Carrier transport in porous silicon

Carrier transport in porous silicon layers has been studied by the time-of-flight method in the strong injection mode at temperatures T=290–350 K and electric field strengths F=(1.5–7)×10⁴ V cm^?1. The electron and hole drift mobilities μ_e≈2×10^?3 cm² V^?1 s^?1 and μ_h≈6×10^?4 cm² V^?1 s^?1 were obtained at T=292 K and F=4×10⁴ V cm^?1. An exponential temperature dependence of drift mobility with activation energy of ～0.38 and ～0.41 eV for, respectively, electrons and holes was established. It is shown that the type of time dependences of the photocurrent associated with carrier drift and the superlinear dependence of the transit time on the reciprocal of the voltage applied to a sample allow use of the concept of space-charge-limited currents under the conditions of anomalous dispersive transport. The experimental data are accounted for in terms of the model of transport controlled by carrier trapping into localized states with energy distribution near the conduction and valence band edges described by an exponential function with a characteristic energy of ～0.03 eV.     

Maximum drift velocity of electrons in selectively doped InAlAs/InGaAs/InAlAs heterostructures with InAs inserts

The dependence of the electron mobility and drift velocity on the growth conditions, thickness, and doping of an InAs insert placed at the center of the quantum well in a selectively doped InAlAs/InGaAs/InAlAs heterostructure has been investigated. Record enhancement of the maximum drift velocity to (2–4) × 10⁷ cm/s in an electric field of 5 × 10³ V/cm has been obtained in a 17-nm-wide quantum well with an undoped 4-nm-thick InAs insert. In the structures with additional doping of the InAs insert, which facilitates an increase in the density of electrons in the quantum well to 4.0 × 10¹² cm^?2, the maximum drift velocity is as high as 2 × 10⁷ cm/s in an electric field of 7 × 10³ V/cm.     

Response Time Measurements in Short-Wave Infrared HgCdTe e-APDs

The impulse response time has been measured as a function of reverse bias, gain, and temperature in backside-illuminated short-wave infrared HgCdTe avalanche photodiodes (APDs) with variable junction geometry. The APD geometry was altered using HgCdTe substrates of variable thickness and by variation of device fabrication parameters. This approach allowed study of the drift–diffusion dynamics of the electrons before entering the junction and the electron and hole dynamics during the junction transition in APDs with different carrier collection distances and junction widths. The response time was typically limited by a double exponential decay, which is attributed to contributions from the impedance mismatch between the interconnection circuit and the 50-Ω radiofrequency probe, and a delayed diffusion response from carriers generated far from the junction. These contributions limited the maximum bandwidth of the diodes to about 600 MHz, independently of gain and temperature. The hot carrier velocities are estimated by fitting the measured response with numerical calculations, taking into account contributions from a direct drift–multiplication response and a delayed diffusion response. This analysis shows that the hot carrier dynamics is close to independent of temperature and that the electron drift velocity saturates at the gain onset to a value of 1 × 10⁷ cm/s, decreasing upon a further increase of the electric field E to a value of about 3 × 10⁶ cm/s at E = 100 kV/cm. The hole velocity shows a slow variation from 3 × 10⁶ cm/s at low electric fields to 1.5 × 10⁶ cm/s at high electric fields.     

Temperature dependence of resistivity and hole conductivity mobility in p-type silicon

A new method is employed to determine the temperature dependence of the resistivity and hole conductivity mobility of p-type silicon. This method involves the use of an aluminum-on-p-Si ohmic diode and a magnesium on p-Si Schottky barrier diode on the same silicon chip. The resistivity is determined from the Al/p-Si ohmic diode. The hole concentration is evaluated from the C-V data of the Mg/p-Si Schottky barrier diode. The conductivity mobility is then computed from the resistivity and hole concentration data. The following ranges are covered: 77–300 K, 0.4–100 ohm-cm and 5 × 10¹⁶?2 × 10¹⁴ holes/cm³ at room temperature.     

The dopant density and temperature dependence of electron mobility and resistivity in n-type silicon

Traditional analysis of electron mobility in n-type silicon neglects the effect of electron-electron scattering in the mobility calculations. As a result, theory fails to conform with experiment when dopant density exceeds 2 × 10¹⁶ cm^?3. In this work, an improved theoretical model for computing mobility and resistivity as functions of dopant density and temperature has been developed for n-type silicon. The model has been applied to phosphorus-doped silicon for dopant densities from 10¹³ to 10¹⁹ cm^?3, and temperatures between 100 and 500 K. The mobility was calculated analytically by appropriately combining lattice, ionized impurity and neutral impurity scattering contributions. The effect of electron-electron scattering was incorporated empirically for dopant densities greater than 2 × 10¹⁶ cm^?3. Additionally, the anisotropic scattering effect was included in the mobility formulations. Resistivity measurements on seven phosphorus-doped silicon wafers with dopant densities from 1.2 × 10¹⁴ to 2.5 × 10¹⁸ cm^?3 were carried out for temperatures from 100 to 500 K. Electron mobility at 300 K was deduced from resistivity and junction C-V measurements for dopant densities from 10¹⁴ to 10¹⁸ cm^?3. Agreement between theoretical calculations and experimental data for both electron mobility and resistivity of phosphorus-doped silicon was within ±7% in the range of dopant densities and temperatures studied.     

Field dependence of the electron drift velocity along the hexagonal axis of 4<Emphasis Type="Italic">H</Emphasis>-SiC

The forward current–voltage characteristics of mesa-epitaxial 4H-SiC Schottky diodes are measured in high electric fields (up to 4 × 10⁵ V/cm) in the n-type base region. A semi-empirical formula for the field dependence of the electron drift velocity in 4H-SiC along the hexagonal axis of the crystal is derived. It is shown that the saturated drift velocity is (1.55 ± 0.05) × 10⁷ cm/s in electric fields higher than 2 × 10⁵ V/cm.     

Evaluation of mobility gaps and density of localized hole states in p-Ge/Ge1−x Six heterostructures in the quantum Hall effect mode

The temperature (0.1 K?T?20 K) and magnetic field (0 T?B?12 T) dependences of the longitudinal (ρ_xx) and Hall (ρ_xy) resistivities have been studied in detail for p-Ge/Ge_1?x Si_x (x=0.07) multilayer heterostructures with hole density p=(2.4–2.6)×10¹¹ cm^?2 and mobility μ=(1.1–1.7)×10⁴ cm² V^?1 s^?1. The energy spectrum parameters of two-dimensional (2D) hole gas in the quantum Hall effect mode have been determined. The mobility gap W=(2–2.5) meV and the background density of localized states g _c =(5–7)×10¹⁰ cm^?2 meV^?1 for the filling factors ν=1 and 2. The results are discussed in terms of long-range impurity potential models for selectively doped 2D systems.     

Transport properties of undoped Cd0.9Zn0.1Te grown by high pressure bridgman technique

The electron drift mobility of undoped Cd_0.9Zn_0.1Te grown by high-pressure Bridgman method is measured by a time-of-flight technique. The sample shows a room temperature mobility and mobility lifetime product of 950 cm²/Vs and 1.6 × 10⁻⁴cm²/V, respectively. The mobility increases monotonically with decreasing temperature to 3000 cm²/Vs at 100 K. The dominant scattering mechanism for the electron transport is discussed by comparing with the theoretical mobility obtained by iterative solution of the Boltzmann equation.     

Optical and transport properties of δ-doped pseudomorphic AlGaAs/InGaAs/GaAs structures

We investigate the effects of spacer layer thickness on the optical and transport properties of the n-typeδ-doped pseudomorphic Al_0.30Ga_0.70As/In_0.15Ga_0.85As / GaAs structures. Aδ-doped AlGaAs/InGaAs/GaAs structure with a 6nm spacer layer yields a sheet carrier concentration of 1.5×10¹² cm^?2 at 77K with electron mobility of 6.4×10³ cm²/Vs, 3.11×10⁴ cm²/Vs, and 3.45×10⁴ cm²/Vs at room temperature, 77 and 20K, respectively. The effects of the different scattering mechanisms on luminescence linewidth and electron mobility have also been discussed.     

The dopant density and temperature dependence of hole mobility and resistivity in boron doped silicon

Theoretical expressions for computing resistivity and conductivity mobility of holes as functions of dopant density and temperature have been derived for boron-doped silicon. The model is applicable for dopant densities from 10¹³ to 3 × 10¹⁸ cm^?3 and temperatures between 100 and 400 K.Using a 3-band [i.e. heavy-hole, light-hole and the spin-orbit splitting (SO) band] model, the hole mobility was calculated by properly combining the contributions from scattering by lattice phonons, ionized impurities and neutral impurities. In addition, the effects of hole-hole (h-h) scattering and nonparabolicity of valence bands were taken into account in the mobility formulation.To verify our theoretical calculations, resistivity measurements on nine boron-doped silicon slices with dopant densities from 4.5 × 10¹⁴ to 3.2 × 10¹⁸ cm^?3 were performed for 100 ≤ T ≤ 400 K, using planar square-array test structure. Agreement between our calculated and measured resistivity values was within 6 percent over the entire range of dopant density and temperature studied here. Excellent agreement (within ±5%) between our calculated hole mobility values and those of Wagner [9] was obtained for N_A ≤ 10¹⁷ cm^?3 for boron-doped silicon, while discrepancies were found for boron densities greater than 10¹⁷ cm^?3. This discrepancy is attributed to neglecting the effect of deionization of boron impurities at higher dopant densities by Wagner (i.e. assuming hole density is equal to the total boron density).     

Microwave Annealing of High Dose Al<Superscript>+</Superscript>-implanted 4H-SiC: Towards Device Fabrication

High-purity semi-insulating 8° off-axis 〈0001〉 4H-SiC was implanted with Al⁺ at different doses and energies to obtain a dopant concentration in the range of 5 × 10¹⁹–5 × 10²⁰ cm^?3. A custom-made microwave heating system was employed for post-implantation annealing at 2,000 °C for 30 s. Sheet resistance and Hall-effect measurements were performed in the temperature range of 150–700 K. At room temperature, for the highest Al concentration, a minimum resistivity of 3 × 10^?2 Ω cm was obtained, whereas for the lowest Al concentration, the measured resistivity value was 4 × 10^?1 Ω cm. The onset of impurity band conduction was observed at around room temperature for the samples implanted with Al concentrations ≥3 × 10²⁰ cm^?3. Vertical p ⁺-i-n diodes whose anodes were made by 1.5 × 10²⁰ cm^?3 Al⁺ implantation and 2,000 °C/30 s microwave annealing showed exponential forward current–voltage characteristics with two different ideality factors under low current injection. A crossover point of the temperature coefficient of the diode resistance, from negative to positive values, was observed when the forward current entered the ohmic regime.     

Analysis of the electrical characteristics of power LDMOSFETs having different design parameters under various temperature

In this paper, we have investigated the electrical characteristics of power lateral double-diffused MOSFETs (LDMOSFETs) having different gate lengths (2.1–3 μm) and drift lengths (6.6–12.6 μm) in the temperature range 100–500 K. The results of this study indicate that gate length and drift region length have a great effect on electrical characteristics, but they have little effect on temperature dependence. The specific on-resistance and the off-state breakdown voltage increase with temperature. The result shows that the specific on-resistance increases exponentially with the exponent 2.2 and, by contrast, the off-state breakdown voltage increases linearly with a slope of 100 mV/K (drift region concentration of measured device: 2×10¹⁵ cm⁻³). As a result, R_on/BV, known for a figure of merit of power device, increases with temperature.     

Dark-current relaxation in MnGa2Se4 single crystals

The results of investigation of isothermal currents and charge accumulation in In-MnGa₂Se₄-In sandwich structures are presented. The obtained data are analyzed on the basis of the theory of isothermal currents and the relay-race mechanism of charge transport. It is shown that the dark-current relaxation in MnGa₂Se₄ single crystals is associated with charge accumulation at deep levels due to injection from the cathode. The following parameters are determined: the contact capacitance C _k = 2 × 10^?13 F, the charge-accumulation-layer thickness d _k = 4 × 10^?6 cm, and the drift charge-carrier mobility μ_ch = 3 × 10^?8 cm² V^?1 s^?1 in MnGa₂Se₄ single crystals.     

Highly transparent and conducting zinc oxide films deposited by activated reactive evaporation

Highly transparent and conducting undoped zinc oxide films have been obtained with a best resistivity of ～1.1 × 10^-3 Ω cm, a carrier density of ～1.5 × 10²⁰ cm^?3 and a mobility of ～38 cm²V^{?1s ?1}. These were produced by activated reactive evaporation at a deposition rate of 2 to 8Å/s with a substrate temperature ≤200° C. The films deposited by this process were found to have resistivities that were thickness independent and also were relatively insensitive to deposition parameters. In terms of conductivity, it was found that films deposited at higher temperatures (T > 300°+ C) were always inferior to the films deposited below 200° C. High temperature vacuum annealing (350° C) significantly degraded the resistivity of the undoped films deposited at low temperature; this was attributable to a drop in both the electron concentration and the mobility. Aluminum doping was found to be able to stabilize the electron concentration while the drop in mobility was found to be related to the choice of substrate.     

Metallic conductivity over an acceptor band of lightly compensated copper-doped p-Hg0.78Cd0.22Te crystals

The conductivity and Hall effect of heavily doped p-Hg_0.78Cd_0.22Te:Cu crystals were studied in the temperature range of 4.2–125 K. The conductivity over the impurity band is of a metallic type for the acceptor concentration N _A>3.8×10¹⁷ cm^?3. The conductivity and the Hall coefficient governed by the delocalized charge carriers in the impurity band are independent of temperature. The sign of the Hall effect is positive in the metallic conductivity range. Near the metal-insulator transition point, the Hall mobility increases linearly with the acceptor concentration and is independent of the acceptor concentration at N _A>1.6×10¹⁸ cm^?3. The metallic conductivity is proportional to N _A in the concentration range under study at N _A<3.1×10¹⁸ cm^?3. The Anderson transition occurs at the Cu concentration N _A=1.4×10¹⁷ cm^?3 in the A ⁺ impurity band, which is formed by positively charged acceptors. Minimum metallic conductivity corresponding to this transition equals 5.1 Ω^?1 cm^?1. It is shown that ?₂ conductivity in the subthreshold region is defined by delocalized carriers in the upper Hubbard band only for fairly heavy doping (N _A>1.4×10¹⁷ cm^?3). For N _A<1.4×10¹⁷ cm^?3, the hopping conductivity is observed.     

Measured In-plane hole drift and hall mobility in heavily-doped strained p-type Si1−xGex

Measured in-plane hole drift and Hall mobilities in heavily boron-doped strained Si_1−xGe_x layers are reported. In the range of boron dopings examined (1.5–2.1 × 10¹⁹ cm⁻³), the drift mobility is seen to increase with increasing germanium fraction. The Hall mobility decreases with increasing germanium fraction. Presented at the 1992 EMC, Boston.     

The mechanisms of hole scattering in p-Hg0.8Cd0.2Te crystals at low temperatures

The resistivity and Hall effect were investigated in p-Hg_0.8Cd_0.2Te crystals that contained from 1.5×10¹⁵ to 1.7×10¹⁸ cm^?3 Cu atoms. The measurements were carried out in the temperature range of 4.2–100 K. It is demonstrated that, in order to correctly determine the Hall mobility of holes at low temperatures, one should exclude the contribution of hopping charge transfer. It was found that heavy holes are scattered at 77 K by each other, by impurity ions, by composition fluctuations, and by lattice vibrations. In compensated crystals, holes are scattered only by lattice vibrations at low temperatures. For uncompensated crystals, when calculating the mobility, it is necessary to make allowance for the hole scattering by positively charged centers formed due to trapping of excess holes by acceptors.     

Optical and electrical properties of porous gallium arsenide

Photoluminescence (PL), Raman scattering, and carrier transport have been studied for the first time in porous GaAs prepared on (111) oriented wafers of n-type crystalline GaAs (faces A and B). Peaks of the main PL band from faces A and B were observed at 1.82 and 1.88 eV, respectively. The electron drift mobility was found to be ～4×10^?4 cm² V^?1 s^?1. The nanocrystallite size in porous GaAs was determined both from PL spectra and from the Raman shift. The obtained values are close or equal to 6–8 nm.     

Effect of high-temperature H2-anneals on the slow-trapping instability of MOS structures

It is shown that the slow-trapping instability upon negative bias-temperature aging of MOS structures (poly-Si and Al-gate) can be significantly reduced by a high temperature (800–900°C) H₂-anneal prior to Al metallization. Additional data are presented on the effects of high (700–900°C) and low temperature (450°C) H₂ annealing of MOS structures containing n-(111) Si, dry HCl oxide and B-doped poly-Si. The midgap N_ss (range: 8×10¹¹ to 1.5×10¹⁰ cm^?2 eV^?1) is reduced by both high and low temperature H₂ anneals whereas the Q_ss (range: 7×10¹¹ to 1×10¹¹ cm^?2 eV^?1) is reduced mainly by the high-temperature H₂-anneals. Presence of B near the interface is believed to cause an abnormal voltage asymmetry of the slow-trapping drift, i.e. ΔV_FB(-BT) < ΔV_FB(+BT). These effects are discussed in the light of Deal's model of the structure of the Si/SiO₂ interface.     

Thermoelectric figure of merit in solid solutions with phonon scattering by off-center impurities

The Seebeck coefficient and the electrical and thermal conductivities (S, σ, and κ) of ternary PbTe_1?xSe_x (x=0.1 and 0.3) and quaternary PbTe_1?2xSe_xS_x (x=0.025, 0.05, 0.1, and 0.15) solid solutions have been studied. Polycrystalline samples with an electron density of (0.5–5.0)×10¹⁸ cm^?3 were used; their quality was monitored by comparing the measured and calculated mobility values at 85 K. A considerable decrease in mobility and an anomalous trend in the σ(T) curve near 77 K were revealed in quaternary alloys with x?0.1; for x=0.15, unusual behavior of κ(T) was also found. According to estimates, the lattice thermal conductivity of this material is temperature-independent in the 80-to 300-K temperature range. This means that a reduction in phonon-phonon scattering with an increase in temperature is completely compensated by an increase in the scattering on impurities. The observed anomalies of σ(T) and κ(T) are considered assuming the possible of off-center location of sulfur atoms at the lattice sites. The thermoelectric figure of merit Z of the studied alloys has been determined in the range 80–300 K. In spite of decreasing mobility, the maximum Z was obtained in a quaternary compound with x=0.1: at 300 K, Z_max=2×10^?3 K^?1 with a carrier density of ～3×10¹⁸ cm^?3; at 175 K, Z_max=1.5×10^?3 K^?1 with the density decreasing to 5×10¹⁷ cm^?3. The obtained data indicate that the introduction of off-center impurities rises Z at T?300 K.