Final Report Summary - NIRAMIDS (Extending the sensitivity of MOVPE grown pyramidal quantum dots into near infrared)
Semiconductor quantum dots have been at the centre of a significant research effort in the recent years. They are viewed as one of the important tools to address quantum information challenges, and overcome technological limitations in terms of device performances. Nevertheless, present technologies are far from having solved all the technical issues involved. The aim of this project was to develop new epitaxial, ordered, position controlled quantum InGaAsN dot structures by metalorganic vapour phase epitaxy (MOVPE) at Tyndall National Institute, a leading institute in the development of new growth capabilities. The emission wavelength of the QDs was intended to move towards infrared to meet the needs of the telecom-oriented devices.
Pyramidal QDs were pseudomorphically grown on (111)B-oriented GaAs and InP substrates. The substrate was prepatterned with 7.5 μm or 1 μm pitch tetrahedrons by standard photolithography and wet chemical etching techniques. Thanks to capillarity and anisotropic decomposition effects, a single QD forms at the centre of each pyramidal recess. The dot shape is determined simply by the self-limited profile of the underneath layers, their thickness and their composition. A complex ensemble of quantum structures (three lateral quantum wires and a vertical one, three lateral quantum wells and three vertical ones, one central and three corner quantum dots) can form due to capillarity and capillarity induced segregation effects. The growth was conducted on lithographically prepatterned substrates (GaAs and InP) which helps to overcome many of the current oddities and difficulties, while allowing to obtain semiconductor quantum dots suitable for telecom applications.
Two routes were investigated to shift the present emission wavelength towards IR: incorporation of the nitrogen impurity into the QD itself on samples grown on the GaAs substrates and the shift of the whole design into the InP matrix, allowing for higher indium content in the QD due to smaller lattice mismatch. The most important results came however from the optical characterization of the samples and opened a very new window of application opportunities to this system: namely, we demonstrated for the first time an entangled photon emission from the array of site controlled dots and identified the spectral features to the new type of nanostructure, which was never observed before.
The most significant results were presented in detail in the peer-review journals listed below. There are also two more manuscript undergoing the reviewing process at the moment, namely:
1. G. Juska, V. Dimastrodonato, T. H. Chung, A. Gocalinska and E. Pelucchi, “Entangled photon emission from site-controlled quantum dots”, submitted to PRB
2. G. Juska, V. Dimastrodonato, L. O. Mereni, T. H. Chung, A. Gocalinska and E. Pelucchi, B. Van Hattem, P. Corfdir, M. Ediger, and R. T. Phillips “Quantum dot nanostructure formation in inverted pyramidal recesses”, submitted to PRB and several others on various stages of preparation.
Here below, a brief summary of the relevant results:
Task 1: Growth of site controlled QDs.
• Incorporation for the first time of a significant (<0.3%) amount of nitrogen into site-controlled quantum dots, resulting in reproducibly narrow emission lines;
The first thick GaAs buffer layer is topped by a gradually variable composition AlxGa1-xAs (0.3 <=x <=0.75) layer and an etch stop film of Al0.75Ga0.25As which enables selective substrate removal during post-growth processing. Here, to enhance the efficiency of pyramidal QDs, on some samples, a new advanced backetching processing practice was applied, with a new gold bonding procedure replacing the usual wax application before substrate removal. Such apex-up geometry acts as a lens which helps to extract more light, enabling highly photon number sensitive measurements, such as correlations, efficient. Different sets of QDs were grown, changing QD thickness and growth temperatures, with optical properties demonstrating relevant nitrogen incorporation.
• Translating the design to the growth on InP substrates
Series of test growths were performed, resulting in optimization of the growth process and successful translation of the basic structural design into the new matrix. The most significant and crucial step turned out to be the reduction of the pyramidal recess size from the standard for GaAs 7.5 μm to 1 μm.
Task 2: Structural characterization.
A standarised protocol for characterizing the grown samples was introduced: all growths were investigated by top view Scanning Electron Microscopy (SEM) and cross sectional Atomic Force Microscopy (AFM). A method to exploit high resolution X-Ray diffraction was developed to assess the layers composition and quality, a novelty for the field.
Task 3: Optical characterization.
• Photoluminescence shift of the QD emission
The structural engineering of the QD (alloy composition – reaching up to x=0.65 in InxGa1-xAs) grown on GaAs matrix allowed for obtaining up to 1 µm emission. On some samples, the nitrogen incorporation resulted in photoluminescence red-shift of at least additional 50 meV. Initial studies of the QDs with high In concentration grown on small pitch pyramids on InP show photoluminescence features that might be identified as dot-like emission, but this will require additional work to clarify.
• Entangled photons emission from site-controlled quantum dots;
We showed that entangled photon emission can be detected from site-controlled In0.25Ga0.75As QDs designed in different ways, such as different QD thickness or shape. Various QD design allowed a coarse tuning of entangled photon emission in an overall range of ~80 meV. Surprisingly, we have found that surfactant effects used in entangled photon emitter fabrication achieved due to nitrogen incorporation are helpful, but not necessary. Also, the dispersion of excitonic patterns over the samples was studied. The QDs with dominant positive charging or neutral excitonic complexes, identified in this work by photon-correlation spectroscopy and theoretical analysis/fitting, appeared to be the only one practically useful. A strong negative charging of non-resonantly excited QDs was shown as the main, however, not fundamental limitation of the current QD system. An efficient solution of this problem was demonstrated by the use of dual wavelength excitation, potentially improving the effectiveness of obtaining a high density of good entangled photon emitters on chip.
• Identification of the new type of nanostructures
The presence of three additional corner quantum dots (CQDs) in the pyramidal nanostructure was identified by in-plane and top-view photoluminescence measurements and by magneto-photoluminescence experiments. We found that that these CQDs form in the lateral QWRs. They present a mean fine structure splitting of 13 µeV that arises presumably from the elongation of the CQD along the lateral QWR axis.
Task 4: Electronic Properties: Theoretical Modelling
To gain a detailed understanding of the electronic properties of the system, an eight band k.p model is was employed together with a continuum elasticity model to account for the influence of strain and piezoelectricity. The models were developed in collaboration with E. O’Reilly group, in the same institution (Tyndall). Both models were rotated analytically to account for the specific numerical problems of modelling (111)-oriented QDs (following the analysis described previously in “Symmetry-adapted calculations of strain and polarization fields in (111)-oriented zinc-blende quantum dots”, Phys. Rev. B 84, 125312, 2011 and with parameters for In(Ga)As in the calculation taken from “Impact of size, shape, and composition on piezoelectric effects and electronic properties of In(Ga)As/GaAs quantum dots” Phys. Rev. B 76, 205324, 2007).
Task 5: Growth modelling
The group has introduced a theoretical model which comprehensively reproduces the main experimentally observable phenomena during the MOVPE growth of pyramidal QDs and VQWRs: material composition and temperature dependence of the self-limiting profile and alloy segregation effects. The reaction-diffusion equations were formulated, accounting for the interplay between precursor decomposition, adatom diffusion and incorporation on the different crystallographic facets of the seeding template, and can be extended to study the morphological evolution of any patterned surface. These results pave the way toward a reproducible on-demand design of seeded low-dimensional nanostructures and represent solid foundations for the future development of quantum based technologies.
Pyramidal QDs were pseudomorphically grown on (111)B-oriented GaAs and InP substrates. The substrate was prepatterned with 7.5 μm or 1 μm pitch tetrahedrons by standard photolithography and wet chemical etching techniques. Thanks to capillarity and anisotropic decomposition effects, a single QD forms at the centre of each pyramidal recess. The dot shape is determined simply by the self-limited profile of the underneath layers, their thickness and their composition. A complex ensemble of quantum structures (three lateral quantum wires and a vertical one, three lateral quantum wells and three vertical ones, one central and three corner quantum dots) can form due to capillarity and capillarity induced segregation effects. The growth was conducted on lithographically prepatterned substrates (GaAs and InP) which helps to overcome many of the current oddities and difficulties, while allowing to obtain semiconductor quantum dots suitable for telecom applications.
Two routes were investigated to shift the present emission wavelength towards IR: incorporation of the nitrogen impurity into the QD itself on samples grown on the GaAs substrates and the shift of the whole design into the InP matrix, allowing for higher indium content in the QD due to smaller lattice mismatch. The most important results came however from the optical characterization of the samples and opened a very new window of application opportunities to this system: namely, we demonstrated for the first time an entangled photon emission from the array of site controlled dots and identified the spectral features to the new type of nanostructure, which was never observed before.
The most significant results were presented in detail in the peer-review journals listed below. There are also two more manuscript undergoing the reviewing process at the moment, namely:
1. G. Juska, V. Dimastrodonato, T. H. Chung, A. Gocalinska and E. Pelucchi, “Entangled photon emission from site-controlled quantum dots”, submitted to PRB
2. G. Juska, V. Dimastrodonato, L. O. Mereni, T. H. Chung, A. Gocalinska and E. Pelucchi, B. Van Hattem, P. Corfdir, M. Ediger, and R. T. Phillips “Quantum dot nanostructure formation in inverted pyramidal recesses”, submitted to PRB and several others on various stages of preparation.
Here below, a brief summary of the relevant results:
Task 1: Growth of site controlled QDs.
• Incorporation for the first time of a significant (<0.3%) amount of nitrogen into site-controlled quantum dots, resulting in reproducibly narrow emission lines;
The first thick GaAs buffer layer is topped by a gradually variable composition AlxGa1-xAs (0.3 <=x <=0.75) layer and an etch stop film of Al0.75Ga0.25As which enables selective substrate removal during post-growth processing. Here, to enhance the efficiency of pyramidal QDs, on some samples, a new advanced backetching processing practice was applied, with a new gold bonding procedure replacing the usual wax application before substrate removal. Such apex-up geometry acts as a lens which helps to extract more light, enabling highly photon number sensitive measurements, such as correlations, efficient. Different sets of QDs were grown, changing QD thickness and growth temperatures, with optical properties demonstrating relevant nitrogen incorporation.
• Translating the design to the growth on InP substrates
Series of test growths were performed, resulting in optimization of the growth process and successful translation of the basic structural design into the new matrix. The most significant and crucial step turned out to be the reduction of the pyramidal recess size from the standard for GaAs 7.5 μm to 1 μm.
Task 2: Structural characterization.
A standarised protocol for characterizing the grown samples was introduced: all growths were investigated by top view Scanning Electron Microscopy (SEM) and cross sectional Atomic Force Microscopy (AFM). A method to exploit high resolution X-Ray diffraction was developed to assess the layers composition and quality, a novelty for the field.
Task 3: Optical characterization.
• Photoluminescence shift of the QD emission
The structural engineering of the QD (alloy composition – reaching up to x=0.65 in InxGa1-xAs) grown on GaAs matrix allowed for obtaining up to 1 µm emission. On some samples, the nitrogen incorporation resulted in photoluminescence red-shift of at least additional 50 meV. Initial studies of the QDs with high In concentration grown on small pitch pyramids on InP show photoluminescence features that might be identified as dot-like emission, but this will require additional work to clarify.
• Entangled photons emission from site-controlled quantum dots;
We showed that entangled photon emission can be detected from site-controlled In0.25Ga0.75As QDs designed in different ways, such as different QD thickness or shape. Various QD design allowed a coarse tuning of entangled photon emission in an overall range of ~80 meV. Surprisingly, we have found that surfactant effects used in entangled photon emitter fabrication achieved due to nitrogen incorporation are helpful, but not necessary. Also, the dispersion of excitonic patterns over the samples was studied. The QDs with dominant positive charging or neutral excitonic complexes, identified in this work by photon-correlation spectroscopy and theoretical analysis/fitting, appeared to be the only one practically useful. A strong negative charging of non-resonantly excited QDs was shown as the main, however, not fundamental limitation of the current QD system. An efficient solution of this problem was demonstrated by the use of dual wavelength excitation, potentially improving the effectiveness of obtaining a high density of good entangled photon emitters on chip.
• Identification of the new type of nanostructures
The presence of three additional corner quantum dots (CQDs) in the pyramidal nanostructure was identified by in-plane and top-view photoluminescence measurements and by magneto-photoluminescence experiments. We found that that these CQDs form in the lateral QWRs. They present a mean fine structure splitting of 13 µeV that arises presumably from the elongation of the CQD along the lateral QWR axis.
Task 4: Electronic Properties: Theoretical Modelling
To gain a detailed understanding of the electronic properties of the system, an eight band k.p model is was employed together with a continuum elasticity model to account for the influence of strain and piezoelectricity. The models were developed in collaboration with E. O’Reilly group, in the same institution (Tyndall). Both models were rotated analytically to account for the specific numerical problems of modelling (111)-oriented QDs (following the analysis described previously in “Symmetry-adapted calculations of strain and polarization fields in (111)-oriented zinc-blende quantum dots”, Phys. Rev. B 84, 125312, 2011 and with parameters for In(Ga)As in the calculation taken from “Impact of size, shape, and composition on piezoelectric effects and electronic properties of In(Ga)As/GaAs quantum dots” Phys. Rev. B 76, 205324, 2007).
Task 5: Growth modelling
The group has introduced a theoretical model which comprehensively reproduces the main experimentally observable phenomena during the MOVPE growth of pyramidal QDs and VQWRs: material composition and temperature dependence of the self-limiting profile and alloy segregation effects. The reaction-diffusion equations were formulated, accounting for the interplay between precursor decomposition, adatom diffusion and incorporation on the different crystallographic facets of the seeding template, and can be extended to study the morphological evolution of any patterned surface. These results pave the way toward a reproducible on-demand design of seeded low-dimensional nanostructures and represent solid foundations for the future development of quantum based technologies.