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InGaN-based light-emitting solar cell (LESC) structure with an inverted pyramidal structure at GaN/sapphire interface was fabricated through a laser decomposition process and a wet crystallographic etching process. The highest light output power of the laser-treated LESC structure, with a 56% backside roughened-area ratio, had a 75% enhancement compared to the conventional device at a 20 mA operating current. By increasing the backside roughened area, the cutoff wavelength of the transmittance spectra and the wavelength of the peak photovoltaic efficiency had a redshift phenomenon that could be caused by increasing the light absorption at InGaN active layer.
©2010 Optical Society of America
1. Introduction
Gallium nitride materials have attracted considerable interest in the development of optoelectronic devices like light-emitting diodes (LEDs), laser diodes, and white LED [1]. However, bright blue LEDs require an increase in their internal and external quantum efficiencies. The lower external quantum efficiency of the InGaN-based LEDs is due to a larger refractive index difference between the GaN layer and the surrounding air (Δn~1.5). Bottom patterned Al2O3 substrates [2,3], top p-type GaN:Mg rough surface processes [4], two-floor air prism arrays as embedded reflectors [5], anisotropically etched GaN-sapphire interface [6], and hexagonal conelike surface by laser-lift-off technique [7] have been used to increase light-extraction efficiency in InGaN-based LEDs on Al2O3 substrates. Stocker et al. [8] reported that crystallographic wet chemical etchings of the n-GaN have (0001), {}, {}, {} and {} stable planes. The multiple-quantum well (MQWs) structures as an absorption layer in InGaN-based solar cell [9–12] exhibited the photovoltaic characteristic. The solar cell with a thin absorption layer bonding onto mirror-coated Si substrate [13] had been reported to enhance light absorption. The InGaN material has a stronger absorption coefficient, a wide bandgap region by varying the Indium content, a radiation hardness property, and a high open-circuit voltage for the photovoltaic applications. Jung et al. [14] reported that the surface texturing of GaN-based light emitting diodes and solar cells by KOH-based PEC etch could enhance the efficiency of GaN-based photonic devices.
In this paper, the inverted pyramidal structures were fabricated on the InGaN-based light-emitting solar cell (LESC) structure through a laser decomposition process and a wet etching process at the GaN/sapphire interface. The treated InGaN LESC structure with an inverted pyramidal structure provided a higher light scattering process to increase the external quantum efficiency. The cutoff wavelength of the transmittance spectra and the peak wavelength of photovoltaic efficiency had strong dependent on the backside roughened-area ratio. Here, the structure and optical characteristics of the laser treated LESCs structures are analyzed in detail.
2. Experiments
InGaN-based LESC structures were grown on a two-sided polished optical-grade C-face (0001) 2”-diameter sapphire substrate by using a metalorganic chemical vapor deposition (MOCVD) system. These LESC structures consisted of a 30nm-thick GaN buffer layer, a 12μm-thick n-type GaN layer, 10 pairs of the InGaN/GaN multiple quantum wells (MQWs) active layers, and a 0.4μm-thick magnesium-doped p-type GaN layer. The active layers consisted of a 30Å-thick InGaN-well layer and a 70Å-thick GaN-barrier layer. The LESC chips were treated by using a triple frequency ultraviolet Nd:YVO4 (355nm) laser for the front-side laser isolation process and the backside GaN decomposition process. The dimension of the chip was 570 × 240 µm2 in size defined by the laser scribing process, and the mesa region of 540 × 210 µm2 was defined by the inductively coupled plasma (ICP) etcher using Cl2 gas. During the laser decomposition process at GaN buffer layer, the laser spot was focus on the bottom GaN/sapphire interface. The periodical striped-line patterns were fabricated through a backside laser scanning process on GaN buffer layer. After the laser decomposition process, the periodical width of each striped-line pattern and the ratio of the roughened area divided by the mesa area were measured at 60µm (29%), 50µm (32%), 40µm (39%), and 30µm (56%) for the RB60-LESC, the RB50-LESC, the RB40-LESC, and the RB30-LESC, respectively. The GaN buffer layer decomposed as Ga metal and N2 gas through the laser decomposition process. The LESC wafer was immersed in a hot KOH solution (KOH, 80°C) for a 15-minute lateral crystallographic wet etching process [15] that occurred at the laser treated striped-line pattern regions. A 240nm-thick indium-tin-oxide layer (ITO) was deposited on the mesa region as a transparent contact layer (TCL). The Cr/Au metal layers were deposited as n-type and p-type contact pads.
The LESC device that was fabricated through this process flow without a laser decomposition process was defined as a standard LESC (ST-LESC). The chosen ST-LESC and RB-LESC devices were both located at the 2” wafer center to allow for analysis of the optical and electrical properties in more similar material properties. The geometric morphology of these LESC structures was observed through a scanning electron microscopy (SEM). The light-output power, electroluminescence (EL) spectra, and electrical properties were characterized by an optical spectrum analyzer (Ando-6315A) and a precision semiconductor parameter analyzer (Agilent 4156C). The light-intensity profiles that went across the whole LESC sample were measured by a beam profiler. The photovoltaic efficiency of the ST-LESCs and the RB-LESCs were analyzed using a keithly 236 source meter, and a monochromatic illumination that was obtained by using 500 W Xe lamp with a monochromator (with a 5nm spectral resolution).
3. Results and discussion
The schematic of the RB-LESC structure was shown in Fig. 1(a) . The fabricated procedures consisted of a front-side laser scribing process, a backside laser decomposition process, and a bottom-up wet etching process. Then, the inverted pyramidal structure at the striped-line region was formed at the GaN/sapphire interface as a roughened backside surface. The laser decomposition region of the RB40-LESC structure was observed at the cross-section SEM micrographs shown in Figs. 1(b) and 1(c). The width and spacing of the striped-line patterns were measured at the values of 15µm and 25µm, with a 40µm periodical width, for the RB40-LESC structure. The bottom-up etching process occurred on the N-face GaN layer vertically along the [0001] direction. The {} face group of the GaN layer has the most stable lattice planes with the lowest surface energy [16], anisotropic etching occurs with a continuous consumption of the () N-face and the gradual exposure of the stable {} terminal faces of the GaN layer. The inverted pyramidal structure acted an embedded wave-like air reflector to increase the light reflected and the light scattering processes in the RB-LESC structures.
Fig. 1 (a) The schematic diagram of the RB-LESC structure with an inverted pyramidal structure at GaN/sapphire interface is shown here. (b)(c) The laser scanning striped-line patterns and the inverted pyramidal structure were observed on the cross-sectional SEM micrographs of the RB-LESC structure.
To analyze the light-intensity distribution over the entire LESC chips at a 20mA operating current, the line-scanning light-intensity profiles of the LED samples are observed in Fig. 2 . The resolution of the beam profiler is 0.35μm/pixel, where the mesa width of the LESC image is 540μm. The width of the higher light intensity striped-line region was measured at the value of 21μm for the RB-LESC structure that the laser treated width is 15μm shown in Fig. 1(c). The light emission intensity of the RB-LESC structure with the backside roughened striped-line region had a 3.5 times higher than the untreated region. In the RB-LESC structure, a higher and wider light-intensity region was observed at the laser-treated striped-line regions that had larger light-scattering process occurred on the roughened backside N-face GaN surface.
Fig. 2 The light-intensity profiles of (a) the RB60-LESC structure, (b) the RB50-LESC structure, (c) the RB40-LESC structure, and (d) the RB30-LESC structure at a 20mA operation current are measured by a beam profiler.
In Fig. 3(a) , the peak wavelength of the EL spectrum was measured at about 456 nm for all LESC structures at a 20 mA operating current. The EL emission intensity was increase by adding the roughened-area ratio at GaN/sapphire interface. In Fig. 3(b), the operating voltage and the light-output power as functions of the pulsed mode injection current were measured. The pulsed mode with 100 μs pulses and a 1% duty cycle was applied on both LESC structures to prevent self-heating [17]. The operating voltages of all the LESC samples were almost the same, at the value of 3.55 V at 20 mA, because the laser-treated striped-line regions were located at the GaN/sapphire interface without any effect on the top active layer. The light output power of the RB30-LESC had a 75% enhancement compared to the ST-LESC at 20 mA. In Fig. 3(c), the peak external quantum efficiency of the RB30-LESC structure was measured at a 2mA operating current that had a 77% enhancement compared to the ST-LESC structure. A similar efficiency droop effect (about 70%) was observed for all the LESC structures at a 100mA operation current that indicated the laser treatment process did not affect the electric property of the RB-LESC structures.
Fig. 3 (a) The EL spectra of the LESC structure were measured at 20 mA. (b) The current-voltage (I-V) characteristics and the light-output power as a function of the operating current are measured. (c) The relativity external quantum efficiency of the LESC structures were measured by varying the pulsed injection current. (d) The transmittance spectra of the LESC structures were measured.
In Fig. 3(d), the transmittance spectra of the LESC structures were measured that the incident Xe light illuminated at the p-type GaN layer and received from the backside sapphire substrate. The cutoff wavelength of the ST-LESC structure was measured at 377.4 nm. By increasing the backside roughened area through the laser decomposition process, the cutoff wavelength was slightly redshifted to 390.2 nm and the transmittance was decreased. This phenomenon could be caused by increasing the light scattering process at the inverted pyramidal structure and by increasing the light absorption at the InGaN active layer. The redshift phenomenon of the cutoff wavelength could be caused by the Rayleigh scattering process on the roughened N-face GaN structure. The shape of the N-face inverted pyramidal structure was continuously tapered from a film layer to a sharp cone-tip structure that increased the Rayleigh scattering process at short wavelength light region.
In Fig. 4(a) , the open-circuit voltage (Voc), short-circuit current density (Jsc) of all the LESC structures were also measured under the illumination of air mass (AM) 1.5G condition. All the LESCs structures had the same Voc values of 2.3 V. The Jsc value of the ST-LESC was measured at a 0.61 mA/cm2. By reducing the periodical width from 60µm to 30µm, the Jsc values of the RB-LESC structures were increased from a 0.83mA/cm2 (BR60-LESC) to a 1.23 mA/cm2 (BR30-LESC) by increasing the backside roughened area at the mesa region. The external quantum efficiency (EQE) of the photovoltaic characteristic as a function of the illuminated wavelength was measured as shown in Fig. 4(b). The peak EQE values were measured at 0.42 (at 371.4 nm), 0.54 (at 372.5 nm), 0.64 (at 373.0 nm), 0.70 (at 373.1nm), and 0.75 (at 373.4 nm) for the ST-LESC, RB60-LESC, RB50-LESC, RB40-LESC, and RB30-LESC, respectively, listed in Table 1 . The peak wavelength of the EQE spectra of the RB-LESC structures had a slightly redshift phenomenon by increasing the backside roughened-area ratio. The short wavelength light has higher scattering process on the sharp cone-tip compared to the long wavelength light that can increase the external quantum efficiency for the LESC devices. By increase the light scattering process at the short wavelength light region on the cone-shaped GaN structure, the peak wavelength of the EQE spectrum was redshifted that caused by the redshift phenomenon of the cutoff wavelength. The peak EQE value and the short-circuit current density of the LESC structures were measured as a function of the backside roughened-area ratio shown in Fig. 4(c). The higher EQE value and the larger short-circuit current density were observed by increasing the roughened-area ratio. The highest EQE value and the Jsc value were measured at 0.75 (at 373.4 nm) and 1.23mA/cm2 for the RB30-LESC structure that had a 56% roughened-area ratio. The short-circuit current densities as a function of the wavelength of an illuminated light source for all the LESC structures were measured shown in Fig. 4(d). The peak Jsc current density and the peak wavelength were measured at 0.61mA/cm2 (389.4nm) for the ST-LESC structure and 1.23mA/cm2 (392.8nm) for the RB30-LESC structures. By increasing the backside roughened-area ratio, the peak Jsc value was increased and the wavelength of the peak Jsc value was slightly redshifted for the RB30-LESC structures. The wavelength of the peak EQE value of the RB-LESC structure was shifted to longer wavelength region that could be caused by a higher light absorption phenomenon at the InGaN well layers of the MQW active layer structure. By forming the N-face inverted pyramidal structures in the LESC structures, the light output power and the external quantum efficiency of the photovoltaic properties has been enhanced through the high light scattering process at the GaN/sapphire interface. The inverted pyramidal structure was continuously tapered from film layer to sharp cone-tip structure through the crystallographic wet etching process. Then, the light scattering process occurred at the nanoscale cone-tip structure was increased through the Rayleigh scattering process at short wavelength light region that can enhance the EQE values of the LESC devices.
Fig. 4 (a) The current density-voltage (J-V) characteristics of the LESC devices measured. (b) The external quantum efficiencies (EQE) of the photovoltaic characteristic were measured as a function of the wavelength of the illuminated light source. (c) The peak EQE value and the short-circuit current density of the LESC structures were measured as a function of the backside roughened-area ratio. (d) The short-circuit current density were measured as a function of the wavelength of the illuminated light source.
Table 1. The experimental data including the cutoff wavelength, the wavelength of the peak EQE values, and the wavelength of peak Jsc values of all the LESC devices
4. Conclusion
The RB-LESC structures with the inverted pyramidal structures were formed through a laser decomposition process and a bottom-up N-face crystallographic etching process. By increasing the roughened-area ratio at the GaN/sapphire interface, the external quantum efficiency of the RB-LESC structure was increased through a higher light scattering process occurred at the inverted pyramidal GaN surface. The cutoff wavelengths of the transmittance spectra and the wavelength of the peak solar cell efficiency had the redshift phenomenon to increase the photovoltaic efficiency of the RB-LESC devices. The LESC devices with the inverted pyramidal structures at GaN/sapphire interface had high light-extraction efficiency for the high efficiency nitride-based LESCs application.
Acknowledgement
The authors gratefully acknowledge the financial support for this research from the National Science Council of Taiwan under grant No. NSC 98-2221-E-005-007-MY3 and No. NSC 98-2622-8-005-002-A2. The authors also thank Professor Pin Han with National Chung Hsing University for his contributions.
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