Investigation and comparison of GaN nanowire nucleation and growth by the catalyst-assisted and self-induced approaches [Elektronische Ressource] / Caroline Cheze. Gutachter: H. Lüth ; W. T. Masselink ; H. Riechert

Frau M.Sc. Caroline Chèze

Sujets

• Physik

Informations

• Publié par: humboldt-universitat_zu_berlin
• Publié le: 01 janvier 2011
• Nombre de lectures: 13
• Langue: English
• Poids de l'ouvrage: 13 Mo

Extrait

Investigation and comparison of GaN nanowire nucleation
and growth by the catalyst-assisted and self-induced
approaches
DISSERTATION
zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
im Fach Physik
eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
Humboldt-Universität zu Berlin
von
Frau M.Sc. Caroline Chèze
geboren am 06.03.1977 in Chambéry, Frankreich
Präsident der Humboldt-Universität zu Berlin:
Prof. Dr. Dr. h.c. Christoph Markschies
Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I:
Prof. Dr. Lutz-Helmut Schön
Gutachter:
1. Prof. Dr. H. Lüth
2. Prof. Dr. W. T. Masselink
3. Prof. Dr. H. Riechert
eingereicht am: 20.08.2009
Tag der mündlichen Prüfung: 04.03.2010Abbreviations
AFM . . . . . . . . . . . . . . . . . atomic force microscopy
C-plane . . . . . . . . . . . . . . . GaN growth along [0001]
CBED . . . . . . . . . . . . . . . . convergent beam electron diffraction
cw-PL . . . . . . . . . . . . . . . . continuous wave photoluminescnece
0(D ,X ) . . . . . . . . . . . . . . . neutral to donor related bound excitonA
EDXS . . . . . . . . . . . . . . . . . energy dispersive x-ray spectroscopy
EELS . . . . . . . . . . . . . . . . . electron energy-loss spectroscopy
ES barrier . . . . . . . . . . . . . Ehrlich-Schwoebel barrier
FFT . . . . . . . . . . . . . . . . . . . fast Fourier transform
FWHM . . . . . . . . . . . . . . . full width at half maximum
Ga . . . . . . . . . . . . . . . . . . . . Gallium
ML . . . . . . . . . . . . . . . . . . . monolayer
m PL . . . . . . . . . . . . . . . . . . micro-photoluminescence
MOVPE . . . . . . . . . . . . . . metal-organic vapor-phase epitaxy
N . . . . . . . . . . . . . . . . . . . . . neutral atomic nitrogen
NW . . . . . . . . . . . . . . . . . . nanowire
PAMBE . . . . . . . . . . . . . . . plasma-assisted molecular beam epitaxy
PL . . . . . . . . . . . . . . . . . . . . photoluminescence
QMS . . . . . . . . . . . . . . . . . quadrupole mass spectrometry
RF . . . . . . . . . . . . . . . . . . . . radio frequency
RBS . . . . . . . . . . . . . . . . . . Rutherford back scattering
RHEED . . . . . . . . . . . . . . . reflection high-energy electron diffraction
RT . . . . . . . . . . . . . . . . . . . . room temperature
SEM . . . . . . . . . . . . . . . . . . scanning electron microscopy
SAED . . . . . . . . . . . . . . . . selected-area electron diffraction
SF . . . . . . . . . . . . . . . . . . . . stacking fault
SK . . . . . . . . . . . . . . . . . . . . Stranski-Krastanov
STM . . . . . . . . . . . . . . . . . . scanning tunneling microscopy
TEM . . . . . . . . . . . . . . . . . . transmission electron microscopy
1D, 2D, 3D . . . . . . . . . . . . one-, two-, three-dimensional
UV . . . . . . . . . . . . . . . . . . . ultra violet
VW . . . . . . . . . . . . . . . . . . . Volmer-Weber
WZ . . . . . . . . . . . . . . . . . . . wurtzite
XEDS . . . . . . . . . . . . . . . . . X-ray energy dispersive spectrometry
XRD . . . . . . . . . . . . . . . . . . X-Ray diffraction
XTEM . . . . . . . . . . . . . . . . cross-sectional transmission electron microscopy
ZB . . . . . . . . . . . . . . . . . . . . zinc blende
iiiAbstract
This work focuses on the nucleation and growth mechanisms of GaN nanowires
(NWs) by molecular beam epitaxy (MBE). The two main novelties of this study are
the intensive employment of in-situ techniques and the direct comparison of self-
induced and catalyst-induced NWs. On silicon substrates, GaN NWs form in MBE
without the use of any external catalyst seed. On sapphire, in contrast, NWs grow
under identical conditions only in the presence of Ni seeds. NW nucleation was
studied in situ by reflection high-energy electron diffraction (RHEED) in correla-
tion with line-of-sight quadrupole mass spectrometry (QMS). The latter technique
allows to monitor the incorporated amount of Ga.
For the catalyst-assisted approach, three nucleation stages were identified: first
incorporation of Ga into the Ni seeds, second transformation of the seed crystal
structure due to Ga accumulation, and last GaN growth under the seeds. The
crystalline structure of the seeds during the first two stages is in accord with the
Ni-Ga binary phase diagram and evidenced that only Ga incorporates into the Ni
particles. GaN forms only after the Ga concentration is larger than the one of Ni,
which is in agreement with the Ni-Ga-N ternary phase diagram. The observation
of diffraction patterns generated by the Ni-Ga seed particles during the whole nu-
cleation evidences the solid state of the seeds. Therefore nucleation is ruled by the
vapor-solid-solid mechanism. Moreover, the QMS study showed that it is not Ga
incorporation into Ni but GaN nucleation itself that limits the growth processes.
For the self-induced NWs, QMS and RHEED investigations indicate very similar
nucleation processes on Si(001) and Si(111) and two nucleation stages were identi-
fied. Transmission electron microscopy on samples grown on Si(001) revealed that
the first stage is characterized by the competition between the nucleation of crys-
talline Si N and GaN. During this stage, the Si surface strongly roughens by thex y
formation of pits and Si mounds. At the same time, very few GaN islands nucleate.
During the second stage, the amorphization of the Si N layer leads to the mas-x y
sive nucleation of GaN islands that are free of the substrate lattice constraint and
therefore form in the wurtzite (WZ) structure.
The processes leading to NW nucleation are fundamentally different for both
approaches. In the catalyst-assisted approach, Ga strongly reacts with the catalyst
Ni particles whose crystal structure and phases are decisive for the NW growth. In
the catalyst-free approach, N forms an interfacial layer with Si before the intense
nucleation of GaN starts, and the lattice-mismatch to the substrate plays the most
important role.
Both approaches are viable to produce NWs within the same range of substrate
temperatures and V/III ratios, provided the latter is larger than one (N-excess). Both
yield monocrystalline GaN NWs of WZ structure, which grow in the Ga-polar di-
rection. However, strong differences are also observed. First, the catalyst-assisted
NWs are longer than the catalyst-free ones after growth under identical conditions
(duration, substrate temperature and V/III ratio), and the former grow at the rate
of the supplied N. This observation can be explained by the local Ga-excess es-
tablished at the Ni-particle position. Therefore, this result is in good agreement
with the catalyst-assisted nucleation model described above. In contrast, the self-
induced NWs grow with an intermediate rate between the supplied Ga- and N-
rates. Second, the catalyst-assisted approach provides GaN NWs that contain many
vstacking faults, while the catalyst-free ones are largely free of defects. Third, the
photoluminescence (PL) of the catalyst-free NWs is narrower and much more in-
tense than the one of the catalyst-assisted NWs. All of these differences can be
explained as effects of the catalyst. The seed captures Ga atoms arriving at the
NW tip more efficiently than the bare top facet in the catalyst-free approach. In
addition, stacking faults could result from both the presence of the additional solid
phase constituted by the catalyst-particles and the contamination of the NWs by
the catalyst material. Finally, such contamination would generate non-radiative
recombination centers and in turn reduce the PL intensity. Thus, the use of cata-
lyst seeds may offer an additional way to control the growth of NWs, but both the
structural and the optical material quality of catalyst-free NWs are superior.
viZusammenfassung
Diese Arbeit befasst sich mit der Keimbildung und den Wachstumsmechanismen
von GaN-Nanodrähten (NWs), die mittels Molekularstrahlepitaxie (MBE) herge-
stellt wurden. Die Hauptneuheiten dieser Studie sind der intensive Gebrauch von
in-situ Messmethoden und der direkte Vergleich zwischen katalysatorfreien und
katalysatorinduzierten NWs. In der MBE bilden sich GaN-NWs auf Silizium ohne
Katalysator. Auf Saphir dagegen wachsen NWs unter den gleichen Bedingungen
nur in der Anwesenheit von Ni-Partikeln. Die Nanodraht-Keimbildung wurde in
situ mittels Beugung hochenergetischer Elektronen in Reflexion (RHEED) sowie
Quadrupol-Massenspektrometrie in Sichtlinie (QMS) studiert. Die letztere Metho-
de ermöglicht die Beobachtung der eingebauten Ga-Menge.
Für den katalysatorinduzierten Ansatz wurden drei Nukleationsstadien identifi-
ziert: erstens der Einbau von Ga in die Ni-Partikel, zweitens die Umwandlung der
Partikelkristallstruktur durch Ga-Anreicherung und drittens das GaN-Wachstum
unterhalb der Ni-Partikel. Die Partikelkristallstrukturen während der zwei ersten
Stadien stimmen mit dem binären Ni-Ga Phasendiagramm überein und bestäti-
gen, dass nur Ga in die Ni-Keime eingebaut wird. GaN wächst erst wenn die Ga-
Konzentration größer als jene von Ni wird, was mit dem ternären Ni-Ga-N Pha-
sendiagramm übereinstimmt. Die Beobachtung von durch die Ni-Ga Partikel ver-
ursachten Beugungsbildern während der gesamten Nukleation beweist den fes-
ten Aggregatszustand der Partikel. Daher ist die durch den