Jenn-Shyong Hwang

Comparison between pulsed terahertz time-domain imaging and continuous wave terahertz imaging

We report an evaluation of pulsed terahertz (THz) time-domain measurement and continuous wave (CW) terahertz measurement for non-destructive testing applications. The strengths and limitations of the modalities are explored via the example of the detection of defects in space shuttle foam insulation. It is decided that CW imaging allows for a more compact and simple system, while pulsed measurements yield a broader range of information.

Raman spectra of Si‐implanted GaSb

The variations of Raman spectra for Si‐implanted (100) GaSb with various doses and energies were investigated. Samples implanted at room temperature showed disorder or amorphous layer. In order to heal the damage layer, furnace annealing as well as rapid thermal annealing were used. We got a better structural recovery with increasing the annealing temperature or time, and rapid thermal annealing showed better results in comparison with conventional furnace annealing. The relative intensities of longitudinal optical phonons from Raman spectra by rapid thermal annealing samples were compared with those of unimplanted GaSb. It is found that a better recovery of damage layer is formed comparable to an unimplanted wafer when the annealing temperature is 600 °C for 30 s.

Study of surface Fermi level and surface state distribution in InAlAs surface-intrinsic- n+ structure by photoreflectance

The built‐in electric field and surface Fermi level in the InAlAs surface‐intrinsic‐n+ structures were studied by room‐temperature photoreflectance. The samples were grown by molecular beam epitaxy with an undoped layer thickness of 1000 A. The undoped layer was subsequently etched to 800, 600, 400, and 200 A. Different chemical solutions were used in the etching process and the built‐in electric field is found independent of the etching process. While the surface Fermi level, in general, varies with the undoped layer thickness, there exists, for each Al concentration, a certain range of thicknesses within which the surface Fermi level is weakly pinned. From the dependence of electric field and surface Fermi level on the undoped layer thickness, we conclude that the surface states distribute over two separate regions within the energy band gap and the densities of surface states are as low as 1.02±0.05×1011 cm−2 for the distribution near the conduction band and 2.91±0.05×1011 cm−2 for the distribution nea...

Determination of surface state density for GaAs and InAlAs by room temperature photoreflectance

Room temperature photoreflectance (PR) was used to investigate the surface state densities of GaAs and In0.52Al0.48As surface intrinsic-n+ structures. The built-in electric field and thus the surface barrier height are evaluated using the observed Franz–Keldysh oscillations in the PR spectra. Based on the thermionic emission theory and current-transport theory, the surface state density as well as the pinning position of the Fermi level can be determined from the dependence of the surface barrier height on the pump beam intensity. Even though this method is significantly simpler, easier to perform, and time efficient compared with other approaches, the results obtained agree with the literature.

Photoreflectance study of surface Fermi level in molecular beam epitaxial grown InAlAs heterostructures

We have studied the band gaps and the surface Fermi level positions of a series of In1−xAlxAs surface‐intrinsic‐n+ structures at room temperature by photoreflectance. Experiments demonstrated that over aluminum concentrations of 0.42–0.57, the surface Fermi level is not pinned at midgap, as commonly believed, but instead varies, respectively, from 0.50±0.01 to 0.81±0.01 eV below the conduction band edge.

Studies of band alignment and two-dimensional electron gas in InGaPN/GaAs heterostructures

Room-temperature photoreflectance (PR) and photoluminescence (PL) spectra are measured for a series of In0.54Ga0.46P1−yNy∕GaAs heterostructures grown on GaAs (100) substrate. Redshifts of the PR and PL peaks indicate that the band gap of In0.54Ga0.46P1−yNy is dramatically reduced as nitrogen is incorporated. The emergence of additional peaks in PR spectra as nitrogen is incorporated indicates that the band alignment switches from type I to type II, due to the lowering of the conduction band, thus forming a two-dimensional electron gas (2DEG) in the interface region between In0.54Ga0.46P1−yNy and GaAs. The band gap energy and transition energies between the confined levels in the 2DEG are determined for samples with various nitrogen concentrations y. The number of confined levels in the 2DEG is found to increase with y; the composition-dependent bowing parameter is determined.

Non-destructive sub-THz CW imaging

A simple, compact CW sub-THz imaging system, utilizing a 0.2 and 0.6 THz Gunn diode source is presented. A silicon beam lead diode detector and a Golay cell are used for the detection. Various results are presented, which show that the CW THz imaging modality is suitable for diverse applications, such as non-destructive testing and security. The key components of the system include the Gunn diode assembly, an optical chopper, a polyethylene lens, a detector, a lock-in amplifier, and two translation stages. The beam from the Gunn diode is focused on the sample being imaged by the polyethylene lens, the transmitted or reflected beam is measured by the detector. The energy transmitted through the sample at each point in the plane of the sample is detected. Since the system has relatively few components compared to pulsed THz imaging systems, it is less expensive and easier to design and operate, although it does not provide depth or spectral information about the sample. Since no time-delay scans take place, scanning can be done quickly compared to a time-domain system, limited by the maximum velocity of the translation stages and response of the detectors. It provides information about the macroscopic features of hidden structures within materials that are transparent to sub THz radiation, such as space shuttle insulating foam, articles of clothing, and luggage.

Dynamic light scattering studies of collagen

Monodisperse rat skin collagen solutions obtained through purification by density gradient separation have been studied by dynamic light scattering. Autocorrelation functions were measured at 31.8° and 90° scattering angles. Calculations of the translational diffusion coefficient DT and the rotational diffusion coefficient DR from the measured correlation functions of scattered light have been carried out in this study. A new approach to simulatanously determining the translational and rotational diffusion coefficients was used to estimate both the length of the macromolecules and the viscosity of the solutions. Our result for L is 2715±100 A.

Studies of interband transitions and thermal annealing effects on ion-implented (100) GaSb by photoreflectance and Raman spectra

Abstract The technique of photoreflectance was employed to investigate the interband transitions (E0, E0 + Δ0, E1 and E1 + Δ1) of GaSb as well as their temperature dependence over the range of 83 to 300 K. The parameters which describe the temperature dependence in terms of the Varshni expression were also evaluated. Through examination of the spectral lineshapes, it was concluded that band to band transition is the main mechanism of E0 + Δ0 transition, while excitonic transition is responsible for E1 and E1 + Δ1 transitions. Both Raman and photo- reflectance spectra were also used to study the damage resulting from ion implantation, as well as the induced “healing” caused by a range of differing thermal annealing temperatures and/or time. Both techniques provide a convenient, powerful and contactless tool in investigating the above-mentioned studies.

Schottky barrier height and interfacial state density on oxide-GaAs interface

Photoreflectance (PR) and Raman spectra were employed to investigate the interfacial characteristics of a series of oxide films on GaAs. The barrier heights across the interfaces and the densities of interfacial states are determined from the PR intensity as a function of the pump power density. The oxide-GaAs structures fabricated by in situ molecular beam epitaxy exhibit low interfacial state densities in the low 1011 cm−2 range. The density of the interface states of the Ga2O3(Gd2O3)–GaAs structure is as low as (1.24±0.14)×1010 cm−2. The Ga2O3(Gd2O3) dielectric film has effectively passivated the GaAs surface. Additionally, Raman spectra were used to characterize the structural properties of the oxide films.