Journal of Crystal Growth 189/190 (1998) 466—470

Deposition of InN thin films by radiofrequency magnetron sputtering

Qixin Guo*, Nobuhiro Shingai, Mitsuhiro Nishio, Hiroshi OgawaDepartment of Electrical and Electronic Engineering, Faculty of Science and Engineering, Saga University, Saga 840, Japan

Abstract

InN thin films were deposited on glass substrates by radio frequency magnetron sputtering. It was found that the c-axisof InN is perpendicular to the surface plane. All films show n-type conduction with a carrier concentration of the order1020 cm~3. The Hall mobility of the films increases monotonically from 18 cm2/V ) s for films deposited at a substratetemperature of 100°C to 115 cm2/V ) s at 500°C. ( 1998 Elsevier Science B.V. All rights reserved.

PACS: 81.05.Ea; 81.15.Cd

Keywords: InN; Glass substrate; RF magnetron sputtering; Deposition; Substrate temperature

1. Introduction

Indium nitride (InN) which has a wurtzite crystalstructure with a direct band gap of 1.9 eV, isa promising material for optoelectronic devices,low-cost solar cells with high efficiency, opticalcoatings, and various types of sensors [1]. Recently,it has received more attention from the propertiesof its alloys with GaN and AlN which permit thefabrication of In

1~x~yGa

xAl

yN based shortest-

wavelength semiconductor laser diode [2,3]. Ascompared with the GaN-based materials, the in-vestigation of InN is still in its infancy though

*Corresponding author: Fax: #81 952 28 8651; e-mail:guoq@cc.saga-u.ac.jp.

several groups have attempted to obtain InN films[4—9]. In order to fully realize the potential of thegroup III—V nitride semiconductor, the growth ofeach member of the family should be established.Tansley and Foley [6] reported InN films withcarrier concentrations near 1016 cm~3 and withmobilities near 4000 cm2/V ) s grown on glass sub-strates by the RF sputtering technique. However,they have not systematically investigated the de-position conditions to obtain films with suchhigh Hall mobilities and low carrier concentra-tions. In this paper, we report on the growth ofInN films by radio frequency magnetron sput-tering. We investigated the dependence of growthrate, crystallization, carrier concentration, andmobility for the InN films on substrate temper-ature.

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 3 3 2 - 7

2. Experimental procedure

Fig. 1 shows a schematic diagram of the sputter-ing apparatus used in this study. The sputteringsystem was evacuated to the order of 10~7 Torrwith a turbomolecular pump before nitrogen gas(99.999% pure) was bled into the chamber. Thedistance between the magnetron sputtering cath-ode with a bonded indium target (100 mm diam,99.99% pure) and the substrate was fixed at 40 mm.The RF sputtering power and the gas pressure werekept constant at 100 W and 15 mTorr, respectively,in all experiments. Corning d7059 and microslideglass substrates were ultrasonically cleaned in or-ganic solvents and mounted on a substrate holder.The substrate temperature during the depositionwas monitored using a thermocouple and control-led from 100 to 550°C using a heater behind thesubstrate. The growth conditions for InN films aresummarized in Table 1. After deposition, the filmthickness was determined by a surface step profileanalyzer. X-ray diffraction was used to study thestructure of these films. Electrical properties weremeasured using the van der Pauw technique.

3. Results and discussion

Fig. 2 shows the deposition rate of InN films asa function of substrate temperature. Two regions of

Fig. 1. Schematic diagram of the sputtering apparatus for thegrowth of InN films.

Table 1Sputtering conditions for InN deposition

Target Indium (99.99%)Target diameter 10 cmTarget—substrate distance 4 cmSubstrate GlassResidual pressure )5]10~7 TorrRF power 100 WSputtering ambient N

2(99.999%)

N2

gas flow rate 3 sccmSputtering pressure 15 m TorrSubstrate temperature 100—550°C

Fig. 2. Dependence of the deposition rate on substrate temper-ature for InN films.

behavior are observed. The deposition rate is essen-tially independent of the substrate temperature inthe range below 400°C, however, it rapidly in-creases as the substrate temperature is furtherincreased. This tendency has also been reportedfor InN films on (0 0 0 1) sapphire substrates byKistenmacher et al. [8], suggesting that the growthcharacteristics are almost independent of the crys-tal structure of the substrate in RF magnetronsputtering.

Fig. 3 shows X-ray diffraction patterns of theInN film grown on glass substrates. Only two

Q. Guo et al. / Journal of Crystal Growth 189/190 (1998) 466–470 467

Fig. 3. X-ray diffraction patterns of InN films.

diffraction peaks corresponding to the (0 0 0 2) and(0 0 0 4) reflections from InN are observed. None ofthe M1 0 0N, M1 0 1N, M1 1 0N, M1 0 3N, M1 1 2N, andM2 0 1N peaks of InN, which are stronger than the(0 0 0 2) peak in powder diffraction intensity, can bedetected. This indicates that the c-axis of InN witha wurtzite structure is perpendicular to the substra-te surface. The lattice constant c which is twice the(0 0 0 2) interplanar spacing for the InN films isfound to be 5.76 A_ , which is in good agreement withthat reported for InN grown on glass substrates[6], and is close to that for InN grown on sapphire[7] and GaAs [9] substrates. Fig. 4 shows the de-pendence of full-width at half-maximum (FWHM)

Fig. 4. Dependence of full-width at half-maximum (FWHM) ofInN(0 0 0 2) diffraction peaks on the substrate temperature.

of InN(0 0 0 2) diffraction peak on the substratetemperature. The FWHM decreases with the in-crease of the substrate temperature. It is wellknown that FWHM of the XRD in the h—2h modeis affected by the grain size and the distribution oflattice constant due to the lattice distortion. Byanalysing the FWHMs for (0 0 0 2) and (0 0 0 4)reflections [10], we obtained that the broadening ofthe diffraction profiles in Fig. 3 is mainly due to thegrain size effect. This indicates that the grain sizegets larger with the increase of the substrate tem-perature.

All the InN films showed n-type conduction.Fig. 5 shows the room temperature Hall mobilityand carrier concentration of the InN films asa function of the substrate temperature. The carrierconcentration is of the order 1020 cm~3, andalmost independent of the substrate temper-ature. The Hall mobility increases smoothly from18 cm2/V )s for films deposited at a substrate tem-perature of 100°C to 115 cm2/V ) s at 500°C. Sim-ilarly, there is an equally smooth decrease in thefilm resistivity with increasing substrate temper-ature, as shown in Fig. 6. In general, the chemicalreaction for reactive sputtering may take place atthe cathode, during transport of ejected atoms, at

468 Q. Guo et al. / Journal of Crystal Growth 189/190 (1998) 466–470

Fig. 5. Dependence of mobility and carrier concentration onthe substrate temperature for InN films.

Fig. 6. Dependence of resistivity on the substrate temperaturefor InN films.

the film surface, or a combination of these possibili-ties. An extensive study of DC diode reactive sput-tering of indium in N

2gas by Natarajan and

co-workers [5] has shown that the metal indium

target does not completely nitride and that nosputtered ejection of InN molecules was observed,indicating that sputtered indium atoms and atomicnitrogen formed in the plasma react at the substratesurface to form InN. An increasing substratetemperature may improve crystallinity of the InNfilms by providing activation energy for adatomsto occupy the positions of potential minima andenhancing recrystallization due to the coalescenceof islands by increasing the surface and volumediffusion, resulting in the improvement of theelectrical properties with increasing substrate tem-perature.

4. Conclusions

InN thin films were grown on glass substratesby radio frequency magnetron sputtering. X-raydiffraction clearly revealed the c-axis of InN is per-pendicular to the substrate surface plane and thefull-width at half-maximum for the (0 0 0 2) planedecreases with increasing substrate temperature.The lattice parameter c was found to be 5.76 A_which was consistent with the reported values. Allfilms showed n-type conduction with a carrier con-centration of the order 1020 cm~3 and the Hallmobility of the films increases monotonically withincreasing substrate temperature.

Acknowledgements

The authors wish to express their appreciation toProf. H. Uehara for his interest and encourage-ment. We also wish to thank Dr. N. Itoh for valu-able discussions on the XRD data and Y. Mitsuishi,T. Furukawa, H. Mori, and N. Fujita for help withthe sample preparation.

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