Kaushal Vora

Structural, compositional and optical properties of PECVD silicon nitride layers

We have investigated the correlation between the various plasma-enhanced chemical vapour deposition (PECVD) process parameters on the structural, compositional and optical properties of SiNx layers. The investigated process parameters are gas composition, radio frequency power and its frequency and deposition temperature. We also investigated SiON and ammonia-free SiNx layers. Refractive index, thickness, residual stress, structure and composition of the dielectric layers were determined using interferometry, wafer bowing, scanning electron microscopy, Fourier transform infra-red and secondary ion mass spectrometry measurements. SiNx films can be deposited to be almost stress-free with relatively low concentration of hydrogen (10 to 19% of atomic H). SiNx layers have the potential to cover a wide range of refractive indices (1.8–2.1) with possible extension down to 1.40 through various SiON layers.

Titanium Nano-Antenna for High-Power Pulsed Operation

While plasmonic nano-antennas can produce intense electric fields in a very small area, in general, these devices cannot handle high power, because of their small footprints. In order to increase the maximum peak power that these devices can withstand, they can be driven by nano-second pulses from a larger diameter Q-switched laser, which reduces the fluence reaching the devices, thus avoiding their destruction. Furthermore, we show that an increase in the power density capacity of the nano-antennas can be achieved by replacing gold with titanium: more than 18 dB greater power density can be handled by titanium based nano-antennas without significant reduction in their electric field enhancement capabilities.

Enhanced luminescence from GaN nanopillar arrays fabricated using a top-down process.

We report the fabrication of GaN nanopillar arrays with good structural uniformity using the top-down approach. The photoluminescence intensity from the nanopillar arrays is enhanced compared to the epilayer. We use finite difference time domain simulations to show that the enhancement in photoluminescence intensity from the nanopillar arrays is a result of anti-reflection properties of the arrays that result in enhanced light absorption and increase light extraction efficiency compared to the epilayer. The measured quantum efficiency of the nanopillars is comparable to that of an epitaxially grown GaN epilayer.

Manipulating second-harmonic light from semiconductor nanocrystals

Among the nonlinear behaviors exhibited by light, secondharmonic generation (SHG)1 is one of the most important. In SHG, the frequency of an incident light beam is doubled inside of a nonlinear crystal: see Figure 1(a) and (b). SHG is nowadays employed in many applications, including laser sources and nonlinear microscopy. SHG usually relies on bulk nonlinear crystals—see Figure 1(b)—such as lithium niobate, potassium titanyl phosphate, or beta barium borate. Unfortunately, these materials are difficult to integrate with other devices (due to the difficulties inherent in their manufacturing and machining) and are not costeffective. Furthermore, special phase-matching conditions are often required in order to obtain useful conversion efficiencies. Although the output beam profile in bulk crystals can be engineered by complex periodic poling,2 this technique is not easily accessible (due to its requirement for a spatially inhomogeneous distribution of high voltages across the crystals). To overcome these issues, it would be useful if we could replace bulk nonlinear crystals with ultrathin surfaces composed of nanocrystals that can generate SHG with high efficiency. Such nonlinear ‘metasurfaces’ could also be used to manipulate the SHG radiation pattern to form complex beams with arbitrary patterns: see Figure 1(c–e). This may sound like science fiction, but optical technology is rapidly advancing toward achieving Figure 1. (a) Schematic of the nonlinear process of second-harmonic generation (SHG), which doubles the frequency of light in a crystal. (b) A conventional SHG process within a bulk nonlinear crystal, generating a blue Gaussian beam in the forward direction. (c) SHG from small objects, such as anisotropic molecules, is emitted in both forward and backward directions, resulting in a dipolar radiation pattern resembling a figure eight. (d) For larger nanocrystals, the emission can differ in forward and backward directions due to the interference of several resonant modes (multipoles) inside the nanocrystal. (e) Our goal of initiating SHG within small nanocrystals to design a radiation pattern that creates a complex beam shape (e.g., a kangaroo) with high conversion efficiency. !: Angular frequency. .2/: Second-order susceptibility.

Charge trapping and storage in SiN x thin films deposited with Oxford PlasmaLab 100 system

Negative charges in silicon nitride films are beneficial for surface passivation of rear side of p type solar cells.[1] Previous studies indicates that N-rich SiNx films in an oxide-nitride-oxide structure display good charge trapping and storage ability. In this work, an Oxford PlasmaLab 100 PECVD system is used to vary the deposition conditions (temperature and RF power). SiNx films deposited at 400°C and RF=60W show an initial negative charge density of 1.2×1013/cm2 after negative charge injection. Modeling results suggest that tunnel oxide may not be necessary for achieving good charge stability, which will make the application more flexible.

Improved GaAs nanowire solar cells using AlGaAs for surface passivation

Semiconductor nanowire solar cells (NWSCs) have attracted significant interests owning to their potential in the applications of high-performance solar cells. Here, we report improved GaAs solar cell performance using a high-quality AlGaAs shell for surface passivation. The device exhibits an open-circuit voltage (Voc) of 0.70V, a short-circuit current density (Jsc) of 9.79 mA/cm2, a fill factor (FF) of 0.52, and a total power conversion efficiency (η) of 4.22%. This work suggests a promising route to optimize NWSCs.

Giant enhancement and control of second-harmonic radiation from AlGaAs nanoantennas

We fabricate AlGaAs nanoantennas on a glass substrate and demonstrate the highest nonlinear conversion efficiency of 10−4 with the capability for shaping the radiation patterns and polarization of the second harmonic emission in both forward and backward directions. We also decode dynamic multipolar contributions to the second harmonic generation within such nanoantennas.

Directional second harmonic generation from AlGaAs nanoantennas (Conference Presentation)

Optical nanoantennas possess great potential for controlling the spatial distribution of light in the linear regime as well as for frequency conversion of the incoming light in the nonlinear regime. However, the usually used plasmonic nanostructures are highly restricted by Ohmic losses and heat resistance. Dielectric nanoparticles like silicon and germanium can overcome these constrains [1,2], however second harmonic signal cannot be generated in these materials due to their centrosymmetric nature. GaAs-based III-V semiconductors, with non-centrosymmetric crystallinity, can produce second harmonic generation (SHG) [3]. Unfortunately, generating and studying SHG by AlGaAs nanocrystals in both backward and forward directions is very challenging due to difficulties to fabricate III-V semiconductors on low-refractive index substrate, like glass. Here, for the first time to our knowledge, we designed and fabricated AlGaAs nanoantennas on a glass substrate. This novel design allows the excitation, control and detection of backwards and forwards SHG nonlinear signals. Different complex spatial distribution in the SHG signal, including radial and azimuthal polarization originated from the excitation of electric and magnetic multipoles were observed. We have demonstrated an unprecedented SHG conversion efficiency of 10-4; a breakthrough that can open new opportunities for enhancing the performance of light emission and sensing [4]. References [1] A. S. Shorokhov et al. Nano Letters 16, 4857 (2016). [2] G. Grinblat et al. Nano Letters 16, 4635 (2016). [3] S. Liu et al. Nano Letters 16, 7191 (2016). [4] R. Camacho et al. Nano Lett. 16, 7191 (2016).

Fabrication and photoluminescence studies of GaN nanopillars

GaN nanopillar arrays are fabricated by inductively coupled plasma reactive ion etching (ICP-RIE) of lithographically patterned GaN epilayers grown on sapphire substrate. The morphology and optical quality of the nanopillars is investigated by scanning electron microscopy (SEM) and micro-photoluminescence (μ-PL) respectively. The PL intensity of the nanopillars is enhanced by a factor of more than four compared to that of the epitaxial layers. However, a small increase in the full width half maximum (FWHM) of the nanopillar PL spectra is observed.

Top-down approach for fabricating InP nanowires with ohmic metal contacts

Electrical contacts between metals and semiconductors are fundamentally important for practical semiconductor devices. This work reports the electrical characterization of nanowire arrays fabricated by a top-down approach, where electron beam lithography (EBL) and a highly anisotropic Cl2/H2/Ar inductively coupled plasma (ICP) reactive ion etching (RIE) is utilised to generate vertical arrays of InP nanowire arrays. Preliminary results show ohmic behavior from these arrays.