Carrier Concentration Of Films Research Articles

Study of the free carrier characteristics and surface morphology of AlGaAs/GaAs thin films deposited using MOCVD

Ultra-low loss optical thin films find broad applications in fields such as vertical-cavity surface-emitting lasers and optical atomic clocks. The main optical losses in AlGaAs/GaAs distributed Bragg reflectors (DBRs) prepared using metal-organic chemical vapor deposition (MOCVD) arise from absorption loss caused by free carriers within the layers and scattering loss caused by surface roughness. In this study, we fabricated AlGaAs and GaAs single-layer thin films with varying Al compositions on substrates of three crystal orientations and under different V/III ratios. The dependence of carrier concentration and surface morphology on different substrates and growth conditions was investigated. Thin films grown on substrates with three different crystal orientations exhibited three distinct growth modes (step-flow mode, SK mode, and FM mode). The impact of the V/III ratio on the growth mode was found to be complex. Higher V/III ratios resulted in poorer morphology for films grown on (100) substrates, while better morphology was observed on (211) B substrates. Furthermore, the surface morphology of films grown on (100) 15° off substrates showed less sensitivity to changes in the V/III ratio. With increasing Al composition, the carrier concentration of the films significantly increased. Elevating the V/III ratio proved effective in suppressing the incorporation of carbon, thereby reducing the carrier concentration of AlGaAs films. GaAs films exhibited a low carrier concentration at an appropriate V/III ratio. Additionally, the distinct abilities of different substrates to adsorb impurities exerted a significant impact on the carrier concentration of the films. This study demonstrates that, under optimal conditions, it is feasible to fabricate AlGaAs/GaAs Bragg mirrors with low carrier concentration and relatively small roughness on (100) 15° off substrates.

Comparison of thermal, plasma-enhanced and layer by layer Ar plasma treatment atomic layer deposition of Tin oxide thin films

In this work, Tin oxide (SnOx) thin films have been prepared by different atomic layer deposition (ALD) methods, namely thermal (T-ALD), plasma enhanced (PE-ALD) and subsequent layer by layer Ar plasma treatment (PE-ALD-ALT) modes. In the thermal plasma mode, ozone acts as the oxygen source and Tetrakis(dimethylamino)tin (TDMASn) provides the metal cation. In the plasma-enhanced mode, oxygen plasma was used as the oxygen source. For comparison, we added an Ar plasma treatment step after each layer deposition in the plasma enhanced ALD mode. Compared to T-ALD, the reaction process with O2 plasma as the oxygen source obtained higher growth per cycle (GPC) and crystallinity as well as higher carrier mobility. layer by layer Ar treatment further increased the film carrier concentration and decreased the film resistivity, which also inevitably caused a further increase in the roughness and residual stress in the film.

Enhancing electrical properties of Cu2ZnSn(S,Se)4 thin films via trace Co incorporation

Substitution of Zn2+ ionic by Co2+ ionic is known to suppress the cation disorder of Cu2ZnSn(S,Se)4 thin films, which is helpful to obtain the excellent Cu2ZnSn(S,Se)4 solar cell devices. In our work, the extra trace Co incorporation was added in the solution without affecting the compositions of other elements (i.e. Cu, Zn, and Sn) to enhance the electrical properties of Cu2ZnSn(S,Se)4 thin films. And Co incorporation does not enter into the lattice nor be found in the selenized films after taking effect, but increases the crystallinity, compactness, carrier concentration, and electrical properties of Cu2ZnSn(S,Se)4 thin films samples. By tuning the doping ratios of Co2+ ionic, the surface roughness of the selenized films can be decreased from 110.2 nm (0 mol %) to 68.90 nm (5 mol %). As 1 mol % of Co2+ ionic is doped, the carrier concentration of films can be also improved to 4.35 × 1017 cm−3 from 2.63 × 1016 cm−3, and average surface current of films applied 1 V AC voltage can be increased from 2.426 nA to 4.454 nA. The enhanced properties of Co-incorporated films brings hope for the excellent performance of Cu2ZnSn(S,Se)4 solar cells.

Achievement of Low Carrier Concentration of High-Uniformity SiC Films Grown by High Speed Wafer Rotation Vertical CVD Tool

N-type 4H-SiC homo-epitaxial films were grown by high speed wafer rotation vertical CVD tool, and effect of surface C/Si ratios on in-wafer uniformity and lower limit of carrier concentration of the SiC films on 150 mm diameter wafers was investigated. From analysis of in-wafer distribution by mapping of whole wafers, it was found that high in-wafer thickness uniformity was obtained for the films grown both at lower introduced C/Si ratio of about 1.35, which corresponds to surface C/Si ratio is lower than 1.0, and at higher introduced C/Si ratio of about 1.80, which corresponds to surface C/Si ratio is higher than 1.0. However, larger fluctuation of the carrier concentration at wafer edge was observed for the film grown at lower introduced C/Si ratio, compared with the film grown at higher introduced C/Si ratio. Lower fluctuation of the carrier concentration for the film grown at higher introduced C/Si ratio is thought to be due to lower sensitivity of carrier concentration on growth temperature, which is observed for the film grown at surface C/Si ratio higher than 1.0. Also, the film grown at higher introduced C/Si ratio showed longer carrier lifetime with higher uniformity, although the film grown at lower introduced C/Si ratio indicated shorter carrier lifetime with lower uniformity. The mean carrier concentration of the films grown at higher introduced C/Si ratio was reduced by two digits compared with those grown at lower introduced C/Si ratio, and low carrier concentration of 1.7 x 1014 cm-3 with high in-wafer uniformity for intentionally doped SiC film grown at higher introduced C/Si ratio was achieved. Based on the results mentioned above, fabrication of thick and low-doped epi layer on 150 mm diameter wafer with high in-wafer uniformity was demonstrated.

Repeatability of Epitaxial Growth of n-Type 4H-SiC Films by High Speed Wafer Rotation Vertical CVD Tool

N-type 4H-SiC homo-epitaxial films were grown by high speed wafer rotation vertical CVD tool, and both short term repeatability and long term repeatability of epitaxial growth were investigated. In the short term repeatability investigated by 6 epitaxial wafers chosen from successively grown 12 epitaxial wafers using same recipe, it was found that total variation of thickness and carrier concentration for all measurement points of 6 wafers is 1.44 and 4.64 % respectively, which indicates excellent repeatability as well as high uniform thickness and carrier concentration. In the long term repeatability investigated by a very large number of growth experiments, probability achieving target uniformity of thickness and carrier concentration of the SiC films was increased up to 1.32 times by optimizing gas nozzle structure in gas inlet compared with in the case of conventional gas nozzles. Moreover, adjustment range of C/Si ratio necessary to achieve target uniformity of thickness and carrier concentration of the films was considerably reduced to 22 % by optimized gas nozzles.

Thermoelectric properties improvement in Mg2Sn thin films by structural modification

Mg-Sn thin films (21 ≤ at. % Sn ≤ 42.5) were deposited by magnetron sputtering in the argon atmosphere. The structure and morphology of the films were characterized as a function of the composition. Mg2Sn structure was changed from stable face-centered cubic to metastable orthorhombic structure while the content of Sn in the films increased. The influence of this structural modification on thermoelectric properties was discussed in a wide range of temperatures (30–200 °C). The film carrier concentration and mobility were measured to explain the electronic transport behavior as a function of the film structural modifications. The maximum figure of merit ZT ≈ 0.26 at 200 °C was reached for the film with 36 at. % Sn while a mixture of cubic and orthorhombic Mg2Sn structures coexisted. An annealing treatment was performed under vacuum (∼10−4 Pa) at different temperatures (up to 600 °C) to determine the limit of structural and morphological stability of this film.

Bulk transport properties of bismuth selenide thin films grown by magnetron sputtering approaching the two-dimensional limit

Transport properties of topological insulator Bi2Se3 thin films are investigated with an emphasis on understanding finite-size effects as the two-dimensional limit is approached. Ultra-smooth, (000l)-orientated Bi2Se3 thin-films fabricated using radio-frequency magnetron sputtering reveal disproportionately large changes in bulk resistivity and Hall mobility below six quintuple layers (QL). We correlate such changes to the bulk (other than surface) electronic structure where a bandgap enlargement is observed below six QL in optical absorption spectroscopy measurements. This effect is strongest at the three and two quintuple layers that show the largest changes in bandgap. Temperature dependent transport measurements reveal the effect of disorder from substrate and high carrier concentration. Films on sapphire substrate demonstrate better transport properties compared to amorphous quartz. While temperature dependence of bulk resistivity is both thickness and substrate sensitive, the temperature dependence of Hall coefficient is determined only by the carrier concentration in films. Our work highlights the influence of the bulk electronic structure on the transport properties of few-layer Bi2Se3 which is of interest in the applied areas of optoelectronics, nanoelectronics, and spintronics.

Optical Properties and Interband Transitions of ZnO and Cu-Doped ZnO Films Revealed by Spectroscopic Ellipsometry Measurement

Optical properties analysis of ZnO and Cu-doped ZnO films on Pt substrate have been probed by using Spectroscopic Ellipsometry (SE) at room temperature. Here, we study the optical transitions and the evolution of excitonic states by means of the critical points (CP) analysis. The second derivative spectra of dielectric function (CP analysis) show the transition of excitonic peak at the fundamental absorption edge (E0). Corresponds to e1 spectra, the peak of refractive index (n) at around 3.3 eV is broaden by introducing Cu dopant. This peak is assigned as excitonic states and the peak broadening is mainly due to the screening of excitons. This excitonic screening is more pronounced in the second derivative spectra of dielectric function where the peaks intensity is decreased upon Cu dopant. In additions, effective number of carrier concentrations of ZnO film shows a high number as high as ~1022 cm-3. Furthermore, the addition of Cu doping -in this case 8 in at.%- does not significantly affect the effective number of ZnO film carrier concentration. This result confirms the strong interaction between ZnO film and Pt substrate and negligible contribution of Cu dopant for carrier concentration.

RF Magnetron Sputtering Grown Cu2O Film Structural, Morphological, and Electrical Property Dependencies on Substrate Type.

We investigated the structural, morphological, and electrical properties of cuprous oxide (Cu2O) film dependency on substrate type. Thin films grown using RF magnetron sputtering were characterized by scanning electron microscopy, X-ray diffraction (XRD), and Hall effect measurements. Cu2O thin films were deposited onto sapphire (0001), Si (100), and MgO (110) substrates, and showed Cu2O single phase only, which was confirmed by XRD measurement. Relatively larger compressive strain existed in Cu2O film grown on sapphire and Si, while a smaller tensile strain appeared in Cu2O film grown on MgO. Cu2O thin film crystallite sizes showed a linear dependence on strain. Moreover, film carrier concentration and mobility increased with increasing strain, while resistivity decreased with decreasing strain. Cu2O film strain due to induced strain opens the possibility of controlling structural and electrical properties in device applications.

Role of annealing temperature on electrical and optical properties of Al-doped ZnO thin films

Undoped ZnO and Al-doped ZnO (AZO) thin films 200 nm thick were grown on polished and fused silica (SiO2) substrates by room temperature RF magnetron sputtering. High-purity ZnO (99.99%) and Al (99.999%) target materials were used. The samples were annealed at 300°C, 400°C, and 500°C for 45min in N2 ambient gas. The effects of the annealing temperature on crystal structure, optical properties, and resistivity of the AZO thin films were systematically explored. As grown samples exhibit single crystalline Würtzite structure with preferred orientation along c axis from substrate surface. The crystal quality decreases with increasing amount of Al concentration. Annealing process increases the long range crystal order of films. Optical transmittance measurements show that all samples annealed at 500°C have average 80% transparency in the visible light. Optical band gap values range between 3.23eV to 3.72eV for as grown and annealed samples. The electric conductivity of the AZO films increases with increasing Al concentration. The annealing process especially at 400°C provides the best conductivity due to the improved crystal structure and carrier concentration of films. The minimum resistivity value of 9.40×10−05 Ωcm was measured for the Al12.30ZnO film after annealing. The mobility decreases due to increase in grain boundary scattering.

Solution-Processed LiF-Doped ZnO Films for High Performance Low Temperature Field Effect Transistors and Inverted Solar Cells

This paper reports that high performance metal oxide thin film transistors (TFTs) can be achieved by using LiF-doped ZnO thin films processed from aqueous solution. It was found that LiF doping at an appropriate amount enhanced the oxide film carrier concentration. The TFTs based on the 10 mol % LiF-doped ZnO thin films annealed at 300 °C revealed a good device performance with an average electron mobility of 8.9 cm(2) V(-1) s(-1) and a high on/off current ratio of 4 × 10(7), superior to the devices based on the nondoped ZnO TFTs (1.6 cm(2) V(-1) s(-1)). Even when annealed at 150 °C, the device still showed good transistor operation with an electron mobility of 0.54 cm(2) V(-1) s(-1). The inverted bulk heterojunction solar cells based on P3HT:PCBM blend system fabricated using 10 mol % LiF doped ZnO as electron selective layer showed higher power conversion efficiency (η = 3.3%) than that from undoped ZnO thin films (η = 2.94%) due to enhanced short circuit current (Jsc = 10.55 mA/cm(2)). Our results suggest that LiF incorporation can be a useful technique to produce high performance and low temperature solution-processed oxide TFTs and interface layer for solar cells.

Effect of the Low-Temperature Annealing on Zn-Doped Indium–Tin-Oxide Films for Silicon Heterojunction Solar Cells

The effects of the low-temperature annealing on Zn-doped indium–tin-oxide (ITO) films such as the electrical, optical and structural properties were investigated. Zn-doped ITO films were fabricated by rf magnetron sputtering of ITO and Al-doped ZnO (AZO) targets on corning glass at room temperature. The content of Zn increased with increasing the power of AZO target. The carrier concentration of films shows the decreasing behaviour with increasing the content of Zn, due to a carrier compensation originating from the substitution of a doped Zn for an In or interstitial site. After the low-temperature annealing at 180 °C in vacuum, all films were slightly decreased a carrier concentration and increased the hall mobility because of the absorption of oxygen on the surface films. In addition, the average transmittance did not show a considerable change and had a high values over 80%. Especially, the Zn-doped ITO with atomic ratio of Zn/(In+Zn) of 6.8 at. % had the resistivity of 4×10-4 Ω cm, the highest hall mobility of 41 cm2 V-1 s-1, and the average transmittance of 82%.

Effect of the Low-Temperature Annealing on Zn-Doped Indium–Tin-Oxide Films for Silicon Heterojunction Solar Cells

The effects of the low-temperature annealing on Zn-doped indium–tin-oxide (ITO) films such as the electrical, optical and structural properties were investigated. Zn-doped ITO films were fabricated by rf magnetron sputtering of ITO and Al-doped ZnO (AZO) targets on corning glass at room temperature. The content of Zn increased with increasing the power of AZO target. The carrier concentration of films shows the decreasing behaviour with increasing the content of Zn, due to a carrier compensation originating from the substitution of a doped Zn for an In or interstitial site. After the low-temperature annealing at 180 °C in vacuum, all films were slightly decreased a carrier concentration and increased the hall mobility because of the absorption of oxygen on the surface films. In addition, the average transmittance did not show a considerable change and had a high values over 80%. Especially, the Zn-doped ITO with atomic ratio of Zn/(In+Zn) of 6.8 at. % had the resistivity of 4×10-4 Ω cm, the highest hall mobility of 41 cm2 V-1 s-1, and the average transmittance of 82%.

Effect of annealing ambient on the structural, optical and electrical properties of (Mg,Al)-codoped ZnO thin films

Nominal 1 at.% Al-doped Zn1−xMgxO (x=0–8 at.%) thin films are prepared by the sol–gel method. The codoping films retain the hexagonal wurtzite structure of ZnO, and show preferential c-axis orientation. The effect of annealing ambient (in vacuum and nitrogen) on the optical and electrical properties of (Mg,Al)-codoped ZnO films is investigated. The transmittance of the codoping films is obviously degraded to 50–60% by vacuum annealing, but is enhanced to 70–80% after nitrogen annealing. Both the carrier concentration and the Hall mobility decrease, and the resistivity increases with the widening of the band gap in films treated in vacuum. However, the carrier concentration of films annealed in nitrogen displays fluctuations with increasing x, and the carrier concentrations of films with x=2 and 4% are higher than those of other films. The initial band gap increase of nitrogen-annealed films might be attributed both to Mg doping and to the Burstein–Moss effect due to the NO–(N2)O complex donors. In view of the transmittance and conductivity, nitrogen annealing might be a more appropriate post-annealing method than vacuum annealing for (Mg,Al)-codoped ZnO films to fulfil the requirements of transparent conducting oxide.

Influences on Optoelectronic Properties of Damp Heat Stability of AZO and GZO for Thin Film Solar Cells

This study investigates the effects of damp heat stability on the optoelectronic properties of ZnO:Al (AZO) and ZnO:Ga(GZO) films with respect to thin-film solar cells. The lowest resistivities of AZO and GZO thin films are 8.2621×10-4 Ω-cm and 2.8561×10-4 Ω-cm, respectively. After damp heat testing for 999h, the resistivities of AZO and GZO thin film increase by 39.72% and 11.97%, respectively. XPS binding energy analysis shows that the AZO thin film has a higher O 1s spectrum than the GZO thin film. Thus, the carrier concentration of films decreases, as a higher binding energy is attributed to the chemisorbed oxygen atoms (O-). Experimental results show that after expousre to a damp heat test at 85°C and 85% relative humidity for electrical, optical, structural, and morphological analysis, GZO films are more stable than AZO films.

Effect of indium concentration on the structural and electrical properties of Al-doped ZnO thin films grown by pulsed laser deposition

The Al-doped ZnO (AZO) films doped with different indium concentrations were grown on glass substrates (Corning 1737) at 200 °C by pulsed laser deposition. Indium doping in AZO films shows a critical effect on the crystallinity, resistivity and optical properties of the films. The AZO films doped with 0.3 atom% indium content exhibit the highest crystallinity, the lowest resistivity of 4.5 × 10−4 Ω cm and the maximum transmittance of 93%. The crystallinity of the indium doped-AZO films is strongly related to the resistivity of the films. The carrier concentration in the indium doped-AZO films linearly increases with increasing indium concentration. The mobility of the AZO films with increasing indium concentration was reduced with an increase in the carrier concentration and the decrease in mobility was attributed to the ionized impurity scattering mechanism. In optical transmittance, the shift of the optical absorption edge to a shorter wavelength strongly depends on the electronic carrier concentration in the films. The figure of merit FTC used for evaluating transparent electrodes reached 0.32 Ω−1 at 550 nm wavelength.

Improvement of Thermoelectric Properties of Bismuth Telluride Thin Films using Rapid Thermal Processing

Effects of rapid thermal annealing of bismuth telluride thin films on their thermoelectric properties were investigated. Films with four different compositions were elaborated by co-sputtering of Bi and Te targets. Rapid thermal treatments in range of <TEX>$300{\sim}400^{\circ}C$</TEX> were carried out during 10 minutes under the reducing atmosphere (Ar with 10% <TEX>$H_2$</TEX>). As the temperature of thermal treatment increased, carrier concentrations of films decreased while their mobilities increased. These changes were clearly observed for the films close to the stoichiometric composition. Rapid thermal treatment was found to be effective in improving the thermoelectric properties of <TEX>$Bi_2Te_3$</TEX> films. Recrystallization of <TEX>$Bi_2Te_3$</TEX> phase has caused the enhancement of thermoelectric properties, along with the decrease of the carrier concentration. Maximum values of Seebeck coefficient and power factor were obtained for the films treated at <TEX>$400^{\circ}C$</TEX> (about <TEX>$-128{\mu}V/K$</TEX> and <TEX>$9{\times}10^{-4}\;W/K^2m$</TEX>, respectively). With further higher temperature (<TEX>$500^{\circ}C$</TEX>), thermoelectric properties deteriorated due to the evaporation of Te element and subsequent disruption of film's structure.

Characterization of thermally evaporated 1,8-diacetoxy-9,10-anthraquinone films

The compound 1,8-diacetoxy-9,10-anthraquinone has been synthesised. Thin films coatings of this material have been prepared by thermal evaporation on glass substrates at different temperatures in vacuum. The experimental conditions have been optimised to obtain better crystallinity of the films. The films prepared thus have been studied for their structural, electrical and optical properties. The comparison of infrared (IR) and nuclear magnetic resonance (NMR) spectra of 1,8-diacetoxy-9,10-anthraquinone and its films shows that the compound remains unaffected during vacuum deposition and annealing processes. Observations reveal that the crystallinity of the films increases at higher substrate temperatures. Grains as large as 3.16 μm are observed in the case of films deposited at 333 K. The current–voltage characteristics of these films show ohmic behaviour of conduction within the investigated field and temperature range (10–60 V, 290–410 K). The conduction appears to take place by thermally activated hopping above intermolecular barriers. The electrical conductivity and carrier concentration of films also increases at higher substrate temperatures. Analysis of optical absorption measurements on the films indicate that the interband transition energies lie in the range 2.80–3.43 eV.

Characterization of AgInTe 2 films grown by a hot wall epitaxy technique on KCl substrates

AgInTe 2 films have been grown by a hot wall epitaxy technique onto KCl substrates kept at different temperatures in a vacuum of 1.3×10 −3 Pa. Experimental conditions were optimized to obtain better crystallinity in the films. The films thus prepared were studied for their electrical conductivity, Hall mobility, carrier concentration, as well as structural and optical properties. Observations reveal that the electrical conductivity and carrier concentration of films increases with increasing temperature of the substrate during deposition, while the Hall mobility decreases. The results indicate that the films are p-type, thus indicating holes as dominant charge carriers. The scanning electron micrographs of the films show an increase in grain size with increasing substrate temperature. Analysis of optical absorption studies on the films indicate the band gap energies lie in the range 1.12–1.26 eV.

Characterization of 1,4-/1,8-dihydroxy-9,10-anthraquinone films

1,4-Dihydroxy-9,10-anthraquinone and 1,8-dihydroxy-9,10-anthraquinone films have been grown by a thermal evaporation technique onto a glass substrate kept at different temperatures in a vacuum of 10−5 torr. The experimental conditions were optimized to obtain better crystallinity of the films. The films prepared thus have been studied for their electrical conductivity, carrier concentration, drift mobility, optical properties and electron microscopy. Observations revealed that the electrical conductivity and carrier concentration of films increased with the increase in temperature (290–370 K), while the drift mobility decreased. The conduction in these films was found to be ohmic in nature. The films deposited at higher substrate temperatures appeared to be more ordered. Optical absorption studies on the films indicated that the band gap energies were in the range of 1.6–2.4 eV. The frequencies of the IR absorption bands for the films were in good agreement with the powder samples taken as a reference.