AlN heterostructures grown on Si substrate by plasma-assisted MBE for MSM UV photodetector applications

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AlN/GaN/AlN heterostructures grown on Si substrate by plasma-assisted MBE for MSM UV photodetector applications M.Z. Mohd Yusoff a,b,n, A. Mahyuddin c, Z. Hassan b, H. Abu Hassan b, M.J. Abdullah b, M. Rusop d, S.M. Mohammad b, Naser M. Ahmed b a

Department of Applied Sciences, Universiti Teknologi MARA (Pulau Pinang), 13500 Permatang Pauh, Penang, Malaysia Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia Universiti Kuala Lumpur, Malaysian Institute of Industrial Technology (MITEC), Persiaran Sinaran Ilmu, Bandar Seri Alam, 81750 Johor, Malaysia d Faculty of Electrical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia b c

a r t i c l e i n f o

Keywords: AlN MBE XRD III-Nitride Silicon

abstract The AlN/GaN/AlN heterostructures were successfully grown on silicon substrate by plasma-assisted molecular beam epitaxy (MBE). High purity gallium (7N) and aluminum (6N5) were used to grow GaN and AlN, respectively. The structural and optical properties of the samples have been investigated by high-resolution X-ray diffraction (HR-XRD), photoluminescence (PL), Raman spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED), dark field scanning transmission electron microscopy (DF STEM), and high-angle annular dark field scanning transmission electron microscopy (HAADF STEM). HR-XRD measurement showed that the sample has a typical diffraction pattern of hexagonal AlN/GaN/AlN heterostructures. Raman spectra revealed all four Raman-active modes, i.e., GaN-like E2 (H), AlN-like A1 (TO), AlN-like E2 (H), and AlN-like A1 (LO) inside the AlN/GaN/AlN heterostructures. Good thickness uniformity of the layers and high-quality hetero-structures without cracking were confirmed by TEM, SAED, DF STEM and HAADF STEM. The fabricated AlN/GaN/AlN heterostructures based metal-semiconductor-metal (MSM) for the UV photodetector shows a rise and fall of photoresponses, suggesting that the AlN/GaN/AlN heterostructures have good carrier transport and crystallinity properties. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction III-nitride semiconductors are promising materials which have great potential for uses in displays, optical data storage, high power, high-frequency electronic devices, ultraviolet detectors, and related technologies [1]. In specific, aluminum nitride (AlN) has attracted considerable interest because of its application as a buffer n Corresponding author at: Department of Applied Sciences, Universiti Teknologi MARA (Pulau Pinang), 13500 Permatang Pauh, Penang, Malaysia. Tel.: þ6012 5811 401; fax: þ 604 657 9150. E-mail address: [email protected] (M.Z. Mohd Yusoff).

layer for the growth of device grade GaN as well as for application in electro-acoustic, acousto-optical and optoelectronic devices [2]. For that purpose, molecular beam epitaxy (MBE) is a technique of choice as a result of in situ monitoring surface roughness and reconstruction during growth. Silicon substrate presents the obvious advantages of well-known technology, low cost, and potential hybrid integration. However, Si (111) has been less investigated than sapphire as a substrate to grow nitride due to the higher lattice and thermal expansion mismatches which produce a higher dislocation density and the potential generation of crack [3]. In this work, the microstructures and optical properties of AlN/GaN/AlN heterostructures on

http://dx.doi.org/10.1016/j.mssp.2014.03.041 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

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Si (111) substrate grown by plasma-assisted molecular beam epitaxy (MBE) were studied. The surface morphology and structural properties of the layers were determined using high-resolution X-ray diffraction (HR-XRD, Model PANalytical X’Pert PRO MRD PW3040), photoluminescence and Raman spectroscopy (Jobin Yvon HR 800 UV spectrometer system), and transmission electron microscopy (TEM), (Model: JEOL). The potential application of this structure for light detection is also included. 2. Experimental The AlN/GaN/AlN heterostructures sample was grown on Si (111) substrate using Gen II MBE system with a radiofrequency plasma source. High purity material sources such as gallium (7 N) and aluminum (6N5) were used in the Knudsen cells. Nitrogen with 7 N purity was channeled to a radio frequency (RF) source to generate reactive nitrogen species. The plasma was operated at typical nitrogen pressure of 1.5  10  5 Torr under a discharge power of 300 W. The 3-in. Si (111) substrate was ex-situ cleaned with the standard cleaning procedure by using RCA method before it was loaded into the MBE system. The substrates were outgassed at 900 1C for 2 h in ultra-high vacuum. In order to remove SiO2 on the surface of silicon, a few monolayers of Ga were deposited on the substrate to form Ga2O3. A clean Si (111) can be seen from the presence of prominent Kikuchi lines in the typical Si (111) 7  7 surface reconstruction pattern. To prevent the SiN formation at the surface, a few monolayers of Al were deposited before the nitrogen source was activated. AlN buffer or wetting layer was first grown by opening both Al (cell temperature at 1120 1C and flux of 2.2  10  7 Torr) and N cell shutters and starting the N plasma simultaneously for 15 min. Then, GaN epilayer was grown on top of the buffer layer at cell temperature of 1085 1C and flux of 7.9  10  7 Torr for 1 h. Subsequently, a thin AlN layer was grown on top of the GaN surface at cell temperature of 1120 1C and flux of 2.2  10  7 Torr for 10 min. The typical procedures for growth of AlN/GaN/AlN on Si (111) substrate are shown in Fig. 1. This is followed by thermal

evaporation of aluminum (Al) metal on the sample to form interdigitated Schottky contact electrodes. The structure of the MSM photodetector consists of two interdigitated Schottky contacts with finger width of 200 mm, finger spacing of 400 mm, and the length of each electrode was about 3300 mm. It consists of four fingers at each electrode. After Al metallization the MSM structure was subjected to annealing at 500 1C for 10 min in a vacuum tube furnace in flowing nitrogen ambient. The high pressure spherical Xenon lamp (Model: LLC-7, Manufacturer: Lambda Scientific) and 460 nm LED (Manufacturer: Sensor Electronic Technology, Inc.) were used as light sources for photocurrent and photoresponse measurements, respectively. The power of those lamps used are 555.8 mW (Xenon lamp) and 29.8 mW (460 nm LED). 3. Results and discussion Fig. 2 shows XRD scans of AlN/GaN/AlN heterostructures layers on Si (111) substrate with a scan range from 201 to 451. XRD results confirm that the structure of AlN/ GaN/AlN was epitaxially grown on Si (111) substrate. It can be seen that the XRD spectra indicate that no sign of cubic phase of sample is found within the detection limit of the XRD, so it is confirmed that the samples possessed hexagonal structure. Table 1 shows the peak positions of AlN (0002), GaN (0002), and Si (111) substrate. Also, the XRD spectrum in Fig. 2 show peaks at 381 and 591 which correspond to Al droplets and impurities, respectively. Formation of Al droplets has previously been reported for Al-rich growth of AlN at temperature of 750–850 1C [4–6]. XRD rocking curve was also carried out to investigate the crystalline quality of the epilayers as shown in Fig. 3. The full width at half maximum (FWHM) of the sample is 0.461 (27.60 ) [7]. This result is better than that previously reported by Meng and Perry (510 ) [8], Yasutake et al. (550 ) [9], Luo et al. (930 ) [10], and Schenk et al. (37.20 ) [11]. Generally, the result of on-axis XRD curve, like (0002) FWHM, shows the information related to the tilt of the sub-grains with respect to the substrate and only the

Growth temperature

900oC

Outgassing

850o

850oC

850oC

850oC

850oC

Ga flush

AlN buffer layer (Al high flux)

AlN buffer layer (Al low flux)

GaN layer

AlN layer

2 min

90 s

10 min

60 min

10 min

Time 20 min

Fig. 1. Growth procedure as well as schematic diagram of growth process for AlN/GaN/AlN heterostructures on Si (111) substrate.

Please cite this article as: M.Z. Mohd Yusoff, et al., Materials Science in Semiconductor Processing (2014), http://dx.doi. org/10.1016/j.mssp.2014.03.041i

M.Z. Mohd Yusoff et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] 1800

1000000

Si (111) GaN (0002)

100000

AlN (0002)

10000

1600 1400

GaN (0004) AlN (0004)

1000 100 10

1200 Intensity (a.u)

Intensity (a.u.)

10000000

1

3

1000 800 600 400

20

30

40

50

60

70

80

2Theta (degree)

0

Fig. 2. XRD scans of AlN/GaN/AlN heterostructures grown on Si (111) substrates taken from the (0002) diffraction plane and measured by the 2Theta-ω scan mode.

Table 1 The 2θ peak position of AlN/GaN/AlN heterostructures grown on Si (111) substrate. Sample

2θ Peak position (deg)

AlN/GaN/AlN on Si (111) substrate

200

Si (111)

AlN (0002)

GaN (0002)

28.475

36.075

34.575

350

400

450

500

550

600

Wavelength (nm) Fig. 4. PL spectra of the AlN/GaN/AlN heterostructures grown on Si (111) substrate.

Table 2 Phonon frequency of AlN/GaN/AlN heterostructures samples, unstrained AlN, and unstrained GaN at room temperature. Phonons

Sample (cm  1)

GaN-like E2 (high) GaN-like A1 (LO) AlN-like E2 (high) AlN-like A1 (TO)

569.94 735.79 651.80 622.03

1200 18.11

o

1000

Intensity (a.u.)

800 600 o

FWHM = 0.46

400 200 0 12

14

16

18

20

22

24

26

Omega (degree) Fig. 3. XRD symmetric RC ω/2θ scans of (0002) plane of AlN/GaN/AlN layers grown on Si (111) substrate.

threading dislocations with a screw component make a contribution [12]. The broadening of the FWHM can be attributed to crystal imperfection (which includes strain, deformation, twinning, composition in-homogeneity, and mosaic structure) and crystallite size [13]. Therefore, we suggest that the narrower (0002) FWHM of AlN contributes to the lower defects of threading dislocations with a screw component [7]. Fig. 4 shows the PL spectrum of AlN/GaN/AlN heterostructures on silicon substrate. The sample obtained at room temperature shows the band-edge (BE) emission of GaN at 349.33 nm (Eg ¼hc/λE1239.8193 eV/λ E3.55 eV), which is comparable with that obtained by Ponce et al. [14] and Chuah et al. [15]. However, the band edge emission of AlN was not obtained due to the limitation

of the excitation source used in this study. The absence of yellow band emission in PL result confirmed that the thin film is of good optical quality. Furthermore, the absence of hydrogen element during growth was also believed to produce no yellow luminescence in this sample [16]. Ogino and Aoki reported that the yellow defect-related emission (at around 563 nm) from PL measurement is often detected in GaN thin film [17] (Table 2). Fig. 5 shows Raman spectra of the AlN/GaN/AlN heterostructures grown on Si (111) substrate. The peaks at 300 cm  1 and 521 cm  1 correspond to Si modes. The Raman spectra of these materials have recently been studied by Davydov et al. [18]. On the whole, our results are comparable with the results of these works. The most important finding of our study is that all four Ramanactive modes, i.e., GaN-like E2 (H), AlN-like A1 (TO), AlNlike E2 (H), and AlN-like A1 (LO), are observed in the spectra of AlN/GaN/AlN heterostructures films. Therefore, the obtained Raman data clearly demonstrate that samples under study can be characterized as good epitaxial layers of AlN/GaN/AlN heterostructures films. This conclusion is consistent with the X-ray data (see Fig. 2). The dominant E2 (high) phonon mode of GaN appears at 569.94 cm  1. Meanwhile, the GaN-like A1 (LO) mode also appears at 735.79 cm  1. It can be seen that most of our results are comparable with the reported results for unstrained GaN layer [18] and thick GaN film [19] as well as bulk GaN [20,21]. The E2 (high) mode of AlN appears at 651.80 cm  1, which deviates from the standard value of 655 cm  1 for unstrained AlN [22]. Finally, the phonon mode at 622.03 cm  1 is attributed to the AlN-like A1 (TO) phonon mode [18]. The appearance of forbidden AlN-like

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AlN/GaN/AlN heterostructures have been successfully grown on silicon substrate. As seen in HAADF-STEM images in Fig. 6(d), the AlN layers appear as dark bands while the GaN layer appear as a bright band, which were confirmed by energy dispersive X-ray spectroscopy (EDS) analysis in our previous report [7]. The growth rate of epilayers can be estimated through the layers thickness and growth time. The estimated thicknesses of AlN/GaN/ AlN layers are 69.94 nm (AlN top layer), 345.3 nm (GaN), and 103.4 nm (AlN buffer layer), with growth rate of 0.41964 mm/h, 0.3453 mm/h and 0.4136 mm/h, respectively (see Fig. 6(e)). The quality of GaN epitaxial layer has been found to be very sensitive to initial surface coating on Si substrates and is being improved by exploring AlN buffer layer between GaN and the Si substrate [31–33]. This is because the lattice and thermal mismatch between AlN and GaN are much smaller than those between Si and GaN, and AlN has good wetting properties on Si [33]. The role of AlN buffer layers is not only to improve the crystalline quality of the GaN layer, but also to electrically insulate the epitaxial film from the conductive substrate of silicon [34]. At room temperature, Van der Pauw– Hall measurement was applied to AlN/GaN/AlN heterostructures using indium as ohmic contact. The carrier concentration value of sample was measured as 3.70  1020 cm  3. The value of bulk resistance and carriers' mobility in sample were 1.73  10  3 Ω cm and 9.72 cm2/V s, respectively. Fig. 7 shows the current voltage (I–V) characteristics of AlN/GaN/AlN MSM photodetector under dark and Xenon lamp illumination conditions, operating under forward and reversed biases. The photo and dark currents were about 10 mA and 9.01 mA for applied voltage of 3 V, respectively. In addition to the I–V characteristics, the gain current (Gain ¼photo-current/dark current) of AlN/GaN/ AlN MSM device was about 1.1096 for applied voltage of 3 V. The photo-response time of the AlN MSM photodetector was investigated by using the pulse tester. The devices were exposed to 460 nm light illumination and the corresponding current at a particular voltage (0.1, 1, and 1.5 V) for the devices was measured. Under 460 nm light illumination, we observed similar characteristics of photo-response (rise and fall) for various

A1 (TO) resulted from the break of selection rules due to the 3D island and disorder near the interface [23]. The E2 (high) modes in the Raman spectra can be used to estimate the stress because it has been proven to be particularly sensitive to biaxial stress in samples. In the linear approximation, the deviation in frequency of a given phonon mode γ under symmetry-conserving stress can be expressed in term of the biaxial stress sxx [24,25]. Δωγ ¼ K γ sxx ;

ð1Þ

Obviously, the biaxial stress can be calculated, according to Eq. (1), from the measured Raman frequency shift of a given phonon mode if the linear stress coefficient Kγ is known. Gleize et al. [25] have reported a stress coefficient of 3.39 cm  1/GPa for the E2-high mode of AlN which falls in between the values reported respectively by Prokofyeva et al. (4.5 cm  1/GPa) [26] and by Sarua et al. (3.0 cm  1/GPa) [27]. Here, we adopt the values from Gleize et al. [25] for the stress coefficient and standard frequency of 655 cm  1 for AlN E2-high mode. The calculated residual stress in the AlN E2-high mode is approximately  0.94 GPa which is less than the value reported by Zhao et al. (1.77 GPa) [28]. By using formula (1), the stress states in GaN buffer layers grown on AlN buffer layer was examined. For the GaN/Si sample, the stress coefficient 4.3 cm  1/GPa reported by Tripathy et al. [29] is adopted and the calculated stress in the GaN buffer layer is 0.68 GPa. The biaxial stress of E2-high mode peak causes the shift of 2.94 cm  1 with respect to the standard value of 567 cm  1 for unstrained GaN [18]. The broadening in alloy semiconductors has been attributed to defects and/or inhomogeneous stress and/or the inhomogeneous distribution of constituent atoms [30]. It is known that the line width E2 (H) reflects the crystalline quality and can be an indicator of the degree of randomness of the alloy [30]. Fig. 6 (a and b) shows the TEM and SAED images of the AlN/GaN/AlN heterostructures grown on Si (111) substrate. Good thickness uniformity of the layers and a high-quality heterointerface without cracking were obtained by optimizing the growth conditions. Diffraction spots from SAED are regular which shows AlN/GaN/AlN heterostructures are hexagonal structures. Meanwhile, dark field STEM micrograph (Fig. 6(c)) reveals that good layers of

GaN-like E2 (H) = 569.94 cm-1

SiO2

-1

Intensity (a.u)

Si = 300 cm

AlN-like A1 (TO) = 622.03 cm-1 AlN-like E2 (H) = 651.80 cm-1 GaN-like A1 (LO) = 735.79 cm-1

2nd band of Si

100 200

400

600

800

1000

-1

Raman Shift (cm ) Fig. 5. Raman spectra of AlN/GaN/AlN heterostructures grown on Si (111) substrate.

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Si

AlN

GaN

5

AlN

AlN

AlN

GaN

GaN

AlN

AlN

Si

Si

AlN GaN AlN

Si

Fig. 6. TEM, SAED, DF STEM, HAADF STEM, and FESEM cross-section image of AlN/GaN/AlN heterostructures grown on Si (111) substrate.

voltages of 0.1, 1, and 1.5 V (see Fig. 8(a–c)). The photocurrent decay under 460 nm light illumination conditions without showing a pronouncing photocurrent tail suggests that the AlN/GaN/AlN heterostructures have good carrier transport properties. The photoresponse spectra of the photodetector can be related to the crystallinity, defect ratio and barrier height of devices [35,36]. Therefore, the result of photoresponse is in agreement with the rocking curve XRD result indicating the good crystallinity of the sample. Moreover, the photo-current of MSM device always showed the same trend over the entire duration of the test indicating the high stability of the AlN/GaN/AlN

heterostructures device. The saturated photo-currents were about 0.364 mA, 3.60 mA, and 5.79 mA for applied voltages of 0.1, 1, and 1.5 V, under 460 nm illuminations, respectively. Meanwhile, the dark currents were about 0.311 mA, 3.46 mA, and 5.64 mA for applied voltages of 0.1, 1, and 1.5 V, respectively. The net photo-current (after subtraction of the dark current) was 52.8 μA, 14.1 μA, and 146 μA for applied voltages of 0.1, 1, and 1.5 V, respectively. Fig. 9(a–d) shows the photoconductivity of the AlN/GaN/ AlN heterostructures MSM photodetector under the illumination of Xenon lamp (555.8 mW). The photoconductivity measurements were performed in the wavelength of

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6

1.50E-02

64.000 1.00E-02

0.00E+00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00

0.50

1.00

-5.00E-03

1.50

2.00

2.50

Photo current

-1.00E-02

Photo Current (µ A)

Current (A)

63.500 5.00E-03

63.000 62.500 62.000 61.500

Dark current

61.000 -1.50E-02

200

250

Fig. 7. I–V characteristics of Al contact on AlN MSM photodetector annealed for 10 min at 500 1C in nitrogen ambient.

Photo current

Current (A)

3.60E-04 3.50E-04 3.40E-04 3.30E-04 3.20E-04

400

2.225 2.220 2.215 2.210

Dark current

3.10E-04 3.00E-04 0.00

350

2.230

Photo Current (m A)

3.70E-04

300 Wavelength (nm)

Voltage (V)

2.205 200

0.50

1.00

1.50

2.00

2.50

250

300

350

400

350

400

350

400

Wavelength (nm)

3.00

Time (s)

Photo current

Photo Current ( m A)

Current (A)

3.62E-03 3.57E-03 3.52E-03 3.47E-03 3.42E-03 0.00

Dark current

0.50

1.00

1.50

2.00

2.50

3.00

5.075 5.070 5.065 5.060 5.055 5.050 5.045 5.040 5.035 5.030 5.025 5.020 200

250

Time (s)

8.250 Photo Current (m A)

Current (A)

8.260

Photo current

5.80E-03 5.75E-03 5.70E-03 5.65E-03 5.60E-03 0.00

Dark current

0.50

1.00

1.50

300 Wavelength (nm)

2.00

2.50

3.00

Time (s) Fig. 8. Photo-response of AlN/GaN/AlN MSM photodetector illuminated by 460-nm light at (a) 0.1 V, (b) 1 V, and (c) 1.5 V bias voltages.

200–400 nm. Generally, the photocurrent increased with increasing wavelength for all applied voltages. However, the detector shows a cut off wavelength at 350 nm wavelength for all applied voltages. The presence of a cut-off wavelength at 350 nm is shown in Fig. 9(a–d), confirming that the device can perform as a good photodetector in the

8.240 8.230 8.220 8.210 8.200 8.190 8.180 8.170 8.160 200

250

300 Wavelength (nm)

Fig. 9. The AlN/GaN/AlN MSM photoconductivity versus wavelength (a) 0 V, (b) 0.5 V, (c) 1 V, and (d) 1.5 V bias voltages.

UV-A (315–400 nm) range. The detector photoconductivity increases almost significantly than at around 350 nm, as presented in Fig. 9(a–d). These results have been previously observed in an AlN MSM photodetector grown by MOCVD on GaN substrate [37]. The data of Fig. 9(a and b) suggest that

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MSM has gain, which may be due to the presence of dislocations or deep level defects in the epilayers [38]. It is also possible that there exist some trap levels in the AlN layer which help to transport electrons through the AlN layer via deep-level assisted tunneling [39]. Zhou et al. [41] reported that the maximum responsivities of the AlN/GaN heterostructures photodetector can be achieved at 360 nm, which is much higher than those of a GaN monolayer photodetector [40]. They suggest that the main reason is that a strong polarization field was formed in the AlN/GaN heterostructure interface [40]. Since AlN has the largest energy band gap ( 6.1 eV) among nitride semiconductors and offers the ability for band gap engineering through the use of alloying and heterostructure design, our results indicate good quality of AlN/GaN/AlN heterostructures on silicon substrate, and the potential for MSM application for these AlN/GaN/AlN heterostructures on silicon substrate. 4. Conclusion In summary, the growth of AlN/GaN/AlN heterostructures on silicon substrate has been successfully performed using plasma-assisted MBE. The microstructures and optical properties of the sample have been revealed by using HR-XRD, photoluminescence, Raman, TEM, SAED, DF STEM and HAADF STEM. XRD spectrum confirmed that the sample was hexagonal structure. Raman spectrum showed all four Raman-active modes inside the sample. The MSM UV photodetector fabricated on AlN/GaN/AlN heterostructures has also been presented. A good photoresponse result indicates that the device can detect UV light to produce photo-current response. Acknowledgments This work was conducted under Research University Grant no. (1001/PFIZIK/814189). The support from Universiti Sains Malaysia and Universiti Teknologi MARA are gratefully acknowledged. References [1] Y. Lu, X. Liu, X. Wang, D.C. Lu, D. Li, X. Han, Z. Wang, J. Cryst. Growth 263 (1) (2004) 4–11. [2] Hadis Morkoc, Aldo Di Carlo, Roberto Cingolani, Solid State Electron. 46 (2002) 157–202. [3] J.L. Pau, E. Monroy, E. Munoz, F.B. Naranjo, F. Calle, M.A. SanchezGarcia, E. Calleja, J. Cryst. Growth 230 (2001) 544. [4] V. Lebedev, B. Schrö ter, G. Kipshidze, W. Richter, J. Cryst. Growth 207 (1999) 266–272. [5] G. Koblmueller, R. Averbeck, L. Geelhaar, H. Riechert, W. Hosler, P. Pongratz, J. Appl. Phys. 93 (2003) 9591–9596. [6] C.D. Lee, Y. Dong, R.M. Feenstra, J.E. Northrup, J. Neugebauer, Phys. Rev. B 68 (2003) 205317–205327.

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Please cite this article as: M.Z. Mohd Yusoff, et al., Materials Science in Semiconductor Processing (2014), http://dx.doi. org/10.1016/j.mssp.2014.03.041i