Shaoling Guo

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...

Ferroelectricity in sol-gel derived Ba0.8Sr0.2TiO3 thin films using a highly diluted precursor solution

Barium strontium titanate films with good ferroelectricity have been obtained by a developed sol-gel processing, using a highly dilute spin-on solution. X-ray diffraction and scanning electron microscopy investigations show that large grains with the size of 100–200 nm in the films are formed from highly dilute spin-on solutions with layer-by-layer homoepitaxy. Polarization-electric field and dielectric constant-temperature (er−T) measurements reveal that the ferroelectricity becomes more evident as the grain size increases. The measurements for quality Ba0.8Sr0.2TiO3 ferroelectric films derived from a 0.05 M solution have shown a remnant polarization of 3.5 μC/cm2, a coercive field of 86 kV/cm, and two distinctive phase transitions.

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.

Backside-illuminated infrared photoluminescence and photoreflectance: Probe of vertical nonuniformity of HgCdTe on GaAs

Vertical uniformity of HgCdTe epilayer is a crucial parameter for infrared detector engineering. In this work, backside illuminated infrared photoluminescence (PL) and photoreflectance (PR) measurements are carried out on an arsenic-doped Hg1−xCdxTe layer molecular-beam epitaxially grown on GaAs substrate, and the alloy composition and impurity states of the HgCdTe near the substrate are evaluated. By comparing to frontside illuminated PL and PR data, the vertical nonuniformity of composition and impurity states are evidenced. The results indicate that backside illuminated PL and PR are good pathway for evaluating contactlessly the nonuniformity of alloy composition and impurity states along the growth direction.

Pyroelectric Ba0.8Sr0.2TiO3 thin films derived from a 0.05 M solution precursor by sol–gel processing

Tetragonal Ba0.8Sr0.2TiO3 thin films with large columnar grains 100–200 nm in diameter have been prepared on Pt/Ti/SiO2/Si substrates using a 0.05 M solution precursor by sol–gel processing. The ferroelectric phase transition in the prepared Ba0.8Sr0.2TiO3 thin films is broadened, and suppressed to 40 °C with a maximum dielectric constant of er (100 kHz)=680. The observed low dissipation factor tan δ=2.6% and high pyroelectric coefficient p=4.586×10−4 C/m2 K at 33 °C render the prepared Ba0.8Sr0.2TiO3 thin films promising for uncooled infrared detector and thermal imaging applications.

Infrared photoluminescence of arsenic-doped HgCdTe in a wide temperature range of up to 290 K

Infrared modulated photoluminescence (PL) spectra are recorded in the temperature range of 11.5-290 K for both as-grown and p-type annealed arsenic-doped narrow-gap HgCdTe epilayers prepared by molecular beam epitaxy. Curve fittings indicate that conduction band-to-valence band transition is just detectable at 77 K but will dominate the PL line shape at temperatures above about 200 K. Below 100 K, transitions are mainly impurity-related. Shallow impurity levels are estimated to be about 12, 20, 17, and 26 meV, respectively, for VHg and AsTe as acceptors, and AsHg and TeHg as donors, which are very similar to those established by theory and/or infrared photoreflectance spectroscopy. The p-type annealing removes deep level-related PL features that are only observed in the as-grown HgCdTe epilayer. Comparison with previous PL study suggests that infrared modulated PL technique can serve as an easily accessible pathway for characterizing impurities in narrow-gap HgCdTe when PL spectra are recorded with good s...

Magnetoresistance oscillations induced by intersubband scattering of two-dimensional electron gas in Al0.22Ga0.78N/GaN heterostructures

Magnetointersubband scattering (MIS) oscillations of two-dimensional electron gas (2DEG) in Al0.22Ga0.78N/GaN heterostructures have been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Double periodic Shubnikov–de Haas oscillations modulated by MIS oscillations have been observed due to the intersubband scattering of the 2DEG at the two lowest subbands in the triangular quantum well at the heterointerface. By using the fast Fourier transform analysis, it is found that the MIS oscillations become slightly weaker with an increase in temperature. From the MIS frequency, the energy separation between the first and the second subbands is determined to be 80 meV. The observation of the MIS effect indicates that the effective masses of the electrons in the first and second subbands are the same in Al0.22Ga0.78N/GaN heterostructures.

Shallow-terrace-like interface in dilute-bismuth GaSb/AlGaSb single quantum wells evidenced by photoluminescence

Photoluminescence (PL) measurements are performed on one GaSb/AlGaSb single-quantum-well (SQW) sample and two dilute-bismuth (Bi) GaSb/AlGaSb SQW samples grown at 360 and 380 °C, at low temperatures and under magnetic fields. Bimodal PL features are identified in the dilute-Bi samples, and to be accompanied by abnormal PL blueshift in the sample grown at 360 °C. The bimodal PL features are found to be from similar origins of band-to-band transition by magneto-PL evolution. Analysis indicates that the phenomenon can be well interpreted by the joint effect of interfacial large-lateral-scale islands and Al/Ga interdiffusion due to Bi incorporation. The interdiffusion introduces about 1-monolayer shrinkage to the effective quantum-well thickness, which is similar to the interfacial islands height, and the both together result in an unusual shallow-terrace-like interface between GaSbBi and AlGaSb. A phenomenological model is established, the Bi content of isoelectronic incorporation and the exciton reduced effective mass are estimated for the GaSbBi sample grown at 380 °C, and a value of about 21 meV/% is suggested for the bandgap bowing rate of GaSbBi. An effective routine is suggested for determining the Bi content and the depth of the shallow-terraces at interface in dilute-Bi SQW structures.

Mechanisms of infrared photoluminescence in HgTe/HgCdTe superlattice

Temperature (11–250 K) and excitation power (5–480 mW) dependent infrared photoluminescence (PL) measurements are conducted on a HgTe/Hg0.05Cd0.95Te superlattice (SL) sample in a spectral range of 5–18 μm with adequate spectral resolution and signal-to-noise ratio. Three PL components are identified from the evolution of the PL lineshape with temperature although the full-width at half-maximum (FWHM) of the whole PL signal is only about 7 meV at 11 K, for which different changes of the energy, FWHM, and integral intensity are evidenced. The mechanisms are clarified that the medium-energy component is due to electron-heavy hole intersubband transition, while the low-energy (LE) component correlates to localized states and the high-energy (HE) one may originate in interfacial inhomogeneous chemical intermixing and Brillouin-zone boundary effects. The LE and HE component-related effects are responsible for the PL quality of the SL at the temperatures well below and above 77 K, respectively.

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.