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(Deep Level Transient Spectroscopy)

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1 (Deep Level Transient Spectroscopy)
Introduction to DLTS (Deep Level Transient Spectroscopy) III. Our DLTS System O. Breitenstein MPI MSP Halle

2 Outline: 1. Basic principles Application field of DLTS Principles of DLTS Basic measurement techniques 2. Advanced techniques Advanced DLTS measurement techniques 3. Our DLTS system - Philosophy - Hardware - User surface

3 Recapitulation: DLTS routine (repeating!) :
reverse reverse reduced or forward Vr bias t e- e- e- band diagram e- e- RF- capacitance t DC t

4 Generation of the DLTS signal
low T opt. T high T t1 t2 t t1 t2 t DCmeas t1 t2 t "rate window": T Tpeak DCpeak DLTS 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/T DLTS This "Arrhenius plot" allows an identification of a deep level defect

6 Advanced techniques DLTS on Schottky diodes only reveals majority carrier taps DLTS on pn junctions also reveals minority carrier traps Optically excited DLRS (MCDLTS) also reveals minority carrier traps in Schottky diodes There 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 data Conventional 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 file Advantage: 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 deconvolution tmin tn tn = n tmin Logarithmic time scale (base 2, also 1.1 possible): Time resolution proportional to elapsed time, drastic savings in file size averaging over differently sized periods! tmin tn = (2n-1) tmin tn

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) t1 t2 t3 good compromise between resolution and sensitivity, many rate windows

10 3.2. Exponential correlation: High sensitivity, but less resolution
K(t) t DLTS T DC(0) 2-point

11 3.3. High resolution correlation: Low sensitivity
K(t) t DLTS T DC(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 choice electronic C- and G- (conductivity) compensation manual or automatic compensation sample bias range: V pulse bias range: V. If pulse bias > bias: injection ! preamplifier separated, connected with main unit by 1 m cable computer controlled via 2x16 bit ADC / DAC interface card T control unit, controlled via RS232 by computer linear or exponential T-ramp at choice, speed adjustable

13 C Meter and T controller

14 DLTS system wiring scheme
bias pulse bias Delta C C-compens computer preamp. ext ext out out rear side C-meter front side delta C bias out Computer preamplifier + Probe - Probe Probe extern INPUT CH.1 INPUT CH EXT.TRIG. Oscilloscope Cryostat T- controller RS 232 "Trig." ADC 0 ADC 2 DAC 0 sample

15 2 different cryostats at choice: 1. Bath cryostat only for samples mounted on TO5 transistor holders manually immersing in liquid nitrogen (cool down), measurement after lifting above LN2 level, quick measurement not optimum for very slow T-ramps or constant T measurements

16 2. Evaporator cryostat for samples mounted on TO5 or TO18 holders or bare samples fully automatic cooling down and heating up (software controlled) slower measurement, larger LN2 consumption

17 Software Made by MSC Technik Halle (

18 “settings” menu:

19 “display” menu

20 What this system can do:
DLTS measurements from 78 K to 400 K sample capacitance < 500 pF sample parallel resistance > 500 W bias and pulse bias range: V samples mounted on transistor holders or as raw chips linear and logarithmic sampling (to base 2 or 1.1) rate window range from < 1 s-1 to 104 s-1 monitoring 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 timp display of up to 10 DLTS traces export of C transients, C0(T) and DLTS traces as ASCII files system 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

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