)A.F.Ioffe Physical-Technical Institute, Politekhnicheskaya st. 26, 194021 St. Petersburg, Russia.
)The problem of silicon optoelectronics, important for semiconductor physics and engineering, was unresolved in the last thirty years of the twentieth century. The quantum efficiency of silicon luminescence at room temperature is restricted by weak radiative recombination. Electroluminescence of silicon at the temperature a little bit higher than room temperature was investigated in our experiments to solve this problem.
A recombination radiation line of electron-hole plasma is observed in electroluminescence spectra of tunneling silicon MOS diodes under tunneling injection of nonequilibrium carriers at the temperature T ~(300 ... 350) K [1,2]. A spectral position of the luminescence line indicates, that a weak overheating of the diode by the diode current results in an anomalously strong reduction of the semiconductor energy gap inside the plasma. A very high quantum efficiency of the luminescence (10-3 ... 10-2) and unusual spectral features of the luminescence line are explained by appearance of dense states of an electron-hole plasma. Condensation of electron-hole plasma into a dense plasma flex or into dense plasma flexes occurs in a strong electrical field in silicon doped by boron ?1,2?. This is accompanied by appearance of a negative differential resistance of the diodes. The negative differential resistance may result from occupation of the valence band GAMMA7 by hot high mobility holes, observed in electroluminescence spectra of the diodes in a strong electrical field, and from an increase of the number of injected carriers. Dense surface plasma is created in silicon doped by phosphorus [1,2]. In this case the negative differential resistance of the diodes is not observed, and the electron-hole plasma exists in the form of a surface plasma drop or drops. A remarkable threshold optical hysteresis, observed in the luminescence intensity with changing diode current, represents a strong evidence of the plasma condensation [2].
A decrease of the semiconductor energy gap with increasing temperature, a negative heat capacity of the electron-hole plasma and weak diffusion of phonons at high temperatures cause the plasma condensation [2]. Generation of phonons by the plasma in an electrical field and under recombination of electrons and holes results in a strong local overheating of the lattice inside the plasma. This is accompanied by a decrease of the semiconductor energy gap and a decrease of the average energy of electron-hole pairs inside the plasma. The local lattice temperature increases with increasing plasma density. This indicates an attraction between carriers changed by the Fermi repulsion at very high plasma densities, and the surprising conclusion follows: the average energy of electron-hole pairs reaches a minimum in its dependence on the plasma density at the density no > 1018 cm-3. Hence, at high temperatures the electron-hole plasma in silicon creates an attractive field producing the self-compression of the plasma.
If the metal gate is fabricated as a small metal spot, condensation of injected electron-hole plasma into a dense surface plasma drop can be used in an effective light emitter, represented by a transistor. The condensed plasma emitter ( CP emitter ) can operate as an element in integrated optoelectronic devices. Observation of a new phenomenon - the self-compression of injected electron-hole plasma in silicon gives an opportunity for realization of silicon optoelectronics.
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