VCSEL
Vertical Cavity Surface-Emitting Laser
Docente: Mauro Mosca
(www.dieet.unipa.it/tfl)
A.A. 2015-16
Ricevimento: alla fine della lezione o per appuntamento
Università di Palermo – Scuola Politecnica - DEIM
Edge-emitting e surface-emitting laser
- Advantage over edge emitting LDs: micrometric size, that
allows larger devices density on substrates
- No additional technology steps for the realization of good
reflective facets (for edge emitting LDs: chemical assisted ion
beam etching, cleaving focused ion beam polishing or wet
chemical polishing)
- Circularly shaped, low numerical aperture beam (ideal
sources for fiber coupling and free space optics), single
longitudinal mode operation (due to a cavity length of the order
of one λ), low power dissipation and significantly lower
operating currents
- Applications: local area networks, color displays, bio-sensing,
printing applications (using VCSEL arrays would increase printing
throughput) and optical data storage
Struttura di una cavità VCSEL planare
99%
epitaxially grown
or dielectric
l/n
ltipically
/2n
The emission wavelength is not determined anymore by the maximum
gain of the active material but rather by the geometry of the cavity. Thus,
Lasing in a VCSEL critically depends on the reflectivity of both the top and
VCSELs
cansingle
lase only
if the QW
emission
wavelength
Only a mirrors.
few
longitudinal
modes
are supported
byapproximately
a VCSEL cavity
bottom
coincides with the cavity mode
Condizione di soglia (threshold)
threshold material gain
threshold modal gain
- The lasing condition in a laser is reached when the amplitude of
the optical field is maintained
’ after a round trip in the cavity
- This condition is reached when the optical gain in the cavity is
sufficient to compensate all the losses the field experiences in the
cavity during a round trip
The confinement factors (Γxy and Γz) accounts for the volume actually
occupiedINTERNAL
by photonsLOSSES
in the cavity, that isEXTERNAL
usually larger
than the active
LOSSES
= 1 in edge-emitting laser
region volume
absorption scattering diffraction
mirror reflectivity
Gz is subject to an enhancement dependent on the position of the
active region into the cavity
standing wave enhancement factor
Distribuzione di campo
longitudinale dentro la cavità
Genh = 2
EZ
z
1
= 0
lact

QW s
2
E0
E0
2
2
dz 
2
lact
 dz  l
QW s
2
act
 lact
Guadagno e corrente di soglia in
funzione della riflettività
’
gain parameter
quantum-wells
number ofper
quantum-wells
transparency current
The transparency
current
density
is
defined
as
the
current
density
semiconductor material becomes “transparent”minimal
(material
transparency)
for
which
becomes
for any
photon energy
larger
when
the the
ratematerial
of absorption
just transparent
equals the rate
of stimulated
emission.
than
or equal photon
to Eg,qwproduces exactly one photon in the output
One incident
The gain upon transparency is g = 0 (Jth = Jtr , for NQW = 1)
Guadagno e corrente di soglia in
funzione della riflettività (InGaAs QW)
Guadagno e corrente di soglia in
funzione della riflettività (InGaAs QW)
very high R
with a single QW we have
the lowest Jth because
it requires the smallest current
density to be pumped to the
transparency
lower R
gth is higher and it is still higher
for only one QW
Jth will be lower with 3 QWs
than one
g th
1
 ln  
R
a large number of QWs decreases
gth, since lact increases
Guadagno e corrente di soglia in
funzione della riflettività (GaN QW)
Guadagno e corrente di soglia in
funzione della riflettività (GaN QW)
DBR (Distributed Bragg Reflectors)
normal incidence
+
+
a mirror with a wide stop-band ensures a higher tolerance with respect to
emission wavelength
the two contributions
variations sum in phase at the design wavelength λ0
DBR: lunghezza efficace della cavità
The penetration of the optical mode into the DBR stack has to be taken
into account, as it defines the effective cavity length and, subsequently, the
wavelength of the lasing mode
The penetration depth leff of the DBR is defined as the depth into the
mirror, at which the optical field intensity is equal to 1/e of its value at
the input of the mirror
DBR: effetti delle perdite per assorbimento
low Dn and high aDBR
and l0 for long
wavelength VCSEL
reflectivity of
an m-pair DBR
absorption
coefficient
of the DBR
high Dn helps to reduce the absorption losses of the mirrors
Effetto interfacce graduali
- Interfaces not abrupt!
- The material composition is
linearly graded over a distance
of some tens of nm in orded to
reduce the electrical resistance
across the interface
Resistenza degli specchi
heating
Resistenza degli specchi
doping
p-doped
barrier for holes
(high R)
fabrication
technology
simpler
Specchi graduali
valence
band
Efficienza differenziale
3.
MECHANISMS THAT REDUCE hd :
1.
fattore di normalizzazione
2.
mirror losses
total losses
Efficienza differenziale
depends on cavity design
depends on mirror design
Wall-plug efficiency
top mirror: 19 periods
thermal roll-over
Wall-plug efficiency
Inoltre… se la riflettività diminuisce troppo,
aumenta la corrente di soglia!!!
W
Confinamento laterale
Injections schemes for GaAs-based VCSELs:
- conductive DBRs with and without current confinement layers;
- annular intra-cavity contacts in combination with one or two current
confinement layers
sacrificial layer
Ossidazione laterale dell’AlAs
.
Ossidazione laterale dell’AlAs
With GaAs (instead of AlAs): DG > 0!!
W
?
T
Energia libera di Gibbs a 698 K
< 0, vuol dire che la reazione è spontanea nella direzione indicata
Ossidazione laterale dell’AlAs
processo lineare…
Controlled Evaporated
and Mixing
system dalla radice
ma diventa
dipendente
quadrata
i gas reagenti penetrano difficilmente
attraverso l’ossido
(per alti spessori dello stesso ossido)
Ossidazione laterale dell’AlGaAs
Fabbricazione di un VCSEL con
ossidazione laterale
Ossidazione laterale in VCSEL a nitruri
Ossidazione laterale in VCSEL a nitruri
Ossidazione laterale in VCSEL a nitruri