J. Böhrer

Radiative recombination in type‐II GaSb/GaAs quantum dots

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Low-threshold injection lasers based on vertically coupled quantum dots

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during successive deposition of several sheets of (In,Ga)As quantum dots separated by thin GaAs spacers. VECODs are introduced in the active region of a GaAs-A1GaAs GRIN SCH lasers. Increasing the number of periods (N) in the VECOD leads to a remarkable decrease in threshold current density ( ~ 100 A/cm 2 at 300 K for N = 10). Lasing proceeds via the ground state of the quantum dots (QD) up to room temperature. Placing the QD array into an external AIGaAs--GaAs quantum well allows us to extend the range of thermal stability of threshold current density (To = 350 K) up to room temperature. Using (In,Ga)As-(A1,Ga)As VECODs in combination with high temperature growth of emitter and waveguide layers results in further reduction of threshold current density (60-80 A/cm 2, 300 K) and increase in internal quantum efficiency (70%). Room temperature continuous wave operation (light output 160 mW per mirror) and lasing via the states of QDs up to I = (6-7) Ith have been demonstrated.

Formation of coherent superdots using metal‐organic chemical vapor deposition

We demonstrate direct growth of electronically coupled zero‐dimensional structures forming a super‐quantum dot using metal‐organic chemical vapor deposition. After the first sheet with InGaAs pyramids is formed on GaAs surface, alternate short‐period GaAs‐InGaAs deposition leads to spontaneous formation of layered structures driven by the energetics of Stranski–Krastanow growth. As a result columnlike InGaAs structures each having a characteristic lateral size of ∼23 nm at the top and composed of many closely packed InGaAs parts are formed. The full width at half maximum of superdot luminescence of 28 meV at 8 K indicates good average uniformity of the superdot ensemble. Absorption is found to be resonant with luminescence.

Interface inequivalence of the InP/InAlAs/InP staggered double heterostructure grown by metalorganic chemical vapor deposition

The optical and structural properties of the normal InAlAs on InP and the inverted InP on the InAlAs staggered band lineup interface grown by metalorganic chemical vapor deposition (MOCVD) are compared by use of transmission electron microscopy (TEM), time integrated, and time resolved photoluminescence. TEM images show that both interfaces are dissimilar. The normal interface is very abrupt. The inverted interface shows an additional graded layer of about 2.5 nm in width of In1−xAlxAsyP1−y with x (0.48–0) and y (1.0–0.0). A large optical anisotropy exists because of the inequivalence of the two interfaces. The larger spatial separation of the carriers at the inverted interface is responsible for a smaller overlap of the electron and hole wave functions and for that reason a one order of magnitude longer e‐h luminescence decay time of 45 ns is observed. The normal interface transition shifts approximately to the third root of excitation while the inverted interface transition shifts logarithmically.

Composition dependence of band gap and type of lineup in In1−x−yGaxAlyAs/InP heterostructures

In1−x−yGaxAlyAs is grown lattice matched by low pressure metalorganic chemical vapor deposition on InP and characterized using low temperature photoluminescence. Compositional information is obtained from energy dispersive x‐ray spectroscopy and the band gap is determined as a function of Al content. We obtain Eg(y)=0.81+0.036y+2.96y2 eV at 2 K. For Al compositions larger than 22% a type II staggered band lineup is observed. At this point the conduction band discontinuity disappears (ΔEc=0). The conduction band discontinuity as a function of the Al composition is ΔEc(y)=0.245−1.179y+0.3y2 eV.

InAsP islands at the lower interface of InGaAs/InP quantum wells grown by metalorganic chemical vapor deposition

The optical properties of narrow InGaAs/InP quantum wells and their dependence on the gas switching procedure during the metalorganic chemical vapor deposition growth process are studied. A transition from In1−xGaxAs monolayer to InAs1−xPx monolayer splitting is observed in the photoluminescence spectrum, if the AsH3 purging time of the InP surface is equal larger than 2 s. This transition is attributed to a P‐As exchange at the lower interface (InGaAs on InP) leading to InAs1−xPx interfacial islands, which are larger than the excitonic diameter.

STRAIN DISTRIBUTION IN INP/INGAAS SUPERLATTICE STRUCTURE DETERMINED BY HIGH RESOLUTION X-RAY DIFFRACTION

Interfacial strain distribution in a short period InP/InGaAs superlattice structure is evaluated by means of high resolution x‐ray diffraction. The diffraction pattern of the structure allows an unambiguous determination of interfacial strain distribution. From the numerical calculation, positively strained interfacial monolayers at the InP→InGaAs and negatively strained interfacial monolayers at the InGaAs→InP interfaces had to be introduced in order to reproduce the experimental data. At the InP→InGaAs interfaces a group V exchange reaction leading to a positively strained InAs or InAs1−xP interfacial layer is compatible with the simulation. At the InGaAs→InP interfaces negatively strained ternary or quaternary InGaAsyP1−y meet these requirements. The results are consistent with low temperature calorimetric absorption measurements which exhibit a wide band gap InGaAsP‐like absorption feature at 1.48 eV beyond the InP energy gap.

Carrier dynamics in staggered‐band lineup n‐InAlAs/n‐InP heterostructures

The temperature and time dependence of the spatially indirect recombination of two‐dimensional (2D) electrons and holes localized at adjacent sides of an isotype n‐InAlAs/n‐InP heterojunction having a staggered band lineup is investigated. With increasing temperature, a much weaker drop of the photoluminescence intensity than in quantum wells and 3D layers is observed. Still more surprising is the observed decrease of the decay time with increasing temperature from 3.8 ns at 6 K to 1.2 ns at 300 K. Both observations are consistently explained by an increasing occupation of higher subband levels of the interface potential well and an activation of new radiative and nonradiative recombination channels including Δn≠0 ones with increasing temperature.

Luminescence properties of semiconductor quantum dots

Abstract Semiconductor quantum dot (QD) heterostructures created using self-ordering phenomena on crystal surfaces exhibit luminescence properties predicted for zero-dimensional systems, e.g. ultrasharp luminescence lines up to high temperatures, giantly increased material gain and practically complete temperature insensitivity of the laser threshold current. Faster than expected exciton capture and energy relaxation processes manifest minor role of the so-called phonon bottleneck effect. Formation of QDs with properties satisfying device requirements became possible.

3D Arrays of Quantum Dots for Laser Applications

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during alternate short-period GaAs-InAs (InGaAs) depositions after InAs (or InGaAs) pyramids are formed on a GaAs (100). The resulting arrangement represents laterally ordered array of nanoscale structures inserted in a GaAs matrix, where each structure is composed of several vertically merging InAs (or InGaAs) parts. VECODs are introduced in the active region of GaAs-AlGaAs double heterostructure laser. The threshold current density remarkably decreases with increase in number of periods (N) of the VECOD (down to 90 A cm-2 at 300K for N=10). The differential efficiency increases with N and the lasing occurs through ground state of quantum dot exciton up to room temperature (λ=1.05 μm).