Defect-related Transitions Research Articles

Transient reflecting grating spectroscopy for defect analysis in surface region of semiconductors

Ultrafast transient reflecting grating (TRG) spectroscopy was utilized for defect analysis in the surface region of ion-implanted silicon for the implantation range from 1011 to 1015 cm−2. To deduce signals due to trapped carriers at defect states, the TRG spectra at the delay time of 30 ps were measured because ultrafast carrier dynamics such as many-body recombination had finished before the delay time. According to the dose quantity, the peak of the interband transition was affected and also defect-related transitions emerged. Using this technique, implantation damage can be detected for samples with their dose larger than 1012 cm−2. It was proposed that TRG spectroscopy can be used as a novel analytical method for characterizing defects in the surface region of semiconductors.

Photoluminescence characterization of type II InGaAs/AlAsSb heterostructures lattice matched to InP grown by molecular beam epitaxy

We report here the photoluminescence characterization of InGaAs/AlAsSb quantum wells lattice matched to InP substrate grown by molecular beam epitaxy. A strong blueshift of the PL peak energy was observed as the excitation power increased. Furthermore, these peaks were shown to belong to an excitonic recombination. We have attributed these peaks to the radiative recombination of spatially separated electron–hole pairs and confirmed the type II band alignment. Impurity- or defect-related transitions were also observed.

AlN on sapphire and on SiC: CL and Raman study

Aluminium nitride (AlN) layers were grown on sapphire (0 0 0 1) and on Si-terminated SiC (0 0 0 1) in a RF-plasma enhanced MBE system with integrated UHV-STM/AFM. The layers have been investigated by cathodoluminescence (CL) and Raman measurements. The CL measurements show defect related transitions in the region of 2–5 eV and resolved excitonic features in the near band-edge region followed by multiple A 1(LO) replica. From both CL and Raman studies a superior quality of the AlN/SiC layers in comparison to the AlN/sapphire layers is deduced.

Electrical and Photoelectrical Characterization of Deep Defects In Cubic GaN on GaAs

Nominally undoped cubic GaN epilayers deposited by rf-plasma assisted molecular beam epitaxy on semi-insulating GaAs substrates were investigated by electric and photoelectric spectroscopical methods. As a consequence of the existence of deep levels in the GaAs-substrate itself, special care has to be taken to separate the contributions of the substrate from that of the cubic GaN epilayer in the various spectra. Two different contact configurations (coplanar and sandwich structures) were successfully used to perform this separation. In the cubic GaN epilayer a trap with a thermal activation energy of (85±20)meV was found by thermal admittance spectroscopy and thermal stimulated currents. Optical admittance spectroscopy and photocurrent measurements furthermore revealed defects at EG-(0.04-0.13) eV, EG-(0.21-0.82) eV and two additional deeper defects at 1.91 Ev and 2.1 eV, respectively. These defect related transitions are very similar to those observed in hexagonal GaN.

Cathodoluminescence and Transmission Electron Microscopy Study of the Influence of Crystal Defects on Optical Transitions in GaN

Defect related states and excitonic transitions in epitaxial GaN have been studied by combining cathodoluminescence and transmission electron microscopy. A series of deep levels with energies at about 2.4, 2.6 and 2.8 eV has been found by low temperature cathodoluminescence on free-standing 150 μm thick epitaxial GaN. These deep levels are characterised by a high recombination efficiency. They are radiative from 5 to 70 K and undergo a nonradiative transition at 70 K. These levels completely quench the near band edge and the conventional yellow emissions. We discuss the structural origin of these defects in terms of formation of VGa–SiGa and VGa–ON complexes. The consequences of our model with respect to non radiative transitions at threading dislocations are also presented. An excitonic transition at 3.41 eV close to the near band edge line on differently grown epitaxial GaN has been correlated to stacking faults. This line can be explained by a model based on the concept of excitons bound to SFs that form a quantum well of cubic material in the wurtzite lattice of the layer.

The annealing effects on ZnCdSe/ZnSe quantum wells and ZnSe/GaAs interfaces

This study investigated the thermally induced interdiffusion in ZnCdSe/ZnSe quantum wells and the outdiffusion of Ga atoms from the GaAs substrate by photoluminescence (PL), secondary ion mass spectrometry (SIMS), and contactless electroreflectance (CER) spectroscopy. The quantum well structures grown by molecular beam epitaxy were thermally annealed at temperatures between 250 and 700 °C. According to our results, the 15 K PL main peak disappeared when the annealing temperature reached 550 °C and above for samples grown with a 0.1 μm ZnSe buffer (sample A). In contrast, the PL main peak disappeared when the annealing temperature reached 600 °C and above for samples grown with 0.85 μm ZnSe buffer (sample C). In addition, for sample A, two extra PL peaks around 2.0 and 2.3 eV were observed when the annealing temperature reached 500 °C and above; those peaks were observed only when the annealing temperature reached 700 °C for sample C. SIMS results indicated the interdiffusion of Cd in the ZnCdSe/ZnSe quantum well regions in both samples. However, only sample A revealed a strong outdiffusion of Ga atoms from the substrate into the epilayer side, but a weaker one in sample C. The main peaks in PL spectra and the intersubband transitions in CER spectra disappeared owing to the Cd interdiffusion in the wells and the defect-related transitions introduced by the Ga outdiffusion onto the epilayer side. Based on the SIMS, PL, and CER spectra results, a thicker ZnSe buffer layer can increase the thermal stability of ZnCdSe/ZnSe quantum wells grown on GaAs substrates because of its effectiveness in mitigating the outdiffusion of Ga atoms into buffer layers and the interdiffusion of quantum well regions.

Near-band-edge optical properties of molecular beam epitaxy grown ZnSe epilayers on GaAs by modulation spectroscopy

A study of near-band-edge optical properties of ZnSe epilayers grown on GaAs substrates using various modulation techniques is presented. We compare the contactless electroreflectance (CER) and piezoreflectance spectra to ascertain that our ZnSe epilayers of 1.2 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. The defect-related transitions near the ZnSe/GaAs interface are also compared by identifying the photoreflectance and other spectra. In addition, in order to observe the temperature-dependent energy splitting and strains, we present a detailed investigation of the heavy-hole and light-hole related transition energies as a function of temperature in the 15–200 K range by identifying the excitonic signatures in the CER spectra. We have also calculated the energy splitting between heavy-hole and light-hole valence bands by utilizing the temperature-dependent elastic constants for ZnSe and the thermal expansion coefficients for ZnSe and GaAs. Both the experimental result and the theoretical calculation have shown a similar trend that the biaxial tensile strains decrease in magnitude with increasing temperatures in the 1.2 μm ZnSe epilayer grown on a GaAs substrate.

Electrical and Photoelectrical Characterization of Deep Defects in Cubic GaN on GaAs

AbstractNominally undoped cubic GaN epilayers deposited by rf-plasma assisted molecular beam epitaxy on semi-insulating GaAs substrates were investigated by electric and photoelectric spectroscopical methods. As a consequence of the existence of deep levels in the GaAs-substrate itself, special care has to be taken to separate the contributions of the substrate from that of the cubic GaN epilayer in the various spectra. Two different contact configurations (coplanar and sandwich structures) were successfully used to perform this separation. In the cubic GaN epilayer a trap with a thermal activation energy of (85±20)meV was found by thermal admittance spectroscopy and thermal stimulated currents. Optical admittance spectroscopy and photocurrent measurements furthermore revealed defects at EG -(0.04-0.13) eV, EG-(0.21-0.82) eV and two additional deeper defects at 1.91 Ev and 2.1 eV, respectively. These defect related transitions are very similar to those observed in hexagonal GaN.

Defect-related optical transitions in GaN.

Sub-band-gap absorption of GaN grown by metal-organic chemical vapor deposition on sapphire was investigated by photothermal deflection spectroscopy (PDS), transmission measurements, and the constant photocurrent method (CPM). We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spectra yields the dependence of the quantum efficiency-mobility-lifetime-product ($\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$) versus energy and gives relevant information about the excitation mechanisms. CPM spectra show a significantly smaller absorption (up to a factor of $\frac{1}{10}$) in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers excited with these photon energies have a relatively small $\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$ product and thus do not contribute to the externally detected photocurrent. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexagonal matrix and by excitation of electrons from occupied acceptors into the conduction band of the main hexagonal crystal modification ($h$-GaN). Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confirm this interpretation and can be correlated with the subgap absorption detected by PDS. Transient photocurrent measurements show a persistent photoconductivity, which can also be explained by the existence of isolated cubic-phase inclusions.

Electronic structure and light induced degradation of amorphous silicon-germanium alloys

A systematic study of the deep defect related electronic transitions on a series of device quality amorphous silicon-germanium alloys (a-Si,Ge:H) produced by the glow discharge method was carried out. The experimental results indicate a unique defect structure in these alloys; specifically, a deep defect band below the Fermi level and a clear majority carrier trap distribution above the Fermi level. More significantly, a defect band disclosed by sub-band-gap spectroscopy exhibits a strong relaxation effect, which suppress the electron thermal emission. It is believed that band may act as a ‘safe electron trap’ to enhance the lifetime of the minority carriers.

Room Temperature Band-Edge Luminescence from Silicon Grains Prepared by the Recrystallization of Mesoporous Silicon

AbstractRoom-temperature photoluminescence (PL) peaking at 1.1 eV has been found in electrochemically etched mesoporous silicon annealed at 950°C. Low-temperature PL spectra clearly show a fine structure related to phonon-assisted transitions in pure crystalline silicon (c-Si) and the absence of defect-related (e.g.P-line) and impurity-related (e.g.oxygen, boron) transitions. The maximum PL external quantum efficiency (EQE) is found to be better than 0.1% with a weak temperature dependence in the region from 12K to 400K. The PL intensity is a linear function of excitation intensity up to 100 W/cm2. The PL can be suppressed by an external electric field ≥ 105 V/cm. Room temperature electroluminescence (EL) related to the c-Si band-edge is also demonstrated under an applied bias ≤ 1.2 V and with a current density ≈ 20 mA/cm2. A model is proposed in which the radiative recombination originates from recrystallized Si grains within a non-stoichiometric Si-rich silicon oxide (SRSO) matrix.

Electron paramagnetic resonance in Zn3P2

Powdered Zn3P2 samples are studied by electron paramagnetic resonance (EPR) technique. Two types of material are used: of high resistivity with hole concentrations of 1014 to 1015 cm−3 as well as low resistivity with hole concentration of 1 × 1017 cm −3. The measurements are performed in the dark and under light illumination of energies equal to defect related transitions. The observed EPR spectra are explained in terms of native defects in Zn3P2. Phosphorus and zinc interstitials of the proper charge state seem to be responsible for the main EPR resonances.

Lateral transport in GaAs/AlGaAs quantum wells

The lateral transport of excess carriers in metal organic chemical vapor deposition grown GaAs/AlxGa1−xAs(x=0.5) 20 Å thick single quantum well structures is studied by cathodoluminescence (CL) between 5 K≤T≤170 K. Impurity and native defect related transitions are found to take place around grown-in dislocations where well thickness variations are also found. The CL intensity of free excitons and/or transitions involving impurities and native defects is measured as a function of distance from the dislocation to the point of excitation. Using a simple diffusion model, we are able to determine the hole diffusion length, L=1.5 μm, in slightly n-doped SQW. This represents a novel method for the direct determination of the diffusion length in sufficiently defect-free material.

Photoluminescence and photoacoustic investigation of residual defects in semi-insulating LEC GaAs

Photoluminescence and photoacoustic spectroscopy were used to investigate the effect of post-growth annealing carried out to improve the uniformity of Czochralski-grown semi-insulating GaAs. Single- and multi-step, ingot- and wafer-annealed crystals were studied. It is shown that wafer-annealed crystals do not present the additional defect-related transitions found in ingot-annealed samples. One of these defects is concluded to originate from arsenic precipitates. A second additional band is asserted to originate from an acceptor-like defect with a binding energy of 69 meV. Besides the improvement in crystalline quality, it is shown that the residual impurities, silicon and zinc, occur in very low concentrations in wafer-annealed samples, probably owing to an out-diffusion process during the thermal treatments.

Defect-related transitions in Zn3P2 studied by means of photovoltaic effect spectroscopy

Transitions at energies of 1.31 eV and 1.41 eV have been observed in Zn3P2 at room temperature (T=295 K) by using surface photovoltage (SPV) spectroscopy. Results obtained from absorption and photoconductivity measurements are also discussed here. The polarity of the PSV signal has been analysed. Two acceptor levels were found to be at 0.29 eV and 0.19 eV above the valence band.

Photoluminescence and Photoacoüstic Spectroscopy of Si Single-and Multi-Step Ingot- and Wafer-Annealed GaAs Crystals

ABSTRACTSI GaAs crystals submitted to single- or multi-step, ingot-or wafer-annealing are investigated using photoluminescence (PL) and photoacoustic spectroscopy (PA). The near-band-edge PL transitions are well resolved, with a neutral acceptor-bound exciton recombination displayed as a split doublet. The improvement induced by wafer-annealing is illustrated by the absence of additional defect-related transitions found after ingot-annealing. For the room temperature PA measurements, the intensity of a peak occuring at 1.39 eV is shown to lead to an estimation of the arsenic micro-defect density as evaluated by AB etching. The 1.39 eV PA band is also asserted to be the non-radiative recombination path of a 1.482 eV band found in the low-temperature PL spectra.

Optical properties of molecular beam epitaxially grown GaAs1−xSbx (0<x<0.5) on GaAs and InP substrates

Undoped and lightly Si doped GaAs1−xSbx bulk layers have been grown on GaAs substrates with Sb composition as high as 0.5 by molecular beam epitaxy. Low temperature (4 K) photoluminescence (PL) and absorption measurements were carried out. For samples with x values smaller than 0.06, PL spectra were dominated by two bands, the emission associated with the bound excitons and with the electron (or donor) to acceptor recombinations. However, the free exciton peak was clearly observable in absorption spectra. For higher Sb compositions (x>0.06), only one peak associated with the impurity or defect related transitions dominates PL spectra. We found that the energy and the line shape of this main peak are closely related to the sample quality for the same x value. The lower peak energy than the band gap, the low signal intensity and the large line broadening are due to the emissions associated with the defect band below the ideal (perfect crystal) energy gap introduced by the lattice mismatch near the heterointerface of GaAs1−xSbx and GaAs. As expected, higher quality epilayers provide a stronger PL intensity, smaller line width, as well as a higher peak energy.

Photoluminescence spectra of some ternary and quaternary chalcopyrite semiconductors

Photoluminescence (PL) emission spectra of single crystals and thin films of CuGaxIn1−xSe2 compounds have been investigated at various measuring temperatures, and compared with the emission from CuGaSe2 and CuInSe2. The observed PL spectra consisted of two groups of emission lines: the near-band-gap group (A) and the lower energy group (B). It was found that the type of emission obtained is determined by the amount of stoichiometry and molecularity deviations. The PL data also showed a correspondence between the defect related transitions from the CuGaxIn1−xSe2 solid solution and CuGaSe2 for x<0.5. Based on the intrinsic defect states in the two components CuInSe2 and CuGaSe2, the predominant defect states transitions in the solid solution are defined for the m<1 and ΔS>0. Interpretation of the PL spectra of Se-deficient compounds with m>1 are rather complicated, and much work remains to be done before the defect chemistry of CuGaxIn1−xSe2 could be fully understood.

Recombination ina−Si:H: Defect luminescence

A study of defect photoluminescence in $a\ensuremath{-}\mathrm{Si}:\mathrm{H}$ is presented. In both doped and undoped samples we observe a broad defect-related transition peaking at 0.9 eV with a width of \ensuremath{\sim}0.35 eV. The line shape, temperature dependence, and excitation energy dependence of this luminescence are described. The recombination is interpreted as a transition between an electron in a doubly occupied dangling bond, and a valence-band tail hole. We estimate that the electron trap depth is \ensuremath{\sim}0.5 eV and involves a distortion energy of about 0.1 eV. The relation between the luminescence data and light-induced electron spin resonance is also discussed.