Electrical properties of n-type GaS single crystals

Hall effect and Thermally Stimulated Currents (TSC) measurements have been carried out with the current flowing perpendicular to the c-axis on n-GaS crystals grown both from the melt by the Bridgman-Stockbarger method and from the vapour by chemical transport with iodine. An impurity hopping conduction with an activation energy of 0.2 eV has been evidenced in the range of temperatures between 200 and 300 K. The results of TSC measurements indicate the iodine as being responsible for a donor level at 0.44 eV from the conduction band.     

Electrical properties of n-type vapor-grown gallium nitride

Hall measurements are reported for undoped and Zn-doped vapor-grown single crystal GaN on (0001) Al₂O₃ layers with 298 K carrier concentrations (n-type) between 1·4×10¹⁷cm^?3 and 9×10¹⁹ cm^?3. Then n～10¹⁷ cm^?3 crystals (undoped) have mobilities up to μ～440 cm²/V sec at 298 K. Their conduction behavior can be described by a two-donor model between 150 and 1225 K and by impurity band transport below 150 K. Crystals with n≥8×10¹⁸ cm^?3 show metallic conduction with no appreciable variation in n or μ between 10 and 298 K.Results of mass spectrographic analyses indicate that the total level of impurities detected is too low to account for the observed electron concentration at the n～10¹⁹ cm^?3 level, and suggest the presence of a high concentration of native donors in these crystals. No significant reduction in carrier concentration was achieved with Zn doping up to concentrations of ～10²⁰ cm^?3 under the growth conditions of the present work, and no evidence was found to indicate that high conductivity p-type behavior may be achieved in GaN. The influence of factors such as growth rate, crystalline perfection and vapor phase composition during growth on the properties of the layers is described.     

Low-temperature chemical vapor deposition of highly doped n-type epitaxial Si at high growth rate

We investigated the growth of in-situ n-type doped epitaxial Si layers with arsenic and phosphorus by means of low-temperature chemical vapor deposition using trisilane as Si-precursor. Indeed, in order to prevent the alteration of the characteristics of the devices which are already present on the wafer, an epitaxy process at low temperature is highly desired for applications such as BiCMOS. In this work, the varying parameters are the deposition temperature, the Si-precursor mass flow and the dopant gas flow. As a result, a process for the deposition of heavily doped epilayers was demonstrated at 600 °C with high deposition rate, which is important for maintaining high throughput and low process cost. We showed that using trisilane as a Si-precursor resulted in a much more linear n-type doping behavior than using dichlorosilane. Therefore it allowed an easier process control and a wider dynamic doping range. Our process is an interesting route for the epitaxy of a low-resistance emitter layer for bipolar transistor application.     

Influence of intervalley transitions on the photoconductivity in n-type Si 〈100〉 inversion layers

Transitions between the different sub-band systems corresponding to the valleys with low and high effective mass in the direction of surface quantization involve large momentum transfer. As a consequence the probabilities for impurity induced transitions between the respective sub-band systems are low. The corresponding transitions by phonon emission are energetically forbidden if the sub-band splitting does not exceed the energy of acoustic phonons near the K-point. Negative photoconductivity due to carrier transfer into the sub-band system with lower mobility and extremely long response times, as observed experimentally,¹ results if the sub-band splitting is less than this energy. Photoconductivity effects in 〈100〉 inversion layers will be weaker and response times will be much shorter for sub-band splitting larger than the critical value. For the corresponding case in 〈100〉 inversion layers, in contrast, strongly enhanced photoresponse is expected and the long response time should persist.     

Temperature dependence of hole mobility in n-type InSb upon electron bombardment

The mobility of the minority carriers, in n-InSb was investigated from the photomagnetoelectric effect as a function of temperature in the range of 12–80 K prior to and after bombardment with 1 MeV average energy beta particles. The hole mobility shows a minimum near 40 K which can be due to the h-e scattering becoming predominant in the region where other scattering mechanisms are relatively weak. After bombardment the curves have the same form but higher mobilities. This is due to a decrease of h-e scattering resulting from the decrease of the electron concentration, as determined from independent measurements.     

Transport properties of conduction electrons in n-type inversion layers in (100) surfaces of silicon

The conductivities of n-type inversion layers in (100) surfaces of p-type silicon were measured extensively as functions of electron density in the inversion layer, the ambient temperature and the applied magnetic field. Measurements were made on the carefully fabricated four “classes” of MOS field-effect transistors whose maximum mobilities at 4·2K were 14,000, 8000, 6800 and 1500 cm²/V·sec, respectively. From the temperature dependence of the mobility, dominant momentum scattering was reasonably ascribed to surfon at 100 ～ 300 K. and degenerate or non-degenerate coulomb scattering at lower temperatures as treated by Stern and Howard. From the curves of conductivity vs temperature at low temperatures and low electron concentration for specimens with high mobilities, an activation energy of 1·2 meV, relating to the shallow bound states associated with the lowest electrin sub-band, was observed. The conductivity σ_xx of the inversion layer in a strong transverse magnetic field showed behaviors like those of completely free electrons without effects belonging to its material in its oscillation pattern. That is, the peak value of σ_xx as a function of the gate voltage V_R dependend only on the Landau index. The σ_xx as a function of the magnetic field H at a constant V_R showed a similar Shubnikov-de Haas (SdH) type oscillation to that of three dimensional one. The SdH oscillation gave an “apparent” g-value g* which ranges from 2 to 5 depending on the surface carrier density n_s, due to the change in the ratios of the widths of the Landau levels to the level separation. The “reasonable” g-value of the conduction electrons in the inversion layer has been determined using a modified tilted magnetic field method. The g-value at the fixed magnetic field was independent of surface carrier density n_s and tended to 2 in the extreme strong magnetic field.Discussion is made of the g-value relating to the Landau level width and the energy gaps in the density of states under strong magnetic field.     

Study of persistent photoconductivity effect in n-type selectively doped AlGaAs/GaAs heterojunction

A detailed study of the persistent photoconductivity effect (PPE) at selectively doped Al_0.3Ga_0.7As/GaAs interface was carried out at low (4.2 and 77 K) temperatures on samples with different original channel concentrations and mobilities. The observed selectiveness of the PPE to the photon energy allowed us to identify two independent mechanisms of the PPE making almost equal contributions to the total effect. These two mechanisms are: (i) electron photoexcitation from DX centers in AlGaAs layer, (ii) electron—hole generation in bulk GaAs with a charge separation at the interface. It has been found that the behavior of the mobility as a function of the channel concentration (altered by light) depends on a setback thickness d. For a sample with small d a marked mobility drop has been found. The well-resolved structure in the dependence of the electron mobility on the channel concentration has been observed. The first peculiarity is explained by free electron population of AlGaAs layer due to the electron photorelease from DX-centers. The second feature, occurring at higher charge densities in the channel is attributed to the effect of intersubband scattering arising due to the electron occupation of an excited subband at the interface.     

Transport parameters of n-type GaSb

The conduction band parameters of GaSb have been investigated in the temperature region 70–280 K. Hall and magnetoresistance data have been analysed according to the two band model to obtain information about the energy separation ΔE₁₂ of the conduction band minima and its temperature coefficient α₂.     

Electrical characterization of deep levels in n-type GaAs after hydrogen plasma treatment

Deep level transient spectroscopy (DLTS) and Laplace-DLTS (L-DLTS) have been used to investigate defects in an n-type GaAs before and after exposure to a dc hydrogen plasma (hydrogenation). DLTS revealed the presence of three prominent electron traps in the material in the temperature range 20-300 K. However, L-DLTS with its higher resolution enabled the splitting of two narrowly spaced emission rates. Consequently four electron traps at, E_C—0.33 eV, E_C—0.36 eV, E_C—0.38 eV and E_C—0.56 eV were observed in the control sample. Following hydrogenation, all these traps were passivated with a new complex (presumably the M3), emerging at E_C—0.58 eV. Isochronal annealing of the passivated material between 50 and 300 °C, revealed the emergence of a secondary defect, not previously observed, at E_C—0.37 eV. Finally, the effect of hydrogen passivation is completely reversed upon annealing at 300 °C, as all the defects originally observed in the reference sample were recovered.     

Electrical properties and proton conduction of gadolinium doped ceria

The electrical properties and proton conduction of Gd_0.1Ce_0.9O_1.95 (10GCO) were investigated via impedance spectroscopy in different atmospheres and various gas concentration cells. In oxygen atmosphere, GCO is nearly a pure oxygen ionic conductor, while in hydrogen GCO behaves as a mixed conductor of oxygen ions, electrons and protons. Depending on the temperature, the total conductivity is usually enhanced by one to two orders of magnitude in hydrogen than in air/oxygen due to mixed conduction. By examining ionic transport properties of oxygen ions and protons using gas concentration cells we have discovered that the ionic transport properties depend largely on the gas atmospheres and change from one type to the other. Proton conduction generally exists in GCOs, and becomes significant in hydrogen atmospheres, which normally results in a contribution between 5 to 10 % of the total conductivity for 10 GCO. A maximum value of 17 % of the contribution by protons has been observed. The reduction of Ce⁴⁺ to Ce³⁺ of the sample in reduced atmospheres causes the formation of additional oxygen vacancies and electrons, associated also with the creation of protons. All these charge carriers are responsible for the electrical and transport properties of the investigated GCO materials. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

On the g factor for electrons in n-type silicon surface inversion layers

Self-consistent calculations for energy levels are performed for n-type inversion layers of silicon with magnetic field perpendicular to the (100) surface. ‘Apparent’ g factor g¹, obtained from the period of oscillation of states density at Fermi level for varying magnetic field, is plotted as a function of the $\text{Γ}\text{2βH}$, where Γ is the width of Landau levels. The results show that $\text{g}{}^1\text{～ g}\text{for Γ ? 2β}\text{H,}\text{and}\text{g}{}^1\text{～}\text{5 for Γ < 2β}\text{H}$. This means that we should be very careful when interpret the g shift of electrons in inversion layers for small surface electron density.     

Valley splitting and valley degeneracy in n-type silicon (110) inversion layers

The magneto-quantum transport phenomena of silicon (110) n-type inversion layers directly reveal the presence of two types of electrons in i = O and O' electric subband states from their respective quantum oscillations. While the higher i = O' states are occupied by only about 6% of the total electron concentration in our samples, the i = O electrons populate two valley pairs each, the level ladders of which are shifted in energy by less than the Landau splitting in the range where quantum oscillations are observed. We propose this effect to originate from a slight misorientation (Δθ < 0.5°) of the surfaces investigated with respect to the (110) axis. Hence, increasing the gate voltage in a quantizing magnetic field all four i = O valleys are alternatingly occupied in pairs with increasing Landau quantum number in our (110) surfaces contrary to former conclusions on samples with comparable properties. The effect of the magnitude of Δθ on the resulting magneto-quantum oscillations is unimportant for 0 < Δθ ? 0.5° as a result of energy minimization for the total electron system.     

Galvanomagnetic properties of n-type Hg0.8Cd0.2Te

Galvanomagnetic properties of low and high mobility n-Hg_0.8Cd_0.2Te are reported. The experiments were carried out in magnetic fields up to 60 kG and between 1.8 and 77 K. The Hall coefficient does not show thermal and magnetic freeze-out of carriers. At 77 K the transversal magnetoresistance shows a proportionality ?⊥ ∝ B as was predicted by Gurevich and Firsov for the case of polar optical scattering in non-degenerated semiconductors. At 4 K where the mobility is governed by impurity scattering ?⊥ ∝ B^2.4 was observed in the extreme quantum limit. A negative longitudinal magnetoresistance was found at 77 K. The experimental results of high and low mobility samples show significant differences.     

Electrical transport properties of hydrogen-exposed p-type PbTe films

Hall coefficient and d.c. conductivity measurements were made on p-type PbTe epitaxial films exposed to molecular hydrogen gas at high pressures (100–500 psi) in the temperature range (100–300K). It is found that hydrogen converts p-type films to n-type at a pressure of about 300 psi. The results are explained by assuming that the action of hydrogen is to provide donor electrons.     

Irradiation-induced compensation of n-type germanium — electrical measurements

The temperature dependence of the Hall effect in an n-type germanium sample following each of a series of 2 MeV electron irradiations at 80 K has been analysed. It is shown that conversion to p-type proceeds by the introduction of defects which are acceptors contrary to the suggestion made recently that conversion proceeds by the removal of donors.     

Impact of ZnTe buffer on the electrical properties of n-type GaSb:Te films

We have investigated on the molecular beam epitaxy (MBE) of Te-doped GaSb films on ZnTe buffer. Te-doped GaSb (GaSb:Te) films with and without ZnTe buffer were grown on (0 0 1) GaAs substrates. GaSb:Te/ZnTe/GaAs film revealed higher mobility (=631 cm²/V s) in comparison to GaSb:Te/GaAs film (=249 cm²/V s). To explain the higher mobility of GaSb:Te on ZnTe buffer, dislocation density and temperature dependence of Hall measurement results were analyzed. Temperature dependence of Hall measurement shows strong influence of the dislocation scattering, which indicates that dislocation reduction by the ZnTe buffer enhances the carrier mobility of GaSb films.     

Optical properties of residual shallow donors in GaN epitaxial layers grown by horizontal LP-MOCVD

Received: 1 May 1998/Accepted: 23 June 1998     

Photoelectronic characterization of n-type silicon wafers using photocarrier radiometry

Photocarrier radiometry (PCR) was used to characterize four n-type silicon wafers with different resistivity values in the 1-20 Ω cm range. Simulations of the PCR signal have been performed to study the influence of the recombination lifetime and front surface recombination velocity on them; besides, the transport parameters (carrier recombination lifetime, diffusion coefficient, and frontal surface recombination) of the wafers were obtained by means of a fitting procedure. The PCR images that are related to the lifetime are presented, and the first photoelectronic images of a porous silicon sample are obtained.     

Electrical properties of doped 3-tetradecylpolypyrrole/metal devices

The electrical properties of devices made of doped 3-tetradecylpolypyrrole (PPy-C₁₄) thin films sandwiched between indium-tin-oxyde (ITO) and gold metal electrodes are reported. The current density–voltage (J–V) curves are asymmetric and nonlinear implying a non Ohmic rectifying contact. Using standard thermionic emission theory (Schottky) J–V characteristics were satisfactorily fitted with a saturation current of J₀=1.5×10^-5 A cm^-2, a barrier height of ϕ_b=0.7 eV, and an ideality factor of n=5.3. Characteristics from the plot of J–V versus 1/T show that the activation energy of the thermionic emission process is higher below the glass transition temperature of PPy-C₁₄ (T_g=45 °C) than above, which seems to indicate that the hopping conduction process is enhanced at T>T_g. The carrier concentration has been calculated from capacitance–voltage (C-V) measurements (N=1.9×10¹⁷ cm^-3) allowing estimation of the carrier mobility μ=2.6×10^-2 cm² V^-1 s^-1. PACS 73.61.Ph; 73.40.Sx; 73.30.+y     

Optical properties of doped graphene layers

The reflectance of a graphene monolayer, as well as of a system of monolayers, is calculated in the infrared range. A quantum expression for the conductivity in the collisionless regime that depends on the frequency, the temperature, and the concentration of carriers is used in the calculations. Above the threshold of the interband electron absorption, the reflectance decreases with increasing frequency. With decreasing temperature, excitation of plasmons in the system of layers is possible in a narrow range near the threshold, which results in the occurrence of a deep and sharp minimum in the frequency dependence of the reflectance.