Flat Conduction Band Research Articles

Ternary Cu3BiY3(Y= S, Se, and Te) for Thin-Film Solar Cells

ABSTRACTVery recently, Cu3BiS3has been suggested as an alternative material for photovoltaic (PV) thin-film technologies. In this work, we analyze the electronic and optical properties of Cu3BiY3with the anion elementsY= S, Se, and Te, employing a first-principles approach within the density function theory. We find that the three Cu3BiY3compounds have indirect band gaps and the gap energies are in the region of 1.2–1.7 eV. The energy dispersions of the lowest conduction bands are small, and therefore the direct gap energies are only ∼0.1 eV larger than the fundamental gap energies. The flat conduction bands are explained by the presence of localized Bip-states in the band gap region. Flat energy dispersion implies a large optical absorption, and the calculations reveal that the absorption coefficient of Cu3BiY3is larger than 105cm−1for photon energies of ∼2.5 eV. The absorption is stronger than other Cu-S based materials like CuInS2and Cu2ZnSnS4. Thereby, Cu3BiY3has the potential to be a suitable material in thin-film PV technologies.

Design of energy band alignment at the Zn1−xMgxO/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells

The electronic band structure at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface was investigated for its potential application in Cd-free Cu(In,Ga)Se(2) thin film solar cells. Zn(1-x)Mg(x)O thin films with various Mg contents were grown by atomic layer deposition on Cu(In(0.7)Ga(0.3))Se(2) absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn(0.9)Mg(0.1)O and Zn(0.8)Mg(0.2)O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn(1-x)Mg(x)O (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn(1-x)Mg(x)O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn(1-x)Mg(x)O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn(1-x)Mg(x)O films results in the flat (0 eV) and cliff (-0.23 eV) conduction band alignments at the Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) and Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) interfaces, respectively. The experimental results suggest that the bandgap energy of Zn(1-x)Mg(x)O films is the main factor that determines the conduction band offset at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface. Based on these results, we conclude that a Zn(1-x)Mg(x)O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn(1-x)Mg(x)O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn(1-x)Mg(x)O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.

Ab initio study of II–(VI)2 dichalcogenides

The structural stabilities of the (Zn,Cd)(S,Se,Te)2 dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe2 pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications.

Silver(ii) triflate with one-dimensional [Ag(ii)(SO3CF3)4/2]∞ chains hosting antiferromagnetism

Silver(II) triflate, previously reported by Leung et al. in 1979 (Can. J. Chem., 1979, 57, 326–329), crystallizes in a triclinic P cell with a = 4.9117(11) A, b = 5.1136(10) A, c = 11.033(3) A, α = 79.955(14)°, β = 75.771(16)°, γ = 61.571(17)° and V = 235.68(10) A3 and is isomorphous to Cu(SO3CF3)2. The compound has a layered structure with an interlayer separation of 10.67 A; the van der Waals gaps open between the CF3 groups from neighbouring sheets. Ag(II) is coordinated by six O atoms in the form of an elongated octahedron; the adjacent Ag2+ cations are linked via –OSO– bridges; direct –O– bridges are absent. The [Ag(II)(SO3CF3)2]∞ layers consist of one-dimensional chains which interact weakly with each other via longer Ag⋯OSO–Ag contacts. This results in a 1D rather than 2D antiferromagnetic ordering, which can be described via the Bonner–Fisher model with a superexchange constant, Jintra-chain, of 104 K (9.0 meV) per pair of interacting Ag2+ cations. The magnetic ordering persists even at room temperature leading to a broad ESR signal with g = 2.199. DFT calculations confirm the 1D character of electronic structure and antiferromagnetism residing within the [Ag(II)(SO3CF3)4/2]∞ chains with a |Jinter-chain|/|Jintra-chain| ratio of ∼10−3 to 10−2. The calculated indirect bandgap at the Fermi level of ∼1 eV opens between rather flat valence and conduction bands, which are predominated by contributions from Ag and O atoms. Ag(SO3CF3)2 is extremely sensitive to moisture and decomposes instantly when exposed to atmosphere. When dry, it rapidly decomposes thermally above 120 °C, but its slow exothermic decay to AgSO3CF3 takes place even at room temperature. Silver(II) triflate is also photosensitive and irradiating it with a 632.8 nm laser radiation at a power greater than 0.17 mW leads to its decomposition to Ag(I) triflate and Ag(I)2S2O7.

Band Alignment Tailoring of InAs1−xSbx/GaAs Quantum Dots: Control of Type I to Type II Transition

We report the growth of InAs(1-x)Sb(x) self-assembled quantum dots (QDs) on GaAs (100) by molecular beam epitaxy. The optical properties of the QDs are investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL). A type I to type II band alignment transition is demonstrated by both power-dependent PL and TRPL in InAs(1-x)Sb(x) QD samples with increased Sb beam flux. Results are compared to an eight-band strain-dependent k x p model incorporating detailed QD structure and alloy composition. The calculations show that the conduction band offset of InAs(1-x)Sb(x)/GaAs can be continuously tuned from 0 to 500 meV and a flat conduction band alignment exists when 60% Sb is incorporated into the QDs. Our study offers the possibility of tailoring the band structure of GaAs based InAsSb QDs and opens up new means for device applications.

Optical absorption of rutile SnO2 and TiO2

AbstractThe electronic band‐edge structure and optical response of rutile SnO2 and TiO2 are studied, employing a fully relativistic projected augmented wave method within the local density approximation (LDA). A quasi‐particle model corrects the LDA band‐gap energies, which is used to fit the on‐site self‐interaction correction (SIC) potential LDA+USIC. We show that inclusion of this k‐dependent SIC‐like potential correction, as well as inclusion of polaronic screening are of outermost importance for accurately determining the electronic structure and optical properties. For both SnO2 and TiO2, the calculated absorption coefficient α (ω) reveals a very small optical band‐edge absorption in the photon energy region Eg ≤ ħω ≤ ∼Eg +Δ (Δ = 0.8 eV and 0.5 eV for SnO2 and TiO2, respectively). The main difference between SnO2 and TiO2 is the presence of the unoccupied low‐energy 3d conduction‐band states in TiO2 which yield flat conduction bands and a very strong optical absorption for ħω > Eg +Δ. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of Band Alignment at CBD-CdS/Cu(In1-xGax)Se2 (x = 0.2 - 1.0) Interfaces by Photoemission and Inverse Photoemission Spectroscopy

AbstractDependence of band alignments at interfaces between CdS by chemical bath deposition and Cu(In1-xGax)Se2 by conventional 3-stage co-evaporation on Ga substitution ratio x from 0.2 to 1.0 has been systematically studied by means of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). For the specimens of the In-rich CIGS, conduction band minimum (CBM) by CIGS was lower than that of CdS. Conduction band offset of them was positive about +0.3 ~ +0.4 eV. Almost flat conduction band alignment was realized at x = 0.4 ~ 0.5. On the other hand, at the interfaces over the Ga-rich CIGS, CBM of CIGS was higher than that of CdS, and CBO became negative. The present study reveals that the decrease of CBO with a rise of x presents over the wide rage of x, which results in the sign change of CBO around 0.4 ~ 0.45. In the Ga-rich interfaces, the minimum of band gap energy, which corresponded to energy spacing between CBM of CdS and valence band maximum of CIGS, was almost identical against the change of band gap energy of CIGS. Additionally, local accumulation of oxygen related impurities was observed at the Ga-rich samples, which might cause the local rise of band edges in central region of the interface.

Characterization of interface nature and band alignment in CBD-CdS/Cu(In,Ga)Se 2 bi-layer structure by photoemission and inverse photoemission spectroscopy

Depth profiles of electronic structure at the interface between Cu(In 1− x Ga x )Se 2 (CIGSe) grown by three-stage process and CdS through chemical bath deposition have been studied by ultraviolet X-ray photoelectron spectroscopy (UPS, XPS) and inverse photoemission spectroscopy (IPES). Especially, a dependence of band alignment on Ga content in the CIGSe was investigated. An intrinsic feature at an arbitrary depth was successfully exposed by etching with an Ar ion beam with a low ion energy of 330 eV. After the removal of surface contamination, the CdS layer exhibited a band gap of 2.4 eV. The band gap started to shrink when XPS core signals of CIGSe became detectable. For the interface over the CIGSe with a Ga substitution ratio x of 0.20%, valence band offset (VBO) was about 0.7 eV, and conduction band offset (CBO) looked finite. An increase of the Ga substitution ratio to 0.40 resulted in an increase of the VBO and a reduction of the CBO. An almost flat conduction band alignment was observed at the interface of CdS/Cu 0.93(In 0.60Ga 0.40)Se 2.

Band alignment at the i-ZnO/CdS interface in Cu(In,Ga)(S,Se)2 thin-film solar cells

The interface between the i-ZnO layer and the CdS buffer in Cu(In,Ga)(S,Se)2 thin-film solar cells from the Shell Solar baseline process has been investigated using ultraviolet- and x-ray photoelectron spectroscopy and inverse photoemission. Combining both techniques, a direct determination of the conduction and valence band offsets at the interface is possible. Different from existing models, we find a flat conduction band alignment (i.e., a conduction band offset of 0.10±0.15 eV), ∼0.5 eV above the Fermi level, and a valence band offset of 0.96±0.15 eV.

STRAINED SILICON GERMANIUM HETEROSTRUCTURES FOR DEVICE APPLICATIONS

Silicon Germanium is lattice mismatched to Silicon by up to 4.2% depending on the Ge content. Up to a critical thickness elastic strain accommodates the mismatch. The band ordering of SiGe/Si interfaces is strongly influenced by the strain shifting the band ordering from flat conduction band to a type II ordering when proper strain adjustment is performed. As device examples the heterobipolartransistor, hetero field effect transistor and room temperature Esaki tunneling are treated. As key questions for further material development are identified the growth and processing of ultrametastable layers and the strain adjustment by thin virtual substrates.

Self-organized nanoclusters of semiconductor compounds on vicinal Si surfaces and its influence on carrier recombination in Si

The self-organized nanocluster ensembles of ZnSe, CdSe and PbS were grown by laser ablation on vicinal Silicon (111) surfaces. The properties of the layers were investigated by XPS (X-rays photoelectron spectroscopy) and Auger spectroscopy. The properties of the semiconductor-clustered layer on single Si crystal structure were investigated by deep level transient spectroscopy (DLTS) and by microwave absorption induced by light pulse excitation. The clustered layer and multilayer structure modified the recombination at the Si surface and created additional carrier capture centres. Absolute values of surface recombination rate, s, were determined by the method of asymptotic lifetime amplitude. An increase of s is caused by formation of PbS clusters, while for CdSe clusters, this dependence was found to be weaker. Structures with barriers or flat conduction bands were grown on the silicon surface using different doping concentrations of layers.

Isotype heterojunctions with flat valence or conduction band

We show that isotype heterojunctions with a perfectly flat majority or minority carrier band edge can be realized by modulation doping of arbitrary continuous alloy grading. The required impurity distribution is obtained analytically from the knowledge of the compositional grading and band structure parameters in the grading. This analytic relationship is exact for heterojunctions in which the grading fields are negligible in comparison with the atomic fields. We illustrate the design of flat valence-band heterojunctions for application in high-reflectivity low-resistance distributed Bragg reflectors for vertical-cavity lasers. The presented formalism enables the design of isotype heterojunctions with arbitrary band-edge profiles.

Design of flat-band AlGaAs heterojunction Bragg reflectors

Bragg reflectors using periodic AlGaAs heterojunction layers can be built into a light emitting diode to reduce the absorption in the substrate, or into a laser diode to form a cavity. In this paper, a numerical model for the design of low resistance flat-band p-type or n-type graded heterojunction reflectors is presented. It computes the required doping profile to achieve a flat conduction band or valence band across a graded junction, such that the reflector will add minimum series resistance to the devices.

Excitons in molybdenum disulphide

Optical spectra of Mo${\mathrm{S}}_{2}$ in the excitonic region were obtained by photoconductivity, photovoltaic effect and wavelength-modulated reflectivity at temperatures of 300 to 4.2 K and in magnetic fields of up to 70 kG. Cleaved natural crystals and synthetic crystals studied in both the Faraday and Voigt configurations showed nearly the same characteristics. The results indicate the possibility of up to four excitonic series in Mo${\mathrm{S}}_{2}$ as in other Mo dichalcogenides, and support band-structure calculations predicting flat conduction and valence bands originating from Mo orbitals. A ground-state anomaly of the $A$ exciton is explained by a central cell correction of the type used by Harbeke and Tosatti to explain a similiar anomaly in Pb${\mathrm{I}}_{2}$. The reduced mass of the $A$ exciton is found to be ${\ensuremath{\mu}}^{*}\ensuremath{\simeq}0.4{m}_{0}$, both from the study of the series itself, and from its magneto-optical properties, thus removing a large discrepancy found in previous work.