Yi-Sheng Lai

Bistable resistance switching of poly(N-vinylcarbazole) films for nonvolatile memory applications

Poly(N-vinylcarbazole) (PVK) has been fabricated by spin-coating to show the bistable resistance switching characteristics. Various resistance states can be made by controlling the on-state current through the PVK films. The resistance of the on-state PVK films also affects the turn-off current, which needs to erase the on state. The filament theory is used to elucidate the observed phenomenon. We demonstrate that the PVK films exhibit good retention and stable “read-write-read-erase” cyclic switching characteristics. The PVK films also show a good switching behavior with on-off ratio of 104, which will be a potential material for nonvolatile memory application.

Charge-transport characteristics in bistable resistive Poly(N-vinylcarbazole) films

Charge-transport characteristics of bistable resistive poly(N-vinylcarbazole) films as a function of temperature have been investigated. It is found that the on-state charge transport is dominated by ohmic conduction, hopping with an energy scale of the order of /spl sim/ 20 meV. The off-state charge transport appears to follow a transition from ohmic to space charge limited conduction with a shallow-trap distribution. Nonetheless, the poly(N-vinylcarbazole) (PVK) film does not possess its memory characteristics when operated at temperatures higher than 410 K. The PVK film can be operated at low voltage (< 2 V) with a high on/off current ratio as large as 10/sup 6/.

Dependence of crystal structure and work function of WNx films on the nitrogen content

The effect of nitrogen content on crystal structure (phase and grain size) and work function (Φm) of WNx films is investigated. The Φm of WNx films is extracted from the plot of flatband voltage versus SiO2 thickness. For W and WN0.4 films, the Φm are 4.67 and 4.39V, and their crystal phases are both body-centered-cubic W. For WN0.6 film, it contains W+W2N mixed phases and the Φm is 4.50V. On the other hand, the Φm of WN0.8 and WN1.5 films are 5.01 and 4.49V, and their crystal phases are both face-centered-cubic W2N. The grain size of W and W2N phases decreases with the increase of the nitrogen content in WNx. It is concluded that the Φm is affected by the crystal phase as well as the grain size of WNx film.

Investigation of the interlayer characteristics of Ta2O5 thin films deposited on bare, N2O, and NH3 plasma nitridated Si substrates

Low-temperature N2O and NH3 plasma nitridations on Si surfaces are conducted to prevent the thermodynamic instability at the Ta2O5/Si interface. The surface and interface of the Ta2O5/Si systems, without or with nitridation, were examined by x-ray photoelectron spectroscopy (XPS), atomic force microscopy, transmission electron microscopy, and secondary ion mass spectrometry. Capacitance–voltage (C–V) measurements were carried out to investigate the electric/defect characteristics of the Ta2O5/Si systems. For the non-nitrided Si substrate XPS detects no surface oxide formed prior to Ta2O5 deposition. In contrast, NH3 plasma nitridation forms a nitrogen-rich SiOxNy layer, while N2O plasma nitridation produces an oxygen-rich SiOxNy layer on Si. C–V measurement reveals high densities of fixed charges, trapping sites, and interface states in the Ta2O5/non-nitrided Si sample, indicative that a defective interlayer was formed during Ta2O5 deposition. Ta2O5 on both nitrided systems exhibits a reduced amount of fi...

Evolution of chemical bonding configuration in ultrathin SiOxNy layers grown by low-temperature plasma nitridation

Ultrathin silicon oxynitride (SiOxNy) less than 25 A has been grown by low-temperature (at 450 °C) N2O and NH3 plasma immersion on a Si surface. The bonding structures and their distribution in depth, as well as the quantity of nitrogen in SiOxNy layers are studied by x-ray photoelectron spectroscopy. A good coincidence is established in SiOxNy thickness extracted from the attenuated Si 2p3/2 photoelectron signal and high-resolution transmission electron micrograph. The increase of SiOxNy thickness is fast at the initial growth and slows down as the oxynitride thickens. It is found that nitrogen atoms are concentrated at the SiOxNy/Si interface. The binding energy (BE) of N 1s core levels shows that N atoms are mostly bonded to three Si atoms (the N–Si3 state). Long-time growth of oxynitride will shift N and O 1s core levels to a higher BE due to increments of oxygen in the SiOxNy layer. Exploration of (N2O+NH3) plasma nitridation suggests that the nitrogen content increases with increasing NH3/N2O ratio,...

Influence of Nitrogen Content in WN x on Its Thermal Stability and Electrical Property as a Gate Electrode

The effects of nitrogen concentration on the thermal stability and electric properties of the WN x film as the gate electrode are investigated. WN x is deposited by using reactive radio frequency sputtering, and films with composition of WN 0.6 , WN 0.8 , and WN 15 are obtained at 10, 25, and 40% of N 2 partial flow ratio, respectively. The crystal structure of the WN 0.6 film indicates that this film is a mixture of W + W 2 N, while WN 0.8 and WN 1.5 films both show the W 2 N phase. After annealing in N 2 + H 2 (N 2 /H 2 =9:1) ambient at 500°C, the surface of the WN 0.6 film reveals only the W-O bonding but no W-N bonding. In addition, oxygen diffuses from SiO 2 into WN 0.6 and leads to the formation of a mixing layer. Subsequently, flatband voltage (v FB ) of the metal oxide semiconductor capacitor shifts positively after annealing at 500°C. After annealing at 500°C, WN 0.8 and WN 1.5 films exhibit better resistance to oxidation than the WN 0.6 film, regardless of the surface of the WN x film or the interface between WN x and SiO 2 . Resistivity of all WN x films increases after annealing and also increases with increasing nitrogen content in the WN x films. However, neither the nitrogen content in the WN x nor the postmetal annealing affect the leakage current of WN x /SiO 2 /Si capacitors at both positive and negative biases.

Comparison of Dielectric Characteristics of Ta2 O 5 Thin Films on RuO2 and Ru Bottom Electrodes

Dielectric properties of Ta 2 O 5 thin films with RuO 2 and Ru as the bottom electrodes were investigated. The Ta 2 O 5 thin films were reactively sputtered on the bottom electrodes and then annealed in oxygen ambient at 700°C for 30 min. Using X-ray diffraction and Auger electron spectrometry, it has been found that the Ru bottom electrode was partially oxidized during annealing, while the RuO 2 electrode remained its structure. However, the annealed Ta 2 O 5 exhibited a higher dielectric constant, as well as a smaller leakage current, on the Ru electrode than on the RuO 2 electrode. Accordingly, the Ru bottom electrode is satisfactory for Ta 2 O 5 storage capacitors, even in a high temperature, oxidizing environment. The divergent electrical performances of two electrodes are attributed to the different crystallinity of annealed Ta 2 O 5 on Ru and RuO 2 .

Comparison of Thermal Stability and Chemical Bonding Configurations of Plasma Oxynitrided Hf and Zr Thin Films

Achieving high-performance complementary metal-oxidesemiconductor field-effect transistors drives the downscaling of silicon technology forward. High dielectric constant materials are becoming increasingly favorable due to the exponential increase in tunneling currents with decreasing SiO2 thickness in the ultrathin regime. Of these, ZrO2 and HfO2 are the most promising candidates for their high relative dielectric constant = 20-25 and good thermal stability. Applying high- materials can maintain the same gate capacitance in terms of the equivalent oxide thickness EOT of SiO2 while decreasing the tunneling current with a thicker physical thickness.

Effects of Plasma Prenitridation and Postdeposition Annealing on the Structural and Dielectric Characteristics of the Ta2 O 5 / Si System

Material and dielectric properties of Ta 2 O 5 layers grown on bare Si, as well as N 2 O and NH 3 plasma nitrided Si substrates, before and after postdeposition annealing in oxygen, were investigated. X-ray photoelectron spectroscopy reveals that NH 3 and N 2 O plasma nitridation on Si at 450°C formed a nitrogen-rich and an oxygen-rich SiO x N y layer, respectively. Capacitance-voltage (C-V) measurements show that Ta 2 O 5 layers deposited on the prenitrided Si exhibit a higher relative dielectric constant and contain a lower density of charge trapping sites (N h ), as compared to the one on bare Si. In contrast, the interface state density (D it ) near the midgap is ∼10 13 cm -2 eV -1 for non-nitrided and NH 3 nitrided samples, but it is ∼10 12 cm -2 eV -1 for the N 2 O nitrided sample. Postdeposition annealing at 650 or 800°C leads to an increment of N h , except to a lesser extent for the N 2 O nitrided sample, and reduces all D it to ∼10 12 cm -2 eV -1 . Concurrently, crystallization of the Ta 2 O 5 layer and depletion of nitrogen in the SiO x N y interlayer formed by NH 3 plasma nitridation are observed after annealing at 800°C. As for current-voltage (I-V) characteristics, all as-deposited samples exhibit large leakage currents, regardless of the prenitridation process. Postdeposition annealing will significantly lower the leakage currents, and the annealed Ta 2 O 5 deposited on N 2 O nitrided Si exhibits better I-V character than the others at high electric field. Effects of various prenitridation and postannealing processes on the structural and electrical characteristics of Ta 2 O 5 /Si systems are discussed.

Interlayer growth and electrical behavior of Ta2O5/SiOxNy/Si gate stacks

Growth and reaction of the interlayer (IL) between ultrathin Ta 2 O 5 films and bare, and N 2 O or NH 3 plasma-nitrided Si substrate, before and after rapid thermal oxidation (RTO), is examined by X-ray photoelectron spectroscopy. The IL thickness extracted from the attenuated Si 2p photoelectron signal shows that the thermal instability between Ta 2 O 5 and Si causes the IL to grow further after RTO annealing. The SiO x N y layer formed on the N 2 O plasma-nitrided Si appears to provide better barrier efficiency in retarding the growth of IL. For current-voltage measurements, an anomalous saturated current is observed for as-deposited Ta 2 O 5 films when stressed positive bias, presumably due to the film/substrate stress-induced Si bandgap widening. After RTO annealing, the leakage current through Ta 2 O 5 /IL stacks is higher under positive bias than under negative bias. Ta 2 O 5 deposited on N 2 O-nitrided Si also exhibits the best leakage behavior among the three systems with the current of 1.7 × 10 -8 A/cm 2 at E = -2.0 MV/cm and 1.9 X 10 -7 A/cm 2 at E = +2.0 MV/cm. The correlation between leakage current as well as IL growth is also discussed.