Relationship between the photoluminescence and conductivity of undoped ZnO thin films grown with various oxygen pressures

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Relationship between the photoluminescence and conductivity of undoped ZnO

thin ﬁlms grown with various oxygen pressures

Chang-Feng Yu

a,

*

, Che-Wei Sung

a

, Sy-Hann Chen

a

, Shih-Jye Sun

b

a_{Department of Applied Physics, National Chiayi University, No. 300 Syuefu Rd, Chiayi 600, Taiwan} b

Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan

1. Introduction

Zinc oxide (ZnO) ﬁlms have been investigated in recent years as transparent conducting oxides (TCO) because of their good electrical and optical properties in combination with their large band gap of 3.3 eV, abundance in nature, and lack of toxicity. It should be noted that this material offers a number of advantages. This material is an II–VI compound semiconductor with a wide variety of applications as electrodes, window materials in display, solar cells, and various optoelectronic devices [1–3]. Various techniques have been used to deposit undoped and doped ZnO ﬁlms on different substrates, including metal-organic chemical vapor deposition (MOCVD)[4], molecular beam epitaxy (MBE)[5], pulsed laser deposition (PLD)[6], and spray pyrolysis deposition (SPD)[7]. Furthermore, compared to other deposition methods, PLD is characterized by several advantages, such as low substrate temperatures, good adhesion on substrates, and the easy deposi-tion of alloys and compounds of materials with different vapor pressures[6,8].

The main aim of this study was to investigate the structure, electrical and optical properties of undoped ZnO ﬁlms synthesized by the PLD technique. In order to investigate the relationship between electrical properties and oxygen vacancies, the

experi-ment in this article was considered to vary the oxygen pressures. The photoluminescence spectra have been used to probe the concentration of oxygen vacancies.

2. Experiment

In this article a reliable method was used to deposit the thin ﬁlms of undoped zinc oxide transparent electrodes on glass substrates by PLD. The ablation PLD targets with dimensions of 1 in. diameter 0.125 in. thick were fabricated by the combustion synthesis reaction technique. The pure (99.99%) ZnO powders were mixed with polyvinyl alcohol binder and water. The mixture was stirred, crushed into powder, dye palletized, and sintered at 1200 8C for 5 h. Undoped ZnO thin ﬁlms were evaporated in a PLD vacuum chamber with a base pressure of 1 10 6_{Torr. Oxygen}

(99.99%) gas was introduced in the chamber as the reactive gas and the working pressure varied from 40 to 150 mTorr during deposition. The glass substrates were cut into standard sizes of 2.5 cm 6 cm. All substrates were ultrasonically cleaned in acetone and dried in an oven until it was loaded into the deposition chamber. A stainless steel substrate holder is capable of being heated up to 400 8C. The deposition temperature was maintained at 150 8C. An Nd:YAG pulsed laser with a visible wavelength of 532 nm, pulse duration of about 7 ns, and 18.5 mJ/ cm2energy density was focused on the target to obtain the ZnO thin ﬁlms. The target to substrate distance was kept at 2.5 cm. The ZnO ﬁlms were characterized for cystallinity and surface

A R T I C L E I N F O

Article history: Received 22 April 2009

Received in revised form 14 August 2009 Accepted 14 August 2009

Available online 21 August 2009 Keywords:

Pulsed laser deposition Undoped ZnO Deep-level-emission Conductivity

A B S T R A C T

The pulsed laser deposition (PLD) technique is used to deposit undoped ZnO thin films on glass substrates at 150 8C with different oxygen pressures of 40, 80, 100 and 150 mTorr. X-ray diffraction (XRD) and atomic force microscopy (AFM) studies indicated that the obtained ZnO thin films were hexagonal wurtzite-type structures with strong (0 0 2) c-axis orientation. The relationship between photoluminescence and the conductivity of the ZnO thin films grown by pulsed laser deposition at various oxygen pressures was also discussed. The intensity of the deep-level-emission (DLE) and conductivity generally increased as the oxygen pressure decreased. The intensity of DLE peak was generally proportional to the conductivity. The band gap energy values, determined from transmittance spectra, were around 3.30–3.34 eV, and decreased when the oxygen pressure increased.

* Corresponding author.

E-mail address:[email protected](C.-F. Yu).

Contents lists available atScienceDirect

Applied Surface Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

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morphology by X-ray diffraction (XRD) and atomic force micro-scopy (AFM). A UV–vis spectrophotometer (Agilent 8453) was used to measure the optical transmission spectra for wavelengths from 300 to 800 nm. Photoluminescence were measured with a He–Cd laser as a light source using an excitation wavelength of 325 nm. All spectra were measured at room temperature. The electrical properties were measured in the four-point probe Van der Pauw conﬁguration. Indium was used as the electrodes.

3. Results and discussions

The experimentally measured XRD data for undoped ZnO ﬁlms deposited with various oxygen pressures are represented inFig. 1. The results of the XRD analysis show that the relatively sharp (0 0 2) peaks of all ZnO thin ﬁlms are observed at 2

u

= 34.28 in the films. It indicates that the ZnO thin films deposited by PLD show a good c-axis orientation, namely, the vertical growth to the substrate. It was observed that the structure of ZnO films depends sensitively on the preparation oxygen pressure. The (0 0 2) peak becomes sharper with increasing oxygen pressure. Undoubtedly, it was caused by the crystallinity of the ZnO thin films that have improved and formed a more stoichiometric oxide with increasing oxygen pressure. After fitting the XRD patterns with a Gaussian line shape, the average particle size was determined from the full width at half maximum (FWHM) of the strongest X-ray diffraction peak (0 0 2) using Scherrer equation[9]which assumes the small crystallite size to be the only case of line broadening. The average grain size of ZnO is about 10 nm.

The surface morphology of the ZnO thin films was characterized by AFM. With a bias of 0.5 V applied to the tip during scanning, the topography (left) and current (right) images of four ZnO samples with different oxygen pressure are shown inFig. 2. From the results of surface morphology as shown inFig. 2(left), all the surfaces of films revealed a granular, polycrystalline and nano-crystal morphology with grain size. It is well known that all of the ZnO films with better texture aligned microstructures have superior transmission to those that are randomly oriented and, therefore, suitable for most optical, electrical and optoelectronic applications. It is also found that the surface morphology is strongly dependent on the oxygen pressure. With increasing oxygen pressure, the surface morphologies of the films reveal a

noticeable transformation. It seems that the grain sizes become larger when the oxygen pressure is increased. However, the average surface root-mean-square (RMS) roughness also increases. Surface RMS roughness values of the ZnO thin ﬁlms with the various deposited oxygen pressures of 40, 80, 100, and 150 mTorr are 18.24, 33.27, 33.37, and 24.31 A˚ for a scanning area of 1

m

m 1

m

m, respectively.

In the current images ofFig. 2(right), both the conducting and non-conducting regions exist on the surfaces, with the former characterized by the bright contrasts. According to the local current–voltage (I–V) measurements, when the tip is located at the regions where the current is greater than 1

m

A, all the measured I– V relationships are linear. The contact is apparently an ohmic

Fig. 1. XRD patterns of ZnO thin ﬁlms as a function of the oxygen pressure.

Fig. 2. Topography (left) and current (right) images of the undoped ZnO thin ﬁlms with: (a) 40 mTorr, (b) 80 mTorr, (c) 100 mTorr, (d) 150 mTorr oxygen pressures. The tip was biased to 0.5 V and the ZnO was grounded.

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contact and these surface regions are called conducting regions. On the other hand, for regions where the current is less than 1

m

A, the I–V relationship is nonlinear. In these regions, a Fowler–Nordheim

[10] type of electron tunneling is responsible for the detected current, these regions are called non-conducting regions. At a high oxygen pressure of 150 mTorr, 100% of the surface area is non-conducting. The variation of the average current values for the four current images inFig. 2(right) is shown inFig. 3as a function of oxygen pressure. The average current values at the oxygen pressures of 40, 80, 100, and 150 mTorr are 6.48, 6.35, 1.64, and 0.006

m

A, respectively. With an increase in the oxygen pressure, the surface conductivity of the ZnO thin films would be reduced. However, with an increase in the oxygen pressure, the increased ambient oxygen atoms fill into the oxygen vacancies resulting in a decrease in carriers, thereby reducing the conductivity of the undoped ZnO thin films. A similar phenomenon of the reduction in the conductivity by increasing the oxygen pressure is confirmed by the Hall measurement in the next section.

The ZnO film generally grows as an n-type semiconductor, due to the presence of native defects in the form of zinc interstitials, oxygen vacancies, or both. The variation of resistivity, mobility and carrier concentration as a function of oxygen pressure are measured and summarized inTable 1. The resistivity of ZnO films shows very sensitive dependence on the oxygen pressure. The Hall measurement shows that the higher conductivity is due to the increase in the carrier concentration and mobility of carriers. The increase in carrier concentration may be due to the formation of metal-rich oxide films. As a result, the resistivity of the as-grown film increases to a certain level of resistivity with higher oxygen pressure. With the increase in the oxygen pressure, the concen-tration of the oxygen vacancies would decrease and the resistivity

of the ZnO thin films would rise up rapidly. Particularly, the film grown above the oxygen pressure of 150 mTorr shows the limitedly measured resistivity, because with a greater increase in the oxygen pressure (above 200 mTorr), the ZnO thin films would be non-conducting.

Photoluminescence (PL) spectra of the ZnO thin films deposited with the various oxygen pressures are shown in Fig. 4. Two emission bands were apparently observed for all samples: one is the near-band-edge emission (NBE) peak at the UV region (3.3 eV), which is due to the carriers recombination from band to band, and the other was the deep-level-emission (DLE) peak around the green-yellow band (2.25–2.52 eV)[11–14], that is corresponding to the transition of the excited optical center from the deep level to the valence band. Many researchers [15–18] have explored the origin of the DLE peak in ZnO thin films and attributed it to the oxygen vacancy. The strong DLE peak is found to be strongly dependent on the oxygen pressure. With the increase in the oxygen pressure, the intensity of the DLE peak decreases remarkably and that of the NBE peak increases rapidly. It is observed that with the increase in the oxygen pressure up to 150 mTorr, the intensity of the NBE peak increases remarkably. Thus, it is expected that the sample grown at the high oxygen pressures probably can improve the stoichiometry of ZnO thin films with less oxygen vacancies. In addition, the relative intensity of the DLE peak (green-yellow) decreases in the ZnO thin films with an increase in the partial oxygen pressure. The stoichiometric effects are similarly consis-tent with the measurement results of the resistivity as listed in

Table 1.

As the samples grown at the lower oxygen pressure have the largest number of oxygen vacancies, correspondingly the electron concentration and the conductivity is higher than those of any other samples grown at the higher oxygen pressure. As the stoichiometry improved with the oxygen supply, the number of oxygen vacancies decreased. This intensity ratio of the DLE to NBE emission may also be related to the electrical conductivity of the thin ﬁlms as shown inFig. 5. A higher ratio of the DLE to NBE

Fig. 3. Variations in the average current analyzed from the current image inFig. 2as a function of the oxygen pressure.

Table 1

Electrical properties of ZnO thin ﬁlms deposited at 150 8C with different oxygen pressures.

Oxygen pressure (mTorr)

40 mTorr 80 mTorr 100 mTorr 150 mTorr n (cm3_{) carrier} concentration 7.36 1020 _{5.27 10}20 _{3.20 10}19 _{4.68 10}18 r(V-cm) resistivity 1.3 10 3 1.74 103 2.25 10 2 4.46 10 2 m(cm2 V1 s 1 ) mobility 8.32 6.83 3.54 1.07

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emission obtained from the PL results in higher electrical conductivity. This means that a good electrical conducting ZnO thin ﬁlm has a high level of DLE peak and a relatively low level of NBE peak.

The effects of oxygen pressure on ZnO thin film growth and optical properties were studied. The optical transmission spectra for the wavelength range between 300 and 800 nm of the deposited ZnO thin films with various oxygen pressures were illustrated graphically as inFig. 6. It is observed that the average transmission of the ZnO thin films improves from 60% to 95% as a result of the increase in oxygen pressure from 40 to 150 mTorr. In other words, a higher oxygen pressure enhances the transparency of the deposited ZnO thin films. The superior optical transmission of the ZnO thin films could be dominantly due to the ability of the film surface roughness to reduce reflectivity, an increase in the structural homogeneity, fine texturing and the approach of the film composition to the stoichiometry. In addition to the DLE elimination at higher oxygen pressures, the surface roughness increased with grain growth at higher pressure. It could reduce the reflectivity and result in superior optical transmission for ZnO thin films. From previous X-ray diffraction and AFM studies as shown in

Figs. 1 and 2, it is revealed that better epitaxially aligned lattice matching and larger surface roughness ﬁlms are suitable to improve the light transmittance. The previous result has been also

proven by the optical transmission spectra measurement in the wavelength range between 300 and 800 nm for the deposited ZnO thin ﬁlms with various oxygen pressures (Fig. 6).

InFig. 6, it is found that the transmission edges around 355– 380 nm shifted to longer wavelength as the oxygen pressure increases. This phenomenon can be observed clearly in Fig. 7, which shows the relationship of the absorption coefﬁcients as a function of the photon energy. Band gap energies can be estimated from the relationship of (

a

h

n

)2_{vs. h}

_n

_{for direct transitions}_[19]_{. The}

energy gap was obtained by extrapolating the linear part of the curve inFig. 7. The optical band gaps of the ﬁlms decrease from 3.34 to 3.30 eV as a result of the increase in oxygen pressure from 40 to 150 mTorr. The widening of band gap with the decrease of the deposited oxygen pressures might be due to the increase of the carrier density in addition to the Moss–Burstein shift. The Burstein–Moss effect is present in direct transition types and it was observed that the optical band gap increases in energy with increasing impurity concentrations[20,21]. Such a widening of the band gap was also noticed earlier in non-stoichiometric ZnO ﬁlms

[22]. The same result is also found inFig. 4. The positions of the NBE peaks (direct energy gap) shifted to longer wavelength as the oxygen pressure increased.

4. Conclusions

In conclusion, ZnO thin films were deposited on glass substrates at 150 8C using a pulsed laser deposition with the oxygen pressures of 40, 80, 100, and 150 mTorr. The c-axis preferred orientation was easily formed by increasing oxygen pressure. Conducting atomic force microscopy was employed to investigate the electrical properties of ZnO films with nanoscale surfaces. The ZnO thin films exhibit low surface roughness, high surface conductivity, and good conductive uniformity with lower oxygen pressure. When the substrate temperature and oxygen background pressure are set at 150 8C and 40 mTorr, respectively, the analysis results indicate that the ohmic-contact-type regions on the ZnO surface are effectively increased and a uniformity effect is achieved. However, it must be emphasized that the ZnO surface conductivity may easily be affected and reduced with increasing oxygen pressure. The same

Fig. 5. Variation in the ratio of the DLE to NBE peak inFig. 4and the resistivity of the ZnO thin ﬁlm as a function of the oxygen pressure.

Fig. 6. Transmission spectra for the ZnO ﬁlms deposited with various oxygen pressures.

Fig. 7. (ahn)2

vs. hnplot for ZnO ﬁlms deposited with various oxygen pressures. The insert shows the optical band gap, determined fromFig. 6, as a function of the oxygen pressure.

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results were also proved by the Hall measurement. When the oxygen pressure was increased, the resistivity of the ZnO thin films would increase. There exist two PL emission peaks in all thin films; one is the NBE emission at about 3.3 eV; the other is the wide DLE emission around 2.25–2.52 eV. With increasing oxygen pressure, the NBE peak was greatly enhanced; the DLE peak was quenched. The PL intensity of DLE peak and the electrical conductivity generally increase as the oxygen pressure decreases. This is probably due to the concentration of oxygen vacancies in the ZnO thin films is increased by reducing oxygen pressure. The electrical conductivity is proportional to the intensity ratio of the DLE to NBE peak. The band gap values of ZnO thin films, which are determined from transmittance spectra, are about 3.30–3.34 eV. The band gap of ZnO thin films decreases when the oxygen pressure increases. Acknowledgment

We acknowledge the useful discussions with Professor C C Hsu. References

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