Fangyu Yue

Modulated photoluminescence spectroscopy with a step-scan Fourier transform infrared spectrometer

Fourier transform infrared (FTIR) spectrometer is a powerful tool for studying the photoluminescence (PL) properties of semiconductors, due to its well-known multiplexing and throughput advantages. However, it suffers from internal He–Ne laser disturbance in near-infrared and∕or environmental background thermal emission in mid- and far-infrared spectral regions. In this work, a modulated PL technique is developed based on step-scan (SS)-FTIR spectrometer. Theoretical analysis is conducted, and applications of the technique are given as examples in the PL study of mid-infrared HgCdTe thin films and near-infrared GaInP∕AlGaInP multiple quantum wells, respectively. The results indicate that the He–Ne laser and∕or thermal emission disturbance can be reduced at least 1∕1000 and∕or even 1∕10000, respectively, by the modulated SS-FTIR PL technique, and hence a rather smooth PL spectrum can be obtained even under room temperature for HgCdTe thin films. A brief comparison is given of this technique with previously...

Photoreflectance spectroscopy with a step-scan Fourier-transform infrared spectrometer: Technique and applications

We report on a new technique of realizing photoreflectance (PR) spectroscopy with a step-scan Fourier-transform infrared spectrometer. The experimental configuration is briefly described and a detailed theoretical analysis is conducted. The results reveal two distinct features of this PR technique that (i) the PR related signal is enhanced by a factor of at least 100 relative to those of the conventional PR techniques and (ii) the unwanted spurious signal introduced by either diffuse reflected pump beam or pump-beam induced materials photoluminescence reaching the photodetector of the PR configuration is eliminated without any special consideration of normalization for deducing the final PR spectrum. Applications are given as examples in the study of GaNAs/GaAs single quantum wells and GaInP/AlGaInP multiple quantum wells, respectively, under different pump-beam excitation energy and/or power. The experimental results approve the theoretically predicted features and illustrate the possibility of investigating weak PR features by using high pump-beam power. A brief comparison of this technique with the conventional PR techniques is given, and the extendibility of this technique to long-wavelength spectral regions is pointed out.

Oxygen-vacancy-related dielectric relaxation in BiFeO3 films grown by pulsed laser deposition

Two kinds of BiFeO3 (BFO) thin films with different oxygen stoichiometry are fabricated on (La, Sr)CoO3 coated silicon substrates. A Debye-like dielectric relaxation was observed in the samples thermally treated at 3 Pa oxygen. The frequency dependence of permittivity of the samples treated at 3 Pa oxygen can be fitted by a model containing the Debye-like dielectric response and the universal dielectric response. According to the model, the dielectric relaxation can be ascribed to the oxygen vacancy, and the possible influences from the interfacial polarization between BFO and electrodes have been excluded by the measurement of the dielectric responses of BFO films at different dc biased voltages. The calculated value of dc electric conductivity in BFO films from this model has the same order of magnitude as the published results. These results indicate that the existence of oxygen vacancy not only influences the leakage performance of BFO films but also affects the dielectric properties of BFO. The electrical performance of BFO films and devices can be improved by decreasing the density of oxygen vacancy.

Temperature dependence of optical band gap in ferroelectric Bi3.25La0.75Ti3O12 films determined by ultraviolet transmittance measurements

Optical properties of ferroelectric Bi3.25La0.75Ti3O12 (BLT) films on quartz have been investigated using ultraviolet-infrared transmittance spectra in the temperature range of 77–500K. The spectra can be divided into three distinctive photon regions between 1.1 and 6.5eV. It is found that the band gap Eg decreases from 3.88to3.77eV with the temperature. The parameters aB and ΘB of the Bose-Einstein model are 30.3meV and 218.7K, respectively. The band narrowing coefficient dEg∕dT is −2.65×10−4eV∕K at room temperature. The present results can be crucial for future application of ferroelectric BLT-based electro-optic and high temperature optoelectronic devices.

Deep/shallow levels in arsenic-doped HgCdTe determined by modulated photoluminescence spectra

The modulated photoluminescence spectra have been performed on as-grown and in situ annealed arsenic-doped Hg1−xCdxTe grown by molecular beam epitaxy. Besides the discussions about shallow levels including VHg, AsHg, VHg–AsHg complex, and TeHg, two deep levels have been observed in as-grown with an ionization energy of ∼77.0 and ∼95.0 meV, respectively, which can be completely eliminated by annealing and temporarily ascribed to As-related clusters or interstitials.

Modulated photoluminescence of shallow levels in arsenic-doped Hg1−xCdxTe (x≈0.3) grown by molecular beam epitaxy

Shallow levels in arsenic-doped Hg1−xCdxTe grown by molecular beam epitaxy have been investigated by temperature- and excitation power-dependent modulated photoluminescence spectroscopy. The ionization energies of the shallow levels of AsTe, AsHg, and the AsHg–VHg complex are preliminarily determined to be about 11.0, 8.5, and 33.5meV, respectively. Correspondingly, the forming energy of the AsHg–VHg complex has been deduced to be approximately 10.5meV. The results could be used as guidelines for the material growth or the fabrication of related devices.

Photomodulated infrared spectroscopy by a step-scan Fourier transform infrared spectrometer

A technique is developed for photomodulated spectroscopy in a long-wavelength region, based on a step-scan Fourier transform infrared spectrometer. The experimental setup is discussed, and photoreflectance (PR) spectra of narrow-gap HgCdTe materials are given as examples at the wavelengths of 5 and 9μm. The photoluminescence spectra suggest that the PR features are related to the material fundamental gap. The signal-to-noise ratio and spectral resolution of the PR spectrum are quite good for line-shape analysis. The results indicate that the PR spectrum can be well fitted by a third-derivative line-shape function. Advantages and extendability are emphasized, and the potential for advancing the study of narrow-gap materials’ band structures is foreseen.

Anomalous temperature dependence of absorption edge in narrow-gap HgCdTe semiconductors

Abnormal temperature dependence of absorption edge is reported for narrow-gap Hg1−xCdxTe semiconductors at low temperature. Infrared absorption spectra are taken for bulk and molecular-beam epitaxial Hg1−xCdxTe, respectively, in the temperature range of 11–300K. The results indicate an abnormal shift of the absorption edge around the temperature range of 30–70K. Analysis suggests that (i) the phenomenon is introduced by Hg vacancies in the samples, of which the energy level locates about 9–12meV above the Hg1−xCdxTe valence band, and (ii) the conventional criterion for the determination of band gap energy, Eg, from absorption spectra is not accurate enough as soon as Hg vacancies exist, especially at a temperature above 77K. It hence provides an explanation why there should exist difference between the cutoff wavelength of the detector and the absorption-edge (Eg) wavelength of the material the detector was made of.

Magnetophotoluminescence study of GaxIn1−xP quantum wells with CuPt-type long-range ordering

Magnetophotoluminescence (PL) measurements are performed on GaxIn1−xP∕AlGaInP quantum wells with CuPt-type long-range ordering. Curve-fitting and second-order derivative operations are conducted, respectively, on the PL spectra, and two different PL components are identified with an energetic separation of about 5∼25meV. Obvious diamagnetic shift can be observed only for the PL component with higher energy, indicating the excitonic feature of the related PL transition. The in-(001)-plane exciton reduced effective mass and binding energy are evaluated based solely on the excitonic PL component, by a quasi-two-dimensional exciton effective mass approximation. The results suggest that the ordering competes with compressive strain, and enlarges the exciton reduced effective mass and binding energy. The enlargement is discussed and a comparison with previous reports is given.

Properties of highly (100) oriented Pb(Mg1∕3,Nb2∕3)O3–PbTiO3 films on LaNiO3 bottom electrodes

The 70%Pb(Mg1∕3,Nb2∕3)O3–30%PbTiO3 (PMNT) films have been fabricated on LaNiO3 (LNO) coated silicon substrate. The conductive LNO films act as a seed layer for the growth of PMNT films, which depresses the formation of pyrochlore phase and induces the high (100) preferred orientation of perovskite PMNT films. Compared with the PMNT films grown on platinum bottom electrode, the ferroelectric properties of PMNT films grown on LNO are enhanced. The frequency dependence of complex permittivity from PMNT films on LNO is the conjunct result of polarization relaxation and movement of oxygen vacancy, which can be fitted by the function containing Debye and universal dielectric response models, respectively.