In the first part of this paper, we present a model that explains and determines quantitatively the twists between nucleation islands in the case of a Volmer–Weber heteroepitaxial growth of tetrahedrally coordinated semiconductors along hexagonal orientations. These twists are caused by the network of the screw components of the 60° misfit dislocations. The orientations of the screw components are distributed randomly, and the maximum twist is obtained when all the screw components have the same orientation. The maximum twists are related to the density of misfit dislocations and, therefore, increase with the mismatch between the deposited materials and their substrate. In the second part of the paper, we study five systems having a large distribution of mismatches from 4% to 19%. For the four systems fulfilling the conditions necessary for the application of the model (plastic relaxation of grown islands), the measured maximum twists fit with the calculated values, thereby validating the model. The twists of nucleation islands are related to the mismatch and are, therefore, intrinsic to the material systems. The defects created at the coalescence of twisted islands determine the initial microstructure/defect distribution of the nucleation layer.

Off-axis electron holography has been used to measure the width of the depletion region in a series of tunnel junction GaN light emitting diodes that have been prepared using different growth processes for blue emission. The total measured potentials are combinations of the mean inner potential, dopant potential, and piezoelectric contributions. The dopant potential has been unmixed from the mean inner potential such that the width of the tunnel junctions in the different diodes can be measured. The experimental results are then compared to secondary ion mass spectrometry, simulations, and opto-electronic testing. We find that the measured tunnel junction widths are consistent with simulations as well as the current density and voltage characteristics. As such, off-axis electron holography has been demonstrated as a unique technique that can be used to reproducibly measure the electrostatic potentials in tunnel junctions with nm-scale resolution in real III–V device specimens.

A multi-microscopy investigation of a GaN tunnel junction (TJ) grown on an InGaN-based light emitting diode (LED) has been performed. The TJ consists of a heavily Ge-doped n-type GaN layer grown by ammonia-based molecular-beam epitaxy on a heavily Mg-doped p-type GaN thin layer, grown by metalorganic vapor phase epitaxy. A correlation of atom probe tomography, electron holography and secondary ion mass spectrometry has been performed in order to investigate the nm-scale distribution of both Mg and Ge at the TJ. Experimental results reveal that Mg segregates at the TJ interface, and diffuses into the Ge-doped layer. As a result, the dopant concentration and distribution differ significantly from the nominal values. Despite this, electron holography reveals a TJ depletion width of ∼7 nm, in agreement with band diagram simulations using the experimentally determined dopant distribution.

The use of tunnel junctions (TJs) is a potential solution in blue light-emitting diodes (LEDs) to poor p-contacts, replacing it with another n-contact. TJs are even more advantageous for UV-emitting structures, which suffer from considerably low injection efficiency in high Al concentration UV LEDs. In this article, we report our work on Ge n-doped GaN and AlGaN TJs grown on top of blue and UV LEDs, respectively, by a hybrid growth method. We have achieved state-of-the-art mobility (67 cm2/V s) and resistivity (1.7 × 10−4 Ω cm) at a free electron concentration of 5.5 × 1020 cm−3 in Ge-doped GaN. With an emission wavelength of 436 nm, the GaN TJ slightly increased the optical power of the blue LED. The AlGaN TJs, on the other hand, improved the optical power of the UV LED (304 nm) by at least a factor of 3, suggesting the enhancement of the hole injection efficiency by the use of TJs in UV-emitting structures.

We demonstrate the growth of almost strain-free (10-11) semipolar GaN on silicon-on-insulator (SOI) substrates, with no meltback etching and with a defect density strongly reduced compared to semipolar templates grown on patterned silicon substrates. This is carried out using SOI substrates with a very thin (∼150 nm) 6° off (001) Si top layer. By resorting to very small nucleation (111) facets, revealed through chemical etching of the topmost thin Si layer, we are able to diminish significantly the overall dislocation density. Cathodoluminescence and scanning electron microscopy images at different stages of the growth illustrate how the defect density reduction operates and confirm the complete suppression of meltback etching over the whole 2 in. wafer. Low temperature photoluminescence and optical reflectivity indicate that complete strain relaxation is closely achieved (D0X at 3.473 ± 0.001 eV), compared to semipolar epilayers grown onto “bulk” silicon (D0X at 3.460 eV). Thanks to this efficient strain relaxation, very thick layers, up to 9 μm, could be obtained crack-free.

•Quantifying the effect of GaN regrown layers on InAlN gallium contamination.•Model explaining quantitatively the behavior of gallium contamination in InAlN barrier layers.•The need of an optimum GaN regrowth thickness to have the best sheet resistance values.

Tunnel junctions (TJs) are envisaged as potential solutions to improve the electrical injection efficiency of nitride emitters in the visible as well as in the UV range. Indeed TJs would solve the issues related to the poor contact with the top p type nitride layer, replacing it by an n type one. But if metal-organic chemical vapor deposition (MOCVD) is chosen to grow the n side of the TJ on a LED, one faces the problem of a potential re-passivation by hydrogen of the underlying p type layer. We propose a TJ epitaxial process whereby low growth temperatures, high growth rates and the type of carrier gas will minimize hydrogen incorporation in the underlying layers. In this view, n++/p++ GaN TJs with and without an (Ga,In)N intermediate layer are grown by MOCVD at varying temperatures (800°C and 1080°C), using N2 as a carrier gas under a very high growth rate of 2.5μm/h on top of blue (Ga,In)N/GaN LEDs. The LEDs made under N2 carrier gas and lower temperature growth conditions are operational without the need for further thermal activation of the Mg acceptors. The light emission intensity from the top surface of the TJ-LEDs is improved compared to the reference LED without TJ: besides the more efficient carrier injection this is also attributable to the larger photon extraction efficiency because of the rough surface of the low temperature grown n-GaN contact layer of the TJ-LEDs.

AlN-on-Si can be considered as a model system for heteroepitaxial growth of highly mismatched materials. Indeed, AlN and Si drastically differ in terms of chemistry, crystalline structure, and lattice parameters. In this paper, we present a transmission electron microscopy and grazing incidence X-ray diffraction study of the microstructure of AlN layers epitaxially grown on Si (111) by molecular beam epitaxy. The large interfacial energy due to the dissimilarities between AlN and Si results in a 3D Volmer-Weber growth mode with the nucleation of independent and relaxed AlN islands. Despite a well-defined epitaxial relationship, these islands exhibit in-plane misorientations up to 6°–7°. We propose a model which quantitatively explains these misorientations by taking into account the relaxation of the islands through the introduction of 60° a-type misfit dislocations. Threading dislocations (TDs) are formed to compensate these misorientations when islands coalesce. TD density depends on two parameters: the islands' misorientation and density. We show that the former is related to the mismatch between AlN and Si, while the latter depends on the growth parameters. A large decrease in TD density occurs during the 3D growth stage by overlap and overgrowth of highly misoriented islands. On the other hand, the TD density does not change significantly when the growth becomes 2D. The proposed model, explaining the misorientations of 3D-grown islands, may be extended to other (0001)-oriented III-nitrides and more generally to any heteroepitaxial system exhibiting a 3D Volmer-Weber growth mode with islands relaxed thanks to the introduction of mixed-type misfit dislocations.

This paper reports on the study of 1/f noise in iron oxide thin films as a function of sheet resistance (R sh ), for materials elaborated through two synthesis processes. It establishes the relationships between process, structural properties and 1/f noise behaviour with R sh , following the approach described in [12] which is suitable for thin films materials.

With the objective of introducing further GaN semipolar orientations adequate for the fabrication of optoelectronic devices on silicon, we report on the selective area growth of semipolar (202̄1) GaN layers on patterned silicon (114) 1° off substrates. The patterning and subsequent anisotropic etching of the Si substrates allows exposing Si {111}-type facets on which GaN grows along the [0001] direction. Upon coalescence of neighboring GaN stripes, a continuous (202̄1) semipolar layer is obtained, displaying a facetted surface morphology dominated by more thermodynamically-stable crystallographic planes, i.e. semipolar (101̄1) and nonpolar (101̄0). Transmission electron microscopy shows that dislocation bending in the early growth stages determines the structural quality of the final semipolar (202̄1) layer. This is confirmed by X-ray rocking curves displaying full width at half-maximum values comparable to those of (202̄1) GaN epilayers of equivalent thickness grown on patterned sapphire. Optical characterization displays regions having threading dislocations and stacking faults alternating with almost defect-free regions, which correlate well with the transmission electron microscopy study.

In this work, we present the fabrication and the characterization of an optical waveguide made of AlN and GaN layers grown by MBE on a Si(111) substrate. For the fundamental mode at 633 nm, the propagation losses are in the order of 2 dB/cm, which is a good number for SC waveguides at this wavelength. The propagation losses dramatically increase with the mode order. A careful comparison of measurements and modeling of the complete structure allows identifying the part of the losses due to absorption in the Si substrate, and showing that propagation losses could be further reduced by using well chosen SOI substrates.

LED reliability and lifetime prediction is a key point for Solid State Lighting adoption. For this purpose, one hundred and fifty LEDs have been aged for a reliability analysis. LEDs have been grouped following nine current-temperature stress conditions. Stress driving current was fixed between 350mA and 1A and ambient temperature between 85C and 120 degrees C. Using integrating sphere and I(V) measurements, a cross study of the evolution of electrical and optical characteristics has been done. Results show two main failure mechanisms regarding lumen maintenance. The first one is the typically observed lumen depreciation and the second one is a much more quicker depreciation related to an increase of the leakage and non radiative currents. Models of the typical lumen depreciation and leakage resistance depreciation have been made using electrical and optical measurements during the aging tests. The combination of those models allows a new method toward a quicker LED lifetime prediction. These two models have been used for lifetime predictions for LEDs.

Current and temperature aging have been conducted on flip chip high power light emitting diodes (LEDs) with Al-NiTi-Au n-contacts. Electrical and optical characteristics have been monitored during aging and a forward voltage increase has been observed. In order to understand this behavior, cross sections have been made on representative aged samples. For LEDs with a forward voltage shift, in the n-contact area, Au-Al inter-diffusion leading to a possible formation of Au-Al intermetallic compounds has been observed. We propose here to analyze the failure modes; the related failure mechanism(s) and consequences on LED flip chip reliability.

The local physical properties of polycrystalline semiconducting films drive their performances in a wide variety of optoelectronic devices but are still not completely elucidated. These properties are investigated and correlated on the same region of polycrystalline CdTe films by combining electron backscattered diffraction, mu-Laue x-ray experiments using synchrotron radiation, electron beam-induced current, and cathodoluminescence. The local band bending is revealed at random grain boundaries: its characteristics vary from one grain to another, depending on the nature of grain boundaries and the doping level in the nearby grains, in agreement with the theoretical approach of the double Schottky potential barriers. In contrast, no local band bending occurs at Sigma 3 growth twins since these extended defects have no dangling bonds in their center. Additionally, the density of unpaired dislocations and the components of the strain and stress tensors are found to be highly nonuniform from one grain to another and within the grains themselves. This reveals that grain-to-grain interactions (i.e., collective effects) occur during the Volmer-Weber-type growth. These findings emphasize the critical importance of grain boundary design engineering. They also highlight how polycrystalline semiconducting films work locally and show the complexity of the local physical processes governing their macroscopic performances in optoelectronic devices.

In this article, we report on ZnO nanowires that were phosphorus doped using a spin on dopant glass deposition and diffusion method. Photoluminescence measurements suggest that this process yields p-doped ZnO. The spatial location of P atoms was studied using x-ray near-edge absorption structure spectroscopy and it is concluded that the doping is amphoteric with P atoms located on both Zn and O sites.

HgCdTe films grown by liquid phase epitaxy with different Cd compositions were post-annealed to control the Hg vacancy concentration. Then temperature-dependent Hall measurements and photoluminescence measurements allowed us to study the evolution of the Hg vacancy acceptor levels with the cadmium composition. For Cd compositions below 33% the Hg vacancies in HgCdTe present a negative-U property with the ionized state V− stabilized compared to the neutral state V0. For Cd compositions higher than 45%, the Hg vacancies in HgCdTe present a more standard level ordering with the ionized state V− at higher energy than the neutral state V0.

Stimulated emission and optical gain in CdTe/CdMnTe graded index separate confinement quantum wells have been investigated as a function of optical excitation powers and temperatures. Maximum gain of about 100 cm-1 is obtained at 95K for a single quantum well under 2-3 kW/cm2 excitation. This value allows to design laser cavities compatible with the microgun pumped laser device concept. The temperature dependence of the gain still remains a problem (T0 = 110K).