Chapter 4. Non-homogeneous SiGe Nanowire Sensors
4.7 Effect of annealing on poly-Si nanowires
We carried out oxidation and annealing processes to poly-Si nanowires as well. Fig. 4-15 is the SEM images of poly-Si nanowires before and after dry oxidation at 900℃ for 4 minutes.
Before oxidation, the average height was about 155nm, and the average width was about 100nm. After oxidation, the average height was about 145nm, and the average width is about 90nm. In our experiment, the oxidation rate of SiGe was faster than poly-Si by a factor between 2~3. Fig. 4-16 shows the conductance change after APTMS modification for samples underwent different annealing process. The response of poly-Si nanowires was barely influenced by annealing process in our experiment. This fact confirmed that the existence of Ge plays an important role in SiGe nanowire, and the Ge concentration at the surface of nanowire was decisive to sensitivity. Fig. 4-16 shows the conductance of poly-Si nanowire underwent different thermal process. It seems like the quality of nanowires was improved after annealing because of defect curing, or grain growth occurred. However, the difference is still within process deviation, so we can not be sure about the effectiveness of curing effect.
Chapter 5
Conclusion
In sum, SiGe and Si nanowire sensors were fabricated by sidewall spacer formation.
Complementary sensing of p-type and n-type SiGe nanowires was demonstrated. P-type nanowires, which operated in depletion mode, provided larger ΔG after APTMS modification.
Both p-type and n-type SiGe nanowires exhibited larger ΔG than Si nanowire, and the sensitivity of n-type SiGe nanowire was the highest. However, the sensitivity of p-type SiGe nanowires was the lowest because the baseline conductance of p-type SiGe nanowires was a lot higher than that of other samples. Therefore, p-type SiGe nanowire may be more suitable for working in differential mode. Furthermore, the fact that blank SiGe nanowires exhibited small response to DI water and BS3 solution explicitly indicates that the conductance variation after APTMS modification was indeed resulted from APTMS molecules binding, and APTMS modification was necessary for BS3 sensing.
SiGe sensors with non-homogeneous structure were successfully fabricated by Ge condensation technique. Thermal processes were investigated in order to improve the quality of SiGe nanowire while retain the Ge-rich region. For the polycrystalline SiGe nanowires we fabricated, Ge diffusion becomes prominent at 1000℃. As the duration of annealing was prolonged, the sensitivity decreased correspondingly. Therefore, annealing at the temperature below 900℃ is recommended because diffusion of Ge atoms is suppressed, and sensitivity degradation is prevented. From the Auger depth profile, we conclude that Ge distribution
became locally uniform after annealing at 900℃. However, from the aspect of defect healing, the temperature of annealing should be as high as possible. Taking these factors into consideration, we concluded that 900℃ is the optimal annealing condition in our experiment.
Annealing at 700℃ for 60 minutes and 900℃ for 10 minutes resulted in about the same amount of improvement by defect healing. Annealing at 900℃ for 30 minutes after ion implantation led to the greatest improvement, and the conductance change was greater than that of homogeneous SiGe nanowires by a factor of 2.5. The optimal condition depends on the duration of oxidation, the amount of defects generated, and the Ge concentration of the nanowire with which to begin.
Chapter 6
Future Work
The amount of Ge remain in oxide layer during oxidation could be reduced by increasing the oxidation temperature. Also, defect generation could be alleviated by increasing the oxidation temperature. However, the temperature should not be higher than 1000℃. Therefore, there should be a optimal temperature between 900℃ and 1000℃. Further investigation of the effect of temperature is necessary to achieve optimum conditions. Moreover, surface chemistry of SiGe nanowire has to be further characterized in order to explore the sensing mechanism of SiGe nanowire and the difference between SiGe nanowire and Si nanowire.
Fig. 1-1. Numerical simulation result of the relationship between the response time (ts) and the detectable concentration (ρ0) of a DNA sensor[1].
Fig. 1-2. Images of sample and setup, with (a) showing the chip layout with gold pads and (b) showing the central device area. The positions of the nanowires are indicated by letters A C. In (c) a schematic cross-section of the etching setup with a mounted sample is shown, and in (d) a schematic cross-section of the nanowire during etching is shown [14].
Fig. 1-3. (A) schematic after anisotropic etch. The silicon-on-insulator active channel (yellow, width w and thickness t) is undercut etched, whereas degenerate leads (red) are etch-resistant. (B) SEM image of a device[15].
Fig. 1-4. (A) Schematic illustrating the conversion of a NWFET into NW pH sensor. (B) Real-time response of an APTES-modified SiNW for pHs from 2 to 9. (C) Plot of the conductance versus pH. (D) The conductance of unmodified SiNW versus pH[20].
Fig. 1-5. (A) A biotion-modified SiNW and subsequent binding of streptavidin to the SiNW surface. (B) Plot of conductance versus time for a biotin-modified SiNW,where region 1 correspond to buffer solution,region 2 corresponds to the addition of 250nM streptavidin[20].
Fig. 1-6. Optical image (top) of a nanowire device array. The schematic illustration (bottom) shows the detail of the red rectangle in the optical image. Golden lines are electrodes connecting nanowires, which is blue lines here[21].
(A)
Fig. 1-7. Complementary sensing of PSAusing p-type (NW1) and n-type (NW2) silicon-nanowire devices in the same array[21].
Fig. 1-8. Conductance-versus-time data recorded for the simultaneous detection of PSA, CEA and mucin-1 on p-type silicon-nanowire array in which NW1, NW2 and NW3 were functional-ized with mAbs for PSA, CEA and mucin-1, respectively. The solutions were delivered to the nanowire array sequentially as follows: 1) 0.9 ng/mL PSA, 2) 1.4 pg/mL PSA, 3) 0.2 ng/mL CEA, 4) 2 pg/mL CEA, 5) 0.5 ng/mL mucin-1, 6) 5 pg/mL mucin-1[21].
Fig. 1-9. Modification scheme of the SiNW surface for the DNA detector: (1) self-assembly of 3-mercaptopropyltrimethoxysilane (MPTMS); (2) covalent immobilization of DNA probes; (3) DNA detection based on hybridization between label-free complementary DNA target and the immobilized DNA probes on the SiNW surfaces. The inset is the SPV signal on a p-type Si surface at different stages of the modification[76].
Fig. 1-10. Conductance of the same p-type SiNW, where the arrow indicates the addition of 25pM complementary DNA (GGA TTA TTG TTA) solution[76].
Fig. 1-11. Conductance (Upper) and optical (Lower) data recorded simultaneously vs. time for a single silicon nanowire device after introduction of influenza A solution[23].
Fig. 1-12. SEM (A), TEM (B), z-axis amplified AFM micrographs of the SiNW array fabricated by a top-down approach proposed by Gao and co-workers[22].
Fig. 1-13. The dependence of resistance change of the PNA-functionalized SiNW array on hybridization time in (1) 1.0nM control, (2) 25fM, (3) 100fM, and (4) 1.0nM target DNA in buffer solution.
Fig. 1-14. (A) I-Vd curves of In2O3 nanowire sensors before and after exposure to 1% NH3. (Inset) Energy band diagrams of heavily doped In2O3 and NH3 molecules. (B) I-Vd
curves of In2O3 nanowire sensors before and after exposure to 1% NH3. (Inset) Energy band diagrams of lightly doped In2O3 and NH3 molecules[34].
Fig. 1-15. (A) Schematic diagram of a PMA-based hydrogen sensor or switch. (B) SEM image of the active area of a PMA-based hydrogen sensor. (Right) Atomic force microscope images of a Pd mesowire on a graphite surface. Images (A) and (C) were acquired in air, and images (B) and (D) were acquired in a stream of hydrogen gas. A hydrogen-actuated break junction is highlighted[40].
Fig. 1-16. (A) Schematic of CdTe-Au-CdTe nanowire field-effect transistor. (B) Schematic illustration of surface receptors modified CdTe-Au-CdTe nanowire FET for the detection of DNA[43].
(A)
(B)
Fig. 1-17. CdTe-Au-CdTe nanowire sensor detecting ssDNA-(II) at different low concentrations[43].
Fig. 1-18. Electrical responses of an unmodified polymer nanowire (A) to 100 nM biotin-DNA (single stranded) and avidin-embedded polypyrrole (200 nm) nanowires to 1 nM (B) and 100 nM (C) biotin-DNA. The responses were recorded on two separate polypyrrole-avidin nanowires[53].
Fig. 1-19. Schema of mechanism of IgG antibody detection.
Fig. 1-20. Typical response of a SiGe nanowire IgG sensor.
APTMS BS3
Fig. 1-21. Simulated conductance values as a function of the surface potential for the 200-nm-wide and 50-nm-wide wires[11].
Fig. 1-22.Variation of current as function of ozone concentration for (a) SnO2, (b) ZnO, and (c) In2O3 nanowire[64].
Fig. 1-23. IDS-VDS characteristics of Si- and SiGe-channel MOSFET[67].
Fig. 1-24. Ternary phase diagram for the Si–Ge–O system at 1000 K and 1 bar, calculated based on the thermochemical data[71].
Si substrate 5000Å SiO
2Fig. 2-1. SiO2 layer is grown on Si substrate. The thickness of SiO2 layer is 5000Å.
Fig. 2-2. Defined active area. The height of oxide step is 3000Å.
Fig. 2-3. Amorphous Si layer is deposited on SiO2 layer. The thickness of α-Si layer is 150Å.
Fig. 2-4. SiGe layer is deposited on α-Si layer.
Fig. 2-5. Defined S/D region and nanowire.
SiGe nanowire
Fig. 2-6. Remove one side of the parallel SiGe spacer.
SiGe nanowire
Al contact
Fig. 2-7. Defined Al contact pad.
Fig. 3-1. SEM image of the cross-section of a poly-crystalline SiGe nanowire.
Fig. 3-2. SEM image of the cross-section of a poly-Si nanowire.
Fig. 3-3. ID-VD curves of a 13μm-long p-type Si0.8Ge0.2 nanowire.
Fig. 3-4. Conductance variation of the 13μm-long p-type Si0.8Ge0.2 nanowire, extracted from Fig. 3-3.
Fig. 3-5. ID-VD curves of a 8μm-long n-type Si0.8Ge0.2 nanowire.
Fig. 3-6. Conductance variation of the 8μm-long n-type Si0.8Ge0.2 nanowire, extracted from Fig. 3-5.
Fig. 3-7. ID-VD curves of a 9μm-long p-type Si nanowire.
Fig. 3-8. Conductance variation of the 9μm-long n-type Si nanowire, extracted from Fig. 3-7.
Fig. 3-9. ID-VD curves of a 17μm-long p-type Si nanowire.
Fig. 3-10. Conductance variation of the 17μm-long n-type Si nanowire, extracted from
Fig. 3-9.
Fig. 3-11. Normalized conductance change measured from nanowires in different length after APTMS modification.
Fig. 3-12. Normalized baseline conductance (G0) of blank nanowires.
Fig. 3-13. Sensitivity of APTMS binding.
Fig. 3-14. The change of conductance after DI water treatment.
Fig. 3-15. The change of conductance after BS3 treatment on blank nanowires.
Fig. 3-16. Sensitivity of blank p-type Si0.8Ge0.2 nanowires exposed to different solutions.
Fig. 3-17. Sensitivity of blank n-type Si0.8Ge0.2 nanowires exposed to different solutions.
Fig. 3-18. Sensitivity of blank p-type Si nanowires exposed to different solutions.
Fig. 3-19. Sensitivity of blank n-type Si nanowires exposed to different solutions.
Fig. 4-1. SEM images of Si0.93Ge0.07 nanowire (A) before oxidation and (B) after oxidation in dry ambient at 900℃ for 4 minutes.
Fig. 4-2. Auger depth profile after dry oxidation at 900℃ for 4 minutes.
-20
Fig. 4-3. ID-VD curves of n-type Si0.93Ge0.07 nanowire after oxidation for 4min at 900℃. The length of nanowire was 8μm.
Fig. 4-4. After the oxidation of 4min at 900℃, the conductance of n-type Si0.93Ge0.07
nanowire changes with different chemical molecules. The length of nanowire was 8μm. The sensitivity of APTMS binding is 10%.
Fig. 4-6. The change of conductance after APTMS modification. 1000℃ annealing resulted in sensitivity degradation.
Ramp up
Oxidation at
900℃ for 4min Ramp up (or ramp down)
Annealing
Ramp down
Fig. 4-5. Procedure of oxidation and annealing process. The only difference between samples is the annealing temperature and duration after oxidation.
Fig. 4-7. Auger depth profile after dry oxidation at 900℃ for 4 minutes and 1000℃
annealing for 10min.
Fig. 4-8. The change of conductance after APTMS modification for samples without annealing and annealed at 900℃.
Fig. 4-9. Auger depth profile after dry oxidation at 900℃ for 4 minutes and 900℃ annealing for 30min.
Fig. 4-10. Normalized conductance for samples under different annealing conditions.
-20
Fig. 4-11. Normalized conductance change of A sample, which was in situ annealed at 900℃
for 30min, and B sample, which underwent 30min annealing at 900℃ after ion implantation.
Fig. 4-12. Normalized couductance of A sample and B sample.
Fig. 4-13. Sensitivity of A sample and B sample.
Fig. 4-14. SEM images of poly-Si nanowire (A) before oxidation and (B) after oxidation in dry ambient at 900℃ for 4 minutes.
(A)
(B)
Fig. 4-15. Conductance change of poly-Si nanowires after APTMS modification.
Fig. 4-16. Conductance for Si nanowires underwent different thermal process.
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