Yu-Chang Lin and Hsin-Ying Lee*
Department of Photonics, Research Center Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan, Republic of China
*Tel: 886-6-2082368 Fax: 886-6-2082368 E-mail: hylee@ee.ncku.edu.tw
Conclusions
Appropriate H2O2 pretreatment generated suitable hydroxyl groups to react with zinc on the initial sapphire surface.
The ZnO films deposited on sapphire substrates with H2O2 pretreatment for 60 min exhibited the best chemical bonding state.
The performance of the ZnO MSM-UPDs with H2O2 pretreatment for 60 min was enhanced. At a bias voltage of 5 V, the UV-visible rejection ratio of ZnO MSM-UPDs was improved to 1.06 × 103.
The H2O2 pretreatment was an effective and promising method to improve the quality of the ZnO films using PE-ALD system.
Schematic diagram of ZnO MSM-PDs structure.
Active layer:
ZnO film, 100 nm
Electrode:
Ni/Au, 20 nm/100 nm
Area of active layer:
100×100 μm
2 Width and spacing of metal fingers were 2 μm
H2O2 Pretreatment Dark current (μA) UV-visible rejection ratio (R365 nm/R450 nm)
0 min 3.64 6.32×102
60 min 0.27 1.06×103
Sample for XPS analysis
Investigation the performance of hydrogen peroxide pretreatment ZnO UV photodetectors using plasma-
enhanced atomic layer deposition
Background
In ALD system, the diethylzinc (DEZn) and water (H2O) precursors are generally used as the zinc and oxygen sources to deposit the ZnO films, respectively. Since both DEZn and H2O contain hydrogen, the Zn-OH bonds were produced in every half cycle of the ZnO deposition processes. The OH- was regarded as a hydrogen impurity in the ZnO films and the inclusion of OH ions would lead to an increase of the electron concentration in the ZnO films. To overcome the above issue, ozone (O3) and O2 plasma were used to substitute for H2O precursor, which decreased the Zn-OH bonds 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. Instead of the H2O precursor, the O2 plasma was utilized as the oxygen source, which could effectively decrease the Zn-OH bonds in the ZnO films. However, the starting surfaces of substrates were not reactive with the precursors during the early stages of the ZnO deposition process regardless of using the ALD or PE-ALD technique. Besides, some chemical reactions took place unsatisfactory because the precursors were not completely decomposed at low deposition temperature. To improve the chemisorption at early cycles, the hydroxyl (OH) group on the initial substrate surface was necessary for the reaction with DEZn precursor. In this work, it was proposed that the sapphire substrates were pretreated by hydrogen peroxide (H2O2) solution to provide sufficient hydroxyl groups on the initial sapphire surface. The effects of H2O2 pretreatment on the crystallinity and chemical bonding properties of the ZnO films deposited 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.
MSM-PD Structure
Performance of ZnO MSM-PDs
XPS spectra of 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.
Hydrogen peroxide (H
2O
2) pretreatment
Results
0 2 4 6 8 10
10-14 10-12 10-10 10-8 10-6 10-4 10-2
Da rk cu rren t ( A)
Voltage (V)
H2O2 pretreatment 0 min
60 min
300 350 400 450 500 550
10-2 10-1 100 101 102 103 104
Re sp on siv ity (A /W )
Wavelength (nm)
H2O2 pretreatment 0 min
60 min
Sapphire substrate 100 μm
Ni/Au
ZnO active layer
Sapphire substrate ZnO (6 nm)
Before the ZnO deposition, sapphire substrates were immersed in the H2O2 solution at room temperature for 15 min, 30 min, 45 min, 60 min, and 75 min, respectively.
526 528 530 532 534 536
1500 2000 2500 3000 3500 4000
4500 O 1s
Int ens ity (a rb. uni ts)
Binding Energy (eV)
H2O2 pretreatment 30 min
Zn-O 530.4 eV
Zn-OH 531.8 eV
526 528 530 532 534 536
1500 2000 2500 3000 3500 4000
4500 O 1s
Int ens ity (a rb. uni ts)
Binding Energy (eV)
H2O2 pretreatment
75 min Zn-O 530.4 eV
Zn-OH 531.8 eV
526 528 530 532 534 536
1500 2000 2500 3000 3500 4000
4500 O 1s
Int ens ity (a rb. uni ts)
Binding Energy (eV)
H2O2 pretreatment 60 min
Zn-O 530.4 eV
Zn-OH 531.8 eV
526 528 530 532 534 536
1500 2000 2500 3000 3500 4000 4500
Zn-OH 531.8 eV
Int ens ity (a rb. uni ts)
Binding Energy (eV)
H2O2 pretreatment 0 min
Zn-O 530.4 eV
O 1s
Schematic illustration of H 2 O 2 pretreament
0 min 60 min 75 min
Limited number of reactive sites causes saturation
Provided sufficient number of reactive sites
Initial state of substrate without pretreatment
