1054 IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 10, OCTOBER 2009 Enhanced Light Extraction in Wafer-Bonded AlGaInP-Based Light-Emitting Diodes via Micro- and Nanoscale Surface Textured Yea-Chen Lee, Hao-Chung Kuo, Senior Member, IEEE, Bo-Siao Cheng, Chia-En Lee, Ching-Hua Chiu, Tien-Chang Lu, Member, IEEE, Shing-Chung Wang, Life Member, IEEE, Tien-Fu Liao, and Chih-Sung Chang Abstract—AlGaInP-based metal-bonding light-emitting diodes (LEDs) with micro- and nanoscale textured surface were inves- tigated. The device surface with microbowls and nanorods were formed by a chemical wet-etching and dry-etching technique for enhancing light-extraction purpose. The luminous intensity could be enhanced 65.8% under 20-mA current injection as compared with the plane surface LEDs. The maximum wall-plug efficiency was achieved 14.1% at 7.5-mA operation. Index Terms—AlGaInP light-emitting diodes (LEDs), light- extraction efficiency, metal bonding, silica nanoparticles, spin coating, surface textured. T HE HIGH-EFFICIENCY light-emitting diodes (LEDs) of AlGaInP-based quaternary materials with a visible spectrum from red to yellow-green are widely applied for many applications such as optical communications light source, TFT-LCD back light, traffic signal, automotive, exterior light- ing, indoor and outdoor display, decorative lighting, etc. [1], [2]. Recent years, the epitaxy quality in AlGaInP material was greatly improved and internal quantum efficiency had already approached to 90% or higher [3] by excellent epitaxy technique. However, the external quantum efficiency is limited owing to the absorbing GaAs substrate and large reflection index different between AlGaInP-based material and outside medium (air or epoxy). According to the Snall’s law, the critical angle (θ c ) in AlGaInP-based LEDs is approximately 18 ◦ , and therefore the most generated photons could be trapped in the device from the total internal reflection (TIR) effect. Many advanced processes have been implemented in AlGaInP LEDs for enhancing light-extraction efficiency. The AlGaInP LEDs structure having a truncated-inverted-pyramid geometry GaP substrate was demonstrated [4]. Various surface rough- ness methods were applied in chip process [5]–[8]. Several Manuscript received May 18, 2009; revised July 13, 2009. First published September 1, 2009; current version published September 29, 2009. This work was supported in part by the MOE ATU Program and in part by the National Science Council of Republic of China in Taiwan under Contracts NSC 97- 2221-E-009-030-MY2, NSC 96-2628-E-009-017-MY3, and NSC 96-2221-E- 009-092-MY3. The review of this letter was arranged by Editor P. K.-L. Yu. Y.-C. Lee, H.-C. Kuo, B.-S. Cheng, C.-E. Lee, C.-H. Chiu, T.-C. Lu, and S.-C. Wang are with the Institute of Electro-Optical Engineering, Na- tional Chiao Tung University, Hsinchu 300, Taiwan (e-mail: alenlee.eo94g@ nctu.edu.tw; hckuo@faculty.nctu.edu.tw; lce.eo94g@nctu.edu.tw; timtclu@ faculty.nctu.edu.tw; scwang@cc.nctu.edu.tw). T.-F. Liao and C.-S. Chang are with the High Power Opto. Inc., Taichung 42827, Taiwan (e-mail: cyclen@hpoled.com.tw; pinechang@hpoled.com.tw). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2009.2028445 films which have transparent, electric conductivity, and lower refractive index properties were deposited on surface [9], [10]. The absorbing GaAs substrate was substituted by a transparent substrate of sapphire which has a geometric shaping sidewall via glue bonding and chemical etching [11], [12]. Since the devices substrate in this structure is transparent, the output light is all directions including chip sidewall and surface. In this investigation, the absorbing GaAs substrate was also substituted by a high thermal dispersion Si substrate with a high-reflection mirror interface. The major distinction of output light path between thin film and transparent substrate LEDs is surface and all directions radiation, respectively. Furthermore, microbowls array and nanorods surface-textured processes were also im- plemented in this letter, which were created using nanoparticle spin coating, and dry- and wet-etching techniques. Detailed experiment steps, device structure, performances, and results will be illustrated in subsequence paragraphs. AlGaInP epistructure was epitaxially grown on 2-in GaAs (100) substrates by a low-pressure metal–organic chemi- cal vapor deposition system. This structure with a domi- nant wavelength (λ d ) at 625 nm comprised a 0.1-μm-thick n-Ga 0.5 In 0.5 P etching stop layer grown on a GaAs buffer layer, a 2-μm-thick Si doped n-(Al 0.5 Ga 0.5 ) 0.5 In 0.5 P, a 0.5-μm-thick Si doped n-Al 0.5 In 0.5 P cladding layer, a 0.5-μm- thick unintentionally doped active layer with 20 periods (Al x Ga 1-x ) 0.5 In 0.5 P/(Al y Ga 1-y ) 0.5 In 0.5 P multiple quantum wells, a 0.8-μm-thick Mg doped p-Al 0.5 In 0.5 P cladding layer, a 5-μm-thick Mg doped p-GaP window layer, and finally an 8-μm-thick double window layer with an ultrathin GaAs layer was inserted between the p-GaP surface window layer and p-Al 0.5 In 0.5 P cladding layer [13]. Before the metal-bonding process, the AuBe/Au array of metal dots were contacted on the p-GaP surface as a function of p-type ohmic contact. The diameter, pitch, and thickness of circular AuBe/Au metal dots are 20, 90, and 0.15 μm, respectively. A SiO 2 layer with thickness equal to AuBe/Au metal was selectively deposited on the p-GaP window layer. A quarter-wave-thick indium tin oxide (ITO) was sequentially deposited. High-reflective mirror of 300-nm silver layer was deposited on the ITO layer to constitute the GaP-SiO 2 -ITO-Ag omnidirectional reflector structure for enhancing output power. Moreover, the Ti/W/Pt/Au multilayer was orderly stacked on Ag layer to serve a function of adhesion, barrier, and bonding metal. The surfaces of p-type Si substrate were successively deposited Ti/Au/In metal for ohmic contact and bonding material. The epiwafer was flipped and bonded to the Si substrate in 220 ◦ C ambience. After metal-bonding 0741-3106/$26.00 © 2009 IEEE Authorized licensed use limited to: National Chiao Tung University. Downloaded on March 29,2010 at 23:08:35 EDT from IEEE Xplore. Restrictions apply.