Awschalom Group

Gallium Nitride

Although no spectroscopic signatures of quantum dots have yet been observed in Gallium Nitride (GaN) based systems, there has been great speculation on the nature of the radiative recombination in these materials. The realization of various laser diodes and light emitting diodes based on InxGaN1-x/GaN quantum wells has proven that they are exceptionally efficient photo emitters in the blue portion of the visible spectrum, despite their very large threading dislocation densities (109 to 1010 cm-2) in InGaN epilayers and quantum wells (QW). While some photoluminescence (PL) and time resolved PL measurements suggest band to band recombination of free carriers, other measurements allude to excitonic recombination localized by alloy fluctuations, and none of these far field experiments have probed with length scales less than one micron.

We have used near field scanning optical microscopy to image a 3 nm wide In0.22Ga0.78N single QW with approximately 100 nm spatial resolution at temperatures between 50 K and 295 K. We observed strong (50%) fluctuations of QW PL as well as an order of magnitude enhancement of deep level-related emission at lower energies which occur at pits (which have a diameter of ~500 nm) in the sample. Although we find regions of smaller (15%) fluctuations in the QW PL not correlated with the presence of pits, the spectrum of the QW PL shows no significant variations on the length scales probed in this experiment.


A majority of the GaN produced is grown on sapphire substrates. Due to sapphire's 13% lattice mismatch with GaN, there is a very large dislocation density (109 - 1010 cm-2) that is four orders of magnitude higher than MBE grown GaAs. There are two distinct features in the AFM image; the threading dislocations, which appear as dots, and the large hexagonal pits (400nm in diameter), which extend into the buffer layer. Spiral growth of the GaN is initiated around these threading dislocations eventually coalescing with other nearby islands.

Quantum well grown by MOVCD:

Quantum Well Photoluminescence

Spatial maps of the photoluminescence intensity can be measured at a fixed detection energy of the photo-multiplier tube. Images of a 2.5 x 8 micron region of the sample at T = 295 K obtained using the near-field technique: (a) Topographic image of the sample measured using the shear-force feedback technique. (b) Transmission image at the excitation energy (3.16 eV). (c),(d),(e) PL images obtained at detection energies of 2.48, 2.88, and 2.95 eV respectively. The colorscale for each image is indicated at the right.

Deep Level-Related Emission

A PL image at 2.48 eV (in the green band) is shown, which demonstrates that green emission occurs only when the tip is over one of the large hexagonal pits. Other regions of the sample are essentially dark (approximately 10 times less bright) at 2.48 eV. This observation is consistent with the existence of a large number of emission sites at (or near) the walls of the pit. Cathodoluminescence studies have shown that the yellow band luminescence in GaN is enhanced at the edges of microcrystallites, presumably due to a larger number of extended defects at the surface. A similar mechanism may play a role at the hexagonal pits in the structure under study here.