Mordehai Heiblum

Tunneling hot electron transfer amplifiers (theta): Amplifiers operating up to the infrared

Abstract A family of novel three-terminal devices which relies on the transfer of a quasi-monoenergetic hot electron beam through a thin base is described. The devices are similar in principle to the proposed tunneling amplifier by Mead in the early sixties (“Cold Cathode” or “Metal Base” amplifiers). Results are reviewed and the probable reasons for the poor performances are pointed out. It is predicted that, with a proper choice of parameters, metal-base amplifiers can operate as switches, negative resistance devices and continuous amplifiers in the subpicosecond range. Two subclasses are described: The tunneling emitter (THETA), in the major part of the work, and the nontunneling emitter (BHETA) amplifiers. In the THETA family the metal-oxide-metal-oxide-metal (MOMOM), the MOM-semiconductor (MOMS), and the heterojunctions devices are described. Members of the BHETA family generate quasi-monoenergetic electron beams by injecting electrons by an n + n − or a metal- n − junctions, and include a variety of metals and semiconductor combinations. Very thin films are required in these devices (oxides ∼15 A, metals ∼100 A, semiconductors ∼100 A). The molecular beam epitaxy technique and lattice matching considerations are required for pinhole free semiconductors and metal films with minimum interface states. Sputter-oxidation methods are needed for thin oxide growth. Systems which combine these features with availability of microfabrication make these devices feasible today.

Use of superlattices to realize inverted GaAs/AlGaAs heterojunctions with low‐temperature mobility of 2×106 cm2/V s

Reproducible realization of high quality inverted interfaces (GaAs on AlGaAs) grown by molecular beam epitaxy is reported. Effective use of thin‐layer GaAs/AlAs superlattices in place of an AlGaAs barrier was made to reduce the number of impurities and the roughness at these interfaces. The low‐temperature (≂4 K) mobility for electrons at these interfaces is as high as 2×106 cm2/V s for an electron density of ≂5×1011 cm−2—a factor of four improvement over the highest mobility reported for inverted interfaces.

Magnetocapacitance measurements in GaAs heterostructures

Abstract Measurements of the magnetocapacitance of a two-dimensional electron gas in high mobility GaAs(Ga, Al)As heterostructure confirm, for the first time, the relationship between the density of states and the capacitance of a two-dimensional electron gas. We present the first magnetocapacitance measurements in the fractionally quantized Hall regime.

The frequency dependence of the diagonal conductivity of a 2DEG in GaAs heterostructure in the quantum Hall regime

Abstract We have measured directly the diagonal conductivity of a 2DEG in GaAs/ A1GaAs heterostructure via a capacitively coupled structure with d.c. bias at magnetic fields up to 6 tesla. From the ungated region of a sample of Corbino geometry, the diagonal conductivity at the Landau gap regions was measured via capacitive coupling to a non-quantized 2DEG in the source and drain capacitors. A strong frequency dependence was observed over a range of frequencies from 100 Hz to 20 kHz at the temperature of 1.3 K. The result is compared to the calculations from the percolation theory. From the measurements of two different temperatures, 1.3 and 4.2 K, the activation energy for the conductivity was estimated to be about 10% of Landau level spacing.

HOT ELECTRON TRANSPORT IN TWO DIMENSIONAL ELECTRON GAS

Abstract We demonstrate ballistic transport of hot electrons in a high mobility two dimensional electron gas (2DEG) over distances as large as two micrometers and tunneling through a barrier induced in the 2DEG. Various inelastic scattering mechanisms are studied experimentally. For electrons with excess energy larger than the longitudinal optical (LO) phonon energy quanta we show sequential emission of up to three phonons. Below that energy we find a surprisingly long mean free path, approximately an order of magnitude longer than expected theoretically for electron-electron scattering. The combination of a very short mean free path for LO phonon emission and the long mean free path below the phonon energy leads to a new type of periodic oscillations in the maximal energy of the collected electrons. Finally we report on a study of the angular distribution of hot electrons injected from a point contact and demonstrate electrostatic steering of the electronic beam.

AC conductivity of (AlGa)AsGaAs heterostructures and silicon MOSFETS in the quantum Hall regime

Abstract We have measured the two-terminal AC conductance of (AlGa)AsGaAs heterostructures and a silicon MOSFET up to 50 MHz. While the ungated heterostructure samples show no frequency dependence in the integer and no additional structure in the fractional quantum Hall regimes, the gated silicon MOSFET shows strong frequency dependence of the two-terminal AC conductance due to coupling of the signal from the 2DEG to the gate. We have also measured the capacitance of the silicon MOSFET under the same circumstances, and thereby confirm the role of the gate in coupling the high frequency signal.

A gallium arsenide ballistic transistor

The authors review the work done in 1985 at the IBM Thomas J. Watson Research Center on measuring the ballistic transport of electrons in a gallium arsenide heterostructure hot-electron transistor. The device is referred to as the tunneling hot-electron transfer amplifier (Theta). The way in which the transistor functions as its own spectrometer is explained. The experimental results obtained with Theta are summarized. It was found that when no voltage is applied across the base-collector heterojunction, essentially only ballistic electrons are collected. These account for about 40% of the total number of electrons injected from the emitter at low temperatures. As the voltage across the collector is increased positively, up to 75% of them are collected.

Capacitively-coupled measurements on the frequency dependent diagonal magnetoconductance of a 2DEG in GaAs heterostructure in the quantum Hall regime

Abstract We have examined the complex capacitance of the 2DEG in GaAs heterostructures at a temperature of 1.3 K, magnetic fields up to 8 T and over a range of frequencies from 200 Hz to 100 kHz. The experiment was performed on a high mobility GaAs/AlGaAs heterostructure from an MBE grown wafer with Corbino geometry. We find that the real and imaginary parts of the complex capacitance of the capacitively-coupled structure, are well explained by a one-dimensional diffusion model and the derived diagonal magnetoconductances in the Landau gap regions are in good agreement with those directly measured via a capacitively coupled structure (triple dip method). Spin splitting was also observed at magnetic fields as low as 2.5 T. The value of the enhanced g -factor at high magnetic fields was larger than 2 which is comparable to those determined by conductivity measurements using ohmic contacts.

Lateral tunneling and ballistic transport in two‐dimensional electron gas devices defined by nanostructure gates

Novel, laterally configured, tunneling hot electron transfer amplifier (THETA) devices have been formed using nanostructure metal gates to induce potential barriers separating emitter, base, and collector regions in a two‐dimensional electron gas. The gates were formed by a liftoff technique using PMMA resist and a high resolution electron beam writing tool. Lateral electron tunneling has been directly observed, using electron energy spectroscopy, through potential barriers under metal gates as wide as 52 nm. The energy distribution of the hot electrons was only ≂5 meV wide. Certain gate configurations were found to increase the fraction of emitted electrons which reached the collector. Current gain as high as 105 was observed. This is better than the best results previously obtained with vertical THETA devices.

First observation of ballistic holes in a p-type THETA device

Novel p-type tunneling hot electron transfer amplifier (THETA) devices have been fabricated for the first time. A tunnel injector is used to separate the light holes from the heavy ones. In the present case, in p GaAs doped to 2*10/sup 18/ cm/sup -3/, the fraction of light holes is only about 6%. After tunneling through a 10-nm-thick AlGaAs barrier, 0.2-eV high, the current due to light holes is more than 10/sup 4/ times greater than that due to the heavy holes. This method of injection has been used to launch primarily light holes into a 30-nm p-GaAs layer, doped as above, and performed energy spectroscopy with another, relatively thick, AlGaAs spectrometer barrier at the exit. It was found that 10% of the injected holes traversed the GaAs layer and the spectrometer ballistically with narrow energy distributions, 35-meV wide. The nature of the ballistic transport was also independently verified to be due to the light holes. This was done through the observation of quantum interference effects of the ballistic holes in the thin GaAs base. The results show that p-type ballistic devices with performance potential approaching that of n-type devices may be possible. >