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.

Unconfined transition doublet peaks have been observed in GaAsAlxGa1−xAs superlattices at 5 K by photoluminescence excitation spectroscopy and by resonance Raman spectroscopy. The Raman scattering experiments were carried out with longitudinal optical phonons of the barrier layers. The doublets arise from the energy subband dispersion along the superlattice growth direction.

We present (I) low temperature excitation-wavelength-dependent photoluminescence studies in GaAs/AlxGai-xAs quantum well structures, which reveal the well thickness variations along the MBE growth direction, and (II) the photoluminescence excitation spectroscopy work carried out in the region of unconfined transitions with a series of GaAs/AlxGal-xAs superlattices which have a fixed well size and aluminum concentration in the barrier. We have found that the changes in the barrier widths of the superlattice samples can drastically affect the strengths and energies of the unconfined transitions.

Doublets from optical transitions with energies greater than the barrier energy gap were observed by photoluminescence excitation spectroscopy in a series of GaAs/AlxGa1−xAs superlattices. The well width and the aluminum concentration in the barrier regions were fixed at 150 Å and 22%, respectively. The barrier widths ranged from 70 to 180 Å. The doublets arise from transitions between the first unconfined heavy-hole and conduction subbands. The separation of the doublet is found to be a sensitive function of the barrier width. It corresponds to the energy difference between the transition at the Brillouin zone center and at the Brillouin zone boundary along the superlattice growth direction. Good agreement was found between the experimental data and theoretical calculations. The sensitivity of the energy splitting to the width of the barrier provides a useful analytical tool in determining this quantity, in much the same way that the confined transition energies relate to the well width.

We have employed nondestructive (optical and x-ray) as well as destructive (transmission electron microscopy) methods in characterizing molecular-beam-epitaxial GaAs/AI x Ga1− x As quantum well structures and superlattices. With photoluminescence excitation (PLE) spectroscopy, we measured optical transitions between the valence and conduction energy levels, which depend on sample parameters. With x-ray double-crystal diffraction measurements, symmetric (400), (200) rocking curves were obtained. The diffraction peak positions and their intensities also depend on the sample parameters. The MBE samples were then destroyed for transmission electron microscopy (TEM).

Low temperature photoluminescence spectra measured from MBE GaAs/AℓxGa1-xAs superlattices with different excitation photon energies were analyzed to reveal superxlattice well thickness variations along the MBE growth direction. Due to their unique electrical and optical properties, semiconducting multi-quantum-well (MQW) structures are receivIng much attention fo; their applications in microstructure 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 i Tany 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.

We present various optical characterization methods of GaAs/AlGaAs multi-quantum-well structures employing tunable pulsed and cw lasers as well as incoherent light sources. The samples we examined were molecular-beam eptiaxy (MBE) grown, with layer thicknesses ranging from 20 to 250 Å. The inhomogeneity along the MBE-growth direction in terms of the impurity species and the well layer thicknesses can be revealed by utilizing the wavelength-dependent beam penetration depth changes into the sample. In one of the samples, the GaAs well layer thickness near the sample surface was found to be two monolayers (5.65 Å) smaller than those inside the sample toward the substrate material. We also discuss the potential usefulness of the high-lying energy states above the AlGaAs barrier energy gap in characterizing superlattice samples.

Optical transitions between confined electron and hole states in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As superlattices have been studied extensively. Transitions involving unconfined states, i.e., states above the conduction- or below the valence-band barriers, have not received much attention. Here, we report the results of photoluminescence-excitation spectroscopy in the energy range of the unconfined transitions. We find a variety of structures in the spectra, which can be interpreted with the aid of theoretically generated absorption spectra. The strength of the transition involving the first unconfined heavy-hole to first unconfined conduction state is found to depend strongly on the barrier width. For superlattices with 150-A\r{} wells and barriers with aluminum concentrations of 20%, the transition is very strong for 150-A\r{} barriers, but insignificant for barrier widths of 70 and 30 A\r{}. Transitions involving the split-off valence bands are also observed.

Current output patterns of a solar cell panel exposed to a scanning light spot are computed for fault-free and for faulty cell containing either cracks with leakage conductances across the exposed junction at the crack, or point shorts in series with various spreading resistances. Low light level, uniform attenuation length, and an external panel termination of low impedance are assumed. It is shown that various boundary conditions can be satisfied by appropriate imaging techniques. A general equivalency theorem for the cell output of an illuminated point with that of an illuminated line through that point parallel to the finger electrode is derived and utilized. The preferred attenuation length of about half the finger electrode spacing can be achieved by modulating the light beam at an appropriate frequency which is typically in the low MHz range. Output patterns generated by a modulated light beam are compared with those obtained by unmodulated light.

The steady-state current through the double dielectric of metal-nitride-oxide-silicon capacitors on p-silicon substrate was measured as a function of temperature for several fixed fields in the silicon oxide of a polarity-promoting hole flow from the silicon. It is shown that the availability of empty recipient trap states for holes tunneling from the silicon into the nitride controls the current. The occupancy of these trap states is governed by Frenkel-Poole detrapping at elevated temperatures and by Fowler-Nordheim tunnel emission from the traps at low temperature. Transient charging measurements support this interpretation.

The principal type of test for which the circuit is designed is the stair-case charging method which charges an MNOS device under constant oxide field. Other tests such as memory retention and endurance are also described.

The charge versus centroid relationship is determined by staircase charging, in which a sequence of identical pulses is applied, the memory device is returned to flat-band condition after each pulse, and the subsequent pulse is superimposed on the flat-band voltage corresponding to the accumulated memory charge distribution resulting from the preceding pulses. Staircase patterns of accumulated negative charge and of device voltage are analyzed, and effects arising from back-tunneling and leakage currents are identified. Comparison of the initial injection current during a voltage pulse with the steady-state current indicates that hole injection from the gate does not contribute significantly to the steady-state oxide tunnel current.

A simple analytical expression is derived for charge retention in MNOS memory devices assuming that retention loss is limited by Frenkel-Poole release from monoenergetic traps. This model shows that charge retention becomes eventually independent of the initial charge distribution. Experimental data obtained at elevated temperatures confirm this model and provide a trap depth of 1.5 eV, Frenkel-Poole coefficient of about 6×10−4 cm1/2 V−1/2 eV, and effective escape attempt rate factor of 1.2×108 sec−1.

The efficiency degradation of solar cells due to sheet resistance-generated losses is computed as a function of light intensity and electrode spacing. The degradation derived by Heizer and Chu is shown to be 33 per cent too large at small electrode spacing. We give an approximate analytic relation for the efficiency degradation and derive from it the electrode spacing which minimizes the combined losses due to sheet resistance and electrode shading. An electrode configuration which prevents extreme power loss by local short circuits in the photojunction is described.

A concise theoretical expression based on pronounced detrapping is derived for the charge-vs-centroid relationship of an MNOS structure subjected to constant current pulses which include the considerable injected charge levels encountered in practical memory device operation. Good agreement is found with available experimental charge-vs-centroid data obtained at various temperatures, thus enabling determination of Frenkel-Poole coefficient and trap depth. Values so obtained agree with those for bulk silicon nitride. An analytical expression valid for any degree of detrapping is derived for change of centroid with charge in the limit of small injected charge levels. Implications of constant voltage versus constant current charging are discussed.