Amorphous Oxide Semiconductor: Factors Determining TFT Performance and Stability

26-2 / T. Kamiya

• IMID 2009 DIGEST

Abstract

Amorphous oxide semiconductors (AOSs) are expected as new channel materials in TFTs for large-area and/or flexible FPDs, and several prototype displays have been demonstrated in these five years since the first report of AOS TFT. In this paper, we review fundamental materials science of AOSs that have been clarified to date in connection with operation characteristics of AOS TFTs.

1. Introduction

In late 2004, we reported that amorphous InGaZnO4 (a-IGZO) produces high mobility, flexible and transparent thin-film transistors (TFTs) [1]. Now, these families of oxide semiconductors are called amorphous oxide semiconductors (AOSs) and studied for TFT back planes in flat-panel displays (FPDs). As seen in Fig. 1, AOS TFTs exhibit superior characteristics such as field-effect mobilities ~ 10 cm2_{/Vs [2], low operation voltages < 5 V, low} off-current, and no inversion operation even if a simple metal electrodes are used for source / drain contacts. In this paper, we show that these features benefit from ionic chemical bonds in AOSs and defect structure specific to the oxides (more details will be found in reviews [3,4]).

2. Results and Discussion

We propose a guiding principle to explore AOS with a large electron mobility in refs. [5,6]. That is, because the electron transport paths (i.e. the conduction band minimum, CBM) is mainly made of spherical s orbitals of metal cations in typical oxides (Fig. 2(a)), incorporation of heavy metal cations with

large principle quantum numbers n ≥ 5 produces good oxide conductors, which is actually the case for a representative transparent conducting oxide (TCO), tin-doped indium oxide (ITO). This CBM structure leads to an important feature of oxide conductors. The overlap of the spherical CBM wave functions is not affected largely by disordered structures (Fig. 2(b)), and therefore the CBM electronic structure in AOS is similar to that in a corresponding crystalline oxide. It is confirmed by the small electron effective mass of a-IGZO, ~0.34 me [7], which is similar to those of ITO and ZnO. This view of the electronic structure is confirmed by first-principles calculations based on density functional theory (DFT) as seen in Fig. 2(c), which supports the above conclusion that the CBM in a-IGZO is mainly made of In 5s orbitals [8].

0 5 10 15 0 50 100 150 200 -5 0 5 10 10-14 10-13 10-12 10-11 10-10 10-09 10-08 10-07 10-06 10-05 10-04 VDS=2,4,6,8,10V S = =0.12 V/dec DS I d dV log VGS(V) VDS(V) VGS=0–15V IDS (µ A) IDS (A ) (a) (b)

Fig. 1. Typical (a) output and (b) transfer characteristics of a-IGZO TFT. It exhibits a large

µsat of 11.8 cm2_{/Vs and low operation voltages < 5 V}

owing to the S value as small as 0.1 V/dec.

We think the schematic electronic structure in Fig. 3 summarizes the specific features of AOSs. We have studied them mainly for a-IGZO by several methods as explained in the following.

Amorphous Oxide Semiconductor:

Factors Determining TFT Performance and Stability

Toshio Kamiya

, Kenji Nomura

and Hideo Hosono

1_{Materials and Structures Laboratory, Tokyo Inst. Tech., Yokohama 226-8503, Japan} Tel.:81-45-924-5357, E-mail: [email protected]

2_{ERATO-SORST, Japan Science and Technology Agency, Yokohama 226-8503, Japan} 2_{Frontier Research Center, Tokyo Inst. Tech., Yokohama 226-8503, Japan}

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IMID 2009 DIGEST •

(a) (b)

(c)

Fig. 2. Characteristic electronic structure of AOS. Illustration of electron transport paths in (a) crystalline and (b) amorphous oxides, which are made of unoccupied spherical s orbitals of metal cations. (c) Conduction band minimum wave function (electron transport path) of a-IGZO.

Log (DOS) En er gy (e V ) CB VB Tauc gap ~ 3.2 eV VB tail Mobility edge Mobility edge ∆E=5–20 meV tail (E_u~ 80 – 150 meV, N_total~ 1017_cm-3₎ Deep traps (~2×1016_cm-3_/eV)

Deep states (>5×1020_cm-3₎ E_center 30–100 meV width ~1.5 eV E_D= 0.1–0.15 eV For as-deposited

Fig. 3. Schematic electronic structure in a-IGZO.

Optical analyses revealed that a-IGZO films have optical bandgaps of Eg = 3.0 – 3.2 eV [9] (Fig. 4(a)), and that optical absorption spectra exhibit

Urbach-type tail states below Eg with a somewhat large

Urbach energy of Eu = 0.1 - 0.15 eV. We observed

that a-IGZO have rather large subgap density of states (DOS) near the balance band maximum (VBM) as seen in x-ray photoemission spectra (XPS) in Fig. 4(b) [10], which substantiates that the large Eu in the optical absorption spectra come from the subgap DOS above VBM. Due to the large subgap DOSs, which pin the Fermi level in the negative gate bias operation, AOS TFTs have not operated in p-channel inversion operation to date, which also explains why the low off-current is obtained although the source / drain contacts are made of a simple metal / a-IGZO structure.

Device simulation [11] and the capacitance-voltage method [12] provided subgap DOS in the

vicinity of the CBM (Fig. 5). These showed that a-IGZO have more than two orders of magnitude smaller subgap DOS compared to a-Si:H. The low subgap DOSs give small subthreshold voltage swings (S) of ~0.1 V/dec as seen in Fig. 1(b), which is responsible for the low operation voltages of AOS TFTs. 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 3.0 3.5 4.0 4.5 5.0 5.5 Photon energy / eV 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 1 2 Photon energy / eV ε2 ε1 LQ-ann HQ-as LQ-as HQ-ann c-IGZO c-IGZO LQ-ann LQ-as HQ,as HQ-ann HQ-as LQ-ann LQ-as HQ-ann (a) (b) EVBM= 3.3 eV EVBM= 3.2 eV EVBM= 3.2 eV EVBM= 3.1 eV EF VBM

Fig. 4. Dielectric functions ε1 + jε2 of crystalline

InGaZnO4 (c-IGZO) and a-IGZO films.

0 0.2 0.4 0.6 0.8 1016 1017 1018 1019 1020 EC - E / eV DO S / cm /e V -3 a-Si:H annealed (0.013-0.102 eV) TCAD, annealed depletion type (Eu=0.14 eV) enhancement type (Eu=0.08 eV)

C-V, unannealed (0.013-0.102 eV)

Fig. 5 Subgap DOSs obtained by device simulations (TCAD) and C-V method.

The low subgap DOS (defect states) would be explained by the ionic nature of chemical bonds in oxides. As illustrated in Fig. 6(a), the CB and VB of Si is made of anti-bonding and bonding states of sp3 hybridized orbitals, respectively. If a Si vacancy is generated, a dangling bond state is formed near the middle of the bandgap and is occupied by one electron, which causes both electron trap and hole trap, and consequently works as a pinning center and deteriorates operation characteristics of both n-channel and p-n-channel TFTs. This is why the Si dangling bonds must be passivated by e.g. hydrogen (Fig. 6(b)). By contrast, CB in oxide is mainly made of unoccupied metal s orbitals and VB of O 2p; therefore, a dangling bond generated by an oxygen

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• IMID 2009 DIGEST

vacancy (VO) is formed in or near the CBM and does not work as an electron / hole trap; this is thought to be the case for TCOs (as illustrated in Fig. 6(c)). Note that such donor states are not stable in many oxides (even in a representative TCO, ZnO); but even in such cases, the stabilized state forms a fully-occupied state as illustrated in Fig. 6(d), which does not trap an electron anymore and inactive for electron transport and n-channel TFTs. Actually, we observed large subgap DOSs above VBM in Fig. 4(b), but these TFTs (especially HQ-as/-ann and LQ-ann TFTs) exhibit good performances such as field-effect mobilities ~ 10 cm2_/Vs. Si 3p Si 3s _VB CB VSi Si 3p Si 3s VB CB H H 1s VO M ns Mm+ VB CB VO Mm+ VB CB Relaxation (a) (c) (b) (d)

Fig. 6. Schematic electronic structures of defects in (a,b) Si and (c,d) oxides. Closed circles show occupied states and open circles unoccupied states.

Another important feature is observed in the subgap DOS for an unannealed a-IGZO TFT (‘C-V, unannealed’ in Fig. 5); i.e., an extra Gaussian-type DOS exists at ~0.2 eV below CBM, while it is

annihilated by thermal annealing at 400o_C

(‘annealed’). The annealing process is examined by in-situ DC conductivity measurements shown in Fig. 7

[13]. It shows that thermal annealing in N2

continuously generates free electrons and increases

the conductivity, suggesting that the N2 annealing

generates donor defects in the whole temperature range from room temperature to 400o_{C. By contrast,} O2 annealing decreases the conductivity at > 300oC, indicating that the oxidizing atmosphere efficiently reduces the donor defects. This result is consistent with the fact that the operation characteristics, uniformity and stability of AOS TFTs are improved

significantly by thermal annealing at ≥ 300o_{C. In}

addition, we found that an appropriate addition of water vapor to the annealing O2 atmosphere further improves the TFT characteristics and their uniformity, which is explained by stronger oxidation power of H2O than that of O2. It is also found that dry and wet O2 annealing improves the long-term stability [14].

Fig. 7 In-situ measurements of DC conductivity

during thermal annealing in dry O2, wet O2, and

dry N2 atmospheres. 1015 1016 1017 1018 1019 1020

1

10

Carrier density / cm

Mo

b

il

it

y /

c

m

/V

s

10

₁₀

₁₀

₁₀

₁₀

Fig. 8 Relationship between Hall mobility and

carrier density for c-InGaZnO4 (IGZO1),

c-InGaO3(ZnO)5 (c-IGZO5), HQ and LQ a-IGZO.

Another interesting, but strange feature of AOSs is the peculiar behavior of electron mobility µe; i.e. µe increases with increasing the carrier density Ne (Fig. 8), which is opposite to crystalline semiconductors. It is explained by the existence of distributed potential barrier above the mobility edge as illustrated in Fig. 2 [15]. The analyses with a percolation conduction model revealed that the potential barriers are characterized by the average barrier height of Ecenter = 30 – 100 meV and the distribution width ∆E of 5 – 20 meV, which explains well the temperature dependence of Hall mobility. This analysis also gives

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IMID 2009 DIGEST •

a donor level of ~0.11 eV at the donor density of 9.0×1015 _cm-3_{. The donor level is shallower than those} in c-IGZO1, which is expressed roughly by Ed = Ed0 - αND1/3 _{with Ed0 = 0.22 eV and α = 2.3×10}-7 _eV⋅cm. The shallower Ed would be related to the smaller Eg.

-3 -2 -1 0 1 2 3 0 50 100 150 200 250 300 350 400 450 Energy / eV Total DOS c-IGZO a-IGZO VOI H EVBM ×1/4 ×1 ECBM EF V_OII

Fig. 9. DOSs of a-IGZO with various defects

calculated by DFT. EF > ECBM indicates that the

defect works as a shallow electron donor. [9,16]

As for defect states and electron doping, DFT calculations have revealed that an oxygen deficiency works as both a deep electron trap (‘VO I’ in Fig. 9) and a shallow electron donor (‘VO II’) [16]. It depends on the local atomic configuration of the oxygen deficiency. That is, if an oxygen deficiency is formed with a large free space, the free space forms an extra electronic state and traps two electrons. While, if such a large free space is not formed, an extra electronic state is not formed in the bandgap, and therefore the oxygen deficiency does not work as an electron trap and generates free electrons. DFT calculations also tell as that incorporation of hydrogen always forms OH bonds and ionizes the hydrogen atom, which consequently releases an electron to the CB and causes electron doping (‘H’). These results indicate that we should consider oxygen deficiency and

hydrogen doping (e.g. from H2 and H2O molecules

existing in the deposition and annealing atmospheres) as an origin of electron doping, while oxygen deficiency not always generates free electrons but can also form a deep fully-occupied state as observed in the XPS spectra in Fig. 4(b).

4. Summary

We have reviewed the origins of the unique

carrier transport properties and superior performances of AOS and their TFTs, which benefit from the strong ionicity of AOSs.

Until the last year, prototype FPDs have been reported by Toppan, LGE and Samsung group. More recently, more companies have joined the R&D of AOS technology.

5. References

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3. T. Kamiya, and H. Hosono, Asia Mater. (2009) submitted.

4. T. Kamiya, K. Nomura, and H. Hosono, J. Display Technology (2009) in print.

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8. K. Nomura, T. Kamiya, H. Ohta, T. Uruga, M. Hirano, and H. Hosono, Phys. Rev. B, 75, p. 035212 (2007).

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16. T. Kamiya, K. Nomura, M. Hirano, and H. Hosono, phys. stat. solidi (c), 5, p. 3098 (2008).