Controlling The Alloy Fluctuations in AlGaN Materials Grown by Plasma-Assisted Molecular Beam Epitaxy: Effect on Ultraviolet Light Emitting Diodes

Molecular beam epitaxy (MBE) has been extensively used for the growth of III-Nitride materials and devices as this technique has the scope of accessing a much wider range of operating parameter-space than competing technologies. These can be exploited to address the problems in the UV-LEDs, such as low internal quantum efficiency, poor p-type doping etc. This work focuses on the effect of group III/V flux ratio variation on the AlGaN alloy properties, and consequently on the characteristics of UV emitters. Furthermore, in-situ monitoring of this parameter which is essential for accurate and reproducible growth process has been addressed.<br/><br/>AlGaN alloys were grown using a VEECO GEN930 plasma-assisted molecular beam epitaxy (PA-MBE) system. The growth was monitored in real-time based on the temporal evolution of RHEED patterns and a custom designed image capture and analysis system. An algorithm was developed for the direct monitoring of excess metallic film on the growth surface using the RHEED pattern, which was found to be the critical parameter. A series of otherwise identical samples were grown under various group III/V flux ratios ranging from excess group III to near-stoichiometric, and UV-LEDs were fabricated.<br/><br/>For series A devices (covering the high group III flux range), a single electroluminescence peak was observed, which was red-shifted from 323 nm under III/V&gt;1 conditions to 348 nm under III/V&gt;&gt;1 growth, while the intensity of the peak increased by nearly 30 times. Furthermore, the droop, that is the reduction of peak intensity with increased injection currents reduced significantly for the samples grown under III/V&gt;&gt;1 conditions.<br/><br/>These results can be attributed to the presence of in-plane compositional inhomogeneities in the active region material. For growth under excess group III conditions, a thin metallic film of Ga always remains on top of the growth surface. The Al atoms are incorporated in this film, and subsequent reaction with plasma nitrogen results in the formation of AlGaN. As the metallic film consists of an inhomogeneous mixture of Al and Ga, the resulting AlGaN film possesses nano-scale fluctuations in the composition. Under conditions of III/V&gt;&gt;1, deep level fluctuations are formed within the AlGaN alloy, that give rise to single red-shifted peaks. These fluctuations are spatially dense, inhibiting the carriers from travelling to non-radiative recombination sites and thus reducing the probability of current droop in the devices.<br/><br/>For series B devices (covering the near-stoichiometric growth regime), however, clear dual-wavelength behavior was observed with two peaks at around 330 nm (P1) and 345 nm (P2). Interestingly, switching between these two peaks was observed when the duty cycle of the excitation was varied from 10% to dc values, without any change in the other excitation parameters. In this series, other devices grown under slightly different deposition conditions indicated a switching between 310 nm and 350 nm.<br/><br/>Under conditions of near-stoichiometry, there is nearly no metallic film on the growth surface. As liquid Ga also acts as a surfactant, the absence of it leads to limited surface adatom mobility, specifically of Al compared to Ga, which eventually result in short range fluctuations in the AlGaN films. These give rise to two peaks of distinctly different wavelengths in the devices of series B. The wavelength-switching phenomenon is due to the difference in thermal dissociation of excitons in fluctuations of different energy depths which was confirmed by carrying out temperature dependent electroluminescence measurements.<br/><br/>We conclude that the emission parameters are influenced by change in growth conditions, specifically the group III/V flux ratio, generating not only single-peak LEDs that displayed significantly reduced droop but also switchable dual wavelength LEDs.