Origin of non-uniformity in MBE grown nanometer-sized InGaAs ridge quantum wires and its removal by atomic hydrogen-assisted cleaning

The origin of non-uniformity of MBE-grown InGaAs ridge quantum wires (QWRs) in sub 10-nm wire width range was investigated in detail by SEM, in situ XPS, TEM and PL measurements. InAlAs/InGaAs/InAlAs wires were selectively grown on InGaAs ridge structures prepared also by MBE on 10 stripe patterned (001) InP substrates. The main source of non-uniformity was thermal cleaning of InP in As₄ done prior to InGaAs ridge formation which produced ridges having irregular sized InGaAs islands due to an initial As–P exchange reaction. Low temperature atomic hydrogen cleaning removed this problem, and led to successful formation of sub 10-nm QWRs.     

Time-dependent photon number discrimination of InGaAs/InP avalanche photodiode single-photon detector

We investigated the photon-number-resolving (PNR) performance of the InGaAs/InP avalanche photodiode (APD) as a function of the electric gate width and the photon arrival time. The optimal electric gate width was around 1 ns for PNR measurements in our experiment, which provided a PNR capability up to three photons per pulse when the detection efficiency was ~20%. And the dependence of the PNR performance on the arrival time of the photons showed that the photon number could be better resolved if the photons arrived on the rising edge of the electric gate than on the falling edge. In addition, we found that with the increase of the electric gate width, PNR performance got worse. The observation would be helpful for improving the PNR performance of the InGaAs/InP APD in the gated mode.     

InGaAsP/InP有源时分光子交换器件

自行设计并研制成功了两种有源时分光子交换器件：ＩｎＧａＡｓＰ／ＩｎＰ ＥＭＰＢＨ双稳激光器ＩｎＧａＡｓＰ／ＩｎＰ ＭＱＷ ＤＣＰＢＨ双稳激光器，对这两种器件的部分性能作了简要报道。     

Threshold Fluence Measurement for Laser Liftoff of InP Thin Films by Selective Absorption

We explore conditions for achieving laser liftoff in epitaxially grown heterojunctions, in which single crystal thin films can be separated from their growth substrates using a selectively absorbing buried intermediate layer. Because this highly non‐linear process is subject to a variety of process instabilities, it is essential to accurately characterize the parameters resulting in liftoff. Here, we present an InP/InGaAs/InP heterojunction as a model system for such characterization. We show separation of InP thin films from single crystal InP growth substrates, wherein a ≈10 ns, Nd:YAG laser pulse selectively heats a coherently strained, buried InGaAs layer. We develop a technique to measure liftoff threshold fluences within an inhomogeneous laser spatial profile, and apply this technique to determine threshold fluences of the order 0.5 J cm^?2 for our specimens. We find that the fluence at the InGaAs layer is limited by non‐linear absorption and InP surface damage at high powers, and measure the energy transmission in an InP substrate from 0 to 8 J cm^?2. Characterization of the ejected thin films shows crack‐free, single crystal InP. Finally, we present evidence that the hot InGaAs initiates a liquid phase front that travels into the InP substrate during liftoff.

     

Self-assembled InAs/GaAs quantum dots covered by different strain reducing layers exhibiting strong photo- and electroluminescence in 1.3 and 1.55 microm bands

The effect of different InGaAs and GaAsSb strain reducing layers on photoluminescence and electroluminescence from self-assembled InAs/GaAs quantum dots grown by metal-organic vapour phase epitaxy was investigated. The aim was to shift their luminescence maximum towards optical communication wavelengths at 1.3 or 1.55 microm. Results show that covering by InGaAs strain reducing layer provides stronger shift of photoluminescence maximum (up to 1.55 microm) as compared to GaAsSb one with similar strain in the structure. This is caused by the increase of quantum dot size during InGaAs capping and reduction of quantum confinement of the electron wave function which spreads into the cap. Unfortunately, the weaker electron confinement in quantum dots is a reason of a considerable blue shift of electroluminescence from these InGaAs structures since optical transitions move to InGaAs quantum well. Although strong electroluminescence at 1300 nm was achieved from quantum dots covered by both types of strain reducing layers, the GaAsSb strain reducing layer is more suitable for long wavelength electroluminescence due to higher electron confinement potential allowing suppression of thermal carrier escape from quantum dots.     

Selective Intermixing of InAs/InGaAs/InP Quantum Dot Structure With Large Energy Band Gap Tuning

Selective postgrowth band gap tuning of self-assembled InAs/InGaAs/InP quantum dot (QD) structures has been investigated. Very large band gap blueshift of over 158 meV of the InAs QD structure has been received through the intermixing by exposing the sample under argon plasma and followed by thermal annealing at 780 °C. Selective intermixing of the InAs QD structure has been studied by depositing a $hbox{SiO}_{2}$ mask layer on the sample for the intermixing. The largest selective band gap shift between the $hbox{SiO}_{2}$ covered and uncovered regions of the sample reaches 77 meV. This intermixing selectivity decreases when the annealing temperature is increased. This reduction in the intermixing selectivity is attributed to the enhanced QDs intermixing of the $hbox{SiO}_{2}$-masked samples because of the out diffusion of different elements from the InAs/InGaAs/InP QD structure into the $hbox{SiO}_{2}$ cover layer. Three different energy band gap shifts of an InAs/InGaAs/InP QD structure across the wafer have been received by this postgrowth selective intermixing. The selective band gap tuning paves a way for monolithic integration of passive and active optoelectronic devices in QD systems.      

Argon-plasma-induced InAs/InGaAs/InP quantum dot intermixing

We report the first study of argon (Ar)-plasma-enhanced intermixing of InAs/InGaAs/InP self-assembled quantum dots (QDs) in an inductively coupled plasma reactive ion etch system. The Ar-plasma exposure creates point defects, which propagate into the QD structure and enhance the intermixing between the QDs and their barrier layers, hence tuning the energy bandgap of the QDs. By optimizing the plasma exposure time and the annealing temperature, we observe (i) a blueshift of 160?nm and an increase in the photoluminescence (PL) intensity of the QD samples immediately after Ar-plasma exposure for 90?s, and (ii) a further increase in the blueshift of 330?nm, accompanied by 2.5-times increase in the PL intensity and 37?nm narrowing in the PL linewidth after subsequent rapid thermal annealing at 720?°C. The ability to generate a large blueshift without degrading the material quality shows that Ar-plasma exposure is an efficient post-growth technique for tuning the energy bandgap of QD structures.     

Investigation of InP/InGaAs superlattice-emitter bipolar transistor with multiple negative-differential-resistance regions

The sequential tunneling behavior in InP/InGaAs superlattice-emitter bipolar transistors is demonstrated by theoretical analysis and experimental results. The tunneling mechanism in the InP/InGaAs superlattice structures is analyzed by theoretical calculation. Due to the weak coupled tunneling mechanism, the interesting multiple negative-differential-resistances (NDRs) resulting from the creation and extension of high-field domain in the superlattice are observed at room temperature. Experimentally, the transistor performances, including a high current gain of 454, a low collector–emitter offset voltage of 80 mV, and a pronounced multiple NDRs, are achieved. The proposed structures may provide good potential for signal amplifiers and multiple-valued logic circuit applications.     

Reliability and degradation of 980 NM InGaAs/GaAs strained quantum well lasers

Degradation behaviours of 980 nm InGaAs/GaAs strained quantum well (QW) lasers are clarified and compared with normal AlGaAdGaAs, InGaAsP/InP and GaAs/GaAs QW lasers. Through various ageing tests, it is confirmed that 980 nm InGaAs/GaAs strained QW lasers are applicable to optical fibre transmission systems where the components are required to be highly reliable.     

Laser pulse-stretching with multiple optical ring cavities

We describe a simple and passive nanosecond-long laser-pulse stretcher using multiple optical ring cavities. We present a model of the pulse-stretching process for an arbitrary number of optical ring cavities. This new model explicitly includes the effects of cavity delay time, beam-splitter reflectivity, total number of optical cavities, and describes the effects of spatial profile sensitivity. Using the model, we optimize the design of a pulse stretcher for use in a spontaneous Raman-scattering excitation system that avoids laser-induced plasma spark problems. From the optimized design, we then experimentally demonstrate and verify the model with a three-cavity pulse-stretcher system that converts a 1000-mJ, 8.4-ns-long input laser pulse into an approximately 75-ns-long (FWHM) output laser pulse with a peak power reduction of 0.10x and an 83% efficiency.     

Increase in electron mobility of InGaAs/InP composite channel high electron mobility transistor structure due to SiN passivation

The influence of silicon nitride passivation on electron mobility of InGaAs/InP composite channel high electron mobility transistor structure has been studied. Different from the structures with single InGaAs channel, an increase in effective mobility μ_e with a negligible change of sheet carrier density n_s after SiN deposition is clearly observed in the composite channel structures. The enhancement of μ_e could be explained under the framework of electrons transferring from the InP sub-channel into InGaAs channel region due to the energy band bending at the surface region caused by SiN passivation, which is further confirmed by low temperature photoluminescence measurements.     

Performance of InGaAs/InP Avalanche Photodiodes as Gated-Mode Photon Counters

We investigate the performance of separate absorption multiplication InGaAs/InP avalanche photodiodes as single-photon detectors for 1.3- and 1.55-mum wavelengths. First we study afterpulses and choose experimental conditions to limit this effect. Then we compare the InGaAs/InP detector with a germanium avalanche photodiode; the former shows a lower dark-count rate. The effect of operating temperature is studied for both wavelengths. At 173 K and with a dark-count probability per gate of 10(-4), detection efficiencies of 16% for 1.3 mum and 7% for 1.55 mum are obtained. Finally, a timing resolution of less than200 ps is demonstrated.     

Optical properties of indium phosphide in the 50-200 Å wavelength region using a reflectivity technique

The optical constants of indium phosphide (InP) in the soft x-ray region of 50-200 ? are determined from angle-dependent reflectivity measurements. The measurements are carried out using the reflectivity beam line at the Indus-1 synchrotron source. The derived optical constants are compared with tabulated values of Henke et al. [At. Data Nucl. Data Tables 54, 181 (1993)]. Experimental values of δ and β are in close agreement with the tabulated values in the lower wavelength region of 50-120 ?. The experimental value indicates an edge shift of 0.4 ? toward the lower wavelength side from the phosphorous L-edge value of 92 ?. However, above the 120 ? region, where the indium N(2) edge falls at 160.7 ?, there is a huge difference between experimental and tabulated values. Both delta and beta values are significantly higher. In contrast to tabulated values of the β/δ ratio, which is more than 1 above the 140 ? region, the experimental measured ratio is found to be less than 1. This study presents the first reported experimental values of optical constants for InP in this wavelength range, to the best of our knowledge.     

Correlation between hot-electron-stress-induced degradation and cathodoluminescence in InP-based HEMTs

Low temperature spectrally resolved cathodoluminescence has been used to study the effects of electrical degradation induced by hot-electron-stress on the optical transitions in lattice matched InAlAs/InGaAs/InP HEMTs. A clear reduction of the cathodoluminescence emission collected from the gate–drain region of stressed devices has been found, indicating a modification of the trap density inside the device. Depth resolved analyses from the gate–drain region before and after stress evidenced that the electric field due to the traps induced by the hot-electron-stress mainly influenced a device region between the highly doped InAlAs and InGaAs cap layers and the n+ doped donor and undoped spacer? InAlAs layers. The effect of the hot electron stress has been evidenced mainly on the cathodoluminescence transitions from the InAlAs layers. No evidence of a possible influence on the intrinsic InGaAs channel has been found.     

Investigations on the nucleation parameters of InGaAs grown on InP during LPE

The initial stages of LPE growth of the InGaAs ternary compound on an InP substrate were analysed using the classical heterogeneous nucleation theory, incorporating lattice mismatch between the grown alloy and the substrate. The explicit expression for the lattice mismatch induced supercooling for the growth of the chosen system was established, and it was used to evaluate the nucleation parameters. It has been proved theoretically that the nucleation barrier for the formation of InxGa_{1 –x} As on InP depends very strongly on the composition of the alloy; the condition for the growth of good quality InGaAs on InP was calculated.     

Study of the kink effect in AlInAs/GaInAs/InP composite channel HFETs

A detailed study is presented on AlInAs/InGaAs/InP composite channels. These devices combine the advantages of high mobility at low voltages and high electric field operations thanks to the use of a composite channel formed by a thin InGaAs layer and a doped InP subchannel. Due to the very low gate leakage currents, it has been possible to precisely study the impact ionization contributions as a function of the temperature. Surprisingly, it is not possible from our measurements to correlate the kink effect observed in the devices with the impact ionization phenomenon. Therefore, a detailed study of the AlInAs deep traps, and the deep levels detected in the devices has been performed using DLTS, CTS, drain lag, and low frequency noise measurements. The observation of the kink effect in our HFETs has been clearly connected to an electron trap located in the AlInAs layers. In order to confirm this result, the optical properties of this deep trap have been studied by I–V measurements under optical excitation for HFETs, and by DLOS on bulk AlInAs. Our measurements show that the same defect is observed in AlInAs and in the HFETs and that it is possible to suppress the kink effect by an optical ionization of this electron trap. Finally, our electro-optical study shows the direct correlation between deep traps in the AlInAs barrier layers and the kink effect in these devices.     

New approach for the design of an optical square pulse generator

What is believed to be a new approach for the design and analysis of a reconfigurable optical square pulse generator using the concept of temporal optical integration and the digital signal processing method is presented. The reconfigurable square pulse generator is synthesized using compact active semiconductor-based waveguide technology, and it consists simply of the cascade of a tunable microring resonator (or a tunable all-pole filter) and a tunable asymmetrical Mach-Zehnder interferometer (or a tunable all-zero filter). The reconfigurable generator can convert an input picosecond pulse (i.e., soliton or Gaussian pulse) into an optical square pulse. The pulse width of the generated square pulse can be adjusted by controlling the time delay of a variable delay element in the tunable all-zero filter. The reconfigurable generator can convert an input picosecond pulse train into return-to-zero (RZ) and non-return-to-zero (NRZ) signals with square pulse shapes. The repetition rates of the generated RZ and NRZ signals can be varied by adjusting the bit period of the input picosecond pulse train, the input pulse width, and the time delay of the variable delay element. The effect of the deviation of the parameter values on the generator performance is also studied.     

Modeling Transient Heat Conduction and Radiative Transport in Semitransparent Media: A Tool for the Interpretation of Reflectivity Data

In this work the problem of interpretation of reflectivity data for semitransparent materials at high temperatures is addressed. A detailed analysis of the transient thermal response of a participating medium subjected to a pulse of finite energy is performed using a new method developed to solve the general equation of energy transfer for a conductive, absorbing, emitting, and scattering medium. The model, previously presented for a material with constant optical properties (Musella, Tschudi, Int J Thermophys 26:981, 2005), has been upgraded to encompass a much wider scenario where the thermophysical and optical properties are temperature dependent. This allows the study of the transient reflectivity of laser-heated samples where high heating rates and strong temperature gradients occur near the surface. Considerable differences of the reflectivity values for the same surface temperature calculated in the heating and cooling phases, both different from the corresponding values for an isothermal sample, are reported in accordance with experimental results.     

Impact of signal filling factor for switching in reflective vertical cavity-based fast semiconductor saturable absorbers

A numerical model on reflective vertical cavity-based fast semiconductor (carrier recombination time in ps regime) saturable absorbers (VC-FSSA) is investigated for all-optical high-speed future applications, where the fluctuation of cavity optical length due to thermo-optic effects and cavity heating, and optically induced carrier density change are considered simultaneously with the increase of input signal intensity. The phase-shift due to thermal-optic effects and cavity heating is very important for high-speed device performances. At first, intensity and wavelength bi-stabilities are modelled with the different intensity time filling factor of RZ modulated pump signals and also, with the different input intensities, where the intensity tuning time is considered at few μs. Then the inverse saturation characteristics of reflectivity for the universal logic operations are shown for probe signal with the proper wavelength, which can control the negative effects of optically induced heating effects. These characteristics can be used for the high-speed thermally stable bi-stable optical switch and all-optical logic applications.     

Effect of the wall thickness on the forming behavior and welding result during magnetic pulse welding

Magnetic pulse welding is a promising technology for the joining of dissimilar metals. Since the input of thermal energy is significantly reduced compared to conventional fusion welding technologies, critical intermetallic phases can largely be avoided. Therefore, proper collision conditions are necessary. Those require a careful adjustment of the energetic and geometric parameters at the impact welding setup. The thickness of the accelerated joining partner (flyer) determines the necessary energy input for a successful weld. However, at the same time, it has an effect on the weld formation. This study utilizes a novel optical measurement system to explain these findings and to gain insights into the forming behavior of the flyer parts. It is shown that the collision angle depends on the flyer tube thickness and, thus, directly has an effect on the welding result.