Efficient near IR photoluminescence from gallium nitride layers doped with arsenic

Photoluminescence in the 1.2–1.4 eV spectral range from GaN:As layers grown on (0001) Al₂O₃ substrates was observed and studied. The photoluminescence is attributed to radiative recombination in GaAs nanocrystallites, self-organized in the GaN matrix during growth. The photoluminescence intensity attains a maximum at a growth temperature of ～780°C, which is explained by the competition between several temperature-dependent processes that affect the formation of GaAs nanocrystallites. Sharp emission lines were observed at the high-energy edge of the photoluminescence band. These lines are caused by an emission of bound excitons in the GaAs nanocrystallites and by phonon replicas of the bound-exciton emission. The energies of the corresponding optical phonons are typical of GaAs. The photoluminescence-excitation spectra exhibit features related to resonantly excited free and bound excitons as well as to excitons formed simultaneously with the emission of optical phonons.     

Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

Structural and luminescent properties of ZnO:P films produced by annealing ZnP<Subscript>2</Subscript> wafers in atomic oxygen

The epitaxial ZnO:P films are fabricated by annealing the ZnP₂ wafers in atomic oxygen (oxygen radicals). The properties of the films are studied by X-ray diffraction analysis, atomic force microscopy, Auger spectroscopy, and photoluminescence measurements. In the X-ray diffraction spectra of the samples, the (002) peak is observed, suggesting that the ZnO:P films are oriented along the c axis. The Auger spectroscopy data show that the ZnO films contain phosphorus. The low-temperature photoluminescence spectrum observed for the ZnO: P films exhibits a 3.356 eV peak presumably corresponding to excitons bound at neutral acceptors and a peak at 3.306 eV (free electron)-acceptor transitions associated with the P_O level. The phosphorus related acceptor level is localized at 128 meV above the top of the valence band. The origin of the 3.312 eV band related to recombination at donor-acceptor pairs is discussed.     

Characterization of undoped and silicon-doped InGaN/GaN single quantum wells

We investigated the influence of doping and InGaN layer thickness on the emission wavelength and full width at half maximum (FWHM) of InGaN/GaN single quantum wells (SQW) of thicknesses between 1 nm and 5 nm by temperature and intensity resolved photoluminescence (PL). The crystalline quality of the GaN claddings was assessed by low temperature PL. The emission energy of 5 nm Si doped SQW could be tuned from 3.24 eV to 2.98 eV by reducing the deposition temperature. An increase of piezoelectric (PE) field screening with increasing deposition temperature is attributed to an increase of the SiH₄ decomposition efficiency. Piezoelectric (PE) fields between 0.5 MV/cm and 1.2 MV/cm in undoped structures of varying SQW thicknesses were calculated. Two activation energies of 15 meV and 46 meV of the SQW emission could be observed in temperature resolved measurements. The higher value was assigned to the confined exciton binding energy, whereas the activation energy of 15 meV is probably due to a decrease in carrier supply from the absorption zone in the GaN cladding into the SQW.     

Modeling of optical gain in InGaN-AlGaN andInxGa1-xN-InyGa1-yNquantum-well lasers

This paper presents computations of the optical gain in In_x Ga_1-xN-In_yGa_1-yN and InGaN-AlGaN quantum-well lasers involving the contributions of excitons as well as free carriers transitions. The behavior of optical gain in GaN based quantum wells due to excitonic transitions is quite similar to that of ZnCdSe-ZnSSe system, as the magnitude of the exciton binding energies (~30 meV) is comparable. The model compares the exciton emission energy with the experimental data reported on In_0.22Ga_0.78N-In_0.06Ga_0.94N multiple quantum wells as well as in GaN layers (cubic grown on 3C SiC), including the effect of strain induced band gap changes. The optical gain is also computed as a function of the injection current density for the InGaN-AlGaN multiple quantum-well lasers. The model evaluates the feasibility of obtaining GaN based blue and ultraviolet lasers. It is shown that the excitonic transitions reduce the threshold current density which is adversely affected by the presence of dislocations and other defects     

Optical spectroscopy of free excitons in a CuInS<Subscript>2</Subscript> chalcopyrite semiconductor compound

The spectra of reflectance and luminescence of high-quality CuInS₂ single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy ～1.5356 eV and doubly degenerate peak BC at the energy ～1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A _UPB at ～1.5361 eV and A _LPB at ～1.5347 eV, with a half-width ～1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be ～18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.     

GaN grown by hydride vapor phase epitaxy on p-type 6H-SiC layers

6H-SiC/GaN pn-heterostructures were grown by subsequent epitaxial growth of p-SiC by low temperature liquid phase epitaxy (LTLPE) and n-GaN by hydride vapor phase epitaxy (HVPE). For the first time, p-type epitaxial layers grown on 6H-SiC wafers were used as substrates for GaN HVPE growth. The GaN layers exhibit high crystal quality which was determined by x-ray diffraction. The full width at a half maximum (FWHM) for the ω-scan rocking curve for (0002) GaN reflection was ∼120 arcsec. The photoluminescence spectra for these films were dominated by band-edge emission. The FWHM of the edge PL peak at 361 nm was about 5 nm (80K).     

Photoluminescence excitation spectroscopy of the dense exciton gasinvolved in the lasing transition in ZnSe epitaxial layers

The lasing transition in ZnSe epitaxial layers has been investigated at 77 K. The lowest lasing threshold (I_th) was achieved when the layers were resonantly excited at the photon energy of the exciton level. It was found that the exciton level at the excitation intensity just above the I_th was red-shifted by about 16 meV compared with the free exciton line (2.792 eV) under weak excitation condition. The energy difference between the exciton line and the lasing peak was about 19 meV at I=I_th and increased with increasing excitation intensity up to I=8×I_th. This suggests that the stimulated emission occurs due to the inelastic exciton-exciton scattering process at this temperature     

Correlation of biaxial strains, bound exciton energies, and defect microstructures in gan films grown on AlN/6H-SiC(0001) substrates

Biaxial strains resulting from mismatches in thermal expansion coefficients and lattice parameters in 22 GaN films grown on A1N buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured via changes in the c-axis lattice parameter. A Poisson’s ratio of ν = 0.18 was calculated. The bound exciton energy (E_BX) was a linear function of these strains. The shift in E_BX with film stress was 23 meV/GPa. Threading dislocations densities of ~10¹⁰/cm² and ~10⁸/em² were determined for GaN films grown on vicinal and on-axis SiC, respectively. A 0.9% residual compressive strain at the GaN/AIN interface was observed by high resolution transmission electron microscopy (HRTEM).     

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

Spontaneous and stimulated UV luminescence of ZnO:N at 77 K

The spontaneous and stimulated UV luminescence spectra of ZnO:N samples possessing different nitrogen contents are measured at a temperature of 77 K. Luminescence peaks resulting from bound and free excitons, biexcitons, and electron recombination via the nitrogen acceptor level are identified. The optical depth (123 meV) of the N_O impurity acceptor level is determined. It is established that stimulated UV luminescence originating from inelastic exciton interaction sets in as the optical pump power increases.     

The role of stacking faults and their associated 0.13 ev acceptor state in doped and undoped ZnO layers and nanostructures

An emission band at 3.31 eV is frequently observed in low-temperature photoluminescence (PL) measurements on ZnO p-doped with group-V elements, and also on nominally undoped ZnO layers and nanostructures. It has alternatively been ascribed to LO- or TO-phonon replicas of free excitons, to acceptor-bound excitons, to donor-acceptor pair transitions, to two-electron satellites of donor-bound excitons, or to free-to-bound transitions. This band frequently dominates the PL of ZnO nanostructures and layers at room temperature. Annealing leads to drastic changes in its intensity.We report on low-temperature cathodoluminescence measurements with very high spatial resolution and high-resolution transmission electron microscope investigations carried out on the same pieces of hetero-epitaxial ZnO samples with unusual layer orientation. These data allow us to correlate this emission unambiguously with c-plane stacking faults. The emission is found to be due to the recombination of a free electron with a hole bound to a relatively shallow acceptor state ≈130 meV above the valence band edge. Locally, these acceptor states occur in high concentrations of up to some 10¹⁸ cm⁻³, and thus lead to strong two-dimensional perturbations of the free carrier concentration. They have severe implications for the conductivity of layers and nanostructures in general, and on the interpretation of Hall and luminescence data in particular. Literature data are critically reviewed in the light of these findings.     

Interface properties of ZnO nanotips grown on Si substrates

ZnO nanotips have been grown on Si (100) using metalorganic chemical vapor deposition (MOCVD). The growth temperature is optimized for good crystallinity, morphology, and optical properties. ZnO nanotips exhibit a strong near band edge emission of ∼376 nm at room temperature with negligible green band emission. Pregrowth substrate treatment using diluted hydrofluoric acid (HF) and minimized oxygen exposure before the initial growth significantly reduces the interfacial SiO₂ thickness, while maintaining good morphology. An n-ZnO nanotips/p-Si diode is fabricated and its I–V characteristic is measured. The threshold voltage of the diode is found to be below 2.0 V with small reverse leakage current. The ZnO/p-Si diodes provide the possibility of integrating the ZnO nanotips with Si-based electronic devices.     

ZnO nanotips grown on Si substrates by metal-organic chemical-vapor deposition

The ZnO nanotips are grown on silicon and silicon-on-sapphire (SOS) substrates using the metal-organic chemical-vapor deposition (MOCVD) technique. The ZnO nanotips are found to be single crystal and vertically aligned along the c-axis. In-situ Ga doping is carried out during the MOCVD growth. The ZnO nanotips display strong near-band edge photoluminescence (PL) emission with negligible deep-level emission. Free excitonic emission dominates the 77-K PL spectrum of the as-grown, undoped ZnO nanotips, indicating good optical properties and a low defect concentration of the nanotips. The increase of PL intensity from Ga doping is attributed to Ga-related impurity band emission. Photoluminescence quenching is also observed because of heavy Ga doping. ZnO nanotips grown on Si can be patterned through photolithography and etching processes, providing the potential for integrating ZnO nanotip arrays with Si devices.     

Preparation and characteristics of ZnO films with preferential nonpolar plane orientation on polar sapphire substrates

The properties of ZnO thin film on sapphire (0001) substrate fabricated by single source chemical vapour deposition (SSCVD) method are studied. X-ray diffraction (XRD) analysis demonstrates that the film exhibits hexagonal structures but with preferential nonpolar (100) plane orientation, which is different from the crystalline structure of substrate, and its formation mechanism is also analyzed. The film has the characteristic of p-type conductivity originating from excess of oxygen, and its p-type conductivity is comparatively stable due to its nonpolar plane orientation. A strong ultraviolet (UV) emission and a high light transmission in visible wavelength region are observed from photo-luminescence (PL) spectrum and transmittance spectra at the room temperature, and the strong ultraviolet emission originates from the recombination of free and bound excitons. Compared with the ZnO film on silicon substrates, the exciton emission peaks of the film on sapphire substrate show a slight blue shift about 50 meV, which might be related to the different crystallite sizes or surface stress of the films.     

Photoluminescent properties of ZnO thin films grown on SiO2/Si(100) by metal-organic chemical vapor deposition

We report on the photoluminescent (PL) properties of ZnO thin films grown on SiO₂/Si(100) substrates using low pressure metal-organic chemical vapor deposition. The growth temperature of the films was as low as 400°C. From the PL spectra of the films at 10–300 K, strong PL peaks due to free and bound excitons were observed. The origin of the near bandedge emission peaks was investigated measuring temperature-dependent PL spectra. In addition, the Zn O films demonstrated a stimulated emission peak at room temperature. Upon illumination with an excitation density of 1 MW/cm², a strong, sharp peak was observed at 3.181 eV.     

Growth of GaN Films on Si (100) Buffered with ZnO by Ion-Beam-Assisted Filtered Cathodic Vacuum Arc Technique

GaN films have been fabricated on Si (100) substrates with ZnO buffer layers by an ion-beam-assisted filtered cathodic vacuum arc (I-FCVA) technique at␣450°C. GaN films are highly (002)-oriented with a hexagonal structure examined by X-ray diffraction. The room-temperature photoluminescence spectrum of the GaN film exhibits a strong and sharp band-edge emission peak at 3.36 eV. The obtained results demonstrate the potential of the I-FCVA technique for the fabrication of high-quality GaN layers on Si substrates.     

Doping of gallium nitride using disilane

The silicon doping of n-type GaN using disilane has been demonstrated for films grown on sapphire substrates by low pressure organometallic vapor phase epitaxy. The binding energy of an exciton bound to a neutral Si donor has been determined from low temperature (6K) photoluminescence spectra to be 8.6 meV. Nearly complete activation of the Si impurity atom in the GaN lattice has been observed.     

Optical spectroscopy of excitonic states in CuInSe2

Optical properties of structurally perfect CuInSe₂ single crystals were studied in the temperature range of 4.2–300 K with the use of photoluminescence, optical absorption, optical reflection, and wavelength-modulated optical reflection (WMOR). The intense lines of free excitons A (～1.0414 eV) and B (～1.0449 eV) with a half-width of ～0.7 meV at 4.2 K are found to be related to two extrema of valence band split by a crystal field. The excitons emission line C (～1.2779 eV) in WMOR spectra are related to a lower valence band split-off by spin-orbit interaction. Within the context of the quasi-cubic Hopfield model, the parameters of valence band splitting Δ_CF=5.2 meV and Δ_SO=234.7 meV defined by the crystal and spin-orbit interaction, respectively, are calculated. In the region of the fundamental absorption edge, the lines of bound excitons are found with a half-width ～0.3 meV that is indicative of a high quality of grown CuInSe₂ crystals.     

Synthesis and photoluminescence properties of the 4H-SiC/SiO2 nanowires

4H-SiC/SiO2 nanowires are synthesized and the temperature-dependent photoluminescence （PL） properties of the nanowires are studied. Their structure and chemical composition are studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and Raman spectra. At room temperature, an ul- traviolet PL peak and a green PL band are observed. From the PL spectrum measured in the temperature range from 80 K to 300 K, the free excition emission, donor bound excition emission and their multiple-phonon replicas have been observed in ultraviolet region, and their origins have been identified. Moreover, it has been found that the temperature dependence of the free exciton peak position can be described by standard expression, and the thermal activation en- ergy values extracted from the temperature dependence of the free exciton and bound exciton peak integral intensity are about 40 meV and 181 meV, respectively.