Tetramethylammonium Hydroxide Treatment Research Articles

Re-evaluation of the reduced heart weights in male rats in a 28-day oral repeated-dose toxicity study of tetramethylammonium hydroxide

We recently conducted a detailed hazard assessment of tetramethylammonium hydroxide (TMAH), a priority chemical substance under the Japan Chemical Substances Control Law. During this assessment, there was debate regarding the reduced heart weight observed in the treated male groups in the 28-day rat oral repeated-dose toxicity study. This finding was not observed in females in this study and in both sexes of oral toxicity studies for tetramethylammonium chloride (TMAC) or tetramethylammonium hydrogen phthalate (TMAHP). Unpublished individual data from the oral TMAH developmental and reproductive toxicity (DART) screening study were also obtained; no effect on heart weight was observed. In addition, background data on rat heart weight from six 28-day oral toxicity studies conducted in the same facility, year, strain, age, and breeder as the TMAH study were obtained from the Japan Existing Chemical Substances Database (JECDB). These investigations suggest that the statistically significant lower heart weight in the treated males in the 28-day toxicity study is likely caused by an incidental skewing of individuals with heavier heart weights toward control male groups and is not due to TMAH treatment. Thus, it is worthwhile to include as much relevant data as possible to confirm or refute unexpected findings in toxicity studies.

Impacts and effectiveness of sidewall treatment on the spatially resolved optical properties and efficiency enhancement for GaN-based blue and green micro-LEDs

InGaN-based micro-light-emitting diodes (micro-LEDs) are crucial for efficient full-color micro-displays, but suffer from severe degradation of external quantum efficiency (EQE) with size reduction. In this study, the impacts of chemical treatment on the sidewall condition and the EQE of the 10 μm InGaN-based blue and green micro-LEDs were demonstrated. The sidewall treatment can effectively alleviate the residual damage caused by dry etching and suppress the nonradiative recombination, facilitating the achievement of high internal quantum efficiency (IQE). Nevertheless, the sidewall light extraction of the micro-LEDs with smoother sidewall morphology after tetramethylammonium hydroxide (TMAH) treatment is lower than those with rough and prismatic sidewall surface. By utilizing microscopic hyperspectral imaging, the difference in chromatic properties between the inner region and the whole micro-LED caused by dry etching can hinder the stability of chromatic properties of InGaN-based blue micro-LEDs, which can be alleviated by sidewall treatment. The optimal duration for TMAH treatment is obtained as 10 min for both blue and green micro-LEDs, which is the consequence of the compromise between sidewall light extraction efficiency (LEE) and IQE, whereas the improvement of IQE by sidewall treatment for green micro-LEDs is less significant compared to that for blue ones, which can be attributed to the higher dislocation density and recombination via localized states of green InGaN micro-LEDs. This work demonstrates the evolution of IQE and LEE for blue and green micro-LEDs with different sidewall treatment strategies to achieving the ultimate EQE.

Comparative analysis of microstructure, electrical and optical performance in sidewall etching process for GaN-based green micro-LED

Micro-LEDs show the size-dependent external quantum efficiency (EQE) reduction problem, mainly owing to increased non-radiative recombination loss at the sidewall for smaller chip size. In this work, the evolution of microstructure, surface potential and optical performance of the green micro-LED sidewall was investigated comparatively after inductively coupled plasma (ICP) and tetramethylammonium hydroxide (TMAH) etching through transmission electron microscopy (TEM), Kelvin probe force microscope (KPFM), cathodoluminescence (CL) and time-resolved photoluminescence (TRPL). As confirmed by TEM and geometric phase analysis (GPA), ICP etching causes sidewalls to form atomically rough semi-polar surfaces and increases 25% compressive strain at the sidewall compared to the inside. TMAH solution introduces new sidewall defects due to excessive etching of three atomic layers of InGaN. Holes accumulate at the surface because of build-in electric field as showed by KPFM. The sidewall defects lead to a decrease in carrier lifetime resulting in uneven luminescence of micro-LED mesa. TMAH treatment removes the damaged layer and reduces the non-radiative recombination rate. ICP causes damage to the nanoscale structure, however the influence of sidewall defects on the carrier behavior is in the micron range due to unavoidable surface dangling bonds and surface lattice relaxation. A non-radiative recombination mechanism is proposed based on strain relaxation.

Enhancement of device performance in β-Ga2O3 Schottky barrier diodes with tetramethylammonium hydroxide treatment

Schottky barrier diodes were fabricated on tetramethylammonium hydroxide (TMAH)-treated β-Ga2O3 surfaces with Ni/Au as Schottky metal contacts. TMAH wet surface treatment efficiently reduced the roughness of the β-Ga2O3 surface. An X-ray photoelectron emission analysis of the TMAH-treated β-Ga2O3 surface chemical binding features indicates the effective removal of unwanted surface oxides and contaminants formed unintentionally by chemisorption during exposure to ambient conditions. The TMAH-treated Au/Ni/β-Ga2O3 Schottky diode displayed improved rectification behavior with an increment of barrier height by 0.30 eV compared to the untreated diode. The breakdown voltages (Vbr) for the untreated and TMAH-treated Au/Ni/β-Ga2O3 Schottky barrier diode are 298 and 358 V, respectively. The decrease in the turn-on resistance (Ron) in the Au/Ni/β-Ga2O3 Schottky barrier diode by TMAH treatment can be ascribed to the improved surface morphology. The improved electrical behavior of the Au/Ni/β-Ga2O3 Schottky barrier diode formed on the TMAH-treated β-Ga2O3 surface can be accredited to the oxygen vacancies reduction, that deceased the carrier concentration in the β-Ga2O3 epilayer and to the improved the surface morphology. These results demonstrate the effectiveness of TMAH wet surface treatment on β-Ga2O3 and its potential as a simple method for fabricating β-Ga2O3-based devices with improved performance.

Investigation of GaN-Based Micro-LEDs with Effective Tetramethylammonium Hydroxide Treatment

This study investigates the effects of TMAH treatment on 5 μm-sized GaN-based Micro LEDs. Compared with untreated GaN Micro LEDs, the optical output power and external quantum efficiency of TMAH treated Micro LEDs are significantly improved. These results can be attributed to the formation of microstructures on the sidewall of Micro LEDs through the TMAH treatment and the effective light reflection is therefore constructed. This research not only improves the characteristics of LEDs, but also paves the way for green and advanced optoelectronic devices.

Room-temperature synthesis of 2D-Ti3C2Tx nano-sheets by organic base treatment.

The growing demand for improved electrochemical performance in energy storage systems has stimulated research into advanced two-dimensional (2D) materials for electrodes. In this work, we obtain a layered MXene compound by exfoliating a titanium aluminum carbide precursor using tetramethylammonium hydroxide (TMAOH) ions in a full room temperature process followed by manual shaking. The hexagonal crystal structure and composition of the layered materials are characterized using different techniques. X-Ray diffraction shows the formation of 2D nano-sheets before and after the TMAOH treatment via its characteristic (002) diffraction peak, bringing to light an increase in the interlayer spacing after treatment. Scanning electron microscopy images confirm the layered morphology, whose composition is determined by energy dispersive x-ray analysis for the bulk material and by x-ray photoelectron spectroscopy for the surface of the obtained compounds. This study demonstrates a promising route to enhance delamination of this MXene 2D material in a low-cost room-temperature approach.

Impact of Sidewall Conditions on Internal Quantum Efficiency and Light Extraction Efficiency of Micro‐LEDs

AbstractThe sidewall condition is a key factor determining the performance of micro‐light emitting diodes (µLEDs). In this study, equilateral triangular III‐nitride blue µLEDs are prepared with exclusively m‐plane sidewall surfaces to confirm the impact of sidewall conditions. It is found that inductively coupled plasma‐reactive ion etching (ICP‐RIE) causes surface damages to the sidewall and results in rough surface morphology. As confirmed by time‐resolved photoluminescence (TRPL) and X‐ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminates the etching damage and flattens the sidewall surface. After ICP‐RIE, 100 µm2‐µLEDs yield higher external quantum efficiency (EQE) than 400 µm2‐µLEDs. However, after TMAH treatment, the peak EQE of 400 µm2‐µLEDs increases by ≈10% in the low current regime, whereas that of 100 µm2‐µLEDs slightly decreases by ≈3%. The EQE of the 100 µm2‐µLEDs decreases after TMAH treatment although the internal quantum efficiency (IQE) increases. Further, the IQE of the 100 µm2‐µLEDs before and after TMAH treatment is insignificant at temperatures below 150 K, above which it becomes considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non‐radiative recombination rate and light extraction.

P‐8.6: Elimination of Nanorods by Tetramethylammonium Hydroxide for the Fabrication of AlGaN‐based UV‐C Micro‐LED Array

Short‐wavelength ultraviolet (UV‐C) micro light‐emitting diode (Micro‐LED) array greatly benefits the optogenetic applications in neuroscience, optical communications, and maskless lithography. In this work, UV‐C Micro‐LED arrays with each pixel size of 10×10 μm2 were fabricated, and the nanorods introduced by inductively coupled plasma (ICP) etching were eliminated by post‐ICP tetramethylammonium hydroxide (TMAH) treatment. The pixel‐level electrical characteristics were analyzed, and the peak wavelength stability under variable injected current densities was demonstrated. The results indicated the potential of applying the TMAH treatment in the fabrication of the UV‐C Micro‐LED array.

Optical and microstructural characterization of Micro-LED with sidewall treatment

The traditional influence on the sidewall damage of Micro-LED was mostly focused on the research of I-V-L. In this paper, we discussed the influence of Micro-LED sidewall damage from the perspective of optical and microstructural characterization. Scanning electron microscopy showed that the Micro-LED structure with smaller size was more irregular after inductively coupled plasma etching. High-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy analysis showed that the area of the upper and lower regions of the quantum well was inconsistent, there was about 2 nm lattice disorder on the surface of the sidewall of the mesa, and oxygen and silicon impurity atoms were enriched. For optical characterization, a method combining laser scanning confocal microscopy and photoluminescence (PL) was proposed to evaluate the optical performance of the mesa. The results showed that the luminescence of Micro-LED mesa was uneven, the luminous intensity at the edge of the mesa was reduced by more than 65%, and the luminous wavelength was shifted by several nanometers. Finally, we optimized the sidewall treatment process, effectively improved the performance of Micro-LED devices by combining tetramethylammonium hydroxide treatment and SiO2 passivation, and increased the luminous intensity of Micro-LED 2 μm away from the edge by about 4.7 times and PL uniformity was greatly improved. These results provided an available reference for the development of Micro-LED.

P‐76: Elimination of Nanorods by Tetramethylammonium Hydroxide for the Fabrication of AlGaN‐based UV‐C Micro‐LED Array

Short‐wavelength ultraviolet (UV‐C) micro light‐emitting diode (Micro‐LED) array greatly benefits the optogenetic applications in neuroscience, optical communications, and maskless lithography. In this work, UV‐C Micro‐LED arrays with each pixel size of 10×10 μm2 were fabricated, and the nanorods introduced by inductively coupled plasma (ICP) etching were eliminated by post‐ICP tetramethylammonium hydroxide (TMAH) treatment. The pixel‐level electrical characteristics were analyzed, and the peak wavelength stability under variable injected current densities was demonstrated. The results indicated the potential of applying the TMAH treatment in the fabrication of the UV‐C Micro‐LED array.

Performance-enhanced NiO/β-Ga2O3 heterojunction diodes fabricated on an etched β-Ga2O3 surface

In this study, performance-enhanced NiO/β-Ga2O3 heterojunction diodes (HJDs) were realized on an etched β-Ga2O3 surface through applying a surface preparation process involving a BCl3/Cl2-based inductively coupled plasma (ICP) etch followed by a hot tetramethylammonium hydroxide (TMAH) treatment. Atomic force microscopy scan and X-ray photoelectron spectroscopy analysis revealed that the ICP etch process created a rough β-Ga2O3 surface with high density small peaks associated with Boron-related etch by-product, which resulted in a dramatic decrease of the HJDs’ breakdown voltage from 1.43 kV to 0.92 kV in average. On the other hand, the TMAH treatment effectively removed the etch-induced damages and contaminations from the β-Ga2O3 surface, therefore obviating its surface instability issue. Consequently, on the processed β-Ga2O3 surface high-quality NiO/β-Ga2O3 heterojunctions with a fully recovered breakdown voltage and an improved turn-on property were successfully formed.

High-performance normally-off recessed tri-gate GaN MIS-FETs in micrometer scale

In this study, a normally-off AlGaN/GaN metal–insulator–semiconductor field-effect transistor (MIS-FET) based on the combination of tri-gate and recessed MIS gate is fabricated and characterized. The recessed tri-gate MIS-FET is manufactured by micro-level trenches, defining the fin-shaped channel and improving the gate control capability. The recessed surface is cleaned by a diluted buffered oxide etch, HCl solution, and tetramethylammonium hydroxide treatment before a 20 nm Al2O3 deposition by atomic layer deposition. After deposition, post-deposition annealing was carried out. Recessed tri-gate MIS-FET demonstrates a high threshold voltage of 3.1 V, a high drain current of 1121 mA mm−1, and an on/off current ratio of 2 × 108. A smaller on-resistance of 5.4 Ω mm compared with recessed planar MIS-FET of 12.7 Ω mm is achieved. Besides, the devices show a low I–V hysteresis. All experimental results confirm micro-level trenches realize the advantages of the recessed tri-gate structures, which supports a promising technique to pursue the normally-off operation of GaN high electron mobility transistors.

Crystal-orientation-dependent flatband voltage of non-polar GaN MOS interfaces investigated using trench sidewall capacitors

We systematically characterized gallium nitride (GaN) metal-oxide semiconductor interfaces formed on trench sidewalls, paying particular attention to eliminating the parasitic capacitance caused by the trench bottom. The flatband voltage (Vfb) was found to be clearly higher on the m-face trench than on the a-face trench, and an interface state density of ∼1 × 1011 cm−2 eV−1 near the conduction band edge was achieved, irrespective of the trench direction. For trench sidewalls treated with a tetramethyl ammonium hydroxide (TMAH) solution, variation in Vfb among the trench direction was suppressed, reflecting that microscopic m-faces were generated after the TMAH treatment, irrespective of the direction of the trench. Therefore, the origin of the Vfb variation was attributed to the GaN crystal face dependence of fixed charge density at SiO2/GaN interfaces.

Achieving a Record Open‐Circuit Voltage for Organic/Si Hybrid Solar Cells by Improving Junction Quality

Nanostructured Si has attracted widespread attention due to its strong light‐harvesting ability, which is beneficial to improve the performance of solar cells. However, the poor contact of the organic/nanostructured Si interface and the large number of recombination centers on the Si surface result in an unsatisfactory open circuit voltage (VOC) and fill factor (FF) of the device. Herein, a simple and versatile method, Si nanowires modified with tetramethylammonium hydroxide (TMAH) and poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) doped with 3‐glycidoxypropyltrimethoxydsilane (GOPS), is proposed to improve the contact quality of the heterojunction. TMAH treatment adjusts the morphology of the Si nanowires and makes the surface smoother, effectively inhibiting surface recombination at the heterojunction interface. Moreover, the GOPS‐doped PEDOT:PSS solution can achieve conformal contact on the surface of nanostructured Si, which is more conducive to the extraction and separation of charges. As a result, a record VOC of 660 mV is obtained for PEDOT:PSS/nanostructured Si hybrid solar cells. In addition, the short circuit current density (JSC) is as high as 33.00 mA cm−2, and the FF is as high as 69.03%; therefore, the efficiency of the device reaches 15.01%. The results provide a simple and feasible strategy to improve the junction quality and performance of organic/Si solar cells.

AlGaN/GaN Superlattice‐Based p‐Type Field‐Effect Transistor with Tetramethylammonium Hydroxide Treatment

To realize the full spectrum of advantages that the III‐nitride materials system offers, the demonstration of p‐channel III‐nitride‐based devices is valuable. The first p‐type field‐effect transistor (pFET) based on an AlGaN/GaN superlattice (SL), grown using metal–organic chemical vapor deposition (MOCVD), is reported. Magnesium is used as the p‐type dopant. A sheet resistance of 11.6 kΩ □−1 and a contact resistance of 14.9 Ω mm are determined using transmission line measurements (TLMs) for Mg doping of 1.5 × 1019 cm−3. Mobilities in the range of 7–10 cm2 (V s)−1 and a total sheet charge density in the range of 1 × 1013–6 × 1013 cm−2 are measured using room temperature Hall effect measurements. Without tetramethylammonium hydroxide (TMAH) treatment, the fabricated pFETs have a maximum drain‐source current (IDS) of 3 mA mm−1 and an on‐resistance (RON) of 3.48 kΩ mm and do not turn off completely. With TMAH treatment during fabrication, a maximum IDS of 4.5 mA mm−1, RON of 2.2 kΩ mm, and five orders of current modulation are demonstrated.

Dual-Surface Modification of AlGaN/GaN HEMTs Using TMAH and Piranha Solutions for Enhancing Current and 1/f-Noise Characteristics

We demonstrated dual-surface modification of GaN/AlGaN/GaN high-electron mobility transistors using tetramethylammonium hydroxide (TMAH) and piranha solutions prior to gate metallization. The TMAH-treated device exhibits improved performances with lower I-V hysteresis, in off-state leakage current and gate leakage current. The device performances were further significantly improved with applies additional piranha solution treatment right after the TMAH treatment, especially in hysteresis and 1/f-noise characteristics. It is found that the Schottky barrier height is high and ideality factor is low measured from I-V characteristics for the TMAH and piranha solution treated device. Reasonable gate leakage mechanisms were also discussed using Poole–Frenkel and Schottky emissions. In addition, it is observed that the magnitude of interface state density for the TMAH treatment after the piranha solution treated device shows significantly low compared to other devices. These excellent device-performances are observed due to the reason of dual-surface treatment which effectively decreases the surface trap density with an appropriate etching and passivation of the device surface exposed prior to the gate metallization.

Capacitance-voltage characterization of Al2O3/GaN-on-insulator (GaNOI) structures with TMAH surface treatment

The capacitance-voltage (C-V) characterizations of Al2O3/GaN-on-insulator (GaNOI) structure, prepared with the Smart Cut™ technology, with and without tetramethylammonium hydroxide (TMAH) surface treatment have been investigated. The GaNOI structure consists of a 150nm-thick GaN layer and a 800μm-thick Si3N4/SiO2 buried insulating layer deposited on sapphire substrate. For fabrication of the MIS capacitor, an Al2O3 layer with thickness of 28nm as a gate dielectric was deposited on the GaNOI wafer by atomic layer deposition (ALD). The calculated carrier concentration of the GaN layer on the buried insulator was increased to 2.8×1017cm−3 from the value of ~5×1016cm−3 for the as-grown undoped GaN layer prior to the wafer transfer. The MIS capacitor with TMAH surface treatment showed a positive threshold voltage shift with negligible hysteresis and low interface trap density compared to the capacitor without TMAH surface treatment. Severe frequency dispersion was observed regardless of the TMAH treatment due to the crystal defects generated during the complicated wafer transfer process.

The effects of tetramethylammonium hydroxide treatment on the performance of recessed-gate AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated using a gate recess process and a surface treatment with tetramethylammonium hydroxide (TMAH) prior to gate metal deposition. Electrical characterizations show improved extrinsic transconductance and saturation current, as well as more uniform off-state behavior with reduced off-current by a factor of 3.5 and gate leakage current by a factor of 4.2 in the devices with TMAH treatment. The analyses based on atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the TMAH treatment effectively reduces the roughness of the recess-etched AlGaN surface and removes the native oxide layer on the AlGaN surface, suggesting a simple and viable route towards the fabrication of gate-recessed HEMTs based on AlGaN/GaN heterostructure with improved controllability and uniformity.

High efficiency conjugated polymer/Si hybrid solar cells with tetramethylammonium hydroxide treatment

Conjugated polymer/Si hybrid solar cells are fabricated based on a Si nanowire array (SiNW) substrate prepared by metal-assisted electroless etching.

Precise Structural Tuning of Porous GaN Using Two-Step Anisotropic Etching for Optical and Photo-Electric Applications

Gallium nitride (GaN) and its alloys are attractive materials for various functional devices, such as light-emitting diodes, laser diodes, solar cells, and so on. One promising approach for improving device performance is forming the nanostructures with unique electrical and optical properties. GaN nanostructures have been commonly formed using selective-area growth or dry etching processes, however, there are severe limitations on increasing the density of nanostructures. And when a dry etching process is involved, the etching damage induced by ion bombardment is not negligible and could significantly degrade the device performance. One alternative approach is an anodic formation of porous structures utilizing low-energy electrochemical (EC) reactions. This process has a big advantage for high-density formation over the conventional lithography process. However, the control of structural dimension such as a pore diameter and depth has not been achieved adequately for the device applications. In this study, we aimed to improve structural controllability of GaN porous structures by combining EC process with conventional chemical etching process. We have developed the two-step etching process for a GaN (0001) substrate. EC etching was first performed in the dark condition using a standard electrochemical cell with three electrodes. Wet chemical etching was subsequently conducted in 25 % tetramethylammonium hydroxide (TMAH) at 90°C. In order to clarify the structural controllability, the observation using scanning electron microscopy (SEM) was conducted on the porous samples formed by changing the applied EC voltage, EC etching time and TMAH etching time. The optical properties were evaluated by photo-reflectance measurement and photo-electrochemical measurement. Figures 1(a) and (b) show the top and cross-sectional SEM images of the GaN porous structure formed by the first EC etching. Straight pores oriented perpendicular to the top surface was observed. Pore depth linearly increased with EC etching time, indicating that superior depth control was achieved. This behavior can be explained as resulting from anisotropy of the EC etching. Namely, the etching reaction proceeded only to [000-1] direction belong to the high-electric field induced by the anodic voltage. On the other hand, the pore diameter kept a constant value throughout the EC etching and could not be controlled by the EC etching time. Figures 1(c) and (d) show the top and cross-sectional SEM images of the GaN porous structure formed by the two-step etching process with a TMAH treatment. The pore shape appeared at the surface drastically changed from circular shape to hexagonal shape. In addition, the pore diameter estimated from SEM observation increased linearly with the TMAH etching time, whereas the pore depth was unchanged. This result denotes that the etching rate for a (1-100) plane is lower than that for other crystal planes. From these results, we can conclude that the pore depth and diameter were independently controlled by the EC etching time and the TMAH etching time, respectively. We investigated the correlation between the structural properties and the optical properties of the porous structures formed by the two-step etching process. The photo-reflectance of GaN surface decreased after the formation of porous structures. In addition, the reflectance spectra showed oscillation behavior as function of light wavelength, where the oscillation periods were changed with TMAH etching time. The oscillation of photo-reflectance can be explained by the interference of two kinds of light that was reflected at air/porous GaN interface and porous/bulk GaN interface. The results obtained here indicate that the refractive index was varied with the structural features such as a pore diameter that were changed by the TMAH etching time. The photo-electrochemical measurements were conducted on two kinds of porous samples formed by the first EC etching and the two-step etching processes. Figure 2 compared the photocurrents of various samples obtained under the UV light with a wavelength of 350 nm and an intensity of 0.1 mW/cm2. The photocurrents of both porous samples increased as compared with those of planar sample, and especially the large increase by 180% was obtained by the two-step etching. From the systematic measurements, it was found that the increase and decrease of photocurrents were sensitively affected by the change of the pore diameter. These strongly suggest that the efficiency of the photo-electric conversion was determined by such factors as the intensity of the electric filed induced at the GaN/electrolyte interface and the width of the depletion layer formed at the pore wall. In conclusion, the two-step etching process is very powerful technique to control the optical properties of GaN porous structures on the basis of the precise structural tuning. Figure 1