Photoluminescence (PL) is the spontaneous emission of light from a material under optical excitation. The excitation energy and intensity are chosen to probe different regions and excitation concentrations in the sample. PL investigations can be used to characterize a variety of material parameters. PL spectroscopy provides electrical (as opposed to mechanical) characterization, and it is a selective and extremely sensitive probe of discrete electronic states. Features of the emission spectrum can be used to identify surface, interface, and impurity levels and to gauge alloy disorder and interface roughness. The intensity of the PL signal provides information on the quality of surfaces and interfaces. Under pulsed excitation, the transient PLintensity yields the lifetime of nonequilibrium interface and bulk states. Variation of the PL intensity under an applied bias can be used to map the electric field at the surface of a sample. In addition, thermally activated processes cause changes in PL intensity with temperature. PL analysis is nondestructive. Indeed, the technique requires very little sample manipulation or environmental control. Because the sample is excited optically, electrical contacts and junctions are unnecessary and high‐resistivity materials pose no practical difficulty. In addition, time‐resolved PL can be very fast, making it useful for characterizing the most rapid processes in a material. The fundamental limitation of PL analysis is its reliance on radiative events. Materials with poor radiative efficiency, such as low‐quality indirect bandgap semiconductors, are difficult to study via ordinary PL. Similarly, identification of impurity and defect states depends on their optical activity. Although PL is a very sensitive probe of radiative levels, one must rely on secondary evidence to study states that couple weakly with light.

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We use confocal photoluminescence microscopy to study carrier diffusion near an isolated extended defect (ED) in GaAs. We observe that the carrier diffusion length varies non-monotonically with carrier density, which we attribute to competition between point defects and the extended defect. High density laser illumination induces a permanent change in the structure of the extended defect, more significantly an apparent change in the effective polarity of the defect, and thus a drastic change in its range of influence. The inferred switch of principal diffusing species leads to a potential design consideration for high injection optoelectronic devices.

Traditionally, spatially-resolved photoluminescence (PL) has been performed using a point-by-point scan mode with both excitation and detection occurring at the same spatial location. But with the availability of high quality detector arrays like CCDs, an imaging mode has become popular for performing spatially-resolved PL. By illuminating the entire area of interest and collecting the data simultaneously from all spatial locations, the measurement efficiency can be greatly improved. However, this new approach has proceeded under the implicit assumption of comparable spatial resolution. We show here that when carrier diffusion is present, the spatial resolution can actually differ substantially between the two modes, with the less efficient scan mode being far superior. We apply both techniques in investigation of defects in a GaAs epilayer – where isolated singlet and doublet dislocations can be identified. A superposition principle is developed for solving the diffusion equation to extract the intrinsic carrier diffusion length, which can be applied to a system with arbitrarily distributed defects. The understanding derived from this work is significant for a broad range of problems in physics and beyond (for instance biology) – whenever the dynamics of generation, diffusion, and annihilation of species can be probed with either measurement mode.

We have used photoexcitation-dependent radiative efficiency measurements to investigate the rates of defect-related, radiative, and Auger recombination in lattice-matched In x Ga 1Ϫx As/InAs y P 1Ϫy double heterostructures on InP substrates. Temperature dependence is used to discern the underlying mechanisms responsible for the nonradiative recombination processes. We find that defect-related recombination decreases with an increase in the temperature when the epistructure is lattice matched to the substrate (xϭ0.53). In contrast, when the epistructure is lattice mismatched to the substrate, defect-related recombination increases slowly with the temperature. The difference between the lattice-matched and mismatched cases is related to fundamental changes in the defect-related density of states function. The temperature dependence in the lattice-mismatched structures is attributed to two competing effects: wider carrier diffusion, which augments the capture rate, and thermally activated escape, which reduces the occupation of shallow traps. The band gap and temperature dependence of the Auger rate demonstrate that the conduction to heavy hole band/ splitoff to heavy hole band mechanism generally dominates Auger recombination in undoped low-band gap In x Ga 1Ϫx As. With this interpretation, our results give a spin-orbit valence split-off band effective mass of m so ϭ(0.12Ϯ0.02)m 0 .

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We have measured the excitation-dependent radiative efficiency in a set of lattice-matched InxGa1−xAs/InAsyP1−y double heterostructures incrementally lattice mismatched to InP substrates. We find that the overall rate of defect-related recombination shows little change from the lattice-matched case. However, the excitation-dependent transition between defect-related and radiative recombination changes dramatically with mismatch. While a simple defect recombination model assuming defect levels concentrated near the middle of the band gap fits well for the lattice-matched material, the model does not fit the shape of the efficiency curve for the mismatched structures. We show that the addition of band edge exponential tails to the defect-related density of states gives a much better theoretical fit.

In order to increase the radiative efficiency and directivity of spontaneous emission from a lattice-matched InGaAs/InP heterostructure, we have polished the substrate into a parabolic reflector. We combine optical and thermal measurements to obtain the absolute external efficiency over a wide range of carrier densities. Using a simple model, the measurement is used to determine interface, radiative, and Auger recombination rates in the active material. At the optimal density, the quantum efficiency exceeds 60% at room temperature. The divergence of the emitted light is less than 20°. In fact, the beam profile is dominated by a 6° wide lobe that can be swept across the field of emission by changing the excitation position. This suggests a way to create an all-electronic scanned light beam.