Photoluminescence Studies of Epitaxial ZnO Thin Films on Si (100) Substrates by Pulsed Laser Deposition

Ashok Kumar, Sathyaharish Jeedigunta, I. Tarasov and S. Ostapenko

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AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract

ZnO thin films were grown on silicon (100) substrates by pulsed laser deposition at various substrate temperatures ranging from room temperature to 600 °C with and without oxygen during the deposition. The effect of deposition parameters on the properties of ZnO thin films was investigated by means of X-ray diffraction (XRD), room temperature photoluminescence (PL), and atomic force microscopy (AFM) measurements. As the deposition temperature is increased, both crystallinity and optical quality of the films have been improved. The XRD measurements have revealed that highly c-axis oriented ZnO films with a preferential orientation of (002) and (004) are grown even at temperatures as low as 300°C. It was also observed from XRD measurements that for those films deposited at higher temperatures, with the increase in the oxygen content, the structural quality of the films degraded. In contrast, the UV luminescence intensity increased, which implies that the optical quality of the film improved. Along with the UV emission peak, a broad unstructured band consisting of a green, orange, and red ones was observed, the origin and cause of each of these emissions are investigated in this paper.

Keywords

ZnO, Pulsed Laser Deposition, Photoluminescence

Introduction

There has been great interest in the growth, synthesis and applications of II-VI compound semiconductors, because these compounds find applications in optoelectronics at shorter wavelengths. ZnO is a II-VI compound semiconductor with a direct band gap of 3.37 eV at room temperature and crystallizes in hexagonal wurtzite structure [1]. It exhibits strong n-type conductivity and has good piezoelectric, piezo-optic, photoelectric, and acousto-optic properties due to which it has numerous applications including sensors, piezoelectric devices, surface acoustic wave devices, optical waveguides, cryovaristors, nanotips [2,3]. ZnO also acts as an ideal buffer layer for GaN thin films. It is an isomorphic material with wurtzite GaN and the lattice mismatch in their basal plane is approximately 2.2% [4]. An interesting feature of ZnO thin films is its large excitonic binding energy of 60 meV at room temperature, which attracted much attention as an optoelectronic material for short wavelength light emitting devices and laser diodes [5].

A variety of techniques have been utilized to grow ZnO thin films of good quality such as rf magnetron sputtering [6-8], MOCVD [9,10], MOVPE [11], spray pyrolysis [12,13], and pulsed laser deposition [14-17]. Among such various deposition techniques, pulsed laser deposition (PLD) has been identified as one of the most promising techniques in the thin film technology. PLD offers high quality crystalline films at relatively lower deposition temperatures and higher deposition rates.

To date, many reported the photoluminescence properties of ZnO thin films. Generally there are two emission peaks centering at UV and visible region in the PL spectra of ZnO, which have been studied extensively. Wang et al. [18] suggested the cause of deep-level emissions to be a zinc vacancy and antisite defect. Bae et al. [17] suggested that the visible emissions are due to defect centers, which correspond to the lattice imperfection from large lattice mismatch and compressive force on the film from the substrate due to thermal expansion coefficient. Few others reported that the oxygen vacancies or zinc interstitials are responsible for the deep level emissions mainly in green region [19-24]. The visible emissions include green, orange and red photoluminescence’s. The origin of the orange luminescence [25,26] and the red luminescence [27] has been reported in very few papers.

In this paper, we have focused our attention on the photoluminescence studies of ZnO films at different deposition parameters. The films show the luminescent properties in UV, green-yellow, orange, and red regions. The orange and the red luminescence properties in ZnO films received less attention. Hence an attempt is made to understand the phenomenon and study these emissions. The effect of the ambient pressure and deposition temperature on the luminescent properties has been investigated. The structural and morphological properties of the films have been characterized by x-ray diffraction (XRD) and atomic force microscopy (AFM).

Experiment

High purity ZnO (99.9999%) target was used to deposit the films on Si(100) substrates. A Lambda Physik LPX 210i KrF excimer laser was operated at a wavelength of 248 nm, a pulse width of 25 ns, a laser fluence of 450 mJ/pulse, and at a repetition rate of 10 Hz. The target and the substrate were separated by 5.0 cm. The deposition was carried out by first evacuating the chamber to a base pressure of 1 X 10^-6 Torr. The laser beam was directed and focused on to the target using various laser optics. The ZnO films were deposited on si(100) substrates at different temperatures ranging from RT to 600 °C under UHV(without any oxygen), 50 mTorr, and 100 mTorr oxygen reactive ambience for 20 min. The target holder is rotated throughout the experiment to have a uniform stoichiometry of the film and to avoid any damage to the target. The target is polished before each deposition. The thicknesses of the films deposited were around 300 – 375 nm. The room temperature PL measurements were carried out by employing a 325 nm cw He-Cd laser. The structural properties of the films were characterized by X-Pert PRO XRD with CuKa radiation. The surface morphology and roughness were measured by Nanoscope AFM.

Results & Discussion

Crystal Structure

Fig. 1-4 shows the XRD patterns of the ZnO thin films deposited at different substrate temperatures and partial pressures. At 100 °C without any oxygen, none of the characteristic peaks of ZnO could be observed. But with the incorporation of oxygen during the deposition at a substrate temperature of 100°C, the ZnO peaks could be noticed. Hence it is evident that ZnO can be grown epitaxially even at temperatures as low as 100 °C with the supply of oxygen. At a constant oxygen pressure of 50mTorr and with the increase in the substrate temperature from 100 °C to 600 °C, the intensity of ZnO (002) peak around 35° has increased tremendously. For temperatures above 300 °C, it was observed that even without any oxygen during the deposition, the preferred orientations i.e., ZnO (002) and ZnO (004) peaks are seen at around 34.01° and 71.36°. The crystallanity of the films improved further as the oxygen content in the films increased. As the oxygen pressure was increased from 50 to 100 mTorr, there is a remarkable decrease in the FWHM value from 0.32 to 0.16 (Fig. 1(a) and 1(b)). The comparison of the crystalline quality of the films from their XRD patterns was performed by taking into account the FWHM values and intensity counts as tabulated in Table 1. As the thickness of the film increased, the crystalline nature of the film improved, which is also in agreement with Zhang et al [28]. It was observed that there was a shift in the 2θ values from 34°. When the diffraction peak shifted towards a larger angle direction, from Bragg’s law it can be implied that there is a decrease in the lattice constant. It was attributed to the generation of oxygen vacancies or zinc interstitials [30]. As shown in the Fig. 3, the films deposited at a temperature of 400 °C in 50 mTorr oxygen partial pressure had the least FWHM value of 0.216° for the ZnO (002) peak. Of the samples prepared for the study, this sample is relatively thicker with a thickness around 372 nm. Both the (002) and (004) peak intensities were high for this sample as can be viewed from Fig 3.

Figure 1. XRD Pattern of ZnO Film Deposited at 100 °C in 50 mTorr and 100 mTorr O₂

Figure 2. XRD Pattern of ZnO Film Deposited at 300 °C in 0 mTorr and 50 mTorr O₂

Figure 3. XRD Pattern of ZnO Film Deposited at 400 °C in 0 mTorr and 50 mTorr O₂

Figure 4. XRD Pattern of ZnO Film Deposited at 600°C in 0 mTorr and 50 mTorr O₂

Table 1. XRD Analysis of ZnO films deposited at various Deposition conditions.

Photoluminescence (PL)

Photoluminescence studies were conducted at room temperature using a He-Cd (325 nm) laser as an excitation source on ZnO films deposited at different temperatures and partial pressures (Figure 5 and 6).

Figure 5. Room temperature PL from the ZnO films deposited on silicon substrate at different deposition temperatures ant constant oxygen pressure 50 mTorr

Figure 6. Room temperature PL from the ZnO films deposited on silicon substrate at different deposition temperatures ant constant oxygen pressure 100 mTorr

Table 2. PL Results of ZnO Thin Films.

The PL spectra of the ZnO films exhibited four peaks: UV emission peak around 378 nm, green-yellow luminescence at 510 nm, orange band at 630 nm, and red luminescence at 738 nm. The first peak due to UV emissions is attributed to band-to-band transitions, excitonic emissions, and donor-acceptor pair transitions. The broad green-yellow band at around 505 nm is due to the deep level emissions in green region, which is attributed to oxygen vacancies, zinc interstitials or zinc vacancies [21]. These two peaks are the most commonly found luminescent peaks in all the ZnO samples.

Apart from these two common peaks, the other two emission peaks observed in the orange and the red regions are in the focus of attention. The orange band around a wavelength of 630 nm is attributed either to the presence of excess local oxygen or is known to be caused by structural imperfection [26]. This band was observed in oxygen rich ZnO films deposited by spray pyrolysis [3] and filtered cathodic vacuum arc [30]. This band contains many overlapped peaks.

The fourth band was observed in the infrared region at a wavelength of 738 nm. The sputtered ZnO films exhibit n-type conductivity, which led to red and IR emission bands [22,31]. It was believed that the centers responsible for this emission are attributed to the formation of native defects rather than from chemical impurity doping such as copper or lithium. The origin of this band is still ambiguous and is under study.

In the first place, the effect of substrate temperature was studied at constant oxygen pressure. Figures 5 and 6 present a room-temperature PL spectra measured for the ZnO films deposited at 100°C, 300°C, and 600°C. At each of these temperatures, an oxygen partial pressure of 50 mTorr and 100 mTorr is maintained. As can be seen from Figure 5, the intensity of UV peak at a oxygen pressure of 50 mTorr increased tremendously with the increase in the deposition temperature. We could not observe any detestable emission corresponding to 505 nm band in our samples, which means that the films deposited had high quality.

At the oxygen pressure of 100 mTorr, it was observed that the strong yellow-orange luminescence from the ZnO films (Figure 10). The intensities of UV peak for the films deposited at 300°C and 600°C are very close to each other, which may be an indication of some type of saturation. For all the temperatures, the yellow-orange peak can be fitted by two Gaussian curves. The position of these Gaussian curves corresponds to 646 nm and 786 nm. The presence of these bands can be attributed to the presence of excess local oxygen or existence of a structural imperfection and to the IR emission from n-type conductivity in ZnO film. The ratio between the intensities of these two peaks varies linearly with temperature (see insert on Figure 6). The yellow and orange bands observed in the films appear to be interrelated because the film stoichiometry is a combination of the target composition and the oxygen gas supplied. The origin of the orange luminescence arising from the structural imperfection due to polycrystallinity of the ZnO films can be ruled out, since the XRD results prove that the ZnO films are highly crystalline and c-axis oriented.

This shows that the UV luminescence intensity is dependent on the ZnO film stoichiometry rather than its microstructural quality, which is in agreement with [1,11] and contrary to that given by Tang et al [28].

Surface Morphology

Figures 7 and 8 show the surface morphology of ZnO films deposited at 100°C and 400 °C as investigated by AFM. The first set of samples was deposited at 100°C with and without oxygen. For those samples with oxygen, the partial pressure was maintained at 50 mTorr. It was observed that those films deposited in presence of oxygen had a smoother surface with fine grains distributed uniformly throughout the surface of the film. The surface roughness of the film deposited at 100°C in 50 mTorr oxygen is found to be around 6.44 nm. As the temperature was increased from 100°C to 400°C, the grain size is found to increase. At 400°C without oxygen, it can be observed that the grain size was not uniform and a surface roughness value of 11.44 nm was obtained. But with the incorporation of oxygen, the grains were uniform and were concentrated at the center of the film with a considerable reduction in the surface roughness value to 5.54 nm. It was also observed that there is particulate formation with a varying particulate size. This might be attributed to the PLD technique employed for depositing the films.

Figure 7(a). Top View of ZnO film deposited at 100°C and 0 mTorr O₂

Figure 7(b). Top View of ZnO film deposited at 400 °C in 0 mTorr O₂

Figure 8(a). Surface plot of ZnO film deposited at 100°C in 50 mTorr O₂.

Figure 8(b). Surface Plot of ZnO film deposited at 400°C in 50mTorr O₂.

Table 3. RMS Roughness of the films deposited at different conditions.

Conclusions

We have deposited ZnO films on Si(100) at various deposition temperatures ranging from room temperature to 600°C with and without oxygen by pulsed laser deposition. With the incorporation of oxygen, there is an improvement in the ZnO phase and the crystalline nature of the films. The XRD results indicated that the films grown at a substrate temperature of 400°C and at 50 mTorr oxygen pressure had the lowest FWHM value of 0.216°, which is also supported by the AFM results by having the least surface roughness value of 5.54 nm. As the oxygen partial pressure is increased from 50 to 100 mTorr, the crystallinity of the films improved. But at higher temperatures (~600 °C) the structural properties of the films degraded with the increase in the oxygen pressure, though the optical properties have improved. The photoluminescence studies show that the UV luminescence increases with the increase in the oxygen pressure. This may be attributed to the improvement of film stoichiometry. The appearance of the orange band in the films may be attributed to the excess local oxygen. The PL results for the film deposited at 300°C and at 50 mTorr oxygen has good photoluminescence properties.

References

Contact Details

Ashok Kumar
Department of Mechanical Engineering
University of South Florida
Tampa FL 33620; USA

Email: [email protected]

Sathyaharish Jeedigunta
Department of Electrical Engineering
University of South Florida
Tampa FL 33620; USA

I. Tarasovc and S. Ostapenkoc
Nanomaterials and Nanomanufacturing Research Center
University of South Florida
Tampa FL 33620; USA

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 10[2] (2008) 95-100.