The requirement of high growth temperature for high-quality epitaxial AlGaN, which is typically around 100 Β°C higher than the growth temperature of GaN, is unfavorable for p-type dopant (Mg) incorporation, representing a grand challenge for AlGaN deep ultraviolet (UV) light-emitting devices. In this context, we show high-quality AlGaN nanowires emitting in the deep UV band grown at merely the growth temperature of GaN nanowires by molecular beam epitaxy. This is enabled by the discovery of a narrow GaN nanowire template growth window. We have further compared the room-temperature internal quantum efficiency of the samples emitting around 255 nm grown in the low-temperature regime and high-temperature regime. It is found that the sample grown in the low-temperature regime can possess optical quality close to the sample grown in the high-temperature regime. This study, therefore, suggests that even with a low growth temperature, using nanowire structures can still lead to AlGaN alloys with a relatively high optical quality, and the use of low substrate temperature could be beneficial for p-type doping.
We fabricate AlGaN nanowires by molecular beam epitaxy and we investigate their field emission properties by means of an experimental setup using nano-manipulated tungsten tips as electrodes, inside a scanning electron microscope. The tip-shaped anode gives access to local properties, and allows collecting electrons emitted from areas as small as 1Β΅m 2 . The field emission characteristics are analysed in the framework of Fowler-Nordheim theory and we find a field enhancement factor as high as Ξ² = 556 and a minimum turn-on field πΈ π‘π’ππβππ = 17 V/Β΅m for a cathode-anode separation distance π = 500 nm. We show that for increasing separation distance, πΈ π‘π’ππβππ increases up to about 35 V/Β΅m and Ξ² decreases to οΎ100 at π = 1600 nm. We also demonstrate the time stability of the field emission current from AlGaN nanowires for several minutes. Finally, we explain the observation of modified slope of the Fowler-Nordheim plots at low fields in terms of non-homogeneous field enhancement factors due to the presence of protruding emitters.
Aluminum gallium nitride (AlGaN) nanowires have become an emerging approach for semiconductor deep ultraviolet light-emitting devices. To further improve the device performance, it is critical to understand the optical quality of AlGaN nanowires. However, today, the room-temperature internal quantum efficiency (IQE) of AlGaN nanowires is predominantly analyzed by the temperature-dependent photoluminescence (PL) approach under one excitation power or taking the PL intensity ratio at the room temperature and low temperature with different excitation powers. In both cases, one needs to assume the low temperature IQE to be 100%, which is not always valid, in particular when the excitation power changes at the low temperature. In this work, we study the room-temperature IQE of AlGaN nanowires through the detailed excitation power-dependent PL experiments and theoretical analysis. This allows us to derive the intrinsic room-temperature IQE of AlGaN nanowires as a function of the excitation power. It is found that for an Al content in the range of 22%β54%, the IQE of all samples increases as the excitation increases, followed by an efficiency droop. Moreover, comparing different samples, the IQE at low excitations increases as the Al content increases, whereas the peak IQE reduces from 73% to 56% as the Al content increases. The underlying mechanisms are also discussed in this paper.
AlGaN in the form of nanowires is an important platform for semiconductor ultraviolet light sources on Si. In the past, significant efforts have been devoted to improving the quality of AlGaN nanowires. In this context, we present a comparative study on the molecular beam epitaxial growth and characterization of AlGaN nanowire structures on the AlN buffer layer on Si and on Si directly. It is found that AlGaN nanowires grown on the AlN buffer layer shows an improved internal quantum efficiency, compared with the nanowires grown on Si directly. This improvement is attributed to the reduced nanowire coalescence due to the improved vertical alignment of the nanowires grown on the AlN buffer layer.
Limited work has been focused on the efficiency droop in AlGaN ultraviolet light-emitting diodes. Herein, we analyzed recombination coefficients in the ultraviolet-emitting AlGaN nanowires, implying the dominant role of carrier transport in the efficiency droop.
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