On exciton absorption, band structure, and phase transformation of GaSe under pressure

Abstract

AbstractThe spectra of the absorption coefficient (α) of GaSe at the photon energy range 1.94 to 2.64eV and at pressures between 1 bar and 12 kbar are investigated at 295°K. Data at 1 to 4360 bar confirm that the edge exciton of GaSe is the direct allowed one. The pressure coefficient of the band gap (dEg/dP = −6.2 × 10−6eV bar−1), one of the Rydberg exciton constant (dR/dP = −0.6 × 10−6eV bar−1), and one of the exciton effective mass [(1/μ) (dμ/dP) = −2.4 × 10−5 bar−1] are approximately determined. It is supposed that two interband exciton transitions at 2.001 and 3.25eV give the contribution into the α spectrum above 2.1eV. The assignment of these transitions is discussed in connection with the band structure calculation of Schlüter. At 5280 bar a discontinuous jump of the absorption edge shape is observed for GaSe, with a change of the sign and a value for dEg/dP of +2.0 × 10−6eV bar−1 within the range of 5280 to 11780 bar. This effect is interpreted as the transformation of one GaSe polytype into another, i.e. as a possible phase transformation close to that of second kind.