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Optoelectronics & Communications

Nanorod photon management in nitride-based devices

Nanostructures fabricated on nitride-based devices improve their performance, enhancing the output of LEDs and the absorption of solar cells.
19 December 2011, SPIE Newsroom. DOI: 10.1117/2.1201112.004008

The unique properties of indium gallium nitride (InGaN) alloys, including their wide electronic band gap spanning 0.7–3.4eV, high absorption coefficient, high carrier mobility, and superior radiation resistance,1, 2 make them ideal candidates for optoelectronic devices. Intense research efforts since the mid-1990s have led to remarkable successes in LEDs. More recently, the promising photovoltaic characteristics of InGaN have also attracted increasing research interest.3–5

In the development of nitride-based optoelectronic devices, one of the main challenges is the difficulty of growing high-crystal-quality InGaN layers on GaN. GaN usually makes up p- and n-contacts. The lattice mismatch between InGaN and GaN can adversely affect device performance once the InxGa1−xN layer grows beyond a critical thickness on the GaN substrate.6 To address the issue, multiple quantum wells (MQWs) are used to form an active layer, ensuring excellent radiative recombination efficiencies for LEDs while also avoiding the undesired tradeoff between crystal quality and absorption efficiency for solar cells. Furthermore, the quantized energy levels in MQWs offer an additional level of control of solar absorption through proper selection of well and barrier widths without changing the indium content.7 Although progress has been made, many challenges still remain. For LEDs, the internal electrical field in MQWs that results from spontaneous and piezoelectric polarization leads to charge separation and reduces internal quantum efficiency.8 The great difference between the refractive index of GaN and that of air also prevents a high proportion of photons from escaping out of the device.9 In addition to the strong surface reflection caused by the large change in refractive index, the thin InGaN active layer also limits the absorption of solar energy, yielding suboptimal photovoltaic operation.

Figure 1. Process schematics for textured gallium nitride (GaN)-based LEDs. (a) SiO2/Ag (silica/silver) layers are deposited on indium tin oxide (ITO). (b) Material following thermal annealing at 270°for a few minutes and (c) following a reactive ion etching process. (d) The silver nanoparticles (NPs) are then removed by nitric acid.

Figure 2. Time-averaged, normalized transverse electric (TE) field distribution within GaN-based LEDs with two different surface conditions: (a) bare and (b) SiO2 nanorod arrays (NRAs)/roughened p-GaN. Ez: Electric field normal to the surface.

Figure 3. (a) Top-view and (b) cross-sectional scanning electron microscopy images of syringe-like zinc oxide (ZnO) NRAs. (c) Enlarged view of ultrasharp tips on ZnO NRAs.

Figure 4. Time-averaged and normalized TE field distribution within InGaN-based multiple quantum well (MQW) solar cells with three surface conditions: (a) bare, (b) flat-top NRAs, and (c) syringe-like NRAs. The MQW region is indicated by the red dashed lines. The insets in (b) and (c) show the enlarged images of the NRA tips.

However, thanks to advances in nanofabrication techniques, the adoption of various forms of nanostructures onto nitride-based optoelectronic devices has been shown to improve their performance. For example, nanorod arrays (NRAs) can lead to enhanced light emission or absorption due to the refractive index gradient they create. In solar cells, this facilitates light traveling across the device interface between air and GaN.

In this work, we will show that antireflective NRAs (AR-NRAs) also yield improved performance, for example, in LEDs. As shown in Figure 1, we first deposited silica (SiO2) and silver metal layers on ITO (indium titanium oxide) by electron beam evaporation. An etching mask of silver nanoparticles was then formed by thermally annealing the sample at 270°C for a few minutes. We obtained SiO2 AR-NRAs by reactive ion etching. Figure 2 shows the steady-state distribution of electromagnetic fields for both flat surfaces and roughened p-GaN/SiO2 NRA surfaces simulated by finite-difference time-domain (FDTD) analyses. We used Maxwell's equations to calculate the propagation of the waves. It can be seen that the light intensity in the integrated regions for the roughened p-GaN/SiO2 NRA surfaces is enhanced, compared with that for the flat surface. We attribute this result to the medium causing strong light scattering and thus increasing the external quantum efficiency (EQE).

We have also shown that ZnO (zinc oxide) AR-NRAs produced by a hydrothermal method hold great promise for InGaN-based MQW solar cells, requiring no lithography, operating well at low temperatures (<100°C), and exhibiting wafer-scale uniformity (>5in-diameter area).10 This layer exhibits superior AR performance when compared to conventional counterparts. Figure 3(a–c) shows scanning electron microscopy (SEM) images of ZnO NRAs grown on MQW solar cells. The average length of the rods in the NRAs is 1μm, and their diameter is 85nm. According to the Figure 3(a), the area density of NRAs is around 2.6×109cm−2. In Figure 3(b), we can see that the space-filling ratio of NRAs is increased in the bottom region. We can also see that NRAs are terminated with ultrasharp tips, around 200nm in height, visible in Figure 3(c). To evaluate light propagation in MQW solar cells, we carried out FDTD analysis: see Figure 4(a–c). The wavelength chosen for all simulations is 415nm, where a noticeable enhancement of EQEs by ZnO NRAs is observed. The values of the steady-state optical power integrated over MQW regions is 0.72 for bare devices, 0.80 for flat-topped NRAs, and 0.83 for the syringe-like NRAs. These results indicate that the NRAs not only facilitate wave propagation across interfaces but also increase the lateral distribution of fields as they do so. Figure 5 shows the current density-voltage curves of solar cells measured when illuminated by an air mass 1.5 international-standard solar simulator at a power level of 100mWcm−2. Compared with the bare surface, the syringe-like NRAs lead to an increase in the short-circuit current density (Jsc), indicating an enhanced solar absorption of the MQWs. The enhanced Jsc increases the conversion efficiency (η) up to 0.6%, an improvement in η of ∼36% as compared with the bare surface.

Figure 5. Current density-voltage (J-V) characteristics of InGaN-based MQW solar cells measured on bare and syringe-like NRA surfaces.

In summary, we have described two situations in which nanostructures can effectively improve the efficiencies of both LEDs and solar cells. We incorporated SiO2 NRAs onto the surfaces of InGaN-based MQW LEDs to improve light extraction from the active regions, and used syringe-like ZnO NRAs to boost the light-collection efficiency of MQW solar cells. Simulations based on FDTD analysis reveal that light propagation across the air/device interface is greatly improved on the application of antireflective nanostructures. We attribute the superior photon management of NRAs to subwavelength dimensions and geometries, improving surface reflection via a graded index profile. The design concept and fabrication techniques adopted here provide a viable scheme for photon collection in a wide variety of optoelectronic devices. We continue to develop novel nanostructures and sophisticated fabrication techniques, and to explore the excellent optoelectronic performance of InGaN-based MQW devices.

Guan-Jhong Lin, Cheng-Han Ho, Po-Han Fu, Jr-Hau He
Graduate Institute of Photonics and Optoelectronics
National Taiwan University
Taipei, Taiwan

Guan-Jhong Lin is working toward an MS. His research focuses on efficiency enhancement of nitride- and silicon-based optoelectronic devices.

Cheng-Han Ho is working toward his MS. His research focuses on efficiency enhancement in nitride-based optoelectronic devices.

Po-Han Fu is working toward his MS. His research focuses on efficiency enhancement in nitride-based optoelectronic devices.

Jr-Hau He is an associate professor at the Graduate Institute of Photonics and Optoelectronics and the Department of Electrical Engineering. His research focuses on designing new nanostructured architectures for nanophotonics and the next generation of nanodevices, including photovoltaics and resistive RAM. This interest is reinforced by his efforts to understand light scattering and trapping in nanostructured materials. In addition, he is interested in the transport of charge carriers across these solar cells and improving light coupling to increase the efficiency of photoinduced charge separation. His group is also concerned with fundamental physical properties of nanomaterials, such as the transport and switching behavior of nanowire field effect transistors. His work has fed back into the design of next-generation solar cells, having been picked up by the photovoltaics industry in Taiwan.

Kun-Yu Lai
Department of Optics and Photonics
National Central University
Taoyuan, Taiwan

Kun-Yu Lai is an assistant professor. His research interests include III-nitride optoelectronic devices and the optical properties of low-dimensional structures, such as quantum wells, quantum dots, and nanowires.

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