Photo-excited Charge-collection Spectroscopy Research Articles

Facile interfacial defect healing in solution-processed In-Ga-Zn-O thin film transistor through rapid intense pulsed light annealing

We introduce intense pulsed light (IPL) annealing as a versatile and cost-effective interface treatment approach for solution-processed In-Ga-Zn-O thin film transistor, focusing on its potential to improve its operational stability. We fabricate solution-processed IGZO-based thin film transistors (IGZO-TFTs) and systematically investigate the dependence of IPL shot numbers on key parameters that dictate the electrical characteristics of the devices. Under identical positive bias stress (PBS) conditions, we compare the operational stability of IGZO-TFTs treated with IPL annealing during ∼5 min to those subjected to conventional high-temperature thermal processes during 2 h, and reveal that IPL-annealed IGZO-TFTs exhibit superior stability as well as significantly suppressed threshold voltage shifts by ∼22 % (from 3.31 to 2.58 V). To gain deeper insights, we employ photo-excited charge collection spectroscopy analysis, which provides quantitative evidence of the rapid recovery of oxygen vacancies and hydrogen-related interfacial defect states (located at 2.1 ∼ 2.5 eV and near 2.79 eV below conduction band minimum) affecting operational device stability. Our findings underscore the technical advantages of IPL annealing over time-consuming thermal processes, positioning it as a promising method for optimizing IGZO-TFT performance.

Investigation of Donor-like State Distributions in Solution-Processed IZO Thin-Film Transistor through Photocurrent Analysis

The density of donor-like state distributions in solution-processed indium–zinc-oxide (IZO) thin-film transistors (TFTs) is thoroughly analyzed using photon energy irradiation. This study focuses on quantitatively calculating the distribution of density of states (DOS) in IZO semiconductors, with a specific emphasis on their variation with indium concentration. Two calculation methods, namely photoexcited charge collection spectroscopy (PECCS) and photocurrent-induced DOS spectroscopy (PIDS), are employed to estimate the density of the donor-like states. This dual approach not only ensures the accuracy of the findings but also provides a comprehensive perspective on the properties of semiconductors. The results reveal a consistent characteristic: the Recombination–Generation (R-G) center energy ET, a key aspect of the donor-like state, is acquired at approximately 3.26 eV, irrespective of the In concentration. This finding suggests that weak bonds and oxygen vacancies within the Zn-O bonding structure of IZO semiconductors act as the primary source of R-G centers, contributing to the donor-like state distribution. By highlighting this fundamental aspect of IZO semiconductors, this study enhances our understanding of their charge-transport mechanisms. Moreover, it offers valuable insight for addressing stability issues such as negative bias illumination stress, potentially leading to the improved performance and reliability of solution-processed IZO TFTs. The study contributes to the advancement of displays and technologies by presenting further innovations and applications for evaluating the fundamentals of semiconductors.

Se-Vacancy Healing with Substitutional Oxygen in WSe2 for High-Mobility p-Type Field-Effect Transistors.

Transition-metal dichalcogenides possess high carrier mobility and can be scaled to sub-nanometer dimensions, making them viable alternative to Si electronics. WSe2 is capable of hole and electron carrier transport, making it a key component in CMOS logic circuits. However, since the p-type electrical performance of the WSe2-field effect transistor (FET) is still limited, various approaches are being investigated to circumvent this issue. Here, we formed a heterostructural multilayer WSe2 channel and solution-processed aluminum-doped zinc oxide (AZO) for compositional modification of WSe2 to obtain a device with excellent electrical properties. Supplying oxygen anions from AZO to the WSe2 channel eliminated subgap states through Se-deficiency healing, resulting in improved transport capacity. Se vacancies are known to cause mobility degradation due to scattering, which is mitigated through ionic compensation. Consequently, the hole mobility can reach high values, with a maximum of approximately 100 cm2/V s. Further, the transport behavior of the oxygen-doped WSe2-FET is systematically analyzed using density functional theory simulations and photoexcited charge collection spectroscopy measurements.

Nano-patterning on multilayer MoS2 via block copolymer lithography for highly sensitive and responsive phototransistors

Indirect bandgap of multilayer molybdenum disulfide has been recognized as a major hindrance to high responsivity of MoS2 phototransistors. Here, to overcome this fundamental limitation, we propose a structural engineering of MoS2 via nano-patterning using block copolymer lithography. The fabricated nanoporous MoS2, consisting of periodic hexagonal arrays of hexagon nanoholes, includes abundant edges having a zigzag configuration of atomic columns with molybdenum and sulfur atoms. These exposed zigzag edges are responsible for multiple trap states in the bandgap region, as confirmed by photo-excited charge-collection spectroscopy measurements on multilayer nanoporous MoS2 phototransistors, showing that in-gap states only near the valence band can result in a photogating effect. The effect of nano-patterning is therefore to significantly enhance the responsivity of multilayer nanoporous MoS2 phototransistors, exhibiting an ultra-high photoresponsivity of 622.2 A W−1. Our nano-patterning of MoS2 for photosensing application paves a route to structural engineering of two-dimensional materials for highly sensitive and responsive optoelectronic devices.

Channel Defect Analysis Method of a-IGZO TFTs on Polyimide for Flexible Displays

In this paper, we introduce a method enabling faster and more innovative analysis of the channel characteristics of amorphous indium-gallium- zinc oxide (a-IGZO) thin film transistors (TFTs) manufactured on flexible substrates through electrical property analysis. The TFTs used in the present evaluation were a-IGZO TFTs with different oxygen flow rates (OFRs), which have been reported on frequently in the past, to confirm the effectiveness of the new evaluation method. The interface characteristics of the IGZO and the insulator were confirmed by density of state (DOS) extraction using photoexcited charge collection spectroscopy (PECCS). Defects of the TFT active layer were confirmed through the capacitance-time (c-t) method. It was confirmed that the smaller the OFR, the smaller the carrier lifetime due to the presence of oxygen vacancies. In conclusion, by obtaining the same results as previously reported physical analyses, the proposed electrical channel characteristics analysis of IGZO TFTs was shown to be a reliable analysis method.

Interband Transitions in Monolayer and Few-Layer WSe2 Probed Using Photoexcited Charge Collection Spectroscopy.

Transition-metal dichalcogenides are currently under rigorous investigation because of their distinct layer-dependent physical properties originating from the corresponding evolution of the band structure. Here, we report the highly resolved probing of layer-dependent band structure evolution for WSe2 using photoexcited charge collection spectroscopy (PECCS). Monolayer, few-layer, and multilayer WSe2 can be probed in top-gate field-effect transistor platforms, and their interband transitions are efficiently observed. Our theoretical calculations show a great coincidence with the PECCS results, proving that the indirect Γ-K and Γ-Λ transitions as well as the direct K-K transition are clearly resolved in multilayer WSe2 by PECCS.

Analysis of Self-Heating Effect on Short Channel Amorphous InGaZnO Thin-Film Transistors

We report on a thin-flim transistor (TFT) degradation encountered in short channel amorphous indium-gallium-zinc-oxide TFTs under high applied power condition leading to self-heating. Negative shift was observed in the initial stage of stress period followed by positive shift with severe degradation. To understand the causes of this phenomenon in depth, trap density-of-states were measured by photo-excited charge-collection spectroscopy and time-dependent recovery of stressed device samples was also studied. As a result, we found that the combination of hot carrier effect and self-heating in channel was responsible for the degradation.

Trap density probing on top-gate MoS₂ nanosheet field-effect transistors by photo-excited charge collection spectroscopy.

Two-dimensional (2D) molybdenum disulfide (MoS₂) field-effect transistors (FETs) have been extensively studied, but most of the FETs with gate insulators have displayed negative threshold voltage values, which indicates the presence of interfacial traps both shallow and deep in energy level. Despite such interface trap issues, reports on trap densities in MoS₂ are quite limited. Here, we probed top-gate MoS₂ FETs with two- (2L), three- (3L), and four-layer (4L) MoS₂/dielectric interfaces to quantify deep-level interface trap densities by photo-excited charge collection spectroscopy (PECCS), and reported the result that deep-level trap densities over 10(12) cm(-2) may exist in the interface and bulk MoS₂ near the interface. Transfer curve hysteresis and PECCS measurements show that shallow traps and deep traps are not that different in density order from each other. We conclude that our PECCS analysis distinguishably provides valuable information on deep level interface/bulk trap densities in 2D-based FETs.

Origin of channel/dielectric interfacial trap states modification by ultraviolet irradiation on organic thin-film transistors

The authors report on the origin of channel/dielectric interfacial states modified by ultraviolet (UV) irradiation on pentacene thin-film transistors (TFTs) with poly-4-vinylphenol and AlOx double gate dielectric. Exposing the device to 352 nm UV irradiation on caused a positive threshold voltage (Vth) shift, and then photo-excited charge-collection spectroscopy (PECCS) was applied to the device to clarify the changes of the interfacial density of states (DOS) as the origin of Vth modification. From the PECCS measurement, it could be probed that there was a drastic decrease in DOS for deep traps at ∼1.31 and ∼1.38 eV above the highest occupied molecular orbital level of pentacene. Consequently, we can conclude that the positive shift of Vth for pentacene-TFT by UV irradiation results from the elimination of deep hole traps, which could be introduced by the interfacial dipoles at the channel/dielectric interface.

Systematic Reliability Study of Top-Gate p- and n-Channel Organic Field-Effect Transistors

We report on a systematic investigation on the performance and stability of p-channel and n-channel top-gate OFETs, with a CYTOP/Al2O3 bilayer gate dielectric, exposed to controlled dry oxygen and humid atmospheres. Despite the severe conditions of environmental exposure, p-channel and n-channel top-gate OFETs show only minor changes of their performance parameters without undergoing irreversible damage. When correlated with the conditions of environmental exposure, these changes provide new insight into the possible physical mechanisms in the presence of oxygen and water. Photoexcited charge collection spectroscopy experiments provided further evidence of oxygen and water effects on OFETs. Top-gate OFETs also display outstanding durability, even when exposed to oxygen plasma and subsequent immersion in water or operated under aqueous media. These remarkable properties arise as a consequence of the use of relatively air stable organic semiconductors and proper engineering of the OFET structure.

Origin of Bias-Stress Induced Instability in Organic Thin-Film Transistors with Semiconducting Small-Molecule/Insulating Polymer Blend Channel

The stabilities of a blending type organic thin-film transistor with phase-separated TIPS-pentacene channel layer were characterized under the conditions of negative-bias-stress (NBS) and positive-bias-stress (PBS). During NBS, threshold voltage (Vth) shifts noticeably. NBS-imposed devices revealed interfacial trap density-of-states (DOS) at 1.56 and 1.66 eV, whereas initial device showed the DOS at only 1.56 eV, as measured by photoexcited charge-collection spectroscopy (PECCS) method. Possible origin of this newly created defect is related to ester group in PMMA, which induces some hole traps at the TIPS-pentacene/i-PMMA interface. PBS-imposed device showed little Vth shift but visible off-current increase as "back-channel" effect, which is attributed to the water molecules trapped on the TFT surface.

Capacitance–Voltage Measurement With Photon Probe to Quantify the Trap Density of States in Amorphous Thin-Film Transistors

We report on a characterization method to quantitatively estimate the interfacial trap density of states (DOS) in thin-film transistors (TFTs): photon-probe capacitance-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</i> ) measurement. The photo- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</i> method was compared to photoexcited charge-collection spectroscopy, which is another photon-probe method using current-voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ) measurement to meet the same purpose. Here, we directly characterized the DOS of amorphous-Si- and amorphous-InGaZnO-based TFTs using the photon-probe <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CV</i> measurement, and the results turned out to mainly focus the trap DOS at channel/dielectric interface. On the one hand, the DOS profile by photon-probe <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> method appeared to give additional trap information from the transistor back channel.

Impact of Interfacial Trap Density of States on the Stability of Amorphous InGaZnO-Based Thin-Film Transistors

The impact of interfacial trap states on the stability of amorphous indium-gallium-zinc oxide thin film transistors is studied under positive gate bias stress. With increasing stress time, the device exhibits a large positive drift of threshold voltage while maintaining a stable sub-threshold swing and a constant field-effect mobility of channel electrons. The threshold voltage drift is explained by charge trapping at the high-density trap states near the channel/dielectric interface, which is confirmed by photo-excited charge-collection spectroscopy measurement.

Photoexcited charge collection spectroscopy of two-dimensional polaronic states in polymer thin-film transistors

The polaronic structures associated with the interchain stacking in the polymer semiconductor bulk or polymer semiconductor-dielectric interface in fully operational thiophene-based thin-film transistors (TFTs) were investigated with a newly developed in situ photoexcited charge collection spectroscopy (PECCS). The in situ PECCS spectra of polymer semiconductors exhibited the \ensuremath{\pi}-\ensuremath{\pi}${}^{*}$ absorption peaks between 2.05 and 2.45 eV, which were well-matched with those in the Ultraviolet-visible (UV-vis) absorption spectra of the corresponding polymer films. The spectra also showed associated charge-induced absorption peaks at 1.32, 1.39, 1.58, and 1.71 eV below the \ensuremath{\pi}-\ensuremath{\pi}${}^{*}$ absorption edge. These charge-induced absorption peaks might be related to the delocalized polaron states located in the both polymer semiconductor bulk and semiconductor-dielectric interface of our polymer-based TFTs. From the spectral results, we were able to extract the polaronic density of states (DOS) of the polymer channel, and we confirmed that the denser interchain packing in the polymer semiconductor channel leads to a larger DOS near the band edge, resulting in higher field-effect mobility.

Stability-improved organic n-channel thin-film transistors with nm-thin hydrophobic polymer-coated high-k dielectrics

We report on the fabrication of N,N'-ditridecyl-perylene-3,4:9,10-tetracarboxylic diimide-C13 (PTCDI-C13), n-channel organic thin-film transistors (OTFTs) with 30 nm Al(2)O(3) whose surface has been un-modified or modified with hexamethyldisilazane (HMDS) and thin hydrophobic CYTOP. Among all the devices, the OTFTs with CYTOP-modified dielectrics exhibit the most superior device performance and stability. The optimum post-annealing temperature for organic n-channels on CYTOP was also found to be as low as 80 °C, although the post-annealing was previously implemented at 120-140 °C for PTCDI domain growth in general. The low temperature of 80 °C hardly damages the CYTOP/n-channel organic interface which is deformed at a temperature higher than the glass transition temperature of CYTOP (∼110 °C). The pentacenequinone passivation layer turned out to be helpful to keep the interfacial trap density minimum, according to the photo-excited charge collection spectroscopy results for our 80 °C-annealed OTFTs with CYTOP-modified dielectrics.

Trap Density of States Measured by Photon Probe on Amorphous-InGaZnO Thin-Film Transistors

Trap density of states (DOS) for two amorphous InGaZnO thin-film transistors are measured by photoexcited charge-collection spectroscopy (PECCS); free charges trapped at a certain energy level are liberated by the corresponding energetic photons and then electrically collected at the source/drain electrodes. During this photoelectric process, the threshold voltage of TFT shifts and its magnitude provides the DOS information of charge traps. According to the PECCS analysis, the TFT with a channel deposited under high sputtering power appeared more stable than the other device prepared with low sputtering power, showing even less than ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> as its totally integrated trap density.