3.46 PHOTONIC MATERIALS AND DEVICES
Lecture 15: III-V Processing
Notes
Lecture
Double Hetero structure laser
AlGaAs
GaAs
(band structure engineering)
AlGaAs
e-
hν
n
E
P
h+
X
n
x
I
d < 1 μm
1. Large refractive index active region
2. Low Eg active region
ηd is increased
• faster inversion for same injection
current
• light concentrated for stimulated
emission
Confinement
light (guided) confinement
carrier (electron and hole) confinement
100× ↓ of Jth
Γ
DH: Double Heterostructure
SQW: Single Quantum Well
SCH: Separate Confinement
Heterostructure
DH
Γ =1
SQW
Γ ∝ Δnd2
SCH
Γ ∝ Δnd
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 1 of 11
Lecture
Notes
nth ↓ as d ↓
DH
g
g(ν) ∝ fg (ν) ⋅ ρbulk (ν)
gp
nth
SQW
g
gp :peak gain
higher T stability
gp
g(ν) ∝ fg (ν) ⋅ ρQW (ν)
nth
Density of States of QW
ρ(ν ) = 1012 states / cm2 for dE = kT
ρ(E)
QW
)
=
E1
E2
E3
2mr
d
E
λ 2mr
gP =
@ threshhold
2τr hd
multilevel gain
g
Threshhold current density
Rth = threshold recombination rate
Jth = eR th
CRnp
10−10 cm3 s−1 ⋅ (2 ×1018 cm−3 )2 = 4×1026 cm−3 s−1
= 1.6 ×10−19 ⋅R th
Jth decreases with d
Jth ≈ 6.4 kA cm-2 ⋅ μm
l×w d
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 2 of 11
Notes
Lecture
d
DH:
Jth = 1.2 kA/cm2
I th
SQW:
0.2 μm
10 − 20 mA
d < 300 Å
Jth < 180 A/cm2
SQW
1. E levels quantized → lasing @ QW
transitions
2. ρ (ν)(2D) more efficient, gP = const (ν)
C
3. g saturates
4. QW 1012 states/cm2
DH 1013 states/cm2 in d = 1000Å
5. Confinement optimized by separation
SCH
V
Strained Layers
Strain (compressive)
• raises the LH sub band
• reduces carriers to invert ⇒ Jth ↓
⇒ ηd ↑
d<300Å
with band filling, transition not @ gp are
useless
(a)
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 3 of 11
Notes
Lecture
III-V Compound Semiconductor Processing
1. Substrate Preparation
GaAs, InP
2. Epitaxial layer growth
LPE, MBE, MOCVD, CVD
3. Etch
Dry (RIE), wet
4. Contacts
Au, silicides, metals
1. Process Constraints
A. CSBH laser provides (CSBH: Channeled-Substrate Buried Heterostructure) lateral optical and electrical confinement. i. grow InP:Fe SI layer
ii. etch channel
iii.
grow InP/InGaAsP/InP DH in channel B. APD detector (SAM)
i. grow InGaAs/InP het.
ii. SiNx dielectric deposition
iii.
etch contact window
iv.
d
iffuse p+ contact/junction
v. implant p- guard ring
Both devices employ deposited dielectrics
for AR coatings (APD) and facet reflectors
(laser).
2. Issues
A. Groups V volatility
i. incongruent vaporization of
P from InP @ T > 360°C ii. as from GaAs @ T > 600°C
Solution: group V overpressure or stable dielectric cap layer. iii.
RIE creates group III rich
suffice Solution: lower T, lower E, high Z (Z: atomic number)
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 4 of 11
Lecture
Notes
B. Preferential etch of V groove Solution: surface prep. C. Metallization reactions
Solution: barriers or stable phases
D. Degradation of ηi
Solution: defect control, life testing
3. Epitaxial Growth
A. Dislocation density
B. Stoichiometry
Concept: Single crystal film bonded to a
single crystal substrate with a
common interface and the
lattice of the film having a
definite orientation w.r.t. the
substrate lattice.
Substrate: semi infinite thickness
Surface:
a
tomically flat
(ledges)
(bond reconstruction)
Si(100) 2 × 1
or
GaAs(100)
→ rows of AS
V-termination
→ flat surface
Film: homogeneous, 2D (x, y >> t)
(phase separation?)
Interface:
sharp (interdiffusion)
Tangential forces:
←a0→
sinusoidal in a0
Growth Modes
Efs = film/substrate bond strength
Eff = film/film bond strength
E
W = fs = relative strength of bonds to
Eff
substrate
a − af
η = lattice misfit = s
af
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 5 of 11
Lecture
Notes
Epitaxy
equilibrium:
• low deposition rate
• high T (surface diffusion)
ΔG (system energy)
• minimize
Nf (film atoms)
Coherency
(dislocations)
1. 1. variables:
a, Eff, h
2. minimize energy
E∈ =∈2 Bh
Film thickness
strain Frank-Vander Merwe 1D harmonic chain
elastic constant Coherent:
Incoherent:
η =∈
η =∈ +δ
(strained)
(relaxed)
δ = strain relief by dislocations
separation of parallel misfit dislocations:
S =
b
δ
η (relaxed) =∈ +
1
b cos λ
S
projection of b on plane of interface
Critical hc
minimize E∈ vs. Edislocation
Matthews–Blakelee
⎡ ⎛ hc ⎞ ⎤
b
⎢ln ⎜⎜ ⎟⎟ +1⎥
hc =
8πη(1+ υ ) ⎢⎣ ⎜⎝ b ⎠⎟ ⎥⎦
hc ≈
b
4η
104
hc 10
(Å)
103
of η = 10−2
102
1
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
1 Å ⇒ hc = 100 Å
b
4
2
η%
3
4
Lecture 15: III-V Processing
Page 6 of 11
Lecture
Morphology (wetting)
μf =
Notes
Deposition
w=1
μ 0−1 ⇒ monolayer coverage G ↓
μ1−2 ⇒ f-f clustering G ↑
∂G
∂N
μ1−2
ML:
w=2
⇒ layer growth G ↓
monolayers
G ↑ after one
monolayer
G ↑ initially (no
wetting)
Stronski-Kranstanov:
Nucleation barrier to clustering
Volmer-Weber:
γ 3c / v
8π
ΔG* =
3 ρ 2 ⎡ΔF (η)⎤ 2
0 ⎣
⎦
density of unstrained film
ΔF (η) ∝ η2
N* =
16πγ 3c / v
3
3 [ΔF(η)] ρ0 2
I = Ns Γ exp (−ΔG * / RT)
Morphology + Coherency are determined
by nucleation barriers ΔG * for dislocation
formation clustering
Metastability is common
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 7 of 11
Lecture
Notes
4. Contacts
•
•
•
•
•
stable
selective
low Rc
low T deposition
adhesion
Eutectics
• Au(Be)
• Au(Ge)
Rc < 10−5 Ω⋅ cm2
for lasers
P
n
• small process window
o RTA
• unreliable
surface defects pin EF
→ contact resistance
(Schottky Barrier)
for n-GaAs
p-InP
⇒ heavily doped epilayer under contact
Silicides
• Stable
o undefined interface
→ Rc ↑
Metals
• reactive with compounds
→ defects, dissociation
→ phase stability
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 8 of 11
Lecture
Notes
AB dominant
500 °C
Ga
MAx dominant
MBy
AB
Ti
MBy dominant
500 °C
TiGa4
MBy
TiGa3
TiAs
As
300 °C
No phase dominant
PtAs2
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
PtGa3
PtAs2 + PtGa
GaAs
Lecture 15: III-V Processing
Page 9 of 11
Lecture
Notes
NiP (conductor)
In
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing Page 10 of 11
Lecture
Notes
Adhesion:
• local structural relaxation
• ion beam mixing
• chemical bonding
(Cu / Al2O3 with excess 2 )
Interdiffusion
• Polycrystal: grain boundary diffusion
3Ea
Dbulk = Ea
Ddisloc
4
Ea
Dgb ∼
2
D = Dbulk + f ⋅
gb
Dbulk (TMP )
⎛1
⎞
Dgb ⎜⎜ TMP ⎟⎟⎟
⎜⎝ 2
⎠
refractory TM:
Cr, Ni, Ta, Ti, Hf
• Diffusion Barrier (Ti / Pt)Au
o high TMP
o chemically stable
• Intermetallic Compound
• Coherent Interface
Dielectric Deposition
SiO2, SiOxNy, SiNx
• sputter
• PECVD
• e-beam
facets, isolation, diffusion masks
Etch
• Wet etch (Br:CH3OH, HCl)
o layer stop
H2SO4 : H2O2 : H2O
o v-groove
• Dry etch (CF2Cl2), (HBr, HI)
o Anisotropy
o Photoelectrochemical etch anisotropy
3.46 Photonic Materials and Devices
Prof. Lionel C. Kimerling
Lecture 15: III-V Processing
Page 11 of 11