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Vertical GaN-on-GaN p-n diodes grown by ammonia molecular beam epitaxy (NH3 MBE) are reported in this letter for prospective high-power application devices. The diodes, consisting of only $4~\mu \text{m}$ drift layer with low unintentional background doping of $\sim 3\times {10}^{{15}}{\mathrm {cm}}^{\text {-3}}$ , showed a maximum breakdown voltage of >1.0 kV and a low differential specific on-resistance of 0.28 $\text{m}\Omega $ -cm2. Moreover, these diodes exhibited an excellent rectifying behavior with a low minimum ideality factor of 1.36 and a punch-through electric field comparable to the state-of-the-art vertical GaN-on-GaN p-n diodes grown by metalorganic chemical vapor deposition (MOCVD). These results suggest that all-MBE vertical GaN-on-GaN p-n diodes grown by NH3 MBE can be promising for small-size high-power electronic devices.

We demonstrate a superluminescent diode fabricated on a nonpolar ${m}$ -plane GaN substrate by employing a linearly tapered waveguide design. A high electrical −3dB modulation bandwidth ( $f_{\mathbf {3dB}}$ ) of 2.5 GHz at a current density of 30 kA/cm2 is achieved. The high modulation bandwidth is attributed to the shorter carrier recombination lifetime, the linear gain curve in the nonpolar ${m}$ -plane quantum wells, and the ability to operate at high current densities while effectively suppressing lasing. We derive a general expression for the −3dB bandwidth as a function of current density for SLDs using a similar approach to that for laser diodes. The −3dB bandwidth of a nonpolar superluminescent diode increases exponentially with current density. The experimental results are consistent with the derived expression for $f_{\mathbf {3dB}}$ vs . current density.

Multiphysics processes such as recombination dynamics in the active region, carrier injection and transport, and internal heating may contribute to thermal and efficiency droop in InGaN/GaN light-emitting diodes (LEDs). However, an unambiguous methodology and characterization technique to decouple these processes under electrical injection and determine their individual roles in droop phenomena is lacking. In this work, we investigate thermal and efficiency droop in electrically injected single-quantum-well InGaN/GaN LEDs by decoupling the inherent radiative efficiency, injection efficiency, carrier transport, and thermal effects using a comprehensive rate equation approach and a temperature-dependent pulsed-RF measurement technique. Determination of the inherent recombination rates in the quantum well confirms efficiency droop at high current densities is caused by a combination of strong non-radiative recombination (with temperature dependence consistent with indirect Auger) and saturation of the radiative rate. The overall reduction of efficiency at elevated temperatures (thermal droop) results from carriers shifting from the radiative process to the non-radiative processes. The rate equation approach and temperature-dependent pulsed-RF measurement technique unambiguously gives access to the true recombination dynamics in the QW and is a useful methodology to study efficiency issues in III-nitride LEDs.

We present the first demonstration of RF characteristics of electrically injected GaN/InGaN core–shell nanowire-based micro light-emitting diodes (μLEDs) for μLED displays and visible-light communi...

We report high-voltage regrown nonpolar ${m}$ -plane p-n diodes on freestanding GaN substrates. A high blocking voltage of 540 V at ~1 mA/cm $^{\textsf {2}}$ (corresponding to an electric field of E ~ 3.35 MV/cm), turn- ON voltages between 2.9 and 3.1 V, specific on-resistance of 1.7 $\text{m}\Omega \cdot \textsf {cm}^{\textsf {2}}$ at 300 A/cm $^{\textsf {2}}$ , and a minimum ideality factor of 1.7 were obtained for the regrown diodes. Our results suggest that Si, O, and C interfacial impurity levels up to $\textsf {2}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , $\textsf {8}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , and $\textsf {1}\times \textsf {10}^{\textsf {19}}$ cm $^{-\textsf {3}}$ , respectively, at the metallurgical junction of ${m}$ -plane, p-n diodes do not result in very early breakdown in the reverse bias although the off-state leakage current in the forward bias is affected. The impact of the growth interruption/regrowth on diode performance is also investigated.

We investigate Tamm plasmon (TP) modes in a metal/semiconductor distributed Bragg reflector (DBR) interface. A thin Ag (silver) layer with an optimized thickness (~ 55 nm from simulation) was deposited on nanoporous GaN DBRs fabricated using electrochemical etching on freestanding semipolar GaN substrates. The reflectivity spectra of the DBRs are compared before and after the Ag deposition and with that of a blanket Ag layer deposited on GaN. The results indicate presence of a TP mode at ~ 455 nm on the structure after the Ag deposition. An active medium can also be accommodated within the mode for optoelectronics and photonics. Moreover, the simulation results predict a sensitivity of the TP mode wavelength to the ambient (~ 4 nm shift when changing the ambient within the pores from air with n = 1 to isopropanol n = 1.3) , suggesting an application of the nanoporous GaN based TP structure for optical sensing., 8 pages, 4 figures

GaN-based vertical-cavity surface-emitting lasers (VCSELs) have drawn interest in recent years for their potential applications in data storage, laser printing, solid-state lighting, optical communications, sensing, and displays. Several research groups have demonstrated electrically injected GaN-based VCSELs utilizing different growth and fabrication techniques to address the many challenges associated with III-nitride materials. One such challenge is fabrication of highquality conductive epitaxial distributed Bragg reflectors (DBRs). A relatively new approach that yields high-index-contrast lattice-matched epitaxial DBRs is to introduce subwavelength air-voids (nanopores) in alternating layers of doped/undoped GaN. These nanoporous layers can be achieved by the controlled anodic electrochemical etching of highly doped n-type GaN in acids. The selective formation of the nanopores in the doped layers effectively lowers the refractive index compared to the adjacent undoped GaN layers, resulting in a refractive index difference of ~0.83, allowing high reflectance (>99%) with only ~16 pairs. Here, we will present electrically injected nonpolar m-plane GaN-based VCSELs with lattice-matched nanoporous GaN bottom DBRs and top dielectric DBRs. Lasing under pulsed operation at room temperature was observed at 409 nm with a linewidth of ~0.6 nm and a maximum output power of ~1.5 mW. The nonpolar m-plane orientation offers low transparency, high material gain, and anisotropic gain characteristics. The VCSELs were linearly polarized with a polarization ratio of ~0.94 and polarization-pinned emission along the a-direction. The mode profiles, thermal properties, and lasing yield of the VCSELs are also discussed.

We experimentally characterize mid-infrared intersubband transitions in identical Al0.5Ga0.5N/GaN heterostructures grown on a- and m-plane GaN substrates. The absorption peaks of the m-plane samples are 10 to 40% narrower than that of the a-plane samples.

We demonstrate a high-speed nonpolar ${m}$ -plane InGaN/GaN micro-scale light-emitting diode (LED) with a record electrical −3 dB modulation bandwidth of 1.5 GHz at a current density of 1 kA/cm2. A differential carrier lifetime (DLT) of 200 ps at 1 kA/cm2 was extracted using a rate-equation model. The short DLT is attributed to the high electron–hole wave function overlap in polarization-free nonpolar InGaN/GaN quantum wells, which leads to a higher spontaneous emission rate at low current densities compared to polar $c$ -plane quantum wells. LEDs with improved high-speed performance at low current densities will help to reduce power dissipation and increase efficiency in Gb/s visible-light communication systems.

We report on ammonia and plasma-assisted molecular beam epitaxy (NH3-MBE and PAMBE) grown GaN layers with a low net carrier concentration (Nnet). Growth parameters, such as growth rate, V–III ratio, and plasma power, were investigated on different substrates to study their impact on surface morphology and background doping levels using atomic force microscopy and capacitance–voltage (C–V) measurements, respectively. The elevated growth rates are especially interesting for vertical power switches, requiring very thick drift regions (over 10 μm) with low background concentrations. For our NH3-MBE-grown layers, Nnet shows an almost linear increase with the growth rate. Using a freestanding substrate and at a fast growth rate of 1.4 μm hr−1, a Nnet value as low as 1 × 1015 cm−3 was achieved. For samples grown via PAMBE, the lowest Nnet among samples grown under a Ga adlayer was 2 × 1016 cm−3 for a growth rate of 0.32 μm h−1 on a GaN-on-sapphire template. The results support the use of MBE for growing high-quality GaN material with reasonably fast growth rates maintaining low background doping levels for high-voltage vertical power electronic devices.

We utilize a rate equation approach and RF measurement technique to study carrier dynamics in InGaN/GaN LEDs. Study of carrier dynamics aids in understanding efficiency challenges and design of high-speed, efficient micro-LEDs for visible-light communication and micro-LED display applications.

III-nitride high-speed optoelectronics are emerging for a variety of applications, including visible-light communication (VLC) in light-fidelity (Li-Fi) networks, optical communication using photonic-integrated circuits, and micro-pixel LED displays. III-nitride materials offer efficient light emission and the potential to route, modulate, and detect light on a single chip. Previous research into III-nitride optoelectronics has mainly focused on developing emitters with high efficiencies and large optical output powers, which are needed for conventional lighting and data storage applications. Here, we present the more recent development of III-nitride high-speed emitters, including planar micro-LEDs, nanowire-based micro-LEDs, and superluminescent diodes. We demonstrate record-high modulation bandwidths by growing devices on nonpolar and semipolar orientations of GaN, which are free from the polarization-related effects that limit the modulation bandwidths of $c$ -plane III-nitride. We also combine a rate equation model with an RF measurement technique to extract the carrier dynamics in the devices [1].

We investigate the impact of high-dose gamma-ray irradiation on the electrical performance of Ga-polar and N-polar GaN-based p–n diodes grown by metalorganic chemical vapor deposition. We compare the current density–voltage (J–V), capacitance–voltage (C–V), and circular transfer length method characteristics of the p–n diodes fabricated on Ga-polar and N-polar orientations before and after irradiation. The relative turn-on voltage increases for the Ga-polar diodes with an increasing irradiation dose, while it increases initially and then starts to decrease for the N-polar diodes. The p-contact total resistance increases for Ga-polar and decreases for N-polar samples, which we attribute to the formation of point defects and additional Mg activation after irradiation. The J–V characteristics of most of the tested diodes recovered over time, suggesting the changes in the J–V characteristics are temporary and potentially due to metastable occupancy of traps after irradiation. X-ray photoelectron spectroscopy and photoluminescence measurements reveal the existence of different types of initial defects and surface electronic states on Ga-polar and N-polar samples. Gallium vacancies (VGa) are dominant defects in Ga-polar samples, while nitrogen vacancies (VN) are dominant in N-polar samples. The presence of a higher concentration of surface states on Ga-polar surfaces than N-polar surfaces was confirmed by calculating the band bending and the corresponding screening effect due to opposite polarization bound charge and ionized acceptors at the surface. The difference in surface stoichiometry in these two orientations is responsible for the different behavior in electrical characteristics after gamma-ray interactions.

Steady-state photocapacitance (SSPC) was conducted on nonpolar m-plane GaN n-type Schottky diodes to evaluate the defects induced by inductively coupled plasma (ICP) dry etching in etched-and-regrown unipolar structures. An ∼10× increase in the near-midgap Ec – 1.9 eV level compared to an as-grown material was observed. Defect levels associated with regrowth without an etch were also investigated. The defects in the regrown structure (without an etch) are highly spatially localized to the regrowth interface. Subsequently, by depth profiling an etched-and-regrown sample, we show that the intensities of the defect-related SSPC features associated with dry etching depend strongly on the depth away from the regrowth interface, which is also reported previously [Nedy et al., Semicond. Sci. Technol. 30, 085019 (2015); Fang et al., Jpn. J. Appl. Phys. 42, 4207–4212 (2003); and Cao et al., IEEE Trans. Electron Devices 47, 1320–1324 (2000)]. A photoelectrochemical etching (PEC) method and a wet AZ400K treatment are also introduced to reduce the etch-induced deep levels. A significant reduction in the density of deep levels is observed in the sample that was treated with PEC etching after dry etching and prior to regrowth. An ∼2× reduction in the density of Ec – 1.9 eV level compared to a reference etched-and-regrown structure was observed upon the application of PEC etching treatment prior to the regrowth. The PEC etching method is promising for reducing defects in selective-area doping for vertical power switching structures with complex geometries [Meyers et al., J. Electron. Mater. 49, 3481–3489 (2020)].

We report on the vertical hole transport through unipolar unintentionally doped (UID) and p-type doped AlGaN heterostructures to evaluate the effectiveness of the UID and doped AlGaN as barriers to the hole transport. Band diagram and current density–voltage (J–V) simulations are conducted in one-dimensional and three-dimensional schemes, with the latter including compositional fluctuations within the alloy AlGaN barrier layer. The simulation results using a self-consistent Poisson-drift diffusion scheme, incorporating the Localization Landscape theory, indicate a large asymmetric barrier to the hole transport by UID AlGaN. The asymmetric J–V characteristics are attributed to the asymmetric band diagrams calculated for the unipolar structure. The simulation results are verified by experiments using unipolar vertical hole transport structures enabled by n-to-p tunnel junctions (TJs) grown by ammonia molecular-beam epitaxy. The TJ structures are utilized to minimize the issues with the high spreading resistance of p-regions and to eliminate the need for its dry etching, which normally results in degraded p-contacts. The experimental results show that even a thin UID AlxGa1−xN (x = 14%, 13 nm) introduces an asymmetric barrier to the hole transport; a nearly 100% increase in the voltage drop induced by a thin UID AlGaN at 50 A/cm2 in the reverse direction is observed compared to an only 25% corresponding increase in the forward direction. Furthermore, p-type doping of the AlGaN layer results in a drastic drop in the potential barrier to hole transport in both directions. The results are beneficial for understanding the behavior of various structure designs within optoelectronics and power electronics.

Mid-infrared intersubband transitions are investigated in nonpolar m-plane and a-plane GaN/AlGaN multi-quantum well heterostructures. Nominally identical heterostructures were grown by ammonia molecular-beam epitaxy on free-standing m-plane and a-plane GaN substrates. A total of 12 well- and barrier-doped samples with intersubband transition energies in the range of 220–320 meV (wavelength range 3.8–5.6 μm) were grown. The intersubband absorption lines of the m-plane samples were 10–40% narrower than those of the a-plane samples, and a very narrow intersubband absorption linewidth of 38 meV (full width at half maximum) at a transition energy of approximately 250 meV (5 μm wavelength) was observed in an m-plane sample. Narrower intersubband absorption linewidths of m-plane samples can be explained by more abrupt heterostructure interfaces revealed by structural characterization, which is attributed to a higher stability of the m-plane compared to the a-plane. No significant difference in the intersubband absorption linewidth was observed between the barrier- and well-doped samples.

The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate, the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (10-10), and semipolar (20-2-1) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. Record modulation bandwidths above 1 GHz are achieved for the nonpolar and semipolar orientations. We also present a small-signal method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection.

GaN is an attractive material for high-power electronics due to its wide bandgap and large breakdown field. Verticalgeometry devices are of interest due to their high blocking voltage and small form factor. One challenge for realizing complex vertical devices is the regrowth of low-leakage-current p-n junctions within selectively defined regions of the wafer. Presently, regrown p-n junctions exhibit higher leakage current than continuously grown p-n junctions, possibly due to impurity incorporation at the regrowth interfaces, which consist of c-plane and non-basal planes. Here, we study the interfacial impurity incorporation induced by various growth interruptions and regrowth conditions on m-plane p-n junctions on free-standing GaN substrates. The following interruption types were investigated: (1) sample in the main MOCVD chamber for 10 min, (2) sample in the MOCVD load lock for 10 min, (3) sample outside the MOCVD for 10 min, and (4) sample outside the MOCVD for one week. Regrowth after the interruptions was performed on two different samples under n-GaN and p-GaN growth conditions, respectively. Secondary ion mass spectrometry (SIMS) analysis indicated interfacial silicon spikes with concentrations ranging from 5e16 cm-3 to 2e18 cm-3 for the n-GaN growth conditions and 2e16 cm-3 to 5e18 cm-3 for the p-GaN growth conditions. Oxygen spikes with concentrations ~1e17 cm-3 were observed at the regrowth interfaces. Carbon impurity levels did not spike at the regrowth interfaces under either set of growth conditions. We have correlated the effects of these interfacial impurities with the reverse leakage current and breakdown voltage of regrown m-plane p-n junctions.

We report the fabrication and electro-optic characterization of nonpolar GaN-based superluminescent diodes with a tapered waveguide design. The devices were grown and fabricated on freestanding m-plane GaN. An electrical −3dB modulation bandwidth of 2.5 GHz is reported at the highest current density.

We demonstrate a high-speed nonpolar InGaN/GaN micro-LED with a record −3dB bandwidth of 1.5 GHz at a current density of 1 kA/cm2. The high-speed at low current density is attributed to the large electron-hole wavefunction overlap in nonpolar QWs leading to a shorter carrier lifetime.

Optically pumped nonpolar GaN-based vertical-cavity surface-emitting lasers (VCSELs) are demonstrated using nanoporous DBRs formed by electrochemical etching. Polarization-pinned single-mode stimulated emission is observed at 462 nm with a threshold power density of ∼5 kW/cm2 and a FWHM of ∼0.12 nm.

Free-standing nonpolar GaN substrates provide an excellent platform for the fabrication of high-speed blue and green light-emitting diodes (LEDs), which are attractive for visible-light communication, plastic optical fiber communication, and short-range under water optical communication. Nonpolar LEDs on free-standing GaN exhibit a large electron-hole wave function overlap, low extended defect density, and favorable thermal properties. Here, we demonstrate high-speed nonpolar InGaN/GaN LEDs with a peak emission wavelength between 455 and 465 nm on free-standing nonpolar GaN substrates. A large frequency modulation bandwidth of 524 MHz is demonstrated at a current density of 10 kA/cm 2 .

A nonpolar GaN-based vertical-cavity surface-emitting laser (VCSEL) using nanoporous bottom epitaxial distributed Bragg reflector (DBR) is demonstrated at room temperature (RT) under continuous-wave (CW) optical pumping. The porous layers enable the epitaxial growth of lattice-matched high-reflectance DBRs without sacrificing the conductive properties needed for high-performance VCSELs. The 2-λ cavity VCSEL reported here employs a hybrid design with top dielectric DBR and bottom nanoporous DBR. Single longitudinal mode lasing is observed at 462 nm with a threshold power density of ~5 kW/cm2 and a FWHM of ~0.12 nm. The emission polarization was pinned in the a-direction at all measured locations.

We demonstrate the first electrically injected nonpolar m-plane GaN-based vertical-cavity surface-emitting lasers (VCSELs) with lattice-matched nanoporous bottom DBRs. Lasing under pulsed operation at room temperature was observed near 409 nm with a linewidth of ~0.6 nm and a maximum output power of ~1.5 mW. The VCSELs were linearly polarized and polarization-locked in the a-direction, with a polarization ratio of 0.94. The high polarization ratio and polarization pinning reveal that the optical scattering from the nanoporous DBRs is negligible. A high characteristic temperature of 357 K resulted from the slightly negative offset between the peak gain and cavity mode wavelengths.

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59 eV emerges, which is attributed to electron transitions from the conduction band to the same defect. In addition to the ZPL, a set of phonon replicas is observed, which is caused by the emission of phonons with energies of 39.5 meV and 91.5 meV. The defect is called the YL1 center. The possible identity of the YL1 center is discussed. The results indicate that the same defect is responsible for the strong YL1 band in undoped and Si-doped GaN samples.

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.

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.

Can, Nuri (Balikesir Author), Among the major obstacles for development of non-polar and semipolar GaN structures on foreign substrates are stacking faults which deteriorate the structural and optical quality of the material. In this work, an in-situ SiNx nano-network has been employed to achieve high quality heteroepitaxial semipolar (11 (2) over bar2) GaN on m-plane sapphire with reduced stacking fault density. This approach involves in-situ deposition of a porous SiNx interlayer on GaN that serves as a nano-mask for the subsequent growth, which starts in the nanometer-sized pores (window regions) and then progresses laterally as well, as in the case of conventional epitaxial lateral overgrowth (ELO). The inserted SiNx nano-mask effectively prevents the propagation of defects, such as dislocations and stacking faults, in the growth direction and thus reduces their density in the overgrown layers. The resulting semipolar (11 (2) over bar2) GaN layers exhibit relatively smooth surface morphology and improved optical properties (PL intensity enhanced by a factor of 5 and carrier lifetimes by 35% to 85% compared to the reference semipolar (11 (2) over bar2) GaN layer) which approach to those of the c-plane in-situ nano-ELO GaN reference and, therefore, holds promise for light emitting and detecting devices., Materials World Network grant from the National Science Foundation - DMR-1210282 German Research Foundation, DFG -FOR 957 Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK)

The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) light-fidelity (Li-Fi) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation due to their lower parasitic capacitance and ability for operation at high current densities where the carrier lifetime is significantly reduced. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate (inversely proportional to the carrier lifetime), the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths, especially at relatively low current densities. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (1010), and semipolar (2021) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. We demonstrate that the modulation bandwidths are well correlated with the wave function overlap in the active region by examining the orientation-dependence of the bandwidth. Record modulation bandwidths for GaN-based LEDs above 1 GHz are achieved for the nonpolar and semipolar orientations. We also show the modulation characteristics of an electrically injected nanowire-based LED, which achieves a modulation bandwidth of 1.2 GHz. Finally, we present a small-signal modeling method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection and discuss the results in the context of efficiency droop. Figure 1: Bandwidth vs. current density for the nonpolar LEDs (open black triangles), semipolar LED (open blue circles), and polar c-plane LED (open green squares) compared to other reported polar c-plane LEDs on sapphire or with a flip-chip design and compared to semipolar (1122) LEDs. The bandwidth characteristics for a nanowire-based LED are also included. Also shown are the crystal orientations used in the orientation-dependent bandwidth study. Figure 1

Semiconductor materials with wide bandgaps, including GaN and AlxGa1-xN, offer many performance advantages for power electronic devices compared to conventional Si-based devices. These include larger critical electric fields enabling higher reverse breakdown voltages, fast turn-off and turn-on transitions allowing higher-frequency operation, and high thermal conductivity making higher power density device operation possible. Various device architectures have been demonstrated in these materials, with laterally-oriented AlxGa1-xN/GaN high electron mobility transistors (HEMTs) maturing first for radio-frequency applications. More recently, lateral architectures are also starting to be seen in power switching applications. Vertical GaN PN diodes have been demonstrated with excellent forward and reverse electrical behavior including high current capacity, low forward specific on-resistance, high reverse breakdown voltage, and fast reverse recovery time. Selective-area doping control in GaN would enable traditional vertical power devices including merged PN-Schottky (MPS) diodes, junction field effect transistors (JFETs), and more advanced device architectures common in Si and SiC, thereby greatly extending the operating functionality and capability of GaN power devices. This talk will report on our work for two advanced GaN device technologies, including fundamental studies of selective-area doping control and the demonstration of a photoconductive semiconductor switch (PCSS). Both areas of work share the common goal of increasing the power density of switching power converters. Selective-area doping control using epitaxial regrowth processes by metal-organic chemical-vapor deposition (MOCVD) will be presented. Literature reports of GaN PN diodes where the p-layer is formed by epitaxial regrowth have, to date, indicated lower breakdown voltages and/or higher reverse leakage currents compared to continuously-grown PN junctions. The measurement of a “Si spike” at the regrown interface is commonly observed for GaN using characterization methods such as secondary ion mass spectroscopy (SIMS). Our initial studies to examine possible Si sources and the effects of interfacial Si on electrical performance have indicated that the MOCVD reactor is not a major source of Si. Additionally, intentional Si doping at the interface, with similar levels as measured by SIMS in regrown structures ([Si] = 6E16 cm-3 and [Si] = 5E17 cm-3), shows no noticeable degradation in forward- or reverse-bias electrical performance in planar c-plane devices fabricated on GaN substrates (Figure 1). However, regrown GaN diodes formed on m-plane GaN substrates have shown worse electrical performance than c-plane diodes. The increased chemical reactivity of the m-plane and the resulting increase in impurity uptake is likely a contributing factor, and studies are underway to identify the specific impurities responsible. Mitigation strategies to manage the challenges of the m-plane are also being investigated. Further, fabrication-induced damage due to dry-etching processes was shown to increase reverse leakage currents by at least an order of magnitude in planar, c-plane diodes. We will report on these results and the study of various surface treatment methods to reduce these leakage currents in planar, c-plane diodes. The details of a laterally-oriented, GaN PCSS switch will also be presented. PCSS technology is of interest for applications including high-voltage pulsed-power, electrical grid protection, and increased electrification through new build-outs of high-efficiency/performance power systems. PCSS devices have been demonstrated in several semiconductor materials, but GaN promises an improvement in switch capability due to its advantageous material properties. The device is operated by optically triggering photocarriers in a high-field area generated between two electrodes. Using commercially-available, semi-insulating GaN wafers, surface electrodes were fabricated with a 600 µm gap, and the device was optically triggered using a sub-bandgap 532 nm laser source. Low-voltage (low field) operation of the device demonstrated a linear photoconductive response. Increasing the field over a threshold value of 10-15 kV/cm demonstrated a “high-gain” or “lock-on” operating mode where current continues to flow after the laser source is turned off, if a sufficient field exists (Figure 2). This operating mode has been observed for other semiconductors, including GaAs, and is desirable for high-efficiency switch operation because of the low optical energy needed to turn on the switch. High-gain switching has been demonstrated with PCSS powers of 150 kW (1250 V, 120 A) triggered by optical energies as low as 35 µJ. Further increases in switch power are expected with device improvements. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the IDEAS and PNDIODES programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. Figure 1

We extract the injection efficiency as a function of current density in single-quantum-well semipolar ( 20 2 ¯ 1 ¯) InGaN/GaN light-emitting diodes (LEDs) using small-signal RF measurements and rate equation analysis of the dominant carrier mechanisms. The rate equations are used to derive a small-signal equivalent circuit that yields expressions for the modulation response and the input impedance of the LED. Simultaneous fitting of the modulation response and the input impedance of the circuit to the measured modulation response and the input impedance of the LED gives the differential carrier lifetime, the net differential carrier escape time from the active region, and the differential recombination lifetime in the cladding layers. The extracted lifetimes are used to calculate the injection efficiency of the LED at various current densities. Carrier leakage from the active region results in an injection efficiency below one at low current densities. At high current densities, the injection efficiency approaches one due to the higher recombination rate in the quantum well (QW) and the lower carrier leakage. Analysis of the lifetimes shows that the higher recombination rate in the QW results in a slower increase in the carrier population in the QW compared to that in the cladding layers. However, the recombination current in the QW is generally higher than that in the cladding, leading to high injection efficiency at high current densities. The data are consistent with a Coulomb-enhanced capture process. The high injection efficiency obtained at high current densities rules out injection-related mechanisms as the primary cause of efficiency droop in semipolar ( 20 2 ¯ 1 ¯) InGaN/GaN LEDs.

InGaN/GaN light-emitting diodes (LEDs) with large modulation bandwidths are desirable for visible-light communication. Along with modulation speed, the consideration of the internal quantum efficiency (IQE) under operating conditions is also important. Here, we report the modulation characteristics of semipolar (202¯1¯) InGaN/GaN (LEDs) with single-quantum well (SQW) and multiple-quantum-well (MQW) active regions grown on free-standing semipolar GaN substrates with peak internal quantum efficiencies (IQEs) of 0.93 and 0.73, respectively. The MQW LEDs exhibit on average about 40–80% higher modulation bandwidth, reaching 1.5 GHz at 13 kA/cm2, but about 27% lower peak IQE than the SQW LEDs. We extract the differential carrier lifetimes (DLTs), RC parasitics, and carrier escape lifetimes and discuss their role in the bandwidth and IQE characteristics. A coulomb-enhanced capture process is shown to rapidly reduce the DLT of the MQW LED at high current densities. Auger recombination is also shown to play little role in increasing the speed of the LEDs. Finally, we investigate the trade-offs between the bandwidth and efficiency and introduce the bandwidth-IQE product as a potential figure of merit for optimizing speed and efficiency in InGaN/GaN LEDs.

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization energy of ∼0.9 eV for the YL1 center. In semi-insulating GaN, an abrupt and tunable quenching of the YL1 band is observed, where the apparent activation energy in the Arrhenius plot is not related to the ionization energy of the defect. In this case, the ionization energy can be found by analyzing the shift of the characteristic temperature of PL quenching with excitation intensity. We conclude that only one defect, namely, the YL1 center, is responsible for the yellow band in undoped and doped GaN samples grown by different techniques.

High-speed InGaN/GaN blue light-emitting diodes (LEDs) are needed for future gigabit-per-second visible-light communication systems. Large LED modulation bandwidths are typically achieved at high current densities, with reports close to 1 GHz bandwidth at current densities ranging from 5 to 10 kA/cm2. However, the internal quantum efficiency (IQE) of InGaN/GaN LEDs is quite low at high current densities due to the well-known efficiency droop phenomenon. Here, we show experimentally that nonpolar and semipolar orientations of GaN enable higher modulation bandwidths at low current densities where the IQE is expected to be higher and power dissipation is lower. We experimentally compare the modulation bandwidth vs. current density for LEDs on nonpolar (101¯0), semipolar (202¯1¯), and polar 0001 orientations. In agreement with wavefunction overlap considerations, the experimental results indicate a higher modulation bandwidth for the nonpolar and semipolar LEDs, especially at relatively low current densities....