ulhas project presentation

8/14/2019 Ulhas Project Presentation

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Introduction

Heterojunction:-Interface between layers or regions of the twodissimilar semiconductors.

Semiconducting materials with unequal bandgaps.

CB1

To engineer the electronic bands in solid state devices likesemiconductor lasers, solar cells and transistorsArrangement of the heterojunctions forms theheterostructures

VB1

VB2

S1 S2


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About III- Nitride Materials:-

Higher Mechanical and thermal stabilityCan Operate in Higher temperature region.Large piezoelectric constant.High electron mobility of GaN used to fabricate highpower transistor based on AlGaN/ GaN

heterostructure

Key for Choosing Gallium Nitride

Pure, clear wafers of this material are the key toblue and ultraviolet semiconductor lasers for

next-generation DVDs


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Quantum heterostructure:-

Heterostructure with size restricting also the movements of thecharge carriers is restricted and forcing them into a quantumconfinement.According to the confinement direction of electrons materials can bedevided as follows.

(a)Bulk Material (b)Quantum well

(c)Quantum wire (d) Quantum dot


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Why Quantum Well, Wire and Dot structure is advantageous?

High Radiative Efficiency:-Small VolumeHigh Carrier Concentration

Low Thresholds Current:

Small volumeCarrier density for population Inversion

How the Quantum Structures are made?Using Heterojunctions.

Heterojunction utilization at the sidewhere quantun confinment is need to bedone.

AlGaN

GaN

AlGaN

AlGaN

XY

Z


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Kronig-Penny Model

Has an analytical solution hence provide simple calculation.The KP model is a strongly simplified one-dimensional quantummechanical model of a crystal.

According to Schrodinger equation


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and

Solutions for eqn. (1) and (2) are

Here A,B,C and D are the constants and can be determinedby the boundary conditions as,At x=0=A+B=C+D and

At x=a


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And

These equations have solutions if A,B,C and vanishes or if

This equation was introduced by the Kronig-penny first to solve forthe allowed energies


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Occupation factor for Quantum Wire:-

Occupation Factor:-Availability of the particle in

Results:-

.

Decrease in occupation factormeans increase in tunneling fromactive region to barrier region.Eigen energy is inverselyproportional to active region.

Hence lower Eigen energymeans more active region andmore active region is capable ofconsisting more electrons.


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2.5

3x 10

11

Scattering rate:-

Possibility of the electron to be tunneled down to barrierregion.Variation of scattering rate w.r.t. to different angle madeby electron moment with the direction of electronpropagation.Maximum scattering upto angle 20 due to higherprobability of electron in quantum region.Further as angle theta increases possibility of directtunneling of electron from the active region. Yields thatelectron escapes from the active region by lowering the

0 20 40 60 80 100 120 140 160 1800

0.5

1

1.5

2

angle, theta (degree)

scattering

rate,

1/tau

(s-1

degree

-1)

scattering rate.

x

Y

Z


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Probability density and structure of square quantum wire:-

Peak=13.42 Dimensions=6X6 (nm)Quantum Wire:-Movement along the z axisand confinement by the xand y axis.

Quantum square wire:-Lengths of confinement axisare same i.e. x and y are

Contour Graphs:- Acontour graph is a two-

dimensional representationof a surface graph.


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Peak= 14.92 Dimensions=7X7 (nm)

Probability density :-Probability density is theprobability of finding a wavepacket in a given region ofphase space.Tells about the findingelectron in the structure ofdevice.

Colors from faint(Blue) todark represents thepossibilities of confinment.


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Peak= 16.3

Dimensions=8X8nmFactors affecting on peakdensity in wire region.Enhanced electronconcentration in active

region.(due to increased areaof active region).Peak density is at themiddle of the active region

.

The contour images shows thespread in the square form.


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Peak= 17.4 Dimensions=9X9(nm)

Blue peaks risen from thebottom of the graph tellslosses in the structure.These indicates the loss of

electron in the form oftunneling towards thebarrier region from activeregion.

Factors affecting lossesActive area regionElectron eigen energy


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Peak= 18.41 Dimensions=10X10(nm)

Using analogy if ohms lawjustification for nature of theincreasing peak density:-

According to Ohms law,

So, as area increasesresistance decreases

which increases theconductivity.


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Square wire width along X and Y direction vs peak amplitude ofprobability density:-

As the dimension of thewire goes on increasingvalue of peak densityincreases.

Increasing the wiredimension means area ofactive region increases.More active region

of consisting moreelectrons with increasingthe peak density.


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FWHM for square quantum wire:-

Tells about the confinement and looses in active region.If FWHM is greater than that of the device dimension, thenlosses increases.


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Probability density and structure of rectangular

quantum wire:- Peak= 15.7 Dimensions=6X10(nm)

Quantum rectangle wire:-Lengths of confinement axis arenot same i.e. x and y are not same

in magnitude.

The surface image of probabilityclearly shows the distribution of

electron in a rectangular manner.The intensity spread in imagefigure has been highlighted by therectangular border which shows aclearer picture of spread of electron

in active region.


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Peak= 16.63 Dimensions=7X10 (nm)

Increasing wire dimensionmeans increasing activeregion.As active region again goeson increasing means Eigenenergy of the electrondecreases.And confinement increases.

Spread in Y direction is dueto the increased dimension inY direction.


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Peak= 17.94Dimensions=9X10(nm)

According to Ohms law,

So, as area increases

resistance decreaseswhich increases theconductivity.


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Peak amplitude of probability densityvs x distance

SummaryPeak density increases

as x distance increasesActive region areaEigen energy


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FWHM for rectangular quantum wire:-

Again in the rectangularquantum wire FWHM

increases as thedimension increases.


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Probability density with varying mole fraction:-

Mole fraction=0.25 Peak value= 21.67

Mole fraction:-Indicating the How much

Aluminum is available in theAlGaN

Mole fraction=0.20 Peak value= Peak=20.97


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M.F.=0.15 P.V.= 20.01 M.F.=0.10 P.V.= 18.41

M.F.=0.05 P.V.= 15.08


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Conclusions:-

Probability Density dependence on the DimensionsDimensions decides the active region area. Capability of confining electrons increases.

Losses depends on the dimensions

Again dimensions decides active regionMore Active regionLesser Eigen energyLess losses

Probability Density dependence on the Mole fraction.More mole fractionMore band offsetMore electron transfer from barrier region to wire regionSo more electrons for confinement


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I am enough of an artist to draw freely upon my imagination.

Imagination is more important than knowledge.

Knowledge is limited.

Imagination encircles the world.

:- Einstein

ulhas project presentation

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