Coevaporation phosphorus doping in Si grown by molecular beam epitaxy

Phosphorus-doped Si epilayers with bulk-like mobilities were grown by molecular beam epitaxy (Si-MBE) by coevaporation of phosphorus from a tin phosphide source. The behaviour of P doping as a function of growth parameters and of potential enhanced doping indicates a non-unity, almost growth-temperature independent incorporation efficiency with negligible surface segregation -a unique combination among coevaporated dopants in Si-MBE.     

Influence of material design parameters on radiative recombination in GaAs doping superlattices grown by MBE

The specific luminescence process in GaAs doping superlattices arises from recombination of electrons populating low-index conduction subbands with holes in the acceptor impurity band across the indirect gap in real space. The luminescence peak energy thus directly reflects the actual value of the tunable gap for the photoexcited state of the superlattice. We have studied the tunability of the effective gap, the recombination rate, and the relative quantum efficiency on superlattice specimen of different material design parameters by means of low-temperature photoluminescence measurements. For optimized design parameters the ratio between luminescence and excitation intensity remains nearly constant over the entire tunability range of the effective gap.     

The electrical properties of doped silicon,grown by Molecular-Beam-Epitaxy (MBE)

Measurements of the Hall coefficients and of the resistivity of MBE-grown Si, doped with P, As, Sb, B, and Ga in the concentration range 10¹⁴ to 10²⁰ cm^–3, were carried out at 77 K and at 300 K. With the exception of Ga-doped Si, the measured mobilities were close to or higher than those of bulk materials at both temperatures. The Mott metal/non-metal transition has been observed in the present epitaxial materials and the measured values for the critical impurity concentration at which the transition occurs, agree with values reported by other workers for bulk silicon.     

Nucleation effects during MBE growth of Sn-Doped GaAs

The morphology, structure and composition of Sn films on GaAs substrates have been investigated in relation to the predeposition technique used with Sn doping of MBE-grown GaAs films. It has been shown that 3-D aggregates of Sn are formed and a similar morphology has been identified in the Sn which accumulates at the surface of all Sn-doped GaAs films. The way in which this relates to donor incorporation is considered and a simple model of incorporation kinetics proposed which is shown to be consistent with observation.     

Incorporation behaviour of manganese in MBE grown Ga0.47In0.53As

Acceptor doping of MBE grown Ga_0.47In_0.53As on InP:Fe substrates utilizing manganese is investigated as an alternative for overcoming problems related to beryllium doping. The incorporation behaviour of manganese is analyzed in detail with respect to its application in optoelectronic device structures. Special emphasis is put on low-level and maximum-level doping relevant for use in buffer and contact layers, respectively. The dependence of activation energy and the degree of ionization on acceptor concentration is determined. At high doping levels the free-hole concentration is markedly lower than the doping concentration. It is attributed to the diffusion of acceptor species across the heterointerface into the substrate. Manganese diffusion is demonstrated to be an important effect for the interpretation of the measurement results. Manganese doping is applied in p⁺/n/p^–-layer structures for junction field effect transistor applications. The intended abruptness of acceptor profiles is deteriorated by diffusion of manganese in the Ga_0.47In_0.53As material. The degraded device characteristics obtained are attributed to the acceptor diffusion behaviour established.     

ArF laser CVD of hydrogenated amorphous silicon: The role of buffer gases

ArF laser-induced CVD has been employed to generate hydrogenated amorphous silicon (a-Si:H) from Si₂H₆ gas dilute with He, Ar, or H₂. The formation of amorphous films or powder is found to depend critically on the kind of buffer gas, the stationary total and partial gas pressures, and the substrate temperature. These dependences have been investigated in the 1–5 Torr pressure and 100–400 °C temperature ranges. They are semiquantitatively discussed in terms of ArF laser photolysis of disilane, gas heating by heat flow from the substrate and laser irradiation, diffusion, and gas phase polymerization. Furthermore, photo ionization has been observed but found irrelevant for the a-Si:H layer properties. The photo and dark conductivities ( _ph, _d) of the semiconductor layers are determined by the substrate temperature. The _ph values range between 10^–7 and 10^{–4 –1} cm^–1 and the _d values between 10^–11 and 10^{–8 –1} cm^–1. The maximum ratio _ph/ _d amounts to 4×10⁴. The layers are further characterized by their optical band gap and activation energy. The layer properties are compared to literature values of amorphous films prepared by various photo, HOMO, and plasma CVD methods.     

Oxidation and thermal annealing effects on native and ion-irradiated PbTe films grown by molecular beam deposition

Investigations are reported into the effect of low-pressure oxygen exposure and thermal annealing on the carrier transport properties of native and 350 eV Ar⁺ bombarded PbTe films. The electrical measurements were madein situ on MBD-grown PbTe films without breaking vacuum. On native surfaces, oxidation was initially sustained by diffusion of a donor species from the film bulk to the surface, where reaction with oxygen occured. This diffusion process was apparently inhibited on ion irradiated films and direct doping of the film surface effected a gradual reduction in the ion-induced electron accumulation resident at the film surface. The native properties and behavioural characteristics of the films could be recovered by thermal annealing of the ion-irradiated and/or oxidized films at 300–350 °C.     

Properties of RF sputtered ZnxCd1−xS thin films

Zn _x Cd_1–x S thin films (0x0.20) were prepared using rf sputtering in argon atmosphere and characterized using X-ray diffraction, optical transmission, electrical resistivity and photoconductive decay measurements. The films were found to possess hexagonal structure. The crystallite size and degree of preferential orientation were found to decrease with the increase ofx and to improve upon annealing in vacuum at 250 °C. The transmission edge shifted towards shorter wavelengths with the increase ofx in agreement with the expected shift in the energy band gap. The films were found to exhibit room temperature resistivity in the range 100–1000 cm. The obtained values of long wavelength transmission (70–80%) and minority carrier diffusion length (30 m) are high enough for the application of these films in the field of solar cells.     

First-principles study of arsenic impurity clusters in molecular beam epitaxy (MBE) grown HgCdTe

The understanding of the microstructures of the arsenic tetramer , dimer , and singlet of HgCdTe is important to explain the high electrical compensation of molecular beam epitaxy (MBE) samples and the conversion to p-type behavior. The stable configurations were obtained from the first-principles calculations for the arsenic cluster defects [ (n=1, 2, and 4)] in as-grown HgCdTe. According to the defect formation energies calculated under Te-rich conditions, the most probable configurations of , , and have been established. For the optimized and the energy is favorable to combine in a nearest neighboring mercury vacancy , and the corresponding configurations can be used to explain the self-compensated n-type characteristics in as-grown materials. is likely to be more abundant than in as-grown materials, but arsenic atoms are more strongly bounded in than in , thus more substantial activation energy is needed for than that for . The atomic relaxations as well as the structural stability of the arsenic defects have also been investigated.     

The photoconductive effect in Pb2CrO5 thin films prepared by an electron-beam evaporation technique

Photoconductivity in Pb₂CrO₅ thin film prepared by an electron-beam evaporation technique is described. Crystallographically, three kinds of thin films are fabricated which depend on substrate temperature. A sample showing a similar x-ray diffraction profile to the evaporation source material gives the highest photoconductive response. Light illumination from the glass substrate onto the sample improves photoconductivity. A pair of interdigital electrodes is more effective than a pair of planar electrodes on the photoconductive measurement. A band gap energy level of Pb₂CrO₅ thin film is around 2.2–2.4 eV as a result of the spectral photoconductive response.     

Characterization of SnO2/Pb2CrO5 thin film/metal photovoltaic device

The photovoltaic behavior in a design of SnO₂/Pb₂CrO₅ thin film/metal (Au or Al) is described as functions of light intensity and wavelength for two different illumination directions. The Pb₂CrO₅ thin film in each device is formed over the SnO₂ coated glass substrate by an electron-beam evaporation technique. When illuminated from the metal side, the device with Au or Al shows the positive or the negative photovoltage, respectively. For the illumination from the SnO₂ side, the device with Au has the negative photovoltage at the short wavelength, and the positive one at the longer, while the device with Al has the negative photovoltage at both wavelength regions. These photovoltaic devices have two junctions at both sides of the Pb₂CrO₅ thin film. The band model of each device is also discussed.     

Preparation and structural properties of MgO films grown on GaAs substrate

Epitaxial MgO thin films have been grown on semiinsulating GaAs (0 0 1) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (0 0 2) reflection only and rocking curve widths ∼2.2-3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.     

Structure and optical properties of ZnS thin films grown by glancing angle deposition

The glancing angle deposition (GLAD) technique was used to deposit ZnS films by electron beam evaporation method. The cross sectional scanning electron microscopy (SEM) image illustrated a highly orientated microstructure composed of slanted column. The atomic force microscopy (AFM) analysis indicated that incident flux angle had significant effects on the nodule size and surface roughness. Under identical nominal thickness, the actual thickness of the GLAD films is related to the incident flux angle. The refractive index and in-plane birefringence of the GLAD ZnS films were discussed, and the maximum birefringence Δn = 0.036 was obtained at incident flux angle of α = 80°. Therefore, the glancing angle deposition technique is a promising way to create a columnar structure with enhanced birefringent property.     

Characterization of amorphous silicon films by contactless transient photoconductivity measurements

Contactless transient photoconductivity measurements of intrinsic a-Si: H films in the microwave frequency range are presented. The measurements are evaluated quantitatively and the electron drift mobility is determined. It is shown that the influence of the surface on the observed decay behaviour can be neglected. A relation between the long time decay behaviour and the position of the Fermi level is observed.     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.     

X-ray photoelectron spectroscopy study of GaAs (001) surface thermocleaning prior to molecular beam epitaxy

X-ray photoelectron spectroscopy has been performed in order to investigate the encapsulent effect of an oxide layer, grown at the end of the chemical etching procedure, on the annealing behaviour of GaAs (001) substrates prior to M.B.E. growth. The oxide layer makes the surface unable to react with arsenic molecules until the desorption of the gallium oxide around 570 °C, preventing the surface against arsenic desorption below this temperature. Otherwise, if unoxidized substrates are used, annealing must be performed under an arsenic flux from 150 °C upwards to avoid the formation of an arsenic depleted surface.     

Microwave detected transient photoconductivity measurements during plasma deposition of intrinsic hydrogenated amorphous silicon

Contactless in-situ measurements of the microwave detected transient photoconductivity during growth of intrinsic a-Si:H films by plasma enhanced chemical vapour deposition are presented. It is shown, that these measurements can be performed without perturbation of the deposition process. The growth of a-Si:H films at 250° C and 120° C substrate temperature is studied and the information obtained from these measurements is discussed. In-situ characterization during growth of a multilayer structure with films deposited subsequently at 120° C, 250° C and again at 120° C is shown.     

Thermal stability of CdZnO thin films grown by molecular-beam epitaxy

CdZnO thin films with near-band-edge (NBE) photoluminescence (PL) emission from 2.39 eV to 2.74 eV were grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates with 800 °C in situ annealing. CdZnO thin films evolve from pure wurtzite (wz) structure, to mixture of wz and rock-salt (rs) structures confirmed by X-ray diffraction studies. Rapid-thermo-annealing (RTA) was performed on in situ annealed CdZnO samples. Pure wz CdZnO shows insignificant NBE PL peak shift after RTA, while mixture structure CdZnO shows evident blue shifts due to phase change after annealing, indicating the rs phase CdZnO changes to wz phase CdZnO during RTA process.     

Surface morphology of highly mismatched InSb films grown on GaAs substrates by molecular beam epitaxy

InSb films on GaAs(001) substrates with and without GaAs buffer layer have been grown by molecular beam epitaxy. Rather than surface undulations, aligned ripples and pyramidal hillocks along the orthogonal 〈110〉 directions were observed on the surface of InSb films. Both the preferential growth and the termination of ripples were proved to be related to strain‐driven mass transport. A model was proposed to elucidate the formation of the hillocks, which are more efficient to relax strain than ripples. Due to the strain relaxation through hillocks with small bases predominantly, the surfaces of the InSb films grown without a GaAs buffer layer are smoother than those of films grown with a GaAs buffer layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Oxygen and monatomic hydrogen interactions with PbTe film surfaces prepared by molecular-beam deposition

The effects on the electrical properties and surface composition of molecular-beam-deposited PbTe films to exposure by oxygen and monatomic hydrogen were investigated. Three stages of oxidation have been identified, depending on the pressure regime employed. Characteristic differences in the response of each of the three oxides to thermal annealing, electron irradiation during Auger analysis and to monatomic hydrogen exposure were observed. The kinetics of n-type doping by monatomic hydrogen, and its reversibility under ultra-high vacuum and oxygen-exposure conditions are presented.