Shallow Energy Levels Research Articles

Oleylamine-functionalized graphene oxide as an electron block layer towards high-performance and photostable fullerene-free polymer solar cells.

Oleylamine-functionalized graphene oxide (GO) has a shallower energy level of conduction band (ECB) and a deeper energy level of the valence band (EVB) as compared to common hole extraction layer (HEL) materials, which make the electron block layer (EBL). Photoluminescence, X-ray photoelectron spectroscopy (XPS), and current density-voltage (J-V) curves with a large reverse bias voltage range obtained under dark conditions are used to determine whether GO layers play important roles in blocking the electron transport to the MoO3/Ag composite anode and prevent MoO3 diffusion into a photoactive layer under light illumination. Moreover, GO inserted between a photoactive layer and an HEL enhances charge carrier transport and collection and avoids the monomolecular recombination between the photoactive layer and HEL. Photovoltaic parameters and photostability measurements of inverted and forward PSCs have shown that upon introduction of GO, the performance and photostability of PSCs are improved. On adding GO to PSCs, the power conversion efficiency (PCE) increases approximately 5% and 4% and reduces the decay ratio to approximately 50% and 65% of the initial value for the inverted and forward PSCs, respectively.

Effects of antimony (Sb) on electron trapping near SiO2/4H-SiC interfaces

To investigate the mechanism by which Sb at the SiO2/SiC interface improves the channel mobility of 4H-SiC MOSFETs, 1 MHz capacitance measurements and constant capacitance deep level transient spectroscopy (CCDLTS) measurements were performed on Sb-implanted 4H-SiC MOS capacitors. The measurements reveal a significant concentration of Sb donors near the SiO2/SiC interface. Two Sb donor related CCDLTS peaks corresponding to shallow energy levels in SiC were observed close to the SiO2/SiC interface. Furthermore, CCDLTS measurements show that the same type of near-interface traps found in conventional dry oxide or NO-annealed capacitors are present in the Sb implanted samples. These are O1 traps, suggested to be carbon dimers substituted for O dimers in SiO2, and O2 traps, suggested to be interstitial Si in SiO2. However, electron trapping is reduced by a factor of ∼2 in Sb-implanted samples compared with samples with no Sb, primarily at energy levels within 0.2 eV of the SiC conduction band edge. This trap passivation effect is relatively small compared with the Sb-induced counter-doping effect on the MOSFET channel surface, which results in improved channel transport.

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.

Nanogap Embedded Transistor for Investigation of Charge Properties in DNA

A new structure to investigate the charge trapping characteristics of a DNA is proposed. The structure of the proposed device is a field effect transistor (FET) with a nanogap in the dielectric layer. With the proposed device, the charge trapping characteristics of the DNA are confirmed without any adhesion problems and electrical contact instability. The extraction of both electron and hole trapping characteristics were analyzed with an n-channel and a p-channel FET, respectively. The results showed that guanine has plenty of hole trap sites with a shallow energy level, which explains why hole hopping dominantly occurs in the guanine base.

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high‐efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep‐defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect‐free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two‐dimensional device simulations suggest that these defects are one origin of the reduced open‐circuit voltage of the photovoltaic devices. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Characterization of the effects of nitrogen and hydrogen passivation on SiO2/4H-SiC interface by low temperature conductance measurements**Project supported by the National Natural Science Foundation of China (Nos. 61106080, 61275042) and the National Science and Technology Major Project of the Ministry of Science and Technology of China (No. 2013ZX02305).

We investigate the effects of NO annealing and forming gas (FG) annealing on the electrical properties of a SiO2/SiC interface by low-temperature conductance measurements. With nitrogen passivation, the density of interface states (DIT) is significantly reduced in the entire energy range, and the shift of flatband voltage, ΔVFB, is effectively suppressed to less than 0.4 V. However, very fast states are observed after NO annealing and the response frequencies are higher than 1 MHz at room temperature. After additional FG annealing, the DIT and ΔVFB are further reduced. The values of the DIT decrease to less than 1011 cm−2 eV−1 for the energy range of EC − ET > 0.4 eV. It is suggested that the fast states in shallow energy levels originated from the N atoms accumulating at the interface by NO annealing. Though FG annealing has a limited effect on these shallow traps, hydrogen can terminate the residual Si and C dangling bonds corresponding to traps at deep energy levels and improve the interface quality further. It is indicated that NO annealing in conjunction with FG annealing will be a better post-oxidation process method for high performance SiC MOSFETs.

First-principles study on the electronic structures and optical properties of Cu, Fe doped LiNbO_3 crystals

The binding energies, electronic structures and optical properties of LiNbO3 and Cu/Fe doped LiNbO3 crystals are investigated by first principles based on the density functional theory in this paper. The supersell structures of crystals are established each with 60 atoms, including five models: pure LiNbO3, LN1 (Cu2+ occupy Li+ site), LN2 (Fe3+ occupy Li+ site), LN3 (Cu2+ occupy Li+site and Fe3+ occupy Li+ site) and LN4 (Cu2+ occupy Li+ site and Fe3+ occupy Nb5+ site). The optimized results show that the total energies of all models can achieve certain stable values, which means that the models accord with the actual crystal structures. The impurity energy levels of Cu and Fe doped LiNbO3 crystals appear within the band gaps, which are contributed by Cu 3d orbital, Fe 3d orbital and O 2p orbital; in co-doped LiNbO3, Cu offers deep energy level and Fe offers shallow energy level within the band gaps. There are two wide absorption peaks appearing respectively at 445 nm and 630 nm in co-doped LiNbO3 crystal, which correspond to the electron transitions from Eg orbital of Cu to Nb 4d orbital and T2g orbital of Fe to Nb 4d orbital respectively; the absorption edge of Cu, Fe mono and co-doped LiNbO3 crystals are red-shift successively, which coincides with the variation of band gape. The light absorption intensity of co-doped LiNbO3 crystal is stronger than that of mono-doped LiNbO3 crystal. The co-doped sample light absorption property is related to Fe site occupation. In this paper, it is suggested that the co-doped sample with Fe at Nb site is more competitive than that with Fe at Li site in optical volume holographic storage applications, and that reducing properly [Fe2+]/[Fe3+] value may be conducible to the formation of this advantage.

Experimentally tailoring s-d and p-d interactions in spin polarization via post deposition annealing conditions

We investigated magneto-electrical properties and the effect of intrinsic point defects in the lattice on film properties of 10 mol% cobalt containing ZnO thin films that were doped with 0.7 ± 0.1 to 1.1 ± 0.2 mol% W atoms. Thin films were deposited on Si (100) substrates. The anomalous Hall Effect seen in magneto-electrical measurements indicated a correlation between the polarized spins and the positive magneto-resistivity due to hole and electron mediated interactions. N-type carriers were dominant in conductivity, as shown by Hall resistance measurements. The 55 % positive magneto-resistivity and the split of 132.3 ± 0.1 Ω and 28.5 ± 0.1 Ω differences in the magneto-hysteresis curves through both negative and positive regions, respectively, proved that a polarized spin current was effectively formed under both s-d and p-d interactions. In addition, the shallow energy levels, close to extreme points of conduction and valance bands, reinforced the polarized spin current.

Efficient Co-B-codoped TiO2 photocatalyst for degradation of organic water pollutant under visible light

Lattice location of B in TiO2 is tuned to determine its effect on the photocatalytic activity of Co-B codoped TiO2. Sol–gel method was used to synthesize the samples. The concentrations of Co and B were first optimized by maximizing the photocatalytic activity for the monodoped (Co or B)-TiO2. In addition to the DFT calculations for discovering new energetic levels introduced in TiO2 by codoping, various characterization techniques were used to determine the dopant lattice sites in TiO2 and interactions between them; and also determining their consequences on electronic, morphological, structural, and optical properties. At low concentration of B-doping (1at.%), B occupies the interstitial site (Bint), but as the concentration increases (2at.% and 3at.%) B also occupies substitutional O position (Bsub) in addition to Bint to form TiO2 containing Bint+sub. Both these B-doped TiO2 showed improved photocatalytic activity attributed to effective charge separation obtained for TiO2–Bint due to the formation of shallow energy level while higher visible light absorption is achieved with TiO2–Bint+sub owing to the presence of two deep energy levels in the band gap as confirmed by DFT calculations. In the case of Co doping, the band gap of TiO2 is reduced but the recombination rates are always high and are caused by the formation of Co states in the band gap. For Co monodoped TiO2, the photocatalytic activity is low for all the concentrations considered, except for very low concentration of Co (0.1at.%). Two opposite effects were observed when small amount of Co (0.1at.%) was codoped with Bint or Bint+sub. In particular, the photocatalytic degradation rate of organic aqueous pollutants (p-nitrophenol and rhodamine B dye) reduces for TiO2–Co–Bint whereas it is enhanced remarkably for TiO2–Co–Bint+sub as compared to (Co or B) monodoped (∼2.1 times) and undoped (∼7.8 times) TiO2. Higher photocatalytic activity observed in Co-doped TiO2–Bint+sub is discussed in terms of the interactions of Co with B at two different lattice positions in TiO2 and the synergistic effect created by higher visible light absorption and the improved charge separation.

Microstructure and Optical Properties of Yttrium-doped Zinc Oxide (YZO) Nanobolts Synthesized by Hydrothermal Method

In this work, yttrium-doped zinc oxide (YZO) nanopowder was synthesized via hydrothermal precipitation-method. The microstructure and optical properties of yttrium-doped zinc oxide nanopowder were characterized, which confirmed the well-crystalline wurtzite hexagonal phase of ZnO. The yttrium-doped zinc oxide nanopowder grains formed the nanobolts of ~400 nm in length and ~900 nm in width. High resolution-transmission electron microscopy (HR-TEM) of the nanobolts revealed uniform lattice fringes and no visible faults and/or distortions. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of yttrium in the zinc oxide lattice, proving the contribution of yttrium on the microstructural and optical properties of the material. A strong ultra violet (UV) emission peak of the YZO exhibited a red shift compared to pure zinc oxide, which was ascribed to the defects and the formation of a shallow energy level caused by the incorporation of yttrium.

Isotropic strains tuned hole-mediated ferromagnetism in nitrogen-doped ZnO

The effects of isotropic strains on the doping and magnetic properties of nitrogen doped ZnO system are investigated by density functional theory calculations. The formation energy and the ionization energy of NO in ZnO are calculated each as a function of strains. We show that compressive strains favor the solubility of nitrogen in ZnO, while the tensile ones render the acceptor level shallower. The shallower energy level means that the acceptor wave function becomes delocalized, which is favorable for mediating long-range coupling between defects. The effects of strains on the coupling strength between two NOs are also studied. Our calculations indicate that the ferromagnetic coupling in ZnO:N can be enhanced by applying tensile strains, but weakened by compressive strains. These manipulation trends of the magnetic couplings under stains are interpreted by the band coupling models.

Trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors**Project supported by the National Natural Science Foundation of China (Grant Nos. 61334002 and 61106106).

Frequency-dependent conductance measurements were carried out to investigate the trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors (HEMTs) quantitatively. For the non-recessed HEMT, the trap state density decreases from 2.48 × 1013 cm−2·eV−1 at an energy of 0.29 eV to 2.79 × 1012 cm−2·eV−1 at ET = 0.33 eV. In contrast, the trap state density of 2.38 × 1013–1.10 × 1014 cm−2·eV−1 is located at ET in a range of 0.30–0.33 eV for the recessed HEMT. Thus, lots of trap states with shallow energy levels are induced by the gate recess etching. The induced shallow trap states can be changed into deep trap states by 350 °C annealing process. As a result, there are two different types of trap sates, fast and slow, in the annealed HEMT. The parameters of the annealed HEMT are ET = 0.29–0.31 eV and DT = 8.16 × 1012–5.58 × 1013 cm−2·eV−1 for the fast trap states, and ET = 0.37–0.45 eV and DT = 1.84 × 1013 − 8.50 × 1013 cm−2·eV−1 for the slow trap states. The gate leakage currents are changed by the etching and following annealing process, and this change can be explained by the analysis of the trap states.

Hydrogen ion-implantation induced low resistive layer in KNbO3 bulk single crystal: Evaluation by elastic recoil detection analysis

Origins of low resistivity in H-ion implanted KNbO3 bulk single crystals are studied by elastic recoil detection analysis (ERDA) and Van der Pauw methods. The H-ion implantation (peak ion fluence: 5.0×1015cm−2) into KNbO3 is performed using a 500keV implanter. The sheet resistance decreases from ∼108Ω/□ for an un-implanted KNbO3 sample to 2.33×105Ω/□ for as-implanted, 2.29×105Ω/□ for 100°C annealed, and 4.25×105Ω/□ for 150°C annealed samples, respectively. The ERDA experiment using the 1.5MeV-4He+ beam can evaluate hydrogen from the surface to around 60nm. The hydrogen concentration near the surface estimated using the 1.5MeV helium beam is 5.1×1014cm−2 for un-implanted KNbO3 sample, 5.6×1014cm−2 for as-implanted, 3.4×1014cm−2 for 150°C annealed samples, respectively, indicating that a part of hydrogen is diffused out by annealing. The low resistive layer induced in H-ion implanted KNbO3 suggests the existence of a shallow energy level related to the complex defect consisting of hydrogen interstitial and the proton induced defect such as oxygen vacancy.

Dielectric property, electric breakdown, and discharged energy density of a poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) copolymer with low temperature processing

ABSTRACTDifferent thermal processing methods were used to fabricate the crystalline properties of poly(vinylidene fluoride‐co‐chlorotrifluoroethylene) [P(VDF‐co‐CTFE)] films. We observed that the crystallinity and crystal grain size of the various samples decreased with the quenching temperature. Compared to that of the annealed P(VDF‐co‐CTFE) sample, a higher dielectric constant of 13.9 at a frequency of 100 Hz was obtained in the film with liquid nitrogen quenching because the increasing small crystalline regions were susceptible to the excitation of external electric field. Meanwhile, the breakdown electric strength of the low‐temperature‐quenched film increased to 530 MV/m when the depth of shallow electronic energy level decreased, as depicted by Fröhlich collective electron approximated electric breakdown theory. Moreover, when we introduced the leakage current density curves, the effect of the space charges on the electric displacement was proven. As a result, the discharged energy density of the liquid‐nitrogen‐quenched P(VDF‐co‐CTFE) film was enhanced to 15.32 J/cm3 at an electric field of 530 MV/m; this provided an effective way in addition to chemical modification to achieve a high energy storage ability in this poly(vinylidene fluoride)‐based fluoropolymer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42794.

Space Charge Behavior in Paper Insulation Induced by Copper Sulfide in High-Voltage Direct Current Power Transformers

The main insulation system in high-voltage direct current (HVDC) transformer consists of oil-paper insulation. The formation of space charge in insulation paper is crucial for the dielectric strength. Unfortunately, space charge behavior changes because of the corrosive sulfur substance in oil. This paper presents the space charge behavior in insulation paper induced by copper sulfide generated by corrosive sulfur in insulation oil. Thermal aging tests of paper-wrapped copper strip called the pigtail model were conducted at 130 °C in laboratory. Scanning electron microscopy (SEM) was used to observe the surface of copper and paper. Pulse electroacoustic (PEA) and thermally stimulated current (TSC) methods were used to obtain the space charge behavior in paper. Results showed that both maximum and total amount of space charge increased for the insulation paper contaminated by semi-conductor chemical substance copper sulfide. The space charge decay rate of contaminated paper was significantly enhanced after the polarization voltage was removed. The TSC results revealed that copper sulfide increased the trap density and lowered the shallow trap energy levels. These results contributed to charge transportation by de-trapping and trapping processes. This improved charge transportation could be the main reason for the decreased breakdown voltage of paper insulation material.

Interaction of oxygen vacancy and its impact on transmission coefficient in oxygen‐deficient titanium dioxide: Ab initio calculations

Ab initio calculations on the interaction of the oxygen vacancy and its impact on the transmission coefficients in oxygen‐deficient TiO2 are reported. It is found that the separated oxygen vacancies or divacancies produce partial shallow defect energy levels below the conduction‐band edge as weak interactions between these vacancies. On the contrary, for the ordering oxygen divacancies, we observe that the defect energy levels fill in the bandgap. A much higher extent is found for the ordering oxygen divacancies in both columns of the supercell due to the appearance of many reduced Ti3+ ions. As a result, these oxygen divacancies expand the transmission range and improve the transmission coefficients. The microscopic origin of the oxygen‐vacancy interaction is helpful to clarify the mechanism of the resistive switching phenomenon.

Maxwell–Wagner relaxation and anomalies of physical properties in Cu0.15Fe1.7PS3 mixed material

In this contribution the broadband dielectric spectra of newly synthesized Cu0.15Fe1.7PS3 polycrystalline material are presented in a wide temperature range. The dielectric/electric properties of Cu0.15Fe1.7PS3 are governed by the Maxwell–Wagner relaxation, arising at the interfaces between crystallites and boundaries. No ferroelectric phase transition was detected by broadband dielectric investigations in the temperature range from 25 K to 300 K. However anomalies of the electrical conductivity, the thermal capacity and the ultrasonic attenuation indicate a structural transition close to 109 K. The activation energy has been calculated from the temperature dependence of the DC conductivity and compared with conductivity activation energy values of FePS3, CuCrP2S6 and CuInP2S6 crystals. The values of the dc conductivity activation energy and the dielectric relaxation are very low, indicating that the hopping between shallow energy levels is the main electrical conductivity mechanism.

Acridine modified dibenzothiophene derivatives as high triplet energy host materials for blue phosphorescent organic light-emitting diodes

Acridine modified dibenzothiophene derivatives were synthesized as host materials of imidazole ligand based Ir emitters for blue phosphorescent devices. The shallow energy levels of the acridine based host materials by the strong electron donating acridine moiety were well matched with the energy levels of Ir blue triplet emitter derived from imidazole. Additionally, strong electron transporting character of the acridine moiety could increase hole density in the emitting layer and balance holes and electrons. The energy level matching between the host and dopant materials and increased hole density in the emitting layer provided a very high external quantum efficiency of 27.8% in the blue phosphorescent organic light-emitting diodes.

Leakage current mechanisms and their dependence on composition in silicon carbonitride thin films

Electrical conduction in amorphous silicon carbonitride (a-SiCN:H) thin films deposited by plasma enhanced chemical vapor deposition (PECVD) is investigated for varying carbon to nitrogen ratios at room temperature. Films deposited with a lower carbon/nitrogen ratio showed two modes of electrical conduction; namely, Schottky emission mode below 2.3 MV cm−1 electric field and Poole–Frenkel mode from 2.3 MV cm−1 up to the breakdown field. Films with higher carbon/nitrogen ratios showed only Poole–Frenkel mode of conduction throughout the entire range of operation up to the breakdown field. The carbon rich films exhibited higher leakage currents attributed to its shallow defect energy levels leading to its higher Poole–Frenkel conductivity.

Facile synthesis of TiO2/trititanate heterostructure with enhanced photoelectric efficiency for an improved photocatalysis

TiO2/trititanate photocatalyst was prepared by alkaline hydrothermal treatment of TiO2, and characterized by transmission electron microscopy, X-ray diffraction, and Raman etc. The photocatalytic activities of catalysts were evaluated by the photocatalytic degradation of rhodamine B (RhB). It is found that the heterostructure can be directly formed via the conversion of surface TiO2 into trititanate. The coupled nanostructure possesses enhanced adsorption ability for RhB as compared with the raw TiO2, owing to the formation of an increased amount of hydroxyl groups on the prepared catalyst surface. Besides, the generated trititanate can successfully introduce a shallow energy level in the coupled composite, which results in the improvement of separation efficiency of photoinduced electron–hole pairs. In the degradation experiments, TiO2/trititanate exhibits much higher photocatalytic activity than the bare TiO2. These advantages of the coupled nanostructure in adsorption capacity and photoelectric efficiency may make it a wider application for the removal of organic pollutants.