The role of intervalley phonons in hot carrier transfer and extraction in type-II InAs/AlAsSb quantum-well solar cells

Abstract

Simultaneous continuous wave (CW) photoluminescence and monochromatic current densityvoltage (J-V) measurements of InAs/AlAsSb QW p-i-n diodes reveal stable hot carriers observed at relatively low power (nearly independent of power). This behavior is attributed to preferential scattering of high energy carriers to the upper satellite L-and X-valleys, which inhibits carrier thermalization via LO phonon emission. Although both high electric field and optical excitation are shown to enable stable hot carrier generation in the quantum wells (QWs), the extraction of these carriers is inhibited by the mismatch in valley degeneracy (L to Γ) across the InAs QW/n-AlInAs layers resulting in carrier localization in the QWs in the operating regime of the solar cell.

Hot-carrier cooling dynamics of type-II InAs/AlA1-xSbx quantum wells

“…Alternatively, even though the onset of the plateau occurs several 100's of meV below the excitation energy required to expect easy inter-valley scattering to the L-valley 3,16,17,34,35 , the electric field of the THz probe is approximately double the required field to result in a Gunn effect in these samples ∼17 kV/cm 16 . However, if hot electrons persist and are even heated through electron phonon interactions after their initial excitation some degree of band tilting due to the THz field may scatter e1 electrons into the L-valley 36 to contribute to the complicated transient response.…”

Hot-carrier dynamics in InAs/AlAsSb multiple-quantum wells

“…Alternatively, even though the onset of the plateau occurs several 100’s of meV below the excitation energy required to expect easy inter-valley scattering to the L-valley 3 , 16 , 17 , 35 , 36 , the electric field of the THz probe is approximately double the required field to result in a Gunn effect in these samples ( 17 kV/cm) 16 . However, if hot electrons persist and are even heated through electron-phonon interactions after their initial excitation some degree of band tilting due to the THz field may scatter e1 electrons into the L-valley 37 to contribute to the complicated transient response.…”

“…Moreover, this metastability is likely to be present in all narrow-well InAs/AlAsSb MQW systems. Dynamics from this work can be applied to device-like systems where electricity extraction can also be affected by tunnelling between the wells 16 .…”

“…Not shown in Fig. 1 b are the L-valley and X-valley local minima of the e1 conduction band 16 which may also play a role in the scattering of the electron once photoexcited.

Figure 2 Transient differential THz-electric field [ ] through the MQW sample at 4 K for the excitation photon energies of 0.85 eV (cyan) and 0.97 eV (red).

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“…These properties all occur in InAs/AlAs Sb multiple-quantum wells (MQWs), where a hot-carrier distribution is shown along with extended carrier lifetimes as a result of inhibited phonon–phonon interactions 8 . In steady-state photoluminescence (PL), these MQWs have shown evidence of hot carriers 14 , non-monotonic emission energy as a function of lattice temperature due to the complicated valence band structure 15 and even intervalley scattering of electrons to the long-lived L-valley states 16 . Transient absorption directly shows long-lived carriers complementing the evidence of hot carriers and the non-monotonic temperature dependence 8 .…”