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Some physical properties of a Si-doped nano-crystalline GaAs thin film grown by thermionic vacuum arc
Vacuum 119 (2015) 228e232
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Some physical properties of a Si-doped nano-crystalline GaAs thin film grown by thermionic vacuum arc a, * € Volkan S¸enay a, b, Soner Ozen , Suat Pat a, S¸adan Korkmaz a a b
Department of Physics, Eskis¸ehir Osmangazi University, Mes¸elik Campus, 26480, Turkey Primary Science Education Department, Bayburt University, 69000, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 25 May 2015 Accepted 26 May 2015 Available online 3 June 2015
A 160 nm thick Si-doped nano-crystalline GaAs film was grown on a glass substrate by means of the thermionic vacuum arc in just 50 s. Tools and techniques such as an optical reflectometer, UveViseNIR spectrophotometer, XRD, FESEM, AFM, Hall effect measurement, and diiodomethane, ethylene glycol, formamide, and water contact angle measurements were employed to investigate some of the physical properties of the produced film. XRD characterization indicated that the film contained GaAs (hexagonal system in atomic ratio of 1) and SiAs (monoclinic system in atomic ratio of 1) phases. The grain size obtained from Scherrer's Formula was in the range of 30e40 nm. The AFM micrographs showed the film topography, revealing a surface root mean square roughness of 14.5 nm and average height of 48.3 nm. From the wetting experiments, it was found that the contact angle value is strongly dependent on the liquid used. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermionic vacuum arc GaAs:Si FESEM AFM Surface free energy
Recently, GaAs has been considered as a technologically important material and used in a large variety of microelectronic and optoelectronic devices [1e10]. In semiconductor production, doping intentionally introduces impurities into an intrinsic semiconductor to modulate its electrical properties. In extrinsic semiconductors, dopant atoms increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. In GaAs doping process, according to the dopant used, both n-type and p-type material can be realized. Silicon is an interesting dopant in GaAs since it acts as a donor or acceptor by occupying the gallium or arsenic site, respectively [11,12]. This amphoteric nature of silicon can expand the flexibility of the device applications [13,14]. However, the doping characteristics strongly depend on the nature of the applied growth technique. Undoped and doped GaAs thin films have been deposited by various techniques such as chemical vapor deposition [2], metal organic chemical vapor deposition [15e17], molecular beam epitaxy [4,18,19], RF magnetron sputtering [20], laser ablation [21] and thermionic vacuum arc (TVA) [22]. TVA can be ignited in high or ultra-high vacuum conditions between a heated filament surrounded by an electron focusing
* Corresponding author. € E-mail address: [email protected] (S. Ozen). http://dx.doi.org/10.1016/j.vacuum.2015.05.030 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Wehnelt cylinder and an anode containing the material to be deposited [23]. Due to the electron bombardment of the anode by the accelerated thermoelectrons from the grounded cathode towards the anode which is at high voltage, anode material first melts and afterwards starts to evaporate ensuring a steady state concentration of the evaporated atoms in the cathodeeanode space [24]. A discharge appears with a subsequent decrease of the voltage over the electrodes and with a significant increase of the current. In this way, TVA is easily established [25]. Since the cathode of TVA is at earth potential and grounded vacuum vessel also, the plasma ions are accelerated toward the vacuum vessel wall by gaining energy equal to the plasma potential drop. As the discharge is generated in vacuum condition and the deposited film is bombarded with energetic ions, nano-structured thin films with high purity, increased adhesion, low roughness, and compact structure are obtained [26e32]. TVA is a rapid, repeatable and easily controlled thin film deposition technique. The energy of the ions can be controlled and changed during the deposition [33]. In this research, the investigated film was deposited on a microscope slide which was located at a distance of 80 mm above the anode. The distance between the anode and the cathode was d ¼ 4 mm. The anode contained 0.13 g of GaAs and 0.03 g of Si pellets. The working pressure inside the vacuum vessel was about P ¼ 4 103 Pa. The filament heating current was If ¼ 18 A. The voltage applied to the space between anode and cathode was 600 V.
V. S¸enay et al. / Vacuum 119 (2015) 228e232
The voltage dropped to 150 V and the current of the discharge ignited in anode material vapors was Idisch ¼ 0.5 A. The deposition process was carried out for 50 s with a deposition rate of 3.2 nm/s at room temperature (RT). The film thickness was measured by an optical reflectometer (Filmetrics F20). The spectral dependencies (between the wavelengths 400 and 1000 nm) of reflectance and optical constants were also obtained with the same apparatus. The transmittance and absorbance spectra were recorded by a spectrophotometer (Shimadzu Solid Spec-3700 DUV) in the wavelength range of 300e3300 nm. Hall effect measurement was used to determine the sheet resistivity, sheet carrier density, and Hall mobility in the temperature range from 25 to 300 K. The crystal structure was analyzed by an X-ray diffractometer (Bruker D2 Phaser). The surface morphology was examined by a field emission scanning electron microscope (Carl Zeiss SUPRA 40) equipped with electron dispersive X-ray spectrometer for elemental chemical analysis, as well as atomic force microscope (Ambios Q-scope). The wetting property of the produced film was investigated by measuring the contact angles of diiodomethane, ethylene glycol, formamide, and water using a tensiometer (Attension Theta Lite). The surface free energy was calculated according to the AcideBase, OWRK/Fowkes, Wu, Equation of State, and Zisman Methods. The measured film thickness was 160 nm. The spectral curves of reflectance R, refractive index n and extinction coefficient k were given in Fig. 1a. The reflectance spectrum depicted an optical reflection of 9e26% for the wavelength range of 400e1000 nm. The refractive index of the film which was observed to be higher than the reported refractive index value of the GaAs films in Ref. [22] decreased from 3.90 to 3.82 with the increasing wavelength. The extinction coefficient also decreased with increasing wavelength and reset to zero when the wavelength was larger than 800 nm. This distribution coincided with the recorded absorption spectrum.
229
Fig. 2. The sheet carrier density as a function of the measurement temperature.
The variations of the real (ε1 ¼ n2 k2) and imaginary (ε2 ¼ 2nk) parts of the dielectric constant with wavelength were shown in Fig. 1b. Both of these parameters decreased with increasing wavelength and the obtained values for the real part were higher than those for the imaginary part. The knowledge of the real and the imaginary parts of the dielectric constant provides information about the dissipation or loss factor which is the ratio of the imaginary part to the real part of the dielectric constant. The variation of dissipation factor with wavelength was also shown in Fig. 1b, indicating that the dissipation factor also decreased with increasing wavelength and reset to zero when the wavelength was larger than 800 nm. The transmittance and absorbance curves of the produced film were presented in Fig. 1c. The transmittance value in the
Fig. 1. Optical properties of the produced film. a) Reflectance, refractive index and extinction coefficient spectra b) Plots of parts of dielectric constant and dissipation factor vs. wavelength c) Transmittance and absorbance spectra d) Tauc plot of (ahv)2 vs. hv to estimate the optical band gap and inset shows the plot of (ahv)1/2 as a function of photon energy hv.
230
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Fig. 3. Compositional, structural and morphological properties of the produced film. a) EDX spectrum b) XRD pattern c) FESEM image at 60 kx magnification d) 2D, 3D AFM images and dimensional analysis.
transparent region was in the range of 45e65% and increased as the wavelength of the incident photons increased. The obtained A spectrum showed just opposite trend of the T spectra. A sudden rise in absorption was observed in the wavelength range between 300 and 800 nm. The absorption in this region was associated with the higher energy of photons than the band gap energy of the film. The absorption coefficient a was computed from the experimentally measured values of absorbance A using the following equation:
a ¼ 2:303ðA=tÞ
(1)
where t is the thickness of the film. The optical energy band gap Eg of the semiconductor materials is related to the absorption coefficient a in the strong absorption region by the relation:
ahv ¼ B hv Eg
n
(2)
where B is a constant, hv is the photon energy, and n is a number which have the values of 1/2 for direct transition or 2 for indirect transition. Fig. 1d showed the plot of (ahv)2 vs. hv, which gave the direct Eg while the inset of Fig. 1d showed the plot of (ahv)1/2 vs. hv giving the indirect Eg. The presence of a linear part confirmed that the optical threshold of absorption was of direct nature. The Eg of the film was estimated to be 1.25 eV. This value was observed to be lower than the Eg value of the GaAs films in Ref. [22]. Tauc's approximation is quite reliable though it might be not totally
correct. The error might be about ±5e10%. From the RT Hall effect measurement, the sheet resistivity, sheet carrier density, and Hall mobility were 1.26 105 U/sqr, 5.73 1011 1/cm2, and 95 cm2/(Vs), respectively. Fig. 2 showed the sheet carrier density as a function of the measurement temperature. One can notice the carrier-type switch from p-type to n-type occurring at 250 K. At lower temperatures than 250 K the sample was p-type while the sample was n-type at temperatures higher than 250 K. The EDX spectrum of the produced film was shown in Fig. 3a. The EDX analysis confirmed the presence of Ga, As and Si in the film. The X-ray diffraction pattern with the peaks identified and labeled was presented in Fig. 3b. The most intense peak at 2q ¼ 29.54 was attributed to the (011) reflection of the hexagonal GaAs and the (310) reflection of the monoclinic SiAs. Besides, the observed weaker peaks indicated the existence a small amount of Ga and Si crystals in the film. The grain size was calculated using Scherrer formula:
D¼
0:94l b cos qB
(3)
where D is the crystallite size, l is the wavelength of the X-ray radiation, b is the FWHM of the observed peak and q is the Bragg's angle of the peak. The grain size of the film was evaluated to be in range of 30e40 nm. Fig. 3c showed a FESEM image of the film, indicating that the film was composed of round grains with grain size varying in the range of 50e80 nm. Different results in the grain
V. S¸enay et al. / Vacuum 119 (2015) 228e232
231
Fig. 4. Digital snapshots of different testing liquid droplets on the produced film's surface.
size of the produced film were obtained from the XRD and FESEM analysis. The larger grain sizes observed in the FESEM image can be attributed to agglomeration of the crystallites leading to larger grains. This finding was also confirmed by the relatively high RMS roughness value. When compared to the FESEM images presented in Ref. [22], the grain size tended to be smaller with Si doping. Fig. 3d showed a 3D AFM micrograph which revealed that the produced film had a dense microstructure as was also depicted by the photomicrograph obtained from the FESEM study. The RMS roughness value of the film was determined to be 14.5 nm and the average height was 48.3 nm. The determined RMS roughness value and average height of the produced film were observed to be higher than the RMS roughness value and average height of the GaAs film in Ref. [34] and Si-doped GaAs film in Ref. [35], respectively. The snapshots of the testing liquids droplets on the produced film were shown in Fig. 4. The contact angle (CA) value was strongly dependent on the liquid used. The highest CA value was obtained for water which is a polar liquid with higher surface tension than ethylene glycol and formamide. The lowest value of CA was obtained for diiodomethane which is a dispersive liquid. The data of surface tensions and their components for the used testing liquids can be found in literature [36e39]. The CA values can be employed to calculate the surface free energy (SFE) of the produced film. There are various approaches in literature to evaluate the SFE of solid materials from wetting experiments. However, the solid SFE obtained can be different depending on the approach chosen for calculation. To determine SFE, five methods were selected for this study: AcideBase, OWRK/ Fowkes, Wu, Equation of State and Zisman approaches. Three methods allowed the calculation of polar (or acidebase) and disperse components of SFE: the AcideBase, OWRK/Fowkes, and Wu approaches. The results were given in Table 1. In conclusion, a Si-doped nano-crystalline GaAs thin film was deposited on a glass substrate using the TVA technique and its optical, electrical, structural, compositional and morphological properties were examined. The observed shrinkage of the band gap can be attributed to the large grains, random grain distribution, and structural modification of the material in the film [40e42]. The Hall effect measurement indicated that the film was a n-type semiconductor with a sheet resistivity of 1.26 105 U/sqr at RT. XRD characterization showed that the produced film composed by a mix Table 1 SFE values of the produced film according to the AcideBase, OWRK/Fowkes, Wu, Equation of State, and Zisman Methods. Method
gTOT (mN/m)
gd (mN/m)
gp (mN/m)
AcideBase OWRK/Fowkes Wu Equation of State Zisman
50.215 47.583 40.801 28.759 44.480
48.934 45.216 43.687
1.282 2.367 2.885
of, GaAs, SiAs, Ga and Si crystals. Different grain sizes obtained from the XRD and FESEM analysis were attributed to agglomeration of the crystallites. This study can be a contribution to the knowledge of Si impurity doping process of GaAs by employing a fast and simple technique. Acknowledgment The authors would like to thank Prof. Dr. Mutlu Kundakçi from Atatürk University for all his help and X-ray diffraction measurements. References [1] Acharya KP, Mahalingam K, Ullrich B. Structural, compositional, and optoelectronic properties of thin-film CdS on p-GaAs prepared by pulsed-laser deposition. Thin Solid Films 2010;518:1784e7. http://dx.doi.org/10.1016/ j.tsf.2009.09.032. [2] Liu CH, Lin TK, Chang SJ, Su YK, Chiou YZ, Wang CK, et al. Photo-assisted thermally oxidized GaAs insulator layers deposited by photo-CVD. Surf Coatings Technol 2006;200:3250e3. http://dx.doi.org/10.1016/ j.surfcoat.2005.07.024. [3] Hoke WE, Lemonias PJ, Kennedy TD, Torabi A, Tong EK, Bourque RJ, et al. Metamorphic heterojunction bipolar transistors and PeIeN photodiodes on GaAs substrates prepared by molecular beam epitaxy. J Vac Sci Technol B Microelectron Nanom Struct 2001;19:1505e9. http://dx.doi.org/10.1116/ 1.1374624. [4] Heiss M, Conesa-Boj S, Ren J, Tseng HH, Gali A, Rudolph A, et al. Direct correlation of crystal structure and optical properties in wurtzite/zinc-blende GaAs nanowire heterostructures. Phys Rev B 2011;83:045303. http:// dx.doi.org/10.1103/PhysRevB.83.045303. [5] Chang AM, Zhang H, Pfeiffer LN, West KW. Fabrication of submicron devices on the (011) cleave surface of a cleaved-edge-overgrowth GaAs/AlGaAs crystal. Appl Phys Lett 2012;100:123106. http://dx.doi.org/10.1063/ 1.3694052. [6] Kosten ED, Atwater JH, Parsons J, Polman A, Atwater HA. Highly efficient GaAs solar cells by limiting light emission angle. Light Sci Appl 2013;2:45. http:// dx.doi.org/10.1038/lsa.2013.1. [7] Fan K, Hwang HY, Liu M, Strikwerda AC, Sternbach A, Zhang J, et al. Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs. Phys Rev Lett 2013;110(21):217404. http://dx.doi.org/10.1103/ PhysRevLett.110.217404. [8] Wang HL, Huang YJ, Wu CH. Optical frequency response analysis of lightemitting transistors under different microwave configurations. Appl Phys Lett 2013;103(5):051110. http://dx.doi.org/10.1063/1.4817545. [9] Hossain N, Marko IP, Jin SR, Hild K, Sweeney SJ, Lewis RB, et al. Recombination mechanisms and band alignment of GaAs1xBix/GaAs light emitting diodes. Appl Phys Lett 2012;100(5):051105. http://dx.doi.org/10.1063/1.3681139. [10] Lee K, Zimmerman JD, Xiao X, Sun K, Forrest SR. Reuse of GaAs substrates for epitaxial lift-off by employing protection layers. J Appl Phys 2012;111(3): 033527. http://dx.doi.org/10.1063/1.3684555. [11] Ohno Y, Taishi T, Yonenaga I, Takeda S. Atomistic structure of stacking faults in a commercial GaAs: Si wafer revealed by cross-sectional scanning tunneling microscopy. Phys B Condens Matter 2007;401:230e3. http://dx.doi.org/ 10.1016/j.physb.2007.08.154. [12] Dimakis E, Ramsteiner M, Tahraoui A, Riechert H, Geelhaar L. Shell-doping of GaAs nanowires with Si for n-type conductivity. Nano Res 2012;5(11): 796e804. http://dx.doi.org/10.1007/s12274-012-0263-9. [13] Boumaraf R, Sengouga N, Mari RH, Meftah A, Aziz M, Jameel D, et al. Deep traps and temperature effects on the capacitance of p-type Si-doped GaAs Schottky diodes on (211) and (311) oriented GaAs substrates. Superlattices Microstruct 2014;65:319e31. http://dx.doi.org/10.1016/j.spmi.2013.11.019.
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
Some physical properties of a Si-doped nano-crystalline GaAs thin film grown by thermionic vacuum arc a, * € Volkan S¸enay a, b, Soner Ozen , Suat Pat a, S¸adan Korkmaz a a b
Department of Physics, Eskis¸ehir Osmangazi University, Mes¸elik Campus, 26480, Turkey Primary Science Education Department, Bayburt University, 69000, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 25 May 2015 Accepted 26 May 2015 Available online 3 June 2015
A 160 nm thick Si-doped nano-crystalline GaAs film was grown on a glass substrate by means of the thermionic vacuum arc in just 50 s. Tools and techniques such as an optical reflectometer, UveViseNIR spectrophotometer, XRD, FESEM, AFM, Hall effect measurement, and diiodomethane, ethylene glycol, formamide, and water contact angle measurements were employed to investigate some of the physical properties of the produced film. XRD characterization indicated that the film contained GaAs (hexagonal system in atomic ratio of 1) and SiAs (monoclinic system in atomic ratio of 1) phases. The grain size obtained from Scherrer's Formula was in the range of 30e40 nm. The AFM micrographs showed the film topography, revealing a surface root mean square roughness of 14.5 nm and average height of 48.3 nm. From the wetting experiments, it was found that the contact angle value is strongly dependent on the liquid used. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermionic vacuum arc GaAs:Si FESEM AFM Surface free energy
Recently, GaAs has been considered as a technologically important material and used in a large variety of microelectronic and optoelectronic devices [1e10]. In semiconductor production, doping intentionally introduces impurities into an intrinsic semiconductor to modulate its electrical properties. In extrinsic semiconductors, dopant atoms increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. In GaAs doping process, according to the dopant used, both n-type and p-type material can be realized. Silicon is an interesting dopant in GaAs since it acts as a donor or acceptor by occupying the gallium or arsenic site, respectively [11,12]. This amphoteric nature of silicon can expand the flexibility of the device applications [13,14]. However, the doping characteristics strongly depend on the nature of the applied growth technique. Undoped and doped GaAs thin films have been deposited by various techniques such as chemical vapor deposition [2], metal organic chemical vapor deposition [15e17], molecular beam epitaxy [4,18,19], RF magnetron sputtering [20], laser ablation [21] and thermionic vacuum arc (TVA) [22]. TVA can be ignited in high or ultra-high vacuum conditions between a heated filament surrounded by an electron focusing
* Corresponding author. € E-mail address: [email protected] (S. Ozen). http://dx.doi.org/10.1016/j.vacuum.2015.05.030 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Wehnelt cylinder and an anode containing the material to be deposited [23]. Due to the electron bombardment of the anode by the accelerated thermoelectrons from the grounded cathode towards the anode which is at high voltage, anode material first melts and afterwards starts to evaporate ensuring a steady state concentration of the evaporated atoms in the cathodeeanode space [24]. A discharge appears with a subsequent decrease of the voltage over the electrodes and with a significant increase of the current. In this way, TVA is easily established [25]. Since the cathode of TVA is at earth potential and grounded vacuum vessel also, the plasma ions are accelerated toward the vacuum vessel wall by gaining energy equal to the plasma potential drop. As the discharge is generated in vacuum condition and the deposited film is bombarded with energetic ions, nano-structured thin films with high purity, increased adhesion, low roughness, and compact structure are obtained [26e32]. TVA is a rapid, repeatable and easily controlled thin film deposition technique. The energy of the ions can be controlled and changed during the deposition [33]. In this research, the investigated film was deposited on a microscope slide which was located at a distance of 80 mm above the anode. The distance between the anode and the cathode was d ¼ 4 mm. The anode contained 0.13 g of GaAs and 0.03 g of Si pellets. The working pressure inside the vacuum vessel was about P ¼ 4 103 Pa. The filament heating current was If ¼ 18 A. The voltage applied to the space between anode and cathode was 600 V.
V. S¸enay et al. / Vacuum 119 (2015) 228e232
The voltage dropped to 150 V and the current of the discharge ignited in anode material vapors was Idisch ¼ 0.5 A. The deposition process was carried out for 50 s with a deposition rate of 3.2 nm/s at room temperature (RT). The film thickness was measured by an optical reflectometer (Filmetrics F20). The spectral dependencies (between the wavelengths 400 and 1000 nm) of reflectance and optical constants were also obtained with the same apparatus. The transmittance and absorbance spectra were recorded by a spectrophotometer (Shimadzu Solid Spec-3700 DUV) in the wavelength range of 300e3300 nm. Hall effect measurement was used to determine the sheet resistivity, sheet carrier density, and Hall mobility in the temperature range from 25 to 300 K. The crystal structure was analyzed by an X-ray diffractometer (Bruker D2 Phaser). The surface morphology was examined by a field emission scanning electron microscope (Carl Zeiss SUPRA 40) equipped with electron dispersive X-ray spectrometer for elemental chemical analysis, as well as atomic force microscope (Ambios Q-scope). The wetting property of the produced film was investigated by measuring the contact angles of diiodomethane, ethylene glycol, formamide, and water using a tensiometer (Attension Theta Lite). The surface free energy was calculated according to the AcideBase, OWRK/Fowkes, Wu, Equation of State, and Zisman Methods. The measured film thickness was 160 nm. The spectral curves of reflectance R, refractive index n and extinction coefficient k were given in Fig. 1a. The reflectance spectrum depicted an optical reflection of 9e26% for the wavelength range of 400e1000 nm. The refractive index of the film which was observed to be higher than the reported refractive index value of the GaAs films in Ref. [22] decreased from 3.90 to 3.82 with the increasing wavelength. The extinction coefficient also decreased with increasing wavelength and reset to zero when the wavelength was larger than 800 nm. This distribution coincided with the recorded absorption spectrum.
229
Fig. 2. The sheet carrier density as a function of the measurement temperature.
The variations of the real (ε1 ¼ n2 k2) and imaginary (ε2 ¼ 2nk) parts of the dielectric constant with wavelength were shown in Fig. 1b. Both of these parameters decreased with increasing wavelength and the obtained values for the real part were higher than those for the imaginary part. The knowledge of the real and the imaginary parts of the dielectric constant provides information about the dissipation or loss factor which is the ratio of the imaginary part to the real part of the dielectric constant. The variation of dissipation factor with wavelength was also shown in Fig. 1b, indicating that the dissipation factor also decreased with increasing wavelength and reset to zero when the wavelength was larger than 800 nm. The transmittance and absorbance curves of the produced film were presented in Fig. 1c. The transmittance value in the
Fig. 1. Optical properties of the produced film. a) Reflectance, refractive index and extinction coefficient spectra b) Plots of parts of dielectric constant and dissipation factor vs. wavelength c) Transmittance and absorbance spectra d) Tauc plot of (ahv)2 vs. hv to estimate the optical band gap and inset shows the plot of (ahv)1/2 as a function of photon energy hv.
230
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Fig. 3. Compositional, structural and morphological properties of the produced film. a) EDX spectrum b) XRD pattern c) FESEM image at 60 kx magnification d) 2D, 3D AFM images and dimensional analysis.
transparent region was in the range of 45e65% and increased as the wavelength of the incident photons increased. The obtained A spectrum showed just opposite trend of the T spectra. A sudden rise in absorption was observed in the wavelength range between 300 and 800 nm. The absorption in this region was associated with the higher energy of photons than the band gap energy of the film. The absorption coefficient a was computed from the experimentally measured values of absorbance A using the following equation:
a ¼ 2:303ðA=tÞ
(1)
where t is the thickness of the film. The optical energy band gap Eg of the semiconductor materials is related to the absorption coefficient a in the strong absorption region by the relation:
ahv ¼ B hv Eg
n
(2)
where B is a constant, hv is the photon energy, and n is a number which have the values of 1/2 for direct transition or 2 for indirect transition. Fig. 1d showed the plot of (ahv)2 vs. hv, which gave the direct Eg while the inset of Fig. 1d showed the plot of (ahv)1/2 vs. hv giving the indirect Eg. The presence of a linear part confirmed that the optical threshold of absorption was of direct nature. The Eg of the film was estimated to be 1.25 eV. This value was observed to be lower than the Eg value of the GaAs films in Ref. [22]. Tauc's approximation is quite reliable though it might be not totally
correct. The error might be about ±5e10%. From the RT Hall effect measurement, the sheet resistivity, sheet carrier density, and Hall mobility were 1.26 105 U/sqr, 5.73 1011 1/cm2, and 95 cm2/(Vs), respectively. Fig. 2 showed the sheet carrier density as a function of the measurement temperature. One can notice the carrier-type switch from p-type to n-type occurring at 250 K. At lower temperatures than 250 K the sample was p-type while the sample was n-type at temperatures higher than 250 K. The EDX spectrum of the produced film was shown in Fig. 3a. The EDX analysis confirmed the presence of Ga, As and Si in the film. The X-ray diffraction pattern with the peaks identified and labeled was presented in Fig. 3b. The most intense peak at 2q ¼ 29.54 was attributed to the (011) reflection of the hexagonal GaAs and the (310) reflection of the monoclinic SiAs. Besides, the observed weaker peaks indicated the existence a small amount of Ga and Si crystals in the film. The grain size was calculated using Scherrer formula:
D¼
0:94l b cos qB
(3)
where D is the crystallite size, l is the wavelength of the X-ray radiation, b is the FWHM of the observed peak and q is the Bragg's angle of the peak. The grain size of the film was evaluated to be in range of 30e40 nm. Fig. 3c showed a FESEM image of the film, indicating that the film was composed of round grains with grain size varying in the range of 50e80 nm. Different results in the grain
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231
Fig. 4. Digital snapshots of different testing liquid droplets on the produced film's surface.
size of the produced film were obtained from the XRD and FESEM analysis. The larger grain sizes observed in the FESEM image can be attributed to agglomeration of the crystallites leading to larger grains. This finding was also confirmed by the relatively high RMS roughness value. When compared to the FESEM images presented in Ref. [22], the grain size tended to be smaller with Si doping. Fig. 3d showed a 3D AFM micrograph which revealed that the produced film had a dense microstructure as was also depicted by the photomicrograph obtained from the FESEM study. The RMS roughness value of the film was determined to be 14.5 nm and the average height was 48.3 nm. The determined RMS roughness value and average height of the produced film were observed to be higher than the RMS roughness value and average height of the GaAs film in Ref. [34] and Si-doped GaAs film in Ref. [35], respectively. The snapshots of the testing liquids droplets on the produced film were shown in Fig. 4. The contact angle (CA) value was strongly dependent on the liquid used. The highest CA value was obtained for water which is a polar liquid with higher surface tension than ethylene glycol and formamide. The lowest value of CA was obtained for diiodomethane which is a dispersive liquid. The data of surface tensions and their components for the used testing liquids can be found in literature [36e39]. The CA values can be employed to calculate the surface free energy (SFE) of the produced film. There are various approaches in literature to evaluate the SFE of solid materials from wetting experiments. However, the solid SFE obtained can be different depending on the approach chosen for calculation. To determine SFE, five methods were selected for this study: AcideBase, OWRK/ Fowkes, Wu, Equation of State and Zisman approaches. Three methods allowed the calculation of polar (or acidebase) and disperse components of SFE: the AcideBase, OWRK/Fowkes, and Wu approaches. The results were given in Table 1. In conclusion, a Si-doped nano-crystalline GaAs thin film was deposited on a glass substrate using the TVA technique and its optical, electrical, structural, compositional and morphological properties were examined. The observed shrinkage of the band gap can be attributed to the large grains, random grain distribution, and structural modification of the material in the film [40e42]. The Hall effect measurement indicated that the film was a n-type semiconductor with a sheet resistivity of 1.26 105 U/sqr at RT. XRD characterization showed that the produced film composed by a mix Table 1 SFE values of the produced film according to the AcideBase, OWRK/Fowkes, Wu, Equation of State, and Zisman Methods. Method
gTOT (mN/m)
gd (mN/m)
gp (mN/m)
AcideBase OWRK/Fowkes Wu Equation of State Zisman
50.215 47.583 40.801 28.759 44.480
48.934 45.216 43.687
1.282 2.367 2.885
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