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.
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.
We have demonstrated the growth of high-quality Al x Ga (1−x) N (x∼50%) nanowires (NWs) for the first time on the sapphire substrate without using GaN NWs as the template, by plasmaassisted molecular beam epitaxy. Our newly developed process elucidates that depending on the substrate temperature and V/III ratio an AlGaN network is formed on sapphire substrate prior to the NWs growth. We find that the ledges of this kinked shaped network act as nucleation sites for the NW growth. The present observations suggest that availability of nucleation sites and higher substrate temperature during growth are the key parameters for the growth of homogeneous AlGaN NWs on the sapphire substrates. Energy dispersive x-ray spectroscopy, high-resolution transmission electron microscopy, Raman spectroscopy, x-ray diffraction, photoluminescence spectroscopy, and scanning electron microscopy analysis show that AlGaN NWs exhibit nearatomic scale compositional uniformity along the length as well as across the diameter.
Strained Si1−xGex (x = 0.1–0.4) layers were grown on Si(111) and Si(001) substrates using molecular beam epitaxy followed by the growth of epitaxial Gd2O3 thin films on Si1−xGex layers using same technique. Pt/Gd2O3/Si1−xGex/Si stacks fabricated by several in situ process steps exhibit excellent electrical properties. Surface and microstructural analysis of both Si1−xGex and Gd2O3 layers carried out by different in situ and ex situ tools reveal a relaxed epi-Gd2O3 layer on a strained Si1−xGex layer on both Si(111) and Si(001) substrates with sharp interfaces between the oxide and the SiGe layer. Standard electrical measurements, such as capacitance-voltage and leakage current analysis, demonstrate promising electrical properties for such metal oxide semiconductor capacitors. A capacitance equivalent thickness as low as 1.20 nm with associated leakage current density of 2.0 mA/cm2 was obtained for devices with 4.5 nm thin oxide films where the density of interface trap (Dit) was only ∼1011 cm−2 eV−1.
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