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We fabricated amorphous oxide semiconductor films, a-(Ga1–xZnx)Oy, at room temperature on glass, which have widely tunable band gaps (Eg) ranging from 3.47–4.12 eV. The highest electron Hall mobility ~ 7 cm2 V− 1 s− 1 was obtained for Eg = ~ 3.8 eV. Ultraviolet photoemission spectroscopy revealed that the increase in Eg with increasing the Ga content comes mostly from the deepening of the valence band maximum level while the conduction band minimum level remains almost unchanged. These characteristics are explained by their electronic structures. As these films can be fabricated at room temperature on plastic, this achievement extends the applications of flexible electronics to opto-electronic integrated circuits associated with deep ultraviolet region.

We investigated the effects of thermal annealing for high-density subgap states in amorphous In–Ga–Zn–O (a-IGZO) films by focusing on low-quality defective films deposited without O 2 supply (LQ films). It was found that most of the subgap states were thermally unstable and decreased dramatically by annealing at ≤ 400 °C in O 2 . These defects (but with different shapes) were further reduced by 600 °C annealing, whose subgap states appeared similar to that of a-IGZO films deposited at an optimum condition (high quality, HQ films) and annealed at 300 °C. However, electron Hall mobilities and field-effect mobilities of their thin-film transistors (TFTs) were low for the LQ films/TFTs even annealed at 600 °C compared to those for the HQ films/TFTs. It implies that not only the subgap states but also heavier structural disorder deteriorated the electron transport in the LQ films. The present results also suggest that although a-IGZO deposition without O 2 supply is sometimes employed in particular for DC sputtering, supplying some O 2 gas would be better to produce good TFTs at lower temperatures.

A device model for amorphous In–Ga–Zn–O thin-film transistors ( a -IGZO TFTs) that explains temperature dependence is proposed. It incorporates a carrier-density dependent mobility and a density of subgap traps of a -IGZO. The model parameters were extracted from only one transfer curve of an a -IGZO TFT at a low drain voltage through a simple analytical model. Device simulation based on this model reproduced current- and mobility-gate voltage characteristics of the a -IGZO TFT well over a wide range of bias voltage and temperature (253–393 K).

We fabricated ZnS:(Cu,Al) thin film phosphors on glass substrates by pulsed laser deposition (PLD) using a KrF 248 nm laser in a H 2 S gas flow. Firstly, we investigated effects of annealing temperature, stress and crystallinity on photoluminescence properties of powder samples. For thin films, photoluminescence intensity increased with increasing the substrate temperature up to 700 °C and then decreased. The former increase is attributed to improvement of crystalline quality and strain, while the latter degradation is attributed to reduction of the Cu acceptors. Intense photoluminescence is observed under an ambient light when excited by 325 nm monochromated light from a fluorescence spectrophotometer. It was confirmed that the PLD process stabilized the wurtzite structure, which is not stable in bulk samples at the thermal equilibrium.

Photovoltaic properties and electronic structures of n-type amorphous In–Ga x –Zn–O/ p -type Si heterojunction solar cells ( x = 1, 2, and 3) were investigated focusing on the effects of Ga content based on expectation that Ga-rich films have larger band gaps and improve open circuit voltages (V OC ) of solar cells. To know the electronic structures such as the conduction band minimums (CBMs) and the valence band maximums (VBMs) of these materials, hard x-ray photoemission spectroscopy (HX-PES) was performed. Contrary to the above expectation, the best result was obtained for x = 1 with an energy conversion efficiency of 5.3%. Although the Ga-rich films had larger optical band gaps and higher CBMs, V OC were remained low and poorer fill factors were obtained due to larger densities of defects. The low V OC were partly resulted from the deep VBM levels of the Ga-rich films. The defect densities are discussed in relation also to near-VBM states and near-CBM states observed in HX-PES and subgap optical absorptions.

The stability of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) was investigated focusing on the effects of passivation layer materials (Y 2 O 3 , Al 2 O 3 , HfO 2 , and SiO 2 ) and thermal annealing. Positive bias constant current stress (CCS), negative bias stress without light illumination (NBS), and negative bias light illumination stress (NBLS) were examined. It was found that Y 2 O 3 was the best passivation layer material in this study in terms of all the stability tests if the channel was annealed prior to the passivation formation (post-deposition annealing) and the passivation layer was annealed at ≥ 250 °C (post-fabrication annealing). Post-fabrication thermal annealing of the Y 2 O 3 passivation layer produced very stable TFTs against the CCS and NBS stresses and eliminated subgap photoresponse up to the photon energy of 2.9 eV. Even for NBLS with 2.7 eV photons, the threshold voltage shift is suppressed well to − 4.4 V after 3 h of test. These results provide the following information; (i) passivation removes the surface deep subgap defects in a-IGZO and eliminates the subgap photoresponse, but (ii) the bulk defects in a-IGZO should be removed prior to the passivation process. The Y 2 O 3 -passivated TFT is not only stable for these stress conditions, but is also compatible with high-frequency operation with the current gain cut-off frequency of 91 kHz, which is consistent with the static characteristics.

Effects of low-temperature annealing were examined for amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). In a previous study, we reported that O2 annealing is effective to improve performances of a-IGZO TFTs when annealed at ≥ 300 °C, but causes large negative threshold voltage shift when annealed at ≤ 200 °C. Here, we examined effects of ozone (O3) annealing on physical properties and TFT characteristics of a-IGZO in comparison with conventional O2 annealing. We found little differences in chemical composition, band gap and photoemission spectra between the O2 and the O3 annealed films. On the other hand, free electron density was suppressed well by the O3 annealing even at low temperatures ≤ 200 °C. Moreover, even at 150 °C, the TFTs characteristics were improved to the subthreshold voltage swing of 217 mV/decade, the saturation mobility of ~ 11.4 cm2(Vs)− 1 and the threshold voltage of 0.1 V by the O3 annealing. It was also found that the effects of the O3 annealing is more effective for thicker channel TFTs, which would be due to stronger oxidation power and the larger diffusion constant of oxygen atoms produced from O3 molecules than those of O2. These results substantiate that the O3 annealing is more effective to improve TFT characteristics in particular for low-temperature processes at ≤ 200 °C.

An operation model for an amorphous In–Ga–Zn–O (a-IGZO) based thin film transistor (TFT) is studied. The model is not based on the exponential tail states employed in hydrogenated amorphous Si (a-Si:H) TFT, but on a power function of the carrier density which is observed in the TFT and Hall mobilities of a-IGZO. A 2D numerical simulator employing this model reproduced current–voltage characteristics under on operation of coplanar homojunction a-IGZO TFTs. Although the mathematical expression of the mobility is similar to the field effect mobility of a-Si:H TFT, the present model explains the temperature dependence of the on characteristics of a-IGZO TFT.

Epitaxial (0001) films of GaN were grown on (111) YSZ substrates using single-crystalline InGaZnO 4 (sc-IGZO) lattice-matched buffer layers by molecular beam epitaxy with a NH 3 source. The epitaxial relationships are (0001) GaN //(0001) IGZO //(111) YSZ in out-of-plane and [112¯0] GaN //[112¯0] IGZO //[11¯0] YSZ in in-plane. This is different from those reported for GaN on many oxide crystals; the in-plane orientation of GaN crystal lattice is rotated by 30° with respect to those of oxide substrates except for ZnO. Although these GaN films showed relatively large tilting and twisting angles, which would be due to the reaction between GaN and IGZO, the GaN films grown on the sc-IGZO buffer layers exhibited stronger band-edge photoluminescence than GaN grown on a low-temperature GaN buffer layer.

Photoresponse was investigated for an amorphous oxide semiconductor, In–Ga–Zn–O, by the steady-state photoconductivity (SSPC) method. All the films exhibited extremely slow reversible photoresponses. Analysis of the transient photocurrent at varied temperatures provided similar activation energies of ~ 0.5 eV for both the time constants and the photoconductivity. Mobility–lifetime (μτ) products were estimated from the photoconductivity spectra measured at the sweep rate of 2 nm/s, which monotonically increased with increasing dark conductivity σD (i.e. the Fermi level EF becomes shallower). The obtained μτ values are larger than those of hydrogenated amorphous silicon even if the EF dependence is considered.

Stability under constant current stress, along with hysteresis characteristics, was studied for a-In-Ga-Zn-O thin-film transistors (TFTs) in several atmospheres and at several temperatures. Unannealed TFTs showed rather large instability; i.e., large hysteresis in transfer curves (ΔV G > 0.8 V) and large positive threshold voltage shift (ΔV th > 10 V for 50 h tests at 5 μA) with deterioration of subthreshold voltage swing was observed. The instability for the unannealed TFT had a strong dependence on the stress atmosphere and the stress temperature, which suggests that trap states generated by the stress test is related to oxygen vacancy formed by breaking weak chemical bonds. Wet annealing improved stability; the hysteresis disappeared and the ΔV th was reduced to

A low-temperature process to improve performances of a-In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) fabricated at room temperature was examined. Two deposition methods, pulsed laser deposition (PLD) and RF magnetron sputtering were employed to deposit the a-IGZO channels. For the PLD case, the TFT characteristics were improved significantly by wet annealing at dew point (d.p.) of 50 °C at the annealing temperature of 200 °C. For the sputtered TFTs, a wider range of annealing temperature from 100 to 200 °C was examined. It was found that annealing at ≥ 150 °C improved the TFT characteristics when dry annealing was employed. On the other hand, wet annealing also improved μ sat and S values, but very large negative threshold voltage ( V th ) shift was observed. These results indicate that the annealing at 150 °C is enough to obtain mobility ( μ sat ) as large as 8 cm 2 Vs − 1 , but annealing temperature as high as 200 °C provides larger μ sat comparable to those obtained by 400 °C annealing. It is speculated that the large negative V th shift originates from compensated donors in as-deposited sputtered films.

Amorphous In–Ga–Zn-O ( a -IGZO) thin-film transistors (TFTs) with a coplanar homojunction structure are demonstrated. The coplanar source and drain regions made of a -IGZO were formed by depositing a hydrogenated silicon nitride (SiN X :H) layer onto the a -IGZO layer. The a -IGZO regions on which the SiN X :H layer was directly deposited showed the low resistivity of 4.7 × 10 −3 Ω cm and degenerated conduction. The fabricated TFT showed excellent transfer and output characteristics with a field-effect mobility of 11 cm 2 V − 1 s − 1 , a subthreshold swing of 0.17 V decade − 1 , and an on-to-off current ratio larger than 1 × 10 9 . The width-normalized source-to-drain resistance ( R sd W ) calculated using a channel resistance method was 51 Ω cm. This TFT also showed good stability over environment change and under electrical stress.

Single-crystalline thin films of ScAlMgO 4 (SCAM) were fabricated on YSZ (111) substrates by reactive solid-phase epitaxy (R-SPE) using ScGaO 3 (ZnO) m (SGZO) single-crystalline buffer layers, which suppress interface reactions between the SCAM layers and the YSZ substrates. First, post-annealing of the SCAM layers fabricated by the R-SPE process was examined. When annealing temperature was raised to > 1200 °C, the formation of a spinel MgAl 2 O 4 phase was observed. This is due probably to out-diffusion of Sc ions from the SCAM layers to the YSZ substrates. By introducing an SGZO buffer layer, a single-phase SCAM layer with a step-and-terrace surface was obtained by annealing at 1450 °C without the spinel formation. The SCAM layer was grown heteroepitaxially with an epitaxial relationship of (000 l) SCAM (111) YSZ and (110) SCAM (11–0) YSZ . Atomically flat surfaces were eventually formed by chemical-mechanical polishing. The SCAM layer was applied to a lattice-matched buffer layer for the growth of ZnO film, leading to marked lateral growth of ZnO domains.

A p-type transparent oxide semiconductor, CuGaO 2 , was grown epitaxially on (111) single-crystalline yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD). Single-phase epitaxial films were obtained in narrow conditions: i.e. 725 ≤ T s ≤ 780 °C at P O 2 = 6 Pa, and 3 Pa ≤ P O 2 ≤ 6 Pa at T s = 750 °C. The electrical conductivity of the as-deposited films was controlled from 3.3 × 10 − 5 S∙cm − 1 to 1.7 × 10 − 2 S∙cm − 1 by increasing the oxygen partial pressure from 3 to 6.5 Pa. The hole concentration and Hall mobility of the most conductive film were 5 × 10 17 cm − 3 and 0.2 cm 2 ∙V − 1 ∙s − 1 , respectively. Estimated from the conductivity, the hole concentration was controlled from ∼ 10 14 cm − 3 to ∼ 10 17 cm − 3 by the oxygen partial pressure. Post-annealing at 1215 °C smoothed the films surface to 0.56 nm in the root-mean-squares roughness and increased the Hall mobility to 0.8 cm 2 V − 1 ∙s − 1 , which is the largest value among CuGaO 2 films reported to date.

Specific contact resistances between an amorphous oxide semiconductor, In-Ga-Zn-O, and various metallic electrodes, Ag, Au, In, Pt, Ti, polycrystalline indium tin oxide (ITO) and amorphous indium zinc oxide (a-IZO), were examined. All the contacts except for Au and Pt showed linear current-voltage characteristics, while Au and Pt did Schottky contacts. Low contact resistances

Thin film transistors (TFTs) using amorphous oxides of post-transition metals: indium, gallium, and zinc for the channel materials are fabricated with radio-frequency magnetron sputtering methods for the deposition of the channel and the gate insulator layers, at room temperature with no high-temperature post-deposition annealing process. The TFTs operate as n -channel field-effect transistors with various structures of top/bottom gate and top/bottom source-and-drain contact including the inverse-stagger types, and with various materials for the gate insulators, the electrodes, and the substrates. The TFTs having smoother channel interfaces show the better performance at the saturation mobility beyond 10 cm 2 V − 1 s − 1 and the on-to-off current ratio over 10 8 than the rough channel interfaces. The ring oscillator circuits operate with five-stage inverters of the top-gate TFTs or the inverse-stagger TFTs. Organic light-emission diode cells are driven by a simple circuit of the TFTs. It is also found by a combinatorial approach to the material exploration that the TFT characteristics can be controlled by the composition ratio of the metals in the channel layers. The amorphous oxide channel TFTs fabricated with sputtering deposition at low temperature could be a candidate for key devices of large-area flexible electronics.

LaZnO Pn ( Pn = P and As) thin films were successfully grown on MgO (001) substrates with an epitaxial relationship of (001) LaZnO Pn || (001) MgO and [110] LaZnO Pn || [110] MgO. The electrical conductivity of undoped LaZnOP epitaxial film was so low, 3.1 × 10 − 6 S/cm at room temperature, that reliable Seebeck and Hall measurement results were not obtained. On the other hand, the electrical conductivities of the Cu-doped LaZnOP and undoped LaZnOAs epitaxial films were 2.7 × 10 − 2 and 2.0 × 10 − 1 S/cm at room temperature, respectively. The Seebeck coefficients of both the epitaxial films were positive, indicating p-type semiconductors. The optical bandgaps of LaZnOP and LaZnOAs were estimated to be ∼ 1.7 eV and ∼ 1.5 eV, respectively.

Effects of lattice-matched substrates on growth of ZnO epitaxial films were studied. ZnO thin films were grown on single-crystalline InGaO 3 (ZnO) 6 (IGZO) layers, which have small lattice mismatches of ∼0.8% and ∼2.2% in a - and c -axes, respectively. Epitaxial ZnO films were grown with the epitaxial relationship between the ZnO film and the single-crystalline IGZO of [0001] ZnO //[0001] IGZO //[111] YSZ and [112¯0] ZnO //[112¯0] IGZO //[11¯0] YSZ . The use of the lattice-matched substrate and optimization of film microstructure and post-annealing condition led to atomically flat surfaces at maximum process temperatures as low as 700 °C. A large Hall electron mobility ∼80 cm 2 (V s) −1 ( N e : ∼2.8×10 18 cm −3 ) was obtained even if the film thickness was only 150 nm although comparable mobilities have been reported on films having much larger thicknesses (∼1000 nm) fabricated at higher temperatures ∼1000 °C.

Carrier transport properties in amorphous oxide semiconductor InGaZnO 4 (a-IGZO) thin films were investigated in detail using temperature dependence of Hall measurements. It was found that Hall mobility increased distinctly as carrier concentration increased. Unlikely conventional amorphous semiconductors such as a-Si/H, definite normal Hall voltage signals were observed on the films with carrier concentrations ( N e )>10 16 cm −3 , and Hall mobilities as large as 15 cm 2 (Vs) −1 were attained in the films with N e >10 20 cm −3 . When N e was less than 10 19 cm −3 , the temperature dependence of Hall mobility showed thermally-activated behavior in spite that carrier concentration was independent of temperature. While, it changed to almost degenerate conduction at N e >10 18 cm −3 . These behaviors are similar to those observed in single-crystalline IGZO, and are explained by percolation conduction through distributed potential barriers which are formed in the vicinity of the conduction band bottom due to the randomness of the amorphous structure. The effective mass of a-IGZO was estimated to be ∼0.34 m e ( m e is the mass of free electron) from optical data, which is almost the same as that of crystalline IGZO (∼0.32 m e ).

Growth conditions of heteroepitaxial thin films of tin-doped Ga 2 O 3 were surveyed from the viewpoint of visible application to field-effect transistors (FETs). Films were deposited by pulsed laser deposition, and post-annealing was examined to improve film structures. Atomically flat surfaces were obtained for films grown on yttria-stabilized zirconia (111) plane and post-annealed at 1400 °C, but they were insulating. Conductive heteroepitaxial films applicable to FETs were obtained on α-Al 2 O 3 (0001) at specific deposition conditions, i.e. substrate temperatures from 500 to 550 °C and oxygen pressures from 5 × 10 − 4 to 1 × 10 − 3 Pa. It was found that the resulting epitaxial films have a crystal structure different from that of β-Ga 2 O 3 . The crystal lattice for the films is determined to be orthorhombic with a large possibility of a higher-symmetry hexagonal or rhombohedral system. The films exhibited high transparency in the near infrared–deep ultraviolet region and had bandgap of ∼ 4.9 eV. The operation of top-gate MISFETs using the Ga 2 O 3 film for the n channel was demonstrated.

Hole transport and optical properties were investigated on undoped and Mg-doped LaCuOS 1− x Se x ( x =0–1) epitaxial films. Both electrical conductivity and Hall mobility were found to increase monotonously with increasing Se content in the films. The increase in Hall mobility is considered to be associated with the increase in valence band dispersion. Mg ion doping increased hole concentrations in the undoped films by an order of magnitude to ∼2×10 20 cm −3 , while Mg doping reduced mobility to merely half that of undoped films. The results suggest that hole scattering due to Mg impurity ions is suppressed by natural modulation doping originating from the layered structure of LaCuOS 1− x Se x . Hole concentrations showed no temperature dependence, indicating degenerate conduction. The largest value for conductivity, 140 S cm −1 , was obtained with Mg-doped LaCuOSe epitaxial film. Accompanying characteristics included moderately high optical transparency in the visible region and blue photoluminescence.

We have investigated the characteristics of transparent metal-insulator-semiconductor field-effect transistors (MISFETs) fabricated using InGaO 3 (ZnO) m ( m =integer) single-crystalline thin films as n-channel layers and amorphous alumina as gate insulator films. The MISFETs exhibit good characteristics such as insensitivity to visible light illumination, off-current as low as ∼1 nA with a positive threshold voltage of ∼3 V and on/off current ratio of 10 5 . The field-effect mobility increased from ∼1 to ∼10 cm 2 (V s) −1 as the m -value increased. Room temperature Hall mobility also increased. However, unexpectedly these values were lower than the field-effect mobility. It is explained by existence of shallow localized state in the homologous compounds.

Thin film fabrication of crystalline 12CaO·7Al2O3 (C12A7) with zeolitic structure was examined, and their electrical and optical properties were measured. Polycrystalline thin films were prepared by post-annealing of amorphous films in oxygen atmosphere at temperatures above 800 °C. Choice of substrates was crucial for obtaining single-phase thin films. Although various oxide substrates (single crystals of Al2O3, Y-stabilized ZrO2, MgO and silica glass) were examined, single-phase films were obtained only for MgO substrates and the other substrates reacted with the CaO component in the films during post-annealing. The optical band gap of C12A7 was evaluated to be 5.9 eV. Hydride ions were incorporated into the film by a thermal treatment in a hydrogen atmosphere at 1200 °C. The resulting transparent thin films were converted into transparent persistent electronic conductors exhibiting an electrical conductivity 6.2×10−1 S cm−1 at 300 K by ultraviolet light illumination. This is the first example of transparent conductive thin film in which conductive areas can be patterned directly by light.

A transparent ultraviolet (UV)-detector was fabricated using a high-quality pn-heterojunction diode composed of transparent oxide semiconductors, p-type NiO and n-type ZnO, and its UV-response was measured at room temperature. Transparent tri-layered oxide films of ZnO/NiO/ITO were heteroepitaxially grown on an YSZ (1 1 1) substrate by a pulsed-laser-deposition combined with a solid-phase-epitaxy technique and they were processed to fabricate a p-NiO/n-ZnO diode. The diodes exhibited a clear rectifying I – V characteristic with an ideality factor of ∼2 and a forward threshold voltage of ∼1 V. Although the photo-responsivity was fairly weak at the zero bias voltage, it was enhanced up to ∼0.3 A W −1 by applying a reverse bias of −6 V under an irradiation of 360-nm light, which is comparable to that of commercial devices.

P-type polycrystalline silicon (poly-Si) thin films are fabricated from SiF4, H2 and SiH4 gas mixtures by plasma-enhanced chemical vapor deposition using in situ doping of B2H6 or BF3. Relationships between deposition condition, film structure and transport properties are focused on. In addition, ionization efficiency is discussed in connection with deposition condition and with B and F concentrations. When poly-Si is doped with B, crystal fractions are lower for 300°C-grown films than for 200°C-grown films. High B doping results in (220) preferential orientation even when no fluorinated gas is used in the gas source. These trends are very similar among films prepared using different doping sources, i.e. B2H6 and BF3. When SiF4 is used, the B ionization efficiency is very low, ∼10%. It is improved to ∼50% by removing SiF4 from the gas source. This low ionization efficiency shows good correlation with atomic concentration ratio of B/F in the poly-Si films, suggesting that some high incorporation of F compared to B inhibits the ionization of B. Using SiH4, H2 and BF3 without SiF4, highly (220) oriented, large grain poly-Si is obtained and it exhibits large mobility of 3.7 cm2/Vs.

The control of grain orientation in polycrystalline silicon thin films on glass substrates by low-temperature techniques was investigated. Either (220) or (400) preferential grain orientation could be attained by control of source gas ratio over substrate temperatures between 250°C and 360°C. A remote type plasma chemical vapor deposition system was used with source gas mixtures of SiF 4 , H 2 and Ar. The (220) preferential films were obtained with Ar/H 2 /SiF 4 gas flow rates of 60/15/30 seem (respectively), while the (400) preferential oriented films were obtained at higher SiF 4 /H 2 ratios (SiF 4 /H 2 = 90/10 sccm). At the higher SiF 4 /H 2 ratio during the crystal nucleation stage, either randomly oriented or (400) grains formed followed by the highly preferred deposition of (400) oriented crystallites. Raman scattering and ellipsometry spectra indicated that the (400) oriented films had a very smooth surface.

9th International Symposium on Transparent Oxide and Related Materials for Electronics and Optics (TOEO9) Preface

Ž. The electrical properties of 400 oriented polycrystalline silicon films fabricated at 300C by 100-MHz plasma enhanced chemical vapor deposition from gaseous mixture of SiF H SiH are reported. A double layered structure of phosphorus-doped 42 4 Ž. poly-SiHF film 200 nm undoped poly-SiHF film was adopted to research the changes in electrical properties in the doped layer induced by the undoped layer thickness. The carrier mobility in the crystalline grain of the doped layer, analyzed by free carrier optical absorption, increased from 10 to 35 cm 2 Vs as the undoped film thickness increased from 0 to 1000 nm. The carrier density in the crystalline grain was 2.5 10 20 cm 3 for each sample. The grain properties in the doped layer improved as the undoped film thickness increased. 2001 Elsevier Science B.V. All rights reserved. Ž.

Applying the concept of materials design for transparent conductive oxides to layered oxychalcogenides, several p-type and n-type layered oxychalcogenides were proposed as wide-gap semiconductors and their basic optical and electrical properties were examined. The layered oxychalcogenides are composed of ionic oxide layers and covalent chalcogenide layers, which bring wide-gap and conductive properties to these materials, respectively. The electronic structures of the materials were examined by normal/inverse photoemission spectroscopy and energy band calculations. The results of the examinations suggested that these materials possess unique features more than simple wide-gap semiconductors. Namely, the layered oxychalcogenides are considered to be extremely thin quantum wells composed of the oxide and chalcogenide layers or 2D chalcogenide crystals/molecules embedded in an oxide matrix. Observation of step-like absorption edges, large band gap energy and large exciton binding energy demonstrated these features originating from 2D density of states and quantum size effects in these layered materials.