R. A. Mair

Time-resolved photoluminescence studies of AlxGa1−xN alloys

The optical properties of AlxGa1−xN alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al content, the low-temperature PL decay lifetime increases with Al content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the AlxGa1−xN alloys. The Al-content dependence of the energy-tail-state distribution parameter E0, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and, consequently, increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.

Time-resolved photoluminescence studies of InxGa1−xAs1−yNy

Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a InxGa1−xAs1−yNy (x∼0.03, y∼0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07–0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, we observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations.Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a In{sub x}Ga{sub 1{minus}x}As{sub 1{minus}y}N{sub y} (x {approximately} 0.03, y {approximately} 0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07--0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, the authors observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations.

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.

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.

Time-resolved photoluminescence studies of an ionized donor-bound exciton in GaN

Time-resolved photoluminescence (PL) spectroscopy has been used to study the radiative recombination of excitons bound to ionized donors in GaN doped with both Mg and Si at concentrations of 5×1018/cm3 and 1.5×1017/cm3, respectively. Low temperature (T∼10K) time-resolved, as well as integrated PL spectra, identify an ionized donor-bound (Si) exciton peak (D+X) approximately 11.5 meV below and a neutral acceptor-bound exciton (A0X) 20.5 meV below the free exciton peak. Rapid decay of the free exciton emission (⩽20 ps) implies that excitons are quickly captured by acceptors and ionized donors. We find the (A0X) emission lifetime is consistent with previous measurements for GaN:Mg epilayers, while the (D+X) lifetime of 160 ps is longer than that of the well studied neutral donor-bound exciton (D0X). The measured (D+X) lifetime, in comparison with (D0X) and (A0X), suggests that the state is stable at low temperature.

Optical transitions in Pr-implanted GaN

Photoluminescence (PL) spectroscopy has been used to investigate praseodymium (Pr) related transitions in Pr-implanted GaN. Wurtzite GaN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates and subsequently ion implanted with Pr to a dose of 5.7×1013/cm2. The implanted samples were annealed in nitrogen to facilitate recovery from implantation related damage. Narrow PL emission bands related to 4f intrashell transitions of the trivalent Pr ion were observed near 650, 950, 1100, and 1300 nm. The dependence of PL emission on sample temperature, excitation intensity, oxygen incorporation, and annealing temperature was systematically studied. We find that the PL efficiency increases exponentially with annealing temperature up to the maximum temperature of 1050 °C applied in the current study. Furthermore, the PL emission shows no evidence of significant thermal quenching over the sample temperature range of 10–300 K. This thermal stability will have particular advantages for ap...

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.

Time-resolved photoluminescence studies of In{sub x}Ga{sub 1-x}As{sub 1-y}N{sub y}

Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a InxGa1−xAs1−yNy (x∼0.03, y∼0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07–0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, we observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations.Time-resolved photoluminescence spectroscopy has been used to investigate carrier decay dynamics in a In{sub x}Ga{sub 1{minus}x}As{sub 1{minus}y}N{sub y} (x {approximately} 0.03, y {approximately} 0.01) epilayer grown on GaAs by metal organic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements, performed for various excitation intensities and sample temperatures, indicate that the broad PL emission at low temperature is dominated by localized exciton recombination. Lifetimes in the range of 0.07--0.34 ns are measured; these photoluminescence lifetimes are significantly shorter than corresponding values obtained for GaAs. In particular, the authors observe an emission energy dependence of the decay lifetime at 10 K, whereby the lifetime decreases with increasing emission energy across the PL spectrum. This behavior is characteristic of a distribution of localized states, which arises from alloy fluctuations.

Optical properties and resonant modes in GaN/AlGaN and InGaN/GaN multiple quantum well microdisk cavities

Optical resonance modes have been observed in optically pumped microdisk cavities fabricated from 50 angstroms/50 angstroms GaN/AlxGa1-xN (x approximately 0.07) and 45 angstroms/45 angstroms InxGa1-xN/GaN (x approximately 0.15) multiple quantum well structures. Microdisks, approximately 9 micrometers in diameter and regularly spaced every 50 micrometers , were formed by ion beam etch process. Individual disk was pumped from 10 K to 300 K with 290 nm laser pulses focused to a spot size much smaller than the disk diameter. Optical properties of these microdisks have been studied by picosecond time-resolved photoluminescence (PL) spectroscopy. From cw PL emission spectra, optical modes corresponding to (1) the radial mode type with a spacing of 49 - 51 meV (both TE and TM) and (2) the Whispering gallery mode with a spacing of 15 - 16 meV were observed in the GaN-based microdisk cavities. The spacings of these modes are consistent with theoretical calculation. The implications of our results to III-Nitride microdisk lasers are discussed.

Exciton localization dynamics in AlxGa1-xN alloys

The optical properties of AlxGa1-xN alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al-content, the low temperature PL decay lifetime increases with Al-content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the AlxGa1-xN alloys. The Al content dependence of the energy tail state distribution parameter, E0, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and consequently increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.