Native disorder potential at the surface of a heavily doped semiconductor

The dependence of native potential inhomogeneities on spatial dispersion of the dielectric response of the two-dimensional electron gas at the surface of a heavily doped semiconductor is discussed. The amplitude and scale of the disorder potential in the case of a strongly degenerate surface electron gas are determined. It is shown that the inhomogeneities considered depend on the surface and bulk parameters.     

Rigid band analysis of heavily doped semiconductor devices

The conventional carrier transport equations used in device analysis must be modified for heavily doped semiconductor regions. The modifications to Shoekley's auxiliary equations relating the carrier densities to their corresponding quasi-Fermi levels are derived for the rigid band model. We include the effects of asymmetric bandgap narrowing and of carrier degeneracy (Fermi-Dirac statistics). Emphasis is placed on writing the equations in a simple form that indicates the effect of changes in the band structure due to heavy doping. In this form they can serve as a basis for computer-aided analysis and design. We show that, in general, the effective intrinsic carrier density nieas well as the electron and hole current densities depend on the asymmetry in bandgap narrowing. However, for the special case of low-level injection, nieand the minority current density depend only on the total bandgap narrowingDeltaE_{g}. Furthermore, we indicate that interpretation of experiments with theory using Boltzmann statistics, instead of Femi-Dirac statistics, will underestimateDeltaE_{g}in degenerate material.     

Electrical properties of heavily doped polycrystallinesilicon-germanium films

The electrical properties of polycrystalline silicon-germanium (poly-Si_1-xGe_x) films with germanium mole fractions up to 0.56 doped by high-dose ion implantation are presented. The resistivity of heavily doped p-type (P⁺) poly-Si_1-x Ge_x is much lower than that of comparably doped poly-Si, because higher levels of boron activation and higher hole mobilities are achieved in poly-Si_1-xGe_x. The resistivity of heavily doped n-type (N⁺) poly-S_1-xGe_x is similar to that of comparably doped poly-Si for x<0.45; however, it is considerably higher for larger Ge mole fractions due to significant reductions in phosphorus activation. Lower temperatures (~500°C), as well as lower implant doses, are sufficient to achieve low resistivities in boron-implanted poly-Si_1-xGe_x films, compared to poly-Si films. The work function of P⁺ poly-Si_1-xGe_x decreases significantly (by up to ~0.4 Volts), whereas the work function of N⁺ poly-Si_1-xGe_x decreases only slightly, as Ge content is increased. Estimates of the energy bandgap of poly-Si_1-xGe_x show a reduction (relative to the bandgap of poly-Si) similar to that observed for unstrained single-crystalline Si_1-xGe_x for a 26% Ge film, and a reduction closer to that observed for strained single-crystalline Si _1-xGe_x for a 56% Ge film. The electrical properties of poly-Si_1-xGe_x make it a potentially favorable alternative to poly-Si for P⁺ gate-material applications in metal-oxide-semiconductor technologies and also for p-channel thin-film transistor applications     

Transport equations for the analysis of heavily doped semiconductor devices

Transport equations for use in analyzing heavily doped semiconductor devices are considered. These transport equations describe the effects of the nonuniform band structure and the influence of Fermi-Dirac statistics, which are important in heavily doped semiconductors. Previous workers [1, 2] have derived transport equations in terms of the nonuniform band structure. These equations, however, are not convenient for use in semiconductor device analysis because the band structure of heavily doped semiconductors is not well known. In this paper, the transport equations of Marshak and van Vliet [1, 3] are recast into a simple, Boltzmann-like form in which the effects associated with the nonuniform band structure and degenerate carrier concentrations are described by two parameters, the effective gap shrinkage, Δ_G, and the effective asymmetry factor, γ. The experimental determination of both of these parameters is also discussed. Finally, Adler's contention [4], that some important features of semiconductor device operation can be modeled accurately by using an electrically measured Δ_G with an arbitrarily chosen γ, is considered. The validity of this procedure, under certain simplifying assumptions, is established.     

Gold-germanium-based ohmic contacts to the two-dimensional electron gas at selectively doped semiconductor heterointerfaces

Results of a study of ohmic contacts to the two-dimensional electron gas (2DEG) at N⁺-n III-V semiconductor heterointerfaces are presented. In a comparison of alloyed metallizations based on the Au-27at. %Ge eutectic system, the addition of Ni and the method of deposition were found to have the largest effects in lowering the contact resistance. The Ni-Ge-Au Ohmic contact reproducibly gives a (width-normalized) contact resistance of less than 0.2 Ω . mm, which is adequate for MESFET applications using these structures. MESFET's fabricated with (Al,Ga) As and (Al,In,Ga) As heterojunction 2DEG channels and Ni-Ge-Au contacts have source-drain resistances (Rsd), which decrease dramatically at low temperature as a result of the mobility enhancement in the 2DEG channel and the small contribution of contact resistances. The transconductance (gm) of the device thus more nearly approaches its high intrinsic value. At 77 K, the best (Al,Ga) As FET's had Rsd= 0.69 Ω . mm and gm= 309 mS/mm with gate lengths of 1.5 µm and a source-drain spacing of 9 µm. A microwave gain of 11 dB at 6.4 GHz has been obtained at room temperature for these devices.     

Scattering of conduction electrons at a spatially correlated system of charges in a heavily doped GaAs: Te semiconductor

The results of studying the absorption of infrared radiation by free charge carriers in the GaAs:Te single crystals, grown by the Czochralski method, had the electron concentration n ₀=5×10¹⁷?6×10¹⁸ cm^?3 are reported. An analysis of the spectral dependences of the absorption coefficient took into account the spatial correlation in the impurity-charge distribution. It is shown that the short-range correlation model makes it possible to account for the decrease in the absorption coefficient and a weakening of its spectral dependence, in the region of the impurity-mediated free-carrier absorption.     

Features of conduction mechanisms in n-HfNiSn semiconductor heavily doped with a Rh acceptor impurity

The crystal structure and electron-density distribution, as well as the energy, kinetic, and magnetic characteristics of n-HfNiSn intermetallic semiconductor heavily doped with a Rh acceptor impurity in the temperature range T = 80–400 K, in the acceptor-concentration range N _A ^Rh ≈ 9.5 × 10¹⁹?1.9 × 10²¹ cm^?3 (x = 0.005–0.10), and in magnetic fields H ≤ 10 kG are investigated. It is established that doping is accompanied by a simultaneous decrease in concentration, the elimination of donor-type structural defects (to x ≈ 0.02), and an increase in the concentration of acceptor-type structural defects (0 < x ≤ 0.10). The dependence of the degree of semiconductor compensation on temperature is revealed. A model of the spatial arrangement of atoms in HfNi_{1 ? x} Rh _x Sn is proposed, and the results of calculating the electron structure based on this model agree with the results of investigations of the kinetic and magnetic characteristics of the semiconductor. The results are discussed within the context of the Shklovskii-Efros model for a heavily doped and compensated semiconductor.     

Band gap widening and narrowing in moderately and heavily doped n-ZnO films

In semiconductors, a widening of the optical band gap occurs because the lower states in the conduction band are blocked. At the same time band gap narrowing also occurs due to many body interactions on doping. This paper reports the analysis of widening and narrowing of optical band gap in sol–gel derived ZnO films moderately doped with Yttrium and heavily doped sputtered ZnO films with aluminum and scandium. The band gap was evaluated using optical transmission data. Carrier concentration was known from the Hall measurements. At high concentrations the effective change in band gap is found to be the difference of the band gap widening and band gap narrowing. At low concentration of dopant the many body theories do not apply and the experiments also show that the band gap narrowing is practically negligible at these concentrations and the effective band gap widening is determined by the band gap widening alone. The chemical nature of the dopant played practically no role.     

Features of conductivity of the intermetallic semiconductor n-ZrNiSn heavily doped with a Bi donor impurity

The crystal structure, distribution of the electron density of states, and the energy, kinetic, and magnetic properties of the intermetallic semiconductor n-ZrNiSn heavily doped with a Bi donor impurity have been investigated in the ranges T = 80?400 K, N _D ^Bi ?? 9.5 × 10¹⁹cm^?3 (x = 0.005)?1.9 × 10²¹ cm^?3 (x = 0.10), and H ?? 0.5 T. It has been established that such doping generates two types of donor-like structural defects in the crystal, which manifest themselves in both the dependence of the variation in the unit cell parameter a(x) and temperature dependence of resistivity ln??(1/T) of ZrNiSn_{1 ? x} Bi _x (x = 0.005). It is shown that ZrNiSn_{1 ? x} Bi _x is a new promising thermoelectric material, which converts thermal energy to electric energy much more effectively as compared to n-ZrNiSn. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and strongly compensated semiconductor.     

Effective electron mass in heavily doped GaAs in the ordering of impurity complexes

Spectra of edge photoluminescence (PL) at 300 K have been studied in a set of Czochralski-grown Te-doped GaAs single crystals with a free carrier density of n₀=10¹⁷–10¹⁹ cm^?3. The carrier density dependences of the chemical potential and band gap narrowing are obtained by analyzing the PL spectral line profiles. The dependence of the effective mass of electrons at the bottom of the conduction band on their density, m ₀ ^* (n₀), is calculated. It is shown that the nonmonotonic m ₀ ^* (n₀) dependence correlates with data on electron scattering in the material under study and results from the ordering of impurity complexes.     

Features of the conduction mechanisms of the n-HfNiSn semiconductor heavily doped with the Co acceptor impurity

The crystalline structure, electron density distribution, energy and kinetic parameters of a HfNi_{1 ? x} Co _x Sn semiconductor heavily doped with a Co acceptor impurity are studied in the ranges T = 80?C1620 K and N _A ^Co from 9.5 × 10²⁰ cm^?3 (at x = 0.05) to 7.6 × 10²¹ cm^?3 (at x = 0.40). It is shown that variations in the activation energy of hopping conduction ? ₃ ^?? (x) and the modulation amplitudes of continuous energy bands ? ₃ ^?? (x) are caused by the appearance of a donor source in the n-type HfNi_{1 ? x} Co _x Sn semiconductor. It is shown that the doping of n-HfNiSn with a Co acceptor impurity is accompanied by a change in the degree of compensation of the semiconductor due to the simultaneous generation of both structural acceptor-type defects during Ni atom substitution with Co atoms and structural donor-type defects during the partial occupation of Ni sites by Sn atoms. The results are discussed within the Shklovskii-Efros model for a heavily doped and compensated semiconductor.     

Thermal stability of Ni silicide films on heavily doped n and p Si substrates

Electrical and structural properties of Ni silicide films formed at various temperatures ranged from 200 °C to 950 °C on both heavily doped n⁺ and p⁺ Si substrates were studied. It was found that surface morphology as well as the sheet resistance properties of the Ni silicide films formed on n⁺ and p⁺ Si substrates at the temperatures higher than 600 °C were very different. Agglomerations of Ni silicide films on n⁺ Si substrates begin to occur at around 600 °C while there is no agglomeration observed in Ni silicide films on p⁺ Si substrates up to a forming temperature of 700 °C. It was also found that the phase transition temperature from NiSi phase to NiSi₂ phase depend on substrate types; 900 °C for NiSi film on n⁺ Si substrate and 750 °C for NiSi film on p⁺ Si substrate, respectively. Our results show that the agglomeration is, especially, important factor in the process temperature dependency of the sheet resistance of Ni silicides formed on n⁺ Si substrates.     

Hole and electron mobilities in heavily doped silicon: comparison of theory and experiment

Most device models for npn or pnp transistors assume that hole (electron) mobilities in n-type and p-type silicon are equal. Partial-wave phase shift calculations for the contributions of carrier-dopant ion scattering to the carrier mobilities lead to unequal minority hole (electron) and majority hole (electron) mobilities at the same doping density. These calculations are valid over the doping range of 2 x 10¹⁹ to 8 x 10¹⁹ cm⁻³ in n-type and p-type silicon and contain the assumptions that the holes and electrons move in isotropic parabolic energy bands and are scattered by the screened Coulomb potentials of the dopant ions. When the effects of carrier-acoustic phonon and carrier-carrier scatterings are included, these calculations agree to within the spread of experimental value for the majority mobilities reported in the literature. This agreement is a substantial improvement by factors of 2–4 over the results of earlier theories such as first order Born and nondegenerate theories. The results of this work, particularly the inequality of minority and majority carrier mobilities, have implications for the modeling of both bipolar and field effect transistors.     

A model of reduction of oxidation-enhanced diffusion in heavily doped Si layers

A model of reduction of oxidation-enhanced diffusion (OED) in heavily doped Si layers via bulk recombination of self-interstitials at centers associated with the dopant is suggested. The allowance made for the recombination of excess self-interstitials, which are generated upon thermal oxidation, allows one to describe the dependence of OED reduction on the doping level. The experimental data on the OED of B and P impurities in uniformly doped Si layers are analyzed. From the analysis, the recombination-rate constants are determined and capture radii are estimated for various variants of interaction of excess self-interstitials with impurity atoms and impurity-vacancy pairs.     

Optical characterization of heavily doped silicon

Out of a variety of optical techniques used to characterize heavily doped semiconductors photoluminescence and Raman spectroscopy will be discussed as tools to study heavy doping effects. Photoluminescence spectroscopy is sensitive to electronic transitions between the conduction and valence band whereas electronic Raman scattering probes transitions within either band. Parameters relevant to device physics such as the band gap shrinkage due to heavy doping are extracted from these measurements. It is further shown that both techniques are applicable to the characterization of thin heavily doped implanted or epitaxial layers.     

Nitrogen doped silicon films heavily boron implanted for MOS structures: Simulation and characterization

In this work we study the boron diffusion and its activation into recrystallized nitrogen doped silicon thin films (NIDOS) and we also discuss the influence of the chemical interaction between boron and nitrogen in NIDOS films. These films are deposited by low pressure chemical vapor (LPCVD) for the development of a P⁺ polysilicon gate for MOS structures. The reduction of boron diffusion with increasing nitrogen content is observed by SIMS profiles. SUPREM IV software is used in order to estimate the boron diffusion coefficients in NIDOS films. FTIR analyses show the appearance of a B–N complex whose density strongly depends on the annealing treatment in terms of temperature and duration. It is deduced through resistivity measurements and SEM observation that the formation of B–N complexes tends to degrade the electrical properties of polysilicon thin layers through the decrease of both electrically active boron and polycrystalline grains growth.     

Minority-carrier transport parameters in heavily doped p-typesilicon at 296 and 77 K

Minority-carrier electron lifetime, mobility and diffusion length in heavily doped p-type Si were measured at 296 and 77 K. It was found that a 296 K μ_n (pSi)≈μ_n (nSi) for N _AA≲5×10¹⁸ cm^-3, while μ_n (pSi)/μ_n (nSi)≈1 to 2.7 for higher dopings. The results also show that for N_AA≲3×10¹⁹ cm^-3, D (pSi) at 77 K is smaller than that at 296 K, while for higher dopings D_n (pSi) is larger at 77 K than at 296 K. μ_n (pSi) at 77 K increases with the increasing doping above N_AA>3×10¹⁸ cm^-3, in contrast to the opposite dependence for μ_n (nSi) in n⁺ Si     

Static dielectric constant of heavily doped semiconductors

One of the major effects of heavy doping in semiconductors is that the dielectric constant κ changes with impurity density and thus with position. The change is significant for impurity densities N greater than 10¹⁷ cm^?3. The low temperature classical relation of Castellan and Seitz is modified by including the contribution to the local field at the atomic site by the polarization of the host atoms due to an electric field. This correction makes an appreciable difference in κ(N). A new model for the polarizability of the impurity atoms is also developed. It is found that there is good agreement between the values of the dielectric constant of Si doped with As, P and Sb predicted by this theory and those found experimentally by Bethin et al. The critical impurity concentration for the metal-nonmetal transition found from this theory is however higher than that given by Castner et al. Reasons for this discrepancy are given.     

Electrical band-gap narrowing in n- and p-type heavily doped silicon at 300 K

Based on previous results band-gap narrowing in heavily doped silicon at 300 K is investigated and expressed in terms of impurity size-and-doping effects. The results obtained for n- and p-type heavily doped silicon are compared with other theories and experiments.