Search

We study thermal annealing effects on the size and composition variations of indium-aggregated clusters in two InGaN thin films with photoluminescence (PL) in the yellow and red ranges. The methods of investigation include optical measurement, nanoscale material analysis, and theoretical calculation. Such a study is important for determining the relation between the band gap and the average indium content of InGaN. In one of the samples, the major part of the PL spectrum is shifted from the yellow band into the blue range upon thermal annealing. In the other sample, after thermal annealing, a broad spectrum covering the whole visible range is observed. To obtain a basic physics picture behind the spectral blue shift upon thermal annealing in the yellow emission sample, we theoretically study the quantum-confinement effects of InGaN clusters based on a quantum box model. [ABSTRACT FROM AUTHOR]

Deep ultraviolet photoluminescence spectroscopy has been employed to study the optical transitions in AlN and GaN epilayers at temperatures from 10 to 800 K, from which the parameters that describe the temperature variation of the energy band gap (α and β or aB and θ) and linewidth broadening have been obtained. These parameters are compared with the previously reported values in AlN and GaN obtained by different methods in narrower temperature ranges. Our experimental results demonstrate that the broader temperature range of measurements is necessary to obtain accurate values of these parameters, particularly for AlN. These results, together with other well-known physical properties of AlN, may expand future prospects for the application of III-nitride materials. [ABSTRACT FROM AUTHOR]

Deep UV photoluminescence spectroscopy has been employed to study the optical properties of AlxGa1-xN alloys (0≤x≤1). The emission intensity with polarization of E⊥c and the degree of polarization were found to decrease with increasing x. This is a consequence of the fact that the dominant band edge emission in GaN (AlN) is with polarization of E⊥c(E∥c). Our experimental results suggest that the decreased emission efficiency in AlxGa1-xN alloys and related UV emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of E⊥c decreases with x. It is thus concluded that UV emitters with AlGaN alloys as active layers have very different properties from InGaN and other semiconductor emitters. [ABSTRACT FROM AUTHOR]

AlN epilayers grown by metalorganic chemical vapor deposition were implanted with cobalt ions and studied by deep UV photoluminescence (PL). A PL emission peak at 5.87 eV (at 10 K) was observed for the Co-implanted AlN epilayers, which was absent in as-grown AlN epilayers. Temperature dependence of the PL intensity of the 5.87 eV emission line revealed an ion-implantation induced defect with energy level of about 260 meV below the conduction band. The 5.87 eV emission line is believed due to a band-to-impurity transition involving the nitrogen vacancy (V[sub N]) in ion-implanted AlN. The experimentally determined energy level of the nitrogen vacancy is in reasonable agreement with the calculated value of 300 meV. From the band-to-impurity transition involving V[sub N], we have deduced the energy band gap of AlN to be 6.13 eV, which is consistent with our previous result. Our results suggest that nitrogen vacancies in AlN cannot make any significant contribution to the n-type conductivity due to the large binding energy as well as the large formation energy. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Si-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep ultraviolet picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in the grown epilayers. The donor bound exciton (or I[sub 2]) transition was found to be the dominant recombination line in Si-doped AlN epilayers at 10 K and its emission intensity decreases with increasing Si dopant concentration. Doping-induced PL emission linewidth broadening and band-gap renormalization effects have also been observed. Time-resolved PL studies revealed a linear decrease of PL decay lifetime with increasing Si dopant concentration, which was believed to be a direct consequence of the doping-enhanced nonradiative recombination rates. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the optical properties of gallium nitride (GaN) pyramids overgrown on hexagonal GaN(0001) epilayers on sapphire and silicon substrates with AlN buffers using picosecond time-resolved photoluminescence (PL). Low-temperature PL spectra measurements; Occurrence of impurity related shape transitions.

We present the results of picosecond time-resolved photoluminescence (PL) measurements for a set of 30 Å well GaN/Al[sub x]Ga[sub 1-x]N (x∼0.2) multiple-quantum-well (MQW) structures with varying barrier widths L[sub B] from 30 to 100 Å, grown by metalorganic chemical-vapor deposition. The PL quantum efficiency and the recombination lifetime of these MQWs were observed to increase monotonously with an increase of the barrier width up to 80 Å. These behaviors were explained by considering two distinct mechanisms that control the radiative recombination efficiencies in MQWs. When the barrier width is below the critical thickness, the nonradiative recombination rate increases with a decrease of the barrier width due to enhanced probabilities of the electron and hole wave functions at the interfaces as well as in the AlGaN barriers. On the other hand, the misfit dislocation density increases as the barrier width approaches the critical thickness, which can result in an enhanced nonradiative interface recombination rate. Our studies here have shown that the optimal GaN/AlGaN (x∼0.2) MQW structures for UV light-emitter applications are those with barrier widths ranging from 40 to 80 Å. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optical properties of Al[sub x]Ga[sub 1-x]N 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 Al[sub x]Ga[sub 1-x]N alloys. The Al-content dependence of the energy-tail-state distribution parameter E[sub 0], 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. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Optical properties of GaN/AlN multiple quantum wells (MQW) have been investigated by Raman scattering, photoluminescence and photoluminescence excitation measurements. A careful examination of the Raman spectrum reveals the fact that the constituent layers of GaN/AlN MQWs are well strained. The experimental results of emission and absorption in MQWs were compared with the calculated solutions of the finite quantum well and the bound states involved in the optical transitions were identified. It is found that the interband transitions up to n=3 bound state can be observed in the strained GaN/AlN MQWs sample. The temperature dependence of the heavy-hole transitions shows an interesting phenomenon, in which the peak energy first increases with increasing temperature and then decreases with the temperature rapidly. The observation can be explained in a consistent way by the strain effects of lattice mismatch due to the interplay between the thermal expansion of GaN and AlN layers. Our results indicate that pseudomorphic GaN/AlN MQWs with good quality can be readily grown, and their applications in optoelectronics can be expected in the near future. [Copyright &y& Elsevier]

Abstract: We report on the high-resolution photoluminescence (PL) and electron spin resonance (ESR) studies of highly Er-doped (2×1020 to 2×1021 cm−3) MOCVD grown GaN epilayers. The high-resolution Fourier transform of the 4I13/2 → 4I15/2 PL of Er3+ near 1.5μm, site-selective PL and PL excitation measurements show that in MOCVD grown GaN only one type of Er-centers exists. This conclusion has been confirmed by ESR measurements. In ESR the axial Er3+ spectrum was observed with g || =2.861 and g ⊥ =7.645 characteristic for substitutional Er ions at Ga sites (C 3V symmetry). Angular dependence of the ESR did not point to the existence of other centers. [Copyright &y& Elsevier]