Piezoelectric Photoacoustic Signal Research Articles

Investigation of deep levels in semi-insulating GaAs by means of a thermally activated piezoelectric photoacoustic measurements

The temperature variation of the piezoelectric photoacoustic (PPA) signal intensity of semi-insulating (SI) GaAs from 20 to 150K was measured. Four peaks at 50, 70, 110 and 125K were observed in the PPA signal. From the theoretical analysis based on the rate equations of electrons in the conduction band and several deep levels, we concluded that the observed four peaks were due to the nonradiative electron recombinations via EL6, EL7, EL15 and an unknown deep level, respectively. Deep levels with extremely low concentrations (1011–1015cm−3) were clearly identified conveniently in SI–GaAs by using the PPA method for the first time.

Temperature variation of nonradiative carrier recombination processes in high-quality CuGaSe2 thin films grown by molecular beam epitaxy

The piezoelectric photoacoustic (PPA) measurements for Cu-rich CuGaSe2(CGS)/GaAs(001) epitaxial layer were carried out between liquid helium and room temperatures. The band gap energies of CGS (A, B, and C bands) were measured to be 1.73, 1.83, and 2.04 eV at liquid nitrogen temperature, respectively. The A band was clearly obtained from 5 to 300 K, and the temperature dependence of the peak energy was fitted with the modified Manoogian–Woolley equation. PPA signals for CGS/GaAs (001) epitaxial layers were obtained between liquid helium and room temperature.

Photoluminescence and photoacoustic spectra of ZnSe bulk crystals and epitaxial layers

Temperature dependence of the piezoelectric photoacoustic (PPA) and photoluminescence (PL) spectra are clearly observed between liquid helium and room temperatures for nondoped ZnSe bulk crystals and epitaxial layers. In both samples, the PPA signals are clearly observed at room temperature. However, any PL emissions are not observed at room temperature. Two kinds of recombination processes, i.e., both radiative and nonradiative processes are observed at 80 K in the bulk crystals.

Optical characterizations of CuInSe2 epitaxial layers grown by molecular beam epitaxy

CuInSe 2 (CIS) films with Cu/In ratios of γ=0.82–1.79 have been grown on a GaAs (001) substrate by molecular beam epitaxy. Piezoelectric photoacoustic (PPA) measurements were carried out from liquid helium to room temperature to investigate nonradiative carrier recombination processes in comparison with photoluminescence (PL) measurements which directly detected radiative carrier recombination processes. Three PPA signal peaks which corresponded to band gap energies of the CIS (AB and C bands) and the GaAs substrate, were clearly obtained between liquid helium and room temperatures. A free-exciton emission line was observed up to 200 K in the PL spectra. Two additional peaks on intrinsic defects which are Cu vacancy (VCu) and interstitial In (Ini) were observed in the In-rich CIS samples. The PPA measurements were useful in investigating the defect levels and the band gap energy in the CIS/GaAs thin films.

Piezoelectric photoacoustic studies of optical recovery of metastable states related to EL2 and EL6 levels in semi-insulating GaAs

Effect of secondary-light illuminations of $h\ensuremath{\nu}=0.9$ and 1.45 eV on photoquenched and photoenhanced states in semi-insulating GaAs are investigated using piezoelectric photoacoustic (PPA) measurements at 80 K. It is found that the secondary-light illuminations of 0.9 and 1.45 eV cause an optical recovery of the PPA signal from $\mathrm{EL}{2}^{*}$ to ${\mathrm{EL}2}^{0}.$ We observed an appearance of a broad band at 0.8--1.0 eV in the PPA spectrum after the secondary-light illumination of 0.9 eV. The most important finding is that the PPA spectra after the secondary-light illumination of 0.9 eV both on the photoquenched and on the photoenhanced states are the same in the whole photon-energy region. We conclude that the secondary-light of $h\ensuremath{\nu}=0.9\mathrm{eV}$ induces both an optical recovery and a generation of metastable state of the EL6 level at the same time. The different rates for the transformation of these two processes explain well the observed complex natures of the PPA signal under the secondary-light illumination.

Photoacoustic spectra of zincselenide thin films grown by molecular beam epitaxy

Piezoelectric photoacoustic (PPA) measurements of nondoped ZnSe epitaxial layers grown by molecular beam epitaxy (MBE) are carried out at room temperature. A distinct peak due to the bandgap of ZnSe and substrate GaAs is clearly observed. The deep level with activation energy of about 0.65 eV is not successfully resolved from the nonradiative electron transition point of view. The PPA signals of nondoped ZnSe/GaAs thin film grown by MBE are successfully observed for the first time.

Temperature dependence of photoacoustic spectra in CuInSe 2 thin films grown by molecular beam epitaxy

CuInSe 2 (CIS) thin films, the thickness of about 1.0 μm and composition of Cu In ratio (γ = 1.79), were grown on (0 0 1)-oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of T s, = 450°C. The samples have been characterized by means of piezoelectric photoacoustic (PPA) measurements between liquid helium (4.2 K) and room temperature (300 K). Two distinct PPA signals due to band gap of CIS and GaAs are observed in the whole temperature range from 4.2 to 300 K and the PPA signals of CIS decrease curvilinearly at the temperature range. Since the PPA signals of CIS thin films can be obtained up to room temperature, the PPA measurements are quite effective to obtain the optical characterizations, especially for the nonradiative recombination processes.

Effect of the carbon acceptor concentration on the photoquenching and following enhancement of the photoacoustic signals of semi-insulating GaAs

The spectral and the time dependent piezoelectric photoacoustic (PPA) measurements under the continuous light illumination were carried out at 85 K to investigate nonradiative recombination processes involving EL2 defect levels in carbon concentration controlled and not intentionally doped semi-insulating (SI) GaAs. The decrease of the PPA signal due to the photoquenching effect of EL2 is observed for a short period of illumination in the photon energy region from 1.0 to 1.3 eV. Since almost all of the carbon acceptors are compensated by deep donor EL2 in SI GaAs, electron occupancy of EL2 level can be controlled by changing the carbon acceptor concentration. It is found that the photoquenching becomes drastic with increasing the carbon concentration. After fully photoquenching, the PPA signal increases again through a local minimum by the continuous light illumination and finally exceeds the initial value before illumination until the saturation level is reached. The deep donor level EL6 and its metastable state are proposed. EL6 level donates electrons to compensate a part of carbon acceptors after EL20 to EL2* transition is accomplished. The nonradiative recombination through this level generates the PPA signal. Since the PPA measurement can detect lower concentration of EL6 than that of EL2, the higher sensitivity of the PPA measurements than the optical absorption measurements is pointed out. The usefulness of the PPA technique for studying the nonradiative transition through deep levels in semiconductors is also suggested.

Photoacoustic Study of The Effect of 0.9 eV Light Illumination in Semi-Insulating GaAs

AbstractThe effect of the secondary light illumination of hv = 0.9 eV on the photoquenched and the enhanced states in semi-insulating GaAs are investigated by using piezoelectric photoacoustic (PPA) measurements at 80 K. It is found that the secondary light causes an optical recovery from EL2* to EL2° and this is in agrezment with the result reported by using infrared optical absorption measurements. We observed a broad peak around 0.9 eV after the secondary light illumination for the first time. The most important finding is that the PPA spectra after the secondary light illumination on the quenched and the enhanced states are the same in the whole photon energy region. We concluded that the secondary light of hv = 0.9 eV induces both an optical recovery and generation of metastable state of the EL6 level. The difference of the transformation rates of these two processes explains well the observed complex natures of the PPA signal under the secondary light illumination.

Effect Of The Carbon Acceptor Concentration On The Photoquenching And Enhancement Of The Piezoelectric Photoacoustic Signals Of Semi-Insulating Gaas

AbstractThe spectral and the time dependent piezoelectric photoacoustic (PPA) measurements under the continuous light illumination were carried out at 85 K to investigate nonradiative recombination processes involving EL2 defect levels in carbon doped semi-insulating (SI) GaAs. The decrease of the PPA signal due to the photoquenching effect of EL2 was observed for a short period of illumination of 1.12 eV. It was found that the photoquenching becomes drastic with increasing the carbon concentration. After fully photoquenching, the PPA signal increased again through a local minimum by the continuous light illumination and finally exceeded the initial value before illumination until the saturation level was reached. The deep donor level EL6 and its metastable state are proposed. EL6 level donates electrons to compensate a part of carbon acceptors after photoquenching. The nonradiative recombination through this level generates the PPA signal. The usefulness of the PPA technique for studying the nonradiative transition through deep levels in semiconductor is suggested.

Photoacoustic signals of n-type GaAs layers grown by molecular-beam epitaxy on semi-insulating substrates.

Piezoelectric photoacoustic (PA) measurements of molecular-beam-epitaxial (MBE)-grown GaAs layers were carried out in the temperature range from 90 to 290 K. A broad D band with a maximum near 1.3 eV and a sharp Q peak at 1.485 eV have been observed in the 90-290 K spectra. They vanish in a presence of secondary light illumination. By comparing with optical-absorption spectra, it is considered that the D band is due to electron transitions involving EL2 deep defect levels in the GaAs substrate. The PA signal is considered to be enhanced by the presence of the electric field at the interface between the MBE layer and the substrate

Piezoelectric detection of the photoacoustic signals of n-type GaAs single crystals

Piezoelectric photoacoustic (PA) measurements on liquid-encapsulated-Czochralski-grown n-GaAs were carried out at room temperature. A continuous broad band below 1.35 eV and a peak at 1.383 eV were observed in the PA amplitude spectra. By comparing with the optical-absorption spectra, it is concluded that the broad band is due to the electron transition involving the EL2 deep-lying defect levels. For the observed peak at 1.383 eV, the origin is considered to be dislocation related. The possibility that this peak is an apparent one expected from the proposed models for the PA signal generation is denied.

Impulse response of piezoelectric photoacoustic signal in semiconductors

A theory of piezoelectric photoacoustic impulse response in semiconductors which takes into account the diffusion, lifetime, and surface recombination of the photogenerated carriers in addition to the thermal diffusion is discussed. It is shown that the impulse response changes considerably with the lifetime and the surface recombination velocity of photogenerated carriers. The impulse response has been measured for Si wafers with various surfaces, i.e. etched, sodium dichromate-coated and black painted. The impulse response depends strongly on the surface treatment, which indicates the importance of the diffusion and the surface recombination of photogenerated carriers in the analysis of the photogenerated carriers in the analysis of the photoacoustic impulse response of semiconductors. >

Analysis of the impulse response of a piezoelectric photoacoustic signal

Impulse responses of the piezoelectric photoacoustic (PA) signal have been measured by using the technique of cross correlation. Experimental impulse responses of aluminum model samples are in good agreement with theoretical prediction. It is shown that thermal diffusivities of opaque materials can be determined by measured zero-crossing times of PA impulse response. Cracks in aluminum samples are detected by measuring a one-dimensional image of the impulse response of the PA signal.