Drop In Mobility Research Articles

Effect of thermal annealing on properties of polycrystalline ZnO thin films

Electrical properties (density, carriers mobility, resistivity), optical absorption and photoluminescence spectra of ZnO, grown by MOCVD and hydrothermal methods, have been investigated depending on the annealing and treatment modes in a hydrogen plasma. It has been shown that the electrical and photoluminescent (PL) properties of ZnO are strongly dependent on gas atmosphere during annealing. The annealing in oxygen atmosphere causes a sharp drop of carrier mobility and films conductivity due to the absorption of oxygen on grain boundaries. The process of ZnO electrical properties recovery by the thermal annealing in inert atmosphere (nitrogen), in oil (2×10−2mbar) and oil-free (1×10−5mbar) vacuum has been investigated. The hydrogen plasma treatment influence on the intensity of near-band-gap emission (NBE) has been studied. The effect of annealing and subsequent plasma treatment on PL intensity depends on the gas atmosphere of preliminary thermal annealing.

Relative Permeability Characterization for Water-Alternating-Gas Injection in Oil Reservoirs

Summary Large quantities of oil usually remain in oil reservoirs after conventional waterfloods. A significant part of this remaining oil can still be economically recovered by water-alternating-gas (WAG) injection. WAG injection involves drainage and imbibition processes taking place sequentially; therefore, the numerical simulation of the WAG process requires reliable knowledge of three-phase relative permeability (kr) accounting for cyclic-hysteresis effects. In this study, the results of a series of unsteady-state two-phase displacements and WAG coreflood experiments were used to investigate the behavior of three-phase kr and hysteresis effects in the WAG process. The experiments were performed on two different cores with different characteristics and wettability conditions. An in-house coreflood simulator was developed to obtain three-phase relative permeability values directly from unsteady-state WAG experiments by history matching the measured recovery and differential-pressure profiles. The results show that three-phase gas relative permeability is reduced in consecutive gas-injection cycles and consequently the gas mobility and injectivity drop significantly with successive gas injections during the WAG process, under different rock conditions. The trend of hysteresis in the relative permeabilty of gas (krg) partly contradicts the existing hysteresis models available in the literature. The three-phase water relative permeability (krw) of the water-wet (WW) core does not exhibit considerable hysteresis effect during different water injections, whereas the mixed-wet (MW) core shows slight cyclic hysteresis. This may indicate a slight increase of the water injectivity in the subsequent water injections in the WAG process under MW conditions. Insignificant hysteresis is observed in the oil relative permeability (kro) during different gas-injection cycles for both WW and MW rocks. However, a considerable cyclic-hysteresis effect in kro is observed during water-injection cycles of WAG, which is attributed to the reduction of the residual oil saturation (ROS) during successive water injections. The kro of the WW core exhibits much-more cyclic-hysteresis effect than that of the MW core. No models currently exist in reservoir simulators that can capture the observed cyclic-hysteresis effect in oil relative permeability for the WAG process. Investigation of relative permeability data obtained from these displacement tests at different rock conditions revealed that there is a significant discrepancy between two-phase and three-phase relative permeability of all fluids. This highlights that not only the three-phase relative permeability of the intermediate phase (oil), but also the three-phase kr of the wetting phase (water) and nonwetting phase (gas) are functions of two independent saturations.

Interface state density of SiO2/p-type 4H-SiC (0001), (112¯), (11¯00) metal-oxide-semiconductor structures characterized by low-temperature subthreshold slopes

Interface properties of heavily Al-doped 4H-SiC (0001) (Si-face), (112¯0) (a-face), and (11¯00) (m-face) metal-oxide-semiconductor (MOS) structures were characterized from the low-temperature gate characteristics of metal-oxide-semiconductor field-effect transistors (MOSFETs). From low-temperature subthreshold slopes, interface state density (Dit) at very shallow energy levels (ET) near the conduction band edge (Ec) was evaluated. We discovered that the Dit near Ec (Ec− 0.01 eV < ET < Ec) increases in MOS structures with higher Al doping density for every crystal face (Si-, a-, and m-face). Linear correlation is observed between the channel mobility and Dit near Ec, and we concluded that the mobility drop observed in heavily doped MOSFETs is mainly caused by the increase of Dit near Ec.

Reliability aspects of hydrogen‐doped indium oxide

We demonstrate an alternative route to reliably prepare hydrogen‐doped indium oxide (In2O3:H). The common hydrogen source, water vapor, was substituted in our process by hydrogen and oxygen gas. The resulting films showed similar optical and electrical properties. Nevertheless, the process using gaseous hydrogen led to a simplification of the deposition process. By replacing the hydrogen source we increased the reproducibility of the electrical film properties significantly, thus, paving the way for a reliable device implementation of the material. Furthermore, we investigated the degradation behavior of In2O3:H under damp heat conditions as preliminary test for long‐term durability in photovoltaic devices. The results revealed a degradation of the electrical properties that differs in detail regarding the amorphous and polycrystalline material. In the amorphous material, the main degradation is caused by loss of charge carriers, whereas in the polycrystalline material a drop of the charge carrier mobility causes a significant rise of the resistivity. However, we show that the relative degradation of the In2O3:H films is similar to other transparent contacts that are implemented in solar cells. Finally, we demonstrate that the degradation of the charge carrier mobility in the polycrystalline In2O3:H films is completely reversible by vacuum annealing at 200 °C.

Performance Analysis of QoS Parameters of MANET on Mobility and Energy based Model with Different MANET Routing Protocols

Objectives: A network is group of devices that are connected to each other called as nodes. The nodes can be mobile or static. The performance of mobile Adhoc wireless networks (MANETs) helps to identify the type of applications that are supported by the network. Our objective is Performance analysis of QoS parameters of MANETs on Mobility & Energy based Model with Routing Protocols. Method/Analysis: The various network scenarios of MANETS are simulated using NS2.35. Protocols used to analyze performance are AODV, DSDV and DSR. Network layer parameters (throughput, packet delivery ratio, normalized routing overhead and average end-to-end delay) are evaluated. Network scenarios are generated through variation in pause time and number of nodes. Area of simulation is formed in 600*600 m*m area. Findings/ Results: The mobiles devices in the network get connected only when there is a demand for it. The reactive gateway discovery algorithm is used in AODV and DSR. With the random movement of nodes in the simulated area (direction) and variation in mobility, the delay and packet drop increases but PDR and throughput decreases. There is a significant differential observed while measuring the performance. Our observation with respect to DSR was it reacted well for two parameters delivery ratio and routing overhead. Average delay was less in AODV and DSDV performed well providing loop free path. Conclusion: After the simulation study and all experimental evaluations we can conclude that the DSR protocol dominates all other protocols like AODV and DSDV. The Dynamic Source Routing protocol in mobility and energy based model for throughput, packet delivery ratio performs well than AODV and DSDV. The adverse result is with the increase of node speed, routing overhead increased for DSR. Positive aspect of DSR was that average energy consumption was quite low in contrast to AODV and DSDV.

Influence of Double Channel Layers on the Performance of Nitrogen Doped Indium-zinc-oxide Thin Film Transistors

The nitrogen-doped amorphous indium-zinc-oxide thin film transistors with double channel layers (a-IZO/IZON-TFTs) were fabricated by RF magnetron sputtering of IZO target on the thermal oxidized p-type Si substrate. Influence of the double channel layers on the electrical performance and thermal stability of the devices were investigated. It is found that a-IZO/IZON-TFTs have high field effect mobility of 23.26 cm/(Vs) and more positively shifted threshold voltage than that of a-IZO-TFTs. This is ascribed to the doped nitrogen which can help reduce oxygen vacancy in the channel layer, suppress carrier concentration and make the devices have a better threshold voltage. Meanwhile, employing a-IZO thin film can avoid the sharp drop of field effect mobility and drain on current caused by nitrogen doping on a-IZON layer, leading to promoting Ion/Ioff ratio effectively. Besides, according to the transfer characteristics measured at temperatures from 298 K to 423 K, devices with a-IZO/IZON double layers have superior performance and thermal stability to TFTs of single channel layer, which can be ascribed to the protective effect of a-IZON thin film on the channel layers. The doped nitrogen can reduce the adsorption/desorption reaction of oxygen molecules on the back channel layer, leading to a significant improvement on thermal stability of the devices. 746 无 机 材 料 学 报 第 31卷

Growth and characterization of insulating ferromagnetic semiconductor (Al,Fe)Sb

We investigate the crystal structure, transport, and magnetic properties of Fe-doped ferromagnetic semiconductor (Al1−x,Fex)Sb thin films up to x = 14% grown by molecular beam epitaxy. All the samples show p-type conduction at room temperature and insulating behavior at low temperature. The (Al1−x,Fex)Sb thin films with x ≤ 10% maintain the zinc blende crystal structure of the host material AlSb. The (Al1−x,Fex)Sb thin film with x = 10% shows intrinsic ferromagnetism with a Curie temperature (TC) of 40 K. In the (Al1−x,Fex)Sb thin film with x = 14%, a sudden drop of the hole mobility and TC was observed, which may be due to the microscopic phase separation. The observation of ferromagnetism in (Al,Fe)Sb paves the way to realize a spin-filtering tunnel barrier that is compatible with well-established III-V semiconductor devices.

On the Extraction of Charge Carrier Mobility in High-Mobility Organic Transistors.

Transistor parameter extraction by the conventional transconductance method can lead to a mobility overestimate. Organic transistors undergoing major contact resistance experience a significant drop in mobility upon mild annealing. Before annealing, strong field-dependent contact resistance yields nonlinear transfer curves with locally high transconductances, resulting in a mobility overestimate. After annealing, a contact resistance below 200 Ω cm is achieved, which is stable over a wide V(G) range.

Nonmonotonous electron mobility due to structurally induced resonant coupling of subband states in an asymmetric double quantum well

We show that sharp nonmonotic variation of low temperature electron mobility μ can be achieved in GaAs/AlxGa1-xAs barrier delta-doped double quantum well structure due to quantum mechanical transfer of subband electron wave functions within the wells. We vary the potential profile of the coupled structure as a function of the doping concentration in order to bring the subbands into resonance such that the subband energy levels anticross and the eigen states of the coupled structure equally share both the wells thereby giving rise to a dip in mobility. When the wells are of equal widths, the dip in mobility occurs under symmetric doping of the side barriers. In case of unequal well widths, the resonance can be obtained by suitable asymmetric variation of the doping concentrations. The dip in mobility becomes sharp and also the wavy nature of mobility takes a rectangular shape by increasing the barrier width. We show that the dip in mobility at resonance is governed by the interface roughness scattering through step like changes in the subband mobilities. It is also gratifying to show that the drop in mobility at the onset of occupation of second subband is substantially supressed through the quantum mechanical transfer of subband wave functions between the wells. Our results can be utilized for performance enhancement of coupled quantum well devices.

A scaling rule for the flow mobility of a power-law fluid through unidirectional fibrous porous media

We propose a scaling rule for the flow mobility prediction of a power-law fluid in unidirectional fibrous porous media by separating rheological contribution from geometrical factor. Starting from the analogy to a simple circular pipe flow, we find mobility dependence on pressure drop with the help of lubrication theory for flows of a power-law fluid through arrays of square and hexagonal packing cylinders. There appears a nearly unique intersection point independent of rheological parameters in the mobility and pressure drop space, once scaled with the consistency index, and the intersection point coordinate is expressed by rheological and geometrical factors separately. Further simplification is achieved by approximating the rheological contribution by a constant that scales linearly with the consistency index. The proposed mobility estimation scheme facilitates a simple but reasonably accurate estimation and it has been verified by extensive numerical simulations for various pressure drops, rheological parameters, porosities and fibrous porous architectures.

Direct determination of the electron effective mass of GaAsN by terahertz cyclotron resonance spectroscopy

We use cyclotron resonance THz-spectroscopy in pulsed magnetic fields up to 63 T to measure the electron effective mass in Si-doped GaAsN semiconductor alloys with nitrogen content up to 0.2%. This technique directly probes the transport properties of the N-modified conduction band, particularly the electron effective mass, which has been discussed controversially in the experimental and theoretical literature. We report a slight increase of the electron effective mass and nonparabolicity with N-content for different photon energies in agreement with the two-level band anticrossing model calculations. Furthermore, we show a pronounced electron mobility drop with increasing N-content.

DC Conduction and Electric Modulus Formulation of Lithium‐Doped Bismuth Zinc Vanadate Semiconducting Glassy System

The conduction parameters related to dc conductivity and electric modulus formulation in the temperature range 313–533 K for xLi2O (100 − x) (50V2O5.20 Bi2O3. 30 ZnO) glass system have been studied. The temperature‐dependant dc conductivity is analyzed using the Mott's model for transition metals and modified Mott's VRH model. It is observed that Mott's model is in good agreement with the experimental data in high‐ and intermediate‐temperature region as a strong electron–phonon interaction seems to be prevalent. The conduction in these glasses has been attributed to small polaron hopping at the vanadium sites in nonadiabatic regime. The ion‐polaron effect (coupling between ions and polarons) leads to drop of the effective mobility and it can manifest itself as decrease in the dc conductivity with the addition of lithium content. The composition and temperature dependence of imaginary part of electric modulus () collapsed onto a single master curve, confirms the presence of same type of relaxation phenomenon in all the studied glass compositions.

Band-like transport down to 20 K in organic single-crystal transistors based on dioctylbenzothienobenzothiophene

Band-like transport has been realized down to 20 K in solution-processed single-crystal transistors based on dioctylbenzothienobenzothiophene. The mobility increases from 16 to 52 cm2/V s as the temperature is lowered from 300 to 80 K. An abrupt mobility drop is observed around 80 K, but even below 80 K, gradually increasing mobility is restored again down to 20 K instead of thermally activated transport. From the observation of a shoulder structure in the transfer curve, the mobility drop is attributed to a discrete trap state.

Magneto-transport of an electron bilayer system in an undoped Si/SiGe double-quantum-well heterostructure

We report the design, the fabrication, and the magneto-transport study of an electron bilayer system embedded in an undoped Si/SiGe double-quantum-well heterostructure. Combined Hall densities (nHall) ranging from 2.6 × 1010 cm−2 to 2.7 × 1011 cm−2 were achieved, yielding a maximal combined Hall mobility (μHall) of 7.7 × 105 cm2/(V ⋅ s) at the highest density. Simultaneous electron population of both quantum wells is clearly observed through a Hall mobility drop as the Hall density is increased to nHall > 3.3 × 1010 cm−2, consistent with Schrödinger-Poisson simulations. The integer and fractional quantum Hall effects are observed in the device, and single-layer behavior is observed when both layers have comparable densities, either due to spontaneous interlayer coherence or to the symmetric-antisymmetric gap.

Effect of grain boundary on the field-effect mobility of microrod single crystal organic transistors.

High-performance microrod single crystal organic transistors based on a p-type 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) semiconductor are fabricated and the effects of grain boundaries on the carrier transport have been investigated. The spin-coating of C8-BTBT and subsequent solvent vapor annealing process enabled the formation of organic single crystals with high aspect ratio in the range of 10 - 20. It was found that the organic field-effect transistors (OFETs) based on these single crystals yield a field-effect mobility and an on/off current ratio of 8.04 cm2/Vs and > 10(5), respectively. However, single crystal OFETs with a kink, in which two single crystals are fused together, exhibited a noticeable drop of field-effect mobility, and we claim that this phenomenon results from the carrier scattering at the grain boundary.

Conductometric determination of ammonium ion with a mobile drop

A mobile drop based approach was utilized for the conductometric determination of aqueous ammonium ion. A liquid drop was manipulated by gravity to remain or move out of a conductivity measurement zone located on a hydrophobic path. A dilute sulfuric acid drop functioned as a micro gas absorber. Gaseous NH3 was liberated from a sample containing NH4+ by adding NaOH and purged to flow over the absorber drop. As H+ in the absorber drop was converted to NH4+, the conductivity decreased. The rate of the conductivity decrease thus ultimately depended on the NH4+ concentration in the sample. While the sample could be complex, the liberation of NH3 gas essentially freed the analyte of interest from the matrix and allowed the use of an otherwise nonselective detector. The system was applied to the determination of NH4+ in some natural waters without prior filtration; the results agreed well with those from a manual indophenol blue method. The approach exhibited a limit of detection (LOD) of 5μM and a relative standard deviation (RSD) of<5%. The mobile drop gave a constant volume of the gas absorbing solution and provided a cost-effective and simple analytical platform.

Novel Crosslinked PVA Without Photoinitiator for Organic Passivation Layers of Pentacene Thin-Film Transistors

We report a new polyvinyl alcohol (PVA)-tetraethoxysilane (TEOS) hybrid layer to protect an organic thin-film transistor (OTFT). PVA/TEOS hybrid thin film was successfully fabricated through hydrolysis and condensation mechanism. The surface roughness of PVA/TEOS film was measured by atomic force miscroscophy (AFM) and it showed a good surface roughness with a root-mean-squae value of about 0.23 nm. Thermally crosslinked PVA with TEOS was successfully adaped as a solution processable passivation layer for pentacene TFT. In case of well known photo-crosslinked PVA/ammonium dichromater (ADC) passivation, extremely large initial performance drop (almost 52% mobility drop) was observed after passivation. PVA/TEOS hybrid passiavtion, however, no significant initial performance drop was found after passivation process. In addition, pentacene TFT with PVA/TEOS passivation layer exhibited very stable TFT operation with almost no field mobility drop or threshold voltage shift up to 980 h.

Plasma Post-Oxidation for High Mobility Strained-Ge pFETs with Aggressively Scaled High-κ Dielectrics

High mobility materials such as Ge and III-V are currently being investigated to improve CMOS performance with continued scaling [1-3]. For p-channel devices, the maximum transport enhancement to silicon is achieved by maximizing both strain and Ge content in SiGe; uniaxially strained-Ge (s-Ge) should provide the highest transport enhancement relative to Si [4]. In order to scale s-Ge to short-channel, a scaled gate dielectric must be developed as well. In this work, we use plasma post-oxidation (PPO) [5] in order to maintain the high mobility of biaxially s-Ge while aggressively scaling to one of the lowest capacitance equivalent thicknesses (CETs) reported [6]. Annular s-Ge p-MOSFETs (Fig. 1) were fabricated on biaxially s-Ge to evaluate the impact of plasma post-oxidation and highly scaled high-κ on mobility. An initial gate stack of 5Å Al2O3/20 Å HfO2 was deposited via ALD and ex-situ PPO was performed on select samples before TiN was deposited and patterned. Ion implantation of boron was used to dope the source and drain regions followed by RTA at 625°C for 10s to activate the dopants. The fabrication was followed by a standard metallization process and a forming gas anneal for 30min at 450°C. Plasma post-oxidation greatly improves the gate control of the channel (Fig. 2). PPO increases the Imax/Imin of the drain current to 104 compared to <102 for a device without PPO. The subthreshold swing is drastically improved and the Vt of the device after PPO is shifted in negative direction. These characteristics suggest that PPO greatly reduces the Dit through oxidation of the Ge interface. Because GeOx is formed in the process, the EOT is increased and the gate leakage is also lowered. The split C-V measurements show that the CET is ~1.1 nm for the PPO treated devices. Here, the CET is taken at VG = -0.5V ~ Vt - 0.7 V. Fig. 3 shows the split-CV hole mobility vs. inversion charge density (Ninv) for a typical device with PPO process and CET~1.1nm. The mobility of a sample with similar structure and ozone surface passivation (CET~1.4nm at VGS-Vt=-0.7V or CET~1.3nm taken from Cmax [7]) is also overlaid. It is well known that CET scaling is required for aggressively scaled devices to maintain the electrostatic integrity. The device with PPO offers ~1.27X capacitance benefit over the sample with ozone passivation, at an over-drive of 0.7V, while slight mobility degradation of ~14% is measured at the same over-drive. To compare at a given Ninv, inset of Fig. 3 shows the hole mobility vs. constant over-drive CET at Ninv=5×1012 and 1013 cm-2. The mobility drop is 20% and 7% at Ninv=5×1012 and 1013 cm-2, respectively. This data indicates that both high mobility and low CET are maintained for s-Ge. The results are competitive with those reported in [6]. These results suggest that PPO could be a suitable method for making aggressively scaled high-κ for nanoscale s-Ge devices. Acknowledgements The authors thank G. Riggott and staff of IBM T.J. Watson for helpful discussions.

The effect of synthetic driving force on the atomic mechanisms associated with grain boundary motion below the interface roughening temperature

The mechanisms associated with grain boundary motion induced by synthetic, crystal-orientation-dependent driving forces are investigated for a large-angle [001] Ni symmetric tilt grain boundary. The application of non-physical forces by this method brings legitimate concern that it could lead to non-physical results. This concern is especially relevant below the interface roughening transition temperature where there is a substantial drop in grain boundary mobility and large driving force dependence. Using slip-vector analysis and examining continuum metrics for microrotation and strain, this work shows that the application of synthetic-driving forces does not alter the fundamental mechanisms leading to grain boundary motion. Results in this work are compared directly to shear driven simulations which reveal that the path and deformation history of grain boundary motion is unbiased by the non-physical nature of the driving force acting on the boundary. Nudged elastic band calculations show that the transition path for grain boundary motion is independent of the driving force magnitude and the energy barriers for motion are not appreciably altered by the application of the synthetic driving force.

InAs量子点引入应力对调制掺杂结构中二维电子气输运特性的影响

Effect of embedding InAs quantum dots( QDs) on the transport properties of two-dimensional gas( 2-DEG) in a modulation-doped AlGaAs / GaAs heterostructure was investigated.Samples with difference thickness( Tch) from QDs layer to 2-DEG were prepared using molecular beam epitaxy.Hall measurements show that the mobility drops dramatically with the decrease of Tch,and the electron concentration shows a same trend.Strain distributions have been measured by means of geometric phase analysis of the high-resolution transmission electron microscope images.The results show that the strain is focused in the edges and above of InAs QDs which are resulted from the extended lattice distortion.And the non-uniform strain field could be another key factor for the drop of the mobility besides coulomb scattering.