Large thermal mismatch between GaN surface and sapphire results in compressive stress in Gallium Nitride (GaN) layer which degrades the device performance. Nanostructuring the GaN can reduce this stress leading to reduction in Quantum Confined Stark Effect. Aligned GaN nanorods based nanodevices have potential applications in electronics and optoelectronics. This paper describes the fabrication of GaN nanorods using Ni nanomasking and reactive ion etching. The morphology of GaN nanorods was studied by field emission scanning electron microscopy. The optical properties of GaN nanorods were studied by Cathodoluminescence (CL) spectroscopy. CL results revealed the existence of characteristic band-edge luminescence and yellow band luminescence.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3099

Christiansen, S.

Kumar, Ashutosh

Latzel, Michael

Singh, R.

Tessarek, C.

English

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 5 No 2, 02001(3pp) (2013) Том 5 № 2, 02001(3cc) (2013) 
 
 
2077-6772/2013/5(2)02001(3) 02001-1  2013 Sumy State University 
Characterization of GaN Nanorods Fabricated Using Ni Nanomasking and  
Reactive Ion Etching: A Top-Down Approach 
 
Ashutosh Kumar1,*, Michael Latzel2, C. Tessarek2, S. Christiansen2, R. Singh1,† 
 
1 Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, 110016 New Delhi, India 
2 Max Planck Institute for the Science of Light, 24, Günther-Scharowsky-Str. 1/Bau, 91058 Erlangen, Germany 
 
(Received 15 February 2013; published online 04 May 2013) 
 
Large thermal mismatch between GaN surface and sapphire results in compressive stress in Gallium 
Nitride (GaN) layer which degrades the device performance. Nanostructuring the GaN can reduce this 
stress leading to reduction in Quantum Confined Stark Effect. Aligned GaN nanorods based nanodevices 
have potential applications in electronics and optoelectronics. This paper describes the fabrication of GaN 
nanorods using Ni nanomasking and reactive ion etching. The morphology of GaN nanorods was studied 
by field emission scanning electron microscopy. The optical properties of GaN nanorods were studied by 
Cathodoluminescence (CL) spectroscopy. CL results revealed the existence of characteristic band-edge lu-
minescence and yellow band luminescence. 
 
Keywords: GaN, Nanorod, Nanomasking, Reactive-ion etching, Cathodoluminescence, Band-edge  
luminescence, Yellow band luminescence. 
 
 PACS numbers: 81.16. – c, 81.16.rf, 78.67. – n, 78.60.Hk 
 
 
                                                          
* ashutosh.kumar@physics.iitd.ac.in 
† rsingh@physics.iitd.ac.in 
1. INTRODUCTION 
 
Semiconductor nanostructures have recently 
emerged as the leading material for the fabrication of 
electronic [1-2] and optoelectronic devices [3]. Over the 
last decade, use of Gallium Nitride (GaN) nanostruc-
tures for high power transistors [4] and solid state 
lightning [5-7] is an interesting area for researchers. 
The main reason behind these interests is the fact that 
nanostructuring can reduce the compressive stress in 
GaN layer grown over sapphire leading to reduction in 
Quantum Confined Stark Effect. Also, due to direct and 
wide band gap of GaN, its nanostructures can be used 
for high temperature and high power electronic devices 
applications [8]. GaN nanorods based nanodevices do 
have some advantages as compared to bulk GaN based 
devices which are listed below [9-11]: 
 High surface to volume ratio: increase in active 
area for LEDs and solar cells. 
 Strain relaxation in GaN nanorods: increase in 
radiative recombination efficiency in multiple quantum 
wells (MQWs) due to reduction in quantum confined 
stark effect. 
 Growth of MQWs on non-polar planes: increase in 
radiative recombination efficiency in MQWs and wave-
length stability. 
 Reduction in threading dislocation density: in-
crease in performance of nanodevices. 
GaN nanorods can be fabricated using top-down 
and down-top approach. The down-top approach which 
is a self organized growth technique includes catalyst 
assisted Vapor-Liquid-Solid (VLS) growth, catalyst free 
or self-induced growth and selective area growth on 
nano-patterned substrate. GaN nanorods are generally 
synthesized using VLS mechanism which was first de-
veloped by Wagner and Ellis [12] in 1964. Han et al. 
[13] in 1997 was the first to demonstrate successfully 
the fabrication of GaN nanowires. VLS growth methods 
do have disadvantages of requiring highly specific 
growth conditions to increase the on-axis growth rate 
while minimizing the lateral growth rate. This can lead 
to non-optimal material quality and less flexibility in 
material design such as doping and heterostructures.  
For fabrication of a device using 1D GaN nanostruc-
ture, it is essential to develop a process to make con-
tacts to individual nanowires or nanorods as well as 
array of nanowires or nanorods which lacks in VLS 
mechanism. One solution to this problem is top-down 
approach where GaN nanorods can be fabricated by 
etching GaN layer using nanomasks. This approach 
includes methods like conventional photolithography, 
e-beam lithography, self-organized nanomasking or 
nanosphere lithography [14, 16-17]. In these tech-
niques, dry etching methods like as RIE-ICP are used 
to obtain high aspect ratio. High aspect ratio of nano-
rods can be achieved using techniques like e-beam li-
thography or photolithography but these techniques 
have limitations in nanofabrication over large area 
substrates. However, techniques like self-assembled 
nanomasking and nanosphere lithography are cost ef-
fective and applicable to nanofabrication over large 
area substrates. Ramesh et al. [15] demonstrated blue 
shift in Photoluminescence (PL) emission peak as well 
as improvement in PL internal quantum efficiency in 
GaN nanoLED fabricated using top-down approach as 
compared to as grown down-top GaN nanoLED. An 
enhancement of PL intensity and blue shift of PL emis-
sion peak (which indicates the strain relaxation) in 
GaN nanorods fabricated using top-down approach are 
also reported by various groups [16-17]. 
So in this work, we employed a top-down approach 
for the fabrication of GaN nanorods using nanomasking 
and reactive ion etching (RIE) for nanoelectronics and 
optoelectronics applications.  
 
 A. KUMAR, M. LATZEL, S. TESSAREK, ET AL. J. NANO- ELECTRON. PHYS. 5, 02001 (2013) 
 
 
02001-2 
2. EXPERIMENTAL 
 
The samples used in this study were cut from nom-
inally undoped GaN wafer of thickness about 2 m 
grown on c-plane sapphire substrate by metalorganic 
chemical-vapor deposition (MOCVD) technique. GaN 
nanorods were fabricated using top-down approach by 
Nickel (Ni) nanomasking and reactive ion etching.  
Prior to nanorods fabrication, samples were ultrasoni-
cally cleaned in trichloroethylene, acetone and iso-
propanol respectively at 70 ºC. The samples were then 
dipped in 1:1 HCl for 2 min to remove the native ox-
ides. Finally the clean samples were rinsed with de-
ionized water and blown with dry nitrogen. A SiO2  
layer of thickness about 100 nm was deposited over  
n-GaN using sputtering followed by sputter deposition 
of Ni metal film. Then rapid thermal annealing (RTA) 
was performed at 800 ºC for 60 sec under nitrogen am-
bient to produce Ni metal nano-islands. In next step, 
reactive ion etching (RIE) of SiO2 was performed fol-
lowed by selective RIE of n-GaN layer. SiO2 layer was 
selective etched using CF4 plasma while selective etch-
ing of GaN layer was performed using Cl2/Ar plasma. 
Finally to fabricate GaN nanorods, Ni and SiO2 were 
etched using HF for 60 sec. The HF etching also re-
moves any other contaminants. The schematic for fab-
rication of GaN nanorods using Ni nanomasking and 
reactive ion etching is shown in Fig. 1. 
 
 
 
Fig. 1 – Schematic diagram for fabrication of GaN nanorods 
using Ni nanomasking and reactive ion etching technique 
 
3. RESULTS AND DISCUSSION  
 
3.1 Morphological Studies of GaN Nanorods 
 
The shape and morphology of the GaN nanorods 
were observed using field emission scanning electron 
microscopy (FESEM). The side view image of GaN na-
norods fabricated using top-down approach taken at 
different magnification (× 20,000 and × 50,000) is 
shown in Fig. 2a, b. From the side-view secondary elec-
tron image of the GaN nanorods cluster, the diameters 
of the nanorods on the sapphire substrate were about 
100 to 250 nm. High vertical alignment of these nano-
rods is also clear from secondary electron images. 
 
3.2 Cathodoluminescence Studies 
 
Cathodoluminescence (CL) measurements were also 
performed in order to investigate the light emission  
 
 
 
 
Fig. 2 – FESEM image of GaN nanorods at two different 
magnifications (× 20,000 and × 50,000) 
 
 
 
Fig. 3 – CL spectrum of GaN nanorods at room temperature 
from mapped region showing two peaks which corresponds to 
band-edge luminescence (at 3.4 eV) and yellow band lumines-
cence (centred about 2.3 eV) 
 
properties of GaN nanorods. The CL measurements 
reported here were performed at room temperature 
using standard CL measurement set-up with accelerat-
ing voltage of 5 kV and electron beam current of 20 A. 
Fig. 3 shows the CL spectrum of GaN nanorods from 
the mapped region. 
The CL spectrum shows a stronger peak from GaN 
band edge luminescence at 3.4 eV (364 nm) and an ad-
ditional broad peak centred about 2.3 eV (550 nm) 
which is commonly known as yellow band luminescence 
(YL). Both the peaks are characteristics of the GaN 
nanostructures and also earlier reported in GaN  
 CHARACTERIZATION OF GAN NANORODS FABRICATED… J. NANO- ELECTRON. PHYS. 5, 02001 (2013) 
 
 
02001-3 
 
 
 
 
Fig. 4 – (a) FESEM image and (b) corresponding panchro-
matic CL image of GaN nanorods 
 
nanostructures fabricated from down-top approach [18-
19]. The existence of YL indicates that there is a deep 
level located at about 2.3 eV below conduction band 
edge and recombination of conduction electrons 
through this deep level can give yellow emission 
A plan-view integrated panchromatic CL image 
from GaN nanorods acquired with an accelerating volt-
age of 5 kV and electron beam current of 20 A is 
shown in Fig. 4a, b. Fig. 4a shows the FESEM image of 
GaN nanorods and Fig. 4b shows the corresponding 
panchromatic CL image. It is clear that CL intensity is 
not uniform and shows strong contrast between dark 
and bright regions. The bright spots in the image rep-
resent high CL emission intensity while dark spots 
represent low emission intensity. The non-uniform CL 
emission is associated with the presence of extended 
crystalline defects such as dislocations which can act as 
the radiative centers. However, comparison between 
monochromatic CL imaging at 364 nm and 550 nm can 
give more information about these defects which is a 
matter of future investigation. 
 
4. CONCLUSIONS 
 
A different route for the fabrication of highly vertical-
ly aligned GaN nanorods on c-plane sapphire is reported 
in this study. We fabricated GaN nanorods using a top-
down approach by nanomasking and reactive ion etch-
ing.  The FESEM images indicated the GaN nanorods 
with diameter ranging between 100-250 nm were grown 
via top-down approach. FESEM images also indicated 
the highly vertical alignment of these nanorods. The CL 
analysis confirmed the formation and high optical prop-
erties of GaN nanorods. These studies give us more un-
derstanding about the fundamental properties of GaN 
nanomaterials and provide useful information in the 
application of GaN nanorods-based devices. 
 
AKNOWLEDGEMENTS 
 
Ashutosh Kumar is grateful to Council of Scientific 
and Industrial Research (CSIR) India for Senior Re-
search Fellowship. The authors are thankful to Prof. 
Vikram Kumar for his valuable suggestion during this 
work. The authors (A.K. and R.S) are thankful to De-
partment of Information Technology, Govt. of India for 
setting up Nanoscale Research Facility at IIT Delhi. 
 
 
REFERENCES 
 
1. Y. Xia, P. Yang, Y. Sun, Y. Bu, B. Mayers, B. Gates, 
Y. Fin, F. Kim, H. Yan, Adv. Mater. 15, 353 (2003). 
2. J. Appenzeller, J. Knoch, T.M. Bjork, H. Riel, H. Schmid, 
W. Riess, IEEE T. Electron Dev. 55, 2827 (2008). 
3. X. Duan, Y. Huang, Y. Cui, J. Wang, C. M. Lieber, Nature 
409, 66 (2001). 
4. Y. Huang, X. Duan, Y. Cui, C.M. Lieber, Nano Lett. 2, 101 
(2002). 
5. S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64, 
1689 (1994). 
6. J. Zhu, L. Wang, S. Zhang, H. Wang, D. Zhao, J. Zhu, 
Z. Liu, D. Jiang, H. Yang, J. Appl. Phys. 108, 074302 
(2010). 
7. C.J. Neufeld, C. Schaake, M. Grundmann, 
N.A. Fichtenbaum, S. Keller, U.K. Mishra, phys. status 
solidi c 4, 1605 (2007). 
8. L. Nguyen, Gallium Nitride Nanowires, COSMOS (2012). 
9. T. Fujii, Y. Gao, R. Sharma, E.L. Hu, S.P. DenBaars, 
S. Nakamura, Appl. Phys. Lett. 84, 855 (2004). 
10. C. Huh, K.-S. Lee, E.-J. Kang, S.-J. Park, J. Appl. Phys. 
93, 9383 (2003). 
11. H.W. Huang, J.T. Chu, C.C. Kao, T.H. Hseuh, T.C. Lu, 
H.C. Kuo, S.C. Wang, C.C. Yu, Nanotechnology 16, 1844 
(2005). 
12. R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4, 89 (1964). 
13. W. Han. S. Fan, Q. Li, Y. Hu, Science 277, 1287 (1997). 
14. S. Li, A. Waag, J. Appl. Phys. 111, 071101 (2012). 
15. V. Ramesh, A. Kikuchi, K. Kishino, M. Funato, 
Y. Kawakami, J. Appl. Phys. 107, 114303 (2010).  
16. C.H. Chiu, T.C. Lu, H.W. Huang, C.F. Lai, C.C. Kao, 
J.T. Chu, C.C. Yu, H.C. Kuo, S.C. Wang, C.F. Lin, 
T.H. Hsueh, Nanotechnology 18, 445201 (2007). 
17. H.W. Huang, J.T. Chu, T.H. Hsueh, M.C. Ou-Yang, 
H.C. Kuo, S.C. Wang, J. Vac. Sci. Technol. B 24, 1909 
(2006). 
18. S.O. Kucheyev, M. Toth, M.R. Phillips, J.S. Williams, 
C. Jagadish, Appl. Phys. Lett. 79, 2154 (2001). 
19. H.D.P. Schenk, S.I. Borenstain, A. Berezin, A. Schön, 
E. Cheifetz, S. Khatsevich, D.H. Rich, J. Appl. Phys. 103, 
103502 (2008). 
 


Characterization of GaN Nanorods Fabricated Using Ni Nanomasking and Reactive Ion Etching: A Top-Down Approach

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS  ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 5 No 2, 02001(3pp) (2013)  Том 5 № 2, 02001(3cc) (2013)   2077-6772/2013/5(2)02001(3)  02001-1   2013 Sumy State University Characterization of GaN Nanorods Fabricated Using Ni Nanomasking and  Reactive Ion Etching: A Top-Down Approach  Ashutosh Kumar1,*, Michael Latzel2, C. Tessarek2, S. Christiansen2, R. Singh1,†  1  Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, 110016 New Delhi, India 2  Max Planck Institute for the Science of Light, 24, Günther-Scharowsky-Str. 1/Bau, 91058 Erlangen, Germany  (Received 15 February 2013; published online 04 May 2013)  Large thermal mismatch between GaN surface and sapphire results in compressive stress in Gallium Nitride (GaN) layer which degrades the device performance. Nanostructuring the GaN can reduce this stress leading to reduction in Quantum Confined Stark Effect. Aligned GaN nanorods based nanodevices have potential applications in electronics and optoelectronics. This paper describes the fabrication of GaN nanorods using Ni nanomasking and reactive ion etching. The morphology of GaN nanorods was studied by field emission scanning electron microscopy. The optical properties of GaN nanorods were studied by Cathodoluminescence (CL) spectroscopy. CL results revealed the existence of characteristic band-edge lu-minescence and yellow band luminescence.  Keywords: GaN, Nanorod, Nanomasking, Reactive-ion etching, Cathodoluminescence, Band-edge  luminescence, Yellow band luminescence.    PACS numbers: 81.16. – c, 81.16.rf, 78.67. – n, 78.60.Hk                                                               * ashutosh.kumar@physics.iitd.ac.in † rsingh@physics.iitd.ac.in 1.  INTRODUCTION  Semiconductor  nanostructures  have  recently emerged as the leading material for the fabrication of electronic [1-2] and optoelectronic devices [3]. Over the last decade, use of Gallium Nitride (GaN) nanostruc-tures  for  high  power  transistors  [4]  and  solid  state lightning  [5-7]  is  an  interesting  area  for  researchers. The main reason behind these interests is the fact that nanostructuring can reduce the compressive stress in GaN layer grown over sapphire leading to reduction in Quantum Confined Stark Effect. Also, due to direct and wide band gap of GaN, its nanostructures can be used for high temperature and high power electronic devices applications  [8].  GaN nanorods  based nanodevices  do have some advantages as compared to bulk GaN based devices which are listed below [9-11]:   High  surface  to  volume  ratio:  increase  in  active area for LEDs and solar cells.   Strain  relaxation  in  GaN  nanorods:  increase  in radiative recombination efficiency in multiple quantum wells  (MQWs)  due  to  reduction  in  quantum  confined stark effect.   Growth of MQWs on non-polar planes: increase in radiative recombination efficiency in MQWs and wave-length stability.   Reduction  in  threading  dislocation  density:  in-crease in performance of nanodevices. GaN  nanorods  can  be  fabricated  using  top-down and down-top approach. The down-top approach which is a self organized growth technique includes catalyst assisted Vapor-Liquid-Solid (VLS) growth, catalyst free or  self-induced  growth  and  selective  area  growth  on nano-patterned substrate. GaN nanorods are generally synthesized using VLS mechanism which was first de-veloped by Wagner and Ellis [12] in 1964. Han et al. [13] in 1997 was the first to demonstrate successfully the fabrication of GaN nanowires. VLS growth methods do  have  disadvantages  of  requiring  highly  specific growth conditions to increase the on-axis growth rate while minimizing the lateral growth rate. This can lead to non-optimal material quality and less flexibility in material design such as doping and heterostructures.  For fabrication of a device using 1D GaN nanostruc-ture, it is essential to develop a process to make con-tacts  to  individual  nanowires  or  nanorods  as  well  as array  of  nanowires  or  nanorods  which  lacks  in  VLS mechanism. One solution to this problem is top-down approach  where  GaN  nanorods  can  be  fabricated  by etching  GaN  layer  using  nanomasks.  This  approach includes  methods  like  conventional  photolithography, e-beam  lithography,  self-organized  nanomasking  or nanosphere  lithography  [14,  16-17].  In  these  tech-niques, dry etching methods like as RIE-ICP are used to obtain high aspect ratio. High aspect ratio of nano-rods can be achieved using techniques like e-beam li-thography  or  photolithography  but  these  techniques have  limitations  in  nanofabrication  over  large  area substrates.  However,  techniques  like  self-assembled nanomasking and nanosphere lithography are cost ef-fective  and  applicable  to  nanofabrication  over  large area substrates. Ramesh et al. [15] demonstrated blue shift in Photoluminescence (PL) emission peak as well as improvement in PL internal quantum efficiency in GaN nanoLED fabricated using top-down approach as compared  to  as  grown  down-top  GaN  nanoLED.  An enhancement of PL intensity and blue shift of PL emis-sion  peak  (which  indicates  the  strain  relaxation)  in GaN nanorods fabricated using top-down approach are also reported by various groups [16-17]. So in this work, we employed a top-down approach for the fabrication of GaN nanorods using nanomasking and reactive ion etching (RIE) for nanoelectronics and optoelectronics applications.    A. KUMAR, M. LATZEL, S. TESSAREK, ET AL.  J. NANO- ELECTRON. PHYS. 5, 02001 (2013)   02001-2 2.  EXPERIMENTAL  The samples used in this study were cut from nom-inally  undoped  GaN  wafer  of  thickness  about  2  m grown  on  c-plane  sapphire substrate  by  metalorganic chemical-vapor  deposition  (MOCVD)  technique.  GaN nanorods were fabricated using top-down approach by Nickel  (Ni)  nanomasking  and  reactive  ion  etching.  Prior to nanorods fabrication, samples were ultrasoni-cally  cleaned  in  trichloroethylene,  acetone  and  iso-propanol respectively at 70 ºC. The samples were then dipped in 1:1 HCl for 2 min to remove the native ox-ides.  Finally  the  clean  samples  were  rinsed  with  de-ionized  water  and  blown  with  dry  nitrogen.  A  SiO2  layer  of  thickness  about  100 nm  was  deposited  over  n-GaN using sputtering followed by sputter deposition of Ni metal film. Then rapid thermal annealing (RTA) was performed at 800 ºC for 60 sec under nitrogen am-bient to produce Ni metal nano-islands. In next step, reactive ion etching (RIE) of SiO2 was performed fol-lowed by selective RIE of n-GaN layer. SiO2 layer was selective etched using CF4 plasma while selective etch-ing of GaN layer was performed using Cl2/Ar plasma. Finally to fabricate GaN nanorods, Ni and SiO2 were etched  using  HF  for  60 sec.  The  HF  etching  also  re-moves any other contaminants. The schematic for fab-rication of GaN nanorods using Ni nanomasking and reactive ion etching is shown in Fig. 1.    Fig. 1 – Schematic diagram for fabrication of GaN nanorods using Ni nanomasking and reactive ion etching technique  3.  RESULTS AND DISCUSSION   3.1  Morphological Studies of GaN Nanorods  The  shape  and  morphology  of  the  GaN  nanorods were  observed  using  field  emission  scanning  electron microscopy (FESEM). The side view image of GaN na-norods  fabricated  using  top-down  approach  taken  at different  magnification  (× 20,000  and  × 50,000)  is shown in Fig. 2a, b. From the side-view secondary elec-tron image of the GaN nanorods cluster, the diameters of the nanorods on the sapphire substrate were about 100 to 250 nm. High vertical alignment of these nano-rods is also clear from secondary electron images.  3.2  Cathodoluminescence Studies  Cathodoluminescence (CL) measurements were also performed in order to investigate the light emission      Fig. 2 – FESEM  image  of  GaN  nanorods  at  two  different magnifications (× 20,000 and × 50,000)    Fig. 3 – CL spectrum of GaN nanorods at room temperature from mapped region showing two peaks which corresponds to band-edge luminescence (at 3.4 eV) and yellow band lumines-cence (centred about 2.3 eV)  properties  of  GaN  nanorods.  The  CL  measurements reported  here  were  performed  at  room  temperature using standard CL measurement set-up with accelerat-ing voltage of 5 kV and electron beam current of 20  A. Fig. 3 shows the CL spectrum of GaN nanorods from the mapped region. The CL spectrum shows a stronger peak from GaN band edge luminescence at 3.4 eV (364 nm) and an ad-ditional  broad  peak  centred  about  2.3 eV  (550 nm) which is commonly known as yellow band luminescence (YL).  Both  the  peaks  are  characteristics  of  the  GaN nanostructures and also earlier reported in GaN   CHARACTERIZATION OF GAN NANORODS FABRICATED…  J. NANO- ELECTRON. PHYS. 5, 02001 (2013)   02001-3     Fig. 4 – (a)  FESEM  image  and  (b)  corresponding  panchro-matic CL image of GaN nanorods  nanostructures fabricated from down-top approach [18-19]. The existence of YL indicates that there is a deep level  located  at  about  2.3 eV  below  conduction  band edge  and  recombination  of  conduction  electrons through this deep level can give yellow emission A  plan-view  integrated  panchromatic  CL  image from GaN nanorods acquired with an accelerating volt-age  of  5 kV  and  electron  beam  current  of  20  A  is shown in Fig. 4a, b. Fig. 4a shows the FESEM image of GaN  nanorods  and  Fig. 4b  shows  the  corresponding panchromatic CL image. It is clear that CL intensity is not uniform and shows strong contrast between dark and bright regions. The bright spots in the image rep-resent  high  CL  emission  intensity  while  dark  spots represent low emission intensity. The non-uniform CL emission  is  associated  with  the  presence  of  extended crystalline defects such as dislocations which can act as the  radiative  centers.  However,  comparison  between monochromatic CL imaging at 364 nm and 550 nm can give more information about these defects which is a matter of future investigation.  4.  CONCLUSIONS  A different route for the fabrication of highly vertical-ly aligned GaN nanorods on c-plane sapphire is reported in this study. We fabricated GaN nanorods using a top-down approach by nanomasking and reactive ion etch-ing.  The FESEM images indicated the GaN nanorods with diameter ranging between 100-250 nm were grown via  top-down  approach.  FESEM  images  also  indicated the highly vertical alignment of these nanorods. The CL analysis confirmed the formation and high optical prop-erties of GaN nanorods. These studies give us more un-derstanding  about  the  fundamental  properties  of  GaN nanomaterials  and  provide  useful  information  in  the application of GaN nanorods-based devices.  AKNOWLEDGEMENTS  Ashutosh Kumar is grateful to Council of Scientific and  Industrial  Research  (CSIR)  India  for  Senior  Re-search  Fellowship.  The  authors  are  thankful  to  Prof. Vikram Kumar for his valuable suggestion during this work. The authors (A.K. and R.S) are thankful to De-partment of Information Technology, Govt. of India for setting up Nanoscale Research Facility at IIT Delhi.   REFERENCES  1.  Y. Xia,  P. Yang,  Y. Sun,  Y. Bu,  B. Mayers,  B. Gates, Y. Fin, F. Kim, H. Yan, Adv. 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Gallium Nitride Nanowires,

phys. status solidi c 4,

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Characterization of GaN Nanorods Fabricated Using Ni Nanomasking and Reactive Ion Etching: A Top-Down Approach

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