl. Phys., 108, 073119 (2010).,Current-Voltage measurement,A typical rectifying behavior was clearly observed by the semiconductor parameter analyzer. .The forward turn-on v
17、oltage and the reverse breakdown voltage were about 3.25 V and -9.4 V, respectively.,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
18、-UV LEDs were much higher, about 3.08 and 1.82 times.,visible region,380 nm,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 re
19、gion 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.,Thanks for your attention!,
