Physical Review B

Role of Structural Factors in the Nonlinear Optical Properties of Phthalocyanines and Related Compounds

Phthalocyanines: old dyes, new materials. Putting color in nanotechnology

Ultraviolet (UV) photodetection has drawn a great deal of attention in recent years due to a wide range of civil and military applications. Because of its wide band gap, low cost, strong radiation hardness and high chemical stability, ZnO are regarded as one of the most promising candidates for UV photodetectors. Additionally, doping in ZnO with Mg elements can adjust the bandgap largely and make it feasible to prepare UV photodetectors with different cut-off wavelengths. ZnO-based photoconductors, Schottky photodiodes, metal–semiconductor–metal photodiodes and p–n junction photodetectors have been developed. In this work, it mainly focuses on the ZnO and ZnMgO films photodetectors. We analyze the performance of ZnO-based photodetectors, discussing recent achievements, and comparing the characteristics of the various photodetector structures developed to date.

https://www.mdpi.com/1424-8220/10/9/8604/pdf

ZnO-Based Ultraviolet Photodetectors

Phthalocyanines (Pcs) and related compounds with their extended two-dimensional pi;-electronic delocalization are important targets to study nonlinear optical responses. The tailorability of these macrocycles allows the fine-tuning of the chemical structure and nonlinear optical response. In this article, the design and main properties of phthalocyanines for second- and third-order nonlinear optical (NLO) and optical limiting applications are discussed, both at the microscopic and macroscopic level. The points of view of synthetic organic chemists and physicists are accorded, the main aim of the review being to highlight the key problems of the field and place them within the general context of NLO materials.

Phthalocyanines and related compounds:organic targets for nonlinear optical applications

N-Schottky and p+–n GaN junctions are currently used for different technologies. A comparison of the deep levels found throughout the entire band gap of n-GaN grown by metal-organic chemical vapor deposition under both configurations is presented. Both deep level optical spectroscopy and deep level transient spectroscopy measurements are used allowing the observation of both majority and minority carrier traps. Deep levels at Ec−Et=0.58–0.62, 1.35, 2.57–2.64, and 3.22 eV are observed for both diode configurations, with concentrations in the ∼1014–1016 cm−3 range. The 0.58–0.62 eV level appears correlated with residual Mg impurities in the n side of the p+–n diode measured by secondary-ion-mass spectroscopy, while the 1.35 eV level concentration increases by a factor of ∼4 for the Schottky junction possibly correlating with the carbon profile. The 2.57–2.64 eV level is a minority carrier hole trap in n-GaN, likely related to the yellow photoluminescence band, and is detected both optically from the conduct...

Optically and thermally detected deep levels in n-type Schottky and p+-n GaN diodes

Differential postgrowth hydrogen passivation of deep levels in n–GaN grown by metal-organic chemical vapor deposition has been directly observed by means of both deep level transient spectroscopy and deep level optical spectroscopy. Two deep levels found at Ec−Et=0.62 and 1.35 eV show strong H passivation effects, with their concentrations decreasing by a factor of ⩾30 and ∼14, respectively. The decrease in the 0.62 eV trap concentration together with its correlation with the presence of Mg in n–GaN is consistent with Mg–H complex formation. A band of closely spaced deep levels observed at Ec−Et=2.64–2.80 eV narrows to Ec−Et=2.74–2.80 eV after hydrogenation, consistent with hydrogen complexing with VGa3− defects as anticipated by earlier theoretical results. Finally, a deep level at Ec−Et=3.22 eV likely related to background acceptors remains unaffected by hydrogen.

Hydrogen passivation of deep levels in n–GaN

The Sb-induced changes in the optical properties of GaAsSb-capped InAs/GaAs quantum dots (QDs) are shown to be strongly correlated with structural changes. The observed redshift of the photoluminescence emission is shown to follow two different regimes. In the first regime, with Sb concentrations up to $\ensuremath{\sim}12\mathrm{%}$, the emission wavelength shifts up to $\ensuremath{\sim}1280\text{ }\text{nm}$ with a large enhancement of the luminescence characteristics. A structural analysis at the atomic scale by cross-sectional scanning tunneling microscopy shows that this enhancement arises from a gradual increase in QD height, which improves carrier confinement and reduces the sensitivity of the excitonic band gap to QD size fluctuations within the ensemble. The increased QD height results from the progressive suppression of QD decomposition during the capping process due to the presence of Sb atoms on the growth surface. In the second regime, with Sb concentrations above $\ensuremath{\sim}12\mathrm{%}$, the emission wavelength shifts up to $\ensuremath{\sim}1500\text{ }\text{nm}$, but the luminescence characteristics progressively degrade with the Sb content. This degradation at high Sb contents occurs as a result of composition modulation in the capping layer and strain-induced Sb migration to the top of the QDs, together with a transition to a type-II band alignment.

/pdf/gaassb-capped-inas-quantum-dots-from-enlarged-quantum-dot-1ogu2skszw.pdf

GaAsSb-capped InAs quantum dots: From enlarged quantum dot height to alloy fluctuations

Schottky photodiodes based on Au-ZnMgO/sapphire are demonstrated covering the spectral region from 3.35 to 3.48 eV, with UV/VIS rejection ratios up to ∼105 and responsivities as high as 185 A/W. Both the rejection ratio and the responsivity are shown to be largely enhanced by the presence of an internal gain mechanism, by which the compensated films become highly conductive as a result of illumination. This causes a large increase in the tunnel current through the Schottky barrier, yielding internal gains that are a function of the incident photon flux.

High responsivity and internal gain mechanisms in Au-ZnMgO Schottky photodiodes

The effect of growth regime on the deep level spectrum of n-GaN using molecular-beam epitaxy (MBE) was investigated. As the Ga/N flux ratio was decreased towards Ga-lean conditions, the concentration of two acceptor-like levels, at Ec−3.04 and 3.28 eV, increased from 1015 to 1016 cm−3 causing carrier compensation in these films. Thus, these two traps behaved as the dominant compensating centers in MBE n-GaN. Furthermore, the increase in trap concentration also strongly correlated with the degradation of both surface morphology and bulk electron mobility towards Ga-lean conditions, where higher pit densities and lower mobility were observed. These results show that the growth regime directly impacts all morphology, bulk transport, and trap states in n-GaN.

Adrian Hierro

Papers

Optically and thermally detected deep levels in n-type Schottky and p+-n GaN diodes

Hydrogen passivation of deep levels in n–GaN

GaAsSb-capped InAs quantum dots: From enlarged quantum dot height to alloy fluctuations

High responsivity and internal gain mechanisms in Au-ZnMgO Schottky photodiodes

Impact of Ga/N flux ratio on trap states in n-GaN grown by plasma-assisted molecular-beam epitaxy