Influence of sputtering conditions on room-temperature fabricated InGaZnO-based Schottky diodes

Qian Xina,b*, Linlong Yana, Lulu Dua, Jiawei Zhangc, Yi Luoa, Qingpu Wanga and Aimin Songa,b,c*

aSchool of Physics, Shandong University, Jinan, 250100, China

bSuzhou Institute of Shandong University, Suzhou, 215123, China

cSchool of Electrical and Electronic Engineering, University of Manchester, Manchester, M13 9PL, United

Kingdom

Corresponding author: xinq@sdu.edu.cn (Qian Xin) and A.Song@manchester.ac.uk (Aimin Song)

Tel: +86-531-88363606

AbstractIndium gallium zinc oxide (InGaZnO or IGZO) has attracted much attention in recent years for flexible and

transparent electronics, because of its superior electric properties, optical transparency and low processing

temperature. In this work, Schottky diodes with a structure of Pd/IGZO(100 nm)/Ti/Au were fabricated by radio-

frequency magnetron sputtering at room temperature without any thermal treatment. The relationship between

different IGZO deposition parameters and the forward and reverse current-voltage characteristics of the diodes was

systematically studied. We have experimentally revealed that an increase in the oxygen partial pressure during the

sputtering process can effectively improve the barrier height of these diodes. However, either high oxygen partial

pressure or high sputtering power leads to a deterioration of the Schottky interface quality and thus an increased

ideality factor and a decreased rectification ratio. By using relatively low sputtering power (≤ 70 W) and low oxygen

content of sputtering atmosphere (~ 2.5%), high-performance diodes have been achieved with high rectification ratio,

low ideality factor, and high barrier height of 1.3 105, 1.14, and 0.73 eV, respectively. All diodes with IGZO layer

sputtered in oxygen-argon atmosphere show a high reversed breakdown voltage ~ -6V.

Keywords: Indium gallium zinc oxide (InGaZnO or IGZO), Schottky diodes, radio-frequency magnetron sputtering

1. Introduction

Indium gallium zinc oxide (InGaZnO or IGZO), one of the most promising oxide semiconductors, has

attracted great attention because of the possibility of uniform deposition over a large area by the

conventional sputtering method at room temperature, high electron mobility (> 10 cm2/Vs), excellent

optical transparency in the visible light region, and long-term stability under bias stress. Recent analysis

showed that replacement of amorphous silicon by IGZO in the backplane driver of LCD displays could

offer an impressive improvement in display power efficiency (by 57%) and contrast (by 23%). To date,

extensive efforts have been paid on IGZO-based transistors, but research on IGZO-based diodes is yet

very limited, although diodes are basic building blocks in most circuits and microwave communications

[1-5]. Up to present, most of Schottky diodes are based on silicon, III-V compounds, SiC, etc. rather than

on metal oxide semiconductors despite their outstanding advantages of flexibility and low cost. One of

the main reasons is the challenge to form stable and high-quality Schottky junctions. For example, metal-

oxide surface is very sensitive to the process conditions, such as the oxygen partial pressure and RF

power during the sputtering process. Yet, the limited studies so far already showed a great potential to

realize high-performance IGZO diodes with large Schottky barrier height [6-8], high rectification ratio [8-

10], small ideality factor [6, 11], high frequency operation [8, 12, 13], low frequency noise comparable to

that of silicon based Shottky diodes [14], and mechanical flexibility [6, 11, 13]. The influence of the

electrode metal, interface oxygen treatment, thermal treatment, and thickness of the active layer on the

performance of the IGZO-based Schottky diodes have been studied [6-9, 11, 15]. However, no systematic

study on the effects of sputtering conditions, such as RF power and oxygen partial pressure, during

IGZO-deposition on the diode performance has yet been reported. Studies on the breakdown behavior of

IGZO-based Schottky diodes are also very limited [7, 11], although Schottky diodes are often subject to a

much larger reverse bias than forward bias in applications, e.g. as a front-end rectifier in an radio

frequency identification card.

Here, we have systematically investigated the impacts of the key processing parameters of IGZO

deposition, including RF sputtering power and oxygen partial pressure (PO2) during the IGZO deposition

process, on both the forward current-voltage (I-V) characteristics and reverse-bias breakdown behavior of

the Pd/IGZO Schottky diodes. The experiments revealed that depending on the detailed deposition

conditions, the device performance may vary significantly and even by orders of magnitude: ideality

factor from 1.14 to 1.58, barrier height from 0.49 to 0.86 eV, rectification ratio from 7.9 101 to 1.3

105, series resistance from 10 to 2430 Ω. These results are analyzed and discussed mainly in terms of the

effects on the barrier interface. All devices were fabricated on glass substrate but at room temperature

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with no thermal treatment carried on all these diodes, so that the results are relevant to possible

applications on flexible substrates.

2. Experiment

The diode fabrication started with the electron-beam-evaporation of a 50-nm Pd anode on a glass

substrate (Corning, 7059). The electrode was then treated by UV-ozone (10 min, ProCleanerTM ) in order

to ensure good oxygen stoichiometry and low defect density at the Pd/IGZO interfaces and hence to

obtain a stable and high-quality Schottky barrier [6, 7]. 100-nm IGZO thin films were deposited by RF

magnetron sputtering at room temperature with a polycrystalline IGZO target (In:Ga:Zn = 1:1:1 in atomic

ratio, Lesker Co.). Various RF sputtering powers (50, 70, 90, 110, 130 W) and oxygen partial pressures

[PO2 = oxygen/(oxygen + argon) = 0, 2.5, 5, 10, 15%] were applied to determine their influence to the

device performance. The flow rate of the argon and oxygen gas mixture supply was fixed at 20 sccm with

a working pressure of 3.3 mTorr. Ti(40 nm)/Au(40 nm) was used as the diode top ohmic contact, which

was formed by electron-beam evaporation at room temperature through a shadow mask. The size of the

top electrode was 9.45510-4 cm2. The cross section of the prepared Schottky diodes is shown in the inset

of figure 1(a). The I-V measurements of the Schottky diodes were performed using an Agilent 2902

source meter.

3. Results and discussion

The I-V curves of the fabricated Schottky diodes are analyzed using the standard thermionic emission

theory of majority carriers over the junction barrier [16]:

I=I S {exp[ q (V−I R s )nkT ]−1}

(1)

I S=A A¿T2 exp (−ΦB

kT)

(2)

where Is is the saturation current, A is the contact area, A* is the effective Richardson constant, ΦB is

the effective barrier height, n is the ideality factor, k is the Boltzmann constant, q is the electron charge,

and Rs is the series resistance. For IGZO, A* has a theoretical value of 41 Acm-2K-2, as calculated from the

relation of A* = 4qm*k2/h3 = 120m*/me by using m* = 0.34 me [17]. Here, m* is the effective mass of the

electron carrier, h is Plank constant, and me is the mass of free electron. By fitting the forward I-V

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characteristics with equations (1) and (2), n and ΦB can be extracted.Rs can be estimated from the linear

differential dV/dI of I-V curves at +1 V where the current is mainly limited by the series resistance.

3.1 InfluenceofRFsputteringpower

Figure 1. (a) The current density-voltage (J-V) characteristics of the Schottky diodes with the IGZO layer sputtered at various RF sputtering powers and an oxygen partial pressure of 2.5%. Inset is the schematic of the diode structure. (b) The dependences of the ideality factor (n) and the Schottky barrier height (B) on the RF sputtering power.

RF sputtering power plays an important role to the quality and the conductivity of the IGZO thin

films. The impact of RF power on the current density-voltage (J-V) characteristics of the as deposited

Schottky diodes with a100-nm IGZO layer sputtered at PO2 = 2.5% is shown in figure 1(a). As the RF

sputtering powers increases, nincreases and the on current decreases, while the off current is essentially

the same for different sputtering powers. A higher RF power leads to stronger argon and oxygen plasma

bombardment, resulting in a higher film growth rate but deteriorated interface and film quality with more

stress and strain defects. This causes the increased ideality factor at high sputtering powers. Stronger

oxygen plasma bombardment is also expected to reduce the density of oxygen vacancies in the IGZO

film, resulting a lower carrier concentration and hence higher series resistance as shown in table 1. As

such, the on current at +1 V, which is largely determined by the series resistance, decreases with the

sputtering power as shown in figure 1(a). On the other hand, the reduced oxygen vacancies and lower

carrier concentration result in the higher barrier height as in figure 1(b) and table 1. The slight drop of

barrier height at the highest power of 130 W may be due to the deterioration of the Schottky barrier

interface and increased barrier inhomogeneity due to the high power plasma bombardment [18]. Overall,

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the experiment shows that the Schottky diodes prepared at relatively low RF sputtering powers, e.g., 50

W and 70 W, have a relatively large Ion/off (1.3 105 ~ 6.8 104 at 1 V), close-to-unit n (1.14 ~ 1.15),

and a reasonable B of 0.73 eV, as shown in figure 1(b) and table 1. In the following experiments, we

have therefore chosen the sputtering power of 70 W to deposit the IGZO film.

3.2Influenceofoxygenpartialpressure

Figure 2. (a) The J-V characteristics of the Schottky diodes with IGZO layers sputtered at various oxygen partial pressures and an RF sputtering power of 70 W; (b) The dependences of the ideality factor (n) and the Schottky barrier height (B) on the oxygen partial pressure.

Figure 2(a) shows the J-V characteristics of the Schottky diodes with IGZO layers sputtered at

various PO2 and a fixed RF sputtering power of 70 W. For the IGZO layer sputtered in a pure argon

atmosphere (PO2 = 0%), Pd and IGZO form a poor Schottky contact with a low Ion/off of only 84 at 1V,

low B of 0.48 eV, and large n of 1.7 as shown in figure 2(b) and table 1. Without oxygen during

sputtering, it is quite easy to form oxygen vacancies and very high carrier concentration within the IGZO

layer, making it difficult to form high quality Schottky contacts [16]. In contrast, diodes with IGZO

sputtered in mixed argon and oxygen atmosphere show good rectification property, and both n and B

generally increase as PO2 increases. The series resistance is also found to increase significantly from 10 to

2430 Ω due to the reduced carrier concentration. With increased PO2, there are fewer oxygen vacancies

but more traps within the IGZO/Pd interfaces [19, 20], which thus leads to a reduced doping density and

free carrier concentration, and an increased n as shown in figure 2(a) and table 1. As the PO2 increases, the

Schottky barrier height B becomes larger. During the initial phase of IGZO deposition, it has been

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shown that the bombardment of negatively charged oxygen ions can have two effects: (a) it acts as an in

situ plasma treatment of the Pd surfaces by removing the hydroxide-induced highly conductive surface

accumulation layer which would reduce B [21]; and (b) Pd can be oxidized in to PdO which has a higher

work function [22]. Comparing with the dependence of B on RF sputtering power, our results reveal that

the increase of oxygen partial pressure has a stronger influence on the Schottky barrier height than the RF

sputtering power. The as deposited Schottky diode fabricated at PO2 = 15% shows the largest B of 0.85

eV, but also the largest n of 1.6 and a low Ion/off of 5.8 103 (at 1 V). The diode fabricated at PO2 of

2.5% shows the smallest n of 1.15, the highest Ion/off of 6.8 104 at 1 V, and a B of 0.73 eV.

Table 1. The ideality factors (n), Schottky barrier heights (B) by J-V, rectification ratios (Ion/off) at 1 V, series resistances (Rs), and breakdown voltages (VBR) of the Schottky diodes prepared at various RF sputtering power (50, 70, 90, 110, 130 W) and an oxygen partial pressure of 2.5%, and at various oxygen partial pressure (0, 2.5, 5, 10, 15%) and a RF sputtering power of 70 W.

Power (W) n B (eV) Ion/off RS (Ω) VBR (V)50 1.14 0.73 1.3 105 15 -5.820.5370 1.15 0.73 7.9 104 23 -6.240.8590 1.25 0.71 3.5 104 29 -6.100.91

110 1.32 0.84 5.7 103 183 -5.450.67130 1.52 0.78 4.0 103 486 -5.710.80

PO2 (%) n B (eV) Ion/off RS (Ω) VBR (V)0 1.48 0.49 7.9 101 10 -3.590.28

2.5 1.15 0.73 7.9 104 33 -6.240.855 1.26 0.75 6.4 103 121 -5.660.8010 1.46 0.78 9.9 103 440 -5.760.9915 1.58 0.86 6.1 103 2430 -5.541.24

3.3Aboutbreakdownvoltage

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Figure 3. The breakdown voltage dependences on RF sputtering powers at a fixed oxygen partial pressure of 2.5% (black solid line with closed dot) and on oxygen partial pressures at a fixed RF sputtering power of 70 W (blue solid line with closed square). The error bar at each measured point is also included. Inset: the large-bias J-V characteristic of the diodes with IGZO layers sputtered at 70 W in pure argon atmosphere (black solid line with open dot) and in mixed atmosphere with oxygen partial pressure of 2.5% (red solid line with open square).

The breakdown voltage (VBR) is one of the key performance parameters of Schottky diodes, since in

many applications Schottky diodes are reversely biased to a large voltage during operation. Here we

investigate the behavior of all the above diodes by measuring their reverse-bias J-V curves as shown in

the inset of figure 3. Tunneling breakdown is regarded as the main mechanism for these diodes. In

general, VBR is determined by B, doping concentration, the width of the depletion layer (around 100 nm

as proven previously [11]), and marginal tunneling effect which can be ignored here [11, 23]. In this

work, it is found that the diode with IGZO layer sputtered in pure argon atmosphere has a low VBR of ~ -

3.6 V as shown in figure 3. This is because of the low B caused by a large number of oxygen vacancies

and high carrier concentration. The diodes with IGZO layers sputtered in oxygen-included atmosphere

show high and nearly constant VBR -6 V. This suggests that the Schottky barrier inhomogeneity of these

diodes may be important in the tunneling breakdown. The barrier height obtained from J-V measurement

is found to be different from the value obtained from C-V measurement: 0.73 eV from J-V and 0.64 eV

from C-V for the diode with IGZO sputtered under RF power of 70 W and PO2 of 2.5% [11]. Such a

difference is attributed to the barrier inhomogeneity at the Pd/IGZO interfaces, suggesting that there are

small patches at the Schottky barrier interface with lower barrier heights where tunneling breakdown

occurs predominantly [18].

4. Conclusion

In summary, high performance Schottky diodes with Pd anodes and 100-nm-IGZO sputtered under

various RF sputtering powers and oxygen partial pressures are fabricated on glass substrate. These diodes

fabricated with low RF sputtering powers (e.g. 50 W) and small oxygen partial pressure (e.g. 2.5%) show

very low ideality factor (~ 1.14), high rectification ratio (~1.3 105), and good barrier height (0.73 eV).

For the Schottky junctions fabricated in oxygen/argon atmosphere, the oxygen partial pressure is found

affect the barrier height significantly. Furthermore, either high RF sputtering power or high oxygen partial

pressure leads to degraded interface quality, increases the ideality factor, and decreases the rectification

ratio. The diode with IGZO sputtered in pure argon atmosphere shows a low breakdown voltage (~ -3.6

V), while the diodes with IGZO sputtered in oxygen-included show a high breakdown voltage (~ -6 V)

independent on the RF sputtering power and oxygen partial pressure. The findings may be useful also for

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flexible diodes on plastic substrate since all the fabrication steps in this work are carried out at room

temperature without any thermal treatment.

Acknowledgements

This work was financed by the National Natural Science Foundation of China (Grant Nos. 11374185 and

11304180), the Natural Science Foundation of Shandong Province (ZR2013EMQ011), the Natural

Science Foundation of Jiangsu Province (BK20151255), and a Suzhou Planning Project of Science and

Technology (SYG201527).

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