Debut for epitaxial InGaPBi
High-resolution X-ray diffraction produces peaks with a full width at half- maximum of 54 arcsec, 49 arcsec and 50 arcsec, for InGaPBi alloys with a bismuth content of 0.5 percent, 1.0 percent and 1.8 percent, respectively.
Researchers from China are claiming to have grown a new III-V epilayer by incorporating bismuth into InGaP to form InGaPBi, a quaternary alloy that is nearly lattice matched to GaAs.
"[InGaPBi] provides a new building material to tune the bandgap and strain of InGaP on GaAs, and thus can be of significance for optoelectronic devices," argues corresponding author Shumin Wang, an academic at Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, and also the holder of a position at Chalmers University of Technology, Sweden.
Adding bismuth to InGaP is expected to lead to a large change in bandgap. That is the case when bismuth is added to InP, with every percent of bismuth producing a 56 meV shift in bandgap.
One particularly interesting possibility, according to Wang, is that the characteristics of InGaPBi might mirror those of InPBi, which produces mid-bandgap photoluminescence signals. "This might be achieved by introducing bismuth clusters forming deep levels," enthuses Wang. According to him, current levels of incorporation of bismuth, which are up to 2 percent, are insufficient for clustering.
"If high bismuth incorporation were successful, it could provide a new route for making intermediate band solar cells on GaAs," adds Wang.
The team produced a series of InGaPBi films by gas source MBE. After loading GaAs substrates into the growth chamber and removing the surface oxide, they deposited an undoped, 100 nm-thick GaAs buffer at 580degC, before lowering the temperature to 300degC and depositing 330 nm-thick InGaPBi films that contained either 0.5 percent, 1.0 percent or 1.8 percent bismuth.
Incorporation of these levels of bismuth was confirmed with Rutherford backscattering spectrometry, while high-resolution X-ray diffraction offered evidence of excellent crystal quality, producing peaks for InGaPBi with low values of full-width at half maximum.
Adding bismuth did not impair material quality, according to atomic force microscopy measurements. InGaP and InGaPBi with varying degrees of bismuth all have a root-mean-square roughness of 0.4 nm for a scan area of 2 µm by 2 µm.
Raman spectroscopy has also been used to scrutinise samples. This technique uncovers phonon modes associated with InBi and GaBi and reveals that atoms of bismuth replace those of phosphor to create covalent bonds. Adding more bismuth creates more InBi and GaBi bonds, leading to stronger signals.
Room-temperature photoluminescence measurements produced a peak at 1.78 eV for the quaternary with 0.5 percent bismuth. The intensity of this peak did not improve after one minute of sample annealing at 450degC and then at 500degC. Wang says that the challenge is to now understand the nature of the photoluminescence "“ whether it is related to the bandgap, or to deep levels.
InGaPBi films with more than 0.5 percent bismuth did not produce detectable photoluminescence. The researchers believe that this is due to a disorder effect induced by bismuth clusters.
Wang admits that the lack of strong photoluminescence is a concern for the use of this quaternary in LEDs and lasers. "If the observed photoluminescence is from carrier recombination at the bandgap, it can be enhanced when the quantum well is formed," argues Wang, who warns that if the luminescence is related to deep levels, enhancement via quantisation may not be as effective.
