ISSN 2078-5453/ Journal of Science and Innovation, Vol. 1, No. 1, 2011, 19-24
Formation of Gallium Nitride Films on Silicon Substrate by
Low-Temperature RF Sputter
Chien-Jung Huang a,1, Dei-Wei Chou b, Kuo-Yuan Li c, Teen-Hang Meen d, Wen-Ray Chen d and Wen-Chi Chen e
a Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan
b Department of Aviation & Communication Electronics, Air Force Institute of Technology, Kaohsiung, Taiwan
c Graduate School of Precision Machinery & Manufacturing Technology, Yungta Institute of Technology an
d Commerce, Pingtung, Taiwan
d Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan
e Department o
f Electronic Engineering, Southern Taiwan University of Technology, Tainan, Taiwan
Received 5 January 2010; received in revid form 16 March 2010; accepted 16 November 2010
ABSTRACT
This study prents the growth of gallium nitride (GaN) on silicon (Si) substrate using a RF sputter
method. A double preprocessing operation is propod, first etching the substrate in NH4F and
then exposing the substrate to ionized N2 for 30minutes. It is shown that this preprocessing forms a
SiN x buffer layer on Si substrate prior to GaN film growth. It is implied that no GaN layer can be
grown on a Si substrate without SiN x between the GaN and the Si substrate. Studied growth
temperature range is from room temperature to 450 . Good quality and reliability are obtained at
deposition rates up to 558Å/hr. The refractive index of the sputtered GaN/Si film is about 1.88
with growth at 300 . When the sputtered GaN/Si film is measured with a four point probe, the
surface sheet resistivity is about 61 /square.
Keywords: Gallium Nitride, RF-Sputter, Silicon.
1. Introduction
The group III nitrides have recently attracted inten attention becau of their potential for application in highly efficient blue light emitting devices [1,2].Gallium nitride (GaN) film has a large direct energy band gap of 3.39eV at room temperature and thus is a candidate for fabrication of blue and near-ultraviolet optoelectronic devices such as light-emitting diodes (LEDs) or lar diodes (LDs) [3,5]. GaN films have been grown heteroepitaxially on sapphire or silicon as a substrate [6,7]. Silicon is thought one of the most promising substrate materials since it has the advantages of high quality, large available sizes and low cost. However, single crystalline GaN films on silicon substrates have not been achieved becau of the large lattice mismatch (17%) and/or the large difference in the thermal expansion coefficients between GaN and Si. Thermal biaxial strain introduced by different thermal expansion coefficients between substrate and GaN film results in stress that can crack the brittle material. In order to minimize the problems, one current method
us a two-step growth method involving a buffer layer such as SiN x [8]. Indeed, GaN grown directly on Si (100) results in a polycrystalline film due to the formation of an interfacial amorphous silicon nitride layer.
Heretofore, GaN has been grown on silicon by various methods including plasma-assisted molecular beam epitaxy (MBE) [9], metal organic chemical vapor deposition (MOCVD) [10],RF-sputter deposition [11], etc. This paper propos a new preprocedure for the RF-sputter fabrication of high-quality GaN films on Si substrates. The propod preprocedure produces a SiN x interfacial layer on Si (100) substrates. The propod method offers the advantages of a low temperature process using simple equipment at low cost with lective
1Corresponding author: C.J. Huang; email: chien@nuk.edu.tw
deposition, and produces a high quality film.
2. Experimental Details
GaN film was deposited using a 13.56MHz RF sputtering apparatus. The source material was a GaN target (2 inch diam.) with 99.999% purity. The n+ Si (100) substrate was cleaned successively by ac
etone and methanol, 10 minutes for each cleaner, and then etched in a pH-controlled solution (pH=10.8, 27 , 29% NH4F:H2O＝1:1), thus constructing a single layer of SiN on the surface of the Si substrate before loading the substrate into the sputter chamber. Once in the sputtering chamber, the RF power is held to 50W. Deposition is begun by evacuating the chamber to a ba pressure of 7*10-7torr. In-chamber preprocessing of the target is done for 30 minutes with pure nitrogen gas. During this pha, the nitrogen gas is ionized and infus into the Si substrate, forming a top layer of amorphous SiN in the substrate. The GaN target is then heated and deposited in a mixed sputtering gas of N2 and Ar maintained at a pressure of 2*10-3torr. After deposition, the substrates are cooled in 99.99% N2 to less 200 .
Test GaN/Si films were grown at different substrate temperatures, which ranged from 25 to 450 . Thickness and refractive index were measured by a Rudolph Auto EL-III ellipsometer (Japan). Auger electron spectroscopy (AES) was employed to investigate the bonding characteristics and film composition. In addition, the film was evaluated by x-ray diffraction (XRD) and Fourier transform infrared (FTIR) to investigate the physical crystallinity and structure.
3. Result and Discussion
Films grown at low growth rate showed extremely low resistivity. Resistivity incread with increasing substrate temperature and then dropped after reaching a maximum resistivity, as shown in Fig. 1. The decrea of resistivity probably originates in an increa of film carrier density as the films become Ga-rich as a result of high substrate temperature. GaN film growth rate increas with substrate temperature and drops after a maximum growth rate. The increa in growth rate is due to the fact that the surface mobility of the deposited atoms increas [12]. On the other hand, both strain and surface energies of the GaN film compensate and balance each other under certain condition [13]. However, the greater the surface energy of the GaN film resulting from substrate temperature, the less structured the crystal, i.e. the more amorphous it is. Thus, GaN film growth rate decreas with the growth temperature after the maximum growth rate. Sheet resistivity remains constant despite variations of GaN film thickness, as shown in Fig. 2. This indicates that the surface of the GaN film is flat and uniform. Furthermore the refractive index of the sputtered GaN/Si film is about 1.88 with growth at 300 . Fig. 3 shows the results from Auger electron spectroscopy (AES) analysis for our 300 GaN/Si. The depth profile shows that oxygen and carbon contamination does not occur on the GaN surface. The AES profile indicates the GaN layer is non-stoichiometric. The atomic ratio of Ga/N for near the surface of GaN film was found to be 7/3. Furthermore, the atomic ratio of Ga/N for near the surface of the Si substrate reasonably reprents stoichiometry of GaN. Th
e variable composition of the region near the GaN/Si interface may be due to presputtering of the target. We thus conjecture that a silicon nitride (SiN x) layer exists at the GaN/Si interface. Although the exact role of the SiN x buffer layer between GaN and Si substrate is not completely clear, the enhancements in growth rate and adhesion of the GaN film may be tentatively attributed to an improved bonding of the GaN film and Si. The AES depth profile also shows that interdiffusion is not obvious between GaN layer and Si substrate. The apparently high nitrogen content in the film could be caud by the preferential sputtering action of nitrogen. All the above data reveal that GaN film can be reliably grown on a Si substrate.
Fig. 4 shows X-ray scan data for the same sample of GaN/Si. The GaN film deposited on low temperature 300 substrate shows only two peaks, one at 2 =35.61o and the other at 48.31o, attributable respectively to GaN (111) cubic structure and GaN (102) hexagonal structure. If the GaN film is properly annealed in ambient N2O [14], the (111) cubic structure is more obvious. The (111) structure is preferred for magnesium doping (with an activation energy of about 0.19 eV) to form p-n junction diodes [15]. Fourier Transform Infrared (FTIR) permits the direct verification of the prence and orientation of GaN, and permits the evaluation of film quality. Fig. 5, obtained from the same sample as Fig. 4, shows the GaN typical FTIR spectrum, with bands at 554 and 735cm-1
due to Ga-N stretching vibrations in the hexagonal and cubic type GaN crystals, respectively. As the substrate temperature increas from room temperature to 300 , the crystallinity of the GaN films is improved. Increasing the substrate temperature to 450 , however, results in the GaN film surface near the center of the sample having a rougher surface than if grown at lower temperature due to the temperature gradient between the center and edge of the substrate. When a thicker SiN x buffer layer is ud, poorer GaN film quality is also obtained for the same deposition conditions. This means that the thickness of the buffer layer together with substrate temperature play very important roles in growing good epitaxial GaN film. In summary, the above results show that good physical and chemical quality GaN films can be grown on Si substrate by low-temperature RF sputtering.
Fig. 1. Resistivity and growth rate of GaN film as a function of substrate growth temperature.
Fig. 2. Sheet Resistivity of GaN film as a function of GaN film thickness.
Fig. 3. Auger depth profile of GaN/Si (100) at substrate growth temperature= .
Fig. 4. X-ray diffraction pattern of GaN film at substrate growth temperature= .
Fig. 5. FTIR spectrum of GaN film at substrate growth temperature= .
4. Conclusion
In summary, we have achieved low temperature growth of GaN on Si substrate by RF sputtering. A double preprocessing operation is propod, first etching the substrate in NH4F and then exposing the substrate to ionized N2 for 30 minutes. It is shown that this preprocessing forms a SiN x buffer layer on Si substrate prior to GaN film growth. It is implied that no GaN layer can be grown on a Si substrate without SiN x between the GaN and the Si substrate. GaN films grown at different substrate temperatures were characterized by AES, XRD, FTIR and ellipsometry. Diver substrate temperatures resulted in different film quality. Good film quality was found for 300 . RF sputtering deposition of GaN on silicon has thus been shown to be a good candidate for future optoelectronic device processing.
Acknowledgment
This work was partially supported by the National Science Council of Republic of China under the contract number NSC 99-2221-E-390-036.
References
[1]S. Nakamura, T. Mukai, M. noh, Appl. Phys. Lett. 64 (1994) 1687.
[2]S. Nakamura, M. noh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoky, Y. Sugimoto, Jpn. J.
Appl. Phys. 35 (1996) L74.
[3]S. Nakamura, M. noh, N. Isawa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797.
[4]M.A. Khan, A. Bhattarai, J.N. Kuznia, D.T. Olson, Appl. Phys. Lett. 63 (1993) 1214.
[5]S. Nakamura, M. noh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoky, Y. Sugimoto, Jpn. J.
Appl. Phys. 35 (1996) L217.
