This paper demonstrates that conventional drain current transient (DCT) measurements fail at identifying the correct activation energy of buffer defects in transistors with p-GaN gate. Based on combined pulsed and transient characterization, we demonstrate that (i) under off-state stress, the analysed devices suffer from moderate dynamic-Ron and from positive shift in the threshold voltage; (ii) the de-trapping kinetics, analysed by DCT, are unexpectedly not thermally-activated; (iii) de-trapping kinetics are significantly accelerated when measured at high gate voltage; (iv) we report a power law correlation between the gate leakage measured during the de-trapping phase and the time constant for recovery. Finally, (v) we propose a new characterization procedure, based on TLM structures, to overcome these issues, thus accurately investigating buffer-related effects.

The performance of normally-off Gallium-Nitride (GaN) High-Electron-Mobility-Transistors (HEMTs) under extended short circuit operation is investigated. A thermal limit is found in the aluminium metallization, where at temperatures around 600 °C a protrusion of the gate metal through the Inter-Level Dielectric (ILD) may form, short-circuiting gate and source metallization and thus resulting in a permanently-off failure state. The present work shows how this particular failure mode can be induced by extreme overload operation, and presents a Finite Element (FE) model which agrees with the experimental observations and gives insights in the mechanical stress-state developing in the device. The deeper thermo-mechanical understanding of the degradation mechanism suggests directions in order to improve the device's robustness.

Exchange of carriers between the GaN channel and the dielectric/AlGaN interface in AlGaN/GaN metal insulator semiconductor high electron mobility transistors was recently attributed to a serial process of electron transport through the AlGaN barrier and electron trapping/emission at the interface. In this paper, the time constant related to barrier transport is evaluated from the measurements of time onset of threshold voltage drift in stress-recovery experiments. Temperature and forward gate bias dependent studies reveal an activation energy of 0.65 eV for the electron transport at zero bias being consistent with the estimated potential barrier of 0.75 eV at the dielectric/AlGaN interface. Thermo-ionic emission and defect assisted tunneling to near interface states are considered as transport mechanisms.

We report on AlGaN/GaN MIS-HEMTs with a thermally stable dielectric/AlGaN interface induced by plasma fluorination. The plasma treatment leads to a modification of the "native" surface donors and a fundamentally different device and defect behavior. The feasibility of III-N surface defect modifications shows a new direction for reducing VTh drifts or the possibility of engineering the surface defects.

Threshold voltage instabilities are investigated in GaN-based metal-insulator-semiconductor (MIS) high-electron-mobility-transistors (HEMTs) with specially designed on-wafer heaters structures. The heaters are based on metal lines or 2-dimensional electron gas (2DEG) resistors and enable to choose the temperature during stress and recovery in the stress-recovery experiments independently. It allows to decouple thermal activation of capture (<; 0.9 eV) and emission (0.4-0.9 eV) processes at the dielectric/nitride interface, which is not possible in stress-recovery experiments performed at a common ambient temperature.

The present paper focuses on the system-level optimization of GaN technology for high voltage applications. We will show that a key requirement for the future success of the GaN technology is the full system-optimization achieved by a simultaneous optimization of technology, packaging and applications. We will also show that Virtual Prototyping (VP) becomes, in GaN technology, a fundamental tool that allows not only to have a fundamental understanding of the device properties but more importantly it allows to strongly link device optimization, technology and system-level performance. In the present paper we will describe our view on the system-level optimization of high voltage GaN technology and present detailed simulations and comparison with experiments for both normally on isolated GaN transistors and cascoded GaN devices in real switching applications.

An embodiment of a cascade diode with a breakdown voltage of about 300 V has a HEMT and a Si Schottky diode. The HEMT has a gate, a drain, a source and a two-dimensional electron gas channel region which connects the source and the drain and which is controlled by the gate. The HEMT has a breakdown voltage of about 300 V. The Si Schottky diode is monolithically integrated with the HEMT. The Si Schottky diode has a cathode connected to the source of HEMT and an anode, which is connected to the gate of the HEMT on. The Si Schottky diode has a breakdown voltage of less than 300 V and a forward voltage of less than or equal to 0.4 V. The anode of the Si Schottky diode is the anode of the diode cascade, and the drain of the HEMT forms the cathode of the diode cascade.

We report on the photoluminescence (PL) measurements of two structurally different CdTe self-assembled quantum dot (QD) samples. These include CdTe QD's connected with the uniform two-dimensional wetting layer (WL) and fully developed CdTe QD's that are isolated from each other. Both temperature and excitation power dependencies found for the ensemble of fully developed QD's reflect that of individual QD's. In this case, no signature of intradot carrier redistribution is observed even at 120 K. In contrast, both continuous-wave and time-resolved PL show the presence of strong thermally induced carrier redistribution for the structure, where the dots are connected by a uniform two-dimensional WL. In particular, three distinct temperature regions are identified. At temperatures below 30 K, dots in the ensemble are isolated from each other: the emission occurs only from QD's occupied initially by both electron and hole. When temperature increases (T>30 K), the carriers from QD's are thermally activated into the uniform WL and are subsequently captured by other QD's. Finally, above 100 K the nonradiative recombination in the WL prevents the capture of thermally activated carriers. These results, while providing a thorough understanding of the optical properties of QD structures, give also important information about structural complexity of epitaxially grown II-VI semiconductor QD's.

The magnetization of single, ultrathin MnTe layers embedded in nonmagnetic quantum wells is studied by magneto-optical spectroscopy as well as by numerical simulations. It is shown to be proportional to the Zeeman splitting and thus it can be directly deduced from the magneto-optical experiments. The inverse of the experimentally determined magnetization measured as a function of temperature clearly demonstrates deviations from Curie-Weiss behavior due to the antiferromagnetic coupling between the Mn ions. By fitting this temperature dependence, an approximate Mn diffusion profile is obtained for each sample. The fitting procedure takes into account the antiferromagnetic coupling between the Mn ions as well as the exchange interactions between the Mn ions and the photoexcited electrons. For this purpose we have numerically solved the two-dimensional Ising model by a Monte Carlo method giving the magnetization of two-dimensional layers as a function of magnetic field, temperature, and Mn concentration.

We report on the optical properties of CdTe/ZnTe quantum dot superlattices with different thickness of the spacer between neighboring layers of quantum dots. It is found that for a spacer width larger than 20 monolayers (ML) of ZnTe, only one emission band is observed which is ascribable to excitonic recombination in isolated CdTe islands. On the other hand, if the ZnTe spacer width is smaller than 20ML, a second luminescence band evolves on the low energy side of the main PL peak, which we attribute to recombination of excitons in spatially correlated CdTe quantum dots. This additional emission is characterized by a small oscillator strength and by a very long (∼20ns) decay time, as evidenced by the photoreflectance and the time resolved luminescence spectroscopy. Thus, it is interpreted as being due to a recombination of electrons and holes localized in two spatially separated but neighboring CdTe quantum dots.

Magnetic-ion-containing self-assembled quantum dots based on II-VI semiconductors are investigated by time-resolved optical spectroscopy. It is found that the dynamical properties of excitons confined in quantum dots depend on the relative position of intra-Mn transition with respect to the quantum-dot-related emission. When the intra-Mn transition energy is smaller than the dot emission energy, then the decay time of excitons in quantum dots increases with increasing magnetic field. When the intra-Mn transition energy is larger, no influence of the magnetic field on the recombination dynamics of excitons is found. The first case corresponds to CdMnSe/ZnSe structures, while the second one is observed in CdMnTe/ZnTe magnetic quantum dots.

The exciton recombination time and the exciton photoluminescence Stokes shift are systematically investigated in a set of CdTe/Cd1-xMgxTe quantum-well samples containing MnTe layers with the thickness below or equal to 1 ML. Similar to diluted magnetic semiconductors, these digital magnetic heterostructures exhibit strongly magnetic-field-dependent photoluminescence lines which are inhomogeneously broadened. Both quantities under investigation depend on the Mn content, on the details of the Mn distribution within the quantum wells, and on the energy of detection. Three sets of samples are investigated: digital magnetic heterostructures containing four MnTe layers with equidistant spacing and various thickness of the MnTe layers, quantum wells containing 1/4-ML MnTe at various positions in the quantum well, and quantum wells containing in total 1-ML MnTe but distributed in the quantum well in different ways. For all samples, the exciton recombination time is strongly correlated to the Stokes shift. This correlation is caused by exciton localization, leading to a mixing of radiatively decaying states with small momentum with nonradiative states with large momentum. In our experiment, however, a linear dependence between the exciton decay time and the Stokes shift is observed, which is not yet explained by theory, to our knowledge. Although, the luminescence properties in semimagnetic semiconductors are affected by exchange interactions leading to the formation of magnetic polarons, this process can be ruled out as the origin of the observed correlation.

We use a method to probe experimentally the probability density of carriers confined in semiconductor quantum structures. The exciton Zeeman splitting in quantum wells containing a single, ultranarrow magnetic layer is studied depending on the layer position. In particular, a system consisting of a 1/4 monolayer MnTe embedded at varying positions in nonmagnetic CdTe/CdMgTe quantum wells is investigated. The sp-d exchange interaction results in a drastic increase of the Zeeman splitting, which, because of the strongly localized nature of this interaction, sensitively depends on the position of the MnTe submonolayer in the quantum well. For various interband transitions we show that the dependence of the exciton Zeeman splitting on the position of the magnetic layer directly maps the probability density of free holes in the growth direction.

The photoluminescence properties of antiferromagnetic EuTe layers grown by molecular-beam epitaxy are reported. At low temperatures, two excitonic photoluminescence peaks are observed at 1.92 and 1.88 eV with a full width at half maximum of about 10 meV. With applied magnetic field, these excitonic transitions shift linearly by -34 meV/T to smaller transition energies with a total shift of more than 240 meV at 7.2 T. This is the largest tuning range observed in any semiconductor. The observed magnetic field and temperature dependence of the luminescence lines is explained by the formation of large magnetic polarons due to exchange interactions between the d-like electrons in the conduction band and localized 4f spins.

EuTe layers with high purity, grown by molecular beam epitaxy, are investigated by magneto-optical spectroscopy. Low temperature photoluminescence (PL) experiments reveal narrow, exciton like emission peaks exhibiting a Stokes shift of 300meV. These emission peaks can be tuned over a giant range of 160meV by applying magnetic fields between zero and 5T. The temperature dependence of the PL transition energies shows a kink at the antiferromagnetic-paramagnetic phase transition shifting to lower temperatures with increasing magnetic field. This unique magnetic field dependence as well as the temperature dependence of the PL transitions results from the formation of magnetic polarons, due to d–f exchange interactions between electrons in the conduction band and localized magnetic moments.

Narrow photoluminescence transitions of excitons in antiferromagnetic layers of EuTe grown by molecular beam epitaxy are reported. At low temperatures, two excitonic peaks are observed at around 1.9 eV with an additional broad emission band at 1.5 eV that is attributed to defects. With applied magnetic field, the excitonic transitions shift linearly by −34 meV/T to smaller energies with a total shift of more than 240 meV at 7.2 T. At T=2 K, the observed magnetic field dependence corresponds to an effective g factor of 1140, which is independent of applied field. The observed magnetic field tunability of the excitonic transitions is explained by the formation of magnetic polarons.

We demonstrate the possibility to fabricate self-organized semimagnetic quantum dots by the growth of two-dimensional layers and subsequent thermal annealing. In particular, CdTe/CdMgTe quantum-well samples containing four narrow MnTe barriers are characterized by optical methods. After annealing well above the growth temperature, from time-resolved photoluminescence (PL), PL experiments under selective excitation, as well as micro-PL measurements, we find clear evidence for the presence of quantum dots. For a sample with a average Mn content of 5%, the dot luminescence shows a redshift of 18 meV under an applied magnetic field of 4 T, due to the giant Zeeman effect.

The properties of superlattices consisting of 2 monolayer wide CdTe insertions into ZnTe spacer barriers with thickness ranging from 3 to 75 monolayers are investigated by means of transmission electron microscopy and photoluminescence spectroscopy. We show that quasi zero-dimensional CdTe islands form in this highly lattice-mismatched system. For spacer thickness smaller than 25 monolayers, the islands are vertically correlated along the axis tilted by 40° with respect to the growth direction, while for thicker ZnTe spacers no correlation is observed. The electronic coupling between the correlated islands manifests itself by the appearing of an additional emission band at energies lower to those corresponding to uncorrelated dots. The optical spectroscopy data reveal zero-dimensional localization of excitons by the electronically coupled islands. The decay time of the excitonic recombination is found to be over an order of magnitude longer in the case of the coupled islands than in the case of isolated ones.

We studied optical properties of CdTe quantum dots (QDs) by steady-state and time-resolved photoluminescence spectroscopy. By changing the excitation power at high temperatures (about T = 70K) we can significantly influence the distribution of excitons within the quantum dot ensemble. The effect manifests itself by a large (100 meV) red shift of the PL emission energy when the excitation power decreases by five orders of magnitude. This red shift is accompanied by a decrease of the linewidth of the emission band. We discuss these effects in the frame of a model of thermally induced redistribution of carriers between the zero-dimensional electronic states within the quantum dot ensemble. Moreover, we found that the exciton decay time of the QD emission increases dramatically when the number of excitons injected into the system is reduced.

Single MnTe (sub)monolayer embedded in nonmagnetic quantum wells are investigated by magneto optical spectroscopy. In particular, the magnetization is probed as a function of temperature. This dependence shows a non Curie-Weiss behavior due to clusters with antiferromagnetic ordering. The experimental data agree with the magnetization predicted by Monte Carlo simulations performed for a diluted two dimensional arrangement of Mn ions.