Cuprous halides, characterized by a direct wide band-gap and a good lattice matching with Si, is an intrinsic p-type I-VII compound semiconductor. It shows remarkable optoelectronic properties, including a large exciton binding energy at room temperature and a very small piezoelectric coefficient. The major obstacle to its application is the difficulty in growing a single-crystal epitaxial film of cuprous halides. We first demonstrate the single crystal epitaxy of high quality cuprous iodide (CuI) film grown on Si and sapphire substrates by molecular beam epitaxy. enhanced photoluminescence on the order of magnitude larger than that of GaN and continuous-wave optically pumped lasing were found in MBE grown CuI film. The intrinsic p-type characteristics of CuI were confirmed using an n-AlGaN/p-CuI junction that emits blue light. The discovery will provide an alternative way towards highly efficient optoelectronic devices compatible with both Si and III-nitride technologies. Cuprous halide atoms form a tetrahedrally coordinated isomorphic structure with the diamond-type lattice 1-16. The Cu atoms are located on a face-centered cubic lattice, and the four nearest neighboring sites are occupied by halogen-type atoms, as shown in Fig. 1(a), which forms a zinc blende structure 8,17-26. The zinc blende structure minimizes the piezoelectric effect resulting from polarization because it has four symmetry-equivalent polar axes and each electrical field contribution cancels each other. A semiconductor is polarized if it has a singular polar axis as in the case of the [0001] axis for wurtzite semiconductors, such as gallium nitride (GaN) and zinc oxide (ZnO) 27-35. It is well known that the luminous efficiency of wurtzite structure is significantly affected by the presence of polarization. Large built-in electrostatic fields of the order of MV/cm in the active layer will result in long radiative lifetimes 28-35. Since the piezoelectric effect of cuprous halides is much smaller than that of III-nitrides, we can easily ignore piezoelectric field effects for cuprous halide semiconductors, which is a great advantage for optoelectronic applications 1,23. Moreover, the large lattice mismatch between the nitride semiconductor and the commercially available substrates, typically sapphire or SiC, inevitably generates a high misfit dislocation density in the order of 10 10 cm −2. The performance and reliability of the device would be degraded. Although a lot of epitaxial growth technologies have been developed to minimize misfit dislocation density, it is still suffered from the high density of defects which prevents realization of high performance electronic and optical devices on nitride semiconductor 2,4,16,20. It would be desirable to have a kind of semiconductor with good lattice match to Si for the purpose.

Using first-principles calculations based on density functional theory, we have systematically studied the influence of in-plane lattice constant and thickness of slabs on the concentration and distribution of two-dimensional hole gas (2DHG) in AlN/GaN superlattices. We show that the increase of in-plane lattice constant would increase the concentration of 2DHG at interfaces and decrease the valence band offset, which may lead to a leak of current. Increasing the thickness of AlN and/or decreasing the thickness of GaN would remarkably strengthen the internal field in GaN layer, resulting in better confinement of 2DHG at AlN/GaN interfaces. Therefore, a moderate larger in-plane lattice constant and thicker AlN layer could improve the concentration and confinement of 2DHG at AlN/GaN interfaces. Our study could serve as a guide to control the properties of 2DHG at III-nitride interfaces and help to optimize the performance of p-type nitride-based devices.