Fabrication of sub-micrometer periodic nanostructures using pulsed laser interference for efficient light trapping in optoelectronic devices

Surface nano-texturing can play an important role for efficiency enhancement of light emission and absorption in optoelectronic devices through reduced surface reflection or enhanced broadband absorption. Periodic and uniform semiconductor nanostructures are highly applicable in bandgap tuning applications but are quite challenging to realize through conventional techniques. We present the fabrication of large area and uniform square lattice based periodic nanostructures with 300 - 400 nm spatial periodicity on a GaAs substrate using pulsed laser interference. Single pulses from a plane-polarized pulsed laser working at 355 nm with 20-50 mJ energy and 7 ns pulse duration are used in a conventional four beam interference geometry at an incidence angle of 36.3° to realize square lattice patterns on photoresist coated over the GaAs substrate. The optical properties of the proposed designs are studied using FDTD simulations and show more than 95% of electromagnetic energy trapping over a broad optical wavelength range. This semiconductor based nanostructuring technology can find applications in improving the efficiency of solar cells or light emitting devices.


INTRODUCTION
Photonic bandgap tunability and modification are important aspects in photonic design of semiconductor materials for enabling unique optoelectronic properties [1]. Periodic nanowires made using surface nanostructuring are an active area of semiconductor research with potential applications in solar cells and optoelectronic components [2][3][4]. Photonics integration is advantageous in semiconductors to utilize a broad solar spectrum for photovoltaic applications [5]. A large surface to volume ratio and reduction in refractive index contrast between the semiconductor and air are achieved through photonics-based wave guiding and confinement of modes in ordered 2D nanowires, where there is a confinement along XY plane and a free space propagation along the third direction.
Nanowires based on III -V semiconductors, specifically GaAs have been widely studied for solar cells [6], light emitting diodes [7], and photodetectors [8]. In general, semiconductor nanowire structures with typically a few hundred nm diameters and one to few micrometers of axial lengths can allow electromagnetic wave localization along the XY plane with the channelization of electromagnetic waves in the axial direction. Furthermore, the lifetime of the semiconductor devices is another important aspect which is limited by the surface recombination alongside the nanowire structures. These structures can be grown using various growth techniques such as selective area metalorganic vapor phase epitaxy [9] and molecular beam epitaxy [10]. Although these techniques are well-established for semiconductors, uniform and regular periodic growth of nanowires are still a challenge. In addition, the growth of such structures is very much dependent on the crystal orientation of the substrate. Fabricating nanowires in different crystal orientations are still a challenge using the existing bottom-up techniques. Therefore, a regular and uniform fabrication technique with proper scalable design parameters over large area are important to semiconductor photonics. A periodic patterning without the dependence of crystal orientations would add versatility to the semiconductor-photonic integration exploring wide applications.
Uniform integration of photonic and optoelectronic devices in semiconductors are also possible through laser interference lithography (IL) as a large area, low-cost and rapid fabrication approach [11][12][13][14][15]. In this work, we propose a single-step, large area pulsed laser-based interference lithography of four beams to realize sub micrometer scale periodic photonic structures over GaAs substrate towards nanoscale structuring or the growth of uniform nanowires. The current study is aimed at design and fabrication of interfaces for enhanced photonic wave guidance and light trapping towards absorption enhancement. We present square lattice based photonic GaAs nanowire designs, simulations and interference lithography-based fabrication approaches towards enhance light trapping mechanism. FDTD based simulation studies are carried out to show the optical absorption properties of GaAs nanowires in photonic lattices. Single-pulsed laser interference lithography is used to achieve nanostructuring on GaAs substrates. In future, this kind of study can find application in solar photovoltaics, where the device fabrication can be initiated with a positive electron beam resist to obtain similar periodic holes in GaAs for growth of nanowires resulting in pn or p-i-n type of solar cells.

FDTD Simulation
We have carried out FDTD based simulation using Lumerical's FDTD solution module for studies on the optical properties of the patterned GaAs nanowire arrays in a square lattice as per the model shown in Figure1(a). The diameter of the nanowires (d) is 160 nm, periodicity of square lattice is 300 nm and the axial height (Z span) is varied from 1000 to 2500 nm for the maximization of optical absorbance. Absorbance is calculated as A = 1-(R+T). In case of a square lattice, we observe 94% absorbance for a Z span of 1µm of the nanowires that increases to 97% in case of a Z span of 2.5 µm as shown in Figure1 (b). It is also inferred that although, the nanowires with maximum Z span of 2.5 µm show improved absorbance, the change in absorbance with respect to the Z span is not significant. Therefore, for a low-cost fabrication approach that uses less material, processing time and fabrication feasibility, we propose Z span of 1 µm as the preferred design for the solar cell applications. The electric field distributions through the GaAs nanowire photonic crystal showing electromagnetic energy trapping through photonic crystal-based wave guiding are presented in Figure1(c) and 1(d) at two different wavelengths 727 nm and 567 nm.

FEM Simulation
We have carried our finite element method-based simulation using COMSOL Multiphysics on the designed square arraybased GaAs photonic crystal with a lattice constant of 400 nm, diameter of 160 nm and height of 1µm. The surface normal plots of the electric field distribution in XY plane or the presence of different eigen modes are shown in Figure2. Figure 2. Surface normal plots of electric fields or eigen modes due to the GaAs nanowires arranged in a square lattice (a-i) presents at different eigen frequencies related to 788 nm, 762 nm 559 nm, 530 nm, 511 nm, 450 nm and 439 nm.

MATLAB Simulation using four beams interference
We have simulated the resultant irradiance profiles due to four plane-polarized interfering beams using MATLAB plane beams and have viewed the effect of interference angle on it. The irradiance profile due to the superposition of planepolarized beams leading to the regular square lattice is given by:  (1) Where, P n is the polarization vector, K n are the propagation vectors associated with each interfering plane beam. Φ n is the initial phase or the phase of the interfering plane beams. The irradiance profile due to interference of the plane beams is presented in Figure. 3 (a -b) for two different interfering angles showing different lattice spacing over same volume. Figure 3 (c) shows the intensity distribution over XY plane.  We have carried out conventional 4 beam laser interference lithography in a vertical geometry as shown in Figure 4 using a nanosecond pulsed laser (Spitlight 1000, INNOLAS Laser GMBH, Germany). We have considered single pulses for laser with powers varying from 15 -40 mJ per pulse to examine the threshold of the photosensitive material. We have used the 3rd harmonic of an Nd: YAG laser at 355 nm, which is a TEM00 mode with horizontal polarization. The beam path in Figure 4 achieves vertical polarization in the plane of the beams impinging the substrate. Figure 5 shows a surface analysis study using SEM of the realized patterns on a photoresist (AZ1514H) coated crystalline substrate. A uniform square lattice of periodicity approximately 300 nm is realized through this technique for an interference half angle of 36.3 º.

EXPERIMENT AND RESULT ANALYSIS
Initially, we have recorded the patterns on positive PR coated GaAs substrate and developed for 5 -30 seconds for an exposure of 15 -30 mJ to realize square array structures as shown in Figure5 (a-d). We have carried out a wet-etching of GaAs using Hydrobromic acid (37%): Potassium dichromate: acetic acid (100%) in a ratio 1:1:1 followed by washing with DI water and the SEM image of the samples are presented in Figure5 (e -f). However, after cleaning the samples with acetone, we could not obtain any patterned region. This may occur due to incomplete exposure of resist till the substrate interface. We have carried out electron beam lithography-based patterning on GaAs substrate and performed ICP etching to obtain nano-structured GaAs interface and performed optical characterization studies through spectroscopy for the sake of verification of simulation results on the absorbance of the nano-structured GaAs. SEM image of the EBL patterned, and etched GaAs interface is presented in Figure6(a) and the optical reflectance (R) absorbance (1-R) properties measured through a visible spectrometer is presented in Figure6(b). It is observed that a reverse patterning in GaAs has reduced the surface reflection up to 11% from 43% for the case of bare GaAs substrate. This shows enhanced optical absorption of 89% from 450 -800 nm wavelength range for the nanostructured GaAs interface for light trapping applications towards solar photovoltaics. .

CONCLUSION
We have carried out a single-pulsed interference lithography for rapid and large area nanostructuring over GaAs substrate. This study is intended towards the surface nano-texturing and ordered growth of nanowires for nanophotonics applications in semiconductor materials. To study the applications, we have carried out FDTD based simulation studies on the optical properties of the designed nanowire array photonic structure. We have observed through simulation and experiment that nanostructuring over GaAs have reduced the surface reflections and enabled high absorption up to more than 97% and 89% respectively over a broad wavelength range (450 -800 nm). We believe, this kind of study is highly helpful towards semiconductor nanophotonics for solar energy harvesting.