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Fabrication of Schottky barrier diodes on clump of gallium nitride nanowires grown by chemical vapour deposition
Applied Surface Science 456 (2018) 526–531
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Fabrication of Schottky barrier diodes on clump of gallium nitride nanowires grown by chemical vapour deposition ⁎
S. Sanjaya, , K. Baskara,b, a b
T
⁎
Crystal Growth Centre, Anna University, Chennai 600 025, India Manonmaniam Sundaranar University, Tirunelveli 627 012, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Gallium nitride nanowires Chemical vapour deposition Vapour liquid solid process Schottky barrier diodes
In this study, Schottky barrier diodes on ‘clump of nanowires’ has been fabricated and its electrical behaviour on variation of distance of separation between Ohmic (Ti/Al/Ni/Au) and Schottky (Ni/Au) contacts has been studied. The variations in current under dark and bright conditions have been investigated. High quality gallium nitride nanowires were grown by chemical vapour deposition technique using gold-palladium alloy as catalyst. The novelty of this work is that, the barrier height remains unaffected irrespective of any variations in the ideality factor. Also, the method of lithography during fabrication did not give rise to any surface related inhomogeneities.
1. Introduction The fascinating properties of gallium nitride nanostructures (GaN NS) include wide direct band gap, huge surface area [1,2], very high light absorption and extraction capability [3,4], high quantum confinement [5], tendency to grow on non-native substrates [6,7], flexibility to doping (both p- and n-type) [8,9] and tolerance towards harsh environments. In the last few years, efforts have been made to grow different forms of GaN NS, especially nanowires (NWs) and to utilize them in various optical and electronic nanoscale devices such as high electron mobility transistors (HEMTs) [10], field effect transistors (FETs) [11], p-n junction diodes [12], laser diodes [13], light emitting diodes (LEDs) [14], nano generators [15], solar cells [16] and photodetectors [17]. For the growth of GaN NWs, chemical vapour deposition (CVD) technique is being conventionally preferred due to its easiness, capability to grow good quality NWs on non-native substrates and ability to tune the NW thickness [1,3,18,19]. However, the NW growth using CVD technique is better achieved only in the presence of a metal catalyst where the growth process is governed by a well-known vapourliquid-solid (VLS) mechanism [20]. There were intense research focussing on the growth of GaN NWs by using single metal catalyst such as gold, platinum, nickel, copper, cobalt, magnesium, indium and iron. In a recent study, we have substituted the single metal catalyst with binary metal alloy (gold-palladium) for the growth of GaN NWs. Unlike single metal catalyst, the usage of binary catalytic alloy was found to tune the diameter and luminescence properties of the NWs and also
⁎
helped in achieving better growth rate at different growth conditions [21,22]. Over the past two decades, efforts were being made to fabricate nanoscale devices comprising of epitaxial film or single NW. However, fabrication of epitaxial film based on nanoscale device is considered easier compared to the other because single NW fabrication suffers from complex sample preparations, lithographic process and less yield. These demerits make them unsuitable for mass production [23]. To overcome these limitations, a method of fabricating clump of NWs was developed. This method of fabrication was found to improve the yield, fabrication procedures and ensured large scale productivity [23,24]. In spite of these merits; the varying lengths and thicknesses of NWs, random alignment of NWs during growth and the charge transport across metalsemiconductor interface (MSI) due to size effect are some of the critical issues. These limitations always degrade the device performance and make them technologically unfit for optical and/or electronic applications. In the present study, the GaN NWs were grown using binary catalyzed Au-Pd alloy in a CVD furnace. Schottky diodes were fabricated on clump of NWs and its device performances were examined. A schematic representation of the fabrication of device and examination thereon is shown in Fig. 1. It was observed that, the fabricated Schottky diode exhibits improved barrier height (ϕB) and ideality factor (η) in comparison to the earlier available reports as shown in Table 1. For clarity, the electrical performance obtained from I-V curve has been considered as the dark current measurement of the diode. The performance of the device was
Address: Crystal Growth Centre, Anna University, Chennai 600 025, India. E-mail addresses: [email protected] (S. Sanjay), [email protected] (K. Baskar).
https://doi.org/10.1016/j.apsusc.2018.06.171 Received 18 March 2018; Received in revised form 11 June 2018; Accepted 19 June 2018 Available online 22 June 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
Fig. 1. Schematic representation of fabrication and examination of GaN NWs based Schottky barrier diodes. (a) cleaned sapphire substrate, (b) catalyst deposition, (c) formation of catalytic metal particles/droplets, (d) formation of GaN NWs by VLS growth process, (e) fabricated Schottky barrier diodes and (f) magnified image corresponding to (d).
literature [22].
Table 1 A comparison with the reported literature on fabrication of Schottky barrier diodes using two dimensional and one dimensional GaN nanostructures. Material
Growth method
Ideality factor
Barrier height
Ref.
Ge-GaN thin film
RF reactive sputtering MOCVD MOCVD MOCVD MOVPE RF reactive sputtering PAMBE OMVPE CVD CVD CVD CVD CVD CVD
4.05–5.47
0.53–0.62
[25]
2.9 1.6 1.06–3.85 1.01–1.09 4.2–6.2
0.45 0.46 0.80–0.93 0.86–1.03 0.64–0.77
[26] [26] [27] [28] [29]
2.1 6.5 7.2–8.6 18 3.3 7.2–11.9 12 1.65–1.78
0.52 0.62 0.32–0.42 0.2 0.68 0.32–0.42 0.48 0.8–0.82
[30] [31] [32] [33] [34] [35] [36] This work
GaN thin film Si-GaN thin film GaN thin film GaN thin film Zn-GaN thin film GaN GaN GaN GaN GaN GaN GaN GaN
thin film nanorods nanowires nanowires nanorods nanowires nanowires nanowires
2.2. Fabrication of GaN NWs Schottky barrier diodes were then formed on clump of GaN NWs using standard optical lithography, metallization and lift-off procedures. Ti (20 nm)/Al (60 nm)/Ni (20 nm)/Au (100 nm) were used as the metals for Ohmic contact and were deposited using thermal evaporation system. Two step rapid thermal annealing (RTA) process was performed at 500 ˚C and 900 ˚C for 30 and 90 s respectively for good Ohmic contacts formation. The Schottky contact was deposited by thermal evaporation system using Ni (100 nm)/Au (100 nm) metals. Three types of diodes namely D1, D2 and D3 were fabricated by varying the distance of separation between Ohmic-Schottky contacts as 30, 45 and 60 μm. 2.3. Material and electrical characterization The structural characteristics of GaN NWs were studied using X-ray diffractometer (XRD) [PAN analytical X’Pert PRO]. The selected area electron diffraction (SAED) pattern was obtained from transmission electron microscopy (TEM) [FEI TITAN Themis] along with an image of single NW and its corresponding lattice fringes pattern. The surface morphologies of the NWs were investigated using scanning electron microscopy (SEM) [Zeiss EVO 18]. The elemental maps were recorded using energy dispersive X-ray (EDX) which revealed the elemental distributions in the NWs. The surface compositions of the samples were obtained using X-ray photoelectron spectroscopy (XPS) [AXIS ULTRA]. The electrical measurements of the samples were carried out using semiconductor parameter analyzer [Agilent Device Analyzer B1500A].
also tested under light (bright current measurement). It was observed that, on illumination, the current increases many folds in comparison to the dark current. To the best of our knowledge, only a few reports are available based on the device fabrication on clump of GaN NWs. 2. Experimental methods 2.1. Growth of GaN NWs Growth of GaN NWs was carried out in a horizontal flow CVD furnace. Initially, the sapphire substrates of size 1 cm2 were ultrasonically cleaned with isopropyl alcohol and ethanol for 10 min, rinsed with deionized water and blow dried under N2 ambience. Then, a 15 nm thin layer of Au-Pd catalyst was sputtered on the sapphire substrates. Gallium metal and liquid ammonia (NH3) were used as gallium (Ga) and nitride (N) precursors. By employing nitrogen (N2) as a carrier gas, liquid ammonia placed in the bubbler was transported into the furnace’s reaction zone. The substrate with catalyst and Ga metal were then loaded into the CVD furnace and sealed. The residual oxygen in the furnace was evacuated by purging carrier gas into the furnace for 15 min at 10 sccm (standard cubic centimetre per minute). The furnace was then ramped up to attain a growth temperature of 900 ˚C. The growth of NWs was carried out in two steps. Firstly, the catalyst was allowed to anneal at the growth temperature for 30 min followed by the growth of NWs for 90 min with a source-to-substrate distance of 5 cm and carrier gas flow rate of 500 sccm. Only at this condition, good quality and dense growth of NWs were achieved as reported in
3. Results and discussions 3.1. Structural characteristics Fig. 2(a) is the XRD data revealing the structural details of GaN NWs. The diffraction peaks were obtained corresponding to GaN, Au-Pd catalyst and sapphire substrate. The XRD results have been briefly discussed already in the reported literature [22]. Fig. 2(b) represents the SAED pattern of GaN NWs. The TEM image of individual NW and its corresponding lattice fringes pattern are shown in Fig. 2(c) and (d). The substrates used for the growth is c-plane sapphire with a crystallography direction of (0 0 1). It has been reported that, for a c-plane sapphire substrate, the growth direction of GaN NWs is along (1 0 0) direction [37]. Hence, the grown NWs in the present case grew along the m-axis (Fig. 2d). From Fig. 2(b) and (d), the SAED diffraction spots and lattice fringes pattern obtained were found to be regular, 527
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
Fig. 2. Room temperature diffraction patterns corresponding to: (a) XRD, (b) SAED of the grown GaN NWs, (c) TEM image of individual NW and (d) lattice fringes of the GaN NW corresponding to (c).
asymmetrical peak broadening as shown in Fig. 4(c) was observed for N 1s [38]. The detection of C 1s and O 1s (Fig. 4d and e) peaks at ∼284.5 eV and ∼532 eV are attributed to the extrinsic contaminations during sample testing. The identified core levels from XPS clearly indicate the wurtzite phase of GaN.
illustrating that the grown NWs exhibit hexagonal crystal structure [38]. The obtained SAED pattern corroborates with the XRD results affirming wurtzite phase GaN. 3.2. Morphological characteristics
3.4. Optical characteristics
The surface morphology and elemental maps of GaN NWs were obtained from SEM and EDX. As an effect of annealing, formation of catalytic metal particles with an average size of 100 ± 15 nm was observed on the sample surface as shown in Fig. 3(a). Post growth, dense formation of GaN NWs with good growth rate [inset (i) in Fig. 3(a)] was observed on the samples. The yellow circles shown in inset (ii) of Fig. 3(a) illustrates the presence of catalytic tip at the apex of NWs confirming the VLS growth process [20]. On an average, the obtained NWs were found to be several micrometres long and 80–100 ± 15 nm thick. The Fig. 3(b) and (c–f) represents the EDX map revealing mixed and the individual elemental distribution corresponding to gallium, nitride and catalyst (Au-Pd) particles respectively.
Fig. 5 is the room temperature Raman spectra of synthesized GaN NWs collected over a range of 350–750 cm−1. The peak positioned at ∼420 cm−1 represents the zone boundary (ZB) which originates due to the finite size of NWs [40]. Two prominent Raman active peaks positioned at 568 cm−1 and ∼725 cm−1 corresponding to E2 (high) and A1 longitudinal optical (LO) modes were observed. A shoulder peak positioned at ∼530 cm−1 was observed which represents A1 transverse optical (TO) mode [41]. The peak position of E2(high) records a shift due to the stress in the GaN NWs. This can be attributed to the lattice mismatch between sapphire substrate – GaN NWs – catalyst particle. Another shoulder peak positioned at 650 cm−1 belonging to the Eg mode of sapphire substrates was observed [42]. The peaks identified were found to shift slightly from their reported values due to the stress induced during the NW growth. From the obtained Raman spectra, the wurtzite crystal structure of GaN is confirmed.
3.3. Surface characteristics The surface composition of the synthesized GaN NWs was ascertained from XPS spectra. Fig. 4(a) corresponds to the XPS spectra collected over a wide range of binding energy (0–1200 eV). The identified peaks represent the core levels associating gallium (Ga 3d, 3p, 3s, 2p, LMM Auger peaks), nitrogen (N 1s), carbon (C 1s) and oxygen (O 1s) respectively. The observed Ga 3d (∼20 eV) peak indicates absence of oxygen traces. It is also clear evidence of absence of GaeO bond formation. The Ga 2p peaks at ∼1116 eV (Ga 2p3/2) shown in Fig. 4(b), illustrates the presence of gallium in compound state [39]. The N 1s peak at ∼397 eV represents the presence of nitrogen and nitride contents in the samples. Due to NeH2 and NeH3 bond formation, an
3.5. Device characteristics Fig. 6(a) represents the measured I-V response of GaN NW based Schottky diodes (indicated as D1, D2 and D3). The obtained I-V response illustrates good contacts between the metal electrodes and GaN NWs. For good device fabrication/performance, an effective charge transfer across a metal-semiconductor interface (MSI) is the prime requirement. This is assessed by calculating the barrier height (ϕB) and 528
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
Fig. 3. (a) SEM image representing the formation of catalytic particles as an effect of pre-growth annealing and (b) Mixed EDX map revealing the elemental distributions of gallium, nitride and catalyst particles. Individual elemental maps revealing: (c) gallium, (d) nitride, (e and f) gold-palladium contents. Inset (i) and (ii) of (a) are the high magnification images of GaN NWs.
−qϕB ⎞ Is = AA∗T 2e ⎛ ⎝ KT ⎠
ideality factor (η) of the Schottky diode. The expression for calculating ϕB and η can be given as [26]:
KT ⎛ AA∗T 2 ⎞ ln q ⎝ Is ⎠
(1)
q ⎡ ∂V ⎤ ⎞· η=⎛ ⎝ KT ⎠ ⎢ ⎦ ⎣ ∂ (lnI) ⎥
(2)
ϕB =
⎜
⎜
(4)
The calculated ϕB and η for the fabricated devices D1, D2, D3 have been tabulated in Table 2. It can be seen from the table that the variation in the distance of separation between Ohmic and Schottky contacts significantly affects the ideality factor as shown in Fig. 6(b). The deviation in the ideality factor from unity illustrates that thermionic emission is not completely dominant. Instead, interfacial surface states and barrier inhomogeneities seem to dominate the electrical characteristics of the Schottky diode significantly [31–34]. It is well known that, the interfacial surface states act as recombination centres favoring trap-assisted tunneling causing deviation in the ideality factor beyond unity. Similarly, due to barrier inhomogeneities, the charge transport across the MSI will deviate by significantly reducing the measured barrier height [34]. However, in our case only the ideality factor is found to deviate
⎟
where A is the area of Schottky contact, q is the electronic charge, V is the applied voltage, T is the temperature in Kelvin, K is the Boltzmann constant (1.38 × 10−23 m2kg/s2K), A* is the Richardson’s constant (26.8 A/cm2 K2 for GaN) and Is the reverse saturation current derived from thermionic emission model as shown below [26]:
qV ⎞ ⎤ I = Is ⎡e ⎛⎜ ⎟−1 ⎢ ηKT ⎥ ⎠ ⎦ ⎣ ⎝
⎟
(3)
Fig. 4. (a) Room temperature XPS survey spectrum with the individual spectra of GaN NWs representing: (b) gallium, (c) nitrogen, (d) oxygen and (e) carbon respectively. 529
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
Table 2 Electrical characteristics obtained from I-V curve of the fabricated Schottky barrier diodes. Device
Ohmic-Schottky separation length (µm)
Ideality factor ‘η’
Barrier height ‘ϕB’ (eV)
Reverse saturation current ‘Is’(A)
D1 D2 D3
30 45 60
1.65 1.70 1.78
0.80 0.80 0.82
1.40 × 10−9 1.60 × 10−9 67.0 × 10−9
due to deep emissions at 670 nm which are attributed to vacancy type point defects.
4. Conclusion High quality GaN NWs were grown on sapphire substrates by employing Au-Pd as catalyst. The synthesized GaN NWs were found to exhibit wurtzite crystal structure. The Schottky barrier diodes with different Ohmic-Schottky separation length were fabricated on the clump of NWs and its electrical characterizations were investigated under dark and light conditions. All diodes exhibited rectifying behaviour under dark condition with a barrier height and ideality factor in the range from 0.8 to 0.82 eV and 1.65 to 1.78 respectively. Upon illumination, many fold increase in diode current was observed. Results have shown that the barrier height remains unaffected irrespective of any variations in the ideality factor. Also, the method of lithography during fabrication did not give rise to any surface related inhomogeneities.
Fig. 5. Room temperature Raman spectra of the synthesized GaN NWs.
while the barrier height is in good agreement with the theoretical value calculated using Schottky Mott model [34]. In comparison to the earlier reports [25–36], results obtained by us show that even when there is change in the ideality factor, the barrier height does not change. Also, the surface related inhomogeneities does not occur during fabrication. I-V response of the devices under light illumination is shown in Fig. 6(a) (indicated as D1′, D2′ and D3′). The adjustable halogen lamp attached to the DC probe station for microscopic illumination was used as the light source. Upon illumination, a significant increase in the photocurrent was observed. The photocurrent increased by ∼16 times the dark current as shown in Fig. 6(a). This is attributed to the immense generation of electron-hole pairs which increases the magnitude of the current [3]. Since the present study is about testing the I-V response of the device under light, the optical characteristics such as sensitivity, responsivity and gain were not examined. The results lead to the conclusion that, when the diodes are exposed to light, the current was found to increase many fold in comparison to the dark current, irrespective of the fact that the excitation energy of light source (range from 3.1 to 2.1 eV) is smaller than the band gap of semiconductor (3.4 eV for GaN). It is worth to note that these nanostructures have shown two sharp emissions at 350 nm and 670 nm in cathodoluminescence measurements [22]. The increase in current is
Conflicts of interest There are no conflicts of interest to declare.
Acknowledgement The authors gratefully acknowledge the financial assistance from Dept. of Science and Technology (DST), Govt. of India [Grant No: DST/ TM/ SERI/2K12/71(G)]. The TEM, XPS, and electrical characterization studies were performed at 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, Bengaluru.
Fig. 6. (a) I-V response corresponding to the fabricated GaN NWs tested under dark (D1, D2 and D3) and light conditions (D1′, D2′ and D3′) respectively. (b) Effect of Ohmic-Schottky separation length on the ideality factor of the diodes. 530
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
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Fabrication of Schottky barrier diodes on clump of gallium nitride nanowires grown by chemical vapour deposition ⁎
S. Sanjaya, , K. Baskara,b, a b
T
⁎
Crystal Growth Centre, Anna University, Chennai 600 025, India Manonmaniam Sundaranar University, Tirunelveli 627 012, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Gallium nitride nanowires Chemical vapour deposition Vapour liquid solid process Schottky barrier diodes
In this study, Schottky barrier diodes on ‘clump of nanowires’ has been fabricated and its electrical behaviour on variation of distance of separation between Ohmic (Ti/Al/Ni/Au) and Schottky (Ni/Au) contacts has been studied. The variations in current under dark and bright conditions have been investigated. High quality gallium nitride nanowires were grown by chemical vapour deposition technique using gold-palladium alloy as catalyst. The novelty of this work is that, the barrier height remains unaffected irrespective of any variations in the ideality factor. Also, the method of lithography during fabrication did not give rise to any surface related inhomogeneities.
1. Introduction The fascinating properties of gallium nitride nanostructures (GaN NS) include wide direct band gap, huge surface area [1,2], very high light absorption and extraction capability [3,4], high quantum confinement [5], tendency to grow on non-native substrates [6,7], flexibility to doping (both p- and n-type) [8,9] and tolerance towards harsh environments. In the last few years, efforts have been made to grow different forms of GaN NS, especially nanowires (NWs) and to utilize them in various optical and electronic nanoscale devices such as high electron mobility transistors (HEMTs) [10], field effect transistors (FETs) [11], p-n junction diodes [12], laser diodes [13], light emitting diodes (LEDs) [14], nano generators [15], solar cells [16] and photodetectors [17]. For the growth of GaN NWs, chemical vapour deposition (CVD) technique is being conventionally preferred due to its easiness, capability to grow good quality NWs on non-native substrates and ability to tune the NW thickness [1,3,18,19]. However, the NW growth using CVD technique is better achieved only in the presence of a metal catalyst where the growth process is governed by a well-known vapourliquid-solid (VLS) mechanism [20]. There were intense research focussing on the growth of GaN NWs by using single metal catalyst such as gold, platinum, nickel, copper, cobalt, magnesium, indium and iron. In a recent study, we have substituted the single metal catalyst with binary metal alloy (gold-palladium) for the growth of GaN NWs. Unlike single metal catalyst, the usage of binary catalytic alloy was found to tune the diameter and luminescence properties of the NWs and also
⁎
helped in achieving better growth rate at different growth conditions [21,22]. Over the past two decades, efforts were being made to fabricate nanoscale devices comprising of epitaxial film or single NW. However, fabrication of epitaxial film based on nanoscale device is considered easier compared to the other because single NW fabrication suffers from complex sample preparations, lithographic process and less yield. These demerits make them unsuitable for mass production [23]. To overcome these limitations, a method of fabricating clump of NWs was developed. This method of fabrication was found to improve the yield, fabrication procedures and ensured large scale productivity [23,24]. In spite of these merits; the varying lengths and thicknesses of NWs, random alignment of NWs during growth and the charge transport across metalsemiconductor interface (MSI) due to size effect are some of the critical issues. These limitations always degrade the device performance and make them technologically unfit for optical and/or electronic applications. In the present study, the GaN NWs were grown using binary catalyzed Au-Pd alloy in a CVD furnace. Schottky diodes were fabricated on clump of NWs and its device performances were examined. A schematic representation of the fabrication of device and examination thereon is shown in Fig. 1. It was observed that, the fabricated Schottky diode exhibits improved barrier height (ϕB) and ideality factor (η) in comparison to the earlier available reports as shown in Table 1. For clarity, the electrical performance obtained from I-V curve has been considered as the dark current measurement of the diode. The performance of the device was
Address: Crystal Growth Centre, Anna University, Chennai 600 025, India. E-mail addresses: [email protected] (S. Sanjay), [email protected] (K. Baskar).
https://doi.org/10.1016/j.apsusc.2018.06.171 Received 18 March 2018; Received in revised form 11 June 2018; Accepted 19 June 2018 Available online 22 June 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
Fig. 1. Schematic representation of fabrication and examination of GaN NWs based Schottky barrier diodes. (a) cleaned sapphire substrate, (b) catalyst deposition, (c) formation of catalytic metal particles/droplets, (d) formation of GaN NWs by VLS growth process, (e) fabricated Schottky barrier diodes and (f) magnified image corresponding to (d).
literature [22].
Table 1 A comparison with the reported literature on fabrication of Schottky barrier diodes using two dimensional and one dimensional GaN nanostructures. Material
Growth method
Ideality factor
Barrier height
Ref.
Ge-GaN thin film
RF reactive sputtering MOCVD MOCVD MOCVD MOVPE RF reactive sputtering PAMBE OMVPE CVD CVD CVD CVD CVD CVD
4.05–5.47
0.53–0.62
[25]
2.9 1.6 1.06–3.85 1.01–1.09 4.2–6.2
0.45 0.46 0.80–0.93 0.86–1.03 0.64–0.77
[26] [26] [27] [28] [29]
2.1 6.5 7.2–8.6 18 3.3 7.2–11.9 12 1.65–1.78
0.52 0.62 0.32–0.42 0.2 0.68 0.32–0.42 0.48 0.8–0.82
[30] [31] [32] [33] [34] [35] [36] This work
GaN thin film Si-GaN thin film GaN thin film GaN thin film Zn-GaN thin film GaN GaN GaN GaN GaN GaN GaN GaN
thin film nanorods nanowires nanowires nanorods nanowires nanowires nanowires
2.2. Fabrication of GaN NWs Schottky barrier diodes were then formed on clump of GaN NWs using standard optical lithography, metallization and lift-off procedures. Ti (20 nm)/Al (60 nm)/Ni (20 nm)/Au (100 nm) were used as the metals for Ohmic contact and were deposited using thermal evaporation system. Two step rapid thermal annealing (RTA) process was performed at 500 ˚C and 900 ˚C for 30 and 90 s respectively for good Ohmic contacts formation. The Schottky contact was deposited by thermal evaporation system using Ni (100 nm)/Au (100 nm) metals. Three types of diodes namely D1, D2 and D3 were fabricated by varying the distance of separation between Ohmic-Schottky contacts as 30, 45 and 60 μm. 2.3. Material and electrical characterization The structural characteristics of GaN NWs were studied using X-ray diffractometer (XRD) [PAN analytical X’Pert PRO]. The selected area electron diffraction (SAED) pattern was obtained from transmission electron microscopy (TEM) [FEI TITAN Themis] along with an image of single NW and its corresponding lattice fringes pattern. The surface morphologies of the NWs were investigated using scanning electron microscopy (SEM) [Zeiss EVO 18]. The elemental maps were recorded using energy dispersive X-ray (EDX) which revealed the elemental distributions in the NWs. The surface compositions of the samples were obtained using X-ray photoelectron spectroscopy (XPS) [AXIS ULTRA]. The electrical measurements of the samples were carried out using semiconductor parameter analyzer [Agilent Device Analyzer B1500A].
also tested under light (bright current measurement). It was observed that, on illumination, the current increases many folds in comparison to the dark current. To the best of our knowledge, only a few reports are available based on the device fabrication on clump of GaN NWs. 2. Experimental methods 2.1. Growth of GaN NWs Growth of GaN NWs was carried out in a horizontal flow CVD furnace. Initially, the sapphire substrates of size 1 cm2 were ultrasonically cleaned with isopropyl alcohol and ethanol for 10 min, rinsed with deionized water and blow dried under N2 ambience. Then, a 15 nm thin layer of Au-Pd catalyst was sputtered on the sapphire substrates. Gallium metal and liquid ammonia (NH3) were used as gallium (Ga) and nitride (N) precursors. By employing nitrogen (N2) as a carrier gas, liquid ammonia placed in the bubbler was transported into the furnace’s reaction zone. The substrate with catalyst and Ga metal were then loaded into the CVD furnace and sealed. The residual oxygen in the furnace was evacuated by purging carrier gas into the furnace for 15 min at 10 sccm (standard cubic centimetre per minute). The furnace was then ramped up to attain a growth temperature of 900 ˚C. The growth of NWs was carried out in two steps. Firstly, the catalyst was allowed to anneal at the growth temperature for 30 min followed by the growth of NWs for 90 min with a source-to-substrate distance of 5 cm and carrier gas flow rate of 500 sccm. Only at this condition, good quality and dense growth of NWs were achieved as reported in
3. Results and discussions 3.1. Structural characteristics Fig. 2(a) is the XRD data revealing the structural details of GaN NWs. The diffraction peaks were obtained corresponding to GaN, Au-Pd catalyst and sapphire substrate. The XRD results have been briefly discussed already in the reported literature [22]. Fig. 2(b) represents the SAED pattern of GaN NWs. The TEM image of individual NW and its corresponding lattice fringes pattern are shown in Fig. 2(c) and (d). The substrates used for the growth is c-plane sapphire with a crystallography direction of (0 0 1). It has been reported that, for a c-plane sapphire substrate, the growth direction of GaN NWs is along (1 0 0) direction [37]. Hence, the grown NWs in the present case grew along the m-axis (Fig. 2d). From Fig. 2(b) and (d), the SAED diffraction spots and lattice fringes pattern obtained were found to be regular, 527
Applied Surface Science 456 (2018) 526–531
S. Sanjay, K. Baskar
Fig. 2. Room temperature diffraction patterns corresponding to: (a) XRD, (b) SAED of the grown GaN NWs, (c) TEM image of individual NW and (d) lattice fringes of the GaN NW corresponding to (c).
asymmetrical peak broadening as shown in Fig. 4(c) was observed for N 1s [38]. The detection of C 1s and O 1s (Fig. 4d and e) peaks at ∼284.5 eV and ∼532 eV are attributed to the extrinsic contaminations during sample testing. The identified core levels from XPS clearly indicate the wurtzite phase of GaN.
illustrating that the grown NWs exhibit hexagonal crystal structure [38]. The obtained SAED pattern corroborates with the XRD results affirming wurtzite phase GaN. 3.2. Morphological characteristics
3.4. Optical characteristics
The surface morphology and elemental maps of GaN NWs were obtained from SEM and EDX. As an effect of annealing, formation of catalytic metal particles with an average size of 100 ± 15 nm was observed on the sample surface as shown in Fig. 3(a). Post growth, dense formation of GaN NWs with good growth rate [inset (i) in Fig. 3(a)] was observed on the samples. The yellow circles shown in inset (ii) of Fig. 3(a) illustrates the presence of catalytic tip at the apex of NWs confirming the VLS growth process [20]. On an average, the obtained NWs were found to be several micrometres long and 80–100 ± 15 nm thick. The Fig. 3(b) and (c–f) represents the EDX map revealing mixed and the individual elemental distribution corresponding to gallium, nitride and catalyst (Au-Pd) particles respectively.
Fig. 5 is the room temperature Raman spectra of synthesized GaN NWs collected over a range of 350–750 cm−1. The peak positioned at ∼420 cm−1 represents the zone boundary (ZB) which originates due to the finite size of NWs [40]. Two prominent Raman active peaks positioned at 568 cm−1 and ∼725 cm−1 corresponding to E2 (high) and A1 longitudinal optical (LO) modes were observed. A shoulder peak positioned at ∼530 cm−1 was observed which represents A1 transverse optical (TO) mode [41]. The peak position of E2(high) records a shift due to the stress in the GaN NWs. This can be attributed to the lattice mismatch between sapphire substrate – GaN NWs – catalyst particle. Another shoulder peak positioned at 650 cm−1 belonging to the Eg mode of sapphire substrates was observed [42]. The peaks identified were found to shift slightly from their reported values due to the stress induced during the NW growth. From the obtained Raman spectra, the wurtzite crystal structure of GaN is confirmed.
3.3. Surface characteristics The surface composition of the synthesized GaN NWs was ascertained from XPS spectra. Fig. 4(a) corresponds to the XPS spectra collected over a wide range of binding energy (0–1200 eV). The identified peaks represent the core levels associating gallium (Ga 3d, 3p, 3s, 2p, LMM Auger peaks), nitrogen (N 1s), carbon (C 1s) and oxygen (O 1s) respectively. The observed Ga 3d (∼20 eV) peak indicates absence of oxygen traces. It is also clear evidence of absence of GaeO bond formation. The Ga 2p peaks at ∼1116 eV (Ga 2p3/2) shown in Fig. 4(b), illustrates the presence of gallium in compound state [39]. The N 1s peak at ∼397 eV represents the presence of nitrogen and nitride contents in the samples. Due to NeH2 and NeH3 bond formation, an
3.5. Device characteristics Fig. 6(a) represents the measured I-V response of GaN NW based Schottky diodes (indicated as D1, D2 and D3). The obtained I-V response illustrates good contacts between the metal electrodes and GaN NWs. For good device fabrication/performance, an effective charge transfer across a metal-semiconductor interface (MSI) is the prime requirement. This is assessed by calculating the barrier height (ϕB) and 528
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S. Sanjay, K. Baskar
Fig. 3. (a) SEM image representing the formation of catalytic particles as an effect of pre-growth annealing and (b) Mixed EDX map revealing the elemental distributions of gallium, nitride and catalyst particles. Individual elemental maps revealing: (c) gallium, (d) nitride, (e and f) gold-palladium contents. Inset (i) and (ii) of (a) are the high magnification images of GaN NWs.
−qϕB ⎞ Is = AA∗T 2e ⎛ ⎝ KT ⎠
ideality factor (η) of the Schottky diode. The expression for calculating ϕB and η can be given as [26]:
KT ⎛ AA∗T 2 ⎞ ln q ⎝ Is ⎠
(1)
q ⎡ ∂V ⎤ ⎞· η=⎛ ⎝ KT ⎠ ⎢ ⎦ ⎣ ∂ (lnI) ⎥
(2)
ϕB =
⎜
⎜
(4)
The calculated ϕB and η for the fabricated devices D1, D2, D3 have been tabulated in Table 2. It can be seen from the table that the variation in the distance of separation between Ohmic and Schottky contacts significantly affects the ideality factor as shown in Fig. 6(b). The deviation in the ideality factor from unity illustrates that thermionic emission is not completely dominant. Instead, interfacial surface states and barrier inhomogeneities seem to dominate the electrical characteristics of the Schottky diode significantly [31–34]. It is well known that, the interfacial surface states act as recombination centres favoring trap-assisted tunneling causing deviation in the ideality factor beyond unity. Similarly, due to barrier inhomogeneities, the charge transport across the MSI will deviate by significantly reducing the measured barrier height [34]. However, in our case only the ideality factor is found to deviate
⎟
where A is the area of Schottky contact, q is the electronic charge, V is the applied voltage, T is the temperature in Kelvin, K is the Boltzmann constant (1.38 × 10−23 m2kg/s2K), A* is the Richardson’s constant (26.8 A/cm2 K2 for GaN) and Is the reverse saturation current derived from thermionic emission model as shown below [26]:
qV ⎞ ⎤ I = Is ⎡e ⎛⎜ ⎟−1 ⎢ ηKT ⎥ ⎠ ⎦ ⎣ ⎝
⎟
(3)
Fig. 4. (a) Room temperature XPS survey spectrum with the individual spectra of GaN NWs representing: (b) gallium, (c) nitrogen, (d) oxygen and (e) carbon respectively. 529
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Table 2 Electrical characteristics obtained from I-V curve of the fabricated Schottky barrier diodes. Device
Ohmic-Schottky separation length (µm)
Ideality factor ‘η’
Barrier height ‘ϕB’ (eV)
Reverse saturation current ‘Is’(A)
D1 D2 D3
30 45 60
1.65 1.70 1.78
0.80 0.80 0.82
1.40 × 10−9 1.60 × 10−9 67.0 × 10−9
due to deep emissions at 670 nm which are attributed to vacancy type point defects.
4. Conclusion High quality GaN NWs were grown on sapphire substrates by employing Au-Pd as catalyst. The synthesized GaN NWs were found to exhibit wurtzite crystal structure. The Schottky barrier diodes with different Ohmic-Schottky separation length were fabricated on the clump of NWs and its electrical characterizations were investigated under dark and light conditions. All diodes exhibited rectifying behaviour under dark condition with a barrier height and ideality factor in the range from 0.8 to 0.82 eV and 1.65 to 1.78 respectively. Upon illumination, many fold increase in diode current was observed. Results have shown that the barrier height remains unaffected irrespective of any variations in the ideality factor. Also, the method of lithography during fabrication did not give rise to any surface related inhomogeneities.
Fig. 5. Room temperature Raman spectra of the synthesized GaN NWs.
while the barrier height is in good agreement with the theoretical value calculated using Schottky Mott model [34]. In comparison to the earlier reports [25–36], results obtained by us show that even when there is change in the ideality factor, the barrier height does not change. Also, the surface related inhomogeneities does not occur during fabrication. I-V response of the devices under light illumination is shown in Fig. 6(a) (indicated as D1′, D2′ and D3′). The adjustable halogen lamp attached to the DC probe station for microscopic illumination was used as the light source. Upon illumination, a significant increase in the photocurrent was observed. The photocurrent increased by ∼16 times the dark current as shown in Fig. 6(a). This is attributed to the immense generation of electron-hole pairs which increases the magnitude of the current [3]. Since the present study is about testing the I-V response of the device under light, the optical characteristics such as sensitivity, responsivity and gain were not examined. The results lead to the conclusion that, when the diodes are exposed to light, the current was found to increase many fold in comparison to the dark current, irrespective of the fact that the excitation energy of light source (range from 3.1 to 2.1 eV) is smaller than the band gap of semiconductor (3.4 eV for GaN). It is worth to note that these nanostructures have shown two sharp emissions at 350 nm and 670 nm in cathodoluminescence measurements [22]. The increase in current is
Conflicts of interest There are no conflicts of interest to declare.
Acknowledgement The authors gratefully acknowledge the financial assistance from Dept. of Science and Technology (DST), Govt. of India [Grant No: DST/ TM/ SERI/2K12/71(G)]. The TEM, XPS, and electrical characterization studies were performed at 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, Bengaluru.
Fig. 6. (a) I-V response corresponding to the fabricated GaN NWs tested under dark (D1, D2 and D3) and light conditions (D1′, D2′ and D3′) respectively. (b) Effect of Ohmic-Schottky separation length on the ideality factor of the diodes. 530
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S. Sanjay, K. Baskar
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