Processing of deeply etched GaAs/AlGaAs quantum cascade lasers with grating structures
- PDF / 389,446 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 99 Downloads / 139 Views
B5.2.1
Processing of deeply etched GaAs/AlGaAs quantum cascade lasers with grating structures S.Golka, M.Austerer, C. Pflügl, W.Schrenk, and G.Strasser Zentrum für Mikro- und Nanostrukturen Technische Universität Wien Floragasse 7 1040 Wien, Österreich ABSTRACT Gratings in GaAs/AlGaAs mid-infrared quantum cascade lasers (QCLs) are fabricated with a structure depth of more than 10 µm. A N2/SiCl4 inductively coupled plasma reactive ion etching (ICP-RIE) process was employed to achieve extremely smooth sidewalls and selectivities to the SiNX etch mask of up to 70:1. EDX spectra measured on as-etched samples show that sidewall etch inhibition is caused by a thin Si containing layer on the sidewalls that is formed simultaneously with ICP etching of GaAs at the bottom of the trenches. To demonstrate device application gratings with a pitch of 1.72 µm are applied to long rib waveguide -based QCLs emitting at λ = 10.7 µm. When etched laterally together with the rib the grating gives rise to stable single mode emission up to 295K from these QCLs. The respective grating coupling coefficient is determined to be κ = 29 cm-1. INTRODUCTION Substrate based planar Photonic Crystals (PhCs) for mid-infrared wavelengths are expected to greatly enhance optical performance of quantum cascade devices. Which is particularly interesting since QCLs form a very convenient model system for novel optical resonators due to their low electric surface recombination losses. Furthermore the strong diffractive properties of deep gratings are important for vertical light emission, in-plane refractive properties are wanted for lasing threshold reduction and single mode operation. In fabrication of such devices etch depth becomes very important in order to prevent leakage of the optical mode into the substrate. Leakage occurs when structures are not much deeper than the vertical light mode profile. This crucial issue needs to be adressed properly since degraded etch profiles are the primary source of optical power loss. Subsequent process challenges caused by high aspect ratios have to be overcome too. Real world applications so far included particularly distributed Bragg reflectors (DBRs) [1,2] and two dimensional Photonic Crystals (PhCs) [3], but as demonstrated here such techniques are also very useful for fabrication of wavelength selective gratings. In a first step we focused on rib waveguide QCLs with distributed feedback (DFB). These devices enhance the performance of mid-infrared QCLs and at the same time reduce the required processing steps.
B5.2.2
Figure 1 SEM pictures of cleaved facets of large area gratings at 10 sccm SiCl4 , 10 sccm N2 Right: hexagonal photonic crystal with pitch 1 µm.
HIGH ASPECT RATIO ETCHING In a first step we solely focus on extremely vertical etching [4]. To achieve the aspired depth of 10 µm a two step etch process was employed for enhancing etch selectivity and avoiding organic contaminants. First a 500 nm thick SiNX intermediate layer was deposited by PECVD (plasma enhanced chemical vapor deposition) on the sa
Data Loading...
