chapter2 introduction to quantum mechanics
TRANSCRIPT
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)
�Position
�Velocity � device’s speed
I ∝ n x q x vCurrent,
Introduction 1
Need to know electron behavior in the crystal and
the material (energy band, etc)
�Velocity � device’s speed
�No. of electronDensity of electron velocity
Introduction of quantum mechanics (Defines electron with wave function)
� Schrodinger equation
Microelectronics I : Chapter 2
�Quantum mechanic becomes more significant as electronic 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
�Here, classical Physics no longer applicable !!
Microelectronics I : Chapter 2
Objective:
To understand the basic of quantum mechanics
�Wave-particle duality
�Schrodinger equation
- equation
- physical meaning
�Application:
-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�Interference 1905, Einstein
“Photon (discrete packet)”
Explains the photoelectric effect
�Interference
�Refraction
�diffraction
light photoelectron
Max kinetic energy of photoelectron
Wave-Particle duality
frequencyνo
Microelectronics I : Chapter 2
Wave-Particle duality of electron
�Electron: charged particle (q=1.6 x 10-19 C)
�De Broglie (1924)
I ∝ n x q x vex:
�De Broglie (1924)
Particle with momentum, p has wavelength, λ
λ = h/p P: Planck’s constant
Ex:
e
Velocity, v=105 m/s
Wave nature
e
λ = h/p =h/m.v
=6.625 x 10-34/(105 x 9.11 x 10-31)
= 7.27 nm
Microelectronics I : Chapter 2
Electron gun
Double slit screen
e
Wave-particle duality : experiment
When electron hits the screen, a dot will appears
Particle naturee
① Shoot electron 4 times
Double slit screen
Particle nature
1st�Electron could go through the slit
Electron gun
e 2nd
3rd
4th�Electron could go through the slit�Position of electron was random
Microelectronics I : Chapter 2
② Shoot electron many time
Electron gun
slit screen
e
http://www.hitachi.com/rd/research/em/doubleslit.html
e
slit screena: 8 electrons, b: 270 electrons, c: 2000 electrons,d: 160,000 electrons
�Interference pattern
Wave nature
d: 160,000 electrons
Interpretations
Microelectronics I : Chapter 2
1. Electron propagates in space like wave2. Each electron pass through both two open slit at the same time3. Electron interfere with itself
Electron gun
Double slit screen
e
e
The experimental results confirm the wave-particle duality of electron
Microelectronics I : Chapter 2
The Uncertainty principle (Heisenberg 1927)
∆p∆x ≥ ħ
①
Impossible to simultaneously describe with absolute accuracy the position and momentum of a particle
∆p∆x ≥ ħ
∆E∆t ≥ ħ
② Impossible to simultaneously describe with absolute accuracy the energy of particle and the instant of time the particle has this energy
∆p: uncertainty in momentum
∆x : uncertainty in position
∆E∆t ≥ ħ∆E: uncertainty in energy
∆t : uncertainty in time
�Cannot determine exact position of electron
use “probability”
Microelectronics I : Chapter 2
Schrodinger Wave Equation
Describe and discuss electron behavior
Wave function� sin, cos
k : wave numberΨ(x,t)= a.cos(kx-ωt+ε))
Ψ(x,t)= exp ( j(kx-ωt))= exp( jkx)exp(-jωt)
k : wave numberω : angular momentumε : initial phase
Ψ(x,t)= a.cos(kx-ωt+ε))
Use “exp”
exp(jθ)=cosθ+jsinθ
ε=0
= exp( jkx)exp(-jωt)
Ψ(x,t)= Φ(t)φ(x)
position-dependenttime-dependent
Hφ(x)=Eφ(x)
Time-independent Schrodinger equation
Hamiltonian
Schrodinger Wave Equation2
Microelectronics I : Chapter 2
Energy of electronHamiltonian
Hamiltonian: total energy operator
H = kinetic energy + potential energy
)(2 2
22
xVxm
H +∂
∂−=h
Energy of electron
)()()(2 2
22
xExxVxm
ϕϕ =
+
∂
∂−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)()(|)(| *2dxxxdxx ϕϕϕ
total probability=1
φ*(x): complex conjugate function
total probability=1
Microelectronics I : Chapter 2
Major quantities: energy, momentum, position of electron
)(
)(
xxi
xH
ϕ
ϕ
∂
∂h
Energy,
Momentum, Result :equation
)(
)(
xx
xxi
ϕ
ϕ∂
Momentum,
Position,
Value of major quantities given by expected value in probability theory
∫+∞
∂* ϕϕh
∫+∞
ϕϕ ∫+∞
dxxxx )()(* ϕϕ
energy momentum position
∫
∫∞+
∞−
∞−
∂
∂
dxxx
dxxxi
x
)()(
)()(
*
*
ϕϕ
ϕϕh
∫
∫∞+
∞−
∞−
dxxx
dxxHx
)()(
)()(
*
*
ϕϕ
ϕϕ
∫
∫∞+
∞−
∞−
dxxx
dxxxx
)()(
)()(
*
*
ϕϕ
ϕϕ
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 continuousCondition 1: φ(x) must be finite, single-valued, and continuousCondition 2: ∂φ(x)/∂x must be finite, single-valued and continuous
ax
II
ax
I
III
xx
aa
==∂
∂=
∂
∂
=
ϕϕ
ϕϕ )()( Condition 1
Condition 2
Microelectronics I : Chapter 2
Basic solution of Schrodinger equation
Consider V: constant
)()(2 2
22
xExVxm
ϕϕ =
+
∂
∂−h
0)()(2)(
22
2
=−
−∂
∂x
EVm
x
xϕ
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
EVm
eq.1ϕαϕ 2"
0=−
Solution: xx
BeAeααϕ −+= A,B: Coefficient
ϕαϕ
ϕαϕ2"
0
=
=−
∞∞
Condition 1( φ must be finite)
xBe
αϕ −=
1. if, E > V
0)(2 2
2>−=
−− β
h
EVm
eq.1ϕβϕ =+
Microelectronics I : Chapter 2
Solution: xixiDeCe
ββϕ −+= C,D: Coefficient
eq.1
ϕβϕ
ϕβϕ2"
2"0
−=
=+
�Wave function is given by the combination of the two type solution
Microelectronics I : Chapter 2
Application 1: Potential well
∞ ∞
region I region II region III
region I and III
V(x)=∞
x=Lx=0
region I region II region IIIV(x)=∞
Electron cannot exist in the regions
region II (0<x<L)
−∂ ϕ
φ=0
……Time-independent equation
V=0
0)(2)(
22
2
=+∂
∂x
mE
x
xϕ
ϕ
h
---eq. 1
0)()(2)(
22
2
=−
−∂
∂x
EVm
x
xϕ
ϕ
h
eq. 1
Microelectronics I : Chapter 2
)(2)(
22
2
xmE
x
xϕ
ϕ
h−=
∂
∂
ϕβϕ 2" −= 2
2 2
h
mE=βϕβϕ −= 2
h
Solution ;xixi
BeAeββϕ −+=
Boundary condition;
0)0()0( ==III
ϕϕ 0)()( == LLIIIII
ϕϕ
0=+ BA 0=+ − LiLiBeAe
ββ...eq. 2 ...eq. 3
Microelectronics I : Chapter 2
From eq. 2 & 3
0)( =− − LiLieeA
ββ
A≠0
0=− −ee
LiLi ββregion II
0)sin(2
0
=
=− −
Li
eeLiLi
β
ββ
n; integer
2nL
mE
nL
=
=
π
πβ
x=Lx=0
En=1
En=2
En=3
2
22
2n
LmE
nL
=
=
π
π
h
hx=Lx=0
The energy of particle is quantized
Particular discrete values
Classical; continuous values
Microelectronics I : Chapter 2
Wave function
xixiBeAe
ββϕ −+=
= x
L
niA
πsin2
=
xL
nC
L
πsin
normalization
1sin0
22 =
∫ dxx
L
nC
L
πTotal probability=1
2
LC
2=
= x
L
n
L
πϕ sin
2n=1,2,3,4……
Microelectronics I : Chapter 2
Corresponding probability functionsWave functions
x=L x=L
Microelectronics I : Chapter 2
Application 2: Potential well
region I region II region III
V0
x=Lx=0
eRegion I
0)(2)(
22
2
=+∂
∂x
mE
x
x
I
I ϕϕ
h
Region II Region III
0)(2)(
22
2
=+∂
∂x
mE
x
x
III
III ϕϕ
h0)(
)(2)(2
0
2
2
=−
−∂
∂x
EVm
x
x
II
II ϕϕ
h
Microelectronics I : Chapter 2
Consider
E<V0, ,)(2
2
02
h
EVm −=α 2
2 2
h
mE=β
Solution; wave functionSolution; wave function
xixi
IBeAe
ββϕ −+=
xixi
IIIFeEe
ββϕ −+=
xx
IIDeCe
ααϕ −+=
region I region II region III
V0
A
B
C
D
E
F
x=Lx=0F=0
xi
IIIEe
βϕ =
Microelectronics I : Chapter 2
Boundary condition;
)()(
)0()0(
LLIIIII
III
ϕϕ
ϕϕ
=
=Continuous wave function
)()( LLIIIII
ϕϕ =
)()(
)0()0(
''
''
LL IIIII
III
ϕϕ
ϕϕ
=
=Continuous first derivative
4 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 I region II region III
V0
A E *
*
AA
EET
⋅
⋅=
x=Lx=0
AA ⋅
)2exp(116 LEE
T β−
−
≈ )2exp(11600
LV
E
V
ET β−
−
≈
�T 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)
e2−
=
+Nucleus; positively charged
r
erV
0
2
4)(
πε
−=
“Quantized
approximation
+Quantum well
Expected results
+Quantum well
“Quantized
energy”
= x
L
n
L
πϕ sin
2
n=1,2,3,…
Microelectronics I : Chapter 2
Solving Schrodinger equation
0),,())((2
),,(2
02 =−+∇ φθψφθψ rrVEm
rh
Wave function & energy of electron in the atomWave function & energy of electron in the atom
result
222
0
4
0 1
2)4( n
emE
n
hπε
−=
n; 1, 2, 3,…
(principal quantum number)
1. Energy of atom is quantized. The value is determined by a quantum number,n
2. Wave function of electron also determined by quantum numbers (n, l, m)
Microelectronics I : Chapter 2
�n: principal quantum number
(determine total electron energy)
Quantum number
Quantum states of electron
(determine total electron energy)
N=1, 2, 3,….
�l: azimuthal quantum number
(specifies the shape of atomic orbital)
l= n-1, n-2,……,0 (s,p,d,..)
�m: magnetic quantum number
( direction)
|m|=l,l-1,…0 S- orbital
z
y
x
z
y
xz
z
y
x
|m|=l,l-1,…0
�s: spin quantum number
( spin of electron)
S=1/2, -1/2
S- orbital
y
x
p- orbital
Microelectronics I : Chapter 2
electron
energyex: C (no. of electron: 14)
l=2p+ n=1
n=2
l=1s
l=2sl=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