Substrate Temperature Tsub Research Articles

Low-temperature preparation of boron-doped nanocrystalline SiC:H films using mercury-sensitized photo-CVD technique

Abstract We investigated the dependence of hydrogenated boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) film characteristics against the substrate temperature (Tsub) by the transmission electron micrograph, Raman spectrum, and dark conductivity measurements. High quality of nanocrystalline growth at the low temperature of 120°C shows that the mercury-sensitized photo-assisted chemical vapor deposition (photo-CVD) technique is promising for a low-temperature fabrication of thin film solar cells onto flexible plastic substrates.

Influence of growth conditions on the lattice constant and composition of (Ga,Mn)As

The lattice constant and the alloy composition of (Ga,Mn)As are investigated by high-resolution x-ray diffraction and secondary ion mass spectroscopy. The (Ga,Mn)As layers are grown by low-temperature molecular-beam epitaxy under various growth conditions. We find that, while the alloy composition is mainly determined by the Mn cell temperature (TMn), the substrate temperature (Tsub) and the arsenic to gallium flux ratio (As/Ga) strongly influence the lattice constant. In particular, layers which have the same composition but different growth parameters have quite different lattice constants, caused by the amount of excess As incorporation in the (Ga,Mn)As crystal. This implies that the lattice parameter of (Ga,Mn)As cannot even serve as a rough measure of the crystal composition. (Ga,Mn)As is therefore an example of a system which does not obey Vegard’s law.

Nucleation of Cu(In, Ga)Se2 on Molybdenum Substrates

AbstractWe investigate growth mechanisms of Cu(In, Ga)Se2 on Mo films on glass, as the typical back contact for Cu(In, Ga)Se2 solar cells. A thermal evaporation process deposits Cu(In, Ga)Se2of nominal 3 nm thickness at different rates R and substrate temperatures TSub. An ultrahigh resolution scanning electron microscope serves to investigate the nucleation behavior. The deposited Cu(In, Ga)Se2 forms three-dimensional isolated nuclei, known as Volmer-Weber growth mode. Deposition rate R and substrate temperature TSub control the areal density n of the Cu(In, Ga)Se2 nuclei. We observe a power law dependence between the island density n and the deposition rate R, and an exponential dependence of the island density n on substrate temperature TSub. The theory of homogeneous nucleation explains the Cu(In, Ga)Se2 cluster formation on polycrystalline Mo and the dependence of the island density on the growth conditions.

On the Role of Surface Diffusion and Its Relation to the Hydrogen Incorporation During Hydrogenated Amorphous Silicon Growth

AbstractThe incorporation of hydrogen in vacancies and at void surfaces during hydrogenated amorphous silicon growth from a remote expanding thermal plasma (ETP) is systematically studied by variation of the mass growth flux Γa-Si:H and substrate temperatureTsub. An evident relation between the void incorporation and the growth parameters Γa-Si:H andTsubhas been observed. We speculate on a possible relation with the surface diffusion processes during deposition. An activation energy for surface diffusion during a-Si:H growth of 0.8-1.1 eV is obtained using this assertion, similar to the activation energy deduced from surface roughness evolution studies. For compact films hydrogen is predominantly present at vacancies, and a possible relation with the hydrogen removal mechanism during deposition is discussed.

Structural-Color-Control by AlN Coating on Rough-Surface Al Film

(99.9995%) and N2 (99.999%) or the pure Ar was used as sputtering gas. The total working gas pressure (Ptot) was 1.33 Pa, which was the sum of the partial pressure of N2 (PN2) and that of Ar. In order to change the composition of the film, the concentration of partial N2 gas pressure (cN2=100� PN2=Ptot) was changed. Transparent AlN film was obtained at cN2=12%N2. 7) Of course the pure Al film was formed without N2. Corning #7059 glass (0.7 mm in thickness) was used as a substrate. Film thickness was measured using a surface texture measurement instrument (Surfcom 200B, Tokyo Seimitu Ltd.). The deposition rates were estimated by using smooth films with the thickness less than 100 nm. The typical deposition rates at input power of 200 W were 0.18 and 0.13 nm/s at 0 and 12%N2, respectively. Then in this study each film (or layer) is named by its substance, deposition film thickness (d) which is estimated by deposition rate, and substrate temperature (Tsub), e.g., a notation of Al(d ,T sub) represents an aluminum film with a thickness of d deposited at Tsub. Surface morphology was observed using an atomic force microscope (AFM: Nanopics, NPX100AFM head, Seiko Instruments Inc.). Optical properties of the specimens were measured using an ultraviolet-visible-near infrared spectrometer (UV-3100PC, Shimadzu Co.) in the wavelength range of 0.24 to 2.60 mm. The specular reflectance (R0) was evaluated by measuring the reflectance at an incidence angle of 12 � . The diffusive reflectance (Rd) at normal incidence was also measured using an integrating sphere with a diameter of 60 mm. The color photograph of specimen is taken using a digital camera (Digital Revio KD-300Z, Konica Co.). 3. Results

Characterization of SrS evaporated materials grown from a new high temperature source with a carbon heater

Strontium sulfide (SrS) was used as the evaporant material to prepare the phosphor layer for a blue electroluminescent (EL) element. SrS evaporation was performed by using a resistively heated carbon heater. No heat treatment of the phosphor layer after deposition was made. A crucible temperature of more than 1400 °C was obtained in this evaporation system which makes this system equally applicable for the evaporation of other materials with high melting temperature. Cerium (Ce) luminescent centers were doped in SrS films and a Ce concentration of 0.3 mol % was determined to be optimum for EL. For SrS:Ce films deposited onto a quartz substrate, x-ray diffraction and photoluminescent measurements indicated an optimum substrate temperature (Tsub) of 500 °C. In the EL fabrication process, Tsub was restricted to ≦410 °C by the softening temperature of the alumino-silicate substrate used in the EL device.

A Study of the Effect of V/III Flux Ratio and Substrate Temperature on the in Incorporation Efficiency in InxGa1−x/GaN Heterostructures Grown by RF Plasma-Assisted Molecular Beam Epitaxy

Laterally resolved high resolution X-ray diffraction (HRXRD) and photoluminescence spectroscopy (PL) have been used to assess In incorporation efficiency in InxGa1−xN/GaN heterostructures grown through rf-plasma-assisted molecular beam epitaxy. Average alloy composition over a set of InxGa1−xN/GaN superlattices has been found to depend systematically upon both substrate temperature (Tsub) and V/III flux ratio during growth. A pronounced thermally activated In loss (with more than an order-of-magnitude decrease in average alloy composition) is observed over a narrow temperature range (590–670°C), with V/III flux ratio fixed. Additionally, the V/III flux ratio is observed to further strongly affect In incorporation efficiency for samples grown at high Tsub, with up to an order-of-magnitude enhancement in In content despite only a minor increase in V/III flux ratio. PL spectra reveal redshifts as In content is increased and luminescence efficiency which degrades rapidly with decreasing Tsub. Results are consistent with In loss arising from thermally activated surface segregation + surface desorption processes during growth.

Growth of spinel zinc thioindionate thin film by single-source MOCVD

Binuclear complex Et2In(S2CNEt2)·ZnEt(S2CNEt2) was prepared by the reaction of indium triethyl (InEt3) with zinc bis(diethyldithiocarbamate) (Zn(S2CNEt2)2). This complex was an excellent single-source precursor for low-pressure MOCVD growth of spinel ZnIn2S4 thin layers. Optimal growth conditions for strongly 〈111〉 oriented spinel ZnIn2S4 thin film were as follows: substrate temperature Tsub, 400°C; source temperature Tso, 60°C; carrier gas N2; carrier flow rate Rflow (total pressure 1.3 kPa), 0.3 l/min. The spinel ZnIn2S4 thin film grown on an Si(111) substrate showed an n-type conduction nature and excellent semiconductor properties, such as optical bandgap energy (Eg)opt, 2.72 eV, mobility μ, 25.4 cm2/Vs, and carrier concentration N, 3.7×1016 cm−3.

A Study of the Effect of V/III Flux Ratio and Substrate Temperature on the in Incorporation Efficiency in inxGa1−x/GaN Heterostructures Grown by Rf Plasma-Assisted Molecular Beam Epitaxy

AbstractLaterally resolved high resolution X-ray diffraction (HRXRD) and photoluminescence spectroscopy (PL) have been used to assess In incorporation efficiency in InxGa1−xN/GaN heterostructures grown through rf-plasma-assisted molecular beam epitaxy. Average alloy composition over a set of InxGa1−xN/GaN superlattices has been found to depend systematically upon both substrate temperature (Tsub) and V/III flux ratio during growth. A pronounced thermally activated In loss (with more than an order-of-magnitude decrease in average alloy composition) is observed over a narrow temperature range (590–670oC), with V/III flux ratio fixed. Additionally, the V/III flux ratio is observed to further strongly affect In incorporation efficiency for samples grown at high Tsub, with up to an order-of-magnitude enhancement in In content despite only a minor increase in V/III flux ratio. PL spectra reveal redshifts as In content is increased and luminescence efficiency which degrades rapidly with decreasing Tsub. Results are consistent with In loss arising from thermally activated surface segregation + surface desorption processes during growth.

Growth of GaN and InGaN prepared by a hot wall beam epitaxy system with NH3

GaN and InGaN epilayers were successfully grown on the sapphire (0001) by hot wall beam epitaxy (HWBE) using Ga and In metals and NH3 gas. For the PL spectra of GaN epilayers, strong band edge (BE) emissions without deep level emissions were obtained. The In mole fraction of InGaN epilayers which were grown for the first time by HWBE was able to be controlled in the range less than 12% by changing only the substrate temperature (Tsub). Moreover, the growth interruption method was found to be effective for improving the crystalline quality of InGaN epilayers.

Analysis of image retention in alternating-current thin-film electro- luminescent devices

Image retention can limit the upper operational frequency of ZnS-based alternating-current thin-®lm electro-luminescent (ACTFEL) devices. We studied the impact of indium-tin-oxide (ITO) electrodes pro®le, ZnS deposition conditions, the moisture content of the source material, and the drive conditions on image retention. Layer-by-layer etching and scanning electron microscopy (SEM) crosssectioning techniques were used for the investigation of this phenomenon. The failure was found to start at the column electrode edges of the pixel. Some peculiarities of the image-retention phenomenon are discussed, on the basis of an equivalent electrical scheme of the device and via crystalline properties of the active layer. The conditions for producing displays with a reduced occurrence of image retention are also discussed. ZnS:Mn-based alternating-current thin-®lm electroluminescent devices (ACTFEL) are being widely applied as high-contrast, lightweight, and rugged displays. Additional signi®cant advantages of these devices are their wide viewing angle, suf®ciently low power consumption, the capability to operate in wide temperature ranges, long life, and potentially high legibility in almost all ambient light conditions [1]. Many reports exist in the literature on the growth techniques of ACTFEL devices and the different ways to improve the ef®ciency of the structure [2, 3]. Much less attention has been paid to the appearance of destructive defects and aging phenomena, such as image retention or differential aging, which limit the operating potential of the device and ultimately hold back the exploitation of the displays. The available information is limited mostly by studies on domain electro-luminescence [4], localized destructive breakdown phenomenon [5], and the effect of moisture presence on some characteristics of the devices [6]. The appearance of the differential aging is usually explained by improper driving methods [7]. In this letter, we concentrate on the failure analysis of ZnS:Mn based ACTFEL devices, and present some important results that expand the understanding of image retention in ACTFEL devices. The typical ACTFEL structure, consisting of the glass substrate, indium-tin-oxide patterned layer, 6000 AE ZnS:Mn active layer sandwiched between two dielectrics, back aluminum electrode and packaging oxide layer, was used for this study. The dielectrics are typically yttrium oxide (Y2O3), aluminum oxide (Al2O3), and silicon oxi-nitride (SiON). The in uence of growth conditions, such as growth rate and substrate temperature Tsub, as well as the ITO shoulders pro®le and ZnS moisture content, were analyzed to clarify the origin of image retention. Before environmental and electrical testing, each structure was stabilized by a burn-in procedure. The optical and SEM studies of cross-sectioned and layer-by-layer etched samples were used as basic failure and structural analysis techniques in this series of experiments. The layer-by-layer etching technique was used to clarify the position of the damage region in the stack. All reactants were properly prepared to attack only each individual layer, without touching the adjacent one. The image retention can be described as undesirable and unrecoverable changes in driven areas of the device, leading to the appearance of the remaining image in non-energized displays (Fig. 1). It was found that the image retention can be brought on by aggressive drive conditions, such as high voltage (V . 180 V for device with threshold voltage 130 V), square wave-form pulse, and=or high frequency (.1 kHz). The ®rst two drive parameters result in the current overshooting at the onset of the pulse. The high frequency of the addressing signal leads to additional stress on the insulator= semiconductor interfaces (ISI), as well as extra heat energy release. Fig. 2a shows the typical ACTFEL ITO electrode pro®le. We suggest that in the case of a steep pro®le, electrode edges are too sharp to provide a uniform thickness distribution of the phosphor layer. As a result, the ZnS layer becomes thinner along the column edges. This results in a higher electric ®eld

Hydrogenated Amorphous Silicon Germanium Alloys Grown by the Hot-Wire Chemical Vapor Deposition Technique

ABSTRACTWe successfully grow high-quality hydrogenated amorphous-silicon-germanium alloys (a-SiGe:H) by the hot-wire chemical-vapor deposition (HWCVD) technique using silane and germane gas mixtures. These alloys display electronic properties as good as those grown by the plasma-enhanced chemical-vapor deposition (PECVD) technique, when comparing materials with the same optical bandgaps. However, we grow materials with good electrical properties at high deposition rates—up to 40 Å/s, compared to 1–4 Å/s for PECVD materials. Our alloys exhibit similar trends with increasing Ge content to alloys grown by PECVD. The defect density, the dark conductivity, and the degree of nanostructural heterogeneity (as measured by small-angle X-ray scattering) all increase with increasing germanium content in the alloy. The nanostructural heterogeneity displays a sharp transition between 9 at.% and 14 at.% germanium. PECVD- grown a-SiGe:H alloys exhibit a similar transition at 20 at.% Ge. The photoconductivity and the ambipolar diffusion length of the alloys decrease with increasing germanium content. For a fixed silane-to-germane gas ratio, all material properties improve substantially when increasing substrate temperature (Tsub) from 220°C to 375°C. Increasing Tsub also narrows the optical bandgap and lowers the hydrogen content in the alloys for the same germane-to-silane gas ratio.

Low filament temperature deposition of a-Si:H by hot-wire chemical vapor deposition

Hydrogenated amorphous silicon, a-Si:H, is deposited from silane and hydrogen by hot-wire chemical vapor deposition using a tungsten wire filament at a temperature Tfil=1200 °C. Film properties depend on whether the films were deposited using filaments with an accumulated deposition time lower than 90 min (‘‘new’’ filaments) or longer than 90 min (‘‘old’’ filaments). The deposition rate for films deposited with ‘‘new’’ filaments is 4 times higher than that for aged filaments. For ‘‘new’’ filaments, a monotonic increase of the growth rate, rd, with the pressure is observed, as well as a maximum of rd for FH2/FSiH4 close to unity. The optoelectronic properties are controlled by the substrate temperature Tsub, and show different dependences for ‘‘new’’ and ‘‘old’’ filaments. The Urbach band tail energy, Eu, is lower for films deposited with ‘‘new’’ filaments. A kinetic growth model for hot-wire chemical vapor deposition in the Tfil∼1200 °C regime is proposed.

Growth of diamond films using an enclosed combustion flame

This paper discusses the growth of diamond thin films using an enclosed oxyacetylene torch operating at 700 Torr. Using response surface methodologies, we have systematically explored the parameter space to construct maps of nucleation density, film quality, growth rate, and orientational texture as functions of conditions. The deposition process has been broken down into a nucleation enhancement step and a growth step, and each step is optimized separately. In the study of the nucleation enhancement, we vary the flow ratio=O2/C2H2 (R), substrate-flame distance (z), and pretreatment time (t) while holding substrate temperature (Tsub) ≤ 550 °C and flow rate (F)=4 slm. Scanning electron microscopy determines the nucleation density and nucleation uniformity. The best nucleation enhancement occurs at R=0.91, z=50% of the feather length, and t=5 minutes. For the growth study, the variables are R, z, and Tsub (F=4 slm), and we employ two different Raman scattering measurements to assess film quality. In one case, we determine quality using the quality fraction=diamond peak/(diamond peak + nondiamond peak); the second indicator is the luminescence (L) determined from the baseline of the spectrum and is related to defects in the film. The highest quality films appear at high R (0.95-0.97), z=1 mm, and high Tsub (900–1000 °C). We have also determined the growth rate as a function of conditions using infrared spectroscopy and find that the growth rate is a factor of 10 less than in the unenclosed flame (i.e., open to the atmosphere). Films grown for one hour show orientational texturing predominantly in the 〈111〉 direction.

Metalorganic molecular beam epitaxy growth characteristics of GaAs using triethylgallium and trisdimethylaminoarsenic

Metalorganic molecular beam epitaxy (MOMBE) growth characteristics of GaAs using triethylgallium (TEGa) and trisdimethylaminoarsenic (TDMAAs) are studied in detail by using reflection high energy electron diffraction intensity oscillations. It is found that the GaAs growth rate variation with substrate temperature (Tsub) is similar to that in the MOMBE growth of GaAs with TEGa and elemental As (As4) except in the high Tsub region, despite the use of uncracked TDMAAs instead of As4. GaAs growth starts at about 350 °C and shows a maximum growth rate at about 500 °C. However, the absolute growth rate is about 15% lower than that using As4 with the same TEGa flow rate in the mass-transport limited growth region. Above 600 °C the growth rate shows a rapid decrease with increasing Tsub. Further, it is found that etching of GaAs occurs when only TDMAAs is supplied to the GaAs surface at Tsub above 500 °C. Unintentionally doped GaAs films show n-type conduction in the whole Tsub range investigated with the highest 77 K electron mobility of 22 000 cm2/V s and a low carrier concentration of 2×1015 cm−3 for the 650 °C grown samples. 4.2 K photoluminescence spectra show an exciton-bound-to-impurity emission peak at the wavelength of 819.3 nm with a full width at half-maximum of less than 3.5 meV indicating good optical qualities.

Effects of activated reactive evaporation process parameters on the microhardness of polycrystalline silicon carbide thin films

Polycrystalline ß-SiC is expected to be an excellent material for thin film temperature sensors for the measurement of the surface temperature of gas turbine engine components operated at high temperatures (up to 1500°C). In this paper, Vickers and Knoop indentation microhardness tests were carried out on the silicon carbide films grown on Si(100) substrates by the activated reactive evaporation (ARE) process to measure one of the important mechanical properties, i.e. hardness of the films. The results of these tests are presented as a function of the ARE process parameters, such as C2H2 pressure PC2H2, substrate temperature Tsub, ARE electrode voltage VARE for the generation of d.c. glow discharge and substrate bias Vsub. It was found that the hardness of the films is very much dependent on these process parameters. A Vickers hardness value of 3680 kgf mm−2 (36.1 GPa) at 25 gf load (0.25 N) and a Knoop hardness value of 2060 kgf mm−2 (20.2 GPa) at 100 gf load (0.98 N) are obtained for polycrystalline ß-SiC films (C-to-Si ratio, 1.17) prepared under the following deposition conditions: C2H2 pressure, 3 mTorr (0.4 Pa); substrate temperature, 700°C; ARE electrode voltage, +150 V; substrate bias, −50 V.Microhardness variation of the ß-SiC films with applied load on the indenter was also studied. It was found that the hardness of the films decreases with increasing load, which is believed to be due to indentation size and substrate hardness effects.

Study of the compositional control of the antimonide alloys InGaSb and GaAsSb grown by metalorganic molecular beam epitaxy

MOMBE (metalorganic molecular beam epitaxy) growth characteristics of Sb containing ternary alloys, InGaSb, and GaAsSb are investigated. In the growth of InGaSb using TEGa (triethylgallium), TMIn (trimethylindium), and Sb4 (elemental antimony), the enhanced desorption of methyl-In molecules at a substrate temperature Tsub of around 500 °C as well as the enhanced desorption of ethyl-Ga molecules at around 515 °C are observed. They are due to the weak bond strength of antimonide compounds. Furthermore, the decrease of Ga solid composition with increasing Sb4 flux and the increase of GaSb partial growth rate with TMIn flow rate are also observed at as high as 500 °C. This is caused by the fact that the site blocking effect of excess Sb atoms exists up to higher Tsub. In the growth of GaAsSb using TEGa, TEAs (triethylarsine), and TESb (triethylstibine), the Sb composition versus TESb/(TEAs+TESb) curve exhibits a bowing characteristic, which is similar to that in the MOVPE (metalorganic vapor phase epitaxy) growth and is different from that in the MBE (molecular beam epitaxy) growth. Mass transport properties of Sb molecules in the MOMBE are considered to be similar to that in the MOVPE. It is found that the Tsub dependence of Sb composition is much weaker than that in the MBE, which is a superior point of MOMBE in the growth of antimonide alloys.

Low Filament Temperature Deposition of a-Si:H by Catalytic Chemical Vapor Deposition

Hydrogenated amorphous silicon, a-Si:H, is deposited from silane (SiH4) and hydrogen (H2) using a tungsten wire at low filament temperatures (Tfil = 1200°C) by catalytic chemical vapor deposition.The deposition rate increases monotonically with the depositions pressures and shows a maximum at an H2 : SiH4 flow ratio of unity. Vanishingly small deposition rates were observed for silane-only depositions and for H2-to-SiH4 flow ratios of 2.5 and above. The optoelectronic properties show complex dependence on substrate temperature (Tsub). Three intervals of Tsub with distinct optoelectronic were observed: as Tsub increases from 180 to 220°C, the optical bandgap, EgTauc increases from 1.9 to 2.4eV, the dark conductivity,σd, decreases from 10-10 to 10-15 Ω-1cm-1and the photoconductivity, σph, decreases from 10-5 to 10-10Ω-1cm-1 (region (i)). As Tsub increases from 220 to 250 °C, EgTauc decreases to 1.8eV and the photosensitivity, σph/σd decreases to ~ 1 due to an increase of both σd and σph (region (ii)). Throughout these two regions, the photoconductivity γ factor remains between 0.6 and 0.9 and the activation energy of the dark conductivity, Ea,σd, remains between 0.7 and 0.9eV. Above 250°C, the σph and σd remain approximately constant at 10-4 Ω-1cm-1 and γ decreases to below 0.5 and Ea,σd ~ 0.3eV.

Preparation and Characterization of YBaCuO Superconducting Films by Low-Temperature Chemical Vapor Deposition Using β-Diketonate Complex and N2O

Low-temperature growth of YBa2Cu3Ox superconducting films by chemical vapor deposition (CVD) has been investigated using dipivaloylmetanato (DPM) chelates of Y, Ba and Cu as starting materials and N2O as an oxidizing agent. YBa2Cu3Ox structure has been prepared at a substrate temperature (Tsub) of 530°C. Superconducting films with superconductive critical temperature (Tc_onset) of 87 K and zero-resistivity critical temperature (Tc_zero) of 15 K have been obtained at Tsub=610°C and Tc_onset=89 K, Tc_zero=83 K at Tsub=650°C. The relationship between surface morphology, chemical composition and superconductivity is discussed.

Mercury-sensitized photo-induced chemical vapor deposition of YBa 2Cu 3O x films

Low-temperature growth of YBaCuO films by photo-induced chemical vapor deposition using β-diketonate complex and N2O is described. The mercury-sensitization technique is employed. Cu-oxide films are deposited at a substrate temperature Tsub=120°C. Highly oriented Cu2O films are obtained on MgO substrate. YBaCuO films are deposited at Tsub=215°C. The film composition as analysed by energy-dispersive X-ray spectrometry is nearly 1:2:3 (= Y:Ba:Cu). The structure of the YBa2Cu3Ox film is the amorphous phase.