Chemical characterization of semiconductor nanostructures by energy filtered PEEM

Transcription

1 Chemical characterization of semiconductor nanostructures by energy filtered PEEM S. Heun TASC-INFM Laboratory, Area di Ricerca di Trieste, Basovizza, SS-14, Km 163.5, Trieste, ITALY

2 Outline A brief introduction to spectromicroscopy The SPELEEM at Elettra SPELEEM = spectroscopic photoemission and low energy electron microscope Application example: AFM local anodic oxidation Si oxides GaAs oxides

3 Motivation Why XPS? chemical state information surface sensitive ease of quantification nondestructive Why spectromicroscopy? (semicond.) nanostructures: self-organization, lithography devices diffusion, segregation alloying (silicide formation) catalysis, chemical waves surface magnetism (XMCD)

4 Location of TASC and Elettra Elettra TASC-INFM

5 Elettra Beamlines exit beamline source 6 1.1L TWINMIC 1.2L Nanospectroscopy id 1.2R FEL (Free-Electron Laser) - 2.2L ESCA Microscopy id 2.2R SuperESCA id short id 3.2L Spectro Microscopy id 5 3.2R VUV Photoemission id 4.2 Circularly Polarised Light id 5.2L SAXS (Small Angle X-Ray Scattering) id 5.2R XRD1 (X-ray Diffraction) id 6.1L Material science bm 6.1R SYRMEP (SYnchrotron Radiation for MEdical Physics) bm 6.2R Gas Phase id 7.1 MCX (Powder Diffraction Beamline) bm 7.2 ALOISA (Advanced Line for Overlayer, Interface and Surface Analysis) id 8.1L BEAR (Bending magnet for Emission Absorption and Reflectivity) bm 1 8.1R LILIT (Lab of Interdisciplinary LIThography) bm 8.2 BACH (Beamline for Advanced DiCHroism) id 9.1 IRSR (Infrared Synchrotron Radiaton Microscopy) bm 9.2 APE (Advanced Photoelectric-effect Experiments) id 10.1L X-ray microfluorescence bm R DXRL (Deep-etch Lithography) bm 10.2L IUVS (Inelastic Ultra Violet Scattering) id 10.2R BAD Elph id 11.1 XAFS (X-ray Absorption Fine Structure) bm 11.2 XRD2 (X-ray Diffraction) id

6 Scanning vs. direct imaging type Monochromatic light Sample (scanned) Detector Photo-emission detector Photon optics is demagnifying the beam: Scanning Instrument Whole power of XPS in a small spot mode. Flexibility for adding different detectors. Rough surfaces can be measured. Limited use for fast dynamic processes. Lower lateral resolution than imaging instruments Sample hv Stigmator Intermediate lens Projective lens MCP Screen Object plane Object lens Backfocal plane Intermediate image Intermediate image Image plane CCD camera Electron optics to magnify irradiated area: Imaging Instrument High lateral resolution (20 nm). Multi-method instrument (XPEEM/PED). Excellent for monitoring dynamic processes. Poorer spectroscopic ability. Sensitive to rough surfaces. e -

7 Outline A brief introduction to spectromicroscopy The SPELEEM at Elettra SPELEEM = spectroscopic photoemission and low energy electron microscope Application example: AFM local anodic oxidation Si oxides GaAs oxides

8 The SPELEEM at Elettra Spectroscopic photoemission and low energy electron microscope

9 The SPELEEM at ELETTRA

10 The energy filter R = 100 mm Pass Energy = 900 V

11 Modes of Operation XPEEM LEEM PES ELS PED LEED

12 XPEEM: Spectroscopic Microscopy Images from a Field Effect Transistor (FET) at different binding energies. Photon energy ev.

13 XPEEM: Core Level Spectroscopy

14 Imaging of Dispersive Plane W{110} clean surface W 4f core level hv = 98 ev Resolution 210 mev parameter Gamma B (ev) Alpha B Gamma S (ev) Alpha S Gauss. Broad. (ev) W 7/2 -W 5/2 BE diff. (ev) SCLS (ev) Ref. (1) (2) Our fit Fixed Fixed Fixed Fixed (1) Riffe et al., PRL 63 (1989) (2) Webelements

15 Imaging of Dispersive Plane W{110} clean surface W 4f core level hv = 98 ev Resolution 210 mev parameter Gamma B (ev) Alpha B Gamma S (ev) Alpha S Gauss. Broad. (ev) W 7/2 -W 5/2 BE diff. (ev) SCLS (ev) Ref. (1) (2) Our fit Fixed Fixed Fixed Fixed (1) Riffe et al., PRL 63 (1989) (2) Webelements

16 Lateral resolution C 1s image (hv= 350 ev, KE = 62 ev) lateral resolution: 0.7 * (4 pixel /435 pixel ) * 5000 (nm)= 32 nm acquisition time: 1min intensity (a.u.) distance (pixel) Lateral resolution: 32 nm

17 Outline A brief introduction to spectromicroscopy The SPELEEM at Elettra SPELEEM = spectroscopic photoemission and low energy electron microscope Application example: AFM local anodic oxidation Si oxides GaAs oxides

18 Motivation Lithography for fabrication of state-of-the-art semiconductor nanostructures Basic research and quantum device applications Approaches: Traditional lithography Proximal probes (STM or AFM) H.C. Manoharan, C.P. Lutz, D.M. Eigler: Nature 403 (2000) 512

19 Local Anodic Oxidation (LAO) Water electrolysis H 2 O H + + OH -. OH - groups migrate towards the sample. Oxide penetration induced by the intense local electric field. Versatile tool at relatively low cost High lateral resolution but small area

20 LAO on GaAs/AlGaAs Before oxidation: Z After oxidation: Z Z Z GaAs AlGaAs GaAs

21 Setup for Lithography on GaAs voltage source Hygrometer AFM camera Plexiglass hood AFM N 2 +H 2 O Thermomicroscope Microcope CP-Resource water bottle

22 Quantum Point Contact Y x Vgate Conductance (2e 2 /h) K 30 K IPG voltage(v) G. Mori et al, JVST B 22 (2004) 570. Electron flow

23 Why Spectroscopic Microscopy? Lack of information on the oxidation process and on the chemical nature of the oxides. Lack of reliable microscopic techniques able to perform chemical analysis on such small structures. Understand the composition of the AFMgrown oxides (electrical and chemical properties, effect of oxidation parameters). Improve the fabrication of devices with LAO.

24 Si Oxide: Sample bias (V) Height (nm) Morphology studied after oxidation by AFM. Thickness increases with writing voltage! M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

25 Si Oxide: Image Contrast at Si 2p Field of view 12 µm, hv = ev, energy resolution: 1 ev -14V a b -12V -10V -8V Binding energy ev Binding energy ev M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

26 Si Oxide: Spectroscopy at Si 2p Intensity (a.u.) binding energy (ev) native oxide AFM oxide Spectra show high unformity of each stripe Bulk silicon peak: position and width constant Native oxide: SiO 2 peak (3.9 ev shifted) AFM oxide shows higher binding energy There is no evidence of broadening Spectra explain contrast inversion M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

27 Si Oxide: Spectroscopy at Si 2p a b 1.0 Intensity (a.u.) native oxide AFM oxide 104.7eV binding energy (ev) 102.3eV M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

28 Si Oxide: Writing Voltage Effect 14V native 12V 10V 8V ΔE = 3.97 ev (native) ΔE = 4.62 ev (U = 14V) ΔE = 4.46 ev (U = 12V) ΔE = ev (U = 10V) ΔE = 4.39 ev (U = 8V) Normalized Photoemission Intensity Si 2p silicon oxide bulk silicon : native oxide : 14 V : 12 V : 10 V : 8 V Binding Energy (ev) Shift (Si bulk -SiO x ) increases with increasing writing voltage (oxide thickness). M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

29 Si Oxide: Charging Effects H. Kobayashi, T. Kubota, H. Kawa, Y. Nakato, and M. Nishiyama: Appl. Phys. Lett. 73 (1998) Intensity decreases due to escape depth effect Charged (yellow) layer contribution is more important Squared points: our data Line: Kobayashi model

30 GaAs Oxide: Photon Exposure AFM before: height 18nm AFM after: height 13nm Images taken with secondary electrons Photon energy: 125 ev Kinetic energy: 4 ev Field of view: 10 µm One image every 2 sec

31 GaAs Oxide: Desorption Before photon exposure After hours of exposure 14V 12V 16V 10V hv 14V 12V 1μm Writing speed 0.5 µm/s Humidity 50 % Photon energy 130 ev Photon flux ph cm -2 sec -1

32 Height reduction vs. exposure time hv = 130 ev = 9.5 nm We observe a linear relation between exposure time and height reduction. A dependence on other oxidation parameters (bias, speed) could not be detected.

33 Spectra From AFM GaAs Oxide Time resolved spectroscopy with SPELEEM using Dispersive Plane (hv = 130 ev) Sample S03B Hole (3,2) Writing voltage 15 V Structure height 3 nm Image taken with secondary electrons: Photon energy: 130 ev Kinetic energy: 0.3 ev Field of view: 10 µm

34 Spectra From AFM GaAs Oxide Time resolved spectroscopy with SPELEEM using Dispersive Plane (hv = 130 ev) Intensity (a. u.) Exposure Time 3 min 5 min 7 min 13 min 20 min 28 min 43 min 63 min As 3d hv = 130 ev Intensity (a.u.) Exposure Time 4 min 6 min 10 min 17 min 25 min 54 min 74 min Ga 3d hv = 130 ev 1000 As oxide GaAs 200 O 2s Ga oxides + GaAs Relative Binding Energy (ev) Relative Binding Energy (ev) The As-oxide signal disappeares with time. The Ga-oxide signal remains unchanged (early stage of exposure).

35 Spectra From AFM GaAs Oxide 4000 As 3d hv = 130 ev after 160 min. 20x10 3 Ga 3d hv = 130 ev after 115 min. Intensity (a. u.) GaAs Intensity (a. u.) O 2s Ga 2 O AsO Ga 2 O 3 GaAs Kinetic energy (ev) Kinetic Energy (ev) 110 The AFM-oxide is mainly composed of Ga 2 O. After 3 hours exposure, only traces of As-oxides observed in As 3d. Absolute ratio As : Ga = 1 : 5

36 A possible model for the desorption Our Observations: The AFM-grown oxide exposed to 130 ev photons desorbs linearly with exposure time. The AFM-oxide is mainly composed of Ga2O with traces of As-oxide. The shape of the Ga peak does not change with exposure time (early stage of desorption). The As-oxides desorb completely after 3 hours of exposure. Not even traces of As-oxides detected in scanning Auger microscopy. Ga 2 O + As-oxides Ga 2 O GaAs

37 The physics underlying the desorption Does the desorpion depend on the total amount of energy delivered to the system? No! We delivered the same amount of energy with electrons and conventional XPS but no desorption took place. Does the photon beam heat up the system enough to locally deoxidize the sample? No! The oxides are stable up to 400ºC. The calculated local heating is less than 10ºC. The desorption depends on photon flux and energy!

38 Thermal stability RT 300C 400C 450C 500C 550C 600C Each annealing step: 10 minutes in N 2 atmosphere

39 The Knotek-Feibelman mechanism E E E Valence electrons Valence electrons Valence electrons 3d 3d 3d E Ga Initial state Valence electrons Ga Final state 3d Ga Ga The valence electrons are mainly localized at O atoms. This Auger decay leads to a final state with two vacancies in valence band weakening the bond between Ga and O.

40 Summary Si oxide: The AFM induced oxidation produces chemically uniform, stoichiometric SiO 2 with dielectric properties comparable to those of thermal SiO 2. GaAs oxide: The AFM-oxide is mainly composed of Ga 2 O with traces of As-oxide at the surface. Photon assisted partial desorption of the AFM-grown oxide was observed. All As oxides and the oxygen-rich Ga oxides are desorbed. We proposed a simple model for the dynamics of the desorption.

41 Acknowledgements M. Lazzarino, G. Mori, D. Ercolani, B. Ressel, and L. Sorba (Laboratorio TASC-INFM, Area di Ricerca, I Trieste, Italy) A. Locatelli, S. Cherifi, A. Ballestrazzi, and K. C. Prince (Sincrotrone Trieste, Area di Ricerca, I Trieste, Italy) P. Pingue (NEST-INFM and Scuola Normale Superiore, I Pisa, Italy)