Structural and optical characterizations of InPBi thin films grown by molecular beam epitaxy
© Gu et al.; licensee Springer. 2014
Received: 13 November 2013
Accepted: 18 December 2013
Published: 13 January 2014
InPBi thin films have been grown on InP by gas source molecular beam epitaxy. A maximum Bi composition of 2.4% is determined by Rutherford backscattering spectrometry. X-ray diffraction measurements show good structural quality for Bi composition up to 1.4% and a partially relaxed structure for higher Bi contents. The bandgap was measured by optical absorption, and the bandgap reduction caused by the Bi incorporation was estimated to be about 56 meV/Bi%. Strong and broad photoluminescence signals were observed at room temperature for samples with xBi < 2.4%. The PL peak position varies from 1.4 to 1.9 μm, far below the measured InPBi bandgap.
Group III-V semiconductors containing small amounts of bismuth (Bi), popularly known as ‘dilute bismide,’ attracted great attention in the past decade. Bismuth is the largest and the heaviest group V element with its isoelectronic energy level that resides in the valence band of most III-V materials. Incorporation of a small amount of Bi atoms in a common III-V compound is expected to lead to a large bandgap reduction and strong spin-orbit splitting. This provides a new degree of freedom to engineering the band structure for potential optoelectronic and electronic device applications. Under such conditions, it is expected that troublesome hot-hole-induced Auger recombination and inter-valence band absorption (IVBA) processes can be suppressed leading to high efficiency and temperature insensitive lasers for optical communications. Most published literatures so far focus on growth and material properties of GaAsBi with improving quality, making GaAsBi closer to device applications. GaAsBi light-emitting diodes (LEDs) and optically pumped and electrically injected laser diodes have been demonstrated recently.
Group III-V semiconductor phosphides are important materials for optoelectronic devices working at visible and near-infrared wavelength range[7, 8]. The incorporation of Bi into InP can further extend transition wavelengths for optoelectronic devices with aforementioned improved device performances as a result of the suppressed Auger recombination and IVBA processes. Berding et al. theoretically compared InPBi, InAsBi, InSbBi, and HgCdTe, and pointed out that InPBi was much more robust than the others, thus making it as a promising candidate for infrared applications. However, their calculations also showed that InPBi was very difficult to synthesize due to a larger miscibility gap than that of InAsBi and InSbBi. So far, a few works on the optical studies of InP/Bi where the incorporated Bi is only in the doping level[10, 11] were reported. The spectroscopy reveals rich sharp transitions at energy levels close to the InP bandgap at low temperatures.
In this work, we investigate the structural and optical properties of InPBi with Bi composition in the range of 0.6% to 2.4%. The Bi-induced bandgap reduction of around 56 meV/Bi% is obtained. Strong and broad photoluminescence (PL) signals have been observed at transition energy much smaller than the InPBi bandgap.
The samples were grown on (100) semi-insulating InP substrates by V90 gas source molecular beam epitaxy (GSMBE). Elemental In and Bi and P2 cracked from phosphine were applied. After the surface oxide desorption of InP substrate at 524°C, a 75-nm undoped InP buffer was grown at 474°C, the normal growth temperature of InP. Then the growth temperature was decreased significantly for InPBi growth. Both the Bi/P ratio and the growth temperature were adjusted to achieve InPBi with various Bi compositions. The thickness of the InPBi epi-layers was kept around 430 nm. An InP reference sample was also grown at the low temperature.
After the growth, the Bi compositions were determined by Rutherford backscattering spectrometry (RBS) with 2.275 MeV 4He2+ ions. The structural qualities were characterized by a Philips X’pert MRD high-resolution x-ray diffractometer (HRXRD) equipped with a four-crystal Ge (220) monochromator (Philips, Amsterdam, Netherlands). The PL and absorption spectra were measured using a Nicolet Magna 860 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA), in which a liquid-nitrogen cooled InSb detector and a CaF2 beam splitter were used. A diode-pumped solid-state (DPSS) laser (λ = 532 nm) was used as the excitation source for PL measurements, and the double modulation mode was used to eliminate the mid-infrared background radiation beyond 2 μm. For the low-temperature PL measurements, the samples were mounted into a continuous-flow helium cryostat, and the temperature was controlled from 8 to 300 K by a Lake Shore 330 temperature controller (Lake Shore Cryotronics, Inc., Westerville, OH, USA).
Results and discussions
The structural and optical properties of 430-nm-thick InPBi thin films have been investigated. The Bi compositions determined by RBS measurements were in the range of 0.6% to 2.4%. A good quality has been demonstrated for the samples with the Bi composition lower than 1.4%, whereas the samples with higher Bi contents become partially relaxed. It was found that the incorporation of Bi caused the bandgap reduction of about 56 meV/Bi%. Strong and broad PL signals containing multiple overlapped peaks were observed at room temperature with peak wavelength that varied from 1.4 to 1.9 μm, which is far from the band-to-band transition. The origins of the long wavelength PL signals were discussed, but further investigation is necessary for unambiguous explanation.
The authors wish to acknowledge the support of National Basic Research Program of China under grant nos. 2014CB643900 and 2012CB619202; the National Natural Science Foundation of China under grant nos. 61334004, 61204133, and 61275113; the Guiding Project of Chinese Academy of Sciences under grant no. XDA5-1; the Key Research Program of the Chinese Academy of Sciences under grant no. KGZD-EW-804; and the Innovation Research Group Project of National Natural Science Foundation under grant no. 61321492.
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