Selective area molecular beam epitaxy of InAs on GaAs (110) masked substrates for direct fabrication of planar nanowire field-effect transistors

We have synthesized InAs nanowires (NWs) by selective area molecular beam epitaxy (SA-MBE) on GaAs masked substrates. In particular, we have obtained in-plane-oriented NWs on the (110) plane, and then directly applied the NWs to planar nanowire field-effect transistors (NWFETs) using conventional electron beam lithography without a NW dispersion process. We have measured output and transfer characteristics of the NWFETs at room temperature, and obtained a current swing but no turning off, and a field-effect mobility peak of 150 cm²/V-s. We have also observed almost no temperature influence on field-effect mobility between 2 K and 300 K, suggesting a high-dense surface accumulation layer even at low temperatures.     

Xerographic spectroscopy of gap states in Se-rich amorphous semiconductors review

One of the most important parameters which determine the performance of many modern devices based on amorphous semiconductors is the drift mobility-lifetime product, μτ. There has been much interest in determination of charge-carrier ranges in amorphous semiconductors by various measurement techniques. Although the mobility, μ, can be measured by the conventional time-of-flight transient photoconductivity technique, the determination of the lifetime, τ, is often complicated by both experimental and theoretical limitations. The present article provides an overview of xerographic measurements as a tool for studying the electrical properties of amorphous semiconductors. First, details of the experimental set-up are discussed. Thereafter, the analysis and interpretation of dark discharge, the first cycle residual potential, cycled-up saturated residual potential are considered. It is shown that from such measurements the charge-carrier lifetime, τ, the range of the carriers, μτ, and the integrated concentration of deep traps in the mobility gap can be readily and accurately determined. Xerographic measurements on Se-rich amorphous photoconductors have indicated the presence of relatively narrow distribution of deep hole traps with integrated density of about 10¹³ cm^? 3. These states are located at ~ 0.85 eV from the valence band. A good correlation was observed between residual potential and the hole range, in agreement with the simple Warter expression. The capture radius is estimated to be r_c = 2–3 Å. Since r_c for pure a-Se and a-As_xSe_1 ? x is comparable to the Se–Se interatomic bond length in a-Se, it can be suggested that deep hole trapping centers in these chalcogenide semiconductors are neutral-looking defects, possibly of intimate valence-alternation pair (IVAP) in nature. The absence of any electron spin resonance signal (ESR) at room temperature seemed to be a strong argument in favor of this suggestion. Finally, photoinduced effects on xerographic parameters are discussed. It has been shown that photoexcitation of a-As_xSe_1 ? x amorphous films with band-gap light alters deep hole and electron states. During room-temperature annealing photosensitized states relax to equilibrium. Recovery process becomes slower with increasing As content. Qualitative explanation of the observed behavior may be based on associating the deep states with C₃⁺ and C₁^?intimate-valence-alternation–pair (IVAP) centers.     

Vapor growth and characterization of pyrite (FeS2) doped with Co,Ni, and As: Variations in semiconducting properties

Pyrite (FeS₂) crystals doped with As, Ni and Co were synthesized with chemical vapor transport over an 18 cm horizontal gradient of 700–600 °C in evacuated quartz tubes, from a mixture of FeS and S, with FeBr₃ as a transport agent. Sulfur fugacity and thus S:Fe stoichiometry was constrained by the Fe_1−xS/FeS_2−y buffer. As, Ni and Co concentrations were ∼3–800 ppm, ∼200–1500 ppm and ∼ 450–3700 ppm, respectively.Semiconducting properties were measured at room temperature using a van der Pauw and Hall measurement system. Ni and Co-doped pyrite are n-type while As-doped pyrite tends to be p-type. Resistivity for Co-doped pyrite ranged from 0.009 to 0.02 Ω cm while for Ni- and As-doped pyrite, resistivity ranged from 2 to 17 Ω cm. Undoped pyrite resistivity ranged from 15 to 85 Ω cm. Carrier concentration was similar for undoped and Ni-doped pyrite, ranging from 10¹⁵ to 10^16.6 cm⁻³, while for Co-doped pyrite it ranged from 10^18.7 to 10^19.3 cm⁻³ and for As-doped pyrite it ranged from 10¹⁴ to 10¹⁸ cm⁻³. Hall mobility was similar for Co and Ni-doped pyrite ranging from 60 to 270 cm² v⁻¹ s⁻¹ while for undoped pyrite it ranged from 8 to 70 cm² v⁻¹ s⁻¹. Hall mobility for As-doped pyrite ranged from 55.0 to 0.2 cm² v⁻¹ s⁻¹ for electrons and from 0.1 to 11.3 cm² v⁻¹ s⁻¹ for holes with the exception of one sample (of 22). These values should be viewed more as trends than as definitive. The results obtained for Ni, Co, and undoped pyrite are similar to those reported in the literature while results for As-doped synthetic pyrite have not previously been reported.     

Optical and kinetic properties of cathodically deposited amorphous tungsten oxide films

Optical properties and the coloration–decoloration kinetics of electrochromic films of amorphous tungsten oxide (a-WO₃), produced by cathodic deposition from a sodium tungstate based aqueous peroxide electrolyte, have been investigated. As films color in 1 N H₂SO₄, sequential appearance of bands with maxima at ∼1 eV, 1.6 eV, 2 eV, and 2.4 eV is observed in their optical absorption and electrosorption spectra, is the same as in the case of reduction of nanosized hydrated-WO₃ colloids with a gradual decrease in their size to that of 12-tungsten polyanions with Keggin structure, indicating the presence of such polytungstates in cathodically deposited a-WO₃, too. When polytungstate is reduced by one electron, an absorption band with a maximum at ∼1.6 eV appears in the optical spectrum of the film. This band corresponds to the optical excitation of charge transfer of the W⁵⁺ → W⁶⁺ type between two adjacent tungsten atoms. The reduction of polytungstate by a second electron with potential shift towards more negative values is accompanied by the appearance of an analogous band with a maximum at ∼2 eV. The reduction of such polytungstates involves participation of the bulk of injected electrons, indicating their dominate role in the nanostructure of the films investigated. The effective co-diffusion coefficient of electrons and protons in cathodically deposited a-WO₃ exhibits a potential dependence with a maximum at 0.1 V against a silver-chloride electrode, where its value is ∼10⁻⁸ cm²/s. It has been shown that the decrease in this coefficient at potential values of over 0.1 V is caused by a decrease in electron mobility.     

Vapor hydration of SON68 glass from 90 °C to 200 °C: A kinetic study and corrosion products investigation

Corrosion of nuclear waste glass in unsaturated conditions is expected to occur upon the closure of the repository galleries during disposal cell saturation in the proposed French disposal site. The objectives of the present work were to determine the alteration kinetics of the SON68 reference in such conditions. Vapor hydration tests were conducted using thin, polished SON68 glass coupons contained in stainless steel autoclaves. Temperatures ranged between 90 °C and 200 °C and the relative humidity (RH) was maintained at 91 ± 1%. Additional experiments at 175 °C and 80, 85, 90 and 95% RH were also conducted to assess the role of RH on the glass corrosion rate. The nature and extent of corrosion have been determined by characterizing the reacted glass surface with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Elemental profiling of the glass hydrated at 90 °C was studied by TOF-SIMS. The chemical composition of the external layer depends on experimental conditions. The hydration rate at 90 °C (TOF-SIMS analysis) is 10 × higher than the generally accepted final rate of SON68 in water at 90 °C (~ 10^? 4 g m^? 2 d^? 1). This may indicate that the glass hydration process cannot be simulated by experiments in aqueous solution with a high S/V ratio. Subsequent leaching (corrosion in an aqueous solution) of samples weathered in water vapor showed dissolution rate values higher than those of pristine glass. This result indicates that mobile elements are trapped within the alteration products during the hydration step and it gives insight into mobility variations of the considered elements.     

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm⁻¹ was found in Raman scattering spectra for all films with buffers, which is corresponding to the E₂ mode.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Infrared semiconductor laser irradiation used for crystallization of silicon thin films

We report the rapid thermal crystallization of silicon films using infrared semiconductor laser. Carbon films were used on silicon films to absorb the laser light. Uniform crystalline regions were achieved by a line shape laser beam with a length of 20 μm. Polycrystalline silicon thin film transistors were fabricated in crystallized regions. The effective electron carrier mobility and threshold voltage were achieved to be 130 cm²/Vs and 0.4 V, respectively, when the crystalline volume ratio of the silicon films was 0.95.     

High quality InAlN/GaN heterostructures grown on sapphire by pulsed metal organic chemical vapor deposition

High quality InAlN/GaN heterostructures are successfully grown on the (0 0 0 1) sapphire substrate by pulsed metal organic chemical vapor deposition. The InAlN barrier layer with an indium composition of 17% is observed to be nearly lattice matched to GaN layer, and a smooth surface morphology can be obtained with root mean square roughness of 0.3 nm and without indium droplets and phase separation. The 50 mm InAlN/GaN heterostructure wafer exhibits a mobility of 1402 cm²/V s with a sheet carrier density of 2.01×10¹³  cm^?2, and a low average sheet resistance of 234 Ω/cm² with a sheet resistance nonuniformity of 1.22%. Compared with the conventional continual growth method, PMOCVD reduces the growth temperature of the InAlN layer and the Al related prereaction in the gas phase, and consequently enhances the surface migration, and improves the crystallization quality. Furthermore, indium concentration of InAlN layer can be controlled by adjusting the pulse time ratio of TMIn to TMAl in a unit cycle, the growth temperature and pressure, as well as the InAlN layer thickness by the number of unit cycle repeats.     

Characterization of electrodeposited Ni and Ni80Fe20 nanowires

In this work we use nanoporous alumina substrates as templates for the growth of Ni and Ni₈₀Fe₂₀ nanowires. Our membranes were obtained by a two-step anodization process of high-purity aluminum foils: first anodizations were performed at 40 V for 15 h in 0.3 M oxalic acid, at 2–6 °C; the second anodization was carried out using the same conditions for 30 min, resulting in pore lengths of ～1.3 μm. After the second anodization, the pore bottom barrier-layer was thinned, to allow the current to flow through electron tunneling. A pulsed electrodeposition method was then used to grow Ni and Ni₈₀Fe₂₀ nanowires. Transport characterization and isothermal magnetization measurements performed on the produced arrays are presented.     

Determination of carrier mobility in MEH-PPV thin-films by stationary and transient current techniques

Charge transport and shelf-degradation of MEH-PPV thin-films were investigated through stationary (e.g. current versus voltage — JxV) and transient (e.g. Time-of-Flight — ToF, Dark-Injection Space-Charge-Limited Current — DI-SCLC, Charge Extraction by Linearly Increasing Voltage — CELIV) current techniques. Charge carrier mobility in nanometric films was best characterized through JxV and DI-SCLC. It approaches 10^? 6 cm²/Vs under a SCLC regime with deep traps for light-emitting diode applications. ToF measurements performed on micrometric layers (i.e. ~ 3 μm) confirmed studies in 100 nm-thick films as deposited in OLEDs. All results were comparable to a similar poly(para-phenylene vinylene) derivative, MDMO-PPV. Electrical properties extracted from thin-film transistors demonstrated mobility dependence on carrier concentration in the channel (~ 10^? 7–10^? 4 cm²/Vs). At low accumulated charge levels and reduced free carrier concentration, a perfect agreement to the previously cited techniques was observed. Degradation was verified through mobility reduction and changes in trap distribution of states.     

Amorphous tungsten-doped In2O3 transparent conductive films deposited at room temperature from metallic target

Amorphous tungsten-doped In₂O₃ (IWO) films were deposited from a metallic target by dc magnetron sputtering at room temperature. Both oxygen partial pressure and sputtering power have significant effects on the electrical and optical properties of the films. The as-deposited IWO films with the optimum resistivity of 5.8 × 10^?4 Ω·cm and the average optical transmittance of 92.3% from 400 to 700 nm were obtained at a W content of 1 wt%. The average transmittance in the near infrared region (700–2500 nm) is 84.6–92.8% for amorphous IWO prepared under varied oxygen partial pressure. The mobility of the IWO films reaches its highest value of 30.3 cm² V^?1 s^?1 with the carrier concentration of 1.6 × 10²⁰ cm^?3, confirming their potential application as transparent conductive oxide films in various flexible devices.     

Beryllium doping and silicon amphotericity in (1 1 0) GaAs-based heterostructures: structural and optical properties

The use of beryllium as an acceptor at high doping levels in (1 1 0)GaAs-based heterostructures is found to be deleterious to the structural and optical properties of these epi-layers. This may limit the use of beryllium as a p-type dopant on the (1 1 0) surface. Because silicon is amphoteric on the (1 1 0), it can be used as an alternative p-type dopant, in addition to its traditional role as an n-type dopant. Transmission electron microscopy, optical absorption, and luminescence data indicate that high quality multiple quantum well structures with p-type GaAs buffer layers doped with silicon, rather than beryllium, can be grown.     

Preparation of transparent cobalt doped glass ceramic and application as saturable absorber Q switch for 1.54 μm Er-glass laser

Glass with composition of 51SiO₂–24.5Al₂O₃–23MgO–1.5K₂O doped with Co²⁺ ions was prepared by conventional melting method. The glass sample was heated at 900 °C for 360 min under atmosphere, and the powder XRD measurement showed that crystalline phase successfully precipitated in the sample. As is compared with standard JCPDS card, the crystalline phase identified as a mixture of zirconium titanate (ZrTiO₄) and one of the compounds of magnesium aluminum oxide. The crystallite size was confirmed by transmission electron microscope (TEM) observation; it could be estimated as 30 nm in diameter from the TEM image. Based on the area ratio of crystalline phase and residual glass phase, the precipitated crystallite phase volume ratio can be estimated to be not higher than 30% in the Co²⁺ doped glass ceramic sample. The absorption coefficient at 1.54 μm for transparent glass ceramic sample is clearly higher than that in base glass, which can be explained by the fact that Co²⁺ ions entered into the precipitated nano-sized crystal phase and led to higher absorption coefficient at 1.54 μm for tetrahedral coordinated Co²⁺ ion. Consequently, the Co²⁺ doped transparent glass ceramic sample with thickness of 0.35 mm was used as a saturable absorber for 1.54 μm Er-glass laser oscillation, and Q switched pulses with pulse energy of 40 mJ, pulse width of 42 ns, and peak power of 0.95 mW were shown in the experiments.     

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3 cm²/(V s) respectively at 300 K and a thickness to electric field ratio of 1.8 × 10⁻⁷ cm²/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13 eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.     

Epitaxial growth of CoO films on semiconductor and metal substrates by constructing a complex heterostructure

Epitaxial growth of CoO films was studied using reflection high-energy electron diffraction (RHEED), electron energy loss spectroscopy (EELS), ultraviolet photoelectron spectroscopy (UPS) and Auger electron spectroscopy (AES). The RHEED results indicated that an epitaxial CoO film grew on semiconductor and metal substrates (CoO (0 0 1)∥GaAs (0 0 1), Cu (0 0 1), Ag (0 0 1) and [1 0 0]CoO∥[1 0 0] substrates) by constructing a complex heterostructure with two alkali halide buffer layers. The AES, EELS and UPS results showed that the grown CoO film had almost the same electronic structure as bulk CoO. We could show that use of alkali halide buffer layers was a good way to grow metal oxide films on semiconductor and metal substrates in an O₂ atmosphere. The alkali halide layers not only works as glue to connect very dissimilar materials but also prevents oxidation of metal and semiconductor substrates.     

Selective crystallization of amorphous silicon thin film by a CW green laser

A simple and effective method for selective CW laser crystallization of a-Si (CLC) without pre-patterning of a-Si has been reported. By using a metallic shadow mask instead of a photolithographic process, we can reduce the process steps and time compared with a conventional CLC process. It shows very high performance – mobility of 173 cm²/s, I_off of ∼10⁻¹³ A @ V_d = −5 V, I_on/I_off of >10⁸ – as a p-channel poly-Si TFT even without any pre-/post-treatment to improve TFT characteristics such as plasma hydrogenation.     

Correlation between the method of preparation and the properties of the sol–gel HfO2 thin films

This work describes the preparation of HfO₂ thin films by the sol–gel method, starting with different precursors such as hafnium ethoxide, hafnium 2,4-pentadionate and hafnium chloride. From the solution prepared as mentioned above, thin films on silicon wafer substrates have been realized by ‘dip-coating’ with a pulling out speed of 5 cm min^?1. The films densification was achieved by thermal treatment for 10 min at 100 °C and 30 min at 450 °C or 600 °C, with a heating rate of 1 °C min^?1. The structural and optical properties of the films are determined employing spectroellipsometric (SE) measurements in the visible range (0.4–0.7 μm), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The main objective of this paper was to establish a correlation between the method of preparation (precursor, annealing temperature) and the properties of the obtained films. The samples prepared from pentadionate and ethoxide precursors are homogenous and uniform in thickness. The samples prepared starting from chloride precursor are thicker and proved to be less uniform in thickness. Higher non-uniformity develops in multi-deposition films or in crystallized films. A nano-porosity is present in the quasi-amorphous films as well in the crystallized one. For the samples deposited on silicon wafer, the thermal treatment induced the formation of a SiO₂ layer at the coating–substrate interface.     

Excimer laser crystallization of microcrystalline silicon for TFTs on flexible substrate

Microcrystalline silicon (μc-Si) films have been deposited on PDMS as well as on PEN substrate. Excimer laser annealing was used to improve the crystalline structure and so to obtain high mobility TFTs. The effect of the laser annealing on the crystalline structure of silicon films is studied using different characterization techniques and discussed. Mobility values of 60 cm²/V s with PDMS and 46 cm²/V s with PEN are obtained.