The deconstructed Standard Model equation _ - symmetry magazine.pdf
Raman spectroscopy of ga mnn codoped with mg 2
1. Raman spectroscopy of dilute
magnetic semiconductor Ga1-xMnxN
co-doped with Mg
Review
2. • Dilute magnetic semiconductor (DMS) GaMnN for spintronic
applications
o Nature of ferromagnetism in transition metals (TM)-doped DMS
o Structures grown by group of prof. Bonanni at Linz
o Infrared photoluminescence of GaN(Mn, Mg)
o Magnetic properties of random ferromagnets GaMnN
• Raman spectroscopy of GaMnN:Mg
o Raman spectrum of wurtzite GaN
o Mn and Mg-induced Raman modes in GaN
o Strain-induced Raman shifts in GaN:Mg
o High-frequency H-related Raman modes
• Can hole concentration be estimated in p-type GaMnN:Mg from
Raman spectroscopy?
o Plasmon-phonon modes in n-type A3B5 and GaN
o Plasmon-phonon modes in p-type A3B5
o Can coupled plasmon-LO-phonon modes be observed in p-type
doped GaN?
o Low-frequency valence inter-subband scattering and hole density
• Paper on Raman spectroscopy published by group of prof. Bonanni
• Conclusions
Outline
2
3. Dilute magnetic semiconductor (DMS) GaMnN for spintronic applications
• GaN doped with Mn was predicted to have room temperature ferromagnetism.
• The nature of ferromagnetism is suppose to be exchange interaction between magnetic ions.
"an element whose atom has a partially filled d sub-shell, or
which can give rise to cations with an incomplete d sub-
shell".
transition or “3d” metals (TM)
3
room temperature
5% of Mn
4. 4
• Exchange interaction is a short-range, so certain concentration of TM is needed to realize it.
• Solubility limit of TM in GaN except for Mn is lower than 1%. So in most cases magnetic moments remain
almost uncoupled at room temperature.
• To solve this problem it is possible to use approach of carrier-mediated ferromagnetism by co-doping
with shallow impurities. In this case changing spin and charge state of TM can alter the magnetic
response.
• In case of doping higher than solubility limit, TM tend to form clusters or secondary phases with different
magnetic properties (antiferromagnetic, paramagnetic, etc.).
• In most reports observation of room-temperature ferromagnetism is related either with clusters of TM,
secondary phases or defects.
Nature of ferromagnetism in TM-doped DMS
TM-doped DMS Possible configurations
6. Structures grown by MOVPE in Linz University
Ga1−xMnxN samples are grown by MOVPE on
GaN/c-sapphire substrate.
Mn is randomly distributed up to concentration of
3% and within a confidence of 90% the Mn ions
occupy exclusively Ga-substitutional positions.
In order to get ferromagnetic behavior Mn should
be in the charge state of Mn2+ . Ratio of Mn3+/Mn2+
can be tuned by introduction of Mg.
Functional Mn-Mgk complexes control
spin and charge state of Mn!!!
GaN:Fe and (Ga,Fe)NGa1-xMnxN(Mg)
Sci Rep. 2012; 2: 722.
GaN:Fe and (Ga,Fe)N are deposited by MOVPE.
Fe concentration in GaN:Fe is up to 2*1019 cm-3
and no secondary phases are revealed.
(Ga,Fe)N have higher Fe concentrations and
contain nanocrystals embeded into host GaN
Ferromagnetic component exceed the
paramagnetic contribution at Fe concentrations
above the solubility limit of 0.4% .
Ratio of Fe3+/Fe2+ can be tuned by introduction
by Si or Mg.
Increase of Fe content lead to:
Appearance of interstitial Fe acting like double
donor and increasing electron concentration.
phase separation with precipitates of Fe and
Fe compounds.
spinodal decomposition with low and high Fe
content.
Fe-rich nanocrystals are responsible for high-
temperature ferromagnetism in (Ga,Fe)N .
6
8. Magnetic properties of random ferromagnets GaMnN
Ferromagnetic superexchange is the dominant coupling mechanism between Ga-substitutional Mn3+ ions
in Ga1−xMnxN, leading to TC<12.5K at x=9.5%.
Theoretical predictions gives room-temperature ferromagnetism at x=50%
Low Curie temperature is due to short-range character of the super-exchange
At high Fe concentration Fe-rich nanocrystals are responsible for high-temperature ferromagnetism in
(Ga,Fe)N
8
Secondary phases
10. Гopt=А1+ 2В1 + E1 + 2Е2
10
A1 E2
low E2
high E1 B1
lowB1
high
R+IR R R R+IR silent
Raman spectrum of wurtzite GaN Six Raman modes are active in different
scattering geometries!
11. Mg-induced Raman modes in GaN:Mg
11
Local vibrational modes in Mg-doped GaN grown by molecular
beam epitaxy Kaschner et al. // Appl. Phys. Lett., Vol. 74, No. 22, 1999
Mg-induced modes
Disorder-activated modes in DOS
LVM of MgIntensity of 657 and 260 cm-1 bands
correlates with activated Mg
concentrations
• For substitution Mg LVM is expected at:
• As-grown samples do not exhibit Mg-LVM mode, because it is passivated with H in Mg-N-H complexes,
which are Raman silent.
12. Mn-induced vibrational modes in GaN:Mn
• LVM of substitutional Mn of 574 cm-1 is close to E2(high) mode and its
observation is difficult.
• Incorporation of Mn induces a series of additional Raman bands (A, B, C, D) in
the region between E2(high) and A1(LO) modes.
• Intensity of these additional modes scales with Mn concentration. Additional
modes reveal resonance behavior.
• The nature of Mn-induced modes is controversy. One of the explanation is
disorder-activated modes in DOS of GaMnN.
All additional modes (A,B,C,D) exhibit A1 symmetry.
Mn-induced modes are associated with the vibrations of N-sublattice.
Mn-LVM:
Impurity-induced Fröhlich type
resonance Raman scattering
A1
Mn-LVM
13. High-frequency H-related Raman modes in GaN(Mg)
H-related bonds, such as Mg-H, Vn-H, Ga-H
Mg-H bonds
N-H bond for Mg-N-H complex
13
14. Strain-induced Raman shifts in GaN:Mg
• Non-polar E2(high) mode is sensitive to strains.
• Incorporation of Mg on Ga site leads to compressive strain due to higher ionic radius, which is
observed for low Mg concentrations up to 2*1018 cm-3
• At higher Mg concentration appearance of defects (nitrogen vacancies) compensates the compressive
strain and convert it to tension. This is also confirmed by sudden increase in FWHM.
• Activation energy of compensating defects decreases upon reduction of Fermi level at high Mg
concentrations .
14
16. Plasmon-phonon modes in n-type A3B5 and GaN
22 2
2 2
( ) 1 ,
( )
pLO TO
TO i i
1/22 2 2 2 2 22 2
2
( ) 4 ( )( )
.
2 2
p LO p LO pLO p
2
*
4
p
e n
m
e
16
17. Plasmon-phonon modes in p-type A3B5
17
• p-type GaN may contain 10-100 times more impurities than
the holes because the electrical activity of the acceptor in
GaN is ussually very low.
• Scattering of carriers on ionized impurities significantly
reduces the mobility.
Low
damping
high
damping
Γ=10 Γ=500
GaAs:Zn
μ=50
ZnO:N-I
Compound μel, cm2 V-1s-1 μhole, cm2 V-1s-1
InSb <77000 <850
GaAs <8500 <400
GaSb <3000 <1000
ZnO <2000 <100
GaP <250 <150
GaN <440 <10 !!!!
μ=150
μ=100
18. • LO-phonon modes in p-type GaN couple with plasmons very
weakly in contrast to the case of n-type.
• Inactive coupling between the LO phonon and the plasmon in
p-type GaN is attributed mainly to heavy damping of the hole
plasmon
Can coupled plasmon-LO-phonon modes be observed in p-type doped GaN?
18
2
*
4
p
e n
m
e
plasmon damping rate γ=1000-3000 cm-1
plasmon frequency ωp=30-140 cm-1
so ωp«γ for p=5*1016-1.1*1018 cm-1
20. Paper on Raman spectroscopy of GaN:(Mn,Mg) published by group of prof. Bonanni
Mn-LVM (579 cm-1)
Mg-LVM (657 cm-1)
20
GaN:Mg do not exhibit Mg-LVM, confirming that H is
bound to Mg.
Incorporation of Mn induces appearance of reach
structure (A,B,C,D,E) in the range between E2(high) and
A1(LO). All these modes were shown earlier to exhibit
A1 symmetry.
Introduction of Mg into GaN:Mn heavily affects Mn-
induced peaks (A, B, D, E).
LVM of Mn
21. 21
Introduction of Mg into GaN(Mn) leads to:
Decrease in intensity of Mn-induced peaks (A,B,D,E and
to less extant of C-peak) with Mg concentration. Less
quenching of the C-peak indicates different nature of this
peak.
Shift in the position of the C-peak with the Mg/Mn ratio,
which is explained by shortening of Mn-N bond length
induced by the presence of Mg.
Appearance of Mg-LVM at 650 cm-1 due to formation of
Mn-Mgk cation complexes eliminating incorporation of H.
LVM is shifted due to distortion of the Mg–N4
tetrahedron in the vicinity of Mn.
Intensity of broad feature at 688 cm-1 correlates with
formation of different Mn-Mgk cation complexes.
Additional modes
C
22. Conclusions:
Raman spectroscopy of GaMnN(Mg) allows for:
• Control of crystal quality and strains
• H-control and control of impurity activation throw high-frequency Mg-H modes
• Control of incorporation throw the LVM modes of Mg and Mn.
• Plasmon-phonon modes have almost no influence on Raman spectra of p-type
GaN and cant be used for estimation of carrier concentration.
• Indirect control of free holes from low-frequency single-particle scattering and
strain throw the position of non-polar E2(high) mode.
22
Notes de l'éditeur
Кристалізується в гексагональну структуру вюрциту з чотирма атомами в елементарній комірці, тип хімічного зв'язку – змішаний йонно-ковалентний
Внаслідок полярності кристалічного зв'язку спостерігається LO-TO розщеплення
За правилами відбору в КР дозволеними є 6 коливних мод