VeCSEL Technology & Nanostructures

, by Arnaud Garnache, Jean-Philippe PEREZ, Laurent Cerutti

Semiconductor Nanostructure of the 1/2 VCSEL

Whatever the emission wavelength, the VECSEL gain structures are composed of an epitaxialhigh-reflectivity (usually 99.9%) bottom Bragg mirror (> 25 pairs) and an active layer on top, designed with some QWs (usually 3 to 10) in a k.λ/2 long active region. The QW’s are distributed among the optical standing-wave antinodes with a distribution function such as the excited carrier density is almost equal in the QW’s, to ensure a low laser threshold and a homogeneous
gain. The QW number is optimized for low threshold with 1/2% cavity losses typically. A confinement layer and a caping layer are usually added on top, depending of the properties targeted for the laser. The bottom substrate is usually be used as a heatsink. The following figure shows a typical GaAs 1/2 VCSEL structure designed for 1 µm emission :

Similar structures can be grown with GaSb materials in order to reach "Graal" wavelengths for gas-analysis spectroscopists, typically 2.3 µm and 2.7 µm.

The first-order properties of these nanostructures, including Bragg backside mirror reflectivity (exhibiting active section resonance) and Photoluminescence spectrum are as follows :

A last important technical point is that the semiconductor material must be of high quality, without traps inducing fast electron capture, in order to obtain a high performance VeCSEL. Indeed, some structures can exhibit dark lines because of too strong strain at the interface of two semiconductor layers, for example in the neighborhood of thin layers such as the quantum wells of the active section. We show in the following movie the real-time pump-beam-energy-induced growth of defects in the neighborhood of a dark-line, that induce fast electron recombinations, thus can increase threshold and perhaps generate modelocking effects.

1/2 VCSEL Technology for Thermal Management

To greatly improve heat dissipation and thus reach high power operation, the semiconductor heatsink substrate has to be improved. For this purpose,
Mainly two solutions have been described in the literature to boost the power performances: first growing the structure upside down and removing the substrate to dissipate the heat through the Bragg mirror bonded on a
highly conductive host substrate; or secondly removing the heat through the top of the semiconductor structure by optical bonding it to a thick window of high thermal conductivity and high optical quality (like sapphire, diamond or SiC). This intracavity optical heatspreader introduces some birefringence, a parasitic Fabry-Perot etalon effect, thermal lensing and non-negligible optical losses. For our VeCSELs we investigated the first solution : the 1/2 VCSEL gain structure is bonded onto a SiC or Gold host substrate , which thermal conductivities are up to 10 times stronger than the one of GaAs or GaSb materials. This strongly impacts the device thermal resistance, as shown in the following Femlab Simulation :

In terms of fabrication, this implies the development of a VCSEL membrane technology that permits to report of the Bragg + Active section on top of a new host substrate. For tis purpose, the fabrication of the 1/2 VCSEL structure is performed in 4 main steps :

  • The 1/2 VCSEL structure is grown in reverse order. During the growth, an etch-stop layer is inserted between the semiconductor substrate and the top of the active section.
  • For SiC technology, a metal bonding is performed. A possible drawback of this technique is a possible low quality bonding/thermal uniformity over a large surface. For that reason, Au technology is preferred, because an Au substrate can be obtained simply by electroplated Au deposit.
  • The substrate is then removed by selective chemical etching.
  • The final step is the evaporation of an AR coating on the top of the structure.

The structure resulting from the fabrication process has been observed with SEM microscopy (example of a GaSb structure obtained by Molecular Beam Epitaxy at IES in Nanomir Group) :

1/2 VCSEL Technology for Electrical Pumping

For high power TEM00 VECSEL operation, a uniform carrier distribution over large mesa (>50µm) together with low thermal impedance are required. In top emitting configuration, active region is very close to the annular electrical contact leading to weak carrier injection in the center of active regionwhere the optical field ismaximum.On the contrary, a uniformcarrier injection can be performed in bottom emitting configuration with a disk contact on the bragg mirror,
directly bonded on a high thermal conductivity mount. Therefore, our 1/2-VCSEL structure is designed for bottom emission at λ = 1µm. It was grown in reverse order on a "thin" low doped n-type GaAs substrate. This intracavity substrate induces rather high optical losses that have to be compensated for low threshold and high efficiency. To strengthen the optical field Emcavity in the active region by mean of a Fabry-Perot micro-cavity, we introduce a low-reflectivity Bragg mirror between the active region and the substrate to increase the longitudinal confinement factor in the micro-cavity, providing a 1/2-VCSEL modal gain per reflection Gm higher than 0.2 and enhancing the spectral selectivity of a single longitudinal mode. The active layer is designed with NQW = 3 strained balanced InGaAs/GaAs(P) QWs in a undoped ? long active region. The gain peak is designed to match the micro-cavity mode wavelength at room temperature. Like in optical pumping structures, the QW’s are distributed on the optical standing-wave antinode.