Deep-Level Transient Spectroscopy
Contents
1. correlator to be less than 2 ms where Correlator 1 having the largest value and each consecutive correlator having consecutively smaller values Ensure the Y outputs of the correlators are connected to the corresponding channels of the NI BNC adaptor Ch1 2 3 4 The Period of pulse repetition needs to be set according to e Period gt Width 10 Initial Delay on all correlators e Period lt 50 Initial Delay on correlators 1 and 2 The pulse generator is now setup for DLTS and the rest of the measurement electronics can be set up The Pre amp output on correlator 1 unit should be connected to the CRO to monitor the capacitance signal of the diode that is fed into the correlators for measurement Increase the Pre amp gain on the correlator to as large as possible without producing a noisy signal The pre amp output should now be connected to the Input of the auxiliary correlators 3 and 4 Connect the Pre amp output on correlator 1 to the CRO Increase the Pre amp gain on the correlator to as large as possible without producing a noisy signal Connect the Pre amp output to the Input of the auxiliary correlators 3 and 4 The TC value of the correlators affects how fast the internal capacitors discharge and hence how fast consecutive measurements can be made TC contributes to the total time constant TCto which should be less than 10 and as close as possible for each correlator TCto depends on many factors h2. stuff The experiment is primarily controlled by a labview program which has a nice GUI into which you can enter the values that you would like certain parameters to assume For example to change the sample temperature just type say 275 into the relevant box hit Idle at initial temp and the electronics will do the rest Your demonstrator will take you through most of what you need to do but a few extra hints will be provided here For example you need to tell the software what scale the capacitance meter is set to so that the correct capacitance value is recorded You will be analysing the data using a command line plotting analysis software called genplot which has some similarities to IDL and C for those who are into such stuff You will be using pre written programs macros to analyse the data you have taken using Labview using ASCII files Your demonstrator will show you how to do this 4 3 The cryostat and sample holder be contact 3 SPR Al Wire Bond SA Au contact steel Quartz n Si Ag paint InGa eutectic Ag paint Cu base Figure 6 Sample schematic Ask your demonstrator to take you through the cryostat and ask them to load a sample The sample mounting setup is shown in figure 6 The InGa is a eutectic mixture of metals which is liquid at room temperature We use it because it forms an Ohmic contact with the sample unlike the gold contact on top which forms a S
3. 2 Table 2 TC settings for Correlators on the SULA DLTS 4 9 Analysing the DLTS data e Calculate the defect concentrations for each defect observed e Use the SULA analysis to determine F and o 4 10 If time permits it won t References 1 P Blood and J W Orton The Electrical Characterization of Semiconductors Majority Carriers and Electron States Academic Press 1992 Page 344 B G Streetman Solid State Electronic Devices 1990 E H Rhoderick Metal Semiconductor Contacts Oxford University Press 1978 1st Ed Page 21 E H Rhoderick Metal Semiconductor Contacts Oxford University Press 1978 1st Ed Page 10 ao e WwW N E H Rhoderick and R H Williams Metal Semiconductor Contacts Oxford University Press 1998 2nd Ed Page 168 6 Dieter K Schroder Semiconductor Material and Device Characterization Wiley Inter Science Acknowledgements 14 These notes were written by Matthew Lay in 2004 and tweaked by David Hoxley in the same year 15
4. Deep Level Transient Spectroscopy Part 3 Labs July 28 2004 Contents 1 Introduction 1 2 Elementary Semiconductor Physics 2 3 Background Electrical Analysis of Semiconductors 3 3 1 Schottky Barrier Diodes soirs ponc 4 bb Oo 54 POR bee eee ee ae es Ba dh ad od 3 3 2 Capacitance Voltage C V Measurements 2 2 00000 eee ee 4 3 3 Deep Level Transient Spectroscopy DLTS 2 00002 eee eee 4 3 3 1 The capacitance transient ecen aa aaa oa BO A add aa e a a E E E e A 6 3 3 2 Variation in DLTS signal with temperature 2 0 0 a T 4 Experimental Work 8 4 1 The racks of electronics siopa i oe aa a a pa a a e a e a e E e a a G a a 9 4 2 The computer stutl 2424 54 44 4 0609 64222025450 a O ee a a ee Gee PRS 9 4 3 The cryostat and sample holder 2 0 2 eee ee ee 9 4 4 Making a room temperature C V measurement 2 0 0 0 0 0 0 eee eee 10 4 5 Making a low temperature C V measurement 0 0 e e ee eee 11 4 6 Analysing the C V data 4 2 2 8 e a a RO OO Be a es 12 4 7 DEIS measurement 460 2 2a wee ch PR ae EY ig eee a ER E 12 4 8 Perfoming a C T measurement 2 00 000 ee ee 13 4 9 Analysing the DLTS data ralen d 844 ope REESE EE oA A eee ae 14 410 It time permits 282 oe Pitna tne Grade a a ee eae eee bd ee ed we deh bed 14 1 Introduction Semiconductor devices have revolutionised the world in which we live and have become such a part of our everyday lives that we often take i
5. al Analysis of Semiconductors Deep level transient spectroscopy DLTS will be the principal technique used in this experiment to evaluate defect concentration levels as well their trap energy and capture cross sections By comparing these characteristics as well as the annealing behaviour of the defects to those outlined in literature it is possible to identify the species of defects present in the samples DLTS uses the fact that the energy levels of the deep level traps are affected by the energy band bending at the interface between the semiconductor sample and a metal contact This metal semiconductor interface forms a Schottky barrier diode By varying the extent of the band bending by applied biases traps can be filled and emptied This has an affect on the capacitance of the diode which can be measured and the signal analysed to evaluate the defect concentration and to characterise the defects present To understand how this is possible it is necessary first to describe the Schottky diode 3 1 Schottky Barrier Diodes Schottky diodes can be fabricated on doped semiconductor surfaces to facilitate electrical characterisation of the sample A Schottky diode is a metal semiconductor interface that exhibits current rectifying properties It is similar to a p n junction diode except that the Schottky diode characteristics only depend on the majority carrier electrons 4 whereas the p n diode characteristics depend on both majority and minority
6. as been calculated in table 2 Set TC so that TC is as close as possible for each correlator according to table 2 The hardware is now setup and the rest of the experiment is controlled by the software Change the front panel Experiment to DLTS and enter all the hardware settings into the window The Initial Temp should be set to 78 K and the Final Temp to room temperature The stray capacitance is irrelevant and the Offset should be set to measure at start The offset measurement is important because it removes any offsets from the correlator readings so that if there are no defects hence no change in capacitance the signal will be zero Change Experiment to DLTS on SULA software front panel Enter all the hardware settings into the window Set The Initial Temp should be set to 78 K and the Final Temp to room temperature should be set to measure at start Once you have completed this you can click Run experiment and the scan will take approximately 1 2 hours to complete Perfoming a C T measurement You will need to know C at various temperatures to determine the defect concentrations Eq 12 You can do this on a case by case basis or perform a scan over the whole temperature range used in DLTS 13 Correlator 1 or 2 Period ms Initial Delay ms 200 200 T 3 7 13 2 4 7 13 2 4 7 eat 1 2 3 1 1 2 3 1 1 1 2 ow H Q oO Ay AWWW A RWWA KB wl OD io
7. as many traps as possible The pulse width affects how many of the defects will be filled during the pulse since they have a characteristic capture cross section and hence capture rate For the defects in ion implanted silicon this needs to be between 10 and 100 ms The electronics does not monitor this and the CRO needs to be used Although a larger pulse fills more defects it also means that it will take longer for the electronics to recover after a pulse The filling of defects also does not scale linearly with pulse width but has an inverse exponential behaviour so it is often not necessary to use a long pulse 12 10 11 12 13 14 15 16 17 18 19 20 21 22 Offset 23 4 8 Set pulse Width and the range knob below it to between 10 and 100 ms as read seen in the applied bias on the CRO The Initial delay t1 of the correlators need to be set to examine the capacitance transient over the correct rate windows There are four correlators so we can simultaneously look at four different rate windows The correlators work with a constant r of 4 3 i e t2 4 3xt For ion implanted silicon we need values of t lt 5 ms Correlator 1 should have the largest initial delay with the next correlator having consecutively smaller initial delays The Y outputs of the correlators should now also be connected to the corresponding channels of the NI BNC adaptor Channels 1 2 3 4 Set Initial delay for each
8. carriers This discussion will focus on the characteristics of a n type semiconductor Schottky diode You can get the charac teristics of the p type semiconductor Schottky diode by simple err extension The rectification is a result of the Schottky barrier formed at the interface due to the different work functions of the metal m and semiconductor s This barrier is characterised by a barrier height see fig 1 Figure 1 The bending of the energy bands for a n type semiconductor Schottky barrier diode 6 In general the work function of a metal m is different to a semiconductor and when the two materials come into electrical contact with each other electrons will from the material with the larger Fermi level or smaller work function to that with a lower Fermi level The work function of a material describes the binding energy of the electrons and is related to their Fermi level see fig 1 When the metal and semiconductor come into contact it becomes energetically favourable for electrons in the material with the higher Fermi level to diffuse across to the other material This builds up a net charge difference over the interface which creates a built in voltage Vp The region in the semiconductor which will now have a net charge is the depletion region and characterised by a width w The width of the depletion region can be varied by an applied field V and is temperature T dependent Due to the band bending there
9. chottky contact The silver paint is there to hold the sample in place The quartz is an insulator to stop current from flowing between the probe contact and the backing plate bypassing the sample Question 4 1 What property of In or Ga makes InGa a suitable Ohmic contact What about the Ag The sample is electrically isolated from the chamber in order to minimise stray capacitance For the same reason the coaxial cables connecting the sample to the electronics are kept as short as possible The sample needs to be cooled in order to minimise the thermally generated currents which act as noise in the measurement We need precise and accurate control of the temperature in order to do some of the funky temp dependent stuff later For this reason the stage is fitted with a heater and two thermocouples electric thermometers Thermocouple A is used for temperature control and is situated at the base of the Cu cold head in the chamber near the heater Thermocouple B is situated at the top of the cold head near the sample and is taken as the measured sample temperature The cooling is provided by drawing liquid nitrogen up the tube at the bottom and through a tube around the base of and in intimate thermal contact with the cold head and out the side through the transparent plastic tube This is acheived by pumping on the transparent plastic tube with a diaphragm pump the same as used for fish tanks Note that liquid nitrogen is in fact very diffi
10. cult to pump and in fact the pump doesn t actually pump on liquid nitrogen as it evaporates before reaching it This diaphragm N2 pump is controlled by a variable voltage source variac this one is red and lives under the electronics 70 V is good for cooling down to 77K takes about 15 mins 20 V will keep it there Beware of liquid nitrogen it is cold It also displaces air upon evaporation so if you keep the doors closed and fill the room with nitrogen you may faint or suffocate Ensure that you have read the safety manual before proceeding don t mess with the liquid nitrogen and keep the doors open The sample chamber is also evacuated using a rotary roughing pump It is kept under vacuum for a couple of reasons It keeps out any moisture oils and dust in the air which may contaminate the sample and sample chamber Most importantly it is needed for thermal isolation of the cold head so that lower temperatures can be maintained The pump is connected to the chamber via a power off venting valve which closes off the port to the tee piece hence chamber and vents the roughing line the line connected to the pump when the pump is switched off This is to prevent suck back of oil from the pump when it is off 4 4 Making a room temperature C V measurement The C V measurement will allow us to determine the barrier height depletion width and free electron density of the sample Equipment e Air Liquide LN2 cryostat dewar rotary r
11. is a region in the semiconductor that has been emptied of charge carriers and has a net charge This is the depletion region characterised by a width w as seen in figure 1 The depletion region width is given by Where is the dielectric constant of the semiconductor n the free electron concentration in the semiconductor Vo is the built in voltage and V the externally applied bias as shown in figure 1 The capacitance of the Schottky diode can be determined by considering the depletion region as a dielectric of width w separating the metal contact of area A parallel to the edge of the depletion region in the semiconductor cA C 2 2 Substituting equation 1 into 2 we have OEA hee eon 3 2 Vai 7 V Armed with this expression we can measure many properties of the semiconductor by slapping a metal layer on top of the sample thus forming a Schottky diode and measuring how the capacitance varies with the applied bias This is the nub of the DLTS measurement 3 2 Capacitance Voltage C V Measurements By varying the applied voltage while measuring the capacitance we can determine the built in voltage of the diode and the concentration of free electrons in the semiconductor which will be important for discussion of the DLTS results later on Exercise 3 1 Rearrange equation 3 to express a as a function of V Question 3 1 How can we now calculate n and Vy from the measured capacitance and the applied bia
12. n nr t nr t 0 e7en 4 Now the capacitance is affected by the trapped charge and the decrease in trapped charge causes an increase in capacitance This is shown in figure 4 and will be explained in the next section The emission rate e depends on temperature T the trap energy level Er and the capture cross section of the trap Oc viz Ec E En en T Er 0c Yn Fc Te B T 5 Yn is a set of constants given by yn 2V3M 20 k m h 6 Where Me is the number of minima in the conduction band of the semiconductor and m is the effective electron mass in the conduction band 1 Since the trap energy level and the capture cross section characterise different defects the emission rate en may be different for each defect 3 3 1 The capacitance transient Figure 4 shows how the capacitance of the Schottky diode changes over time as a result of the traps emptying This capacitance transient can be expressed as Gee 7 n For the case where np lt lt n we have ey _ nr t C Co 1 om 8 C C V V C 0 AC t4 t2 Figure 4 The time variation of capacitance as traps empty 6 With DLTS we monitor the change in capacitance over some time interval measurement or rate window t1 t2 Where t is the initial delay after the pulse The change in capacitance over the rate window is C C ti C t2 9 We divide this by the final capacitance Co to form our DLTS signal S which can be written a
13. nd of range of the uniform free electron concentration region identified in the previous section 1 0 1 2 microns wide between 6 and 10 V Monitor the voltage with the V setting on the Capacitance meter Since the bias output of the pulse generator should also be connected to the CRO you should also be able to observe this on the CRO To ensure proper operation check the leakage current by pressing the i setting on the Capacitance meter The reading is in uA if it is more than 10 uA reduce the reverse bias 2 Set Offset on the Pulse generator to between 6 and 10 V as read on the V setting on the Capacitance meter 3 Check the leakage current by pressing the i setting on the Capacitance meter reading is is uA Reduce bias if reading is more than 10 uA reduce the reverse bias 4 Next turn on the pulse generator with the On Off toggle switch Press the _ _ button on the Capacitance meter The display now indicates the voltage at which the bias is pulsed to To examine as much of the sample as possible set the pulse Amplitude on the pulse generator so that it pulses to 0 V You should be able to now see the pulse on the CRO If not ensure that the CRO is set to the DC mode and external trigger If the trigger is set to normal you may need to adjust the trigger level 5 Set Amplitude on the Pulse generator to 0 V as read on the _ _ setting on the Capacitance meter 6 The pulse Width needs to be set to fill
14. nity to gain a deeper understanding of semiconductor diodes and the band structure description of semiconductors From the analysis of the results you will be able to determine the nature and extent of radiation damage that the silicon chip has been exposed to In understanding the techniques applied you will gain a deeper understanding of semiconductor diodes and the band structure description of semiconductors 2 Elementary Semiconductor Physics The sections that follow assume a working knowledge of elementary semiconductor physics in particular band diagrams and doping Some of you will be comfortable with these concepts because you have met them in other courses but others may me meeting them for the first time A good introduction is given in Streetman a photocopy of which can be found in the lab Also feel free to ask your demonstrators any questions no matter how dumb they sound either the questions or the demonstrators Here is a list of jargon words the meaning of which you need to know in order to understand the rest of the prac notes e Semiconductor e n type e p type e Work Function e Fermi Level e Capacitance e Effective mass e Diode e Rectify e Energy Band Conduction band valence band e Band gap e Band Bending e Dopant e Depletion region e Energy Level e Deep level e Majority carrier Minority carrier Carrier concentration e Trap Capture cross section Defect 3 Background Electric
15. nted or irradiated Si sample If time permits do one of the following Examine samples implanted irradiated under different conditions Perform quasi isothermal DLTS with different biases and analyse data to determine depth profile of the defect Involves more complicated calculations The equipment consists of three bunches of stuff the racks of electronics the computer stuff and the cryostat Most of the components aren t cheap off the shelf items so treat them with respect Furthermore the DLTS system is also used for reseach by the MARC COE in the school If you re curious about the research work talk to your demonstrator 4 1 The racks of electronics The box at the top sitting on the PC is the temperature controller This is primarily under the remote con trol of the PC and software Do not play with it if there is an error message or other problem consult your demonstrator Next comes the digital CRO for monitoring the appled bias on the sample Then there is the SULA DLTS electronics Further information on these units can be found in the SULA DLTS user manual Underneat this is a rack of BNC connectors Computers don t come with BNC sockets so this rack is purely to interface the PC with all the electronics The last tray at the bottom is the variac for the diaphragm pump for adjusting the pump speed and the thermocouple vacuum gauge for the roughing line pressure see section 4 3 4 2 The computer
16. oughing pump N2 diaphram pump temp controller e National instruments BNC adaptor DAC SULA Labview software e SULA Pulse generator and capacitance meter units The bias is applied to the top diode contact of sample via the output BNC connection on the Pulse generator unit Labelled as _ _ and the capacitance is read from the back contact which is connected to the In of the Capacitance meter unit 1 Connect the Pulse generator output to the Probe A coax of the cryostat 2 Connect the Back contact to the In of the Capacitance meter 3 The black buttons on the Capacitance meter control what is displayed on the LED C shows the capacitance divided by whatever Crange is set to e g Crange is 300 pF and the reading is 0 50 then the capacitance is 150 pF The reading should always be positive and less than about 1 3 if it isn t contact your demonstrator or change the Crange 4 Ask your demonstrator to load a sample into the cryostat chamber and show you how the sample is connected inside the chamber and how the temperature is controlled and measured inside the chamber 10 10 11 12 13 14 15 16 17 18 19 20 21 4 5 To maintain a stable temperature for measurement the cryostat is simultaneously heated and cooled with liquid nitrogen Before you proceed you need to evacuate the chamber to 300 400 mTorr with the roughing pump to remove as much air and in particular any water vapour in the ai
17. r as possible Open the chamber valve close the venting valve Turn on the Vac Gauge via the bottom power board Turn on the Roughing pump via the power board Once it has reached the operating pressure switch it off but leave the Vac Gauge on and make sure it reaches the blue line true atm pressure If it doesn t contact your demonstrator Typically the output of the pulse generator is tee d off to the CRO so that the applied bias can be observed The applied DC bias can be controlled in two ways with the Offset knob and via an input into the Ext Bias connection The software will use the digital to analogue converter DAC in the PC to apply a DC analogue bias to the sample via this input from DACOUTO of the BNC adaptor Connect the Ext bias to the DACOUT0 BNC of the NI BNC adaptor Set Offset bias on the Pulse generator to 0 This completes the hardware setup and the rest of the setup is via the SULA software For starters you will perform a C V measurement at room temperature biasing the diode from 0 V to 10 V An interval of around 0 2 V will be needed for a nice resolution data set for analysis To correct for stray background capacitance from the wires and sample chamber the software needs to subtract this off the raw capacitance reading this has been measured for various capacitance range settings and appears in table 1 Flip the Measurement type to C V Set the Initial temp to something aro
18. s g C j 5 nT Pas Sa 2n Py Wy where wo and wy are the widths of the depletion region under zero and reverse bias respectively The bias voltage is pulsed to the higher Vo value for the filling pulse time tp to fill the traps again and the cycle is repeated This allows many readings to be taken to average the signal over Exercise 3 3 Prove the above equation Eq 10 Hard Exercise 3 4 Write an expression for the concentration of filled traps at the end of the measurement window in terms of t and t2 3 3 2 Variation in DLTS signal with temperature Figure 5 shows how the signal changes as a function of temperature when a single trap is present This occurs due to the temperature dependence of the emission rate and it is the plot of the DLTS signal as a function of temperature that forms a DLTS spectrum From equation 10 it is in fact the emission rate that governs what temperature the signal is largest at Since this depends on the trap energy Eq 5 it is hence possible to seperate the signals from different traps in the band gap Exercise 3 5 Differentiate equation 10 with respect to T to obtain an expression for the peak signal temperature Hard Capacitance Transients at Various Temperatures Temperature Time C C t C t Figure 5 The temperature dependence of the DLTS signal 6 Note that in equation 10 if the trap concentration nr is zero then the signal is also zero This is to say tha
19. s Write an expression for them N B n may not be constant and may vary with depth hence V through the sample 3 3 Deep Level Transient Spectroscopy DLTS DLTS uses a changing bias to fill and empty charge traps to examine the traps over a given depth in the semiconductor Figure 2 shows an applied bias pulse cycle required for DLTS Figure 3 shows the effect of changing the bias on the trap population in the sample Note also how the depletion region changes b Figure 3 Deep level traps fill and empty depending on the applied bias 6 When the bias voltage is pulsed to the higher Vo value for some filling pulse time tp the traps in regions II and III are exponentially filled with electrons from the dopants or conduction band Exercise 3 2 Write an expression for the concentration of filled traps in region II after the pulse nr t 0 in terms of tp and a capture rate te Assume all the traps are empty before the pulse The sample however spends most of it s time under a lower bias V stead state reverse bias Fig 3b where the traps in region II begin to empty as the band bending makes it energetically favourable for the electrons to spill over into the conduction band of the semiconductor We call this electron emission from the traps The concentration of filled traps in region II after the end of the pulse undergoes an exponential decay that depends on the concentration of filled traps n7 t 0 and the emission rate e
20. strator Since the pump is quite loud you may wish to leave the room at this stage Once you have reached the initial temperature reduce the diaphram pump voltage to 10 20 V and make sure that the temperature controller can maintain a stable temperature within 0 5 K Now repeat the C V measurement 4 6 Analysing the C V data Calculate the free electron concentration depth profile You will need to re examine your answers to exercise 3 1 and question 3 2 and convert the voltage to a corresponding depletion width depth Your demonstrator will show you how to use genplot to calculate and plot the profile from your equations From your profile determine a range over which the concentration is relatively constant Use this range to perform DLTS over i e Choose a reverse bias and pulse bias to vary the depletion width over this range 4 7 DLTS measurement Equipment e Air Liquide LN2 cryostat dewar rotary roughing pump N2 diaphram pump temp controller e National instruments BNC adaptor DAC SULA Labview software e SULA Pulse generator capacitance meter 4 correlator units Again the chamber should still be under vacuum and inserted into the dewar If not make it so 1 Setting up the filling pulse and correlators The electronics can now be setup for a DLTS measurement The first thing to do is set the Offset on the Pulse generator to place the diode under reverse bias This should be set so that the depletion region is at the e
21. t the magnitude of the DLTS signal is also proportional to the concentration of defects present nr and can be expressed as ic Co np l r 11 imax 2N rr Where r 2 We can then write for the trap concentration 12 It is also important to know the concentration of electrically active dopants in the same region that is being probed to determine the defect concentration in that region This is determined with the C V measurements and is outlined in that section By taking DLTS signals at various rate windows we may obtain a range of values for the peak temperature associated with each defect Using these values and noting that at the peak temperature the emission rate is given by 13 Exercise 3 6 Substitute the above expression Eq 13 into equation 5 to obtain In lt in terms of If we now take the temperature at each of the DLTS peaks and the associated emission rate as given by equation 13 we have a method of determining the trap energy level and capture cross section of the defect This is achieved by producing an Arrhenius plot of ln 4 vs The trap energy level can then be found from the slope and the capture cross section from the intercept 4 Experimental Work A rough experimental outline is as follows e Do C V measurements on a n Si Schottky diode at room temp and 79K Analyse data to determine free electron concentration and barrier height Do DLTS and C T measurements on a ion impla
22. t for granted that they can and will perform the task that they were designed for However due to their high quality the single crystal semiconductors wafers used are in fact extremely sensitive to defects created arising from contamination from impurities and bombardment from high energy ions In this experiment you will examine how the static and transient capacitance of a metal semiconductor diode Schottky diode varies with applied bias temperature and time using Capacitance Voltage C V measurements and Deep Level Transient Spectroscopy DLTS These techniques are routinely used for wafer characterisation by the semiconductor industry DLTS is one of the most sensitive techniques available with the ability to measure defect concentrations down to 1 defect per 101 silicon atoms Wow Analysing semiconductors eg Silicon wafers using Deep Level Transient Spectroscopy DLTS reveals vital information about the nature and effect of defects present in the semiconductor DLTS is one of the few techniques that probes the traps in the band gap introduced by ion implantation of dopants The experiment involves analysing a Silicon wafer which has been irradiated or implanted with a low dose of ions and characterising the resulting defects The sample is cooled with liquid nitrogen a metal contact is deposited to form a Schottky diode and the capacitance measured under a variety of voltage pulses and temperatures Students will have the opportu
23. und room temp turn on the diaphragm pump and set the voltage to around 10 V click Idle at initial temp to set the temperature controller Set Initial bias to 0 Final bias to 10 The Stray capacitance is given in table 1 Set the Background capacitance to Use current value The Crange should be set to be the same as the capacitance meter setting The Preamp gain is irrelevant I think Click Run experiment to star the measurement N B The measurement will not start until the temper ature of thermocouple A is at the setpoint Crange pF Background Capacitance pF 100 5 40 300 5 26 1000 5 09 Table 1 Table of Stray Background Capacitances for Cu mounting plate and AirLiquide cyrostat Making a low temperature C V measurement The chamber should still be under vacuum and inserted into the dewar at this stage If not make it so It is important to ensure the chamber is at the correct low vacuum otherwise water vapour will condense onto the sample and the cold head The chamber valve should be closed and roughing pump turned off to reduce any electrical noise and vibrations Set the Initial Temp to around 79 K now and click Idle at Initial temp The Spoint on the temp controller 11 should now be at this value Turn on the power for the N2 diaphram pump and increase the voltage to 70 V The sample should now be cooling and will take approximately 15 minutes to reach 78 K If it doesn t contact your demon
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