digital etching of iii v semiconductors with a monolayer...
TRANSCRIPT
Digital etching of III‐V semiconductors with a monolayer precision induced by electropositive and
electronegative atoms
O.E. Tereshchenko, V.L. Alperovich, K.V. Toropetsky
Institute of Semiconductor Physics and Novosibirsk State University
Russian Federation
S. V. Eremeev, S. E. Kulkova
bInstitute of Strength Physics and Materials Science and Tomsk State University, Russian Federation
Motivation: To develop the method of self‐limited etching of solidsurfaces with atomic layer precision.
Atomic layer depositionMolecular beam epitaxy
Layer by layer growth
(controlled by surface reconstructions)
Growth Etching
Layer by layer etching
Reactive ion etching:(plasma)
dry etching(gas etching)
intermixing, stoichiometric modification, surface roughening
It is difficult to control the etch depth precisely through the reactive ion etching due to the
fluctuation of the etch process
Cl (F) –diffuses into III-V and forms III-Clx (III-Fx)
Self-limited etching
How to apply self-limited etching for III-V?
As
Ga
Sb
InPreparation of reconstructed
III-V surfaces without V-fluxes1.
Adsorbate-induced selective interaction with the group III and V atoms2.
Structure and stoichiometryof GaAs(001) surface
As/GaAs(001):As/GaAs(001):HClHCl--iPAiPA--treatmenttreatment
(1x1) (2x4)/c(2x8) (4x2)/c(8x2)(3x6)/(2x6)
HCl-treatment
iPA
Temperature ( C)
3dA
s/3dG
a (a
rb. u
n.)
o200 300 400 500 600
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4(1x1)
c(2x8)
(4x6)
(2x6)(3x6)
/
c(8x2)As/Ga ratio on GaAs(001)after chemical treatment inHCl-iPA и subsequentannealing in UHV
(4x6)
480 °С 550 °С 580 °С
∆T ∼ 100°
420 °С
LowEnergyElectronDiffraction
Is it possible to reduce the transition temperature from As- to Ga-rich?
[001]
How the reversible transition between the Ga-rich and As-rich surfaces
at low temperature can be realized?
Questions we tried to answer:
O.E. Tereshchenko et al., JVST A17 (1999) 26551. Irreversible transition
2. Thermal defects generation
Idea: the self-limited etchingcontrolled by surface reconstructionsSURFACE REACTION CONTROL
(001) III-Velectronegative (filled dangling bonds)
As
Criterions for adsorbates:non-intermixing; selective interaction
with the substrate atoms
electropositive (empty dangling bonds)Surface stabilizationGa
?
Adsorbate-induced charge transfer: weakening of the backbonds
For polar faces of III-V: searching for adsorbates which selectively react with the elements of III and V groups
As-rich
Cs
Ga-rich
I - iodine
GaAs(001):
Goal: to reveal the mechanism providing the s elec tive interac tion of I andC s with Ga and As on the GaAs (001) s urfac e and to us e this s elec tivity to implement the revers ible recons truc tion trans itions on this s urfac e.
Electronegativity concept
W.K. Wang et al. PRB, 61 (2000) 2164K.N. Eltsov et al. Proc. Int. Conf. Scanning Probe Microscopy (SPM-2002), March 2-6, 2002, Nizhny Novgorod, Russia, p.99.
I2/GaAs, InAs :
Cs/GaAs : O.E. Tereshchenko et al. PRB, 69 (2004) 125332
Experimental setup: electron spectrometer ADES-500 with nitrogen box for chemical treatment
Solid state electrochemical iodine source
Advantages of the electrochemical source:
Directed flow of iodine on a sample allows to avoid contamination of vacuum chamber
It is possible to calculate number of iodine molecule generated by the iodine source using ion current through the sourceExperimental procedures:
Preparation of clean As-rich and Ga-rich GaAs(001) surfaces by removing oxides in HCl-iPA solution with subsequent annealing in ultra-high vacuum
Cesium and iodine adsorption on GaAs(001) surface with subsequent annealing
Investigation of structural and electronic properties of GaAs(001) surface using LEED, XPS and photoemission
As-rich (2x4)/c(2x8)
Ga-rich (4x2)/c(8x2)
Kinetics of Cs and I adsorption on GaAs(001)
• Langmuir adsorption model with constant sticking coefficient.
• The sticking coefficients for As- и Ga-rich surfaces of GaAscoincide to within 5%.
0.0
0.4
0.5
0.8
1.2
1.6
2.01.0
Cesium deposition time (min)
Cs3
nten
sity
i
n
d (a
rb. u
).
Cs c
over
age
(ML
)
- As-stabilized
Photocurrentmax.
- Ga-stabilized
0 20 40 60 80 1000.0
Cs /GaAs(001) I /GaAs(001)
ICs
The difference in Cs adsorption on As-and Ga-rich surfaces is due to the difference in the interaction mechanism of Cs with arsenic and gallium surface atoms!
Cs/GaAs(001)
Wor
k fu
nctio
n ch
ange
s(eV
)
Cesium coverage (ML)0.0 0.2 0.4 0.6 0.8 1.0
I3nt
ensit
y (a
rb. u
)
i
n.d
- As-stabilized
- Ga-stabilized
8 106420Iodine deposition time (min)
0
765432
1
+- +
-
Work function variation of GaAs(001) under Cs and I deposition
non-intermixing; selective interaction
Cs deposition on As and Ga – rich GaAs(001)
LEED:
CsAs-(2x4) Ga-(4x2)
0.5 ML Cs(1x1) (4x1)
Ga-(4x2)T=450oC
As-(2x4) Ga-(4x2)
(1x1) (4x1)
0.5 ML Cs550 °С
T=450oC
Ga-(4x2)Ga-(4x2)Ga-(4x2)
I2 deposition on As and Ga – rich GaAs(001)
LEED:
Ga-(4x2) I2As-(2x4)
As-(2x4) Ga-(4x2)0.5 ML I2
(1x1)/(2x1)(1x1)/(2x1) 0.5 ML I2
(1x1)/(2x1)(1x1)/(2x1)
As-(2x4)T=400oC T=450oC
As-(2x4)
As-(2x4)As-(2x4)
Cs ‐ induced low temperature
(2х4) → (4х2) transition
Iodine ‐ induced(4х2) → (2х4) transition
0.5 ML Cs
(2x4)/c(2x8) (4x2)/c(8x2)
450°C
560°C
Alternative method of preparation of Ga- rich (4х2) surface at reduced temperature.
TAs-rich (2x4)
Iodine
T=400oC
Selective interaction of Cs and I with As and Ga – rich GaAs(001) surfaces
Cs
As-(2x4)Ga-(4x2)
(1x1)(4x1)
T=450oC
0.5 ML Cs
Ga-(4x2)Ga-(4x2)
As-(2x4)Ga-(4x2)
I2
0.5 ML I2
(1x1)/(2x1)(1x1)/(2x1)
T=400oC
As-(2x4)As-(2x4)
removal of 1 ML
Ga-rich As-rich
Layer‐by‐layer etching of GaAs(001)
0.00.10.20.30.40.50.60.70.8
321
(2x4)0.9 ML I 2
(2x4)
Number of (Cs, I ) cycle2
(4x2)
Cs+4 0 C5 o
(2x4)0.5 ML I 2
(4x2)Cs+4 0 C5 o
(2x4)0.25 ML I 2
4 0+ 0 oC 4 0+ 0 oC 4 0+ 0 oC
(4x2)Cs+4 0 C5 o
LEE
D sp
ot in
tens
ity
Quality of GaAs(001) surface during adsorption and desorption of cesium and
iodine
Photoemission experiment Core-levels and surfacechemical shifts
sample
Cs (I)‐induced charge redistribution:photoemission measurements
Cs I2
73 75 7 77 9 73 75 7 77 9
50 51 52 53 54 55 56
72 73 74 75 76 77 78
50 51 52 53 54 55 56 5
72 73 74 75 76 77 78
As-(2x4)Ga-(4x2) As-(2x4)Ga-(4x2)
As 3d As 3d
Ga 3d Ga 3d
Kinetic energy, eV Kinetic energy, eV
1.0 1.5 2.0 2.5 3.0-0.1
0.0
0.1
0.2
0.3
0.4
0.5
∆ρ,
el.
R, Ang
Cs Ga(2a) Ga(2b) As(1)
Cs-D/As-rich_β2
∆ρ>0 ∆ρ<0∆ρ>0 ∆ρ<0D T2’
β2
T1
H3
D3
1.0 1.5 2.0 2.5 3.0-0.3-0.2-0.10.00.10.20.3
Cs Ga As
∆ρ, e
l.
R, A
As-richT2'
Cs on β2-GaAs(001)-(2x4): DFT
Charge integration at D position
)()()()( / rrrr GaAsIIGaAs ρρρρ −+=∆
I on GaAs(001): first principal calculation
)()()()( / rrrr GaAsIIGaAs ρρρρ −+=∆
β2-GaAs(001)-(2x4)
∆ρ<0∆ρ>0
ζ-GaAs(001)-(4x2)T4
∆ρ>0 ∆ρ<0S1
+-
Unpinned behavior of Fermi level at Cs/GaAs(001)
0.2
0.4
0.6
20 400 500 600Temperature (°C )
ϕ s (
eV)
- Cs/As rich- without Cs
CsCs--induced surface states induced surface states passivationpassivation and unpinned and unpinned behavior behavior of the Fermi levelof the Fermi level
Summary
• The selective interaction of the iodine and cesium atoms with the GaAs(001) surface leads to a decrease in the bond energy of the Gaand As surface atoms, respectively, owing to the redistribution of the electron density in the near-surface region under the effect of electronegative and electropositive adsorbates.
• This selective interaction makes it possible to remove alternately the Gaand As monolayers in the iodine and cesium adsorption followed by heating at T ≤ 450°C and, thus, to implement reversible low-temperature transitions between the Ga- and As-stabilized reconstructions, as well as the atomic layer etching of the semiconductor with the physically ultimate monolayer accuracy.