Determination of the Minority Carrier Surface Thermogeneration Rate in Metal-Oxide-Semiconductor Structures

A metal-oxide-semiconductor (MOS) structure with the common field electrode insulated from the semiconductor by oxide layers h of different thicknesses allows the surface generation rate of minority charge carriers from current I(t) of nonequilibrium depletion state relaxation to be found. At the same time, it is possible in similar structures to observe the kinematics of electron-hole pair generation at the periphery of the field electrode (the edge generation effect). Measurements performed on an n-Si MOS structure with a stepwise change in oxide layer thickness (h ₁ = 100 Å and h ₂ = 3200 Å) allowed the generation rate at both the initial transient (t ∼ 10⁻⁵ s) stage of surface generation (4.34 × 10¹⁰ cm⁻² s⁻¹) and at the basic extremely slow stage (10.4 cm⁻² s⁻¹) to be reliably determined for the first time. The estimated peripheral generation rate of minority charge carriers (holes) was 7.8 × 10¹¹ cm⁻² s⁻¹.__________Translated from Pribory i Tekhnika Eksperimenta, No. 4, 2005, pp. 84–88.Original Russian Text Copyright © 2005 by Chucheva, Zhdan, Akhmedov, Kukharskaya.     

Calibration of AFM cantilever stiffness: a microfabricated array of reflective springs

Calibration of the spring constant of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules.

We have developed a compact and easy-to-use reference standard for this calibration. The new artifact consists of an array of 12 dual spiral-cantilever springs, each supporting a mirrored polycrystalline silicon disc of 160 μm in diameter. These devices were fabricated by a three-layer polysilicon surface micromachining method, including a reflective layer of gold on chromium. We call such an array a Microfabricated Array of Reference Springs (MARS). These devices have a number of advantages. Cantilever calibration using this device is straightforward and rapid. The devices have very small inertia, and are therefore resistant to shock and vibration. This means they need no careful treatment except reasonably clean laboratory conditions.

The array spans the range of spring constant from around 0.16 to 11 N/m important in AFM, allowing almost all contact-mode AFM cantilevers to be calibrated easily and rapidly. Each device incorporates its own discrete gold mirror to improve reflectivity. The incorporation of a gold mirror both simplifies calibration of the devices themselves (via Doppler velocimetry) and allows interferometric calibration of the AFM z-axis using the apparent periodicity in the force–distance curve before contact. Therefore, from a single force–distance curve, taking about one second to acquire, one can calibrate the cantilever spring constant and, optionally, the z-axis scale. These are all the data one needs to make accurate and reliable force measurements.     

Determination of the absolute value of the semiconductor surface potential by the quasi-static capacitance-voltage characteristics of an MIS structure

A simultaneous analysis of the derivatives C_V′V_g of the experimental and ideal quasi-static capacitance-voltage characteristics (plotted as a function of the normalized differential capacitance of a metal-insulator-semiconductor (MIS) structure) allows identification of regions within the semiconductor band gap E_g, in which interface states are virtually aTSent and the relation between the surface potential ψ_S of the real semiconductor and the voltage V_g applied to the MIS structure may be readily ascertained. This allows an accurate enough determination of the additive constants ψ_S0(V_g0) necessary to calculate the dependence ψ_S(V_g) in the entire range of V_g by numerical integration of the experimental quasi-static C-V characteristic. The comparison of this dependence with the ideal one characterizes in detail the integral electronic properties of the semiconductor-insulator heterojunction: the E_g-averaged density of interface states, the qualitative pattern of their distribution over the band gap, and the flat-band voltage V_FB and its components caused by a charge fixed in the undergate insulator and a charge localized at boundary states. A high accuracy of the V_FB measurements allows detection of even a weak physical response of MIS structures to external factors or to variations in the heterojunction technology. Results of such an analysis for a typical SiO₂/Si interface of an n-Si-MOS (metal-oxide-semiconductor) structure are considered. The application of C_V′-C_V diagrams for analyzing the high-frequency C-V characteristics is considered.     

Reconstruction of dependences of the tunneling current on the oxide voltage using the dynamic current-voltage characteristics of the n +-Si-SiO2- n -Si heterostructures

Precision measurements of dynamic current-voltage characteristics of an Al-n ⁺-Si-SiO₂-n-Si structure with a thin (<50 ?) oxide make it possible to separate the active (I _a ) and capacitive (I _c ) components from the total current. An algorithm for the analysis of the capacitive component is developed; this algorithm makes it possible to determine in a single experiment the doping level of n-Si, the oxide capacitance C _i , and also the density and sign of the charge fixed in the oxide. Dependences of the surface potential in n-Si and the voltage drop across the oxide on the gate potential V _g in the transverse electric fields |F| ≤ 10 MV/cm were calculated based on the above data without using any adjustable parameters. At maximum values of |F|, the sheet density of electrons (holes) in n-Si does not exceed 10¹³ cm⁻², which is indicative of the degeneracy and size quantization of electron gas. The dependences I _t (V _g ) and V _i (V _g ) were used to recover the current-voltage characteristics of the tunneling current I _t (V _i ) ≡ I _a (V _i ); these characteristics were measured within more than ten orders of magnitude of their range of variation in the conditions of both the enhancement of the n-Si surface and the inversion. The observed I _t (V _i ) characteristics are not quantitatively described in the context of existing concepts of the tunnel effect. Original Russian Text ? A.G. Zhdan, N.F. Kukharskaya, V.G. Naryshkina, G.V. Chucheva, 2007, published in Fizika i Tekhnika Poluprovodnikov, 2007, Vol. 41, No. 9, pp. 1135–1142.     

Reactive thrust generated by continuous detonation in the air ejection mode

Processes of continuous spin detonation and pulsed detonation, as well as combustion of a hydrogen-air mixture in an annular combustor 306 mm in diameter in the regime of air ejection are studied experimentally. The specific flow rates of hydrogen are 0.6–9.8 kg/(s ·m²). It is found that the greatest specific impulses of thrust generated by the combustor are reached in the case of continuous spin detonation. On the average, they are greater than the corresponding values by a factor of 1.5 in the case of burning the mixture in streamwise detonation waves, by a factor of 2 in the case of conventional combustion (by a factor of 3 at the maximum thrust impulse of 2200 m/s), and by a factor of 10 in the case of exhaustion of cold hydrogen. A change in the specific flow rate of hydrogen beginning from ≈1.2 kg/(s·m²) corresponding to the maximum thrust impulse decreases its value, and this decrease is more profound as the detonation limits in terms of the specific flow rate of hydrogen are approached. The maximum reactive thrust (83 N) is developed in the examined detonation chamber near the upper limit at the specific flow rate of hydrogen equal to 3 kg/(s·m²).