Mechanism of Defect Reactions in Semiconductors

We investigate the dependence of the electronic band structure on the localized phonon mode at a Ga vacancy in GaN. The electronic states and phonon modes are both calculated using a first-principles method based on the density functional theory. Comparing the calculated electronic band structures and phonon-frequency densities of states of GaN without and with a Ga vacancy, we find that 1) there are localized electronic midgap states closely above the top of the valence band, 2) localized phonon modes appear above the acoustic and optical phonon bands, 3) one of these localized phonon modes contains asymmetric distortions in which one of the four N atoms around a Ga vacancy strongly oscillates, and 4) the localized electronic midgap states strongly couple with the localized asymmetric mode. These results indicate that a Ga vacancy can act as a nonradiative recombination center and may trigger defect reactions in GaN-based devices.

Section: Electron-phonon Couplingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

First-principles calculation of electron–phonon coupling at a Ga vacancy in GaN

Tsujio¹,

Oda²,

Shinozuka³

2017

“…5) In the case of off-center-type distortion, which is found at some deep-level defects, the symmetry in Q 1 and ÀQ 1 does not exist. [27][28][29][30][31] In that case, it is important to determine which corresponds to the off-center configuration, Q…”

Section: Defect Reactionmentioning

confidence: 99%

Phonon-Kick Mechanism for Defect Reactions in Semiconductors

Shinozuka¹,

Wakita²,

Suzuki³

2012

The possibility of a feedback mechanism between nonradiative carrier captures by a deep-level defect and induced transient lattice vibrations is discussed using appropriate configuration coordinate diagrams for many carriers. By treating the lattice motion classically, we self-consistently simulate the time evolution of the interaction mode and a series of athermal captures of electron(s) and hole(s). When both the activation energies E e act and E h act are low, a series of successive nonradiative carrier-captures are possible for high carrier densities. However, we find that the possibility of inflation in the amplitude of lattice vibration critically depends on minority carrier capture rate and the relative width of the phonon frequency distribution. Referring to the obtained results, we discuss the phonon-kick mechanism for defect reactions.

“…This process is called the phonon-kick mechanism. [22][23][24] The necessary conditions for the phonon-kick mechanism were also reported: (i) there must be an existing localized phonon mode, (ii) there must be an existing localized mid-gap electronic state, and (iii) the e-p coupling between the mode and the mid-gap state must be strong. 21) However, it has yet to be found whether these conditions are satisfied or not in actual materials because they are generally derived with simple systems.…”

Section: Introductionmentioning

confidence: 99%

Investigation of the electron–phonon interactions around Ga vacancies in GaN and their role in the first stage of defect reactions

Oda¹

2019

Defects in GaN can cause degradation in the active regions of GaN-based devices; however, the origin of such defects remains unclear. Here, the electron–phonon interaction around Ga vacancies (VGa) in GaN is investigated quantitatively using first-principles calculations. It is shown that the energy shift in an electronic system with a localized phonon mode has the same order of magnitude as that in an atomic system. Next, the phononic structures of a system in a positively charged state are calculated. It is demonstrated that localized modes in the neutral state are still localized under slightly shifted frequencies. These results indicate that a VGa satisfies all conditions for the phonon-kick mechanism. This characteristic is expected to result in enhancement of the amplitude of the localized modes, which should cause N atoms to migrate toward VGa sites and form a NGa–VN complex defect. Hence, the migration energies of N atoms from VGa sites to NGa–VN complex defects in both neutral and positively charged states are investigated. It is revealed that the migration barrier is smaller in the positively charged state than in the neutral state and that the NGa–VN complex is more stable than a VGa. In light of these findings, the conformation change from a VGa to a NGa–VN complex may serve as a microscopic mechanism for the first stage of the defect reactions around VGa sites in GaN. These findings are expected to be useful in the future development of light-emitting diodes and transistors with longer lifetimes and higher efficiencies.