Friedrich-Leonhard Schein

Transparent p-CuI/n-ZnO heterojunction diodes

Transparent and electrically conducting p-type copper(I)-iodide thin-films form highly rectifying p-CuI/n-ZnO diodes. Sputtered copper thin films on glass were transformed into polycrystalline γ-CuI by exposing them to iodine vapor. The electrical parameters extracted from Hall effect are p=5×1018 cm−3, μh,Hall=6 cm2/Vs, and ρ=0.2 Ωcm for hole concentration, mobility, and electrical resistivity, respectively. Heterostructures consisting of p-CuI and pulsed-laser deposited n-ZnO were fabricated on a-plane sapphire substrates. The p-CuI/n-ZnO diode exhibits a current rectification ratio of 6×106 at ±2 V and an ideality factor of η=2.14.

Highly rectifying p-ZnCo2O4/n-ZnO heterojunction diodes

We present oxide bipolar heterojunction diodes consisting of p-type ZnCo2O4 and n-type ZnO fabricated by pulsed-laser deposition. Hole conduction of ZnCo2O4 (ZCO) was evaluated by Hall and Seebeck effect as well as scanning capacitance spectroscopy. Both, ZCO/ZnO and ZnO/ZCO type heterostructures, showed diode characteristics. For amorphous ZCO deposited at room temperature on epitaxial ZnO/Al2O3 thin films, we achieved current rectification ratios up to 2 × 1010, ideality factors around 2, and long-term stability.

Room-temperature Domain-epitaxy of Copper Iodide Thin Films for Transparent CuI/ZnO Heterojunctions with High Rectification Ratios Larger than 10(9).

CuI is a p-type transparent conductive semiconductor with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm2 V−1 s−1 in bulk), and large room-temperature exciton binding energy (62 meV). The difficulty in epitaxy of CuI is the main obstacle for its application in advanced solid-state electronic devices. Herein, room-temperature heteroepitaxial growth of CuI on various substrates with well-defined in-plane epitaxial relations is realized by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated in accordance with existing theoretical study of domain-epitaxy. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type semiconductors with the purpose to fabricate epitaxial thin film heterojunctions. Such heterostructures have superior properties to structures without or with weakly ordered in-plane orientation. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2 × 109 (±2 V), a 100-fold improvement compared to diodes with disordered interfaces. Also a low saturation current density down to 5 × 10−9 Acm−2 is formed. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material.

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.

Oxide bipolar electronics: materials, devices and circuits

We present the history of, and the latest progress in, the field of bipolar oxide thin film devices. As such we consider primarily pn-junctions in which at least one of the materials is a metal oxide semiconductor. A wide range of n-type and p-type oxides has been explored for the formation of such bipolar diodes. Since most oxide semiconductors are unipolar, challenges and opportunities exist with regard to the formation of heterojunction diodes and band lineups. Recently, various approaches have led to devices with high rectification, namely p-type ZnCo2O4 and NiO on n-type ZnO and amorphous zinc-tin-oxide. Subsequent bipolar devices and applications such as photodetectors, solar cells, junction field-effect transistors and integrated circuits like inverters and ring oscillators are discussed. The tremendous progress shows that bipolar oxide electronics has evolved from the exploration of various materials and heterostructures to the demonstration of functioning integrated circuits. Therefore a viable, facile and high performance technology is ready for further exploitation and performance optimization.

ZnO-Based n-Channel Junction Field-Effect Transistor With Room-Temperature-Fabricated Amorphous p-Type

ZnO-based junction field-effect transistors were fabricated by pulsed-laser deposition using room-temperature-deposited amorphous p-type ZnCo₂O₄ as heterojunction gate on top of a n-ZnO channel layer. A channel mobility of 8.4 cm²/(Vs), current on/off ratio of 1.3 ×10⁷, and a subthreshold swing of 91 mV/dec were achieved for a transistor with a 40 nm thin channel layer. The devices are normally on and show excellent bias-stress stability, exhibiting a negligible threshold-voltage shift. Elevated temperatures up to 150^°C changed the device performance slightly, but the transistor remains fully operative.

\hbox{ZnCo}_{2}\hbox{O}_{4}

The electrical properties of identically fabricated PtOx Schottky contacts on -oriented gallium oxide thin films and bulk crystals were investigated using current–voltage measurements at room temperature. The homogeneous barrier height of the Schottky contacts on thin films is 1.55 ± 0.15 eV, which is significantly smaller than that of those fabricated on bulk single crystals, 2.01 ± 0.12 eV. This large difference indicates an upward band bending of 0.4–0.5 eV at the surface of the bulk crystals in the as-received state, which is explained by the larger net doping density of the thin films compared to the single crystals.

Gate

We compare key properties of zinc oxide (ZnO)-based junction field-effect transistors (JFETs), metal-semiconductor field-effect transistors (MESFETs), and metal-insulator-semiconductor field-effect transistors (MISFETs) prepared from a common ZnO:Mg thin film. The JFETs are fabricated with a ZnCo2O4-gate, the MESFETs with reactively sputtered Pt-gate and the MISFETs with WO3 as gate insulator. The three FET types are compared with regarding dc characteristics, frequency dependence, and stability at temperatures up to 150°C. All devices can be switched within a similar gate voltage range of less than 3 V, making a direct comparison of the device characteristics possible. Measurements above room temperature show a common shift of the transfer curves to higher gate voltages, which seems to be a distinguishing property of ZnO compared with other semiconductors. All electric measurements show major differences between the devices, which can be attributed to the different gate structures.

Comparison of Schottky contacts on β-gallium oxide thin films and bulk crystals

We present integrated inverter circuits based on junction FETs (JFETs) with ZnO channels and amorphous ZnCo2O4 gate contacts. The inverters reach high gain values up to 276 and uncertainty ranges down to 0.3 V for an operating voltage of 3 V. The magnitude of the gain is traced back theoretically to the slope of the JFET saturation current. The use of a level shifter is demonstrated, in order to obtain full inverters, which can be integrated into logic circuits.

Comparison of ZnO-Based JFET, MESFET, and MISFET

The stability of the figures of merit of transparent inverter circuits at temperatures up to 150°C and under illumination by light in the visible spectral range is investigated. The inverter circuits consist of two transparent metal-semiconductor field-effect transistors. For temperatures up to 150°C, the inverters remain operational; the gate electrode degradation affects the voltage transfer characteristic (VTC) for temperatures above 90°C. Except for blue light, which slightly alters the VTC, visible light does not influence the device operation.