Blue-green laser diode grown by photo-assisted MOCVD

Operation of the first blue-green laser diode grown by metalorganic chemical vapor deposition has been demonstrated at 77 K under pulsed current injection. The precursors were dimethylzinc, dimethylcadmium, diethylsulfide, bismethyl-cyclopentadienyl-magnesium, and dimethylselenide. Diisopropylamine and ethyliodide were used for a p-type and n-type doping under irradiation with ultraviolet light generated by a high-pressure mercury lamp, respectively. A 1 × 10¹⁸ cm⁻³ nitrogen-atom concentration, which was measured by secondary ion mass spectroscopy, was obtained in the p-ZnSe contact layer. The 4.2 K photoluminescence spectrum was dominated by strong donor-acceptor pair emission and the net acceptor concentration was 1.4 × 10¹⁶ cm⁻³.     

Nitrogen as carrier gas for the growth of ZnSe and ZnSe:N in plasma enhanced metalorganic vapor phase epitaxy

For the precursor combination dimethylzinc-triethylamine and ditertiarybutylselenide the use of nitrogen carrier gas was investigated for the metalorganic vapor phase epitaxy (MOVPE) of ZnSe and ZnSe:N. The nitrogen doping was carried out with a separate nitrogen flow which was activated by a dc-plasma. In the photoluminescence spectra of undoped layers grown at 340°C with a VI/II ratio of 0.18 only excitonic emissions, separated into free and donor bound excitons, can be observed. The background carrier concentration was of the order of 1 × 10¹⁶ cm⁻³. The growth rate of epilayers grown in nitrogen is reduced by about 75% in comparison to the value obtained by using hydrogen as carrier gas. The doping can be regulated by the dopant flow and by the total pressure in the reactor. With increasing flow of plasma activated nitrogen and a reduced total pressure, the PL spectra showed broadened DAP emission without excitonic emissions. The electrical and optical properties obtained with nitrogen carrier gas are comparable with the results obtained with hydrogen carrier gas. So far, no p-type conductivity was measured. Therefore, the problem of compensation of p-type conductivity of ZnSe : N doped by dc-plasma enhanced N₂ was not solved by the use of N₂ carrier gas.     

Boron-doped a-SiOx:H films prepared by photo-CVD technique

The optoelectronic and structural properties of p-type a-SiO_x:H films have been studied. The deposition parameters e.g. chamber pressure and diborane to silane ratio are optimized to get a film with dark conductivity (σ_d) 7.9×10⁻⁶ S cm⁻¹ and photoconductivity 9.3×10⁻⁶ S cm⁻¹ for an optical gap (E₀₄) of 1.94 eV. The decrease of optical gap accompanied by the increase of conductivity is due to less oxygen incorporation in the film, which is substantiated by the decrease of the intensity of SiO absorption spectra. The properties are very much effected by the chamber pressure and diborane to silane ratio.     

Optical properties of hydrogenated amorphous silicon films doped with fluorinated gases

Doped amorphous silicon films were prepared by plasma-enhanced chemical vapour deposition of silane and hydrogen mixtures, using phosphorus pentafluoride (PF₅) and boron trifluoride (BF₃) as dopant precursors. The films were studied by UV-vis spectroscopy and their photo and dark conductivity were measured, the latter as a function of temperature. The optical gap of the n-type samples, doped with PF₅, diminished as the concentration of this gas in the plasma was increased. However, the optical gap of p-type samples, doped with BF₃, did not show any appreciable optical gap decrease as the concentration of BF₃ was varied from 0.04% to 4.7%. The dark conductivity of the p-type films at these extremes of the doping range were 7.6 × 10⁻¹⁰ and 3.5 × 10⁻¹ Ω⁻¹ cm⁻¹, respectively.     

Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Heavily carbon-doped p-type In_xGa_1−xAs (0≤x<0.49) was successfully grown by gas-source molecular beam epitaxy using diiodomethane (CH₂I₂), triethylindium (TEIn), triethylgallium (TEGa) and AsH₃. Hole concentrations as high as 2.1×10²⁰ cm⁻³ were achieved in GaAs at an electrical activation efficiency of 100%. For In_xGa_1−xAs, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x, increased from 0.41 to 0.49. Hole concentrations of 5.1×10¹⁸ and 1.5×10¹⁹ cm⁻³ for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As₄ pressure), the hole concentration increased to 6.2×10¹⁸ cm⁻³ for x = 0.49, probably due to the activation of hydrogen-passivated carbon accepters.     

Growth of nitrogen-doped p-type ZnO films by spray pyrolysis and their electrical and optical properties

Nitrogen-doped ZnO films were deposited on silicon (1 0 0) substrate using zinc acetate and ammonium acetate aqueous solution as precursors by ultrasonic spray pyrolysis. Successful p-type doping can be realized at optimized substrate temperature. The p-type ZnO films show excellent electrical properties such as hole concentration of 10¹⁸ cm⁻³, hole mobility of 10² cm² V⁻¹ s⁻¹ and resistivity of 10⁻² Ω cm. In the photoluminescence measurement, a strong near-band-edge emission was observed, while the deep-level emission was almost undetectable in both undoped and N-doped ZnO films. The growth and doping mechanism of N-doped ZnO films were discussed.     

A study of the chemical reactions involved in Li–Ca–N system

The chemical reactions and phases involved in the potential flux system of Li–Ca–N for the growth of bulk GaN crystals have been investigated under varying conditions. It is found that no preferential nitrification of Li or Ca by N₂ in Li–Ca melts at 500 °C. Only the ternary compound LiCaN is identified in the Li–Ca–N system under the present experimental conditions. Static N₂ pressures are found to enhance the formation of LiCaN compared to an N₂ stream. LiCaN forms from two possible pathways: one is a modified metathesis chemical reaction represented by Li₃N+Ca→LiCaN+2Li, and the other is a combination chemical reaction represented by Li₃N+Ca₃N₂→3LiCaN. The formation of LiCaN by the metathesis reaction is thermodynamically favored over the other pathway. In addition, the formation of LiCaN might benefit from a slightly larger initial amount of Li₃N compared with Ca or Ca₃N₂.     

Viscosity and IR investigations in the Li2O---B2O3 system

The high viscosity in melts of the Li₂O---B₂O₃ system makes it very difficult to grow large crystals of lithium triborate. The viscosity and IR characteristics of molten li₂O---B₂O₃ system are reported in this paper. When the temperature increases the viscosity of li₂O---B₂O₃ system decreases and follows an Arrhenius-type relationship. With an increasing 13203 ratio in Li₂O---B₂O₃ melts, the viscosity rises gradually to a maximum with a composition Li₂O: 3.513203 then it falls rapidly. In order to find active agents to reduce the viscosity, Na₂O, NaCl, LiF, P205, M003, W03 etc oxides were added to Li₂O---B₂O₃ samples respectively and investigated using the orthogonal method. The experimental results show that the addition of acidic oxides can significantly decrease the viscosity in the Li₂O---B₂O₃ system. For Li₂O: 4.513203, an ideal additive agent is 20wt% Li₂O:: 2MoO₃. Near the composition for crystal growth, the percentage reduction of viscosity is 62.2%. The IR spectra of Li₂O---B₂O₃ system revealed that the BO₄⁻/NO₃⁻ ratio is reduced in the melt using Li₂O: 2MoO₃ as an additive. It is proposed that the M003 reduced the concentration of bridging oxygen atoms of BO₄⁻. The change of structure explains the decline in the viscosity. In the crystal structures of lithium triborate, the matrix spaces are so small that larger other cations than Li+ are very difficult to enter the crystal matrix. So the use of additive agents to reduce the viscosity is a possible method if no new phase appears.     

On the mechanism of electrical relaxation in Na-conductive phosphate glasses

The crossover from a frequency independent to a frequency dependent ac response has been studied in glasses with the composition 37.2Na₂O-12.8CaO₅ · 50P₂O₅ and 30Na₂O-5CaO-7.5Al₂O₃-57.5P₂O₅ (mol%) containing 5 × 10⁻³ ≤ mol% Ag₂O ≤ 5 × 10⁻¹. Recently, we have established that in these glasses the diffusion coefficient of guest silver ions varies in space. In this case, as a first approximation, the diffusion coefficient may be considered as a constant within regions whose size is no less than 10 nm across. We assume that the diffusion of sodium ions can be given by the D(r) coefficient with the same spatial dispersion as that of silver ions. It is demonstrated that the frequency dependence of ac response is in fair agreement with the assumption.     

SiC epitaxial layer growth in a novel multi-wafer vapor-phase epitaxial (VPE) reactor

Experimental results are presented for SiC epitaxial layer growths employing a unique planetary SiC-VPE reactor. The high-throughput, multi-wafer (7×2″) reactor, was designed for atmospheric and reduced pressure operation at temperatures up to and exceeding 1600°C. Specular epitaxial layers have been grown in the reactor at growth rates ranging from 3–5 μm/h. The thickest layer grown to date is 42 μm thick. The layers exhibit minimum unintentional n-type doping of 1×10¹⁵ cm⁻³, and room temperature mobilities of 1000 cm²/V s. Intentional n-type doping from 5×10¹⁵ cm⁻³ to >1×10¹⁹ cm⁻³ has been achieved. Intrawafer layer thickness and doping uniformities (standard deviation/mean at 1×10¹⁶ cm⁻³) are typically 4 and 7%, respectively, on 35 mm diameter substrates. Moderately doped, 4×10¹⁷ cm⁻³, layers, exhibit 3% doping uniformity. Recently, 3% thickness and 10% doping uniformity (at 1×10¹⁶ cm⁻³) has been demonstrated on 50 mm substrates. Within a run, wafer-to-wafer thickness deviation averages 9%. Doping variation, initially ranging as much as a factor of two from the highest to the lowest doped wafer, has been reduced to 13% at 1×10¹⁶ cm⁻³, by reducing susceptor temperature nonuniformity and eliminating exposed susceptor graphite. Ongoing developments intended to further improve layer uniformity and run-to-run reproducibility, are also presented.     

Flux growth of BPO4 crystals

Single crystals of BPO₄ with sizes up to 15×10×12 mm³ were grown by top-seeded solution growth method using Li₂O–Li₄P₂O₇ as fluxes. The components volatilized from the melt were characterized by the method of X-ray powder diffraction. The defects of grown crystals have also been investigated. The measured ultraviolet cutoff edge of BPO₄ was about 130 nm. Its density was 2.82 g/cm³ determined using drainage method.     

p-Type GaAs doped by diiodomethane (CI2H2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI₂H₂) as a carbon source. In the low 10¹⁹ cm⁻³ range, almost all carbon atoms are electrically activated and at 9×10¹⁹ cm⁻³, 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.     

Efficiency optimization of p-type doping in GaN:Mg layers grown by molecular-beam epitaxy

The Mg-doping efficiency in GaN layers grown by molecular-beam epitaxy has been studied as a function of the growth temperature, the growth rate, and the Mg beam flux. The Mg cell temperature window for efficient p-type doping is rather narrow, being limited by the GaN n-type background doping density (lower limit) and by the Mg surface coverage that, beyond a threshold, induces a layer polarity inversion (N-polarity), leading to a reduction of the Mg incorporation (upper limit). An increase of the growth temperature avoids this polarity inversion, but the Mg flux must be increased to compensate the strong desorption rate. Thus, a trade-off between both temperatures has to be reached. A reduction of the growth rate has a strong effect on the p-type doping level, yielding up to 7×10¹⁷ holes/cm³ for a total Mg concentration of 1×10¹⁹ cm⁻³. This high Mg concentration does not seem to generate Mg-related defects or deep traps.     

Growth and spectral properties of Nd:LaVO4 crystal

This paper reports the growth and spectral properties of 3.5 at% Nd³⁺:LaVO₄ crystal with diameter of 20×15 mm² which has been grown by the Czochralski method. The spectral parameters were calculated based on Judd–Ofelt theory. The intensity parameters Ω_λ are: Ω₂=2.102×10⁻²⁰ cm², Ω₄=3.871×10⁻²⁰ cm², Ω₆=3.235×10⁻²⁰ cm². The radiative lifetime τ_r is 209 μs and calculated fluorescence branch ratios are: β₁(0.88μm)=45.2, β₂(1.06μm)=46.7, β₃(1.34μm)=8.1. The measured fluorescence lifetime τ_f is 137 μm and the quantum efficiency η is 65.6%. The absorption band at 808 nm wavelength has an FWHM of 20 nm. The absorption and emission cross sections are 3×10⁻²⁰ and 6.13×10⁻²⁰ cm², respectively.     

Multiple-step annealing for 50% enhanced p-conductivity of GaN

In this article, multiple-step rapid thermal annealing (RTA) processes for the activation of Mg doped GaN are compared with conventional single-step RTA processes. The investigated multiple-step processes consist of a low temperature annealing step at temperatures between 350°C and 700°C with dwell times up to 5 min and a short time high temperature step. With optimized process parameters, and multiple-step processes, we achieved p-type free carrier concentrations up to 1–2×10¹⁸ cm⁻³. The best achieved conductivity, so far, lies at 1.2 Ω⁻¹ cm⁻¹. This is a 50% improvement compared to conventional single-step process at 800°C, 10 min.     

Chemical strengthening of Na2O---Al2O3---SiO2 system glasses by surface controlled crystallization

Appropriate glass compositions of the Na₂O---Al₂O₃---SiO₂ system and glass melting technology were investigated. The dependence of some factors, such as composition of molten salts, time and temperature of ion exchange treatment on the modulus of rupture was studied. As expected, the coefficient of interdiffusion is a function of concentration and has the mixed-alkali effect. The order of the interdiffusion coefficient is 10⁻⁶ cm²/s. The effect of binary and ternary salt baths on the surface controlled crystallization was reported. The experimental data showed that there is a series of technological advantages from applying molten salts of the ternary system Li₂SO₄---Na₂SO₄---K₂SO₄ in comparison with the binary system Li₂SO₄---Na₂SO₄.     

Determination of diffusion coefficient of titanium ion in TiO2---SiO2 wet gel prepared from metal alkoxides during leaching

It was found that the radial gradient refractive index (r-GRIN) glass rod could be produced by the leaching of wet gel prepared by a sol-gel method from metal alkoxides and heat-treatment of the gel. The concentration gradient of the doped component resulting from its diffusion in the wet gel was studied. In order to estimate the exact diffusion coefficient of the titanium ion in the binary TiO₂---SiO₂ wet gel, its value was measured by using two methods. The observed diffusion coefficient by colorimetric analysis was about 1.1 × 10⁻⁶ cm²/s, while the value obtained from analysis of EPMA was about 1.2 × 10⁻⁶ cm²/s, and both values were nearly equal to each other. The activation energy of titanium ion diffusion in the wet gel was estimated to be about 11 kcal/mol.     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na₂O · xMgO · (15−x)CaO · yAl₂O₃ · (80−y)SiO₂ with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe₂O₃ and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe³⁺ to Fe²⁺, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na⁺ and Ca²⁺ on FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.     

Effects of ion implantation on intermediate range order: IR spectra of silica

The reflectance spectra of ion implanted SiO₂ glasses has been measured from 5000 cm⁻¹ to 400 cm⁻¹. The silica was implanted with Ti, Cr, Mn, Fe, Cu and Bi to nominal doses ranging from 1×10¹⁵ ions/cm² to 1.2×10¹⁷ ions/cm² at an energy of 160 keV and currents of approximately 2.6 μA/cm². Changes in the intensity of the 1232 cm⁻¹ and 1015 cm⁻¹ vibrational modes are attributed to changes in the intermediate range order (IRO) and to changes in the concentration of non-bridging oxygen (NBO) defects in the implanted layer. These changes are ion and dose dependent. The differing effects on IRO and NBO are attributed to the chemical interaction of the implanted ions with the substrate.     

Influence of oxygen contamination during Si low presure vapour phase epitaxy on epitaxial layer quality

The influence of oxygen contamination on Si low pressure vapour phase epitaxy (LPVPE) at 800°C in the SiCl₂H₂−H₂ system has been investigated. O₂ was added intentionally with partial pressures between 10⁻⁸ and 2×10⁻⁴ mbar. The quality of the epitaxially grown silicon layers was determined by comparing surface morphology, defect density, Schottky diode characteristics and SIMS measurements. These four parameters are correlated and they reveal a drastic decrease in epitaxial layer quality for O₂ pressures above 15×10⁻⁶ mbar. The critical oxygen pressure which has been until now considered as a limit for epitaxial growth can therefore be exceeded by one order of magnitude.