Owing to wide range bandgap tunability to more than 5 eV, the quaternary (Be,Mg)ZnO solid solutions are attractive for a variety of UV optoelectronic applications, inclusive of solar blind photodetectors, and intersubband transition devices. The mutual compensation effects of Be and Mg on the formation energy and strain allows a wide range of compositions and bandgaps beyond those achievable by MgZnO and BeZnO ternaries. Localization effects are well pronounced in such wide-bandgap semiconductor alloys due to large differences in metal covalent radii and the lattice constants of the binaries, resulting in strain-driven compositional variations within the film and consequently large potential fluctuations, in addition to that possibly caused by defects. However, carrier localization may suppress recombination through nonradiative channels, and thus, facilitate high-efficiency optoelectronic devices. To investigate potential fluctuations and localization in Be<sub>x</sub>Mg<sub>y</sub>Zn(<sub>1-x-y</sub>)O films grown by plasma-assisted molecular beam epitaxy, optical absorption and steady-state and time-resolved photoluminescence (PL) measurements were performed. O-polar Be<sub>x</sub>Mg<sub>y</sub>Zn(<sub>1-x-y</sub>)O samples grown on GaN templates with compositions up to x = 0.04 and y = 0.18 were used for timeresolved studies, and O-polar Be<sub>x</sub>Mg<sub>y</sub>Zn(<sub>1-x-y</sub>)O samples grown on sapphire with compositions up to x = 0.19 and y = 0.52 were used for absorption measurements. From spectrally resolved PL transients, BeMgZnO samples with higher Mg/Be content ratio were found to exhibit smaller localization depth, Δ<sub>0</sub>=98 meV for Be<sub>0.04</sub>Mg<sub>0.17</sub>Zn<sub>0.79</sub>O and Δ<sub>0</sub>=173 meV for Be<sub>0.10</sub>Mg<sub>0.25</sub>Zn<sub>0.65</sub>O, compared to samples with smaller Mg/Be ratio, Δ<sub>0</sub>=268 meV for Be<sub>0.11</sub>Mg<sub>0.15</sub>Zn<sub>0.74</sub>O. Similar correlation is observed in temporal redshift of the PL peak position of 8 meV, 42 meV and 55 meV for Be<sub>0.04</sub>Mg<sub>0.17</sub>Zn<sub>0.79</sub>O, Be<sub>0.10</sub>Mg<sub>0.25</sub>Zn<sub>0.65</sub>O and Be<sub>0.11</sub>Mg<sub>0.15</sub>Zn<sub>0.74</sub>O, respectively, that originates from potential fluctuations and removal of band filling effect in the localized states. PL transients indicate that emission at low temperature is dominated by recombination of localized excitons, which exhibit decay times as long as τ = 0.36 ns at the peak position. The Sshaped behavior of PL peak with change in temperature was observed for the quaternary alloy Be<sub>0.04</sub>Mg<sub>0.17</sub>Zn<sub>0.79</sub>O. The degree of localization σ was determined to be 22 meV. Relatively high potential fluctuations and localization energy lead to a strong Stokes shift, which increased with bandgap reaching ~0.5 eV for O-polar BeMgZnO on sapphire with 4.6 eV absorption edge.
Near-field scanning optical microscopy was applied to investigate the spatial variations of extended defects and their effects on the optical quality for semi-polar (1-101) and (11-22) InGaN light emitting diodes (LEDs). (1-101) and (11-22) oriented InGaN LEDs emitting at 450-470 nm were grown on patterned Si (001) 7° offcut substrates and m-sapphire substrates by means of nano-epitaxial lateral overgrowth (ELO), respectively. For (1-101) structures, the photoluminescence (PL) at 85 K from the near surface c+ wings was found to be relatively uniform and strong across the sample. However, emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations (TDs) and basal plane stacking faults (BSFs) as revealed from the local PL spectra. In case of (11-22) LED structures, near-field PL intensity correlated with the surface features and the striations along the direction parallel to the c-axis projection exposed facets where the Indium content was higher as deduced from shift in the PL peak energy.
Heteroepitaxial semipolar and nonpolar GaN layers often suffer from high densities of extended defects including basal plane stacking faults (BSFs). BSFs which are considered as inclusions of cubic zinc-blende phase in wurtzite matrix act as quantum wells strongly affecting device performance. Band alignment in BSFs has been discussed as type of band alignment at the wurtzite/zinc blende interface governs the response in differential transmission; fast decay after the pulse followed by slow recovery due to spatial splitting of electrons and heavy holes for type- II band alignment in contrast to decay with no recovery in case of type I band alignment. Based on the results, band alignment is demonstrated to be of type II in zinc-blende segments in wurtzite matrix as in BSFs.
The effect of compressive strain in buffer layer on strain relaxation and indium incorporation in InGaN multi-quantum wells (MQWs) is studied for two sets of samples grown side by side on both relaxed GaN layers and strained 10-pairs of AlN/GaN periodic multilayers. The 14-nm AlN layers were utilized in both multilayers, while GaN thickness was 4.5 and 2.5 nm in the first and the second set, respectively. The obtained results for the InGaN active layers on relaxed GaN and AlN/GaN periodic multilayers indicate enhanced indium incorporation for more relaxed InGaN active layers providing a variety of emission colors from purple to green.
Enhancement of coherent zone folded longitudinal acoustic phonon (ZFLAP) oscillations at terahertz frequencies was demonstrated in InGaN multiple quantum wells (MQWs) by using wavelength degenerate time resolved differential transmission spectroscopy. Screening of the piezoelectric field in InGaN MQWs by photogenerated carriers upon femtosecond pulse excitation gave rise to terahertz ZFLAPs, which were monitored at the Brillouin zone center in the transmission geometry. MQWs composed of 10 pairs In<sub>x</sub>Ga<sub>1-x</sub>N wells and In<sub>0.03</sub>Ga<sub>0.97</sub>N barriers provided coherent phonon frequencies of 0.69-0.80 THz depending on the period of MQWs. Dependences of ZFLAP amplitude on excitation density and wavelength were also investigated. Possibility of achieving phonon cavity, incorporating a MQW placed between two AlN/GaN phonon mirrors designed to exhibit large acoustic gaps at the zone center, was also explored.
Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p- GaN or 1300 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photogeneration near the surface region by above bandgap excitation. Taking into consideration the absorption in the active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be about 92 ± 7 nm and 68 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively. Cross-sectional cathodoluminescence line-scan measurement was performed on a separate sample and the diffusion length in n-type GaN was measured to be 280 nm.
Carrier transport in double heterostructure (DH) InGaN light emitting diodes (LEDs) was investigated using photocurrent measurements performed under CW HeCd laser (325 nm wavelength) excitation. The effect of electron injector thicknesses was investigated by monitoring the excitation density and applied bias dependent escape of photogenerated carriers from the active region and through energy band structure and carrier transport simulations using Silvaco Atlas. For quad (4x) 3-nm DH LED structures incorporating staircase electron injectors (SEIs), photocurrent increased with SEI thickness due to reduced effective barrier opposing carrier escape from the active region as confirmed by simulations. The carrier leakage percentile at -3V bias and 280 Wcm<sup>-2</sup> optical excitation density increased from 24 % to 55 % when In<sub> 0.04</sub>Ga<sub>0.96</sub>N + In<sub>0.08</sub>Ga<sub>0.92</sub>N SEI thickness was increased from 4 nm + 4 nm to 30 nm + 30 nm. The increased leakage with thicker SEI correlates with increased carrier overflow under forward bias.
The carrier recombination dynamics in bulk m-plane GaN were investigated by excitation and temperature dependent time-resolved photoluminescence (TRPL) spectroscopy. Polarization-resolved measurements of photoluminescence (PRPL) spectra were performed to evaluate the individual contributions of excitons and free carriers to the radiative recombination. The polarization degree and PL lifetime were strongly correlated and dependent on the populations of free carriers and excitons at different excitation density and temperature levels. The free carrier concentration was found to increase due to the dissociation of excitons at high excitation density and temperatures. The excitonic PL life time was found to be ~ 0.7 ns at 10 K at the lowest excitation density used 0.04 μJ/cm<sup>2</sup>, where no exciton screening was present as confirmed by the 100% polarization degree. The polarization degree obtained at different excitation levels and temperatures by comparing the PL decay times to the excitonic PL lifetime correlated very well with the polarization degree obtained from the excitation dependent PRPL measurements. Finally, it was shown that TRPL and PRPL can be used to separate the excitonic and free carrier contributions to the recombination dynamics in m-plane GaN at any temperature and excitation density.
For high efficiency at high current injection InGaN light emitting diodes (LEDs) necessitate active regions that can mitigate the aggravating electron overflow. Multi double-heterostructures (DHs), 3D active regions separated by low energy barriers, were investigated as optimum solutions for high efficiency as they can accommodate a larger number of states compared to multiple quantum wells (MQWs). However, the number of DH active regions is limited as the material degrades with increasing thickness; therefore, carrier cooling should be partially achieved before the active region using staircase electron injector (SEI) layers. Using electroluminescence (EL) efficiency measurements supported by simulations, active regions and electron injectors were optimized to minimize the electron overflow and the associated efficiency drop at high injection. For a single 3 nm DH LED, the electron overflow was nearly eliminated by increasing the two-step staircase electron injector layer thickness from 4+4 nm to 20+20 nm, whereas the change in SEI thickness had nearly no effect for the DH LEDs with thicker active region. Temperature and excitation density dependent photoluminescence (PL) spectroscopy allowed determination of the material quality and the internal quantum efficiency of device structures with varying active region and SEI thickness.
GaN-based vertical cavity structures containing bottom AlN/GaN DBRs with top dielectric DBRs on freestanding c-GaN and all dielectric DBRs on GaN on c-sapphire were investigated. Epitaxial lateral overgrowth (ELO) technique allowed the use of both top and bottom all dielectric reflector stacks without substrate removal and the fabrication of the active region containing InGaN multiple quantum wells entirely on the nearly defect-free laterally grown wing regions to avoid nonradiative centers caused by extended and point defects. Compared with the cavity containing hybrid-DBRs on freestanding GaN, the cavity with all dielectric DBRs exhibited quality factors up to 1200 at high optical
excitation and an order of magnitude lower stimulated emission threshold density (nearly 5 μJ/cm<sup>2</sup>). Vertical to lateral growth ratio for ELO could be enhanced up to 5 by increasing the V/III ratio and employment of NH3 modulation, which minimizes the use of dry etching to reduce the cavity thickness and therefore is promising for high quality vertical cavities with all dielectric DBRs.