[Show abstract][Hide abstract] ABSTRACT: Nanoscale periodic patterning on insulating materials using focused-ion beam (FIB) is challenging because of charging effect, which causes pattern distortion and resolution degradation. In this paper, we used a charging suppression scheme using electron conducting polymer for the implementation of FIB patterned dielectric subwavelength grating (SWG) reflector. Prior to the FIB patterning, we numerically designed the optimal structure and the fabrication tolerance for all grating parameters (period, grating thickness, fill-factor and low refractive index layer thickness) using the rigorous-coupled wave analysis computation. Then, we performed the FIB patterning on the dielectric SWG reflector spin-coated with electron conducting polymer for the anti-charging purpose. We also performed similar patterning using thin conductive film anti-charging scheme (30 nm Cr coating) for comparison. Our results show that the electron conducting polymer anti-charging scheme effectively suppressing the charging effect during the FIB patterning of dielectric SWG reflector. The fabricated grating exhibited nanoscale precision, high uniformity and contrast, constant patterning and complied with fabrication tolerance for all grating parameters across the entire patterned area. Utilization of electron conducting polymer lead to a simpler anti-charging scheme with high precision and uniformity for FIB patterning on insulator materials.
Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 11/2015; 33(6):06F701-1. DOI:10.1116/1.4929152 · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate a record high-speed underwater wireless optical communication (UWOC) over 7 m distance using on-off keying non-return-to-zero (OOK-NRZ) modulation scheme. The communication link uses a commercial TO-9 packaged pigtailed 520 nm laser diode (LD) with 1.2 GHz bandwidth as the optical transmitter and an avalanche photodiode (APD) module as the receiver. At 2.3 Gbit/s transmission, the measured bit error rate of the received data is 2.23×10<sup>-4</sup>, well below the forward error correction (FEC) threshold of 2×10<sup>-3</sup> required for error-free operation. The high bandwidth of the LD coupled with high sensitivity APD and optimized operating conditions is the key enabling factor in obtaining high bit rate transmission in our proposed system. To the best of our knowledge, this result presents the highest data rate ever achieved in UWOC systems thus far.
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate an underwater wireless optical communications (UWOC) employing 450-nm TO-9 packaged and fiber-pigtailed laser diode (LD) directly encoded with an orthogonal frequency division multiplexed quadrature amplitude modulation (QAM-OFDM) data. A record data rate of up to 4.8 Gbit/s over 5.4-m transmission distance is achieved. By encoding the full 1.2-GHz bandwidth of the 450-nm LD with a 16-QAM-OFDM data, an error vector magnitude (EVM) of 16.5%, a signal-to-noise ratio (SNR) of 15.63 dB and a bit error rate (BER) of 2.6 × 10<sup>-3</sup>, well pass the forward error correction (FEC) criterion, were obtained.
[Show abstract][Hide abstract] ABSTRACT: The coherent amplified spontaneous emission and high photoluminescence quantum efficiency of organolead trihalide perovskite have led to research interest in this material for use in photonic devices. In this paper, the authors present a focused-ion beam patterning strategy for methylammonium lead tribromide (MAPbBr3) perovskite crystal for subwavelength grating nanophotonic applications. The essential parameters for milling, such as the number of scan passes, dwell time, ion dose, ion current, ion incident angle, and gas-assisted etching, were experimentally evaluated to determine the sputtering yield of the perovskite. Based on our patterning conditions, the authors observed that the sputtering yield ranged from 0.0302 to 0.0719 lm3/pC for the MAPbBr3 perovskite crystal. Using XeF2 for the focused-ion beam gas-assisted etching, the authors determined that the etching rate was reduced to between 0.40 and 0.97, depending on the ion dose, compared
with milling with ions only. Using the optimized patterning parameters, the authors patterned binary and circular subwavelength grating reflectors on the MAPbBr3 perovskite crystal using the focused-ion beam technique. Based on the computed grating structure with around 97% reflectivity, all of the grating dimensions (period, duty cycle, and grating thickness) were patterned with nanoscale precision (>63 nm), high contrast, and excellent uniformity. Our results provide a platform for utilizing the focused-ion beam technique for fast prototyping of photonic nanostructures or nanodevices on organolead trihalide perovskite.
Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 08/2015; 33(5):051207-1. DOI:10.1116/1.4927542 · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a detailed study on the effects of dangling bond passivation and the comparison of different sulfides passivation process on the properties of InGaN/GaN quantum-disk (Qdisk)-in-nanowire based light emitting diodes (NW-LEDs). Our results demonstrated the first organic sulfide passivation process for nitride nanowires (NWs). The results from Raman spectroscopy, photoluminescence (PL) measurements, and X-ray photoelectron spectroscopy (XPS) showed octadecylthiol (ODT) effectively passivated the surface states, and altered the surface dynamic charge, thereby recovered the band-edge emission. The effectiveness of the process with passivation duration was also studied. Moreover, we also compared the electro-optical performance of NW-LEDs emitting at green wavelength before and after ODT passivation. We have shown that the Shockley-Read-Hall (SRH) non-radiative recombination of NW-LEDs can be greatly reduced after passivation by ODT, which led to a much faster increasing trend of quantum efficiency, and higher peak efficiency. Our results highlighted the research opportunity in employing this technique for further design and realization of high performance NW-LEDs and NW-lasers.
[Show abstract][Hide abstract] ABSTRACT: With increasing interest in visible light communication, the laser diode (LD) provides an attractive alternative, with higher efficiency, shorter linewidth and larger bandwidth for high-speed visible light communication (VLC). Previously, more than 3 Gbps data rate was demonstrated using LED. By using LDs and spectral-efficient orthogonal frequency division multiplexing encoding scheme, significantly higher data rates has been achieved in this work. Using 16-QAM modulation scheme, in conjunction with red, blue and green LDs, data rates of 4.4 Gbps, 4 Gbps and 4 Gbps, with the corresponding BER/SNR/EVM of 3.3 × 10<sup>-3</sup>/15.3/17.9, 1.4 × 10<sup>-3</sup>/16.3/15.4 and 2.8 × 10<sup>-3</sup>/15.5/16.7were obtained over transmission distance of ~20 cm. We also simultaneously demonstrated white light emission using red, blue and green LDs, after passing through a commercially available diffuser element. Our work highlighted that a tradeoff exists in operating the blue LDs at optimum bias condition while maintaining good color temperature. The best results were obtained when encoding red LDs which gave both the strongest received signal amplitude and white light with CCT value of 5835K.
[Show abstract][Hide abstract] ABSTRACT: The fabrication of orange-emitting semiconductor laser on interdif-fused InGaP/InAlGaP structure is reported. The lasers lased at 22°C at a wavelength as short as 608 nm with threshold current density of 3.4 KAcm −2 and a maximum output power of ∼46 mW. This is the shortest wavelength electrically pumped semiconductor laser emission from the InGaP/InAlGaP structure. Introduction: Semiconductor visible laser diodes (LDs) cover a wide spectrum of wavelengths. For example, the InGaN/GaN based LDs cover the violet to green spectrum (∼405–530 nm), and InGaP/ InAlGaP system based LDs cover the red spectrum (635–690 nm). The wavelength from ∼530–635 nm is not covered by any commercial LDs yet, which has some important applications in medicine [1, 2], horticulture , displays  and in optical communication using plastic fibres . LDs in the green-yellow-orange range (530–635 nm) can be grown ideally either by InGaN/GaN or InGaP/InAlGaP based material system. For the InGaN/GaN quantum well (QW) structure, large strain and indium segregation prevent the growth of high quality light emitting devices in yellow and orange spectrum regions. In the case of the InGaP/InAlGaP system, small band offset between the QW and barriers leads small carrier confinement and large carrier leakage to prohibit the growth of high quality QW structures for yellow and orange emissions. The only access to orange, yellow and green regions has been achieved by frequency doubling of diode-pumped solid state lasers  or infrared LDs  or through the application of high external pressures which cause large blue-shifts of the emission wavelength of diode lasers . However, the frequency doubled diode-pumped semiconductor lasers use non-linear crystals for inefficient second-harmonic generation and require externally distributed Bragg reflector and good heat sink, which makes the overall system more complex. Although InGaN based vertical-external-cavity surface-emitting lasers, also known as optically pumped semiconductor lasers, are worthy contenders for wavelength tuning, high optical output power and a nearly diffraction-limited beam quality means electrical pumping in these devices is challenging . Moreover, the lasers produced by application of external pressure technique are non-practical for any commercial applications. Therefore, there is huge demand for replacements of these complex, expensive and power consuming lasers. In this letter we demonstrate the first room-temperature (RT) orange emission at 608 nm from the interdiffused InGaP/InAlGaP structure. Red laser (∼640 nm) InGaP/InAlGaP structure is known to be very hard to have its bandgap blue-shifted using quantum well intermixing (QWI) technique . Here, a novel QWI technique utilising strain-induced from a thick dielectric cap with cycles annealing at elevated temperature to promote interdiffusion. With this QWI technique, we have successfully tuned the bandgap of InGaP/InAlGaP structure from 640 to 565 nm. Experiment: The single quantum well (SQW) InGaP/InAlGaP laser structure was grown on 10° offcut GaAs substrate using metal-organic chemical vapour deposition as shown in Fig. 1. The structure consists of a 200 nm Si-doped GaAs buffer layer with carrier concentration of 1-2 × 10 18 cm −3 , 1 µm thick n-In 0.5 Al 0.5 P lattice-matched lower cladding layer with carrier concentration of 1 × 10 18 cm −3 , a SQW InGaP sandwiched between two 80 nm undoped In 0.5 Al 0.3 Ga 0.2 P waveguide layers, 1 µm thick Zn-doped In 0.5 Al 0.5 P lattice-matched upper cladding with carrier concentration of 1 × 10 18 cm −3 , 75 nm lattice matched pIn 0.5 Ga 0.5 P barrier reduction layer with carrier concentration of 3 × 10 18 cm −3 and 200 nm highly doped p-GaAs contact layer with carrier concentration of 2-3 × 10 19 cm −3. The emission of the laser was designed to be at 638 ± 2 nm. For the novel QWI process, we studied the effect of the thickness of dielectric encapsulant (external strain), annealing temperature, anneal-ing duration and number of cycles of annealing to identify the optimal process conditions for preserving the surface morphology, photolumi-nescence (PL) characteristics and electrical properties. For the purpose of this work a 1 µm thick SiO 2 cap, 950°C annealing temperature, five cycles of 30 s duration was applied to achieve the desired emission wavelength and optimal process conditions. The bandgap shifts induced by the above procedure were measured at RT using PL spectroscopy equipped with a 473 nm cobalt laser as the excitation source. Wafers were then processed using conventional processing and 1 mm long and 75 µm wide ridge devices were used for opto-electronic characteris-ation. All the devices were mounted on ceramic tiles and probed directly. The measurements were carried out at a tile temperature of 295 K, while pulsed operation (0.5 µs pulsed duration, 0.1% duty cycle) was used to minimise self-heating effects.
[Show abstract][Hide abstract] ABSTRACT: Polarized thin film mirror with high reflectivity and broadband characteristics for optoelectronic devices can be realized using subwavelength grating (SWG). 1 For nanoscale SWG, most of the out-of plane emission devices utilized electron beam lithography, however this technique is challenging for SWG fabrication on the facets of in-plane emission devices. Direct SWG patterning at facet of in-plane devices using focused ion beam (FIB) technique has been reported for antireflector in quantum cascade laser (QCL) 2 and plasmonic collimator for QCL 3 , besides the subwavelength slit grating 4 for typical out-of plane emission case. However, these FIB patterned SWG are only demonstrated for III-V compounds and metallic based devices at infra-red spectrum. Here we propose SWG reflector fabrication using FIB technique on dielectric multilayers (Si3N4/SiO2) using GaN-sapphire substrate at visible wavelength. We spin-coated electron dissipation polymer (ESPACER 300Z) on the sample surface to reduce the charging effect of the dielectrics, GaN and sapphire during milling.
The 59th International Conference on Electron, Ion, and Photon Beam Technology and Nanofabrication (EIPBN 2015), San Diego, USA; 05/2015
[Show abstract][Hide abstract] ABSTRACT: We investigated the design and growth of compositionally-graded InGaN multiple quantum wells (MQW) based light-emitting diode (LED) without an electron-blocking layer (EBL). Numerical investigation showed uniform carrier distribution in the active region, and higher radiative recombination rate for the optimized graded-MQW design, i.e. In0→xGa1→(1-x)N / InxGa(1-x)N / Inx→0Ga(1-x)→1N, as compared to the conventional stepped-MQW-LED. The composition-grading schemes, such as linear, parabolic, and Fermi-function profiles were numerically investigated for comparison. The stepped- and graded-MQW-LED were then grown using plasma assisted molecular beam epitaxy (PAMBE) through surface-stoichiometry optimization based on reflection high-energy electron-diffraction (RHEED) in-situ observations. Stepped- and graded-MQW-LED showed efficiency roll over at 160 A/cm2 and 275 A/cm2, respectively. The extended threshold current density roll-over (droop) in graded-MQW-LED is due to the improvement in carrier uniformity and radiative recombination rate, consistent with the numerical simulation.
[Show abstract][Hide abstract] ABSTRACT: We report on a unique area-selective, post-growth approach in engineering the quantum-confined potential-energy profile of InGaN/GaN quantum wells (QWs) utilizing metal/dielectric-coating induced intermixing process. This led to simultaneous realization of adjacent regions with peak emission of 2.74 eV and 2.82 eV with a high spatial resolution (~1 μm) at the coating boundary. The potential profile softening in the intermixed QW light-emitting diode (LED) was experimentally and numerically correlated, shedding light on the origin of alleviated efficiency droop from 30.5% to 16.6% (at 150 A/cm2). The technique is advantageous for fabricating high efficiency light-emitters, and is amenable to monolithic integration of nitride-based photonic devices.
[Show abstract][Hide abstract] ABSTRACT: We analyze an epitaxially grown
heterostructure composed of InGaN nanodisks inserted in GaN
nanowires in order to relate indium concentration to the electronic properties. This study was achieved with spatially resolved low-loss electron energy-loss spectroscopy using monochromated electrons to probe optical excitations—plasmons—at nanometer scale. Our findings show that each nanowire has its own indium fluctuation and therefore its own average composition. Due to this indium distribution, a scatter is obtained in plasmon energies, and therefore in the optical dielectric function, of the nanowire ensemble. We suppose that these inhomogeneous electronic properties significantly alter band-to-band transitions and consequently induce emission broadening. In addition, the observation of tailing indium composition into the GaN barrier suggests a graded well-barrier interface leading to further inhomogeneous broadening of the electro-optical properties. An improvement in the indium incorporation during growth is therefore needed to narrow the emission linewidth of the presently studied heterostructures.
[Show abstract][Hide abstract] ABSTRACT: We investigated the mechanisms of radiative recombination in a CH3NH3PbBr3 hybrid perovskite material using low-temperature, power-dependent (77 K), and temperature-dependent photoluminescence (PL) measurements. Two bound-excitonic radiative transitions related to grain size inhomogeneity were identified. Both transitions led to PL spectra broadening as a result of concurrent blue and red shifts of these excitonic peaks. The red-shifted bound-excitonic peak dominated at high PL excitation led to a true-green wavelength of 553 nm for CH3NH3PbBr3 powders that are encapsulated in polydimethylsiloxane. Amplified spontaneous emission was eventually achieved for an excitation threshold energy of approximately 350 μJ/cm2. Our results provide a platform for potential extension towards a true-green light-emitting device for solid-state lighting and display applications.
[Show abstract][Hide abstract] ABSTRACT: The small signal modulation characteristics of an InGaN/GaN nanowire array edge- emitting laser on (001) silicon are reported. The emission wavelength is 610 nm. Lattice matched InAlN cladding layers were incorporated in the laser heterostructure for better mode confinement. The suitability of the nanowire lasers for use in plastic fiber communication systems with direct modulation is demonstrated through their modulation bandwidth of f-3dB,max = 3.1 GHz, very low values of chirp (0.8 Å) and α-parameter, and large differential gain (3.1 × 10−17 cm2).
[Show abstract][Hide abstract] ABSTRACT: We report on a flat-top and ultrawide emission bandwidth of 125 nm from InGaAsP/InP multiple quantum-well (MQW) superluminescent diode with antireflection coated and tilted ridge-waveguide device configuration. A total output power in excess of 70 mW with an average power spectral density of 0.56 mW/nm and spectral ripple <= 1.2 +/- 0.5 dB is measured from the device. Wall-plug efficiency and output power as high as 14% and 80 mW, respectively, is demonstrated from this batch of devices. We attribute the broad emission to the inherent inhomogeneity of the electron-heavy-hole (e-hh) and electron-light-hole (e-lh) recombination of the ground state and the first excited state of the MQWs and their simultaneous emission.
[Show abstract][Hide abstract] ABSTRACT: We have studied enhanced carrier injection by having an electron blocking layer (EBL) based on a graded superlattice (SL) design. Here, we examine, using a selfconsistent 6 × 6 k.p method, the energy band alignment diagrams under equilibrium and forward bias conditions while also considering carrier distribution and recombination rates (Shockley-Read-Hall, Auger, and radiative recombination rates). The graded SL is based on AlxGa1-xN (larger bandgap) Al0:5Ga0:5N (smaller bandgap) SL, where x is changed from 0.8 to 0.56 in steps of 0.06. Graded SL was found to be effective in reducing electron leakage and enhancing hole injection into the active region. Due to our band engineering scheme for EBL, four orders-of-magnitude enhancement were observed in the direct recombination rate, as compared with the conventional bulk EBL consisting of Al0:8Ga0:2N. An increase in the spatial overlap of carrier wavefunction was obtained due to polarization-induced band bending in the active region. An efficient single quantum-well ultraviolet-B light-emitting diode was designed, which emits at 280 nm. This is the effective wavelength for water disinfection application, among others.
[Show abstract][Hide abstract] ABSTRACT: The advances in lasers, electronic and photonic integrated circuits (EPIC), optical interconnects as well as the modulation techniques allow the present day society to embrace the convenience of broadband, high speed internet and mobile network connectivity. However, the steep increase in energy demand and bandwidth requirement calls for further innovation in ultra-compact EPIC technologies. In the optical domain, advancement in the laser technologies beyond the current quantum well (Qwell) based laser technologies are already taking place and presenting very promising results. Homogeneously grown quantum dot (Qdot) lasers and optical amplifiers, can serve in the future energy saving information and communication technologies (ICT) as the work-horse for transmitting and amplifying information through optical fiber. The encouraging results in the zero-dimensional (0D) structures emitting at 980 nm, in the form of vertical cavity surface emitting laser (VCSEL), are already operational at low threshold current density and capable of 40 Gbp s error-free transmission at 108 fJ/bit. Subsequent achievements for lasers and amplifiers operating in the O–, C–, L–, U–bands, and beyond will eventually lay the foundation for green ICT. On the hand, the inhomogeneously grown quasi 0D quantum dash (Qdash) lasers are brilliant solutions for potential broadband connectivity in server farms or access network. A single broadband Qdash laser operating in the stimulated emission mode can replace tens of discrete narrow-band lasers in dense wavelength division multiplexing (DWDM) transmission thereby further saving energy, cost and footprint. We herein reviewed the progress of both Qdots and Qdash devices, based on the InAs/InGaAlAs/InP and InAs/InGaAsP/InP material systems, from the angles of growth and device performance. In particular, we discussed the progress in lasers, semiconductor optical amplifiers (SOA), mode locked lasers, and superluminescent diodes, which are the building blocks of EPIC and ICT. Alternatively, these optical sources are potential candidates for other multi-disciplinary field applications.
[Show abstract][Hide abstract] ABSTRACT: The ~25nm thick threading dislocation free GaN nanomembrane was prepared using ultraviolet electroless chemical etching method offering the possibility of flexible integration of (Al,In,Ga)N optoelectronic and electronic devices.
[Show abstract][Hide abstract] ABSTRACT: We report on the high performance characteristics from superluminescent diodes (SLD) based on four-stack InAs/InGaAlAs chirped-barrier thickness quantum dash (Qdash) in a well structure. The active region exhibits a measured broad gain spectrum of ~140 nm with peak modal gain ~41 cm-1. The non-coated two section gain-absorber broad area and ridge-waveguide device configuration exhibits an output power >20 mW and >12 mW, respectively. The corresponding -3 dB bandwidths span ~82 nm and ~72 nm, with small spectral ripple <0.2 dB, related largely to the contribution from dispersive height dash ensembles of the highly inhomogeneous active region. These C-L communication band devices will find applications in various cross-disciplinary fields of optical metrology, optical coherent tomography, etc.
[Show abstract][Hide abstract] ABSTRACT: A silicon-based laser, preferably electrically pumped, has long been a scientific and engineering goal. We demonstrate here, for the first time, an edge-emitting InGaN/GaN disk-in-nanowire array electrically pumped laser emitting in the green (λ=533nm) on (001)silicon substrate. The devices display excellent dc and dynamic characteristics with values of threshold current density, differential gain, T0 and small signal modulation bandwidth equal to 1.76kA/cm2, 3x10-17cm2, 232K and 5.8GHz respectively under continuous wave operation. Preliminary reliability measurements indicate a lifetime of 7000 hours. The emission wavelength can be tuned by varying the alloy composition in the quantum disks. The monolithic nanowire laser on (001)Si can therefore address wide-ranging applications such as solid state lighting, displays, plastic fiber communication, medical diagnostics and silicon photonics.