A. zyxwvutsrqpon Nespola T.Chau M.C.Wu G.Ghione zyxwvutsrqp Abstract: zyxwvutsrqpo The thermal-runaway process in long- wavelength velocity-matched distributed photodetectors (VMDP) with metal- semiconductor-metal photodiodes has been investigated. A three-dimensional numerical electrothermal model has been developed which takes into account the nonlinear thermal properties of the substrate and the nonuniform temperature rise due to self heating. The model shows that, owing to its distributed nature, the photodetector is able to operate at high optical- power level before catastrophic failure occurs. When this happens, a highly localised hot spot appears within the device and the zyxwvutsr I-V characteristic exhibits a typical ‘current crush point’, where the current rapidly increases with increasing bias voltage. Examples are discussed to highlight the thermal behaviour of the distributed detector and to compare the model with experimental data. High-power high-speed photodetectors are attractive devices for applications in optical-heterodyne detection in the microwave- and millimetre-wave range. More- over, they are key components in microwave fibre-optic links whose performance parameters (such as the link gain, signal-to-noise ratio and spurious free dynamic range) benefit from receiving the maximum available optical power [l]. Significant progress has been made in this kind of photodetector using both surface-illumi- nated [2, 31 and waveguide approaches [4]; however, the small absorption volume, required for high-speed oper- ation, causes the density of photogenerated carriers to be high even at low input optical power. The ensuing electric-field screening effects lead to low saturation power, nonlinear behaviour and harmonic distortion. Enlarging the effective absorption volume is the most direct way of increasing the saturation power. To achieve high-power capabilities with high bandwidth zyxwvu 0 IEE, 1999 IEE zyxwvutsrqponmlk Proceeding.s online no. 19990457 DOI: 10.IO49hp-opt:19990457 Paper first received 9th June and in revised form 28th October 1998 A. Nespola and G. Ghione are with Polutecnico di Torino, Dipartimento di Elettronica, Corso Duca degli Abruzzi 24, 1-100129 Torino, Italy T.Chau and M.C. Wu are with UCLA, Electrical Engineering Depart- ment, 66-147-D Engineering IV, Los Angeles, CA 90095-1594, USA and efficiency, travelling-wave photodetectors, first suggested in [SI, have been used [6, 71. Although their increased absorption volume enables good saturation levels to be obtained, very low bandwidth has been reported (4.8GHz) because of the difficulty of match- ing the velocities of the copropagating optical and elec- trical waves [7]. To overcome this problem and to take advantage of the travelling-wave concept, a GaAs/AlGaAs velocity- matched distributed photodetector (VMDP) is pro- posed which operates at 860nm [8]; a high saturaton photocurrent of 56mA and a bandwidth of 49GHz have been achieved. However, InP-based long-wave- length photodetectors, operating at 1.3 and 1 S5pn-1, are required in long-haul photonic systems. Recently, the first experimental results on InGaAs/InAlAs/InP VMDP were reported [9]. Its structure and physical behaviour are discussed in detail below. Since VDMPs are high-power detectors, device breakdown induced by thermal runaway is a poten- tially serious problem, and a quantitative failure model is needed to improve the device performance and relia- bility. In this paper, the thermal runaway process that leads to catastrophic damage in InP-based photodetec- tor is investigated through a self-consistent steady-state electrothermal model. To obtain high-resolution sur- face-temperature maps at a moderate computational cost, Green’s-function approaches, coupled with effi- cient numerical implementations, have been exploited. Thermal runaway is assumed as an indicator for poten- tial device failure, since the actual failure mechanism (metal diffusion into the semiconductor, which ulti- mately leads to photodiode short-circuiting), is acti- vated by a temperature increase. zyxw 2 Design end ffdWi@a~iQUI The schematic structure of the VMDP is illustrated in Fig. 1. High-speed metal-semiconductor-metal (MSM) photodiodes, chosen because of their easy integration with microwave transmission lines, are periodically dis- tributed on top of a passive optical waveguide. The output photocurrents are collected by a 50Q coplanar- strip (CPS) microwave transmission line that is velocity matched to the optical waveguide. Each active MSM photodiode consists of an InGaAs absorption layer, InGaAsIInAlAs graded superlattice layers, InAlAs Schottky-barrier enhancement layer, and TiIAu fingers with thickness of 200&2000~. The mesa is defined by wet etching down to the InAlAs upper cladding 11. Fig. 2 shows the cross-section of VMDP after mesa etching and an enlarged image of graded superlattice which consists of 11 pairs of alter- 25 IEE zyxwvutsrqponml Pruc.-Optoel~clron,, Vol. zyxwvutsrqponmlk 146, Nu. zyxwvutsrqpon 1, Ftlmiary 1999