Cubic nitrides are candidate materials for next-generation optoelectronic applications as they lack internal fields and promise to cover large parts of the electromagnetic spectrum from the deep UV towards the mid infrared. This demands high-quality epitaxial growth of c-GaN as base material. We demonstrate the influence of pre-growth treatments and c- AlN buffer layers on the quality of c-GaN grown on 3C-SiC/Si substrates by molecular beam epitaxy (MBE). Optimized parameters yield extremely small surface roughness values below 1 nm of phase pure c-GaN layers with very limited stacking fault densities. Structural properties have been studied by X-ray diffraction and atomic force microscopy and surpasses the current standards, which allows for growth of more complex quantum structures for device application.
Cubic InxGa1-xN alloys are a candidate material for optoelectronic applications because they lack internal polarization fields and promise to cover a vast range of emission wavelengths. However, the large discrepancy in interatomic spacing and growth temperatures of c-GaN and c-InN hinder InxGa1-xN-growth. We report cubic InxGa1-xN layers grown by plasmaassisted MBE and achieve continuous miscibility of the indium content x(In) over the whole composition range. X-ray diffraction precisely monitors the composition, phase purity and miscibility of the thin films. Furthermore, we discuss the impact of the indium content on the crystallinity. Complementary, low-temperature photoluminescence studies elucidate the optical response of cubic InxGa1-xN layers.
A strong limitation for the quantum efficiency of group III-nitride based light emitters is the spatial electron-hole separation due to the quantum-confined Stark effect (QCSE). To overcome this problem, Hönig et al. [1] proposed a concept, the Internal-Field-Guarded-Active-Region Design (IFGARD), which enables quasi electric-field free active regions in polar heterostructures. Here, we show how the encapsulation of the active region by additional guard layers results in a strong reduction of the built-in electric field in c-plane wurtzite nanostructures. In particular, we demonstrate experimental evidence for the successful realization of an IFGARD structure based on GaN/AlN heterostructures embedded in GaN nanowires. By means of power-dependent and time-resolved µ-photoluminescence (µ-PL) we experimentally proof the validity of the IFGARD structure. We managed to tune the emission of 4-nm-thick GaN nano-discs up to 3.32 eV at low excitation powers, which is just 150 meV below the bulk GaN bandgap [2]. Our results demonstrate an almost complete elimination of the QCSE in comparison to conventional structures which show approximately 1 eV red-shifted emission. The reduction of the QCSE results in a significant increase of the radiative exciton decay rates by orders of magnitude and demonstrates the potential of IFGARD structures for future light sources based on polar heterostructures.
[1] Hönig et al., Phys. Rev. Applied 7, 024004 (2017)
[2] Schlichting et al., arXiv:1707.06882 (2017).
Research on III-nitride intersubband (ISB) transitions in the THz spectral range is motivated by the large LO-phonon energy of GaN, which should permit device operation with limited thermal interference, and at infrared wavelengths inaccessible to other III-V compounds due to Reststrahlen absorption. A main challenge to extend the polar GaN-ISB technology towards the THz region comes from the polarization-induced internal electric field, which imposes an additional confinement that increases the energetic distance between the electronic levels. In order to surmount this constraint, we propose alternative multi-layer quantum well designs that create a pseudo-square potential profile with symmetric wavefunctions [1]. The robustness of these designs and their integration in device architectures requiring tunneling transport will be discussed.
An alternative approach to obtain square potential profiles is the use of nonpolar crystallographic orientations. In this contribution, we present an experimental study of THz ISB transitions in m-plane GaN/AlGaN quantum wells grown on free-standing m-GaN [2]. For Al contents below 15%, such structures can be grown without epitaxially-induced extended defects. We demonstrate nonpolar quantum wells which display ISB transitions in the 7-10 THz band, and we will discuss the effect of the doping density in the quantum wells on the transition energy and line width. Finally, we will present a comparative study using silicon and germanium as n-type dopants.
[1] M. Beeler, et al., Appl. Phys. Lett. 105, 131106 (2014)
[2] C.B. Lim, et al., Nanotechnology 26, 435201 (2015); Nanotechnology 27, 145201 (2016).
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