Hole Trap Density Research Articles

Depth profiles of hole traps in the tail region of Al ion implantation into p-type 4H-SiC

Abstract Hole traps generated in the tail region of Al ion implantation in p-type 4H-SiC were characterized by deep level transient spectroscopy measurements. Hole traps energetically located at Ev + 0.51 eV, Ev + 0.72 eV, Ev + 0.77 eV, and Ev + 1.40 eV (Ev : energy of the valence band top) were detected. The hole trap densities were roughly 7–40 times smaller than the implanted Al atom density in the tail region, and the densities of the observed traps exponentially decreased with the decay lengths of 84–150 nm.

Reducing Hole Trap Density in Sn–Pb Perovskite Solar Cells via Molecular Phenylhydrazine

Narrow‐bandgap Sn–Pb‐alloyed perovskite is potential for a bottom cell in perovskite tandem solar cells. However, Sn‐contained perovskites tend to be readily oxidized in air atmosphere, which has ill influence on stability and photovoltaic performance. Herein, a method to suppress oxidation of divalent tin via post‐treatment of perovskite film with phenylhydrazine is reported. Phenylhydrazine‐treated FA0.5MA0.5Pb0.5Sn0.5I3 perovskite films effectively reduce the hole traps caused by the oxidation of Sn2+ on the perovskite surfaces. In addition, surface energetics are well aligned by post‐treatment, which is beneficial for voltage gain and charge transport. As a result, power conversion efficiency (PCE) is increased from 18.16% to 20.67% after post‐treatment due mainly to the significant improvement of open‐circuit voltage from 0.75 to 0.83 V. Furthermore, the device stability is improved, where 91.67% of initial PCE is maintained after 1000 h.

Changes in Hole and Electron Injection under Electrical Stress and the Rapid Electroluminescence Loss in Blue Quantum-Dot Light-Emitting Devices.

Blue quantum dot light-emitting devices (QLEDs) suffer from fast electroluminescence (EL) loss when under electrical bias. Here, it is identified that the fast EL loss in blue QLEDs is not due to a deterioration in the photoluminescence quantum yield of the quantum dots (QDs), contrary to what is commonly believed, but rather arises primarily from changes in charge injection overtime under the bias that leads to a deterioration in charge balance. Measurements on hole-only and electron-only devices show that hole injection into blue QDs increases over time whereas electron injection decreases. Results also show that the changes are associated with changes in hole and electron trap densities. The results are further verified using QLEDs with blue and red QDs combinations, capacitance versus voltage, and versus time characteristics of the blue QLEDs. The changes in charge injection are also observed to be partially reversible, and therefore using pulsed current instead of constant current bias for driving the blue QLEDs leads to an almost 2.5× longer lifetime at the same initial luminance. This work systematically investigates the origin of blue QLEDs EL loss and provides insights for designing improved blue QDs paving the way for QLEDs technology commercialization.

Modeling the impacts of energetic disorder and charge transport on the exciton formation zone in organic light-emitting diodes

The width of exciton formation zone plays a vital role in determining the long-term stability of organic light-emitting diodes (OLEDs). Here, based on the general model of carrier device lifetimes, the dependences of the width and location of exciton formation zone on energetic disorder and charge transport have been simulated in OLEDs with emissive layer featuring hole and electron transport on host and guest molecules, respectively. The width of exciton formation zone increases with emissive layer’s energetic disorder increasing. The emissive layer with properly imbalanced carrier mobilities enables larger width of exciton formation zone than that with balanced carrier mobilities, due to the different hole and electron trap densities. Moreover, increasing hole transport layer’s hole mobility or decreasing electron transport layer’s electron mobility helps increase the width of exciton formation zone. There is no width of exciton formation zone obtained, when hole (electron) transport layer’s hole (electron) mobility is smaller than a certain value. The location of exciton formation relies on not only emissive layer’s carrier mobilities but also hole (electron) transport layer’s hole (electron) mobility. The general model provides the comprehensive picture to elucidate the roles of energetic disorder and charge transport in enhancing the stability of OLEDs, beneficial to improve the OLEDs designs towards the high stability at high luminance.

Estimation of trap distribution by the temperature spectrum of space charge decay

The energy band structure is crucial for solid insulation research. The traditional thermally stimulated current is used mostly for direct trap estimation but has some limitations, which can be overcome by the space charge decay method. With the help of a modified pulsed-electro acoustic system, we perform the test under different low temperatures and obtain a more comprehensive band spectrum including shallow traps and different polarities. This study chooses isotactic polypropylene (PP) to investigate in a temperature range of −120 to 20 °C. Signal processing and recovery are considered. Two basic trap estimation methods are compared and the one based on the modified isothermal relaxation current theory is used here. The electron and hole trap densities near different electrode interfaces are considered in the experiment. In the preliminary results, it is found that the traps tend to move toward a shallow direction with a decrease in temperature. A model of the retrapping process caused by phonon scattering is put forward to explain the results. The retrapping possibility is, therefore, assumed to follow a power relation with temperature. An attempt to rectify the trap depth is performed on such a model by numerical approximation and reaches the result that the retrapping possibility may be dependent upon the second power of temperature. Finally, the broadband energy spectrum is calculated after correction. In the normalized coordinate system, the traps in PP are mainly distributed in a deep range; few shallow traps are found maybe because the space charge density is low.

Dopant-free hole conductor with hybrid multisite passivation for perovskite solar cells

• Hybrid multisite passivation were introduced for multi-functional small molecular HTMs. • The passivation effect of carbonyl can be strengthened by introducing nitrogen. • Hole mobility is 71% higher and hole trap-density is 49% lower by the strategy. Polymer hole-transporting material (HTM) with multi-site anchors is an interesting idea for defect passivation in perovskite solar cells (PSC). In this work, a small molecular HTM coded as TPA-DN with hybrid multisite passivation, that is two chelated pyridine nitrogen atoms and carbonyl double anchor sites, is developed. Two nitrogen atoms with higher electronegativity in TPA-DN can chelate with uncoordinated lead in perovskite, and the electron cloud density of oxygen in carbonyl can be increased by introducing nitrogen in fluorene-9-one core. The hole mobility is 71% higher and the hole trap-density is 49% lower after introducing the hybrid multisite passivation strategy. Comparing with the analogue without chelated nitrogen atoms, TPA-DN causes the power conversion efficiency (PCE) increasing from 18.37% to 19.45% with increased short-circuit current density and open-circuit voltage, respectively. This work proves that hybrid multisite passivation is an effective strategy in the design of organic p-type semiconductors.

Universal Surface Passivation of Organic-Inorganic Halide Perovskite Films by Tetraoctylammonium Chloride for High-Performance and Stable Perovskite Solar Cells.

The power conversion efficiency (PCE) of perovskite solar cells has been showing rapid improvement in the last decade. However, still, there is an unarguable performance deficit compared with the Schockley-Queisser (SQ) limit. One of the major causes for such performance discrepancy is surface and grain boundary defects. They are a source of nonradiative recombination in the devices that not only causes performance loss but also instability of the solar cells. In this study, we employed a direct postsurface passivation strategy at mild temperatures to modify perovskite layer defects using tetraoctylammonium chloride (TOAC). The passivated perovskite layers have demonstrated extraordinary improvement in photoluminescence and charge carrier lifetimes compared to their control counterparts in both Cs0.05(FAPbI3)0.83(MAPbBr3)0.17 and MAPbI3-type perovskite layers. The investigation on electron-only and hole-only devices after TOAC treatment revealed suppressed electron and hole trap density of states. The electrochemical study demonstrated that TOAC treatment improved the charge recombination resistance of the perovskite layers and reduced the charge accumulation on the surface of perovskite films. As a result, perovskite solar cells prepared by TOAC treatment showed a champion PCE of 21.24% for the Cs0.05(FAPbI3)0.83(MAPbBr3)0.17-based device compared to 19.58% without passivation. Likewise, the PCE of MAPbI3 improved from 18.09 to 19.27% with TOAC treatment. The long-term stability of TOAC-passivated perovskite Cs0.05(FAPbI3)0.83(MAPbBr3)0.17 devices has retained over 97% of its initial performance after 720 h in air.

The Effects of Dopant Concentration on the Performances of the a-SiOx:H(p)/a-Si:H(i1)/a-Si:H(i2)/µc-Si:H(n) Heterojunction Solar Cell

In this work, the imbalances in band gap energy between p-window layer and intrinsic layer (p/i interface) in p-i-n type solar cells to suppress charge recombination adopting with the addition of buffer layer, at p/i interface, namely solar cell structures without buffer (Cell A) and with buffer (Cell B). Using well-practiced AFORS-HET software, performances of Cell A and Cell B structures are evaluated and compared to experimental data. A good agreement between AFORS-HET modelling and experimental data was obtained for Cell A (error = 1.02%) and Cell B (error = 0.07%), respectively. The effects of dopant concentrations of the p-type and n-type were examined with respect to cell B for better performance by analysing the energy band diagram, the electric field distribution, the trapped hole density, the light J-V characteristics, and the external quantum efficiency. The simulated results of an optimised Cell B showed that the highest efficiency of 8.81% (VOC = 1042 mV, JSC = 10.08 mA/cm2, FF = 83.85%) has been obtained for the optimum dopant values of NA = 1.0 x 1019 cm-3 and ND = 1.0 x 1019 cm-3, respectively. A comparison between experimental data and simulation results for Cell B showed that the conversion efficiency can be enhanced from 5.61% to 8.81%, using the optimized values

An Electron and Hole Trap Energy Distribution Estimation Model and Its Application in Polypropylene Nanocomposites

To differentiate the electron and hole traps inside polymers, a model based on the pulsed electro-acoustic (PEA) method is proposed. It is premised that under short-term DC electric field polarization at relatively low temperature, the homocharge is injected into the sample to a shallow depth and dominates the space charge distribution. Thus, the homocharge near the ground electrode and the high voltage electrode can be separated. During depolarization, only the homocharge near the ground electrode is analyzed to reduce the influence of ultrasonic wave attenuation, which also results in unipolar space charge decay behavior being obtained. The isothermal relaxation current (IRC) theory is then employed to obtain the unipolar quasi-continuous trap energy distribution based on the space charge decay. Thereby, the electron and hole traps can be differentiated by applying positive and negative DC electric field polarization, respectively. The experimental results of low density polyethylene (LDPE) verify the validity of the proposed model. Based on the model, polypropylene (PP) and PP/MgO nanocomposites are studied to explore the effect of MgO doping on the trap characteristics. The results show that numerous electron deep traps are introduced into the nanocomposites due to the intrinsic properties of nanoparticles. Besides, the heterogeneous nucleation of the nanoparticles increases the crystal/amorphous and crystal/vacuum interfaces, which may be the main reason for the increase of hole trap density of nanocomposites.

Correlating Structure and Detection Properties in HgTe Nanocrystal Films.

HgTe nanocrystals (NCs) enable broadly tunable infrared absorption, now commonly used to design light sensors. This material tends to grow under multipodic shapes and does not present well-defined size distributions. Such point generates traps and reduces the particle packing, leading to a reduced mobility. It is thus highly desirable to comprehensively explore the effect of the shape on their performance. Here, we show, using a combination of electron tomography and tight binding simulations, that the charge dissociation is strong within HgTe NCs, but poorly shape dependent. Then, we design a dual-gate field-effect-transistor made of tripod HgTe NCs and use it to generate a planar p-n junction, offering more tunability than its vertical geometry counterpart. Interestingly, the performance of the tripods is higher than sphere ones, and this can be correlated with a stronger Te excess in the case of sphere shapes which is responsible for a higher hole trap density.

Post dry etching treatment of nanopillar GaN/InGaN multi-quantum-wells

Time-resolved photoluminescence (PL), current-voltage characteristics and deep trap spectra of nanopillar GaN/InGaN multi-quantum-well (MQW) light emitting diodes (LEDs) prepared by reactive ion etching (RIE) from planar samples were studied as a function of post-RIE surface treatment. Immediately after RIE, we observe a sharp drop of intensity of the 460 nm MQW PL band and a strong increase of intensity of the defect-related 600 nm MQW band similar to the yellow band in n-GaN. KOH etching and (NH4)2S sulfur passivation treatments increased the MQW PL band intensity above the level of the planar structure and decreased the intensity of the defect 600 nm PL band, but was not effective in decreasing the RIE-induced excessive leakage current of the LEDs. Only annealing at ≥ 700 °C following RIE was able to strongly suppress the excessive leakage. Deep trap spectra suggest that the RIE-induced drop of the 460 nm PL intensity and increase of the 600 nm PL intensity are accompanied by the increase of the Ec-0.7 eV electron trap density and of Ev+0.8 eV hole trap density in the quantum wells. The first of these defects is attributed to nitrogen interstitial acceptors believed to be effective nonradiative recombination centers, while the hole traps are attributed to gallium vacancy acceptor complexes with shallow donors. Both types of defects are produced on the sidewalls of nanopillars by RIE and can be largely annealed at 700 °C, although higher annealing temperatures are needed to fully suppress their negative impact. These results are also relevant to curing the effects of dry-etch damage for micro-LEDs with small chip dimensions, a serious problem in the micro-LED field.

Design of ultra-scaled-channel N-polar GaN HEMTs with high charge density: A systematic study of hole traps and their impact on charge density in the channel

It has been previously shown that hole/donor traps at the (Al, Ga) N/GaN interface cause DC-RF dispersion and output conductance in N-polar GaN high electron mobility transistors (HEMTs). In this work, we systematically studied the impact of hole trap energy and density on two-dimensional electron gas (2DEG) density using Silvaco Atlas. Simulation results revealed that the exclusion of the hole traps in the model results in an underestimation of 2DEG density compared to experimentally obtained 2DEG density. By comparing simulations with the experimental results, a hole trap with a density of 3 × 1013 cm−2 at 280 meV above the valence band at the AlN/GaN negative polarization interface was estimated. Three different silicon doping schemes were then examined to suppress the effect of traps. Delta doping (15 nm) at the GaN buffer and barrier interface (doping Scheme-A) is effective in compensating traps present at that interface but insufficient to compensate traps near the GaN channel. Similarly, doping the back-barrier (doping scheme-B) is sufficient to neutralize traps in the middle of the back-barrier and close to the channel but inadequate to neutralize traps at the buffer–barrier interface. Series-C doping employs a combination of doping schemes A and B that effectively neutralizes traps present at all interfaces while simultaneously modulating the 2DEG charge density. An ultra-scaled 5-nm-thick GaN HEMT epitaxial structure was also designed by band engineering that can maintain high 2DEG density in the channel (2 × 013 cm−2) with less than 5% parasitic charge and trap ionization over a wide range of doping from 6 × 1018 cm−3 to 1 × 1019 cm−3.

Assessment of high-k gate stacked In2O5Sn gate recessed channel MOSFET for x-ray radiation reliability

This work reports the effect of x-ray radiation on In2O5Sn based Transparent Gate Recessed Channel (TGRC) MOSFET with the high-k dielectric at the sub-20 nm regime. Reliability of TGRC-MOSFET with a high-k dielectric in harsh radiation environment (x-ray radiation in the 1k to 10k rad dose range after irradiation) is the main aim of this analysis using TCAD simulation. Results reveal that HfO2 as a gate stack on SiO2 improves the device reliability and enhances the drain current, hole trap density, threshold voltage shift, and radiation sensitivity as compared to Al2O3 and ZrO2 gate stack with 1k rad to 10 k rad radiation doses. Trap/de-trap model has been used for interface charging as well as insulator along with the electron-hole pair generation and recombination. Further, the thermal effect on threshold voltage and sensitivity has also been evaluated. Results suggest that the proposed device with a high-k dielectric is more reliable in the x-ray radiation environment at the sub 20 nm scale. This device finds enormous applications in the clinical and space environment along with signal amplification and a processing circuit.

Dual-color-sub-bandgap-light-excited isothermal capacitance transient spectroscopy for quick measurement of carbon-related hole trap density in n-type GaN

A quick method is proposed for measurement of the carbon-related hole trap (H1: +0.87 eV) density in an n-type GaN homoepitaxial layer using dual-color-sub-bandgap-light-excited isothermal capacitance transient spectroscopy. Shorter wavelength (390 nm) light irradiation is employed to cause the hole traps to be in the hole-occupied state. Longer wavelength (660 nm) light irradiation is then used to emit the hole from the trap to the valence band. The photoemission of holes is much quicker than the thermal emission, which reduces the measurement time. The trap density can be calculated from the capacitance transient.

Effect of Zr doping and lattice oxygen release on the resistive switching properties of ZrxHf1−xO2-based metal-oxide-semiconductor devices

The effect of Zr content on resistive switching properties of ZrxHf1–xO2-based metal-oxide-semiconductor (MOS) devices has been studied. The electrical characteristics and x-ray photoelectron spectroscopy studies reveal that the resistive switching property performs its best when the Zr content in the film is 9.11%. The XPS data reveals presence of non-lattice oxygen in all the devices. The differential scanning calorimetry (DSC) study shows an endotherm peak at ~ 145.9 °C only for the sample at a particular Zr content in the ZrxHf1–xO2 film indicating release of lattice oxygen. Thus, besides presence of non-lattice oxygen in the film, further generation of hole trap density in the form of lattice oxygen release is required to achieve a better low resistance state (LRS) and the same is reflected in the double logarithmic plot of I − V. It is believed that some sort of microstructural arrangements take place at a critical Zr content that lead to the lowest resistive state of the MOS device.

Accurate Threshold Voltage Reliability Evaluation of Thin Al2O3 Top-Gated Dielectric Black Phosphorous FETs Using Ultrafast Measurement Pulses.

Few-layer black phosphorus (BP) has attracted significant interest in recent years due to electrical and photonic properties that are far superior to those of other two-dimensional layered semiconductors. The study of long term electrical stability and reliability of black phosphorus field effect transistors (BP-FETs) with technologically relevant thin, and device-selective, gate dielectrics, stressed under realistic (closer to operation) bias and measured using state-of-the-art ultrafast reliability characterization techniques, is essential for their qualification and use in different applications. In this work, air-stable BP-FETs with a thin top-gated dielectric (15 nm Al2O3, SiO2 equivalent thickness of 5 nm) were fabricated and comprehensively characterized for threshold voltage ( Vth) instability under negative gate bias stress at various measurement delays ( tm), stress biases ( VGSTR), temperatures ( T), and stress times ( tstr) for the first time. Thin top-gated oxide enables low VGSTR that is closer to the operating condition and ultrafast Vth measurements with low delay ( tm = 10 μs, due to high drain current) that ensure minimal recovery. The resultant time kinetics of Vth degradation (Δ Vth) shows fast saturation at longer stress times and low-temperature activation energy. Vth instability in these top-gated devices is suggested to be dominated by hole trapping, which is modeled using first-order equations at different VGSTR and T. It is shown that measurements using larger tm show lower degradation magnitude that do not saturate due to recovery artifacts and give inaccurate estimation of hole trap densities. Conventional, thick, and global back-gated oxide BP-FETs were also fabricated and characterized for varying tm (1 ms being the lowest due to a low drain current level for thick oxide), VGSTR, and T to benchmark our top-gated results. Nonsaturating Δ Vth in the back-gated devices is shown to result from recovery artifacts due to the large tm (1 ms and greater) values. Finally, using a VGSTR and T-dependent first-order model, we show that the top-gated Al2O3 BP-FETs with scaled gate oxide thickness can match state-of-the-art Si reliability specifications at operating voltage and room/elevated temperature.

Defects responsible for charge carrier removal and correlation with deep level introduction in irradiated β-Ga2O3

Carrier removal rates and electron and hole trap densities in β-Ga2O3 films grown by hydride vapor phase epitaxy (HVPE) and irradiated with 18 MeV α-particles and 20 MeV protons were measured and compared to the results of modeling. The electron removal rates for proton and α-radiation were found to be close to the theoretical production rates of vacancies, whereas the concentrations of major electron and hole traps were much lower, suggesting that the main process responsible for carrier removal is the formation of neutral complexes between vacancies and shallow donors. There is a concurrent decrease in the diffusion length of nonequilibrium charge carriers after irradiation, which correlates with the increase in density of the main electron traps E2* at Ec − (0.75–0.78) eV, E3 at Ec − (0.95–1.05) eV, and E4 at Ec − 1.2 eV. The introduction rates of these traps are similar for the 18 MeV α-particles and 20 MeV protons and are much lower than the carrier removal rates.

Identification of Interface States and Shallow and Deep Hole Traps under NBTI Stress using Fast, Normal, and Charge-pumping Measurement Techniques

We propose an identification method of interface state density (NSUBit/SUB), shallow hole trap density (NSUBht.S/SUB), and deep hole trap density (NSUBht.D/SUB), which were generated by negative bias temperature instability (NBTI) stress. NSUBht.S/SUB, NSUBht.D/SUB, and Nit were decoupled into recoverable traps and permanent traps through the proposed method. We analyzed which traps among hole-trapping charges and interface states constitute the main permanent damage component and have the largest power-law exponent with various stress voltages and temperatures. The power-law exponent of each trap showed that NSUBht.D/SUB and NSUBit/SUB lead to considerable timedependent degradation. Moreover, based on the recovery characteristics of each trap, 31% of the total traps became permanent traps and 77% of the permanent traps were NSUBht.D/SUB, which implies that NSUBht.D/SUB is a crucial factor, constituting the main component of the permanent damage.

Solution-Processed Organic-Inorganic Perovskite Field-Effect Transistors with High Hole Mobilities.

A very high hole mobility of 15 cm2 V-1 s-1 along with negligible hysteresis are demonstrated in transistors with an organic-inorganic perovskite semiconductor. This high mobility results from the well-developed perovskite crystallites, improved conversion to perovskite, reduced hole trap density, and improved hole injection by employing a top-contact/top-gate structure with surface treatment and MoOx hole-injection layers.

Simulation study of the a-Si:H/nc-Si:H solar cells performance sensitivity to the TCO work function, the band gap and the thickness of i-a-Si:H absorber layer

In this paper, two solar cells n–i–p+ and n–i–p–p+ based on hydrogenated amorphous silicon (a-Si:H) and hydrogenated nanocrystalline silicon (nc-Si:H) have been simulated by using AMPS-1D (Analysis of Microelectronic and Photonic Structures) simulator. Various factors that affect cell performance have been studied, such as work function of Transparent Conducting Oxide (WTCO), and band gap and thickness of the i-a-Si:H absorber layer. At first the effect of work function WTCO on the performances of both structures was studied. The best cell’s external parameters where obtained in the case of n–i–p–p+ structure. For this latter, the energy band diagram, current–voltage characteristics J(V), the trapped hole density, the distribution of built-in electric field, and the quantum efficiency are calculated and analyzed in depth to understand the effect of WTCO. It is demonstrated that, for high efficiency of solar cell, the WTCO value should be high enough in order to enhance built-in potential, and gives the electric field negative and the front hole barrier height for a neutral (i.e., no band bending) contact at TCO/p+nc-Si:H interface. Later, band gap and thickness of i-a-Si:H absorber layer are optimized. Simulation results showed that the highest efficiency of 9.35% (JSC=13.96mA/cm2; FF=71.4%; VOC=936mV) has been obtained in the case of n–i–p–p+ structure, when values of WTCO, i-a-Si:H band gap and i-a-Si:H layer thickness are 5.45eV, 1.78eV, and 550nm, respectively. A comparison between simulation results and experimental data, showed that the conversion efficiency of n–i–p–p+ solar cell can be enhanced from 7.44% to 9.35%, by using the optimized values.