M. C. Tamargo

Optical spectroscopy of ultrasmall structures etched from quantum wells

We have studied the photoluminescence and photoexcitation spectra of ultrasmall structures, of approximately 500 A in dimension, which we refer to as quantum ribbons and quantum disks. These are fabricated from GaAs‐AlGaAs quantum wells grown by molecular beam epitaxy and patterned by electron beam lithography. Contrary to our expectation, photoluminescence from these structures is extremely efficient. The excitation spectra of the two types of small structures differ greatly from each other and from that of the as‐grown quantum wells. These differences may be a result of the confinement of the carriers in these small structures.

Molecular beam epitaxy of GaAs/AlGaAs superlattice heterostructures on nonplanar substrates

GaAs/AlGaAs superlattice heterostructures with layer thicknesses ≲100 A were grown by molecular beam epitaxy on nonplanar GaAs substrates. The resulting superlattices exhibit different periods, depending on the crystal plane on which they grow. Period variation of more than 50%, from 180 to 80 A, was obtained for adjacent superlattice sections. The transition between regions of different periodicity was mostly smooth and occurred within lateral dimensions ≲100 A. Our results suggest that molecular beam epitaxy of superlattice heterostructures on patterned substrates provides a method for obtaining controllable lateral variations in physical properties which depend on the superlattice period. In particular, by growing quantum well heterostructures on nonplanar substrates, it might be possible to utilize the strong dependence of the carrier confinement energy on the well thickness in order to achieve lateral carrier confinement.

X-point excitons in AlAs/GaAs superlattices

We have found a long‐lived emission at low temperatures in AlAs/GaAs superlattices with approximately equal thicknesses of AlAs and GaAs and with periods in the range 18–60 A. The emission shows the nonexponential time decay characteristic of an indirect exciton made allowed by disorder. The exciton is found to be at the zone boundary, and to consist of a Γ hole localized in the GaAs and an AlAs X‐point electron. The disorder is at the AlAs‐GaAs interfaces. There is no ‘‘camel’s back’’ in the exciton dispersion.

Growth of AlxGa1−xAs parabolic quantum wells by real‐time feedback control of composition

Epitaxial AlxGa1−xAs structures whose compositions x vary continuously with thickness according to a given input function have been grown by chemical‐beam epitaxy under closed‐loop ellipsometric control. 200‐ and 500‐A parabolic quantum wells analyzed by photoreflectance and secondary‐ion mass spectrometry, respectively, show that actual compositions follow target values to within 0.02 in x. Growth of the 200‐A profile was controlled using compositions ellipsometrically determined for the outermost running 3.1 A (∼1 monolayer) of depositing material.Epitaxial AlxGa1−xAs structures whose compositions x vary continuously with thickness according to a given input function have been grown by chemical‐beam epitaxy under closed‐loop ellipsometric control. 200‐ and 500‐A parabolic quantum wells analyzed by photoreflectance and secondary‐ion mass spectrometry, respectively, show that actual compositions follow target values to within 0.02 in x. Growth of the 200‐A profile was controlled using compositions ellipsometrically determined for the outermost running 3.1 A (∼1 monolayer) of depositing material.

Structural characterization of GaAs/ZnSe interfaces

We have studied, using reflection high energy electron diffraction (RHEED), the initial growth stages of ZnSe layers grown by molecular beam epitaxy (MBE) on GaAs exhibiting various surface terminations. The structural quality of the ZnSe layers was assessed by transmission electron microscopy (TEM). We have observed a preference for two‐dimensional nucleation on As‐terminated GaAs substrates, whether epitaxial or bulk. On Ga‐terminated surfaces a transitional region of three‐dimensional growth forms initially. The different behaviors may reflect the electronic imbalance present in the growing GaAs/ZnSe interface. While defect free, thin layers of ZnSe are best obtained on As‐terminated GaAs, the initial three‐dimensional growth region on the Ga‐rich surfaces appears to reduce the density of extended faults formed in thick ZnSe layers due to the lattice mismatch with GaAs.

Optical properties and band structure of short-period GaAs/AlAs superlattices

Abstract We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 A and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.

Backilluminated GaN/AlGaN heterojunction ultraviolet photodetector with high internal gain

We report on a backilluminated GaN/Al0.18Ga0.82N heterojunction ultraviolet (UV) photodetector with high internal gain based on metal-semiconductor-metal structures. A narrow band pass spectral response between 365 and 343 nm was achieved. When operating in dc mode, the responsivity reaches up to the order of 102 A/W under weak UV illumination, which is due to enormous internal gain up to 103. The linear dependence of photocurrent on bias and its square root dependence on optical power are found and explained by a trapping and recombination model. The high photocurrent gain is attributed to trapping and recombination centers with an acceptor character induced by dislocations in GaN.

Raman scattering measurements of decreased barrier heights in GaAs following surface chemical passivation

We present Raman scattering data from GaAs samples whose surfaces had been treated with thin films of sodium sulfide nonahydride (Na2S⋅9H2O). Raman scattering provides a quantitative, contactless means of measuring the reduced barrier height associated with decreased density of GaAs surface states. For GaAs samples doped at levels of n≊1018 cm−3, the barrier height is reduced to 0.48±0.10 eV.

Aluminum ion‐implantation enhanced intermixing of GaAs‐AlGaAs quantum‐well structures

We have studied aluminum‐implantation enhanced intermixing of GaAs‐AlGaAs quantum‐well structures using low‐temperature photoluminescence. The energy shift of the heavy‐hole exciton was determined for Al doses varying from 2×1013 cm−2 to 1×1015 cm−2 after either furnace annealing at 800 °C or optical rapid thermal annealing at 925 °C. A variational calculation yields the diffusion length from the energy shift of the exciton. This shift is due both to the increase of Al in the center of the well and to the change in electron and heavy‐hole confinement energies. The ion‐implantation enhancement of the diffusion length depends on Al‐ion dose but not significantly on annealing time or temperature. This work indicates that Al‐ion implantation should be useful for the fabrication of structures of reduced dimensionality by patterned implantation of AlGaAs‐GaAs quantum wells.

Full-color light-emitting diodes from ZnCdMgSe/ZnCdSe quantum well structures grown on InP substrates

We have grown p-type ZnCdMgSe quaternary layers and have fabricated light-emitting diodes (LEDs) from pseudomorphic quantum well (QW) structures of ZnCdSe/ZnCdMgSe grown on InP substrates that emit throughout the visible range. Nearly identical structures, di!ering only in the ZnCdSe QW layer thickness and/or composition can produce light ranging from blue to red. Good current}voltage characteristics are obtained from the diodes using p#ZnSeTe lattice matched to InP as the p-type contact layer. These structures have potential applications as integrated full-color display elements. ( 2000 Elsevier Science B.V. All rights reserved.