Jochen Sigmund

Structure investigation of low-temperature-grown GaAsSb, a material for photoconductive terahertz antennas

The formation of precipitates after an annealing process in low-temperature-grown GaAs0.6Sb0.4 is observed. We use high-resolution transmission electron microscopy for a detailed structure investigation and demonstrate the functionality of the material, which has a band gap of 0.86 eV, for ultrafast optical switches. The average diameter of the clusters is 5.7 nm in the bulk region, while larger clusters are observed near the interface to a 1.3% lattice mismatched Al0.77In0.23As buffer layer. After an annealing treatment, the sheet resistance is increased by five orders of magnitude to 1.7×108Ω/sq, which is sufficiently high to enable Auston switch operation. We report terahertz (THz) emission from the material measured in a THz time-domain spectroscopy setup. THz generation is demonstrated by using a six interdigital finger metal-semiconductor-metal structure at the feed-point of a log-periodic antenna.

Spectral characterization of broadband THz antennas by photoconductive mixing: toward optimal antenna design

The spectral characterization of a broadband antenna using a pump-probe photomixing continuous-wave (CW) terahertz (THz) system is presented. The high dynamic range of the system, comparable to or better than that of similar pump-probe systems reported in the literature, provides an accurate means of antenna characterization. The planar antenna exhibits a log-periodic behavior at low frequencies, a bow-tie behavior at high frequencies, and a resonance characteristic in between, well in agreement with the antenna geometry. It is predicted that an improved geometry that extends the log-periodic behavior to higher frequencies would contribute significantly in enhancing the broadband performance of antenna-coupled photomixers.

Microstructure analysis of ohmic contacts on MBE grown n-GaSb and investigation of sub-micron contacts

We investigated the thermally induced solid state interdiffusion of Au Ge Pd and Ge Pd ohmic contacts on MBE grown n-GaSb. Furthermore, the electrical behavior of these contacts for different contact sizes down to 540 nm in diameter was compared. A specific contact resistivity as low as 4.9 × 10 6 Ωcm 2 was measured for the Au/Ge/Pd metallization. After annealing, polycrystalline AuSb 2 was observed by grazing incident X-ray diffraction (GIXD). Compared to Ge/Pd metallizations a gold top layer reduces the specific contact resistivity. The atomic structure or microstructure of the Au/Ge/Pd metallization showed a significant reduction of the thickness of amorphous Ge and led to a more spiky interface, which was observed by cross-sectional transmission clectron microscopy (TEM). Furthermore, an epitaxial regrowth of GaSb occurs, which is estimated to lead to a n - GaSb layer. The atomic microstructure has a significant effect on the current voltage (I V) characteristic up to a contact size of 950 nm in diameter, which shows a wide spread from ohmic to a more Schottky like behavior.

Magnetotransport properties of two-dimensional electron gas in AlSb∕InAs quantum well structures designed for device applications

The mobility and the sheet electron density of two-dimensional electron gas in AlSb∕InAs quantum well structures optimized for device applications were measured in the temperature range 4.2K<T<90K. A maximum electron mobility μ=3.24×105 was found at 4.2K at a sheet electron density n2D=1.1×1012cm−2. Measurements of the integral quantum Hall and Shubnikov-de Haas oscillations in the temperature range 0.07–9K were also carried out to obtain additional information on the characteristics of the two-dimensional electron gas. The electron effective mass m* and the effective electron g-factor g* were determined from these measurements and found to be, respectively, 0.032m0 and 14.6. The latter is in good agreement with the recent experimental data obtained from cyclotron resonance and titled magnetic-field experiments.

Planar Terahertz antenna optimisation

The presented antennas make use of advanced technologies such as low-temperature molecular beam epitaxy and micromachining for optimised antenna performance in the THz frequency range with respect to the specific requirements. Sophisticated technology in combination with state-of-the-art computational electromagnetic CAD software has been utilised to optimise THz antennas for the application with photomixers.

Efficient THz-emitters for low-temperature-grown GaAs photomixers

Different design techniques have been applied to low-temperature grown (LT-) GaAs photomixers and the respective antennas to increase the efficiency and output power of these devices. High DC-photocurrents have been achieved with interdigitated structures and transmission lines have been used to improve matching between antenna and photomixer. The input impedances of antenna structures on GaAs substrate have been investigated and optimised for matching and technological feasibility.

Binary optical mixing for broadband THz communication

A log-periodic antenna has been realised on LTG-GaAs (low-temperature-grown gallium arsenide) with an interdigitated photomixer for the generation of THz radiation. The signal is generated by optical heterodyning using the difference frequency of the lasers. The response is measured from 55 GHz up to 1200 GHz and compared with calculations. This approach leads to an optical modulation facility for THz communication of ultra broadband capability.

Investigation of a growth interruption under an As flux at AlSb/InAs interfaces with InSb bonds

We report on an optimized growth interruption time under an As flux during the AlSb/ InAs interface growth of unintentionally doped InAs quantum well structures. The migration-enhanced epitaxy method is used to create InSb interface bonds. After the deposition of the first In monolayer the surface is exposed to an As flux for different time periods, before the In shutter is opened again. Time-resolved reflection high-electron-energy diffraction (RHEED) and high-resolution transmission electron microscopy are used to characterize the interface. In combination with Hall measurements of single quantum well structures the interruption time is optimized. The interface quality depends on the RHEED intensity recovering during the interruption time under an As flux. If the interruption time is sufficient to reach the same RHEED intensity level as it is for the InAs growth, the electron mobility rises from 180,000 to 255,OOOcm 2 /Vs at 77 K. Furthermore, we observed that a disordered surface during the interface growth can be incorporated into the layer, leading to amorphous regions directly above the interface. Moreover, the disordering depends on the crystal orientation and is larger in the [III:0] azimuth.

Low-Temperature Grown GaAsSb with Sub-Picosecond Photocarrier Lifetime for Continuous-Wave Terahertz Measurements

We report on continuous-wave optoelectronic terahertz (THz) measurements using low-temperature grown (LTG) GaAsSb as photomixer material. A broadband log-periodic antenna and a six interdigital finger photomixer with 1μm gap is fabricated on LTG-GaAsSb for THz generation and detection. At 0.37THz, the resonance frequency of the inner most antenna tooth, we obtained a power of 6.3nW. A Golay cell was used as detector. The photocarrier lifetime of the material was determined to be 700fs by pump-probe experiments with an optical wavelength close to the band gap of LTG-GaAsSb. The band gap was 1.0eV, measured by wavelength dependent pump-probe measurements.

Reliability investigations on LTG-GaAs photomixers for THz generation based on optical heterodyning

Optical heterodyne conversion, or photomixing, is a frequency-agile technique that generates continuous-wave radiation at THz frequencies using thin films of low-temperature-grown (LTG) GaAs at high electrical bias. Very high electric fields (>300 kV/cm) and thermal excitation by focused laser illumination require reliability assessments prior to implementation in THz systems. Pulsed electrical stress and DC measurements under optical illumination are used to characterise the reliability of photomixers for THz generation based on optical heterodyning. An improvement in terms of reliability is observed by replacing Ti/Au by Pt metallisation.