Ferdinand Scholz

Tunable polarization converter based on ordered AlGaInP waveguide structures

An electrically tunable transverse electric/transverse magnetic polarization converter using ordered AlGaInP waveguide structures is presented. The structures are grown on (001) GaAs substrates with the surface tilted 6° towards {111}B. This selects one of the two possible {111}B variants for the ordering planes. Thus, also on a macroscopic scale, the symmetry of the planar waveguide is broken. The properties of the guided modes are very sensitive to the size and orientation of the axes of the dielectric tensor. So a slight modification of their magnitudes due to electrorefraction leads to a remarkable change in the propagation constants. This change is used to accomplish tunable rotation of polarization by 90°.

Birefringence and tilted modes in ordered GaInP/AlGaInP waveguides and lasers

We have studied the influence of chemical ordering on the properties of GaInP/AlGaInP optical waveguide structures and lasers. We have observed mode‐conversion between transverse electric (TE) and transverse magnetic (TM) modes within typically 60 μm. We show that an ordering‐induced birefringence with the optical axis tilted with respect to the growth direction is responsible for the coupling of the TE and TM modes. A new linear combination of TE and TM modes, the ‘‘super modes,’’ can propagate in the waveguide without change. We show that these super modes govern the polarization of lasers made from ordered material.

Single variant ordering in GaInAs/InP

Spontaneously ordered Ga0.47In0.53As grown on substrates with the (001) surface tilted 4° towards {111}B are studied using spectroscopic methods as well as x-ray diffraction, transmission electron diffraction and dark-field transmission electron microscopy. The single variant ordering is proved by the absence of one class of the ordering induced 12{111}B superlattice spots in transmission electron diffraction patterns as well as by the tilted polarization of the photoluminescence emerging from the samples cleaved edge. The temperature dependence of the luminescence peak position shows an anomalous behavior at low temperatures and a strong dependence of the peak position on the excitation power. From low temperature absorption measurements, we find a band gap reduction of 37 meV and a valence band splitting of 13.2 meV.

A mechanism for low‐power all‐optical switching in multiple‐quantum‐well structures

We demonstrate a novel type of optical switching mechanism in pin separate confinement multiple quantum well (SCMQW) structures. By introducing additional large barriers into conventional InGaAs(P)/InP SCMQW structures, the transport of photogenerated holes can be controlled in such a way that they accumulate in the intrinsic region. This positive space charge leads to a local screening of the internal field in the optical confinement layer and to an enhancement of the internal field in the MQW region. We characterize the optical nonlinearity, which is based on the quantum confined Stark effect (QCSE), experimentally and theoretically. As the nonlinearity is observed at input powers <1 W/cm2 in the basic nonoptimized structures presented here, we propose to use our structure especially for low‐power optical switches.

Effect of Ar ion beam channeling on AlGaN/GaN heterostructures during the ion beam etching process

The effect of Ar+ ion beam etching of AlGaN/GaN heterostructures at a bias voltage of 250 V was investigated with respect to different ion incident angles. The samples were measured before and after etching with respect to mobility, sheet electron concentration, and sheet resistance. We found a pronounced dependency of the electrical characteristics after etching on the ion incident angle. Especially at zero degree, the mobility of the two-dimensional electron gas (2DEG), which is located at the AlGaN/GaN interface, decreases dramatically after etching. The sheet resistance increases in the same way. At larger ion incidence angles, the effect vanished. We attribute this behavior predominantly to channeling of the ions through the AlGaN layer down to the 2DEG. An annealing step after etching shows improvement of the electrical characteristic. These results show that gate-recessed GaN field effect transistors can be limited in their device performance by etch-process-induced ion channeling effects. The resu...

Intrinsic modulation bandwidth of strained GaInP/AlGaInP quantum well lasers

We have investigated the intrinsic modulation response of GaxIn1-xP/AlGaInP quantum well lasers. The differential gain resulting from the measurement of the frequency response, is in good agreement with calculations based on a 6-band kp-theory and direct measurements of the optical gain. We find a maximum value of 3.8×10−16 cm2 for a strongly compressively strained sample, which is less than in InGaAs lasers according to the larger effective masses. The maximum bandwidth we observed is 9.3 GHz at −3 dB. We show that the bandwidth is limited by catastrophic optical damage and not by intrinsic mechanisms.We have investigated the intrinsic modulation response of GaxIn1-xP/AlGaInP quantum well lasers. The differential gain resulting from the measurement of the frequency response, is in good agreement with calculations based on a 6-band kp-theory and direct measurements of the optical gain. We find a maximum value of 3.8×10−16 cm2 for a strongly compressively strained sample, which is less than in InGaAs lasers according to the larger effective masses. The maximum bandwidth we observed is 9.3 GHz at −3 dB. We show that the bandwidth is limited by catastrophic optical damage and not by intrinsic mechanisms.

Size effect upon emission dynamics of 1.5 μm quasi‐quantum wire distributed feedback semiconductor lasers

Emission dynamics of 1.5 μm InGaAs/InGaAsP quasi‐quantum wire lasers excited by ultrashort optical pulses were investigated. Wire distributed feedback lasers with different nanometer wire widths and distances between the wires, which were realized by dry‐etching technique, were compared in detail. The dependence of dynamic behavior on wire width was observed. Detailed theoretical simulations show that, the improvement of dynamic response with decrease of wire width is attributed to a superlinear increase of differential gain with decrease of wire width, which counteracts the negative effects due to decrease of packing density of the active region and increase of electron relaxation time.

Control of field screening effects in GaInAs(P)/GaInAsP quantum‐confined Stark effect modulator structures

We compare the field screening behavior of quantum‐confined Stark effect modulator structures where the GaInAsP/InP heterojunction is in the intrinsic (standard structure) or in the doped regions (modified structure) at working wavelengths of 1.55 and 1.3 μm. The modified structures are obtained by expanding the GaInAsP confinement layers into the p‐doped and n‐doped regions without changing the total intrinsic layer thickness. The effectiveness of the InP heterobarriers for the holes on the p‐side and for the electrons on the n‐side is thereby lowered. A significant reduction of field screening for the modified structures is achieved.

Waveguiding in (quantum) wire structures: Impact on the polarization characteristics and the slope of the optical gain

This work discusses the role of quantum wires as electromagnetic waveguides. The optical gain of buried InGaAs/In(GaAs)P wire structures is investigated as a function of the wire width. We find that the relative contribution of the light and heavy hole transition as well as the optical polarization of the gain are strongly dependent on the wire width. This corresponds to the behavior expected as a result of quantum confinement. Based on calculations of the waveguiding behavior of the wires, however, we attribute these effects to changes of the waveguide properties rather than to quantum confinement effects of the carriers.

Magnetotransport in periodically modulated quantum wires

Quantum wires with an artificially designed periodic potential have been fabricated by etching a chain of antidots along the central wire axis. The resistivity ρxx in the presence of a perpendicular magnetic field shows distinct peaks in the low magnetic field regime that can be explained by localized orbits around one antidot or between two neighboring antidots. By changing the lateral position of the antidot chain on the wire from the central axis to the wire side walls, a localized orbit in the region between four antidots has been detected.