Free-Electron Screening Mechanism of the Shallow Impurity Breakdown in n-GaAs: Evidences from the Photoelectric Zeeman and Cyclotron Resonance Spectroscopies

摘要

A novel breakdown (BD) mechanism of shallow impurity (SI) under the electric field at low temperatures is suggested for n‐GaAs samples with the donor concentrations 1014?cm?3≤ND≤1016?cm?3 and the compensation degree 0.3≤K=NA/ND≤0.8 with acceptors of concentration NA in the external magnetic fields up to H=6.5T, oriented parallel or perpendicular to the external electric field. Diagnosis of the BD mechanism was performed by SI Zeeman (mainly from the ground state 1s to 2p+1 and other excitation states) and cyclotron resonance photoelectric spectroscopy (PES) methods in the wide interval of the electric field including the BD region too. The obtained results reveal that the BD electric field εBD does not correlate with K and the carrier’s mobility μ of the samples, which contradict to the well-known impact ionization mechanism (IIM). A serious discrepancy with IIM is that εBD does not almost depend on the magnetic field up to H=6.5T when εH though the SI ionization energy increases two times. The cyclotron resonance (CR) measurements show that the line width does not depend on the electric field for εεBD, indicating the lack of free-carrier (FC) heating in contradiction with IIM. A considerable decrease of the free carriers’ capture cross section (CCS) area by ionized SI centers with a subsequent increase in the FC concentration n is observed by means of PES investigation of the 1s?2p+1 and CR lines in the electric fields ε≤εBD and at different magnetic fields, applied along (Hε) the electric field or perpendicular (H⊥ε) to the electric field. The slope of the 1s?2p+1 line intensity on the electric field for εH does not depend on the magnetic field, which is valid for εBD too. Various effects determined in the PES measurements at ε=εBD, such as a drastic narrowing of the 1s?2p+1 and CR lines, a shift of the CR line to higher magnetic fields, and disappearing of the lines to higher excited SI states, were clarified to be a result of screening of the SI Coulomb potential by free carriers. The FC screening at the BD reduces the potential fluctuation and its influence to the PES line shape of 1s?2p+1 and other excited states. It is shown that an increase in the FC concentration reduces the CCS, which can be assumed as the main factor along with the increase in the ionization coefficient for the SI breakdown in the electric field. The screening length rs of the SI Coulomb potential decreases with the increasing FC concentration, reducing the CCS; the latter seems to vanish completely at rs=aB? (aB? is the effective Bohr radius), when high screening results in vanishing of all the bound states of the Coulomb potential. Note that this limit is similar to the Mott transition. Many experimental facts and our calculation of the CCS support the suggested mechanism for the SI breakdown. The well-known IIM is valid for samples with SI concentrations N?1013?cm?3 and takes place at very high electric fields.