Wurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia. The resulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron microscopy and transmission electron microscopy revealed both thin smooth and thick corrugated nanowires. The growth axes of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (\u3c0001\u3e), while the corrugated nanowires and the large polyhedra grew parallel to \u3c0001\u3e. We propose a model to explain these morphology variations in terms of the Ga/N ratio and the different characteristic lengths of {0001} polar surfaces in the different nanostructures

Fischer, John E

Nam, Chang-Yong

Tham, Douglas

English

Kosmopolis

University of Pennsylvania
ScholarlyCommons
Departmental Papers (MSE) Department of Materials Science & Engineering
December 2004
Effect of the polar surface on GaN nanostructure
growth and morphology
Chang-Yong Nam
University of Pennsylvania
Douglas Tham
University of Pennsylvania
John E. Fischer
University of Pennsylvania, fischer@seas.upenn.edu
Follow this and additional works at: http://repository.upenn.edu/mse_papers
Postprint version. Published in Applied Physics Letters, Volume 85, Issue 23, December 6, 2004, pages 5676-5678.
Publisher URL: http://dx.doi.org/10.1063/1.1829780
This paper is posted at ScholarlyCommons. http://repository.upenn.edu/mse_papers/50
For more information, please contact libraryrepository@pobox.upenn.edu.
Recommended Citation
Nam, C., Tham, D., & Fischer, J. E. (2004). Effect of the polar surface on GaN nanostructure growth and morphology. Retrieved from
http://repository.upenn.edu/mse_papers/50
Effect of the polar surface on GaN nanostructure growth and morphology
Abstract
Wurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia. The
resulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were obtained,
while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron microscopy and
transmission electron microscopy revealed both thin smooth and thick corrugated nanowires. The growth
axes of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (<0001>), while
the corrugated nanowires and the large polyhedra grew parallel to <0001>. We propose a model to explain
these morphology variations in terms of the Ga/N ratio and the different characteristic lengths of {0001}
polar surfaces in the different nanostructures.
Comments
Postprint version. Published in Applied Physics Letters, Volume 85, Issue 23, December 6, 2004, pages
5676-5678.
Publisher URL: http://dx.doi.org/10.1063/1.1829780
This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/50
Applied Physics Letters Dec. 6 2004 issue 
 
Effect of the polar surface on GaN nanostructure growth and morphology 
C. Y. Nam, D. Tham and J. E. Fischer*
Department of Materials Science and Engineering, University of Pennsylvania, 
3231 Walnut Street, Philadelphia, PA 19104-6272 
• fischer@seas.upenn.edu 
 
 
Wurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia.  
The resulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were 
obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron 
microscopy and transmission electron microscopy revealed both thin smooth and thick corrugated   
nanowires. The growth axes of most of the smooth ones, as well as the nanobelts, were perpendicular to 
the c-axis (<0001>), while the corrugated nanowires and the large polyhedra grew parallel to <0001>. 
We propose a model to explain these morphology variations in terms of the Ga/N ratio and the different 
characteristic lengths of {0001} polar surfaces in the different nanostructures. 
 
Despite numerous reports on gallium nitride (GaN) nanostructures, such as nanowires1 and nanobelts 
(or nanoribbons)2, only a few studies have concentrated on controlling these morphologies within a 
given synthesis method. Synthesis schemes mostly involve one of two typical growth mechanisms. The 
vapor-liquid-solid (VLS) mechanism utilizes a transition metal catalyst while the vapor-solid (VS) 
mechanism relies on direct crystallization from the vapor. For VLS we expect the dominant morphology 
to be nanowires because nucleation and growth are defined by a liquid catalyst particle.1 Without this 
constraint, i.e. for VS, one might expect the nanostructure morphology to be more easily influenced by 
growth conditions and thus more varied. 
ElAhl et. al. recently reported a morphology variation of GaN structures under different growth 
conditions during direct reaction of Ga and ammonia (NH3).3 They observed nanowires and microrods 
but did not observe a transition to the nanobelt morphology. In this letter, we report the morphological 
evolution of GaN nanostructures from nanowires to polyhedral crystals to nanobelts by varying the NH3 
flow rate in the thermal reaction of gallium oxide (Ga2O3) and NH3 at 1100℃. We propose that the 
Ga/N ratio in the vapor phase plays an important role in determining the resulting morphology, based on 
the observed relationship between the morphology and the characteristic length of {0001} polar 
surfaces.  
No catalyst was used in the synthesis. Micro Raman spectroscopy (Renishaw 1000) and X-ray 
diffraction (XRD) confirmed that all morphologies were wurtzite GaN. Scanning electron microscopy 
(SEM, JEOL 6300FV) and high-resolution transmission electron microscopy (TEM, JEOL 2010F) were 
used to investigate the detailed morphology and crystallography of grown structures. 
0.3 g of Ga2O3 powder (Alfa Aesar 99.999%, mesh size 350, ~40 µm) in an alumina boat was placed 
at the center of a 36” long quartz tube with 3/4” diameter in a horizontal three-zone tube furnace (figure 
1). Cleaned Si substrates with only a native SiOx layer were placed downstream 7-10” from the source. 
The furnace was ramped to 300℃ and held for 1 hr while purging with 60 sccm N2. Next, the gas was 
switched to NH3, and the temperature of the center zone was ramped to 1100℃ and held for 2 hr while 
the flow rate was maintained at either 75 or 175 sccm. Substrate temperatures at different positions were 
determined from a previously measured temperature profile, and the pressure in the reactor was ~1 atm 
at all conditions. 
Figure 2 shows SEM images of three typical morphologies obtained at different growth conditions; 
table 1 provides a summary. At 75 sccm, nanowires were observed for all substrate positions, the 
temperature being in the range 921-1034℃. Lengths up to 50 µm, and diameters from 50 nm to 300 nm, 
1
 
were observed (figure 2a). In contrast, at 175 sccm different products were obtained on different 
substrate positions. For 7-8” downstream (1034-1000℃), a mixture of nanowires, nanobelts (figure 2b) 
and polyhedral crystals (figure 2c) was obtained, while for 9-10” (970-921℃) only nanobelts were 
obtained. Nanobelts could be more than 500 nm wide, with similar lengths to the nanowires and smooth 
broad surfaces. Polyhedral crystals were up to 10 µm long with diameters from ~500 nm to ~5 µm, and 
well-defined six side facets suggesting growth orientation parallel to the c-axis (<0001>).  
TEM revealed two distinct nanowire morphologies: relatively coarse with corrugated surfaces (figure 
3a), or thin and smooth-surfaced (figure 3b). Diameters of smooth-surfaced nanowires were uniform 
along their lengths, while for the corrugated ones the diameters oscillated along the length. Selected area 
diffraction patterns (SADP) showed unambiguously that both types of nanowires were single crystals, 
the corrugated surfaces being in fact crystal facets. More interestingly, the two surface morphologies 
correlated well with two different growth orientations. Most of the smooth-surfaced nanowires had 
growth orientations perpendicular to <0001>, e.g. <2-1-10> and <10-10>, while the corrugated ones had 
growth orientations parallel to <0001>, or occasionally containing a c-axis component, e.g. <-1101>. It 
is significant that smooth wires were much more prevalent than corrugated ones for all growth 
conditions. Nanobelts had the same growth orientations as smooth wires (figure 3b,c), orientations 
perpendicular to <0001> with the broad face corresponding to {0001}. 
Wurtzite GaN contains polar and non-polar surfaces. In the following we suggest that the difference in 
the growth rates of these chemically and structurally distinct surfaces have a major influence on the 
resulting nanostructure morphologies. It should be noted that unlike other III-V semiconductors such as 
GaAs, polar surfaces in GaN, e.g. {0001}, always tend to be Ga-stabilized independent of the chemical 
environment (Ga- or N-rich), whereas non-polar surfaces generally maintain their Ga-N 
stoichiometry.4,5 Ga atoms thus have a significantly lower diffusion barrier on the polar surface because 
the adsorbate-surface interaction is predominantly realized by very weak delocalized metallic Ga-Ga 
bonds (Tmelt=30℃ ).4 An important consequence is that, similar to GaN thin film growth6, the 
nanostructure morphology will mainly display broad and smooth two-dimensionally grown polar 
surfaces under Ga-rich growth conditions, while under N-rich conditions it will mainly exhibit rough 
surfaces or facets composed of polar and non-polar surfaces. It is known that excess N (or N reactant) 
reduces the Ga effective diffusion length on the polar surface, and this in turn induces rough or facetted 
surfaces.4 We apply this reasoning to understand the observed morphology variations. First, we estimate 
the Ga/N reactant ratios vs. substrate position from the reaction chemistry and the reactant’s dispersion 
in the reactor.   
In the thermal reaction of Ga2O3 and NH3, Ga2O3 will first be converted to Ga2O(g): 7
Ga2O3(s) + 2H2(g)  Ga2O(g) + 2H2O(g) 
or Ga2O3(s)  Ga2O(g) + O2(g). 
The Ga2O(g) then reacts with NH3 to form GaN either on the surface of growing GaN or in vapor by the 
following reaction:  
Ga2O(g) + 2NH3(g)  2GaN(s) + H2O(g) + 2H2(g) 
We expect direct reaction of Ga and NH3 to be also possible because Ga2O(g) can back react with H2 to 
form Ga on the GaN surface, i.e.:   7
Ga2O(g) + H2  2Ga(l) + H2O(g) 
2Ga(l) + 2NH3(g)  2GaN(s) + 3H2(g) 
For simplicity, we use the former GaN formation reaction in the discussion. To estimate Ga/N reactant 
ratio vs. substrate position, we need to understand the distribution of Ga2O(g) in the stream of NH3. 
Since the equilibrium vapor pressure of Ga2O(g) at the reaction temperature (T=1100℃) is very small 
(~10-6 atm)7 while the total pressure in the reactor is 1 atm, the dispersion of Ga2O(g) in our experiment 
can be treated as a continuous Ga2O(g) point source diffusion under NH3 advection, which implies that  
the forced diffusion of Ga2O(g) by NH3 “carrier gas” is more important than the molecular diffusion 
driven by a concentration gradient. The Reynolds number for our reactor at 175 sccm NH3 and at 1100
℃ is less than 30, so we expect the NH3 flow to be laminar at all conditions. Then, we can apply the 
2
 
mass-balance equation to estimate the steady state Ga2O(g) concentration profile. A general solution for 
uniform flow field in Cartesian coordinates is available in the literature.8 Although more rigorous 
treatment is required to account for the cylindrical geometry of the reactor and the shear dispersion of 
flow field, the solution still predicts that in this type of diffusion there generally exists a maximum in 
the Ga2O(g) concentration downstream of the source, and as the flow rate increases the maximum 
moves farther downstream. 
From the above, we expect the maximum Ga2O(g) concentration to be slightly downstream from the 
source at 75 sccm; increasing to 175 sccm shifts the whole profile farther downstream, leaving the 
region adjacent to the source relatively N-rich (i.e. depleted of Ga2O(g)) while Ga-rich conditions apply 
farther downstream. We assume that this variation in the Ga2O(g) concentration profile is somehow 
connected to the sequential appearance of facetted polyhedral crystals and broad and smooth-surfaced 
nanobelts at the higher flow rate. Conversely, analogous to GaN thin film growth, as reaction conditions 
evolve from relatively Ga-rich to N-rich (i.e. Ga/N ratio higher to lower), the dominant growth 
morphology changes from broad and smooth-surfaced nanobelts to facetted polyhedral crystals. We 
introduce the notion of a variable “characteristic length” of polar surface to explain the connection 
between reaction conditions and nanostructure morphologies. 
Note that the Ga diffusion is, as introduced, more significant on the polar surfaces than on the non-
polar surfaces, and the diffusion length on the polar surfaces varies depending on the reaction condition 
(Ga- or N-rich) while that on the non-polar surfaces is expected to be relatively constant regardless of 
the reaction conditions due to Ga-N stoichiometry on these surfaces. Then, we expect the morphology 
variation at different reaction conditions to be dominated by the change of the “relative length scale” of 
polar surface, which increases under Ga-rich conditions (longer Ga diffusion length and more Ga flux), 
and vice versa. For instance, if the reaction condition is relatively more Ga-rich, we expect significant 
anisotropy in the morphology due to the larger difference of Ga diffusion length between the polar 
surface and the non-polar surface. That is, polar surfaces might be expected to display larger lengths, or 
surface areas, under high Ga flux. In contrast, under more N-rich reaction conditions, a less anisotropic 
morphology with smaller lengths of polar surfaces is expected owing to the decreased diffusion length 
difference and Ga flux. This length scale of polar surface in the morphologies defines a characteristic 
length.  
Interestingly, we observed a unique variation in the characteristic length in our nanostructure 
morphologies. In the TEM data, the nanobelt’s broad face was always {0001} polar surface with the 
growth orientation perpendicular to <0001> direction. As illustrated in figure 4, the {0001} surface thus 
extends lengthwise along the growth direction of the nanobelt. For the thin smooth-surfaced nanowire 
grown perpendicular to <0001>, several polar surfaces, including {0001}, exist on the nanowire surface 
and extend lengthwise along the growth direction. Then, the characteristic length for these two 
morphologies is ~50 µm, i.e. the length of nanobelt or smooth-surfaced nanowire. However, for the 
polyhedral crystals and most of the corrugated nanowires grown parallel to <0001>, the characteristic 
lengths of the {0001} polar surfaces are no more than ~5 µm and ~1 µm respectively, i.e. the diameters 
of two morphologies.  
This variation in the characteristic length reveals the relationship between reaction conditions and 
favored nanostructure morphologies. The characteristic length of the {0001} surfaces decreases as the 
reaction condition varies from Ga-rich to N-rich, and concurrently, the favored morphology evolves in 
the sequence: nanobelt, thin smooth-surfaced nanowire, polyhedral crystal and coarse corrugated 
nanowire (figure 4). We can safely exclude the temperature effect on the morphology variation since the 
difference between the highest and the lowest substrate temperatures was only ~100℃, which leads to 
virtually no difference in the surface diffusivity for any given surface.  
3
In summary, the morphology of wurtzite GaN nanostructures can be controlled by changing the NH3 
flow rate during thermal reaction of Ga2O3 and NH3. When the lower NH3 flow rate (75 sccm) was used, 
only nanowires were observed, while polyhedral crystals and nanobelts were obtained when the higher 
NH3 flow rate (175 sccm) was applied. Based on SEM and TEM observations, and considering Ga2O(g) 
profile and the variation of the characteristic length of {0001} polar surfaces, we propose the variation 
 
of Ga/N reactant ratio with substrate positions determines the resultant GaN nanostructure morphology. 
Specifically, as the reaction condition varies from relatively Ga-rich to N-rich condition, the favored 
growth morphology changes from nanobelts, thin smooth-surfaced nanowires, and polyhedral crystals to 
coarse corrugated nanowires 
 
ACKNOWLEDGMENT  
 This research was supported by the NSF/NIRT Program, Grant No. DMR03-04178.  We also 
acknowledge the use of shared facilities supported by NSF/MRSEC: DMR02-03378. 
 
REFERENCES  
Duan, X.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. 
Li, J. Y.; Qiao, Z. Y.; Chen, X. L.; Cao, Y. G.; Lan, Y. C.; Wang, C. Y. J. Appl. Phys. A 2000, 71, 587. 
ElAhl, A. M. S.; He, M.; Zhou, P.; Harris, G. L.; Salamanca-Riba, L.; Felt, F.; Shaw, H. C.; Sharma, A.; 
Jah, M.; Lakins, D.; Steiner, T.; Mohammad, S. N. J. Appl. Phys. 2003, 94, 7749. 
Zywietz, T.; Neugebauer, J.; Scheffler, M. Appl. Phys. Lett. 1998, 73, 487. 
Smith, A. R.; Feenstra, R. M.; Greve, D. W.; Shin, M. S.; Skowronski, M.; Neugebauer, J.; Northrup, J. 
E. J. Vac. Sci. Technol. B 1998, 16, 2242  
Botchkarev, A.; Salvador, A.; Sverdlov, B.; Myoung, J.; Morkoç, H. J. Appl. Phys. 1995, 77, 4455. 
Butt, D. P.; Park, Y.; Taylor, T. N. J. Nucl. Mater. 1999, 264, 71. 
Fischer, H. B.; List, E. J.; Koh, R. C. Y.; Imberger, J.; Brooks, N. H. Mixing in Inland and Coastal 
Waters, Academic Press: San Diego, CA, 1979; pp 50-54.  
4
 
 
 
Table 1. Summary of growth conditions and corresponding GaN morphologies. 
Substrate position  
(inch away from the source)NH3 flow rate 
7, 8 9, 10 
75 sccm NW NW 
175 sccm NW+XL+NB NB 
NW: Nanowire, NB: Nanobelt, XL: Polyhedral crystal 
 
 
 
Figure 1. Schematic illustration of a GaN reactor. The substrate temperature is varying from 1034℃ to 
921 ℃  as the substrate position goes downstream. 
 
 
 
Figure 2. SEM micrographs of (a) nanowires, (b) nanobelts and (c) polyhedral crystals grown on a Si 
substrate. No metal coating was applied for SEM imaging. 
 
 
5
 
Figure 3. TEM micrographs of typical (a) coarse corrugated nanowire, (b) thin smooth-surfaced 
nanowire and (c) nanobelt. In the image, B denotes zone axis, and the indices next to arrows indicate 
growth orientations. Note that while corrugated naowire’s growth orientation is parallel to the c-axis 
(<0001>) of a wurtzite structure, both smooth-surfaced nanowire and nanobelt’s growth orientations are 
perpendicular to <0001> direction. In the nanobelt’s image, the fringes on the surface are due to strain.  
 
 
 
 
 
Figure 4. Model for the relationship between the GaN nanostructure morphology and the reaction 
condition. The shaded area indicates {0001} polar surface. As the reaction condition becomes relatively 
more N-rich the characteristic length of {0001} surface decreases in the morphologies. In the smooth-
surfaced nanowire, there should be only two tangential {0001} polar surfaces running parallel to the 
growth orientation, but extra shaded lines were inserted to indicate other possible tangential polar 
surfaces on the nanowire surface, e.g. {1-101}. Note in a wurtzite structure there is no polar plane 
running parallel to <0001> direction, the growth orientation of polyhedral crystal or corrugated 
nanowire, i.e. any surfaces running parallel to <0001> is always non-polar. 
 
 
 
 
6
 


Effect of the polar surface on GaN nanostructure growth and morphology

ScholarlyCommons@Penn

University of PennsylvaniaScholarlyCommonsDepartmental Papers (MSE) Department of Materials Science & EngineeringDecember 2004Effect of the polar surface on GaN nanostructuregrowth and morphologyChang-Yong NamUniversity of PennsylvaniaDouglas ThamUniversity of PennsylvaniaJohn E. FischerUniversity of Pennsylvania, fischer@seas.upenn.eduFollow this and additional works at: http://repository.upenn.edu/mse_papersPostprint version. Published in Applied Physics Letters, Volume 85, Issue 23, December 6, 2004, pages 5676-5678.Publisher URL: http://dx.doi.org/10.1063/1.1829780This paper is posted at ScholarlyCommons. http://repository.upenn.edu/mse_papers/50For more information, please contact libraryrepository@pobox.upenn.edu.Recommended CitationNam, C., Tham, D., & Fischer, J. E. (2004). Effect of the polar surface on GaN nanostructure growth and morphology. Retrieved fromhttp://repository.upenn.edu/mse_papers/50Effect of the polar surface on GaN nanostructure growth and morphologyAbstractWurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia. Theresulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were obtained,while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron microscopy andtransmission electron microscopy revealed both thin smooth and thick corrugated nanowires. The growthaxes of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (<0001>), whilethe corrugated nanowires and the large polyhedra grew parallel to <0001>. We propose a model to explainthese morphology variations in terms of the Ga/N ratio and the different characteristic lengths of {0001}polar surfaces in the different nanostructures.CommentsPostprint version. Published in Applied Physics Letters, Volume 85, Issue 23, December 6, 2004, pages5676-5678.Publisher URL: http://dx.doi.org/10.1063/1.1829780This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/50Applied Physics Letters Dec. 6 2004 issue  Effect of the polar surface on GaN nanostructure growth and morphology C. Y. Nam, D. Tham and J. E. Fischer*Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272 • fischer@seas.upenn.edu   Wurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia.  The resulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron microscopy and transmission electron microscopy revealed both thin smooth and thick corrugated   nanowires. The growth axes of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (<0001>), while the corrugated nanowires and the large polyhedra grew parallel to <0001>. We propose a model to explain these morphology variations in terms of the Ga/N ratio and the different characteristic lengths of {0001} polar surfaces in the different nanostructures.  Despite numerous reports on gallium nitride (GaN) nanostructures, such as nanowires1 and nanobelts (or nanoribbons)2, only a few studies have concentrated on controlling these morphologies within a given synthesis method. Synthesis schemes mostly involve one of two typical growth mechanisms. The vapor-liquid-solid (VLS) mechanism utilizes a transition metal catalyst while the vapor-solid (VS) mechanism relies on direct crystallization from the vapor. For VLS we expect the dominant morphology to be nanowires because nucleation and growth are defined by a liquid catalyst particle.1 Without this constraint, i.e. for VS, one might expect the nanostructure morphology to be more easily influenced by growth conditions and thus more varied. ElAhl et. al. recently reported a morphology variation of GaN structures under different growth conditions during direct reaction of Ga and ammonia (NH3).3 They observed nanowires and microrods but did not observe a transition to the nanobelt morphology. In this letter, we report the morphological evolution of GaN nanostructures from nanowires to polyhedral crystals to nanobelts by varying the NH3 flow rate in the thermal reaction of gallium oxide (Ga2O3) and NH3 at 1100℃. We propose that the Ga/N ratio in the vapor phase plays an important role in determining the resulting morphology, based on the observed relationship between the morphology and the characteristic length of {0001} polar surfaces.  No catalyst was used in the synthesis. Micro Raman spectroscopy (Renishaw 1000) and X-ray diffraction (XRD) confirmed that all morphologies were wurtzite GaN. Scanning electron microscopy (SEM, JEOL 6300FV) and high-resolution transmission electron microscopy (TEM, JEOL 2010F) were used to investigate the detailed morphology and crystallography of grown structures. 0.3 g of Ga2O3 powder (Alfa Aesar 99.999%, mesh size 350, ~40 µm) in an alumina boat was placed at the center of a 36” long quartz tube with 3/4” diameter in a horizontal three-zone tube furnace (figure 1). Cleaned Si substrates with only a native SiOx layer were placed downstream 7-10” from the source. The furnace was ramped to 300℃ and held for 1 hr while purging with 60 sccm N2. Next, the gas was switched to NH3, and the temperature of the center zone was ramped to 1100℃ and held for 2 hr while the flow rate was maintained at either 75 or 175 sccm. Substrate temperatures at different positions were determined from a previously measured temperature profile, and the pressure in the reactor was ~1 atm at all conditions. Figure 2 shows SEM images of three typical morphologies obtained at different growth conditions; table 1 provides a summary. At 75 sccm, nanowires were observed for all substrate positions, the temperature being in the range 921-1034℃. Lengths up to 50 µm, and diameters from 50 nm to 300 nm, 1 were observed (figure 2a). In contrast, at 175 sccm different products were obtained on different substrate positions. For 7-8” downstream (1034-1000℃), a mixture of nanowires, nanobelts (figure 2b) and polyhedral crystals (figure 2c) was obtained, while for 9-10” (970-921℃) only nanobelts were obtained. Nanobelts could be more than 500 nm wide, with similar lengths to the nanowires and smooth broad surfaces. Polyhedral crystals were up to 10 µm long with diameters from ~500 nm to ~5 µm, and well-defined six side facets suggesting growth orientation parallel to the c-axis (<0001>).  TEM revealed two distinct nanowire morphologies: relatively coarse with corrugated surfaces (figure 3a), or thin and smooth-surfaced (figure 3b). Diameters of smooth-surfaced nanowires were uniform along their lengths, while for the corrugated ones the diameters oscillated along the length. Selected area diffraction patterns (SADP) showed unambiguously that both types of nanowires were single crystals, the corrugated surfaces being in fact crystal facets. More interestingly, the two surface morphologies correlated well with two different growth orientations. Most of the smooth-surfaced nanowires had growth orientations perpendicular to <0001>, e.g. <2-1-10> and <10-10>, while the corrugated ones had growth orientations parallel to <0001>, or occasionally containing a c-axis component, e.g. <-1101>. It is significant that smooth wires were much more prevalent than corrugated ones for all growth conditions. Nanobelts had the same growth orientations as smooth wires (figure 3b,c), orientations perpendicular to <0001> with the broad face corresponding to {0001}. Wurtzite GaN contains polar and non-polar surfaces. In the following we suggest that the difference in the growth rates of these chemically and structurally distinct surfaces have a major influence on the resulting nanostructure morphologies. It should be noted that unlike other III-V semiconductors such as GaAs, polar surfaces in GaN, e.g. {0001}, always tend to be Ga-stabilized independent of the chemical environment (Ga- or N-rich), whereas non-polar surfaces generally maintain their Ga-N stoichiometry.4,5 Ga atoms thus have a significantly lower diffusion barrier on the polar surface because the adsorbate-surface interaction is predominantly realized by very weak delocalized metallic Ga-Ga bonds (Tmelt=30℃ ).4 An important consequence is that, similar to GaN thin film growth6, the nanostructure morphology will mainly display broad and smooth two-dimensionally grown polar surfaces under Ga-rich growth conditions, while under N-rich conditions it will mainly exhibit rough surfaces or facets composed of polar and non-polar surfaces. It is known that excess N (or N reactant) reduces the Ga effective diffusion length on the polar surface, and this in turn induces rough or facetted surfaces.4 We apply this reasoning to understand the observed morphology variations. First, we estimate the Ga/N reactant ratios vs. substrate position from the reaction chemistry and the reactant’s dispersion in the reactor.   In the thermal reaction of Ga2O3 and NH3, Ga2O3 will first be converted to Ga2O(g): 7Ga2O3(s) + 2H2(g) ? Ga2O(g) + 2H2O(g) or Ga2O3(s) ? Ga2O(g) + O2(g). The Ga2O(g) then reacts with NH3 to form GaN either on the surface of growing GaN or in vapor by the following reaction:  Ga2O(g) + 2NH3(g) ? 2GaN(s) + H2O(g) + 2H2(g) We expect direct reaction of Ga and NH3 to be also possible because Ga2O(g) can back react with H2 to form Ga on the GaN surface, i.e.:   7Ga2O(g) + H2 ? 2Ga(l) + H2O(g) 2Ga(l) + 2NH3(g) ? 2GaN(s) + 3H2(g) For simplicity, we use the former GaN formation reaction in the discussion. To estimate Ga/N reactant ratio vs. substrate position, we need to understand the distribution of Ga2O(g) in the stream of NH3. Since the equilibrium vapor pressure of Ga2O(g) at the reaction temperature (T=1100℃) is very small (~10-6 atm)7 while the total pressure in the reactor is 1 atm, the dispersion of Ga2O(g) in our experiment can be treated as a continuous Ga2O(g) point source diffusion under NH3 advection, which implies that  the forced diffusion of Ga2O(g) by NH3 “carrier gas” is more important than the molecular diffusion driven by a concentration gradient. The Reynolds number for our reactor at 175 sccm NH3 and at 1100℃ is less than 30, so we expect the NH3 flow to be laminar at all conditions. Then, we can apply the 2 mass-balance equation to estimate the steady state Ga2O(g) concentration profile. A general solution for uniform flow field in Cartesian coordinates is available in the literature.8 Although more rigorous treatment is required to account for the cylindrical geometry of the reactor and the shear dispersion of flow field, the solution still predicts that in this type of diffusion there generally exists a maximum in the Ga2O(g) concentration downstream of the source, and as the flow rate increases the maximum moves farther downstream. From the above, we expect the maximum Ga2O(g) concentration to be slightly downstream from the source at 75 sccm; increasing to 175 sccm shifts the whole profile farther downstream, leaving the region adjacent to the source relatively N-rich (i.e. depleted of Ga2O(g)) while Ga-rich conditions apply farther downstream. We assume that this variation in the Ga2O(g) concentration profile is somehow connected to the sequential appearance of facetted polyhedral crystals and broad and smooth-surfaced nanobelts at the higher flow rate. Conversely, analogous to GaN thin film growth, as reaction conditions evolve from relatively Ga-rich to N-rich (i.e. Ga/N ratio higher to lower), the dominant growth morphology changes from broad and smooth-surfaced nanobelts to facetted polyhedral crystals. We introduce the notion of a variable “characteristic length” of polar surface to explain the connection between reaction conditions and nanostructure morphologies. Note that the Ga diffusion is, as introduced, more significant on the polar surfaces than on the non-polar surfaces, and the diffusion length on the polar surfaces varies depending on the reaction condition (Ga- or N-rich) while that on the non-polar surfaces is expected to be relatively constant regardless of the reaction conditions due to Ga-N stoichiometry on these surfaces. Then, we expect the morphology variation at different reaction conditions to be dominated by the change of the “relative length scale” of polar surface, which increases under Ga-rich conditions (longer Ga diffusion length and more Ga flux), and vice versa. For instance, if the reaction condition is relatively more Ga-rich, we expect significant anisotropy in the morphology due to the larger difference of Ga diffusion length between the polar surface and the non-polar surface. That is, polar surfaces might be expected to display larger lengths, or surface areas, under high Ga flux. In contrast, under more N-rich reaction conditions, a less anisotropic morphology with smaller lengths of polar surfaces is expected owing to the decreased diffusion length difference and Ga flux. This length scale of polar surface in the morphologies defines a characteristic length.  Interestingly, we observed a unique variation in the characteristic length in our nanostructure morphologies. In the TEM data, the nanobelt’s broad face was always {0001} polar surface with the growth orientation perpendicular to <0001> direction. As illustrated in figure 4, the {0001} surface thus extends lengthwise along the growth direction of the nanobelt. For the thin smooth-surfaced nanowire grown perpendicular to <0001>, several polar surfaces, including {0001}, exist on the nanowire surface and extend lengthwise along the growth direction. Then, the characteristic length for these two morphologies is ~50 µm, i.e. the length of nanobelt or smooth-surfaced nanowire. However, for the polyhedral crystals and most of the corrugated nanowires grown parallel to <0001>, the characteristic lengths of the {0001} polar surfaces are no more than ~5 µm and ~1 µm respectively, i.e. the diameters of two morphologies.  This variation in the characteristic length reveals the relationship between reaction conditions and favored nanostructure morphologies. The characteristic length of the {0001} surfaces decreases as the reaction condition varies from Ga-rich to N-rich, and concurrently, the favored morphology evolves in the sequence: nanobelt, thin smooth-surfaced nanowire, polyhedral crystal and coarse corrugated nanowire (figure 4). We can safely exclude the temperature effect on the morphology variation since the difference between the highest and the lowest substrate temperatures was only ~100℃, which leads to virtually no difference in the surface diffusivity for any given surface.  3In summary, the morphology of wurtzite GaN nanostructures can be controlled by changing the NH3 flow rate during thermal reaction of Ga2O3 and NH3. When the lower NH3 flow rate (75 sccm) was used, only nanowires were observed, while polyhedral crystals and nanobelts were obtained when the higher NH3 flow rate (175 sccm) was applied. Based on SEM and TEM observations, and considering Ga2O(g) profile and the variation of the characteristic length of {0001} polar surfaces, we propose the variation  of Ga/N reactant ratio with substrate positions determines the resultant GaN nanostructure morphology. Specifically, as the reaction condition varies from relatively Ga-rich to N-rich condition, the favored growth morphology changes from nanobelts, thin smooth-surfaced nanowires, and polyhedral crystals to coarse corrugated nanowires  ACKNOWLEDGMENT   This research was supported by the NSF/NIRT Program, Grant No. DMR03-04178.  We also acknowledge the use of shared facilities supported by NSF/MRSEC: DMR02-03378.  REFERENCES  Duan, X.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. Li, J. Y.; Qiao, Z. Y.; Chen, X. L.; Cao, Y. G.; Lan, Y. C.; Wang, C. Y. J. Appl. Phys. A 2000, 71, 587. ElAhl, A. M. S.; He, M.; Zhou, P.; Harris, G. L.; Salamanca-Riba, L.; Felt, F.; Shaw, H. C.; Sharma, A.; Jah, M.; Lakins, D.; Steiner, T.; Mohammad, S. N. J. Appl. Phys. 2003, 94, 7749. Zywietz, T.; Neugebauer, J.; Scheffler, M. Appl. Phys. Lett. 1998, 73, 487. Smith, A. R.; Feenstra, R. M.; Greve, D. W.; Shin, M. S.; Skowronski, M.; Neugebauer, J.; Northrup, J. E. J. Vac. Sci. Technol. B 1998, 16, 2242  Botchkarev, A.; Salvador, A.; Sverdlov, B.; Myoung, J.; Morkoç, H. J. Appl. Phys. 1995, 77, 4455. Butt, D. P.; Park, Y.; Taylor, T. N. J. Nucl. Mater. 1999, 264, 71. Fischer, H. B.; List, E. J.; Koh, R. C. Y.; Imberger, J.; Brooks, N. H. Mixing in Inland and Coastal Waters, Academic Press: San Diego, CA, 1979; pp 50-54.  4   Table 1. Summary of growth conditions and corresponding GaN morphologies. Substrate position  (inch away from the source)NH3 flow rate 7, 8 9, 10 75 sccm NW NW 175 sccm NW+XL+NB NB NW: Nanowire, NB: Nanobelt, XL: Polyhedral crystal    Figure 1. Schematic illustration of a GaN reactor. The substrate temperature is varying from 1034℃ to 921 ℃  as the substrate position goes downstream.    Figure 2. SEM micrographs of (a) nanowires, (b) nanobelts and (c) polyhedral crystals grown on a Si substrate. No metal coating was applied for SEM imaging.   5 Figure 3. TEM micrographs of typical (a) coarse corrugated nanowire, (b) thin smooth-surfaced nanowire and (c) nanobelt. In the image, B denotes zone axis, and the indices next to arrows indicate growth orientations. Note that while corrugated naowire’s growth orientation is parallel to the c-axis (<0001>) of a wurtzite structure, both smooth-surfaced nanowire and nanobelt’s growth orientations are perpendicular to <0001> direction. In the nanobelt’s image, the fringes on the surface are due to strain.       Figure 4. Model for the relationship between the GaN nanostructure morphology and the reaction condition. The shaded area indicates {0001} polar surface. As the reaction condition becomes relatively more N-rich the characteristic length of {0001} surface decreases in the morphologies. In the smooth-surfaced nanowire, there should be only two tangential {0001} polar surfaces running parallel to the growth orientation, but extra shaded lines were inserted to indicate other possible tangential polar surfaces on the nanowire surface, e.g. {1-101}. Note in a wurtzite structure there is no polar plane running parallel to <0001> direction, the growth orientation of polyhedral crystal or corrugated nanowire, i.e. any surfaces running parallel to <0001> is always non-polar.     6 

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Effect of the polar surface on GaN nanostructure growth and morphology

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