Enhanced Doping Efficiency of Al-Doped ZnO by Atomic Layer

Nov 6, 2013 - (4, 5) One of the commonly introduced dopants is Al which is used to improve the ... distribution of Al atoms, thereby increasing the do...
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Enhanced Doping Efficiency of Al-Doped ZnO by Atomic Layer Deposition Using Dimethylaluminum Isopropoxide as an Alternative Aluminum Precursor Y. Wu,†,§ S. E. Potts,† P. M. Hermkens,† H. C. M. Knoops,† F. Roozeboom,†,§ and W. M. M. Kessels*,† †

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Holst Centre, P.O. Box 8550, 5605 KN Eindhoven, The Netherlands

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ABSTRACT: Atomic layer deposition offers the unique opportunity to control, at the atomic level, the 3D distribution of dopants in highly uniform and conformal thin films. Here, it is demonstrated that the maximum doping efficiency of Al in ZnO can be improved from ∼10% to almost 60% using dimethylaluminum isopropoxide (DMAI, Al(CH3)2(OiPr)) as an alternative Al precursor instead of the conventionally used trimethylaluminum (TMA, Al(CH3)3). Due to the steric hindrance of the isopropoxyl ligand of the precursor, the Al atoms can be deposited more widely dispersed, which enables higher active-dopant densities and hence a higher conductivity of the Al-doped films. KEYWORDS: transparent conducting oxide, Al-doped ZnO, atomic layer deposition, doping efficiency, steric hindrance

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INTRODUCTION Intrinsic and doped ZnO thin films have a growing number of prominent applications in electronic devices. In solar cells and displays they are used as transparent conducting oxides (TCO),1,2 in transparent thin-film transistors for displays they are used as semiconducting layers,3 and in low power gas sensors they serve as gas sensitive layers.4,5 One of the commonly introduced dopants is Al which is used to improve the conductivity of ZnO films.6 Atomic layer deposition (ALD) is a deposition technique which allows for the preparation of high quality, highly conformal, and uniform thin films with precise growth control.7,8 Due to the cyclic nature of the technique, ALD is also particularly well suited to deposit doped ZnO films when employing supercycles in which ALD cycles with Zn and dopant precursors are alternated. In principle, under carefully chosen conditions, ALD would allow for the precise, atomic scale control over the concentration, position, and spacing of the dopant in the ZnO lattice. However, when the Al-doped films are deposited from the commonly applied precursors diethylzinc [DEZ, Zn(C2H5)2] and trimethylaluminum [TMA, Al(CH3)3], in combination with water, typically a nanolaminate structure is obtained. This means that ZnO layers are alternated by layers of AlOx clusters9 that have a relatively large areal density of Al atoms. The latter leads to overlapping effective electric fields from adjacent Al dopant atoms10 while the AlOx clusters also act as scattering centers for electrons limiting their mobility and, consequently, the film conductivity.11 In order to alleviate this problem, efforts have been undertaken to improve the control of the spatial distribution of Al atoms. For example, © 2013 American Chemical Society

Yanguas-Gil et al. dosed organic molecules prior to dosing the TMA to reduce the saturation growth rate of the Al.12 However, this method introduces additional steps to the ALD cycles while it also did not lead to a sufficient improvement in conductivity of the films. In this work, we present another, very straightforward approach to increase the conductivity of Aldoped ZnO films. We demonstrate that an alternative Al precursor, with bulkier ligands leading to steric hindrance, affords control of the spatial distribution of Al atoms, thereby increasing the doping efficiency of the Al atoms. When ALD cycles of ZnO are alternated by a cycle of Al2O3, the number of the Al precursor molecules that adsorb and react on the ZnO surface determines the density of the Al atoms in the Al-doped ZnO. This implies that if a larger Al precursor is employed than TMA, fewer Al atoms would be incorporated due to steric hindrance caused by the precursor ligands.13 With the Al atoms wider spaced laterally, the formation of AlOx clusters will be less pronounced and this reduces the overlap of the effective electric field from adjacent Al atoms as well as the scattering of electrons by AlOx clusters. This should increase the doping efficiency and the electron mobility of the films. To test this hypothesis, we used dimethylaluminum isopropoxide [DMAI, Al(CH3)2(OiPr)] as an alternative Al precursor to the conventional TMA. Each DMAI monomer contains one isopropoxyl ligand (OiPr), which is estimated to be 4.9 Å in size and therefore 1.9 times bulkier than a methyl (CH3) ligand Received: September 4, 2013 Revised: October 14, 2013 Published: November 6, 2013 4619

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(2.6 Å in size).14,15 Consequently, during the Al2O3 cycles, each adsorbed −Al(CH3)(OiPr) surface group16 can shield a greater surface area than a corresponding −Al(CH3)2 surface group, thereby sterically hindering further reactions between the adjacent −OH surface groups and incoming DMAI molecules. Such a more pronounced influence of steric hindrance for DMAI when compared to TMA is also indicated by a reduced growth per cycle (in terms Al atoms deposited per cycle) obtained for Al2O3 films prepared with DMAI.16

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ZnO were required after an Al2O3 cycle. This effect can possibly be related to the Brønsted acidity of ZnOH* and AlOH* (asterisk denotes surface-bound species).9,18 During the transition between Al2O3 and ZnO cycles, ZnOH* and AlOH* coexist on the surface. Since ZnOH* is relatively basic compared to AlOH*, a proton-transfer surface reaction can occur: ZnOH* + AlOH* → ZnOH+2 ···AlO− *

EXPERIMENTAL SECTION

Al-doped ZnO films were deposited using an Oxford Instruments OpAL ALD reactor at a substrate temperature of 250 °C. Si wafers with thermally grown SiO2 layers with a thickness of 450 nm were used as substrates. DEZ, DMAI, TMA, and deionized water (DI H2O) vapor were used as precursors.16 Films with a thickness of 40 ± 3 nm were deposited, as monitored by in situ spectroscopic ellipsometry, and their doping level was varied by employing different ratios between the ZnO and Al2O3 cycles.11 The doping levels were expressed by the aluminum fraction (AF),

Al atom % AF = × 100% Al atom % + Zn atom %

ZnOH* + Al(CH3)3 → Al(OH)(CH3)* + Zn(CH3)2 (4)

(1)

Such “etching” was not observed in the DMAI case, where the GPC of the Al2O3 cycle was in good agreement with the value of the pure Al2O3 films, as indicated by the dashed line in Figure 1a. The resistivity of the Al-doped ZnO films is shown in Figure 2a, which shows two regions for both DMAI and TMA separated by a minimum resistivity obtained at a certain AF value. In the case of DMAI, a lower resistivity (1.1 mΩ cm) was achieved than for TMA (2.4 mΩ cm) at a lower AF (4.6% for DMAI and 6.9% for TMA). Figure 2b reveals the carrier density and mobility of the Al-doped ZnO films. At low AF values (region I), the carrier density increases significantly with increasing AF as a result of the effective Al doping. In region II, at higher AF values, the carrier density saturates. For the case of DMAI, this happens at a value of ∼1.0 × 1021 cm−3, which is significantly higher than the maximum value of 3.7 × 1020 cm−3 for the TMA case.11 The mobility shows a decrease with AF over that largest part of the AF range. The improvement in the conductivity of the films prepared with DMAI is particularly prominent at lower Al doping levels. This indicates a large difference in doping efficiency, i.e., the percentage of Al atoms which effectively donate a free electron to the ZnO films. With one Al atom donating at maximum one free electron, this doping efficiency η can be calculated from the data by

which was calculated from the atomic fractions of Al (Al atom %) and Zn (Zn atom %) as measured by depth-profiling X-ray photoelectron spectroscopy (XPS).11 The areal density of the Al atoms was obtained from Rutherford backscattering spectrometry (RBS) experiments. The resistivity ρ and carrier density n of the films were obtained by fourpoint probe and optical measurements, respectively. The electron mobility μ was obtained by the expression11,17

ρ = (e × n × μ)−1

(3)

−

The formation of the ZnOH2 ··· AlO * complex consumes surface hydroxyl groups, leaving fewer reactive sites on the surface for the growth of ZnO in the subsequent cycles. Furthermore, besides the nucleation delay for ZnO, it was also observed that the GPC of the Al2O3 cycle was smaller than the value for pure Al2O3 films in the case that TMA was employed as precursor. This effect was reported before9,18 and was attributed to the etching of surface Zn atoms via ligand exchange during the Al2O3 cycles: +

(2)

This procedure for extracting μ has been validated by a comparison with data from Hall measurements.11

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RESULTS AND DISCUSSION First, the values of the growth per cycle (GPC) for the ZnO and Al2O3 cycles were investigated by in situ spectroscopic ellipsometry (SE). In supercycles, these values can differ from the values obtained for pure ZnO and Al2O3 films. As shown in Figure 1, a nucleation delay was observed for ZnO after one cycle of Al2O3. Both for DMAI and TMA, about four cycles of

atomic density of active dopants × 100% atomic density of Al n − n0 = × 100% NZn × AF

η=

(5)

where n and n0 are the carrier densities of Al-doped and intrinsic ZnO, respectively. NZn is the atomic density of Zn as measured by RBS, which was 4.0 × 1022 cm−3. n − n0 is the increase of the carrier density due to active Al dopants, or, equivalently, it is the active dopant density. As shown in Figure 2c, the films prepared with DMAI show a significantly higher doping efficiency (up to 60%) than the films prepared with TMA (∼10%). The higher doping efficiency for the case of DMAI explains the fact that the carrier density increases faster with increasing AF than for the case of TMA. It also implies that a larger active

Figure 1. Growth per cycle of consecutive ZnO and Al2O3 ALD cycles using (a) DMAI and (b) TMA as the Al precursor and as measured by in situ spectroscopic ellipsometry. Dashed lines indicate the growth per cycle of pure ZnO and Al2O3 films. 4620

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the mobility decreases with increasing active dopant density, mainly due to ionized impurity scattering by the ionized Al dopants.19 This effect is particularly pronounced for the DMAI case, where the mobility decreases monotonically for active dopant densities in the range (1−9) × 1020 cm−3 as a result from the high carrier density. In this region the mobility follows a similar trend as reported by Ellmer et al. on the basis of the work by Masetti et al.19,20 However, in region II, the mobility drops much faster with the active dopant density, especially for the case of TMA. This can be understood from the relatively large density of ineffective Al atoms that are present in the films in these cases. These ineffective Al atoms act as additional scattering centers for electrons reducing the mobility of the films significantly. Figure 3 indicates therefore clearly the impact of the higher doping efficiency for the case of DMAI. The higher doping efficiency implies that it is possible to increase the density of Al atoms in the ZnO leading to a higher active dopant density without the adverse effect of significantly increasing the density of ineffective Al atoms which drastically reduces the mobility. In this way, much larger active dopant densities at reasonable mobility levels can be achieved with DMAI than with TMA. The results presented in Figure 2 confirm that the doping efficiency and hence the conductivity of Al-doped ZnO can be improved by using DMAI as alternative Al precursor. To verify the hypothesis that this is related to more steric hindrance for DMAI than for TMA (both precursors differ only by one ligand), the areal densities of Al atoms deposited in one Al2O3 cycle were determined by RBS for films of 40 nm thick (the number of Al2O3 cycles was kept the same in both cases such that the cycle ratio ZnO:Al2O3 had to be adjusted). As shown in Table 1, the number of Al atoms deposited per cm−2 in one

Figure 2. (a) Resistivity, (b) carrier density and mobility, and (c) doping efficiency of Al-doped ZnO films prepared with DMAI and TMA as Al precursors as a function of Al fraction (AF). In both cases two regions can be distinguished (as indicated by dashed and dotted lines), a region in which the resistivity decreases and a region in which the resistivity increases.

Table 1. Conditions and Results from Rutherford Backscattering Spectrometry (RBS) for Al-Doped ZnO Films Prepared by DMAI or TMA as an Al Precursor DMAI

dopant density is obtained at a certain AF value when comparing the DMAI with the TMA case. In turn, a larger percentage of active dopants means also a smaller percentage of Al atoms that are ineffective, such as Al atoms located within ZnO grains in the form of neutral impurities6,19 or at grain boundaries in the form of AlOx clusters.11 Such a smaller percentage of ineffective Al atoms has an effect on the mobility of the films as shown in Figure 3 in which the mobility has been plotted as a function of the active dopant density. In region I,

number of Al2O3 cycles cycle ratio ZnO:Al2O3 Al fraction by RBS: AFRBS Al atoms deposited per Al2O3 cycle

19 14:1 2.9% (0.29 ± 0.01) × 1015 cm−2

TMA 19 12:1 13.4% (1.10 ± 0.04) × 1015 cm−2

Al2O3 cycle in the DMAI case was 3.8 times lower than that in the TMA case. This is in line with the aforementioned fact that the OiPr ligand is considerably larger than the CH3 ligand. This corroborates that the Al atoms deposited were more widely dispersed for ZnO films prepared by DMAI explaining the higher doping efficiency obtained by DMAI.

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CONCLUSIONS In conclusion, it has been demonstrated that ALD is well suited to prepare Al-doped ZnO films with low resistivity values by controlling the spatial distribution of the Al dopant atoms through careful selection of the Al precursor. When using DMAI instead of TMA, the bulky isopropoxyl ligand leads to more widely dispersed Al atoms in the ZnO due to the selflimiting nature of the ALD surface reactions and the effect of steric hindrance. This significantly improves the doping efficiency of the Al for the case of DMAI compared to TMA and this, in turn, means that much higher active dopant densities can be obtained while reducing the adverse influence

Figure 3. Mobility of Al-doped ZnO films as a function of active dopant density. The films have been prepared with DMAI and TMA as Al precursors. The regions as indicated in Figure 2 are shown as well. 4621

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of ineffective Al atoms that act as scattering centers for electrons and hence reduce the mobility of the Al-doped films. Being very straightforward, it is expected that this approach can also be used to prepare other doped films by ALD for which an atomic level control of the 3D distribution of the dopant atoms is vital.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Holst Centre/IMEC-NL, The Netherlands, for financially supporting this project. The work of S.E.P. is supported by NanoNextNL, a micro- and nanotechnology programme of the Dutch ministry of economic affairs, agriculture and innovation (EL&I) and 130 partners. Air Liquide is acknowledged for providing the DMAI precursor.

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