Colloidal Quantum Dots (CQD), due to their extremely large optical absorption coefficient and tunability of the absorption bands, are very promising for the realization of photodetectors. PbS quantum dots, in particular, can be effectively employed as a material for near infrared photodetectors with sensitivity peaks ranging from 1 to 2μm. CQD photodetectors, nevertheless, present still many unsolved issues when it comes to fast detection and noise performance. Thanks to the recent advances in CQD material synthesis and treatment, photodetectors achieved unprecedented performance but the aforementioned issues could still not be fully addressed. Concerning photodetectors, however, material quality is only the starting point for the realization of performing devices: CQD technology came to the point where an engineering approach is needed in order to fully comprehend the behavior of the photodetectors, to define proper strategies for the enhancement of their performance and introduce them in practical applications. In this work we analyze the optical and electrical characteristics of PbS CQD near infrared photodetectors fabricated on SiO2 substrate and demonstrate how even a simple, fully passive readout circuit topology could be employed in order to obtain a dramatic enhancement of the characteristics of the devices.
We report on the integration of Ge p-i-n heterojunction photodiodes on Si substrates. The crucial role of interface defects at the Ge/Si interface on the performance of photodetectors is analyzed and taken into account in the design of the devices. We have designed and fabricated high performance p-i-n Ge photodiodes for the near infrared. Pure Ge is grown by ultra-high-vacuum CVD followed by a cyclic thermal annealing and ion implantation. Devices are fabricated using standard photolithography. The photodiodes exhibit maximum responsivity of 0.8 A/W at 1.3 micrometers and 0.7 A/W at 1.55 micrometers , reverse dark currents in the 20 mA/cm2 range at 1V and response times as short as 520 ps. Our devices are the first p-i-n Ge on Si photodetectors fabricated by CVD and exhibit high performances for a wide range of applications.
We present a technology for the integration of high performance near-infrared Ge P-I-N photodetectors on Si for Si microphotonics. High quality Ge epilayers were grown on Si by a two-step ultrahigh-vacuum / chemical-vapor-deposition (UHV/CVD) process. Two-step UHV/CVD allows the epitaxial growth of Ge on Si without islanding. Threading-dislocations in Ge epilayers were reduced by cyclic thermal annealing. The reduction of threading-dislocations can be understood in terms of thermal stress induced dislocation glide and reactions. We found that sessile threading-dislocations are not permanent and can be removed.
Integration of near infrared (NIR) photodetectors on a silicon substrate is a key step for the fabrication of an all silicon based NIR transceiver. To this extent, polycrystalline germanium (poly-Ge) technology is attractive due to the low deposition temperature and cost. Poly-Ge detectors demonstrated broad response, covering the whole NIR spectrum to 1.55 micron, fast, subnanosecond, speed and excellent versatility. In this work we present our recent results on the integration of a poly-Ge photodetector on a SOI silicon waveguide. The use of a waveguide for light in-coupling is appealing for telecom applications where signal is transported on an optical fibre, but, at the same time, it allows to increase detector responsivity. In fact, in this device the light is absorbed into the thin sensitive layer of the poly-Ge/Si heterojunction in a distributed way, during propagation. This releases the strong constraint of the absorption length being smaller than photocarrier collection length typical of normal incidence photodetectors. In the paper, both design issues and experimental results are reported.
Nowadays refractive-index engineering has become a challenging area for experimentalists in semiconductor integrated optics, whereas design constraints are often more strict than both standard technology tolerances and model accuracies. In fact, it is crucial to non-destructively evaluate thicknesses and refractive indices of a multilayer waveguide independently, and to this aim we resorted to X-ray reflectometry and effective index measurements on MBE-grown AlGaAs waveguides, respectively. With the first technique interference effects (Kiessig fringes) arise, which are related to layer thicknesses. By standard data processing, thickness accuracies of +/- 0.05 nm are readily achieved. Effective index measurements were performed at several wavelengths on both slab and rib waveguides, through grating-assisted distributed coupling with both photoresist and etched gratings. Effective indices were determined with an absolute precision as good as 1/2000, adequate for phase matching in parametric devices. Merging thickness and effective index evaluations, the refractive indices of the constituent layers were determined with unprecedented accuracies, in substantial agreement with existing models.
We report on a novel solid state wavelength meter in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength sensitive detector. We describe the design, the fabrication and the characterization of such device and we demonstrate its capability in the measurement of the wavelength of quasi- monochromatic light beams.
We report the fabrication of fast heterojunction Ge/Si photodetectors which, to the best of our knowledge, exhibit the highest near infrared responsivity at normal incidence reported to date. Such performances are related to the quality of the epitaxial Ge film grown by a two-step UHV-CVD process followed by cyclic thermal annealing. We have measured a fast (FWHM equals 850 ps at 1.3 micrometers ) and efficient (R equals 0.55 A/W at 1.3 micrometers and 0.25 A/W at 1.55 micrometers ) photoresponse. Our technology makes these devices suitable for integration with other electronic and optoelectronic components on Si chips. In the paper we discuss processing technology, material quality, device fabrication and performance measurements.
We report on the fabrication of a detector array for the near infrared on silicon substrate. Thermally evaporated polycrystalline germanium is used as the active layer in the device which consists of 16 pixel with dot-pitch of about 100 micron; the single pixel has a metal-semiconductor-metal structure. We demonstrate a responsivity of 16 mA/W at 1.3 micron and extending down to 1.55 micron. At the same wavelength an operation speed in the nanoseconds range is demonstrated. The overall fabrication process, including substrate cleaning and preparation, requires temperatures lower than 350 degrees Celsius being fully compatible with silicon electronics.