Superlattices and Microstructures 65 (2014) 146–151

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Superlattices and Microstructures

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 . co m / l o c a t e / s u p e r l a t t i c e s

Effect of thickness on the microstructure,surface morphology and optical propertiesof N-incorporated b-Ga2O3 films

0749-6036/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.11.004

⇑ Corresponding authors. Tel.: +86 755 2603 2709; fax: +86 755 2603 3504.E-mail addresses: ruisun0803@163.com (R. Sun), wanggghit@yahoo.com (G.-G. Wang).

Rui Sun a,⇑, Hua-Yu Zhang a, Gui-Gen Wang a,⇑, Jie-Cai Han a,b,Xin-Zhong Wang c, Lin Cui a, Xu-Ping Kuang a, Can Zhu a, Lei Jin a

a Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, PR Chinab Center for Composite Materials, Harbin Institute of Technology, Harbin 150080, PR Chinac Shenzhen Institute of Information Technology, Shenzhen 518172, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 October 2013Received in revised form 26 October 2013Accepted 8 November 2013Available online 16 November 2013

Keywords:RF magnetron sputteringb-Ga2O3

Nitrogen incorporatedFilm thicknessTransmittanceBand gap

To investigate the effect of thickness on the microstructure, surfacemorphology and optical properties of N-incorporated b-Ga2O3

films, a series of N-incorporated b-Ga2O3 samples with differentthicknesses were prepared by radio frequency (RF) magnetronsputtering method. X-ray diffraction measurements show thatthe crystallinity quality was improved by increasing the film thick-ness. Atomic force microscopy results show that the surface exhib-its hillock and island morphology under different film thicknesses.The optical transmittance of all samples is more than 80% in thevisible range. The UV absorption edge shifts to longer wavelength,and the small change in the optical band gap value is due to theimproved crystallinity of the films.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Monoclinic gallium oxide (b-Ga2O3), a new window material with a wide and direct band gap semi-conductor material, has received considerable attention due to its excellent thermal and chemicalproperties. The band gap of 4.9 eV makes it potentially useful for ultraviolet (UV) optoelectronic de-vices, such as solar-blind photodetector [1]. Also, it is a suitable candidate for high temperature gassensors due to the high melting point (1780 �C) and sensitivity to reducing gases (H2, CO and CHx)[2,3]. Moreover, the dopants or impurities in b-Ga2O3 can considerably change their electrical and

R. Sun et al. / Superlattices and Microstructures 65 (2014) 146–151 147

optical properties, which make it widely used as transparent electrodes for flat panel displays and so-lar cells [4,5].

b-Ga2O3 thin films can be prepared by several methods, such as screen-printing [6], pulsed laserdeposition (PLD) [7], metal organic chemical vapor deposition (MOCVD) [8], sol–gel method [9], mag-netron sputtering deposition [10], and molecular beam epitaxy (MBE) [11]. Because of the advantagesof high deposition rate, low substrate temperature, strong adhesion and easy control of growthparameters, the magnetron sputtering deposition is considered to be a suitable technique forpreparing b-Ga2O3 thin films. Chen et al. has reported that the thickness affected the electrical andgas-sensitive characteristics of b-Ga2O3 films [12]. However, the effect of thickness on the microstruc-ture, surface morphology and optical characteristics of b-Ga2O3 films is less reported. Therefore, in thisstudy, we prepared the N-incorporated b-Ga2O3 films by RF magnetron sputtering method underAr–N2 gas mixtures on sapphire substrates. The effect of thickness on the microstructure, surface mor-phology and optical properties of N-incorporated b-Ga2O3 films was analyzed in detail.

2. Experimental

The N-incorporated b-Ga2O3 films were deposited on 1 � 1 cm2 double-side polished c-plane sap-phire substrates by RF magnetron sputtering deposition. The Ga2O3 (99.99%) target was 6 cm in diam-eter. Prior to deposition, sapphire substrates were ultrasonically cleaned in acetone, rinsed indeionized water, and subsequently dried in flowing nitrogen gas before deposition. The sputteringwas performed in a mix atmosphere of Ar and N2 with a target to substrate distance of 5 cm. The sput-tering pressure and power were 2 Pa and 90 W, respectively. The deposition temperature was 500 �Cand the background pressure was 3 � 10�3 Pa. Different thicknesses were obtained by adjusting thegrowth time.

The crystallinity properties were characterized by X-ray diffraction (XRD). The chemical composi-tion was analyzed using X-ray photoelectron spectroscopy (XPS). The surface morphologies were ana-lyzed by atomic force microscope (AFM). A UV–Vis spectrophotometer was used to measure theoptical transmittance in the region of 200–800 nm.

3. Results and discussion

Fig. 1 shows the XRD patterns with different thicknesses. The sharp diffraction peaks of (-201),(-402) and (-603) are all indexed to the b-Ga2O3. All the diffraction peak intensities increase gradually

Fig. 1. XRD spectra of N-incorporated b-Ga2O3 films with different thicknesses.

Fig. 2. FWHM and grain sizes of (�402) peak as a function of film thickness.

Fig. 3. Typical XPS spectra of the N-incorporated b-Ga2O3 film with a 125 nm thickness: (a) wide scanning spectrum,(b) Ga 2p3/2 spectrum, (c) N 1s spectrum and (d) O 1s spectrum.

148 R. Sun et al. / Superlattices and Microstructures 65 (2014) 146–151

with increasing the film thickness. The lattice parameters of N-incorporated b-Ga2O3 are evaluated tobe a = 12.277 Å, b = 3.046 Å, c = 5.8 Å and b = 103.70� [13]. The lattice mismatch between (�201) planeof N-incorporated b-Ga2O3 and the (0001) c-plane sapphire substrate is about 6.6% [14]. Fig. 2 shows

Fig. 4. AFM images of N-incorporated b-Ga2O3 films with different thicknesses.

R. Sun et al. / Superlattices and Microstructures 65 (2014) 146–151 149

the full width at half maximum (FWHM) of XRD (�402) peaks and the grain sizes of the samples as afunction of thickness. According to Fig. 2, the FWHM values decrease and the grain sizes increase withincreasing the film thickness. It is evident that the crystallinity quality was improved by increasing thefilm thickness, which may be caused by two reasons: (1) in the initial stage of N-incorporated b-Ga2O3

film deposition, the large lattice mismatch between N-incorporated b-Ga2O3 and sapphire substratedegrades the crystallinity quality. As the deposition proceeds, the stress between new deposited filmand former layer diminishes, therefore the crystallinity quality is improved, and (2) the more (�402)crystal plane is developed, the stronger the (-402) diffraction peak is [15,16].

The typical XPS spectra of the N-incorporated b-Ga2O3 film with a 125 nm thickness are shown inFig. 3. In Fig. 3(a), the peaks located at 1117.9 eV, 397.2 eV and 531.2 eV correspond to Ga 2p3/2, N 1sand O 1s states [17,18], respectively. It is obvious from Fig. 3(b) that the Ga 2p level is split into a dou-blet with an energy separation due to spin orbit angular momentum interaction. The Ga 2p3/2 bindingenergy is significantly different from Ga 2p3/2 of the gallium oxide (1120.2 eV) [19]. After incorporat-ing the nitrogen, more oxygen atoms will be substituted by nitrogen, thus the binding energy of Ga2p3/2 decreases due to the smaller electronegativity of nitrogen than that of oxygen. The N 1s peakin Fig. 3(c), located at around 402.5 eV, may be due to a small amount of N–O/N–N bonds [13,20].According to the analysis of XPS, the concentration of N is around 1.7 at.%.

AFM measurements, with a scanning area of 5 lm � 5 lm, were carried out to investigate the sur-face morphology. The AFM images of N-incorporated b-Ga2O3 films with different thicknesses areshown in Fig. 4. As shown in Fig. 4, the surface morphologies change greatly with increasing the filmthicknesses. In the view of Fig. 4(a), the film thickness is relatively thin and the surface exhibits hillockmorphology. As the film thickness reaches 75 nm, the surface morphology exhibits many islands. Thensome islands coalesce together, as shown in Fig. 4(c), and finally the films are formed. The evolution ofsurface morphologies of N-incorporated b-Ga2O3 films with different thicknesses might reflect the dif-ferent film growth stage. In the initial stage, crystal grains grow vertically when the film thickness isless than 40 nm. After the film thickness is more than 75 nm, the small grains and unstable grains tendto aggregate to form large islands. As the deposition proceeds, these large islands gradually coalescetogether laterally and then form high quality N-incorporated b-Ga2O3 films. The evolution of surfacemorphologies indicates the mode transition from three dimensions to two dimensions as increasingthe film thickness.

The optical transmission spectra of the N-incorporated b-Ga2O3 films were measured at room tem-perature using UV–Vis spectrometer. In visual inspection, all the films appear transparent and color-less. As shown in Fig. 5, the optical transmittance of all samples is more than 80% in the visible rangeand shows a sharp drop around the wavelength of 250 nm. The UV absorption edge shifts to longerwavelength with increasing the film thickness. The optical band gap can be obtained from the inter-cept of (ahm)2 vs. hm for direct allowed transitions [21]. The inset figure shows the dependence of N-incorporated b-Ga2O3 film band gap on the film thickness. The band gap of N-incorporated b-Ga2O3

film has a small change from 4.9 to 4.86 eV with increasing the film thickness from 40 to 125 nm.In Fig. 1, the crystallinity of the N-incorporated b-Ga2O3 films with thin thicknesses is poor and someamorphous phase appears, especially in the thickness of 40 nm. As the deposition proceeds, the amor-

Fig. 5. Optical transmission spectra of N-incorporated b-Ga2O3 films. The inset figure shows the optical band gap ofN-incorporated b-Ga2O3 films with different thicknesses.

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phous phase decreases and disappears. Therefore, the small change in the optical band gap value isdue to the improved crystallinity of the films [22]. Moreover, through Hall effect measurements,the typical electrical resistivity and the carrier concentration of the N-incorporated b-Ga2O3 film witha thickness of 125 nm are 4.78 � 102 X cm and 1.01 � 1012 cm�3, respectivity. The poor electricalproperty is due to the low concentration of N.

4. Conclusions

In summary, N-incorporated b-Ga2O3 films were deposited on sapphire substrates by RF magne-tron sputtering. The effect of film thickness on the microstructure, surface morphology and opticalproperties of N-incorporated b-Ga2O3 films was evaluated. The XRD patterns indicated that the crys-tallinity crystalline quality was improved by increasing the film thickness. The film thickness also af-fects the surface morphology greatly. In addition, the optical transmittance of all samples in the visiblerange is more than 80% and the UV absorption edge shifts to longer wavelength with increasing thefilm thickness. The small change in the optical band gap value is due to the improved crystallinityof the films.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (Grant No.61240015), the Natural Science Foundation of Guangdong Province (Grant No. S2012010010030),the Basic Research Plan Program of Shenzhen City in 2012 (Grant Nos. JCYJ20120613134210982,JCYJ20120615101957810), the Natural Scientific Research Innovation Foundation in Harbin Instituteof Technology (Grant No. HIT.NSFIR.2011123).

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