1、Introduction to DLTS (Deep Level Transient Spectroscopy)II. Advanced TechniquesO. Breitenstein MPI MSP Halle,Outline:1. Basic principlesApplication field of DLTSPrinciples of DLTSBasic measurement techniques2. Advanced techniquesAdvanced DLTS measurement techniques3. (next time) Our DLTS system- Phi
2、losophy- Hardware- User surface,Recapitulation: DLTS routine (repeating!) :,t,Vr,e-,e-,e-,e-,e-,t,DC,t,0,0,bias,band diagram,RF- capacitance,reverse,reduced or forward,reverse,Generation of the DLTS signal,low T,opt. T,high T,If T is slowly varying, at a certain temperature a DLTS peak occures,“rate
3、 window“:,DLTS measurements at different rate windows allow one to measure Et,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,reverse bias,V = 0,forward bias,pn-junctio
4、n,xe,majority carrier flow,minority carrier injection,Schottky diodes:Standard, easy to prepare, high quality demand !Only majority carrier traps visible, even under forward biaspn-junctions:reverse bias reduction up to 0V: “majority carrier pulse“forward bias (injection): “minority carrier pulse“ (
5、MC)MC pulse may reveal both minority and majority carrier trapsHowever, if opposite carrier capture dominates, traps may remain uncharged (invisible in DLTS) = basic limitation !Asymmetric doping concentration: signal from lower doped sideOther sample types:Grain boundary (anti-serial Schottky diode
6、s) = bonded wafersMIS devicesFETs (“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,thermal equilibrium traps emptied (from holes),filling pulse hole
7、 capture,measurement hole emission,allows investigation of minority carrier traps in Schottky diodes,2.3. Optical DLTS (ODLTS)trap filling by bias pulsescontinuous irradiation of IR light ( Eg)optical emission additional to thermal emissionstrong dependence on intensity and l,dark,illuminated,ODLTS
8、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,Vr,Vr,Vr,t,t,t,t,linear dependence on Vp: homogeneous concentration !,2.5. Measurement of
9、field dependence of en;p (pulse height scan),Vr,Vr,Vr,Vr,t,t,t,t,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,“real“ capture cross sectionmeasurement at different
10、 rate windows: T-dependence of CCSinjection: 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 pro
11、bability depends on average occupation statebarrier-controlled capture, depending on occupation state,low occupation,high occupation,DLTS,T,point defect,extended defect,DLTS,log(timp),tc,extended defects show logarithmic capture behaviour,SummaryDLTS on Schottky diodes only reveals majority carrier
12、tapsDLTS on pn junctions also reveals minority carrier trapsOptically excited DLRS (MCDLTS) also reveals minority carrier traps in Schottky diodesODLTS 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 holesExtended 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,
