B. Knüpfer

Above band gap absorption spectra of the arsenic antisite defect in low temperature grown GaAs and AlGaAs

Room temperature absorption spectra of low temperature molecular beam epitaxy grown GaAs (LT‐GaAs) and AlGaAs (LT‐AlGaAs) are reported. We performed measurements in an extended spectral range from 0.8 eV to photon energies of 2.8 eV far above the band gap. For as‐grown LT‐materials, the absorption coefficients at the band gap are twice as high as for high temperature grown materials. By annealing the samples, we obtained a drastic reduced absorption coefficient below as well as above the band gap. We observed absorption changes up to 17 000 cm−1 for LT‐GaAs and 9000 cm−1 for LT‐AlGaAs taking place in a two phase process.

Polarization dependence of the electroabsorption in low‐temperature grown GaAs for above band‐gap energies

We have measured the electroabsorption in low‐temperature grown GaAs by performing room‐temperature transmission experiments in the spectral range from 1.3 to 1.9 eV for different electric fields induced by a voltage applied to a metal‐semiconductor‐metal structure. The devices were separated from the substrate by using an epitaxial liftoff technique. Therefore, we have been able to observe the electro‐optic effect at the fundamental band gap as well as at the split‐off band edge. The absorption is clearly polarization dependent at the fundamental band gap but only weakly at the split‐off band gap, in agreement with the theory of the Franz–Keldysh effect.

Polarization-insensitive high-contrast GaAs/AlGaAs waveguide modulator based on the Franz-Keldysh effect

A GaAs/AlGaAs p-i-n double heterostructure waveguide modulator based on the Franz-Keldysh effect is reported. On/off ratios up to 40 dB are obtained over a broad range of wavelengths (>or=32 dB between 905 nm and 960 nm), while absorption loss in the on-state is very low (<1 cm/sup -1/). The polarization dependence of the Franz-Keldysh effect is relatively weak and only causes a shift of the transmission-voltage characteristics of TE-polarized light towards higher reverse bias compared to the corresponding TM curves. Therefore, very high contrast ratios are achievable over the whole range of wavelengths even for arbitrary polarization of the light.<<ETX>>

Annealing induced refractive index and absorption changes of low‐temperature grown GaAs

Large changes of the refractive index (Δn≊0.25) and absorption coefficient (Δα≊16 000 cm −1) of GaAs grown by molecular beam epitaxy (MBE) at low substrate temperature (LT‐GaAs) induced by annealing are reported. The refractive index difference between the LT‐GaAs layer and the GaAs substrate are determined from both, the amplitude of the Fabry–Perot oscillations and the shift of their extrema towards shorter wavelengths yielding nearly the same results. With increasing anneal temperature the excess refractive index as well as the strong absorption at photon energies below the GaAs band gap (determined by transmission measurements) disappear around 700  °C.

Low power (bistable) opto‐electrical threshold switches with high gain based on n‐i‐p‐i doping superlattices

We report on the realization of an electro‐optical switch on n‐i‐p‐i doping superlattices for both bistable and step‐like threshold operation at very low input power levels (≂200 pW) and with high opto‐electrical gain (6.8×106). The configuration presented in this letter consists of a (two terminal) photodiode with negative current‐voltage characteristic and a novel three‐terminal photoconductive detector structure. The minimum optical switching power is determined by the reverse dark currents of the devices that are below pA. The switches are suitable for monolithical integration into arrays.

Electroabsorption in InGaAsP: electro‐optical modulators and bistable optical switches

We report Franz–Keldysh absorption spectra of InGaAsP obtained from transmission and photoresponse measurements in InP/InGaAsP/InP p‐i‐n double heterostructures. The values of the absorption changes (Δαmax≊3000 cm−1) are comparable with those obtained from the quantum‐confined Stark effect. Optical bistability is observed at very low light intensities (40 μW/cm−2), due to very low leakage currents and the strong absorption changes above the band gap.

Bistable opto‐optical switches with high optical gain based on n‐i‐p‐i doping superlattices

We report on optical logic gates and bistable opto‐optical switches with high gain operating at very low threshold power. These devices are ‘‘smart pixels’’ composed of n‐i‐p‐i threshold switches and modulators. Very high values of 3×105 for the opto‐optical gain (‘‘fan‐out’’) and extremely low switching powers of less than 500 pW in combination with a broad hysteresis width of 200 pW are obtained. The switching power, the width of the hysteresis, and the opto‐optical gain are all externally adjustable. The observed contrast ratio of 4.6 for the output signal corresponds to an optically induced voltage swing of 2.5 V.

SMART PIXEL USING A VERTICAL CAVITY SURFACE-EMITTING LASER

We present a smart pixel with surface-normal optical input and output beams using a vertical cavity surface-emitting laser. In its present nonoptimized hybrid version, the smart pixel exhibits opto-optical switching with a contrast of 35 dB, an optical gain of 55 dB and a 3 dB bandwidth of 90 MHz. The optical switching energy is 715 fJ. NOR, and NAND logic operations at 4 Mbit/s with a fan-out of 5 are also demonstrated.

Dynamical switching behavior of n‐i‐p‐i modulator structures

We report on the ac switching behavior of surface‐normal electro‐optical modulators based on hetero‐n‐i‐p‐i structures. We have investigated systematically the way the transition times scale with device dimensions and present a straightforward theoretical model including sheet resistivities of n and p layers as well as contact resistances. The model is in excellent agreement with experimental data. With a voltage swing as low as 3.7 V, we achieved a single path contrast ratio in excess of 2:1 and maximum 3 dB frequencies larger than 250 MHz.

Optical and electro-optical investigation of low-temperature grown GaAs

Abstract We report on the effects of growth temperature, arsenic source and post growth annealing treatment on the optical properties of low temperature grown GaAs. We have determined the absorption coefficient over a wide spectral range of energies between 0.8 and 2.0 eV. Special interest was dedicated to the shape of the band edge absorption for the different growth conditions and post growth treatments. The band edge is sharper for samples grown with an As 4 source than for samples grown using As 2 . However, the band edge becomes sharper due to the annealing process for all samples. A sharp band edge is essential for a high-contrast electro-optical modulator. We also measured the electro-absorption of a metal-semiconductor-metal structure based on annealed LT-GaAs and achieved a contrast ratio of 1:1.35 corresponding to an absorption change of 2300 cm −1 .