Investigating gate metal induced reduction of surface donor density in GaN/AlGaN/GaN heterostructure by electroreflectance spectroscopy

The internal field of GaN/AlGaN/GaN heterostructure on Si-substrate was investigated by varying the thickness of an undoped-GaN capping layer using electroreflectance spectroscopy. The four samples investigated are AlGaN/GaN heterostructure without a GaN cap layer (reference sample) and three other samples with GaN/AlGaN/GaN heterostructures in which the different thickness of GaN cap layer (2.7 nm, 7.5 nm, and 12.4 nm) has been considered. The sheet carrier density (n_s) of a two-dimensional electron gas has decreased significantly from 4.66 × 10¹² cm⁻² to 2.15 × 10¹² cm⁻² upon deposition of a GaN capping layer (12.4 nm) over the reference structure. Through the analysis of internal fields in each GaN capping and AlGaN barrier layers, it has been concluded that the undiminished surface donor states (n_s) of a reference structure and the reduced n_s caused by the Au gate metal are approximately 5.66 × 10¹² cm⁻² and 1.08 × 10¹² cm⁻², respectively.     

Ohmic contact and space-charge-limited current in molybdenum oxide modified devices

The effect of indium-tin oxide (ITO) surface treatment on hole injection of devices with molybdenum oxide (MoO₃) as a buffer layer on ITO was studied. The Ohmic contact is formed at the metal/organic interface due to high work function of MoO₃. Hence, the current is due to space charge limited when ITO is positively biased. The hole mobility of N, N′-bis-(1-napthyl)-N, N′-diphenyl-1, 1′biphenyl-4, 4′-diamine (NPB) at various thicknesses (100–400 nm) has been estimated by using space-charge-limited current measurements. The hole mobility of NPB, 1.09×10⁻⁵ cm²/V s at 100 nm is smaller than the value of 1.52×10⁻⁴ cm²/V s at 400 nm at 0.8 MV/cm, which is caused by the interfacial trap states restricted by the surface interaction. The mobility is hardly changed with NPB thickness for the effect of interfacial trap states on mobility which can be negligible when the thickness is more than 300 nm.     

Preparation and characterization of semiconductor GNR-CNT nanocomposite and its application in FET

So far, little is known about the experimental potential of graphene nanoribbon-carbon nanotube (GNR-CNT) heterostructure as a semiconductor nanocomposite. The present work examined the structural features, topography and electronic properties of GNR-CNT nanocomposite by using Raman spectroscopy, transmission electron microscopy, scanning tunneling microscopy and spectroscopy (STS). The homogenous semiconductor GNR-CNT nanocomposites were produced under optimized synthesis conditions. The narrow band gap was exhibited by optimization of the reduction step. The STS of the micro-scale surface of the nanocomposite shows local density of state in selected areas that represent the 0.08 eV band gap of a homogenous nanocomposite. The potential of the semiconductor nanocomposite was considered for application in stacked graphene nanoribbon-field effect transistors (SGNR-FETs). A simple method of device fabrication is proposed based on a semiconductor stacked GNR nanocomposite. The high hole mobility and rectifying effect of the p–n junction of the SGNR nanocomposite on TiO₂ are demonstrated. The optimal thickness for the back gate TiO₂ dielectric for the tested devices was 40 nm. This thickness decreased leakage current at the p–n junction of the SGNR/TiO₂ interface, which is promising heterojunction for optoelectronics. The thickness of gate dielectric and quantum capacitance of the gate was investigated at the low 40 nm thickness by calculating the mobility. In the proposed SGNR-FET, holes dominate electrical transport with a high mobility of about 1030 cm²/V s.     

Image quality of microns-thick specimens in the ultra-high voltage electron microscope

Image quality of MeV transmission electrons is an important factor for both observation and electron tomography of microns-thick specimens with the high voltage electron microscope (HVEM) and the ultra-HVEM. In this work, we have investigated image quality of a tilted thick specimen by experiment and analysis. In a 3 MV ultra-HVEM, we obtained transmission electron images in amplitude contrast of 100 nm gold particles on the top surface of a tilted 5 μm thick amorphous epoxy-resin film. From line profiles of the images, we then measured and evaluated image blurring, contrast, and the signal-to-noise ratio (SNR) under different effective thicknesses of the tilted specimen and accelerating voltages of electrons. The variation of imaging blurring was consistent with the analysis based on multiple elastic scattering. When the effective thickness almost tripled, image blurring increased from ～3 to ～20 nm at the accelerating voltage of 3 MV. For the increase of accelerating voltage from 1 to 3 MV in the condition of the 14.6 μm effective thickness, due to the reduction of multiple scattering effects, image blurring decreased from ～54 to ～20 nm, and image contrast and SNR were both obviously enhanced by a factor of ～3 to preferable values. The specimen thickness was shown to influence image quality more than the accelerating voltage. Moreover, improvement on image quality of thick specimens due to increasing the accelerating voltage would become less when it was further increased from 2 to 3 MV in this work.     

Study of two-dimensional hole gas concentration and hole mobility in zinc delta-doped GaAs and pseudomorphic GaAs/In0.2Ga0.8As heterostructures

Zinc delta-doped GaAs and pseudomorphic GaAs/In}_0.2Ga_0.8As heterostructures grown by low-pressure metalorganic chemical vapour deposition have been demonstrated. The influence of delta-doping period and spacer thickness on two-dimensional hole gas concentrations and hole mobility was studied. From secondary-ion mass spectroscopy and Hall measurement, we conclude that zinc delta-doping can form an excellent abrupt profile (full-width at half maximum is of 10 nm) and offer a high two-dimensional hole gas sheet density (as high as 1 × 10¹³cm⁻²) By adopting a strained InGaAs material as the active channel and by carefully modulating the spacer layer thickness, one can obtain a significantly enhanced hole mobility.     

Anisotropic behaviour of the magnetoresistance in single crystalline iron films

Magnetoresistive properties of single crystalline Fe(0 0 1) films with thickness in the range 5–100 nm are reported. The films possess low coercive fields (∼100 A/m) but a weak irreversible behaviour of the magnetization remains to fields of the order of the anisotropy field. The anisotropic behaviour of the magnetoresistance is investigated as a function of temperature and film thickness. A reversal in sign of the anisotropic magnetoresistance from negative to positive values is found at low temperatures on decreasing the film thickness from 100 to 5 nm, or by increasing the temperature from 10 to 300 K of a sufficiently thick film. The reversal in sign is associated with a crossover from Lorentz force (ordinary) to spin–orbit (extraordinary) dominated scattering processes governing the anisotropic magnetoresistance as the length scale of the electron mean free path, λ, decreases.     

Nanoflake CoN as a high capacity anode for Li-ion batteries

CoN films with nanoflake morphology are prepared by RF magnetron sputtering on Cu and oxidized Si substrates and characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) techniques. The thickness and composition of the films are determined by the Rutherford back scattering (RBS) technique confirming the stoichiometric composition of CoN with a thickness, 200 (± 10) nm. Li-storage and cycling behavior of nanoflake CoN have been evaluated by galvanostatic discharge–charge cycling and cyclic voltammetry (CV) in cells with Li–metal as counter electrode in the range of 0.005–3.0 V at ambient temperature. Results show that a first-cycle reversible capacity of 760 (± 10) mAhg^? 1 at a current rate 250 mAg^? 1(0.33 C) increases consistently to yield a capacity of 990 (± 10) mAhg^? 1 after 80 cycles. The latter value corresponds to 2.7 mol of cyclable Li/mol of CoN vs. the theoretical, 3.0 mol of Li. Very good rate capability is shown when cycled at 0.59 C (up to 80 cycles) and at 6.6 C (up to 50 cycles). The coloumbic efficiency is found to be 96–98% in the range of 10–80 cycles. The average charge and discharge potentials are 0.7 and 0.2 V, respectively for the decomposition/formation of Li₃N as determined by CV. However, cycling to an upper cut-off voltage of 3.0 V is essential for the completion of the “conversion reaction”. Based on the ex-situ-XRD, -HR-TEM and -SAED data, the plausible Li-cycling mechanism is discussed. The results show that nanoflake CoN film is a prospective anode material for Li-ion batteries.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Analysis of microscopic parameters of surface charging in polymer caused by defocused electron beam irradiation

The relationship between microscopic parameters and polymer charging caused by defocused electron beam irradiation is investigated using a dynamic scattering-transport model. The dynamic charging process of an irradiated polymer using a defocused 30 keV electron beam is conducted. In this study, the space charge distribution with a 30 keV non-penetrating e-beam is negative and supported by some existing experimental data. The internal potential is negative, but relatively high near the surface, and it decreases to a maximum negative value at z = 6 μm and finally tend to 0 at the bottom of film. The leakage current and the surface potential behave similarly, and the secondary electron and leakage currents follow the charging equilibrium condition. The surface potential decreases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. The total charge density increases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. This study shows a comprehensive analysis of microscopic factors of surface charging characteristics in an electron-based surface microscopy and analysis.     

Optical properties of amorphous Si/SiO2superlattice

Amorphous Si/SiO₂(a-Si/SiO₂) superlattices have been fabricated by the magnetron sputtering technique. The superlattice with an Si layer thickness of 1.8 nm has been characterized by transmission electron microscopy (TEM). The result indicates that most of the regions in the Si layer consist of amorphous phase, while regular structure appears in some local regions. This is in agreement with the Raman scattering spectroscopy. The optical absorption spectrum and photoluminescence (PL) spectrum have been measured. Moreover, the third-order optical nonlinearity χ⁽³⁾of this superlattice has been measured. To our knowledge, this is the first investigation of the nonlinear absorption and refractive index of an a-Si/SiO₂superlattice using the Z -scan technique. The real and imaginary parts of χ⁽³⁾have been found to be 1.316  ×  10 ^− 7eus and  − 5.596  ×  10 ^− 7eus, respectively, which are about two orders of magnitude greater than those of porous silicon. The results may be attractive for potential application in electro-optics devices.     

The surface potential of insulating thin films negatively charged by a low-energy focused electron beam

We report on the surface potential characteristics in the equilibrium state of the grounded insulating thin films of several 100 nm thickness negatively charged by a low-energy (<5 keV) focused electron beam, which have been simulated with a newly developed two-dimensional self-consistent model incorporating electron scattering, charge transport and charge trapping. The obtained space charge is positive and negative within and outside the region, respectively, where the electron and hole densities are greater than the trap density. Thus, the surface potential is relatively high around the center, then it decreases to a maximum negative value and finally tends to zero along the radial direction. The position of the maximum value is far beyond the range of e-beam irradiation as a consequence of electron scattering and charge transport. Moreover, a positive electric field can be generated near the surface in both radial and axial directions. The surface potential at center exhibits a maximum negative value in the condition of the ～2 keV energy non-penetrating e-beam in this work, which is supported by some existing experimental data in scanning electron microscopy. Furthermore, the surface potential decreases with the increase in beam current, trap density and film thickness, but with the decrease in electron mobility.     

Development of scattering near-field optical microspectroscopy apparatus using an infrared synchrotron radiation source

We report the status of a scattering near-field microspectroscopy apparatus developed at SPring-8 using an infrared synchrotron radiation (IR-SR) source. It consists of a scattering type scanning near-field optical microscope and a Fourier transform infrared spectrometer. The IR-SR is used as a highly brilliant and broad-band IR source. This apparatus has potential for application in near-field spectroscopy with high spatial resolution beyond the diffraction limit. In order to eliminate background scatterings from the probe shaft and/or sample surface, we used higher harmonic demodulation method. The near-field spectra were observed by 2nd harmonic components using the lock-in detection. The spatial resolution of about 300 nm was achieved at around 1000 cm^? 1 (10 μm wavelength).     

Highly efficient and stable organic light-emitting diode by balancing drift current of charge

Highly efficient and stable OLED device in which hole-drift current and electron-drift current are balanced was fabricated. Drift current characteristics according to the thickness of organic layer were examined using the device with ITO/m-MTDATA/NPB/Al structure that can only move the hole and the device with Al/LiF/Alq₃/LiF/Al structure that can only move the electron. Using the result of such examination, green device with balanced drift current was produced. Device with the structure of m-MTDATA (80 nm)/NPB (20 nm)/C-545T (3%) doped Alq₃ (5 nm)/Alq₃ (59 nm)/LiF (1 nm)/Al (200 nm) showed color purity of (0.309, 0.643) and high efficiency of 7.0 lm/W (14.4 cd/A). Most of light emission was observed inside the green emitting layer. Through the result of EL spectrum for the device also including red emitting layer, same result could be obtained. The device with balanced drift current also showed half life-time of 175 h for initial luminance of 3000 cd/m², which is more stable in comparison to the device without balanced drift current.     

Facile preparation and characterization of hollow poly(St-co-MMA-co-BA-co-MAA) core–double shell latex nanoparticles for electrophoretic displays

Novel core–double shell particles with poly(methyl methacrylate-co-butyl acrylate) (PMMA-co-BA) as the cores, poly(methyl methacrylate-co-butyl acrylate-co-methacrylic acid) (PMMA-co-BA-co-MAA) as the inner shells, poly(styrene-co-methyl methacrylate) (PS-co-MMA) as the outer shells were prepared by soap-free emulsion polymerization. The acid–alkali osmotic swelling processes were made before the outer shells wrapped for bigger aperture. The optimal experiment conditions were summarized. The morphology and size of the hollow latex particles were observed by transmission electron microscopy. The results showed that the uniform sizes of the hollow latex particles were about 230 nm. The electrophoretic mobility of them in tetrachloroethylene was 0.91 × 10⁻¹⁰ m² V⁻¹ s⁻¹, and the Zeta-potential was 5.87 mV. The results showed that the hollow polymer particles can used as background particles.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Electron channel with high carrier mobility at the interface of type-II broken-gapp-GaInAsSb/p-InAs single heterojunctions

We have studied experimentally the magneto-transport properties of type-II broken-gap Ga_1 − xIn_xAsSb/p-InAs heterostructures with various doping levels of the quaternary layer by Te or Zn. A strong electron channel with high electron mobility was observed at the interface of the heterostructures. Interface roughness scattering was found to dominate the electron mobility atT = 4.2–47 K in samples with an undoped or a slightly doped quaternary layer. A drastic mobility drop with increasing Zn doping level was observed. Shubnikov–de Haas oscillations at low temperatures (1.5–20 K) were studied and a weak anisotropy of magnetoresistance was found. Some important parameters of the heterostructures under study were determined.     

Enhanced boron adsorption onto synthesized MgO nanosheets by ultrasonic method

MgO nanosheets with high adsorption performance were fabricated by an ultrasonic method. It was revealed that, nest-like MgO was formed from the magnesium salt solution precipitation and further calcination. Then the nest-like MgO was exfoliated by ultrasonic waves to obtain MgO nanosheets with approximately a lateral of 200–600 nm and a thickness of 10 nm. Adjusting the ultrasonic time and power, the specific surface areas of MgO nanosheets could be tuned in a range of 79–168 m²/g. The synthesized MgO nanosheets were used as adsorbents to remove boron from aqueous solution, and the maximum boron adsorption capacity of these MgO nanosheets reached 87 mg g⁻¹. The high uptake capability of the MgO nanosheets makes it potentially adsorbent for the removal of boron from wastewaters.     

Picosecond molecular switch based on the influence of photogenerated electric fields on optical charge transfer transitions

A concept for a picosecond molecular switch is demonstrated using a photoinduced electron transfer reaction in a covalently linked, fixed distance donor–acceptor molecule D–A linked to a perylene-3,4-dicarboximide chromophore, C. The chromophore C possesses a strong charge transfer transition in its optical spectrum. Selective excitation of C within D–A–C using 530 nm, 130 fs laser pulses produces^1 * C, which undergoes singlet–singlet energy transfer to produce^1 * D, which in turn transfers an electron to A. If the D–A–C system is selectively excited with 416 nm, 130 fs laser pulses to produce D ⁺ – A ⁻ –C prior to excitation of C with 530 nm, 130 fs laser pulses, a 25% lower yield of^1 * C is generated. The intense local electric field produced by D ⁺ – A ⁻ causes a 15 nm electrochromic red shift of the charge transfer absorption of C. Thus, the absorption of C at 530 nm is significantly diminished by the presence of D ⁺ – A ⁻ . The need to use two laser pulses with different wavelengths to observe these effects, and the resulting picosecond time response makes it possible to consider applications of this concept in the design of molecular switches.     

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for Al_xGa_1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13 K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.     

Part 2: Ultra-short pulse laser patterning of very thin indium tin oxide on glass substrates

We investigate selective patterning of ultra-thin 20 nm Indium Tin Oxide (ITO) thin films on glass substrates, using 343, 515, and 1030 nm femtosecond (fs), and 1030 nm picoseconds (ps) laser pulses. An ablative removal mechanism is observed for all wavelengths at both femtosecond and picoseconds time-scales. The absorbed threshold fluence values were determined to be 12.5 mJ cm⁻² at 343 nm, 9.68 mJ cm⁻² at 515 nm, and 7.50 mJ cm⁻² at 1030 nm for femtosecond and 9.14 mJ cm⁻² at 1030 nm for picosecond laser exposure. Surface analysis of ablated craters using atomic force microscopy confirms that the selective removal of the film from the glass substrate is dependent on the applied fluence. Film removal is shown to be primarily through ultrafast lattice deformation generated by an electron blast force. The laser absorption and heating process was simulated using a two temperature model (TTM). The predicted surface temperatures confirm that film removal below 1 J cm⁻² to be predominately by a non-thermal mechanism.