Originalitatea

ORIGINALITATEA ȘI CONTRIBUȚIA INOVATIVĂ A PROIECTULUI

In cadrul general al efortului stiiintific de obtinere de structuri fotovoltaice cu eficienta buna, acest proiect propune sistemul AlxGa1-xSb/GaSb, o heterostructura obtinuta prin tehnica MBE. Fotocurentii in domeniul 1-10 A sunt tipici pentru dezvoltarea generatorilor PV in baza GaSb, o problema care va fi rezolvata in acest proiect va fi design-ul convertorilor PV cu rezistenta serie mica. Facilitatile experimentale legate de obtinerea si caracterizarea acestei structuri sun disponibile in INCDFM-Magurele. Evolutia calitatii suprafetei pregatite prin corodare chimica pentru cresterea epitaxiala, va fi investigata prin tehnicile AFM  si STM. Tinand cont de asteptarile legate de eficienta de conversie la radiatie monocromatica de putere mare, structura fotosenzitiva GaSb va fi investigata la iradiere laser iar interactia de suprafata va fi investigata prin AFM si SEM impreuna cu o caracterizare electrica completa. Mentionam ca caracteristicile initiale ale celulelor solare din compusi III-V sunt excelente din punct de vedere teoretic. Datorita structurii electronice de banda directa ele pot absorbi pana la 97% din radiatia solara in conditii de sol AM1 intr-o grosime de 2μm.

Parametrul principal care este marimea gap-ului poate fi modificat in compusi ternari si cuaternari prin selectarea proportiilor relative ale elementelor in compus- se realizeaza ceea ce numim inginerie de banda. Dupa cum a fost prezentat, fotoconversia radiatiei produse in domeniul gap-ului mic (Eg = 0.5-0.8 eV) este eficienta in cazul dezvoltarii emitorilor selectivi care se potrivesc cu spectrul PV (ex: filtre optice selective) iar proiectul nostru  propune o eficienta mai mare de 30% ceea ce este in tendinta literaturii actuale (V.P.Khvostikov et al-2006 [14]).

Diferenta dintre literatura prezentata si munca noastra experimentala este legata de posibilitatea de a obtine o structura activa in baza GaSb intr-un proces tehnologic in situ. Parametrii clasici pentru celulele solare din GaSb sunt : densitatea de curent de scurt-circuit JSC= 4.5 A/cm2, tensiunea de circuit deschis VOC= 0.48 V si factorul de umplere FF=0.65 la temperatura maxima a modulului. Acesti parametrii sunt influentati in mare parte de grosimea celulei, nivelul de dopare al substratului si design-ul contactului de spate al celulei. In acest proiect o parte a originalitatii va fi structura epitaxiala a celulei impreuna cu design-ul contactului pe emitorul p ceea ce implica un studiu al diferitelor sisteme posibile (ex; Cr/Au, Cr/Ag, Ti/Pt/Au) si  pe n-GaSb (ex: Au/Ge/Ni, Au/Te/Au).

Un avantaj important al echipei noastre este experienta in tehnologia de contactare a compusilor III-V impreuna cu posibilitatea de caracterizare in situ a suprafetei (ex: XPS, STM). Contributia inovativa a acestui proiect este legata de proiectarea structurii celulei fotosenzitive de vreme ce este necesara performanta la nivelul de iluminare AM0 pentru aplicatia ca celula solara, si de asemenea sa fie performanta in raspunsul in infrarosu (IR). Mentionam ca design-ul grilei de contact destinata folosirii mastilor fotolitografice trebuie sa fie adaptat unei depuneri in situ a metalelor in sistemul performant de camere XPS-MBE. In cursul anului 2009 (completate ulterior in 2010-2012), au fost puse la punct doua instalatii complexe de fizica suprafetelor si interfetelor, reprezentate in Figura.

Fig1. Instalatii complexe

Instalatiile constau din trei unitati : (a) incinte de epitaxie din fascicul molecular (MBE); (b) incinte de analiza prin spectroscopii de electroni (fotoelectroni si electroni Auger); incinte de analiza prin microscopie cu efect tunel (STM). Folosind numai incintele MBE, se pot desfasura preparari de sisteme complexe(suprafete atomic curate, straturi subtiri, multistraturi), care pot fi caracterizate in situ prin urmatoarele tehnici : (a) difractie de electroni lenti (LEED); (b) spectroscopie de electroni Auger(AES); (c) difractie de electroni rapizi reflectati (RHEED); (d) analiza de desorbtie termica; (e) spectroscopia de masa a ionilor imprastiati (SIMS). Instalatiile STM permit realizarea (a) experientelor de microscopie cu efect tunel la temperatura variabila (80-600 K) (b) de masuratori de spectroscopie de efect tunel (STS). Incintele de spectroscopie de fotoelectroni permit urmatoarele metode de analiza: (a) spectroscopie de fotoelectroni excitati cu raze X (XPS); (b) difractie de fotoelectroni (XPD); (c) spectroscopie de fotoelectroni excitati cu radiatie in domeniul ultraviolet (UPS); (d) UPS cu rezolutie unghiulara (ARUPS); (e) Ups cu rezolutie de spin; (f) ARUPS cu rezolutie de spin; (g) AES; (h) difractie de electroni Auger(AED); (i) spectroscopie de pierdere de energie a electronilor (EELS); (j) spectroscopie de imprastiere inelastica a ionilor (MEIS). Aceste aparate functioneaza in regim continuu, 24 de ore pe zi si 7 zile pe saptamana. Sistemele sunt relativ complete  si nu necesita up-grade-uri importante. Pe de alta parte, putem anticipa ca pe durata urmatorilor 2 ani pot apare diverse disfunctii si defectiuni, in special in ceea ce priveste sistemele de pompare (pompele uscate, cele turbomoleculare si cele ionice au o durata de viata finita), asa ca este posibil sa fie necesara inlocuirea/repararea (pretul mediu al unei pompe uscate este de 2 500 euro, al unei pompe turbomoleculare intre 4000 si 14 000 euro, al unei pompe ionice de cca. 5 000 euro; cei mai multi producatori propun o procedura de buy-back, recuperand pompa defecta si punand la dispozitia clientului una noua, la jumatate de pret).

De asemenea, alte elemente cu durata de viata finita sunt detectorii multiplicatori de electroni (channeltrons), care trebuie schimbati la fiecare 2 ani in regimul actual de functionare (o baterie de 9 channeltrons costa cca. 10 000 euro). Alte elemente care sunt schimbate cam o data pe an sunt filamentele tunurilor de raze X si varfurile microscoapelor tunel. Este necesar ca un buget de cca. 30-50 000 lei sa fie prevazut pentru a acoperi partial aceste cheltuieli de functionare pentru sistemele implicate in proiect la INCDFM. Dupa cum s-a afirmat, o alta parte originala va fi investigarea structurii privind raspunsul la radiatia laser ceea ce va constitui un studiu extins legat si de fizica suprafetei (ex: caracteristici de degradare).

O bariera care trebuie luata in considerare este legata de costul acestor celule unde elementul principal este costul plachetei substrat pentru ca productia este mica si deci substratul este scump (aprox. : 400-600 euro functie de dopaj si diametru). Dar, costul real al unei celule fotosenzitive de GaSb este dat de costul de procesare. Din acest punct de vedere, folosirea tehnologiei in situ permite un control mai bun al diferitelor etape, cum ar fi corodare ionica cu ioni de Ar+, cresterea epitaxiala MBE si depunerea de contacte, ceea ce conduce la un cost de fabricatie competitiv. Am amintit ca ca parametrii buni ai diferitelor structuri de dispozitiv sunt un rezultat al unui design adaptat scaderii vitezei de recombinare la suprafata.

Pentru compusul ternar in baza GaSb, este avantajosa prezenta unui emitor p de grosime mai mare pentru ca se obtine o lungime de difuzie a purtatorilor minoritari mai mare. Grupul nostru are experienta in domeniul pasivarii GaAs sau GaP in vederea obtinerii Schottky si ohmice bune R.V.Ghita et al -2010 [18]; R.V.Ghita et al-2008 [19], iar privitor la compusii III-V avem avantajul inceputului de drum in Romania in ceea ce priveste know-how-ul. Mentionam de asemenea, ca la INCDFM exista facilitatea de infrastructura destinata unui flux tehnologic competitiv, respectiv Camera Curata Clasa 1000 (vezi Figura) pentru necesitati de fotolitografie. In caracterizarea dispozitivelor PV sunt importante curbele I-V in conditii de intuneric cum ar fi I01, pentru ca acest parametru depinde nu numai de nivelele de dopare si lungimile de difuzie a purtatorilor minoritari, dar si de viteza de recombinare la suprafata S (ex: S> 10 cms-1). Pe langa alte probleme, trebuie proiectat design-ul structurii in baza GaSb avand o rezistenta serie mica pentru a asigura o eficienta buna (peste 30%). Din acest punct de vedere, se propune fabricarea unei heterostructuri GaSb/AlGaAsSb cu un compus cuaternar de banda larga ceea ce reprezinta un tel al unei cercetari viitoare.

Problemele fundamentale ridicate de dezvoltarea unei tehnologii in situ pentru structura fotosenzitiva de GaSb vor fi rezolvate in cadrul acestui proiect. O parte a acestor probleme sunt: design-ul structurii, caracterizarea de suprafata, pasivarea suprafetei, design-ul grilei de contact, depunerea metalica si pot fi rezolvate ca o contributie inovativa intr-un lant tehnologic in situ.

Caracterizarea in conditii de iluminare puternica impreuna cu  comportarea principalilor parametrii legat de efecte de degradare poate aduce o contributie noua din punct de vedere fundamental privind functionarea dispozitivului. O alta parte a contributiei originale este legata de studiul suprafetei convertorului  GaSb  pentru suprafata expusa conditiilor extreme. 

References

[1] http://en.wikipedia.org/wiki/Photovoltaics

[2] T.J.Couts, Renew. Sustainable Energy, 3, pp.77 (1999)

[3] T.Markert „Photovoltaic Solar Energy Conversion „, European Summer Energy for Europe, Strasbourg, 7-14 July, 2002

[4] L.M.Fraas, J.E.Avery, V.S. Sundaram, V.T Dinh, T.M. Davenport, J.M.Yerkes, „Photovoltaic Specialists Conference 1990”, Conference Record of the Twenty First IEEE, Vol.1,pp. 190, 21-25 May 1990, Kissimmee, Fl. USA

[5] C.Algora & D.Martin, AIP Conference Proceedings 653, pp.452 (2003)

[6] A.Mitric, Th.Duffar, A.Amariei, X.Chatzistavrou, E.Pavlidou, K.M.Paraskevopoulos, E.K.Polychroniadis, J.Optoelectron. Adv.Mater, 7(2), pp.659 (2005)

[7] V.M.Andreev, V.P.Khvostikov, E.V.Paleeva et al Proceeedings of 25 th IEEE Photovoltaic Specialists Conference, Washington, 1996 (IEEE New York, 1996), pp.143

[8] V.M.Andreev, V.A.Grilikhes and V.D. Rumyantsev, Photovoltaic Conversion of Concentrated  Solar Radiation (Nauka, Leningrad 1989, Wiley, Chichester, 1997)

[9] D.Z.Garbuzov, R.V.Martinelli, V.Khalfin et al, Proceedings of Space Technology and Applications, International Forum (Albuquerque, NM, 1998) pp.1400

[10] A.Subekti, V.W.L.Chin and T.L.Tansley, Solid-State Electron, 39, pp.329 (1996)

[11] M.Rolland, S.Gaillard, E.Villeman et al, J.Phys. III, 3 pp.1825 (1993)

[12] K. I. Kossi, M.Goldenberg and J.Mittereder, Solid-State Electron, 46, pp.1627 (2002)

[13] C.C.Negrila, C.Logofatu, R.V.Ghita, C.Cotirlan, F.Ungureanu, A.S.Manea, M.F.Lazarescu, Journal of Crystal Growth 310, pp.1576 (2008)

[14] V.P.Khostikov, M.G.Rastegaeva, O.A.Khvostikova, S.V.Sorokina, A.V.Malevskaya, M.Z.Shvarts, A.N.Andreev, D.V.Davydov and V.M.Andreev, Physics of Semiconductor Devices, ISSN 1063-7826, Semiconductors, 2006, 40(10), pp.1242

[15] Y. Rouillard, B.Jenichen, L.Daweritz, K.Ploog, C.Gerardi, C.Giannini, L.De Caro, L.Tapfer, Journal of Crystal Growth 204(3), pp.263 (1999)

[16] M.Yano, Y.Suzuki, T.Ishii, Y.Matsushima, M.Kimata, Jpn. Journal of Appl.Phys. 17 (1978), pp.2091

[17] C.Cotirlan, C.Logofatu, C.C.Negrila, R.V.Ghita, A.S.Manea, M.F.Lazarescu, J.Optoelectron Adv. Mater. 11(4), pp.386 (2009)

[18] R.V.Ghita, C.C.Negrila, F.Ungureanu, C.Logofatu, Optoelectronics and Advanced Materials-Rapid Communications , 4(11), pp.1736 (2010)

[19] R.V.Ghita, V.Lazarescu, C.Logofatu, C.C.Negrila, M.F.Lazarescu, Materials Science in Semiconductor Processing 11, pp.394 (2008)

[20] R.V.Ghita, C.Logofatu, C.Negrila et al, J.Optoelectron Adv.Mater. 7(6) pp.3033 (2005)

[21] R.V.Ghita, C.Logofatu, C.Negrila et al, Physica Status Solidi (a), 204(4), pp.1025 (2007)

[22] C.C.Negrila, F.Ungureanu, R.V.Ghita et al  IEEE-CAS-2009, International Semiconductor Conference 1-2, Proceedongs, pp.455 (2009)

[23] R.V.Ghita, C.Negrila, A.S.Manea et al, J.Optoelectron. Adv.Mater. 5(4) pp.859 (2003)

 

Original and innovative contributions of the project

In the general frame of the scientific effort of obtaining photovoltaic sensitive structure with good efficiency this project proposes the system: GaSb/AlxGa1-xSb system, a heterostructure obtained by Molecular Beam Epitaxy technique. The photocurrents in the range 1-10A are typical for the development of PV generators in GaSb base, a problem to be solved in this project being the design of GaSb photoconvertors with lower spreading resistance. The facilities related to the obtaining and partial  characterization  of this structure are available in National Institute of Materials Physics (NIMP)-Magurele. The evolution of  surface quality for starting the growth process after a chemical etching will be investigate by Atomic Force Microscopy (AFM) and STM  facilities  Due to a expected conversion efficiency to high-power monochromatic radiation the GaSb based structure will be investigated to laser irradiation and the surface interaction will be investigated by AFM, and SEM facilities together with a complete electrical characterization. It is worth to mention that the initial characteristics of III-V solar cells are excellent. Due to their direct bandgap electronic structure they are able to absorb 97% of the AM1 radiation in a thickness of 2μm. The main parameter that is the size of band gap can be changed in ternary or quaternary compounds by selecting the relative proportion of element in the compound.  As it was already presented solar photoconversion of produced radiation in low-bad gap (Eg=0.5-0.8 eV) is efficient in the case of the development of selective emitters matched to the PV cell spectrum (e,g. selective optical filters) and this experiment proposing an efficiency higher than 30% is in the trend of literature (V.P.Khvostikov et al -2006 [14]). The difference from presented papers and our experimental work is the possibility to obtain an GaSb base active structure in an approximate in situ technological process. The classical parameters for GaSb solar cells were a short circuit photocurrent density JSC= 4.5 A/cm2, open circuit voltage VOC=0.48 V and fill factor FF=0.65 at the maximum temperature of module. These parameters are influenced in the main part by the cell thickness, substrate doping level and back surface contact design. In this project a part of originality will be the cell epitaxial structure together with the contact design on p-emitter that implies a study of different possible systems (e.g. Cr/Au, Cr/Ag, Ti/Pt/Au) and on n-GaSb (e.g Au/Ge/Ni, Au/Te/Au) An important advantage of our team is the experience in contact technology on III-V compounds together with the possibility of in situ surface characterization. (e.g. XPS, STM. The innovative contribution of this project is related to the photosensitive cell design characteristics as they had to have efficiency performance at AM0 illuminated levels for solar cell application, and also to have efficiency performance in IR response. The applied design of contact grid using photolithography masks has to be adapted to the in situ deposition of metals in the XPS-MBE performance system. Mainly in 2009 (but with some upgrades during 2010-2012) two complex surface science clusters have been set up in NIMP, as presented in the Figure.

The setups are each one formed by three units (a) molecular beam epitaxy (MBE) chambers; (b) chambers dedicated to electron spectroscopies (XPS, Auger); (c) chambers for scanning tunnelling microscopy (STM). By using the MBE chambers only, onemay achieve preparation of complex systems ( atomically clean surfaces, thin fims, multilayers, surface alloys), which may be in situ characterized by techniques as : (a) low energy electron diffraction (LEED); (b) Auger electron spectroscopy (AES); (c) reflection high energy electron diffraction (RHEED);(d) secondary ion mass spectroscopy (SIMS); (e) thermal desorption analysis by using a quadrupole mass spectrometer. The STM chambers allow experiments of (a) variable temperature (80-600K) scanning tunnelling microscopy; (b) scanning tunnelling spectroscopy (STS). The chambers dedicated to X-ray photoelectron spectroscopies allow the following analysis methods (a) X-ray photoelectron spectroscopy (XPS); (b) X-ray photoelectron diffraction (XPD); (c) ultraviolet  photoelectron spectroscopy UPS; (d) angle-resolved UPS (ARUPS); (e) spin-resolved UPS; (f) spin-resolved ARUPS; (g) AES; (h) Auger electron diffraction (AED);(i) electron energy loss spectroscopy (EELS); (j) medium energy ion scattering. These setups are actually running continuously, 24 hours a day and 7 days a week. The systems are relatively complete and do not need important upgrades. On the other hand, one may anticipate that during the following 3 years some malfunctions or breakdown of some devices might occur, especially concerning the pumping systems (all dry, turbo or ion getter pumps have a finite lifetime), thus it is possible that replacement or fixing of some of these pumps might be necessary (the average price is 2 500 euros for a dry pump, that of a turbomolecular pump ranges between 4000 and 14 000 euros and that of an ion getter pump is about 5 000 euros; most manufacturers propose a buy-back procedure, retrieving the broken pump and offering a completely new pump at half of its price.) Also, other elements with finite lifetime are electron multipliers (channeltrons), which have to be replaced each 2 years in the actual regime of operation, and a 9-channeltron battery costs 10 000 euros. Other elements that must be replaced each year are X-ray gun filaments and STM tips. It is necessary a budget of  abot 30-50 000 lei to ensure the operation costs of the system involved in this project.  As was stated another original part will be the investigation of the structure to the response to a monochromatic light of a laser with an extended study of surface characteristics. One barrier that will be consider in order to over come it, is the cost of these cells where the main cost driver is the substrate itself-the production quantity is small and hence wafers are expensive. But the real cost of the photosensitive GaSb cell is given by the processing cost. In this view the use a in situ processing technology allows a better control of different technological events i.e  Ar+ ion beam etching , MBE growth process and contact deposition. We mention that the good parameters of different device structures are the result of an appropriate design that reduces the value for the surface recombination velocity.

For the proposed ternary antimonide material, a thick p-doped emitter is advantageous because a higher minority carrier diffusion length is expected. Our group has experience in the field of passivation of GaAs or GaP in order to obtain good ohmic and Schottky contacts R.V.Ghita et al-2010 [18]; R.V.Ghita  et al 2008 [19], and from this point of view regarding III-V compounds we have the advantage of starting step in know-how development. In this view it is worth to mention that in NIMP does exist the facility of infrastructure dedicated to competitive technology flux, namely clean room Class 1000 (see the Figure) for lithography needs.  In the characterization of PV devices are important I-V curves in dark-current conditions as I01, because this parameter depends not only on the doping levels and minority carrier diffusion length of the emitter and base, but also on the surface recombination velocity S (e.g. S>10 cms-1).

Besides other problems, one to be solved is the design of the structure of GaSb photoconverter with a lower spreading resistance in order to have a good efficiency (over 30%). In a large perspective, this problem can be solved with the fabrication of GaSb/AlGaAsSb heterostructure, with a quaternary wide-gap compound, a task that represents another scientific level of a future project.

The basic problems of the in situ technology of photosensitive GaSb structures have to be solved in the present projects. Part of these problems is: structure design, surface characterization, surface passivation, contact grid design, metal deposition, and they can be solved as an innovative contribution   of an in situ technological chain.

The characterization in high light illumination condition coupled with the behaviour of main parameters related to degradation effects can bring new contribution to fundamental aspects of the device operating. Another part of original contribution will be related to a competent surface study of front-side surface of PV GaSb converter in extreme conditions. 

 

 

 

 

Fig2

 

Fig.3.

 

 




Pagină actualizată la 04 Noiembrie 2014.