Donor-acceptor Pair Emission Research Articles

Effect of Mg Doping on the Photoluminescence of GaN:Mg Films by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

We investigate undoped GaN and Mg-doped GaN grown by rf plasma-assisted molecular beam epitaxy (MBE) with different Mg concentrations by photoluminescence (PL) at low temperature, Hall-effect and XRD measurements. In the PL spectra of lightly Mg-doped GaN films, a low intensity near band edge (NBE) emission and strong donor-acceptor pair (DAP) emission with its phonon replicas are observed. As the Mg concentration is increased, the DAP and NBE bands become weaker and a red shift of these bands is observed in the PL spectra. Yellow luminescence (YL) is observed in heavily Mg-doped GaN. The x-ray diffraction is employed to study the structure of the films. Hall measurement shows that there is a maximum value (3.9 × 1018 cm−3) of hole concentration with increasing Mg source temperature for compensation effect. PL spectra of undoped GaN are also studied under N-rich and Ga-rich growth conditions. Yellow luminescences of undoped Ga-rich GaN and heavily Mg-doped GaN are compared, indicating the different origins of the YL bands.

Dynamics of Bound Exciton Complexes in CdS Nanobelts

Intrinsic defects such as vacancies, interstitials, and anti-sites often introduce rich luminescent properties in II-VI semiconductor nanomaterials. A clear understanding of the dynamics of the defect-related excitons is particularly important for the design and optimization of nanoscale optoelectronic devices. In this paper, low-temperature steady-state and time-resolved photoluminescence (PL) spectroscopies have been carried out to investigate the emission of cadmium sulfide (CdS) nanobelts that originates from the radiative recombination of excitons bound to neutral donors (I(2)) and the spatially localized donor-acceptor pairs (DAP), in which the assignment is supported by first principle calculations. Our results verify that the shallow donors in CdS are contributed by sulfur vacancies while the acceptors are contributed by cadmium vacancies. At high excitation intensities, the DAP emission saturates and the PL is dominated by I(2) emission. Beyond a threshold power of approximately 5 μW, amplified spontaneous emission (ASE) of I(2) occurs. Further analysis shows that these intrinsic defects created long-lived (spin triplet) DAP trap states due to spin-polarized Cd vacancies which become saturated at intense carrier excitations.

The p-type ZnO film realized by a hydrothermal treatment method

Reproducible and high quality N-doped ZnO (ZnO:N) films were achieved by a hydrothermal treatment method. The ZnO:N films exhibited p-type characteristics by means of the Hall-effect and the photoluminescence measurements. At room temperature, the electrical properties of ZnO:N film showed a hole concentration of 1×1016 cm−3 and hole mobility of 8.6 cm2 V−1 s−1. At 83 K two acceptor related emission peaks could be observed located at 3.353 and 3.237 eV, which were assigned to the acceptor-bound exciton and the donor-acceptor pair emissions. This result gave a direct evidence for the generation of the acceptor energy level after the hydrothermal treatment process. Also, a ZnO homojunction diode was fabricated by this method, which displayed a good rectification characteristic at room temperature. This study revealed that the hydrothermal treatment method was effective and practicable in producing p-type ZnO.

Synthesis of p-type ZnSe nanowires by atmosphere compensating technique

The atmosphere compensating technique with an individual selenium source is, first, used in the growth of phosphorus-doped p-type ZnSe nanowires. The morphology and structure characterisations reveal that the as-synthesised ZnSe nanowires have a wurtzite structure with a diameter of about 160 nm, a growth direction of [001]. The electrical properties’ characterisations demonstrate that the selenium atmosphere compensation technique assisted with phosphorus-doping leads to a substantial action in p-type conductivity of ZnSe nanowires with a high mobility of 1.25 cm2 V−1 S−1 and carrier concentration of 1.47×1018 cm−3. The photoluminescence measurements show a dominant emission and two donor–acceptor pair emission.

Photoluminescence Study of Damage Introduced in GaN by Ar- and Kr-Plasmas Etching

ABSTRACTWe investigated, by employing a photoluminescence technique, the etching damage introduced in near-surface regions of GaN by Ar and Kr plasmas and clarified the differences between the damage characteristics of these regions for the two plasma etching cases. For Ar plasma, the shallow donor-acceptor pair emission at ~3.28 eV was significantly weakened; additionally, a broad blue luminescence band arose at approximately ~3.0 eV. In contrast, for Kr plasma under high gas pressure, we found the recovery of the damage to the same level as the as-grown crystallinity. These differences in the damage characteristics for the two plasma etching cases probably depend upon which atom (N or Ga) is preferentially etched in these cases.

Ionization energy of the phosphorus donor in 3C–SiC from the donor-acceptor pair emission

Donor-acceptor pair luminescence of P–Al and N–Al pairs in 3C–SiC is analyzed. The structures in the spectra corresponding to recombination of pairs at intermediate distances are fitted with theoretical spectra of type I (P–Al pairs) and type II (N–Al pairs). It is shown that in the regions chosen for fitting the line positions obey the equation ℏω(R)=EG−ED−EA+e2/εR, where ℏω(R) is the energy of the photon emitted by recombination of a pair at a distance R, e is the electron charge, ε is the static dielectric constant, and EG, ED, and EA are the electronic band gap and the donor and acceptor ionization energies, respectively. The fits yield the values EG−ED−EA for the N–Al (2094 meV) and P–Al (2100.1 meV) cases. Using the known value of the nitrogen ionization energy, 54.2 meV, phosphorus ionization energy of 48.1 meV is obtained. Identification of the sharp lines corresponding to recombination of close pairs in the P–Al spectrum is suggested.

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.

Fabrication and characterization of magnetron sputtered arsenic doped p-type ZnO epitaxial thin films

Arsenic doped p-type ZnO thin films were grown on sapphire substrate by magnetron sputtering. As grown films reveal p-type conduction confirmed by Hall-effect and photoluminescence measurements. The p-type film with a hole concentration of 2.16× 10 17 cm −3, mobility of 1.30 cm 2/V.s and resistivity of 22.29 Ω-m were obtained at substrate temperature of 700 °C. ZnO homojunction synthesized by in-situ deposition of As doped p-ZnO layer on Al doped n-ZnO layer showed p-n diode like characteristics. X-ray pole figure and Transmission Electron Microscope studies confirm epitaxial nature of the films. Photoluminescence results exhibit the peaks associated with donor acceptor pair emission.

Surface morphology and photoluminescence studies of Sb-doped ZnO layers grown using MOCVD

ZnO and ZnO:Sb films were deposited using low pressure metal organic chemical vapor deposition on C- and R-oriented sapphire and O-polar ZnO substrates. Surface morphologies were studied using scanning electron microscopy. Whereas ZnO films grown on C-sapphire show a rough surface and hexagonal rods, the doped ZnO:Sb layers exhibit a relatively smoother surface, indicating a possible surfactant effect of antimony. The secondary ion mass spectrometry measurements permit to determine the antimony profile in the doped layers. Sb concentrations from 10 18 to 10 20 at/cm 3 were measured, depending on the growth conditions and substrate nature. Photoluminescence spectra exhibit donor-acceptor pair emission at 3.22 eV.

A-plane GaN layer grown on (3 0 2) (-LiAlO 2 by MOCVD

Single phase a-GaN layer has been grown on the small lattice mismatch (3 0 2) (-LiAlO 2 (LAO) substrate. The epitaxial relationships between a-plane GaN and (3 0 2) LAO are [ 1 1 ¯ 0 0 ] GaN | | [ 2 0 3 ] LAO = − 2.86 % and [ 0 0 0 1 ] GaN | | [ 0 1 0 ] LAO = − 0.31 % . The diffraction angle for GaN ( 1 1 2 ¯ 0 ) layer presents a 0.12% extension parallel to the growth direction [ 1 1 2 ¯ 0 ] due to lattice mismatch and thermal mismatch. The compressive stress is gradually released with the a-GaN layer growth indicated by the E 2 frequency of Raman spectra. The a-GaN layer has high transmittance (85%), sharp absorption edge and observed excitonic absorption. The photoluminescence (PL) shows that the donor acceptor pair emission peak energy is ∼3.41 eV with full width at half maximum value (FWHM) of 105 meV and weak yellow peaks originate from the incorporation of Li into GaN as a result of the decomposition of the substrate while ramping up temperatures. These results show that (3 0 2) LAO is a promising substrate for fabricating high-quality a-GaN layer and device.

Optical charaterization of Ag/Ga composition ratio in AgGaSe2 thin film

AbstractAgGaSe2 thin films, changing the Ag/Ga ratio from 0.4 to 1.5, on glass substrates were successfully grown by vacuum evaporation method. Fundamental absorption bandedges were clearly observed except for Ag/Ga ratios of 0.4 and 1.5 in the optical transmittance spectra at RT. The optical bandgap increased with increasing Ag/Ga ratio. This was due to the Bernstein‐Moss shifts. Photoluminescence spectrum was strongly observed in the stoichiometric sample in comparison to Ag‐ and Ga‐rich samples. This means that nonradiative recombination transition was a few in the stoichiometric sample because there were few defects in the samples Two distinct peaks at 1.77 and 1.70 eV were clearly observed in the stoichiometric sample. They were due to bound exciton emission and donor‐acceptor pair (DAP) emission, respectively. The DAP emission is due to recombination between Se‐ and Ga vacancies. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure and optical properties of non‐doped and N‐doped ZnO films with non‐polar surfaces grown homoepitaxially on single crystal ZnO (11$ \bar 2 $0) substrates

AbstractNon‐doped ZnO films and nitrogen‐doped ZnO films (ZnO:N films) were grown epitaxially on non‐polar single crystal ZnO (11$ \bar 2 $0) substrates by using the plasma‐assisted reactive evaporation (PARE) method. The value of FWHM of the ω‐scan rocking curve of non‐doped ZnO (11$ \bar 2 $0) films were 26 to 62 arcseconds. Crystallinity of ZnO (11$ \bar 2 $0) films deteriorated with increase in N2 gas flow rate, but crystallinity of the films was better than that of ZnO (11$ \bar 2 $0) films grown on sapphire (11$ \bar 2 $0) substrates by the PARE method. Independently of N2 gas flow rate, AFM images of non‐polar ZnO films showed a smooth surface with about 1 nm of root mean square roughness. In near band‐edge PL spectra from these non‐doped ZnO films measured at 5 K, free‐exciton emission (FE) at about 3.378 eV, ionized‐donor‐bound excitons (D+X) at about 3.368 eV and neutral‐donor‐bound excitons (D0X) at about 3.362 eV were clearly observed. PL intensity of D+X of non‐polar ZnO:N films was decreased but that of donor‐acceptor pairs (DAP) emission at around 3.308 eV of the films increased with an increase in N2 gas flow rate. It is thought that N atoms occupied oxygen sites and surface state density were decreased with increase in N2 gas flow rate and that N atoms act as acceptors in ZnO. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation of Cu(In,Al)Se2 thin films by selenization using diethylselenide

AbstractSelenization growth of purely single‐phase, polycrystalline CuIn1–x Alx Se2 (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less‐hazardous metal‐organic selenide, diethylselenide [(C2H5)2Se: DESe]. Without additional thermal annealing, approximately 2.0‐μm‐thick alloy films exhibiting X‐ray diffraction peaks originating exclusively from the chalcopyrite structure were obtained. Their low temperature photoluminescence spectra were dominated by characteristic donor‐acceptor pair emissions usually seen in the state‐of‐the‐art CuInAlSe2 photoabsorbing layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heteroepitaxial growth and luminescence properties of non-polar (1 1 0) orientation ZnO films on Si(0 0 1) substrates by pulsed laser deposition

Epitaxial growth of a ZnO(1 1 0)/STO(0 0 1)/TiN(0 0 1)/Si(0 0 1) heterostructure has first been demonstrated on a Si(0 0 1) substrate by pulsed laser deposition. The growth process was monitored in situ by reflection high-energy electron diffraction, and the epitaxial orientation relationship was further confirmed by ex situ x-ray diffraction analysis. Results show that the high (1 1 0) oriented ZnO films are in domain-growth mode, and composed of two kinds of domains with their c-axes perpendicular to each other along STO⟨1 1 0⟩. The excitation-density-dependent photoluminescence spectra analyses at 83 K indicate that the ZnO films have good optical quality with low defect density, and no significant deep-level emission is detected. The strong near-band-edge luminescence consists of several acceptor-related emissions such as the acceptor–bound exciton emission (3.352 eV), the free electron–acceptor emission (3.318 eV) and the donor–acceptor-pair emission (3.248 eV). The presence of these acceptor-related emissions in undoped ZnO films should be attributed to structural acceptor defects, which are mainly caused by the domain-growth mode in this case.

Optical investigation of nitrogen ion implanted bulk ZnO

Nitrogen ions were implanted into melt-grown ZnO (0001) substrates and subsequently annealed at 800 °C under an oxygen ambient. The photoluminescence spectrum of N +-implanted ZnO excited by a He–Cd laser exhibited donor–acceptor pair (DAP) transition emission at 385 nm with a full width at half maximum of 30 nm at 10 K. The DAP emission is associated with the acceptor energy of nitrogen in ZnO, calculated to be 170 meV. Defect-related red emission at about 610 nm observed in N +-implanted ZnO was due to the residual damage from the implantation step because it was also observed in Ar +-implanted ZnO but not in un-implanted ZnO annealed at 800 °C under the same oxygen ambient.

Activation energies of the acceptor-bound excitons and the donor-acceptor pairs in nitrogen-doped p-type ZnSe, p-type ZnS ySe 1− y, and p-type Zn 1− xMg xS ySe 1− y epitaxial films grown on GaAs (1 0 0) substrates

Nitrogen-doped p-type ZnSe, p-type ZnS ySe 1− y , and p-type Zn 1− x Mg x S y Se 1− y epilayers were grown on n-type GaAs (1 0 0) substrates by molecular beam epitaxy. Photoluminescence (PL) spectra for the p-type ZnSe and the lattice-matched p-type ZnS 0.06Se 0.94, and p-type Zn 0.92Mg 0.08S 0.12Se 0.88 epilayers showed a deep acceptor bound exciton emission and a donor–acceptor pair emission. Temperature-dependent PL measurements were carried out to determine the activation energies of these states. The activation energies of the acceptor-bound excitons and the donor–acceptor pairs were determined to be 40 and 65 meV in the p-type ZnSe epilayer, 20 and 45 meV in the p-type ZnS 0.06Se 0.94, and 45 and 43 meV in the p-type Zn 0.92Mg 0.08S 0.12Se 0.88 epilayers.

Investigation on the ZnO:N films grown on (0 0 0 1) and (0 0 0 1¯) ZnO templates by plasma-assisted molecular beam epitaxy

Nitrogen-doped ZnO films (ZnO:N) were grown on (0 0 0 1) and (0 0 0 1¯) ZnO templates by plasma-assisted molecular beam epitaxy (PA-MBE). High nitrogen concentration (>5×10 20/cm 3) was achieved from ZnO:N/(0 0 0 1)ZnO films, while relatively low nitrogen concentration (∼10 19/cm 3) was observed from ZnO:N/(0 0 0 1¯)ZnO layers at the same growth temperature ( T g). High-resolution X-ray diffraction (HRXRD) revealed considerable lattice deformation. At T g=300 °C, the c-lattice expansion in ZnO:N/(0 0 0 1)ZnO film is almost 36% larger than that in ZnO:N/(0 0 0 1¯)ZnO due to the high nitrogen concentration in the film. Electron concentration of the ZnO:N/(0 0 0 1)ZnO films gradually decreases down to ∼10 16/cm 3 as the nitrogen concentration increases. Moreover, donor–acceptor-pair emission dominates the photoluminescence (PL) spectrum (3.23 eV) of ZnO:N/(0 0 0 1)ZnO films grown at 300 °C and that emission is 3.1 times stronger than that of ZnO:N/(0 0 0 1¯)ZnO.

Growth of single-phase Cu(In,Al)Se 2 photoabsorbing films by selenization using diethylselenide

Selenization growth of purely single-phase, polycrystalline CuIn 1 − x Al x Se 2 (0 ≤ x ≤ 0.26) alloy films was demonstrated using a less-hazardous metal-organic selenide, diethylselenide [(C 2H 5) 2Se: DESe], by simple thermal annealing of metal precursor. Approximately 2.0 µm thick alloy films exhibited X-ray diffraction peaks originating exclusively from the chalcopyrite structure. Transitions seen in the low temperature photoluminescence spectra were attributed to characteristic donor–acceptor pair emissions of the state-of-the-art CuIn 1 − x Al x Se 2 photoabsorbing layers.

Study of steady-state photoluminescence of AgInSe 2 crystals

The steady-state photoluminescence (PL) spectra of AgInSe 2 grown by the Hot-Press method have been examined. The broad peaks located at 1.19 eV and 1.16 eV are very likely to be the donor–acceptor-pair emissions. The energies of the donor levels participating in these emissions correspondingly are 25 eV and 50–52 meV, as found from the analyses of the activation energies of the PL intensity. The peaks observed in the obtained PL spectra in the band-edge region are attributed to free and bound excitons. The binding energy of the free exciton is estimated to be 29 meV.

Optical and structural properties of Mg-ion implanted GaN nanowires

Mg + ions (60 keV) were implanted into GaN nanowires (NWs) with total fluxes of 5 × 10 12–5 × 10 14 cm −2 followed by thermal annealing at 700 °C in N 2 ambient. Transmission electron microscopic images showed amorphous layer formation and defect accumulation in the higher dose Mg-implanted GaN NWs after annealing. Photoluminescence spectra (300 K) of the annealed Mg-implanted GaN NWs exhibited near-band-edge (NBE) emission, donor–acceptor pair (DAP) emission, and defect-related yellow luminescence. With increasing dose, the NBE and DAP emissions are red shifted. Similar phenomena were observed in samples implanted with Ar to produce similar amounts of lattice disorder. The NWs show a much higher sensitivity to defect accumulation than GaN thin films.