Nathan G. Young

High performance thin quantum barrier InGaN/GaN solar cells on sapphire and bulk (0001) GaN substrates

We demonstrate high performance InGaN/GaN multiple quantum well solar cells with thin quantum barriers and spectral response extending to 460 nm. Devices grown on bulk (0001) GaN substrates with up to 50 quantum wells (QWs) outperform those grown simultaneously on sapphire due to the lower threading dislocation density. Increasing the number of QWs eventually leads to performance degradation of devices grown on both substrates. Solar cells are demonstrated with peak external quantum efficiencies up to 60%, open circuit voltages up to 2.28 V, fill factors up to 80%, and conversion efficiencies up to 2.4% under 1 sun AM0 equivalent illumination.

High-performance broadband optical coatings on InGaN/GaN solar cells for multijunction device integration

We demonstrate InGaN/GaN multiple quantum well solar cells grown by metalorganic chemical vapor deposition on a bulk (0001) substrate with high-performance broadband optical coatings to improve light absorption. A front-side anti-reflective coating and a back-side dichroic mirror were designed to minimize front surface reflections across a broad spectral range and maximize rear surface reflections only in the spectral range absorbed by the InGaN, making the cells suitable for multijunction solar cell integration. Application of optical coatings increased the peak external quantum efficiency by 56% (relative) and conversion efficiency by 37.5% (relative) under 1 sun AM0 equivalent illumination.

Origin of electrons emitted into vacuum from InGaN light emitting diodes

The mechanism responsible for efficiency droop in InGaN light-emitting diodes (LEDs) has long been elusive due to indirect measurement techniques used for its identification. Auger recombination is unique among proposed efficiency droop mechanisms, in that it is the only mechanism capable of generating hot carriers. In a previous study [J. Iveland et al., Phys. Rev. Lett. 110, 177406 (2013)], we performed electron energy analysis of electrons emitted into vacuum from a forward biased InGaN LED that had been brought into negative electron affinity by cesiation. Three peaks were observed in the energy spectrum of vacuum emitted electrons. In this Letter, we unambiguously identify the origin of the peaks. The two higher energy peaks correspond to accumulation of electrons transported to the surface in the bulk Γ and side L conduction band valleys. The L-valley peak is a direct signature of a hot Auger electron population. The lower energy peak results from surface photoemission induced by the internal LED light emitted from the InGaN quantum wells. Two control experiments were performed. In the first, a simple GaN pn junction generated only a single Γ peak in electroemission. In the second, selective detection of the photoemission from an LED under modulated light excitation and DC electrical injection confirms that only the low energy peak is photogenerated and that LED light is incapable of generating Γ or L-valley peaks, the latter only occurring due to the Auger effect in the LED active region.

Carrier dynamics and Coulomb-enhanced capture in III-nitride quantum heterostructures

A detailed study of the small-signal response of III-Nitride quantum well (QW) light-emitting diodes is presented, in which the electrical and optical responses are simultaneously measured. A complete transport-recombination model is introduced to account for measurements. This allows for a proper evaluation of the recombination lifetime and for the accurate quantification of thermionic carrier escape from the QW. Further, a yet-unreported carrier capture mechanism is identified and quantified; it increases with the carrier density in the QW and bears the signature of a Coulomb in-scattering process.

Electrical properties of III-Nitride LEDs: Recombination-based injection model and theoretical limits to electrical efficiency and electroluminescent cooling

The current-voltage characteristic and ideality factor of III-Nitride quantum well light-emitting diodes (LEDs) grown on bulk GaN substrates are investigated. At operating temperature, these electrical properties exhibit a simple behavior. A model in which only active-region recombinations have a contribution to the LED current is found to account for experimental results. The limit of LED electrical efficiency is discussed based on the model and on thermodynamic arguments, and implications for electroluminescent cooling are examined.

Polarization field screening in thick (0001) InGaN/GaN single quantum well light-emitting diodes

We demonstrate through simulation that complete screening of polarization-induced electric fields in c-plane InGaN/GaN quantum wells (QWs) is possible by equal n- and p-doping of 10 nm layers immediately adjacent to the QW at a level of 7 × 1019 cm−3. Single QW light-emitting diodes with varying QW thickness are grown using the simulated structure. Biased photoluminescence (PL) measurements show no wavelength shift, indicating complete screening of the polarization field. The behavior of PL peak intensity as a function of bias can be explained as a competition between radiative recombination and carrier escape through tunneling or thermionic emission.

All-optical measurements of carrier dynamics in bulk-GaN LEDs: Beyond the ABC approximation

An all-optical measurement of differential carrier lifetimes is performed in a specially designed single-quantum-well structure. The measurement reveals the complex carrier-dependence of radiative and non-radiative recombinations, which directly manifest wavefunction-overlap and field-screening effects. This analysis clarifies the range of applicability of the common ABC model and its limitations.

Influence of the Structure Parameters on the Relaxation of Semipolar InGaN/GaN Multi Quantum Wells

The influence of semipolar (201) InGaN/GaN multi quantum well (MQW) structure parameters such as well composition and thickness (dw), barrier thickness, as well as total number of periods on the structural and optical properties of the MQWs grown on (201) GaN by metal organic chemical vapor deposition was investigated. At dw < 3 nm, the MQW stacks were very robust with respect to changes in the barrier thickness or the number of periods in the MQW stack, and 30 period (2.5 nm In0.25Ga0.75N/8.5 nm GaN) MQWs exhibiting bright luminescence at 465 nm were demonstrated. For all samples with dw<3 nm in this study, one-dimensional relaxation via misfit dislocations did not lead to any deterioration of the optical properties of the films, and a decrease in the photoluminescence intensity was only observed after the on-set of two-dimensional relaxation via non-basal plane defects.

Low damage dry etch for III-nitride light emitters

We have developed a dry etch process for the fabrication of lithographically defined features close to light emitting layers in the III-nitride material system. The dry etch was tested for its effect on the internal quantum efficiency of c-plane InGaN quantum wells using the photoluminescence of a test structure with two active regions. No change was observed in the internal quantum efficiency of the test active region when the etched surface was greater than 71 nm away. To demonstrate the application of the developed dry etch process, surface-etched air gaps were fabricated 275 nm away from the active region of an m-plane InGaN/GaN laser diode and served as the waveguide upper cladding. Electrically injected lasing was observed without the need for regrowth or recovery anneals. This dry etch opens up a new design tool that can be utilized in the next generation of GaN light emitters.

Field-assisted Shockley-Read-Hall recombinations in III-nitride quantum wells

The physical process driving low-current non-radiative recombinations in high-quality III-Nitride quantum wells is investigated. Lifetime measurements reveal that these recombinations scale with the overlap of the electron and hole wavefunctions and show weak temperature dependence, in contrast with common empirical expectations for Shockley-Read-Hall recombinations. A model of field-assisted multiphonon point defect recombination in quantum wells is introduced, and shown to quantitatively explain the data. This study provides insight on the high efficiency of III-Nitride light emitters.