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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. [ABSTRACT FROM AUTHOR]

Growth of wurtzite ScxAl1−xN (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 Sc0.18Al0.82N/GaN multi-quantum wells grown under these conditions display strong and narrow near-infrared intersubband absorption lines that confirm advantageous optical and electronic properties. [ABSTRACT FROM AUTHOR]

Anomalous growth rate reduction and associated composition divergence with increasing aluminum flux in m-plane AlGaN grown by plasma-assisted molecular beam epitaxy at low temperature (565 °C) are observed and investigated. We find that the AlGaN growth rate under conventional gallium-rich conditions decreases rapidly with increasing aluminum flux, contrary to expectations. Moreover, the aluminum fraction of these layers increases super-linearly with aluminum flux, indicating substantial nitrogen and gallium loss from the crystal surface. Indium surfactant assisted epitaxy (ISAE) is found to mitigate this effect significantly. ISAE AlGaN layers do not exhibit a significant decrease in the growth rate with increasing aluminum flux, and their aluminum compositions increase linearly with aluminum flux. Transmission electron microscopy (TEM) images reveal the presence of high-aluminum composition defects within the conventionally grown AlGaN layers, which are significantly reduced in ISAE AlGaN layers. Spatial correlation of these defects with local areas of reduced growth rate observed in an (In)Al0.30Ga0.70N/In0.16Ga0.84N multiple quantum well (MQW) structure suggests that these phenomena have a causal relationship. We attribute the growth rate reduction to the loss of nitrogen and gallium due to site-blocking effects of aluminum adatoms. TEM imaging indicates that high-quality, virtually defect-free (In)Al0.24Ga0.76N/In0.16Ga0.84N MQWs can be grown at 565 °C with negligible indium incorporation into the barriers. [ABSTRACT FROM AUTHOR]

The use of an indium surfactant considerably alters the composition and morphology of low-temperature non-polar m-plane (10 1 ¯ 0) AlxGa1−xN (x ∼ 0.2) and of silicon-doped AlGaN/InGaN multiple quantum wells grown by plasma-assisted molecular beam epitaxy. This paper compares heterostructures grown with indium surfactant with those grown under conventional stoichiometric and gallium-rich conditions at the relatively low temperature necessary for growth of In0.16Ga0.84N quantum wells (565 °C). Stoichiometric growth results in rough, inhomogeneous AlGaN layers that are unsuitable for optical devices. Gallium-rich growth produces a smoother AlGaN layer, reduced inhomogeneities, and sharper interfaces as compared to stoichiometric growth. However, due to the low temperature, gallium-rich growth leads to the formation of an unintentional GaN layer on top of each AlGaN barrier, reducing the energies of confined electronic states in the quantum wells. An indium surfactant enables two-dimensional AlGaN growth at low temperature, producing atomically flat surface morphology and sharp heterostructure interfaces. Indium surfactant assisted epitaxy also eliminates the high aluminum alloy inhomogeneities observed with conventional stoichiometric and gallium-rich growth. Even though partial indium incorporation into the AlGaN layer is found at the studied temperatures, the high-quality, uniform non-polar In0.055Al0.19Ga0.755N/In0.16Ga0.84N quantum wells grown with indium surfactant display bright and narrow photoluminescence that is essential for device applications. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectroscopy of nonpolar m-plane InGaN thin films with indium composition up to 21% and nearly strain-balanced In0.09Ga0.91N/Al0.19Ga0.81N superlattices grown by plasma-assisted molecular beam epitaxy was performed as a function of temperature. The experimental transition energies are consistently lower than the calculation based on structural parameters extracted from x-ray diffraction measurements. This indicates the presence of indium composition fluctuations in InGaN and hence local bandgap reduction that produces charge localization centers. The spectral width of the low-temperature PL of our m-plane InGaN/AlGaN superlattices is narrower than previously reported for m-plane InGaN/GaN quantum wells grown by MOCVD. The PL integrated intensity drops rapidly, though, as the temperature is increased to 300 K, indicating strong non-radiative recombination at room temperature. Time-resolved PL at low temperatures was performed to characterize the relaxation time scales in an undoped and a doped superlattice. [ABSTRACT FROM AUTHOR]