Photocurrent Excitation Spectroscopy Research Articles

Probing room temperature indirect and minimum direct band gaps of h-BN

Hexagonal boron nitride (h-BN) has attracted considerable interest as an ultrawide bandgap (UWBG) semiconductor. Experimental studies focused on the detailed near band-edge structure of h-BN at room temperature are still lacking. We report a direct experimental measurement of the near band-edge structure performed on h-BN quasi-bulk wafers via photocurrent excitation spectroscopy (PES). PES resolved the band-to-band transitions near M- and K-points in the Brillion zone (BZ), from which the room temperature indirect band gap of EgMK ∼6.02 eV, minimum direct bandgap at M-point of EgM=6.36eV and next lowest direct energy bandgap at K-point of EgK=6.56eV, have been simultaneously determined for the first time experimentally. The measured energy differences between K- and M-points in the conduction band minimum (CBM) and valence band maximum (VBM) are Δ ECMK=0.54eV and Δ EVMK = 0.34 eV, respectively, in good agreement with the calculation results. Significantly differing from its III-nitride wurtzite counterparts, in which only electrons and holes in the conduction and valence band extremes at the Γ-point are predominantly involved in the optical and transport processes, the results highlighted that charge carriers associated with both M- and K-valleys control to the optical excitation, recombination and charge transport processes in h-BN.

Probing Boron Vacancy Complexes in h-BN Semi-Bulk Crystals Synthesized by Hydride Vapor Phase Epitaxy

Hexagonal BN (h-BN) has emerged as an important ultrawide bandgap (UWBG) semiconductor (Eg~6 eV). The crystal growth technologies for producing semi-bulk crystals/epilayers in large wafer sizes and understanding of defect properties lag decades behind conventional III-nitride wide bandgap (WBG) semiconductors. Here we report probing of boron vacancy (VB)-related defects in freestanding h-BN semi-bulk wafers synthesized by hydride vapor phase epitaxy (HVPE). A photocurrent excitation spectroscopy (PES) was designed to monitor the transport of photoexcited holes from deep-level acceptors. A dominant transition line at 1.66 eV with a side band near 1.62 eV has been directly observed, which matches well with the calculated energy levels of 1.65 for the VB-H deep acceptor in h-BN. The identification of VB complexes via PES measurement was further corroborated by the temperature-dependent dark resistivity and secondary ion mass spectrometry measurements. The results presented here suggested that it is necessary to focus on the optimization of V/III ratio during HVPE growth to minimize the generation of VB-related defects and to improve the overall material quality of h-BN semi-bulk crystals. The work also provided a better understanding of how VB complexes behave and affect the electronic and optical properties of h-BN.

Effects of unique band structure of h-BN probed by photocurrent excitation spectroscopy

By employing a photocurrent excitation spectroscopy measurement, a direct bandgap of ∼6.46 eV has been resolved for the first time in thick B-10 enriched h-BN films. Together with previous band calculations, an unconventional energy diagram has been constructed to capture the unique features of h-BN: h-BN has a minimum direct bandgap of ∼6.5 eV and a bandgap of ∼6.1 eV which is indirect with the conduction band minimum (CBM) at M-point and valence band maximum (VBM) at K-point in the Brillouin zone, and the energy levels of the donor and acceptor impurities are measured relative to CBM and VBM, respectively.

(Invited) Hexagonal Boron Nitride Epilayers

As a member of the III-nitride wide bandgap semiconductor family, BN has received much less attention in comparison with other nitride semiconductors. The stable phase of BN synthesized at any temperature under ambient pressure is hexagonal. Due to its wide bandgap (> 6 eV) and layered structure, hexagonal BN (h-BN) is an ideal platform for probing fundamental 2D properties of wide bandgap semiconductors. In this talk, a brief overview of the synthesis and electrical and optical properties of wafer-scale h-BN epilayers will be presented [1-7]. It was shown that the unique 2D structure of h-BN induces exceptionally high density of states, large exciton binding energy and high optical absorption and emission intensity. By growing h-BN under high V/III ratios, epilayers exhibiting pure free exciton emission have been obtained [4]. Photocurrent excitation spectroscopy results directly provided a room temperature bandgap value for h-BN in between 6.4 and 6.5 eV and the free exciton binding energy (Ex) of 0.73 eV [5]. Layer-structured B-rich BGaN alloys, heterostructures, and quantum wells have also been successfully grown and their properties will be discussed. Thermal neutron detectors fabricated from 100% B-10 enriched h-BN epilayers of thicknesses exceeding 50 mm have attained the highest detection efficiency to date among solid-state detectors at about 58% [6, 7]. These solid-state neutron detectors have become increasingly desirable for a wide range of applications from fissile materials sensing to well logging, because 3He gas detectors are inherently bulky, require high pressurization and high voltage application, slow response time, and expensive. It is our belief that h-BN will lead to many potential applications from deep UV optoelectronics, radiation detectors, to novel layered-structured photonic and electronic devices. [1] R. Dahal, J. Li, S. Majety, B.N. Pantha, X. K. Cao, J. Y. Lin, and H.X. Jiang, Appl. Phys. Lett. 98, 211110 (2011). [2] S. Majety, J. Li, X. K. Cao, R. Dahal, B. N. Pantha, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 100, 061121 (2012). [3] H. X. Jiang, and J. Y. Lin, Semicon. Sci. Technol. 29, 084003 (2014). [4] X. Z. Du, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 106, 021110 (2015); 108, 052106 (2016). [5] T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 122101 (2016). [6] A. Maity, T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 072101 (2016). [7] A. Maity, S. J. Grenadier, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 111, 033507 (2017) and J. Appl. Phys. 123, 044501 (2018).

Probing carbon impurities in hexagonal boron nitride epilayers

Carbon doped hexagonal boron nitride epilayers have been grown by metal organic chemical vapor deposition. Photocurrent excitation spectroscopy has been utilized to probe the energy levels associated with carbon impurities in hexagonal boron nitride (h-BN). The observed transition peaks in photocurrent excitation spectra correspond well to the energy positions of the bandgap, substitutional donors (CB, carbon impurities occupying boron sites), and substitutional acceptors (CN, carbon impurities occupying nitrogen sites). From the observed transition peak positions, the derived energy level of CB donors in h-BN is ED ∼ 0.45 eV, which agrees well with the value deduced from the temperature dependent electrical resistivity. The present study further confirms that the room temperature bandgap of h-BN is about 6.42–6.45 eV, and the CN deep acceptors have an energy level of about 2.2–2.3 eV. The results also infer that carbon doping introduces both shallow donors (CB) and deep acceptors (CN) via self-compensation, and the energy level of carbon donors appears to be too deep to enable carbon as a viable candidate as an n-type dopant in h-BN epilayers.

Bandgap and exciton binding energies of hexagonal boron nitride probed by photocurrent excitation spectroscopy

Photocurrent excitation spectroscopy has been employed to probe the band structure and basic parameters of hexagonal boron nitride (h-BN) epilayers synthesized by metal-organic chemical vapor deposition. Bias dependent photocurrent excitation spectra clearly resolved the band-to-band, free exciton, and impurity bound exciton transitions. The energy bandgap (Eg), binding energy of free exciton (Ex), and binding energy of impurity bound exciton (Ebx) in h-BN have been directly obtained from the photocurrent spectral peak positions and comparison with the related photoluminescence emission peaks. The direct observation of the band-to-band transition suggests that h-BN is a semiconductor with a direct energy bandgap of Eg = 6.42 eV at room temperature. These results provide a more coherent picture regarding the fundamental parameters of this important emerging ultra-wide bandgap semiconductor.

Ultrafast decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells.

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the twodimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20 fs or less.

2D coherent photocurrent excitation spectroscopy

2D coherent photocurrent excitation spectroscopy

Photocurrent and picosecond photoluminescence spectroscopy in GaAs/AlGaAs quantum wells

Photocurrent and photoluminescence excitation spectra of GaAs/AlGaAs quantum wells exposed to an electric field perpendicular to the layers have been studied at different temperatures between 8 and 300 K. Strong quenching of the photoluminescence and a corresponding increase of the photocurrent is found for a 5 nm single quantum well at electric fields of ≈ 5 × 10 4 V/cm. The field mainly changes the number of carriers decaying via the radiative recombination channel whereas the respective recombination mechanisms are almost field independent. It is shown that photocurrent excitation spectra reveal the same features as observed in photoluminescence spectroscopy at low temperatures. In contrast to photoluminescence, the photocurrent spectrum can be easily recorded at room temperature. The independence of low-temperature equipment and the extremely high sensitivity distinguish photocurrent excitation spectroscopy as a versatile tool for characterization of quantum well structures.

Influence of electric fields on the hot carrier kinetics in AlGaAs/GaAs quantum wells

Abstract The carrier dynamics in a 5nm Al 0.4 Ga 0.6 As/GaAs single quantum well which is exposed to an electric field perpendicular to the quantum well plane have been studied at low temperature by means of picosecond luminescence, photoluminescence excitation as well as photocurrent excitation spectroscopy. Three different relaxation channels of the photoexcited carriers in different regimes of the electric field strength are observed. Excitons are formed at low electric fields. A strong quenching of the exciton luminescence and an increase of the impurity or defect related luminescence is observed for fields up to 10 4 V/cm. The application of higher electric fields of ∼5×10 4 V/cm leads to an efficient extraction of free carriers out of the quantum well as demonstrated by the increase of the photo-current. The carrier escape explains the accompanying quenching of the extrinsic luminescence at any excitation energy between 1.63eV and 1.77eV. Time-resolved measurements show that the extrinsic and intrinsic luminescence decay times are not significantly changed with increasing electric field. The quenching of the intrinsic and extrinsic luminescence is thus explained by the decrease of the density of carriers which can recombine via exciton and impurity or defect related luminescence.

Comparison of different methods for the detection of ppm impurities in organic molecular crystals

Some interesting physical properties of organic molecular crystals such as exciton and change carrier transport are found to depend sensitively on trace impurities even if these molecules are present only on a ppm or sub-ppm scale. Impurities often exhibit close molecular similarity to the matrix compound, a fact which makes their analytical detection and their separation even more difficult. We wish to summarize some of the analytical techniques which can be applied under these circumstances: gas chromatography, mass spectrometry, liquid chromatography, sensitized fluorescence emission, delayed fluorescence, triplet exciton lifetime, thermally activated current spectroscopy, including time-resolved measurements of trap-controlled charge carrier mobilities, and radical ion photocurrent excitation spectroscopy. The different methods are compared under the aspects of sensitivity, selectivity, universality and lower detection limit.