Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition
© Hou et al.; licensee Springer. 2013
Received: 4 November 2012
Accepted: 4 March 2013
Published: 28 March 2013
ZnO/Al2O3 multilayers were prepared by alternating atomic layer deposition (ALD) at 150°C using diethylzinc, trimethylaluminum, and water. The growth process, crystallinity, and electrical and optical properties of the multilayers were studied with a variety of the cycle ratios of ZnO and Al2O3 sublayers. Transparent conductive Al-doped ZnO films were prepared with the minimum resistivity of 2.4 × 10−3 Ω·cm at a low Al doping concentration of 2.26%. Photoluminescence spectroscopy in conjunction with X-ray diffraction analysis revealed that the thickness of ZnO sublayers plays an important role on the priority for selective crystallization of ZnAl2O4 and ZnO phases during high-temperature annealing ZnO/Al2O3 multilayers. It was found that pure ZnAl2O4 film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al2O3 sublayers, which could only be deposited precisely by utilizing ALD technology.
In the ZnO-Al2O3 composite material system, Al-doped zinc oxide (AZO) and zinc aluminate (ZnAl2O4) spinels are well known for their applications in optoelectronic devices and chemical industry. AZO was considered as an alternative low-cost transparent conductive oxide material instead of indium tin oxide in photovoltaic cells and displays[1, 2]. ZnAl2O4 material has been used in many catalytic reactions, such as cracking, dehydration, hydrogenation, and dehydrogenation reactions[3, 4]. As a wide-bandgap semiconductor material, ZnAl2O4 was also used as host of phosphors doping with Mn and rare earth ions[5, 6]. AZO and ZnAl2O4 thin films have been deposited by different techniques, such as sol–gel coating, pulsed laser deposition, chemical vapor deposition, radio-frequency sputtering, and atomic layer deposition (ALD)[12, 13]. Recently, ALD technology has been employed to grow transparent conductive AZO films with low resistivity in the order of 10−3 Ω·cm[14, 15]. However, the correlation between the optical and the electrical properties in the ALD of AZO films has not yet been understood very well. Meanwhile, ZnAl2O4 film deposited on porous or nanostructure supporting materials by ALD technology may have large surface area and potential applications in catalysts and phosphors. However, since the ZnAl2O4 films need to be synthesized by annealing ZnO/Al2O3 composite films at elevated temperatures, the preferable crystallization of ZnO in the ALD of ZnO/Al2O3 composite films may strongly influence the purity of the synthesized ZnAl2O4 films. A detailed study on the correlation between the ZnO/Al2O3 cycle ratios in the multilayers and the formation of ZnO and ZnAl2O4 crystal phases during the subsequent thermal annealing would be crucial for synthesizing high purity ZnAl2O4 films.
In this paper, the ALD processes of the Al2O3 and ZnO thin films were studied using diethylzinc (DEZn), trimethylaluminum (TMA), and water with a variety of substrate temperatures. The growth temperature of the ZnO/Al2O3 composite films was determined by optimizing the growth temperature of ZnO layer according to the photoluminescence (PL) spectroscopy analysis. Then AZO films were prepared by adding a small fraction of Al2O3 doping cycles in the ALD process of ZnO films. The dependences of the crystalline structure, resistivity, and optical band gap of the AZO films on the Al doping concentration were studied in detail. Afterwards, multiple crystalline ZnAl2O4 films were synthesized by annealing the ALD ZnO/Al2O3 multilayers with a high fraction of Al2O3 layers. The influences of the ALD cycle ratio of the ZnO/Al2O3 sublayers and the annealing temperature on the formation of ZnO and ZnAl2O4 phases were studied by X-ray diffraction analysis. PL spectroscopy was used in conjunction with X-ray diffraction (XRD) to analyze traceable ZnO phase in thermal processed samples. It was found that pure ZnAl2O4 film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al2O3 sublayers, which could only be deposited precisely utilizing ALD technology.
ZnO/Al2O3 composite films were deposited on quartz glass substrates or n-type Si substrates with (100) orientation. Before the film deposition, the Si substrates were cleaned through the Radio Corporation of America process, and the quartz glass substrates were treated by ultrasonic cleaning in alcohol and acetone. The ALD equipment is a 4-in. small chamber ALD system (Cambridge NanoTech Savannah 100, Cambridge NanoTech Inc., Cambridge, MA, USA). Diethylzinc (DEZn Zn(C2H5)2) and TMA Al(CH3)3 were used as the metal precursors for ZnO and Al2O3, respectively, while water vapor was used as oxidant. During the ALD process, the DEZn and TMA sources were not intentionally heated, and the precursor delivery lines were kept at 150°C. Nitrogen (99.999%) was used as carrier and purge gas with a flow rate of 20 sccm. One ZnO cycle consists of 0.015 s DEZn pulse time, 5 s N2 purge, 0.02 s H2O pulse time, and 5 s N2 purge. One Al2O3 cycle has 0.015 s TMA pulse time, 5 s N2 purge, 0.02 s H2O pulse time and 5 s N2 purge. First, pure ZnO and Al2O3 films were deposited on Si substrates with a variety of the growth temperature from 100°C to 350°C to determine the ALD windows. Then AZO films were deposited on quartz glass substrates at a temperature of 150°C. The total ALD cycles of ZnO plus Al2O3 layers are 1,090 for all the AZO samples, and the ALD cycles of the ZnO and Al2O3 sublayers in AZO films are varied with 50/1, 22/1, 20/1, 18/1, 16/1, 14/1, 12/1, and 10/1, respectively. For the ZnO/Al2O3 composite films with high fraction of Al2O3 sublayers, the total ALD cycles of the multilayers are 1,002, and the ALD cycles of the ZnO and Al2O3 sublayers are varied with 5/1, 4/1, 3/1, 2/1, 1/1, and 1/2, respectively. In order to synthesize crystalline ZnAl2O4 spinel films, the as-grown composite films were annealed subsequently in air at 400, 600, 700, 800, 1,000, and 1,100°C for 30 min, respectively.
The crystal structures of the samples were characterized by XRD analysis with Cu K α radiation. The resistivity of the AZO films deposited on quartz substrate was measured using four-point probe technique. Transmission spectra were taken by a spectrometer with a 150 W Xe lamp. The thickness and the refractive index of the ZnO/Al2O3 composite films were measured by an ellipsometer with a 632.8-nm He-Ne laser beam at an incident angle of 69.8°. The average film growth per cycle was calculated by dividing the film thickness by the total number of ALD cycles. PL spectra from the films were measured at room temperature under the excitation of the 266 nm line of a Q-switch solid state laser (CryLas DX-Q; CryLaS GmbH, Berlin, Germany). The PL signal was collected by a 1/2-m monochromator and detected by a photomultiplier (model H7732-10) connected to a computer controlled Keithley 2010 multimeter (Keithley Instruments Inc., Cleveland, OH, USA). The topography of the ZnAl2O4 films was observed using a scanning electron microscope (SEM).
Results and discussion
Growth temperature of the ZnO/Al2O3 composite films
The AZO films
where is the percentage of Al2O3 cycles, ρAl, and ρZn are the densities of Al and Zn atoms deposited during each ALD cycle for the pure Al2O3 and ZnO films, respectively. The densities of Al2O3 and ZnO growth by ALD are 2.91 and 5.62 g/cm3, So ρAl and ρZn were calculated to be 5.89 × 10−10 mol/cm2/cycle and 1.27 × 10−9 mol/cm2/cycle, respectively.
AZO and ZnAl2O4 films were prepared by alternating atomic layer deposition (ALD) of ZnO/Al2O3 laminates using DEZn, TMA and water. A deposition temperature of 150°C was selected for the ZnO/Al2O3 composite films. The growth per cycle, structure, electrical, and optical properties of the ZnO/Al2O3 laminates were studied at different Al concentration, which was controlled by varying the cycle ratio of ZnO/Al2O3 from 1:2 to 50:1. It is shown that the growth rate of the ZnO is reduced during the ALD of ZnO/Al2O3 multilayers due to the etching of the ZnO surface layer during exposure to TMA precursor in Al2O3 cycle. Conductive transparent AZO films were obtained at low Al doping concentration with the minimum resistivity of 2.38 × 10−3 Ω·cm and transmittance above 80% in the visible range. The PL spectroscopy in conjunction with XRD reveals that pure ZnAl2O4 film was synthesized from the composite with alternative monocycle of ZnO and Al2O3 deposited by precise ALD technology. SEM and XRD studies indicate that the crystalline ZnAl2O4 films can be synthesized at annealing temperature from 800°C to 1,100°C.
atomic layer deposition
scanning electron microscope
One of the authors would like to acknowledge Dr. Jun Qian for assisting in X-ray diffraction analysis. This work was supported by Chinese ‘973’ project (no. 2013CB632102) and National Natural Science Foundation of China NSFC (nos. 61275056 and 60977036).
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