Spectroscopic and electrical properties of ultrathin SiO2 layers formed with nitric acid

Spectroscopic and electrical properties of ultrathin silicon dioxide (SiO₂) layers formed with nitric acid have been investigated. The leakage current density of the as-grown SiO₂ layers of 1.3 nm thickness is high. The leakage current density is greatly decreased by post-oxidation annealing (POA) treatment at 900 °C in nitrogen, and consequently it becomes lower than those for thermally grown SiO₂ layers with the same thickness. X-ray photoelectron spectroscopy measurements show that high density suboxide species are present before POA and they are markedly decreased by POA. Fourier transformed infrared absorption measurements show that water and silanol group are present in the SiO₂ layers before POA but they are removed almost completely by POA above 800 °C. The atomic density of the as-grown chemical SiO₂ layers is 4% lower than that of bulk SiO₂ layers, while it becomes 12% higher after POA. It is concluded that the high atomic density results from the desorption of water and OH species, and oxidation of the suboxide species, both resulting in the formation of SiO₂. The valence band discontinuity energy at the Si/SiO₂ interface increases from 4.1 to 4.6 eV by POA at 900 °C. The high atomic density enlarges the SiO₂ band-gap energy, resulting in the increase in the band discontinuity energy. The decrease in the leakage current density by POA is attributed to (i) a reduction in the tunneling probability of charge carriers through SiO₂ by the enlargement of the band discontinuity energy, (ii) elimination of trap states in SiO₂, and (iii) elimination of interface states.     

Stimulated photoluminescence emission and trap states in Si/SiO2 interface formed by irradiation of laser

Stimulated photoluminescence （PL） emission has been observed from an oxide structure of silicon when optically excited by a radiation of 514nm laser. Sharp twin peaks at 694 and 692nm are dominated by stimulated emission, which can be demonstrated by its threshold behaviour and linear transition of emission intensity as a function of pump power. The oxide structure is formed by laser irradiation on silicon and its annealing treatment. A model for explaining the stimulated emission is proposed, in which the trap states of the interface between an oxide of silicon and porous nanocrystal play an important role.     

Properties of thick SiO2/Si structure formed at 120 °C by use of two-step nitric acid oxidation method

Thick (i.e., ∼10 nm) SiO₂/Si structure has been formed at 121 °C by immersion of Si in relatively low concentration HNO₃ followed by that in 68 wt.% HNO₃ (i.e., two-step nitric acid (HNO₃) oxidation method of Si, NAOS) and spectroscopic properties and electrical characteristics of the NAOS SiO₂ layers are investigated. The SiO₂ thickness strongly depends on the concentration of HNO₃ aqueous solutions employed in the initial oxidation, and it becomes the largest at the HNO₃ concentration of 40 wt.%. The MOS diodes with the ∼9 nm SiO₂ layer formed by the NAOS method possess a relatively low leakage current density (e.g., 10⁻⁸ A/cm² at the forward bias of 1 V) and it is further decreased by more than one order of magnitude by post-metallization annealing (PMA) in hydrogen at 250 °C. The good leakage characteristic is attributable to atomically flat SiO₂/Si interfaces and high atomic density of 2.30-2.32 × 10²² atoms/cm³ of the NAOS SiO₂ layers. High-density interface states are present in as-prepared SiO₂ layers and they are eliminated by PMA in hydrogen.     

Passivation of aluminium–n+ silicon contacts for solar cells by ultrathin Al2O3 and SiO2 dielectric layers

Ultra‐thin thermally grown SiO₂ and atomic‐layer‐deposited (ALD) Al₂O₃ films are trialled as passivating dielectrics for metal–insulator–semiconductor (MIS) type contacts on top of phosphorus diffused regions applicable to high efficiency silicon solar cells. An investigation of the optimum insulator thickness in terms of contact recombination factor J_{0_cont} and contact resistivity ρ_c is undertaken on 85 Ω/□ and 103 Ω/□ diffusions. An optimum ALD Al₂O₃ thickness of ～22 Å produces a J_{0_cont} of ～300 fAcm^–2 whilst maintaining a ρ_c lower than 1 mΩ cm² for the 103 Ω/□ diffusion. This has the potential to improve the open‐circuit voltage by a maximum 15 mV. The thermally grown SiO₂ fails to achieve equivalently low J_{0_cont} values but exhibits greater thermal stability, resulting in slight improvements in ρ_c when annealed for 10 minutes at 300 °C without significant changes in J_{0_cont}. The after‐anneal J_{0_cont} reaches ～600 fAcm^–2 with a ρ_c of ～2.5 mΩ cm² for the 85 Ω/□ diffusion amounting to a maximum gain in open‐circuit voltage of 6 mV. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Band alignment of SiO2/Si structure formed with nitric acid vapor below 500 °C

A relatively thick (i.e., ∼9 nm) SiO₂ layer can be formed by oxidation of Si with nitric acid (HNO₃) vapor below 500 °C. In spite of the low temperature formation, the leakage current density flowing through the SiO₂ layer is considerably low, and it follows the Fowler-Nordheim mechanism. From the Fowler-Nordheim plots, the conduction band offset energy at the SiO₂/Si interface is determined to be 2.57 and 2.21 eV for HNO₃ vapor oxidation at 500 and 350 °C, respectively. From X-ray photoelectron spectroscopy measurements, the valence band offset energy is estimated to be 4.80 and 4.48 eV, respectively, for 500 and 350 °C oxidation. The band-gap energy of the SiO₂ layer formed at 500 °C (8.39 eV) is 0.68 eV larger than that formed at 350 °C. The higher band-gap energy for 500 °C oxidation is mainly attributable to the higher atomic density of the SiO₂ layer of 2.46 × 10²²/cm³. Another reason may be the absence of SiO₂ trap-states.     

Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr-O valence electron matching to the number of dangling bonds per surface Si atom. The Zr-O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C-O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.     

Surface reactions and kinetically-driven patterning scheme for selective deposition of Si and Ge nanoparticle arrays on HfO2

We demonstrate a kinetically-driven patterning scheme to selectively position arrays of Ge or Si nanoparticles within lithographically defined HfO₂ windows. The surface reactions enabling patterning are revealed through temperature programmed desorption experiments and selectivity of the deposition is verified by X-ray photoelectron spectroscopy and scanning electron microscopy. Patterning is possible by exploiting the different reactivity of Ge and Si on HfO₂ and SiO₂ surfaces and employing a sacrificial SiO₂ mask on which adatoms etch the SiO₂ surface and do not accumulate to form nanocrystals.     

Experimental and theoretical studies of Si-CN bonds to eliminate interface states at Si/SiO2 interface

Cyanide treatment, which includes the immersion of Si in KCN solutions followed by a rinse, effectively passivates interface states at Si/SiO₂ interfaces by the reaction of CN⁻ ions with interface states to form Si-CN bonds. X-ray photoelectron spectroscopy (XPS) measurements show that the concentration of the CN species in the surface region after the cyanide treatment is ∼0.25 at.%. Take-off angle-dependent measurements of the XPS spectra indicate that the concentration of the CN species increases with the depth from the Si/SiO₂ interface at least up to ∼2 nm when ultrathin SiO₂ layers are formed at 450 °C after the cyanide treatment. When the cyanide treatment is applied to metal-oxide-semiconductor (MOS) solar cells with 〈ITO/SiO₂/n-Si〉 structure, the photovoltage greatly increases, leading to a high conversion efficiency of 16.2% in spite of the simple cell structure with no pn-junction. Si-CN bonds are not ruptured by air mass 1.5 100 mW cm⁻² irradiation for 1000 h, and consequently the solar cells show no degradation. Neither are Si-CN bonds broken by heat treatment at 800 °C performed after the cyanide treatment. The thermal and irradiation stability of the cyanide treatment is attributable to strong Si-CN bonds, whose bond energy is calculated to be 1 eV higher than that of the Si-H bond energy using a density functional method.     

Analyses of temperature-dependent interface states,series resistances,and AC electrical conductivities of Al/p Si and Al/Bi_4Ti_3O_(12)/p Si structures by using the admittance spectroscopy method

In this study, Al/p-Si and Al/Bi4Ti3O12/p-Si structures are fabricated and their interface states （Nss）, the values of series resistance （Rs）, and AC electrical conductivity （σac） are obtained each as a function of temperature using admit- tance spectroscopy method which includes capacitance-voltage （C-V） and conductance-voltage （G-V） measurements. In addition, the effect of interfacial Bi4Ti3012 （BTO） layer on the performance of the structure is investigated. The voltage- dependent profiles of Nss and Rs are obtained from the high-low frequency capacitance method and the Nicollian method, respectively. Experimental results show that Nss and Rs, as strong functions of temperature and applied bias voltage, each exhibit a peak, whose position shifts towards the reverse bias region, in the depletion region. Such a peak behavior is attributed to the particular distribution of Nss and the reordering and restructuring of Nss under the effect of temperature. The values of activation energy （Ea）, obtained from the slope of the Arrhenius plot, of both structures are obtained to be bias voltage-independent, and the Ea of the metal-ferroelectric-semiconductor （MFS） structure is found to be half that of the metal-semiconductor （MS） structure. Furthermore, other main electrical parameters, such as carrier concentration of acceptor atoms （NA）, built-in potential （Vbi）, Fermi energy （EF）, image force barrier lowering （△φb）, and barrier height （φb）, are extracted using reverse bias C 2-V characteristics as a function of temperature.     

Influence of Si/SiO2 interface properties on electrical performance and breakdown characteristics of ultrathin stacked oxide/nitride dielectric films

In this work, the influence of Si/SiO₂ interface properties, interface nitridation and remote-plasma-assisted oxidation (RPAO) thickness (<1 nm), on electrical performance and TDDB characteristics of sub-2 nm stacked oxide/nitride gate dielectrics has been investigated using a constant voltage stress (CVS). It is demonstrated that interfacial plasma nitridation improves the breakdown and electrical characteristics. In the case of PMOSFETs stressed in accumulation, interface nitridation suppresses the hole traps at the Si/SiO₂ interface evidenced by less negative V_t shifts. Interface nitridation also retards hole tunneling between the gate and drain, resulting in reduced off-state drain leakage. In addition, the RPAO thickness of stacked gate dielectrics shows a profound effect in device performance and TDDB reliability. Also, it is demonstrated that TDDB characteristics are improved for both PMOS and NMOS devices with the 0.6 nm-RPAO layer using Weibull analysis. The maximum operating voltage is projected to be improved by 0.3 V difference for a 10-year lifetime. However, physical breakdown mechanism and effective defect radius during stress appear to be independent of RPAO thickness from the observation of the Weibull slopes. A correlation between trap generation and dielectric thickness changes based on the C-V distortion and oxide thinning model is presented to clarify the trapping behavior in the RPAO and bulk nitride layer during CVS stress.     

Formation of 10-30 nm SiO2/Si structure with a uniform thickness at ∼120 °C by nitric acid oxidation method

Silicon dioxide (SiO₂) layers with a thickness more than 10 nm can be formed at ∼120 °C by direct Si oxidation with nitric acid (HNO₃). Si is initially immersed in 40 wt.% HNO₃ at the boiling temperature of 108 °C, which forms a ∼1 nm SiO₂ layer, and the immersion is continued after reaching the azeotropic point (i.e., 68 wt.% HNO₃ at 121 °C), resulting in an increase in the SiO₂ thickness. The nitric acid oxidation rates are the same for (1 1 1) and (1 0 0) orientations, and n-type and p-type Si wafers. The oxidation rate is constant at least up to 15 nm SiO₂ thickness (i.e., 1.5 nm/h for single crystalline Si and 3.4 nm/h for polycrystalline Si (poly-Si)), indicating that the interfacial reaction is the rate-determining step. SiO₂ layers with a uniform thickness are formed even on a rough surface of poly-Si thin film.     

Electrical and optical characterization of SiOxNy and SiO2 dielectric layers and rear surface passivation by using SiO2/SiOxNy stack layers with screen printed local Al-BSF for c-Si solar cells

In c-Si solar cells, surface recombination velocity increases as the wafer thickness decreases due to an increase in surface to volume ratio. For high efficiency, in addition to low surface recombination velocity at the rear side, a high internal reflection from the rear surface is also required. The SiO_xN_y film with low absorbance can act as rear surface reflector. In this study, industrially feasible SiO₂/SiO_xN_y stack for rear surface passivation and screen printed local aluminium back surface field were used in the cell structure. A 3 nm thick oxide layer has resulted in low fixed oxide charge density of 1.58 × 10¹¹ cm⁻² without parasitic shunting. The oxide layer capped with SiO_xN_y layer led to surface recombination velocity of 155 cm/s after firing. Using single layer (SiO₂) rear passivation, an efficiency of 18.13% has been obtained with V_oc of 625 mV, J_sc of 36.4 mA/cm² and fill factor of 78.7%. By using double layer (SiO₂/SiO_xN_y stack) passivation at the rear side, an efficiency of 18.59% has been achieved with V_oc of 632 mV, J_sc of 37.6 mA/cm², and fill factor of 78.3%. An improved cell performance was obtained with SiO₂/SiO_xN_y rear stack passivation and local BSF.     

Comparative study of the oxidation of thin porous silicon layers studied by reflectometry, spectroscopic ellipsometry and secondary ion mass spectroscopy

We present a systematic study on ultrathin porous silicon (PS) layers (40–120 nm) of different porosities, formed by electrochemical etching and followed by thermal oxidation treatment (300°C and 600°C) and by electrochemical oxidation. The oxidised and non-oxidised PS layers have been analysed by spectroscopic reflectometry (SR), spectroscopic ellipsometry (SE) and secondary ion mass spectroscopy (SIMS). The SR and SE spectra were fitted by a multiparameter fit program and the composition and the thickness of the PS layers were evaluated by different optical models. PS layers, formed electrochemically in the outermost layer of a p/n⁺ monocrystalline silicon junction were successfully evaluated using a gradient porosity optical model. The non-oxidised PS, formed in p-type silicon, can be well described by a simple optical model (one-layer of two-components, silicon and voids). The spectra of the oxidised PS layers can be fitted better using an optical model with three interdependent components (crystalline-silicon, silicon-dioxide, voids). The SIMS results give a strong support for the optical model used for SR and SE.     

Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures

We have developed low temperature formation methods of SiO₂/Si and SiO₂/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68 wt% HNO₃ aqueous solutions at 120 °C), an ultrathin (i.e., 1.3-1.4 nm) SiO₂ layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200 °C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1 V becomes 1/4-1/20 of that of an ultrathin (i.e., 1.5 nm) thermal oxide layer usually formed at temperatures between 800 and 900 °C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO₂ gap-state density, and (iii) high band discontinuity energy at the SiO₂/Si interface arising from the high atomic density of the NAOS SiO₂ layer.For the formation of a relatively thick (i.e., ≥10 nm) SiO₂ layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40 wt% HNO₃ and azeotropic HNO₃ aqueous solutions, respectively. In this case, the SiO₂ formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO₂ layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO₂ layer is slightly higher than that for thermal oxide. When PMA at 250 °C in hydrogen is performed on the two-step NAOS SiO₂ layer, the current-voltage and capacitance-voltage characteristics become as good as those for thermal oxide formed at 900 °C.A relatively thick (i.e., ≥10 nm) SiO₂ layer can also be formed on SiC at 120 °C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400 °C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO₂/SiC interface and a uniform thickness SiO₂.     

On formation of thin SiO2/a-Si:H interface when biased oxidized semiconductor surface interacts with plasma or liquid solution

In this paper we present the results of research into a relation(s) between the bias voltage of an oxide/a-Si:H/c-Si sample during formation of very-thin and thin oxides and the resulting distribution of oxide/semiconductor interface states in the a-Si:H band gap. Two oxygen plasma sources were used for the first time in our laboratories for formation of oxide layers on a-Si:H: i) inductively coupled plasma in connection with its application at plasma anodic oxidation; ii) rf plasma as the source of positive oxygen ions for the plasma immersion ion implantation process. The oxide growth on a-Si:H during plasma anodization is also simply described theoretically. Properties of plasmatic structures are compared to ones treated by chemical oxidation that uses 68 wt% nitric acid aqueous solutions. We have confirmed that three parameters of the oxide growth process — kinetic energy of interacting particles, UV-VIS-NIR light emitted by plasma sources, and bias of the samples — determine the distribution of defect states at both the oxide/a-Si:H interface and the volume of the a-Si:H layer, respectively. Additionally, a bias of the sample applied during the oxide growth process has a similar impact on the distribution of defect states as it can be observed during the bias-annealing of similar MOS structure outside of the plasma reactor. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

Defect reduction and surface passivation of SiO2/Si by heat treatment with high-pressure H2O vapor

Heat treatment with high-pressure H₂O vapor was applied to improve interface properties of SiO₂/Si and passivate the silicon surface. Heat treatment at 180–420 °C with high-pressure H₂O vapor changed SiO_x films, 150 nm thick formed at room temperature by thermal evaporation in vacuum, into SiO₂ films with a Si-O-Si bonding network similar to that of thermally grown SiO₂ films. Heat treatment at 130 °C with 2.8×10⁵ Pa H₂O for 3 h reduced the recombination velocity for the electron minority carriers from 405 cm/s (as-fabricated 150-nm-thick SiO_x/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiO_x films on the SiO₂ films formed by heat treatment at 340 °C with high-pressure H₂O vapor. The SiO_x deposition reduced the recombination velocity from 100 cm/s to 48 cm/s. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

High-k Al2O3 MOS structures with Si interface control layer formed on air-exposed GaAs and InGaAs wafers

This paper attempts to realize unpinned high-k insulator-semiconductor interfaces on air-exposed GaAs and In_0.53Ga_0.47As by using the Si interface control layer (Si ICL). Al₂O₃ was deposited by ex situ atomic layer deposition (ALD) as the high-k insulator. By applying an optimal chemical treatment using HF acid combined with subsequent thermal cleaning below 500 °C in UHV, interface bonding configurations similar to those by in situ UHV process were achieved both for GaAs and InGaAs after MBE growth of the Si ICL with no trace of residual native oxide components. As compared with the MIS structures without Si ICL, insertion of Si ICL improved the electrical interface quality, a great deal both for GaAs and InGaAs, reducing frequency dispersion of capacitance, hysteresis effects and interface state density (D_it). A minimum value of D_it of 2 × 10¹¹ eV⁻¹ cm⁻² was achieved both for GaAs and InGaAs. However, the range of bias-induced surface potential excursion within the band gap was different, making formation of electron layer by surface inversion possible in InGaAs, but not possible in GaAs. The difference was explained by the disorder induced gap state (DIGS) model.     

Reaction products and oxide thickness formed by Ti out-diffusion and oxidization in poly-Pt/Ti/SiO2/Si with oxide films deposited

In the paper, we present experimental results to enhance the understanding of Ti out-diffusion and oxidization in commercial poly-Pt/Ti/SiO₂/Si wafers with perovskite oxide films deposited when heat-treated in flowing oxygen ambient. It indicates that when heat-treated at 550 and 600 °C, PtTi₃+PtTi and PtTi are the reaction products from interfacial interaction, respectively; while heat-treated at 650 °C and above, the products become three layers of titanium oxides instead of the alloys. Confirmed to be rutile TiO₂, the first two layers spaced by 65 nm encapsulate the Pt surface by the first layer with 60 nm thick forming at its surface and by the next layer with 35 nm thick inserting its original layer. In addition, the next layer is formed as a barrier to block up continuous diffusion paths of Ti, and thus results in the last layer of TiO_2−x formed by the residual Ti oxidizing.     

Fabrication of p-Si/β-FeSi2 balls/n-si structures by MBE and their electrical and optical properties

We report on the formation technique of single-crystalline β-FeSi₂ balls (<100 nm) embedded in a Si p–n junction region by Si molecular beam epitaxy (MBE). β-FeSi₂ films grown on Si (0 0 1) by reactive deposition epitaxy (RDE) aggregated into islands after annealing at 850°C in ultrahigh vacuum. The islands of β-FeSi₂ aggregated further into a ball shape by following the Si MBE overgrowth at 750°C. It was found from X-ray diffraction (XRD) patterns that the epitaxial relationship between the two materials, and single-crystalline nature were preserved even after the annealing and the Si overgrowth. Capacitance–voltage (C–V) characteristics and transmission electron microscope (TEM) images revealed that a lot of defects were introduced around the embedded β-FeSi₂ balls with an increase of embedded β-FeSi₂ quantity.     

Characterization of novel cation-ordered compounds with fluorite and α-PbO2 related structures prepared by oxidation of Sn-Nb-O pyrochlore

Two cation-ordered phases were prepared by the oxidation of a pyrochlore phase as a precursor in the Sn-Nb-O system. One was a cation-ordered fluorite related phase, whose unit cell volume was eight times larger than that of the fluorite structure, similar to the κ-CeZrO₄ phase. The other was a cation-ordered α-PbO₂ phase, in which the ordered arrangement of cations was distinctly different from the well-known cation-ordered α-PbO₂ structures such as wolflamite and columbite. The chemical compositions of the phases, including valence states of cations, local structure and electronic structure near the energy band gap were studied. The compositions of the precursor and its oxidized phases were evaluated, respectively, to be Sn^II_1.62(Nb^V_1.86Sn^IV_0.14)O_6.55 and Sn^IV_0.81(Nb^V_0.93Sn^IV_0.07)O_4.085, i.e. Sn^IV_1.62(Nb^V_1.86Sn^IV_0.14)O_8.17, by TG-DTA and ICP analyses, ¹¹⁹Sn Mössbauer spectroscopy and X-ray photoemission spectroscopy. It was ascertained using IR and Raman spectroscopies that the structural framework of the cation-ordered fluorite related phase was close to that of the precursor pyrochlore phase, and the structural framework of the cation-ordered α-PbO₂ phase was distinct from that of the precursor pyrochlore. The Mössbauer parameters obtained showed a strong deviation of oxygen atoms contained in the SnO₈ polyhedron in the cation-ordered fluorite related phase from the cubic arrangement for the ideal fluorite related structure, and a lower site symmetry of Sn^IV in the cation-ordered α-PbO₂ than in rutile SnO₂. An increase in the energy band gap from ∼2.5 eV for the precursor pyrochlore to ∼3.5 eV for the oxidized phases was attributed to the vanishing of Sn 5s² lone-pair states upon oxidation of Sn^II in the precursor to Sn^IV in the oxidized phases.