Realization of an extremely low reflectance surface based on InP porous nanostructures for application to photoelectrochemical solar cells

Sato, Taketomo; Yoshizawa, Naoki; Hashizume, Tamotsu

doi:10.1016/j.tsf.2010.02.029

Cited by 21 publications

(24 citation statements)

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“…This phenomenon is very similar to the case of InP porous nanostructures. 19 In samples C and B , pores with enlarged openings appeared on the surface, where air holes are closely aligned between the thin GaN walls. This kind of Figure 8 Photoelectrochemical characteristics of non-porous and porous GaN: sample A formed at t a = 300 s, sample B formed at t a = 600 s, sample C formed at t a = 1800 s, and sample B formed at t a = 600 s after H 3 PO 4 treatment.…”

Section: Resultsmentioning

confidence: 94%

“…The most well-known application of an electrochemical process is the formation of porous nanostructure by anodic etching in which a high-density array of nanometer-or micrometer-sized pores is formed with high productivity over a large area on the semiconductor surface. 17 We have recently reported that InP porous nanostructures showed low photoreflectance and high photoabsorption, 18,19 which are very promising features for porous nanostructures used in the photoelectric conversion devices such as photodetectors and solar cells. Besides, the electrochemical process is applicable to various semiconductors, [20][21][22][23][24] even chemically stable materials such as GaN, [25][26][27][28][29] without causing processing damage.…”

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confidence: 99%

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Correlation between Structural and Photoelectrochemical Properties of GaN Porous Nanostructures Formed by Photo-Assisted Electrochemical Etching

Kumazaki¹,

Watanabe²,

Yatabe³

et al. 2014

J. Electrochem. Soc.

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We investigated the correlation between structural and photoelectrochemical properties of GaN porous nanostructures formed by photo-assisted electrochemical etching. The porous nanostructures were formed during light irradiation of the top-surface of homo-epitaxial layers grown on freestanding GaN substrates. The pore depth, wall thickness, and surface morphology of porous nanostructures were strongly influenced by the way holes generated by the light irradiation were supplied. Such structural features influenced the optical properties of GaN porous nanostructures. The photoluminescence peaks measured on GaN porous nanostructures were shifted to higher energies because of the quantum confinement in the thin GaN walls between pores. Formation of porous nanostructure decreased the photoreflectance of the GaN surface, and the smallest reflectance was obtained from the porous sample having large pores on its surface after the ultrathin layer with small pores had been removed by surface-etching. The photoelectrochemical response measured on GaN porous nanostructures in a NaCl electrolyte were drastically enhanced by the unique features of those structures, such as low photoreflectance and large surface area. The largest photocurrents were obtained from the sample from which H 3 PO 4 treatment had removed the ultrathin layer without thinning the pore walls. Photoelectrochemical systems based on semiconductor photoelectrodes have recently attracted much attention due to their potential use in the next generation of green technologies such as water splitting for fuel cells, artificial photosynthesis, and so on.1-5 Among the photoelectrode materials, GaN is one of the most attractive because of its chemical stability and its potential to achieve direct photoelectrolysis by solar power without the consumption of electric power. [6][7][8] In addition, the bandgap energy of GaN-based materials can be varied from about 0.65 to 6.0 eV by alloying them with InN and AlN, which enables us to design various functional photoelectrodes not only for spectral matching of solar light but also for the electrochemical reduction of CO 2 to carbohydrate. 9 One of the common approaches to improving conversion efficiency is to form nanostructures on the photoelectrode surface in order to increase its surface area. Most reported GaN nanostructures have been made using selective-area growth 10,11 or a dry etching process such as reactive ion etching. 12,13 There are, however, severe limitations on increasing the density of nanostructures because most approaches use lithography for defining the size and position of the nanostructures. And when a dry etching process is involved, the etching damage induced by ion bombardment is not negligible [14][15][16] and could significantly degrade the photoelectrochemical efficiency.One alternative approach is an electrochemical-fabrication process, which can form various semiconductor nanostructures in a self-assembled fashion. The most well-known application of an electrochemical process is the formation...

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Section: Resultsmentioning

confidence: 94%

mentioning

confidence: 99%

Correlation between Structural and Photoelectrochemical Properties of GaN Porous Nanostructures Formed by Photo-Assisted Electrochemical Etching

Kumazaki¹,

Watanabe²,

Yatabe³

et al. 2014

J. Electrochem. Soc.

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“…5,6 In addition, this technique is performed at room temperature and does not require any complicated process such as lithography, indicating lower damage and higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth.…”

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confidence: 99%

“…[1][2][3][4] Among the various techniques for improving their conversion efficiency, porosification utilizing EC reactions is one of the most powerful because a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. 5,6 In addition, this technique is performed at room temperature and does not require any complicated process such as lithography, indicating lower damage and higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth. [7][8][9][10] Many reports involving porosification use photo-assisted EC etching utilizing charge carriers generated by band-edge absorption.…”

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confidence: 99%

Precise Structural Control of GaN Porous Nanostructures Utilizing Anisotropic Electrochemical and Chemical Etching for the Optical and Photoelectrochemical Applications

Kumazaki

Matsumoto

Sato

2017

J. Electrochem. Soc.

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A low-damaged wet process utilizing electrochemical (EC) etching and subsequent chemical etching has been developed for the fabrication of GaN porous structures. Superior controllability in depth and diameter could be obtained by achieving anisotropic nature of the vertical direction to the substrate by EC etching and horizontal direction by tetramethylammonium hydroxide (TMAH) etching, respectively. The optical and photoelectrochemical properties of GaN porous structures were very sensitive to the structural properties. Photoreflectance measurement revealed that porous sample had an effective refractive index that could be controlled by TMAH etching time. In photoelectrochemical measurement, the incident-photon-to-current conversion efficiency (IPCE) was dramatically enhanced to as high as 91% by the formation of porous structures. A series of experimental results were consistently explained by the change of thickness of pore wall and width of space charge region. Gallium nitride (GaN) and its alloys are getting much attention as building block materials for electrochemical (EC) energy conversion systems such as chemical sensors, water splitting, and artificial photosynthesis because of their attractive features including superior chemical stability, direct transition, and widely tunable bandgap by alloying.1-4 Among the various techniques for improving their conversion efficiency, porosification utilizing EC reactions is one of the most powerful because a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. 5,6 In addition, this technique is performed at room temperature and does not require any complicated process such as lithography, indicating lower damage and higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth. 7-10Many reports involving porosification use photo-assisted EC etching utilizing charge carriers generated by band-edge absorption. 11-14Our group has also succeeded in forming GaN porous nanostructures by photo-assisted EC etching and found the energy-conversion efficiency to be enhanced compared with the planar substrate. 15 However, the pore depth could not be controlled linearly by etching time, resulting in difficulty with forming pores deeper than a micro-meter. This was because charge carriers, most of which were generated near the top surface, were expended preferentially in the lateral etching of pore walls that appeared on the top surface. Such insufficient anisotropic nature makes it difficult to improve structural controllability and form pores deeper than a micro-meter.In this study, we aimed to develop anisotropic EC etching by utilizing charge carriers generated by the avalanche effect as a porosification process. We also investigated a combination of EC etching and subsequent wet chemical etching to further improve the structural controllability. Optical characterization such as photoluminescence (PL), photoreflectance, and photoelectrochemical measure...

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“…Electrochemically formed porous structures, which have been applied for various materials, [4][5][6][7] are promising nanostructures for the above-mentioned applications. Many researchers have reported the electrochemical formation of GaN-porous structures.…”

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Formation of GaN-Porous Structures Using Photo-Assisted Electrochemical Process in Back-Side Illumination Mode

Watanabe

Kumazaki

Yatabe

et al. 2015

ECS Electrochemistry Letters

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We investigated the structural features of gallium-nitride-porous structures formed using the photo-assisted electrochemical process in the back-side illumination (BSI) mode. The pore diameter and depth were strongly affected by the direction of illumination, where higher controllability was achieved compared with front-side illumination. The spectroscopic measurements revealed that illumination with photon energy below the bulk bandgap plays an important role in pore formation. We propose a formation model by considering the Franz-Keldysh effect that can consistently explain the obtained experimental data in which anodic etching occurs only at the pore tips under the high electric field induced in the depletion region. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0031505eel] All rights reserved.Manuscript submitted December 26, 2014; revised manuscript received February 5, 2015. Published March 14, 2015 Gallium nitride (GaN) is a III/V direct bandgap semiconductor that exhibits high-thermal, chemical, and mechanical stability and is relatively harmless to humans and the environment because it does not contain toxic substances such as arsenic (As). Most notably, GaN is a wide-bandgap material with 3.4 eV of energy, which can be varied from 0.65 to 6.0 eV by alloying it with indium nitride (InN) and aluminum nitride (AlN). On the basis of these excellent properties, various functional devices have been designed, for example ultraviolet (UV) laser diodes, 1 chemical sensors, 2 and photo-electrodes for water splitting.3 The well known challenge to improve these device's performance is to use nanostructures in which the unique optical and electrical properties, such as quantum effects and increased surface sensitivity, appear with a large surface area to volume ratio.Electrochemically formed porous structures, which have been applied for various materials, 4-7 are promising nanostructures for the above-mentioned applications. Many researchers have reported the electrochemical formation of GaN-porous structures.8-10 However, their formation mechanism has not been fully understood. One of the reasons is that the photo-assisted electrochemical process is commonly used in which the formation process becomes more complicated due to the supply of photo-carries generated by illumination. We previously argued that it is difficult with front-side illumination (FSI) to control the structural properties of GaN-porous structures. 11We believe that the optimization of the supply of photo-carriers is one of the key issues for controlling of the structural properties.In this study, we formed GaN-porous structures using the photoassisted electrochemical process in the back-side illumination (BSI) mode, for the first time, and compared with it that in FSI mode. From both experimental and theoretical aspects, we discuss the formation mechanism of GaN-porous structures in BSI mode.The electrochemical setup used in this study is schematically shown in Figure 1. A custom-made cell equipped with a crystal window and Indium Tin Oxi...

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Realization of an extremely low reflectance surface based on InP porous nanostructures for application to photoelectrochemical solar cells

Cited by 21 publications

References 20 publications

Correlation between Structural and Photoelectrochemical Properties of GaN Porous Nanostructures Formed by Photo-Assisted Electrochemical Etching

Correlation between Structural and Photoelectrochemical Properties of GaN Porous Nanostructures Formed by Photo-Assisted Electrochemical Etching

Precise Structural Control of GaN Porous Nanostructures Utilizing Anisotropic Electrochemical and Chemical Etching for the Optical and Photoelectrochemical Applications

Formation of GaN-Porous Structures Using Photo-Assisted Electrochemical Process in Back-Side Illumination Mode

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