Ballistic Electron Emission Microscopy Current Research Articles

Schottky barrier and attenuation length for hot hole injection in nonepitaxial Au on p-type GaAs

Ballistic electron emission microscopy (BEEM) was performed to obtain current versus bias characteristics of nonepitaxial nanometer-thick Au on p-type GaAs in order to accurately measure the local Schottky barrier height. Hole injection BEEM data were averaged from thousands of spectra for various Au film thicknesses and then used to determine the attenuation length of the energetic charge carriers as a function of tip bias. The authors report an increase in attenuation length at biases near the Schottky barrier, providing evidence for the existence of coherent BEEM currents in Schottky diodes. These results provide additional evidence for the conservation of the parallel momentum of charge carriers at the metal–semiconductor interface.

Subthreshold characteristics of ballistic electron emission spectra

We report upon a comprehensive investigation of the subthreshold characteristics of the ballistic electron emission microscopy (BEEM) current in ballistic electron emission spectroscopy. Starting from the Bell-Kaiser model, we derive an analytical equation to describe the subthreshold behavior of the BEEM current. It is found that the BEEM current in this region should exhibit a subthreshold swing of ∼60 mV/decade at room temperature, which we experimentally verified. This finding provides a rule of thumb for the detectability of the subthreshold behavior in a spectrum. For spectra where the subthreshold behavior is discernible above the signal noise, it is demonstrated that significant deviations in the near-threshold region can occur when fitting with a simple quadratic model that ignores the subthreshold behavior. To take the subthreshold behavior into account, a simple analytical model is proposed. This model not only fits significantly better in the near threshold region than the square model, but also gives a barrier height closer to the one extracted from the Bell-Kaiser model. More significantly, this model provides a quick method to estimate the subthreshold BEEM current amplitude based on the BEEM current above the barrier height.

Conservation of the Lateral Electron Momentum at a Metal-Semiconductor Interface Studied by Ballistic Electron Emission Microscopy

We report on ballistic electron emission microscopy and spectroscopy studies on epitaxial (3-5 nm thick) Bi(111) films, grown on n-type Si substrates. The effective barrier heights of the Schottky barrier observed are 0.58 eV for the Bi/Si(100)-(2x1) and 0.68 eV for the Bi/Si(111)-(7x7). At the step edges of the epitaxial films a strong increase of the ballistic electron emission microscopy current is observed for Bi/Si(111)-(7x7), while no increase occurs for Bi/Si(100)-(2x1). These observations can be explained by the conservation of the lateral momentum of the electron at the metal-semiconductor interface.

Two dimensionally patterned GaNxAs1−x/GaAs nanostructures using N+ implantation followed by pulsed laser melting

We present measurements on two dimensionally patterned GaNxAs1−x dots fabricated in a GaAs matrix using ion implantation followed by pulsed laser melting and rapid thermal annealing. The lithographically patterned GaNxAs1−x regions are imaged by ballistic electron emission microscopy (BEEM). By analyzing the BEEM spectra of the locally confined dots, we observe the decrease in the Schottky barrier height with nitrogen incorporation. The second derivatives of BEEM currents from unpatterned GaNxAs1−x films exhibit a decrease in Γ-like thresholds as the nitrogen concentration increases. The composition dependence of the thresholds agrees well with that of previously studied low temperature molecular beam epitaxy grown alloys.

Measuring spin dependent hot electron transport through a metal-semiconductor interface using spin-polarized ballistic electron emission microscopy

Spin-polarized ballistic electron transport has been studied in $\mathrm{Fe}∕\mathrm{Si}(001)$ Schottky diodes using ballistic electron emission microscopy (BEEM). Spin dependent scattering of polarized ballistic electrons injected from an Fe coated Au tip into the Fe films has been shown to affect the BEEM current. The spin dependent attenuation lengths were determined by measuring this effect with Fe thickness and found to be $1.8\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the minority spin electrons and $2.5\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the majority spin electrons at a tip bias of $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. In addition, the attenuation lengths were measured as a function of tip bias, which indicated that the $\mathrm{Fe}∕\mathrm{Si}(001)$ interface band structure has an effect on the hot electron transport through the diode.

Cubic inclusions in 4H-SiC studied with ballistic electron-emission microscopy

High-temperature-processing-induced “double-stacking fault” cubic inclusions in 4H-SiC were studied with ballistic electron emission microscopy (BEEM). Large BEEM current and a ∼0.53 eV local reduction in the Schottky barrier height (SBH) were observed where the inclusions intersect a Pt interface, confirming the quantum-well nature of the inclusions and providing nanometer scale information about local electronic behavior. Measured spatial variations in the BEEM current are related to the inclusion orientation and local surface step structure. An observation of an anomalously low SBH is discussed, suggesting the existence of a triple- or quadruple-stacking fault inclusion.

Observation of a scanning tunneling microscopy induced photocurrent during ballistic electron emission microscopy

We report that a scanning tunneling microscope induced photocurrent (STM–PC) can occur during ballistic electron emission microscopy (BEEM) measurements on Schottky-barrier samples with typical efficiency of 10−4–10−6 and apparent BEEM threshold slightly above the substrate semiconductor band gap. This STM–PC resembles normal BEEM current, and hence, can interfere with BEEM measurements of structures with intrinsically low signal and/or intrinsic BEEM threshold voltage larger than the substrate band gap. We discuss a simple test for the existence of STM–PC during BEEM measurements.

Electronic transport on Au/Si structures: Electron-electron, electron-phonon, and band structure effects

Ballistic electron emission microscopy currents on Au/Si(111) and Au/Si(100) are analyzed using a Keldysh's formalism. The relative effect of the electron-electron interaction, quasielastic scattering (phonons), and band structure effects are shown to be relevant to interpret available experiments. The electron-phonon interaction can be introduced in the theory by computing an appropriate self-energy and solving Keldysh's equations self-consistently. The main effect of phonons is to create a more homogeneous current distribution in reciprocal espace, favoring injection on Si(111) over Si(100). Estimates for the electron-electron mean free path on Au are also obtained from this formalism.

Direct observation of local hot electron transport through Al2O3 tunnel junctions

A modified ballistic electron emission microscopy (BEEM) technique using local transport of hot electrons through a buried interface, was successfully applied to study the Al2O3 barrier in the Co/Al2O3/Ru tunnel junction. This technique enabled us to straightforwardly measure an effective barrier height of 1.7 eV and to observe the rise of the barrier height due to continuous current injection into a single point of the junction attributed to charging effects and/or degradation of the barrier structure. Scanning over an area of 510 nm×510 nm showed a spatial inhomogenity of the barrier resulting in different dependencies of the BEEM current on the energy of the injected electrons.

Controlled modifications of electron injection on Au/Si and Au/SiO2/Si contacts using ballistic electron emission microscopy

In this article, ballistic electron emission microscopy (BEEM) induced modifications on Au/Si and Au/SiO2/Si contacts are presented. BEEM current can be locally enhanced or reduced in a controlled manner. These observations are attributed to tip induced modifications on the gold surface. According to Au thickness, x-ray reflectivity experiments show different surface evolutions correlated to the size variations of the modifications introduced as a function of time.

Hot-electron effects and oxide degradation in MOS structures studied with ballistic electron emission microscopy

The application of the STM-based technique of ballistic electron emission microscopy (BEEM) to the study of transport properties of SiO2 gate oxide layers is reviewed. Oxide degradation observed on a local scale of nanometer dimensions ranges from the filling of electron traps with low-kinetic-energy electrons injected just above the oxide barrier, to trap generation and filling triggered by electrons with kinetic energies ≥2 eV. BEEM provides means to determine the spatial distributions of the traps. Only positively charged traps are observed for thin (≤4 nm) SiO2 layers upon electrical stressing. Oxide breakdown is seldom induced by local stressing, suggesting that it occurs at extrinsic defect sites that are widely separated. BEEM also provides unique opportunities to address fundamental issues. In that regard, examples of the dynamic response of the SiO2 dielectric to a moving electron are presented, as well as the determination of the dispersion of the electron mass of the conduction-band electrons. The latter is achieved by modeling quantum interference oscillations in the BEEM current.

Scattering theory of ballistic-electron-emission microscopy at nonepitaxial interfaces

We present an interface scattering model to describe ballistic-electron-emission microscopy (BEEM) at nonepitaxial metal/semiconductor interfaces. The model starts with a Hamiltonian consisting of the sum of two terms: one term, ${H}_{0},$ describes an ideal interface for which the interface parallel component of wave vector is a good quantum number, and the second term, $\ensuremath{\delta}H,$ describes interfacial scattering centers. The eigenstates of ${H}_{0}$ consist of an incident and a reflected part in the metal and a transmitted part in the semiconductor. The three components of each eigenstate have the same interface parallel wave vector. Because tunneling preferentially weights forward-directed states, the interface parallel component of wave vector is small for the ${H}_{0}$ eigenstates that are initially populated with high probability in BEEM. $\ensuremath{\delta}H$ scatters electrons between the eigenstates of ${H}_{0}.$ The scattering conserves energy, but not the interface parallel wave vector. In the final state of the scattering process, states with a large interface parallel wave vector can be occupied with reasonable probability. If scattering is weak, so that the parallel wave vector is nearly conserved, the calculated collector current into conduction-band valleys with zero parallel wave vector at the minimum, such as the \ensuremath{\Gamma} valley for GaAs(100), is much larger than the calculated collector current into conduction-band valleys with a large parallel wave vector at the minimum, such as the L valleys for GaAs(100). However, if scattering is strong, the injected electron flux distribution is redistributed and valleys with zero interface transverse wave vector at their energy minimum are not preferentially weighted. Instead, the weighting varies as the density of final states for the scattering process so that, for example, the calculated L-channel collector current is much larger than the calculated \ensuremath{\Gamma}-channel collector current for GaAs(100). Interfacial scattering reduces the overall magnitude of the calculated BEEM current near threshold for GaAs. We generalize the model to describe buried heterostructures and apply it to the Au/GaAs(100) interface and ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures buried beneath this interface. Experimental results on these materials are presented and compared with the model. Strong scattering is required to describe the observed BEEM currents for Au/GaAs(100) and buried ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures.

Green's function calculation of Ballistic Electron Emission Microscopy currents (BEEM v2.1)

We describe the FORTRAN-90 program BEEM v2.1 for the computation of real and reciprocal space currents in Ballistic Electron Emission Microscopy (BEEM), and Spectroscopy (BEES). Based on a LCAO Hamiltonian, the equilibrium one-particle Green's functions of a semi-infinite crystal are obtained using a decimation technique. By means of Keldysh's formalism, the STM tip is coupled to this crystal, and expressions for BEEM currents in real and reciprocal space are obtained. Finally, a Dyson-like equation and a self-consistent charge neutrality condition are applied to incorporate reconstructed or relaxed layers in the surface region. Running demos for Au and CoSi 2 are provided.

Evolution ofGaAs1−xNxconduction states and giantAu/GaAs1−xNxSchottky barrier reduction studied by ballistic electron emission spectroscopy

The evolution of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ band structure at low nitrogen concentrations (up to $x=0.021)$ is studied by ballistic electron emission microscopy (BEEM) spectra of ${\mathrm{A}\mathrm{u}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ heterostructures. Two peaks observed in the second derivative BEEM spectra are identified with the contribution from the \ensuremath{\Gamma}- and L-like bands of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}.$ As the nitrogen concentration increases, the energetic separation between these peaks increases, with a relative decrease of the L-like band contribution to the BEEM current. In addition, we found a strong decrease of the ${\mathrm{A}\mathrm{u}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ Schottky barrier with the nitrogen incorporation, from $\ensuremath{\sim}0.92 \mathrm{eV}$ at $x=0$ down to $\ensuremath{\sim}0.55 \mathrm{eV}$ at $x=0.021.$ The observed Schottky barrier reduction approximates the ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ band-gap reduction.

An inexpensive up-gradation of scanning tunneling microscope for ballistic electron emission microscopy and spectroscopy

Ballistic electron emission microscopy (BEEM) and Ballistic electron emission spectroscopy (BEES) have become highly useful tools for studying the transport property of electrons and holes across metals, metal–semiconductor interfaces and in semiconductors. Although the techniques are developed, but there is hardly any detailed description of experimental set-ups in this regard. We have carried out an inexpensive upgradation of commercial STM (model STM635 of RHK Technologies) for BEEM and BEES studies on metal–semiconductor interfaces. Along with the associated electronics and a suitable sample holder, we have also developed a technique to produce high quality STM tips necessary for these studies. We have carried out BEEM studies on gold–silicon interface and the results are in conformity with those reported earlier. The minimum threshold of obtaining ballistic current is found to be 0.7 V for Au–n-Si interface whereas it may rise up to 4 V if a layer of oxide is permitted to grow on silicon before deposition of gold film. In the case of a 100 Å gold film over n-Si(100), the BEEM current is more intense at the grain boundaries and relatively less over the grains of greater heights.

Hot-electron transport through Au/CaF2/Si(111) structure studied by ballistic electron emission spectroscopy

The influence of calcium fluoride (CaF2) [about two monolayers (ML)] in Au/CaF2/Si(111) heterostructure on hot-electron transport across the CaF2 intralayers has been studied using ballistic electron emission microscopy (BEEM) and Fourier transform infrared spectroscopy (FTIR). The BEEM current–voltage spectra show that the electron transport property is strongly affected by the CaF2 intralayers. The threshold voltage V0 for the onset of the BEEM current for an insulating CaF2 intralayer, which is about 3.6 V, is obtained only for the sample in which CaF2 was deposited at 700 °C. In contrast, the threshold voltage of the sample in which CaF2 was deposited at 550 °C is determined to be about 0.74 V. The FTIR spectra of these CaF2 layers show that Ca–Si–F bonds exist in the latter, but not in the former. The existence of Ca–Si–F bonds implies that the CaF2 heteroepitaxial growth at 550 °C is unsuccessful in obtaining a high-quality CaF2 layer and will induce many defects in the CaF2 layer and/or at the interface. The defect-induced states in the CaF2 intralayers allow hot electrons to travel through the intralayers even below 3.6 eV and lie in a position of the threshold voltage for the onset of the BEEM current to be about 0.74 V.

Ballistic electron transport through Au(111)/Si(111) and Au(111)/Si(100) interfaces

Ballistic electron emission microscopy (BEEM) measurements have been made, as a function of Au thickness, of Au(111)/Si(111) and Au(111)/Si(100) interfaces, fabricated and studied in ultrahigh vacuum. The measured ballistic electron attenuation length for Au is in excess of 230 \AA{} for both Si orientations. In accord with metal band-structure considerations, the BEEM current is consistently more than 50% higher for Si(111) than for Si(100) substrates. Surface electron states have been observed on atomically flat surfaces of the Au grains, and their presence correlated with a spatially varying BEEM current.

Ballistic Electron Emission Microscopy onCoSi2/Si(111)Interfaces: Band Structure Induced Atomic-Scale Resolution and Role of Localized Surface States

Applying a Keldysh Green`s function method it is shown that hot electrons injected from a STM-tip into a CoSi${}_2$/Si(111) system form a highly focused beam due to the silicide band structure. This explains the atomic resolution obtained in recent Ballistic Electron Emission Microscopy (BEEM) experiments. Localized surface states in the $(2 \times 1)$-reconstruction are found to be responsible for the also reported anticorrugation of the BEEM current. These results clearly demonstrate the importance of bulk and surface band structure effects for a detailed understanding of BEEM data.

Current oscillations in thin metal–oxide–semiconductor structures observed by ballistic electron emission microscopy

Quantum interference oscillations of electrons in a thin SiO2 layer were observed by ballistic electron emission microscopy (BEEM). With BEEM, electrons are injected across the gate of a metal–oxide–semiconductor (MOS) structure and directly into the conduction band of the SiO2. The MOS capacitor consisted of a 5 nm thick Pd film deposited on a 2.8±0.2 nm oxide thermally grown on Si(100). Oscillations with up to four peaks in an energy range of 0–3 eV above the injection threshold were noted. Their magnitude is of the order of 30% of the underlying BEEM current. The oscillations were most salient and their energy location repeatable at points of the sample that were previously not exposed to the electron beam. Even modest exposures caused a buildup of positive charge. This charge resulted in energy shifts, as well as a weakening of the oscillations, both of which are a consequence of the added scattering and local field inhomogeneities associated with the random distribution of the positive charge. Solutions of the Schrödinger equation that included a built-in oxide potential of 0.20 V and image force effects at both interfaces gave excellent fits to the experimental data for an effective electron mass in the oxide mox=0.63±0.09mo. The uncertainty in mox arises from an uncertainty of ±0.2 nm in the determination of the oxide thickness by ellipsometric methods. Nevertheless, the obtained value is well above the generally accepted value of 0.5mo.

Ballistic electron emission microscopy spectroscopy study of AlSb and InAs/AlSb superlattice barriers

Due to its large band gap, AlSb is often used as a barrier in antimonide heterostructure devices. However, its transport characteristics are not totally clear. We have employed ballistic electron emission microscopy (BEEM) to directly probe AlSb barriers as well as more complicated structures such as selectively doped n-type InAs/AlSb superlattices. The aforementioned structures were grown by molecular beam epitaxy on GaSb substrates. A 100 Å InAs or 50 Å GaSb capping layer was used to prevent surface oxidation from ex situ processing. Different substrate and capping layer combinations were explored to suppress background current and maximize transport of BEEM current. The samples were finished with a sputter deposited 100 Å metal layer so that the final BEEM structure was of the form of a metal/capping layer/semiconductor. Of note is that we have found that hole current contributed significantly to BEEM noise due to type II band alignment in the antimonide system. BEEM data revealed that the electron barrier height of Al/AlSb centered around 1.17 eV, which was attributed to transport through the conduction band minimum near the AlSb X point. Variation in the BEEM threshold indicated unevenness at the Al/AlSb interface. The metal on semiconductor barrier height was too low for the superlattice to allow consistent probing by BEEM spectroscopy. However, the superlattice BEEM signal was elevated above the background noise after repeated stressing of the metal surface. A BEEM threshold of 0.8 eV was observed for the Au/24 Å period superlattice system after the stress treatment.