Structural properties of ZnSy Se1−yZnSe/GaAs (001) heterostructures grown by photoassisted metalorganic vapor phase epitaxy

ZnS_ySe_1−yZnSe/GaAs (001) heterostructures have been grown by photoassisted metalorganic vapor phase epitaxy, using the sources dimethylzinc, dimethylselenium, diethylsulfur, and irradiation by a Hg arc lamp. The solid phase composition vs gas phase composition characteristics have been determined for ZnSy_ySe_1−y grown with different mole fractions of dimethylselenium and different temperatures. Although the growth is not mass-transport controlled with respect to the column VI precursors, the solid phase composition vs gas phase composition characteristics are sufficiently gradual so that good compositional control and lattice matching to GaAs substrates can be readily achieved by photoassisted growth in the temperature range 360°C ≤ T ≤ 400°C. ZnSe/GaAs (001) single heterostructures were grown by a two-step process with ZnSe thicknesses in the range from 54 nm to 776 nm. Based on 004 x-ray rocking curve full width at half maximums (FWHMs), we have determined that the critical layer thickness is h_c ≤200 nm. Using the classical method involving strain, lattice relaxation is undetectable in layers thinner than 270 nm for the growth conditions used here. Therefore, the rocking curve FWHM is a more sensitive indicator of lattice relaxation than the residual strain. For ZnS_ySe_1−y layers grown on ZnSe buffers at 400°C, the measured dislocation density-thickness product Dh increases monotonically with the room temperature mismatch. Lower values of the Dh product are obtained for epitaxy on 135 nm buffers compared to the case of 270 nm buffers. This difference is due to the fact that the 135 nm ZnSe buffers are pseudomorphic as deposited. For ZnS_ySe_1−y layers grown on 135 nm ZnSe buffers at 360°C, the minimum dislocation density corresponds approximately to room-temperature lattice matching (y ∼ 5.9%), rather than growth temperature lattice matching (y ∼ 7.6%). Epitaxial layers with lower dislocation densities demonstrated superior optical quality, as judged by the near-band edge/deep level emission peak intensity ratio and the near band edge absolute peak intensity from 300K photoluminescence measurements.     

MOVPE of ZnMgSSe heterostructures for optically pumped blue-green lasers

We report on the growth of ZnMgSSe/ZnSSe/ZnSe heterostructures in a low pressure metalorganic vapor phase epitaxy (MOVPE) system at 400 hPa and a growth temperature of 330°C. The precursor combination was dimethylzinc(triethylamine adduct), ditertiarybutylselenium, ditertiarybutylsulphur, and bismethylcyclopentadienylmagnesium. This combination allows the reproducible adjustment of the alloy composition in a wide range (currently up to 40% S and 32% Mg) maintaining high crystal homogeneity and almost lattice matched growth. Undoped separate confinement heterostructure (SCH) lasers with ZnMgSSe cladding and ZnSSe guiding layers were deposited on GaAs substrates. X-ray diffraction (reciprocal space mapping), photoluminescence (PL) at 14–300K, PL excitation, and optical pumping experiments were performed. The quantum wells show a high luminescence efficiency up to room temperature. Optical pumping experiments were carried out at various temperatures (77, 300–375K) and excitation densities using a nitrogen laser. The lasing threshold could be determined to be less than 20 kW/cm² at 77K, and even room temperature lasing was observed at an excitation density which was below 200 kW/cm².     

Growth method,composition, and defect structure dependence of mercury diffusion in CdxHg1–xTe

Mercury radiotracer diffusion results are presented, in the range 254 to 452°C, for bulk and epitaxial Cd_xHg_1–xTe, and we believe this to be the first report for metalorganic vapor phase epitaxy (MOVPE) grown Cd_xHg_1–xTe. For all growth types studied, with compositions of x_Cd=0.2±0.04, the variation of the lattice diffusion coefficient, D_Hg, with temperature, under saturated mercury partial pressure, obeyed the equation: D_Hg=3×10⁻³ exp(−1.2 eV/kT) cm² s⁻¹. It was found to have a strong composition dependence but was insensitive to changes of substrate material or crystal orientation. Autoradiography was used to show that mercury also exploited defect structure to diffuse rapidly from the surface. Dislocation diffusion analysis is used to model defect tails in MOVPE Cd_xHg_1–xTe profiles.     

Effect of annealing temperature on 1.5 μm photoluminescence from Er-lmplanted 6H-SiC

The effect of post-implantation anneal on erbium-doped 6H-SiC has been investigated. 6H-SiC has been implanted with 330 keV Er⁺ at a dose of 1 × 1013 /cm2. Er depth profiles were obtained by secondary ion mass spectrometry (SIMS). The as-implanted Er-profile had a peak concentration of∼1.3 × 10¹⁸/cm³ at a depth of 770Å. The samples were annealed in Ar at temperatures from 1200 to 1900°C. The photoluminescence intensity integrated over the 1.5 to 1.6 μm region is essentially independent of annealing temperature from 1400 to 1900°C. Reduced, but still significant PL intensity, was measured from the sample annealed at 1200°C. The approximate diffusivity of Er in 6H SiC was calculated from the SIMS profiles, yielding values from 4.5 × 10⁻¹⁶ cm²/s at 1200°C to 5.5 × 10⁻¹⁵ cm²/s at 1900°C.     

Low temperature growth of (100) HgCdTe layers with DtBTe in metalorganic vapor phase epitaxy

Growth characteristics of (100) HgCdTe (MCT) layers by MOVPE at low temperature of 275°C were studied using ditertiarybutyltelluride as a tellurium precursor. Growths were conducted in a vertical narrow-spacing growth cell at atmospheric pressure. Cd composition of MCT layers were controlled from 0 to 0.98 using dimethylcadmium (DMCd) flow. The growth rate was constant for increase of DMCd flow. During the growth, Cd was incorporated preferentially into the MCT layers. Enhancement of Cd incorporation in the presence of Hg was also observed. Crystal quality and electrical properties were also evaluated, which showed that high quality MCT layers can be grown at 275°C. Strain in CdTe layers grown at 425 and 275°C was also evaluated. Lattice parameter of layers grown at 425°C approached bulk value at thickness of 5 μm, while layers grown at 275°C relaxed at 1 μm. The rapid strain relaxation of layers grown at 275°C was considered due to the layer growth on the strain relaxed buffer layer. The effect of the thermal stress on the relaxation of CdTe lattice strain was also discussed.     

InxGa1−xAs (x=0.25–0.35) grown at low temperature

In_xGa_1−xAs (x=0.25–0.35) grown at low temperature on GaAs by molecular beam epitaxy is characterized by Hall effect, transmission electron microscopy, and ultrafast optical testing. As with low temperature (LT) GaAs, the resistivity is generally higher after a brief anneal at 600°C. High-resolution transmission electron microscopy shows all the as-grown epilayers to be heavily dislocated due to the large lattice mismatch (2–3%). When the layers are annealed, in addition to the dislocations, precipitates are also generally observed. As with LT-GaAs, the lifetime shortens as growth temperature is reduced through the range 300–120°C; also, the lifetime in LT-In_xGa_1−xAs is generally shorter in as-grown samples relative to annealed samples. Metal-semiconductor-metal photodetectors fabricated on the material exhibit response times of 1–2 picoseconds, comparable to results reported on GaAs grown at low temperature, and the fastest ever reported in the wavelength range of 1.0–1.3 μm.     

Optimization of GaAsP/AlGaAs-based QW laser structures for high power 800 nm operation

We present a detailed study of the MOVPE growth of 800 nm diode laser structures based on the combination of a GaAsP quantum well with well-established AlGaAs waveguide structures. By optimizing the strain and thickness of the quantum well highly-reliable diode lasers with low threshold current and high efficiency were demonstrated. 100 μm aperture “broad area” devices mounted epi-side up achieve a CW output power of 8.9 W with a wall-plug efficiency of 50%. These output powers represent record values for diode lasers in this wavelength range. Reliability measurements at 1.5 W and 50°C ambient temperature suggest lifetimes >10 000 h.     

Multi-wafer growth of highly uniform InGaP/GaAs by low pressure MOVPE

This paper reports on the large area growth of InGaP/GaAs heterostructures for short wavelength applications (λ ∼ 650 nm) by low pressure MOVPE in a vertical, high speed, rotating disk reactor. Highly uniform films were obtained both on a single 50 mm diam wafer at the center of a 5 inch diam wafer platter and on three, 50 mm diameter GaAs wafers symmetrically placed on a 5 inch diam platter. Characterization was performed by x-ray diffraction, SEM, and room temperature photoluminescence (PL) mapping. For the single wafer growth, PL mapping results show that the total range on wavelength was ±2 nm with a 2 mm edge exclusion. The standard deviation of the peak wavelength,σ _w , is 0.7 nm. Thickness uniformity, measured by SEM, is less than 2%. Similar results were obtained for the multi-wafer runs. Each individual wafer has aσ _w of 1.1 nm. The wafers have nearly identical PL maps with the variation of the average wavelength from the three wafers within ±0.1 nm.     

Carbon doped GaAs grown in low pressure-metalorganic vapor phase epitaxy using carbon tetrabromide

Carbon tetrabromide was used as carbon source for heavily p-doped GaAs in low pressure metalorganic vapor phase epitaxy (MOVPE). The efficiency of carbon incorporation was investigated at temperatures between 550 and 670°C, at V/III ratios from 1 to 50 and carbon tetrabromide partial pressures from 0.01 to 0.03 Pa. Hole concentrations from 8 × 10¹⁷ to 5 × 10¹⁹ cm⁻³ in as-grown layers were obtained. After annealing in nitrogen atmosphere at 450°C, a maximum hole concentration of 9 × 10¹⁹ cm⁻³ and a mobility of 87 cm²/Vs was found. At growth temperatures below 600°C, traces of bromine were detected in the layers. Photoluminescence mapping revealed an excellent doping homogeneity. Thus, CBr₄ is found to be a suitable carbon dopant source in MOVPE.     

Growth condition of iodine-doped n+-CdTe layers in metal-organic vapor phase epitaxy

Iodine doping of CdTe layers grown on (100) GaAs by metal-organic vapor phase epitaxy (MOVPE) was studied using diethyltelluride (DETe) and diisopropyltelluride (DiPTe) as tellurium precursors and ethyliodine (EI) as a dopant. Electron densities of doped layers increased gradually with decreasing the growth temperature from 425°C to 325°C. Doped layers grown with DETe had higher electron densities than those grown with DiPTe. When the hot-wall temperature was increased from 200°C to 250°C at the growth temperature of 325°C, doped layers grown with DETe showed an increase of the electron density from 3.7×10¹⁶ cm⁻³ to 2.6×10¹⁸ cm⁻³. On the other hand, such an increase of the electron density was not observed for layers grown with DiPTe. The mechanisms for different doping properties for DETe and DiPTe were studied on the basis of the growth characteristics for these precursors. Higher thermal stability of DETe than that of DiPTe was considered to cause the difference of doping properties. With increasing the hot-wall temperature from 200°C to 250°C, the effective ratio of Cd to Te species on the growth surface became larger for layers grown with DETe than those grown with DiPTe. This was considered to decrease the compensation of doped iodine and to increase the electron density of layers grown with DETe. The effective ratio of Cd to Te species on the growth surface also increased with decreasing growth temperature. This was considered to increase the electron density with decreasing growth temperature.     

Chemical beam epitaxy and laser-modified chemical beam epitaxy of InGaAs using tris-dimethylaminoarsenic

The growth of In_xGaj_1−xAs (x = 0.13–0.25) on GaAs by chemical beam epitaxy (CBE) and laser-modified CBE using trimethylindium (TMIn), triethylgallium (TEGa), and tris-dimethylaminoarsenic (TDMAAs) has been studied. Reflection high-energy electron diffraction measurements were used to investigate the growth behavior of InGaAs at different conditions. X-ray rocking curve and lowtemperature photoluminescence (PL) measurements were used to characterize the InGaAs/GaAs pseudomorphic strained quantum well structures. Good InGaAs/GaAs interface and optical property were obtained by optimizing the growth condition. As determined by the x-ray simulation, laser irradiation during the InGaAs quantum well growth was found to enhance the InGaAs growth rate and reduce the indium composition in the substrate temperature range studied, 440–500°C, where good interfaces can be achieved. These changes, which are believed to be caused by laser-enhanced decomposition of TEGa and laser-enhanced desorption of TDMAAs, were found to depend on the laser power density as well. With laser irradiation, lateral variation of PL exciton peaks was observed, and the PL peaks became narrower.     

The photoassisted MOVPE growth of ZnSSe using tertiary-butylmercaptan

We have conducted a study of the compositional control of epitaxial ZnS_ySe_1-y grown by photoassisted metal organic vapor phase epitaxy (MOVPE) (250 torr, 340°C, UV=14 mW/cm²) on GaAs (100) substrates. We have achieved lattice matched ZnSSe films on GaAs substrates using photoassisted growth using dimethylzinc (DMZn), dimethylselenide (DMSe), and tertiary-butylmercaptan (t-BuSH) as precursors. In addition, we have obtained sulfur compositions (y), ranging from 0.023 to unity (ZnS). The growth rate of the ZnS was 1 μm/h, which was previously unattainable by our group using diethylsulfur. The closely lattice matched sample (y=0.07 as determined by high resolution x-ray diffraction) showed a near band edge peak intensity (NBE) to deep level emission intensity (DLE) ratio of 77 to 1, as determined by room temperature photoluminescence measurements. We have examined the sulfur incorporation as a function of source mole fractions, UV intensity, and growth temperature and found that optimized growth conditions (optimized for range of compositions possible, and NBE/DLE ratio) are X_DMZn=1.5 × 10⁻⁴, X_DMSe=3×10⁻⁴, UV=14 mW/cm², growth temperature=340°C. X_DMZn and X_DMSe are the mole fractions of DMZn and DMSe, respectively. We have found the growth rate to be 1 μm/h for y=0.023 to 0.24 for these optimized conditions. It was found that to achieve sulfur compositions of less than 0.9, the t-BuSH mole fractions had to be kept low. Higher UV intensities increased the incorporation of selenium, while also lowering the material quality (NBE/DLE ratios). We have shown that the optical material qualities of ZnSSe films grown with t-BuSH are much better than ZnSSe films grown with DES.     

High-Dose Phosphorus-Implanted 4H-SiC: Microwave and Conventional Post-Implantation Annealing at Temperatures ≥1700°C

Semi-insulating 4H-SiC ⟨0001⟩ wafers have been phosphorus ion implanted at 500°C to obtain phosphorus box depth profiles with dopant concentration from 5 × 10¹⁹ cm⁻³ to 8 × 10²⁰ cm⁻³. These samples have been annealed by microwave and conventional inductively heated systems in the temperature range 1700°C to 2050°C. Resistivity, Hall electron density, and Hall mobility of the phosphorus-implanted and annealed 4H-SiC layers have been measured in the temperature range from room temperature to 450°C. The high-resolution x-ray diffraction and rocking curve of both virgin and processed 4H-SiC samples have been analyzed to obtain the sample crystal quality up to about 3 μm depth from the wafer surface. For both increasing implanted phosphorus concentration and increasing post-implantation annealing temperature the implanted material resistivity decreases to an asymptotic value of about 1.5 × 10⁻³ Ω cm. Increasing the implanted phosphorus concentration and post-implantation annealing temperature beyond 4 × 10²⁰ cm⁻³ and 2000°C, respectively, does not bring any apparent benefit with respect to the minimum obtainable resistivity. Sheet resistance and sheet electron density increase with increasing measurement temperature. Electron density saturates at 1.5 × 10²⁰ cm⁻³ for implanted phosphorus plateau values ≥4 × 10²⁰ cm⁻³, irrespective of the post-implantation annealing method. Implantation produces an increase of the lattice parameter in the bulk 4H-SiC underneath the phosphorus-implanted layer. Microwave and conventional annealing produce a further increase of the lattice parameter in such a depth region and an equivalent recovered lattice in the phosphorus-implanted layers.     

The effects of the growth temperature on AlxGal-xAs (0≤ x ≤0.37) LED materials grown by OM-VPE

The epitaxial growth of AlGaAs of LED quality by OM-VPE is achieved either by using high growth temperatures (≥780°C) or by using oxygen gettering methods and low growth temperatures (≤750°C). For 6% AlGaAs, the most studied composition in this work, graphite baffles and a molecular sieve, are used at low growth temperatures (≈680°C) to improve both the normalized PL intensity of epi layers and the EL efficiency of LEDs. Growth at high temperature, however, does not require oxygen gettering methods to achieve the same material quality. The improvement in both cases is achieved by reducing the concentration of the oxygen-related defect that is the source of the 0.8 eV peak, which limits the performance of 6% AlGaAs LEDs. Al_xGa_1−xAs alloys with x≥0.06 also show a similar behavior relative to the growth temperature. The defect peak itself remains invariant with x. This 0.8 eV PL peak is likely to be associated with Al, since the reaction between Al and oxygen is strong and the 0.8 eV peak is seldom observed in GaAs epi layers, regardless of the growth temperature.     

MOCVD growth of GaBN on 6H-SiC (0001) substrates

B_xGa_1−xN films were deposited on 6H-SiC (0001) substrates at 1000°C by low pressure MOVPE using diborane, trimethylgallium, and ammonia as precursors. The presence of boron was detected by Auger scanning microprobe, the shift of the (00.2) x-ray diffraction peak, and low-temperature photoluminescence. A single-phase B_xGa_1−xN alloy with x=1.5% was produced at the gas phase B/Ga ratio of 0.005. Phase separation into wurtzite BGaN and the B-rich phase occurred for a B/Ga ratio in the 0.01–0.2 range. Only BN was formed for B/Ga >0.2. The B-rich phase was identified as h-BN with sp² bonding based on the results of Fourier transform infrared spectroscopy. As the diborane flow exceeds the threshold concentration, the growth rate of BGaN decreases sharply, because the growth of GaN is poisoned by the formation of the slow growing BN phase. The bandedge emission of B_xGa_1−xN varies from 3.451 eV for x=0% with FWHM of 39.2 meV to 3.465 eV for x=1.5% with FWHM of 35.1 meV. The narrower FWHM indicates that the quality of GaN epilayer is improved with a small amount of boron incorporation. The PL linewidths become broader as more boron is introduced into the solid solution.     

Growth and characterizations of InP self-assembled quantum dots embedded in InAIP grown on GaAs substrates

We report the characteristics of InP self-assembled quantum dots embedded in In_0.5Al_0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 10⁸ mm⁻². These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.     

Impact of growth rate on the quality of ZNS-MQW InGaAsP/InP laser structures grown by LP-MOVPE

We investigated the influence of the growth rate on the quality of zero-net-strained InGaAsP/InGaAsP/InP multiquantum well structures for 1.55 μm emission grown by low pressure metalorganic vapor phase epitaxy. The samples consisted of fixed compressive strained wells (ɛ=+1%) and tensile strained barriers (ɛ=−0.5%) grown with different quaternary bandgap wavelengths (λ_B=1.1–1.4 μm). Using higher growth rates, we obtained for the first time high quality zero net strained multi quantum well structures, regardless having constant group V composition in the well and barriers. The samples were analyzed by x-ray diffraction, photoluminescence and atomic force microscopy techniques. The amplitude of surface modulation roughness along [011] direction decreased from 20 nm to 0.53 nm with increasing growth rate and/or quaternary compositions grown outside the miscibility gap. A new deep PL broad emission band strongly correlated with the onset of wavy layer growth is also reported. Broad area and ridge waveguide lasers with 10 wells exhibited low losses (34 cm⁻¹) and low threshold current densities at infinite cavity length (1020 A·cm⁻² and 1190 A·cm⁻², respectively).     

Further Observations on Tin Pest Formation in Solder Alloys

The most recent observations of the response of bulk samples of several commercial solder alloys, exposed to temperatures below the allotropic transition for tin (13°C) for extended periods, are reported. Damage associated with tin pest development has been arbitrarily graded into six levels, and the formation of visible α-phase warts used for comparative purposes. Since the previous examination, some 2 years ago, tin pest has been observed for the first time in the traditional Sn-37Pb solder alloy after exposure at −18°C and −40°C, and actual warts were apparent in as-cast Sn-0.5Cu stored at −40°C and in as-cast Sn-3.5Ag after exposure at −18°C. No tin pest was detected in Sn-Zn-3Bi after exposure for periods up to 6 years. Tin pest continued to develop in those lead-free alloys in which it had previously been observed, indicating the probability that all would eventually disintegrate in time. In general, prior thermal or mechanical treatment accentuated tin pest formation. The influence of exposure temperature was unclear, since some alloys (Sn-0.5Cu and Sn-3.8Ag-0.7Cu) experienced more damage at −18°C, but others (Sn-37Pb and Sn-3.5Ag) were more susceptible at −40°C. From a consideration of the findings and other published information, it is contended that impurity levels below 0.1 mass% (1000 ppm) play a vital role in determining whether tin pest develops in realistic timescales. A major factor in the absence of tin pest, to date, on actual joints may be simply the mismatch between the timescales experienced in service and those in long-duration laboratory tests.     

Influences of ZnO buffer layers on the quality of ZnO films synthesized by the metal-organic chemical vapor deposition process

High-quality ZnO thin films were prepared by metal-organic chemical vapor deposition (MOCVD) on a sapphire (a-Al₂O₃) substrate. The synthesis of ZnO films was performed over a substrate temperature of 400–700°C and at chamber pressures of 0.1–10 torr. The structural and optical properties of ZnO films were investigated in terms of deposition conditions, such as substrate temperature, working pressure, and the ratio of Zn precursor (Diethylzinc (DEZn)) to oxygen. The ZnO films, preferentially oriented to 34.42° diffraction because of the (002) plane, were obtained under processing conditions of 700°C and 3 torr. This film shows a full-width at half-maximum (FWHM) of 0.4–0.6°. The results of photoluminescence (PL) spectroscopy also show a strong near band-edge emission at 3.36 eV at 10 K as well as a very weak emission at deep levels around 2.5 eV at room temperature. In addition, we are interested in the introduction of ZnO buffer-layer growth by the sputtering process to reduce lattice mismatch stress. This paper addresses how to advance the crystalline and optical properties of film. The ZnO film grown with the aid of a buffer layer shows a FWHM of 0.06–0.1° in the x-ray diffraction (XRD) pattern. This result indicates that crystalline properties were highly improved by the ZnO buffer layers. The PL spectroscopy data of ZnO film also shows a strong near band-edge emission and very weak deep-level emission similar to films synthesized without a buffer layer. Accordingly, synthesized ZnO films with buffer layers indicate fairly good optical properties and low defect density as well as excellent crystallinity.     

Growth of AlxGa1−x as by metalorganic chemical vapor deposition using trimethylgallium and trimethylamine alane

High-quality Al_xGa_1−xAs layers with aluminum arsenide contentx up to 0.34 have been grown in a low pressure metalorganic chemical vapor deposition (MOCVD) system using trimethylgallium (TMG), trimethylamine alane (TMAA) and arsine. The carbon content in these films depended on growth conditions but was in general lower than in those obtained with trimethylaluminum (TMA) instead of TMAA in the same reactor under similar conditions. Unlike TMA grown layers, the TMAA grown Al_xGa_1−xAs layers, (grown at much lower temperature—down to 650° C), exhibited room temperature photolu-minescence (PL). Low temperature (25 K) PL from these films showed sharp bound exciton peaks with a line width of 5.1 meV for Al_0.25Ga_0.75As. A 39 period Al_0.28Ga_0.72As (5.5 nm)/GaAs (8.0 nm) superlattice grown at 650° C showed a strong PL peak at 25 K with a line width of 5.5 meV attesting to the high quality of these layers.