M.-H. Meynadier

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.

Molecular‐beam epitaxial growth and characterization of pseudomorphic InAs/In0.52Al0.48 As quantum wells

The evolution of the structural and optical quality of molecular‐beam epitaxial (MBE) grown InAs/In0.52Al0.48As pseudomorphic quantum wells with increasing well thickness has been analyzed by transmission electron microscopy (TEM) and photoluminescence measurements. TEM was used to assess the commensurability of the samples as well as to confirm the build up of stress as the InAs layer is made thicker. Under our growth conditions, intense intrinsic photoluminescence is observed for InAs layers as thick as 30 A. Beyond this thickness a deterioration of the photoluminescence characteristics occurs due to roughening of the interfaces rather than to relaxation of the InAs lattice.

High gain InP/InGaAs heterojunction bipolar transistors grown by OMCVD

Abstract Atmospheric pressure organometallic chemical vapor deposition has been used to grow InP/GaInAs heterojunction bipolar transistors from trimethylgallium, trimethylindium, arsine and phosphine sources. The doping characteristics of the layers comprising the device has been studied using diethylzinc and diethyltelluride as the dopants. It is shown that both Zn and Te, while behaving nearly ideally in GaInAs, exhibit anomalous behavior in InP. In spite of this, InP/GaInAs heterojunction bipolar transistors with gains as high as 5000 have been fabricated. The estimated minority carrier diffusion length in the base is 7.5 μm which indicates that the GaInAs is of extremely high quality.

Photoconductivity and photoreflectance studies of quantum-wells and superlattices

Abstract We have performed room-temperature in-plane photoconductivity and photoreflectance measurements on various types of GaAs/AlAs quantum-wells and superlattices. The photoconductivity spectra of samples with even a few (1 to 7) quantum-wells show sharp excitonic structures superimposed on the bulk GaAs signal, and provide a useful alternative to absorption spectra. The photoreflectance spectra are demonstrated experimentally to follow the third derivative of the photoconductivity spectra, as previously reported for bulk materials, even though the mechanism of photoreflectance must be different in the present two-dimensional systems.

ZNSE/III–V Heterostructures Grown in a Multichamber MBE System

We have grown ZnSe epitaxial layers on bulk GaAs substrates and on GaAs epitaxial layers, with both As-rich and Ga-rich surface terminations. We have also grown ZnSe on AlAs epitaxial surfaces with different As to Al ratios. In all cases, abrupt, layer-by-layer growth is observed on the As-rich surfaces, while 3-dimensional nucleation is observed on the group III-rich surfaces. GaAs was also grown on ZnSe layers. In this case, microtwins form at the interface whose density diminishes as the layer is made thicker. A growth model is proposed consistent with these results which requires over-all electronic balance at the interface.

Optical properties of thin layer AlAs/GaAs superlattices

Abstract We have used photoreflectance and low temperature photoluminescence to study the optical properties of short period AlAs/GaAs superlattices with periods ranging from 18 to 60 A and varying AlAs to GaAs layer thickness ratios. For superlattices with AlAs to GaAs thickness ratios significantly less than one, the lowest energy transitions are direct in character, that is associated with the Γ state in the GaAs well; while for those with AlAs to GaAs ratios of approximately one the lowest energy transitions are indirect in character. These results are interpreted in the light of recent tight binding calculations which predict that for these indirect superlattices the lowest state for electrons is associated with the X-state in the AlAs barrier, while holes remain predominantly localized in the GaAs layers.

Multiple Chamber Molecular Beam Epitaxy Growth System: Growth Of GaAs/ZnSe Heterostructures

A multiple chamber molecular beam epitaxy (MBE) system has been used to investigate a novel material system: GaAs/ZnSe heterostructures. Growth of ZnSe on GaAs shows that two dimensional nucleation of ZnSe occurs only on As rich GaAs surfaces while island growth occurs on the Ga rich surfaces. Studies of the inverted interface, GaAs on ZnSe, reveal a special disorder and roughening at the interface. These results are explained as manisfestations of the electronic imbalance which exists at the ZnSe/GaAs interface. Also, improved ZnSe crystalline quality is achieved by the incorporation of thin epitaxial layers of AlAs or InGaAs between the GaAs substrate and the ZnSe layer. Finally, an assessment of the interface quality resulting from the transfer between growth chambers confirms that extremely high quality interfaces can be obtained by this multiple chamber process.

Type II indirect gap superlattices under intense optical excitation

Abstract We have made cw and time-resolved photoluminescence measurements at high excitation intensity on type II GaAs-AlAs superlattices at low temperature. A new band appears between the Γ and X exciton lines at a threshold power which is dependent on the sample, but is ordorder 10 12 electron-hole pairs per cm 2 per layer for cw pumping, and 10 13 for pulsed excitation at 50 pps. Below threshold only the exciton lines are visible, somewhat broadened, in the luminescence spectrum, but just above threshold the new band completely dominates the spectrum. Lineshape analysis suggests that the new band is due to the didirect recombination of an electron-hole plasma. The fitted plasma density is found to be approximately constant over a wide range of excitation, and varies little during decay after pulsed excitation, while the integrated emission intensity changes by more than an order of magnitude. The renormalized band gap increases with excitation and decreases with time after pulsed excitation, an effect which may be connected with the internal field generated by charge separation in the type II superlattice.

Regrowth on molecular‐beam epitaxial layers by transferring in ultrahigh vacuum between growth chambers: An assessment of the interface quality

The effect of transferring in ultrahigh vacuum conditions between molecular‐beam epitaxy chambers on the quality of regrown interfaces is addressed. In particular, the Al0.35Ga0.65As/GaAs heterointerface is probed by low‐temperature photoluminescence using a single quantum well (SQW) configuration, where the growth of the second interface has been interrupted and resumed after taking the sample back from the UHV transferring modules. Comparison of the emission characteristics of these SQW structures to others grown uninterruptedly indicates the feasibility of the UHV multichamber configuration for the realization of high‐quality heterostructures involving incompatible materials or different processing steps.