HAL Id: jpa-00249720
https://hal.archives-ouvertes.fr/jpa-00249720
Submitted on 1 Jan 1997
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Heteroepitaxy of Cubic GaN
A. Trampert, O. Brandt, H. Yang, K. Ploog
To cite this version:
A. Trampert, O. Brandt, H. Yang, K. Ploog. Heteroepitaxy of Cubic GaN. Journal de Physique III, EDP Sciences, 1997, 7 (12), pp.2309-2316. �10.1051/jp3:1997260�. �jpa-00249720�
Heteroepitaxy of Cubic GaN
A. Trampert (*), O. Brandt, H. Yang and K.H. Ploog
Paul-Drude-Institut fur Festkorperelektromk, Haujvogteiplatz 5-7, 1011? Berlin, Germany
(Received 3 October 1996, revised 9 May1997, accepted 29 August 1997)
PACS.61.16.Bg Transmission, reflection and scanning electron microscopy (including EBIC)
PACS 68.55.Jk Structure and morphology; thickness
PACS.81.15.Hi Molecular, atomic, ion and chemical beam epitaxy
Abstract. We report on the epitaxial growth and the microstructure of cubic GaN. The
layers investigated are deposited by plasma-assisted molecular beam epitaxy on GaAs (001) and (311)A substrates. Transmission electron microscopy reveals that, despite ofthe extreme lattice
mismatch between these two materials, GaN grows in the metastable cubic phase with a well- defined orientation-relationship to the substrate and a sharp heteroboundary. This preference of the metastable phase and its epitaxial orientation originate in the initial stage of growth which is discussed in connection with a coincidence lattice for the investigated interface structures
1. Introduction
The realization of highly efficient blue light-emitting diodes has stimulated the recent activity in research on group-III nitrides. Because of the lack of large single crystals of GaN, heteroepitaxy
is necessary to produce these devices. Sapphire and-6H-SiC serve as the common substrate materials for growing the thermodynamically stable wurtzite modification of GaN. Cubic GaN
on suitable substrates, such as GaAs or Si, is scarcely payed attention to because of the very
high lattice mismatch between these materials. Such a high mismatch might be expected to result m a breakdown of ideal epitaxial growth leading to a high defect density and eventually
to polycrystalline growth which prevents the layers to be useful for applications.
It is well known that the initial stages of growth are most important for the epitaxial relation-
ship and the resulting micro-structure of heteroepitaxial systems. In this context, nitridation of the GaAs substrate surfaces prior to growth, or deposition of a thin amorphous layer promot- ing nucleation during subsequent solid-state epitaxy, are discussed in the literature as possible
routes to prepare cubic GaN. The results, however, are quite unsatisfactory, in that rough in-
terfaces, polycrystalline features as well as phase mixtures are found in these structures [1-3j.
In this paper, we demonstrate by conventional as well as high-resolution transmission electron microscopy that cubic GaN can be grown epitaxially on (001) and (311)A GaAs substrates with comparatively high crystalline perfection. In contrast to previous approaches, we grow
GaN on GaAs ma the immediate nucleation of cubic crystallites exhibiting the desired epitaxial
orientation. The resulting microstructure is a direct consequence of the atomic configuration
(* Author for correspondence (e-mail: tramperttlpdi.wias-berlin.de)
@ Les (ditions de Physique 1997
2310 JOURNAL DE PHYSIQUE III N°12
of the interface which can be explained in terms of a near coincidence lattice model. The
application of this model to different interface orientations underlines its general validity.
2. Experimental Details
The GaN/GaAs(001) heterostructures are grown in a plasma-assisted molecular beam epitaxy (MBE) system [4-7,9j. Prior to the deposition of GaN, a GaAs buffer layer is grown under conditions suitable for obtaining an atomically smooth surface. The substrate temperature is set to 580 °C at the beginning of GaN growth and increased to 650 °C after deposition of
a few monolayers (MLS) The initial nucleation and subsequent growth is monitored m situ
by reflection high-energy electron diffraction (RHEED). Various GaN samples with different nominal layer thicknesses are grown to investigate the successive stages of nucleation and the
evolution of the microstructure. The preparation of transmission electron microscopy (TEM) specimen from the as-deposited GaN layers is done by mechanical pre-thinning and final argon ion milling using a specimen cooling unit. The TEM observations are carried out in a Jeol
JEM 4000FX microscope operating at 400 kV and a JEM ARM 1250 with an acceleration
voltage of 1250 kV.
3. Results and Discussion
The TEM analysis reveals that the growth of cubic GaN on (001) GaAs takes place ma a
three-dimensional (3D) growth mode which can be subdivided into the following three stages describing the initial nucleation and relaxation process. A detailed study of the direct ob- servation of the initial stage of growth is given in a previous paper [8j. The results can be
summarized as follows:
ii) Initially, 3D-critical nuclei are formed with an average radius of m 1-2 nm. These nanoscale nuclei exhibit a strong epitaxial orientation-relationship and are relaxed by the in-
corporation of misfit dislocations which are instantaneously generated during their formation.
(ii) Further growth appears to be almost exclusively lateral with only little growth at this stage along the [001j-direction. Moreover, highly spatially periodic misfit dislocations are introduced at the GaN/GaAs interface without any climb or glide motion. Their number (at
every sth GaN lattice plane) is sufficient to relax all of the lattice mismatch between GaN and GaAs. Because of the high density of GaN nuclei conditional on the selected growth conditions, a connected film-like morphology is reached at a layer thickness of little more than 5 ML (Fig. I), i.e. at a very early stage of growth.
(iii) During the coalescence stage of the GaN islands, planar defects are generated which
were not observed in isolated nuclei [8j. In fact, since the spacing of the dislocations of the individual nuclei will not necessary be in phase with each other, the locations of coalescence
correspond to centers of high stress which are responsible for the generation of secondary defects, namely, stacking faults. This type of stacking fault is then caused by the easy glide of
partial dislocations at the stress-concentrated regions provided a low stacking fault energy.
In the case of 3D-growth, a di~erent mechanism of forming planar defects applies which is discussed in the following [10j. First, it has to be assumed that the island nuclei readily produce (ill) facets. During further growth, these (ill) planes can be easily mis-stacked, generating
a stacking fault. This mechanism is not stress-induced and occurs in isolated islands, which is, however, in contrast to our observations [8j. Furthermore, during the island coalescence this
stacking fault formation must produce "V"-shaped bundles of stacking faults ill]. However,
most of the defect bundles which are detected at the GaN/GaAs interface are running along only one set of (ill) planes making this mechanism of stacking fault formation rather unlikely.
(a)
1_
(b)
<((
h
Fig I. a) Cross-sectional HRTEM image of the GaN/GaAs (001) heterostructure taken along the
(l10) direction The very smooth layer appears nearly connected with
an atomically flat interface b) Magnified micrograph showing the presence of misfit dislocations (arrows)
Despite these planar defects, the GaN epilayer is characterized by a high epitaxial alignment throughout its whole thickness. The occurrence of epitaxy of metastable cubic GaN must be connected with a special interface configuration exhibiting a low total interfacial energy. Being
confident that the strain energy represents the most important part of the total interfacial energy (the chemical portion is assumed to be comparable between the cubic and hexagonal phase and can thus be neglected), the occurrence of the cube-on-cube oriented interface can be
explained by a near coincidence lattice model. Perfect coincidence sites between the epilayer
lattice (ae) and substrate lattice (as) would occur when aelas
= m/n, where m and n are
integers. If m
= n + I, there is one extra lattice plane in each unit cell of the coincidence site lattice i-e-, a geometrical (~) edge dislocation is generated. In general, however, the
(~) We want to point out that we use the expression "geometrical" misfit dislocation in order to
distinguish these from conventional misfit dislocations as defined by Matthews. This latter type of misfit dislocation is, in general, of different physical nature because it corresponds to a bulk dislocation
which
moves to the interface.
2312 JOURNAL DE PHYSIQUE III N°12
Fig. 2. )133] cross-sectional HRTEM image of the GaN/GaAs(311) interface The 5/4-periodicity
of the matching (022) planes is clearly visible.
tiny iiooj
As
~~ [311]
~
[233]
] [011]
N
,~t
Ga As
Fig 3. a) Schemihtic ideal atomic bonding diagram of the (311)A GaAs surface viewed in the [01i]
projection. b) Ball-and-stick model of the tilted boundary.
Fig. 4 a) [011] cross-sectional HRTEM image of the GaN/GaAs(311) interface demonstrating the
epilayer tilt (inset: magnified part of an interfacial region). b) Selected area diffraction pattern of a
large interfacial region (T: twin spots).
epitaxial hetero-system is not expected to be at true coincidence, and the coincidence-lattice mismatch f expresses this deviation from perfect coincidence as f
= (mas nae)/mas. This deviation introduces elastic strain at the interface in addition to the strain accommodated by
the geometrical misfit dislocations. Therefore, the energy of heteroboundaries is expected to be
2314 JOURNAL DE PHYSIQUE III N°12
small and epitaxy is favored if f does not deviate substantially from true coincidence. For the heterosystem under investigation, we obtain f
= -0 0002 + 0.0020 by taking m/n
= 4/5 and
the most accurate values for the lattice constants available at growth temperature, namely,
aGaN " 0.455 + 0.01 nm and aGaAs
" 0.568886 nm. Thus, this system is close to perfect
coincidence "magic mismatch") and a square array of geometrical edge dislocations running along the < l10 >-directions with a period of 5 GaN lattice planes will indeed account for the entire lattice mismatch. The occurrence of an almost perfect coincidence between cubic GaN and GaAs provides an explanation of the phenomenon of epitaxial growth for a strain at which
epitaxy of covalently bonded materials is usually no longer achieved, particularly so for the
case of a metastable phase. There are further examples given in the literature verifying our simple explanation of epitaxial growth in highly lattice-mismatched covalently bonded material
systems, but none with a higher mismatch than GaN /GaAs. For example, a 5-to-4 coincidence of lattice planes at the interface is observed by HRTEM [12j for metastable cubic SiC grown
on Si(001) (19.7Sl [13j) with a coincidence-lattice mismatch of -010024. Furthermore, at the
AIN/sapphire interface, having a lattice mismatch of13.29Sl, a correspondence of 8 AI-AI interatomic distances of AIN with 9 AI-AI distances in sapphire is found [14,lsj.
The argument of nearly perfect coincidence is generally valid and must be applicable to any interface orientation of the GaN/GaAs system. Figure 2 shows a [§33j cross-sectional HRTEM
image of a GaN/GaAs(311) interface. Again, the periodicity of perfectly matched planes,
which are here the (220) planes running perpendicular to the interface, is directly visible.
The spacing of the matching sites correspond to the expected 5/4 ratio. However, we should
take into account that the (311) surface exhibits a lower symmetry compared to the (001)
surface, and that the coincidence site lattice must share the symmetry of the adjacent planes
of both materials at the interface. Since the [§33j and the [01ij axes are directions with a high density of coincident sites, we may assume that the principal directions of strain relaxation are
perpendicular to these directions. Therefore, the cross-section HRTEM in the [133j projection
reflects not only the periodicity of the coincidence sites, but also a component of the Burgers
vector of one type of misfit dislocations.
In addition, the (311) surface is not atomically flat, but consists of a step array running along the [01ij-direction which consists of (ill) risers and (100) terraces cf. Fig. 3a). This
corrugation of the surface is also expected to influence the nature of misfit relieving defects, the
Burgers vector of which have to be inclined to the (311) surface resulting in a tilt component.
Hence it follows from the coincidence lattice that a periodic arrangement of inclined misfit dislocations of the same nature must result in a macroscopic tilt comparable to the case of
a low-angle tilt boundary (Fig. 3b). Applying the formula 9 1 b~ID, where b~ is the tilt component of the Burgers vector and D the periodic dislocation distance, the tilt angle 9 can be calculated to about 3.4° In fact, Figure 4 represents a HRTEM micrograph of the interface
viewed along the [011j projection In spite of the high density of nanotwms and stacking faults,
the tilting of the (ill) planes of the epilayer with respect to the substrate is clearly observable and can be estimated to be 4° +1°. This tilt angle is still observed in an interfacial region,
where less planar defects start from the heteroboundary (see inset in Fig. 4a)
The same tilt value is measured from the selected area di~raction pattern taken from an interfacial area of a few hundred nanometers (Fig. 4b). Therefore, we conclude that an ex-
planation of the epilayer tilt based only on the appearance of planar defects is more unlikely
because of their highly irregular arrangemept on this mesoscopic scale However, the origin of the planar defects and their possible influence on the epilayer orientation is still an open question. It should be taken into consideration that the lattice-mismatched heteroepitaxial growth on (311) surfaces does not only introduce a tetragonal distortion in the epilayer but
also a shear deformation [16,17j. This shear stress component could~ act as the driving force for
the generation of the planar defects, such as deformation twins and stacking faults, in addition to the geometrical misfit dislocations. To clarify this assumption, the initial GaN growth on (311) surfaces and the onset of plastic deformation must be investigated in more detail.
In conclusion, we have demonstrated that the nucleation and the corresponding interface
structure are important for the resulting microstructure of the GaN epilayer. The interface
structure between cubic GaN and GaAs is determined by the strain state and can be explained
by an extended coincidence site lattice model which takes into account the importance of the deviation from perfect coincidence. The magic mismatch is a prerequisite for epitaxy in systems of such high lattice mismatch.
Acknowledgments
Part of this work was sponsored by the Bundesministerium fur Forschung und Bildung of the Federal Republic of Germany. The authors would like to thank the Max-Planck-Institut fur
Metallforschung in Stuttgart and the Forschungszentrum Jiibch for providing their microscope facilities.
References
ill Strite S., Ruan J., Li Z., Salvador A., Chen H., Smith D.J., Choyke W.J. and Morkog H., An investigation of the properties of cubic GaN grown on GaAs by plasma-assisted
molecular-beam epitaxy, J. Vac. So. Technol. B 9 (1991) 1924.
[2j Kuwano N., Nagatomo Y., Kobayashi K., Oki K., Miyoshi S., Yaguchi H., Onabe K.
and Shiraki Y., Transmission Electron Microscope Observation of Cubic GaN Grown by Metalorganic Vapor Phase Epitaxy with Dimethylhydrazine on (001) GaAs, Jpn. J. Appl.
Phys. 33 (1994) 18.
[3j Chandrasekhar D., Smith D.J., Strite S., Lin M.E. and Morkog H., Characterization of group III-nitride semiconductors by high-resolution electron microscopy, J. Cryst. Growth 152 (1995) 135.
[4j Brandt O., Yang H., Jenichen B., Suzuki Y., Dhweritz L. and Ploog K.H., Surface re- constructions of zincblende GaN/GaAs(001) in plasma-assisted molecular beam epitaxy, Phys. Rev B 52 (1995) R2253.
[5j Yang H., Brandt O. and Ploog K.H., MBE Growth of Cubic GaN on GaAs Substrates, Phys. Stat. Sol. (b)194 (1996) 109.
[6j Yang H., Brandt O., Wassermeier M., Behrend J. and Ploog K.H., Evaluation of the
surface stoichiometry during molecular beam epitaxy of cubic GaN on (001) GaAs, Appl.
Phys. Lett. 68 (1996) 244.
[7j Yang H., Brandt O., Trampert A. and Ploog K.H
,
Initial stage of growth of GaN/GaAs (001) in plasma-assisted molecular beam epitaxy, Appl. Surf. Sci. lo4/los (1996) 461.
[8j Trampert A., Brandt O., Yang H. and Ploog K.H., Direct observation of the initial nucle- ation and epitaxial growth of metastable cubic GaN on (001) GaAs, Appl. Phys. Lett. 70
(1997) 583.
[9j Brandt O., Yang H. and Ploog K.H., Surface kinetics of zinc-blende (001) GaN, Phys.
Rev. B 54 (1996) 4432.
[10j Ernst F. and Pirouz P., The formation mechanism of planar defects in compound semi- conductors grown epitaxially on (100) silicon substrates, J. Mater. Res. 4 (1989) 834.
2316 JOURNAL DE PHYSIQUE III N°12
[llj Kim S.-D. and Harris J.S. Jr., Stacking fault stability in GaAs/Si hetero-epitaxial growth
J. Cryst. Growth 123 (1992) 439.
[12j Nutt S.R., Smith D.J., Kim H.J. and Davis R.F., Interface structure in beta-silicon carbide thin films, Appl. Phys. Lett. 50 (1987) 203.
[13j Kitabatake M. and Greene J.E., Surface-Structure-Controlled Heteroepitaxial Growth of
3C-SiC(001)3 x 2 on Si(001): Simulation and Experiment, Jpn. J. Appl. Phys. 35 (1996)
5261.
[14j Sun C.J., Saxler A., Ohsato H., Haritos K. and Razeghi M., A crystallographic model of
(00.I) aluminum nitride epitaxial thin film growth on (00.I sapphire substrate, J. Appl.
Phys. 75 (1994) 3964.
[lsj Masu K., Nakamura Y., Yamazaki T., Shibata T., Takahashi M. and Tsubouchi K., Trans- mission Electron Microscopic Observation of AIN / a-A1203 Heteroepitaxial Interface with Initial-Nitriding AIN Layer, Jpn. J. Appl. Phys. 34 (1995) L760.
[16j Hinckley J.k. and Singh J., Influence of substrate composition and crystallographic orien- tation on the band structure of pseudomorphic Si-Ge alloy films, Phys. Rev. B 42 (1990)
3546.
[17j De Carlo L. and Tapfer L
,
Elastic lattice deformation of semiconductor heterostructures grown on arbitrarily oriented substrate surfaces, Phys. Rev. B 48 (1993) 2298.
