Institute of Ultra-High Frequency Semiconductor Electronics
Russian Academy of Sciences
Creation of Sn nanowires inserted in GaAs
crystall by molecular beam epitaxy and
electrical properties of nanowires
Klochkov A., Senichkin A., Bugaev A., Yachmenev A., Galiev G.
Budapest, 2012
Institute of Ultra-High Frequency Semiconductor Electronics
Russian Academy of Sciences
Specialization
Fundademental and applied research in the field of microwave
frequency semiconductor electronics
Field of interest
• Physics and technology of AIIIBV semiconductor heterostructures
• Electronic phenomena in MW devices based on low-dimensional
heterostructures
• Micro- and nano-technology of fabrication of short-channel high
electron mobilty transistors (HEMT)
• Development of the MW monolithic integrated circuits based on
GaAlInAs and GaN materials
• Investigation of new MW device types (for example, MW
microelectromechanical systems)
• Investigation of new materials for MW electronics
www.isvch.ru, isvch@isvch.ru
Institute of Ultra-High Frequency Semiconductor Electronics
Russian Academy of Sciences
Full technological process of MW integrated circuits production
Molecular beam epitaxy of AlGaInAs heterostructures
CNA-24
Riber-32P
Institute of Ultra-High Frequency Semiconductor Electronics
Russian Academy of Sciences
Plasmachemical dielectric layers
deposition system
Plasmalab-100-ICP 180, Oxford
Instruments
Plasma etching
SI-500 ICP, Sentech Instruments
Institute of Ultra-High Frequency Semiconductor Electronics
Russian Academy of Sciences
Lithography and microscopy
Precision contact
photolithography
Inspection optic microscope
(visible light and UV)
SUSS MJB4
Leica INM100
Electron beam
nanolithography system
Raith150-TWO
Microvawe devices performance
fT = vinj/2πLg
fT – cutoff frequency
vinj - injection velocity
Lg - gate length
Two ways of increasing fT:
1) Decrease the gate length
Lg
2) Use different material or modulate its electronic
properties (low-dimensional systems)
Electron saturation velocity
Hot carrier scattering mechanisms liming drift velocity:
Optical phonon scattering
Intervalley scattering
Band structure of GaAs
Field dependences of the electron drift velocity
(Blakemore, J. Appl. Phys. 53, 10 (1982) R123-R181)
Peculiarities of electron-optical phonon interaction in
1D systems
Singularity of the density of states of
1D systems
Phonon absorption and emission scattering rates
as a function of the electron energy for transitions
into different 1D subbands, T = 300 K
Singularity in phonon emission rate for 1D systems J.P. Leburton, J. Appl. Phys. 56, 2850 (1984)
may result in decreasing of mean energy of drifting
hot electrons and in suppression of intervalley
scattering
Vicinal surface – a template for nanostructure
constructing
Misorientation angle
tg(α) = h/d
h – step height, d – terrace width
Doping of vicinal GaAs (111)A surface by Si
Orientation dependent impurity
properties:
(100) GaAs – n-type of conductivity
(111)B GaAs – n-type
(111)A GaAs – either n-type or p-type depending
on
the
growth
condictions
(substrate
temperature and flux ratio of Ga and As atoms)
Reason – Si
compensation
amphoteric
character,
self-
Doping of the vicinal (111)A surface will result in
inhomogeneous impurity surface distribution and
rearrangement of electrons between Si-donors
and Si-acceptors resulting in strong lateral
electric fields and anisotropic potential
Delta layers of Si grown on vicinal GaAs (111)A
surface
Experimental structures diagram
Samples resistivity along vicinal terraces
Rpa, resistivity anisotropy k = Rpe/ Rpa
n+ 100 Å : Si, Nd~1018 cm-3
Sample
i-GaAs 50 nm
GaAs substrate (vicinal or
singular)
77 K
Rpa,
Om/□
k
nH,101
2 cm-2
Rpa,
Om/□
k
nH,1012
cm-2
1, (100)
305
1.0
-12
280
1.0
-11.6
2, α=0.5˚
1750
2.3
28
3910
6.0
3.3
3, α=1.5˚
2600
1.1
21
23100
1.5
3.6
4, α=3˚
3540
1.0
21
23700
1.0
3.6
δ-doped GaAs
i-GaAs 0,42 um
300 K
Terrace
width
for
different
misorientation angles of GaAs
(111) A
α
d, nm
0.5˚
37.5
1.5˚
12.4
3˚
6.2
Conductivity anisotropy of delta layers of Si grown
on vicinal GaAs (111)A surface
Temperature dependence of sample resistivity across terraces (1), along
terraces (2) and anisotropy coefficient (3).
α = 0.5˚
α = 1.5˚
Temperature dependence of resistivity of samples 2-4 at T < 50 K:
Mott’s law for hopping conduction of two-dimensional systems:
ρ = ρ0 exp {(T0/T)1/3}
Application of tin for doping of GaAs vicinal surfaces
Peculiarities of tin doping of GaAs compared to the
silicon:
1) Sn don’t have amphoteric properties, it always occupies Ga
lattice sites dureng MBE
2) Sn solubility limit (1019 cm-3) is higher than Si (~5∙1018 cm-3)
3) Sn atomic radius exceeds radius of Ga and As atoms. In
combination with high surface diffusion rate it results in
segregation of tin atoms at surface inhomogeneties, pits,
humps and steps
Negative feature
Tendensy of tin atoms to segregate at the sample surface
during MBE, wide profile of Sn delta-doping layers.
Segregation of tin atoms at steps of the vicinal
surface
Experimental samples and technological details of
sample preparation
Sample structure
n+ 100 Å : Si
For vicinal surfaces of GaAs (100):
i-GaAs 400 Å
d = 0.283 nm / tg(α)
δ-Sn 7,5×1012 см-2
i-GaAs 0,6 мкм
Sample
α
d
Vicinal substrate GaAs
A
0.3˚
53 nm
B
3˚
5.3 nm
Technological details of sample preparation
n+ 100 Å : Si
i-GaAs 400 Å
δ-Sn 7,5×1012 cm-2
Doped contact layer is formed for low-resistivity metallic Ohmcontact creation
Covering layer is grown at low temperature (prevent Sn
diffusion snearring) and at high speed growth mode, at
high As/Ga flux ratio
Increased growth temperature for maximum Sn adatom
surface diffusion
Growth interruption after Sn deposition for Sn atoms
redistribution along steps to occur
i-GaAs 0,6 um
Vicinal substrate GaAs
Buffer layer consists of two parts, grown at different
conditions for purposes:
1) smoothing the substrate roughness
2) forming the atomic-smooth terraces of vicinal surface
Growth interruption at high T for perfect surface
After removal of natural oxide substrate surface is highly
rough
Samples topology
Contact materials Ni/Ge/Au/Ni/Au
Contacts were formed by lift-off
method with help of the contact
photolithography and annealing in N2
atmosphere afterwards
Contact width W = 20 nm, intercontact
distance L = 6 nm
Contact topology for current-voltage
characteristic measurements in two
different directions perpendicular and
parallel to the atomic steps.
Current-voltage characteristics
14
12
I, мА
10
8
1
6
2
4
2
0
0
1
2
3
4
5
6
7
E, kВ/см
8
9
Sample B, misorientation angle 3˚
10
11
12
Current-voltage characteristics
14
12
I, мА
10
8
1
6
2
4
2
0
0
1
2
3
4
5
6
7
E, kВ/см
8
9
10
11
12
Sample A, misorientation angle 0.3˚, curve 1 – along steps, 2 – across steps
Sn doping nanowires
Observed phenomena:
1) Saturation current anisotropy lacking for 3˚ samples
2) Current fluctuations in 0.3˚ samples for direction across atomic steps lacking for
direction along steps (up to thermal breakdown) and for 3˚ samples
Result:
Inhomogeneous impurity distribution correlating with direction of atomic steps of vicinal
surface
Tin diffusion broadening forms impurity cylindrical clouds, which both generate
electrons in the structure and create electrostatic potential for electron localization
Energy spectrum of doping nanowires
Cylindrical nanowire radius – 5
nm impurity density – 3.9∙106
cm-1 distance between the
wires,
d = 200 A
Electrostatic potential, energy levels and wave
functions of electrons in doping quantum wires
Anjos, Marletta, J. Phys.: Condens. Matter, 18, 8715
(2006)
Thank you for your
attention!