The effects of arsenic source contamination on doped GaAs grown by MBE

The presence of parts per million (ppm) levels of impurities in the low-temperature arsenic charge used in molecular beam epitaxy (MBE) is shown to have a dramatic effect on the quality of GaAs grown. Source charge impurities play a vital role in determining the apparent variation in reported doping behaviour in MBE:GaAs.     

Measurements of residual stress in semi-insulating GaAs by Cr-related luminescence lines

We have measured systematically the Cr-related zero-phonon lines in the 0.839 eV region in a series of plastically-bent semi-insulating GaAs:Cr with compressive or tensile stress along various bending axes. As a result, it has been found that the residual stress in semi-insulating GaAs:Cr wafers can be sensitively characterized from a splitting and energy shift of the 0.839 eV Cr-related luminescence lines in the low-temperature photoluminescence spectra. Furthermore, we have applied this method to the characterization of the interface stress of OMVPE-grown ZnSe/GaAs:Cr heterostructure and found that anomalous stress exists at the ZnSe/GaAs interface, which is inconsistent with stress predicted by the lattice mismatch of the heterojunctions.     

Dislocation-related photoluminescence in silicon

Photoluminescence is studied in silicon, deformed in a well-defined and reproducible way. Usual deformation conditions (high temperature, low stress) result in sharp spectra of the D1 through D4 lines as recently described in the literature. New lines D5 and D6 emerge for predeformation as above and subsequent low-temperature, high-stress deformation. Another new sharp line, D12, is observed when both the familiar and the novel lines appear simultaneously. Annealing for 1 h atT _A 300 °C causes all new lines to disappear and the D1–D4 spectra to reappear. Quantitative annealing and TEM micrographs suggest that D5 is related to straight dislocations and D6 to stacking faults, whereas D1–D4 are due to relaxed dislocations. Photoluminescence under uniaxial stress shows that D1/D2 originate in tetragonal defects with random orientation relative to 100 directions, whereas D6 stems from triclinic centers, preferentially oriented — as are the D3/D4 centers. We conclude that the D3/D4 and the D5 and D6 defects are closely related, whereas the independent D1/D2 centers might be deformation-produced point defects in the strain region of dislocations.     

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.     

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.     

Influence of pH value on the quality of sol-gel derived ZnO films

In sol-gel derived ZnO films the pH value of the sol plays an important role in controlling their properties. In this study, the influence of adding monoethanolamine to zinc acetate solution in methanol on structural, surface morphology and optical properties has been investigated. Addition of monoethanolamine to zinc acetate solution transforms the nature of the sol from acidic to alkaline by changing the pH value from 6.4 to 10.6. The investigations indicate that high quality ZnO films are obtained by using sol having monoethanolamine to zinc acetate ratio of 1:1 and pH value of 10.6.     

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.     

Effects of sic buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates

ZnO films have been grown by a sol-gel process on Si (1 1 1) substrates with and without SiC buffer layers. The influence of SiC buffer layer on the optical properties of ZnO films grown on Si (1 1 1) substrates was investigated. The intensity of the E₂ (high) phonon peak in the micro-Raman spectrum of ZnO film with the SiC buffer layer is stronger than that of the sample without the SiC buffer layer, and the breadth of E₂ (high) phonon peak of ZnO film with the SiC buffer layer is narrower than that of the sample without the SiC buffer layer. These results indicated that the crystalline quality of the sample with the SiC buffer layer is better than that of the sample without the SiC buffer layer. In photoluminescence spectra, the intensity of free exciton emission from ZnO films with the SiC buffer was much stronger than that from ZnO film without the SiC buffer layer, while the intensity of deep level emission from sample with the SiC buffer layer was about half of that of sample without the SiC buffer layer. The results indicate the SiC buffer layer improves optical qualities of ZnO films on Si (1 1 1) substrates.     

Absence of dislocation-induced luminescence in GaAs

Photoluminescence measurements on highly deformed GaAs samples in the near band edge region as well as in the region far below the bandgap are reported. The radiative recombination gives no evidence for dislocation-induced states in the energy gap.     

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As₂) vs tetrameric (As₄) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As₂ and As₄ sources, respectively, and from a single source providing an adjustable As₂/As₄ flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As₄/As₂ flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As₂ molecular beam species which provide a higher steady-state arsenic surface population.     

Auger and photoluminescence analysis of ZnO nanowires grown on AlN thin film

ZnO nanowires were grown on AlN thin film deposited on the glass substrates using a physical vapor deposition method in a conventional tube furnace without introducing any catalysts. The temperature of the substrates was maintained between 500 and 600 °C during the growth process. The typical average diameters of the obtained nanowires on substrate at 600 and 500 °C were about 57 and 22 nm respectively with several micrometers in length. X-ray diffraction and Auger spectroscopy results showed Al diffused from AlN thin film into the ZnO nanowires for the sample grown at 600 °C. Photoluminescence of the nanowires exhibits appearance of two emission bands, one related to ultraviolet emission with a strong peak at 380-382 nm, and the other related to deep level emission with a weak peak at 503-505 nm. The ultraviolet peak of the nanowires grown at 500 °C was blue shifted by 2 nm compared to those grown at 600 °C. This shift could be attributed to surface effect.     

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.     

Acceptor levels in indium selenide. An investigation by means of the Hall effect,deep-level-transient spectroscopy and photoluminescence

Acceptor levels related to I, II, IV, and V group impurities in indium selenide are studied by means of the Hall effect, deep-level-transient spectroscopy (DLTS) and photoluminescence. Activation energies for hole concentrations in the range from 200 to 300 meV have been measured. A reversible change of sign of the Hall voltage has been observed below 215 K. This behaviour can be explained through a model in which acceptor levels are assumed to be shallow and interlayer planar precipitates of ionized shallow donors create potential wells that behave as deep donors and in which a low concentration of bidimensional free electrons can exist. This model also explains the capacitance-voltage characteristics of both ITO/p-InSe and Au/p-InSe barriers. DLTS results are coherent with this model: hole traps in high concentration located about 570 meV above the valence band are detected. Photoluminescence also confirms the shallow character of acceptor levels. A broad band whose intensity is related to p conductivity appears in the PL spectra of low resistivity p-InSe. The shape and temperature dependence of this band can be explained through self-activated photoluminescence in a complex center in which the ground acceptor level must be at about 50 meV above the valence band.     

As-doped p-type ZnO films grown on SiO2/Si by radio frequency magnetron sputtering

p-Type ZnO:As films with a hole concentration of 10¹⁶-10¹⁷ cm⁻³ and a mobility of 1.32-6.08 cm²/V s have been deposited on SiO₂/Si substrates by magnetron sputtering. XRD, SEM, Hall measurements are used to investigate the structural and electrical properties of the films. A p-n homojunction comprising an undoped ZnO layer and a ZnO:As layer exhibits a typical rectifying behavior. Our study demonstrates a simple method to fabricate reproducible p-type ZnO film on the SiO₂/Si substrate for the development of ZnO-based optoelectronic devices on Si-based substrates.     

Donor-acceptor pair luminescence in nitrogen-doped ZnO films grown on lattice-matched ScAlMgO4 (0001) substrates

We have grown nitrogen-doped ZnO (ZnO:N) films by laser molecular-beam epitaxy. The use of lattice-matched ScAlMgO₄ substrates prevented the degradation of crystallinity induced by the nitrogen incorporation to the films. Despite this improvement, we have not obtained ZnO:N films which showed p-type conductivity. We studied the optical properties of these ZnO:N films. Donor-acceptor pair (DAP) luminescence was observed. The results indicate the formation of an acceptor state. The energy position of the DAP luminescence is lower than that reported by Look et al. [Appl. Phys. Lett. 81 (2002) 1830]. The DAP luminescence band shifts to lower energy with increasing nitrogen concentration. A photoluminescence recombination possibly due to the free-electron-to-acceptor (FA) transition was observed at temperatures higher than 40 K. The acceptor ionization energy was estimated from the energy position of the FA luminescence to be 266 meV.     

Luminescence properties of ZnSe films grown by hot wall epitaxy

Received: 10 June 1996/Accepted: 20 October 1996     

Population inversion between the H4 and the F4 excited states of Tm investigated by means of numerical solutions of the rate equations system in Tm-doped and Tm, Ho-codoped fluoride glasses

Population inversion between the ³H₄ and the ³F₄ excited states of Tm³⁺ ions responsible for the 1.5 μm emission in Tm³⁺ singly doped (0.5%) and Tm³⁺, Ho³⁺-codoped fluoride (ZBLAN) glasses and its dependence on the Ho³⁺ concentration (x=0.2-1%) was investigated by means of numerical solution of the rate equations system for continuous pumping at 797 nm. Mean lifetimes of donor and acceptor states were evaluated by using the integration method applied to the best fitting of fluorescence curves previously reported. Lifetime values were used to obtain the rate constants of all non-radiative energy-transfer processes involved and a complete set of rate equations better describing the observations was given. The rate equations were solved by numerical method and the population inversion between the ³H₄ and the ³F₄ excited states of Tm³⁺ was calculated to examine the beneficial effects on the gain associated with Ho³⁺ codoping. The results have shown that Tm³⁺ population inversion is reached only for high Ho³⁺-codoping (?0.3 mol%). Highest population inversion (∼1.6×10¹⁸ Tm³⁺ ions cm⁻³) was obtained in Tm(0.5%), Ho(1%)-codoped (ZBLAN) pumped by 2.8 kW cm⁻². This population inversion density is ∼6.4 times higher than that one observed in Tm:Tb:GLKZ, Tm:Tb:Ge-Ga-As-S-CsBr and Tm:Ho:Ge-Ga-As-S-CsBr for a similar pumping condition (∼2.5×10¹⁷ cm⁻³). In addition, Tm(0.5%):Ho(1%):ZBLAN presents the highest population inversion that linearly increases with the pumping intensity; this behavior does not show saturation effect at least for the maximum intensity of 12 kW cm⁻² employed. The use of 1 mol% of Ho³⁺-codoping maximizes the potential gain of Tm³⁺-doped (0.5%) ZBLAN to produce stimulated emission near 1.5 μm, making this material suitable for using it as fiber optical amplifier and/or fiber laser operating in 1.4-1.5 μm region of the spectrum.     

Stoichiometry effects on surface properties of GaAs{100} grown in situ by MBE

In this paper, we report results dealing with the effects of stoichiometry on surface properties of GaAs(001) layers grown by MBE. Three aspects of surface properties were investigated: crystallography, electronic properties and chemical reactivity. Surface crystallography was studied mainly by LEED. The reconstruction of the surface was found to be drastically dependent on the composition of the uppermost atomic layer, i.e. the surface stoichiometry. According to the arsenic surface coverage, many structures from the c(8 × 2) Ga rich to the (1 × 1) arsenic saturated surface have been observed. The influence of stoichiometry on surface electronic properties has been studied by electron loss spectroscopy (ELS) and contact potential difference (CPD) measurements. In the electron loss spectra, two peaks, at about 10.3 and 20.2 eV are very sensitive to the surface composition: they gradually disappear when the arsenic coverage increases, and consequently are associated with surface states on gallium atoms. On the other hand, the CPD measurements have shown that the variation of the work function with the arsenic surface coverage is not monotonic: in particular, an abrupt change of work function of about 300 meV occurs between the (1 × 6) and c(2 × 8) structures which are very similar as far as the arsenic surface coverage (about 0,5 and 0,6 respectively) is concerned. Therefore, it seems that the work function is strongly dependent on the atomic reconstruction occurring at the surface, and not only on its stoichiometry. The connection between stoichiometry and chemical reactivity of the surface is illustrated by the study of H₂S adsorption: a large difference (factor of 10³) in sticking coefficient has been found between surfaces with different arsenic coverages.     

Hydrostatic-pressure-induced degradation of MBE CdTe/CdMgTe heterostructures grown on GaAs substrate

Abstract

Modulation doped CdTe/Cd_1?xMg_xTe heterostructures grown on GaAs substrates were studied by means of magnetotransport measurements performed under hydrostatic pressure, as well as X-ray diffraction and cross-sectional transmission electron microscopy completed before and after pressure experiments. We have shown that hydrostatic pressure leads to the creation of dislocations in the CdTe/Cd_1?xMg_xTe structure in the vicinity of the interface between the II-VI structure and the substrate. The dislocation-enhanced internal stress leads to internal microfractures, resulting in a permanent damage of the heterostructure.     

Fabrication of BDD quantum node switches on embedded GaAs quantum wires grown by selective MBE

In view of applications to hexagonal binary decision diagram (BDD) LSIs, a first attempt is made to form quantum BDD node switches on selectively grown (SG) embedded quantum wires (QWRs) by molecular beam epitaxy (MBE). SG branch switches controlled by a Schottky wrap gate (WPG) were successfully fabricated by MBE growth and subsequent device processing. Gate control characteristics were studied by gate-dependent Shubnikov–de-Haas measurements, and the behavior was found to be similar to that of devices fabricated on wires by etching. The switch exhibited clear conductance quantization at low temperature, and temperature dependence of the voltage slope of conductance jump was clarified. A Y-branch BDD node device using two SG branch switches was successfully fabricated, and realized clear path switching characteristics.