Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

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High-K materials and metal gates for CMOS applications

Employing density-functional theory in combination with scanning tunneling microscopy, we demonstrate that a thin metallic film on a semiconductor surface may open an efficient and hitherto not expected diffusion channel for lateral adatom transport: adatoms may prefer diffusion within this metallic layer rather than on top of the surface. Based on this concept, we interpret recent experiments: We explain why and when In acts as a surfactant on GaN surfaces, why Ga acts as an autosurfactant, and how this mechanism can be used to optimize group-III nitride growth.

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Adatom kinetics on and below the surface: The existence of a new diffusion channel

We have studied the electronic confinement in hexagonal (0001) $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ multiple quantum wells by means of structural (high-resolution x-ray diffraction and transmission electron microscopy) as well as optical characterizations, namely intersubband absorption and interband photoluminescence spectroscopies. Intense intersubband absorptions covering the $1.33--1.91\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wavelength range have been measured on a series of samples with well thicknesses varying from $1\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The absorption line shape exhibits either a pure Lorentzian shape or multiple peaks. In the first case the broadening is homogeneous with a state-of-the-art low value of $67\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. We deduce a dephasing time of the electrons in the excited subband ${T}_{2}$ of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. For structured spectra the absorption can be perfectly reproduced with a sum of several Lorentzian curves; the individual peaks originate from absorption in quantum well regions with thickness equal to an integer number of monolayers. We have also carried out simulations of the electronic structure which point out the relevance of the nonparabolicity and many-body corrections on the intersubband absorption energy. The intersubband absorption exhibits a blue shift with doping as a result of many-body effects dominated by the exchange interaction. An excellent agreement with the experimental data is demonstrated. The best fit is achieved using a conduction band offset at the $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ heterointerfaces of $1.7\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and a polarization discontinuity $\ensuremath{\Delta}P∕({ϵ}_{0}{ϵ}_{r})$ of $10\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{M}\mathrm{V}∕\mathrm{cm}$.

Systematic experimental and theoretical investigation of intersubband absorption in Ga N ∕ Al N quantum wells

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. Fr...

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GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed.

https://hal-cea.archives-ouvertes.fr/cea-01989598/document

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

We propose a procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum, after evaporation of the Ga bilayer stabilizing the two-dimensional GaN layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the nominal amount of GaN deposited. We demonstrate that the QD density can be controlled in the 3×1010 cm−2–2×1011 cm−2 range. It is shown that beyond a given amount of GaN nominally deposited, there is a coexistence between elastic and plastic relaxation, with GaN islands being formed on a partially relaxed two-dimensional GaN layer thicker than two monolayers.

Structure of GaN quantum dots grown under “modified Stranski–Krastanow” conditions on AlN

We report experimental and theoretical results on interband and intersubband transitions in GaN quantum wells with strained AlN barriers. All of the samples are grown by molecular-beam epitaxy on sapphire (0001) substrates. The results show that even at room temperature, strong electron localization occurs in the plane of the quantum wells due to the combined effect of monolayer thickness fluctuations and the high internal field in the GaN layers. We also demonstrate that the intersubband absorption is systematically blueshifted in n-doped quantum wells with respect to nominally undoped samples as a result of strong many-body effects, namely the exchange interaction. The results for both undoped and doped quantum wells are in good agreement with simulations.

Intersubband spectroscopy of doped and undoped GaN/AlN quantum wells grown by molecular-beam epitaxy

In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.

Surfactant effect of In for AlGaN growth by plasma-assisted molecular beam epitaxy

We have investigated the strain relaxation mechanisms in short-period polar GaN/AlN superlattices deposited by plasma-assisted molecular-beam epitaxy, and designed to display intersubband transitions at 1.55 μm. In a first stage, we have identified the growth conditions to minimize strain relaxation, using a Ga excess to reduce the (0001) surface free energy of both GaN and AlN. Under these growth conditions, crack propagation is not observed, even for the tensile-strained superlattices grown on GaN templates. The initial misfit relaxation in the vicinity of the buffer occurs by the formation of a-type dislocations. The final strain state of the superlattice, reached after 10–20 periods, is independent of the substrate (either GaN or AlN templates). Once the steady-state conditions are reached, we observe a periodic partial relaxation of quantum wells and barriers. High-resolution transmission electron microscopy indicates that the periodic relaxation can be related to the presence of basal and prismatic ...

D. Jalabert

Papers

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

Structure of GaN quantum dots grown under “modified Stranski–Krastanow” conditions on AlN

Intersubband spectroscopy of doped and undoped GaN/AlN quantum wells grown by molecular-beam epitaxy

Surfactant effect of In for AlGaN growth by plasma-assisted molecular beam epitaxy

Strain relaxation in short-period polar GaN/AlN superlattices