Creation and annealing of point defects in germanium crystal lattices by subthreshold energy events

Transcription

1 Creation and annealing of point defects in germanium crystal lattices by subthreshold energy events University of Sevilla 203 Sergio M. M. Coelho, Juan F. R. Archilla 2 and F. Danie Auret Physics Department, University of Pretoria, South Africa 2 Group of Nonlinear Physics, University of Sevilla, Spain

2 Our team in Pretoria Juan FR Archilla (Project leader) Group of Nonlinear Physics Lineal (GFNL) University of Sevilla, Spain With Sergio Coelho, F Danie Auret Department of Physics University of Pretoria, South Africa Vladimir Dubinko Kharkov Physical-Technical Institute, Kharkov, Ukraine Vladimir Hizhnyakov Institute of Physics, University of Tartu Tartu, Estonia sergio@up.ac.za Sevilla, 203 Slide

3 Outline Why study this? Low energy? Really? Germanium ultrapure material Hydrogen in germanium DLTS Deep Level Transient Spectroscopy An experiment: ICP annealing of E-center Summary and conclusions Sevilla, 203 Slide 2

4 Why low energy? Why study this? Low energy? Really? Impacts generate Moving Intrinsic Localized Modes (ILMs) or Discrete Breathers (DBs) in Ge ILMs in Ge Defect annealing 2600 nm deep Very efficient process Technology applications Transferable to other systems Sevilla, 203 Slide 3

5 Germanium Niche applications: Opto-electronics Purdue Ultra pure Ge Low impurities Ohmic contact easy (Au-Sb) Au SBD s Resistive evaporation No defects introduced good current voltage characteristics sergio@up.ac.za Sevilla, 203 Slide 4

6 Electrical Characterization of Defects Classic DLTS measurements use majority carrier capture and emission process to obtain a fingerprint of the defect. By monitoring the change of emission rate with temperature an activation energy is obtained. By observing the capture rate a cross section can be obtained. stage when n >> 0 capture of electrons c = n V n n th stage 2 when n ~0 emission of electrons e n = A exp (-E a/kt) deep states (ideal point defects) sergio@up.ac.za Sevilla, 203 Slide 5

7 The DLTS Spectrum Monitor transient as function of temperature,t Plot S = C(t ) C(t 2 ) vs T DLTS peak More than one level DLTS spectrum At the maximum of a peak: e max ln( t t t t 2 2 ) Typical rates: /s D. V. Lang, JAP 45, 2023 (974) or Schroder, Semiconductor Material and Device Characterization

8 Deep Level Transient Spectroscopy DLTS requires a depletion (space charge) region p-n, metal-semiconductor, MOS,.. Apply zero / forward bias reverse bias pulse sequence Monitor C-t, I-t, as function of T. Transient behaviour indicates the Presence of defects with levels in the band gap Analysis of transient yields: Distiction between majority and minority carrier defects Activation enthalpy (level position), E T Capture cross section, Defect concentration, N T (x) Electric field: defect type (donor, acceptor, ) Uniaxial stress: defect orientation and symmetry DLTS Signal: C/C (x0-3 ) 3 2 As-grown ZnO 5.4 MeV -particle irradiation E2 460 Hz 46 Hz 46 Hz E3 E E4 E 2 0. Hz E0 E Temperature (K) sergio@up.ac.za Sevilla, 203 Slide 7

9 L-DLTS of (H H 0.30 ) peak The (H H 0.30 ) DLTS peak is asymmetric It consists of more than one superimposed peaks H 0.30 L-DLTS revealed that these peaks belong to H 0.30 (-/0) state of V-Sb H 0.26 unknown structure L-DLTS signal T = 50 K V r = -.0 V V p = +3.0 V H 0.26 This is similar to what has been reported for electron & gamma irradiation of Ge Emission rate (/s) V. P. Markevich et al, J. Appl. Phys. 95, 4078 (2004). sergio@up.ac.za Sevilla, 203 Slide 8

10 DLTS: different metallization methods Resistive deposition does not introduce defects Curve (a) Sputter deposition introduced five electron traps Curve (b) EBD deposition introduced five electron traps Curve (c) DLTS Signal (pf) (d) (c) (b) x x 2 x 2 E 0.5 E E 0.2 E 0.3 ES ES 0.2 ES 0.4 E 0.24 E 0.3 x E 0.38 Temperature (K) All the SD induced defects are also introduced by MeV electron irradiation Curve (d) F. D. Auret et al, J. Electron. Mat. 36, 604 (2007). (a) x 0 H 0.30 ES0.24 ES 0.3 sergio@up.ac.za Sevilla, 203 Slide 9 x x x 2

11 DLTS: Example, two RW: Defect E center RW n n T 2 80s exp( e n ET kt ( T ) ) RW n 2 n T s exp( e ET kt 2 n ) ( T 2 ) n and E T can be determined 0

12 Number of traps for n-type semiconductor: N D :concentration of donors C C 0 N 2N The number of traps N t can be determined The depletion layer with W increases with the bias potential: The profile N t (x) can be obtained t D

13 Annealing at room temperature dnt - dt k(t)n t k(t) E0 exp( k T B ) Annealing rate constant k(t) can be determined Isochronal annealing: same time, different temperatures Annealing activation energy E 0 can be determined 2

14 Sample Preparation Chemical cleaning, ie degreasing Chemical etch oxide removal RF Sputter etch Ar ICP Use COPRA plasma source Inductively coupled plasma (ICP) Low energy Ar ions: - 20 ev Fluence rate: 0 5 cm -2 s - Etch rates: 0. nm s - for Ge Area: several tens of square cm Sevilla, 203 Slide 3

15 Caution 4 ev Ar ICP: Average at sample 3.7 ev transfer to Ge atom Sample temperature increases 3 x 0 minute ICP (40 C) Annealing also in time and at increased temperature Sample temperature increases 3 x 0 minute ICP (40 C) Hydrogen passivation of defects Sevilla, 203 Slide 4

16 Our basic experiment: 4 ev Ar-ICP plasma annealing.-sb doped Ge is damaged with 5 MeV alpha particles Rest 24 hours 2.-Au diode is evaporated in half the sample (half A) 3.- DLTS on A (black) 4.-ICP on A and B: 3 x 0 min 5.- Au RE - diode B 6.- DLTS on A (red-dashed) 7.- DLTS on B (blue) 5

17 Observations 4 ev ICP: E-center concentration -30% Effect depth in Ge: exceeds 2600 nm ICP through metal less annealing Annealing diminished with higher sample temperature. x 30 minute ICP (70 C) 8 ev Ar ICP (T?) Compare to anneal with phonons: Low T (40 C) ~ 50 C High efficiency Lower efficiency sergio@up.ac.za Sevilla, 203 Slide 6

18 Our hypothesis: Ar ions impacting on Ge produce Intrinsic Localised Modes that travel through Ge and anneal defects. Why?.-ILMs with MD in metals by Hyzhnyakov group have ev 2.-The maximum energy transfer from Ar to Ge is 3.6 ev 3.- The activation energy for annealing an E center is about.36 ev 4.- Energy remains localized exceeding 4000 lattice units 5.- Increasing the energy of the plasma does not enhance the effect, this is because ILMs typically have a definite range or energies. 6. Increasing sample temperature effect diminished. 7.- At least stationary ILMs have been obtained for Si and Ge with MD. 7

19 Conclusions: Plasma of 4eV produces annealing of defects very deep in Ge. The energy delivered to E-center is ~ ev. Likely conclusions:. 4 ev Ar hit produces an ILM in Ge with very high efficiency. 2. ILM of energy ~3eV travel distances of the order of 0 4 lattice units or more. 3. The annealing efficiency of ILM with respect to phonons is extremely large. 8

20 Gracias Acknowledgements J. F. R. Archilla many suggestions & hospitality South African NRF Financial assistance Group members for listening & helpful comments. Sevilla, 203 Slide 9

21 Our basic experiment: Facts.-Sb concentration: cm -3 (n i = cm -3 ) ; Sb per 0 8 Ge 2.- Metal (Au) thickness: 25nm 3.- After ICP on Ge the E center concentration drops 29% from N T = cm If ICP is done on Au, the E center reduction is smaller, but exists. 5.- ICP is done for 30 in intervals to prevent heating 6.- Defect annealing occurs up to 2600 nm or 4600 lattice units 7.- If the plasma energy is increased the effect is smaller 8.- Thermal annealing has to be done at 50 C to obtain a similar effect. 20

22 Some numbers.-ion current can be measured, 2.- DB creation efficiency: Ar cm s 0.04a s : DB Ar ; 3.-Number of breathers: 4. Phonons: E ph 0.035eV n ph DB ndb cs 3nGe exp( E / k ph B cm 4.6 T ) cm -3 For N T dn T, DB dt s N T dn T, RT dt 2 0 s Relative annealing efficiency per DB or phonon: ~0 22 Relative annealing efficiency per ev of DB or phonons: ~0 20 2

23 Interaction cross-section and energy delivered by a breather Interaction cross-section σ dn dt T N T DB E0 exp( k B T ) Interaction cross-section 0 ; Minimal interacion cross-section 2/ n Ge 0 сm Apparent diminution of the activation energy because of DB interaction: Δ dn dt dn dt T T ions 0 N N T T For ions ions E0 exp( k, B T ) ions.2ev 50,, unknown cm 22

24 References. 23

25 Advantages: Electron beam deposition Easily evaporates high melting point metals. Highly controllable deposition rates. Good adhesion. Disadvantage: Introduces defects at and below the surface of semiconductors. Chen et al Mooney et al Sevilla, 203 Slide 2

26 E vs AMU for e or H.5 ev to create Frenkel pair st : electron-atom 2 nd atom to crystal Example: e to H to Ge sergio@up.ac.za Sevilla, 203 Slide 2

27 The Defect Concentration Defect concentration from peak height, C/C: N T ( x m ) 2N D( x) C C t 0 x x 2 x p x 2 x m ( x x p ) / 2 x 2 Vbi qn D V 2 ( E q F ET) 2 N D N T as low as 0 0 defects /cm 3 Y. Zohta et al, JAP. 53, 809 (982) sergio@up.ac.za Sevilla, 203 Slide 2

28 Depth Distribution of EBD defects Use fixed bias, variable pulse DLTS method With correction. Not possible to profile the hole traps: Hole concentration is not known. V-Sb (E 0.38 ): Concentration decreases rapidly away from the surface. Approaches 0 4 cm -3 at surface. Diffusion of vacancies from the surface? Y. Zohta et al. J. Appl. Phys. 53, 809 (98) Defect Concentration (cm -3 ) E 0.38 below a Pt SBD formed by EBD V r = -2.0 V T = 205 K dv p = 0. V Depth below junction (microns) sergio@up.ac.za Sevilla, 203 Slide 2

29 E-beam deposition - Pt 0 kev electrons Beam path Reflected electrons Particles Experiment sergio@up.ac.za Sevilla, 203 Slide 2

30 E-beam shielding e-traps High vacuum-2 shields F/gas 2 shields F/gas shield F/gas no shields Standard EBD sergio@up.ac.za Sevilla, 203 Slide 2

31 E-beam shielding h-traps High vacuum-2 shields F/gas 2 shields F/gas shield F/gas no shields Standard EBD sergio@up.ac.za Sevilla, 203 Slide 2

32 Energy levels E 0.37 no surprise! E.3 new E 0.28 new E 0.3 E 0.0 Use L-DLTS Conclusions sergio@up.ac.za Sevilla, 203 Slide 2

33 Experiment 2 - EBE Deposition vs exposure No similarities sergio@up.ac.za Sevilla, 203 Slide 2

34 Arrhenius plots L-DLTS No hole traps in common New defects impurity related? Sevilla, 203 Slide 2

35 Experiment 2 Au & e-traps E-traps dominant Defect concentration sergio@up.ac.za Sevilla, 203 Slide 2

36 Arrhenius plots L-DLTS E 0.38 Common to both New defects perhaps not impurity related sergio@up.ac.za Sevilla, 203 Slide 2

37 Summary and Conclusions. Shielding lowers the defects introduced by EBD. Damage caused by impacts with ions / particles not e. 2. Damage caused in st 0.5 µm, at / near defect site. How was the energy transferred? 3. Damage caused by E <.3 ev transfer to Ge. Only enough to displace H or light atoms single bond. Will DLTS be more useful in future? sergio@up.ac.za Sevilla, 203 Slide 2

38 Thank you Acknowledgements J. F. R. Archilla many suggestions & hospitality South African NRF Financial assistance Group members for listening & helpful comments. Sevilla, 203 Slide 2

39 MeV electrons.0 E 0.24 E 0.3 E 0.37 V r = 2 V V P = 0 V t p = ms RW = 80 s DLTS Signal (pf) 0.5 x x E 0.0 E 0.5 E 0.20 E 0.2 E 0.24 x0 E E 0.3 x5 MeV electrons Alpha irradiation (d) (c) 0.0 x 0 x 0 E? E? E 0.3 RF sputter etch (b) (a) Temperature (K) sergio@up.ac.za Sevilla, 203 Slide 2

40 Electron beam deposition 0.02 V r = - V V P = 3 V t p = ms * 08 K * H 0.22 L-DLTS signal H * * H K 02 K * 99 K Fig. 7 (of 7 ), S. M. M. Coelho et al Emission rate (s - ) sergio@up.ac.za Sevilla, 203 Slide 2

41 E vs AMU for e or H kev electrons 24eV hydrogen.5 ev to create Frenkel pair Energy (ev) Atomic mass (AMU) sergio@up.ac.za Sevilla, 203 Slide 2

42 I-V: Pt diodes 0-2 Forward bias Ideality = No shield Standard vacuum No shields with forming gas 2 shields & high vacuum Previous lowest:.05 Current (A) Reverse bias Current lowest measured 0-6 Defects linked to e-beam Voltage (V) Next experiment sergio@up.ac.za Sevilla, 203 Slide 2

43 Defects in Semiconductors Defects can be good or bad Solar cells: bad : eliminate them!!! Fast switches: good : deliberately introduce them! Defects are introduced during Crystal growth, sawing / cutting and polishing Critical processing steps» Surface cleaning by particle processing (sputter etching)» Metallization Radiation» Space, reactors» Accelerators / implanters Important defect parameters Energy level, E T, in bandgap Capture cross section, Concentration, N T sergio@up.ac.za Sevilla, 203 Slide 2