SEMI MF391 - Test Method for Minority Carrier Diffusion Length in Extrinsic Semiconductors by Measurement of Steady-State Surface Photovoltage -

Abstract

1  Purpose
1.1  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.
1.2  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. Three test methods are described:
1.2.1  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.
1.2.2  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.
1.2.3  Test Method C — Digital oscilloscope recording (DOR) method. This test method provides the direct control of relaxation processes of surface photovoltage (SPV) appearance and disappearance on minimum radiation intensity and eliminates any non-linear and other distorting effects, which is feasible on these conditions.
1.3  These test methods are suitable for use in research, process control, and materials acceptance.
1.4  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.
1.5  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.
1.6  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 cannot be interpreted as the diffusion length (refer to ¶ 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.
2  Scope
2.1  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 lifetime  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.
2.2  All three test methods covered are nondestructive.
2.3  The limits of applicability with respect to specimen material, resistivity, and carrier lifetime have not been determined; however, measurements have been made on 0.01 to 50 Ω·cm n- and p-type silicon specimens with carrier lifetimes as short as 2 ns.
2.4  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.
2.5  These test methods also have been applied to the determination of the width of the denuded zone in silicon wafers.
2.6  These test methods measure diffusion lengths at room temperature (22°C) only. Lifetime and diffusion length are a function of temperature.

Referenced SEMI Standards (purchase separately)
SEMI C23 — Specification for Buffered Oxide Etchants
SEMI C30 — Specification and Guide for Hydrogen Peroxide
SEMI C34 — Specification and Guide for Mixed Acid Etchants
SEMI M59 — Terminology for Silicon Technology
SEMI MF28 — Test Method for Minority Carrier Lifetime in Bulk Germanium and Silicon by Measurement of Photoconductivity Decay
SEMI MF84 — Test Method for Measuring Resistivity of Silicon Wafers with an In-Line Four-Point Probe
SEMI MF95 — Test Method for Thickness of Lightly Doped Silicon Epitaxial Layers on Heavily Doped Silicon Substrates Using an Infrared Dispersive Spectrophotometer
SEMI MF110 — Test Method for Thickness of Epitaxial or Diffused Layers in Silicon by the Angle Lapping and Staining Technique
SEMI MF533 — Test Method for Thickness and Thickness Variation of Silicon Wafers
SEMI MF673 — Test Method for Measuring Resistivity of Semiconductor Slices or Sheet Resistance of Semiconductor Films with a Noncontact Eddy-Current Gauge
SEMI MF1535 — Test Method for Carrier Recombination Lifetime in Silicon Wafers by Noncontact Measurement of Photoconductivity Decay by Microwave Reflectance

Revision History
SEMI MF391-0310E (Reapproved 0425)
SEMI MF391-0310E (editorial revision)
SEMI MF391-0310 (technical revision)
SEMI MF391-0708 (technical revision)
SEMI MF391-1106 (technical revision)
SEMI MF391-02 (first SEMI publication, transferred from ASTM)

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