Deep ultraviolet (DUV) optoelectronic devices (<280 nm) have widely applications in high-density information storage, environmental protection, medical treatment, sterilization and other fields. Especially in nowadays, the outbreak of a new coronavirus is threatening over ten millions of lives globally. The investigation and development of new generation semiconductor material and optoelectronic devices with wavelengths in DUV range become increasingly attractive for both academic and industry worldwide. AlGaN as a wide band gap semiconductor spanning the UVA, UVB, and UVC range with appealing properties, including high peak electron velocity, saturation velocity, thermal stability and breakdown field, allow device operation below the visible spectrum range suitable for solid state DUV emitters. Despite their recognized prospective applications, Al-rich AlGaN optical devices are still subject to the limited efficiency. In the previous study, it is observed in high-Al-content AlGaN quantum well (QW) on the c-plane that the primary radiative emission at the band edge exhibits an abnormal behavior, which is different from the other emission that is sensitive to the external electric fields, as shown in figure 1. It is generally believed that the lowest quantum level can offer better carrier confinement than higher quantum level. As the valence band maximum (VBM) of high-Al-content AlGaN is dispersive crystal field split-off hole band (CH band) consist of pz orbital, the abnormal quantum confinement become one of the limitations of efficiencient DUV emission. On account of the unique band structure and orbital configuration, the properties of radiative transition in quantum structure is strongly correlated to pz orbital. The induced energy gain in the con?nement direction should be taken into account in addition to the band-offset. The limited understanding of orbital coupling mechanism in quantum confine direction impeding the development of high-Al-content AlGaN optoelectronic devices.
To understand the underlying mechanism of orbital intercoupling based on the quantum con?nement direction, high-Al-content AlGaN QW models were constructed and studied by first-principle simulations. The band structure analysis shows that valence band is consist of p orbitals. p orbitals overlapping in a head-over-head fashion tend to form ppσ coupling with positive energy gain, while p orbital overlapping with one another in a side-by-side fashion tend to bring about ppπ coupling with negative energy variation. In the c-plane polar AlGaN QW, the compensation of CH band potential barrier at VBM is dominantly contributed from the ppσ coupling. In the case of the HH/LH band, the barrier is enhanced by the ppπ coupling. The carrier distribution and the orbital projected density of states at VBM along the confinement direction in figure 2 exhibit that the dominant ppσ coupling of pz state lower the potential barrier, resulting in the delocalization of the pz orbital. The situation is reversed upon the the ppπ coupling that px orbital have higher barrier for quantum confinement to confine carriers in the quantum well.
In the light of the relationship between the orbital coupling and the confinement direction, orbital engineering was proposed by incline quantum well plane to modify the energy variations in duced by the orbital coupling. Al0.75Ga0.25N/AlN quantum well models were constructed with the well plane inclined from 0° to 90° at a step of 30° referred to the c plane. The inclination angle of 30° and 60° correspond to the (10¯13) and (10¯11), two 90° models of (10¯10) and (11¯20) plane nonpolar quantum well. As increasing the inclination angles, the ppσ component decreases while the ppπ component increases. The distribution of p states at VBM further confirm the enhancement of quantum confinement with the increase in radiative transition rate at the band edge. Experimentally, the quantum well were grown on different facets of the hexagonal microrods with pyramid-shaped tops , as shown in figure 3. According to scanning electron microscope and cathodoluminescence (CL) spectra, even the characterization are taken from the top of the pyramid, the emission from the quantum well on the nonpolar plane is stronger than that on the semipolar and polar plane, which demonstrates the feasibility of the orbital engineering.
The research effort in the present work has led to the evidence that the orbital intercoupling plays a pivotal role especially in the materials with strong polarization. The valence p orbitals in the quantum structure are sensitive to the confinement direction, which distorts the symmetric the rectangular well potential. Awareness of the orbital intercoupling and its influence on the well potential provides a new perspective on the construction of heterostructures and superlattices. It has been reported in the literature that high-temperature superconductivity is observed in charge-transfer compounds characterized by strong hybridization between oxygen 2p and transition metal 3d states and with complex electronic configurations, which gives hints for the active presence of the orbital coupling. It is natural to expect that the orbital engineering is not just limited to AlGaN but also to extending the applications in novel semiconductor material and structure with unique functional properties.