[IEEE 2005 International Semiconductor Device Research Symposium - Bethesda, Maryland, USA (Dec....
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Role of Low O 2 Pressure and Growth Temperature on Electrical Transport of PLD Grown ZnO Thin Films on Si Substrates Ch. Pandis 1 , Ν. Brilis 1 , D. Tsamakis 1 , H. Ali 2 , S. Krishnamoorthy 2 and A. A. Iliadis 2,3 1: School of Electrical Engineering and Computer Science, National Technical University of Athens, Iroon Polytechniou 9 Zografou, 15773 Athens, Greece 2: Department of Electrical & Computer Engineering, University of Maryland, College Park, Maryland 20742, USA 3: Department of Information and Communication Systems Engineering, University of the Aegean, Karlovasi, 83200 Samos, Greece I. INTRODUCTION Recently there has been noticeable interest in the fabrication and study of ZnO films for a variety of electrical and optical devices. Reports indicate that the electrical properties of undoped ZnO thin films can be controlled by changing the growth parameters [1] such as: O 2 partial pressure during growth, deposition temperature as well as the fluence of the laser beam. The effects of very low oxygen pressure however have not been well investigated. In this work we have investigated the effect of very low oxygen pressure and deposition temperature during growth on electrical and structural characteristics of PLD grown ZnO films on Si. II. EXPERIMENTAL DETAILS Undoped ZnO thin films were grown on (100) Si substrates by pulsed laser deposition (PLD) in a vacuum chamber with a base pressure of 1x10 -7 Torr. During deposition, the Si substrate was held between 150 and 450 o C, and was rotated in order to obtain uniform growth. Ablation of a ZnO target was achieved by a Lambda Physic KrF excimer laser at a wavelength λ=248nm, and a pulse duration τ=25 ns, at energy density of 0.8 and 1.8 J/cm 2 . The ablated material was deposited onto the Si substrate under an oxygen partial pressure ranging from of 1x10 -3 to 8x10 -7 Torr. X-Ray diffraction (XRD) measurements were carried out in order to identify the structure and the phase of the grown ZnO films. The surface morphology was examined using atomic force microscopy (AFM). Electrical resistivity ρ and the Hall coefficient R H of all films were measured using the 4-point technique of Van der Pauw at room temperature. For ZnO films grown at temperatures of 150 and 200 o C and O 2 pressure lower than 10 -6 Torr, electrical measurements were performed in the temperature range from 80 to 340 o K using a liquid- N2 cryostat. The Hall mobility values (μ H ) were calculated by the well-known relation: μ H =R H /ρ. 30 31 32 33 34 35 36 37 38 39 40 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 2000 4000 6000 8000 grain size=8,3nm 10 -3 Torr Si (200) ZnO (002) ZnO (101) ZnO (100) 2Θ (deg) intensity (a.u) 1,5 x 10 -6 Torr laser fluence : 0,8J/cm 2 sub. temperature : 200 0 C grain size=4,5nm Fig 1. XRD patterns for PLD grown ZnO thin films on Si at 200 o C under O 2 pressures of 10 -3 and 1,5x10 -6 Torr. III. RESULTS-DISCUSSION XRD patterns of ZnO thin films grown on (100) Si were obtained for various substrate temperatures ranging from 150 to 450 o C at 10 -4 Torr and laser fluence of 1,8 J/cm 2 . Crystallite size, extracted from XRD using the Scherrer formula, increases as the growth temperature increases. Crystallinity and alignment of the ZnO thin films is also dependent on the O 2 partial pressure inside the growth chamber. In Figure 1 the XRD patterns for 1-4244-0084-8/05/$20.00/©2005 IEEE
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Role of Low O2 Pressure and Growth Temperature on Electrical Transport of PLD Grown ZnO Thin Films on Si Substrates
Ch. Pandis1, Ν. Brilis1, D. Tsamakis1, H. Ali2, S. Krishnamoorthy2 and A. A. Iliadis2,3
1: School of Electrical Engineering and Computer Science, National Technical University of Athens, Iroon Polytechniou 9 Zografou, 15773 Athens, Greece
2: Department of Electrical & Computer Engineering, University of Maryland, College Park, Maryland 20742, USA
3: Department of Information and Communication Systems Engineering, University of the Aegean, Karlovasi, 83200 Samos, Greece
I. INTRODUCTION
Recently there has been noticeable interest in the fabrication and study of ZnO films for a variety of electrical and optical devices. Reports indicate that the electrical properties of undoped ZnO thin films can be controlled by changing the growth parameters [1] such as: O2 partial pressure during growth, deposition temperature as well as the fluence of the laser beam. The effects of very low oxygen pressure however have not been well investigated. In this work we have investigated the effect of very low oxygen pressure and deposition temperature during growth on electrical and structural characteristics of PLD grown ZnO films on Si.
II. EXPERIMENTAL DETAILS
Undoped ZnO thin films were grown on (100) Si substrates by pulsed laser deposition (PLD) in a vacuum chamber with a base pressure of 1x10-7 Torr. During deposition, the Si substrate was held between 150 and 450oC, and was rotated in order to obtain uniform growth. Ablation of a ZnO target was achieved by a Lambda Physic KrF excimer laser at a wavelength λ=248nm, and a pulse duration τ=25 ns, at energy density of 0.8 and 1.8 J/cm2. The ablated material was deposited onto the Si substrate under an oxygen partial pressure ranging from of 1x10-3 to 8x10-7 Torr. X-Ray diffraction (XRD) measurements were carried out in order to identify the structure and the phase of the grown ZnO films. The surface morphology was examined using atomic force microscopy (AFM). Electrical resistivity ρ and the Hall coefficient RH of all films were measured using the 4-point technique of Van der Pauw at room temperature.
For ZnO films grown at temperatures of 150
and 200oC and O2 pressure lower than 10-6 Torr, electrical measurements were performed in the temperature range from 80 to 340oK using a liquid-N2 cryostat. The Hall mobility values (µH) were calculated by the well-known relation: µH=RH/ρ.
30 31 32 33 34 35 36 37 38 39 400
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nsity
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laser fluence : 0,8J/cm2
sub. temperature : 2000C
grain size=4,5nm
Fig 1. XRD patterns for PLD grown ZnO thin films on Si at 200oC under O2 pressures of 10-3 and 1,5x10-6 Torr.
III. RESULTS-DISCUSSION
XRD patterns of ZnO thin films grown on (100) Si were obtained for various substrate temperatures ranging from 150 to 450oC at 10-4 Torr and laser fluence of 1,8 J/cm2. Crystallite size, extracted from XRD using the Scherrer formula, increases as the growth temperature increases. Crystallinity and alignment of the ZnO thin films is also dependent on the O2 partial pressure inside the growth chamber. In Figure 1 the XRD patterns for
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ZnO films grown on Si substrates under O2 pressure of 10-3 and 1,5x10-6 Torr at 200oC and a fluence of 0,8 J/cm2 are shown. The observed preferential alignment at 2Θ = 34,4o corresponds to the (002) direction. As the oxygen pressure increases, the intensity of the peaks decreases, while the directions (100) and (101) appear. It seems that the crystalline quality of the films degrades to polycrystalline, while average grain size increases from 4,5 to 8,3 nm.
Figure 2 is a graphic representation of the sheet resistance values versus growth temperature in the range of 150 – 450oC under O2 partial pressure of 10-4 Torr and laser fluence of 1,8 J/cm2. For substrate temperatures higher than 300oC, sheet resistance shows a rapid increase. Since non-stoichiometry is the main cause of n-type electrical conductivity in ZnO films, electron conduction may be attributed to O vacancies or Zn interstitials [2]. Resistivity increase is probably caused by the increased incorporation of O in the films’ matrix, resulting in the decrease of O vacancies and thus reduction of electron concentrations. This behavior of the electrical parameters of ZnO films, grown at O2 pressure lower than 10-6 Torr can be explained by the main role of the residual H2 incorporated during growth rather than by natural defects (O vacancies or Zn interstitials) [3].
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et R
esis
tanc
e (Ω
/sq)
Substrate Temperature (oC)
oxygen pressure : 10-4 Torrlaser fluence : 1,8J/cm2
Fig 2. Sheet resistance versus growth temperature for PLD grown ZnO thin films on Si under O2 ambient of 10-4 Torr
REFERENCES
[1] Ji Nan Zeng, Juay Kiang Low, Zhong Min Ren, Thomas Liew and Yong Feng Lu, Appl. Surface Science, 197-198, 362-367, (2002) [2] D.C. Look, Semicond. Sci. Technol. 20 S55-S61 (2005) [3] C.G. Van der Walle, Phys. Rev. 85, 1012-1015 (2000)
