Applications of AAO technology on Optoelectronics

1. Applications of AAO technology on Optoelectronics AAO技術應用於光電元件 Prof. Mau-Phon Houng （洪茂峰 教授） Department of Electrical Engineering National Cheng Kung University

2. Introduction Experiment AAO on LED AAO on Solar cell AAO on AR Conclusion Outline

3. Introduction:LED package

4. Introduction (Cont’d) • The internal quantum efficiency of AlGaInP light-emitting diodes (LEDs) can achieve near 100% and GaN LEDs can achieve over 80%, but extraction efficiency is much small. • The main reason for a low ηext is resulted by the total internal reflection (TIR) happened at the interface of semiconductor and air. Smart Lighting

5. Introduction (Cont’d) • The narrow escape cone for spontaneous emission covers a solid angle of • A mere 2% and 4% of the internally generated light can escape into free space in AlInGaP and GaN LEDs. • The Critical angle reflection and Fresnel reflection → Low light extraction efficiency Compound Semiconductor, 8, 39, 2002.

6. Introduction (Cont’d) LEDs with Textured (Rough) Surface Nanomask + Etching Mater. Sci. Eng., B, 136, 182, 2007. Thin Solid Films, 515, 768, 2006. Low Temp. Growth PEC Wet Etching Mater. Sci. Eng., B, 112, 10, 2004. Appl. Phys. Lett., 84, 855, 2004.

7. Introduction (Cont’d) 陽極氧化鋁(anodic aluminum oxide)，簡稱為AAO，是一種具有六邊形(hexagon)高規則孔洞陣列結構之氧化鋁，其孔道筆直且均勻，並且可以用人工的方式來控制孔洞的大小、深度以及密度，因此多孔氧化鋁可以作為蝕刻或沉積光罩以及資訊的儲存。

8. Formation: Al(s) → Al3+ + 3 e- H2O → 2 H+ + O2- 2 Al3+ + 3 O2- → Al2O3 Dissolution: Al2O3 + 6 H+ → 2 Al3+ + 3 H2O About 30% of the current produced dissolved species, while 70% of the current produced solid oxide. Mechanisms of PAA Formation

9. In traditional anodizing experiment, the aluminum is put into the electrolyte vertically by an alligator clip. Due to the much stronger electric field gathering in the surface, it makes the aluminum foil broken. We designed a sample holder made by Teflon, it can cause a uniform electric field and prevent the electrolyte from soaking, and a low temperature experiment condition. Ordered PAA Mask Fabrication

10. Experiment (Chemical Polishing and Electropolish） • The solution of chemical polishing is a mixture H3PO4 (86 %) : HNO3 (50 ~ 70 %) : H2O = 8 : 1 : 1 (by volume ratio) for 10 minutes and the solution temperature was kept to be 80 °C. • The electrolyte of the electrolytic polishing is a mixture of HClO4 (aq) and C2H5OH in a volume ratio of 1 : 4. A constant voltage of 20 V was applied between the cathode and the anode for 60 ~ 90 sec, and the solution temperature was kept to be 5 °C during electropolishing. Two-step Anodization

11. Two-step Anodization (Polished Al) Unpolished: Polished:

12. Wafer Cleaning Anodization Using evaporation system to evaporate aluminum on LED’s p-side with 200 nm thickness. Anodizing in 20 V, 25 °C, 0.3 M phosphoric acid for 8 ~ 11 minutes. Pore widening in 25 °C, 5 wt% phosphoric acid for 0, 40, 80 minutes. (The “0 minute” means PAA film without pore-widening) Photolithography Experiment AlGaInP LEDs with PAA Films

13. At 0 pore-widening time: The highest values than that of the others Forward voltage Dynamic resistance The contact resistance and the surface defects formed at the interface between the PAA and GaP window layer, Hinder the lateral current spreading Enhance the surface recombination current. The curves corresponding to the samples pore-widening at 40 min and 80 min pore-widening time more than 40 min, a removal of PAA film inside the nano-pores eventually causes the GaP window layer direct exposure to the air. IV Curves

14. Avoid a correspondingly high level of light power to be internally reflected within the device. Inserting PAA between air and the GaP window layer roughly Increasing the critical angle Decreasing the Fresnel reflection Using the refractive indices 3.5, 1.7, and 1 for GaP, PAA, and the air. LI Curves Fresnel Reflection (Normal Incidence):

15. Another reason is due to the natural porosity of the PAA film Regarded as light scattering centers The sample with 0 pore-widening time is 29 % which is lower than 39 % of the same sample followed by pore-widening at 40 min. As being a scattering center, larger pore size lowers the light to be effectively emitted through the surface by scattering. The highest forward voltage and the dynamic resistance to lessen the radiative recombination. The samples with pore-widening at 40 min and 80 min have the same electrical performances, LI Curves (Cont’d)

16. The intensity of the scattered light is inversely proportional to the fourth power of the wavelength EL & CIE

17. Summary I • PAA-LEDs • Depositing a porous anodic alumina (PAA) film at the top surface of GaP window layer could enhance the AlGaInP LED to have 29 % light output power improvement as compared with the conventional device. • Nnanopores of porous alumina also can be enlarged (~0.4nm/per min) in the phosphoric acid. • The diameter of nano-pores varies form 30 nm to 60 nm comparing with conventional LED. We obtained the best improvement is 39 % at 20 mA in light output intensity with appropriate pore distribution after 40 minutes pore-widening treatment.

18. AAO on Solar Cell Institute of Microelectronics and Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan

19. Institute of Microelectronics and Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan

20. Institute of Microelectronics and Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan

21. Institute of Microelectronics and Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan

22. Institute of Microelectronics and Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan

29. AAO on AR Institute of Microelectronics and Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan

30. AR Layer • The bare Silicon in intimate contact with air reflectsabout 35% of the incident power over all wavelengths of interest in solar cell work. • Reflectance depends on the indices of refraction ofthe materials involved, the angle ofincidence and the polarization state ofthe incident light. • Increasing optical path length is important: Randomly roughened surfaces Periodic gratings Bragg reflectors

31. (1) Design of AAO/TiO2 DUAL LAYER ANTI REFLECTION • In practical process, the refractive index of silicon layer would be 3.9 (at 640 nm). • According as two-quarterwavelength coatingsequation (1), n1=1.57, n2=2.46 d1=102nm, d2=65nm • nAAO=1.51, nTiO2=2.49 dAAO=100 nm, dTiO2=65 nm • Why double-layer? • (1) two minima can be achieved in the reflectance spectrum, leading to a lower spectral weighted reflectance. • (2) their optical performance is less sensitive to layer thickness variations.

32. Reflectance/Transmittance of AAO (1) (3) (2)

33. Reflectance of TiO2 (2) (1)

34. SEM

35. Reflectance Average reflectance : 8.5 % Average reflectance : 4.3% S. Winderbaum, F. Yun, and O. Reinhold, “Application of plasma enhanced chemical vapor deposition silicon nitride as a double layer antireflection coating and passivation layer for polysilicon solar cells”, J. Vac. Sci. Technol. A 15(3), May/Jun 1997

36. SummaryII • The DLAR structure of AAO/TiO2 has the advantage of decreased reflectance by the light trapping (increase of the light path length), interference or diffraction effect. • Compared to SiNx double-layer antireflection coating, AAO does not have the problem of high absorption for short wavelength. It is attributed to the very low extinction index (nearly zero) of AAO.

37. Conclusion 光罩 奈米點 規則性結構 模板 奈米線 AAO 提升LED強度 表面粗化 光子晶體 全角度反射器 折射率周期性變化 某個頻率範圍

38. Conclusion (Cont’d) 氧化鋁的製作成本低、生產快、步驟簡單並且可以用人工的方式來控制孔洞的大小、深度以及密度，因此多孔氧化鋁可以作為蝕刻光罩、沉積光罩、光子晶體和資訊儲存等多項的應用。