The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett. 18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors 2, 624 (1968)], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)], ZnO∕ZnSe n-p junctions (1975) by Tsurkan et al. [Semiconductors 6, 1183 (1975)], and Al∕Au Ohmic contacts by Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B 241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.

Multiple quantum well (MQW) InGaN light emitting diodes with and without electron blocking layers, with relatively small and large barriers, with and without p-type doping in the MQW region emitting at ∼420nm were used to determine the genesis of efficiency droop observed at injection levels of approximately ⩾50A∕cm2. Pulsed electroluminescence measurements, to avoid heating effects, revealed that the efficiency peak occurs at ∼900A∕cm2 current density for the Mg-doped barrier, near 550A∕cm2 for the lightly doped n-GaN injection layer, meant to bring the electron injection level closer to that of holes, and below 220A∕cm2 for the undoped InGaN barrier cases. For samples with GaN barriers (larger band discontinuity) or without p-AlGaN electron blocking layers the droop occurred at much lower current densities (⩽110A∕cm2). In contrast, photoluminescence measurements revealed no efficiency droop for optical carrier generation rates corresponding to the maximum current density employed in pulsed injection measurements. All the data are consistent with heavy effective mass of holes, low hole injection efficiency (due to relatively lower p-doping) leading to severe electron leakage being responsible for efficiency droop.

Compared with conventional display technologies, liquid crystal display (LCD), and organic light emitting diode (OLED), micro-LED displays possess potential advantages such as high contrast, fast response, and relatively wide color gamut, low power consumption, and long lifetime. Therefore, micro-LED displays are deemed as a promising technology that could replace LCD and OLED at least in some applications. While the prospects are bright, there are still some technological challenges that have not yet been fully resolved in order to realize the high volume commercialization, which include efficient and reliable assembly of individual LED dies into addressable arrays, full-color schemes, defect and yield management, repair technology and cost control. In this article, we review the recent technological developments of micro-LEDs from various aspects.

We derive a simple analytical formula for the low field electron mobility which uses the 2‐d degenerate statistics of the 2‐d electron gas. This takes into account the finite width of the depletion layer in (Al,Ga)As for the scattering by remote donors, scattering by the interface charge, and the polar‐optical and acoustic deformation potential and piezoelectric scattering. The largest measured value of mobility is determined by scattering due to interface charge in some cases. The ultimate value of the mobility which may be achieved is limited by the acoustic deformation potential and piezoelectric scattering at about 6.5×10⁶ cm²/V s for an interface carrier density of the 2‐d electron gas n_s0 ≂4×10¹¹ cm⁻². Our results agree very well with experimental data obtained in our laboratory as well as other laboratories.

A procedure based on the stacking sequences of semiconducting materials structures is proposed in order to determine the possible dislocation character of the interface steps between two different polytypes. It is next applied to the interface between the 2H and the 3C, 4H and 6H polytypes. Three different possible dislocations are found with ⅙〈2023〉, ⅓〈1010〉 and ½〈0001〉 Burgers vectors. In a high-resolution electron microscopy study of the particular case 2H-AlN/6H-SiC, only two kinds of interface step could be observed. This is in agreement with the assumption that the growth of AlN systematically starts at the wurtzite position on the surface of the substrate. One kind of interface step is observed to generate systematically an extended defect in the AlN layer which folds from the basal to the prismatic plane. Although the prismatic defect is not seen edge on, a careful study by image simulation allows one to identify a ½ 〈1011〉 fault vector contained in the fault plane. The formation of such a defect is related to the growth mode of AlN on α-SiC.