MBE Layers Research Articles

Magnetic properties of electrodeposited Fe/Au(1 1 1) layers: : in situ AGFM measurements

The magnetization state of electrodeposited Fe/Au(1 1 1) layers is investigated during the growth using in situ alternating gradient force magnetometry. It is shown that perpendicular magnetization anisotropy occurs as long as the thickness t Fe<2 ML. The effect of depositing Cu on top is also studied. Results are compared to those obtained with MBE layers and discussed in the light of preliminary STM investigations of Fe growth.

He ++-irradiated beryllium-doped Al 0.5Ga 0.5As MBE layers

In the present paper the deep-level transient spectroscopy (DLTS) method was applied to investigate defects in Be-doped Al 0.5Ga 0.5As created during He ++-ion bombardment. The samples were irradiated at room temperature with two fluences of 0.3 MeV He ++ ions: 10 11 cm −2 and 10 12 cm −2 . The studied samples had a net acceptor concentration equal to 10 16 cm −3 . It was found that irradiation increases the concentration of three traps and produces at least two new traps. It was possible to determine the activation energy for one of them as being equal to 0.54 eV.

Electrically stable p-type doping of ZnSe grown by molecular beam epitaxy with different nitrogen activators

Nitrogen doping of ZnSe MBE layers is studied using two original nitrogen activators which controllably produce output beams with different partial content of an atomic nitrogen (N) and metastable electronically excited molecules ( N 2 ∗ ). The most electrically stable doping with a net acceptor concentration up to 5×10 17 cm -3 is achieved by an RF capacitively coupled magnetron activator in the regimes corresponding to maximal partial concentration of N 2 ∗ while a dominance of N, typical for DC discharge with a vacuum anode sheath, results in the electrical instability of the doping parameters. Low-temperature photoluminescence spectra reveal DAP bands related to shallow and deep donor levels, where the intensities strongly depend on the partial composition of the activator output beam. The results demonstrate that N 2 ∗ selective dissociative adsorption at a Zn atom is a favorable p-type doping mechanism for ZnSe, whereas the atomic nitrogen, although providing a similar net acceptor concentration, appears to be responsible also for the unstable shallow donor center. A 〈1 0 0〉 split interstitial complex is considered as the most probable candidate for this donor.

Characterization of semiconductor heterostructures by acousto-optical perturbation technique

A new technique is developed for investigating defects at substrate/layer semiconductor interface. Acoustic waves (AW) are used to induce changes in the spectral dependence of the optical reflectance and transmission of semiconductor heterostructures together with photoacoustic (PA) and photoelectric (PE) imagings of the layers. The method is illustrated with experiments performed on n-type doped GaAs MBE layers of different quality (unintentionally doping and Si or Te controlled doping). The spectral peaks observed at about 37 and 29 meV inside the band gap are attributed to energy levels of Si and Te impurities, respectively. Two peaks ranged from 54 to 67 and 80–90 meV below the band gap in different samples are suggested to be due to two energy levels of As-vacancy related defects localized at the interface and influenced by electric fields and mechanical stresses in the boundary region. The exact energy position of these peaks is interpreted to be indicative of the interface quality. PA and PE imagings are employed to qualitatively determine electric charge conditions of the interface. It is demonstrated that the nonuniformity of the PE image increases for lower quality substrate/layer interface. Direct evidence is found that relates the interface charge to the spectral peak position of the As-vacancy related defect in acoustically perturbed reflectance spectra.

Deep levels in He++ irradiated Be-doped Al0.5Ga0.5As MBE layers

He++ irradiation induced defects in Be-doped Al0.5Ga0.5As MBE layers have been studied using the DLTS method. The samples were irradiated by He++ ions of an energy equal to 0.5MeV and fluence of 1012cm−2. In irradiated samples, persistent photocapacitance was observed after illumination of the samples at the liquid nitrogen ambient. Defect (or defects) introduced during irradiation can be responsible for the observed persistent photocapacitance effect.

Surfactant-mediated growth of Ge/Si(0 0 1) studied by Raman spectroscopy and TEM

Growth of Ge/Si follows the Stranski–Krastanov growth mode (2D→3D) because of the 4.2% lattice mismatch between Ge and Si. However, the growth mode can be altered to Frank–Van der Merwe (layer-by-layer) by depositing a suitable surfactant. In this study, we have grown thin and thick Ge(0 0 1)/Si(0 0 1) MBE layers with and without As deposition prior to Ge growth in order to investigate the details of surfactant-mediated epitaxy. Raman spectroscopy shows that without As surfactant use, Ge and Si interdiffuse at the interface and form an alloy. Furthermore, peak shift measurements as well as cross-sectional electron micrographs reveal that layers grown with As slowly relieve misfit strain through the accommodation of Lomer edge dislocations at the interface leaving a relaxed, nearly defect-free layer.

Elastic interaction between defects on a surface

We study elastic interactions between defects in thin MBE layers. To date, the classical repulsive law (1/ r 3, r being the defects separation) between point defects (deposited atoms) at the surface of a semi-infinite substrate has been adopted. In this paper, we report that atoms deposited on a thin layer may either attract or repel each other depending on the force distribution induced by the crystal symmetry. Monte Carlo simulations allow one to observe the effect of these elastic interactions on the distribution and shapes of islands.

MBE Grown Planar Doped Barrier Diodes

The realization of Planar Doped Barrier Diode (PDBD) presented in this paper starts out from the limits of the MBE layer growth technology. The limitations due to the background doping concentration and the diffusion of the n and p type dopants during the epitaxial growth are considered. The next parameter is the height of the potential barrier. The choice of this value depends on the requirements of the application. It must take into consideration the current transport mechanisms and the current limitation appearing at higher bias levels.

Characterization of homoepitaxial SiC layers

An n-type 3C-SiC MBE layer is grown on an off-oriented p-type 6H-SiC substrate with alternating supply of gas sources at a growth temperature of 1100°C. The crystalline quality of the 3C-SiC layer is characterized by high resolution transmission electron microscopy and the electrical properties of this layer are investigated by a series of analytical techniques (current-voltage measurement, capacitance-voltage measurement, deep level transient spectroscopy).

Surface reflectance anisotropy of indium-terminated GaAs(100) surfaces

The growth of thin indium-layers on the GaAs(100) As-rich (2 × 4) c(2 × 8) surface has been investigated by reflectance anisotropy spectroscopy (RAS), LEED and AES. Clean surfaces of the (2 × 4) c(2 × 8) reconstruction were prepared in UHV by thermal desorption of a protective arsenic layer deposited on homoepitaxially grown MBE layers. Room temperature deposition of indium on the (2 × 4) c(2 × 8) surface and subsequent annealing at 450°C leads to a 90° rotation of symmetry in the LEED pattern at a threshold coverage of 0.5 monolayers, i.e. a change from the (2 × 4) c(2 × 8) to the (4 × 2) c(8 × 2) reconstruction. The RAS spectra show the evolution of a distinct negative feature at 1.8 eV, that shifts to 2.1 eV after annealing, corresponding to optical transitions attributed to In-dimers orientated along the [011]-direction. AES analysis shows a change in growth mode beyond 0.5 ML indicating saturation of all available adsorption sites at this coverage and RAS spectra show a contribution from additional disordered In. The AES spectra display no evidence of a surface exchange reaction between gallium and arsenic atoms, thus indicating a surface termination by In-dimers adsorbed on a layer of As.

Surfactant-mediated growth of indium on GaAs(001)

The influence of antimony as surfactant on the growth mode of indium on GaAs(001) has been investigated for deposition at various temperatures. Clean surfaces of (2 × 4) reconstruction were prepared in UHV by thermal desorption of a protective arsenic cap deposited on top of homoepitaxially grown MBE layers. Before In deposition, 2 ML Sb were pre-deposited on top of the clean GaAs surface of part of the samples. The In growth mode was then monitored by AES and compared with that occurring during In growth on GaAs without an Sb interlayer. After removal of the samples from UHV, SEM investigations show regularly shaped, flat In islands oriented along the 〈110〉 substrate directions. In comparison, In islands grown without an Sb interlayer also show a preferential orientation along the 〈110〉 substrate directions, but differ in shape and size. Low-temperature In deposition (120 K) leads to the formation of a continuous In layer, which tears during warm up to room temperature. An exchange between surfactant material and indium during growth is found for both low- and room-temperature In deposition. X-ray diffraction measurements show a strong influence of the Sb surfactant on the crystallinity of the In film.

Matrix and impurity element distributions in CdHgTe (CMT) and (Cd,Zn)(Te,Se) compounds by chemical analysis

This review describes several of the main techniques used to determine matrix element distributions and those which can provide a survey of impurity levels and assess deliberate doping concentrations in Cd x Hg 1 − x Te and CdTe-based substrate materials. The most widely used method to non-destructively determine x is that of Fourier transform infrared (FTIR) spectrometry and lateral x variations in current bulk, LPE and MOVPE material measured by this technique will be presented. Auger electron spectrometry (AES) has been used on bevelled samples to assess variations in x with depth and interface widths in LPE, MOVPE and MBE layers and examples will be given. Near IR spectrometry is also now being used to monitor the variations in Zn and Se content, in CdZnTe and CdTeSe respectively, and results in this area will be described along with measurements of Zn on the micro-scale using AES. All of these techniques need to be calibrated against an absolute chemical analysis technique and we have used atomic absorption spectrometry (AAS). The latter technique also provides the accurate measure of dopant and impurity elements to standardise other techniques. Secondary ion mass spectrometry (SIMS) is mainly used for the determination of dopant depth distributions while laser scan mass spectrometry (LSMS) has the unique capability of providing a survey of low levels of impurities in thin epitaxial layers. Depth profiles of arsenic and iodine in MOVPE heterostructures, using SIMS, will be given. Impurity surveys, using LSMS, in bulk CMT and substrate materials and in CMT epitaxial layers grown by LPE, MOVPE and MBE will be described. Reported glow discharge mass spectrometry (GDMS) results on substrate materials will be compared to the present results.

Dimensional dependence of magneto-optical properties of ZnMnSe MBE layers

A set of samples have been grown consisting of ZnMnSe MBE layers with xMn = 0.12 and various layer thicknesses between 500 nm and 3 monolayers. Luminescence, excitation and reflection spectroscopy were performed in a magnetic field up to 7.5 T. Whereas for thick ZnMnSe layers of 500 and 50 nm direct excitons are to be seen, in the case of thin layers d < 10 nm only spatially indirect excitons are observable in luminescence with holes localized mainly in the ZnMnSe layers and the electrons localized in the adjacent ZnSe layers. These interface localized spatially indirect excitons exhibit a reduced Zeeman splitting compared to the direct excitons observable in the thick layers, due to the reduced number of Mn ions inside of the exciton volume. The corresponding ZnMnSe gap energy shift shows, however, a strong increase with decreasing layer thickness. The enhancement of paramagnetic response is only partly due to the interface. We suggest a strong increase of the exchange integrals for the dimensional crossover.

Interface reaction between Ir films and relaxed SiGe MBE layers by rapid thermal annealing

The thermal evolution of Ir p-Si x Ge 1 − x interface reactions was investigated for the first time for the potential application in far infrared image sensors. A 20 nm iridium film was deposited in vacuum by electron beam evaporation onto p-Si 0.75Ge 0.25 fully relaxed MBE grown layers. Rapid thermal annealing in the temperature range of 500–1000°C have been performed. Cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering spectrometry (RBS) have been used in characterizing the reacted layers. The morphology of the interface at the early stages of the reaction has been shown to be very dependent on the annealing temperature. For the annealing times investigated it was found that Ir reacts mainly with Si to form silicides, while the Ge is expelled from the reacted layer and piles up at the silicide SiGe interface. The effect of a double step annealing on the redistribution of the elements in the reacted layer is also described.

Exciton-polariton edge of GaAs: MBE layers between multiple-quantum-well structures

We have studied the absorption threshold of GaAs layers sandwiched between two multiple-quantum-well structures, in the 15–400K range, with the aim to eliminate macroscopic electric field in the layers. The reflectance lineshapes display pronounced spatial dispersion effects and are explained quantitatively using an effective dielectric function. We have observed remarkably strong photoluminescence from the GaAs layers up to the temperature of 400K.

Optically Pumped GaN-AlGaN Double-Heterostructure Lasers Grown by ECR-GSMBE and HVPE

ABSTRACTWe have observed laser emission with well-defined cavity modes in optically pumped GaN-Al0.1Ga0.9N double-heterostructure (DH) lasers. The laser structures were grown using an electron-cyclotron-resonance nitrogen-discharge source and gas-source molecular beam epitaxy (ECR-GSMBE) on thick (≥10 µm) GaN buffers grown by hydride vapor-phase epitaxy (HVPE) on c-plane sapphire. Transversely pumped cavities using a 337.1 nra nitrogen laser pump source exhibit a threshold pump fluence ranging from 0.15 to 0.3 mJ/cm2 at 77 K, a linear light output above threshold, a lasing wavelength of 358 nm, and an estimated differential quantum efficiency of 1%. The room-temperature threshold is about 1.7 times higher. Longitudinal mode structure has been resolved in a shorter-cavity (23 µn) device at 77 K. The measured mode spacing of 0.56 nm corresponds to a group index of 5.0. Far-field measurements in a plane perpendicular to the plane of the heterostructure indicate a double-lobed pattern for a 1000 Ǻ thick GaN active region, and a single lobe with a FWHM of 60° for a 4000 Ǻ active region. The thick HVPE GaN buffer layer provides for a lattice-matched growth and results in improved nucleation in MBE, as indicated by a high-quality reflection-electron-diffraction pattern of the as-loaded wafers. The surface morphology of the MBE layers on the HVPE buffer shows improved optical smoothness as compared to layers grown directly on sapphire using a low-temperature, MBE-grown GaN buffer. Laser facets were formed either by saw cutting or cleaving of the GaN buffer and epilayer along crystal planes. Details of the material development and laser performance are described.

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.

In-situ control of Ga(A1)As MBE layers by pyrometric interferometry

Pyrometric interferometry is used for the in-situ optical thickness control of Ga(A1)As MBE layers. Real time computer evaluation of the oscillations in temperature radiation from the heated substrate allows easy control of quarter wavelength layers for vertical cavity surface emitting laser diodes. Effects due to variation of the substrate surface temperature and reflection of radiation from hot evaporation cells are discussed as well as influences from optical phase shift or phase matching.

Nanometer scale local epitaxy with silicon MBE

A method for the mesa growth of silicon MBE layers with lateral dimensions down to the nanometer scale is presented. The use of micro-shadow masks, which are compatible to ultra-high-vacuum conditions, allows the epitaxial deposition of laterally structured MBE layers. The electrical characteristics of locally grown p-i-n diodes demonstrate an excellent crystal quality.

Stress in Thin Layers Applied by Substrate Bending; Calibration and Potentialities

ABSTRACTUniaxial stresses up to 0.3GPa, both compressive and tensile, can be generated in thin epitaxial layers by substrate bending. We demonstrate the feasibility of the method and calibrate the stress, measuring piezoresistance at temperatures below the impurity freeze-out, on lightly B-doped silicon MBE layers, using the known expression for a surface stress in a bent cantilever beam. The ground state energy displacement of B in Si thus obtained can vary by ±5meV, suggesting potential applications of the technique in tunable devices, involving doped semiconductors or heterostructures.