Deep-Level Transient Spectroscopy
Contents
1. preferably the ground 2 3 Ventilation The room in which the dewar is stored and experiments are carried out must be kept well ventilated and the door open when people are inside This is to ensure that the oxygen content of the room does not drop below 18 due to ANY spill including the worst case scenario where the entire contents of the dewar evaporates 2 4 Personal protective equipment The following safety equipment is required when there is exposed liquid nitrogen present e g when the cap or cryostat is not inserted into the dewar during refilling Gloves Appropriate gloves should be worn when handling the dewar Face and eye protection A full face shield conforming to AS 1337 must be worn Clothing Loose dry clothing that completely covers the arms and legs must be worn Trouser legs preferably should not have cuffs that may trap liquid nitrogen Footwear Shoes that completely cover the feet must be worn Matthew D H Lay July 2004 Borrowed from EHS Manual Safe Work Procedure Liquid nitrogen School of Botany 3 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 it 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 impuriti2. THE UNIVERSITY OF MELBOURNE Deep Level Transient Spectroscopy Part 3 Laboratories March 23 2005 Contents Some words about third year labs Safe handling of liquid nitrogen 2 1 General safety information lt lt 4 44 ee ee 2 2 Vocations PGS A RAC AE dv ee wee Koko kot oa e Ek ete ee A ee ee U E 29 Wentilation 4 3 438284 eth ei A ale oh 2 4 Personal protective equipment sooo e ee Introduction Elementary Semiconductor Physics Background Electrical analysis of semiconductors 5 1 Schottky barrier diodes of ates a e o phe Gk dn AAA a Wa Ake 4 5 2 Deep Level Transient Spectroscopy DLTS 0 2 0 0 eee eee 5 2 1 The capacitance transient and DLTS signal 2 2 2 lt 44 5 2 2 Calculating trap energy levels and capture cross sections lt lt lt lt lt 5 2 3 Calculating trap concentrations lt lt lt lt 44 444 a 5 3 Deep level traps in implanted silicon lt lt lt lt 444 Experimental Work 6 1 The racks of electronics 4 is o ea koky ea ddd dd RR we See A venda An dl 8 u 6 2 The computer stuf nd it bee a ge A A eae a pe ba Y 6 3 The cryostat and sample holder lt lt lt lt lt 44 e 6 4 Making a room temperature C V measurement 6 5 Making a low temperature C V measurement lt lt lt lt lt 44 4 ee 6 6 Analysing the CV data 04 444 alive eee ee ly Je Je dk be bE ee eed AA e
3. 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 on the 1 V div scale When checking the noise in the pre amp gain signal capacitance transient use the 1 V division scale on the CRO 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 TC which should be less than 12 ms and as close as possible for each correlator TCtot depends on many factors and has been calculated for you Refer to the table on the wall of the lab The TC ot value should be kept less than 12 seconds Set TC so that TC or is as close as possible for each correlator according to the table on the wall in the lab 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 mode should be on Step The stray capacitance is irrelevant and th
4. of the Capacitance meter unit EVACUATING THE SAMPLE CHAMBER 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 capac itance 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 14 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 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 air as possible Open the chamber valve close the venting valve being careful not to overtighten the valve as this will damage the bellows inside Disconnect the tube connecting the venting valve to the dewar this allows the pressure build up in the dewar to escape Turn on the Vac Gauge via the bottom power board Turn on the Roughing pump via the power board and note down in the log book how long it takes to reach the operating pressure below 400 mTorr Then switch it off but leave the
5. 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 C V and DLTS measurements Question 3 From Equation 3 how can we calculate n and Vyp from measured capacitances and applied biases Check your answer with your demonstrator as you will need to use this later Question 4 The calculated value for n at a given bias V can be shown to be equal to the uncompensated donor concentration at the edge of the depletion region corresponding the applied bias V 10 From your answer to question 5 how is it possible to calculate the uncompensated donor concentration n as a function of depth Check your answer with your demonstrator as you will need to use this later Note that although the native oxide on the surface silicon is etched off some of it may grow back when the samples are being transferred in air into the metal deposition chamber Electrons readily tunnel through this thin oxide layer but there is a small contribution to the barrier height 5 2 Deep Level Transient Spectroscopy DLTS Figure 2 Required pulse cycle for DLTS 12 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
6. Ed Page 10 10 E H Rhoderick and R H Williams Metal Semiconductor Contacts Oxford University Press 1998 2nd Ed Page 159 11 E H Rhoderick and R H Williams Metal Semiconductor Contacts Oxford University Press 1998 2nd Ed Page 168 12 D K Schroder Semiconductor Material and Device Characterization Wiley Inter Science Acknowledgements These notes were written by Matthew Lay in 2004 and tweaked by David Hoxley in the same year Samples were prepared and mounted by Matthew Lay with wire bonding done by Sean Hearne 20
7. 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 6 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 12 Then there is the SULA DLTS electronics Further information on these units can be found in the SULA DLTS user manual Underneath 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 6 3 6 2 The computer 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 wha
8. Vac Gauge on and make sure it reaches the blue line true atm pressure If it doesn t contact your demonstrator write down the final chamber pressure and how long it took to pump down in the user log book 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 DACOUTO BNC of the NI BNC adaptor For starters perform a C V measurement at room temperature biasing the diode over as large a range as possible Click on i on the Capacitance meter and adjust the offset DC bias applied to the diode to determine a voltage range where i lt 5 wA Read the bias by pressing V Perform C V measurements over this voltage range instead of 0 to 10 V Set Offset bias on the Pulse generator to 0 Flip the Experiment switch to C V Record the pumping time and operating voltage in the user log Set the initial bias and Final bias to the values you determined above This completes the hardware setup and the rest of the setup is via the SULA software A voltage interval of around 0 2 V will be needed for a nice resolution data set for analysis To correct for stray ba
9. close to zero This is because will find that the active dopant concentration is not uniform over depth Furthermore you should not calculate values for each data point and it is much more accurate to perform a linear fit Exercise 5 To determine the built in voltage Ve you should only fit the data over biases close to zero This is because the free carrier concentration as you will see is not uniform over depth and there is more leakage current at larger biases making the capcacitance measurements less accurate Do not calculate Vy for each data point this is not an accurate method The SULA labview program can be used to calculate the built in voltage but you also be able to calculate this yourself using excel Compare your answer to what the SULA program gives you Exercise 6 Calculate the barrier height which can be expressed as P Voi Ef 14 16 Where Eg is the position of the Fermi level relative to the conduction band and is given by ne is the Density of states in the conduction band of silicon kyT Ne 2 x 10 2mm 75 ie 16 N B Your answer should be expressed in eV and greater than the built in voltage but less than the band gap of silicon Now that you have the built in voltage and the barrier height sketch a labeled band diagram for the diode Exercise 7 Calculate the active dopant concentration as a function of depth for both of your scans You will need to re examine your answers to exercise 5 and ques
10. peak temperature the emission rate is given by your answer to Equation 11 it is possibe to calculate both the trap energy level and the capture cross section Question 5 Re arrange Equation 5 to obtain In 4 in terms of Since the temperature at which the DLTS signal peaks changes with the emission rate window we can generate a plot of the above equation for various temperatures and emission rates A plot of a temperature dependant rate against is known as an Arhhenius plot How can this be used to calculate the trap energy and capture cross section Again check with your demonstrator as this will be important later in the DLTS analysis 5 2 3 Calculating trap concentrations Note that in equation 10 if the trap concentration ny is zero then the signal is also zero This is to say that the magnitude of the DLTS signal is also proportional to the concentration of defects present nr and can be expressed as C telat 12 Co fre 2n r A Where r i We can then write for the trap concentration 10 y C pra NT n di Co re 1 r 13 Note that the trap concentration is dependant on electrically active dopant concentration in the region that is being probed This is determined with C V measurements and is outlined in that section Exercise 4 Derive equation 12 5 3 Deep level traps in implanted silicon You will also find a good introduction to ion implantation in Streetman 2 It has been well established tha
11. 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 5 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 9 whereas the p n diode characteristics depend on both majority and minority carriers The diodes fabricated for this lab were first cleaned with solvents to remove any organic contaminates on the surfa
12. C group Part 3 laboratory apparatus and is thus also used for research The research of the MARC group and perhaps 11 Defect type Trap Energy Capture cross section ev cm H related H related HC VO y 7 4 x107 CC 8x10718 V related 3 5x10 717 V 5 4 x10716 V20 V30 VO H other H related 0 3x10715 V related 5x10716 H related 1x10017 H related y VP 5 3 x7 15 5 3 x7 15 1x10717 V2 related H related Unknown Table 1 Defect characteristics as published in the literature for defects relevant to ion implanted n type silicon even one of your demonstrators depends on the correct functionality of the eguipment so it is especially important to treat it with respect A rough experimental outline is as follows e Do C V measurements on a n Si Schottky diode at room temp and 79K e Analyse data to determine active dopant concentration and barrier height e Do DLTS and C T measurements on a ion implanted or irradiated Si sample e 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
13. aces 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 6 4 Making a room temperature C V measurement The C V measurement will allow us to determine the barrier height depletion width and active dopant density of the sample Equipment e Air Liquide LN cryostat dewar rotary roughing pump N 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
14. apacitance 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 difficult 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 77 K takes about 15 mins 20 V will keep it there Beware of liquid nitrogen it is cold It also displ
15. ce degreasing Next the thin native oxide 10 20A that exists on the surface of bare silicon is etched off with HF acid and the sample is promptly transferred to thermal evaporator for metal deposition through an Al mask The evaporation chamber is evacuated to around 5X1079 mbar This discussion will focus on the characteristics of a n type semiconductor Schottky diode You can get the characteristics 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 0 see fig 1 Figure 1 The bending of the energy bands for a n type semiconductor Schottky barrier diode 12 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 bui
16. ckground 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 2 Flip the Experiment type to C V Set the Initial temp to something around room temp turn on the diaphragm pump and set the voltage to around 10 V record the length of time and voltage you have this on in the logbook Click Idle at initial temp to set the temperature controller Set Initial bias and Final bias to the minimum and maximum biases you previously determined The Stray capacitance is given in table 2 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 start the measurement N B The measurement will not start until the temper ature of thermocouple A is at the setpoint 15 Crange pF Background Capacitance pF 100 5 40 300 5 26 1000 5 09 Table 2 Table of Stray Background Capacitances for Cu mounting plate and AirLiquide cyrostat 6 5 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 it will water vapour will condense in the chamber and it will take a long time to pump down to a lo
17. e 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 to 78 K the Final Temp to room temperature and the mode to Step 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 13 You can do this on a case by case basis or perform a scan over the whole temperature range used in DLTS 6 9 Analysing the DLTS data 6 9 1 Calculate the defect concentrations Calculate the defect concentration for each of the defect peaks observed 19 6 9 2 Characterise the defects observed Use the SULA analysis program to determine the peak DLTS signal temperatures for each of the defects You should not rely on the program to calculate F and o however since it will not give you any useful information on how good the linear fit is Therefore use Excel or genplot N B A good fit gives R close to 1 or x close to 0 Important constants Diameter of diode 780 um with an uncertainty of 20 um q 1 60218E 19 elec
18. eated This allows many readings to be taken to average the signal over 5 2 2 Calculating trap energy levels and capture cross sections 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 reaches its peak value Since this depends on the trap energy Eq 5 traps with different energy levels in the band gap will produce a peak at different temperatures Exercise 3 Differentiate Equation 10 with respect to T and show that the emission rate at the temperature that the dits signal is greatest is given by the following equation 11 N B You will NOT NEED to explicitly differentiate en just carry it through Capacitance Transients at Various Temperatures Temperature Time C C t C t Figure 5 The temperature dependence of the DLTS signal 12 Equation 11 shows that the measurement times t and tz set what is known as a rate window for the emission rate It can be shown that different rate windows cause the peak to shift in temperature By taking DLTS spectra at various rate windows we may obtain a range of values for the peak temperature associated with each emission rate and hence each defect Using these values and noting that at the
19. ee bu 6 6 1 Determine the built in voltage of the diode lt lt lt lt lt 4 6 7 DLTS measurement si tai ak n a el db dz d da bad las la EEA ee 6 8 Perfoming a C T measurement lt lt 444 eee ee 6 9 Analysing the DLTS data s 26594 d d 4 a ee a ee ed 6 9 1 Calculate the defect concentrations lt lt lt lt 4 lt 4 4 4 4 eee 6 9 2 Characterise the defects observed lt lt lt lt lt lt lt ee 6 10 time permits y a shade A ee Ae Se Sree biel ee BSS or Ge eS Oe 1 Some words about third year labs These notes don t contain all the information you need they re here to define the terms for you and to give you an idea of the direction you should be taking You ll find that you need to go foraging for more detailed information in other places often in the references and even more often in the rubble of your demonstrators minds You re going to learn a lot while you re doing that so don t waste the effort share all the work you re doing with us in your report Remember that your report has to be self contained It doesn t have to be beautiful but it should be clear and detailed Perhaps the most important thing to remember is that when you write your report you re trying to teach your reader what you did why you did it and what you learned That way we get to learn something too Enjoy By the way these notes have recenlty undergone a major revision and may be litte
20. es 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 10 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 opportunity 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 t
21. he capacitance transient over the correct rate windows There are four correlators so we can simultaneously look at four different rate windows The correlators work so that the rate window is equal to 4 3xt1 i e t2 5 3xt1 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 correlator to be less than 5 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 Chl 2 3 4 The Period of pulse repetition needs to be carefully set to ensure the electronics have enough time to recover Follow the rules 18 12 13 14 15 16 17 18 19 20 21 22 23 Offset 24 6 8 e Period gt Width 10 Initial Delay on all correlators e Period lt 200 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
22. heir annealing characteristics In the case of ion implanted phosphorus doped silicon the interstitial silicon does not form a trap but the vacan cies can migrate and combine with impurities such as oxygen and carbon found in even top grade commercially available silicon wafers as well as with the phosphorus dopant to form vacancy complexes Chemical cleaning of the silicon can also lead to incorporation of hydrogen which is believed to be the constituent of many defect complexes Defects can be characterised by their position in the band gap of a semiconductor relative to the conduction band trap energy E and their cross section for trapping capture cross section o The values reported for electron traps in ion implanted silicon are found in table 1 This table shows the average value for the trap energies found in the literature Absolute errors in the last digit are shown in parentheses no error is quoted for occasions where there is only one reference for the defect nor is the error quoted for capture cross sections that have an error that is over an order of magnitude difference V indicates vacancy other letters indicate elements subscripts 2 amp 3 indicate the number of vacancies in a cluster and superscripts indicate the charge state of the trap Other subscripts indicate either substitutional or interstitial atomic positions in the lattice 6 Experimental Work The SULA DLTS system and its associated electronics is a joint MAR
23. in the sample Note also how the depletion region changes 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 2 Write an expression for the concentration of filled traps in region II after the pulse nr t 0 in terms of tp a capture rate re and a total trap concentration Nr Assume all the traps are empty before the pulse and don t forget that there is a finite number of traps to be filled b Figure 3 Deep level traps fill and empty depending on the applied bias 12 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 en nr t nr t je et 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 en depends on temperature T the trap energy level Er and the ca
24. lt in voltage Vbi In n type silicon if the work function of the metal is larger than that for silicon we get the situation shown in figure 1 The electrons in silicon drift across to the metal and the region left behind will now have a net positive charge and is the depletion region depleted of majority charge carriers 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 Question 1 In terms of current flow what is the difference between the built in voltage Vi and the barrier height dp Question 2 What happens if we put a metal with a smaller work function on top of n type silicon Due to the band bending there 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 e is the dielectric constant of the semiconductor n the active dopant concentration in the semiconductor Vp 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 Substituting equation 1 into 2 we have eqn 2 Vii 424 V C A 3 Armed with this expression
25. o In understanding the techniques applied you will gain a deeper understanding of semiconductor diodes and the band structure description of semiconductors 4 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 Fermi Level 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 e Minority carrier e Carrier concentration e Trap e Capture cross section e Defect Exercise 1 Write a paragraph explaining each item in the above list Show it to your demonstrator and check that you both agree 5 Background Electrical analysis of semiconductors Deep level transient spectroscopy DLTS will be the principal technique used in this experiment
26. pture cross section of the trap Ge Viz A En Yn Oc T e FET 5 Where y is a set of constants given by Yn 2V3M 21 k2m h 3 6 Where Me is the number of minima in the conduction band 6 for silicon 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 e may be different for each defect 5 2 1 The capacitance transient and DLTS signal 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 nT t C Coy 1 7 For the case where np lt lt n we have t NR nel 8 C C V M C s AC t4 t2 Figure 4 The time variation of capacitance as traps empty 12 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 t1 C t2 9 We divide this by the final capacitance Co to form our DLTS signal S which can be written as C nT en T ta e Co 2n ca S 10 where wo and w 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 rep
27. red with typos and possibly more serious errors We in part3 would love it if you could bring any mistakes to the attention of your demonstrators or email suggestions to part3 physics unimelb edu au 2 Safe handling of liquid nitrogen 2 1 General safety information Liguid nitrogen has two main hazards associated with it frostbite and asphyxiation These are related to its low boiling point 196 C 77 K and its large gas to liquid volume ratio 682 at 1 atm and 15 C Frostbite The eyes are particularly at risk from frostbite cold burns due to their moisture content Liguid nitrogen rapidly boils when it comes in contact with an object at room temperature and will splatter or spit There is a risk of frostbite not only from direct contact but also from indirect contact with a surface that has been cooled by liquid nitrogen Objects less than about 20 C will cause cold burns and if there is sufficient moisture on your skin you may become frozen to the object Asphyxiation When liguid nitrogen evaporates it displaces air and reduces the oxygen concentration in the air Symptoms of asphyxia can be sudden in the case of a deep inhalation of air with a low oxygen content or gradual asphyxia where the symptoms are more subtle For further information please consult the MSDS Material Safety Data Sheets for liguid nitrogen in the safety folder 2 2 Location The dewar must be placed on level surface at all times except during transport
28. t ion implantation introduces many defects into the substrate 3 4 5 In particular Frenkel defects are created which consist of a host atom that has been knocked off its substitutional lattice site into an interstitial position i e between crystal lattice sites leaving a vacancy behind in the lattice 5 At room temperature the vacancy and interstitial can migrate and separate without recombination This occurs for 4 10 of the Frenkel defects created during ion implantation 5 These defects can go on to cluster together or form stable defect complexes with impurities About 10 25 of the Frenkel defects that survive recombination form a di vacancy cluster 6 which is a characteristic defect for ion implanted silicon 3 7 The defects of prime importance for electrical devices are those that are electrically active meaning that they can trap charge carriers in the device Electrically active traps are basically unoccupied states in the band gap of a semiconductor that trap charge carriers This occurs mostly due to defects having dangling bonds which are unpaired electrons and the traps exist at some energy level depending on the structure of the defect Identification of defect structures and atomic constituents is achieved most reliably with Electron Para magnetic Resonance EPR measurements although not every defect complex has an EPR signature The identity of defects which do not show up in EPR are often inferred from detailed analysis of t
29. t 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 6 3 The cryostat and sample holder probe contact Al Wire Bond al Au contact stee Quartz n Si l Ag paint InGa eutectic Ag paint Cu base Figure 6 Sample holder schematic Ask your demonstrator to loas a sample and to show you how the sample is connected and how the temperature is controlled and measured inside the chamber The sample mounting setup is shown in figure 6 The InGa is a eutectic 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 Schottky 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 6 What property of In or Ga makes InGa a suitable Ohmic contact What about the Ag Hint It has nothing to do with resistivity conductivity 13 The sample is electrically isolated from the chamber in order to minimise stray c
30. tinuing 17 6 7 DLTS measurement Equipment e Air Liquide LN cryostat dewar rotary roughing pump Nz 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 Furthermore the sample should be at 78 K If not make it so 10 11 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 end of range you decided in the previous section 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 yA it should not be more than 5 yA Set Offset on the Pulse generator to the value you have determined above as read on the V setting on the Capacitance meter Check the leakage current by pressing the i setting on the Capacitance meter reading is is uA Reduce bias if reading is more than 5 yA reduce the reverse bias Next turn on the pulse generator with the On Off
31. tion 5 and convert the capacitance to a corresponding depletion width depth You will need these results for analysing the DLTS data You can use Excel or genplot for data analysis but use genplot to differentiate x y data sets It will take an ASCII text file that you save with excel and will output it as another data file with a file name that you can specify Step by step 1 First copy diffdat mac to the same directory your data file is in 2 Open the XGenplot program and change to the directory where your data is 3 Then type xeq diffdat mac to execute the differentiation macro Genplot commands you may find useful are pwd lists current directory ls lists contents of directory cd xxx changes to directory xxx cd goes up a directory level read reads your data file show show the data pl plot the data trans dy dx differentiates the data write saves data file to disk fit lin to perform a linear fit lt n to plot with a line type n lt 0 sy n to plot with symbol type n pen n to plot with colour n ov fit to overlay the fit You can now import the file into Excel to calculate the free carrier concentration You should obtain values on the order of 1015 cm Comment on any differences between your scans From your profile determine a depth range over which you want to perform DLTS over and establish what biases are needed for depletion depths over this range Discuss this with your demonstrator before con
32. toggle switch Press the _ button on the Capacitance meter The display now indicates the voltage at which the bias is pulsed to Set the pulse Amplitude on the pulse generator so that the depletion width will be at the start of the region of uniform free carriers 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 Set Amplitude on the Pulse generator to the bias reading as determined avove as read on the J setting on the Capacitance meter The pulse Width needs to be set to fill 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 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 t
33. tron charge C eps0 8 85418E 14 permittivity of free space F cm epss 11 9 Dielectric constant for Si kb 1 38066E 23 Boltzmann s constant J K h 6 62617E 34 Planck s constant J s me0 9 1095E 31 electron rest mass kg me 0 98x0 192 1 3 effective electron mass factor in Si at 300K 6 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 B G Svensson et al Divacancy acceptor levels in ion irradiated silicon Phys Rev B 43 1991 Page 2292 P Hazdra et al Nelr Instrmts and Mthds in Physcs Rsrch B 55 1990 Page 637 ao AeA WwW N S Coffa and F Priolo Electrical properties of ion implanted and electron irradiated c Si in R Hull et al editors Properties of Crystalline Si Inspec EMIS 1999 Page 746 6 B G Svensson and others Generation of vacancy type point defects in single collision cascades during swift ion bombardment of silicon Phys Rev B 55 1997 10 498 7 A Hallen et al Deep level transient spectroscopy analysis of fast ion tracks in silicon J Appl Phys 67 1990 1266 8 E H Rhoderick Metal Semiconductor Contacts Oxford University Press 1978 1st Ed Page 21 9 E H Rhoderick Metal Semiconductor Contacts Oxford University Press 1978 1st
34. w temperature COOLING THE SAMPLE COLD HEAD 1 Ensure that the chamber is within the operating pressure less than 400 mTorr Record the chamber pressure and how long it took to pump down in the user log book 2 The chamber valve should be closed and roughing pump turned off to reduce any electrical noise and vibrations 3 Set the Initial Temp to around 79 K now and click Idle at Initial temp 4 The Spoint on the temp controller should now be at this value 5 Ensure that the venting tube between the venting valve and the dewar collar is disconnected to allow Na build up in the dewar to escape 6 Turn on the power for the Na diaphram pump and increase the voltage to 70 V The sample should now be cooling record how long it takes to cool down in the user log book It should take approximately 15 minutes to reach 79 K If it doesn t contact your demonstrator Since the pump is quite loud you may wish to leave the room at this stage 7 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 8 Record the pumping time and operating voltage of the diaphram pump in the log book 6 6 Analysing the C V data 6 6 1 Determine the built in voltage of the diode You will need to re examine your answers to exercise 3 and question 3 You should only fit the data over biases
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