Bias Cycle Research Articles

Electrical instabilities of a-IGZO TFTs under different conditions of bias and illumination stress

Amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) have been widely applied in display technologies. An important factor of these devices is the electrical stability to avoid misinterpretations of current or voltage values under different conditions such as: bias stress, recovery time, and illumination. This work presents the effects of ΔVT under positive bias stress, negative bias stress, positive bias illumination stress, negative bias illumination stress (PBS, NBS, PBIS, NBIS), stress recovery, cyclic bias stress, hysteresis, fixed bias, and aging effects. It was found that a full recovery was achieved only under illumination because of the de-trapping effect (PBS) and ionization of oxygen vacancies (NBS). Some tests showed a low ΔVT over time, however, some considerations must be taken to carry out this purpose. Finally, fixed polarization under illumination was shown to find a stable operating region over time showing negligible variations upon aging.

Evolution of Interface State Density and Near Interface Oxide Traps under Controlled Nitric Oxide Annealing in SiO<sub>2</sub>/SiC Lateral MOSFETs

In this paper, the effect of different post oxide deposition nitridation processes in NO on n-channel lateral MOSFETs fabricated on implanted 4H-SiC were investigated. In particular, the electrical behavior of the MOSFETs was deeply investigated not only in terms of SiO2/SiC interface state density and field effect mobility, but also considering the threshold voltage stability effect. The aim of this work was to explore to which extent post oxide deposition annealing in NO is beneficial for the MOS interface behavior and when their detrimental effects start to become predominant on the device performances. Here, the separation of the trapping states at the interface – either close to the conduction and valence band edges – and the near interface oxide traps are reported for the different duration of the post oxide deposition annealing. In fact, cyclic gate bias stress was employed in order to analyze the behavior of the trapping states and to correlate them with the variation of the benefits in terms of the channel mobility (that saturates at about 80 cm2V-1s-1), and on the threshold voltage instability effect. In particular, prolonged PDAs may induce an increase of the amount of trapping states close to the valence band edge and inside the insulator of about 20% and 50 %, respectively.

Charge Trapping Mechanisms in Nitridated SiO<sub>2</sub>/ 4H-SiC MOSFET Interfaces: Threshold Voltage Instability and Interface Chemistry

Silicon dioxide (SiO2) layers deposited on 4H-SiC and subjected to different post deposition annealing (PDA) in NO and N2O were studied to identify the key factors influencing the channel mobility and threshold voltage stability in lateral implanted 4H-SiC MOSFETs. Cyclic gate bias stress measurements allowed to separate the contributions of interface states (Nit) and near interface oxide traps (NIOTs) in the two oxides. The reduction of these traps in the NO annealed sample is due to the lower amounts of sub-stoichiometric silicon oxide (~1nm) and carbon-related defects (<1nm) at the interface, as could be demonstrated by Electron Energy Loss Spectroscopy. The experimental results indicate that limiting the SiC re-oxidation during post-deposition annealing in MOSFET technology is a key factor to improve the mobility and threshold voltage stability.

Single-Dislocation Schottky Diodes.

Dislocations often exhibit unique physical properties distinct from those of the bulk material. However, functional applications of dislocations are challenging due to difficulties in the construction of high-performance devices of dislocations. Here we demonstrate unidirectional single-dislocation Schottky diode arrays in a Fe2O3 thin film on Nb-doped SrTiO3 substrates. Conductivity measurements using conductive atomic force microscopy indicate that a net current will flow through individual dislocation Schottky diodes under forward bias and disappear under reverse bias. Under cyclic bias voltages, the single-dislocation Schottky diodes exhibit a distinct resistive switching behavior containing low-resistance and high-resistance states with a high resistance ratio of ∼103. A combined study of transmission electron microscopy and first-principles calculations reveals that the Fe2O3 dislocations comprise mixed Fe2+ and Fe3+ ions due to O deficiency and exhibit a one-dimensional electrical conductivity. The single-dislocation Schottky diodes may find applications for developing ultrahigh-density electronic and memory devices.

Interfacial electrical and chemical properties of deposited SiO2 layers in lateral implanted 4H-SiC MOSFETs subjected to different nitridations

In this paper, SiO2 layers deposited on 4H-SiC and subjected to different post deposition annealing (PDA) in NO and N2O were studied to identify the key factors influencing the channel mobility and threshold voltage stability in lateral implanted 4H-SiC MOSFETs. In particular, PDA in NO gave a higher channel mobility (55 cm2V−1s−1) than PDA in N2O (20 cm2V−1s−1), and the subthreshold behavior of the devices confirmed a lower total amount of interface states for the NO case. On the other hand, cyclic gate bias stress measurements allowed to separate the contributions of interface states (Nit) both on the upper and bottom parts of the 4H-SiC band gap and near interface oxide traps (NIOTs) in the two oxides. In particular, it was found that NO annealing reduced the total density of charges trapped at the interface states down to 3 × 1011 cm−2 and those trapped inside the oxide down to 1 × 1011 cm−2. Electron energy loss spectroscopy demonstrated that the reduction of these traps in the NO annealed sample is due to the lower amounts of sub-stoichiometric silicon oxide (~1nm) and carbon-related defects (<1nm) at the interface, respectively. This correlation represents a further step in the comprehension of the physics of the SiO2/4H-SiC interface explaining the mobility and threshold voltage behavior of 4H-SiC MOSFETs. Furthermore, the experimental results indicate that an accurate control of SiC re-oxidation during post-deposition annealing in MOSFET technology is a key factor to improve the mobility and threshold voltage stability.

Identification of two trapping mechanisms responsible of the threshold voltage variation in SiO2/4H-SiC MOSFETs

A method based on cyclic gate bias stress followed by a single point drain current measurement is used to probe the interface or near-interface traps in the SiO2/4H-SiC system over the whole 4H-SiC bandgap. The temperature-dependent instability of the threshold voltage in lateral MOSFETs is investigated, and two separated trapping mechanisms were found. The experimental results corroborate the hypothesis that one mechanism is nearly temperature independent and it is correlated with the presence of near-interface oxide traps that are trapped via tunneling from the semiconductor. The second mechanism, having an activation energy of 0.1 eV, has been correlated with the presence of intrinsic defects at the SiO2/4H-SiC interface.

Investigating the impact of the defect dynamic characteristics on the PBTI in the high-κ gate device

Recent device reliability studies on the metal/high-κ device observed the inter-convertible characteristics of the electron trap levels between the shallow and deep defect states under cyclic positive-bias temperature stressing. Although the oxygen vacancy and oxygen interstitial defect, being two typical types of defects in the high-κ oxide, have been criticized as the culprit for the device reliability issue and investigated in many simulations, all results have indicated that the defect levels induced by them were either too shallow or too deep and failed to explain the above experimental observation. Nevertheless, studies on the static characteristics of vacancy-interstitial (VO-Oi) model showed scattering distributed electron trap levels within the bandgap, making it as a promising defect type that can account for the above experimental observation. In this work, we investigated the dynamic characteristics of the VO-Oi defect pair under PBTI stress by tuning the relative position of VO and Oi. Our simulation results show multiple energy barriers for the structure transformation along the Oi migration path, and the charge trap level of the specific defect pair during the Oi migration is shown to be adjustable within the HfO2 band gap, depending on the Oi positions. These results depict an atomic picture to help us understand the defect electrical behavior under cyclic positive bias stress condition.

Electrical measurements of dielectric nonlinearities in ferroelectric bilayer thin films

Frequency dependence of the dielectric response is studied for ferroelectric bilayer thin-film capacitors under a cyclic bias voltage. The approach used in this work enables probing strong dielectric nonlinearities at millisecond and microsecond time scales during polarization switching in thin films. The frequency-dependent changes that are observed in the relative dielectric permittivity of PbZr0.8Ti0.2O3/PbZr0.6Ti0.4O3 and PbZr0.6Ti0.4O3/PbZr0.2Ti0.8O3 epitaxial bilayers without applying the cyclic bias are small in the kHz-to-MHz frequency range. When the bias is applied, a strong frequency dependence of the relative dielectric permittivity is observed during polarization switching. For instance, the permittivity of the PbZr0.6Ti0.4O3/PbZr0.2Ti0.8O3 bilayer changes from εr≈300 at 100 kHz to εr≈1600 at 2 kHz. The observed frequency dependence of the permittivity can be described by [1+TT+Tcln(1ωt0)]2 that has been proposed for dynamic response in systems involving pinning at two-dimensional domain walls.

Enhancement of Electrochemiluminescence of Porous Silicon with Tri‐n‐propylamine as Co‐reactant

AbstractVisible light emission can be stimulated by applying a cyclic bias to a porous silicon/tri‐n‐propylamine (PS/TPrA) solid/liquid system. With the presence of 2.5 mM TPrA, the electrochemiluminescence intensity of the in situ formed PS electrode is 450 times higher than the blank solution. An applied voltage as low as 1.2 V is capable of generating light emission, indicating that current‐induced chemical reactions are an aid to the generation of charge carriers. A mechanism involving oxidation of TPrA followed by hole injection is proposed. The system also shows good ECL stability. In addition, the ECL emission can be inhibited by diphenols rather than phenol.

Effect of biasing at elevated temperature on the electronic structure of Pt/HfO 2/Si stacks

The spatial distribution of charges in a Pt/HfO 2/Si stack has been manipulated by applying a cyclic bias voltage ±2.5 V in combination with moderate ( T ∼ 630 K) heating. The modifications were monitored in situ by room temperature capacitance–voltage ( C–V) and current–voltage ( I–V) measurements and analyzed ex situ by hard X-ray photoelectron spectroscopy which additionally provides information on possible chemical changes at the interfaces. The experimental data on the charge/potential distributions resulting from the different steps of bias-temperature stress (BTS) are consistent with the model that additional oxygen vacancies, which are generated in HfO 2 and positively charged by charge transfer across the interface with a high work function metal (Pt), are driven across the HfO 2 layer. These vacancies ultimately control the observed growth/dissolution of SiO x at the bottom interface upon negative/positive BTS, respectively.

The effect of an external electric field on solid-state phase transition of (P(VDF/TrFE)(72/28)

The copolymers of vinylidene fluoride and trifluoroethylene (P(VDF/TrFE)) with VDF content of 50–80 mole % can be applied to the field of nonvolatile ferroelectric polymeric random access memory (FePoRAM) devices, since they exhibit stable ferroelectricβ-phase at room temperature with spontaneous polarization of the C-F dipoles towards an external electric field greater than the coercive field. Many researchers have already reported the molecular structures and dynamics of the ferroelectric (F) crystalline phase and the unique change in chain conformation between polarF phase and non-polar paraelectric (P) phase near their Curie transition temperature (T c) which is dependent on factors such as VDF content and annealing treatment conditions. The effect of external electric field strength on theF⇔P crystalline phase transition in P(VDF/TrFE)(72/28) random copolymer samples of nanometer thickness was investigated. Capacitance of 250 nm thick sample measured as a function of heating-cooling under varying external electric field strength exhibited increasingT c’s during heating (T ↑ ) and cooling (T ↓ ) under an applied electric field of more than 0.03 MV/cm. Applying cyclic bias electric field (+1 to −1 MV/cm) for samples kept isothermally at just above theirT c(T ↓ ) during cooling, we were able to observe the field-inducedP→F phase transition. With increasing cycles of the applied electric field for sample maintained just above (T ↓ ), the bistableC-E hysteresis was observed and the phase change fromP→F is irreversible even after the electric field is removed. However, for samples kept well above (T ↓ ) and nearT m (100 °C and 120°C respectively) during cooling, theF-phase initially formed through the field-induced phase transition is reversibly transformed to theP-phase when the applied electric field is removed. Drastic changes were observed in both coercive field (E c) and remanent polarization (P r) values during heating and cooling near theT c range due to theF⇔P phase transition and the results are reported in detail here.

Tracking frequency laser distance gauge

Advanced astronomical missions with greatly enhanced resolution and physics missions of unprecedented accuracy will require laser distance gauges of substantially improved performance. We describe a laser gauge, based on Pound–Drever–Hall locking, in which the optical frequency is adjusted to maintain an interferometer’s null condition. This technique has been demonstrated with pm performance. Automatic fringe hopping allows it to track arbitrary distance changes. The instrument is intrinsically free of the nm-scale cyclic bias present in traditional (heterodyne) high-precision laser gauges. The output is a radio frequency, readily measured to sufficient accuracy. The laser gauge has operated in a resonant cavity, which improves precision, can suppress the effects of misalignments, and makes possible precise automatic alignment. The measurement of absolute distance requires little or no additional hardware, and has also been demonstrated. The proof-of-concept version, based on a stabilized HeNe laser and operating on a 0.5 m path, has achieved 10 pm precision with 0.1 s integration time, and 0.1 mm absolute distance accuracy. This version has also followed substantial distance changes as fast as 16mm∕s. We show that, if the precision in optical frequency is a fixed fraction of the linewidth, both incremental and absolute distance precision are independent of the distance measured. We discuss systematic error sources, and present plans for a new version of the gauge based on semiconductor lasers and fiber-coupled components.

Hysteretic contact potential changes in nanometallic tunnel junctions in air

We have used a hybrid scanning tunneling \char21{} atomic force microscope to measure current and forces generated between nanotunnel junction (NTJ) electrodes. Cyclic bias voltage scans performed in air reveal hysteretic increases in force above a \ensuremath{\sim}1 V threshold. Current measurements exhibit simultaneous increases in magnitude and noise. Our results are consistent with hysteretic changes in contact potentials and indicate that, in addition to tunneling and electrochemistry, molecules can mediate widespread occurrences of ion trapping and sudden detrapping in NTJ's.

Room temperature scanning Hall probe microscopy of localized magnetic field fluctuations on the surfaces of magnetic recording media, permanent magnets and crystalline garnet films in external bias fields

A sub-micron room temperature scanning Hall probe microscope (RT-SHPM) was used for real-time imaging of surface magnetic domains of floppy disks, Sr ferrite magnets and Bi-substituted iron garnets placed in large external bias fields. Domain wall nucleation was observed in the garnets where bubble lattices expanded, collapsed and transformed into stripe domains in cyclic bias fields. Evolution of RT-SHPM images was compared with conventional vibrating sample magnetometer measurements.

Ultrasonic attenuation study on the interaction between dislocations and point defects in 99.999 wt% Al and Al–0.025 wt% Mg

The variation of ultrasonic attenuation Δα versus bias strain is measured during cyclic bias deformation of polycrystalline 99.999 wt% Al and Al-0.025 wt% Mg alloy at room temperature. The interaction between dislocations and point defects is analysed in the light of the dislocation string model. Δα is caused by the vibration of dislocations. The point defects and the cyclic strain amplitude influence the variation of Δα together.

Cyclic Fluvial Processes and Bias in Environmental Monitoring, Colorado River in Grand Canyon

The Colorado River in Grand Canyon is one of the most intensively studied regulated rivers in the world where discharge is controlled to optimize peak load hydropower production. Sandy channel margin deposits that occur along the Colorado River have been monitored for nearly three decades to determine the effects of flow regulation by Glen Canyon Dam. Recent results from remote daily monitoring show that it is common for large areas of fluvial sand deposits to be eroded in less than one day, followed by redeposition of the area within a few weeks or months. These remote observations were confirmed directly. A review of the maximum erosion and deposition measurements from all sources shows that the recently measured daily changes equal or exceed the magnitude of changes measured over time spans from 2 weeks to 10 years. Consequently, erosion and deposition rates are significantly greater than previously reported because erosion and deposition occur in cycles that repeat several times each year at individual sites, each with unique recurrence intervals. These findings show that monitoring environmental changes downstream of a peaking power dam such as at Glen Canyon presents temporal and spatial sampling problems that lead to data biasing. This has implications for past interpretations as well as for future investigations on this and other regulated rivers.

Study of the dislocation dynamics by ultrasonic cyclic bias stress experiments Gremaud, G. Ultrasonic methods in evaluation of inhomogeneous materials, proceedings, Erice, (Italy), 15–25 Oct. 1985, pp. 187–196. Edited by A. Alippi and W.G. Mayer, Nato Advanced Study Institute Series E Applied Sciences, vol. 126, Martinus Nijhoff, 1987

Study of the dislocation dynamics by ultrasonic cyclic bias stress experiments Gremaud, G. Ultrasonic methods in evaluation of inhomogeneous materials, proceedings, Erice, (Italy), 15–25 Oct. 1985, pp. 187–196. Edited by A. Alippi and W.G. Mayer, Nato Advanced Study Institute Series E Applied Sciences, vol. 126, Martinus Nijhoff, 1987

Study of the dislocation dynamics by ultrasonic cyclic bias stress experiments: Gremaud, G. Ultrasonic methods in evaluation of inhomogeneous materials, proceedings, Erice, (Italy), 15–25 Oct. 1985, pp. 187–196. Edited by A. Alippi and W.G. Mayer, Nato Advanced Study Institute Series E Applied Sciences, vol. 126, Martinus Nijhoff, 1987

Study of the dislocation dynamics by ultrasonic cyclic bias stress experiments: Gremaud, G. Ultrasonic methods in evaluation of inhomogeneous materials, proceedings, Erice, (Italy), 15–25 Oct. 1985, pp. 187–196. Edited by A. Alippi and W.G. Mayer, Nato Advanced Study Institute Series E Applied Sciences, vol. 126, Martinus Nijhoff, 1987

Laser measurement techniques applied to turbulent combustion in piston engines

The application of experimental laser diagnostic techniques to the study of turbulent flame propagation in homogeneous-charge internal combustion engines is demonstrated. A variety of laser refraction and scattering techniques are employed: the qualitative shape and detailed internal turbulence structure of the flame surface is investigated using laser schlieren photography; the flame location and local flame speed are measured using multibeam laser refraction; the instantaneous flame thickness is measured using Rayleigh scattering; gas temperatures are measured using Rayleigh and Raman scattering; gas velocity, turbulence intensities, and time scales of turbulence are measured using laser Doppler velocimetry. The problem of cyclic variation bias in turbulence measurements is addressed using conditional sampling procedures that involve the simultaneous implementation of multiple diagnostics. This paper focuses on the thermodynamic and fluid mechanic properties of interest to turbulence combustion studies, illustrating how laser diagnostics can be used to research the critical areas of interest.