Tj Tom Eijkemans

Formation of InAs quantum dot arrays on GaAs (100) by self-organized anisotropic strain engineering of a (In,Ga)As superlattice template

We demonstrate the formation of well-defined InAs quantum dot (QD) arrays by self-organized engineering of anisotropic strain in a (In,Ga)As/GaAs superlattice (SL). Due to the accumulation and improvement of the uniformity of the strain-field modulation along [011], formation of InAs QD arrays along [0-11] with 140 nm lateral periodicity is clearly observed on the SL template when the number of SL periods is larger than ten. By enhancing the In adatom surface migration length at low growth rates, clear arrays of single InAs QDs are obtained. The QD arrays exhibit strong photoluminescence efficiency that is not reduced compared to that from InAs QD layers on GaAs. Hence, ordering by self-organized anisotropic strain engineering maintains the high structural quality of InAs QDs.

Wavelength tuning of InAs quantum dots grown on InP (100) by chemical-beam epitaxy

We report on an effective way to continuously tune the emission wavelength of InAs quantum dots (QDs) grown on InP (100) by chemical-beam epitaxy. The InAs QD layer is embedded in a GaInAsP layer lattice matched to InP. With an ultrathin GaAs layer inserted between the InAs QD layer and the GaInAsP buffer, the peak wavelength from the InAs QDs can be continuously tuned from above 1.6 μm down to 1.5 μm at room temperature. The major role of the thin GaAs layer is to greatly suppress the As/P exchange during the deposition of InAs and subsequent growth interruption under arsenic flux, as well as to consume the segregated surface In layer floating on the GaInAsP buffer layer.

Lasing of wavelength-tunable (1.55μm region) InAs∕InGaAsP∕InP (100) quantum dots grown by metal organic vapor-phase epitaxy

The authors report lasing of InAs∕InGaAsP∕InP (100) quantum dots (QDs) wavelength tuned into the 1.55μm telecom region. Wavelength control of the InAs QDs in an InGaAsP∕InP waveguide is based on the suppression of As∕P exchange through ultrathin GaAs interlayers. The narrow ridge-waveguide QD lasers operate in continuous wave mode at room temperature on the QD ground state transition. The low threshold current density of 580A∕cm2 and low transparency current density of 6A∕cm2 per QD layer, measured in pulsed mode, are accompanied by low loss and high gain with an 80-nm-wide gain spectrum.

Large redshift in photoluminescence of p-doped InP nanowires induced by Fermi-level pinning

We have studied the effect of impurity doping on the optical properties of indium phosphide (InP) nanowires. Photoluminescence measurements have been performed on individual nanowires at low temperatures (5–70 K) and at low excitation intensities (0.5–10W∕cm2). We show that the observed redshift (200 meV) and the linewidth (70 meV) of the emission of p-type InP wires are a result of a built-in electric field in the nanowires. This bandbending is induced by Fermi-level pinning at the nanowire surface. Upon increasing the excitation intensity, the typical emission from these p-InP wires blueshifts with 70meV∕decade, due to a reduction of the bandbending induced by an increase in the carrier concentration. For intrinsic and n-type nanowires, we found several impurity-related emission lines.

Wavelength-tunable (1.55‐μm region) InAs quantum dots in InGaAsP∕InP (100) grown by metal-organic vapor-phase epitaxy

Growth of wavelength-tunable InAs quantum dots (QDs) embedded in a lattice-matched InGaAsP matrix on InP (100) substrates by metal-organic vapor-phase epitaxy is demonstrated. As∕P exchange plays an important role in determining QD size and emission wavelength. The As∕P exchange reaction is suppressed by decreasing the QD growth temperature and the V∕III flow ratio, reducing the QD size and emission wavelength. The As∕P exchange reaction and QD emission wavelength are then reproducibly controlled by the thickness of an ultrathin [zero to two monolayers (MLs)] GaAs interlayer underneath the QDs. An extended interruption after GaAs interlayer growth is essential to obtain well-defined InAs QDs. Submonolayer GaAs coverages result in a shape transition from QD to quantum dash at low V∕III flow ratio with a slightly shorter emission wavelength. Only the combination of reduced growth temperature and V∕III flow ratio with the insertion of GaAs interlayers above ML thicknesses allows wavelength tuning of QDs at r...

Direct imaging of self-organized anisotropic strain engineering for improved one-dimensional ordering of (In,Ga)As quantum dot arrays

Single (In,Ga)As quantum dot (QD) arrays are formed on GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs quantum wire (QWR) superlattice (SL) template in molecular beam epitaxy. The crucial steps in QWR template evolution, i.e., elongated QD formation at elevated temperature, thin GaAs capping, annealing, and stacking, are directly imaged by atomic force microscopy (AFM). AFM reveals a very smooth connection of the QDs into QWRs upon annealing. In addition, AFM shows the presence of height and width fluctuations of the QWRs with a significant number of bends and branches. These are attributed to excess strain accumulation during formation of the QWR template. By reducing the amount of (In,Ga)As and increasing the GaAs separation layer thickness in each SL period, a dramatic improvement of the uniformity of the QWR template is achieved. On the improved QWR template, well-defined one-dimensional single (In,Ga)As QD arrays are formed which are straight over more than...

Self Assembled InAs/InP Quantum Dots for Telecom Applications in the 1.55 µm Wavelength Range: Wavelength Tuning, Stacking, Polarization Control, and Lasing

Wavelength-tunable InAs quantum dots (QDs) embedded in lattice-matched InGaAsP on InP(100) substrates are grown by metalorganic vapor-phase epitaxy (MOVPE). As/P exchange, which causes a QD size and an emission wavelength that are very large, is suppressed by decreasing the QD growth temperature and V–III flow ratio. As/P exchange, QD size and emission wavelength are then reproducibly controlled by the thickness of ultrathin [0–2 monolayers (ML)] GaAs interlayers underneath the QDs. Submonolayer GaAs coverages result in a shape transition from QDs to quantum dashes for a low V–III flow ratio. It is the combination of reduced growth temperature and V–III flow ratio with the insertion of GaAs interlayers of greater than 1 ML thickness which allows the tuning of the emission wavelength of QDs at room temperature in the 1.55 µm wavelength range. Temperature-dependent photoluminescence (PL) measurements reveal the excellent optical properties of the QDs. Widely stacked QD layers are reproduced with identical PL emission to increase the active volume while closely stacked QD layers reveal a systematic PL redshift and linewidth reduction due to vertical electronic coupling, which is proven by the fact that the linear polarization of the cleaved-side PL changes from in-plane to isotropic. Ridge-waveguide laser diodes with stacked QD layers for their active regions exhibit threshold currents at room temperature in continuous-wave mode that are among the lowest threshold currents achieved for InAs/InP QD lasers operating in the 1.55 µm wavelength range.

Stacking and polarization control of wavelength-tunable (1.55 μm region) InAs / InGaAsP / InP (100) quantum dots

Stacking and polarization control of wavelength-tunable InAs quantum dots (QDs) embedded in lattice-matched InGaAsP on InP (100) grown by metalorganic vapor-phase epitaxy is demonstrated. Wavelength control over the 1.55μm region at room temperature is achieved by inserting ultrathin GaAs interlayers underneath the QDs and adjusting the amount of InAs. For widely stacked QDs with a 40nm separation layer, the linear dependence of the emission wavelength on the GaAs interlayer thickness coincides with that of single QD layers revealing the reproduction of identical QD layers. For closely stacked QDs with 4nm separation layer, the emission wavelength as a function of the GaAs interlayer thickness is systematically redshifted and the linewidth is reduced indicating vertical electronic coupling which is proven by the linear polarization of the cleaved-side luminescence changing from in-plane to isotropic.

Formation of columnar (In,Ga)As quantum dots on GaAs(100)

Columnar (In,Ga)As quantum dots (QDs) with homogeneous composition and shape in the growth direction are realized by molecular-beam epitaxy on GaAs(100) substrates. The columnar (In,Ga)As QDs are formed on InAs seed QDs by alternating deposition of thin GaAs intermediate layers and monolayers of InAs with extended growth interruptions after each layer. The height of the columnar (In,Ga)As QDs is controlled by varying the number of stacked GaAs/InAs layers. The structural and optical properties are studied by cross-sectional scanning tunneling microscopy, atomic force microscopy, and photoluminescence spectroscopy. With increase of the aspect ratio of the columnar QDs, the emission wavelength is redshifted and the linewidth is reduced.

Complex quantum dot arrays formed by combination of self-organized anisotropic strain engineering and step engineering on shallow patterned substrates

One-dimensional (In,Ga)As quantum dot (QD) arrays are created on planar singular, vicinal, and shallow mesa-patterned GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As∕GaAs quantum wire (QWR) superlattice template in molecular beam epitaxy. On planar singular substrates, highly uniform single QD arrays along [0−11] are formed. On shallow [0−11] and [011] stripe-patterned substrates, the generated type-A and -B steps distinctly affect the surface migration processes which are crucial for QWR template development, i.e., strain-gradient-driven In adatom migration along [011] and surface-reconstruction-induced Ga∕In adatom migration along [0−11]. In the presence of both type-A and -B steps on vicinal substrates misoriented towards [101], the direction of adatom migration is altered to rotate the QD arrays. This establishes the relationship between self-organized anisotropic strain and step engineering, which is exploited on shallow zigzag-patterned substrates for the reali...