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We present a torsion pendulum dual oscillator sensor designed toward the direct detection of Newtonian noise. We discuss the sensitivity limitations of the system, experimental performance characterization results, and prospectives to improve performance. The sensor is being developed to contribute to the mitigation of Newtonian noise impacts in the sensitivities of next generation terrestrial gravitational-wave detectors.

For a biased trap-filled insulator, the current transport is governed by the physics of charge injection from the metallic electrode and also the trap-limited space-charge conduction in the insulator. With a Schottky barrier at the interface of the metal-insulator junction, charge tunneling injection from a metal into the trap-filled insulator is different from an Ohmic contact. At sufficiently large amount of charge injection at high voltages, the charge transport becomes the trap-limited space-charge conduction. In this paper, we develop a consistent model to calculate the correct IV characteristics up to a breakdown field strength of 1 V/nm. Using this model, we analyze the transport characteristic of three different metal-insulator junctions (ITO/PPV, Al/h-BN, and Al/ZrO2) and identify the conduction mechanisms over a wide range of the applied voltage, insulator's thickness, and properties of the traps. Our findings report the interplay between various transport mechanisms, which is useful to characterize the correct current transport for novel insulators such as organic semiconductors, 2D insulators, and metal-oxide electronics.

An InP-based quantum cascade laser structure emitting at a wavelength of 13.6 μm is proposed and demonstrated. The active region is based on a diagonal transition and three-phonon-resonance design. A 5 mm long, 30 μm wide high-reflection coated device with a double channel ridge waveguide structure has shown a threshold current density of 3.0 kA/cm2, a dynamic range of 4.4 kA/cm2, a peak output power close to 1 W, and an average optical power up to 11.7 mW at 3% duty cycle, at 293 K. The laser shows a characteristic temperature T0 of 314 K and T1 of 336 K over a temperature range from 283 to 313 K. National Research Foundation (NRF) Published version Part of this work was conducted within the Delta-NTU Corporate Lab for Cyber-Physical Systems with funding support from Delta Electronics Inc. and the National Research Foundation (NRF) Singapore under the Corp Lab@University Scheme. This research was also supported partially by National Research Foundation Singapore programme (Nos. NRF-CRP18-2017-02 and NRF-CRP19-2017-01).

Amorphous Se is a well-known photoconductor from its early applications in xerography and ultra-sensitive photodetectors like the High-gain Avalanche Rushing Photoconductor (HARP) device. The established way of fabricating the photoconductor for the HARP is rotational thermal evaporation using multilayers of Se and As2Se3. However, the electronic effects of multilayering have not yet been clarified. In this report, we investigated the multilayer structure as a superlattice of Se and As2Se3 fabricated using rotational evaporation and show that the superlattice structure results in the uniformization of the defect levels in the base materials. We found four energy levels associated with defects in As2Se3 and three levels in amorphous Se. In comparison, the superlattice structure of the two materials shows two clear energy levels at EC,Se − 0.533 eV and EV,Se + 0.269 eV. The resulting two occupied energy levels explain the photoelectronic and transport properties observed in multilayer amorphous Se. This result “reinvents” the multilayer structure as a material with observed quantum effects, which significantly improves the material performance in photodetection.

Tin germanium oxide, (Sn,Ge)O2, films were prepared using atomic layer deposition and tailored to a SnS absorber layer by incorporating various amounts of germanium into tin oxide to adjust band alignments at the interfaces of SnS/(Sn,Ge)O2 photovoltaic devices. Carrier concentrations of (Sn,Ge)O2 were suppressed from 1020 to 1018 cm−3 with germanium incorporation, with nitrogen doping further reducing carrier concentrations by another order of magnitude. Excellent tunability of both band energy levels and carrier concentrations of (Sn,Ge)O2 allowed optimizing SnS-based solar cells. SnS/(Sn,Ge)O2:N devices were demonstrated, with an open-circuit voltage as high as 400 mV, due to the effective mitigation of interfacial recombination of photogenerated carriers at the SnS/(Sn,Ge)O2:N absorber-buffer heterojunction interface.Tin germanium oxide, (Sn,Ge)O2, films were prepared using atomic layer deposition and tailored to a SnS absorber layer by incorporating various amounts of germanium into tin oxide to adjust band alignments at the interfaces of SnS/(Sn,Ge)O2 photovoltaic devices. Carrier concentrations of (Sn,Ge)O2 were suppressed from 1020 to 1018 cm−3 with germanium incorporation, with nitrogen doping further reducing carrier concentrations by another order of magnitude. Excellent tunability of both band energy levels and carrier concentrations of (Sn,Ge)O2 allowed optimizing SnS-based solar cells. SnS/(Sn,Ge)O2:N devices were demonstrated, with an open-circuit voltage as high as 400 mV, due to the effective mitigation of interfacial recombination of photogenerated carriers at the SnS/(Sn,Ge)O2:N absorber-buffer heterojunction interface.

Copper sulfides (Cu2−xS) comprises a family of sulfides which possess good electrical and photovoltaic properties due to their self-doping (p-type) nature, attributed from the copper vacancies in their structure. Recently, metal sulfide thin films have been investigated for their resistive RAM behaviour due to the reversible formation of conductive bridges between crystalline structures. In this letter, 2D self-assembled finned covellite (CuS) was prepared via RF Magnetron Sputtering. This 2D finned CuS was found to be polycrystalline via TEM, exhibiting ReRAM behaviour with resistances having two orders of magnitude differences between 0 and −0.5 V. This fin structure eliminates the need for a multi-layered device which until now is the primary method of harnessing the ReRAM behaviour in copper sulphide. It gives rise to very low voltages of −3 V and 1 V for writing and reset, respectively, with a writing time of 20 ms.

Based on the differential conformal transformation in the fractional order, we defined the fractional memristor in contrast to the traditional (integer-order) memristor. As an example, a typical spintransfer torque (STT) memristor (with the asymmetric resistance hysteresis) was proved to be a 0.8 fractional memristor. In conclusion, many memristors should not be treated as ideal ones due to the fractional interaction between flux and charge. Indeed, unless a non-ideal memristor is properly modelled as a fractional memristor, no deep physical understanding would be possible to develop a reliable commercial product.

A method to enhance the responsivity of a transistor terahertz (THz) detector by introducing a Fabry-Perot (FP) cavity was proposed. A theoretical model for the voltage responsivity of the transistor THz detector with a resonant cavity was derived and verified experimentally using a GaN high electron mobility transistor (HEMT) with the FP cavity formed between the GaN HEMT substrate and an indium tin oxide mirror. The measured detector responsivity shows 2.5 times enhancement compared to the device without the cavity and lower noise equivalent power. The performance of the GaN HEMT THz detector with a FP cavity, which has a designed quality factor of 16, is dependent on the incident angle of the THz radiation.

In this study, the effects of periodic Si delta-doping on the morphological and optical properties of GaN films grown on Si (111) substrate have been investigated. It is found that the flow rate of Si dopant during growth significantly affects the surface morphology, structural and optical quality of GaN. Compared to undoped GaN on Si(111), films grown using periodic delta-doping show a significant reduction of the in plane tensile stress, which is confirmed by the blueshift of the E2(TO) phonon and band edge photoluminescence peaks. The crack density in GaN films also reduces due to delta-doping.

An intelligent and monolithic integration of optical power splitter with optically switchable cross-connect has been proposed. It is fabricated on the multimode interference principle in Si-based SiGe material system and configured for a 3×2 symmetrical structure of the three input waveguides and the two output waveguides. The central waveguide section is based on a multimode interference and incorporated with an activated carrier injection element. The operating wavelengths of the device are specially designed for 1545, 1550, and 1555nm conventional-band wavelengths. The measured crosstalk is at around −17dB and the average insertion loss is about 5.25dB. At switch-ON state, the measured injection current is 370mA corresponding to an injection current density of 950A∕cm2.

Optical absorption spectra of intersubband transitions in In0.3Ga0.7As∕GaAs multiple quantum dots were investigated using the optical absorption as a function of the number of In0.3Ga0.7As monolayers deposited using the molecular-beam epitaxy Stranski–Krastanow technique. The peak position energy reached 13.7μm for a sample containing 50 monolayers of In0.3Ga0.7As. The lack of the observation of intersubband transitions in small quantum dots, where the number of the deposited monolayer is less than 15 monolayers, is an indication of the absence of quantum confinement. On the other hand, the presence of high dislocations density in larger quantum dots, where the deposited number of monolayers exceeds 50, could be the reason of why the intersubband transitions are degraded.

Nanopore arrays with pore diameters of approximately 75nm were fabricated in GaN films by inductively coupled plasma etching using anodic aluminum oxide (AAO) films as etch masks. Nanoporous AAO films were formed on the GaN surface by evaporating an Al film onto a GaN epilayer and subsequently anodizing the aluminum. To minimize plasma-induced damage, the template was exposed to CF4-based plasma conditions. Scanning electron microscopy analysis shows that the diameter and the periodicity of the nanopores in the GaN were directly transferred from the original anodic alumina template. The pore diameter in the AAO film can be easily controlled by tuning the anodization conditions. Atomic force microscopy, photoluminescence, and micro-Raman techniques were employed to assess the etched GaN nanopore surface. This cost-effective, nonlithographic method to produce nano-patterned GaN templates is expected to be useful for growth and fabrication of nitride-based nanostructures and photonic band gap materials.

A two-stage “nucleation-augmented” growth method for producing InAs self-assembled quantum dots (QDs) using molecular-beam epitaxy on GaAs (100) substrates has been investigated in detail. Photoluminescence (PL) measurements show that a 1.8-monolayer-(MLs) InAs QD “nucleation” layer grown at a fast rate, followed by a 2.6-MLs-InAs “augmented” layer grown under pulsed conditions at a slow rate, dramatically increases the dot density and improves the PL intensity for the InAs QDs. It was found that, when the effective growth rate of the InAs augmented layer was reduced, the PL peak emission shifts to a longer wavelength and the PL intensity is enhanced. These changes in characteristics were attributed to improved optical quality and greater dot density.

We unambiguously identify and derive the irreversibilities and entropy fluxes of the quantum and classical processes that intrinsically limit the performance of thermionic and tunneling chillers. The fundamental tradeoff between cooling flux and efficiency can then be evaluated, including the ramifications for feasible cooling regimes. Quantitative illustrations are provided for real solid-state devices. We cast these results as trajectories in the temperature-entropy plane and unify the thermodynamic perspective of these quantum chillers with that of conventional cooling cycles.

Using a thermal-crosslinkable siloxane bisbenzocyclobutene, high quality spin-on (solutionprocessable) gate dielectric layers as thin as 50 nm have been fabricated over the semiconductor layer for polymer field-effect transistors. This was demonstrated on a poly(9,9-dialkylfluorene-alt-triarylamine) as p-channel semiconductor, with a surfactantion-exchanged poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate complex as top-gate electrode. The devices operate at a low voltage with a field-effect mobility of few 10−4 cm2/Vs, and can be continuously operated at 120 °C.

An ultracompact, multifunctional, and highly integrated photonic switch with a 3×2 configuration has been designed and fabricated with SiGe/Si material by using silicon-optical bench technology. This kind of switch can be used in fiber optic communications systems, photonic integrated circuits and wavelength division multiplexed networks as an optical power splitter, optical crossconnect, optical add-drop multiplexer, and wavelength division multiplexer simultaneously or individually. The function of the device is to combine multiwavelengths from different input channels and to switch them to different output channels. The operating wavelength range of the device is designed in C band, i.e., 1530–1570 nm. The device was characterized at 1540, 1550, and 1560 nm wavelengths. The performance at these wavelengths is found satisfactory. The measured insertion loss is less than 2 dB, ON/OFF ratio is greater than 30 dB, crosstalk is between −20 and −25 dB, and switching speed is 100–200 ns.

In this letter, we report on temperature-dependent anti-Stokes photoluminescence (ASPL) at an interface between partially ordered GaInP2 epilayer and GaAs substrate. It is found that the intensity of the ASPL depends strongly on temperature accompanying with a clear blueshift in energy. A localized-state luminescence model was employed to quantitatively interpret temperature dependence of the ASPL. Excellent agreement between the theory and experiment was obtained. Radiative recombination mechanism of the up-converted carriers was discussed.

2-dimensional (2D) inorganic analogues of graphene such as MoS2 and WS2 present interesting opportunities for field emission technology due to their high aspect ratio and good electrical conductivity. However, research on 2D MoS2 and WS2 as potential field emitters remains largely undeveloped compared to graphene. Herein, we present an approach to directly fabricate ultrathin MoS2 and WS2 onto Ag nano-tips using pulsed laser deposition at low temperatures of 450–500 °C. In addition to providing a layer of chemical and mechanical protection for the Ag nano-tips, the growth of ultrathin MoS2 and WS2 layers on Ag led to enhanced emission properties over that of pristine nano-tips due to a reduction of the effective barrier height arising from charge injection from Ag to the overlying MoS2 or WS2. For WS2 on Ag nano-tips, the phasic mixture was also an important factor influencing the field emission performance. The presence of 1T-WS2 at the metal-WS2 interface in a hybrid film of 2H/1T-WS2 leads to improvement in the field emission capabilities as compared to pure 2H-WS2 on Ag nano-tips.

Graded InGaN buffers were employed to relax the strain arising from the lattice and thermal mismatch in GaN/InGaN epilayers grown on sapphire. An enhanced strain relaxation was observed in GaN grown on a stack of five InGaN layers, each 200 nm thick with the In content increased in each layer, and with an intermediate thin GaN layer, 10 nm thick inserted between the InGaN layers, as compared to the conventional two-step growth of GaN epilayer on sapphire. The function of the intermediate layer is to progressively relax the strain that builds up in the InGaN layer. If the InGaN layers were graded too rapidly, more dislocations will be generated. This increases the probability of the dislocations getting entangled, thereby impeding the motion of the dislocations to relax the strain in the InGaN layer. The optimum growth conditions of the intermediate layer play a major role in promoting the suppression and filling of the V-pits in the GaN cap layer, and were empirically found to be a thin 10-nm GaN layer grown at 750 °C and annealed at 1000 °C.

InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.InGaN/GaN multiple quantum wells (MQWs) with emission peaks in the green light region were grown by metalorganic chemical vapor deposition. Photoluminescence (PL) measurements on these samples showed extra peaks in the spectrum beside the green emission due to reactor temperature ramping up to 1000 °C after the MQWs growth. It was found that growth interruption at the upper interface of MQWs combining with interface treatment by H2 flow could eliminate these extra peaks. Our analysis of the PL spectra suggested that these extra emission peaks originate due to the occurrence of In segregation in the upper part of the quantum wells induced by partial strain relaxation in the processes of high temperature treatment, and the interface treatment with H2 flow can suppress this phase separation phenomenon.

Using tertiarybutylphosphine (TBP) as phosphorus precursor, high-quality AlGaInP epilayers and AlGaInP/GaInP multiple-quantum-well (MQW) structures have been grown by metalorganic chemical vapor deposition. The photoluminescence results indicate that the AlGaInP materials are as good as those grown using PH3 in terms of optical quality. Finally, AlGaInP MQW red laser structures have been grown, and the electrically pumped AlGaInP red lasers grown by TBP have been demonstrated with the emission wavelength of 647 nm, indicating that TBP can be used to grow high-quality AlGaInP epilayers and AlGaInP-based red lasers, which presently is dominated by the highly toxic gas source PH3.

It has been found, by using photoluminescence (PL) studies, that both the localized states and nonradiative recombination centers in In0.06Ga0.94N/GaN multiple quantum wells (MQWs) can be greatly suppressed by inserting a monolayer of AlN before the growth of each well layer. While inserting a monolayer of AlN before each well layer does not have any effect on the growth rate or on the indium content of MQWs, it does improve room-temperature PL intensity of the In0.06Ga0.94N/GaN MQWs. The physics behind the suppression of localized states and nonradiative recombination centers is discussed.

A thin 3-nm-thick parylene layer is deposited by chemical vapor deposition at room temperature on the indium tin oxide (ITO) coated glass substrate to form a bilayer anode of an organic light emitting diode. The parylene layer forms a conformal film to cover the spikes present in the ITO film. This parylene film presents a smoother surface to the subsequent organic layers. The parylene film not only reduces the occurrence of dark spots, acting as a barrier for oxygen diffusion from either the ITO or from the atmosphere and stabilizing the migration of the electrodes during electrical stress, but also improves the injection of holes from the anode. By inserting another parylene layer in between the organic and cathode layers, the probability of formation of nonemissive areas is further reduced.

We report the use of an in situ four-point probe method to investigate the relation between oxygen plasma treatment on indium tin oxide (ITO) and the variation in its sheet resistance. Analyses on the ITO surface composition made with time-of-flight secondary ion mass spectroscopy probe a dual-layer parallel resistor model for oxygen plasma-treated ITO anodes. We have shown that the increase in the ITO sheet resistance can be attributed to the reduction of oxygen deficiency near the surface. The improvement in carrier injection in phenyl-substituted poly(p-phenylenevinylene)-based light-emitting diodes correlates directly with a layer of low conductivity, several nanometers thick. This was induced on the ITO surface and serves as an efficient hole injecting anode.

The degradation and failure of organic light-emitting device are observed via optical microscopy. The “degraded area” has been identified to be made up of three regions: (1) a dark spot at the center, (2) a nonemitting area forming the core, and (3) a weakly emitting area surrounding the core. It is found that due to metal migration, as evidenced from the secondary ion mass spectrometry profiles, the indium tin oxide/polymer interface roughens during operation. The intense local current at sharp points degrades the polymer causing the formation of the dark center. Further current stress caused the central core to carbonize which may lead to short and/or open circuits accompanied by fluctuations in the device current.

Polymer degradation under electrical stress was studied using the device structure indium tin oxide/poly(p-phenylene vinylene)/Ca/Ag. The devices that were used in this study do not show any significant decrease in electroluminescent intensity for up to several thousand hours of operation. However for devices that were electrically stressed and then exposed to normal ambient, there was a significant decrease in the photoluminescence. Those that were not electrically stressed but exposed to normal ambient or were electrically stressed but not exposed to normal ambient, there was no significant degradation in the photoluminescence. These results showed that both conditions of electrical stress and photo-oxidation are necessary for a significant decrease of the photoluminescence intensity. Degradation is accompanied by an increase in the x-ray crystallization peak. We interpret that the photoluminescence intensity degradation is due to the change in polymer crystallinity caused by electrical stress followed by photo-oxidation.

Nanometer-scale InGaN self-assembled quantum dots have been formed in an InGaN single-quantum-well structure on a (0001) sapphire substrate with In as the antisurfactant using low-pressure metalorganic chemical-vapor deposition. High-resolution transmission electron microscopy reveals that the average dimensions of InGaN nanometer-scale structures are as small as 4 nm wide and 1.5 nm high. Strong photoluminescence emission of the InGaN quantum dots was observed at room temperature with an emission peak of about 2.56 eV (485 nm) and a full width at half maximum of about 150 meV (30 nm). The choice of In as the antisurfactant also avoids the incorporation of foreign atoms in the active layers.

The degradation in electroluminescence of poly(p-phenylene vinylene)-based organic light-emitting devices is studied using optical microscopy, scanning electron microscopy, and secondary ion mass spectroscopy. “Bubbles” are formed at the polymer and indium tin oxide interface or in the polymer layer within the nonemissive area. This formation, which occurs during device electrical stress, is accompanied by a fluctuation of the device current. The bubbles are formed by the degraded polymer and/or the gas released from disintegration of the polymer. High local current density flowing near the dark spot center and the resultant heating, decomposes the polymer layer. The resultant carbonized area causes either local short circuit and/or open circuit leading to the final light-emitting device failure.

Based on the two-mode interference principle, the high carrier injection effect, and the free-carrier plasma dispersion effect, a directional coupling active optical switch in SiGe/Si alloy has been fabricated. Its insertion loss and crosstalk were measured to be 2.74 and −15.5 dB, respectively, at the wavelength of 1.3 μm and the total switching current of 110 mA. The fastest response time of the switch is up to 30 ns.

This work reports on the high-frequency capacitance–voltage (C–V) measurements of metal-insulator-semiconductor structures fabricated by depositing SiO2 film on an n-type GaN epitaxial layer. The SiO2 film was grown by plasma-enhanced chemical vapor deposition at 310 °C, and the GaN layer was grown by low-pressure metalorganic chemical vapor deposition on a sapphire substrate. High-frequency C–V measurements have been carried out in darkness with different bias ranges and sweep rates. With a bias between ±20 V, the small flatband shift and the very small hysteresis indicate that the interface trap concentration in the sample is low, and the interface state density is 2.1×1011 eV−1 cm−2. However, a pronounced increase of hysteresis with an extended bias range was observed. When the bias is over ±50 V, the increase of the hysteresis is much larger, indicating the nonuniform distribution of different slow deep states in the structure. The extracted interface state density is in the range of 1012 eV−1 cm−2.

Phenyl-substituted polymer electroluminescent (EL) devices using an insulating lithium–fluoride (LiF) layer between indium tin oxide (ITO) and poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT) hole transporting layer have been fabricated. By comparing the devices made without this layer, the results demonstrate that the former has a higher EL brightness operated at the same current density. At a given constant current density of 20 mA/cm2, the luminance and efficiency for devices with 1.5 nm LiF-coated ITO were 1600 cd/m2 and 7 cd/A. These values were 1170 cd/m2 and 5.7 cd/A, respectively, for the same devices made with only an ITO anode. The ultrathin LiF layer between ITO and PEDOT modifies the hole injection properties. A more balanced charge carrier injection due to the anode modification by an ultrathin LiF layer is used to explain this enhancement.

Our in situ experimental observations of dark spot growth in organic light-emitting diodes using optical microscopy show a linear rate of growth for the area of all the dark spots. We used uniformly sized silica micro particles to intentionally create size-controllable pinholes on the cathode protective layer. Subsequently, we observed initial formation of dark spots as a result of these pinholes and then monitored their growth. Due to usage of particles of various diameters, we were able to linearly correlate the growth rate with pinhole size. This allows us to estimate the original pinhole sizes that gave rise to the dark spots, which we believe were initiated by “dust” particles. Our studies verify that dark spot formation is due to pinholes on the protective layer that creates pathways for water or oxygen permeation, and that dark spot growth is dependent on the pinhole sizes.

Self-organized InP quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix by metalorganic chemical vapor deposition. Strong photoluminescence centered at 986 nm is observed for the sample of InP grown at 490 °C, which can be attributed to radiative recombination of zero-dimensional (0D) electrons located in the InP dots and holes located in the surrounding regions. The indirect recombination of photogenerated carriers has been confirmed by the measurement of luminescence at different excitation densities and temperatures. If the InP is grown at 600 °C, experimental results show that a thicker and much smoother wetting layer is formed which results in much stronger and narrower luminescence located at 875 nm. In addition, state filling of the 0D electrons is also observed for the type-II quantum dots.

Ultrashallow p+/n junctions formed by B+-ion implantation and annealed by spike rapid thermal annealing (RTA) or laser annealing were studied. The effect of the preamorphizing depth on the redistribution of boron atoms after annealing has also been investigated. Our results show that for ultrashallow junctions formed by ultra-low-energy ion implantation and spike RTA, the depth of the preamorphizing implant has very little impact on the junction depth. By optimizing the laser fluence and preamorphization depth, a highly activated, ultrashallow, and abrupt junction can be obtained using a 248 nm excimer laser. The secondary-ion-mass spectrometry results clearly indicate that a step-like profile with a junction depth of 370 A (for a B+ implant at 1 keV) can be formed with a single-pulse laser irradiation at 0.5 J/cm2.

A InGaAsP/InP self-aligned, native oxidized buried heterostructure (BH) distributed feedback (DFB) laser is proposed. It is as easy to process as the ridge waveguide DFB laser and has superior performance. The current aperture can be easily controlled without selective regrowth. The laser exhibits a low threshold of 5.0 mA with 36 dB side mode suppression ratio at the emission wavelength of 1.562 mu m. It emits in a single lobe with full width at half maximum angles of 33.6 degrees and 42.6 degrees for the lateral and vertical fields, respectively. Its beam is more circular than that of the as-grown BH laser because the lower refractive index of oxide compared to the as-grown layer and results in a larger lateral optical confinement. Its characteristic temperature (T-0) is 50 K at room temperature but increases in value at the higher temperature range. (C) 2000 American Institute of Physics. [S0003-6951(00)00812-3].