Piezoelectric Photothermal Signal Research Articles

Photothermal investigation for optimizing a lattice strain relaxation condition of InGaAs/GaAsP superlattice photovoltaic structures from a nonradiative transition point of view

The carrier collection efficiencies of InGaAs/GaAsP superlattice (SL) photovoltaic structures were optimized by choosing adequate manufacturing parameters, such as the composition and thickness of the quantum wells (QWs) and barrier layers. However, no insights have been observed from the viewpoint of the nonradiative transition of photoexcited carriers. In this study, piezoelectric photothermal (PPT) and photoluminescence (PL) measurements were performed as a function of temperature from 100 to 340 K. Using a piezoelectric transducer, the PPT signal detected the heat generated by nonradiative recombination (NR). The indium composition of the QW layer was fixed at 0.3, and the phosphorus composition x[P] in the barrier layer was changed from 0.4 to 0.6. The observed temperature dependences of the PPT and PL signal intensities were analyzed using a rate equation for the photoexcited carriers in e1 and hh1 quantized levels. Four carrier dissipating processes, namely, radiative recombination, NR, thermal escape from the QW thermal excitation (TE), and tunneling after thermal excitation (TATE), were considered for both electrons and holes. Thermal activation energies were included in the NR, TE, and TATE processes. Because nonradiative and radiative transition components cause PPT and PL signals, curve fitting of the temperature behavior enabled us to determine the activation energies. We then found that the activation energy of the NR process reached a maximum at x[P] = 0.45. No such maxima were observed for the TE and TATE process. This result was explained by a trade-off between the strain valance condition over the entire range of the SL structure and the local residual strain at the interfaces between the QW, interlayer, and barrier layer when x[P] increased. Because no software can theoretically calculate the activation energy of the NR process, we demonstrated the usefulness of the present PPT experimental methodology for investigating carrier transport properties.

Reduction of non-radiative recombination by inserting a GaAs strain-relaxation interlayer in InGaAs/GaAsP superlattice solar cells investigated by photo-thermal spectroscopy

The role of a GaAs strain-relaxation interlayer inserted into InGaAs/GaAsP superlattice solar cells was evaluated by measuring the piezoelectric photothermal (PPT) signals in the temperature range from 100 K to a device operation temperature of around 340 K. The PPT signals caused by the non-radiative recombination of electrons photo-excited to the first quantized level were observed. The temperature-dependent PPT signal intensities were assessed using an electron carrier relaxation model comprising four processes: radiative recombination, non-radiative recombination, thermionic emission, and tunneling of carriers through the e2-miniband after thermal excitation from the e1-level. The contribution of holes in the hh1 state was also included in this model, in which e1 and e2 are the first and second electron levels in the conduction band, respectively, and hh1 is the first heavy hole level in the valence band of the quantum wells. A similar analysis was conducted using photoluminescence (PL) spectra to elucidate the carrier transition dynamics in greater detail, because PPT and PL measurements are complementary to each other in terms of non-radiative and radiative electron transitions. Consequently, although the non-radiative recombination remained dominant around room temperature, the quantum yield of the carrier tunneling process increased and became comparable to that of non-radiative recombination. This implies that the recombination loss of the photo-excited carriers is suppressed by the insertion of the GaAs interlayer. By clarifying the role of the inserted interlayer with respect to the non-radiative recombination process, the usefulness of the PPT method is demonstrated.

Decreasing of the thermal conductivity of Si nanopillar/SiGe composite films investigated by using a piezoelectric photothermal spectroscopy

To investigate the decrease of thermal conductivity () of nanoscale Si materials, we conducted the piezoelectric photothermal (PPT) method for the highly periodic Si nanopillar arrays embedded in Si0.7Ge0.3. The PPT is an electrode free method that can measure a heat propagation in the parallel to the nanopillars direction. A distinctive dip was observed in the frequency-dependent PPT signal intensity. By focusing the dip frequency, was estimated from the comparison with the model analysis based on the one-dimensional multilayer thermal diffusion equation. The estimated was 0.19 ± 0.07 W m–1 K, in the parallel to the nanopillars direction. Since the considerable decrease of was confirmed from the non-radiative recombination point of view, we found the present non-destructive PPT method is very useful to estimate in the nanostructured devices for the thermoelectric application.

Effects of barrier thickness on carrier non-radiative relaxation process in InGaAs/GaAsP superlattice solar cells by piezoelectric photothermal and surface photovoltage spectroscopies

We investigated the carrier non-radiative relaxation process of InGaAs/GaAsP superlattice (SL) solar cells with different barrier thicknesses by combining piezoelectric photothermal (PPT) and surface photovoltage (SPV) measurements. The former technique detected heat generated by non-radiative relaxation and the latter detected the surface potential change induced by the carrier accumulation. Although the mechanisms of these measurements were different, corresponding signals of the transition between the quantum levels were observed in both spectra. The SPV signal intensities were independent of the barrier thicknesses. The carrier tunneling process functioned poorly under open-circuit conditions. Conversely, the PPT signal intensities increased with decreasing barrier thickness. We experimentally demonstrated that the non-radiative relaxation component increased as the barrier thicknesses decreased owing to the lattice relaxation at interfaces between the quantum well and barrier layers. We concluded that the non-radiative relaxation process in the SL structure must be directly evaluated to improve the solar cell performance.

Investigation of miniband formation and optical properties of strain-balanced InGaAs/GaAsP superlattice structure embedded in p–i–n GaAs solar cells

To improve the superlattice (SL) solar cell performance, we carried out an accurate estimation of transition energies and miniband widths and focused on understanding of the optical properties of the SL structure using piezoelectric photothermal (PPT), photoreflectance (PR), and photoluminescence (PL) methods. Solar cell structure samples with different barrier thicknesses from 2.0 to 7.8 nm in quantum wells were prepared. From the PR and theoretical calculation, the formation of a miniband was confirmed. The PL peak showed a redshift and a decrease in signal intensity with decreasing barrier thickness, which were explained by carrier separation as a consequence of electron transportation through the miniband without recombination. The PPT signal intensities of the SL were still large even for the 2.0-nm-barrier-thickness sample. It is conceivable that the multiple-phonon emission during carrier transport through the miniband was detected. The usefulness of multidimensional investigation by using the above three methods is clearly demonstrated.

Nitrogen Related Deep Levels in GaAsN Films Investigated by a Temperature Dependence of Piezoelectric Photothermal Signal

The temperature dependences of the piezoelectric photo-thermal (PPT) signals from unintentionally doped p-type GaAsN films grown on semi-insulating GaAs substrate were measured from 80 to 300 K. From the theoretical analysis based on the rate equation for the recombination of photo exited carriers to the localized levels, we identified five majority hole traps, P1-P5 in GaAsN films. Among them, estimated concentrations of the P3 and P5 traps increased with the nitrogen contents. Therefore, we concluded that these two traps were due to nitrogen-related recombination centers in GaAsN.

Investigation of the photovoltaic performance of the polycrystalline silicon p–n junction by a photothermal measurement

For establishing a new methodology for evaluating an effect of the grain boundaries, both the piezoelectric photo-thermal (PPT) and the surface photo-voltage (SPV) measurements of polycrystalline Si p–n junction samples with different volume fractions of grain boundaries were carried out. We could define the signal intensity ratio of SPV/PPT as the key indicator of photovoltaic performance. This is because the PPT signal implies the phonon emitting carrier loss, whereas the SPV denotes the photo-excited carrier accumulation at the surface and the junction interface. It was found that the SPV/PPT ratio and solar cell efficiency decreased with increasing volume fraction of the grain boundaries. Present experimental results demonstrated that one can directly estimate the photovoltaic performance of in-process polycrystalline Si p–n junction wafer by adopting the combination of the PPT and the SPV methodologies without electrodes. Since the PPT detects the non-radiative recombination process, present methodology and the laser-beam-induced current and the photoluminescence imaging methods are complementary. By complementary use of these methods, it becomes possible to investigate the characteristic of grain boundary.

Temperature dependence of the optical gap of diamond-like carbon films investigated by a piezoelectric photothermal spectroscopy

Abstract Temperature dependences of the optical gap of diamond-like carbon (DLC) films were investigated by using a piezoelectric photothermal (PPT) spectroscopy, that detects the nonradiative transition of photoexcited electrons. Since the PPT signal intensity is expected to be proportional to its optical absorption coefficient, the optical gap was estimated by a Tauc plot. The temperature coefficient of the optical gap energy was found to be very small compared with that of other semiconductor such as Si. This result implies that the device performance is expected to be stable by temperature change and the DLC films are excellent as the next generation solar cells materials.

Nonradiative Investigation of Photodecomposition of Poly(di-n-hexylsilane) Thin Films Using Piezoelectric Photothermal Spectroscopy

To investigate the photodecomposition mechanism of poly(di-n-hexylsilane) (PDHS), the ultraviolet (UV) light irradiation time dependence of the optical absorption (OA) and the piezoelectric photothermal (PPT) signals were measured in vacuum. A strong OA peak at 370 nm and a broad OA band at approximately 320 nm related to the all-trans and helical conformations, respectively, were observed. With increasing UV light irradiation time, their intensities decreased and blue shifts were observed because of the decomposition of Si–Si bonds in the silicon backbone. In the PPT spectrum, a nonradiative recombination peak associated with the all-trans conformation was also observed at 370 nm. When UV light irradiated the sample, in addition to the decrease in the intensity and the blue shift of the PPT peak at 370 nm, new PPT signals at approximately 290 nm were observed. We concluded that these signals were caused by the silicon and/or carbon dangling bonds generated by photodecomposition in vacuum.

Investigation of Carrier Recombination Processes in GaAs/AlAs Multiple Quantum Wells Using Piezoelectric Photothermal and Surface Photovoltage Techniques

We investigate the confinement, radiative, and nonradiative recombinations of photoexcited carriers in each subband of GaAs/AlAs multiple quantum wells (MQWs) using two nondestructive techniques, namely, the surface photovoltage (SPV) and the piezoelectric photothermal (PPT) techniques. In the temperature range from 160 to 300 K, there is no difference between the temperature dependences of carrier confinement in 1HH, 2HH, and 3HH subband transitions. As temperature decreased, increases in the value of the nonradiative recombination component in 2HH and 3HH were observed. These are attributed to the phonon-emitting intersubband relaxation. It was found that the number of emitted phonons by this relaxation increased with increasing order of the subband and with decreasing sample temperature. We conclude that this phonon-emitting intersubband relaxation is the most important process for electron transition in the higher-order subband in MQWs and for PPT signal generation. This is the first time that phonon-emitting intersubband relaxation was directly detected from the nonradiative recombination viewpoint.

Temperature and thickness dependences of electron nonradiative transitions in GaInNAs/GaAs SQW investigated by piezoelectric photothermal spectroscopy

Recently, GaInNAs/GaAs single quantum well (SQW) structures have been considered as a new candidate material for an effective infrared LED for optical fibre communications. In order to investigate the electron nonradiative transitions in the GaInNAs SQW layer, piezoelectric photothermal (PPT) spectroscopy was used. The thickness- and the temperature-dependences of the PPT signal for the samples of thickness 10, 5 and 3 nm and from 300 K down to 77 K were investigated. The PPT spectra show a blue shift of the exciton and band-to-band transition with decreasing sample thickness. This was explained by quantum mechanical discussions for the confinement of the charge carriers supposing an electron effective mass of 0.08 m0 and band offset ratio of 6:4. The effect of the induced strain in the quantum well on the electronic structure is also discussed. The decreasing temperature also exhibits a blue shift of the PPT signal peaks and a decrease of the PPT signal amplitude. The temperature variation shows that the nonradiative intraband transition between the heavy and the light hole bands causes the additional contribution to the signal. It is concluded that the present PPT technique is an effective methodology for measuring the electron transitions in the SQW.

Optical properties of GaAsNSe/GaAs superlattice investigated by means of piezoelectric photothermal spectroscopy for nonradiative electron transitions

Optical properties of the GaAsNSe/GaAs superlattice (SL) were investigated using piezoelectric photothermal (PPT) and photoluminescence (PL) spectroscopies. The sample was grown by metalorganic molecular beam epitaxy on a semi-insulating (SI-)GaAs substrate. The SL consists of five periods of GaAsNSe and GaAs layers of thickness 11 nm. The temperature variation of the PPT spectrum of this sample was measured down to liquid nitrogen temperature and three peaks were observed at 0.7, 0.90 and 1.25 eV at 80 K. The 1.25 eV peak seems to be due to the bandgap transition of GaAsNSe. Comparing the PPT spectra with the PL results, it was considered that the localised levels caused the PPT signal peaks at 0.7 and 0.90 eV. The intensity of the 0.90 eV peak drastically changed with secondary light illumination. The effect of the electron nonradiative recombination at the interface states should be considered.

Effect of Surface States on Piezoelectric Photothermal Specta of Silicon Single Crystals

The effect of surface states on the piezoelectric photothermal (PPT) spectra of p- and n-type, <100> oriented, single crystal silicon was investigated. We showed that a PPT signal bearing a lower energy edge around 1.04 eV at room temperature was due to the bulk effect. This bulk signal was greatly affected by another signal bearing a broad peak around 1.18±0.01 eV. The broad peaked signal was found to be due to the surface states. The variation in amplitude and phase of the 1.18±0.01 eV signal revealed that the surface states were affected by the ambient illumination and low-temperature thermal annealing at 350 K. The results also indicated that a very low absorption coefficient in indirect semiconductors can be effectively observed by using the proposed PPTS technique.

Investigation of the annealing effect on the nonradiative carrier recombination in AlGaAs/GaAs utilizing the piezoelectric photothermal technique

The electron nonradiative recombination process of photoexcited carriers in as-grown and annealed n=Al0.2Ga0.8As/GaAs heterostructure samples is investigated by using piezoelectric photothermal (PPT) spectroscopy. The PPT signal above the band-gap energy of GaAs substrate decreased when the sample was annealed at 815 °C. In the frequency dependent measurements, the deviations from the 1/f linear function are clearly observed in the AlGaAs/GaAs samples. This critical deviation frequency was found to shift to the lower frequency region by annealing. Our experimental results are explained by assuming that the sample annealing generates an unknown deep level in the AlGaAs epitaxial layer region and this level effectively traps the photoexcited carriers nonradiatively.

Investigation of EL6 Deep Level in Semi-Insulating GaAs by Means of a Temperature Dependence of Piezoelectric Photothermal Signal

The temperature dependence of the piezoelectric photothermal (PPT) signal intensity of semi-insulating (SI) GaAs from 160 to 297 K was measured. One peak at 205 K was observed in the temperature dependence of the PPT signal. Curve fitting to experimental results by a theoretical analysis based on the rate equations of electrons in the conduction band and a corresponding deep level was carried out. The temperature dependence of the quasi-Fermi level in the band gap of SI-GaAs was also taken into account in the present theoretical model for the first time. We then identified that the observed PPT peak was due to the nonradiative electron transitions through the deep level EL6 in GaAs. It is demonstrated that the deep level was clearly characterized in SI-GaAs by using the PPT measurement.

Investigation of deep levels in semi-insulating GaAs by means of the temperature change piezoelectric photo-thermal measurements

The temperature variation of the piezoelectric photo-thermal (PPT) signal intensity of semi-insulating (SI) GaAs from 20 to 150 K was measured. Four peaks at 50, 70, 110, and 125 K were observed in the PPT signal. From the theoretical analysis based on the rate equations of electrons in the conduction band and deep levels, we concluded that the observed four peaks were due to the nonradiative electron transitions through EL6, EL7, EL15, and an unspecified deep level, respectively. Deep levels with extremely low concentration (1012–1015 cm−3) were clearly identified in SI GaAs by using the PPT method.

Optical characterization of native defects in ZnSe substrate

The photoluminescence (PL) and piezoelectric photothermal (PPT) spectroscopy of the nondoped and Al-doped ZnSe single crystals were carried out between liquid helium and room temperatures. Two signals were obtained in the Al-doped ZnSe crystal in the PPT spectrum at liquid nitrogen temperature. These signals were due to the Al-related defects because the signals were not observed in the nondoped ZnSe crystal. The emission band which was due to the Al-related defects were observed in the PL spectrum at liquid nitrogen temperature. Therefore, it was deduced that the Al atom in the ZnSe crystal acted as the nonradiative and radiative carrier recombination centers at liquid nitrogen temperature. At room temperature, one broad band, Al-related defects, was clearly observed in the PL spectrum of the Al-doped ZnSe crystal. On the other hand, no emission peak was present in the nondoped ZnSe crystal. Therefore, the PL emission, the radiative carrier recombination process, could be observed by the Al-doping. In addition, it was clear that the PPT signal, the nonradiative carrier recombination process, decreased by the Al-doping.