I nstitute of Ultra-High Frequency Semiconductor Electronics Russian Academy of Sciences

1. 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

2. 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

3. 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

4. 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

5. 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

6. Microvawe devices performance fT– cutoff frequency vinj-injection velocity Lg - gate length fT = vinj/2πLg • Two ways of increasing fT: • Decrease the gate length • Use different material or modulate its electronic properties (low-dimensional systems) Lg

7. Electron saturation velocity Hot carrier scattering mechanisms liming drift velocity: Optical phonon scattering Intervalley scattering Field dependences of the electron drift velocity (Blakemore, J. Appl. Phys. 53, 10 (1982) R123-R181) Band structure of GaAs

8. 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 J.P. Leburton, J. Appl. Phys. 56, 2850 (1984) 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

9. Vicinal surface – a template for nanostructure constructing Misorientation angle tg(α) = h/d h – step height, d – terrace width

10. 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 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

11. 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 Terrace width for different misorientation angles of GaAs (111) A

12. 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}

13. Application of tin for doping of GaAs vicinal surfaces • Peculiarities of tin doping of GaAs compared to the silicon: • Sn don’t have amphoteric properties, it always occupies Ga lattice sites dureng MBE • Sn solubility limit (1019 cm-3) is higher than Si (~5∙1018 cm-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.

14. Segregation of tin atoms at steps of the vicinal surface

15. Experimental samples and technological details of sample preparation Sample structure For vicinal surfaces of GaAs (100): d = 0.283 nm / tg(α)

16. Technological details of sample preparation 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

17. 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.

18. Current-voltage characteristics Sample B, misorientation angle 3˚

19. Current-voltage characteristics Sample A, misorientation angle 0.3˚, curve 1 – along steps, 2 – across steps

20. 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 cylindricalclouds, which both generate electrons in the structure and create electrostatic potential for electron localization

21. 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)

22. Thank youfor your attention!