Barrier Height Fluctuations Research Articles

Stochasticity in ferroelectric memory devices with different bottom electrode crystallinity

This study comprehensively analyzes of the influence of bottom electrode crystallinity on the stochastic noise in ferroelectric tunnel junctions (FTJs). We perform a comparative analysis of stochastic read noise in FTJ devices with crystalline silicon (FTJc-Si) and poly-silicon (FTJpoly-Si) bottom electrodes by utilizing low-frequency noise spectroscopy and multi-frequency conductance measurements. The results indicate that FTJpoly-Si exhibits a 50 % higher read current fluctuation in the high-resistance state (HRS) at high read currents compared to FTJc-Si due to differences in interface trap density and the subsequent barrier height fluctuations. Although the crystallinity of the bottom electrode affects the noise in the HRS, the noise in the low-resistance state (LRS) remains consistent with the consequence of bulk-limited Poole-Frenkel emission. These findings clarify the relationship between bottom electrode crystallinity and stochastic noise in FTJs, providing crucial insights for the design and operation of future ferroelectric-based memory.

Electrical Behavior of Vertical Pt/Au Schottky Diodes on GaN Homoepitaxy

Schottky barrier diodes on GaN on GaN substrates are fabricated for the purpose of material and technology characterization. The epitaxial layers are grown by MOCVD. I–V measurements as a function of the temperature in the range 80–480 K show ideality factor (n) and barrier height (ϕB) variations not following a thermionic (TE) model. Consequently, barrier height fluctuations are considered. In the temperature range 280–480 K, an average barrier height of 1.31 eV with a relatively large standard deviation (σ) of 0.15 eV is extracted using this model. The n(T) variation is also analyzed in order to extract the field sensibility of 1) the mean barrier height variation (ρ2 = −0.1) and 2) the barrier height standard deviation (ρ3 = −15 mV). The corrected Richardson plot using ϕB and σ values is linear and gives a Richardson constant of 31.5 A cm−2 K−2 close to the theoretical value of 26.4 A cm−2 K−2. For a deeper understanding of ϕB fluctuation origins, micro‐Raman mapping of the epitaxial layers and deep‐level transient spectroscopy (DLTS) are used. μ‐RS mappings show compressive strain for diodes having suffered electrical breakdown. DLTS analysis shows the presence of nine levels whose signatures are extracted and nature discussed.

Phototransistors Based on hBN-Encapsulated NiPS3

Transition metal phosphorous trichalcogenides (MPX3) have been extensively investigated as photodetectors due to their wide-bandgap semiconductor properties. However, the research involved in the photoresponses at low temperatures remain blank. Here, hexagonal boron nitride (hBN)-encapsulated NiPS3 field effect transistors were fabricated by using the dry-transfer technique, indicating strong stability under atmospheric environments. The NiPS3 devices with the thickness of 10.4 nm, showed broad photoresponses from near-infrared to ultraviolet radiation at the liquid nitrogen temperature, and the minimum of rise time can reach 30 ms under the wavelength of 405 nm. The mechanism of temperature-dependent photoresponses can be deduced by competition between Schottky barrier height and thermal fluctuation. Our findings provide insights into superior phototransistors in few-layered NiPS3 for ultrasensitive light detection.

Understanding random telegraph noise in two-dimensional BP/ReS2 heterointerface

Black phosphorus (BP)-based broken gap heterojunctions have attracted significant attention mainly owing to its wide thickness-dependent Fermi level, offering opportunities to demonstrate various carrier transport characteristics and high performing optoelectronic applications. However, the interfacial effects on the carrier scattering mechanism of the two-dimensional (2D) broken gap heterojunctions are unclear. Herein, we discuss the origin of random telegraph noise of multilayer BP/ReS2 heterojunction diode, in particular, at the direct tunneling (DT) conduction regime. The gate-tunable diode characteristic of BP/ReS2 heterojunction allows one to unveil systematically the transition of the charge fluctuation mechanism from drift-diffusion to the DT regime. Unlike individual BP and ReS2 devices, the current noise histogram obtained from the BP/ReS2 heterojunction device exhibits exclusively two dominant peaks at the DT regime. We ascribed this distinct low-frequency noise feature representing the presence of random telegraph signal to the BP/ReS2 interfacial traps by taking into account of the inherent direct tunneling current conduction mechanism. In addition, the electrostatic bias-dependent power spectrum density manifests clearly that the dominant scattering mechanism is the carrier number fluctuation rather than tunneling barrier height fluctuation at the BP/ReS2 heterointerface. This study elucidates the carrier transport and the charge fluctuation mechanism at the 2D heterostructure interface.

Effects of Schottky barrier height fluctuations on conductivity: Consequences on power-law response in tin oxide gas sensors

Potential barriers formed at the intergrain of polycrystalline metal-oxide semiconductors, such as tin oxide, present fluctuations that are usually ignored in the study of gas sensors responses. Fluctuations in the Schottky-type barriers arise from the punctual character and distribution of the charges at the depletion region. Accordingly, we present a numerical model that allows to determine the distribution of the barrier heights. We then analyze the influence of fluctuations on the electrical intergrain conductivity as a function of oxygen pressure, since the signal detection in polycrystalline metal-oxide semiconductors gas sensors comes from the change of material conductivity after interaction with target gas molecules. To do so, we incorporate the fluctuation distribution found in our numerical model in the calculations of conductance. Based on these results, we could also relate the oxygen pressure with the adsorbate coverage and the fluctuating intergranular barrier. Finally, consequences on the known power-law response for gas sensors are discussed.

Investigation of Low-Frequency Noise Characteristics in Gated Schottky Diodes

In this work, the low-frequency noise (LFN) characteristics of the 3-terminal ${n}$ -type gated Schottky diode (GSD) were investigated. The GSD shows the 1/ ${f}$ noise behavior, and the noise origin depends on the operating current ( ${I}_{\text{D}}$ ) of the GSD. The 1/ ${f}$ noise is mainly generated by the barrier height fluctuation near the metal-semiconductor interface at the reverse Schottky diode (SD) in the low ${I}_{\text{D}}$ region, and by the carrier number fluctuation (CNF) at the ${n}$ -type FET in the high ${I}_{\text{D}}$ region. The LFN characteristics in the low $I_{\text{D}}$ region are strongly affected by the temperature. With increasing temperature, the transition of the $1/f$ noise origin from the reverse SD to the $n$ -type FET occurs at lower $I_{\text{D}}$ .

A Highly Sensitive Temperature Sensor Based on Au/Graphene-PVP/n-Si Type Schottky Diodes and the Possible Conduction Mechanisms in the Wide Range Temperatures

We report that the sensitive temperature response and possible Conduction Mechanisms (CMs) of Au/graphene-PVP/ ${n}$ -Si type Schottky diodes (SDs) are investigated using the standard Thermionic Emission (TE) theory at low temperatures (LTs) and high temperatures (HTs). The obtained results indicate that the zero-apparent barrier height ( $\phi _{\textit {Bo}}$ - $\phi _{\textit {ap}}$ ) increases while the ideality factor ( ${n}$ ), series and shunt resistances ( ${R} _{s}$ , ${R} _{\textit {sh}}$ ), rectifying rate (at ±2V) and surface states ( ${N} _{\textit {ss}}$ ) decrease with increasing temperature. The $\phi _{\textit {Bo}}$ , ${n}$ and ${R} _{s}$ values are also extracted from Cheung’s functions and, then compared with those obtained TE theory. The conventional Richardson plot ( $\ell {n}$ ( ${I} _{o}$ /T 2)- q/kT ) displays the deviation from the linearity at low-temperatures ( $T\le140$ K). Besides, the experimental value of Richardson constant ( ${A} ^{\ast }$ ) deduced from the intercept of plot was found to be several orders lower than the theoretical value. The discrepancies and higher values for the parameter of ${n}$ are important evidences for the deviation from TE theory. This is mainly attributed to the spatial inhomogeneities of the barrier height and potential fluctuations at the interface including low/high barrier areas. Hence the CMs across diode preferentially flows through these lower barriers/patches at the regions of LTs. The decrement in the ${N} _{\textit {ss}}$ with the enhancement in the temperature is in relation to the molecular restructuring-reordering under temperature and voltage effects. The SDs fabricated with graphene-PVP interlayer exhibit a higher sensitivity ( ${S}$ ) rather than many silicon/SOI-based structures. Numerically, the ${S}$ values are found to be in a range of 1.3 mV/K (LTs)/−1.93mV/K (HTs) in case of ${I} =0.1\,\,\mu \text{A}$ as against much greater values of −8.2 mV/K (LTs)/−7.9mV/K (HTs) for ${I} = 10\,\,\mu \text{A}$ .

Temperature dependent Schottky barrier characteristics of Al/n-type Si Schottky barrier diode with Au–Cu phthalocyanine interlayer

The Al/n-type Si Schottky barrier diodes (SBDs) were prepared with Au-Cu phthalocyanine (Au-CuPc) interlayers with different Au and CuPc compositional ratios, probed their temperature dependent current-voltage (I–V) and low frequency noise properties. The Schottky barrier properties like barrier height and the ideality factor are strongly temperature dependent, implying the existence of Schottky barrier inhomogeneities. The barrier inhomogeneities was elucidated using thermionic emission theory based on the assumption of Gaussian distribution of the barrier heights. Further, a divergence in the barrier heights evaluated from the I–V characteristics and Norde's method indicates a deviation of the Al/Au–CuPc/n-type Si SBDs behavior from thermionic emission. The Richardson plot evaluated with the Gaussian distribution of barrier heights method yielded Richardson constant values close to theoretical Richardson constant of n-type Si. The room temperature current noise power spectral density (SI) of the Al/Au–CuPc/n-type Si SBDs having various Au and CuPc concentrations showed a reciprocal of frequency (1/f)γ behavior having γ values near to unity. The decrease in SI and γ with increasing Au concentration in Au-CuPc interlayers was indicative of lessening barrier inhomogeneity. This was also correlated with the decrease in the value of standard deviation derived using temperature dependent I–V characteristics. Further investigations revealed that origins of 1/f noise could be associated with the fluctuations of barrier height and carrier mobility for Al/Au–CuPc/n-type Si SBDs with lower and higher Au compositions in Au-CuPc interlayers, respectively.

Chaotic Potential on the Degenerated Semiconductor Surface

In this paper, natural heterogeneities of the potential on the degenerated semiconductor surface are discussed as well as the barrier height fluctuations in metal-semiconductor contacts. In the case of electroactive dopant linear screening, characteristic values of chaotic potential amplitudes have been defined. The dependence is shown of these heterogeneities on semiconductor electrophysical parameters.

Electron Mobility due to Surface Roughness Scattering in Depleted GaAs Free-Standing Thin Ribbon

Abstract:Electron mobility limited by surface roughness scattering in free-standing GaAs thin ribbon with an internal parabolic quantum well caused by surface state is investigated in detail. Based on analyzing the parabolic quantum well including the energy subband level, wave function and the confined potential profile in the thin ribbon by solving Schrödinger and Poisson equations self-consistently, the electron mobility could be investigated. Conclusion indicates that remote surface roughness (RSR) of the thin ribbon will change the two dimensional electron gas (2DEG) mobility through the medium of barrier height fluctuation of the parabolic well in atomic scale. Calculation results reveal that the 2DEG mobility decreases with increasing roughness amplitude, which is characterized in terms of the surface roughness height and the roughness lateral size.

Graphene electrode with tunable charge transport in thin-film transistors

Graphene, a single atomic layer of sp2-hybridized carbon, has immense potential as a transparent conducting material in electronic applications owing to its superior properties, including optical transparency and high conductivity. Particularly, the tunable work function of graphene enables the integration of graphene electrodes with various electronic devices. To achieve high performance in graphene-based devices, effective charge transport between the graphene electrode and the semiconducting material needs to be optimized; this is closely related to the modulation of the Schottky barrier (SB). In this study, we investigate the tunable charge transport properties as a function of graphene doping in n-channel thin-film transistors (TFTs) in terms of the electrical characteristics and low-frequency noise (LFN) behaviors. Alkali metal carbonates tuned the work function of graphene, resulting in a dramatic decrease in the SB and an improvement of the carrier injection in n-channel TFTs. The electrical performance of the TFTs was evaluated by extraction of the field-effect mobilities and ratio of contact resistance to total resistance. Furthermore, the level of contact noise created by the barrier height fluctuation and relative contribution of channel noise and contact noise in the TFTs was investigated by LFN measurements to demonstrate the tunable charge transport. Our findings therefore provide new insights into the tunable charge transport mechanism in graphene-based devices and reveal the immense potential of graphene as electrodes in high performance flexible and transparent displays.

Intergranular barrier height fluctuations in polycrystalline semiconductors

The punctual character and random nature of the impurity positions in depletion regions lead to inhomogeneities that can significantly affect the potential intergranular barriers at polycrystalline semiconductors and, in general, of potential barriers at any semiconductor interface. This would reflect in Arrhenius plots for the electrical conductivity that become curved due to fluctuations of the intergranular barrier heights. Experimental results for polycrystalline tin oxide can be fitted assuming thermionic emission conduction at grain boundaries with a Gaussian distribution of barrier height fluctuations. However, resorting to a computational numerical model, we found that spatial fluctuations in barrier heights due to the discreteness of the donors and their statistical distribution at the depletion region differ from a Gaussian distribution. The type of obtained fluctuations, considering thermionic emission conduction, cannot explain the Arrhenius plots for the electrical conductivity found experimentally, especially at low temperature. Conversely, the tunneling contribution to conduction, without resorting to fluctuations, presents the observed trends.

Nanocube In2O3@RGO heterostructure based gas sensor for acetone and formaldehyde detection

Here, we demonstrated the formation of active sites O2− and O− on the surface of nanocube In2O3@RGO heterostructure, gas sensing mechanisms and fluctuations in potential barrier height before and after insertion of acetone and formaldehyde.

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.

Temperature dependent current transport of Pd/ZnO nanowire Schottky diodes

Zinc oxide (ZnO) nanowire based Schottky barrier diodes are fabricated by depositing Pd metal contact on top of vertically well-aligned ZnO nanowire arrays. A vertical array of ZnO nanowires on indium tin oxide (ITO) coated glass substrates is synthesized by hybrid wet chemical route. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurement confirm the formation of stoichiometric well-aligned hexagonal (h-ZnO) nanowire arrays with wurtzite structure. Temperature dependent current–voltage (I–V) measurements on palladium-ZnO (Pd/ZnO) nanowire Schottky junctions in the temperature range 303–383 K exhibit excellent rectifying character. From these nonlinear I–V plots, different electrical parameters of diode-like reverse saturation current, barrier height and ideality factor are determined as a function of temperature assuming pure thermionic emission model. The ideality factor is found to decrease while the barrier height increases with the increase in temperature. The series resistance values calculated from Cheung’s functions also show temperature dependency. Such behavior can be attributed to the presence of defects that traps carriers, and barrier height inhomogeneity at the interface of the barrier junction. After barrier height inhomogeneity correction, considering a Gaussian distributed barrier height fluctuation across the Pd/ZnO interface, the estimated values of mean barrier height and modified Richardson constant are more closely matched to the theoretically predicted value for Pd/ZnO Schottky barrier diodes. The variation of density of interface states as a function of interface state energy is also calculated.

Spatial fluctuations in barrier height at the graphene–silicon carbide Schottky junction

When graphene is interfaced with a semiconductor, a Schottky contact forms with rectifying properties. Graphene, however, is also susceptible to the formation of ripples upon making contact with another material. Here we report intrinsic ripple- and electric field-induced effects at the graphene semiconductor Schottky junction, by comparing chemical vapour-deposited graphene transferred on semiconductor surfaces of opposite polarization-the hydrogen-terminated silicon and carbon faces of hexagonal silicon carbide. Using scanning tunnelling microscopy/spectroscopy and first-principles calculations, we show the formation of a narrow Schottky dipole barrier approximately 10 Å wide, which facilitates the observed effective electric field control of the Schottky barrier height. We further find atomic-scale spatial fluctuations in the Schottky barrier that directly follow the undulation of ripples on both graphene-silicon carbide junctions. These findings reveal fundamental properties of the graphene/semiconductor Schottky junction-a key component of vertical graphene devices that offer functionalities unattainable in planar device architecture.

Universality of transport properties of ultrathin oxide films

We report low-temperature measurements of current–voltage characteristics for highly conductive Nb/Al–AlOx–Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultrathin barriers formed by diffusive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance compared with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the aluminum oxide barrier proposed earlier by Schep and Bauer (1997 Phys. Rev. Lett. 78 3015). We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations in the nearest to conducting electrode layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.

Dynamic effects of dipolar interactions on the magnetic behavior of magnetite nanoparticles

Isothermal magnetization and initial dc susceptibility of spheroidal, nearly monodisperse magnetite nanoparticles (typical diameter: 8 nm) prepared by a standard thermo-chemical route have been measured between 10 and 300 K. The samples contained magnetite nanoparticles in the form of either a dried powder (each nanoparticle being surrounded by a stable oleic acid shell as a result of the preparation procedure) or a solid dispersion in PEGDA-600 polymer; different nanoparticle (NP) concentrations in the polymer were studied. In all samples the NPs were not tightly agglomerated nor their ferromagnetic cores were directly touching. The high-temperature inverse magnetic susceptibility is always found to follow a linear law as a function of T, crossing the horizontal axis at negative temperatures ranging from 175 to about 1,000 K. The deviation from the standard superparamagnetic behavior is related to dipolar interaction among NPs; however, a careful analysis makes it hard to conclude that such a behavior originates from a dominant antiferromagnetic character of the interaction. The results are well explained considering that the studied samples are in the interacting superparamagnetic (ISP) regime. The ISP model is basically a mean field theory which allows one to straightforwardly account for the role of magnetic dipolar interaction in a NP system. The model predicts the existence of specific scaling laws for the reduced magnetization which have been confirmed in all studied samples. The interaction of each magnetic dipole moment with the local, random dipolar field produced by the other dipoles results in the presence of a large fluctuating energy term whose magnitude is comparable to the static barrier for magnetization reversal/rotation related to magnetic anisotropy. On the basis of the existing theories on thermal crossing of a barrier whose height randomly fluctuates in time it is predicted that the rate of barrier crossing is substantially driven by the rate of barrier fluctuations, which is fast (108–109 Hz) and almost independent of temperature. As a consequence, the standard picture of superparamagnetic NPs which undergo single-particle blocking by a static barrier below the blocking temperature should be substantially revised, at least in the present materials. The ISP model is perfectly matching with the view of activated magnetization rotation whose kinetics is significantly modified by barrier height fluctuations.

Resistance state-dependent barrier inhomogeneity and transport mechanisms in resistive-switching Pt/SrTiO3 junctions

We investigated the current–voltage (I−V) and photocurrent characteristics of Pt/Nb-doped SrTiO3 (001) single-crystal junctions that exhibit resistive-switching behaviors. The temperature-dependent I−V data and the photocurrent spectra showed that the barrier height fluctuation depended on the resistance state but the mean barrier height was nearly constant regardless of the junctions’ resistance state. In addition, local barrier height variations allowed transitions from thermionic to tunneling transport for the low-resistance state.

Impact of carbon concentration on 1∕f noise and random telegraph signal noise in SiGe:C heterojunction bipolar transistors

The influence of carbon concentration on the low-frequency noise (LF noise) of Si∕SiGe:C∕Si heterojunction bipolar transistors (HBTs) is investigated. When carbon is incorporated into these HBTs, representative noise spectra of the input current spectral density SIB show significant generation-recombination (GR) components. On the other hand, for transistors without carbon incorporation, no GR components were observed. When only 1∕f noise component is observed, the 1∕f noise level is found to be independent of the carbon concentration and the associated figure of merit of the normalized noise magnitude KB has a very good value of ∼4×10−10μm2. In order to relate the 1∕f noise and the high-frequency performance of the transistor, we studied and modeled the figure of merit defined as the ratio fc∕fT (fc is the low-frequency corner frequency and fT the unity current-gain frequency). Then we performed a detailed analysis of the GR components associated with the presence of the carbon. We found that the observed Lorentzian spectra are associated with random telegraph signal (RTS) noise. However, no RTS noise was measured in carbon-free devices. It is believed that the RTS noise is due to electrically active defects formed by the addition of carbon, typically observed for concentrations above the bulk solid solubility limit in silicon. The RTS amplitude (ΔIB) is found to scale with the base current, to decrease exponentially with temperature, and to be independent of the carbon concentration. The mean pulse widths (tH,tL) of the RTS are found to decrease rapidly with bias voltage, as 1∕exp(qVBE∕kT) or stronger. Our results confirm that electrically active C-related defects are located in the base-emitter junction, and the RTS amplitude is explained by a model based on voltage barrier height fluctuations across the base-emitter junction induced by trapped carriers in the space charge region. The observed bias dependence of mean pulse widths seems to indicate that two capture processes are involved, electron and hole capture. These C-related defects behave like recombination centers with deep energy levels rather than electron or hole traps involving trapping-detrapping processes.