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Background Statement for SEMI Draft Document 4618
NEW STANDARD: TEST METHOD FOR THE CARBON ACCEPTOR CONCENTRATION IN SEMI-INSULATING (SI) GALLIUM ARSENIDE SINGLE CRYSTALS BY INFRARED ABSORPTION SPECTROSCOPY
Notice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in reaching an informed decision based on the rationale of the activity that preceded the creation of this document.
Background statement:
Semi-insulating GaAs crystal growth has reached in the last decade a high level of performance, when considering the major electrical characteristics, i.e. carrier concentration (resistivity) and carrier mobility. This includes the possibility to obtain wafer material with well-defined concentrations of the most relevant electrically active defects, carbon acceptor and EL2 deep donor. The old standard test method for carbon (SEMI M30) - covering only material with high carbon content – is technically outdated and has been withdrawn. The present document is intended to fill this gap and to provide a standard exploiting the full potential of state–of-the-art infrared spectroscopy.
Review and Adjudication Information
Task Force Review / Committee AdjudicationGroup: / Carbon in gallium arsenide, FTIR determination / Europe Compound Semiconductor Materials
Date: / Tuesday 13 March 2012 / Tuesday 13 March 2012
Time & Timezone: / TBD / TBD
Location: / CS Europe / CS Europe
City, State/Country: / Frankfurt, Germany / Frankfurt, Germany
Leader(s): / Hans Christian Alt(University of
Applied Sciences – Muenchen) / A. Weber (SiCrystal)
Standards Staff: / Kevin Nguyen (SEMI NA)
408.943.7997
/ Kevin Nguyen (SEMI NA)
408.943.7997
This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact the task force leaders or Standards staff for confirmation.
Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will not be able to attend these meetings in person but would like to participate by telephone/web, please contact Standards staff.
SEMI Draft Document 4618
NEW STANDARD: TEST METHOD FOR THE CARBON ACCEPTOR CONCENTRATION IN SEMI-INSULATING GALLIUM ARSENIDE SINGLE CRYSTALS BY INFRARED ABSORPTION SPECTROSCOPY
1 Purpose
1.1 The purpose of this document is to specify a method for the measurement of the carbon acceptor concentration in semi-insulating (SI) gallium arsenide (GaAs) by infrared absorption.
2 Scope
2.1 This standard test method covers the determination of the carbon content in GaAs using infrared (IR) absorption. The test method utilizes the linear relationship between the carbon content and the integrated absorption of the localized vibration of substitutionally bonded carbon (CAs). At a measurement temperature of 300 K (room temperature) this absorption band is observed at 580 cm-1, at 77 K between 582 and 583 cm-1.
2.2 This test method covers the determination of substitutional carbon mainly in SI single crystal GaAs. It may also be used for polycrystalline GaAs and conducting GaAs up to a free carrier concentration of about 11016 cm-3.
2.3 This test method is applicable for carbon content between 11013 cm-3 and the solubility limit (>11016 cm-3). The lower limit depends on the measurement temperature, the sample geometry and the quality of the spectrometer.
2.4 Measurement temperatures of both 300 K and 77 K may be used.
2.4.1 Advantages of the 300 K measurement are:
- the necessary spectral resolution is 1 cm-1 as compared to 0.1 cm-1 required at 77 K;
- cryostat is not necessary.
- Advantages of the 77 K measurement are:
- sensitivity is higher (and therefore the detection limit lower);
- no reference sample with a low carbon content is necessary;
- commercial thin wafers can be measured.
- The chemical carbon content in GaAs may be higher than the substitutional carbon content CAs as measured by this standard. According to the present knowledge, however, state-of-the-art single crystal GaAs contains CAs only.
- The document follows the roadmap laid out by SEMI M54 (Guide for Semi-Insulating (SI) GaAs Material Parameters), defining the carbon concentration as an essential material parameter.
NOTICE:SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and determine the applicability of regulatory or other limitations prior to use.
3 Referenced Standards and Documents
3.1 SEMI Standards and Safety Guidelines
SEMI M54 — Guide for Semi-Insulating (SI) GaAs Material Parameters
SEMI M64 — Test Method for the EL2 Deep Donor Concentration in Semi-Insulating (SI) Gallium Arsenide Single Crystals by Infrared Absorption Spectroscopy
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
4 Terminology
NOTE 1:Refer to SEMI’s Compilation of Terms (COT) for a list of the most current terms and their definitions.
4.1 Terms, acronyms, and symbols relating to compound semiconductor material technology are defined in SEMI M64.
4.2 Other Terms Used in this Standard
4.2.1 Single beam spectrum I – intensity of IR radiation impinging on the detector as a function of the wavenumber.
4.2.2 Wavenumber – reciprocal wavelength of IR radiation, measured in units of cm-1.
5 Physical background and methodology
5.1 Case 300 K/room temperature (sample temperature between 290 and 310 K – The localized mode of carbon in gallium arsenide leads to an absorption band with the main line at 579.8 cm-1 (full width at half maximum (FWHM) 1.3 cm-1) and a weak side line at 576.6 cm-1 (see Fig. 1(a)). The carbon band is superimposed on a three-phonon band of the GaAs matrix that has to be subtracted using a carbon-lean reference sample.
5.2 Case 77 K (sample temperature between 75 and 85K) – At temperatures < 100 K the carbon band shows a fine structure due to the different masses of the isotopes 69Ga and 71Ga[1]. The center of the band is situated at 582.4 cm-1. FWHM of the whole band amounts to 0.65 cm-1. FWHM of the single lines of the fine structure is about 0.1 cm-1 (see Fig. 1(b)). Due to the nearly constant absorption of the matrix in the range of the carbon band no reference sample is needed in this case.
5.3 Baseline method – A straight baseline is used to separate the carbon band from the remaining contributions to the absorption spectrum. It connects two regions of the absorption spectrum to the left and right of the carbon band. These regions should be chosen as close to the carbon band as possible; however outside the interval from 581.9 cm-1 to 582.9 cm-1 at 77 K and outside the interval from 573 cm-1 to 583 cm-1 at 300 K, respectively (see Fig. 1).
5.4 The baseline method can only be used if the absorption spectrum without the carbon band is a reasonably linear function of the wavenumber. Careful control of proper baseline settings is recommended.
5.5 Integrated absorption method– Quantitative determination of the area of the carbon band (integrated absorption) requires that the line shape of the band is not distorted by the convolution with the resolution function of the spectrometer[2]. At 300 K, therefore, the resolution must be at least 1 cm-1. At 77 K, the resolution must be at least 0.1 cm-1. If maxd0.1, where max is the maximum absorption coefficient of the band and d is the sample thickness, a reduced spectral resolution of about 0.5 cm-1 is sufficient.
5.6 Measurement temperature lower than 77 K is discouraged, as the FWHM of the lines of the fine structure becomes smaller[3] and, therefore, the spectral resolution must be further increased.
Figure 1
Transmittance spectra of a single-crystalline, SI GaAs sample (d=0.50 cm) at 300 K (a) and at 77 K (b), recorded with a spectral resolution of 1 cm-1 (a) and 0.06 cm-1 (b), respectively.
Carbon content is 3.21015 cm-3.
Figure 2
Absorption bands of carbon resulting from the spectra plotted in Fig. 1. (300 K: after subtraction of the absorption spectrum of the reference sample). Light grey area: Spectral range of the carbon band. Dark grey line: Baseline (fitting regions are indicated by arrows).
6 Interferences
6.1 Beam shadingcan have a deleterious effect on the accuracy for the determination of the sample transmittance. The area of the sample should be larger than the cross section of the IR beam impinging on the sample.
6.2 Detector nonlinearity – can be a problem of mercury cadmium telluride (MCT) detectors. A long pass filter in the IR beam usually leads to an improvement (see also ¶ 7.1).
6.3 Distortions of the transmittance spectrum – lead to ambiguities in the choice of the baseline and/or determination of the area of the carbon absorption band.
7 Apparatus
7.1 Fourier transform infrared spectrometer (FTIR)must be equipped with suitable optics and detector for use in the wavenumber range from 500 cm-1 to 700 cm-1. The use of a long pass filter (cut-on wavenumber of about 1000 cm-1) is recommended. Spectral resolution must be at least 1.0 cm1 for measurements at 300 K, 0.25 cm-1 for measurements at 77 K using thin wafers, and 0.1 cm-1 using thick samples, respectively.
7.2 Cryostathas to be used for measurements at 77 K. Optical windows of the cryostat must be made from material with constant transmittance between 500 cm-1 and 700 cm-1 (e.g. cesium iodide, KRS-5 or zinc selenide).
7.3 Sample holderSamples must be mounted in the sample holder with the sample surface roughly perpendicular to the IR beam (Angle of 2 to 5° between the optical axis and the normal to the sample surface is favorable).
8 Sample preparation
8.1 For measurements at 300K a test sample thickness d of 0.2 to 1.0 cm, depending on carbon concentration, is recommended. For measurements at 77K the sample thickness should not exceed 0.4 cm.
8.2 Any crystallographic orientation of the sample can be used. The size should be large enough that the IR beam does not hit the edge of the sample. Surfaces must be mirror polished or at least etch-polished. For measurements at 77Kwith high spectral resolution, wedging of the sample is recommended to avoid interference fringes. For instance, a wedge angle of about0.5° is suitable for an IR beam diameter of 6 mm (see ¶ 10.2 for details).
8.3 Alternatively, a sample of thickness d may be assembled by stacking thin rectangular slices of width d, typically cleaved from commercial wafers. The IR beam is directed onto the surface composed of the cleaved edges.
8.4 Reference sample (only for measurements at 300K) is a GaAs sample with a carbon concentration cC < 31013 cm-3. Other geometrical or physical parameters (thickness, preparation of surfaces, and concentration of free carriers) should be similar to the test sample. Thickness may deviate from the test sample by 5 % at a maximum.
9 Testing the instrument
9.1 Prior to the measurement, the stability of the spectrometer has to be checked by recording the 100 % transmittance baseline in the wavenumber range between 500 cm-1 and 700 cm-1 (at intended operation conditions). It is recommended to carry out this check with a spectral resolution of 1 cm-1 or 2 cm-1. Deviations from the 100 % baseline must be less than 0.5 %.
10 Measurement procedure and data reduction
- The following flowchart helps to find the appropriate steps within the document to perform a measurement according to this standard.
NOTE:#1:The “ideal” sample has geometry according to ¶8.1 and ¶8.2, optically perfect surfaces, and no free carrier absorption (semi-insulating).
Figure 3
Flow Chart
10.1 Single beam spectra - Single-beam spectra of the background I0(), test sample Im,s(), and reference sample Im,r() (only for 300 K) are recorded.
10.2 Transmittance spectrum - The transmittance spectra of the test sample Tm,s()=Im,s()/I0(), and of the reference sample Tm,r()= Im,r()/I0() are calculated (usually using spectrometer software), by taking the ratio of the corresponding single-beam spectra, in the range between 570 and 590 cm-1. The number of scans in the single beam spectra has to be chosen large enough to reduce the peak-to-peak value of the noise level in the transmittance spectrum to 510-3.
10.2.1 T() of a sample with parallel, optically ideal surfaces and thickness d and a perpendicularly impinging IR beam is given by:
T() = (1 – R)2 exp(-()d)/ [1 + R2 exp(-2()d) – 2Rcos(()) exp(-()d)] . (1)
10.2.2 For the interface GaAs/vacuum or GaAs/gas the reflectance R is 0.276 at 300 K and 0.271 at 77 K[4]. If the sample is immersed into liquid nitrogen R is 0.2122. R can be assumed constant in the wavenumber range of interest.
10.2.3 The absorption coefficient = () contains the absorption due to carbon C(), the absorption due to matrix phonons and the free carrier absorption in the case of conducting samples.
10.2.4 The spectrum of samples with parallel surfaces is modulated periodically due to multiple internal reflections (interference fringes). The modulation period is determined by the phase angle ():
() = 4 n d ,(2)
where the refractive index n of GaAs is 3.22 at 300 K and 3.17 at 77 K4. Therefore, the modulation period is 0.3 cm-1 for a sample thickness of 0.5cm and 3 cm-1 for a sample of 0.05 cm.
10.2.5 Wedged samples are treated by averaging equation (1) over the phase angle (), resulting in
T() = (1 – R)2 exp(-()d)/ [1 - R2 exp(-2()d)] . (3)
10.2.6 In the following, only transmittance spectra free of periodical modulations are considered.
10.3 Absorption spectrum
10.3.1 Exact calculation
10.3.1.1 The absorption spectrum () is calculated from equation (3):
() = 1/d ln{ (1 – R)2 /T() [1/2 + [1/4 + (R T())2 / (1 – R)4]1/2] } . (4)
10.3.1.2 The use of this equation is recommended for samples polished on both sides which have optically perfect surfaces and show no absorption due to free carriers.
10.3.1.3 If the experimentally determined transmittance Tm deviates from the true value given by equation (3), a correction must be applied. This can happen due to a nonlinear response of the detector, due to a shift of the beam focus at the position of the detector originating from a large sample thickness, etc. The check has to be performed at
= 570 cm-1 by comparing Tmwith the true transmittance T. T is calculated by equation (3) using the sample thickness d and the matrix absorption caused by phonons. The absorption coefficient is 1.4 cm-1 at 300 K and 0.66 cm-1 at 77 K[5].
10.3.1.4 Hence
T = 0.524 exp(-1.4d/cm)/ [1 – 0.076exp(-2.8d/cm)] at 300 K,(5)
and
T = 0.531 exp(-0.66d/cm)/ [1 – 0.073exp(-1.32d/cm)] at 77 K.(6)
10.3.1.5 In the case of a difference |Tm– T| > ad (with a = 0.03 cm-1), prior to the calculation of the absorption spectrum according to equation (3), the transmittance spectrum has to be multiplied with a parameter C1, determined by
C1 = T/Tm (7)
10.3.1.6 Therefore the absorption spectrum is finally calculated by:
() = 1/d ln{ (1 – R)2/(C1Tm()) [1/2 + [1/4 + (RC1 Tm())2 / (1 – R)4]1/2] } , (8)
where Tm() is the experimentally determined transmittance spectrum.
10.3.2 Approximate calculation
10.3.2.1 The approximate calculation neglects the effect of multiple internal reflections, leading to the following relation for the transmittance spectrum T:
T() = (1 – R)2 exp(-()d) (9)
10.3.2.2 Therefore, the absorption spectrum is given by:
() = 1/d ln[(1 – R)2/Tm()] = – 1/d lnTm() + const.(10)
10.3.2.3 Use of equation (9) is recommended for samples with optical surface conditions not precisely known (e.g. etched surfaces), thick samples and samples showing absorption due to free carriers.
10.4 Carbon absorption spectrum
10.4.1 Reference sample method (only for tests at 300 K)
10.4.1.1 The carbon absorption spectrum c() of the test sample is calculatedtaking into account the influence of multiple internal reflections - from equation (8) by
c()=s()(11)
neglecting of the influence of multiple internal reflections - from equation (10) by
c()=1/d [ lnTm,r()– lnTm,s()]. (12)
10.4.1.2 The indices s“ and r“ designate the test sample and the reference sample, respectively.
10.4.1.3 If the carbon absorption spectrum is not equal to zero outside the wavenumber range from 573 cm-1 to 583 cm-1, a baseline must be drawn according to the procedure described in ¶ 5.3.
10.4.2 Baseline method (only for tests at 77 K)
10.4.2.1 The carbon absorption spectrum c() of the test sample is calculated according to equations (8) or (10) from the absorption spectrum using the baseline method (¶ 5.3)
10.5 Integrated absorption of the carbon band
10.5.1 The integrated absorption I of the carbon absorption band is calculated from the carbon absorption spectrum c() of the test sample as
I =c() d .(13)
10.5.2 Alternatively, at 300 K, the product of the peak absorption max and the FWHM of the main line at 579.8cm-1, max, can be calculated. This product is proportional to the integrated absorption:
I* =max .(14)
10.5.3 The product max can sometimes be determined more precisely than the value I, as the weak side line at
576.6 cm-1 (¶ 5.1) is only detectable in test samples with a carbon content cC21015 cm-3. For a correct determination of max it is important to set the baseline correctly, as described in (¶ 5.3).
10.6 Calculation of the carbon content
10.6.1 The carbon content cC of the test sample measured at 300 K is calculated by multiplying either I with the calibration factor F300, or I* with the calibration factor F*300.If the sampleis measured at 77 K I is multiplied by F77.
10.6.2 For this test method the following values have to be used[6]:
F300 = 7.41015 cm-1(15)
F*300 = 10.61015 cm-1(16)
F77 = 7.21015 cm-1(17)
11 Reporting results
11.1 The following information must be included in the report
11.1.1 Instrument used
11.1.2 Characteristics of the GaAs sample (crystal growth technique, conductivity type, resistivity, dopants, surface condition)
11.1.3 Sample geometry and thickness, wedge angle, diameter of measurement beam
11.1.4 Measurement temperature, spectral resolution, apodization function used
11.1.5 Evaluation procedure
11.1.6 Corrections applied
11.1.7 Integrated absorption of the carbon band (I or I*)
11.1.8 Carbon content cc
11.1.9 For 300K measurement transmittance spectrum of the test sample at least in the wavenumber range from 570 to 590 cm-1 and carbon absorption spectrum at least in the wavenumber range from 570 to 585 cm-1 ; for measurement at 77 K carbon absorption spectrum at least from 580 to 585 cm-1.
11.1.10 Operator, place, and date
12 Precision and bias of the test method
12.1 The statement given below only refers to the calculation of the integrated absorption of the carbon band and does not address the trueness of the calibration factors (see ref. 6 for further information).
12.2 At 300 K, a noise level in the transmittance spectrum of 510-3 leads to a precision - dependent on the sample thickness d - of 210-2 cm-1/d for Iand of 21014 cm-3/d for cc, respectively. At 77 K, the numbers are 710-3 cm-1/d for I and 51013 cm-3/d for cc.
APPENDIX 1
NOTICE: The material in this Appendix is an official part of SEMI doc. 4618 and was approved by full letter ballot procedures on [A&R approval date].
A1-1.1 Sets of GaAs samples with graded carbon concentrations between 11015 and 11016cm-3 have been prepared and tested in round robin measurements performed according to this standard. Those sets can be borrowed by users of the standard to check their FTIR instrument. The request should be addressed to SEMI.
Table A1-1 Recommended Physical Parameters of GaAs300 K / 77 K
Reflectance R / 0.276 (GaAs/vacuum) / 0.271 (GaAs/vacuum)
0.212 (GaAs, liquid nitrogen)
Refractive index n / 3.22 / 3.17
Phonon absorption coefficient ph at 570 cm-1 / 1.4 cm-1 / 0.66 cm-1
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This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.
Page 1Doc. 4618 SEMI
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