Hussein Ayedh PhD Studet Department of Physics
OUTLINE Introduction Semiconductors Basics DLTS Theory DLTS Requirements Example Summary
Introduction Energetically "deep trapping levels in semiconductor space charge region affect semiconductor performance - Shortening of carrier life-time - Enhanced recombination of minority carriers - Output power limitations etc. Characterization of impurities and traps is essential to analyze the performance of semiconductor DLTS was Introduced by D. V. Lang in 1974 It is commonly Used for characterizing point defects in semiconductors
Introduction DLTS Principle Emission of trapped charge carriers change the depletion capacitance of a pn-junction or Schottky diode. The transient measurement provides information on the defect levels in the band gap. Electrical properties of defects: Energy position in band gap Capture cross section Concentration of defects Very sensitive: It can detect traps down to 10 8 cm -3 in very good samples Non destructive
Semiconductor Basics pn- junction p E c E f E v n E c E f E v p - - W ++ ++ ++ ++ F n qv 0 E c C A W Connection of p- and n-type regions: Diffusion of charge carriers into the opposite regions. This will give rise to an electric field across the depletion region (W), with a capacitance C No free charge carriers in W as the field will sweep them across the junction E v
Equilibrium Forward Bias Reverse Bias V F V R p - - W ++ ++ ++ ++ F n qv 0 E c p W - + n ++ p - +++ +++ - - n - - - +++ F q(v 0 V F ) W +++ F q(v 0 + V R ) E v The SCR width (W) Changes with applied voltage and doping concentration High doping small W Low doping large W The depletion region width (W) will extend mostly into low-doped material in order to keep charge balance
Point Defects - Substitution impurity: extra impurity atom in an origin position - Vacancy: missing atom at a certain crystal lattice position - Interstitial impurity atom: extra impurity atom in an interstitial position - Self-interstitial atom: extra atom in an interstitial position; Introduce energy level in the band structure Shallow level Close to the edges of the bandgap Use mainly as a dopant Deep level Close to the middle of the bandgap Act as generation/recombination or trap center. http://cnx.org/content/m16927/latest/
Capture & Emission Processes Deep levels in the band gap act as - Recombination centers: can interact with both edges of bandgap c n = c p. - Electron traps: If they mostly interact with the conduction band c n c p. - Hole traps: If they mostly interact with the valence band c n c p. Thermal (electron) emission rate: Conduction Band e n e nvnn g c exp ( EC E kt v n T 1 2 N c T 3 2 n n T 2 E exp C E kt t t ) e n c n c p c p Valence Band c n e p Electron Trap center Recombination center Hole Trap Center
DLTS Theory Principle of measurement Diode kept at fixed reverse bias. Filling Pulse to fill the traps. Return to the reverse bias: - Change of the W - Emission of charge carriers changes the capacitance of the depletion region as a function of time C t = C rb C o exp e n t V V r C C(rb) V p DC t Repeated through a temperature scan 0 t
DLTS Theory DLTS Measurement: (A) Equilibrium state (B) Filling pulse (C) Return to the reverse bias with change in the capacitance (D) Emission case.
Capacitance Transient (pf) Temperature (K) DLTS Theory -The carrier concentration of the traps is changed exponentially Built of the DLTS spectra C(t 2 ) C(t 1 ) vs Temperature C(t 2 ) C(t 1 ) max at certain T n T ( t) N exp( e t) T n - The trap concentration can be deduced from the maximum amplitude of the transient C o = N T 2N d C rb t 1 t 2 Time (s)
Capacitance Transient (pf) DLTS Spectra Build of the DLTS spectra DLTS transient is multiplied by a weighting function W(t) S = C t W t dt = C rb N T 2N d Lock-in weighting function: -1,1 exp e n t W t dt We record the transient for each temperature, then create the spectra after the measurement 1-1 Time (s) Temperature (K)
Build of the DLTS spectra Lock-in gives a good signal to noise ratio But wide spectra Use of different weighting function in order to separate close defect levels Like GS4 and GS6. Lock-in: high SNR, wide peak GS4: low SNR, narrow peak (DLTS) Presentation, MENA9510 Course,2013. D. Åberg, PhD thesis, KTH (2001)
Capacitance Transient (pf) Extraction of defect properties By varying the length of the rate-window, the peak is shifted in temperature Emission rate (e n ) of each window can be numerically calculated. A set of T max and e n for all DLTS spectra will be obtained. T 3, e n3 T 2, e n2 T 1, e n1 W 1 W 2 W 3 Time (s) Temperature (K)
Extraction of defect properties - The emission rate e n n T 2 E exp C E kt Arrhenius plot of ln e n T2 against 1/T t ln e n T 2 = ln βσ n E c E T kt E c = 0.67 ev σ n = 4 10 14 cm 2
DLTS Requirement Samples Rectifying junction (Schottky or pn-junction) Junction capacitance 1-1000pF, (100pF-range most ideal) Trap concentration less than 10-15% of doping DC C rb or N T N d Low leakage current and low conductance Lower limit for detectable trap concentrations: Depends on the sensitivity of the C-bridge and S/N ratio E.g. for DC 0,min 5 ff, C R 50 pf (N t /N s ) min 2(DC 0,min /C R ) 2 10-4
DLTS Requirement Required Equipment Capacitance meter Pulse generator Temperature controller Liquid Nitrogen Helium cryostat Heater Temperature Reader Equipment at MiNaLab Two setups in temperature range 15K-300K One setup in temperature range 77K-400K One setup in temperature range 77K-600K (DLTS) Presentation, MENA9510 Course,2013.
Example DLTS measurements PN-diodes (Si) Irradiated by protons with dose 2 10 10 cm 2 Defect Identity Energy Position Capture Cross Section VO [78K] E C E t = 0.17 ev σ = 7.2 10 15 cm 2 V 2 (=/ ) [114K] E C E t = 0.23 ev σ = 3.5 10 15 cm 2 V 2 ( /0) [196K] E C E t = 0.41 ev σ = 2 10 15 cm 2
Examples Window 6 of three PNdiodes are irradiated by protons at different doses: - Sample #1 3 10 9 cm 2 - Sample #2 6 10 9 cm 2 - Sample #3 2 10 10 cm 2 Defect Identity N t /N d in Sample #1 N t /N d in Sample #2 N t /N d in Sample #3 VO [78K] 0.0053 0.012 0.042 V 2 (=/ ) [114K] 0.0014 0.0034 0.012 V 2 ( /0) [196K] 0.0020 0.0045 0.017
Summary Deep Level Transient spectroscopy Characterization of electrically active defects Energy position in band gap Capture cross section Concentration of defects with accuracy up to (~10 8 cm -3 ) No information about the chemical composition. Signal is obtained by filling pulse in applied bias, and observing a transient decay of trapped charge carriers in the depletion region. Requirement: DLTS requires rectifying junction with capacitance in 1-1000pF range Low leakage current is important to get good measurements Trap concentrations between 0.0001-0.2 of doping concentrations
REFERENCES [1] D.V.Lang, JAP, 45, 7, 1974. [2] P. Blood and J. W. Orton, The Electrical characterization of Semiconductors: Majority Carriers and Electron States (ACADIMIC PRESS, USA, 1992). [3] W. E. Meyer, Digital DLTS Studies on radiation induced defects in Si, GaAs and GaN, PhD dissertation, University of Pretoria, 2007. [4] F. D. Auret and P. N. K. Deenapanray, Deep Level Transient Spectroscopy of Defects in High-Energy Light-Particle Irradiated Si, Solid State and Materials Sciences, 29:1 44, 2004. [5] Deep level transient spectroscopy (DLTS) Presentation, Advanced Characterization Methods Course MENA9510, 2013. [6] Dieter K. Schroder: Semiconductor Material and Device Characterization, John Wiley & Sons, 2006.