AlN substrates: Fabrication via vapor phase growth and characterization

Article abstract of DOI:10.1002/pssa.200303522

A new vapor phase crystal growth technique was applied to fabricate single crystal AlN wafers up to 1.75-inch diameter. The wafers were sliced from AlN crystals and polished. Fabricated AlN wafers were investigated by X-ray diffraction, TEM, and cathodoluminescence. X-ray diffraction and TEM studies confirmed single crystal structure of grown material. High electrical resistivity of these AlN wafers was verified. AlN homoepitaxial layers exhibiting sharp near-band-edge emission were grown on fabricated AlN substrates. These results open the door for the commercialization of AlN substrates for advanced high-power mm-wave devices and optoelectronic devices based on Group III-nitride semiconductors.

Full text of DOI:10.1002/pssa.200303522

phys. stat. sol. (a) 200, No. 1, 22–25 (2003) / DOI 10.1002/pssa.200303522
AlN substrates: fabrication via vapor phase growth
and characterization
*
, 1
1
Yu. Melnik , V. Soukhoveev , V. Ivantsov , V. Sizov , A. Pechnikov , K. Tsvetkov ,
1
1
1
1
1
1
2
2
O. Kovalenkov , V. Dmitriev , A. Nikolaev , N. Kuznetsov , E. Silveira
3, 4
,
3
and J. Freitas, Jr.
1
TDI, Inc., 12214 Plum Orchard Dr., Silver Spring, MD 20904, USA
2
A.F. Ioffe Physico-Technical Institute, 26 Polytechnicheskaya Str., St. Petersburg, 124091, Russia
Naval Research Laboratory, Washington, DC 20375-5347, USA
3
4
Departamento de Física, Universidade Federal do Paraná, 81531-990, Curitiba_PR, Brazil
Received 22 May 2003, revised 23 June 2003, accepted 14 August 2003
Published online 12 November 2003
PACS 61.10.Nz, 78.60.Hk, 81.05.Ea, 81.15.Kk
A new vapor phase crystal growth technique was applied to fabricate single crystal AlN wafers up to
1.75-inch diameter. The wafers were sliced from AlN crystals and polished. Fabricated AlN wafers were
investigated by X-ray diffraction, TEM, and cathodoluminescence. X-ray diffraction and TEM studies
confirmed single crystal structure of grown material. High electrical resistivity of these AlN wafers was
verified. AlN homoepitaxial layers exhibiting sharp near-band-edge emission were grown on fabricated
AlN substrates. These results open the door for the commercialization of AlN substrates for advanced
high-power mm-wave devices and optoelectronic devices based on Group III-nitride semiconductors.
©
2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction High quality single crystal GaN and AlN substrates are needed for further break-
through in the field of III-nitrides advanced electronic and optoelectronic devices.
Recently, significant progress has been achieved in the development and fabrication of GaN substrates
1, 2]. The growth method accepted for the fabrication of these GaN substrates is hydride vapor phase
[
epitaxy (HVPE).
Alternative native substrate material for AlGaN-based devices is aluminum nitride. Having energy gap
of 6.2 eV, AlN is an attractive substrate material for optoelectronic devices operating in UV spectral
–1
–1
region. AlN has thermal conductivity of about 3.3 WK cm [3], which is more than two times higher
than that for GaN. Due to high intrinsic electrical resistivity, AlN substrates may be used not only for
optoelectronic devices, but also for new generation of high frequency high power GaN-based electronic
devices including high electron mobility transistors (HEMTs) and monolithic microwave integral circuits
(MMICs).
So far, progress in AlN bulk crystal growth has been limited. Due to the low nitrogen solubility in
Al-melt, AlN crystal growth from liquid phase is extremely difficult task [4]. Sublimation AlN crystals
growth has reported by several groups [5–7], however the maximum size of AlN wafers fabricated by the
sublimation technique is still less than 1 inch. At the same time, at least 2-inch diameter AlN substrates
are required for optoelectronic applications and 4-inch diameter AlN wafers are preferred for electronic
applications.
In this paper we report on the fabrication of single crystal AlN wafers employing novel vapor phase
crystal growth technique.
*
Corresponding author: e-mail: yuvm@tdii.com, Phone: +1 301 572 7834, Fax: +1 301 572 6438
©
2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 200, No. 1 (2003) / www.physica-status-solidi.com
23
Fig. 1 The 1-inch diameter AlN wafers produced from
an AlN single crystal boule. The wafers are not polished.
Fig. 2 (online colour at: www.interscience.wiley.com)
Photo of 44 mm (1.75-inch) diameter bulk AlN wafer.
The wafer thickness is 300 µm.
2
Experimental procedure AlN crystals were grown on AlN seeds using vapor phase crystal tech-
nique developed at TDI. Growth temperature was varied from 1000 to 1100 °C. Crystals were grown in
the [0001] direction on (0001)Al face of AlN seeds. As initial seeds, thick AlN layers removed from Si
or SiC substrates were used [8]. Size of AlN seeds ranged from 0.5 to 1.75 inch. Typically AlN seeds
had round shape.
Typical diameter of grown bulk AlN crystals was about 1-inch. The boules were a few millimeters
thick. Grown AlN bulk crystals were sliced using diamond wire saw to fabricate AlN wafers. Thickness
of sliced AlN wafers ranged from 200 to 500 µm. The lapping and polishing procedures were applied to
the AlN wafers to accomplish the wafer fabrication. Some of AlN wafers were used as seeds for next
technological cycle to grow AlN bulk crystals. By this way the quality of resulted AlN crystals was im-
proved. Figure 1 shows AlN wafers sliced from the same AlN boule. Largest size of AlN single crystal
wafer fabricated to date is 1.75-inch as it can be seen in Fig. 2. Properties of fabricated wafers were stud-
ied by means of x-ray diffraction methods, transmission electron microscopy (TEM), and cathodolumi-
nescence (CL). Temperature dependence of electrical resistivity of fabricated AlN wafers was measured.
Fabricated AlN wafers were used as substrates for AlN homoepitaxy. Homoepitaxial AlN layers of
about 2 microns thick were grown on (0001)Al face of polished AlN wafers by hydride vapour phase
epitaxy (HVPE) in a conventional horizontal-flow HVPE reactor [9]. The epitaxial growth was per-
formed at 1050 °C and atmospheric pressure with argon as a carrier gas. Ammonia and hydrogen chlo-
ride (HCl) were used as precursors. For AlN layers growth, HCl was passed over Al source to form
gaseous aluminum chloride. The basic reaction at the Al source was Al + 3 HCl → AlCl + 3/2 H . The
3
2
aluminum chloride was then carried by the Ar flow to the growth zone where it was mixed with ammo-
nia, providing deposition of AlN layers on the AlN substrate. The gas flow rates were maintained to
obtain AlN growth rates in the range of 0.2 – 1.0 µm/min.
Fig. 3 (online colour at: www.interscience.wiley.com)
X-ray pole figure measured for (0002) AlN reflex.
Fig. 4 Cross sectional TEM image for AlN wafer.
©
2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

24
Yu. Melnik et al.: AlN substrates: fabrication via vapor phase growth and characterization
3
Material characterization Structural characterization of fabricated AlN wafers was done using
X-ray diffraction techniques and transmission electron microscopy (TEM). Figure 3 shows pole figure
measurement. The result indicated that AlN material has single crystal structure without polycrystalline
inclusions and mis-oriented blocks. The minimum value for the full width at a half maximum (FWHM)
of X-ray diffraction ω-scan rocking curves measured for (0002) AlN reflection using wide (10 × 1 mm)
X-ray beam and opened slits was about 700 arcsec. Cross sectional TEM view of AlN wafer is given in
Fig. 4. Results of TEM measurements confirmed single crystal structure of grown AlN materials.
Cathodoluminescence (CL) experiment was carried out with excitation current of about 5 µA and
electron-beam energies between 5 and 10 keV. Figure 5 shows a 6K CL spectrum of an AlN wafer. Fig-
ure 6 represents 6 K CL spectra taken at three different positions of an AlN homoepitaxial layer. The
pronounced broad luminescence bands between ~2.0 eV and 5.5 eV has been reported previously and
attributed to oxygen-related complexes or carbon-related defects in AlN [10, 11]. Besides the deep lumi-
nescence bands we also observe near-band-edge (NBE) emission around 6 eV. This emission is more
pronounced in the homoepitaxial AlN film spectra. Slight differences in intensities are observed for
difference positions on the homoepitaxial layer. The presence of NBE emissions, even for the AlN
wafer, is an indication of good optical quality. In order to identify the nature of these emissions observed
on the AlN bulk and AlN homoepitaxial layer a more detailed study is planed for the near future.
Electrical measurements were done for 300 µm thick AlN wafer. To measure the specific resistivity,
Ni contact pads were evaporated on the wafer surface. The diameter of the contact pads was 300 µm.
The backside contact was formed using Ga/In paste. One electrical probe was placed on Ni contact pad
and the another one was placed to the backside contact.
Specific resistivity of the AlN bulk material was evaluated at the conditions of the direct current pass-
ing through the sample at a fixed bias in the temperature range from 250 to 650 K. The absolute value of
current was limited by the resistivity of the AlN bulk material. Thus, specific resistivity of the AlN was
evaluated using the equation: ρ = (V*S)/(I*L), where V is fixed voltage, I is the measured current, S is
the area of the Ni contact and L is the thickness of the AlN bulk material.
Temperature dependence of the specific electrical resistivity measured for an AlN wafer is shown in
Fig. 7. According to the model for the conductivity phenomena in highly resistive semiconductors, the
slope of an exponential part of the log (ρ) = f(1/T) dependency corresponds to an ionization energy (EA)
of a deep level. The plot shown in Fig. 7 identifies single deep level with E ~ (0.45 ± 0.01) eV. Material
A
8 6
resistivity exceeds 10 Ohm cm at room temperature and 10 Ohm cm at 500 K.
Fig. 5 CL spectra measured for AlN wafer. The
sharp features around 2.0 and 2.5 eV are due to the
spectrometer gratings.
Fig. 6 (online colour at: www.interscience.wiley.com)
CL spectra measured for AlN homoepitaxial layer grown
on AlN bulk substrate at three different locations on the
wafer.
©
2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 200, No. 1 (2003) / www.physica-status-solidi.com
25
1E+10
Fig. 7 Temperature dependence of specific resistivity measured
on AlN sample having thickness of 300 µm.
1E+9
1E+8
1E+7
1E+6
1E+5
0
.45 +- 0.01 eV
1
.0
2.0
3.0
4.0
1
000/T (K-1
)
4
Conclusion Largest size of the single crystal AlN wafer fabricated to date is 1.74 inch. AlN wafers
8
6
resistivity exceeds 10 Ω cm at room temperature and 10 Ω cm at 500 K. CL measurements revealed
near band edge emission around 6 eV and broad luminescence bands between 2.0 eV and 5.5 eV as well.
The results discussed in the present work show the potential of AlN substrates for advanced high-power
mm-wave devices and optoelectronic devices based on Group III-nitride semiconductors.
Despite on considerable progress several challenges remain to be solved. The growth process must be
optimized to improve crystal quality and increase wafer size to 2-inch. The polishing process needs to be
optimized.
Acknowledgments This work at TDI was partly supported by MDA and managed by ONR (contract manager
Colin Wood). E. Silveira and J. A. Freitas were partially supported by Navy Grants N00014-02-1-4087 (IFO) and
N00014WR20015, respectively.
References
[1] M. Albrecht, I. Nikitina, A. Nikolaev, Yu. Melnik, V. Dmitriev, and H. Strunk, phys. stat. sol. (b) 176, 453
(1999).
[
[
[
[
2] L. Liu and J. H. Edgar, Mater. Sci. Eng. R 37, 61 (2002).
3] D. I. Florescu, V. M. Asnin, and F. H. Pollak, Compound Semicond. 7 (2), 62 (2001).
4] M. Bockowski, M. Wróblewski, B. Łucznik, and I. Grzegory, Mater. Sci. Semicond. Process. 4, 543 (2001).
5] J. C. Rojo, L. J. Schowalter, G. Slack, K. Morgan, J. Barani, S. Schujman, S. Biswas, B. Raghothamachar,
M. Dudley, M. Shur, R. Gaska, N. M. Johnson, and M. Kneissl, Mater. Res. Soc. Symp. Proc. Vol. 722,
K1.1.1. (2002).
[
[
[
[
6] A. S. Segal, S. Yu. Karpov, Yu. N. Makarov, E. N. Mokhov, A. D. Roenkov, M. G. Ramm, and Yu. A. Voda-
kov, J. Cryst. Growth 211, 68 (2000).
7] C. M. Balkas, Z. Sitar, T. Zheleva, L. Bergman, R. Nemanich, and R. F. Davis, J. Cryst. Growth 179, 363
(1997).
8] A. Nikolaev, I. Nikitina, A. Zubrilov, M. Mynbaeva, Yu. Melnik, and V. Dmitriev, MRS Internet J. Nitride
Semicond. 595, W6.5.1. (2000).
9] Yu. Melnik, D. Tsvetkov, A. Pechnikov, I. Nikitina, N. Kuznetsov, and V. Dmitriev, phys. stat. sol. (a) 188,
4
63 (2001).
10] G. A. Slack, L. J. Schowalter, D. Morelli, and J. A. Freitas Jr., J. Cryst. Growth 246, 287 (2002).
11] X. Tang, F. Houssain, K. Wongchotigul, and M. Spencer, Appl. Phys. Lett. 72, 1501 (1998).
[
[
©
2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim