S. L. Tyan

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...

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 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.

Photoreflectance and photoluminescence spectroscopy of the lattice-matched InGaAs/InAlAs single quantum well

A lattice-matched In0.53Ga0.47As/In0.52Al0.48As single quantum well (SQW) structure grown by gas source molecular beam epitaxy has been investigated by photoreflectance (PR) and photoluminescence (PL). The PR measurements allowed the observation of interband transitions from the heavy- and light-hole valence subbands to the conduction subbands. The transition energies measured from the PR spectra agree with those calculated theoretically. Two features corresponding to the ground state transition coming from the SQW and the band gap transition generated from the buffer layer are observed in the PL spectra and are in good agreement with the PR data. The effect of the temperature on the transition energies is essentially same as that in the gap transition of the bulk structure. The values of the Varshni coefficients of InGaAs/InAlAs were obtained from the relation between the exciton transition energy and the temperature. The built-in electric field could be determined and located from a series of PR spectra...

Investigation of modulation-doped GaAs/AlGaAs single quantum well by photoreflectance

The interband transitions, built‐in electric field, and energy of the two‐dimensional electron gas (2DEG) of the modulation‐doped GaAs/AlGaAs single quantum well at different temperatures (100–300 K) have been studied by the modulation spectroscopy of photoreflectance (PR). The built‐in electric field was evaluated using the observed Franz–Keldysh oscillations and found to be increasing from 20±1 to 75±3 kV/cm with an increase in temperature from 100 to 300 K. In addition to the allowed quantum transitions, the forbidden transition 12L in the quantum well was also observed. Moreover, the energy level of the 2DEG was determined from the PR spectra and was found to decrease with the increasing temperature. The energy of the 2DEG of the higher doping sample is greater than that of the lower one.

Oxide-GaAs interfacial electronic properties characterized by modulation spectroscopy of photoreflectance

Built-in electric fields and interfacial state densities (Dit) in a series of oxide–GaAs heterostructures fabricated by in situ molecular beam epitaxy were studied using room temperature photoreflectance. The samples investigated were air-, Al2O3–Ga2Ox–, and Ga2O3(Gd2O3)–GaAs. We found that the built-in electric fields are 48, 44, and 38 kV/cm for air-, Al2O3-, and Ga2Ox–GaAs samples, respectively. For the Ga2O3(Gd2O3)–GaAs sample, the built-in electric field is negligibly small, indicating a very low interfacial state density. Estimated by the low field limit criterion, Dit is less than 1×1011 cm−2 eV−1. Our results on the Ga2O3(Gd2O3)–GaAs sample are consistent with the data obtained previously using capacitance–voltage measurements in quasistatic/high frequency modes and photoluminescence measurements.

Unambiguous photoreflectance determination of electric fields using phase suppression

Photoreflectance measurements have been performed on a δ‐doped GaAs homojunction. Two Franz–Keldysh oscillation features originating from two different regions (a buffer layer and a top layer) of the structure superimpose with each other in the photoreflectance spectrum. By properly selecting the reference phase, one of the features can be suppressed, thus enabling us to determine the electric fields from the two regions unambiguously. The electric field in the top layer is 3.5±0.2×105 V/cm, which is in good agreement with theoretical calculation. The electric field in the buffer layer is 1.2±0.1×104 V/cm.

Photoluminescence and electrical properties of bidirectional ZnO nanowires on Zn foils via a thermal oxidation method

ZnO nanowires (NWs) were directly grown on ductile zinc foils through a two-step process: (a) large, thin, and ductile Zn foils were fabricated from a mixture of Zn and ZnO powders; and (b) ZnO NWs were produced by thermal oxidation at temperatures of 300–600 °C. The ZnO NWs presented preferential growth in the [101] orientation. The highly crystalline NWs synthesized at 500 °C exhibited a bidirectional mode with an angle of approximately 60° between their longitudinal axes. On foils oxidized at 600 °C, the NWs evolved into nanotowers. The photoluminescence (PL) spectra showed strong peaks at approximately 368 nm and weaker peaks at approximately 374 nm in the UV region; moreover, a broad deep-level-related green emission peak was recorded at approximately 520 nm in the visible region. The PL green emission line was strongly suppressed for the samples produced at higher oxidation temperatures, which indicated good optical qualities. These good optical qualities, combined with the bidirectional mode and the ductility of the foil, are expected to be useful for flexible planar device applications. The electrical properties of a single ZnO NW were investigated. I–V measurements revealed the Schottky characteristics of the NWs and the resistivity of the ZnO NWs was measured to be ∼93 Ω cm.

Magneto-Photoluminescence Study of InGaAs/GaAs Quantum Wells and Quantum Dots Grown on (111)B GaAs Substrate

InGaAs/GaAs quantum well (QW) and quantum dot (QD) structures grown on GaAs (111)B substrates under different growing temperatures are investigated by magneto-photoluminescence (PL) up to 15 T in both Faraday and Voigt configurations. The spatial extents of the carrier wave functions (ECWFs) are deduced from the diamagnetic shift of the PL peak energy. The binding energies of the InGaAs/GaAs QWs are evaluated to be about 5 meV. The QW ECWFs in the growth direction obtained by the diamagnetic shift are consistent with those calculated by the k p theory. The heights and radii of the InGaAs/GaAs QDs are also estimated from the ECWFs. In addition, we found that the in-plane ECWFs decreased slightly as the growth temperature was varied from 525 to 450°C. The ECWFs in the growth direction decreased when the growth temperature was varied from 525 to 480°C and then increased as the temperature was decreased to 450°C.