Observed Barrier Height Research Articles

Characterization of budding twigs of flower-type zinc oxide nanocrystals for the fabrication and study of nano-ZnO/p-Si heterojunction UV light photodiode

In this work, we focused on the study of the structural and optical properties of chemical precipitation-derived special shape (budding twigs of flower-type) ZnO nanostructures for optoelectronic and photodetection applications. The structural and optical properties of the budding twigs of Jasminum flower-like ZnO nanocrystals have been studied and discussed in detail from the XRD, HRTEM, UV–Vis, and photoluminescence spectra. The grown ZnO nanocrystals have been coated on the p-Si substrate to fabricate nano-ZnO/p-Si heterojunction photodiode. The junction properties of the fabricated photodiode were examined by measuring ultraviolet (UV)-light-dependant (λ ~ 366 nm) and dark condition current (I)–voltage (V) as well as capacitance (C)–voltage (V) characteristics. The photodetection properties of the diode in the UV light region have been examined. The diode has well-defined rectifying behavior with a photoresponsivity and external photodetection efficiency of 0.065 and 21.5%, respectively. The observed barrier height and donor concentration under dark condition were ≈ 0.25 eV and 2 × 1017 cm−3, respectively. The change in heterojunction capacitance, barrier height, the depletion width, and other parameters of the heterojunction photodiode under UV illumination has been discussed. The qualities of the device demonstrate that it tends to be used for UV photodetection applications in nano-optoelectronic and photonic devices.

Surface Al doping of 4H-SiC via low temperature annealing

We present a method of forming shallow p-doping on a 4H-SiC surface by depositing a thin Al layer (d = 5 nm) and then thermally annealing it at 1000 °C for 10 min. A secondary ion mass spectrometry analysis of the annealed Al/SiC sample reveals an Al concentration in excess of 1017 cm−3 up to a depth of d ≤ 250 nm. I–V measurements and CV characterizations of Ti-SiC Schottky barrier diodes (SBDs) fabricated on a n-type SiC epi-wafer indicate that the shallow Al doping increases the built-in potential of the junction and the barrier height by ΔVbi=0.51 eV and ΔϕB=0.26 eV, respectively. Assuming a rectangular doping profile, calculations of the built-in voltage shift and the Schottky barrier height indicate that partial dopant activation (activation ratio ∼2%) can induce the observed barrier height shift. The shallow doping method was then used to fabricate junction terminations in SBDs which increased the breakdown voltage and reduced the reverse leakage current. Technology CAD simulations of the SBD with and without doping verify that a reduction of peak electric field can explain the improvement of the breakdown voltage.

Significant effect of the molecular formula in silver nanoparticle ink to the characteristics of fully gravure-printed carbon nanotube thin-film transistors on flexible polymer substrate

Single-walled carbon nanotubes (SWNTs) are a valuable material for use in not only nanoelectronics but also printed electronics because of their stability, tunable operation speed, and scalability. However, the device characteristics of fully printed, SWNT-based thin film transistors (SWNT-TFTs) often have large variations, and the fundamental cause of these inconsistencies are not yet well understood. Therefore, fully printed SWNT-TFT-based electronic devices have not been practically realized in the market. In this study, the significant variation in the electrical parameters of printed SWNT-TFTs that is caused by minor molecular variations in the formulation of silver nanoparticle-based ink is reported. Strikingly, a very small difference in the chemical structure between ethylene glycol and diethylene glycol in the silver nanoparticle-based ink, which is used to print drain-source electrodes in the SWNT-TFTs, with everything else identical, induced a difference of approximately 70 meV in the barrier height between the drain-source electrodes and the SWNT layer at 300 K. The modification of the absorbed polymer binder in the silver nanoparticle ink due to the additive is the major cause of the observed barrier height difference. These results allow for a better understanding of the relationship between the ink rheology at the molecular level and the printed device properties, and enable a more precise design and control of device properties which will have profound impacts on printed electronic devices.

Barrier height fluctuations in InGaN polarization dipole diodes

We present an analysis of the effects of lateral barrier height fluctuations on the transport properties of an InGaN polarization dipole diode (PDD). Typical diode analysis proceeds by performing a linear fit to the data contained in a Richardson plot in order to extract the zero-bias barrier height (ϕB) and the Richardson constant (A*). The experimental PDD Richardson plot, however, is highly nonlinear and we attribute this to a spatially nonuniform distribution of barrier heights. After modeling the barrier height distribution using a Gaussian, we fit the Richardson data with a modified, second-order function from which we extract the Richardson constant, mean barrier height, and standard deviation simultaneously. We propose that the physical cause of the observed barrier height inhomogeneity in the PDD is statistical nanoscale fluctuations in indium composition.

Barrier height inhomogeneity in electrical transport characteristics of InGaN/GaN heterostructure interfaces

We have grown InGaN/GaN heterostructures using plasma-assisted molecular beam epitaxy and studied the temperature dependent electrical transport characteristics. The barrier height (φb) and the ideally factor (η) estimated using thermionic emission model were found to be temperature dependent. The conventional Richardson plot of ln(Js/T2) versus 1/kT showed two temperature regions (region-I: 400–500 K and region-II: 200–350 K) and it provides Richardson constants (A∗) which are much lower than the theoretical value of GaN. The observed variation in the barrier height and the presence of two temperature regions were attributed to spatial barrier inhomogeneities at the heterojunction interface and was explained by assuming a double Gaussian distribution of barrier heights with mean barrier height values 1.61 and 1.21 eV with standard deviation (σs2) of 0.044 and 0.022 V, respectively. The modified Richardson plot of ln(Js/T2) − (q2σs2/2k2T2) versus 1/kT for two temperature regions gave mean barrier height values as 1.61 eV and 1.22 eV with Richardson constants (A∗) values 25.5 Acm−2K−2 and 43.9 Acm−2K−2, respectively, which are very close to the theoretical value. The observed barrier height inhomogeneities were interpreted on the basis of the existence of a double Gaussian distribution of barrier heights at the interface.

Electrical characterization of Au/ZnO/Si Schottky contact

In this study, temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements have been performed on Au/ZnO/Si Schottky barrier diode in the range 150 – 400K. The room temperature values for ideality factor and barrier height found to be 2.68 and 0.68 eV respectively. From the temperature dependence of I–V, the ideality factor was observed to decrease with increasing temperature and barrier height increased with increasing temperature. The observed barrier height trend was disagreeing with the negative temperature coefficient for semiconductor. A deep defect with activation energy 0.57 eV below the conduction band was observed using the saturation current plot and deep level transient spectroscopy.

Impact of fixed charge on metal-insulator-semiconductor barrier height reduction

Recently, the insertion of ultrathin insulators to form metal-insulator-semiconductor (MIS) contacts has been used extensively to reduce the Schottky barrier height and to shift the Fermi level pinning. In this paper, we investigate the physical non-idealities of the ultrathin insulator in Al/Al2O3/n-GaAs MIS through stoichiometry, density, and bandgap measurements. These structural non-idealities electrically manifest as bulk and interface fixed charges that are found to contribute to the observed barrier height reduction. The effect of fixed charge has not been considered before, and when combined with the previously reported interface dipoles, it provides a more thorough understanding of the MIS contacts.

Coupling of structural fluctuations to deamidation reaction in triosephosphate isomerase by Gaussian network model

We study the structural fluctuations of triosephosphate isomerase (TIM) by an elastic model, namely, the Gaussian network model (GNM), to identify a network of coupled motions in the allosteric communication between its deamidation and catalytic sites, and the promoting motions for the deamidation activity. For this, three TIM structures have been studied: one crystal structure and two model structures designed to describe different putative models for the deamidation reaction taking place at the subunit interface. The structural fluctuations have been mapped on the functional properties; then the differences in the fluctuations between the two models in relation to the deamidation reaction have been considered. The results demonstrate that the qualitative picture of the mean-square fluctuations and the correlations between the fluctuations are similar in both, but the differences may affect the observed barrier height of the deamidation reaction. The higher packing density at regions close to deamidation sites, reflected by the high-frequency fluctuating residues in the respective regions, the stronger positive correlation between the fluctuations of the deamidation sites, and enhanced positive correlation of the primary deamidation site with the extended vicinity of the catalytic region on the juxtaposed unit promote the probability of the deamidation reaction. The results in general emphasize the importance of structural fluctuations in enzyme reactions, as well as proposing the present methodology as a plausible approach for studies on the network of coupled promoting motions in protein functions.

Measurements of potential barrier height of grain boundaries in polycrystalline silicon by Kelvin probe force microscopy

In recent years, the importance of polycrystalline silicon has been recognized in electronic device technology, although grain boundaries present in the material often exert a detrimental influence on the electrical properties because of the potential barriers associated with them. However, it is not true that all grain boundaries have similar properties, since they have their own character depending on the orientation relationship between two adjoining grains. We report here the first determination of the potential barrier height for well-characterized grain boundaries in polycrystalline silicon, using Kelvin probe force microscopy. The observed barrier height of the grain boundaries was found to vary in the range 10 to 100 meV depending on the grain boundary character. The most important finding is that the potential barrier height is approximately twice as high at random boundaries as at low-energy coincidence boundaries.

A frequency analysis of the ν9 band of CD3CD3: experiment and ab initio calculations

The infrared gaseous spectrum of CD3CD3 has been measured in the range of 530–670cm−1 to investigate vibration—torsion effects in the ν9 band. Three separate spectra all taken under different experimental conditions were recorded. The lines with (ΔK = −1) and with high values of K show torsional splittings that are substantially larger than expected from the observed barrier height. These splittings are caused primarily by Coriolis-type interactions between the torsional stack of ν9 = 1 and the corresponding stack for the ground vibrational state. Because of a near-degeneracy that exists between the states (ν9 = 0, ν4 = 3) and (ν9 = 1, ν4 = 0), three subbands (K, σ) = (15,1), (16,2), (17,3) are resonantly perturbed. For these cases, perturbation-allowed 3ν4 torsional transitions have been identified. Here σ= 0, 1, 2 or 3 labels the torsional sublevels. Measurements from the ν9 and 3ν4 bands, frequencies from the far-infrared torsional spectra in the ground vibrational state, and lower state combination differences from the ν9 + ν4 − ν4 band were fitted to within experimental uncertainty using an effective Hamiltonian which considered three torsional stacks; one for the ground vibrational state and two for ν9 = 1. In all, 22 parameters were determined using a total of 2001 lines. Of these, three parameters were the interstack couplings, eight are from the ground vibrational state and 11 are from the excited vibrational state. Two barrier-dependent torsion—rotation parameters, which were essential for obtaining a satisfactory fit, were calculated by ab initio methods.

A Combined Frequency Analysis of the ν3, ν9, 3ν4 and the Far-Infrared Bands of Ethane: A Reassessment of the Torsional Parameters for the Ground Vibrational State

The lowest frequency parallel fundamental band ν3 of ethane is Raman active. A stimulated Raman spectrum of the Q branch for this band at a resolution of 0.0055 cm−1 has been measured by D. Bermejo et al. (1992, J. Chem. Phys.97, 7055). The torsion–rotation series in this band with σ=3, where σ=0, 1, 2, and 3 labels the torsional sublevels, is perturbed by over 1 cm−1. The lowest frequency-degenerate fundamental ν9 is infrared active. A high-resolution (0.0014 cm−1) Fourier transform spectrum of this band has been measured by N. Moazzen-Ahmadi et al. (1999, J. Chem. Phys.111, 9609). The observed torsional splittings for this band are substantially larger than expected from the observed barrier height. Because of a near-degeneracy of the upper level in the ν9 band with its interacting partner (v9=0, v4=3) a perturbation allowed band 3ν4 has also been observed. We have carried out a combined analysis of ν3, ν9, and 3ν4 together with the far-infrared torsional spectra in the ground vibrational state (gs). A fit to within the experimental error was achieved using 37 parameters. The large torsional splittings in the ν9 band are attributed to Coriolis-type interactions between the torsional stacks of gs and v9=1 whereas the large shift for the torsion–rotation series with σ=3 in the ν3 band is attributed to Fermi-type interactions between the torsional stacks of the gs and v3=1. The introduction of the Fermi-type interactions causes a considerable change in the leading terms in the torsional Hamiltonian for the gs. These changes are quantitatively explained.

Correlation of microstructure with electrical behavior of Ti/GaN schottky contacts

We correlate structural and electrical characteristics of as-deposited and low-temperature annealed Ti contacts on GaN. Temperature dependent currentvoltage measurements are used to determine the effective barrier heights of the respective contacts, while high-resolution transmission electron microscopy is utilized for structural characterization. As-deposited Ti contacts are slightly rectifying with an effective barrier height of ∼200 meV. After annealing at 230°C, the barrier height increases to values of ∼450 meV. A similar behavior of Schottky contacts with more strongly rectifying diodes upon low-temperature annealing is observed for Zr metal contacts on GaN. As-deposited Ti already forms a thin TiN layer at the GaN interface. After annealing at 230°C, the average thickness and the distribution of TiN grains remain practically unchanged, but the interface with GaN roughens. We correlate the observed barrier height changes with interface roughness and phase formation and we discuss the results in terms of interface damage and the Schottky-Mott theory.

Interfacial Fermi level and surface band bending in Ni/semi-insulating GaAs contact

For nickel on the chemically clean surface of undoped semi-insulating GaAs at room temperature, an upward surface band bending of 0.062 eV and a barrier height of 0.690 eV have been observed by the photovoltage and the internal photoemission techniques, respectively. The observed surface band bending is in excellent agreement with its predicted value, and the observed barrier height also agrees very well with its value from the very careful analysis of reversed I-V data. It has been determined that the interfacial Fermi level lies at 0.690 eV below the GaAs conduction band minimum at the interface. The interfacial Fermi level is found to coincide with the energy level of the EL2 native defect, indicating the importance of the EL2 in the Fermi level pinning at the interface.

Ab initio H 2 desorption pathways for H/Si(100): the role of SiH 2(a)

We present ab initio calculations examining two previously proposed mechanisms for H 2 desorption from the Si(100)-2 × 1 monohydride phase: (i) the “prepairing” mechanism, where H 2 desorbs directly in a one-step process via two hydrogen atoms paired on one silicon dimer and (ii) a stepwise mechanism in which H 2 desorbs from a dihydride intermediate formed via isomerization of the monohydride. Both pathways are predicted to be 66 kcal mol endothermic. A detailed search of the transition state region rules out the direct one-step mechanism, as only one saddle point was found and a search of the reaction path showed that it evolves from the dihydride intermediate rather than the monohydride. This saddle point for the second pathway corresponds to a desorption activation barrier of 94 kcal mol , which is much higher than those measured by thermal desorption experiments (45–66 kcal mol ) . Other prepairing desorption pathways involving H 2 desorption from two neighboring hydrogen atoms on adjacent dimers are argued to be inconsistent with the observed first-order kinetics. Thus, no previously proposed mechanism appears consistent with both the observed barrier height and reaction order. We propose an alternative mechanism involving H atom diffusion prior to H 2 desorption. In particular, our calculations suggest two constraints on the mechanism: (i) H 2 must desorb from a SiH 2(a) species that either has no memory of how it was formed or is formed by means of a step no more than ~ 10 kcal mol endothermic and (ii) H atom surface diffusion to form SiH 2(a) plays a key role in determining the reaction orders on different Si surfaces. Our results indicate that H 2 always desorbs solely from SiH 2(a) independent of θ H or surface structure. At low coverages (θ H ⩽ 1 ML), where the monohydride phase is present, the activation barrier for H 2, desorption from SiH 2(a), is predicted to be 55 kcal mol . We predict that the activation barrier decreases with increasing θ H, reaching a minimum of 38 kcal mol at θ H = 2 ML, in good agreement with experiment.

Barrier height temperature dependence of Au/InP from photoreflectance spectroscopy

The barrier height of Au/InP was measured at various temperatures from Franz–Kelydsh oscillations (FKO) of photoreflectance spectroscopy. It appears that the photoinduced voltage can not be neglected especially at low temperature. The presently observed barrier height temperature dependence confirms the theory of Hecht [M. Hecht, Phys. Rev. B 41, 7918 (1990)]. Also, the mixture between FKO and E0+Δ0 oscillations was observed. The degree of mixture varies with temperature.

Structural dynamics of proteins investigated by hole-burning spectroscopy

Low-frequency vibrations in a conformational substate and thermal crossing over conformational barriers have been studied in Zn-substituted and free-base myoglobins by persistent spectral hole-burning. First, in order to investigate the nature of the vibrational modes (or phonons) and the electron-phonon coupling, phonon sideband profiles have been calculated on the basis of a normal-mode analysis for myoglobin, and compared with the observed sideband hole spectra. It has been found that the calculated and the experimental results agree well in the region above 10 cm-1. Next, the distribution of barrier heights between the conformational substates has been obtained by a heat cycle experiment and found to depend on the species that substitute the heme iron and on the presence of a solvent. The primary factor that determines the observed barrier height distributions will be discussed.

Schottky-limit barrier heights for CO-coated metal clusters on GaAs(110)

This letter discusses band bending induced by the deposition of metal clusters coated with onto GaAs(110). The layer of CO between the metallic clusters and the undisrupted semiconductor simulates a metal-insulator-semiconductor junction. The observed barrier height shows Schottky-limit-like dependence on the work function of the metal.

Thermionic currents and acceptor diffusion in p+-In0.53Ga0.47As/n-InP heterojunctions

p +-In0.53Ga0.47As/n-InP heterojunctions grown by liquid-phase epitaxy have been investigated through I-V and I0(T) measurements. It is shown that the injection of electrons across the heterointerface is well described by a thermionic emission model characterized by a barrier height which is strongly affected by p-type dopant diffusion. Heterojunction bipolar transistors with Schottky collectors were used as tools to measure the injection current in as-grown p+-In0.53Ga0.47As/n-InP diodes, and the results were compared to the predictions of the thermionic emission model. In this way, the relative diffusivity in InP at 600 °C of three p-type dopants, Mn, Mg, and Be, have been evaluated. The results can be summarized as follows: DBe/DMn≊20, DMg/DBe≊1. The effects of postgrowth heat treatments on the observed barrier height are also discussed.

The use of Au-Cd alloys to achieve large Schottky barrier heights on CdTe

The Schottky barrier height of Cd, Au, and Au-Cd alloys on vacuum cleaved surfaces of n-CdTe was determined by I-V, C-V, and photoresponse techniques. A large barrier of 0.92 eV was found in the case of the Au-Cd alloy contacts. Contacts made with elemental Cd or Au gave barrier heights of 0.45 and 0.65 eV, respectively. The increased barrier height found on Au-Cd alloy contacts may be related to recent UHV observations on Schottky barrier formation where crystal defects play a role in determining the observed barrier height.

Influence of a thin oxide layer between metal and semiconductor on Schottky diode behavior

Two frequently observed problems with Schottky diodes are soft current–voltage characteristics and low avalanche breakdown voltages. These problems are sometimes found immediately upon fabrication, or they may develop during use. It is proposed that these phenomena can be explained by the existence of a thin layer (25–250 Å thick) between the metal and semiconductor which (i) has a high charge density, (ii) has a high trap density, and (iii) is conducting. Observed barrier height changes are explained by the trapping of charge carriers flowing through the layer and the lowered avalanche breakdown voltage by heating of carriers in the high field in the intervening layer. These explanations have been confirmed by measurements on GaP and GaAs diodes having a deliberately grown thin interfacial oxide ∠100 Å thick. Such an interfacial layer can be detected using a pulsed current–voltage technique described in the paper.