R. Heer

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.

Floating electrometer for scanning tunneling microscope applications in the femtoampere range

In this work, a floating, high precision current–voltage converter, applicable to scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM) measurements, is described. The electrometer circuit presented shows a bias independent output offset, adds low noise, and has low thermal drift. The amplifier is useful for any floating applications where an ultrasmall current has to be measured with high resolution (±20 fA). The circuit is fast enough for typical sampling rates required for BEEM or STM image measurements and can also be used for fA measurements related to ground.

A highly transmittive semiconductor base for ballistic electron emission microscopy

Ballistic electron emission spectroscopy and ballistic electron emission microscopy offer the unique possibility of probing subsurface quantum states. To improve the spectroscopic sensitivity, it is important to increase the amount of electrons, which are able to penetrate into the sample. In this work, we show that the transmission coefficient and the attenuation length of the base layer can be enhanced by more than one order of magnitude, if the commonly used thin metal film is replaced by a molecular beam epitaxy grown InAs layer. At low temperatures (T=100 K), a passivated InAs layer yields an attenuation length in the order of 70–90 nm instead of 5 nm obtained on Au films.

Metal-insulator-metal injector for ballistic electron emission spectroscopy

We introduce a solid-state version of ballistic electron emission microscopy/spectroscopy (BEEM/BEES) on GaAs–AlGaAs heterostructures using a metal–insulator–metal (MIM) injector structure that replaces the tip of the scanning tunneling microscope (STM). In the present work, the MIM injector is realized by an Al–Al2O3–Al tunnel junction yielding an easy-to-fabricate three-terminal device for ballistic electron spectroscopy. The device principle is applied to several GaAs–AlGaAs structures. The barrier heights obtained from the onsets of the ballistic current spectra are in good agreement with self-consistent calculations as well as earlier experimental results achieved with STM-based BEES.

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

Abstract 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.

Ballistic transport through GaAs–AlGaAs superlattices in transverse magnetic fields

In this work, ballistic electron transport through the lowest miniband of a biased GaAs–AlGaAs superlattice is investigated in transverse magnetic fields. As method we employ a solid-state version of ballistic electron emission microscopy/spectroscopy using a metal-insulator-metal injector structure that replaces the tip of the scanning tunneling microscope (STM). The ballistic electron current measured as a function of the collector bias shows a peak at flatband conditions indicating coherent transport through the superlattice miniband. With increasing transverse magnetic fields, this peak is quenched and evidence of sequential LO-phonon scattering inside the superlattice is found. Using an extended transfer matrix method, the observed effects are quantitatively explained; differences to previous STM based measurements are discussed.

Highly transmittive semiconductor base for ballistic electron emission microscopy

In this work we introduce a molecular beam epitaxy (MBE)-grown InAs layer as base in ballistic electron emission spectroscopy/microscopy (BEES/BEEM). Compared to the commonly used thin metal film as base, the transmission coefficient and the attenuation length can be enhanced by more than one order of magnitude. At low temperatures (T = 100 K), a passivated InAs layer yields an attenuation length of the order of 70-90 nm, instead of the 5 nm obtained on Au films. This feature makes InAs a promising new base material for BEEM. To clarify the mechanism of this behaviour, temperature-dependent BEEM studies on InAs-GaAs heterostructures were performed. Unlike 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 observed.

k//=0 filtering effects in ballistic electron transport through sub-surface GaAs–AlGaAs double barrier resonant tunneling structures

Abstract In ballistic electron emission microscopy on Au–GaAs double barrier resonant tunneling diodes, electrons are transferred across an interface between an area of high and low effective mass and subsequently through a low-dimensional state. Experimentally, the resonant level in the double barrier structure becomes evident as clear step in the ballistic current measured as a function of sample bias. To analyze the spectrum, an extended transfer matrix method, together with the commonly accepted Bell Kaiser model is used. In terms of this model we show that only electrons with zero wave vector parallel to the barriers can be transmitted resonantly.

Biased GaAs/AlGaAs superlattices employed as energy filter for ballistic electron emission microscopy

In this work, buried Al 0.4 Ga 0.6 As/GaAs superlattices are employed as an energy filter in order to study the energy distribution of the ballistic electron current in ballistic electron emission microscopy (BEEM). As the measured total transmission of the superlattice is in excellent agreement with the calculated transmission, the superlattice is a promising tunable energy filter for studying the energy distribution of ballistic electrons. We further show that due to the large difference in electron masses between the Au base layer and the GaAs collector, parallel momentum conservation leads to considerable electron refraction at the Au/GaAs interface. As a consequence, the energy distribution of ballistic electrons is inverted beyond the Au/GaAs interface.

k||=0 filtering effects in ballistic electron transport through sub-surface resonant tunneling diodes

Abstract In ballistic electron emission microscopy on Au–GaAs double barrier resonant tunneling diodes, electrons are transferred across an interface between an area of high and low effective mass and subsequently through a low-dimensional state. Using an extended transfer matrix method, we show that in this situation only electrons with zero wave vector parallel to the barriers can be transmitted resonantly.