Ultrafast excitonic radiative recombination in Al-rich semipolar AlGaN quantum wells was characterized by photoluminescence and analyzed with a non-equilibrium Green's function (NEGF) model. The heterostructures consisted of Al0.69Ga0.31N/Al0.9Ga0.1N multiple quantum wells grown on (202¯1) bulk AlN substrates by MOVPE. The quantum wells were 3 nm thick, separated by 10-nm thick barrier layers, and possessed a low (<100 kV/cm) polarization field. Radiative lifetimes as low as ∼60 ps were recorded at 75 K. Experimental results validated the NEGF model, which incorporated excitons and free carriers and enabled calculation of the radiative lifetimes for different growth planes, temperatures, and carrier densities. Including intersubband scattering in the NEGF model significantly reduced the calculated radiative lifetimes for QW widths larger than the Bohr radius of the excitons in semipolar and nonpolar orientations, for which the polarization field is reduced or eliminated. These wide wells with low polarization fields and high radiative recombination are less susceptible to the deleterious effects of alloy and interface fluctuations, making them an attractive possibility for future devices.

Chip-based, single-frequency and low phase-noise integrated photonic laser diodes emitting in the violet (412 nm) and blue (461 nm) regime are demonstrated. The GaN-based edge-emitting laser diodes were coupled to high-Q on-chip micro-resonators for optical feedback and mode selection resulting in laser self-injection locking with narrow emission linewidth. Multiple group III-nitride (III-N) based photonic integrated circuit chips with different waveguide designs including single-crystalline AlN, AlGaN, and GaN were developed and characterized. Single-frequency laser operation was demonstrated for all studied waveguide core materials. The best side-mode suppression ratio was determined to be ∼36 dB at 412 nm with a single-frequency laser emission linewidth of only about 3.8 MHz at 461 nm. The performance metrics of this novel type of laser suggest potential implementation in next generation, portable quantum systems.

Low phase noise lasers based on the combination of III–V semiconductors and silicon photonics are well established in the near-infrared spectral regime. Recent advances in the development of low-loss silicon nitride-based photonic integrated resonators have allowed them to outperform bulk external diode and fiber lasers in both phase noise and frequency agility in the 1550 nm-telecommunication window. Here, we demonstrate for the first time a hybrid integrated laser composed of a gallium nitride-based laser diode and a silicon nitride photonic chip-based microresonator operating at record low wavelengths as low as 410 nm in the near-ultraviolet wavelength region suitable for addressing atomic transitions of atoms and ions used in atomic clocks, quantum computing, or for underwater LiDAR. By self-injection locking of the Fabry–Pérot diode laser to a high-Q (0.4 × 106) photonic integrated microresonator, we reduce the optical phase noise at 461 nm by a factor greater than 100×, limited by the device quality factor and back-reflection.

We demonstrate for the first time a hybrid integrated low-noise laser composed of a gallium nitride (GaN) based laser diode and a silicon nitride photonic chip based microresonator operating at record low wavelengths as low as 410 nm in the near ultraviolet wavelength region.

Compact, single-frequency and low-noise integrated photonic laser diodes emitting in the violet (~412 nm) and blue (461 nm) regime are demonstrated. Hybrid-integration of the III-Nitride edge-type laser diodes to a butt-coupled on-chip high-Q resonator was used to achieved ultra-low phase-noise laser operation with ~36 dB side-mode suppression ratio. Both a CMOS-compatible PIC chip with SiN core as well as a new class of PIC platform with crystalline III-Nitride heterostructures was developed and evaluated. Successful demonstration of laser self-injection locking with ultra-narrow emission linewidth was demonstrated for both material platforms. Emission linewidth of only ~ 1 MHz was determined with an external-cavity reference laser implying state-of-the-art phase noise performance and offering a vastly improved form factor. The performance metrics of this novel type of laser suggest the potential implementation of this new technology in next generation, portable quantum systems.

The effect of hydrostatic pressure on optical transitions in InGaN/GaN multiple quantum wells (MQWs) has been studied. Photoluminescence (PL) and photomodulated transmission (PT) measurements were performed under applied pressure to examine the pressure dependence of optical transitions associated with confined states in MQWs. The PL emission from the MQWs was found to shift linearly to higher energy with applied pressure but exhibit a significantly weaker pressure dependence compared to epilayer samples with similar bandgap energies. Similar pressure coefficients obtained by PT measurements rule out the possibility of PL resulting from deep localized states. We show that the difference in the compressibility of InGaN and of GaN induces a tensile strain in the compressively strained InGaN well layers that partially compensates the applied hydrostatic pressure. This mechanical effect is the primary factor for the smaller pressure dependence of the optical transitions in the InGaN/GaN MQWs. At pressure above 100 kbar, the PL signal in MQWs samples is quenched, indicating that the carriers involved in the radiative recombination processes in the well layers originate primarily from the adjacent GaN layers.

A systematic dispersion engineering approach is presented toward designing a III-nitride micro-resonator for a blue frequency comb. The motivation for this endeavor is to fill the need for compact, coherent, multi-wavelength photon sources that can be paired with, e.g., the 171Yb+ ion in a photonic integrated chip for optical sensing, time-keeping, and quantum computing applications. The challenge is to overcome the normal material dispersion exhibited by the otherwise ideal (i.e., low-loss and large-Kerr-coefficient) AlGaN family of materials, as this is a prerequisite for bright-soliton Kerr comb generation. The proposed approach exploits the avoided-crossing phenomenon in coupled waveguides to achieve strong anomalous dispersion in the desired wavelength range. The resulting designs reveal a wide range of dispersion response tunability, which is expected to allow access to the near-UV wavelength regime as well. Numerical simulations of the spatio-temporal evolution of the intra-cavity field under continuous-wave laser pumping confirm that such a structure is capable of generating a broadband blue bright-soliton Kerr frequency comb. The proposed micro-resonator heterostructure is amenable to the current state-of-the-art growth and fabrication methods for AlGaN semiconductors.

Nitride-based device structures for electronic and optoelectronic applications usually incor-porate layers of AlxGa1−xN, and n- and p-type doping of these alloys is typically required. Experimental results indicate that doping efficiencies in AlxGa1−xN are lower than in GaN. We address the cause of these doping difficulties, based on results from first-principles density-functional-pseudopotential calculations. For n-type doping we will discuss doping with oxygen, the most common unintentional donor, and with silicon. For oxygen, a DX transition occurs which converts the shallow donor into a negatively charged deep level. We present experimental evidence that oxygen is a DX center in AlxGa1−xN for x>∼0.3. For p-type doping, we find that compensation by nitrogen vacancies becomes increasingly important as the Al content is in-creased. We also find that the ionization energy of the Mg acceptor increases with alloy composition x. To address the limitations on p-type doping we have performed a comprehensive investigation of alternative acceptor impurities; none of the candidates exhibits characteristics that surpass those of Mg in all respects.

Nuclear magnetic resonance (NMR) has provided essential information on the local atomic bonding and microstructure of hydrogen in hydrogenated amorphous silicon (a-Si:H). Here we describe results from NMR and Raman spectroscopy on the hydrogen distribution and bonding in a-Si:H prepared by remote hydrogen plasma (RHP) deposition and contrast the results with those from a-Si:H prepared by conventional glow discharge (GD) deposition. The films prepared by the two techniques have similar H bonding except for the presence in the RHP sample of about 1 atomic % molecular hydrogen, a factor of ten higher than in GD material. For RHP samples prepared from a deuterium plasma rather than a hydrogen plasma, substantial differences in the hydrogen NMR spectra, hydrogen spin lattice relaxation time and Raman spectra are observed. The hydrogen which necessarily originates from the silane has a dramatically altered spectra.

Temperature-dependent photoluminescence (PL) lifetimes were measured for a series of ultra-thin c-plane Al0.61Ga0.39N/AlN multiple quantum wells (QWs) on bulk AlN substrates with the well thickness varying from 0.6 to 2 nm. At temperatures below 75 K, estimates of the internal quantum efficiency indicate that the recombination is primarily radiative, with a lifetime of ∼160 ps for the 0.6 nm QWs, comparable to the low temperature PL lifetime observed in bulk AlGaN films of a similar Al content. This short lifetime is observed despite the presence of layer thickness fluctuations and the quantum-confined Stark effect associated with the large polarization field in the heterostructures, which tend to increase the radiative lifetime. This behavior is explained using many-body calculations of radiative recombination rates that extend beyond the conventional ABC rate equation model by accounting for both excitons and free carriers within a nonequilibrium Green's function formalism. The results indicate that the combination of the large wave function overlap integral (∼0.65) and exciton binding energy (1.82 times the 3D Rydberg) for the 0.6 nm QWs leads to an ∼20-fold increase in the radiative recombination rate relative to that obtained for the 2 nm QWs. This greater radiative recombination rate competes favorably with trapping at interface fluctuations and defect-induced nonradiative recombination that dominates recombination at higher temperatures.

Moore’s Law — realized via a combination of device physics advances, technology investments, and economic returns— allowed the number of transistors on a chip to double roughly every two years for over five decades. During those five decades, the cost of a unit of computing dropped by eight orders of magnitude. Those declines and the associated computing advances have dramatically affected every aspect of society, from science and technology through business and health to national security. Today, further computing advances are limited by the economics of chip fabrication and the physical limits on transistors, electrical interconnects, and memory elements at the nanoscale. If we are to evolve new generations of computing systems over the next decades that are both faster and more energy efficient, a complete reorganization of the science and technology underlying computing is needed. Within the Department of Energy’s (DOE’s) mission, which includes a pivotal and historical role in the evolution of computing, the needs are critical as well. Advanced computing and simulation underpin all aspects of DOE missions in energy, the environment, and national security, requiring energy-efficient computing beyond the exascale range. Edge computing, low-power computing technologies, and computers optimized for artificial intelligence are key to next-generation scientific facilities for high-energy physics research, as well as neutron and x-ray facilities. Finally, microelectronics will also play a major role in reshaping the U.S. electricity grid: from its current state to one that is cleaner, more efficient, cyber-secure, and resilient to widespread events, both natural and manmade. Realizing this vision will require advancements in both system architecture and power electronics. To enable continued advances in computing technologies, a fundamental rethinking is needed of the science behind the materials, synthesis and placement technologies, architectures, and algorithms. This cannot be modular and linear, as it has been in the past. Rather, these advances must be developed collectively, in a spirit of co-design, where each scientific discipline informs and engages the other to achieve orders of magnitude improvements in system-level performance. To explore these challenges, the Office of Science convened a Basic Research Needs Workshop for Microelectronics in October 2018 and charged workshop participants to conduct a thorough assessment of the scientific issues associated with advanced microelectronics technologies for applications relevant to the DOE mission. The workshop examined research relevant to the extension of complementary metal oxide semiconductor (CMOS) and beyond CMOS technologies; however, topics of direct relevance to quantum information science and quantum computing were outside the scope of this workshop.

The dependence of resonator length on the threshold power and emission wavelength of electron-beam-pumped UV-A edge-type emitting lasers is demonstrated. The lowest pump power thresholds are achieved for 100-200 mu m long resonators, where there is good overlap of the focused e-beam spot with the resonator cavity. For longer resonator lengths the focused circular e-beam spot excites only a segment of the resonator cavity, yet lasing is observed. Absorption of the e-beam generated photons in the un-pumped sections of the resonator results in increased absorption losses and, consequently, higher laser thresholds. In addition, a significant wavelength shift from 375 nm for the 50 mu m resonators to 385 nm for the 600 mu m resonators is observed due to absorption of the higher energy photons and re-emission at longer wavelengths in the un-pumped sections.

Sylwester Porowski, Bulk and Homoepitaxial GaN-growth and Characterisation, Journal of Crystal Growth, 1998, 153–158, 189/190, Elsevier Science B.V., Warsaw, Poland. Y. Naoi. K. Kobatake, S. Kurai, K. Nishino, H. Sato, M. Nozaki, S. Sakai, Y. Shintani, Characterization of bulk GaN grown by Sublimation technique, Journal of Crystal Growth, 1998, 163–166, 189/190, Elsevier Science B.V., Warshaw, Poland.

Heterostructures of AlGaN with multiple quantum wells were grown by metal-organic vapor phase epitaxy on semipolar (20-21) bulk AlN substrates. Smooth epitaxial surfaces with excellent heterostructure interfaces were demonstrated. Luminescence from the AlGaN multiple quantum wells emitting at λ = 237 nm show a substantial degree of polarization of about 35% as determined by low-temperature photoluminescence measurements.

AlGaN-based multiple-quantum-well (MQW) heterostructures were irradiated with a pulsed electron beam. Excitation with a beam energy of 12 keV and a beam current of 4.4 mA produced cathodoluminescense at λ=246 nm with a measured peak output power of >200 mW. The emission is dominated by radiative recombination from the MQW up to the maximum tested excitation power density of 1 MW/cm2, as evidenced by unity slope in a double-logarithmic plot of the light output power vs. excitation power density. Monte Carlo simulations of the depth distribution of deposited energy for different beam energies produced good agreement with the measured peak output power vs. beam energy for an assumed carrier diffusion length of ∼200 nm.

The current status of UV lasers and laser diodes (LDs) based on group III-nitrides is reviewed. The focus is on the design, fabrication, and performance of AlGa(In) N laser heterostructures grown by metal-organic vapor phase epitaxy (MOVPE) on high-quality bulk AlN substrates. The review begins with the fundamentals of laser diode operation and identifies the challenges to realize short-wavelength devices with wide band gap materials. In particular, simultaneously achieving high material quality and good p-type conductivity becomes increasingly challenging with higher aluminum concentrations in the epitaxial films. Using low defect density bulk AlN substrates is a good strategy to realize high internal quantum efficiencies and, ultimately, high gain within the active zone. Polarization-assisted hole generation with a short-period superlattice for the cladding layer is a viable approach to overcome the limitations of thermally activated p-type doping. Topics include LD processing considerations that are relevant for the high band gap materials, issues related to efficient carrier injection at the high current densities required for LD operation, and specific approaches to improve the functionality of the electron blocking layer. Next, results are presented for optically pumped UV lasers, with wavelengths down to lambda = 237 nm and low lasing thresholds, and design options are described to manipulate the polarization of the emitted laser light. The review concludes with a discussion of alternative laser designs to realize deep-UV laser emission with nitride semiconductors.