Electron Transport In Devices Research Articles

Robust Computational Models of Quantum Transport in Electronic Devices

A family of efficient quantum transport models for simulation of modern nanoscale devices is presented. These models are used for quantitative calculations of quantum currents in nanoscale electronic devices within our device simulator software. Specifically, we used them to simulate the tunneling current through thin barrier in vertical-cavity surface-emitting laser (VCSEL), direct and reverse tunnel currents through the tunnel junction, Schottky contact characteristics, and gate induced drain leakage (GIDL).

1:30 000 cryogenic current comparator with optimum squid readout

We have developed a 1:30000 winding ratio cryogenic current comparator (CCC) for measurement of small currents, e.g., generated by a single electron transport device. A dedicated low-noise SQUID is used for the readout, which allows reaching a sensitivity close to ideal, that is without loss of sensitivity in the flux transformer coupling the CCC and the SQUID. The CCC-SQUID equivalent current input noise using a 1:30 000 winding ratio is 2.1 fA/Hz/sup 1/2/, measured down to 0.1 Hz. Room temperature impedance measurements on CCCs appear to be a useful method for the determination of the screening by shields surrounding the CCC as well as for the detection of internal CCC resonances.

Electrical characteristics of light-emitting diode based on poly( p-phenylenevinylene) derivatives: CzEH-PPV and OxdEH-PPV

In the light-emitting devices (LEDs) based on the π-conjugated polymers, the relationship between the quantum efficiency and the balance of hole ( μ h) and electron ( μ e) mobility has been investigated. In order to measure the μ h and μ e of the LEDs based on π-conjugated polymers, we fabricated the hole transport device (HTD) and the electron transport device (ETD) by using various metal electrodes with different work functions. For the materials of light emitting layer, we synthesized poly[2-( N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV) and poly[2-{4-[5-(4- tert-butylphenyl)-1,3,4-oxadiazolyl]-phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (OxdEH-PPV) with electron-rich groups. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), which is well known material for the polymer-based LED, was synthesized for the reference. We measured the current density vs. applied field ( J– E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J– E curves were analyzed by using the space charge limited conduction (SCLC) model. Based upon the SCLC model, μ h and μ e of MEH-PPV sample was measured to be ∼10 −6 cm 2/V s and ∼10 −8 cm 2/V s, respectively. For CzEH-PPV and OxdEH-PPV samples with electron-rich groups, μ h was similar to μ e with 10 −10–10 −11 cm 2/V s. The μ h and μ e of CzEH-PPV and OxdEH-PPV samples was lower than that of MEH-PPV sample, but more balanced. The quantum efficiency of the LED by using CzEH-PPV or OxdEH-PPV materials was ∼10 times higher than that prepared from MEH-PPV. The balance of the μ h and μ e plays an important role for the quantum efficiency. We analyze the balance of the μ h and μ e and the relatively low mobilities of CzEH-PPV and OxdEH-PPV samples in terms of the heavier effective mass due to the asymmetric dipole distribution in the side chains. The results of photocurrent of the systems qualitatively agreed with the result of the electrical measurements. From AC impedance measurement of the LEDs, we observed that the relaxation time of MEH-PPV was shorter than that of OxdEH-PPV sample because of the higher mobility of MEH-PPV sample.

Electrical and optical characteristics of various PPV derivatives (MEH-PPV, CzEH-PPV, OxdEH-PPV)

To investigate the electrical characteristics of polymer based light emitting diode (LED) devices, we fabricated the hole transport device (HTD) and the electron transport device (ETD). The ITO and Au with high work function were used as electrodes for the HTD, and the Al and Li:Al with low work function were used for the ETD. The active layer materials were poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV), and poly[2-(4-tert-butylphenyl)-5-phenyl-1,3,4-oxadiazole-5(2-ethylhexoxy)-1,4-phenylene vinylene] (OxdEH-PPV). We measured the current density–applied field (J–E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J–E curves were analyzed by using tunneling model, space charge limited conduction (SCLC) model, etc. In the SCLC model, the mobility of the hole and the electron of MEH-PPV is ∼10−6 and ∼10−8cm2/Vs, respectively. For CzEH-PPV and OxdEH-PPV, the hole mobility is similar to the value of the electron mobility with ∼10−10cm2/Vs. The luminescent efficiency of CzEH-PPV or OxdEH-PPV is higher than that of MEH-PPV. The results of photoconductivity of the systems qualitatively agrees with the result of the electrical measurement. We analyze that the balance of the electron and the hole mobility plays an important role for the efficiency of the LEDs.

Fabrication and properties of dot array using electron-beam-induced deposition

Electron-beam(EB)-induced deposition using precursor molecule of WF6 is applied to making metal/insulator/metal tunnel junctions for single electron transport devices. The SiO2 substrate pre-treated by oxygen plasma is contaminated with hydro-carbon material and the deposits show rapid decrease in resistance with the increase in EB dose and higher resistivity of about 100 Ω cm at 300 K which decreases by 20% at 130 K. In contrast, the thickness of deposits on clean substrates is 120 as small as the case of contaminated substrate for the same exposure dose. Also deposits with a lower resistivity of 4×10−3 Ω cm can be obtained. Under such deposition conditions, single electron transistor structure is fabricated which is composed of several dots connected to single line deposits with deposits for gate electrode 300 nm distant from the dot region. The source-drain voltage dependence of source current for 7 dots with 10 nm pitch exhibits Coulomb blockade characteristics at 12 K.

The impedance of the near ballistic electron transport device in n-GaAs

The ac impedance of a two-terminal near-ballistic electron device is calculated by solving the equation of motion, Poisson’s equation, and the equation for current density. The influences of diffusion and collision drag are discussed. Our calculations show that this particular device has an equivalent circuit characterized by an antiresonance whose frequency shifts with increasing collision effect away from the plasma frequency towards lower values, and that diffusion and collision effects reduce substantially the magnitude and phase of the impedance. Even so, the impedance appears to be measurable.