Brelon J. May

Tunnel junction enhanced nanowire ultraviolet light emitting diodes

Polarization engineered interband tunnel junctions (TJs) are integrated in nanowire ultraviolet (UV) light emitting diodes (LEDs). A ∼6 V reduction in turn-on voltage is achieved by the integration of tunnel junction at the base of polarization doped nanowire UV LEDs. Moreover, efficient hole injection into the nanowire LEDs leads to suppressed efficiency droop in TJ integrated nanowire LEDs. The combination of both reduced bias voltage and increased hole injection increases the wall plug efficiency in these devices. More than 100 μW of UV emission at ∼310 nm is measured with external quantum efficiency in the range of 4–6 m%. The realization of tunnel junction within the nanowire LEDs opens a pathway towards the monolithic integration of cascaded multi-junction nanowire LEDs on silicon.

Nanowire LEDs grown directly on flexible metal foil

Using molecular beam epitaxy, self-assembled AlGaN nanowires are grown directly on Ta and Ti foils. Scanning electron microscopy shows that the nanowires are locally textured with the underlying metallic grains. Photoluminescence spectra of GaN nanowires grown on metal foils are comparable to GaN nanowires grown on single crystal Si wafers. Similarly, photoluminescence lifetimes do not vary significantly between these samples. Operational AlGaN light emitting diodes are grown directly on flexible Ta foil with an electroluminescence peak emission of ∼350 nm and a turn-on voltage of ∼5 V. These results pave the way for roll-to-roll manufacturing of solid state optoelectronics.

Nanoscale current uniformity and injection efficiency of nanowire light emitting diodes

As an alternative to light emitting diodes (LEDs) based on thin films, nanowire based LEDs are the focus of recent development efforts in solid state lighting as they offer distinct photonic advantages and enable direct integration on a variety of different substrates. However, for practical nanowire LEDs to be realized, uniform electrical injection must be achieved through large numbers of nanowire LEDs. Here, we investigate the effect of the integration of a III-Nitride polarization engineered tunnel junction (TJ) in nanowire LEDs on Si on both the overall injection efficiency and nanoscale current uniformity. By using conductive atomic force microscopy (cAFM) and current-voltage (IV) analysis, we explore the link between the nanoscale nonuniformities and the ensemble devices which consist of many diodes wired in parallel. Nanometer resolved current maps reveal that the integration of a TJ on n-Si increases the amount of current a single nanowire can pass at a given applied bias by up to an order of mag...

Nanoscale Electronic Conditioning for Improvement of Nanowire Light Emitting Diode Efficiency

Commercial III-Nitride LEDs and lasers spanning visible and ultraviolet wavelengths are based on epitaxial films. Alternatively, nanowire-based III-Nitride optoelectronics offer the advantage of strain compliance and high crystalline quality growth on a variety of inexpensive substrates. However, nanowire LEDs exhibit an inherent property distribution, resulting in uneven current spreading through macroscopic devices that consist of millions of individual nanowire diodes connected in parallel. Despite being electrically connected, only a small fraction of nanowires, sometimes <1%, contribute to the electroluminescence (EL). Here, we show that a population of electrical shorts exists in the devices, consisting of a subset of low-resistance nanowires that pass a large portion of the total current in the ensemble devices. Burn-in electronic conditioning is performed by applying a short-term overload voltage; the nanoshorts experience very high current density, sufficient to render them open circuits, thereby forcing a new current path through more nanowire LEDs in an ensemble device. Current-voltage measurements of individual nanowires are acquired using conductive atomic force microscopy to observe the removal of nanoshorts using burn-in. In macroscopic devices, this results in a 33× increase in peak EL and reduced leakage current. Burn-in conditioning of nanowire ensembles therefore provides a straightforward method to mitigate nonuniformities inherent to nanowire devices.

Simultaneous molecular beam epitaxy growth at multiple uniform substrate temperatures

A substrate holder is demonstrated for molecular beam epitaxy (MBE) growth at four calibrated substrate temperatures in the same growth run. On a standard 3-in. substrate block, the substrate face plate can hold simultaneously four substrates, each at a uniform and isolated temperature. The samples are otherwise under identical growth conditions, providing a fourfold increase in sample throughput per growth run. Therefore, the multi-temperature-zone substrate holder is particularly suited for materials research and development by MBE, where it enables rapid mapping of the growth phase diagram.

Tunnel junction integrated ultraviolet nanowire LEDs

Efficiency of ultraviolet (UV) and Deep UV LEDs are mainly hindered by poor dopant ionization in wide bandgap AlGaN. Polarization induced doping via composition grading has been used to increase the amount of ionized dopants. Composition gradient over the full composition range is limited by the small critical thickness of the epitaxial graded layer in planar thin films. However, nanowires can accommodate significantly large amount of strain due to large surface to volume ratio. Taking this advantage, previously, polarization induced doping in whole composition range is demonstrated to fabricate UV and deep UV LEDs using catalyst-free nanowires grown by plasma assisted molecular beam epitaxy on silicon wafers. Due to majority N-face (0001̅) crystallographic direction of the nanowires polarization induced nanowire light emitting diodes (PINLEDs) needs to be grown on p-type silicon substrate which suffers from large turn-on voltage and poor hole injection at the p-Si/p-GaN interface due to large valence band discontinuity. In this work, we integrate an InGaN tunnel junction (TJ) at the base of the PINLEDs to lower the turn-on voltage and increase hole injection into the p-graded region of the PINLEDs.