Silicon carbide thin films with different processing growth as an alternative for energetic application

Optical Materials xxx (2016) 1e7

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Silicon carbide thin films with different processing growth as an alternative for energetic application A.M. Ouadfel a, b, A. Keffous a, *, A. Kheloufi a, A. Cheriet a, C. Yaddaden a, N. Gabouze a, M. Kechouane b, Y. Belkacem a, A. Boukezzata a, S. Kaci a, L. Talbi a, Y. Ouadah a, I. Bozetine a, B. Rezgui c, L. Guerbous d, H. Menari a, B. Mahmoudi a, I. Menous a a Centre de Recherche en Technologie des Semi-conducteurs pour l’Energ etique (CRTSE), Division Couches Mines Surfaces et Interfaces, 02 Bd Frantz Fanon, BP. 140, Alger, Algeria b Facult e de Physique, Universit e Houari Boumediene (USTHB), Bab ezzouar, Alger, Algeria c Laboratoire Photovoltaique, Centre des Recherches et des Technologies de l’Energie (CRTEn) B. P N
95 - 2050 Hammam Lif, Tunisia d Centre de Recherche Nucl eare d’Alger (CRNA), Alger, Algeria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 April 2016 Received in revised form 6 September 2016 Accepted 8 September 2016 Available online xxx

Different SiC thin film structures were obtained depending on the deposition techniques. Crystalline films were grown using a Pulsed laser deposition (PLD), in contrary the sputtering DC magnetron method allow to have an amorphous films (a-SiC:H and a-Si1xCx:H). A comparative study of the structural and optical characteristics of the elaborated films has been performed. The energetic application possibilities such as blue or multicolor LEDs have been explored. Different techniques have been used to investigate the elaborated films such as SEM-EDS, SIMS, photoluminescence and spectral response. © 2016 Elsevier B.V. All rights reserved.

Keywords: Silicon carbide Thin films Plasma Crystalline structure Amorphous films

1. Introduction Silicon carbide has been attracted much attention in recent years because of its potential application in many kinds of optoelectronic devices, such as solar cells, images sensors [1e3], gas sensors [4] and photodiodes [5], applications in optics (earth observation) [6,7], Micromachined SiC optic fiber as pressure sensors for high-temperature aerospace applications [8] and SiC-SiC composites optics for UV applications [9]. The silicon carbide (SiC) has become the focus of considerable attention due to its excellent material properties, promising it for different applications. Several reports on p-type SiC were published, but not much effort has been focused on a hot-pressed 6H-SiC material with granular structure or on different structure of crystalline or amorphous thin SiC films [10e13]. To date no works have been

* Corresponding author. Tel.: þ213 (0) 21 43 35 11, þ213 (0) 21 43 26 30; fax: þ213 (0) 21 43 10 49. E-mail addresses: [email protected] (A.M. Ouadfel), [email protected], [email protected] (A. Keffous).

performed on low-doped polycrystalline 6H-SiC and no chemical polishing solution has been developed on the material [14,15]. In this paper, we present results on elaborate thin silicon carbide films by the PLD and sputtering DC magnetron and thin SiC films, then the application of SiC thin films as a gas sensor. In the first part of the work, we describe a comparative study of the structural and optical properties of polycrystalline silicon carbide (6H-SiC) and SiC thin films grown onto p-type (100) silicon by pulsed laser deposition (PLD) using 6H-SiC as a target and p-type Si(100). Then, we demonstrated the interest and the application of SiC in Schottky photodiode based onto SiC thin films [16,17]. 2. Experimental procedure The measurements presented here were performed on square samples (10  5 mm2) cut from an unpolished 6HeSiC wafer of 2 mm thickness with a resistivity of 30 kUcm. The deposition of SiC thin films by Pulsed Laser Deposited (PLD) method has been described in our previous work [17,18]. The thickness of the deposited layers was varying by remote the deposition time. The

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Fig. 1. Plan view SEM image (a) and EDS Spectra of polished polycrystalline 6H-SiC (b).

amorphous a-Si1xCx:H films were prepared in a sputtering DC magnetron system using 32e86 chips of a silicon carbide (6HeSiC) of 10 mm  5 mm placed onto a high pulverisation region of single monocrystalline silicon as a target. The polycrystalline 6HeSiC chips were cut from a hot-pressed wafer (supplied by Goodfellow) and radially fixed on silicon target. The a-Si1xCx:H thin films of 0.2e1.8 mm thicknesses were deposited on single crystalline silicon(100) wafers and corning glass 9075, which were cleaned with ethanol before deposition. Deposition rate of 2 Å/s was achieved in argon and hydrogen plasma mixture, using an operating pressure of 1 105 mbar with constant gas flow rates of 10 and 2 sccm for H2 and Ar, respectively. The atomic hydrogen is used for its important role in controlling the film network and in turns the physical properties of amorphous semiconductors [14]. All samples were deposited with a 130 W power and temperature deposition of almost 300
C. To investigate the structural properties, the samples were analysed with scanning electron microscopy “SEM-EDS” (JEOL JSM 6360LV), secondary ion mass spectrometry “SIMS” (CAMECA

4FE7-CRTSE-Algiers) and photoluminescence “PL” Perkin Elmer Spectrometer LS 50B00 for optical properties were used. After the elaboration of a-SiC:H thin films, an ohmic Aluminum contact is realized on the rear p-Si(100) substrate, then the samples were placed into a deposition chamber in order to evaporate a thin gold layer (99.995% purity) on the a-SiC:H/p-Si to form a Schottky contact. The thickness of the gold layer measured by a Tencor 250 profilometer was 250 nm. The spectral response (SR) measurements were carried out by using a tungsten filament lamp with an incident power (Pinc) of 170 Watts with a JOBIN YVON monochromatic in the range 350e1000 nm wavelength [19,20]. 3. Results and discussion 3.1. Structural properties 3.1.1. SEM observations Fig. 1 depicts a SEM plan view of the polished bare 6H-p-type SiC

Fig. 2. Cross sectional, plan view SEM image (a) and EDS Spectra of c-SiC thin film.

Fig. 3. Cross sectional, Plan view SEM image and EDS spectra of a-Si1xCx:H thin film.

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Fig. 4. X-ray diffraction patterns of polycrystalline 6HeSiC Target (a), crystalline SiC thin film elaborated by PLD method (b) and amorphous structure of SiC by sputtering Method (c).

surface where it shows that the material exhibits a granular form with an average grain size diameter assessed to be around 5 mm. Fig. 2 shows a plan view and cross-sectional SEM image of a thin SiC layer deposited on the Si(100) substrate which revealed a planar

surface with a low defect density due to the high flux of matter induced by the high energy of the laser used in the growth process. Fig. 3 shows a plan view and cross-sectional SEM image of a thin a-Si1xCx:H layer deposited on the Si(100) substrate by sputtering

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DC magnetron, it reveals a planar surface with a low defect density due to the high flux of matter induced by the high energy of the laser used in the growth process. 3.1.2. X-Rays diffraction measurements The XRD analysis allows identifying the nature single-phase of the sample, crystalline quality of the material analysed and to measure the crystalline parameters. Fig. 4 shows XRD pattern of SiC target and thin SiC layer deposited onto Si. The structure of SiC target is identified using the XRD pattern of ASTM 72e0018 as hexagonal (6H) with a preferential orientation (102). The comparison between the XRD patterns of polycrystalline 6H-SiC and that of thin SiC layer clearly indicates the crystalline structure of the later and a same structure 6H [19]. 3.1.3. Secondary ion mass spectrometry (SIMS) measurements A camera 4FE7 from CRTSE e Algiers, has been used to characterize the samples, in our case to analyze the Si and C elements, we

used a dynamic SIMS to determine the concentration of the various constituents of the Si-C alloy. Fig. 5(a) and (b) show a SIMS depth profile of the 6H-SiC target and thin SiC layer deposited onto pSi(100). The ratio 28Si/12C obtained here is 1.06 for the target whereas it is found equal to 1.15 for the thin SiC layer. These ratios should be close to unit (1). This difference in ratio values is due to the degree of the SIMS sensitivity and to the higher ionisation efficiency of silicon by report to carbon. Also, the SIMS Analysis indicates that the first atomic plan surface was carbon with a p-type conduction revealed by the presence of aluminium. Finally, we can confirm that the macroscopic properties of the films are the same that of the target. Fig. 6(a) and (b) show a SIMS depth profile of the 6H-SiC target and thin SiC layer deposited onto p-Si(100) at 500
C. As it can be seen the ratio 28Si (signal) /12C (signal) ¼ 2.02, when using 86 chips of 6H-SiC deposited onto p-type silicon target and equal to 1.66, the analysis indicate that the elaborated films are Sirich films and the first atomic plan surface was silicon. Fig. 7(a) and (b) depict SIMS Profil of a-SiC thin films elaborated by Bottom-Up

Fig. 5. SIMS profile of polycrystalline 6H-SiC target (a), crystalline SiC film elaborated by PLD method (b).

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Fig. 6. SIMS profile of a-Si1xCx:H film elaborated by two targets: Si single crystal with chips of 6H-SiC target with 65 chips (a) and 86 chips (b) 6H-SiC under sputtering DC magnetron under Bottom e Up method.

(a) and Up-Down (b) sputtering DC magnetron. In this last case, the Ratio 28Si (signal)/12C (signal) ¼ 1.73 with the first atomic plan on the surface is composed by silicon, For Up-Down process a ratio 28Si (signal) /12C (signal) gives a value of 0.98 and the carbon atomic plan is the first plan on surface. The two methods Bottom- Up and Up-Down can allow choosing the nature of the first atomic plan on the surface such as silicon. 3.2. Optical properties 3.2.1. Photoluminescence measurements The silicon carbide is a material with an excellent and a high blue luminescence when is used as a substrate for GaN, in our case the polycrystalline 6H-SiC target exhibits two bands, a blue and green band centered at 415 nm (2.99 eV) which is ascribed probably to the radiative recombination from some direct transitions such as self-trapped excitons in the surface of nanoclusters and 560 nm (2.21 eV) which could be attributed to the defects in the film, respectively, as presented in Fig. 8 (curve (a)). In contrary, the crystalline SiC thin films elaborated by Pulsed Laser Deposition (PLD) exhibit only a blue light centered at 438 nm (2.83 eV), which can be attributed to carbon cluster present onto the thin films (Fig. 8 curve (b)). A same phenomenon has been observed on the amorphous SiC films obtained from 6H-SiC chips which are

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Fig. 7. SIMS profile of a-SiC:H film elaborated by sputtering DC magnetron using 6HSiC target: Bottom e Up method, (b) Top e Down method.

deposited onto silicon target and when used only 6H-SiC target with sputtering DC magnetron (Fig. 8 curve (c) and curve (d)). It was found that such emission is similar to that observed on hydrogenated amorphous SiC. Wang et al. [21] found that the PL intensities are enhanced by UV irradiation (3.80 eV) at room temperature and the luminescence center with peak energy in the range 2.10e2.20 eV is induced by the UV light for the porous-like SiC samples, the authors suggested that UV irradiation may induce some stable or metastable states as luminescence centers in the sample [22e26]. We can note that the relative increase in PL intensity with the thickness of the thin film (Fig. 8 curve (c)) suggests the depletion of grains, which plays a major role in increasing defect densities, in other words a reduction in the surface/volume ration. It has been reported also, that the apparent strong and blue luminescence obtained from the hydrogenated amorphous SiC films (curve (c) and curve (d)) elaborated by sputtering DC magnetron are also indicative of the size quantization attributed to quantum confinement of charge carriers in the restricted volume of the SiC particles. 3.2.2. Application: SiC photodiode Spectral response measurements were carried out by using a tungsten filament lamp with incident power 170 W with a JOBIN YVON monochromatic in the range 350e1100 nm wavelength. The spectral response (SR) is given by the formula [20]:

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12

12

(a) PL Intensity [Arb. Unit.]

PL Intensity [Arb. Unit.]

6H-SiC Bulk

560 nm (2.20 eV) 438 nm (2.83 eV) 386 nm (3.20 eV)

8

Bleu Band

4

0 300 350 400

Green Band

450 500 550 600 λ [nm]

650

386 nm (3.21 eV) 438 nm (2.83 eV) 1.6 μm c-SiC / p-Si(100)

8

4

0 300

700

(b)

Blue Band

520 nm (2.36 eV)

350

400

450

500

550

600

650

700

λ [nm]

800

386 nm (3.21 eV) 438 nm (2.83 eV)

250

E290111 - 1350 nm E310111 - 950 nm

(c)

600

200 PL Intensity [a. u.]

PL Intensity [Arb.Unit.]

1000 438 nm

150

400

(d)

386 nm 410 nm 475 nm

100

Blue Band 200 300 350 400

520 nm (2.36 eV)

450 500 550 λ [nm]

600 650 700

520 nm 50

Blue Band

0 300 350 400 450 500 550 600 650 700 λ [nm]

Fig. 8. Room temperature Photoluminescence spectra of SiC thin films. (a) 6H-SiC polycrystalline target (b) c-SiC crystalline thin film by PLD method. (c) a-Si1xCx:H thin film using Si single crystal þ 86 chips of 6H-SiC by DC magnetron sputtering. (d) a-SiC:H thin film using only 6H-SiC by DC magnetron sputtering.



aW

Iph ¼ I0 1  e

 q $ l hc

(1)

where Iph is the photocurrent, I0 is the maximum photocurrent for the total photon absorption, k is the wavelength, a is the absorption coefficient of the SiC film, h is the Planck's constant (6.62 1034 J s), c is the velocity of light, W is the depletion width and q is the electron charge (1.6 1019 C). The spectral response (SR) is given by the formula:

SR½A=W ¼

Iph Pinc $ q hn

(2)

where Pinc is the incident power and hy is the photo energy. The results presented in this work show the role of the nature and the structure of the film on the spectral response (SR) based onto crystalline (c-SiC) and amorphous (a-SiC:H SiC) films Schottky photodiodes. As seen in Fig. 9 (curve a) and (curve b),

which were fabricated using two different metals such as palladium (Pd) and gold (Au). For a sensitive surface of 10 mm2, a relative high spectral response value of 0.065 mA/W in the blue region at 400 nm was obtained for Au/a-SiC:H structure, this is due to the increase of the specific area of the thin SiC film, compared to the photodiode obtained with Pd/c-SiC with two bands at 400 nm and 850 nm with a spectral response value of 10 mA/W and 13 mA/W, respectively. Finally, in order to increase the spectral response, we propose to use a semi-transparent gold (Au) layer metal with a thickness of 7.5 nm instead of palladium, operating under ideal conditions of minimum reflectance [27], high quality crystal structure allows to approaching a spectral which will be our next preoccupation. 4. Conclusion The results show that the thin SiC layer realized by PLD has a same macroscopic properties as the used target material

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Acknowledgements This work was supported by Funds National Research of DGRSDT/MESRS, Algeria. References [1] [2] [3] [4]

[5] [6]

[7] [8]

[9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] Fig. 9. Spectral response vs. wavelength based onto hydrogenated amorphous SiC thin films (a) and crystalline SiC thin films (b).

[19] [20] [21]

(polycrystalline 6H-SiC), clearly indicating that the deposited SiC layer by PLD was stoichiometric, results in agreement with those given in the literature. However, we noted a shift and a decrease in the luminescence intensity for the thin SiC layer in comparison with the polycrystalline 6H-SiC one. Finally, Schottky photodiodes realized from polycrystalline and thin SiC films shows a good rectifying behaviour from the fabricated diodes and a high spectral sensitivity.

[22] [23] [24] [25] [26] [27]

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