Thin Solid Films 518 (2010) 7377–7380
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
InGaN-based light-emitting solar cells with a pattern-nanoporous p-type GaN:Mg layer Kuei-Ting Chen, Chia-Feng Lin ⁎, Chun-Min Lin, Chung-Chieh Yang, Ren-Hao Jiang Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan, ROC
a r t i c l e
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Available online 8 May 2010 Keywords: InGaN-based optoelectronic device Photoeletrochemical (PEC) process
a b s t r a c t InGaN-based light-emitting solar cells (LESCs) with a nanoporous micro-pattern array (NMPA) p-type GaN: Mg structures were fabricated through a photoeletrochemical (PEC) process. The photovoltaic property of these NMPA devices was analyzed. The higher light output power and light absorption properties were observed from the NMPA structure compared with the standard devices. The nanoporous structures acted as an anti-reflection layer to increase the light-coupling process at the light propagation surface. The light output power of the NMPA-LESCs had a 41% enhancement at 20-mA operating current, compared to standard LESCs (ST-LESCs). The peak external quantum efficiencies (EQE) were measured as the values of 42% (at 365 nm) and 27% (at 370 nm) for the NMPA-LESCs and the ST-LESCs structures, respectively. The photovoltaic device structure with the NMPA structure had the higher EQE at the ultraviolet (UV) region for higher efficiency nitride-based solar cell devices applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The indium gallium nitride-based (InGaN-based) optoelectronic devices have recently attracted extensive investigation because of their wide applications in different industries such as light-emitting diodes (LEDs), laser diodes, and photovoltaic devices. The InGaNbased LED with the patterned sapphire substrates [1], the photonic crystal structures [2], the textured p-type GaN:Mg surface from nanoimprint technology [3], the nano-sphere lithography [4], and the vertical-type LED covered with a patterned high-index layer [5] were all used to improve the light extraction efficiency. Mostly, the surface texturing processes were fabricated through an inductively couple plasma (ICP) dry etching process that affected the electrical properties of the LED structures. A bias-assisted photoelectrochemical (PEC) wet etching method was reported to roughen the p-GaN surface effectively in potassium hydroxide (KOH) solution [6]. Otherwise, the photovoltaic devices fabricated from InGaN alloys had been reported in recent years [7]. The multiple-quantum well (MQW) structures as an absorption layer in InGaN-based solar cell [8] also exhibit a desirable photovoltaic characteristic. In order to maximize the conversion efficiency of InGaN solar cell, methods like bonding onto mirror-coated Si substrate [9], textured top p-type InGaN layer with an anti-reflective coating indium tin oxide (ITO) layer [10] were proposed to achieve this objective. ⁎ Corresponding author. E-mail addresses:
[email protected] (K.-T. Chen), cfl
[email protected] (C.-F. Lin),
[email protected] (C.-M. Lin),
[email protected] (C.-C. Yang),
[email protected] (R.-H. Jiang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.007
In this paper, the nanoporous micro-pattern array (NMPA) structures on the p-type GaN:Mg surface were fabricated through a PEC wet oxidation process in deionized water and an oxide-removing process in a diluted hydrochloric acid (HCl) solution. The continuous-nanoporous structures distributed around the micro-pattern region could enhance the light output power and light absorption of the InGaN/GaN MQW active layer for the LED and photovoltaic device. The light output power and the photovoltaic characteristics of the PEC treated InGaN-based light-emitting solar cells (LESCs) are discussed in more detail here. 2. Experimental details The LESC structures consisted of a 1 μm-thick unintentionally doped GaN layer, a 3 μm-thick n-type GaN layer, ten pairs of InGaN/ GaN MQW active layers, a 0.05 μm-thick p-type AlGaN:Mg layer, and a 0.3 μm-thick p-type GaN:Mg layer. The active layers consisted of ten pairs each with a 3 nm-thick InGaN well layer and a 7 nm-thick GaN barrier layer in the InGaN/GaN MQW structures. A 2-in. LESC wafer was divided into two half, and one of the half was used to fabricate the conventional devices (without the PEC treatment), and the other was the nanoporous micro-pattern array devices. A 200 nm-thick titanium (Ti) metal was deposited on the top p-type GaN:Mg surface by an electron beam (E-gun) evaporation system, and 5 μm-diameter disk array patterns were defined through a photolithography process. An 800 W mercury (Hg) lamp was used as the front-side illumination source during the PEC process. An external direct current (DC) bias fixed at a positive 15 V was applied to the p-type GaN:Mg layer surface as the anode contact, and platinum was applied as the cathode. The LED structure with a nanoporous micro-pattern array
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structure was defined as the NMPA-LESCs structure, and the schematic diagram of the NMPA-LESCs structure consisted of a micro-disk array pattern and the a continuous-nanoporous p-type GaN:Mg surface as shown in Fig. 1. The mesa region of the LESCs was 260 × 260 μm2 in size. An ITO layer was deposited on the p-GaN surface as a transparent contact layer (TCL). Cr/Au metal layers were deposited on the p-type GaN:Mg and n-type GaN:Si surfaces as the contact electrodes. The conventional LESCs without the PEC oxidation process was defined as the standard LESCs (ST-LESCs). Surface morphology was observed through a field-emission scanning electron microscopy (FE-SEM). The electroluminescence (EL) spectra and light output power were characterized on the LED chips by an optical spectrum analyzer (Ando-6315A) and a precision semiconductor parameter analyzer (Agilent 4156C). The EQE of the ST-LESCs and NMPA-LESCs were analyzed using a microscopy-probe station, a Keithly 236 source meter, and a monochromatic illumination that was obtained by using 500 W xenon lamp with a monochromator (with a spectral resolution of about 5 nm). 3. Results and discussion After a 40 min PEC pattern-oxidation process, the NMPA structures were observed on the mesa region shown in Fig. 2(a) and (b). By removing the gallium oxide (GaOx) layer in the diluted HCl solution and the Ti mask in the hydrofluoric acid (HF) solution, the average diameters were measured at about 4.0 μm for disk-pattern of the NMPA structure. The average etching depth of the NMPA samples was measured at 0.2 μm after four sequential PEC processes. The oxidation rates were calculated as the values of 0.3 μm/h in the vertical direction and 0.75 μm/h in the lateral direction. The average pore size of the PEC treated nanoporous structures was measured at a value of about 80 nm estimated from the higher magnification SEM micrographs as shown in Fig. 2(b). From the SEM micrograph shown in Fig. 2(a), the nanoporous structure was obtained through the PEC oxidation process except the individual 4 μm-diameter disk-pattern array that was protected by Ti metal disk patterns. The percentages of the nanoporous area on the mesa region were calculated as the value of 90.4%. By forming the patterned-nanoporous structure, the higher light extraction efficiency during the LED operation and the higher light-coupling property increased the peak EQE for the solar cell operation. The forward voltages and light output powers of both LESCs structures were measured as functions of the injected current as shown in Fig. 3(a). The forward voltages and series resistances (Rs) with a 20-mA operating current were measured as 3.33 V (21.1 Ω) and 3.84 V (28.4 Ω) for the ST-LESCs and NMPA-LESCs, respectively. The slightly higher operation voltage in the NMPA-LESC structure could be caused by the higher contact resistance between the
Fig. 1. The schematic diagram of the LESCs structure with the nanoporous micro-pattern structure on p-type GaN:Mg surface.
Fig. 2. (a) A patterned-nanoporous structure was observed on the mesa region that was distributed around the micro-disk array pattern. (b) The average pore size is about 80 nm that was estimated from the SEM micrographs.
roughened p-type GaN:Mg layer and the ITO layer at the NMPA regions. The light output power of the NMPA-LESCs had a 41% enhancement compared to that of the ST-LESCs at a 20-mA operating current. The higher light output power of the NMPA-LESCs was caused by increasing light scattering and extraction processes occurring at the nanoporous micro-pattern array GaN:Mg surface. Fig. 3(b) shows the current density–voltage (J–V) characteristics of the ST-LESCs and the NMPA-LESCs measured under the illumination of air mass (AM) 1.5G condition. The open-circuit voltage (Voc), shortcircuit current density (Jsc) and fill factor (FF) of both LESCs were also shown and listed in Fig. 3(b). Both of the LESC structures had the same Voc values of 2.3 V. The Jsc values were measured as 1.5 mA/cm2 and 2.4 mA/cm2 for the ST-LESCs and NMPA-LESCs, respectively. The higher Jsc of the NMPA-LESCs could be attributed to enhance the light absorption and reduce the light reflectance in InGaN/GaN active layers by forming the patterned-nanoporous p-type GaN:Mg surfaces. This argument could be confirmed by the reflection spectra as shown in Fig. 4(b). The FF values of the ST-LESCs and the NMPA-LESCs are 62% and 58% respectively. Sheu et al. [11] reported that the high Rs of their photovoltaic device could be determined around 74 Ω due to the GaN/ InGaN superlattice absorption layer with the number of heterointerfaces and resulted in a low FF value. The Rs values were calculated at 100 Ω for the ST-LESCs and 220 Ω for the NMPA-LESCs. Therefore, the lower FF value of the NMPA-LESCs could be attributed to the higher Rs value during the solar cell operation by forming the pattern-
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Fig. 3. (a) The light output power and the operation voltage as a function of the operating current and (b) J–V characteristics under the illumination of AM 1.5G were measured for both LESC structures.
nanoporous structure on p-type GaN:Mg surface. This result is consistent with the forward voltage characteristic of the NMPALESCs in LED operation. Fig. 4(a) shows the emission wavelength of the EL spectra were measured at 460.3 nm for ST-LESCs and 457.7 nm for NMPA-LESCs with a 20-mA operating current. The blueshift phenomenon of the EL emission spectra may be caused by the partial compress strain release on the InGaN active layer by forming the nanoporous structure on the top p-type GaN:Mg layer. The relative reflection spectra of both LESC structures were measured shown in Fig. 4(b). The relative reflectivity of the NMPA-LESCs with the patterned-nanoporous structure was lower compared with the ST-LESCs. This lower reflectivity at shorter wavelength range (360 nm to 500 nm) of the NMPA-LESCs indicated that the patterned-nanoporous p-GaN:Mg surface provided a higher light-coupling phenomenon and a higher external quantum efficiency (EQE). Fig. 5 shows the EQE spectra of the ST-LESCs and the NMPA-LESCs measured as a function of excitation wavelength from 300 nm to 500 nm. The peak EQE values of ST-LESCs and NMPA-LESCs were measured as the values of 27% (at 370 nm) and 42% (at 365 nm), respectively. The higher response was observed at the shorter wavelength region in the NMPA-LESCs structure. The peak EQE response of the LESCs with NMPA structure was shifted to the shorter
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Fig. 4. (a) The EL spectra and (b) relative reflection spectra of both LESCs were measured.
wavelength region that could be caused by the higher light absorption phenomenon at the GaN barrier layers and lower piezoelectric effect at the InGaN well layers for InGaN/GaN MQW active layers.
Fig. 5. The external quantum efficiency (EQE) for the ST-LESCs and NMPA-LESCs were measured by varying the wavelength of excitation light at room temperature.
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4. Conclusions The NMPA structures on the p-type GaN:Mg layers were fabricated through the PEC oxidation and the oxide removal processes. The high EQE of the NMPA-LESCs was observed at the ultraviolet (UV) region that was caused by forming the NMPA structures as the anti-reflection and the light-coupling layer. The light output power and the photovoltaic conversion efficiency were increased by forming the nanoporous micropattern structures on the p-type GaN:Mg layer that can be used for higher efficiency optoelectronic device applications. Acknowledgement The authors gratefully acknowledge the financial support for this research by the National Science Council of Taiwan under grant Nos. NSC 95-2221-E-005-132-MY3, NSC 98-2221-E-005-007-MY3, and 982622-E-005-009-CC3.
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