Björn Stålnacke

Modulation response measurements and evaluation of MQW InGaAsP lasers of various designs

Results from modulation measurements of 40 high-speed multi quantum well (MQW) lasers ((lambda) equals 1.55 micrometer) of various designs are presented. By fitting the careful calibrated measurements, both magnitude and phase, to an analytical transfer function we were able to determine if a certain laser was limited by thermal effects, parasitic-like effects, or nonlinear gain effects. We found that most of the devices in the study were limited by thermal effects and/or contact parasitics. The parasitics were found to be determined by the width of the high-doped contact layer and cladding layers below the metallic contact. It was also found that a high doping of the separate confinement heterostructure (SCH) layers decreases the damping of the relaxation peak since it facilitates the carrier transport. Improved contact design and high doped SCH-layers resulted in modulation bandwidths of around 24 GHz.

Interwell carrier transport in InGaAsP multiple quantum well laser structures

We present direct measurements of interwell carrier transport in InGaAsP quantum well (QW) laser structures performed by time‐resolved photoluminescence. Conditions of originally empty and filled wells are explored. In both cases, the time for the hole transport across the structure is found to be of the order of tens of picoseconds. Comparison of experimental results and simulations allowed us to develop an adequate interwell carrier transport model that includes thermionic capture/emission over the QW interfaces and drift/diffusion in the barrier regions. We show that dynamic consideration of carrier densities and band bending for each QW are essential.

Morphological and compositional variations in strain-compensated InGaAsP/InGaP superlattices

We have investigated the properties of strain-compensated InGaAsP/InGaP superlattices, grown by metalorganic vapor phase epitaxy, with and without InP interlayers inserted in the InGaP barrier. Using cross-sectional scanning tunneling microscopy and spectroscopy, we observe lateral variations in layer thickness and electronic properties. When the number of superlattice periods is increased from 8 to 16, the growth front develops large undulations in the top two to four superlattice periods. For structures with InP layers inserted in the InGaP barrier, only slight undulations of the top superlattice periods occur. We discuss the origins of the growth front undulations in terms of the elastic relaxation of strain arising from thickness and/or composition variations in the superlattice layers. Finally, we observe a fourfold periodicity of the [001] atomic spacing, presumably arising from atomic ordering in the alloys.

Demonstration of quantized conductance in deeply reactive ion etched In0.53Ga0.47As/InP electron waveguides with in-plane gates

Clear conductance quantization at T=4.2 K has been demonstrated in a 140 nm wide and 200 nm long trench-isolated In0.53Ga0.47As/InP electron waveguide with in-plane gates, using the surrounding two-dimensional electron gas as the gate. It was fabricated using metalorganic vapor phase epitaxy, electron beam lithography, and CH4/H2 reactive ion etching. In a T=20 K measurement on a 60 nm wide and 100 nm long device, only the first conductance plateau of 2e2/h was reached.

MOVPE growth of strain-compensated 1300 nm In1−xGaxAsyP1−y quantum well structures

Different concepts for achieving strain-compensated quantum well structures emitting at 1300 nm have been investigated. Structures employing up to eight compressively strained wells with the same x in well and barrier exhibits excellent structural and optical properties, including very high photoluminescence efficiency. Increased number of quantum wells beyond 8 resulted in deteriorated materials quality, most likely due to accumulated strain-induced roughness of the growing surface. Good laser characteristics, including T 0 values of 64 K, were demonstrated for strain-compensated structures with tensile wells.

Two-section InGaAsP DBR-lasers at 1.55 /spl mu/m wavelength with 31 GHz direct modulation bandwidth

The small signal modulation response of two-section InGaAsP DBR-lasers at 1.55 /spl mu/m wavelength was investigated. The response was fitted to a general transfer function and it was found that for almost all lasers the response could be described by a three pole model consisting of the laser response from the standard rate equations and an additional first order low pass roll-off. The lasers exhibited reduced damping and increased resonance frequency due to what we believe is detuned loading. This led to a maximum bandwidth of 30 GHz for lasers described by the three pole model. Some lasers exhibited an additional effect which we believe is cavity resonant enhancement of one of the modulation side-bands. This effect increased the maximum -3dB bandwidth to 31 GHz but could not be described by a three pole model.

Ballistic transport and gate control mechanism in deeply etched electron-waveguide based devices

Abstract We prepared In 0.53 Ga 0.47 As/InP electron waveguides with in-plane side gates by deep etching through a two-dimensional electron gas. In these structures we investigated the conductance dependent on the voltages applied at both gates. Our experiments show a remarkable asymmetry in the gate efficiency: negative gate voltages seem to be more efficient than positive ones, and the pinch-off voltage of the waveguide depends mainly on the more negative of the two gate voltages. A model is given which takes the etched surfaces into account. The conductance in the waveguide is essentially determined by the potentials at the etched surfaces, which are via Schottky-diode transitions coupled with the gates and the channel. With the help of this model we are able to explain the behavior of the electron waveguides.

Enhanced modulation bandwidth and self-pulsations in detuned loaded InGaAsP DBR-lasers

A strong undamping of the relaxation peak leading to self-pulsations and a record high modulation bandwidth (26 GHz) were observed in 1.55 /spl mu/m InGaAsP DBR QW lasers. These results are attributed to the mechanism of detuned loading.

Electron Transport in InGaAsP/InP Quantum Well Laser Structures

Carrier transport in quantum well (QW) lasers is one of the factors affecting the high frequency performance of these devices. Usually the transport of holes is assumed to be of major significance. However, if the active region of a laser is shifted towards the p contact of the device,1 or this region is p-doped,2 the electron transport may become an important factor.

Carrier Transport in Multiple Quantum Well Region of InGaAsP/InP Structures

After injection by a current pulse, initial carrier distribution between the quantum wells (QW) of a multiple QW laser is highly nonuniform1. For a laser to operate effectively, it is desirable that all the QWs equally contribute to the lasing action. As far as the high frequency modulation is concerned, the rate of the carrier transport and redistribution between the QWs should be considerably faster than the modulation rate.