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Transmission Electron Microscopy of III-V Nanowires and Nanotrees

Karlsson, Lisa LU (2007)
Abstract (Swedish)
Popular Abstract in Swedish

I denna avhandling har morfologin och kristallstrukturen av epitaxiellt växta nanotrådstrukturer undersökts med hjälp av svepelektronmikroskopi (SEM), transmissionselektronmikroskopi (TEM) och multi-slice (MS) simuleringar. Bland annat har vi funnit att de lamellära tvillingar som ofta observeras i III-V nanotrådar växta i <111>B riktningarna tenderar att begränsas av enbart {111} fasetter. Om en sådan nanotråd betraktas i en <-110> riktning, parallell med tvillingplanen, uppvisar nanotråden mikrofasetterade kanter eftersom två av {111} planen ligger i zonen. Om samma nanotråd roteras 30 grader runt dess egen axel, motsvarande en <11-2> riktning, ses däremot helt parallella... (More)
Popular Abstract in Swedish

I denna avhandling har morfologin och kristallstrukturen av epitaxiellt växta nanotrådstrukturer undersökts med hjälp av svepelektronmikroskopi (SEM), transmissionselektronmikroskopi (TEM) och multi-slice (MS) simuleringar. Bland annat har vi funnit att de lamellära tvillingar som ofta observeras i III-V nanotrådar växta i <111>B riktningarna tenderar att begränsas av enbart {111} fasetter. Om en sådan nanotråd betraktas i en <-110> riktning, parallell med tvillingplanen, uppvisar nanotråden mikrofasetterade kanter eftersom två av {111} planen ligger i zonen. Om samma nanotråd roteras 30 grader runt dess egen axel, motsvarande en <11-2> riktning, ses däremot helt parallella sidofasetter utan någon mikrofasettering. Detta beror på att kanterna mellan {111} fasetterna ger upphov till projicerade sidofasetter i en sådan projektion. Dessutom är tvillingplanen klart urskiljbara i <-110> men mindre tydliga i <11-2>, eftersom atomkolumnerna överlappar varandra i det senare fallet. Två tvillingtyper har tidigare rapporterats i litteraturen med endera heteroatomära (III-V) eller monoatomära (III-III eller V-V) bindningar över tvillingplanet. I denna studie har vi funnit, utifrån MS-simuleringar, att den heteroatomära tvillingtypen dominerar.



Om nanotrådarna betraktas i kristallriktningar som inte är parallella med tvillingplanen uppvisar angränsande tvillingsegment två olika kristallriktningar. Dessa riktningar är kristallografiskt relaterade eftersom de delar ett gemensamt (111) tvillingplan och kan därför indexeras. MS-simuleringar har visat att båda tvillingtyperna genererar samma typ av högupplösande TEM-bilder i zonerna <100>, <-1-10>, <1-1-1> och <1-1-2>. De motsvarande kristallriktningarna i angränsande tvillingsegment har bestämts till <1-2-2>, <11-4> och <11-5>. Eftersom varje segment är begränsat av {111} ytor kommer regioner av ickeöverlappande tvillingsegment att ha en specifik morfologi.



Vid växt av nanotrådar används ofta guldnanopartiklar för att definiera nanotrådarnas diameter och position. Interaktionen mellan dessa nanopartiklar och III-V substratet samt nanotråden har i denna avhandling undersökts för GaAs. Effekterna av upphettning innan växt samt olika atmosfärer vid nedkylning har studerats genom sammansättningsanalys av guldnanopartiklarna efter växt. Det har visat sig att guldpartiklarna inte reagerar med GaAs i As-rik atmosfär under upphettning, växt och nedkylning. Däremot om en Ga-rik atmosfär används ökar halten Ga i nanopartiklarna efter växt eftersom legeringsreaktioner sker mellan Au och Ga. Dessa observationer stämmer väl överens med det ternära Au-Ga-As fasdiagrammet. Baserat på detta drog vi slutsatsen att guldpartiklarna förblir i fast fas så länge växten sker under As-rika förhållanden i vårt system. Om däremot legering tillåts ske mellan Au och Ga är det möjligt att nanopartiklarna smälter.



När det gäller Au-In-As systemet är situationen annorlunda eftersom Au och InAs inte är stabila tillsammans. Detta har fått till följd att växt av InAs nanotrådstrukturer stabiliseras av växt med förlegerade Au-In nanopartiklar.



Mer komplexa nanotrådstrukturer kan växas genom att använda tidigare generationer av nanotrådar som fristående substrat. På grund av morfologin benämns dessa strukturer nanoträd, med stam, grenar och löv motsvarande tre olika generationer av nanotrådväxt. För III-V material där zinkbländestruktur dominerar bibehålls växtriktningarna till <111>B oavsett växtgeneration. Detta gäller även de lamellära tvillingarna med några få undantag. I övergången mellan stam och gren hittas ofta en enkristallin region som tros bero på låg övermättnad av Ga i det initiala stadiet av växt. Om nukleering av ett löv sker på en sådan region bildas en helt enkristallin nanotråd.



Vid heteroepitaxiell växt av InP på GaP fås ett annat växtbeteende. Växten sker då topotaxiellt med InP grenar eller löv krypande parallet med eller i spiral runt om tidigare generationer av GaP nanotrådar. Undantaget är toppen av tidigare nanotrådgenerationer där växtriktningen bibehålls men ett byte av material sker. Skillnaden mellan dessa två fall är troligen olika förhållanden för nukleering.



Tack vare den begränsade storleken på nanotrådstrukturerna kan högupplösande TEM-bilder med näst intill atomär upplösning erhållas. Dessa bilder har tillsammans med MS-simuleringar kunnat användas för att rekonstruera den tredimensionella strukturen av nanotrådarna. Dessutom har Moiré mönster och dubbeldiffraktion i TEM-bilderna, orsakade av överlappande kristallint material, varit användbara för att avgöra det relativa förhållandet mellan såväl nanotrådgenerationer i nanoträd som angränsande tvillingsegment i en nanotråd. (Less)
Abstract
In this work, the morphology and crystal structure of epitaxial semiconductor nanowire structures grown by metal-organic vapour phase epitaxy (MOVPE) are studied by electron microscopy methods. In particular, the three-dimensional structure of nanowires and nanotrees has been characterised by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and multi-slice (MS) simulations. It has been found that the repeated lamellar twinning often observed in III-V <111>B nanowires tends to form a morphology bound by {111} facets only. When this structure is viewed parallel to the twin planes in <-110>, a zig-zag appearance is found as two of the {111} facet types are in the zone. When rotated 30... (More)
In this work, the morphology and crystal structure of epitaxial semiconductor nanowire structures grown by metal-organic vapour phase epitaxy (MOVPE) are studied by electron microscopy methods. In particular, the three-dimensional structure of nanowires and nanotrees has been characterised by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and multi-slice (MS) simulations. It has been found that the repeated lamellar twinning often observed in III-V <111>B nanowires tends to form a morphology bound by {111} facets only. When this structure is viewed parallel to the twin planes in <-110>, a zig-zag appearance is found as two of the {111} facet types are in the zone. When rotated 30 degrees around the growth axis to a <11-2> direction the edges between the {111} facets align to seemingly flat side facets. Also, the twin planes are clearly distinguishable in <-110> but are less pronounced in <11-2>, due to alignment of the atomic columns in the latter case. Two possible twin types have previously been reported in literature, a rotation twin 60 degrees around the growth axis of the nanowire and a mirror twin 180 degrees over the twin plane. The dominating type in this study, as found from MS-simulations, is the rotation twin type with III-V bonding over the twin plane.



Viewing directions of the nanowires non-parallel to the twin plane will show two different crystal orientations in neighbouring twin segments, as a consequence of twinning. These are crystallographically related as they share a common (111) twin plane and can be indexed based on stereographic projections. MS-simulations showed that both twin types would have the same appearance in HRTEM-images of the studied low index zones <-100>, <-1-10>, <1-1-1> and <1-1-2>. The corresponding zone axes in the neighbouring segments were indexed to <1-2-2>, <11-4> and <11-5> in the first three cases. Moreover, the appearance of the nonoverlapping regions in these viewing directions confirmed a suggested three-dimensional model with only {111} facets.



To mediate the nanowire growth, Au seed nanoparticles are often used to define the position and diameter of the nanowires. The influence of the interaction between the Au seed nanoparticles and the substrate and nanowire structures during growth was investigated for GaAs. The effects of annealing prior to growth and of different termination procedures during cooling after growth were studied by ex situ EDS-analysis. It was found that the Au seed particles are stable in contact with GaAs nanowires when kept at As-rich conditions during annealing, growth and cooling. However, if Ga-rich conditions are used, alloying between Au and Ga occurs readily and the composition of the seed nanoparticles changes with growth temperature. These observations are consistent with the ternary Au-Ga-As phase diagram where a Au-GaAs-As tie-triangle dominates at As-rich conditions and GaAs alloy tie-lines are found on the Ga-rich side. Therefore, it was concluded that the Au seed particles are solid during growth, in our MOVPE-system, but that the situation can change if alloying is allowed to occur during annealing. For Au-In-As the situation is different as there is no tie-line of Au-InAs and the seed particles will alloy more readily with the substrate. In fact, to stabilise the growth of InAs nanotrees pre-alloyed Au-In seed nanoparticles were used.



By sequential nanowire growth using previously grown nanowires as free-standing substrates more complex, so called nanotree structures, can be grown. Due to the similarity of a tree the respective growth sequences are labelled trunk, branch, and leaf. For III-V nanotrees of predominantly zinc blende structure the nanowire growth was found to adopt the <111>B growth directions irrespective of growth sequence. However, a single crystalline transition section was found close to the base of branches and/or leaves nucleating on a twinned region. This section was believed to form during low supersaturation conditions in the initial stage of growth. If nucleation occurred on a single crystalline section, fully single crystalline leaves would form. Similarly, when nucleating at the very base of a branch, fully twinned leaves would grow. Heteroepitaxial growth of InP on GaP resulted in a topotaxial growth behaviour with InP branches or leaves crawling parallel to or spiralling around the previously grown nanowires. Only the top section of the previously grown GaP nanowires



would form InP in the <111>B growth direction. This led to the conclusion that the nucleation conditions of the two generations are different.



Thanks to the limited diameter of the nanowire structures, HRTEM-images with atomic resolution can be obtained on as grown samples with minimal sample preparation. Based on the appearance of these images, the 3D structure of the nanowires can be characterised using MS-simulations. Also, Moiré patterns and double diffraction of overlapping crystalline materials can be used to determine the relative orientation of sequentially grown nanowire structures as well as neighbouring twin segments. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr. Liliental-Weber, Zuzanna, Materials Sciences Division, Lawrence Berkely National Laboratory, University of California
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Technological sciences, Naturvetenskap, Natural science, nanowires, TEM, III-V, Teknik
pages
148 pages
publisher
Kemiska institutionen
defense location
Fysiska institutionen, Sölvegatan 14, hörsal B, Lund
defense date
2007-12-03 09:15
ISBN
978-91-7422-174-9
language
English
LU publication?
yes
id
4c98f856-1482-4f1b-bd32-b90d662fc576 (old id 599295)
date added to LUP
2007-11-13 08:13:11
date last changed
2016-09-19 08:45:12
@misc{4c98f856-1482-4f1b-bd32-b90d662fc576,
  abstract     = {In this work, the morphology and crystal structure of epitaxial semiconductor nanowire structures grown by metal-organic vapour phase epitaxy (MOVPE) are studied by electron microscopy methods. In particular, the three-dimensional structure of nanowires and nanotrees has been characterised by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and multi-slice (MS) simulations. It has been found that the repeated lamellar twinning often observed in III-V &lt;111&gt;B nanowires tends to form a morphology bound by {111} facets only. When this structure is viewed parallel to the twin planes in &lt;-110&gt;, a zig-zag appearance is found as two of the {111} facet types are in the zone. When rotated 30 degrees around the growth axis to a &lt;11-2&gt; direction the edges between the {111} facets align to seemingly flat side facets. Also, the twin planes are clearly distinguishable in &lt;-110&gt; but are less pronounced in &lt;11-2&gt;, due to alignment of the atomic columns in the latter case. Two possible twin types have previously been reported in literature, a rotation twin 60 degrees around the growth axis of the nanowire and a mirror twin 180 degrees over the twin plane. The dominating type in this study, as found from MS-simulations, is the rotation twin type with III-V bonding over the twin plane.<br/><br>
<br/><br>
Viewing directions of the nanowires non-parallel to the twin plane will show two different crystal orientations in neighbouring twin segments, as a consequence of twinning. These are crystallographically related as they share a common (111) twin plane and can be indexed based on stereographic projections. MS-simulations showed that both twin types would have the same appearance in HRTEM-images of the studied low index zones &lt;-100&gt;, &lt;-1-10&gt;, &lt;1-1-1&gt; and &lt;1-1-2&gt;. The corresponding zone axes in the neighbouring segments were indexed to &lt;1-2-2&gt;, &lt;11-4&gt; and &lt;11-5&gt; in the first three cases. Moreover, the appearance of the nonoverlapping regions in these viewing directions confirmed a suggested three-dimensional model with only {111} facets.<br/><br>
<br/><br>
To mediate the nanowire growth, Au seed nanoparticles are often used to define the position and diameter of the nanowires. The influence of the interaction between the Au seed nanoparticles and the substrate and nanowire structures during growth was investigated for GaAs. The effects of annealing prior to growth and of different termination procedures during cooling after growth were studied by ex situ EDS-analysis. It was found that the Au seed particles are stable in contact with GaAs nanowires when kept at As-rich conditions during annealing, growth and cooling. However, if Ga-rich conditions are used, alloying between Au and Ga occurs readily and the composition of the seed nanoparticles changes with growth temperature. These observations are consistent with the ternary Au-Ga-As phase diagram where a Au-GaAs-As tie-triangle dominates at As-rich conditions and GaAs alloy tie-lines are found on the Ga-rich side. Therefore, it was concluded that the Au seed particles are solid during growth, in our MOVPE-system, but that the situation can change if alloying is allowed to occur during annealing. For Au-In-As the situation is different as there is no tie-line of Au-InAs and the seed particles will alloy more readily with the substrate. In fact, to stabilise the growth of InAs nanotrees pre-alloyed Au-In seed nanoparticles were used.<br/><br>
<br/><br>
By sequential nanowire growth using previously grown nanowires as free-standing substrates more complex, so called nanotree structures, can be grown. Due to the similarity of a tree the respective growth sequences are labelled trunk, branch, and leaf. For III-V nanotrees of predominantly zinc blende structure the nanowire growth was found to adopt the &lt;111&gt;B growth directions irrespective of growth sequence. However, a single crystalline transition section was found close to the base of branches and/or leaves nucleating on a twinned region. This section was believed to form during low supersaturation conditions in the initial stage of growth. If nucleation occurred on a single crystalline section, fully single crystalline leaves would form. Similarly, when nucleating at the very base of a branch, fully twinned leaves would grow. Heteroepitaxial growth of InP on GaP resulted in a topotaxial growth behaviour with InP branches or leaves crawling parallel to or spiralling around the previously grown nanowires. Only the top section of the previously grown GaP nanowires<br/><br>
<br/><br>
would form InP in the &lt;111&gt;B growth direction. This led to the conclusion that the nucleation conditions of the two generations are different.<br/><br>
<br/><br>
Thanks to the limited diameter of the nanowire structures, HRTEM-images with atomic resolution can be obtained on as grown samples with minimal sample preparation. Based on the appearance of these images, the 3D structure of the nanowires can be characterised using MS-simulations. Also, Moiré patterns and double diffraction of overlapping crystalline materials can be used to determine the relative orientation of sequentially grown nanowire structures as well as neighbouring twin segments.},
  author       = {Karlsson, Lisa},
  isbn         = {978-91-7422-174-9},
  keyword      = {Technological sciences,Naturvetenskap,Natural science,nanowires,TEM,III-V,Teknik},
  language     = {eng},
  pages        = {148},
  publisher    = {ARRAY(0xb0d30c0)},
  title        = {Transmission Electron Microscopy of III-V Nanowires and Nanotrees},
  year         = {2007},
}