It has been shown recently [1] that under uniaxial compression up to P = 4 kbar in [110] and [1-10] directions electroluminescence spectra of strained p-AlxGa1-xAs/GaAs1-yPy/n-AlxGa1-xAs double heterostructures, usually used in TM emitting 808 nm high-power diode lasers, demonstrate a blue shift, while the electroluminescence intensity shows 2-3 times increase. If the emitted light wavelength is definitely determined by increase of the energy gap in GaAs0.84P0.16 quantum well, the noticeable increase of luminescence intensity under compression remains uncertain and can not be explained neither by decrease of non-radiative recombination under compression nor by arising piezoelectric field or a potential barrier lowering. In the present research, we have extended the pressure interval of the electroluminescence study up to P = 5 kbar (Fig.1). Besides, we search the explanation of the intensity increase under compression in the shifts of heavy hole (HH) and light hole (LH) eigen energies in the biaxially strained GaAs0.84P0.16 quantum well that could result in valence band states mixing and change of transition probabilities.figureFigure 1. Electroluminescence spectra measured at 77 K and forward current 5.5 mA. Insert shows pressure dependence of photon energy shift.figureFigure 2. Calculated TM-mode optical gain spectra. Insert shows energy shifts of light (LH1) and heavy (HH1) holes ground states under pressure.The valence band and conduction band quantum size levels in the investigated GaAs0.84P0.16 quantum well were numerically calculated for different values of the external uniaxial compression along [110] direction. The Luttinger-Kohn Hamiltonian with strain terms was self-consistently solved together with Poisson’s equation for the electrostatic potential using the finite-difference k×p method in the framework of the model developed in [2]. The necessary parameters were taken from literature [3]. According to the calculations, in the strained GaAs0.84P0.16 quantum well under investigation at P = 0 the LH1 level is the ground state instead of the HH1 level in contrast to lattice- matched A3B5 structures. Under uniaxial compression, LH1 and HH1 levels move toward each other; and after P ~ 4 kbar the HH1 becomes again the hole ground state in the GaAs0.84P0.16 quantum well (Fig.2, insert). The crossover of LH1 and HH1 levels naturally explains the nonlinear character of the optical energy gap shift under an applied uniaxial stress (Fig.1, insert). Further, around the crossover point at P ~ 4 kbar, a strong LH1-HH1 state mixing is detected. The calculated optical gain shows a well pronounced growth under the pressure up to P = 5 kbar in a rather wide energy range (Fig.2). It is consistent with increased oscillator strength and joint density of states due to LH1 and HH1 merger in the crossover vicinity, and may explain the electroluminescence enhancement under uniaxial compression.