Nurul Fadzlin Hasbullah

Correlation between defect density and current leakage in InAs/GaAs quantum dot-in-well structures

We present a study of InAs∕GaAs quantum dot-in-well (DWELL) material using transmission electron microscopy and leakage current-voltage measurements. The spacer layers between the DWELL layers have a variety of annealing and growth temperatures. We show that there is a strong correlation between spacer layer, annealing temperature, defect density, and these leakage currents, with the most defective sample having 30 times more defects and a leakage current several orders of magnitude above that of the least defective. Cross section transmission electron microscope (TEM) shows that surface roughness above defective dots is responsible for the high defect densities. However, even in the best sample the reverse bias leakage current is several orders of magnitude above that typically seen in quantum well materials and a measurable density of defective dots are observed in planar view TEM.

Dependence of the Electroluminescence on the Spacer Layer Growth Temperature of Multilayer Quantum-Dot Laser Structures

Electroluminescence (EL) measurements have been performed on a set of In(Ga)As-GaAs quantum-dot (QD) structures with varying spacer layer growth temperature. At room temperature and low injection current, a superlinear dependence of the integrated EL intensity (IEL) on the injection current is observed. This superlinearity decreases as the spacer layer growth temperature increases and is attributed to a reduction in the amount of nonradiative recombination. Temperature-dependent IEL measurements show a reduction of the IEL with increasing temperature. Two thermally activated quenching processes, with activation energies of ~ 157 meV and ~ 320 meV, are deduced and these are attributed to the loss of electrons and holes from the QD ground state to the GaAs barriers. Our results demonstrate that growing the GaAs barriers at higher temperatures improves their quality, thereby increasing the radiative efficiency of the QD emission.

Dark Current mechanisms in quantum dot laser structures

Current-voltage measurements have been performed on InAs/InGaAs/GaAs quantum dot structures with varying growth and design parameters. These measurements show that the forward and reverse bias dark currents decrease with increasing spacer growth temperature, however, they are relatively insensitive to the number of periods of the quantum dot layers. Temperature dependent current-voltage measurements show that the mechanism that governs the reverse bias leakage current is due to generation-recombination via mid-band traps assisted by the Frenkel-Poole emission of carriers from these traps.

Neutron Radiation Effects on the Electrical Characteristics of InAs/GaAs Quantum Dot-in-a-Well Structures

This paper studies the effects of neutron radiation on the electrical behaviour and leakage current mechanism of quantum dot-in-a-well (DWELL) semiconductor diodes with fluence ranging from 3 to 9 ×1013 neutron/cm2. After neutron irradiation, the forward bias and reverse bias leakage currents showed significant rise approximately of up to two orders of magnitude which is believed to be attributed to the presence of displacement damage induced traps. The ideality factor of the forward bias leakage current corresponding to all neutron fluence irradiations were found to be close to 2, suggesting that the forward bias current mechanism is largely due to trap-assisted generation-recombination (TAGR) of carriers. Subsequently, it is also observed that the capacitances reduced after irradiations which were further shown to be due to the deep carrier trapping effects and the Neutron Transmutation Doping effects (NTD). From the temperature dependence measurements, it is found that the reverse bias leakage current mechanisms of the irradiated samples are primarily attributed to two process; TAGR of carriers with emission from the traps assisted by the Frenkel-Poole (F-P). The traps due to both mechanisms were derived and shown to increase with neutron fluence.

Electrical performances of commercial GaN and GaAs based optoelectronics under neutron irradiation

This paper aims to demonstrate the effects of displacement damage caused by high energetic neutron particle towards the electrical performances of gallium arsenide (GaAs) and gallium nitride (GaN) p-n based diodes. The investigations are carried out through current-voltage (I-V) and capacitance-voltage (C-V) measurements using Keithley 4200 SCS. Two different commercial optoelectronics diodes; GaN on SiC light emitting diode (LED) and GaAs infrared emitting diode (IRED) were radiated with neutron using pneumatic transfer system (PTS) in the PUSPATI TRIGA Mark II research reactor under total neutron flux of 1×1012 neutron/cm2.s. Following the neutron exposure for 1, 3 and 5 minutes, the I-V forward bias and reverse bias leakage current increase for GaAs IREDs, but minimal changes were observed in the GaN LEDs. The C-V measurements revealed that the capacitance and carrier concentration of GaAs IREDs decrease with increasing radiation flux.

Fuzzy logic based temperature control of thermoelectric cooler (TEC) for single photon avalanche diode (SPAD) application

Single photon avalanche diode (SPAD) is a temperature sensitive device. Even a slight variation of temperature can cause unstable performance in quantum efficiency, responsitivity and dark counts. Due to these reasons, unstable temperature could cause overall poor performance of SPAD. It is common for thermoelectric cooler (TEC) to be used as cooling of photodetectors. SPAD was mounted onto the TEC where it needs to be maintained at a constant low temperature under variation of ambient temperature. The system is simulated using Fuzzy Logic Toolbox in MATLAB Simulink. Simulated using P-type fuzzy logic with the set point temperature of −20°C and ambient temperature of 16°C, produce a result of −19.44°C. The P-type fuzzy logic control design has shown a good overall performance where the steady state error is ±0.56°C, which is equivalent to ±2.8% and the settling time for the output simulation, ts, is 35.91s.

Radiation Damage Study of Electrical Properties in GaN LEDs Diode after Electron Irradiation

Nitride-based light emitting diodes (LEDs) is an attractive material due to its high temperature tolerance and suitable to be used in extreme environment. The irradiation process of Gallium Nitride (GaN) diode was carried out by electron irradiation with 1000 kGy and 1500 kGy doses with a conveyor speed of 50 kGy per pass. Capacitance-voltage (C-V) and current-voltage (I-V) characterization for both pre and post irradiation samples was done. Both current and capacitance show decreasing while reverse leakage current increased after irradiation. The reverse leakage current revealed that the current were start leakage at 1.0 x10 -7 A and 1.0 x10-9 A for 1000 kGy and 1500 kGy irradiations respectively. The current-voltage graph indicated that the effect of electron irradiation on diode produced weak spots as defect cause leakage current. The traps and bulk defect is believed to contributed to the leakage current increased.

Electrical properties of neutron-irradiated silicon and GaAs commercial diodes

Neutron radiation testing was performed on silicon and GaAs diodes to investigate changes in the device parameters after neutron exposure. Californium-252 source was used to irradiate these diodes up to total dose of 1117.87mSv. The effects of nuclear radiation on the forward and reverse current-voltage (I-V) characteristics of Silicon and GaAs diodes were studied at room temperature. It was found that the magnitudes of forward bias electrical characteristics were in most instances unaffected by irradiation in both materials. The increments in TSKS5400S GaAs infrared emitting diode reverse currents were large after irradiation. These changes were interpreted as effects of displacement damage generating generation-recombination currents due to defects created. However, reverse bias (RB) characteristics of 1N4148 silicon diodes showed decrement in dark current. This is attributed to the type of diodes used.

Gain Investigation for commercial GaAs and SiGe HBT LNA's under Electron irradiation

In this paper, a characterization and comparison between the effects of Electron irradiation on the gain of low noise amplifiers (LNAs) implemented Silicon-Germanium (SiGe) Heterojunction Bipolar Transistor (HBT) and Gallium-Arsenide (GaAs) HBT technologies was carried out respectively. Previous studies have shown that the properties of SiGe and GaAs HBTs are very tolerant to gamma, neutron, and proton irradiation without additional radiation hardening. Nowadays, commercial on the shelves (COTS) LNAs have been used in CubeSat communication system lunched in Low and Medium Earth Orbits. It therefore believed that the electron radiation in space may degrade the LNAs performance and lead to its failure. This is shows the importance of such investigation in evaluating and comparing the performance of the GaAs and SiGe LNAs which represent an important module in the front end of the communication receiver system. Two samples of GaAs and SiGe have been selected: the ADL 5523 GaAs and the SiGe BFU730F LNAs which are respectively cover a frequency range of 400MHz to 4 GHz and 2.3 to 2.7 GHz. Samples were irradiated with 3 MeV Electron doses ranging from 50 kGy to 250 kGy in the Electron Beam Irradiation Centre (Alutron), Nuclear Malaysia Agency. Results measurement have been carry out in the RF Laboratory in the faculty of engineering (IIUM), using the vector network analyzer 50 GHz. The results indicate that both SiGe and GaAs HBT technologies have been affected by the electron Irradiation.

Radiation Characteristics and SEU Rates in NEqO Environment Using SPENVIS

RazakSat1 was launched at Near Equatorial Orbit (NEqO) where Trapped Protons / Electrons (TP/TE) and Galactic Cosmic Rays (GCR) has intensive effect on satellites memory like SRAM based FPGA. Due to this devastating effect, it is important to investigate the radiation environment of the orbit to predict the SEU rate for 6T SRAM, which is the building block of SRAM based FPGA. This study investigates the radiation environment in NEqO specifically for the orbit of RazakSat satellite. Solar Event Particles (SEPs), Trapped Protons / Electrons (TP/TE) and Galactic Cosmic Rays (GCR) are the three main sources of radiation which are taken to consideration in this study. The fluxes spectra of these three types are simulated and SEU rates for 180nm 6T SRAM are predicted using SPENVIS models for NEqO orbit. The results show that GCR fluxes are the most dominant at NEqO which reached to 105 MeV whereas TE has 4MeV and TP has the second dominant fluxes with 400 MeV. However, if the magnetic shielding atmosphere is on, there are no solar particles fluxes and almost no SEU was detected. Results also illustrate that SEU rates at NEqO is 0.5 upset / bit day when there is no shielding to the device (6T SRAM) but this rates reduced by 1.6 x 106 times when the device shielded by 0.5 g/cm2 of Aluminium. Comparisons of NEqO with polar orbit in terms of shielding effect and SEU rates are also presented in this study.