Growth and characterization of dilute bismide GaAs based alloys for high efficiency infra red laser diodes
A lot of energy in today's optical communication is wasted due to the inefficiency of optoelectronic devices operating at the telecommunication wavelength of 1.55 µm. The novel Ga(AsBi) material system is very promising to address this as it could enable the fabrication of high efficiency IR ph...
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|A lot of energy in today's optical communication is wasted due to the inefficiency of optoelectronic devices operating at the telecommunication wavelength of 1.55 µm. The novel Ga(AsBi) material system is very promising to address this as it could enable the fabrication of high efficiency IR photonic devices such as laser diodes and EAM.
In this work the growth of Ga(AsBi) and Ga(NAsBi) on GaAs substrates using MOVPE was investigated, and thereby MQW as well as bulk-like structures were deposited. Several growth parameters were varied systematically applying pulsed as well as continuous precursor flows. Structural analysis such as HR-XRD, AFM, SEM, (S)TEM of the crystals were performed and PL spectroscopy has been carried out. Furthermore, the first electrically pumped Ga(AsBi) containing laser diode was demonstrated.
The surface of the first Ga(AsBi) samples were covered by metallic droplets consisting either of Ga, Bi or both in phase separated droplets. Drastically reducing the amounts of TMBi offer and carefully adjusting the TBAs/TEGa ratio enabled the growth of droplet free samples with measurable Bi fractions applying pulsed as well as continuous precursor flow. Bi segregates to the surface and it was found that the Bi incorporation depends on the Bi surface coverage during growth. In the case the coverage is too small, Bi only floats at the surface and does not get incorporated. At higher amounts the Bi fraction scales with the surface coverage up to a certain maximum and when it becomes too high Bi droplets begin to form. The maximum Bi fraction was found to increase with decreasing growth temperature that was varied in the range of 350 °C to 475 °C. However, at lower temperatures more defects occur and the precursor decomposition is reduced. Thus, 375 °C and 400 °C were stated to be most suitable for the MOVPE growth of Ga(AsBi) so far and maximum Bi fractions of about 7% and 5% were realized, respectively. The incorporation efficiency also increases with the growth rate and is inversely proportional to the TBAs/TEGa ratio within a range around unity were droplet free growth of Ga(AsBi) is possible. What makes the optimization and investigation of the growth conditions more complicated is that the before mentioned parameters are not independent. For example, the presence of Bi or the not fully decomposed TMBi at the surface reduces the growth rate, which is a hint that it hinders either the decomposition of the TEGa or its approach to the surface. Furthermore, lowering the growth temperature reduces the decomposition of the precursors and, hence, has an impact on the optimum TBAs/TEGa and TMBi/V ratios and reduces the growth rate.
However, chemically homogenous Ga(AsBi) samples were realized and, if the subsequent layer was grown at temperatures as high as 625 °C, sharp hetero interfaces were found. The Bi floating at the surface acts as surfactant that quenches the unintentional C incorporation that usually occurs at the low applied temperatures in MOVPE and it reduces the point defect density. Hence, strong bandgap PL was found for samples that are grown in the regime at which the Bi saturation sets in. The PL peak fits perfectly to the prediction from theory with linewidths (FWHM) of about 80 to 90 meV that are related to the disorder in dilute bismides.
To investigate whether the dilute bismides are suitable for optoelectronic devices, broad area Ga(AsBi) QW lasers were fabricated with Bi fractions of 2.2% and 4.4%. Electrical injection lasing of dilute bismides was demonstrated for the first time on a Ga(AsBi0.022) SQW laser with (AlGa)As barriers and cladding layers that showed room temperature lasing operation. The lowest achieved threshold current density of Ith=1.0 kA/cm² at pulsed current injection is very promising for such a new material system, however about 80% of Ith is lost by non-radiative recombination through defects. For devices with 4.4% Bi lasing was only found at low temperatures up to 180 K showing the necessity of further improving the growth of Ga(AsBi), especially when increasing the Bi fraction.
For the growth of Ga(NAsBi) it was stated that at constant Bi fraction the N fraction can easily be controlled by the UDMHy supply. Samples containing up to 4% Bi and N were realized, however, it was not possible to observe room temperature PL from those structures. Hence, photo reflection measurements were carried out, showing that at constant Bi fraction the bandgap reduction due to N is about 140 meV/%N confirming that N and Bi act independently on the band structure of GaAs.