Techniques for determination of deep level trap parameters in irradiated silicon detectors AUTHOR: Irena Dolenc ADVISOR: prof. dr. Vladimir Cindro

Motivation  Silicon detectors usually used as inermost part of tracking system in particle physics detectors → can receive severe levels of radiation → cause of bulk damage which deteriorates detector performance  Radiation damage effects caused by lattice deformation:  change in effective dopant concentration which effects full depletion voltage  increase of leakage current  deterioration of charge collection efficiency: part of the drifting charge, created by ionizing particle, is temporarly trapped by the defects introduced with irradiation  Damage mechanism:  irradiation particles knock off Si atoms  dislocated Si atoms (interstitials), empty lattice sites (vacancies), impurities (eg. O, C) form defect complexes → introduction of energy levels (traps) in the band gap  For the developement of more radiation hard Si detectors the knowledge of defect kinetics and of correlation between microscopic defects and macroscopic properties of the detector is needed  The main tools for characterization of deep level defects:  Deep Level Transient Spectroscopy (DLTS)  Thermally Stimulated Current (TSC) technique

Operation principles of silicon detectors  Capacitance  Si detector: a diode operated under reverse bias where the depleted region acts like ionization chamber  Desired detector operation voltage V > full depletion voltage V FD  Usually p + -n junction →  Neutrality of the system →  If N D,n comparable with N A,n than N D,n must be replaced with effective dopant concentration N eff =│ N D,n - N A,n │

Shockley-Read-Hall statistics  Occupation of a defect states wth concentration N t, energy level E t and average occupation probability P t density of occupied defects density of non occupied defects  Change of a defect occupancy possible by  electron capture with rate  electron emission with rate  hole capture with rate  hole emission with rate  Defect parameters (besides E t ) that are determened by DLTS or TSC method:  capture coefficients →  emission probabilities → e n,p  The rate of change of the defect ocupancy

Shockley-Read-Hall statistics  Thermal equilibrium  P t = Fermi function  steady state → dn t /dt =0  no current → no net flow of electrones or holes between conduction and valence band → R p =G p, R n =G n Relations between e n,p and c n,p remain valid non-equilibrium conditions → defect is fully described by E t and  n,p  Space charge region in steady state  carrier concentration negligible: p,n ~ 0 → capture processes can be neglected  steady state → dn t /dt =0 extraction of P t → E i acts like E F in thermal equilibirium electron traps hole traps generation centers (leakage current)

DLTS: principle of operation Deep level transient spectroscopy (DLTS): uses capacitance transient signals resulting from relaxation processes following an abrupt change of bias voltage or light applied to the sample being investigated. 1.Electron trap → located in the upper half of the band gap: During the measurement the device must be partially depleted!

DLTS: principle of operation 2.Hole trap → located in the lower half of the band gap:

DLTS: principle of operation 2.Hole trap → located in the lower half of the band gap: Filling of the traps:  By forward bias: electrons and holes are injected !! When a high enough number of electrons and holes is injected the occupation of defect is governed by capture processes. → trap must have a feature of c p >>c n to be filled with holes !  By short - laser (e.g. red) from the n-side: if the penetration of laser used is very short compared to detector thickness, only holes are drifting to their adjacent electrode while electron drift is negligible. → only holes are injected in SCR and hole traps with c p >c n can be detected too

DLTS: determination of trap parameters Electron trap of acceptor type:  Effective dopant concentration after the filling pulse  Density of occupied traps after the filling pulse  Capacitance after the filling pulse 1.Capacitance after the filling pulse

DLTS: determination of trap parameters 2.DLTS spectrum Low T : emission process to slow to be observed High T : emission process to fast to be observed Peak observed at T max where emission time satisfies  Several temperature scans with different t W = t 2 - t 1 → Arrhenius plot : 3.Extraction of parameters  Connection between e n and  n  N t extracted form DLTS peak since n t (0)  N t  E t extracted from slope of Arrhenius plot  n from the intercept of Arrhenius plot with ordinate

DLTS: example of a DLTS spectrum

TSC: principles of operation ♦ DLTS: observing the change in depth of SCR, due to emission of trapped charge, by measuring capacitance transient ♦ Thermally Stimulated Current technique (TSC): observing release of trapped charge directly, by measuring the current due to emission of trapped carriers Measurement process: 1.Cooling: Sample is coold to a low T. Cooling under reverse bias → traps are not filled with carriers 2.Filling: −Switching to zero bias → filling with electrons −Switching to forward bias → electrone and hole injection −Illumination with short-λ laser of n-side (filling only with electrons) or p + -side (filling only with holes) 3.Recording: Heating under reverse bias with constant heating rate. At some T emission probability is no longer negligible → trapped charge is rapidly emitted and swept out of SCR → peaks in current signal

TSC: example of TSC spectrum

TSC: determination of trap parameters  Density of occupied traps during the heating  Current due to emitted trapped charge during the heating − e n >> e p −During whole TSC scan device fully depleted → w ( t )=detector thickness D Electron trap  Determination of N t −From the area under TSC peak −From the peak height

TSC: determination of trap parameters 1.Variable heating method Position of the TSC peak at T max which satisfies →ΔE= E C – E t → extracted from the slope of the plot → σ n → extracted from the intercept with the ordinate By repeating TSC temperature scans with different heating rate β, a plot is obtained 2.Delayed heating method  Several TSC scans performed from starting point T 0 with different delay times τ d between end of the filling pulse and start of the heating → plot ln( I TSC,max ) versus τ d  I TSC (t)  n t (0)  exp(- e n (T 0 ) τ d ) → TSC peak decreases with increasing τ d from the slope of a plot emission probability e n ( T 0 ) is obtained  Repeating procedure at different T 0 → Arrhenius plot  Connection between e n and  n : →ΔE= E C – E t → extracted from the slope of the plot → σ n → extracted from the intercept with the ordinate 3.Deconvolution method: numerical fit to the whole spectrum

Summary  TSC and DLTS → techniques for determination of deep level traps parameters, based on observing reversely biased detector response to applied light or an abrupt change of biased voltage (filling of traps with holes and/or electrons)  Difference between DLTS and TSC  DLTS method:  Capacitance transient after the filling process is measured  Capacitance transient caused by the change of the width of SCR due to emission of carriers that were trapped during the filling  During the measurement device must be biased with the voltage lower than full depletion voltage  TSC method:  Sample is cooled to a low temperature before filling  Emission of trapped carriers observed directly by measuring the current while heating the sample  Device is fully biased during the measurement