Manoj Nag

Low-voltage gallium–indium–zinc–oxide thin film transistors based logic circuits on thin plastic foil: Building blocks for radio frequency identification application

In this work a technology to fabricate low-voltage amorphous gallium-indium-zinc oxide thin film transistors (TFTs) based integrated circuits on 25 µm foils is presented. High performance TFTs were fabricated at low processing temperatures (<150 °C) with field effect mobility around 17 cm2 /V s. The technology is demonstrated with circuit building blocks relevant for radio frequency identification applications such as high-frequency functional code generators and efficient rectifiers. The integration level is about 300 transistors.

High-performance a-In-Ga-Zn-O Schottky diode with oxygen-treated metal contacts

High-performance Schottky diodes based on palladium blocking contacts were fabricated upon depositing indium-gallium-zinc oxide (IGZO) with high oxygen content. We find that an oxygen treatment of the palladium contact is needed to achieve low off currents in the Schottky diode, and rationalize this by relating an increased oxygen content at the Pd/IGZO interface to a lower interfacial trap density. Optimized IGZO films were obtained with a record high ratio of free charge carrier density to subgap traps. The rectification ratios of diodes with such films are higher than 107 with current densities exceeding 103 A/cm2 at low forward bias of 2 V.

A 6b 10MS/s current-steering DAC manufactured with amorphous Gallium-Indium-Zinc-Oxide TFTs achieving SFDR > 30dB up to 300kHz

Amorphous Gallium-lndium-Zinc-Oxide (GIZO or IGZO) has been recently pro- posed [1] as an interesting semiconductor for manufacturing TFTs because of its mobility (μ~20cm7Vs), superior to other common materials for large-area elec- tronics like organic semiconductors and a-Si (μ~1cm7Vs). The amorphous nature of GIZO grants also a good uniformity, contrary to Low Temperature Polycrystalline Silicon (LTPS), which still offers the best mobility among large- area TFT technologies (μ~100cm2Λ/s). The optical transparency and the relative- ly low fabrication temperature (<;150°C) make this technology especially suitable for display backplanes and relative driving electronics [2], as well as for any kind of large-area applications on plastic foils, e.g. biomedical sensors, non-volatile memories [3], RFIDs [4], etc.

High-Performance a-IGZO Thin Film Diode as Selector for Cross-Point Memory Application

We present amorphous indium-gallium-zinc oxide Schottky diodes with unprecedented current densities of 104 and 105 A/cm2 at forward biases of 1.5 and 5 V, respectively. The diode presents a high rectification ratio of 1010 at ±2 V, which is essential for suppressing the sneak current of not-selected cells in the memory array. In addition, we show that the diode complies with the demanding performance of memory applications. The device degradation, given by a 30% reduction of its forward current after 104 s of continuous bias stress or 109 pulses cycles, was studied via I-V and C-V measurements and can be attributed to trapping of electrons at deep acceptor levels, which increases the diode built-in potential. Finally, we show that the device is stable upon thermal stress at 300 °C for 1 h, which opens the possibility of further processing and integration with the memory cell.

Low-temperature formation of source–drain contacts in self-aligned amorphous oxide thin-film transistors

We demonstrated self-aligned amorphous-Indium-Gallium-Zinc-Oxide (a-IGZO) thin-film transistors (TFTs) where the source–drain (S/D) regions were made conductive via chemical reduction of the a-IGZO via metallic calcium (Ca). Due to the higher chemical reactivity of Ca, the process can be operated at lower temperatures. The Ca process has the additional benefit of the reaction byproduct calcium oxide being removable through a water rinse step, thus simplifying the device integration. The Ca-reduced a-IGZO showed a sheet resistance (RSHEET) value of 0.7 kΩ/sq., with molybdenum as the S/D metal. The corresponding a-IGZO TFTs exhibited good electrical properties, such as a field-effect mobility (μFE) of 12.0 cm2/(V s), a subthreshold slope (SS−1) of 0.4 V/decade, and an on/off current ratio (ION/OFF) above 108.

Back-channel-etch amorphous indium–gallium–zinc oxide thin-film transistors: The impact of source/drain metal etch and final passivation

We report on the impact of source/drain (S/D) metal (molybdenum) etch and the final passivation (SiO2) layer on the bias-stress stability of back-channel-etch (BCE) configuration based amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs). It is observed that the BCE configurations TFTs suffer poor bias-stability in comparison to etch-stop-layer (ESL) TFTs. By analysis with transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS), as well as by a comparative analysis of contacts formed by other metals, we infer that this poor bias-stability for BCE transistors having Mo S/D contacts is associated with contamination of the back channel interface, which occurs by Mo-containing deposits on the back channel during the final plasma process of the physical vapor deposited SiO2 passivation.

An Integrated a-IGZO UHF Energy Harvester for Passive RFID Tags

We present an ultrahigh frequency energy harvester based on low temperature processed a-IGZO (amorphous indium-gallium-zinc oxide) semiconductor on a glass substrate. The harvester is composed of a dipole antenna, matching network, and a double half-wave rectifier and is capable of delivering more than 1 Vdc at a distance of 2 m from the transmitter antenna. In the proposed wireless system, this sensitivity corresponds to 2.75-m distance harvesting at 4-W (36 dBm) emitted power from a base station, which is within EPC regulations. The main element of the rectifier is the high-performance a-IGZO Schottky diode on glass, with a rectification ratio of 107 at ±1 V, a low threshold voltage of 0.6 V and a cutoff frequency of 3 GHz at 0 V bias.

Deep-level transient spectroscopy on an amorphous InGaZnO4 Schottky diode

The first direct measurement is reported of the bulk density of deep states in amorphous IGZO (indium-gallium-zinc oxide) semiconductor by means of deep-level transient spectroscopy (DLTS). The device under test is a Schottky diode of amorphous IGZO semiconductor on a palladium (Pd) Schottky-barrier electrode and with a molybdenum (Mo) Ohmic contact at the top. The DLTS technique allows to independently measure the energy and spatial distribution of subgap states in the IGZO thin film. The subgap trap concentration has a double exponential distribution as a function energy, with a value of ∼10 19 cm-3 eV-1 at the conduction band edge and a value of ∼1017 cm-3 eV-1 at an energy of 0.55 eV below the conduction band. Such spectral distribution, however, is not uniform through the semiconductor film. The spatial distribution of subgap states correlates well with the background doping density distribution in the semiconductor, which increases towards the Ohmic Mo contact, suggesting that these two properties share the same physical origin. cop. 2014 AIP Publishing LLC.

Integrated Line Driver for Digital Pulse-Width Modulation Driven AMOLED Displays on Flex

An integrated scan-line driver, driving half a QQVGA flexible AMOLED display using amorphous-IGZO backplane technology on foil, has been designed and measured. A pulse-width modulation technique has been implemented, enabling to drive the OLEDs with a duty cycle up to almost 100%. The digital driving method also results in a 40% static power reduction of the display. Dynamic logic and bootstrapping techniques enabled the use of clock frequencies up to 300 kHz in unipolar amorphous-IGZO technologies on foil.

Analysis of frequency dispersion in amorphous In–Ga–Zn–O thin-film transistors

It is shown in this paper that the finite resistance of the accumulation channel in amorphous In–Ga–Zn–O thin-film transistors (a-IGZO TFTs) is the main cause of the frequency dispersion of the capacitance–voltage curves in these devices. A transmission line model, accounting for the distributed nature of channel resistance, is used to explain this. Multi-frequency analysis techniques for trap density distribution use a lumped series resistance model and attribute dispersion solely to the charging and discharging of trap states. As the resistance–capacitance (RC) time constant values of the IGZO TFTs are in the range of 10–100 µs, a distributed RC network is better suited for the measured frequency range (1 kHz–1 MHz).