Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

Photoluminescence of InAs quantum dots embedded in graded InGaAs barriers

The effects of the top barrier and the dot density on photoluminescence (PL) of the InAs quantum dots (QDs) sandwiched by the graded In_xGa_1−xAs barriers grown by metal-organic vapor phase epitaxy (MOVPE) have been studied. Two emission peaks corresponding to the ground state and the 1st excited state transitions of the QD structures have been observed, which matches well to the theoretical calculation. The PL emission linewidth and intensity of the InAs QDs structure are improved by reducing the Indium/Gallium composition variation of the graded In_xGa_1−xAs top barrier layer of the structure. The QDs’ ground states filling excitation power depends on the crystal quality of the InGaAs barrier layer and the QD density. The extracted thermal activation energy for the QDs’ PL emission is sensitive to the QD size.     

Tailoring of the Wave Function Overlaps and the Carrier Lifetimes in InAs/GaAs1−xSbx Type-II Quantum Dots

Effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs_1−xSb_x layer are investigated by photoluminescence (PL) and time-resolved PL measurements. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAs_1−xSb_x layer. We find that the type-II QD structure can sustain thermal annealing up to 850 °C. In particular, we find that it is possible to manipulate between type-I and type-II recombinations in annealed QDs by using different excitation powers. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAs_1−xSb_x type-II QDs.     

InAs/AlAs quantum dots with InGaAs insertion layer: dependence of the indium composition and the thickness

We have systematically studied the effect of an In_xGa_1−xAs insertion layer (IL) on the optical and structural properties of InAs quantum dot (QD) structures. A high density of 9.6×10¹⁰ cm⁻² of InAs QDs with an In_0.3Ga_0.7As IL has been achieved on a GaAs (1 0 0) substrate by metal organic chemical vapor deposition. A photoluminescence line width of 25 meV from these QDs has been obtained. We attribute the high density and high uniformity of these QDs to the use of the IL. Our results show that the InGaAs IL is useful for obtaining high-quality InAs QD structures for devices with a 1.3 μm operation.     

Dependence on temperature of homogeneous broadening of InGaAs/InAs/GaAs quantum dot fundamental transitions

We report systematic temperature-dependent measurements of photoluminescence spectra in self-assembled InGaAs/InAs/GaAs quantum dots (QDs). We have studied the rise in temperature of the ground-state homogeneous linewidth.A theoretical model is presented and accounts for the phonon-assisted broadening of this transition in individual QD. We have estimated the homogeneous linewidth of an individual QD from PL spectra of self-organized InAs/GaAs QDs by isolating the PL of each individual QD and fitting the narrow line associated with self-organized QDs through a Lorentzian convoluted by a Gaussian. We have observed a strong exciton–LO–phonon coupling (γ_LO) which becomes the dominating contribution to the linewidth above the temperature of 45 K. We have also derived the activation energy (ΔE) of the exciton–LO–phonon coupling, zero temperature linewidth (Γ₀) and the exciton-LA-phonon coupling parameter (γ_Ac). We report that our values are close to the values found in the literature for single InGaAs QD and InAs QD.     

Effect of deposition period on structural and optical properties of InGaAs/GaAs quantum dots formed by InAs/GaAs short-period superlattices

Structural and optical properties of In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown at 510 °C by atomic layer molecular beam epitaxy technique are studied as a function of n repeated deposition of 1-ML-thick InAs and 1-ML-thick GaAs. Cross-sectional images reveal that the QDs are formed by single large QDs rather than closely stacked InAs QDs and their shape is trapezoidal. In the image, existence of wetting layers is not clear. In 300 K-photoluminescence (PL) spectra of InGaAs QDs (n=5), 4 peaks are resolved. Origin of each peak transition is discussed. Finally, it was found that the PL linewidths of atomic layer epitaxy (ALE) QDs were weakly sensitive to cryostat temperatures (16–300 K). This is attributed to the nature of ALE QDs; higher uniformity and weaker wetting effect compared to SK QDs.     

Antimonide type-II “W” lasers: growth studies and guided-mode leakage into substrate

The lasing characteristics of mid-IR type-II “W” [InAs/GaInSb/InAs/AlAsSb] structures are found to correlate strongly with the growth conditions and low-temperature photoluminescence (PL) properties. The highest PL intensities and narrowest PL lines are obtained when the wafers are grown at ≈480–510°C with mixed interface bonds. A number of structures grown at a non-optimal lower temperature (≈425°C) nonetheless yielded lower lasing thresholds, lower internal losses, and longer Shockley-Read lifetimes than any grown previously on the present Riber 32P MBE system. All of the laser spectra display regularly-spaced multiple peaks that are consistent with periodic modulation of the cavity loss due to mode-leakage into the GaSb substrate.     

Exciton capture and thermal escape in InAs dot-in-a-well laser structures

The photoluminescence (PL), its temperature and power dependences have been studied in InAs quantum dots (QDs) embedded in asymmetric In_xGa_1?y As/GaAs quantum wells (QWs) with variable In_xGa_1?x As compositions in the capping layer. Three stages for thermally activated decay of QD PL intensity have been revealed. A set of rate equations for exciton dynamics (relaxation into QWs and QDs, and thermal escape) are solved to analyze the mechanism of PL thermal decay. The variety of PL intensities and peak positions, as well as the activation energies of PL intensity decay in DWELL structures with different compositions of a capping layer are discussed.     

The emission wavelength tuning of InAs/InP quantum dots with thin GaAs, InGaAs, InP capping layers by MOCVD

InAs quantum dots (QDs) were grown on InP substrates by metalorganic chemical vapor deposition. The width and height of the dots were 50 and 5.8 nm, respectively on the average and an areal density of 3.0×10¹⁰ cm⁻² was observed by atomic force microscopy before the capping process. The influences of GaAs, In_0.53Ga_0.47As, and InP capping layers (5–10 ML thickness) on the InAs/InP QDs were studied. Insertion of a thin GaAs capping layer on the QDs led to a blue shift of up to 146 meV of the photoluminescence (PL) peak and an InGaAs capping layer on the QDs led to a red shift of 64 meV relative to the case when a conventional InP capping layer was used. We were able to tune the emission wavelength of the InAs QDs from 1.43 to 1.89 μm by using the GaAs and InGaAs capping layers. In addition, the full-width at half-maximum of the PL peak decreased from 79 to 26 meV by inserting a 7.5 ML GaAs layer. It is believed that this technique is useful in tailoring the optical properties of the InAs QDs at mid-infrared regime.     

Optical transitions and carrier dynamics in self-organized InAs quantum dots grown on In0.52Al0.48As/InP(0 0 1)

Optical transitions in self-organized InAs quantum dots (QDs) grown on In_0.52Al_0.48As layer lattice matched to InP(0 0 1) substrate, have been studied by continuous wave (cw) photoluminescence (PL) and time-resolved PL. The dependence of the PL transition on excitation power and photoluminescence excitation measurements clearly shows that the multi-component cw-PL spectrum is related to emission coming from ground and related excited states of QDs with heights varying by monolayer fluctuations. While decay times measured by time-resolved PL are in the nanosecond range for the ground states, shorter decay times related to relaxation of carriers down directly to the ground state are determined for the excited states.     

High-Throughput Robotic Synthesis and Photoluminescence Characterization of Aqueous Multinary Copper–Silver Indium Chalcogenide Quantum Dots

The feasibility of a high-throughput robot-assisted synthesis of complex Cu_1-xAg_xInS_ySe_1-x (CAISSe) quantum dots (QDs) by spontaneous alloying of aqueous glutathione-capped Ag–In–S, Cu–In–S, Ag–In–Se, and Cu–In–Se QDs is demonstrated. Both colloidal and thin-film core CAISSe and core/shell CAISSe/ZnS QDs are produced and studied by high-throughput semiautomated photoluminescence (PL) spectroscopy. The silver-copper-mixed QDs reveal clear evidence of a band bowing effect in the PL spectra and higher average PL lifetimes compared to the counterparts containing silver or copper only. The photophysical analysis of CAISSe and CAISSe/ZnS QDs indicates a composition-dependent character of the nonradiative recombination in QDs. The rate of this process is found to be lower for mixed copper-silver-based QDs compared to Cu- or Ag-only QDs. The combination of the band bowing effect and the suppressed nonradiative recombination of CAISSe QDs is beneficial for their applications in photovoltaics and photochemistry. The synergy of high-throughput robotic synthesis and a high-throughput characterization in this study is expected to grow into a self-learning synthetic platform for the production of metal chalcogenide QDs for light-harvesting, light-sensing, and light-emitting applications.     

Materials and device properties of pseudomorphic In x Ga1−x As/Al0.3Ga0.7As/GaAs high electron mobility transistors (0<x<0.5)

Modulation doped Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with a constant In _x Ga_1–x As quantum well width was investigated as a function of InAs mole fraction. If the In _x Ga_1–x As quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided -doped structures was obtained for the structures with -doping on both sides of the In _x Ga_1–x As quantum well. Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0–0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25–0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 m a f _T of 115 GHz was measured.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Mn-including InAs quantum dots fabricated by Mn implantation

Mn-including InAs quantum dots (QDs) were fabricated by Mn-ion implantation and subsequent annealing. The optical, compositional, and structural properties of the treated samples were analyzed by photoluminescence (PL) and microscopy. Energy dispersive X-ray (EDX) results indicate that Mn ions diffused from the bulk GaAs into the InAs QDs during annealing, and the diffusion appears to be driven by the strain in the InAs QDs. The temperature dependence of the PL of Mn-including InAs QD samples exhibits QDs PL characteristics. At the same time, the heavy Mn-including InAs QD samples have ferromagnetic properties and high T_c.     

不同组分InxGa1-xAs(0≤x≤0.3)覆盖层对自组织InAs量子点的影响

研究了不同In组分的In_xGa_1-xAs(0≤x≤0.3)覆盖层对自组织InAs量子点的结构及发光特性的影响.透射电子显微镜和原子力显微镜表明,InAs量子点在InGaAs做盖层时所受应力较GaAs盖层时有所减小,并且x＝0.3时,InGaAs在InAs量子点上继续成岛.随x值的增大,量子点的光荧光峰红移,但随温度的变化发光峰峰位变化不明显.理论分析表明InAs量子点所受应力及其均匀性的变化分别是导致上述现象的主要原因. 关键词： 量子点 盖层 应力 红移     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

Study of InAs/GaAs quantum dots grown by MOVPE under the safer growth conditions

InAs quantum dots (QDs) have been formed on GaAs (001) substrate by metal-organic vapor phase epitaxy (MOVPE) under the safer growth conditions: using tertiarybutylarsine (TBA) to replace AsH₃ as the arsenic source and replacing hydrogen by pure nitrogen as the carrier gas. Effects of growth conditions on the QD formation have been investigated. It is observed that the wetting layer is stabilized with some material being transferred to form the QDs due to the strain relaxation process during the QD formation. Dot size dispersion becomes broader when the post-growth interruption is more than 20 s. Compared with normal one-step grown QDs, dot density increases greatly by 213% after employing two-step deposition for QD growth. This is explained by considering the indium-flux-dependent nucleation density at step 1 and kinetically self-limiting growth at step 2. The two photoluminescence (PL) emission peaks, 1.203 μm and 1.094 μm, from the two-step grown QDs are attributed to E1–HH1 and E1–LH1 transitions of the QDs, respectively. The measured results agree well with those received by an 8 k·p theoretical calculation. The narrow PL linewidth of ~50 nm shows high quality of the QDs. This paves the way to develop safer MOVPE process, using TBA/N₂ instead of AsH₃/H₂, to grow QDs for device application.     

Photoluminescence scanning on InAs/InGaAs quantum dot structures

The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of In_xGa_1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm²) as the excitation source. The structures with x = 0.15 In/Ga composition in the In_xGa_1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well.     

Polarization field effects on the recombination dynamics in low-In-content InGaN multi-quantum wells

The interplay of polarization fields and free carrier screening in In_xGa_1−xN/GaN (0.03<x<0.07) multiple quantum wells is studied by combining photoluminescence (time-integrated and time-resolved) and cathodoluminescence studies, in an excitation density range from 10⁸ to 10¹² cm⁻² of generated e–h pairs. For such low In content, the quantum-confined Stark effect is verified to rule the recombination dynamics, while effects of carrier localization in potential fluctuations have a minor role. Efficient field screening is demonstrated in CL steady-state high-injection conditions and in PL time-resolved experiments at the maximum excitation density. Under recovered nearly flat band conditions, quantum confinement effects are revealed and a high and possibly composition-dependent bowing parameter is extrapolated. Information on radiative and non-radiative rates for carrier recombination in the wells is obtained, both from steady-state and from time-resolved experiments, modelling the carrier dynamics in the framework of a theoretical rate equation model, which calculates electronic states and recombination rates in the nanostructure by coupling complete self-consistent solutions of Schrödinger and Poisson equations.     

Competition in the carrier capture between InGaAs/AlGaAs quantum dots and deep point defects

The presence of an extrinsic photoluminescence (PL) band peaked at 1.356 eV at low temperature is observed, on a large number of self-assembled InAs and In_0.5Ga_0.5As quantum dot (QD) structures, when exciting just below the GaAs absorption edge. A detailed optical characterization allows us to attribute the 1.356 eV PL band to the radiative transition between the conduction band and the doubly ionized Cu _Ga acceptor in GaAs. A striking common feature is observed in all investigated samples, namely a resonant quenching of the QD-PL when exciting on the excited level of this deep defect. Moreover, the photoluminescence excitation (PLE) spectrum of the 1.356 eV emission turns out to be almost specular to the QD PLE. This correlation between the PL efficiency of the QDs and the Cu centers evidences a competition in the carrier capture arising from a resonant coupling between the excited level of the defect and the electronic states of the wetting layer on which the QDs nucleate. The estimated Cu concentration is compatible with a contamination during the epitaxial growth. Received 13 November 2001 / Received in final form 28 May 2002 Published online 19 July 2002     

Strain accumulation in InAs/In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As quantum dots

The effect of strain accumulation in the InAs/In _x Ga_1−x As quantum dots (QDs) system was studied in this work. It was found that strain in the In _x Ga_1−x As layer accumulation in the QD layer. This effect resulted in a dramatic reduction of growth mode transition thickness of the QD layer. For InAs/In_0.25Ga_0.75As QDs, critical thickness is measured to be as low as 1.08 ML. The experimental results in this work highlight the importance of strain accumulation in the design and fabrication of QD-based devices with metamorphic buffer layer involved.