S. Bauer

Low temperature MOVPE growth of ZnSe with ditertiarybutylselenide

Abstract The results on the synthesis of the selenium alkyl ditertiarybutylselenide and its application in atmospheric pressure MOVPE are presented. In combination with dimethylzinc-triethylamine, single crystalline ZnSe layers were grown on GaAs at temperatures lower than 350°C. Good morphology, crystalline and interface quality are demonstrated by optical and electron microscopy, X-ray diffraction and Raman spectroscopy. Photoluminescence at 2 K reveals chlorine as an impurity. The electron mobility of 500 cm 2 /V·s at room temperature supports a fairly low compensation.

Investigation of strained ZnTe epilayers by high resolution electron microscopy

Abstract Structural properties and strain relaxation at the ZnTe/GaAs (001) interface are investigated by high resolution electron microscopy (HREM). Images taken from ZnTe MOVPE-grown epilayers with a resolution of 0.2 nm show an array of misfit dislocations at the interface. Their analysis is facilitated by using a digital image processing method. The most common line defects at the ZnTe/GaAs interface are Lomer and 60° dislocations. Also a small number of stacking faults, limited by partial dislocations, can be observed at the interface. The spacing between the misfit dislocations allows an estimate of the strain relaxation.

Growth and vertical correlation of CdSe/ZnSe quantum dots

The growth and vertical organization of CdSe quantum dots in three-layer stacks consisting of CdSe with a nominal thickness of 2.5 monolayers (ML) and ZnSe spacers with thicknesses between 10 and 20 ML was investigated by reflection high energy electron diffraction during the growth and different transmission electron microscopy techniques. The samples were grown by molecular beam epitaxy at 400u200a°C. It was found that up to 10 ML spacer thickness all three CdSe layers and ZnSe spacers form one broad (Cd,u200aZn)Se alloy layer with a small Cd concentration containing Cd-rich islands with a size of ∼15 nm. For spacers with a larger thickness (12–20 ML) three separated ternary (Cd,u200aZn)Se layers are observed which contain Cd-rich inclusions (small islands) with a size of less than 10 nm. A preferential vertical correlation of the small islands occurs for the 12 ML spacer thickness. With increasing spacer thickness, the number of the correlated small islands is reduced displaying a tendency to uncorrelated growth.

The metal organic vapour phase epitaxy of ZnTe: III. Correlation of growth and layer properties

Abstract The crystalline structure, surface morphology, optical properties and purity of ZnTe layers grown by MOVPE were investigated. Various substrates, different combinations of metalorganics and various growth conditions were studied. The results of three different MOVPE growth systems and reactor cells are compared. A variety of methods were used to study the structure and morphology (e.g. TEM, HRTEM, X-ray diffraction, Nomarski microscopy, photo reflection, Raman scattering). The preparation of the GaAs and ZnTe surfaces is well advanced but problematic for GaSb. For heterostructures like ZnTe on GaAs (001) with about 7% mismatch, the crystalline structure is mainly dominated by interfacial misfit dislocations and threading dislocations penetrating about 300 nm into the ZnTe layer. With better matched substrates (GaSb or ZnTe) or different orientations (GaAs (111)), the threading dislocations can nearly be eliminated. The comparison of growth studies with MOVPE, MBE and ALE reveals that the initial growth of ZnTe on GaAs (001) is mainly determined by the misfit and its relaxation. The first ≈4 monolayers are characterized by a pseudomorphous, 2D growth mode. The following surface roughening is caused by a relaxation through a 3D growth mode with islands. Once the relaxation is completed, the surface smoothens and recovers the 2D growth mode. The initial growth stages, representing the Stranski-Krastanov mechanism, are dependent on the surface termination of the substrate and on the growth method and parameters. The surface morphology of thicker ZnTe layers (>0.1 μm on GaAs (001)) grown by MOVPE is solely determined by the conditions at the growing interface (adsorption and decomposition of the precursors, desorption of undesired species, quantities of released Zn and Te, surface stoichiometry). These interface conditions are dependent on the precursor combination, the kinetic or mass transport limitation and the reactor hydrodynamics. The studies of the layer properties were sometimes bothered by a thin native oxide with the structure ZnTe/Te/ZnO. Purity and optical quality were tested by low temperature PL. The strain induced by the cooling of the ZnTe/GaAs structures grown at 300–400°C renders the identification of the radiative transitions difficult. However, many transitions and radiative centers are now identified. Under many growth conditions, the (A0Asl, X1) transition which is due to As from substrate autodoping, dominates the spectra. Hence, stoichiometry and inhomogeneity of the GaAs substrates are reflected in the spectra. But also precursor combination, partial pressures and growth temperatures have a significant influence on the PL spectra. The substrate type (GaAs, GaSb, ZnTe) is reflected in the spectra by transitions due to outdiffusion and by the Y-lines which are related to the misfit dislocations. Transitions induced by layer contaminations like Cu, Li, O and N were found. With the alkyl combination DEZn/DIPTe, reproducible growth of samples is possible showing PL spectra dominated by free excitons. The frequently appearing I1c and I1c transitions might be due to a VZn vacancy or a vacancy donor complex. Complex relations to the growth stoichiometry were found. SIMS measurements helped to further identify the layer impurities. The layer purity is affected by extrinsic impurities due to substrate outdiffusion (As, Ga), contaminations from growth system (O, Cu) and precursors (Li). Impurities like H and C (and N) which are intrinsic to the MOVPE process, depend on the dissociation kinetics of the precursors which are themselves dependent on the growth parameters (e.g. temperature, carrier gas). The origins of the extrinsic impurities are identified as far as possible.

On the origin of the “coffee-bean” contrast in transmission electron microscopy images of CdSe/ZnSe quantum dot structures

The origin of the “coffee-bean” strain contrast is studied, that is observed in the plan-view transmission electron microscopy (TEM) images of CdSe/ZnSe quantum dot structures. The samples were grown by two different methods: standard molecular-beam epitaxy at 350u200a°C and atomic layer epitaxy at 230u200a°C with annealing at 340u200a°C after the CdSe deposition. The nominal CdSe thickness was above 3 ML. In situ reflection high energy electron diffraction during the growth or during the annealing shows the transition from the two- (2D) into the three-dimensional (3D) surface morphology for both samples. The coffee-bean contrast is usually assigned to three-dimensional islands which are generated after the morphological 2D/3D transition. It is found that the coffee-bean contrast in plan-view TEM images is alternatively associated with pairs of stacking faults on {111} lattice planes which are inclined against each other. The stacking faults, which are bound by Shockley partial dislocations, are preferably generated ...

Misfit dislocations in epitaxial ZnTe/GaAs (001) studied by HRTEM

Abstract Single-crystalline ZnTe epilayers were grown by atmospheric-pressure metal organic vapor phase epitaxy (MOVPE) and hot wall epitaxy (HWE) on (001) GaAs. The misfit of lattice constants is -7.5% at the growth temperature of 350°C. Conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) have been used to investigate the misfit dislocations in the ZnTe/GaAs (001) system. The types and distribution of the observed dislocations are independent of growth techniques used. The most common line defects at the ZnTe/GaAs interface are 60° and Lomer dislocations with Burgers vectors of 1 2 a〈110〉. Their respective abundance ratio is 2:1. Less than 3% of the 60° dislocations dissociate into partial dislocations limiting a stacking fault. The distances between parallel {111} planes terminating at the interface have been determined from an analysis of about 1 μm of the projected ZnTe/GaAs interface. The distribution fits by a Gaussian with an average distance of 57 A. This allows an estimate of the residual biaxial strain at the interface which is found to be compressive with a magnitude less than 0.6%. The distances of the observed dislocation cores form a histogram. Its special shape shows four maxima at about 0, 5, 9 and 12× 1 2 as〈110〉. They can be explained by a model for the correlated nucleation of misfit dislocations in highly mismatched heterostructures like ZnTe/GaAs.

Raman studies of the growth mechanism of ZnSe/GaAs(001) heterostructures

Monocrystalline ZnSe/GaAs(001) heterostructures have been grown by atomic layer epitaxy with typical thicknesses up to 200 nm. The 1LO Raman spectrum of ZnSe and GaAs, respectively, was recorded in situ during growth interruptions. The spectra were analyzed with respect to Raman intensity, phonon frequency, and bandwidth. Two different growth modes are distinguishable: (i) The biaxial compressive strain in the epilayers is increased in the thickness range up to 40 nm due to lattice mismatch of the heterosystem. This can be interpreted as a transition from an initial 3D island to the 2D layer by layer growth. (ii) When larger thicknesses (40–170 nm) are reached the strain is constant and the 1LO Raman intensity from the epilayer and from the substrate oscillates significantly with thickness. These periodic oscillations are quantitatively described in the framework of interference‐enhanced Raman scattering and verify the 2D growth mode in this thickness range. The crystalline quality and the final thickness...

Optical assessment of reactive ion etched ZnTe and ZnSe for nanostructures

Abstract We have developed a reactive ion etching (RIE) process capable of producing nanostructures in ZnTe and ZnSe epitaxial layers. Wires of width down to 65 nm and dots with diameter down to 50 nm have been obtained using CH 4 /H 2 . The optical properties of (RIE) etched epitaxial layers have been investigated. The deep level emission of control and etched samples decreases substantially after annealing, revealing much finer structure especially for ZnTe. Raman scattering has shown no detectable damage induced by the etching process in both ZnTe and ZnSe.

The absolute efficiency of resonant Raman scattering by longitudinal optical phonons in ZnTe near the E0 gap

The absolute efficiency of resonant Raman scattering by longitudinal optical phonons is determined for an orientated ZnTe bulk crystal and single-crystalline ZnTe epilayers with incident photon energies near the E0 band gap at a temperature of 2 K. The ZnTe layers were grown by metal-organic vapour-phase epitaxy (MOVPE) on (001) GaAs and (001) GaSb, respectively, with a thickness of about 2 mu m. Absolute values of the Raman scattering efficiency (RSE) have been determined in z(x,x)z and z(y,x)z backscattering configuration using a sample-substitution method and correcting the measured intensities with respect to absorption, reflection, and refraction. Measurements of the absorption coefficient and the reflectivity were performed on the same samples and at the same temperature. Well resolved maxima of the RSE appear at the m=1 and m=2 discrete energy levels of the band-gap exciton for incoming resonance and at the m=1 level for outgoing resonance. Valence-band splitting due to residual biaxial strain in the ZnTe epilayers results in a splitting of the m=1 resonance maximum for both incoming and outgoing resonance.

Resonant Raman scattering by LO phonons in Cd0.8Mn0.2Te near the E0 gap in external magnetic fields

Abstract The efficiency of resonant Raman scattering by LO phonons in Cd0.8Mn0.2Te is studied near the E0 gap at T = 2 K in external magnetic fields. Normalized Raman intensities for scattering via deformation-potential and Frohlich electron-phonon interaction were measured in Faraday configuration. The application of an external magnetic field causes a giant Zeeman splitting of the valence and conduction band states which changes the resonance behaviour of the scattering efficiency drastically. Existing model calculations for the absolute Raman scattering efficiency in zincblende-type semiconductors, taking into account Wannier-Mott excitons as intermediate electronic states in the scattering process, were modified for the special case of semimagnetic semiconductors in external magnetic fields and compared with the experimental results.