S. M. Islam

Sub-230 nm deep-UV emission from GaN quantum disks in AlN grown by a modified Stranski–Krastanov mode

We report tunable deep-ultraviolet (DUV) emission over the 222–231 nm range from 1–2 monolayer (ML) GaN quantum disks (QDs) grown in an AlN matrix. The linewidth of the emission were as narrow as ~10 nm at 5 K. The disks were grown in modified Stranski–Krastanov (mSK) mode. High resolution scanning transmission electron microscopy (STEM) images confirmed insertion of 1–2 MLs of GaN between 3 nm AlN barriers. The internal quantum efficiency was estimated from low temperature photoluminescence measurements for the disks, and compared with 1 and 2 ML GaN quantum wells/AlN barriers. The internal quantum efficiency (IQE) of the GaN QDs was found to be ~35% for 222 nm emission, ~200% higher than 1 ML GaN QWs.

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.

Physics and polarization characteristics of 298 nm AlN-delta-GaN quantum well ultraviolet light-emitting diodes

This work investigates the physics and polarization characteristics of 298 nm AlN-delta-GaN quantum well (QW) ultraviolet (UV) light-emitting diodes (LEDs). The physics analysis shows that the use of the AlN-delta-GaN QW structure can ensure dominant conduction band (C) to heavy-hole (HH) subband transition and significantly improve the electron and top HH subband wave function overlap. As a result, up to 30-times enhancement in the transverse-electric (TE)-polarized spontaneous emission rate of the proposed structure can be obtained as compared to a conventional AlGaN QW structure. The polarization properties of molecular beam epitaxy-grown AlN/GaN QW-like UV LEDs, which consist of 3–4 monolayer (QW-like) delta-GaN layers sandwiched by 2.5-nm AlN sub-QW layers, are investigated in this study. The polarization-dependent electroluminescence measurement results are consistent with the theoretical analysis. Specifically, the TE-polarized emission intensity is measured to be much larger than the transverse-ma...

New tunneling features in polar III-nitride resonant tunneling diodes

Double barrier GaN/AlN resonant tunneling heterostructures have been grown by molecular beam epitaxy on the (0001) plane of commercially available bulk GaN substrates. Resonant tunneling diodes were fabricated; room temperature current-voltage measurements reveal the presence of a negative differential conductance region under forward bias with peak current densities of ~6.4

High-quality InN films on GaN using graded InGaN buffers by MBE

kA/cm^2

Deep-UV emission at 219 nm from ultrathin MBE GaN/AlN quantum heterostructures

and a peak to valley current ratio of ~1.3. Reverse bias operation presents a characteristic turn-on threshold voltage intimately linked to the polarization fields present in the heterostructure. An analytic electrostatic model is developed to capture the unique features of polar-heterostructure-based resonant tunneling diodes; both the resonant and threshold voltages are derived as a function of the design parameters and polarization fields. Subsequent measurements confirm the repeatability of the negative conductance and demonstrate that III-nitride tunneling heterostructures are capable of robust resonant transport at room temperature.

600 V GaN vertical V-trench MOSFET with MBE regrown channel

The growth of high-quality thick InN films is challenging because of the lack of native substrates. In this work, we demonstrate the use of a linearly graded InGaN buffer layer for the growth of InN films on GaN substrates. A 500 nm InN film with <0.1 nm RMS roughness is obtained with a peak mobility of 1410 cm2/(Vs) at 300 K. A strong room temperature photoluminescence showing a bandgap of 0.65 eV with 79 meV linewidth is observed. A graded InGaN buffer is found to lead to extremely smooth and high-quality InN films.

234 nm and 246 nm AlN-Delta-GaN quantum well deep ultraviolet light-emitting diodes

Deep ultraviolet (UV) optical emission below 250 nm (∼5 eV) in semiconductors is traditionally obtained from high aluminum containing AlGaN alloy quantum wells. It is shown here that high-quality epitaxial ultrathin binary GaN quantum disks embedded in an AlN matrix can produce efficient optical emission in the 219–235 nm (∼5.7–5.3 eV) spectral range, far above the bulk bandgap (3.4 eV) of GaN. The quantum confinement energy in these heterostructures is larger than the bandgaps of traditional semiconductors, made possible by the large band offsets. These molecular beam epitaxy-grown extreme quantum-confinement GaN/AlN heterostructures exhibit an internal quantum efficiency of 40% at wavelengths as short as 219 nm. These observations together with the ability to engineer the interband optical matrix elements to control the direction of photon emission in such binary quantum disk active regions offer unique advantages over alloy AlGaN quantum well counterparts for the realization of deep-UV light-emitting d...

III-N heterostructure devices for low-power logic

GaN vertical power transistors have gained increasing interest in recent years due to the advantages over lateral transistors in high voltage/high current applications. To date, two major topologies have been studied most: gate-on-epi-surface (GoE) and gate-on-sidewall (GoS). The GoE devices include CAVET [1] and VDMOSFET-like transistors [2, 3]. The GoS devices include U-MOS or trench-MOSFETs with inversion channel [4, 5] or regrown AlGaN/GaN semi-polar channel [6], as well as depletion-mode MISFET [7]. The vertical MISFET is the simplest to fabricate, however, it does not have avalanche capabilities inherently besides being difficult to achieve sufficiently large Vth. It is easier for trench MOSFETs to achieve normally-off operation, high breakdown voltage (BV) and small footprint. However, it is challenging to achieve high mobility in the inversion channel. In contrast, CAVETs, VDMOS-like transistors and PolarMOS [3] utilize high mobility AlGaN/GaN channel to achieve low Ron, but the channel regrowth posts challenges in achieving low off-state leakage in un-gated regrowth interface. Recently, a novel design based on trench MOSFET is realized by MOCVD regrowth of a thin GaN interlayer [8]. Low Ron and high BV is achieved in the gated regrown channel. Similar to the other MOCVD regrown devices, the buried Mg-doped p-GaN needs to be re-activated by exposing the p-GaN surface during high temperature anneal. This leads to high thermal budget and poses limitations on device geometry. Furthermore, any incomplete activation of buried p-GaN leads to reduced BV. In this work, we design a V-shaped trench MOSFET with MBE regrown UID GaN channel. −600 V breakdown voltage with normally-off operation is demonstrated without the need for re-activation of the buried p-GaN. To our knowledge, this is the highest BV achieved in GaN vertical transistors with MBE regrown channel.

Demonstration of AlGaN-delta-GaN QW by plasma-assisted molecular beam epitaxy for 260-nm ultraviolet light emitting diodes

Deep ultraviolet (DUV) AlN-delta-GaN quantum well (QW) light-emitting diodes (LEDs) with emission wavelengths of 234 nm and 246 nm are proposed and demonstrated in this work. Our results reveal that the use of AlN-delta-GaN QW with ∼1–3 monolayer GaN delta-layer can achieve a large transverse electric (TE)-polarized spontaneous emission rate instead of transverse magnetic-polarized emission, contrary to what is observed in conventional AlGaN QW in the 230–250 nm wavelength regime. The switching of light polarization in the proposed AlN-delta-GaN QW active region is attributed to the rearrangement of the valence subbands near the Γ-point. The light radiation patterns obtained from angle-dependent electroluminescence measurements for the Molecular Beam Epitaxy (MBE)-grown 234 nm and 246 nm AlN-delta-GaN QW LEDs show that the photons are mainly emitted towards the surface rather than the edge, consistent with the simulated patterns achieved by the finite-difference time-domain modeling. The results demonstra...