We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well ͑MQW͒ structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3-1.9 m wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures ͑Ͼ720°C͒ due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to 10 20 cm −3 . By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.

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We overview recent progress in growth aspects of group III-nitride heterostructures for deep ultraviolet (DUV) light-emitting diodes (LEDs), with particular emphasis on the growth approaches for attaining high-quality AlN and high Al-molar fraction AlGaN. The discussion commences with the introduction of the current status of group III-nitride DUV LEDs and the remaining challenges. This segues into discussion of LED designs enabling high device performance followed by the review of advances in the methods for the growth of bulk single crystal AlN intended as a native substrate together with a discussion of its UV transparency. It should be stated, however, that due to the high-cost of bulk AlN substrates at the time of this writing, the growth of DUV LEDs on foreign substrates such as sapphire still dominates the field. On the deposition front, the heteroepitaxial growth approaches incorporate high-temperature metal organic chemical vapor deposition (MOCVD) and pulsed-flow growth, a variant of MOCVD, with the overarching goal of enhancing adatom surface mobility, and thus epitaxial lateral overgrowth which culminates in minimization the effect of lattice-and thermal-mismatches. This is followed by addressing the benefits of pseudomorphic growth of strained high Al-molar fraction AlGaN on AlN. Finally, methods utilized to enhance both p-and n-type conductivity of high Al-molar fraction AlGaN are reviewed.

We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n-and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources. V

We present a temperature-dependent time-resolved photoluminescence study of the nonradiative processes in polar GaN/AlN quantum dots and quantum wells. The photoluminescence decay times of quantum wells drop above 50 K due to the presence of nonradiative recombination centers. In contrast, the three-dimensional carrier confinement in quantum dots efficiently suppresses nonradiative processes up to room temperature, even for radiative decay times reaching the microsecond range.

We have investigated the strain relaxation mechanisms in short-period polar GaN/AlN superlattices deposited by plasma-assisted molecular-beam epitaxy, and designed to display intersubband transitions at 1.55 μm. In a first stage, we have identified the growth conditions to minimize strain relaxation, using a Ga excess to reduce the (0001) surface free energy of both GaN and AlN. Under these growth conditions, crack propagation is not observed, even for the tensile-strained superlattices grown on GaN templates. The initial misfit relaxation in the vicinity of the buffer occurs by the formation of a-type dislocations. The final strain state of the superlattice, reached after 10–20 periods, is independent of the substrate (either GaN or AlN templates). Once the steady-state conditions are reached, we observe a periodic partial relaxation of quantum wells and barriers. High-resolution transmission electron microscopy indicates that the periodic relaxation can be related to the presence of basal and prismatic stacking faults creating clusters with an in-plane length of tens of nanometers. The effect of these defects on the optical performance of the superlattices is discussed by simulation of electronic structure using an 8×8 k⋅p Schrödinger–Poisson solver. In the presence of basal stacking faults at the quantum well interfaces, the deviation of the e1-e2 intersubband transition with respect to the nominal value is expected to be smaller than the measured absorption line width.

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities ($10 10 cm À3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.