For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:400-13).

Price 75 cents

Preface

Semiconductor Measurement Technology:

Improved Infrared Response Technique for Detecting Defects and Impurities in Germanium and Silicon p-i-n Diodes

Table of Contents

PREFACE

commerce.

The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance, interchangeability, and reliability of discrete semiconductor devices and integrated circuits through improvements in measurement technology for use in controlling device fabrication processes and in specifying materials and devices in national and international Its major thrusts are the development of carefully evaluated and well documented test procedures and associated technology, for use on production lines and in the exchange of devices and materials, and the dissemination of such information to the electronics community. Application of the output by industry is expected to contribute to higher yields, lower cost, and higher reliability of semiconductor devices. In addition, the improvements in measurement technology will lead to greater economy in government procurement and will provide a basis for controlled improvements in fabrication processes and in essential device characteristics. Cooperation with industrial users and suppliers of semiconductor devices is achieved through NBS participation in standardizing organizations; through direct consultations with device and material suppliers, government agencies, and other users; and through periodically scheduled symposia and workshops. In addition, progress reports are regularly prepared for issuance in the NBS Special Publication 400 sub-series. More detailed reports such as state-of-the-art reviews, literature compilations, and summaries of technical efforts conducted within the Program are issued as these activities are completed. Reports of this type which are published by NBS also appear in the Special Publication 400 sub-series. ments of availability of all publications in this sub-series are sent by the Government Printing Office to those who have requested this service. A request form for this purpose may be found at the end of this report.

SEMICONDUCTOR MEASUREMENT TECHNOLOGY:

IMPROVED INFRARED RESPONSE TECHNIQUE FOR DETECTING DEFECTS
AND IMPURITIES IN GERMANIUM AND SILICON p-i-n DIODES*

A. H. Sher

Institute for Applied Technology
National Bureau of Standards
Washington, D.C. 20234

An infrared response (IRR) technique was
evaluated for its utility in qualifying germanium
for radiation detector use. Because of several improve-
ments in the sensitivity and interpretation of

the technique made during the evaluation, it

was possible to observe a number of discrete energy
levels lying within the forbidden energy gap of
germanium which had passed unobserved in previous
studies. These levels correlate with the type of
defects and vacancies introduced by radiation
damage into germanium as measured using such
techniques as photoconductivity and Hall effect
measurements after irradiation. Furthermore,
the improved infrared response measurement method
was used to identify impurities, such as copper,
gold, and iron, and dislocations resulting from
heat treatments in germanium. A major advance was
made when it was determined that the IRR spectra
could be grouped into five distinct types on the
basis of spectral features observed in the energy
range from 0.6 to 0.7 eV. One of the spectrum
types represented crystals from which good quality
detectors could be fabricated; the other four
represented crystals that yielded poorer quality
detectors due to carrier trapping, or crystals that
presented problems such as low lithium drift mobility
in detector fabrication. Three of the four spectrum
types representative of poor crystal quality could
be duplicated by suitably degrading specimens of a
good quality crystal. The material and detector
characteristics of crystals within each spectrum
type were found to be similar.

Key Words:

Carrier trapping; gamma-ray detector; germanium; Ge (Li) detector; infrared response; silicon.

Work supported by the AEC Division of Biomedical and Environmental Research

Armantrout introduced a method that made use of the response of p-i-n diodes to monochromatic infrared radiation to identify trapping centers in material intended for use in fabricating Ge (Li) detectors [1]. Further investigation of this infrared response (IRR) technique was undertaken in order to corroborate the previously published findings and determine the applicability of the method. In the process, modifications were made that enhanced the sensitivity and scope of the measurement. This is manifested by increased observation of details in the IRR spectra and led to a better interpretation of energy levels arising from impurities and defects in both germanium and silicon crystals.

2. INITIAL MEASUREMENTS

In the IRR measurement, photons incident on a reverse-biased p-i-n diode induce the formation of free carriers in the depleted region of the device by excitation of carriers into or out of energy levels within the band gap. The current due to the generation of free carriers and the subsequent transport of these carriers to the electrodes of the diode is measured as a function of the incident photon energy. The diodes used in this study were, in most cases, lithium-drifted germanium gamma-ray spectrometer structures. These structures are produced by first thermally diffusing lithium, a donor impurity, into single crystal p-type germanium doped with an acceptor impurity, usually gallium. The resulting p-n junction is reverse-biased so that the mobile lithium ions drift into the crystal under the influence of the applied field. The lithium-ion donors compensate the charge of the acceptors present and produce a compensated region that contains few free charge carriers. This compensated or depleted region, sandwiched between the n-type diffused layer and remaining bulk p-type crystal, is sensitive to radiation. Figure 1 shows IRR spectra obtained from four lithium-compensated p-i-n germanium diodes using apparatus similar to that used in the original work [1] except that a grating (rather than a prism) monochromator and a phasesensitive amplifier were employed [2]. The results are similar to those previously observed. Diodes NBS 83-3 and NBS 83-4 were fabricated from specimens of the same p-type germanium single crystal; the former was subjected only to the typical lithium-drifted detector fabrication process while the latter was, in addition, heated to 800°C and quenched to room temperature. Diodes NBS 13 and NBS 13Cu were fabricated from specimens of a germanium crystal with an initially high dislocation density (104 cm-3). Diode NBS 13Cu was also doped with electrically active copper to a density of approximately 1 x 1014 cm-3. All that could be inferred from the IRR spectra of the four diodes on the basis of the previous studies [1] is that all the diodes except diode NBS 83-3 would show some degree of carrier trapping due to their observed response below approximately 0.7 eV and that diode NBS 13 exhibits the lithiumdefect interaction as manifested by the shelflike response at 0.50 eV.

Considerably more information was obtained by improving the apparatus used to measure diode IRR by using 1-mm thick germanium filter windows