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We investigate carrier localization in Al-rich AlGaN/AlN quantum well (QW) structures. Low temperature time-resolved photoluminescence (PL) experiments reveal a strong variation of the carrier decay times with detection photon energy, suggesting a strong impact of carrier localization, which is found to depend primarily on the QW width. In combination with time-integrated PL measurements and numerical band structure calculations, we are able to provide conclusive evidence that the localization strength in AlGaN-based QW structures is directly coupled to the oscillator strength, providing an explanation for its strong dependence on the QW width. This is further supported by the observation of a strong polarization field dependency of the carrier localization, which excludes excitons and may be explained by the accumulation of electrons close to the QW interface, while holes are independently localized across the QW. We complete our discussion by proposing a model to explain the well-known phenomenon of efficiency droop in accordance with our findings, suggesting delocalization-induced Auger recombination as the responsible loss channel.

Carrier dynamics in AlGaN-based single quantum well (QW) structures grown on sapphire are studied by means of time-integrated and time-resolved photoluminescence spectroscopy (PL) in a wide temperature range from 5 K to 350 K. The samples cover a broad compositional range, with aluminum contents ranging between 42% and 60% and QW widths between 1.5 nm and 2.5 nm. All samples reveal the characteristic “S”-shape temperature dependence of the PL emission energy as frequently reported in InGaN-based systems, albeit with significantly larger localization strengths of up to 60 meV. It is shown that in the compositional range investigated, carrier localization is determined primarily by the QW width and, in contrast, exhibits a much weaker dependence on aluminum concentration. By the combination of time-integrated and time-resolved PL measurements, the localization of carriers is demonstrated to have a significant impact on the recombination dynamics of AlGaN/AlN QWs grown on sapphire, heavily affecting the internal quantum efficiency and efficiency droop even in standard LED operation conditions.

We report on a systematic study of the determination of the internal quantum efficiency (IQE) in AlGaN-based multiple-quantum-well (MQW) structures using different optical evaluation methodologies and experimental conditions, in order to derive a standard set of measurement conditions for reliable IQE determination. Several potential sources of error that may distort the IQE obtained by optical measurements are discussed, such as carrier transport effects, excitation conditions failing to fulfill ideal resonance conditions, and morphology issues. A series of nominally identical AlGaN-based MQW structures is grown on an AlGaN layer separated by an AlN interlayer of varying thickness. The MQW structures are studied both by resonant and quasiresonant photoluminescence spectroscopy, and IQEs are determined via different commonly employed methods. The obtained values are shown to be significantly affected by the employed excitation conditions, as well as the evaluation techniques. In addition, growth morphology issues and carrier transport effects need to be considered in the interpretation of the measured data, with the latter being investigated in greater detail. The results emphasize the need for an appropriate choice of both experimental conditions and evaluation methodology in order to extract reliable and comparable IQE values.

Atomic layer deposited aluminum oxide (ALD-Al2O3) is a dielectric material, which is widely used in organic light emitting diodes in order to prevent their organic layers from humidity related degradation. Unfortunately, there are strong hints that in some cases, ALD-Al2O3 itself is suffering from humidity related degradation. Especially, high temperature and high humidity seem to enhance ALD-Al2O3 degradation strongly. For this reason, the degradation behavior of ALD-Al2O3 films at high temperature and high humidity was investigated in detail and a way to prevent it from degradation was searched. The degradation behavior is analyzed in the first part of this paper. Using infrared absorbance measurements and X-ray diffraction, boehmite (γ-AlOOH) was identified as a degradation product. In the second part of the paper, it is shown that ALD-Al2O3 films can be effectively protected from degradation using a silicon oxide capping. The deposition of very small amounts of silicon in a molecular beam epitaxy syst...

Recent photoluminescence experiments presented by M. Binder et al. [Appl. Phys. Lett. 103, 071108 (2013)] demonstrated the visualization of high-energy carriers generated by Auger recombination in (AlInGa)N multi quantum wells. Two fundamental limitations were deduced which reduce the detection efficiency of Auger processes contributing to the reduction in internal quantum efficiency: the transfer probability of these hot electrons and holes in a detection well and the asymmetry in type of Auger recombination. We investigate the transport and capture properties of these high-energy carriers regarding polarization fields, the transfer distance to the generating well, and the number of detection wells. All three factors are shown to have a noticeable impact on the detection of these hot particles. Furthermore, the investigations support the finding that electron-electron-hole exceeds electron-hole-hole Auger recombination if the densities of both carrier types are similar. Overall, the results add to the evidence that Auger processes play an important role in the reduction of efficiency in (AlInGa)N based LEDs.

We have studied the electrical and optical characteristics of (AlGaIn)N multiple quantum well light-emitting diodes. Minimizing contact effects by utilizing platinum as p-contact metal, ideality factors as low as 1.1 have been achieved. In agreement with basic semiconductor theory, a correlation between ideality factor and small-current efficiency was found. We were able to emulate the experimental current-voltage characteristic over seven orders of magnitude utilizing a two diode model. This model enables a very good prediction of internal quantum efficiency at moderate current densities out of purely electrically derived parameters.

In this paper, the carrier transport in (Al)InGaN based test structures with In-rich quantum wells on c-plane substrates is investigated under high current operation. To get access to the injection efficiency, the devices are processed as ridge waveguide lasers and examined above threshold. The slope efficiency reveals a slight decrease as a function of current even under pulsed operation that can be related to a reduction of the injection efficiency based on carrier leakage. As the test structure contains an InGaN detection layer on the n-side, it is possible to verify hole overflow across the active region. Moreover, by analysing the current dependence of the radiative recombination in the detection layer, the reduction of slope efficiency can be correlated to increasing hole leakage.