Thin CdTe Layers Research Articles

AFM Characterization of the Initial Growth Stages of CdTe on Si(111) Substrates

We report on the growth and characterization of very thin CdTe layers on Si(111). The samples were grown by hot wall epitaxy on Si substrates treated by a diluted HF solution. Using this process, good quality CdTe layers can be grown, with the whole process being conducted at temperatures below 300 °C. The samples were grown at a very low growth rate of about 0.02 Å/s in order to investigate the nucleation process using atomic force microscopy. Our measurements show that this system follows the Volmer–Weber growth mode, with the nucleation of isolated CdTe islands on the Si substrate surface. We describe the size and density distribution of these islands as a function of substrate temperature. The results show that the Volmer–Weber growth mode can be successfully used to obtain self-assembled quantum dots of CdTe on Si using an inexpensive growth technique.

Zero-dimensional excitons in CdTe/ZnTe nanostructures

Thin CdTe layers embedded in ZnTe matrix grown by atomic layer epitaxy have been studied by time resolved spectroscopy and spatially resolved spectroscopy. The presence of Cd-rich dotlike islands in these CdTe nanostructures is shown by both atomic force microscopy and high resolution transmission electron microscopy. Zero-dimensional nature of excitons is shown both by the temperature dependence of the decay time and observation of sharp exciton lines in microphotoluminescence spectra. Zero-dimensional excitons probed by microphotoluminescence present a doublet structure linearly polarized along two orthogonal directions. This doublet structure is attributed to bright heavy-hole exciton states split by the local asymmetry of the localization potential. Reversible spectral shifts in the emission of some single quantum dots are observed on a time scale of hundreds of milliseconds. These small shifts can be attributed to the Stark effect caused by fluctuating electric fields and can significantly affect time-integrated transition linewidths.

Preparation of CdTe Quantum Structures on ZnTe by Hot-Wall Epitaxy

The formation of quantum structure of CdTe on ZnTe is investigated. Thin CdTe layers were deposited by hot-wall epitaxy. The thickness of CdTe layer was precisely controlled by alternate deposition of Cd and Te in an auto regulated manner. It is shown that the CdTe quantum structure is appreciably improved by depositing CdTe on thermally annealed ZnTe. We found as experimental result the ripening of CdTe surface.

Observation and first-principles calculation of buried wurtzite phases in zinc-blende CdTe thin films

We report direct observation of the existence of buried thin wurtzite CdTe layers in nominally pure zinc-blende CdTe thin films using high-resolution transmission electron microscopy. The formation of the buried wurtzite layers is a result of the formation of high density of planar defects in the zinc-blende films—the wurtzite layers are formed by closely spaced lamellar twins. First-principles calculations reveal that the presence of the buried wurtzite layers may be responsible for the poor electrical properties of the polycrystalline zinc-blende CdTe films.

Surface reconstructions of (001) CdTe and their role in the dynamics of evaporation and molecular-beam epitaxy growth

We present studies of the surface reconstruction and chemical analysis of the uppermost atomic layer of thick and thin (001) CdTe layers grown on (001) CdTe and ZnTe by (MBE). Techniques include reflection high energy electron diffraction and x-ray photoelectron spectroscopy (XPS). All the surfaces obtained in the temperature range used (220–300 °C) in MBE are Cd covered with about 50% of undimerized Cd atoms; the peak of the Cd surface is clearly identified by XPS on thin CdTe layers. The surfaces can be (2×1), c(2×2) or (2×1)+c(2×2) reconstructed. Heating these surfaces above 300 °C leads to a (2×1) Te-stabilized surface. A reversible transition occurs between this last surface and the mixed (2×1)+c(2×2) structure: thermal cycling between these two reconstructed surfaces induces the evaporation of half a monolayer per cycle. Tentative models to account for the results are presented and correlation between these results and the properties of the MBE layers is discussed.

Depth non-uniformities in thin CdTe layers grown by MBE on InSb substrates

Measurements have been made of the carrier concentration as a function of depth through CdTe layers grown by MBE on InSb (001) substrates. Both In-doped and undoped layers are n-type but the free carrier concentration diminishes in the vicinity of the CdTeInSb interface, possibly due to the presence of an increased density of extended defects in this region resulting from lattice mismatch at the interface. In order to provide more detailed information concerning the origin of these depth non-uniformities, double crystal X-ray diffraction (DCXRD) techniques have been employed to provide information about the crystal quality and lattice strain within the layers. By studying a variety of samples with different structure and quality, it is clearly demonstrated that regions of reduced carrier concentration correspond to those regions where significant strain relaxation has occured.

Structural and electrical non-uniformities in thin CdTe layers grown on InSb by MBE

Electrical measurements made on CdTe layers grown on (001) InSb substrates reveal significant non-uniformities as a function of depth. As there is a small mismatch between the lattice constants for CdTe and InSb, extended defects resulting from strain relief in the epilayer are believed to have an important influence on the electrical properties and, with the aid of computer modelling to simulate X-ray diffraction rocking curves, the distribution of structural defects is being explored. Examples are presented of carrier concentration depth profiles in the vicinity of the CdTe-InSb interface for samples grown under a variety of different conditions and the relationship between the observed electrical non-uniformity and the distribution of dislocations resulting from strain relief is discussed.

A novel technique for the MBE growth of twin-free HgCdTe

A novel photon-assisted MBE technique, compositionally modulated structures (CMSs), has been developed to suppress twinning in MBE HgCdTe growth. Strain effects, introduced by thin CdTe layers, suppress planar twinning as well as lowering dislocation densities. A CMS epilayer consists of multiple repeats of a basic unit comprised of a thick (500–1500 Å) layer of HgCdTe and a thin (20–50 Å) layer of CdTe. The CMS epilayer is then interdiffused in a sealed quartz ampoule at temperatures consistent with n-type (or p-type) annealing producing isotropic HgCdTe suitable for infrared detector array fabrication. CMS has also been successfully applied to the low defect density growth of HgCdTe on the slightly lattice-mismatched CdTe substrates. This is an important result because CdTe substrates are less expensive, more chemically pure, and more readily available than the lattice-matched substrate CdZnTe.

RBS studies of superlattices

The possibilities and limitations of the Rutherford backscattering spectrometry and ion channeling in the high and medium ion energy region for the characterization of heteroepitaxial structures are demonstrated. The thickness and change of the stoichiometry at buried interfaces were determined for thin CdTe layers grown on (001) GaAs. A multilayer twin structure of a (111) CdTe layer with single twin layer thickness less than 3.5 nm was proved by an azimuthal-dependent (111) channeling analysis with 300 keV protons. The stoichiometry, superlattice period and crystalline quality have been determined for superlattices with Sb delta-doped layers in silicon and for InGaAs/GaAs strained-layer superlattices (SLS's). For thin layered SLS's the application of a new channeling method for strain measurements is shown. The results are in good agreement with computer simulations of ion channeling and with XRD measurements. Results of the location of misfit dislocation networks are obtained by a backscattering-angle-dependent dechanneling analysis.

Observation of negative differential resistance from a single barrier heterostructure

We report the first experimental observation of negative differential resistance (NDR) due to electron tunneling in a single barrier heterostructure. The largest peak-to-valley current ratio attained is slightly greater than 2:1. The single barrier structure studied here consists of a thin CdTe layer sandwiched between two Hg0.78Cd0.22Te electrodes. In this particular material system, NDR can only be achieved at low temperatures (T=4.2 K) due to the dominance of thermionic hole currents at high temperatures. The observation of NDR in this system suggests that the low-temperature valence-band discontinuity at the HgTe-CdTe interface is small (less than 100 mV). Room-temperature operation of single barrier NDR structures may be possible in other semiconductor systems.

I n s i t u spectroscopic ellipsometry during molecular-beam epitaxy of cadmium mercury telluride

To improve the control of the growth, a II–VI molecular-beam epitaxy chamber has been equipped with an in situ spectroscopic ellipsometer. Using this characterization technique we checked the interdiffusion between CdTe and HgTe. A thin CdTe layer was deposited on a HgTe layer. The initial abrupt interface (less than 1 nm) evolved during successive annealings. Spectroscopic measurements were performed at each step. A modeling permits the evaluation of the thickness of a graded composition layer resulting from the interdiffusion at 260 °C.