Tailoring the Structural and Optoelectronic Properties of Al-Doped Nanocrystalline ZnO Thin Films

This paper describes the effect of Al doping (0 at.% to 6 at.%) on the structural and optoelectronic properties of nanocrystalline ZnO thin films deposited by thermal evaporation. X-ray diffraction patterns confirm that an increase in Al concentration (from 0% to 6%) in ZnO lowers the crystallinity of the films and reduces grain size. Al doping is also found to influence the optical properties of the ZnO thin films. Visible transmittance above 85% was obtained by increasing the Al doping to 6%. The optical bandgap was found to vary from 3.20 eV to 3.97 eV with changing Al content from 0% to 6%, which is in accordance with the Burstein–Moss shift. The mobility of ZnO thin films can be varied from 5.60 cm²/Vs to 24.25 cm²/Vs, the carrier concentration from 5.93 × 10¹⁸/cm³ to 9.11 × 10²⁰/cm³, and the resistivity from 4.62 × 10⁻⁴ Ω cm to 4.34 × 10⁻² Ω cm, depending on the Al doping concentration (0% to 6%). This study suggests that ZnO:Al films can be tailor-made to meet the requirements for various optoelectronic applications such as flexible photocells or ultraviolet (UV) photodetectors covering a wide range of short wavelengths.     

Room-temperature ferromagnetic ordering in Mn-doped ZnO thin films grown by pulsed laser deposition

The ferromagnetic ordering in Mn-doped ZnO thin films grown by pulsed laser deposition (PLD) as a function of oxygen pressure and substrate temperature has been investigated. Room-temperature ferromagnetic behaviors in the Mn-doped ZnO films grown at 700°C and 800°C under 10⁻¹ torr in oxygen pressure were found, whereas ferromagnetic ordering in the films grown under 10⁻³ torr disappeared at 300 K. The large positive magnetoresistance (MR), ∼10%, was observed at 5 K at low fields and small negative MR was observed at high fields, irrespective of oxygen pressure. In particular, anomalous Hall effect (AHE) in the Mn-doped ZnO film grown at 700°C under 10⁻¹ Torr has been observed up to 210 K. In this work, the observed AHE is believed to be further direct evidence demonstrating that the Mn-doped ZnO thin films are ferromagnetic.     

Study on the structural, electrical and optical properties of Al-F co-doped ZnO thin films prepared by RF magnetron sputtering

Al and F co-doped ZnO(ZnO:(Al,F)) thin films on glass substrates are prepared by the RF magnetron sputtering with different F doping contents.The structural,electrical and optical properties of the deposited films are sensitive to the F doping content.The X-ray analysis shows that the films are c-axis orientated along the(002) plane with the grain size ranging from 9 nm to 13 nm.Micrographs obtained by the scanning electron microscope(SEM) show a uniform surface.The best films obtained have a resistivity of 2.16×10-3Ω·cm,while the high optical transmission is 92.0% at the F content of 2.46 wt.%.     

The effect of introducing Al ions in cationic deposition bath on as-prepared PbS thin film through SILAR deposition method

Undoped and Al doped lead sulfide (PbS) thin films were grown on soda lime glass substrates by Successive Ionic Layer Adsorption and Reaction (SILAR) deposition method. Al content in aqueous cationic solution was varied by adding 0.5–2% of aluminum nitrate in step of 0.5. The characterization of the film was carried out using X-ray diffraction, scanning electron microscopy, and optical and electrical measurement techniques. X-ray diffraction analysis revealed that both the undoped and doped films were polycrystalline and exhibited galena type cubic structure with average crystallite size in the range of 15.5–30.9 nm. The compositional analysis results indicated that Pb, S and Al were present in the samples. Optical studies revealed prominent blue-shift in the absorption edge of as-deposited samples upon doping as compared to that of bulk PbS and this shift was due to a quantum confinement effect. The room temperature conductivity of the PbS thin films was in the range of 1.343×10⁻⁷–1.009×10⁻⁶ (Ω cm)⁻¹for doped samples and 5.172×10⁻⁸ for undoped PbS thin film sample. The optical band gap energy has inverse relation with grain size and electrical conductivity is closely related to structural parameters like grain size, crystallinity and microstrain. The estimated lattice parameter, grain size, optical band gaps and electrical properties were correlated with Al concentration in the cationic solution.     

InP-on-CdS thin film heterostructures

Thin films of InP were deposited on single crystals and thin films of CdS by the planar reactive deposition technique. Good local epitaxy was observed on single crystals of CdS as well as InP and GaAs. The electrical evaluation of unintentionally doped films on semi-insulating InP substrates show them to be n-type with room temperature electron concentrations ranging from 5 × 1016 cm⁻³ to 5 × 10¹⁷ cm⁻³ and mobilities up to 1350 cm²/Vsec. For films intentionally doped with Mn and Be, p-type films were obtained. For Mn doping (deep acceptor level), room temperature mobilities as high as 140 cm²/Vsec and free carrier concentrations as low as 5 × 10¹⁶ cm⁻³ (with dopant level of 3 × 10¹⁸ cm⁻³) were obtained. For Bedoped films, free carrier concentrations of about 5 × 10¹⁸ cm⁻³ and mobilities of 20 cm²/Vsec were found. Scanning electron microscope and microprobe pictures show appreciable interdiffusion between the InP/CdS thin-film pair for InP deposited at 450°C. The loss of Cd from the CdS and the presence of an indium-cadmium-sulfur phase at the InP/CdS interface were observed. Interdiffusion is alleviated for InP deposition at lower temperatures. Supported in part by ERDA and AFOSR.     

Field Emission and Electric Discharge of Nanocrystalline Diamond Films

Nanocrystalline diamond (NCD) films were produced by microwave plasma-enhanced chemical vapor deposition (MPECVD) using gas mixtures of Ar, H₂, and CH₄. The structural properties, electron emission, and electric discharge behaviors of the NCD films varied with H₂ flow rates during MPECVD. The turn-on field for electron emission at a pressure of 2.66 × 10⁻⁴ Pa increased from 4.2 V μm⁻¹ for the NCD films that were deposited using a H₂ flow rate of 10 cm³ min⁻¹ to 7 V μm⁻¹ for films deposited at a H₂ flow rate of 20 cm³ min⁻¹. The NCD film with a low turn-on field also induced low breakdown voltages in N₂. The grain size and roughness of the NCD films may influence both the electron emission and the electric discharge behaviors of the NCD cathodes.     

Dielectric properties of low loss Ba<Subscript>6−3x</Subscript>Nd<Subscript>8+2x</Subscript>Ti<Subscript>18</Subscript>O<Subscript>54</Subscript> thin films prepared by pulsed laser deposition for microwave applications

Ba_6−3xNd_8+2xTi₁₈O₅₄ with x=0.25 (BNT-0.25, or simply, BNT) dielectric thin films with a thickness of 320 nm have been prepared on Pt-coated silicon substrates by pulsed laser deposition (PLD) at the substrate temperature of 650°C in 20 Pa oxygen ambient. X-ray analysis showed that the as-deposited films are amorphous and the films remain amorphous after a postannealing at 750°C for 30 min. The dielectric constant of the BNT films has been determined to be about 80 with a low loss tan δ of about 0.006 at 1 MHz. The capacitance-voltage (C-V), capacitance-frequency, and capacitance-temperature characteristics of a BNT capacitor with Pt top electrode were measured. A low leakage-current density of 4×10⁻⁶ A/cm² at 6 V was measured, and a preliminary discussion of the leakage-current mechanism is also given. It is proposed that amorphous BNT-0.25 thin films will be a potential dielectric material for microwave applications.     

High temperature stability of chromium boride ohmic contacts to p-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC using CrB₂. Two hundred nanometer thick films were sputter-deposited on substrates of doping concentration 1.3×10¹⁹ cm⁻³ in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and transmission electron microscopy. The as-deposited CrB₂ contacts exhibited rectifying characteristics and contained oxygen as a major contaminant. Ohmic behavior with linear current-voltage characteristics was observed following short anneals at 1100°C for 2 min at a pressure of 5×10⁻⁷ Torr. The oxygen in the CrB₂ films was removed by the annealing process, and the lowest value of the specific contact resistance (r_c) measured at room temperature was 8.2×10⁻⁵ Ω-cm². Longer anneals at 1100°C for 3.5 h and 1200°C for 2 h reduced the room temperature values of r to 1.4×10⁻⁵ Ω-cm². A thin reaction region has been identified at the CrB₂/SiC interface; however, the interface remains essentially stable. Thermal stressing at 300°C in vacuum for over 2200 h produced only a slight increase in the specific contact resistance. The low value of the specific contact resistance and the excellent high temperature stability of the CrB₂/SiC interface make this contact a candidate for high power/high temperature SiC device applications.     

Oxygen and carbon incorporation in low temperature epitaxial Si films grown by rapid thermal chemical vapor deposition (RTCVD)

Using SIMS analysis, we have measured oxygen and carbon concentrations in epitaxial Si films grown between 550 and 900° C. The films were grown by rapid thermal chemical vapor deposition from SiH₄ as well as several different SiH₂Cl₂ sources. We have found that at low deposition temperatures (∼750° C or lower), oxygen incorporation is first dictated by source gas impurities and then by residual chamber gases. For the case of SiH₂Cl₂, which can have substantial oxygen content due to its reactivity with H₂O, oxygen concentrations of about 10²⁰ cm^-3 are typical at low deposition temperatures. SiH₄, however, can be obtained in higher purity, and oxygen concentrations of 10¹⁸ cm^-3 can be realized at low temperatures. At higher deposition temperatures (750-900° C), SiO volatilizes, leaving the films grown from all sources with low oxygen concentrations, typically less than 5 × 10¹⁷ cm^-3. Carbon incorporation is much less of a problem since it is present to a lesser extent both in the chamber background and in the source gases. Carbon levels less than or equal to 10¹⁸ cm^-3 can be obtained at all deposition temperatures greater than about 650° C. The performance ofp/n junctions is shown to degrade significantly for junctions grown below 850° C. We conclude that for growth of long lifetime Si films in the temperature range <800° C, that low residual H₂O partial pressures (<10^-10 Torr) are desired. Therefore, CVD chambers should be loadlocked and also capable of base pressures as low as about 10^-9 Torr.     

Improvement in Power Durability of Al Electrode Films Used in SAW Devices by Zr Additive and Ti Underlayer

In order to improve the power durability of Al electrode films and obtain fine-dimensional control in high-frequency surface acoustic wave (SAW) devices, two-layered Al-Zr/Ti electrode films were investigated on 128°Y−X LiNbO₃ substrates by sputter deposition. The results indicated that Al-Zr/Ti electrode films had improved electromigration (EM) reliability compared to Al/Ti electrode films and their lifetime was strongly dependent on the microstructure and Zr concentration. Al-Zr/Ti electrode films with a strong Al (111) texture exhibited longer EM lifetime than polycrystalline films. The Al-Zr/Ti electrode film with an ideal Zr concentration of 0.2 wt.% and a sputtering pressure of 0.25 Pa had a strong Al (111) texture, and its lifetime was approximately four times longer than that of Al/Ti films tested at a current density of 5 × 10⁷ A/cm² at 200°C. Furthermore, the Al-Zr/Ti films were easily etched in reactive ion etching and fine-dimensional control was realized during pattern replication for high-frequency SAW devices.     

Growth of (GaAs)1_x(Ge2)x by metalorganic chemical vapor deposition

We report deposition of (GaAs)_{1_x}(Ge₂)_x on GaAs substrates over the entire alloy range. Growth was performed by metalorganic chemical vapor deposition at temperatures of 675 to 750°C, at 50 and 760 Torr, using trimethylgallium, arsine, and germane at rates of 2–10 μ/h. Extrinsic doping was achieved using silane and dimethylzinc in hydrogen. Characterization methods include double-crystal x-ray rocking curve analysis, Auger electron spectroscopy, 5K photoluminescence, optical transmission spectra, Hall-effect, and Polaron profiling. Results achieved include an x-ray rocking curve full-width at half maximum as narrow as 12 arc-s, Auger compositions spanning the alloy range from x = 0.03 to x = 0.94, specular surface morphologies, and 5K photoluminescence to wavelengths as long as 1620 nm. Undoped films are n type, with n ≈ 1 × 10¹⁷ cm⁻³. Extrinsic doping with silane and dimethylzinc have resulted in films which are n type (10¹⁷ to 10¹⁸ cnr⁻³) or p type (5 × 10¹⁸ to 1 × 10²⁰ cm⁻³). Mobilities are generally ≈ 50 cm²/V-s and 500 cm²/V-s, for p and n films, respectively.     

The use of ICP to determine delivery rates of transportable metal contaminants from metal organic liquid sources

Direct gas analysis was performed on metal organic (MO) liquid source bubblers em-ployed for MOCVD growth of vanadium doped and undoped GaAs and AlGaAs thin films. Using either direct or standard addition methods, the lower limit of quantitation (LLQ) for transportable Si was 5 × 10^s−11 moles/min with precision better than 10% (25 cc/min H₂ carrier). The other transportable contaminants sought during a typical anal-ysis were P, S, Cd, Cu, Zn, Fe, Cr, Ni, Mo, Mn, Sn, Pb, Ca, Mg, and Ge. None of these were detected with a LLQ estimated to be less than 10^s−10 moles/min.     

Synthesis and characterization of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria-stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high-quality <001> textured Pb(Zr_0.54Ti_0.46)O₃ (PZT) thin films on (00l)Si with interposing <001> textured YBa₂Cu₃O_7−δ (YBCO) and yttria-stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ, = 248 nm, τ = 20 nanosecs). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9 × 10⁻⁴ Torr) at 775°C on (001)Si substrate having <001>YSZ // <001>Si texture. The YBCO thin films were deposited in-situ in oxygen ambient (200 mTorr) at 650°C. The temperature and oxygen ambient for the PZT deposition were optimized to be 530°C and 0.4-0.6 Torr, respectively. The laser fluence to deposit this multilayer structure was 2.5-5.0 J/cm². The <001> textured perovskite PZT films showed a dielectric constant of 800-1000, a saturation polarization of 37.81 μC/cm², remnant polarization of 24.38 μC/cm² and a coercive field of 125 kV/cm. The effects of processing parameters on microstructure and ferroelectric properties of PZT films and device implications of these structures are discussed.     

Electrical characterization of semiconducting diamond thin films and single crystals

Naturally occurring semiconducting single crystal (type IIb) diamonds and boron doped polycrystalline thin films were characterized by differential capacitance-voltage and Hall effect measurements, as well as secondary ion mass spectroscopy (SIMS). Results for natural diamonds indicated that the average compensation for a type IIb diamond was >17%. Mobilities for the natural crystals varied between 130 and 564 cm²/V·s at room temperature. The uncompensated dopant concentration obtained by C-V measurements (2.8 ± 0.1 × 10¹⁶ cm⁻³) was consistent with the atomic B concentration measured by SIMS performed on similar samples (3.0 ± 1.5 x 10¹⁶ cm⁻³). Measurement of barrier heights for three different metals (platinum, gold, and aluminum) found essentially the same value of 2.3 ± 0.1 eV in each case, indicating that the Fermi level was pinned at the diamond surface. Polycrystalline semiconducting diamond thin films demonstrated a complex carrier concentration behavior as a function of dopant density. This behavior may be understood in terms of a grain boundary model previously developed for polycrystalline silicon, or by considering a combination of compensation and impurity band conduction effects. The highest mobility measured for a polycrystalline sample was 10 cm²/V·s, indicating that electrical transport in the polycrystalline material was significantly degraded relative to the single crystal samples.     

The influence of OMVPE growth pressure on the morphology, compensation, and doping of GaN and related alloys

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10⁵ Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10¹³ cm⁻² and mobilities of 1500 cm²/Vs, and reduced trapping effects in fabricated devices.     

Dependence of electrical properties of polycrystalline cvd si films on grain size and impurity doping concentrationa,b

The electrical properties of sets of simultaneously grown p-type polycrystalline Si films, deposited by SiH₄ pyrolysis on polycrystalline high-purity alumina substrates and B-doped during growth, were determined by Hall-effect measurements in the temperature range 77-420K as functions both of impurity doping concentration N (~10^l5 to ~10²⁰cm⁻³) and average grain size (≈1 to ≈125μm) in the film. Room temperature data showed rapidly increasing resistivities and rapidly decreasing free-carrier concentrations for doping below a critical concentration N_m and distinct mobility minima at that concentration, with the value of N_m being larger the smaller the average grain size. Measurements as a function of sample temperature showed the intergrain barrier height E_b, decreasing from a maximum value of ~0.4eV at the critical concentration to very small values (~0.01eV) for concentrations above 10¹⁹cm⁻³, with a functional dependence close to E_b ∝l/N^1/2 and E_b for any given concentration being larger the smaller the average grain size. Results are interpreted in terms of the grain-boundary trapping model. Trapped carrier densities in the grain boundaries were calculated to range from ~5×l0¹¹cm⁻² at N≈N_m to ~5×l0¹²cm⁻² for N>10¹⁹cm⁻³, the density being higher the smaller the grain size, and evidence was found for an energy distribution of traps in the Si bandgap, rather than a fixed density at a single discrete energy level. The observed relationship between N_m and average grain size nearly coincides with that of the model for films with ~lμm grain size but sharply departs from it for larger grain sizes, indicating probable applicability of the model for grain sizes up to that range. ^aThis work was supported by the U.S. Department of Energythrough its San Francisco Operations Office under Contract DE-AC03-79ET23045 and monitored by the Solar Energy Research Institute, Golden, CO. bThese results were first described at the 22nd Electronic Materials Conference, Ithaca, NY, June 21–27, 1980, Paper No. M4.     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

The growth of AlInSb by metalorganic chemical vapor deposition

We have grown Al_xIn_1−xSb epitaxial layers by metalorganic chemical vapor deposition using tritertiarybutylaluminum (TTBAl), trimethylindium (TMIn), and triethylantimony (TESb) as sources in a high speed rotating disk reactor. Growth temperatures of 435 to 505°C at 200 Torr were investigated. The V/III ratio was varied from 1.6 to 7.2 and TTBAl/(TTBAl+TMIn) ratios of 0.26 to 0.82 were investigated. Al_xIn_1−xSb compositions from x=0.002 to 0.52 were grown with TTBAl/(TTBAl+TMIn) ratios of 0.62 to 0.82. Under these conditions, no Al was incorporated for TTBAl/(TTBAl+TMIn) ratios less than 0.62. Hall measurements of Al_xIn_1−xSb showed hole concentrations between 5×10¹⁶ cm⁻³ to 2 × 10¹⁷ cm⁻³ and mobilities of 24 to 91 cm²/Vs for not intentionally doped Al_xIn_1−xSb.     

Simultaneous formation of p- and n-type ohmic contacts to 4H-SiC using the ternary Ni/Ti/Al system

Fabrication procedures for silicon carbide power metal oxide semiconductor field effect transistors (MOSFETs) can be improved through simultaneous formation (i.e., same contact materials and one step annealing) of ohmic contacts on both the p-well and n-source regions. We have succeeded with the simultaneous formation of the ohmic contacts for p- and n-type SiC semiconductors by examining ternary Ni/Ti/Al materials with various compositions, where a slash symbol “/” indicates the deposition sequence starting with Ni. The Ni(20 nm)/Ti(50 nm)/Al(50 nm) combination provided specific contact resistances of 2 × 10⁻³ Ω-cm² and 2 × 10⁻⁴ Ω-cm² for p- and n-type SiC, respectively, after annealing at 800°C for 30 min, where the doping level of Al in the SiC substrate was 4.5 × 10¹⁸ cm⁻³ and the level of N was 1.0 × 10¹⁹ cm⁻³.     

Refractory metal boride ohmic contacts to P-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC (1.3×10¹⁹ cm⁻³) using CrB₂, W₂B, and TiB₂. The boride layers (∼100–200 nm) were sputter-deposited in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry. All as-deposited contacts exhibited rectifying characteristics. Ohmic behavior was observed following short anneals (2–10 min) at 1100°C and 5×10⁻⁷ Torr. Current-voltage characteristics were linear for CrB₂ and W₂B and quasi-linear for TiB₂. The lowest values of the specific contact resistance (r_c in Ω-cm²) measured at room temperature for CrB₂ and W₂B were 8.2×10⁻⁵ and 5.8×10⁻⁵, respectively. The specific contact resistance for TiB₂ was not determined accurately. Longer anneals (30 min for W₂B and 90 min for CrB₂) reduced the room temperature values of r_c to 6.1×10⁻⁵ for W₂B and 1.9×10⁻⁵ for CrB₂. Backscattering spectra revealed substantial concentrations of oxygen in all as-deposited boride films. The short anneal cycle removed the oxygen in the CrB₂ films and reduced the concentration substantially in the W₂B films; however, annealing had no affect on the oxygen concentration in the TiB₂ films. The CrB₂/SiC interface remained stable during annealing; i.e., Si and carbon were not observed in the boride layers after annealing. In contrast, W₂B and TiB₂ reacted with the SiC epilayers, and after annealing, Si and carbon were observed at the surface of each boride layer.