High Transmittance Ti doped ITO Transparent Conducting Layer Applying to UV-LED. Y. H. Lin and C. Y. Liu

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1 High Transmittance Ti doped ITO Transparent Conducting Layer Applying to UV-LED Y. H. Lin and C. Y. Liu Department of Chemical Engineering and Materials Engineering, National Central University, Jhongli, Taiwan 32001, R. O. C. Abstract: In this experiment, the optical and electrical properties of Indium Tin Oxide (ITO) thin films with different weight percentage of Ti were investigated. By controlling the weight percentage of Ti and the annealing ambient, the transmittance at 380 nm of Ti doped ITO thin film is 90.2 % and the resistivity is Ω-cm after 10 % Ti doping and annealing the thin film at 500 in vacuum. Besides basic physical properties measurements, the Ti doped ITO thin film was applied to UV-LED, too. The output power was enhanced 52.1 % by applying Ti doped ITO thin films to be the transparent conducting layer for p-contact metal layer. Introduction: In many optoelectronic devices, transparent conducting layer (TCL) plays an important role due to it supplies a way to be a conducting electrode or a current spreading layer but it does not absorb the light. Commonly TCL are ITO, ZnO, and AZO which have low resistivity and high transmittance in visible region. Especially, ITO which contains 90 % In 2 O 3 and 10 % SnO 2 is a widely used transparent conducting layer. Nowadays, ultra violet optical devices, such like UV-LED, are investigated to enhance the light output power or some spectacular purpose that the optical properties in UV or near UV region are more important 1,2. According to Kuo s results 3, the light output power of near-uv LED (at 400 nm) could be enhanced 1

2 36 % by applying to ITO instead of Ni/Au as ohmic contact layer due to ITO has higher transmittance performance at 400 nm. Yet, the band gap of ITO is about 3 ev that it absorbs the light which wavelength shorter than 400 nm seriously that the transmittance of ITO in UV region would be low 4. In our previously study, ITO transmittance diagram would be affected by doping metals, and we found the band gap is enlarged by Experiment: In this experiment, Ti:ITO thin film was made by depositing ITO and metal thin films alternatively by sputtering system and the concentration of Ti was controlled by different thickness of Ti. Before the thin films deposition, quartz substrates were immersed in ACE, IPA, and DI water and ultra-sonically cleaned for 5 minutes, 2 minutes, and 5 minutes, respectively, and then Ti and ITO thin films were deposited on quartz substrates by sputtering system. 50 W DC power was used to deposit the Ti (99.99 %) and 100 W RF power was used to deposit ITO (In 2 O 3 90 % and SnO 2 10 %). After thin films deposition, all samples were annealed at 300 to 500 in oxygen and vacuum ambient for 30 minutes. Reflectivity was measured by JASCO 470 spectrometer and Van der Pauw method was used to determine the carrier concentration, hall mobility, and resistivity of ITO/Ti multi-layers. Fig. 1 shows the resistivity of (a) pure ITO and (b) Ti:ITO (10 wt.%) annealed in the oxygen and the vacuum ambient at different temperatures. The lowest resistivity of Ti:ITO (10 wt.%) is Ω-cm when annealed at 400 in vacuum. For both cases, the resistivity of samples annealed in the vacuum ambient are lower than the resistivity of the samples annealed in the oxygen ambient due to ITO thin film would create more free carriers when partial pressure of oxygen in the ambient is low. 2

3 Two electrical conduction mechanisms of ITO are reported; (1) substitution of Sn atoms by In atoms, and (2) oxygen vacancies formation, both mechanisms create the free electrons. While ITO annealed at the oxygen ambient, the high oxygen concentration at the ambient caused the decrease of the oxygen vacancies in ITO. As a result, the carrier concentration in the ITO samples annealed in oxygen ambient would be greatly decreased, as shown in Fig. 2. Although carrier concentration of the thin films were enhanced slightly or remained the same with as-deposited thin films, the mobility of the thin films were increasing with increasing temperatures that the resistivity could be lowered, as shown in Fig. 3. Figure 4 shows the transmittance of ITO and Ti:ITO with different annealing temperatures. The transmittance of the as-deposited Ti:ITO thin film is small due to the absorption of sandwiched Ti layers. Upon the annealing process, the sandwiched Ti layers diffused and reacted with ITO to form TiO 2. Therefore, the transmittance of Ti:ITO increases with increasing annealing temperature and the transmittances of thin films could be enhanced by Ti doping into ITO film and annealing. The highest transmittance could reach 90.2 % after annealing at 500. According to the transmittance results, the band gap of thin films could be calculated from the follow relations T=B exp(- αt) and (αhυ) 2 =B (hυ-eg). Where T is transmittance, B is edge width parameter, α is absorption coefficient, h is Planck s constant, υ is wave frequency, and Eg is band gap. When the incident light energy is higher than the band gap of the thin film, the band transition would occur, and the transmittance would be decreased seriously that we can calculate the band gap energy by plotting (αhυ) 2 versus hυ. From the 3

4 calculation results, the band gap of pure ITO and Ti:ITO thin films annealed at 500 are 3.1 ev and 3.3 ev, respectively 4. Furthermore, when the free electrons were created, they would occupy the bottom energy level of the conduction band, and then the electrons in the valence band have to absorb higher energy of the incident light to transit to the higher energy level of the conduction band, so called Burstein Moss effect 5. The Burstein Moss effect also observed in Figure 4, the band edge of Ti:ITO samples annealed in vacuum are smaller than the Ti:ITO samples annealed in oxygen. From carrier concentration results, the band edge blue shifted is due the higher carrier would be created when Ti:ITO thin films under lower oxygen ambient. To evaluate the Ti:ITO used as the current spreading layer for UV LED, Ti:ITO TCL is processed on 380 nm UV LED. Ni/ITO (10 nm/200 nm) and Ni/Ti:ITO (10 %) (2 nm/200 nm) were deposited on p-gan as the current spreading layer. The Ni layer serves as adhesion layer and contact layer. Then, Ti/Al/Ti/Au (200 nm/1 μm/500 nm/200 nm) multi-layer was deposited on the n-gan layer for the n-pad. Figure 5 is the L-I-V curve of the UV LED chips with ITO and Ti:ITO current spreading layers. LED chip with Ti:ITO current spreading layer has lower forward voltage than that of the pure ITO current spreading layer might due to p-gan/ni/ti:ito has lower contact resistance than p-gan/ni/ito. Yet, remarkably, the photo current of UV LED chip with Ti:ITO TCL is enhanced by 52.1 %. Because of the transmittance difference between ITO and Ti:ITO at 380 nm is around 20 % but the light output power is enhanced by 52.1 %, the transmittance difference should not be the only reason for the enhancement, there might be some other electrical properties affect the light output power results. Figure 6 are the emission intensity distribution of UV LED chips with Ti:ITO 4

5 and ITO current spreading layer under 250 ma. Suppose to the intensity is proportional to the current injecting into multi quantum well 6, it shows that ITO thin film could spread the injecting current better than Ti:ITO thin film due to p-gan/ni/ito has higher contact resistance because of the resistivity of these two films are similar. Figure 7 show the thermal images of UV-LED chips with different current spreading layer at 250 ma current injecting, the highest temperature of chip with ITO and Ti:ITO current spreading layer are 39.5 and 39.2, respectively. Comparing to chip with Ti:ITO thin film, chip with ITO thin film has higher temperature, since the heat spreading ability should be the same between these two schemes that the higher temperature and poor temperature distribution should be due to ITO/p-GaN has worse contact resistance and produces more Joule heat and then increases the temperature. The higher junction temperature results in the peak wavelength of chip red shifted more seriously 7, as shown in Figure 8. The higher junction temperature would also decrease the recombination efficiency of the quantum well and result in the UV LED chips with Ti:ITO current spreading layer has higher output power under the same injecting current than chips with ITO current spreading layer, as shown in Figure 9. Conclusion: In this study, after annealing, the transmittance of ITO thin film can be enhanced after inserting Ti layers and also remaining the low resistivity. During the thermal process, the oxygen partial pressure in the annealing ambient is a variable to affect the carrier concentration, and it also affects the transmittance spectrum. Applying Ti:ITO (10%) annealed at 500 in vacuum, which has highest transmittance at 380 nm(90.2 %) and lowest resistivity ( Ω-cm), to be the current spreading layer for UV LED (380 nm), the photo current could be enhanced 52.1 % comparing 5

6 with pure ITO. From light distribution and thermal image, the transmittance enhancement and lower Joule heat generation are the main reasons for the light output power enhancement. Acknowledgement: Reference: 1. Enhanced light output of GaN-based power LEDs with transparent Al-doped ZnO current spreading layer, IEEE Photonics Technology Letters, 18, No. 1 (2006) 2. AlGaN-based ultraviolet light-emitting diodes grown on AlN epilayers, Applied Physics Letters, 85, No. 20 (2004) 3. Nitride-based near-ultraviolet LEDs with an ITO transparent contact, Materials Science and Engineering, B106,69 72, (2004) 4. Highly reflective and low-resistant Ni/Au/ITO/Ag ohmic contact on p-type GaN, Electrochemical and Solid-state Letters, 7, G102-G104 (2004). 5. Effects of electron concentration on the optical absorption edge of InN, Applied Physics Letters, 84, No Efficiency of GaN/InGaN light-emitting diodes with interdigitated mesa geometry Applied Physics Letters, 79, No.13, 1936 (2001) 7. Junction and carrier temperature measurements in deep-ultraviolet light-emitting diodes using three different methods, Applied Physics Letters, 86, (2005) 6

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14 Captions: Figure 1: Resistivity of (a) ITO and (b) Ti:ITO (10 %) annealed in oxygen and vacuum ambient at different temperatures. Figure 2: Carrier concentration of (a) ITO and (b) Ti:ITO (10 %) annealed in oxygen and vacuum ambient at different temperatures. Figure 3: Hall measurement results of (a) ITO and (b) Ti:ITO (10 %) annealed in oxygen and vacuum ambient at different temperatures. Figure 4: Transmittance results of Ti:ITO (10 %) thin films annealed in (a) oxygen and (b) vacuum. Figure 5: LIV curve of UV LED with ITO and Ti:ITO current spreading layer Figure 6: Relative photo current distribution of UV LED chip under injecting 250 ma current. Figure 7: Thermal image of UV LED chip with (a) ITO and (b) Ti:ITO current spreading layer. Figure 8: Peak emission wavelength versus different injecting current of different TCL. Figure 9: Photo current distribution of UV LED chip under injecting 250 ma current. 14

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