Seongil Im

Improving the Gate Stability of ZnO Thin-Film Transistors with Aluminum Oxide Dielectric Layers

We report on the fabrication of gate-stable ZnO thin-film transistors (TFTs) with aluminum oxide dielectric. When an off-stoichiometric AlO x was deposited at room temperature, the ZnO-TFT revealed unreliable transfer characteristics: a large drain current-gate bias (I D -V G ) hysteresis and a large amount of threshold voltage (V T ) shift under gate-bias stress. As rapid thermal annealing (RTA) in O 2 ambient was applied onto AIO X at 300°C prior to ZnO channel deposition, the gate-bias reliability of the ZnO device was improved. The RTA might cause our AlO x surface to be more stoichiometric and thus to be resistant against ZnO sputter-induced damage. When the bottom-gate ZnO-TFT was fabricated with a stoichiometric Al 2 O 3 dielectric grown by atomic layer deposition (ALD), our device showed much more stable electrical characteristics than with the sputter-deposited off-stoichiometric AlO x . Last, as an ultimate effort to improve the gate reliability, we fabricated a top-gate ZnO-TFT device adopting the same thick ALD-grown stoichiometric Al 2 O 3 as in the bottom-gate device. Our top-gate device with the Al 2 O 3 dielectric then showed no hysteresis and no V T shift after several times of gate bias sweep. We conclude that both the high quality dielectric and optimized device structure are necessary to realize electrically stable ZnO-TFTs.

42.3: Transparent ZnO Thin Film Transistor for the Application of High Aperture Ratio Bottom Emission AM-OLED Display

We have fabricated 2.5″ QCIF+ bottom emission AM-OLED with aperture ratio of 59.6% using fully transparent ZnO-TFT array and highly conductive oxide/metal/oxide electrode for the first time. The bias stability of ZnO TFT was improved by optimizing ZnO deposition and first gate insulator process. Plasma free process for the gate insulator makes ZnO TFT very stable under electrical bias stress. The Vth shift was less than 0.3V after VDS=25 V and VGS=15 V application for 60 hours. Transparent ZnO TFT characteristics did not change noticeably under irradiation of visible light.

MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap

We report on the fabrication of top-gate phototransistors based on a few-layered MoS(2) nanosheet with a transparent gate electrode. Our devices with triple MoS(2) layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the number of MoS(2) layers. The photoelectric probing on working transistors with the nanosheets demonstrates that single-layer MoS(2) has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS(2) reduce to 1.65 and 1.35 eV, respectively.

Violet and UV luminescence emitted from ZnO thin films grown on sapphire by pulsed laser deposition

Abstract Visible violet photoluminescence (PL) has been achieved at room temperature (RT) from ZnO films grown on sapphire (001) substrate by pulsed laser deposition (PLD). Substrate temperatures of 200, 300, and 400°C have been used in an oxygen pressure of 1 mTorr during the PLD. As the oxygen pressure for the thin film deposition increases over 20 mTorr at a substrate temperature of 400°C, the violet luminescence vanishes. Instead ultra-violet (UV) and green–yellow luminescence appear. The most intense UV and green–yellow luminescence is obtained from a sample grown in an oxygen pressure of 200 mTorr at 400°C. It is concluded that the intensity of the UV luminescence strongly depends on the stoichiometry of the film as well as the crystalline quality, while the violet PL is due to a defect level in the grain boundaries of the ZnO x crystals.

Ultraviolet-enhanced photodiode employing n-ZnO/p-Si structure

We report on the photoelectric properties of n-ZnO/p-Si photodiodes which detect UV photons in the depleted n-ZnO and simultaneously detect visible photons in the depleted p-Si. As characterized by I–V measurements in the photon range of 310 to 650 nm our photodiodes exposed to UV photons show a linear increase in photocurrent with reverse bias. In the visible range, the photocurrent rises rapidly with bias but saturates beyond a critical voltage. Our diodes exhibit strong responsivities of 0.5 and 0.3 A/W for UV (310-nm) and red (650-nm) photons, respectively, under a 30-V bias with a weak minimum near 380 nm, the wavelength corresponding to the band gap of ZnO. It is concluded that our n-ZnO/p-Si diode can be a UV-enhanced photodiode that simultaneously detects UV and visible photons by employing two related photoelectric mechanisms in parallel.

Effects of native defects on optical and electrical properties of ZnO prepared by pulsed laser deposition

ZnO thin film has been deposited on a sapphire (001) at a temperature of 400°C using a pulsed laser deposition (PLD) with oxygen pressures of 50, 200, 300 and 500 mTorr. As the oxygen pressure for the thin film deposition increases, the crystallinity of the samples degrades as measured by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In contrast, the photoluminescence (PL) intensity of ultra-violet (UV) luminescence increases as the oxygen pressure increases up to 300 mTorr. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. According to the results from Hall measurements, the oxygen vacancy as a native donor defect in the ZnO decreases in concentration as the pressure increases. It is concluded that the UV luminescence intensity strongly depends on the stoichiometry in the ZnO film rather than the micro-structural quality of the crystal.

Optimizing n-ZnO/p-Si heterojunctions for photodiode applications

Abstract N-ZnO/p-Si heterojunction photodiodes have been fabricated by sputter deposition of n-ZnO films on p-Si substrates. The substrate temperatures of 300, 400, 480 and 550°C were taken for the n-ZnO film deposition using an Ar/O 2 ratio of 6:1. All the diodes show typical rectifying behaviors as characterized by the current–voltage ( I – V ) measurement in a dark room and their photoelectric effects from the diodes have been observed under illumination using monochromatic red light with a wavelength of 670 nm. Maximum amount of photo-current or responsivity is obtained under reverse bias conditions from a n-ZnO/p-Si heterojunction when the ZnO film was deposited at 480°C while the ZnO films deposited at 550°C show the best stoichiometric and crystalline quality. Junction leakage or dark current is much higher in the diode with n-ZnO deposited at 550°C than in the other diodes. It is thus concluded that for a photodiode the quality of the diode junction is as important as that of the n-ZnO film deposited on p-Si.

Photodetecting properties of ZnO-based thin-film transistors

We report on the photodetecting properties of a ZnO-based thin-film transistor (TFT) that has been fabricated on a SiO2/p-Si substrate by rf magnetron sputtering at room temperature. Our ZnO-based TFT exhibited a saturation current level of about 6.5 μA under a gate bias of 40 V, decent electron mobility of 0.1 cm2/V s, and on/off current ratio of ∼106 in the dark. Illuminated by ultraviolet (λ=340 nm), blue (λ=450 nm), and green (λ=540 nm) light with intensity of 0.7 mW/cm2, our TFT displays high photocurrent gain of 50, 32, and 15 μA, respectively, under a gate bias of 40 V. In the channel depletion state with gate bias of −30 V, the photodetecting sensitivity becomes much higher than in the accumulation state. It is thus concluded that our ZnO-based TFT can be a good UV photodetecting device as well as an electronic device.

Comparative studies on the stability of polymer versus SiO2 gate dielectrics for pentacene thin-film transistors

The authors report on the electrical reliabilities of poly-4-vinyl phenol (PVP) and SiO2 gate dielectrics for pentacene thin-film transistors (TFTs). SiO2 films were grown by dry oxidation and PVP films were prepared by spin coating and subsequent cross-linking at 175°C for 15min. The pentacene TFTs with the PVP cured for 15min exhibited a large hysteresis and an abnormal drain-current increase under a gate bias stress over time, while the other TFT with SiO2 displayed a small hysteresis but its drain current decreases with time. The hysteresis behaviors induced by PVP and SiO2 were opposite to each other in the gate bias swing direction, due to the difference in hysteresis mechanism between the two types of TFTs. Comparing their hysteresis behavior, the authors fabricated a far more reliable pentacene TFT with PVP by extending the PVP curing time to 1h. Our improved device with PVP exhibited no hysteresis and persistent toughness to the gate bias stress.

Hysteresis mechanisms of pentacene thin-film transistors with polymer/oxide bilayer gate dielectrics

We have studied the electrical stability of organic poly-4-vinyl phenol (PVP)/inorganic oxide bilayer gate dielectrics for low-voltage pentacene thin-film transistors (TFTs). Curing conditions of spin-cast PVP influence on the drain current-gate bias hysteresis behavior; long term curing reduces the magnitude of the hysteresis, which can also be reduced by decreasing the PVP thickness. The electron charge injection from gate electrode plays as another cause of the electrical hysteresis. These instabilities are categorized into the following three: channel/dielectric interface-induced, slow polarization-induced, and gate charge injection-induced hystereses. By examining the hysteresis behavior of pentacene TFTs with five different combinations of bilayer dielectric, we clarified the instability mechanisms responsible for the electrical hysteresis.