Xianfeng Lu

Composition dependence of photoluminescence of GaAs1-xBix alloys

Room temperature photoluminescence (PL) spectra have been measured for GaAs1−xBix alloys with Bi concentrations in the 0.2%–10.6% range. The decrease in the PL peak energy with increasing Bi concentration follows the reduction in bandgap computed from density functional theory. The PL peak energy is found to increase with PL pump intensity, which we attribute to the presence of shallow localized states associated with Bi clusters near the top of the valence band. The PL intensity is found to increase with Bi concentration at low Bi concentrations, peaking at 4.5% Bi.

Effect of molecular beam epitaxy growth conditions on the Bi content of GaAs1−xBix

We describe how the Bi content of GaAs1−xBix epilayers grown on GaAs can be controlled by the growth conditions in molecular beam epitaxy. Nonstandard growth conditions are required because of the strong tendency for Bi to surface segregate under usual growth conditions for GaAs. A maximum Bi content of 10% is achieved at low substrate temperature and low arsenic pressure, as inferred from x-ray diffraction measurements. A model for bismuth incorporation is proposed that fits a large body of experimental data on Bi content for a wide range of growth conditions. Low growth rates are found to facilitate the growth of bismide alloys with a low density of Bi droplets.

Clustering effects in Ga(AsBi)

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

Disorder and the Urbach edge in dilute bismide GaAsBi

The characteristic emission from tail states below the bandgap of GaAsBi/GaAs quantum wells is studied using photoluminescence spectroscopy over a 10–300 K temperature range and a 0.1–1000 mW pump power range. The tail states exhibit two characteristic energies: A deeper one that is temperature independent at 29 meV and one nearer to bandgap that is temperature dependent, broadening from 17 meV at 10 K–29 meV at room temperature. The tail states are thought to originate from localization of the Bi states and disorder effects due to alloy fluctuations and clustering on the group-V sublattice.

Bi-induced p-type conductivity in nominally undoped Ga(AsBi)

We report p-type conductivity in nominally undoped GaAs1–xBix epilayers for a wide range of Bi-concentrations (0.6% ≤ x ≤ 10.6%). The counterintuitive increase of the conductivity with increasing x is paralleled by an increase in the density of free holes by more than three orders of magnitude in the investigated Bi-concentration range. The p-type conductivity results from holes thermally excited from Bi-induced acceptor levels lying at 26.8 meV above the valence band edge of GaAs1−xBix with concentration up to 2.4 × 1017 cm−3 at x = 10.6%.

Quantitative study of localization effects and recombination dynamics in GaAsBi/GaAs single quantum wells

Localization effects on the optical properties of GaAs1−xBix/GaAs single quantum wells (SQWs), with Bi contents ranging from x = 1.1% to 6.0%, are investigated using continuous-wave and time-resolved photoluminescence. The temperature- and excitation density dependence of the PL spectra are systematically studied, and the carrier recombination mechanisms are analyzed. At low temperatures, the time-integrated PL emission is dominated by the recombination of localized electron-hole pairs due to the varying content and clustering of Bi in the alloy. The extracted energy scales fluctuate tremendously when the Bi content is varied with a weak tendency to increase with Bi content. Relatively low energy scales are found for the SQW with x = 5.5%, which makes it a potential candidate for long-wavelength optoelectronic devices.

Luminescence dynamics in Ga(AsBi)

The temporal evolution of the spectrally resolved luminescence is measured for a Ga(AsBi) sample at low temperatures. The results are analyzed with the help of kinetic Monte Carlo simulations incorporating two disorder scales attributed to alloy disorder and Bi- clustering. An average time of 5 ps is identified as the upper limit for carrier capture into the Bi clusters whereas the extracted hopping rate associated with alloy fluctuations is much faster than the transitions between the individual cluster sites.

Effects of hydrogen on the electronic properties of Ga(AsBi) alloys

The effects of hydrogen incorporation on the electronic properties of Ga(AsBi) alloys are investigated in a wide range of Bi-concentration (0.6% ≤ x ≤ 10.6%) by Hall effect measurements in magnetic fields up to 14 T and by photoluminescence spectroscopy. For all the investigated Bi-concentrations, we report the passivation of Bi-induced shallow acceptor levels—responsible for the native p-type conductivity in Ga(AsBi)—and a tenfold increase of the hole mobility upon hydrogen incorporation in the host lattice. The emission energy is, instead, negligibly affected by hydrogenation, indicating that the narrowing of the band-gap energy with Bi and the native p-type conductivity are two uncorrelated effects arising from different Bi-induced electronic levels. Passivation by hydrogen of the shallow Bi-acceptor levels makes also possible to identify deep Bi-acceptor states.

Bismuth-containing III–V semiconductors: Epitaxial growth and physical properties

The growth, surface, and bulk properties of GaAsBi and related III-V alloys are examined and the potential benefits of these materials are explored in terms of device applications. The methods used include molecular beam epitaxy growth, scanning tunneling microscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, deep-level transient spectroscopy, dynamic modeling, and theoretical analysis. The results show that considerable progress has been made in alloying bismuth with GaAs and that the structural, optical, and electronic quality is very good for the alloys investigated.

Carrier?phonon coupling in GaAs1?xBix/GaAs quantum wells

The phonon-assisted emission of GaAs1 − xBix quantum wells with Bi concentrations up to x = 0.055 is investigated by continuous-wave photoluminescence as a function of lattice temperature and excitation density. Strong carrier–phonon coupling is observed manifesting itself in a Huang–Rhys factor of up to 0.3 depending on the lattice temperature and Bi content. Carriers bound to Bi-clusters are identified to be the origin of the efficient carrier–phonon scattering.