Abstract
Results are presented of high pressure studies of some optical, electrical and band-structure characteristics of the quaternary semiconductor alloy In[1-x] Ga[x] Asy P[1-y]. The first pressure measurements on lasers with a 1.3 um (In Ga) (As P) active layer have been made in order to determine the cause of the extreme temperature sensitivity of the threshold current in these devices. The threshold current in 20 um stripe (In Ga) (As P) lasers, operating at 1.3 um wavelength, decreases with increasing pressure, whereas the opposite effect occurs in (Ga Al) As lasers. At high temperatures the threshold of quaternary lasers shows a more marked decrease with pressure than at room temperature. The temperature sensitivity parameter To increases from about 65 K at atmospheric pressure to over 100 K at about 7 kbars. In order to interpret these results a high pressure photoconductivity technique was used in determining the pressure coefficient of the direct energy band-gap, dE[o]/dP, across the alloy composition. The samples were grown on InP at 659 C. The resulting epitaxial layers were 5-10 pm thickness and had mid 10 16 cm[-3] carrier concentration The first measured values of dEo/dP for the quaternary alloy was found to vary from 8.5 +/- 0.5 meV kbar[-1], at y = 0, to 12.5 +/- 0.5 meV kbar[-1] near y = 1 alloy composition. Good theoretical agreement was obtained with these rather surprising results. From the calculated pressure dependence of the hand structure, in addition to the values, the variation of the effective masses of electrons and holes were also calculated. Hence the presence of alloy scattering rather than space charge scattering was determined from the pressure dependence of the mobility. Analysis of the first pressure measurements of the quaternary lasers, indicates that the results can best be interpreted in terms of an intervalence band absorption loss mechanism that decreases with increasing pressure. Since To increases with increasing pressure it may reasonably be assumed that the mechanism responsible for the decrease in threshold current with pressure is the loss mechanism causing the extreme temperature sensitivity of the lasers.