Analysis of Bragg reflectors by lateral photoluminescence spectroscopy

Abstract

A new method for the rapid determination of layer thicknesses in semiconductor Bragg mirrors for vertical cavity surface emitting lasers (VCSEL) has been developed. The photoluminescence spectrum of a complete VCSEL structure for a wavelength of 980 nm is measured in the wafer plane at room temperature with the excitation being normal to the specimen. In the spectrum from 850 to 1000 nm three regions can be distinguished; the long wavelength part of the spectrum (λ > 950 nm) gives iriformation about the quantum well emission, the shorter part (λ > 850 nm) stems from luminescence of heavily doped p type GaAs. A pronounced structure in the range from 900 to 950 nm arises from constructive interference of light generated in the GaAs layers and leaving the specimen parallel to the surface near the angle of total reflection. From the wavelength of these peaks the layer thicknesses within the Bragg mirror can be calculated with high precision.     

Luminescence and lasing in GaN epitaxial layers and InGaN/GaN quantum well heterostructures under optical and electron-beam excitation

Interrelation between stimulated and excitonic emission intensity of GaN epitaxial layers and yellow luminescence intensity as well as correlation between photoluminescence and laser properties of InGaN based multiple quantum well heterostructures was investigated. It was found among all studied undoped GaN epitaxial layers that the higher intensity of the yellow luminescence and so the higher concentration of the yellow luminescence related centres the higher is the excitonic, electron–hole plasma and stimulated emission intensity. It was shown that a small Stokes shift and a high ratio of the luminescence intensity from InGaN quantum well layers to the photoluminescence intensity from GaN barrier layers indicate high laser quality of the multiple quantum well heterostructures. The lowest full width at half maximum of the laser line was 0.04 nm, the highest operating temperature was 585 K, the lowest threshold was 100 kW cm⁻², the highest characteristic temperature was 164 K and the highest wavelength was 442.5 nm. The far-field patterns of the laser emission from the MQW lasers consist of two approximately symmetrical high brightness spots localized at angles =±30–35°.     

Photoluminescence of heterostructures with highly strained GaAs quantum wells in Al0.48In0.52As and Ga0.47In0.53As layers

Heterostructures comprising highly strained GaAs quantum wells in Al_0.48In_0.52As and Ga_0.47In_0.53As layers have been grown by the metalorganic chemical vapor deposition method on InP(100) substrates. The photoluminescence spectra of these structures have been studied, and it is established that GaAs quantum wells form type-I heterojunctions with Al_0.48In_0.52As layers and type-II heterojunctions with Ga_0.47In_0.53As layers.     

Influence of the nature of oxide matrix on the photoluminescence spectrum of ion-synthesized silicon nanostructures

The photoluminescence of various Si ion implanted oxide layers annealed at high-temperature has been studied in the range of 350-1500 nm. The set of investigated oxide materials includes thermal SiO₂, deposited SiO₂, Si_0.9Ge_0.1O₂, GeO₂ films on silicon substrate, and sapphire wafers. The results are discussed in terms of generation and modification of the defect centers and nanoclusters formation taking into account several factors related to composition and structure of the original oxide matrices.     

Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers

The effect of ultrathin silicon nitride (Si₃N₄) barrier layers on the formation and photoluminescence (PL) of Si nanocrystals (NCs) in Si-rich nitride (SRN)/Si₃N₄ multilayer structure was investigated. The layered structures composed of alternating layers of SRN and Si₃N₄ were prepared using magnetron sputtering followed by a different high temperature annealing. The formation of uniformly sized Si NCs was confirmed by the transmission electron microscopy and X-ray diffraction measurements. In particular, the 1 nm thick Si₃N₄ barrier layers was found to be sufficient in restraining the growth of Si NCs within the SRN layers upon high annealing processes. Moreover, X-ray photoelectron spectroscopy spectra shown that films subjected to post-anneal processes were not oxidized during the annealing. X-ray reflection measurements revealed that high annealing process induced low variation in the multilayer structure where the 1 nm Si₃N₄ layers act as good diffusion barriers to inhibit inter-diffusion between SRN layers. The PL emission observed was shown to be originated from the quantum confinement of Si NCs in the SRN. Furthermore, the blue shift of PL peaks accompanied by improved PL intensity after annealing process could be attributed to the effect of improved crystallization as well as nitride passivation in the films. Such multilayer structure should be advantageous for photovoltaic applications as the ultrathin barrier layer allow better electrical conductivity while still able to confine the growth of desired Si NC size for bandgap engineering.     

Photoluminescence properties of nanocrystalline,wide band gap nitrides (C3N4, BN,AIN, GaN)

Nanocrystalline layers of C₃N₄, BN, AlN and GaN were grown by Impulse Plasma Assisted Chemical Vapor Deposition method on silicon substrates kept at 300K. Optical absorption studies of the layers revealed broad bands below the fundamental absorption edge, which were ascribed to transitions involving defect levels. When excited by a 3 mW He-Cd laser (325 nm line) all samples showed a wide photoluminescence spectrum (2.0 to 3.5 eV), but the disordered crystalline structure quenched the excitonic photoluminescence observed in monocrystals. The efficient visible luminescence of nanocrystalline AlN layers in the 2–3 eV region observed under non optimal excitation conditions (energy bandgap of AlN is twice the excitation energy) is promising for optoelectronic use of this material as a visible light source.     

One-step synthesis of water-soluble ZnSe quantum dots via microwave irradiation

A facile strategy has been developed for the synthesis of glutathione-capped ZnSe quantum dots (QDs) in aqueous media. The reaction was carried out in air atmosphere with a single step by using Na₂SeO₃, a stable and commercial Se source, to replace the commonly adopted NaHSe or H₂Se. Moreover, microwave irradiation improved the photoluminescence quantum yield (PLQY) as well as lowered the trap emission of as-prepared ZnSe QDs. The obtained QDs performed strong band-edge luminescence (PLQY reached 18%), narrow size distribution (full width at half maximum was 26-30 nm) and weak trap emission without post-treatments. The results of transmission electron microscopy and X-ray diffraction demonstrated the small particle size (2-3 nm), good monodispersity and ZnSe(S) alloyed structure of as-prepared QDs. The experimental variables including precursors and stabilizer amounts as well as pH value had significant influence on the PL properties of the ZnSe QDs.     

Numerical study on optical properties of Ag/SiO2SiO2/Ag sandwich nanocrescents substrate

A novel structure of a metal/dielectric/metal (Ag/SiO₂SiO₂/Ag) sandwich nanocrescent has been proposed and studied. We make a detailed numerical analysis on the extinction efficiency and LSPR property of the sandwich nanocrescent by using the finite difference time domain (FDTD) method. It clearly demonstrates that a comparable field enhancement can be achieved by varying the thickness of the SiO₂SiO₂ layer at different incident polarizations. Excited in the Y-polarization, the maximum electric field enhancement factor reaches 600 at the peak wavelength 1108.9?nm, which is six times higher than previous reported single layer nanocrescent. The refractive index sensitivity of this new sandwich nanocrescent is 375.5?nm/RIU (refractive index unit). The structure is shown to produce a high local field enhancement as well as wide plasmon resonance tunabilities. Besides, compared with adjusting the shape and size of the single layer nanocrescent structure, it is much more convenient and easier to change the thickness of the sandwich nanocrescent. Due to its excellent properties, this structure is very suitable for LSPR and SERS nanosensing substrate.     

Formation of a two-dimensional electron gas in ZnO/MgZnO single heterostructures and quantum wells

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112?0) and ZnO (0001?) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance-voltage (C-V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C-V measurements the sheet carrier density in a Zn_0.8 Mg_0.2O/ZnO/Zn_0.8 Mg_0.2O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10¹⁰ cm^− 2. For the films deposited on sapphire 2D growth is observed in the Burton-Cabrera-Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.     

Structure,phase composition,and nanohardness of vacuum-annealed multilayer fullerite/aluminum films

The influence of the number of layers and thermal annealing on the structure, elemental and phase compositions, and nanohardness of multilayer fullerite/aluminum films has been studied by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray microanalysis, and nanoindentation. The results demonstrate that sequential growth of five aluminum layers and four fullerite layers, each 50 nm in thickness, on oxidized single-crystal silicon substrates leads to the formation of textured films, which retain 111 texture after vacuum annealing at 620 K (τ = 5 h). In the case of the growth of bilayer films of greater thickness, C₆₀(200 nm)/Al(300 nm), the fullerite and aluminum have a polycrystalline structure with no growth texture. Thermal annealing of the bilayer films leads to the formation of a new phase, Al_xC₆₀. The materials studied here possess enhanced nanohardness compared to pure aluminum and fullerite films.     

Time-resolved photoluminescence from Si-in-SiNx/Si-in-SiC quantum well-dot structures

Si-in-SiN_x/Si-in-SiC quantum well-dot structures, with Si quantum dots slightly larger than 1.0 nm embedded in both amorphous SiN_x and SiC sublayers, were grown at nearly room temperature by using PECVD. Time-resolved photoluminescence for three samples in a period of 100/30 nm, 60/10 nm and 20/10 nm, respectively, has been measured at emission lengths ranging from 430 nm to 490 nm, and fitted with a stretched-exponential function. Typical decay time was at the order of one nanosecond, which could be attributed to the core-state emission. The matrix materials forming the well provide a non-uniform potential background which induces a modulation to the carrier diffusion process, thus resulting in an emission-wavelength dependent decay time. When confinement effect from the well comes into play as in the sample of smaller well width, the decay time can be below 1.0 ns and indifferent to the varied emission wavelength, and the carrier diffusion is dominated by hopping. These quantum well-dot systems of strong and fast decaying light emission in blue–violet colors might find potential utilization in GHz optical connection and other photoelectronic devices.     

A Study of the Crystal Structure of Co<Subscript>40</Subscript>Fe<Subscript>40</Subscript>B<Subscript>20</Subscript> Epitaxial Films on a Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Topological Insulator

Laser molecular-beam epitaxy has been used to form Co₄₀Fe₄₀B₂₀ layers on Bi₂Te₃ topological insulator substrates, and their growth conditions have been studied. The possibility of growing epitaxial ferromagnetic layers on the surface of a topological insulator is demonstrated for the first time. The CoFeB layers have a body-centered cubic crystal structure with the (111) crystal plane parallel to the (0001) plane of Bi₂Te₃. 3D mapping in the reciprocal space of high-energy electron-diffraction patterns made it possible to determine the epitaxial relationships between the film and the substrate.     

Photoluminescence of Ca(Ba)Ga2S4:RE poly- and nanocrystals

The photoluminescence and photoluminescence excitation spectra and luminescence decay kinetics of CaGa₂S₄:Eu²⁺ bulk crystals and powders ranging in particle size from 100 to 600 nm have been studied in the temperature range 77–300 K. The results indicate that the full width at half maximum of the photoluminescence band of the CaGa₂S₄:Eu²⁺ nanopowders is about twice that of the bulk crystals. Analysis of the photoluminescence spectra shows that the energy position of the emission band is almost independent of the particle size, temperature, and excitation intensity in the ranges 77–300 K and 10^?3 to 10⁶ W/cm², respectively. The shape of the photoluminescence band is well represented by a Gaussian. The excited state lifetime of the Eu²⁺ ion is ～1000 ns as evaluated from the exponential portion of the luminescence decay curve.     

Properties of nanostructured oxide formed during oxidation of a zirconium wire by supercritical water

Synthesis of ZrO₂ during oxidation of a zirconium wire by supercritical water at P = 25 MPa and T = 500 or 525°C has been investigated. It is established that an inhomogeneous nanostructured ZrO₂ layer is formed as a result of oxidation. Rate of 〈Zr〉 oxidation, oxide porosity and morphology, and average size and structure of crystallites are determined. The thermal conductivity of the synthesized ZrO₂ layer in supercritical water and in nitrogen is determined by pulsed electric heating of a partially oxidized wire. The low values of thermal conductivity (about 0.2 W/(m K)) correspond to a layered structure of porous material, with layers orientated parallel to the oxidized-metal surface.     

Effect of annealing in external magnetic field on the microstructure and magnetic properties of FePt films

We have studied the influence of annealing in an external magnetic field on the microstructure and magnetic properties of a multilayer Si/Fe(2 nm)/Fe₅₀Pt₅₀(20 nm)/Pt(2 nm) structure synthesized by means of sequential RF magnetron sputtering of the components. The magnetic field was oriented perpendicular to layers of the structure. It is established that annealing in the external magnetic field leads to the formation of predominant (001) texture in the multilayer structure with L1₀-FePt phase. Thus, a method of obtaining multilayer structures based on FePt films required for the perpendicular magnetic recording has been developed.     

Wet chemical preparation of YVO4:Eu thin films as red-emitting phosphor layers for fully transparent flat dielectric discharge lamp

Highly transparent YVO₄:Eu thin films were deposited via dip coating of liquid nanoparticle dispersions on glass substrates. Annealing of the nanoparticle layers resulted in restructuring of the material into oriented crystalline films. The crystallinity was confirmed using powder X-ray diffraction. Film thickness was adjusted to 467 nm by multiple deposition. The resulting coatings show > 99% absorbance for wavelength below 300 nm and > 90% transmission in the visible spectral range. Under UV-light excitation a bright red photoluminescence with a quantum efficiency of 20% is observed. A planar, transparent dielectric barrier discharge lamp was constructed using YVO₄:Eu coated glasses and transparent electrodes made from antimony-doped tin dioxide thin films.     

Selective modification of the band gaps of GaInNas/GaAs structures by quantum well intermixing techniques

We report the unambiguous demonstration of controlled quantum well intermixing (QWI) in the technologically important GaInNAs/GaAs 1.3 μm material system. QWI is a key technique to selectively modify the band gap of quantum wells, which has found broad application in semiconductor lasers and photonic integrated circuits (PICs). Extending such technology to GaInNAs/GaAs structures is highly desirable due to the technologically advantageous properties of this material system. Here, we investigate well-characterized GaInNAs quantum well material which has been annealed “to saturation” before QWI processing to allow unambiguous interpretation of results. After RTA at 700 °C for ∼180 s, controlled shifts in band-gap at room temperature of up to 200 nm have been observed in sputtered SiO₂-capped samples, whilst uncapped and PECVD SiO₂-capped samples demonstrated negligible shift. This selective modification of the band gap has been confirmed by detailed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Analysis of composition profile by SIMS revealed that the QWI is due to the interdiffusion of In–Ga between the quantum wells and the barriers enhanced by the point defects generated during the sputtering process. Investigation of a series of samples of differing N concentrations will be presented, which provides extra information about the intrinsic properties of GaInNAs.     

Fabrication and characterisation of Cu(In,Ga)Se2 solar cells on polyimide

Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se₂/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se₂ (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E_g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.     

Porous silicon upon multicrystalline silicon: Structure and photoluminiscence

Ultrathin porous silicon layers have been stain-etched upon multicrystalline silicon (multi-Si) substrates. We studied optical and structural properties of porous silicon by photoluminescence, photo-luminescence excitation, reflection, atomic force microscopy and scanning tunnel microscopy methods. It was observed that the thickness of porous silicon did not exceed 20 nm. The photoluminescence method has shown that photoluminescence spectra of porous silicon of different grains have shown that they differ insignificantly (∼10%) in intensity. It was found that por-Si layers with optimal antireflection characteristics was obtained during etching time 7 min. In the paper the comparison of the reflection characteristics of investigated samples por-Si with industrial antireflection coating is presented.     

Photoionization coefficient of thin calcium films

The photoionization coefficient of thin calcium films was studied by means of photoemission measurements. The incident photon energies ranged from 2 eV to 5.6 eV. The films were obtained by thermal evaporation and condensation on a silica substrate in ultrahigh vacuum. The mass thicknesses ranged from 2 nm to 50 nm. The photoionization coefficient β exhibited two maxima and reached a constant value for films thicker than 15 nm. The first maximum corresponds to a granular structure (d₁ = 3 nm; β₁ = 62 × 10^?31 per absorbed quantum in a second). The second maximum corresponds to the granular-lacunar structure transition (d₀ ≈ 5.5 nm; β₀ = 36 × 10^?31 per absorbed quantum in a second). An attempt is made to explain the result in terms of an electronic double layer located near the surface.