Alexander Lajn

Recent Progress on ZnO‐Based Metal‐Semiconductor Field‐Effect Transistors and Their Application in Transparent Integrated Circuits

Metal-semiconductor field-effect transistors (MESFETs) are widely known from opaque high-speed GaAs or high-power SiC and GaN technology. For the emerging field of transparent electronics, only metal-insulator-semiconductor field-effect transistors (MISFETs) were considered so far. This article reviews the progress of high-performance MESFETs in oxide electronics and reflects the recent advances of this technique towards transparent MESFET circuitry. We discuss design prospects as well as limitations regarding device performance, reliability and stability. The presented ZnO-based MESFETs and inverters have superior properties compared to MISFETs, i.e., high channel mobilities and on/off-ratios, high gain, and low uncertainty level at comparatively low operating voltages. This makes them a promising approach for future low-cost transparent electronics.

Properties of reactively sputtered Ag, Au, Pd, and Pt Schottky contacts on n-type ZnO

Highly rectifying Ag, Au, Pd, and Pt Schottky contacts have been fabricated on heteroepitaxial pulsed-laser deposited ZnO-thin films by reactive sputtering. X-ray photoelectron spectroscopy revealed an oxidation of the Ag, Pd, and Pt contact material; the gold contacts are purely metallic. The necessity of a conductive capping of the oxidized contacts is proven by photocurrent measurements of AgxO contacts. The ideality factors and the effective barrier heights were determined by current-voltage measurements. Capacitance-voltage and temperature dependent current-voltage measurements were furthermore carried out to determine the mean barrier height, the standard deviation and the respective voltage dependencies taking lateral fluctuations of the barrier height into account.

ZnO metal-semiconductor field-effect transistors with Ag-Schottky gates

Metal-semiconductor field-effect transistors (MESFETs) were fabricated by reactive dc sputtering of Ag-Schottky gate contacts on ZnO thin-film channels grown by pulsed-laser deposition on sapphire. The n-type conductivity (normally on) of typical MESFETs is tunable over 8 decades in a voltage range of 2.5V with an off voltage of −1.5V and very low off-current density in the range of 10−6A∕cm2. Channel mobilities of up to 27cm2∕Vs have been achieved.

Low-temperature processed Schottky-gated field-effect transistors based on amorphous gallium-indium-zinc-oxide thin films

We have investigated the electrical properties of metal-semiconductor field-effect transistors (MESFET) based on amorphous oxide semiconductor channels. All functional parts of the devices were sputter-deposited at room temperature. The influence on the electrical properties of a 150 °C annealing step of the gallium-indium-zinc-oxide channel is investigated. The MESFET technology offers a simple route for processing of the transistors with excellent electrical properties such as low subthreshold swing of 112 mV/decade, gate sweep voltages of 2.5 V, and channel mobilities up to 15 cm2/V s.

High-gain integrated inverters based on ZnO metal-semiconductor field-effect transistor technology

We report on the design and fabrication of ZnO-based integrated inverters consisting of normally-on metal-semiconductor field-effect transistors and AgxO Schottky diodes as level shifters. The inverters show high gain values up to 197 at 3 V operating voltage and low uncertainty levels in the range of 0.13 V. The influence of the level shifter and the channel material/thickness on the performance of the inverters has been investigated. Using Zn0.997Mg0.003O for the channel thin film leads to high reproducibility (90%) of the devices. A logic NOR-gate has been implemented showing the possibility to fabricate a complete logic.

ZnO-based metal-semiconductor field-effect transistors on glass substrates

We investigate the influence of quartz glass and borosilicate glass substrates on the electrical properties of ZnO-based metal-semiconductor field-effect transistors (MESFETs). The n-type ZnO thin-film channels were grown by pulsed-laser deposition and MESFETs were processed by reactive dc sputtering of AgxO-Schottky gate contacts. All devices are in the normally-off state. They exhibit very low off-currents in the range of 10−13A and on/off ratios of maximum 6 decades. The channel mobilities are highest for ZnO on quartz with 1.3cm2/Vs. The glass substrates introduce a compensating effect on the conduction of the ZnO channel resulting in higher on/off-voltages and lower on-current.

Comparative study of transparent rectifying contacts on semiconducting oxide single crystals and amorphous thin films

We demonstrate fully transparent, highly rectifying contacts (TRC) on amorphous GaInZnO and compare them to TRC fabricated on single crystalline bulk ZnO and heteroepitaxial ZnO thin films. The contacts’ transparency in the visible spectral range exceeds 70%. From numerical simulations, we conclude that thermionic emission is the dominating transport mechanism, however, for several samples with low net doping density diffusion theory must be applied. The detailed investigation of the rectification properties of the TRC using temperature-dependent current-voltage and capacitance-voltage measurements reveals that barrier inhomogeneities govern the IV-characteristics of all diodes irrespective of the sample crystallinity. Assuming a Gaussian barrier height distribution, the extracted mean barrier heights typically range between 1.1 and 1.3 V. The width of the barrier distribution correlates with the mean barrier height and ranges from 110 to 130 mV. By compiling literature data, we found that this correlatio...

Oxide Thin Film Heterostructures on Large Area, with Flexible Doping, Low Dislocation Density, and Abrupt Interfaces: Grown by Pulsed Laser Deposition

Advanced Pulsed Laser Deposition (PLD) processes allow the growth of oxide thin film heterostructures on large area substrates up to 4-inch diameter, with flexible and controlled doping, low dislocation density, and abrupt interfaces. These PLD processes are discussed and their capabilities demonstrated using selected results of structural, electrical, and optical characterization of superconducting (YBa 2Cu 3O 7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO 3-based) and dielectric (wide-gap oxide) thin films and multilayers. Regarding the homogeneity on large area of structure and electrical properties, flexibility of doping, and state-of-the-art electronic and optical performance, the comparably simple PLD processes are now advantageous or at least fully competitive to Metal Organic Chemical Vapor Deposition or Molecular Beam Epitaxy. In particular, the high flexibility connected with high film quality makes PLD a more and more widespread growth technique in oxide research.

Ultrathin gate-contacts for metal-semiconductor field-effect transistor devices: An alternative approach in transparent electronics

Transparent metal-insulator-semiconductor field-effect transistors (TMISFETs) are commonly designated as one keystone of transparent circuitry. TMISFETs were demonstrated using carbon nanotubes, organics, or oxides. The optimization of their gate-insulator as well as the field-effect mobility and switching voltages is of major interest in this research field. We present an alternative approach based on metal-semiconductor field-effect transistors (MESFETs) circumventing these problems of TMISFET technology. We use ultrathin transparent rectifying contacts (TRCs) consisting of non-insulating AgxO or PtxO layers and a highly conducting capping layer realized by room-temperature sputtering. The process is compatible with other low-temperature, low-cost manufacturing steps. We demonstrate the potential of such TRCs for ZnO-based MESFETs having a transparency of 70% in the visible, on/off-ratios higher than 106 within a gate-voltage sweep of only 2.7 V and mobilities up to 12 cm2/V s. Inverters fabricated from...

Gate- and drain-lag effects in (Mg,Zn)O-based metal-semiconductor field-effect transistors

The dynamic properties of (Mg,Zn)O-based metal-semiconductor field-effect transistors (MESFETs) with the gate materials AgxO, PtOy, and Au were investigated. The AgxO-gated FETs exhibit the best static properties but are limited in their switching speed, probably due to Ag atoms present in the channel after diffusion during the transistor fabrication. Devices with PtOy and Au gates did not exhibit any significant delay in frequency-dependent gate lag measurements in the studied frequency range up to 1 MHz.