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Photothermal atomic force microscopy (PT-AFM) enables us to examine non-radiative recombination property with high spatial resolution. In order to realize variable frequency modulation of incident light in PT-AFM, we have proposed a multi-pulse modulation method, where the duration for light illumination is divided into multiple light pulses and their period is varied. We have assessed the validity of the proposed method through acquisition of photothermal signal images on microcrystalline materials under various modulation frequencies.

The efficiency potential of chalcopyrite and kesterite solar cells including CIGSe (CuInGaSe2), CIGS (CuInGaS2), CZTS (Cu2ZnSnS4) and CZTSSe [Cu2ZnSn(S,Se )4] solar cells is discussed based on external radiative efficiency (ERE), open-circuit voltage loss, fill factor loss, non-radiative recombination and resistance loss. CIGSe cells achieve efficiency potential of 26.8% and 27.5% by improving the ERE from around 1% to 10% and 20%, respectively. CIGS and CZTS(Se) cells achieve the efficiency potential of 25% and 22%, respectively, by improvement in ERE from around 1 × 10−4% to 3%–5%. The effects of non-radiative recombination and resistance loss upon the properties of wide-bandgap CIGSe, CIGS and CZTS(Se) cells are discussed. In the case of wide-bandgap CIGSe cells, lattice mismatching between the buffer layer and CIGSe active layer and deep-level defects are thought to originate from non-radiative recombination loss. CIGS and CZTS(Se) cells are shown to have lower ERE and higher resistance loss compared to that of CIGSe cells.

A model calculation on optical gain and co-stimulated emission of photons and phonons in indirect bandgap semiconductors such as silicon is presented. An analytical expression for optical gain via phonon-assisted optical transitions in indirect bandgap semiconductors is presented. Population inversion can occur when the difference between the quasi-Fermi levels for electrons and holes is greater than the photon energy. The rate equations and their steady state solutions for electron, photon, and phonon involved in the phonon-assisted optical transitions are presented. It is shown that co-stimulated emissions of photons and phonons will occur when the threshold condition for laser oscillation is satisfied. The magnitude of optical gain in bulk crystalline silicon is calculated and shown to be smaller than the free carrier absorption at room temperature. However, it is shown, for the first time, that the optical gain is greater than the free carrier absorption in bulk crystalline silicon at the temperature below 23 K. Thus, the calculation predicts that the co-stimulated emissions of photons and phonons could take place in bulk crystalline silicon at the low temperature.

Piezoelectric photothermal spectroscopy (PPTS) measurements were conducted on p- and n-type silicon, orientation samples. We showed that a PPTS signal bearing a lower energy edge around 1.048 eV at room temperature was due to band-to-band transitions with optical mode phonon (0.064 eV) absorption. Similarly, a signal bearing an edge above 1.19 eV, observed at temperature less than 200 K was due to band-to-band transitions involving the phonon emission. The two signals were observed in both p- and n-type samples. These results indicated that very low absorption coefficient in indirect semiconductor can be effectively observed by using the proposed PPTS technique. An additional signal bearing a broad peak around 1.18±0.01 eV at room temperature was also observed for both types of samples. We proposed that the broad-peak signal was due to surface states.

Thermal quenching of luminescence of Er dopants in Si nano-crystals (Si-nc's) was investigated employing an impedance model for the analysis of photo-injected charges. Relaxation response indicated that Er doping forms not only optical centers but also trapping centers near the Si-nc's. The response time constant of trapped charges was dependent on temperature, with the dependence correlating to thermal quenching. These findings indicate that quenching occurs by trapping followed by consumption of charges. The complex analyses revealed that the response represents nonradiative recombination at the centers rather than release of confined charges from the Si-nc through the centers. We propose a possible energy diagram for the non-radiative recombination. � 2014 The Japan Society of Applied Physics.

We investigate the effects of inductively coupled O2 plasma ashing, capacitively coupled C3F8 plasma etching, and capacitively coupled C3F8/O2 plasma etching on the optical and electrical characteristics of GaAs-related optical device structures. The O2 plasma ashing produces non radiative recombination centers close to the sample surface. The sample with a cap AlGaAs layer prevents the degradation of photoluminescence (PL) intensity, probably inhibiting the penetration of damaging ions into the buried quantum well. The C3F8 plasma etching also degrades the PL intensity; however, under moderate conditions, the effect of non radiative surface recombination can be suppressed by the formation of a surface oxide layer. On the other hand, the formation of oxide increases the specific contact resistance (ρc) of the electrode. The etching with C3F8/O2 mixed plasma also deteriorates the optical characteristics presumably owing to the synthesized impact of the constituent gases, while it has limited effect on ρc.

Plan-view transmission electron microscopy (TEM) and cathodoluminescence (CL) images were taken for the same sample at exactly the same location in n-type GaN grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). There was a clear one to one correspondence between the dark spots observed in CL images and the dislocations in TEM foils, indicating that the dislocations are non-radiative recombination centers. The hole diffusion length in n-type GaN was estimated to be neighboring 50 nm by comparing the diameters of the dark spots in thick samples used for CL and samples that were thinned for TEM observation. The efficiency of light emission is high as long as the minority carrier diffusion length is shorter than the dislocation spacing.

Photoreflectance (PR) spectroscopy and capacitance-voltage (C-V) measurements are used to characterize the surface damage caused by BCl3/Ar plasma in barrier layer of GaInP/GaInAs/GaAs pseudomorphic high electron mobility transistors (PHEMT). When the BCl3/Ar flow ratio is either lower or higher than 6:4, PR spectra indicate plasma causes damage such as nonradiative recombination centers, scattering centers and strain which lead to decreased signal intensity, broadened linewidth, and the spectral shift, respectively. The plasma-induced surface damage also results in a positive voltage shift of the C-V curve. By using a BCl3/Ar mixed gas, the positive voltage shift of the C-V curve is less than that of samples with pure BCl3. Both PR and C-V measurements are in good agreement. These results show that photoreflectance spectroscopy is a powerful, nondestructive tool for investigating surface damage and can be used to improve the performance of PHEMTs.

We have studied the quality of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double hetero (DH) structures, whose lower hetero interface is formed by regrowing GaAs directly on an etched Al0.3Ga0.7As surface. The Al0.3Ga0.7As surface is prepared by two-step in situ etching, which is comprised of a low temperature treatment and high temperature etching with HCl gas, just prior to the regrowth by metalorganic chemical vapor deposition. Time resolved photoluminescence measurements have revealed that the optical quality of the regrown DH structure is significantly improved by applying the two-step HCl gas etching process compared to the wet processed one. The PL decay curve of the in situ processed sample, however, shows a relatively shorter carrier lifetime than that of the continuously grown DH sample. An interface recombination velocity of 6.8×103 cm/s is obtained for the in situ processed AlGaAs interface. This value for the AlGaAs with a high Al content is comparable to the best results for the in situ processed GaAs and InGaAs surfaces reported so far. The relatively high recombination velocity compared to continuously grown interfaces is mainly due to residual oxygen at the in situ processed interface of concentrations as low as 2.2×1011 cm-2.

Recombination at p-type Si(100) surfaces electrochemically treated in NH4F solution is investigated in situ by photoluminescence (PL) under excitation with short pulses of a N2-laser. By registration of the integrated PL intensity we detect sensitively changes in the surface defect concentration. Periodic variations of the nonradiative recombination rate during electropolishing in the oscillating regime together with surface passivation by hydrogenation are observed. The estimated density of the surface nonradiative centers changes from 1013 cm-2 during the electropolishing to 5×1010 cm-2 for quiescent hydrogenated surfaces.

It has been found byin situmeasurements that immersion in deionized water and simultaneous uv irradiation of porous silicon can cause a considerable change in emission intensity and a blue shift in visible (yellow) photoluminescence. It was determined that the blue shift is mainly due to reduction in size and surface alteration by oxidation of porous Si and that an initial increase and subsequent decrease in intensity is due to competition between surface passivation and generation of nonradiative recombination centers by the oxidation. After prolonged irradiation, a new blue emission was observed in place of the yellow emission. Excitation spectra for the yellow photoluminescence were also investigated.

A nonradiative transition in ion-implanted and epitaxially grown layers on a Si substrate was measured by a piezoelectric photoacoustic (PA) spectroscopy. It is considered that the observed peak at 1.07 eV is caused by a nonradiative recombination of optically excited electrons from boron acceptor levels. The decrease of the 1.07 eV peak intensity by ion implantation is considered to be due to the formation of a surface damaged layer with deep defect levels. The PA spectra of epitaxially grown samples are well explained by summing the spectra of p- and n-type bulk samples. The electric field at the p-n junction may be effective in the PA signal generation mechanism in the higher photon- energy region.

Submicron heterojunction bipolar transistors (HBTs) with maximum frequency of oscillation, f MAX, of 91 GHz have been fabricated using a self-aligned technique and a very heavily carbon-doped (1020 cm-3) base layer. Since the quenching of photoluminescence (PL) intensity in heavily-doped GaAs is mainly due to nonradiative recombination in the bulk material, while contribution from surface recombination is negligible, the use of a heavily carbon-doped base layer in AlGaAs/GaAs HBTs minimizes the influence of surface recombination in the extrinsic base region. Thus, for HBTs with a heavily doped base layer, the ”emitter size effect” (degradation of HBT current gain) is greatly reduced when the emitter width is scaled down to submicron (0.6 µm) dimensions.

Photoacoustic (PA) measurements of p-Si single crystals annealed at 450°C were carried out at room temperature near an optical absorption edge. The height of the pronounced peak at 1.07 eV, which appears only in the p-type samples, becomes smaller and disappear with an increase in the annealing time. Since annealing facilitates the thermal donor formation, the decrease in the peak height is explained by the compensation mechanism of the boron acceptors. The reappearance of the 1.07 eV peak with further annealing at 800°C for 30 minutes, where thermal donors are swept away, firmly supports our explanation.

Photoacoustic (PA) measurements were carried out at 95 K for semi-insulating (SI) GaAs. A broad peak at 0.92 eV and humps in the higher-energy region are observed. Onsets of the humps are at 1.04 and 1.25 eV. Among these structures in the PA spectra, the broad peak at 0.92 eV is partly photoquenched upon illumination of a secondary light (hν=1.12 eV) and thermal recovery occurs after heating to around 150 K. The spatial profiles of this photoquenching along the wafer diameter show a W-shaped distribution. These results indicate that the quenching component of the peak at 0.92 eV in the PA spectra is due to nonradiative recombination of electrons excited from the EL2 level to the Γ-conduction band minimum. The electron transitions involving the EL2 level can be resolved by the PA measurements for the first time.

We have investigated the optical properties of wet chemically etched InP/InGaAs-wires with widths between 100 nm and 10 µm for a wide range of excitation densities and energies. We observe that the non radiative recombination centers at the etched sidewalls can be saturated already at moderate excitation densities of 100 W/cm2. Avoiding saturation effects we obtain a surprisingly large sidewall recombination velocity of 107 cm/s at 77 K. The comparison of luminescence measurements at excitation energies above and below the InP band gap energy shows, that the non radiative recombination centers are located in the InGaAs quantum well.

Time-resolved luminescence spectra and electron spin resonance (ESR) spectra were measured at 300 K and 77 K in CdSSe-doped glass. Intense light irradiation accelerates the decay of luminescence. The change in ESR spectra is indicative of the creation of recombination centers by irradiation.

Nonradiative recombination processes have been studied in Cd1-x Mn x Te of x=0.4 and 0.6 by highly sensitive photoacoustic spectroscopy. The photoacoustic spectrum is found to be extended deeply into the energy-band-gap region by 150-300 meV and shows a remarkable decrease of the photoacoustic intensity at 2.07 eV. The results are interpreted in terms of the nonradiative recombination process which takes place in the fluctuating band-edge region of the material, and the suppression of the nonradiative process due to the energy transfer from the band electrons to the Mn ion levels. The relationship with the photoluminescence process is also discussed.

The trap density, electron and hole capture rates of the dominant nonradiative recombination center in heavily Si-doped ([Si]=1.6×1019 cm-3) GaAs/Al0.22Ga0.78As multiple quantum well structures were determined by examining the excitation power dependence of time-resolved photoluminescence characteristics. Sufficient power excitation allowed us to observe the increase of the band emission lifetime due to the trap-filling effect. A theoretical analysis based on the deep-level statistics agreed fairly well with the experiment, which revealed the recombination mechanism in such a heavily doped condition for the first time.

Nonradiative electron-hole(e-h) recombination characteristics have been investigated for a dominant mid-gap electron trap (0.86 eV) related to oxygen-complex defect center in MBE-grown Al x Ga1-x As films(x=0.4) by means of an extended capacitance-transient spectroscopy technique. Direct evidence is presented to demonstrate that the 0.86 eV defect level plays an important role as an efficient electron-hole nonradiative recombination channel as well as an electron capture and emission center role accompanied with strong lattice relaxation.

Large increases in nonradiative recombination lifetimes were observed for the first time in a In-doped semi-insulating GaAs. The variation ratio of the lifetime increase is over 180 for the incident optical power of 8 mW/cm2. The increase in the diffusion length associated with the lifetime increase in the bulk is also indicated.

Transient photoluminescence measurements for pulsed electric field at room temperature on AlGaAs quantum well structures have been carried out to clarify the characteristics of luminescence switching by an electric field. With our method, the radiative lifetime and nonradiative lifetime can be separately evaluated. Luminescence switching by electric field shows a high speed capability that is free from lifetime limitation. Based on the relationships between radiative and nonradiative processes, the intensity modulation of luminescence under the condition of a constant carrier density is demonstrated.

Interaction of non-radiative recombination centers with dislocations in a LEC-grown GaAs crystal is investigated by observing the change in cathodoluminescence image caused by various heat-treatments. Impurities related to non-radiative recombination centers are gettered by dislocations most effectively at a temperature of around 750°C. The cause of the gettering is concluded to be due to the reaction incorporating the impurities that takes place at the dislocation core.