Thin Solid Films 518 (2010) 1658–1660

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Thin Solid Films

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Synthesis and characterization of 4H-SiC on C-plane sapphire by C60 and Si molecularbeam epitaxy

JianChao Li ⁎, Paolo Batoni, Raphael TsuDepartment of Electrical and Computer Engineering, University of North Carolina at Charlotte, Charlotte, NC 28223, USA

⁎ Corresponding author. Tel.: +1 704 687 3447; fax:E-mail address: jcli0002@gmail.com (J.C. Li).

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.11.088

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 May 2009Received in revised form 24 November 2009Accepted 27 November 2009Available online 3 December 2009

Pacs:81.15. Hi81. 05. Hd81.05. Tp

Keywords:Molecular beam epitaxy (MBE)Silicon carbide (SiC)FullerenesAdhesion

4H-SiC (silicon carbide) films were grown on (0001) sapphire substrate at rather low temperatures(1000–1100 °C) with relative high deposition rate by using fullerene (C60) and silicon solid sources molecular beamepitaxy with substrate nitridation and aluminum nitride (AlN) buffer layer deposition prior to the SiCdeposition. The effects of substrate nitridation and AlN buffer layer to the adhesion of the SiC thin films onsapphire have been studied. X-Ray diffraction, pole figure, atomic force microscope, Fourier transforminfrared spectroscopy and photoluminescence were employed for the analysis of composition, orientation ofthe film and surface morphology. Relative high deposition rate at ∼165 nm/h was achieved.

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1. Introduction

SiC is an IV–IV compound semiconductor which shows polytyp-ism. SiC is recognized to crystallize in more than 200 polytypes.Among many polytypes, technologically important ones are 3C-, 4H-,and 6H-SiC. In general 3C-SiC is known as a low-temperature stablepolytype, which often appears at low temperatures, Whereas 4H- and6H-SiC are known as high-temperature stable polytypes, which needrelatively high temperature to grow [1,2]. Molecular beam epitaxy(MBE) growth of homoepitaxy of 6H-SiC and heteroepitaxy of 3C-SiCon 6H-SiC (0001) and Si (111) using solid-source silicon and fullerene(C60) have shown potential at relatively low substrate temperature[3,4].

The adhesion of SiC epitaxial layer on sapphire is poor due to therelative large lattice mismatch. Nakamatsu et al. reported adherent3C-SiC films growth directly on (0001) sapphire with an ArF excimerlaser chemical vapor deposition system [5]. Since the lattice mismatchbetween AlN and SiC is less than 1%, the use of AlN buffer layer seemsmost obvious. B.S. Sywe et al. proved that by pre-depositing a thin AlNbuffer layer, the nucleation and adherence of the 6H-SiC epilayers onAlN/sapphire substrate are vastly improved [6]. Nitridation ofsapphire substrate has been reported as an effective way to enhance

adherence during AlN deposition on sapphire substrate by MBE [7].However, we found negative effectiveness when subsequent SiC isgrown. The crucial criterion depends on exceeding certain minimumthickness of the AlN buffer layer for the MBE growth of SiC on C-planesapphire substrates.

We have grown of 4H-SiC on C-plane sapphire substrates andinvestigated the effect of growth of AlN prior to the adhesion of SiCfilms through the nitridation of sapphire substrates and AlN bufferlayer process. X-Ray diffraction (XRD) analysis confirmed thecrystallinity of the as-deposited SiC films. Good surface morphologywas observed by atomic force microscope (AFM). Moreover, Fouriertransform infrared spectroscopy (FTIR) and photoluminescence (PL)were used to identify their polytype.

2. Experimental details

A customized SVT Associate MBE system with base pressure lessthan 4×10−8 Pa was employed for these experiments. The substratemanipulator is capable of reaching 1200 °C. Growth temperature wasmeasure by integrated thermal couple and calibrated by IS4K dualwavelength pyrometer. The C60 source material is 99.9% purefullerene powder and is deposited by a conventional effusion cell attemperatures of 500–550 °C. The Si source material is a portion of a99.999% pure boule evaporated in a high-temperature effusion cell attemperatures ∼1500–1600 °C. Temperature of the both effusion cellswere measured by integrated thermal couple inside the cells. The

Fig. 2. The XRD θ–2θ scan for sample 1 (a) and sample 5 (b).

Table 1Preparation conditions of the different SiC/sapphire samples described in the text.

Sample 1 Sample 2

Substrate temperature (°C) 1100 1100Temperature of Si/C60 (°C) 1600/550 1600/550Deposition time (hour) 1.5 2Nitridation/AlN buffer layer Nitridation NitridationAverage thickness (µm) 0.093 0.335FWHM of XRD (degree) 0.29 0.21RMS of AFM (nm) 5.48 Peeling

Fig. 1. The FTIR reflection spectra of sample 1.

1659J. Li et al. / Thin Solid Films 518 (2010) 1658–1660

substrates are C-plane (0001) sapphire with 30 min of 800 °C thermalclean treatment before deposition. An aluminum effusion cell and aNitrogen plasma RF generator were employed for the substratenitridation and AlN buffer layer processes. Two hours nitridation withsubstrate temperature at 500 °C was processed prior to SiC filmdeposition for some of the samples, while half hour AlN buffer layerdeposition was processed for the rest of the samples. For AlN bufferlayer process, the substrate temperature and Al cell temperature areat 800 °C and 1100–1200 °C respectively. The thickness of the AlNbuffer layers are typically ∼100 nm.

3. Results and conclusion

FTIR reflection measurement was performed by using FTIRspectrophotometer (Thermo Nicolet NEXUS 670) to determine thefilm composition. The setup of the resolution, number of scan and thespectrum range are 2 cm−1, 128 and from 400 to 4000 cm−1

respectively. Fig. 1 shows that the typical absorption peak at about800 cm−1 from transverse optical (TO) mode of the SiC lattice whichindicates the SiC composition is formed [8]. Similar results wereobtained from the rest of the samples.

The films were characterized by PANalysis X'Pert PRO X-raydiffractionmeter with 2θ goniometer scans. The diffractometer wasoperated at 45 kV and 40 mAwith Cu Kα X-ray (λ Cu Kα=0.154 nm)at the grazing incidence angle of 1°. XRD results of sample 1 andsample 5 are shown in Fig. 2 which has the possible (0004) and(0008) 4H-SiC peaks at 35.6° and 75.61° for both samples. The fullwidth at half maximum (FWHM) of the possible 4H-SiC peak are 0.29and 0.24° for sample 1 and sample 5, respectively. The preparationcondition of sample 1 and 5 are shown in Table 1. Note that it showsthe AlN (0002) peak at 36. 07° for sample 5 which is as result of AlNbuffer layer of ∼100 nm thickness. Lack of any other peaks indicatesthat the 4H-SiC obtained is crystalline aligned to the sapphiresubstrate as expected. All the samples have similar XRD results withvarious intensities of both peaks. To further prove the epitaxy to thesapphire substrate and to determine the distribution of crystalorientations, XRD pole figure measurement and PL were preformed.The measured {110} 4H-SiC XRD pole figure result of sample 5 wasemployed where 2θ and omega are fixed to the diffraction angle of4H-SiC (110), the phi axis is scanned from 0° to 360°, and the psi axisruns from 0° to 85°. A 6-fold symmetry was obtained which indicateshexagonal structure of the film.

Since the bandgaps of the 4H-SiC (3.18, and 3.37 eV) [9,10], 3C-SiC(2.39 eV) [11] and 6H-SiC (3.0 eV) [11] are sufficiently different, roomtemperature photoluminescence was performed to determine thepolytype of the SiC film using 325 nm HeCd laser (Kimmon IK5652R-G, 30mW, Kimmon Koha Co., Ltd, Tokyo, Japan). The spectrometer(Ocean Optics USB 2000, Ocean Optics Inc., Dunedin, FL) was used toobtain the PL spectrum utilizing the Ocean Optics SpectroSuitesoftware. The sample was accommodated within the cavity of acryogenic cold finger (Cryogenic Control Systems, Rancho Santa Fe,CA). The subtraction of PL results of the sapphire substrate and sample5 were obtained. Fig. 3 shows the PL peak of sample 5 at around373.5 nm (3.3 eV) which is due to the recombination of carriers

Sample 3 Sample 4 Sample 5

1000 1000 11001500/520 1600/550 1600/5502 1.3 4Nitridation AlN BL AlN BL0.22 0.11 0.6220.26 0.29 0.24Peeling 5.78 17.73

Fig. 4. The AFM images of the sample 1 (a) and sample 5 (b).

Fig. 3. The room temperature photoluminescence spectra of sample 5.

1660 J. Li et al. / Thin Solid Films 518 (2010) 1658–1660

between the bottom of the secondary conduction band edge and thetop of the valence band of 4H-SiC [9,10] and demonstrates that the4H-SiC film was obtained.

In Fig. 4, the surface morphology of sample 1 (a) and sample 5 (b)are depicted by Atomic Force Microscope (Veeco Dimension 3100with Nanoscope SPM V5 software). The microscope operated attapping mode with etched silicone cantilever probe (Model TESP)with scan rate at 1 Hz. The RMS value of the AFM baseline noise is0.1 nm. As it shows, the grain size for the sample 1 and sample 5 areabout 50 nm and 100 nm, respectively. The RMS of the scan area forsample 1 and sample 5 are 5.7 nm and 17.7 nm.

Table 1 gives the preparation conditions of the difference SiC/sapphire samples. For the samples with nitridation, peeling occurredfor the thickness of the SiC film above 0.2 μm. The nitridationprocesses were performed with 800 °C for the substrate temperatureat an RF power of 375 W for 1 h. This shows that nitridation mayprovide chemical compatibility but cannot improve lattice match. Thesubstrate temperature used is 850 °C and Al cell temperature used is1100–1200 °C with RF power at 380 W for the AlN buffer layer. Allsamples with AlN buffer layer show good adhesion.

Our growth rate of 4H-SiC layer on sapphire is ∼145–165 nm/h, 4times higher than the reported rates for 6H-SiC[3] and 3C-SiC [4]. Thesubstrate nitridation provides poor adhesion for SiC film particularlywith films thicker than 0.1 μm. However, 0.2–0.3 μm AlN buffer layerprovides satisfactory adhesion. This is because AlN forms anintermediate layer providing lattice matched growth of SiC. Thegrowth of 4H-SiC on sapphire with AlN as buffer with relatively highgrowth rate opens the door for SiC devices at high temperature andhigh power.

Acknowledgements

We acknowledge the support of this work by Northrop Grummanand the Defense Microelectronics Activity, Sacramento CA.

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