Disorder Effects in Aluminum-Gallium - Arsenide Quantum-Well Heterostructures.

The effects of disorder on the luminescence and laser properties of Al(,x)Ga(,1-x)As-GaAs (or AlAs-GaAs) quantum-well heterostructures (QWH's) grown by metalorganic chemical vapor deposition (MO-CVD) are investigated. Various forms of disorder in QWH's are studied and compared by employing photoluminescence techniques. It is possible to disorder a QWH by thermal annealing at temperatures (800-1000(DEGREES)C) significantly higher than the crystal growth temperature. A proper choice of annealing time and temperature results in the interdiffusion of the thin GaAs and AlAs (or Al(,x)Ga(,1-x)As) layers in the QWH active region and forms a compositionally disordered Al(,x)Ga(,1-x)As active layer. This resulting double heterostructure may be either direct or indirect band gap depending on the specific sizes and compositions of the "as-grown" layers. Additional information concerning phonon effects in QWH's may be gained by annealing a QWH with an active region of one large GaAs layer coupled to an array of smaller layers. Recombination from the "as-grown" QWH is phonon-assisted, however, after the smaller layers are damaged by thermally-induced interdiffusion, emission is no longer phonon-assisted but characteristic of a single layer of GaAs. Similar effects are seen in Zn-diffused QWH's. It is shown that even at relatively low temperatures (500 -600(DEGREES)C), well below the crystal growth temperature, conventional Zn diffusion greatly enhances the Al-Ga interdiffusion process. Thus, AlAs (or Al(,x)Ga(,1-x)As) and GaAs layers may be converted to single-crystal homogeneous Al(,x)Ga(,1 -x)As that is now doped p-type. Since the Zn diffusions may be masked by a thin layer of Si(,3)N(,4), it is possible to disorder only selected portions of a QWH, leaving the remaining areas in their "as-grown" form. An example of a red superlattice laser integrated in a yellow indirect -band-gap Al(,x)Ga(,1-x)As cavity is shown, establishing a basis for monolithically integrating QWH lasers (and other devices) on an Al(,x)Ga(,1-x)As optoelectronic "chip". Another form of disorder is that of clustering in a ternary alloy such as Al(,x)Ga(,1-x)As. Although alloy clustering may be affected by many variables, it must be regarded at least to some extent as intrinsic to a ternary alloy. Data are presented that show QWH luminescence to be sensitive to alloy clustering when Al(,x)Ga(,1-x)As barrier sizes approach the maximum cluster size. It is shown that alloy clustering and its associated problems (spectral broadening and lower-energy laser emission) are avoided by substituting AlAs for Al(,x)Ga(,1-x)As in the QWH active region. This allows fluctuations in layer thicknesses to be as small as (TURN)5 (ANGSTROM) and in addition results in high-energy (visible) laser operation (TURN)400 meV above E(,g)(GaAs) at room temperature.