Bragg Reflector Stack Research Articles

High-velocity and large-coupling surface wave mode using acoustic Bragg reflector stack with matched excitation condition

High-velocity, large-coupling acoustic mode is the eternal pursuit of microacoustic research. Surface mode on piezoelectric films supported by an acoustic Bragg reflector (ABR) stack is one possible solution for realizing such a goal. However, some acoustic modes exhibit lower couplings in experiments than theoretically predicted, especially when metal layers are used as high-impedance layers in the ABR. The mechanism of this performance deterioration is still unclear. In this work, the electric field distribution in a piezoelectric plate mounted to an ABR stack is analyzed by considering the inductive current effect in conductive ABR layers. The field distribution required by one mode is found to be distorted by the effect, which results in a reduction in the excitation efficiency for the mode. We propose the use of another acoustic mode with a concentrated electric field in the top layers. As analyzed in comparison, the impact of the inductive effect on the mode is negligible. On an optimized crystal cut of lithium niobate, the selected mode exhibits 22.64% coupling and an acoustic velocity of 6006 m/s.

Design and fabrication of double AlGaN/GaN distributed Bragg reflector stack mirror for the application of GaN-based optoelectronic devices

In this work, a near-ultraviolet (380 nm) double Al0.2Ga0.8N/GaN distributed Bragg reflectors (DBRs) stack mirror was designed and fabricated. The double DBRs stack mirror consists of a 30-pair Al0.2Ga0.8N/GaN DBRs centered at 375 nm and a 20-pair Al0.2Ga0.8N/GaN DBRs centered at 385 nm. Our simulation results show that the method of double DBRs stack mirror design can broaden the stopband width greatly and increase the reflected angle efficiently, compared with the single Al0.2Ga0.8N/GaN DBRs mirror. In experiment, the double Al0.2Ga0.8N/GaN DBRs stack mirror and the reference Al0.2Ga0.8N/GaN DBRs mirror were grown on sapphire substrate by metalorganic chemical vapor deposition. The measured stopband width of the double DBRs stack mirror (~ 25 nm) is more than two times that of the reference DBRs mirror (~ 11 nm), which consists well with our simulation results. It is reasonable to believe that this work could provide a valuable information to obtain AlGaN/GaN DBRs with wide stopband width that can be used in the fabrication of GaN-based resonant cavity light-emitting diodes and vertical cavity surface emitting lasers.

Tunable bulk acoustic wave resonators based on Ba<sub>0.25</sub>Sr<sub>0.75</sub>TiO<sub>3</sub> thin films and a HfO<sub>2</sub>/SiO<sub>2</sub> bragg reflector

A switchable and tunable bulk acoustic wave resonator based on a paraelectric phase Ba(0.25)Sr(0.75)TiO(3) thin film and an all-dielectric HfO(2)/SiO(2) Bragg reflector is presented. The achieved tuning range (3.8%) and effective electromechanical coupling coefficient (7.1%) are the highest reported for solidly mounted tunable bulk acoustic wave resonators. The non-conductive Bragg reflector stack provides excellent integration possibilities.

Tunable Solidly Mounted Thin Film Bulk Acoustic Resonators Based on Ba$_{x}{{\hbox{Sr}}_{1-x}}{\hbox{TiO}}_{3}$ Films

Electrically tunable solidly mounted thin film bulk acoustic resonators based on Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films are reported for the first time. The films are acoustically isolated from the silicon substrate by a Bragg reflector stack. Applying DC bias induces piezoelectric effect and an acoustic resonance at approximately 4 GHz. Under 10 V applied DC bias the resonance frequency of the resonators based on Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films is tuned 1.2% to lower frequencies. The Q-factor of these resonators is approximately 120, and the electromechanical coupling coefficient is 0.5%. The resonant frequency of the BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> based resonators shifts upwards 1.3% under 10 V DC bias, and the -factor is approximately 30, with an electromechanical coupling coefficient of 6.2%.

Whispering-gallery mode lasing in GaN microdisks

The wide range of wavelengths covered by the Gallium nitride (GaN) group of materials is probably the major reason for its growing use by the semiconductor industry. GaN light emitting diodes are used extensively for illumination and display purposes,1 while UV emitters have been employed in a wide range of biomedical applications.2 High-performance GaN transistors have also been demonstrated. The laser diodes emitting at 405nm and found in the latest high-density DVD players are also GaN-based. Shorter wavelengths allow data to be packed more tightly, so it becomes possible to fit more data on a disc even though it is the same size as a CD/DVD. GaN laser diodes are currently fabricated in the conventional edge-emitting geometry, where the laser light propagates parallel to the wafer surface of the semiconductor. However, the newer surface-emitting versions, where the light propagates in the direction perpendicular to the semiconductor surface, offer more advantages. The first is their lower fabrication cost since the chips can be tested on-wafer, unlike edge-emitters that can only be tested at the end of the fabrication process. They also offer beams with lower divergence due to large output apertures, low power consumption, and wavelength tuning. However, the transition from edge to surface emission is not trivial in GaN structures. In principle, a surface-emitting laser requires a vertical cavity with top and bottom mirrors. In practice, high quality GaN materials are grown on sapphire substrates, which makes the insertion of bottom mirrors inherently difficult. One common solution is the deposition of AlGaN/GaNdistributed Bragg reflector stacks between the device structure and the sapphire substrate.3 Alternatively, undercut microdisk lasers that address mirror insertion challenges have also been fabricated using selective photoelectrochemical etching techniques.4 Recently, we have also developed a microdisk fabrication method based on GaN grown on Si substrates. Pivoted GaN Figure 1. Scanning-electron-microscope images showing (a) the cross section of a single pivoted microdisk and (b) a microdisk array.

Process study and optimization for fabrication of poly-Si thin-film transistors on plastic substrates

— A complete poly-Si thin-film transistor (TFT) on plastic process has been optimized to produce TFT arrays for active-matrix displays. We present a detailed study of the poly-Si crystallization process, a mechanism for protecting the plastic substrate from the pulsed laser used to crystallize the silicon, and a high-performance low-temperature gate dielectric film. Poly-Si grain sizes and the corresponding TFT performance have been measured for a range of excimer-laser crystallization fluences near the full-melt threshold, allowing optimization of the laser-crystallization process. A Bragg reflector stack has been embedded in the plastic coating layers; its effectiveness in protecting the plastic from the excimer-laser pulse is described. Finally, we describe a plasma pre-oxidation step, which has been added to a low-temperature (<100°C) gate dielectric film deposition process to dramatically improve the electrical properties of the gate dielectric. These processes have been integrated into a complete poly-Si TFT on plastic fabrication process, which produces PMOS TFTs with mobilities of 66 cm2 /V-sec, threshold voltages of −3.5 V, and off currents of approximately 1 pA per micron of gate width.

Distributed Bragg reflectors based on AlN/GaN multilayers

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.

A comparison of 1.55 mu m distributed Bragg reflector stacks for use in multi quantum well micro resonator modulators

We have investigated p- and n-doped distributed Bragg reflector stacks (DBRs) in several material systems in order to evaluate their use in multi quantum well (MQW) micro resonator modulators. We have shown that over 80% reflectivity can be obtained using 12 periods in a p-type Ga0.35In0.65As0.75P0.25/InP or Al0.08Ga0.39In0.53As/Al0.48In0.52As. Lattice-matched n-doped Ga0.47In0.53As/InP and Ga0.47In0.53As/Al0.48In0.52As DBRs are shown to give 98% reflectivity with only 20 periods. We have found that n doping of greater than 1*1018 cm-3 shifts the absorption edge of lattice-matched GaInAs to enable its use in 1.55 mu m DBRs.

Long-wavelength GaAs/AlAs distributed Bragg reflectors for use in GaSb-based resonant cavity detectors

Two GaAs/AlAs Bragg reflector stacks were designed for operation at 1.68 µm and 2.2 µm to be used in resonant cavity photodetectors. The mirrors were grown by solid source MBE, and measured reflectivities were in excess of 95 and 85%, respectively.

High-frequency, high-efficiency MSM photodetectors

Metal-semiconductor-metal (MSM) photodiodes with submicron spaced interdigitated Schottky barrier fingers have been developed for applications in monolithic integrated optical receiver circuits capable of detecting a millimeter-wave modulation signal. Each photodetector layer, is designed for optimal absorption about a narrow linewidth centered on a specific wavelength between 700 and 800 nm. The MBE grown layers consist of an Al/sub x/Ga/sub 1-x/As cap layer, to prevent any surface recombination of carriers and to minimize top surface reflections; a thin GaAs absorption layer (375 nm), to achieve a high-frequency response (>39 GHz) by minimizing the collection times of optically generated carriers; and a buried Bragg reflector stack which reflects unabsorbed light back into the GaAs absorption layer. Using this layer design, we are able to fabricate detectors that have millimeter-wave bandwidths without sacrificing quantum efficiency. The measured internal quantum efficiency of an MSM photodiode, fabricated on such a layer structure, was 82% at 5 V and close to 94% at 10 V. >

Highly doped 1.55 µm GaxIn1-xAs/InP distributed Bragg reflector stacks

The authors have investigated 20 period lattice matched mirror stacks doped to 5*10/sup 18/ cm/sup -1/ and 10/sup 19/ cm/sup -1/ at 1.55 mu m in Ga/sub x/In/sub 1-x/As/InP and have obtained greater than 95% reflectivity over 100 nm and peak reflectivities up to 98%. >

Process control system for improved run-to-run control of molecular-beam epitaxial growth of GaAs/AlxGa1−xAs heterostructures

A system was developed that significantly improves the accuracy of molecular-beam epitaxial flux calibration and provides ±1.0% run-to-run control of film thickness as demonstrated by double-crystal x-ray diffraction analysis of 980 nm GaAs/AlAs Bragg reflector stacks. The system combines PC-based data acquisition and analysis of reflection high-energy electron diffraction (RHEED) wave forms and beam flux gauge signals with mechanical improvements in RHEED sample preparation and mounting. In developing this system, an extensive series of experiments were performed to determine the sources and magnitudes of error in RHEED flux measurements. Proper flux transient corrections, precisely centered 1 mm×1 mm RHEED slices, and improved signal processing and analysis of RHEED wave forms were found to significantly enhance run-to-run flux control.

Fabry–Perot reflectance modulator for 1.3 μm from (InAlGa)As materials grown at low temperature

We report the first all-semiconductor Fabry–Perot-cavity reflectance modulators operating at wavelengths of 1.32–1.33 μm. These devices were grown on a GaAs substrate using an intermediate, linearly graded InGaAs buffer layer terminating in an In0.33Ga0.67As layer. The Bragg reflector stacks of the Fabry–Perot structure are composed of InGaAs and InAlAs layers lattice matched to the buffer, and the active cavity region is an In0.4Ga0.6As/In0.26Al0.35Ga0.39As strained-layer superlattice. The key to obtaining device-quality material was low temperature growth (∼400 °C) of the entire structure. For a device with a 0.38-μm-thick active region and a 4 dB insertion loss, we obtained a contrast ratio of ∼3:1 at 4 V bias.

Passively mode-locked Er/sup 3+/ fiber laser using a semiconductor nonlinear mirror

A nonlinear mirror consisting of an InGaAs/InP multiple-quantum-well saturable absorber integrated with a Bragg reflector stack has been used to passively mode-lock a fiber laser in an extremely simple and advantageous linear-cavity configuration. To date, pulses as short as 7.6 ps have been observed. >

High reflectivity and low resistance 1.55μm Al0.65In0.35As/Ga0.63In0.37As strained quarter wave Bragg reflector stack

Reflector stacks grown for use in devices operating at 1.55μm on InP substrates normally use lattice matched quaternary alloys and a large number of periods to obtain a high reflectivity. In this work therefore we have investigated the use of Al0.65In0.35As/Ga0.63In0.37As ternary layers, which are ∼ 1% strained with respect to the substrate. Results show that over 80% reflectivity and a low resistance can be obtained with only a 12 period structure.

Growth of an InGaAs/Alingaas Mqw Non-Linear Etalon with Integrated 1.5μm GaInAsP/InP Epitaxial BRAGG Reflector

The Gas Source MBE growth of high quality GalnAsP/InP Bragg reflector stacks for operation at 1.5μm followed by growth of an MOVPE AlInGaAs/InGaAs MQW structure is demonstrated. An asymmetric Fabry-Perot non-linear etalon structure was completed by dielectric mirror deposition. A clear bistable response was observed for the device as a function of incident light intensity, with low critical switching power (<1mW) and excellent thermal stability.