Abdul Rahman Mohmad

The effect of Bi composition to the optical quality of GaAs1−xBix

GaAs1−xBix alloys grown by molecular beam epitaxy for x up to 0.06 were studied by photoluminescence (PL). The results indicate that dilute fractions of bismuth (Bi) with x < 0.025 improve the material quality of this low temperature growth alloys by reducing the density of gallium (Ga) and/or arsenic related defects. The crystal quality starts to degrade at higher Bi concentration probably due to significant amount of Bi-related defects, BiGa. However, the room temperature PL intensity continues to increase with Bi content for the range studied due to greater band-gap offset between GaAs and GaAs1−xBix. Analysis carried out shows no correlation between localization effects and the room temperature PL enhancement.

Absorption Characteristics of

The absorption properties of a series of GaAs₁_-_xBi_x layers with ~ 6% Bi have been systematically investigated by measuring photocurrent spectra in p-i-n diode structures grown by molecular beam epitaxy. The GaAs₁_-_xBi_x layers varied in thickness from 50 to 350 nm and showed a photoresponse in the near-infrared up to almost 1.3 μm. The absorption coefficients of the layers were obtained from the responsivity data. Below the band gap, the absorption coefficients showed an exponential dependence on the photon energy (Urbach tailing).

{\rm GaAs}_{1-x}{\rm Bi}_{x}/{\rm GaAs}

The effects of rapid thermal annealing on the optical and structural properties of GaAs1-xBix alloys for x ranging from 0.022 to 0.065 were investigated. At room temperature, the annealed GaAs1-xBix showed modest improvement (∼3 times) in photoluminescence (PL) while the PL peak wavelength remained relatively unchanged. It was found that bismuth related defects are not easily removed by annealing and the PL improvement may be dominated by the reduction of other types of defects including arsenic and gallium related defects. Also, the optimum annealing temperature is Bi composition dependent. For samples with x < 0.048, the optimum annealing temperature is 700 °C but reduces to 600 °C for higher compositions.

Diodes in the Near-Infrared

Photoluminescence (PL) of GaAs0.97Bi0.03 alloy was measured over a wide range of temperatures and excitation powers. Room temperature PL with peak wavelength of 1038 nm and full-width-half-maximum of 75 meV was observed which is relatively low for this composition. The improved quality is believed due to reduced alloy fluctuations by growing at relatively high temperature. The temperature dependence of PL peak energy indicated significant exciton localization at low temperatures. Furthermore, the band gap temperature dependence was found to be weaker than GaAs. An analysis of dominant carrier recombination mechanism(s) was also carried out indicating that radiative recombination is dominant at low temperature.

Effects of rapid thermal annealing on GaAs1-xBix alloys

The optical and structural properties of GaAsBi bulk and quantum well (QW) samples grown under various conditions were studied by photoluminescence (PL), high resolution x-ray diffraction (HR-XRD) and transmission electron microscopy (TEM). At 10 K, the 90 nm bulk sample shows two PL peaks at 1.18 and 1.3 eV. The temperature and power dependent PL data suggest that both PL peaks originate from the GaAsBi layer which consists of two regions with different Bi concentrations. The TEM images verify that the Bi concentration decreases monotonically across the layer, showing a high Bi concentration (~0.053) close to the bottom interface which then reduces to ~0.02 for thicknesses >25 nm. Besides, the high Bi content region cannot be detected by HR-XRD due to a broad and weak diffraction intensity. For multiple QW samples, a similar Bi profile was also observed in which the first well has a significantly higher Bi content compared to the other wells. The energy separation between the PL peaks is 0.12 eV and is consistent with the energy difference observed for the bulk sample. However, two PL peaks were not observed in the other GaAsBi bulk sample which was grown under different conditions, showing the importance of growth optimizations.

Photoluminescence investigation of high quality GaAs1-xBix on GaAs

A GaAsBi/GaAs multiple quantum well diode with a p-i-n structure including a 0.6μm i-region containing a 40 periods of 8nm well and 7.4nm barriers has been grown by molecular beam epitaxy. The photocurrent of the fabricated device was measured and compared to that of a strain-balanced 65 period InGaAs/GaAsP MQW p-i-n diode grown by metal-organic vapour phase epitaxy. The GaAsBi device exhibits an ideality factor of 1.8 and a photoluminescence peak at 1055nm. The photocurrent of the GaAsBi device was comparable to that of the InGaAs/GaAsP device at around 920nm but continued to produce current out to ~1100nm.

Bismuth concentration inhomogeneity in GaAsBi bulk and quantum well structures

The quality of GaAsBi samples grown under various conditions were investigated by photoluminescence (PL) and atomic force microscopy (AFM). The samples were grown by molecular beam epitaxy at a rate of 0.36 and 0.61 μm/h. For each growth rates, three samples were grown under different Bi fluxes. For samples grown at a rate of 0.36 μm/h, the PL peak wavelength was red-shifted from 1103 to 1241 nm as the Bi flux was increased from 0.53 to 1.0 × 10-7 mBar. However, for sample grown with the highest Bi flux, the optical quality degraded showing a weak and broad PL spectrum. The AFM image shows that the sample grown with Bi flux of 0.53 × 10-7 mBar has a smooth surface with rms roughness of 0.78 nm. However, the presence of Bi droplets was observed for samples grown with higher Bi fluxes. A similar PL trend was also observed for samples grown at 0.61 μm/h. The results indicate that high Bi flux may increase the incorporation of Bi into GaAs but it is limited by the formation of Bi droplets.

GaAsBi MQWs for multi-junction photovoltaics

The structural and optical properties of GaAs1-xBix samples with x = 0.048 and 0.06 were investigated by high resolution X-ray diffraction (HR-XRD) and photoluminescence (PL). The HR-XRD results show that both samples have a smooth and coherent GaAs1-xBix/GaAs interface. The PL peak energy shows an S-shape behavior with temperature (10 K to room temperature) which is an evidence of carrier localizations. An Arrhenius analysis reveals that the thermal quenching activation energy is 30 and 19 meV for the GaAs0.952Bi0.048 and GaAs0.94Bi0.06 sample, respectively. It was found that the activation energy is not necessarily originated from alloy fluctuations but may also due to CuPt ordering-induced band gap fluctuations. The power dependent PL indicates that the localized energy states are continuously present and located up to 40-47 meV from the valence band.