C. Silfvenius

Strain variations in InGaAsP/InGaP superlattices studied by scanning probe microscopy

Strain-compensated InGaAsP/InGaP superlattices are studied in cross section by atomic force microscopy and scanning tunneling microscopy. Undulations in the morphology of the {110} cross-sectional faces are observed, and are attributed to elastic relaxation of this surface due to underlying strain arising from thickness and compositional variations of the superlattice layers. Finite element computations are used to extract a quantitative measure of the strain variation.

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.

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.

Hole distribution in InGaAsP 1.3-/spl mu/m multiple-quantum-well laser structures with different hole confinement energies

We have investigated the hole distribution in strained InGaAsP multiple-quantum-well (MQW) structures by direct hole transport measurements with time-resolved photoluminescence spectroscopy. The results show that the hole transport time over the MQW primarily depends on the hole confinement energy in the wells and increases sharply with the well depth. A simple thermionic emission model indicates that the heavy holes escape predominantly over the light-hole barrier edge for strain-compensated MQW structures. The results are corroborated with observed laser performance data.

Compositional variations in strain-compensated InGaAsP/InAsP superlattices studied by scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy (STM) and scanning tunneling spectroscopy are used to study strain-compensated InGaAsP/InAsP superlattices grown by metalorganic vapor phase epitaxy, with or without an InP layer inserted in the InAsP barrier. A difference of contrast in the STM images is observed between the InAsP barrier grown over an InP layer compared with the InAsP grown over the InGaAsP well. The first ≈4 nm of the InAsP barrier layers grown over the wells are found to be compositionally intermixed, containing significant enrichment of both arsenic and gallium atoms. This intermixing is believed to be due to some carryover or surface segregation of these species when the growth is switched from well to barrier.

Carrier Transport Effects in 1.3 µm Multiple Quantum Well InGaAsP Laser Design

We have investigated the influence of the barrier height on the performance of InGaAsP MQW lasers emitting at 1.3 µm via simulations, direct hole transport time measurements and laser evaluation. The results from the simulation showed that very high barriers result in an increase in hole and electron concentrations at the p-side of the multiple quantum well (MQW). This leads to a severe increase in the Auger recombination rate. The measured hole transport time is clearly dependent on the barrier height in the MQW. Metal organic vapour phase epitaxy fabricated lasers with up to twelve periods and optimised barrier heights showed internal efficiency values above 95%, internal losses below 10 cm-1, threshold densities as low as 60 Acm-2/well and T0 values as high as 79 K in the 20–80°C temperature range. Our conclusion is that attention should be given to the valence band carrier distribution when designing low threshold, high optical output MQW lasers suitable for elevated temperature operation.

Design, growth and performance of different QW structures for improved 1300 nm InGaAsP lasers

We have investigated the material quality of three alternative InGaAsP 1.3 μm wavelength multiple quantum well structures with strained wells, fabricated by low pressure metal organic vapour phase epitaxy. The designs have radically different compositions but similar calculated properties concerning gain, carrier distribution, laser threshold and optical output power. The structures considered all employ compressively strained wells and have constant-As, constant-Ga or InAsP-InGaAsP materials in wells and barriers. Growth conditions were optimised for each design. Evaluation of the constant-As multiple quantum well (MQW) resulted in poor X-ray diffraction (XRD) and photoluminescence (PL) response. The InAsP MQW exhibited clearly defined XRD-satellites but as the As-content was increased to reach 1.3 μm, PL properties degraded severely. The constant-Ga MQW indicated superior material quality with excellent PL and XRD properties. Fabricated lasers with up to 12 periods and lattice matched barriers, showed internal efficiency values above 95%, internal losses below 10 cm -1 , threshold densities as low as 60 A cm -2 /well and temperature constant, T 0 , values as high as 79 K in the temperature range 20-80°C, The constant-Ga structure allows a variable barrier height. strain compensation and simultaneously avoids the problem with growth undulation and interdiffusion, typically encountered for the InAsP and constant-As cases and should therefore be an excellent candidate for active layers in 1.3 μm lasers.

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.