Short Carrier Research Articles

The legacy of Rosalind E. Franklin: Landmark contributions to two Nobel Prizes

Rosalind Franklin's short scientific carrier produced brilliant contributions to the structure of carbon, DNA, and helical and spherical viruses. At 30, she was a recognized authority who switched from carbon to DNA research and, a few years later, to nucleic-acid-protein complexes known as viruses. She made landmark contributions that led to two Nobel Prizes. She did not receive or witness either of them. 100 years after her birthday, Franklin's scientific contributions are even more important now than during her lifetime.

Nanoporous Cubic Silicon Carbide Photoanodes for Enhanced Solar Water Splitting.

Cubic silicon carbide (3C-SiC) is a promising photoelectrode material for solar water splitting due to its relatively small band gap (2.36 eV) and its ideal energy band positions that straddle the water redox potentials. However, despite various coupled oxygen-evolution-reaction (OER) cocatalysts, it commonly exhibits a much smaller photocurrent (<∼1 mA cm–2) than the expected value (8 mA cm–2) from its band gap under AM1.5G 100 mW cm–2 illumination. Here, we show that a short carrier diffusion length with respect to the large light penetration depth in 3C-SiC significantly limits the charge separation, thus resulting in a small photocurrent. To overcome this drawback, this work demonstrates a facile anodization method to fabricate nanoporous 3C-SiC photoanodes coupled with Ni:FeOOH cocatalyst that evidently improve the solar water splitting performance. The optimized nanoporous 3C-SiC shows a high photocurrent density of 2.30 mA cm–2 at 1.23 V versus reversible hydrogen electrode (VRHE) under AM1.5G 100 mW cm–2 illumination, which is 3.3 times higher than that of its planar counterpart (0.69 mA cm–2 at 1.23 VRHE). We further demonstrate that the optimized nanoporous photoanode exhibits an enhanced light-harvesting efficiency (LHE) of over 93%, a high charge-separation efficiency (Φsep) of 38%, and a high charge-injection efficiency (Φox) of 91% for water oxidation at 1.23 VRHE, which are significantly outperforming those its planar counterpart (LHE = 78%, Φsep = 28%, and Φox = 53% at 1.23 VRHE). All of these properties of nanoporous 3C-SiC enable a synergetic enhancement of solar water splitting performance. This work also brings insights into the design of other indirect band gap semiconductors for solar energy conversion.

Defect Engineering in Earth‐Abundant Cu2ZnSn(S,Se)4 Photovoltaic Materials via Ga3+‐Doping for over 12% Efficient Solar Cells

AbstractThe efficiency of earth‐abundant Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is considerably lower than the Shockley–Queisser limit. One of the main reasons for this is the presence of deleterious cation disordering caused by SnZn antisite and 2CuZn+SnZn defect clusters, resulting in a short minority carrier lifetime and significant band tailing, leading to a large open‐circuit voltage deficit, and hence, low efficiency. In this study, Ga‐doping is used to increase the CZTSSe solar cell efficiency to as high as 12.3%, one of the highest for this type of cells. First‐principles calculations show that the preference of Ga3+ occupying Zn and Sn sites has a benign effect on suppressing the formation of the SnZn deep donor defects by upwardly shifting the Fermi level, which is further confirmed by deep‐level transient spectroscopy characterization. Besides, the Ga dopants can also form defect‐dopant clusters, such as GaZn+CuZn and GaZn+GaSn, which also have positive effects on suppressing the band‐tailing states. The defect engineering via Ga3+‐doping may suppress the band‐tailing defect with a decreased Urbach energy, elevate the minority carrier lifetime, and in the end, enhance the VOC from 473 to 515 mV. These results provide a new route to further increase CZTSSe‐based solar cell efficiency by defect engineering.

High-Responsivity Solar-Blind Ultraviolet Photodetectors Based on Carbon Nanodots/Graphene Hybrids

The intrinsically poor optical absorption and short carrier lifetime of graphene have restricted its practical application in photodetectors. Carbon nanodots (CNDs) have received much attention due to their fascinating optical properties, environmentally friendly nature, small size, and superiority of a large optical absorptivity. Thus, in this article, a hybrid photodetector based on CNDs and graphene has been proposed for the solar-blind ultraviolet (UV) photodetection, in which CNDs were integrated with graphene to form a hybrid structure which was capable of detecting UV light with 254 nm. Here, this article also demonstrated that a high photoresponsivity of ~433.33 A/W and a detectivity of ~1.08 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> Jones can be obtained in the hybrid photodetector. UV illumination causes photogenerated electrons and holes in CNDs. Holes transfer to graphene and then recirculate many times, which originates from the high carrier mobility of graphene and the prolonged lifetime of excited electrons that remain trapped in CNDs. Our result demonstrates that the integration of graphene and CNDs shows better optical absorption and wavelength selectivity. Together with the mechanism, it paves a step for practical applications of quantum dots sensitized graphene-based photodetectors.

Orientation Identification of Peptide Translocation Through an Aerolysin Nanopore

Nanopores have been proven to have a wide application in biosensing at the single-molecule level. Nanopore-based protein sequencing is getting more and more attention. Recently it has been reported that 13 of the 20 natural amino acids with a short polycationic carrier can be distinguished by aerolysin nanopore via the current blockades, demonstrated that aerolysin nanopore is a suitable tool for protein sequencing. However, there are still more challenges that are needed to overcome while nanopore-based protein sequencing such as the uncertain orientation of the peptide through the pore and the non-uniform charge of the polypeptidic chain, etc. To determine the direction of the protein translocating through the aerolysin nanopore, a mutant aerolysin nanopore was used to detect the Aβ25-35 and Aβ35-25 fragments modified by specific charged tags at C-terminal or N-terminal in our experiments. The results showed that the duration time of N-terminal modified peptide Aβ25-35 and Aβ35-25 is nearly 6 times and 4 times respectively of the C-terminal modified polypeptides at +100 mV, indicating the N-terminal modified peptide have a stronger interaction with aerolysin nanopore. The difference in the interaction behavior between the N-terminal and C-terminal and aerolysin nanopore provides a new idea for designing the threading orientation of peptide for peptide sequencing.

Eco-Friendly AgBiS2 Nanocrystal/ZnO Nanowire Heterojunction Solar Cells with Enhanced Carrier Collection Efficiency.

AgBiS2 nanocrystals (NCs) are nontoxic, lead-free, and near-infrared absorbing materials. Eco-friendly solar cells were constructed using interdigitated layers of ZnO nanowires (NWs) and AgBiS2 NCs, with the aim of elongating the otherwise short carrier diffusion length of the AgBiS2 NC assembly. AgBiS2 NCs were uniformly infiltrated into the ZnO NW layers using a low-cost and easily scalable dip coating method. The resulting ZnO NW/AgBiS2 NC interdigitated structures provided efficient carrier pathways in constructed nanowire solar cells (NWSCs), composed of a transparent electrode/ZnO NW/AgBiS2 NC interdigitated layer/P3HT hole transport layer/Au. The photocurrent external quantum efficiency (EQE) in the visible to near-infrared regions was enhanced compared to those of the control solar cells made with ZnO/AgBiS2 tandem layered structures. The maximum EQE for the NWSCs reached 82% in the visible region, which is higher than the EQE values previously reported for solar cells fabricated with ZnO/AgBiS2 NCs. Air stability tests on unsealed NWSCs demonstrated that 90% or more of the initial power conversion efficiency was maintained even after 6 months.

Progress of long wavelength infrared focal plane arrays based on antimonide compounds superlattice

<p indent=0mm>The research of antimonide compounds began with 1950s. With the appearing of superlattice concept and energy band engineering in the 1970s, the potentials in infrared detection of antimonide compounds gradually emerged. Due to the reality demand, epitaxy and preparation technologies of antimonide-based materials have achieved rapid progress. The rapid advance of T2SL also benefits from the previously five decades ago of development of III-V group materials and devices research. The development of InAs/GaSb type-II superlattices (T2SLs) results from two primary motivations: the InAs/GaSb superlattice has more apparent advantages than HgCdTe in low cost, reproducibility, operability and high uniformity, especially for long wavelength detector. Secondly, the Auger recombination probability of type II superlattices is lower than that of HgCdTe, which means that the infrared detector of type II superlattice has a lower dark current or a higher working temperature than that of HgCdTe detector. And improving the working temperature of the long-wave focal plane is crucial for reducing the power consumption, size and weight of the imaging system. In addition, the atmosphere window with highest transmittance is located at the range of <sc>8−14 μm.</sc> And the emitted infrared radiation by an object at room temperature <sc>(300 K)</sc> has a wavelength of about <sc>10 μm.</sc> Therefore, long wavelength infrared detection is extremely valuable for InAs/GaSb type-II superlattices. Compared with HgCdTe device, T2SL detectors can provide equivalent or better performance under the same cutoff wavelength. But the short minority carrier lifetime of InAs/GaSb superlattice cause a high level of generation-recombination current (<italic>J</italic>_gr). In order to suppress <italic>J</italic>_gr and other dark current, the different device structures like W-structure, PπMN structure, CBIRD and unipolar barrier were put forward and employed in the detectors. As a result, the dark current and impedance was greatly improved. Furthermore, with the putting forward of InAs/InAsSb superlattice, the defects level in band gap introduced by Ga were avoided, and the minority carriers lifetime were effectively improved. The reproducibility, operability and uniformity of T2SL bring more advantages than HgCdTe detector with the sustainable developement of T2SL technologies and theories.

Dendritic CuBi2O4 Array Photocathode Coated with Conformal TiO2 Protection Layer for Efficient and Stable Photoelectrochemical Hydrogen Evolution Reaction

Short carrier diffusion length and poor photostability are two major bottlenecks limiting the application of CuBi2O4 as a competitive candidate for photoelectrochemical (PEC) hydrogen evolution reaction (HER). To overcome the bottlenecks, we develop a novel template-assisted synthesis strategy to prepare the CuBi2O4 dendrites and then protect the dendritic structure with a conformal nanometer-thick TiO2 passivation layer. The obtained CuBi2O4/TiO2 composite dendrites exhibit high activity for the PEC HER with a photocurrent density of ca. 0.90 mA cm–2 at 0.200 V vs reversible hydrogen electrode (RHE) in neutral solution, and this value is among the highest reported on CuBi2O4-based photocathodes at the same potential. Moreover, the CuBi2O4/TiO2 dendrites show a high photostability with the photocurrent density being stabilized at ca. 0.36 mA cm–2 (88.8% of its original value) after 6 h measurements at a high applied potential of 0.600 V vs RHE, which is superior to all CuBi2O4-based photocathodes reported recently in the literature. This work provides an effective strategy to improve the activity and stability of the copper-oxide-based materials for the HER via constructing a dendritic structure and protecting the structure with a nanoscale TiO2 passivation layer.

Carrier lifetime control by intentional boron doping in aluminum doped p-type 4H-SiC epilayers

The carrier lifetime control in p-type 4H-SiC epilayers with intentional aluminum (Al) and boron (B) doping is demonstrated as part of work to develop a p-type “recombination-enhancing layer” for n-channel insulated gate bipolar devices fabricated on p-type substrates. The (Al + B)-doped epilayers (Al: 5 × 1017, B: 4 × 1016 cm−3) showed a very short minority carrier lifetime of less than 20 ns at 293 K, resembling that of highly Al-doped epilayers (Al: 1 × 1019 cm−3). Besides, the minority carrier lifetimes in (Al + B)-doped epilayers are stable against post-annealing in Ar and H2 ambient, while that of Al-doped epilayers varied considerably. PiN diodes with a 10 μm-thick (Al + B)-doped buffer layer inserted on p-type substrates showed no evident degradation after a stress test under a pulse current density of 2000 A/cm2.

Ultrafast control of slow light in THz electromagnetically induced transparency metasurfaces

In this paper, we experimentally demonstrate ultrafast optical control of slow light in the terahertz (THz) range by combining the electromagnetically induced transparency (EIT) metasurfaces with the cut wire made of P+-implanted silicon with short carrier lifetime. Employing the optical-pump THz-probe spectroscopy, we observed that the device transited from a state with a slow light effect to a state without a slow light effect in an ultrafast time of 5 ps and recovered within 200 ps. A coupled oscillator model is utilized to explain the origin of controllability. The experimental results agree very well with the simulated and theoretical results. These EIT metasurfaces have the potential to be used as an ultrafast THz optical delay device.

Constructing a 2D/2D interfacial contact in ReS2/TiO2via Ti–S bond for efficient charge transfer in photocatalytic hydrogen production

The 2D/2D intimate interfacial contact and short carrier transport distance were achieved by Ti–S bond in ReS2/TiO2. Dramatic photocatalytic hydrogen activity relies on the coexistence of homojunction and heterojunction.

Flexible Diodes/Transistors Based on Tunable p-n-Type Semiconductivity in Graphene/Mn-Co-Ni-O Nanocomposites.

We report a novel Mn-Co-Ni-O (MCN) nanocomposite in which the p-type semiconductivity of Mn-Co-Ni-O can be manipulated by addition of graphene. With an increase of graphene content, the semiconductivity of the nanocomposite can be tuned from p-type through electrically neutral to n-type. The very low effective mass of electrons in graphene facilitates electron tunneling into the MCN, neutralizing holes in the MCN nanoparticles. XPS analysis shows that the multivalent manganese ions in the MCN nanoparticles are chemically reduced by the graphene electrons to lower-valent states. Unlike traditional semiconductor devices, electrons are excited from the filled graphite band into the empty band at the Dirac points from where they move freely in the graphene and tunnel into the MCN. The new composite film demonstrates inherent flexibility, high mobility, short carrier lifetime, and high carrier concentration. This work is useful not only in manufacturing flexible transistors, FETs, and thermosensitive and thermoelectric devices with unique properties but also in providing a new method for future development of 2D-based semiconductors.

Boron quantum dots all-optical modulator based on efficient photothermal effect

All-optical devices without external electronic components have drawn extraordinary attentions in all-optical communication. In this work, boron quantum dots (BQDs) were synthesized by a facile liquid-phase exfoliation method. The as-prepared BQDs showed good structural homogeneity and crystallinity, broadband optical absorption as well as excellent photothermal properties. Femtosecond-resolved transient absorption further revealed the short carrier relaxation time of BQDs. Inspired by the outstanding photothermal properties and ultrafast carrier dynamic of BQDs, we fabricated BQDs-based all-optical modulator. The phase shift with a slope efficiency of 0.032 π/mW and response time of 0.97 ms can be achieved. The modulator was used in laser resonance cavity to achieve all-optical actively Q-switched laser operation with control repetition rate. This prototypical BQDs-based all-optical modulator shows a great potential to be applied in all-optical information processing and communication.

Enhancing the efficiency of polymer solar cells by embedding Au@Ag NPs Durian shape in buffer layer

Organic semiconducting polymers are useful in several photonic applications due to their tunable optical and electronic properties and ease of fabrication. However, due to the short exciton diffusion length and low carrier mobility of polymer solar cell (PSC) materials, their power conversion efficiency (PCE) is comparably lower than their inorganic counterparts. Therefore, this study introduces plasmonic effects into these organic polymer-based solar cells by incorporating Au@Ag nanoparticles (NPs) to improve their performance. The plasmonic NPs (Au@Ag durian-shaped NPs), which can be placed in the hole transport layer (HTL) of the PSC, scatter light into the active layer thereby increasing the optical path length of the incident light, leading to higher absorption and short circuit current density of the PSCs. The shape of nanoparticles is an important factor since it directly affects surface plasmonic resonance (SPR) and the incident light’s scattering. The proposed Au@Ag NPs (with many sharp spikes) confirmed the effect of small radii of the spike ends of the durian-shaped NPs on the performance PSCs. The results reveal that the embedded Au@Ag NPs improve the electrical properties of both the active layer and HTL in PSCs. In more detail, Au@Ag NPs create plasmon-electrical effects which shorten transport path length of the low-mobility holes and lengthen that of high-mobility electrons for better-balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au@Ag NPs. Therefore Jsc and PCE of up to 17 mA/cm2 and 3.73%, respectively, implies a 44.4% improvement in short-circuit photocurrent density and 49% enhancement of the power conversion efficiency compared to the reference cell.

Scalable fabrication of long-wave infrared PtSe2-G heterostructure array photodetectors

Two-dimensional (2D) materials with excellent optoelectronic properties have attracted tremendous research interest in recent years. The promising performances of photodetectors based on 2D materials, such as ultrafast photoresponse and ultrahigh photoresponsivity, have been demonstrated in the visible to short-wavelength infrared spectrum range (0.8–2 μm). However, high performance, room temperature operation long-wavelength infrared (LWIR) photodetection is challenging. The detectors based on graphene usually exhibit low photoresponsivity due to the low optical absorption and short carrier lifetime. In addition, the relatively large bandgap of transition metal dichalcogenides limited the photoresponse bandwidth. Here, we report a way to fabricate a scalable device array of room-temperature operation LWIR PtSe2-G heterostructure detectors. The photoresponsivity at 10.6 μm up to ∼300 mA/W is obtained. The long-wave infrared light in the pico-watt range could be detected at room temperature by the PtSe2-G heterostructure detector. This result indicates that the PtSe2-G heterostructure device could be a highly competitive candidate for an uncooled LWIR detector. It also opens a way for a scalable array infrared focus plane device for the LWIR image.

Eco-Friendly AgBiS2 Nanocrystal / ZnO Nanowire Heterojunction Solar Cells with Enhanced Carrier Collection

Solution-process compatibility and size-dependent bandgap tunability of colloidal quantum dots (CQDs) make them low-cost promising materials for next generation solar cells. Among CQD-based solar cells, heterojunction devices formed with PbS CQDs and ZnO exhibit impressive solar cell performance. However, the hazardous impact of Pb is causing concern. In addition, researches on CQD-based solar cells fabricated using non-toxic and abundant elements are still very limited. Therefore, eco-friendly CQDs for solar cell have drawn much attention. AgBiS2 colloidal nanocrystals with a wide absorption band (0.4-1.2 μm) are one of the alternatives to PbS CQDs. However, the power conversion efficiencies of planar-type heterojunction solar cells using AgBiS2 nanocrystals (NCs) (ITO / ZnO dense layer / AgBiS2 NC layer/ hole transport layers (eg. PTB7:MoOx) / Ag) have been at most approximately 6% mainly due to the short carrier diffusion length of AgBiS2 [2].In this work, thus, we paid our attention to nanowire (NW) structures, with the aim of elongation of carrier diffusion length, and also applied widely-used and less-expensive hole transport materials. We then fabricated NW-type solar cells (ITO / ZnO NW: AgBiS2 mixture / AgBiS2 overlayer / P3HT / Au) by a dip-coating method.We independently tailored ZnO nanowire length and the active device layer thickness to study charge collection in AgBiS2 NW-type solar cell. ZnO NWs were synthesized by a hydrothermal method, and the length of ZnO NW was controlled by the reaction time [3]. As a reference, planar-type solar cells using dense ZnO layers with different thicknesses (NPSC) were also constructed. Jsc values of NPSCs increase as AgBiS2 layer thickness becomes thicker and reach to a maximum value of 12 mA/cm2 at 100 nm. And Jsc then decreases with thickening AgBiS2 layers due to the limited carrier diffusion length of AgBiS2 layer. Whereas in the case of Jsc of NWSCs, the highest Jsc of 20.51 mA / cm2 can be obtained at 200 nm-ZnO NWs. This Jsc value is higher than previously-reported AgBiS2 solar cells (eg. JSC = 15.10 mA/cm2, VOC = 0.46 V, FF = 57%, and PCE = 3.99% [1]), and correspoinding PCE reached 3.97%, approximately doubled the value of planar cell (PCE = 2.06%).Figure 1 shows the EQE spectra obtained with the best performing NWSC and NPSC. It can be seen that NWSC gives high EQE in almost all the spectral region studied. The ZnO NW / AgBiS2 heterojunction enabled achievement of more efficient charge carrier transportation than a planar ZnO NP / AgBiS2 cell. The cell gives a maximum EQE of 80% and shows a wide photocurrent response range of 300-1200 nm. In particular, the EQE is greatly enhanced in the near-infrared region from 35.6% (800 nm) in NPSC to 60.0% in NWSC, indicating improved carrier collection in near-infrared region for NWSC. NWSC also shows higher power conversion efficiency than NPSC because of the higher Jsc of NWSC (inset table in Fig. 1). The length of ZnO NW dependence of NWSC performance was also studied to get a better understanding of diffusion mechanism in NWSC. The structure of NWSCs was optimized by varying nanowire length and active overlayer thickness, carrier transport mechanisms related to the AgBiS2 ZnO-NW blend region and planar AgBiS2 overlayer region were clarified. The solar cell with the 320 nm-thick active layer (200 nm-NW region plus 120 nm-overlayer region) gave the best performance.These results show that ZnO nanowire / AgBiS2 nanocrystal mixtures are useful to achieve more efficient carrier collection. Long-term stability is equally important; at least 3-month air stabilty has been confirmed in NWSCs.[1] M. Bernechea, N. C. Miller, G. Xercavins, et al. Nature Photonics 2016, 10, 521.[2] H. Wang, T. Kubo, J. Nakazaki, T. Kinoshita, H. Segawa. J. Phys. Chem. Lett. 2013, 4, 2455.[3] H. Wang, V. Gonzalez-Pedro, Kubo, T.; F. Fabregat-Santiago, J. Bisquert, Y. Sanehira, J. Nakazaki, H. Segawa. The Journal of Physical Chemistry C 2015, 49, 27265. Figure 1

Photoelectrochemical water oxidation performance promoted by a cupric oxide-hematite heterojunction photoanode

Hematite is a promising material for photoelectrochemical (PEC) water oxidation due to its narrow bandgap and chemical stability in alkaline electrolytes. However, the PEC performance of hematite is known to be inhibited by a short carrier diffusion length and slow kinetics for the oxygen evolution process. The rational control of morphology, crystallinity, and interfacial charge transfer plays an important role in tuning the PEC performance of α-Fe2O3 photoelectrodes. In this study, different iron oxide nano/microstructures are synthesized by the sintering of iron sheets in air at different temperatures. The synergetic effect of the surface morphology and crystallinity on the photocurrent and onset potential of the photoanode is investigated. The coral-like structure with high index (104) facets prepared at a high temperature shows a decreased onset potential, suggesting a strong correlation between the onset potential and crystalline orientation. To further improve the photocurrent density of this hematite photoanode, cupric oxide-hematite heterostructures are explored. An obvious improvement in the photocurrent density is observed from 1.1 to 1.6 V. The resulting α-Fe2O3/CuO photoanode exhibits a photocurrent density of 1.5 mA cm−2 at 1.6 V, 25% higher than that of the pristine hematite. The Mott-Schottky plot and EIS measurement indicate that the α-Fe2O3/CuO heterojunction facilitates the extraction of the accumulated holes in the hematite through a type-II band alignment and the CuO can serve as a catalyst for promoting the oxygen evolution reaction.

Growth and Characterization of Arsenic-Doped CdTe1−xSex Single Crystals Grown by the Cd-Solvent Traveling Heater Method

The photovoltaic performance of CdTe solar cells is mainly limited by low doping and short minority carrier lifetime. Group-V element doping and Se-alloying have a significant impact on tuning these fundamental CdTe properties. In this paper, we report the growth of p-type As-doped, Cd-rich CdTe1−xSex single crystals using metallic Cd as the solvent in the traveling-heater method. The structural and electrical properties of CdTe1−xSex are examined for different Se concentrations. CdTe1−xSex single crystals (0 ≤ x ≤ 0.5) with zincblende structure indicate homogeneous composition. The 1017 cm−3 As-doping activation efficiency can be maintained at close to 50% for x ≤ 0.2. Se alloying leads to bulk minority carrier lifetime exceeding 30 ns for samples doped near 1017 cm−3. These results help us to overcome the current roadblocks in device performance.

Achievement of Low Carrier Concentration of High-Uniformity SiC Films Grown by High Speed Wafer Rotation Vertical CVD Tool

N-type 4H-SiC homo-epitaxial films were grown by high speed wafer rotation vertical CVD tool, and effect of surface C/Si ratios on in-wafer uniformity and lower limit of carrier concentration of the SiC films on 150 mm diameter wafers was investigated. From analysis of in-wafer distribution by mapping of whole wafers, it was found that high in-wafer thickness uniformity was obtained for the films grown both at lower introduced C/Si ratio of about 1.35, which corresponds to surface C/Si ratio is lower than 1.0, and at higher introduced C/Si ratio of about 1.80, which corresponds to surface C/Si ratio is higher than 1.0. However, larger fluctuation of the carrier concentration at wafer edge was observed for the film grown at lower introduced C/Si ratio, compared with the film grown at higher introduced C/Si ratio. Lower fluctuation of the carrier concentration for the film grown at higher introduced C/Si ratio is thought to be due to lower sensitivity of carrier concentration on growth temperature, which is observed for the film grown at surface C/Si ratio higher than 1.0. Also, the film grown at higher introduced C/Si ratio showed longer carrier lifetime with higher uniformity, although the film grown at lower introduced C/Si ratio indicated shorter carrier lifetime with lower uniformity. The mean carrier concentration of the films grown at higher introduced C/Si ratio was reduced by two digits compared with those grown at lower introduced C/Si ratio, and low carrier concentration of 1.7 x 1014 cm-3 with high in-wafer uniformity for intentionally doped SiC film grown at higher introduced C/Si ratio was achieved. Based on the results mentioned above, fabrication of thick and low-doped epi layer on 150 mm diameter wafer with high in-wafer uniformity was demonstrated.

Coarticulatory Aspects of the Fluent Speech of French and Italian People Who Stutter Under Altered Auditory Feedback.

A number of studies have shown that phonetic peculiarities, especially at the coarticulation level, exist in the disfluent as well as in the perceptively fluent speech of people who stutter (PWS). However, results from fluent speech are very disparate and not easily interpretable. Are the coarticulatory features observed in fluent speech of PWS a manifestation of the disorder, or rather a compensation for the disorder itself? The purpose of the present study is to investigate the coarticulatory behavior in the fluent speech of PWS in the attempt to answer the question on its symptomatic or adaptive nature. In order to achieve this, we have studied the speech of 21 adult PWS (10 French and 11 Italian) compared to that of 20 fluent adults (10 French and 10 Italian). The participants had to repeat simple CV syllables in short carrier sentences, where C = /b, d, g/ and V = /a, i, u/. Crucially, this repetition task was performed in order to compare fluent speech coarticulation of PWS to that of PWNS, and to compare the coarticulation of PWS under a condition with normal auditory feedback (NAF) and under a fluency-enhancing condition due to an altered auditory feedback (AAF). This is the first study, to our knowledge, to investigate the coarticulation behavior under AAF. The degree of coarticulation was measured by means of the Locus Equations (LE). The coarticulation degree observed in fluent PWS speech is lower than that of the PWNS, and, more importantly, in AAF condition, PWS coarticulation appears even weaker than in the NAF condition. The results allow to interpret the lower degree of coarticulation found in fluent speech of PWS under NAF condition as a compensation for the disorder, based on the fact that PWS’s coarticulation is weakening in fluency-enhancing conditions, further away from the degree of coarticulation observed in PWNS. Since a lower degree of coarticulation is associated to a greater separation between the places of articulation of the consonant and the vowel, these results are compatible with the hypothesis that larger articulatory movements could be responsible for the stabilization of the PWS speech motor system, increasing the kinesthetic feedback from the effector system. This interpretation shares with a number of relatively recent proposal the idea that stuttering derives from an impaired feedforward (open-loop) control system, which makes PWS rely more heavily on a feedback-based (closed loop) motor control strategy.