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.     

On the behavior of Bi in a CdTe lattice and the compensation effect in CdTe:Bi

CdTe crystals of two types have been grown by the vertical Bridgman method: (i) crystals doped with Bi to ～10¹⁸ cm^?3 and (ii) double-doped (Bi + Cl) crystals with a Bi concentration of ～10¹⁸ cm^?3 and a Cl concentration of ～10¹⁷ cm^?3. The temperature dependences of the resistivity, photoconductivity, and low-temperature photoluminescence are investigated for the crystals grown. Analysis has shown that doping with Bi (crystals of the first type) leads to compensation of the material. The resistivity of the CdTe:Bi samples at room temperature, depending on the doping level, is varied in the range of 10⁵–10⁹ Ω cm. The hole concentration is determined by the acceptor level at E _v + 0.4 eV in lightly doped CdTe:Bi samples and by the deep center at E _v + 0.72 eV in heavily doped CdTe:Bi samples. Double doping leads to inversion of the conductivity type and reduces the resistivity to ～1 Ω cm. Heavily doped CdTe:Bi crystals and double-doped crystals exhibit the presence of acceptors with an ionization energy of 36 meV, which is atypical of CdTe.     

Highly transparent conductive films of thermally evaporated In2O3

Highly transparent (over 90% transmission in the visible range) and highly conductive (resistivity ≈2 × 10^-4 ohm-cm) indium oxide (undoped) films have been produced by thermal evaporation from In₂O₃ + In source in a vacuum chamber con-taining low pressures of O₂, . Film properties are comparable or superior to the best tin-doped indium oxide films that have ever been reported, and excellent reproducibility has been achieved. Hall effect measurements have revealed that the observed low resistivity is primarily a result of the excellent electron mobility (? 70 cm²/V-sec), although the electron concentration is also rather high (≥4 × l0²⁰/cm³). X-ray diffraction measurements show distinctly polycrystal-line In₂O₃ structure with a lattice constant ranging from 10.07Å to 10.11Å. Electrolytic electroreflectance spectra exhibit at least four critical transitions, from which we have determined the direct and indirect optical band gaps (3.56eV and 2.69eV, respectively). Burstein shifts due to the population of electrons in the condition band are also observed. From an internal photoemission study, the work function of the In₂O₃ film has been determined to be 5.0eV. These and other results, along with a discussion of the processing details are reported.     

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.     

Effects of surface treatment using aqua regia solution on the change of surface band bending of p-type GaN

Effects of surface treatment on the change of band bending at the surface of p-type GaN were studied using synchrotron radiation photoemission spectroscopy, and the results were used to interpret the reduction of contact resistivity by the surface treatment. The contact resistivity on p-type GaN decreased from (5.1±1.2)×10⁻¹ to (9.3±3.5)×10⁻⁵Ω cm² by the surface treatment using aqua regia prior to Pt deposition. Surface band bending was reduced by 0.58 eV and 0.87 eV after the surface treatments by HCl and aqua regia solutions, respectively. The atomic ratio of Ga/N decreased as the photoelectron detection angle was decreased, indicating that the surface oxide was mainly composed of Ga and O, GaO_x, formed during high-temperature annealing for the generation of holes, and Ga vacancies, V_Ga, were produced below the GaO_x layer. Consequently, the aqua regia treatment plays a role in removing GaO_x formed on p-type GaN, leading to the shift of the Fermi level toward the energy levels of V_Ga located near the valence band edge. This causes the decrease of barrier height for the transport of holes, resulting in the good ohmic contacts to p-type GaN.     

Low resistance contacts to p-type cadmium telluride

A survey of metal contacts to p-type CdTe has revealed that the best contact is one prepared by vacuum deposition of CuAu onto a surface that has been etched with K₂Cr₂O₇:H₂SO₄ . The contact resistivity on 0.5 ohm-cm p-type CdTe is between 0.1 and 0.5 ohm-cm² . The contact resistivity does increase if subjected to heat treatment, but is stable in air at room temperature for at least several months. Contacts of CuAu and Au on a chromate etched surface were compared in detail with similar metal contacts on a methanol-bromine etched surface through measurements of current vs. voltage as a function of temperature, photovoltaic response, and Auger analysis. CuAu contacts on a chromate etched surface are describable in terms of a thermally assisted tunneling model with a barrier height of about 0.60 eV. Au contacts on either a chromate etched or a methanol-bromine etched surface can be described equally well by a thermally assisted tunneling model or by a thermionic emission model. In either case the barrier height is about 0.1 eV larger on the methanol-bromine etched surface than on the chromate etched surface. Auger analysis indicates that chromate etching produces a tellurium rich surface that plays a key role in determining subsequent contact properties.     

Specific resistivity of a metal-n GaAs OHMIC contact with an intermediate N+ layer

The specific contact resistivity ρ_c of a metal-n GaAs structure, over a wide range of carrier concentrations in the intermediate layer (5 × 10¹⁸–5 × 10²⁰ cm^?3) and in the substrate (10¹⁵?10¹⁷ cm^?3) is calculated. The results which are presented graphically demonstrate the dependence of ρ_c on metal-semiconductor barrier height from 0.2 eV–0.8 eV. A comparison with the measured data of an alloyed AuGeNi-n GaAs system, suggests that the calculations corresponding to a concentration of 5 × 10¹⁸ cm^?3 in the layer and barrier height of 0.4 eV give the best fit based on the experimental observations. It is hoped that the results can be used as a guideline in developing ohmic contacts of ultra low resistance values.     

Realization of intrinsic p-type ZnO thin films by metal organic chemical vapor deposition

P-type ZnO thin films were grown on sapphire substrates with and without nitrous oxide (N₂O) by metal organic chemical vapor deposition (MOCVD). The intrinsic p-type ZnO films were achieved by controlling the Zn:O ratio in the range of 0.05–0.2 without N₂O flow. Secondary ion mass spectroscopy (SIMS) showed that the films contained little or no nitrogen (N) impurities for all samples. The p-type behavior of the samples should be due to the intrinsic acceptor-like defects V_Zn, for ZnO film grown without nitrous oxide, and N, occupying O sites as acceptors for ZnO film grown with nitrous oxide. The best p-type ZnO film has low resistivity of 0.369 Ω-cm, high carrier density of 1.62×10¹⁹ cm⁻³, and mobility of 3.14 cm²/V-s. The obtained p-type ZnO films possess a transmittance of nearly 100% in the visible region and strong near-band-edge emission.     

Growth of CdGeAs2 single crystals by the chemical vapor transport method

We have grown CdGeAs₂ single crystals by chemical vapor transport (CVT), a method not previously applied for this compound. The crystallographic data of this chalcopyrite (cell parametersa ₀ = 5.9456 ± 0.0001Å, c₀ = 11.2131 ± 0.0007Å) and its electrical transport properties are reported. Predominantly n-type crystals are obtained (at RTn = 1 · 10¹⁷cm?3, μⁿ = 2000 cm²(Vs)^?1). Vacuum heat treatment at 500° C yields a type conversion fromn- to p-type. In all p-type samples the minority carrier mobility is calculated to be larger than 10000 cm²(Vs)^?1.     

Thermally stimulated current measurements on proton implanted gallium phosphide

The thermally stimulated current technique has been used to investigate deep levels introduced into high resistivity GaP as a result of 300 keV proton bombardment. It was found that levels at 0.19, 0.22, 0.35, 0.40, 0.48 and 0.61 eV below the conduction band were present in the unimplanted material and that after implantation the number of centres responsible for the 0.19, 0.40 and 0.61 eV levels increased and additional levels at 0.24 and 0.75 eV below the conduction band were detectable. Trapping centre capture cross sections were in the range 10^?15 cm² – 10^?11 cm^?2.     

Preparation and properties of Mn-doped epitaxial gallium arsenide

The preparation of manganese-doped GaAs epitaxial layers growth from the liquid-phase, and Hall effect and resistivity measurements in the temperature range from 60 to 300°K, are described. An anomalous solubility of GaAs in the Ga + GaAs + Mn solution was observed and occurrence of a Mn₂As-phase in solution was identified. From the analysis of the Hall effect measurements the thermal activation energy of the manganese acceptor was determined. The energy decreases from 0·092 to 0·084 eV with the Mn-concentration in the epitaxial layers increasing from 6 × 10¹⁷ cm^?3 to 3 × 10¹⁹ cm^?3. The solubility of Mn in GaAs at the temperature of the preparation (850°C) was found to be about 4 × 10¹⁹ cm^?3.     

Hopping conduction in heavily doped bulk n-type SiC

The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 10¹⁷ cm⁻³ or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy ε₃ obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The ε₃ value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy E_a for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.     

Effects of Ni, Pd, and Pt Substitutions on Thermoelectric Properties of CoSi Alloys

Ni-, Pd-, and Pt-substituted CoSi samples have been prepared by an arc melting and annealing procedure. The x-ray diffraction and scanning electron microscopy results show that Ni and Pd are effective n-type dopants for CoSi, while Pt is immiscible with CoSi and forms an impurity phase with a possible chemical formula of PtCoSi₂. The thermoelectric properties were measured from 80 K to 300 K. For Ni- and Pd-doped samples, the electrical resistivity and Seebeck coefficient decrease simultaneously due to the increasing carrier concentration. For Pt-substituted samples, the electrical resistivity also decreases. However, this reduction is thought to be not due to an increase in carriers but rather to originate from the effect of the Pt-rich impurity phase at the grain boundaries. The Seebeck coefficient is not affected by 1% Pt substitution; however, further increase of the Pt level also causes a decrease in the Seebeck coefficient. The room-temperature power factor is 63 μW K^?2 cm^?1 for pure CoSi and 73 μW K^?2 cm^?1 for the Co_0.99Pt_0.01Si sample. Although the thermal conductivity is reduced for both n-type-doped and Pt-substituted samples around 80 K, the room-temperature values are still close to that of pure CoSi. As a result, ZT of 0.13 is obtained at room temperature for Co_0.99Pt_0.01Si, an 18% increase compared with CoSi.     

CVD-based tungsten carbide schottky contacts to 6H-SiC for very high-temperature operation

In this study, tungsten carbide, with its hardness, chemical inertness, thermal stability and low resistivity (25 μΩ cm)¹ is shown as a reliable contact material to n- and p-type 6H-SiC for very high temperature applications. WC films with thicknesses of 100–150 nm were deposited by chemical vapor deposition (CVD) from a WF₆/C₃H₈/H₂ mixture at 1173 K. A method to pattern CVD-tungsten carbide is suggested. TEM analysis of as deposited samples displayed a clear and unreacted interface. The electrical investigations of the p-type 6H-SiC Schottky contacts revealed a high rectification ratio and a low reverse current density (6.1 × 10⁻⁵ A cm⁻², −10 V) up to 773 K. On n-type, a low barrier (Φ_Bn=0.79 eV) at room temperature was observed. The low Φ_Bn value suggests WC to be promising as an ohmic contact material on highly doped n-type epi-layers. We will show a temperature dependence for the barrier height of tungsten carbide contacts that can be related to the simultaneous change in the energy bandgap, which should be considered when designing SiC devices intended for high temperature operation.     

Submillimeter wave photoconductivity observation of shallow donors in In0.53Ga0.47As

The hydrogen like 1s → 2p (m=?1,0,+1) transitions of two donors have been observed in high intensity magnetic fields up to 8.5T. The m=?1 transitions ocurred between 2 cm^?1 and 25 cm^?1. The signature curves for donors in ternary semiconductor In_0.53Ga_0.47As have now been established.     

Effect of features of the technology of polycrystalline CdTe growth on the conductivity and deep level spectrum after annealing

The conductivity, morphology, and deep levels in polycrystalline CdTe are studied. Undoped p-CdTe is grown from the vapor phase by low-temperature methods of direct Cd and Te chemical reaction and CdTe vacuum sublimation at P _min. Chlorine-doped CdTe is also grown. The resistivity of the grown samples is ～105–109 Ω cm. After annealing in liquid cadmium or in cadmium vapor at ～500°C, the conductivity type changes, the free-carrier concentration in the undoped and doped samples increases to 4 × 10¹⁵ and ～2 × 10¹⁶ cm^?3, respectively. For all samples, a defect ground level of ～0.84 eV and continuous background are observed in DLTS spectra after annealing. A correlation between the primary-defect and free-carrier concentrations in undoped and doped CdTe is observed. Chlorine is a main residual impurity in the undoped samples. It is assumed that the defect is a complex including chlorine and observed structural defects in CdTe.     

Electrical characteristics of thin Ni2Si,NiSi, and NiSi2 layers grown on silicon

We have determined the resistivity, carrier concentration, and Hall mobility as a function of thickness (700–3000 Å) of Ni₂Si, NiSi, and NiSi₂ layers formed by vacuum annealing at 270÷v300°C, ≈ 400°C, and ≈ 800°C, respectively, of nickel films vacuum-deposited on a silicon substrate (111 n-type and 100 p-type Si ρ ≈ 1KΩ). The layer thicknesses were measured by 2 MeV⁴He⁺ backscattering spectrometry. The silicide phase was confirmed by x-ray measurements. The electrical measurements were carried out using van der Pauw configuration. We found the electrical transport parameters to be independent of the film thickness within the experimental uncertainty. The Hall factors were assumed to be unity. The majority carriers are electrons in NiSi and holes in Ni₂Si and NiSi₂. The resistivity values are 24±2, 14±1, and 34±2 μΩcm, the electron concentrations are 9±3, 10 and 7±1, and ≈ 2 × 10²² cm^?3, and the Hall mobilities are 3±1, ≈ 4.5 and 6, and ≈ 9 cm²/Vs for Ni₂Si, NiSi (〈100〉 and 〈111〉), and NiSi₂, respectively. The systematic error in the measured values caused by currents in the high resistivity substrate is estimated to be less than 6% for the Hall coefficient. The results show that Ni₂Si, NiSi, and NiSi₂ layers formed by a thin film reaction are electrically metallic conductors, a result which concurs with those reported previously (1) for refractory metal silicides. The Hall mobility increases with the Si content in the silicide. The electron concentration is lowest for NiSi₂ leading to the highest resistivity for the epitaxial phase of NiSi₂.     

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.     

Ni(Pt) 锗硅化合物/Si1-xGex 接触电阻的测试

The electrical properties of Ni0.95Pt0.05-germanosilicide/Si1_xGex contacts on heavily doped p-type strained Sil-xGex layers as a function of composition and doping concentration for a given composition have been investigated. A four-terminal Kelvin-resistor structure has been fabricated by using the conventional com- plementary metal-oxide-semiconductor （CMOS） process to measure contact resistance. The results showed that the contact resistance of the Ni0.95Pt0.05-germanosilicide/Sil-xGex contacts slightly reduced with increasing the Ge fraction. The higher the doping concentration, the lower the contact resistivity. The contact resistance of the samples with doping concentration of 4×10^19 cm^-3 is nearly one order of magnitude lower than that of the sam- ples with doping concentration of 5 × 10^17 cm^-3. In addition, the influence of dopant segregation on the contact resistance for the lower doped samples is larger than that for the higher doped samples.     

Optical properties of Ca4Ga2S7:Eu2+

The features of optical transitions in the photon-energy range of 1.85–3.00 eV in single-crystal Ca₄Ga₂S₇:Eu²⁺ were determined from optical measurements in the temperature range of 77–300 K. It was ascertained that indirect and direct optical transitions with band gaps of E _gi=1.889 eV and E _gd=2.455 eV, respectively, occur at 300 K in the photon-energy ranges of 2.2–2.6 and 2.6–3.0 eV. The temperature coefficients of E _gi and E _gd are equal to ?5.15×10^?4 and ?14.86×10^?4 eV/K, respectively.