Introduction to DLTS(Deep Level Transient Spectroscopy)

II. Advanced Techniques

O. BreitensteinMPI MSP Halle

Outline:

1. Basic principles

• Application field of DLTS• Principles of DLTS• Basic measurement techniques

2. Advanced techniques

• Advanced DLTS measurement techniques

3. (next time) Our DLTS system• - Philosophy• - Hardware• - User surface

Recapitulation: DLTS routine (repeating!) :

t

Vr

e-

e-

e-

e-

e-

t

C

0

0

bias

banddiagram

RF-capacitance

reverse reducedor forward

reverse

Generation of the DLTS signal

t1 t2t t1 t2 t

Cmeas

t1 t2t

T

Tpeak

Cpeak

DLTS signal = C(t1)-C(t2)

low T opt. T high T

If T is slowly varying, at a certain temperature a DLTS peak occures

12

1

2

1peakp;n tt

t

tln

se

"rate window":

DLTS measurements at different rate windows allow one to measure Et

e01

e02

e03

T1 T2 T3T

ln(en)

1000/TT1T2T3

e01

e02

e03

DLTS kT2

T

1000n*198

meVE t

This "Arrhenius plot" allows an identification of a deep level defect

2. Advanced DLTS measurement techniques2.1. Possible samples: Schottky diodes or pn-junctions

e- e-

Schottky diode

reversebias

V = 0

forwardbias

e- e-

pn-junction

e

e-

h+

h+

e- e-

h+ h

e- e-

h+ h+

majority carrier flow minority carrier injection

e-

Schottky diodes:• Standard, easy to prepare, high quality demand !• Only majority carrier traps visible, even under forward bias

pn-junctions:• reverse bias reduction up to 0V: "majority carrier pulse"• forward bias (injection): "minority carrier pulse" (MC)• MC pulse may reveal both minority and majority carrier traps• However, if opposite carrier capture dominates, traps may remain uncharged (invisible in DLTS) => basic limitation !• Asymmetric doping concentration: signal from lower doped side

Other sample types:• Grain boundary (anti-serial Schottky diodes) => bonded wafers• MIS devices• FETs ("conductivity DLTS")• point contacts at nanowires ? ...

2.2. Optically excited DLTS (minority carrier DLTS, MCDLTS)

• trap filling by optically excited minority carriers (h > Eg)• reverse bias remains constant

e-e-

h+

h+

h+

e-

h+

h+

thermal equilibriumtraps emptied(from holes)

filling pulsehole capture

measurementhole emission

• allows investigation of minority carrier traps in Schottky diodes

2.3. Optical DLTS (ODLTS)

• trap filling by bias pulses• continuous irradiation of IR light (< Eg)• optical emission additional to thermal emission• strong dependence on intensity and

T

Tpeakdark

T

Tpeakillumin.

dark illuminated

• ODLTS allows to measure optical capture cross sections opt()• connection between deep levels electrically detected (DLTS) and optically detected (absorption)

2.4. Concentration depth profiling (pulse height scan)Vr

Vr

Vr

Vr

t

t

t

t

e-

e-

e-

e-

T

Tpeak

T

Tpeak

T

Tpeak

T

Tpeak

• linear dependence on Vp: homogeneous concentration !

2.5. Measurement of field dependence of en;p (pulse height scan)Vr

Vr

Vr

Vr

t

t

t

t

T

Tpeak1

T

Tpeak2

T

Tpeak3

T

Tpeak4

• quantitative evaluation: difference spectra (DDLTS)• field depencence indicates charged occupied state

2.6. Measurement of capture cross sections (pulse width scan)

Vr

t

Vr

t

Vr

t

Vr

t

T

Tpeak

T

Tpeak

T

Tpeak

T

Tpeak

c

impimp

texp1C)t(C

nv

1

nc

1

nnnc

• "real" capture cross section• measurement at different rate windows: T-dependence of CCS• injection: measurement of minority carrier CCS

2.7. Point defects and extended defects

• all previous considerations referred to isolated point defects• "extended defects": dislocations, grain boundaries, precipitates ...• continuum of states, "broadened states"• emission probability depends on average occupation state• barrier-controlled capture, depending on occupation state

e - e -

e - e -

lowoccupation

highoccupation

DLTS

T

pointdefect

extendeddefect

DLTS

log(timp)c

• extended defects show logarithmic capture behaviour

Summary

• 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• ODLTS reveals optical trap parameters opt()• There are special DLTS procedures for measuring:- concentration depth profiles- electric field dependence of en;p

- capture cross sections for electrons and holes• Extended defects are usually characterized by a logarithmic capture behaviour and often show non-exponential emission (broadened peaks)

Next time: Introduction of our own DLTS system