Lin, Ronghui; Galan, Sergio Valdes; Sun, Haiding; Hu, Yangrui; Alias, Mohd Sharizal; Janjua, Bilal; Ng, Tien Khee; Ooi, Boon S.; Li, Xiaohang(Photonics Research, The Optical Society, 2018-04-21)[Article]
A nanowire (NW) structure provides an alternative scheme for deep ultraviolet light emitting diodes (DUV-LEDs) that promises high material quality and better light extraction efficiency (LEE). In this report, we investigate the influence of the tapering angle of closely packed AlGaN NWs, which is found to exist naturally in molecular beam epitaxy (MBE) grown NW structures, on the LEE of NW DUV-LEDs. It is observed that, by having a small tapering angle, the vertical extraction is greatly enhanced for both transverse magnetic (TM) and transverse electric (TE) polarizations. Most notably, the vertical extraction of TM emission increased from 4.8% to 24.3%, which makes the LEE reasonably large to achieve high-performance DUV-LEDs. This is because the breaking of symmetry in the vertical direction changes the propagation of the light significantly to allow more coupling into radiation modes. Finally, we introduce errors to the NW positions to show the advantages of the tapered NW structures can be projected to random closely packed NW arrays. The results obtained in this paper can provide guidelines for designing efficient NW DUV-LEDs.
Ebaid, Mohamed; Min, Jungwook; Zhao, Chao; Ng, Tien Khee; Idriss, Hicham; Ooi, Boon S.(Journal of Materials Chemistry A, Royal Society of Chemistry (RSC), 2018-03-09)[Article]
Water splitting using InGaN-based photocatalysts may have a great contribution in future renewable energy production systems. Among the most important parameters to solve are those related to substrate lattice-matching compatibility. Here, we directly grow InGaN nanowires (NWs) on a metallic Ti/Si template, for improving water splitting performance compared to a bare Si substrate. The open circuit potential of the epitaxially grown InGaN NWs on metallic Ti was almost two times that of those grown on Si substrate. The interfacial transfer resistance was also reduced significantly after introducing the metallic Ti interlayer. An applied-bias-photon-to-current conversion efficiency of 2.2% and almost unity Faradic efficiency for hydrogen generation were achieved using this approach. The InGaN NWs grown on Ti showed improved stability of hydrogen generation under continuous operation conditions, when compared to those grown on Si, emphasizing the role of the semiconductor-on-metal approach in enhancing the overall efficiency of water splitting catalysts.
Janjua, Bilal; Priante, Davide; Prabaswara, Aditya; Alanazi, Lafi M.; Zhao, Chao; Alhamoud, Abdullah; Alias, Mohd Sharizal; Rahman, Abdul; Alyamani, Ahmed; Ng, Tien Khee; Ooi, Boon S.(IEEE Photonics Journal, Institute of Electrical and Electronics Engineers (IEEE), 2018-03-16)[Article]
Group-III nitride-based ultraviolet (UV) quantum-disks (Qdisks) nanowires (NWs) light-emitting diodes grown on silicon substrates offer a scalable, environment-friendly, compact, and low-cost solution for numerous applications such as solid-state lighting, spectroscopy, and biomedical. However, the internal quantum efficiency, injection efficiency, and extraction efficiency need to be further improved. The focus of this paper encompasses investigations based on structural optimization, device simulation, and device reliability. To optimize a UV-A (320-400 nm) device structure we utilize the self-assembled quantum-disk-NWs with varying quantum-disks thickness to study carrier separation in active-region and implement an improved p-contact-layer to increase output power. By simulation, we found a 100° improvement in the direct recombination rate for samples with thicker Qdisks thickness of 1.2 nm compared to the sample with 0.6 nm-thick Qdisks. Moreover, the sample with graded top Mg-doped AlGaN layer in conjunction with thin Mg-doped GaN layer shows 10° improvement in the output power compared to the samples with thicker top Mg-doped GaN absorbing contact layer. A fitting with ABC model revealed the increase in non-radiative recombination centers in the active region after a soft stress-test. This work aims to shed light on the research efforts required for furthering the UV NWs LED research for practical applications.
Shen, Chao; Ng, Tien Khee; Lee, Changmin; Nakamura, Shuji; Speck, James S.; DenBaars, Steven P.; Alyamani, Ahmed Y.; El-Desouki, Munir M.; Ooi, Boon S.(Optics Express, The Optical Society, 2018-02-14)[Article]
GaN-based semiconductor optical amplifier (SOA) and its integration with laser diode (LD) is an essential building block yet to be demonstrated for III-nitride photonic integrated circuits (PICs) at visible wavelength. This paper presents the InGaN/GaN quantum well (QW) based dual-section LD consisting of integrated amplifier and laser gain regions fabricated on a semipolar GaN substrate. The threshold current in the laser gain region was favorably reduced from 229mA to 135mA at SOA driving voltages, VSOA, of 0V and 6.25V, respectively. The amplification effect was measured based on a large gain of 5.7 dB at VSOA = 6.25V from the increased optical output power of 8.2 mW to 30.5 mW. Such integrated amplifier can be modulated to achieve Gbps data communication using on-off keying technique. The monolithically integrated amplifier-LD paves the way towards the III-nitride on-chip photonic system, providing a compact, low-cost, and multi-functional solution for applications such as smart lighting and visible light communications.
Sun, Xiaobin; Cai, Wenqi; Alkhazragi, Omar; Ooi, Ee-Ning; He, Hongsen; Chaaban, Anas; Shen, Chao; Oubei, Hassan M.; Khan, Mohammed Zahed Mustafa; Ng, Tien Khee; Alouini, Mohamed-Slim; Ooi, Boon S.(Optics Express, The Optical Society, 2018-05-04)[Article]
For circumventing the alignment requirement of line-of-sight (LOS) underwater wireless optical communication (UWOC), we demonstrated a non-line-of-sight (NLOS) UWOC link adequately enhanced using ultraviolet (UV) 375-nm laser. Path loss was chosen as a figure-of-merit for link performance in this investigation, which considers the effects of geometries, water turbidity, and transmission wavelength. The experiments suggest that path loss decreases with smaller azimuth angles, higher water turbidity, and shorter wavelength due in part to enhanced scattering utilizing 375-nm radiation. We highlighted that it is feasible to extend the current findings for long distance NLOS UWOC link in turbid water, such as harbor water.
Zhao, Chao; Ebaid, Mohamed; Zhang, Huafan; Priante, Davide; Janjua, Bilal; Zhang, Daliang; Wei, Nini; Alhamoud, Abdullah; Shakfa, M. Khaled; Ng, Tien Khee; Ooi, Boon S.(Nanoscale, Royal Society of Chemistry (RSC), 2018-05-29)[Article]
P-type doping in wide bandgap and new classes of ultra-wide bandgap materials has long been a scientific and engineering problem. The challenges arise from the large activation energy of dopants and high densities of dislocations in materials. We report here, a significantly enhanced p-type conduction using high-quality AlGaN nanowires. For the first time, the hole concentration in Mg-doped AlGaN nanowires is quantified. The incorporation of Mg into AlGaN was verified by correlation with photoluminescence and Raman measurements. The open-circuit potential measurements further confirmed the p-type conductivity; while Mott-Schottky experiments measured a hole concentration of 1.3×1019 cm-3. These results from photoelectrochemical measurements allow us to design prototype ultraviolet (UV) light-emitting diodes (LEDs) incorporating the AlGaN quantum-disks-in-nanowire and optimized p-type AlGaN contact layer for UV-transparency. The ~335-nm LEDs exhibited a low turn-on voltage of 5 V with a series resistance of 32 Ω, due to the efficient p-type doping of the AlGaN nanowires. The bias-dependent Raman measurements further revealed the negligible self-heating of devices. This study provides an attractive solution to evaluate electrical properties of AlGaN, which is applicable to other wide bandgap nanostructures. Our results are expected to open doors to new applications for wide and ultra-wide bandgap materials.
This paper presents the first demonstration of InGaN multiple quantum well (MQW) based micro-photodetectors (µPD) used as the optical receiver in orthogonal frequency-division multiplexing (OFDM) modulated visible communication system (VLC). The 80-µm diameter µPD exhibits a wavelength-selective responsivity in the near-UV to violet regime (374 nm - 408 nm) under a low reverse bias of −3 V. The modulation scheme of 16-quadrature amplitude modulation (16-QAM) OFDM enables the use of frequency response beyond −3 dB cutoff bandwidth of µPD. A record high data rate of 3.2 Gigabit per second (Gpbs) was achieved as a result, which provides the proof-of-concept verification of a viable high speed VLC link.
Alias, Mohd Sharizal; Alatawi, Abdullah; Wong, Ka Chun; Tangi, Malleswararao; Holguin Lerma, Jorge Alberto; Stegenburgs, Edgars; Shakfa, Mohammad Khaled; Ng, Tien Khee; Rahman, Abdul; Alyamani, Ahmed; Ooi, Boon S.(IEEE Photonics Journal, Institute of Electrical and Electronics Engineers (IEEE), 2018-02-15)[Article]
A distributed Bragg reflector (DBR) composed of Y2O3-doped HfO2 (YDH)/SiO2 layers with high reflectivity spectrum centered at a wavelength of ~240 nm is deposited using radio-frequency magnetron sputtering. Before the DBR deposition, optical properties for a single layer of YDH, SiO2, and HfO2 thin films were studied using spectroscopic ellipsometry and spectrophotometry. To investigate the performance of YDH as a material for the high refractive index layer in the DBR, a comparison of its optical properties was made with HfO2 thin films. Due to larger optical bandgap, the YDH thin films demonstrated higher transparency, lower extinction coefficient, and lower absorption coefficient in the UV-C regime (especially for wavelengths below 250 nm) compared to the HfO2 thin films. The deposited YDH/SiO2 DBR consisting of 15 periods achieved a reflectivity higher than 99.9% at the wavelength of ~240 nm with a stopband of ~50 nm. The high reflectivity and broad stopband of YDH/SiO2 DBRs will enable further advancement of various photonic devices such as vertical-cavity surface-emitting lasers, resonant-cavity light-emitting diodes, and resonant-cavity photodetectors operating in the UV-C wavelength regime.
Alheadary, Wael; Park, Kihong; Alfaraj, Nasir; Guo, Yujian; Stegenburgs, Edgars; Ng, Tien Khee; Ooi, Boon S.; Alouini, Mohamed-Slim(Optics Express, The Optical Society, 2018-03-05)[Article]
Over the years, free-space optical (FSO) communication has attracted considerable research interest owing to its high transmission rates via the unbounded and unlicensed bandwidths. Nevertheless, various weather conditions lead to significant deterioration of the FSO link capabilities. In this context, we report on the modelling of the channel attenuation coefficient (β) for a coastal environment and related ambient, considering the effect of coastal air temperature (T), relative humidity (RH) and dew point (TD) by employing a mobile FSO communication system capable of achieving a transmission rate of 1 Gbps at an outdoor distance of 70 m for optical beam wavelengths of 1310 nm and 1550 nm. For further validation of the proposed models, an indoor measurement over a 1.5 m distance utilizing 1310 nm, 1550 nm, and 1064 nm lasers was also performed. The first model provides a general link between T and β, while the second model provides a relation between β, RH as well as TD. By validating our attenuation coefficient model with actual outdoor and indoor experiments, we obtained a scaling parameter x and decaying parameter c values of 19.94, 40.02, 45.82 and 0.03015, 0.04096, 0.0428 for wavelengths of 1550, 1310, 1064 nm, respectively. The proposed models are well validated over the large variation of temperature and humidity over the FSO link in a coastal region and emulated indoor environment.
Prabaswara, Aditya; Min, Jung-Wook; Zhao, Chao; Janjua, Bilal; Zhang, Daliang; Albadri, Abdulrahman M.; Alyamani, Ahmed Y.; Ng, Tien Khee; Ooi, Boon S.(Nanoscale Research Letters, Springer Nature, 2018-02-06)[Article]
Consumer electronics have increasingly relied on ultra-thin glass screen due to its transparency, scalability, and cost. In particular, display technology relies on integrating light-emitting diodes with display panel as a source for backlighting. In this study, we undertook the challenge of integrating light emitters onto amorphous quartz by demonstrating the direct growth and fabrication of a III-nitride nanowire-based light-emitting diode. The proof-of-concept device exhibits a low turn-on voltage of 2.6 V, on an amorphous quartz substrate. We achieved ~ 40% transparency across the visible wavelength while maintaining electrical conductivity by employing a TiN/Ti interlayer on quartz as a translucent conducting layer. The nanowire-on-quartz LED emits a broad linewidth spectrum of light centered at true yellow color (~ 590 nm), an important wavelength bridging the green-gap in solid-state lighting technology, with significantly less strain and dislocations compared to conventional planar quantum well nitride structures. Our endeavor highlighted the feasibility of fabricating III-nitride optoelectronic device on a scalable amorphous substrate through facile growth and fabrication steps. For practical demonstration, we demonstrated tunable correlated color temperature white light, leveraging on the broadly tunable nanowire spectral characteristics across red-amber-yellow color regime.
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