Transcript Document
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 A III B V 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, [email protected]
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 Plasma etching Plasmalab-100-ICP 180, Oxford Instruments SI-500 ICP, Sentech Instruments
Institute of Ultra-High Frequency Semiconductor Electronics Russian Academy of Sciences
Lithography and microscopy
Precision contact photolithography SUSS MJB4 Inspection optic microscope (visible light and UV) Leica INM100 Electron beam nanolithography system Raith150-TWO
f T = v inj /2 πL g
Microvawe devices performance
f T
– cutoff frequency
v inj L g
- injection velocity - gate length Two ways of increasing f T : 1) Decrease the gate length 2) Use different material or modulate its electronic properties (low-dimensional systems)
L
g
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 Singularity in phonon emission rate for 1D systems may result in decreasing of mean energy of drifting hot electrons and in suppression of intervalley scattering Phonon absorption and emission scattering rates as a function of the electron energy for transitions into different 1D subbands, T = 300 K J.P. Leburton, J. Appl. Phys. 56, 2850 (1984)
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 on the – either n-type or p-type depending growth condictions (substrate temperature and flux ratio of Ga and As atoms)
Reason
– Si amphoteric character, self compensation 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 n+ 100 Å : Si, N d ~10 18 cm -3 i-GaAs 50 nm δ-doped GaAs i-GaAs 0,42 um GaAs substrate (vicinal or singular) Samples resistivity along vicinal terraces R pa , resistivity anisotropy k = R pe / R pa Sample 1, (100) 2, α=0.5˚ 3, α=1.5˚ 4, α=3˚ R pa , Om/ □ 300 K k n H ,10 1 2 cm -2 305 1.0
-12 1750 2.3
28 2600 3540 1.1
1.0
21 21 R pa , Om/ □ 280 3910 23100 23700 77 K k n H ,10 12 cm -2 1.0
6.0
1.5
1.0
-11.6
3.3
3.6
3.6
Terrace width misorientation (111) A for angles of different GaAs
α
0.5˚ 1.5˚ 3˚
d, nm
37.5
12.4
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 = ρ 0 exp {(T 0 /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 (10 19 cm -3 ) is higher than Si (~5 ∙10 18 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 i-GaAs 400 Å δ-Sn 7,5 × 10 12 см -2 i-GaAs 0,6 мкм Vicinal substrate GaAs
For vicinal surfaces of GaAs (100):
d
= 0.283 nm /
tg( α)
Sample A B α 0.3˚ 3˚ d 53 nm 5.3 nm
n+ 100 Å : Si i-GaAs 400 Å δ-Sn 7,5 × 10 12 cm -2 i-GaAs 0,6 um Technological details of sample preparation Vicinal substrate GaAs Doped contact layer is formed for low-resistivity metallic Ohm contact 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 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 method were with formed help of by the lift-off contact photolithography and annealing in N 2 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 10 8 6 4 2 0 0 1 2 3 4 5 6
E, kВ/см
7 8 9 10 11 12 1 2 Sample B, misorientation angle 3˚
Current-voltage characteristics
4 2 8 6 14 12 10 1 2 0 0 1 2 3 4 5 6
E, kВ/см
7 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 nm impurity density – 5 – 3.9∙10 6 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)