Rachel A. Oliver

Advances in AFM for the electrical characterization of semiconductors

Atomic force microscopy (AFM) is a key tool for nanotechnology research and finds its principal application in the determination of surface topography. However, the use of the AFM tip as a probe of electrical properties allows enormous insights into material functionality at the nanoscale. Hence, a burgeoning suite of techniques has been developed to allow the determination of properties such as resistivity, surface potential and capacitance simultaneously with topographic information. This has required the development of new instrumentation, of novel probes and of advanced sample preparation techniques. In order to understand and quantify the results of AFM-based electrical measurements, it has proved important to consider the interplay of topographic and electrical information, and the role of surface states in determining a materials electrical response at the nanoscale. Despite these challenges, AFM-based techniques provide unique insights into the electrical characteristics of ever-shrinking semiconductor devices and also allow us to probe the electrical properties of defects and self-assembled nanostructures.

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.

InGaN quantum dots grown by metalorganic vapor phase epitaxy employing a post-growth nitrogen anneal

We describe the growth of InGaN quantum dots (QDs) by metalorganic vapor phase epitaxy. A thin InGaN epilayer is grown on a GaN buffer layer and then annealed at the growth temperature in molecular nitrogen inducing quantum dot formation. Microphotoluminescence studies of these QDs reveal sharp peaks with typical linewidths of ∼700 μeV at 4.2 K, the linewidth being limited by the spectral resolution. Time-resolved photoluminescence suggests that the excitons in these structures have lifetimes in excess of 2 ns at 4.2 K.

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.

Growth modes in heteroepitaxy of InGaN on GaN

The morphology of InGaN epilayers grown by metal-organic vapor phase epitaxy on GaN pseudosubstrates has been examined by atomic force microscopy. The composition of the epilayers has been measured using a combination of secondary ion mass spectrometry and x-ray photoelectron spectroscopy. The dependence of the growth mode on the growth conditions has been investigated. At the lowest temperatures and NH3 fluxes, a two-dimensional island nucleation growth mode is described, in which flat islands form stacks which align along underlying GaN terraces. As the growth temperature is increased a transition to a step-flow growth mode is observed. A transition from two-dimensional island nucleation to step-flow growth may also be achieved by increasing the NH3 flux, or by decreasing the trimethylindium flux. Each transition is discussed in terms of both surface kinetics and indium incorporation into the growing film. A transition from two-dimensional to three-dimensional growth may be induced by an increase in the...

The consequences of high injected carrier densities on carrier localization and efficiency droop in InGaN/GaN quantum well structures

There is a great deal of interest in the underlying causes of efficiency droop in InGaN/GaN quantum welllight emitting diodes, with several physical mechanisms being put forward to explain the phenomenon. In this paper we report on the observation of a reduction in the localization induced S-shape temperature dependence of the peak photoluminescence energy with increasing excitation power density. This S-shape dependence is a key fingerprint of carrier localization. Over the range of excitation power density where the depth of the S shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.

Role of gross well-width fluctuations in bright, green-emitting single InGaN∕GaN quantum well structures

Gross well-width fluctuations have been observed by transmission electron microscopy (TEM) in single InGaN∕GaN quantum wells (QWs) grown by metal-organic vapor phase epitaxy. Similar thickness variations are observed in commercial, green InGaN∕GaN multi-QW light emitting diodes. Atomic force microscopy studies of equivalent epilayers suggest that these fluctuations arise from a network of interlinking InGaN strips, which are found (using TEM) to be indium rich at their centers. Plan-view TEM indicates that 90±8% of all threading dislocations (TDs) intersect the QW plane between the InGaN strips. Excitons may be localized at the strips’ centers, preventing nonradiative recombination at TDs.

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...

Time-resolved dynamics in single InGaN quantum dots

We present measurements of photoluminescence decay dynamics for single InGaN quantum dots. The recombination is shown to be characterized by a single exponential decay, in contrast to the nonexponential recombination dynamics seen in the two-dimensional wetting layer. The lifetimes of single dots in the temperature range 4 to 60 K decrease with increasing temperature.