New Approach to Technology of Local Anodic Oxidation by AFM Tip

1. New Approach to Technology of Local Anodic Oxidation by AFM Tip Ing. Jozef Martaus, dept. of optoelectronics, Institute of Electrical Engineering SAS • Outline: • Motivation of the work– quantum structures and devices • Local anodic oxidation (LAO) by an AFM tip – method for quantum structures and devices preparation • Common versus our approach to technology of LAO by AFM • Experiments, results • 5. Conclusion, future work J. Martaus CENG ‘07

2. 1. Motivation of the work – quantum structures and devices 1 drain in-plane gate Conductivity (2e2/h) source Gate voltage (V) Conductivity (nS) Gate voltage (V) Quantum Point Contact (QPC) quantized conductivity QPC GaAs/AlGaAs 0.6 K G. Mori et al., J. Vac. Sci. Technol. B22 (2), (2004) B. J. van Weeset al.,Phys. Rew. Lett.60, 848 (1988) Single Electron Transistor (SET) Coulomb’s oscillations U. F. Keyseret al., Appl. Phys. Lett.76 (4), (2000) J. Martaus CENG ‘07

3. 2. LAO by an AFM tip – method for quantum structures and devices preparation 2 AFM tip water film sample Basic principle (cathode) OH-ions sample atoms oxide (electrolyte) (anode) water film – because of ambient humidity J. Martaus CENG ‘07

4. 2. LAO by an AFM tip – method for quantum structures and devices preparation 3 I-V AFM tip motion vector oxide line d depleted region potential barrier w qΦ(0) charge carriers in 2DEG w qΦ(V) w qV Local Anodic Oxidation applied to the semiconductor heterostructure with the 2DEG applying LAO on semiconductor heterostructure, one can shape 2DEG, thus prepare quantum structures and devices simplified cross-section of semiconductor heterostructure with a two-dimensional electron gas (2DEG) 2DEG Ohmic contact electrons from 2DEG trapped in oxide create effective potential barrier in 2DEG layer d – oxide line width w – potential barrier width qΦ(0) – potential barrier height with zero voltage applied qΦ(V) – potential barrier height with voltage applied J. Martaus CENG ‘07

5. 3. Common versus our approach to technology of LAO by AFM 4 InGaP 3.5 nm AlGaAs 3 nm 5 nm GaAs Si Δ AlGaAs 17 nm 1 2 nm AlGaAs Si Δ 2DEG GaAs buffer 17 nm 500 nm AlGaAs 2DEG 500 nm GaAs buffer substr a t e substr a t e E E F C E E F C commonly used: we are using: 2DEG 23 nm beneath the surface 2DEG 34 nm beneath the surface GaAs/AlxGa1-xAsmaterial system GaAs/AlxGa1-xAs/InGaPmaterial system InGaP is a barrier material relative to AlGaAs J. Martaus CENG ‘07

6. 3. Common versus our approach to technology of LAO by AFM 5 AFM tip AFM tip 5 nm GaAs 12 nm AlGaAs Δ-Si doping 17 nm AlGaAs 2DEG 500 nm GaAs buffer common approach: our approach: native oxides etching solution oxide line w w h h 3.5 nm InGaP 3 nm AlGaAs 34nm 23nm Δ-Si doping 17 nm AlGaAs 2DEG 500 nm GaAs buffer oxide line has to be as high as h ≈ 15 nm to affect Δ-Si doping h ≈ 2 nm high oxide line is enough to fully oxidize InGaP cap layer it can lead to its base width w ≈ 40 nm it leads to its base width w ≈ 130 nm D. Graf et al., J. Appl. Phys.99, 053707 (2006) J. Martaus CENG ‘07

7. 4. Experiment, results 6 AFM topographies of prepared oxide line or trenches for measurement purposes after LAO h=2.4±0.3 nm, wo=74.3±3.3 nm after 1st etch run d1=3.4±1.1 nm w1=52.0±8.2 nm after 2nd etch run d2=17.1±0.9 nm w2=95.9±8.4 nm J. Martaus CENG ‘07

8. 4. Experiment, results 7 IV characteristics measured on a two-terminal device a) b) c) 380 mV 750 mV Breakdown voltage (at the current of 200 pA) Martaus et al. Ultramicroscopy, sent J. Martaus CENG ‘07

9. 4. Experiment, results 8 dots prepared by LAO at humidity = 45 % holes emerged by wet etching base width ≈ 80 nm pithead width ≈ 60 nm height ≈ 3 nm depth ≈ 6 nm spacing: 500 nm 200 nm 100 nm J. Martaus CENG ‘07

10. 4. Experiment, results 9 dots prepared by LAO at humidity = 20% holes emerged by wet etching base width ≈ 60 nm pithead width ≈ 40 nm height ≈ 2 nm depth ≈ 4 nm spacing: 500 nm 200 nm 100 nm J. Martaus CENG ‘07

11. 4. Experiment, results 10 QPCs and rings prepared by LAO at humidity = 20 %, then etched oxide line base width ≈ 60 nm, pithead width of trench ≈ 40 nm oxide line height ≈ 2 nm, trench depth ≈ 4 nm concentric rings QPC topography 2 QPC topography 1 70 nm 90 nm 40 nm 70 nm 30 nm J. Martaus CENG ‘07

12. 5. Conclusion, future work11 conclusion • Shallow InGaP/AlGaAs/GaAs semiconductor heterostructures for the LAO were prepared by MOVPE (2DEG 23 nm beneath the surface) • New approach in LAO technology for semiconductor heterostructures was used: • only 2-nm high oxide lines with 60 nm base width were prepared in InGaP cap layer by AFM tip • oxide lines were removed by wet etching, created 4-nm deep trenches with 40 nm pithead width to uncover underlying AlGaAs, • InGaP unaffected by LAO serve like an etching mask for further repeated etching of oxidized AlGaAs, thus preparing high aspect ratio trenches • Trenches defined represent high potential barriers in the 2DEG • This new approach allows us to prepare narrow patterns that represent high energy barriers in 2DEG, which is promising for quantum structures fabrication future work • Study of the mechanism of potential barrier creation • Prepare and characterize sub-micrometer Hall probe • Prepare and characterize the quantum structures (QPC, SET) J. Martaus CENG ‘07

13. Acknowledgement 12 Thank you for your attention. jozef.martaus@savba.sk Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Republic J. Martaus CENG ‘07