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In this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference, hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices.

International audience; Quantum well systems based on semiconductors with the wurtzite crystalline structure have found widespread applications in photonics and optoelectronic devices, such as light-emitting diodes, laser diodes, or single-photon emitters. In these structures, the radiative recombination processes can be affected by (i) the presence of strain and polarization-induced electric fields, (ii) quantum well thickness fluctuations and blurring of a well–barrier interface, and (iii) the presence of dislocations and native point defects (intentional and unintentional impurities). A separate investigation of these phenomena is not straightforward since they give rise to similar effects, such as a decrease of luminescence efficiency and decay rate, enhancement of the Stokes shift, and strong blueshift of the emission with increasing pump intensity. In this Perspective article, we review the usefulness of measurements of the quantum well luminescence as a function of the hydrostatic pressure for both scientific research and the development of light-emitting technologies. The results presented here show that high-pressure investigations combined with ab initio calculations can identify the nature of optical transitions and the main physical factors affecting the radiative efficiency in quantum well systems. Finally, we will discuss an outlook to the further possibilities to gain new knowledge about the nature of recombination processes in quantum wells using high-pressure spectroscopy.

We analyze the evolution of the exciton recombination energy ${E}_{\mathrm{PL}}$ and its pressure coefficient $d{E}_{\mathrm{PL}}/dp$, with the laser power density (LPD) exciting excitonic photoluminescence. Two $\mathrm{I}{\mathrm{n}}_{0.17}\mathrm{G}{\mathrm{a}}_{0.83}\mathrm{N}/\mathrm{GaN}$ quantum well (QW) structures are used: sample A consisting of a 5.2-nm-wide single QW and sample B consisting of two 2.6-nm-wide QWs separated by a 0.78-nm-thick barrier. Changes of ${E}_{\mathrm{PL}}$ and $d{E}_{\mathrm{PL}}/dp$ with LPD in both structures cover a wide range of magnitude. They switch character in the vicinity of the indirect to direct exciton induced by increasing LPD. In sample B, at low excitation, a negative pressure coefficient of ${E}_{\mathrm{PL}}$ has been found. Moreover, pressure dependences of ${E}_{\mathrm{PL}}$ are different for the two samples under study. We examined pressure dependence of the threshold LPD for indirect-direct exciton switching in both samples. The reported observations originate to a high extent from screening of a built-in electric field by carriers externally introduced into the quantum wells. We also note that the universal relation between transition energies and their pressure coefficients (observed previously in InGaN/GaN quantum wells) does not hold for samples modified by screening.

HgTe quantum wells (QWs) are two-dimensional semiconductor systems that change their properties at the critical thickness dc, corresponding to the band inversion and topological phase transition. The motivation of this work was to study magnetotransport properties of HgTe QWs with thickness approaching dc, and examine them as potential candidates for quantum Hall effect (QHE) resistance standards. We show that in the case of d > dc (inverted QWs), the quantization is influenced by coexistence of topological helical edge states and QHE chiral states. However, at d ≈ dc, where QW states exhibit a graphene-like band structure, an accurate Hall resistance quantization in low magnetic fields (B ≤ 1.4 T) and at relatively high temperatures (T ≥ 1.3 K) may be achieved. We observe wider and more robust quantized QHE plateaus for holes, which suggests—in accordance with the “charge reservoir” model—a pinning of the Fermi level in the valence band region. Our analysis exhibits advantages and drawbacks of HgTe QWs for quantum metrology applications, as compared to graphene and GaAs counterparts.

International audience; This paper discusses the results of high temperature resistivity and Hall effect studies of Mg-doped GaN and AlxGa1−xN epilayers (0.05

Studies of internal electric fields in GaN/AlxGa1-xN (3 nm/4 nm) multi-quantum-wells (MQWs) with x = 0.25, 0.5, and 1 are presented. The structures were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction, transmission electron microscopy, and secondary ion mass spectroscopy. Optical properties of these structures were investigated by ambient and high-pressure photoluminescence (PL) measurements. They were strongly affected by polarization-induced electric fields giving rise to a quantum confined Stark effect, depending on the composition of the AlGaN barrier. The optical emission energy redshifts by > 100 meV when the Al content in the barrier increases from x = 0.25 up to x = 1, while the pressure coefficients of the PL energy are significantly reduced in comparison with bulk GaN. The transition energies and their pressure dependencies are modelled for tetragonally strained structures with the same geometry using a full tensorial representation of the strain in the MQWs under external pressure. The same MQWs are also simulated using density functional theory calculations. Additionally, influence of blurring of well-barrier interface on transition energy has been modelled. A good agreement between experimental results and theoretical analysis indicates that nonlinear effects induced by the tetragonal strain due to the lattice mismatch between the substrates and the polar MQWs are responsible for a drastic decrease of the pressure coefficients of PL energy, and that these effects are well described by ab initio calculation procedures. Our results reflect the role of composition and geometry on the basic optical properties of nitride quantum wells.

International audience; The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

This work presents epitaxial growth and optical spectroscopy of CdTe quantum dots (QDs) in (Cd,Zn,Mg)Te barriers placed on the top of (Cd,Zn,Mg)Te distributed Bragg reflector. The formed photonic mode in our half-cavity structure permits to enhance the local excitation intensity and extraction efficiency of the QD photoluminescence, while suppressing the reflectance within the spectral range covering the QD transitions. This allows to perform coherent, nonlinear, resonant spectroscopy of individual QDs. The coherence dynamics of a charged exciton is measured via four-wave mixing, with the estimated dephasing time $T_2=(210\,\pm\,40)$ ps. The same structure contains QDs doped with single Mn$^{2+}$ ions, as detected in photoluminescence spectra. Our work therefore paves the way toward investigating and controlling an exciton coherence coupled, via $s$,$p$-$d$ exchange interaction, with an individual spin of a magnetic dopant., 6 pages, 5 figures

Spontaneous and piezoelectric polarization in the nitrides is analyzed. The slab model was designed and proved to be appropriate to obtain the spontaneous polarization in AlN, GaN and InN. The spontaneous polarization and polarization related electric fields in AlN, GaN and InN were determined using DFT slab calculations. The procedure generates single value of spontaneous polarization in the nitrides. It was shown that Berry phase polarization may be applied to determination of spontaneous polarization by appropriate addition of polarization induced electric fields. The electric fields obtained from slab model are consistent with the Berry phase results of Bernardini et al. The obtained spontaneous polarization values are: 8.69*10-3 C/m2, 1.88*10-3 C/m2, and 1.96*10-3 C/m2 for AlN, GaN and InN respectively. The related Berry phase polarization values are 8.69*10-2 C/m2, 1.92*10-2 C/m2, and 2.86*10-2 C/m2, for these three compounds, respectively. The GaN/AlN multiquantum wells (MQWs) were simulated using ab intio calculations. The obtained electric fields are in good agreement with those derived from bulk polarization values. GaN/AlN MQWs structures, obtained by MBE growth were characterized by TEM and X-ray measurements. Time dependent photoluminescence measurements were used to determine optical transition energies in these structures. The PL obtained energies are in good agreement with ab initio data confirming overall agreement between theoretical and experimental data.

The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape o...

Crystallization of GaN by High Nitrogen Pressure Solution method in multi-feed–seed (MFS) configuration without intentional doping [1] results in: (1) Growth of strongly n-type crystals with free electron concentration increasing with growth temperature in ranges of 2–6×10 19 cm −3 and 1350–1430 °C, (2) stable growth on Ga-polar surface and unstable growth on N-polar surface, crystals slightly brown, (3) improvement of (0001) crystallographic planes curvature (flattening) with respect to bowing of these planes in the seed crystals. The addition of magnesium into the growth solution causes strong compensation of free electrons in the crystals. Therefore, highly resistive GaN crystals can be grown. In this work, the crystallization of Mg doped GaN on flat ∼1 in. seeds (substrates) grown by HVPE in MFS configuration has been studied. It is shown that: (1) Highly resistive GaN:Mg crystals with resistivity higher than 10 7 Ω cm were grown, (2) the growth is stable on N-polar surfaces of the seeds whereas it is unstable on the Ga-polar surfaces, which is opposite to the HNPS growth of the n-type crystals. The GaN:Mg crystals are fully transparent with no visible color, (3) shape of (0001) crystallographic planes improves (flattens) with respect to bowing of these planes in the seed crystals (HVPE substrates).

We have measured thermoelectric power in two series of polar InN:Mg samples with wide range of Mg content having In- as well as N-growth polarities. We have observed essential differences between both polarities: In the “p-type window” centered at about 1 × 1019 cm−3 of [Mg], reported recently, the thermoelectric power changed its sign from n to p-type, only for In-growth polarity samples. These results have been confirmed by the so-called mobility spectrum analysis. It strongly supports the suggestion that In-growth polarity is more propitious to p-type conduction in InN:Mg than the N one.

We have grown GaN films and GaN-AlGaN quantum wells (QWs) on homoepitaxial substrates, by molecular beam epitaxy using ammonia. Both the GaN film and the QW are found to have superior excitonic recombination properties which are extremely promising for the development of indium free ultra-violet lasers based on nitrides. (C) 2001 Published by Elsevier Science Ltd.

GaN epilayers and GaN/AlGaN quantum wells (QWs) were grown by molecular beam epitaxy on GaN(0001) single crystal substrates. Transmission electron microscopy (TEM) was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 10(5) cm(-2) is deduced from TEM imaging. Low temperature (1.8 K) photoluminescence (PL) of homoepitaxial GaN reveals PL linewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxial GaN/Al2O3 layer. A 2 nm thick GaN/Al0.1Ga0.9N QW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxial QW is shown to be comparable to that measured on heteroepitaxial QWs grown either on sapphire or silicon substrates. (C) 2000 American Institute of Physics. [S0021-8979(00)07513- 7].

High-pressure and time-resolved studies of the optical emission from n-type doped GaN/AlN multi-quantum-wells (MQWs) with various well thicknesses are analysed in comparison with ab initio calculations of the electronic (band structure, density of states) and optical (emission energies and their pressure derivatives, oscillator strength) properties. The optical properties of GaN/AlN MQWs are strongly affected by quantum confinement and polarization-induced electric fields. Thus, the photoluminescence (PL) peak energy decreases by over 1 eV with quantum well (QW) thicknesses increasing from 1 to 6 nm. Furthermore, the respective PL decay times increased from about 1 ns up to 10 μs, due to the strong built-in electric field. It was also shown that the band gap pressure coefficients are significantly reduced in MQWs as compared to bulk AlN and GaN crystals. Such coefficients are strongly dependent on the geometric factors such as the thickness of the wells and barriers. The transition energies, their oscilla...

International audience; We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320K range. We first show that the effective lifetime of QW excitons tau increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing tau depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs.

The coexistence of two types of carriers (free electrons and free holes) in InN:Mg and their competition is demonstrated by the temperature and magnetic-field-induced change of the sign of thermopower (a) as well as the maximum entropy mobility spectrum analysis. The results confirm the existence of alternative carrier channels in addition to the n-type surface inversion layer and p-type bulk. They also show that In-polarity can be propitious for occurrence of p-type conductivity.

International audience; Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm−2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K theQWemission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described.

International audience; The competition between radiative and nonradiative recombination of electron-hole pairs in (Al,In,Ga)N/(Al,In,Ga)N quantum wells of two different compositions and various well widths was investigated in order to understand the microscopic mechanisms of the highly intense light emission from this type of structures. By using time-resolved photoluminescence, we have verified that one can adjust the compositions of the quaternaries so as to optimize the confinement effects and to minimize the built-in electric field which is present in such hexagonal group-III nitride based QWs. The role of local potential fluctuations in the quaternary alloy on the localization of carriers was studied and analyzed by measuring the photoluminescence energy, intensity and dynamics versus temperature.

International audience; Measurements of (i) the time-resolved photoluminescence and (ii) pressure dependence of the emitted light energy in two series of quaternary (AlInGa)N multiquantum wells have been used to point out the small magnitude of internal electric field as promoting highly efficient luminescence in these specially designed heterostructures. Findings of the experimental examination give the magnitude of built-in electric field few times smaller than that predicted theoretically. Electrical transport measurements support the explanation of the obtained results consisting of the screening of the built-in electric field by means of unintentional doping.

In this work we review studies of the pressure dependence of the light emission from quantum structures and thick epitaxial layers of hexagonal and cubic group-III nitrides. In these semiconductors, an association of the band gap magnitude with the energy of light emission should be treated with caution. Usually, anomalously large Stokes shifts between light emission and absorption are observed, which can reach a value of 0.5-1.0 eV. Two effects are taken into account to explain this phenomenon: (i) in the hexagonal (wurtzite) nitride quantum structures large built-in electric fields caused by spontaneous and piezoelectric polarization dominate their optical properties; (ii) in hexagonal as well as in cubic nitrides In-distribution fluctuations lead to a strong Stokes shift effect, too. We show that both effects influence the pressure coefficient of the emitted light energy, dE E /dP, making it different from dE G /dP. In the final part we show that the built-in biaxial strain caused by a lattice mismatch can also lead to a reduction of dE E /dP.

We investigate the microscopic mechanisms of the highly intense light emission from (Al,Ga,In)N/ (Al,Ga,In)N quantum wells (QW's). We concentrate on the competition between radiative and nonradiative recombination of electron-hole pairs for Al 0 . 1 2 Ga 0 . 8 4 In 0 . 0 4 N/Al 0 . 3 0 Ga 0 . 6 9 In 0 . 0 1 N multiple quantum wells of various widths. These nominal compositions correspond to the maximization of the luminescence intensity. By using time-resolved photoluminescence, we verify that such particular compositions of the quaternaries are capable of minimizing the magnitude of the longitudinal electric field which usually appears in such hexagonal group-III nitride based QW's. We find that the radiative lifetime is nearly independent of the well width, whereas it should show an exponential dependence in the case of a strong electric field. We discuss our results via a theoretical estimation of the internal electric field. We also analyze the role played by local potential fluctuations in the quaternary alloy on the localization of carriers, by measuring the change of the photoluminescence energy, intensity, and dynamics versus the temperature.

International audience; A study of light emission of InAlGaN based multi quantum wells (with different well widths) has been performed by means of hydrostatic pressure and time resolved measurements. Both techniques are very sensitive to the presence of the large internal electric fields in hexagonal InGaN/GaN and GaN/AlGaN heterostructures. In the present studies, we found that independently of the quantum well width the pressure shift of the light emission energy and the photoluminescence decay time show almost constant values. This observation is interpreted as an evidence of the lack of built-in electric field in the used quaternary quantum wells.

This article presents the dynamics of excitons in a-plane (Al,Ga)N/GaN single quantum wells of various thicknesses grown on bulk GaN substrates. For all quantum well samples, recombination is observed to be predominantly radiative in the low-temperature range. At higher temperatures, the escape of charge carriers from the quantum well to the (Al,Ga)N barriers is accompanied by a reduction in internal quantum efficiency. Based on the temperature-dependence of time-resolved photoluminescence experiments, we also show how the local disorder affects the exciton radiative lifetime at low temperature and the exciton non-radiative lifetime at high temperature.

International audience; The authors have applied modulation spectroscopy to study intersubband (IS) and interband (IB) transitions in InGaN/AlInN multi quantum well (QW) structures with the QW width varying from 1.3 to 1.8 nm, and proposed a simple method to analyze the broadening of IS and IB transitions. Within this method the measured broadenings of IS and IB transitions have been compared with theoretical calculations performed within the effective mass approximation for a QW system with various QW width fluctuations. In the framework of this comparison it has been found that the QW width fluctuation in the investigated InGaN/AlInN QW system is close to 2 monolayers.

International audience; We investigated the relation between structural properties and carrier recombination processes in InGaN∕GaNmultiple quantum well(MQW) structures with quantum well widths of 3 and 9nm, grown by metal-organic chemical-vapor deposition on bulk GaN crystals. Quantum barriers of the samples are heavily n-type doped in order to effectively screen the large polarization-induced electric fields which commonly occur in hexagonal InGaN∕GaN quantum structures. High thermal stability in these structures, reflected by strong photoluminescence(PL) even above 400K, is attributed to a combination of low-dislocation densities and potential profile fluctuations in the InGaN∕GaNquantum wells. The role of potential profile fluctuations is further investigated by time-resolved photoluminescence and cathodoluminescence(CL) mapping. Comparison of both samples shows that the sample with 3‐nm-wide QWs exhibits (i) a larger width of the PL peak in the temperature range of 8–420K, (ii) a higher amplitude of potential profile fluctuations as measured by CL mapping, and (iii) higher radiative and nonradiative PL recombination times. At the same time a much weaker drop of PL intensity with temperature is recorded on the sample with 9‐nm-wide QWs. Our results show that, contrary to intuitive expectation, a decrease of the potential profile fluctuation amplitude can be helpful in enhancing the radiative recombination efficiency, particularly at high temperatures.