Electromechanical coupling in free-standing AlGaN/GaN planar structures

The strain and electric fields present in free-standing AlGaN/GaN slabs are examined theoretically within the framework of fully-coupled continuum elastic and dielectric models. Simultaneous solutions for the electric field and strain components are obtained by minimizing the electric enthalpy. We apply constraints appropriate to pseudomorphic semiconductor epitaxial layers and obtain closed-form analytic expressions that take into account the wurtzite crystal anisotropy. It is shown that in the absence of free charges, the calculated strain and electric fields are substantially differently from those obtained using the standard model without electromechanical coupling. It is also shown, however, that when a two-dimensional electron gas is present at the AlGaN/GaN interface, a condition that is the basis for heterojunction field-effect transistors, the electromechanical coupling is screened and the decoupled model is once again a good approximation. Specific cases of these calculations corresponding to transistor and superlattice structures are discussed.Comment: revte

Birefringence and refractive indices of wurtzite GaN in the transparency range

Birefringence and anisotropic refractive indices of wurtzite GaN within the spectral range from 0.58 eV to 3.335 eV were determined combining optical retardation and spectroscopic ellipsometry measurements on a series of undoped m- and c-plane GaN bulk substrates grown by hydride vapor phase epitaxy. It is observable that the birefringence has a maximum close to the absorption edge and a weak broad minimum in near-IR range. A quantitative explanation of the whole data is given in terms of contributions to the optical response of GaN due to discrete excitons, Coulomb enhanced band-to-band optical transitions near the E(0) critical point of the band structure, high-energy optical transitions, and infrared active optical phonon modes which are different for the ordinary and extraordinary waves both in magnitude and in spectral dependence

Anisotropic optical constants, birefringence, and dichroism of wurtzite GaN between 0.6 eV and 6 eV

We report the room-temperature anisotropic dielectric functions (DFs), refractive indices, and absorption coefficients as well as birefringence and dichroism of wurtzite GaN in the spectral range between 0.6 eV and 6 eV. They have been determined by combined spectroscopic ellipsometry, optical retardation, and transmission measurements on a series of m- and c-plane bulk substrates prepared from crystals grown by hydride vapor phase epitaxy. The accuracy of the derived DFs is estimated by investigation of the role of mosaicity-related crystal imperfections, self-consistency test based on a Kramers-Kronig analysis, and examination of the influence of kind of overlayer. We also briefly discuss optical properties of a highly defective near-surface layer of GaN crystals introduced by their mechanical polishing

Thienopyrazine-based low-bandgap polymers for flexible polymer solar cells

The optical gaps of the low-bandgap PPVs (PM-20, PM-19, PM-18) are decreased down to 1.6-1.7 eV compared with that of MDMO-PPV (2.2 eV). The best lateral hole mobility was determined to be 2.1 × 10-3 cm2/V s (PM-18) in field effect transistors and exceeds that of MDMO-PPV (poly-[ 2-methoxy-5-(3'.7'-dimethyloctyloxy)-1.4-phenylenevinylene], 8.5 × 10-4 cm2/V s). This allows to reduce the PCBM ([6.6]-phenyl-C$_{61(71)}$-butanoic acid methyl ester) content in solar cell devices down to 1:2 w/w giving a better $\eta_{\rm AM1.5}$ than for MDMO-PPV:[60]-PCBM cells (PM-19:[60]-PCBM 2.32% on ITO-PET, 2.86% on ITO glass). The charge transfer to PCBM as acceptor occurs quite normally and shows an effective charge separation using light-induced spin resonance spectroscopy (LESR). The [70]-PCBM$^{-\bullet}$ signals are shifted to lower field related to those of [60]-PCBM$^{-\bullet}$ and overlap more with the polaron signal of PM-19. The LESR g-factor components of [70]-PCBM$^{-\bullet}$ are reported for the first time. The external quantum efficiency peak values achieve up to 42% at ~350–400 nm and 26% at ~640 nm (PM-19:[60]-PCBM)