Indium nitride is one of the very few semiconductors which is known to have a superconducting phase at temperatures of Tc > 1 K. Superconductivity occurs in a window of carrier densities of approximately 1018–1020 cm−3. This is a very low density when compared to other superconductors (i.e. metals, alloys, high Tc oxides) and thus raises interesting fundamental questions as well as technological possibilities. In this paper we address one key question about the dimensionality of the superconducting state of InN by using angle dependent critical field measurements. Our samples were grown by two different growth techniques (chemical vapour deposition and plasma-assisted molecular beam epitaxy) on c-oriented sapphire, with and without a GaN buffer layer. In both cases we find that for film thicknesses much larger than the coherence length d ≫ ξ, the angle dependence of the critical field (down to T < 280 mK) with respect to the c-axis continues to be clearly two-dimensional, demonstrating a characteristic cusp when the angle crosses 90° with respect to the c-axis. This indicates that the superconducting electrons are most likely confined to a layer much thinner than the thickness of the InN film. Further we find the magnitude of the gap to be 2Δ(0)/kBTc = 3.6, very close to the BCS prediction.
Growth of InGaN, having high Indium composition without compromising crystal quality has always been a great challenge to obtain efficient optical devices. In this work, we extensively study the impact of non-radiative defects on optical response of the plasma assisted molecular beam epitaxy (PA-MBE) grown InGaN nanowires, emitting in the higher wavelength regime (
λ
>
520
nm). Our analysis focuses into the effect of defect saturation on the optical output, manifested by photoluminescence (PL) spectroscopy. Defect saturation has not so far been thoroughly investigated in InGaN based systems at such a high wavelength, where defects play a key role in restraining efficient optical performance. We argue that with saturation of defect states by photo-generated carriers, the advantages of carrier localization can be employed to enhance the optical output. Carrier localization arises because of Indium phase segregation, which is confirmed from wide PL spectrum and analysis from transmission electron microscopy (TEM). A theoretical model has been proposed and solved using coupled differential rate equations in steady state to undertake different phenomena, occurred during PL measurements. Analysis of the model helps us understand the impact of non-radiative defects on PL response and identifying the origin of enhanced radiative recombination.
Non-radiative defects play a deterministic role in regulating the performance of LEDs. Yet, defect saturation in LEDs is relatively unexplored in the literature. Here, we establish the theoretical background of carrier-induced defect saturation from the band structure of quantum well (QW)-based InGaN LEDs after solving Poisson and Schrodinger's equations self-consistently. Time dynamics of defect saturation are demonstrated through solving a set of coupled differential rate equations iteratively, considering carrier transitions between different energy levels in the QW region. They indicate an increasing degree of defect saturation with higher carrier injection at steady state. Capacitance versus voltage (CV) measurements on fabricated InGaN MQW LEDs, conducted at low frequencies clearly demonstrate the considerable effect of defect saturation at higher bias. We propose a correction term in the typical RC circuit model for LEDs, considering defect saturation, and solved it analytically to explain the frequency-dependent CV characteristics. Analytical calculation of CV response, based on the modified RC model, shows a fairly satisfactory matching with the experimental data at different frequencies. Also, the frequency dependence of negative capacitance at a higher bias regime is explained through the conductance versus voltage (GV) characteristics.
The
rapidly increasing interest in nanowires (NWs) of GaN and associated
III-Nitrides for (opto-)electronic applications demands immediate
address of the technological challenges associated with NW-based device
processing. Toward this end, we demonstrate in this work an approach
to suppress the thermal decomposition of GaN NWs, which also serves
to passivate the surface states. Both of these effects are known to
be significant challenges in the development of GaN-NW-based devices.
The approach entails AlN capping of the as-grown GaN NWs, in the same
molecular beam epitaxy growth step. We show that the epitaxial AlN
crest that grows on the top facet of the NW arrests thermal decomposition,
while the AlN shell on the sidewalls (together with the crest) protects
the NW surface from the generation of oxygen-induced surface states.
This simple approach can be used for the development of GaN-NW-based
devices.
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