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Cost-effective selective-area growth of GaN-based nanocolumns on silicon substrates by molecular-beam epitaxy
Journal of Crystal Growth 514 (2019) 124–129
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Cost-effective selective-area growth of GaN-based nanocolumns on silicon substrates by molecular-beam epitaxy
T
Yukun Zhaoa, Wenxian Yanga, Shulong Lua, , Yuanyuan Wua, Xin Zhangb, Lifeng Biana, Xuefei Lia, Ming Tana ⁎
a b
Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, PR China Optical & Microwave Technology Research Dept., Huawei Technologies Co. Ltd., Shenzhen 518129, PR China
ARTICLE INFO
ABSTRACT
Communicated by Dr C Charles W Tu
In this paper, the selective-area growth (SAG) of GaN-based nanocolumns (NCs) on silicon (Si) has been investigated experimentally. A low-cost and facile method has been proposed and implemented to fabricate the SAG NCs, which contains the processes of nanosphere lithography (NSL) and plasma-assisted molecular beam epitaxy (MBE). SAG NCs grown by this method indicates a higher ability to modulate the NC distributions than those grown by the conventional patterned method. According to photoluminescence results, InGaN/GaN NCs grown on Si nanopillars produced by NSL and etching show the more uniform indium (In) distribution than those grown on the openings of patterned Si substrate. Furthermore, according to the kinetic processes during MBE growth, it is proposed that the SAG of NCs results from the combination of the shadow effect of Si nano-pillars and the growth suppression of titanium (Ti) film.
Keywords: A1. Etching A3. Molecular beam epitaxy A3. Selective epitaxy B1. Nitrides B2. Semiconducting III-V materials
1. Introduction GaN-based nanocolumns (NCs) have the advantages of dislocationand strain-free growth on different substrates [1–2]. The high crystal quality yields a new generation of advanced devices intended for different applications, such as light emitting diodes (LEDs), batteries, solar cells and thermoelectric devices [2–5]. By means of plasma-assisted molecular beam epitaxy (MBE), many efforts have been dedicated to the growth of self-assembled NCs, allowing a better knowledge of the material’s physical properties and growth mechanisms [2,6]. However, the strong morphology dispersion of a self-assembled approach hinders the processing of nano-device arrays and deteriorates their electrical behavior [2]. Arrays of NCs grown by selective-area growth (SAG) provide a much better homogeneity in terms of morphology, electrical and optical characteristics [2]. Besides the excellent control of NCs’ formation site and size uniformity, using SAG can lead to a nearly defect-free, quasi-epilayer template with arbitrary alloy composition [1]. Furthermore, the large range of band gap has made InGaN a desirable material for color tunable (from ultraviolet to infrared) LEDs and laser diodes (LDs) by tuning the In content appropriately [7]. Therefore, the SAG of InGaN NCs could be a promising route to realize a new generation of advanced optoelectronics. So far, the most common approach of SAG is using the patterned substrate as a mask layer [1]. Katsumi Kishino et al. have utilized the
⁎
Corresponding author. E-mail address: [email protected] (S. Lu).
https://doi.org/10.1016/j.jcrysgro.2019.02.036
Available online 16 February 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
focused ion beam (FIB) milling to pattern titanium (Ti) films on GaN/ sapphire templates [8,9]. Ti films on GaN/sapphire templates patterned by e-beam lithography (EBL) have also been adopted as masks for the NC SAG [6,10]. On the other hand, silicon (Si) is a proven semiconductor platform for a broad range of devices from logic and memory integrated circuits (ICs) to sensors and micro- or nano-electromechanical systems [11]. Since NCs on Si indeed demonstrate certain advantages in low cost, available large-scale (up to 12 in.) and simplified electrode process, GaNbased NC growth directly on Si is advantageous [11–13]. However, GaNbased NC SAG is mainly performed on GaN/sapphire templates, not Si substrates. One of the major reasons is that the heteroepitaxial growth of GaN on Si has a larger lattice and thermal mismatch compared to the homoepitaxy on GaN/sapphire templates [14,15]. Thanks to the huge amount of free surface of NCs in relaxing the strain [14], GaN NCs on Si by MBE have also been investigated by some groups [16–19]. However, to date, GaN-based NCs on Si are mainly self-assembled, not SAG, which remains an active focus of research. Besides, from the view of device processing, the common SAG approach is usually combined with EBL or FIB technology, which requires expensive apparatuses and complicated processes [8–10]. Those can limit the promotion and industrialization of NC SAG. Therefore, an effective fabrication approach with low-cost and facile processes is still imperative and challenging for GaN-based NC SAG, especially for that
Journal of Crystal Growth 514 (2019) 124–129
Y. Zhao, et al.
Fig. 1. Schematic of the processes for fabricating patterned Si for SAG. (a) PS spheres self-assembled on the substrate. (b) O2 etching process. (c) Deposit Ti film and remove PS spheres. (d) PS etching process. (e) Deposit Ti film. (f) PS sphere removal. Patterned substrate completed in (c) is used for sample A, while that in (f) is used for sample B.
Fig. 2. (a) Experimental top-view SEM image of PS spheres etched by oxygen plasma for sample A. (b) Top-view SEM image of patterned Ti film for sample A. (c) Top-view and (d) side-view SEM images of PS spheres etched by RIE for sample B.
on Si substrates. However, at present, very few papers have been published with the aim of simplifying the fabrication processes of GaNbased NC SAG on Si. In this paper, the well-known method of two-dimensional ordering of latex particles is used as a basis for nanosphere lithography (NSL) due to its cost-efficiency and stability [20,21]. According to our previous work, colloidal spherical particles of polystyrene (PS) beads were self-assembled to form a mask for fabricating nanostructures [22,23]. Here, a facile and low-cost method with NSL in combination with mask etching has been utilized for InGaN NC SAG on Si. Meanwhile, the relevant growth mechanism has also been investigated.
deionized water. A hexagonally-ordered monolayer template of ∼500nm-diameter PS spheres was self-assembled coated onto the cleaned substrates [Fig. 1(a)]. Then, the dried samples were etched by oxygen plasma to reduce the PS diameters [Fig. 1(b)]. In this step, we use the etching time to control the PS diameters. After that, a ∼20-nm-thick Ti film was deposited on the substrate by electron beam evaporation. Next, the chloroform solution was utilized to remove the PS spheres [Fig. 1(c)]. According to the red arrows shown in Fig. 1, the processes of Fig. 1(d)–(f) are utilized for fabricating the patterned Si of sample B. In Fig. 1(d), reactive ion etching (RIE) was used to etch the substrate for ∼135 s. After that, as illustrated in Fig. 1(e) and (f), a Ti film was deposited and the PS spheres were removed. For the MBE growth of samples A and B, the growth conditions for GaN/InGaN NC heterostructures included the following steps. Firstly, a ∼2-nm-thick AlN buffer layer was grown with a substrate temperature of 830 °C. Subsequently, GaN NCs were grown with a Ga flux of ∼2.0 × 10−8 Torr for ∼2.5 h. Then the InGaN NCs were grown at the substrate temperatures of 610 °C. After that, a thin GaN NCs were
2. Experiments Fig. 1 is a schematic of the fabrication procedure for making patterned substrate. Processes of Fig. 1(a)–(c) are used to fabricate the patterned Si for the SAG of sample A. Firstly, the original Si(1 1 1) substrates were ultrasonically cleaned by acetone, alcohol, and 125
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grown as a cladding layer for ∼4 min. The substrate temperature mentioned here refers to the thermocouple reading on the backside of the substrate. Scanning electron microscopy (SEM) was used to characterize the quality of PS spheres and NCs, and scanning transmission electron microscopy (STEM) was used to characterize the InGaN/GaN NC heterostructures. Room-temperature PL measurements were done using a fluorescence lifetime spectrometer excited by a 325 nm laser. A spectrometer and a charge-coupled device (CCD) were also used to collect the luminescence.
intensity of green peak is much higher than the other one. To quantitatively identify the difference between the two PL peaks, the ratio (η) of two peak emission intensities (P1 and P2) is calculated as follows.
= max (P1, P2 )/ min (P1, P2 )
(1)
Max(P1,P2) means the larger one among the data P1 and P2, while Min(P1,P2) represents the smaller one. By the data from Fig. 4(c), ηB (η of sample B) is 9.3, which is much larger than ηA (3.1). It reveals that SAG InGaN sections may have a more uniform In distribution than those InGaN sections grown on self-assembled sample. In addition, sample B is also investigated by STEM (Fig. 5). GaN NCs are grown on the top surface of Si nano-pillars, while those are significantly limited on the bottom surface of Si nano-pillars. As shown in Fig. 3(d) and 5(d), a very limited GaN can grow on the up sidewalls of Si nano-pillars. One reason is that few Ti could deposit on the up sidewalls as being blocked by PS spheres [Fig. 2(d)]. Structural schematic illustrations and enlarged cross-section schematic diagrams of samples A and B are illustrated in Fig. 6. Due to the larger bonding energy of Si-N (4.5 eV) than Ga-N (2.2 eV), a very thin amorphous SiNx film forms at the interface when growing GaN NCs directly on Si substrates [12]. This amorphous layer causes NCs oriented randomly and tilted away from the substrate surface normal, which lowers the electronic and optical quality of the device [12,25]. To overcome this problem, a AlN seeding layer is grown into nanoscale growth template prior to GaN NC growth [12,25,26]. As Al and N atoms impinge all over the surface in MBE, the growth process begins with the incubation stage, where AlN nuclei are spontaneously forming and disintegrating on the whole surface. Stable spherical cap-shaped three-dimensional (3D) islands grow until reaching a certain critical radius (∼5 nm) [27]. Subsequently, the shape transits to the NC-like morphology [27]. As time evolves, the NCs elongate all over the surface, including both Si surfaces covered and uncovered by the Ti film [Fig. 6(b)]. Thus, self-assembled NCs can grow on the Ti film based on the AlN pedestals. In other words, AlN pedestals could improve the crystal quality of NCs as a buffer layer, but deteriorate the limited effect of Ti film, resulting in the NCs grown on Ti film. To further study the growth mechanism of sample B, Fig. 6(d) displays the kinetic processes such as atomic impinge, adsorption,
3 . Results and discussions Fig. 2 is a collection of the SEM images of patterned Si(1 1 1) substrate. As shown in Fig. 2(a), the PS spheres etched by oxygen plasma were utilized as a mask for sample A. After Ti film deposition and PS removal [Fig. 2(b)], the patterned substrate can be achieved for the NC SAG of sample A. Fig. 2(c) and (d) show the top-view and side-view SEM images of Si substrates with PS spheres etched by RIE for sample B, respectively. It is shown that uniform hexagonally-ordered PS spheres can be obtained on Si(1 1 1) substrate, which are utilized as the mask to pattern the substrate for NC SAG. After MBE growths on the corresponding substrates, samples A and B can be obtained, which are illustrated in Fig. 3. As clearly shown in Fig. 3(a) and (b), no obvious SAG NCs can be achieved in sample A. It indicates that GaN NCs are not only grown on the patterned Si regions, but also grown on the Ti film. In comparison, obvious SAG NCs are able to be obtained in Fig. 3(c) and (d). NCs are grown along the uniform Si nano-pillars in sample B, while the GaN NC growths are significantly limited on the Ti film. With respect to Fig. 3(a) and (b), inhomogeneous nano-umbrellas with InGaN sections are obtained at the top of NCs. Fig. 4(a) and (b) shows the STEM images of InGaN/GaN NC heterostructures of sample A. For a single InGaN section, the core is richer in In than that of the shell [16,19]. Furthermore, different sizes of umbrella-like InGaN sections have the different In compositions, leading to emit photons with different wavelengths [24]. Hence, two obvious peaks (∼382 nm and ∼542 nm) of sample A have been achieved during the photoluminescence (PL) measurements [Fig. 4(c)]. Sample B also has two PL peaks, while the
Fig. 3. Experimental (a) top-view and (b) side-view SEM images of sample A. (c) Top-view and (d) side-view SEM images of sample B. 126
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Fig. 4. (a) (b) STEM image of InGaN/GaN NC heterostructures with the In (Green) and Ga (Red) concentration maps obtained by energy dispersive x-ray (EDX) spectroscopy. (c) Normalized PL spectra of samples A and B. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) STEM image of sample B. (b)–(d) Enlarged STEM images of the corresponding regions in (a).
diffusion and desorption. To describe the processes involved in kinetic model, atomic flux (Ф) expressing the atomic quantity per unit surface and unit time. Фtop,impinge and Фside,impinge mean the impinging fluxes at top surface and sidewall, respectively. Фtop,desorption and Фside,desorption represent the desorption loss at top surface and sidewall, respectively. Фdiff is the diffusion flux from sidewall to top surface. Under a steady state growth, the comprehensive equations for effectively-adsorbed atomic flux (Фeff) are calculated as follows [13]. top, eff
=
top, impinge
side,eff
=
side, impinge
+
diff
diff
top, desorption
side, desorption
spacing. Derived from Fig. 2(d), αB is about 45°. Due to αB < αA, compared with those of sample A, less impinging fluxes of Al and N atoms can reach the Ti surface of sample B. That means a large amount of Al and N atoms are blocked because of the shadow effect of Si nano-pillars. Thus, less AlN pedestals could be nucleated on the Ti surface of sample B, leading to improve the limited effect of Ti film. Therefore, GaN-based NCs are mainly grown on the top surfaces of Si nano-pillars, not on the entire Si surface [Fig. 3(c) and (d)]. In addition, as αB is similar with θ, less impinging fluxes of Ga and N atoms could reach the Si nano-pillar sidewalls [Region I in Fig. 6(d)] when the NC height increases. Therefore, it is proposed that GaN materials can grow on Si nano-pillar sidewalls, but the growth rate is lower than that of GaN NC on the top surfaces of Si nano-pillars [shown in Fig. 3(d) and 5(a)].
(2) (3)
As the NC radius can increase during growth but with a growth rate that is more than 1 order of magnitude lower than the axial growth rate [27], Фside,eff is much smaller than Фtop,eff (Фside,eff ≪ Фtop,eff). θ represents the angle of impinging flux, which is about 47° in the experiments. As shown in Fig. 6(b) and (d), α shows the angle of the top and bottom surfaces of nanostructures, which depends on the nanostructural size and
4 . Conclusions We have fabricated the GaN-based SAG NCs on Si with a cost-effective and facile method. The method requires the process of 127
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Fig. 6. Structural schematic illustrations of (a) sample A, and (c) sample B. Enlarged cross-section schematic diagrams of (b) sample A, and (d) sample B.
nanosphere lithography (NSL) and Ti deposition, which is utilized to pattern the Si substrates for SAG growth. To simplify the SAG processes, the Ti film is directly formed on Si instead of on a previously grown GaN or AlN buffer layer. According to the SEM images, it is very difficult to achieve SAG NCs on Si substrate patterned by the conventional method because AlN pedestals could deteriorate the limited effect of Ti film. When fabricating the Si substrate by the proposed method, GaNbased NCs are mainly grown in specific regions defined by Si nanopillars, not on the whole Si surface. Due to the shadow effect of Si nanopillars, less impinging fluxes of Al and N atoms can reach the Ti surface, leading to the improvement of Ti growth-limited effect. Furthermore, with the analysis of EDX and PL results, a more uniform In distribution is probably obtained in SAG InGaN section than that of NCs self-assembled grown. It is proposed the ability to fabricate GaN-based SAG NCs on Si is of great potential significance to some NC-device applications.
[5]
[6]
[7]
[8]
[9]
Acknowledgments The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 61804163, 61574161, 61574130 and 61674051), the Natural Science Foundation of Jiangsu (Grant Nos. BK20180252 and BK20151455), and Natural Science Foundation of Jiangxi (Grant No. 20181BAB211021). The authors also would like to acknowledge the support from SINANO (Y4JAQ21001, Y6AAQ11001), and the External Cooperation Program of BIC of Chinese Academy of Sciences (Grant No. 121E32KYSB20160071). We are thankful for the technical support from the Vacuum Interconnected Nano-X Research Facility (No. H042-2018), Platform for Characterization & Test of SINANO, CAS.
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[11]
[12]
[13]
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Cost-effective selective-area growth of GaN-based nanocolumns on silicon substrates by molecular-beam epitaxy
T
Yukun Zhaoa, Wenxian Yanga, Shulong Lua, , Yuanyuan Wua, Xin Zhangb, Lifeng Biana, Xuefei Lia, Ming Tana ⁎
a b
Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, PR China Optical & Microwave Technology Research Dept., Huawei Technologies Co. Ltd., Shenzhen 518129, PR China
ARTICLE INFO
ABSTRACT
Communicated by Dr C Charles W Tu
In this paper, the selective-area growth (SAG) of GaN-based nanocolumns (NCs) on silicon (Si) has been investigated experimentally. A low-cost and facile method has been proposed and implemented to fabricate the SAG NCs, which contains the processes of nanosphere lithography (NSL) and plasma-assisted molecular beam epitaxy (MBE). SAG NCs grown by this method indicates a higher ability to modulate the NC distributions than those grown by the conventional patterned method. According to photoluminescence results, InGaN/GaN NCs grown on Si nanopillars produced by NSL and etching show the more uniform indium (In) distribution than those grown on the openings of patterned Si substrate. Furthermore, according to the kinetic processes during MBE growth, it is proposed that the SAG of NCs results from the combination of the shadow effect of Si nano-pillars and the growth suppression of titanium (Ti) film.
Keywords: A1. Etching A3. Molecular beam epitaxy A3. Selective epitaxy B1. Nitrides B2. Semiconducting III-V materials
1. Introduction GaN-based nanocolumns (NCs) have the advantages of dislocationand strain-free growth on different substrates [1–2]. The high crystal quality yields a new generation of advanced devices intended for different applications, such as light emitting diodes (LEDs), batteries, solar cells and thermoelectric devices [2–5]. By means of plasma-assisted molecular beam epitaxy (MBE), many efforts have been dedicated to the growth of self-assembled NCs, allowing a better knowledge of the material’s physical properties and growth mechanisms [2,6]. However, the strong morphology dispersion of a self-assembled approach hinders the processing of nano-device arrays and deteriorates their electrical behavior [2]. Arrays of NCs grown by selective-area growth (SAG) provide a much better homogeneity in terms of morphology, electrical and optical characteristics [2]. Besides the excellent control of NCs’ formation site and size uniformity, using SAG can lead to a nearly defect-free, quasi-epilayer template with arbitrary alloy composition [1]. Furthermore, the large range of band gap has made InGaN a desirable material for color tunable (from ultraviolet to infrared) LEDs and laser diodes (LDs) by tuning the In content appropriately [7]. Therefore, the SAG of InGaN NCs could be a promising route to realize a new generation of advanced optoelectronics. So far, the most common approach of SAG is using the patterned substrate as a mask layer [1]. Katsumi Kishino et al. have utilized the
⁎
Corresponding author. E-mail address: [email protected] (S. Lu).
https://doi.org/10.1016/j.jcrysgro.2019.02.036
Available online 16 February 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
focused ion beam (FIB) milling to pattern titanium (Ti) films on GaN/ sapphire templates [8,9]. Ti films on GaN/sapphire templates patterned by e-beam lithography (EBL) have also been adopted as masks for the NC SAG [6,10]. On the other hand, silicon (Si) is a proven semiconductor platform for a broad range of devices from logic and memory integrated circuits (ICs) to sensors and micro- or nano-electromechanical systems [11]. Since NCs on Si indeed demonstrate certain advantages in low cost, available large-scale (up to 12 in.) and simplified electrode process, GaNbased NC growth directly on Si is advantageous [11–13]. However, GaNbased NC SAG is mainly performed on GaN/sapphire templates, not Si substrates. One of the major reasons is that the heteroepitaxial growth of GaN on Si has a larger lattice and thermal mismatch compared to the homoepitaxy on GaN/sapphire templates [14,15]. Thanks to the huge amount of free surface of NCs in relaxing the strain [14], GaN NCs on Si by MBE have also been investigated by some groups [16–19]. However, to date, GaN-based NCs on Si are mainly self-assembled, not SAG, which remains an active focus of research. Besides, from the view of device processing, the common SAG approach is usually combined with EBL or FIB technology, which requires expensive apparatuses and complicated processes [8–10]. Those can limit the promotion and industrialization of NC SAG. Therefore, an effective fabrication approach with low-cost and facile processes is still imperative and challenging for GaN-based NC SAG, especially for that
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Fig. 1. Schematic of the processes for fabricating patterned Si for SAG. (a) PS spheres self-assembled on the substrate. (b) O2 etching process. (c) Deposit Ti film and remove PS spheres. (d) PS etching process. (e) Deposit Ti film. (f) PS sphere removal. Patterned substrate completed in (c) is used for sample A, while that in (f) is used for sample B.
Fig. 2. (a) Experimental top-view SEM image of PS spheres etched by oxygen plasma for sample A. (b) Top-view SEM image of patterned Ti film for sample A. (c) Top-view and (d) side-view SEM images of PS spheres etched by RIE for sample B.
on Si substrates. However, at present, very few papers have been published with the aim of simplifying the fabrication processes of GaNbased NC SAG on Si. In this paper, the well-known method of two-dimensional ordering of latex particles is used as a basis for nanosphere lithography (NSL) due to its cost-efficiency and stability [20,21]. According to our previous work, colloidal spherical particles of polystyrene (PS) beads were self-assembled to form a mask for fabricating nanostructures [22,23]. Here, a facile and low-cost method with NSL in combination with mask etching has been utilized for InGaN NC SAG on Si. Meanwhile, the relevant growth mechanism has also been investigated.
deionized water. A hexagonally-ordered monolayer template of ∼500nm-diameter PS spheres was self-assembled coated onto the cleaned substrates [Fig. 1(a)]. Then, the dried samples were etched by oxygen plasma to reduce the PS diameters [Fig. 1(b)]. In this step, we use the etching time to control the PS diameters. After that, a ∼20-nm-thick Ti film was deposited on the substrate by electron beam evaporation. Next, the chloroform solution was utilized to remove the PS spheres [Fig. 1(c)]. According to the red arrows shown in Fig. 1, the processes of Fig. 1(d)–(f) are utilized for fabricating the patterned Si of sample B. In Fig. 1(d), reactive ion etching (RIE) was used to etch the substrate for ∼135 s. After that, as illustrated in Fig. 1(e) and (f), a Ti film was deposited and the PS spheres were removed. For the MBE growth of samples A and B, the growth conditions for GaN/InGaN NC heterostructures included the following steps. Firstly, a ∼2-nm-thick AlN buffer layer was grown with a substrate temperature of 830 °C. Subsequently, GaN NCs were grown with a Ga flux of ∼2.0 × 10−8 Torr for ∼2.5 h. Then the InGaN NCs were grown at the substrate temperatures of 610 °C. After that, a thin GaN NCs were
2. Experiments Fig. 1 is a schematic of the fabrication procedure for making patterned substrate. Processes of Fig. 1(a)–(c) are used to fabricate the patterned Si for the SAG of sample A. Firstly, the original Si(1 1 1) substrates were ultrasonically cleaned by acetone, alcohol, and 125
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grown as a cladding layer for ∼4 min. The substrate temperature mentioned here refers to the thermocouple reading on the backside of the substrate. Scanning electron microscopy (SEM) was used to characterize the quality of PS spheres and NCs, and scanning transmission electron microscopy (STEM) was used to characterize the InGaN/GaN NC heterostructures. Room-temperature PL measurements were done using a fluorescence lifetime spectrometer excited by a 325 nm laser. A spectrometer and a charge-coupled device (CCD) were also used to collect the luminescence.
intensity of green peak is much higher than the other one. To quantitatively identify the difference between the two PL peaks, the ratio (η) of two peak emission intensities (P1 and P2) is calculated as follows.
= max (P1, P2 )/ min (P1, P2 )
(1)
Max(P1,P2) means the larger one among the data P1 and P2, while Min(P1,P2) represents the smaller one. By the data from Fig. 4(c), ηB (η of sample B) is 9.3, which is much larger than ηA (3.1). It reveals that SAG InGaN sections may have a more uniform In distribution than those InGaN sections grown on self-assembled sample. In addition, sample B is also investigated by STEM (Fig. 5). GaN NCs are grown on the top surface of Si nano-pillars, while those are significantly limited on the bottom surface of Si nano-pillars. As shown in Fig. 3(d) and 5(d), a very limited GaN can grow on the up sidewalls of Si nano-pillars. One reason is that few Ti could deposit on the up sidewalls as being blocked by PS spheres [Fig. 2(d)]. Structural schematic illustrations and enlarged cross-section schematic diagrams of samples A and B are illustrated in Fig. 6. Due to the larger bonding energy of Si-N (4.5 eV) than Ga-N (2.2 eV), a very thin amorphous SiNx film forms at the interface when growing GaN NCs directly on Si substrates [12]. This amorphous layer causes NCs oriented randomly and tilted away from the substrate surface normal, which lowers the electronic and optical quality of the device [12,25]. To overcome this problem, a AlN seeding layer is grown into nanoscale growth template prior to GaN NC growth [12,25,26]. As Al and N atoms impinge all over the surface in MBE, the growth process begins with the incubation stage, where AlN nuclei are spontaneously forming and disintegrating on the whole surface. Stable spherical cap-shaped three-dimensional (3D) islands grow until reaching a certain critical radius (∼5 nm) [27]. Subsequently, the shape transits to the NC-like morphology [27]. As time evolves, the NCs elongate all over the surface, including both Si surfaces covered and uncovered by the Ti film [Fig. 6(b)]. Thus, self-assembled NCs can grow on the Ti film based on the AlN pedestals. In other words, AlN pedestals could improve the crystal quality of NCs as a buffer layer, but deteriorate the limited effect of Ti film, resulting in the NCs grown on Ti film. To further study the growth mechanism of sample B, Fig. 6(d) displays the kinetic processes such as atomic impinge, adsorption,
3 . Results and discussions Fig. 2 is a collection of the SEM images of patterned Si(1 1 1) substrate. As shown in Fig. 2(a), the PS spheres etched by oxygen plasma were utilized as a mask for sample A. After Ti film deposition and PS removal [Fig. 2(b)], the patterned substrate can be achieved for the NC SAG of sample A. Fig. 2(c) and (d) show the top-view and side-view SEM images of Si substrates with PS spheres etched by RIE for sample B, respectively. It is shown that uniform hexagonally-ordered PS spheres can be obtained on Si(1 1 1) substrate, which are utilized as the mask to pattern the substrate for NC SAG. After MBE growths on the corresponding substrates, samples A and B can be obtained, which are illustrated in Fig. 3. As clearly shown in Fig. 3(a) and (b), no obvious SAG NCs can be achieved in sample A. It indicates that GaN NCs are not only grown on the patterned Si regions, but also grown on the Ti film. In comparison, obvious SAG NCs are able to be obtained in Fig. 3(c) and (d). NCs are grown along the uniform Si nano-pillars in sample B, while the GaN NC growths are significantly limited on the Ti film. With respect to Fig. 3(a) and (b), inhomogeneous nano-umbrellas with InGaN sections are obtained at the top of NCs. Fig. 4(a) and (b) shows the STEM images of InGaN/GaN NC heterostructures of sample A. For a single InGaN section, the core is richer in In than that of the shell [16,19]. Furthermore, different sizes of umbrella-like InGaN sections have the different In compositions, leading to emit photons with different wavelengths [24]. Hence, two obvious peaks (∼382 nm and ∼542 nm) of sample A have been achieved during the photoluminescence (PL) measurements [Fig. 4(c)]. Sample B also has two PL peaks, while the
Fig. 3. Experimental (a) top-view and (b) side-view SEM images of sample A. (c) Top-view and (d) side-view SEM images of sample B. 126
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Fig. 4. (a) (b) STEM image of InGaN/GaN NC heterostructures with the In (Green) and Ga (Red) concentration maps obtained by energy dispersive x-ray (EDX) spectroscopy. (c) Normalized PL spectra of samples A and B. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) STEM image of sample B. (b)–(d) Enlarged STEM images of the corresponding regions in (a).
diffusion and desorption. To describe the processes involved in kinetic model, atomic flux (Ф) expressing the atomic quantity per unit surface and unit time. Фtop,impinge and Фside,impinge mean the impinging fluxes at top surface and sidewall, respectively. Фtop,desorption and Фside,desorption represent the desorption loss at top surface and sidewall, respectively. Фdiff is the diffusion flux from sidewall to top surface. Under a steady state growth, the comprehensive equations for effectively-adsorbed atomic flux (Фeff) are calculated as follows [13]. top, eff
=
top, impinge
side,eff
=
side, impinge
+
diff
diff
top, desorption
side, desorption
spacing. Derived from Fig. 2(d), αB is about 45°. Due to αB < αA, compared with those of sample A, less impinging fluxes of Al and N atoms can reach the Ti surface of sample B. That means a large amount of Al and N atoms are blocked because of the shadow effect of Si nano-pillars. Thus, less AlN pedestals could be nucleated on the Ti surface of sample B, leading to improve the limited effect of Ti film. Therefore, GaN-based NCs are mainly grown on the top surfaces of Si nano-pillars, not on the entire Si surface [Fig. 3(c) and (d)]. In addition, as αB is similar with θ, less impinging fluxes of Ga and N atoms could reach the Si nano-pillar sidewalls [Region I in Fig. 6(d)] when the NC height increases. Therefore, it is proposed that GaN materials can grow on Si nano-pillar sidewalls, but the growth rate is lower than that of GaN NC on the top surfaces of Si nano-pillars [shown in Fig. 3(d) and 5(a)].
(2) (3)
As the NC radius can increase during growth but with a growth rate that is more than 1 order of magnitude lower than the axial growth rate [27], Фside,eff is much smaller than Фtop,eff (Фside,eff ≪ Фtop,eff). θ represents the angle of impinging flux, which is about 47° in the experiments. As shown in Fig. 6(b) and (d), α shows the angle of the top and bottom surfaces of nanostructures, which depends on the nanostructural size and
4 . Conclusions We have fabricated the GaN-based SAG NCs on Si with a cost-effective and facile method. The method requires the process of 127
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Fig. 6. Structural schematic illustrations of (a) sample A, and (c) sample B. Enlarged cross-section schematic diagrams of (b) sample A, and (d) sample B.
nanosphere lithography (NSL) and Ti deposition, which is utilized to pattern the Si substrates for SAG growth. To simplify the SAG processes, the Ti film is directly formed on Si instead of on a previously grown GaN or AlN buffer layer. According to the SEM images, it is very difficult to achieve SAG NCs on Si substrate patterned by the conventional method because AlN pedestals could deteriorate the limited effect of Ti film. When fabricating the Si substrate by the proposed method, GaNbased NCs are mainly grown in specific regions defined by Si nanopillars, not on the whole Si surface. Due to the shadow effect of Si nanopillars, less impinging fluxes of Al and N atoms can reach the Ti surface, leading to the improvement of Ti growth-limited effect. Furthermore, with the analysis of EDX and PL results, a more uniform In distribution is probably obtained in SAG InGaN section than that of NCs self-assembled grown. It is proposed the ability to fabricate GaN-based SAG NCs on Si is of great potential significance to some NC-device applications.
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Acknowledgments The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 61804163, 61574161, 61574130 and 61674051), the Natural Science Foundation of Jiangsu (Grant Nos. BK20180252 and BK20151455), and Natural Science Foundation of Jiangxi (Grant No. 20181BAB211021). The authors also would like to acknowledge the support from SINANO (Y4JAQ21001, Y6AAQ11001), and the External Cooperation Program of BIC of Chinese Academy of Sciences (Grant No. 121E32KYSB20160071). We are thankful for the technical support from the Vacuum Interconnected Nano-X Research Facility (No. H042-2018), Platform for Characterization & Test of SINANO, CAS.
[10]
[11]
[12]
[13]
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