Y. T. Oh

Diminution of the surface states on GaAs by a sulfur treatment

The surface chemical properties of a GaAs layer grown by molecular beam epitaxy were investigated by photoluminescence (PL) and photoreflectance (PR) measurements. While the intensity of the PL spectra for the sulfur‐treated GaAs, using a (NH4)2Sx solution, increased 75 times compared to that for the as‐grown GaAs, the peaks for the as‐grown GaAs measured by PR vanished after a sulfur treatment. These results indicate that the surface state acting as the nonradiative recombination centers was passivated by the sulfur. The chemical adsorption behavior resulting from the sulfur is discussed.

The relation between Ga vacancy concentrations and diffusion lengths in intermixed GaAs/Al0.35Ga0.65As multiple quantum wells

Abstract Photoluminescence (PL) measurements were performed in order to investigate the intermixing behavior of GaAs/Al0.35Ga0.65 As multiple quantum wells due to thermal treatment. The PL spectra for the annealed samples show the step like signals, and show that the magnitude of the intermixing of Al and Ga induced by thermal annealing in GaAs/Al0.35Ga0.65As MQWs is different according to the depths. The results of the PL spectra obtained using a procedure of successive layer-by-layer chemical etching show that the atomic mixing is maximum near the sample surface and decreases monotonically with the depth. This result is caused by the different distribution of the diffused Ga vacancy. The potential profile of the GaAs/Al0.35Ga0.65As as a function of the diffusion length was investigated, and electron and hole energy subbands in the MQWs were calculated by a variational method making use of the calculated potential profile. The Ga vacancy concentration corresponding to the depth of the MQWs was investigated from the Al interdiffusion length and the distribution of the diffused Ga vacancy, and this result indicates that the diffusion length of Al is proportional to the logarithmic function of the Ga vacancy concentration.

Carbon impurity effects in Al‐Ga interdiffused GaAs/AlAs multiple quantum wells

Photoluminescence (PL) measurements were performed in order to investigate the carbon impurity effects on the intermixing behavior of GaAs/AlAs multiple quantum wells (MQWs) grown by molecular‐beam epitaxy. The GaAs/AlAs MQWs were annealed with a carbon source in a furnace annealing system. The PL spectra show that the magnitude of the intermixing of Al and Ga induced by thermal annealing in GaAs/AlAs MQWs increases with depth. This behavior is not in agreement with the intermixing mechanism considering vacancy injection of the surface. The nonuniformity of the intermixing as a function of the depth originated from the carbon impurities which were injected during thermal treatment.

Stark effect and Stark‐ladder effect in Al0.4Ga0.6As/GaAs asymmetric coupled multiple quantum wells

Photoluminescence (PL) measurements at 10 K were performed in order to investigate the influence of electric fields on the exciton states in Al0.4Ga0.6As/GaAs asymmetric coupled multiple quantum wells (ACMQW) grown by molecular‐beam epitaxy. The coupling of the electronic energy levels in the wells led to an enhancement of the quantum‐confined Stark effect. The PL intensity decreased as the electric field increased. Calculated values of the intersubband transition energies were in good agreement with the experimental values for the ACMQW, and these values showed a similar behavior as those for the step quantum well. When the external applied field was very strong, Stark‐ladder transitions were observed, and the measured dependence of the field‐induced energy shifts of the Stark‐ladder transitions for the ACMQW agreed with theory. These results indicate that the Stark effect and the Stark‐ladder transitions in a unique ACMQW based on the AlxGa1−xAs/GaAs structure were observed simultaneously.

Photoluminescence and photoreflectance from GaAs/AlAs multiple quantum wells

Photoluminescence (PL) and photoreflectance (PR) measurements have been performed to investigate the intermixing behavior of Al and Ga in GaAs/AlAs multiple quantum wells (MQWs) grown by molecular‐beam epitaxy and treated by rapid thermal annealing. These results indicate that the magnitude of the disordering for a GaAs/AlAs MQW increases as the layer thickness increases. When the GaAs/AlAs MQWs with layer thicknesses of 34 A are annealed at 950 °C for 10 s, the GaAs/AlAs MQWs are totally intermixed, resulting in a formation of an Al0.45Ga0.55As alloy. For the intermixed GaAs/AlAs MQWs, PL spectra show dominantly the Γ‐valley direct transition, and PR signals show that the peaks originating from the interband transitions disappear. The observed increases of the full width at half‐maximum (FWHM) in the PL spectra for the annealed GaAs/AlAs MQWs originate from the nonuniformity of the intermixing as a function of depth, and the decrease of FWHM in the PL spectra for the GaAs/AlAs MQWs annealed for longer ti...

Confirmation of the Franz-Keldysh oscillation due to a two-dimensional electron gas in GaAsAl0.3Ga0.7As modulation-doped single heterostructures

Abstract Shubnikov-de Haas and Van der Pauw Hall effects, and temperature and modulation source wavelength-dependence photoreflectance (PR) measurements on GaAs Al 0.3 Ga 0.7 As heterostructures grown by molecular beam epitaxy have been carried out to investigate the electrical and optical properties of a two-dimensional electron gas (2DEG) in a GaAs layer. The results of the PR measurements showed that the 2DEG was transferred from the modulation-doped Al0.3Ga0.7As layer to the GaAs layer.

Subband energy studies in intermixed GaAsAl0.35Ga0.65As multiple quantum wells

Abstract Photoluminescence (PL) measurements were performed in order to investigate the deformation of the GaAs Al 0.35 Ga 0.65 As multiple quantum wells (MQWs) due to thermal treatment. Rapid thermal annealing was performed at 950°C for 10, 20, and 30 s after the Si3N4 capping layer was evaporated on the sample. The PL spectra for the as-grown and annealed samples show the transitions from the 1 st electronic subband to the 1 st heavy hole (E1-HH1) and from the 2nd electronic subband to the 2nd heavy hole (E2-HH2), and the PL signals shift to the high-energy side as the annealing time increases. The potential profile of the quantum well as a function of the Al diffusion length was investigated, and subband energy levels in the GaAs Al 0.35 Ga 0.65 As MQMs were calculated by a variational method making use of the potential profile. The Al diffusion lengths for the annealed MQWs at 950°C for 10, 20, and 30 s determined from the (E1-HH1) PL peaks were 2.15, 4.45, and 6.3 nm, respectively. The behavior of the difference between the experimental values of (E2-HH2) and (E1-HH1) as a function of the Al diffusion length is in good agreement with theoretical results.

Surface State Behavior and Neutralization of Impurities in Sulfur‐Treated, Hydrogenated, and Annealed GaAs

Surface state behavior and neutralization of impurities in sulfur-treated, hydrogenated, and annealed Si-doped bulk GaAs and nominally undoped GaAs layers have been studied. While the intensity of the photoluminescence (PL) spectra for the sulfur-treated GaAs dramatically increased in comparison to that for as-grown GaAs, the signal for the as-grown GaAs measured by photoreflectance (PR) vanished after sulfur treatment. After the GaAs was hydrogenated and annealed, the relative PL intensity ratio between the donor-bound exciton and the carbon acceptor increased, and a variation in the PR broadening parameter was induced by neutralization of the carbon acceptors. Variations of the surface states and neutralization of the carbon ions affect the intensity of the PL spectra and the intensity and the broadening parameter of the PR signal. These results indicate that the sulfur treatment removes the dangling bonds at the GaAs surface and that the thermal treatment breaks the bonds between hydrogen atoms and carbon ions in GaAs.

Comparison of the interdiffusion lengths in GaAs/Al0.35Ga0.65As multiple quantum wells determined from photoluminescence and double crystal X-ray rocking curve measurements

Abstract Photoluminescence (PL) and double crystal X-ray rocking curve (DCRC) measurements were performed in order to determine the interdiffusion length in GaAs/AlGaAs multiple quantum wells (MQWs) grown by molecular beam epitaxy (MBE) treated by rapid thermal annealing. The PL spectra show that the ground state energy level in the GaAs quantum well increases after thermal annealing, and the interdiffusion length is calculated using this result. After the potential profile as a function of the diffusion length was determined, an Al mole fraction, an structure factor and an satellite peak of the m=+1 in the well and barrier sides were determined. The diffusion length in the quantum well interface was also determined from the experimental and theoretical values from the satellite peak intensities of the m=±1. The agreement of the interdiffusion length value between PL and DCRC results is reasonable.

Dominant direct transitions in annealed GaAs/AlAs multiple quantum wells

Photoluminescence (PL) and photoreflectance (PR) measurements were performed in order to investigate the formation of the Al0.45Ga0.55As alloys in the GaAs/AlAs multiple quantum wells (MQWs) grown by molecular beam epitaxy (MBE) and treated by rapid thermal annealing. The results of the PL measurements show that a Γ-valley direct transition is dominant in annealed GaAs/AlAs MQWs while an X-valley indirect transition is typical in as-MBE-grown Al0.45Ga0.55As/GaAs. This result indicates that the PL spectrum for the AlxGa1−xAs alloy structure with a high Al mole fraction formed by annealing GaAs/AlAs MQWs shows direct transitions, which holds promise for potential applications in optoelectronic devices.