J. Lilja

A comparative study of growth of ZnSe films on GaAs by conventional molecular‐beam epitaxy and migration enhanced epitaxy

Growth of high‐quality ZnSe films on GaAs(100) substrates by conventional molecular‐beam epitaxy (MBE) and migration enhanced epitaxy (MEE) have been examined. We have found a self‐regulatory MEE process in the temperature range between 350 and 400 °C where a complete monolayer of ZnSe was achieved per three operational cycles of Zn+Sen. Thin films (<1 μm) grown by self‐regulatory MEE exhibited much higher structural perfection than MBE thin ZnSe films grown under closely identical conditions. The explanation for these structural differences must lie in details of how initial nucleation takes place in MBE and MEE. For thicker films (≂2 μm) no significant differences in structural or electrical properties were found for the two methods.

Exciton complexes in ZnSe layers: a tool for probing the strain distribution

Differently acting strain components caused by lattice mismatch or deviating thermal expansion coefficients of ZnSe and any substrate material yield a typical thickness-dependent strain profile in ZnSe epitaxial layers. For the analysis of strain in those layers, the investigation of free and bound excitons is very suitable. In the present work, samples are used exhibiting surface strain magnitudes ϵ between -5×10-4 (compressive in-plane strain) and +15×10-4 (tensile in-plane strain), determined from reflection loops of the strain-split free excitons Xlh and Xhh. Whereas reflection spectroscopy scans the strain situation at the layer surface only, excitation and resonant Raman spectroscopy, in particular of donor-bound excitons (D0, X), give information about the strain distribution in the whole film. Based on the observed set of different (D0, X) transitions I2i, originating from excited single-hole states, we are able to predict the strain profile and the distribution of the main donors in the film.

Growth of ZnSe films on GaAs 〈100〉 substrates by conventional and pulsed molecular beam epitaxy

Abstract Growth of ZnSe films on GaAs by conventional and pulsed modes of MBE has been examined as a function of substrate temperature. For pulsed MBE, we find four different regions in the growth rate versus substrate temperature diagram, while for MBE, the growth rate is proportional to the beam fluxes and increases steadily as temperature decreases. In particular, a self-regulatory process is found to govern layer stacking in the pulsed mode at temperatures between 350 and 400°C.

Growth of Li-doped ZnSe by molecular beam epitaxy using an alkali metal dispenser

Abstract An alkali metal dispenser was employed as a lithium dopant source in the growth of ZnSe films by molecular beam epitaxy in an attempt to obtain p -type conductivity. Depending on the doping level, these films exhibited either dominant acceptor-bound exciton emission in low-temperature photoluminescent spectra, indicative of p -type conversion, or strong donor-acceptor-pair emission, likely due to transitions between interstitial Li donors and substitutional Li acceptors. Electrical resistivity was very high preventing a direct measurement of the majority carriers.

Measurement of the ZnSe:MnSe:ZnSe heterojunction valence‐band discontinuities

UV‐photoemission spectroscopy was used to measure directly the valence‐band discontinuity ΔEv for both sides of a MnSe layer which was sandwiched between two ZnSe layers by the molecular beam epitaxy method. ΔEv is 0.16±0.05 eV for each interface; the valence‐band edge Emaxv of the wider gap MnSe semiconductor lies within the ZnSe gap. The interface‐pinning position of the Fermi level appears at 1.74 eV above Emaxv of ZnSe. It is concluded that interfacial electrostatic dipoles are small compared to the observed shift in Emaxv of MnSe, which lends a qualitative support to Tersoff’s model [Phys. Rev. Lett. 52, 465 (1984); Phys. Rev. B 30, 4874 (1984)] of heterojunction band offsets.

Electrical, optical, and structural properties of Ga-doped ZnSe grown by molecular beam epitaxy

Abstract Undoped and Ga-doped ZnSe films have been grown on (100) GaAs substrates by molecular beam epitaxy. Measurements of photoluminescence, Hall mobilities, and X-ray diffraction for these films have been made. Uniform Ga-doping is possible up to the net carrier concentration of 5 × 10 17 cm −3 , resulting in low-resistivity films. The compensation ratio ranges from 0.8 to 0.96. If higher doping concentration is attempted an excessive Ga incorporation into the lattice generates a large number of defects. These defects are evidenced by photoluminescence as strong emission from Ga Zn -V Zn complexes and donor-acceptor-pair bands. At the same time, the carrier concentration and Hall mobility drastically decrease from their values for less compensated ZnSe. The lattice mismatch for ZnSe of less than 4 μn in thickness varies from 1100 to 1300 ppm, indicating that the lattice is not fully relaxed. This lattice mismatch is independent of Ga doping. The (100) Bragg planes of ZnSe are slightly tilted, about 100–200 arc sec, with respect to those of GaAs.

Optical study of ZnSe and (Zn,Mn)Se MBE layers on GaAs

Abstract MBE grown (100) ZnSe and (Zn,Mn)Se epilayers on (100) GaAs substrates with thicknesse between 0.2 and 4 μm are studied by photoluminescence and reflection spectroscopy. The observed splitting of the exciton reflection peak caused by the lattice misfit is found to decrease with increasing layer thickness. The energy gap as a function of the Mn concentration (0 ≤ x Mn ≤ 0.36) is found to vary non-monotonously due to exchange interaction. Besides the usual yellow Mn 2+ luminescence additional lower-energy bands are observed for x Mn > 0.1 which can be ascribed to Mn 2+ ions in modified environments. Band-gap related luminescence from the GaAs substrate is measured through the epilayers, indicating a Mn influence on the heterointerface.

Measurement Of Band Discontinuities For The Double Heterojunction ZnSe:MnSe:ZnSe

UV-photoemission spectroscopy was used to measure directly the valence-band discontinuity, ΔEv, for both sides of a MnSe layer which was sandwiched between two ZnSe layers by the Molecular Beam Epitaxy method. ΔEv is 0.16±0.05 eV for each interface; the valence band edge EVmax of the wider-gap MnSe semiconductor lies within the ZnSe gap. The interface-pinning position of the Fermi level appears at 1.74 eV above Evmax of ZnSe. It is concluded that interfacial electrostatic dipoles are small compared to the observed shift in Evmax of MnSe, which lends a qualitative support to Tersoffs model [Phys. Rev. Lett 52, 465 11984); Phys. Rev. B 30, 4874 (1984)] of heterojunction band offsets. Photoemission from MnSe shows that the Mn-derived 3d states, which are responsible for the semiconductor magneto-optical properties, lay 4.2 ± 0.1 eV below EVax.

Electrical characterization of Li-doped ZnSe grown by molecular beam epitaxy

Abstract Effects of Li doping on the electrical properties of ZnSe grown by the molecular beam epitaxy have been studied. The net carrier density in ZnSe: Li was observed to depend strongly and nonlinearly on doping density. At small concentration, Li occupies interstitial sites acting as a donor. At somewhat higher doping level Li starts to occupy substitutional acceptor sites and results in high-resistivity layers. As doping concentration is further increased, Li again adopts preferentially the interstitial sites.

Electrical and optical properties of As- and Li-doped ZnSe films

Luminescense and photoconductivity measurements were performed on MBE grown ZnSe layers with various arsenic concentrations. Two shallow acceptor levels with energies of 125 meV and 260 meV were found. Increasing the As content in order to increase the number of shallow acceptor states resulted in highly compensated samples. For Li the acceptor binding energy was found to be 113 meV. Also in the case of Li a higher doping concentration did not augment the shallow levels. Electrical characterization of the Li doped samples was done by C-V and I-V measurements. The films were found to be p-type.