Moderní experimentální metody Rentgenová a elektronová spektroskopie III
Fotoelektronová spektroskopie
• Fotoelektronová spektroskopie (XPS) a spektroskopie Augerových elektronů (AES)
• Experimentální aspekty
• Zdroje: ARPES, ARUPS
• Detektory
• Příprava vzorků
• Úhlově rozlišená fotoelektronová spektroskopie (ARPES)
• EELS
Fotoemise a Augerův jev
Fotoelektrony - přímo vyražené fotonem
Augerovy elektrony - sekundární emise - alternativní proces ke
vzniku charakteristického rtg záření.
photoemission Auger
Elektronová spektroskopie měří kinetickou energii elektronů.
Charakteristické rtg záření
O       £0      40      60      60     100 120
Atomic number
Fotoemise a Augerův jev
Fotoelektrony - přímo vyražené fotonem
Augerovy elektrony - sekundární emise - alternativní proces ke
vzniku charakteristického rtg záření.
K
fi
\ - r
vac
€B(PE)
kin
^b(A)
kin(sp)
photoemission
Auger
vzorek
spektrometr
XPS - ESCA
Electron spectroscopy for chemical analysis
• Reálne všechny prvky pro Z>=3, od Li
• Citlivost - jednotky promile
• Povrchová citlivost - povrchová kontaminace
• Chemický stav povrchu
• Profily podél povrchu
• Do hloubky při kombinaci s iontovým svazkem
Fotoelektronová spektroskopie
Podmínky experimentu:
Energie fotonů: 10 až 2000eV Laboratorní zdroje:
• He výbojka 21 eV
• Mg lampa 1254 eV
• AI lampa 1486 eV
Synchrotron - laditelný zdroj
Vysoké vakuum (tlak max ÍO6 Pa)
Vysoká kvalita a čistota povrchu vzorků
Optika - jen na odraz - zrcadla, difrakční mřížky, žádná okénka!
Hloubka vniku 10° - 101 nm
XPS
Fotoelektronová spektroskopie
Gas	Emission Line	Energy (eV)	Wavelength (nm)	Relative Intensity (%)
H	Lyman a	10 20	121.57	100
	Lyman (3	12 09	102.57	10
He	1 a	2122	5843	100
	1 p	23.09	53.70	approx 1.5
	1 V	23.74	52.22	0.5
	2 a	40.81	30.38	100
	2p	48 37	25.63	<10
	2V	51 02	24.30	negligible
Ne	1 a	16.67	7437	15
	1 a	16 85	73.62	100
	1 p	19.69	6297	<1
	1 p	19.78	6268	<1
	2 a	2681	46.24	100
	2 a	26.91	46.07	100
	2p	27.69	44.79	20
	2p	27.76	4466	20
	2P	27.78	44.63	20
	2p	2786	44.51	20
	2V	30.45	40.71	20
	2V	30.55	40.58	20
Ar	1	11.62	106.70	100
	1	11.83	104.80	50
	2	13.30	93.22	30
	2	13.48	91 84	15
ARPES
Hemisférický analyzátor + CCD
Hemisférický analyzátor
V(r) = -
(R2 - At)
(R1R2)
+ const
\E(r)\ = -
(Vi - Vi) {R2 - R1)
(R1R2)
V(r) = ^r°^°^ const.
R0=(R1+R2)/2
ARPES
Hemisfericky analyzator VG Scienta R4000
Scienta R4000 Spectrornicroscopy Performance Optics:
Enecgy resolution:
Theoretical resolving power using 0.2 mm entrance slit:
Kinetic energy range:
Spatial resolution:
Magnification: Imaging Mode Transmission Mode
Ultimate angular resolution:
Widest angular range for multiplexed angle recordings:
< 2 meV
1 Ox, 20x, 40x 5x
0.1 degrees
10 degcees
R4000 Spectrometer
XPS
Detekce: CCD
Channeltron
XPS
h—r i t t i
1—I f i 11 f-r-1—r l l t t t i
100
I— Alftl
50
CL
io
\
At*
Theory
\
J_I_I   L '
Hi* Ai*
J_I_i  J i i n .
Of*
A*" "»
4ua
.......
-i-
10 50      100 500
Electron energy ( eV )
1000 2000
......1_I_I_■   i  ■ i i « I_I_I_I_I ■ ' |' I_I_I_......'_I_I_I_l_L
10 UK) 1000
Kinetic I energy (eV)
XPS
Příprava vzorků
Atomárně čistý povrch a hladký povrch
• Štípání ve vakuu
• Depoziční aparatura v UHV propojená s XPS komorou
• Vakuový kufřík
• Ochranné vrstvy
Fotoemise a Augerův jev
Fotoelektrony - přímo vyražené fotonem
Augerovy elektrony - sekundární emise - alternativní proces ke
vzniku charakteristického rtg záření.
K
fi
\ - r
vac
€B(PE)
kin
^b(A)
kin(sp)
photoemission
Auger
vzorek
spektrometr
XPS
Rentgenová fotoelektronová spektroskopie
X-ray Photoelectron Spectroscopy-XPS, PES
Kinetická energie elektronu dopovídá rozdílu energie fotonu a
ionizační energie slupky.
in
Tilu- [eB(PE)+<f>s
p J
Umožňuje detekovat změny vazebné energie chemickým stavem atomu.
XPS na germaniu s velmi tenkou oxidovou vrstvou.
c
OJ
•il—i—i—i—i—i—i—i—i—r
Ge 2p
2.3 eV
3/2
Ge3d
GeO
j_i_
J_I_1_I_ft_I_I_I_I_I_I_J__3_l
1224    1220    1216    34 30
Binding energy (eV)
26
XPS
Priklad: Bi2(Se,Te)3 excitace 125 eV.
Kinetic energy (eV)
Fotoemise a Augerův jev
Fotoelektrony - přímo vyražené fotonem
Augerovy elektrony - sekundární emise - alternativní proces ke
vzniku charakteristického rtg záření.
K
fi
\ - r
vac
€B(PE)
kin
^b(A)
kin(sp)
photoemission
Auger
vzorek
spektrometr
AES
Spektroskopie Augerových elektronů Auger Electron Spectroscopy - AES
Energie elektronu při přechodu na uvolněnou hladinu se může předat Augerovu elektronu.
ug
efi - [eB(A) + <Ž>S
pJ
Charakteristické energie se značí FIB. Přechod I - F, ionizuje se B.
Pravděpodobnost emise Augerova elektronu a rtg záření závisí na protonovém čísle.
Augerova spektroskopie je nejcitlivější na lehké prvky.
1.0 0.9 0.8 £ 0.7 5 0.6
0.5 -
Často se měří v kombinaci s XPS, používá se v elektronové mikroskopii.
<u 0.4 Cl
? 0.3
0.2 0.1
0
i-r
Auger electron yield
X-ray yield
5       10      15      20      25      30      35      40 45 Atomic Number
XPS + AES
Fotoelektronova a Augerova spektroskopie na Mg2Sn. Budici rtg svazek 1486 eV - cara Al Ka
VB
core levels
			
■			CO
	T3		CO
c	C CO	s	CD
o O			
(A C
KLY Auger |
Mg2Sn
KL"V KLV
1400 1300
Kinetic energy (eV)
1200
ARPES - úhlově rozlišená
ARPES
Zákony zachování
• Kvaziimpulz - zachovává se tečná složka
hk
foton,
ľík ó
hk
fhh
hk
f
hk
X
v2mĚsm6
v2mĚ sin 0 cos </>
hky = v2mĚ sin 0 sin (j> Tik? = v2mE cos 0
energie
ound — hbJi     £ldn, elektron
(sin (0)x + cos (0) sin (0)y)
2meEk
in
Dispersive plane High
\Pi       Non dispersive plana
SamplQ
Measurement is
<c)     Phosphor screen
ARPES at BESSY
/(£kill, k) oc \Mfi(kf, ki)\2f(hv - Ekin - <D, T) j A(hv - £kill - O. k)C(kz, ftf) d*z,
where k, and kf are the three-dimensional wave vectors for the initial and final states in the solid, k is the wave vector for the free electron outside the solid, hv is the photon energy, /is the Fermi distribution, <i> is the work function. A is the hole spectral function and C is a Lorentzian distribution to account for the real-space damping of the outgoing electron wave, with being the perpendicular wave vector for the un-damped final state wave. Mf{ is the matrix element for the pho toe mission process. The integration is over all possible perpendicular wave vector components k2r For QWS or surface states, this integration becomes irrelevant.
The quantity of most interest is often the spectral function A. In simple terms, this is a measure of finding an electron with a certain energy and crystal momentum, i.e. an image of the band structure, including lifetime broadening effects. For two-dimensional systems such as surface states and QWS, the photoemission intensity is directly proportional to A, as no final state broadening has to be considered. For a detailed further analysis, it is only necessary to assume that the matrix element does not depend strongly on the binding energy or the wave vector.
I OP Publishing
Semiconductor Science and Technology
Semicond. Sei. Techno]. 27 (.2012") 12.4O01 C14pp> doi: 10.1088/0268-1242/27/12/1 24001
INVITED PAPER
The electronic structure of clean and adsorbate-covered Bi2Se3: an angle-resolved photoemission study
Marco Etiaiitlii1, Richard C Hatch1, Damian Giian1, Tilo Planke1 , Jianli Mi2, Bo Brummerstedt Iverseii2 and Philip Hofmann1
ARPES
Příklad: pásová struktura Bi2Se3
Y-Scale [d eg]
V-Scale [degj
k, (A1) K (A1) (A1)
Figure 5. ARPES spectra for the pristine surface of Bi2Se^ High photoemission intensity is displayed in bright, (a) Energy dispersion in the KTK direction of the SBZ and (b) Fermi surface for the stoichiometric Bi^Sei sample, (c) and (</) Energy dispersion and Fermi surface for the Ca-doped sample, respectively. The Dirac point is at a binding energy of % 50 meV.
2.4
: 3.0
Z
Figure 6. Photon energy scan on the pristine surface of intrinsic BiiSe.i crystal illustrating the dispersion of the states at normal emission as a function of L. The data shown are a subset of a larger photon energy scan between hv = 14 eV and hv = 32 eV. The drift of the Dirac point with photon energy is due to the ageing effect that occurs during the scan. The CB and valence band (VB) (highlighted with blue and magenta Jines as a guide to the eye) disperse, revealing the bulk T and Z points. The dashed magenta line is a shifted replica of the VB dispersion caused by a surface uniklapp process.
ARPES
Příklad: pásová struktura Bi2Se3
Te 8%, hv = 15.0 eV Te 10%, hv = 15.0 eV Te 27%, hv = 15.0 eV Te 30%, hv = 15.0 eV
Te 34%, hv = 15.0 eV Te 38%, hv = 15.0 eV Te 38%, hv = 15.0 eV Te 39%, hv = 15.0 eV
ARPES
Příklad: pásová struktura Bi2Se3
Te 27%, hv = 15.0 eV Te 39%, hv = 15.0 eV
-0.1 -0.05   S)  ,0.05   0.1 -0.1 -0.05   j0  < 0.05 0.1
k (A ~1) k (A _1)
Topological crystalline insulators:
Topological surface states (TSS) protected
by point group crystal symmetry
Band inversion is required
Phase diagrams
Materials:
(i) Rock salt SnTe and SnSe:
Conduction and valence bands inverted
TSS protected by (110) mirror plane symmetry
Disadvantages:
High intrinsic p-doping due to Se/Te vacancies. Thus, only valence band can be seen by ARPES.
SnSe has orthorombic lattice at normal conditions
(ii) Ternary Pbi^Sn.Te and Pbi^Sn^Se:
Pseudobinary systems with rock salt structure Band inversion occurs for xSn > critical value Topological transition can be tuned by temperature and Sn content
p-type carrier concentration decreases with increasing of Pb content
—I-1-1---1——
O OTA  present   aa la  ( heating)
*> data o , & ant-  ond two - phasf samples ----a---- OTA d*t*r™iinfltL^fi of 2fwl ärdör
Band inversion In RbSniTe(Se)
—i—S—I—11 -°"
o.e o.4
Mole Fracrit
0.2 Si»ö_ S«„
Stoichiometric FbTe liquidus ^_
10" 1022 TO23 Electron
Brillouin zone projections onto (001), (111), anr^j (110) planed
Pb^SnJe
88,241303(R) (2013)
Angle Resolved Photoemission Spectroscopy
Photo-excitation
hv > lOeV/
♦ ♦
Sample
Spectrum
¥""K"/y
Valence
Core levcis
N(E)
hv- const
Angle resolved photoemission (ARPES)
Photoemission N(EPE)~D(EBE)
EK = hv-ED-0
Ekm =hv-{EB)-0A
hv —const, z*
N(E)
analyzer
detector
We need:
binding energy - Eb initial momentum - k1
a or b
sample
Eb Y E - hv + W
■=> E(k,)-dispersion
k\ =kf„ =7 2m£/h^^) k=kf-<3=7 2mE/fr cose-G
Energy Conservation KB=h<o-KK-*A
obtained from angular and energy
spectrum of photoexcited electrons
Undulator
Detector
Sample
Exit slit
> 3D E{k) dispersion obtained by hemispherical electron analyzer and 2D detector array
> Resolution: AE ~10meV, A6 ~0.1C
Angular Resolved Photoemission: Examples
Examples:
Bi2Se3
•BCB SSB
EF
En <?J
1' -<— Dirac point
BUTe
ARPES experiment
Ekm =hv-(£)-<PA
hv —const, z
*ll.x (1/Ä) °-2-°-2
ll.y
ll.x      0.2-0.2 (1/A) (1/A) (1/A)
analyzer
detector
We need:
binding energy - Eb initial momentum - k'
E - hv + W
sample
^ E(k)-dispersion
k1' ll=kfll =7 2mE/f\^in0) k'=kf,-GW 2mE/fr cosG-G
^ Measurement of angular dependent energy spectrum N(E) of photoexcited electrons
100
5- 50
10
—I j i J 111 j <AjuaAu
I    I  I i i I i 11--1-P   i I 1 i llj-P—
Electron mean free path in solids
Ma b>
I    ' I I I 1111_i_.......'
2        5     10 50   100 500 1000 2000
ELECTRON ENERGY (eV)
Zoom-in around the r - Point: PbSnSe (111) Epilayers
(a) PbSnSe with xSn = 10%: Normal band structure
-0.05
j j j :DE
:= i j ;i iE
£ 0.20
■
-m 0.25 m° 0.30 j ?E
;i iE
180m eV
jj
DE 10.101 0.1E 10.20 I
.
10 30
141meV
-005    0     0 05 K.{A"1)
-0.05    0 0.05
84m eV
j1j
D.1E
j 23 "i /f
K,|{A-1!
-0.05    0 0.05 K,|{A"1!
-0.05 0
K„
K„|A"']
^>Eo(x,T) = 125 - 1021x + (480 + 0.256 T2)
1/2
300
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i I I I I
>
® 200 150
CL
CO 100
CD
-o 50
00 -50 CD
CO -100 c
CO -150 -Q
CL -200 -250 -300
250 |E (x,T) of PbSnSe
xsn = 10%
xsn = 20%
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
2D Structures: TCI Surface Quantum Wells
^ MBE of asymmetric quantum wells (vacuum barrier): discreet 2D QW states
^ Opening of a gap due to coupling between top & bottom topological surface state
(a) PbSe QW pT3Q50^=o%] pT23QO^=P%
dpw=100A
xsn = 0% (M2969a)
PbEuSe
E
o
>
PbSe
>
cq
LU
0.05 =-
o.io h
0.15 h 0.20 p 0.25 r 0.30 r 0.35 h 0.40 h 0.45 1= 0.50 h 0.55 h 0.60 h 0.65 B
LI= 176^x^=0%
Ei_L
1
1
LI = 85X^x5^0%
-ii
~180meV
0 ^.05
Km (A)
-0.05    0 0.05
K„ (A1)
-0.05    0 0.05
K„ (A)
NanoESCA- kombinace mikroskopie a
spektroskopie
PEEM - photoelectron microscopy
Imaging
ESCA-
zobrazení
jen určité
energie
elektronů
XPS
-■ ■ - -1 ■ -11 ^
5|fitem    MCP Sawi
I"um nt
Rozlišení 50nm
Clean Surface Preparation by Capping / Decapping
^ Surface needs to be protected against oxidation during transfer from MBE to ARPES in air: Use of an easily desorbable capping layer
I. Selenide Compounds:
Amorphous Se cap layer (~ 100 nm ) deposited at RT: Volatile surface oxide
Se-cap
PbSnSe
substrate
1. PbSnSe growth	II. Se capping
After growth/350°C	100 nm Se cap 50°C
•	
•	
Decapping: Heating RT - 300C (P=const.)
T = 229°C
T = 300°C
■=> Clean surface recovered at Tdes < Tgrowth
II. Tellurides:  ^ More difficult
Tellurium oxide is too stable for desorption & Se strongly intermixes with tellurides
^ Te / Se double cap layer structure used.
T =300°C
1 aim JUU ^
Se
Te
Pbi-xSnJe
BaF^