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SEMI MF391 Document Information:
Title
TEST METHODS FOR MINORITY CARRIER DIFFUSION LENGTH IN EXTRINSIC SEMICONDUCTORS BY MEASUREMENT OF STEADYSTATE SURFACE PHOTOVOLTAGE
Semiconductor Equipment and Materials International
Publication Date:
Jul 1, 2008
Scope:
These test methods are based on the measurement of surface
photovoltage (SPV) as a function of energy (wavelength) of the
incident illumination.
NOTE 1: The minority carrier lifetime is the square of the
diffusion length divided by the minority carrier diffusion constant
that is assumed or can be determined from drift mobility
measurements. SPV measurements are sensitive primarily to the
minority carriers; the contribution from majority carriers is
minimized by the use of a surface depletion region. As a result,
lifetimes measured by the SPV method are often shorter than the
lifetimes measured by the PCD method because the photoconductivity
can contain contributions from majority as well as minority
carriers. When both majority and minority carrier lifetimes are the
same, both the SPV and PCD methods yield the same values of
lifetime1 provided that the correct values of absorption
coefficient are used for the SPV measurements and that the
contributions from surface recombination are properly accounted for
in the PCD measurement.
Both test methods covered are nondestructive.
The limits of applicability with respect to specimen material,
resistivity, and carrier lifetime have not been determined;
however, measurements have been made on 0.1–50 O·cm n- and
p-type silicon specimens with carrier lifetimes as short
as 2 ns.
These test methods were developed for use on single crystal
specimens of silicon. They may also be used to measure an effective
diffusion length in specimens of other semiconductors such as
gallium arsenide (with suitable adjustment of the wavelength
(energy) range of the illumination and specimen preparation
procedures) and an average effective diffusion length in specimens
of polysilicon in which the grain boundaries are normal to the
surface.
These test methods also have been applied to the determination
of the width of the denuded zone in silicon wafers.
These test methods measure diffusion lengths at room temperature
(22°C) only. Lifetime and diffusion length are a function of
temperature.
NOTICE: This standard does not purport to
address safety issues, if any, associated with its use. It is the
responsibility of the users of this standard to establish
appropriate safety and health practices and determine the
applicability of regulatory or other limitations prior to use.
Purpose
Minority carrier lifetime is one of the essential
characteristics of semiconductor materials. In epitaxial layers and
in thin single crystal wafers, the surface recombination
corrections necessary to derive the minority carrier lifetime from
the photoconductive decay (PCD) method covered by SEMI MF28 and
SEMI MF1535 are excessively large.
Therefore, other test methods are required to cover the
measurement of minority carrier diffusion lengths in specimens of
extrinsic single-crystal semiconducting materials or in
homoepitaxial layers of known resistivity deposited on more heavily
doped substrates of the same type, provided that the thickness of
the specimen or layer is greater than four times the diffusion
length. Two test methods are described:
Test Method A — Constant magnitude surface photovoltage
(CMSPV) method. This test method circumvents the influence of
surface recombination on the lifetime measurement by maintaining
constant front surface conditions.
Test Method B — Linear photovoltage, constant photon
flux (LPVCPF) method. This test method utilizes only conditions and
data points that are not influenced by surface recombination and
other non-linear effects.
These test methods are suitable for use in research, process
control, and materials acceptance.
These test methods are particularly useful in testing materials
to be used in photovoltaic cells and other optical device
applications since the diffusion length is derived by methods that
are closely related to the functioning of the device.
Because carrier lifetime is directly influenced by the presence
of metallic impurity contamination, these test methods can be
interpreted to establish the presence of such contamination.
However, such interpretation is beyond the scope of these test
methods.
If a very thin surface region with long lifetime, such as an
epitaxial layer or a denuded zone, is on a bulk region with very
short lifetime, such as a heavily doped substrate or an internally
gettered wafer with oxide precipitates, respectively, the intercept
can not be interpreted as the diffusion length (see ¶ 3.2). Under
certain circumstances, the intercept can be related to the layer
thickness, providing a nondestructive means for determining the
thickness of the layer.
1 Saritas, M., and McKell, H. D., "Comparison of
Minority-Carrier Diffusion Length Measurements in Silicon by the
Photoconductive Decay and Surface Photovoltage Methods," J. Appl.
Phys. 63 (1988): pp. 4562–4567.
Keywords:
- diffusion length
- minority carriers
- polysilicon
- silicon
- single crystal silicon
- surface photovoltage
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