InGaAs/InP P-I (MQW)-N Surface Normal Electroabsorption Modulators Exhibiting Better Than 8:1 Contrast Ratio For 1.55/spl mu/m Applications Grown By Gas Source MBE

Traditional bi-directional optical fiber systems utilizing two lasers, two fibers and two detectors are unattractive for fiber to the home communication applications due to the high component cost. A system for medium range applications has recently been demonstrated in which a surface normal p-i (MQW)-n modulator was successllly used to convert downstream light into upstream data fiom the subscriber terminal [l]. A requirement for these devices to be used in such a system is that they exhibit a contrast ratio on the order of 10: 1. Such contrast ratios have been achieved in devices operating at 860 nm but much lower contrast ratios have been reported for surface normal devices operating at 13001550 nm, corresponding to the low loss-low dispersion window of optical fibers, for long haul communication applications. The highest contrast ratio reported for surface normal devices in this wavelength regime to the best of our knowledge is 2.6: 1 (4.1 dB) at an applied reverse bias of 40 V for a device incorporating a 150 period InGaAs-InP MQW [2] and 3 dB for an Asymmetric Fabry Perot (ASFP) modulator albeit at a low drive voltage of 5 V [3]. The reason for the low contrast ratios is the low value of the absorption coefficient exhibited by this material system (only about 40% of that exhibited by the AIGaAs-GaAs material system). The device structure was grown on top of an n type Inp (Sn doped) wafer by Gas Source MBE using ASH3 and PH3 as the Group V source gases at a growth temperature of 500 C. The structure consisted of a 1.5 pm n-type InP clad layer (Si doped to 3 x 1018 cm-3) followed by an intrinsic region composed of 200, 10 nm &.53%.47As wells lattice matched to 8 nm InP barriers, followed by a 1 pm thick p-type InP clad layer (Be doped to 3 x lo** cm-3). Post growth processing consisted of a mesa etch and p and n type contact metallizations. An Si0 anti reflection coating was then applied to the backside of the wafer. Since the InP substrate is transparent at the wavelength of interest no substrate thinning was performed.