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Epitaxy of gallium nitride pyramids on few layer graphene for metal-semiconductor-metal based photodetectors
Materials Chemistry and Physics 240 (2020) 122189
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Epitaxy of gallium nitride pyramids on few layer graphene for metal-semiconductor-metal based photodetectors S. Sanjay a, b, *, K. Prabakaran b, K. Baskar b a b
Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation, Hyderabad, 500 075, Telangana, India Crystal Growth Centre, Anna University, Chennai, 600 025, Tamil Nadu, India
H I G H L I G H T S
� Fabrication of MSM photodetectors on GaN pyramids grown by CVD. � The barrier height emphasizes the absence of any surface or barrier related inhomogeneities. � The ideality factor illustrates that there are no interfacial surface states. � The fabricated photodetector exhibits enhanced electrical and optical performances. A R T I C L E I N F O
A B S T R A C T
Keywords: Gallium nitride pyramids Few layer graphene Electrospinning Chemical vapour deposition
Growth of gallium nitride pyramids was carried out on few layer graphene substrates with different growth parameters such as growth time and precursor-to-substrate distance. The few layer graphene was synthesized by ultra-sonicating graphite and deposited on c-plane sapphire substrates by electrospinning technique. The indexed diffraction peaks obtained from X-ray diffractometer revealed good crystalline quality gallium nitride with a hexagonal crystal structure. The surface compositions of the samples have been studied by the X-ray photo electron spectroscopy. The scanning electron microscopy results revealed the formation of gallium nitride pyr amid structures all over the samples. The cathodoluminescence spectra showcased an improved band edge emission at 355 nm with weak deep level emissions corresponding to gallium nitride. The quality of gallium nitride obtained in the presence of few layer graphene had a positive effect on the luminescence properties, electrical and optical device performances.
1. Introduction The wide direct bandgap (Eg ¼ 3.4 eV) of gallium nitride (GaN) and its ability to practically tune the bandgap between near infra-red and deep ultra-violet regime when formed as ternary alloy with indium nitride (InN, Eg ¼ ~0.7 eV) and aluminium nitride (AlN, Eg ¼ 6.2 eV), has made GaN the most widely explored semiconductor material. Other fascinating properties of GaN include high carrier mobility, chemical stability and electrical breakdown field. All these innate qualities make GaN a suitable material for applications in electronic and optical devices [1–4]. Usually GaN is hetero-epitaxially grown on non-native substrates (silicon, sapphire and silicon carbide) with a certain lattice and thermal mismatch. Conventionally, sapphire is most commonly preferred for the
epitaxial growth of GaN. The hexagonal crystal structure of sapphire which is like that of crystal structure of GaN is the main reason for preferring sapphire substrates despite their high lattice mismatch with GaN [5–7]. These mismatches tend to induce defects and dislocations, thereby degrading material properties and device performances when fabricated. To avoid such constraints, it is beneficial to grow GaN in the form of nanostructures rather than as epilayers [8–10]. Despite scaling in the dimensionality, the insulating nature of sap phire substrates restricts the electrical performance of the GaN layer. To overcome this challenge, a simple solution could be to employ twodimensional materials such as graphene or boron nitride [11,12]. Over the past decade, intense research is being focussed on utilizing graphene, as a substrate material for enhancing the material properties of GaN [11]. This type of growth process was found to improve the electrical
* Corresponding author. Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation, Hyderabad, 500 075, Telangana, India. E-mail address: [email protected] (S. Sanjay). https://doi.org/10.1016/j.matchemphys.2019.122189 Received 3 July 2019; Received in revised form 7 September 2019; Accepted 15 September 2019 Available online 17 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 240 (2020) 122189
Fig. 1. Schematic representation of fabrication and examination of GaN pyra mids based MSM photodetectors. (a) sapphire substrate, (b) FLG deposited by electrospinning, (c) GaN pyramids grown by CVD technique, (e) fabricated MSM photodetector and (d) magnified image corresponding to (c) representing GaN pyramids grown on FLG substrates.
Fig. 4. SEM images representing: (a) sample S1, (b) S2, (c) S3, (d) S4, (e) S5 and (f) S6 respectively.
characteristics and help in overcoming the lattice and thermal mismatch problems between substrate and GaN. Unlike the conventional procedures involving synthesis of single layer graphene (SLG) on copper foil by chemical vapour deposition (CVD) followed by its transfer on desired substrate, the present study utilizes few layer graphene (FLG) instead of SLG. The FLG was synthe sized by liquid exfoliation technique and was electro-spun on sapphire substrates. This type of synthesis and deposition process proved to be economic and does not require any post growth transfer process. The GaN pyramids growth was carried out on FLG substrates at different growth conditions. The photoconductive devices were fabricated on these GaN samples to evaluate their electronic and optical perfor mances. A schematic representation of the device fabrication and ex amination thereon is shown in Fig. 1. To the best of our knowledge, this method of synthesis route and experimental conditions have not been reported earlier. The effect of FLG on GaN growth and the resultant growth conditions are briefly discussed in this manuscript.
Table 1 Experimental conditions employed for the growth of GaN pyramids on FLG substrates by CVD. Type
1 2
Samples
S1 S2 S3 S4 S5 S6
Growth Conditions Growth Temperature (� C)
N2 Flow Rate (SCCM)
Precursor-toSubstrate Distance (cm)
Growth Time (min)
900
500
3 3 3 3 5 7
60 90 120 90 90 90
2. Experimental methods 2.1. Synthesis of few layer graphene Graphite powder (99.999%) was used as the source of carbon. Ethanol was used as solvent for the preparation of FLG. The FLG was prepared by liquid exfoliation technique as described elsewhere [13]. In a typical process, 0.5 g of graphite powder was dispersed in 10 ml of ethanol and the mixture is subjected to ultra-sonication for 120 min. The obtained product was dried and used for electrospinning. 2.2. Deposition of few layer graphene by electrospinning technique The FLG powder, acetic acid, N,N-dimethylformamide (DMF), poly vinyl pyrrolidone (PVP) and ethanol were the materials used for pre paring the electrospinning solution. The solution was prepared in two steps [i.e.] 0.5 g of FLG powder is first dispersed into a mixture con taining ethanol (10 ml) and acetic acid (3 ml) and subjected to magnetic stirring for 60 min. Simultaneously a mixture containing ethanol (10 ml), DMF (2 ml) and PVP (1.5 g) was separately prepared under magnetic stirring for 30 min. Both the solutions were mixed together and stirred further for another 120 min in order to obtain a uniform blend. The obtained mixture was then transferred to a 5 ml syringe attached to a syringe pump. The syringe consists of stainless steel needle which serves as a positive terminal. The c-plane sapphire substrates of size 1 cm2 was initially cleaned by standard solvent evaporation technique as described elsewhere [13] and placed on an electrically grounded aluminium foil at the collector end. The flow rate of the syringe pump was maintained at 1 ml/h. The electrospinning voltage and distance of separation between the needle and collector was 20 kV and 15 cm respectively. The charged FLG solution was then electrospinned on to the sapphire substrates for 120 min (hereafter referred as FLG substrates).
Fig. 2. XRD pattern of the GaN pyramids grown by varying the growth time (S1, S2 and S3) and precursor-to-substrate distance (S4, S5 and S6) respectively.
Fig. 3. SEM images representing the: (a) electrospun FLG and (b) effect of pregrowth annealing process at 900 � C.
2
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Materials Chemistry and Physics 240 (2020) 122189
Fig. 5. Elemental maps revealing (a) gallium, (b) nitrogen, (c) carbon and (d) oxygen contents in sample S3.
Fig. 6. (a) XPS survey spectrum with the individual spectra of GaN pyramids representing: (b) gallium, (c) nitrogen, (d) carbon and (e) oxygen respectively.
to anneal prior to GaN growth. The temperature of the reaction zone and carrier gas flow rate was maintained at 900 � C and 500 standard cubic centimetre per minute (SCCM) respectively. Post growth, the furnace was allowed to cool down naturally under nitrogen ambience. 2.4. Fabrication of photodetectors Metal-semiconductor-metal (MSM) photodetectors were then formed on the GaN pyramids using standard optical lithography, metallization and lift-off procedures. Au metal of 100 nm thickness was used for the Schottky contact and was deposited using sputtering system. 2.5. Material characterization The structural characteristics of the GaN pyramids were studied using X-ray diffractometer (XRD, PAN analytical X’Pert PRO). The sur face morphologies of the GaN pyramids were investigated using scan ning electron microscopy (SEM, Zeiss EVO 18). The elemental contents and maps were recorded using energy dispersive X-ray (EDX) which revealed the elemental compositions and their distributions in the samples. The surface compositions of the samples were obtained using X-ray photoelectron spectroscopy (XPS) beamline. The cath odoluminescence (CL) studies were performed using field emission scanning electron microscopy (FESEM, Ultra 55). The electrical and optical performances of the fabricated photodetectors were performed using a solar simulator (Newport 91160A).
Fig. 7. CL spectra of GaN pyramids grown by varying the growth time (S1, S2 and S3) and precursor-to-substrate distance (S4, S5 and S6) respectively.
2.3. Growth of GaN pyramids by chemical vapour deposition technique Growth of GaN pyramids on FLG substrates was carried out in a horizontal flow CVD reactor by manipulating the growth time and precursor-to-substrate distance (Table 1). Gallium (Ga) metal (99.99%) and liquid ammonia (NH3) were used as the precursors for gallium and nitrogen (N). Nitrogen (N2) gas was employed as a carrier gas. About 0.3 g of Ga and FLG substrates were loaded into the reactor and sealed to prevent the samples from extrinsic contaminations. The reactor was then gradually ramped at 5 � C/min to attain a growth temperature of 900 � C and is maintained at that temperature for 30 min for the FLG substrates
3. Results and discussion 3.1. Structural characteristics The crystalline information of the GaN pyramids grown by varying the growth time and precursor-to-substrate distance were examined using XRD. Fig. 2 represents the 2θ-scans of the samples, where domi nant diffraction peaks corresponding to GaN at (1 0 0), (0 0 2), (1 0 1) orientations were detected in the 25� -85� measurement range. 3
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Materials Chemistry and Physics 240 (2020) 122189
Fig. 8. (a) I–V characteristics in linear scale and (b) photocurrent response of the fabricated GaN photodetector at a bias voltage of 0.4 V. Fig (b) inset represents the rise and fall time of the fabricated photodetector. Table 2 Electrical and optical performances of the GaN photodetectors utilized in the present study. Material
FLG/GaN based photodetector
Electrical Performances
Optical Performances
ϕB (eV)
η
Sensitivity %
Responsivity (A/W)
Detectivity (cm Hz0.5 W-1)
Rise time (s)
Fall time (s)
0.83
0.93
40
14 (0.4V)
3 x 1010
9
49
Table 3 A comparison of the optical performances of different GaN photodetector available in literature. Photodetectors Without graphene interlayer
With graphene interlayer
GaN Nanowire GaN Nanowire GaN Nanowire GaN Nanowire RGO/GaN Graphene/GaN FLG/GaN pyramids
Optical Characteristics
Ref
Responsivity (A/W)
Detectivity (cm Hz0.5 W-1)
Rise time
Fall time
0.003 2.2 x 104 – 70.4 0.00154 – 14
– – – – 1.45 x 1010 – 3 x 1010
0.3 s <26 m s – – 60 s 0.2 m s 9
– <26 m s >120 s – 267 s 0.4 m s 49
All the indexed peaks represent hexagonal crystal structure of GaN. It was seen that, the full width at half maximum of the GaN samples grown on FLG provide better crystallinity in comparison to our previous report [14]. This is a clear confirmation that the presence of FLG certainly improved the crystalline quality. There were no additional XRD peaks relating to FLG and supporting substrates being detected, indicating that the grown GaN pyramids are comparatively thick.
[28] [29] [30] [31] [32] [33] This work
The formation of GaN pyramids can be attributed to the decompo sition of FLG layer underneath GaN. This is because, prior to GaN growth, the pre-growth annealing process at 900 � C could have been the reason for self-decomposition of FLG. This phenomenon tends to pro duce pyramid shaped graphene masks [15]. During the GaN growth process, the Ga precursor used, favours the decomposition of FLG. In order to control the decomposition of FLG, NH3 was used. This is because, in the present study, NH3 is employed as N precursor for the growth of GaN. When NH3 gets decomposed into N at the growth tem perature, it not only facilitates the growth of GaN but also contributes in controlling the FLG from getting decomposed completely. The growth process formulated is in good agreement with the literature [15]. In respect of C atoms, it is worth to note that, during FLG decom position at the GaN/FLG interface, the C atoms from the underlying FLG layer gets incorporated into the top GaN layer. In such a case, C atoms tend to replace N atoms present in the GaN. Another possibility could be, when the NH3 reacts with Ga, it directly or indirectly acts as N dopant to the underlying FLG layer as reported elsewhere [16,17]. Consequently, both these processes (incorporation of N and C atoms) tend to produce deep level emissions (DLE) in GaN as shown in the CL spectra (Fig. 7). In respect of O, it is important to take into account the initial growth process for clear understanding. This is because, there is a possibility that, the residual oxygen present in the CVD reactor could have contributed towards the presence of O. Consequently, this transforms the Ga metal (precursor) into gallium oxide (GaOx) complex with vari ations in stoichiometries prior to the growth of GaN. Since Ga metal melts at room temperature, incorporation of O into molten Ga occurs
3.2. Morphological characteristics Fig. 3(a) shows the surface morphology of the FLG substrates ob tained from SEM. The SEM image emphasize that, the obtained morphology was mostly smooth. Prior to the GaN growth, the pregrowth annealing process of FLG substrates for 30 min at 900 � C in the CVD reactor, tend to transform the morphology from fibers to film like pattern as shown in Fig. 3(b). Fig. 4 shows the surface morphologies of the GaN pyramids obtained from SEM. The SEM images revealed numerous GaN pyramids of submicrometer size covering the entire FLG substrates. As mentioned earlier, growth was carried out in two types by varying: (a) growth time (samples S1, S2 and S3) and (b) precursor-to-substrate distance (samples S4, S5 and S6) respectively. Irrespective of changes in the growth pa rameters, no perceptible changes in the GaN pyramids were noticed. However, the distribution of pyramids across each surface (samples S1–S6) were typically found to be non-uniform. The EDX maps shown in Fig. 5, compliments the XRD data. In addition, carbon (C) and oxygen (O) traces were also observed from the EDX maps. 4
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100 mW/cm2 was utilized for the measurement of photo current. Since the work function of Au metal (5.1 eV) utilized is higher than that of the electron affinity of GaN (4.2 eV), only a Schottky contact is expected to form. The effective charge transfer across the metal and semiconductor interface (MSI) were determined by calculating the ide ality factor (η) and barrier height (ϕB) between the metal and semi conductor junctions by utilizing the same parameters as reported elsewhere [14,27]. The barrier height and ideality factor of the GaN photodetector was estimated as 0.83 eV and 0.93 respectively (Table 2). The barrier height obtained is in good agreement with the theoretical limit determined using Schottky Mott model, which emphasizes the absence of any surface or barrier related inhomogeneities [27]. Simi larly, the ideality factor obtained is close to unity [14], which illustrates that there are no interfacial surface states which act as recombination centres and favour trap-assisted tunnelling. The photocurrent behaviour in respect of the GaN photodetector during photoconductivity measurements is shown in Fig. 8(b). In order to quantitatively determine the efficiency of an optoelectronic device, it is necessary to take into consideration the three figure of merits such as sensitivity (S), responsivity (R) and detectivity (D) of an optoelectronic device [34]. From the obtained photocurrent measurement, the sensi tivity of the photodetector was estimated as 40%. A detector respon sivity of about 14 AW-1 was measured for an active light illumination area of 1 � 0.2 cm2 with a light intensity of 100 mW/cm2. Using the photo response, the detectivity of the device was estimated around 3 � 1010 cm Hz0.5 W-1. The optical performances of the fabricated GaN photodetector in terms of their sensitivity, responsivity, detectivity, rise time and fall time (Fig. 7(b) inset) have been tabulated in Table 2. Also, comparison of different one-dimensional and two-dimensional GaN based photodetector both in the presence and absence of graphene interlayer available in the literature as well as estimated in the present study are tabulated in Table 3.
rapidly. This process tends to produce GaOx droplets on FLG substrates during the initial growth process. It is to be noted that, these GaOx droplets play a significant role as nucleation sites for the growth of GaN. With elapse in the growth time, N atoms replace the O atoms leading to the formation of GaN as reported elsewhere [18]. In practical cases, the replacement will not occur completely [i.e.] there will be some amount of O atoms still remains in GaN. In such a case, there will be both O and N atoms present in GaN thereby generating DLE as shown in CL spectra (Fig. 7). 3.3. Surface characteristics The surface elemental composition of the GaN pyramids was deter mined from XPS data. Fig. 6 shows dominant diffraction peaks in terms of binding energy collected over a wide scan range of 0–1200 eV, rep resenting the core levels of Ga (Ga-3d, 3p, 3s and 2p), N (1s), C (1s) and O (1s) respectively. The 2p core level of Ga corresponding to 2p3/2 and 2p1/2 states were identified at 1117 eV and 1144 eV respectively. The 2p state of Ga indicates the existence of predominant Ga–N bonding. Due to the presence of Ga–Ga metallic bonding on the surface, a slight shift in the elemental Ga was observed as shown in Fig. 5(b) [19]. In addition, LMM auger peaks in respect of Ga was also observed. This can be attributed to the radiation less transition occurring from the electrostatic interaction between two electrons in a singly ionized atom [20]. The core level of C at 284.7 eV can be attributed to the C–N bonding. The presence of N at 398 eV reveals the N dopant in graphene. The O core-level from GaN reveal a little amount of Ga–O bonding. This type of bonding (metal-oxygen) arise from the native oxide present on surface of the GaN nucleation sites [19]. The identified core levels indicate that the GaN pyramids grown at different growth conditions exhibit hexagonal crystal structure. 3.4. Luminescence characteristics
4. Conclusion
The GaN pyramids grown on FLG at different growth conditions were tested using CL spectra as shown in Fig. 7. Two emission bands corre sponding to band edge emission (BEE) at 355 nm and DLE between 600 and 800 nm were noticed. The BEE peak arises due to the band to band transition resulting from electron-hole pair recombination. The DLE occurs as a consequence of point defects relating to O and Ga vacancies. The strong BEE peak reveals that the GaN pyramids possess good optical quality. Whereas, the presence of FLG underneath GaN pyramids could also be a reason for observing weak DLE related peak [21]. It was noticed that, in respect of variation in both growth time (samples S1–S3) and precursor-to-substrate (samples S4–S6), the GaN pyramid did not reveal any significant change in the peak position of BEE. Variations related to peak intensities were observed which can be ascribed to the size effect (high surface-to-volume ratio). Based on Ogino and Aoki proposal, doping GaN with C increases DLE resulting in the parasitic luminescence effect [22]. They proposed that the DLE is attributed to a shallow donor (Ga vacancy) and a deep acceptor (C) transition. Later Glaser et al. [23] proposed an alternate model where, the DLE is attributed to a deep donor (Ga vacancy) and a shallow acceptor (C) transition. In both the cases, C substitutes for one of the N atoms around the Ga vacancy. It is worth to note that, the presence of FLG underneath GaN not only decreased the DLE but also has improved the optical quality (BEE) in GaN [24], when compared with the reports published earlier with respect to the growth of GaN NWs on sapphire substrate (without graphene interlayer) [25].
Gallium nitride pyramids were grown on few layer graphene sub strates at different growth conditions. This method of few layer gra phene synthesis and deposition were found to be economic and simple. The quality of gallium nitride on few layer graphene substrates had a positive effect on the luminescence properties. The effect of few layer graphene has improved the band edge emission and helped in sup pressing the deep level emission in gallium nitride. The barrier height and ideality factor of the GaN photodetectors emphasizes the absence of any surface or barrier related inhomogeneities and interfacial surface states. In addition, the fabricated photodetector exhibits enhanced responsivity, detectivity and fast rise time and fall time. This emphasizes that the synthesized gallium nitride pyramids can be directly utilized for next era optical and electronic devices. Acknowledgment The CL studies were performed at Centre for Nano Science and En gineering (CeNSE), under Indian Nanoelectronics Users Program (INUP), funded by Ministry of Electronics and Information technology (MeitY), Govt. of India, located at the Indian Institute of Science, Ben galuru. The XPS studies were performed at Raja Ramanna Centre for Advanced Technology (RRCAT) Indus-2 (BL-14), Indore, Madhya Pradesh. References
3.5. Electrical characteristics
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Epitaxy of gallium nitride pyramids on few layer graphene for metal-semiconductor-metal based photodetectors S. Sanjay a, b, *, K. Prabakaran b, K. Baskar b a b
Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation, Hyderabad, 500 075, Telangana, India Crystal Growth Centre, Anna University, Chennai, 600 025, Tamil Nadu, India
H I G H L I G H T S
� Fabrication of MSM photodetectors on GaN pyramids grown by CVD. � The barrier height emphasizes the absence of any surface or barrier related inhomogeneities. � The ideality factor illustrates that there are no interfacial surface states. � The fabricated photodetector exhibits enhanced electrical and optical performances. A R T I C L E I N F O
A B S T R A C T
Keywords: Gallium nitride pyramids Few layer graphene Electrospinning Chemical vapour deposition
Growth of gallium nitride pyramids was carried out on few layer graphene substrates with different growth parameters such as growth time and precursor-to-substrate distance. The few layer graphene was synthesized by ultra-sonicating graphite and deposited on c-plane sapphire substrates by electrospinning technique. The indexed diffraction peaks obtained from X-ray diffractometer revealed good crystalline quality gallium nitride with a hexagonal crystal structure. The surface compositions of the samples have been studied by the X-ray photo electron spectroscopy. The scanning electron microscopy results revealed the formation of gallium nitride pyr amid structures all over the samples. The cathodoluminescence spectra showcased an improved band edge emission at 355 nm with weak deep level emissions corresponding to gallium nitride. The quality of gallium nitride obtained in the presence of few layer graphene had a positive effect on the luminescence properties, electrical and optical device performances.
1. Introduction The wide direct bandgap (Eg ¼ 3.4 eV) of gallium nitride (GaN) and its ability to practically tune the bandgap between near infra-red and deep ultra-violet regime when formed as ternary alloy with indium nitride (InN, Eg ¼ ~0.7 eV) and aluminium nitride (AlN, Eg ¼ 6.2 eV), has made GaN the most widely explored semiconductor material. Other fascinating properties of GaN include high carrier mobility, chemical stability and electrical breakdown field. All these innate qualities make GaN a suitable material for applications in electronic and optical devices [1–4]. Usually GaN is hetero-epitaxially grown on non-native substrates (silicon, sapphire and silicon carbide) with a certain lattice and thermal mismatch. Conventionally, sapphire is most commonly preferred for the
epitaxial growth of GaN. The hexagonal crystal structure of sapphire which is like that of crystal structure of GaN is the main reason for preferring sapphire substrates despite their high lattice mismatch with GaN [5–7]. These mismatches tend to induce defects and dislocations, thereby degrading material properties and device performances when fabricated. To avoid such constraints, it is beneficial to grow GaN in the form of nanostructures rather than as epilayers [8–10]. Despite scaling in the dimensionality, the insulating nature of sap phire substrates restricts the electrical performance of the GaN layer. To overcome this challenge, a simple solution could be to employ twodimensional materials such as graphene or boron nitride [11,12]. Over the past decade, intense research is being focussed on utilizing graphene, as a substrate material for enhancing the material properties of GaN [11]. This type of growth process was found to improve the electrical
* Corresponding author. Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation, Hyderabad, 500 075, Telangana, India. E-mail address: [email protected] (S. Sanjay). https://doi.org/10.1016/j.matchemphys.2019.122189 Received 3 July 2019; Received in revised form 7 September 2019; Accepted 15 September 2019 Available online 17 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic representation of fabrication and examination of GaN pyra mids based MSM photodetectors. (a) sapphire substrate, (b) FLG deposited by electrospinning, (c) GaN pyramids grown by CVD technique, (e) fabricated MSM photodetector and (d) magnified image corresponding to (c) representing GaN pyramids grown on FLG substrates.
Fig. 4. SEM images representing: (a) sample S1, (b) S2, (c) S3, (d) S4, (e) S5 and (f) S6 respectively.
characteristics and help in overcoming the lattice and thermal mismatch problems between substrate and GaN. Unlike the conventional procedures involving synthesis of single layer graphene (SLG) on copper foil by chemical vapour deposition (CVD) followed by its transfer on desired substrate, the present study utilizes few layer graphene (FLG) instead of SLG. The FLG was synthe sized by liquid exfoliation technique and was electro-spun on sapphire substrates. This type of synthesis and deposition process proved to be economic and does not require any post growth transfer process. The GaN pyramids growth was carried out on FLG substrates at different growth conditions. The photoconductive devices were fabricated on these GaN samples to evaluate their electronic and optical perfor mances. A schematic representation of the device fabrication and ex amination thereon is shown in Fig. 1. To the best of our knowledge, this method of synthesis route and experimental conditions have not been reported earlier. The effect of FLG on GaN growth and the resultant growth conditions are briefly discussed in this manuscript.
Table 1 Experimental conditions employed for the growth of GaN pyramids on FLG substrates by CVD. Type
1 2
Samples
S1 S2 S3 S4 S5 S6
Growth Conditions Growth Temperature (� C)
N2 Flow Rate (SCCM)
Precursor-toSubstrate Distance (cm)
Growth Time (min)
900
500
3 3 3 3 5 7
60 90 120 90 90 90
2. Experimental methods 2.1. Synthesis of few layer graphene Graphite powder (99.999%) was used as the source of carbon. Ethanol was used as solvent for the preparation of FLG. The FLG was prepared by liquid exfoliation technique as described elsewhere [13]. In a typical process, 0.5 g of graphite powder was dispersed in 10 ml of ethanol and the mixture is subjected to ultra-sonication for 120 min. The obtained product was dried and used for electrospinning. 2.2. Deposition of few layer graphene by electrospinning technique The FLG powder, acetic acid, N,N-dimethylformamide (DMF), poly vinyl pyrrolidone (PVP) and ethanol were the materials used for pre paring the electrospinning solution. The solution was prepared in two steps [i.e.] 0.5 g of FLG powder is first dispersed into a mixture con taining ethanol (10 ml) and acetic acid (3 ml) and subjected to magnetic stirring for 60 min. Simultaneously a mixture containing ethanol (10 ml), DMF (2 ml) and PVP (1.5 g) was separately prepared under magnetic stirring for 30 min. Both the solutions were mixed together and stirred further for another 120 min in order to obtain a uniform blend. The obtained mixture was then transferred to a 5 ml syringe attached to a syringe pump. The syringe consists of stainless steel needle which serves as a positive terminal. The c-plane sapphire substrates of size 1 cm2 was initially cleaned by standard solvent evaporation technique as described elsewhere [13] and placed on an electrically grounded aluminium foil at the collector end. The flow rate of the syringe pump was maintained at 1 ml/h. The electrospinning voltage and distance of separation between the needle and collector was 20 kV and 15 cm respectively. The charged FLG solution was then electrospinned on to the sapphire substrates for 120 min (hereafter referred as FLG substrates).
Fig. 2. XRD pattern of the GaN pyramids grown by varying the growth time (S1, S2 and S3) and precursor-to-substrate distance (S4, S5 and S6) respectively.
Fig. 3. SEM images representing the: (a) electrospun FLG and (b) effect of pregrowth annealing process at 900 � C.
2
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Fig. 5. Elemental maps revealing (a) gallium, (b) nitrogen, (c) carbon and (d) oxygen contents in sample S3.
Fig. 6. (a) XPS survey spectrum with the individual spectra of GaN pyramids representing: (b) gallium, (c) nitrogen, (d) carbon and (e) oxygen respectively.
to anneal prior to GaN growth. The temperature of the reaction zone and carrier gas flow rate was maintained at 900 � C and 500 standard cubic centimetre per minute (SCCM) respectively. Post growth, the furnace was allowed to cool down naturally under nitrogen ambience. 2.4. Fabrication of photodetectors Metal-semiconductor-metal (MSM) photodetectors were then formed on the GaN pyramids using standard optical lithography, metallization and lift-off procedures. Au metal of 100 nm thickness was used for the Schottky contact and was deposited using sputtering system. 2.5. Material characterization The structural characteristics of the GaN pyramids were studied using X-ray diffractometer (XRD, PAN analytical X’Pert PRO). The sur face morphologies of the GaN pyramids were investigated using scan ning electron microscopy (SEM, Zeiss EVO 18). The elemental contents and maps were recorded using energy dispersive X-ray (EDX) which revealed the elemental compositions and their distributions in the samples. The surface compositions of the samples were obtained using X-ray photoelectron spectroscopy (XPS) beamline. The cath odoluminescence (CL) studies were performed using field emission scanning electron microscopy (FESEM, Ultra 55). The electrical and optical performances of the fabricated photodetectors were performed using a solar simulator (Newport 91160A).
Fig. 7. CL spectra of GaN pyramids grown by varying the growth time (S1, S2 and S3) and precursor-to-substrate distance (S4, S5 and S6) respectively.
2.3. Growth of GaN pyramids by chemical vapour deposition technique Growth of GaN pyramids on FLG substrates was carried out in a horizontal flow CVD reactor by manipulating the growth time and precursor-to-substrate distance (Table 1). Gallium (Ga) metal (99.99%) and liquid ammonia (NH3) were used as the precursors for gallium and nitrogen (N). Nitrogen (N2) gas was employed as a carrier gas. About 0.3 g of Ga and FLG substrates were loaded into the reactor and sealed to prevent the samples from extrinsic contaminations. The reactor was then gradually ramped at 5 � C/min to attain a growth temperature of 900 � C and is maintained at that temperature for 30 min for the FLG substrates
3. Results and discussion 3.1. Structural characteristics The crystalline information of the GaN pyramids grown by varying the growth time and precursor-to-substrate distance were examined using XRD. Fig. 2 represents the 2θ-scans of the samples, where domi nant diffraction peaks corresponding to GaN at (1 0 0), (0 0 2), (1 0 1) orientations were detected in the 25� -85� measurement range. 3
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Fig. 8. (a) I–V characteristics in linear scale and (b) photocurrent response of the fabricated GaN photodetector at a bias voltage of 0.4 V. Fig (b) inset represents the rise and fall time of the fabricated photodetector. Table 2 Electrical and optical performances of the GaN photodetectors utilized in the present study. Material
FLG/GaN based photodetector
Electrical Performances
Optical Performances
ϕB (eV)
η
Sensitivity %
Responsivity (A/W)
Detectivity (cm Hz0.5 W-1)
Rise time (s)
Fall time (s)
0.83
0.93
40
14 (0.4V)
3 x 1010
9
49
Table 3 A comparison of the optical performances of different GaN photodetector available in literature. Photodetectors Without graphene interlayer
With graphene interlayer
GaN Nanowire GaN Nanowire GaN Nanowire GaN Nanowire RGO/GaN Graphene/GaN FLG/GaN pyramids
Optical Characteristics
Ref
Responsivity (A/W)
Detectivity (cm Hz0.5 W-1)
Rise time
Fall time
0.003 2.2 x 104 – 70.4 0.00154 – 14
– – – – 1.45 x 1010 – 3 x 1010
0.3 s <26 m s – – 60 s 0.2 m s 9
– <26 m s >120 s – 267 s 0.4 m s 49
All the indexed peaks represent hexagonal crystal structure of GaN. It was seen that, the full width at half maximum of the GaN samples grown on FLG provide better crystallinity in comparison to our previous report [14]. This is a clear confirmation that the presence of FLG certainly improved the crystalline quality. There were no additional XRD peaks relating to FLG and supporting substrates being detected, indicating that the grown GaN pyramids are comparatively thick.
[28] [29] [30] [31] [32] [33] This work
The formation of GaN pyramids can be attributed to the decompo sition of FLG layer underneath GaN. This is because, prior to GaN growth, the pre-growth annealing process at 900 � C could have been the reason for self-decomposition of FLG. This phenomenon tends to pro duce pyramid shaped graphene masks [15]. During the GaN growth process, the Ga precursor used, favours the decomposition of FLG. In order to control the decomposition of FLG, NH3 was used. This is because, in the present study, NH3 is employed as N precursor for the growth of GaN. When NH3 gets decomposed into N at the growth tem perature, it not only facilitates the growth of GaN but also contributes in controlling the FLG from getting decomposed completely. The growth process formulated is in good agreement with the literature [15]. In respect of C atoms, it is worth to note that, during FLG decom position at the GaN/FLG interface, the C atoms from the underlying FLG layer gets incorporated into the top GaN layer. In such a case, C atoms tend to replace N atoms present in the GaN. Another possibility could be, when the NH3 reacts with Ga, it directly or indirectly acts as N dopant to the underlying FLG layer as reported elsewhere [16,17]. Consequently, both these processes (incorporation of N and C atoms) tend to produce deep level emissions (DLE) in GaN as shown in the CL spectra (Fig. 7). In respect of O, it is important to take into account the initial growth process for clear understanding. This is because, there is a possibility that, the residual oxygen present in the CVD reactor could have contributed towards the presence of O. Consequently, this transforms the Ga metal (precursor) into gallium oxide (GaOx) complex with vari ations in stoichiometries prior to the growth of GaN. Since Ga metal melts at room temperature, incorporation of O into molten Ga occurs
3.2. Morphological characteristics Fig. 3(a) shows the surface morphology of the FLG substrates ob tained from SEM. The SEM image emphasize that, the obtained morphology was mostly smooth. Prior to the GaN growth, the pregrowth annealing process of FLG substrates for 30 min at 900 � C in the CVD reactor, tend to transform the morphology from fibers to film like pattern as shown in Fig. 3(b). Fig. 4 shows the surface morphologies of the GaN pyramids obtained from SEM. The SEM images revealed numerous GaN pyramids of submicrometer size covering the entire FLG substrates. As mentioned earlier, growth was carried out in two types by varying: (a) growth time (samples S1, S2 and S3) and (b) precursor-to-substrate distance (samples S4, S5 and S6) respectively. Irrespective of changes in the growth pa rameters, no perceptible changes in the GaN pyramids were noticed. However, the distribution of pyramids across each surface (samples S1–S6) were typically found to be non-uniform. The EDX maps shown in Fig. 5, compliments the XRD data. In addition, carbon (C) and oxygen (O) traces were also observed from the EDX maps. 4
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100 mW/cm2 was utilized for the measurement of photo current. Since the work function of Au metal (5.1 eV) utilized is higher than that of the electron affinity of GaN (4.2 eV), only a Schottky contact is expected to form. The effective charge transfer across the metal and semiconductor interface (MSI) were determined by calculating the ide ality factor (η) and barrier height (ϕB) between the metal and semi conductor junctions by utilizing the same parameters as reported elsewhere [14,27]. The barrier height and ideality factor of the GaN photodetector was estimated as 0.83 eV and 0.93 respectively (Table 2). The barrier height obtained is in good agreement with the theoretical limit determined using Schottky Mott model, which emphasizes the absence of any surface or barrier related inhomogeneities [27]. Simi larly, the ideality factor obtained is close to unity [14], which illustrates that there are no interfacial surface states which act as recombination centres and favour trap-assisted tunnelling. The photocurrent behaviour in respect of the GaN photodetector during photoconductivity measurements is shown in Fig. 8(b). In order to quantitatively determine the efficiency of an optoelectronic device, it is necessary to take into consideration the three figure of merits such as sensitivity (S), responsivity (R) and detectivity (D) of an optoelectronic device [34]. From the obtained photocurrent measurement, the sensi tivity of the photodetector was estimated as 40%. A detector respon sivity of about 14 AW-1 was measured for an active light illumination area of 1 � 0.2 cm2 with a light intensity of 100 mW/cm2. Using the photo response, the detectivity of the device was estimated around 3 � 1010 cm Hz0.5 W-1. The optical performances of the fabricated GaN photodetector in terms of their sensitivity, responsivity, detectivity, rise time and fall time (Fig. 7(b) inset) have been tabulated in Table 2. Also, comparison of different one-dimensional and two-dimensional GaN based photodetector both in the presence and absence of graphene interlayer available in the literature as well as estimated in the present study are tabulated in Table 3.
rapidly. This process tends to produce GaOx droplets on FLG substrates during the initial growth process. It is to be noted that, these GaOx droplets play a significant role as nucleation sites for the growth of GaN. With elapse in the growth time, N atoms replace the O atoms leading to the formation of GaN as reported elsewhere [18]. In practical cases, the replacement will not occur completely [i.e.] there will be some amount of O atoms still remains in GaN. In such a case, there will be both O and N atoms present in GaN thereby generating DLE as shown in CL spectra (Fig. 7). 3.3. Surface characteristics The surface elemental composition of the GaN pyramids was deter mined from XPS data. Fig. 6 shows dominant diffraction peaks in terms of binding energy collected over a wide scan range of 0–1200 eV, rep resenting the core levels of Ga (Ga-3d, 3p, 3s and 2p), N (1s), C (1s) and O (1s) respectively. The 2p core level of Ga corresponding to 2p3/2 and 2p1/2 states were identified at 1117 eV and 1144 eV respectively. The 2p state of Ga indicates the existence of predominant Ga–N bonding. Due to the presence of Ga–Ga metallic bonding on the surface, a slight shift in the elemental Ga was observed as shown in Fig. 5(b) [19]. In addition, LMM auger peaks in respect of Ga was also observed. This can be attributed to the radiation less transition occurring from the electrostatic interaction between two electrons in a singly ionized atom [20]. The core level of C at 284.7 eV can be attributed to the C–N bonding. The presence of N at 398 eV reveals the N dopant in graphene. The O core-level from GaN reveal a little amount of Ga–O bonding. This type of bonding (metal-oxygen) arise from the native oxide present on surface of the GaN nucleation sites [19]. The identified core levels indicate that the GaN pyramids grown at different growth conditions exhibit hexagonal crystal structure. 3.4. Luminescence characteristics
4. Conclusion
The GaN pyramids grown on FLG at different growth conditions were tested using CL spectra as shown in Fig. 7. Two emission bands corre sponding to band edge emission (BEE) at 355 nm and DLE between 600 and 800 nm were noticed. The BEE peak arises due to the band to band transition resulting from electron-hole pair recombination. The DLE occurs as a consequence of point defects relating to O and Ga vacancies. The strong BEE peak reveals that the GaN pyramids possess good optical quality. Whereas, the presence of FLG underneath GaN pyramids could also be a reason for observing weak DLE related peak [21]. It was noticed that, in respect of variation in both growth time (samples S1–S3) and precursor-to-substrate (samples S4–S6), the GaN pyramid did not reveal any significant change in the peak position of BEE. Variations related to peak intensities were observed which can be ascribed to the size effect (high surface-to-volume ratio). Based on Ogino and Aoki proposal, doping GaN with C increases DLE resulting in the parasitic luminescence effect [22]. They proposed that the DLE is attributed to a shallow donor (Ga vacancy) and a deep acceptor (C) transition. Later Glaser et al. [23] proposed an alternate model where, the DLE is attributed to a deep donor (Ga vacancy) and a shallow acceptor (C) transition. In both the cases, C substitutes for one of the N atoms around the Ga vacancy. It is worth to note that, the presence of FLG underneath GaN not only decreased the DLE but also has improved the optical quality (BEE) in GaN [24], when compared with the reports published earlier with respect to the growth of GaN NWs on sapphire substrate (without graphene interlayer) [25].
Gallium nitride pyramids were grown on few layer graphene sub strates at different growth conditions. This method of few layer gra phene synthesis and deposition were found to be economic and simple. The quality of gallium nitride on few layer graphene substrates had a positive effect on the luminescence properties. The effect of few layer graphene has improved the band edge emission and helped in sup pressing the deep level emission in gallium nitride. The barrier height and ideality factor of the GaN photodetectors emphasizes the absence of any surface or barrier related inhomogeneities and interfacial surface states. In addition, the fabricated photodetector exhibits enhanced responsivity, detectivity and fast rise time and fall time. This emphasizes that the synthesized gallium nitride pyramids can be directly utilized for next era optical and electronic devices. Acknowledgment The CL studies were performed at Centre for Nano Science and En gineering (CeNSE), under Indian Nanoelectronics Users Program (INUP), funded by Ministry of Electronics and Information technology (MeitY), Govt. of India, located at the Indian Institute of Science, Ben galuru. The XPS studies were performed at Raja Ramanna Centre for Advanced Technology (RRCAT) Indus-2 (BL-14), Indore, Madhya Pradesh. References
3.5. Electrical characteristics
[1] T. Hashimoto, F. Wu, J.S. Speck, S. Nakamura, A GaN bulk crystal with improved structural quality grown by the ammonothermal method, Nat. Mater. 6 (2007) 568–571. [2] S. Krishna, N. Aggarwal, A. Gundimeda, A. Sharma, S. Husale, K.K. Maurya, G. Gupta, Correlation of donor-acceptor pair emission on the performance of GaN based UV photodetector, Mater. Sci. Semicond. Process. 98 (2019) 59–64.
The current vs voltage (I–V) measurements at an applied bias voltage of 0.4 V were carried out in respect of the GaN photodetector under dark and bright conditions as shown in Fig. 8(a). As reported elsewhere [26], solar simulator with a white light source, with a light intensity of 5
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