Zinc oxide is a breakthrough multifunctional material of emerging interest applicable in the areas of electronics, computing, energy harvesting, sensing, optoelectronics, and biomedicine. ZnO has a direct and wide bandgap and high exciton binding energy. It is nontoxic, earth-abundant, and biocompatible. However, the growth and characterization of high-quality ZnO has been a challenge and bottleneck in its development. Efforts have been made to synthesize device-quality zinc oxide and unleash its potential for multiple advanced applications. ZnO could be grown as thin films, nanostructures, or bulk, and its properties could be optimized by tuning the growth techniques, conditions, and doping. Zinc oxide could be a suitable material for next generation devices including spintronics, sensors, solar cells, light-emitting diodes, thermoelectrics, etc. It is important and urgent to collate recent advances in this material, which would strategically help in further research and developments in ZnO. This paper provides a coherent review of developments in ZnO growth, leading to its advancing applications. Recent developments in growth technologies that address native defects, current challenges in zinc oxide, and its emerging applications are reviewed and discussed in this article.

A series of AlGaN/GaN high-electron-mobility transistor (HEMT) structures, with an AlN thin buffer, GaN thick layer and Al_0.25Ga_0.75N layer (13–104 nm thick), is prepared by metal–organic chemical vapor deposition and investigated via multiple techniques. Spectroscopic ellipsometry (SE) and temperature-dependent measurements and penetrative analyses have achieved significant understanding of these HEMT structures. Bandgaps of AlGaN and GaN are acquired via SE-deduced relationships of refraction index n and extinguish coefficient k vs. wavelength λ in a simple but straightforward way. The optical constants of n and k, and the energy gap E_g of AlGaN layers, are found slightly altered with the variation in AlGaN layer thickness. The Urbach energy E_U at the AlGaN and GaN layers are deduced. High-resolution X-ray diffraction and calculations determined the extremely low screw dislocation density of 1.6 × 10⁸ cm⁻². The top AlGaN layer exhibits a tensile stress influenced by the under beneath GaN and its crystalline quality is improved with the increase in thickness. Comparative photoluminescence (PL) experiments using 266 nm and 325 nm two excitations reveal and confirm the 2DEG within the AlGaN-GaN HEMT structures. DUV (266 nm) excitation Raman scattering and calculations acquired carrier concentrations in compatible AlGaN and GaN layers.

GaN on Si plays an important role in the integration and promotion of GaN-based wide-gap materials with Si-based integrated circuits (IC) technology. A series of GaN film materials were grown on Si (111) substrate using a unique plasma assistant molecular beam epitaxy (PA-MBE) technology and investigated using multiple characterization techniques of Nomarski microscopy (NM), high-resolution X-ray diffraction (HR-XRD), variable angular spectroscopic ellipsometry (VASE), Raman scattering, photoluminescence (PL), and synchrotron radiation (SR) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. NM confirmed crack-free wurtzite (w-) GaN thin films in a large range of 180–1500 nm. XRD identified the w- single crystalline structure for these GaN films with the orientation along the c-axis in the normal growth direction. An optimized 700 °C growth temperature, plus other corresponding parameters, was obtained for the PA-MBE growth of GaN on Si, exhibiting strong PL emission, narrow/strong Raman phonon modes, XRD w-GaN peaks, and high crystalline perfection. VASE studies identified this set of MBE-grown GaN/Si as having very low Urbach energy of about 18 meV. UV (325 nm)-excited Raman spectra of GaN/Si samples exhibited the GaN E₂(low) and E₂(high) phonon modes clearly without Raman features from the Si substrate, overcoming the difficulties from visible (532 nm) Raman measurements with strong Si Raman features overwhelming the GaN signals. The combined UV excitation Raman–PL spectra revealed multiple LO phonons spread over the GaN fundamental band edge emission PL band due to the outgoing resonance effect. Calculation of the UV Raman spectra determined the carrier concentrations with excellent values. Angular-dependent NEXAFS on Ga K-edge revealed the significant anisotropy of the conduction band of w-GaN and identified the NEXAFS resonances corresponding to different final states in the hexagonal GaN films on Si. Comparative GaN material properties are investigated in depth.

A series of indium nitride (InN) thin films have been grown on sapphire substrates by molecular beam epitaxy (MBE) technology under different growth conditions of temperature and plasma power. Their structural, surface, and optical properties are studied by a variety of techniques of scanning electron microscopy, Hall effect, x-ray diffraction, photoluminescence (PL), Raman scattering, x-ray photoelectron spectroscopy (XPS), synchrotron radiation x-ray absorption near edge structure (XANES), and so on. The lower carrier concentration in InN can be obtained with a higher MBE growth temperature and a lower plasma power. As the plasma power increases, the PL peak energy is observed to shift toward the higher energy side and the Raman E2 (high) and A1 (LO) modes are shifted to the lower frequency. The residual compressive strain in epitaxial InN is relaxed. The lower plasma power and the higher growth temperature are preferred for the MBE growth of high-quality InN films. The influencing factors on the InN PL peak and band gap Eg have been revealed. It is evidenced that the InN PL peak and Eg can be shifted from high down to ∼0.65 eV with the carrier concentration down to low E19 cm−3 and the plasma power down to ∼80 W. Both the XPS and N K-edge XANES revealed the antisite defect of N on the In site, NIn. The XANES In L-edge measurements on the In L3-edge of InN films with various carrier concentrations has indicated the fourfold InN intermediate crystal structures. These obtained results are significant and useful to deepen the understanding and promote further investigation in InN and III-N materials.

Ultrawide bandgap (UWBG) AlN c- and m-face crystals have been prepared using the physical vapor transport (PVT) method and studied penetratively using temperature-dependent (TD) Raman scattering (RS) measurements under both visible (457 nm) and DUV (266 nm) excitations in 80–870 K, plus correlative atomic force microscopy (AFM) and variable-angle (VA) spectroscopic ellipsometry (SE). VASE identified their band gap energy as 6.2 eV, indicating excellent AlN characteristics and revealing Urbach energy levels of about 85 meV. Raman analyses revealed the residual tensile stress. TDRS shows that the E₂(high) phonon lifetime decayed gradually in the 80–600 K range. Temperature has the greater influence on the stress of m-face grown AlN crystal. The influence of low temperature on the E₂(high) phonon lifetime of m-plane AlN crystal is greater than that of the high-temperature region. By way of the LO-phonon and plasma coupling (LOPC), simulations of A₁(LO) modes and carrier concentrations along different faces and depths in AlN crystals are determined. These unique and significant findings provide useful references for the AlN crystal growth and deepen our understanding on the UWBG AlN materials.