1. Trapping Phenomena in Nanocrystalline Semiconductors Magdalena Lidia Ciurea Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007- National Institute of Materials Physics Bucharest-Magurele, Romania.

2. 1. Introduction Contents: 2. Traps in group IV nanocrystalline semiconductors 3. Traps in II-VI nanocrystalline semiconductors 4. OCS modeling 5. Conclusions Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea

3. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea 1. Introduction The traps - in nanowires and nanolayers: similar with those in bulk semiconductors. - in nanodots, supplementary trapping appears on the quantum confinement (QC) levels. Several methods:  deep level transient spectroscopy (DLTS);  photoinduced currents transient spectroscopy (PICTS);  thermally stimulated currents (TSC);  optical charging spectroscopy (OCS).

4. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea  zero bias method;  step one: the sample is cooled down at T 0 and then it is illuminated with monochromatic light; λ is chosen in the absorption band; photogenerated carriers diffuse with different velocities (v n ≠ v p ) into the film (L D >d). light intensity sufficiently high  uniform filling of traps (if not, the filling decreases with depth).  the trapped carriers generate a frozen-in electric field  step two: after switch-off of the light, sample is heated (at a constant rate, quasistatic process)  the detrapped carriers move under the field of the still trapped ones  discharge current. Optical charging spectroscopy (OCS)

5. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea 2. Traps in group IV nanocrystalline semiconductors Traps in nc-Si proposed as tools for quantum computers and memory devices. The schematics of the nitrided nc-Si dot memory. Traps of SiN x (1 nm thickness) are exactly located on each nc-Si dot. Both of them offer dual memory-nodes. (a) writing and (b) erasure processes in nitrided nc-Si dot based memory devices. (1) Direct tunneling from channel to nc-Si dot; (2) polarization to the top of the dot; (3) drop into defect traps at nc-Si/silicon- nitride interface; (I) delocalized states in nc-Si dots; (II) localized states in defect traps. [5] S. Huang and S. Oda, Charge storage in nitrided nanocrystalline silicon dots, Appl. Phys. Lett. 2005; 87:173107 – 1-3.

6. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea Displacement current characteristics of the samples under identical gate-voltage scan rate at room temperature. (a) Charge/discharge current peaks of Samples A and B. (b) No current peaks in Sample C.

7. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea Traps centers in SiO 2 films containing nc-Si AFM images: (a) An etched SiO 2 film containing nc-Si (Si ion implantation and annealing) before charging. (b) The same sample after charge transfer. The injected charge is imaged as a protrusion on the surface. The lateral size for (a) and (b) is 0.5 μm. The vertical scale (black to white) is 15 nm for (a) and 25 nm for (b). (c) and (d) - AFM images before and after charging of an etched SiO 2 film implanted with Ar + ions and annealed. In this sample no localized charging is seen  charge is not trapped in defects from the implantation process. The lateral size for (c) and (d) is 1 μm. The vertical scale (black to white) for both is 1.5 nm. CONCLUSION: charge trap centers in annealed, Si-implanted SiO 2 films are due to the presence of nanocrystals. Elizabeth A. Boer, a) Mark L. Brongersma, and Harry A. Atwater, Richard C. Flagan, L. D. Bell, Localized charge injection in SiO 2 films containing silicon nanocrystals, Appl. Phys. Lett. 2001; 79(6), 791.

8. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea OCS discharge current in a fresh nc-PS sample: a) λ = 1.0 μm; b) λ = 0.5 μm Ea 1 = 0.29 eV, Ea 3 = 0.47 eV and Ea 4 = 0.61 eV (from. fractional heating procedure). Ea 2 = 0.37 – 0.41 eV !! cannot be determined with enough precision) M. L. Ciurea, M. Draghici, S. Lazanu, V. Iancu, A. Nasiopoulou, V. Ioannou, and V. Tsakiri, Trapping levels in nanocrystalline porous silicon, Appl. Phys. Lett. 2000; 76:3067-9. Traps in nc-PS Oxidized films:  the first three maxima/shoulders are flattened→due to surface trapping centers.  a new maximum, Ea 5 = 0.82 eV: bulk traps? or surface/interface strain?

9. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea SEM images of the cross sections of PS film (fractured silicon wafer). a 1 and a 2 : cross sections nearly parallel with (100) plane of Si (perpendicular to PS film); b 1 and b 2 : an oblique cross section nearly parallel to the (110) cleavage plane of Si. First level porosity: honeycomb-like system of alveolar columnar macropores (1.5 – 3 µm diameter, 100 – 200 nm alveolar walls thickness) – 70 % porosity.

10. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea Second level porosity: nanowires network (mean diameter 3.25 nm) – 50 % porosity. (a) TEM conventional contrast and (b) high- resolution detail of the wall surface of PS nanostructure, revealed by the lattice fringes contrast with respect to the amorphous structure of the silicon oxide and glue. M. L. Ciurea, V. Iancu, V. S. Teodorescu, L. C. Nistor, M. G. Blanchin, J. Electrochem. Soc. 146, 3516 (1999).

11. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea OCS discharge current in MQW structure: a) zero (no illumination) curve; b) OCS (λ = 0.5 μm) curve. Fractional heating procedure for OCS in MQW (λ = 0.5 μm). Multi-quantum well (MQW) (nc-Si/CaF 2 ) 50  thickness of nc-Si and CaF 2 layers – g = 1.6 nm IMPORTANT: the second maximum disappears in curve c, but it reappears in curve d, with opposite sign. the third one reappears with opposite sign in curve d  strong retrapping process  the third trapping level correspond to opposite sign carriers than the other ones. Ea 1 = 0.30 eV, Ea 2 = 0.42 eV, Ea 3 = 0.44 eV, Ea 4 = 0.75 eV. 3

12. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea 3.Traps in II-VI nanocrystalline semiconductors Absorption (left) and photoluminescence (right) spectra of the CdSe nanocrystals (3.9 nm core diameter), with and without passivation (OA = octylamine). D. E. Gómez, J. van Embden, J. Jasieniak, T. A. Smith, and P. Mulvaney, Blinking and Surface Chemistry of Single CdSe Nanocrystals, Small 2006; 2:204-8. Trapping phenomena in single CdSe nanodots - studied by absorption and blinking photoluminescence.  red shifts after the coating with CdS may be related to the quantum confinement  the increase of the diameter implies a decrease of the QC levels.  CONCLUSION: coating with CdS eliminates the surface traps (curves + statistics of the blinking PL).

13. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea 4. OCS modeling Sandwich configuration – semitransparent top electrode / nc-film / substrate / bottom Ohmic electrode. Assumptions: i. all the traps located in the nc-film; d << l; σ film << σ substrate where d and l – nc-film and sample thicknesses, respectively. ii. the heating is quasistatic (β small). iii. the trapping levels have zero width.  if the illumination time is long enough  steady state.  d < L D, L λ << l (L D - bipolar diffusion length; L λ - light penetration depth).  if the illumination is strong enough  all the traps are filled up.

14. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea Detrapping-retrapping equations in quasistatic regime:, , are the concentrations of the carriers trapped on the levels i and k;  N ti and P tk are trap concentrations. for MQW: traps generated by local stresses (different thermal expansion coefficients) , where T s = 300 K storage temperature and γ a critical exponent.  capture coefficients, - capture cross-sections and, - the thermal velocities for electrons and holes.

15. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea,,,  Frozen-in electric field (trapped carriers): - built-in field,;,    E ti, E tk - activation energies of the trapping levels i, j and - non-equilibrium carriers detrapped from the levels i and j and τ ni, τ pk their lifetimes. System. (1, 1’) - non-linear coupled differential equations  can be solved only numerically. For weak retrapping (τ ni c ni N ti, τ pk c pk P tk << 1), Eqs. (1, 1’) can be decoupled and analytically solved. This happens for nc-PS, but not for MQW.,  Electrical conductivity (Ohmic conduction nc-PS): - equilibrium carriers - non-equilibrium carriers

16. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea  The total OCS discharge current: - mean value of current density; The current is the sum of five contributions: (i) The non-equilibrium carriers conduction current density: A quadratic function of the trap concentrations (ii) The equilibrium carriers current density: One to two orders of magnitude smaller than j ne (iii) The diffusion current density: Its mean value is

17. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea (iv) The displacement current density:  j d sign given by its temperature derivative, not by the electric field.  in nc-PS j d is negligible  in MQW this is the main contribution  Correlation between the surface and bulk concentrations:  (v) The tunneling current density:  in nc-PS, the tunneling current is much smaller than the conduction one  it is neglected.  in MQW:, (φ - the barrier height),  Simmons high field-assisted tunneling formula,, ;.

18. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea Applications:  fresh nc-PS λ = 0.5 μm, Fitting of the OCS results for fresh nc-PS sample: solid line – model; dotted line – experimental data., V. Iancu, M. L. Ciurea, and M. Draghici, Modeling of optical charging spectroscopy investigation of trapping phenomena in nanocrystalline porous silicon, J. Appl. Lett. 2003; 49:216-23. Max. No. Max. Type  (10 -22 m 2 ) N t (P t ) (10 17 m -3 ) τ (ns) E t (eV) ModelExp. 1p S3.018.0500.300.29 2’p S3.015.0500.37 0.37 – 0.41 2’’n S1.52.5500.41 3n S0.914.0500.47 4p B3.00.851500.61 Parameter values for the nc-PS trapping levels:

19. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea 1 2 3 Fitting of the OCS results for MQW structure: solid line – model; dotted line – experimental data. Critical exponent for the spikes is γ = 4., Max. No. Max. Type  (10 -22 m 2 ) N t (0) (P t (0) ) (10 20 m -3 ) τ (ns) γ E t (eV) ModelExp. 1n S1.7066.0040040.30 2n S0.4126.0040040.42 3p S1.000.2918000.44 4n S1.5055.0040000.720.75 Parameter values for the MQW trapping levels:

20. Nano and Giga Challenges in Electronics and Photonics Phoenix, Arizona, March 12-16, 2007 National Institute of Materials Physics Bucharest-Magurele, Romania. Magdalena Lidia Ciurea 5. Conclusions  The traps in nanocrystalline semiconductors are mainly located at the surface / interface of the nanocrystals.  The nanodots introduce supplementary trapping on the quantum confinement levels.  A general model for the OCS trapping-detrapping-retrapping processes was proposed and discussed. The model allows to determine the trap parameters that are not directly measurable. It was successfully applied to nc-PS and MQW structures.