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- Propriedades ópticas e transporte electrónico transiente, relacionados com a desordem, em semicondutores de grande hiatoPublication . Niehus, ManfredNeste trabalho estudamos as propriedades ópticas e o transporte electrónico em semicondutores de grande hiato, usando uma abordagem relacionada com a desordem estrutural frequentemente encontrada nestes materiais. Observamos uma variedade de características da fotocorrente transiente (TPC) e fotoluminescência transiente (TPL) que não podem ser explicados no âmbito dos mecanismos “canónicos” de relaxação electrónica, que são, para TPC, o modelo de Captura Múltipla (MT) de portadores livres e, para TPL, o modelo de recombinação entre pares de dadores e aceitadores (DAP) localizados. Em consequência, desenvolvemos um novo modelo analítico (Modelo TR) que integra a evidência experimental complementar de dispersão espacial (DAP) e dispersão energética (caudas das bandas). O modelo baseia-se na competição entre a termalização e recombinação (TR) de portadores minoritários capturados em estados energéticos, distribuídos exponencialmente em energia, e localizados no espaço. O modelo descreve a dependência da TPL da energia e do tempo, em excelente concordância com os dados experimentais, e revela a correlação entre TPL e TPC.
- Optical properties and transport in PLD-GaNPublication . Niehus, Manfred; Sanguino, P.; Monteiro, T.; Soares, M. J.; Pereira, E.; Vieira, Manuela; Koynov, S.; Schwarz, R.We present structural, optical and transport data on GaN samples grown by hybrid, two-step low temperature pulsed laser deposition. The band gap of samples with good crystallinity has been deduced from optical spectra. Large below gap band tails were observed. In samples with the lowest crystalline quality the PL spectra are quite dependent on spot laser incidence. The most intense PL lines can be attributed to excitons bounded to stacking faults. When the crystalline quality of the samples is increased the ubiquitous yellow emission band can be detected following a quenching process described by a similar activation energy to that one found in MOCVD grown samples. The samples with the highest quality present, besides the yellow band, show a large near band edge emission which peaked at 3.47 eV and could be observed up to room temperature. The large width of the NBE is attributed to effect of a wide distribution of band tail states on the excitons. Photoconductivity data supports this interpretation.
- Non-linear optical spontaneous photoluminescence emission enhancement effect in wide gap gallium nitride thin filmsPublication . Niehus, Manfred; Schwarz, ReinhardWith two interfering pulses from the 4th harmonic of a Nd-YAG laser we burnt a periodic lattice structure into the surface of GaN thin films. The lattice period of this permanent grating could be controlled between less than one and several tens of microns. Above the decomposition threshold, nitrogen evades from the sample surface, and the residual metallic gallium accumulates in the form of tiny droplets at the surfaces. The patterned structure shows structural similarities with microcavities. The question arises if the residual metallic gallium may act as a partially reflecting mirror. To test this hypothesis, we studied the steady-state and transient photoluminescence through the modulation of light emerging from the ubiquitous broad “yellow” photoluminescence band. The microlattice shows up by energy-equidistant spontaneous emission enhancement peaks in the steady-state photoluminescence spectra. We suggest that the partial reflection due to the residual metallic gallium leads to the observed enhancement effect.