Si homojunction structured nearinfrared laser bas

Authors: T Kawazoe M Ohtsu K Akahane N Yamamoto

Publish Date: 2012/05/25

Volume: 107, Issue: 3, Pages: 659-663

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Abstract

We fabricated several nearinfrared Si laser devices wavelength ∼1300 nm showing continuouswave oscillation at room temperature by using a phononassisted process induced by dressed photons Their optical resonators were formed of ridge waveguides with a width of 10 μm and a thickness of 2 μm with two cleaved facets and the resonator lengths were 250–1000 μm The oscillation threshold currents of these Si lasers were 50–60 mA From nearfield and farfield images of the optical radiation pattern we observed the high directivity which is characteristic of a laser beam Typical values of the threshold current density for laser oscillation the ratio of powers in the TE polarization and TM polarization during oscillation the optical output power at a current of 60 mA and the external differential quantum efficiency were 11–20 kA/cm2 81 50 μW and 1  respectivelyBecause silicon Si is an indirecttransitiontype semiconductor it is difficult to use it as a material for optical devices such as lightemitting diodes LEDs and lasers Nevertheless Si has been the subject of extensive research for use in fabricating lasers since it shows excellent compatibility with electronic devices 1 For example there are reports in the literature on Raman lasers 2 and lasers utilizing quantum size effects 3 however parameters such as the operating temperature efficiency wavelength and so forth are still not adequate for practical adoption of these devices To solve these problems in the research described here we developed a Si laser showing continuouswave operation at room temperature To do so we applied the same fabrication method and operating principle used for a SiLED that we previously developed which used a Si crystal having a p–n homojunction 4 We report the results hereFirst step Dressed photons are generated by the light irradiation in regions where the dopant concentration in the p–n junction has a nonuniform spatial distribution A dressed photon is a quasiparticle representing a coupled state due to the mutual interaction between a photon and an electron on the nanometer scale The dressed photon then couples with a multimode phonon generating stimulated emission that causes a conductionband electron to transition from an initial state E mathrmexmathitel rangleotimes E mathrmex mathrmthermalmathitphonon rangle to an intermediate state E g el〉⊗E exphonon〉 Here E exel〉 and E g el〉 respectively represent the excited state conduction band and ground state valence band of the electron E mathrmex mathrmthermalmathitphonon rangle and E exphonon〉 respectively represent the thermal equilibrium state and excited state of the phonon and the symbol ⊗ represents the direct product of the ket vectors Because this transition is an electricdipole–allowed transition propagating light and a dressed photon are generatedSecond step A transition from the intermediate state E g el〉⊗E exphonon〉 to the final state E g el〉⊗E ex′phonon〉 occurs producing stimulated emission Here E ex′phonon〉 is the excited state of the phonon but it differs from E exphonon〉 in process i above Because this is an electricdipole–forbidden transition only a dressed photon is generated After this transition the phonon relaxes to the thermal equilibrium stateThe twostep transition process described above referred to as a phononassisted process has already been applied to photochemical vapor deposition 6 photolithography 7 photoetching 8 optical frequency upconversion 9 photovoltaic devices 10 and so onWhen the electron number densities in the initial state E mathrmexmathitel rangleotimes E mathrmex mathrmthermalmathitphonon rangle and the intermediate state E g el〉⊗E exphonon〉n ex and n inter satisfy the Bernard–Duraffourg inversion condition n exn inter 11 optical amplification gain occurs In Si which is an indirecttransitiontype semiconductor the spontaneous emission probability is low and the probability of the firststep transition in process i occurring in the absence of externally incident light is low However if the fabricated devices have an optical cavity structure for confining the emission energy in the p–n junction and if the optical amplification gain is larger than the cavity loss there is a possibility of laser oscillation occurring as a result of spontaneous emissionTo examine this possibility we fabricated Si lasers by the following method We used an Asdoped ntype Si crystal with an electrical resistivity of 10 Ω cm and a thickness of 625 μm as a device substrate This substrate was doped with boron B by ion implantation to form a ptype layer The implantation energy for the B doping was 700 keV and the ion dose density was 5×1013 cm−2 After forming a p–n homojunction an indium tin oxide ITO film with a thickness of 150 nm was deposited on the player side of the Si substrate and an aluminum Al film with a thickness of 80 nm was deposited on the nsubstrate side both by RF sputtering for use as electrodes Next the Si substrate was diced to form the device The device area was about 400 mm2 Similarly to 4 the substrate was irradiated with laser light having a wavelength of 1320 nm and a power density of 200 mW/cm2 during which annealing was performed by applying a forwardbias current of 12 A to generate Joule heating causing the B to disperseWith this method the spatial distribution of the B concentration changes forming microdomain boundaries in a selforganized manner which allows efficient generation of the phononassisted process These domain boundaries have a shape and distribution suitable for efficiently inducing the phononassisted process described above during light emission 4 5

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