ECS Solid State Letters

Wafer geometry and residual stress go through significant changes at different points in the semiconductor manufacturing process flow. Precise wafer geometry measurement is very important to assess process induced wafer geometry change (PIWGC) and minimize pattern overlay in lithography steps of advanced node devices and 3-D (3-dimensional) packaged devices. However, the precise wafer geometry measurement is very difficult due to gravitational wafer sag and interaction between the anisotropy of mechanical properties of Si and wafer supporting configurations. Effects of anisotropy and supporting configuration on 300 mm Si (001) wafer profile measurements were investigated for pattern overlay estimation and process optimization.

We investigate the hysteresis in the transfer characteristic of amorphous indium-gallium-zinc-oxide thin-film transistor by controlling the sweep waveform of the gate voltage (Vg) provided by parameter measure unit. It is conventionally studied by double sweeping Vg with the default setup of the source measure units, which speed may vary with the current level. By manipulating the step time of sweeping Vg, we found that the response time of charge traps or donor-like states is in the range that overlaps with the conventional time step in the measurement and must be considered.

The switching uniformity and reliability of the TaO_x based resistive random access memory (RRAM) device were investigated with varying nitrogen doping concentration. The nitrogen doped samples shows excellent electrical and reliability characteristics such as small switching variability for 3-bit multilevel per cell (MLC), low power operation and good retention properties. Compared with control sample, improved device characteristics of nitrogen doped device can be explained by nitrogen induced filament confinement.

Room temperature ferroelectricity in pulsed laser deposited rare-earth doped hafnium oxide (HfO₂) thin films is discussed. Maximum values of remnant polarizations (P_r) ∼13.5 and 12 μC/cm² along with coercive fields (E_C) ∼334 and 384 kV/cm are observed in 6 mol. % of rare-earth (Sm or Gd) doped-HfO₂ thin films (Sm:HfO₂ and Gd:HfO₂), respectively. Piezoresponse force microscopy measurements confirmed ferroelectric nature of films by showing phase hysteresis and butterfly amplitude loops. It is noticed that wake-up cycles improved the remnant polarization and found to be necessary for the forming of well saturated hysteresis loops. Our results showed potential toward realization of future highly scaled non-volatile ferroelectric memories.

Uniform and crack-free cubic Ga₂S₃ thin films have been epitaxially synthesized on n-GaAs (111) substrates by sulfurization. Atomic-force microscopy revealed that the Ga₂S₃ surface is dominated by nanoparticles of smaller than 50 nm in diameter. The nanoparticles, clustered into regular triangle structures that hierarchically packaged on GaAs, significantly reduced the reflectance of GaAs. Low-temperature photoluminescence revealed typical acceptor-like defects while X-ray photoemission spectroscopy revealed type-I heterojunction with a valence-band offset of 0.6 eV for the Ga₂S₃/GaAs heterostructure. Photoelectrochemical properties of the Ga₂S₃/n-GaAs (111) heterojunction are studied and compared with those of bare n-GaAs (111) substrate in a typical three-electrode setup.

The formation of edge bonding voids during hydrophilic direct wafer bonding is investigated. These defects are linked to the bonding wave propagation and to fluid dynamics between the wafers. Fluid mechanics modeling shows that a gas pressure drop occurs at the wafer edge described by a Joule-Thomson expansion. This adiabatic process results in a gas temperature change which can lead to the condensation of small water droplets close to the wafer edge. Therefore, controlling the gas atmosphere during the bonding process is needed to avoid such bonding defects. Several solutions are proposed to avoid the formation of edge bonding voids.

We demonstrate a wet etching method to reduce the thickness of thin germanium-on-insulator (GOI) films using a dilute solution (a NH₄OH:H₂O₂:H₂O 2:1:4000 mixture) at a low temperature (5°C). The etch rate and thickness uniformity were well controlled. The root mean square roughness after wet etching was less than 0.5 nm and did not degrade compared with the original sample. Finally, back gate junctionless transistors were fabricated using the GOI wafers with 15-nm-thick Ge films, thinned by the developed method. The transistors had good I_on/I_off ratio and mobility qualities, indicating that the wet etching process effectively thinned the Ge films.

In this work an effective diffusion suppression of low energy – high dose (11 keV/10¹⁵ cm⁻²) phosphorous in 10 keV/5 × 10¹⁴cm^{− 2} N₂⁺-implanted p-germanium is demonstrated. Diffusion of both species after isochronal 30 sec RTA annealing at 550–650°C has been studied by SIMS in the presence of Al₂O₃ as surface capping layer. Nitrogen shows anomalous non-Fickian diffusion towards the Al₂O₃/Ge interface, while phosphorous dose loss and diffusion to the bulk are strongly suppressed in comparison to Ge substrate without nitrogen implantation. Possible physical mechanisms (involving phosphorous-vacancies or phosphorous-nitrogen complexes, and end-of-range interstitials) that explain this result are presented and discussed.

Three-dimensional network structures with interconnected micro-channels were formed through the carbonization of three different varieties of wood. Performance of these carbonized woods was tested for their application as supercapacitor electrodes. From charge-discharge cycling in a KOH electrolyte solution, a maximum energy density of ∼45.6 Wh/kg (discharge current of 200 mA/g) and a maximum power density of ∼2000 W/kg (discharge current of 4000 mA/g) were obtained. The carbonized wood electrodes exhibited excellent cyclability, with 99.7% of the specific capacitance being retained after 2000 cycles. These remarkable results demonstrate the exciting potential for carbonized wood materials as inexpensive, high performance supercapacitor electrodes.

The performance of bottom-gate ZnO thin film transistors (TFTs) using MgO gate dielectrics evaporated with and without introducing oxygen have been investigated. The oxygen introduced during MgO deposition improves the field-effect mobility significantly as compared to the device without introducing oxygen during MgO deposition. The oxygen-introduced MgO exhibits a dielectric constant of 10.9 and the field-effect mobility of the TFT device is enhanced to 78.3 cm²/V s. The threshold voltages can also be related to whether the oxygen is introduced into MgO or not. The interface between oxygen-introduced MgO and ZnO is examined and its connection with mobility enhancement is discussed.

Thin films of Li₇La₃Zr₂O₁₂ were deposited on SrTiO₃ (100) and Sapphire (0001) substrates at room-temperature using a pulsed-laser-deposition technique. Detailed structural, compositional, optical, and electrochemical characterizations of the films were performed. The films deposited at room-temperature had amorphous structure, and exhibited a lithium-ion conductivity of 3.35 × 10⁻⁷ S/cm. The effects of thermal annealing and pulsed laser annealing on the properties of the films were investigated. Pulsed laser annealed films were found to have a superior lithium-ion conductivity value of 7.36 × 10⁻⁷ S/cm. Moreover, the Li₇La₃Zr₂O₁₂ films were found to be electrochemically stable against lithium metal.

Amorphous In-Ga-Zn-O (a-IGZO) based metal-semiconductor field-effect transistors (MESFETs) were fabricated at 200°C. A bottom Pt structure was employed to form a good Schottky gate, and channel thickness was optimized to 230 nm to obtain high on-currents I_on > 10⁻⁶ A, on-to-off current ratios > 10⁶ and low subthreshold voltage swing of 129 mV/decade. The field-effect mobility (μ_FE) was as small as 0.5 cm²(Vs)⁻¹, which is explained quantitatively by the subgap trap density in the 200°C-annealed a-IGZO. The present result suggests that a double-layered channel with different donor densities will be effective to improve I_on and μ_FE of a-IGZO MESFETs.

Vacuum-annealed semiconducting single walled nanotubes are n-type, but become p-type when exposed to ambient air. Here we present evidence that this effect arises from pinning of the Fermi energy by the four-electron oxygen redox couple in an adsorbed water film, a type of surface transfer doping. The pronounced electrical sensitivity to O₂, O₃, NO₂, SO₂, NH₃ and HNO₃ vapors results from electron exchange between the redox couple and the nanotube. This effect must be considered when using nanotubes for any application in humid air. We also suggest that changes in the electrical properties of graphene and multi-walled nanotubes can be explained by this phenomenon.

Single crystalline Sb₂S₃ nanorods were fabricated through a soft chemical route without the assistance of any templates or surfactants. The as-prepared samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). TEM results indicate the Sb₂S₃ nanorods grow along with [001] direction and were composed of orthorhombic structure. The magnetic properties of Sb₂S₃ nanorods were studied. In addition, Sb₂S₃ nanorods were evaluated as electrode materials in lithium secondary batteries, and the first discharge capacity reached about 850 mA h g⁻¹.

Tin oxide thin films are of great interest for device applications, but in spite of their good optical transparency and outstanding semiconductor properties, stable p-type tin oxide needs to be developed. In this study both p-type SnO and n-type SnO₂ thin films were deposited by reactive rf magnetron sputtering using Sn target in Ar and O₂ gas mixture. The structural, electrical, and optical properties of both p-type SnO and n-type SnO₂ thin films were analysed. The transparent homo-junction pn diode was also fabricated and it showed a fairly good rectification behavior with a turn-on voltage of 2.3 V.

A processing failure of void formation has been observed in 3D IC microbumps due to small solder volume. We prepared the sandwiched Ni/Sn2.3Ag/Ni microbumps with 4 μm and 11 μm thick solders and reflowed them at 260°C to study the mechanism of void formation in the processing. Due to the thin solder, intermetallic compound formation of Ni₃Sn₄ from the two interfaces of the solder joint can physically bridge each other. When that happens, the degree of freedom of motion in the direction normal to the interfaces is removed. Consequently, when the remaining molten solder is drained by side wall reaction, large voids form in the joint. This is a unique mode of processing failure because of the smaller and smaller volume of solder joints in the trend of miniaturization.

Two-dimensional materials like graphene have great potential to excess the mobility limit of conventional silicon devices. Particularly, MoS₂ FETs have high mobility and low off-current simultaneously due to their sizable band-gap. However, large hysteresis in its transfer curve is an obstacle for implementation to logic circuits and switching devices. Here, we report on a multi-layer MoS₂ FET using atomic layer deposition Al₂O₃ as a gate insulator, showing small hysteresis of 0.86 V. It is thought that, such improvement in the hysteresis attributes to the small trap at the MoS₂/Al₂O₃ interface, and it is confirmed through a constant current stress test.

Three-dimensional network structures with interconnected micro-channels were formed through the carbonization of three different varieties of wood. Performance of these carbonized woods was tested for their application as supercapacitor electrodes. From charge-discharge cycling in a KOH electrolyte solution, a maximum energy density of ∼45.6 Wh/kg (discharge current of 200 mA/g) and a maximum power density of ∼2000 W/kg (discharge current of 4000 mA/g) were obtained. The carbonized wood electrodes exhibited excellent cyclability, with 99.7% of the specific capacitance being retained after 2000 cycles. These remarkable results demonstrate the exciting potential for carbonized wood materials as inexpensive, high performance supercapacitor electrodes.

The performance of bottom-gate ZnO thin film transistors (TFTs) using MgO gate dielectrics evaporated with and without introducing oxygen have been investigated. The oxygen introduced during MgO deposition improves the field-effect mobility significantly as compared to the device without introducing oxygen during MgO deposition. The oxygen-introduced MgO exhibits a dielectric constant of 10.9 and the field-effect mobility of the TFT device is enhanced to 78.3 cm²/V s. The threshold voltages can also be related to whether the oxygen is introduced into MgO or not. The interface between oxygen-introduced MgO and ZnO is examined and its connection with mobility enhancement is discussed.

The charge balance mechanism in green fluorescent organic light-emitting diodes is investigated for different electron transport layers (ETLs) and electron mobilities. Carrier accumulation and an increase in the exciton recombination probability are shown to be critical for improving the current and power efficiencies by aligning the bands at the interface between the emitting layer (EML) and ETL. The peak in the electroluminescence (EL) spectra was found to shift slightly in response to changes in the width of the emission zone and reflected the electron mobility of the ETL. Higher electron mobility resulted in a wider recombination zone in the EML that was manifested by a blue-shift of the EL peak.