A nonlocal semiempirical pseudopotential calculation of the electronic structure of wurtzite ZnO is proposed. The local and nonlocal components of the atomic effective potentials have been sequentially optimized and an excellent quantitative agreement has been achieved with a wide range of band features (energy gaps at high symmetry points, valence band ordering, in-plane and perpendicular components of the effective masses for electrons and holes at Γ), selected not only from available experimental and ab initio results, but also from new calculations performed with the code developed by the ABINIT project. The valence band description has been further improved through the inclusion of spin-orbit corrections. The complex dielectric function along the main crystallographic directions corresponding to the optimized electronic structure is also presented, along with extensive comparisons of all the computed quantities with the literature data.
A full-band Monte Carlo model has been developed for understanding the carrier multiplication process in HgCdTe infrared avalanche photodiodes. The proposed model is based on a realistic electronic structure obtained by pseudopotential calculations and a phonon dispersion relation determined by ab initio techniques. The calculated carrier-phonon scattering rates are consistent with the electronic structure and the phonon dispersion relation, thus removing adjustable parameters such as deformation potential coefficients. The computation of the impact ionization transition rate is based on the calculated electronic structure and the corresponding wavevector-dependent dielectric function. The Monte Carlo model is applied to investigate key performance figures of long-wavelength infrared (LWIR) and mid-wavelength infrared (MWIR) HgCdTe avalanche photodetectors such as carrier multiplication and noise properties. Good agreement is achieved between simulations and experimental results. The multiplication process in LWIR (k c = 9.0 lm at 80 K) and MWIR (k c = 5.1 lm at 80 K) devices is found to be initiated only by electrons, as expected from excess noise measurements. This single-carrier multiplication behavior can be traced back to the details of the computed valence-band structure and phonon scattering rates.
Two alternative approximations of the electronic structure of CdTe and HgTe are proposed, both suited to the needs of accuracy and numerical efficiency of full-band carrier transport simulation: a local empirical pseudopotential (EPM) parametrization including relativistic corrections, and an original fullBrillouin-zone (FBZ) k Á p model using two expansion points (C and W). The EPM and k Á p band structures closely match the available experimental and ab initio information, complemented with the results of new density functional theory (DFT)-local density approximation (LDA) calculations, for the conduction and valence bands relevant in transport phenomena. The EPM description of the binary compounds, featuring transferable Te pseudopotentials, is the basis for a computation of the electronic structure of the ternary alloy Hg 1Àx Cd x Te in the framework of disorder-corrected virtual crystal approximation. The composition dependence of energy gaps, effective masses, and high-frequency dielectric constants are discussed and compared with available experimental data, and the novel FBZ approach is applied to the case of x = 0.7.
We present a k⋅p model for wurtzite semiconductors that allows the accurate approximation of the electronic structure over the entire Brillouin zone. The inclusion of an additional expansion point besides Γ allows significant improvements over standard full-Brillouin-zone approaches while keeping a manageable number of model parameters. We provide complete information about the Hamiltonian matrices of both expansion points and discuss the details of the optimization process used to determine the matrix parameters. As a demonstration of our scheme, we propose an approximation of the electronic structure of wurtzite ZnO, optimized for application to full-band Monte Carlo electron transport simulation. (A MATLAB implementation of the k⋅p model for ZnO is available from the authors.)
Objective. Recent SiPM developments and improved front-end electronics have opened new doors in TOF-PET with a focus on prompt photon detection. For instance, the relatively high Cherenkov yield of Bismuth-Germanate (BGO) upon 511 keV gamma interaction has triggered a lot of interest, especially for its use in total body PET scanners due to the crystal’s relatively low material and production costs. However, the electronic readout and timing optimization of the SiPMs still poses many questions. Lab experiments have shown the prospect of Cherenkov detection, with coincidence time resolutions (CTRs) of 200 ps FWHM achieved with small pixels, but lack system integration due to an unacceptable high power uptake of the used amplifiers. Approach. Following recent studies the most practical circuits with lower power uptake (<30 mW) have been implemented and the CTR performance with BGO of newly developed SiPMs from Fondazione Bruno Kessler (FBK) tested. These novel SiPMs are optimized for highest single photon time resolution
(SPTR). Main results. We achieved a best CTR FWHM of 123 ps for 2 × 2 × 3 mm³ and 243 ps for 3 × 3 × 20 mm³ BGO crystals. We further show that with these devices a CTR of 106 ps is possible using commercially available 3 × 3 × 20 mm³ LYSO:Ce,Mg crystals. To give an insight in the timing properties of these SiPMs, we measured the SPTR with black coated PbF2 of 2 × 2 × 3 mm³ size. We
confirmed an SPTR of 68 ps FWHM published in literature for standard devices and show that the optimized SiPMs can improve this value to 42 ps. Pushing the SiPM bias and using 1 x 1 mm² area devices we measured an SPTR of 28 ps FWHM. Significance. We have shown that advancements in readout electronics and SiPMs can lead to improved CTR with Cherenkov emitting crystals. Enabling TOF with BGO will trigger a high interest for its use in low-cost and total-body PET scanners. Furthermore, owing to the prompt nature of Cherenkov emission, future CTR improvements are conceivable, for which a low-power electronic implementation is indispensable. In an extended discussion we will give a roadmap to best timing with prompt photons.
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