G. Leo

MOVPE growth of wide band-gap II-VI compounds for near-UV and deep-blue light emitting devices

We report on the growth by metalorganic vapour phase epitaxy of high structural and optical quality ZnS, ZnSe and ZnS/ZnSe multiple quantum well (MQW) based heterostructures for applications to laser diodes operating in the 400 nm spectral region. High purity t BuSH, t Bu 2 Se and the adduct Me 2 Zn : Et 3 N were used as precursors of S, Se and Zn, respectively. The effect of the different MOVPE growth parameters on the growth rates and structural properties of the epilayers is reported, showing that the crystallinity of both ZnS and ZnSe is limited by the kinetics of the incorporation of Zn, S and Se species at the growing surface. Very good structural and optical quality ZnS and ZnSe epilayers are obtained under optimized growth conditions, for which also dominant (excitonic) band-edge emissions are reported. The excellent ZnS and ZnSe obtained by our MOVPE growth matches the stringent requirements needed to achieve high quality ZnS/ZnSe MQWs. Their structural properties under optimized MOVPE conditions are shown to be limited mostly by the formation of microtwins, a result of the intrinsic high lattice mismatch involved into the ZnS/ZnSe heterostructure. Despite the large amount of defects found, the optical quality of the MWQs turned out to be high, which made possible the full characterization of their electronic and lasing properties. In particular, photopumped lasing emission up to 50 K in the 3.0 eV energy region are reported for the present MQWs heterostructures under power excitation density above 100 kW/cm 2 .

Inhomogeneous Strain Relaxation and Defect Distribution of Znte Layers Deposited on (100)Gaas by Metalorganic Vapor-Phase Epitaxy

The structural characterization of ZnTe epilayers grown on (100)GaAs by metalorganic vapor‐phase epitaxy is reported. A detailed study of the ZnTe/GaAs heterostructure based on both high‐resolution and conventional electron microscopy and ion channeling Rutherford backscattering spectrometry allows correlation of the type and spatial distribution of the extended defects occurring at or close to the ZnTe/GaAs interface with the amount of residual lattice strain into the ZnTe epilayers. Both pure edge Lomer and 60°‐mixed misfit dislocations were identified at the interface along with partial dislocations bounding stacking faults, their overall density and distance distribution indicating the occurrence of a residual compressive strain at the heterostructure interface. By comparing this interface strain to the corresponding surface value of the same samples the occurrence of an inhomogeneous strain relaxation along the growth direction is clearly demonstrated. It is shown that such a strain gradient should b...

Determination of surface lattice strain in ZnTe epilayers on (100)GaAs by ion channeling and reflectance spectroscopy

We report on the direct measurements of surface lattice strain in ZnTe epitaxial layers on {100}GaAs substrates by ion channeling Rutherford backscattering spectrometry and low‐temperature (10 K) reflectance spectroscopy measurements. The measured ZnTe strain is the superposition of the expected thermal (tensile) strain and a thickness‐dependent residual compressive strain. Our data indicate that the removal of this residual strain is slower than the rate predicted by the equilibrium theory, following an apparent h−1/2 power‐law dependence on the epilayer thickness h, above ∼100 nm.

Influence of a ZnTe buffer layer on the structural quality of CdTe epilayers grown on (100)GaAs by metalorganic vapor phase epitaxy

The influence of a ZnTe buffer layer on the structural quality of CdTe epilayers grown on (100)GaAs substrates by metalorganic vapor phase epitaxy has been investigated by both x‐ray diffraction and ion channeling Rutherford backscattering spectrometry measurements. Single‐crystal (100) oriented CdTe epilayers of good structural quality have been obtained after inserting a ZnTe buffer layer of a thickness ranging between 300 and 500 nm. The influence of the buffer layer thickness on the crystalline quality and the morphology of the CdTe epilayer has been related to the defect distribution and the surface roughness of the ZnTe buffer layer. The crystalline quality and the surface strain have been thus studied as a function of the CdTe thickness on samples having optimal ZnTe layer thickness. The initial compressive mismatch between CdTe and ZnTe, f=−5.8%, appears to be almost fully relaxed for a CdTe thickness around 200 nm. A residual compressive in‐plane strain (about −0.02%), independent of the CdTe epi...

Investigation of growth mode behavior and surface morphology evolution of metalorganic vapor phase epitaxy grown ZnTe layers on (001) GaAs

A systematic investigation on the mechanisms of nucleation and surface morphology evolution was performed on ZnTe epilayers, deposited on chemically etched GaAs(001) by metalorganic vapor phase epitaxy. A 2D–3D growth mode transition was observed at around two ZnTe equivalent monolyers (ML), which was ascribed to a Stransky–Krastanow growth mode. The 3D growth behavior was correlated to the development of {n11}-type planes, leading to a surface ridging effect along the [110] direction for 4000-ML-thick ZnTe epilayers. The use of a solid-on- solid kinetic roughening model allowed the identification of a mechanism that limits the self- organization of ZnTe nanosized islands, namely, the high density of kink sites found in non- atomically flat GaAs substrates.

Optimization of the structural and optical properties of ZnS epilayers grown on (100)GaAs by MOVPE

Abstract The crystalline and optical quality of ZnS epilayers grown on (100)GaAs by MOVPE was investigated by channeling-RBS, 10 K photoluminescence and absorption spectroscopy for different growth conditions. The measurements point out that the crystalline and optical quality of the epilayers strongly depends on the VI II precursor vapor phase stoichiometry as well as on GaAs surface treatments before the growth. Optimized MOVPE growth conditions have been determined.

Structural study of (100)CdTe epilayers grown by MOVPE on ZnTe buffered and unbuffered (100)GaAs

Abstract We report on the structural assessment of metalorganic vapour phase epitaxy grown (100)-oriented CdTe epilayers on both (100)GaAs and (100) ZnTe GaAs . Ion channelling Rutherford backscattering spectrometry and cathodoluminescence (CL) measurements are used to study the effect of inserting a proper ZnTe buffer layer [G. Leo et al., J. Vac. Sci. and Technol. B 14 (1996) 1739] between CdTe and GaAs. The insertion of a ZnTe buffer layer improves the surface crystalline and optical quality of the CdTe: CL images show that non-radiative recombination regions, associated with extended defects, strongly decrease when a ZnTe buffer layer is used. Also, enhanced excitonic emissions are observed in the case of CdTe/ZnTe/GaAs samples.

Structural characterization of CdS epilayers by channeling Rutherford backscattering spectrometry

We report on the structural characterization by Rutherford backscattering spectrometry in channeling geometry of CdS epitaxial layers grown on the highly lattice‐mismatched CdTe substrates by chemical vapor deposition. The overall layer quality has been investigated for different CdS thicknesses, as well as the occurrence of small misalignments of the crystallographic directions from point‐to‐point in the layers. A crystallographic study of the hexagonal (wurtzite) lattice is also presented: two principal planes parallel to the c axis have been determined experimentally, i.e., (1010) and (1120), together with their in‐plane axial directions. This allows investigation of the occurrence of 30° rotation effects in the crystal lattice as observed by scanning electron microscopy.

SELF-ORGANIZED GROWTH OF ZNTE NANOSCALE ISLANDS ON (001)GAAS

The Stransky–Krastanow metalorganic vapor phase epitaxy growth of self-organized ZnTe islands on homoepitaxial (001)GaAs is demonstrated. The −7.4% lattice mismatch of the ZnTe/GaAs heterostructure leads to a strain-driven distribution of nanoscale ZnTe islands on top of a two-dimensionally (2D) grown wetting layer. Atomic force microscopy and Rutherford backscattering spectrometry are used to determine the island dimensions and the thickness of the wetting layer. The density of the islands, their average diameter, and aspect ratio turn out to be about 520 μm−2, 13.6 nm, and 0.20, respectively, for a 1.2 ML thick 2D layer. Furthermore, the average aspect ratio of the islands decreases by increasing the thickness of the wetting layer, as expected by the progressive extinction of the strain-driven island nucleation.

Structural characterization and surface lattice strain determination of ZnS/GaAs heterostructures grown by metalorganic vapour phase epitaxy

Abstract We report on the structural characterization of ZnS epilayers grown on (1 0 0)GaAs by metalorganic vapour-phase epitaxy (MOVPE). The crystalline quality at the ZnS epilayer surface and the defect depth distribution was studied by Rutherford Backscattering Spectrometry (RBS)-ion channeling measurements as a function of the epilayer thickness. Transmission electron microscopy (TEM) observations were performed on selected ZnS/GaAs heterostructures. Misfit dislocations (MD) were observed at the ZnS/GaAs interface. In addition, a high density of planar defects such as stacking faults (SF) and microtwins (MT) were identified into the epilayer up to 200–300 nm. The density of these defects decreases by increasing the epilayer thickness, but a quite high and constant density of microtwins still occurs in epilayers thicker than 400 nm. However, absorption measurements point out a high optical quality for all the measured ZnS epitaxial layers. Finally, surface lattice strain was determined in the ZnS/GaAs samples by ion channeling measurements. Our data indicate that the initial lattice misfit is already fully relaxed in epilayers as thick as 400 nm and only a small residual thermal strain is measured in thicker samples.