Impact of electron beam propagation on high-resolution quantitative chemical analysis of 1-nm-wide GaN/AlGaN quantum wells.

The objective of this work is to simulate a single quantum well ultraviolet light emitting diode (LED) based on AlGaN/GaN/AlGaN and AlGaN/BGaN/AlGaN, by using TCAD Silvaco simulator. The first structure has a GaN quantum well taken between two layers, of n-AlGaN and p-AlGaN. The second one has a BGaN quantum well instead of GaN. We fix the concentration of the boron in BGaN to only 1% and we vary the thickness of GaN and BGaN quantum well layer from 7 to 20 nm, for the two structures. As results, we obtain respectively for GaN-LED and BGaN-LED, a maximum current of 0.52 and 0.27 mA, a maximum power spectral density of 1.935 and 6.7 W cm−1 eV−1, a maximum spontaneous emission of 3.34 × 1028 and 3.43 × 1028 s−1 cm−3 eV−1, and a maximum Light output power of 0.56 and 0.89 mW.

Room-temperature linear optical gain of a GaN quantum well is estimated qualitatively and is compared with optical gains of the ZnSe and GaAs quantum wells in order to examine the feasibility of GaN quantum-well lasers. Our analysis is based on the simple model using the density matrix method with parabolic band approximation. For 100 Å quantum wells, the calculated transparency level of a GaN quantum well (8.3×1018 cm−3) is slightly higher than that of a ZnSe structure (7.4×1018 cm−3). But the transparency levels of GaN and ZnSe quantum wells are much higher than the transparency level of a GaAs quantum well (2.3×1018 cm−3). It is expected that gain spectra of a GaN quantum well be narrower than those of a ZnSe quantum well.

For hybrid light emitting devices (LEDs) consisting of GaN quantum wells and colloidal quantum dots, it is necessary to explore the physical mechanisms causing decreases in the quantum efficiencies and the energy transfer efficiency between a GaN quantum well and CdSe quantum dots. This study investigated the electro-luminescence for a hybrid LED consisting of colloidal quantum dots and a GaN quantum well patterned with photonic crystals. It was found that both the quantum efficiency of colloidal quantum dots on a GaN quantum well and the energy transfer efficiency between the patterned GaN quantum well and the colloidal quantum dots decreased with increases in the driving voltage or the driving time. Under high driving voltages, the decreases in the quantum efficiency of the colloidal quantum dots and the energy transfer efficiency can be attributed to Auger recombination, while those decreases under long driving time are due to photo-bleaching and Auger recombination.

We report on the experimental determination of the photoluminescence mechanism in a set of In0.25Ga0.75N quantum wells. Instead of studying the photoluminescence for different In contents, we have investigated it as a function of the quantum well width in combination with a similar study performed on GaN quantum wells. In this way we show that the photoluminescence is not coming from quantum dots in the quantum well but from the quantum well itself under the influence of an internal electric field induced by strain.

We report on the experimental determination of the photoluminescence mechanism in a set of In0.25Ga0.75N quantum wells. Instead of studying the photoluminescence for different In contents, we have investigated it as a function of the quantum well width in combination with a similar study performed on GaN quantum wells. In this way we show that the photoluminescence is not coming from quantum dots in the quantum well but from the quantum well itself under the influence of an internal electric field induced by strain.

The pronounced enhancement of indium incorporation efficiency for InGaN∕GaN quantum wells due to the rough, faceted surface of the GaN template grown in situ by ammonia-molecular-beam epitaxy is reported. The InGaN∕GaN quantum wells are grown by plasma-assisted molecular-beam epitaxy. Unlike the smooth (0002) surface of GaN template layers grown by metalorganic chemical vapor deposition, the surface of the template layers grown by ammonia-molecular-beam epitaxy is defined by {10-1m} pyramidal facets causing significant surface roughness. The drastically enhanced indium incorporation rate associated with the rough templates allows the InGaN∕GaN quantum wells to be grown at higher temperatures as it compensates for the increased thermal decomposition. High luminescence efficiency is achieved as a result. Using such efficient InGaN∕GaN quantum wells, light-emitting diodes have been grown entirely by molecular-beam epitaxy on sapphire substrates, demonstrating output power of 0.22mW for 20mA injection current.

We present analytical expressions for internal electric field and strain in single and multiple quantum wells, incorporating electromechanical coupling, spontaneous polarization, and periodic boundary conditions. Internal fields are typically 2% lower than the fields calculated using an uncoupled model. We point out two possible interpolation routes to calculate the piezoelectric (PZ) constants eij of an alloy from the PZ constants of the constituent materials and show that, for an In0.2Ga0.8N∕GaN quantum well system, the respective internal electric fields differ by 10%. Using an effective-mass model, we explore the effect of the uncertainty in the elastic and PZ constants of GaN on the internal field and optical transitions of InGaN∕GaN quantum wells, and find that the range of published values of eij produces an uncertainty of more than ±20% in the internal field and of more than ±30% in the blueshift in optical transition energy between zero bias and flatband conditions (when the applied field is equal and opposite to the internal field). Using the PZ constants of Shimada et al. [J. Appl. Phys. 84, 4951 (1998)] in our model gives the best fit to results in the literature for internal field and optical transition energy in InGaN∕GaN quantum wells. We find that a well with a smooth In gradient along the growth direction has similar optical properties to a well with constant composition, if the average In content of the two wells is the same.

In this paper we present room temperature electroreflectance studies of GaN quantum wells (QWs) with different well width. The electroreflectance measurements were performed with external voltage applied to the structure therefore it was possible to tune the electric field inside QW up to its completely screening and furthermore even reversing it. The analysis of QW spectral lines showed the Stark shift dependence on applied voltage and well width reaching about 35 meV for highest voltage and widest well width. It was possible to obtain the condition of zero electric field in QW. Both broadening and amplitude of QW lines are minimal for zero electric field and increases for increasing electric field in QW. The energy transition is maximum for zero electric field and for increasing electric field it decreases due to Stark effect. Neither amplitude and broadening parameter nor energy transition does not depend on the direction of electric field. Only parameter that depends on the direction of electric field in QW is phase of the signal. The analysis of Franz-Keldysh oscillations (FKOs) from AlGaN barriers allowed to calculate the real electric field dependence on applied voltage and therefore to obtain the Stark shift dependence on electric field. The Stark shift reached from −12 meV to −35 meV for 450 kV/cm depending on the well width. This conditions were established for highest forward voltages therefore this is the value of electric field and Stark shift caused only by the intrinsic polarization of nitrides.

Body: In this work, we investigate the properties of spatially indirect excitons (IXs) confined in pairs of atomically think GaN wells, separated by polar AlN barriers. Atomically thin GaN is a promising material for realizing strongly bound excitons because of its extreme quantum confinement effect. Also, the spontaneous polarization fields in nitride heterostructures allow IXs to form in atomically thin GaN quantum wells even without external electric fields. We performed first-principles calculations based on density functional theory and many-body perturbation theory to investigate the properties of IXs and DXs in pairs of atomically thin GaN quantum wells separated by polar AlN layers with varying thickness. Our calculation shows that the overlap of the electron and hole wavefunctions, the degree of electron-hole interaction, and the character (IX or DX) of the lowest-energy exciton can be controlled by changing the thickness and the resulting electrical polarization of the separating AlN barrier. We demonstrate that room-temperature stable IXs with radiative decay rates several orders of magnitude lower than DXs can be realized in these atomically thin polar nitride heterostructures for potential excitonic applications at room temperature based on a commercial semiconductor platform. For realization of IXs in atomically-thin GaN quantum wells, molecular beam epitaxial growth was performed on an n-type Si substrate using a Veeco GEN II system with radio frequency plasma-assisted nitrogen source. The epitaxy includes ~100 nm long GaN nanowires which serves as a template for the subsequent epitaxy of ~150 nm long AlN nanowires. Then, monolayer GaN quantum well/AlN barrier/monolayer GaN quantum well heterostructure was grown on top of AlN nanowires. The thickness of AlN barrier was varied by changing the growth duration. Figure 1 shows the photoluminescence spectra of two samples with 1 ML and 7 MLs thick AlN barriers incorporated, respectively. The emission peak around 5.93 eV can be attributed to the excitonic emission from AlN. DX emission dominates in the spectrum of sample with 7 MLs thick AlN barrier. In comparison, there’s a clear shoulder emerging on the lower energy side observed in the sample with 1 ML thick AlN barrier, which can be assigned to indirect exciton (IX) emission. The work is supported by the University of Michigan College of Engineering Blue Sky Research Program. W.L. was partially supported by the Kwanjeong Educational Foundation Scholarship. Computational resources were provided by the DOE NERSC facility.

Strongly enhanced lateral diffusion of photogenerated carriers was directly observed in the luminescent image of the InGaN∕GaN quantum wells. Such an effect was quantitatively modeled using diffusion equation and the ambipolar diffusion coefficient derived by K. H. Gulden and his co-workers [Phys. Rev. Lett. 66, 373 (1991)]. Our simulation shows that the vertical piezoelectric field existing in strained InGaN∕GaN quantum wells is the original “driving force” for the enhancement of lateral diffusion. Influence of the density of photogenerated carriers and their average mobility on the enhancement was discussed.

We experimentally demonstrate the formation of a superradiant optical mode in the room-temperature reflection spectra from a resonant Bragg structure composed of 30 equidistant GaN quantum wells separated by (Al,Ga)N barriers. The mode arises when the condition of the Bragg diffraction is fulfilled at the wavelength corresponding to the energy of the quasi-two-dimensional excitons in the quantum wells. It manifests itself as a significant increase in the amplitude and a change in the shape of the resonant optical reflection due to the electromagnetic coupling of the excitons. By modeling of the optical spectra, we evaluate the radiative and non-radiative broadening parameters of the excitonic states in the GaN quantum wells, which appear to be 0.4 ± 0.02 and 40 ± 5 meV, correspondingly, for the resonant exciton energy of 3.605 eV. The resonant Bragg structure based on the periodic sequence of the GaN quantum wells demonstrates an efficient coupling of photons and excitons at room temperature, which makes it promising for device applications.

The authors report the observation of the enhancement of photoluminescence excitation through the couplings of an InGaN∕GaN quantum well (QW) with localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs), which are generated on a Ag nanostructure deposited on the SiN-coated QW epitaxial sample. At the wavelengths corresponding to the LSP modes, the excitation light is first absorbed by the LSPs. The LSP energy is then transferred into the QW such that the effective QW absorption is enhanced. Meanwhile, the application of the LSP local field to the QW may increase its absorption coefficient. Then, the coupling of the relaxed carriers with the SPPs enhances light emission that becomes stronger as temperature increases because of the increased carrier momentum.

In this article, we study the effects of strong spin–orbit (SO) split-off band coupling on the valence-band structure and the optical gain of 70 Å strained InGaP–In(AlGa)P quantum-well lasers and a 100 Å cubic GaN quantum well using the 6×6 Luttinger–Kohn model. We first calculate the optical gain of InGaP quantum wells by comparing the 6×6 and 4×4 Luttinger–Kohn models. In the case of InGaP–In(AlGa)P quantum wells which have a SO split-off energy of 0.1 eV, the peak gain of the strained quantum well is overestimated in the low carrier injection region and is underestimated in the high injection, in the 4×4 model. On the other hand, the peak gain of an unstrained quantum well is overestimated in the 4×4 model over the wide range of carrier densities. Second, we obtain the Luttinger valence-band parameters γ1, γ2, and γ3 for a cubic phase of GaN using a semiempirical five level k⋅p model. Calculated valence-subband structures show that the subbands originated from the ‘‘light hole’’ and the ‘‘SO’’ are strongly coupled even at the zone center because of the very narrow SO split-off energy. It is expected that a very narrow separation (10 meV) between the SO band and the heavy- and light-hole bands causes two undesirable effects on the lasing of the GaN quantum well: (1) the TE and the TM polarizations have comparable magnitudes over the wide range of carrier densities and (2) the SO band will be easily occupied by the injected holes which in turn reduces the injection efficiency or increases the lasing threshold. Band-structure engineering is proposed to reduce the hole and the electron masses and to increase the SO band separation in order to reduce the lasing threshold.

Using metal-organic chemical vapor deposition, we have fabricated fully epitaxial nitride microcavties with AlGaN∕GaN distributed Bragg reflectors and InGaN quantum wells as the light emitter. To solve the problem of cracking, a thin AlN anticracking layer was used. The samples were characterized using transmission electron microscope, reflectivity spectroscopy, and photoluminescence spectroscopy. A cavity quality factor of 200 was obtained and the spontaneous emission of cavity mode was measured from a 1λ GaN microcavity, with 40-pair Al0.24Ga0.76N∕GaN distributed Bragg reflectors as the bottom and top reflectors and three period In0.10Ga0.90N∕GaN quantum wells in the GaN cavity layer.

We apply first-principles calculations to study the effects of extreme quantum confinement on the electronic, excitonic, and radiative properties of atomically thin (1–4 atomic monolayers) GaN quantum wells embedded in AlN. We determine the quasiparticle bandgaps, exciton energies and wave functions, radiative lifetimes, and Mott critical densities as a function of well and barrier thickness. Our results show that quantum confinement in GaN monolayers increases the bandgap up to 5.44 eV and the exciton binding energy up to 215 meV, indicating the thermal stability of excitons at room temperature. Exciton radiative lifetimes range from 1 to 3 ns at room temperature, while the Mott critical density for exciton dissociation is approximately 1013 cm−2. The luminescence is transverse-electric polarized, which facilitates light extraction from c-plane heterostructures. We also introduce a simple approximate model for calculating the exciton radiative lifetime based on the free-carrier bimolecular radiative recombination coefficient and the exciton radius, which agrees well with our results obtained with the Bethe–Salpeter equation predictions. Our results demonstrate that atomically thin GaN quantum wells exhibit stable excitons at room temperature for potential applications in efficient light emitters in the deep ultraviolet as well as room-temperature excitonic devices.