C. Gaspar

Temperature behaviour of the yellow emission in GaN

Even in good quality undoped GaN samples, as assessed by the intense excitonic emission, the yellow band is present. This band has been attributed either to a shallow donor to deep double donor pair recombination [1] , to a deep donor to a shallow acceptor [2] or to a shallow donor and a deep state [3] . However, its origin is not yet clear. We present data on time resolved spectroscopy compared with steady state results. These results indicate that there is no difference in band shape between steady state and time resolved spectra at all temperatures. However, in some samples there is an increase in intensity of the yellow band. It is concluded that besides a fast emission, due to prompt excitation of the centre, an indirect path from a trap 13.7 meV below the shallow donor is responsible for the long component of the decay and the intensity increase. An emission with a lifetime of ca. 300 ms is also present with a maximum at 2.35 eV.

Optical characterization of ZnO

The green (2.19 eV) and yellow (2.00 eV) luminescence bands in ZnO polycrystalline samples were studied by photoluminescence, excitation luminescence, time-resolved spectroscopy and lifetime measurements. A shift towards higher energies of the green emission band is observed for temperatures above 35 K indicating that at least two excited levels 74 meV apart are involved in the recombination process.

Time resolved spectroscopy of mid-band-gap emissions in Si-doped GaN

Silicon is until now the most promising dopant for n-type GaN. Besides the near-band-gap emission centred at 3.461 eV, which becomes broader with increasing Si concentration also transitions at lower energies are observed. These emissions decrease strongly when the silicon concentration increases. While the near-band-gap emission is due to fast transitions (with decays shorter than 10 μs) the lower energy emissions have longer components whose behaviour is discussed.

Optical and electrical characterisation of iron-doped ZnO

The 1.7855 eV luminescence and electrical DC behaviour in polycrystalline ZnO are studied. Luminescence transient decay times of 1.4 ms and 23 ms are found at 11 K. While the first one remains constant with temperature the slower one decreases reaching a value of 5 ms at 160 K with an activation energy of 76 meV. A fast decrease of the intensity above 30 K with activation energy of 8 meV was found. Electrical DC measurements show a space charge limited current behaviour with an activation energy of 40 meV above 30 K. For lower temperatures a variable range hopping was found. Based on this data an excitonic model for the centre is proposed.

Steady-state and time-resolved luminescence in InGaN layers

Abstract InGaN layers are used as active layers of high brightness in nitride-based LEDs and lasers. Despite the progress in device development many of the fundamental optical properties are not completely understood. InGaN samples with different In content are studied by steady-state and time-resolved photoluminescence. The low-temperature photoluminescence spectra show a near band edge emission that shifts to lower energies with increasing In content, excitation wavelength and delay times. The emission is broader than typical excitonic emission from binary material. Temperature dependent measurements indicate that the near band edge emission is an overlap of various emission bands with different quenching behaviour.

Time Resolved Photoluminescence of Cubic Mg Doped GaN

Mg doped cubic GaN layers were studied by steady state and time resolved photoluminescence. The blue emission due to Mg doping can be decomposed in three bands. The decay curves and the spectral shift with time delays indicates donor-acceptor pair behavior. This can be confirmed by excitation density dependent measurements. Furthermore temperature dependent analysis shows that the three emissions have one impurity in common. The authors propose that this is an acceptor level related to the Mg incorporation and the three deep donor levels are due to compensation effects.