Evaluation of the concentration of point defects in GaN

Atomistic Origins of Various Luminescent Centers and n-Type Conductivity in GaN: Exploring the Point Defects Induced by Cr, Mn, and O through an Ab Initio Thermodynamic Approach

GaN is a technologically indispensable material for various optoelectronic properties, mainly due to the dopant-induced or native atomic-scale point defects that can create single photon emitters, a range of luminescence bands, and n- or p-type conductivities. Among the various dopants, chromium and manganese-induced defects have been of particular interest over the past few years, because some of them contribute to our present-day light-emitting diode (LED) and spintronic technologies. However, the nature of such atomistic centers in Cr and Mn-doped GaN is yet to be understood. A comprehensive defect thermodynamic analysis of Cr- and Mn-induced defects is essential for their engineering in GaN crystals because by mapping out the defect stabilities as a function of crystal growth parameters, we can maximize the concentration of the target point defects. We therefore investigate chromium and manganese-induced defects in GaN with ab initio methods using the highly accurate exchange-correlation hybrid functionals, and the phase transformations upon excess incorporation of these dopants using the CALPHAD method. We also investigate the impact of oxygen codoping that can be unintentionally incorporated during crystal growth. Our analysis sheds light on the atomistic cause of the unintentional n-type conductivity in GaN, being ON-related. In the case of Cr doping, the formation of CrGa defects is the most dominant, with an E +/0 charge transition at E VBM + 2.19 eV. Increasing nitrogen partial pressure tends to enhance the concentration of CrGa. However, in the case of doping with Mn, several different Mn-related centers can form depending on the growth conditions, with MnGa being the most dominant. MnGa possesses the E 2+/+, E +/0, and E 0/- charge transitions at 0.56, 1.04, and 2.10 eV above the VBM. The incorporation of oxygen tends to cause the formation of the MnGa-VGa center, which explains a series of prior experimental observations in Mn-doped GaN. We provide a powerful tool for point defect engineering in wide band gap binary semiconductors that can be readily used to design optimal crystal growth protocols.

Dislocation characterization inc-plane GaN epitaxial layers on 6 inch Si wafer with a fast second-harmonic generation intensity mapping technique

Second harmonic generation (SHG) intensity, Raman scattering stress, photoluminescence and reflected interference pattern are used to determine the distributions of threading dislocations (TDs) and horizontal dislocations (HDs) in thec-plane GaN epitaxial layers on 6 inch Si wafer which is a structure of high electron mobility transistor (HEMT). The Raman scattering spectra show that the TD and HD result in the tensile stress and compressive stress in the GaN epitaxial layers, respectively. Besides, the SHG intensity is confirmed that to be proportional to the stress value of GaN epitaxial layers, which explains the spatial distribution of SHG intensity for the first time. It is noted that the dislocation-mediated SHG intensity mapping image of the GaN epitaxial layers on 6 inch Si wafer can be obtained within 2 h, which can be used in the optimization of high-performance GaN based HEMTs.

Highly efficient blue InGaN nanoscale light-emitting diodes

Indium gallium nitride (InGaN)-based micro-LEDs (μLEDs) are suitable for meeting ever-increasing demands for high-performance displays owing to their high efficiency, brightness and stability^1-5. However, μLEDs have a large problem in that the external quantum efficiency (EQE) decreases with the size reduction^6-9. Here we demonstrate a blue InGaN/GaN multiple quantum well (MQW) nanorod-LED (nLED) with high EQE. To overcome the size-dependent EQE reduction problem^8,9, we studied the interaction between the GaN surface and the sidewall passivation layer through various analyses. Minimizing the point defects created during the passivation process is crucial to manufacturing high-performance nLEDs. Notably, the sol-gel method is advantageous for the passivation because SiO₂ nanoparticles are adsorbed on the GaN surface, thereby minimizing its atomic interactions. The fabricated nLEDs showed an EQE of 20.2 ± 0.6%, the highest EQE value ever reported for the LED in the nanoscale. This work opens the way for manufacturing self-emissive nLED displays that can become an enabling technology for next-generation displays.

Point defect detection and strain mapping in Si single crystal by two-dimensional multiplication moiré method

Although defect detection is critical for evaluating the manufacturing processes of semiconductor materials and metals, the detection of crystal defects, especially point defects, over a large field of view still faces considerable challenges. Herein, we report on the development of a two-dimensional (2D) multiplication moiré method using digital image processing to simultaneously detect point and line defects in a wide field of view. Defect locations were automatically detected by employing the concept of a hybrid strain, that is, the absolute value of the product of the strain distributions in different principal directions. To demonstrate a typical application of the proposed method, the hybrid strain distribution in a Si single crystal was measured, and point defects were successfully detected by transmission electron microscopy. The effectiveness of the proposed method was experimentally verified based on the enlarged views of atomic structures at several detected defect locations. This method is capable of visualizing defects by magnifying the lattice distortion in situ, which is a good solution to the problem faced by traditional methods in detecting point defects. This study paves the way for the detection of vacancies, interstitial atoms, substitutional atoms, dislocations, slips, and interfaces in various crystal structures and 2D materials.

Shallow and Deep States of Beryllium Acceptor in GaN: Why Photoluminescence Experiments Do Not Reveal Small Polarons for Defects in Semiconductors

Currently, only one shallow acceptor (Mg) has been discovered in GaN. Here, using photoluminescence (PL) measurements combined with hybrid density functional theory, we demonstrate that a shallow effective-mass state also exists for the Be_{Ga} acceptor. A PL band with a maximum at 3.38 eV reveals a shallow Be_{Ga} acceptor level at 113±5  meV above the valence band, which is the lowest value among any dopants in GaN reported to date. Calculations suggest that the Be_{Ga} is a dual-nature acceptor with the "bright" shallow state responsible for the 3.38 eV PL band, and the "dark," strongly localized small polaronic state with a significantly lower hole capture efficiency.

Synthesis and photoluminescence properties of hybrid 1D core-shell structured nanocomposites based on ZnO/polydopamine

In the present work, we report on the modelling of processes at the zinc oxide and polydopamine (ZnO/PDA) interface. The PDA layer was deposited onto ZnO nanorods (NRs) via chemical bath deposition. The defect concentrations in ZnO before and after PDA deposition were calculated and analysed. The ZnONRs/PDA core–shell nanostructures were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Fourier-transform infrared (FTIR) spectroscopy, photoluminescence (PL) measurements, and diffuse reflectance spectroscopy. The TEM and electron energy loss spectroscopy (EELS) measurements confirmed the conformal coating of PDA, while the PL emission from ZnO and ZnONRs/PDA samples showed a reduction of intensity after the PDA deposition. The decrease of defect concentration participating in PL and quantum efficiency explains the PL reduction. Finally, the observed decrease of activation energies and a shift of the PL peaks are attributed to the formation of an additional local electrical field between the PDA and ZnO nanostructures.

The results shown in this study provide a unique insight into the optical and electronic processes of the ZnO/PDA interface.

Determination of the concentration of impurities in GaN from photoluminescence and secondary-ion mass spectrometry

Photoluminescence (PL) was used to estimate the concentration of carbon in GaN grown by hydride vapor phase epitaxy (HVPE). The PL data were compared with profiles of the impurities obtained from secondary ion mass spectrometry (SIMS) measurements. Comparison of PL and SIMS data has revealed that apparently high concentrations of C and O at depths up to 1 µm in SIMS profiles do not represent depth distributions of these species in the GaN matrix but are rather caused by post-growth surface contamination and knocking-in impurity species from the surface. In particular, PL analysis supplemented by reactive ion etching up to the depth of 400 nm indicates that the concentration of carbon in nitrogen sites is below 2-5 × 10¹⁵ cm^-3 at any depth of GaN samples grown by HVPE. We demonstrate that PL is a very sensitive and reliable tool to determine the concentrations of impurities in the GaN matrix.

Two yellow luminescence bands in undoped GaN

Two yellow luminescence bands related to different defects have been revealed in undoped GaN grown by hydride vapor phase epitaxy (HVPE). One of them, labeled YL1, has the zero-phonon line (ZPL) at 2.57 eV and the band maximum at 2.20 eV at low temperature. This luminescence band is the ubiquitous yellow band observed in GaN grown by metalorganic chemical vapor deposition, either undoped (but containing carbon with high concentration) or doped with Si. Another yellow band, labeled YL3, has the ZPL at 2.36 eV and the band maximum at 2.09 eV. Previously, the ZPL and fine structure of this band were erroneously attributed to the red luminescence band. Both the YL1 and YL3 bands show phonon-related fine structure at the high-energy side, which is caused by strong electron-phonon coupling involving the LO and pseudo-local phonon modes. The shapes of the bands are described with a one-dimensional configuration coordinate model, and the Huang-Rhys factors are found. Possible origins of the defect-related luminescence bands are discussed.