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Current mode atomic force microscopy study of Si films grown

on 6H-SiC(0001)

L.B. Li1, 2, , L.X. Song1, Y. Zang2, Z.M. Chen2, H.B. Pu2, Z.Y. Tu1, Y.L. Han2, Q. Chu1

1 School of Science, Xi’an polytechnic University, Xi’an, China

2 Department of Electronic Engineering, Xi’an University of technology, Xi’an, China

Abstract: Si/SiC heterostructures with different growth temperatures were prepared on

6H-SiC(0001) by LPCVD. Current mode atomic force microscopy and transmission electron

microscopy were employed to investigate the electrical properties and crystalline structure of

Si/SiC heterostructures. An FCC-on-HCP parallel epitaxy is achieved for the

Si(111)/SiC(0001) heterostructure with a growth temperature of 900oC. As the growth

temperature increases to 1050oC, the <110> preferential orientation of the Si film appears. It

is shown that the Si films with different growth orientations on 6H-SiC(0001) have two types

of distinctive crystalline grain structures: quasi-spherical grains with a general size of 20μm,

and columnar grains with a typical size of 7×20μm. The electrical properties are greatly

influenced by the grain structures. The Si film with <110> orientation on SiC(0001) consists

of columnar grains. With a low current fluctuation and relatively uniform current distributions,

Si(110)/6H-SiC(0001) heterostructure is more suitable to prepare the Si/SiC devices with

better electrical properties.

Keywords: Si/6H-SiC heterostructure; Electrical properties; Current mode AFM; Chemical

vapor deposition

1. Introduction

With a wide bandgap of 3.25–2.2eV, SiC has attracted much attention because of its wide

applications for various optoelectronic and electronic devices[1-4]. However, due to its wide

bandgap, SiC is not sensitive to long-wavelength light ranging from most of the visible to the

corresponding author. Tel.: +86-29-82312135. E-mail address: xpu_lilianbi@163.com

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infrared region of the optical spectrum. This essentially limits its applications for detection of

visible and infrared light[5]. A promising way to solve this problem is to adopt a Si/SiC

heterostructure, in which Si is used as a non-UV light absorption layer[6, 7]. At present, the

SiC-based Si/SiC heterostructure is comparatively less studied[8-11], and the studies just focused on

using Si/SiC heterostructure to improve the performance of the SiC SBD[9], or using Si/SiC

heterostructure to solve the problem of SiC/SiO2 interface defect states in SiC MOSFET[10, 11], the

non-UV photoelectric applications of the Si/SiC heterostructure are rarely reported.

In our previous work, it was found that the Si films on SiC substrates always have a

polycrystalline structure with multiple preferential orientations at different growth temperatures[12,

13]. Preferential growth orientation of <111> can be achieved in a temperature range of

825~1000oC, the <110> preferential orientation of the Si film appears when the growth

temperature increases to 1050oC[11]. However, the local electrical properties of the epitaxial Si

films with different orientations on SiC have not been investigated, which are closely related to

the carrier transportation and recombination, and determine some important parameters of the

heterostructure devices such as the reverse leakage current and conducted current. By exploring

the local electrical properties of the epitaxial Si films, the relations between the current

distribution and the crystalline structure of the Si/SiC heterostructure can be revealed and the

device performance can be optimized.

Current mode atomic force microscopy (C-AFM) is a powerful method for characterizing

local electrical properties of the semiconductor thin films. This method can probe the overall

microstructure of the thin film since the voltage is applied between the sample stage and the

C-AFM cantilever to induce the current flowing across in the direction of the film thickness[14-16].

With applying the voltage, the local current through the Si/SiC heterostructure can be measured by

C-AFM with the topographic scan.

In this paper, the Si/SiC heterostructures with different growth temperatures were prepared

on 6H-SiC (0001) by low-pressure chemical vapor deposition (LPCVD). C-AFM, transmission

electron microscopy (TEM) and X-ray diffraction (XRD) were employed to investigate the Si/SiC

heterojunctions.

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2. Experimental

The n-type isotype Si/SiC heterostructure was prepared on 6H-SiC(0001) substrate by

LPCVD. An n-type doped (doping concentration of ~1017cm-3) 6H-SiC wafer with a thickness of

300μm was purchased from II-VI Inc.. The Si films were grown on 6H-SiC substrates at

750oC~1050oC. Silane (SiH4) and hydrogen (H2) are used as a silicon source and a carrier,

respectively. Prior to deposition, the 6H-SiC substrates were cleaned using the standard RCA

method, and then treated in H2 atmosphere at 1050oC for 10min. The growth pressure is

maintained at 300Pa during the Si/SiC heterostructure growth. In the present work, we describe

results of local topography and electrical measurements with C-AFM on Si/SiC heterostructure. A

bias voltage between the substrate and the conducting cantilever (which is grounded) was 1.5V

during all imaging experiments, and the scheme of the experimental setup for C-AFM

measurements is shown in Fig. 1. The crystal structure of the Si films was determined using a

Rigaku SmartLab high-resolution X-ray diffractometer (XRD) with Cu Kɑ radiation (λ=1.5406Å).

The heterostructure interface was investigated by cross-sectional TEM (JEM-3010).

3. Results and discussion

The low magnification cross-sectional TEM bright-field image of the Si thin film grown on

6H-SiC(0001) at 900oC is shown in Fig. 2(a). In this image, the lower part belongs to the 6H-SiC

substrate, while the upper part represents the Si thin film. Near-spherical grains running across the

entire Si film and the coalescence of these grains are observed. The Si film with an

inhomogeneous thickness of 0.38~0.60μm shows irregular heterogeneous diffraction contrast,

which suggests the existence of some structural defects such as grain boundaries, stacking faults

and twins in the film. And these defects could lead to the differences of the current distributions.

The SAED patterns at the Si/6H-SiC interface corresponding to Si[-110]SiC[-12-10] zone axes are

shown in Fig. 2(b). The diffraction spots can be categorized into two sets. One has a hexagonal

close-packed structure with a lattice constant of 3.08Å, which is identical with the corresponding

lattice constant of the 6H-SiC. The other belongs to the Si film with a face-centered cubic

structure and <111> growth orientation. SAED patterns confirm that the Si film has epitaxial

connection with the 6H-SiC substrate and the orientation relationship of Si/6H-SiC heterostructure

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is Si(111)//6H-SiC(0001). Fig. 2(c) is a low magnification cross-sectional TEM image of the

Si/6H-SiC(0001) heterostructure grown at 1050oC. The Si/SiC heterostructure haves a sharp

interface and consist of columnar grains. SAED patterns at the Si/6H-SiC interface corresponding

to Si[001]SiC[1-100] zone axes in Fig. 2(d) clearly show the FCC-on-HCP orientation relationship

of Si(110)//6H-SiC(0001), confirming the epitaxial growth of the Si films with [110] growth

orientation.

Figure 3 shows the XRD θ-2θ scans for Si/SiC(0001) heterostructures prepared at 900 oC and

1050oC, respectively. As shown in Fig. 3(a), apart from the SiC(0006) reflection of the substrate,

only the Si(111) reflection was observed. No trace of the Si(220) reflection was detected. When

the Si was grown at 1050oC the diffraction pattern was different. Figure 3(b) shows the prominent

presence of the Si(220) reflection. The intensity of the Si(111) reflection, on the other hand, is

much reduced. It is shown that when the Si was deposited at the lower temperatures of 900 oC, the

Si film is <111> oriented, but when the Si layer was grown at 1050oC, it is mainly <110> oriented,

which agrees with the SAED characterizations.

The surface morphologies of the Si/6H-SiC(0001) heterostructures were carefully examined

using AFM, as shown in Fig. 4. The grain growth mode is observed in Si layers deposited on

6H-SiC(0001) substrate with different temperatures. Crystalline grain with a lateral size of 1~3 μm

slightly stick out of the Si surface. And the presence of these grains is due to the large lattice

mismatch of the Si/SiC heterostructure. Fig. 5(a) shows C-AFM measurements of the

Si(111)/6H-SiC(0001) heterostructure with applying positive 1.5V at the 6H-SiC substrate. The

heterogeneous current distributions indicate quasi-spherical grains with a typical size of 20μm

present in the Si(111)/6H-SiC(0001) heterostructure. Compared with the grain size of 1~3μm

observed in Fig. 4(a), it is deduced that the crystalline grains have been coalesced. This is

consistent with the TEM observations. The electrical properties are greatly influenced by the

coalesced grains. There are obvious positive current of up to 4nA at the boundaries of the

coalesced grains and negative current of about 15nA on the coalesced grains. Fig. 5(c) shows the

C-AFM images of Si(110)/6H-SiC(0001) heterostructure grown at 1050oC. The sample consists

of columnar grains with a typical size of 7×20μm. Compared with the Si(111)/6H-SiC(0001)

heterostructure, the positive current at the grain boundaries and the negative current on the

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coalesced grains are relatively low, and the current distributions are more uniform. It is

demonstrated that the Si(110)/6H-SiC(0001) heterostructure with a low current fluctuation is

more suitable to prepare the Si/SiC devices with better electrical properties.

4. Conclusions

In this article, Si/SiC heterojunctions with different growth temperatures were prepared on

6H-SiC(0001) by LPCVD. C-AFM and TEM were employed to investigate the electrical

properties and crystalline structure of Si/SiC heterojunctions. An FCC-on-HCP parallel

epitaxy is achieved for the Si(111)/SiC(0001) heterostructure with a growth temperature of

900oC. As the growth temperature increases to 1050oC, the <110> preferential orientation of

the Si film appears. It is shown that the Si films with different growth orientations on

6H-SiC(0001) have two types of distinctive crystalline grain structures: quasi-spherical grains

with a general size of 20μm, and columnar grains with a typical size of 7×20μm. The

electrical properties are greatly influenced by the grain structures. The Si film with <110>

orientation on SiC(0001) consists of columnar grains. With a low current fluctuation and

relatively uniform current distributions, it is more suitable to prepare the Si/SiC devices with

better electrical properties.

Acknowledgements

This work was Supported financially by the National Natural Science Foundation of

China (Grant No. 51402230, 51177134, 21503153), the Project Supported by Natural Science

Basic Research Plan in Shaanxi Province of China (Grant No. 2015JM6282), Scientific

Research Program Funded by Shaanxi Provincial Education Department (Grant No.

14JK1302) and China Postdoctoral Science Foundation funded project (Grant No.

2013M532072).

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Produced by L.B. Li et al.

Si epitaxial layer

6H-SiC substrate

Current profile

Conductive tip

Fig. 1 Si/6H-SiC heterostructure and scheme of the C-AFM experiments.

V

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Fig. 2 Low magnification cross-sectional TEM image and the SAED patterns of Si/6H-SiC(0001) interface. (a, b)

Si/SiC heterostructure grown at 900oC, (c, d) Si/SiC heterostructure grown at 1050oC. The SAED patterns at the

Si/6H-SiC interface corresponding to Si[-110]SiC[-12-10] and Si[001]SiC[1-100] zone axes, respectively.

(c) (d)

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Si(111)6H-SiC(0006)

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6H-SiC(0006)

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Fig. 3 X-ray specular θ-2θ scans for Si/SiC(0001) heterostructures with the Si layer grown at (a) 900oC, (b) 1050oC

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Fig. 4 AFM images of Si/SiC(0001) heterostructures with the Si layer grown at (a) 900oC, (b) 1050oC

(a) (b)

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(a)

(b)

(c)

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Fig. 5 C-AFM images of Si/SiC(0001) heterostructures with the Si layer grown at (a, b) 900oC, (c, d) 1050oC