Avalanche generation is a physical mechanism responsible for the breakdown at extremely high field, such as in the reverse bias conditions typical of ESD discharges. In this work, for the first time we provide experimental evidence that avalanche generation can take place in state-of-the-art InGaN based blue LEDs. We measured the current-voltage and electroluminescence curves of the devices while pulsing them with increasing reverse voltages. We investigated a wide span of temperatures (from cryogenic to room temperature) in order to verify that the increase in leakage current detected below -80 V is related to avalanche generation (positive temperature-coefficient). Numerical simulations show that in this bias condition the band-to-band tunneling barrier thickness is low, leading to the possible injection of highly-energetic electrons from the p-side to the n-side that can start the avalanche process. The spectral shape shows a broad emission, covering the spectral range between 1.25 and 3.5 eV; the low energy side slowly decreases below 2.2 eV, and two sharp edges are seen at the high-energy side. Since an avalanche generation process is present, we can interpret the spectrum as follows: (i) hole and electron pairs generated by the avalanche process recombine, emitting photons; (ii) high-energy side: reabsorption of the emitted photons in the In-containing layers and nGaN side, confirmed by the red-shift at higher temperature; (iii) low-energy side: internal photoluminescence of the defects in the n-GaN layer, confirmed by PL measurements with external excitation. A theoretical computation based on this model is able to reproduce the experimental data.

The thermal droop (reduction of the optical power when the temperature is increased) is a phenomenon that strongly limits the efficiency of InGaN-based light-emitting diodes. In this paper we analyze the role of Shockley-Read-Hall (SRH) recombination and of the electron blocking layer (EBL) in the process by using numerical simulations and literature data. The benefic impact of EBL suggests that carrier escape from the quantum wells gives a significant contribution to the thermal droop, therefore we review some of the mechanisms described in the literature (thermionic emission, phonon-assisted tunneling, thermionic trap-assisted tunneling). Since no formulation is able to fit the behavior of the measured SQW devices, we develop a new model based on two phonon-assisted tunneling steps through a defective state, extended in order to take into account zero-field emission. By using experimental data, material constants from the literature and only two fitting parameters the model is able to reproduce the experimental behavior.

This paper reports an investigation of the physical origin of the thermal droop (the drop of the optical power at high temperatures) in InGaN-based light-emitting diodes. We critically investigate the role of various mechanisms including Shockley-Read-Hall recombination, thermionic escape from the quantum well, phonon-assisted tunneling, and thermionic trap-assisted tunneling; in addition, to explain the thermal droop, we propose a closed-form model which is able to accurately fit the experimental data by using values extracted from measurements and simulations and a limited set of fitting parameters. The model is based on a two-step phonon-assisted tunneling over an intermediate defective state, corrected in order to take into account the pure thermionic component at zero bias and the field-assisted term.

This paper describes the degradation of InGaN-based LEDs submitted to constant current stress; based on combined electroluminescence, photoluminescence and deep-level transient spectroscopy we show that: (i) when submitted to constant current stress, LEDs can show a measurable decrease in the optical power, which is more prominent in the low current regime; (ii) the decrease in optical power is strongly correlated to the increase in the Shockley–Read–Hall recombination coefficient A, as estimated by differential lifetime measurements; (iii) stress induces the increase in the concentration of a trap level, with activation energy between 0.6 and 0.7eV, which is supposed to be located next to/within the active region. The results suggest that the optical degradation can be ascribed to the increase in non-radiative recombination, rather than to a decrease in carrier injection efficiency.

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.

A monolithic integrable capacitive humidity sensing method to determine water vapour transmission rates (WVTRs) of dielectric thin films is presented. The capacitive sensor, being used to detect transmission of water vapour, as well as the dielectric thin film to be tested can be processed subsequently with standard semiconductor technology. First measurements yield a reliable value of the well investigated dielectric silicon dioxide (SiO2). A 330nm thick plasma enhanced chemical vapour deposited film of SiO2 showed a WVTR of ∼1.6∗10-2±0.7∗10-2gm2∗d at 124°C and a step in surrounding relative humidity from 65% to 85%. The working principle of the sensor, its drawbacks and improvements are discussed and compared with other methods.

This paper presents an extensive investigation of the deep levels related to non-radiative recombination in InGaN/GaN light-emitting diodes (LEDs). The study is based on combined optical and deep-level transient spectroscopy measurements, carried out on LEDs with identical structure and with different values of the non-radiative recombination coefficient. Experimental data lead to the following, relevant, results: (i) LEDs with a high non-radiative recombination coefficient have a higher concentration of a trap (labeled as “e2”) with an activation energy of 0.7 eV, which is supposed to be located close to/within the active region; (ii) measurements carried out with varying filling pulse duration suggest that this deep level behaves as a point-defect/dislocation complex. The Arrhenius plot of this deep level is critically compared with the previous literature reports, to identify its physical origin.

The interplay of different photoluminescence degradation mechanisms in InGaAlP light emitting diode structures is studied. We investigate the dependence of the photoluminescence degradation behavior on different stress and detection laser power densities. The aging experiments are carried out in an optical overstress setup via high power laser excitation. The experimental data are analyzed by a multicomponent defect evolution approach. Simultaneous growth and annealing of different kinds of defects during the photoluminescence degradation of the device are described by individual characteristic time constants for each of the defect evolution processes. This defect evolution approach is combined with a rate equation model covering radiative and non-radiative processes in the active layer as well as leakage effects into the confining layers.

We examine the influence of an applied reverse bias on the optically induced and measured photoluminescence degradation characteristics of an InGaAlP light-emitting diode (LED) structure. We show that a reverse bias applied simultaneously to laser excitation of the sample has a strong impact on the observable photoluminescence degradation properties of the structure investigated via intense laser excitation. With the help of this approach, it is possible to control the carrier density and the internal electric field of the diode independently. By doing this, a distinction of several usually interfering photoluminescence degradation mechanisms from each other is achievable. Further, a comparison of the experimental data with simulated data delivers some indication on the local origin of the defect evolution processes within the light-emitting diode structure.

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.

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.

We report the direct observation of hot carriers generated by Auger recombination via photoluminescence spectroscopy on tailored (AlGaIn)N multiple quantum well (QW) structures containing alternating green and ultra-violet (UV) emitting (GaIn)N QWs. Optically pumping solely the green QWs using a blue emitting high power laser diode, carrier densities similar to electrical light-emitting diode (LED) operation were achieved, circumventing possible leakage and injection effects. This way, luminescence from the UV QWs could be observed for excitation where the emission from the green QWs showed significant droop, giving direct evidence for Auger generated hot electrons and holes being injected into the UV QWs. An examination of the quantitative relation between the intensity of the UV luminescence and the amount of charge carriers lost due to drooping of the QWs supports the conclusion that Auger processes contribute significantly to the droop phenomenon in (AlGaIn)N based light-emitting diodes.

Recent experiments on the DX behaviour of Si in AlGaN alloys are reviewed. From persistent photoconductivity, light-induced electron spin resonance, and thermal quenching experiments as well as from the thermal activation of the generation-recombination noise, a quantitative configuration coordinate diagram is constructed for pure AlN and Al0.75Ga0.25N. The effects of DX centres on the properties of AlGaN alloys with lower Al concentration are discussed.

The electronic noise properties of Si-doped AlN and Al0.3Ga0.7N are investigated. In AlN:Si, generation–recombination (g–r) noise is observed and shown to be linked to DX-centers. The potential energy barriers for capture into and emission from the DX− ground state are quantitatively determined from the noise measurements. In Al0.3Ga0.7N:Si, in addition to 1/f noise, we find two g–r noise processes. However, an unambiguous identification of their origin proves to be difficult.

Silicon doped AlxGa1-xN alloys with high aluminum content show a large persistent photoconductivity if exposed to light with photon energies above 1.5 eV at temperatures below 60 K. In combination with a persistent electron spin resonance signal, which again can be detected only after illumination at low temperatures, this infers a DX-like behavior of silicon in these materials. The height of the energy barrier EB impeding the transition from a shallow donor state to the deep DX state is deduced from the decay dynamics of the persistent photoconductivity.

Generation–recombination noise is observed in Si-doped aluminum nitride (AlN:Si). Both the magnitude and the characteristic frequency of the generation–recombination noise power density are found to be thermally activated. Using a model based on charge carrier number fluctuations in a two-level system, transition energies and potential barriers of the DX center formed by Si donors in AlN are quantitatively determined.

Silicon doped wurtzite Al(x)Ga(1)-(x)N alloys with x = 0, 0.15, 0.32, 0.52, 0.75, and 1 were characterized with electron spin resonance experiments. For Al contents x < 0.4 the g-values measured for the shallow Si donor are identical to literature values obtained from nominally undoped material. For x > 0.4 g-values are reported for the first time. The experimental results can be fitted within a five-band k (.) p model in the cubic approximation if a spin orbit splitting of the higher conduction bands of at least 50 meV is considered.

Capacitance–voltage (CV) and deep level transient spectroscopy (DLTS) experiments (thermally and optically excited) are applied to explore various properties of boron-doped type IIb high pressure high temperature diamond and of homoepitaxially grown boron doped CVD diamond: (a) properties of Al Schottky contacts; (b) acceptor densities and hydrogen boron interactions; and (c) energy distributions and densities of compensating defects. Two distinct excitation energies at 0.9 and 1.25 eV are detected by DLTS and optically excited DLTS. Carbon implantation experiments reveal that the defect with optical excitation energy of 1.25 eV is the positively charged vacancy. Annealing measurements show that in the temperature regime of 525–600 K a significant part of the vacancy density is annealed which is attributed to recombination of carbon interstitial with vacancies.