M. Mikulics

Large-area traveling-wave photonic mixers for increased continuous terahertz power

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs (LT-GaAs). It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 μW at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices.

Impact of the contact metallization on the performance of photoconductive THz antennas

Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. Here, we present the first systematic comparison between these two metallization types. A series of antennas of both kinds is excited by femtosecond laser pulses and by the emission from two diode lasers, i.e. we test the structures as pulsed THz emitters and as photomixers. In both cases, coherent and incoherent detection schemes are employed. We find that the power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model we conclude that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.

Traveling-wave photomixer with recessed interdigitated contacts on low-temperature-grown GaAs

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal (MSM) contacts based on low-temperature-grown GaAs. The new recessed MSM geometry led to an improved electric-field distribution inside the photomixer structure and resulted in an up-to-100% increase in the output power of continuously operated devices, compared to conventional MSM devices with standard surface electrodes fabricated on an identical material. The recessed electrode structure also resulted in lower saturation of output power at higher input powers, enabling it to take advantage of higher input powers.

Ultrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown (LT) GaN. The photodetector devices exhibit up to 200kV∕cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720fs in our LT GaN material.

Fiber-coupled THz spectroscopy for monitoring polymeric compounding processes

We present a compact, robust, and transportable fiber-coupled THz system for inline monitoring of polymeric compounding processes in an industrial environment. The system is built on a 90cm x 90cm large shock absorbing optical bench. A sealed metal box protects the system against dust and mechanical disturbances. A closed loop controller unit is used to ensure optimum coupling of the laser beam into the fiber. In order to build efficient and stable fiber-coupled antennas we glue the fibers directly onto photoconductive switches. Thus, the antenna performance is very stable and it is secured from dust or misalignment by vibrations. We discuss fabrication details and antenna performance. First spectroscopic data obtained with this system is presented.

Ultrafast and highly sensitive photodetectors with recessed electrodes fabricated on low-temperature-grown GaAs

We have fabricated and characterized ultrafast metal-semiconductor-metal (MSM) photodetectors with recessed electrodes, based on low-temperature-grown GaAs. The new recessed-electrode MSM geometry led to an improved electric-field distribution inside the photodetector structure and resulted in a 25% breakdown voltage and sensitivity increase with simultaneous four-fold reduction of capacitance, as compared to the identical MSM devices with planar electrodes. Time-resolved studies performed using 100-fs-duration laser pulses showed that recessed-electrode MSMs exhibited 1.0-ps-wide photoresponse transients with no slow after-pulse tails and their photoresponse time was 0.9 ps. The improved transient photoresponse parameters are the main advantages of the recessed-electrode geometry.

Growth and properties of GaN and AlN layers on silver substrates

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN(11-22) orientation. An intermetallic phase of Ga3Ag is found at the GaN∕Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of (0002). Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current (∼10−3A∕cm2). These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices.

Ultrafast low-temperature-grown epitaxial GaAs photodetectors transferred on flexible plastic substrates

We demonstrate low-temperature (LT)-grown GaAs photodetectors transferred on flexible polyethylene terephthalate (PET) plastic substrates. The LT-GaAs layer was patterned into 20/spl times/20 /spl mu/m/sup 2/ chips, which after placing on the PET substrates were integrated with coplanar strip transmission lines. The devices exhibit low dark currents (/spl les/2/spl times/10/sup -8/ A), subpicosecond photoresponse time, and signal amplitudes up to /spl sim/0.9 V at the bias voltage of /spl les/80 V and under laser beam excitation power of /spl les/8 mW at 810-nm wavelength. At the highest bias (/spl sim/80 V) level, an increase of the response time (up to 1.3 ps) was observed and attributed to the influence of heating effects due to low thermal conductivity of PET. Our LT-GaAs-on-PET photodetectors withstand hundredfold mechanical bending of the substrate and are intended for applications in hybrid optoelectronic circuits fabricated on noncrystalline substrates, in terahertz imaging, and in biology-related current-excitation tests.

Ultrafast and highly sensitive photodetectors fabricated on high-energy nitrogen-implanted GaAs

We have fabricated and tested metal–semiconductor–metal (MSM) photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial GaAs films at an ion concentration of 3×1012u2002cm−2. Ti/Au MSM photodetectors with 1-μm-wide fingers were fabricated on top of the implanted GaAs. In comparison to low-temperature-grown GaAs photodetectors, produced in parallel in identical MSM geometry, the 880 keV N+-implanted photodetectors exhibited almost two orders of magnitude lower dark current (10 nA at 1 V bias) and the responsivity more than doubled (>20 mA/W at 20 V bias). Illumination with 100-fs-wide, 810 nm wavelength laser pulses, generated ∼2.5-ps-wide photoresponse signals with amplitudes as high as 2 V. The 2.5 ps relaxation time was the same for both the ion-implanted and low-temperature-grown devices and was limited by the MSM capacitance time constant.

Traveling-wave photomixers fabricated on high energy nitrogen-ion-implanted GaAs

The authors report on fabrication and measurement of traveling-wave photomixers based on high energy and low dose nitrogen-ion-implanted GaAs. They used 3MeV energy to implant N+ ions into GaAs substrates with an ion concentration dose of 3×1012cm−2. The N+-implanted GaAs photomixers exhibit improvements in the output power in comparison with their counterparts, photomixers fabricated on low-temperature-grown GaAs. The maximal output power was 2.64μW at 850GHz. No saturation of the output power with increased bias voltage and optical input power was observed. These characteristics make N+-implanted GaAs the material of choice for efficient high power sources of terahertz radiation.