Luminescent properties of AlGaAs grown by transient-mode liquid epitaxy
Transient-mode liquid epitaxy (TMLE), a method of non-equilibrium LPE growth, has been used to prepare thin n-type single-crystal layers of AlGaAs on GaAs substrates for the fabrication of infrared-emitting LED’s. It is well known that the concentration of Al in an epitaxial layer grown from the liquid phase is primarily dependent on solution composition, growth temperature and growth rate. During conventional LPE, the growth rate is usually made low enough that the equilibrium distribution coefficient can be assumed to prevail and the Al concentration for direct bandgap compositions will decrease as a layer grows. This, however, is not the case for material produced by TMLE. Since the growth rate during TMLE continually drops from an initially high value towards zero, the amount of Al incorporated in the epitaxial layer, and hence the bandgap energy, is not reduced as the layer is generated. The luminescence created at a diffused p-n junction is emitted through the p-side with very little absorption. Photoluminescence measurements using an argon-ion laser source (4579 Å) were made to determine the bandgap energies of AlGaAs epitaxial layers grown by TMLE for various lengths of time. Diodes fabricated from from this material had peak electroluminescence between 7900 and 8250 Å With half-intensity bandwidths of 200 A. In addition to a discussion of diode preparation, the spectral characteristics of diodes are compared with results obtained from photoluminescence measurements.