ZrO2 insulator modified by a thin Al2O3 film to enhance the performance of InGaZnO thin-film transistor

A high-performance InGaZnO (IGZO) thin-film transistor (TFT) with ZrO₂–Al₂O₃ bilayer gate insulator is fabricated. Compared to IGZO-TFT with ZrO₂ single gate insulator, its electrical characteristics are significantly improved, specifically, enhancement of I_on/I_off ratios by one order of magnitude, increase of the field-effect mobility (from 9.8 to 14 cm²/Vs), reduction of the subthreshold swing from 0.46 to 0.33 V/dec, the maximum density of surface states at the channel-insulator interface decreased from 4.3 × 10¹² to 2.5 × 10¹² cm⁻². The performance enhancements are attributed to the suppression of leakage current, smoother surface morphology, and suppression of charge trapping by using Al₂O₃ films to modify the high-k ZrO₂ dielectric.     

Temperature-dependent resistive switching characteristics for Au/n-type CuAlOx/heavily doped p-type Si devices

Bipolar switching phenomenon is found for Au/n-type CuAlO_x/heavily doped p-type Si devices at temperatures above 220 K. For high or low resistive states (HRS or LRS), the electrical resistance is decreased with increasing temperature, indicating a semiconducting behavior. Carrier transport at LRS or HRS is dominated by hopping conduction. It is reasonable to conclude that the transition from HRS to LRS due to the migration of oxygen vacancies (V_O) is associated with electron hopping mediated through the V_O trap sites. The disappearance of the resistive switching behavior below 220 K is attributed to the immobile V_O traps. The deep understanding of conduction mechanism could help to control the device performance.     

Enhanced charge storage characteristics by ZrO2 nanocrystallites precipitated from amorphous (ZrO2)0.8(SiO2)0.2 charge trapping layer

Charge trapping memory capacitors using (ZrO₂)_0.8(SiO₂)_0.2 film as charge trapping layer and amorphous Al₂O₃ as the tunneling layer and blocking layer were fabricated for nonvolatile semiconductor memory application. The ZrO₂ nanocrystallites with a size of 3–5 nm precipitated from amorphous (ZrO₂)_0.8(SiO₂)_0.2 during rapid thermal annealing at 800 °C can serve as the storage nodes, with which a large hysteresis memory window of 7.5 V at a sweeping gate voltage of 8 V has been achieved. At 150 °C bake temperature, the memory capacitor exhibited an excellent endurance up to 10⁵ write/erase cycles, after which a small charge loss of about 12% was achieved.     

Fabrication and electrical characterization of Al/p-ZnIn2Se4 thin film Schottky diode structure

Polycrystalline thin films of ternary ZnIn₂Se₄ compound with p-type conductivity were deposited on a pre-deposited aluminium (Al) film by a flash evaporation technique. A Schottky diode comprising of Al/p-ZnIn₂Se₄ structure was fabricated and characterized in the temperature range 303–323 K in dark condition. The Schottky diode was subjected to current (I)-voltage (V) and capacitance (C)-voltage (V) characterization. The Al/p-ZnIn₂Se₄ Schottky diode showed behaviour typical of a p-n junction diode. The devices showed very good diode behaviour with the rectification ratio of about 10⁵ at 1.0 V in dark. The Schottky diode ideality factor, barrier height, carrier concentration, etc. were derived from I-V and C-V measurements. At lower applied voltages (V≤0.5 V), the electrical conduction was found to take place by thermionic emission (TE) whereas at higher voltages (V>0.5 V), a space charge limited conduction mechanism (SCLC) was observed. An energy band diagram was constructed for fabricated Al/p-ZnIn₂Se₄ Schottky diode.     

Characteristics of RF reactive sputter-deposited Pt/SiO2/n-InGaN MOS Schottky diodes

All RF sputtering-deposited Pt/SiO₂/n-type indium gallium nitride (n-InGaN) metal–oxide–semiconductor (MOS) diodes were investigated before and after annealing at 400 °C. By scanning electron microscopy (SEM), the thickness of Pt, SiO_2, n-InGaN layer was measured to be ~250, 70, and 800 nm, respectively. AFM results also show that the grains become a little bigger after annealing, the surface topography of the as-deposited film was smoother with the rms roughness of 1.67 nm and had the slight increase of 1.92 nm for annealed sample. Electrical properties of MOS diodes have been determined by using the current–voltage (I–V) and capacitance–voltage (C–V) measurements. The results showed that Schottky barrier height (SBH) increased slightly to 0.69 eV (I–V) and 0.82 eV (C–V) after annealing at 400 °C for 15 min in N₂ ambient, compared to that of 0.67 eV (I–V) and 0.79 eV (C–V) for the as-deposited sample. There was the considerable improvement in the leakage current, dropped from 6.5×10⁻⁷ A for the as-deposited to 1.4×10⁻⁷ A for the 400 °C-annealed one. The annealed MOS Schottky diode had shown the higher SBH, lower leakage current, smaller ideality factor (n), and denser microstructure. In addition to the SBH, n, and series resistance (R_s) determined by Cheungs׳ and Norde methods, other parameters for MOS diodes tested at room temperature were also calculated by C–V measurement.     

Compositional dependence of work function and Fermi level position of the HfNx/SiO2 system

In situ Kelvin Probe (KP) measurements performed on HfN_x showed an increase of the vacuum work function as a function of N content from a value of 3.9 eV (silicon conduction band edge) for pure Hf to a value of 5 eV (silicon valence band edge) for x ～ 2. In contrast, capacitance–voltage (C–V) measurements showed that the effective work function increased only until x < 1 and saturated around a value of 4.6 eV (silicon midgap). This behavior is attributed to Fermi level pinning, which is probably due to oxidation of the HfN_x during the reactive sputtering deposition step.     

Effect of Al content on the performance of Cu(In,Al)Se2 powders prepared by mechanochemical process

Cu(In,Al)Se₂ (CIAS) powders with varying Al/(In+Al) ratios x (x=0, 0.15, 0.23, 0.38, 0.57) were synthesized by mechanochemical process (MCP). Composition, morphology of CIAS powders was investigated by energy dispersive analysis and scanning electron microscopy, respectively. Influence of Al content on structural and optical properties of CIAS was detected by X-ray diffraction and UV–Vis spectroscopy, respectively. The results show molecularity and stoichiometry deviation of composition were small. Particle aggregation and inhomogeneous distribution of Cu, In, Al and Se element is observed at early milling stage. Field emission transmission electron microscopy image confirms that the particles can be milled to several hundred nanometers. Chalcopyrite characteristic peaks of CuInSe₂ are clearly observed. CIAS crystalline size decreases as Al₂O₃ increases in the products. CIAS (1 1 2) peaks shifts to high value and lattice constants for CIAS samples decrease with the Al additions. The band gap enlarged from 1.01 eV to 1.29 eV by adding Al. Al₂O₃ impurities impact CIAS powder grain size and the band gap seriously. All CIAS samples detected by hot probe method show the p-type conductivity.     

On the current conduction mechanisms of CeO2/La2O3 stacked gate dielectric

It was found that the electrical properties of CeO₂/La₂O₃ stack are much better than a single layer La₂O₃ film. A thin CeO₂ capping layer can effectively suppress the oxygen vacancy formation in the La₂O₃ film. This work further investigates the current conduction mechanisms of the CeO₂ (1 nm thick)/La₂O₃ (4 nm thick) stack. Results show that this thin stacked dielectric film still has a large leakage current density; the typical 1−V leakage can exceed 1 mA/cm² at room temperature. The large leakage current should be due to both the oxide defect centers as well as the film structure. Results show that at low electric field (<0.2 MV/cm), the thermionic emission induced current conduction in this stacked structure is quite pronounced as a result of interface barrier lowering due to the capping CeO₂ film which has a higher k value than that of the La₂O₃ film. At higher electric fields, the current conduction is governed by Poole–Frenkel (PF) emission via defect centers with an effective energy level of 0.119 eV. The temperature dependent current–voltage characteristics further indicate that the dielectric defects may be regenerated as a result of the change of the thermal equilibrium of the redox reaction in CeO₂ film at high temperature and the drift of oxygen under the applied electric field.     

Ultra-low-energy ion-beam-synthesis of Ge nanocrystals in thin ALD Al2O3 layers for memory applications

Structural and electrical properties of ALD-grown 5 and 7 nm-thick Al₂O₃ layers before and after implantation of Ge ions (1 keV, 0.5–1 × 10¹⁶ cm^?2) and thermal annealing at temperatures in the 700–1050 °C range are reported. Transmission Electron Microscopy reveals the development of a 1 nm-thick SiO₂-rich layer at the Al₂O₃/Si substrate interface as well as the formation of Ge nanocrystals with a mean diameter of ～5 nm inside the implanted Al₂O₃ layers after annealing at 800 °C for 20 min. Electrical measurements performed on metal–insulator–semiconductor capacitors using Ge-implanted and annealed Al₂O₃ layers reveal charge storage at low-electric fields mainly due to location of the Ge nanocrystals at a tunnelling distance from the substrate and their spatial dispersion inside the Al₂O₃ layers.     

Mechanism of mobility enhancement in Ge p-channel metal-oxide-semiconductor field-effect transistor due to introduction of Al atoms into SiO2/GeO2 gate stack

In this paper, we present comprehensive results on Al-postmetallization annealing (Al-PMA) effect for the SiO₂/GeO₂ gate stack on a Ge substrate, which were fabricated by a physical vapor deposition method. The effective oxide thickness of metal-oxide-semiconductor (MOS) capacitor (CAP) was ~7 nm, and the Al-PMA was performed at a temperature in the range of 300–400 °C. The flat band voltage (V_FB), the hysteresis (HT), the interfacial states density (D_it), and the border traps density (D_bt) for MOSCAPs were characterized by a capacitance–voltage method and a constant-temperature deep-level transient spectroscopy method. The MOSCAP without Al-PMA had an electrical dipole of ~−0.8 eV at a SiO₂/GeO₂ interface, which was disappeared after Al-PMA at 300 °C. The HT, D_it, and D_bt were decreased after Al-PMA at 300 °C and were maintained in the temperature range of 300–400 °C. On the other hand, the V_FB was monotonically shifted in the positive direction with an increase in PMA temperature, suggesting the generation of negatively charged atoms. Structural analyses for MOSCAPs without and with Al-PMA were performed by a time-of-flight secondary ion mass spectroscopy method and an X-ray photoelectron spectroscopy method. It was confirmed that Al atoms diffused from an Al electrode to a SiO₂ film and reacted with GeO₂. The dipole disappearance after Al-PMA at 300 °C is likely to be associated with the structural change at the SiO₂/GeO₂ interface. We also present the device performances of Al-gated p-channel MOS field-effect transistors (FET) with PMA treatments, which were fabricated using PtGe/Ge contacts as source/drain. The peak field-effect mobility (μ_h) of the p-MOSFET was reached a value of 468 cm²/Vs after Al-PMA at 325 °C. The μ_h enhancement was explained by a decrease in the total charge densities at/near the GeO₂/Ge interface.     

Frequency and voltage dependency of interface states and series resistance in Al/SiO2/p-Si MOS structure

The capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics of Al/SiO₂/p-Si metal-oxide-semiconductor (MOS) Schottky diodes have been measured in the voltage range from ?3 to +3 V and frequency range from 5 KHz to 1 MHz at room temperature. It is found that both C and G/ω of the MOS capacitor are very sensitive to frequency. The fairly large frequency dispersion of C–V and G/ω–V characteristics can be interpreted in terms of the particular distribution of interface states at SiO₂/Si interface and the effect of series resistance. At relatively low frequencies, the interface states can follow an alternating current (AC) signal that contributes to excess capacitance and conductance. This leads to an anomalous peak of C–V curve in the depletion and accumulation regions. In addition, a peak at approximately ?0.2 V appears in the R_s–V profiles at low frequency. The peak values of the capacitance and conductance decrease with increasing frequency. The density distribution profile of interface state density (N_ss) obtained from C_HF–C_LF capacitance measurement also shows a peak in the depletion region.     

Photophysical properties of nano Si/SiOx composites in Al/composite/mono Si structures for green light emitting and photodetector-Schottky diodes

The concept of the self-formation of a nanocrystallite (nc-) Si/SiO_x : Si_zO_yAl nanocomposite at the Al/oxidized porous silicon interface in the result of solid-phase processes between Al and oxidized porous Si (PS) and the influence of its composition on photophysical properties were developed and experimentally confirmed for the Si chip with optical intra-chip interconnect consisting of light emitting and photodetector diodes and alumina waveguide on oxidized PS surface with aluminum electrodes. The peculiarities of nanocomposite photophysical properties (the refractive index, photoluminiscence (PL) peak situation, PL spectrum shape in the green range) have been shown to be due to the quantum confinement effects (revealed by XPS, Raman spectroscopy) and depend on the Al presence in the nanocomposite (obtained by XPS, IR spectroscopy). The experimental confirmation of this concept is (i) the shift of the nc-Si valence band relatively to that of monocrystalline Si (c-Si) on 0.2–0.7 eV for nc-Si size in 2.5–6.5 nm range; (ii) the decrease of Si nanocrystallite size in the Al presence; (iii) the approach of the value of the refractive index of nc-Si : SiO : Si₂O₃ : Si_zO_yAl nanocomposite at λ=236 nm to that of porous Si with 45% porosity and (iv) the stable green PL spectra in the Si_zO_yAl presence in the nanocomposite.     

Memory characteristics of Al2O3/La2O3/Al2O3 multi-layer films with various blocking and tunnel oxide thicknesses

In order to investigate charge trap characteristics with various thicknesses of blocking and tunnel oxide for application to non-volatile memory devices, we fabricated 5 and 15 nm Al₂O₃/5 nm La₂O₃/5 nm Al₂O₃ and 15 nm Al₂O₃/5 nm La₂O₃/5, 7.5, and 10 nm Al₂O₃ multi-stack films, respectively. The optimized structure was 15 nm Al₂O₃ blocking oxide/5 nm La₂O₃ trap layer/5 nm Al₂O₃ tunnel oxide film. The maximum memory window of this film of about 1.12 V was observed at 11 V for 10 ms in program mode and at ?13 V for 100 ms in erase mode. At these program/erase conditions, the threshold voltage of the 15 nm Al₂O₃/5 nm La₂O₃/5 nm Al₂O₃ film did not change for up to about 10⁴ cycles. Although the value of the memory window in this structure was not large, it is thought that a memory window of 1.12 V is acceptable in the flash memory devices due to a recently improved sense amplifier.     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

Influence of processing and annealing steps on electrical properties of InAlN/GaN high electron mobility transistor with Al2O3 gate insulation and passivation

We report on preparation and electrical characterization of InAlN/AlN/GaN metal–oxide–semiconductor high electron mobility transistors (MOS HEMTs) with Al₂O₃ gate insulation and surface passivation. About 12 nm thin high-κ dielectric film was deposited by MOCVD. Before and after the dielectric deposition, the samples were treated by different processing steps. We monitored and analyzed the steps by sequential device testing. It was found that both intentional (ex situ) and unintentional (in situ before Al₂O₃ growth) InAlN surface oxidation increases the channel sheet resistance and causes a current collapse. Post deposition annealing decreases the sheet resistance of the MOS HEMT devices and effectively suppresses the current collapse. Transistors dimensions were source-to-drain distance 8 μm and gate width 2 μm. A maximum transconductance of 110 mS/mm, a drain current of ～0.6 A/mm (V_GS = 1 V) and a gate leakage current reduction from 4 to 6 orders of magnitude compared to Schottky barrier (SB) HEMTs was achieved for MOS HEMT with 1 h annealing at 700 °C in forming gas ambient. Moreover, InAlN/GaN MOS HEMTs with deposited Al₂O₃ dielectric film were found highly thermally stable by resisting 5 h 700 °C annealing.     

Use of Al2O3 as inter-poly dielectric in a production proven 130 nm embedded Flash technology

We have successfully integrated 2 Mb arrays with SiO₂/Al₂O₃ stacks as inter-poly dielectric (IPD) fabricated in a proven 130 nm embedded Flash technology. Gate stack write/erase high voltages (HV) can be reduced by 3 V. Write/erase distributions show evidence of bit pinning which can be explained by barrier lowering along Al₂O₃ grain boundaries. Reliability assessment of the 2 Mb array reveals promising data retention and cycle endurance, indicating the absence of charge trapping in the high-k IPD. Despite several integration issues, these results demonstrate the high potential of Al₂O₃ IPDs in embedded Flash technologies.     

Electronic structure of highly crystalline polyaniline by study of tunneling conduction in n+-Si/self-assembled monolayer/polyaniline heterostructures

Highly crystalline polyaniline (PANI) films were deposited on degenerated silicon (n⁺-Si) substrates covered with its native oxide (SiO₂), surface modified with amino-silane self-assembled monolayers (SAM). Scanning electron microscopy studies reveal formation of single crystal domains scattered all over the surface of film. Height and current images obtained using current-sensing AFM (C-AFM) exhibit pyramidal topography of crystallites, and inhomogeneous conductivity. As the native oxide and SAM acts as a very thin insulating layer (<2 nm) between the metal-like substrate (degenerated Si) and the PANI film, it forms n⁺-Si/SiO₂/SAM/PANI metal-insulator-semiconductor (MIS) heterostructure. C-AFM probe was used for I–V measurements on the MIS structures and study the tunneling conduction across it. The conductance spectra derived from I–V characteristics corroborates well with the polaron-lattice band structure of doped PANI with presence of polaron bands between the HOMO-LUMO energy gap. These polaron bands are well-resolved from our C-AFM measurements and they are located about 0.25 eV below the LUMO and above the HOMO.     

Photo-electrochemical characterization of porous material Fe-FSM-16. Application for hydrogen production

We describe in the present work the photo-electrochemical characterization of iron/folded-sheets mesoporous materials (Fe-FSM-16, Si/Fe=60) synthesized by microwave-assisted hydrothermal (M-H) method and its application for the hydrogen evolution upon visible light. The mesoporous catalyst consists of small Fe₂O₃ particles (～2 nm) spread on SiO₂ with specific surface area of ～800 m² g^?1. The capacitance measurements reveal an iron deficiency and the oxide exhibit p type conductivity with activation energy of 0.07 eV. The optical gap of the hematite (α-Fe₂O₃) is evaluated at 3.24 eV from the diffuse reflectance spectrum. The flat band potential V_fb (?0.54 V_SCE) and the holes density N_D (9.56×10¹⁴ cm^?3) of the hematite are obtained respectively by extrapolating the linear part to C^?2=0 and the slope of the Mott Schottky plot. At pH=7, the conduction band (?0.47 V_SCE) is suitably positioned with respect to the H₂O/H₂ level (?0.59 V_SCE) leading to a spontaneous water reduction. The oxide is stabilized by hole consumption involving SO₃^2? and S₂O₃^2? species and spectacular improvement of the hydrogen evolution is reported with evolution rates of ～461 and 163 μ mol respectively.     

IGZO thin film transistors with Al2O3 gate insulators fabricated at different temperatures

Thin film transistors (TFTs) with bottom gate and staggered electrodes using atomic layer deposited Al₂O₃ as gate insulator and radio frequency sputtered In–Ga–Zn Oxide (IGZO) as channel layer are fabricated in this work. The performances of IGZO TFTs with different deposition temperature of Al₂O₃ are investigated and compared. The experiment results show that the Al₂O₃ deposition temperature play an important role in the field effect mobility, I_on/I_off ratio, sub-threshold swing and bias stability of the devices. The TFT with a 250 °C Al₂O₃ gate insulator shows the best performance; specifically, field effect mobility of 6.3 cm²/Vs, threshold voltage of 5.1 V, I_on/I_off ratio of 4×10⁷, and sub-threshold swing of 0.56 V/dec. The 250 °C Al₂O₃ insulator based device also shows a substantially smaller threshold voltage shift of 1.5 V after a 10 V gate voltage is stressed for 1 h, while the value for the 200, 300 and 350 °C Al₂O₃ insulator based devices are 2.3, 2.6, and 1.64 V, respectively.     

Charge transport mechanism of Al/Bi2Te3/Al thin film devices

Results of dielectric and conduction properties of vacuum evaporated Bi₂Te₃ thin film capacitors (Al/Bi₂Te₃/Al) have been reported in the frequency range 12 Hz to 100 kHz at various temperatures (303–423 K). The variation of capacitance and dielectric constant was found to be temperature and frequency dependent. The capacitance of the film decrease with increasing temperature which may be due to the expansion of the lattice and also due to the excitation of charge carriers at the sites of imperfection. The dielectric constant decreases with frequency at all temperature. This can be attributed to an interfacial polarization caused by space charge. The prevailing ac conduction mechanism in these films is hopping type. The activation energies were evaluated for various thicknesses and it is found to be 0.016 and 0.014 eV for the frequencies 200 Hz and 1 kHz, respectively.