Ballistic Electron Emission Microscopy Images Research Articles

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.

Nanometer-resolution measurement and modeling of lateral variations of the effective work function at the bilayerPt/Al/SiO2interface

A ballistic electron emission microscopy (BEEM) comparison of the dependence on gate voltage of the average energy barrier of a metal bilayer $\text{Pt}/\text{Al}/{\text{SiO}}_{2}/\text{Si}$ sample and a $\text{Pt}/{\text{SiO}}_{2}/\text{Si}$ sample suggests that the metal/oxide interface of the $\text{Pt}/\text{Al}/{\text{SiO}}_{2}/\text{Si}$ sample is laterally inhomogeneous at nm length scales. However, BEEM images of the bilayer sample do not show significantly larger lateral variations than observed on a (uniform) $\text{Pt}/{\text{SiO}}_{2}/\text{Si}$ sample, indicating that any inhomogeneous ``patches'' of lower-energy barrier height have size smaller than the lateral resolution of BEEM, estimated for these samples to be $\ensuremath{\sim}10\text{ }\text{nm}$. Finite element electrostatic simulations of an assumed inhomogeneous interface with nm size patches of different effective work function can fit the experimental data of the bilayer sample much better than an assumed homogenous interface, indicating that the bilayer film is laterally inhomogeneous at the nm scale.

Charge transport across metal molecule interfaces probed by BEEM

We use Ballistic Electron Emission Microscopy (BEEM) technique to determine directly the Schottky barrier distribution over silver /H-T3-(CH2)4-HS (abbreviated as T3C4SH) self assembled monolayer interface area with nanometer scale spatial resolution. The selfassembled monolayer is absorbed on template stripped gold. BEEM images show spatially non-uniform carrier injection. A Wentzel Kramel Brillouin (WKB) calculation is performed and compared with BEEM spectra. The results show that the measured currents are four orders of magnitude larger than the direct tunnelling contribution, indicating molecular levels being accessed. To further substantiate the findings, characterization by STM distance versus potential spectroscopy is carried out to determine injection barriers at the interface. The results from these two techniques are compared and the implications of which will be discussed.

Switching in organic devices caused by nanoscale Schottky barrier patches

We have identified a possible electronic origin of metal filaments, invoked to explain the switching behavior of organic devices. Interfaces of two representative organics polyparaphenylene (PPP) and poly(2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) with Ag are investigated using ballistic emission microscopy. Nanometer scale spatial nonuniformity of carrier injection is observed in ballistic electron emission microscopy images of both interfaces. The measured Schottky barrier (SB) appears to be consistent with metal states tailing into the gap of the PPP. We find that the SB values exhibit a distribution, even for the diodes with low ideality factors. The implications of this distribution on the measured physical properties of the diode are discussed, in light of work on devices of similar geometry, published in the literature. We also demonstrate that patches of low SB are likely to nucleate current filaments which can cause local ionization and are reported to be responsible for the switching behavior observed in metal-organic, metal-CuS and Ag-AgSe structures.

Ballistic Electron Emission Microscopy Studies of Au/Molecule/n-GaAs Diodes

We present nanometer-scale resolution, ballistic electron emission microscopy (BEEM) studies of Au/octanedithiol/n-GaAs (001) diodes. The presence of the molecule dramatically increases the BEEM threshold voltage and displays an unusual transport signature as compared to reference Au/GaAs diodes. Furthermore, BEEM images indicate laterally inhomogeneous interfacial structure. We present calculations that address the role of the molecular layer at the interface. Our results indicate that spatially resolved measurements add new insight to studies using conventional spatial-averaging techniques.

Surface and bulk band-structure effects onCoSi2/Si(111)ballistic-electron emission experiments

A theoretical model of ballistic-electron-emission microscopy (BEEM) based on linear combination of atomic orbitals Hamiltonians and Keldysh Green's functions is applied to analyze experimental data obtained for ${\mathrm{CoSi}}_{2}/\mathrm{Si}(111)$ contacts. Hot electrons injected from a scanning tunneling microscope tip into the silicide film form a highly focused beam, which even after propagation through films of moderate thickness is narrow enough to allow the observed atomic resolution of interfacial point defects. On $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed domains a certain fraction of the initial current is injected into localized surface states, leading to the reported contrast in BEEM images, reflecting the topography at the surface. These results confirm that band-structure effects, both in the bulk and at the surface of the metallic overlayer, intricately influence the interface-related information contained in BEEM data. It is found that for a careful analysis of experimental results, a theoretical model going beyond the ballistic hypotesis is required.

Hot-electron transport processes in ballistic-electron emission microscopy at Au–Si interfaces

Abstract Ballistic-electron emission microscopy (BEEM) allows to study the transport of hot electrons in thin films and at buried interfaces with a lateral resolution in the nanometer range. In this review, the relevant transport processes are described in detail. For the particular case of the interface between thin Au films and n -type Si(111), BEEM results are presented. In BEEM imaging, it is observed that the collector current increases when the tip is located at terrace edges of the Au film. This effect demonstrates an improved matching of the populated electron states in the metal with the conduction-band states in the semiconductor in the case of electron injection at a terrace edge. It indicates a strong influence of the electronic structure of the metal on hot-electron transport as well as a predominant conservation of lateral momentum at the interface.

Microscopy and spectroscopy of buried nanostructures

Ballistic-electron-emission microscopy (BEEM) has been performed on epitaxial CoSi 2 /Si(111) and CoSi 2 /Si(100) films, and on CoSi 2 /Si/Ge/Si(100) heterostructures containing buried Ge quantum dots. At CoSi 2 /n-Si(111) and CoSi 2 /p-Si(111) interfaces the spatial variation of hot carrier transmission is dominated by scattering at dislocations and point defects, while the Schottky barrier height is not measurably affected by defects. The excellent spatial resolution of 1.3 nm, achieved for films as thick as 5 nm on n-Si(111) and of 1.6 nm on p-Si(111) can be explained only by taking into account the theoretically predicted focussing effect of the hot electron beam, induced by the special constant energy surfaces of CoSi 2 . Scanning tunneling spectroscopy (STS) studies of Ge quantum dots buried in a Si matrix have revealed a substantial lowering of the Si surface band gap due to the strain imposed by the dots. In CoSi 2 /Si/Ge/Si(100) heterostructures the Ge-induced strain has been found to modify the defect structure at the CoSi 2 /Si interface, leading to pronounced contrast variations in BEEM images. The BEEM contrast gives direct evidence for the rotation of Ge islands by 45° during capping with epitaxial Si.

Modification of GaN Schottky barrier interfaces probed by ballistic-electron-emission microscopy and spectroscopy

Ballistic-electron-emission microscopy (BEEM) and spectroscopy have been used to investigate the properties of Au/GaN interfaces. The effects of in situ and ex situ annealing on the starting GaN surface were examined, with the aim of increasing the surprisingly low value of interface electron transmission observed in previous BEEM measurements. BEEM imaging and spectroscopy have demonstrated that much higher, more uniform transmission across the Au/GaN interface can be achieved. However, while methods were identified that increase transmission by more than an order of magnitude, BEEM spectroscopy indicates that annealing can substantially alter the Schottky barrier height. These barrier height changes at moderate temperatures are attributed to vacancy diffusion.

Determination of the electron mean free path in the 1–1.8 eV energy range in thin gold layers using ballistic electron emission microscopy

Electron mean free path (λ a ) has been investigated using Ballistic Electron Emission Microscopy (BEEM). Using the average collector current computed from large scale BEEM images and a model in which the current exponentially decreases in terms of metal thickness, a constant value of λ a = 11 nm has been calculated in the 1-1.8 eV electron energy range. On small scale images, the study of well-defined BEEM domains shows that either λ a or the interface transmission factor (or both) may differ from their average values. These local variations from one grain to another are interpreted as interface defects and channeling of the electron beam due to the electronic and crystallographic of the gold layer.

Investigation of ultrathin SiO2 film thickness variations by ballistic electron emission microscopy

We investigate the feasibility of using ballistic electron emission microscopy (BEEM) to study possible thickness variations of ultrathin SiO2, which might exist at substrate defects, such as steps. We find that simple BEEM imaging of the oxide film sandwiched into a metal–oxide–semiconductor (MOS) structure does not reveal any features that could be related to the oxide film. We further present initial results suggesting that hot-electron resonance in the oxide conduction band could be observed by BEEM and could be sensitive to local film thickness. We also confirm the ability of oxide film to sustain injection of very high densities of hot electrons without any observable damage. In some cases we observe local damage of the MOS structure induced by BEEM measurements, but we conclude that it is most likely related to failure of the metal overlayer and probably not related to hot-electron breakdown of the oxide.

In-situ BEEM study of interfacial dislocations and point defects

Epitaxial CoSi 2 /n-Si(1 1 1) interfaces have been studied by in-situ ballistic-electron-emission microscopy (BEEM) at 77 K. The scattering of hot electrons by individual misfit dislocations and point defects leads to significant contrast in the BEEM images, proving that interfacial defects of atomic dimensions can be probed by BEEM. The Schottky barrier height is not measurably affected by these defects. When present, grains of a metastable CoSi2 phase with a defect-CsCl structure do, however, lower the barrier appreciably.

Atomic and mesoscopic scale characterization of semiconductor interfaces by ballistic electron emission microscopy

Ballistic electron emission microscopy (BEEM) is a powerful new low energy electron microscopy in materials physics for nondestructive local electronic characterization of semiconductor heterostructures. In this article, low energy electron imaging of buried semiconductor heterostructures will be explored, with particular emphasis on BEEM imaging of buried objects in metal–GaAs based heterostructures.

Imaging and Spectroscopy of Single InAs Self-Assembled Quantum Dots using Ballistic Electron Emission Microscopy.

Single InAs self-assembled quantum dots buried spatially beneath a Au/GaAs interface are probed for the first time using the imaging and spectroscopic modes of ballistic electron emission microscopy (BEEM). BEEM images show enhanced current through each dot. Spectra taken with the tip positioned on a dot show shifted current thresholds when compared with the off dot spectra, which are essentially the same as those of Au on bulk GaAs. Shifts in the $\ensuremath{\gamma}$ and $L$ conduction band thresholds are attributed to strain in the GaAs cap layer. Fine structure below the $\ensuremath{\gamma}$ threshold is consistent with resonant tunneling through zero-dimensional states within the quantum dots.

Conduction band offsets in ordered-GaInP/GaAs heterostructures studied by ballistic-electron-emission microscopy

Ordered-GaInP/GaAs heterostructures have been studied using ballistic-electron-emission microscopy (BEEM). The GaInP/GaAs conduction band offset was found to decrease with increasing order. Samples were grown simultaneously on different misoriented substrates to vary the degree of order in the GaInP. Concurrent scanning tunneling microscopy and BEEM images show ridge structures in the topography and contrast in the BEEM current that may correspond to ordered domains in the GaInP. Room temperature conduction band offsets of 137 and 86 meV were measured using BEEM spectroscopy for GaInP with 2 K band gaps of 1.97 and 1.89 eV, respectively.

Ballistic-electron emission microscopy as a probe for surface science

In situ ballistic-electron emission microscopy (BEEM) and spectroscopy (BEES) have been performed at 77 K on epitaxial CoSi 2 Si(100) and Si(111). BEEM images reflect the atomic-scale periodicity of the surface topography. This effect is due to variations of the energy distribution of the tunneling electrons on an atomic scale. The ability of BEEM to image individual interfacial point defects and dislocations allows us to study the angular distribution of the tunneling electrons as well, in particular its dependence on the atomic structure of the sample and the tip. Our results show that with BEEM useful information can be obtained about the tunneling process on an atomic scale, which is complementary to the one accessible in a conventional STM experiment.

Characterisation of buried lateral period superlattices by ballistic electron emission microscopy

Ballistic electron emission microscopy (BEEM), a derivation of scanning tunnelling microscopy (STM), is a recently developed technique that is most often used to measure local potential barrier height variations in metal-semiconductor systems (i.e. Schottky barriers). Here, for the first time, the successful application to the study of buried lateral period superlattices, produced by molecular beam epitaxial growth on a vicinal GaAs(001) substrate, is reported. The structure consists of thin Au and GaAs films on top of the buried AlAs GaAs superlattice and finally a GaAs substrate. Simultaneously obtained STM topographical images of the Au film and BEEM images of the ballistic electrons reaching the GaAs substrate reveal the expected periodic modulation only in the BEEM image. In addition, spectroscopic examination of the potential barrier at various places over the samples reveals a bimodal distribution consistent with a modulation of Al x Ga 1− x As mole fraction perpendicular to the surface steps. More detailed analysis reveals a compositional modulation larger than previously detected in such samples.

Epitaxial metal silicides: interface mapping by scanning probe techniques

The defect structures at epitaxial CoSi 2 interfaces were imaged by means of scanning tunneling microscopy, scanning tunneling spectroscopy (STS), modulation spectroscopy (MS), and ballistic electron-emission microscopy (BEEM). Interfacial dislocations and point defects in thin films give rise to significant contrast in BEEM images. The lateral variations of the film thickness can be determined with monolayer precision by making use of the quantum size effect in STS or MS, and of the inelastic hot electron scattering within CoSi 2 in BEEM.

Hot electron transport through metal–oxide–semiconductor structures studied by ballistic electron emission spectroscopy

The tip of a scanning tunneling microscope (STM) was used to inject electrons into thin Pt layers of metal–oxide–semiconductor (MOS) structures. The collector currents emanating from the n-type Si(100) substrates were measured as a function of the electron energy, determined by the STM tip bias VT, for different oxide biases Vox applied independently across the oxide layers. The SiO2 layers were thermally grown in a device processing line and ranged from 27 to 62 Å in thickness. A current threshold near VT=3.90 V is interpreted in terms of current transport through the SiO2 conduction band. The current transport through the MOS structure was modeled in a single band description for zero oxide thickness, and fitted to the collector currents that had been corrected for impact ionization effects in the Si. Deviations between the two curves represent the influence of the transmission probability Tox through the SiO2 film of finite thickness. Tox can thus be determined from the experimental data. Within an eV of threshold the magnitude of Tox was observed to be particularly sensitive to small changes in oxide bias in the range 0.3 V≳Vox≳−0.1 V. The transmission probabilities were also calculated by integrating the Boltzmann equation using Monte Carlo techniques that incorporate energy dependent effective masses and electron phonon scattering rates. Agreement between the two approaches is quite good, including the observed sensitivity on oxide bias in the threshold region, which is a direct consequence of the strong electron-optical phonon scattering in the oxide. The 27 Å thick oxide structures exhibited in the ballistic electron emission microscopy images scattered patches of high transmittance of only 1–2 nm in extent. The collector currents arising from injection at these patches indicated thresholds as low as 1.1 eV, but the observed modest currents above that threshold argue against local shorts that would arise from pinholes in the oxide.

Ballistic electron emission microscopy of strained and relaxed In0.35Ga0.65As/AlAs interfaces

We report here our investigations upon both strained and relaxed In0.35Ga0.65As/AlAs interfaces using the ballistic electron emission microscopy (BEEM) and current–voltage (I–V) techniques. Six samples, of different InGaAs thickness and doping density, were grown by molecular beam epitaxy and were designed to assess the importance of strain relaxation/dislocations in barrier formation. In the heavily dislocated situation (thicker InGaAs film) the obtained barrier most likely resulted from Fermi level pinning. However, this was obviously not the case for both the pseudomorphic and lightly dislocated situations (thinner InGaAs films). It was also noted that the measured barriers varied more widely over space in the lightly dislocated layer than that in the pseudomorphic case and that the simultaneously obtained topographic and BEEM images points to a better interface in the pseudomorphic layer than that in the partially relaxed situation.