Heavy doping effects in silicon

The different mechanisms causing bandgap narrowing in heavily doped silicon are reviewed. A distinction is made between many-body effects and the effects due to random impurity distribution. The values of bandgap narrowing, calculated using a theoretical model, are compared with the experimental results. Recombination in heavily-doped silicon is discussed and the different recombination mechanisms, present at high doping levels, are explained. Experimental values for the minority-carrier lifetime as a function of the doping level are given. Surface recombination at the heavily doped Si/SiO₂ interface is discussed, the trnasport equations in the case of a position dependent bandgap are derived, and finally the influence of heavy-doping effects on the performance of several devices is discussed.     

Temperature effects in silicon avalanche diodes

Gain-voltage curves for reach-through avalanche photodiodes are computed using the ionization rate data of van Overstraeten and de Man and Lee et al. and compared with measured curves. Noise spectral density measurements are also made and compared with McIntyre's avalanche noise theory. Agreement between computed and measured k-values as obtained from the noise measurements and between the computed and measured gain-voltage relationship can only be obtained with the ionization rate data of Lee et al. Gain-voltage curves were measured over the temperature range ?17 to +55°C and compared with computed curves in which temperature dependence of the ionization rate data of Lee et al. is computed using the modified Baraff theory. Excellent agreement between theory and experiment is obtained.     

Temperature effects in silicon solar cells

The temperature variation of the relative spectral response RSR, short circuit current I_sc and open circuit voltage V_oc is measured and the results are theoretically discussed. The RSR at wavelengths larger than the peak wavelengths always increases with temperature. The observed increase is found to agree with theory when temperature variation of the absorption coefficient of light and of L_n the carrier diffusion length in the base is taken into account properly. The temperature variation of short wavelength RSR depends on S, the surface recombination velocity and $\text{L}{}_{\mathrm{p}}\text{d}\text{, L}{}_{\mathrm{p}}$ is the carrier diffusion length in the diffused layer and d is the junction depth. If S is small and $\text{L}{}_{\mathrm{p}}\text{d}$ is large, the RSR is practically independent of temperature. On the other hand if S is large and/or $\text{L}{}_{\mathrm{p}}\text{d}$ is small, the RSR decreases as the temperature increases.A new relation between L_n, d and the peak position of RSR is derived. The observed temperature shift in the peak position agrees well with that predicted by this relation.The temperature increase of I_sc and decrease of V_oc are different for AM0 and AM1 illuminations. The observed differences can be explained at least qualitatively on the basis of the results obtained in this paper.     

Near-surface effects of gold in silicon

Gold was introduced into silicon wafers by diffusion from the back surface. The voltage threshold for inversion, near-surface doping density, carrier-generation lifetime, and surface generation velocity were studied by C–VandC-t techniques. Carrier-density profiles were obtained from spreading resistance measurements. The threshold-voltage shifts found are consistent with the introduction by gold of negative charges at the Si/SiO₂ interface. By preparing samples where strong carrier-compensation effects are measured, one enters a regime of high sensitivity of silicon resistance to gold-concentration changes. We infer a negative gold-concentration gradient as one proceeds from the surface into the bulk. The gold concentration continues to change at least up to 17 h at 900°C, with a time constant characteristic of the substitutional diffusion of gold in silicon. The reciprocal of the generation lifetime and the surface generation velocity increase approximately as the gold concentration. Lifetime decreases are limited substantially (by a factor of about 10 for a shallow doping density of 10¹⁴ cm⁻³) for samples of low doping density if strong compensation effects are to be avoided. In spite of the inferred subtle rearrangements in gold concentration lasting many hours, the generation lifetime is independent of gold diffusion time at 900°C within experimental uncertainty in the interval sampled (10–1000 min).     

Electrooptical processing for radio astronomy

A recent report of the first computer-generated radio-derived photographs of the Whirlpool Galaxy will no doubt provide impetus for additional radio photography of astronomical events. Electro-optical techniques, developed in the past decade for obtaining high resolution range, range-rate, and angular target data with array radars, may be highly suitable for generating visible images of astronomical events. The electrooptical processor can furnish continuously and unambiguously, in the form of an optical image, the distribution of radio emission present in the entire effective scan sector of an array without the necessity of electronic steering.     

Electrooptical modulation in multiple quantum well hetero nipiwaveguides

An optical intensity modulator based on multiple quantum well hetero (MQW-H) nipi waveguides is reported. In the low optical power regime (~10^-5 W), the modulator exhibits an extinction ratio in excess of 100:1 at low drive voltage (4 V) and 5-B attenuation. Modelling and experimental results of the time response of the waveguide modulator are presented, and it is shown that the response is limited by the RC time constant of the drive circuit and the photocurrent charging up a dielectric capacitance. The modelling shows that with a loss penalty of ~1.5 dB, the speed of the present waveguide modulator is limited solely by the RC time constant that would allow modulation speeds of ~100 ps     

Electrooptical Characterization of MWIR InAsSb Detectors

InAs_1?x Sb _x material with an alloy composition of the absorber layer adjusted to achieve 200-K cutoff wavelengths in the 5-μm range has been grown. Compound-barrier (CB) detectors were fabricated and tested for optical response, and J _dark–V _d measurements were taken as a function of temperature. Based on absorption coefficient information in the literature and spectral response measurements of the midwave infrared (MWIR) nCBn detectors, an absorption coefficient formula α(Ε, x, T) is proposed. Since the presently suggested absorption coefficient is based on limited data, additional measurements of material and detectors with different x values and as a function of temperature should refine the absorption coefficient, providing more accurate parametrization. Material electronic structures were computed using a k·p formalism. From the band structure, dark-current density (J _dark) as a function of bias (V _d) and temperature (T) was calculated and matched to J _dark–V _d curves at fixed T and J _dark–T curves at constant V _d. There is a good match between simulation and data over a wide range of bias, but discrepancies that are not presently understood exist near zero bias.     

Recombination saturation effects in silicon solar cells

Significant interest has recently been shown in the use of dark and illuminated current-voltage (I-V) measurements for the characterization of high-efficiency silicon solar cells. Similar nonideal behavior, in the form of “humps” in dark I-V curves, has been observed by various research groups but apparently different interpretations of this effect given. In this paper we present detailed computer simulations of solar cells with defects (producing recombination centers within the bandgap) at a number of specific positions in the devices. It is found that a distinct shoulder (or “hump”) occurs in the I-V characteristics when the recombination centers exhibit unequal electron and hole capture rates. Furthermore, it is shown that these shoulders are a result of the saturation of (Shockley-Read-Hall) recombination via the defect levels, which dominates behavior at low forward bias. As the bias voltage is increased, recombination in the defected region increases again, beyond the saturation level. The simulations show conclusively that the shoulders in the measured dark I-V curves of high-efficiency silicon solar cells produced at the University of New South Wales arise from the rear Si-SiO₂ interface     

Charge storage effects in silicon dioxide films

The dc properties of the system Si:SiO2(6000 Å), metal have been investigated at 200-400°C. The system shows unidirectional conduction at 400°C. Little or no current is observed when the silicon electrode is positive. There is a region of reversible charge storage and discharge at negative silicon voltages; the center of this region is around -2 volts when the contact metal is gold, but more positive when it is aluminum. The quantities of charge stored range from 4 × 10¹²to 2 × 10¹⁴charges/cm². At still more negative silicon voltages there is a region of ohmic conduction, the resistivity at 400°C ranging from 10¹²to 10¹⁴Ω-cm. The charge storage, which is far greater than that anticipated from the oxide capacitance, can be shown to occur principally in the oxide under the metal contact, and not over a broad adjacent area. Storage of charge in the oxide causes ann-type shift of the underlying silicon surface. Charges can develop in the oxide as a result of nonelectrical processes, such as high-temperature gas baking treatments. Conversion of the upper 600 Å of the oxide to a phosphate glass eliminates both charge storage and ohmic conductivity in the oxide. The results described are interpreted in terms of an electrochemical potential existing across the oxide, and the presence of mobile charged species within the oxide. Charge storage can occur as the result of double-layer formation by the mobile charged species, or of its electrochemical discharge at the electrodes. The unidirectionality of the ohmic conduction is considered to result from electrolytic rectification at the silicon-oxide boundary.     

Charge instability effects in the system silicon carbide-insulator

The influence of laser radiation on the electric strength and characteristics of the interface of the system silicon carbide-insulator is investigated by nondestructive inspection according to the noise characteristics and by means of the capacitance-voltage characteristics. It is found that a laser beam can be used to control the fixed charge in the insulating layer and to modify the distribution of the density of surface states. Possible mechanisms underlying the observed effects are discussed. Fiz. Tekh. Poluprovodn. 31, 53–55 (January 1997)     

High-field effects in silicon nitride passivated GaN MODFETs

This paper presents a detailed study of high-field effects in GaN MODFETs. Degradation of DC characteristics and change of flicker noise due to hot electron and high-reverse current stresses in Si/sub 3/N/sub 4/ passivated GaN MODFETs have been investigated. The authors observe that during hot electron stress, electron trapping in the barrier layer and interface state creation occur. These cause a positive shift of V/sub t/, reduce I/sub D/, skew the transfer characteristics, and degrade g/sub m/. Flicker noise (1/f) measurements show that after hot electron stress, the scaled drain current noise spectrum (S/sub I(D)//I/sub D//sup 2/) decreases in depletion, but increases only slightly in strong accumulation, corroborating the creation of interface states but only a small creation of transition-layer tunnel traps that contribute to 1/f noise. During high-reverse current stress, electron trapping dominates for the first 50-60 s and then hole trapping and trap creation begin to manifest. However, there still is net electron trapping under the gate after one hour of stress. The degradation processes bring about a positive shift of V/sub t/, degrade I/sub D/ and g/sub m/, and increase reverse leakage. After high-reverse current stress, S/sub I(D)//I/sub D//sup 2/ increases substantially in strong accumulation, indicating the creation of transition layer tunnel traps.     

Hot electron effects in single-injection silicon SCL diodes

A theory is presented on hot electron effects on the noise in single-injection SCL diodes. It is shown that the noise temperature T_n of such diodes can be written as T_n = T₀(1 + BV_a²), where T₀ is the lattice temperature, V_a the anode voltage and B a constant depending on the diode geometry and material constants. Experimental data presented for Si SCL diodes made on high-resistivity p-type material show excellent agreement with the theoretical expressions for T_n. Making certain assumptions, the measured values of B give a value of 1400 V/cm for the critical field strength E_c of high-resistivity p-type silicon.     

Electrooptical generation and detection of femtosecond electricaltransients

Recent work on the generation of ultrashort electrical pulses by optical rectification of femtosecond optical pulses in electrooptic materials is summarized. This technique, which is called electrooptic Cherenkov radiation, is described in detail with particular emphasis on the effects of dispersion due to coupling to lattice vibrational resonances. Recent experimental results in lithium tantalate are described which illustrate these effects. When the duration of the exciting femto second optical pulse is comparable to the period of the lattice vibrations, a pronounced ringing is observed in the electrical waveforms. A coupled oscillator model is used to develop a theory that accurately accounts for these effects and can be used to predict the limiting speed of response and generation efficiency of different electrooptic materials     

Thermal effects on the integrity of aluminum to silicon contacts in silicon integrated circuits

Serious discontinuities in aluminum interconnections at contact windows of integrated circuits have been observed with the scanning electron microscope. In some instances the discontinuities have produced catastrophic failure. Two types of discontinuities have been observed and simulation of one type has been possible through exposure to temperatures 25° to 50°C below the aluminum-silicon eutectic of 577°C. Preliminary electron probe analysis has revealed the presence of silicon in the interconnection regions overlying the silicon dioxide steps bordering the contact windows.     

Photodetection using photomagnetoelectric and Dember effects in gold-doped silicon

Photodetection in the visible and near IR wavelength region using photomagneto-electric and Dember effects in gold-doped silicon has been demonstrated in this note. Response time trof 0.2 µs and noise equivalent power (NEP) of 6 × 10^-10for a photomagnetoelectric (PME) cell, and tr< 0.03 µs and NEP = 2.5 × 10^-10for a Dember cell, were obtained from the present results.     

Photolectric effects in silicon switching structures utilizing rare-earth fluorides

The photoelectric characteristics of silicon metal-insulator-semiconductor switching structures are investigated; cerium, dysprosium, and erbium fluorides are used for the insulator layer. It is shown that the photoelectric characteristics of the structures in the low-resistance state are similar to those of a metal-(tunneling insulator)-semiconductor structure; in particular, a mechanism of injection amplification of the photocurrent comes into play. Fiz. Tekh. Poluprovodn. 33, 795–800 (July 1999)     

载流子吸收对SOIMach-Zehnder干涉型电光调制器性能的影响

分析了载流子吸收对SOI材料制作的Y分支型Mach-Zehnder干涉型电光调制器/开关性能的影响,并提出了改进器件性能的一些措施.     

Experimental evidence of transit-time effects in silicon punch-through diodes

Small-signal frequency characteristics indicating the effect of hole transit times were measured for `punch-through? p?v?p silicon diodes. The transit time was sufficiently long to allow measurements in a relatively low frequency range (0.5?18MHz), thus avoiding parasitic effects. Experimental results for several operating points in the square-law region of the d.c. characteristic agree very well with the theoretical frequency characteristics.