Fabrication and characterization of semiconductor nanowires for thermoelectric application 4

4.3 Results and discussion 4.3.1 Ge nanowire GeNw growth on Si 111 substrate 4.3.1.1 Different GeNw structure fabricated with more Ge powder loaded The GeNw growth is sensitive to sev

Chapter 4 Thermal conductivity

characterization of Ge Nanowires

4.1 Introduction

Germanium (Ge) has potential to be used as a thermoelectric material because of its higher electron mobility (or lower electrical resistivity) and lower thermal conductivity compared to silicon (Si) The electrical resistivity of bulk Ge is typically about 100

!"cm, about 1000 times less than that of bulk Si The thermal conductivity of bulk Ge

is 60.2 Wm-1K-1, which is less than half of that of bulk Si It has been predicted that semiconductors such as Si and Ge can have a thermal conductivity that is two orders lower than its bulk value when shrunk to nanoscale dimensions This work will

investigate the extent to which the thermal conductivity (k) can be decreased in Ge

nanowires produced by the VLS process The investigation will also show the feasibility of using the 3! method, which was used in characterizing the thermal

conductivity of carbon nanotubes [7], to measure the k value of the Ge nanowires Si nanowires were also fabricated for comparison with the result from Ge nanowires

4.2 Verification of the 3 ! method setup with platinum microwire

When the thermal conductivity of the platinum microwire was initially tested with a home-made current source circuit with op-amp TL074, capacitors and a few resistors, there was some problem in obtaining a thermal conductivity that was close to the theoretical value Since other researchers who published on the 3# method also used

platinum microwire of the same diameter to test their setup, the first priority was to make sure that the setup can measure the thermal conductivity of a platinum microwire that was close to the theoretical value Only after the home-made op-amp current source circuit was replaced with the Keithley 6221 AC and DC current source, the 3! characterization setup was then able give results of thermal conductivity of the platinum microwire that match with the theoretical value

Figure 31 shows the platinum sample that was placed into a CERDIP package The contacts were made by silver paste The whole package was put in an oven and kept at 90"C for 30 minutes to drive out the moisture

!"#$%&'()'*+,-".$/'/"0%12"%&'3,/4+&'".','56789*'4,0:,#&;'

Listed below are some theoretical values of platinum which are needed for the thermal conductivity measurement:

Stefan–Boltzmann constant ! = 5.67*10-8

Emissivity " = 1 (for true blackbody object, <1 for any real object)

C p (Platinum) = 132.55 J kg-1 K-1 (at 300K)

# (Platinum) = 21450 kg m-3

Theoretical thermal conductivity k (platinum) = 68 W m-1 K-1

Electrical resistivity # o = 9.8*10-8 !m (at 273 K)

Figure 32 shows the 3! voltage (V 3w ) versus current bias (I) measurement obtained on

the platinum microwire Careful selection of the time constant is required to minimize the noise level and to obtain stable signals However, too high a time constant would filter off important signal components A time constant of 1 second was found to be suitable for a current bias range of 25 mA to 100 mA because the slope was closest to

theoretical expected value of 3 in the log versus log plot of V 3w versus I

!"#$%&'()'* (+ ',-'.'/&0-$%&/&12'31'24&'5602"1$/'/"7%3+"%&'

At I = 80 mA, measured V 3! = 30.15mA

Substituting all the values from above,

measurement result of the platinum microwire verified that the 3! method thermal conductivity characterization setup was working

4.3 Results and discussion

4.3.1 Ge nanowire (GeNw) growth on Si (111) substrate

4.3.1.1 Different GeNw structure fabricated with more Ge powder loaded

The GeNw growth is sensitive to several parameters in the process For example, Ge precipitates can be found on the Ge nanowires when the amount of Ge powder used for the GeNw growth was doubled (see Figure 33) All the other growth conditions remained the same as the case with normal GeNw growth A 6 nm thick Au film was deposited on a Si (111) substrate with resistivity of 0.85-1.15 !"cm The growth temperature for the 3 zones (Ge source, center of the quartz tube and the substrates)of the furnace (as shown in Figure 24(a) in Chapter 3) was 900#C, 450 #C, and 350 #C respectively, The ramp up time and holding time for the temperature profile (see Figure 24(b) in Chapter 3) was also 60 minutes each

As discussed in the literature review section, roughness of the nanowire surface was

found to have a great impact on the thermal conductivity k Since the rate of

phonon–phonon Umklapp scattering scales as !2, where ! is the phonon frequency, low-frequency acoustic phonons have long mean free paths and contribute

significantly to k at high temperature [19-22] The Ge precipitates may contribute to

the surface roughness of the nanowires and act as a form of phonon-scattering

elements at several length scales If precipitates are incorporated carefully, the k of Ge

nanowires could decrease dramatically, possibly raising the overall thermoelectric efficiency provided the electrical conductivity is not adversely affected by the Ge precipitates However, the process of how these nanostructures (precipitates) were formed along the GeNw is not fully understood It remains of interest to compare the thermoelectric properties of these wires with properties of standard GeNw without the

Ge precipitates

!"#$%&'((')&'*%&+"*",-,&.'/0')&'0-0/1"%&.'12&0',2&'-3/$0,'/4')&'*/15&%'./$%+&'1-.' 5/$67&58'

Unsuccessful GeNw growth with contamination of the Au catalyst

When contamination was introduced to the Au catalyst, the Ge nanowire growth mechanism can also get disrupted Although Au is not reactive and it does not react with most substances at room temperature, Au could still get contaminated After 6 nm

of Au was deposited on a cleaned Si (111) wafer under the same conditions, the Si sample was left at room temperature and pressure for two days before the GeNw growth All the other conditions and preparation for the nanowire growth was the same

as the normal GeNw growth process However, the GeNw fabricated were much less than a normal growth; this was observed in all 4 substrates used in the experiment Most of the areas on the Si substrate was bare without much nanowire growth as shown in Figure 34 Very short and thin GeNw was observed occasionally The Au film had agglomerated to form Au colloids but there was minimum nanowire growth Some of the Au colloids only showed traces of germanium underneath

The mystery of how unreactive Au can get contaminated lies in the silicon substrate that Au was deposited on After the cleaning process and oxide strip, the Si surface is

in close contact with the Au atoms Si had probably diffused into the Au layer since Si outdiffusion into Au is well known [92] Many chemical models and studies [93] suggest that the Au atoms bond with the Si atoms (Au-Si surface dangling bonds) for less than 1 monolayer of Au coverage; for a thicker Au layer, the Au-Si bonds facilitate Si-Si bond disruption, the diffusion of Si atoms, and the formation of Au-Si alloy The presence of Si in the Au layer could result in the formation of a silicon oxide layer at the gold surface when the Si is subsequently oxidized when left for some time The presence of a silicon oxide layer will affect the GeNw growth in the VLS

process

Another more direct way of contamination was when the ceramic furnace tube was not used for a long time and contamination started to accumulate in the furnace tube Since the furnace tube was not kept in a vacuum environment during growth, there could be redeposition of the contaminant particles which get incorporated into the Au colloids Therefore, the nanowire growth process should be arranged immediately after the Au catalyst deposition Also, the furnace tube and quartz tube must be kept clean for growth

!"#$%&'()'*+,'"-.#&'/0'1$'2/33/"4'.54'6.%7".3'8&'5.5/9"%&'#%/97:;'

Successful GeNw growth

Figures 35 and 36 show the SEM images of GeNw grown successfully The temperature of the 3 zones of the growth furnace were ramped up to 900!C, 450!C and 350!C respectively in 60 minutes (see Figure 24 in Chapter 3) The constant temperature regime was then maintained for 60 minutes However, it was observed that other than zone 1 which stayed at 900!C, the temperature of zone 2 and zone 3 continued to rise slowly At the end of the constant temperature regime, zone 2 and zone 3 reached a temperature of 538 !C and 423 !C respectively The higher than expected temperature could be due to the continued heat transfer from zone 1 by conduction through the furnace tube and convection of gas in and around the furnace tube Therefore, it is important to note the actual temperature of each zone during the growth process since the actual growth temperature can be very different from the temperature set for growth

For the purpose of electrode deposition, longer GeNw would be suitable Attempts of fabricating longer wires were made by increasing the duration of the constant temperature regime of the growth from 60 minutes to 90 minutes From SEM examination after growth, longer GeNws were observed However, the GeNw observed after dispersing on the oxidized Si substrate was shorter Also, more clumps and clusters of GeNws were observed It was hard to find a single separate GeNw of suitable length This was due to the characteristic of GeNws grown by the VLS process Typically, the GeNws do not grow in a vertical manner Even when the beginning orientation of the GeNw was in the (100) direction, it is very common for

the GeNw to bend and entangle with one another after the GeNws have attained a certain length The entangled GeNws can easily break when they were dispersed in a sonication bath The typical length of GeNws after being dispersed ranges from 10 !m

to 38 !m

!"#$%&'()'*+,'"-.#&'/0'1&234'#%/35'67'89&':;*'<%/=&44>'

4.3.2 Effect of annealing on contact resistance

Figures 37(a) and (b) show the GeNw sample contacted with 4 electrodes that was used for the current-voltage (I-V) testing The HP 4155B parameter analyzer and probe station were used to measure the current with voltage sweeping from 0 to 2V The I-V test result is shown in Figure 38 A current compliance was set at 1 nA to protect the nanowire from damage during the I-V test At 1 V, there was almost a 100% increase

in the current In other words, the total resistance including the contact resistance and

the resistance of the GeNw was almost halved Although the contact resistance seemed

to have decreased for all points of contacts between the 4 electrodes, not all contacts will show the significant improvement in contact resistance Some of the contacts only show a 10% decrease in total resistance

!"#$%&' ()' *+,' -./' "0+#&' 12' 34&' 5&67' 8+09:&' +23&%' 34&%0+:' +;;&+:<' *=,' 83"0+3"1;' 12' 34&'

>"+0&3&%'12'34&'5&6?'8+09:&<'

!"#$%&'()' ' *'+,'-'./01'02'13&'4&56',78./&'8&7,$%&9':;'13&'<='>?@@A'.7%78&1&%'7B7/;C&%D'

From Figure 38, a slightly non-linear graph was observed from 0 to 0.5V The electrodes contacting the GeNw are Cr/Au (10/100 nm) This is because contacts to homostructure nanowires are usually dominated by Schottky barriers in both room temperature and low temperature measurements [94, 95] Wang stated that the conductance of nanowire FETS are dominated primarily by the modulation of the Schottky barrier formed between the nanowire and source/drain contacts, which depends on dopant type and concentration [95] [Quote ref by Wang]

Verification for leakage

To verify that the conductivity is due to the Ge Nw sample and not due to a leakage path through the substrate, a control sample with 4 electrodes contacting the substrate, but without any GeNw, was probed and tested As shown in Figure 39, only current at the background noise level of picoampere or tenths of picoamperes could be detected under the sweeping voltage bias

!"#$%&'()' ' *'+,'-'./01'02'13&'4051%0/',67./&'850'56509"%&:'93&%&'13&';'&/&41%0<&,'9&%&'501' 4055&41&<'=>'?&@9A

!"#$#%&'(")*+)%,")-"./)0"1#2#)3245)-") )

All possible permutations of two point contacts on the Ge Nw sample was investigated

to ensure that all 4 electrodes have good electrical contact to the GeNw The two-point resistivity of the GeNw were measured and extracted accordingly as below

Electrode 1 to 2, 2 to 3 and 3 to 4 : 0.349 ! m

Electrode 1 to 3 and 2 to 4 : 0.413 ! m

Electrode 1 to 4 : 0.482 ! m

Literature value for resistivity of Ge (bulk) : 0.305 ! m

The two-point resistivity values measured were slightly higher than the theoretical value from literature as these included the effects of contact resistance and the resistance of the GeNw The measured two-point resistivity generally matches with the literature value of the resistivity of bulk Ge, thus verifying that there are no high resistance issues for the four electrode contacts to the GeNw For the 3! thermal conductivity measurement, the 4-point probe measurement technique will be used so that the effect of the contact resistance is reduced

4.3.3 Effect of the process time on GeNw sample

Unlike Si, the oxidation of Ge is not self limiting, which means that the GeNw could

be fully oxidized after it is left in air for a long time An experiment was done to test how long a duration was allowed before the GeNw sample becomes fully oxidized Figures 40(a) and 40(b) shows the SEM images of the GeNw sample that was annealed

after 60 hours and 24 hours respectively from the completion of nanowire fabrication The SEM image clearly shows that the Ge Nw in Figure 40(a) had disappeared after the annealing while the Ge Nw sample in Figure 40(b) still remained after the annealing As discussed in the experimental details chapter, annealing is capable of activating the chemical reaction that transforms GeO2 to GeO followed by the complete desorption of GeO [91] This was why the GeNw sample in Figure 40(a) had disappeared as it is likely the nanowire had been substantially oxidized to GeO2 after being left for 60 hours after fabrication The subsequent annealing of the nanowire will cause the GeO2 to react with remaining Ge to form GeO, which is then desorbed To further confirm that there was no nanowire contacting the 4 electrodes, the sample in Figure 40(a) was also tested with the parameter analyzer and no conductivity was detected between pairs of electrodes

Since GeNw oxidized so readily, the total process time from the completion of nanowire fabrication to the device integration to form the 4-electrode contact to the dispersed nanowire and final testing is critical It is best to keep the total process and measurement time down to within 24 hours after completion of the nanowire fabrication