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We report on the near-infrared intersubband (ISB) absorption properties of strain-free Sc_0.14Al_0.86N/GaN multiple quantum wells (MQWs) grown on c-plane GaN substrates by molecular beam epitaxy. These MQWs exhibit strong, sharp, and tunable absorption energies between 515 meV and 709 meV, for well widths ranging from 7 nm to 1.5 nm, respectively. Observation of ISB absorption in ultra-thin Sc_0.14Al_0.86N/GaN MQWs not only extends the near-infrared range accessible with Sc-containing nitrides but also highlights the challenges of growing nanometer-thick GaN quantum wells. We explore the effects of growth temperature on absorption characteristics and find that substrate temperatures above 600°C significantly enhance ISB absorption intensity but also introduce an energy redshift for the narrowest wells. The redshift is attributed to increased interface roughness due to ScAlN surface morphology degradation at higher temperatures. Additionally, a comparison of experimental results with simulated band-structures indicates that the magnitude of net polarization rises faster with Sc-composition than previously suggested by theoretical calculations. This study advances the prospects of ScAlN/GaN heterostructures for novel photonic devices in the technologically important near-infrared range. 

Using comprehensive x-ray reciprocal space mapping, we establish the precise lattice-matching composition for wurtzite ScxAl1−xN layers on (0001) GaN to be x = 0.14 ± 0.01. 100 nm thick ScxAl1−xN films (x = 0.09–0.19) were grown in small composition increments on c-plane GaN templates by plasma-assisted molecular beam epitaxy. The alloy composition was estimated from the fit of the (0002) x-ray peak positions, assuming the c-lattice parameter of ScAlN films coherently strained on GaN increases linearly with Sc-content determined independently by Rutherford backscattering spectrometry [Dzuba et al., J. Appl. Phys. 132, 175701 (2022)]. Reciprocal space maps obtained from high-resolution x-ray diffraction measurements of the (101¯5) reflection reveal that ScxAl1−xN films with x = 0.14 ± 0.01 are coherently strained with the GaN substrate, while the other compositions show evidence of relaxation. The in-plane lattice-matching with GaN is further confirmed for a 300 nm thick Sc0.14Al0.86N layer. The full-width-at-half-maximum of the (0002) reflection rocking curve for this Sc0.14Al0.86N film is 106 arc sec and corresponds to the lowest value reported in the literature for wurtzite ScAlN films. 

Wurtzite ScxAl1−xN/GaN (x = 0.13–0.18) multi-quantum wells grown by molecular beam epitaxy on c-plane GaN are found to exhibit remarkably strong and narrow near-infrared intersubband absorption in the technologically important 1.8–2.4 μm range. Band structure simulations reveal that, for GaN wells wider than 3 nm, the quantized energies are set by the steep triangular profile of the conduction band caused by intrinsic polarization fields. As a result, the intersubband transition energies provide unique and direct access to essential ScAlN polarization parameters. Measured infrared absorption indicates that the spontaneous polarization difference of the presumed lattice-matched Sc0.18Al0.82N/GaN heterostructure is smaller than the theoretically calculated value. The intersubband transition energies are relatively insensitive to the barrier alloy composition indicating negligible variation of the net polarization field in the probed 0.13–0.18 Sc composition range. 

Growth of wurtzite Sc x Al 1−x N (x < 0.23) by plasma-assisted molecular-beam epitaxy on c-plane GaN at high temperatures significantly alters the extracted lattice constants of the material due to defects likely associated with remnant phases. In contrast, ScAlN grown below a composition-dependent threshold temperature exhibits uniform alloy distribution, reduced defect density, and atomic-step surface morphology. The c-plane lattice constant of this low-temperature ScAlN varies with composition as expected from previous theoretical calculations and can be used to reliably estimate alloy composition. Moreover, lattice-matched Sc 0.18 Al 0.82 N/GaN multi-quantum wells grown under these conditions display strong and narrow near-infrared intersubband absorption lines that confirm advantageous optical and electronic properties. 

Band structure, strain, and polarization engineering of nitride heterostructures open unparalleled opportunities for quantum sensing in the infrared. Intersubband absorption and photoluminescence are employed to correlate structure with optical properties of nonpolar strain-balanced InGaN/AlGaN nanostructures grown by molecular-beam epitaxy. Mid-infrared intersubband transitions in m-plane (In)AlxGa1-xN/In0.16Ga0.84N (0.19x0.3) multi-quantum wells were observed for the first time in the range of 3.4-5.1 μm (244-360 meV). Direct and attenuated total-reflection infrared absorption measurements are interpreted using structural information revealed by high-resolution x-ray diffraction and transmission electron microanalysis. The experimental intersubband energies are better reproduced by calculations using the local-density approximation than the Hartree-Fock approximation for the exchange-correlation correction. The effect of charge density, quantum well width, and barrier alloy composition on the intersubband transition energy was examined to evaluate the potential of this material for practical infrared applications. Temperature-dependent continuous-wave and time-resolved photoluminescence (TRPL) measurements are also investigated to probe carrier localization and recombination in m-plane InGaN/AlGaN quantum wells. Average localization depths of 21 meV and 40 meV were estimated for the undoped and doped structures, respectively. Using TRPL, dual localization centers were identified in undoped structures, while a single type of localization centers was found in doped structures. At 2 K, a fast decay time of approximately 0.3ns was measured for both undoped and doped structures, while a longer decay time of 2.2 ns was found only for the undoped sample. TRPL in magnetic field was explored to examine the effect of doping sheets on carrier dynamics. 

Mid-infrared intersubband transitions in strain-balanced m-plane (In)Al_xGa_1-xN/In_0.16Ga_0.84N (0.19≤x≤0.3) multi-quantum wells are reported for the first time in the range of 3.4–5.1 µm (244–360 meV). Direct and attenuated total-reflection infrared absorption measurements are interpreted using structural information revealed by high-resolution x-ray diffraction and transmission electron microanalysis. The experimental intersubband energies are better reproduced by calculations using the local-density approximation than the Hartree-Fock approximation for the exchange-correlation correction. The effect of charge density, quantum well width, and barrier alloy composition on the intersubband transition energy is also investigated to evaluate the potential of this material for practical device applications. 

Temperature-dependent continuous-excitation and time-resolved photoluminescence are studied to probe carrier localization and recombination in nearly strain-balanced m-plane In0.09Ga0.91N/Al0.19Ga0.81N multi-quantum wells grown by plasma-assisted molecular-beam epitaxy. An average localization depth of 21 meV is estimated for the undoped sample. This depth is much smaller than the reported values in polar structures and m-plane InGaN quantum wells. As part of this study, temperature and magnetic field dependence of time-resolved photoluminescence is performed. At 2 K, an initial fast decay time of 0.3 ns is measured for both undoped and doped structures. The undoped sample also exhibits a slow decay component with a time scale of 2.2 ns. The existence of two relaxation paths in the undoped structure can be attributed to different localization centers. The fast relaxation decays are relatively insensitive to external magnetic fields, while the slower relaxation time constant decreases significantly with increasing magnetic fields. The fast decay time scale in the undoped sample is likely due to indium fluctuations in the quantum well. The slow decay time may be related to carrier localization in the barriers. The addition of doping leads to a single fast decay time likely due to stronger exciton localization in the InGaN quantum wells.