Fabrication and characterization of semiconductor nanowires for thermoelectric application 3

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 the

Chapter 3 Experimental Details

3.1 Four-point probe structure test device

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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

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

R l 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

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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

(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)

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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

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

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

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

residence time of the Ge vapour over the substrate, allowing the formation of high-density Ge nanowires

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3.4 Scanning electron microscope examination

The scanning electron microscope (SEM) is a type of microscope that uses a beam of high energy electrons to image a sample by scanning it in a raster scan manner The interaction of the electrons with the atoms of the sample produces signals and information of the sample surface such as composition, topography and electrical conductivity A SEM has the capability to image a sample with magnification from 10 times to more than 500,000 times

The SEM used for this work is the Philips Nova Nanosem 230 This SEM was used in

many occasions to check the micro-sized or nano-sized sample to ensure this has the desired structures before proceeding to the next step of experiment For the Ge nanowire growth, the SEM was important for checking the structure of the Ge nanowires produced since a slight change in the growth temperature or duration could potentially cause very different morphology of the nanowires fabricated Figure 25 shows a SEM image of typical Ge nanowires grown in the three-zone furnace as mentioned in section 3.3 A charging phenomenon was observed when certain spots or certain nanowires were viewed under SEM for a long time This is due to the dielectric nature of the native oxide on the silicon substrate and germanium nanowire samples One possible way that SEM can affect the sample is through carbon contamination Although the SEM chamber was pumped down to an extremely low pressure level, traces of residual gas such as hydrocarbons can still be found in the SEM chamber The energy from the electron beam could cause the hydrocarbons to be deposited on the spot of the sample being viewed The contamination will be worse when the beam energy is high with high magnification viewing To eliminate the possibility of carbon contamination, the nanowires of the sample viewed under SEM will not be used for the subsequent experiment Only the neighbouring die from the same batch of nanowires will be used for the next step of dispersing the nanowires onto the oxidized substrate Typical electron beam energy to view nano-sized features is between 10 to 20 keV and the sample has to be placed at a distance of about 5 mm from the final lens of the SEM for best focus

3.5 Nanowire integration

After SEM examination to ensure that the Ge nanowires grown from the three-zone furnace are of the desirable length, diameter and uniformity, a neighbouring Si sample with Ge nanowires that has not been viewed under SEM before was transferred to a small 30 ml glass bottle Since the Ge nanowires are very fragile, care had to be taken

to ensure the side of the Si die with the grown Ge nanowires did not face downwards and come into contact with the bottom of the bottle throughout the transfer 5 drops of MOS-grade ethanol were transferred into the bottle by a micro-pipette The bottle, with the nanowires sample soaked in ethanol, was then subjected to sonication for 10 seconds to disperse the Ge nanowires in the ethanol solution The bottle was placed in

a water bath during the sonication process to reduce the power of the vibration that the nanowires were exposed to Otherwise, the nanowires can further break into shorter segments which will make the subsequent electrode deposition for contact formation

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 was needed, merely dipping the micro-pipette into the solution would be able to draw enough nanowires solution up the micro-pipette by capillary force With the pipette tip in contact with a cleaned oxidized Si substrate, the Van der Waals forces will pull the nanowires solution out from the pipette to coat the oxidized Si substrate evenly The oxidized substrate with dispersed nanowires was immediately put into an oven at 120!C for 10 seconds to ensure that the solvent evaporates quickly so that the nanowires would not have time to move and join to form clumps

After the substrate with dispersed Ge nanowires was cooled to room temperature, poly(methyl methacrylate) or PMMA will be spin-coated onto the substrate using the Cookson Spincoater P6700 at about 2000 revolutions per minute (rpm) PMMA was used as a type of organic resist for the electron beam lithography process described later in section 3.7 After the substrates were spin-coated with PMMA, the substrates were put into the oven at 120 !C for 15 minutes to drive out excess moisture in the PMMA

3.6 Optical microscope examination

Optical microscopy is a convenient and non-destructive method to obtain an image of

a small feature with dimensions larger than a quarter micron The optical microscope

used in this work is the Leica DMLM microscope The microscope is able to give magnification 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 relative to the markers on the substrate will be made and recorded as shown in Figure 26 The markers were pre-fabricated to help in locating a nanowire on a large area substrate With markers and information on the relative coordinates of the nanowires available, it was possible

to move the electron beam (e-beam) to the desired location for subsequent e-beam lithography without exposing the surrounding area

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The markers were pre-fabricated on the oxidized Si substrate by the optical lithography process Positive photoresist AZ5412 was first spin-coated onto the oxidized Si substrate using the Cookson Spincoater P6700 at about 6000 rpm for 45 seconds The photoresist was then baked at 90!C for 30 minutes to remove excess moisture After the substrates had cooled down to room temperature, the photoresist was put in direct contact with the chrome mask and exposed for 1 minute 50 seconds The substrates were subsequently developed in AZ1512 for 1 minute 5 seconds After the standard thermal evaporation of Cr (10 nm) followed by Au (40 nm) in the thermal evaporator (Edwards auto 306), the 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 process This e-beam system was capable of fabricating feature sizes of larger than 300 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 was put into the chamber of the Phillips XL30 FEG SEM for the e-beam lithography process A dummy sample was used here for calibration purpose to find out the right dose, resolution and developing time Over-exposure will increase the total process time while under-exposure will lead to