Carrier Lifetime Analysis by Photoconductance Decay and Free Carrier Absorption Measurements

Abstract

High-power devices require a thick silicon layer with high resistivity to achieve high breakdown voltages. In order to optimize the turn-on and turn-off behavior of these devices, precise adjustment of the carrier lifetime is necessary, together with knowledge of its dependence on process parameters and operation conditions ͑e.g., temperature and injection level͒. We have therefore analyzed the carrier lifetime of the starting material ͑float-zone silicon͒ as well as processed devices that have both been either doped with transition metals ͑Pt, Au, Fe͒ or irradiated with electrons, using two different measurement techniques based on photoconductivity decay and free carrier absorption measurements.The fabrication of high-power devices usually requires n-type silicon with high resistivity. For a blocking capability of 8 kV, for example, material with a resistivity of about 500 ⍀ cm and a wafer thickness of about 1.5 mm is necessary. 1 Furthermore, high-power devices should also be able to switch high currents of several kiloamperes, requiring large device diameters of up to 6 in. In contrast to integrated circuits, almost the total volume of the wafer is electrically active in the case of high-power devices. Therefore, the concentration of contaminating atoms in the starting material must be very low (10 11 atom/cm 3 ), typically two orders of magnitude smaller than the fairly small n-doping concentration. In addition, the lateral and vertical distribution of the doping atoms must be very uniform. Since the electrical performance of the devices, as, e.g., the switching behavior, can often be optimized by reducing the charge carrier lifetime in the electrically active region, the active area ͑or part of it͒ is doped with transition metals such as Pt or Au, or irradiated with electrons or/and light ions. Sometimes it is advantageous to combine these techniques; a typical example is reducing the carrier lifetime uniformly by diffusion of Pt atoms and locally in a certain depth below the wafer surface by means of ion irradiation. Since the operation of power devices involves a wide range of temperature ͑230-450 K͒ and current density (10 Ϫ5 to 400 A/cm 2 ), understanding the temperature and injection-level dependence of the carrier lifetime is important for device optimization. Although a number of investigations has already been performed on temperature and injection-level dependence of as-grown silicon, 2 of silicon doped with metal impurities ͑e.g., Ref. 3-7͒, and of irradiated silicon ͑e.g., Ref. 8-10͒, the characterization and understanding of lifetime dependence is still incomplete. 11 In this paper we present carrier lifetime measurements on silicon wafers and processed devices using two different experimental techniques: microwave photoconductive decay ͑-PCD͒ measurements and free carrier absorption ͑FCA͒ measurements. Both the temperature and injection-level dependence of the carrier lifetime can be measured with either of these contactless techniques. Since bulklifetime measurements on pure as-grown silicon o...

Analysis of Radiation-Induced Defects and Performance Conditioning in High-Power Devices

“…Capture cross sections and thermal velocities were adopted from Ref. 12 and a value of ␥ ϭ 1 was assumed for the degeneracy factor. The total thickness of the sample is about 720 m, where the p ϩ -n junction is located at a depth of approximately 100 m. The resistivity of the low-doped float zone ͑FZ͒ silicon ͑111͒ starting material amounts to about 530 ⍀ cm.…”

Analysis of Excess Carrier Concentration Control in Fast-Recovery High Power Bipolar Diodes at Low Current Densities

“…27 Therefore, the spatial resolution and radiation power transmission were enhanced. After that, the strips were finally soldered and bonded on a power substrate ͑insulated metal substrate͒.…”

“…Experimental conditions.-Concerning the sample preparation, the diodes were cut in strips of 1.5 ϫ 10 mm, and afterward, their lateral walls were polished following physical and chemical processes. 27 Therefore, the spatial resolution and radiation power transmission were enhanced. After that, the strips were finally soldered and bonded on a power substrate ͑insulated metal substrate͒.…”

Z-scanning laser photoreflectance as a tool for characterization of electronic transport properties

“…Moreover, evaluation of these parameters is essential in the semiconductor manufacturing industry because semiconductor device performance directly depends upon these electronic transport properties [1]. The techniques used to determine carrier transport properties may be divided into two classes: those requiring direct contact with the sample such as four-point probe (4PP) and Hall effect measurements; and noncontact methods such as surface photovoltage (SPV) [2,3], photoconductance decay [4], photoluminescence [5], and time-resolved THz spectroscopy [6,7]. The main advantage of the contact techniques is their standardization [8].…”