F. Renner

Efficient terahertz emission from ballistic transport enhanced n-i-p-n-i-p superlattice photomixers

The authors report on photomixing terahertz sources that overcome the transit time versus RC-time trade-off and allow for independent optimization of both of them, using a n-i-p-n-i-p superlattice. Furthermore, they take advantage of ballistic transport for reduced transit times. Apart from more favorable material parameters, In(Al)GaAs photomixers benefit from the advanced telecommunication laser technology around 1.55μm as compared to GaAs. In such devices, a terahertz-power output of 1μW has been achieved at 0.4THz at a photocurrent of 3.8mA. A comparison between corresponding GaAs- and InGaAs-based n-i-p-n-i-p photomixers reveals an improvement of performance by at least an order of magnitude for the latter one.

THz-photomixer based on quasi-ballistic transport

We report on a novel concept for THz photomixers with high conversion efficiency up to several THz. In contrast to the conventional pin photomixer we can overcome the trade-off between either optimizing transit-time or RC-roll-off. Using quasi-ballistic transport in nano-pin-diodes the transport path can be optimized regarding both path length and transit time. Independently, the capacitance can be kept small by using a sufficiently large number of optimized nano-pin-diodes in series. The concept is presented in detail and first experimental results are reported which corroborate our theoretical expectations.

Enhanced recombination tunneling in GaAs pn junctions containing low-temperature-grown-gaas and ErAs layers

We report electrical conductivity studies of highly-doped GaAs pn diodes containing a strongly n-doped low-temperature-grown (LT)–GaAs layer and pn junctions containing an approximately one monolayer thick ErAs layer. At room temperature, current densities of 1 kA/cm2 for the n-LT–GaAs samples and 6u2002kA/cm2 for the ErAs samples at 1 V forward bias have been measured. The I–V characteristics under forward bias for the n-LT–GaAs and ErAs samples exhibit significantly different behavior. At low temperatures, the n-LT–GaAs samples reveal a shoulder in the I–V characteristics, which can be explained by a model taking into account tunneling of carriers into LT midgap states. A similar model was able to explain the current transport in the ErAs diodes as tunneling of carriers into metallic regions inside the pn junction.

Optical Far-IR wave generation - state-of-the-art and advanced device structures

A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.

A new THz-photomixer based on a n-i-p-n doping superlattice

We investigate a THz-photomixer based on quasi-ballistic transport in an asymmetric n-i-p-n superlattice. The measured frequency roll-off is due to the device RC time constant alone whilst the transit-time limitation is overcome in this device. Measured power levels compare well with those from standard LT-GaAs photomixers.

Efficient THz Source Using GaAs and InGaAs nipnip Photomixers

We report on efficient ballistic-transport enhanced GaAs and InGaAs nipnip superlattice CW-THz sources with a transit-time 3 dB-frequency up to 1 THz and independently designable RC-roll-off. 1 muW output power at 400 GHz has been achieved.

Interference experiments with a coherent tunable THz nipnip superlattice photomixer

To date, THz radiation is mainly generated by pulsed sources. However, these are very expensive and bulky as strong solid-state pump sources are required to provide sufficient power. Additionally, the linewidth of the THz signal from pulsed systems is always Fourier-transform-limited in contrast to a CW THz source. We present a semiconductor based, room-temperature operating and frequency tunable CW system, based on photomixing with InGaAs nip nip-devices to generate the THz radiation.

Ballistic transport in nanostructures used for novel THz emitters

We report on a novel concept for THz-photomixers with strongly increased conversion efficiency. In the familiar THz photomixers, based on photoconductivity in Low-temperature-grown-GaAs (LT-GaAs) the performance depends critically on an extremely short lifetime of photo-generated charge carriers in LT-GaAs and is typically limited by the (very low) photoconductive gain. In our photomixer the gain does not depend on the recombination lifetime but only on the transit time and on the path length of the photo-generated electrons within suitably designed p-i-n /spl I.bar/ nanostructures, which both can be optimized if the transport is (primarily) ballistic. In addition, impedance matching to the attached antenna can be achieved.