n =1 heavy -hole exciton transitions. The changes observed in spectral shapes, however, are considered to arise from the difference in the penetration

Low temperature Abstract Low temperature photoluminescence spectra measured from MBE GaAs/Afc Ga., As superlatt i ces with different excitation photon energies were analyzed to reveal superlattice well thickness variations along the MBE growth direction. Due to their unique electrical and optical properties, semiconducting multi-quant urn-well (MQW) structures are receiving much attention foe their applications in microstrueture devices such as HEMT , SEED , and tunable lasers . The properties of MQW structures are greatly affected by the MQW heterointerface qualities, impurity contents, and layer thickness irregularities. So far, molecular-beam-epitaxy (MBE) prepared MQW structures are found to exhibit superior sample qualities in many respects, and a great deal of effort is being made to further advance MBE technology ' . The characterization of multi-layered structures, however, has not progressed as fast as the development of thin film growth methods. Among the various sample characterization methods, optical techniques offer certain advantages since the test results can be obtained in a relatively quick and simple manner. Besides, the nondestructive (or non-contact) nature of optical methods provides opportunities for repetitive evaluations, which improves accuracy in sample characterizations. One of the most common optical characterization methods of superlattice samples is based on6photoluminescence (PL) spectroscopy carried out at a fixed laser excitation wavelength X. In general, the PL spectrum of superlattice (SL) at low temperatures arises from exciton transitions involving n=1 electron states and n=1 heavy-hole states . The spectral position of the PL peak is related to the layer thickness of the SL, whereas the PL spectral halfwidth is considered to reflect the heter©structure interface quality. We have found that the PL spectrum from GaAs/AJl Ga, As MQW samples may depend on the PL excitation wavelength, X. Our observation points to the fact that caution is needed in characterizing SL samples based on their PL spectra taken at one fixed X. The GaAs/A& ^ai_ As superlattice samples used in this work were molecular beam exitaxy (MBE) - grown on n substrate at 600 C. Both the wej.1 arid the barrier layers were undoped. Details of the growth procedures were reported in reference 7. The sample parameters were determined from growth conditions as well as x-ray double crystal diffraction measurements accompanied by curve-fitting procedures. The photoluminescence signals were detected by a cooled GaAs photomultipiier (RCA C31034) in conjunction with a double monochromator (Spex 1403) and a computer-inter faced photon counting system. Superlattice samples were mounted on the copper cold finger of a cryostat (Airproducts Helitran LT-3-110 and a temperature controller model APD-E) using silicon grease. Backscattering geometry with the beam-incident angle of -20° was employed. The excitation beam source was an incoherent lamp and the wavelength selection was accomplished with a 1/4 m spectrometer. The power density on the sample surface was purposely kept low to avoid any saturation effects, and estimated to be 10 -10 Watt/cm 2.