Tunnett

Conclusion The p⁺–n⁺–i(v)–n⁺ GaAs Tunnett diodes have been fabricated with control of the growing diffusion technique in TDM CVP liquid phase epitaxial growth. The threshold current density (J_th) of 2.8×10⁴ A/cm², the threshold voltage (V_th) of 5.3V for the oscillation and the 1% duty cycle operation have been succeeded. The more reduction of J_th and V_th will be able to realize the CW operation.     

Optical absorption spectrum of Ni2+ ions doped in sodium potassium sulphate single crystal

Single crystals of nickel-doped sodium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the observed bands could be made assuming the octahedral symmetry for the Ni^{2 +} ion in the crystal. The bands have been ascribed to transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The experimental and calculated energies are in good agreement. The crystal field parameters derived areDq= =880 cm^–1,B= 900cm^–1 andC=3600 cm^–1.One of the authors, Sujatha John expresses her thanks to the Secretary and the Principal, R. B. V. R. Reddy College, Hyderabad for according her permission to pursue the M. Phil. course.     

Electronic absorption spectrum of Ni2+ in lithium potassium sulphate single crystal

Single crystals of nickel-doped lithium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the bands could be made assumingO _h symmetry for the Ni²⁺ ion in the crystal. The bands have been assigned transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The crystal field parameters derived areDq=910cm^–1,B=890cm^–1 andC=3560cm^–1.The authors wish to express their thanks to Prof. K. Sreerama Murthy for his constant encouragement throughout this investigation. The authors are also thankful to Prof. Mihir Chowdhury, Indian Association for the cultivation of Science, Calcutta for giving permission to take the spectra.     

Energy levels of vanadium ions in CdTe

In CdTe doped with vanadium the photoluminescence due to the ³ T ₂(F) ³ A ₂(F) transition of V³⁺(d ²) is detected. Its decay time is determined as (630±20) s, a result comparable to the analogous emissions in various host lattices. Further emissions around 5000 cm^–1 and 9000 cm^–1 are caused by charge-transfer transitions or bound-exciton decay. Excitation and sensitization spectra yield information on the positions of the energy levels within the gap, which are discussed using two different models. At T=4.2 K, the distance of the V²⁺/V³⁺ donor level is 7300 cm^–1 and 5700 cm^–1 referred to the valence and the conduction band edges, respectively. The absence of V²⁺(d ³) centres is tentatively ascribed to the existence of deeply bound excitons.     

Formation of thermally stable low-resistance Ti/W/Au ohmic contacts on n-type GaN

A Ti(12 nm)/W(20 nm)/Au(50 nm) metallization scheme has been investigated for obtaining thermally stable low-resistance ohmic contacts to n-type GaN (4.0×10¹⁸ cm^-3). It is shown that the current–voltage (I–V) characteristics of the samples are abnormally dependent on the annealing temperature. For example, the samples that were annealed at temperatures below 750 °C for 1 min in a N₂ ambient show rectifying behavior. However, annealing the samples at temperatures in excess of 850 °C results in linear I–V characteristics. The contact produces a specific contact resistance as low as 8.4×10^-6 Ω cm² when annealed at 900 °C. It is further shown that the contacts are fairly thermally stable even after annealing at 900 °C; annealing the samples at 900 °C for 30 min causes insignificant degradation of the electrical and structural properties. Based on glancing angle X-ray diffraction and Auger electron microscopy results, the abnormal temperature dependence of the ohmic behavior is described and discussed. PACS 72.80.Ey; 73.40.Cg; 73.20.At; 79.60Bm; 73.40.Gk     

Shallow acceptor population and free hole concentration in Si: In and Si:Ga with IR-photoexcitation

Hall measurements at low temperaturesT<50 K have been performed on Si:In (N _In10¹⁷ cm^–3) and Si:Ga (N _Ga10¹⁸ cm^–3) with infrared photoexcitation of holes into the valence band. It is shown in quantitative agreement with a theoretical model that the population of shallow acceptors, e.g. B and Al, which are present as impurities in concentrations ofN _B,Al10¹²-10¹⁴ cm^–3 strongly affects the photoexcited hole concentration. Photo-Hall measurements can, therefore, serve as a tool for the determination of low impurity acceptor concentrations in the case of high In- or Ga-doping. Hole capture coefficientsB _In=6×10^–4 (T/K)^–1,8 cm³ s^–1 andB _Ga=2×10^–4 (T/K)^–1 cm³ s^–1 have been determined.     

The sub-micrometer thickness n-InSb/i-GaAs epilayers for magnetoresistor applications at room temperatures of operation

Magnetotransport at fields up to 500 mT and LF-noise characteristics are reported for miniature magnetoresistors with ferrite concentrators based on Sn-doped n-InSb/i-GaAs heterostructures grown by MBE. The thickness of the InSb epilayers lie in the range 0.55–1.5 μm giving room temperature mobilities of 2.5–5.5 m² V⁻¹ s⁻¹ with carrier densities of (0.5–1.5)×10¹⁷ cm⁻³. The room temperature magnetoresistance (MR) for our two terminal devices could be as high as 115% at 50 mT which is comparable to the extraordinary MR (ExMR) recently reported in microscopic composite van der Pauw disks four terminal devices [Science 289 (2000) 1530]. In addition, a high signal-to-noise ratio and a good temperature stability of R(B)/R₀=0.5–0.83% K⁻¹ was observed for B<60 mT (below the saturation field B_sat for ferrite). Device resistance stability R₀(T) was equal to 0.27–0.66% K⁻¹ in zero field with a nominal device resistance R₀=197–224 Ω for DC currents in the range I=0.01–1.0 mA. The minimum detectable magnetic field is estimated from the reduced differential MR (∂R/∂B)/R=2000% T⁻¹ at B=31 mT and normalised 1/f current noise power spectral density measured at the same field. The resolution limit B_min=2.6 nT at 10² Hz and B_min=0.82 nT at 10³ Hz. These resolution limits are seven times better than those recently reported for the same material n-InSb/i-GaAs and ferrite fabricated Hall sensors [Magnetotransport and Raman characterization of n-InSb/i-GaAs epilayers, for Hall sensors applications over extremely wide ranges of temperature and magnetic field, Proceedings NGS 10, IPAP Conference Series 2, IPAP, Tokyo, 2001, pp. 151–154].     

The fabrication and characterization of 4H SiC power UMOSFETs

The fabrication of 4H-SiC vertical trench-gate metal-oxide-semiconductor field-effect transistors(UMOSFETs) is reported in this paper.The device has a 15-μm thick drift layer with 3×1015 cm-3 N-type doping concentration and a 3.1μm channel length.The measured on-state source-drain current density is 65.4 A/cm2 at Vg = 40 V and VDS = 15 V.The measured threshold voltage(Vth) is 5.5 V by linear extrapolation from the transfer characteristics.A specific on-resistance(Rsp-on) is 181 mΩ·cm2 at Vg = 40 V and a blocking voltage(BV) is 880 V(IDS = 100 μA@880V) at Vg = 0 V.     

AC-hopping conductance of self-organized Ge/Si quantum dot arrays

Dense (n=4×10¹¹ cm^-2) arrays of Ge quantum dots in a Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ΔΓ=Γ(ω,H)-Γ(ω,0), and change of velocity of SAW, ΔV/V=(V(H)-V(0))/V(0), were measured in the temperature interval T=1.5–4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f=30–300 MHz. Based on the dependences of ΔΓ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed based on existing theoretical concepts.     

Characterization of a a-Si:H thin-film transistors and the effect of thermal annealing

An analysis of characteristics of a-Si:H thin-film transistors were performed. The mobility of electrons in the accumulation layer induced by a gate voltage was in a order of 0.5 cm²/V · s at a field strength lower than 1×10⁴V/cm, and proportional toE ^–r at higher electric field, wherer was 0–0.2.The effect of thermal annealing at the temperatures 100–160°C on the parametersV _T andr are discussed. The activation energies for the variation of both parameters were 0.31 eV and 0.33 eV, respectively, that suggests the mechanism influencing both parameters may be the same. The mechanism is discussed in relation to the carrier hopping through the network of localized states.     

Excited Even-Symmetry States of Metallocomplexes of Porphin and Its Derivatives

Based on calculations by the CNDO/S method, data on the excited molecular states of even parity of the magnesium complexes of porphin (P), tetraazaporphin (TAP), tetrabenzoporphin (TBP), and phthalocyanine (Phc) are obtained. It is only in MgP that the first excited g-state 1¹ B _2g (29,000 cm^–1) is located 300 cm^–1 higher than the B level (28,700 cm^–1). In MgTBP, the two states 1¹ B _1g (24,700 cm^–1) and 1¹ B _2g (25,500 cm^–1) are found to be near the B level (27,500 cm^–1), while the states 1¹ B _2g (25,500 cm^–1) in MgTAP and 1¹ B _2g (21,000 cm^–1) and 1¹ B _1g (23,100 cm^–1) in MgPhc are located much lower than the B level; the energy of the latter is 31,900 and 32,400 cm^–1 in MgTAP and MgPhc respectively. The results obtained are in good agreement with experimental data on two-photon absorption: in the zinc complex of tetraphenylporphin (TPhP), the g-state is detected in the region of the B level, while in ZnPhc, two bands at 20,400 and 21,700 cm^–1 show up.     

High-resolution infrared spectra of HCF3 in the ν6 (500 cm) and 2ν6 (1000 cm) regions: rovibrational analysis and accurate determination of the ground state constants C0 and DK

The high-resolution infrared spectrum of HCF₃ was studied in the ν₆ fundamental (near 500 cm⁻¹) and in the 2ν₆ overtones (near 1000 cm⁻¹) regions. The present study reports on the analysis of the hot bands in the ν₆ region, as well as the first observation and assignment of the 2ν₆² perpendicular band. Using ν₆, 2ν₆^±2–ν₆^±1 and 2ν₆² experimental wavenumbers, accurate coefficients C₀ and D_K⁰ of the K-dependent ground-state energy terms were obtained, using the so-called “loop method.” Ground-state energy differences Δ(K,J)=E₀(K,J)−E₀(K−3,J) were obtained for K=3–30. A least-squares fit of 81 such differences gave the following results (in cm⁻¹): C₀=0.1892550(15); D_K⁰=2.779(26) × 10⁻⁷.     

Analysis of the 2ν3 band of CD3F at 5.06 μm recorded under high resolution

The 2ν₃(A₁) band of ¹²CD₃F near 5.06 μm has been recorded with a resolution of 20–24 × 10⁻³ cm⁻¹. The value of the parameter (α^B − α^A) for this band was found to be very small and, therefore, the K structure of the R(J) and P(J) manifolds was unresolved for J < 15 and only partially resolved for larger J values. The band was analyzed using standard techniques and values for the following constants determined: ν₀ = 1977.178(3) cm⁻¹, B″ = 0.68216(9) cm⁻¹, D″_J = 1.10(30) × 10⁻⁶ cm⁻¹, α^B = (B″ − B′) = 3.086(7) × 10⁻³ cm⁻¹, and β^J = (D″_J − D′_J) = −3.24(11) × 10⁻⁷ cm⁻¹. A value of α^A = (A″ − A′) = 2.90(5) × 10⁻³ cm⁻¹ has been obtained through band contour simulations of the R(J) and P(J) multiplets.     

Fabrication and electrical characterization of p-Sb2S3/n-Si heterojunctions for solar cells application

Antimony trisulphide (Sb₂S₃) films were prepared by thermal evaporation technique on n-type single crystal Si substrates to fabricate p-Sb₂S₃/n-Si heterojunctions. The electrical transport properties of the p–Sb₂S₃/n-Si heterojunctions were investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by multi-step tunnelling current and the activation energy of saturation current was about 0.54 eV. The 1/C²–V plots indicated the junction was abrupt and the junction built-in potential was 0.6 V at room temperature and decreased with increasing temperature. The solar cell parameters have been calculated for the fabricated cell as V_oc = 0.50 V, J_sc = 14.53 mA cm⁻², FF = 0.32 and η = 4.65% under an illumination of 50 mW cm⁻².     

Molecular magnetism of a linear Fe(III)-Mn(II)-Fe(III) complex. Influence of long-range exchange interaction

The magnetic properties of [L-Fe(III)-dmg₃Mn(II)-Fe(III)-L] (ClO₄)₂ have been characterized by magnetic susceptibility, EPR, and Mössbauer studies. L represents 1,4,7-trimethyl-, 1,4,7-triazacyclononane and dmg represents dimethylglyoxime. X-ray diffraction measurements yield that the arrangement of the three metal centers is strictly linear with atomic distancesd _Fe-Mn=0.35 nm andd _Fe-Fe=0.7 nm. Magnetic susceptibility measurements (3–295 K) were analyzed in the framework of the spin-Hamiltonian formalism considering Heisenberg exchange and Zeeman interaction:=J _Fe-Mn(S _Fe1+S _Fe2)S _Mn +J _Fe-Fe(S _Fe1 S _Fe2) +g_B S _total B. The spinsS _Fe1=S _Fe2 =S _Mn=5/2 of the complex are antiferromagnetically coupled, yielding a total spin ofS _total=5/2 with exchange coupling constantsF _Fe-Mn=13.4 cm^–1 andJ _Fe-Fe= 4.5 cm^–1. Magnetically split Mössbauer spectra were recorded at 1.5 K under various applied fields (20 mT, 170 mT, 4T). The spin-Hamiltonian analysis of these spectra yields isotropic magnetic hyperfine coupling withA _total/(g _{N N})=–18.5 T. The corresponding local componentA _Fe is related toA _total via spin-projection:A _total=(6/7)A_Fe. The resultingA _Fe/(g _NN)=–21.6 T is in agreement with standard values of ferric high-spin complexes. Spin-Hamiltonian parameters as obtained from Mössbauer studies and exchange coupling constants as derived from susceptibility measurements are corroborated by temperature-dependent EPR studies.     

High-resolution infrared and millimeterwave spectra of the v3=1 vibrational state of NF3 at 907 cm

The ν₃^±1 perpendicular band of ¹⁴NF₃ ( cm⁻¹) has been studied with a resolution of 2.5 × 10⁻³ cm⁻¹, and 3682 infrared (IR) transitions (J_max=55, K_max=45) have been assigned. These transitions were complemented by 183 millimeterwave (MMW) rotational lines (J_max=25, K_max=19) in the 150–550 GHz region (precision 50–100 kHz). The kl=+1 level reveals a strong A₁/A₂ splitting due to the l(2,2) rotational interaction (q=−4.05 × 10⁻³ cm⁻¹) while the kl=−2 and +4 levels exhibit small A₁/A₂ splittings due to l(2,−4) and l(0,6) rotational interactions. All these splittings were observed by both experimental methods. Assuming the v₃=1 vibrational state as isolated, a Hamiltonian model of interactions in the D reduction, with l(2,−1) rotational interaction (r=−1.96 × 10⁻⁴ cm⁻¹) added, accounted for the observations. A set of 26 molecular constants reproduced the IR observations with σ_IR=0.175 × 10⁻³ cm⁻¹ and the MMW data with σ_MMW=134 kHz. The Q reduction was also performed and found of comparable quality while the QD reduction behaved poorly. This may be explained by a predicted Coriolis interaction between v₃=1 and v₁=1 (A₁, 1032.001 cm⁻¹) which induces a slow convergence of the Hamiltonian in the QD reduction but has no major influence on the other reductions. The experimental equilibrium structure could be calculated as: r_e(N–F)=1.3676 Å and (FNF)=101.84°.     

Nearest neighbor effect on capture cross sections measured in IR phototransients on Si:In

The extrinsic photoconductive decay at T=20–100 K is analyzed in FZ-grown Si: In material after pulsed irradiation by a PbSSe infrared laser (=4 m). Trapping time constants (=10 ns-100 s) are resolved for the prevalent In acceptor (N _In=10¹⁶–10¹⁷ cm^–3) and for additional shallow acceptors B, Al, and the X(In)-center present at low concentrations (N=10¹²–10¹⁴ cm^–3). Hole capture cross sections determined for the acceptor levels show a large scatter over up to 4 orders of magnitude. It is shown that the capture cross section is dependent on all the dopant concentrations present in the sample due to nearest neighbor interaction. Due to the formation of donor-acceptor dipoles, the capture cross section assumes low values. A model calculation of the interaction based on only fundamental parameters of Si is in accordance with the experimental data within the experimental error. The hole capture cross sections for isolated acceptors are _p=1×10^–12, 1×10^–14, 1×10^–13, 2.5×10^–13 cm² for indium, X-center, aluminum, and boron at the temperatures T=95 K, 100 K, 70 K, 45 K, respectively.     

Influence of magnetic field on cesium thermionic energy converter

This article deals with the calculation of the influence of the magnetic field upon the electric current of a thermionic converter presupposing the approach to conditions in a low-pressure cesium converter. The distribution of the starting velocities of the emitted electrons is considered firstly as independent of the angle from the perpendicular to the emitter plane, and secondly according to the cosine law.The magnetic field effect from the converter current is calculated and compared with the calculations in the papers by Schock [1] and Block [2]; the effect of the external magnetic field is verified by measurements on a solar thermionic converter prototype.Symbols F=I/I ₀ factor of current reduction from magnetic field effect - ¯F value of factorF (when the magnetic field is not constant) - I [A/m²] density of collector current (real current influenced by magnetic field) - I ₀ [A/m²] theoretical density of collector current (in ideal case equals electron emission current) - T _e [°K] electron gas temperature; assumed equal to emitter temperatureT _E [°K] - B[Wb/m²] magnetic induction (field) - D[m] distance from emitter to collector - R[m] radius of electrodes, emitter and collector - r[m] variable radius in the limits 0 toR - V [m/s] random velocity of electron - v _xz [m/s] component of the vectorV inx-z plane - v _m =2kT _E /m most probable velocity in the velocity distribution according to Maxwell and Boltzmann - w-v _xz /v _m relatively expressed electron velocityv _xz - the angle of any vectorV - [m] radius of circular electron path - n [m^–3] number (density) of electrons with certain value of random velocity - n ₀ [m^–3] total electron number (density) - n ₁ [m^–3] number of electrons returned to emitter by means of magnetic field - N ₀ [m^–2s^–1] total flow of thermionic electrons emitted from a unit surface - N ₁ [m^–2s^–1] partial flow of electrons returned to emitter - P=N ₁/N₀ relatively expressed flow of electrons returned to emitter (whenB = const.) - ¯P mean value ofP (whenB const.) - F _cos, ,P _cos, values asF,¯F,P,¯P in case of velocity distribution according to cosine law - m=9·107×10^–31 [gk] electron mass - e=1·60×10^–19 [C] electron charge - k×1·38×10^–23 [J/grad] Boltzmann's constant - ₀ 1·257×10^–6 [H/Vs] permeability of vacuum     

Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.