Current laser-based display and lighting applications are invariably using blue laser diodes (LDs) grown on free-standing GaN substrates, which are costly and smaller in size compared with other substrate materials.^1–3 Utilizing less expensive and large-diameter Si substrates for hetero-epitaxial growth of indium gallium nitride/gallium nitride (InGaN/GaN) multiple quantum well (MQW) structure can substantially reduce the cost of blue LDs and boost their applications. To obtain a high crystalline quality crack-free GaN thin film on Si for the subsequent growth of a blue laser structure, a hand-shaking structure was formed by inserting Al-composition step down-graded AlN/Al_xGa_1−xN buffer layers between GaN and Si substrate. Thermal degradation in InGaN/GaN blue MQWs was successfully suppressed with indium-rich clusters eliminated by introducing hydrogen during the growth of GaN quantum barriers (QBs) and lowering the growth temperature for the p-type AlGaN/GaN superlattice optical cladding layer. A continuous-wave (CW) electrically pumped InGaN/GaN quantum well (QW) blue (450 nm) LD grown on Si was successfully demonstrated at room temperature (RT) with a threshold current density of 7.8 kA/cm².

We demonstrate the growth of pure wurtzite phase Sc_xAl_1−xN with a Sc composition as high as x = 0.34 on GaN and AlN templates using plasma-assisted molecular beam epitaxy. The wurtzite structure is well maintained even at high growth temperatures up to 900 °C for Sc_0.2Al_0.8N. Smooth surface morphology (root mean square roughness less than 1 nm) and excellent crystal quality [(002) plane rocking curve full-width at half maximum below 450 arc sec] are achieved over the range of x ≤ 0.34. Optical absorption studies indicate a decreasing bandgap with increasing Sc with a linear relationship of E_g(x) = 6.1 − 3.39x, which is in good agreement with the theoretical prediction. A monotonically tunable refractive index between AlN and GaN is further measured for Sc_xAl_1−xN with various Sc compositions. This work provides a viable path for the epitaxy of wurtzite Sc_xAl_1−xN with high Sc compositions. The distinct effect of substitutional Sc on bandgap and refractive index could be used in designing high-performance optoelectronic, electronic, and piezoelectric devices, and III-nitride integrated photonics and optical cavities.

Molecular beam epitaxy of single-phase wurtzite N-polar Sc_xAl_1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc_0.18Al_0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc_0.11Al_0.89N and Sc_0.30Al_0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc_0.18Al_0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm²/V s is measured for a 15-nm-thick Sc_0.18Al_0.82N barrier sample with a sheet electron concentration of 4.1 × 10¹³ cm⁻², and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of Sc_xAl_1−xN provides promising opportunities for applications in high-frequency and high-power electronic and ferroelectric devices.

Aluminum-nitride-on-sapphire has recently emerged as a novel low-loss photonics platform for a variety of on-chip electro-optics as well as linear and nonlinear optics applications. In this Letter, we demonstrate ultrahigh quality factor (${\mathrm{Q}}_{\mathrm{int}}$) microring resonators using single-crystal aluminum nitride grown on a sapphire substrate with an optimized design and fabrication process. A record high intrinsic ${\mathrm{Q}}_{\mathrm{int}}$ up to $2.8\times {10}^6$ at the wavelength of 1550 nm is achieved with a fully etched structure, indicating a low propagation loss less than 0.13 dB/cm. Such high ${\mathrm{Q}}_{\mathrm{int}}$ aluminum-nitride-on-sapphire resonators with their wide bandgap and electro-optical and nonlinear optical properties is promising for a wide range of low-power and high-power compact on-chip applications over a broad spectral range.

The efficiency of conventional quantum well light-emitting diodes (LEDs) decreases drastically with reducing areal size. Here we show that such a critical size scaling issue of LEDs can be addressed by utilizing N-polar InGaN nanowires. We studied the epitaxy and performance characteristics of N-polar InGaN nanowire LEDs grown on sapphire substrate by plasma-assisted molecular beam epitaxy. A maximum external quantum efficiency ∼ 11 % was measured for LEDs with lateral dimensions as small as 750 nm directly on wafer without any packaging. The effect of electron overflow and Auger recombination on the device performance is also studied. This work provides a viable approach for achieving high-efficiency nano and micro LEDs that were not previously possible.