Influence of laser recrystallization and hydrogen annealing on electrical characteristics of polysilicon

 Asenic ions are implanted with doses of 5×10~(11)—5×10~(15)/cm~2 into LPCVD polysilicon films on SiO_2 isolating substrate.The polysilicon films have been recrystallized with CW Ar~+ laser before implantation.Electrical measurements show that the resistivity is lowered and the mobility is increased significantly at low doping concentration(～10~(17)As~+/cm~3).Plasma hydrogen annealing is performed on laser-recrystallized samples.The electrical characteristics of plasma hydrogen annealed samples are close to that of single crystalline silicon.It is found that the resistivity decreases from 1.2 Ω.cm to 0.45 Ω.cm,the mobility rises from 62 cm~2/V.s to 271 cm~2/V.s,the electrical activation energy reduces from 0.03 eV to -0.007 eV and the trapping state density at the grain boundary drops from 3.7×10~(11)/cm~2 to 1.7×10~(11)/cm~2.Based on the existing theoretical models for conduction in polysilicon, a new formula for large grain polysilicon has been proposed,with the help of which,a good agreement between theory and experimental results is achieved within the doping concentration range from 10~(16)/cm~3 to 10~(20)/cm~3.     

Effect of oxygen on the properties of encapsulated polycrystalline CdSe films

This paper presents a comprehensive study of the effects of annealing silicon dioxide encapsulated CdSe films in oxygen on the microstructure, resistivity, photosensitivity and energy levels. The energy levels were investigated by using the independent methods of thermally stimulated current, photocurrent spectral response, and Hall measurements. The film structure is wurtzite with grains of average size 0.35 μm, which extend through the thickness of the films. Annealing the films in oxygen at 450°C increases the resistivity from 10 ohm cm to 10⁶ ohm cm. The electron mobility, which has an activation energy of 0.08 eV, remains constant at about 100 cm² V⁻¹ s⁻¹ during the anneal steps. The change in the resistivity is due to a combination of thermal rearrangement and oxygen diffusing uniformly into the films. Various energy levels ranging from 0.11 eV to 1.3 eV were detected and the density of all these decreased on annealing.     

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.     

Annealing effects on physical properties of doped CdTe thin films for photovoltaic applications

Polycrystalline Cadmium Telluride (CdTe) thin films were prepared on glass substrates by thermal evaporation at the chamber ambient temperature and then annealed for an hour in vacuum ~1×10⁻⁵ mbar at 400 °C. These annealed thin films were doped with copper (Cu) via ion exchange by immersing these films in Cu (NO₃)₂ solution (1 g/1000 ml) for 20 min. Further these films were again annealed at different temperatures for better diffusion of dopant species. The physical properties of an as doped sample and samples annealed at different temperatures after doping were determined by using energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), Raman spectroscopy, transmission spectra analysis, photoconductivity response and hot probe for conductivity type. The optical band gap of these thermally evaporated Cu doped CdTe thin films was determined from the transmission spectra and was found to be in the range 1.42–1.75 eV. The direct energy band gap was found annealing temperatures dependent. The absorption coefficient was >10⁴ cm⁻¹ for incident photons having energy greater than the band gap energy. Optical density was observed also dependent on postdoping annealing temperature. All samples were found having p-type conductivity. These films are strong potential candidates for photovoltaic applications like solar cells.     

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.     

The structural,optical, and electrical properties of vacuum evaporated Cu-doped ZnTe polycrystalline thin films

We have studied the structural, optical, and electrical properties of thermally evaporated, Cu-doped, ZnTe thin films as a function of Cu concentration and post-deposition annealing temperature. X-ray diffraction measurements showed that the ZnTe films evaporated on room temperature substrates were characterized by an average grain size of 300Å with a (111) preferred orientation. Optical absorption measurements yielded a bandgap of 2.21 eV for undoped ZnTe. A bandgap shrinkage was observed for the Cu-doped films. The dark resistivity of the as-deposited ZnTe decreased by more than three orders of magnitude as the Cu concentration was increased from 4 to 8 at.% and decreased to less than 1 ohm-cm after annealing at 260°C. For films doped with 6–7 at.% Cu, an increase of resistivity was also observed during annealing at 150–200°C. The activation energy of the dark conductivity was measured as a function of Cu concentration and annealing temperature. Hall measurements yielded hole mobility values in the range between 0.1 and 1 cm²/V·s for both as-deposited and annealed films. Solar cells with a CdS/CdTe/ZnTe/metal structure were fabricated using Cudoped ZnTe as a back contact layer on electrodeposited CdTe. Fill factors approaching 0.75 and energy conversion efficiencies as high as 12.1% were obtained.     

Optical and electrical properties of AlGaN films implanted with Mn,Co, or Cr

The electrical and optical properties of undoped n-AlGaN films with Al mole fraction close to x=0.4 were studied before and after implantation of 3×10¹⁶ cm⁻² 250-keV Mn, Co, and Cr ions. The electrical properties of the virgin samples are shown to be dominated by deep donors with the level near E_c-0.25 eV and concentration of about 2×10¹⁸ cm⁻³. The microcathodoluminescence (MCL) spectra of the virgin samples were dominated by two strong defect bands at 2.5 eV and 3.7 eV. After implantation, the resistivity of the implanted films increased but could not be accurately measured because of the shunting influence of the unimplanted portions of the films. Their resistivity was increased by more than an order of magnitude compared to the virgin samples because of the compensation by defects coming from the implanted layer during the post-implantation annealing. The absorption and luminescence spectra of the implanted samples were dominated by two strong bands near 2 eV and 3.5 eV. The latter are attributed to the electron transitions from the Mn, Co, or Cr acceptors to the conduction band.     

Neutralization of electrically active aluminum in recrystallized silicon-on-sapphire films

We report the effect of steam oxidation at 875° C on the electrical resistivity, crystalline quality (measured by ion channeling), and Al concentration (measured by secondary ion mass spectrometry) in 0.25 μm thick, Si-implanted and recrystallized, Si-on-sapphire films. After a deep Si implantation (180 keV, 1.4×l0¹⁵ Si/cm²) at room temperature, and solid-phase epitaxial regrowth from the non-amorphized, 0.03 μm thick surface region, the initially undoped SOS films become doped p-type, and their resistivity decreases from (1−5)xl0¹⁴ ficm to 0.5 Ωcm. The doping is due to electrically active Al, released from the A1₂O₃ by the Si implantation, and present in the recrystallized films at a concentration of ≃2×l0¹⁶ Al/cm³ . After a 75 min steam oxidation at 875 °C, which consumes 0.06 Μm of Si, the resistivity of the recrystallized films increases to over 40 Ωcm, but the Al concentration is unchanged. The oxidation also uncovers higher quality material below the non-recrystallized surface layer. A semi-quantitative model is proposed to explain the electrical data, based on the diffusion of oxygen from the Si/SiO₂ interface into the SOS film during oxidation, and the formation of Al-O-Si neutral complexes. Data on the stability of the high-resistivity films against high-temperature annealing or re-amorphization and annealing is given.     

Chlorine-doped CdS thin films from CdCl<Subscript>2</Subscript>-mixed CdS powder

The CdS:Cl thin films have been prepared using thermally evaporated, CdCl₂-mixed CdS powder at 200°C substrate temperature. The percentage of CdCl₂ in the mixture varied from 0% to 0.20%. The electrical properties and the grain size of the deposited films were investigated. The results show that light doping, resistivity, carrier concentration, and mobility follow Seto’s model for polycrystalline material. However, with heavy doping, these properties undergo a saturation trend. The saturation behavior can be understood in terms of the rapid formation of the A-center complexes in the films. The deposited films were annealed at 250°C and 300°C. The resistivity of pure and lightly doped CdS films increased with annealing temperature, whereas carrier concentration and mobility in these films decreased. However, for the higher doping concentrations, the resistivity decreased, whereas carrier concentration and mobility showed improvement. These changes in electrical properties of the deposited films with annealing and doping concentration are attributed to a reduction in the lattice defect sites in CdS upon annealing. The experimental results are interpreted in terms of a modified version of Seto’s model for polycrystalline materials.     

P-type as-doping of Hg1−xCdxTe grown by MOMBE

This paper presents a study of both as-grown and annealed p-type Hg_1−xCd_xTe layers that were doped using a cadmium arsenide source. It is shown that by using a metalorganic molecular beam epitaxy system stable and reproducible p-type HgCdTe:As layers were obtained through direct homogeneous doping. The hole concentrations in the as-grown and annealed samples were 8 × 10¹⁶ to 3 × 10¹⁷ cm⁻³ with mobilities of 120∼300 cm²/V-s. The as-grown HgCdTe:As layers had very good crystalline quality with double crystal x-ray rocking curve line-widths ranging from 27 to 42 arc sec. Experimental data demonstrated a strong correlation of hole concentration and mobility with the surface morphology and crystalline quality as a function of Hg flux. The optimum growth window was defined by a narrow range of Hg flux values that gave a smooth film with fewer voids, and higher hole concentrations and mobilities than were obtained at lower or higher Hg fluxes. This correlation between the growth window defined by the surface morphology and the dopant behavior was very important for the successful growth of p-type As-doped HgCdTe materials.     

Mechanism investigation of NiO<Subscript>x</Subscript> in Au/Ni/p-type GaN ohmic contacts annealed in air

Recently, Au/Ni/p-type GaN ohmic contacts annealed in an air ambient have been widely investigated. However, to obtain a low specific-contact resistance, the annealing window is limited. In this study, to understand the oxidation function of metallic Ni, the Au/Ni/p-type GaN structure was annealed in an air ambient for 10 min at various temperatures. Using x-ray photoelectron spectroscopy (XPS) analysis, the metallic Ni was oxidized into NiO and NiO_1.3 compositions at annealing temperatures of 500°C and 600°C, respectively. However, metallic Ni still existed on the interface of the Ni/p-type GaN annealed at 400°C. The associated barrier heights of 0.42 eV, 0.21 eV, and 0.31 eV were obtained with p-type GaN for the Ni, NiO, and NiO_1.3 contacts, respectively. The hole concentrations of p-type NiO and p-type NiO_1.3 were 2.6×10¹⁶ cm⁻³ and 2.0×10¹⁸ cm⁻³, respectively. The lower hole concentration of the p-type NiO would lead to reducing the valence-band bending of the p-type GaN, as well as the barrier height for holes crossing from the p-type NiO to the p-type GaN. The formation of NiO was thus an important issue for lowering the specific-contact resistance of the Au/Ni/p-type GaN ohmic contacts annealed in an air ambient.     

Selective doping of N-type ZnSe layers with chlorine grown by molecular beam epitaxy

N-type ZnSe with electron concentration up to 3 × 10²⁰ cm⁻³ and low resistivity down to 1 × 10⁻⁴ ohm-cm, has been grown using a selective doping technique with chlorine during molecular beam epitaxy. The photoluminescence evaluation shows that the selectively doped ZnSe layers are superior to uniformly doped ones, especially for the case of high-concentration chlorine doping. The in-depth profile of chlorine concentration in a selectively doped sample was measured with secondary-ion mass spectroscopy (SIMS). The SIMS analysis shows only slight diffusion of the incorporated chlorine atoms even in highly doped samples.     

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.     

Optical and electrochromic properties of annealed lithium-molybdenum-bronze thin films

Optical band gaps, Urbach inverse slopes, and coloration bands of various samples of annealed, microcrystalline Li_xMoO₃-bronze thin films in the concentration range 0<x<0.6 were determined over the photon energy range from 0.4 eV to 4.2 eV. On investigation, it is learned that the measured, optical band gaps do not shift rigidly over the annealing temperature range 293≤T≤423 K and, therefore, do not reveal the Burstein-Moss effect or reflect any stable, crystallographic phase transformation during any investigated annealing cycle. A model relating the temperature-dependent optical gap to the real part of the refractive index has also been developed, and this model fits very well to the annealed data within a maximum error of about 20%. Next, using an oscillator model, a phonon energy of ∼0.08 eV was obtained, which is very close to the characteristic phonon energy of the material, MoO₃. Using this model, it becomes more certain that the contributions to the Urbach absorption edge for the annealed-molybdenum bronzes are coming from the structural and compositional disorder. In another finding, it was found that the absorption-peak energy for the annealed data was about 1.5–1.6 eV, which is still broad and asymmetrical, and therefore, it is almost of the Mo⁶⁺ (or Mo⁴⁺)-Mo⁵⁺ intervalence or polaronic type. Using the polaron model, the half-bandwidth of the coloration bands of investigated, annealed Li_xMoO₃-thin films was found to be almost constant, which is consistent with the nonrigid band behavior.     

Electrical characteristics and structural properties of ohmic contacts to p-type 4H-SiC epitaxial layers

Epitaxial films grown by low-temperature liquid phase epitaxy on p-type 4H-SiC were used as strongly doped subcontact layers for making low-resistance contacts to the p-type material. The layers had a bulk resistivity of ∼0.02 Ω · cm and an aluminum atom concentration of ∼1.5×10²⁰ cm⁻³. The absence of polytype inclusions and the distinct crystalline quality of the strongly doped subcontact layers was confirmed by x-ray diffraction methods. Ohmic contacts with resistivities less than 10⁻⁴ Ω · cm² were prepared by depositing and then annealing multilayer metal mixtures containing Al and Ti. The structural properties and energy characteristics of the resulting ohmic contacts are discussed. Fiz. Tekh. Poluprovodn. 33, 1334–1339 (November 1999)     

Electrical properties and luminescence of CuInSe2

We have studied the electrical properties and luminescence spectra of melt-grown CuInSe₂ single crystals and the doping of the crystals with intrinsic defects and extrinsic impurities. The preparation and the properties of p-n junctions are also discussed. We find that the low temperature photoluminescence spectra of crystals as-grown or annealed between 400 and 700 C are characteristic of the conductivity type. At 77 K, p-type crystals emit in a band peaked at 1.00 eV (type A spectrum) whereas the emission of n-type crystals peaks at 0.93 eV (type B spectrum). Type A and type B spectra can be interchanged by alternate anneals in minimum or maximum Se pressure. Type B emission dominates the electroluminescence spectrum of p-n junctions. These features are explained by the appearing and disappearing of a donor level as a consequence of annealing in vacuum or in Se atmosphere. We find that Zn and Cd act as donors. Crystals doped with these impurities have electron concentrations above 10¹⁸ cm⁻³. Zn-dopped samples exhibit a very broad recombination band below the bandgap in photoluminescence both at 77 and 300 K.     

Analysis of low doping limitation in molecular beam epitaxially grown HgCdTe(211)B epitaxial layers

We report the results of the transport properties and the recombination mechanisms of indium-doped HgCdTe(211)B (x ≈ 23.0% ± 2.0%) layers grown by molecular beam epitaxy. We have investigated the origin(s) of the background doping limitation in these layers. Molecular beam epitaxially grown layers exhibit excellent Hall characteristics down to indium levels of 2 x 10¹⁵ cm⁻³. Electron mobilities ranging from (2-3) x 10⁵ cm²/v-s at 23K were obtained. Measured lifetime data fits very well with the intrinsic band-to-band recombinations. However, below 2 x 10¹⁵ cm⁻³ doping levels, mobility vs temperature curves starts to reflect nonuniformity in carrier distribution. Also, when we reduced the Hg vacancy concentration down to 10¹² cm⁻³ range, by annealing at 150°C, Hall characteristics shows an increase in the nonuniformity in the epilayers. It was found that after annealed at 150°C, the obtained SR defect level has a different origin than the previously obtain Hg-vacancy related defect level.     

Si- and Mg-implanted InP,GaInAs and short-time proximity cap annealing

Si- and Mg-ions with energies of 180 keV have been implanted into semi-insulating InP substrates and low doped n- and p-type GalnAs epitaxial layers (3 · l0¹⁶cm⁻³). Sheet resistances and doping profiles are analyzed and compared with LSS theory. Post-implantation annealing is studied with respect to encapsulation, time and temperature. We have tested as new encapsulation techniques for InP the simple proximity cap annealing and for GalnAs the As-doped spun-on SiO₂. Proximity cap annealing yields decomposition-free surfaces when using a recessed capsubstrate. At annealing temperatures of around 800 °C less activation is obtained than with conventional PSG annealing and a surface accumulation of charge-carriers is established. A time limit of around 3 min is found for Si- and Mg-implanted InP, beyond which the sheet resistance no longer decreases and the doping saturates. For Si in InP, short-time annealing yields to a 68 % activation of carriers, not significantly higher than with conventional long-time annealing. In the case of Si in GalnAs, however, short-time annealing is much more effective. A 100 % activation is obtained for a dose of 2.10¹⁴ cm⁻², while only 7 % is found for long annealing. Even at such a high dose of 1. 10¹⁶cm⁻² we have achieved about an order of magnitude higher activation with short annealing than with long annealing. Most information contained in this paper was presented at the 1984 Electron Materials Conference as paper L-l.     

MeV energy sulfur implantation in GaAs and InP

Room temperature and elevated temperature sulfur implants were performed into semi-insulating GaAs and InP at variable energies and fluences. The implantations were performed in the energy range 1–16 MeV. Range statistics of sulfur in InP and GaAs were calculated from the secondary ion mass spectrometry atomic concentration depth profiles and were compared with TRIM92 values. Slight in-diffusion of sulfur was observed in both InP and GaAs at higher annealing temperatures for room temperature implants. Little or no redistribution of sulfur was observed for elevated temperature implants. Elevated temperature implants showed higher activations and higher mobilities compared to room temperature implants in both GaAs and InP after annealing. Higher peak electron concentrations were observed in sulfur-implanted InP (n ≈ 1 × 10¹⁹ cm⁻³) compared to GaAs (n ≈ 2 × 10¹⁸ cm⁻³). The doping profile for a buried n⁺ layer (n ≈ 3.5 × 10¹⁸ cm⁻³) of a positive-intrinsic-negative diode in GaAs was produced by using Si/S coimplantation.     

Shallow and Deep Centers in As-Grown and Annealed MgZnO/ZnO Structures with Quantum Wells

Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied before and after annealing in oxygen atmosphere at high temperatures of 850°C to 950°C. In both as-grown and annealed structures, microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13 eV compared with bulk ZnO films, indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of shallow uncompensated donors from ~10¹⁷ cm⁻³ to ~10¹⁶ cm⁻³ and makes it possible to probe the region of the QW by capacitance–voltage (C–V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are the well-known traps with activation energies of 0.3 eV and 0.8 eV. After annealing, the electron traps observed in the structure have activation energies of 0.14 eV, 0.33 eV, and 0.57 eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of band-edge luminescence and produce a deep luminescence defect band at 2.2 eV. In addition, a defect vibrational band becomes visible in Raman spectra near 650 cm⁻¹. No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed.