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Comparison of the structural and optical properties of porous In0.08Ga0.92N thin films synthesized by electrochemical etching
Journal of Solid State Chemistry 212 (2014) 242–248
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Comparison of the structural and optical properties of porous In0.08Ga0.92N thin films synthesized by electrochemical etching Saleh H. Abud a,b,n, Z. Hassan a, F.K. Yam a, A.J. Ghazai c a
Nano-Optoelectronics Research and Technology (N.O.R.) Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia Department of Physics, College of Science, University of Kufa, Iraq c Department of Physics, College of Science, Thi-Qar University, Iraq b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2013 Received in revised form 2 October 2013 Accepted 7 October 2013 Available online 16 October 2013
This paper presents the structural and optical study of porous (1 mm) In0.08Ga0.92N synthesized by photoelectrochemical etching under various conditions. Field emission scanning electron microscope and atomic force microscope images showed that the pre-etched thin films have a sufficiently smooth surface over a large region with wurtzite structure. The roughness increased with an increase in etching duration. The blue shift phenomenon was measured for photoluminescence emission peaks at 300 K. The energy band gap increased to be 3.18 and 3.16 eV for post-etched films at ratios of 1:4 and 1:5, respectively. At the same time, the photoluminescence intensities of the post-etched thin films indicated that the optical properties have been enhanced. & 2013 Elsevier Inc. All rights reserved.
Keywords: III-nitride semiconductor Porous InGaN Atomic force microscopy Photoluminescence
1. Introduction
2. Experimental procedure
Ternary alloy systems, particularly InGaN, have attracted interest because of their ability to tune direct band gap from infrared to ultraviolet (0.7–3.4 eV) [1] through control of their In/Ga ratio. This property of InGaN makes it a promising material for various energy applications [2]. Porous III-nitride compounds are particularly desirable for optoelectronics [3] and chemical and biochemical sensors [4] because of their unique optical properties compared with their bulk counterparts [5,6]. The porous structure of III-nitride compounds can reduce the defect density and strain relief caused by lattice mismatch [5], as well as shed light on the fundamental properties of nanoscale structures for nanotechnology development. Researchers [7–9], have used photoelectrochemical (PEC) etching to synthesize porous GaN. The PEC technique is cheaper and more suitable than other techniques for the production of high-density nanostructures with controlled pore size and shape [10]. Electrolyte, current density, illumination, and time are the main factors that affect electrochemical etching. In this study, we attempt to synthesize porous InGaN structure by choosing suitable factors to enhance the structural and optical properties of this material in order to use it in different applications.
We used commercially and unintentionally doped n-type In0.08 Ga0.92N grown on low resistivity p-type Si(1 1 1) substrate by plasmaassisted molecular beam epitaxy with AlN buffer layer. The native oxide of the samples was initially removed with NH4OH:H2O (1:20), followed by HF:H2O (1:50). Boiling aqua regia HCl:HNO3 (3:1) was subsequently used to clean the samples. 2.1. Synthesis of porous InGaN nanostructures Porous InGaN was produced with the UV-PEC etching technique. The etching cell was made from Teflon with platinum wire as cathode and InGaN wafer as an anode. The samples were then etched with two different ratios (1:4 and 1:5) of HF (49%):C2H5OH (99.99%) under the illumination of 500 W UV lamp and a constant current density of 25 mA/cm2 for 5, 10, 15, and 20 min. All experimental processes were conducted at room temperature. Similar to anodic etching of GaN [11,12], the anodic etching of InGaN can be explained as follows: þ The formation of holes (h ) at the surface induces the emergence of dangling bonds: þ
γ
2InGaN þ 12h ⟹2In3 þ þ 2Ga3 þ þ N2 ↑ n
Corresponding author at: Nano-Optoelectronics Research and Technology (N.O.R.) Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia. Tel.: þ 60 134743701; fax: þ 60 46579150. E-mail address: [email protected] (S.H. Abud). 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.10.007
ð1Þ
The bubbles emerging from the reaction is evidence of the release the nitrogen gas. A certain number of dangling bonds will react with nucleophilic species (such as OH ) from the electrolyte
S.H. Abud et al. / Journal of Solid State Chemistry 212 (2014) 242–248
243
before being saturated by the electrons from the semiconductor:
3. Results and discussion
2In3 þ þ 2Ga3 þ þ 12OH 1 -In2 O3 þ Ga2 O3 þ 6H2 O
Fig. 1a shows the FESEM image of the n-In0.08Ga0.92N thin film grown on the Si(1 1 1) substrate. The thin film has a sufficiently smooth and uniform surface over a large region. Fig. 1b shows the cross section of the pre-etched film, in which the thickness is 1000 and 100 nm for InGaN and AlN buffer layer, respectively. Fig. 1c shows the EDX spectrum of the thin film. The intensity of the peak refers to the concentration and atomic composition of the elements in the film, this spectrum refers to the absence of contaminating elements. Whereas the EDX stack spectra of the cross section was shown in Fig. 1d. Fig. 2 shows the top view FESEM images of the porous In0.08Ga0.92N surfaces prepared at a ratio of 1:4 HF:C2H5OH under various durations. The morphology of the thin film etched for 5 min (Fig. 2a) shows that pores are beginning to form, whereas that of the thin film etched for 10 min (Fig. 2b) exhibits pores with sizes of 30–70 nm. On the thin film etched for 15 min (Fig. 2c), the pores become more regular, with sizes of 60–100 nm, and the pore walls became thinner. The increase of etching duration to 20 min (Fig. 2d) led to deform the pores shape. Figs. 3a–d show FESEM images of the porous In0.08Ga0.92N surfaces with a ratio of (1:5). Coral-like ridges started to form and the surface became rough during the etching duration of 5 min (Fig. 3a); irregular shapes of ridges with different sizes were found. Increasing the duration to 10 min (Fig. 3b) and 15 min (Fig. 3c) led to increase in the number of forming corals-like ridges, which were the greatest at 15 min.
ð2Þ
The new compounds have no or less direct chemical connection to the semiconductor. If these compounds are soluble in the etching solution, then it can dissolve chemically and thus the surface of the sample will be free and ready for the next interaction with nucleophilic species from the solution: In2 O3 þGa2 O3 þ 8H þ 1 -In2 O þ Ga2 O þ 4H2 O
ð3Þ
2.2. Characterizations We presented the structural parameters and optical properties of the pre- and post-etched In0.08Ga0.92N thin films. Field emission scanning electron microscope (FESEM, Model FEI Nova NanoSEM 450), atomic force microscope (AFM, Model Dimension EDGE, BRUKER), and high-resolution X-ray diffractometer system (XRD, Model PANalytical X’Pert PRO MRD PW3040) were used to determine the surface morphology and structural parameters of thin films. Optical properties were investigated by photoluminescence spectroscopy system (PL, Model Jobin Yvon HR 800 UV) and excited by a He–Cd laser at 325 nm.
Fig. 1. (a) FESEM image of the as-grown InGaN thin film; (b) cross section of the thin film; (c) EDX spectrum; and (d) EDX spectra depend on cross section.
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Fig. 2. FESEM images of the porous InGaN at a ratio of 1:4 and under various etching durations: (a) 5 min; (b) 10 min; (c) 15 min; and (d) 20 min. The inset is the cross section.
However, a 20 min duration yielded to deform these structures as shown in Fig. 3d. Thus the etching duration affects the shape and size of the formed pores. Fig. 4 depicts 3D-AFM image of the thin film with a root mean square (RMS) roughness of 2.2 nm. This observation consistents with the FESEM image shown in Fig. 1a. Figs. 5a–d and 6a–d show the 3D-AFM images of the porous InGaN thin films at ratios of 1:4 and 1:5, respectively, at different etching durations of 5, 10, 15, and 20 min. The RMS roughness of the porous thin films increased with an increase in etching duration for all films. This observation can be further supported by the FESEM images shown in Figs. 2 and 3. Fig. 7a and b shows the XRD patterns of the pre- and postetched InGaN thin films at ratios of 1:4 and 1:5, respectively. For the ratio of 1:4 (Fig. 7a), the diffraction peaks were located at 34.241, 34.361, 34.411, 34.481, and 34.531 relative to the (0 0 0 2) InGaN pre- and post-etched at 5, 10, 15, and 20 min, respectively. Compared with that of the pre-etched sample, the diffraction peaks of the post-etched samples shifted to high-angle region (approach to the GaN diffraction peak), indicating that the indium content gradually decreased progressively as etching duration increased, subsequently decreasing the lattice constant c of the alloys as shown in Table 1. The peak located at 36.171 relatives to the (0 0 0 2) AlN buffer layer. No diffraction peak was observed for InN, which indicate that phase segregation did not occur [1]. With the increase in value along with longer duration, the intensity of the porous thin films reached maximum value at 15 min and then
decreased as the duration was lengthened to 20 min. This decrease is attributed to the pore damages on the surface. Fig. 7b shows the XRD patterns of the pre- and post-etched films under a ratio of 1:5, in which, the behaviour of the post-etched patterns is similar to that under a ratio of 1:4, but the diffraction peaks were located at 34.281, 34.331, 34.361, and 34.371 relative to the (0 0 0 2) InGaN post-etched at 5, 10, 15, and 20 min, respectively. The slight change in the value of lattice constant at 1:5 ratio due to the decrease in concentration of HF, which in turn leads to a decrease in the ratio of etched indium. Lattice constant c was determined using XRD symmetric 2θ scans of the (0 0 0 2) plane [13]: c¼
λl ; 2 sin θ
ð4Þ
where λ is the wavelength of the X-ray radiation (0.1540 nm), θ is the Bragg angle, and l is the Miller index. The mean crystallite size ðDÞ of the thin films can be calculated with the Debye–Scherrer formula [14,15]: D¼
0:9λ ; B cos θ
ð5Þ
where B is the full width at half maximum. The values of the lattice constant and mean crystallite size of the pre- and postetched thin films at a ratio of 1:4 and 1:5 are summarized in Tables 1 and 2, respectively.
S.H. Abud et al. / Journal of Solid State Chemistry 212 (2014) 242–248
245
Fig. 3. FESEM images of the porous InGaN at a ratio of 1:5 and under various etching durations: (a) 5 min; (b) 10 min; (c) 15 min; and (d) 20 min. The inset is the cross section.
Fig. 4. AFM (3-D) view of the as-grown InGaN.
The strain ε along the c-axis is given by the following equation [16]:
ε¼
c c0 100%; c0
ð6Þ
where c is the lattice constant of the strained InGaN thin films calculated from Eq. 1 and c0 is the unstrained lattice constant of the InGaN. The unstrained lattice constant of bulk GaN and InN, is respectively, 0.5185 nm and 0.5703 nm [17]. The positive value (tensile strain) of the ε indicates that the lattice constant is
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S.H. Abud et al. / Journal of Solid State Chemistry 212 (2014) 242–248
Fig. 5. AFM images of the porous InGaN at a ratio of 1:4 and under various etching durations: (a) 5 min (RMS ¼113 nm); (b) 10 min (RMS ¼160 nm); (c) 15 min (RMS ¼ 200 nm); and (d) 20 min (RMS ¼370 nm).
Fig. 6. AFM images of the porous InGaN at a ratio of 1:5 and under various etching durations: (a) 5 min (RMS ¼ 27 nm); (b) 10 min (RMS ¼ 193 nm); (c) 15 min (RMS ¼ 220 nm); and (d) 20 min (RMS ¼ 430 nm).
S.H. Abud et al. / Journal of Solid State Chemistry 212 (2014) 242–248
247
Fig. 7. XRD spectra of the as-grown and porous InGaN at a ratio of (a) 1:4 and (b) 1:5 under various etching durations.
Table 1 Peak position, FWHM, lattice constant, mean crystallite size, and strain of pre- and post-etched thin films at a ratio of 1:4. Etching time (min)
Peak 2θ (deg)
FWHM (deg)
c (nm)
D (nm)
ε (%) 10 3
As-grown 5 10 15 20
34.24 34.36 34.41 34.48 34.53
0.196 0.246 0.246 0.252 0.261
0.5234 0.5216 0.5208 0.5198 0.5191
44.29 35.30 35.31 34.47 33.29
143 195 336 532 672
Table 2 Peak position, FWHM, lattice constant, mean crystallite size, and strain of pre- and post-etched thin films at a ratio of 1:5. Etching time (min)
Peak 2θ 2θðdegÞ
FWHM ðdegÞ
c (nm)
D (nm)
ε (%) 10 3
As-grown 5 10 15 20
34.24 34.28 34.33 34.36 34.37
0.196 0.246 0.247 0.252 0.266
0.5234 0.5228 0.5220 0.5216 0.5214
44.29 35.30 35.16 34.46 32.65
143 30 111 195 224
Fig. 8. PL spectra of the as-grown and porous InGaN at a ratio of (a) 1:4 and (b) 1:5 under various etching durations.
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S.H. Abud et al. / Journal of Solid State Chemistry 212 (2014) 242–248
elongated compared to that of the bulk sample. The negative value (compressive strain) arises from shrinking the lattice constant. The crystallite size for all post-etched films are decreased with increasing the etching duration accompanied by an increase in the compressive strain as shown in Tables 1 and 2. Fig. 8 shows the PL spectra of the thin films at pre- and postetching for 5, 10, 15, and 20 min. In Fig. 8a (ratio of 1:4), The PL intensity of the etched films increased with longer etching duration, reached maximum value at 15 min, and then decreased at 20 min. Blue shifts were observed in the spectra of the post-etched films (compared to the as-grown). Abud et al. [1] also observed and reported similar blue shifts. The high porosity-induced PL intensity can be explained by the extraction of strong PL via light scattering from the sidewalls of the sample crystallites [18]. Compared with the as-grown films, porous films have higher surface area per unit volume; thus, porous InGaN provides much exposure to the illumination of PL excitation lights for the InGaN molecules. This phenomenon may result in a higher number of electrons in excitation and recombination in porous films compared with the case involving a small surface area of as-grown films [19]. The energy gap was increased from 3.08 eV (403 nm) for the pre-etched thin film to 3.18 eV (390 nm) for the postetched thin film at 15 min. Under a ratio of 1:5 (Fig. 8b), the behaviour of the post-etched films was similar to that under a ratio of 1:4; however, the energy gap shifted to 3.16 eV (392 nm). The energy gap thus shifted from the visible to the ultraviolet region of the electromagnetic spectrum. Thus we can fabricate an optoelectronics device that operates in both regions (UV and visible) by selecting appropriate etching factors. 4. Conclusion The structural analysis revealed a wurtzite structure of the In0.08Ga0.92N thin film. This film exhibits smooth surface morphology, as revealed by field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM) measurements. The roughness of the porous films was increased compared to the asgrown film. XRD measurements show that the c-lattice parameter of the post-etched films was decreased with compressive strain compared to the pre-etched film. The blue shift changed the energy gap of the films from 3.08 eV for the pre-etched film to
3.18 eV and 3.16 eV for the etched films at a ratio of 1:4 and 1:5, respectively, as revealed by PL measurements. This study found that nanostructures can open a new and promising area in ternary III-nitride materials through the use of suitable etching factors to enhance the structure and optical properties of optoelectronic devices.
Acknowledgements The authors gratefully acknowledge support from Reseach University (RU) Grant, Universiti Sains Malaysia, and University of Kufa. References [1] S.H. Abud, Z. Hassan, F. Yam, Int. J. Electrochem. Sci. 7 (2012) 10038–10046. [2] R. Padma, B. Prasanna Lakshmi, M.Siva Pratap Reddy, V.Rajagopal Reddy, Superlattices Microstruct. 56 (2013) 64–76. [3] Z. Hassan, Y. Lee, F. Yam, K. Ibrahim, M. Kordesch, W. Halverson, P. Colter, Solid State Commun. 133 (2005) 283–287. [4] F.K. Yam, Z. Hassan, Mater. Lett. 63 (2009) 724–727. [5] C.F. Lin, J.H. Zheng, Z.J. Yang, J.J. Dai, D.Y. Lin, C.Y. Chang, Z.X. Lai, C. Hong, Appl. Phys. Lett. 88 (2006) 083121. [6] C.B. Soh, W. Liu, H. Hartono, N.S.S. Ang, S.J. Chua, S.Y. Chow, C.B. Tay, A. P. Vajpeyi, Appl. Phys. Lett. 98 (2011) 191903–191906. [7] C.F. Lin, Z.J. Yang, J.H. Zheng, J.J. Dai, J. Electrochem. Soc. 153 (2006) G39–G43. [8] M. Mynbaeva, A. Titkov, A. Kryganovskii, V. Ratnikov, K. Mynbaev, H. Huhtinen, R. Laiho, V. Dmitriev, Appl. Phys. Lett. 76 (2000) 1113–1115. [9] A. Mahmood, Z. Hassan, F. Yam, L. Chuah, Adv. Mater. Rapid Commun. 4 (2010) 1316–1320. [10] A. Ramizy, Z. Hassan, K. Omar, Sens. Actuators B 155 (2011) 699–708. [11] T. Rotter, D. Mistele, J. Stemmer, F. Fedler, J. Aderhold, J. Graul, V. Schwegler, C. Kirchner, M. Kamp, M. Heuken, Appl. Phys. Lett. 76 (2000) 3923–3925. [12] E. Trichas, C. Xenogianni, M. Kayambaki, P. Tsotsis, E. Iliopoulos, N.T. Pelekanos, P.G. Savvidis, Phys. Status Solidi A 205 (2008) 2509–2512. [13] A. Hussein, S. Thahab, Z. Hassan, C. Chin, H. Abu Hassan, S. Ng, J. Alloys Compd. 487 (2009) 24–27. [14] S.H. Abud, Z. Hassan, F.K. Yam, A.J. Ghazai, Adv. Mater. Res. 620 (2013) 368–372. [15] A.W. Burton, K. Ong, T. Rea, I.Y. Chan, Microporous Mesoporous Mater. 117 (2009) 75–90. [16] H. Ong, A. Zhu, G. Du, Appl. Phys. Lett. 80 (2002) 941–943. [17] H. Morkoç, Growth and Growth Methods for Nitride Semiconductors, in: Handbook of Nitride Semiconductors and Devices, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2009. [18] A. Vajpeyi, S. Tripathy, S. Chua, E. Fitzgerald, Physica E 28 (2005) 141–149. [19] K. Al-Heuseen, M.R. Hashim, N.K. Ali, Appl. Surf. Sci. 257 (2011) 6197–6201.
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Comparison of the structural and optical properties of porous In0.08Ga0.92N thin films synthesized by electrochemical etching Saleh H. Abud a,b,n, Z. Hassan a, F.K. Yam a, A.J. Ghazai c a
Nano-Optoelectronics Research and Technology (N.O.R.) Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia Department of Physics, College of Science, University of Kufa, Iraq c Department of Physics, College of Science, Thi-Qar University, Iraq b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 May 2013 Received in revised form 2 October 2013 Accepted 7 October 2013 Available online 16 October 2013
This paper presents the structural and optical study of porous (1 mm) In0.08Ga0.92N synthesized by photoelectrochemical etching under various conditions. Field emission scanning electron microscope and atomic force microscope images showed that the pre-etched thin films have a sufficiently smooth surface over a large region with wurtzite structure. The roughness increased with an increase in etching duration. The blue shift phenomenon was measured for photoluminescence emission peaks at 300 K. The energy band gap increased to be 3.18 and 3.16 eV for post-etched films at ratios of 1:4 and 1:5, respectively. At the same time, the photoluminescence intensities of the post-etched thin films indicated that the optical properties have been enhanced. & 2013 Elsevier Inc. All rights reserved.
Keywords: III-nitride semiconductor Porous InGaN Atomic force microscopy Photoluminescence
1. Introduction
2. Experimental procedure
Ternary alloy systems, particularly InGaN, have attracted interest because of their ability to tune direct band gap from infrared to ultraviolet (0.7–3.4 eV) [1] through control of their In/Ga ratio. This property of InGaN makes it a promising material for various energy applications [2]. Porous III-nitride compounds are particularly desirable for optoelectronics [3] and chemical and biochemical sensors [4] because of their unique optical properties compared with their bulk counterparts [5,6]. The porous structure of III-nitride compounds can reduce the defect density and strain relief caused by lattice mismatch [5], as well as shed light on the fundamental properties of nanoscale structures for nanotechnology development. Researchers [7–9], have used photoelectrochemical (PEC) etching to synthesize porous GaN. The PEC technique is cheaper and more suitable than other techniques for the production of high-density nanostructures with controlled pore size and shape [10]. Electrolyte, current density, illumination, and time are the main factors that affect electrochemical etching. In this study, we attempt to synthesize porous InGaN structure by choosing suitable factors to enhance the structural and optical properties of this material in order to use it in different applications.
We used commercially and unintentionally doped n-type In0.08 Ga0.92N grown on low resistivity p-type Si(1 1 1) substrate by plasmaassisted molecular beam epitaxy with AlN buffer layer. The native oxide of the samples was initially removed with NH4OH:H2O (1:20), followed by HF:H2O (1:50). Boiling aqua regia HCl:HNO3 (3:1) was subsequently used to clean the samples. 2.1. Synthesis of porous InGaN nanostructures Porous InGaN was produced with the UV-PEC etching technique. The etching cell was made from Teflon with platinum wire as cathode and InGaN wafer as an anode. The samples were then etched with two different ratios (1:4 and 1:5) of HF (49%):C2H5OH (99.99%) under the illumination of 500 W UV lamp and a constant current density of 25 mA/cm2 for 5, 10, 15, and 20 min. All experimental processes were conducted at room temperature. Similar to anodic etching of GaN [11,12], the anodic etching of InGaN can be explained as follows: þ The formation of holes (h ) at the surface induces the emergence of dangling bonds: þ
γ
2InGaN þ 12h ⟹2In3 þ þ 2Ga3 þ þ N2 ↑ n
Corresponding author at: Nano-Optoelectronics Research and Technology (N.O.R.) Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia. Tel.: þ 60 134743701; fax: þ 60 46579150. E-mail address: [email protected] (S.H. Abud). 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.10.007
ð1Þ
The bubbles emerging from the reaction is evidence of the release the nitrogen gas. A certain number of dangling bonds will react with nucleophilic species (such as OH ) from the electrolyte
S.H. Abud et al. / Journal of Solid State Chemistry 212 (2014) 242–248
243
before being saturated by the electrons from the semiconductor:
3. Results and discussion
2In3 þ þ 2Ga3 þ þ 12OH 1 -In2 O3 þ Ga2 O3 þ 6H2 O
Fig. 1a shows the FESEM image of the n-In0.08Ga0.92N thin film grown on the Si(1 1 1) substrate. The thin film has a sufficiently smooth and uniform surface over a large region. Fig. 1b shows the cross section of the pre-etched film, in which the thickness is 1000 and 100 nm for InGaN and AlN buffer layer, respectively. Fig. 1c shows the EDX spectrum of the thin film. The intensity of the peak refers to the concentration and atomic composition of the elements in the film, this spectrum refers to the absence of contaminating elements. Whereas the EDX stack spectra of the cross section was shown in Fig. 1d. Fig. 2 shows the top view FESEM images of the porous In0.08Ga0.92N surfaces prepared at a ratio of 1:4 HF:C2H5OH under various durations. The morphology of the thin film etched for 5 min (Fig. 2a) shows that pores are beginning to form, whereas that of the thin film etched for 10 min (Fig. 2b) exhibits pores with sizes of 30–70 nm. On the thin film etched for 15 min (Fig. 2c), the pores become more regular, with sizes of 60–100 nm, and the pore walls became thinner. The increase of etching duration to 20 min (Fig. 2d) led to deform the pores shape. Figs. 3a–d show FESEM images of the porous In0.08Ga0.92N surfaces with a ratio of (1:5). Coral-like ridges started to form and the surface became rough during the etching duration of 5 min (Fig. 3a); irregular shapes of ridges with different sizes were found. Increasing the duration to 10 min (Fig. 3b) and 15 min (Fig. 3c) led to increase in the number of forming corals-like ridges, which were the greatest at 15 min.
ð2Þ
The new compounds have no or less direct chemical connection to the semiconductor. If these compounds are soluble in the etching solution, then it can dissolve chemically and thus the surface of the sample will be free and ready for the next interaction with nucleophilic species from the solution: In2 O3 þGa2 O3 þ 8H þ 1 -In2 O þ Ga2 O þ 4H2 O
ð3Þ
2.2. Characterizations We presented the structural parameters and optical properties of the pre- and post-etched In0.08Ga0.92N thin films. Field emission scanning electron microscope (FESEM, Model FEI Nova NanoSEM 450), atomic force microscope (AFM, Model Dimension EDGE, BRUKER), and high-resolution X-ray diffractometer system (XRD, Model PANalytical X’Pert PRO MRD PW3040) were used to determine the surface morphology and structural parameters of thin films. Optical properties were investigated by photoluminescence spectroscopy system (PL, Model Jobin Yvon HR 800 UV) and excited by a He–Cd laser at 325 nm.
Fig. 1. (a) FESEM image of the as-grown InGaN thin film; (b) cross section of the thin film; (c) EDX spectrum; and (d) EDX spectra depend on cross section.
244
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Fig. 2. FESEM images of the porous InGaN at a ratio of 1:4 and under various etching durations: (a) 5 min; (b) 10 min; (c) 15 min; and (d) 20 min. The inset is the cross section.
However, a 20 min duration yielded to deform these structures as shown in Fig. 3d. Thus the etching duration affects the shape and size of the formed pores. Fig. 4 depicts 3D-AFM image of the thin film with a root mean square (RMS) roughness of 2.2 nm. This observation consistents with the FESEM image shown in Fig. 1a. Figs. 5a–d and 6a–d show the 3D-AFM images of the porous InGaN thin films at ratios of 1:4 and 1:5, respectively, at different etching durations of 5, 10, 15, and 20 min. The RMS roughness of the porous thin films increased with an increase in etching duration for all films. This observation can be further supported by the FESEM images shown in Figs. 2 and 3. Fig. 7a and b shows the XRD patterns of the pre- and postetched InGaN thin films at ratios of 1:4 and 1:5, respectively. For the ratio of 1:4 (Fig. 7a), the diffraction peaks were located at 34.241, 34.361, 34.411, 34.481, and 34.531 relative to the (0 0 0 2) InGaN pre- and post-etched at 5, 10, 15, and 20 min, respectively. Compared with that of the pre-etched sample, the diffraction peaks of the post-etched samples shifted to high-angle region (approach to the GaN diffraction peak), indicating that the indium content gradually decreased progressively as etching duration increased, subsequently decreasing the lattice constant c of the alloys as shown in Table 1. The peak located at 36.171 relatives to the (0 0 0 2) AlN buffer layer. No diffraction peak was observed for InN, which indicate that phase segregation did not occur [1]. With the increase in value along with longer duration, the intensity of the porous thin films reached maximum value at 15 min and then
decreased as the duration was lengthened to 20 min. This decrease is attributed to the pore damages on the surface. Fig. 7b shows the XRD patterns of the pre- and post-etched films under a ratio of 1:5, in which, the behaviour of the post-etched patterns is similar to that under a ratio of 1:4, but the diffraction peaks were located at 34.281, 34.331, 34.361, and 34.371 relative to the (0 0 0 2) InGaN post-etched at 5, 10, 15, and 20 min, respectively. The slight change in the value of lattice constant at 1:5 ratio due to the decrease in concentration of HF, which in turn leads to a decrease in the ratio of etched indium. Lattice constant c was determined using XRD symmetric 2θ scans of the (0 0 0 2) plane [13]: c¼
λl ; 2 sin θ
ð4Þ
where λ is the wavelength of the X-ray radiation (0.1540 nm), θ is the Bragg angle, and l is the Miller index. The mean crystallite size ðDÞ of the thin films can be calculated with the Debye–Scherrer formula [14,15]: D¼
0:9λ ; B cos θ
ð5Þ
where B is the full width at half maximum. The values of the lattice constant and mean crystallite size of the pre- and postetched thin films at a ratio of 1:4 and 1:5 are summarized in Tables 1 and 2, respectively.
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Fig. 3. FESEM images of the porous InGaN at a ratio of 1:5 and under various etching durations: (a) 5 min; (b) 10 min; (c) 15 min; and (d) 20 min. The inset is the cross section.
Fig. 4. AFM (3-D) view of the as-grown InGaN.
The strain ε along the c-axis is given by the following equation [16]:
ε¼
c c0 100%; c0
ð6Þ
where c is the lattice constant of the strained InGaN thin films calculated from Eq. 1 and c0 is the unstrained lattice constant of the InGaN. The unstrained lattice constant of bulk GaN and InN, is respectively, 0.5185 nm and 0.5703 nm [17]. The positive value (tensile strain) of the ε indicates that the lattice constant is
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Fig. 5. AFM images of the porous InGaN at a ratio of 1:4 and under various etching durations: (a) 5 min (RMS ¼113 nm); (b) 10 min (RMS ¼160 nm); (c) 15 min (RMS ¼ 200 nm); and (d) 20 min (RMS ¼370 nm).
Fig. 6. AFM images of the porous InGaN at a ratio of 1:5 and under various etching durations: (a) 5 min (RMS ¼ 27 nm); (b) 10 min (RMS ¼ 193 nm); (c) 15 min (RMS ¼ 220 nm); and (d) 20 min (RMS ¼ 430 nm).
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Fig. 7. XRD spectra of the as-grown and porous InGaN at a ratio of (a) 1:4 and (b) 1:5 under various etching durations.
Table 1 Peak position, FWHM, lattice constant, mean crystallite size, and strain of pre- and post-etched thin films at a ratio of 1:4. Etching time (min)
Peak 2θ (deg)
FWHM (deg)
c (nm)
D (nm)
ε (%) 10 3
As-grown 5 10 15 20
34.24 34.36 34.41 34.48 34.53
0.196 0.246 0.246 0.252 0.261
0.5234 0.5216 0.5208 0.5198 0.5191
44.29 35.30 35.31 34.47 33.29
143 195 336 532 672
Table 2 Peak position, FWHM, lattice constant, mean crystallite size, and strain of pre- and post-etched thin films at a ratio of 1:5. Etching time (min)
Peak 2θ 2θðdegÞ
FWHM ðdegÞ
c (nm)
D (nm)
ε (%) 10 3
As-grown 5 10 15 20
34.24 34.28 34.33 34.36 34.37
0.196 0.246 0.247 0.252 0.266
0.5234 0.5228 0.5220 0.5216 0.5214
44.29 35.30 35.16 34.46 32.65
143 30 111 195 224
Fig. 8. PL spectra of the as-grown and porous InGaN at a ratio of (a) 1:4 and (b) 1:5 under various etching durations.
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elongated compared to that of the bulk sample. The negative value (compressive strain) arises from shrinking the lattice constant. The crystallite size for all post-etched films are decreased with increasing the etching duration accompanied by an increase in the compressive strain as shown in Tables 1 and 2. Fig. 8 shows the PL spectra of the thin films at pre- and postetching for 5, 10, 15, and 20 min. In Fig. 8a (ratio of 1:4), The PL intensity of the etched films increased with longer etching duration, reached maximum value at 15 min, and then decreased at 20 min. Blue shifts were observed in the spectra of the post-etched films (compared to the as-grown). Abud et al. [1] also observed and reported similar blue shifts. The high porosity-induced PL intensity can be explained by the extraction of strong PL via light scattering from the sidewalls of the sample crystallites [18]. Compared with the as-grown films, porous films have higher surface area per unit volume; thus, porous InGaN provides much exposure to the illumination of PL excitation lights for the InGaN molecules. This phenomenon may result in a higher number of electrons in excitation and recombination in porous films compared with the case involving a small surface area of as-grown films [19]. The energy gap was increased from 3.08 eV (403 nm) for the pre-etched thin film to 3.18 eV (390 nm) for the postetched thin film at 15 min. Under a ratio of 1:5 (Fig. 8b), the behaviour of the post-etched films was similar to that under a ratio of 1:4; however, the energy gap shifted to 3.16 eV (392 nm). The energy gap thus shifted from the visible to the ultraviolet region of the electromagnetic spectrum. Thus we can fabricate an optoelectronics device that operates in both regions (UV and visible) by selecting appropriate etching factors. 4. Conclusion The structural analysis revealed a wurtzite structure of the In0.08Ga0.92N thin film. This film exhibits smooth surface morphology, as revealed by field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM) measurements. The roughness of the porous films was increased compared to the asgrown film. XRD measurements show that the c-lattice parameter of the post-etched films was decreased with compressive strain compared to the pre-etched film. The blue shift changed the energy gap of the films from 3.08 eV for the pre-etched film to
3.18 eV and 3.16 eV for the etched films at a ratio of 1:4 and 1:5, respectively, as revealed by PL measurements. This study found that nanostructures can open a new and promising area in ternary III-nitride materials through the use of suitable etching factors to enhance the structure and optical properties of optoelectronic devices.
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