Tin-doping ofn + InP OMVPE layers

Tin as a donor dopant in OMVPE epilayers of InP has been studied for its dependence on growth parameters to assess the nature of the dopant incorporation from a tetraethyltin (TESn) source and to establish reproducible conditions for tailoring carrier concentrations. Free carrier concentrations depend linearly on Sn/In ratio and are independent of PH₃ concentration consistent with the impurity incorporation on In-lattice sites. The carrier concentration saturates at 3 × 10¹⁹ cm³ and is accompanied by excess Sn accumulation on the layer surface. X-ray diffraction shows an expansion of the lattice proportional both to the measured free carrier concentration and to the TESn gas concentration up to solid saturation. The lattice expansion is larger than expected from Sn for In radius substitution. SIMS profiles for abrupt turn-on and turn-off of the dopant source show transient changes of Sn concentration consistent with a Sn surface layer buildup.     

Carbon tetrachloride doped Al x Ga1−x As grown by metalorganic chemical vapor deposition

A dilute mixture of CCl₄ in H₂ has recently been shown to be a suitable carbon doping source for obtainingp-type GaAs grown by metalorganic chemical vapor deposition (MOCVD) with carbon acceptor concentrations in excess of 1 × 10¹⁹cm⁻³. To understand the effect of growth parameters on carbon incorporation in CCl₄ doped Al _x Ga_1−x As, carbon acceptor concentration was studied as a function of Al composition, growth temperature, growth rate, and CCl₄ flow rate using electrochemical capacitance-voltage profiling. The carbon incorporation as a function of Al composition, growth temperature and CCl₄ flow rate was also measured by secondary ion mass spectroscopy (SIMS). All layers were grown by low pressure MOCVD using TMGa and TMAl as column III precursors, and 100% AsH₃ as the column V source. Increased Al composition reduced the dependence of carbon concentration on the growth temperature. Reduced growth rate, which resulted in substantially decreased carbon acceptor concentrations in GaAs, had an insignificant effect on the carrier concentration of Al_0.4Ga_0.6As. A linear relationship between hole concentration and CC1₄ flow rate in Al_xGa_1−x As for 0.0 ≤x ≤ 0.8 was observed. These results are interpreted to indicate that adsorption and desorption of CCl _y (y ≤ 3) on the Al _x Ga_1-x As surface during crystal growth plays an important role in the carbon incorporation mechanism.     

Growth and characterization of undoped and N-type (Te) doped MOVPE grown gallium antimonide

The growth of GaSb by MOVPE and itsn-type doping using a dimethyltellurium dopant source are investigated. The results of growth rate, morphology and Te incorporation as a function of growth parameters are given. Increasing growth temperature and V/III reactant ratio were found to reduce the Te incorporation. The lowest Hall carrier concentrations obtained at room-temperature, onp-type andn-type MOVPE GaSb are respectively:p _H= 2.2 × 10¹⁶cm⁻³ with a Hall mobility ofμ _H= 860 cm²/V.s andn _H= 8.5 × 10¹⁵cm⁻³ withμ _H= 3860 cm²/V.s. Furthermore, Hall mobilities as high as 5000 cm²/V.s were measured onn-type GaSb samples.     

Epitaxial Growth and Characterization of p-Type ZnO

N-doped p-type ZnO thin films were grown on c-sapphire substrates, semi-insulating GaN templates, and n-type ZnO substrates by metal organic chemical vapor deposition (MOCVD). Diethylzinc and oxygen were used as precursors for Zn and O, respectively, while ammonia (NH₃) and nitrous oxide (N₂O) were employed as the nitrogen dopant sources. X-ray diffraction (XRD) studies depicted highly oriented N-doped ZnO thin films. Photoluminescence (PL) measurements showed a main emission line around 380 nm, corresponding to an energy gap of 3.26 eV. Nitrogen concentration in the grown films was analyzed by secondary ion mass spectrometry (SIMS) and was found to be on the order of 10¹⁸ cm⁻³. Electrical properties of N-doped ZnO epilayers grown on semi-insulating GaN:Mg templates were measured by the Hall effect and the results indicated p-type with carrier concentration on the order of 10¹⁷ cm⁻³.     

Quantitative oxygen measurements in OMVPE Al x Ga1−x As grown by methyl precursors

Oxygen has always been considered to be a major contaminant in the organo-metallic vapor phase epitaxy (OMVPE) of Al _x Ga_1−x As. Oxygen incorporation has been invoked as a contributor to low luminescence efficiency, dopant compensation and degradation of surface morphology among other deleterious effects. This study presents quantitative measurements of oxygen concentration in nominally high purity Al _x Ga_1−x As. The oxygen concentration was measured as a function of alloy composition, growth temperature, andV/III ratio. Quantitative secondary ion mass spectroscopy (SIMS) measurements were used to determine the oxygen content as well as the carbon concentration in the film. The oxygen concentration increases with decreased growth temperature and V/III ratio while increasing superlinearly with Al content in the epitaxial layer.     

Structural and Electrical Properties of Atomic Layer Deposited Al‐Doped ZnO Films

Structural and electrical properties of Al‐doped ZnO (AZO) films deposited by atomic layer deposition (ALD) are investigated to study the extrinsic doping mechanism of a transparent conducting oxide. ALD‐AZO films exhibit a unique layer‐by‐layer structure consisting of a ZnO matrix and Al₂O₃ dopant layers, as determined by transmission electron microscopy analysis. In these layered AZO films, a single Al₂O₃ dopant layer deposited during one ALD cycle could provide ≈4.5 × 10¹³ cm^?2 free electrons to the ZnO. The effective field model for doping is suggested to explain the decrease in the carrier concentration of ALD‐AZO films when the interval between the Al₂O₃ layers is reduced to less than ≈2.6 nm (>3.4 at% Al). By correlating the electrical and structural properties, an extrinsic doping mechanism of ALD‐AZO films is proposed in which the incorporated Al atoms take oxygen from the ZnO matrix and form doubly charged donors, such as oxygen vacancies or zinc interstitials.     

Vapor Annealing as a Post-Processing Technique to Control Carrier Concentrations of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Thin Films

This article demonstrates that carrier concentrations in bismuth telluride films can be controlled through annealing in controlled vapor pressures of tellurium. For the bismuth telluride source with a small excess of tellurium, all the films reached a steady state carrier concentration of 4 × 10¹⁹ carriers/cm³ with Seebeck coefficients of −170 μV K⁻¹. For temperatures below 300°C and for film thicknesses of 0.4 μm or less, the rate-limiting step in reaching a steady state for the carrier concentration appeared to be the mass transport of tellurium through the gas phase. At higher temperatures, with the resulting higher pressures of tellurium or for thicker films, it was expected that mass transport through the solid would become rate limiting. The mobility also changed with annealing, but at a rate different from that of the carrier concentration, perhaps as a consequence of the non-equilibrium concentration of defects trapped in the films studied by the low temperature synthesis approach.     

Nitrogen doping of ZnSe and CdTe epilayers: A comparison of two rf sources

Nitrogen is the most promising dopant for p-type ZnSe, and is attractive for CdTe p-type doping, but unwanted compensating centers have limited the maximum achievable carrier densities in both of these material systems. One important factor is the nitrogen source. The characteristics of the nitrogen source (i.e., ratio of charged to neutral species) influence both the nitrogen incorporation rate and the types of centers which may be produced in epilayers. We have performed a study of ZnSe:N and CdTe:N using two different rf plasma sources: an Oxford CARS-25 and an EPI Unibulb. The EPI source has a very high efficiency for producing atomic nitrogen (∼70%) while maintaining a very low total ion content. Both sources gave a high incorporation of nitrogen; however, Hall measurements indicate that compensation is still a problem in p-type doping of these materials. A detailed liquid-helium-temperature photoluminescence study has been performed to monitor changes in the defect structure produced using different source operating parameters.     

The preparation of (Al2O3)x(SiO2)y thin films using [al(OSiEt3)3]2 as a single-source precursor

Amorphous (Al₂O₃)_x(SiO₂)_y thin films have been grown by atmospheric pressure metal-organic chemical vapour deposition using the single-source precursor [Al(OSiEt₃)₃]₂. Characterisation by X-ray photoelectron spectroscopy indicated that the films consisted of a mixture of Al₂O₃, SiO₂ and an aluminosilicate. The relative amount of each species was dependent on the deposition temperature and the carrier gas composition. Use of NH₃ as the carrier gas resulted in the increased volatility of the precursor by the in situ formation of the low-melting Lewis acid–base adduct Al(OSiEt₃)₃(NH₃); however, no nitrogen incorporation was observed in these deposited films.     

Properties of In-Doped ZnO Films Grown by Metalorganic Chemical Vapor Deposition on GaN(0001) Templates

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 10¹⁹ cm⁻³ as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 10¹⁹ cm⁻³ was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.     

Characterization of vapor phase epitaxial ZnSe films

Properties of ZnSe films doped with donor impurities were investigated. The ZnSe films were grown at 350°C by using metallic zinc and selenium as the source materials; their vapors were transported separately by H₂ gas under atmospheric pressure. Iodine-doped ZnSe films were grown using CH₃I (1000 ppm, diluted in helium) as a dopant source. However, it was necessary to stop this dopant flow during the film growth to obtain epitaxial films. HC1 gas etching and evacuation of the reaction apparatus before the film growth began were employed to obtain epitaxial films and to avoid redistribution of impurities without heat-treatment at higher temperature. Secondary ion mass spectroscopy analysis indicated that both chlorine and gallium were included in the layers, as well as iodine, because of residual HC1 gas. Optically high-quality and rather highly conductive n-type ZnSe films were obtained. Maximum electron concentration was 3.3 × 10¹⁷ cm⁻³.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te epitaxial films for terahertz detectors

Liquid-phase epitaxy is used to fabricate Pb_0.8Sn_0.2Te films, undoped or doped with indium to different levels. The depth profiles of the carrier density and dopant concentration in the films are measured and examined. A uniform dopant concentration to a depth of 15 μm is obtained. Electrical-conduction inversion is observed at a temperature of 77.3 K as the doping level is varied. The liquid-phase epitaxial method is shown to be a more suitable technology for the reproducible manufacture of epitaxial films with a given carrier density, such as the ones used in terahertz detectors.     

An Initial Investigation of Nitrogen Doping of Wide-Bandgap HgCdTe During Molecular-Beam Epitaxy Using Ar/N Plasmas

An initial investigation of the use of atomic nitrogen for controlled p-type doping of wide-bandgap Hg_0.3Cd_0.7Te (x = 0.7) is reported. Mixtures of argon and nitrogen, ranging in nitrogen concentration from 0.1% to 100%, have been utilized to demonstrate well-controlled nitrogen incorporation in the 10¹⁶ cm⁻³ to 10²⁰ cm⁻³ range using total gas flow rates of 0.3 sccm to 4.0 sccm and radiofrequency (RF) powers of 100 W to 400 W. Nitrogen doping exhibits several desirable attributes including abrupt turn-on and turn-off and minimal sensitivity to variations in growth temperature and HgCdTe composition, with no negative effects on HgCdTe dislocation density and morphology. Preliminary electrical measurements indicate primarily n-type behavior in the 10¹⁴ cm⁻³ to 10¹⁵ cm⁻³ range in as-grown x = 0.7 HgCdTe and CdTe films doped with nitrogen at 10¹⁸ cm⁻³ to 10²⁰ cm⁻³ concentrations, while ZnTe films have exhibited p-type electrical activity with hole concentrations approaching 10²⁰ cm⁻³.     

In situ B-doped Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition: Physical and electrical characterization

In situ boron doping of Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition (RPCVD) has been studied using secondary ion mass spectroscopy (SIMS), Hall effect measurements, defect etching in conjunction with Nomarski microscopy, cross-sectional transmission electron microscopy (XTEM), and current-voltage measurements. Boron incorporation is shown to be controllable and electrically active from 7 × 10¹⁷ to over 10²⁰ cm^-3, with no dependence on process parameters (temperature, rf power, and substrate bias) in the ranges studied, other than the B₂H₆/SiH₄ gas-phase ratio. No change in deposition rate upon introduction of B₂H₆ dopant gas is seen, contrary to what has been observed in several higher-temperature CVD processes. No defects such as stacking faults are seen under Nomarski microscopy, but a visible haze covers some areas ofin situ B-doped wafers. This haze appears to consist of amorphous cone-shaped structures with their apexes at the substrate-epilayer interface. The origin of the conical defects is believed to be related to some phenomenon at the initiation of growth. In order to evaluate the electrical quality ofin situ B-doped epilayers,P ⁺/N mesa diodes have been fabricated using both homoepitaxial and heteroepitaxial (Ge_xSi_1-x)p-type epitaxial films. The electrical junction in these diodes coincides with the (epi-substrate)—interface in the grown films. To avoid interdiffusion or annealing effects during diode fabrication, all processing temperatures were kept at or below 450‡ C. Ideality factors are 1.2-1.3 for all diodes, indicating diffusion-limited transport rather than recombination in the depletion region.     

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.     

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 10¹⁸ cm⁻³ in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N₂ dopant flow. Due to the high n-type background carrier concentration of the order of 10¹⁷ cm⁻³ in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 10¹⁸ cm⁻³. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H₂ was used as carrier gas.     

Optimization of Digital Growth of Thick N-Polar InGaN by MOCVD

Smooth 200 nm thick N-polar InGaN films were grown by metal–organic chemical vapor deposition (MOCVD) on sapphire using a digital approach consisting of a constant In, Ga, and N precursor flow with pulsed injection of H2 into the N2 carrier gas. Using this growth scheme, the H2 injection time was altered and the effect on the morphology and indium incorporation in the films observed. The effect of periodic insertion of additional GaN inter-layers on the surface morphology of the InGaN layers was also studied.     

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.     

Characterization of AlxInyGa1−x−yN quaternary alloys grown on sapphire substrates by molecular-beam epitaxy

High-resolution X-ray diffraction (HR-XRD) with rocking curve, atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy have been performed on high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using a molecular beam epitaxy (MBE) technique with aluminum (Al) mole fractions x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. HR-XRD measurements confirmed the high crystalline quality of these alloys without any phase separation. The X-ray rocking curve of Al_xIn_yGa_1−x−yN films typically shows full widths at half maximum (FWHM) intensity between 14.4 and 28.8 arcmin. AFM measurements revealed a two-dimensional (2D) growth mode with a smooth surface morphology of quaternary epilayers. PL spectra exhibited both an enhancement of the integrated intensity and an increasing blueshift with increased Al content with reference to the ternary sample In_0.1Ga_0.90N. Both effects arise from Al-enhanced exciton localization. PL was used to determine the behavior of the energy band gap of the quaternary films, which was found to increase with increasing Al composition from 0.05 to 0.2. This trend is expected since the incorporation of Al increases the energy band gap of ternary In_0.1Ga_0.90N (3.004 eV). We have also investigated the bowing parameter for the variation of energy band gaps and found it to be very sensitive on the Al composition. A value of b=10.4 has been obtained for our quaternary Al_xIn_yGa_1−x−yN alloys.