Electronic structure and defect states of transition films from amorphous to microcrystalline silicon studied by surface photovoltage spectroscopy

In this paper, surface photovoltage spectroscopy （SPS） is used to determine the electronic structure of the hydrogenated transition Si films. All samples are prepared by using helicon wave plasma-enhanced chemical vapour deposition technique, the films exhibit a transition from the amorphous phase to the microcrystalline phase with increasing temperature. The film deposited at lower substrate temperature has the amorphous-like electronic structure with two types of dominant defect states corresponding to the occupied Si dangling bond states （D^0/D^-） and the empty Si dangling states （D＋）. At higher substrate temperature, the crystallinity of the deposited films increases, while their band gap energy decreases. Meanwhile, two types of additional defect states is incorporate into the films as compared with the amorphous counterpart, which is attributed to the interface defect states between the microcrystalline Si grains and the amorphous matrix. The relative SPS intensity of these two kinds of defect states in samples deposited above 300℃ increases first and decreases afterwards, which may be interpreted as a result of the competition between hydrogen release and crystalline grain size increment with increasing substrate temperature.

Low temperature photoluminescence study of Ga As defect states

Low temperature(77 K)photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality.Several defect-related luminescence peaks have been observed,including 1.452 eV,1.476 eV,1.326 eV peaks deriving from 78 meV GaAs antisite defects,and 1.372 eV,1.289 eV peaks resulting from As vacancy related defects.Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states.The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.

WAVELET METHOD FOR CALCULATING THE DEFECT STATES OF TWO-DIMENSIONAL PHONONIC CRYSTALS

Based on the variational theory, a wavelet-based numerical method is developed to calculate the defect states of acoustic waves in two-dimensional phononic crystals with point and line defects. The supercell technique is applied. By expanding the displacement field and the material constants （mass density and elastic stiffness） in periodic wavelets, the explicit formulations of an eigenvalue problem for the plane harmonic bulk waves in such a phononic structure are derived. The point and line defect states in solid-liquid and solid-solid systems are calculated. Comparisons of the present results with those measured experimentally or those from the plane wave expansion method show that the present method can yield accurate results with faster convergence and less computing time.

Influence of defect states on band gaps in the two-dimensional phononic crystal of 4340 steel in an epoxy

The band structures of a new two-dimensional triangle-shaped array geometry of 4340 steel cylinders of square cross section in an epoxy resin were studied by the plane-wave expansion and supercell calculation method. The band gaps of this type of phononic crystals with different defects were calculated such as defect-free, 60° crystal linear defect states, 120° crystal linear defect states, and 180° crystal linear defect states. It was found that the band gap will emerge in different linear defects of the phononic crystals and the bandwidth of linear defect states is larger than that of the free-defect crystal by about 2.14 times within the filling fraction F = 0.1-0.85. In addition, the influence of the filling fraction on the relative width of the minimum band gap is discussed.

Decade Milestone Advancement of Defect-Engineered g-C_(3)N_(4) for Solar Catalytic Applications

Over the past decade, graphitic carbon nitride(g-C_(3)N_(4)) has emerged as a universal photocatalyst toward various sustainable carbo-neutral technologies. Despite solar applications discrepancy, g-C_(3)N_(4) is still confronted with a general fatal issue of insufficient supply of thermodynamically active photocarriers due to its inferior solar harvesting ability and sluggish charge transfer dynamics. Fortunately, this could be significantly alleviated by the “all-in-one” defect engineering strategy, which enables a simultaneous amelioration of both textural uniqueness and intrinsic electronic band structures. To this end, we have summarized an unprecedently comprehensive discussion on defect controls including the vacancy/non-metallic dopant creation with optimized electronic band structure and electronic density, metallic doping with ultraactive coordinated environment(M–N_(x), M–C_(2)N_(2), M–O bonding), functional group grafting with optimized band structure, and promoted crystallinity with extended conjugation π system with weakened interlayered van der Waals interaction. Among them, the defect states induced by various defect types such as N vacancy, P/S/halogen dopants, and cyano group in boosting solar harvesting and accelerating photocarrier transfer have also been emphasized. More importantly, the shallow defect traps identified by femtosecond transient absorption spectra(fs-TAS) have also been highlighted. It is believed that this review would pave the way for future readers with a unique insight into a more precise defective g-C_(3)N_(4) “customization”, motivating more profound thinking and flourishing research outputs on g-C_(3)N_(4)-based photocatalysis.

Effects of defect states on the performance of perovskite solar cells

We built an ideal perovskite solar cell model and investigated the effects of defect states on the so- lar cell＇s performance. The verities of defect states with a different energy level in the band gap and those in the absorption layer CH3NH3PbI3 （MAPbI3）, the interface between the buffer layer/MAPbI3, and the interface be- tween the hole transport material （HTM） and MAPbI3, were studied. We have quantitatively analyzed these effects on perovskite solar cells＇ performance parameters. They are open-circuit voltage, short-circuit current, fill factor, and photoelectric conversion efficiency. We found that the performances of perovskite solar cells change worse with defect state density increasing, but when defect state density is lower than 1016 cm^-3, the effects are small. Defect states in the absorption layer have much larger effects than those in the adjacent interface layers. The per-ovskite solar cells have better performance as its working temperature is reduced. When the thickness of MAPbI3 is about 0.3μm, perovskite solar cells show better comprehensive performance, while the thickness 0.05μm for Spiro-OMeTAD is enough.

Effects of defect states on the performance of CuInGaSe_2 solar cells

Device modeling has been carried out to investigate the effects of defect states on the performance of ideal CulnGaSe2 （CIGS） thin film solar cells theoretically. The varieties of defect states （location in the band gap and densities） in absorption layer CIGS and in buffer layer CdS were examined. The performance parameters： open-circuit voltage, short-circuit current, fill factor, and photoelectric conversion efficiency for different defect states were quantitatively analyzed. We found that defect states always harm the performance of CIGS solar cells, but when defect state density is less than 10 14 cm-3 in CIGS or less than 10 18 cm-3 in CdS, defect states have little effect on the performances. When defect states are located in the middle of the band gap, they are more harmful. The effects of temperature and thickness are also considered. We found that CIGS solar cells have optimal performance at about 170 K and 2 μm of CIGS is enough for solar light absorption.

Identification and elimination of inductively coupled plasma-induced defects in Al_xGa_(1-x)N/GaN heterostructures

By using temperature-dependent Hall, variable-frequency capacitance-voltage and cathodoluminescence （CL） measurements, the identification of inductively coupled plasma （ICP）-induced defect states around the AlxGa1-xN/GaN heterointerface and their elimination by subsequent annealing in AlxGa1-xN/GaN heterostructures are systematically investigated. The energy levels of interface states with activation energies in a range from 0.211 to 0.253 eV below the conduction band of GaN are observed. The interface state density after the ICP-etching process is as high as 2.75× 10^12 cm^-2.eV^-1. The ICP-induced interface states could be reduced by two orders of magnitude by subsequent annealing in N2 ambient. The CL studies indicate that the ICP-induced defects should be Ga-vacancy related.

Large-scale photonic crystals with inserted defects and their optical properties

Deliberately introducing defects into photonic crystals is an important way to functionalize the photonic crystals. We prepare a special large-scale three-dimensional （3D） photonic crystal （PC） with designed defects by an easy and low-cost method. The defect layer consists of photoresist strips or air-core strips. Field emission scanning electron microscopy （FESEM） shows that the 3D PC is of good quality and the defect layer is uniform. Different defect states shown in the ultraviolet-visible spectra are induced by the photoresist strip layer and air-core strip layer. The special large-scale 3D PC can be tested for integrated optical circuits, and the defects can act as optical waveguides.

Tracing the formation of oxygen vacancies at the conductive LaAlO3/SrTiO3 interface via photoemission

The two-dimensional electron gas(2DEG)generated at the LaAlO3/SrTiO3 interface has been in the focus of oxides re-search since its first discovery.Although oxygen vacancies play an important role in the generation of the insulator-to-metal transition of the SrTiO3 bare surface,their contribution at the LaAlO3/SrTiO3 interface remains unclear.In this work,we investigated a LaAlO3/SrTiO3 heterostructure with regional distribution of defect-based localized polar sites at the interface.Using static and time-resolved threshold photoemission electron microscopy,we prove that oxygen vacan-cies are induced near those polar sites,resulting in the increase of carrier density of the 2DEG states.In addition,oxy-gen-related surface states were uncovered,which we attributed to the release of lattice oxygen during the formation of oxygen vacancies.Such effects are mainly found spatially located around the defect sites at the buried interface,while other regions remain unaffected.Our results confirm that the itinerant electrons induced by oxygen vacancies can coex-ist with the charge transfer mechanism in the LaAlO3/SrTiO3 heterostructure,together leading to the formation of the metallic interface.These observations provide fundamental insights into the nature of LaAlO3/SrTiO3 interface based 2DEG and unique perspectives for potential applications.

Hydrothermal synthesis and characterization of carbon-doped TiO2 nanoparticles

We report the hydrothermal growth of pure and doped TiO2 nanoparticles with different concentrations of carbon.The microstructure of the as-synthesized samples is characterized by x-ray diffraction(XRD),field emission scanning electron microscopy(FESEM),energy dispersive x-ray spectroscopy(EDX),and Raman spectroscopy to understand the structure and composition.The XRD patterns confirm the formation of anatase phase of TiO2 with the average crystallite size is calculated to be in the range of 13 nm to 14.7 nm.The functional groups of these nanostructures are characterized by Fourier transformed infrared(FT-IR)spectroscopy,which further confirms the single anatase phase of the synthesized nanostructures.UV-visible absorption spectroscopy is used to understand the absorption behavior,which shows modification in the optical bandgap from 3.13 eV(pure TiO2)to 3.74 eV(1.2 mol%C-doped TiO2).Furthermore,the Ti^3+centers associated with oxygen vacancies are identified using electron paramagnetic resonance spectroscopy(EPR).

Novel ultra-wideband fluorescence material:Defect state control based on nickel-doped semiconductor QDs embedded in inorganic glasses

In recent years,the development of an environmentally friendly quantum dots(QDs)embedded luminous solid by a simple method has attracted considerable attention.In this study,semiconductor ZnS QDs were successfully prepared in an inorganic matrix of amorphous glass,which yielded beneficial broadband emission in the long-wavelength region of the visible range.The strong red emission belonged to the defect state energy level of the ZnS QDs,which could be enhanced by incorporation of nickel ions into the fixed matrix to regulate the defects state.The novel material had a small self-absorption,wide excitation and emission ranges,and thus potential applications in light-conversion devices,luminescent solar concentrators,and solar cell cover glasses.

PLE spectra analysis of the sub-structure in the absorption spectra of CdSeS quantum dots

The semiconductor CdSeS quantum dots （QDs） embedded in glass are analysed by means of absorption spectra, photoluminescence （PL） spectra and photoluminescence excitation （PLE） spectra. The peaks of absorption spectra shift to lower energies with the size of QD increasing, which obviously shows a quantum-size effect. Using the PLE spectra, the physical origin of the lowest absorption peak is analysed. In PLE spectra, the lowest absorption peak can be deconvoluted into two peaks that stem from the transitions of 1S3/2-1Se and 2S3/2-1Se respectively. The measured energy difference between the two peaks is found to decrease with the size of QD increasing, which agrees well with the theoretical calculation for the two transitions. The luminescence peak of defect states is also analysed by PLE spectra. Two transitions are present in the PLE, which indicates that the transitions of 1S3/2 1Se and 2S3/2 1Se are responsible for the defect states luminescence.

First principle study of nitrogen vacancy in aluminium nitride

In the framework of density functional theory, using the plane-wave pseudopotential method, the nitrogen vacancy （VN） in both wurtzite and zinc-blende AlN is studied by the supercell approach. The atom configuration, density of states, and formation energies of various charge states are calculated. Two defect states are introduced by the defect, which are a doubly occupied single state above the valance band maximum （VBM） and a singly occupied triple state below the conduction band minimum （CBM） for wurtzite AlN and above the CBM for zinc-blende AlN. So VN acts as a deep donor in wurtzite AlN and a shallow donor in zinc-blende AlN. A thermodynamic transition level E（3＋/＋） with very low formation energy appears at 0.7 and 0,6eV above the VBM in wurtzite and zinc-blende structure respectively, which may have a wide shift to the low energy side if atoms surrounding the defect are not fully relaxed. Several other transition levels appear in the upper part of the bandgap. The number of these levels decreases with the structure relaxation. However, these levels are unimportant to AlN properties because of their high formation energy.

Theoretical Study on the Structural and Electronic Properties of the Reduced SnO_2 (110) Surface

The reduced SnO2（110） surface has been investigated by using first-principles method with a slab model. By examining the vacancy formation energy of three kinds of reduced SnO2（110） surfaces, the most energetically favorable defect surface is confirmed to be the surface with the coexistence of bridging and in-plane oxygen vacancies, which is different with the traditional model by only removing bridging oxygen. The results of band structure calculations indicate that the electronic structure of this defect surface is similar to the SnO surface.

Extraction of the defect density of states in microcrystalline silicon from experimental results and simulation studies

The constant photocurrent method in the ac-mode （ac-CPM） is used to determine the defect density of states （DOS） in hydrogenated microcrystalline silicon （μc-Si：H） prepared by very high frequency plasma-enhanced chemical vapor deposition （VHF-PECVD）. The absorption coefficient spectrum （ac-α （h v））, is measured under ac- CPM conditions at 60 Hz. The measured ac-α（hv） is converted by the CPM spectroscopy into a DOS distribution covering a portion in the lower energy range of occupied states. We have found that the density of valence band- tail states falls exponentially towards the gap with a typical band-tail width of 63 meV. Independently, computer simulations of the ac-CPM are developed using a DOS model that is consistent with the measured ac-α（hv） in the present work and a previously measured transient photocurrent （TPC） for the same material. The DOS distribution model suggested by the measurements in the lower and in the upper part of the energy-gap, as well as by the numerical modelling in the middle part of the energy-gap, coincide reasonably well with the real DOS distribution in hydrogenated microcrystalline silicon because the computed ac-α（h v） is found to agree satisfactorily with the measured ac-α （h v）.

Improved luminescence and photocatalytic properties of Sm^(3+)-doped ZnO nanoparticles via modified sol-gel route:A unified experimental and DFT+U approach

In the current study,a modified sol-gel route was used to produce undoped and Sm^(3+)doped(1 mol%,3 mol%and 5 mol%)ZnO nanoparticles(NPs).The study of opto-structural properties of Sm^(3+)doped NPs was carried out both experimentally and theoretically.Complete dissolution of Sm^(3+)ions into the ZnO lattice is obviously seen from X-ray diffraction(XRD)analysis.Morphological evolution with doping was studied using field emission scanning electron microscopy(FESEM)and transmission electron microscopy(TEM).X-ray photoelectron spectroscopy(XPS)was carried out to confirm the prese nce of Sm~(3+)on the doped NPs.Increasing dopant quantity results in a redshift of the NPs along with a reduction in bandgap with increasing abso rption in the visible range,and a minimum of 3.18 eV of optical bandgap for Zn_(0.97)Sm_(0.03)O is found.Photoluminescence spectroscopy reveals a drop in the recombination rate of electron-hole with increasing doping content till 3 mol%,followed by an increase of Zn_(0.95)Sm_(0.05)O.Photogenerated electron-hole pair recombination is revealed by the orange band in the luminescence spectra.Theoretical analysis was also carried out with density functional theory(DFT).This work also unfolds the fundamental understanding of the structural properties of the synthesized NPs to enhance photocatalytic activity successfully.Later,photocatalytic activity for the optimum composition,i.e.,3 mol%,was assessed experimentally.

Investigation of an a-Si/c-Si interface on a c-Si(P) substrate by simulation

We investigate the recombination mechanism in an a-Si/c-Si interface,and analyze the key factors that influence the interface passivation quality,such as Q_s,δ_p/δ_n and D_（it）.The polarity of the dielectric film is very important to the illustration level dependent passivation quality;when nδ_n = pδ_p and the defect level E_t equal to E_i（c-Si）,the defect states are the most effective recombination center,AFORS-HET simulation and analysis indicate that emitter doping and a-Si/c-Si band offset modulation are effective in depleting or accumulating one charged carrier.Interface states（D_（it）） severely deteriorate V_（oc） compared with J_（sc） for a-Si/c-Si HJ cell performance when D_（it） is over 1×10^（10） cm^（-2）·eV^（-1）.For a c-Si（P）/a-Si（P~＋） structure,φ_（BSF） in c-Si andφ_0 in a-Si have different performances in optimization contact resistance and c-Si（P）/a-Si（P~＋） interface recombination.

Correlated barrier hopping of CuO nanoparticles

The ac conduction mechanism in copper oxide nanoparticles with 8 nm size, synthesized by a precipitation method was studied by analyzing ac conductivity in the frequency range of 50 Hz-1 MHz and in the temperature range of 373-573 K. X-ray diffraction and transmission electron microscopy （TEM） were employed for the structural and morphological characterization of CuO nanoparticles. The experimental and theoretical in- vestigations suggested that the ac conduction mechanism in CuO nanoparticles can be successfully explained by a correlated barrier hopping model, which provided reasonable values for the maximum barrier height and characteristic relaxation time. It was also found that bipolaron hopping become prominent up to a particular temperature and beyond that single polaron hopping predominates. Physical parameters such as hopping distance and density of defect states were also calculated. Photoluminescence studies confirm the presence of a surface defect in CuO nanoparticles.

High transparency Pr:Y_(2)O_(3) ceramics:A promising gain medium for red emission solid-state lasers

Highly transparent 0.5 and 1.0 at%Pr-doped Y_(2)O_(3)ceramics were fabricated by vacuum sintering plus hot isostatic pressing(HIP)treatment.The selection of suitable pre-sintering temperatures and right microstructures before HIP was critical to obtain high density of the final sintered bodies.The well-densified ceramics had pore-free microstructures with an average grain size of about 1μm.It was also found that the charge states of the Pr ions could be changed through regulating the annealing atmospheres,resulting in different absorption and emission characteristics in the visible wavelength region.Annealing in reducing atmosphere(5%H_(2)/95%Ar)favored the formation of Pr^(3+),resulting in stronger red emissions,while annealing in oxygen atmosphere led to the rise of lattice constant due to the concentration increase of oxygen interstitials.The H_(2)/Ar-annealed 0.5 at%Pr:Y_(2)O_(3)ceramics exhibited strong red emission at 600–675 nm,which may be a promising gain material for red solid-state lasers.