VCSEL Vertical Cavity Surface-Emitting Laser Docente: Mauro Mosca (www.dieet.unipa.it/tfl) A.A. 2013-14 Ricevimento: alla fine della lezione o per appuntamento Università di Palermo – Facoltà di Ingegneria (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: color displays, bio-sensing, printing applications (using VCSEL arrays would increase printing throughput) and optical data storage

Sezione di una cavità VCSEL planare 99% epitaxially grown or dielectric l/n The emission wavelength is not determined anymore by the maximum gain of the active material but rather by the geometry of the cavity. Thus, VCSELs can lase only if the QW emission wavelength approximately coincides with the cavity mode Lasing in a VCSEL critically depends on the reflectivity of both the top and bottom mirrors. Only a few single longitudinal modes are supported by a VCSEL cavity

Condizione di soglia (threshold) threshold modal gain threshold material 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 INTERNAL LOSSES EXTERNAL LOSSES absorption scattering diffraction mirror reflectivity The confinement factors (Γxy and Γz) accounts for the volume actually occupied by photons in the cavity, that is usually larger than the active region volume small… standing wave enhancement factor

Distribuzione di campo longitudinale dentro la cavità Genh = 2

Guadagno e corrente di soglia in funzione della riflettività

Guadagno e corrente di soglia in funzione della riflettività (InGaAs QW)

Guadagno e corrente di soglia in funzione della riflettività (InGaAs QW)

Guadagno e corrente di soglia in funzione della riflettività (GaN QW)

Guadagno e corrente di soglia in funzione della riflettività (GaN QW)

Altre perdite (scattering alle interfacce e diffrazione)

DBR (Distributed Bragg Reflectors) normal incidence + + a mirror with a wide stop-band ensures a higher tolerance with respect to emission wavelength variations the two contributions 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 reflectivity of an m-pair DBR absorption coefficient of the DBR low Dn and high aDBR and l0 for long wavelength VCSEL high Dn helps to reduce the absorption losses of the mirrors

Effetto interfacce graduali Interfaces not abrupt! The material is linearly graded over a distance of some tens of nm in orded to reduce the electrical resistance across the interface

Resistenza degli specchi

Resistenza degli specchi doping barrier for holes (high R) fabrication technology simpler

Specchi graduali valence band

Efficienza differenziale 3. 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

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… ma diventa dipendente dalla radice quadrata Controlled Evaporated and Mixing system 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