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Differing morphologies of textured diamond films with electrical properties made with microwave plasma chemical vapor deposition
Applied Surface Science 257 (2010) 1729–1735
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Differing morphologies of textured diamond films with electrical properties made with microwave plasma chemical vapor deposition Wen Chi Lai a , Yu-Shiang Wu b,∗ , Hou-Cheng Chang c , Yuan-Haun Lee a a b c
Department of Material Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC Department of Mechanical Engineering, China University of Science and Technology, Taipei 11581, Taiwan, ROC Department of Electronic Engineering, China University of Science and Technology, Taipei 11581, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 24 January 2010 Received in revised form 22 August 2010 Accepted 2 September 2010 Available online 15 September 2010 Keywords: {1 1 0} preferred orientation Rectangular structure Tier Ohmic contact
a b s t r a c t This study investigates the orientation of textured diamond films produced through microwave plasma chemical vapor deposition (MPCVD) at 1200 W, 110 Torr, CH4 /H2 = 1/20, with depositions times of 0.5–4.0 h. After a growth period of 2.0–4.0 h, this particular morphology revealed a rectangular structure stacked regularly on the diamond film. The orientation on {1 1 1}-textured diamond films grew a preferred orientation of {1 1 0} on the surface, as measured by XRD. The formation of the diamond epitaxial film formed textured octahedrons in ball shaped (or cauliflower-like) diamonds in the early stages (0.5 h), and the surface of the diamond film extended to pile the rectangular structure at 4.0 h. The width of the tier was approximately 200 nm at the 3.0 h point of deposition, according to TEM images. The results revealed that the textured diamond films showed two different morphological structures (typical ball shaped and rectangular diamonds), at different stages of the deposition period. The I–V characteristics of the oriented diamond films after 4.0 h of deposition time showed good conformity with the ohmic contact. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diamond has many excellence characteristics for industrial applications, such as good wear, the highest Young’s modulus (a high degree of hardness) optical characteristics, and semiconducting properties. There are many methods for describing the orientation and texture of diamond films under different deposition conditions, and the individual properties of (1 1 1) and (0 0 1) facets have been discussed in several previous studies [1–4]. For example, diamond films comprising (1 0 0) facets have better wear than those comprising (1 1 1) facets, because the surfaces of (1 1 1) facets are too sharp, causing the diamond films to break down easily [5]. Polycrystalline diamond films at (1 0 0) orientation are superior to (1 1 1) in their optical and electrical properties [2,6,7]. However, research on the properties of rectangular and stacked texture diamond films is limited, compared with (1 1 0) polycrystalline diamond films. The temperature of the substrate and the hydrocarbon concentration are deposition parameters that influence the orientation of
∗ Corresponding author at: 245, Sec. 3, Yen-Chiu-Yuan Road, Nankang, Taipei 115, Taiwan. Tel.: +886 2 27867048x37; fax: +886 2 27867253. E-mail address: [email protected] (Y.-S. Wu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.006
diamond films [8]. In previous studies, by controlling these two parameters, the distinctive features of orientation were investigated [3,4]. In a microwave plasma chemical vapor deposition (MPCVD) system, the concentration of methane and pressure within the chamber influenced the morphology of the diamond films. It was also found that nucleation density decreased with an increase in pressure, while it increased with an increase in the concentration of CH4 [9]. The manner of growth was clearly dependent on ambient pressure: i.e., at higher pressure, diamond {1 0 0} was the favorable growth surface, whereas, {1 1 1} was the favorable growth surface at low pressure [10]. Diamond quality depends greatly on pressure and methane concentration with certain combinations of the two parameters; the maximum allows the methane concentration to increase with pressure, thus allowing higher growth rates [11]. In an MPCVD system, to make high quality diamond films, the carbonaceous species contained 0.1–2% hydrogen and the pressure was 20–50 Torr [11–15]. However, the growth of diamond films under specific deposition conditions was unclear. Ball shaped diamonds were also denominated by polycrystalline diamonds and cauliflower-like diamonds in some studies [16–18]. The ball shaped (ballases) diamonds were formed under non-optimal growth conditions [19], and were generally sensitive to temperature under different methane concentrations. However, preview researchers did not provide a clear, detailed discussion of
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Fig. 1. SEM images of the diamond films prepared with different periods of deposition: (a) 0.5 h; (b) 1.0 h; (c) 2.0 h; (d) 3.0 h; (e) 4.0 h.
the stacked rectangular growth of diamond films under particular deposition conditions (in a high methane concentration and high pressure). In this work, we prepared the diamond films under high pressure and at high methane concentrations to observe the deposition characteristics of textured diamond films grown by MPCVD. To understand the changes in the (1 1 0) diamond films, the study used various deposition times to observe changes in the morphology. The results were analyzed by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, atomic force
microscopy (AFM), and electrical tests, to identify the distinguishing characteristics of the diamond films. 2. Experimental All experiments were carried out using a 2.45 GHz MPCVD system (ITRI, MP2000-DH2) with a 2 kW microwave source. The substrate was a (1 1 1) silicon wafer with an area of 1 cm × 1 cm, prepared with 1 m power suspension in an ultrasonic bath for 3.0 h. After the ultrasonic bath, it was cleaned in an ultrasonic bath of acetone for 20 min. The diamond films were deposited in a contin-
W.C. Lai et al. / Applied Surface Science 257 (2010) 1729–1735
1731
(111)
Intensity (a.u.)
(220) (311)
(400)
3.0 h
0.5 h 40
60
80
100
120
2θ (degree) Fig. 2. XRD patterns of diamond films prepared at a deposition time of 3.0 h.
uous flow of mixed gas made up of 15 sccm methane and 300 sccm hydrogen. The microwave power was 1200 W, the pressure within the chamber was kept at 110 Torr, and the deposition times were 0.5, 1.0, 2.0, 3.0, and 4.0 h. The morphology of the diamond films was examined with FESEM (Leo, 1530). The crystallization and orientation of the diamond films were estimated by XRD (Rigaku, Ttrax III) and the lattice structure was observed by TEM (Philips, Technai F30). Their physical properties and characteristic qualities were analyzed by Raman spectroscopy (Jobin Yvon, iHR-320) with a 514.5 nm He–Ne green light laser. The roughness of diamond film surface was evaluated by AFM (Veeco, MultiMode 8 scanning probe microscope). The electrical properties were measured by a semiconductor characterization system (Keithley, 2430). Prior to the electrical tests, both sides of the samples were coated in platinum by sputtering, and annealed at 500 ◦ C for 1 h. 3. Results and discussion The SEM images of the textured diamond films are shown in Fig. 1(a)–(e) representing deposition times of 0.5–4.0 h. In the diamond growth stage, ball shaped diamonds with a particle size of approximately 2 m were heterogeneously deposited on the substrate, and many nucleation sites appeared on the surfaces of ball shaped diamonds, as shown in Fig. 1(a). There were many secondary crystallites on the top faces at a deposition time of 1.0 h, as shown in Fig. 1(b). The clearer diamond structures piled atop one another. The size of pyramidal forms deposited in a high methane concentration increased, and the substrate was covered with diamonds, as shown in Fig. 1(c). Fig. 1(d) shows the diamond films with rectangular structures on top at a deposition time of 4.0 h. At a deposition time of 4.0 h, square diamonds formed at approximately 2 m × 2 m. Nevertheless, the square (or rectangular) structure was the (1 1 0) orientation, as discussed in previous studies. In addition, the cross section of the diamond films at 3.0 h deposition time was approximately 10 m, for a calculated growth rate of 3.3 m/h. The XRD patterns for the deposited diamond films and diffraction peaks correspond to the (1 1 1), (2 2 0), (3 1 1), and the (4 0 0) as indexed by the Joint Committee on Powder Diffraction Standards (JCPDS) diffraction data card (card 0675 from Set 06) for diamond films. In Fig. 2, the peaks at 2 angles of 44.05◦ , 75.50◦ , 91.75◦ , and 119.50◦ were identified as (1 1 1), (2 2 0), (3 1 1), and (4 0 0), respectively [20]. Table 1 shows the intensity ratios of diamond characterized by XRD patterns. Intensity ratios of I(2 2 0) /I(1 1 1) , I(3 1 1) /I(1 1 1) and I(4 0 0) /I(1 1 1) for the JCPDS diffraction data card were
Fig. 3. (a) TEM image of pyramidal diamond film deposited for 3.0 h; (b) the diffraction pattern of the top position, indicated by a white arrow from (a).
0.25, 0.16, and 0.08, respectively. At a deposition time of 0.5 h, the I(2 2 0) /I(1 1 1) was 0.17; the main diamond structures had (1 1 1) textures in the initial growth stage. During deposition times of 1.0 and 2.0 h, the standard value of the intensity ratios of I(2 2 0) /I(1 1 1) did not change substantially. The structure of the diamond films maintained the (1 1 1) facet as the deposition time increased. The intensity ratio of I(2 2 0) /I(1 1 1) for diamond deposition times of 3.0 and 4.0 h was calculated to be 0.40 and 0.46, at which the point the percentage of diamond films exhibiting (1 1 0) textures increased. Kim et al. [3] pointed out a considerably greater {1 1 1} diffraction intensity for the {1 1 1} then the diffraction intensity of the isotropic sample, in relation to the diffraction intensities of other low index planes. The {2 2 0} and {4 0 0} peaks were much larger than those of an isotropic sample in relation to the diffraction intensities of the other low index planes [21]. Thus, the textures of the diamond films had orientations of 1 1 0 and 1 0 0. Table 1 XRD intensity ratios of diamond films prepared with different deposition times. Intensity ratio I(2 2 0) I(1 1 1) I(3 1 1) I(1 1 1) I(4 0 0) I(1 1 1)
JCPDS
0.5 h
1.0 h
2.0 h
3.0 h
4.0 h
0.25
0.17
0.25
0.27
0.40
0.46
0.16
0.08
0.12
0.14
0.15
0.18
0.08
0.03
0.05
0.08
0.08
0.08
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(a)
D-band ∇
G-band ♦
Intensity (a.u.)
4.0 h
3.0 h 2.0 h 1.0 h 0.5 h
1000
1100
1200
1300
1400
1500
1600
1700
Wave number ( cm -1 ) 28
1.0
(b) 24
0.8 20
-1
0.7
FWHM (cm
IG / ID ratio
0.9
) 16 0.6
12
0.5 0
1
2
3
4
Time (hour) Fig. 5. (a) Raman spectra of diamond films prepared with different deposition times; (b) IG /ID ratios and FWHM obtained from Raman spectra.
Fig. 4. (a) TEM image of the bevel of the pyramidal diamond film deposited after 3.0 h and its position (indicated by a white arrow); (b) HRTEM image between the two tiers in image (a).
The morphology of the textured diamond films was supposed to be an orientation of {1 1 1}, but the structure of the facets was the rectangular, which had not been shown or discussed in previous studies. Fig. 3 shows the morphology and diffraction pattern of the top of a pyramidal diamond layer deposited at 3.0 h. The diffraction pattern in Fig. 3(b) shows how the zone axis is in the orientation of [1 1 0], proving that rectangular diamond films had the {1 1 0} texture. The intensity of the I(2 2 0) /I(1 1 1) ratio was increased at deposition times of 3.0 and 4.0 h. The pyramidal diamond of the image shows a distinctive bevel structure, which we studied by TEM. Fig. 4(a) shows the bevel of the pyramidal facet of the diamond film (the white indicator in the TEM image). The figure shows the two tiers: the black layers are approximately 200 nm thick and the distance between them is approximately 200 nm. Based on the morphology shown in the image, these diamond films should consist of twin structures [4], but the interface between the black and white layers in the high resolution TEM (HRTEM) image (Fig. 4(b)) exhibits the same atomic arrangement, without dissimilar domains. The carbonaceous species had possibly overcome the surface free energy by stacking faults to form the following tier thanks to the increased pressure promoting higher plasma efficiency in the development of diamond film layers.
Raman spectroscopy is the most widely used technique for characterizing the composition of diamond films, as it is capable of defining diamond, graphite, and amorphous carbon. The spectra contained both the D-band of diamond at 1332 cm−1 , and the G-band of graphite at 1580 cm−1 [11,22–24]. Fig. 5(a) shows the Raman spectra of diamond films at different deposition times. It clearly distinguishes between the D-band (diamond) at 1332 cm−1 and G-band (amorphous graphite) at 1585 cm−1 range, following an increase in deposition time, and board peak (at 1585 cm−1 ). Similarly, Bühlmann et al. [19] obtained ballas diamonds under various deposition conditions, showing that the D-band was lesser and the graphite peak greater than that of faceted diamonds. The low intensity of the Raman spectra also related to the diminished content of diamond at a deposition time of 0.5 h. At a deposition time of 4.0 h, the D-band increased and the peak of the diamond growth was more fully formed. As the diamond films had overgrown, the crystallinity was the highest after 4.0 h. The IG /ID ratios and full width of diamond films at half maximum (FWHM) with different deposition times shown in Fig. 5(b) were obtained from Raman spectra. With an increase in deposition time, IG /ID ratios decreased, and the percentage of the graphite phase was lower than that of the diamond phase. The FWHM of diamond films at 4.0 h was of higher quality. As the pyramid of the diamond grew (see Fig. 1(c) and (d)), square diamonds formed after longer deposition periods, as the residual methane in the experimental process was depleted. Fig. 6 shows an AFM of the morphology of the diamond films deposited for different periods of times. Roughness data of tex-
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Fig. 6. AFM of the diamond films deposited for deposition periods of (a) 0.5 h; (b) 1.0 h; (c) 2.0 h; (d) 3.0 h; (e) 4.0 h.
tured diamond films is listed in Table 2. The surface of the diamond films at a deposition time of 0.5 h was observably smoother than for other periods of deposition. In the initial stages, the ball-like diamonds did not cover the surface; therefore, the roughness was less pronounced in the nucleation and growth sites. The roughest surface texture (583.87 and 412.56 nm) was due to a large number of pyramidal structures on the surface of the ball-like diamonds
after 1.0 and 2.0 h of deposition. During the period of crystal period, the surface of the diamond film changed from pyramidal to rectangular structures, therefore the roughness changed from 232.36 to 241.78 nm. In the I–V curve, the linear symmetrical curve, referred to as the ohmic contact, played an important role in semiconducting characteristics. The stable ohmic contact influenced the key properties
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Table 2 Roughness of diamond films prepared with different deposition times. Time
Roughness (nm)
0.5 h
1.0 h
2.0 h
3.0 h
4.0 h
287.6
583.9
417.1
232.4
241.8
and stability of electrical transmission. Fig. 7 shows the I–V curves of textured diamond films at deposition times of 2.0, 3.0, and 4.0 h. These I–V curves are related to the morphology shown by AFM and SEM. As illustrated in the SEM image and AFM data, for deposition times of 0.5 and 1.0 h, the surface was not covered with the initial growth characteristic of textured diamond films, and the electrical characteristics of diamond films were not detected. For this reason Fig. 6 does not include the curves for films at deposition times of 0.5 and 1.0 h. At 4.0 h, the diamond films had a linear curve, suggesting good ohmic contact. The I–V characteristics of diamond films produced at a deposition time of 4.0 h showed the best ohmic contact. The characteristics were very similar to those obtained in previous studies [25]. These I–V characteristics of (0 0 1)-oriented diamond films showed a desirable linear curve, suggesting the formation of good ohmic contact, thanks to the smoothness of the surface of the (0 0 1)-oriented diamond film exceeding that of the (1 1 1) film [6]. SEM images show that the diamond films produced at 0.5 h had formed ball shaped diamonds, and the morphology was cubooctahedral, typical of the square structures produced in high methane concentrations at 1.0–4.0 h. Schematic diagrams showing the growth textured diamonds are shown in Fig. 8. The diamond is preferentially nucleated and heteroepitaxially grown on SiC or Si substrates [26,27]. In the initial growth stages, the epitaxial growth was thought to form diamond with an orientation of {1 1 1}, which was dependent on the morphology of the crystals in the Si substrate. XRD determined that the facets on the surface of the diamond film had preferentially grown to an orientation of {1 1 0} as the MPCVD system was operated at 1200 W, 110 Torr and CH4 /H2 = 1/20, for 3.0 and 4.0 h. First, the diamond film formed the {1 1 1} texture by heteroepitaxial growth. The {1 1 0} textured diamond formed over the {1 1 1} textured diamond. The surface morphology of the rectangular structures was different from that described in previous studies, with the square structure of diamond films on (0 0 1) textured crystallizations of growth conditions
50
4.0 h 3.0 h 2.0 h
40 30
Current (nA)
20
Fig. 8. Growth schematic diagrams of surface of textured diamonds (a) 0.5 h (final state); (b) 1.0 h; (c) 2.0 h; (d) 3.0 h (top view).
[6,28,29]. TEM showed that the rectangular structure exhibited {1 1 0} orientation. The tops of the octahedral structures on the diamond films gradually grew into rectangular forms, as shown in Fig. 8(b)–(d). Accompanying this growth, a secondary nucleation of the diamond appeared to have accumulated on the uppermost areas. This related to a gradual increase (with time) in the active carbonaceous species. Because the high pressure promoted the plasma to deposit more efficiently create new nucleation sites, the diamond films were able to overcome the surface free energy and develop layered structures by stacking faults. Thus, the morphology of diamond films changed to pyramidal structures (Fig. 8(c)), where the top surface comprised rectangular diamond flakes, stacked in regular 200 nm width tiers. At a deposition time of 4.0 h, the saturated carbonaceous species had grown to extend the area of the {1 1 0} plane (see Figs. 8(d) and 1(d)), as the cubo-octahedral state was the most stable. The carbonaceous species had been depleted in that period, thus, at this point the Raman spectra shows the highest crystallinity of the diamond films. More graphite formed at 4.0 h. One aspect of the ball shaped diamond films in the CVD system was that much of the amorphous graphite at 1500–1600 cm−1 formed on the surface of the diamond film. In electrical applications, the I–V characteristics of diamond films conformed to the ohmic contact after 4.0 h of deposition time, making the diamond films suitable for semiconducting purposes. The pyramid form produced an excessively rugged surface of rectangular diamonds, and the uneven surface decreased the ohmic contact.
4. Conclusion
10 0 -10 -20 -30 -40 -50 -80
-60
-40
-20
0
20
40
60
80
100
Voltage (V) Fig. 7. I–V curves of textured diamond films produced at deposition times of 2.0 h, 3.0 h, and 4.0 h. Diamond films with deposition times of 0.5 and 1.0 h exhibit no electrical characteristics.
Textured diamond films were made with MPCVD with a microwave power of 1200 W, pressure of 110 Torr, and CH4 /H2 = 1/20, for deposition periods of 0.5, 1.0, 2.0, 3.0, and 4.0 h. Produced in a high concentration of methane, the morphology of the diamond films was rectangular with a preferred orientation of {1 1 0}, with a high proportion of {1 1 1} textured facets. Under high pressure and a high concentration of carbon, the diamond films maintained amorphous graphite on the surface, which supported the growth of pyramidal structures, which later expanded to a square structure following continued deposition over time. A large amount of amorphous graphite formed on the surface of the diamond film. Tiers of diamonds, caused by stacking faults, exhibited the same atomic arrangement. The textured diamond films produced at a deposition time of 4.0 h had the best ohmic contact.
W.C. Lai et al. / Applied Surface Science 257 (2010) 1729–1735
Acknowledgement The authors are thankful for the financial support of National Science Council of Taiwan R.O.C. under grant no. NSC 97-2511-S157-002-MY3. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Differing morphologies of textured diamond films with electrical properties made with microwave plasma chemical vapor deposition Wen Chi Lai a , Yu-Shiang Wu b,∗ , Hou-Cheng Chang c , Yuan-Haun Lee a a b c
Department of Material Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC Department of Mechanical Engineering, China University of Science and Technology, Taipei 11581, Taiwan, ROC Department of Electronic Engineering, China University of Science and Technology, Taipei 11581, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 24 January 2010 Received in revised form 22 August 2010 Accepted 2 September 2010 Available online 15 September 2010 Keywords: {1 1 0} preferred orientation Rectangular structure Tier Ohmic contact
a b s t r a c t This study investigates the orientation of textured diamond films produced through microwave plasma chemical vapor deposition (MPCVD) at 1200 W, 110 Torr, CH4 /H2 = 1/20, with depositions times of 0.5–4.0 h. After a growth period of 2.0–4.0 h, this particular morphology revealed a rectangular structure stacked regularly on the diamond film. The orientation on {1 1 1}-textured diamond films grew a preferred orientation of {1 1 0} on the surface, as measured by XRD. The formation of the diamond epitaxial film formed textured octahedrons in ball shaped (or cauliflower-like) diamonds in the early stages (0.5 h), and the surface of the diamond film extended to pile the rectangular structure at 4.0 h. The width of the tier was approximately 200 nm at the 3.0 h point of deposition, according to TEM images. The results revealed that the textured diamond films showed two different morphological structures (typical ball shaped and rectangular diamonds), at different stages of the deposition period. The I–V characteristics of the oriented diamond films after 4.0 h of deposition time showed good conformity with the ohmic contact. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Diamond has many excellence characteristics for industrial applications, such as good wear, the highest Young’s modulus (a high degree of hardness) optical characteristics, and semiconducting properties. There are many methods for describing the orientation and texture of diamond films under different deposition conditions, and the individual properties of (1 1 1) and (0 0 1) facets have been discussed in several previous studies [1–4]. For example, diamond films comprising (1 0 0) facets have better wear than those comprising (1 1 1) facets, because the surfaces of (1 1 1) facets are too sharp, causing the diamond films to break down easily [5]. Polycrystalline diamond films at (1 0 0) orientation are superior to (1 1 1) in their optical and electrical properties [2,6,7]. However, research on the properties of rectangular and stacked texture diamond films is limited, compared with (1 1 0) polycrystalline diamond films. The temperature of the substrate and the hydrocarbon concentration are deposition parameters that influence the orientation of
∗ Corresponding author at: 245, Sec. 3, Yen-Chiu-Yuan Road, Nankang, Taipei 115, Taiwan. Tel.: +886 2 27867048x37; fax: +886 2 27867253. E-mail address: [email protected] (Y.-S. Wu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.006
diamond films [8]. In previous studies, by controlling these two parameters, the distinctive features of orientation were investigated [3,4]. In a microwave plasma chemical vapor deposition (MPCVD) system, the concentration of methane and pressure within the chamber influenced the morphology of the diamond films. It was also found that nucleation density decreased with an increase in pressure, while it increased with an increase in the concentration of CH4 [9]. The manner of growth was clearly dependent on ambient pressure: i.e., at higher pressure, diamond {1 0 0} was the favorable growth surface, whereas, {1 1 1} was the favorable growth surface at low pressure [10]. Diamond quality depends greatly on pressure and methane concentration with certain combinations of the two parameters; the maximum allows the methane concentration to increase with pressure, thus allowing higher growth rates [11]. In an MPCVD system, to make high quality diamond films, the carbonaceous species contained 0.1–2% hydrogen and the pressure was 20–50 Torr [11–15]. However, the growth of diamond films under specific deposition conditions was unclear. Ball shaped diamonds were also denominated by polycrystalline diamonds and cauliflower-like diamonds in some studies [16–18]. The ball shaped (ballases) diamonds were formed under non-optimal growth conditions [19], and were generally sensitive to temperature under different methane concentrations. However, preview researchers did not provide a clear, detailed discussion of
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W.C. Lai et al. / Applied Surface Science 257 (2010) 1729–1735
Fig. 1. SEM images of the diamond films prepared with different periods of deposition: (a) 0.5 h; (b) 1.0 h; (c) 2.0 h; (d) 3.0 h; (e) 4.0 h.
the stacked rectangular growth of diamond films under particular deposition conditions (in a high methane concentration and high pressure). In this work, we prepared the diamond films under high pressure and at high methane concentrations to observe the deposition characteristics of textured diamond films grown by MPCVD. To understand the changes in the (1 1 0) diamond films, the study used various deposition times to observe changes in the morphology. The results were analyzed by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, atomic force
microscopy (AFM), and electrical tests, to identify the distinguishing characteristics of the diamond films. 2. Experimental All experiments were carried out using a 2.45 GHz MPCVD system (ITRI, MP2000-DH2) with a 2 kW microwave source. The substrate was a (1 1 1) silicon wafer with an area of 1 cm × 1 cm, prepared with 1 m power suspension in an ultrasonic bath for 3.0 h. After the ultrasonic bath, it was cleaned in an ultrasonic bath of acetone for 20 min. The diamond films were deposited in a contin-
W.C. Lai et al. / Applied Surface Science 257 (2010) 1729–1735
1731
(111)
Intensity (a.u.)
(220) (311)
(400)
3.0 h
0.5 h 40
60
80
100
120
2θ (degree) Fig. 2. XRD patterns of diamond films prepared at a deposition time of 3.0 h.
uous flow of mixed gas made up of 15 sccm methane and 300 sccm hydrogen. The microwave power was 1200 W, the pressure within the chamber was kept at 110 Torr, and the deposition times were 0.5, 1.0, 2.0, 3.0, and 4.0 h. The morphology of the diamond films was examined with FESEM (Leo, 1530). The crystallization and orientation of the diamond films were estimated by XRD (Rigaku, Ttrax III) and the lattice structure was observed by TEM (Philips, Technai F30). Their physical properties and characteristic qualities were analyzed by Raman spectroscopy (Jobin Yvon, iHR-320) with a 514.5 nm He–Ne green light laser. The roughness of diamond film surface was evaluated by AFM (Veeco, MultiMode 8 scanning probe microscope). The electrical properties were measured by a semiconductor characterization system (Keithley, 2430). Prior to the electrical tests, both sides of the samples were coated in platinum by sputtering, and annealed at 500 ◦ C for 1 h. 3. Results and discussion The SEM images of the textured diamond films are shown in Fig. 1(a)–(e) representing deposition times of 0.5–4.0 h. In the diamond growth stage, ball shaped diamonds with a particle size of approximately 2 m were heterogeneously deposited on the substrate, and many nucleation sites appeared on the surfaces of ball shaped diamonds, as shown in Fig. 1(a). There were many secondary crystallites on the top faces at a deposition time of 1.0 h, as shown in Fig. 1(b). The clearer diamond structures piled atop one another. The size of pyramidal forms deposited in a high methane concentration increased, and the substrate was covered with diamonds, as shown in Fig. 1(c). Fig. 1(d) shows the diamond films with rectangular structures on top at a deposition time of 4.0 h. At a deposition time of 4.0 h, square diamonds formed at approximately 2 m × 2 m. Nevertheless, the square (or rectangular) structure was the (1 1 0) orientation, as discussed in previous studies. In addition, the cross section of the diamond films at 3.0 h deposition time was approximately 10 m, for a calculated growth rate of 3.3 m/h. The XRD patterns for the deposited diamond films and diffraction peaks correspond to the (1 1 1), (2 2 0), (3 1 1), and the (4 0 0) as indexed by the Joint Committee on Powder Diffraction Standards (JCPDS) diffraction data card (card 0675 from Set 06) for diamond films. In Fig. 2, the peaks at 2 angles of 44.05◦ , 75.50◦ , 91.75◦ , and 119.50◦ were identified as (1 1 1), (2 2 0), (3 1 1), and (4 0 0), respectively [20]. Table 1 shows the intensity ratios of diamond characterized by XRD patterns. Intensity ratios of I(2 2 0) /I(1 1 1) , I(3 1 1) /I(1 1 1) and I(4 0 0) /I(1 1 1) for the JCPDS diffraction data card were
Fig. 3. (a) TEM image of pyramidal diamond film deposited for 3.0 h; (b) the diffraction pattern of the top position, indicated by a white arrow from (a).
0.25, 0.16, and 0.08, respectively. At a deposition time of 0.5 h, the I(2 2 0) /I(1 1 1) was 0.17; the main diamond structures had (1 1 1) textures in the initial growth stage. During deposition times of 1.0 and 2.0 h, the standard value of the intensity ratios of I(2 2 0) /I(1 1 1) did not change substantially. The structure of the diamond films maintained the (1 1 1) facet as the deposition time increased. The intensity ratio of I(2 2 0) /I(1 1 1) for diamond deposition times of 3.0 and 4.0 h was calculated to be 0.40 and 0.46, at which the point the percentage of diamond films exhibiting (1 1 0) textures increased. Kim et al. [3] pointed out a considerably greater {1 1 1} diffraction intensity for the {1 1 1} then the diffraction intensity of the isotropic sample, in relation to the diffraction intensities of other low index planes. The {2 2 0} and {4 0 0} peaks were much larger than those of an isotropic sample in relation to the diffraction intensities of the other low index planes [21]. Thus, the textures of the diamond films had orientations of 1 1 0 and 1 0 0. Table 1 XRD intensity ratios of diamond films prepared with different deposition times. Intensity ratio I(2 2 0) I(1 1 1) I(3 1 1) I(1 1 1) I(4 0 0) I(1 1 1)
JCPDS
0.5 h
1.0 h
2.0 h
3.0 h
4.0 h
0.25
0.17
0.25
0.27
0.40
0.46
0.16
0.08
0.12
0.14
0.15
0.18
0.08
0.03
0.05
0.08
0.08
0.08
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(a)
D-band ∇
G-band ♦
Intensity (a.u.)
4.0 h
3.0 h 2.0 h 1.0 h 0.5 h
1000
1100
1200
1300
1400
1500
1600
1700
Wave number ( cm -1 ) 28
1.0
(b) 24
0.8 20
-1
0.7
FWHM (cm
IG / ID ratio
0.9
) 16 0.6
12
0.5 0
1
2
3
4
Time (hour) Fig. 5. (a) Raman spectra of diamond films prepared with different deposition times; (b) IG /ID ratios and FWHM obtained from Raman spectra.
Fig. 4. (a) TEM image of the bevel of the pyramidal diamond film deposited after 3.0 h and its position (indicated by a white arrow); (b) HRTEM image between the two tiers in image (a).
The morphology of the textured diamond films was supposed to be an orientation of {1 1 1}, but the structure of the facets was the rectangular, which had not been shown or discussed in previous studies. Fig. 3 shows the morphology and diffraction pattern of the top of a pyramidal diamond layer deposited at 3.0 h. The diffraction pattern in Fig. 3(b) shows how the zone axis is in the orientation of [1 1 0], proving that rectangular diamond films had the {1 1 0} texture. The intensity of the I(2 2 0) /I(1 1 1) ratio was increased at deposition times of 3.0 and 4.0 h. The pyramidal diamond of the image shows a distinctive bevel structure, which we studied by TEM. Fig. 4(a) shows the bevel of the pyramidal facet of the diamond film (the white indicator in the TEM image). The figure shows the two tiers: the black layers are approximately 200 nm thick and the distance between them is approximately 200 nm. Based on the morphology shown in the image, these diamond films should consist of twin structures [4], but the interface between the black and white layers in the high resolution TEM (HRTEM) image (Fig. 4(b)) exhibits the same atomic arrangement, without dissimilar domains. The carbonaceous species had possibly overcome the surface free energy by stacking faults to form the following tier thanks to the increased pressure promoting higher plasma efficiency in the development of diamond film layers.
Raman spectroscopy is the most widely used technique for characterizing the composition of diamond films, as it is capable of defining diamond, graphite, and amorphous carbon. The spectra contained both the D-band of diamond at 1332 cm−1 , and the G-band of graphite at 1580 cm−1 [11,22–24]. Fig. 5(a) shows the Raman spectra of diamond films at different deposition times. It clearly distinguishes between the D-band (diamond) at 1332 cm−1 and G-band (amorphous graphite) at 1585 cm−1 range, following an increase in deposition time, and board peak (at 1585 cm−1 ). Similarly, Bühlmann et al. [19] obtained ballas diamonds under various deposition conditions, showing that the D-band was lesser and the graphite peak greater than that of faceted diamonds. The low intensity of the Raman spectra also related to the diminished content of diamond at a deposition time of 0.5 h. At a deposition time of 4.0 h, the D-band increased and the peak of the diamond growth was more fully formed. As the diamond films had overgrown, the crystallinity was the highest after 4.0 h. The IG /ID ratios and full width of diamond films at half maximum (FWHM) with different deposition times shown in Fig. 5(b) were obtained from Raman spectra. With an increase in deposition time, IG /ID ratios decreased, and the percentage of the graphite phase was lower than that of the diamond phase. The FWHM of diamond films at 4.0 h was of higher quality. As the pyramid of the diamond grew (see Fig. 1(c) and (d)), square diamonds formed after longer deposition periods, as the residual methane in the experimental process was depleted. Fig. 6 shows an AFM of the morphology of the diamond films deposited for different periods of times. Roughness data of tex-
W.C. Lai et al. / Applied Surface Science 257 (2010) 1729–1735
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Fig. 6. AFM of the diamond films deposited for deposition periods of (a) 0.5 h; (b) 1.0 h; (c) 2.0 h; (d) 3.0 h; (e) 4.0 h.
tured diamond films is listed in Table 2. The surface of the diamond films at a deposition time of 0.5 h was observably smoother than for other periods of deposition. In the initial stages, the ball-like diamonds did not cover the surface; therefore, the roughness was less pronounced in the nucleation and growth sites. The roughest surface texture (583.87 and 412.56 nm) was due to a large number of pyramidal structures on the surface of the ball-like diamonds
after 1.0 and 2.0 h of deposition. During the period of crystal period, the surface of the diamond film changed from pyramidal to rectangular structures, therefore the roughness changed from 232.36 to 241.78 nm. In the I–V curve, the linear symmetrical curve, referred to as the ohmic contact, played an important role in semiconducting characteristics. The stable ohmic contact influenced the key properties
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Table 2 Roughness of diamond films prepared with different deposition times. Time
Roughness (nm)
0.5 h
1.0 h
2.0 h
3.0 h
4.0 h
287.6
583.9
417.1
232.4
241.8
and stability of electrical transmission. Fig. 7 shows the I–V curves of textured diamond films at deposition times of 2.0, 3.0, and 4.0 h. These I–V curves are related to the morphology shown by AFM and SEM. As illustrated in the SEM image and AFM data, for deposition times of 0.5 and 1.0 h, the surface was not covered with the initial growth characteristic of textured diamond films, and the electrical characteristics of diamond films were not detected. For this reason Fig. 6 does not include the curves for films at deposition times of 0.5 and 1.0 h. At 4.0 h, the diamond films had a linear curve, suggesting good ohmic contact. The I–V characteristics of diamond films produced at a deposition time of 4.0 h showed the best ohmic contact. The characteristics were very similar to those obtained in previous studies [25]. These I–V characteristics of (0 0 1)-oriented diamond films showed a desirable linear curve, suggesting the formation of good ohmic contact, thanks to the smoothness of the surface of the (0 0 1)-oriented diamond film exceeding that of the (1 1 1) film [6]. SEM images show that the diamond films produced at 0.5 h had formed ball shaped diamonds, and the morphology was cubooctahedral, typical of the square structures produced in high methane concentrations at 1.0–4.0 h. Schematic diagrams showing the growth textured diamonds are shown in Fig. 8. The diamond is preferentially nucleated and heteroepitaxially grown on SiC or Si substrates [26,27]. In the initial growth stages, the epitaxial growth was thought to form diamond with an orientation of {1 1 1}, which was dependent on the morphology of the crystals in the Si substrate. XRD determined that the facets on the surface of the diamond film had preferentially grown to an orientation of {1 1 0} as the MPCVD system was operated at 1200 W, 110 Torr and CH4 /H2 = 1/20, for 3.0 and 4.0 h. First, the diamond film formed the {1 1 1} texture by heteroepitaxial growth. The {1 1 0} textured diamond formed over the {1 1 1} textured diamond. The surface morphology of the rectangular structures was different from that described in previous studies, with the square structure of diamond films on (0 0 1) textured crystallizations of growth conditions
50
4.0 h 3.0 h 2.0 h
40 30
Current (nA)
20
Fig. 8. Growth schematic diagrams of surface of textured diamonds (a) 0.5 h (final state); (b) 1.0 h; (c) 2.0 h; (d) 3.0 h (top view).
[6,28,29]. TEM showed that the rectangular structure exhibited {1 1 0} orientation. The tops of the octahedral structures on the diamond films gradually grew into rectangular forms, as shown in Fig. 8(b)–(d). Accompanying this growth, a secondary nucleation of the diamond appeared to have accumulated on the uppermost areas. This related to a gradual increase (with time) in the active carbonaceous species. Because the high pressure promoted the plasma to deposit more efficiently create new nucleation sites, the diamond films were able to overcome the surface free energy and develop layered structures by stacking faults. Thus, the morphology of diamond films changed to pyramidal structures (Fig. 8(c)), where the top surface comprised rectangular diamond flakes, stacked in regular 200 nm width tiers. At a deposition time of 4.0 h, the saturated carbonaceous species had grown to extend the area of the {1 1 0} plane (see Figs. 8(d) and 1(d)), as the cubo-octahedral state was the most stable. The carbonaceous species had been depleted in that period, thus, at this point the Raman spectra shows the highest crystallinity of the diamond films. More graphite formed at 4.0 h. One aspect of the ball shaped diamond films in the CVD system was that much of the amorphous graphite at 1500–1600 cm−1 formed on the surface of the diamond film. In electrical applications, the I–V characteristics of diamond films conformed to the ohmic contact after 4.0 h of deposition time, making the diamond films suitable for semiconducting purposes. The pyramid form produced an excessively rugged surface of rectangular diamonds, and the uneven surface decreased the ohmic contact.
4. Conclusion
10 0 -10 -20 -30 -40 -50 -80
-60
-40
-20
0
20
40
60
80
100
Voltage (V) Fig. 7. I–V curves of textured diamond films produced at deposition times of 2.0 h, 3.0 h, and 4.0 h. Diamond films with deposition times of 0.5 and 1.0 h exhibit no electrical characteristics.
Textured diamond films were made with MPCVD with a microwave power of 1200 W, pressure of 110 Torr, and CH4 /H2 = 1/20, for deposition periods of 0.5, 1.0, 2.0, 3.0, and 4.0 h. Produced in a high concentration of methane, the morphology of the diamond films was rectangular with a preferred orientation of {1 1 0}, with a high proportion of {1 1 1} textured facets. Under high pressure and a high concentration of carbon, the diamond films maintained amorphous graphite on the surface, which supported the growth of pyramidal structures, which later expanded to a square structure following continued deposition over time. A large amount of amorphous graphite formed on the surface of the diamond film. Tiers of diamonds, caused by stacking faults, exhibited the same atomic arrangement. The textured diamond films produced at a deposition time of 4.0 h had the best ohmic contact.
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