Translated Title

Translated Abstract

Aluminium　nitride　(AlN)　belongs　to　the　third　generation　semiconductor　materials,　exhibiting
prominent　features　such　as　wide　bandgap,　high　electronic　saturation　mobility　and　low
permittivity.　Thus　AlN　has　potential　applications　in　fabricating　deep　ultra-violet　emitters　and
optoelectronic　detectors,　and　attracts　much　attention.　Due　to　hexagonal　wurtzite　structure
and　strong　ionic　bonding　between　the　atoms,　AlN　possesses　large　spontaneous　polarization,
significantly　influencing　the　optoelectronic　properties　of　devices.　Thus　the　spontaneous
polarization　is　an　inevitable　issue　in　the　study　of　wide　bandgap　semiconductors.　In　this
dissertation,　based　on　the　modern　polarization　theory　and　employing　the　first　principles,　the
spontaneous　polarization　of　bulk　and　one　dimension　nanostructure　is　systematically
researched.　The　findings　are　of　guiding　significance　help　for　improving　the　properties　of
optoelectronic　and　piezoelectronic　devices.
The　spontaneous　polarization　and　piezoelectric　constants　of　wurtzite　AlN,　ZnO　and　GaN　are
calculated　by　using　the　Berry　phase　method　and　maximally　localized　Wannier　functions,
respectively.　The　relations　between　the　spontaneous　polarization　and　the　crystal　structures
are　also　investigated.　The　result　indicates　that　the　spontaneous　polarization　and　piezoelectric
constants　of　AlN　are　the　1argest　one　among　the　three　semiconductors.　The　spontaneous
polarization　of　AlN　is　two　times　more　than　that　of　the　other　two　semiconductors.　Analyzing
the　contribution　of　electronic　part　and　ionic　part,　we　found　that　it　is　the　maximum　deviation
from　the　parameter　u　of　ideal　wurtzite　structure　and　high　ionicity　of　N-Al　bond　that　lead　to
the　strong　spontaneous　polarization.
We　point　out　the　deficiencies　of　the　approach　used　in　the　literature　to　calculate　the
spontaneous　polarization.　On　the　basis　of　the　discussion　of　local　dipole,　a　correction　formula
is　given.　By　using　the　charge　depth　of　the　formula　unit,　we　also　proposed　a　new　method　to
directly　calculate　the　spontaneous　polarization　of　wurtzite　structure　without　building　the
reference　structure.　The　production　of　the　spontaneous　polarization　can　be　intuitively
explanation　in　terms　of　this　method.　Comparing　with　the　Berry　phase　method,　owning　to　the
production　of　the　Wannier　centers,　the　maximally　localized　Wannier　functions　has　a　unique
advantage　in　analyzing　spontaneous　polarization,　electric　structures　and　atomic　bonding,
which　will　give　a　more　clear　physical　picture.
The　electronic　structure　of　the　intrinsic　point　defects　is　firstly　calculated　through　the
maximally　Wannier　functions.　Because　vacancy　is　one　of　the　most　common　point　defect　in
the　crystal　growth,　which　influences　many　properties　of　the　crystal.　By　using　first-principles
plane-wave　ultrasoft　Pseudopotential　method　and　based　on　the　density　functional　theory,　the
maximally　localized　Wannier　functions　of　the　vacancy　with　different　charge　state　are
calculated.　With　the　shape　and　center　of　the　computed　Wannier　function,　the　electronic
structure　and　the　spontaneous　polarization　change　in　the　[　0001],[-1010]　and　[-12-10]
orientation　are　studied　respectively.　It　is　found　that　the　electrons　of　the　dangling　bonds
displace　to　the　positions　of　the　vacancies　in　N　vacancy　structure,　while　the　electrons　keep
away　from　the　vacancy　and　move　to　the　other　side　of　N　atoms　in　Al　vacancy　structure.　Since
the　vacancy　breaks　the　centrosymmetry　of　the　
[-1010　]　and　[-12-10]　orientation,　polarization
will　be　induced　in　these　two　directions,　and　the　polarization　increase　as　the　charge　increase.
In　the　[　0001]　orientation　of　the　N　vacancy　structure,　the　spontaneous　polarization　will
reverse　as　the　charge　state　of　vacancy　increase　because　of　the　dramatic　variation　of　the
electronic　structure.　Contrary　to　the　N　vacancy,　although　the　spontaneous　polarization　of　Al
vacancy　increases　as　the　charge　state　increases,　the　polarization　reversion　does　not　occur　in
the　Al　vacancy　structure.　For　Comparison,　the　spontaneous　polarization　variation　of　ZnO
caused　by　O　vacancy　and　Zn　interstitial　is　also　computed.　It　is　found　that　the　polarization
properties　induced　by　the　vacancy　is　similar　to　that　of　AlN.　The　polarization　change　induced
by　the　vacancy　interaction　in　ZnO　is　further　researched.　It　is　found　that　the　system　energy　is
more　stable　when　the　two　vacancies　get　close　to　each　other,　and　the　strong　interaction　causes
will　decrease　the　spontaneous　polarization,　which　can　be　used　to　regulate　the　polarization.
The　prominent　properties　of　the　nano-structure　of　AlN　are　attracted　much　attention　in　recent
years.　By　building　the　nanowires　with　different　sizes,　the　spontaneous　polarization　and
piezoelectric　properties　of　AlN　nanowires　(NWs)　are　investigated　in　terms　of　local　dipole
obtained　by　maximally　localized　Wannier　functions.　It　is　found　that　the　value　of　spontaneous
polarization　is　larger　in　NWs　than　in　the　bulk,　and　will　increase　by　decreasing　the　NWs　size.
A　further　analysis　is　performed　from　the　point　of　view　of　lattice　and　surface　relaxation
occurring　in　the　NWs.　We　show　that　surface　relaxation　enhances　the　spontaneous
polarization,　while　the　change　of　lattice　parameter　plays　a　negative　role.　For　the　piezoelectric
property,　we　find　that　it　is　slightly　improved　compared　with　bulk　and　is　similar　in　a　certain
range　of　NW　diameters.　These　findings　are　of　guiding　significance　help　for　the　fabrication　of
piezoelectric　devices　based　on　AlN　NWs.

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