A review is presented of the various theoretical methods that have thus far been developed for the study of states introduced by impurities and other point defects in semiconductors. The main body of the paper is prefaced with brief sections on the role of impurities and defects in semiconductors and on the general aspects of experimental techniques, as an appropriate setting for the theoretical discourse. Theoretical methods, including those of the effective-mass type, and a wide range of methods appropriate to deep levels are then presented. Applications of these methods are discussed critically. Finally, the relative merits of the various approaches are compared and the prospects for future work are assessed.

The electronic structure of impurities and other point defects in semiconductors

Under suitable circumstances imperfections in semiconductors can bind electrons (holes) with a binding energy small compared to the intrinsic energy gap of the host; the wavefunctions characterising the energy levels of the imperfection are extended over many lattice spacings. This review discusses the electronic energy levels of chemical impurities in the classic group IV elemental and the III-V and II-VI compound semiconductors. The large dielectric constant of the host, the anisotropic effective mass tensor and/or the small effective mass of the charge carrier are the factors which play a significant role in the description of the electronic energy levels; they can be viewed as scaled-down versions of the hydrogen atom with bound states having binding energies orders of magnitude smaller than those of the hydrogen atom. The authors present the experimental results on the spectroscopy of donors and acceptors in semiconductors together with the theory necessary for their interpretation. They discuss the experimental results and the theory of the bound states of impurities in the context of the symmetry and the effective-mass parameters of the band extrema with which they are associated. Effects of external perturbation-piezo- and magneto-spectroscopy-are presented both from experimental and theoretical points of view. The review concludes with the experimental observations on the linewidths of the excitation spectra of donors and acceptors in semiconductors and an analysis of the causes underlying them.

https://iopscience.iop.org/article/10.1088/0034-4885/44/12/002/pdf

Spectroscopy of the solid-state analogues of the hydrogen atom: donors and acceptors in semiconductors

Light-emitting diodes (LEDs) are devices designed to efficiently convert electrical energy into electromagnetic radiation, most of which is visible to the human eye. Some of the disciplines involved in the understanding and utilization of LEDs are reviewed, with emphasis on the III-V semiconducting compounds and GaP LEDs in particular. Salient features of photometry, the physics of electrical injection and luminescence, and the design of LEDs are discussed in detail, followed by a survey of prominent applications for the various LEDs.

Light-emitting diodes

The theory of the electronic states of crystals containing point defects in small concentration is reviewed and its application to the case of semiconductors is discussed. The basic properties of scattering and resonant and bound states are related to the band structure of the semiconductor. The form of the impurity potentials and the effect of the dielectric screening and deep levels are also discussed. Applications to the cases of donor acceptor and isoelectronic impurities are described, making use of the effective mass approximation and taking into account interband mixing and central cell corrections. Existing calculations are reviewed and compared with some of the experimentally known levels. Optical processes involving transitions between all types of electronic levels are described and experimental evidence for them is reported. Effects of external perturbations like uniaxial pressure, electric and magnetic fields are finally examined.

Electronic impurity levels in semiconductors

A study of the conductance and capacitance associated with traps in Schottky barriers is described. A sample calculation of these two quantities is developed and compared with experimental data obtained in GaP VPE saples. It is shown that traps introduce peaks on the conductance‐vs‐temperatures curves; from them the concentration of each trap can be calculated. A study at different frequencies permits the determination of the characteristics of the traps. In GaP we have observed trapping effects in donor levels at Ec−Et?100 and 86 meV, the emission rates were found, respectively, to equal 2×1011 and 2×109 s−1 for these two levels. It is also shown that photoconductance effects occur at the oxygen deep levels.

Conductance and capacitance studies in GaP Schottky barriers

Infrared transmission measurements of GaP with S or Si donor impurities at liquid-helium temperature reveal absorption lines due to excitation of the donor electron. Strong absorption peaks corresponding to the $1s\ensuremath{\rightarrow}2{p}_{\ifmmode\pm\else\textpm\fi{}}$ transition are observed, and their spectral positions are used to determine optical ionization energies of 104.2 and 82.5 meV (\ifmmode\pm\else\textpm\fi{}0.3 meV), respectively, for S and Si donors in GaP. A fitting of Faulkner's effective-mass calculation for donor states to the observed levels yields the effective masses of the conduction band: ${m}_{\ensuremath{\perp}}=(0.191\ifmmode\pm\else\textpm\fi{}0.005){m}_{0}$ and ${m}_{\mathrm{II}}=(1.7\ifmmode\pm\else\textpm\fi{}0.2){m}_{0}$. The $p$-like final states observed in these measurements are found to differ significantly in binding energy from the excited donor-electron states observed in the "two-electron" recombination of the exciton bound to neutral sulfur in GaP.

A. Onton

Papers

Optical Absorption Due to Excitation of Electrons Bound to Si and S in GaP