BİLKENT UNIVERSITY NANOTECHNOLOGY RESEARCH CENTER

HIGH PERFORMANCE NEAR-INFRARED SEMICONDUCTOR

PHOTODETECTORS and LASERS

LT-GaAs FOR 1.55 MICRON APPLICATIONS

_ In Nanotechnology Research Center, we have been studying High-Performance photodetectors ranging in wavelength from deep-UV, through solar-_blind, visible, near-infrared, infrared and to far-infrared regions.

Fig.1: (False Colored) SEM Images of fabricated photodetectors

Semiconductor photodetectors operating in 1.3-1.5 micrometer (um) wavelength range are important components for long distance fiber-optic communications. Gallium Arsenide (GaAs), however, is not suitable for these wavelengths, because of its bandgap limitation, though it has superior optoelectronic properties. Its bandgap is 1.42 eV, corresponding to a cut-off wavelength of 870 nm. So, GaAs can not be used as a photodetection instrument above this wavelength, practically.

To overcome this limitation and to use GaAs-based detectors in the 1.3-1.6 um wavelength region, mainly two detector structures were offered:

· Schottky-barrier internal photo-emission photodetectors and

· Low-Temperature-Grown-GaAs-based (LT-GaAs) photodetectors

It was shown in late 80s that LT-GaAs was able to absorb long-wavelength signals due to mid-gap defects or As precipitates. LT-GaAs-based high- _speed photodetectors operating in the 1.3-1.6 mm range have been reported.

Wafer Design and Fabrication:

Structures are designed using Transfer Matrix Method (TMM) based theoretical simulations. Although LT-GaAs absorbs light at longer wavelengths, there is a low absorption problem. An offered solution to this problem is to put active layer inside a Fabry-Perot cavity (Resonant Cavity Enhancement (RCE) effect), so that photons passes through active layer more than once. Effectively, we obtain more absorption in smaller devices.

The RCE p-i-n photodiode was designed to achieve a resonance at 1.55 um. To meet this condition, a highly reflecting 15 pair GaAs/AlAs Bragg mirror centered at 1.55 um was designed as the bottom mirror of the detector cavity. Air/GaAs interface acts as the top mirror for the cavity. After fabrication, an extra DBR would be grown on top of the device for further increasing the absorption.

Structures were grown on 2-inch semi-insulating GaAs wafers (001) by molecular beam epitaxy (MBE). All GaAs/AlAs layers were grown around 550-600 °C except the active LT-GaAs layer. During the growth of LT region, temperature is decreased to 200 °C (This is where Low-Temperature term comes from.)

Then we fabricated devices in Advanced Research Laboratory Class-100 Clean Room. The fabrication is about 5-8 lithography steps, depending on the devices’ required performances.

Fig.2: Optical Microscope image of a fabricated photodetector and epitaxial structure.

Electrical/Optical Characterization of devices

Electrical characterization of diodes consist of Current-Voltage (IV) measurement. The resulting RCE p-i-n photodiodes had breakdown voltages around 15 V and turn-on voltages around 1 V. Small area devices exhibited a few tens of nA dark current at 1 V reverse bias voltage. These values were much higher than the devices we fabricated later; possibly due to fabrication related contamination or the like.

Spectral measurements include Quantum Efficiency(QE) and Frequency Response measurements.

We measure QE using a computer controlled 1550 nm tunable external cavity laser and lock-in. Frequency response of devices are measured by a femtosecond fiber laser and 50 GHz digitizing oscilloscope.

The fabricated devices exhibited a resonance around 1548 nm. When compared to the efficiency of a conventional single-pass detector, an enhancement factor of 7.5 was achieved. Temporal pulse-response measurements were carried out at 1.55 um. Fast pulse responses with 30 ps pulse-_width and a corresponding 3-dB bandwidth of 11.2 GHz was measured.

Further details on the process and measurement results can be found in the references below.

Ongoing Research:

We are working on optimization of Quantum Efficiency of the devices by different growth conditions and epitaxial structures. Also, other electric characterizations are being investigated, such as noise performance and temperature dependence.

Fig.3: Quantum efficiency of a 80 um diameter diode.

Fig.4: Frequency response of a 7umx7um size diode

Related Publications:

Bayram Butun, Necmi Biyikli, Ibrahim Kimukin, Orhan Aytur, and Ekmel Ozbay “High-speed 1.55 mm operation of low-temperature-grown GaAs-_based resonant-cavity-enhanced p– i– n photodiodes,” Applied Physics Letters, volume 84, 4185 (2004).

Kimukin, N. Biyikli, E. Ozbay, and G. Tuttle “High-Speed GaAs Based Resonant-Cavity-Enhanced 1.3 Micron photodetector,” Applied Physics Letters, volume 77, No. 23, 3890 (2000).

InP Based Low Dark Current 1.55 Micron Photodiodes

LT-GaAs for 1.55 Micron Applications

Fabrication and Characterization of Higher Power Laser Diodes

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

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