Ion beam modification of porous silicon using high energy Au ions and its impact on photoluminescence spectra

This paper presents investigation of impact of high-energy ion-irradiation on properties of light emitting porous silicon (PS) through photoluminescence (PL) spectroscopy. Irradiation was performed with 100 MeV Au⁺⁷ ions from a pelletron accelerator at ion doses in 10¹⁰-10¹⁴ cm⁻² range. The effect was associated with a blueshift (∼40 nm) and an enhancement of the PL intensity, in general. The efficiency and stability of PL with respect to ambients was seen to be relatively improved. The PL properties of PS were found to be stable against low to medium dose irradiation (<10¹³ cm⁻²), whereas, higher dose led to further degradation of the optical properties. The effects have been explained in terms of a decrease in the non-radiative recombination probability of electron-hole pairs due to chemical restructuring of the surface and a reduced crystallite size as a result of irradiation.     

Preparation and characterization of CdS/Si coaxial nanowires

CdS/Si coaxial nanowires were fabricated via a simple one-step thermal evaporation of CdS powder in mass scale. Their crystallinities, general morphologies and detailed microstructures were characterized by using X-ray diffraction, scanning electron microscope, transmission electron microscope and Raman spectra. The CdS core crystallizes in a hexagonal wurtzite structure with lattice constants of a=0.4140 nm and c=0.6719 nm, and the Si shell is amorphous. Five Raman peaks from the CdS core were observed. They are 1LO at 305 cm⁻¹, 2LO at 601 cm⁻¹, A₁-TO at 212 cm⁻¹, E₁-TO at 234 cm⁻¹, and E₂ at 252 cm⁻¹. Photoluminescence measurements show that the nanowires have two emission bands around 510 and 590 nm, which originate from the intrinsic transitions of CdS cores and the amorphous Si shells, respectively.     

As-doped p-type ZnO films grown on SiO2/Si by radio frequency magnetron sputtering

p-Type ZnO:As films with a hole concentration of 10¹⁶-10¹⁷ cm⁻³ and a mobility of 1.32-6.08 cm²/V s have been deposited on SiO₂/Si substrates by magnetron sputtering. XRD, SEM, Hall measurements are used to investigate the structural and electrical properties of the films. A p-n homojunction comprising an undoped ZnO layer and a ZnO:As layer exhibits a typical rectifying behavior. Our study demonstrates a simple method to fabricate reproducible p-type ZnO film on the SiO₂/Si substrate for the development of ZnO-based optoelectronic devices on Si-based substrates.     

Effects of different ions implantation on yellow luminescence from GaN

The influence of C, N, O, Mg, Si and co-implants (Mg+Si) ions implantation with fluences in the wide range 10¹³-10¹⁷ cm⁻² on the yellow luminescence (YL) properties of wurtzite GaN has been studied by photoluminescence (PL) spectroscopy. Two types of n-type GaN samples grown by metal-organic chemical vapor deposition method (MOCVD) and labeled as No-1 and No-2 were studied. In their as-grown states, No-1 samples had strong YL, while No-2 samples had weak YL. Results of the frontside and backside PL measurements in one of the as-grown GaN epifilms are also presented. Comparing the intensity of YL between frontside and backside PL spectra, the backside PL spectrum shows the more intense YL intensity. This implies that most of the intrinsic defects giving rise to YL exist mainly near the interface between the epilayer and buffer layer. Our experimental results show that the intensity ratio of YL to near-band-edge UV emission (I_YL/I_UV) decreases gradually by increasing the C implantation fluence from 10¹³ to 10¹⁶ cm⁻² for No-1 samples after annealing at 900 °C. When the fluence is 10¹⁷ cm⁻², a distinct change of the I_YL/I_UV is observed, which is strongly increased after annealing. For No-2 samples, after annealing the I_YL/I_UV decreases gradually with increase in the C implantation fluence from 10¹³ to 10¹⁵ cm⁻². The I_YL/I_UV is gradually increased with increasing C fluence from 10¹⁶ to 10¹⁷ cm⁻² after annealing, while I_YL/I_UV for other ions-implanted GaN samples decreases monotonically with increase in the ions implantation fluences from 10¹³ to 10¹⁷ cm⁻² for both No-1 samples and No-2 samples. It is noted that for annealed C-implanted No-2 samples I_YL/I_UV is much higher than that of the as-grown one and other ion-implanted ones. In addition, I_YL/I_UV for the Mg, Si, and co-implants (Mg+Si) implanted No-2 samples with a fluence of 10¹³ cm⁻² after being annealed at 900 °C is higher than that of the as-grown one. Based on our experimental data and literature results reported previously, the origins of the YL band have been discussed.     

Investigation of p-type behavior in Ag-doped ZnO thin films by E-beam evaporation

Ag-doped ZnO (ZnO:Ag) thin films were grown on glass substrates by E-beam evaporation technique. The structural, electrical and optical properties of the films were investigated as a function of annealing temperature. The films were subjected to post annealing at different temperatures in the range of 350-650 °C in an air ambient. All the as grown and annealed films at temperature of 350 °C showed p-type conduction. The films lost p-type conduction after post annealing treatment temperature of above 350 °C, suggesting a narrow post annealing temperature window for the fabrication of p-type ZnO:Ag films. ZnO:Ag film annealed at 350 °C revealed lowest resistivity of 7.25 × 10⁻² Ω cm with hole concentration and mobility of 5.09 × 10¹⁹ cm⁻³ and 1.69 cm²/V s, respectively. Observation of a free-to-neutral-acceptor (e,A^o) and donor-acceptor-pair (DAP) emissions in the low temperature photoluminescence measurement confirms p-type conduction in the ZnO:Ag films.     

Fabrication of high hole-carrier density p-type ZnO thin films by N-Al co-doping

In order to obtain p-type ZnO thin films, effect of atomic ratio of Zn:N:Al on the electronic and structural characteristic of ZnO thin films was investigated. Hall measurement indicated that with the increase of Al doping, conductive type of as-grown ZnO thin films changed from n-type to p-type and then to n-type again, reasons are discussed in details. Results of X-ray diffraction revealed that co-doped ZnO thin films have similar crystallization characteristic (0 0 2 preferential orientation) like that of un-doping. However, SEM measurement indicated that co-doped ZnO thin films have different surface morphology compared with un-doped ZnO thin films. p-type ZnO thin films with high hole concentration were obtained on glass (4.6 × 10¹⁸ cm⁻³) and n-type silicon (7.51 × 10¹⁹ cm⁻³), respectively.     

Investigation of 2.0 μm emission in Tm and Ho co-doped oxyfluoride tellurite glass

This paper reports 2.0 μm emission properties of Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass exited by 808 nm laser diode (LD). Mid-infrared transmittance property of glass was investigated by Fourier transform infrared (FTIR) spectrometer. The real chemical composition of investigated glass was identified by X-ray photoelectric spectroscopy (XPS). Thermal stability of the glass was determined by differential thermal analysis (DTA) measurement. The Judd-Ofelt parameters, spontaneous radiative transition probabilities, branching ratios and radiative lifetime of Ho³⁺ were calculated based on the absorption spectra by using Judd-Ofelt theory. Results indicate that the maximum 2.0 μm emission intensity attributed to the ⁵I₇→⁵I₈ transition of Ho³⁺ was achieved at 1.5 mol% Tm₂O₃ and 1 mol% Ho₂O₃ concentrations in oxyfluoride tellurite glass. OH⁻ absorption at 3000 cm⁻¹ was greatly depressed by introduction of 10 mol% F⁻. The maximum absorption and stimulated emission cross-section of Ho³⁺ near 2.0 μm are 7.0×10⁻²¹ cm² at 1950 nm and 8.8×10⁻²¹ cm² at 2048 nm, respectively. The calculated radiative lifetime of 4.4 ms for ⁵I₇→⁵I₈ transition and large stimulated emission cross-section of the Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass indicate that the glass has a potential application in efficient 2.0 μm laser.     

Spectral properties of Nd:Ca2Nb2O7 crystal

This paper reports the spectral properties of Nd³⁺:Ca₂Nb₂O₇. The spectral parameters of Nd³⁺ in Nd³⁺:Ca₂Nb₂O₇ crystal have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained. The parameters of line strengths Ω_λ are Ω₂=4.967×10⁻²⁰ cm², Ω₄=5.431×10⁻²⁰ cm², Ω₆=5.693×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 122 μs, 103 μs and 84.4%, respectively. The fluorescence branch ratios calculated: β₁=0.425, β₂=0.479, β₃=0.091, β₄=0.004. The emission cross section at 1068 nm is 6.204×10⁻²⁰ cm².     

Effect of annealing atmosphere on the photoluminescence of ZnO nanospheres

ZnO nanospheres were synthesized by a wet-chemical method. X-ray diffraction and field-emission scanning electron microscopy confirmed the formation of wurtzite-structured ZnO with regular sphere shape. Two Raman modes located at 333 cm⁻¹ and 437 cm⁻¹ with two additional Raman humps centered at 577 cm⁻¹ and 1077 cm⁻¹ were observed. Photoluminescence spectra showed ultraviolet, green, orange and red emissions, which changed significantly after the samples were annealed in air, oxygen, argon and forming gas four different ambiences. All the evidence indicates that surface states are responsible to orange and red emissions in addition to excitonic recombination (3.18 eV) and oxygen vacancy (2.25 eV) emission.     

Lasing at 1065 nm in bulk Nd-doped telluride-tungstate glass

We have achieved, for the first time to our knowledge, lasing in a new type of telluride-tungstate glass host doped with neodymium: Nd³⁺:(0.8)TeO₂-(0.2)WO₃. Lasing was obtained at 1065 nm with two samples containing 0.5 mol% and 1.0 mol% Nd₂O₃. During gain-switched operation, slope efficiencies of 12% and 10% were obtained with the 0.5 mol% and 1.0 mol% doped samples, respectively, at a pulse repetition rate of 1 kHz. Judd-Ofelt analysis was further employed to determine the emission cross section σ_e at 1065 nm from the absorption spectra and lifetime data. The emission cross section from the Judd-Ofelt analysis came to 3.23 ± 0.09 × 10⁻²⁰ cm², in reasonable agreement with the value of 2.0 ± 0.13 × 10⁻²⁰ cm² obtained from the analysis of laser threshold data.     

Formation of Al-N co-doped p-ZnO/n-Si (1 0 0) heterojunction structure by RF co-sputtering technique

Al-N co-doped ZnO (ZnO:Al-N) thin films were grown on n-Si (1 0 0) substrate by RF co-sputtering technique. As-grown ZnO:Al-N film exhibited n-type conductivity whereas on annealing in Ar ambient the conduction of ZnO:Al-N film changes to p-type, typically at 600 °C the high hole concentration of ZnO:Al-N co-doped film was found to be 2.86 × 10¹⁹ cm⁻³ and a low resistivity of 1.85 × 10⁻² Ω-cm. The current-voltage characteristics of the obtained p-ZnO:Al-N/n-Si heterojunction showed good diode like rectifying behavior. Room temperature photoluminescence spectra of annealed co-doped films revealed a dominant peak at 3.24 eV.     

Microphotoluminescence study of local temperature fluctuations in n-type (Cd,Mn)Te quantum well

Microphotoluminescence mapping measurements were performed in a magnetic field on a (Cd,Mn)Te quantum well, modulation n-doped with iodine at about 10¹⁰ cm⁻². Photoluminescence spectra contain neutral (X⁰) and negatively charged (X⁻) exciton lines. The Zeeman effect shows a significant role of heating of the Mn system even under lowest excitation densities. The effective temperature of the magnetic system exhibits strong fluctuations anticorrelated with the total intensity of PL signal. An interpretation of these fluctuations in terms of the influence of non-radiative recombination centers is proposed (in two alternative versions). Maps of the local X⁰/X⁻ intensity ratio indicate a minor role of electrostatic potential fluctuations.     

Optical study of implantation damage recovery from Si-implanted GaN

Comprehensive and systematic optical activation studies of Si-implanted GaN grown on sapphire substrates have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×10¹³ to 5×10¹⁵ cm⁻² at room temperature. The samples were proximity cap annealed from 1250 to 1350 °C with a 500-Å-thick AlN cap in a nitrogen environment. The results of photoluminescence measurements made at 3 K show a very sharp neutral-donor-bound exciton peak along with a sharp donor-acceptor pair peak after annealing at 1350 °C for 20 s, indicating excellent implantation damage recovery. The results also indicate the AlN cap protected the implanted GaN layer very well during high temperature annealing without creating any significant anneal-induced damage. This observation is consistent with the electrical activation results for these samples.     

Ar irradiation of Si nanocrystal-doped SiO2: Evolution of photoluminescence

We report the evolution of photoluminescence (PL) of Si nanocrystals (nc-Si) embedded in a matrix of SiO₂ during Ar⁺ ion bombardment. The integrated intensity of nc-Si PL falls down drastically before the Ar⁺ ion fluence of 10¹⁵ ions cm⁻², and then decreases slowly with the increasing ion fluence. At the meantime, the PL peak position blueshifts steadily before the fluence of 10¹⁵ ions cm⁻², and then changes in an oscillatory manner. Also it is found that the nc-Si PL of the Ar⁺-irradiated sample can be partly recovered after annealing at 800 °C in nitrogen, but can be almost totally recovered after annealing in oxygen. The results confirm that the ion irradiation-induced defects are made up of oxygen vacancies, which absorb light strongly. The oscillatory peak shift of nc-Si can be related to a size-distance distribution of nc-Si in SiO₂.     

Low temperature fabrication of 5-10 nm SiO2/Si structure using advanced nitric acid oxidation of silicon (NAOS) method

We have developed the advanced nitric acid oxidation of Si (NAOS) method to form relatively thick (5-10 nm) SiO₂/Si structure with good electrical characteristics. This method simply involves immersion of Si in 68 wt% nitric acid aqueous solutions at 120 °C with polysilazane films. Fourier transform infrared absorption (FT-IR) measurements show that the atomic density of the NAOS SiO₂ layer is considerably high even without post-oxidation anneal (POA), i.e., 2.28 × 10²² atoms/cm², and it increases by POA at 400 °C in wet-oxygen (2.32 × 10²² atoms/cm²) or dry-oxygen (2.30 × 10²² atoms/cm²). The leakage current density is considerably low (e.g., 10⁻⁵ A/cm² at 8 MV/cm) and it is greatly decreased (10⁻⁸ A/cm² at 8 MV/cm) by POA at 400 °C in wet-oxygen. POA in wet-oxygen increases the atomic density of the SiO₂ layer, and decreases the density of oxide fixed positive charges.     

Trap levels in layered semiconductor Ga2SeS

Trap levels in nominally undoped Ga₂SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga₂SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10⁻²³, 1.8×10⁻²³ and 2.8×10⁻²² cm² with concentrations of 1.3×10¹², 5.4×10¹² and 4.2×10¹² cm⁻³, respectively.     

Growth and spectral properties of Nd:GdVO4 laser crystal

In this paper, the Czochralski growth, absorption spectra, and photoluminescence spectra of Nd:GdVO₄ crystals are studied. From its absorption spectra, Nd:GdVO₄ is found to exhibit an anisotropic optical absorption effect, and its effective Judd-Ofelt parameters are calculated: Ω₂=10.281×10⁻²⁰ cm², Ω₄=5.426×10⁻²⁰ cm² and Ω₆=9.943×10⁻²⁰ cm². By these parameters, the absorption oscillator strengths, emission oscillator strengths, transition probabilities, fluorescence branch ratios, energy lifetimes, and integrated emission cross-sections are also derived. The photoluminescence spectra of Nd:GdVO₄ crystal consist of a wide emission band of host and the characteristic emission bands of Nd³⁺. Based on the excitation spectrum, both the two evident peaks locating at 345 and 371 nm are ascribed to the characteristic excitation of Nd³⁺, and an energy transfer from the host to its doping Nd³⁺ ions is indicated.     

Recrystallization behavior of high-fluence N-implanted GaAs studied by Raman spectroscopy

Raman spectroscopy was used to study the evolution of host lattice recrystallization in high-fluence N⁺-implanted GaAs. A high-fluence of N⁺ ions (>10¹⁵ cm⁻²) was introduced into semi-insulating GaAs by the combinatorial implantation method. Subsequent thermal annealing at 800 °C was carried out to re-grow the implantation-induced amorphous layers. The dependence of Raman parameters on N contents was systematically observed for each recrystallized cell. The volume of the newly formed crystallites with original orientation decreases with increasing fluences, whereas that of crystallites of other orientations increases after high-fluence implantation and annealing. The correlation length L, representing the size of crystalline regions with preserved translational symmetry, was determined by fitting the LO phonon signal with spatial correlation model. For 10¹⁶ cm⁻² implantation, the recrystallized layer consists of nano-meter-sized crystallites (∼30 nm). The dimension of the recrystallized crystallites decreases with increasing N⁺ fluences, in good agreement with the model.     

Single crystal EPR and optical absorption study of Cr doped l-histidine hydrochloride monohydrate

EPR study of the Cr³⁺ ion doped l-histidine hydrochloride monohydrate single crystal is done at room temperature. Two magnetically inequivalent interstitial sites are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The zero field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations and the parameters are: D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.9108±0.0002, g_y=1.9791±0.0002, g_z=2.0389±0.0002, A_x=(252±2)×10⁻⁴ cm⁻¹, A_y=(254±2)×10⁻⁴ cm⁻¹, A_z=(304±2)×10⁻⁴ cm⁻¹ for site I and D=(300±2)×10⁻⁴ cm⁻¹, E=(96±2)×10⁻⁴ cm⁻¹, g_x=1.8543±0.0002, g_y=1.9897±0.0002, g_z=2.0793±0.0002, A_x=(251±2)×10⁻⁴ cm⁻¹, A_y=(257±2)×10⁻⁴ cm⁻¹, A_z=(309±2)×10⁻⁴ cm⁻¹ for site II, respectively. The optical absorption studies of single crystals are also carried out at room temperature in the wavelength range 195-925 nm. Using EPR and optical data, different bonding parameters are calculated and the nature of bonding in the crystal is discussed. The values of Racah parameters (B and C), crystal field parameter (Dq) and nephelauxetic parameters (h and k) are: B=636, C=3123, Dq=2039 cm⁻¹, h=1.46 and k=0.21, respectively.     

Population inversion between the H4 and the F4 excited states of Tm investigated by means of numerical solutions of the rate equations system in Tm-doped and Tm, Ho-codoped fluoride glasses

Population inversion between the ³H₄ and the ³F₄ excited states of Tm³⁺ ions responsible for the 1.5 μm emission in Tm³⁺ singly doped (0.5%) and Tm³⁺, Ho³⁺-codoped fluoride (ZBLAN) glasses and its dependence on the Ho³⁺ concentration (x=0.2-1%) was investigated by means of numerical solution of the rate equations system for continuous pumping at 797 nm. Mean lifetimes of donor and acceptor states were evaluated by using the integration method applied to the best fitting of fluorescence curves previously reported. Lifetime values were used to obtain the rate constants of all non-radiative energy-transfer processes involved and a complete set of rate equations better describing the observations was given. The rate equations were solved by numerical method and the population inversion between the ³H₄ and the ³F₄ excited states of Tm³⁺ was calculated to examine the beneficial effects on the gain associated with Ho³⁺ codoping. The results have shown that Tm³⁺ population inversion is reached only for high Ho³⁺-codoping (?0.3 mol%). Highest population inversion (∼1.6×10¹⁸ Tm³⁺ ions cm⁻³) was obtained in Tm(0.5%), Ho(1%)-codoped (ZBLAN) pumped by 2.8 kW cm⁻². This population inversion density is ∼6.4 times higher than that one observed in Tm:Tb:GLKZ, Tm:Tb:Ge-Ga-As-S-CsBr and Tm:Ho:Ge-Ga-As-S-CsBr for a similar pumping condition (∼2.5×10¹⁷ cm⁻³). In addition, Tm(0.5%):Ho(1%):ZBLAN presents the highest population inversion that linearly increases with the pumping intensity; this behavior does not show saturation effect at least for the maximum intensity of 12 kW cm⁻² employed. The use of 1 mol% of Ho³⁺-codoping maximizes the potential gain of Tm³⁺-doped (0.5%) ZBLAN to produce stimulated emission near 1.5 μm, making this material suitable for using it as fiber optical amplifier and/or fiber laser operating in 1.4-1.5 μm region of the spectrum.