MOVPE metamorphic lasers and nanostructures engineering at telecom wavelengths
Mura, Enrica E.
University College Cork
In recent years, considerable attention has been drawn to the design of heterostructures on GaAs substrates emitting in the 1.3-µm spectral range for replacing InP injection lasers in medium range fiber-optic communication links. Scaling considerations apart, the enhanced electronic confinement in GaAs-based devices can be expected to reduce carrier leakage at high temperatures, thereby overcoming one of the limiting factors associated with InP-based technologies. InGaAs metamorphic buffer heterostructures constitutes an alternative to the conventional routes relying on quantum dots or dilute nitride approaches, all with their own technical challenges and drawbacks. Metamorphic growth techniques provide compositionally graded buffer layers where the dislocations caused by strain relaxation are confined to the graded layers. However, when grown by metal-organic vapour phase epitaxy (MOVPE), it has been shown as extremely challenging to achieve ∼ 1.3µm emission in InGaAs metamorphic quantum well (QW) lasers (on GaAs substrate), due to a variety of strong, growth related issues, fundamentally linked to the overall epilayer thickness. In this contribution we demonstrate a > 1.3 µm-band laser grown by MOVPE on an engineered metamorphic parabolic graded InxGa1 –xAs buffer. A metamorphic multiple-quantum well structure containing cladding, active, and contact layers was grown. In the cladding, we exploit/control the correlation between epilayer thickness and defect generation and, importantly, demonstrate that the limiting factors introduced by surface instabilities during epitaxy can be managed by an innovative design. The bottom and the upper cladding are built as a combination of AlInGaAs and InGaP alloys in a superlattice (SL) structure. The improved quality of the material was confirmed, for example, by extensive Atomic Force Microscopy (AFM) analyses, showing low roughness (and no direct evidence of defect lines). The heavily compressive strain in QWs and in the metamorphic buffer layer (in combination with the surface step bunched ordering) promoted three-dimensional (3D) features formation under certain growth temperatures and for certain percentage of indium in the QWs. To avoid and control the 3D nanostructuring we proposed as a possible solution the insertion of a GaAs layer deposited before the QW. Moreover, we individuated a range of growth temperature and indium content in the QWs 3D-nanostructures and defects free, verifying the emission of interest. Building on these results, stripe waveguide lasers were fabricated, then characterized electro-optically. Best electro-optical result are reached with modified lower and upper SL cladding structures, adding a graded composition layers at the interfaces following the aim to improve the carrier transport. A 500 µm long and 2.5 µm wide stripe waveguide exhibited a threshold current (Ith) of ∼ 152 mA, corresponding to a density threshold current (Jth) of ∼ 127 mA/cm2 per QWs , operating at room temperature in pulse mode. The turning voltage was ∼ 0.8 V and the resistance series was 4.5 Ω. The emission wavelength was peaked at ∼ 1.34 µm, registered in pulse mode at low duty cycle. With shorter stripes laser, 10 µm and 20 µm wide, with different cavity lengths, we achieved the Light-current-voltage (L-I-V) curves in pulse and continuous wave (CW) mode. The threshold current varied from 130 mA to 170 mA in the operating temperature range of 30 ◦C-80 ◦C, and a characteristic temperature (T0) of 95 K was calculated. The internall loss (αi) and internal quantum efficiency (ηi) extrapolated were ∼ 30 cm−1 and ∼ 57% respectively. Those results prove that the epitaxial structure developed in this thesis work allow to fabricate one the few (specifically the second one, referring to that proposed by a Nippon Telegraph and Telephone Corporation (NTT) Japanese group in 2015 year) InGaAs metamorphic QW laser GaAs based, operating at > 1.3 µm using the MOVPE technology.
InGaAs strained quantum wells , Metamorphic , Telecom lasers , Metal-organic vapour phase epitaxy
Mura, E. 2019. MOVPE metamorphic lasers and nanostructures engineering at telecom wavelengths. PhD Thesis, University College Cork.