MSM photodiodes attracted attention due to their high-speed performance and ease of integration, but this interest has waned recently. This paper endeavors to explore why this occurred and tries to address these issues. MSM photodiodes have a much lower capacitance per unit area than p-i-n photodiodes, and are often transit time limited. MSM photodiodes are comprised of back-to-back Schottky diodes using an interdigitated electrode configuration on top of an active light collection region. The transit time is related to the spacing between these interdigitated electrodes. MSM photodiodes are more easily integrated with pre-amplifier circuitry than p-i-n photodiodes. One reason is that MSM photodiodes do not require doping which eliminates any parasitic capacitive coupling between the photodiode and doped regions within the active transistors. Another reason is that the Schottky electrodes of the MSM photodiodes are essentially identical to the gate metallization of field effect transistors (FET), which might eliminate one photolithography step. But, MSM photodiodes suffer from very low external quantum efficiency (EQE) and high leakage currents. MSM photodiodes exhibit low EQE because the metallization for the electrodes shadows the active light-collecting region. Shadowing can limit the incident light from reaching the active region of the MSM detector and prevent an ideal MSM from achieving high EQE. Transparent conductors have been shown to nearly double responsivity. Leakage currents are determined primarily by the Schottky barrier heights. These can be unreliable. However, thin wide bandgap cap layers can be inserted below the Schottky and different metals used for the anode and cathode to break symmetry and to circumnavigate these concerns.
In this paper we introduce a micro-optical architecture that uses meso-scopic diffractive optical elements (DOEs) as 3D interconnects in a memory system for high level instruction level parallelism (ILP) processors. By using meso-scopic DOEs we can rescue the scale of integration to the VLSI scale, ie.e., the micron scale, achieve submicron alignment tolerance, improve reliability due to monolithic integration, facilitate integration into the current manufacturing infrastructure, and offer the ability for higher bandwidth and high interconnect densities. To this end we are developing the component technologies needed to realize this system. In this paper we present our work in the development of a theoretical and experimental framework for the design and characterization of meso-scopic DOEs, preliminary experimental results of meso-scopic beam splitters, and a large scale demonstration of the ILP memory system.
We report on the fabrication and characterization of a photodiode made from a heterojunction of epitaxial p-type Ge1-xCx on an n-type Si substrate. Epitaxial Ge1-xCx layers with carbon percentages of 0.2, 0.8, 1.4 and 2% were grown on (100) Si substrates by solid source molecular beam epitaxy. The p-GeC/n-Si junction exhibits diode rectification with low reverse saturation current (2 at -1 volt) and high reverse breakdown voltage in excess of -40 volts. Despite the large number of dislocations and defects at the heterojunction, photoresponsivity was observed from the p- Ge1-xCx/n-Si diodes using laser excitation at a wavelength of 1.3 micrometers . External quantum efficiency was measured between 1.2 and 2.3%.
Metal-semiconductor-metal (MSM) photodiodes with electrodes fabricated from the transparent conductor cadmium tin oxide (CTO) have been shown to double photoresponsivity. Their bandwidths, however, are significantly lower than those of MSMs fabricated with standard Ti/Au contacts. Though MSMs are generally believed to be limited by the transit time of electrons, it is possible the larger resistivity of CTO has become a significant factor, making the MSMs RC time constant limited instead. Previous models of MSMs only account for one of the two back-to-back Schottky diodes. A new model which takes into account both the forward and reverse biased junctions has been developed from the small signal model of a Schottky diode. This new model was fit to data obtained from S-parameter measurements, and incorporates both the transit time response and RC time constant response.
We will present high quality In0.53Ga0.47As which has been grown on semi- insulating (100) InP:Fe substrates by rare earth doped (Yb, Gd, and Er) liquid phase epitaxy using a graphite boat. The new earth ions, which are highly reactive, are thought to better impurities like O, C, and Si by reacting with these impurities and precipitating out in the melt, but not incorporating into the epitaxial layer to any significant amount.
We have investigated monolithic pin-FET photoreceivers in both the InP and GaAs systems. The GaAs-based circuits consisted of a single growth step in which the p-i-n diode was grown on top of the MESFET. The circuits exhibited flatband gains as high as 17 dB and bandwidths of 2.0 GHz. The InP circuits featured regrown MODFETs integrated with p-i-n diodes. These devices exhibited a gain of 17 dB and a bandwidth of 10 GHz.
A single-step metal organic chemical vapor deposition (MOCVD) growth has been used to fabricate a buried heterostructure InGaAs/GaAs multi-quantum well laser over a patterned GaAs substrate. The pattern used here is a re-entrant mesa formed by wet chemical etching oriented along  direction. Growth over the mesa results in isolated buried heterostructures. The 250 micrometers long lasers have threshold currents of 30 mA and emit > 100 mW/facet at room temperature. The external differential quantum efficiency is found to be almost independent of temperature in the temperature range of 10 degree(s)C to 90 degree(s)C which suggests a low temperature dependence of leakage current. The threshold current of the laser as a function of temperature can be represented by the usual expression Ith approximately Io exp(T/To) with a characteristic temperature (To) of about 120 K in the temperature range 10 degree(s)C to 90 degree(s)C.