GaAsBi Layers

We have investigated the effect of long thermal annealing (3 h) at 200 ◦C on the physical
properties of GaBiAs layers with a Bi concentration of 3% by using photoluminescence (PL)
and polarized resolved PL under magnetic fields up to 14 T. The PL intensity and the
diamagnetic shift of the annealed samples increased substantially. This indicates a reduction of
carrier/exciton localization due to defects. In addition, it was found that the degree of the
electron spin polarization increases up 41% at 14 T after thermal annealing due to the
reduction of the density of defects.

We report room-temperature Raman studies of strained (100) and (311)B GaAs1xBix epitaxial
layers for x0.039. The Raman spectra exhibit a two-mode behavior, as well as disorder-activated
GaAs-like phonons. The experimental results show that the GaAs-like LO(C) mode experiences a
strong composition-dependent redshift as a result of alloying. The peak frequency decreases
linearly from the value for pure GaAs (293 cm1) with the alloyed Bi fraction x and the
introduced in-plane lattice strain ek, by DxLO ¼ Dxalloy  Dxstrain. X-ray diffraction measurements
are used to determine x and ek allowing Dxalloy to be decoupled and is estimated to be 12(64)
cm1/x for (100) GaAs1xBix. DxLO is measured to be roughly double for samples grown on
(311)B-oriented substrates to that of (100) GaAs. This large difference in redshift is accounted for
by examining the Bi induced strain, effects from alloying, and defects formed during high-index
(311)B crystal growth.

The effects of long time thermal annealing at 200 C on the optical
and structural properties of GaAs1xBix alloys were investigated by
X-ray diffraction (XRD), field emission scanning electron microscopy
(FESEM), high resolution transmission electron microscopy
(HRTEM) and photoluminescence (PL). FESEM images show that
bismuth islands nucleate on the surface and their diameter
increases after annealing. It was observed a PL intensity enhancement
and a small blue shift in PL peak energy after thermal annealing
at 200 C for 3 h of GaAs1xBix alloys which was associated to
the reduction of the density of defects. However these defects are
not completed removed by thermal annealing although an important
PL intensity improvement is observed.

We report room-temperature Raman scattering studies of nominally undoped (100) GaAs1−xBix epitaxial layers exhibiting Bi-induced (p-type) longitudinal-optical-plasmon-coupled (LOPC) modes for 0.018 ≤ x ≤ 0.048. Redshifts in the GaAs-like optical modes due to alloying are evaluated and are paralleled by strong damping of the LOPC. The relative integrated Raman intensities of LO(Γ) and LOPC ALO/ALOPC are characteristic of heavily doped p-GaAs, with a remarkable near total screening of the LO(Γ) phonon (ALO/ALOPC → 0) for larger Bi concentrations. A method of spectral analysis is set out which yields estimates of hole concentrations in excess of 5 × 1017cm−3 and correlates with the Bi molar fraction. These findings are in general agreement with recent electrical transport measurements performed on the alloy, and while the absolute size of the hole concentrations differ, likely origins for the discrepancy are discussed. We conclude that the damped LO-phonon-hole-plasmon coupling phenomena plays a dominant role in Raman scattering from unpassivated nominally undoped GaAsBi.