Generic origin of subgap states in transparent amorphous semiconductor oxides illustrated for the cases of In-Zn-O and In-Sn-O

Körner, Wolfgang; Urban, Daniel F.; Elsässer, Christian

doi:10.1002/pssa.201431871

Cited by 27 publications

(21 citation statements)

References 48 publications

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“…1) [22]. In the case of these amorphous TOSs, structural imperfections led to the formation of an antiparabolic shaped extra state just below the original conduction band bottom, which played an essential role in the transistor switching of TOS-based TFTs [27,28]. In the present case, the almost linear shaped extra DOS (i.e., tail states) formed just below the conduction band bottom, possibly due to an oxygen deficiency.…”

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confidence: 68%

Thermopower modulation clarification of the intrinsic effective mass in transparent oxide semiconductor BaSnO3

Sanchela

Onozato

Feng

et al. 2017

Phys. Rev. Materials

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The exact intrinsic carrier effective mass m * of a well-studied transparent oxide semiconductor BaSnO 3 is unknown because the reported m * values are scattered from 0.06m 0 to 3.7m 0 . This paper identifies the intrinsic m * of BaSnO 3 , m * = 0.40 ± 0.01m 0 , by the thermopower modulation clarification method and determines the threshold of the degenerate/nondegenerate semiconductor. At the threshold, the thermopower values of both the La-doped BaSnO 3 and BaSnO 3 thin-film transistor structures are 240 μV K −1 , the bulk carrier concentration is 1.4 × 10 19 cm −3 , and the two-dimensional sheet carrier concentration is 1.8 × 10 12 cm −2 . When the Fermi energy E F is located above the parabolic shaped conduction band bottom, the mobility is rather high. In contrast, E F below the threshold exhibits a very low carrier mobility, most likely because the tail states suppress the carrier mobility. The present results are useful to further develop BaSnO 3 -based oxide electronics. DOI: 10.1103/PhysRevMaterials.1.034603Transparent oxide semiconductors (TOSs) with a relatively high electrical conductivity and a large band gap (E g > 3.1 eV) are commonly used as transparent electrodes and channel semiconductors for thin-film transistor (TFT) driven flat panel displays (FPDs) such as liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) [1]. TOS materials include Sn-doped In 2 O 3 (ITO) and InGaZnO 4 -based oxides. Novel TOSs exhibiting higher carrier mobilities have been intensively explored since the TFT performance strongly depends on the carrier mobility of the channel semiconductor. In 2012, Kim et al. reported that a La-doped BaSnO 3 (space group P m3m, cubic perovskite structure, a = 4.115Å, E g ∼ 3.1 eV) single crystal grown by the flux method exhibits a very high mobility (μ Hall ∼ 320 cm 2 V −1 s −1 ) at room temperature [2,3]. This report inspired the current interest in BaSnO 3 films and BaSnO 3 -based TFTs [4][5][6][7][8][9].Since the mobility is expressed as μ = eτ m * −1 , where e, τ , and m * are the electron charge, carrier relaxation time, and carrier effective mass, respectively, the high mobility of the Ladoped BaSnO 3 single crystal should be due to both a small m * and a large τ . Generally, the τ value of epitaxial films is smaller than that of the bulk single crystal due to the fact that the carrier electrons are scattered at dislocations, which originated from the lattice mismatch (δ) and at other structural defects, in addition to optical phonon scattering. The estimated misfit dislocation spacing d is 7.4 nm because δ between BaSnO 3 and SrTiO 3 (a = 3.905Å) is +5.3%. We hypothesized that the experimental values include errors since the substrate contributes to the optical spectra of the BaSnO 3 films. To overcome this difficulty, we use the thermopower S modulation clarification method to determine the intrinsic m * of BaSnO 3 as S clearly reflects the energy derivative of the density of states (DOS) at the Fermi energy E F . This study measures the S of La-doped BaSnO 3 ...

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mentioning

confidence: 68%

Thermopower modulation clarification of the intrinsic effective mass in transparent oxide semiconductor BaSnO3

Sanchela

Onozato

Feng

et al. 2017

Phys. Rev. Materials

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“…These tail states [73,75,87,91] originate from the charge localization at undercoordinate oxygen atoms (with a coordination number 3 or below; c.f., Figure 1e) and contribute to the optical absorption within the visible range, that is, at 2-3 eV (Figure 3c). The degree of localization and the energy location of the tail states with respect to the valence band edge depends strongly on the oxygen/metal ratio (as discussed in more details later in this section) and on the cation composition (see Section 5.4).…”

Section: Carrier Generation and Defect Formation In Aossmentioning

confidence: 99%

“…The research area of transparent conducting oxides (TCOs) dates back to 1907 when CdO was reported to combine both optical transparency in the visible range and good electrical of amorphous indium oxide appeared in 2009, [61] followed by models of electron transport in multi-cation AOSs, [62][63][64][65][66][67] DFT calculations of defect formation, [68][69][70][71][72][73][74][75] and statistical descriptions of amorphous network. [76][77][78] However, several key questions regarding the nanostructure and morphology, crystallization, carrier generation, and conductivity mechanisms in AOSs remain unanswered and require a unified theoretical framework capable of handling all these aspects in tandem.…”

Section: Introductionmentioning

confidence: 99%

Recent Advances in Understanding the Structure and Properties of Amorphous Oxide Semiconductors

Medvedeva

Buchholz

Chang

2017

Adv Elect Materials

138

144

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conductivity. [1] Following the discovery of SnO 2 with a similar unique combination of properties, [2] several patents were filed in the 1940s to employ TCOs as antistatic coatings and transparent heaters-long before the discovery of the now well-known Sn-doped In 2 O 3 (ITO) and Al-doped ZnO, [3] widely employed as flat panel display electrodes in the past decades. Despite great technological demand for TCOs [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and extensive experimental efforts to improve the conductivity via impurity doping, [21,22] to tune the work function and carrier concentration via cation composition, [23][24][25][26][27][28] to achieve two-dimensional transport via heterointerfaces, [29] and to p-dope the oxides toward active layers of transparent electronics, [30][31][32] theoretical understanding of these fascinating materials has lagged behind significantly. The first electronic band structure of ITO was calculated in 2001; [33] the role of native defects in prototype TCOs was understood after 2002; [34][35][36][37] the properties of multi-cation TCOs were first considered in 2004 [37][38][39][40][41][42] followed by modeling of novel TCO hosts [43,44] and spin-dependent transport in transition-metal-doped TCOs; [45] the nature of the band gap in In 2 O 3 was clarified in 2008; [46] and a first highthroughput search for p-type TCOs was performed in 2013. [47] Complex oxides that consist of multiple post-transition metals, such as InGaZnO 4 , have recently become competitive with silicon as the active transistor layer to drive arrays of pixels in large area displays. [9,[13][14][15][16][17][18][19]24] As the billion-dollar display industry moves forward, the amorphous phase of the complex oxides is favored both for flexible and high-resolution display applications. [13][14][15][16][48][49][50][51][52][53][54][55][56][57][58][59] The unique properties of AOSs were first demonstrated in 1990, [60] and the research area has been growing exponentially since then. Unlike Si-based semiconductors, AOSs were shown to exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. [48][49][50][51][52][53][54][55][56][57][58] Table 1 summarizes the key physical properties of best-performing crystalline TCOs and AOSs; the differences (or the lack thereof) between the two will be discussed in detail in the respective sections below.

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“…Thus, the physics behind such a system should be identical. Since metallic agglomerates are in fact the intrinsic n-type dopants of ZTO [40] (which corresponds to oxygen vacancies in the more common language employed by the scientific community) the valence change mechanism maintains its validity. The observed 1D RS can be explained by conventional filament formation from the oxygendeficient region close to the titanium top electrode toward the platinum bottom electrode.…”

Section: Wwwadvelectronicmatdementioning

confidence: 99%

2D Resistive Switching Based on Amorphous Zinc–Tin Oxide Schottky Diodes

Branca

Deuermeier

Martins

et al. 2019

Adv Elect Materials

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of less destructive switching events, since the device reliability is independent of the reproducibility of the filament formation. [6] Frequently, an analog switching is observed, which is applicable for neuromorphic systems. [7][8][9][10] Reports exist in the literature, which explore area-dependent switching involving the homogeneous migration of defects or ions through the thickness of the switching matrix. [11,12] This can be referred to as 3D RS. In a device with asymmetric electrodes, both 1D RS and 3D RS result in counter eightwise (c8w) switching loops, when the bias is applied to the current blocking electrode. [13] Another type of area-dependent RS exists when ions are exchanged (or trapping occurs at the interface) between the switching matrix and the electrode. If the current transport through this interface is modulated as a consequence of the ion exchange/trapping, then this can be referred to as 2D RS. The direction of the switching loops is opposite to 1D RS and 3D RS, that is, eightwise (8w) when the bias is applied to the current blocking electrode. Frequently, 2D volatile switching has been observed as a secondary process, affecting the high resistive state (HRS) of an otherwise 1D RS device. [14][15][16][17] In addition, the choice between digital and analog operation modes by different device initialization schemes has been reported for NiO-based devices. [18] However, the resistance window for the analog mode was below one order of magnitude and the mechanism was discussed as 3D RS.Both in 3D RS and 2D RS, the active interface represents a barrier to the current flow, for example, a Schottky junction. [19] In the case of Ti/Pr 0.7 Ca 0.3 MnO 3 (PCMO) devices the underlying mechanism is based on a redox reaction between the titanium oxide interlayer and the PCMO. [20,21] In oxide-based devices with a platinum Schottky barrier bottom electrode, electronic trapping at interface states was proposed as a mechanism. [15] To allow a sizeable data retention, structural stabilization of the trapped charge needs to occur. [22] To achieve a high ratio between HRS and low resistance state (LRS) in 3D and 2D RS, a highly mobile Fermi level, that is, a highly variable charge carrier concentration is desirable. In thin-film transistors (TFT), AOS are known to yield tremendously high on/off ratios. [23] Diodes of AOS have rectification ratios of 10 6 . [24] These applications show how the material is able to change from an insulator to a metallic conductor by A room-temperature-processed resistive switching Schottky diode that can be operated in two distinct modes, depending solely on the choice of device initialization mode, is presented. Electroforming in the diode's reverse polarity leads to an abrupt filamentary switching with inherently long data retention at the expense of rectification. After this electroforming process, the devices may work in either a bipolar or unipolar manner with a resistance window of at least two orders of magnitude. Device initialization in the forward direction shows a smoo...

show abstract

Generic origin of subgap states in transparent amorphous semiconductor oxides illustrated for the cases of In-Zn-O and In-Sn-O

Cited by 27 publications

References 48 publications

Thermopower modulation clarification of the intrinsic effective mass in transparent oxide semiconductor BaSnO3

Thermopower modulation clarification of the intrinsic effective mass in transparent oxide semiconductor BaSnO3

Recent Advances in Understanding the Structure and Properties of Amorphous Oxide Semiconductors

2D Resistive Switching Based on Amorphous Zinc–Tin Oxide Schottky Diodes

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