Background Statement for SEMI Draft Document #5734
Revision to SEMI M64-0306
TEST METHOD FOR THE EL2 DEEP DONOR CONCENTRATION IN
SEMI-INSULATING (SI) GALLIUM ARSENIDE SINGLE CRYSTALS BY
INFRARED ABSORPTION SPECTROSCOPY
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Background
The document is due for five year review. The EU Compound Semiconductor Materials TC Chapter
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Review and Adjudication Information
Task Force Review
Group:
Date:
Committee Adjudication
Task Force for Reapproval of SEMI M63-0306, EU Compound Semiconductor Materials
M46-1101E (reapproved 0309), and M64-0306 TC Chapter
April 15, 2014
April 15, 2014
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SEMI Draft Document #5734
Revision to SEMI M64-0306
TEST METHOD FOR THE EL2 DEEP DONOR CONCENTRATION IN
SEMI-INSULATING (SI) GALLIUM ARSENIDE SINGLE CRYSTALS BY
INFRARED ABSORPTION SPECTROSCOPY
This standard was technically approved by the global Compound Semiconductor Committee. This edition
was approved for publication by the global Audits and Reviews Subcommittee on November 29, 2005. It
was available at www.semi.org in January 2006 and on CD-ROM in March 2006.
1 Purpose
1.1 The purpose of this document is to specify a method to measure the concentration of the deep donor EL2 in SI
GaAs by infrared absorption.
2 Scope
2.1 This test method covers a procedure for measuring the concentration of the deep donor EL2 in SI GaAs. This
method focuses on improving the accuracy and repeatability of the measurement by standardizing the test conditions
and reporting and by routine calibration of the measurement.
2.2 This test method is intended to cover SI GaAs samples with an electrical resistivity in the range from 1  106 to
5  108 cm, determined by doping with carbon. The concentration of EL2 must be greater than 5  1015 cm-3.
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
SEMI M39 — Test Method for Measuring Resistivity and Hall Coefficient and Determining Hall Mobility in Semiinsulating GaAs Single Crystals
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
4 Terminology
4.1 Definitions
4.1.1 transmittance — the ratio of the intensity of transmitted to incident light at a certain wavelength.
4.1.2 reflectance — the ratio of the intensity of reflected to incident light at a certain wavelength.
4.1.3 ionization degree — the ratio of the concentration of singly positive charged to total EL2 defects.
5 Properties of EL2
5.1 SI GaAs crystals, grown under arsenic overpressure, contain the double donor EL2. EL2 is an intrinsic point
defect, consisting of an arsenic antisite atom. By adding a controlled amount of a compensating shallow acceptor
(usually carbon, CAs), the Fermi level is pinned at the first electronic level of EL2. At 300 K, this level is at 0.69 eV
below the conduction band.
5.2 By variation of crystal growth parameters and by thermal treatment of the grown crystal, the concentration of
the EL2 defect can be controlled in the range between 5  1015 and 3  1016 cm-3. To obtain SI GaAs, complete
ionization of shallow acceptors and shallow donors (if present) and partial single ionization of EL2 must be achieved,
requiring that
0 < cA  cD < cEL2
(1)
where cA, cD, and cEL2 are the concentration of shallow acceptors, shallow donors, and EL2, respectively.
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5.3 The determination of cEL2 is carried out by measuring the near-infrared (NIR) absorption due to the electronic
transitions from the first level of EL2 to the conduction band (CB) and valence band (VB). At room temperature,
these transitions cause a broad absorption band, starting at about 1700 nm and increasing in strength towards shorter
wavelengths.
5.4 The partial ionization of EL2 leads to two contributions to the NIR absorption:
(i) The electron transition from the occupied level to the CB
(ii) The hole transition from the unoccupied level to the VB
5.5 The contributions of both transitions are dependent on the ionization degree fI of the EL2 defect. fI is related to
the electron concentration n by

 E 
n
fI  1 
exp  EL 2 
 kBT 
 2NCB
1
(2)
where NCB is the effective density of states in the CB (4.21  1017 cm-3 at 300 K), EEL2 the activation energy of the
electron transition from the first level of EL2 to the CB (0.69 eV at 300 K), kB the Boltzmann constant, and T the
temperature. By measuring n according to SEMI M39 (s.o.), the ionization degree fI can be determined for a given
temperature T.
5.6 The NIR absorption coefficient EL2 at the wavelength  is given by
 EL 2    c EL 2 1  fI   n    fI   p  
(3)
where n and p are the optical absorption cross sections for the electron transition to the CB and the hole transition
to the VB, respectively. At the wavelength  = 1000 nm, the cross sections, used for this standard, have the values
n = 1.29  10-16 cm2 and p = 0.35  10-16 cm2. For further information, see [¶13.1] to [¶13.3].
5.7 The concentration cEL2 is calculated with the help of equations (2) and (3)
cEL 2  EL 2
1  xn
xn  n   p
(4)
with
x
E 
1
 exp EL 2 
2NCB
 kBT 
(5)
5.8 In the case of a low ionization degree (fI << 1 or xn >> 1), equation (4) can be simplified
cEL 2 
 EL 2
n
(6)
6 Interferences
6.1 Electron Concentration n — is strongly dependent on the sample temperature (Near room temperature the
change is about 10 % per 1 °C). Therefore, for calculation of cEL2 from equation (4), n must be measured at the same
temperature as EL2.
6.2 Optical Absorption Cross Sections n and p — There is some discussion in the literature about the proper
value for p ([13.4] and [13.5]). Any error of this value leads to bias of the EL2 concentration, becoming
increasingly important for large ionization degree.
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6.3 Multiple Internal Reflections — Multiple internal reflections at the surfaces of the sample can lead to errors in
the measured transmittance. This effect can be avoided if the coherence length of the measuring NIR light is small
compared to the sample thickness. The proposed spectral width of 40 nm limits the coherence length to about 25 m.
NOTE 1: Laser light is obviously not appropriate for this purpose.
6.4 External Reflections — The tilt of the sample surface with respect to the optical axis must be selected carefully
in order to hinder any external reflections at the surfaces of the sample from hitting the detector directly or indirectly.
6.5 Beam Shading — Can have a deleterious effect on the accuracy for the determination of the sample
transmittance. In particular, the area of the sample must be larger than the beam impinging on the sample.
6.6 Stray Light — Coming from the NIR light source or from external sources in the laboratory can lead to bias of
the measured sample transmittance if it hits the detector. Appropriate shielding of the apparatus is necessary to block
stray light.
7 Apparatus
7.1 NIR Light Source — Broadband quartz halogen lamp with electrical power of (typically) 100 W.
7.2 Optical Imaging System — Lens system to generate a parallel light beam between light source and sample and
an image on the detector.
7.3 NIR Detector — Photodiode made from silicon, germanium or indium gallium arsenide with a high linearity
and homogeneity. The detector area must be large enough to collect the measuring light completely. Optional: Array
or matrix detector for simultaneous measurement of one- or two-dimensional EL2 distributions.
7.4 Sample Holder — Capable of maintaining an angle of 2 to 5° between the optical axis and the sample surface.
Optional: Translation stage in one or two directions vertically to the optical axis for scanning purposes.
7.5 1000-nm Optical Band Pass Filter — with a bandwidth of 40nm. Used to select the wavelength band of NIR
light: 980 nm <  < 1020 nm. Must be placed between light source and sample.
7.6 Aperture Stop — Optional; used to increase the spatial resolution. Must be placed in front of the sample.
7.7 Long-wavelength (1800 – 2000 nm) Band Pass Filter — Optional; used to calibrate the apparatus according to
procedure A. The filter must be exchangeable with the 1000 nm filter for the measuring light.
7.8 Calibrated Grey Filter — Optional; used to calibrate the apparatus according to procedure B. The filter must
have a calibrated transmittance G at the wavelength of 1000 nm in the range between 0.45 and 0.55. G must be
known within  0.005.
7.9 SI GaAs Reference Sample — Optional; used to calibrate the apparatus according to procedure C. The reference
sample must have the same thickness as the sample to be measured (maximum deviation: 2 %) and the same optical
surface quality. The EL2 concentration must be between 1.0 and 1.5  1016 cm-3. The transmittance R at the
wavelength of 1000 nm must be known within 0.005.
8 Specimen Preparation
8.1 The sample must be prepared from SI GaAs single crystals with an electrical resistivity in the range from
1  106 to 5  108 cm, determined by doping with carbon. The measurement of the electrical resistivity is
performed according to SEMI M39.
8.2 The sample must have parallel surfaces and a thickness between 0.5 and 5.0 mm.
8.3 The surfaces must be polished mirrorlike.
8.4 The sample should be cleaned with organic solvent and deionized water and dried.
9 Calibration
9.1 The calibration of the apparatus must be carried out according to one of the following three procedures.
9.1.1 Procedure A — The transmittance  of the sample is measured at a wavelength between 1800 and 2000 nm
using the long-wavelength band pass filter. In this wavelength range no absorption in the sample takes place; the
transmittance is determined by the reflectance R of the surfaces
 = (1 – R) 2 / (1 – R2)
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(1800 nm: R = 0.292,  = 0.548; 2000 nm: R = 0.291,  = 0.550)
9.1.2 Procedure B — The light intensity IS transmitted by the sample at the wavelength of 1000 nm is referred to
the light intensity IG transmitted by the calibrated grey filter at the same wavelength.
9.1.3 Procedure C — The light intensity IS transmitted by the sample at the wavelength of 1000 nm is referred to
the light intensity IR transmitted by the reference sample at the same wavelength.
10 Measurement Procedure
10.1 Preparation of Apparatus
10.1.1 The sample temperature must be kept fixed in the range 25  5 °C. Care must be taken with the warming-up
time of the apparatus, particularly of the light source.
10.2 Calibration Procedure
10.2.1 Procedure A — The light intensity at a wavelength between 1800 and 2000 nm is measured without and with
the sample in position. The measured transmittance must match the range of 0.548  0.005 at 1800 nm or
0.550  0.005 at 2000 nm; otherwise the influence of interferences has to be removed.
10.2.2 Procedure B — The transmitted light intensity IG at 1000 nm is measured, when the calibrated grey filter is
inserted in the sample holder.
10.2.3 Procedure C —The transmitted light intensity IR at 1000 nm is measured, when the reference sample is
inserted in the sample holder.
10.3 Sample Measurement Procedure
10.3.1 The transmitted light intensity IS at 1000 nm is measured, when the sample is inserted in the sample holder.
10.3.2 If calibration procedure A is used, then also the light intensity I0 at 1000 nm without sample has to be
measured.
10.3.3 If the sample holder is equipped with a translation stage or array/matrix detectors are used, lateral
distributions of the EL2 concentration can be measured. Examples of one- and two-dimensional distributions of EL2
are shown in Figure 1.
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EL2 concentration [cm-3]
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Radial distance from wafer center [mm]
(a)
EL2 concentration [1016 cm-3]
(b)
Figure 1
Examples of Lateral Distributions of the EL2 Concentration.
（a）Profile of the EL2 Concentration in a VGF GaAs Wafer (Diameter 100 mm) Along the Radius.
（b）Radial EL2 Topogram of a VGF GaAs Wafer.
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11 Calculations and Interpretations of Results
11.1 Absorption Coefficient, EL2
11.1.1 The calculation is based on the absorption of NIR light at 1000 nm due to EL2. The sample transmittance 
is given by
 



(1  R ) 2  exp   EL 2 d
IS

I0
1  R 2  exp  2 EL 2 d

(7)
where R = 0.310 is the reflectance of GaAs at the wavelength of 1000 nm and d is the sample thickness.
11.1.2 The absorption coefficient EL2 is calculated from equation (7)
 EL 2 

2
1
 1  R 
ln 
d
 2 


1  4R 2 2
 1 

(1  R )4









(8)
11.2 Sample transmittance, 
11.2.1 Procedure A   is calculated by
 = IS / I0
11.2.2 Procedure B   is calculated by
 = IS / IG  G
(10)
11.2.3 Procedure C   is calculated by
 = IS / IR  R
(11)
(9)
11.3 EL2 concentration, cEL2
11.3.1 The concentration of the deep donor EL2 is calculated by
cEL 2   EL 2 
1  x n
x  n  1.29  10 cm2  0.35  10 16 cm2
16
(12)
with
 8006 
x  1.19  10 18 cm- 3  exp

 T 
(13)
where T is the sample temperature in Kelvin.
11.3.2 If the electron concentration n of the sample is larger than 1.5  107 cm-3, i.e. the electrical resistivity is
smaller than 6  107 cm, then the calculation of the EL2 concentration can be performed by
cEL2  EL 2  0.78  1016 cm3
(14)
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12 Reporting results
12.1 The following information shall be included in the report:
12.1.1 Identification of Specimen — Including electrical resistivity, size (thickness, area), surface condition.
12.1.2 Apparatus — Description of the major components (including light source, detector, filters).
12.1.3 Calibration Procedure — Use of procedure A, B or C.
12.1.4 Measuring Spot — Size and position of the measuring spot on the sample surface.
12.1.5 EL2 Concentration — Calculated EL2 concentration. Optional: One- or two-dimensional distributions of the
EL2 concentration.
13 Related Documents
13.1 G. M. Martin, “Optical Assessment of the Main Electron Trap in Bulk Semi-Insulating GaAs”, Appl. Phys.
Lett. 39, 747 (1981)
13.2 P. Dobrilla and J. S. Blakemore, “Experimental Requirements for Quantitative Mapping of Midgap Flow
Concentration in Semi-Insulating GaAs Wafers by Measurement of Near-Infrared Transmittance”, J. Appl. Phys. 58,
208 (1985)
13.3 P. Silverberg, P. Omling, and L. Samuelson, “Hole Photoionization Cross Sections of EL2 in GaAs”, Appl.
Phys. Lett. 52, 1689 (1988)
13.4 G. A. Baraff and M. A. Schluter, “Electronic Aspects of the Optical-Absorption Spectrum of the EL2 Defect in
GaAs”, Phys. Rev. B 45, 8300 (1992)
13.5 U. Kretzer and H. Ch. Alt, “Influence of Compensation Level on EL2 Concentration in Semi-Insulating
Gallium Arsenide”, Phys. Stat. Sol. (c) 0, 845 (2003)
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