M. Kneissl

Novel shadow mask molecular beam epitaxial regrowth technique for selective doping

We present a novel molecular beam epitaxial regrowth technique which provides a simple and convenient way for the in situ lateral structuring of the doping profiles and growth rates on a μm scale. We achieve excellent selective contacts to the respective doping layers for device dimensions varying from several 100 μm down to several μm. Keldysh based n‐i‐p‐i modulator structures, fabricated with our new method, exhibit an on/off ratio of 6:1 for a voltage swing of 7 V without enhancement by additional Bragg mirrors.

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≊16u2009000 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 700u2009 °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.

REVERSE BIASED PHOTOCONDUCTIVE DETECTORS AND SWITCHES WITH SEPARATE ABSORPTION AND DETECTION AREA

We report on reverse biased photoconductive detectors with novel design and improved high‐frequency performance. Taking advantage of the ‘‘giant ambipolar diffusion constant’’ which has been observed previously in n‐i‐p‐i doping superlattices very fast carrier transfer from the inner absorption area to the outer detection area is achieved. The combination of narrow contact spacings and small RC and diffusion time constants results in very high gain‐bandwidth products (≳20 GHz) with adjustable 3 dB frequencies.

N-i-p-i-based high-speed detectors and bistable switches with gain

We report on recent theoretical and experimental results on reverse biased photoconductive detectors and bistable optical switches. Using a special design and taking advantage of the giant ambipolar diffusion constant of n-i-p-i structures, the photogenerated carriers are transferred from the absorption area into a small detection area within very short times. The combination of small contact separations and small RC- and diffusion time constants results in very high gain-bandwidth values with adjustable 3-dB frequencies.

Linearity of double heterostructure electroabsorptive waveguide modulators

The linearity of electrooptical waveguide modulators based on the Franz-Keldysh effect with respect to the voltage-transmission characteristic has been investigated experimentally and theoretically. For a GaAs-AlGaAs double heterostructure modulator the values of the signal-to-noise ratio due to anharmonic and intermodulation distortion have been found to be small enough to suffice the requirements for analog optical transmission in multichannel broad-band fiber network systems compatible to existent cable networks. The results are also applicable to materials in the bandgap region of the optimum fiber transmission wavelengths around 1.3 /spl mu/m and 1.5 /spl mu/m. The advantages in comparison with directly modulated semiconductor lasers will be discussed. >

Optical bistability of p-i-n and n-i-p-i structures at very low optical power

We report on opto-electrical and opto-optical bistability with high electrical/optical gain occurring at very low optical power. In p-i-n and n-i-p-i structures large field induced changes of the absorption coefficient up to 5000 cm-1 can be observed due to the Franz- Keldysh effect. Slightly above the bandgap the absorption decreases with increasing internal field. This leads to n-shape current voltage characteristics which can be used for bistable opto- electrical switching. At an optical power smaller than 200 pW we achieve an on/off ratio of more than 107 for switching the n-layer current corresponding to an opto-electrical gain of 6.8 (DOT) 106. A smart pixel consisting of an electro-optical n-i-p-i modulator controlled by the bistable switch allows the observation of opto-optical bistability. With our smart pixel concept we have achieved an opto-optical gain of up to 3 (DOT) 105. This bistable switching is observed at extremely low switching powers smaller than 500 pW and exhibits a very broad hysteresis width of 200 pW. The minimum switching power is only limited by the dark current of the switch and can be reduced further.

Constructive superposition of field- and carrier induced absorption changes in hetero-n-i-p-i- structures

Abstract By applying a voltage U pn to hetero- n - i - p - i structures with n - and p -contacts, both the internal fields in the intrinsic layers and the carrier concentrations in the quantum wells are changed. Therefore, large changes of the absorption coefficient can be achieved simultaneously by field effects and phase space filling. In this paper we demonstrate that a constructive superposition can be achieved by shifting the bandfilling contribution to lower photon energies in type-I-hetero n - i - p - i crystals. A 25 period δ-doped InGaAs/GaAs hetero- n - i - p - i structure with 10 nm pseudomorphic In 0.07 Ga 0.93 As quantum wells was grown by “shadow mask MBE” in order to obtain selective contacts to the n - and p -layers. At room temperature a relative transmission change of more than 2:1 at photon energies of 1.39 eV with a voltage swing from −2.5 to +0.8 V could be demonstrated. This corresponds to a total change of the absorption coefficient of about 2000 cm −1 referred to the total sample thickness of 3.55 μm. These large absorption changes (Δα) are a consequence of the constructive superposition of the Franz-Keldysh effect in the intrinsic layers and phase space filling in the InGaAs quantum wells. Comparing our experimental results with theory we deduce a maximum Δα of more than 10000 cm −1 (referred to the total quantum well thickness) by bandfilling, whereas the Franz-Keldysh effect contributes a Δα of about 1450 cm −1 (referred to the total intrinsic layer thickness).

Refractive index changes of low temperature grown GaAs depending on its annealing history

as a function of the time delay, the pulsewidth could be measured. Measurements were taken of Q-switched semiconductor lasers (Fig. (2) ) . Interferometric autocorrelation measurements have also been taken. We also report another novel device for measuring pulse widths, a multicontact autocorrelator that consists of a reverse biased p-i-n ridge waveguide similar in design to the autocorrelator previously mentioned. Along the length of the waveguide is a series of contacts electrically isolated from each other. The beam was split into two pulses and propagated through the waveguide from both ends where the pulses form a standing wave pattern, which was measured by monitoring the photocurrent at each contact along the waveguide. The contacts were spaced every 50 pm, which corresponds in time to about 500 fs, the approximate resolution of this particular device (Fig. 3). The main advantages of this device are that the pulsewidth can be obtained in a single measurement with no moving parts, and the device could be integrated with a semiconductor laser. *Present address, School of Physics, University of Bath, Claverton Down, Bath, England BA2 7AY CThM6 1615

Demonstration of extremely low switching energies using new n-i-p-i-based smart pixels

We report on recent theoretical and experimental results on n-i-p-i based smart pixels. We present first results on dynamical switching in these devices. In particular, we present 1.9 ns switching times at an optical power of 880 (mu) W for the photoconductive switch. This corresponds to a total switching energy of 1.7 pJ or 2.4 fJ/micrometers 2 relative to the device area. A contrast of the electronic output signal larger than 107 and a maximum dc gain exceeding 106 is achieved. For an optical NOR gate -- composed of a photoconductive switch and an electroabsorptive n-i-p-i modulator -- we were able to demonstrate operation at optical switching energies of 2 pJ and switching times of 1 microsecond(s) . This corresponds to an opto-optical gain (fan out) of 750. The switching contrast of the optical output signal is 4.5:1.