Dylan Bayerl

Visible-Wavelength Polarized-Light Emission with Small-Diameter InN Nanowires

Group III nitrides are widely used in commercial visible-wavelength optoelectronic devices, but materials issues such as dislocations, composition fluctuations, and strain negatively impact their efficiency. Nitride nanostructures are a promising solution to overcome these issues and to improve device performance. We used first-principles calculations based on many-body perturbation theory to study the electronic and optical properties of small-diameter InN nanowires. We show that quantum confinement in 1 nm wide InN nanowires shifts optical emission to the visible range at green/cyan wavelengths and inverts the order of the top valence bands, leading to linearly polarized visible-light emission. Quantum confinement on this scale also leads to large exciton binding energies of 1.4 eV and electronic band gaps in excess of 3.7 eV. Our results indicate that strong quantum confinement in InN nanostructures is a promising approach to developing efficient visible-wavelength light emitters.

Deep ultraviolet emission from ultra-thin GaN/AlN heterostructures

We present the theoretical and experimental results for the electronic and optical properties of atomically thin (1 and 2 monolayers) GaN quantum wells with AlN barriers. Strong quantum confinement increases the gap of GaN to as high as 5.44 eV and enables light emission in the deep-UV range. Luminescence occurs from the heavy and light hole bands of GaN yielding E ⊥ c polarized light emission. Strong confinement also increases the exciton binding energy up to 230 meV, preventing a thermal dissociation of excitons at room temperature. However, we did not observe excitons experimentally due to high excited free-carrier concentrations. Monolayer-thick GaN wells also exhibit a large electron-hole wave function overlap and negligible Stark shift, which is expected to enhance the radiative recombination efficiency. Our results indicate that atomically thin GaN/AlN heterostructures are promising for efficient deep-UV optoelectronic devices.

First-principles calculations of the near-edge optical properties of β-Ga2O3

We use first-principles calculations based on many-body perturbation theory to investigate the near-edge electronic and optical properties of β-Ga2O3. The fundamental band gap is indirect, but the minimum direct gap is only 29 meV higher in energy, which explains the strong near-edge absorption. Our calculations verify the anisotropy of the absorption onset and explain the range (4.4–5.0 eV) of experimentally reported band-gap values. Our results for the radiative recombination rate indicate that intrinsic light emission in the deep-ultra-violet (UV) range is possible in this indirect-gap semiconductor at high excitation. Our work demonstrates the applicability of β-Ga2O3 for deep-UV detection and emission.

Electronic and optical properties of two-dimensional GaN from first principles

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

Indium nitride nanowires for efficient light emitters

Inorganic light-emitting diodes (LEDs) are the essential components of many display and illumination technologies. However, the energy efficiency of LEDs suffers both at high power and at green wavelengths. Improving the efficiency of green LEDs is therefore a major goal of current research in solid-state lighting technology. Our recent findings predict that indium nitride nanowires just a nanometer wide are a promising solution to this ‘green-gap’ problem, and that changing the nanowire width or cross-sectional shape could tailor optical emission to other visible wavelengths. Many approaches to bridging the green gap in LED technology focus on materials like indium nitride (InN) and gallium nitride (GaN).1 These semiconductors have direct band gaps near opposite ends of the visible spectrum (0.6eV in InN and 3.4eV in GaN). Mixing them in different ratios tunes the band gap between those of the pure materials, in principle achieving light emission of any color in the visible range. However, in practice, fluctuations in the alloy composition, lattice mismatch with the substrate, and strain-induced polarization fields severely limit the efficiency of InGaN-based LEDs.2 These issues are especially prevalent in alloys with composition ratios corresponding to green light emission. To circumvent the problems faced by InGaN alloys, we explored the effects of extreme quantum confinement on nanowires of pure InN. Quantum confinement typically increases the optical emission energy of a semiconductor, and so we expected that the light emitted by InN could be boosted into the visible range in very small diameter nanowires. We found that InN nanowires approximately 1nm in diameter will emit green (2.3eV) or cyan (2.5eV) light, depending on the shape of the nanowire cross-section.3 Moreover, using a nanostructured pure-phase semiconductor instead of an alloy avoids many of the efficiency loss mechanisms occurring in InGaN alloys. Figure 1. Electronic band structures of indium nitride (InN) nanowires with (a) hexagonal and (b) triangular cross-sections. Quantum confinement opens the band gaps to 3.7 and 3.9eV in the hexagonal and triangular nanowires, respectively. (Reprinted with permission.3 Copyright 2014 American Chemical Society.)

Radiative and Auger recombination processes in indium nitride

InN and In-rich InGaN alloys emit in the infrared range desirable for telecommunication applications. However, the droop problem reduces their efficiency at high power. Nonradiative Auger recombination is a strong contributor to this efficiency loss. Here, we investigate radiative and Auger recombination in InN and In-rich InGaN with first-principles calculations. We find that the direct eeh process dominates Auger recombination in these materials. In the degenerate carrier regime, the Auger and radiative rates are suppressed by different mechanisms: the radiative rate is affected by phase-space filling while Auger recombination is primarily reduced by free-carrier screening. The suppression of the radiative rate onsets at lower carrier densities than that of the Auger rate, which reduces the internal quantum efficiency of InN devices. Droop in InN can be mitigated by increasing the bandgap through alloying with GaN. We demonstrate that the peak efficiency of In0.93Ga0.07N alloys (which emit at 1550 nm) is 33% higher than that of InN and occurs at higher carrier densities.