E. Łusakowska

Extremely low temperature growth of ZnO by atomic layer deposition

We report on the zinc oxide (ZnO) thin films obtained by the atomic layer deposition (ALD) method using diethyl zinc and water precursors, which allowed us to lower deposition temperature to below 200 °C. The so-obtained “as grown” ZnO layers are polycrystalline and show excitonic photoluminescence (PL) at room temperature, even if the deposition temperature was lowered down to 100 °C. Defect-related PL bands are of low intensity and are absent for layers grown at 140−200 °C. This is evidence that extremely low temperature growth by ALD can result in high quality ZnO thin films with inefficient nonradiative decay channels and with thermodynamically blocked self-compensation processes.

ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions

We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect.

Misfit strain anisotropy in partially relaxed lattice-mismatched InGaAs/GaAs heterostructures

Partially relaxed InGaAs/GaAs heterostructures with a small lattice mismatch have been studied by means of atomic force microscopy and high-resolution x-ray diffractometry. Additionally, electron-beam induced current in a scanning electron microscope and transmission electron microscopy have been employed to investigate misfit dislocations formed at the (001) heterostructure interface. The measurements revealed a direct correlation between the surface cross-hatched morphology and the arrangement of interfacial misfit dislocations. The reciprocal lattice mapping and the rocking curve techniques employed for the samples aligned with either the or the [110] crystallographic direction perpendicular to the diffraction plane revealed anisotropic misfit strain relaxation of the InGaAs layers. This anisotropy results from an asymmetry in the formation of the α and β types of misfit dislocations oriented along the and [110] directions, respectively, which differ in their core structures. The misfit strain anisotropy causes a distortion of the unit cell of the layer and lowers its symmetry to orthorhombic.

Parameters of substrates-single crystals of ZnTe and Cd1-xZnxTe (x < 0.25), obtained by physical vapor transport technique (PVT)

Abstract It is shown that very high quality (in particular – very good real crystal structure) of the single crystals of ZnTe and Cd 1− x Zn x Te, intended to be substrates for MBE and other techniques of epitaxy, can be reached by an optimized physical vapor transport (PVT) technique. The obtained crystals are twin-free, the narrowness of the rocking curve nearly reaches the theoretical limit and the density of dislocations is low. The results of characterization of the crystals are presented.

ZnO, ZnMnO and ZnCoO films grown by atomic layer deposition

Despite many efforts, the origin of a ferromagnetic (FM) response in ZnMnO and ZnCoO is still not clear. Magnetic investigations of our samples, not discussed here, show that the room temperature FM response is observed only in alloys with a non-uniform Mn or Co distribution. Thus, the control of their distribution is crucial for the explanation of contradicted magnetic properties of ZnCoO and ZnMnO reported till now. In this paper, we discuss advantages of the atomic layer deposition (ALD) growth method, which enables us to control the uniformity of ZnMnO and ZnCoO alloys. Properties of ZnO, ZnMnO and ZnCoO films grown by the ALD are discussed.

Ferromagnetic Transition in Ge_{1-x}Mn_{x}Te Layers

Ferromagnetic transition temperature in thin layers of diluted magnetic (semimagnetic) semiconductor Ge1−xMnxTe was studied experimentally by SQUID magnetometry method and analyzed theoretically for a model Ising-type diluted magnetic system with Ruderman–Kittel–Kasuya–Yosida indirect exchange interaction. The key features of the experimentally observed dependence of the Curie temperature on Mn content (x ≤ 0.12) and conducting hole concentration p = (1–10)× 10 cm−3 were reproduced theoretically for realistic valence band and crystal lattice parameters of p-Ge1−xMnxTe taking into account short carrier mean free path encountered in this material and Ruderman–Kittel–Kasuya–Yosida mechanism with both delta-like and diffused character of spatial dependence of the exchange coupling between magnetic ions and free carriers.

ZnCoO Films by Atomic Layer Deposition - Influence of a Growth Temperature οn Uniformity of Cobalt Distribution

We report on the structural, electrical and magnetic properties of ZnCoO thin films grown by Atomic Layer Deposition (ALD) method using reactive organic precursors of zinc and cobalt. As a zinc precursor we applied either dimethylzinc or diethylzinc and cobalt (II) acetyloacetonate as a cobalt precursor. The use of these precursors allowed us the significant reduction of a growth temperature to 300oC and below, which proved to be very important for the growth of uniform films of ZnCoO. Structural, electrical and magnetic properties of the obtained ZnCoO layers will be discussed based on the results of SIMS, SEM, EDS, XRD, AFM, Hall effect and SQUID investigations.

Low defect density, substrate quality crystals of the wide-gap II–VI compounds, obtained by physical vapour transport technique (PVT)

Abstract It is demonstrated that substrate quality, 1 in. size single crystals of the wide-gap II–VI semiconductors: ZnSe, Zn(Se,S), ZnTe and (Cd,Zn)Te can be obtained by a simple, horizontal, low temperature, physical vapour transport (PVT) technique. The results of the characterization of the obtained crystals are presented.

Growth by molecular beam epitaxy and magnetooptical studies of (100)- and (120)-oriented digital magnetic quantum well structures

Abstract We report on growth by molecular beam epitaxy and magnetooptical investigations of digital alloys of diluted magnetic semiconductors. The digital alloys, i.e., very short-period superlattices, of interest here, were grown from nonmagnetic CdTe and magnetic Cd1−xMnxTe. In particular, quantum well structures containing the digital alloys in the regions of the quantum wells were fabricated. We searched for theoretically expected modifications of magnetic properties of digitally grown materials compared to those of random substitutional alloy counterparts. We monitored the magnetic behavior in our samples by studying their magnetooptical properties. We detected clear differences between magnetizations in random and digital alloys in samples grown along the 〈100〉 crystallographic direction. Unexpectedly, no modification of the spin-splitting (and, thus, of the magnetization) were observed in the case of digital quantum wells grown in the 〈120〉 crystallographic direction. On the other hand, exciton magnetic polaron binding energy in the latter structures was consistently greater by 10–20% than this quantity in similar samples grown along the 〈100〉 direction. The increase can be qualitatively understood in terms of the heavy hole mass anisotropy.

Magnetic anisotropy induced by crystal distortion in Ge1−xMnxTe/PbTe//KCl (001) ferromagnetic semiconductor layers

Ferromagnetic resonance (FMR) study of magnetic anisotropy is presented for thin layers of IV-VI diluted magnetic semiconductor Ge1−xMnxTe with x = 0.14 grown by molecular beam epitaxy on KCl (001) substrate with a thin PbTe buffer. Analysis of the angular dependence of the FMR resonant field reveals that an easy magnetization axis is located near to the normal to the layer plane and is controlled by two crystal distortions present in these rhombohedral Ge1−xMnxTe layers: the ferroelectric distortion with the relative shift of cation and anion sub-lattices along the [111] crystal direction and the biaxial in-plane, compressive strain due to thermal mismatch.