Electrostatic built-in fields in wurtzite III-N nanostructures: impact of growth plane on second-order piezoelectricity

In this work we present a detailed analysis of the second-order piezoelectric effect in wurtzite III-N heterostructures, such as quantum wells and quantum dots, grown on different substrate orientations. Our analysis is based on a continuum model using a here derived analytic expression for the second-order piezoelectric polarization vector field as a function of the incline angle theta to the wurtzite c axis. This expression allows for a straightforward implementation in existing quantum well and quantum dot codes. Our calculations on III-N quantum well systems reveal that especially for semipolar structures with high incline angle values (55 degrees <= theta <= 80 degrees and 105 degrees <= theta <= 120 degrees), second-order piezoelectricity noticeably contributes to the overall electric built-in field. For instance, in an InGaN/GaN multiple quantum well system with 22% In, the electric field increases by approximately 20% due to second-order piezoelectricity. Overall, when including second-order piezoelectric effects in the calculation of electric fields in GaN/AlN and InGaN/GaN quantum well systems an improved agreement between our theory and experimental literature data is observed. When studying quantum dots, at least for the here considered model geometry and growth planes, we observe that for GaN/AlN structures second-order effects are of secondary importance. The situation is different for non-c-plane In0.2Ga0.8N/GaN quantum dots. For example, inside a nonpolar In0.2Ga0.8N/GaN dot the built-in potential arising from second-order piezoelectricity is comparable in magnitude to the built-in potential originating from spontaneous and first-order piezoelectric polarization, but opposite in sign. This feature leads to a change in the built-in potential profile both in and around the In0.2Ga0.8N/GaN quantum dot structure, which in general is relevant for electronic and optical properties of these systems

Electronic and optical properties of III-Nitride nanostructures

Quantum dots (QDs) based on gallium nitride (GaN), indium nitride (InN), aluminium nitride (AlN) and their respective alloys (e.g. InGaN, AlGaN) have attracted significant interest for “non-classical” light emitters such as single-photon or entangled-photon sources. This originates from the fact that these emitters form the cornerstone for quantum cryptography and quantum computing applications. Thanks to their large band offsets and exciton binding energies, when compared to “standard” III-V based systems (indium gallium arsenide), nitride QDs are attractive to realize non-classical light emission near room temperature. By utilizing InGaN QDs, in principle the emission wavelength regime of these emitters can be tailored; an important feature given that commercial single-photon detectors operate in the visible spectral range. However, a major drawback of conventional nitride-based QD systems originates from the "standard" growth along the polar c-axis of the underlying wurtzite crystal lattice, which results in very strong electrostatic built-in fields. These fields significantly affect the radiative recombination rate of these systems, consequently limiting their efficiency. In this thesis, in a first step, we have targeted the electronic and optical properties of InGaN QD structures grown along a so-called non-polar crystallographic direction. Such an approach allows to keep the benefits of the nitride system, e.g. large band offsets, but at the same time offering distinct new features such as significantly reduced electrostatic built-in fields. We have shown, in conjunction with experiment, that these non-polar InGaN QDs indeed exhibit much faster radiative carrier recombination when compared to a c-plane counterpart. Furthermore, we found here that fundamental changes in the underlying electronic structure, when compared to c-plane systems, lead to strongly linearly polarized light emission with a deterministic axis, from nonpolar InGaN QDs. This feature is of great interest for quantum cryptography applications. Additionally, we were able to show that this high degree of optical linear polarization survived up to temperatures of up to 200K and even beyond. Therefore, on-chip operating conditions are within reach. Our theoretical predictions are in excellent agreement with measurements carried out by our colleagues at the University of Oxford (UK) on structures grown at the University of Cambridge (UK). In addition to single-photon emission properties, we also studied the potential of InGaN based QDs for entangled-photon emission. Here, we demonstrated that InGaN QDs are in principle ideal candidates for such applications. Our theoretical studies, combining fully atomistic electronic structure calculations with many-body theory, show that while intrinsic properties of InGaN alloys forming the QD on the one hand side give significant advantages over conventional indium gallium arsenide emitters, these features, on the other hand, could present a major roadblock for polarization entanglement. Overall, in this work, we have shown that these nanostructures are promising systems for achieving next-generation non-classical light emitters required for quantum cryptography applications

Impact of second-order piezoelectricity on electronic and optical properties of c-plane InxGa12xN quantum dots: consequences for long wavelength emitters

In this work, we present a detailed analysis of the second-order piezoelectric effect in c-plane InxGa1xN/GaN quantum dots and its consequences for electronic and optical properties of these systems. Special attention is paid to the impact of increasing In content x on the results. We find that in general the second-order piezoelectric effect leads to an increase in the electrostatic built-in field. Furthermore, our results show that for an In content 30%, this increase in the built-in field has a significant effect on the emission wavelength and the radiative lifetimes. For instance, at 40% In, the radiative lifetime is more than doubled when taking second-order piezoelectricity into account. Overall, our calculations reveal that when designing and describing the electronic and optical properties of c-plane InxGa1xN/GaN quantum dot based light emitters with high In contents, second-order piezoelectric effects cannot be neglected

Exploring the potential of c-plane indium gallium nitride quantum dots for twin-photon emission

Nonclassical light emission, such as entangled and single-photon emission, has attracted significant interest because of its importance in future quantum technology applications. In this work, we study the potential of wurtzite (In,Ga)N/GaN quantum dots for novel nonclassical light emission, namely, twin-photon emission. Our calculations, based on a fully atomistic many-body framework, reveal that the combination of carrier localization due to random alloy fluctuations in the dot, spinâ orbit coupling effects, underlying wurtzite crystal structure, and built-in electric fields leads to an excitonic fine structure that is very different from that of more â conventionalâ zinc-blende (In,Ga)As dots, which have been used so far for twin photon emission. We show and discuss here that the four energetically lowest exciton states are all bright and emit linearly polarized light. Furthermore, three of these excitonic states are basically degenerate. All of these results are independent of the alloy microstructure. Also, our calculations reveal large exciton binding energies (>35 meV), which exceed the thermal energy at room temperature. Therefore, (In,Ga)N/GaN dots are very promising candidates for achieving efficient twin photon emission, potentially at high temperatures and over a wide emission wavelength range

Tunable semiconductor slotted lasers for near-infrared optical coherence tomography

The use of Optical Coherence Tomography in the field of clinical diagnosis is significant. There are different types of swept source lasers available on the market today, however, their design and associated complex fabrication process increase their cost. In the work presented here, an economical six-section slotted tunable laser operating near 850 nm has been designed and fabricated using a UV optical lithography process. The laser is monolithically integrable without a need for any regrowth step. Initial characterization has confirmed the high quality of the slot geometry and stable single mode operation within its tuning range

Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures.

We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions

Direct generation of linearly polarized single photons with a deterministic axis in quantum dots

We report the direct generation of linearly polarized single photons with a deterministic polarization axis in self-assembled quantum dots (QDs), achieved by the use of non-polar InGaN without complex device geometry engineering. Here, we present a comprehensive investigation of the polarization properties of these QDs and their origin with statistically significant experimental data and rigorous k·p modeling. The experimental study of 180 individual QDs allows us to compute an average polarization degree of 0.90, with a standard deviation of only 0.08. When coupled with theoretical insights, we show that these QDs are highly insensitive to size differences, shape anisotropies, and material content variations. Furthermore, 91% of the studied QDs exhibit a polarization axis along the crystal [1–100] axis, with the other 9% polarized orthogonal to this direction. These features give non-polar InGaN QDs unique advantages in polarization control over other materials, such as conventional polar nitride, InAs, or CdSe QDs. Hence, the ability to generate single photons with polarization control makes non-polar InGaN QDs highly attractive for quantum cryptography protocols

Multiscale simulations of the electronic structure of III-nitride quantum wells with varied indium content: Connecting atomistic and continuum-based models

Carrier localization effects in III-N heterostructures are often studied in the frame of modified continuum-based models utilizing a single-band effective mass approximation. However, there exists no comparison between the results of a modified continuum model and atomistic calculations on the same underlying disordered energy landscape. We present a theoretical framework that establishes a connection between atomistic tight-binding theory and continuum-based electronic structure models, here a single-band effective mass approximation, and provide such a comparison for the electronic structure of (In,Ga)N quantum wells. In our approach, in principle, the effective masses are the only adjustable parameters since the confinement energy landscape is directly obtained from tight-binding theory. We find that the electronic structure calculated within effective mass approximation and the tight-binding model differ noticeably. However, at least in terms of energy eigenvalues, an improved agreement between the two methods can be achieved by adjusting the band offsets in the continuum model, enabling, therefore, a recipe for constructing a modified continuum model that gives a reasonable approximation of the tight-binding energies. Carrier localization characteristics for energetically low lying, strongly localized states differ, however, significantly from those obtained using the tight-binding model. For energetically higher lying, more delocalized states, good agreement may be achieved. Therefore, the atomistically motivated continuum-based single-band effective mass model established provides a good, computationally efficient alternative to fully atomistic investigations, at least at when targeting questions related to higher temperatures and carrier densities in (In,Ga)N systems

Dataset: Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures

These data were created in order to assess the high temperature polarisation properties of a-plane InGaN quantum dots, in micro-photoluminescence experiments and kp band simulations, from 2015 to 2017. OriginPro has been used to analyse the data

Direct generation of linearly polarized single photons with a deterministic axis in quantum dots: Dataset

The data were created from k.p simulation, polarization-resolved microphotoluminescence, and Hanbury Brown and Twiss experiments, from 2015 to 2016. The data were created to demonstrate the rigorous generation of polarised single photons with a deterministic axis, and to explain the origin of high polarisation degree and fixed axis, in a-plane InGaN quantum dots. All data were hence used in Figures 2-5 in the publication “Direct generation of linearly polarized single photons with a deterministic axis in quantum dots