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growth temperatures compared with conventional III-Vs. What’s more, the photoluminescence signal is always very sensitive to the bismuth content and the growth conditions.


Both growth technologies have produced some noteworthy success. MBE is, to date, capable of alloys with higher bismuth content, while MOCVD has produced epiwafers that have been processed to yield the world’s first electrically pumped, dilute- bismide laser.


Using MBE, we have grown ternaries with a bismuth content greater than 10 percent. Higher values are possible – Tom Tiedje’s group from University of Victoria, Canada, have recently reported values in excess of 20 percent.


Figure 3: Comparison of the experimental and theoretical values of the energy gap (Eg


energy (ΔSO ) for epitaxially grown GaBix


) and spin-orbit-splitting As1-x


samples on


GaAs substrates. Auger losses are suppressed when the bismuth composition is greater than about 10 percent,


because at concentrations at this level and higher, ΔSO exceeds Eg


.


spin-orbit-splitting energies are also possible with bismide alloys, such as GaBiAs, InGaBiAs and GaBiNAs. If this spin-orbit-splitting energy exceeds the bandgap energy of the telecom laser, which is typically below 1 eV, the law of conservation of energy dictates that there will be a significant reduction in Auger recombination in this device.


We are aiming to design and fabricate a device that behaves just like this. Our efforts kick-started with a study of the bandstructure of epitaxially grown GaBix


As1-x samples on a GaAs substrate.


Photo-modulated reflectance spectroscopy and atomistic theoretical calculations undertaken by us have revealed that the introduction of bismuth into GaAs has the desired effect on the band structure.


This combined theoretical and experimental effort by our team has garnered three key insights: the band gap energy of the alloy GaBix


As1-x


Fibre-coupled photoconductive terahertz detector featuring a GaBiAs layer


decreases


dramatically with bismuth composition, thereby offering a possibility to achieve 1550 nm emission on a GaAs substrate; spin-orbit-splitting energy increases rapidly with bismuth richness and exceeds the bandgap energy at a content of 9-10 percent; and GaBix


As1-x has a type-I band


offset relative to the GaAs substrate, a condition favourable for realising large optical gain and ultimately an efficient laser.


Building bismide lasers Our next step has been to form bismide quantum wells with high optical quality on a GaAs substrate. We have adopted a two-pronged approach, using both MOCVD and MBE to try and obtain high-quality heterostructures. Producing these structures is challenging because the epitaxial growth of metastable GaBiAs requires very low


Meanwhile, with MOCVD, we have grown a laser structure in a commercially available AIX 200-GFR reactor system, using palladium-purified hydrogen as the carrier gas at a reduced reactor pressure of 50 mbar. For the quantum well growth, triethyl gallium is used as a group III precursor, while tertiarybutyl arsine and trimethyl bismuth are used as the group V precursors, since low growth temperatures (around 400 °C) are required. By


Efficient terahertz generation using dilute bismide alloys


Dilute bismide layers are not just promising for the fabrication of telecom lasers and modulators – they also offer significant opportunities for the development of low-cost, efficient terahertz technologies. At the Center for Physical Sciences and Technology (FTMC) in Vilnius, Lithuania, Arunas Krotkus’ group are exploiting very short


photoexcited electron trapping times, which were typical in the first GaBiAs epitaxial layers grown with large bismuth content.


The sub-picosecond trapping times in GaBiAs, together with the narrow bandgap and relatively high electron mobility, make this material very attractive for manufacturing photoconductive antennas for terahertz emission and detection. Such devices are in demand for spectroscopic, imaging, and security applications. Until recently, terahertz emitters and detectors were mainly manufactured from epitaxial layers of GaAs grown by MBE at low substrate temperatures.


This approach is not ideal, because GaAs is transparent to wavelengths beyond 850 nm, so bulky and expensive femtosecond Ti:sapphire lasers are required for carrier excitation in this class of optoelectronic terahertz radiation system. In contrast, compact fibre or diode lasers emitting in the 1.0-1.5 µm range can activate optoelectronic terahertz emitters and detectors developed at Vilnius. Bismide-based systems have already been commercialized. They are available from TERAVIL, a spin-off company of the FTMC.


August / September 2013 www.compoundsemiconductor.net 55


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