(Deep Level Transient Spectroscopy) II. Advanced Techniques Introduction to DLTS (Deep Level Transient Spectroscopy) II. Advanced Techniques O. Breitenstein MPI 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!) : reverse reverse reduced or forward Vr bias t e- e- e- band diagram e- e- RF- capacitance t DC t

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

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

2. Advanced DLTS measurement techniques 2.1. Possible samples: Schottky diodes or pn-junctions Schottky diode pn-junction e- reverse bias e- h+ e- h+ e- V = 0 xe e- h+ xh e- forward bias majority carrier flow minority carrier injection

Schottky diodes: pn-junctions: Other sample types: 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 (hn > Eg) reverse bias remains constant e- e- h+ e- h+ thermal equilibrium traps emptied (from holes) filling pulse hole capture measurement hole emission allows investigation of minority carrier traps in Schottky diodes

trap filling by bias pulses 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 l dark illuminated T Tpeakdark T Tpeakillumin. ODLTS allows to measure optical capture cross sections sopt(l) connection between deep levels electrically detected (DLTS) and optically detected (absorption)

2.4. Concentration depth profiling (pulse height scan) Vr Tpeak e- t e- Vr T Tpeak t Vr e- T Tpeak t T Tpeak Vr e- t linear dependence on Vp: homogeneous concentration !

2.5. Measurement of field dependence of en;p (pulse height scan) Vr Tpeak1 t Vr T Tpeak2 quantitative evaluation: difference spectra (DDLTS) field depencence indicates charged occupied state t Vr T Tpeak3 t Vr T Tpeak4 t

2.6. Measurement of capture cross sections (pulse width scan) Vr T Tpeak t Vr T Tpeak t T Tpeak Vr "real" capture cross section measurement at different rate windows: T-dependence of CCS injection: measurement of minority carrier CCS t Vr T Tpeak t

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 point defect extended defect DLTS low occupation e - T DLTS e - high occupation tc log(timp) 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 sopt(l) 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