ASTM_F_28_-_91_1997.pdf

Designation:F 28 91(Reapproved 1997)Standard Test Methods forMinority-Carrier Lifetime in Bulk Germanium and Silicon byMeasurement of Photoconductivity Decay1This standard is issued under the fixed designation F 28;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 These test methods cover the measurement of minoritycarrier lifetime appropriate to carrier recombination processesin bulk specimens of extrinsic single-crystal germanium orsilicon.1.2 These test methods are based on the measurement of thedecay of the specimen conductivity after generation of carrierswith a light pulse.The following two test methods aredescribed:1.2.1 Test Method APulsed Light Method,that is suitablefor both silicon and germination.21.2.2 Test Method BChopped Light Method,that is spe-cific to silicon specimens with resistivity$1 Vcm.31.3 Both test methods are nondestructive in the sense thatthe specimens can be used repeatedly to carry out the mea-surement,but these methods require special bar-shaped testspecimens of size(see Table 1)and surface condition(lapped)that would be generally unsuitable for other applications.1.4 The shortest measurable lifetime values are determinedby the turn-off characteristics of the light source while thelongest values are determined primarily by the size of the testspecimen(see Table 2).NOTE1Minority carrier lifetime may also be deduced from thediffusion length as measured by the surface photovoltage(SPV)methodmade in accordance with Test Methods F 391.The minority carrierlifetime is the square of the diffusion length divided by the minority carrierdiffusion constant which can be calculated from the drift mobility.SPVmeasurements are sensitive primarily to the minority carriers;the contri-bution from majority carriers is minimized by the use of a surfacedepletion region.As a result lifetimes measured by the SPV method areoften shorter than lifetimes measured by the photoconductivity decay(PCD)method because the photoconductivity can contain contributionsfrom majority as well as minority carriers.In the absence of carriertrapping,both the SPV and PCD methods should yield the same values oflifetime(1)4providing that the correct values of absorption coefficient areused for the SPV measurements and that the contributions from surfacerecombination are properly accounted for in the PCD measurement.1.5 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.Specific hazardstatements are given in Section 9.2.Referenced Documents2.1 ASTM Standards:D 1125 Test Method for Electrical Conductivity and Resis-tivity of Water5F 42 Test Method for Conductivity Type of ExtrinsicSemiconducting Materials6F 43 Test Method for Resistivity of Semiconductor Materi-als6F 391 Test Methods for Minority Carrier Diffusion Lengthin Extrinsic Semiconductors by Measurement of Steady-State Surface Photovoltage62.2Other Standards:DIN 50440/1 Measurement of Carrier Lifetime in SiliconSingle Crystals by Means of Photoconductive Decay:Measurement on Bar-Shaped Test Specimens31These test methods are under the jurisdiction of ASTM Committee F-1 onElectronicsand are the direct responsibility of Subcommittee F01.06 on SiliconMaterials and Process Control.Current edition approved Oct.15,1991.Published December 1991.Originallypublished as F 28 63 T.Last previous edition F 28 90.2This test method is based in part on IEEE Standard 225,Proceedings IRE,Vol49,1961,pp.12921299.3DIN 50440/1 is an equivalent test method.It is the responsibility of DINCommittee NMP 221,with which Committee F-1 maintains close liaison.DIN50440/1,is available from Beuth Verlag GmbH,Burggrafenstrasse 4-10,D-1000Berlin 30,FRG.4The boldface numbers in parenthesis refer to a list of references at the end ofthese test methods.5Annual Book of ASTM Standards,Vol 11.01.6Annual Book of ASTM Standards,Vol 10.05.TABLE 1 Dimensions of Three Recommended Bar-ShapedSpecimensTypeLength,mmWidth,mmThickness,mmA15.02.52.5B25.05.05.0C25.010.010.0TABLE 2 Maximum Measurable Values of Bulk Minority CarrierLifetime,tB,sMaterialType AType BType Cp-type germanium32125460n-type germanium64250950p-type silicon903501300n-type silicon240100038001AMERICAN SOCIETY FOR TESTING AND MATERIALS100 Barr Harbor Dr.,West Conshohocken,PA 19428Reprinted from the Annual Book of ASTM Standards.Copyright ASTMIEEE Standard 225 Measurement of Minority-Carrier Life-time in Germanium and Silicon by the Method of Photo-conductive Decay23.Terminology3.1 Definitions:3.1.1 minority carrier lifetime of a homogeneous semi-conductor,the average time interva