A comparison of Nd:YAG fundamental and second-harmonic Q-switched laser-beam lifetime doping in single-crystal silicon

Silicon devices including bipolar transistors, junction diodes, and MOS capacitors were scanned by aQ-switched Nd:YAG (1.06 µm) and frequency-doubled Nd:YAG (0.53 µm) radiations under various conditions. The electrical characteristics of these devices were measured before and after scanning and again after thermal annealing. The data includes transistor gain versus laser power; junction diode leakage current versus junction depth; MOSC-Vlifetime versus laser power and the effects of subsequent thermal anneals on all of these. The results are that bulk minority-carrier lifetime decreases of several orders of magnitude will be produced by either of these radiations at peak power levels below those which will produce any visible surface damage. The changes in minority-carrier lifetime are stable for post scanning thermal anneals up to 400°C and are almost completely removed from an 800°C anneal. The depths within which minority-carrier lifetime changes significantly are 0.7 and 1.8 µm for 0.53- and 1.06-µm wavelength laser radiations, respectively. The results indicate that the recombination centers produced by the scanning are point defects and their density decreases exponentially with the distance into the silicon. The average power thresholds for point defect production (for both 0.53- and 1.06-µm wavelengths) were determined and are observed to increase with increased laser wavelength and pulse width. Potential applications in silicon devices and integrated circuits such as selective lifetime doping, β trimming, and selective-link making without passivation damage are possible.     

Doping and temperature dependences of minority-carrier diffusion length and lifetime deduced from the spectral response measurements of p-N junction solar cells

The minority-carrier diffusion length in the base region of p⁺?n (n⁺?p) junction solar cells has been deduced from the relative spectral response in the long wavelength. The doping and temperature dependences of minority-carrier diffusion length have also been characterized. It has been shown that the minority-carrier diffusion length is slightly increased with increasing temperature and is decreased with increasing doping concentration. Based on the known minority-carrier diffusivity as functions of doping concentration and temperature, the doping and temperature dependences of minority-carrier lifetimes have been deduced. It has been verified that the empirical relationship between minority-carrier lifetime and doping concentration deduced by other method is in good agreements with our experimental measurements. Moreover, it has been shown that the minority-carrier lifetime is increased with increasing temperature, which is consistent with that measured by the open-circuit voltage decay (OCVD) method.     

Modeling and measurement of minority-carrier lifetime versus doping in diffused layers of n+-p silicon diodes

The results of minority-carrier lifetime measurements in heavily phosphorus-doped n⁺diffused layers of p-n junction diodes using a spectral response technique are reported in this paper. Exact modeling of current-flow equations, modified to include bandgap reduction due to high carrier concentration and Auger recombination, is used to compute the dependence of diffused-layer photocurrent Jpthon the incident light energy and intensity. The photocurrent in the diffused layer is also obtained by subtracting the theoretical value of the space charge and uniformly doped p-region component from the experimentally measured photocurrent of the diode at each wavelength. Note that all calculated values based on light intensity include computed transmittance/reflectance through the oxide layer at each wavelength. The comparison of the values of Jpthwith Jpexp, using nonlinear least square techniques, then directly gives the lifetime profile in the diffused layer. A simple expression is given for lifetime as a function of doping which may be used in modeling and prediction of device performance. Using this experimental technique it was found that the lifetime in the diffused layer is an order of magnitude less than that corresponding to uniformly doped bulk-silicon values and is very much process dependent; its value being 3.72 × 10^-11s for surface concentration of 3.0 × 10²⁰cm^-3and increases to 2.9 × 10^-8s at doping concentration of 1.0 × 10¹⁷cm^-3.     

High brightness GaP green LED's

High brightness GaP green LED's have been developed by optimizing growth conditions to obtain a high quality p-n junction. In particular, the n-type GaP layer near the junction is greatly lowered in carrier concentration in order to decrease the nonradiative recombination center. The carrier concentration in the n-type GaP layer is decreased to 5-6 × 10¹⁵/cm³near the p-n junction and a minority-carrier lifetime of 800 ns (measured through the EL decay time) is obtained. The best device has a quantum efficiency of 0.45 percent at 12.5 A/cm^{2} with encapsulation. Excellent high-brightness LED lamps of 400 m Cd at a driving current of 20 mA can be obtained by using the high efficiency GaP green LED's.     

CMOS ROM arrays programmable by laser beam scanning

A CMOS PROM array with a novel programming method has been developed. Memory cells consist of two n-channel transistors and a p-n junction diode which is built by a p-moat in an n-well. Programming is accomplished by scanning green laser beams in diodes to decrease minority-carrier lifetime, without disturbing passivation after the fabrication is complete. This innovation has application where post-fabrication personalization of circuits is desirable. Also, the memory contents are not easily discovered through visual inspection or reverse engineering.     

重掺杂发射区中禁带宽度和少子复合寿命的确定方法

禁带宽度和少子复合寿命是硅晶体管发射区中重要的物理参数。本文利用p-n结反向扩散电流的温度特性和借助于线性外推法,提出了一种确定绝对零度时禁带宽度的新方法。由于发射区重掺杂,本文考虑了载流子的费米-狄拉克统计分布。提出了确定发射区中少子复合寿命的方法。该方法简便实用。     

Nondestructive SEM measurement of minority-carrier transport parameters of CuxS/CdS solar cells as a function of heat treatment

Electron-beam-induced-current techniques of a scanning-electron microscope have been extended to allow nondestructive measurements to be performed on p-n heterojunction devices consisting of thin layers sensitively influenced by surface effects and under conditions where junction collection efficiency is less than perfect. When applied to CuxS/CdS solar cells formed on polycrystalline CdS with an epitaxial CuxS layer that was heat treated at 180°C in a hydrogen-argon ambient, the dominant change was found to be the greater than two increases in junction collection efficiencies to a maximum and then a decrease for treatment times up to 120 min. No significant variations were found in the minority-carrier diffusion lengths which remained in the 0.20- to 0.26-µm range for the CuxS and in the 0.41- to 0.46-µm range in the CdS. The Cuxsurface-recombination velocity retained a constant magnitude equal to its diffusion velocity. Optimization of the collection efficiency changes should lead to improved device performance.     

A refined step-recovery technique for measuring minority carrier lifetimes and related parameters in asymmetric p-n junction diodes

Minority-carrier lifetime in a forward-biased asymmetrical p-n junction diode can be measured by observing the time response of the diode to a sudden reversing step voltage. An approximate but general theory for p-n junctions with almost arbitrary impurity gradients is developed, and its results are within about 25 percent of those previously obtained for the special cases of ideal step and exponentially graded junctions. A relatively simple experimental technique is described which is suitable for measuring lifetimes down to less than 1 ns. Measurements at extreme ambients are facilitated by the fact that the test diode is mounted at the end of a single coaxial line which can be arbitrarily long. The raw data from the experiment are in the form of an oscilloscope trace, which provides an immediate qualitative and semiquantitative indication of the minority-carrier lifetime and the penetration length for the injected carriers. A graphical presentation of the theoretical results leads quickly to a more precise quantitative evaluation of these parameters. In addition, the technique can be used to measure an average junction depletion capacitance and the device series resistance.     

Minority-carrier diffusion coefficients and mobilities in silicon

A new method for accurate measurement of minority-carrier diffusion coefficients in silicon is described. The method is based on a direct measurement of the minority-carrier transit time through a narrow region of the p-n junction diode. The minority-carrier mobility is obtained from the diffusion coefficient using the Einstein relation. The method is demonstrated on low-doped n- and -p-type Si (dopings ∼10¹⁵cm^-3) and is compared with the literature data for the majority-carrier mobilities. The results show that in low-doped Si the electron minority- and -majority-carrier mobilities are comparable, but the hole minority-carrier mobility is significantly higher (∼30 percent) than the corresponding majority-carrier value. The results confirm earlier data of Dziewior and Silber.     

A method for determining the emitter and base lifetimes in p-n junction diodes

A method is described that provides an experimental means for the first time to separate and determine the emitter and base lifetimes in a p-n diode after the junction has been fabricated. In the method, several static and transient measurements are analyzed using physical models of the diode characteristics. To illustrate the method, diffused silicon diodes are fabricated having substrate (base) impurity concentrations ranging from 10¹⁴to nearly 10¹⁷phosphorous atoms per cubic centimeter. The results show an emitter lifetime that is much smaller than the base lifetime in the diode having the highest base doping concentration. In this diode, the recombination current from the emitter is 65 percent of the recombination current from the base, demonstrating the significance of the emitter in governing the static current-voltage dependence. The importance of emitter recombination to the transient characteristics is also demonstrated. The paper emphasizes the techniques by which the base and emitter lifetimes are distinguished. It also demonstrates the need for carefully basing the quantitative analysis of the measurements on the underlying diode physics. The method described here applies not only to p-n diodes but also to junction solar cells and transistors.     

On the dark currents in germanium Schottky-barrier photodetectors

It is shown that for experimentally observed values of Schottky-barrier height of metal-n-type germanium photodetector structures, the dominant component of dark current can be due to the injection of minority carriers, rather than to the usual majority-carrier component. For barrier heights approaching 0.6 eV, for doping concentrationsN_{d} lsim 10^{15}cm^-3, for short minority-carrier lifetimetau_{p} lsim 1µs, for narrow base widthW_{b} lsim 10µm, or combinations of these conditions, the minority-carrier injection ratio approaches unity and these devices behave in the same way as p⁺-n junctions, with identical dark currents. The low-temperature fabrication requirements and processing simplicity of germanium Schottky barriers makes these devices more attractive under these conditions than germanium p-n junction photodetectors.     

Measurement of hole mobility in heavily doped n-type silicon

Accurate measurements of the mobility (and diffusion coefficient) of minority-carrier holes in Si:P with doping in the 10¹⁹cm^-3range have been done. The technique employed the measurement of diffusion length by means of lateral bipolar transistors of varied base widths, and the measurement of minority-carrier lifetime on the same wafers from the time decay of luminescence radiation after excitation with a short laser pulse. Minority-carrier hole mobility is found to be about a factor of two higher than the mobility of holes as majority carriers in p-type Si of identical doping levels.     

Method for simultaneous measurement of diffusivity lifetime, anddiffusion length, with application to heavily doped silicon

A novel experimental method is presented and demonstrated that allows simultaneous determination of minority-carrier diffusivity, lifetime, and diffusion length in semiconductors. This method is based on the lateral collection of photogenerated carriers by a semi-infinite junction. The semi-infinite nature of the problem makes possible the use of closed-form expressions that greatly simplify the analysis of the experimental results. The main advantages of this method are its analytical simplicity and the self-consistency test provided by the simultaneous measurement of all three transport parameters. The effects of the lifetime and the diffusion coefficient are separated by varying the distance between the illumination edge and the collecting junction. Results of measurements for heavily doped n⁺ silicon with doping density of 2.4×10¹⁹ cm^-3 are demonstrated     

Simultaneous determination of minority-carrier lifetime and deep doping profile using a double-sweep MOS-C technique

A double-sweepCVtechnology is introduced to determine the minority-carrier generation rates and the doping concentrations for non-uniform doping profile devices. The measurement time is typically less than 1 min. The observed minority-carrier lifetime decreased by one decade due to a boron implant. The Zerbst method leads to erroneous minority-carrier generation rate if a doping gradient exists in the deep-depletion region. Such error can be corrected by considering a doping concentration factor in the original Zerbst plot.     

Measurement circuits for silicon-diode and solar-cell lifetime andsurface recombination velocity by electrical short-circuit current decay

Two improved switching circuits for transient electrical short-circuit decay are presented that allow more accurate determination of base-region minority-carrier lifetime and back-surface recombination velocity of silicon p-n junction solar cells and diodes. In one circuit, metal-oxide-semiconductor transistors replace the bipolar switching circuit used in the original implementation of the method as described by T.W. Jung, et al. (ibid., vol.ED-31, p.588, 1984). In the other circuit, a pulse generator directly excites the device under study. Comparison of the two circuits by illustrative measurements shows that, in comparison to the original implementation of the method, these versions allow measurement of shorter effective lifetimes, such as those characteristic of low-resistivity (about 0.1 Ω-cm) silicon solar cells     

Determination of surface- and bulk-generation parameters from dark-current measurements in surface-channel CCD's

A technique for determining surface-generation velocity and bulk minority-carrier generation lifetime from measurements in surface-channel charge-coupled devices (SCCD) is described. Depleted surface-generation velocity of 1.1 cm/s and bulk minority-carrier generation lifetime of 130 µs have been determined in the channel region for the devices used, and are in good agreement with data obtained by other techniques.     

p-n结的隧道击穿模型研究

建立了抽象化的理论模型对p-n结隧道击穿机理进行定量研究,在理论模型的基础上探讨了电子势垒的形状以及势垒形状随外加电压的变化,并进行定量计算,得出隧穿电压随杂质掺杂浓度的变化规律。所得结论与硅、锗p-n结实验数据相吻合,证明了所建立的理论模型在定量研究p-n结的隧道击穿中的合理性与实用性。该理论模型对研究一般材料或器件的隧道击穿具有重要的借鉴意义。     

Analysis of the small-signal voltage decay technique in thecharacterization of Si concentrator solar cells

Detailed analyses of the small-signal voltage decay (SSVD) method of lifetime measurement have been performed. The main difficulty in voltage decay techniques is that the boundary conditions at the junction are coupled. A solution to the time-dependent diffusion equations has been obtained using the quasi-static emitter (QSE) approximation. Assuming a power-law emitter doping profile, a solution to the coupled diffusion equations has also been obtained using a dynamical approach without the QSE approximation. The extent to which effects such as emitter recombination and bandgap narrowing affect the SSVD lifetime has been examined using this solution. The SSVD technique has been applied in investigating the variation of minority-carrier recombination rates in the emitter, base, and back surface of concentrator solar cells with illumination intensities. The experimentally observed degradation in SSVD lifetime with increasing illumination intensity has been accounted for quantitatively     

Characteristics and performance of silicon solar cells between low-and high-level injection—Part II: Results of the study

Applying the extension of the transport velocity transformation method described in Part I of this paper [1], the I-V characteristics of several types of solar-cell structures have been studied in the range between the validity of the low-level or of the high-level injection assumptions. The structures investigated were a more heavily doped "wide-base diode," a lowly doped "narrow-base diode," and a structure with a high-low junction in its base. Various effects have been seen to dominate in different ranges of the I-V characteristic, causing observable changes in its slope. Depending on the particular structure, the major effects are: changes in the majority-carrier distribution, ohmic effects and changes in conductivity modulation, changes in minority-carrier lifetime, and reduction of the high-low junction barrier height, both of the latter resulting from increasing carrier concentrations. In addition, the influence of the impurity concentration in the more lowly doped layer of the base in solar cells with a high-low junction, on the conversion efficiency and on its dependence on the light intensity was investigated for optical concentration ratios up to 1000. In these studies, the doping range of 5E15-5E17/cm³was seen to yield a broad efficiency maximum, with the higher doping more favorable except for its limitation by the onset of the heavy doping effects. It has also been seen that the collection efficiency is increased or decreased by some of the effects investigated. In consequence, the collection efficiency can no longer be rigorously considered as independent of the light intensity. In some cases, this dependency occurs even when the low-level injection condition is still fulfilled.     

Reverse transient characteristics of a P-N junction diode due to minority carrier storage

Analysis of the transient switching characteristics of a p-n junction diode is considered a boundary value problem; solution of this problem yields mathematical equations applicable to the design of high-speed computer components. This analytical technique is used to establish the transient current of a semiconductor diode when an external biasing potential is rapidly switched from the forward to the reverse direction. Using a one-dimensional model of finite geometry, minority-carrier storage is assumed within a region of arbitrary lifetime, bounded on one side by the junction and on the other side by an ohmic contact of arbitrary recombination velocity. Further, this region of carrier storage is assumed to contain a drift field of constant magnitude as would result from an exponential type of conductivity grading. Mathematical equations are presented which characterize this transient situation from its initiation until the junction current has decayed to some arbitrary magnitude. Applications of this analysis are illustrated in graphical form throughout a range of parameters characterizing practical semiconductor devices.