A. O. Zakhar’in

Terahertz electroluminescence under conditions of shallow acceptor breakdown in germanium

The spectrum of spontaneous terahertz electroluminescence was obtained near the breakdown threshold of a shallow acceptor (Ga) in germanium. The emission spectra were recorded by the Fourier spectroscopy method at a temperature of ∼5.5–5.6 K. The emission spectrum exhibits narrow lines with maxima at ∼1.99 THz (8.2 meV) and ∼2.36 THz (9.7 meV), corresponding to the optical transitions of nonequilibrium holes from the excited impurity states to the ground state of impurity center. A broad line with a maximum at ∼3.15 THz (13 meV) corresponding to the hole transitions from the valence band to the impurity ground state is also seen in the spectrum. The contribution of the hole transitions from the states of the valence band increases upon an increase in the electric-field strength. Simultaneously, the optical transitions of nonequilibrium holes between the subbands of the valence band appear in the emission spectrum. The integral terahertz-emission power is ∼17 nW per 1 W of the input power.

Terahertz photoluminescence from GaAs doped with shallow donors at interband excitation

We report on the observation of efficient generation of terahertz radiation at continuous-wave interband excitation of n-GaAs at low temperatures. The radiative transitions, accompanying relaxation and trapping of photoexcited electrons to localized donor states or to empty states in impurity subband, lead to the emission of terahertz photons with a relatively high external quantum yield up to 0.3%.

Terahertz electroluminescence from 6Н-SiC structures with natural superlattice

We report on efficient terahertz emission in the region of 1.5-2 THz from high electric field biased 6H-SiC structures with a natural superlattice (NSL) at liquid helium temperatures. The shape of the THz emission spectrum, the linear dependence of its maximum on the bias, the characteristic field strength required to achieve the emission, and the linear polarization of the emission along the natural superlattice axis allow it to be attributed to steady-state Bloch oscillations of electrons in the 6H-SiC natural superlattice.

Terahertz luminescence in strained GaAsN:Be layers under strong electric fields

The authors report on the experimental studies of terahertz emission from strained GaAsN∕GaAs microstructures doped with beryllium at the conditions of electric breakdown of the shallow impurity. The terahertz emission spectrum demonstrates several distinctive signatures that are related to spontaneous optical transitions between resonant and localized states of the acceptor. The energy spectrum calculated for the Be acceptor in the GaAsN∕GaAs heterostructure fits reasonably well with the experimentally observed peaks.The authors report on the experimental studies of terahertz emission from strained GaAsN∕GaAs microstructures doped with beryllium at the conditions of electric breakdown of the shallow impurity. The terahertz emission spectrum demonstrates several distinctive signatures that are related to spontaneous optical transitions between resonant and localized states of the acceptor. The energy spectrum calculated for the Be acceptor in the GaAsN∕GaAs heterostructure fits reasonably well with the experimentally observed peaks.

Terahertz impurity luminescence under the interband photoexcitation of semiconductors

The observation of intense terahertz luminescence under the interband photoexcitation of n-GaAs and p-Ge semiconductors at low temperatures is reported. The terahertz photoluminescence is caused by radiative transitions, which accompany the capture of nonequilibrium carriers by impurity centers. These centers are in turn created in the crystal due to the impurity-assisted electron-hole recombination. The external quantum yield of the luminescence reaches 0.1%.

Terahertz radiation induced by the Wannier-Stark localization of electrons in a natural silicon carbide superlattice

Intense terahertz electroluminescence from SiC structures with a miniband electron spectrum caused by the natural superlattice has been observed. The shape of the terahertz radiation line, the linear dependence of the position of its maximum on the bias voltage, the typical value of the field required to induce the radiation, and the prevailing polarization of the radiation along the superlattice axis indicate that the observed radiation results from to the excitation of stationary Bloch oscillations of electrons in the natural silicon carbide superlattice.

Terahertz Emission upon the Interband Excitation of GaN Layers

The experimental observation and study of terahertz photoluminescence upon the steady-state interband excitation of epitaxial n-GaN(Si) layers are reported. The properties of the terahertz emission spectrum and the dependence of the spectrum on temperature and photoexcitation intensity suggest that emission occurs due to the trapping of nonequilibrium electrons at charged donors. In the n-type material at low temperatures, charged donors can be formed as a result of the recombination of nonequilibrium holes with electrons localized at donor centers. The main contribution to terahertz photoluminescence is made by 2P → 1S optical transitions between the first excited state and ground state of the donors. In addition, optical transitions of electrons from the conduction band states to the ground state as well as to the excited states of donors are evident in the terahertz emission spectrum.

Terahertz emission and photoconductivity in n-type GaAs/AlGaAs quantum wells: the role of resonant impurity states

Terahertz emission spectra in a longitudinal electric field and lateral photoconductivity spectra under terahertz illumination have been studied in structures with GaAs/AlGaAs quantum wells (QWs). It is shown that the spectra contain features associated with electron transitions involving resonant impurity states related to the second quantum-well subband. Calculations of the energy spectrum of impurity states and matrix elements of optical transitions made by taking into account various positions of the impurity relative to the QW center confirm the assumptions made.

Terahertz-emission generation caused by new effects in the 6H-SiC natural superlattice

It is demonstrated that the 6H-SiC natural superlattice has two simultaneously electroluminescence channels in the terahertz range: narrow lines at frequencies of 1.6–2.3 THz and broader lines at frequencies of about 3.25 THz. The first channel is formed by transitions between the localized states of the Wannier-Stark ladder in the Bloch oscillation mode in the first conduction miniband at point M of the hexagonal Brillouin zone. The second channel is probably caused by transitions between Wannier-Stark ladders formed by electric-field mixing of degenerate states of the conduction band at point K of the hexagonal Brillouin zone. The conduction-band minimum at point K is higher than that at point M by 100–200 meV.

Terahertz intracenter photoluminescence of silicon with lithium at interband excitation

Terahertz radiation has been revealed at interband photoexcitation of lithium-doped silicon crystals at liquid helium temperatures. It has been shown that the lines caused by optical transitions of electrons from the 2P excited states of lithium centers to the 1S(A1) state of the impurity prevail in the radiation spectrum. The strong suppression of terahertz radiation associated with transitions to the lowest state of the donor 1S(E + T2) as compared to radiation associated with transitions to the 1S(A1) state is explained by the reabsorption of radiation. The radiation spectrum also includes weaker terahertz lines, which can be attributed to the intracenter transitions in donors that are caused by Li-O complexes. The radiation spectrum also exhibits lines at ∼12.7 and ∼15.3 meV, which are possibly due to intraexcitonic radiative transitions and transitions from continuum to the ground state of excitons.