Dielectrophoretically aligned GaN nanowire rectifiers

We demonstrate GaN nanowire (NW) current rectifiers which were formed by assembling n-GaN nanowires on a patterned p-Si substrate by means of alternating current (ac) dielectrophoresis. The dielectrophoresis was accomplished at a frequency of 10 kHz with three different ac bias voltages (5, 10, and 15 V_p–p), indicating that the number of aligned GaN nanowires increased with increasing ac bias voltage. The n-GaN NW/p-Si diodes showed well-defined current rectifying behavior with a forward voltage drop of 1.2–1.5 V at a current density of 200 A/cm². We observed that the GaN NW diode functioned well as a half-wave rectifier. PACS 71.20.Nr; 73.40.Cg; 73.40.Ei; 73.40.Kp     

An electrical characterization of a hetero-junction nanowire (NW) PN diode (n-GaN NW/p-Si) formed by dielectrophoresis alignment

This is a report on the electrical characterization of gallium nitride (GaN) nanowire (NW) p–n junction diodes. These diodes were formed by assembling n-GaN NWs on p-Si (1 0 0) substrates using alternating current (AC) dielectrophoresis (DEP). The AC DEP was optimized with a bias voltage of 15 V_p−p at a frequency of 1 kHz. The hetero-junction single GaN nanowire p-n diode (n-GaN NW/p-Si) showed well-defined current rectifying behavior with a forward voltage drop of 1.2–2.0 V at a current density of 10–60 A/cm². The GaN nanowire p–n diodes had a high parasite resistance in the range of >470 kΩ. We observed that these high resistances were mostly the result of the metal contacts to the n-GaN NWs. We also found that these parasite resistances were reduced by the formation of an additional capping layer on the top of the n-GaN NW as well as high temperature annealing.     

Study and applications of plasma-modified Si and porous Si surfaces

In this work we have studied the electrical, chemical and physical properties of CH_x/silicon and CH_x/porous silicon (PS). The hydrocarbon (CH_x) layer has been deposited by plasma of methane under argon atmosphere. Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) Spectroscopy and photoluminescence have been used to characterize the CH_x/p-Si, CH_x/PS interfaces and, electrolyte/(CH_x/p-Si) and CH_x/PS/p-Si structures. The results show that in the case of bare silicon, the CH_x layers act as a resistant film to HF electrolyte and can be used as a potential tool for ultra-low thickness for masking and patterning. The deposition of CH_x layer on PS shows that CH_x/PS/p-Si structure presents a rectifying behaviour and can be used for detecting low concentration of large variety of gases. In addition CH_x coated PS samples exhibit more intense luminescence than that observed from an uncoated PS surface where red luminescence is shown .In conclusion, the results clearly demonstrate the interest and applications of Si and PS electrodes coated with hydrocarbon groups.     

Temperature dependence of the current-voltage characteristics of the Al/Rhodamine-101/p-Si(1 0 0) contacts

The current-voltage (I-V) characteristics of Al/Rhodamine-101/p-Si/Al contacts have been measured at temperatures ranging from 280 to 400 K at 20 K intervals. A barrier height (BH) value of 0.817 eV for the Al/Rh101/p-Si/Al contact was obtained at the room temperature that is significantly larger than the value of 0.58 eV of the conventional Al/p-Si Schottky diode. While the barrier height Φ_b0 decreases the ideality factors (n) become larger with lowering temperature. The high values of n depending on the sample temperature may be ascribed to decrease of the exponentially increase rate in current due to space-charge injection into Rh101 thin film at higher voltage. Therefore, at all temperatures, it has been seen that the I-V characteristics show three different regions, the ohmic behavior at low voltages, and the space charge limited current with an exponential distribution of traps at high voltages.     

Molecular control over Ag/p-Si diode by organic layer

Electrical transport properties of Ag metal-fluorescein sodium salt (FSS) organic layer-silicon junction have been investigated. The current-voltage (I-V) characteristics of the diode show rectifying behavior consistent with a potential barrier formed at the interface. The diode indicates a non-ideal I-V behavior with an ideality factor higher than unity. The ideality factor of the Ag/FSS/p-Si diode decreases with increasing temperature and the barrier height increases with increasing temperature. The barrier height (φ_b=0.98 eV) obtained from the capacitance-voltage (C-V) curve is higher than barrier height (φ_b=0.72 eV) derived from the I-V measurements. The barrier height of the Ag/FSS/p-Si Schottky diode at the room temperature is significantly larger than that of the Ag/p-Si Schottky diode. It is evaluated that the FSS organic layer controls electrical charge transport properties of Ag/p-Si diode by excluding effects of the SiO₂ residual oxides on the hybrid diode.     

Preparation and characterisation of crystalline tris(acetylacetonato)Fe(III) films grown on p-Si substrate for dielectric applications

Thin tris(acetylacetonato)iron(III) films were prepared by sublimation in vacuum on glass and p-Si substrates. Then comprehensive studies of X-ray fluorescence (XRF), X-ray diffraction (XRD), optical absorption spectroscopy, AC-conductivity, and dielectric permittivity as a function of frequency and temperature have been performed. The prepared films show a polycrystalline of orthorhombic structure. The optical absorption spectrum of the film was identical with that of the bulk powder layer. For electrical measurements of the complex as insulator, sample in form of metal-insulator-semiconductor (MIS) structure was prepared and characterised by the measurement of the capacitance and AC-conductance as a function of gate voltage. From those measurements, the state density D_it at insulator/semiconductor interface and the density of the fixed charges in the complex film were determined. It was found that D_it was of order 10¹⁰ eV⁻¹/cm² and the surface charge density in the insulator film was of order 10¹⁰ cm⁻². The frequency dependence of the electrical conductivity and dielectric properties of MIS structures were studied at room temperature. It was observed that the experimental data follow the correlated barrier-hopping (CBH) model, from which the fundamental absorption edge, the cut off hopping distance, and other parameters of the model were determined. It was found that the capacitance of the complex increases as temperature increases. Generally, the present study shows that the tris(acetylacetonato)iron(III) films grown on p-Si is a promising candidate for low-k dielectric applications, it displays low-k value around 2.0.     

Room temperature electroluminescence from ZnO/Si heterojunction devices grown by metal-organic chemical vapor deposition

Heterojunction light-emitting diodes with ZnO/Si structure were fabricated on both high-resistivity (p) and low-resistivity (p⁺) Si substrates by metal-organic chemical vapor deposition technology. Fairly good rectifications were observed from the current-voltage curves of both heterojunctions. Ultraviolet (UV) and blue-white electroluminescence (EL) from ZnO layer were observed only from ZnO/p⁺-Si heterojunction under forward bias at room temperature (RT), while strong infrared (IR) EL emissions from Si substrates were detected from both ZnO/p-Si and ZnO/p⁺-Si heterojunctions. The UV and IR EL mechanisms have been explained by energy band structures. The realization of RT EL in UV-visible and IR region on Si substrate has great applicable potential for Si-based optoelectronic integrated circuits.     

Hot electron population inversion and cyclotron resonance negative differential conductivity in semiconductors

The Monte-Carlo calculation is used to support the suggestion⁴ that the hot electrons in semiconductors, the spontaneous optical phonon emission being the main scattering process, may exhibit a negative differential conductivity (NDC) at the cyclotron resonance (CR). A brief discussion of the possibility of NDC at the high frequencies (ω ? 2–5 × 10¹²sec^-1) under the population inversion in n-GaAs in a strong electric field¹ is also given.     

X-ray diffraction and photoelectron spectroscopy study of swift heavy ion irradiated Mn/p-Si structure

The electronic structure and interfacial chemistry of thin manganese films on p-Si (1 0 0) have been studied by photoelectron spectroscopy measurements using synchrotron radiation of 134 eV and from X-ray diffraction data. The Mn/p-Si structures have been irradiated from swift heavy ions (∼100 MeV) of Fe⁷⁺ with a fluence of 1 × 10¹⁴ ions/cm². Evolution of valence band spectrum with a sharp Fermi edge has been obtained. The observed Mn 3d peak has been related to the bonding of Mn 3d-Si 3sp states. Mn 3p (46.4 eV), Mn 3s (81.4 eV) and Si 2p (99.5 eV) core levels have also been observed which show a binding energy shift towards lower side as compared to their corresponding elemental values. From the photoelectron spectroscopic and X-ray diffraction results, Mn₅Si₃ metallic phase of manganese silicide has been found. The silicide phase has been found to grow on the irradiation.     

Laterally inhomogeneous barrier analysis of the methyl violet/p-Si organic/inorganic hybrid Schottky structures

In this work, we have investigated the electrical characteristics, such as current-voltage (I-V) and capacitance-voltage (C-V) measurements, of identically prepared crystal violet/p-Si Organic/Inorganic (OI) Schottky structures formed by evaporation of organic compound solution to directly p-Si semiconductor substrate. It has been seen that the crystal violet organic dye thin film on the p-Si substrate has exhibited a good rectifying behavior. The barrier heights (BHs) and ideality factors of all devices have been calculated from the electrical characteristics. Although the diodes were all identically prepared, there was a diode-to-diode variation: the effective barrier heights ranged from 0.6 ± 0.1 to 0.8 ± 0.1 eV, and the ideality factor from 1.6 ± 0.4 to 3.5 ± 0.4. The barrier height versus ideality factor plot has been plotted for the OI devices. Lateral homogeneous BH was calculated as a value of 0.7 eV from the observed linear correlation between BH and ideality factor, which can be explained by laterally inhomogeneities of BHs. The values of barrier height and acceptor doping concentration yielded from the reverse bias C-V measurements ranged from 0.7 ± 0.1 to 1.3 ± 0.1 eV and from (4.7 ± 0.8) × 10¹⁴ to (8.1 ± 0.8) × 10¹⁴ cm⁻³, respectively. The mean barrier height and mean acceptor doping concentration from C-V characteristics has been calculated 1.0 eV and 5.9 × 10¹⁴ cm⁻³, respectively. It has been seen that the mean BH value of 0.7 eV obtained for the Al/methyl violet/p-Si contact is significantly larger than BH values of the conventional Al/p-Si Schottky diodes. Thus, modification of the interfacial potential barrier for metal/Si diodes has been achieved using a thin interlayer of the methyl violet organic semiconductor; this has been ascribed to the fact that the methyl violet interlayer increases the effective barrier height by influencing the space charge region of Si.     

Changes in optical and electrical properties and surface recombination velocity of n-type GaN due to (NH4)2Sx treatment

We have employed the photoluminescence (PL), surface photovoltage spectroscopy (SPS) and Hall effect measurements to study the effects of (NH₄)₂S_x treatment on the optical and electrical properties of n-type GaN (n-GaN) in this study. (NH₄)₂S_x treatment of n-GaN led to the decrease of the surface recombination velocity and the increase of the band-edge emission intensity, due to the accumulation of majority carriers and the repulsion of minority carriers near the (NH₄)₂S_x-treated n-GaN surface, the removal of the native oxide existed on the n-GaN, and sulfur passivation.     

Correlation between barrier heights and ideality factors of Cd/n-Si and Cd/p-Si Schottky barrier diodes

Our goal is to experimentally investigate whether or not the effective Schottky barrier heights (SBHs) and ideality factors obtained from the current-voltage (I-V) and capacitance-voltage (C-V) characteristics differ from diode to diode even if the samples were identically prepared. For this purpose, we prepared Cd/n-Si (33 dots) and Cd/p-Si (15 dots) diodes. The SBH for the Cd/n-Si diodes ranged from 0.701 to 0.605 eV, and ideality factor n from 1.913 to 1.213. Φ_b value for the Cd/p-Si diodes ranged from 0.688 to 0.730 eV, and ideality factor n value from 1.473 to 1.040. The experimental SBH distributions obtained from the C⁻²-V and I-V characteristics were fitted by a Gaussian function and their mean SBH values were calculated. Furthermore, the laterally homogeneous barrier heights were also computed from the extrapolation of the linear plot of experimental barrier heights versus ideality factors.     

Effect of annealing temperature on the electrical properties of Au/Ta2O5/n-GaN metal–insulator–semiconductor (MIS) structure

We report on the effect of an annealing temperature on the electrical properties of Au/Ta₂O₅/n-GaN metal–insulator–semiconductor (MIS) structure by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The measured Schottky barrier height (Φ _bo) and ideality factor n values of the as-deposited Au/Ta₂O₅/n-GaN MIS structure are 0.93 eV (I–V) and 1.19. The barrier height (BH) increases to 1.03 eV and ideality factor decreases to 1.13 upon annealing at 500 ^°C for 1 min under nitrogen ambient. When the contact is annealed at 600 ^°C, the barrier height decreases and the ideality factor increases to 0.99 eV and 1.15. The barrier heights obtained from the C–V measurements are higher than those obtained from I–V measurements, and this indicates the existence of spatial inhomogeneity at the interface. Cheung’s functions are also used to calculate the barrier height (Φ _bo), ideality factor (n), and series resistance (R _s ) of the Au/Ta₂O₅/n-GaN MIS structure. Investigations reveal that the Schottky emission is the dominant mechanism and the Poole–Frenkel emission occurs only in the high voltage region. The energy distribution of interface states is determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height. It is observed that the density value of interface states for the annealed samples with interfacial layer is lower than that of the density value of interface states of the as-deposited sample.     

Electrical behaviour of lateral Al/n-GaN/Al structures

The electrical behaviour of lateral Al/n-GaN/Al structures has been studied by current-voltage measurements between a large pad with an area of 22 mm² and small contacts with different areas in the range of 0.01-1 mm². The results indicated that near room temperature the current was limited by the GaN layer exhibiting linear I-V characteristics for large contacts around 1 mm², while it was contact limited for small contacts around 0.1 mm² and below. This indicates that the same metal contact can behave as ohmic or rectifying depending on the contact area and so on the ratio of contact resistance to the series resistance of the structure.Near liquid nitrogen temperature, the current through the lateral Al/n-GaN/Al structures was limited by space charges. The Al/n-GaN contacts exhibited a very low Schottky barrier height below or around 0.2 eV. A new possible mechanism responsible for the temperature dependence of the ideality factor is proposed.     

Differential capacitance of hybrid organic/inorganic CdSe/ZnS quantum dots light-emitting device

We have studied the differential capacitance of self-assembly hybrid organic/inorganic quantum dot (QD) light-emitting devices (QD-OLEDs) at room temperature in the frequency range of 1×10³ to 1×10⁶ Hz. Self-assembly CdSe/ZnS QDs monolayer was used to fabricate trilayer hybrid QD-OLEDs on indium tin oxide coated glass substrates. We observed negative differential capacitance (NDC) in both OLED and QD-OLEDs for the first time in the intermediate frequency range of 5×10³ to 5×10⁵ Hz. The onset and frequency range of NDC are strongly depended on the applied bias voltage. This behavior is described by a mathematical model. The simulated results showed that the NDC was due to the time-dependent transient current from the QDs and organic/metal interfacial states in the device. The probable mechanisms of NDC in organic devices are discussed.     

Influences of thermal annealing, the electrolyte pH, and current density on the interface state density distribution of anodic MOS structures

2 /p-Si MOS structures were prepared in 0.1 M K₂SO₄ electrolyte with a pH of 7 (the 0.1 M KOH solution was buffered with H₂SO₄) at current densities of 3, 5, and 7 mA/cm² and with four different pH values of the electrolyte at 3 mA/cm². It is found that thermal annealing at a relatively low temperature can be used to improve the anodic MOS characteristics. Moreover, of the pH and current density it followed that the pH has a dominant role in the interface electrical properties. The lowest interface state densities at the maximum and the midgap positions are 7.1×10¹¹ and 2.7×10¹⁰ eV^-1cm^-2 for a sample made with pH=7, J=3 mA/cm². The characteristics of this sample seem satisfactory for device applications of anodized p-Si. Received: 8 July 1996/Accepted: 22 January 1997     

Comparison of confinement characters between porous silicon and silicon nanowires

Confinement character and its effects on photoluminescence (PL) properties are theoretically investigated and compared between porous silicon (p-Si) and silicon nanowires (Si-NWs). The method is based on the application of the tight-binding technique using the minimal sp³-basis set, including the second-nearest-neighbor interactions. The results show that the quantum confinement (QC) is not entirely controlled by the porosity, rather it is mainly affected by the average distance between pores (d). The p-Si is found to exhibit weaker confinement character than Si-NWs. The confinement energy of charge carriers decays against d exponentially for p-Si and via a power-law for Si-NWs. This latter type of QC is much stronger and is somewhat similar to the case of a single particle in a quantum box. The excellent fit to the PL data demonstrates that the experimental samples of p-Si do exhibit strong QC character and thus reveals the possibility of silicon clustering into nano-crystals and/or nanowires. Furthermore, the results show that the passivation of the surface dangling bonds by the hydrogen atoms plays an essential role in preventing the appearance of gap states and consequently enhances the optical qualities of the produced structures. The oscillator strength (OS) is found to increase exponentially with energy in Si-NWs confirming the strong confinement character of carriers. Our theoretical findings suggest the existence of Si nanocrystals (Si-NCs) of sizes 1-3 nm and/or Si-NWs of cross-sectional sizes in the 1-3 nm range inside the experimental p-Si samples. The experimentally-observed strong photoluminescence from p-Si should be in favor of an exhibition of 3D-confinement character. The favorable comparison of our theoretical results with the experimental data consolidates our above claims.     

Comparison of electrical parameters of Zn/p-Si and Sn/p-Si Schottky barrier diodes

In this study, current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-semiconductor (MS) Zn/p-Si and Sn/p-Si Schottky diodes, with high resistivity silicon structures, are investigated. The parameters of series resistance (R_S), the ideality factor (n) and the barrier height (Φ_b) are determined by performing different plots from the forward bias current-voltage (I-V) and reverse bias capacitance-voltage (C-V) characteristics. Thus, the barrier heights (Φ_b) for the Si Schottky diodes obtained between 0.725 and 1.051 eV, the ideality factor (n) between 1.043 and 1.309, and the series resistance (R_S) between 12.594 and 12.950 kΩ. The energy distribution of interface states density was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. It was concluded that the density of interface states in the considered energy range are in close agreement with each other values obtained for Zn/p-Si and Sn/p-Si Schottky diodes.     

The influence of interface states and bulk carrier lifetime on the minority carrier behavior in an illuminated metal/insulator/GaN structure

An influence of electronic states at an insulator/GaN interface on the behavior of excess holes in an ultraviolet-illuminated metal/ SiO₂/n-GaN structure has been studied by numerical simulations for weak (gate bias of −0.1 V ) and strong (−1 V ) depletion, in a wide range of excitation light intensities (from 10¹⁰ to 10²⁰ photons cm⁻² s⁻¹) and for various bulk carrier lifetimes (from 1 to 100 ns). It has been found that the interface states with densities of 10¹² eV ⁻¹ cm⁻² dramatically reduce the total (integrated in the whole GaN layer) density of photogenerated holes and thus degrade the sensitivity of the metal/insulator/GaN-based photodetector.     

Electrical characteristics of hole resonant tunneling diodes with high Ge fraction (x > 0.4) Si/strained Si1−xGex/Si(1 0 0) heterostructure

Effectiveness of a Ge fraction modulated spacer in hole resonant tunneling diodes (RTDs) with Si/strained Si_1−xGe_x heterostructures epitaxially grown on Si(1 0 0) was investigated to improve the electrical characteristics at higher temperatures. Electrical characteristics measured for 30 RTDs, with the modulated spacer at higher Ge fraction (x = 0.48) on a single wafer, show that the deviation of the peak current and voltage at the resonant peak falls in ranges of ±25% and ±10%, respectively. For the RTDs, negative differential conductance (NDC) characteristics are obtained even at higher temperatures around 230 K than that for the RTDs with x = 0.42. The result indicates that the introduction of higher Ge fraction is effective for NDC in RTD at higher temperature.