81-tsf44320039_GaN

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					                                                    Thin Solid Films 443 (2003) 9–13




       GaN growth on single-crystal diamond substrates by metalorganic
          chemical vapour deposition and hydride vapour deposition
                                        P.R. Hageman*, J.J. Schermer, P.K. Larsen
      Department of Experimental Solid State Physics III, Mathematics and Computing Science, University of Nijmegen, Toernooiveld 1,
                                                   Nijmegen 6525 ED, The Netherlands

                        Received 26 November 2002; received in revised form 28 May 2003; accepted 7 June 2003



Abstract

   In this study a thick hexagonal GaN layer has been grown on a (110) single crystalline diamond substrate utilising two
different deposition techniques. Using an AlN nucleation layer, metal–organic chemical vapour deposition (MOCVD) has been
used to deposit an initial GaN layer on a (110) single crystal diamond substrate. The layer consists of closely packed GaN grains
with a thickness of approximately 2.5 mm and with different orientations with respect to the substrate. Low temperature
photoluminescence indicates a poor optical quality of the layer due to poor structural properties andyor a high incorporation of
impurities. This layer was used as a template in a hydride vapour phase epitaxy (HVPE) growth experiment. As a result of this,
the GaN grain size has increased enormously and the layer consists of large, hexagonal shaped pillars with a diameter of
approximately 50 mm and a height of more than 100 mm protruding from a polycrystalline background having a more uniform
thickness. PL spectra of this film show a strongly increased intensity of the exciton related emissions when compared to the
MOCVD deposited film. X-Ray diffraction analyses revealed that the dominant orientation of the GaN crystallites perpendicular
to the substrate changed from w001x for the thin MOCVD film to w112x for the HVPE layer.
    2003 Elsevier B.V. All rights reserved.

PACS: 61.82 Bg; 68.55 Jk; 78.30 Fs; 78.55 Cr

Keywords: Chemical vapor deposition; Diamond; Epitaxy; Nitrides


1. Introduction                                                         diamond crystal lattice to that of silicon insinuates the
                                                                        possibility of depositing GaN on diamond. This is
   The development of nitride based technology for both                 strengthened by some recent reports about AlN growth
optical and electronic applications has showed the last                 on diamond w2,3x.
decade an enormous progress. Besides further improve-                      It is clear that lattice constants and thermal expansion
ment of the material quality, specifically for electronic               coefficients of substrate and film should match as close
devices, addressing the thermal management of the                       as possible for hetero-epitaxial deposition to be success-
substrate is required for performance improvement.                      ful. In Table 1 the lattice constants of GaN, sapphire, Si
Although use of alternative substrates may be a solution                and diamond are given together with their thermal
for this, one has to bear in mind that the development                  expansion coefficients w4x. From this table it follows
of device quality material on these substrates may be a                 that the lattice mismatch of GaN on diamond is 11.8%.
difficult and time-consuming task. With this in mind,                   Although this is a considerable value, GaN of device
growth of GaN on diamond substrates, especially type                    quality can be deposited on sapphire substrates using an
IIa diamond w1x, may be attractive because it has the                   appropriate nucleation layer, in spite of the lattice
highest thermal conductivity of all materials ()2000                    mismatch of nearly 16.1% w5,6x between sapphire and
Wym K at room temperature). The resemblance of the                      GaN. The use of an appropriate nucleation layer could
  *Corresponding author. Tel.: q31-24-3653158; fax: q31-24-
                                                                        overcome the problems associated with the lattice mis-
3652620.                                                                match between GaN and diamond. Nevertheless, to the
  E-mail address: paulh@sci.kun.nl (P.R. Hageman).                      best of our knowledge, the growth of GaN on diamond

0040-6090/03/$ - see front matter 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0040-6090(03)00906-4
10                                       P.R. Hageman et al. / Thin Solid Films 443 (2003) 9–13

Table 1                                                                graphic orientation of the deposits was determined from
                  ˚
Lattice constant (A) and thermal expansion coefficient (=10y6          u –2u curves as obtained using a Bruker D8 Discovery
Ky1) of GaN (a-axis), sapphire, diamond and Si w4x                                                                           ˚
                                                                       X-ray diffractometer with a Cu target (ls1.54060 A)
Substrate                 GaN        Sapphire   Si      Diamond        and a 4-bounce monochromator Ge (022). The phase
                          (a-axis)                                     purity of the material was assessed by photolumin-
                  ˚
Lattice constant (A)      3.189      4.758      5.420   3.567          escence (PL) measurements performed at 4 K with a
Thermal expansion         5.59       7.5        4.70    4.38           HeCd laser (325 nm) as excitation source with power
coefficient (=10y6 Ky1)                                                densities up to I0s50 Wycm2 and incident at approxi-
                                                                       mately 308 to the normal of the sample surface. The PL
                                                                       emission was dispersed by a 0.6-m monochromator and
has not been reported. However, diamond deposition on                  detected by a cooled GaAs photomultiplier. The spectral
a GaN substrate was found to result in the growth of a                 resolution was 0.4 meV in the region from 3.2 to 3.55
limited number of individual grains only. This was                     eV. The set-up was calibrated by using a mercury lamp.
argued to be a consequence of the poor wetting of
diamond on GaN w4,7x. Depending on the growth                          3. Results and discussions
conditions a similar behaviour may be expected for the
growth of GaN on diamond.                                                 MOCVD growth on the sapphire substrate resulted in
   In this paper we report on the growth of GaN on a                   a smooth 2.2 mm thick (001) GaN layer as described
(110) single crystal diamond substrate with MOCVD                      in previous work w11x. In contrast with the deposition
using an AlN nucleation layer w7x and subsequent                       of GaN on sapphire or silicon, where the substrate
enlargement of the layer thickness by HVPE. The results                determines the orientation of the epilayer, the GaN layer
are compared with those of GaN layers, which were                      on the diamond substrate consists of differently oriented,
grown on sapphire substrates.                                          closely packed GaN grains. In Fig. 1 a SEM picture is
                                                                       shown of this approximately 2.5 mm thick continuous
2. Experimental procedure                                              polycrystalline layer, which indicates a high nucleation
                                                                       density as was intended by the use of the AlN nucleation
   In the growth experiments type IIa (110) natural                    layer. The different orientations of the grains in the layer
diamond and (001) sapphire crystals were used as                       are confirmed by X-ray diffraction measurements. The
substrates. The diamond substrates were polished to                    X-ray u –2u curve of this layer, as shown in Fig. 3a,
within 38 of the exact orientation and have a surface                  shows that that the GaN layer is hexagonal and that the
roughness less than 20 nm. Immediately before growth                   use of a (110) diamond substrate has not resulted in the
the diamond substrates were cleaned in boiling H2SO4                   growth of a (110) GaN layer. In fact, the spectrum in
with sodium nitrate, followed by an etch in HCly                       Fig. 3a shows a very strong w002x GaN peak, indicating
H2SO4 (3:1) w8x. The nitride layers were grown in a                    that a relatively large fraction of the grains is aligned
horizontal, low-pressure metal–organic chemical vapor                  with the w001x orientation perpendicular to the substrate.
deposition (MOCVD) reactor using trimethylgallium                      This is eighter imposed by the (110) substrate, or the
(TMG), trimethylaluminium (TMA) and ammonia                            result of evolutionary growth and selection of (001)
(NH3) as precursors. A 10 nm thick AlN nucleation
layer is deposited in a H2 carrier gas stream at 50 mbar
at 850 8C using a TMA and a NH3 partial pressure of
4.9=10y6 bar and 1.0=10y1 bar, respectively w9x. After
deposition of the nucleation layer, the temperature was
raised to 1170 8C and GaN was grown for 1 h. Next
the MOCVD pre-grown samples were used as templates
for the growth of thick GaN layers using hydride vapour
phase epitaxy (HVPE). The GaCl growth species were
in-situ synthesised by passing a 30 sccm flow of pure
HCl over liquid gallium (99.9999%) at 890 8C in a
home-built reactor w10x. HVPE deposition took place
for 1 h at a temperature of 910 8C. For the MOCVD
growth of GaN on sapphire the procedure as described
in w11x was used.
   After MOCVD as well as after additional HVPE, the
surface morphology of the layers was examined using
optical differential interference microscopy (DICM) and                Fig. 1. SEM picture of a GaN layer deposited on a (110) single crystal
scanning electron microscopy (SEM). The crystallo-                     diamond substrate using MOCVD.
                                           P.R. Hageman et al. / Thin Solid Films 443 (2003) 9–13                                     11

                                                                         than 100 mm (see Fig. 2b) protrude from a more
                                                                         homogeneous background layer showing grains with a
                                                                         diameter of approximately 20 mm. The hexagonal struc-
                                                                         ture of the GaN is reflected in the appearance of these
                                                                         pillars. The X-ray u –2u curve measurements, as shown
                                                                         in Fig. 3b, indicate that the preferential alignment of the
                                                                         GaN crystallites with respect to the substrate is changed
                                                                         from (001) to (112) and has increased considerably.
                                                                         Also, it is an indication of the preservation of the
                                                                         hexagonal orientation. In GaN the w112x orientation is
                                                                         at an angle of 17.18 with the w001x orientation of the
                                                                         hexagonal top face of the pillar-shaped structures, which
                                                                         correlates well with the fact that the pillars are not
                                                                         perpendicular to the substrate (see Fig. 2).
                                                                            From this morphology it can be deduced that during
                                                                         HVPE the conditions are such that the highest growth
                                                                         rate is in the w112x direction. As a result those grains in
                                                                         the polycrystalline GaN template which are aligned with
                                                                         their w112x orientation perpendicular to the substrate
                                                                         start to protrude from their immediate surroundings.
                                                                         Under the high deposition rate gas phase conditions of
                                                                         HVPE the extended grains encounter a higher concen-
                                                                         tration of the Ga growth precursor. This further enlarges
                                                                         the growth rate of these particular grains while the
                                                                         differently oriented grains in the background more and
                                                                         more suffer from a GaCl depleted gas phase. Recently,
                                                                         a similar growth mechanism was reported to yield N001M



Fig. 2. SEM pictures of a GaN layer deposited using HVPE on a
template consisting of a (110) single crystal diamond substrate cov-
ered by a MOCVD GaN film. (a) overview and (b) detailed image
of the hexagonal, columnar structures protruding the more uniform
background.

GaN from initially randomly oriented GaN nuclei w12x.
Since there is no obvious direct 1:1 relation between the
diamond (110) surface and the GaN (001) surface, the
latter mechanism is more likely. Besides reflections from
GaN, the spectrum shows the reflections of w220x dia-
mond originating from the single crystal (110) diamond
substrate below the relatively thin GaN layer. The
approximately 10 nm thick AlN nucleation layer is too
thin to show up in the spectrum. As for the growth of
GaN on Si(111) substrates w9x, the AlN nucleation layer
should be further optimised with respect to its layer
thickness and growth temperature to improve the GaN
material quality.
   After HVPE growth, the thickness of the uniform
GaN layer deposited on the MOCVD grown GaN layer
on a sapphire substrate has increased to 127 mm. As
shown in Fig. 2 the roughness of the polycrystalline
GaN layer on the diamond substrate has increased                         Fig. 3. X-ray diffraction spectra (u-2u curves) of (a) the MOCVD
enormously. Large, hexagonal shaped pillars with a                       GaN layer layer grown on a (110) diamond substrate and (b) after
diameter of approximately 50 mm and a height of more                     enlargement of the GaN layer thickness using HVPE.
12                                          P.R. Hageman et al. / Thin Solid Films 443 (2003) 9–13

                                                                          sapphire w14x. The most likely candidate for the impurity
                                                                          is carbon from the diamond substrate that incorporates
                                                                          in the MOCVD grown GaN film. Carbon acts as a deep
                                                                          acceptor and quenches the band edge emissions when
                                                                          present in GaN w15x.
                                                                             After HVPE growth the sample exhibits a very intense
                                                                          and detailed PL spectrum (see Fig. 4b). For the HVPE
                                                                          layer on a diamond substrate the intensity of the spec-
                                                                          trum is approximately 20 times higher than that of the
                                                                          initial MOCVD layer, thereby implying a considerable
                                                                          reduction of the amount of non-recombinative radiation
                                                                          centres andyor a large improvement of the structural
                                                                          quality. The peak at the high-energy side of the spectrum
                                                                          (at 3.472 eV) is attributed to excitons bound to a neutral
                                                                          donor (D0BE). This position confirms that the structure
                                                                          of the GaN layer is hexagonal and not cubic. In the
                                                                          latter case the D0BE peak is expected at a position of
Fig. 4. Low temperature photoluminescence spectra of (a) the
MOCVD GaN layers grown on (110) diamond and (001) sapphire
                                                                          3.26 eV due to the reduced bandgap of cubic GaN
substrates and (b) after enlargement of the GaN layer thickness using     compared to hexagonal GaN w16x.
HVPE.                                                                        The position of the D0BE peak at 3.472 eV is almost
                                                                          identical with that found for a 400 mm thick HVPE
textured diamond layers as a result of a gas phase                        grown, free-standing GaN layer w10x. This reveals that
gradient of the growth precursor concentration w13x.                      in contrast to the HVPE layer grown on sapphire (D0BE
   For GaN growth on Si substrates, epilayers in excess                   at 3.449 eV), the layer on the diamond substrate is
of 0.4 mm thickness were found to suffer from severe                      almost stress free. This agrees well with the absence of
cracking due to the large tensile strain in the GaN layer                 cracks in the layer indicating that the thermal stresses
formed when cooling down from growth temperature to                       are effectively accommodated at grain boundaries of the
room temperature w9x. A similar result was obtained in                    polycrystalline layer. In the DAP recombination region
the present work for the GaN layer as obtained by                         between 3.05–3.27 eV the zero-phonon peak is found
HVPE on the sapphire substrate. For GaN growth on                         at 3.262 eV with a strongly reduced intensity compared
diamond the difference in thermal expansion coefficients                  to the D0BE signal. At the low energy side two longi-
is even larger than for GaN on Si or sapphire (see Table                  tudinal-optical (LO) phonon replicas are found at,
1). However, only a few cracks are observed in the                        respectively, 92 meV and 184 meV from the DAP peak,
HVPE grown layer grown on the diamond substrate (see
                                                                          being one and two times the LO-phonon energy of GaN.
Fig. 2a). As will be shown below, the PL data indicate
                                                                          The presence of these phonon replicas indicates a nearly
that the HVPE GaN layer is almost stress-free. The fact
                                                                          perfect crystallinity of the individual crystallites. The
that the GaN forms no closed layer and its crystallites
are almost ‘free-standing’ ensures that the thermally                     difference in PL signature between the MOCVD and
induced strain is effectively absorbed at the grain bound-                HVPE grown GaN layer on the diamond substrate is
aries of a polycrystalline GaN layer resulting in an                      explained by the fact that, during HVPE growth the
almost stress-free layer.                                                 incorporation of carbon originating from the substrate,
   In Fig. 4 the PL spectra of the MOCVD grown GaN                        is eliminated by the initial GaN layer.
layers on the diamond and the sapphire substrate are                         The broad band between 2.0 and 2.4 eV in the PL
given. In contrast to the signal obtained from the                        spectra is known as ‘yellow luminescence’, and was
reference sample on sapphire, the PL signal from the                      commonly detected regardless of the growth technique.
MOCVD GaN layer on a diamond substrate has a very                         It is generally believed that this yellow luminescence
low intensity (see Fig. 4a). This implies that the con-                   involves electronic states associated with intrinsic
centration of non-radiative recombination centres is                      defects in the material, such as vacancies andyor inter-
high. The spectrum reveals no peaks in the excitonic                      stitials w17,18x and is due to the radiative recombination
region. In fact, the only features observed lay in the                    from a shallow donor to a deep localised acceptor state
region where signal is expected from donor-acceptor-                      w19x. In diamond a similar broad band between 2.4 and
pair (DAP) recombination. These characteristics point                     3.2 eV referred to as ‘band A luminescence’, was argued
to a bad structural quality of the material or to a high                  to be a result of donor–acceptor pair recombination w20x
concentration of incorporated donors and acceptors,                       and later was demonstrated to be dislocation related
which is not found for the MOCVD grown sample on                          w21x.
                                           P.R. Hageman et al. / Thin Solid Films 443 (2003) 9–13                                          13

4. Conclusions                                                             w3x M. Ishihara, T. Manabe, T. Kumagai, T. Nakamura, S. Fujiwara,
                                                                               Y. Ebata, S. Shikata, H. Nakahata, A. Hachigo, Y. Koga, Jpn.
                                                                               J. Appl. Phys. 40 (2001) 5065.
   In this study the deposition of GaN on diamond                          w4x M. Oba, T. Sugino, Diamond Relat. Mater. 10 (2001) 1343.
substrates was demonstrated for the first time. As a                       w5x H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys.
nucleation layer for the MOCVD experiments we have                             Lett. 48 (1986) 353.
used a low temperature AlN layer. The resulting                            w6x Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J.
MOCVD GaN layer is polycrystalline and hexagonal                               Cryst. Growth 98 (1989) 209.
with, as a result of carbon incorporation originating                      w7x M. Oba, T. Sugino, Jpn. J. Appl. Phys. 39 (2000) L1.
from the substrate, a very poor optical quality as                         w8x W.J.P. van Enckevort, G. Janssen, W. Vollenberg, M. Chermin,
determined by photoluminescence. Additional enlarge-                           L.J. Giling, Surf. Coat. Technol. 47 (1991) 39.
                                                                           w9x P.R. Hageman, S. Haffouz, V. Kirilyuk, A. Grzegorczyk, P.K.
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                                                                               Larsen, Phys. Stat. Sol. A 188 (2001) 523–526.
preserved the hexagonal structure and resulted in layer                   w10x V. Kirilyuk, P.R. Hageman, P.C.M. Christianen, M. Zielinski,
of good optical quality, i.e. with a strongly reduced                          P.K. Larsen, Appl. Phys. Lett. 79 (2001) 4109.
intensity of the donor-acceptor pair recombination relat-                 w11x F.K. de Theije, A.R.A. Zauner, P.R. Hageman, W.J.P. Enckevort,
ed peaks in the PL spectrum. X-Ray diffraction analyses                        P.K. Larsen, J. Cryst. Growth 197 (1999) 37.
revealed that the dominant orientation of the GaN                         w12x A. van der Drift, Philips Res. Rep. 22 (1967) 267.
crystallites perpendicular to the substrate changed from                  w13x J.J. Schermer, F.K. de Theije, W.A.L.M. Elst, J. Cryst. Growth
w001x for the thin MOCVD film to w112x for the HVPE                            243 (2002) 302.
layer.                                                                    w14x S. Haffouz, V. Kirilyuk, P.R. Hageman, L. Macht, J.L. Weyher,
                                                                               P.K. Larsen, Appl. Phys. Lett. 79 (2001) 2390.
                                                                          w15x H. Tang, J.B. Webb, J.A. Bardwell, S. Raymond, Appl. Phys.
Acknowledgments
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and P. Mulder for the SEM pictures and XRD measure-                            (1996) 503.
ments and V. Kirilyuk, L. Macht and T. Ochalski for                       w18x P. Boguslawski, E. Briggs, J. Bernholc, Phys. Rev. B51 (1995)
performing the PL measurements.                                                17255.
                                                                          w19x P. Perlin, T. Suski, H. Teisseyre, M. Leszczynski, I. Grzeory,
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