M. J. Galtrey

Three-dimensional atom probe studies of an InxGa1−xN∕GaN multiple quantum well structure: Assessment of possible indium clustering

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.

Carrier localization mechanisms in InxGa1-xN/GaN quantum wells

Localization lengths of the electrons and holes in InGaN/GaN quantum wells have been calculated using numerical solutions of the effective mass Schrodinger equation. We have treated the distribution of indium atoms as random and found that the resultant fluctuations in alloy concentration can localize the carriers. By using a locally varying indium concentration function we have calculated the contribution to the potential energy of the carriers from band gap fluctuations, the deformation potential, and the spontaneous and piezoelectric fields. We have considered the effect of well width fluctuations and found that these contribute to electron localization, but not to hole localization. We also simulate low temperature photoluminescence spectra and find good agreement with experiment.

Atom probe tomography today

This review aims to describe and illustrate the advances in the application of atom probe tomography that have been made possible by recent developments, particularly in specimen preparation techniques (using dual-beam focused-ion beam instruments) but also of the more routine use of laser pulsing. The combination of these two developments now permits atomic-scale investigation of site-specific regions within engineering alloys (e.g. at grain boundaries and in the vicinity of cracks) and also the atomic-level characterization of interfaces in multilayers, oxide films, and semiconductor materials and devices.

Three-dimensional atom probe analysis of green- and blue-emitting InxGa1−xN∕GaN multiple quantum well structures

The three-dimensional atom probe has been used to characterize green- and blue-emitting InxGa1−xN∕GaN multiple quantum well structures with subnanometer resolution over a 100nm field of view. The distribution of indium in InxGa1−xN samples with different compositions is analyzed. No evidence is found wherein the indium distribution deviates from that of a random alloy, which appears to preclude indium clustering as the cause of the reported carrier localization in these structures. The upper interface of each quantum well layer is shown to be rougher and more diffuse than the lower interface, and the existence of monolayer steps in the upper interfaces is revealed. These steps could effectively localize carriers at room temperature. Indium is shown to be present in the GaN barrier layers despite the absence of indium precursor flux during barrier layer growth. A strong evidence is produced to support a mechanism for the presence of indium in these layers, namely, that a layer of indium forms on the surfac...

Compositional inhomogeneity of a high-efficiency InxGa1−xN based multiple quantum well ultraviolet emitter studied by three dimensional atom probe

An InxGa1−xN based multiple quantum well structure emitting in the ultraviolet, which has the highest reported efficiency (67%) at its wavelength (380nm), was analyzed with the three-dimensional atom probe. The results reveal gross discontinuities and compositional variations within the quantum well layers on a 20–100nm length scale. In addition, the analysis shows the presence of indium in the AlyGa1−yN barrier layers, albeit at a very low level. By comparing with analogous epilayer samples, we suggest that the quantum well discontinuities we observe may play an important role in improving the efficiency of these structures.

Three-Dimensional Atom Probe Characterisation of III-Nitride Quantum Well Structures

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, was in good agreement with X-ray diffraction measurements. The distribution of indium in the MQWs was analysed: no evidence for either high indium concentration regions or indium clustering was found, in contrast with transmission electron microscopy studies in the literature. We conclude that indium clustering is not necessary for bright luminescence in InGaN.

The Puzzle of Exciton Localisation in GaN-Based Structures: TEM, AFM and 3D APFIM Hold the Key

The InGaN/GaN quantum well system emits intense light even though the dislocation density is high. This is a puzzle since dislocations should quench the light emission. Photoluminescence (PL) experiments show that the excitons in the InGaN quantum well are localised on a nanometre scale, thus separating the carriers from most of the dislocations. Many papers report transmission electron microscopy (TEM) results showing that this localisation is caused by gross indium clustering in the InGaN quantum wells, but our TEM reveals no gross indium clustering. Three-dimensional atom probe field ion microscopy confirms that InGaN is a random alloy. Mechanisms are given for localisation on a nm scale. Confinement on a broader length scale (50 – 100 nm) can also occur in some InGaN quantum wells.

The atomic structure of GaN-based quantum wells and interfaces

We have used a combination of high resolution electron microscopy (HREM), three dimensional atom probe (3DAP) microscopy and atomic force microscopy (AFM) to reveal the atomic structure of InGaN quantum wells (QWs) and InGaN interfaces. Such quantum wells and interfaces are of considerable scientific and technological importance because they form the basis of GaN-based LEDs and lasers.