Photoluminesence studies of CdTe/SnO2 and CdTe/CdS heterojunctions: The influence of oxygen and the CdCl2 heat treatment

The influence of oxygen and annealing in the presence of CdCl₂ on the photoluminescence (PL) spectra of CdTe, component of SnO₂/CdTe heterojunction (HJ), has been studied in a temperature range of 17-100 K. The changes in the photoluminescence spectra were studied as a function of excitation intensity. Analysis of the PL spectra was carried out with considerations of spectra obtained from CdS/CdTe heterojunctions. CdTe side PL (SnO₂/CdTe HJ) consisted of 1.450 eV-DA defect band and 1.243 eV band (17 K). Annealing resulted in the disappearance of 1.243 eV band in oxygen containing samples. Interface PL for the unannealed samples consisted of mainly the 1.264 eV and a trace of the defect band. The CdCl₂ treatment is responsible for an almost symmetrical 1.416 eV band.     

Photoluminescence studies of CdTe films and junctions

Device quality CdTe films and junctions have been studied using low-temperature photoluminescence (PL) measurements. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. The CdTe films and junctions were prepared under various deposition conditions to determine the effect of film deposition and solar cell fabrication parameters, such as the effect of oxygen, and chloride treatment. A PL band located at 1.232 eV has been attributed to the presence of oxygen. This band is present only in as-deposited samples excited at the CdTe surface. Samples annealed in the presence of CdCl₂ exhibit a single PL band located at 1.42 eV. A model explaining the behavior of these bands is presented.     

Synthesis and characterization of 10% Sb doped CdTe thin films by stacked elemental layer (SEL) method

Antimony doped CdTe thin films have been prepared by stacked elemental layer (SEL) method. The X-ray diffraction spectra have demonstrated that the structure of the annealed films are polycrystalline in nature and mixed CdTe and Sb₂Te₃ phases have been observed at high annealing temperature (500 °C). The increased texture coefficient has been observed for planes (311) and (220) rather than (111) plane of CdTe in annealed stack (Te/Cd/Sb). Transmission spectra have been recorded and the calculated band gap lies between 1.45 and 1.68 eV. A pronounced PL spectrum has been noticed at 532 nm and depicted the presence of nano size particles in annealed thin films.     

Role of thermal treatment on the luminescence properties of CdTe thin films for photovoltaic applications

The efficiency of CdTe based solar cells is strongly enhanced by a thermal treatment in HCF₂Cl ambient. CdTe thin films deposited on CdS/ZnO/ITO/glass by Closed Space Sublimation before and after the annealing are characterised. The CdTe morphology is studied by atomic force microscopy and scanning electron microscopy. In the treated films the non-homogeneous distribution of the grain size disappears, in addition an increasing of the dimensions of the grains is observed. Cathodoluminescence analyses show a remarkable difference in the spectra between the treated and untreated structures. A strong increase in the intensity of the 1.4 eV band is observed by increasing the HCF₂Cl content. A model of the electronic levels inside the CdTe band gap, due to incorporation of Cl (or F) is proposed.     

1.4 eV photoluminescence in chlorine-doped polycrystalline CdTe with a high density of defects

The 1.4 eV photoluminescence (PL) band in a polycrystalline CdTe : Cl with a high density of defects was studied as a function of the chlorine dopant concentration and temperature. For a material with a high density of defects this band has a smooth, non-symmetrical shape without any apparent phonon structure. The energy of the intensity maximum of the 1.4 eV band depends on the concentration of chlorine and varies from 1.389 to 1.408 eV. The temperature quenching of the PL intensity for all samples was measured and the activation energy (ET) was found. Minimum and maximum values of the ET were 0.10 and 0.20 eV, respectively. A configurational – coordinate model is proposed for the 1.4 eV PL band in which the excited state of the recombination centre lies within the conduction band. In this model the temperature quenching of the PL is pictured as being essentially due to an electronic transition from the excited state directly to the ground state (the internal mechanism). This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of thermal annealing on the structural and optical properties of CdTe (111)/GaAs (100) heterostructures

Reflection high-energy electron diffraction (RHEED), double-crystal X-ray rocking curve (DCRC), and photoluminescence (PL) measurements were performed to investigate the effect of thermal annealing on the structural and the optical properties of CdTe (111) epilayers grown on GaAs (100) substrates by molecular beam epitaxy (MBE) at low temperature. The results of the RHEED patterns showed that the oxidized layer on the GaAs substrate was removed in a Te atmosphere, and that the 20-Å CdTe layer was grown by three-dimensional process. When the rapid thermal annealing (RTA) was performed at 500°C for 14 s, the FWHM of the DCRC for the CdTe layer had the smallest value. After the RTA process, the luminescence intensity of the exciton remarkably increased, and the peak at 1.476 eV was dominant. As the RTA temperature increased, the luminescence intensity of the exciton peak related to neutral acceptors (A°, X) increased. The temperature dependence of the spectra showed that the (A°, X) peak originated from the recombination of the excitons bound in high-density defects. The excitation power intensity dependence of the PL spectra showed that the peaks of the transitions due to donor-acceptor pairs shifted to larger energies. These results indicate that the structural and the optical properties of the CdTe epilayers grown on GaAs (100) are improved by RTA, and that the RTA process is very useful for the growth of Hg_xCd_1−xTe on CdTe/GaAs heterostructures.     

Utilization of residual CdCl2 in CBD-CdS to realize grain growth in CdTe: A novel route

CdTe films were deposited by thermal evaporation onto chemical bath deposited CdS (CBD-CdS) films. The composite films were subjected to rapid thermal annealing (RTA) to observe simultaneous grain growth in both the CdS and CdTe layers. The films were characterized by measuring the compositional, microstructural and photoluminescence (PL) properties. PL spectra is dominated by the characteristic peaks (∼1.42 eV and ∼1.26 eV) associated with the virgin CdTe film. Additional features located at ∼2.56 eV and ∼1.99 eV could also be detected. The Fourier Transform Infra Red (FTIR) peak at ∼482 cm⁻¹ appeared due to the simultaneous presence of absorption peaks for CdTe stretching mode as well as Cd-S modes. Appearance of the broad peak between 1000 cm⁻¹ and 1165 cm⁻¹ may be an indication of interfacial alloying. Secondary ion mass Spectroscopy (SIMS) measurements were done to observe the compositional uniformity in the film and to measure the interfacial mixing behaviour.     

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO₂ (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd_1 − xZn_xTe ternary compound films have been studied. In order to improve the back contact layer, Cd_0.4Zn_0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.     

Yellow luminescence band in undoped GaN revealed by two-wavelength excited photoluminescence

The below-gap emission components including yellow luminescence (YL) band of an MOCVD grown undoped GaN have been studied by the two-wavelength-excited photoluminescence (TWEPL). The nature of each emission line has been investigated by using an intermittent below-gap excitation (BGE) light of 1.17 eV on an above-gap excitation (AGE) light of 3.49 eV. The intensity of DAP and the YL decreased while it increased for I_OX after irradiation of the BGE. The intensity change in PL after addition of the BGE implies the presence of defect levels in the energy position corresponding to the photon energy of the BGE. Possible recombination models are listed and examined. Only the recombination model in which the YL corresponds to the transition from a shallow donor to a deep state at about 1 eV above the valence band maximum satisfies our experimental result. The possible origin of this defect state is discussed.     

CdTe thin film technology: Leading thin film PV into the future

CdTe is a near perfect material for PV application with a direct band gap of ～1.5 eV that is closely matched to the terrestrial solar spectrum and a high optical absorption coefficient where less than 1 μm thickness is adequate to absorb the incident light. CdTe thin film solar cell and module technology has validated the economies of scale that were projected for thin film PV technologies since the early 1980s where manufacturing costs are now below $0.84 with module efficiencies of 11.1%. Additionally, the low-temperature coefficient of CdTe modules results in a high annualized output. A critical issue for CdTe manufacturers is that there is not a clear pathway to increase the module performance to 15% or beyond based on current laboratory results and efficiency improvements will require fundamental improvements in the CdTe semiconductor properties and/or developing an alternative device structure.     

Growth of cubic and hexagonal CdTe thin films by pulsed laser deposition

The paper reports the growth of cadmium telluride (CdTe) thin films by pulsed laser deposition (PLD) using excimer laser (KrF, λ=248 nm, 10 Hz) on corning 7059 glass and SnO₂-coated glass (SnO₂/glass) substrates at different substrate temperatures (T_s) and at different laser energy pulses. Single crystal target CdTe was used for deposition of thin films. With 30 min deposition time, 1.8- to ∼3-μm-thick films were obtained up to 200 °C substrate temperature. However, the film re-evaporates from the substrate surface at temperatures >275 °C. Atomic force microscopy (AFM) shows an average grain size ∼0.3 μm. X-ray diffraction analysis confirms the formation of CdTe cubic phase at all pulse energies except at 200 mJ. At 200 mJ laser energy, the films show hexagonal phase. Optical properties of CdTe were also investigated and the band gap of CdTe films were found as 1.54 eV for hexagonal phase and ∼1.6 eV for cubic phase.     

Preparation of CdTe thin films by laser irradiation of electrodeposits

We describe here the first attempts to electrodeposit cadmium and tellurium layers onto a glass substrate covered with indium tin oxide with the intention of synthesizing CdTe by laser irradiation of the deposit.We successively deposited tellurium and cadmium cathodically in aqueous solutions of H₂SO₄ and TeO₂ or of CdSO₄ respectively. The deposits were then laser irradiated and CdTe was formed.The as-formed CdTe, of cubic crystallographic structure, exhibits a band edge for direct transitions of about 1.5 eV in agreement with the band gap of CdTe.     

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.     

Luminescence study of donors and acceptors in CdGeAs2

A photoluminescence (PL) study has been performed on a set of p-type bulk single crystalline samples of CdGeAs₂. At liquid-helium temperatures, a PL band peaking near 0.55 eV was observed in samples with enhanced absorption at 5.5 μm. This PL band was observed to vary as much as 40 meV in peak position in our sample set. The PL peak position depends on the net acceptor concentration (i.e., hole concentration) and the PL peak shifts to higher energy as the excitation intensity increases. We show that this behavior is well explained by the model of donor–acceptor pair (DAP) recombination in the presence of potential fluctuations.     

Optical properties of the low-energy Ge-implanted and annealed SiO2 films

Ultraviolet–visible light emissions from nanocrystalline (nc) Ge-embedded SiO₂ films fabricated by ion implantation and rapid annealing techniques are studied as a function of different fabricating conditions (implanting dose and annealing temperature). The samples exhibit seven photoluminescence (PL) peaks peaked at 1.68, 1.75, 1.84, 1.93, 2.00, 2.70, and 3.10 eV. There are also two excitation bands in the PL excitation (PLE) spectra peaked at 4.90 and 5.19 eV. Raman spectra are employed to observe and understand structural variations in SiO₂ matrix during the formation of nc-Ge and defects. Within the frame of the quantum confinement (QC) theory and Oswald ripening growth model, the origin and evolution of the seven PL peaks and two PLE bands are identified clearly demonstrated by the proposed energy-level and generated process diagrams. Our results indicate that both the implanting dose and annealing temperature play a dominant role in modulating the optical properties of the nc-Ge-embedded SiO₂ films.     

Enhancement of photoluminescence in Er-doped Ag-SiO2 nanocomposite thin films: A post annealing study

Silver-silica (with Ag 2.8 at.% and 8.7 at.%) nanocomposite (NC) thin films doped with Er⁺³ (0.1-0.9 at.%) were synthesized by atom beam co-sputtering using 1.5 keV Ar atoms. Optical absorption and photoluminescence (PL) studies of pristine and annealed films were performed, together with Rutherford backscattering and secondary ion mass spectroscopy studies for elemental characterization of the NC films. Optical absorption results of pristine and annealed NC film (with Ag ∼8.7 at.%) confirmed the formation of Ag nanoparticles evidenced by the appearance of characteristic surface plasmon resonance absorption features. Photoluminescence (PL) studies, carried out using Ar Laser pumping at 0.488 μm, the wavelength that the Er ions can absorb resonantly, indicated the presence of PL emission around 1.54 μm in the case of all the as-synthesized samples. The observed PL peak corresponds to the atomic transitions of Er as reported in literature. A relative enhancement in the intensity of PL peak has been observed after annealing the NC films. In the case of NC film with 0.9 at.% Er and 8.7 at.% Ag, the enhancement in PL intensity is almost twice, with respect to the as-deposited sample, for a heat treatment of about 1 h at 600 °C in a nitrogen atmosphere. However for a NC film with Er 0.1 at.% + Ag 2.8 at.%, the PL intensity is enhanced by approximately 3.7 times after annealing at 400 °C for 1 h in nitrogen atmosphere. Since the samples with surface plasmon resonance (SPR) did not show the PL enhancement, the role of SPR in the enhancement of the PL is ruled out. The enhanced PL emission from Er and Ag codoped silica indicates that the Er photo-stimulation is mediated by the energy transfer from Ag nanostructures or ions to Er. Lifetimes of PL peaks for the pristine and annealed samples were also studied. The observed lifetime ∼10 ms is a good indication of excellent PL efficiency.     

Synthesis,strong room-temperature PL and photocatalytic activity of ZnO/ZnWO4 rod-like nanoparticles

Zinc oxide (ZnO)/zinc tungstate (ZnWO₄) rod-like nanoparticles with diameters in the range of 6–11 nm and length of about 30 nm were synthesized by a low temperature soft solution method at 95 °C in the presence of non-ionic copolymer surfactant. It was found that their crystallinity was enhanced with the increase of heating time from 1 h up to 120 h. The photoluminescence (PL) measurements showed very strong, narrow UV band peaked at 3.30 eV and a broad visible band peaking at 2.71 eV with a shoulder at about 2.53 eV, for λ_exc < 300 nm. Quite large variations in the intensities of the two PL bands were observed for different excitation wavelengths. The intensity of the main visible band decreases with decreasing excitation energy and disappears when samples are excited λ = 320 nm (E_exc = 3.875 eV). We found that observed optical properties originate from ZnO phase. UV band gap PL had high intensity for all applied excitations, probably induced by ZnWO₄ phase presence on the surface. In addition, two values were found for direct band-gap energy of ZnO/ZnWO₄ rod-like nanoparticles 3.62 and 3.21 eV, determined from reflectance spectrum. The photocatalytic behaviour of ZnO is strongly dependent on the formation of ZnWO₄ phase, of the obtained rod-like nanoparticles.     

Facile fabrication of CdTe/montmorillonite nanocomposite films with stable photoluminescence properties

We report herein the facile synthesis of CdTe/montmorillonite (MMT) nanocomposite films via charge-charge interactions between the CdTe quantum dots (QDs) and MMT platelets. Firstly, negatively charged CdTe QDs were prepared with the use of thioglycolic acid (TGA) as ligand. Then hybrid of the as-prepared TGA-stabilized CdTe QDs with sodium montmorillonite (Na-MMT) and cetyltrimethylammonium-modified montmorillonite (CTA-MMT), respectively, afforded novel CdTe/MMT nanocomposite films. The structure and optical properties of CdTe/MMT nanocomposite films were thoroughly investigated by scanning electron microscope (SEM), ultraviolet transmittance reflection and photoluminescence (PL) measurements. Results showed that CdTe/(CTA-MMT) nanocomposite films exhibited highly enhanced PL intensity compared with CdTe/(Na-MMT) nanocomposite films. More importantly, CdTe QDs in CdTe/(CTA-MMT) nanocomposite films well maintained PL properties even after thermal annealing at 100 °C for 10 h.     

Synthesis of highly luminescent CdTe/ZnO core/shell quantum dots in aqueous solution

The synthesis and photoluminescence (PL) properties of aqueous CdTe/ZnO core/shell quantum dots (QDs) have been investigated by using thiolglycolic acid as a capping reagent. The highlighted contribution of the present study was CdTe QDs coated with a ZnO shell by controlling the hydrolysis process of Zn(OAc)₂. The QDs benefitted from overcoming the high lattice mismatch between CdTe and ZnO. The PL peak wavelength of the CdTe/ZnO QDs with high PL quantum yields up to 88% was located in a range between 547 and 596 nm by adjusting the size of CdTe cores and the thickness of ZnO shells. The results of X-ray diffraction analysis and transmission electron microscopy observation indicate that the dot-shaped CdTe/ZnO QDs (566 nm) with an average size of 2.2 nm in diameter belong to the cubic CdTe crystal structure. Due to the passivation of surface defects, it is found that the luminescence decay curves accord with a biexponential decay model of exciton and trap radiation behavior. The average PL lifetimes of CdTe (571 nm) and CdTe/ZnO (596 nm) QDs at room temperature are 27.3 and 35.1 ns, respectively.     

Effect of high-temperature annealing on the optical and photoluminescence properties of nanocrystalline SiC films

The optical and photoluminescence (PL) properties of nanocrystalline 3C-SiC films and the effect of the boundary regions between the nanocrystals were studied for two sets of films: (a) films with 10-15 nm nanocrystal size obtained by direct ion deposition method and (b) similar films annealed in oxygen at 850-950 °C. It was shown that annealing of the nanocrystalline SiC films resulted in weaker absorption in a broad spectral range, and to the increase of the optical band gap from 1.8 to 2.2 eV. On the contrary, the edge PL bands in the UV range (2.2 to 2.4 eV) remained similar. In the IR range, three maxima absent in the as-grown films, appeared at 1.52 eV, 1.56 eV and 1.63 eV. Measurement of the intensity of PL maxima as a function of the excitation power showed a nonlinear dependence that was attributed to the onset of stimulated emission.