PowerPoint Presentation

Quantum confinementBY R. DINGLE, W.WIEGMANN & C. H. HENRY

MY 5900 MATERIALS ENGINEERING SEMINARDR. PETER MORAN 1AGENDAPURPOSE OF THIS PRESENTATION

BACKGROUND & INTRODUCTION TO QUANTUM CONFINEMENT

HYPOTHESIS

EXPERIMENTAL SETUP

OBJECTIVE OF THE EXPERIMENT

MEASURED RESULTS

CONCLUSIONS

REFERENCES

PURPOSE OF THIS PRESENTATIONTHE CONCEPT OF QUANTUM CONFINEMENT HAS REMARKABLE PROSPECTIVE IMPLEMENTATIONS IN OPTOELECTRONICS AND LOGIC DESIGN FOR ADVANCED COMPUTING SYSTEMS

THE TALK ALSO DEFINES THE CRUCIAL ASPECT OF RELATIONSHIP BETWEEN THE THICKNESS (THINNESS) OF THE CENTRAL HETEROSTRUCTURAL LAYER IN ORDER TO GENERATE A PRONOUNCED STRUCTURE OF OPTICAL ABSORPTION SPECTRUM

FINALLY IT EQUIPS US TO BETTER DESIGN A HETEROSTRUCTURE SYSTEM THAT COULD ENHANCE THE CONFINEMENT PROPERTY WHICH, IN TURN, COULD BE EXPLOITED FOR VARIOUS APPLICATIONS SUCH AS LEDs AND LASERS

BACKGROUND AND INTRODUCTION TO QUANTUM CONFINEMENTIN 1970 ESAKI & TSU PROPOSED FABRICATION OF AN ARTIFICIAL STRUCTURE, WHICH WOULD CONSIST OF ALTERNATE LAYERS OF TWO DIFFERENT SEMICONDUCTORS CALLED QUANTUM HETEROSTRUCTURE

IN THIS EXPERIMENT, HOWEVER, THE APPLIED ELECTRIC FIELD DISTORTS THE RECTANGULAR QUANTUM WELL INTO TRAPEZOIDAL SHAPE4

QUANTUM CONFINEMENT IS RESPONSIBLE FOR THE INCREASE OF THE ENERGY DIFFERENCE BETWEEN ENERGY STATES AND BAND GAP

THIS PHENOMENON IS TIGHTLY RELATED TO OPTICAL AND ELECTRONIC PROPERTIES OF THE MATERIALS

HYPOTHESISIF THE CARRIERS COULD BE CONFINED IN THE QUANTUM LEVELS PRESENT IN VERY THIN LAYER OF HETEROSTRUCTURES GROWN USING MBE (Molecular Beam Epitaxy), THEN WE COULD SEE A PRONOUNCED STRUCTURE OF GaAs OPTICAL ABSORPTION SPECTRUM AS A RESULT OF THE HETEROSTRUCTURE BEHAVING AS A RECTANGULAR POTENTIAL WELL

THIS SUGGESTS THAT THE APPARENT THINNESS IN THE DESIGN OF THE ACTIVE LAYER OF THE HETEROSTRUCTURE COULD EFFECTIVELY LEAD TO PRONOUNCED STRUCTURE OF GaAs OPTICAL ABSORPTION SPECTRUM

EXPERIMENTAL SETUP

USING MBE, THE MULTILAYER HETEROSTRUCTURE OF GaAs-AlxGa(1-x)As IS DEVELOPED WITH GROWTH CONDITIONS SUCH AS: Vacuum during growth ~ 1 x 10-9mm; Vacuum during growth ~ 1 x 10-7 mm ArsenicTWO FACTORS THAT ENABLE HIGH DEGREES OF FABRICATION PRECISION ARE MBE-FABRICATION TECHNIQUE AND DEVELOPMENT OF SELECTIVE CHEMICAL ETCHESEXPERIMENTAL SETUPTHE USE OF {100} GaAs SUBSTRATE AT 6000C WITH SHUTTERING OCCURING IN THE SEMIAUTOMATIC Al-OVEN RESULTED IN GROWTH RATE OF 1m PER HOUR

FIFTY GaAs LAYERS HAVE BEEN GROWN IN A SINGLE STRUCTURE

THESE GaAs LAYERS ARE SEPERATED BY AlxGa(1-x)As LAYERS OF > 250 THICHKESS

OBJECTIVE OF THE EXPERIMENTIN ORDER TO PROVE WITH CERTAINTY THAT THE OPTICAL ABSORPTION OF GaAs LAYER IS CLEARLY ENHANCED DUE TO THE HIGH DEGREE OF THINNESS OF THE ACTIVE LAYER

MEASURED RESULTSIN ORDER TO QUANTITATIVELY ANALYZE THE DATA AT HAND, THE EIGEN VALUES OF QUANTUM WELL DEPTH POTENTIAL (V0) WERE CALCULATED USING COMPUTER

IT WAS DISCOVERED THAT BOTH THE LEVEL SPACING AND THE NUMBER OF BOUND STATES WENT ON TO DECREASE AS V0 DECREASED BUT n =1 STATE EXISTED FOR ALL POSITIVE VALUES OF V0

THIS MEANT THAT, FOR ALL ATTRACTIVE POTENTIAL WELLS, AT LEAST ONE BOUND STATES WOULD HAVE EXISTED FOR EACH TYPE OF CARRIER (HOLES AND ELECTRONS)

MEASURED RESULTS

COULOMB ATTRACTION CORRELATES THE MOTION OF THE CARRIERS IN THE x AND y DIRECTIONS

THIS RESULTS IN FORMATION OF EXCITON STATES WITH PEAKS IN OPTICAL ABSORPTION SPECTRUM

SINCE STATES WITH SAME QUANTUM NUMBER (n) HAVE A SUBSTANTIALLY GREATER ELECTRON-HOLE OVERLAP; THE EXCITONS OF THESE STATES DOMINATE THE OPTICAL ABSORPTION SPECTRUM

THUS, TWO SERIES OF EXCITON PEAKS, ONE WITH EQUAL-n STATES OF THE ELECTRON AND HEAVY HOLE AND THE OTHER ASSOCIATED WITH EQUAL-n STATES OF ELECTRON AND LIGHT HOLE ARE TO BE FOUND IN THE OPTICAL ABSORPTION SPECTRUMMEASURED RESULTSTHE FIGURE DISPLAYS TYPICAL ABSORPTION SPECTRA OF THE HETEROSTRUCTURES IN THE BAND-EDGE REGION OF GaAs AT 2K TAKEN INTO ACCOUNT IN THIS EXPERIMENT

IT SHOWS THE DOMINANT EXCITONIC CONTRIBUTION TO THE BULK GaAs BAND-EDGE ABSORPTION

MEASURED RESULTSTHIS FIG. SHOWS THE PLOT OF LZ VERSUS THE MEASURED EXCITON ENERGIES

ABSOLUTE ENERGIES WERE DETERMINED BY EXTRAPOLATING THE MEASURED ENERGIES TO n=0

THE CALCULATED ENERGIES OF THE BOUND STATES FOR n=1,2, WERE THEN ADDED TO THESE ENERGIES

MEASURED RESULTSBY VARYING THE DEPTH OF POTENTIAL WELL FOR ELECTRONS, IT WAS FOUND THAT A DEPTH OF 22030 meV WAS REQUIRED TO FIT THE DATA IN THIS FIG.

THE ENERGY OF THE SQUARES DECREASES SLOWLY WITH LZ AND EVENTUALLY SATURATES AT LZ < 200 AT 1.5120.001 eV, ABOUT 3meV BELOW THE BULK EXCITON ENERGY OF 1.515 eV (2 K)

14MEASURED RESULTSTHE ENERGY PLOT INDICATES THAT THE EXCITON BINDING ENERGY INCREASES FROM 4meV TO ~7meV AS A CONSEQUENCE OF CARRIER CONFINEMENT

THIS INCREASE OF BINDING ENERGY AGREES QUITE WELL WITH THAT EXPECTED FOR A 3-D EXCITON AS IT APPROACHES 2-D LIMIT

15CONCLUSIONTHE HETEROSTRUCTURE BHEAVES AS A SIMPLE RECTANGULAR POTENTIAL WELL WITH A DEPTH OF ~0.88Eg FOR CONFINING ELECTRONS AND ~0.12 Eg FOR CONFINING HOLES, WHERE Eg IS THE DIFFERENCE IN THE SEMICONDUCTOR ENERGY GAPS

THIS WOULD, THEREBY, EXPLAIN THE PRONOUNCED STRUCTURE OF THE GaAs OPTICAL ABSORPTION SPECTRUM AS SHOWN BY THE GRAPH BETWEEN ENERGY AND ABSORPTION INTENSITY

AS A CRITIQUE, I REALIZE, THAT THE PAPER HAS SUCCESSFULLY DEMONSTRATED THE PROPERTIES OF HETEROSTRUCTURES WITH RECTANGULAR QUANTUM WELL CONCEPT (UNLIKE TSUS WITH TRAPAZOIDAL OUTCOME DUE TO E-FIELD)

REFERENCES QUANTUM STATES OF CONFINED CARRIERS IN VERY THIN HETEROSTRUCTURES (By R. Dingle, W. Wiegmann, and C. H. Henry)FIG. FROM www.nanohub.orgwww.wikipedia.org