Completion of the software required for a high-temperature
Contents
1. fi coms xi rti ERE dm Ex Bias Voltage Internal Bias V Save data to file IR CA Users labb Desktop DLTS Project Rasmus Euobb tests Vtest2 bxt 500 b 5 30 00 EE Averaged transient p E a 77 Ex Pulse Voltage Internal Pulse V A 500 7000 ilu Pulse width Gn ms i temp before measurement SV ulse width in ms j v ear suh d 11002 1100 0 ET 5 00E 0 fscans min windowlength in ms td measurement interval s 1 420 d 21 Bec 3000 5 00E 3 0 0E 0 20E 1 y STOP Windows measurements I ha 4 pa il DLTS GS4 increament Temperature precision she E Start temp C Endtemp C scans measuremet jo aio High Resolutid IS de TROR SEAN A 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 20 25 30 35 40 45 50 55 60 65 70 75 uw S Window Display windows WE eax ES 2 36 Mx vw Hl I Update graphs 0 10 20 30 40 50 60 7078 Discrete Time BEA 3 2 1 Controls The controls shown in region A of Figure 3 2 are set before running the VI because when pushing run the program reads the values of the controls and initializes the meas urement with these settings Of course if anything goes wrong the stop butt2. s Functions a Search Programming gt EZ ae Structures Array Cluster Clas d 7 4 gt A gt Numeric Boolean String gt gt gt B k Comparison Timing Dialog amp Use gt gt cz E File YO Waveform Application gt Era h Be J B dy Synchronizat Graphics amp S Report Gener Mathematics Signal Processing Express vov vv Favorites Select a VI Y Figure 2 10 LabVIEW functions palette If a certain desired function does not exist in the palette the user can construct their own sub VI with the function and add it to the palette for easy access When creating larger VI s the use of sub VI s is a great tool for hiding low level functions in the block dia gram thus creating hierarchies with a main VI in the top 2 5 1 VISA GPIB Virtual Instrument Software Architecture VISA is a protocol used for communication between a PC and external hardware It includes serial communication through COM ports and more specific I O interfaces such as IEEE 488 or commonly known as Gen eral Purpose Interface Bus GPIB which was developed by Hewlett Packard for easier interconnection with their instruments In LabVIEW there are sub VI s for both VISA and GPIB protocols It is mostly up to the user which one to choose since VISA in cludes GPIB communication as well compare Figure 2 11 with Figure 2 12 However in National Instruments NI VISA user
3. E 2 So if the total trap concentration is Nr and all traps are filled at time 0 the concentration of filled traps for t gt 0 if re trapping is neglected is given by Eq 2 1 n N yexp S 2 1 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP Since the trapped charge concentration changes with time it will give rise to a time de pendent space charge concentration and therefore a time dependent capacitance This capacitance is referred to as the transient and can be obtained by applying a voltage across the sample DUT as shown in Figure 2 5 Voltage ov Vpulse Vo as Capacitance H H H Cp Steady state Time Figure 2 5 The transient starts after the falling edge of the voltage pulse In a n type semiconductor the approach is to provide enough thermal energy to the sample so that the captured electrons in possible existing traps can get enough energy to be emitted The technique is to apply a reversed bias voltage across the sample in order to empty the traps of all electrons that have received enough thermal energy to be able to be emitted Then a short forward pulse filling pulse is applied of a magnitude that makes it possible for the traps to recapture the electrons The pulse is withdrawn and the bias is again applied At this point the transient is at its beginning and will change over time with proportion to the emission of the trapped electrons
4. easily controlled and thus they may fit in many different applications But no electrical device is flawless For example when a semiconductor is doped in order to get some desired property the semiconductor also obtains some chemical impurities called deep level defects The deep level defects affect the forward and reverse current significantly and increases the noise in transistors Identification of defects and characterisation of their impact on semiconductors properties is a very important issue since they so radic ally can change the electrical transport properties of a semiconductor due to thermal heating for example 1 There is a constant struggle going on to build electronic devices that operate with as small losses as possible The most fundamental and first step in the construction of an electronic device is the choice of material One of the most common semiconducting material is silicon and different compounds of silicon but for more extreme cases for example where electronic devices are exposed to very high temperatures a harder ma terial that can withstand the extreme environment is needed Therefore the interest of using diamond for more extreme electronic applications has increased in the past few years Synthetic diamond has been developed for decades but was not pure enough to be con sidered for electrical applications This however is not the case today where a new method has made it possible to grow diamond with
5. 2 1 2 Col A 3 2 TNC ORY drip 2 1 Atomic amp Band structure in solid state physics eres 4 2 1 1 Valence band amp conduction Band s c eos eR een dede vas tto aes 5 22 rcgno m P 5 De DA Deep level defbete zi uei tec i e hid ipd aE E r fut bI esa a tege xu 6 2 2 2 ST APS NE E TN R R RARE 6 2 3 Deep Level Transient SpectrOoSCOPY ssessssssssesssoosssosssoossoossrrersoesrrerrssnrrrer 6 2 3 1 The capacitance Easter s censeo eit Conan a oa o necis T 2 3 2 Window amp weighting TUDCEOFTS s euer cratere trennt tr e ena een even 8 2 5 5 DELS SIGMA a auti spikas ssd inboe ee t Mibi A E inb bit rd daphne NEAR Dep aes 9 2 4 Analysis of DLTS amp Arrhenius plot e eeeeeee eerte eee eene eerte netten eee tnnun 11 DS LabVIEW ere H M 11 2 b VISAUGPIB hacen en a namie dvd Bis seamen alata aia tutu m pt Mus a dus 13 3 1 Experimental SetUpP ssssssossos0ovsossosovscoosssorsseossoo00sserssoo0s0s000s000000000000000s0000000000 16 3227 Pri P ISITE CATION ere PR R 17 3 2 lC ODfEO So opis Seattle tates sc eei atu RAA Ode dies AU II RU dec 18 32 2 Indie tots asa oet ae ble i edeqi dete ERR VG M EE b RE E EDEN RA is 21 d Resulisz s es esr evtl ease d estote eios iuc 3 5 Discussion amp Analysis eee e eee e eee e eee eee eene eee
6. N is the number of measurements for window i and C C t is the ca pacitance value of the j th measurement point at The number of measurement points for window i is decided by N 2 Mz m SES Cywile 2 3 i J By inserting eq 2 2 in eq 2 3 it can be rewritten as NJ2 M seg 2 C E C i j4NJ2 1 2 4 To obtain the DLTS signal the lock in function is used with the transient as shown in Figure 2 6 The figure only shows one step and will result in one point per window of the DLTS signal In order to get a spectrum this is done for multiple temperatures Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP EDDIE Measurement points C Cp Steady state There are three lock in signals used in Figure 2 6 The windows are using eight meas urement points of the capacitance transient and the first window w locks only two measurement points W locks four and w locks eight With eq 2 4 the DLTS signal s value of w w and w can be calculated as S C C C C Sec Cu CEU E a E The transient in Figure 2 6 will be a value around the peak of the DLTS signal for w as shown in Figure 2 7 where the whole spectrum is illustrated 10 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP DLTS Temperature Figure 2 7 A typical DLTS signal with three windows where the values for each window from the transient in Fi
7. STOP button The content of Figure 3 7 can be found to the right of the transient indicators in the front panel It displays the SV C which is the set value of the temperature controller in de grees Celsius Since the current temperature varied due to a bad PID parameter choice of the temperature controller it was interesting to see what the temperature was before and after the measurement so that an understanding of how well the measurement was made at wanted temperature could be done In the figure the STOP button could also be found which aborts the program and finishes the current measurement at any point 21 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 24 80 1 I 1 1 1 1 1 20 30 40 50 60 70 80 90 100 Figure 3 8 The measured capacitance during the bias voltage as a function of temperature Figure 3 8 displays the steady state capacitance measured with a fixed reverse bias V pjas applied at each temperature Next to this indicator to the right on the front panel is the measured conductance during the bias voltage see Figure 3 2 There are also indicators in the front panel which plots the DLTS signals using lock in and GS4 which is not described in this thesis weighting functions 22 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 4 Results After the basic GPIB communication VI s had been replaced with VISA a successful CV
8. are also SHOWI ssssssrsorssrrssrrssrrssnnt 11 Figure 2 8 Block diagram of a simple VI PrOgr allL sesssrsssrsrrssrrssrrsrrsnrnssrerrnrsrnsrrrnnr sr rr nns 12 Figure 2 9 Front panel of a simple VI progt ni e iere pa tret coe i eee een 12 Figure 2 10 LabVIEW functions palette 13 Figure 2 11 Block diagram of a test using GPIB protocol with an external device 14 Figure 2 12 Block diagram of a test using VISA protocol with an external device 15 Figure 3 1 LabVIEW VI hierarchy of the DLTS software eee 17 Figure 3 2 DLTS software overview of the front panel in LabVIEW 18 Figure 3 3 Controls of the DLTS software enne 19 Figure 3 4 The pulse s parameters Vbias Vpulse and pulse width for point 2 3 and 6 In this thesis Vpulse was chosen to be 5V and Vbias is 5V 20 Figure 3 5 For point 7 amp 8 Lock in function with three windows displaying the min imum window length and relation between the window s lengths The next following window is always twice as large as the current window 20 Figure 3 6 Two transients one with the raw data from the CV meter and the other with the signal processed ata tic end D a o xeEap bp MN E INS dass 21 Figure 3 7 Temperature indicators of the front panel and the STOP button 21 Figure 3 8 The measured capacitance during the bias voltage as a function of temperat UNS s
9. communications stopped working Since no solution was proposed 2 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP to get everything running again the whole project was put aside 1 2 Assignment The main purpose of this thesis was to restore the communications between the com puter program written in LabVIEW and the instruments and to implement the correct communication syntax with the instruments here in the division of electricity at ng str m If this was done within the time schedule further work on DLTS measurement would be possible Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 2 Theory 2 1 Atomic amp Band structure in solid state physics An atom consists of protons neutrons and electrons Each electron occupies different discrete energy levels called orbitals The outermost occupied orbitals contains the valence electrons which are forming bonds with other atoms and thus create molecules or crystals To form a molecule two atoms form a bond with each other by sharing their valence electrons Silicon for example which is a common semiconductor has four valence electrons When studying a silicon crystal in 2D a single atom bonds to four neighbouring silicon atoms by sharing each others valence electrons see Figure 2 1 Figure 2 1 Silicon crystal At 0 degree Kelvin the valence electrons of silicon are in some sense bound to the at
10. deep depending on their position relative the valence and conduction band edge Shallow defects have allowed energy states close to the edges and deep level defects closer to the middle of the bandgap 6 7 2 2 1 Deep level defects Deep level defects are the product of chemical impurities and strongly intervene with the electrical and atomic transport properties of semiconductors 1 They also increase the noise in photodiodes and transistors and reduce the minority carrier lifetime 8 2 2 2 Traps The majority charge carriers in a n type semiconductor are electrons and the minority carriers are holes while in p type semiconductors it is the opposite If a carrier spends a relatively long time at the defect it is said to be trapped A trap captures either electrons or holes it is highly unlikely for it to capture both 7 2 3 Deep Level Transient Spectroscopy Deep Level Transient Spectroscopy DLTS is a method to detect deep level defects in a semiconductor The method was first introduced by D V Lang who obtained a DLTS signal simply by measuring the difference of two capacitance values at two fixed times during the transient 9 This method is called boxcar and is illustrated in Figure 2 4 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP Capacitance transient Temperature t t C t C t Time DLTS signal Figure 2 4 The construction of a DLTS signal from the capacitance tran
11. has reached and is stable at its set point before entering the measurement loops As well as remove the high low resolution choice and other controls that are not in use with the current setup so that no one gets confused about them being in the front panel Or at least set them as a constant in the block diagram in case if any of the instruments being replaced and the controls then would be useful The hardware settings could also be considered for improvement The PID parameters of the temperature controller for example could be better chosen in order to stabilize the regulation and get it more accurate Other things that could be done is to implement more data acquisition functions such as depth profiling and Arrhenius plots in order to analyse the results more easily Even a FAQ or manual could be most appropriate for future work and troubleshooting 26 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP Appendix A C NI 488 C VISA LabVIEW NI 488 LabVIEW VISA Device Function INSTR Operation Device Function INSTR Operation viOpen lt no equivalent gt k ue A VISA Open ibonl viClose lt no equivalent gt viWrite viRead viClear VI n AC ose VISAN B na A 3 ite mus BAS n A zx ead VI n Q A Clear ibconfig viAssertTrigger viReadSTB viWaitOnEvent Wait for GPIB RQS S GPIB Initialization
12. manual they recommend using VISA sub VI s for the entire system when programming multiple devices that communicate over more than one bus type 12 Even Agilent technologies states on their website that it would be better to use the more general VISA protocol instead of GPIB protocol 13 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP A Test using VISA vi Front Panel File Edit View Project Operate Tools Window Help STE c iS Font Box ase BB o s VISA resource name X GPIBO 28 INSTR NCM 0043 42E 12 NGM 0 0034E 3 In Figure 2 11 the standard command IDN is sent to an external device using GPIB protocol in LabVIEW and the response is read as a string shown in the box labelled data The IDN command asks the targeted device for its identity and the response string in this case contains information about the capacitance and conductance 14 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP HI Test using VISA vi Front Panel o 8 x File Edit View Project Operate Tools Window Help gt JT DIN 15pt Application Font Lor i e Search VISA resource name NG ath String GPIBD 28 INSTR 12 errot out GPIB C am read buffer NCM 0048 42E 12 NGM 0 0034E 3 In Figure 2 12 the same command is sent to the same device as in Figure 2 11
13. of the bandgap If a semiconductor is introduced with impurity atoms the concentration of electrons versus electron holes changes This will move the position of Ey and Ec in the energy diagram in Figure 2 3 but the fermi level will be at the same position Thus the fermi level will be closer to a band edge which edge Ec or Ev depends on the doping 4 5 This will of course change the probabil ity of an electron to occupy the fermi level at any given time At T OK in a semiconductor all states in the valence band are occupied and the con duction band is completely empty At higher temperatures some of the electrons in the valence band could get enough thermal energy to be excited into the conduction band thus moving freely within the semiconductor The vacancy in the valence band is called an electron hole and can be regarded as a positively charged carrier 2 2 Defects To control the electronic properties of a semiconductor such as conductivity and con ductivity type n or p type impurity atoms are introduced into the semiconductor material this process is called doping One quite accurate and often used technique is Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP ion implantation by which ions are accelerated in an electric field and impacted into a solid However this method will also cause creation of deep level defects 6 Defects can be classified as shallow or
14. r DLTS m tningar p ngstr ms Laboratorium i Uppsala Uppst ll ningen bestod utav en kapacitansm tare en pulsgenerator en temperaturkontroll samt en PC med integrerat kontrollprogram utvecklat i programmeringsmilj n LabVIEW Problemet bestod i att kontrollprogrammet ej hade blivit uppdaterat enligt det instru ment som anv ndes p ngstr m Programmet var konstruerat utav en forskningsgrupp i Norge som anv nde m tinstrument av andra m rken och d rf r skiljde sig de kontroll kommandon som skulle skickas fr n programmet Uppgiften bestod allts av att integrera de korrekta kommandona f r uppst llningen p ngstr m samt se till att kom munikationen fungerade Key words Semiconductors DLTS LabVIEW VISA GPIB 488 2 Preface This thesis had not been finished without Markus Gabrysch s supervision and guidance nor without Florian Burmeister s support and encouragement during my time with the group I also would like to thank my topic examiner Saman Majdi for his determination to al ways help in all ways possible Supervisor Markus Gabrysch Topic examiner Saman Majdi Examiner Nora Masszi Author Rasmus Jansson ngstr m Laboratory Uppsala Sweden 2013 06 II Contents A DD FO ViAIO ETT E I DMEF GUC UIO Meee ose cok secwens eatestoadzccancseveshewseasccdceoasestsenswsseeek a L3 Baek or Oui ssispscsscccctuscsscsicssoucdsasansspicateocdasicausencanssesaacosansacasesssasosedssecseaauasdoecenezeuss
15. ISRN UTH INGUTB EX E 2013 04 SE Examensarbete 15 hp Juni 2013 UNPIPSALA UNIVERSITET Completion of the software required for a high temperature DLTS setup Rasmus Jansson UNIVERSITET Teknisk naturvetenskaplig fakultet UTH enheten Bes ksadress Angstr mlaboratoriet L gerhyddsv gen 1 Hus 4 Plan 0 Postadress Box 536 751 21 Uppsala Telefon 018 471 30 03 Telefax 018 471 30 00 Hemsida http www teknat uu se student Abstract Completion of the software required for a high temperature DLTS setup Rasmus Jansson The main purpose of this thesis was to examine the communication problems with the DLTS set up in the Division for Electricity at Angstrom Laboratory in Uppsala Sweden and to make the DLTS software complete The set up consisted of a C V meter a pulse generator a temperature controller and a PC with a control program written in LabVIEW It was found that the software had been constructed to fit another set of instruments than the set up currently used at Angstrom Laboratory The task was therefore to properly integrate the correct control commands of those instruments into the software Handledare Markus Gabrysch Amnesgranskare Saman Majdi Examinator Nora Masszi ISRN UTH INGUTB EX E 2013 04 SE Tryckt av Uppsala Universitet Uppsala Sammanfattning a summary in Swedish Halvledare anv nds inom flera omr den i elektrotekniska sammanhang Detta f r att de ras elekt
16. This process is done mul tiple times for each temperature and the transient is averaged to reduce possible noise The process can be associated with that of a simple parallel plate capacitor During the filling pulse the distance between the capacitor s plates decreases and the capacitance increases With the reversed bias one can imagine that the distance between the plates has increased causing the capacitance to decrease But at this point the emission of the trapped electrons takes place and will cause the capacitance to successively increase with time 2 3 2 Window amp weighting functions Windowing is a method used for observation of a mathematical function within a certain time interval The windowed function is obtained by multiplying the function of obser 8 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP vation with a weighting function that has a given shape and amplitude within the inter val but is zero outside In some sense it can be recognized as a filter There are many different types of weighting functions all giving slightly different results of shape and quality of the windowed function One example is the ock in function w t which is basically a rectangular signal and is given by Eq 2 2 I 1 0 lt t lt 2 Y EDIT 2 2 0 otherwise 2 3 3 DLTS signal For discrete time the DLTS signal can be described with eq 2 3 where w t is the window i
17. a small concentration of defects Be cause of this the interest of using diamond for electrical applications has become greater but as well as in any other semiconducting materials there are always some de fects present that need to be characterized 2 In the Division for Electricity at ngstr m Laboratory there is an ongoing project focusing on characterizing deep level defects in synthetic diamond that are situated deep in the bandgap Such deep levels have a long lifetime and might have a big impact on the electrical properties of diamond devices The method that is being used is called deep level transient spectroscopy DLTS It is a sensitive quantitative characterization method for deep level defects 1 1 Background In the Division of Electricity the DLTS setup consisted of a C meter a pulse generator and a temperature controller These instruments were controlled with a computer pro gram which had originally been developed by a Ph D student at KTH and revised by a research group in Norway In Norway they had a similar setup to the one here in the Di vision for Electricity but with some minor differences The C meter was the same but the heating controller and the pulse generator were slightly different hence some changes in the program were necessary The program from Norway was created using GPIB communications with the instru ments see section 2 5 1 concerning GPIB However when the program was moved to a new computer the
18. but in stead with VISA protocol in LabVIEW and the response is exactly the same The conclusion of this is that even though the external devices is connected with GPIB buses to the PC LabVIEW still can handle this with VISA VI s What is expected since VISA contains GPIB communication 15 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 3 Method 3 1 Experimental setup The instruments that were used in the experiments are listed in Table 3 1 Device Model C meter C V plotter HP 4280A Pulse generator HP 8112A Temperature controller Tectra HC 3500 with Yudian AI 518P Programmable artificial intelligent temperature controller The C meter was used to measure the capacitance across the sample the pulse generator controlled the voltage pulses and with the temperature controller the temperature was controlled and measured at the sample The pulse generator and the C meter was con nected together in order to synchronize the measurements so that no pulses was applied across the sample during a capacitance read and no read of the capacitance was made during a voltage pulse In order to control the instruments they were all connected to a PC on which a Lab VIEW program had been constructed to perform DLTS measurements and collect data The DLTS control program had been built by a research group in Norway Since they were using another type of pulse generator and temperatu
19. cimal mark was changed to be a point instead of a comma but just to make sure that every value in LabVIEW used point and not comma the change were also made in the OS 25 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 6 Summary amp conclusions Both NI and Agilent technologies recommend the users of GPIB and 488 protocols to change to using the VISA protocol instead The reason for this is according to Agilent s website that Agilent 488 and NI 488 2 is proprietary libraries Another reason would be that it makes the program much more generalized For example if a device is replaced for one with another brand model and the new uses another type of communication it will be much easier to make the change if the program is written using VISA VI s rather than GPIB Because then the only thing that needs to be changed is the addresses of the ports that are being used and perhaps correct the commands that is sent to the device As of this the main purpose of this thesis to get the communication between LabVIEW and the external instruments to work was achieved and proved with the CV measure ment in Figure 4 1 and the DLTS measurement in Figure 4 2 6 1 Outlook As for the future there are still some work with the DLTS software and making it more user friendly A lot of the things can be updated in order simplify the code and make it more effective As for example wait until the temperature controller
20. e offset meas ured while in steady state the capacitance across the sample with bias voltage applied after the transient has converged and become stable When OFF the offset is set to zero Voltage Voltage min window length Time puse width Figure 3 5 For point 7 amp 8 Lock in function with three windows displaying the minimum window length and relation between the window s lengths The next following window is always twice as large as the current window 20 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 3 2 2 Indicators The indicators of the front panel are located in region C of Figure 3 2 Here follows a description of some of the indicators Transient counts Averaged transient pF MEM W WA VAM m l IT l ui Vu 420 1 0 0E 0 I 21 750 1 i 7 7 1 20E1 0 0E 0 1 0E 1 2 0E 1 3 0E 1 4 0E The left transient of Figure 3 6 consists of raw data called counts It is the measured ca pacitance in binary coded format Together with this data from the CV meter a status byte is sent which contains information of how to convert the capacitance data into val ues represented in Farads In the right transient the raw data has been converted into representation of pF Temperature after measurement ki temp before measurement SV C 100 1 100 0 scans LE STOP Figure 3 7 Temperature indicators of the front panel and the
21. eese sess 2 ST Communications cacense uso o eas doo een aeneo aoa neei eoe Na P sc ovn pend Use o saa ue iue Paus Lean Ness aUe 24 5 2 Yudian COnmtrolAT 510 scssssoses vsisss vess sss sesssadssosssssseesssv dsnsssssssnosssdrsssssosssessses 24 5 3 DTS main VIcrss vsvesses osossevsovsevsoso ssovstoresvousnsvederssdsrvrsdorersssnindsrsensessssesisssssnsdenees 25 6 Summary amp conclusions eeeeeeeee eee eee eee eee e esee 2 0 6 1 gin cc 26 ENQUDIDIE WT RE Referent s PL E E s s L HI List of figures Figure 2 15 STIICON crystal e Code RU t nkare i Rr dde tu i in tet sne rr Rd 4 Figure 2 2 Bonds in Silicon crystal at T 0K and T 0K see 4 Figure 2 3 Band structure for metals semiconductors amp insulators 5 Figure 2 4 The construction of a DLTS signal from the capacitance transients 7 Figure 2 5 The transient starts after the falling edge of the voltage pulse 8 Figure 2 6 Capacitance transient with eight measurement points weighted with three windows of the lock in function where Cj is the capacitance value of the j th point The windows value for this particular transient are represented by the symbols to the right of the windows sse 10 Figure 2 7 A typical DLTS signal with three windows where the values for each win dow from the transient in Figure 2 6
22. ent is made End temp the last temperature at which a measurement is made Sets the increment step of the temperature Sets the number of scans per measurement i e the number of scans per temper ature A measurement at a temperature is averaged over the number of scans in order to reduce noise C range is only active in the program when high resolution 20 is OFF It tells the C meter in what range the capacitance is expected to be measured within If it is chosen too low an overflow will occur and the data will be invalid How ever choosing a high value will reduce the accuracy of the measurement data 19 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 17 C speed specifies if the measurement of the capacitance is to be made in fast mode or slow mode As the names suggests fast mode provides optimum speed but also less accuracy compared to slow mode 18 Specifies the sample mode of how the C meter will perform the data collection of a measurement In BURST mode the 4280A makes one measurement per measurement point In SAMPLE mode however it repeats several partial meas urements at each measurement point 19 This specifies the connection mode between the C meter the DUT and the ex ternal pulse generator i e to which one of the inputs slow or fast the pulse generator s output is connected and how the DUT is grounded 20 With high resolution ON the capacitance is related to a capacitanc
23. er it was discovered that the GPIB card in the PC from Agilent technologies was not compatible by default with National Instruments NI 488 2 protocol But Agilent tech provided a configuration for enabling communication with NI 488 2 through their pro gram Agilent Connection Expert that are included in Agilent IO Libraries Suite 16 3 When enabling this the GPIB VI s in LabVIEW started to work Nevertheless VISA VI s were still chosen to be used due to the recommendations from NI and Agilent and the fact that it would be much easier to replace one of the instrument if the replacement were using another type of communication 5 2 Yudian control AI 518 The next step was to understand the heater and why it did not work in the DLTS pro gram The communication with the Yudian controller was made through a USB to serial adapter and therefore VISA VI s had to be used in LabVIEW The first thing that was added to the existing VI s for Yudian in the DLTS main program was a VISA CLOSE function at the end of each VI s execution to properly close the opened session for that port If the port is not closed it stays open for the task it was opened for and locked for other uses When this was done the communications with Yudian worked quite well However when a value was sent to Yudian to set a parameter the answer could some times be ten times larger or ten times less than the wanted value This was no fault merely an effect of how the Yudian controller was p
24. gure 2 6 are also shown The usage of windows makes it possible to study all different shapes of the transient For faster increasing decreasing transients a narrow window will give rise to a peak of the DLTS signal and for slow transients a wide window is needed 2 4 Analysis of DLTS amp Arrhenius plot Each deep level defect gives rise to a peak in the deep level transient spectrum and the height of the peak is proportional to the defect concentration From the peak it is also possible to determine the energy position in the bandgap and the carrier capture cross section This is done with an Arrhenius plot which will result in a straight line of which the activation energy can be extracted from the slope and the carrier capture cross sec tion from the intersection of the line with the y axis 11 For further research in this subject see chapter 4 of Capacitance Transient Measurements on Point Defects in Sil icon and Silicon Carbide by H Kortegaard Nielsen 2 5 LabVIEW LabVIEW Laboratory Virtual Instrument Engineering Workbench is a graphical pro gramming environment from National Instruments NI It is a tool for easy communication with and development of measurement and control systems for ex ternal hardware The user creates so called Virtual Instrument VT blocks to obtain the wanted functionality LabVIEW is created so that a VI consists of a block diagram which contains the source code and a front panel which basicall
25. measurement was made and is shown in Figure 4 1 The measurement was made by sweeping a reverse voltage from OV to 10V and meas ure the capacitance of the sample at room temperature When applying a reverse voltage across the sample the depletion region was expanded and the capacitance decreased with proportion to the decreasing voltage as seen in Figure 4 1 At the final step of this thesis a fully successful DLTS measurement was made using six lock in windows at 25 C to 100 C and is shown in Figure 4 2 DLTS lock in I I I I I I I I I I I 50 55 60 65 70 75 80 85 90 95 100 23 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 5 Discussion amp Analysis 5 1 Communications The first problem to solve was to understand why the communication with the devices did not work The only time they gave a response was when using NI MAX s VISA test panel which allowed the user to send simple commands to the connected device in or der to see if it responded Since the VISA test panel used standard VISA commands to write and read to the device the problem seemed to be the GPIB VI s in the LabVIEW programs All the GPIB VI s were then replaced with VISA VI s instead according to Table A 1 in Appendix A and the connected devices now gave responses even in Lab VIEW A successful CV measurement was then made at room temperature in order to see that everything worked properly with the C meter Lat
26. oa ashe tea eat ute utendo tameu wepiar gaa VASS Tok sees 22 Figure 4 1 CV measurement capacitance in mF on y axis and voltage in V on x axis 23 Figure 4 2 DLTS signal using 6 different lock in windows Temperature on the x axis and DLTS signal on y axis Quies detu seitdem tena da theaeedaweceadereads 23 List of tables Table 3 1 Instrument SET aoctor eR Mese DA aon fd attt ntes 16 Table A 1 NI 488 2 VISA edumvalents s i eade t de t elem bred io tn alas 21 IV Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP Abbreviations C The capacitance of the j th measurement point F DLTS Deep level transient spectroscopy DUT Device Under Test Ec Energy level of the bottom of the conduction band referred to as the conduction band in the text eV E Forbidden energy range between the Ec and Ey called bandgap eV Ey Energy level of the top of the valence band referred to as the valence band in the text eV GPIB General Purpose Interface Bus HP Hewlett Packard Ni The number of measurements measurement points for the i th window It is decided by N 2 Si DLTS signal for the i th window T Temperature K VISA Virtual Instrument Software Architecture Wi The i th window weighting function Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 1 Introduction Semiconductors are widely used in electronic devices Their electrical properties are
27. om but at degrees above OK these bonds may be broken due to thermal vibrations thus causing valence electrons to become free electrons resulting in a hole where the valence electron should have been 3 see Figure 2 2 O Electron hole e Electron T 0K T gt 0K Figure 2 2 Bonds in Silicon crystal at T 0K and T gt 0K Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 2 1 1 Valence band amp conduction band In the band theory of solids the valence band Ey is located in the last occupied energy level and contains the valence electrons The following band directly above the valence band is called the conduction band In insulators and semiconductors the valence and conduction band are separated by a forbidden energy gap called bandgap but in metals they overlap see Figure 2 3 The bandgap E is referred to as the difference between the valence and conduction band E Ec E 3 Conduction band Conduction band Conduction band AA ne MUERE E Bandgap Electron energy Valence band Valence band Metal Semiconductor Insulator The fermi level which is drawn in Figure 2 3 has a 50 chance of being occupied by an electron at any given time assuming thermodynamic equilibrium In semiconduct ors which has the same concentration of electrons and electron holes the position of the fermi level lies near the middle
28. on would abort the program when pushed A closer view of the controls on the front panel is shown in Figure 3 3 18 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP Figure 3 3 Controls of the DLTS software 11 12 13 14 15 16 1 This specifies the COM port to which the temperature control is connected 2 Sets the pulse generator s bias voltage see Figure 3 4 3 Sets the pulse generator s pulse voltage see Figure 3 4 4 When using an internal pulse set by the C meter this sets the bias voltage 5 When using an internal pulse set by the C meter this sets the pulse voltage 6 Sets the pulse width in ms see Figure 3 4 7 Minimum window length in ms sets the first window s length the next window has automatically the double length of the previous window see Figure 3 5 8 Sets the number of windows see Figure 3 5 9 This one is an indicator related to 7 It is not controllable by the user instead it gets the value that is half of the min win dowlength 10 An indicator showing two to the power of number of windows This sets the accuracy of the temperature at which a measurement can be made This relates to how well the PID parameters are set in the temperature controller i e a measurement is started when the temperature is within the interval of T Temperature precision 2 T Temperature precision 2 Start temp the first temperature at which a measurem
29. re controller which meant that the control commands of those instruments were different and needed to be adjusted in the LabVIEW program to fit the instruments at ngstr m Laboratory For the Yudian controller a number of VI s had been created by the research group at Angst m Laborat ory that contained most of the commands and functions needed for the DLTS main program Figure 3 1 shows the hierarchy of the DLTS software with the main VI on top and its sub VI s below The box at the bottom to the right contains the library of driver VI s for Yudian s temperature controller 16 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP IDIAN READ GET ECC A TNDATA READ ASCII 10 HER A detailed description of most of the VI s in Figure 3 1 except the driver library for Yu dian s temperature controller can be found in A LabVIEW program for Deep level transient spectroscopy by W Liu 6 3 2 Implementation The front panel of the DLTS software is displayed in Figure 3 2 in which the following parts are marked by circles A The controls of the settings to initialize a measurement B The path of the save file in which the measurement data is stored C The indicators that displays the results of the measurements and the stop button 17 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP resource name
30. riska egenskaper med l tthet kan formas och styras genom en process som kallas dopning Ett vanligt material med halvledande egenskaper som anv nds f r att tillverka olika elektroniska komponenter r kisel och olika f reningar med kisel Vid sammans ttningen dopningen av dessa kiself reningar bildas ocks defekter i materia let Dessa defekter kan beskrivas som kemiska orenheter som markant p verkar den elektriska str mmen genom materialet Defekterna har den egenheten att de f ngar upp laddningsb rare tex elektroner och f r att fria dem kr vs en viss m ngd energi i denna rapport anv nds v rmeenergi En metod f r att detektera djupa defekter p engelska deep level defects i en halvledare r deep level transient spectroscopy eller kort DLTS Metoden g r ut p att f rst t mma defekterna i provet p de laddningsb rare som er h llit tillr ckligt med energi och sedan l ta dessa falla tillbaka till defekterna f r att sedan utf ra en t mning igen Processen f r att fylla defekterna sker relativt snabbt me dan f r att t mma dem p laddningsb rare sker ver en viss tid ver denna tid kommer kapacitansen i provet att ndras som funktion av tid och det r denna kapacitans ndring som genom signalbehandling ger upphov till en DLTS signal Detta projekts huvudsyfte var att unders ka problemen med samt f rdigst lla kommu nikationen mellan m tinstrumenten och kontrollprogrammet som utgjorde uppst llningen f
31. rogrammed to read and write data According to the datasheet of Yudian the response of all parameters with the same unit as PV process value had to be divided by 10 when the dPt decimal point was set to 1 or higher in order to show the correct value Also noticeable is that the written value to a parameter with the same unit as PV had to be multiplied with 10 before sent to the Yudian control to get the desired value correctly set Just to make sure most of the para meters were tested and processed accordingly so that it became user friendly i e if the user sent 10 to SP1 setpoint 1 the Yudian control would interpret it as 10 and set SPI 24 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP to the same value All these changes led to quite a number of backup files and different versions of the Yu dian VI s They were scattered in different directories on the computer which made LabVIEW lose track of the ones that really mattered Therefore it was decided to col lect the working and updated versions in a user specified driver and add it in the LabVIEW palette for easy access whenever needed thus preventing LabVIEW to link a sub VI to the wrong VI Later it was discovered that there were some values of some parameters that could not be read in LabVIEW Yudian did write the value correctly to the specified parameter but when trying to do a read of the same parameter value the answer from Y
32. roscopy of Point Defects in Silicon and Silicon Carbide KTH Stockholm 2001 H Kortegaard Nielsen Ph D thesis Capacitance Transient Measurements on Point Defects in Silicon and Silicon Carbide KTH Stockholm 2005 VISA NI VISA user manual 1996 2001 edition National Instruments Corporation 28
33. sients Figure 2 4 shows two plots eight different transients with different time constants to the left and to the right is the corresponding DLTS signal the transients give rise to By cal culating the difference in capacitance for time t and t of each transient the DLTS signal can be obtained Nowadays the technique has been refined yet keeping the basics of Lang s method un changed 9 By measuring the capacitance transient of the sample for different temperatures and multiply it with a weighting function the DLTS signal is obtained through the integration of that product 10 2 3 1 The capacitance transient The purpose of DLTS is to characterize the deep level defects in the samples This is done by measuring capacitance transients The influence of deep level defects on the ca pacitance of the device under test DUT is as follows The capacitance of the semiconductor s space charge region the region of the semiconductor which is free of mobile charge carriers also called depletion region depends on the space charge con centration Space charges are fixed charges such as impurity ions or electrically charged deep level defects The deep level defects can trap mobile charge carriers and therefore change the space charge concentration The lifetime t of a trapped charge carrier is pro portional to exp 4E k T such that the re emission becomes more likely for higher temperature T and less likely for traps deep in the bandgap 4E
34. udian could not be recognized as a true value by the computer It was discovered that the received binary bytes from Yudian sometimes contained the same value as what the termination charac ter of the port was set to represent This meant that when LabVIEW encountered the value representing the termination character LabVIEW thought that the reading was done and therefore ended it thus skipping the rest of the bytes Realizing this the solu tion was to disable the termination character and to change the property Serial End Modes for Reads from default to None 0 which meant that the read would not stop when the termination character was encountered 5 3 DLTS main VI When all this was done the Yudian VI s worked properly and put as a driver in the Lab VIEW palette it was time to see to the main DLTS VI Since it originally came from a group in Norway who used a different setup with different brands and models for their devices some changes needed to be done in the main VI It concerned the changing of the instrument commands to the corresponding ones that could be applied to this setup When the commands had been changed another problem was revealed The Operating System used by the computer was using Swedish standards thus using a comma for decimal mark not point as of the English standard This lead to parsing errors in the devices that used the English standard By adding a format string in LabVIEW to every written command the de
35. viSetAttribute 27 S In VISA Asse a Trigger jg VISA Read STB Lo Wai or RQS GPIB Instr amp Property VISA Property Node References 1 2 3 4 5 6 7 8 9 10 11 12 H J von Bardeleben J C Bourgoin D Deresmes A Huber and D Stievenard Identification of a defect in a semiconductor Phys Rev B 34 7192 1986 J Isberg et al High carrier mobility in single crystal plasma deposited diamond Science 297 5587 2002 1670 1672 J P Colinge amp C A Coolinge Physics of Semiconductor Devices Print ISBN 1 4020 7018 7 eBook ISBN 0 306 47622 3 2002 S M Sze Physics of Semiconductor Devices ISBN 0 471 05661 8 Wiley 1964 C Kittel Introduction to solid state physics 7 edition ISBN 0 471 11181 3 Wiley 1996 W Liu Master of Science Thesis 4 LabVIEW program for Deep level transient spectroscopy KTH Stockholm 2009 S Majdi Master Thesis Deep level transient spectroscopy investigation of silicon carbide diodes KTH Stockholm 2005 B Schumacher H G Bach P Spitzer J Obrzut Chapter 9 Electrical Properties in Springer Handbook of Materials Measurement Methods ISBN 978 3 540 20785 6 Springer Verlag Berlin Heidelberg 2006 p 431 P Pellegrino Ph D thesis Point defects in ion implanted silicon and silicon carbide KTH Stockholm 2001 D berg Ph D thesis Capacitance Spect
36. y is the user interface where the user can display the results and sometimes controlling the functionality of the VI 11 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP 33 Untitled 1 Block Diagram em Eile Edit View Project Operate Tools Window Help In Figure 2 8 the block diagram of a simple VI is illustrated It contains a built in func tion that generates a random number and send the result through the wire to an indicator labelled Result The result is not shown in the block diagram instead it is shown in the front panel where the indicator automatically has been placed when adding it to the block diagram as shown in Figure 2 9 To run the program the user simply pushes the arrow in the top left corner of the window either in the block diagram or in the front panel 0 0525558 b i NATIONAL INSTRUMENTS epe Student Edition 12 Bachelor thesis COMPLETION OF THE SOFTWARE REQUIRED FOR A HIGH TEMPERATURE DLTS SETUP A VI is made with different components and or sub VI s wired together in the block dia gram in a manner of the users choice One example of components are standard mathematical operations such as multiplier adder etc For more advanced mathematics and programming there are predefined sub VI s integrated in LabVIEW All sub VI s and components are accessed through the LabVIEW functions palette which is show in Figure 2 10
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