We have studied the low-temperature (T=6K) optical properties of a series of InGaN∕GaN single-quantum-well structures with varying indium fractions. With increasing indium fraction the peak emission moves to lower energy and the strength of the exciton–longitudinal-optical (LO)-phonon coupling increases. The Huang–Rhys factor extracted from the Fabry–Pérot interference-free photoluminescence spectra has been compared with the results of a model calculation, yielding a value of approximately 2nm for the in-plane localization length scale of carriers. We have found reasonable agreement between this length scale and the in-plane extent of well-width fluctuations observed in scanning transmission electron microscopy high-angle annular dark-field images. High-resolution transmission electron microscopy images taken with a short exposure time and a low electron flux have not revealed any evidence of gross indium fluctuations within our InGaN quantum wells. These images could not, however, rule out the possible existence of small-scale indium fluctuations, of the order of a few at. %.

InGaN quantum wells have been found to be extremely sensitive to exposure to the electron beam in the transmission electron microscope (TEM). High-resolution TEM images acquired immediately after first irradiating a region of quantum well indicates no gross fluctuations of indium content in the InGaN alloy. During only a brief period of irradiation, inhomogeneous strain is introduced in the material due to electron beam damage. This strain is very similar to that expected from genuine nanometer-scale indium composition fluctuations which suggests there is the possibility of falsely detecting indium-rich “clusters” in a homogeneous quantum well.

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Dilute magnetic semiconductors (DMSs) that are formed by the partial replacement of cations in semiconductors by magnetic transition metal ions have drawn considerable attention because of their potential use in spintronic devices. [1,2] Many reports on the observation of ferromagnetism (FM) at room temperature in transition-metal-doped ZnO, TiO 2 , and other semiconducting oxides have been published.[3±14] However, the origin of FM in these materials is still not well understood. Mechanisms based on carrier-mediated FM [15,16] and percolation of bound magnetic polarons [3,17,18] have been proposed. The presence of a certain level of free carriers is required for carrier-mediated FM, whereas the bound magnetic polaron model is also applicable to the insulating state. In order to explore the mechanism of FM, epitaxial thin films of Co-doped ZnO have been grown using ultrasonic-assisted solution chemical vapor deposition (UASCVD). The technique is a simple, soft, combinatorial process that allows for growth at low temperatures using either inorganic [19] or organic precursors. [20] Deposition can be carried out in the absence of a vacuum and this is particularly useful for preventing evaporation of cations of high volatility. First, the influence of growth temperature on the presence or absence of a well-known broad feature in undoped ZnO films at ca. 500 nm (green band), which arises because of structural defects, namely Zn interstitials (Zn_ i ) [21] or oxygen vacancies (V_ o ), [22] is investigated. While simple convention would suggest doubly charged Zn_ i _ and V_ o _, here the convention of singly charged defects is used because the conductivity of ZnO due to native defects is found to be correlated to singly charged defects of Zn_ i [23] Figure 2 shows magnetization (M) versus field (H) curves at room temperature for Zn 0.98 Co 0.02 O films prepared at 400 and 500 C. The film prepared at 400 C shows a saturation M value (M s ) of over 0.4 l B /Co (l B : Bohr magneton), whereas the film grown at 500 C has barely a trace signal above the background. Both the films are highly resistive (resistivity q » 10 4 X cm), which is indicative of near-oxygen stoichiometry and suggests that FM does not depend upon the presence of a significant carrier concentration. The net moment per Co atom, although an order of magnitude lower than the absolute Co moment, is comparable to that found in many previous reports and can be interpreted either in terms of a predominately paramagnetic response (most Co ions are decoupled) or whether Co ions can couple antiferromagnetically or ferromagnetically depending on the local environment. Venkatesan et al. [24] present clear evidence for the latter, which implies that the magnetism is likely to be macroscopically homogenous.

An InxGa1−xN∕GaN multiple quantum well (MQW) structure that exhibited bright photoluminescence was examined with the three-dimensional atom probe. The quantum wells were clearly imaged and the indium fraction x measured to be 0.19±0.01, in good agreement with x-ray diffraction measurements. The distribution of indium in the MQWs was analyzed: no evidence for either high indium concentration regions or indium clustering was found, in contrast with many of the transmission electron microscopy studies in the literature. The authors conclude that indium clustering is not necessary for bright luminescence in InGaN.

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Impact of nitrogen incorporation on pseudomorphic site-controlled quantum dots grown by metalorganic vapor phase epitaxy Appl. Phys. Lett. 97, 072115 (2010); 10.1063/1.3481675InGaN self-assembled quantum dots grown by metalorganic chemical-vapor deposition with indium as the antisurfactant Appl.