Fabrication and characterization of semiconductor nanowires for thermoelectric application 3

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Fabrication and characterization of semiconductor nanowires for thermoelectric application 3

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Experimental Details 48 Chapter 3 Experimental Details 3.1 Four-point probe structure test device !"#$%&'()'*+,&-./"+'+%011'1&+/"02'03'/,&'4560"2/'6%07&'/&1/'8&9"+&:' The four-point (4-point) probe method is used to measure the electrical impedance of a thin film or nanostructure. This method uses separate pairs of electrodes to carry the current and to sense the resulting voltage drop. The advantage of the 4-point probe measurement or Kelvin sensing is that the separation of the current and voltage electrodes eliminates the resistance contribution of the connecting wires and contact resistance to the measurement. In a 4-point connection measurement, the current (I) is supplied through a pair of connecting wires and electrodes (force connections). This will lead to a voltage drop (V) across the impedance (resistance R) to be measured according to Ohm’s law, given Experimental Details 49 as V = IR. However, the current will also generate a voltage drop across the connecting wires/electrodes (force connections) that supply the current too since they carry certain resistance. To prevent including that unwanted voltage drop in the measurement, an extra pair of connections is made to measure the voltage as shown in Figure 21. Since Rl is not included in the voltage measurement loop, the voltage drop across Rl will not be measured. The voltmeter has almost infinite impedance (i.e., ideal voltmeter) so there is negligible current flowing through Rv, which means a negligible voltage drop across Rv. This will allow accurate measurement of the voltage difference across the sample with resistance R. Therefore, the 4-point probe configuration is used in the resistance measurement as well as the 3!! thermal conductivity measurement of the nanowires in this project. !"#$%&'()'*+,&-./"+'&0&+/%"+.0'1".#%.-'23'.'4562"7/'6%28&'-&.9$%&-&7/:' Experimental Details 50 3.2 Sample preparation Three types of Si substrate samples were used in this work for the Ge nanowire growth, dispersing the Ge nanowires and depositing the contacting electrodes for thermal conductivity measurement, and the investigation on the catalytic etching mechanism. 3.2.1 Si substrate for Ge nanowire growth The Si substrate sample selected for Ge nanowire growth has the following specifications: Material: Silicon (Si) Doping type: Boron – p type Resistivity: 0.85-1.15 !"cm Crystallographic orientation: (111) Sample size: 6mm x 6mm Wafer thickness 275um Wafer diameter: 2 inches After being diced, the Si substrates were cleaned by ultra-sonication in acetone and then isopropyl alcohol (IPA) for 15 mintues each. The acetone is to remove organic contaminants such as grease from the sample surface while IPA serves to eliminate any possible contaminant residues that might be re-deposited onto the sample surface when acetone was blown dry. The degreased Si wafers were then dipped in 2 wt% hydrofluoric acid (HF) for 1 minute to remove the native oxide (SiOx) formed on its surface. The Si substrates were then immediately transferred into a thermal evaporator Experimental Details 51 (Edwards auto 306) containing high purity Au wire (99.99+% purity, Goodfellow) as an evaporation source. A blanket Au deposition was then carried out onto the Si substrate which was maintained at room temperature (see Figure 22). The base pressure before the evaporation process was at the level of 10-7 mbar and the pressure during evaporation was about 6 x 10-6 mbar. Proper out-gasing (of both the boat and the source) prior to the Au evaporation onto the Si surface was carried out to minimize incorporation of impurities into the evaporated Au film. The thickness of the evaporated Au was typically 4-5 nm, as monitored in-situ by a quartz crystal microbalance (QCM) and verified ex-situ by an atomic force microscopy (JEOL JSPM 5200). !"#$%&'((')*+,-'."/#%/0'+1'23&'&4/5+%/2+%'6762&08' The thin gold film agglomerated and was broken into individual dots (see Figure 23 (a)) by annealing the Au-coated Si substrates in vacuum (0.005 mbar) at 400oC for 30 minutes. The size of the Au-dots spreads over a large range of 5 to 90 nm with a mean diameter of 50-60 nm. Experimental Details 52 !"#$%&'()'*+,'-".%/#%012'/3'"45"6"5$07'8$95/:;'/'4-'8$'3"7-?' 3.2.2 Si substrate for dispersing Ge nanowire and depositing the contacting electrodes The Si substrate sample selected for dispersing the Ge nanowires and depositing the contacting electrodes is similar to that used for the Ge nanowire growth except that it has a thick layer of thermal oxide (1 !m) grown on it. A thick oxide is necessary for isolation since the Ge nanowires are dispersed onto the Si substrate sample. Since minimum transfer of heat from the Ge nanowire to the underlying substrate is needed for the thermal conductivity measurement, a material of low thermal conductivity (k) is needed as the substrate. The k of silicon oxide is 1.38 W/m/K as compared to bulk germanium which has a k of 60.2 W/m/K. Thus, a Si substrate with thick oxide would be suitable for the thermal conductivity measurement. The resistivity of the p type Si under the oxide is 10 – 30 "#cm. Similar to the substrate for Ge nanowire growth, the oxidized Si substrates were Experimental Details 53 cleaned by ultra-sonication in acetone and then isopropyl-alcohol (IPA) for 15 mintues each to remove organic contaminants and any possible contaminant residues that might be re-deposited onto the sample surface when acetone was blown dry. However, the oxidized Si substrate was not subjected for further HF treatment as the oxide had to be preserved in this case. The degreased oxidized Si substrate was then ready for dispersing of Ge nanowires which will be described in a later section. 3.2.3 Si substrate for investigation on the catalytic etching mechanism The Si substrate sample selected for catalytic etching has the following specifications: Material: Silicon (Si) Doping type: Boron – p type Resistivity: 4-8 !"cm Crystallographic orientation: (100) Sample size: 6mm x 6mm Wafer thickness: 275um Wafer diameter: 2 inches Silicon with (100) orientation is chosen since catalytic etching is well-known to occur anisotropically along the (100) directions. Etching in inclined directions on non-(100) substrates will complicate the investigation. The preparation steps for the Si (100) substrates is similar to the substrates used for Ge nanowire growth. After cleaning, the substrates will be put into the evaporator for deposition of various metals before Experimental Details 54 immersing them in the hydrofluoric acid/hydrogen peroxide (HF/H2O2) solution for chemical etching. 3.3 Ge nanowire growth The process chosen for Ge nanowire growth is VLS as discussed in section 2.3.1.1. Growth of GeNWs was carried out in a three-zone furnace (Lindberg/Blue STF55346C). A small amount (about 200 mg) of high purity Ge powder (99.999+% purity, Sigma-Aldrich) was loaded at the close-end of a small quartz tube while the Au-dotted Si substrates were placed near to the open-end of the tube as indicated in Figure 24(a). Before the wire growth, the chamber was pumped down to a base pressure of 0.01 mbar to minimize the presence of unintended contaminating gases such as H2O and O2. A counter-flow of Argon (Ar) gas, at a flow rate of 150 sccm, was then introduced during the wire growth. The pressure during growth was constantly maintained at 2.66 mbar by an automatic metallic-throttle valve (MKS Type 253B) with PID feedback control. The temperature profiles used for various sections of the furnace are shown in Figure 24(b). Ge powders and the Au-dotted Si substrates were heated from room temperature to 900!C in 60 minutes at a uniform ramp up rate. The temperature of the Ge source and the substrates were then maintained at 900!C for another 60 minutes before the system was allowed to cool down to room temperature. At 900!C, Ge vapour would be produced from the Ge powder. The Ge vapour would then be transported to and condensed onto the Au-dotted Si substrates, contributing to Ge nanowire (GeNW) growth in the VLS process. A counter-flow of Ar in this case serves to increase the Experimental Details 55 residence time of the Ge vapour over the substrate, allowing the formation of high-density Ge nanowires. !"#$%&'()'*+,'-&.$/'01%'2&34'#%15.67'+89'*:,'.&;/&%+.$%&'/%10"[...]... neglected if Eq (3. 1) is satisfied Equation (2.10) (repeated here as Eq (3. 2)) limits the temperature of the experimental environment, length and diameter of the specimen as follows: (3. 2) where g = 16!"T0 3/ (#CpD), $ = L2/(%2&) and & = k/(#Cp) Equation (3. 3) should be satisfied for the thermal conductivity calculated to be within +/ -3. 5% of the actual value 0 < 2'$ < 4 (3. 3) The parameters of the lock-in... composition, empirical formula, chemical state and electronic state of the elements that exist within a material When a beam of X-rays is shone on a material, the kinetic energy and number of electrons that escape from the top 1 to 10nm of the material is being analysed With the information, XPS spectra can be obtained Ultra high vacuum of 10-9 mbar is required for this technique 3. 13 Auger electron spectroscopy... difficult Since the nanowires can easily clump together in the solution, the bottle of nanowires solution had to be put through sonication for another 5 seconds if the bottle of solution was left to stand for more than 60 seconds After the nanowires were dispersed into the ethanol solution in the bottle, the nanowires solution was extracted by a micro-pipette Since only a very small amount of nanowires solution... substrates were soaked in acetone for more than 60 minutes during the liftoff process The substrates with markers were cleaned in acetone and IPA as mentioned in chapter 3. 2.2 before Ge nanowires were dispersed on them 3. 7 Electron beam lithography With the coordinates of the two ends of the Ge nanowires, the e-beam system, the Elphy Quantum Plus Advanced SEM/FIB system, was setup for the e-beam lithography... reserve was set in the instrument Since the working frequency for the Ge nanowire sample was 1000 Hz, 0. 03 s of time constant was sufficient for stability 3. 11 Ceramic heater setup for temperature dependence characterization of thermal conductivity The feed-throughs on the right end of the in-house constructed vacuum chamber (Figure 29) for thermal conductivity measurement were connected to a thermocouple... evaporation had to be performed in steps to ensure that the PMMA film did not change when the temperature was too high Preferably, 10 minutes of break was needed for every 15 minutes of evaporation After the evaporation was performed and substrates were cooled to room temperature, the samples were soaked in acetone for 2 hours The liftoff process was performed by gently swirling the beaker of samples in acetone... from 50x to 1000x After the Ge nanowires samples were cooled to room temperature, optical microscope examination was used to search for single straight nanowires The selected nanowires had to be long enough (25 µm or longer) for the 4 electrode contacts to be deposited across and clear of other nanowires which could otherwise short the 4 electrodes After that, measurements of the selected nanowire location... transforms GeO2 to GeO; the latter will then be desorbed [91] For complete desorption of GeO, the temperature was raised to 500 !C for another 10 minutes 3. 9 Probe station and parameter analyzer characterization The probe station was used to check the I-V characteristic of the Ge nanowire sample and to find out the electrical resistance of the sample The probe station used is a typical black chamber... capable of fabricating feature sizes of larger than 30 0 nm on PMMA The coordinates of the Ge nanowires were first keyed into the system so that the software program knows where the selected nanowires are located Then the desired pattern, or 4 electrodes with bond pads in this case, were drawn by the software with the drawing tools given Once the settings were determined, a dummy sample without nanowires. .. typical pattern with 4 square bond pads of 250 µm width and electrodes will take about 1 hour 10 minutes for the exposure to complete The exposed substrates were developed in a solution of the PMMA developer, Methyl isobutyl ketone (MIBK) and IPA in a ratio of 1:1, for 75 seconds Figure 27 shows the pattern of the 4 electrodes drawn after the e-beam lithography process and PMMA development It can be clearly ... 3/ (#CpD), $ = L2/(%2&) and & = k/(#Cp) Equation (3. 3) should be satisfied for the thermal conductivity calculated to be within +/ -3. 5% of the actual value < 2'$ < (3. 3) The parameters of the lock-in... working frequency for the Ge nanowire sample was 1000 Hz, 0. 03 s of time constant was sufficient for stability 3. 11 Ceramic heater setup for temperature dependence characterization of thermal conductivity... hydrofluoric acid/hydrogen peroxide (HF/H2O2) solution for chemical etching 3. 3 Ge nanowire growth The process chosen for Ge nanowire growth is VLS as discussed in section 2 .3. 1.1 Growth of GeNWs

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