662 Nuclear Instruments and Methods in Physics Research B24/25 (1987) 662-666 North-Holland, Amsterdam MEASUREMENT OF ABSOLUTE AI CONCENTRATION IN AI,`Ga 1-,,As D. YAN *, J. Paul FARRELL, P.M.S. LESSER * and F.H. POLLAK * Department of Physics, Brooklyn College of CUNY, Brooklyn, N Y 11210, USA T.F. KUECH and D.J. WOLFORD IBM Thomas J. Watson Research Center, Yorktown Heights, N Y 10598, USA A technique has been developed to measure the absolute AI concentration x in AI~GaI_~,As. The technique involves simultaneous measurements of the 27Al(p, v)ZSsi resonant nuclear reaction and Rutherford backscattering (RBS), and comparisons with an AlAs reference sample. A detailed description of the experimental procedure is given. Samples analyzed in this study were prepared by LPE and MOVPE growth on GaAs substrates, with epitaxial layer thicknesses in the range 1-3 /.tm and AI concentrations in the range 0.10 < x < 0.85. Measurements of x with an absolute error < 0.02 were obtained. Factors limiting the precision obtainable with this technique are discussed. 1. Introduction Among the newer semiconductor materials currently under development, AlxGax_xAs/GaAs has dearly demonstrated its advantageous electrical and optical properties. Applications include devices such as cw room temperature lasers [1], quantum well lasers [2], modula- tion doped high electron mobility transistors [3], and high efficiency solar cells [4,5], to mention a few. A number of other semiconductor materials containing aluminum, including the ternary compounds Al~,Gal_x Sb, Al:,In l_:~As and Al~,In1_~,P, show promise of being similarly very useful for device applications. The prop- erties of wafers and heterojunction devices, made from these materials, depend sensitively on the alloy com- position and for this reason it is important to have precise and reliable methods of measuring the absoIute aluminum concentration, x, in materials such as A1x Gal_xAs. To date, only two primary techniques have been available for determining alloy composition: chemical analysis and electron microprobe analysis. The electron microprobe technique [6,7], when used for analyzing heterostructures having epilayer thicknesses in excess of a few thousand hngstr~Sms, gives results with a claimed relative accuracy of 2-5%. However, this technique is time consuming at best and somewhat difficult to use for very thin films. Therefore, secondary analysis tech- * Also at Physics Department, Graduate School and Univer- sity Center, City University of New York, New York, NY 10036, USA. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) niques such as photoluminescence [7], electroreflectance [8,9], and Raman spectroscopy [10] are commonly utilized to determine x in Al~,Gal_xAs and related compounds. Secondary techniques rely on the fact that optical band gaps or phonon frequencies (or some quantities dependent on them) are monotonic functions of x. The exact relationship between these quantities and x can only be determined experimentally on the basis of primary measurements of x, and therefore the reliability of secondary techniques is limited by the reliability of primary measurements. For example, there are some significant discrepancies in the available litera- ture on photoluminescence peak energy versus alumi- num concentration in AlxGal_,`As [7]. It is important, therefore, to look for alternative primary techniques for measuring aluminum concentration, and to compare results obtained by different methods, in order to establish reliable calibrations of secondary measure- ment techniques. Ion-beam analysis, via Rutherford backscattering (RBS) and/or nuclear reaction analysis (NRA), pro- vides another primary technique for measuring com- positions [11]. In particular, NRA provides a.sensitive, direct method of measuring the absolute AI concentra- tion by measuring the y-ray yield from the 27Al(p, y)2SSi reaction. This reaction has many (well-known) isolated, narrow resonances at low energies [12], the dominant one occurring at an incident proton energy of 992 keV. In the present work, we have employed a combination of NRA and RBS to obtain precise, reliable measure- ments of x in AI~,Gal_,`As. The technique is not, however, without some difficulties and limitations which will be discussed in this paper.