Distribution Of Nonradiative Recombination Centers Research Articles

Development of laser heterodyne photothermal displacement method for mapping carrier nonradiative recombination centers in semiconductors

The laser heterodyne photothermal displacement (LH-PD) method was used to characterize the nonradiative recombination centers of semiconductors, such as defects and deep-lying electronic levels. When a semiconductor surface is irradiated with a modulated continuous wave laser, the irradiated area is periodically heated and expanded owing to the nonradiative recombination of the photoexcited carriers. The LH-PD can measure an absolute value of surface displacement and its time variation at various excitation beam frequencies (fex). Si and GaAs substrate samples were used to confirm the usefulness of the proposed method. The obtained time variation of the surface displacement was well explained by theoretical calculations considering the carrier generation, diffusion, recombination, heat diffusion, and generated thermal strain. Because nonradiative carrier recombination generates local heat at defects in semiconductors, the LH-PD technique is useful for analyzing defect distributions. Additionally, measurements of intentional Fe-contaminated Si samples confirmed that this technique is suitable for defect mapping. Displacement mapping with changing fex suggests the potential to measure the distribution of nonradiative recombination centers in the sample depth direction.

Detection of Nonradiative Recombination Centers in GaPN by Combining Two‐Wavelength Excited Photoluminescence and Time‐Resolved Photoluminescence

Presence and influence of nonradiative recombination (NRR) centers in an intermediate band (IB)‐type material, GaP1–xNx (), are studied by two‐wavelength excited photoluminescence (TWEPL) method and time‐resolved photoluminescence (TRPL) measurement at 77 K. With the use of below‐gap excitation (BGE) light in addition to an above‐gap excitation (AGE), the PL peak intensity is found to increase which indicates the presence of NRR centers and a secondary excitation from the IB to conduction band (CB). Depending on the effect of different BGE energies, an energy diagram on the distribution of NRR centers and NRR process is interpreted. The saturation of PL increase is attributed to the trap‐filling effect in NRR centers, which allows us to modify the rate equation. The NRR parameters are evaluated by a qualitative simulation of the modified rate equations of one‐level model together with the lifetime determined by TRPL. In continuation of evaluating NRR parameters by rate equation analysis, the addition of TRPL measurement improves accuracy and approaches the determination of NRR parameters. A successful characterization of NRR centers leads to a proper optimization of IB‐type solar cells (IBSCs).

Impact of surface morphology on the properties of light emission in InGaN epilayers

Scanning near-field optical microscopy was used to study the influence of the surface morphology on the properties of light emission and alloy composition in InGaN epitaxial layers grown on GaN substrates. A strong correlation between the maps of the photoluminescence (PL) peak energy and the gradient of the surface morphology was observed. This correlation demonstrates that the In incorporation strongly depends on the geometry of the monolayer step edges that form during growth in the step-flow mode. The spatial distribution of nonradiative recombination centers — evaluated from PL intensity maps — was found to strongly anticorrelate with the local content of In atoms in the InGaN alloy.

Photo-etching of GaN: Revealing nano-scale non-homogeneities

Nano-scale non-homogeneities in MOCVD- and HVPE-grown gallium nitride were revealed by means of photo-etching in K2S2O8–KOH solution and by chemical etching in hot KOH solution. These non-homogeneities result in formation of protruding etch features and in-depth elongated nano-pits, respectively. High spatial resolution cathodoluminescence evidenced a non-homogeneous distribution of non-radiative recombination centers, which correlates well with photo-etched pattern of protruding etch features. The density of these features is two orders of magnitude higher than this of threading dislocations. A possible association between the non-uniform in nano-scale distribution of native point-type defects, such as VGa, ON and related VGa–ON complexes, and the observed etch features is presented. Existence of such fluctuations may have a large influence on the reliability of nitride-based electronic and optoelectronic devices.

High spectral uniformity of AlGaN with a high Al content evidenced by scanning near-field photoluminescence spectroscopy

Scanning near-field photoluminescence (PL) spectroscopy was applied to study spatial variations of emission spectra of AlxGa1−xN epilayers with 0.6≤x≤0.7. PL spectra were found to be spatially uniform with peak wavelength standard deviations of only ∼2 meV and ratios between peak intensity standard deviations and average peak intensity values of 0.06. The observed absence of correlation between the PL peak wavelength and intensity shows that spatial distribution of nonradiative recombination centers is not related to band potential fluctuations. Our results demonstrate that the homogeneous broadening and the random cation distribution primarily determine PL linewidths for layers grown under optimized conditions.

Characterization of partially ordered GaInP/GaAs heterointerfaces by the quantum Hall effect

The new approach to the characterization of semiconductor interfacial properties by the quantum Hall effect (QHE) and the scanning near field optical microscopy (SNOM) is demonstrated to the heterointerfaces of partially ordered GaInP/GaAs grown by low-pressure Metal Organic Vapor Phase Epitaxy. The Shubnikov de-Haas (SdH) oscillations and the Hall plateaus are observed in the heterointerfaces of both the less-ordered and more-ordered GaInP/GaAs samples with a large clover-shape, but these samples exhibit both 2D and 3D electron behaviors. In contrast to large clover-shaped samples, the distinct SdH oscillations and the Hall plateaus in the less-ordered sample, while the single SdH oscillation and the corresponding large plateau in the more-ordered small Hall-bar sample are observed. These results suggest that there may be many domains, each having a different carrier density and sizes in the less-ordered sample, while one or few large domains with uniform carrier concentration and sizes in the more-ordered sample. In SNOM measurements, PL intensity varies in the mapping of the more-ordered sample and it is concluded that the variation of the PL intensity may result from an inhomogeneous distribution of non-radiative recombination centers in the more-ordered sample.

Time-Resolved Microphotoluminescence Study of Cu(In,Ga)Se2

The carrier recombination processes in Cu(In1-x,Gax)Se2 (CIGS) thin films were investigated by time-resolved microscopic-photoluminescence (µ-PL) measurement at room temperature. For films with x = 0.45, the spatial distribution of the donor–acceptor pair luminescence is much larger than the grain size. The PL decay lifetime is directly correlated with the µ-PL intensity, but the spectral shape is identical regardless of the sample position. These results suggest that the nonuniform distribution of nonradiative recombination centers mainly affects the carrier recombination in CIGS thin films. At relatively high Ga concentrations (x ≥0.7), the spatial inhomogeneity in µ-PL intensity is enhanced and decay accelerated, suggesting an increase in the density of nonradiative recombination centers. Thus, suppression of nonradiative recombination is critical to enhancing the performance of CIGS-based solar cells.

EBIC study of nonradiative recombination in silicon LEDs with near-band-edge luminescence

Silicon light-emitting diode structures with a near-band-edge luminescence have been studied by the EBIC method. The structures were fabricated by ion implantation with different temperatures of the final stage of the postimplantation annealing (950 and 1000°C). Upon raising the annealing temperature, the diffusion length of minority carriers increases from 1–2 to 25–35 μm in the light-emitting diode region in which the near-band-edge luminescence occurs. Also, the distribution of nonradiative recombination centers in this region becomes less nonuniform in the lateral direction, parallel to the plane of the p-n junction. It is these factors that lead to the observed substantial increase in the intensity of the near-band-edge luminescence.

Spatially‐resolved photoluminescence study of high indium content InGaN LED structures

AbstractSpatially and spectrally resolved photoluminescence (PL) of green light‐emitting diode (LED) structure with InGaN multiple quantum wells was studied using confocal and scanning near field optical microscopy techniques. Inhomogeneous distribution of PL intensity on the scale of 200 nm has been observed. The PL bands peaked at approximately 492 nm and 502 nm in bright and dark areas, respectively. The spot‐to‐spot variation of the peak position of the main PL band was in the range of ∼30 nm for the bright and ∼50 nm for the dark areas. The highest PL intensity in the bright areas was observed in the spots with PL band peaked at ∼492 nm, while emission intensity in dark areas had no correlation with the PL band spectral position. The inhomogeneous PL intensity distribution is explained by inhomogeneous distribution of nonradiative recombination centers, in contrast to a fairly constant tail states density (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Detailed defect study in proton irradiated InP/Si solar cells

A detailed study of the effects of proton irradiation-induced defects in heteroepitaxially grown InP/Si solar cells has been made through a combination of cathodoluminescence (CL), electron beam induced current (EBIC), and electrochemical capacitance versus voltage (ECV) carrier profiling measurements. The CL data indicate the distribution of nonradiative recombination centers both before and after proton irradiation, and temperature dependent and spectroscopic analysis of the CL signal give an estimate of the energies of the dominant defect levels. The EBIC data yield an estimate of the magnitude and spatial variation of the minority carrier diffusion length (L) in the base region. Values of L determined from EBIC measurements made on solar cells irradiated by protons ranging in energy from 0.1 up to 4.5 MeV follow a single curve when plotted versus displacement dose, Dd, allowing a single proton damage coefficient to be determined. The ECV measurements show the evolution of the carrier concentration profile in the cell under irradiation, as carrier removal first depletes and eventually type converts the base region. From an in-depth analysis of the combined data, the physical defects that give rise the radiation-induced energy levels are suggested, and a detailed understanding of the physical mechanisms causing the radiation response of InP/Si solar cells is developed.

Photoluminescence Imaging of III-V Substrates and Epitaxial Heterostructures

AbstractThe characterization of III-V compound semiconductor substrates and epitaxial layers with photoluminescence imaging is reviewed. The luminescence patterns of semi-insulating GaAs are dominantly determined by the concentration and distribution of nonradiative recombination centers, as shown by comparison with spectroscopic temperature and lifetime topography of photoexcited carriers. Wafers fabricated with various growth and annealing procedures are evaluated. Presently available informations on nonradiative centers in GaAs are summarized and discussed. The correlation of luminescence, absorption and resistivity topograms of InP substrates shows various interrelated influences of the Fe acceptor distribution. High resolution luminescence images of growth induced, strain induced and substrate induced defects in epitaxial heterostructures are obtained. The generation of relaxation dislocations in pseudomorphic layers is influenced by growth parameters, layer structures, layer doping and also by substrate properties. Nonradiative recombination center patterns replicate the arrangement of threading dislocations in the substrate.