Investigation the performance of hydrogen peroxide pretreatment ZnO UV photodetectors using plasma-enhanced atomic layer
deposition
Yu-Chang Lin, Hsin-Ying Lee*
1Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan, Republic of China
Corresponding Author: Prof. Hsin-Ying Lee
Address: Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan
Tel: 886-6-2082368 Fax: 886-6-2082368
E-mail: hylee@ee.ncku.edu.tw
Abstract
The ZnO films were deposited using a plasma-enhanced atomic layer deposition (PE-ALD) system and applied to metal-semiconductor-metal ultraviolet photodetectors (MSM-UPDs) as an active layer. In chemisorption of the ALD system at early cycles, the hydroxyl (OH) group on the initial substrate surface was necessary for the reaction with diethylzinc (DEZn). To improve the quality of the ZnO films and further enhanced the performance of the ZnO MSM-UPDs, the sapphire substrates were pretreated by hydrogen peroxide (H2O2) to increase the chemical bonding between ZnO film and substrate prior to the deposition. According to the X-ray photoelectron spectroscopy (XPS) analysis at the binding energy of 530.4 eV and 1021.6 eV in O 1s and Zn 2p orbit, the ZnO films exhibited the strongest Zn-O bonds under the H2O2 pretreatment for 60 min. The results indicated that H2O2 pretreatment effectively made more nucleation opportunity on the initial surface of the substrate to create more Zn-O bonds. Besides, the dark current of the ZnO MSM-UPDs without and with H2O2 pretreatment for 60 min operated at a bias voltage of 5 V was 3.64 μA and 0.27 μA, respectively. Since the better structural property and bonding condition caused the reduction of defects, the dark current was improved. In addition, the UV- visible rejection ratio (R365/R450) of the ZnO MSM-UPDs with H2O2 pretreatment for 60 min increased from 6.32×102 to 1.06×103 in comparison with the ZnO MSM- UPDs without H2O2 pretreatment.
Keywords: Plasma-enhanced atomic layer deposition, ZnO film, Hydrogen peroxide, metal-semiconductor-metal ultraviolet photodetectors, UV-visible ratio.
1. Introduction
The zinc oxide (ZnO) as a II-VI compound semiconductor possess many excellent optical and electrical characteristics. Besides, several advantages including the low cost, non-toxic, wide bandgap of 3.37 eV, and large exciton binding energy of 60 meV lead ZnO becomes a promising material for developing various devices. As a result, the ZnO is highly desirable and widely studied for thin-film transistors [1], light-emitting diodes [2], ultraviolet (UV) photodetectors [3], pH sensors [4], and so on. Recently, the UV photodetectors are highly concerned due to their wide applications, such as flame detection, chemical agent detection, and solar astronomy [3]. The quality of active layer is a main index for the influence on the UV photodetectors performance. In recent years, the ZnO films have been deposited using various techniques including radio frequency (RF) sputtering [5], pulsed laser deposition (PLD) [6], and vapor cooling condensation system [7]. Comparing with other techniques, atomic layer deposition (ALD) was a promising technique for deposition of the ZnO films due to the precise thickness control, high step-coverage, excellent uniformity, and lower growth temperature. Generally, in thermal ALD process, the diethylzinc (DEZn) precursor and water (H2O) precursor were used as the zinc and oxygen sources to deposit the ZnO films, respectively. In thermal ALD process for the ZnO films, both DEZn and H2O contained hydrogen, and the Zn-OH bonds were produced in every half cycle. However, several literatures pointed out that the hydrogen impurities played the role as shallow donors which increased the electron concentration of the ZnO films [8,9]. The hydrogen impurity was incorporated into an interstitial site (OH--like) or into the O site [10-12]. To overcome the above issue, the H2O precursor was replaced by ozone (O3) [13] and O2 plasma [14-16] to decrease Zn-OH bond and created more Zn-O bonds in the ZnO films.
In this work, the ZnO films were deposited using plasma-enhanced ALD (PE- ALD) with O2 plasma which could effectively decrease the Zn-OH bonds. However, some starting surfaces were not reactive with the precursors during the early stages of the deposition process [17]. Besides, there were possibilities that some chemical reactions were not ideal because the precursors were not completely decomposed and bonded at low deposition temperature [18]. To improve the chemisorption at early cycles, the hydroxyl (OH) group on the initial surface of substrates was necessary for the reaction with DEZn. The sapphire substrates were pretreated by hydrogen peroxide (H2O2) solution to provide the sufficient hydroxyl on the initial surface of the substrates. The effects of H2O2 pretreatment on the structural and chemical bonding properties of the ZnO films using PE-ALD with O2 plasma were investigated.
Finally, the performance of ZnO metal-semiconductor-metal ultraviolet photodetectors (MSM-UPDs) with H2O2 pretreatment was also investigated.
2. Experimental procedures
The PE-ALD was utilized to deposit the ZnO films on the sapphire substrates with H2O2 pretreatment for various time durations. The ZnO films were applied to MSM-UPDs as the active layers. Figure 1 shows the schematic configuration of the ZnO MSM-UPDs. At first, the sapphire substrates were cleaned for 5 min by acetone, methanol, and de-ionized (DI) water in ultrasonic bath, respectively. Next, the sapphire substrates were immersed in the H2O2 solution at room temperature for 15 min, 30 min, 45 min, 60 min, and 75 min, respectively. After H2O2 pretreatment for various time durations, the H2O2-pretreated sapphire substrates were rapidlyput into the PE-ALD chamber to deposit 100-nm-thick ZnO films. The DEZn precursor and O2 plasma were used as zinc (Zn) and oxygen (O) sources, respectively. The power of remote plasma (radio frequency of 13.56 MHz) for O2 gas was fixed at 400 W. The chamber pressure and substrate temperature were fixed at 0.6 Torr and 100 oC, respectively. During the deposition process, the flow rate of argon (Ar) gas and O2 gas was fixed at 200 sccm and 50 sccm, respectively. Figure 2 shows the chart of time sequence for the deposition process of ZnO film using PE-ALD. The DEZn precursor injected and chemisorbed on substrate for 3 s and Ar gas carried residual reactant for 3 s. After metallization, O2 plasma as oxidizing source pulsed into chamber for 6 s, and Ar gas carried residual reactant for 6 s. The growth rate of the ZnO films was stabilized at about 0.72 Å/cycle. The active region of 100×100 μm2 for the MSM- UPDs was defined by conventional photolithography, and then the unnecessary ZnO films were etched by diluted hydrochloric acid solution (HCl:H2O = 1:1500). The interdigital Schottky metals Ni/Au (20 nm/100 nm) were deposited on the ZnO active layer using an electron-beam evaporator. Both the width and spacing of the metal fingers were 2 μm.
The crystallinity and chemical bonding of the ZnO films were analyzed by X-ray diffraction (XRD, Cu Kα radiation with a wavelength of 0.154 nm) and X-ray photoelectron spectroscopy (XPS). The electrical characteristics of the ZnO MSM- UPDs were characterized by Agilent 4156C semiconductor parameter analyzer. The photoresponsivity spectra of the ZnO MSM-UPDs were measured by the monochromator with the Xe lamp source.
3. Results and discussion
To verify the above statements, the crystallinity of the ZnO films deposited on sapphire substrates without and with H2O2 pretreatment for various time durations were carried out by XRD and shown in Fig. 3. From the XRD spectra, all the ZnO films exhibited (002) diffraction peak around 34.4 o, which indicated that the crystal structure of the ZnO films was wurtzite structure. The (002) diffraction peak intensity increased as the time duration of H2O2 pretreatment was increased from 15 min to 60
min. The full width at half maximum (FWHM) of (002) diffraction peak decreased from 0.51 o to 0.47 o as the time duration of H2O2 pretreatment was increased from 0 min to 60 min. The better crystal structure of the ZnO films with H2O2 pretreatment was constructed from the better nucleation caused by more hydroxyl bonds on the initial sapphire surface. However, while the time duration of H2O2 pretreatment was increased to 75 min, the crystallinity transformed into polycrystalline with (002) diffraction peak (34.4 o), (100) diffraction peak (31.7 o), and (101) diffraction peak (36.2 o). At the same time, the FWHM of (002) peak increased to 0.53 o as the time duration of H2O2 pretreatment extend to 75 min. This phenomenon indicated that too much hydroxyl bonds might limited the number of bonding sites on initial sapphire surface. The number of bonding sites is less than required for getting full ligand coverage [19]. Worse bonding condition caused less accessible OH bonding sites and changed the crystal orientation. Therefore, the XRD results indicated that appropriate H2O2 pretreatment could enhance the crystalline quality of the ZnO films.
The chemical binding of the resulted ZnO films was analyzed using XPS. For the XPS measurements, the 6-nm-thick ZnO thin films were deposited on sapphire substrates with H2O2 pretreatment for various time durations. Figure 4 shows the XPS spectra of O 1s core-level for the ZnO films deposited on sapphire substrates without and with H2O2 pretreatment for 0 min, 30 min, 60 min, and 75 min, respectively. As shown in Fig. 4, the Zn-O bond (530.4 eV) [20-22] and the Zn-OH bond (532.0 eV) [21-23] were presented in the XPS spectra of all the deposited ZnO films. The intensity of the Zn-O bond and the Zn-OH bond for the ZnO films increased with an increase of the pretreatment time duration until 60 min. The results indicated that H2O2 pretreatment effectively made more nucleation opportunity on the initial surface of the substrate to create more Zn-O bonds, which caused the rearrangement of the structure at the surface within the beginning of few deposition cycles. However, both intensities of the Zn-O bond and the Zn-OH bond and the ratio of the Zn-O bond and the Zn-OH bond were decrease as H2O2 pretreatment time duration was increased to 75 min. As a result, the appropriate H2O2 pretreatment time duration could provide suitable hydroxyl bonds on the initial sapphire surface to produce more nucleation, and excess hydroxyl bond limited number of bonding sites [23]. Consequently, the ZnO films deposited on sapphire substrates with H2O2
pretreatment for 60 min exhibited the best structural property and chemical bonding.
Figure 5 (a) shows the current-voltage (I-V) characteristics of the ZnO MSM- UPDs without and with H2O2 pretreatment for 60 min, respectively. As shown in Fig. 5 (a), the dark current of ZnO MSM-UPDs without and with H2O2 pretreatment for 60 min operated at a bias voltage of 5 V were 3.64 μA and 0.27 μA, respectively. In general, the dark current was influenced by the quality of active layer. Since the better
structural property and bonding condition caused the reduction of defects, the dark current was improved to 0.27 μA. The photoresponsivity of the ZnO MSM-UPDs without and with H2O2 pretreatment was measured at 5 V and shown in Fig. 5 (b). In the photoresponsivity spectra, the UV-visible rejection ratio (R365/R450) of the ZnO MSM-UPDs with H2O2 pretreatment for 60 min increased from 6.32×102 to 1.06×103 in comparison with the ZnO MSM-UPDs without H2O2 pretreatment. The promotional performance of the ZnO MSM-UPDs and results mentioned above testify that the H2O2 pretreatment effectively improve the structural property and bonding condition of the ZnO films.
4. Conclusions
The PE-ALD system using O2 plasma has been developed to deposit the ZnO films on the sapphire substrates. The H2O2 pretreatment was utilized to provide more hydroxyl bonds on the initial surface of the sapphire substrates before deposition. The H2O2 pretreatment greatly enhanced the c-axis crystalline orientation of ZnO films as well. On the other hand, appropriate H2O2 pretreatment generated suitable hydroxyl bonds to react with zinc on the initial surface of the sapphire substrates. The ZnO films deposited on sapphire substrates with H2O2 pretreatment for 60 min exhibited the best structural property and chemical bonding. Moreover, the performance of the MSM-UPDs using the ZnO films with H2O2 pretreatment for 60 min as the active layer was enhanced. At a bias voltage of 5 V, the UV-visible ratio of ZnO MSM- UPDs was improved to 1.06×103. Consequently, the H2O2 pretreatment was an effective and promising method to improve the quality of the ZnO films.
Acknowledgment
This work was supported by the Ministry of Science and Technology of Taiwan under Grant MOST 101-2923-E-006-002-MY3, and the Advanced Optoelectronic Technology Center, National Cheng Kung University, Taiwan.
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Figure captions
Figure 1 The schematic configuration of the ZnO MSM-UPDs.
Figure 2 Chart of time sequence for the deposition process of ZnO film using PE-ALD.
Figure 3 XRD spectra of PE-ALD ZnO thin films without and with H2O2 pretreatment
for various time durations.
Figure 4 XPS sptecta of O 1s core-level for (a) without and with H2O2 pretreatment
for (b) 30 min, (c) 60 min, and (d) 75 min.
Figure 5 The (a) current-voltage characteristics and (b) photoresponsivity spectra of
ZnO MSM-UPDs without and with H2O2 pretreatment for 60 min.
Figure. 1
Figure. 2
Figure. 3
Figure. 4
Figure. 5
