InGaZnO4 heterojunctions

Vacuum 136 (2017) 137e141

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Band offsets in sputtered Sc2O3/InGaZnO4 heterojunctions David C. Hays a, B.P. Gila a, S.J. Pearton a, *, Ryan Thorpe b, F. Ren c a

Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA c Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 November 2016 Received in revised form 27 November 2016 Accepted 1 December 2016 Available online 1 December 2016

We measured the band offsets of sputtered Sc2O3 on thin film InGaZnO4 (IGZO) using X-Ray Photoelectron Spectroscopy and obtained the bandgaps of the materials using reflection electron energy loss spectroscopy and UV/Vis absorption. The valence band offset was determined to be
1.33 eV ± 0.13 eV for Sc2O3 on sputtered IGZO (bandgap 3.16 eV). The conduction band offset for Sc2O3/IGZO was then determined to be 4.07 eV. The Sc2O3/IGZO system has a staggered, type II alignment, meaning that it is not suitable for thin film transistors but it may still be useful for surface passivation to prevent the commonly observed changes in conductivity of the IGZO resulting from atmospheric exposure. © 2016 Elsevier Ltd. All rights reserved.

Keywords: IGZO Band offset X-ray photoelectron spectroscopy

1. Introduction InGaZnO4 (IGZO) is an n-type conducting oxide with excellent optical transparency and high electron mobility even in the amorphous state, making it of obvious interest for flexible transparent thin film transistors (TFTs) [1e22]. Despite very impressive device demonstrations, many IGZO TFTs still exhibit bias and illumination instabilities that result from mechanisms such as trapping at the aIGZO/dielectric interface or conductivity changes in the IGZO resulting from exposure to water vapor or hydrogen [1,18,19]. Thus these devices require both high quality gate dielectrics that also ensure adequate carrier confinement in the channel, as well as surface passivation. To prevent injection of electrons (and holes during illumination under bias), the valence and conduction band offsets of the gate dielectric with the IGZO should optimally be of the order of 1eV [23]. Towards this goal, a large number of dielectrics for IGZO TFTs have been demonstrated [24e50], with the most common based on SiO2, Al2O3, AlN, ZrO2 and HfO2. A variety of band alignments have been reported, including both nested (type I) and straddling (type II) [41e49]. Even if a particular dielectric cannot be used for the gate, it may be a suitable surface passivation layer, which is particularly important for IGZO-based TFTs because of their sensitivity to surface instabilities [18]. Given this need for stable passivation layers on IGZO-based

* Corresponding author. E-mail address: [email protected]fl.edu (S.J. Pearton). http://dx.doi.org/10.1016/j.vacuum.2016.12.001 0042-207X/© 2016 Elsevier Ltd. All rights reserved.

devices, a potential candidate is Sc2O3, which has proven very successful with GaN-based devices, where it provides a low interface state density (~1011 eV
1 cm
2) [51e54] and is effective in reducing the effect of surface states on current collapse in AlGaN/ GaN High Electron Mobility Transistors (HEMTs). In that case, the Sc2O3 can be used simultaneously as a gate oxide and as a surface passivation layer. The Sc2O3 has high environmental stability, a high dielectric constant [14] and reasonable bandgap (5.9e6.3 eV) [51e53]. In this paper, we report on the determination of the band offsets in the Sc2O3/IGZO system and find it has a staggered type-II alignment. This precludes its use as a gate dielectric on TFT structures but it may still be an effective passivation material. 2. Experimental The Sc2O3 was deposited by RF magnetron sputtering on IGZO and quartz substrates at room temperature using a 3-in. diameter target. The RF power was 125W and the working pressure was 5 mTorr in a pure Ar ambient. We produced both thick films (100 nm) as well as thin films (3 nm) of Sc2O3 on IGZO. The films are all amorphous. Higher deposition temperatures or post-deposition annealing are undesirable because the IGZO is intended for low temperature applications on substrates such as plastic, tape and paper. We did observe small concentrations of metal contaminants (Cr (<5%) and Ti (<2%) which originate from cross contamination in the sputtering system because the deposition rates are very low (~0.1 nm per minute), leading to very long deposition times for the

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thick films. The IGZO was deposited in the same system by sputtering on both Si and quartz by RF magnetron sputtering using a 3in. diameter single target of InGaZnO4. The RF power was 150 W, while the working pressure was constant at 5 mTorr in a pure Ar ambient. Note that the samples were not exposed to air prior to the subsequent X-Ray Photoelectron Spectroscopy (XPS) measurements to avoid complications from surface contamination. The latter may lead to less accurate band gap measurements when using reflection electron energy loss spectroscopy [1,27]. To obtain the valence band offsets, X-Ray Photoelectron Spectroscopy (XPS) survey scans were performed to determine the chemical state of the InGaZnO, Sc2O3 thick films and 3 nm Sc2O3/ IGZO samples and identify peaks for high resolution analysis [55]. A Physical Electronics PHI 5701 LSci XPS with an aluminum x-ray source (energy 1486.6 eV) with source power 300W was used, with an analysis area of 2 mm  0.8 mm and exit angle of 50 . The electron pass energy was 35.75 eV. The approximate escape depth (3l sin q) of the electrons was 80 Å. Charge compensation was performed using an electron flood gun, due to the dielectric nature of the films. The charge compensation flood gun is often not sufficient at eliminating all surface charge, and additional corrections must be performed [56]. Using the known position of the adventitious carbon (CeC) line in the C 1s spectra at 284.8 eV, charge correction was performed. A simple peak model involving a single CeC peak at 284.8 eV was insufficient to fit the spectrum and additional peaks were added and the first was constrained to 1.5 eV above the main peak and of equal FWHM. This higher binding energy peak is ascribed to alcohol (CeOH) and/or ester (CeOeC) functionality. A further high binding energy peak, attributed to OeC]O, was added with a position constraint of 3.7 eV above the main peak. All peaks were constrained to a peak area ratio of 2:1:1, based on extensive experience with the cross-calibration of unexposed versus unexposed surfaces in our system, which has an insitu XPS capability. Reflection electron energy loss spectroscopy (REELS) was employed to measure the bandgap of the Sc2O3 [57]. REELS spectra were obtained using a 1 kV electron beam and the hemispherical electron analyzer that is part of the XPS system. The full width of the elastic peak is ~0.7e0.8 eV. The bandgap of the IGZO was determined by conventional UV/Vis absorption measurements.

Fig. 1. XPS survey scans of IGZO, 3 nm Sc2O3 on IGZO and Sc2O3 thick film.

3. Results and discussion Fig. 1 shows an example of the type of XPS survey scans we employed to measure the band offsets. In this case we show the scans for IGZO, 2 nm Sc2O3 on IGZO and Sc2O3 thick films (150 nm). The spectra are consistent with past published XPS data on these materials [51e54]. The valence band maximum (VBM) was determined by linearly fitting the leading edge of the valence band and the flat energy distribution, and finding the intersection of these two lines [55,56], as shown Fig. 2. The VBM was measured to be 0.71 eV for Sc2O3 and 2.31 eV for IGZO. We used UV/Vis to determine the bandgap of IGZO. This was determined to be 3.16 eV from the absorption data, as shown in the Tauc plot at the top of Fig. 3 and this is consistent with previous reports [25e27,31]. Fig. 3 (bottom) shows the REELS spectra to determine the bandgaps of the Sc2O3 films. The band gap was determined from the onset of the energy loss spectrum [59] and was found to be 5.9 eV, which again is consistent with past reports [58,59]. There is a clear nonzero intensity in the REELS spectra in between the elastic peak and the on-set of the loss peak. This is likely a result of the metal contaminants discussed earlier. The REELS technique is a good choice for to determining bandgap for higher band gap (>5 eV) materials where there is a distinct energy

Fig. 2. XPS spectra of core levels to valence band maximum (VBM) for (top) IGZO and (bottom) Sc2O3 thick film.

D.C. Hays et al. / Vacuum 136 (2017) 137e141

139

Fig. 3. (Top) Reflection electron energy loss spectra to determine the bandgap of thick film Sc2O3, (bottom) UV/Vis absorption data to determine bandgap of IGZO.

loss transition. The difference in bandgap between the Sc2O3 films and IGZO is therefore 2.74 eV. To determine the actual band alignment and the respective valence and conduction band offsets, we examined the core level spectra for the samples. High resolution XPS spectra as shown in Fig. 4 for (top) IGZO VBM-core delta levels and (bottom) Sc2O3 VBM-core delta levels. and in Fig. 5 for IGZOeSc2O3 core delta levels were used to determine the selected core level peak positions. These values are shown in Table 1 and were then inserted into the following equation to calculate DEv [56]:

Fig. 4. XPS transitions for (top) IGZO VBM-core delta levels and (bottom) Sc2O3 VBMcore delta levels.

DEV ¼ ðECore
EVBM ÞRef $ IGZO
ðECore
EVBM ÞRef $ Sc2 O3 Sc2 O3  Sc2 O3 IGZO
ECore
ECore IGZO

Fig. 6 shows both a simplified and detailed band diagram of the Sc2O3/ZnO heterostructure. Our data shows this is a staggered, type II alignment, with a valence band offset of ~
1.33 eV and the conduction band offset is then ~4.07 eV using the following equation:

DEC ¼ EgSc2 O3
EgIGZO
DEV ; i:e: DEC ¼ 5:9eV
3:16eV
ð
1:33eVÞ ¼ 4:07eV The Sc2O3/ZnO heterostructure is therefore not suitable as a gate for TFTs where we need positive offsets in both valence and

Fig. 5. XPS transitions for IGZOeSc2O3 core delta levels.

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Table 1 Values of valence band offsets determined in these experiments (eV). Reference IGZO

Reference Sc2O3

Thin Sc2O3

IGZO metal core

IGZO VBM

Metal Core level

Metal core - IGZO VBM

Sc2O3 VBM

Sc 2p3 core level

Sc 2p3 -Sc2O3 VBM

D CL IGZO core -

Zn2p3 Ga2p3 In3d5

2.31

1021.9 1117.7 444.6

1019.59 1115.39 442.29

0.71

401.7

401.0

619.8 715.8 42.7

Valence band offset

Average offset for all core levels


1.20
1.40
1.40


1.33

Sc2p3

O

Fig. 6. Summary and detailed band diagrams for Sc2O3/IGZO.

conduction bands. However, the demonstrated stability of Sc2O3 as a passivation layer on GaN suggests it may also be useful to protect the surface of IGZO from the instabilities induced by exposure to atmosphere [18].

[3]

[4]

3.1. Summary and conclusions [5]

The Sc2O3/IGZO heterojunction was found to have a type II alignment of band offsets with a valence band offset of ~1.67 eV ± 0.16 eV and a conduction band offset determined to be 4.07 eV from XPS measurements. This means that Sc2O3 would be an effective barrier for electrons but not for holes on IGZO and would not be a good choice as a gate dielectric on transparent TFTs based on IGZO. However, it still may have application as a passivation layer to prevent exposure of the IGZO surface to hydrogen and oxygen.

[6]

[7]

[8]

[9]

Acknowledgments The work at UF was partially supported by NSF grant 1159682.

[10]

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