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`2 1 Introduction
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`1.2 Pre-history 3
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`sz.A441)“:
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`ent electronics ‘killer apps‘ are admittedly either not yet well-defined or
`are presently unrealizable due to current limitations in transparent electron-
`ics or in a requisite auxiliary technology. However, this topical ordering
`inversion is meant to be intentionally provocative. Since transparent elec—
`tronics is a nascent technology, we believe that its development will be
`most rapidly and efficiently accomplished if it
`is strongly application—
`driven, and if it is undertaken in a parallel fashion in which materials, de—
`vices, circuits, and system development are pursued concurrently. Hope-
`quy, such a product-driven concurTent development strategy will lead to
`rapid technology assessment, the identification of new and most-likely un—
`expected applications, and an expeditious commercial deployment of this
`technology.
`
`1.2 Pro-history
`
`Two primary technologies which preceded and underlie transparent elec-
`tronics are briefly overviewed. These topics are transparent conductive
`oxides (TCOs) and thin-film transistors (TFTs).
`
`1.2.1 Transparent conductlng oxldes (TCOs)
`
`TCOs constitute an unusual class of materials possessing two physical
`properties - high optical transparency and high electrical conductivity -
`that are generally considered to be mutually exclusive (Hartnagel et al.
`1995). This peculiar combination of physical properties is only achievable
`if a material has a sufficiently large energy band gap so that it is non-
`absorbing or transparent to visible light, i.e., > ~11 eV, and also possesses
`a high enough concentration of electrical carriers, i.e., an electron or hole
`concentration > ~10” cm'], with a sufficiently large mobility, > ~l cm2 V~
`‘s", that the material can be considered to be a ‘good’ conductor of elec-
`tricity.
`
`The three most common TCOs are indium oxide ln203, tin oxide SnOz,
`and zinc oxide ZnO, the basic electrical properties of which are summa—
`rized in Table l.l. All three ofthese materials have band gaps above that
`required for transparency across the full visible spectrum.
`
`Note that although the TCOs listed in Table 1.1 are considered to be
`‘good’ conductors from the perspective of a semiconductor, they are actu-
`ally very poor conductors compared to metals. For example, the conduc—
`
`tivities of tungsten W, aluminum Al, and copper Cu, are approximately
`100,000, 350,000, and 600,000 S cm], indicating that the best In203 conv
`ductivity (for indium tin oxide or ITO) is about a factor of 10 to 60 lower
`than that of a typical integrated circuit contact metal. The low conduc-
`tance of TCOs compared to metals has important consequences for both
`TCO and transparent electronics applications, some of which are explored
`in this book. The theoretical absolute limit of the conductivity for a TCO
`has been estimated to be 25,000 S cm'‘ (Bellingham 1992).
`Table 1.1. Electrical properties of common transparent conducting oxides
`(TCOs) Conductivitiea reported are for best-else polycrystalline films.
`Material
`Bandgnp
`Conductivity
`Electron
`Mobility
`(eV)
`(S cm")
`concentration
`(cml V"
`
`......._
`my
`__
`_
`<ch
`s“)
`tnzoJ
`3.75
`10,000
`>10 '
`35
`ZnO
`3.35
`>10“
`20
`3,000
`3.6SnO; 15 5,000 >1020
`
`
`
`
`
`Returning to Table l.l, notice that all three of the TCOs included in this
`table are n-type, i.e., conductivity is a consequence of electron transport,
`and that the electron carrier concentration is strongly degenerate, i.e., the
`electron density exceeds that of the conduction effective band density of
`states by an appreciable amount (Pierret 1996; Sze and Ng 2007). All of
`the well-known and commercially relevant TCOs are n-type. p-type TCOs
`are a relatively new phenomenon and their conductivity performance is
`quite poor compared to that of n-type TCOs. To a large extent, the poor
`conductivity of p-type TCOs is due to the very low mobility of these mate-
`rials, typically less than ~l cm2 V‘s", compared to mobilities in the range
`of~10-40 cm2 V's" for n-type TCOs.
`
`The n—type mobilities indicated in Table Ll are quite small compared to
`those representative single crystal silicon materials and devices, which
`range from ~250-1,500 cm2 V's". However, this mobility comparison be-
`tween TCOs and single crystal silicon is a bit misleading since single crys-
`tal silicon mobility is not usually specified at doping concentrations as
`large as those typical of TCOs.
`In fact,
`it is reported that single crystal
`silicon mobility is independent of doping concentration above ~10” cm},
`with an electron mobility of ~90 cm2 V's" and a hole mobility of ~50 cm2
`V'Is" (Baliga [995). A low mobility at high carrier concentrations is, to a
`large extent, a consequence of intense ionized impurity scattering associ-
`ated with high doping concentrations (Hannagel et al. [995).
`
`
`
`SEL 2004
`Bluehouse v. SEL
`|PR2018—01405
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`SEL 2004
`Bluehouse v. SEL
`IPR2018-01405
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