Axial Inhomogeneity of Mg-Doped GaN Rods: A Strong Correlation among Componential, Electrical, and Optical Analyses

Abstract

We systematically characterized the inhomogeneous doping properties along the c-axis of Mg-doped p-GaN microrods. Axial variation of doping concentration and electrical resistance on the p-GaN rod were measured by time-of-flight secondary-ion-mass-spectrometry and four-point probe measurements, respectively. Defects-related optical information was obtained from photoluminescence spectra together with Raman experiments revealing the change of crystal quality and strain along the rod. On the basis of a correlation of these analyses, we confirmed that Mg concentration decreased along the axial direction of the rod, leading to increasing electrical resistance. This axial Mg concentration change was revealed by green luminescence because the intensity of green luminescence sensitively varied with the doping density in both high-doping and low-doping rods. Interestingly, all the resistances at the highly doped rods were higher than the lowly doped rods due to overall mobility degradation at the high-doping rods caused by a scattering effect of increased Mg impurities and strain. All analyses provided complementary information on the p-type doping process and contribute to understanding the p-doping properties of GaN rod based photonic devices. Furthermore, our axially resolved optical spectroscopic (photoluminescence and Raman) methods can provide a facile, fast, and nondestructive way to estimate the axial doping and conductivity inhomogeneity of a Mg-doped p-GaN rod without having complex, time-consuming, and destructive structural and electrical measurements.

Understanding the p-Type GaN Nanocrystals on InGaN Nanowire Heterostructures

“…The vertical thickness of the p-GaN nanocrystal layer can also be expected to be ∼200 nm from this CL mapping image. In the case of the nanowire grown at a Mg cell temperature of 370 °C (Mg BEP of ∼2.1 × 10 –7 Torr), strong optical emissions were observed near the entire surface region of the single nanowire, shown in Figure d . Interestingly, a significantly weaker emission was detected at the bottom of the nanowire than at the top of the nanowire.…”

“…A strong GaN near-band-edge (NBE) emission that peaked at 3.51 eV was observed from the undoped GaN nanowire. The p-GaN nanocrystals grown under low Mg doping, mid Mg doping, and high Mg doping conditions show emission peaks of a broad band around 3.0 to 3.25 eV, generally termed as the ultraviolet luminescence (UVL), along with low-intensity peaks around 2.83 eV, termed the blue luminescence (BL), and 3.45 eV. ,, Low Mg-doping GaN showed a peak of 3.24 eV that related to a deep Mg ground state. The energy peak at 3.16 eV for mid Mg-doping GaN has been attributed to the donor–acceptor pair (DAP) transition related to the corresponding Mg Ga deep acceptor.…”

“…In the case of the nanowire grown at a Mg cell temperature of 370 °C (Mg BEP of ∼2.1 × 10 −7 Torr), strong optical emissions were observed near the entire surface region of the single nanowire, shown in Figure 3d. 12 Interestingly, a significantly weaker emission was detected at the bottom of the nanowire than at the top of the nanowire. It is expected that p-GaN thin layers were formed as a core−shell structure on the lateral surfaces of the InGaN/ GaN active region and the n-GaN nanowire, as illustrated in Figure 3c.…”

“…9−11 Furthermore, one of the most important parameters in achieving high-efficiency devices based on GaN materials is the control of p-type doping elements as well as the GaN structure. 12 Magnesium has been an efficient p-type dopant for GaN-based optoelectronic and electronic devices up to now. 13 Several studies have been reported on Mg-doped GaN structures, including the influence of Mg doping on the structural properties, 14 the surface charge properties with Mg incorporation, 15 and the photoelectrochemical measurements on Mg-doped GaN nanowires.…”

“…Compared with conventional planar structures, nanowire heterostructures offer distinct advantages, including significantly reduced polarization fields , and dislocation densities and efficient heat dissipation . Typical GaN-based nanowires are grown randomly on a substrate either by a catalyst or by a catalyst-free growth method, using molecular beam epitaxy (MBE) or metal–organic chemical vapor deposition (MOCVD) techniques. ,− Their application in visible band gap devices, such as GaN- and InGaN-based light-emitting diodes (LEDs) and laser diodes (LDs), has been limited due to the inhomogeneous distribution of indium by the random structure formation of the nanowires. − Furthermore, one of the most important parameters in achieving high-efficiency devices based on GaN materials is the control of p-type doping elements as well as the GaN structure . Magnesium has been an efficient p-type dopant for GaN-based optoelectronic and electronic devices up to now .…”