Ballistic Electron Emission Microscopy Spectra Research Articles

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.

Nanoscale mapping of the W/Si(001) Schottky barrier

The W/Si(001) Schottky barrier was spatially mapped with nanoscale resolution using ballistic electron emission microscopy (BEEM) and ballistic hole emission microscopy (BHEM) using n-type and p-type silicon substrates. The formation of an interfacial tungsten silicide is observed utilizing transmission electron microscopy and Rutherford backscattering spectrometry. The BEEM and BHEM spectra are fit utilizing a linearization method based on the power law BEEM model using the Prietsch Ludeke fitting exponent. The aggregate of the Schottky barrier heights from n-type (0.71 eV) and p-type (0.47 eV) silicon agrees with the silicon band gap at 80 K. Spatially resolved maps of the Schottky barrier are generated from grids of 7225 spectra taken over a 1 μm × 1 μm area and provide insight into its homogeneity. Histograms of the barrier heights have a Gaussian component consistent with an interface dipole model and show deviations that are localized in the spatial maps and are attributed to compositional fluctuations, nanoscale defects, and foreign materials.

Composition dependence of Schottky barrier heights and bandgap energies of GaNxAs1−x synthesized by ion implantation and pulsed-laser melting

We present a systematic investigation on the band structure of the GaNxAs1−x alloys synthesized using nitrogen ion implantation followed by pulsed-laser melting and rapid thermal annealing. The evolution of the nitrogen-concentration depth profile is consistent with liquid-phase diffusion, solute trapping at the rapidly moving solidification front, and surface evaporation. The reduction of the Schottky barrier height of the Γ-like threshold at nitrogen composition up to x=0.016 is studied by ballistic electron emission microscopy (BEEM) and determined quantitatively using the second voltage derivative BEEM spectra to be −191±63 meV per x=0.01, which is close to the corresponding slope for samples grown by low-temperature molecular beam epitaxy. This slope is also consistent with the bandgap narrowing measured on the same samples by photomodulated reflectance and is consistent with the band anticrossing model for the splitting of the conduction band in the GaNxAs1−x alloys. Lithographically patterned GaNxAs1−x dots are imaged by BEEM. Analysis of BEEM spectra of the locally confined dots indicates an alloying-induced decrease in the Schottky barrier height of four times the thermal energy at room temperature.

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.

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.

Hot electron transport across manganese silicide layers on the Si(001) surface

Ballistic electron emission microscopy (BEEM) has been performed on MnSi∕Si(001) Schottky diodes at 80K to study the hot electron transport properties. The BEEM spectra best fit the thermally broadening 5∕2 power law model with two threshold heights at 0.71 and 0.86eV, indicating a complex interface band structure. In addition, the normalized BEEM current in the MnSi overlayer was found to be approximately seven times less than is observed in Au∕Si(001) samples of similar thicknesses, indicating a larger amount of hot electron scattering in the MnSi∕Si(001) samples.

Ballistic electron emission microscopy on spin valve structures

Spin valve structures, as employed in base layers of spin valve transistor devices, are characterized by ballistic electron emission microscopy (BEEM). In detail, Co–Cu–Permalloy–Au layers sputtered onto n-type GaAs bulk substrates were studied. BEEM spectra taken on these multilayers show that magnetocurrents on the order of 600% can be achieved even at room temperature. Small area images (400nm×400nm) show that the spin filtering effect of the spin valves is quite homogeneous on the submicron scale. On larger scales, magnetic domains were imaged close to the switching field of the spin valve structure.

High-energy ballistic transport in hetero- and nano-structures

Ballistic electron emission microscopy (BEEM) is a three terminal extension of scanning tunneling microscopy and yields topographic and spectroscopic information on high-energy electron transport in semiconductors at nm-resolution. In BEEM on GaAs–AlGaAs double barrier resonant tunneling diodes (DBRTDs) ballistic electrons which tunnel through a resonant state inside the DBRTD result in a characteristic linear behavior in the BEEM spectrum. On DBRTDs nanostructured into narrow quantum wires, however, this tunneling is quenched for electron energies below the AlGaAs barrier heights. This quenching of the ballistic current can be explained in terms of a transfer Hamiltonian formalism applied to tunneling processes between electron systems of different dimensionality. We measured BEEM spectra on InAs self-assembled quantum dots (SAQDs) for positions on the dots and for “off-dot” regions on the so-called InAs wetting layer. From these data, we determined the local InAs–GaAs band offsets on the dots and on the wetting layer and investigated the temperature dependence of the InAs–GaAs barrier height.

Magnetic field effects and k∥-filtering in BEEM on GaAsAlGaAs resonant tunneling structures

In this work, GaAs-AlGaAs double barrier res- onant tunneling diodes (RTDs) are investigated by ballistic electron emission microscopy (BEEM). RTDs grown directly below the sample surface exhibit characteristic steplike fea- tures in the BEEM spectrum, whereas for buried RTDs, a lin- ear spectrum is observed. Moreover, the BEEM spectra of sub-surface RTDs show Shubnikov-de Haas-like oscillations in magnetic fields. To investigate the origin of these effects, the BEEM spec- tra were calculated using a scattering formalism within the framework of a semi-empirical tight binding method. As a main result we found that, independent of the applied bias, only electrons within a narrow kdistribution are transferred resonantly through the RTD. Hence, a kfilter is established for ballistic electrons close to k� = 0. The calculated filter width is consistent with the magnetic field data.

Mechanism of nonideality in nearly ideal Si Schottky barriers

The origin of nonideality in an actual nearly ideal Schottky barrier is an inhomogeneous Schottky barrier height (SBH). A high density of point defects is generated in the neighborhood of the interface by the fabrication process of the metal/Si interface. Local SBH lowering by positively ionized defects close to the interface is considered the cause of inhomogeneity based on the property of the metal-induced gap states. Results of analysis by this mechanism are in excellent agreement with ballistic electron emission microscopy (BEEM) observation of low-SBH spots. A Gaussian distribution of inhomogeneous SBH explains the BEEM spectrum, as well as the temperature dependence of both effective SBH and ideality factor, i.e., the so-called T0 anomaly. The spatial distribution of the ionized donor and its variation under applied voltage are obtained. This result indicates that the origin of the ideality factor is preferential neutralization of the donor close to the interface in equilibrium with the Fermi level. Thus, the proposed mechanism explains the various properties of nearly ideal Si Schottky barriers.

Observation of resonant tunneling through a quantized state in InP quantum dots in a double-barrier heterostructure

A direct spectroscopic signature associated with the quantized state of the charge carriers in three-dimensionally confined InP quantum dots (QDs) is reported using a ballistic electron emission microscopy (BEEM)/spectroscopy technique. The self-assembled InP QDs are sandwiched in an AlInP double-barrier heterostructure. The excellent nanometer-scale lateral resolution of the BEEM technique is used to investigate the current transport mechanism by the direct injection of electrons into a single quantum dot. The BEEM spectra taken on and off the dot revealed the presence of a localized state at around 0.1±0.02 eV above the ground state.

Interfacial scattering of hot electrons in ultrathin Au/Co films

We have used room-temperature, ballistic electron emission microscopy (BEEM) to measure hot-electron transport through ultrathin Au/Co multilayer structures deposited onto Si. The samples consist of Au/Co/Si or (Au/Co)n/Au/Si diodes, sputter deposited at 175 or 300 K, where n is the number of repeat layers. The thin-film Co attenuation length, λCo, is extracted from the BEEM spectra as a function of Co thickness, in single Co layer samples. Similarly, the interface attenuation number, or the number of Co/Au interfaces required for a 1/e attenuation, is determined from the multi-interface samples. BEEM barrier heights of Au/Co/Si decrease with increasing Co thickness (for thicknesses <1 nm), as the film becomes continuous and develops a Schottky barrier for Co or CoSi2(<0.7 eV). For these diodes, λCo, increases from 0.3 to 0.5 nm, each with an estimated uncertainty of 0.1 nm, when the deposition temperature is decreased from 300 to 175K. This result is associated with decreased silicide formation at the lower deposition temperature. When Co is isolated from the Si with a 2 nm Au layer, the barrier height is stable (0.82 eV), and λCo, increases further to 0.8±0.1 nm. The λCo values are independent of electron energy over the range measurable (1–1.8 eV). The interface attenuation number was 1.8 and 25 interfaces for 0.6 and 1.2 nm total Co thickness, respectively (in structures with a constant total Au thickness of 8 nm). Thus, significant interface scattering is observed in the thinner sample and negligible scattering when the Co thickness was doubled. Further BEEM measurements on similar superlattices are being carried out to understand these results.

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.

Imaging and local current transport measurements of AlInP quantum dots grown on GaP

Individual AlInP self-assembled quantum dots grown on a (100) GaP substrate are imaged and probed using ballistic electron emission microscopy (BEEM). The excellent nanometer scale lateral resolution of BEEM is utilized to inject carriers directly into a single quantum dot, and thus, current transport through the dot investigated without any direct electrical contact. The BEEM spectra taken on and off the dot revealed a local conduction-band offset between GaP and AlInP with a barrier height of ΔEc∼0.13±0.01 eV.

A BEEM study of Schottky barrier height distributions of ultrathin CoSi2/n-Si(100) formed by solid phase epitaxy

The spatial distributions of the Schottky barrier heights of ultrathin CoSi2 films (~10 nm) on n-Si(100), obtained by multilayer solid state reaction of Co/Ti/n-Si, Co/a-Si/Ti/n-Si, Ti/Co/a-Si/Ti/n-Si and Co/n-Si systems, are studied by ballistic electron emission microscopy (BEEM) and spectroscopy (BEES) at low temperature (~-80 °C). The barrier heights determined from BEEM spectra range between 520 meV and 700 meV, with an approximate Gaussian distribution. The mean barrier heights of the epitaxial CoSi2/Si contacts are 0.60-0.61 eV, lower than the 0.64 eV for polycrystalline CoSi2/Si contacts. Adding a thin amorphous Si interlayer (1 nm) slightly increases the probability of higher barrier heights, while a thin Ti capping layer (1 nm) has no significant influence on the mean barrier height. The BEEM results are compared to those from I-V/C-V measurements.

Ballistic electron emission microscopy of Au/n-ZnSe contacts and local density of states spectroscopy

Ballistic electron emission microscopy (BEEM) has been used to investigate the Au/n-ZnSe contact at high voltage. A statistical barrier height value of 1.63±0.05 eV is obtained. The metal–insulator–semiconductor structure is invoked to explain domains of low electron transmission. Features appear in BEEM spectra at higher voltages and can be attributed to the density of empty states in the semiconductor. Impact ionization effects are observed when the electron kinetic energy exceeds the band-gap energy.

Second Derivative Ballistic Electron Emission Spectroscopy in Au/(AlGa)As

AbstractOur current study focuses on an analysis of the ballistic electron emission microscopy (BEEM) spectra of Au/(AIGa)As heterostructures to estimate quantitatively the effect of the carrier scattering in the metal, at the metal-semiconductor (m-s) interface and in the semiconductor on the multivalley carrier transport. The second derivative (SD)-BEEM spectra, representing the heterostructure transmission coefficient, show explicit partitioning of the contribution of different transport channels (Γ, L and X conduction valleys). Our analysis of SD-BEEM spectra by the developed theoretical model indicates that about 85–92% of the BEEM electrons are scattered at the nonepitaxial Au/GaAs interface. We also show that initial electron distribution among the conduction bands of the semiconductor, specified by the m-s interface scattering, is modified by the further hot-electron transport inside the semiconductor.

Ballistic electron emission microscopy on biased GaAs–AlGaAs superlattices

In this work, ballistic electron transport through the lowest miniband of a biased GaAs–AlGaAs superlattice is investigated by ballistic electron emission microscopy (BEEM). In the BEEM spectra the miniband manifests itself as clear peak in the second derivative of the ballistic electron current. Biasing the superlattice results in a shift of the miniband position and the corresponding peak position. It is shown that the measured total transmission of the superlattice is in excellent agreement with the calculated transmission, which makes the superlattice a promising tunable energy filter for studying the energetic distribution of ballistic electrons.

Quantum-mechanical analysis of the elastic propagation of electrons in the Au/Si system: Application to ballistic-electron-emission microscopy

We present a Green's function approach based on a LCAO scheme to compute the elastic propagation of electrons injected from a STM tip into a metallic film. The obtained 2D current distribution in real and reciprocal space furnish a good representation of the elastic component of Ballistic Electron Emission Microscopy (BEEM) currents. Since this component accurately approximates the total current in the near threshold region, this procedure allows --in contrast to prior analyses-- to take into account effects of the metal band structure in the modeling of these experiments. The Au band structure, and in particular its gaps appearing in the [111] and [100] directions provides a good explanation for the previously irreconcilable results of nanometric resolution and similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).

Electron refraction in ballistic electron-emission microscopy studied by a superlattice energy filter

Buried ${\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ superlattices on Au-GaAs Schottky diodes have been used as an energy filter to study the energetic current distribution in ballistic electron-emission microscopy (BEEM) at room temperature and $T=100\mathrm{K}.$ Due to the large difference in electron masses in Au and GaAs we find that parallel momentum conservation leads to considerable electron refraction at the Au-GaAs interface. As a consequence, the energetic distribution of the ballistic electron current is inverted beyond the Au-GaAs interface and an almost linear behavior of the BEEM spectrum is observed in the energetic regime of the superlattice miniband.