4 Generation of the DLTS signal low Topt. Thigh Tt1t2tt1t2tDCmeast1t2t"rate window":TTpeakDCpeakDLTS signal = C(t1)-C(t2)If T is slowly varying, at a certain temperature a DLTS peak occures
5 DLTS measurements at different rate windows allow one to measure Et ln(en)1000/TDLTSThis "Arrhenius plot" allows an identification of a deep level defect
6 Advanced techniquesDLTS on Schottky diodes only reveals majority carrier tapsDLTS on pn junctions also reveals minority carrier trapsOptically excited DLRS (MCDLTS) also reveals minority carrier traps in Schottky diodesThere are special DLTS procedures for measuring:- concentration depth profiles (Vimp scan)- electric field dependence of en;p (Vimp scan)- capture cross sections for electrons and holes (timp scan)Extended defects are usually characterized by a logarithmic capture behavior and often show non-exponential emission (broadened DLTS peaks)
7 Philosophy of our DLTS system 1. We don’t save DLTS data but transient dataConventional approach: On-line conversion of transient data to DLTS data, saving DLTS(T) (1-dimensional vector of data).Advantage: Small file sizes. Disadvantage: No flexibility with respect to different correlation techniques (see below)Our approach: C(t, T) is saved as a 2-dimensional data fileAdvantage: Flexible DLTS correlation. Disadvantage: Larger file sizes (see below).
8 2. We have both linear and logarithmic time scale at choice Linear time scale: time resolution for large times is the same as immediately after the filling pulse. May be advantageous for software-based multiexponential transient deconvolutiontmintntn = n tminLogarithmic time scale (base 2, also 1.1 possible): Time resolution proportional to elapsed time, drastic savings in file sizeaveraging over differently sized periods!tmintn = (2n-1) tmintn
9 3. We have three different kinds of DLTS correlation at choice 3.1. Modified 2-Point correlation: DLTS = C(t1)-C(t2)Mathematical formulation:C(t)K(t)t1t2t3good compromise between resolution and sensitivity, many rate windows
10 3.2. Exponential correlation: High sensitivity, but less resolution K(t)tDLTSTDC(0)2-point
11 3.3. High resolution correlation: Low sensitivity K(t)tDLTSTDC(0)2-point
12 Hardware C-Meter working at 1 MHz, made at our electronic workshop applied HF signal: 100 mV (pk-pk) or 1 V (pk-pk) at choiceelectronic C- and G- (conductivity) compensationmanual or automatic compensationsample bias range: Vpulse bias range: V. If pulse bias > bias: injection !preamplifier separated, connected with main unit by 1 m cablecomputer controlled via 2x16 bit ADC / DAC interface cardT control unit, controlled via RS232 by computerlinear or exponential T-ramp at choice, speed adjustable
14 DLTS system wiring scheme bias pulse bias Delta C C-compens computer preamp.ext ext out outrear sideC-meterfront sidedelta C bias outComputerpreamplifier+ Probe- ProbeProbeexternINPUT CH.1 INPUT CH EXT.TRIG.OscilloscopeCryostatT-controllerRS232"Trig."ADC 0ADC 2DAC 0sample
15 2 different cryostatsat choice:1. Bath cryostatonly for samples mounted on TO5 transistor holdersmanually immersing in liquid nitrogen (cool down), measurement after lifting above LN2 level, quick measurementnot optimum for very slow T-ramps or constant T measurements
16 2. Evaporator cryostatfor samples mounted on TO5 or TO18 holders or bare samplesfully automatic cooling down and heating up (software controlled)slower measurement, larger LN2 consumption
20 What this system can do: DLTS measurements from 78 K to 400 Ksample capacitance < 500 pFsample parallel resistance > 500 Wbias and pulse bias range: Vsamples mounted on transistor holders or as raw chipslinear and logarithmic sampling (to base 2 or 1.1)rate window range from < 1 s-1 to 104 s-1monitoring and storage of C0(T) (basic capacitance)sensitivity < 10-4pF for 0.1V HF (pk-pk), < 10-5pF for 1 V HF“batch” measurements for bias, pulse bias, tmin, and timpdisplay of up to 10 DLTS tracesexport of C transients, C0(T) and DLTS traces as ASCII filessystem is available in room B.2.05, to be used only after personal introduction by O.B. !
21 Plans for the future:Establishment of Minority Carrier DLTS (optical excitation)DLTS peak evaluation software (parameter fitting etc.)3 ppt Files of this introduction and the DLTS operation manual are available on-line