Influence of oxygen partial pressure on optical and ...

Influence of oxygen partial pressure on optical and structural properties of RF sputtered ZnO thin films P. Murkutea, S. Sahaa, S. K. Pandeya, A. Chatterjeeb, D. Dattac . And S. Chakrabartia* a

Department of Electrical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. b

Jadavpur University, Kolkata-700032, India.

c

Heritage Institute of Technology, Anandapur, Kolkata, West Bengal 700107, India. ABSTRACT In this paper we report a detailed investigation of ZnO thin film properties deposited on Si substrate

o

at 400 C using RF sputtering. To reduce oxygen induced vacancies and interstitial defects in samples, variable oxygen flow rate during deposition followed by post growth annealing in oxygen ambient were carried out. Four samples were deposited under constant temperature condition but with variable oxygen partial pressure of 0%, 20%, 50% and 80% in Argon and Oxygen mixture, namely sample S1, S2 , S3 and S4 respectively. Deposited films were further annealed at 700, 800, 900 and 1000˚C in oxygen ambient for 10s. Photoluminescence (PL) measurements carried at low temperature (18K) demonstrated near band edge emission peak of ZnO at 3.37eV. Increment in PL intensity was observed with increasing annealing temperature and a particular sample S4 annealed at 900 measured narrowest full width half maxima (FWHM) of ~0.1272eV.

Defects peaks observed at lower energies were

suppressed with increasing oxygen flow and post growth annealing, indicating improvement in film quality. From HRXRD measurement it was observed S4 sample annealed at 900˚C has the highest peak intensity and narrowest FWHM compared to other samples, demonstrating the best crystalline property of annealed film at 900˚C. Highest XRD peak intensity measured at 34.53˚ corresponds to (002) crystal orientation reveals that the films were highly caxis oriented. AFM results show increase in grain size with increasing oxygen flow and annealing temperature which ensures improvement in morphological properties of the film.

1. INTRODUCTION There has been an extensive research on ZnO based thin films for electronics and opto-electronic devices. ZnO being an II-VI semiconductor with direct band gap of 3.37 eV is more suitable for photonic devices. It has larger exciton binding energy (60 meV) resulting in an efficient exciton emission at room temperature [1-2]. Hence ZnO finds applications in UV or blue emitting LEDs. It is a general assumption that the UV emission center could be an exciton transition. On account of ZnO being chemically stable and environmentally friendly it is recognized as

Oxide-based Materials and Devices VII, edited by Ferechteh H. Teherani, David C. Look, David J. Rogers, Proc. of SPIE Vol. 9749, 974921 · © 2016 SPIE CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2214006 Proc. of SPIE Vol. 9749 974921-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/12/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

a strong candidate for thin film transistors, transparent electrode in liquid crystal displays and light emitting diodes [3-4]. ZnO has a hexagonal wurtzite crystal structure with a = 0.32 nm and c = 0.512 nm [5]. It is naturally an n-type semiconductor because of deviations from stoichiometry associated with donor state, such as oxygen vacancies (VO) and Zn interstitials (Zni) defect state [7]. The presence of vacancies/defects has strong impact on optical and structural properties of the grown film. Therefore, most of the studies are motivated on controlling defects and improving the quality of thin film in order to enhance UV emission. Various methods have been reported for growth of ZnO thin films such as metal organic chemical vapor deposition [4], molecular beam epitaxy [6], pulsed laser deposition [3],spray pyrolysis [8], RF- sputtering [9] and thermal evaporation [10]. Among these RF sputtering has drawn significant attention for ZnO growth since the resulting film properties can be tailored /altered by varying the sputtering conditions such as substrate temperature, growth ambient, deposition time, pressure, and RF power [11]. In this work, we report the influence of growth ambient (Ar/Ar+O2) on ZnO film deposited by RF sputtering on Si (1 0 0) substrates followed by post growth annealing treatment in oxygen ambient. The optical, structural and surface morphology of deposited ZnO thin films, were investigated by photoluminescence measurements, high resolution X-ray diffraction HRXRD) measurement and atomic force microscopy (AFM).

2. EXPERIMENTAL DETAILS A 2-inch RCA cleaned semi insulating silicon substrate (100) was used for the growth process. Prior to deposition substrate was immersed into 2 % HF (30 seconds) for removal of native oxide. ZnO thin films were grown on this substrate by means of RF- sputtering. A 2- inch ZnO target (99.99%) was used with substrate temperature maintain at 400˚C and sputtering power of 100W during the entire process of deposition for 1hr. Four samples were grown with variable oxygen partial pressure of 0%, 20%, 50% and 80% in argon and oxygen mixture, namely sample S1, S2, S3 and S4. Deposited films were subjected to rapid thermal annealing for 10 sec in oxygen ambient at 700, 800, 900 and 1000˚C to reduce oxygen vacancies and interstitial related defects. Low temperature photoluminescence measurements (PL) were carried out using He-Cd laser of excitation wavelength 325nm and Silicon detector array was used to record emission spectrum. High resolution X-ray diffraction with wavelength of λ=1.5420 nm was carried out using Rigaku (Cu- Kα radiation) within a 2-theta range of 20 to 80 degree. Atomic force microscopy was done in tapping mode for surface imaging to find the surface morphology of deposited film.

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3. RESULT AND DISCUSSION Photoluminescence (PL) measurements carried at low temperature (18K) demonstrated near band edge emission peak of ZnO at 3.37eV. For asgrown samples S1 and S2 defect level peaks were dominant whereas for S3 and S4 defect peaks intensity reduces indicating increase in oxygen content reduces zinc interstitial defects and passivated oxygen vacancies. Increment in PL intensity was observed with increasing annealing temperature and for a particular sample S4 annealed at 900oC measured lowest full width half maxima (FWHM) of ~0.01272eV. Defects peaks observed at lower energies were suppressed with increasing oxygen flow during deposition and post growth annealing, indicating enhancement in film stoichiometry and thereby film quality. With increasing annealing temperature a reduction in PL intensity in between 400-700 nm was observed. Annealing above 700 oC resulted in bimodal PL peak. Thus in these two peaks, first peak at 3.37eV correspond to free exciton transition and second peak at 3.28eV corresponds to donor to bound exciton transition. FWHM values from PL spectra of as grown and 900°C annealed film are 0.624 and 0.012meV respectively. It shows that S4 annealed samples at 900 and 1000ºC have the minimum FWHM indicating better emission properties. Further, we notice that S2 shows sharpest PL spectrum which can be attributed to the reduced oxygen vacancies and interstitials defects due to highest oxygen percentage in growth ambient thereby improving the film stoichiometry.

As_grown 000°c

{-S4 S3

1-aoo°c

I-

IT00`c

S2I

Ias gown

30

3,1

3.2

3.3

3.4

30

3.5

3.5

3.1

3.2

3.3

3.4

3.5

3.6

37

Energy (eV)

EnergyieV)

(A)

(B)

Fig.1. Demonstrate low temperature (18K) PL response A-comparison of all as grown samples, B- Comparison of S 4as grown and their annealed


37

Fig. 2 displays the HRXRD results for asgrown film A and S4 annealed at different temperatures. Crystal orientation was observed at (002) indicating the films were highly c-axis oriented perpendicular to the substrate surface. X-ray peak intensity increased with annealing temperature. Highest peak intensity at 34.53˚ was measures from S4 sample annealed at 900C. With increasing oxygen partial pressure in growth ambient FWHM increases which was tabulated in table 1.

As grown samples

2_theta

FWHM

Grain Size

(Deg)

(Deg)

(nm)

S1

34.30

0.347

43.67

S2

34.33

0.5052

40.34

S3

34.21

0.6767

22.38

S4

34.34

0.5052

29.99

Table1. Comparison of FWHM, Grain Size for all as grown samples using XRD spectra

Highest X-ray peak intensity and lowest FWHM value of 0.2645˚ was observed for sample S4 annealed at 900C. Thermal treatment enhance the intensity of HRXRD peak at (002) axis thereby increasing the grain size.

1000C S4

a s_ g ro w n

900C

Intensity (a.u.)

Intensity (a.u.)

S3

S2

800C

700C

S1

30

31

32

S 4_ as _g ro w n

33

34

35

36

37

38

39

40

30

31

32

2-T h eta (D e g )

33

34

35

36

37

38

39

40

2 -T h e ta (D e g )

(A)

(B)

Figure 2 : 2-theta-omega scan for ZnO films, A -For as grown samples, B- for S4 annealed at different temperature


75

S4

0.34

S4 70

0.32

65

Grain Size (nm)

FWHM (Deg)

0.30

0.28

0.26

60

55

0.24

50 0.22

45

0.20 700

750

800

850

900

950

700

1000

750

800

850

900

950

1000

Annealed Temperature (Deg)


(a)

(b)

Fig. 3. Shows the plot of (a) FWHM (b) Grain Size for S2 sample annealed at 700C, 800C, 900C, 1000C

Fig 4 illustrates AFM images of the surface morphology for as grown and annealed ZnO films deposited. Average surface roughness increases with annealing temperature due to increase in grain size. High temperature annealing can stimulate the migration of grain boundaries and causes coalescence of more grains. Maximum grain size was observed from S4 sample annealed at 900C.

As grown

700C

800C

1: Height 5.0 pin 15

900C

1000C

S4

14

Roughness (nm)

13 12 11 10 9 8 7 700

750

800

850

900

950

1000


Figure. 4 AFM images of S2 as grown and annealed at 700C, 800C, 900C, 1000C and roughness as a function of annealing temperature


4. CONCLUSION In this study, optical and structural of ZnO has been investigated with oxygen partial pressure variation. Low temperature PL (18K) spectra demonstrated near band edge emission peak of ZnO at 3.37eV. Thermal treatment of sample at the range 700-1000 oC causes the increasing in PL peak intensity, decrease the defect states related to oxygen vacancies and zinc interstitial. At higher annealing temperature (above 700 oC), two peaks observed in PL spectra. First peak at 3.37eV correspond to free exciton transition and second peak at 3.28eV corresponds to donor to bound exciton transition. Sample S4 which was grown in oxygen rich ambient annealed at 900 oC measured narrowest FWHM) of ~0.1272eV for peak at 3.37eV, indicating high quality ZnO film. Increment in PL intensity was observed with increasing annealing temperature and zinc vacancies and oxygen interstitial a particular defects peaks (400-700nm) observed at lower energies were suppressed with increasing oxygen flow and post growth annealing, indicating improvement in film quality. From HRXRD measurement it was observed S4 sample annealed at 900˚C has the highest peak intensity and narrowest FWHM compared to other samples, demonstrating the best crystalline property to (002) crystal orientation reveals that the films were highly c-axis oriented. AFM results show increase in surface roughness with increasing oxygen flow and annealing temperature which ensures improvement in morphological properties of the film. These findings will be helpful for improving performance of ZnO based devices on Si substrate.

ACKNOWLEGMENT Authors would like to acknowledge the financial support provided by the Department of Science and Technology, India, and as well as partial financial support provided by the Department of Information Technology, Nano Fabrication facility (IITBNF). SPM Physics, IIT Bombay is also acknowledged.

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