Growth of High Power Ge p-i-n Detectors

There is a need for high power, high current, high speed photodetectors, for efficient, high gain RF photonic links, which can operate with 1-5W RF power in the DC to 20 GHz range. This need is currently being met by state-of-art InGaAs photodetectors [1], but there are several material-dependent limitations. It is well known that InGaAs is a very poor thermal conductor. Based strictly on thermal considerations since Ge is a 9.5x better thermal conductor than InGaAs, Ge p-i-n detectors should be able to operate at 2x the RF power, especially at high frequencies. Not as well known is the effect of impact ionization as a limiting nonlinear mechanism for these photodetectors [2,3]. In the low field regime (5V/μm - 10V/μm) which is the typical operating range for p-i-n diodes, the electron ionization coefficient of InGaAs is more than an order of magnitude greater than that of Ge. The voltage dependent responsivity of the InGaAs detector results in increased 2^nd- and 3^rd- order harmonic distortion. It has been calculated that there should be 100x less 2^nd-order harmonic distortion and 1000x less 3rd-order harmonic distortion with a Ge photodetector. This allows the Ge detector to be used in a broad-band receiver to detect weak signals without being affected by harmonic distortion (spurs) from stronger signals. To fully exploit the material properties of Ge, we have designed the Ge p-i-n detector to be fabricated on Si. Heterogeneous integration with Si permits backside illumination, the optimum configuration for thermal control, allowing the use of front-side heatsinks. In addition, there are the advantages of inexpensive, large Si substrates, compatibility with the multi-B$ Si industry, and potential monolithic integration with Si circuits. The substrate was double-side polished float-zone Si(100) having a resistivity greater than 1.5×10⁴ ohm-cm. The structures were grown using molecular beam epitaxy.- The 4% lattice mismatch between the Si and Ge was accommodated by growth of a 90 nm "virtual" Ge substrate grown at low temperatures. The n+ and p+ contact layers were doped at a concentration of 5×10¹⁹/cm³ using Phosphorus and Boron, respectively. For ease in process development the initial Ge p-i-n on Si structures were fabricated in the typical topside illumination configuration. Fig. 1 shows the carrier concentration profile of a device having an "I" layer thickness of 0.65 mm. The transmission electron micrograph, Fig. 2, shows the threading dislocation density to be about 10⁹/cm². The dark current of a 100 μm diameter diode, Fig. 3, is typical of other Ge-on-Si p-i-n diodes [4]. The optical responsivity of this structure is reported in Fig. 4. Efforts are continuing to improve the detector performance by modifications in the epitaxial growth process.