Preparation and characterization of Ga2xIn2(1-x)O3 films

deposited on αααα-Al2O3 (0001) by MOCVD

Lingyi Kong, Fan Yang, Jin Ma, Caina Luan, Zhen zhu

School of Physics, Shandong University, Jinan, Shandong 250100, P R China

E-mail address: yangfanemily@mail.sdu.edu.cn

Keywords: Ga2xIn2(1-x)O3 films; MOCVD; Band gap

Abstract. Ga2xIn2(1-x)O3 thin films with different gallium content x [x = Ga/(Ga+In) atomic ratio] have been prepared on α-Al2O3 (0001) substrates at 650ºC by metalorganic chemical vapor deposition (MOCVD). Structural, electrical and optical properties of these films have been investigated in detail. The XRD analysis revealed that, as the gallium content increased, the crystalline quality of the films decreased. The highest Hall mobility of the films was 41.32 cm2v−1s−1. The absolute average transmittance of the Ga2xIn2(1-x)O3 thin films in the visible range exceeded 91%. The band gap could be tuned from 3.59 to 4.87 eV as gallium content increased. 1. Introduction

Transparent conductive films have attracted much attention in recent years, because of their high electrical conductivity and excellent optical transparency. These thin films have been widely used in many fields, such as optoelectronic devices, touch panels and antistatic conductive films[1-3]. But recent technologies, such as laser lithography and solar cell fabrication, require new ultraviolet transparent conducting films.[4] Ga2O3 is a promising transparent conductor for the new generation of optoelectronic devices in deep ultraviolet (UV) wavelength region.[5] In2O3 film is an important transparent conductive oxide film. Both Ga2O3 and In2O3 are n-type semiconductor with excellent transparency. Their direct band gap are 4.9 eV and 3.6 eV [6,7], respectively.

According to the theory of R. Hill [8], by controlling the composition of Ga2xIn2(1-x)O3 film, the band gap can be tuned from 3.6 to 4.9eV. Modulation of the band gap of this transparent conductive film is very important for this material to higher potential in the field of UV optoelectronics. In this study, Ga2xIn2(1-x)O3 (0≤x≤1) films were prepared on α-Al2O3 (0001) substrates at 650 ºC by MOCVD. The structural, electrical and optical properties were investigated in detail.

2. Experimental The deposition of the Ga2xIn2(1-x)O3 thin films was carried out on α-Al2O3 (0001) substrates at

650 ºC by MOCVD. In(CH3)3 and Ga(CH3)3 (99.9999% in purity) were used as organometallic (OM) sources. The two kinds of vapor were transported into a reactor by ultra high purity N2 (99.9999999% in purity) which was used as carrier gas. High purity O2 (99.999% in purity) with a flow rate of 50 sccm was injected into the reactor as oxidant. During the deposition, the growth pressure was kept at 50 torr and the substrate temperature was kept at 650 °C. The structural properties were determined by X-ray Diffraction (XRD), in which a RIGAKU D/MAX-γB X-ray diffractometer was used. The scanning electron microscopy (SEM) morphology of the films was measured using S-4800 Ultra-high Resolution Scanning Electron Microscope. The sheet resistivities were measured using a conventional four-probe instrument. The Hall mobilitis

Advanced Materials Research Vols. 79-82 (2009) pp 1535-1538Online available since 2009/Aug/31 at www.scientific.net© (2009) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.79-82.1535

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were measured using Van der Pauw technique at room temperature. The optical transmittance properties were measured by a Shimadzu TV-1900 double-beam UV-vis-NIR spectrophotometer in the wavelength range of 200-800 nm. 3. Results and discussion Fig. 1 shows the X-ray diffraction spectra of the Ga2xIn2(1-x)O3 films with different gallium content. It can be seen that, when x=0.1, two diffraction peaks corresponding to In2O3 (222) and (444) of body centered cubic (bcc) structure are observed. For the films with 0.2≤x≤0.5, two diffraction peaks corresponding to In2O3 (222) and (400) of bcc structure are observed. For the film with x=0.6, only one diffraction peak corresponding to In2O3 (222) is observed. When x≥0.7, no diffraction peak is detected. As x increases from 0.1 to 0.6, the full width at half maximum (FWHM) of peak (222) increases from 0.19° to 1.11°. The results indicate that the crystalline quality of the films decreases with gallium content increases.

20 30 40 50 60 70 80

In2O3 (444)

In2O3 (222)

In2O3 (222)

x=0.1

In2O3 (400)

x=0.4

x=0.3

x=0.2

In2O3 (400)

In2O3 (222)

x=0.7

x=0.6

x=0.5

In2O3 (222)

Intensity (a.u.)

2θθθθ (degree)

0 200 400 600 800 1000 1200

Ga In

Channels

x=0.1

GaIn

x=0.5

HIn

HGa

RBS Yield (arb. units)

x=0.9

Fig. 2 shows the RBS spectra of the Ga2xIn2(1-x)O3 films with enacted values x = 0.1, 0.5 and 0.9.

The arrows indicate the element signals of the films. Corresponding to x=0.1, 0.5 and 0.9, the estimated [9]gallium content in the films is 0.09, 0.41 and 0.78, respectively. The calculated gallium content in the films is less than the enacted value. It is because that gallium is less reactive and more resistive to oxidation compared to indium during the deposition process.

Fig. 3 shows the scanning electron microscopy (SEM) micrographs of the surface of the Ga2xIn2(1-x)O3 films. Images (a), (b) and (c) correspond to x=0.1, 0.5 and 0.9, respectively. The films with gallium content of 0.1 and 0.5 show uniform surface and well covered to the α-Al2O3 substrates. Triquetrous grains with well-defined grain boundaries are observed in image (a). The surface of the film with x=0.5 is smoother than x=0.1. The grain size of the film with gallium content of 0.9 is much smaller than the other two films, and agglomerations of the grains of this film are clearly observed. These results are in good agreement with the XRD measurement.

Fig.1 XRD spectra of the Ga2xIn2(1-x)O3 films with different gallium content.

Fig.2 RBS spectra of the Ga2(1-x)In2xO3 films with enacted values x = 0.1, 0.5 and 0.9.

1536 Multi-Functional Materials and Structures II

Fig. 4 shows the electrical properties of the films as a function of gallium content. It can be seen that, the Hall mobility monotonously decreases from 41.32 to 1.24 cm2v−1s−1 as x increases from 0.1 to 0.9. In the mean time, the carrier concentration of the films decreases from 6.36×1019 to 6.74×1016 cm-3 and the resistivity of the films increases from 2.38×10-3 to 7.48 Ω·cm. The results are attributed to the crystalline quality of the films became worse and the band gap of the films became wider as the gallium content increases.

1016

1017

1018

1019

1020

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

X (gallium content)

Hall Mobility (cm

2 v-1s-1 )

Hall Mobility Resistivity Carrier concentration

10-3

10-2

10-1

100

101

102

Resistivity (

Ω

Ω

Ω

Ω •• ••cm)

Carrier concentration (cm-3)

Fig. 5 shows the transmittance spectra of the Ga2xIn2(1-x)O3 samples with different gallium content

as a function of wavelength. Curves a, b, c, d, e and f correspond to x = 0, 0.2, 0.4, 0.6, 0.8 and 1, respectively. It exhibits that the average transmittance of these samples in the visible range is over 77%. Deducting the influence of substrate (86% for the substrate), the absolute average transmittance of the Ga2xIn2(1-x)O3 films exceeds 91%. The absorption edge of the films shifts to the shorter wavelength as x increases. This can be attributed to the band gap of the films becames wider as the gallium content increases. The optical band gap (Eg) of the films can be obtained by plotting (hνα) 2 vs. hν, (α is the absorption coefficient and hν is the photon energy) and extrapolating the straight-line portion of the plots to the energy axis. The Eg as a function of gallium content is shown in Fig. 6. The Eg increases monotonously from 3.59 to 4.87 eV as x increases from 0 to 1.

Fig.4 The electrical properties of the Ga2xIn2(1-x)O3 films as a function of gallium content from 0.1 to 0.9.

Fig.3 The SEM micrographs of the surface of Ga2xIn2(1-x)O3 films. Images (a), (b) and (c) correspond to x = 0.1, 0.5, 0.9.

Advanced Materials Research Vols. 79-82 1537

200 300 400 500 600 700 8000

10

20

30

40

50

60

70

80

90

100

Transmittance (%)

Wavelength (nm)

fedcba

0.0 0.2 0.4 0.6 0.8 1.0

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

x (gallium content)

bandgap (eV)

4. Conclusions Ga2xIn2(1-x)O3 films with different gallium content were prepared on α-Al2O3 (0001) substrates by MOCVD. The film with x=0.1 exhibited best crystalline quality. The crystalline quality of the film decreased with increasing gallium content. The highest Hall mobility of the films was 41.32 cm2v−1s−1. The absolute average transmittance of the Ga2xIn2(1-x)O3 films in the visible range was over 91%. The band gap could be tuned from 3.59 to 4.87 eV as x increases from 0 to 1. These results indicate that Ga2xIn2(1-x)O3 is a ultraviolet transparent conducting material, which has high potential in the construction of superlattice quantum well, UV detectors, short-wavelength laser devices and so on. Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grant No. 50672054).

References [1] J.L. Vossen: Physics of Thin Films Vol. 9 (1977), p. 1

[2] D.S. Ginley, C. Bright: MRS Bull Vol. 8 (2000), p. 15

[3] K.Utsumi, O. Matsunaga, T. Takahata: Thin Solid Films Vol. 334 (1998), p. 30

[4] U. Naoyuki, H. Hideo: Appl. Phys. Lett Vol. 70 (1997), p. 30

[5] H.H. Tippins: Physical Review Vol. 140 (1965), p. 316

[6] Y. Shigesato, S. Takasi, T. J. Coutts: J. Appl. Phys. Vol. 71 (1992), p. 3356

[7] M. Orita, H. Hiramatsu, H. Ohta: Thin Solid Films Vol. 411 (2002), p. 134

[8] R. Hill: Solid State Phys Vol. 7 (1974), p. 521

[9] W.K. Chu, J.W. Mayer, M.A. Nicolet: Backscattering Spectrometry (Academic Press, America 1978).

Fig.5 The transmittance spectra of the Ga2xIn2(1-x)O3 samples. Curves a, b, c, d, e and f correspond to x = 0, 0.2, 0.4, 0.6, 0.8 and 1, respectively.

Fig.6 The band gap of the Ga2xIn2(1-x)O3 films as a function of gallium content.

1538 Multi-Functional Materials and Structures II

Multi-Functional Materials and Structures II 10.4028/www.scientific.net/AMR.79-82 2xIn2(1-x)O3 Films Deposited on α-Al2O3 (0001) by MOCVD 10.4028/www.scientific.net/AMR.79-82.1535

DOI References

[5] H.H. Tippins: Physical Review Vol. 140 (1965), p. 316

doi:10.1103/PhysRev.140.A316 [7] M. Orita, H. Hiramatsu, H. Ohta: Thin Solid Films Vol. 411 (2002), p. 134

doi:10.1016/S0040-6090(02)00202-X