Dalong Zhao

Fermi level unpinning of GaSb (100) using plasma enhanced atomic layer deposition of Al2O3

N-type and p-type GaSb metal-oxide-semiconductor capacitors (MOSCAPs) with atomic-layer-deposited (ALD) and plasma-enhanced-ALD (PEALD) Al2O3 dielectrics are studied to identify the optimum surface preparation and oxide deposition conditions for a high quality oxide-semiconductor interface. The ALD Al2O3/GaSb MOSCAPs exhibit strongly pinned C-V characteristics with high interface state density (Dit) whereas the PEALD Al2O3/GaSb MOSCAPs show unpinned C-V characteristics (low Dit). The reduction in Sb2O3 to metallic Sb is suppressed for the PEALD samples due to lower process temperature, identified by x-ray photoelectron spectroscopy analysis. Absence of elemental Sb is attributed to unpinning of Fermi level at the PEALD Al2O3/GaSb interface.N-type and p-type GaSb metal-oxide-semiconductor capacitors (MOSCAPs) with atomic-layer-deposited (ALD) and plasma-enhanced-ALD (PEALD) Al2O3 dielectrics are studied to identify the optimum surface preparation and oxide deposition conditions for a high quality oxide-semiconductor interface. The ALD Al2O3/GaSb MOSCAPs exhibit strongly pinned C-V characteristics with high interface state density (Dit) whereas the PEALD Al2O3/GaSb MOSCAPs show unpinned C-V characteristics (low Dit). The reduction in Sb2O3 to metallic Sb is suppressed for the PEALD samples due to lower process temperature, identified by x-ray photoelectron spectroscopy analysis. Absence of elemental Sb is attributed to unpinning of Fermi level at the PEALD Al2O3/GaSb interface.

Fast Flexible Plastic Substrate ZnO Circuits

We report thin-film transistors (TFTs) and circuits fabricated on flexible plastic substrates using ZnO thin films deposited by plasma-enhanced atomic layer deposition at 200°C. Crossover test structures fabricated on flexible substrates have a good yield, and ZnO TFTs have a field-effect mobility of 20 cm2/V · s. The 15-stage ring oscillators are operated at > 2 MHz with a supply voltage of VDD = 18 V, corresponding to a propagation delay of < 20 ns/stage. To the best of our knowledge, these are the fastest oxide-semiconductor circuits on flexible substrates reported to date.

ZnO Thin-Film Transistor Ring Oscillators with 31-ns Propagation Delay

We have fabricated ring oscillators (ROs) using ZnO thin films deposited by using a spatial atomic layer deposition process at atmospheric pressure and low temperature (200degC). Bottom-gate thin-film transistors with aluminum source and drain contacts were fabricated with a field-effect mobility of > 15 cm2/V ldr s. Seven-stage ROs operated at a frequency as high as 2.3 MHz for a supply voltage of 25 V, corresponding to a propagation delay of 31 ns/stage. These circuits also had propagation delays of ~100 ns/stage at a supply voltage of 15 V. To the best of our knowledge, these are the fastest ZnO circuits reported to date.

Designing piezoelectric films for micro electromechanical systems

Piezoelectric thin films are of increasing interest in low-voltage micro electromechanical systems for sensing, actuation, and energy harvesting. They also serve as model systems to study fundamental behavior in piezoelectrics. Next-generation technologies such as ultrasound pill cameras, flexible ultrasound arrays, and energy harvesting systems for unattended wireless sensors will all benefit from improvements in the piezoelectric properties of the films. This paper describes tailoring the composition, microstructure, orientation of thin films, and substrate choice to optimize the response. It is shown that increases in the grain size of lead-based perovskite films from 75 to 300 nm results in 40 and 20% increases in the permittivity and piezoelectric coefficients, respectively. This is accompanied by an increase in the nonlinearity in the response. Band excitation piezoresponse force microscopy was used to interrogate the nonlinearity locally. It was found that chemical solution-derived PbZr0.52Ti0.48O3 thin films show clusters of larger nonlinear response embedded in a more weakly nonlinear matrix. The scale of the clusters significantly exceeds that of the grain size, suggesting that collective motion of many domain walls contributes to the observed Rayleigh behavior in these films. Finally, it is shown that it is possible to increase the energy-harvesting figure of merit through appropriate materials choice, strong imprint, and composite connectivity patterns.

Fermi level unpinning of GaSb(100) using Plasma Enhanced ALD Al 2 O 3 dielectric

This paper discusses arsenic-antimonide based MOS-HEMTs which have great potential to enable complementary logic operation at low supply voltage. The effects of various surface passivation approaches on the capacitance-voltage characteristics (C-V) and the surface chemistry of n-type and p-type GaSb(100) MOS capacitors made with ALD and Plasma Enhanced ALD (PEALD) Al2O3 dielectrics studied in this paper. This paper also proposed for the first time unpinned Fermi level in GaSb MOS system with high-κ PEALD Al2O3 dielectric using admittance spectroscopy and XPS analysis.

F-TES ADT Organic Integrated Circuits on Glass and Plastic Substrates

Using the small-molecule organic semiconductor fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (F-TES ADT), we have fabricated spin cast organic thin film transistors (OTFTs) and integrated circuits on glass and plastic substrates. Our F-TES ADT spin cast films are deposited at room temperature from toluene or chlorobenzene solutions with no annealing steps. Film growth is considerably more ordered on pentafluorobenzenethiol (PFBT) treated Au electrodes surfaces than on oxide and on samples with patterned PFBT-Au structures grains appear to grow out from the PFBT-Au areas into the oxide areas. OTFTs fabricated on silicon wafers have mobility of 0.1 - 0.4 cm2/V·s and 0.2 - 1.0 cm2 / V·s spin cast from toluene and cholobenzene solutions respectively. Results for operational and environmental stability of spin cast F-TES ADT OTFTs and circuits on both glass and plastic substrates are also presented.

Gamma-ray irradiation of ZnO thin film transistors and circuits

The radiation tolerance of electronic devices and circuits is of interest for space and some other harsh environment applications. Properly designed deep submicron gate length Si MOSFETs can have small threshold voltage shift and leakage increase for doses of 100 kGy (10 Mrad) or even larger [1], however polysilicon thin film transistors (TFTs) show significant changes at much lower dose (< 1 kGy) [2] and a-Si:H TFTs have volt-range threshold voltage shift for 10 kGy dose [3]. We report here the effects of gamma-ray irradiation on plasma enhanced atomic layer deposition (PEALD) ZnO TFTs and circuits. Devices and circuits function even after 1 MGy 60Co gamma ray exposure and radiation induced device changes are removed by a modest temperature (200 °C) anneal.

ZnO Thin Film Transistor Ring Oscillators with sub 75 nsec Propagation Delay

We have fabricated simple circuits, including ring oscillators, using ZnO thin films deposited by low temperature (200degC) atmospheric pressure chemical vapor deposition. Bottom gate TFTs with aluminum source and drain contacts typically had field effect mobility >15 cm2/Vs. Seven stage ring oscillators operated at frequency as high as 1 MHz for a supply voltage of 32 V, corresponding to a propagation delay less than 75 nsec/stage. These circuits also had propagation delay less than 150 ns/stage at a supply voltage of 18 V. To our knowledge, these are the fastest ZnO circuits reported to date.

Fast ZnO Thin-Film Transistor Circuits

We report here five-stage ring-oscillator-integrated circuits fabricated using ZnO TFTs with signal propagation delays as low as 100 ns (>1 MHz oscillation frequency) for a 45 V supply voltage. These circuits also operate at a supply voltage as low as 10 V. To our knowledge, these are the fastest ZnO integrated circuits reported to date. We also designed ring oscillators with different source/gate and drain/gate overlap to investigate aspects of the circuit speed.

High-Speed, Low-Temperature Integrated ZnO/Organic CMOS Circuits

The demand for low-cost, high mobility, thin-film technologies has generated particular interest in low-temperature- processed organic and metal oxide semiconductors. In addition, the development of CMOS circuits is likely to be important for low-power and battery-driven technologies. We have focused on ZnO, one of the most promising n- type semiconductors for thin-film application, but to date the formation of stable, high-mobility p-type at low temperatures has been challenging. We have previously demonstrated that by using a low-temperature (200degC) atmospheric pressure spatial ALD process, we can deposit uniform A1203 and ZnO films that result in high-mobility transistors (>15 cm2/V-s) and fast ring oscillators (<50 ns/stage). We have also demonstrated simple circuits using the high-mobility, solution-deposited, p-type organic semiconductor difluoro 5,ll-bis(triethylsilylethynyl) anthradithiophene (diF TES-ADT). We now report an integrated approach that combines these two technologies in a simple way to form low-temperature CMOS circuits.