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Polarization doping in a GaN-InN system with a graded composition layer was studied using ab initio simulations. The electric charge volume density in the graded concentration part was determined by spatial potential dependence. The emerging graded polarization charge was determined to show that it could be obtained from a polarization difference and the concentration slope. It was shown that the GaN-InN polarization difference is changed by piezoelectric effects. The polarization difference is in agreement with the earlier obtained data despite the relatively narrow bandgap for the simulated system. The hole generation may be applied in the design of blue and green laser and light-emitting diodes.

In this work, we study the emergence of polarization doping in AlxGa1-xN layers with graded composition from a theoretical viewpoint. We demonstrate that the charge conservation law applies for fixed and mobile charges separately, leading to nonlocal compensation phenomena involving bulk fixed and mobile charge and polarization sheet charge at the heterointerfaces. The magnitude of the effect allows obtaining technically viable mobile charge density for optoelectronic devices without impurity doping (donors or acceptors). Therefore, it provides an additional tool for the device designer, with the potential to attain high conductivities: high carrier concentrations can be obtained even in materials with high dopant ionization energies, and the mobility is not limited by scattering at ionized impurities., Comment: 16 pages, 4 figures

In this paper, we present a comparative analysis of the optical properties of non-polar and polar GaN/AlGaN multi-quantum well (MQW) structures by time-resolved photoluminescence (TRPL) and pressure-dependent studies. The lack of internal electric fields across the non-polar structures results in an improved electron and hole wavefunction overlap with respect to the polar structures. Therefore, the radiative recombination presents shorter decay times, independent of the well width. On the contrary, the presence of electric fields in the polar structures reduces the emission energy and the wavefunction overlap, which leads to a strong decrease in the recombination rate when increasing the well width. Taking into account the different energy dependences of radiative recombination in non-polar and polar structures of the same geometry, and assuming that non-radiative processes are energy independent, we attempted to explain the ‘S-shape’ behavior of the PL energy observed in polar GaN/AlGaN QWs, and its absence in non-polar structures. This approach has been applied previously to InGaN/GaN structures, showing that the interplay of radiative and non-radiative recombination processes can justify the ‘S-shape’ in polar InGaN/GaN MQWs. Our results show that the differences in the energy dependences of radiative and non-radiative recombination processes cannot explain the ‘S-shape’ behavior by itself, and localization effects due to the QW width fluctuation are also important. Additionally, the influence of the electric field on the pressure behavior of the investigated structures was studied, revealing different pressure dependences of the PL energy in non-polar and polar MQWs. Non-polar MQWs generally follow the pressure dependence of the GaN bandgap. In contrast, the pressure coefficients of the PL energy in polar QWs are highly reduced with respect to those of the bulk GaN, which is due to the hydrostatic-pressure-induced increase in the piezoelectric field in quantum structures and the nonlinear behavior of the piezoelectric constant.

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; Zinc is often regarded as an alternative to magnesium p-type dopants in gallium nitride. However, besides many theoretical predictions, at present, there are poor data on experimentally revealed p-type conductivity and evaluation of Zn activation energy by means of electrical transport measurements. In this paper, ammonothermal crystallization of bulk GaN:Zn monocrystals is reported. Despite a high doping level of Zn (up to 2 × 1020 cm−3), p-type conductivity with hole concentration as low as of 4 × 1015 cm−3 at room temperature and mobility about 3 cm2/V s was observed. A deep nature of the Zn acceptor was proved, as the ionization energy can exceed 260 meV. In addition, conduction in the impurity band appeared at temperatures as high as room temperature

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.

A multiple-quantum-well structure consisting of 40 periods of AlN/GaN:Si was investigated using a transmission electron microscope equipped with energy-dispersive X-ray spectroscopy. The thicknesses of the AlN barriers and the GaN quantum wells were 4 nm and 6 nm, respectively. The QW layers were doped with Si to a concentration of 1.3 × 10 19 c m - 3 (0.012 % at). The procedure for quantifying such a doping level using AlN as a standard is presented. The EDS results (0.013 % at) are compared with secondary ion mass spectrometry measurements (0.05 % at).

An analysis of the main recombination modes in nitrides, based on new method of data treatment is proposed for the determination of the carrier recombination processes in optically excited matter measured by time-resolved photoluminescence (PL). The analysis includes basic recombination modes: nonradiative Shockley-Read-Hall (SRH), radiative and Auger recombination in relation to monomolecular, bi-molecular, and tri-molecular processes of optical relaxation. The method is based on the introduction of instantaneous PL decay rate r P L plotted as a function of the PL intensity or of the time. Such an approach provides deep insight into the time evolution of the recombination of the optically excited semiconductor systems and can be applied to the time evolution of a variety of optically excited systems. The demonstration of its strength is given by the application to III-nitride based systems, including nitride highly doped and semi-insulating thick layers, polar and non-polar multi-quantum wells (MQWs). At low temperatures (5 K), the mono- and bi-molecular processes determine the carrier relaxation, and the tri-molecular Auger recombination contribution is negligible. At room temperature the data indicate an important contribution of Auger processes. It is also shown that asymptotic (low excitation), one-exponential recombination rate has different character depending on the presence of the electric fields across the structure.

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

International audience; We study the inter-well excitonic coupling in a series of In0.17Ga0.83N/GaN Double QWs (DQWs) with varying central barrier width. We observe the switching between indirect IX (inter-well) and direct DX (intra-well) excitons, for thin barriers (2 nm or less), depending on the exciton density. This density is controlled, in cw-PL, by the pumping laser power density (LPD). Above a certain threshold, we observe a sudden change of the PL blue-shift, when switching from IXs (large slope) to DXs (weak slope). In time-resolved PL, the exciton density evolves as the PL intensity decays with time, and the switching occurs from DXs to IXs, after a certain characteristic time. The decay time of IXs is of the order of hundreds of s, whereas the decay time of DXs is shorter by three orders of magnitude. The switching thresholds in both cw- and TR-PL present clear exponential dependences upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching processes. The described effects were studied at T10K but we found that IXs persist up to T=300K pointing out the importance of large exciton biding energy in nitride QWs.

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.

Physical mechanism responsible for hydrogen sensing by AlGaN/GaN transistors was investigated. Original resistance studies versus magnetic fields (up to 8T) were used to determine precisely hydrogen induced change of carrier mobility and density. Results clearly show that the carrier mobility change is responsible for less than five percent and that the hydrogen sensitivity is mainly related (more than 95 percent) to the carrier density change. These results support the physical model that relates hydrogen sensing with a change of surface charges resulting in modification of the GaN depletion layer.

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.

International audience; In recent years indirect (dipolar) excitons have emerged as highly interesting objects for fundamental studies like Bose-Einstein condensation as well as a concept of excitonic devices for development of excitonic signal processing. An indirect exciton (IX) is a bound pair of an electron and a hole (e-h) in spatially separated quantum wells (QWs). Studies of IX have been concentrated almost entirely on (Al,Ga)As/GaAs coupled double QWs (CDQWs). Due to the large exciton binding energy polar heterostructures of Group III nitrides seem to be more suitable for observation of IX. In the present work we have studied IXs formed by a e-h pair in InGaN/GaN CDQWs. Very thin barrier of 1-2 atomic monolayer (ML) separating QWs enables a tunneling of electrons and holes after optical excitation to the adjacent QWs. The samples consisted of DQWs of In0.17Ga0.83N/In0.02Ga0.98N grown by MBE technique. Intended width of QWs was 2.6 nm and the barrier thickness varied between 1- 8 ML (025-2nm). GaN crystals were used as substrates. The XRD and TEM measurements showed that the obtained structures agree well with the intended ones. Modeling of their band structure using the NEXTNANO software revealed that the structures with the thinnest barriers (1-2 ML) formed one electron (hole) state located in the conduction (valence) band of adjacent QWs. The corresponding emission wavelength was λ≈510 nm. Simulations performed for structures with the thicker barriers demonstrated presence of e- and h-states in each QW and the emission at λ≈450nm. PL measured in these samples showed such an emission. For structures with 1-2 ML barrier we found a peak at λ≈505-520nm which we associated with IX in the system of CDQW. To prove strongly indirect character of the long-wavelength PL-band we performed TRPL characterization of the studied structures. At T=4K, the band at λ≈510nm showed the decay time, τD, approaching msec range. Whereas the band at λ≈450nm is characterized by τD of about 100 ns.

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].

International audience; In contrast to GaN-based structures grown along [0001], nonpolar growth allows for the realization of thick quantum wells (QWs) with an optimal overlap between electron and hole wavefunctions. However, nonpolar GaN layers grown on non-lattice-matched substrates exhibit high densities of defects. Then, even at low temperature (T), the carrier recombination is mainly nonradiative.Here, we present the dynamics of charge carriers in AlGaN/GaN QWs of various widths and Al-contents grown on the a-facet of GaN crystals. The QW dislocation density and the exciton diffusion length at 300K are 2.105 cm-2 and 100 nm, respectively. Therefore, dislocations play a minor role in the carrier dynamics at 300K. For all samples, we observe an increasing QW PL lifetime with T, indicating the absence of nonradiative phenomena in the low-T range. Purely radiative recombination is observed up to 240K for a 7 nm thick QW. This suggests the possibility of realizing UV emitters with narrow emission and high internal quantum efficiency at 300K.The observed decrease in QW PL lifetime in the high-T range arises from the thermal escape of carriers from the QW to the AlGaN barriers and their subsequent nonradiative recombination. The efficiency of nonradiative recombination at 300K is thus reduced by limiting the thermal escape of carriers from the QW. This is obtained by growing thick QWs rather than increasing the barrier Al-content, which is important from point of view of defect generation.

International audience; In contrast to GaN-based structures grown along [0001], nonpolar growth allows for the realization of thick quantum wells (QWs) with an optimal overlap between electron and hole wavefunctions. However, nonpolar GaN layers grown on non-lattice-matched substrates exhibit high densities of defects. Then, even at low temperature (T), the carrier recombination is mainly nonradiative.Here, we present the dynamics of charge carriers in AlGaN/GaN QWs of various widths and Al-contents grown on the a-facet of GaN crystals. The QW dislocation density and the exciton diffusion length at 300K are 2.105 cm-2 and 100 nm, respectively. Therefore, dislocations play a minor role in the carrier dynamics at 300K. For all samples, we observe an increasing QW PL lifetime with T, indicating the absence of nonradiative phenomena in the low-T range. Purely radiative recombination is observed up to 240K for a 7 nm thick QW. This suggests the possibility of realizing UV emitters with narrow emission and high internal quantum efficiency at 300K.The observed decrease in QW PL lifetime in the high-T range arises from the thermal escape of carriers from the QW to the AlGaN barriers and their subsequent nonradiative recombination. The efficiency of nonradiative recombination at 300K is thus reduced by limiting the thermal escape of carriers from the QW. This is obtained by growing thick QWs rather than increasing the barrier Al-content, which is important from point of view of defect generation.

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; Contrary to GaN-based structures grown along [0001], nonpolar heterostructures allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions. However, the use of lattice-mismatched substrates induces high densities of stacking faults and dislocations. Then, even at low temperature (T), the exciton decay is dominated by the capture on defects [1]. We investigate here the dynamics of excitons in AlGaN/GaN QWs deposited on the a-facet of GaN crystals. We first determine by cathodoluminescence QW dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations do not affect QW excitons at room-temperature [2]. We then study the time-resolved photoluminescence (PL) of excitons in the 10-320 K range for QW samples with various widths and barrier Al-contents. For all samples, the effective lifetime of QW excitons increases with T, indicating the absence of nonradiative phenomena in the low-T range. We observe purely radiative recombination up to 240 K for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [3]. This observation therefore demonstrates the possibility of achieving UV emitters with a rather narrow emission line and a good radiative efficiency at 300 K. In the high-T range, a drop in the QW PL lifetime is accompanied by an increase in the barrier PL lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the equilibrium between excitons and free carriers in the QWs and the barriers, we show that the nonradiative recombination of charge carriers in the AlGaN barriers is the mechanism limiting the PL lifetime of excitons in the QWs [3]. The nonradiative recombinations at 300 K are suppressed by reducing the thermal escape of carriers from the QW. This is achieved by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010). [2] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [3] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).

International audience; Currently, a growing interest is paid to the study of nonpolar nitride-based heterostructures, as they allow for the growth of thick QWs, while keeping an optimal overlap between electron and hole wave functions [1]. The growth of wide QWs is indeed a key issue to produce high-power nitride-based optoelectronic devices, as they allow reducing the carrier density in the QW, lessening the efficiency of Auger-like mechanisms. However, non-lattice matched foreign substrates are generally used to grow nonpolar GaN, inducing strain in the heteroepitaxial layers. Even when processing techniques such as epitaxial lateral overgrowth are used, strain relaxation through the generation of dislocations or basal stacking faults leads to a drastic reduction of exciton lifetime [2]. In this work, we therefore investigate the dynamics of excitons in single (Al,Ga)N/GaN QWs deposited directly on the a-facet of GaN crystals. We first extract by cathodoluminescence experiments dislocation density and exciton diffusion length at 300 K of 2.105 cm-2 and 100 nm, respectively, demonstrating that dislocations should not play any significant role in the recombination of excitons at room-temperature [3]. We then study by time-resolved photoluminescence the dynamics of excitons in the 10-320 K range for QW samples with various width and barrier Al-content. We first show that, for all samples, the effective lifetime of QW excitons increases with temperature, evidencing the absence of nonradiative phenomena in the low-temperature range. The largest temperature range of purely radiative recombination (up to 240 K) has been observed for a 7 nm thick Al0.06Ga0.94N/GaN QW, i.e. a QW with small exciton localization energy (2 meV) [4]. This observation therefore evidences the possibility of achieving nonpolar room-temperature UV emitters combining a rather narrow emission line with a good radiative efficiency at 300 K. In the high temperature range, a drop in the QW photoluminescence lifetime is always accompanied by an increase in the barrier emission lifetime, until both emissions follow the same dynamics. Supported by a model accounting for the thermodynamic equilibrium between excitons and free carriers in the QWs and the (Al,Ga)N barriers, we demonstrate that at high temperatures, the nonradiative recombination of charge carriers in the (Al,Ga)N barriers is the mechanism limiting the photoluminescence lifetime of excitons confined in the QWs [4]. We finally propose to tackle the thermal escape of carriers by the growth of thick QWs rather than increasing the barrier Al-content, which is important from the defect/strain generation point of view. [1] P. Waltereit et al., Nature 406, 865 (2000). [2] P. Corfdir et al., J. Appl. Phys. 107, 043524 (2010); T. J. Badcock et al., Appl. Phys. Lett. 93, 101901 (2008). [3] P. Corfdir et al., Phys. Rev. B 83, 245326 (2011). [4] P. Corfdir et al., J. Appl. Phys. 111, 033517 (2012).

International audience; Pressure studies of multicarrier conduction in undoped InN grown on GaN buffer

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; Contrary to c-plane heterostructures, the absence of built-in electric fields in non-polar nitride-based heterostructures allows for the growth of thick quantum wells (QWs) without any decrease in the overlap between electron and hole wave functions [1]. However, lattice mismatched substrates are generally used to grow non-polar GaN, inducing strain in the heteroepitaxial layers that relax through the generation of non-radiative recombination centers. Here, we investigate by time-resolved photoluminescence the temperature (T) dependence of charge carriers recombination mechanisms in non-polar (Al,Ga)N/GaN quantum wells (QW) grown by 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 - 320 K range.

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

International audience

International audience; Since Waltereit et al. demonstrated ten years ago the realization of polarization free (Al,Ga)N/GaN quantum wells (QW) a lot of work has been carried out on non-polar nitride-based heterostructures. The current interest in non-polar GaN is indeed motivated by the absence of built-in electric fields, allowing the growth of wide QWs without any decrease in the overlap between electron and hole wave functions. However, the use of non lattice-matched substrates induces strain that relaxes through the generation of dislocations or stacking faults. The dynamics of QW excitons is then dominated by their trapping on these extended defects. In this work, we have adopted a different approach consisting in the homoepitaxial growth of a 4 nm-thick non-polar Al0.06Ga0.94N/GaN single QW. We first show that the resulting heterostructure is free of stacking fault and presents a low dislocation-density that we estimate to 2.105 cm-2. From 10 to 150 K, the QW effective decay time increases from 110 to 350 ps, evidencing the absence of non-radiative recombination channel in this temperature range. The temperature dependence of exciton decay time is instead ascribed to the combined delocalization of excitons bound to interface roughness and to the thermal distribution of free excitons in k-space. From these results, we deduce an oscillator strength of 2.1013 / cm2 for QW free excitons, in agreement with values reported for thin polar (Al,Ga)N/GaN MQWs where the influence of electric fields is negligible.

We report on room temperature terahertz generation by a submicron size AlGaN/GaN-based high electron mobility transistors. The emission peak is found to be tunable by the gate voltage between 0.75 and 2.1 THz. Radiation frequencies correspond to the lowest fundamental plasma mode in the gated region of the transistor channel. Emission appears at a certain drain bias in a thresholdlike manner. Observed emission is interpreted as a result of Dyakonov–Shur plasma wave instability in the gated two-dimensional electron gas.

International audience; In this paper we present the resistivity and Hall-effect measurements on p-type GaN doped with Mg. The experiments were carried out as a function of hydrostatic pressure Lip to 1200 MPa in the temperature range 260-400 K. Both bulk GaN crystals as well as GaN: Mg epilayers were studied. In the investigated samples the decrease of resistivity and increase of hole concentration under pressure was observed. Such a behavior, which is contrary to the n-type material strongly suggests a decrease of the ionization energy of Mg acceptor (E-a = 183 meV) with pressure. This shift is very weak, less than -2 meV/GPa. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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International audience

International audience

International audience

We demonstrate a double heterostructure (DH) nitride laser diode (LD) with an untypically wide 9.5 nm InGaN active region instead of the commonly used narrow InGaN quantum wells. Structures were grown on bulk GaN, which ensures low dislocation densities and therefore low concentrations of nonradiative recombination centers. The efficient screening of polarization induced electric fields in the structures under investigation, which contained heavily (1×1019 cm-3) Si doped barriers, was demonstrated by means of hydrostatic pressure dependent photoluminescence measurements on a simplified sample with identical active region. Since the detrimental separation of carriers by electric fields becomes more pronounced for wider InGaN active regions, efficient screening of polarization induced electric fields is essential for the investigated heterostructures. Optical and electrical parameters of this LD were comparable to those of comparable devices with typical QWs of ∼4 nm as active region. We observed a high thermal stability of the photoluminescence intensity and, via time-resolved photoluminescence, a relatively temperature-independent radiative decay time. These observations support the significance of carrier localization phenomena for the radiative recombination processes of the investigated structure. The implications of these results for DH LD structures are discussed.

International audience

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.

National audience

InGaAsSb∕AlGaAsSb type-I midinfrared diode lasers emitting continuous wave at 2.4μm at room temperature have been studied under high hydrostatic pressure. When the pressure was increased up to 19kbar, the threshold current varied from 240to400A∕cm2, showing a minimum of 200A∕cm2 close to 8kbar, and the emission spectra shifted to shorter wavelengths by up to 700nm (i.e., from 2.4μmto1.7μm). This exceptional tuning range could be very useful in tunable diode laser absorption spectroscopy.

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.

International audience