R. E. Nahory

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.

Room‐temperature blue lasing action in (Zn,Cd)Se/ZnSe optically pumped multiple quantum well structures on lattice‐matched (Ga,In)As substrates

We report on studies of optically pumped laser action in (Zn,Cd)Se/ZnSe multiple quantum well structures prepared by molecular beam epitaxy on lattice‐matched bulk (Ga,In)As substrates. Room‐temperature lasing under pulsed excitation with threshold pump intensity at I≊500 kW/cm2 has been achieved, together with high repetition ‘‘quasi‐continuous’’ mode operation at temperatures so far up to 100 K.

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.

Interface Control in GaAs/GalnP Superlattices Grown by OMCVD

Abstract In this paper, we show that the deleterious effect of the reaction of arsine with the underlying GaInP, when growing GaAs on GaInP, can be overcome by the growth of a thin (≈0.8 nm) GaP interfacial layer. In addition, we show the usefulness of X-ray rocking curve simulations in aiding the understanding and thereby control of heterointerfaces. Variable low temperature photoluminescence measurements were used to confirm the existence of a low bandgap interfacial layer at the GaInP to GaAs interface, predicted from X-ray simulations, by comparing the energy separation between heavy- and light-hole emissions in samples with and without such interfacial layers. The X-ray simulations and photoluminescence measurements also indicated an In contamination of the GaAs quantum well. The source of the In contamination was shown to be the susceptor, which is coated with In based compounds. Both In and As contamination can be a problem in reactors where the carrier gas flows over the susceptor before arriving at the substrate.

Evolution of the band gap and the dominant radiative recombination center versus the composition for ZnSe1−xTex alloys grown by molecular beam epitaxy

We report a systematic study of the optoelectronic properties of ZnSe1−xTex alloys grown by molecular beam epitaxy over the entire range of compositions. The band‐gap energy as a function of the composition presents a minimum at x≂0.65. The main luminescence emission observed at 5 K becomes narrower and closer to the band‐gap energy as we increase the Te content. The linewidth and the difference between the emission peak and band‐gap energy decrease significantly with increasing x and present a break in the slope at x≂0.65.

Optical investigation of atomic steps in ultrathin InGaAs/InP quantum wells grown by vapor levitation epitaxy

Ultrathin InGaAs/InP single quantum well structures, grown by chloride transport vapor levitation epitaxy, have been investigated by low‐temperature photoluminescence (PL). Well‐resolved multiple peaks are observed in the PL spectra, instead of an expected single peak. We attribute this to monolayer (a0/2=2.93 A) variations in quantum well (QW) thickness. Separate peak positions for QW thicknesses corresponding to 2–6 monolayers have been determined, providing an unambiguous thickness calibration for spectral shifts due to quantum confinement. The PL peak corresponding to two monolayers occurs at 1.314 eV, corresponding to an energy shift of 524 meV. Experimental data agree very well with a simple effective mass theory.

Arsenic doped ZnSe grown by molecular‐beam epitaxy

We have been able to incorporate As into molecular‐beam epitaxy (MBE) grown ZnSe in the range of 1017–1021 atoms/cm3 using Zn3As2 as the As source. This contrasts with very low As levels we obtained using an As cracker cell. The As incorporation is highly nonlinear with Zn3As2 flux and depends on the excess Se used. Several samples doped with Zn3As2 show low temperature photoluminescence with near band edge emission dominated by shallow acceptor levels. We will describe the details of several growth variations studied and their influence on As incorporation.

Optical transitions and chemistry at the In0.52Al0.48As/InP interface

We report properties of the InAlAs/InP interface and its formation during growth by organometallic molecular beam epitaxy. Taking advantage of the photoluminescence emission occurring at this type II interface, we were able to directly investigate the interface characteristics for different growth conditions. A shift is observed in the energy of the interface recombination transition which we interpret as evidence of a P‐As exchange effect dependent on the specific growth sequence. This effect was further investigated by growing interfaces with thin layers (InAs, AlAs, AlP) between the InP and InAlAs. The results can be understood in terms of a model based on bond strength considerations. We predicted and demonstrated that the most stable interface is obtained with incorporation of a thin AlP interfacial layer.

Large lateral photovoltaic effect in modulation‐doped AlGaAs/GaAs heterostructures

We describe a large lateral photovoltage that develops in AlGaAs/GaAs modulation‐doped structures in response to excitation by spot illumination. The effect is observed in wafers where the equilibrium inversion channel electron density is negligibly small. This effect is sensitive to the position of the illuminated spot. The dependence of the induced photovoltage on the power and the spectral composition of the excitation as well as its temperature dependence has been carefully studied. A resistive two‐channel transmission line model has been developed to explain the experimental results.

Raman scattering from electronic excitations in periodically δ-doped GaAs

Abstract We report photoluminescence and Raman scattering measurements of periodically δ-doped Si : GaAs. The spectra of short-period structures are similar to those of uniformly doped material, but new lines appear in the Raman spectra of longer-period structures that arise from inter-subband transitions between confined electron levels in a single δ-layer or between minibands in the δ-doping superlattice.