Ballistic Electron Emission Microscopy Research Articles

Going ballistic: a novel characterization for the electronic energy gap.

The energy gap of molecular semiconductors is a critical parameter in molecule-based devices because it fundamentally determines the operating principle and device performance, such as the charge transfer driving force. Hence, an accurate quantification of the energy gap of a molecular semiconductor is essential. The hot electron technique, which is implemented using a hot electron transistor and ballistic electron emission microscope, has been verified as the most powerful characterization in recent years. By monitoring the charge transport on the metal/molecule interface, an electron-injection barrier (or LUMO) and hole-injection barrier (or HOMO) can be achieved, which contributes to the electronic energy gap determination. In this review, a comprehensive comparison of these two techniques was made. It helps us to select a suitable method in specific situations and provides a profound comprehension of the charge transport process in molecular semiconductor and molecule-based devices.

Hot electron spectroscopy: A novel method to study molecular semiconductor

Molecular semiconductors, as the core component, determine the performance of different devices. It is essential to gain more knowledge of such materials. Hot electron spectroscopy, by applying a ballistic electron emission microscope and hot-electron transistor, has been demonstrated to be the most powerful method. This review concentrates on the achievements in several emerging fields, such as the interfacial energy level alignment and the charge transport characteristics in molecular film. As a versatile technique, hot electron spectroscopy has been found to be helpful in exploring the metal/molecule interface energy, the molecular levels, the electronic transport gap, and even the group structures. In comparison to the traditional techniques, such as ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and scanning tunneling spectroscopy, this method offers in-situ characterizations, which makes the detection much more accurate and convenient. This review contributes to a deep comprehension of the intrinsic characteristics of molecular semiconductors and molecule-based devices. • Hot electron spectroscopy has recently been employed to determine the intrinsic lowest unoccupied molecular orbital. • Hot electron spectroscopy has been developed to determine the highest occupied molecular orbital. • Hot electron spectroscopy has applied to acquire more informations about molecular levels, such as the LUMO+1 level. • Hot electron spectroscopy provides a powerful approach to achieve a high-efficient spin injection.

Determination of the energetic resolution of Schottky barrier visualization via interface band structure and parallel momentum conservation

The energetic resolution of Schottky barrier visualization is determined by utilizing differences in interface band structures between the Au/Si(001) and Au/Si(111) non-epitaxial interfaces and parallel momentum conservation of the carriers. The visualization technique is based on ballistic electron emission microscopy and spectroscopy, where tens of thousands of spectra are collected on a grid and then fit to extract a spatially resolved map and histogram of the electrostatic barrier height. A resolution of 10 meV is determined from the minimal splitting and eventual merging of the histograms as the gold thickness decreases for the Au/Si(001) and Au/Si(111) samples. This splitting is below previously measured differences in barrier heights extracted from computational modeling of measured barrier height distributions from other interfaces.

Benchmarking β‐Ga2O3 Schottky Diodes by Nanoscale Ballistic Electron Emission Microscopy

AbstractMonoclinic beta‐phase gallium oxide (β‐Ga2O3) is an ultrawide‐bandgap semiconductor, intensively studied as a viable candidate for next‐generation power electronics, optoelectronics, and extreme environment electronics. Schottky contacts to β‐Ga2O3 are of paramount importance to this end; however, they are not yet fundamentally understood. Intrinsic sources of interfacial disorder, including oxygen‐related defects and extrinsic fabrication factors, are thought to greatly determine the properties of such contacts, for example by originating Fermi level pinning and causing patches with different Schottky barrier heights (SBHs). Ballistic electron emission microscopy (BEEM) is used to probe band bending and interfacial inhomogeneity at the nanoscale for prototypical Au/ and Pt/(100)β‐Ga2O3 single crystal Schottky barrier diodes. It is shown that SBH fluctuations amount to 40–60 meV under vacuum, occurring over length scales of tens of nanometers. Furthermore, a remarkable SBH modulation of ≈0.2 eV takes place upon exposure of devices from vacuum to ambient air. Such findings—better obtained by BEEM than by macroscale approaches—point to the existence of an ubiquitous inhomogeneous interfacial layer, controlling band bending and ambient sensitivity via oxygen ionosorption and interface redox chemistry. This study ascribes a key role to interfacial oxygen vacancies, and has practical implications for transport modelling and interface engineering.

Accurate ab initio determination of ballistic electron emission spectroscopy: Application to Au/Ge

Ab initio nonequilibrium Keldysh formalism based on an $N$-order renormalization technique is used to compute $I(V)$ ballistic electron emission microscopy characteristics at the Au/Ge(001) interface. Such a formalism quantitatively reproduces precise experimental measurements under ultrahigh vacuum and low-temperature conditions. At $T=0$ K, the ballistic current follows the law ${(V\ensuremath{-}{V}_{\mathrm{SB}})}^{2.1}$, ${V}_{\mathrm{SB}}$ being the Schottky barrier. At $Tg0$ K, temperature effects become significant near the onset and must be taken into account to identify an accurate value for ${V}_{\mathrm{SB}}$ from a best-fit procedure. We find two values for ${V}_{\mathrm{SB}}$, 0.67 and 0.75 eV, which we associate with two different atomic registries at the interface.

Temperature- and doping-dependent nanoscale Schottky barrier height at the Au/Nb:SrTiO3 interface

We use ballistic electron emission microscopy to investigate prototypical Au/Nb-doped SrTiO3 (NSTO) Schottky barrier diodes for different temperatures and doping levels. To this end, ultrathin Au overlayers are thermally evaporated onto TiO2-terminated NSTO single crystal substrates. We show that at room temperature, regardless of the nominal doping, rectification is controlled by a spatially inhomogeneous Schottky barrier height (SBH), which varies on a length scale of tens of nanometers according to a Gaussian distribution with a mean value of 1.29–1.34 eV and the standard deviation in the range of 80–100 meV. At lower temperatures, however, doping effects become relevant. In particular, junctions with a low Nb content of 0.01 and 0.05 wt. % show an ∼300 meV decrease in the mean SBH from room temperature to 80 K, which can be explained by an electrostatic analysis assuming a temperature-dependent dielectric permittivity for NSTO. In contrast, this model fails to predict the weaker temperature dependence of SBH for junctions based on 0.5 wt. % NSTO. Our nanoscale investigation demands to reassess conventional models for the NSTO polarizability in high-intensity electric fields. Furthermore, it contributes to the comprehension and prediction of transport in metal/SrTiO3 junctions and devices.

Nanoscale Schottky barrier visualization utilizing computational modeling and ballistic electron emission microscopy

The electrostatic barrier at a metal semiconductor interface is visualized using nanoscale spatial and meV energetic resolution. A combination of Schottky barrier mapping with ballistic electron emission microscopy and computational modeling enables extraction of the barrier heights, the hot electron scattering, and the presence of localized charges at the interface from the histograms of the spectra thresholds. Several metal semiconductor interfaces are investigated including W/Si(001) using two different deposition techniques, Cr/Si(001), and mixed Au-Ag/Si(001). The findings demonstrate the ability to detect the effects of partial silicide formation in the W and Cr samples and the presence of two barrier heights in intermixed Au/Ag films upon the electrostatic barrier of a buried interface with nanoscale resolution. This has potential to transform the fundamental understanding of the relationship between electrostatic uniformity and interface structure for technologically important metal semiconductor interfaces.

Out-of-plane electron transport in finite layer MoS2

Ballistic electron emission microscopy (BEEM) has been used to study the processes affecting electron transport along the [0001] direction of finite layer MoS2 flakes deposited onto the surface of Au/Si(001) Schottky diodes. Prominent features present in the differential spectra from the MoS2 flakes are consistent with the density of states of finite layer MoS2 calculated using density functional theory. The ability to observe the electronic structure of the MoS2 appears to be due to the relatively smooth density of states of Si in this energy range and a substantial amount of elastic or quasi-elastic scattering along the MoS2/Au/Si(001) path. Demonstration of these measurements using BEEM suggests that this technique could potentially be used to study electron transport through van der Waals heterostructures, with applications in a number of electronic devices.

Detection of silicide formation in nanoscale visualization of interface electrostatics

The ability to detect localized silicide formation at a buried metal semiconductor Schottky interface is demonstrated via nanoscale measurements of the electrostatic barrier. This is accomplished by mapping the Schottky barrier height of the Cr/Si(001) interface by ballistic electron emission microscopy (BEEM). Monte-Carlo modeling is employed to simulate the distributions of barrier heights that include scattering of the electrons that traverse the metal layer and a distribution of electrostatic barriers at the interface. The best agreement between the model and the data is achieved when specifying two barrier heights less than 60 meV from one another instead of a singular barrier. This provides strong evidence that localized silicide formation occurs that would be difficult to observe in averaged BEEM spectra or conventional current voltage measurements.

Probing kinetically excited hot electrons using Schottky diodes

Hot electron generation was measured under the impact of energetic Ar and Rb ions on Ag thin film Schottky diodes. The energy- and angular-dependence of the current measured at the backside of the device due to ion bombardment at the frontside is reported. A sharp upturn in the energy dependent yield is consistent with a kinetic emission model for electronic excitations utilizing the device Schottky barrier as determined from current–voltage characteristics. Backside currents measured for ion incident angles of ±30° are strongly peaked about 0° (normal incidence) and resemble results seen in other contexts, e.g., ballistic electron emission microscopy. Accounting for the increased transport distance for excited charges at non-normal incidence, the angular results are consistent with the accepted mean free path for electrons in Ag films.

Electron transport in ultra-thin films and ballistic electron emission microscopy

We have developed a calculation scheme for the elastic electron current in ultra-thin epitaxial heterostructures. Our model uses a Keldysh’s non-equilibrium Green’s function formalism and a layer-by-layer construction of the epitaxial film. Such an approach is appropriate to describe the current in a ballistic electron emission microscope (BEEM) where the metal base layer is ultra-thin and generalizes a previous one based on a decimation technique appropriated for thick slabs. This formalism allows a full quantum mechanical description of the transmission across the epitaxial heterostructure interface, including multiple scattering via the Dyson equation, which is deemed a crucial ingredient to describe interfaces of ultra-thin layers properly in the future.We introduce a theoretical formulation needed for ultra-thin layers and we compare with results obtained for thick Au(1 1 1) metal layers. An interesting effect takes place for a width of about ten layers: a BEEM current can propagate via the center of the reciprocal space () along the Au(1 1 1) direction. We associate this current to a coherent interference finite-width effect that cannot be found using a decimation technique. Finally, we have tested the validity of the handy semiclassical formalism to describe the BEEM current.

Effective Metal Top Contact on the Organic Layer via Buffer-Layer-Assisted Growth: A Multiscale Characterization of Au/Hexadecanethiol/n-GaAs(100) Junctions

In the field of organic and molecular electronics at monolayer coverage, the need for abrupt and well-controlled top metal contacts is a key point. A general method which provides reliable molecular junctions with most metals remains to be found. In this paper we show that reliable molecular junctions Au/hexadecanethiol/n-GaAs(100) are obtained using buffer-layer-assisted growth (BLAG). They show in hot electron transport measurements at the nanoscale a tunnel regime through the organic monolayer with a full spatial uniformity. Using ballistic electron emission microscopy (BEEM) in the spectroscopic mode as well as photoemission and C(V)-transport measurements, we draw a coherent band alignment scheme of the whole heterostructure at the nanoscale and at the macroscopic scale. Through this study, the BLAG method appears as a general method that should work for contacting organic monolayers with most metals. © 2016 American Chemical Society.

Ballistic electron emission microscopy and spectroscopy: Recent results and related techniques

Interfaces play a central role in determining properties of optical and electronic devices. Many mature techniques exist for surface characterization, providing a great deal of detailed, local information. Interface methods with equivalent capabilities have not developed as fully, due primarily to the inaccessibility of buried interfaces to traditional surface sensitive probes. Ballistic electron emission microscopy (BEEM) is a class of microscopies and spectroscopies that uses energetic electrons injected by a scanning tunneling microscopy tip as a probe of subsurface electronic structure. This article reviews the growth of BEEM methods and summarizes recent areas of investigation using these techniques. Research areas span a range of materials, such as metals, semiconductors, insulators, magnetic materials, and organic layers, as well as a variety of physical properties that include interface barrier height, hot-carrier scattering, interface heterogeneity, magnetic domain structure, and electronic band structure.

Nanoscale Schottky barrier mapping of thermally evaporated and sputter deposited W/Si(001) diodes using ballistic electron emission microscopy

Ballistic electron emission microscopy has been utilized to demonstrate differences in the interface electrostatics of tungsten-Si(001) Schottky diodes fabricated using two different deposition techniques: thermal evaporation using electron-beam heating and magnetron sputtering. A difference of 70 meV in the Schottky barrier heights is measured between the two techniques for both p- and n-type silicon even though the sum of n- and p-type Schottky barrier heights agrees with the band gap of silicon. Spatially resolved nanoscale maps of the Schottky barrier heights are uniform for the sputter film and are highly disordered for the e-beam film. Histograms of the barrier heights show a symmetric Gaussian like profile for the sputter film and a skewed lognormal distribution for e-beam film. A Monte-Carlo model is developed to simulate these histograms which give strong indication that localized elastic scattering is causing this skewing as forces the hot electrons to need a greater total energy to surmount the barrier. These differences are attributed to silicide formation from the unintentional substrate heating during the e-beam deposition, which is confirmed with transmission electron microscopy.

Ballistic electron and photocurrent transport in Au/organic/Si(001) diodes with PDI8-CN2 interlayers

The authors use ballistic electron emission microscopy (BEEM) to probe hot-electron and photocurrent transport in Au/organic/n-Si(001) diodes incorporating the n-type perylene diimide semiconductor PDI8-CN2. For the case of an ultrathin organic interlayer, hot-electron injection is weak and can be detected only at randomly distributed nanosized domains, where BEEM provides electronic barrier heights of ∼0.67 and ∼0.94 eV, respectively. No ballistic transport is detected for devices with a 10 nm-thick interlayer. Regardless of the organic layer thickness, BEEM reveals laterally uniform contributions due to scanning tunneling microscopy-induced photocurrent (STM-PC), with a characteristic energy onset at ∼1.2 eV and a broad intensity peak in the 2–4 eV range. The authors give insight on such spectroscopic features by examination of temperature-dependent spectra and of literature data. This study shows that PDI8-CN2 limits the penetration of Au toward Si, likely due to stiff intermolecular interactions and reactivity of the cyano groups. Moreover, ballistic transmittance is remarkably suppressed and photocurrent transport takes place via defects or recombination centers. Our analysis of electronic and STM-PC fingerprints appears useful for the characterization of several organic-on-inorganic interfaces of interest for heterostructures and devices.

Relating spatially resolved maps of the Schottky barrier height to metal/semiconductor interface composition

The Schottky barrier height (SBH) is mapped with nanoscale resolution at pure Au/Si(001) and mixed Au/Ag/Si(001) interfaces utilizing ballistic electron emission microscopy by acquiring and fitting spectra every 11.7 nm × 11.7 nm over a 1 μm × 1 μm area. The SBH distributions for the mixed interfaces are about four times broader than the pure gold interface and have a complex structure arising from the incomplete intermixing and its effects on the scattering of hot electrons. The maps of the barrier heights and amplitudes for the mixed samples are inhomogeneous with localization attributed to the disordered mixture of the Au and Ag. A method to calculate ratio of the Au to Ag Schottky barrier heights is presented and discussed in relationship to the interface composition. These results demonstrate how the Schottky barrier height is a function of position at the interface, and how mapping can capture the electrostatic nature of these and possibly other mixed metal–semiconductor interfaces.

Nanoscale characterization of oxidized ultrathin Co-films by ballistic electron emission microscopy

In anticipation of devices scaling down further to the few nanometer regime, the ability to characterize material localized within the few nm of a critical device region poses a current challenge, particularly when the material is already buried under other material layers such as under a metal contact. Conventional techniques typically provide indirect information of the nanoscale material quality through a surface or volume averaging perspective. Here we present a study of local (nm range) oxidation in few nanometer thick Co-films using Ballistic Electron Emission Microscopy/Spectroscopy (BEEM/BEES). Co films were grown on n-Si(111) substrates, oxidized in ambient atmosphere before capping with a thin Au film to prevent further oxidation and enable BEEM measurements. In addition to BEES, the temporal progression of Co oxidation was also tracked by X-ray Photoelectron Spectroscopy. At room temperature, we report that the electron injection thresholds are sufficiently different for local regions with Co and oxidized-Co enabling their distinction in BEEM measurements. Our results demonstrate the possibility of using BEEM for nanoscale spatial mapping of the oxidized regions in Co-films, and this can provide critical information toward the successful fabrication of next generation Co-based nano-devices.

Spatially resolved band alignments at Au-hexadecanethiol monolayer-GaAs(001) interfaces by ballistic electron emission microscopy

We study structural and electronic inhomogeneities in Metal—Organic Molecular monoLayer (OML)—semiconductor interfaces at the sub-nanometer scale by means of in situ Ballistic Electron Emission Microscopy (BEEM). BEEM imaging of Au/1-hexadecanethiols/GaAs(001) heterostructures reveals the evolution of pinholes density as a function of the thickness of the metallic top-contact. Using BEEM in spectroscopic mode in non-short-circuited areas, local electronic fingerprints (barrier height values and corresponding spectral weights) reveal a low-energy tunneling regime through the insulating organic monolayer. At higher energies, BEEM evidences new conduction channels, associated with hot-electron injection in the empty molecular orbitals of the OML. Corresponding band diagrams at buried interfaces can be thus locally described. The energy position of GaAs conduction band minimum in the heterostructure is observed to evolve as a function of the thickness of the deposited metal, and coherently with size-dependent electrostatic effects under the molecular patches. Such BEEM analysis provides a quantitative diagnosis on metallic top-contact formation on organic molecular monolayer and appears as a relevant characterization for its optimization.

Time dependent changes in Schottky barrier mapping of the W/Si(001) interface utilizing ballistic electron emission microscopy

The W/Si(001) Schottky barrier height is mapped to nanoscale dimensions using ballistic electron emission microscopy (BEEM) over a period of 21 days to observe changes in the interface electrostatics. Initially, the average spectrum is fit to a Schottky barrier height of 0.71 eV, and the map is uniform with 98% of the spectra able to be fit. After 21 days, the average spectrum is fit to a Schottky barrier height of 0.62 eV, and the spatial map changes dramatically with only 27% of the spectra able to be fit. Transmission electron microscopy shows the formation of an ultra-thin tungsten silicide at the interface, which increases in thickness over the 21 days. This increase is attributed to an increase in electron scattering and the changes are observed in the BEEM measurements. Interestingly, little to no change is observed in the I-V measurements throughout the 21 day period.

Penn Algorithm Including Damping for Calculating the Electron Inelastic Mean Free Path

We present an approach for introducing damping into the Penn algorithm by using the Mermin dielectric function instead of the Lindhard dielectric function. We find that for a damping of 1.5 eV, the electron inelastic mean free path calculated by the present algorithm for Al is in excellent agreement with experimental values in the energy range 5–9 eV. Meanwhile, for a damping of 2.0 eV, our result for Au is consistent with the GW+T ab initio calculation at several electronvolts. In particular, at an energy of 1 eV, our result for Au is 297 A and lies within the range 220–330 A obtained from measurements by ballistic electron emission microscopy.