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Journal of the Taiwan Institute of Chemical Engineers 86 (2018) 185–191

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Study on nanocrystalline silicon thin films grown by the filtered cathodic vacuum arc technique using boron doped solid silicon for fast photo detectors Ravi Kant Tripathi a,∗, O.S. Panwar b,∗, Ishpal Rawal c, B.P. Singh d, B.C. Yadav a a

Nanomaterials and Sensors Research Laboratory, Department of Applied Physics, Babasaheb Bhimrao Ambedkar University, Vidya Vihar, Raebareli Road, Lucknow 226025, U.P., India Department of Physics, School of Engineering and Technology, BML Munjal University, 67 KM Stone, NH-8, Sidhrawali, Gurgaon 122413, Haryana, India c Department of Physics, Kirorimal College, University of Delhi, Delhi 110007, India d Physics and Engineering of Carbon Materials, Division of Material Physics and Engineering, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India b

a r t i c l e

i n f o

Article history: Received 31 July 2017 Revised 20 January 2018 Accepted 31 January 2018 Available online 27 February 2018 Keywords: Amorphous/nanocrystalline silicon thin film Filtered cathodic vacuum arc Raman spectroscopy Photoresponse

a b s t r a c t This paper reports the synthesis and properties of as grown and hydrogenated nanocrystalline silicon (ncSi or nc-Si:H) thin films deposited by the filtered cathodic vacuum arc technique using boron doped solid silicon as a cathode. No hazardous gases like silane, diborane etc. (which are used in the conventional growth techniques) were used for the growth of nc-Si or nc-Si:H films in this process. These films have been characterized by X-ray diffraction (XRD), scanning electron microscopy, UV–visible spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and photo detection measurements. The XRD patterns show the amorphous and nanocrystalline coexisting nature of the films deposited under different deposition conditions. Raman spectra also reveal the amorphous nature of the film deposited at room temperature, whereas the films deposited at high temperature and under hydrogen environment silicon films showed the nanocrystalline nature. The evaluated values of dark conductivity (σ D ), activation energy (E), optical band gap (Eg ) vary from 3.6 × 10−5 to 7.2 × 10−3 ohm−1 cm−1 , from 0.55 to 0.24 eV and from 1.24 to 2.12 eV, respectively, in nc-Si or nc-Si:H films. The fast response and recovery time as 4.92 and 4.06 s have been observed for the photo detectors developed from the nc-Si:H films deposited at room temperature. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The amorphous silicon thin film plays a vital role in the fabrication of various photovoltaic cells and renewable energy devices, but it has a light induced degradation problem which affects the device performance. During the last few years, the researchers have extensively studied on the nanocrystalline silicon (nc-Si) thin films for different energy applications, which have high stability under illumination as well as higher conductivity [1–6]. As far as nc-Si is concerned, it has been proved to be a basic material for thin film solar cells which help in preventing the light induced degradation problem [2]. Recently, hydrogenated nanocrystalline silicon (nc-Si:H) thin films with boron doping have attracted great interest in the scientific community due to their unique



Corresponding authors. E-mail addresses: [email protected] bml.edu.in (O.S. Panwar).

(R.K.

Tripathi),

os.panwar@

electrical and optical properties [7,8]. For further improvement in the properties of these films other process parameters like gas dilution, substrate temperature, delivered power etc. have also been used [9–12]. Samanta and Das [9] gave a detailed investigation on the impact of variation of gas pressure of silane with H2 and He dilution in RF PECVD grown nanocrystalline silicon thin films at 250 °C temperature and 200 W RF power. Altmannshofer et al. [10] reported that increasing silane flow rate in microwave plasma achieved high deposition rate of 60 nm/min with excellent order of crystallinity. Frequently, plasma enhanced chemical vapor deposition (PECVD) [2,7], radio frequency-magnetron sputtering (RF-magnetron sputtering) [11] hot wire chemical vapor deposition (HWCVD) [12,13], and electron cyclotron resonance chemical vapor deposition techniques (ECR-CVD) [14] are being used in the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films. These processes require the dilution of SiH4 , B2 H6 and PH3 gases with hydrogen gas at a relatively high substrate temperature for the growth of nc-Si:H thin films, which are flammable and toxic in nature. In order to overcome this severe problem, these

https://doi.org/10.1016/j.jtice.2018.01.051 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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thin films have been grown from the solid silicon target using filtered cathodic vacuum arc (FCVA) technique without using such hazardous gases. The FCVA is a plasma based popular industrial technology for the deposition of the variety of films like metals, ceramics, diamond like carbon and tetrahedral amorphous carbon [15–18]. The FCVA method has been used earlier to deposit phosphorous doped silicon and phosphorous doped silicon carbide thin films [19–23]. In this technique, the properties of grown film can be controlled by the various process parameters such as arc current, substrate temperature, substrate bias and gaseous pressure in the reactive mode. This technique has also been used to deposit boron doped tetrahedral amorphous carbon film as p-type window materials in the inline production of large area amorphous silicon solar cell for improving the efficiency [24]. Hydrogenated p-type silicon is an attractive material for the use as a window layer in p-i-n amorphous/nanocrystalline silicon solar cells [25]. The hydrogenated nanocrystalline silicon (nc-Si:H) thin film, a mixed phase material consisting of nanocrystals embedded in the amorphous matrix, is a subject of extensive research in the field of semiconductor thin film technology because of its specific properties for applications in thin film devices such as solar cells, electrostatic microresonators etc. [25,26]. Benlakehal et al. [11] discussed the electronic transport mechanism in pure and doped nanocrystalline silicon thin film deposited at room temperature by RF magnetron sputtering method. With increasing interest in recent years the intrinsic and doped nanocrystalline silicon films have been precisely investigated, for structural and electrical properties with another important parameter of deposition rate, to improve the performance of solar cell [2,5,6,11–13,24–26]. In this paper, we have reported a simple FCVA technique for the growth of hydrogenated/unhydrogenated amorphous/nanocrystalline silicon films employing a boron doped silicon ingot as cathode with and without hydrogen incorporation at room temperature and 200 °C substrate temperature. These films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), dark conductivity (σ D ), activation energy (E), optical band gap (Eg ), UV–visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and photoconduction measurements. In the outcome of this work one can also use an industrialized FCVA technique for hard coatings as an environment friendly technique for the growth of nc-Si:H films with the fast deposition rate in the large area. Further, these FCVA grown nc-Si:H films can be easily used as the non-toxic materials in the fabrication of the Schottky heterojunction and photo detecting devices.

Fig. 1. The schematic of the FCVA system used to deposit nc-Si or nc-Si:H films.

Fig. 2. The arcing mechanism used to deposit nc-Si or nc-Si:H films.

pressure of ∼10−6 mbar. The films were deposited sequentially for 5 s and then cooled for 50 s (Fig. 2). The process was repeated till the required thickness was obtained. The samples have been prepared with and without H2 gas in the vacuum chamber. The deposition pressure used was 1.4 × 10−5 mbar during the arc without using H2 gas and with H2 gas it was 7.4 × 10−3 mbar. The as grown and hydrogenated amorphous/nanocrystalline silicon thin film (nc-Si:H) deposited under different deposition conditions are designated as S1 [as grown at room temperature (Ts = RT)], S2 [as grown at a substrate temperature 20 0 °C (Ts = 20 0 °C)], S3 [at room temperature with hydrogen gas (Ts = RT, H2 )] and S4 [at a substrate temperature of 200 °C with hydrogen gas (Ts = 200 °C H2 )].

2. Experimental details 2.2. Details of the measurements 2.1. FCVA technique and deposition of the film The schematic of the FCVA system used to deposit unhydrogenated or hydrogenated nanocrystalline silicon (nc-Si or nc-Si:H) thin film is shown in Fig. 1. The detailed description of the system has been described elsewhere [27,28]. Briefly, the vacuum arc produces a high temperature at the cathode surface and the evaporation of source material into the form of a plasma jet. The FCVA process is based on striking the arc (arc voltage of 35–40 V with an arc current of 30–100 A) between the two electrodes. One electrode is of boron doped silicon ingot of (resistivity 0.01  cm) 50 mm diameter and 5 mm thickness used as a cathode (purity 99.999%), that also works as a silicon source in order to deposit thin film. Second electrode is a retractable tungsten rod acts as a striker. The silicon thin films were deposited on the variety of substrates like 7059 glass and silicon substrate placed at a distance of 35 cm away from the cathode. Prior to the deposition, the chamber had evacuated to a base

The deposition rate of the silicon films in the FCVA system was found to be approximately 40–50 nm/min which was derived from the film thickness and deposition time. All the samples were 100 ± 10 nm thick as measured by Talystep (Rank Taylor and Hobson) thickness profiler. The X-ray diffraction experiments on the films were carried out by Philips X’Pert PRO PANanalytical diffrac˚ X-ray source in the scanned region tometer using a CuKα (1.542 A) of 20°–60°. A scanning step of 0.02° is used at a time of 1 s/step. Raman measurements were conducted by a Renishaw In Via reflex micro-Raman spectrometer using a 514.5 nm Ar+ laser for excitation. To measure the electrical properties, the coplanar gap cell method (with a separation of 0.079 cm and width of 0.8 cm) was used and silver as top-electrode was thermally evaporated in a vacuum better than ∼10−5 mbar onto the thin films through a shadow mask. Keithley 4200 semiconductor characterization system was utilized for the I–V measurements under the dark and light illumination of AM1.5 condition (100 mW/cm2 , 25 °C). Optical

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where λ is the wavelength of X-ray radiation, β is the full width at half maximum of XRD peak at diffraction angle θ . The crystallite size evaluated from the Scherrer formula is found to be in the range of 30–50 nm. Both the deposition parameters of the temperature and hydrogenation increase the crystallinity in the amorphous boron doped silicon film, but the temperature is more dominant factor. We evaluated the maximum crystalline size of ∼50 nm in the nc-Si:H films deposited at 200 °C. The FCVA produces a high temperature at the cathode surface and the evaporation of source material into a form of the plasma plume and favors the crystallization. Then, the plasma plume expands in the vacuum and reaches a substrate holder which was fixed with a heater to condense into the thin films. The deposition process of the FCVA consists of plasma generation of the cathode material, transformation and condensation. 3.2. Surface morphology

Fig. 3. XRD patterns of S1–S4 films.

transmittance measurements were performed in the 250–10 0 0 nm spectral range with a Perkin Elmer spectrophotometer (Lambda 950). The room temperature photoresponses of the prepared samples have also been recorded at Keithley’s 2410 source measure unit (SMU) in the resistance mode at an illumination intensity of ∼30 mW/cm2 .

3. Results and discussions 3.1. Structural analysis Fig. 3 shows the XRD patterns of as grown and hydrogenated amorphous/nanocrystalline silicon thin film (nc-Si:H) at various deposition conditions. The XRD patterns show the amorphous and nanocrystalline coexisting nature of all the silicon films deposited by the FCVA technique. The crystallite size (d) from the sharp reflections due to the nanocrystalls in the films is calculated from the well known Scherrer formula [29] expressed as

d=

0.9λ β cos θ

(1)

Fig. 4 shows the typical SEM images of the S1 and S3 films deposited by the FCVA technique. The surface of the S1 film shows the amorphous nature while in the S3 film nearly spherical nanocrystals with the uniformaly nucleated background is clearly visible. In some region these nanocrystals seem to have some regular pattern marked here in circle. The average evaluated size of these nanocrystals varies from 20 to 30 nm. Preliminary studies of the energy dispersive X-ray analysis (EDAX) made on nc-Si and nc-Si:H films deposited at RT and 200 °C indicates that the B content in all the films was ≤0.1 at.%. 3.3. Raman analysis The crystalline nature of the deposited silicon films were investigated by the Raman spectroscopy. Fig. 5 shows the Raman spectra of the S1–S4 films with the inset showing the deconvolution of the Raman spectrum of S2 film into three different peaks at near 470, 500 and 521 cm−1 , which corresponds to the amorphous, intermediate and crystalline phase, respectively [30]. One single broad peak at around ∼480 cm−1 can be observed in the S1 film, which indicates that the film is in a purely amorphous phase. A broad peak centered at around ∼480 cm−1 and a sharp peak at around ∼520 cm−1 are attributed to the transverse optical (TO) mode of Si–Si vibrations in the amorphous and crystalline phases, respectively. The crystalline volume fraction XRaman of the nc-Si/nc-Si:H thin films can be calculated by using XRaman = (Ic + Im )/(Ic + Im + Ia ) from the deconvoluted Raman spectra, here Ic , Im and Ia are intensities of crystalline, intermediate and amorphous phases, respectively [31]. The XRaman evaluated in these nc-Si/nc-Si:H films are found in range of 68–71%.

Fig. 4. The SEM images of the S1 and S3 films.

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Fig. 7. Variation of temperature dependent dark conductivity of the S1–S4 films. Fig. 5. Raman spectra of the S1–S4 films.

The films S2–S4 show the coexisting amorphous and crystalline phases of silicon and S1 films show only the amorphous nature which clearly indicates that both the substrate temperature and H2 dilution in the deposition process favor the nanocrystalline formation in the film. 3.4. FTIR studies Fig. 6 shows the FTIR spectra from (a) 540 to 900 cm−1 and (b) 190 0 to 220 0 cm−1 wave number of the S1 and S3 films, deposited without and with the hydrogen gas, respectively. Two broad peaks centered at 20 0 0 and 2100 cm−1 confirm the presence of hydrogen in the nc-Si film with (SiH2 )n bonds. The peaks near 630 cm−1 are present due to the wagging mode of Si–H bonds in the S3 film. The FTIR spectrum of as grown film S1 (black lines) does not contain any such bands located at 630 and 2100 cm−1 discarding the presence of hydrogen in the film. Further, the hydrogen content has been estimated using the integrated intensity of the peak at 630 cm−1 using the following relation [30,32]:



NH = A

α (ω ) dω ω

(2)

where A = 1.6 × 1019 cm−3 for the wagging mode. The hydrogen content of the S3 film evaluated is found to be ∼4.1 at.%. This is

in agreement with the reported results in the literature by other techniques [30]. 3.5. Electrical and optical properties Fig. 7 shows the variation of dark conductivity (σ D ) versus inverse of temperature for S1–S4 films. It is evident from the figure that the variation of σ D is a thermally activated process that follows a relation of the form:

σD = σ0 exp(−E/kT )

(3)

where σ 0 is the conductivity pre-exponential factor, E is the activation energy and k is the Boltzmann constant and T is temperature in Kelvin. Fig. 8 shows the variation of (α hυ )1/2 versus hυ curve of the S1–S4 films deposited by the FCVA technique at various deposition parameters where the symbols have their usual meanings. At short wavelengths, transmission spectra gradually decrease and at higher wavelength the transmittance is very high (70–95%). We observed the lowest transmittance of 40–75% in the S3 film deposited at room temperature in the presence of hydrogen gas. The values of optical band gap Eg have been evaluated using Tauc’s plot (variation of (α hυ )1/2 versus hυ curve) and by extrapolating the curve to the hυ axis.

Fig. 6. Typical FTIR spectra of the S1 and S3 films from (a) 500 to 900 cm−1 and (b) 1900 to 2200 cm−1 wave number.

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Table 1 The comparison of the electrical and structural properties of the nc-Si/nc-Si:H films. Dark conductivity σ D (ohm−1 cm−1 )

Activation energy E (eV)

Band gap Eg (eV)

Crystalline fraction XRaman

Ref.

3.6 × 10−5 –7.2 × 10−3 6.1 × 10−5 –2.7 × 10−2 9.2 × 10−4 –1.5 × 10−3 1.5 × 10−4 –1.8 × 10−3 1.28 × 10−3 0.1–0.36 8.0 × 10−4 –10−9 0.01–0.07

0.55–0.24 0.11–0.2 – – 0.3 – – –

1.24–2.12 – 1.72–2.02 – 1.76 1.45–1.75 1.77–1.82 2.07–2.18

68–71 35–74 67–71 51–77 95 30–52 50–80 43–80

This work [11] [36] [6] [37] [12] [9] [38]

Fig. 8. Variation of (α hυ )1/2 versus hυ curve of the S1–S4 films.

Fig. 9. Typical I–V curve of S2 film under the dark and light condition.

The values of σ D , E and Eg evaluated for the S1–S4 films are found to be in the range 3.6 × 10−5 to 7.2 × 10−3 ohm−1 cm−1 , 0.24 to 0.55 eV and 1.24 to 2.12 eV, respectively. Table 1 shows the comparison of the electrical and structural properties of nanocrystalline silicon films deposited by various conventional method [6,9,11,12,36–38] and also of the present work. The electrical and optical properties of the FCVA deposited boron doped micro/nano crystalline silicon films are comparable to the PECVD and HWCVD grown films [30–38]. 3.6. Photoconduction measurements Fig. 9 shows a typical I–V curve of the nc-Si film (deposited by the FCVA technique) and n-Si hetrostructure device under the dark and light condition. It is observed that I–V curve of the hetrostructure device of nc-Si/ n-Si heterojucntion is completely symmetrical with respect to the polarity of the applied voltage. This non-linear increase in the current with the applied voltage gives the information about the conduction mechanism of nanocrystalline silicon film. The I–V curve of the nc-Si/n-Si heterojucntion was taken at room temperature and it indicates that the hetrostructure device behaves as a Schottky junction. The Schottky equation, which is described below [39]:

I = Is [exp(eV/nkB T ) − 1],

(4)

Is = A T exp(−e b /kB T ),

R = R0 exp



and ∗ 2

Series resistance has been determined from the I–V curve and is found to be ∼0.63 kΩ. We calculated the reverse saturation current (Is ) by the interpolation of exponential slope of I at V = 0 and the value of diode ideality factor has been calculated using Eq. (4). The value of reverse saturation current (Is ) and the diode ideality factors are found to be 1.315 × 10−6 A and 2.55, respectively. The value of n greater than 2 can be attributed to the recombination of electrons and holes in the depletion region and it might be associated with a relatively large voltage drop in the interface region. Breitenstein et al. [40] reported that when n < 2 in the I–V curve, the extended defects have a low local density of defect states, which follows the Shockley–Read–Hall (SRH) recombination theory of isolated point defects. In contrast, when n > 2 extended defects in combination with defect pair recombines to form multi-level recombination states. The photoconduction behavior of the prepared films on the glass substrate has also been investigated so that they can be employed for the light dependent resistor (LDR) applications. Fig. 10 shows the change in the electrical resistance of the S3 films measured as a function of time in the dark as well as under illumination of ∼30 mW/cm2 . The dynamic resistance of the samples under the light and dark condition can be given by the relations:

(5)

where V is the applied voltage, e is the charge of electron, n is the diode ideality factor, T is the temperature, kB is the Boltzmann constant, A∗ is effective Richardson constant, b is effective barrier height, and Is is the reverse saturation current.

 −t  τres

Rd = R0 1 − exp

when t < tx

 −t  τrec

when t < tx + ty

(6) (7)

where R0 is the steady state resistance in dark, τ res and τ rec are the time constants of response and recovery, respectively. tx (∼65 s for the present case) is the time required to achieve the lowest value of the resistant (Rx = Rmin ) and ty (∼65 s for the present

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Fig. 10. Typical variation of the change in the electrical resistance of the S3 film under the dark and light illumination condition.

case) is the recovery time or time required to achieve the original resistance value (Rd = R0 ). The photo response of the film is observed at the 90% of maximum response as the response time is generally defined by the time taken by the detector to acquire 90% of the maximum resistance. The fast response and recovery time of 4.92 and 4.06 s have been evaluated from the photo detectors made using the S3 films deposited at room temperature. The photoresponse of the sample is governed by the several types of transitions such as valence band to conduction band generally called band to band transition, defect state to conduction band or valence band to unoccupied states, etc. On the other hand, the recovery of the sample depends on the recombination process via de-excitation of the electrons to the ground state. 4. Conclusion The nc-Si thin films have been deposited by the FCVA technique with and without hydrogen gas at two different substrate temperature varying at room temperature (RT) and 200 °C. The XRD patterns shows the amorphous and nanocrystalline coexisting phase of the films. Raman spectra also reveal the amorphous nature of the film deposited at RT and crystalline nature at higher temperatures. The properties of the FCVA deposited nano crystalline silicon films are comparable to the PECVD and HWCVD grown films. The earlier reports and present investigation clearly reveal that the nc-Si/nc-Si:H films having dark conductivity in range of 10−2 – 10−3 ohm−1 cm−1 , optical band gap of the order 1.8–2.1 eV and crystalline fraction close to 70% of nc-Si/nc-Si:H films are suitable candidate for photodetection. The nc-Si/n-Si heterojucntion diode was fabricated and the value of diode quality factor evaluated was 2.5. The visible light photoresponse has been observed in the nc-Si/nc-Si:H films, which may lead to make them the potential candidates for the futuristic non-toxic LDR applications. Acknowledgments The authors are gratefully acknowledged for useful discussion with Dr. Sushil Kumar and Mr. A. K. Kesarwani of CSIR-National Physical Laboratry, New Delhi. Dr. R. K. Tripathi is grateful to the

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