DNA-based organic-on-inorganic semiconductor Schottky structures

A sandwich device has been fabricated from DNA molecular film by solution processing located between Al and p-type silicon inorganic semiconductor. We have performed the electrical characteristics of the device such as current–voltage (I–V), capacitance–voltage (C–V) and capacitance–frequency (C–f) at room temperature and in dark. The DNA-based structure has showed the rectifying behavior. From its optical absorbance spectrum, it has been seen that DNA has been a semiconductor-like material with wide optical band energy gap of 4.12 eV and resistivity of 1.6 × 10¹⁰ Ω cm representing a p-type conductivity.     

Effects of oxygen flow rate on the properties of HfO2 layers grown by metalorganic molecular beam epitaxy

The investigations on the properties of HfO₂ dielectric layers grown by metalorganic molecular beam epitaxy were performed. Hafnium-tetra-tert-butoxide, Hf(C₄H₉O)₄ was used as a Hf precursor and pure oxygen was introduced to form an oxide layer. The grown film was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and capacitance–voltage (C–V) and current–voltage (I–V) analyses. As an experimental variable, the O₂ flow rate was changed from 2 to 8 sccm while the other experimental conditions were fixed. The XPS spectra of Hf 4f and O 1s shifted to the higher binding energy due to the charge transfer effect and the density of trapped charges in the interfacial layer was increased as the oxygen flow rate increased. The observed microstructure indicated the HfO₂ layer was polycrystalline, and the monoclinic phases are the dominant crystal structure. From the C–V analyses, k = 14–16 and EOT = 44–52 were obtained, and the current densities of (3.2–3.3) × 10⁻³ A/cm² were measured at −1.5 V gate voltage from the I–V analyses.     

Effect of oxide thickness on the capacitance and conductance characteristics of MOS structures

In this work, the investigation of the interface states density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics in Au/SnO₂/n-Si (MOS) structures prepared at various SnO₂ layer thicknesses by spray deposition technique have been reported. It is fabricated five samples depending on deposition time. The thicknesses of SnO₂ films obtained from the measurement of the oxide capacitance in the strong accumulation region for MOS Schottky diodes are 37, 79, 274, 401, and 446 Å, for D1, D2, D3, D4, and D5 samples, respectively. The C–V and G–V measurements of Au/SnO₂/n-Si MOS structures are performed in the voltage range from −6 to +10 V and the frequency range from 500 Hz to 10 MHz at room temperature. It is observed that peaks in the forward C–V characteristics appeared because of the series resistance. It has been seen that the value of the series resistance R_s of samples D1 (47 Ω), D2 (64 Ω), D3 (98 Ω), D4 (151 Ω), and D5 (163 Ω) increases with increasing the oxide layer thickness. The interface state density D_it ranges from 2.40×10¹³ cm⁻² eV⁻¹ for D1 sample to 2.73×10¹² cm⁻² eV⁻¹ for D5 sample and increases with increasing the oxide layer thickness.     

Electrical characterization of low temperature deposited TiO2 films on strained-SiGe layers

Thin films of titanium dioxide have been deposited on strained Si_0.82Ge_0.18 epitaxial layers using titanium tetrakis-isopropoxide [TTIP, Ti(O-i-C₃H₇)₄] and oxygen by microwave plasma enhanced chemical vapor deposition (PECVD). The films have been characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Dielectric constant, equivalent oxide thickness (EOT), interface state density (D_it), fixed oxide charge density (Q_f/q) and flat-band voltage (V_FB) of as-deposited films were found to be 13.2, 40.6 Å, 6×10¹¹ eV⁻¹ cm⁻², 3.1×10¹¹ cm⁻² and −1.4 V, respectively. The capacitance–voltage (C–V), current–voltage (I–V) characteristics and charge trapping behavior of the films under constant current stressing exhibit an excellent interface quality and high dielectric reliability making the films suitable for microelectronic applications.     

Analysis of frequency-dependent series resistance and interface states of In/SiO2/p-Si (MIS) structures

In this work, the investigation of the interface state density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G/ω−V) characteristics in In/SiO₂/p-Si metal–insulator–semiconductor (MIS) structures with thin interfacial insulator layer have been reported. The thickness of SiO₂ film obtained from the measurement of the oxide capacitance corrected for series resistance in the strong accumulation region is 220 Å. The forward and reverse bias C–V and G/ω−V characteristics of MIS structures have been studied at the frequency range 30 kHz–1 MHz at room temperature. The frequency dispersion in capacitance and conductance can be interpreted in terms of the series resistance (R_s) and interface state density (D_it) values. Both the series resistance R_s and density of interface states D_it are strongly frequency-dependent and decrease with increasing frequency. The distribution profile of R_s–V gives a peak at low frequencies in the depletion region and disappears with increasing frequency. Experimental results show that the interfacial polarization contributes to the improvement of the dielectric properties of In/SiO₂/p-Si MIS structures. The interface state density value of In/SiO₂/p-Si MIS diode calculated at strong accumulation region is 1.11×10¹² eV⁻¹ cm⁻² at 1 MHz. It is found that the calculated value of D_it (≈10¹² eV⁻¹ cm⁻²) is not high enough to pin the Fermi level of the Si substrate disrupting the device operation.     

The dependence of I–V and C–V characteristics on temperature in the H-terminated Pb/p-Si(1 0 0) Schottky barrier diodes

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of H-terminated Pb/p-Si/Al contacts fabricated by us have been measured in the temperature range of 77–300 K. The experimental values of the barrier height (BH) Φ_bo and the ideality factor n for the device range from 0.674 and 1.072 eV (at 300 K) to 0.352 and 2.452 eV (at 77 K), respectively. The ideality factors become larger with lowering temperature while the barrier height decreases. The Φ_bo(n) plot shows a linear dependence in the temperature range of 77–300 K that can be explained by the barrier inhomogeneity at the metal/semiconductor interface. The extrapolation of the linear Φ_bo(n) plot to n = 1 has given a homogeneous barrier height of approximately 0.713 eV for the Pb/p-Si(1 0 0) contact. A Φ_bo versus 1/T plot was drawn to obtain evidence of a Gaussian distribution of the BHs, and values of and σ_s = 80.5 mV for the mean BH and zero-bias standard deviation have been obtained from this plot, respectively. Then, a modified versus 1/T plot gives and A^* as 0.828 eV and 54.89 A/cm² K², respectively. Furthermore, an average value of −0.687 meV/K for the temperature coefficient has been obtained, the value of −0.687 meV/K for hydrogen terminated p-type Si differs from those given for p-type Si without hydrogen termination in the literature.     

On the barrier inhomogeneities of polyaniline/p-Si/Al structure at low temperature

The current–voltage (I–V) characteristics of Au/polyaniline(PANI)/p-Si/Al structures were determined at various temperatures in the range of 90–300 K. The evaluation of the experimental I–V data reveals a decrease of the zero-bias barrier height (BH) and an increase of the ideality factor (n) with decreasing temperature. It was shown that the occurrence of a Gaussian distribution of then BHs is responsible for the decrease of the apparent BH, increase of the ideality factor n due to barrier height imhomogeneities that prevail at the interface. A Φ_b0 versus 1/T plot has been drawn for evidence of the Gaussian distribution of the barrier height, and and σ₀ = 0.0943 V for the mean barrier height and zero-bias standard deviation, respectively, have been obtained from this plot. Thus, a modified versus 1/T plot gives and A^* as 0.885 eV and A^* = 55.80 A/K² cm², respectively. Hence, it can be concluded that Au/PANI/p-Si/Al structure has a good rectifying contact and the temperature dependence of I–V characteristics of the rectifying contact on p-Si successfully have been explained on the basis of TE mechanism with Gaussian distribution of the barrier heights.     

Barrier height engineering of Ag/GaAs(100) Schottky contacts by a thin organic interlayer

Thin films of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were used as an interlayer for the electronic modification of Ag/n-GaAs(100) Schottky contacts. The electronic properties were investigated recording in situ current–voltage (I–V) and capacitance–voltage (C–V) characteristics. For H-plasma treated substrates the effective barrier height decreases from 0.81 to 0.64 eV as a function of the PTCDA layer thickness (d_PTCDA). In the case of the sulphur passivated GaAs the effective barrier height first increases and then decreases, the overall range being 0.54–0.73 eV. The substrate treatment leads to a different alignment between the band edges of the GaAs and the molecular orbitals of the PTCDA, making it possible to determine the energy position of the LUMO transport level.     

Electronic and photovoltaic properties of p-Si/C70 heterojunction diode

Electronic and photovoltaic properties of p-Si/C₇₀ heterojunction diode have been investigated. The ideality factor n and barrier height φ_b values of the diode were found to be 1.86 and 0.69 eV, respectively. The diode indicates a non-ideal current–voltage behaviour due to the ideality factor being higher than unity. This behaviour results from the effect of series resistance and the presence of an interfacial layer. The series resistance R_S and ideality factor n values were determined using Cheng's method and the obtained values are 2.21×10⁵ Ω and 1.86, respectively. The device shows photovoltaic behaviour with a maximum open-circuit voltage of 0.22 V and a short-circuit current of 0.35 μA under 6 mW/cm² light intensity.     

AFM and electronic transport studies of swift heavy ion irradiated Mn/p-Si bilayer structure

Mn/p-Si structures have been realised by electron beam evaporation of manganese on etched and cleaned p-Si wafers. Bilayer structures have been irradiated by swift heavy ions (of 100 MeV Fe⁷⁺ having a fluence of 1 × 10¹³ ions/cm²). The electronic transport features across the bilayer of the structure (i.e. I–V characteristics across the Mn/p-Si interface) show a significant increase of current (by two orders of magnitude) for the irradiated ones as compared to un-irradiated ones. I–V characteristics across the interface has also been recorded in presence of in-plane (i.e., along the plane of the interface) magnetic field which show a significant magnetic field sensitivity for the irradiated ones. The surface morphological studies from AFM show a granular structure with open face having micro-particles in it, prior to the irradiation and round shaped embedded granular structure after the irradiation. XRD data show the formation of manganese silicide (Mn₅Si₂). The results are understood in the realm of interfacial intermixing which is tailored by the swift heavy ion irradiation.     

Field emission from quantum size GaN structures

Whisker structures and quantum dots fabricated by photoelectrochemical (PEC) etching of undoped and doped metalorganic chemical vapor deposition (MOCVD)-grown GaN (2×10¹⁷ or 3×10¹⁸ cm⁻³) are investigated in relation with their field-emission characteristics. Different surface morphologies, corresponding to different etching time and photocurrent, results in different field-emission characteristics with low turn-on voltage down to 4 V/μm and the appearance of quantum-size effect in the I–V curves.     

Mechanism of current leakage through metal/n-GaN interfaces

Detailed current–voltage–temperature (I–V–T) measurements were performed on the Schottky diodes fabricated on MOVPE-grown n-GaN layers. A large deviation from the thermionic emission (TE) transport was observed in the reverse I–V curves with a large excess leakage. From the calculation based on the thermionic-field emission (TFE) model, it was found that the tunneling plays an important role in the carrier transport across the GaN Schottky barrier even for doping densities as low as 1×10¹⁷ cm⁻³. A novel barrier-modified TFE model based on presence of near-surface fixed charges or surface states is proposed to explain the observed large reverse leakage currents.     

Investigation of the static and dynamic magnetic properties of CoFe2O4 nanoparticles

We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300 K under zero external DC field for frequencies ranging from f=10 to 10,000 Hz. A prominent peak appears in both χ′ and χ″ as a function of T. The peak temperature T₂ of χ″ depends on f following the Vogel–Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at T_B^m94 K in ZFC and 119 K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2 K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295 K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K=6×10⁶ erg/cm³. Mössbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mössbauer spectrum was fitted with two magnetic subspectra with internal fields H_int of 490, 470 and 515 kOe, corresponding to Fe³⁺ ions in A and B sites.     

Reduction of the concentration of slow insulator states in SiO2/InP-MIS structures

The well-known drift phenomena usually found in the InP-metal-insulator-semiconductor (MIS) devices can be explained by the assumption of a spatial and energetical distribution of slow states located within the insulator. The concentration of these states can be reduced by far more than one order of magnitude if a suitable technique of insulator deposition is applied. In this paper we will discuss the influence of the deposition temperature, the spatial separation of sample and plasma (“indirect plasma method”), and the addition of phosphorus into the reaction chamber during the initial period of insulator deposition on the properties of n-type and p-type InP-MIS capacitors. Plasma-enhanced chemical vapor deposited silicon dioxide is used as insulator. The samples were characterized by means of capacitance/voltage (C(V)) and deep level transient spectroscopy (DLTS) measurements. Only minor hysteresis of the C(V) curves and concentrations of slow insulator states of only (1–2)×10¹¹ cm^-2 eV^-1 are measured for the best of our samples.     

Structural and compositional analysis of InBixAsySb(1−x−y) films grown on GaAs(0 0 1) substrates by liquid phase epitaxy

The growth of epitaxial InBi_xAs_ySb_(1−x−y) layers on highly lattice mis-matched semi-insulating GaAs substrates has been successfully achieved via the traditional liquid phase epitaxy. Orientation and single crystalline nature of the film have been confirmed by X-ray diffraction. Scanning electron micrograph shows abrupt interface at micrometer resolution. Surface composition of Bi(x) and As(y) in the InBi_xAs_ySb_(1−x−y) film was measured using energy dispersive X-ray analysis and found to be 2.5 and 10.5 at.%, respectively, and was further confirmed with X-ray photoelectron spectroscopy. Variation of the composition with depth of the film was studied by removing the layers with low current (20 μA) Ar⁺ etching. It was observed that with successive Ar⁺ etching, In/Sb ratio remained the same, while the As/Sb and Bi/Sb ratios changed slightly with etching time. However after about 5 min etching the As/Sb and Bi/Sb ratios reached constant values. The room temperature band gap of InBi_0.025As_0.105Sb_0.870 was found to be in the range of 0.113–0.120 eV. The measured values of mobility and carrier density at room temperature are 3.1×10⁴ cm² V⁻¹ s⁻¹ and 8.07×10¹⁶ cm⁻³, respectively.     

The electrical characterization of Zn(Phen)q/p-type Si/Al diode with interfacial layer by current–voltage characteristics

The current–voltage characteristics of Zinc (II) [(8-hydroxyquinoline)(1,10-phenanthroline)] complex (Zn(phen)q)/p-type Si/Al diode with interfacial layer have been investigated. The barrier height and ideality factor of the diode were found to be 0.71 eV and 2.05. Zn(phen)q/p-type Si/Al diode shows a metal–insulator–semiconductor structure resulted from presence of series resistance and an interfacial layer. The n and φ_B values obtained the presence of interfacial layer are 1.02 and 0.70 eV, respectively. The effect of series resistance was evaluated using a method developed by Cheung. The R_s and n values were determined from the d ln(I)/dV plot and were found to be 30.43 kΩ and 2.16, respectively. The barrier height and R_s values were calculated from the H(I)–I plot and were found to be 0.70 eV and 30.99 kΩ. The density of the interface states of the Zn(phen)q/p-type Si/Al diode was calculated and was found to be an order of 10¹³ eV⁻¹ cm⁻².     

Photoluminescence and capacitance–voltage characterization of GaAs surface passivated by an ultrathin GaN interface control layer

A novel surface passivation technique for GaAs using an ultrathin GaN interface control layer (GaN ICL) formed by surface nitridation was characterized by ultrahigh vacuum (UHV) photoluminescence (PL) and capacitance–voltage (C–V) measurements. The PL quantum efficiency was dramatically enhanced after being passivated by the GaN ICL structure, reaching as high as 30 times of the initial clean GaAs surface. Further analysis of PL data was done by the PL surface state spectroscopy (PLS³) simulation technique. PL and C–V results are in good agreement indicating that ultrathin GaN ICL reduces the gap states and unpins the Fermi level, realizing a wide movement of Fermi level within the midgap region and reduction of the effective surface recombination velocity by a factor of 1/60. GaN layer also introduced a large negative surface fixed charge of about 10¹² cm⁻². A further improvement took place by depositing a Si₃N₄ layer on GaN ICL/GaAs structure.     

Electrical peculiarities in Al/Si/Ge/…/Ge/Si and Al/SiGe/Si structures

The current–voltage (I–V) and capacitance–voltage (C–V) behaviour of different Si/Ge multilayers and SiGe single layers prepared on p-type Si substrates by magnetron sputtering and annealing, has been studied in the temperature range of 80–320 K by using Al Schottky contacts as test structures. Although a significant influence of the microstructure of the Si/Ge multilayers and SiGe layers was obtained on the electrical behaviour of the structures, the structures exhibited similar specific features.     

Electronic properties of nanocrystalline tin oxide dispersed in monolithic mesoporous silica

This paper is devoted to the study of the electrical properties of nanocrystalline tin oxide dispersed in the mesoporous silica. By immersing the silica in precursor solutions with different concentrations and heat-treatment, different samples were obtained. With precursor concentrations increasing from 0.1 to 4.0 M, the resistivities of the samples decrease from 3.15×10⁶ to 2.43×10³ Ω cm. The resistivity changes with the measurement time, and the deviations from Ohm's law in the voltage–current (V–I) measurements illustrate the capacitance property of these nanocomposites. For this new kind of nanocomposites, the obtained results provide experimental evidence of the conducting mechanism for tin oxide nanoparticles.     

Growth and characterization of MOMBE grown HfO2

HfO₂ dielectric layers were grown directly on the p-type Si (1 0 0) by metalorganic molecular beam epitaxy (MOMBE). Hafnium tetra-butoxide was used as a Hf precursor and pure oxygen was introduced to form an oxide layer. The properties of the layers with different thicknesses were evaluated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and capacitance–voltage (C–V) and current–voltage (I–V) analyses. XRD and HRTEM results showed that the HfO₂ films thinner than 12 nm were amorphous while the films thicker than 12 nm began to crystallize in the tetragonal and the monoclinic phases. The XPS spectra of O 1s show that the O---Si binding energies shifted to the lower binding energy with increasing the HfO₂ layer thickness. Moreover, the snap back phenomenon is observed in accumulation capacitance. These changes are believed to be linked with the decomposition of SiO and the crystallization of HfO₂ layer during the film growth.