Spontaneous emission of holes excited by an electric field in germanium

Abstract

In a number of studies (see, e.g., (1) and references therein), the stimulated emission excited by an electric field in uniaxially compressed Gadoped Ge crystals was reported. It was assumed that the emission is pro� duced by holes accumulated in the Gadopant slike states shifted to the continuous spectrum by strains. The study of the energy spectrum of slightly or highly excited holes may be helpful in understanding the pro� cesses occurring in the above conditions. The hole states in Gadoped Ge were studied by absorption and photoelectric spectroscopy of crystals compressed with the pressure P along the (111) and (001) direc� tions (2, 3). pLike resonance impurity states were detected in the lighthole subband near its edge, and the energies of splitting of lightand heavyhole bands were determined as functions of P. However, these experiments provide data only on the transitions of holes from the ground slike state to the excited plike states, whereas the stimulated transitions from pto s� like states can manifest themselves only in the emis� sion of holes. To determine the energy region, wherein the spon� taneous emission of hot holes is produced, we studied the dependences of the integrated emission intensity and current in the Ge samples doped with Ga to the concentration N = 10 14 cm -3 on an electric field with the strength E as high as 3 kV cm -1 with no effect of roomtemperature background emission. We used samples 1 × 1 × 10 mm in dimensions. The In or InGaSn contacts were deposited onto the end faces of the samples. To suppress the effect of injection of elec� trons, the samples were roughly ground. The emission signal was recorded by photoresistors (PRs) fabricated of Ge or Si doped with impurities with different ion� ization energies Ei in the range from 10.5 to 45 meV (4). The PRs were placed into a common helium bath with the emitting samples. We used also cooled optical filters nontransparent for photons with high energies. It was established that, in high fields, the major contri� bution to emission was made by holes with energies up to ~90 meV. Abstract—The dependence of the integrated intensity of spontaneous emission of holes in Gadoped Ge excited by a pulsed electric field with the strength E up to 3 kV cm -1 under uniaxial compression in the (001) and (111) directions with pressure P up to 12 kbar is studied. It is established that the emission intensity in a high electric field is controlled by the processes of excitation and relaxation of light holes even at P = 0. It is found that a substantial contribution to the emission is made by hot holes with energies much higher than the optical phonon energy in Ge. The spontaneous emission spectra of hot holes in a high electric field are recorded and interpreted for the first time. At P = 0 and E > 500 V cm -1 , the energy of holes is not limited by the optical phonon energy and the emission spectrum of holes in the range 45-85 meV exhibits a structure similar to that of the absorption spectra of optical phonons. This is indicative of the accumulation of hot holes on strong interaction with crystal lattice vibrations in Ge. The features of this interaction are discussed.