Enhanced Luminescence Efficiency Research Articles

Luminescence enhancement of InGaAs/InP surface quantum wells by room-temperature ion-gun hydrogenation

An InGaAs/InP surface quantum well has been used as a very effective probe of surface states before and after room-temperature low-energy ion-gun hydrogenation. We found that the luminescence efficiency from the surface quantum well (QW) does not improve for ion exposures less than 1015 ions/cm2. Above this value, the luminescence efficiency increases monotonically, up to two orders of magnitude at the ion exposure of 1017 ions/cm2. The luminescence efficiency for samples hydrogenated with exposures of 1015 and 1016 ions/cm2 degraded slightly after two months in ambient air; however, a further enhancement of luminescence efficiency by ∼20% was observed for samples hydrogenated with an exposure of 1017 ions/cm2.

Luminescence efficiency enhancement in laser soaked hydrogenated amorphous carbon films

An unexpected effect of laser soaking was observed on the properties of annealed a-C:H films. The photoluminescence (PL) intensity was increased up to six times and, the spectrum was blue shifted in samples exposed to an unfocused Ar+ laser beam. The time dependence of the PL intensity increase was found to be exponential up to saturation with characteristic times around 103 s. The observed changes were influenced by illumination intensity and sample properties. These results are corroborated by a PL mechanism when excitation and recombination take place on π-bonded clusters.

Enhanced luminescence efficiency in growth interrupted single quantum wells by atomic hydrogen

Several groups have demonstrated smoothing of (Al,Ga)As/GaAs quantum well heterointerfaces by growth interrupts (GI). Such quantum wells (QW) usually exhibit multiple narrow line emission spectra where the energy and separation of the peaks is consistent with large lateral islands varying in height by a monolayer. Unfortunately, the integrated emission can be reduced by an order of magnitude as a result of GI. Time decay measurements suggest that GI leads to the incorporation of non-radiative centres. The lifetime and luminescence efficiency can be recovered by annealing the samples for 2h in a hydrogen plasma. The H passivates non-radiative centres but does not degrade the smoothness of the interfaces. These results may have important consequences for the optical and electrical properties of quantum devices

Luminescence efficiency of near-surface quantum wells before and after ion-gun hydrogenation

We have studied the effects of the proximity of a bare Al0.3Ga0.7As surface on the luminescence of an underlying GaAs quantum well (QW) before and after hydrogenation. The mechanism which is affected by H is tunneling to surface states through the surface barrier. Its thickness was varied by wet etching from 60 to 350 Å. Our experiments reveal that the degradation of luminescence efficiency from the QW is dependent on the surface barrier thickness and the excitation energy used in the photoluminescence measurements. A complete recovery or even further enhancement of luminescence efficiency was observed in the near-surface QW after low-energy ion-beam hydrogenation, even at room temperature.

Picosecond time resolved photoluminescence in MEHPPV - solution film, gel and blend

We present the results of time resolved luminescence studies of poly(2-methoxy,5-(2′-ethylhexloxy)-p-phenylene-vinylene), MEH-PPV, as a pure film, in solution, in an ultra-high molecular weight polyethylene (UHMW-PE) gel, and in a blend with UHMW-PE. The room temperature luminescence has a characteristic lifetime of 200–300ps, and the 80K luminescence has a characteristic lifetime of 400–600ps. These times are independent of the physical form of the material, contrary to our expectations of enhanced luminescence efficiencies (and therefore lifetimes), in solution. The short lifetimes (compared to intrinsic lifetimes calculated from absorption and emission spectra) indicate the importance of non-radiative decay routes. The decay profile of the PL is found to differ from a single exponential for MEH-PPV at room temperature. Integrated luminescence verses pump intensity data are also presented.

Electroluminescence and photoluminescence from Si1−xGex alloys grown on (100) silicon by molecular beam epitaxy

Abstract Photoluminescence has been observed from Si 1− x Ge x alloy layers, superlattices and non-periodic multilayers where x was varied from 0 to 0.6 A p-type Si 0.82 Ge 0.18 alloy layer 200 nm thick, grown by molecular beam epitaxy (MBE), has been fabricated into a mesa diode which operated as a light-emitting diode, emitting at 1.4 μm at temperatures up to 80 K. This diode was selected from a series of heterostructures which exhibited intense photoluminescence with internal quantum efficiencies in the range 1%–10% at low temperatures. Photoluminescence in the wavelength range 1.2–1.7 μm has been observed from thick (100–200 nm) Si 1− x Ge x alloys and Si 1− x −Si strained layer superlattices with a range of dimensions which was large compared with the unit cell; i.e. where Brillouin zone folding effects were negligible. The intense Si 1− x Ge x alloy photoluminescence peak had a halfwidth of about 80 meV and the peak energy was found to shift consistently and predictably with the germanium fraction. Photoluminescence peak energies at 4.2 K varied from 620 to 990 meV for germanium fractions where 0.53 > x > 0.06. The photoluminescence and electroluminescence peaks were consistently about 120 meV below the established bandgap for tetragonally strained Si 1− x Ge x . In general, Si 1− x Ge x strained layers grown at low temperatures typical of MBE (below 500°C) exhibited low photoluminescence intensity. However, post-growth annealing in the 500–700°C temperature range enhanced luminescence efficiency by up to two orders of magnitude. A comparison is also made of the broad intense photoluminescence observed from MBE material with the weak near-band-edge spectra obtained from Si 1− x Ge x -Si grown by low temperature chemical vapour deposition.