Trapping Time Constants Research Articles

Investigation of interface traps in β-(AlGa)2O3/Ga2O3 heterostructures by dynamic capacitance dispersion technique

In this work, interface traps in β-(AlGa)2O3/Ga2O3 modulation-doped field effect transistor (MODFET) were investigated qualitatively and quantitatively by means of dynamic capacitance dispersion technique. The fabricated β-(AlGa)2O3/Ga2O3 MODFET showed drain current modulation demonstrating the transistor behavior. The existence of two-dimensional electron gas was confirmed by the plateau region in the capacitance–voltage (C–V) characteristics. The difference in capacitance profile dispersion at multifrequency C–V measurement suggested the loss mechanism due to trapping and de-trapping effects of carriers. An interface analysis of β-(AlGa)2O3/Ga2O3 heterostructures estimated the interface states with density of trap states and time constants of 0.56×1012–5.92×1012 cm−2 eV−1 and 59–27 μs, respectively.

Phase-sensitive analysis of a two-color infrared photodetector using photoreflectance spectroscopy

The phase diagrams of photoreflectance spectra were investigated for an InGaAs two-color infrared photodetector. The diagrams for a high excitation intensity revealed that the spectrum is multi-component. The origin of these components was investigated, and the photoreflectance spectra and phase diagrams were also measured for an angle-polished version at different depths. With the help of the polished sample, the variation of the phase delay angles and the trapping time constants was tracked for different depths. Additionally, the polished version enables us to find a confirmation for the origins of the multi-component nature of the whole phase diagram. It can be concluded that when the phase delays or time constants of various components are very close, more attention should be paid to interfering with the phase-sensitive investigations of layered materials. As a main result, the consistency of the phase delay with interface trap densities was confirmed qualitatively. Using a reciprocal space map of the sample, this result can be a piece of experimental evidence for a correlation between the photoreflectance time constant and trap densities in the junctions. This non-contact method enables the characterization of layered devices, offering a valuable tool for achieving high-performance devices.

The effects of stress on interfacial properties and flatband voltage instability of 4H-SiC MOS structures

Strain engineering technology is expected to improve the poor SiO2/4H-SiC interface. The paper investigated the effects of curvature induced stress on interfacial properties and flatband voltage instability of 4H-SiC MOS capacitors. The slightest stress sample demonstrated the best calculation results and the superior properties in terms of interface state density, FN barrier, and effective near-interface traps density, which means the “stress free” or small compressive stress oxide film might be helpful for the realization of high performance of 4H-SiC MOS structures. The behavior of Vfb instability under various bias temperature stress and voltage stress at different durations was also studied. The more significant compressive/tensile stress led to worse reliability at high temperatures, which was caused by the combined effect of mobile ions and charge trapping. The trapping and de-trapping time constants of interface and oxide traps were calculated to clarify the relationship between temperature and Vfb instability. Moreover, the results of the measured Raman shift of the SiC bond implied that stresses tend to be unanimous in the subsequent processes while the influences on interfacial properties and Vfb instability still exist. These results are considered necessary for strain engineering technology, which is expected to lower the interface states density in SiO2/4H-SiC interface and reach the optimal performances of 4H–SiC.

SiC MOSFET Trap Characterization Based on the Self-built Test Platform

The interface state of SiC/SiO2 is one of the key factors limiting the reliability and performance of SiC MOSFETs. In this paper, we built a dedicated detrap measurement platform to improve the accuracy of trap measurement. Based on the excellent voltage switching time of this platform, the entire voltage switching process can be controlled within 1 µs, and the time resolution is also improved. The original millisecond-level acquisition accuracy is improved to a microsecond level, and the identification range of traps is expanded. The traps were extracted by using the transient current method based on the Bayesian deconvolution algorithm. With this approach, we investigated the trapping mechanism of SiC MOSFETs and mainly characterized the trap locations, energy levels, and trapping time constants. The findings revealed the existence of three different types of traps/defects, Dp1, Dp2, and Dp3, with activation energies of 0.28 eV, 0.035 eV, and 0.084 eV, respectively. The non-destructive characterization of SiC MOSFET defects can be realized through the test platform and Bayesian deconvolution algorithm in this paper, which brings great convenience for trap characterization.

A Coupling Mechanism between Flicker Noise and Hot Carrier Degradations in FinFETs.

A coupling mechanism between flicker noise and hot carrier degradation (HCD) is revealed in this work. Predicting the flicker noise properties of fresh and aged devices is becoming essential for circuit designs, requiring an understanding of the fundamental noise behaviors. While certain models for fresh devices have been proposed, those for aged devices have not been reported yet because of the lack of a clear mechanism. The flicker noise of aged FinFETs is characterized based on the measure-stress-measure (MSM) method and analyzed from the device physics. It is found that both the mean and deviations of the noise power spectral density increase compared with the fresh counterparts. A coupling mechanism is proposed to explain the trap time constants, leading to the trap characterizations in their energy profiles. The amplitude and number of contributing traps are also changing and are dependent on the mode of HCD and determined by the position of the induced traps. A microscopic picture is developed from the perspective of trap coupling, reproducing well the measured noise of advanced nanoscale FinFETs. The finding is important for accurate flicker noise calculations and aging-aware circuit designs.

Competition between oxygen and water molecules on SiO2/P-doped Si surface: The electrical dipole evolution on water/oxygen-adsorbed oxide surface

The performance of gas sensors is greatly influenced under humidity environment due to the dynamic correlation between water and oxygen molecules. In this work, we utilize the charge trapping characteristic to study surface gas adsorption and demonstrate the competitive adsorption behaviors between oxygen and water vapor from the perspective of electrical dipole by observing the evolution of second harmonic generation (SHG). By analyzing the trapping time constant of time-dependent SHG spectrum, we propose a dynamic model of the way water molecules weaken oxygen adsorption and develop water adsorption layer. Results show that the electric field created by charge trapping would further align the water molecules and enhance the SHG intensity. Here, a viewpoint of analyzing the net surface dipole change due to the adsorption–desorption on SiO2 surface to realize the energy preferable state is presented and provides a pathway for solving the water–sensor interaction problems in gas sensor.

AC RTN: Testing, Modeling, and Prediction

Random telegraph noise (RTN) adversely induces time dependent device-to-device variations and requires modeling to optimize circuit design. Many early works were focused under dc test conditions, although digital circuits typically operate under ac conditions and it has been reported that ac RTN is substantially different from dc RTN. Tests on ac RTN were carried out mainly on individual traps, and a reliable statistical distribution of trap time constants for ac RTN is still missing. This prevents verifying the statistical accuracy of Monte Carlo ac RTN simulation based on compact models, especially in terms of their ability to predict ac RTN as time window increases. Recently, an integral methodology has been proposed for dc RTN, which can not only model it at short time but also predict it at long time. By introducing the concept of effective charged traps, the need for statistical distribution of trap time constants is removed, making RTN prediction similar to aging prediction. The objectives of this work are to report statistical experimental ac RTN data and to test the applicability of integral methodology to them. For the first time, it will be shown that a model extracted from a time window of 7.8 s can be used to predict the statistical distribution of long-term ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times 10^{{4}}$ </tex-math></inline-formula> s) ac RTN. The dependence of ac RTN on frequency and time window is analyzed, and the contributions of carrier tunneling from gate and substrate are assessed.

Design considerations for gallium arsenide pulse compression photoconductive switch

In this paper, we present the physics and design-space exploration of a novel pulse compression photoconductive switch (PCPS) using semi-insulating gallium arsenide (GaAs) operating in the negative differential mobility (NDM) regime of electron transport. We systematically quantify the relationship between the PCPS performance and various design options, including contact separation, laser energy and placement, and trap dynamics. Specifically, we report the full-width at half-maximum and the peak output current generated by the PCPS as a function of applied electrical and optical bias. We discuss the optimal spacing between the electrodes and the distance of the laser spot to the anode to achieve higher electron confinement and superior radio-frequency (RF) metrics. Reducing the laser energy is important to prevent the appearance of secondary peaks due to diffusive transport, but there exists a trade-off between the bandwidth and the maximum current of the PCPS. We also compare the PCPS response with and without trap dynamics and find that the electrostatic screening from the trap-induced space charge is time-independent when the trapping time constant is set larger than the recombination lifetime. Overall, trap dynamics are detrimental to performance, unless the compensation doping scheme to achieve semi-insulating GaAs is carefully selected. Results presented in this paper can be used by experimentalists to fine-tune the PCPS design parameters to meet the specifications of various RF applications. Moreover, our results will provide a strong theoretical basis to the measurements of PCPS devices using GaAs and other NDM materials under investigation.

The interface trap analysis of AlGaN/GaN high electron mobility transistors with temperature based on conductance method

In this paper, the characteristics of AlGaN/GaN high electron mobility transistors were tested and analyzed at high temperature. The experimental temperature range was 25~500°C. Frequency-dependent capacitance and conductance were adopted to investigate high temperature characteristics of interface trap. Results show that there is a kind of trap at the device interface. The trap density and time constant are (8.41×1010~1.40×1011)eV-1cm2/(0.398~0.636)μS and (1..03×1011~1.15×1011)eV-1cm2/(0.455~0.532)μS at different voltages and temperatures. With the increase of temperature, the trap density and time constant increase. High density interface traps are one of the reasons why device characteristics deteriorate with increasing temperature.

Defect Engineering in Thickness-Controlled Bi2O2Se-Based Transistors by Argon Plasma Treatment.

We present a simple, effective, and controllable method to uniformly thin down the thickness of as-exfoliated two-dimensional Bi2O2Se nanoflakes using Ar+ plasma treatment. Atomic force microscopy (AFM) images and Raman spectra indicate that the surface morphology and crystalline quality of etched Bi2O2Se nanoflakes remain almost unaffected. X-ray photoelectron spectra (XPS) indicate that the O and Se vacancies created during Ar+ plasma etching on the top surface of Bi2O2Se nanoflakes are passivated by forming an ultrathin oxide layer with UV O3 treatment. Moreover, a bottom-gate Bi2O2Se-based field-effect transistor (FET) was constructed to research the effect of thicknesses and defects on electronic properties. The on-current/off-current (Ion/Ioff) ratio of the Bi2O2Se FET increases with decreasing Bi2O2Se thickness and is further improved by UV O3 treatment. Eventually, the thickness-controlled Bi2O2Se FET achieves a high Ion/Ioff ratio of 6.0 × 104 and a high field-effect mobility of 5.7 cm2 V-1 s-1. Specifically, the variation trend of the Ion/Ioff ratio and the electronic transport properties for the bottom-gate Bi2O2Se-based FET are well described by a parallel resistor model (including bulk, channel, and defect resistance). Furthermore, the Ids-Vgs hysteresis and its inversion with UV irradiation were observed. The pulsed gate and drain voltage measurements were used to extract trap time constants and analyze the formation mechanism of different hysteresis. Before UV irradiation, the origin of clockwise hysteresis is attributed to the charge trapping/detrapping of defects at the Bi2O2Se/SiO2 interface and in the Bi2O2Se bulk. After UV irradiation, the large anticlockwise hysteresis is mainly due to the tunneling between deep-level oxygen defects in SiO2 and p++-Si gate, which implies the potential in nonvolatile memory.

Excess noise in high-current diamond diodes

We report the results of an investigation of low-frequency excess noise in high-current diamond diodes. It was found that the electronic excess noise of the diamond diodes is dominated by the 1/f and generation-recombination noise, which reveals itself as Lorentzian spectral features (f is the frequency). The generation-recombination bulges are characteristic of diamond diodes with lower turn-on voltages. The noise spectral density dependence on forward current, I, reveals three distinctive regions in all examined devices—it scales as I2 at the low (I &amp;lt; 10 μA) and high (I &amp;gt; 10 mA) currents and, rather unusually, remains nearly constant at the intermediate current range. The characteristic trap time constants, extracted from the noise data, show a uniquely strong dependence on current. Interestingly, the performance of the diamond diodes improves with the increasing temperature. The obtained results are important for the development of noise spectroscopy-based approaches for device reliability assessment for high-power diamond electronics.

A Novel Approach for the Modeling of the Dynamic ON-State Resistance of GaN-HEMTs

A compact model approach to enhance the accuracy and facilitate the parameter extraction of trapping models is presented. The proposed model replaces the conventional superposition of discrete trapping time constants by a Gaussian distribution of a particular range of time constants. This approach reflects the physically non-constant energy levels of the trap distribution of surface, interface, and bulk traps. Also, we propose a new equivalent circuit that takes the time-dependent charging and discharging of traps into account. We show that our model reduces the model complexity by 52%. The model is verified against dynamic ON-state resistance ( R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS,ON</sub> ) measurements of a commercial 100-V gallium-nitride (GaN) power transistor in soft-switching operation.

Trapping Dynamics in GaN HEMTs for Millimeter-Wave Applications: Measurement-Based Characterization and Technology Comparison

Charge trapping effects represent a major challenge in the performance evaluation and the measurement-based compact modeling of modern short-gate-length (i.e., ≤0.15 μm) Gallium Nitride (GaN) high-electron mobility transistors (HEMT) technology for millimeter-wave applications. In this work, we propose a comprehensive experimental methodology based on multi-bias large-signal transient measurements, useful to characterize charge-trapping dynamics in terms of both capture and release mechanisms across the whole device safe operating area (SOA). From this dataset, characterizations, such as static-IV, pulsed-IV, and trapping time constants, are seamlessly extracted, thus allowing for the separation of trapping and thermal phenomena and delivering a complete basis for measurement-based compact modeling. The approach is applied to different state-of-the-art GaN HEMT commercial technologies, providing a comparative analysis of the measured effects.

Effects of GaN Buffer Resistance on the Device Performances of AlGaN/GaN HEMTs

We investigated the effects of GaN buffer resistance of AlGaN/GaN high-electron-mobility transistors (HEMTs) on direct current (DC), low-frequency noise (LFN), and pulsed I-V characterization performances. The devices with the highest GaN buffer resistance were grown on sapphire substrate using two-step growth temperature method without additional compensation doping. The proposed device exhibited the degraded off-state leakage current due to the improved GaN buffer quality compared to the reference devices with relative low buffer resistance, which is confirmed by high resolution X-ray diffraction (HRXRD). However, the proposed device with deep-level defects in GaN buffer layer showed the reduced hysteresis (∆Vth), increased breakdown voltage (BV), and enhanced pulse I-V characteristics. Regardless of GaN buffer resistance, all devices clearly showed 1/f behavior with carrier number fluctuations (CNF) at on-state but followed 1/f2 characteristic at off-state. From the 1/f2 noise characteristics, the extracted trap time constant (τi) of the proposed device can be obtained to be 10 ms, which is shorter than those of the reference devices because of the full compensation of deep-level defects in the GaN buffer layer.

Investigation of 1/f and Lorentzian Noise in TMAH-treated Normally-Off GaN MISFETs

A tetramethyl ammonium hydroxide (TMAH)-treated normally-off Gallum nitride (GaN) metal-insulator-semiconductor field-effect transistor (MISFET) was fabricated and characterized using low-frequency noise (LFN) measurements in order to find the conduction mechanism and analyze the trapping behavior into the gate insulator as well as the GaN buffer layer. At the on-state, the noise spectra in the fabricated GaN device were 1/fγ properties with γ ≈ 1, which is explained by correlated mobility fluctuations (CMF). On the other hand, the device exhibited Lorentzian or generation-recombination (g-r) noises at the off-state due to deep-level trapping/de-trapping into the GaN buffer layer. The trap time constants (τi) calculated from the g-r noises became longer when the drain voltage increased up to 5 V, which was attributed to deep-level traps rather than shallow traps. The severe drain lag was also investigated from pulsed I-V measurement, which is supported by the noise behavior observed at the off-state.

Impact of Traps on the Adjacent Channel Power Ratios of GaN HEMTs

In this work, the impact of traps on a system level parameter, ACPR (adjacent channel power ratio), of commercial RF AlGaN/GaN HEMTs were studied. ACPR usually depends on the transistor linearity and stability, and in this study it was measured in two different signal duplexing schemes: time division duplexing (TDD) and frequency division duplexing (FDD) after using digital predistortion to minimize static nonlinearities. It is theorized traps lead to time-dependent nonlinearity due to the devices changing operating points, which degrades ACPR. Constant drain current deep level transient spectroscopy (CI $_{D}$ -DLTS) measurements were performed to quantitatively characterize the trap energies and concentration and correlate with the different time sensitivities of the TDD and FDD schemes. Linked by the trap concentration and time constant, the $\text{E}_{C}$ -0.57 eV trap was correlated to the FDD ACPR, and the $\text{E}_{C}$ -0.72 eV trap was correlated to the TDD ACPR. Finally, it is shown that both traps have been widely reported, suggesting that many GaN systems’ ACPR can be potentially improved by reducing the trap concentration.

Gamma irradiation impact on GaN quasi-vertical Schottky barrier diodes

The impact of gamma irradiation on GaN quasi-vertical Schottky barrier diodes (SBDs) has been investigated for the first time. Compared with original GaN SBDs, the ideality factor decreases from 1.2 to 1.01–1.02 and the on/off ratio increases from 109 to 1012 for the irradiated GaN SBDs. The reduction of dynamic on-resistance from 0.67 mΩ · cm2 to 0.54 mΩ · cm2 was observed at the quiescent bias of −100 V compared with the original SBDs. The temperature dependent current characteristics has revealed the existence of Schottky barrier inhomogeneities at the anode/GaN Schottky interface, and the level of barrier inhomogeneities can be effectively suppressed after gamma irradiation. Based on x-ray photoelectron spectroscopy, cathodoluminescence and x-ray diffraction results, the interface state density at the interface and the deep electron trap in the bulk GaN are reduced after gamma irradiation. The voltage dependent capacitance-frequency measurements further confirm that the interface trap states dominate the degraded characteristics of pre-irradiated devices and the trap time constant is estimated to be in the range of 5 μs to 50 μs.

Determination of current transport and trap states density in AlInGaN/GaN heterostructures

The energy distribution and the relaxation time constant of the trap states with respect to conduction bands in the (Ni/Au) Schottky contact on AlInGaN/GaN heterostructures were investigated using the admittance technique. The potential dependent capacitance/conductance measurements were done in the frequency range of 5 kHz to 5 MHz at a temperature of 300 K. We found strong frequency dispersions at the accumulation regions and at the sharp transition regions (depletion region) in the capacitance curves. High frequency dispersion at the accumulation regions in C-V characteristics indicates that there is a high-density of surface traps between the metal–AlInGaN quaternary layer interfaces. Furthermore, the frequency dispersion at the sharp transition regions behavior can be attributed to the interface traps state between the AlInGaN quaternary layer and GaN layer. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming the models in which traps are located between the metal–AlInGaN interface (surface traps) and between AlInGaN/GaN interfaces (interface traps). The trap states density and time constants of the traps states were calculated as a function of energy separation from the conduction-band edge. The trap states' densities change between 1.3 × 1011 eV−1 cm−2 and 6.2 × 1011 eV−1 cm−2. Also, 4.8 to 5.3 μs time interval calculated for the relaxation times.

Impact of Varying Strain Energy in Oxide on Random Telegraph Noise and Associated Time Constants in Silicon Nanowire pMOSFETs

Multilevel random telegraph noise in gate-all-around silicon nanowire pMOSFET shows gate voltage overdrive-dependent interstate transitions, statistical count of which indicates the existence of oxide traps with different time constants. Following the detail extraction of the capture and emission time with gate bias, both have been found to deviate from the usual trend, as predicted by the conventional Shockley–Read–Hall statistics. Two traps in the gate–oxide have been found, in which the near interface is found to respond through the altered RTN activation energy barrier by spatially varying oxide residual strain energy, whereas the farther responds under the weak influence of the oxide strain energy. On the other hand, the emission time of near interface trap has been found to follow similar strain influenced activation barrier, but the magnitude of the emission time of the farther trap shows the hole emission through the p+ polysilicon gate–oxide interfacial trap, while favored by the electrostatic field in oxide. From RTN distributions and time lag plots, interstate transitions and state suppression in RTN are explained under the framework of two-trap postulate, in order to elucidate the unusual gate bias dependence of the measured trap time constants in silicon nanowire pMOSFETs.

Methylammonium lead tribromide semiconductors: Ionizing radiation detection and electronic properties

Abstract The response of single crystal methylammonium lead tribromide (MAPbBr 3 or MAPB) semiconductors to alpha particles at low voltage is presented. Through analysis of the preamplifier traces induced by 210Po alpha particles and collecting holes, we were able to determine the mobility–lifetime product, apparent mobility, trapping time constant, and the detrapping time constant. In addition, a quantitative study of the rate of positive polarization and device breakdown time at different applied voltages is presented. Finally, using a 239 Pu/Be fast neutron source, we were able to demonstrate the response of MAPB to fast neutrons for the first time via benchmarking experiment with Monte Carlo simulations.