Composition-dependent structural and transport properties of amorphous transparent conducting oxides

Abstract

Structural properties of amorphous In-based oxides, In-X-O with X=Zn, Ga, Sn, or Ge, are investigated using ab-initio molecular dynamics liquid-quench simulations. The results reveal that

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

“…In striking contrast to crystalline TCOs where external aliovalent doping is the most viable way to generate free carriers and to control the carrier concentration over a wide range (from 10 17 to 10 21 cm −3 ), additional cations in AOSs do not act as carrier donors or acceptors. This is evident from the following: i) for amorphous In-X-O with as much as 30% substitution of X = Ga, Zn, or Sn, grown by PLD using a deposition temperature of T d = -25 °C and an oxygen partial pressure of 8 mTorr, the measured carrier concentration, n = 0.8 × 10 20 cm −3 , 1.2 × 10 20 cm −3 , or 1.6 × 10 20 cm −3 , respectively, is similar or, in the case of a-ITO, identical to that in undoped amorphous indium oxide grown under the same deposition conditions, n = 1.6 × 10 20 cm −3 ; [93] and ii) our calculated electronic band structure of stoichiometric amorphous ternary In-X-O or quaternary In-Ga-Zn-O, Zn-In-Sn-O, etc, oxides corresponds to an insulator-independent of the level of fractional substitution and the valence of added cation(s) with respect to the valence of the host cation.…”

“…Moreover, the majority (over 60%) of the introduced edge-shared connections with short M-M distances, 3.0-3.2 Å (Figure 5c) involve Sn atoms. This result signifies that tin has a tendency to promote clustering, especially at high concentration of Sn (that also limits the carrier mobility of a-ITO, [93] as discussed in Section 5.3 below). In marked contrast to Sn, the addition of both Ga and Zn not only suppresses the edge-sharing peak in the M-M distribution, but also leads to a much broader range of the corner-shared M-M pairs.…”

“…[93] The electronic structure calculations for the amorphous non-stoichiometric In-X-O with 20% of X show that the cation composition has little effect on the conduction states below the Fermi level and on the free carrier concentration: i) similar to a-IO (Figure 3), the defect state remains to be shallow as seen from the low value of the inverse participation ratio for the occupied states in the conduction band of amorphous In-X-O ( Figure 6); and ii) the Burstein-Moss (BM) shift; that is, the Fermi level shift into the conduction band, is 1.40 eV in a-ITO, 1.46 eV in a-IGO, and 1.51 eV in a-IZO, that is only slightly smaller than the shift in a-IO, 1.61 eV, with the same oxygen stoichiometry. Moreover, additional calculations for an amorphous structure with larger Ga fraction, In 1.2 Ga 0.8 O 2.96 , where a BM shift of 1.55 eV and a shallow donor state are obtained, further illustrate the weak dependence of the number of free carriers on the cation composition in AOSs with high oxygen stoichiometry (> 2.96).…”

“…It is found that the addition of Ga 2 O 3 , ZnO, or SnO 2 into the indium oxide host i) increases the local distortions of the InO polyhedra as signified by a larger variance for the first-shell In-O distances, 1.07 × 10 −2 Å 2 , 1.05 × 10 −2 Å 2 , or 1.24 × 10 −2 Å 2 in a-IGO, a-IZO, or a-ITO, respectively-as compared to σ 2 = 8.42 × 10 −3 Å 2 in amorphous indium oxide; and ii) suppresses the number of high-coordinate In atoms (the effective coordination number 5.5 or above) from 44% in amorphous indium oxide to 23% in a-IGO and a-ITO and to 9% in a-IZO (note that a single amorphous structure from an MD simulation at 300 K or from a DFT-optimized solution at 0 K tend to overestimate the In-O coordination in a-In-X-O with X = Ga or Zn as compared to amorphous indium oxide [92,93] in contrast to the time average presented here). The smallest amount of the fully coordinate In atoms in a-IZO is in accord with the highest crystallization temperature measured experimentally in In-Zn-O with 10% of zinc; namely, 630 °C.…”

“…The non-uniform charge density distribution in the conduction states of multi-cation AOSs is associated with i) different strength of the metal-oxygen bonds; ii) different M-O coordination and the degree of local distortions in the MO polyhedra; as well as iii) spatial distribution of the MO polyhedra, e.g., clustering of GaO x vs. chain formation of InO 6 -SnO 6 vs. random distribution of ZnO 4 . [93] All the above local and medium-range structural features ultimately determine the energy profile of the conduction states and are responsible for the complex nature of the carrier mobility in multicomponent AOSs.…”