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Raman spectroscopy of dilute
magnetic semiconductor Ga1-xMnxN
co-doped with Mg
Review
• 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
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
• 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
5
Nature of ferromagnetism in TM-doped III-nitrides
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
7
Infrared photoluminescence of GaN(Mn, Mg)
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
Raman spectroscopy of GaN (Mn,Mg)
9
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!
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.
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
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
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
15
Can hole concentration be estimated in p-type
GaMnN:Mg from Raman spectroscopy?
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
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
• 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
Low-frequency valence inter-subband scattering and hole density
19
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
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
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

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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
  • 5. 5 Nature of ferromagnetism in TM-doped III-nitrides
  • 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
  • 9. Raman spectroscopy of GaN (Mn,Mg) 9
  • 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
  • 15. 15 Can hole concentration be estimated in p-type GaMnN:Mg from Raman spectroscopy?
  • 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
  • 19. Low-frequency valence inter-subband scattering and hole density 19
  • 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

  1. 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