HIGH PERFORMANCE NEAR-INFRARED SEMICONDUCTOR

 PHOTODETECTORS and LASERS  

InP-BASED LOW DARK-CURRENT 1.55μm PHOTODIODES

     The evolution of long-haul optical fiber communications systems has created the need for high-sensitivity, low-noise, high quantum efficiency, low-cost   photodetectors. We design, fabricate, and characterize InP/InAlAs/InGaAs based p-i-n type low-noise photodiodes. The thickness of the active layer is   increased to 15000 Å to decrease the dark current and to increase the quantum efficiency. On the surface, a thin InGaAs layer is employed for p-ohmic   contact formation. In the simulations, the quantum efficiency of the designed structure is calculated as 70%, and the corresponding responsivity is 0.88   A/W at 1550 nm wavelength. Since the fabrication steps on an N-type substrate are different than previously performed fabrications on semi-isolating   substrates, a new photomask is designed. Figure 1 shows a view of the designed mask. The samples are fabricated by using an eight-step   microwave- _compatible fabrication process in a class-100 clean room environment. First, the Ti/Au (100 Å /1000Å) p-ohmic contacts are deposited on   the _surface. The next step is the definition of circularly shaped p-mesa regions which defines the detector active area around the p-ohmic contacts by   means of a wet etch. This step is followed by a similar yet larger mesa etch that mainly serves to recess the sample surface within a few thousand Å of   the n-contact layer. N-type contact areas are first etched to the N+ InP layer, and afterwards a second mask layer is used to deposit Au/Ge/Ni/Au   contact metallization. Device mesas are etched away around the n-contacts. The contacts are annealed at 400oC for 60 s in a rapid thermal annealing   system. Then, a 200 nm thick Si3N4 is deposited via plasma enhanced chemical vapour deposition (PECVD) for passivation and isolation. The   thickness of this layer is also chosen to form an anti-reflection coating.  Finally, a ~ 0.6 μm thick Ti/Au interconnect metal is deposited and lifted off to   connect the device contacts to the coplanar waveguide transmission line pads.

Figure 2 shows the schematic cross-section of a fabricated photodetector.

Fig.1 Photodetector Mask

Fig.2 Schematic cross-section of a fabricated photodiode.

Fig. 3 (a) Photomicrograph showing a part of fabricated sample

Fig. 3 (b) Photomicrograph showing 50 µm active area device.

Figure 3 (a) and (b) show photomicrographs of photodetectors after the fabrication.

Figure 3 (a) is the general view of a few photodetectors, whereas Fig. 3 (b) is the photomicrograph of a 50 µm active area device.

  After the completion of fabrication, the devices are tested for dark current and photoresponse. First, dark current is measured on a probe station by   means of a semiconductor parametric analyzer. The devices had breakdown voltages higher than 50V. Figure 4 shows the dark I-V graph obtained from   a 100 mm diameter device. Up to 10 V reverse bias, the dark current was less than 7nA.  For photoresponse measurements, 240 mm diameter   photodiodes are tested by using a tunable laser tuned to 1550 nm wavelength. Photodiodes were illuminated by the laser light with the help of a fiber   probe on a  probe station. Dark current and photocurrent are measured in the 0 to -5V interval for different incident optic power values which was   calibrated by a calibrated powermeter. The results of these measurements are shown in Fig. 5. Photocurrent increases linearly with increasing incident   optic power and a constant responsivity of 0.48 A/W is obtained for reverse bias values larger than 0.8 V. This responsivity value corresponds to a   quantum efficiency of %38.

Fig. 4 Dark current of a 100 μm diameter device.

Fig. 5 Photoresponse of a 240 μm diameter device.

   InP Based Low Dark Current 1.55 Micron Photodiodes

 

   LT-GaAs for 1.55 Micron Applications

 

   Fabrication and Characterization of Higher Power Laser Diodes

 

    Indium Solder Bumps

 

Metamaterials   -   Photonic Crystals  -  GaN/AlGaN Devices  -  Other Semiconductor Devices and Fabrication Techniques

 

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