Neutral Acceptor-bound Exciton Research Articles

PL Study on the Effect of Cu on the Front Side Luminescence of CdTe/CdS Solar Cells

The effect of Cu on highly efficient CdTe thin solid film cells with a glass/TCO/CdS/CdTe structure subjected to CdCl2 treatment was investigated by low-temperature photoluminescence (PL). The PL of the CdS/CdTe junction in samples without Cu deposition revealed a large shift in the bound exciton position due to the formation of CdSxTe1−x alloys with Eg (alloy) ≅ 1.557 eV at the interface region. After Cu deposition on the CdTe layer and subsequent heat treatment, a neutral acceptor-bound exciton (A0Cu,X) line at 1.59 eV and two additional band-edge peaks at 1.54 and 1.56 eV were observed, indicating an increase in the energy gap value in the vicinity of the CdTe/CdS interface to that characteristic of bulk CdTe. These results may suggest the disappearance of the intermixing phase at the CdTe/CdS interface due to the presence of Cu atoms in the junction area and the interaction of the Cu with sulfur atoms. Furthermore, an increase in the intensity of CdS-related peaks in Cu-doped samples was observed, implying that Cu atoms were incorporated into CdS after heat treatment.

Role of Pzn-2Vzn centre on the luminescence properties of phosphorus doped ZnO thin films by varying doping concentration

Zinc oxide (ZnO) thin films were deposited using RF sputtering followed by plasma immersion ion implantation for 10–70 s to dope phosphorus on ZnO. Optical and structural characteristics from particular experiments (Photoluminescence (PL)), High resolution X-ray diffraction (HRXRD) and energy dispersive x-ray spectroscopy (EDX) confirms presence of phosphorus doping in implanted ZnO thin films. Dominant peak at around 3.35 eV measured from low temperature PL (18 K) measurement which assign as neutral acceptor-bound exciton peak. Two peaks: free electron acceptor and donor bound acceptor were observed at around 3.32 and 3.25 eV. The acceptor level was observed at 107 meV above the valence band. All samples exhibited dominant (002) crystal orientation peak in HRXRD measurements, indicating perfect c-axis orientation. Highest carrier (hole) concentration of 9.94 × 1017 cm−3 was measured from 60 s implanted sample. Formation of complex acceptor PZn-2VZn was justified by EDX measurements.

Low-temperature photoluminescence study of ZnO:Ni nanowires

ZnO:Ni nanowires have been fabricated through high-temperature vapor–solid deposition process. The morphology of the as-prepared samples was characterized by scanning electron microscopy. The temperature dependent of photoluminescence spectra was investigated. The low-temperature photoluminescence spectrum at 10–150 K shows that there is a multi-peak emission in the UV region. The multi-peak emissions are attributed to neutral acceptor-bound exciton, free electron to acceptor transition (FA), and first- and second-longitudinal optical phonon replicas emission (FA-1LO and FA-2LO), respectively. With increasing temperature, these bands show different temperature dependences. The FA and FA-1LO bands show a normal redshift with the increasing temperature, while the FX and FA-2LO bands exhibit an anomalous behavior. The origins of these bands and their temperature-dependent shifts are discussed.

Zn vacancies creation via (2 × 2) surface reconstruction

Zinc vacancies have been incorporated in ZnO via (2 × 2) surface reconstruction during the epitaxy growth. Evolution of in situ reflection high-energy electron diffraction (RHEED) patterns indicate Zn-polar surface (2 × 2) reconstruction was realized under extreme O-rich conditions. Atom sized triangular pits and islands were presented by the high resolution scanning tunneling microscopy (STM) images, which stand for a single Zn vacancy per (2 × 2) surface unit cell. The neutral acceptor-bound exciton (A0,X), peak located at 3.344 eV, and red luminescence (RL), maximum emission intensity at ~1.76 eV, which related to Zn vacancy, have been studied by variable-temperature/power steady-state photoluminescence (PL) spectroscopy.

Annealing effects on Cd0.96Zn0.04Te crystals with Te inclusions probed by photoluminescence spectroscopy

With effectively improved spectral resolution and signal-to-noise ratio, PL features are well resolved in the Te- and Cd-rich CdZnTe crystals and ascribed to the neutral donor-bound exciton (DX), acceptor-bound exciton (AX), A-center, and deep donor–acceptor pair. A detailed analysis indicates that (i) Cd pressure annealing drives the Te inclusions to the surface region of the crystal, destroys the inclusions by excess Cd atoms, and releases abundant donor-like and acceptor-like impurities; (ii) the PL features of the deep energy levels at 1.30–1.55 eV originate in the A-center consisting Cd vacancy and one type shallow donor, and the PL feature at 1.474 eV is the so-called SA luminescence of a donor–acceptor transition between an A-center and another shallow donor. A quantitative description of the recombination scheme is established, and by which the effects of the annealing are clarified.

PREPARATION AND PHOTOLUMINESCENCE PROPERTIES OF RF-SPUTTERED ZnO FILMS

<p class="TRANSAffiliation"><span>ZnO/Si films were prepared by radio frequency (RF) magnetron sputtering at room temperature. By optimizing the heat treatment conditions, we obtained a good quality film annealed at 700 ºC for longer 60 minutes. This process was monitored carefully by Raman scattering spectroscopy, and X-ray diffraction. The photoluminescence study on this film revealed that only ultraviolet emissions due to donor-acceptor pair (DAP), neutral acceptor-bound exciton (AºX) and donor-bound exciton (DºX) were observed. The intensity and peak position of these emissions depend on the measurement temperature and excitation power density.</span></p>

Phosphorus Concentration Dependent Microstructure and Optical Property of ZnO Nanowires Grown by High-Pressure Pulsed Laser Deposition

Phosphorus-doped ZnO (ZnO:P) nanowires were grown by the high-pressure pulsed laser deposition process (HP-PLD), where phosphorus pentoxide is used as the dopant source. The morphology, composition, and microstructural changes of ZnO nanowires after phosphorus doping were investigated with scanning electron microscopy, X-ray diffraction spectrum, energy-dispersive X-ray spectrum, transmission electron microscope, and Raman scattering spectrum. Optical fingerprints of ZnO:P nanowires like neutral acceptor-bound exciton emission (3.357 eV, A0X), free-electron to neutral-acceptor emission (3.311 eV, FA), and their longitudinal optical (LO) phonon replicas were observed, and their dependence on the phosphorus doping concentration was investigated with room/low-temperature photoluminescence spectra. It indicates that acceptor levels with a binding energy of about 130 meV were formed, and the optimized phosphorus concentration was realized with the ZnO:P2O5 (2 wt %) target.

P-type Sodium-doped Zinc Oxide Nanowire Arrays Grown by High-pressure Pulsed Laser Deposition

Sodium-doped ZnO (ZnO:Na) nanowire arrays were grown with high-pressure pulsed laser deposition (HP-PLD). The influence of growth pressure and thickness of gold catalyst layer on the growth of ZnO:Na nanowires were systemically studied. It is found that c-orientated ZnO nanowire arrays grow on single crystal silicon substrates under the optimized condition, e.g. gold catalyst layer’s thickness of 4.2 nm, growth pressure of 3.33×10 Pa and growth temperature of 875 °C. X-ray diffraction pattern and X-ray photoelectron spectroscope analyses indicate that Na is introduced into ZnO nanowires successfully. Optical fingerprints of sodium-related acceptors in the low-temperature (15 K) photoluminescence spectrum are observed, such as neutral acceptor-bound exciton emission (3.356 eV, AX), free-electron to neutral-acceptor emission (3.312 eV, (e, A)), and donor-to-acceptor pair emission (3.233 eV, DAP). ZnO:Na nanowire arrays grown on ZnO:Al/sapphire substrates form the pn junction. The corresponding I-V curve measurements exhibit a clear rectifying behavior of pn homojunction, which further indicates that such ZnO:Na nanowire is of p-type conductivity. 156 无 机 材 料 学 报 第 29卷

Characterizations of arsenic-doped zinc oxide films produced by atmospheric metal-organic chemical vapor deposition

p-type ZnO films were prepared by atmospheric metal-organic chemical vapor deposition technique using arsine (AsH3) as the doping source. The electrical and optical properties of arsenic-doped ZnO (ZnO:As) films fabricated at 450–600°C with various AsH3 flow rates ranging from 8 to 21.34μmol/min were analyzed and compared. Hall measurements indicate that stable p-type ZnO films with hole concentrations varying from 7.2×1015 to 5.8×1018cm−3 could be obtained. Besides, low temperature (17K) photoluminescence spectra of all ZnO:As films also demonstrate the dominance of the line related to the neutral acceptor-bound exciton. Moreover, the elemental identity and chemical bonding information for ZnO:As films were examined by X-ray photoelectron spectroscopy. Based on the results obtained, the effects of doping conditions on the mechanism responsible for the p-type conduction were studied. Conclusively, a simple technique to fabricate good-quality p-type ZnO films has been recognized in this work. Depositing the film at 550°C with an AsH3 flow rate of 13.72μmol/min is appropriate for producing hole concentrations on the order of 1017cm−3 for it. Ultimately, by increasing the AsH3 flow rate to 21.34μmol/min for doping and depositing the film at 600°C, ZnO:As films with a hole concentration over 5×1018cm−3 together with a mobility of 1.93cm2V−1s−1 and a resistivity of 0.494ohm-cm can be achieved.

Induction of p-type conduction in sputtered deposited Al-N codoped ZnO thin films

Al and N codoped ZnO thin films were grown on n-Si (100) substrate by sputtering technique. Hall effect measurements of as-grown films exhibited n-type conduction, however 500 °C Ar annealed codoped films showed p-type conductivity with a hole concentration of 9.9 × 10 16 cm − 3 , resistivity of 15.95 Ω-cm and hole mobility of 3.95 cm 2/Vs, respectively. Codoped ZnO thin films were found to be highly c-axis oriented with good crystal quality. A neutral acceptor-bound exciton and donor–acceptor-pair emissions that appeared at room temperature photoluminescence measurement verify p-type conduction in Al and N codoped ZnO film. The current–voltage characteristics of p–n heterojunction evidently showed a diode like rectifying behaviour.

Determination of electrical types in the P-doped ZnO thin films by the control of ambient gas flow

Phosphorus (P)-doped ZnO thin films with amphoteric doping behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering with various argon/oxygen gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio without post-annealing. The P-doped ZnO films grown at a argon/oxygen ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of 1.5 × 10 17 cm −3 and 2.5 cm 2/V s, respectively. X-ray diffraction showed that the ZnO (0 0 0 2) peak shifted to lower angle due to the positioning of P 3− ions with a larger ionic radius in the O 2− sites. This indicates that a p-type mechanism was due to the substitutional P O. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/ n-Si heterojunction light emitting diode showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.

Plasma-free nitrogen doping and homojunction light-emitting diodes based on ZnO

The authors develop a plasma-free metalorganic chemical vapour deposition method to grow N-doped p-type ZnO films. The incorporation of the N acceptor and the corresponding change in the Fermi level are well confirmed by x-ray photoelectron spectroscopy. Temperature-dependent photoluminescence reveals the acceptor-related emissions, namely, neutral acceptor-bound exciton and probably donor–acceptor pair transition. In addition, typical rectifying I–V characteristics and room-temperature electroluminescence from ZnO homojunction light-emitting diodes are demonstrated.

Phosphorus acceptor doped ZnO nanowires prepared by pulsed-laser deposition

Phosphorus-doped ZnO (ZnO:P) nanowires were successfully prepared by a novelhigh-pressure pulsed-laser deposition process using phosphorus pentoxide as the dopantsource. Detailed cathodoluminescence studies of single ZnO:P nanowires revealedcharacteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission(A0, X, 3.356 eV), free-to-neutral-acceptor emission (e,A0, 3.314 eV), and donor-to-acceptor pair emission (DAP,∼3.24 and ∼3.04 eV). This means that stable acceptor levels with a binding energy of about 122 meV havebeen induced in the nanowires by phosphorus doping. Moreover, the induced acceptors aredistributed homogeneously along the doped nanowires.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method

Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 10 19–20 cm −3. Current–voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A 0X) emission and a donor–acceptor pair (DAP) emission due to doped nitrogen, respectively.

Coexistence of bound and localized exciton magnetic polarons inCd0.8Mn0.2Te

Fundamental magneto-optical properties of single crystals of the semimagnetic semiconductorCd0.8Mn0.2Te are carefully measured with the use of band-to-band excitation. Two new phenomena arepresented: the peculiar magnetic field dependence of the magnetic polaron luminescenceand the existence of two components for the polaron luminescence. Theoreticalconsideration with regard to the Zeeman effect ascribes the two components ofluminescence to the exciton magnetic polaron spectrum and the neutral acceptor-boundexciton magnetic polaron spectrum. The calculated result for the luminescence peakpositions is also consistent with the experimental result of the circularly polarizedluminescence under the presence of an applied magnetic field. In other words, theσ+ luminescence is due to the exciton magnetic polaron, whereas theσ− luminescence is due to the neutral acceptor-bound exciton magnetic polaron.

Growing the epitaxial undoped and N-doped ZnO films by radical beam gettering epitaxy

Epitaxial undoped and N-doped ZnO films were obtained using the method of radical beam gettering epitaxy. Structural and luminescent properties of the obtained films were researched. In both cases, there can be seen orientation of the films along c- axis. In the spectrum of low-temperature photoluminescence of N-doped ZnO films, observed was a peak 3.31 eV probably of a neutral acceptor-bound exciton NO. The nature of donor-acceptor band 3.23 eV and green band 2.56 eV was discussed.

The mechanism of formation and properties of Li-doped p-type ZnO grown by a two-step heat treatment

Li-doped p-type ZnO was fabricated by heat treatment of Zn–Li alloy film with 2 at% Li on a quartz substrate in N2 flow at 500 °C for 2 h, and then in O2 flow at 700 °C for 1 h. The room-temperature resistivity was measured to be 678.34 Ω cm with a Hall mobility of 1.03 cm2 V−1 s−1 and a carrier concentration of 8.934 × 1015 cm−3. Three emission peaks centred at 3.347, 3.302 and 3.234 eV are observed in the photoluminescence spectrum measured at 12 K and are due to neutral acceptor-bound exciton emission, conduction band to acceptor level transition and donor–acceptor pair recombination emission, respectively. The p-type conduction of the Li-doped ZnO may be attributed to the formation of a LiZn–N complex acceptor. The optical level of the acceptor is estimated to be about 137 meV. The mechanism of formation of the Li-doped p-type ZnO is discussed in the present work.

Superdiffusion in semiconductors: Dynamics of radiation-enhanced superdiffusion

A three-layered system composed of GaAs (layer 3)/Si(Zn)//Si(Zn)/GaAs (layer 1) was used in this study. Si(Zn)/GaAs consists of Si(Zn) evaporation-deposited on GaAs wafers. The overlying layer was in contact with only one other Si(Zn) layer. The surface of layer 3 (GaAs) was irradiated with 750 keV and 7 MeV electrons, respectively. Electron irradiation of the wafer (layer 3) created Frenkel defects and electron–hole pairs. Interstitials of displaced atoms in the overlayer migrated to the substrate interface. Mobility-enhanced diffusion by recombination of electron–hole pairs can also occur. Residual defects in layers 1 and 3 after irradiation were athermally annealed by electron–hole recombination-enhanced defect reactions. Furthermore, new, sharp photoluminescence spectra of the neutral acceptor-bound exciton peaks appear in both layers 1 and 3, without thermal annealing.

MOCVD growth of monomethylhydrazine-doped ZnO layers

Nitrogen (N)-doped ZnO layers were grown using diethylzinc (DEZ), water (H 2O) and monomethylhydrazine (MMHy) as Zn, O and N precursors, respectively. Raman spectra revealed N-related vibrational modes at 275, 510, 582 and 643 cm −1 in addition to the host phonons of ZnO. The intensity of these additional modes was increased with increasing the amount of MMHy. These ZnO samples were thermally annealed under an oxygen ambient to activate N as an acceptor. By increasing the annealing temperature, the intensity of additional modes was decreased, which indicates the dissociation and re-evaporation of N complexes. By capacitance–voltage ( C– V) measurements, it was observed that the capacitance was increased by increasing the positive bias voltage for N-doped as-grown ZnO layer, which indicates that this layer is n-type with donor concentration of 1×10 16 cm −3. In contrast, the capacitance was decreased by increasing the positive bias voltage for N-doped ZnO layer annealed at 800 °C. In addition, the neutral acceptor-bound exciton (A 0X) emission and donor–acceptor pair (DAP) emission were observed in N-doped ZnO layer annealed above 800 °C by low-temperature photoluminescence (PL) measurements.