Low voltage hetero-nipi waveguide modulators

We have made waveguide modulators that give large modulation depth and phase shift with applied voltages around 1 to 2 V. Quantum-well modulators' work using a red shift of the absorption edge or the change in refractive index that occurs when an electric field is applied across the quantum wells in the waveguide core. In normal p-i-n structures the doped cladding layers act as electrical contacts. In our "hetero-nip?' design the multiple-quantum-well region is split into several thin regions separated by doped layers, alternating between nand ptype. When ohmic contacts are applied, connecting all the n-type layers together and likewise the p-type layers, the necessary large electric field across the quantum wells is obtained with a small applied voltage. Making the selective contacts is a problem. A shadowed regrowth technique' has been demonstrated on a reflection modulator operating by band-filling in quantum wells. A simpler method, demonstrated on a tunable Bragg mirror, is to etch a mesa with sloping walls through the entire structure and to apply lateral contacts on the walls3 We have, for the first time, made a waveguide 'modulator with a hetero-nipi core. It performed more efficiently than any reported p-i-n structure. Our device, grown by molecular beam epitaxy on a semi-insulating GaAs substrate, had a waveguide core containing 2 complete n-i-p-i periods. Each intrinsic layer contained ten 75 8, GaAs quantum wells with 45 A AlAs barriers. AlAs has an advantage over AlGaAs as a barrier material because it gives greater confinement at high electric fields! Each nor pdoped layer comprised 8 periods of 40 8, GaAs/22 AlAs superlattice, doped at 2 x 10" ~ m ~ . The doped superlattice was used rather than AlGaAs to avoid contacting problems. The waveguide cladding layers were undoped M.,Ga,,As, 1.5 pm thick. Waveguide ridges were formed by wet etching. A wider mesa was then etched through to the lower cladding layer. Selective lateral ohmic contacts, Sn/Au for n-type and Zn/Au for p-type, were deposited on the sloping walls of the outer mesa. Satisfactory diode characteristics were obtained. A typical 300 pm device gave a reverse current of 50 ,uA at -1 V, compared to 1.4 mA at 1 V forward bias. Working in electroabsorption mode with 844 nm =-polarized light from a Ti:Sapphire laser, a 300 pm device gave a modulation ratio of 10: 1 at 1.0 V applied reverse bias, 100: 1 at 1.8 V. The zero-bias absorption loss was 4 dB. A 400 pm device was then tested as a phase modulator. It was placed in one arm of Mach-Zehnder interferometer, and an infra-red TV camera was used to view the resulting fringes. A phase shift of 180" was obtained at 860 nm with negligible absorption losses at 1.5 V applied, and at 845 nrn with 3 dB of electroabsorption loss and only 0.8 V applied. The "figure of merit", defined as phase shift per mrn length and Volt, was as high as 560"/Vmm. To our knowledge, this value is four times greater than that for any previously reported device. The combination of short length and low voltage results from the use of the hetero-nipi structure, which allows a large field to be applied across the quantum wells with a small voltage.