Effect of atomic layer deposition temperature on the performance of top-down ZnO nanowire transistors
© Sultan et al.; licensee Springer. 2014
Received: 27 May 2014
Accepted: 12 September 2014
Published: 21 September 2014
This paper studies the effect of atomic layer deposition (ALD) temperature on the performance of top-down ZnO nanowire transistors. Electrical characteristics are presented for 10-μm ZnO nanowire field-effect transistors (FETs) and for deposition temperatures in the range 120°C to 210°C. Well-behaved transistor output characteristics are obtained for all deposition temperatures. It is shown that the maximum field-effect mobility occurs for an ALD temperature of 190°C. This maximum field-effect mobility corresponds with a maximum Hall effect bulk mobility and with a ZnO film that is stoichiometric. The optimized transistors have a field-effect mobility of 10 cm2/V.s, which is approximately ten times higher than can typically be achieved in thin-film amorphous silicon transistors. Furthermore, simulations indicate that the drain current and field-effect mobility extraction are limited by the contact resistance. When the effects of contact resistance are de-embedded, a field-effect mobility of 129 cm2/V.s is obtained. This excellent result demonstrates the promise of top-down ZnO nanowire technology for a wide variety of applications such as high-performance thin-film electronics, flexible electronics, and biosensing.
Zinc oxide thin-film transistors are receiving increasing attention because high values of field-effect mobility (3 to 15 cm2/Vs) can routinely be achieved in layers deposited at low temperature (<200°C) [1–6]. The values of mobility achievable are significantly higher than those in more well-researched materials such as α-Si/H (approximately 1 cm2/V.s), pentacene single crystals (approximately 2.7 cm2/V.s), and pentacene thin films (approximately 1.5 cm2/V.s) . This makes ZnO-based thin-film transistors very attractive for application in displays, where the higher mobility would provide higher switching speeds or lower power operation. For display applications, ZnO has the additional advantage of high optical transparency, whereas α-Si/H does not. Furthermore, ZnO-based thin-film transistors have considerable potential in emerging applications such as wearable and flexible electronics.
A variety of approaches have been used for the low-temperature deposition of ZnO-based materials, including sputtering [4–6], pulsed laser deposition , solution-based processes , and atomic layer deposition (ALD) [1–3, 9–12]. Of these methods, ALD is particularly attractive because it offers the prospect of an accurate control of material structure in a manufacturing environment. ALD ZnO layers with reasonable electrical and optical properties can be obtained at deposition temperatures below 100°C  and even down to room temperature [14, 15]. ZnO thin films deposited in ALD exhibit excellent mobility (6 to 30 cm2/s) with good stability against stress [10–13]. A good-quality TFT transistor with controlled carrier concentrations was also often obtained with an ION/IOFF ratio of 107[3, 16].
Recently, there has been increasing interest in ZnO-based nanowire transistors fabricated by top-down approaches [17–19] as opposed to the more common bottom-up self-assembly approach . The top-down approach involves material deposition and anisotropic plasma etching to create a nanowire. The advantage of top-down fabrication is that it provides nanowire transistors in well-defined locations on a wafer and enables transistors with different channel lengths to be produced on the same chip. This latter feature is important for the design of practical electronic circuits.
In our previous work , we demonstrated a top-down technology that produced transistors with well-behaved electrical characteristics at different channel lengths and with excellent values of breakdown voltage. However, the value of field-effect mobility (0.5 cm2/V.s) was at the bottom range of expected values for ALD ZnO thin-film transistors. In this paper, we show how the top-down ZnO nanowire transistor technology can be optimized to give considerably improved values of mobility and drain current. The effects of the ALD deposition temperatures on field-effect mobility are systematically investigated. Nanowire transistor characteristics are compared with the ZnO material properties to determine how the ALD processes influence the transistor electrical characteristics. A field-effect mobility of 10 cm2/V.s is obtained at an ALD deposition temperature of 190°C. When the contact resistance is considered, the extracted field-effect mobility of 129 cm2/V.s is achieved under VD = 1 V.
Our top-down ZnO nanowire transistors were fabricated using the technology described in , which used an ALD ZnO layer deposited over a SiO2 pillar. The ZnO deposition temperature was systematically varied, while all other parameters were kept constant. The atomic layer deposition used 200 cycles of a process comprising an initial Ar purge of 2 s, a 4-s exposure to oxygen plasma, a 1-s (constant) exposure to DEZ, and a final Ar purge of 4 s. The radio-frequency (RF) power and pressure were 100 W and 15 mTorr, respectively. All ALD ZnO films were terminated with the oxygen plasma cycle at the end of each deposition. The thicknesses of the ALD films were measured by ellipsometry and varied somewhat with deposition temperature, from 16 nm at 100°C to 23 nm at 210°C. The ZnO layer was then anisotropically dry etched in an Oxford Instruments Plasma Technology System 100 Inductively Coupled Plasma (ICP) 380 (Oxford Instruments, Yatton, UK) using 25 sccm CHF3, 300 W RF power, 1,000 W ICP power, and a pressure of 10 mTorr. The ZnO ICP etch rate was 50 nm/min.
The deposited ZnO layers were characterized using Hall effect measurements of bulk mobility and carrier concentration, sheet resistance measurements for resistivity, and X-ray photoelectron spectroscopy (XPS) measurements of film stoichiometry. The stoichiometry was based on our previous work in  which is determined from the ratio of the atomic percentages of the main level Zn-2p3/2 peak and the O-1s binding energy peak observed at 1,022 and 531 eV, respectively. Measurements of nanowire transistor transfer and output characteristics were made on a semiconductor parameter analyzer using the silicon substrate as the back gate. The field-effect mobility was determined from the transconductance using the standard method and the threshold voltage was determined by extrapolation of the linear transfer characteristic. The value of the mobility is determined based on the best performance from each sample.
Results and discussion
Summary of parameters obtained for ZnO nanowire transistors fabricated using ALD layers deposited at different temperatures
ALD growth temperature (°C)
Threshold voltage, VTH(V)
Field-effect mobility (cm2/V.s)
Drain current, ID(nA) at VD = 1 V and VG − VTH = 6 V
The Hall effect mobility of 120 cm2/V.s obtained in our ZnO thin films is comparable with values in the range 120 to 155 cm2/V.s obtained for single-crystal ZnO thin films grown on sapphire substrates using molecular beam epitaxy or pulsed laser deposition . The field-effect mobility of 10 cm2/V.s obtained in our ZnO nanowire transistors compares with values of 12.5, 6.7, and 1 cm2/V.s reported by Levy et al. , Lim et al. , and Huby et al., respectively , in ZnO thin-film transistors fabricated using atomic layer deposition. ICP etching can therefore be used to produce ZnO nanowire transistors with comparable values of field-effect mobility as obtained in ZnO thin-film transistors, indicating that the ICP etch does not significantly degrade the device performance.
The total resistance, composed of channel and contact resistances, is measured across the nanowire’s (NW’s) output terminals at VG = 40 V. The channel resistance in the linear region exhibits a purely ohmic behavior  while contact resistance consists of ohmic and a non-ohmic components. The measured total resistances initially reduced from 68.5 GΩ (150°C), 8.5 GΩ (170°C), 111 MΩ (190°C), and finally increased to 531 MΩ for film deposited at 210°C. The doping concentration achieved in our ZnO nanowire transistors is in the range from 1.5 × 1016 to 3.0 × 1016 cm−3 for a deposition at 190°C; so the effect of the source-drain contact resistance (Rcon) and channel resistance (RNW) can limit the drain current and extraction of the field-effect mobility.
Considerable research has been published in the literature on bottom-up ZnO NW FETs [26–33], with widely varying values of field-effect mobility. Extremely high mobility values (>1000 cm2/V.s) have been reported in passivated ZnO nanowire transistors [26, 27] but much lower values (75 to 80 cm2/V.s) in unpassivated devices. The results presented in this work were obtained on unpassivated ZnO nanowire transistors. There may also be a scope to further increase the mobility in our devices by using surface passivation.
This paper has studied the effect of the atomic layer deposition temperature on the performance of top-down, ZnO nanowire field-effect transistors. The ZnO deposition temperature has been systematically varied, with all other deposition conditions kept constant. A deposition temperature of 190°C gives the maximum field-effect mobility of 10 cm2/V.s and also corresponds with the maximum Hall effect mobility of 120 cm2/V.s. This result is explained by the good stoichiometry of the ZnO films at a deposition temperature of 190°C. The optimized field-effect mobility of 10 cm2/V.s is approximately ten times higher than can typically be achieved with thin-film amorphous silicon transistors. Furthermore, device simulations show that the field-effect mobility is limited by contact resistance and when this is de-embedded, the field-effect mobility increases to 129 cm2/V.s. It is clear therefore that top-down fabricated ZnO nanowire transistors show considerable potential for high-performance, transparent, thin-film electronics on either glass or polymer substrates.
S.M. Sultan would like to acknowledge the financial support from Ministry of Higher Education (MOHE) under the Fundamental Research Grant Scheme (FRGS) (Votes nos. R.J130000.7823.4F482).
N.J. Ditshego would like to acknowledge the support of the Government of Botswana. The authors would like to acknowledge the fabrication and experimental support from the Southampton Nanofabrication Centre.
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