K. C. Zeng

Optical modes within III-nitride multiple quantum well microdisk cavities

Optical resonance modes have been observed in optically pumped microdisk cavities fabricated from 50 A/50 A GaN/AlxGa1−xN(x∼0.07) and 45 A/45 A InxGa1−xN/GaN(x∼0.15) multiple quantum well structures. Microdisks, approximately 9 μm in diameter and regularly spaced every 50 μm, were formed by an ion beam etch process. Individual disks were pumped at 300 and 10 K with 290 nm laser pulses focused to a spot size much smaller than the disk diameter. Optical modes corresponding to (i) the radial mode type with a spacing of 49–51 meV (both TE and TM) and (ii) the Whispering Gallery mode with a spacing of 15–16 meV were observed in the GaN microdisk cavities. The spacings of these modes are consistent with those expected for modes within a resonant cavity of cylindrical symmetry, refractive index, and dimensions of the microdisks under investigation. The GaN-based microdisk cavity is compared with its GaAs counterpart and implications regarding future GaN-based microdisk lasers are discussed.

Effects of well thickness and Si doping on the optical properties of GaN/AlGaN multiple quantum wells

Effects of well thickness and Si doping on the optical properties of GaN/AlGaN (MQWs) have been investigated by picosecond time-resolved photoluminescence (PL) measurements. Our results have yielded that (i) the optical transitions in nominally undoped MQWs with narrow well thicknesses (Lw 40 A) at low temperatures, and (iii) Si doping improved significantly the crystalline quality of MQWs of large well thicknesses (Lw>40 A). The implications of these results on the device applications based on III-nitride MQWs have been discussed.

Optical resonance modes in GaN pyramid microcavities

An array of GaN hexagonal pyramids with a side length of 8.0 μm was fabricated by selective epitaxial overgrowth. These microsized pyramids are highly efficient microcavities. Three types of optical resonance modes with mode spacings of 10, 5.0, and 6.0 A were observed when a single pyramid was pumped optically by an intense ultraviolet laser beam. An optical ray tracing method has been developed for calculating the optical resonance modes inside the pyramid microcavities. It was shown that a single pyramidal cavity can support several different types of optical resonance modes. The calculated mode spacing agrees very well with the observations. The uniqueness and advantages of this class of hexagonal pyramidal microcavities over the other microcavities are discussed. The implications of our finding on the future GaN microcavity light emitters including micro-light-emitting diodes, microcavity lasers, and vertical-cavity-surface emitting lasers are also discussed.

Optical properties of GaN pyramids

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN(0001) epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: (i) the release of the biaxial compressive strain in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak position with respect to the strained GaN epilayer grown under identical conditions; (ii) in the GaN pyramids on GaN/AlN/sapphire substrate, strong band edge transitions with much narrower linewidths than those in the GaN epilayer have been observed, indicating the improved crystalline quality of the overgrown pyramids; (iii) PL spectra taken from different parts of the pyramids revealed that the top of the pyramid had the highest crystalline quality; and (iv) the presence of strong band-to-impurity transitions in the pyramids were primarily due to the incorporation of the oxygen and silicon impurities from the SiO2...

Optical resonance modes in InGaN/GaN multiple-quantum-well microring cavities

Microrings of varying sizes have been fabricated from InxGa1−xN/GaN (x∼0.15) multiple quantum wells (MQWs). Photolithography and dry etching techniques including both ion-beam and inductively coupled plasma etching were employed to pattern the III–nitride MQW microrings. Individual microrings were optically pumped and optical resonance modes were observed. The observed mode spacings were consistent with those expected for whispering-gallery (WG) modes within a resonant cavity of cylindrical symmetry, refractive index, and dimensions of the rings under investigation. The results obtained from the microring cavities were compared with those of the III–nitride MQW microdisk cavities. Our results have indicated that resonance modes corresponding to the radial and the WG modes are simultaneously present in microdisk cavities, but only WG modes are available from the microring cavities. Implications of our results on future GaN-based microcavity light emitters have been discussed.

OPTICAL PROPERTIES OF GAN/ALGAN MULTIPLE QUANTUM WELL MICRODISKS

An array of microdisks with a diameter of about 9 μm and spacing of 50 μm has been fabricated by dry etching from a 50 A/50 A GaN/AlxGa1-xN (x∼0.07) multiple quantum well (MQW) structure grown by reactive molecular beam epitaxy. Optical properties of these microdisks have been studied by picosecond time-resolved photoluminescence (PL) spectroscopy. Photoluminescence emission spectra and decay dynamics were measured at various temperatures and pump intensities. With respect to the original MQWs, we observe strong enhancement of the transition intensity and lifetime for both the intrinsic and barrier transitions. The intrinsic transition is excitonic at low temperatures and exhibits an approximate tenfold increase in both lifetime and PL intensity upon formation of the microdisks. This implies a significant enhancement of quantum efficiency in microdisks and a bright future for III-nitride microcavity lasers.

Well-width dependence of the quantum efficiencies of GaN/AlxGa1−xN multiple quantum wells

A set of GaN/AlxGa1−xN(x≈0.2) multiple quantum wells (MQWs) with well widths, Lw, varying from 6 to 48 A has been grown by metalorganic chemical vapor deposition under the optimal GaN-like growth conditions. Picosecond time-resolved photoluminescence spectroscopy has been employed to probe the well-width dependence of the quantum efficiencies (QE) of these MQWs. Our results have shown that these GaN/AlGaN MQW structures exhibit negligibly small piezoelectric effects and hence enhanced QE. Furthermore, GaN/AlxGa1−xN MQWs with Lw between 12 and 42 A were observed to provide the highest QE, which can be attributed to the reduced nonradiative recombination rate as well as the improved quantum-well quality. The decreased QE in GaN/AlxGa1−xN MQWs with Lw 42 A is associated with an increased nonradiative recombination rate as Lw approaching the critical thickness of MQWs. The implications of our re...

Collective effects of interface roughness and alloy disorder in InxGa1−xN/GaN multiple quantum wells

The collective effects of alloy disorder and interface roughness on optical properties of InxGa1−xN/GaN multiple quantum wells (MQWs) have been studied. The results are compared with those of GaN/AlGaN MQWs and InGaN epilayers. InxGa1−xN/GaN MQWs emit a broad and asymmetrical photoluminescence (PL) band, while GaN/AlGaN MQWs and InGaN epilayers emit narrower and Gaussian-shaped PL bands. Furthermore, the decay of excitons at low temperatures in InxGa1−xN/GaN MQWs follows a nonexponential function even at the lower-energy side of the PL spectral peak, while those in GaN/AlGaN MQWs and in InGaN epilayers follow a single exponential function. Both alloy disorder and interface roughness have to be included in order to interpret the PL emission spectrum and the decay dynamics in InxGa1−xN/GaN MQWs. Important parameters of the InxGa1−xN/GaN MQWs, σx, σL, and dτ/dL, denoting the alloy disorder, the interface roughness, and the rate of changing of the exciton decay lifetime with well width, respectively, have bee...

Optical properties of a high-quality insulating GaN epilayer

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of an insulating GaN epilayer grown by metalorganic chemical vapor deposition on a sapphire substrate. Two emission lines at 3.503 and 3.512 eV in the continuous wave (cw) PL spectra observed at 10 K under a low excitation intensity (∼23 W/cm2) were identified as the band-to-band transitions involving the A and B valence bands, respectively. A third emission line at 3.491 eV was identified as a band-to-impurity transition involving a shallow donor. The PL decay behavior can be well understood with a model taking into account both the free carriers and impurities. The effective recombination lifetime of the band-to-band transition in GaN was found to be about 3.7 ns. Possible mechanisms for the band-to-band transition being dominant in this high quality insulating GaN epilayer have also been discussed.

Plasma heating in highly excited GaN/AlGaN multiple quantum wells

Time-resolved photoluminescence (PL) spectroscopy was used to investigate carrier distributions in a GaN/AlGaN multiple quantum well (MQW) sample under high excitation intensities necessary to achieve lasing threshold. Room temperature PL spectra showed optical transitions involving both confined and unconfined states in the quantum well structure. Analysis of the experimental results using a microscopic theory, indicates that at high excitation the carrier distributions are characterized by plasma temperatures which are significantly higher than the lattice temperature. The implications of our findings on GaN MQW laser design are also discussed.