Donor-acceptor Transitions Research Articles

Defect Luminescence in Heavily Mg Doped GaN

Behavior of the photoluminescence band at about 2.8 eV in heavily Mg doped GaN has been studied at different temperatures and excitation intensities. The 2.8 eV band is attributed to donor-acceptor transitions involving a Mg acceptor. The large blue shift of the band with increasing excitation intensity is explained by variation in the contribution of close and distant pairs to the luminescence. The red shift of the band with increasing temperature under high excitation intensity conditions results from thermal release of carriers from close pairs. The thermal activation energy of the deep donor, about 0.4 eV, is determined from the quenching of the 2.8 eV luminescence band at high temperatures.

Optical and structural characterization of ZnSe films grown by molecular beam epitaxy on GaAs substrates with and without GaAs buffer layers

ZnSe films with thickness between 800 and 7500 Å were grown on GaAs(100) by molecular beam epitaxy (MBE), and characterized by photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM), and high resolution x-ray diffraction. A first set of films was prepared with ZnSe directly grown on the GaAs substrate. Another set was prepared using an arsenic capped GaAs buffer layer grown on the GaAs substrate in a separated MBE system. PL studies at 18 K showed that the ZnSe films have more defects for samples grown directly on the GaAs substrate. The behavior of stacking faults and dislocations as a function of film thickness were investigated by TEM, and by the variation of the intensity of PL signals related to these defects. For both sets of samples the intensity of these signals decreased with increasing film thickness, but the decrease is steeper for films grown on GaAs buffer layers. A signal in PL spectra at ∼2.7 eV was observed only for the samples grown directly on GaAs substrates, it was associated with donor–acceptor transitions involving GaZn and VZn. The room temperature PR spectra showed, besides the GaAs and ZnSe band-gap signals, oscillations associated with the Franz–Keldysh effect due to internal electric fields. The strength of these fields was obtained by employing the asymptotic Franz–Keldysh model. A signal 22 meV below the GaAs band-gap energy was observed only in the PR spectra of the samples grown directly on GaAs substrates. This signal was associated with Zn interdiffused into the GaAs, and is correlated to the PL signal observed at 2.7 eV for the same set of samples.

Structural defects and microstrain in GaN induced by Mg ion implantation

The optical and structural characteristics of GaN films implanted with Mg and Be ions, grown by low-pressure metalorganic chemical vapor deposition were studied. The low temperature (20 K) photoluminescence (PL) spectra of annealed Mg implanted GaN show a 356 nm near band edge emission, a 378 nm donor-acceptor (D-A) transition with phonon replicas, and a 528 nm green band deep level emission. The origin of the 528 nm green band emission and the 378 nm D-A emission might be attributed, respectively, to the Mg implantation induced clustering defect and the vacancy defect in GaN film. Observations of in-plane and out-of-plane x-ray diffraction spectra for as-grown undoped, Mg implanted and rapid thermal annealed GaN suggest that ion implantation induced anisotropic strain may be responsible for the observed PL emission characteristics.

Electrical deactivation of interstitial Zn in heteroepitaxial InP by hydrogen and its effect on electronic properties

Hydrogen passivation of InP layers grown on lattice-mismatched substrates can achieve thermally stable deactivation of dislocation-related deep levels, making this a promising process for improving the performance of heteroepitaxial InP space solar cells. However, in addition to dislocation-related defects, interstitial Zn (Zni) defects that are characteristic of Zn-doped InP and which form deep donor states within the InP band gap, are important considerations for optimizing the electronic quality of these layers. Here, we show that hydrogen forms complexes with and deactivates Zni donor states within Zn-doped InP grown by metalorganic chemical vapor deposition. A combination of photoluminescence (PL), electrochemical capacitance–voltage dopant profiling, secondary ion mass spectroscopy and current–voltage (I–V) measurements are applied to a set of samples receiving systematic hydrogenation and annealing treatments. We find that the deactivation of Zni deep donors, as detected by monitoring the evolution of the donor–acceptor transition using PL measurements, causes an increase of ∼50% in the net acceptor concentration of heavily Zn-doped heteroepitaxial InP by elimination of the acceptor compensation effect due to active Zni donors. Analysis of I–V characteristics indicates that Zni passivation sharply reduces depletion region recombination and shunt currents within heteroepitaxial diodes, causing an increase in the diode turn-on voltage from 680 to 960 mV. Subsequent annealing above 500 °C reactivates the Zni defects, resulting in a systematic increase in doping compensation as well as a decrease in VTO toward the original, as-grown value. A study of the reactivation kinetics for the H–Zni complex reveals a greater thermal stability than that of H–Zn acceptor complexes but less than that of H-dislocation complexes in InP, with an estimated dissociation energy for the H–Zni complex of 2.3 eV. While these effects are observed for both homoepitaxial and heteroepitaxial Zn-doped layers, the effect is far more pronounced for the heteroepitaxial layers due to the relatively high Zni concentration in the latter.

2 K photoluminescence of indirect GAP Al xGa 1− xAs alloys grown under near-equilibrium conditions by LPE

Non-intentionally doped Al x Ga 1− x As epitaxial layers with 0.38≤ x≤0.84 were grown on (100) GaAs substrates by liquid-phase epitaxy under near-equilibrium conditions. Making use of photoluminescence measurements at 2 K it was possible to report, breaking down the spectra into multiple gaussians, the quadratic relationship that followed by the indirect bound exciton line, the free electron–acceptor transition and the recombination of donor–acceptor pair with the increase of the aluminum content. The behavior of the intensity of both bound exciton and donor–acceptor transitions is explained from the evolution of the layer thickness according to the aluminum composition, given that all the samples were grown under the same conditions with fixed growth time. It is also possible to observe the excitonic transition corresponding to the Al x Ga 1− x As/GaAs interface, despite the lattice mismatch and other inconveniences, which are involved when growth high aluminum content with this technique. A very narrow new line (2.8 meV) can be observed in the energetic region of deep levels and defects: 1.0385 eV, whose origin is being studied.

Crystal Morphology and Optical Emissions of GaN layers grown on Si(111) substrates by Molecular Beam Epitaxy

Crystal morphology of GaN layers grown on Si(111) evolves from whisker-like microcrystals to compact films as a function of the III/V ratio. Small changes in the III/V ratio (from Ga-rich to N-rich) during the growth of a compact layer result in the appearance of microcrystals on the top of the layer, indicating a sharp transition between the two growth regimes.Four different morphologies are obtained by increasing the III/V ratio: a) completely columnar whisker-like samples exhibiting a pair of intense excitonic emissions at 3.471−3.478 eV; b) a mixture of compact regions with columnar microcrystals showing two pairs of excitonic emissionsnbsp c) compact layers with very small microcrystals on the top surface with a weaker dominant transition at 3.415 eV (±5meV) and, d) full compact and smooth layers with a single dominant excitonic emission at 3.466 eV. A combination of PL measurements with SEM photographs and CL imaging reveals that both pairs of emissions in samples b) come from the columnar microcrystals. The high energy pair (3.471−3.478eV) is attributed to the free-exciton A and a donor-bound exciton while the low energy pair (3.452−3.458eV) is assigned to acceptor-bound excitons associated to valence bands Γ9v and Γ7uv. Power and temperature dependence together with time-resolved data show that the dominant peak at 3.415eV (± 5meV) present in samples c) correspond to a donor-acceptor transition. CL measurements as a function of electron beam energy (depth) also indicate that this emission is more intense towards the interface between the layer and the sample. Finally, the excitonic emission in samples d) is shifted to lower energies due to residual biaxial tensile strain of thermal origin.

Secondary ion mass spectroscopy investigations of magnesium and carbon doped gallium nitride films grown by molecular beam epitaxy

The incorporation of magnesium and carbon in GaN grown by molecular beam epitaxy (MBE) has been investigated by secondary ion mass spectroscopy (SIMS) and other techniques. We have grown Mg:GaN in a wide range of chemical concentrations -. Low temperature photoluminescence of Mg:GaN is dominated by the donor-acceptor transitions associated with Mg at eV. Carrier concentration for Mg:GaN in the range - with mobilities were measured by the Hall effect technique. In the C:GaN layers, it was found that carbon can be uniformly incorporated into the layer at a concentration . However, at this high concentration there is a tendency for carbon to diffuse into the undoped GaN buffer layer.

Defect Luminescence in Heavily Mg Doped GaN

AbstractBehavior of the photoluminescence band at about 2.8 eV in heavily Mg doped GaN has been studied at different temperatures and excitation intensities. The 2.8 eV band is attributed to donor-acceptor transitions involving a Mg acceptor. The large blue shift of the band with increasing excitation intensity is explained by variation in the contribution of close and distant pairs to the luminescence. The red shift of the band with increasing temperature under high excitation intensity conditions results from thermal release of carriers from close pairs. The thermal activation energy of the deep donor, about 0.4 eV, is determined from the quenching of the 2.8 eV luminescence band at high temperatures.

The Incorporation and Complex Formation of Ag Acceptors in CdTe

AbstractUsing the radioactive isotope 111Ag, the incorporation of Ag into CdTe is investigated by photoluminescence spectroscopy (PL) and the perturbed yy-angular correlation technique (PAC). PL is used to demonstrate the incorporation as a substitutional acceptor (AgCd) by monitoring the intensity of the donor-acceptor transition, corresponding to an acceptor level of EV + 108 meV, which decreases just with the half life of the radioactive decay of the 111Ag isotope. Since the isotope 111Ag also serves as a probe atom for PAC experiments, it is possible to confirm the incorporation of Ag atoms on lattice sites with cubic symmetry. Additionally, in CdTe a defect complex is detected, which is assigned to a AgCd-VTe pair. A second defect complex is observed in CdTe doped with In, which is assigned to an InCd-AgCd pair. By PAC experiments with the probe 111In that are performed in Ag doped CdTe, the formation of the InCd-AgCd pair is confirmed besides the well known formation of the InCd-VCd pair. The migration energy of the cation vacancy is estimated on the basis of an isothermal annealing sequence.

Analysis of the Visible and UV Electroluminescence in Homojunction GaN LED's

The electrical and electroluminescent properties of MOVPE GaN p-n homojunctions have been analyzed as a function of temperature and bias. Electroluminescence is observed for V>3 V under dc and ac conditions. The main emission at low T is a donor-acceptor transition involving shallow acceptors, though it disappears at higher T due to the ionization of the acceptors and compensation by ionized donors. Room temperature dc and ac electroluminescence spectra evolve under increasing bias from a blue-shifting visible band involving deep states at the p-type side of the p-n junction, to a band-to-band UV recombination at high bias. In agreement, the superlinear dependence of light-current characteristics at low current injection becomes linear when the defects are saturated. Time analysis of the spectra vs pulse duration and duty cycle allows the determination of the visible radiative recombination and relaxation times associated to the Mg-related deep states, which are found to behave as acceptors lying 0.55 eV above the valence band. A simple 3-level model is able to explain the visible emission, which involves the conduction band (or shallow donor) and those deep acceptors in the p-layer. Optimum UV/visible ratio emission requires intense and relatively long pulses, with a high duty cycle to impede visible recombination.

Radiative recombination in CuInSe2 thin films

We perform a detailed analysis of the radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition. The photoluminescence and photoluminescence excitation spectra are investigated as a function of stoichiometry, temperature and excitation intensity. Using samples with a large composition gradient, we are able to obtain a coherent picture of the optical transitions in the films. The broad–band photoluminescence spectrum typical for In–rich films breaks into a number of well–defined emission lines in Cu–rich CuInSe2. At low temperatures, emission peaks due to free–exciton, bound–exciton, and free–to–bound recombination are identified in Cu–rich films. The spectra of In–rich films tend to be dominated by donor–acceptor transitions. From the optical spectra, exciton ionization energies and the temperature dependence of the band gap are determined. The observed optical transitions are related to intrinsic defects.

Electrical and Optical Characteristics of the Layer Semiconductor p-GaSe Doped with Ag

Impurity levels in Ag-doped p-GaSe have been studied by using photoluminescence and Hall effect measurements. The PL spectra at 77 K are dominated by two new emission bands at 2.02 and 1.74 eV. From the combined results of Hall effect and PL measurements, it is found that the 2.02 and 1.74 eV emission bands are associated with the same acceptor level at 0.07 eV above the valence band, moreover, the 2.02 and 1.74 eV emission bands are caused by conduction band–acceptor and donor–acceptor transitions, respectively.

Effect of sulfur doping on optical anisotropy of CdSiAs2

Polarization dependent photoluminescence and photocurrent measurements were carried out on undoped and sulfur doped single crystals of CdSiAs2. For undoped crystal the interband transition A shows a 40 meV blueshift under perpendicular light polarization with respect to the parallel one. This shift is about twice as large for sulfur doped crystal. A pronounced photoabsorption band at 1.53 eV, related to the sulfur donor, is observed in the low temperature photocurrent spectrum. A significant enhancement of the optical anisotropy of donor-acceptor transition is seen in the low temperature photoluminescence spectra of CdSiAs2 doped with sulfur.

Limitations to n-type doping in diamond: The phosphorus-vacancy complex

In spite of large concentrations of phosphorus being incorporated into diamond, the material often remains insulating. It is argued that this occurs through the formation of phosphorus-vacancy complexes which are deep acceptors and compensate any donor. The complex is analyzed using a first-principles cluster method. In the ionized state, the defect is diamagnetic and cannot give rise to any internal optical transitions although broad band donor–acceptor transitions are expected—and observed—in material codoped with nitrogen.

Optical transitions in cubic GaN investigated by spatially resolved cathodoluminescence

Spatially resolved cathodoluminescence (CL) spectroscopy in connection with scanning electron microscopy performed on cubic (c) GaN between 5 and 300 K reveals that at low temperatures the CL spectra of c-GaN single crystals consist of four well-separated lines. The two lines highest in energy were previously identified as excitonic and donor-acceptor transitions, respectively. Here, we show that the lines lowest in energy are due to an additional free-to-bound transition, involving an impurity different from those related to the donor-acceptor transition, and its phonon replica. The CL spectra of c-GaN layers, while being rather broad, are composed of these four lines. Moreover, at 300 K the spectra of the layers and of the crystals are both dominated by the excitonic transition and closely resemble each other.

Electron-phonon coupling in a delta -doped n-i-p structure in GaAs.

The donor-acceptor transition in the photoluminescence (PL) spectra of \ensuremath{\delta}-doped n-i-p structures in GaAs is studied in detail. Unpolarized PL spectra exhibit an unexpected large linewidth; p-polarized PL spectra reveal that this line broadening is due to electron-phonon coupling. Raman data suggest that the 25-meV phonon involved belongs to the edge of the first Brillouin zone. The Huang-Rhys figure (S), which measures the electron-phonon coupling, shows a remarkable dependence both on the effective dopant charge density ${\mathit{eN}}_{\mathit{u}}$=e(${\mathit{N}}_{\mathit{A}}^{\mathrm{\ensuremath{-}}}$+${\mathit{N}}_{\mathit{D}}^{+}$)/2 as well as on the interplanar distance ${\mathit{R}}_{0}$. Since under strong irradiation ${\mathit{N}}_{\mathit{u}}$\ensuremath{\rightarrow}(${\mathit{N}}_{\mathit{A}}$-${\mathit{N}}_{\mathit{D}}$)/2, both the S value and the Stark shift \ensuremath{\Delta} can be used to determine the magnitude of the miscompensation in these samples. A theoretical model will be presented that explains the observed dependences S(${\mathit{N}}_{\mathit{u}}$) and S(${\mathit{R}}_{0}$) in a qualitative way. \textcopyright{} 1996 The American Physical Society.

Evidence for Shallow Acceptor Levels in MBE Grown GaN

We report the results of photoluminescence measurements on a number of GaN thin films grown by MBE on GaAs (111)B substrates. In particular, we draw attention to a new observation of a line at approximately 3.40eV which is accompanied by complex fine structure and interpret it as due to a donor-acceptor (DA) transition. Assuming a donor energy of 30meV, we derive an acceptor binding energy of approximately 80meV which is very much smaller than the accepted value of 250meV for the well established Mg acceptor. However, our result is in agreement with a recent estimate of the hydrogenic acceptor energy as being 85meV.

MBE Growth and Characterization of Zns/Gan Heterostructures

ABSTRACTHeterostructures involving ZnS/GaN show promise for the injection of holes from p-GaN into n-ZnS. This combination could result in multi-color electroluminescent displays. We have grown single crystal ZnS on GaN and sapphire (0001) by MBE using elemental sources. The ZnS was grown at temperatures from 150°C–400°C, with beam flux equivalent pressures of (0.3 – 2.0) × 10−7 torr. Growth rates of up to 0.4 μm per hour were observed for the lower growth temperatures, with rapidly diminishing rates for temperatures above 350μC. The GaN substrate consisted of a 3 μm epilayer grown on sapphire by MOCVD. XPS analysis revealed the presence of carbon surface contamination on the GaN, which was removed by in situ exposure to an RF nitrogen plasma. RHEED observations indicate that the zincblende ZnS layers commonly contain (111) twins, although twin free films may be grown at a high substrate temperature. The samples were characterized using photoluminescence and X-ray diffraction. X-ray peaks typically had FWHM of 400 arcsec for ω/2θ scans, and somewhat worse for ω scans. Photoluminescence spectra of the ZnS films doped with Ag and Al demonstrated the well known blue donor acceptor transition at 440 nm.

Properties Of Inp Simultaneously Doped With Zinc And Sulfur Grown By Mocvd

AbstractAs the p-type dopant most often used in metalorganic chemical vapor deposition (MOCVD) of Group III - Group V compound semiconductors, Zn presents problems in device design and performance because of its high diffusivity in these materials. While Zn diffusion into n-type layers such as InP:S has been observed frequently, there is little known as to the electronic and optical properties of the resultant material. We have grown InP samples by MOCVD which are doped with both Zn and S to levels as high as 3×1018 cm−3. These samples were analyzed by electrochemical C-V profiling, van der Pauw-Hall analysis, secondary ion mass spectroscopy (SIMS), and low temperature (10K) photoluminescence spectroscopy (PL). We have determined that good hole mobility is maintained in InP:Zn samples that are simultaneously doped with S up to a level of 4×1017 cm−3. PL analysis of co-doped samples shows peaks between 0.91 and 0.92 μm which are indicative of donor-acceptor transitions, and broad peaks with energy levels of approximately 1.0 μm which may be indicative of ZnS complexes or precipitates. SIMS analysis of Zn diffusion into Fe doped substrates shows that Zn diffusion is reduced in the presence of S in the lattice.

Luminescence properties pf p-type thin CdS films prepared by laser ablation

Investigations of the luminescence of p-type CdS:Cu thin (less than or equal to 2 μm) films on glass substrate prepared by laser ablation were performed for the first time. The dependences of the luminescence on the Cu content in the thin films were studied at 300 K with argon laser lines at 457.9 nm, 488.0 nm and 514.5 nm. It is demonstrated that the luminescence excited with the 514.5 nm line corresponds to the donor-acceptor transition. Furthermore, it is shown that the intensity of the red emission of CdS:Cu films can be efficiently bleached by Cu doping.