I have always been attracted by the concept of Feedback which is widely present either in God–made-systems or in Man-made-systems. The roots of this attraction lay probably on the ecological concerns of the sixties which rose the paradigm of a Systems View of the World cared by scientists like Joël de Rosnay in his book The Macroscope or James Lovelock with his Gaya hypothesis.

Feedback is also what living beings use against the deadly faith of the second law of Thermodynamics.

Models and analogy may be revealing, since “what is ^up is similar to what is _down ” (Hermes Trismegistus).

... These are the homeopathic dr₀ps inside my courses on fundamentals of Control theory ... although the System is becoming too much complex for the medicine !

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Feedback Teaching and Touching

I give especial importance to the real world demonstration of concepts. Well chosen experiments are condensation nucleus of the concepts. Experimentation reveals the approximate nature of the theory, besides its usefulness.

Two funny products of that pursuit are shown in the photos.

- One is a new version of the classical Ball and Beam system which uses an optical infrared sensor to detect the position of the ball, what one may call the Ball, beam and light-beam system.

- The other artefact is an electromechanical system that intends to simulate the dynamic coupling of the main and tail rotors of a Helicopter and allows to study feedback control in a MIMO system.

These didactic kits have been designed and constructed in the Control Systems Section of the Department of Electrical and Computer Engineering of IST with cheap materials and components and show satisfactory performance in a pedagogical environment (I acknowledge the collaboration of colleague J. Sanches and students .J. Pires and R. Rebelo).

“BALL-BEAM and LIGHT BEAM”

Features:

Opto-electronic position sensor

Signal processing electronics (synchronous detection)

P, P-I, P-D control actions

Analogical outputs for system variables display and record

____________________________________________________________________________________________________________________

“HELICOPTER”

Features:

MIMO system

Interface for computer control

α: pitch; β: yaw; v_α,β : motor voltage

decoupling control law:

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Research

Life needs Feedback and Energy.

Life is what Energy is for.

Energy comes from Ra, the Egyptian Sun God

,… especially when Oil is going to be scarce as Rome Club predicted in the seventies.

And there is beauty when Light meets Matter,… whatever these categories might be;

and …

a Solar Cell is the right meeting place.

My research work lies in the area of Thin Film Semiconductors and Devices for Photovoltaics and Optoelectronics. A short Chronology tells the time and the mood, a mixture of real-world (experimental) and virtual-world (modelling) experiments, which I try to be isomorphic. On the way is Current Research, supported on local developed Facilities, Simulation software and Characterisation techniques with external Collaborations.

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Chronology:

# - In 1976 I joined Centro de Física Molecular (Lisbon). Since 1976 up to 1981, after implementing the deposition system, I have produced and characterised CdS-Cu₂S (cadmium sulphide-copper sulphide) polycrystalline thin films and solar cells obtained by thermal evaporation in vacuum, (the first solar cells produced from raw materials in a Portuguese laboratory).

CdS-Cu₂S photovoltaic cells and panel prototype (CFM - 1981)

and respective thin film deposition system

# - In 1982 I moved to the Amorphous/Microcrystalline Silicon field, an interesting and by then emerging material for Photovoltaics and Macroelectronics.

Hydrogenated amorphous silicon (a-Si:H) become interesting for applied electronics when material doping was achieved by Spear and Le Comber in 1975 followed by the first a-Si:H based photovoltaic junction device produced by Carlson and Wronsky in 1976. The challenges were, essentially, twofold. One of improving the electronic quality of the semiconductor which, unlike his crystalline brother, has almost all the carriers trapped in defect states (no more a forbidden energy gap but, instead, a continuous distribution of states over a pseudo-gap). And, second, the challenge of increasing the knowledge and understanding of the properties of this novel amorphous semiconductor beyond the frontiers of Mott and Davis reference book on “Electronic Processes in Non-Crystalline Materials”.

# - My Ph.D. thesis (1989) “Production and characterisation of hydrogenated amorphous silicon nitride films (a-Si:N:H), optical, electrical and structural properties”, a contribution to the so-called band-gap engineering, benefited from long stays in Centre National de Recherche Scientifique (CNRS) in Grenoble, in a collaboration with Dr. Étienne Bustarret at LEPES.

# - After Ph.D., I concentrated on Transport and Recombination properties of amorphous semiconductors and developed electronic Recombination models and related software for numerical resolution, taking into account the specific features of band-gap structure in a-Si:H: the dangling-bonds deep defects and the band-tail states. These models have been used in the study of the Fermi level, temperature and light intensity dependences of photoconductivity, majority and minority carriers in a-Si:H (namely the Rose g factor, including supralinearity and associated thermal quenching).

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Current Research interests

A special research interest is the photo-degradation of hydrogenated amorphous Silicon (a-Si:H) films, the so-called Staebler-Wronsky effect, a phenomenon that limits the performance of amorphous silicon based devices.

Light-induced (LID’s) and radiation-induced (RID´s) metastable defects are an opportunity to study, via recombination models, how changes in the electronic density of states are related to changes in the optoelectrical properties.

Minority carrier properties are an important topic of our research since they determine the quality of photovoltaic devices.

Gallium Nitride (GaN) and Zinc Oxide (ZnO) thin films and devices are recent interests, in collaboration with Prof. R. Schwarz’s team in the Physics Department of IST.

Fig - Mobility lifetime products of electrons and holes in a-Si:H as functions

of light-exposure time and annealing time.

A lot of Characterisation techniques have been implemented over the years, directed to the investigation of electronic recombination and defects, majority- and minority-carrier charge transport, in amorphous/microcrystalline semiconductors and related devices. These techniques can also be applied to organic semiconductors.

See Publications for results.

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Facilities

Vacuum System – for Physical Vapour Deposition of powders and metals

Software – package “DBTAIL” for simulation of recombination processes in semiconductors whose electronic density of states N(E) is composed by discrete correlated defects (for instance, dangling-bonds) and continuous distribution of states (for instance, tail states).

Inputs: N_i(E), capture cross sections, mobility gap, Fermi level, photo-generation rate, temperature.

Outputs: electron concentration, hole concentration, gap-states electronic occupation, majority- and minority-carrier lifetimes, photoconductivity.

Characterization techniques

Photothermal Deflection Spectroscopy (PDS)

Steady State Photocarrier Grating technique (SSPG)

Modulated Photocarrier Grating technique (MPG)

Spectral Photoconductivity (SPC) - Spectral Response (SR)

Modulated Photo-Current (MPC)

Constant Photocurrent Method (CPM)

Description and pictures

Photothermal Deflection Spectroscopy (PDS) (since 1996) for measurement of defect related optical absorption. The PDS technique detects the heat produced by light absorption in the sample due to non-radiative recombination. The sample is introduced in a fluid (CCl₄). A laser beam (He-Ne) probes the gradient of the fluid index of refraction, induced by non-radiative recombination heating of the sample; deflexion of the laser beam is measured by a position sensitive detector.

The power normalized PDS signal, for thermally thin samples, can be expressed by:

where α is the optical absorption coefficient, d the film thickness, and S₀ is the saturated signal at high photon energy where αd>>1.

Data acquisition and record are fully automated by computer control of the set-up.


		PDS spectra of a-Si:H for increasing 1.5 MeV He⁴ particle irradiation fluence

- Technical note -

PDS measures light absorption in the bulk of the material, being insensitive to reflected and scattered light unlike the more common Optical Transmission Spectroscopy. This feature allows an accurate measurement of the very low absorbance related to deep defects in the pseudo-band-gap of disordered semiconductor like amorphous Si and Ge alloys, and electronic polymers.

Optical absorption coefficient a (hn) is a function of the convolution of the initial N(E) and final N(E+hn) density of states involved in the electronic transition produced by photons of energy hn.

a (hn) µ ò N(E) N(E+hn) dE

Deconvolution of the PDS spectrum allows the estimate of the density of states distribution N(E).

Steady State Photocarrier Grating technique (SSPG) (since 1998), for measurement of the ambipolar diffusion length, L_amb. and of the mobility-lifetime product (mt) of the minority carriers.

b is the ratio of the photocurrents in the grating and non-grating modes.

L Is the spatial period of the interference light grating:

L_amb is the ambipolar diffusion length: for (mt)_e >> (mt)_h

Modulated Photocarrier Grating technique (MPG) (1999) for assessment of ambipolar transport.

These two techniques, SSPG and MPG, are based on microscopic light gratings produced by laser interference with spatial period L comparable to the diffusion length L of the material under analysis, and yield information on minority carrier properties.

The SSPG technique relies on the assumption that t >> t_d , where t_d is the dielectric relaxation time. The MPG technique is an attempt to obtain good estimates of the minority carrier transport parameters when that relation does not work.

Spectral Photoconductivity of photo-sensible films and Spectral Response of devices.

Data acquisition and record are fully automated by computer control of the set-up.

Modulated Photo-Current (MPC) (since 1998) (Fourier transform of transient photocurrent). In the small-signal mode, under optical bias, MPC gives the small-signal lifetime or response time, t . In the total signal mode, on the basis of the multi-trapping model, MPC yields the density of states distribution close to the dominant conduction channel; in undoped or n-doped a-Si:H, it estimates the conduction band-tail states distribution, thus complementing the valence band-tail states distribution deduced from PDS or CPM.

Constant Photocurrent Method (CPM) (since 1999) – Photocurrent magnitude in the sample is kept constant by feedback control of the impinging photon flux F , along the investigated spectral photon energy range. Invariance of the photocurrent assures invariance of the recombination lifetime during the measurement. For optically thin samples the optical absorption coefficient a is inversely proportional to the measured F . This technique probes the charged states that contribute to the photocurrent, mainly in the bulk, unlike PDS which probes all non-radiative transitions and is sensitive to surface states.

These techniques are useful for optical and electronic studies in Thin Films of Semiconductors, Polymers, Insulators and related Devices.

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Contracts and collaborations

- "Projecto de desenvolvimento no domínio da conversão fotovoltaica da energia solar", (Project and development on photovoltaic solar energy conversion), contract nº 03.78.08, JNICT, 1978-1981.

- "Desenvolvimento de um módulo solar usando células fotovoltaicas de a-Si e CdS/Cu₂S", (Development of a solar panel using a-Si and CdS/Cu₂S photovoltaic cells), contract nº 321.81.71, JNICT, 1981-1985.

- "Defect Studies in Amorphous Silicon by Photothermal Deflection Spectroscopy" contract nº PBIC/C/CTM/1385/92, JNICT, 1993-1997.

- " Transient Grating Methods for Transport Studies in Thin Films of Semiconductors, Polymers and Insulators", contract PRAXIS/PCEX/P/FIS/7/96, prime contractor IST (Physics Department), in collaboration with Faculty of Sciences of University of Lisbon and Instituto Superior de Engenharia de Lisboa - 1997-1999.

- " Time Modulated CVD Processes for Preparation of Advanced Thin Film Materials - TIMOC", contract nº ERB-IC15-CT98-0819, COPERNICUS program, prime contractor IST-Portugal, in collaboration with Bulgarian, German and Russian Institutions, begin November 1998.

- “Pulsed Laser Deposition (PLD) of Wide Bandgap Semiconductors”, contract PRAXIS/P/FIS/10178/1998 – prime contractor IST (Physics Department), in collaboration with University of Aveiro and Instituto Superior de Engenharia de Lisboa – begin 1999.

- “Non-linear Optical Effects in Wide Band Gap Semiconductors (OPTAR)”, contract POCTI / FAT / 48822 / 2002 - prime contractor IST (Physics Department) – in collaboration with University of Aveiro, Instituto Superior de Engenharia de Lisboa and Universidade do Algarve - begin 2004.

Publications (a selection)

- L. Guimarães et E. Morgado, "Fabrication de cellules solaires au Sulfure de Cadmium-Sulfure de Cuivre", Proceedings of Portuguese-French Seminar on Solar Energy, Lisboa, Mars l979 (Édition Service Culturel et de Coopération Scientifique et Technique de l'Ambassade de France, 1979).

- L. Guimarães, R. Martins e E. Morgado, "Photovoltaic Solar Energy Research in Portugal” (in Portuguese: Investigação científica em Portugal no campo da Energia Solar Fotovoltaica), Las Ciências, Vol. XLVI, No. 1/2 (l981) p. 54-59.

- E. Morgado e L. Guimarães, "Research and development of Cadmium Sulphide-Copper Sulphide photovoltaic cells” (in Portuguese: Estudo e desenvolvimento da célula fotovoltaica de Sulfureto de Cádmio-Sulfureto de Cobre (CdS-Cu₂S)), 3^rd Meeting of Portuguese Physical Society , FISICA 82, Coimbra, June l982, Abstracts book, G 34.

- E. Morgado e L.Guimarães, " CdS-Cu₂S photovoltaic cell by thermal evaporation in vacuum” (in Portuguese: Célula fotovoltaica CdS-Cu₂S por evaporação térmica em vácuo), Proceedings of 1^st Iberian Solar Energy Conference ISES 82, Madrid, l982 (editors. M. Collares Pereira, A. Luque e A. Silvério), Vol.2, pp.VI.19 - VI.22.

- E. Morgado, R.M. Almeida and L. Guimarães, "Large optical gap photoconductive a-Si:N:H by r.f. glow-discharge in SiH₄-Ar-N₂", Proceedings of the 6th European Photovoltaic Solar Energy Conference, London, (ed. D. Reidel, Dordrecht, l985) pp.693-696.

- E. Bustarret and E. Morgado, "Angular distortions and the exponential absorption edge of Silicon-rich a-Si_xN_yH_z alloys", Solid State Communications, Vol. 63 (l987) pp. 581-585.

- E. Morgado and E. Bustarret, "Relations between Raman scattering and optical absorption parameters in PECVD a-Si:N:H alloys", Proceedings of the 8th European Photovoltaic Solar Energy Conference, Firenza, (ed. D. Reidel, Dordrecht, l 988) p.898-901.

- E. Morgado, "Recombination at correlated dangling-bonds and the effects of Fermi level position on steady-state photoconductivity in amorphous silicon", Philosophical Magazine B, Vol. 63 (1991) pp. 529-542.

- E. Morgado, “Electronegativity effects on the frequency of the N-H stretching mode in plasma deposited a-Si:N:H alloys", Journal of Non-Crystalline Solids, Vol. 139 (1992) pp. 248-256.

- E. Morgado, “Mobility-lifetime products in a-Si:H and the dangling bond recombination model", Amorphous Silicon Technology - 1992, Materials Research Society Symposium Proceedings Series, Vol. 258 (MRS, Pittsburgh, 1992), pp. 765-770.

- E. Morgado, “Electron and hole mt products in a-Si:H and the standard dangling-bond model”, Journal of Non-Crystalline Solids, Vol. 164/166 (1993) pp.627-630.

- E. Morgado, “Recombination in a-Si:H: results from the standard and defect-pool models”, Proceedings of the 1994 IEEE First World Conference on Photovoltaic Energy Conversion (24th IEEE Photovoltaic Specialists Conference), Waikoloa, Hawaii, December 1994, Vol. I (199 4) pp. 591-594.

- E. Morgado, “Thermal quenching and photo-enhancement of mt products in a-Si:H - the role of dangling bonds and band tails”, Amorphous Silicon Technology - 1994, Materials Research Society Symposium Proceedings Series, Vol. 336, (MRS, Pittsburgh, 1994), pp. 419-424.

- E. Morgado, "Modelling of Supralinearity and Thermal Quenching of Photoconductivity in Amorphous Silicon", Defect and Diffusion Forum, Vol. 134/135 (1996) pp. 39-46.

- E. Morgado, “Supralinearity and thermal quenching of photoconductivity in a-Si:H : the role of gap state structure", 1996 Chelsea Amorphous and Organic Semiconductors Meeting, King's College, Londres, April 1996.

- M. Vieira, A. Maçarico, E. Morgado, S. Koynov, R. Schwarz, "A two dimensional visible/infrared detector based on microcristallyne Si:H p-i-n structure", 17th International Conference on Amorphous and Microcrystalline Semiconductors, Budapest, Hungria, 1997, and Journal of Non-Crystalline Solids, Vol. 227-230 (1998) pp. 1311-1315.

- E. Morgado, M. Rebelo da Silva and R.T. Henriques, "Light-soaking effects on photoconductivity in a-Si:H thin films" Amorphous Silicon Technology - 1997, Materials Research Society Symposium Proceedings Series, Vol. 467 (MRS, Pittsburgh, 1997) pp. 109-114.

- E. Morgado and R.T. Henriques, "Metastable defects and recombination in hydrogenated amorphous silicon", 19th International Conference on Defects in Semiconductors, Aveiro, Julho 1997, and Materials Science Forum, Vol. 258-263.

- M. Vieira, E. Morgado, A. Maçarico, S. Koynov, R. Schwarz, "Microcrystalline silicon thin films for optical sensor applications", VACUUM 52 (1999) pp. 67-71.

- E. Morgado, J. Díez, R. Schwarz, A. Maçarico, S. Koynov, “New results on the Modulated Photocarrier Grating Technique”, 18th International Conference on Amorphous and Microcrystalline Semiconductors (ICAMS 18), August 1999, Snowbird, USA and Journal of Non-Crystalline Solids, Vol. 266-269 (2000) pp. 290-293.

- E. Morgado, “Defect-related photoinduced absorption in amorphous silicon”, Physica B: Physics of Condensed Matter, 308-310 (2001) pp. 174-177.

- E. Morgado, “Light-soaking and annealing kinetics of majority and minority carrier mobility-lifetime products in a-Si:H”, Journal of Non-Crystalline Solids 299-302 (2002) 471-475.

- E. Morgado, “Influence of light-soaking and annealing on electron and hole mobility-lifetime products in a-Si:H”, Journal of Non-Crystalline Solids 338-340 (2004) 386-389.

- E. Morgado, R. Schwarz, T. Braz, C. Casteleiro, A. Maçarico, M. Vieira, E. Alves, “Radiation induced defects in a-Si:H by 1.5 MeV He⁴ beam particles studied by photoconductivity and photothermal deflection spectroscopy”, Journal of Non-Crystalline Solids 352 (2006) 1071-1074.

- R. Schwarz, R. Cabeça, E. Morgado, M. Niehus, O. Ambacher, C.P. Marques, E. Alves, “Stability of GaN films under intense MeV He ion irradiation”, DIAMOND & RELATED MATERIALS 16 (2007)1437-1440.

- P. Queiroz, R. Ayouchi, M. Niehus, E. Morgado, A. Fedorov, J. Martinho, F. Wuensch, M. Kunst and R. Schwarz , “Photoinduced Excess Carrier Dynamics in PLD-Grown ZnO”, Superlattices and Microstructures 42 (2007) 270-277

Student opportunities

Opportunities for student training in the topics of Optoelectronic materials and devices, Control of experimental set-ups and data acquisition, Laser and Optical Instrumentation, Modelling of semiconductor electronic transport and recombination, are available in the frame of Graduation and Post-graduation work, namely SOCRATES-ERASMUS program.

Cooperation is Welcome

Mail to: emorgado@ist.utl.pt

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