Varying Substrate Temperature Research Articles

사파이어 기판 위에 증착된 ZnO 박막의 기판온도와 산소 가스량에 따른 특성

ZnO thin films on (001) <TEX>$Al_2O_3$</TEX> substrates have been deposited by pulsed laser deposition(PLD) technique using an Nd:YAG laser with a wavelength of 266 nm. The influence of the deposition parameters, such as oxygen gas flow, substrate temperature and laser energy density variation on the properties of the grown film, was studied. The experiments were performed for substrate temperatures in the range of <TEX>$300\~450^{\circ}C$</TEX> and oxygen gas flow rate of <TEX>$100\~900$</TEX> sccm. We investigated the structural and optical properties of ZnO thin films using X-ray diffraction(XRD) and photoluminescence(PL).

Predicting Thermal Contact Resistance Between Molten Metal Droplets and a Solid Surface

Thermal contact resistance between molten metal droplets (aluminum alloy 380 and bismuth) and solid plates (steel and brass) was measured experimentally. The diameter of the droplets was 4mm, and droplet impact velocity ranged between 1 and 3m∕s. Substrate temperature was varied from 25to300°C and roughness from 0.06to5.0μm. Substrate temperature variation under impacting droplets was measured using fast temperature sensors that had a response time of 40ns and recorded substrate temperatures at five different radial locations under each droplet. Thermal contact resistance during the first few milliseconds of impact was obtained by matching measured surface temperature variation with an analytical solution of the one-dimensional transient heat conduction equation. An analytical model of the deformation of a free liquid surface in contact with a rough solid was used to calculate the true area of contact between them and, thereby, the thermal contact resistance. Test results agreed well with predictions from the analytical model. Thermal contact resistance values ranged from 10−7to3×10−6m2K∕W, increasing with surface roughness and decreasing with rising impact velocity.

Lateral uniformity in HgCdTe layers grown by molecular beam epitaxy

The fabrication of high-quality focal plane arrays from HgCdTe layers grown by molecular beam epitaxy (MBE) requires a high degree of lateral uniformity in material properties such as the alloy composition, doping concentration, and defect density. While it is well known that MBE source flux nonuniformity can lead to radial compositional variation for rotating substrates, we have also found that composition can be affected significantly by lateral variations in substrate temperature during growth. In diagnostic experiments, we systematically varied the substrate temperature during MBE and quantified the dependence of HgCdTe alloy composition on substrate temperature. Based on these results, we developed a methodology to quickly and nondestructively characterize MBE-grown layers using postgrowth spatial mapping of the cutoff wavelength from the Fourier transform infrared (FTIR) transmission at 300 K, and we were able to obtain a quantitative relationship between the measured spatial variations in cutoff and the substrate temperature lateral distribution during growth. We refined this methodology by more directly inferring the substrate temperature distribution from secondary ion mass spectroscopy (SIMS) measurements of the As concentration across a wafer, using the fact that the As incorporation rate in MBE-grown p-type layers is highly sensitive to substrate temperature. Combining this multiple-point SIMS analysis with FTIR spatial mapping, we demonstrate how the relative contributions from flux nonuniformity and temperature variations on the lateral composition uniformity can be separated. This capability to accurately map the lateral variations in the substrate temperature has been valuable in optimizing the mounting and bonding of large substrates for MBE growth, and can also be valuable for other aspects of MBE process development.

Investigation of structural properties of amorphous carbon nitride thin films prepared by xenon cloride pulsed laser deposition of camphoric carbon precursor

Raman scattering analysis revealed that the structure of carbon (C) films prepared by pulsed laser deposition at room temperature is predominantly amorphous and the structure of amorphous C nitride (a-CN x ) films can be changed with varying substrate temperatures (ST) from 20 to 500 °C. The deposited a-CN x films are composed of C-N, C-N and C-O bonded materials and the C-N and C=N bonds are increased with ST. We have found no other obvious peaks can be distinguished in the range 900 to 2300 cm−1 in which several peaks always appear in a-CN x films. The spectra were deconvoluted into Raman D and G peaks and the structural parameters are determined. The upward shifts of Raman G peak towards 1592 cm−1 shows the evidence of a progressive formation of crystallites in a-CN x films upon increase of ST. While, the upward shifts of Raman D peak towards 1397 cm−1 have been related to the decreased of bond-angle disorder and sp3 tetrahedral bonding in its structure. Raman FWHM and IDIG also indicate that N incorporation with increased of ST caused an increase in the number and/or size of graphitic domains in the a-CN x films.

Optical and chemical characterization of expanding thermal plasma deposited silicon dioxide-like films

In this paper we report on the use of expanding thermal plasma technique for the deposition of carbon-free silicon dioxide films by means of argon/hexamethyldisiloxane/oxygen mixtures, at growth rates in the range of 6–12 nm/s, as a function of the substrate temperature. The variation of substrate temperature has been investigated as an alternative route to ion bombardment in promoting the densification of SiO 2-like films. Information concerning film chemical composition was obtained by means of Fourier transform IR absorption and X-ray photoelectron spectroscopies. The films are stoichiometric and a residual hydrogen content of 3.5·10 21 at/cm 3 is detected in the film deposited at 300 °C (SiO 2H 0.17). The film optical characterization by means of UV–VIS and IR spectroscopic ellipsometry has highlighted the beneficial role of substrate temperature in the densification of the SiO 2 film matrix and the decrease of film porosity.

Fabrication and characterisation of high quality ZnO thin films by reactive electron beam evaporation technique

High quality zinc oxide thin films have been deposited on silicon substrates by reactive e-beam evaporation in an oxygen environment. The effect of the growth temperature and air annealing on the structural, optical and electrical properties has been investigated. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented and that the linewidth of the (002) peak is sensitive to the variation of substrate temperature. The optimum growth temperature has been observed at 300°C. Raman spectroscopy has been found to be an efficient tool to evaluate the residual stress in the as-grown ZnO films from the position of the E2 (high) mode. On the other hand, the vanishing of the 574cm−1. Raman feature after annealing has been explained as due to an increase of grain size and the reduction of O-vacancy and Zn interstitial. The SEM images have shown that the surfaces of the electron beam evaporated ZnO became smoother for the growth temperatures higher than 300°C. The optical transmittance is the highest at 300°C and has been increased after annealing in air showing an improvement of the optical quality. Finally, the maximum electrical resistivity has been found at 300°C, which explains its relation with the crystal quality and increased from 5.8×10−2Ωcm to reach an approximate value of 109Ωcm after annealing at 750°C.

RAMAN SCATTERING STUDIES ON THE STRUCTURAL PROPERTIES OF PULSED LASER DEPOSITED AMORPHOUS CARBON NITRIDE THIN FILMS

Raman scattering analysis revealed that the structure of carbon (C) films prepared by pulsed laser deposition at room temperature is predominantly amorphous and the structure of amorphous C nitride ( a-CN x) films can be changed with varying substrate temperatures (ST) from 20 to 500°C. The deposited a-CN x films are composed of C–N , C≡N and C–O bonded materials and the C–N and C≡N bonds are increased with ST. We have found no other obvious peaks that can be distinguished in the range of 900 to 2300 cm-1 in which several peaks always appear in a-CN x films. The spectra were deconvoluted into Raman D and G peaks and the structural parameters were determined. The upward shifts of Raman G peak towards 1592 cm-1 show evidence of a progressive formation of crystallites in a-CN x films upon increase of ST, while the upward shifts of Raman D peak towards 1397 cm-1 have been related to the decrease of bond-angle disorder and sp 3 tetrahedral bonding in its structure. Raman FWHM and I D /I G also indicate that N incorporation with increase of ST caused an increase in the number and/or size of graphitic domains in the a-CN x films.

Characterization and Raman investigations on high-quality ZnO thin films fabricated by reactive electron beam evaporation technique

High-quality zinc oxide thin films have been evaporated by reactive e-beam in an oxygen environment. Photoluminescence (PL) on ZnO evaporated at 300 °C reveals an enhancement of the ultraviolet near band edge emission at 3.28 eV and shows the smallest full-width at half-maximum of the NBE peak about 150 meV, proving that the optimum growth temperature is 300 °C. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented and that the linewidth of the (0 0 2) peak is sensitive to the variation of substrate temperature. The best crystallinity occurred at 300 °C, correlating with the PL measurements. The optical transmittance is the highest at 300 °C and has been increased after annealing in air, showing an improvement of the optical quality. Raman spectroscopy has been found to be an efficient tool to evaluate the residual stress in the as-grown ZnO films from the position of the E 2 (high) mode. On the other hand, the vanishing of the 574 cm −1 Raman feature after annealing has been explained as due to an increase of grain size and the reduction of O vacancy and Zn interstitial. A model has been developed in this work to determinate the normal mode frequencies from Fourier analysis by considering the five force constants starting from the five elastic constants of the material. The vibrational modes at about 100, 435 and 1123 cm −1 have been found from the model for a ZnO thickness of 0.3 μm.

Surface structures of Sn on Mo(1 1 0) surface investigated by RHEED

The surface structures of tin deposited on a Mo(1 1 0) surface have been investigated by reflection high-energy electron diffraction (RHEED), and a phase diagram of two-dimensional surface structures for variations of substrate temperature and coverage of several monolayers was obtained. Five kinds of surface structures were observed according to changes of coverage and annealing temperature. Less than 0.8 ML of Sn thickness, the α , β , γ and δ structures appeared in order with elevating temperature of the substrate, and above 0.8 ML, the α , γ , δ and ε structures similarly appeared. They are all commensurate structures. Dense direction of each structure is parallel to the [0 0 1]Mo in the α and ε structure, while those in the γ and δ structure is parallel to [1 1 0]Mo. The ε structure has the same atomic arrangement of bcc(1 1 0) plane rotated by 54 . 7 ∘ from Mo(1 1 0) plane with elongation by 6 / 3 times.

Influence of process parameters on the properties of TEOS–PECVD-grown SiO 2 films

The influence of chamber pressure, substrate temperature and mass flow ratio on growth rate, refractive index and stress of plasma-enhanced chemical vapor deposition (PECVD) grown SiO 2 films has been investigated. In this study, the silicon dioxide films were grown by homemade PECVD system by carrying three different sets of processes for variation in chamber pressure (70–240 Pa), substrate temperature in the range of 523–673 K and flow rate ratio (O 2/TEOS) of 2–10. In these processes, liquid tetraethoxysilane (TEOS) has been used as a source of Si instead of SiH 4 to avoid difficult handling of SiH 4. The growth rate (by measuring thickness for specified time process), refractive index and stress of the deposited SiO 2 films have been determined by ellipsometry technique. The present study reveals that, in order to grow good quality SiO 2 films for their microelectronics applications, where stress is of prime importance, deposition processes of TEOS–PECVD should be carried out with substrate temperature and flow ratio variations. However, for optoelectronics applications, the growth processes carried out with varying chamber pressure are believed to be highly desirable, they allow precise control of the refractive index and a higher thickness.

Modelling spatio-temporal near-surface temperature variation in high mountain landscapes

A surface energy balance model (SEB), describing the fluxes of latent and sensible heat as well as the ground heat flux at different surfaces, was developed to simulate substrate, surface and air temperature variations in high mountain landscapes at a high temporal resolution of one hour. Classical atmospheric forcing was imposed at a reference level. The SEB model was calibrated and tested using micro-meteorological measurements collected at different sites within four catchments of the low and middle alpine belt of western and eastern central Norway. These sites represent the most common alpine vegetation types. Agreement between the model predictions and measurements was good: in all cases at least 80% of the variance in the measurements was explained by the model. In the present stage, the model contains limitations concerning the effects of a snow pack and should be applied to the snow-free season, only. Nevertheless, comparably simple SEB models, like this one, can be employed to intensify knowledge about detailed-scaled micro-spatial temperature differentiation and its effects on high mountain ecosystem functioning. Due to its simplicity it is suitable for remote sensing utilization enabling the application along broad-scale altitudinal and oceanic-continental gradients.

Optoelectronic and Structural Properties of Undoped Microcrystalline Silicon Thin Films: Dependence on Substrate Temperature in Very High Frequency Plasma Enhanced Chemical Vapor Deposition Technique

The effects of substrate temperature on optoelectronic and structural properties of undoped microcrystalline silicon thin films have been investigated. The undoped silicon films have been deposited by the very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique using a SiH4 and H2 gas mixture at 105 MHz plasma excitation frequency and moderately low power density of 70 mW/cm2. The effect of the systematic variation of substrate temperature (from 180°C to 370°C) on film properties has been studied, while keeping the other parameters constant. The deposition rate is considerably high (2.6 Ås-1) at 180°C and remains almost constant over the whole temperature range. Dark conductivity for all the films lies around ∼10-6 Scm-1. Low subband gap absorption has been observed by photothermal deflection spectroscopy for these microcrystalline films. Increase of substrate temperature improves the microcrystallinity of the film, which is confirmed from structural studies. Crystalline grain size also increases with increase of substrate temperature and a maximum of 460 Å has been achieved at 370°C. A satisfactory correlation is observed among the results of different structural studies: Raman spectroscopy, infrared spectroscopy, X-ray diffraction, transmission electron microscopy and atomic force microscopy.

Study of GaAs layers grown on Ge substrates by MOVPE and in situ monitored by laser reflectometry

Metalorganic vapor phase epitaxy (MOVPE) of GaAs on Ge substrate has been studied by laser reflectometry (LR) with 632.8 nm laser beam. The layers were grown by varying substrate temperature and the V/III ratio. The relative difference between refractive index of film and substrate results in pronounced easily detected interference oscillations in the reflected beam intensity. The oscillations period provides an accurate and immediate measure of growth rate. In addition, the variations of extrema of oscillations and the average value of reflectance provide an estimate of the quality and surface roughness. A procedure for quantifying the roughening observed during the in situ LR monitoring of growth was presented. Furthermore, the measured interference effects were analyzed by calculating the reflectance. In order to explain the reflectometry behavior, the film was also characterized by X-ray diffraction and scanning electronic microscopy.

Characteristics of Surface Reaction of SnO2 Thin Films Prepared by MOCVD

Tin dioxide(<TEX>$_SnO2$</TEX>) thin films were deposited on alumina substrate by metal-organic chemical vapor deposition (MOCVD) as a function of temperature and time. Thin films were fabricated from di-n-butyltin diacetate as a precursor and oxygen as an oxidation. The microstructure of deposited films was characterized by X-ray diffraction and field emission scanning electron microscopy(FE-SEM). The thickness was linearly increased with deposition time and <TEX>$SnO_2$</TEX>structure was found from <TEX>$375^{\circ}C$</TEX> for the deposition time of 32 min. The maximum sensitivity to 500ppm CO gas was observed for the specimens deposited at <TEX>$375^{\circ}C$</TEX> for 2 min at the operating temperature of <TEX>$350^{\circ}C$</TEX>. Gas sensitivity to CO increased with decreasing the film thickness. The sensing properties of response time, recovery and sensitivity of CO were changed with variations of substrate temperature and time.

Microcrystalline silicon for solar cells deposited at high rates by hot-wire CVD

Using two tungsten (W) filaments and a filament–substrate spacing of 3.2 cm, we have explored the deposition of microcrystalline silicon (μc-Si) solar cells, with the i-layer deposited at high deposition rates ( R d), by the hot-wire CVD (HWCVD) technique. These cells were deposited in the n–i–p configuration on textured stainless steel (SS) substrates, and all layers were deposited by HWCVD. Thin, highly crystalline seed layers were used to facilitate crystallite formation at the n–i interface. Companion devices were also fabricated on flat SS substrates, enabling structural measurements (by XRD) to be performed on i-layers used in actual device structures. Using a filament temperature of 1750 °C, device performance was explored as a function of i-layer deposition conditions, including variations in i-layer substrate temperature ( T sub) using constant H 2 dilution, and also variations in H 2 dilution during i-layer deposition. The intent of the latter is to affect crystallinity at the top surface of the i-layer (i–p interface). We report device performance resulting from these studies, with all i-layers deposited at R d>5 Å/s, and correlate them with i-layer structural studies. The highest device efficiency reported is 6.57%, which is a record efficiency for an all-hot-wire solar cell.

Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased, (c) the low-energy optical absorption at photon energy &amp;lt;1.4 eV increased one to two orders of magnitude, (d) the low-energy PL band at ∼1.0 eV emerged with a decrease of total PL intensity, and (e) the conductivity activation energy decreased. The aforementioned changes correlated well with the crystallinity of the material. We attribute the observations mainly to the formation of the c-Si gain boundaries during crystallization.

Indium segregation and reevaporation effects on the photoluminescence properties of highly strained In xGa 1− xAs/GaAs quantum wells

Temperature dependence of the effective band gap (BG) energy of strained In x Ga 1− x As/GaAs single-quantum well and multi-quantum well structures grown by solid source MBE at varied substrate temperature is investigated by photoluminescence spectroscopy between 10 K and room temperature. For low-temperature-grown heterostructure, the temperature-induced BG shrinkage exhibits a good correlation with that of unstrained material. However, no consensus is shown to occur for a relatively high-temperature-grown quantum wells (QWs). This discrepancy is interpreted in terms of indium segregation and reevaporation during epitaxy. The low-temperature range, where the well-known Varshni law fails to fit PL peak positions, is found to decrease with increasing QW width and is attributed to the interface-roughness-induced exciton localization. This study was propped by numerical solving of Schrödinger equation taking into account strain, indium segregation and desorption effects.

Application of DLC films as masks for integrated circuit fabrication

In order to evaluate the applicability of diamond-like carbon (DLC) films to the above-mentioned processes, the durability of such films against ion implantation has been investigated. Therefore, DLC films deposited on n-type Si (1 1 1) substrates under varying substrate temperature were exposed to boron (B) ion implantation during the integrated circuit (IC) fabrication process. The implantation of B ions was carried out at an acceleration voltage of 30 keV and a dose volume of 5×10 14 ion cm −2. The surface morphology of DLC films before and after implantation was investigated by atomic force microscopy. IR spectra of the DLC films were obtained with an FTIR spectrometer. Concentration and diffusion depth of the implanted dopants in the films were determined by Auger electron spectroscopy (AES). IR spectra indicate that the structure of DLC films is greatly changed by the implantation process. AES spectra reveal the existence of traces of B at the interface between the DLC film and the Si substrate surface. The yield of excellent pn junctions prepared using DLC films as mask materials was measured as a function of the deposition temperature. From the results of I–V characteristics of pn junction and the yield, the optimum condition of the DLC film is at 400 °C of deposition temperature.

Effects of process parameters on trap distributions in organic semiconductors

The effect of different process parameters on the energetic trap structure of organic light emitting diodes (OLEDs) has been investigated. For trap level detection the method of thermally stimulated currents (TSC) has been applied. Energetic trap distributions were derived by means of a fractional TSC technique. The experiments have been carried out on aluminum tris(8-hydroxyquinoline) (Alq 3) single-layer devices that have been prepared by vacuum deposition with evaporation rates of 0.2, 2 and 10 Å/s. In all cases broad trap distributions were found. However, with increasing deposition rate the mean trap energy level shifts to lower energies. This behavior indicates a structural origin of the trap states. A correlation between the I– V and L– V characteristics and the TSC spectra is shown and discussed. Further on, the TSC spectra of N, N′-di(1-naphtyl)- N, N′-diphenylbenzidine (α-NPD) single-layer devices processed at varying substrate temperatures have been measured. These spectra exhibit significant differences, such as the disappearance of one TSC peak, in the case of a heated sample.

Influence of substrate temperature and bias voltage on the optical transmittance of TiN films

Titanium nitride (TiN) thin films were prepared by means of reactive DC sputtering on quartz and sapphire substrates. Structural, electrical and optical effects of deposition parameters such as thickness, substrate temperature, substrate bias voltage were studied. The effect of substrate temperature variations in the 100–300°C range and substrate bias voltage variations in the 0–200 V DC range for 45–180 nm thick TiN films were investigated. Temperature-dependent electrical resistivity in the 100–350 K range and optical transmission in the 300–1500 nm range were measured for the samples. In addition, structural and morphological properties were studied by means of XRD and STM techniques. The smoothest surface and the lowest electrical resistivity was recorded for the optimal samples that were biased at about V s=−120 V DC. Unbiased films exhibited a narrow optical transmission window between 300 and 600 nm. However, the transmission became much greater with increasing bias voltage for the same substrate temperature. Furthermore, it was found that lower substrate temperatures produced optically more transparent films. Application of single layers of MgF 2 antireflecting coating on optimally prepared TiN films helped increase the optical transmission in the visible region to more than 40% for 45 nm thick samples.