Some Key Materials Issues in Semiconductor Quantum Dots Based Nano-electronics

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USA Advisor: Prof. Robert Hull Collaborators: J. A. Floro, UVa; J. F. Graham, FEI; J. L.

1 Some Key Materials Issues in Semiconductor Quantum Dots Based Nano-electronics Prabhu Balasubramanian PhD Graduate Student Department of Materials Science and Engineering Rensselaer Polytechnic Institute, Troy, NY, USA Advisor: Prof. Robert Hull Collaborators: J. A. Floro, UVa; J. F. Graham, FEI; J. L. Gray, PSU NYSTAR: SUNY/INDEX & RPI FOCUS CENTER 2013 Rensselaer Nanotechnology Center Research Symposium Wednesday, November 6, 2013

Background We have expertise in self-assembled nanostructures that form during strained epitaxial growth, here in SSSS xx GGGG (11 xx) films on Si QD: Semiconductor nanostructures that form during

2 Background We have expertise in self-assembled nanostructures that form during strained epitaxial growth, here in SSSS xx GGGG (11 xx) films on Si QD: Semiconductor nanostructures that form during strained epitaxial growth, here in Ge/Si(100) system. Epitaxial Ge Film Substrate QDM Height Profile 5 <510> < > <311> Quantum Dot (Pyramid) Quantum Dot (Dome) QDM: A nanostructure comprising of four island ridges (QDs) and a central pyramidal pit; observed in epi- SixGe(1-x)/Si(100) under conditions of limited adatom mobility. AFM 750 nm Height (nm) Position (nm)

541-557, APRIL 1997 Spin Exchange Switch (SES) Coherent Spin Switch (CSS) V V V V V Tang et al, PRL 97, 119903 (2006) Two Key

3 Potential Nanologic Applications Potential Nanologic Applications Based on Charge and Spin Localizations Inside QDs Possible Charge Configurations (logic states) of a QDM Majority Gate that implements QDMs 0 1 C.S. Lent, P.D. Tougaw, Proc. IEEE, vol 85, pp , APRIL 1997 Spin Exchange Switch (SES) Coherent Spin Switch (CSS) V V V V V Tang et al, PRL 97, (2006) Two Key Requirements: 1) Favorable band structure alignments within QDs and QDMs for carrier localization. 2) Doping of QDs and QDMs for the creation of holes.

1) Local Nano-chemistry Nanochemistry Key Materials Issues Electronic Band-Gap Charge Localization Property To investigate composition distribution across QDMs 2) FIB Damage FIB Beam Diameter: Few

4 1) Local Nano-chemistry Nanochemistry Key Materials Issues Electronic Band-Gap Charge Localization Property To investigate composition distribution across QDMs 2) FIB Damage FIB Beam Diameter: Few tens of nm Ion Current Density: AA cccc 2 (at least three orders of magnitude higher relative to conventional, broad ion beams) To study structural damage recovery of FIB implanted Si FIB Column Ion Beam Modified surface Sample

5 Goal I: QDM Nano-chemistry

6 Experimental Measurements Characterization Technique: Auger Electron Spectroscopy (Resolution: less than 10 nm under optimum conditions);phi 700 Auger Signal Strength Dependent on: 1. Local surface angle 2. Geometry of the feature X-sectional View Incident To Detector To Detector electrons Auger Sample electrons Self-shadowing Neighbor-shadowing To Detector Beam obstruction Topography Corrected Auger Signals (TCAS) 1 used to determine variation of c-si and c-ge within QDMs. The TCAS were then suitably converted to composition. M. Prutton et. al JAP 54 (1), 374 (1983)

7 Approach Spatial distribution of emitted Auger electrons depends on spatial distributions of primary and backscattered electrons Construction of an AED Function j Total Sequence N e j Total Convolution of AED with Model Composition Profiles Comparison of Simulated Profiles with the Experimental Profile = j Primary + j BSE = N e 2 e x 2 σ 2 ( R 1) N + e 2 x 2 σ 2 πσ 1 πσ 2 : Number of Primary Electrons;σ 1 : Primary Beam; σ 2 : Backscattered Electrons; R : Backscattered Electron Correction Factor J. Cazaux, Surf. Sci. 125 (2), 335(1983); M. P. Seah, Surf. Interface Anal. 33 (12), 950 (2002) 1 e 2

8 j Total Surface Angle ( ) Auger Spatial Resolution = j Primary + j BSE Measured on thin sample edges σ 1 (nm) x 2 N σ 2 e 1 = e πσ 2 1 σ 2 (nm) R ( R 1) N + e πσ 2 2 CASINOv2.42 x 2 σ 2 2 e d o, Effective Spatial Resolution, 80%-20% metric (nm) NIST Backscattering Correction Factor FWHM, Primary Beam = 2.35*σ 1 ~ d o BSEs do not significantly degrade the resolution

5 (b) Experimental Results Height (nm) Composition (%) 0-5 -10-15 0 100 200 300 400 500 100 (c) 80 60 40 20 Position (nm) QD QD Pit (a) AFM %Si %Ge 2.

9 5 (b) Experimental Results Height (nm) Composition (%) (c) Position (nm) QD QD Pit (a) AFM %Si %Ge 2.0µm Silicon (%) 100 Position (nm) Remnant topography effects and potential desorption effects have increased measurement errors inside pits This Work Position (nm) Leite et al. Leite et al. (Convoluted) M. S. Leite et. al, PRB 73 (12), 4 (2006) Pits are the most Ge enriched regions within QDMs Pit Bases at 17±9 % Si (Leite et al.) Pit Bases at 58±4 % Si (Our Measurement)

10 (a) Silicon (%) (b) Silicon (%) Simulations Vs. Experiment QD Pit QD Experimental Profile I(Before Conv.) Profile I(After Conv.) Position (nm) Position (nm) QD Pit QD Experimental Profile II(Before Conv.) Profile II(After Conv.) Position (nm) Position (nm) Profile ID Profile I (Convoluted) Profile II(Convoluted) Δ%Si (RMS ) 0.7 Compositional contrast across the QDM = 12% Si Composition continually decreases towards pit cusps starting from exterior edges of QDMs 2

11 Goal II: Structural Damage Recovery of FIB Implanted Silicon

Background Energy Loss Mechanisms: Electron energy loss (EEL) and nuclear energy loss (NEL) NEL results in interstitials through binary collisions ~13eV needed for lattice atom displacement KEY

12 Background Energy Loss Mechanisms: Electron energy loss (EEL) and nuclear energy loss (NEL) NEL results in interstitials through binary collisions ~13eV needed for lattice atom displacement KEY Target Surface Target Atoms Implanted Ion Implanted Ion Trajectory Damage Volume Collision cascade, sec Fast Relaxation of collision cascade, to sec Time Slower Relaxation (Thermal)

Fabrication Technique Broad Beam Ion Implanter Mass Selecting Focused Ion Beam (MS-FIB) Ion Energy 60keV Si+ 30keV Si++ (MS-FIB); 30keV Ge++ (MS-FIB); and 30keV Ga+ (FIB) Dose Range to Ion Current

13 Fabrication Technique Broad Beam Ion Implanter Mass Selecting Focused Ion Beam (MS-FIB) Ion Energy 60keV Si+ 30keV Si++ (MS-FIB); 30keV Ge++ (MS-FIB); and 30keV Ga+ (FIB) Dose Range to Ion Current 1-10 µa 22 pa (Si++); 28 pa (Ge++); and 53 pa (Ga+) Beam Diameter Ion Current Density to 1/2 to 5/8 in. 10s of nm MS-FIB Column Schematic LMIS Extraction electrodes Condenser lens (L1) Ion current selection apertures Wien filter Mass selection apertures Blanking plates Faraday cup Scanning and stigmation octupoles Objective lens (L2) Sample

14 Characterization Technique Raman Spectroscopy (RS) is an optical spectroscopy technique to probe vibrational states in a material At sufficiently high implant doses the vibrational density of states would be affected significantly by damage which would produce a noticeable change in Raman signals The acquired Raman signals are averaged over the probe depth of the incident laser From SRIM (Simulations) Type of Implant 60 kev Si 60 kev Ge 30 kev Ga For 405 nm Laser Laser Probe Depth (nm) Ion Range (nm) Straggle (nm) In Crystalline Si In Amorphous Si

15 Measurement Technique RS was used for measurements of structural damage produced by implantation Peak Height, H Peak Position Structural Damage= 1 HH HH 0 Stress inferred from peak shifts From Implant From unimplanted Si Non-amorphizing implants: Fitted by a Voigt function ( a mix of Gaussian and Lorentzian functions) Partially amorphizing implants: Fitted using a combination of 100% Gaussian and Voigt functions Fully amorphizing implants: Fitted using Gaussian alone

16 Raman Spectroscopy: Ion Dose Rate Effect Implant Energy: 60keV Ion Dose Rates: µa cm 2 (Broad Beam) and 0.1 A cm 2 Annealing Conditions: 730 C, 10 minutes in nitrogen ambient (FIB) As-implanted Post-annealing Si-FIB implants produce a relatively higher damage. Implantation at a higher dose rate (FIB Vs. broad beam) leads to a greater structural damage in the as-implanted state, but produces a greater structural damage recovery upon annealing.

17 Raman Spectroscopy: Ion Species Effect Implant Energy: 30keV, Ge2+; 30keV Ga+ Ion Dose Rates: A cm 2 (FIB) Annealing Conditions: 730 C, 10 minutes in nitrogen ambient As-implanted Post-annealing In the as-implanted state Ge implants lead to a greater structural damage. Ge implants recover structural damage better upon annealing. Presumably a higher solid solubility of Ge in Si when compared to that of Ga leads to a higher structural damage recovery during the annealing.

18 QDM Nano-chemistry Summary QDM pit bases are regions lowest in Si (highest in Ge) with a composition of 58±4 % Si. Holes would localize at pit bases. Composition increases monotonically towards pit bases starting from the exterior edges of QDMs. FIB Damage Recovery in Si Relative to broad beam Si implantation, Si-FIB implantation causes a higher structural damage, but, leads to a better damage recovery upon annealing. Presumably a higher solid solubility of Ge than Ga in Si aids in the better (100% vs. 63% for the highest dose, 5xxxx 15 cccc 2 ) structural damage recovery for the former implant upon annealing.

19 Acknowledgements Prof. Dan Lewis, RPI for discussions on spatial distribution of backscattered electrons M. David Frey, RPI for AFM instrumentation Denny Paul, PHI and Rob Planty, RPI for instrumentation of Auger electron spectroscopy Ray Dove, RPI for Raman instrumentation Dr. Jeremy Graham, FEI and Hamed Parvaneh, RPI for FIB implants Hamed Parvaneh and Kiran Sasikumar at RPI for various discussions

Ion Implantation. alternative to diffusion for the introduction of dopants essentially a physical process, rather than chemical advantages:

Ion Implantation. alternative to diffusion for the introduction of dopants essentially a physical process, rather than chemical advantages: Ion Implantation alternative to diffusion for the introduction of dopants essentially a physical process, rather than chemical advantages: mass separation allows wide varies of dopants dose control: diffusion