UV Photodetector Based on Graphene −−−−ZnO Nanowire UV Photodetector Based on Graphene −−−−ZnO Nanowire Hybrid: 2 Fabrication, Photoresponse and Photoluminescence Studies

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UV Photodetector Based on GrapheneZnO Nanowire Hybrid: 1

Fabrication, Photoresponse and Photoluminescence Studies 2

Ravi K. Biroju1, P. K. Giri1, 2* 3

1Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-781039, India 4 2Department of Physics, Indian Institute of Technology Guwahati, Guwahati-781039, India. 5

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Abstract 7

Herein, we demonstrate a graphene based efficient UV photodetector by using vertically aligned ZnO 8

NWs on chemical vapor deposited graphene with and without a ZnO buffer layer. The effect of rapid 9

thermal annealing (RTA) on grapheneZnO thin film hybrid prior to the growth of ZnO NWs by physical 10

vapor deposition technique is investigated from the Raman spectroscopy and high resolution 11

transmission electron microscopy. The change in the relative intensities of 2D and D band Raman modes 12

and a red shift in 2D band (~ 13 cm-1) after RTA treatment in ZnO coated graphene substrate reveals the 13

strong interaction between graphene and ZnO hexagonal lattice, covalent sp2 carbon lattice. The vertical 14

ZnO NWs with high aspect ratio on the grapheneZnO thin film hybrid exhibit enhanced UV absorption, 15

strong UV and visible photoluminescence and improved UV photoresponse. There is a 6 fold 16

enhancement in the photocurrent in ZnO NWs grown on graphene as compared to the ZnO NWs grown 17

on bare ZnO seed layer. The improved performance of these graphene based 2D1D integrated hybrid ZnO 18

nanostructures is an important step towards the fabrication of hybrid photodetectors and phototransistors 19

with enhanced UV response. 20

Keywords: CVD graphene; GrapheneZnO NWs, UV photodetector, Rapid Thermal Annealing 21

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1. INTRODUCTION 24 *Email of corresponding author: giri@iitg.ernet.in

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A tremendous research interest has grown recently on the fabrication and optoelectronic properties of 1

graphene based semiconducting nanowire (NW) hybrid structures for applications in various 2

optoelectronic devices, such as ultrafast photodetectors (PDs) and light emitting diodes (LEDs) etc.1-3 On 3

the other hand, investigating the photo physical properties from 2D single layer (SLG) and few layer 4

graphene or boron nitride (h-BN) with high quality, wide band gap semiconductor (ZnO, GaN, GaAs etc.) 5

epitaxial layers is a very important aspect in multiquantumwell hetero structures for light emission 6

devises such as LEDs and ultrafast PDs.4-7 Further, incorporating the pretreated epitaxial 7

graphenesemiconductor thin film substrates for the growth of catalyst free, vertically aligned 8

semiconducting nanowires (NWs) for the enhanced photoluminescence (PL) and photoconductivity (PC) 9

properties is a challenging task. Among all, fabrication of ZnO thin film using RF magnetron sputtering 10

followed by ZnO NW growth on grapheneZnO layer substrates by low temperature physical vapor 11

deposition (PVD) technique is very simple technique and controlled fabrication of 2D1D integrated 12

hybrid nanostructures with improved PD device characteristics are challenging. Understanding the 13

epitaxial relation between sp2 hybridized single crystalline graphene layer and hexagonal wurtzite phase 14

ZnO is very important for the growth of catalyst free ZnO NW hetero structures on graphene.1 Further 15

these 2D1D integrated grapheneZnO hybrids will improve the optical properties such as UVvis 16

absorption, PL and photoconductivity (PC) as compared the ZnO NWs only.1,8,9 The advantage of 17

combining ZnO over the graphene, that can catalyze the growth of ZnO NW for the vertical alignment in 18

the presence of ZnO seed layer and due to its very high carrier mobility, very low optical absorption and 19

high thermal conductivity can enhance the optical property of the resultant integrated hybrid 20

nanostructure which is suitable for the improved photosensitivity and ultrafast UV light response.10,11 21

There are no reports on the effect of RTA treatment of the ZnO seed layer on the fabrication of 22

ZnO NWs on graphene-ZnO hybrid layer and the role of graphene layer in the vertical alignment of ZnO 23

NWs on these substrates. In the present work, we have grown the vertical ZnO NWs on graphene 24

substrate with sputter deposited ultra-thin ZnO seed layer and achieved enhanced optical properties from 25

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these hybrids, such as, optical absorption, photoconductivity and PL. The mechanism of catalyst free 1

growth of ZnO NWs over the graphene layer with and without ZnO seed layer was systematically studied 2

from the Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). 3

Characteristic Raman features of grapheneZnO thin film hybrid, such as change in the relative 4

intensities and full width at half maxima (FWHM) of 2D, D Raman bands with respect to the G band 5

reveals the epitaxial relation between graphene and ZnO that originates the growth of vertically aligned 6

ZnO NWs. The structural and optical properties of these grapheneZnO NWs hybrid nanostructures were 7

studied by high resolution FESEM, HRTEM and optical absorption and Raman spectroscopy. Further 8

enhanced UV PL was achieved in case of ZnO NWs grown on grapheneZnO seed layer substrate. These 9

PL results are compared with the bare ZnO seed layer substrates. Photoresponse studies are carried out on 10

the hybrid nanostructures. 11

2. EXPERIMENTAL DETAILS 12

2.1. Synthesis of Graphene by Chemical Vapor Deposition 13

Large area single layer graphene was synthesized on a Cu foil (Alfa Aesar, 99.999% pure, 11 14

inch area) by a thermal CVD technique. As grown graphene on the Cu foil was transferred on to 15

Si/SiO2 and quartz substrates by standard wet transfer method. Initially, the Cu foil was inserted 16

in a quartz boat, which was kept at the center of a tubular quartz chamber (diameter 1 inch, 17

length 1 meter) placed inside a muffle furnace. The base vacuum of 4.0 10-4 mbar inside the 18

tube was created using a turbo molecular pump. Subsequently, Ar gas (300 standard cubic 19

centimeters per minute (SCCM)) was purged into the quartz tube for 15 min. The ratio of 20

precursor gas sources Ar: CH4: H2 was 300:10:50 SCCM during the graphene growth and the 21

duration of growth was 30 min. The growth temperature and pressure were maintained at 1045C 22

and 2.2 mbar, respectively. 23

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Asgrown graphene on the Cu foil was transferred onto various substrates by standard 1

wet transfer process, which involved spin coating of a layer of Poly methyl methacrylate 2

(PMMA)/Toluene on one side of the Cu foil and then etching away the other side by keeping it 3

floating on Fe(NO3)3 solution (etchant solution) overnight. The Fe(NO3)3 solution was then 4

repeatedly diluted with millipore water by using a syringe in such a way that it turns orange color 5

to complete transparent, i.e. in the end only water remained. The layer of PMMA with graphene 6

attached to it was scooped out using various substrates. The substrates were then rinsed with DI 7

water and dried in the oven at 180C for 15 min. The PMMA layer was removed by repeated 8

rinsing of graphene substrates with Acetone for several times. In order to remove the PMMA and 9

residual Cu catalyst particles adhered on graphene layer, hydrogen (H2) annealing was 10

performed. Further details of the H2 annealing of GR were reported in our earlier work.12 11

2.2. Deposition of ZnO Thin Films on Graphene 12

High quality ZnO film with two different thicknesses: ~300 nm (sample codeZ1), ~10 nm 13

(sample codeZ2) were deposited by RF magnetron sputtering on various graphene substrates 14

for the growth of ZnO NWs. Initially, a base pressure of 6.7x10-6 mbar was created inside the 15

chamber and during the RF plasma induced sputtering, it was maintained at 1x10-2 mbar. The RF 16

power and substrate temperature were kept at 100 W and 200C, respectively. Further, the ZnO 17

thin films were subjected to rapid thermal annealing (RTA) treatment at 600C in Ar gas ambient 18

(flow rate of 200 SCCM) atmosphere for 3 minutes using a commercial RTA system 19

(MILA3000, Ulvac, Japan) in order to further improve the crystalline quality as well as to 20

remove the excess oxygen defects in the ZnO films on various substrates. Improvements in the 21

crystallinity of ZnO grains and their phases were confirmed from XRD and HRTEM analyses. 22

Note that the results and discussions are made only for the ZnO NWs grown on the RTA treated 23

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GRZ2 substrate (10nm thick ZnO seed layer on graphene/SiO2), Z2 substrate (10nm thick ZnO 1

seed layer on SiO2) and GR (pristine single layer CVD graphene) substrates only. 2

2.3. Growth of ZnO NWs on Graphene Substrates 3

Commercial nanosized activated Zn powder (purity ~ 99%, Aldrich) was taken as a source 4

material in an alumina boat and placed at the center of a horizontal quartz tube kept inside a 5

muffle furnace. The above prepared substrates were placed downstream ~ 5 cm away from the 6

source material. Initially the quartz tube was pumped down to a pressure of ~ 10-3 mbar. In order 7

to prevent the oxidation of the graphene layer, 300 SCCM of Ar gas was flushed into the 8

chamber until it