Chapter2 introduction to quantum mechanics

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About This Presentation

solid states physics


Slide Content

Microelectronics I
Chapter 2: Introduction to
Quantum Mechanics
2.1 Principles of Quantum Mechanics
2.2 Schrodinger’s Wave Equation
2.3 Applications of Schrodinger’s Wave equation
2.4 Extensions of the Wave Theory to Atoms

Microelectronics I : Chapter 2
Control electron in the solid (crystal)
MPosition
M
Velocity
i
device’s speed
I

n
x q x
v
Current,
Introduction 1
Need to know
electron behavior
in the crystal and
the material (energy band, etc)
M
Velocity
i
device’s speed
MNo. of electron
Density of electron velocity
Introduction of quantum mechanics (Defines electron with wave function)
iSchrodinger equation

Microelectronics I : Chapter 2
cQuantum mechanic becomes more significant as electr onic device becomes smaller
Appearance of “
quantum effect

Introduction 2
Current, I
Voltage, V
Current, I
Width, W
I
Ohm’s law
“classical”
Smaller W
I
?
Electron channel
V
“classical”
V
MHere, classical Physics no longer applicable !!

Microelectronics I : Chapter 2
Objective:
To understand the basic of quantum mechanics
MWave-particle duality
MSchrodinger equation
-equation
-physical meaning
MApplication:
-quantized energy
-tunneling effect
Chapter 3: energy band theory of solids

Microelectronics I : Chapter 2
propagates as wave (frequency, ν)
Particle having energy, hν
*h, Planck's constant = 6.625 x 10
-34
Js
light
1905, Einstein
M
Interference
1905, Einstein “Photon (discrete packet)”
Explains the
photoelectric effect
M
Interference
MRefraction
Mdiffraction
light
photoelectron
Max kinetic energy
of photoelectron
Wave-Particle duality
frequency
ν
o

Microelectronics I : Chapter 2 Wave-Particle duality of electron
MElectron: charged
particle
(q=1.6 x 10
-19
C)
M
De Broglie (1924)
I

n
x q x v
ex:
M
De Broglie (1924)
Particle with momentum, p has
wavelength
, λ
λ= h/p
P: Planck’s constant
Ex:
e
Velocity, v=10
5
m/s
Wave nature
e
λ= h/p =h/m.v
=6.625 x 10
-34
/(10
5
x 9.11 x 10
-31
)
= 7.27 nm

Microelectronics I : Chapter 2 Electron gun
Double slitscreen
e
Wave-particle duality : experiment
When electron hits the screen,
a dot
will appears
Particle nature
e
①Shoot electron 4 times
Double slitscreen
Particle nature
1st
M
Electron could go through the slit
Electron gun
e
2nd
3rd
4th
M
Electron could go through the slit
MPosition of electron was random

Microelectronics I : Chapter 2
②Shoot electron many time
Electron gun
slitscreen
e
http://www.hitachi.com/rd/research/em/doubleslit.ht ml
e
slitscreen
a: 8 electrons, b: 270 electrons, c: 2000 electrons , d: 160,000 electrons
MInterference pattern
Wave nature
d: 160,000 electrons

Interpretations Microelectronics I : Chapter 2
1. Electron propagates in space like wave
2. Each electron pass through both two open slit at the same time
3. Electron interfere with itself
Electron gun
Double slitscreen
e
e
The experimental results confirm the wave-particle duality of electron

Microelectronics I : Chapter 2 The Uncertainty principle (Heisenberg 1927)
i
p
i
x ≥
ħ

Impossible to simultaneously describe with absolute accuracy the position and
momentum of a particle
i
p
i
x ≥
ħ
i
E
i
t ≥
ħ
②Impossible to simultaneously describe with absolute accuracy the energy of
particle and the instant of time the particle has t his energy
ip: uncertainty in momentum
ix : uncertainty in position
i
E
i
t ≥
ħ
iE: uncertainty in energy
it : uncertainty in time
MCannot determine exact position of electron
use “probability”

Microelectronics I : Chapter 2 Schrodinger Wave Equation
Describe and discuss electron behavior
Wave functionisin, cos
k : wave number
Ψ
(x,t)= a.cos(kx
-
ω
t+
ε
))
Ψ(x,t)= exp ( j(kx- ωt))
= exp( jkx)exp(
-

t)k : wave number ω: angular momentum
ε: initial phase
Ψ
(x,t)= a.cos(kx
-
ω
t+
ε
))
Use “exp”
exp(jθ)=cosθ+jsinθ
ε=0
= exp( jkx)exp(
-

t)
Ψ(x,t)= Φ(t)φ(x)
position-dependent time-dependent

Hφ(x)=Eφ(x)
Time-independent Schrodinger equation
Hamiltonian
Schrodinger Wave Equation2 Microelectronics I : Chapter 2
Energy of electron
Hamiltonian
Hamiltonian: total energy operator
H = kinetic energy + potential energy
)(
2
2
2 2
xV
xm
H+


−=
h
Energy of electron
)( )()(
2
2
2 2
x E x xV
xm
ϕ ϕ
=








+



h

Microelectronics I : Chapter 2
Physical meaning of wave equation
φ(x): wave function
|φ(x)|
2
:
probability
of existence of electron at x
position of electron cannot be determined precisely
∫ ∫
+∞
∞−
+∞
∞−
= =1 )()( |) (|
* 2
dxx x dx x
ϕ ϕ ϕ
total probability=1
φ*(x): complex conjugate function
total probability=1

Microelectronics I : Chapter 2 Major quantities: energy, momentum, position of electron
)
(
)
(
x
x
i
x
H
ϕ
ϕ ∂∂h
Energy,
Momentum,
Result :equation
)(
)
(
x x
x
x

ϕ

Momentum,
Position,
Value of major quantities given by
expected value
in probability theory

+∞



*
ϕ
ϕ
h

+∞
ϕ
ϕ

+∞
dx
x
x
x
)
(
)
(
*
ϕ
ϕ
energymomentumposition


∞+
∞−
∞−





∂∂
dxx x
dxx
xi
x
)()(
)( )(
*
*
ϕ ϕ
ϕ
ϕ
h


∞+
∞−
∞−
dxx x
dxx Hx
)()(
)( )(
*
*
ϕ ϕ
ϕ
ϕ
∫∫
∞+
∞−
∞−
dxx x
dx
x
x
x
)()()
(
)
( *
*
ϕ ϕϕ
ϕ
Result :real number

Microelectronics I : Chapter 2
Region I
Region II
Boundary Condition
x=a
Condition 1:
φ(x) must be finite, single
-
valued, and
continuous
Condition 1:
φ(x) must be finite, single
-
valued, and
continuous
Condition 2: ∂φ(x)/∂x must be finite, single-valued and
continuous
ax
II
ax
I
II I
x x
a a
= =


=


=
ϕ ϕ
ϕ
ϕ
)( )(
Condition 1
Condition 2

Microelectronics I : Chapter 2 Basic solution of Schrodinger equation
Consider V: constant
)( )(
2
2
2 2
x E x V
xm
ϕ ϕ
=








+



h
0)(
) (2)(
2 2
2
=




x
E Vm
x
x
ϕ
ϕ
h
--eq.1
Second order differential equation

x
h
constant

Microelectronics I : Chapter 2
1. if, E < V
0
) (2
2
2
> =

α
h
E Vm
eq.1
ϕ
α
ϕ
2 "
0
=

Solution:
x x
Be Ae
α α
ϕ

+ =
A,B: Coefficient
ϕα ϕϕ
α
ϕ
2 "
0
=
=

∞∞
Condition 1(
φmust be finite
)
x
Be
α
ϕ

=

1. if, E > V
0
) (2
2
2
> −=


β
h
E Vm
eq.1
ϕ
β
ϕ
=
+
Microelectronics I : Chapter 2
Solution:
xi xi
De Ce
β β
ϕ

+ =
C,D: Coefficient
eq.1
ϕβ ϕ
ϕ
β
ϕ
2 "
2 "
0
−=
=
+
MWave function is given by the combination of the tw o type solution

Microelectronics I : Chapter 2 Application 1: Potential well
∞∞
region I
region II
region III
region I and III
V(x)=∞
x=L
x=0
region I
region II
region III
V(x)=∞
Electron cannot exist in the regions
region II (0<x<L)


ϕ
φ=0
……Time-independent equation
V=0
0)(
2)(
2 2
2
= +


x
mE
x
x
ϕ
ϕ
h
---eq. 1
0)(
) (2)(
2 2
2
=




x
E Vm
x
x
ϕ
ϕ
h

eq. 1
Microelectronics I : Chapter 2
)(
2 )(
2 2
2
x
mE
x
x
ϕ
ϕ
h
−=


ϕ
β
ϕ
2 "

=
2
2
2
hmE
=
β
ϕ
β
ϕ

=
2
h
Solution ;
xi xi
Be Ae
β β
ϕ

+ =
Boundary condition;
0 )0( )0(
=
=
II I
ϕ
ϕ
0 )( )(
=
=
L L
III II
ϕ
ϕ
0
=
+
B A0= +
−Li Li
Be Ae
β β
...eq. 2...eq. 3

Microelectronics I : Chapter 2
From eq. 2 & 3
0 ) (
= −

Li Li
e eA
β β
A≠0
0
=


e
e
L
i
L
i
β
β
region II
0 ) sin( 20
==


L i
e
e
L
i
L
i
β
β
β
n; integer
2
n
L
mE
n L
=
=
π
π
β
x=L
x=0
E
n=1
E
n=2
E
n=3
2
2
2
2
n
L m
E
n
L






=
= π
π
h
h
x=L
x=0
The energy of particle is quantized
Particular discrete values
Classical; continuous values

Microelectronics I : Chapter 2
Wave function
xi xi
Be Ae
β β
ϕ

+ =



=x
L
n
iA
π
sin 2






=


x
L
n
C
L
π
sin
normalization
1 sin
0
2 2
= 






dxx
L
n
C
L
π
Total probability=1
2L
C
2
=






=x
L
n
L
π
ϕ
sin
2
n=1,2,3,4……

Microelectronics I : Chapter 2
Corresponding probability functions
Wave functions
x=L
x=L

Microelectronics I : Chapter 2 Application 2: Potential well
region Iregion II
region III
V
0
x=L
x=0
e
Region I
0)(
2)(
2 2
2
= +


x
mE
x
x
I
I
ϕ
ϕ
h
Region II
Region III
0)(
2)(
2 2
2
= +


x
mE
x
x
III
III
ϕ
ϕ
h
0)(
) (2)(
2
0
2
2
=




x
E Vm
x
x
II
II
ϕ
ϕ
h

Microelectronics I : Chapter 2
Consider
E<V
0
,
,
) (2
2
0 2
h
E Vm

=
α
2
2
2
h
mE
=
β
Solution; wave function Solution; wave function
xi xi
I
Be Ae
β β
ϕ

+ =
xi xi
III
Fe Ee
β β
ϕ

+ =
x x
II
De Ce
α α
ϕ

+ =
region Iregion II
region III
V
0 A
B
C
D
E
F
x=L
x=0 F=0
xi
III
Ee
β
ϕ
=

Microelectronics I : Chapter 2
Boundary condition;
)
(
)
(
)0( )0(L
L
III
II
II I
ϕ
ϕ
ϕ
ϕ
=
=
Continuous wave function
)
(
)
(
L
L
III
II
ϕ
ϕ
=
)( )(
)0( )0(
' '
' '
L L
III II
II I
ϕ ϕ
ϕ ϕ
=
=
Continuous first derivative4 equation
Can solve for the
4 coefficients
B, C, D, E in term of A

Microelectronics I : Chapter 2
Parameter of interest;
Transmission coefficient, T
region Iregion II
region III
V
0 AE
*
*
A
A
EE
T⋅⋅
=
x=L
x=0
A
A
⋅ )
2
exp(
1
16
L
E
E
T
β







)
2
exp(
1
16
0 0
L
VE
VE
T
β







MT is not zero
Electron penetrate the barrier
“tunneling”

Microelectronics I : Chapter 2
L
Wave function through the potential barrier

Microelectronics I : Chapter 2 Extensions of the wave Theory to Atoms
Potential function (coulomb attraction)
e
2

=
+
Nucleus; positively charged
r
e
rV
02
4
)(
πε

=
“Quantized
approximation
+
Quantum well
Expected results
+
Quantum well
“Quantized energy”






=x
L
n
L
π
ϕ
sin
2
n=1,2,3,…

Microelectronics I : Chapter 2
Solving Schrodinger equation
0),,())( (
2
),,(
2
0 2
= − + ∇
φθ ψ φθ ψ
r rVE
m
r
h
Wave function & energy of electron in the atom Wave function & energy of electron in the atom
result
2 2 2
0
4
0
1
2) 4(n
em
E
n
h
πε−
=
n; 1, 2, 3,…
(principal quantum number)
1. Energy of atom is quantized. The value is determi ned by a quantum number,n
2. Wave function of electron also determined by quan tum numbers (n, l, m)

Microelectronics I : Chapter 2
Mn: principal quantum number (determine total electron energy)
Quantum number Quantum states of electron
(determine total electron energy) N=1, 2, 3,….
Ml: azimuthal quantum number
(specifies the shape of atomic orbital)
l= n-1, n-2,……,0 (s,p,d,..)
Mm: magnetic quantum number
( direction)
|m|=l,l
-
1,…0
S
-
orbital
z
y
x
z
y
x
z
z
y
x
|m|=l,l
-
1,…0
Ms: spin quantum number
( spin of electron)
S=1/2, -1/2
S
-
orbital
y
x
p- orbital

Microelectronics I : Chapter 2 electron
energy
ex: C (no. of electron: 14)
l=2p
+
n=1
n=2
l=1s
l=2s
l=2p
n=1
n=2
As n increases, energy of
quantum state increases
m
++
n=1
Pauli Exclusion Principle
No two electrons may occupy the same quantum state
quantum state increases
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