Properties of aluminum gallium arsenide native oxides for integrated photonics and optoelectronics applications

The properties of AlGaAs native oxides have been investigated and single heterostructure (SH) and double heterostructure (DH) native oxide planar waveguides have been realized. Prism coupling, secondary ion mass spectrometry (SIMS), Fourier transform infrared (FTIR) transmission spectroscopy and other techniques are used to characterize the oxide waveguides. Propagation losses are measured for SH native oxide waveguides. The presence of hydroxyl (OH) groups in AlGaAs native oxides is shown to slightly increase the waveguide loss at λ = 1.55 μm. The wet thermal oxidation process has been extensively investigated for Al_xGa_1−xAs over a wide range of Al compositions (0.3 < x < 0.9). An improvement in the process for oxidation of low Al composition Al_xGa_1−xAs (x < 0.8) has been achieved by controllably adding trace quantities of O₂ to the N ₂ + H₂O process gas. The complicated effects of O₂ + N₂ “mixed carrier gas” on oxidation rates and the surface quality of oxides have been investigated and applied to reduce the propagation loss of a SH waveguide. The role of added O₂ has been analyzed in relation to the possible chemical reactions involved. The effects of mixed carrier gas on the lateral oxidation of Al_0.98Ga _0.02As is also explored but shown to be negligible. Two modified AlGaAs SHs designed for reduced planar oxide waveguide propagation loss have also been processed and characterized, with losses as low as 4 dB/cm at λ = 1.55 μm achieved. Finally, in other experimental results it is shown that “deep-oxidation” (i.e., through a quantum well heterostructure (QWH) containing a low Al composition waveguide and GaAs quantum well) can be attained by using controllably-mixed O₂ + N₂ carrier gas, which effectively modifies the oxidation rate selectivity between high and low x Al_xGa_1−xAs. This achievement eliminates the need for the additional impurity induced layer disordering (IILD) process step used in prior deep-oxidation technology to intermix high and low x Al_xGa_1−xAs in preparation for oxidation. Deep-oxidation enables the realization of strongly-confined, curved optical waveguides required for routing signals around an optical “chip.” This discovery greatly simplifies the process to a more manufacturable and, thus, commercially viable level, and may stimulate further advances in optoelectronics devices and photonic integrated circuits.