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AMELIORATION OF ELECTRONIC PROPERTIES OF THIN FILMS A GAAS IN THE VIEW OF REALISATION OF SOLAR CELLS.

Objective

AMORPHOUS GAAS(A-GAAS) IS A PROMISING MATERIAL FOR PHOTOVOLTAIC CONVERSION. INDEED, ITS HIGH ABSORPTION COEFFICIENT AND ITS DIRECT GAP ALLOW TO ATTAIN HIGH EFFICIENCY (23%) WITH ONLY FEW MICRONS OF SUBSTANCE. FURTHERMORE, AMORPHOUS SEMICONDUCTORS ARE WELL ADAPTED TO THE REALIZATION OF LARGE AREAS OF THIN FILMS; THIS WOULD LEAD TO A DECREASE IN THE PHOTOCELLS COST.

OUR OBJECTIVE WAS TO PRODUCE A GOOD MATERIAL, TO IMPROVE ITS PROPERTIES BY HYDROGENATION AND TO TRY ITS DOPING WITH THE IAM OF REALIZING A SCHOTTKY DIODE AND STUDYING ITS PHOTOELECTRIC PROPERTIES. A-GAAS THIN FILMS (1"MU") WHERE PREPARED BY R.F. SPUTTERING OF A CRYSTALLINE TARGET IN AR OR AR:H2 MIXTURE.

THE MATERIAL IS STOICHIOMETRIC AND INTRINSIC WHEN THE DEPOSITION IS ACHIEVED AT A SUBSTRATE TEMPERATURE OF ABOUT 290 C. IT PRESENTS A BETTER SHORT RANGE ORDER AND CONTAINS LESS CHEMICAL DEFECTS THAN THE MATERIAL PREPARED AT LOWER TEMPERATURE.

HYDROGENATION PROVOKES ALSO A DECREASE OF THE NUMBER OF DEFECTS AND AN IMPROVEMENT OF THE ELECTRICAL PROPERTIES, BUT ITS EFFECT IS LIMITED BY DEFECTS THAT CAN'T BE COMPENSATED BY HYDROGEN INCORPORATION (WRONG BONDS). FURTHERMORE, HYDROGENATION IS ONLY POSSIBLE AT A SUBSTRATE TEMPERATURE LOWER THAN 50C. HIGHER TEMPERATURE CAUSES THE CRYSTALLISATION OF THE MATERIAL.

INCORPORATION OF MO USING COSPUTTERING PROVOKES AN INCREASE OF THE ROOM TEMPERATURE ELECTRICAL CONDUCTIVITY OF ABOUT 6 ORDERS OF MAGNITUDE.

WE ARE ACTUALLY STUDYING THE OHMIC AND RECTIFYING CONTACTS TO A-GAAS WITH VIEW OF REALIZATION OF A SHOTTKY STRUCTURE.
The character of the gallium arsenide compound makes studies of amorphous gallium arsenide more complex than those on amorphous silicon. In particular, when forming thin films, relatively strict conditions of preparation are made necessary as the presence of the 2 constituents may introduce additional defects. Research has been directed towards improving the quality of material using hydrogenation and by hot deposition.
AFTER STUDYING THE EFFECT OF STOICHIOMETRY DEVIATION EXPLAINED BY A SELF DOPING PHENOMENON (AS EXCESS OR GA EXCESS GIVES RISE TO N-TYPE OR P-TYPE MATERIAL RESPECTIVELY), WE HAVE DETERMINED THE EXPERIMENTAL CONDITIONS WHICH ALLOW TO OBTAIN AN INTRINSIC AMORPHOUS SEMICONDUCTOR. IN THIS WAY, THE SPUTTERED A-GAAS HAS THE FOLLOWING CHARACTERISTICS: RESISTIVITY P = 4X10 TO THE POWER 8 EV, DENSITY OF LOCALISED STATES AROUND FERMI LEVEL = 3X10 TO THE POWER 16 EV-1 CM-3.

NEXT WE HAVE TRIED TO DOPE THIS MATERIAL BY COSPUTTERING WITH MO. THE RESISTIVITY OF THE OBTAINED A-GAAS : MO IS SIX ORDERS OF MAGNITUDE LOWER THAN THE INTRINSIC ONE. THE ACTIVATION ENERGY EA IS ALSO LOWERED BY 0.5 EV BY MO INCORPORATION. THIS RESULTED IN N-TYPE MATERIAL.
DOPING WITH TRADITIONAL ELEMENTS LIKE TE AND S, THE FIRST IS INCORPORATED BY CONSPUTTERING WHILE THE SECOND BY MIXING THE H2S WITH THE DISCHARGE GAS, IS ALSO TRIED.
THE PLOT OF ELECTRICAL CONDUCTIVITY IS A FUNCTION OF TEMPERATURE FOR A-GAAS CONTAINING TE AND S RESPECTIVELY.
UNFORNUTATELY THERE IS NO MODIFICATION NEITHER IN NOR IN EA. THE SMALL INCREASE DITECTED IN EA AND THE CORRESPONDING DECREASE IN P FOR SAMPLES CONTAINING S IS DUE TO THE INCORPORATION OF LOW AMOUNT OF HYDROGEN. OUR RESULTS ABOUT DOPING WITH S AND TE CONFIRM LATEST THEORETICAL WORK OF ROBERTSON WHICH SHOWS THE ABSENCE OF DOPING IN A-GAAS BECAUSE OF THE LOW COST TO CREATE COMPENSATING DEFECTS IN THIS MATERIAL. HOWEVER THIS IS WHAT HAPPENED HISTORICALLY WITH A-SI:H SO OTHER WORKS ARE REACTED TO PROVE THE CONTRARY.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Université d'Aix-Marseille III (Université de Droit d'Économie et des Sciences)
Address
3 Avenue Robert Schuman
13628 Aix-en-provence
France

Participants (1)

Université d'Aix-Marseille III (Université de Droit d'Économie et des Sciences)
France
Address
3 Avenue Robert Schuman
13621 Aix-en-provence