Abstract
Black phosphorous (BP) is a layered semiconductor with high carrier mobility, anisotropic optical response and wide bandgap tunability. In view of its application in optoelectronic devices, understanding transient photo-induced effects is crucial. Here, we investigate by time- and angle-resolved photoemission spectroscopy BP in its pristine state and in the presence of Stark splitting, chemically induced by Cs ad-sorption. We show that photo-injected carriers trigger bandgap renormalization, and a concurrent valence band flattening caused by Pauli blocking. In biased samples, photo-excitation leads to a long-lived (ns) surface photovoltage of few hundreds mV that counterbalances the Cs-induced surface band bending. This allows us to disentangle bulk from surface electronic states, and to clarify the mechanism underlying the band inversion observed in bulk samples.
Highlights
Black phosphorous (BP) is a layered semiconductor with interesting physical properties, such as high carrier mobility (up to 104 cm2 V−1 s−1 in the monolayer (1L)) [1], large electronic/optical anisotropies [2, 3] and excellent mechanical properties (1L-BP can sustain tensile strain up to ∼30%) [4]
This gap tunability is attributed to the so-called giant Stark effect [19, 20], i.e. an electric field-induced shift of electronic states, named ‘giant’ since it can lead to gap closure, as confirmed by modeling [6, 10, 17, 18] and photoemission experiments [17, 21], along with a pronounced surface depletion at the valence band (VB) and a concurrent surface confinement of the conduction band (CB)
BP crystallizes in the orthorhombic structure [7, 33], where sp3 orbital hybridization leads to buckled layers normal to the z-axis, figure 1(a)
Summary
Black phosphorous (BP) is a layered semiconductor with interesting physical properties, such as high carrier mobility (up to 104 cm V−1 s−1 in the monolayer (1L)) [1], large electronic/optical anisotropies (reflectance and DC conductance can vary by a factor ∼2–4 with in-plane orientation) [2, 3] and excellent mechanical properties (1L-BP can sustain tensile strain up to ∼30%) [4]. References [16,17,18] demonstrated that surface doping by alkali atoms allows to engineer the gap of BP, leading to surface band inversion at a critical dopant concentration ∼0.4 1L (∼9 × 1013 cm−2) [16] This gap tunability is attributed to the so-called giant Stark effect [19, 20], i.e. an electric field-induced shift of electronic states, named ‘giant’ since it can lead to gap closure, as confirmed by modeling [6, 10, 17, 18] and photoemission experiments [17, 21], along with a pronounced surface depletion at the valence band (VB) and a concurrent surface confinement of the conduction band (CB). ARPES cannot ignore the presence of underlying bulk states, and the comparison with 1L- or FL-BP theoretical predictions might be misleading
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