Structural, photoluminescence, and field emission properties of vertically well-aligned ZnO nanorod arrays

The self-assembled growth of vertically well-aligned ZnO nanorod arrays with uniform length and diameter on Si substrate has been demonstrated via thermal evaporation and vapor-phase transport. The structural, photoluminescence (PL), and field emission properties of the as-prepared nanorod arrays were investigated. The PL spectrum at 10 K shows a strong and sharp near-band gap emission (NBE) peak ( full width at half-maximum (FWHM) = 4.7 meV) and a weak neglectable deep-level emission (DL) peak (I-NBE/I-DL= 220), which implies its good crystallinity and high optical quality. The room-temperature NBE peak was deduced to the composition of free exciton and its first-order replicas emissions by temperature-dependent PL spectra. The field emission measurements indicate that, with a vacuum gap of 400 Am, the turn-on field and threshold field is as low as 2.3 and 4.2 V/mu m. The field enhancement factor beta and vacuum gap d follows a universal equation.

High-power electroabsorption modulator using intrastep quantum well

An electroabsorption modulator using the intrastep quantum well (IQW) active region is fabricated for optical network systems. The strain-compensated InGaAsP/InGaAsP IQW shows good material quality and improved modulation properties, high extinction ratio elliciency 10 dB/V and low capacitance (< 0.42 pF), with which an ultra high frequency (> 15 GHz) can be obtained. High-speed measurement under high-power excitation shows no power saturation up to excitation power of 21 dBm. To our knowledge, the input optical power is the highest reported for multi-quantum well EAMs without heat sinks.

An n-type SiGe/Ge QC structure utilizing the deep Ge quantum well for electron at the Gamma point

In this paper, an n-type Si1-xGex/Ge (x >= 0.85) quantum cascade (QC) structure utilizing a deep Ge quantum well for electrons at the Gamma point is proposed. Based on linear interpolation, a conduction band offset at the Gamma point in a Si1-xGex/Ge ( x >= 0.85) heterostructure is presented, which is suitable for designing a QC laser. This approach has the advantages of a large conduction band offset at the Gamma point, a low lattice mismatch between the Si1-xGex/Ge ( x >= 0.85) active layers and the Si1-yGey ( y > x) virtual substrate, a small electron effective mass in the Gamma band, simple conduction energy band structures and a simple phonon scattering mechanism in the Ge quantum well. The theory predicts that if high-energy electrons are continuously injected into the Gamma band, a quasi-equilibrium distribution of electrons between the Gamma and L bands can be reached and held, i.e., electrons with a certain density will be kept in the Gamma band. This result is supported by the intervalley scattering experiments. In n-type Si1-xGex/Ge ( x >= 0.85) QC structures, population inversion between the laser's upper and lower levels is demonstrated.

Pseudospin in Si delta-doped InAlAs/InGaAs/InAlAs single quantum well

Magneto-transport measurements have been carried out on double/single-barrier-doped In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well samples from 1.5 to 60 K in an applied magnetic field up to 13 T. Beating Shubnikov-de Haas oscillation is observed for the symmetrically double-barrier-doped sample and demonstrated due to a symmetric state and an antisymmetric state confined in two coupled self-consistent potential wells in the single quantum well. The energy separation between the symmetric and the antisymmetric states for the double-barrier-doped sample is extracted from experimental data, which is consistent with calculation. For the single-barrier-doped sample, only beating related to magneto-intersubband scattering shows up. The pesudospin property of the symmetrically double-barrier-doped single quantum well shows that it is a good candidate for fabricating quantum transistors. (c) 2007 Elsevier Ltd. All rights reserved.

Electroabsorption modulator integrated with buried-ridge-stripe dual-core spot-size converter by quantum-well intermixing

A single shallow ridge electroabsorption modulator monolithically integrated with a buried-ridge-stripe dual-core spot-size converter at the input and output port was fabricated by combining quantum-well intermixing and dual-core integration techniques simultaneously, using only a two-step low-pressure metal-organic vapor phase epitaxial process, conventional photolithography, and a chemical wet etching process. The optical insertion loss of the modulator in the on-state and the dc extinction ratio between 0 and -3 V at 1550 nm was -7.5 and 16 dB, respectively. The 3-dB modulation bandwidth was more than 10.0 GHz in electrical-optical response.

Violet electroluminescence of AlInGaN-InGaN multiquantum-well light-emitting diodes: Quantum-confined stark effect and heating effect

Electroluminescence (EL) from AlInGaN-InGaN multiquantum-well violet light-emitting diodes is investigated as a function of forward bias. Two distinct regimes have been identified: 1) quantum-confined Stark effect at low and moderately high forward biases; 2) heating effect at high biases. In the different regimes, the low-temperature EL spectra exhibit different spectral features which are discussed in detail.

Energy of a polaron in a wurtzite nitride finite parabolic quantum well

The effects of electron-phonon interaction oil energy levels of a. polaron in a wurtzite nitride finite parabolic quantum well (PQW) are studied by using a modified Lee-Low-Pines variational method. The ground state, first excited state, and transition energy of the polaron in the GaN/Al0.3Ga0.7N wurtzite PQW are calculated by taking account of the influence of confined LO(TO)-like phonon modes and the half-space LO(TO)-like phonon modes and considering the anisotropy of all kinds of phonon modes. The numerical results are given and discussed. The results show that the electron phonon interaction strongly affects the energy levels of the polaron, and the contributions from phonons to the energy of a polaron hi a wurtzite nitride PQW are greater than that in all AlGaAs PQW. This indicates that the electron-phonon interaction in a wurtzite nitride PQW is not negligible.

Metamorphic growth of 1.25-1.29 mu m InGaAs quantum well lasers on GaAs by molecular beam epitaxy

We demonstrate 1.25-1.29 mu m metamorphic laser diodes grown on GaAs by molecular beam epitaxy (MBE) using an alloy-graded buffer layer (GBL). Use of Be in the GBL is effective to reduce surface/interface roughness and improves optical quality. The RMS surface roughness of the optimized metamorphic laser is only two atomic monolayers for 1 x 1 mu m(2). Cross-sectional transmission electron microscopy (TEM) images confirm that most dislocations are blocked in the GBL. Ridge waveguide lasers with 4 mu m wide ridge were fabricated and characterized. The average threshold current under the pulsed excitation is in 170-200 mA for a cavity length of 0.9-1.5 mm. This value can be further reduced to about 100 mA by high-reflectivity coating. Lasers can work in an ambient temperature up to at least 50 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

A short carrier lifetime semiconductor optical amplifier with n-type modulation-doped multiple quantum well structure

Semiconductor optical amplifiers (SOAs) with n-type modulation-doped multiple quantum well structure have been investigated. The shortened carrier lifetime is derived from the PL spectrum and electrical modulation frequency response measurement. The carrier lifetime in semiconductor optical amplifiers with any n-type-2-modulated doping multiple quantum well structure is less than 60% of that in the undoped partner. The shortest measured carrier lifetime of 236 ps in the MD-MQW SOA with sheet carrier density of 3 x 10(12) cm(-2) was only 38% of that in the undoped MQW SOA, which can increase the wavelength conversion efficiency via four wave mixing by a factor of about 7 and switching speed via XGM and XPM applications by a factor of 2.63.

The influence of internal electric fields on the transition energy of InGaN/gaN quantum well

We present a new way to meet the amount of strain relaxation in an InGaN quantum well layer grown on relaxed GaN by calculating and measuring its internal field. With perturbation theory, we also calculate the transition energy of InGaN/GaN SQWs as affected by internal fields. The newly reported experimental data by Graham et al. fit our calculations well on the assumption that the InGaN well layer suffered a 20% strain relaxation, we discuss the differences between our calculated results and the experimental data. Our calculation suggests that with the increase of indium mole fraction in the InGaN/GaN quantum well, the effect of polarization fields on the luminescence of the quantum well will increase. Moreover, our calculation also suggests that an increase in the quantum well width by only one monolayer can result in a large reduction in the transition energy. (c) 2006 Elsevier B.V. All rights reserved.

Characteristics of InGaN multiple quantum well blue-violet laser diodes

Studies on InGaN multiple quantum well blue-violet laser diodes have been reported. Laser structures with long-period multiple quantum wells were grown by metal-organic chemical vapor deposition. Triple-axis X-ray diffraction (TAXRD) measurements show that the multiple quantum wells were high quality. Ridge waveguide laser diodes were fabricated with cleaved facet mirrors. The laser diodes lase at room temperature under a pulsed current. A threshold current density of 3.3 kA/cm(2) and a characteristic temperature To of 145 K were observed for the laser diode.

Unselective regrowth of 1.5-mu m InGaAsP multiple-quantum-well distributed-feedback buried heterostructure lasers

Unselective regrowth for fabricating 1.5-mu m InGaAsP multiple-quantum well (MQW) distributed-feedback (DFB) buried heterostructure (BH) lasers is developed. The experimental results exhibit superior characteristics, such as a low threshold of 8.5 mA, high slope efficiency of 0.55 mW/mA, circular-like far-field patterns, the narrow line-width of 2.5 MHz, etc. The high performance of the devices effectively proves the feasibility of the new method to fabricate buried heterostructure lasers. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

Thermal lensing effect in ridge structure InGaN multiple quantum well laser diodes

Time-resolved light-current curves, spectra, and far-field distributions of ridge structure InGaN multiple quantum well laser diodes grown on sapphire substrate are measured with a temporal resolution of 0.1 ns under a pulsed current condition. Results show that the thermal lensing effect clearly improves the confinement of the higher order modes. The thermal lens leads to a lower threshold current for the higher order modes, a higher slope efficiency, and a change in the lasing mode of the device. The threshold current for the higher modes decreases by about 5 mA in every 10 ns in a pulse, and the slope efficiency increases by 7.5 times on the average when higher modes lase. (c) 2006 American Institute of Physics.

Influence of GaAsP insertion layers on performance of InGaAsP/InGaP/AlGaAs quantum-well laser

We report on the use of very thin GaAsP insertion layers to improve the performance of an InGaAsP/InGaP/AlGaAs single quantum-well laser structure grown by metal organic chemical vapour deposition. Compared to the non-insertion structure, the full width at half maximum of photoluminescence spectrum of the insertion structure measured at room temperature is decreased from 47 to 38 nm indicating sharper interfaces. X-ray diffraction shows that the GaAsP insertion layers between AlGaAs and InGaP compensates for the compressive strain to improve the total interface. The laser performance of the insertion structure is significantly improved as compared with the counterpart without the insertion layers. The threshold current is decreased from 560 to 450mA while the slope efficiency is increased from 0.61 to 0.7W/A and the output power is increased from 370 to 940mW. The slope efficiency improved is very high for the devices without coated facets. The improved laser performance is attributed to the suppression of indium carry-over due to the use of the GaAsP insertion layers.

Thermal stress analysis for GaInAsP multiple quantum well wafer chemically bonded to Si (100)

An n-InP-based InGaAsP multiple-quantum-well wafer was bonded with p-Si by chemical surface activated bonding at 70 degrees C, and then annealed at 450 degrees C. Different thermal expansion coefficients between InP and Si will induce thermal stresses in the bonded wafer. Planar and cross-sectional distributions of thermal stress in the bonded InP-Si pairs were analyzed by a two-dimensional finite element method. In addition, the normal, peeling, and shear stresses were calculated by an analytic method. Furthermore, x-ray double crystalline diffraction was applied to measure the thermal strain and the strain caused by the mismatching of the crystalline orientation between InP (100) and Si (100). The wavelength redshift of the photoluminescence (PL) spectrum due to thermal strain was investigated via the calculation of the band structure, which is in agreement with the measured PL spectra.