A. Sedhain

Nature of deep center emissions in GaN

Photoluminescence (PL) emission spectroscopy was employed to probe the nature of deep center emissions in GaN. The room temperature PL spectrum measured in the infrared (IR) region revealed an emission band centered around 1.23 eV. Based on detailed analysis of both the IR and visible emission spectra, we suggest that this emission band is a band-to-impurity transition involving a deep level complex consisting of a gallium vacancy and an oxygen atom sitting on one of the neighboring nitrogen sites; the (VGa–ON)2− charge state of (VGa–ON)2−/1−. Two electronic structures, which arise due to two different configurations of (VGa–ON)2−/1−, with ON either along the c-axis (axial configuration) or in one of the three equivalent tetrahedral positions (basal configuration), were observed. Our result also provides explicit evidence that both the yellow luminescence band and the 1.23 eV emission line in GaN are related to a common deep center, which is believed to be (VGa–ON)2−/1−.

Electrical and Optical Properties of P-Type InGaN

Mg-doped InxGa1−xN alloys were grown by metal organic chemical vapor deposition on semi-insulating c-GaN/sapphire templates. Hall effect measurements showed that Mg-doped InxGa1−xN epilayers are p-type for x up to 0.35. Mg-acceptor levels (EA) as a function of x, (x up to 0.35), were experimentally evaluated from the temperature dependent hole concentration. The observed EA in Mg-doped In0.35Ga0.65N alloys was about 43 meV, which is roughly four times smaller than that in Mg doped GaN. A room temperature resistivity as low as 0.4 Ω cm (with a hole concentration ∼5×1018 cm−3 and hole mobility ∼3 cm2/V s) was obtained in Mg-doped In0.22Ga0.78N. It was observed that the photoluminescence (PL) intensity associated with the Mg related emission line decreases exponentially with x. The Mg energy levels in InGaN alloys obtained from PL measurements are consistent with those obtained from Hall-effect measurements.

The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence (PL) spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ∼0.3 eV (at 10 K) was observed at ∼2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, (VAl2−), of isolated aluminum vacancies, (VAl)3−/2− described by VAl2−+hν=VAl3−+h+. In such a context, the reverse process is responsible for the 2.78 eV PL emission.

Nature of optical transitions involving cation vacancies and complexes in AlN and AlGaN

Photoluminescence spectroscopy was employed to probe the nature of optical transitions involving Al vacancy (VAl) and vacancy-oxygen complex (VAl-ON) in AlN. An emission line near 2 eV due to the recombination between the 2− charge state of (VAl-ON)2−/1−, and the valence band was directly observed under a below bandgap excitation scheme. This photoluminescence (PL) band was further resolved into two emission lines at 1.9 and 2.1 eV, due to the anisotropic binding energies of VAl-ON complex caused by two different bonding configurations–the substitutional ON sits along c-axis or sits on one of the three equivalent tetrahedral positions. Moreover, under an above bandgap excitation scheme, a donor-acceptor pair like transition involving shallow donors and (VAl-ON)2−/1− deep acceptors, which is the “yellow-luminescence” band counterpart in AlN, was also seen to split into two emission lines at 3.884 and 4.026 eV for the same physical reason. Together with previous results, a more complete picture for the opti...

Enhancing erbium emission by strain engineering in GaN heteroepitaxial layers

Much research has been devoted to the incorporation of erbium (Er) into semiconductors aimed at achieving photonic integrated circuits with multiple functionalities. GaN appears to be an excellent host material for Er ions due to its structural and thermal stability. Er-doped GaN (GaN:Er) epilayers were grown on different templates, GaN/Al2O3, AlN/Al2O3, GaN/Si (111), and c-GaN bulk. The effects of stress on 1.54 μm emission intensity, caused by lattice mismatch between the GaN:Er epilayer and the substrate, were probed. The emission intensity at 1.54 μm increased with greater tensile stress in the c-direction of the GaN:Er epilayers. These results indicate that the characteristics of photonic devices based on GaN:Er can be optimized through strain engineering.

Probing the relationship between structural and optical properties of Si-doped AlN

Much efforts have been devoted to achieve conductivity control in the ultrahigh band gap (∼6.1 eV) AlN by Si doping. The effects of Si-doping on the structural and optical properties of AlN epilayers have been investigated. X-ray diffraction studies revealed that accumulation of tensile stress in Si-doped AlN is a reason for the formation of additional edge dislocations. Photoluminescence (PL) studies revealed that the linewidths of both band-edge and impurity related transitions are directly correlated with the density of screw dislocations, Nscrew, which increases with the Si doping concentration (NSi). Furthermore, it was formulated that the band-edge (impurity) PL emission linewidth increases linearly with increasing Nscrew at a rate of ∼3.3±0.7 meV/108 cm−2 (26.5±4 meV/108 cm−2), thereby establishing PL measurement as a simple and effective method to estimate screw dislocation density in AlN epilayers.

Probing exciton-phonon interaction in AlN epilayers by photoluminescence

Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy has been employed to investigate the exciton-phonon interaction in AlN. Longitudinal optical (LO) phonon replicas of free exciton recombination lines were observed in PL emission spectra, revealing the coupling of excitons with LO phonons. We have quantified such interaction by measuring Huang–Rhys factor based on polarization resolved DUV PL measurements. It was observed that the exciton-phonon coupling strength in AlN depends on the polarization configuration and is much larger in the direction with the electrical field (E) of the emitted light perpendicular to the wurtzite c-axis (E⊥c) than in the direction of E∥c. Furthermore, a larger coupling constant was also measured in AlN than in GaN. The large effective hole to electron mass ratio in AlN, especially in the E⊥c configuration, mainly accounts for the observed results.

Formation energy of optically active Er3+ centers in Er doped GaN

Erbium doped GaN (GaN:Er) and low In-content InxGa1−xN (x∼0.05) epilayers were synthesized by metal organic chemical deposition. The 1.54 μm PL emission intensity was monitored for GaN:Er epilayers grown at different growth temperatures and utilized to establish a value of 1.8 ± 0.2 eV for the formation energy (EF) of the optically active Er3+ centers in GaN. The optically active Er+ centers are presumably Er and nitrogen vacancy (Er-VN) complexes. The experimentally measured value of the EF of the optically active Er3+ centers is about 0.98 eV larger than the calculated formation energy of Er ions at Ga sites; however, it is 1.1–2.2 eV lower than the formation energy of VN in GaN. Due to the large EF values, relatively high growth temperatures are required to improve the 1.54 μm emission efficiency in GaN:Er.

Photoluminescence properties of erbium doped InGaN epilayers

We report on the photoluminescence properties of erbium (Er) doped InxGa1−xNa epilayers synthesized by metal organic chemical vapor deposition. The crystalline quality and surface morphology of Er doped In0.05Ga0.95N were nearly identical to those of Er doped GaN. The photoluminescence intensity of the 1.54 μm emission in Er doped In0.05Ga0.95N was an order of magnitude lower than in Er doped GaN and decreased with the increase of the In content. The reduction in 1.54 μm emission intensity was accompanied by enhanced emission intensities of deep level impurity transition lines.

Deep ultraviolet photoluminescence of Tm-doped AlGaN alloys

The ultraviolet (UV) photoluminescence (PL) properties of Tm-doped AlxGa1−xN (0.39≤x≤1) alloys grown by solid-source molecular beam epitaxy were probed using above-bandgap excitation from a laser source at 197 nm. The PL spectra show dominant UV emissions at 298 and 358 nm only for samples with x=1 and 0.81. Temperature dependence of the PL intensities of these emission lines reveals exciton binding energies of 150 and 57 meV, respectively. The quenching of these UV emissions appears related to the thermal activation of the excitons bound to rare-earth structured isovalent (RESI) charge traps, which transfer excitonic energy to Tm3+ ions resulting in the UV emissions. A model of the RESI trap levels in AlGaN alloys is presented.