Diode Current Research Articles

Carrier-envelope offset frequency stabilization of a gigahertz semiconductor disk laser

Optical frequency combs based on ultrafast lasers have enabled numerous scientific breakthroughs. However, their use for commercial applications is limited by the complexity and cost of femtosecond laser technology. Ultrafast semiconductor lasers might change this issue as they can be mass produced in a cost-efficient way while providing large spectral coverage from a single technology. However, it has not been proven to date if ultrafast semiconductor lasers are suitable for stabilization of their carrier-envelope offset (CEO) frequency. Here we present what we believe to be the first CEO frequency stabilization of an ultrafast semiconductor disk laser (SDL). The optically pumped SDL is passively modelocked by a semiconductor saturable absorber mirror. It operates at a repetition rate of 1.8 GHz and a center wavelength of 1034 nm. The 273 fs pulses of the oscillator are amplified to an average power level of 6 W and temporally compressed down to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a photonic crystal fiber. The CEO frequency is detected in a standard f–to–2f interferometer and phase locked to an external reference by feedback applied to the current of the SDL pump diode. This proof-of-principle demonstrates that ultrafast SDLs are suitable for CEO stabilization and constitutes a key step for further developments of this comb technology expected in the coming years.

Carrier-envelope offset stabilization of a GHz repetition rate femtosecond laser using opto-optical modulation of a SESAM.

We demonstrate, to the best of our knowledge, the first carrier-envelope offset (CEO) frequency stabilization of a GHz femtosecond laser based on opto-optical modulation (OOM) of a semiconductor saturable absorber mirror (SESAM). The 1.05-GHz laser is based on a Yb:CALGO gain crystal and emits sub-100-fs pulses with 2.1-W average power at a center wavelength of 1055nm. The SESAM plays two key roles: it starts and stabilizes the mode-locking operation and is simultaneously used as an actuator to control the CEO frequency. This second functionality is implemented by pumping the SESAM with a continuous-wave 980-nm laser diode in order to slightly modify its nonlinear reflectivity. We use the standard f-to-2f method for detection of the CEO frequency, which is stabilized by applying a feedback signal to the current of the SESAM pump diode. We compare the SESAM-OOM stabilization with the traditional method of gain modulation via control of the pump power of the Yb:CALGO gain crystal. While the bandwidth for gain modulation is intrinsically limited to ∼250 kHz by the laser cavity dynamics, we show that the OOM provides a feedback bandwidth above 500kHz. Hence, we were able to obtain a residual integrated phase noise of 430mrad for the stabilized CEO beat, which represents an improvement of more than 30% compared to gain modulation stabilization.

Fabrication and Characterization of Ultra-wide Bandgap AlN-Based Schottky Diodes on Sapphire by MOCVD

AlN Schottky diodes with various device geometries were fabricated on sapphire substrate and their temperature-dependent current–voltage characteristics were analyzed. At forward bias, high ideality factors were obtained, indicating a large deviation from the ideal thermionic emission model. At reverse bias, the breakdown voltage showed a negative temperature dependence, and the leakage current was well described using a 2-D variable-range hopping conduction model. Furthermore, the breakdown voltages and leakage currents of the devices showed a strong dependence on the surface distance between the ohmic and Schottky contacts, but a relatively small dependence on the area of the Schottky contacts. These results suggest surface states between ohmic and Schottky contacts play a more important role than the metal/AlN interface in determining the reverse breakdown and leakage current of AlN Schottky diodes. A quantitative study of AlN Schottky diodes at high temperature reveals a geometry-dependent surface breakdown electric field and surface leakage current. Surface passivation and treatments may enhance the device performances and impact the reverse breakdown and current leakage mechanisms. These results will serve as the guidance for the design and fabrication of future AlN electronic devices.

Remote sensing of atmospheric NO2 by employing the continuous-wave differential absorption lidar technique.

Differential absorption lidar (DIAL) technique employed for remote sensing has been so far based on the sophisticated narrow-band pulsed laser sources, which require intensive maintenance during operation. In this work, a continuous-wave (CW) NO2 DIAL system based on the Scheimpflug principle has been developed by employing a compact high-power CW multimode 450 nm laser diode as the light source. Laser emissions at the on-line and off-line wavelengths of the NO2 absorption spectrum are implemented by tuning the injection current of the laser diode. Lidar signals are detected by a 45° tilted area CCD image sensor satisfying the Scheimpflug principle. Range-resolved NO2 concentrations on a near-horizontal path are obtained by the NO2 DIAL system in the range of 0.3-3 km and show good agreement with those measured by a conventional air pollution monitoring station. A detection sensitivity of ± 0.9 ppbv at 95% confidence level in the region of 0.3-1 km is achieved with 15-minute averaging and 700 m range resolution during hours of darkness, which allows accurate concentration measurement of ambient NO2. The low-cost and robust DIAL system demonstrated in this work opens up many possibilities for field NO2 remote sensing applications.

Significant increase of quantum efficiency of green InGaN quantum well by realizing step-flow growth

Two-dimensional (2D) island morphologies have been widely reported for green light-emitting InGaN quantum well (QW) layers, but the step-flow morphology has not been obtained for a green InGaN QW layer to date. In this Letter, we first investigate the cause of the 2D island morphology of green InGaN QWs via a comparison study with blue InGaN QWs. The short diffusion lengths of adatoms at low growth temperatures were found to be the cause of the 2D island morphology for the green InGaN QW. Step-flow growth of green InGaN QWs was obtained by increasing the miscut angle of the c-plane GaN substrates from 0.20° to 0.48°, which reduces the atomic terrace width. Green InGaN/GaN multiple quantum wells (MQWs) with step-flow morphologies were found to have sharper well/barrier interfaces than MQWs with 2D island morphologies. The internal quantum efficiency of the green InGaN/GaN MQWs with the step-flow morphology is double that of the corresponding MQWs with the 2D island morphology at an excitation power density of 6.4 kW/cm2. Additionally, the emission linewidth of the green InGaN/GaN MQWs with the step-flow morphology is greatly reduced. As a result, the threshold currents of green laser diodes with larger miscut angles are greatly reduced.

Wavelength Modulating Self-Mixing Interferometer

Self-mixing interference (SMI) was used to measure the displacement with a resolution of λ/2 by counting the interference signal peaks. In order to increase the measurement accuracy beyond λ/2, sinusoidal wavelength modulation (SWM) technique is introduced in SMI in this paper. Wavelength modulation of the laser beam is obtained by sinusoidal modulating the injection current of the laser diode. Fourier analysis method is proposed to demodulate the phase. Experimentally, the micro-movements of a high precision commercial PZT have been reconstructed, which can obtain a displacement measurement resolution of a few nanometers. It provides a potential displacement sensor with high accuracy and quite compact configuration.

Method of Extracting Solar Cell Parameters From Derivatives of Dark I–V Curves

A method is presented to extract solar cell parameters from the derivatives of dark current–voltage curves with a three-diode model, using the monotonic properties of the current–voltage characteristic associated with each diode or resistive current. The method yields an improved accuracy of the solar cell parameters when compared with that used on the actual current–voltage curves. Despite the complexity of the three-diode model with seven fit parameters, a good accuracy can be reached using the proposed method with a relatively small computational effort. A hypothetical case is presented using various approaches to obtain the solar cell parameters, with a root-mean-square error for the logarithm of the current–voltage curve ( $\text{RMS}_{\text{log}_{10}I}$ ) of $4{\,\,\times\, 10}^{-3}$ . An example fitting is then demonstrated on a multicrystalline passivated emitter rear contact solar cell, yielding $\text{RMS}_{\text{log}_{10}I}=1.45{\,\,\times\, 10}^{-1}$ .

Fabrication and characterization of sputtered Cu2O:N/c-Si heterojunction diode

Nitrogen doped cuprous oxide (Cu2O:N) deposited by reactive sputtering was applied to a crystalline silicon (c-Si) based heterojunction diode. The current density–voltage (J–V) characteristics of the fabricated diode showed rectifying characteristics with a high rectifying ratio of 105 at ±1 V. The capacitance–voltage measurements revealed the existence of a large conduction band offset and a small valence band offset at the Cu2O:N/c-Si interface, which implies that Cu2O:N is a suitable material for a hole selective emitter layer in n-type c-Si based heterojunction solar cells. Detailed analysis of the temperature dependent J–V characteristics showed that the diode current was limited by interface recombination originated from Fermi level pinning at the Cu2O:N/c-Si interface.

High Sensitivity Tuning of Work Function of Self-Assembled Monolayers Modified Electrodes Using Vacuum Ultraviolet Treatment.

We demonstrate systematic work function tuning of thiol-based SAM-modified gold electrodes with high controllability and sensitivity as high as 0.05 eV using vacuum ultraviolet technique (VUV). Under different irradiation times, both work function and wettability of the metal surface is modified. Fine tuning of the electrode work function is demonstrated by observable changes in the reverse current of a polymer Schottky diode. Additionally, the change in SAM chemical functionality validates the work function changes of VUV-irradiated electrodes. Our selective work function patterning on a single Au electrode via VUV could also reduce the required fabrication steps for more complex circuits.

Quasistationary magnetic field generation with a laser-driven capacitor-coil assembly.

Recent experiments are showing possibilities to generate strong magnetic fields on the excess of 500 T with high-energy nanosecond laser pulses in a compact setup of a capacitor connected to a single turn coil. Hot electrons ejected from the capacitor plate (cathode) are collected at the other plate (anode), thus providing the source of a current in the coil. However, the physical processes leading to generation of currents exceeding hundreds of kiloamperes in such a laser-driven diode are not sufficiently understood. Here we present a critical analysis ofprevious results and propose a self-consistent model for the high current generation in a laser-driven capacitor-coil assembly. It accounts for three major effects controlling the diode current: the space charge neutralization, the plasma magnetization between the capacitor plates, and the Ohmic heating of the external circuit-the coil-shaped connecting wire. The model provides the conditions necessary for transporting strongly super-Alfvenic currents through the diode on the time scale of a few nanoseconds. The model validity is confirmed by a comparison with the available experimental data.

Reduced phase noise in an erbium frequency comb via intensity noise suppression.

We present a coherent erbium fiber frequency comb that achieves low phase noise operation through the active suppression of amplitude fluctuations within the laser oscillator. The amplitude noise servo has a bandwidth of 550 kHz and is achieved by current actuation of the laser pump diode. This servo reduces the integrated phase noise of the carrier envelope offset frequency of the comb, fceo, due to the strong coupling of amplitude and phase noise in the laser oscillator. Additionally, we use a composite error signal that utilizes information from both the amplitude noise and the fceo error signal to actuate the pump diode current, which further increases the coherence of the comb. With this locking scheme, the integrated phase noise on fceo is measured to be 270 mrad from 10 Hz to 1.5 MHz, indicating 93% of the optical carrier power is in the coherent signal. A simultaneous phase lock to a narrow-linewidth continuous-wave laser is achieved by actuating on the cavity length, and shows an integrated phase noise of 44 mrad.

A simple input current wave-shaping technique for three-phase diode and SCR converters

ABSTRACTThree-phase converters using diode or silicon-controlled rectifier (SCR) are widely employed to convert the commercial AC supply to DC. Such converters inject harmonics into the power supply system and thereby distort supply system voltage waveform. A simple input current wave-shape improvement technique using a shunt-connected harmonic current compensator is presented in this work, intended to reduce the total harmonic distortion (THD) of input current of three-phase diode and SCR phase-controlled rectifiers operating with inductive loads, by matching them to the specific converter as a combined package. The compensator proposed here comprises of a three-limb voltage source converter using insulated-gate bipolar transistor, working on instantaneous current and voltage measurements of the compensator only and not of the load. The technique uses a simple feedforward control for AC source current harmonic compensation of rectifiers without monitoring the AC line currents, i.e. use of online computation. The proposed system is simulated and tested on a laboratory prototype. The measured input current THD values without additional line filters are found to be below 8.3%, which is within acceptable limits, proving that the new technique is capable of compensating predetermined current harmonics of diode or SCRs.

A Subthreshold Voltage Reference With Scalable Output Voltage for Low-Power IoT Systems

This paper presents a subthreshold voltage reference in which the output voltage is scalable depending on the number of stacked PMOS transistors. A key advantage is that its output voltage can be higher than that obtained with conventional low-power subthreshold voltage references. The proposed reference uses native NMOS transistors as a current source and develops a reference voltage by stacking one or more PMOS transistors. The temperature coefficient of the reference voltage is compensated by setting the size ratio of the native NMOS and stacked pMOS transistors to cancel temperature dependence of transistor threshold voltage and thermal voltage. Also, the transistor size is determined considering the trade-off between diode current between n-well and p-sub and process variation. Prototype chips are fabricated in a 0.18- $\mu \text{m}$ CMOS process. Measurement results from three wafers show $3\sigma $ inaccuracy of ±1.0% from 0 °C to 100 °C after a single room-temperature trim. The proposed voltage reference achieves a line sensitivity of 0.31%/V and a power supply rejection of −41 dB while consuming 35 pW from 1.4 V at room temperature.

Electrical Derivative Measurement of High‐Power InGaAs LDs Under Scanning Current with Variable Step

For the large driving current of high-power semiconductor Laser diodes (LDs), a modified method to measure the electrical derivative of LDs under scanning driving current with variable step length is proposed, which is to achieve the fast and accurate measurement of optical and electrical characteristic parameters of LDs with a relatively small data acquisition. The experimental results show that, with fewer measurements, this method can effectively and accurately measure and extract the LDs corresponding parameters including threshold current (Ith ), voltage-current characteristic (V-I ), luminous power-current relation (P-I ), electrical derivative curve (IdV/dI-I ). The wavelet transformation singularity testing results of the threshold current also verify the accuracy, reliability, and advantage of this method.

A Dual Series-Resonant DC–DC Converter

A dual series-resonant dc-dc converter with zero-voltage switching and zero-current switching features is proposed in this paper. The topology consists of two switches and a clamping capacitor on the primary side of an isolating transformer. The two switches are operated in a complementary mode under a pulse width modulation (PWM) scheme. The secondary side of the transformer is connected to the load through two series-resonant circuits and a half-bridge diode rectifying stage, in which the rise and fall slopes of the diode currents are limited by the slope of the currents in the resonant circuits, resulting in reduced switching losses in the diodes. The two series-resonant circuits provide power transfer to the output load without interruption throughout the positive and negative cycles of operation. It is shown that the output voltage of the proposed converter can be regulated using either PWM control or frequency modulation control. Both step-down and step-up voltage conversions can be achieved using the proposed topology. Programmable system-on-chip is used as the controller platform to implement a 40 W laboratory prototype. A complete steady-state analysis is presented and the experimental results of the prototype of the proposed converter are discussed.

MIS(p) Saturation Tunneling Current Controlled by Neighboring MIS Inversion Level Via Coupling Effect

To study the current coupling effect, novel neighboring MIS tunnel diodes were fabricated and examined. It is found that the saturation dark or light current of MIS tunnel diode can be elevated with more neighboring floating MIS structures, resulting from the lateral diffusion. And coupling also makes saturation current of those neighboring structures the same despite their various sizes. An irradiance of 6 mW/cm2 and gradually-minimizing distance both give rise to the greater saturation current for central MIS diode. TCAD simulation also shows the existence of lateral diffusion; furthermore, the fringing field would increase the efficacy of drift current under nano-scale distance among devices.

Temperature dependence of commercial 4H-SiC UV Schottky photodiodes for X-ray detection and spectroscopy

Two commercial-off-the-shelf (COTS) 4H-SiC UV photodiodes have been investigated for their suitability as low-cost high temperature tolerant X-ray detectors. Electrical characterisation of the photodiodes which had different active areas (0.06mm2 and 0.5mm2) is reported over the temperature range 0°C to 140°C together with measurements of the X-ray photocurrents generated when the detectors were illuminated with an 55Fe radioisotope X-ray source. The 0.06mm2 photodiode was also investigated as a photon counting spectroscopic X-ray detector across the temperature range 0°C to 100°C. The depletion widths (at 120V reverse bias) of the two diodes were found to be 2.3µm and 4.5µm, for the 0.06mm2 and 0.5mm2 detectors respectively, at 140°C. Both devices had low leakage currents (<10 pA) at temperatures ≤40°C even at high electric field strengths (500kV/cm for 0.06mm2 diode; 267kV/cm for 0.5mm2 diode). At 140°C and similar field strengths (514kV/cm for 0.06mm2 diode; 269kV/cm for 0.5mm2 diode), the leakage currents of both diodes were <2nA (corresponding to leakage current densities of 2.4µA/cm2 and 0.3µA/cm2 for each diode respectively). The results demonstrated that both devices could function as current mode detectors of soft X-rays at the temperatures <80°C and that when coupled to a low noise charge sensitive preamplifier, the smaller diode functioned as a photon counting spectroscopic X-ray detector at temperatures ≤100°C with modest energy resolution (1.6keV FWHM at 5.9keV at 0°C; 2.6keV FWHM at 5.9keV at 100°C). Due to their temperature tolerance, wide commercial availability, and the radiation hardness of SiC, such detectors are expected to find utility in future low-cost nanosatellite (cubesat) missions and cost-sensitive industrial applications.

Cross sections of X-ray production induced by C and Si ions with energies up to 1 MeV/u on Ti, Fe, Zn, Nb, Ru and Ta

X-ray production differential cross sections induced by C and Si ions with energies from 1MeV/u down to 0.25MeV/u, produced by the CMAM 5MV tandem accelerator, have been measured for thin targets of Ti, Fe, Zn, Nb, Ru and Ta in a direct way. X-rays have been detected by a fully characterized silicon drift diode and beam currents have been measured by a system of two Faraday cups. Measured cross sections agree in general with previously published results. The ECPSSR theory with the united atoms correction gives absolute values close to the experimental ones for all the studied elements excited by C ions and for Ta, Nb and Ru excited by Si ions. For Ti, Fe and Zn excited by Si, the matching with theory is poor since even the ionization cross section is below the measured data.

Effect of layer thickness on the electrical parameters and conduction mechanisms of conjugated polymer‐based heterojunction diode

ABSTRACTIn this study, thickness‐dependent current density–voltage (J–V) characteristics obtained for poly{(9,9‐dioctylfluorene)−2,7‐diyl‐(4,7‐bis(thien‐2‐yl) 2‐dodecyl‐benzo[1,2,3] triazole)} (PFTBT) conjugated copolymer based heterojunction diode fabricated on ITO were investigated in terms of electrical characteristics. In order to analyze J–V plots with ITO/PEDOT:PSS/PFTBT:PC61BM/LiF/Al configuration, the thickness‐dependent J–V measurements were applied in the thickness range between 90 and 200 nm. The effect of PFTBT:PC61BM layer thickness on the forward J–V characteristics were investigated by evaluating electrical parameters such as zero bias barrier height (ΦBo), ideality factor (n), shunt resistance (Rsh), series resistance (Rs), the interface states density (Nss), and space‐charge limited mobility. The results show that at PFTBT:PC61BM layer thickness of 90 and 200 nm, ideality factor for ITO/PEDOT:PSS/PFTBT:PC61BM/LiF/Al heterojunction diodes ranged from 2.726 to 3.121 and the thermionic emission over the heterojunction diodes is crucial at low current densities and the intrinsic thermally generated charge carriers controlled the forward current this region of the heterojunction diode. At relatively higher voltage, the current mechanism of ITO/PFTBT:PC61BM/PEDOT:PSS/LiF/Al heterojunction diodes were found to obey a space charge limited (SCLC) conduction mechanism. The values of Nss and Rs in heterojunction diodes increase with increasing PFTBT:PC61BM layer thickness and effect the main electrical parameters of diodes. In addition, the leakage current of heterojunction diodes are taken and interpreted via Poole‐Frenkel emission and Schottky emission. The leakage current was controlled in ITO/PEDOT:PSS/PFTBT:PC61BM/LiF/Al heterojunction diodes by Poole‐Frenkel emission above 140 nm and by Schottky emission under 140 nm. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44817.

Heavy Ion Induced Degradation in SiC Schottky Diodes: Incident Angle and Energy Deposition Dependence

Heavy-ion-induced degradation in the reverse leakage current of SiC Schottky power diodes exhibits a strong dependence on the ion angle of incidence. This effect is studied experimentally for several different bias voltages applied during heavy-ion exposure. In addition, TCAD simulations are used to give insight on the physical mechanisms involved.