Band Gap Gradient Research Articles

Charge-trap non-volatile memories fabricated by laser-enabled low-thermal budget processes

We fabricated charge-trap non-volatile memories (NVMs) using low thermal budget processes, including laser-crystallization of poly-Si thin film, chemical vapor deposition deposition of a stacked memory layer, and far-infrared-laser dopant activation. The thin poly-Si channel has a low defect-density at the interface with the bulk, resulting in a steep subthreshold swing for the NVM transistors. The introduction of the stacked SiO2/AlOxNy tunnel layer and the SiNx charge-trap layer with a gradient bandgap leads to reliable retention and endurance at low voltage for the NVMs. The low thermal budget processes are desirable for the integration of the nano-scaled NVMs into system on panels.

Modified co-evaporation process for fabrication of 4 cm × 4 cm large area flexible CIGS thin film solar cells on polyimide substrate

A modified three-stage co-evaporation process has been studied in our work for the fabrication of large area Cu(In,Ga)Se2 (CIGS) thin-film solar cells on flexible polyimide substrates. According to our results, the open circuit voltage and fill factor are improved significantly by using the modified process. In order to quantitatively analyze the effects of the modified band-gap gradient in CIGS thin film, a modeling investigation is carried out by employing a simulation program, wxAMPS. The simulation indicates that the device improvement is not only attributed to the refined Ga gradient profile, but also to the better crystalline quality. The elemental evaporation rates and the content of Cu are found to be key factors for the large-area solar cell preparation. Finally, 16 cm2 CIGS thin-film solar cells on PI substrates are fabricated, and the highest cell efficiency has achieved 7%.

Band-gap deformation potential and elasticity limit of semiconductor free-standing nanorods characterized in situ by scanning electron microscope-cathodoluminescence nanospectroscopy.

Modern field-effect transistors or laser diodes take advantages of band-edge structures engineered by large uniaxial strain εzz, available up to an elasticity limit at a rate of band-gap deformation potential azz (= dEg/dεzz). However, contrary to aP values under hydrostatic pressure, there is no quantitative consensus on azz values under uniaxial tensile, compressive, and bending stress. This makes band-edge engineering inefficient. Here we propose SEM-cathodoluminescence nanospectroscopy under in situ nanomanipulation (Nanoprobe-CL). An apex of a c-axis-oriented free-standing ZnO nanorod (NR) is deflected by point-loading of bending stress, where local uniaxial strain (εcc = r/R) and its gradient across a NR (dεcc/dr = R(-1)) are controlled by a NR local curvature (R(-1)). The NR elasticity limit is evaluated sequentially (εcc = 0.04) from SEM observation of a NR bending deformation cycle. An electron beam is focused on several spots crossing a bent NR, and at each spot the local Eg is evaluated from near-band-edge CL emission energy. Uniaxial acc (= dEg/dεcc) is evaluated at regulated surface depth, and the impact of R(-1) on observed acc is investigated. The acc converges with -1.7 eV to the R(-1) = 0 limit, whereas it quenches with increasing R(-1), which is attributed to free-exciton drift under transversal band-gap gradient. Surface-sensitive CL measurements suggest that a discrepancy from bulk acc = -4 eV may originate from strain relaxation at the side surface under uniaxial stress. The nanoprobe-CL technique reveals an Eg(εij) response to specific strain tensor εij (i, j = x, y, z) and strain-gradient effects on a minority carrier population, enabling simulations and strain-dependent measurements of nanodevices with various structures.

Outermost tensile strain dominated exciton emission in bending CdSe nanowires

Artificial modulation of electronic structures and control of the transport dynamics of carriers and excitons in CdSe nanowire are important for its application in optoelectronic nanodevices. Here, we demonstrate the aggregative flow of excitons by bending CdSe nanowires. The bending strain induces spatial variance of bandgap, and the energy bandgap gradient will result in the flow of excitons towards the bending outer edge of CdSe nanowire. The exciton emission energy shows a uniform redshift in the bending region due to the aggregative flow of excitons, and the energy redshift increases linearly with increasing the strain at the outer edge of the CdSe nanowire. Our results show an effective method to drive, concentrate, and utilize the excitons in CdSe nanowires, which provides a guide for the design of high performance and flexible optoelectronic nanodevices.

Research Scheme Design of Copper-Based Chalcogenide Crystal Thin Film Materials for Solar Cells

The keys for development of copper-based chalcogenide crystal thin film solar cells are to improve efficiency and reduce costs. Oriented growth and band gap grading of its absorption layer crystal film are important ways to improve the efficiency. It proposed controllable preparation of this type thin film by chemical solution deposition and co-reduction, and study on the crystal orientation and regulation of the band gap gradient. It will design the composition and band gap gradient of multi-component and multilayer films, and establish the models for design, theoretical calculation and performance prediction; It will explore the effect of band gap gradient on electrical and optical properties and reveal the effecting laws and mechanism on electrical and optical properties.

Wideband Fabry-Perot Resonator Antenna With Two Complementary FSS Layers

This paper presents a novel design of a Fabry-Perot (FP) resonator antenna with a wide gain bandwidth in X band. The bandwidth enhancement of the antenna is attributed to the positive reflection phase gradient of an electromagnetic band gap (EBG) structure, which is constructed by the combination of two complementary frequency selective surfaces (FSSs). To explain well the design procedure and approach, the EBG structure is modeled as an equivalent circuit and analyzed using the Smith Chart. Experimental results show that the antenna possesses a relative 3 dB gain bandwidth of 28%, from 8.6 GHz to 11.4 GHz, with a peak gain of 13.8 dBi. Moreover, the gain bandwidth can be well covered by the impedance bandwidth for the reflection coefficient ( ${\rm S} _{11}$ ) below $-10~{\rm dB}$ from 8.6 GHz to 11.2 GHz.

Nanocrystal Skins with Exciton Funneling for Photosensing

Highly photosensitive nanocrystal (NC) skins based on exciton funneling are proposed and demonstrated using a graded bandgap profile across which no external bias is applied in operation for light-sensing. Four types of gradient NC skin devices (GNS) made of NC monolayers of distinct sizes with photovoltage readout are fabricated and comparatively studied. In all structures, polyelectrolyte polymers separating CdTe NC monolayers set the interparticle distances between the monolayers of ligand-free NCs to <1 nm. In this photosensitive GNS platform, excitons funnel along the gradually decreasing bandgap gradient of cascaded NC monolayers, and are finally captured by the NC monolayer with the smallest bandgap interfacing the metal electrode. Time-resolved measurements of the cascaded NC skins are conducted at the donor and acceptor wavelengths, and the exciton transfer process is confirmed in these active structures. These findings are expected to enable large-area GNS-based photosensing with highly efficient full-spectrum conversion.

Device Modeling of the Performance of Cu(In,Ga)Se2Solar Cells with V-Shaped Bandgap Profiles

The effect of Cu(In,Ga)Se2(CIGS) with V-shaped bandgap on device performance is investigated in detail. A series of Ga/(In+Ga) ratio are set to study the influence of V-shaped bandgap profile on the electricity of CIGS thin film solar cells. The modeling of device current density-voltage (J-V) curve and bandgap grading profile corresponded well to measurement results. Detailed characteristic and modeling results show that an increased gradient of bandgap from valley to the buffer layer CdS will result in a barrier and lead to an enhanced recombination in the valley. This phenomenon can be modified by the back electric field resulting from a gradient bandgap from valley (bandgap minimum) to the Mo back contact. These results indicate CIGS-based solar cells can achieve higher performance by optimizing the V-shaped bandgap profile.

Influence of band gradients on Cu(In,Ga)Se2 solar cell diode factors

The influence of band gap gradients on the charge collection and diode quality factor of solar cells is investigated by device simulation. A back surface band gap gradient manifested as a gradient of the conduction band is found to lead to an increased diode quality factor. Thus, the positive influence of the gradient on the fill factor is partially counterbalanced by the diode quality factor increase. The reason for the latter is the enhanced contribution of space charge region recombination. If the cell is equipped with a double gradient at front and back surfaces, the detrimental diode factor increase can be suppressed. The relevance of the findings is investigated using different carrier lifetimes and doping levels.

A red anatase TiO2 photocatalyst for solar energy conversion

Narrowing the bandgap of wide-bandgap semiconductor photocatalysts (for instance, anatase TiO2) by introducing suitable heteroatoms has been actively pursued for increasing solar absorption, but usually suffers from a limited thermodynamic/kinetic solubility of substitutional dopants in bulk and/or dopant-induced recombination centres. Here we report a red anatase TiO2 microsphere with a bandgap gradient varying from 1.94 eV on its surface to 3.22 eV in its core by a conceptually different doping approach for harvesting the full spectrum of visible light. This approach uses a pre-doped interstitial boron gradient to weaken nearby Ti-O bonds for the easy substitution of oxygen by nitrogen, and consequently it substantially improves the nitrogen solubility. Furthermore, no nitrogen-related Ti3+ was formed in the red TiO2 due to a charge compensation effect by boron, which inevitably occurs in common nitrogen doped TiO2. The red anatase TiO2 exhibits photoelectrochemical water splitting activity under visible light irradiation. The results obtained may shed light on how to increase high visible light absorbance of wide-bandgap photocatalysts.

In-depth analysis of the CuIn1−xGaxSe2 film for solar cells, structural and optical characterization

Space-resolved x-ray diffraction measurements of gradient-etched CuIn1−xGaxSe2 (CIGS) solar cells provide information about stress and texture depth profiles in the absorber layer. An important parameter of CIGS layer growth dynamics, the absorber thickness-dependent stress in the molybdenum back contact, is analyzed. Texturing of grains and quality of the polycrystalline absorber layer are correlated with the intentional composition gradients (band gap grading). The band gap gradient is determined by space-resolved photoluminescence measurements and correlated with composition and strain profiles.

Analysis of CIGS‐based thin film solar cells with graded band gap

AbstractZnO/n‐CdS/p‐Cu(In,Ga)Se2 thin film solar cells with graded band gap absorber layer have been modelled. The cells are designed to be fabricated on flexible substrates, so, additional requirements and restrictions to metal contacts and interlayer mechanical and electronic properties should be taken into account. The aim is to calculate parameters of Cu(In,Ga)Se2 (CIGS)‐based thin film solar cells, taking into account the buffer layer effect, zone bending and recombination at grain boundaries and another defects, and to introduce band gap gradient in the absorber layer. The model developed is based on one‐dimensional drift‐diffusion approach using the Poisson's equation and continuity equations for electrons and holes. Modelling for various band gap gradient profiles was performed: positive (from 1.08 up to 1.5 eV), zero (1.21 eV) and inverted (from 1.5 down to 1.08 eV) profiles for CIGS layer, and cells' output parameters have been calculated.The nanostructure effects in solar cells formed on a substrate using porous anodic alumina were considered taking into account processes at grain boundaries and nanoordered back contact. Charge carriers accumulation at the interfaces leads to local effective band gap narrowing at the interfaces reducing the conversion efficiency. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoluminescence dynamics in GaAs/AlAs quantum wells modulated by one-dimensional standing surface acoustic waves

The effects of standing surface acoustic waves (SAWs) on carrier dynamics in GaAs/AlAs quantum wells are investigated by spatially and time-resolved photoluminescence (PL) spectroscopy. We found that the PL spectra vary considerably depending on the position and the phase of the standing SAW field. The PL spectra are characterized by oscillations in the PL intensity and emission energy due to the motion of free carriers and excitons driven by the piezoelectric fields as well as by the strain-induced band-gap gradient. It is also demonstrated that the positions of the nodes and antinodes of the standing SAW are precisely controlled.

Stacked-structure-dependent photoelectrochemical properties of CdS nanoparticle/layered double hydroxide (LDH) nanosheet multilayer films prepared by layer-by-layer accumulation

Inorganic multilayer films were prepared by layer-by-layer accumulation of positively charged layered double hydroxide (LDH) nanosheets and negatively charged CdS nanoparticles of different sizes. Nanoparticles were densely immobilized on LDH sheets to form a monolayer without coalescence into larger particles. The absorbance and photoluminescence intensity of immobilized CdS particles were enlarged with an increase in the accumulation number of the film. Hybrid films produced by accumulation of both monolayers of CdS particles (diameter: 5 nm) and those of smaller CdS particles (2.1 nm) exhibited characteristic photoluminescence spectra indicating the efficient energy transfer of photogenerated excitons from nanoparticle layers of smaller CdS particles to those of larger ones. LDH/CdS multilayers deposited on an F-doped SnO(2) (FTO) electrode behaved as an n-type semiconductor photoelectrode in an acetonitrile solution regardless of the size of the CdS particles immobilized, but their efficiency for photocurrent generation was greatly dependent on the stacked structure of the films. Accumulation of CdS particles of 2.1 nm in diameter on pre-coated LDH/CdS layers of 5 nm-sized CdS particles on FTO remarkably enhanced the photocurrent intensity in comparison to that in the case of accumulation of these two kinds of CdS particles in the opposite sequence. These observations can be explained by photoinduced electron transfer and energy transfer along with the band gap gradient in the films.

Long-range pair transport in graded band gap and its applications

The transport of electron–hole pairs in graded band gap of mixed crystal semiconductors is examined. It is estimated that a pair can be transferred for about 30 μm within its lifetime in the band gap gradient of 1 meV/μm in the low-temperature AlGaAs quantum well. Various opto-electronic devices by means of the pair transport are proposed. As one of exciting applications, the author proposes the quantum up-converter, where incoherent low-energy photons are converted to high-energy photons.

Exciton Recycling in Graded Gap Nanocrystal Structures

We propose a cascaded energy transfer structure made of semiconductor nanocrystals. Funnel-like band gap profiles are realized applying layer-by-layer assembly to CdTe nanocrystals of distinct sizes. Optical excitations efficiently transfer along the band gap gradient and are finally captured by the largest nanocrystals. The photoluminescence yield from the center layer is surprisingly high. The stepwise passing on of excitation energy avoids hot carriers, and the funnel structure recycles otherwise trapped and lost electron−hole pairs.

Properties of MBE Cd x Hg 1− x Te/GaAs structures modified by ion‐beam milling

Modification of electrical properties of n- and p-type MBE mercury–cadmium-telluride (MCT) structures with passivation gradient band gap layers under ion milling was investigated. Main features of the CdxHg1−xTe electrical properties modification with such treatment also hold for MBE MCT structures in comparison with bulk samples. It is shown that a significant decrease of the p–n conversion rate for such structures in comparison with homogeneous bulk samples at other equal conditions is caused by the presence of the wide band gap passivation layer. The diffusion-like character of the p–n conversion front movement is confirmed. The nature of additional “heavy” electrons in MBE structures is proposed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Steps toward industrialization of Cu-III-VI 2 thin-film solar cells: a novel full in-line concept

In order to manufacture Cu-III-VI 2 thin-film solar cells in much larger area and full speed as compared to the conventional evaporation method with the CdS layer deposited by CBD, a novel full in-line fabrication concept was developed, which includes in-line cleaning of substrates, sequential in-line sputtering/RTP steps for the fabrication of absorber, buffer layer and window-layer. A novel method named ‘MO sputter’ was developed in which the metal–organic (MO) TMGa used as a reactant gas, was introduced into the sputter for very fine-tuning the sputter-deposited metal precursor layers compositions, which could fulfill the requirement of Cu(In x Ga 1− x )(S,Se) 2 with the gradient bandgap, which help to obtain the precisely controlled, designated electrical characteristics for the device making. Then rapid thermal process (RTP) was utilized for sulfurization or selenization of the metal precursor layers. It has been found that Ga/(Ga+In) ratio incorporated in the films was increased linearly from 0.47 to 0.60 with increasing the TMGa gas flow rate at 450 °C substrate temperature. Single phase Cu(In x Ga 1− x )S 2 films have been obtained by optimizing the deposition parameters. Using the in-line sputtering/RTP sulfurization techniques, the ZnS buffer layer can also be prepared which can ensure not to use the hazardous CdS layer, and together with the sputter-deposited ZnO electrode, this could promote genuine realization of the large-scale production and lay down foundation for the full in-line manufacture of the ultra-high efficiency Cu-III-VI 2 thin film solar cells.

Reverse current in graded-gap p-n structure with nonmonotonic coordinate dependence of the band gap

The specific features of the reverse current of a graded-gap p-n structure with the band gap linearly increasing with the distance from the interface plane are analyzed theoretically. It is shown that the graded gap makes it possible to substantially reduce the reverse current of the p-n junction associated with the thermal generation of carriers in the base regions of the structure and at ohmic contacts. The possible appearance of a region with negative differential resistance in the reverse current-voltage characteristic in the case of a large band-gap gradient is predicted.

Anisotropic diffusion and drift of photogenerated carriers near coreless dislocations in InGaN quantum well

We report on a photoluminescence (PL) study on pinholes and their effects on the carrier recombination process in an InGaN/GaN single quantum well sample. By directly imaging the luminescence from the sample, we observed an anisotropic distribution of photogenerated carriers due to diffusion and drift of the carriers near pinholes. The anisotropy, together with micro-PL spectra at different locations near a pinhole, shows evidence of narrowing of the band gap of the quantum well layer near pinholes. The existence of the band gap gradient is confirmed by reflective and photoluminescence images of the sample under global illumination. Possible causes of the band gap gradient include indium concentration inhomogeneity and partial strain relaxation near pinholes.