Mid-infrared ultrafast carrier dynamics in thin film black phosphorus

Abstract

Black phosphorus is emerging as a promising semiconductor for electronic and optoelectronic applications. To study fundamental carrier properties, we performed ultrafast femtosecond pumpprobe spectroscopy on thin film black phosphorus mechanically exfoliated on a glass substrate.Carriers (electrons and holes) were excited to high energy levels and the process of carrier relaxation through phonon emission and recombination was probed. We used a wide range of probing wavelengths up to and across the band gap to study the evolution of the relaxation dynamics at different energy levels. Our experiments revealed a plethora of important physical phenomena. The fast relaxation time constants, associated with carrier-phonon scattering, steadily increase as the energy of the probe beam approaches the band gap energy, which was determined to be 0.31 eV, and the carrier recombination rate was obtained when the probe wavelength was 2 tuned to match the band gap energy. The carrier-phonon scattering rates were found to be similar along the armchair and zigzag directions, therefore, the anisotropic carrier mobility reported in literature is mainly due to the difference in effective mass of carriers along different directions.The ultrafast spectroscopy data further revealed the oxidation induced surface charges. Our results highlight the importance of using the spectroscopy technique, in this case, in the mid-IR range, to uncover useful physical processes.Since the last decade there has been an increasing interest in the study of two-dimensional materials, especially graphene and transition metal dichalcogenides (TMDC) for fabricating many electronic and optoelectronic devices [1][2][3][4][5][6][7][8][9][10][11] . Black phosphorus has emerged as a recent material in this field and has shown promises with its direct bandgap, anisotropic electronic, optical, and thermal properties, and good device performance [12][13][14][15][16][17][18][19] . Black phosphorus has a direct bandgap in the bulk of around 0.35 eV and can become as large as 1.5 eV in the monolayer form according to first principles calculations, photoluminescence studies, and electrical measurements 13, 20-24 . The direct bandgap is attractive for use in electronic devices such as solar cells 25,26 and photodetectors [27][28][29][30] . Black phosphorus has also been successfully tested as a transistor channel material [31][32][33] .Given the vast interest in black phosphorus, it is important to investigate its properties at a fundamental level. Ultrafast optical spectroscopy is very useful in understanding carrier scattering, relaxation, and recombination processes. Many ultrafast studies have been carried out on graphene [34][35][36][37][38][39][40] and TMDC such as MoS2 41-44 . There have also been a few ultrafast optical studies on black

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