Excess Carrier Generation Research Articles

Photoacoustic study of elastic deformations in silicon membranes

In an optically excited semiconductor micromechanical structure, photogenerated carriers (plasma waves) can produce elastic deformations (local strains and stresses)–plasmaelastic (PE) deformations. On the other hand, the generation of excess carriers will produce heat due to carrier thermalization and recombination processes. The generated heat can produce other elastic deformations–thermoelastic (TE) deformations. For these two components of elastic deformation, it is possible to consider two types of elastic displacements (two mods of elastic vibrations): elastic expanding and elastic bending. A theoretical model of the photoacoustic (PA) signal for optically excited Si membranes is given, which includes thermal diffusion (TD), TE, and PE mechanisms, in order to study elastic expansion and bending. The relations for the PA amplitude and the phase of elastic expanding and bending in the excited membrane are derived. Analysis of the PA signal indicates that the TD component is dominant for all thicknesses and practically the entire range of observed frequencies. The calculated PA amplitude and phase spectra show that the PA elastic expanding component has a significant influence on the total PA signal at low frequencies. On the other side, the calculated PA spectra show that the PA elastic bending component has a significant influence on the total PA signal at high frequencies. Experimental PA signals of Si membranes were measured using the sample-gas-microphone detection technique with a transmission configuration in relation to the modulation frequency of the optical excitation for different membrane thicknesses. The experimental PA spectra were compared with the theoretical ones.

Role of excess-carrier generation and recombination in memristivity

We incorporate the role of excess carrier generation and recombination phenomena in nanostructured material and develop an approach to obtain V-I characteristic for a memristor. The V-I characteristic curve for the memristor is pinched hysteresis loop which is an important signature of memristor. This analysis suggests that our approach may be an alternate viable in realising the underlying theory behind memristivity as an inherent property of nanostructured materials.

Influence of temperature on measurements of the Two Photon Absorption – Transient Current Technique in silicon planar detectors using a 1550 nm femtosecond fibre laser

The effect of temperature on the excess charge carrier generation and collection, injected using two photon absorption at a wavelength of 1550 nm, is investigated in a 285 μm thick, p-type silicon planar detector, with a bulk resistivity of 3.7 kΩ cm. Charge collection measurements are performed for temperatures between −20 °C to 20 °C. The collected charge decreases by about 8% when the temperature is lowered from 20 °C to −20 °C, which is linked to a decreasing excess charge carrier generation. Further, a decreasing time over threshold is observed for decreasing temperatures, which is associated to an increasing charge carrier mobility.

Giant Switchable Persistent Photoconductivity in Soft Chemistry Reduced SrTiO3

AbstractHigh tunability of photoconductivity is highly desired for applications in optical memories, sensors, and bioelectronics. Recently, room temperature persistent photoconductivity (PPC) in SrTiO3 (STO) has been revealed and has attracted great attention. However, reversible switching of the PPC in STO with a large on/off ratio remains challenging to date. Here, a giant switchable PPC in soft chemistry reduced STO is reported. An initial insulator‐to‐metal transition with on/off ratio up to 7 orders of magnitude is observed and about 5 orders of magnitude transition is found to be reversible. Via nuclear magnetic resonance measurements, it is uncovered that such unusual PPC is driven by the generation of excess carriers accompanied with a configuration evolution of the incorporated hydrogen from hydridic HO+ to protic Hi+ upon illumination. The work demonstrates giant switchable PPC transition in soft chemistry reduced perovskite oxides, providing a new platform for pursuing high performance sensors and nonvolatile optoelectronic memory devices.

Visible Light Illumination Effects on Instability of MoS2 Thin‐Film Transistors for Optical Sensor Application

With the advent of the trillion sensor era, molybdenum disulfide (MoS2) offers intriguing future optical device opportunities due to qualities such as its novel optoelectronic properties and preferable form factors. However, while a fundamental understanding of device reliability under illumination is significantly important, most of the previous research has focused on persistent photocurrent (PPC) effects. For this reason, understanding of a clear mechanism remains elusive. Herein the work reported, electrical‐bias‐dependent device instability and net optical illumination effects of MoS2 thin‐film transistors (TFTs) are experimentally evaluated to decouple the origins of device instability coming from either net electrical bias stress or illumination stress. In addition, time‐dependent monitoring of relaxation behaviors under illumination at different wavelength hints that excess carrier generation and remnant electrons after annihilation during illumination play a key role in determination of photo‐initiated instability of a device. For practical suppression of device instability under ambient light, an operation scheme (using trapping time increase under positive bias) is adroitly utilized to provide reliable device performance. Systematic understanding of device instability of MoS2 TFTs, and of their suppression principles, is potentially beneficial for the design of next‐generation devices on the basis of 2D materials.

Physical origin of kink in GaN HEMTs

The kink in the source-drain current of GaN devices has been not clarified. Based on the Maxwell-Boltzmann distribution of hot channel electrons, the generation of excess carriers in the L valleys of GaN and the origin of the kink had been modeled. This proposed model can predict how the kink depends on the gate voltage in experiments. It provides the reason why the kink partially disappears or disappears by sweeping the source-drain voltage from high voltage to low voltage, disappears under illumination, is more obvious at a lower temperature, and can be affected by different traps. All above experimental phenomena are associated with excess carriers in the L valleys of gain due to the hot channel carrier via internal transport. This model gives physical insight into how physical parameters of GaN devices affect the kink in the source-drain current. Thus, it can be used to optimize the physical parameters of GaN devices for reducing the kink.

Simulation and Modeling of Time-Resolved X-Ray Detector for the Saturn Accelerator

We present the technology-aided computer design (TCAD) device simulation and modeling of a silicon p-i-n diode for detecting time-dependent X-ray radiation. We show that the simulated forward and reverse breakdown current–voltage characteristics agree well with the measured data under nonradiation environment by only calibrating carrier lifetimes for the forward bias case and avalanche model critical fields for the reverse bias condition. Using the calibrated parameters and other nominal material properties, we simulated the radiation responses of the p-i-n diode and compared with experimental data when the diode was exposed to X-ray radiation at Sandia’s Saturn facility and the Idaho State University (ISU) TriMeV facility. For Saturn’s Gaussian dose-rate pulses, we show three findings from TCAD simulations. First, the simulated photocurrents are in excellent agreement with the measured data for two dose-rate pulses with peak values of $1.16 \times 10 ^{10}$ and $1.88 \times 10 ^{10}$ rad(Si)/s. Second, the simulation results of high dose-rate pulses predict increased delayed photocurrents with longer time tails in the diode electrical responses due to excess carrier generation. Third, simulated peak values of diode radiation responses versus peak dose rates at different bias conditions provide useful guidance to determine the dose-rate range that the p-i-n diode can reliably detect in experiment. For TriMeV’s non-Gaussian dose-rate pulse, our simulated diode response is in decent agreement with the measured data without further calibration. We also studied the effects of device geometry, recombination process, and dose-rate enhancement via TCAD simulations to understand the higher measured response in the time after the peak dose-rate radiation for the p-i-n diode exposed to TriMeV irradiation.

Modeling Funneling Effect With Generalized Devices for SPICE Simulation of Soft Errors

Recent advances in CMOS scaling have made circuits more and more sensitive to errors and dysfunction caused by ionizing radiation, even at ground level, requiring accurate modeling of such effects. Besides generation, transport, and collection of radiation-induced excess carriers, another phenomenon, called funneling, has to be modeled for an accurate prediction of soft errors. The funneling effect occurs when the radiation track crosses a space charge region and generates excess carriers with a density higher than the doping close to it. These carriers distort the electric field of the space charge region, deeply changing the transport mechanism, from diffusion in a field-free semiconductor to drift. The objective of this work is to include funneling as part of the generalized lumped devices model in order to obtain a complete tool for SPICE-compatible simulations of single-event effects (SEEs). The latter approach has been recently proposed to simulate radiation-induced charges in the silicon substrate and is based on the so-called generalized lumped devices that simulate charge generation, propagation, and collection using standard circuit simulators. The generalized devices are here extended to include funneling and used to simulate an alpha particle impinging on the bulk of nMOS and pMOS transistors. The results obtained are validated with TCAD numerical simulations. Finally, an static random-access memory (SRAM) struck by an alpha particle is analyzed. The model predicts that the occurrence of a soft error, i.e., flipping of memory state, may depend on whether or not there is funneling. This justifies the need for accurate modeling of funneling phenomena to predict SEEs in ICs.

Surface Recombination and Excess Current of Anode-Gate Mesa Sidewall in 4H-SiC Gate Turn-Off Thyristor

The forward and reverse I-V characteristics of Anode-Gate diodes passivated with pyrogenic and dry oxide in4H-SiC gate turn-off thyristor (GTO) were investigated by device size dependent measurement. The bulk and surface current component were manifested and separated, where the surface recombination velocity was accurately evaluated as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm/s. Pyrogenic oxide was found more effective than dry oxide to suppress surface traps induced excess carrier recombination and carrier generation at etched p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> n mesa sidewall. Moreover, temperature dependent measurement revealed that excess current of Anode-Gate diodes is primarily attributed to carrier recombination and generation from surface traps. Surface traps dominates the reverse leakage below 125 °C, while electrically active bulk defects contributes to the reverse leakage simultaneously at elevated temperature. Finally, the influence of passivation on the 4H-SiC GTOs forward characteristics was demonstrated. Higher gate trigger current needed to switch from blocking to conduction state for the 4H-SiC GTOs with dry oxide passivated Anode-Gate mesa sidewall.

Wavelength dependent photosensitivity modulation of Aluminium/Lead sulphide/Indium tin oxide back-to-back diode

The photosensitivity of aluminium (Al)/lead sulphide (PbS)/indium tin oxide (ITO) thin layered structure is investigated considering the photon wavelength dependent current-voltage and capacitance-voltage characteristics of the device. The current-voltage characteristics of the test structure are analyzed adopting the back-to-back Schottky barrier diode model. The diode possesses low dark current in contrast to the high value of photocurrents measured under different illumination wavelengths. The change in photocurrents with different illumination wavelengths clearly indicates the change in photo-sensitivity of the device. The capacitance-voltage characteristics of Al/PbS/ITO structure demonstrate a definite improvement of the device capacitance with the higher wavelength exposures. The matter is explained in terms of the additional capacitance owing to the excess carrier generation within the device under illumination. The photosensitivity modulation of the device can be exploited in photo-sensor or photo-detector applications in various electronic devices.

Impact of Varied Buffer Layer Designs on Single-Event Response of 1.2-kV SiC Power MOSFETs

In this article, the single-event response of the 1.2-kV silicon-carbide (SiC) power MOSFETs with varied buffer layer designs is investigated by the 2-D numerical simulations. The structural parameters of the buffer layers are compared and analyzed to understand the transient response after the heavy ion strike and the related physical mechanisms comprehensively. Simulation results reveal that an optimized single buffer structure can be acquired by using a relatively thicker buffer layer (T $\mu \text{m}$ ) and a moderate doping concentration (D cm−3). It demonstrates that the single-event-burnout (SEB) performance can be improved significantly under the worst case bias conditions [a drain bias voltage of 1.2 kV and a linear energy transfer (LET) of 1 pC/ $\mu \text{m}$ ]. In addition, the optimized structural combinations adopted in the dual or triple buffer layers can strengthen the SEB performance further. The simulation results show that a step electric field distribution is established inside the optimized dual or triple buffers, where the electric field peak is mitigated from 3 to 2.2 MV/cm. Moreover, the excess carrier generation is suppressed and the local temperature rise is weakened during the transient electrothermal process. Consequently, the structure with the optimal buffer layer designs can enlarge the SEB safe operating area (SOA) from 30%–50% to 100% rated breakdown voltage at high LET bias, which makes the SiC power MOSFETs used in aerospace and aviation power electronic systems become possible.

Development of advanced hydrogenation processes for silicon solar cells via an improved understanding of the behaviour of hydrogen in silicon

AbstractThe understanding and development of advanced hydrogenation processes for silicon solar cells are presented. Hydrogen passivation is incorporated into virtually all silicon solar cells, yet the properties of hydrogen in silicon are still poorly understood. This is largely due to the complex behaviour of hydrogen in silicon and its ability to exist in many different forms in the lattice. For commercial solar cells, hydrogen is introduced into the device through the deposition of hydrogen‐containing dielectric layers and the subsequent metallisation firing process. This process can readily passivate structural defects such as grain boundaries but is ineffective at passivating numerous defects in silicon solar cells such as the boron‐oxygen complex, responsible for light‐induced degradation in p‐type Czochralski silicon. This difficulty is due to the need to first form the boron‐oxygen defect and also due to atomic hydrogen naturally occupying low‐mobility and low‐reactivity charge states. However, these challenges can be overcome using advanced hydrogenation processes incorporating excess carrier generation from illumination or current injection that increase the concentration of the highly mobile and reactive neutral charge state. As a result, after fast firing, additional low‐temperature advanced hydrogenation processes incorporating illumination can be implemented to enable the passivation of difficult defects like the boron‐oxygen complex. With the implementation of such processes for industrial silicon solar cells, efficiency improvements of 1.1% absolute can be obtained.

Simulation of CMOS APS Operation and Crosstalk in SPICE With Generalized Devices

We present SPICE simulations of a CMOS APS pixel element consisting of the optical sensor with the sensing circuit. The photodiode is simulated at the physics level by means of the so-called Generalized Devices, without any predefined compact model. Conversely, regular compact models are used for MOSFETs present in the circuit. Modeling with Generalized Devices takes into account in SPICE simulations both the layout and the physics of the photodiode, i.e., drift-diffusion transport, optical generation and recombination of excess carriers, capacitive effects and surface recombination. Moreover, electrical coupling between two adjacent pixels, i.e., the electrical crosstalk, is well predicted by the network of Generalized Devices. This approach is supported by TCAD Sentaurus simulations and opens the way to full SPICE simulation of Active Pixel Sensor from the circuit down to the semiconductor level within the same circuit simulation tool.

A Stretchable Strain-Insensitive Temperature Sensor Based on Free-Standing Elastomeric Composite Fibers for On-Body Monitoring of Skin Temperature.

To realize the potential applications of stretchable sensors in the field of wearable health monitoring, it is essential to develop a stable sensing device with robust electrical and mechanical properties in the present of varying external conditions. Herein, we demonstrate a stretchable temperature sensor with the elimination of strain-induced interference via geometric engineering of the free-standing stretchable fibers (FSSFs) of reduced graphene oxide/polyurethane composite. The FSSFs were formed in serpentine structures and enabled the implementation of a strain-insensitive stretchable temperature sensor. On the basis of the controlled reduction time of graphene oxide, we can modulate the response and thermal index of the device. These results are attributed to the variation in the density of oxygen-containing functional groups in the FSSFs, which affect the hopping charge transport and thermal generation of excess carriers. The FSSF temperature sensor yields increased responsivity (0.8%/°C), stretchability (90%), sensing resolution (0.1 °C), and stability in response to applied stretching (±0.37 °C for strains ranging from 0 to 50%). When the sensor is sewn onto a stretchable bandage and attached to the human body, it can detect the temperature changes of the human skin during different body motions in a continuous and stable manner.

Single-shot pulse duration and intensity diagnostic for 10-ps MeV gamma pulses based on interferometry

To measure the temporal width and the intensity evolution versus time of a MeV gamma pulse generated by a Compton Scatter Source, a time–space conversion method is proposed. This design is based on the consideration that the temporal length of the MeV pulse is proportional to the spatial length of the pulse in a certain semiconductor. The spatial length and the intensity evolution versus time of the MeV pulse can be obtained by recording the region of the refractive index change that is induced by the MeV pulse. The simulation suggests that the equivalent temporal spread of a mono-energy MeV δ pulse in a bulk semiconductor is on the order of picoseconds and does not vary significantly with photon energy and material type. According to our analysis, the excess carrier generation time, excess carrier diffusion and recombination do not significantly influence the temporal resolution of this method. The temporal response of the refractive index change to a MeV pulse is also fast enough to meet the measurement requirements. The signal generation process for measuring a 10-ps MeV pulse with a 200-fs probe beam is analyzed, revealing that the transverse size of the MeV pulse does not influence the temporal resolution of this method.

The effect of sputter growth conditions on the charge transport and stability of In-Ga-Zn-O semiconductors

Thin film transistors (TFTs) incorporating high mobility In-Ga-Zn-O (IGZO) semiconductors were fabricated and evaluated. The Ar:O2 gas flow rates were varied during the sputter growth of IGZO layers, which had a substantial influence on the device performance and stability under negative bias stress (NBS) and negative bias illumination stress (NBIS). As the Ar gas flow rate is increased, the TFTs exhibit inferior performance and stability. Thin film analyses indicate that the relative content of oxygen deficient sites in IGZO increases with increasing Ar gas flow rate, which is accompanied with more pronounced device degradation under bias stress. Relatively high oxygen deficiency usually leads to the generation of excess free carriers in oxide semiconductors, which normally results in high field effect mobility. However a counter-intuitive decrease in mobility is observed in this work, which is suggested to originate from the overall decrease in IGZO film density as a result of elevated sputtering pressure at high Ar gas flow rates. It is conjectured that disruptions of conduction paths occur in low density IGZO layers, owing to the lack of overlap between metal 5s orbitals.

Design and optimization of collection efficiency and conversion gain of buried p-well SOI pixel X-ray detector

Buried P-Well (BPW) technology was used in silicon-on-insulator pixels (SOIPIX) to suppress the back-gate effect, the major challenge in SOIPIX. In this work, we have designed and optimized two novel pixel structures, which are based on different BPW design layouts, to study the carrier collection efficiency and conversion gain of the pixel unit used in SOIPIX X-ray detectors. The first structure has an extended BPW region connected with a P+ node. In the second structure, a separated BPW ring region is formed surrounding the P+ node. Two X-ray sources with different photon energies have been applied in the simulation of excess carrier generation. The results indicated that the first structure had higher collection efficiency while the second structure had a slightly better conversion gain. As a result, the total photoelectric voltage of the first structure is about two times that of the second structure, where low doping concentration (&lt;1 × 1016 cm‒3) in the BPW region is preferred. Such a study of design and optimization of BPW technology is very important for applications in SOIPIX detectors

Point defect reduction in wide bandgap semiconductors by defect quasi Fermi level control

A theoretical framework for a general approach to reduce point defect density in materials via control of defect quasi Fermi level (dQFL) is presented. The control of dQFL is achieved via excess minority carrier generation. General guidelines for controlling dQFL that lead to a significant reduction in compensating point defects in any doped material is proposed. The framework introduces and incorporates the effects of various factors that control the efficacy of the defect reduction process such as defect level, defect formation energy, bandgap, and excess minority carrier density. Modified formation energy diagrams are proposed, which illustrate the effect of the quasi Fermi level control on the defect formation energies. These formation energy diagrams provide powerful tools to determine the feasibility and requirements to produce the desired reduction in specified point defects. An experimental study of the effect of excess minority carriers on point defect incorporation in GaN and AlGaN shows an excellent quantitative agreement with the theoretical predictions. Illumination at energies larger than the bandgap is employed as a means to generate excess minority carriers. The case studies with CN in Si doped GaN, H and VN in Mg doped GaN and VM-2ON in Si doped Al0.65Ga0.35N revealed a significant reduction in impurities in agreement with the proposed theory. Since compensating point defects control the material performance (this is particularly challenging in wide and ultra wide bandgap materials), dQFL control is a highly promising technique with wide scope and may be utilized to improve the properties of various materials systems and performance of devices based upon them.

Suppression of compensating native defect formation during semiconductor processing via excess carriers.

In many semiconductors, compensating defects set doping limits, decrease carrier mobility, and reduce minority carrier lifetime thus limiting their utility in devices. Native defects are often responsible. Suppressing the concentrations of compensating defects during processing close to thermal equilibrium is difficult because formation enthalpies are lowered as the Fermi level moves towards the majority band edge. Excess carriers, introduced for example by photogeneration, modify the formation enthalpy of semiconductor defects and thus can be harnessed during crystal growth or annealing to suppress defect populations. Herein we develop a rigorous and general model for defect formation in the presence of steady-state excess carrier concentrations by combining the standard quasi-chemical formalism with a detailed-balance description that is applicable for any defect state in the bandgap. Considering the quasi-Fermi levels as chemical potentials, we demonstrate that increasing the minority carrier concentration increases the formation enthalpy for typical compensating centers, thus suppressing their formation. This effect is illustrated for the specific example of GaSb. While our treatment is generalized for excess carrier injection or generation in semiconductors by any means, we provide a set of guidelines for applying the concept in photoassisted physical vapor deposition.

Erratum to: A study on the competition between bias-induced charge trapping and light-induced instability in In-Ga-Zn-O thin-film transistors

Thin-film transistors (TFTs) were fabricated using In-Ga-Zn-O (IGZO) semiconductor layers deposited under different oxygen partial pressures. The devices were subjected to negative bias stress (NBS), negative bias illumination stress (NBIS), positive bias stress (PBS) and positive bias illumination stress (PBIS). While device degradation is negligible under NBS, negative shifts in the threshold voltage (Vth) are observed in the presence of light (NBIS), of which the magnitude (ΔVth) decreases with increasing oxygen partial pressure during IGZO growth. Under PBS, the devices undergo positive Vth shifts, which become more severe with increasing oxygen content in IGZO. However, negative ΔVth values are observed under PBIS, of which the magnitude decreases with increasing oxygen content in the semiconductor. When positive gate bias is applied, the trapping of negative charge by interstitial oxygen atoms in IGZO is presumed to be the driving force inducing positive Vth shifts. On the other hand, when light is present, the generation of photo-induced excess carriers from oxygen-deficient defect sites is anticipated to be the driving force inducing negative Vth shifts. A balance between the competing mechanisms inducing either positive or negative Vth shifts must therefore be established when the devices are subjected to PBIS, for example in operating active matrix organic light emitting diode (AMOLED) displays using transparent panel arrays.