High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots

We compare the quality factor values of the whispering gallery modes of microdisks (μ-disks) incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor resonant modes on the whole spectrum, which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor, which reflects the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs-based μ-disks show very high Q values (Q>7000) whereas the Q factor is only up to 2000 in μ-disks embedding QDs grown on the AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.

Recombination Dynamics in Nitride Quantum Boxes and Quantum Wells for Colors Ranging from the UV to the Red

Time-resolved photoluminescence experiments at varying temperature are performed on a series of InxGa1−xN/GaN quantum well and quantum box samples of similar compositions (0.15 < x < 0.20). The results are analyzed by using envelope-function calculations of transition energies and oscillator strengths, accounting for internal electric fields. The respective influences of localization and electric fields on radiative and nonradiative lifetimes and on the Stokes shift are deduced. The results indicate that the spatial extension of localization centers is much smaller than the size of the quantum boxes (∼10 × 3 nm, typically). The room-temperature radiative efficiency of both quantum well and quantum box samples is enhanced by replacing the topmost GaN barrier by an AlGaN one.

Growth Kinetics and Structural Quality in GaN Epitaxy by Low Pressure MOVPE

GaN epilayers have been grown using low pressure MOVPE (76 Torr) onto (0001) sapphire substrates. Triethylgallium (TEGa) and Ammonia (NH3) were used as precursors. The growth rate has been determined versus growth parameters. The growth rate versus NH3 flow displays a rather unusual behavior, in particular at low growth temperature (buffer layer), where the growth rate decreases strongly when the NH3 flow increases. A growth mechanism involving a competitive adsorption step onto the surface is proposed, and results in a strong dependence of the growth rate on the V/III ratio.

The structural quality of the layers has been assessed by X-ray diffraction versus the growth parameters, and will be reported here. A substrate nitridation step, prior to buffer layer deposition has been introduced in our growth process. The effect of substrate nitridation on the structural and optical properties of the epilayers is studied and we propose an optimized pre-treatment for the MOVPE growth of GaN onto (0001) sapphire substrates.

Radiative And Nonradiative Relaxation Of Excitons In GaN

We report on investigations of the excitonic quantum efficiency in GaN epilayers as function of buffer layer thickness, buffer layer material, sample thickness and residual oxygen content. These values are compared to that of GaN bulk material. The quantum efficiency of the free excitons rises with increasing buffer layer thickness, increasing sample thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. Additionally, the homoepitaxial growth of GaN shows higher quantum efficiencies than the growth with an AIN buffer layer. In general, the observed quantum efficiencies in GaN epilayers are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range. Only, GaN bulk material shows quantum efficiencies of 25 % for the free A-exciton XA and of 50 % for the donor-bound exciton complex D0,X).

Influence of Surface Defects on the Characteristics of GaN Schottky Diodes

We have studied the fabrication of Pt/Au Schottky diodes on n-type GaN. We show that the electrical characteristics of the diodes are strongly dependent on the surface chemical treatment before the metal deposition. Lowest leakage currents were obtained by the use of a HC1 solution. We also show that annealing the diode at a moderate temperature (400°C) leads to reduced reverse currents. In order to explain these results, we measured the density of deep levels in the Schottky diode depletion region before and after the annealing process. We did not observe any significant difference in the bulk density of defects due to the anneal. We also studied the temperature dependence of the reverse currents and found a low activation energy. Our results are interpreted in terms of electrical defects at the metal-GaN surface.

Do patients with recurrent ovarian cancer benefit from relative tumor reduction? Results from a prospective study

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Background: Primary cytoreduction is the cornerstone in the management of epithelial ovarian cancer (OC). However, the roles of salvage surgery and of tumor reduction are still discussed controversially. The present study was conducted to assess the impact of secondary tumor reduction surgery on progression-free survival and on overall survival. Methods: Between September 2000 and April 2006, 177 consecutive patients (pts) with a first relapse of OC underwent secondary tumor reduction surgery at our department. The achieved tumor reduction was categorized as 1/5 (20% tumor debulked), 2/5 (40%), 3/5 (60%), 4/5 (80%), or 5/5 (macroscopic tumor-free) and the maximal tumor diameter was also denoted (tumor free, < 1cm, ≥ 1cm). Results: The median age was 56 years (95% CI: 23–83), median follow-up was 10.8 months (95% CI: 1–65). In 79 pts (44.6%) complete macroscopic tumor resection was achieved (median overall survival (OAS) 60.6 months, 21.3–99.8 and median progression-free survival (PFS) 14.9 months, 11.7–18, p < 0.001). In 56 pts (31.6%) 4/5 of the tumor was removed (OAS 15.6 months, 10.3–20.8 and PFS 9 months, 7.2–10.7, p < 0.001), in 13 (7.3%) 3/5 (OAS 21.7 months, no interval and PFS 12 months, 0.0–24.5, p < 0.001) and in 7 (4%) each 2/5 tumor reduction (OAS was 14.2 months, 4.1–24.2 and PFS 11 months, 5.9–16, p < 0.001) and if 1/5 tumor reduction was achieved OAS was 11.1 months, 2–20.1 and PFS 7 months, 2.5–11.4, p < 0.001. Fifteen pts (8.5%) had a bulky unresected disease (OAS 4.7 months, 1.0–8.3 and PFS of 3.7 months, 0–7.6, p < 0.001). From these 98 (55.4%) pts without complete macroscopic tumor resection, 46 were left with <1cm tumor diameter (OAS 17.2 months, 13–21.4 and PFS 9 months, 7.4–10.6 p < 0.001) and 52 with ≥1cm tumor diameter (OAS 8.7 months, 4.2–13.2 and PFS 7 months, 5.8–8.2 p < 0.001). All in all, the median postoperative survival for pts with tumor residuals and any tumor reduction (4/5, 3/5, 2/5. and 1/5 tumor reduction) were better when compared to pts with no tumor reduction (24.8 months vs. 4.7, p < 0.001). Conclusions: Our data demonstrate a significant benefit for salvage surgery if a macroscopic complete tumor resection can be achieved. We could not see any effect of relative tumor reduction on PFS or OAS.

No significant financial relationships to disclose.

Quantification of water and silanol species on various silicas by coupling IR spectroscopy and in-situ thermogravimetry

Five silica samples (four precipitated silicas provided by commercial suppliers and one with the MCM-41 structure) have been studied by infrared spectroscopy and by a homemade thermogravimetry-infrared spectrum (TG-IR) setup. The silanol amount, accessibility to water, and different alcohols, and the affinity to water of these various silicas were compared and quantified. TG-IR measurements allowed the precise determination of the integrated molar absorption coefficient of the (nu+delta)OH band, epsilon(nu+delta)OH=(0.16+/-0.01) cm micromol(-1). It is independent of the sample origin and the concentration of silanol groups on silicas. For the precipitated dried samples evacuated at room temperature, the silanol concentration COH varies between 3.6 and 7.0 mmol g(-1). It is 5.3 mmol g(-1) in the case of the MCM-41 sample. Exchange experiments with D2O, followed by back-exchanges with different alcohols (methanol, propan-2-ol, 2-methyl-propan-2-ol, and 3-ethyl-pentan-3-ol) have been followed by infrared spectroscopy. All of the silanols of the MCM-41 sample are accessible to water and alcohol molecules. By contrast, about 20% of the silanols in precipitated samples are not exchanged by D2O (internal silanols). Accessibility decreases with alcohol size; the main effect is relative to methanol. Taking into account the sample specific surface areas and the silanol accessibility to D2O, the surface silanol density of precipitated silicas is close to 8 OH per nm2, at maximum coverage. At variance, the silanol surface density of the MCM silica is much lower, 4 OH per nm2. The TG-IR setup has also been used to determine the amount of water adsorbed on silicas through the intensity of the deltaH2O band. It varies linearly with the concentration of adsorbed water, whatever the silica sample. The integrated molar absorption coefficient of two bands, epsilondeltaH2O=(1.53+/-0.03) cm micromol(-1) and epsilon(nu+delta)H2O=(0.22+/-0.01) cm micromol(-1), have been determined. The number of H2O molecules adsorbed per nm2 has been compared on the five samples under an equilibrium pressure of 13 hPa at room temperature. Taking into account the number of silanols accessible to D2O for each sample, the silica-water affinity has been defined by the H2O/(SiOHsurf) ratio. It is close to 0.8-0.9 for the precipitated samples but lower (0.7) in the case of the MCM one. This result is explained by the more important amount of isolated silanol groups presented by this sample.

Efficacy and pharmacogenomic correlation of the combination of oxaliplatin and paclitaxel in first line advanced non-small cell lung cancer (NSCLC). Preliminary results of the Spanish Lung Cancer Group (SLCG) 0103 phase II trial

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Background: Ox is an attractive platinum analogue that has not been fully explored in NSCLC. We tested the combination of ox/pac as first-line therapy in NSCLC. We also assessed single nucleotide polymorphisms (SNPs) in several DNA repair genes (XRCC1 Arg399Gln, XRCC3 Met241Thr, XPD Lys751Gln and XPD Asp312Asn) and correlated the results with outcome in these patients (p). Methods: 68 stage IIIB (pleural effusion) and IV NSCLC p received up to 6 cycles of p 200 mg/m2 on day 1 plus ox 130 mg/m2 on day 2 every 21 days. Response was assessed after the third and sixth cycles. SNPs were determined by TaqMan assay in peripheral blood extracted at baseline Results: p characteristics: median age 62; 82% male; performance status 0, 54.5%; adenocarcinoma 48%; CNS metastases (mets), 15%, liver mets, 17%. 259 cycles delivered. Grade (G) 3–4 toxicity profile: neutropenia 11%; febrile neutropenia 8%; neurosensory (G3) 5%; asthenia (G3) 8%; diarrhea (G3) 5%. Laryngodysesthesia was observed in 1 p. There were 2 deaths due to neutropenic fever. In 52 evaluable p, overall response rate was 48% (PR 46%, CR 2%, SD 6%, PD 21%). Median time to progression was 4.4 months (m) (95% CI, 2.9–5.9 m). Median survival was 7.9 m (95% CI, 5.5–10.3 m). Conclusions: The combination of ox/pac offers similar results to other platinum-based regimens with an acceptable different toxicity profile and without a significant increase in neurotoxicity. Final data on response, time to progression and survival according to SNPs will be presented.

No significant financial relationships to disclose.

Photon BLOCH oscillations in porous silicon optical superlattices

We report the first observation of oscillations of the electromagnetic field in an optical superlattice based on porous silicon. These oscillations are an optical equivalent of well-known electronic Bloch oscillations in crystals. Elementary cells of our structure are composed by microcavities whose coupling gives rise to the extended collective modes forming optical minigaps and minibands. By varying thicknesses of the cavities along the structure axis, we have created an effective electric field for photons. A very high quality factor of the confined optical state of the Wannier-Stark ladder may allow lasing in porous silicon-based superlattices.