Ballistic Electron Emission Microscopy Studies Research Articles

Electron injection barrier and energy-level alignment at the Au/PDI8-CN2 interface via current–voltage measurements and ballistic emission microscopy

We probe electron transport across the Au/organic interface based on oriented thin films of the high-performance n-type perylene diimide semiconductor PDI8-CN2. To this purpose, we prepared organic-on-inorganic Schottky diodes, with Au directly evaporated onto PDI8-CN2 grown on n-Si. Temperature-dependent current–voltage characteristics and complementary ballistic electron emission microscopy studies reveal that rectification at the Au/PDI8-CN2 interface is controlled by a spatially inhomogeneous injection barrier, that varies on a length scale of tens of nanometers according to a Gaussian distribution with mean value ∼0.94eV and standard deviation ∼100meV. The former gradually shifts to ∼1.04eV on increasing PDI8-CN2 thickness from 5nm to 50nm. Experimental evidences and general arguments further allow to establish the energetics at the Au/PDI8-CN2 interface. Our work indicates injection-limited current flow in PDI8-CN2-based devices with evaporated Au electrodes. Furthermore, it suggests chemical reactivity of PDI8-CN2 with both Au and Si, driven by the lateral isocyano groups.

Probing electron transport across a LSMO/Nb:STO heterointerface at the nanoscale

We investigate electron transport across a complex oxide heterointerface of La${}_{0.67}$Sr${}_{0.33}$MnO${}_{3}$ (LSMO) on Nb:SrTiO${}_{3}$ (Nb:STO) at different temperatures. For this, we employ the conventional current-voltage method as well as the technique of ballistic electron emission microscopy (BEEM), which can probe lateral inhomogeneities in transport at the nanometer scale. From current-voltage measurements, we find that the Schottky barrier height (SBH) at the LSMO/Nb:STO interface decreases at low temperatures accompanied by a larger than unity ideality factor. This is ascribed to the tunneling dominated transport caused by the narrowing of the depletion width at the interface. However, BEEM studies of such unbiased interfaces do not exhibit SBH lowering at low temperatures, implying that this is triggered by the modification of the interface due to an applied bias and is not an intrinsic property of the interface. Interestingly, the SBH at the nanoscale, as extracted from BEEM studies, at different locations in the device is found to be spatially homogeneous and similar both at room temperature and at low temperatures. Our results highlight the application of BEEM in characterizing electron transport and its homogeneity at such unbiased complex oxide interfaces and yield insights into the origin of the temperature dependence of the SBH at biased interfaces.

Effect of surface contamination on electron tunneling in the high bias range

The effect of surface contamination on the electron tunneling in the high bias range is investigated from the perspective of ballistic electron emission microscopy (BEEM). A comparative BEEM study on the Au/SiO2/Si devices shows that there is a significant difference in the high bias range between the experiments performed with in situ and ex situ deposited Au. Detailed studies show that the difference arises from the contaminations during air exposure. These contaminations significantly accelerated the material transfer between the tip and the sample during tunneling and lead to the unreliability of BEEM studies in the high bias range on the ex situ prepared sample.

Ambipolar ballistic electron emission microscopy studies of gate-field modified Schottky barriers

Four-terminal ambipolar ballistic electron emission microscopy studies are conducted on Au/Si and Cu/Si Schottky contacts fabricated on back-gated silicon-on-insulator wafers, allowing the electric field to be varied so that both electron (n)- and hole (p)-Schottky barrier heights can be measured at the same sample location. While the individual n- and p-Schottky barrier heights varied by more than 200 meV between the Au/Si and Cu/Si contacts, for a given sample they sum to within 15 meV of the same value, indicating that the individual variations are due to variations in a local surface dipole as compared with tip effects or variations in local composition.

Tunneling spectroscopy studies of treated aluminum oxide tunnel barrier layers

We report scanning tunneling microscopy and ballistic electron emission microscopy studies of the electronic states of the uncovered and chemisorbed-oxygen covered surface of AlOx tunnel barrier layers. These states change when chemisorbed oxygen ions are moved into the oxide by either flood gun electron bombardment or by thermal annealing. While untreated samples exhibit band tails extending to zero bias, the former, if sufficiently energetic, results in locally well defined conduction band onsets at ∼1V, while the latter results in a progressively higher local conduction band onset, exceeding 2.3 V for 500 and 600 °C thermal anneals.

Confinement-Enhanced Electron Transport across a Metal-Semiconductor Interface

We present a combined scanning tunneling microscopy and ballistic electron emission microscopy study of electron transport across an epitaxial Pb/Si(111) interface. Experiments with a self-assembled Pb nanoscale wedge reveal the phenomenon of confinement-enhanced interfacial transport: a proportional increase of the electron injection rate into the semiconductor with the frequency of electron oscillations in the Pb quantum well.

Ballistic electron emission microscopy studies of the temperature dependence of Schottky barrier height distribution in CoSi 2/n-Si(1 0 0) diodes formed by solid phase reaction

Ballistic electron emission microscopy (BEEM) and ballistic electron emission spectroscopy have been performed on polycrystalline and epitaxial CoSi 2/n-Si(1 0 0) contacts at temperatures ranging from −144°C to −20°C. The ultra-thin CoSi 2 films (∼10 nm) were fabricated by solid state reaction of a single layer of Co (3 nm) or a multilayer of Ti (1 nm)/Co (3 nm)/amorphous-Si(1 nm)/Ti (1 nm) with a Si substrate, respectively. The spatial distribution of barrier height over the contact area obeys a Gaussian function at each temperature. The mean barrier height increases almost linearly with decreasing temperature with a coefficient of −0.23±0.02 meV/K for polycrystalline CoSi 2/Si diodes and −0.13±0.03 meV/K for epitaxial diodes. This is approximately equal to one or one-half of the temperature coefficient of the indirect energy gap in Si, respectively. It suggests that the Fermi level is pinned to different band positions of Si. The width of the Gaussian distribution is about 30–40 meV, without clear dependence on the temperature. The results obtained from conventional current–voltage and capacitance–voltage ( I– V/ C– V) measurements are compared to BEEM results.

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.

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.

BEEM studies of the PtSi/Si(100) interface electronic structure

Abstract The aim of the present work is a comparative study by ballistic electron emission microscopy (BEEM) of PtSi/n-Si(100) interface electronic structures using the assumption of an energy-dependent transmission coefficient or an energy-independent transmission coefficient and the possible explanation of the standard deviation of the barrier height from the most probable barrier height. PtSi/n-Si(100) interfaces have been studied for PtSi layers with an average estimated thickness of 2–3 nm. Locally measured barrier heights show distributions with the most probable values of barrier heights between 0.734 eV and 0.829 eV and with standard deviations from these most probable values of barrier heights in the range 0.010–0.066 eV. The assumption of an energy-independent coefficient for transmission of electrons through the interface is more preferable since the treatment of experimental results using a square-law leads to a most probable value of the barrier height in accordance to other measurements. The standard deviations of barrier heights from the most probable barrier heights may be explained by the heterogeneity of the PtSi layer thickness. The experimental results show lower barrier height for a higher PtSi layer thickness. The decreasing barrier height is accompanied by an increasing amount of Pt interstitial atoms.

A ballistic electron emission microscopy study of barrier height inhomogeneities introduced in Au/III-V semiconductor Schottky barrier contacts by chemical pretreatments

The distribution of Schottky barrier heights over the contact area in Au/III-V semiconductor (GaAs, InP, AlxGa1-xAs, InxGa1-xAs) diodes was determined using ballistic electron emission microscopy. Samples which received a chemical pretreatment in aqueous HF or HCl solutions showed changes in the barrier height distribution. In some cases, short rinses in deionized water could remove these effects. Additional XPS measurements and our former work on Si enabled us to propose a model wherein negatively charged species containing F or Cl at the interface are assumed to be responsible for these changes in barrier height distribution. However, in some cases, these effects were shadowed by more drastic influences due to the chemical processing such as changes in the stoichiometry of the surface region.

Temperature-dependent studies of InAs base layers for ballistic electron emission microscopy

InAs is a promising base material for ballistic electron emission microscopy (BEEM), since in this material the attenuation length of ballistic electrons is more than one order of magnitude larger than for metal base layers and the corresponding transmission factor for ballistic electrons is strongly enhanced. In this work, temperature-dependent BEEM studies on InAs base layers grown on GaAs substrates are performed. Unlike on samples with metal base layers, it is found that the transmission coefficient of the InAs base decreases with decreasing temperature. In addition, a strongly increasing conduction band offset at the InAs-GaAs interface with decreasing temperature is also observed.

Ballistic electron emission microscopy study of barrier height inhomogeneities introduced in Au/n-Si Schottky contacts by a HF pretreatment

The distribution of Schottky barrier heights over the contact area in Au/n-Si diodes was determined by ballistic electron emission microscopy. For samples on which an aqueous HF pretreatment of the Si substrate was applied, the histogram contains several high barrier Gaussian distribution components. After a short rinse, in de-ionized water or methanol, it was mainly the most important lower Gaussian component which was left. Using additional x-ray photoemission spectroscopy and atomic force microscopy measurements allowed us to propose a model, wherein negatively charged species containing F at the interface, are thought to be responsible for the high barrier Gaussian components.

Ballistic electron emission microscopy study of Schottky contacts on 6H- and 4H-SiC

We have performed ballistic-electron emission microscopy (BEEM) on (Pd,Pt)/(6H,4H)-SiC(0001) Schottky contacts. Schottky barrier heights (SBHs) determined from the BEEM data using the Bell–Kaiser model are 1.27 eV/1.34 eV (Pd/Pt) for 6H- and 1.54 eV/1.58 eV for 4H-SiC. Our measurements also give the first direct evidence of a second conduction band minimum (CBM) on 4H-SiC about 0.14 eV above the overall CBM. Spatial inhomogeneity of SBHs were examined and shown to be no larger than the fitting error due to the system noise. Additionally, enhanced ballistic transmittance was observed over a region intentionally stressed by injecting high kinetic energy (10 eV above EF) hot electrons using BEEM in the Pt/4H-SiC sample.

The origin of the integral barrier height in inhomogeneous Au/Co/GaAs67P33-Schottky contacts: A ballistic electron emission microscopy study

In this work we investigated the relationship between the integral Schottky barrier height (SBH) obtained from conventional current–voltage (I–V) measurement and the distribution of the local SBH measured by ballistic electron emission microscopy (BEEM) on a nanometer scale length. For this purpose, we investigated inhomogeneous Au/Co/GaAs67P33-Schottky contacts. The samples were prepared by the deposition of a discontinuous Co film on the semiconductor followed by the deposition of a continuous Au film. This provided regions with local presence of one or the other metal (Au or Co) at the metal-semiconductor interface, resulting in mesoscopically extended SBH inhomogeneities. The local SBH distribution as well as the integral SBH depended on the preparation parameter of the Co layer, i.e., on the combination of the substrate temperature (300 or 500 K) and the nominal Co thickness (0, 0.25, 0.5, 0.8, 1.0 nm). For the different preparation parameters, statistical distributions of the local SBH were measured by BEEM. Treating these SBH distributions in terms of a parallel conduction model for the electron transport across the MS interface, we calculated for each preparation parameter an integral SBH and compared it with the measured integral SBH obtained from conventional I–V measurement. The calculated and measured integral SBH’s were in very good agreement, demonstrating clearly the strong influence of the low SBH regions on the electron transport across the interface and therefore on the integral SBH. The SBH values for homogeneous Au/GaAs67P33- and Co/GaAs67P33-Schottky contacts, i.e., with only one sort of metal at the interface, were determined to be ΦSBAu=1180±10 meV and ΦSBCo=1030±10 meV. As with regard to the inhomogeneous Schottky contacts the fraction of area of the MS interface covered by Co increased, the local SBH distributions as well as the integral SBH’s decreased gradually from the value of ΦSBAu to ΦSBCo.

BEEM study of electron tunnelling across single AlAs barriers

Ballistic electron emission microscopy (BEEM) has been used to study the electron transport across single potential barriers. The system used was a Au/GaAs/AlAs/GaAs mixed heterostructure, in which the Au forms a Schottky contact to the GaAs and the AlAs layer acts as a buried potential barrier to the electron conduction. The barrier thickness has been changed in a wide range from 0 to 300Åfor which tunnelling probabilities were calculated theoretically, compared with the BEEM data and found to be in good agreement with the measured lowest thresholds. The suitability of the ‘square law’ as a fitting theory in dealing with this tunnelling situation was examined and some modification was found necessary.

Ballistic electron emission microscopy study of transport in GaN thin films

Ballistic electron emission microscopy (BEEM) measurements on GaN grown on sapphire substrates reveal a second conduction band minimum ∼340 meV above the absolute band minimum at the zone center (Γ point). A significant lateral variation of the energy difference between the two band minima, ±50 meV, was observed which may result from nonuniform strain in the material. The existence of two conduction bands in close proximity may affect device applications, i.e., GaN based lasers and electronic devices.

BEEM studies on GaAs/Al xGa 1–xAs quantum wire structures

Ballistic electron emission microscopy (BEEM) was used to study laterally patterned GaAs/Al xGa 1–xAs heterostructures. The measurements were carried out at room temperature in air as well as in liquid helium. Wet chemically etched quantum wires were identified both in topographic and BEEM current imaging. We find that the BEEM current is enhanced if ballistic electronis are injected directly into the quantum wire. The subsurface Al xGa 1–xAs barrier influences the collector current by determining the BEEM current threshold and by influencing the Fermi level pinning position.

Ballistic electron emission microscopy studies of electron scattering in Au/GaAs Schottky diodes damaged by focused ion beam implantation

Ballistic electron emission microscopy is used to investigate the electron scattering properties of undamaged and focused ion beam implanted Au/GaAs diodes. Implanted regions show decreased ballistic electron transmission attributed to increased scattering. A quantitative model of electron transport is developed that includes quantum mechanical transmission at the interface, scattering effects from optical phonons and implantation induced defects in the semiconductor, and scattering in the metal layer. Model predictions of scattering in undamaged regions show good agreement with the data. However, we conclude additional scattering effects must be included to describe the implanted structures.

Quantum transmittance from low-temperature ballistic electron emission spectroscopy of Au/Si(100) Schottky interfaces.

Ballistic electron emission microscopy studies of the Au/Si(100) interface have been performed at temperatures below 77 K for the first time. Spectra below 10 K show collector charging due to the high semiconductor resistivity. Results above 10 K demonstrate unambiguously that both quantum-mechanical phase and amplitude effects are required to describe electron transmission across a metal-semiconductor interface. Furthermore, the ballistic (collisionless) model of electron transmission across this interface is shown to be valid for a wide energy range