Quantum Dot Intermixing Research Articles

The 1200 nm‐Band InAs/GaAs Quantum Dot Intermixing by Dry Etching and Ion Implantation

In this article, the quantum dot intermixing (QDI) technique previously developed for 1550 nm‐band InAs/InAlGaAs QD is applied to 1200 nm‐band InAs/GaAs QD. Three methods of defect introduction for triggering the QDI are used such as inductively coupled plasma reactive ion etching (ICP‐RIE) (Ar+) and ion implantation (Ar+ and B+). As a result, about 80 nm photoluminescence (PL) peak wavelength shift is obtained for ICP‐RIE when annealing is performed at 575 °C, after etching down to 450 nm to the QD layer. On the contrary, about 110 nm PL peak wavelength shift is obtained for B+ ion implantation at an acceleration energy of 120 keV and a dose of 1.0 × 1014 cm−2 and subsequent annealing. Cross‐sectional image analyses by scanning transmission electron microscope (STEM) and energy‐dispersive X‐ray spectroscopy (EDX) clarified the modification of InAs QD structures by the QDI process.

Regional Bandgap Tailoring of 1550 nm‐Band InAs Quantum Dot Intermixing by Controlling Ion Implantation Depth

The regional control of the bandgap energies using the highly stacked quantum dot (QD) on InP(311)B substrate changing ion implantation depths in the process of the quantum dot intermixing (QDI) technique is investigated. The QDI process involves B+ implantation and rapid thermal annealing (RTA) around 600 °C, in which the ion implantation depths are controlled regionally with a combination of SiO2 and polymer (AZ) films. Controlled blue shifts of the photoluminescence (PL) spectra verify the effectiveness of the regionally controlled QDI process for application to semiconductor photonic integrated circuits using 1550 nm‐band QD such as integrated wavelength division multiplexing (WDM) light sources.

InAs/GaAs quantum-dot intermixing: comparison of various dielectric encapsulants

We report on the impurity-free vacancy-disordering effect in InAs/GaAs quantum-dot (QD) laser struc- ture based on seven dielectric capping layers. Compared to the typical SiO2 and Si3N4 films, HfO2 and SrTiO3 dielectric layers showed superior enhancement and suppression of intermixing up to 725°C, respectively. A QD peak ground-state differential blue shift of >175 nm (>148 meV) is obtained for HfO2 capped sample. Likewise, investigation of TiO2 ,A l2O3, and ZnO capping films showed unusual characteristics, such as intermixing-control caps at low annealing temperature (650°C) and interdiffusion-promoting caps at high temperatures (≥675°C). We qualitatively compared the degree of intermixing induced by these films by extracting the rate of intermixing and the temperature for ground-state and excited-state convergences. Based on our systematic characteriza- tion, we established reference intermixing processes based on seven different dielectric encapsulation materi- als. The tailored wavelength emission of ∼1060─1200 nm at room temperature and improved optical quality exhibited from intermixed QDs would serve as key materials for eventual realization of low-cost, compact, and agile lasers. Applications include solid-state laser pumping, optical communications, gas sensing, biomedi- cal imaging, green-yellow-orange coherent light generation, as well as addressing photonic integration via area- selective, and postgrowth bandgap engineering. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Intermixing of InP-based quantum dots and application to micro-ring resonator wavelength-selective filter for photonic integrated devices

In this study, we investigated quantum dot intermixing (QDI) for InAs/InGaAlAs highly stacked QDs on an InP(311)B substrate with low-temperature annealing at 650 °C in order to realize integrated photonic devices with QDs and passive waveguides. In particular, we adopted the method of introducing point defects by ICP-RIE to realize a blue shift of the PL peak wavelength by about 150 nm. Moreover, we successfully fabricated double micro-ring resonators by QDI. The output power contrasts of the devices were found to be 9.0 and 8.6 dB for TE and TM modes, respectively.

DIRECT IMPRINTING OF SiO2 WAVEGUIDE STRUCTURES ON GaAs AND ITS APPLICATION IN InAs/GaAs QUANTUM DOT INTERMIXING

A novel approach for intermixing of InAs / GaAs quantum dot (QD) structure by direct imprinting of SiO 2 strips using silicafilm is presented. The silicafilm SiO 2 strips were imprinted on a InAs / GaAs QD structure by an Obducat nanoimprinter using a polyethylene terepthalate soft mold. Inductively coupled plasma etching of GaAs waveguide structures using the imprinted SiO 2 strips as hard mask has been demonstrated. By using the silicafilm SiO 2 as a capping layer, the effects of impurity-free vacancy disordering intermixing on optical characteristics of the InAs / GaAs QD structure have been examined. The measured photoluminescence spectra of the InAs / GaAs QD structure suggest that differential wavelength blueshift of up to 105 nm for region with and without the SiO 2 film can be achieved. A cost effective one step SiO 2 hard mask with graded thickness profile for quantum well or QD intermixing by imprinting technique is proposed for application in broadband lasers and superluminescent diodes.

Selective disordering of InAs/InGaAs dots-in-a-well structure patterned with sol-gel derived SiO2 strips imprinted by soft mold technique

Selective impurity free vacancy disordering of InAs/InGaAs quantum dot (QD) structures imprinted with sol-gel derived SiO2 strips via a polyethylene terepthalate soft mold has been investigated. Wavelength blueshift of up to 220 nm for areas capped with the sol-gel derived SiO2 is demonstrated. The imprinted SiO2 strips were used as hard mask for plasma etching of GaAs ridge waveguide structures and were found to have similar hardness as the SiO2 prepared by plasma-enhanced chemical vapor deposition. QD intermixing using sol-gel derived SiO2 with step-thickness profile was demonstrated, and a one-step SiO2 imprinting technique for multiple band gap QD intermixing is proposed.

Nitrogen-induced intermixing of InAsN quantum dots with the GaAs matrix

We investigated the influence of nitrogen incorporation on the growth of InAsN∕GaAs quantum dots (QDs) using cross-sectional scanning tunneling microscopy. Nitrogen exposure during InAs growth leads to a rather strong dissolution and the formation of extended almost spherical InGaAs QDs with a very low nitrogen content. Nitrogen atoms are instead observed in the surrounding GaAs matrix, and indium atoms are even found underneath the nominal base plane of the QDs. These effects are related to a rather low solubility of nitrogen within InAs, leading to high strain between indium-rich QDs and the surrounding nitrogen-rich matrix.

Argon-plasma-induced InAs/InGaAs/InP quantum dot intermixing

We report the first study of argon (Ar)-plasma-enhanced intermixing of InAs/InGaAs/InPself-assembled quantum dots (QDs) in an inductively coupled plasma reactive ionetch system. The Ar-plasma exposure creates point defects, which propagateinto the QD structure and enhance the intermixing between the QDs and theirbarrier layers, hence tuning the energy bandgap of the QDs. By optimizing theplasma exposure time and the annealing temperature, we observe (i) a blueshift of160 nm and an increase in the photoluminescence (PL) intensity of the QD samplesimmediately after Ar-plasma exposure for 90 s, and (ii) a further increase in theblueshift of 330 nm, accompanied by 2.5-times increase in the PL intensity and37 nm narrowing in the PL linewidth after subsequent rapid thermal annealing at720 °C. The ability to generate a large blueshift without degrading the material quality showsthat Ar-plasma exposure is an efficient post-growth technique for tuning the energybandgap of QD structures.

Intermixing of InGaAs quantum dots grown by cycled monolayer deposition

We investigate the thermal induced intermixing and the diffusion kinetics of InGaAs quantum dots grown by cycled monolayer deposition subjected to the overgrowth and ex situ annealing. The group-III intermixing, that obeys the Fickian law, reaches a steady state after ex situ annealing up to 850°C. An identical activation energy (Ea=1.5±0.3eV) obtained with and without subjecting to overgrowth implies that the intermixing is primarily governed by the dynamic annealing of intrinsic defects during the epitaxial overgrowth. The intrinsic intermixing is attributed to the instability of the interface morphology driven by the atomic migration during self-formation of quantum dot.

Proton implantation-induced intermixing of InAs∕InP quantum dots

Proton implantation-induced intermixing of InAs quantum dots (QDs) capped with InP, GaInAsP, and InP and InGaAs layers grown by metal-organic chemical vapor deposition is investigated. The samples are annealed at 750, 800, 850, and 900°C for 30s and thermal stability of the QDs is studied. The optimum annealing temperature is around 800°C which gives maximum implantation-induced energy shift. The QDs capped with InP layers show the highest implantation-induced energy shift due to strong group V interdiffusion whereas the QDs grown on and capped with GaInAsP layers show the least implantation-induced energy shift due to weak group V and group III interdiffusion. The QDs capped with InP and InGaAs layers show intermediate implantation-induced energy shift and are less thermally stable compared to the QDs grown on and capped with GaInAsP layers. The QDs capped with InP layers show enhanced photoluminescence (PL) intensity when implanted with lower proton dose (less than 5×1014ions∕cm2). On the other hand higher proton dose (more than 1×1014ions∕cm2) reduces the PL linewidth in all samples.

Neutral ion-implantation-induced selective quantum-dot intermixing

High spatial band-gap tuning has been observed from an InGaAs∕GaAs quantum-dot (QD) structure implanted with electrically neutral species, As and P ions, at 200°C followed by a rapid thermal annealing. Phosphorous was found to be a more effective species to induce QD intermixing than the As at similar dose level. A blueshift as large as 126meV has been observed from the P+-implanted and intermixed sample, while only ∼14meV has been measured from the SixNy-capped sample.

The Definition of Multiple Bandgaps in Quantum-Dot Material by Intermixing

ABSTRACTWe present the recent progress in the intermixing of InGaAs/GaAs quantum dot (QD) material. Quantum dot intermixing (QDI) allows the tuning of the energy bandgap in selected areas of the wafer or optoelectronic device, thus modifying its emission or absorption properties, in much the same way as in quantum well (QW) material. QDI has recently received increasing interest, as it combines bandgap engineering with the predicted advantages that quantum dots offer, such as low temperature-sensitivity of threshold current, high modulation frequency and low chirp.We have applied the dielectric-cap-based techniques that were originally developed for QW structures, to the intermixing of InGaAs/GaAs/AlGaAs QD material with an emission wavelength of 1280 nm. Intermixing was achieved by sputtering a QDI-enhancing cap in some areas, and a QDI-suppressing cap in other areas, followed by a high-temperature anneal cycle. Extremely large bandgap blue-shifts of up to 280 nm have been obtained with an anneal temperature of 800 °C. The shifts were inferred from the photoluminescence (PL) spectra measured at 77 K under red-laser excitation.To be of use in many applications, QDI must be able to provide a multiplicity of bandgaps on a single substrate. Multiple bandgaps can be created by varying the thickness of the QDI-enhancing cap, repeating the anneal cycle several times, or varying the coverage density of QDI-enhancing features over that of QDI-suppressing ones. The latter approach, termed selective intermixing in selected areas (SISA), involves the deposition of QDI-enhancing patterns of various area fill factors, which, upon annealing, will cause different degrees of intermixing in the underlying regions.To demonstrate the SISA process in the QD material, we defined patterns containing lines and squares of various sizes (3 - 100 μm) and area fill factors (5% - 95%). The wafer was then annealed at 725 °C for 1 minute. As expected, the observed bandgap shifts were commensurate with the fill factor, with a 5% coverage providing a minimum shift (0 - 10 nm) and ∼40% a maximum one (∼ 120 nm). At fill factors above 40%, the shifts appeared to saturate and even decrease slightly. The effect of the feature size and shape was not very pronounced, with smaller features generating somewhat larger shifts. This may be due to the fabrication-related size bias that will have the strongest effect on the fill factor of smaller features. The PL spectra measured from patterns of large-size features (20 μm or more) often had a lopsided shape and broader peak width, which may be attributed to the limited spatial resolution of the measurement probe.

Enhancement of room-temperature photoluminescence in InAs quantum dots

We report pronounced enhancement of room-temperature photoluminescence up to 80-fold induced by proton implantation and the rapid thermal annealing process in a multilayer InAs/GaAs quantum-dot structure. This effect is studied by a combination of material methods and resulted from both proton passivation and carrier capture enhancement effects. The maximum photoluminescence peak shift is about 23 meV, resulting from the intermixing of quantum dots. Linear dependence behavior as observed for both the nonradiative recombination time and carrier relaxation time on the ion-implantation dose. Maximum enhancement of the photoluminescence is observed for a proton implantation dose of 1.0×1014 cm−2 followed by rapid thermal annealing at 700 °C. These effects will be useful for quantum dot optoelectronic devices.

Proton-irradiation-induced intermixing of InGaAs quantum dots

Proton irradiation was used to create interdiffusion in In0.5Ga0.5As quantum dots (QDs), grown by low-pressure metalorganic chemical vapor deposition. After 25-keV proton irradiation, the QD samples were annealed at two temperatures (700 or 750 °C) for 30 s. It was found that much lower annealing temperatures were needed to recover the photoluminescence signals than in the quantum-well case. Large blueshifts (120 meV) and narrowing of the photoluminescence spectra were seen. Various doses (5×1013–1×1015 cm−2) and implant temperatures (20–200 °C) were used to study the interdiffusion processes in these samples. In QD samples, much lower doses were required to achieve similar energy shifts than reported in quantum-well samples.

InAs/GaAs quantum dot intermixing induced by proton implantation

We have investigated the intermixing effect of multilayer self-assembled InAs/GaAs quantum dots on photoluminescence (PL) spectra. Proton implantation combined with rapid thermal annealing is used to induce intermixing at the interface of InAs and GaAs. Intermixing results in a change of both the optical transition energy and the linewidth of the PL emission peaks. A blueshift up to 94.3 meV is obtained in the PL emission peaks. Our results show that proton implantation is an efficient method to tune the electronic states in self-assembled InAs/GaAs quantum dots.

Laser-induced InAs/GaAs quantum dot intermixing

Laser annealing of InAs/GaAs quantum-dot (QD) microstructures has been investigated for selective area tuning of their electronic shell structure. Extensive blueshifts of the QD excited states were observed following 20–40 s laser irradiation. In the most extreme case, we were able to shift the position of the ground state transition by 298 meV, i.e., to the spectral region where the photoluminescence signal originates from the as-grown InAs wetting layer. A reduction from ∼50 to 8 meV of the full width at half maximum of the PL peak corresponding to this transition indicates a drastic change in the structural characteristics of the investigated QD ensemble. The attractive feature of the laser-QD-intermixing technique is that it offers the possibility of obtaining targeted blueshifts and inter-sublevel energy spacing on the lateral scale required in the fabrication of QD-based integrated optoelectronic devices and, possibly, photonic band gap crystals.

Selective control of self-organized In0.5Ga0.5As/GaAs quantum dot properties: Quantum dot intermixing

Selective postgrowth control of the photoluminescence (PL) wavelength has been demonstrated for a single layer self-organized In0.5Ga0.5As/GaAs quantum dot (QD) structure. This was achieved by rapid thermal processing of dots using different dielectric caps. Selective band gap shifts of over 100 meV were obtained between samples capped with sputtered and plasma enhanced silica deposition, with the band gap shift under regions covered with plasma enhanced chemical vapor deposition SiO2 less than 70 meV. The effects of different caps on the PL linewidth were also observed. The differential band gap shift will enable the integration of passive and active devices in QD systems.

Strain-induced material intermixing of InAs quantum dots in GaAs

A systematic investigation of the stacking behavior of InAs quantum dots (QDs) with varying GaAs interlayer thickness d is presented. We compare two-fold stacks of large QDs (≈25 nm base width), which emit at 1.30 μm, to small QDs (≈20 nm base width) emitting at 1.14 μm. For large islands photoluminescence yields an energetic blueshift of the second layer islands with decreasing d, although transmission electron microscopy clearly reveals a ≈70% larger dot size in the second layer, whereas for small islands a similar size increase of the dots in the upper layer and an energetic redshift are observed. A detailed analysis of confinement and material intermixing effects suggests that for large QDs strain driven material intermixing is dominant. For small QDs the confinement effect plays the major role and causes the observed photoluminescence energy redshifts.

Engineering quantum-dot lasers

We discuss recent progress in the engineering of quantum-dot (QD) lasers, focusing on the spectral output, dynamics and techniques for integration. Two approaches to such engineering are discussed. Firstly, it is suggested that control of lasing spectra in QD lasers is possible by making use of waveguiding-related phenomena (substrate leakage and reflection) which, in unoptimised laser structures, result in the mode grouping effect (quasiperiodic spectral modulation). Secondly, first experimental studies of quantum dot intermixing are reported, suggesting that this technique is capable of both improving the performance of active QD media and integrating active QD sections with passive waveguides.

In 0.5 Ga 0.5 As quantum dot intermixing and evaporation in GaAs capping layer growth

Our study of GaAs growth over self-assembled In0.5Ga0.5As quantum dots grown by metalorganic vapor-phase epitaxy showed that GaAs capping layer surface morphology at the onset strongly depended on temperature. Incompletely capped In0.5Ga0.5As islands were elongated toward [110], indicating anisotropy in intermixing. During higher-temperature growth interruption, islands show craters in quantum dot centers. Craters become hexagonal holes whose depth matches GaAs capping layer thickness. Postannealing photoluminescence spectra show no peak corresponding to overly large quantum dot radiation, indicating that growth interruption after capping layer formation at a certain thickness eliminates overly large quantum dots.