W.C. van der Vleuten

Polarization stabilization in vertical-cavity surface-emitting lasers through asymmetric current injection

We present experimental evidence that asymmetric current injection in intracavity contacted vertical-cavity surface-emitting lasers (VCSELs) stabilizes the polarization of the emitted light. Anisotropies in the gain and loss mechanisms introduced by asymmetric current injection are considered to explain this effect. The design scheme opens perspectives to obtain actual polarization control in VCSELs.

Broadband single‐electron tunneling transistor

A single‐electron tunneling transistor has been directly coupled on‐chip to a high electron mobility transistor. The high electron mobility transistor (HEMT) is used as an impedance matching circuit with a gain close to unity. The HEMT transformed the 1.4 MΩ output impedance of the single electron tunneling (SET) transistor by two orders of magnitude down to 5 kΩ, increasing its bandwidth to 50 kHz. This circuit makes it possible to observe the motion of individual electrons at high frequencies. The requirements for the bandwidth in high frequency applications is discussed.

New polytypes in vapour grown GaSe

Abstract The occurrence of new polytypes in GaSe is reported. They have been denoted 9R, 12R and 15R following the Ramsdell notation. These crystals are obtained under certain conditions during sublimation experiments. One of the polytypes (9R) is described more extensively. Apart from X-ray determination techniques these polytypes are detected by measuring the angles of the planes using optical-goniometer techniques. The habit of the different rhombohedral polytypes turned out to be similar to each other.

Analysis of 6-nm AlGaAs SQW low-confinement laser structures for very high-power operation

This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation. The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-/spl mu/m-wide stripes was as high as 6.4 W before catastrophic optical degradation. If scaled to continuous-wave (CW) conditions, this value would be 800-1100 MW, which would mean a factor of 22.7 times more than reported for the best devices with normal design for threshold minimization. The absorption coefficient for the symmetrical structure is as low as 1.1 cm/sup -1/, in spite of the low trapping efficiency of carriers in the quantum well (QW). The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions). Threshold current densities were 800 A/cm/sup 2/ for 5-mm-long devices in the symmetrical case and 2200 A/cm/sup 2/ in the asymmetrical one. The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed.

Low-loss low-confinement GaAs-AlGaAs DQW laser diode with optical trap layer for high-power operation

A low-confinement asymmetric GaAs-AlGaAs double-quantum-well molecular-beam-epitaxy grown laser diode structure with optical trap layer is characterized, The value of the internal absorption coefficient is as low as 1.4 cm/sup -1/, while keeping the series resistance at values comparable cm with symmetrical quantum-well gradient index structures in the same material system. Uncoated devices show COD values of 35 mW//spl mu/m. If coated, this should scale to about 90 mW//spl mu/m. The threshold current density is about 1000 A/cm/sup 2/ for 2-mm-long devices and a considerable part of it is probably due to recombination in the optical trap layer. Fundamental mode operation is limited to 120-180 mW for 6.5-/spl mu/m-wide ridge waveguide uncoated devices and to 200-300 mW for 13.5-/spl mu/m-wide ones, because of thermal waveguiding effects. These values are measured under pulsed conditions, 10 /spl mu/s/l ms.

Andreev reflection at superconducting contacts to GaAs/AlGaAs heterostructures

Highly transmissive ohmic contacts to the two‐dimensional electron gas in GaAs/AlGaAs heterostructures have been made. For these contacts, which are of μm scale, a newly developed process of Ti/Sn evaporation and diffusion has been used. Devices consisting of these superconducting contacts combined with gate structures have shown clear evidence for the occurrence of Andreev reflection. Moreover a marked effect of the gate voltage on the dV/dI−V characteristics has been found, which proves that superconductivity has been induced into the semiconductor.

Fabrication of short GaAs wet-etched mirror lasers and their complex spectral behavior

A versatile fabrication technique for GaAs-AlGaAs wet-etched mirror lasers is presented. This technique works independently of the Al concentration in the cladding layers up to a value of 70%, and it requires four photolithography steps. Ridge waveguide lasers have been successfully processed using a double heterostructure (DHS) as well as graded index separate confinement heterostructures (GRINSCH) having different quantum-well (QW) active layers. This technique is used to fabricate short-cavity lasers in GRINSCH structures having GaAs multiple-quantum-well (MQW) or bulk active layers. Laser operation was obtained in a 29-/spl mu/m-long device using a 5-QW structure. Short lasers with QW active layers show a complex spectral behavior. These lasers operate at higher current densities (/spl sim/20 kA/cm/sup 2/) and emit light at more than one wavelength. This implies that higher order transitions are involved which is not the case when using a bulk GaAs active layer. Besides the two peaks corresponding to the n=1 and n=2 transitions, we found an intermediate peak which corresponds presumably to the forbidden transition E1-HH2.

A RHEED study of the dynamics of GaAs and AlGaAs growth on a (001) surface by MBE

Abstract In this paper we report on RHEED investigations of the damping and recovery of the specular-beam intensity during MBE growth. Our results on the damping behaviour during growth of GaAs and Al0.33Ga0.67As indicate that at a temperature of approximately 610°C the average step density on the surface is at a minimum. At this temperature the change from a (2 × 4) As-stabilized surface to the (1 × 1) (3 × 1) metal-rich surface takes place. We attribute the larger degree of damping at lower temperatures to insufficient ad-atom mobility and thus a high step density on the surface. At temperatures above 610°C we attribute the larger degree of damping to adsorption of gallium atoms on arsenic sites. A purely analytical procedure is used to calculate the recovery rate of the specular-beam intensity. Results are presented for growth termination at various temperatures and at various fractions of monolayer coverage, θGa. It can be concluded that the initial, fast recovery process on the arsenic-stabilized surface is due to Group III kinetics with activation energies generally greater than 1.5 eV. However, a lower activation energy is observed on termination of growth at 1 2 of a complete monolayer (θGa = 0.5), indicating the recovery process to be then limited by As4 physisorption. The second, slow step in the recovery has activation energies of the order of 5 eV. The initial recovery process on the gallium-rich surface is characterized by lower activation energies, of the order of 0.5 eV. The mathematical analysis presented here allows one to calculate the minimum growth interruption times to obtain smooth surfaces in a straightforward manner.

Further studies on polytypism in GaSe

Abstract Some new rhombohedral polytypes and a hexagonal polytype are reported. They have been denoted 18R, 21R and 6H. These crystals are obtained during sublimation experiments. A model for the 6H-structure is introduced. The phenomenon of syntactic coalescence, viz. the growth of 6H and 18R in one GaSe needle crystal, is shown. A new feature observed in GaSe polytypes, viz. the rotation of the slabs, which build those polytypes, is discussed.

Low‐voltage hetero‐nipi waveguide modulators with GaAs/AlAs quantum wells

We present a waveguide modulator with a GaAs/AlAs multiple‐quantum‐well hetero‐nipi core. Selective lateral ohmic contacts connect all the p‐type layers together and likewise all the n‐type layers. A small applied voltage creates a large change in the field across each thin undoped quantum well region, causing large changes in the absorption coefficient and refractive index. A 300‐μm‐long device gave a 10:1 modulation ratio at 1‐V reverse bias with 4‐dB zero‐bias absorption loss. 400‐ and 900‐μm‐long devices gave a 180° phase shift with negligible absorption loss at 1.5 and 0.9 V, respectively.