We present brief overview of our study on the low temperature flux growth of two very important novel wide bandgap materials 2H-SiC and Beta-gallium oxide (Beta-Ga2O3). We have synthesized and grown 5 millimeter to 1 centimeter size single crystals of Beta-gallium oxide (Beta-Ga2O3). We used a flux and semi wet method to grow transparent good quality crystals. In the semi-wet method Ga2O3 was synthesized with starting gallium nitrate solution and urea as a nucleation agent. In the flux method we used tin and other metallic flux. This crystal was placed in an alumina crucible and temperature was raised above 1050 degrees Centigrade. After a time period of thirty hours, we observed prismatic and needle shaped crystals of gallium oxide. Scanning electron microscopic studies showed step growth morphology. Crystal was polished to measure the properties. Bandgap was measured 4.7electronvolts using the optical absorption curve. Another wide bandgap hexagonal 2H-SiC was grown by using Si-Al eutectic flux in the graphite crucible. We used slight AlN also as the impurity in the flux. The temperature was raised up to 1050 degrees Centigrade and slowly cooled to 850 degrees Centigrade. Preliminary characterization results of this material are also reported

Arnold, Bradley

Choa, Fow-Sen

Kelly, Lisa

Singh, N. B.

Su, Ching-Hua

NASA Technical Reports Server

  
 
 
Low Temperature Flux Growth of 2H-SiC and -Gallium Oxide  
 
N. B. Singh, Fow-Sen Choa, Ching-Hua Su*, Bradley Arnold and Lisa Kelly 
University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 
*EM31 NASA/Marshall Space Flight Center, Huntsville, AL 35812 
 
Keywords: Silicon carbide, Gallium oxide, Morphology, Wide bandgap, Crystal, Growth, Solubility 
 
ABSTRACT 
We present brief overview of our study on the low temperature flux growth of two very important novel 
wide bandgap materials 2H-SiC and -gallium oxide (Ga2O3). We have synthesized and grown 5mm to 
1cm size single crystals of -gallium oxide (Ga2O3). We used a flux and semi wet method to grow 
transparent good quality crystals. In the semi-wet method Ga2O3 was synthesized with starting gallium 
nitrate solution and urea as a nucleation agent. In the flux method we used tin and other metallic flux. This 
crystal was placed in an alumina crucible and temperature was raised above 1050 ºC. After a time period 
of thirty hours, we observed prismatic and needle shaped crystals of gallium oxide. Scanning electron 
microscopic studies showed step growth morphology. Crystal was polished to measure the properties. 
Bandgap was measured 4.7eV using the optical absorption curve. Another wide bandgap hexagonal 2H-
SiC was grown by using Si-Al eutectic flux in the graphite crucible. We used slight AlN also as the impurity 
in the flux. The temperature was raised up to 1050C and slowly cooled to 850C. Preliminary 
characterization results of this material are also reported.   
 
INTRODUCTION 
Since past several decades researchers are investigating substrates and devices for high power and low noise 
microwave devices for improving commercial and military systems.  Great demands for the efficient, broad-band 
power RF transmitters with high linearity, as well as low noise rugged receivers for T/R modules cannot be achieved 
without great improvements in wide bandgap materials.  Inspite of tremendous progress in GaN development and film 
technology defective nitride film technologies can only be looked upon as a ‘stop gap’ until high quality reliable 
defect-free structures are obtained. The poor quality of GaN film is the biggest problem in transitioning these practical 
devices to large production. There are many reasons for this show stopper. The unavailability of lattice-matched and 
chemically-matched substrate for GaN is perhaps the most important factor.  The substrate choices which have been 
considered are MgO, Si, SiC, GaAs, AlN, GaN and ZnO.  Several experiments [1-5] have been performed to grow 
pure and doped AlN large diameter crystals.  Edgar and his coworkers [1-2] and group at Northrop Grumman have 
studied pure and SiC-AlN alloys to achieve cm size AlN crystals. Due to very high temperature involved and reactivity 
of Al with crucible materials, it was observed that growth of large crystals always had incorporation of impurities in 
the AlN bulk materials (Figure 1a). Some of our experiments using pure aluminum and nitrogen [5, 6] resulted in 
transparent large colorless hexagonal and needles with long aspect ratio (Figure 1b).  It was very difficult to control 
seeded growth due to several factors including low vapor pressure of AlN. During the growth long size needles, we 
observed growth of hexagonal small crystallites (Figure 2). For the growth of large crystals, when temperature was 
above 2150C the AlN single crystal changed to a yellow color due to impurities. All these problems indicate that very 
high quality large transparent materials of this composition could not be achieved by physical vapor transport. On the 
other hand large 4H-SiC and 6H-SiC have been grown in different sizes and are widely used for the hexagonal GaN 
thin and thick films with some surface modification. Since very recently it has been thought that 2H-SiC which has 
largest bandgap and mobility compared to other polytypes of SiC is not stable. However recent work in Japan flux 
using Lithium and nucleation with AlN has demonstrated that 2H-SiC growth is possible as a hexagonal substrate. 
Another exciting and new material gallium oxide, -Ga2O3 is hexagonal and crystals have better chemical 
computability and favorable properties as a substrate for epitaxial growth of gallium nitride (GaN). The details of 
available commercial substrates such as AlN, SiC and Al2O3 along with -Ga2O3 are given in Table 1. We will 
summarize the nucleation and morphology of 2H-SiC and describe the recent development in growth of -Ga2O3 
crystals. 
 
https://ntrs.nasa.gov/search.jsp?R=20170000621 2019-08-29T15:45:55+00:00Z
  
 
 
 
   
(a)                                         (b) 
Figure 1- (a) AlN bulk fabricated  and (b) cm size needle crystal grown using Physical vapor transport method. 
 
 
Figure 2- Growth of hexagonal small crystallites on the surface of large crystallites 
 
As shown in Table 1, the bandgap of b- Ga2O3 is 4.9 eV, which corresponds to the second largest bandgap 
after that of diamond among semiconductors. The preliminary reported thermal conductivity and mobility 
are comparable to SiC and GaN. This combination of properties will enable to develop - Ga2O3 deep 
ultraviolet photo detectors and high power amplifiers for variety of commercial applications in daily life.  
We will present preliminary results on a novel wide bandgap material -Ga2O3 which also has much 
favorable properties for low temperature growth and fabrication.  
 
  
 
 
Table 1. Properties of commercial substrates are compared with -Ga2O3 
Figure of merit for carious semiconductor materials 
 Si 4H-SiC GaN Diamond -Ga2O3 
Bandgap (eV) 1.1 3.18 3.3 5.5 4.85 
Electron Mobility 
(cm2/Vs.) 
1400 700 1200 2000 400 
Breakdown Field 
Eb (MV/cm) 
0.3 2.5 3.3 10 8 
Dielectric Constant 11.8 9.7 9.0 5.5 9.5 
Baliga’s Figure of 
Merit 
1 340 870 24664 3444 
  
 
2. EXPERIMENTAL METHOD AND RESULTS  
2.1 Growth of 2H-SiC Material:  In the past many years we have used the physical vapor transport method in a 
manner similar to that used for silicon carbide and aluminum nitride described in references 3-5. All the growth 
experiments were performed in a vertical transport geometry in which the source material was transported upward 
into a cooler zone. In this investigation we used a combination of Czochralski and flux growth method. A graphite 
crucible was used as container and Al-Si eutectic was used as nutrient and flux. We used a typical flux of 100 gm 
and mixed by melting. Eutectic completely melted above 660C. However, we raised the temperature to 1050C and 
added 1-2 gm AlN powder. The stirring was performed using a ceramic rod with very thin tip containing SiC 
substrate. The silicon reacted with graphite and when we cooled the crucible, small crystals grew on the SiC 
substrate. In the beginning SiC substrate dissolved quickly. Several test runs were performed to avoid dissolution 
and Si concentration was maintained to keep near eutectic.  
 
    
(a)                                                                 (b)                                                            (c) 
Figure 3- Morphology of (a) nucleation (b) small crystallites and thick film of SiC  
 
The normally-unstable 2H-SiC crystal type appear to be stabilized by AlN deposits. In the presence of 
AlN the SiC nucleates and deposits begin as discs, growing to become pyramids, before finally 
coalescing to become a continuous layer. 2H is the natural crystal orientation of AlN. The SiC 
deposits begin as discs and grow to become pyramids, which forms small crystals before finally 
coalescing to become a continuous layer. There is a continuous growth and etching on the substrate 
surface. This type of behavior was observed by Edger as well as Swedish researchers also. Presence 
of AlN forces nucleation as the hexagonal. Some very old researches based on SiC-AlN composites 
have indicated that hexagonal morphology was always observed even in small concentration range of 
AlN. Further analysis by transmission electron microscope (TEM) is required to resolve this issue. 
However previous TEM studies by Wagner, Singh and Berghmans shown in Figure 4 shows 
evidence of 2H-SiC on 6H-SiC substrate. 
 
 
 
 
  
 
 
 
 
Figure 4- TEM of grown 2H-SiC on 6H-SiC substrate (Unpublished results of B. Wagner, A. 
Berghmans, D. Knuteson and N.B.Singh) 
 
2.2 Growth of -Ga2O3 crystals:  In past five years crystal growth of -Ga2O3 has become very 
interesting subject. The problem lies in the nature of the phase-diagram of this material. A very flat 
congruent compound has been reported at the 60atomic % of the oxygen in the Ga-O phase diagram 
above 2300C. This requires control of oxygen pressure, a stable crucible and control of volatile 
impurities. Because of these problems we adapted low temperature flux growth method. In both cases 
w used semi-wet methods for this material.  Crystal growth studies to demonstrate 2H-SiC [7-9] in 
Japan have been made using tin flux for the growth. But process involving tin causes heavy doping 
and growth of high purity sample is very difficult by this method. Because of this reason we used co-
crystallization at low temperature and high temperature grain growth methods.  
 
In the case of co-crystallization method to grow -Ga2O3 crystal, we used urea as nucleating agent. 
The gallium salt and urea were dissolved in the concentrate nitric acid. Since urea was a flux, we used 
urea and gallium oxide in 5:1 ratio. The temperature of the solution was raised to 50C in a small 
thermostat. After saturation of 24 hours, the temperature of the solution was decreased to crystallize 
the -Ga2O3 crystal. The urea was removed by washing with water after crystal growth. The gallium 
oxide and urea or gallium oxide produced a green color partially dissolved material was observed. 
However, concentrated HNO3 produced clear solution (Shown in Figure 5 ).  There is traces of 
gallium chloride in presence of of HCl and excess HNO3 was added to avoid chloride formation.   
 
6H-SiC 
2H-SiC 
  
 
 
   
(a)                                                          (b) 
Figure 5- (a) Green color partially dissolved gallium oxide nutrient (b) Transparent solution of Gallium 
oxide and urea in concentrate nitric acid that is suitable for crystallization and morphology of crystals 
obtained from this solution. 
 
 
 
 
 
 
 
 
 
 
 
(a)                                                                 (b) 
Figure  6 - (a) Large number of small crystals nucleated together and (b) needle and prism crystals 
and bubbles were not observed. 
    
Figure 7- Prism and needle shaped crystals produced by co-crystallization method 
  
 
 
Figure 7 shows as grown needles and prism type of -Ga2O3 crystal. These needles in dilute concentration 
nutrient have very large aspect ratio. Experiments to optimize conditions to produce large crystals and alter 
morphology are continuing by this method. Crystals grown using urea at high temperature showed fat 
needles and prism morphology crystals.  SEM morphology of these crystals is shown in Figure 3. We 
observed several small crystallites and step growth on the surface of large crystals. Gross defects such as 
voids, precipitates  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8- Growth of crystals using crucible and Ga-Sn melt as flux 
 
In the second growth approach we placed the gallium salt in the alumina crucible with small amount of Ga-
Tin mixture and raised the temperature slowly to 300 C. After soaking for a period 8-10 hours we raised 
the temperature to 1050 and left it for 25 hours. Small crystals grew on the surface of the crucible.  These 
prismatic crystals were translucent. On the top portion of the flux, we observed needles. We observed 
precipitates in the bottom of the crucible. In this process it was difficult to control the morphology of 
crystals since we did not use prefabricated seeds. This result was similar to that reported in reference 6.  
 
2.3 Fabrication and Characterization of crystals: As grown crystals cm size crystals were polished using 
water-glycol mixture. Crystals were characterized for the morphology by optical microscope and scanning 
electron microscope (SEM). The bandgap of the material was determined by studying the optical 
transparency and absorbance. 
 
 
 
 
 
 
 
 
 
Precipitates 
Prism  
Needles 
  
 
 
 
 
 
 
 
 
 
 
(a)                                                                            (b) 
Figure 9-  SEM morphology of (a) Needle and (b) plate crystals. Needles showed large 
number of ridges on the surface.  
A careful morphological examination of the morphology of the prism crystal showed growth steps 
(Figure 10) on the flat surfaces. Some of the steps were elongated as needles and some steps were flat 
with large facets. It appears that shape changes were due to variation in the growth rates. 
      
(a)                                                                   (b) 
Figure 10-  Growth steps shown on the surface of (a) needle and (b) prism (arrowed) crystals.  
 
Preliminary X-ray diffraction data showed that lattice parameters a= 5.80A, b=305A and c=12.20A were 
in agreements with previously reported values.  The bandgap of as grown crystal was measured by 
transmission and absorption data. We determined a value of 4.70eV a value slightly lower than predicted 
values of 4.9 eV. Further study in this area is continuing and we expect to achieve pure and better crystals 
for resistivity and bandgap measurements.  
 
SUMMARY 
We used low temperature flux method to grow 2H-SiC and (Ga2O3 crystals. SiC crystals nucleated as a 
disc in presence of AlN and then grew in the form of 2H-SiC thin film on the 6H-SiC substrate. Centimeter 
size single crystals of a novel bandgap material Gallium Oxide (Ga2O3) were grown by solution method. 
In the flux method we used tin and other metallic flux. This crystal was placed in an alumina crucible and 
temperature was raised above 1050 ºC. Crystals showed prismatic and needle shaped morphology. We 
observed faceted growth steps on the surfaces of large prism shape crystals. Bandgap measurement using 
the optical absorption curve indicated a bandgap of 4.7eV.  Further study on purification and growth is 
continuing. 
 
We used a flux and semi wet method to grow transparent good quality crystals. In the semi-wet method 
Ga2O3 was synthesized with starting gallium nitrate solution and urea as a nucleation agent.  
 
ACKNOWLEDGEMENTS 
  
 
 
We are grateful to Andre Berghmans, Brian Wagner, Matt King, Dr. Darren Thomson (now at AFRL, 
Dayton, OH) and Dr. David Knuteson of Northrop Grumman Corporation and several students of the 
UMBC for their advice and help.  
 
 
 
REFERENCES 
1. N. B. Singh B. Wagner, M. King, A. Berghmans, D. Knuteson, S. McLaughlin and D. Kahler, 
“SiC) x(AlN)1-x solid-solution substrates for high temperature and high power devices” , Crystal 
Growth and Design,  10 (8), (2010) 3508-3514. 
2. N. B. Singh, B. Wagner, M. King, A. Berghmans, D. Knuteson and D. Kahler  “Effect of AlN 
doping on the growth morphology of SiC”, J. Crystal Research and Technology, 44, (2009) 903-
909. 
3. B. Wagner, N. B. Singh, N. B. Singh, M. King, A. Berghmans, D. Knuteson and D. Kahler 
”Morphology of thick SiC epitaxial film grown by PVT method”, J. Electronic Materials 37 (2008) 
379-384. 
4. N. B. Singh, A. Berghmans, H. Zhang, T. Waite and R. C. Clarke , “Physical vapor transport growth 
of large AlN crystals” Journal of Crystal Growth, 250 (2003) 107-112.  
5. N. B. Singh, A. Berghmans, J. Golombeck, C. Clarke and H. Zhang “PVT growth and 
characteristics of AlN single  crystals”, Physics of Semiconductor Devices 1, (2003) 117-123. 
6. Masataka Higashiwaki, Kohei Sasaki, Akito Kuramata, Takekazu Masui, and Shigenobu 
Yamakoshi,” Gallium oxide (Ga2O3) metal-semiconductor field-effect transistors on single-
crystal β-Ga2O3 (010) substrates” Applied Physics Letter, 100, (2012) 013504-013507 
7. N. B. Singh, “Development of novel substrates”, National conference on power management, 
BHU, India December 2012.  
8. D. Roehrens, J.Brendt, D.Samuelis 1, M. Martin, “On theammonolysis of Ga2O3: An XRD 
,neutron diffraction and XAS investigation of the oxygen-rich part of the system Ga2O3–GaN” 
Journal of Solid State Chemistry 183 (2010) 532–541 
 


Low Temperature Flux Growth of 2H-SiC and Beta-Gallium Oxide

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