NBSIR 74-626

Semiconductor Nuclear Radiation

Detector Studies - A Final Report

A. H. Sher

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

September 1974

Final

Prepared for U. S. Atomic Energy Commission

Division of Biomedical and Environmental Research

Washington, D. C. 20545

NBSIR 74-626

SEMICONDUCTOR NUCLEAR RADIATION

DETECTOR STUDIES -

A FINAL REPORT

A. H. Sher

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

September 1974

Final

Prepared for U. S. Atomic Energy Commission

Division of Biomedical and Environmental Research

Washington, D. C. 20545

s^"

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

SEMICONDUCTOR NUCLEAR RADIATION DETECTOR STUDIES - A FINAL REPORT*

A. H. Sher

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

In response to a problem that arose with regard

to the availability of germanium for lithium-drifted

germanium detectors [Ge(Li) detectors], a comprehensive program was undertaken aimed toward the

development of a method for the rapid specification

of germanium quality for nuclear radiation detector

use, and the determination of factors affecting

germanium quality. Measurements on a large number of

germanium crystals, most of which had been rejected

for detector use, and intercomparison of these measurements and the methods employed led to significant

developments in the measurement of lithium mobility

and drif tability , carrier trapping, and semiconductor

defect and impurity determination via an improved

infrared response (IRR) technique. The present improvement of the infrared response technique resulted in

the observation of a number of discrete energy levels

lying within the forbidden gap of germanium unobserved

in previous studies. It was possible to assign the

proper position of energy levels detected by IRR in

the upper or lower half of the energy gap. It was

thus possible, in some instances, to determine the

nature of the defects responsible for the observed

energy level from results reported in the literature.

The goal of developing a method for the rapid specification of germanium quality was achieved.

Key Words: Carrier trapping; gamma-ray detector; germanium; Ge(Li)

detector; infrared response; silicon.

k

Work supported by the AEC Division of Biomedical and Environmental

Research.
1. INTRODUCTION

The situation with regard to germanium for high resolution, lithiumdrifted gamma-ray spectrometers [Ge(Li) detectors] was of particular

concern to the scientific community involved in making, studying, and using

these devices in late 1965. At that time serious difficulties in

attempting to fabricate germanium gamma-ray detectors were encountered

at many laboratories. The origin of these difficulties was traced to

the germanium supplied by a domestic source used in the fabrication of

the detector, but the exact causes could not be determined. Subsequently,

suitable material became available, primarily from foreign suppliers, but

selection was based ultimately on the characteristics of detectors

fabricated from specimens of the crystals. Many of the crystals had to

be rejected after this selection process, which might take up to two

months, because satisfactory means to identify detector-grade germanium

prior to the start of the lithium-drift process were not available.

In 1964, the NBS Electronic Instrumentation Division (currently the

Electronic Technology Division) undertook for the AEC a comprehensive

research program which had as its overall objective the identification of

those parameters that would be most useful in specifying detector-grade

germanium and silicon. Because of the immediate problem in 1965, the NBS

program emphasized the study of germanium. The specific objectives of

this program were the development of a method for the rapid specification

of germanium quality for nuclear radiation detector use, and the determination of factors affecting germanium quality.

During the course of the program, the character and use of Ge(Li)

detectors changed markedly. Detectors with larger sensitive volumes

than those used in nuclear physics, the initial application, were required

for an increasing number of different uses, including nuclear medicine,

analytical applications, environmental pollution monitoring, and reactor

fuel monitoring and safeguarding. The rapid determination of starting

crystal suitability was especially critical in regard to the large-volume

Ge(Li) detectors. Where once the cost of the crystal made up perhaps

one-tenth the selling price of the finished detector, it was not uncommon

for large-volume detectors that the cost of the starting material was one-third or more of the selling price. With a large investment in the

starting material, the device producer expended considerable effort in

reworking a crystal in order to obtain a saleable detector. Often, because

of material loss during such reprocessing, the final sensitive volume

of the detector would be less than that anticipated. Also, the fact that

the sources of germanium for commercial detector fabrication were exclusively

foreign was of concern to several government agencies.

The NBS study proceeded on two fronts, characterization of germanium

and evaluation of detector performance. Specification of parameters

customarily used to characterize semiconductors for devices such as transistors or rectifiers appeared to be insufficient for the purpose of

insuring the fabrication of high quality Ge(Li) detectors. A series

of isolated experiments at various laboratories had suggested that some other parameters such as lithium precipitation rate [1] or low-temperature electron drift mobility [2] might be correlated with

2
material quality. The approach adopted at NBS [3] was to collect and

characterize a sizable group of germanium crystals as fully as possible

with respect to the standard parameters such as resistivity, etch pit

density, and lifetime, as well as other parameters such as Hall mobility

at various temperatures, electron drift mobility at 77 K, infrared

absorption due to interstitial oxygen, and lithium precipitation and

drift rates. After a detector was fabricated from the characterized

crystal by means of a standardized procedure, its performance was deter-

« mined by measuring a number of device characteristics such as leakage

current, noise, capacitance, energy resolution, and charge collection

efficiency, in order to determine whether correlations existed between

detector quality and initial material properties. In all, 85 germanium

crystals were collected, most of which had previously been rejected for

detector use by the contributors.

In addition to this comprehensive in-house research effort at NBS,

Committee F-1 on Electronics of the American Society for Testing and

Materials was assisted by NBS in setting up a task force to study germanium properties and to develop standard methods for measuring pertinent

crystal parameters. Staff members also cooperated with the Nuclear

Instruments and Detectors Committee of the Institute for Electrical and

Electronic Engineers in developing standards for Ge(Li) characterization.

Significant accomplishments of the NBS research effort were the

development of a rapid test procedure for determining lithium-ion drift

mobility in germanium and an improved infrared response technique for

detecting defects and impurities in semiconductors. The purpose of this

report is to describe and summarize the results of the NBS study and

show the impact they had on the Ge(Li) detector situation. Major emphasis

will be given to the infrared technique (detailed in the Appendix - NBS Spec. Publ. 400-13), both for its application to material qualification

for radiation detector use and its broader potential for semiconductor

material characterization.

2. EARLY ACCOMPLISHMENTS

In this section will be briefly described the results of NBS research

carried out prior to the infrared response study. Almost all of these

results were published separately in the open technical literature during

the course of the study.

2.1 MATERIALS CHARACTERIZATION

Measurements of lithium-ion drift mobility in germanium, a primary

materials acceptance criterion, were performed in the temperature region

in which the lithium drift process was carried out, between room temperature and approximately 335 K [4]. Prior to this work, such values of lithium

mobility had been obtained by extrapolating values measured at 425 K and

above into this region [5]. It was found that in the temperature region

of interest, the directly measured mobility values were higher than those

obtained by extrapolation. This study also confirmed for germanium some theoretical predictions concerning the two regimes of lithium drift

3
rate [6] that led to NBS introduction of a very rapid technique, subsequently adopted by the American Society for Testing and Materials [7],

for measuring lithium mobility and utilized in industrial practice. Use

of this technique based on diode capacitance measurements early in the

drifting period, cut the drift time required for the test by a factor

of 33 over a previous method [6] based on measurements made in later

phases of the drifting period.

A more realistic model of the lithium-ion drift process was developed [8]. This model postulated loss of mobile lithium ions during

drift. Calculations made using this model were in good agreement with

the experimentally-observed behavior of many germanium crystals during

lithium drift.

Because of the importance of oxygen contamination in germanium as it

affects driftability, three methods for measuring this impurity were

investigated: infrared absorption [9], lithium precipitation [1], and

lithium mobility [4], It was determined that oxygen concentration measured

by both infrared absorption and lithium precipitation agreed to within

10 percent with that determined from lithium mobility measurements [10],

Whereas infrared absorption measurements require apparatus that is not

normally found in a detector fabrication facility, and the precipitation

technique was essentially destructive so that the germanium specimen could

not be used for detector purposes, the mobility measurement, carried out

as part of the detector fabrication process, could be used to rapidly

measure oxygen concentration between 0.2 and 20 parts per billion

(1 X 10^^ to 1 X 10^^ cm~^). This technique also had the best precision

of the three methods, a relative sample standard deviation of 10 percent

as determined in a multi-laboratory experiment [10].

In the course of the research program, it was determined that the

semiconductor properties usually measured to characterize the material could

not be used to qualify germanium for detector use. This became evident in

scatter diagrams, 129 in all, of the possibly meaningful combination of 21

parameters under study (such as bulk resistivity. Hall resistivity, photoconductive decay lifetime, diode recovery lifetime, etch pit density,

electron mobility at both 300 and 77 K, Hall mobility, lithium mobility,

infrared absorption at both 300 and 20 K, and lithium precipitation rate)

that were generated by computer using data on some or all, depending on

the measurement, of the 65 specimens collected at that time [11].

Another parameter, electron drift mobility, was studied as a candidate

for a predictor of acceptable material. Based on measurements of oxygen

concentration, electron drift mobility, and lithium mobility for germanium

specimens with resistivity ranging from 4 to AO fi'cm, correlation was not

observed between decreasing electron drift mobility and increasing oxygen

concentration or between increasing electron drift mobility and increasing

lithium mobility [12]. Thus electron drift mobility measurements did

not appear to be useful in specifying suitability of germanium for detector

fabrication.

2.2 DETECTOR CHARACTERIZATION

While measurements of lithium drift mobility and oxygen concentration

gave information on the probability that a given germanium crystal could

4
be drifted to some desired depth, there was, as yet, no guarantee that the

resulting detector would be of high quality. The performance parameter of concern was the trapping of charge carriers that reduced collection efficiency, degraded energy resolution, and made quantitative determinations

using the detector (such as in activation analysis) difficult. Thus, to

qualify germanium for fabrication of high-quality detectors; a detector

still had to be fabricated from a specimen of the germanium crystal and

its performance evaluated.

A series of planar Ge(Li) detectors was fabricated that had performance characteristics considered, at the time, to be typical of high

performance devices [13], These detectors served as the basis for comparison

for subsequent test detectors and were also used in a series of measurements on trapping. One criterion used in judging the degree of carrier

trapping in Ge(Li) detectors was the Fano factor, F. Prior experimental

measurements of F using uncollimated radiation sources at NBS and other

laboratories indicated that F - 0.13. Since the true Fano factor, F, is

associated only with the charge production process, corrections must be

made to experimental data for additional statistical and loss contributions

from the collection of charge in the detector as well as by factors

contributed by the electronic components of the system. Because of the

difficulty in sorting out these additional statistical and loss contributions, F cannot be measured directly. Therefore, the concept of an effective Fano factor, F', [14] was introduced. The effective Fano factor,

which is measured using collimated gamma-radiation so that the effects

of electron and hole collection can be separately assessed, represents an upper limit for the true Fano factor. Study of two Ge(Li) detectors

showed that F' <_ 0.11. That the true Fano factor was less than or equal

to 0.11 indicated that the performance, with regard to carrier trapping,

of Ge(Li) detectors characterized by F = 0.13 was not the optimum as previously thought. This result was later experimentally confirmed at

another laboratory where detectors were fabricated that had effective

Fano factors of less than 0.10 [15].

Another criterion used to judge the degree of carrier trapping in

Ge(Li) detectors was the shape of the gamma-ray photopeak obtained in a pulse-height vs energy spectrum. Trapping is usually manifested as a broadening on the low energy side of the ideal, sjnmnetrical, gaussianshaped peak. However, models of the trapping process proposed at the

time could not satisfactorily reproduce experimentally-obtained gammaray photopeaks. A new model was developed [16] that could predict the

variation in peak shape resulting from gamma-ray irradiation of planar

Ge(Li) detectors using experimentally obtainable input parameters. The

key to the success of the model was the incorporation of a variation of

peak amplitude as a function of charge collection efficiency (shown to be

responsible for the observed increase in tailing with a decrease in field)

and of the variation of carrier lifetime, for both carriers, with applied

field (responsible for the observed shift in peak positions with field)

.

In order to aid in the fabricating, testing, and use of Ge(Li)

detectors, a series of six nomographs was produced and published covering

calculations routinely used in fabricating detectors and measuring detector

quality [17]. These nomographs found wide use; one thousand copies of

5
this docxjment were sold through the Government Printing Office, and some

400 were distributed directly by the author.

Contributions were also made to efforts aimed at standardizing detector

measurements. NBS staff members served as project leaders for the development of a voluntary standard test procedure for specifying Ge(Li) detectors

at the buyer-seller interface [18]. Prepared in cooperation with the

Nuclear Instruments and Detectors Committee of the Institute for Electrical

and Electronic Engineers and later accepted by the American National

Standards Institute, this standard included several test procedures developed

at NBS.

3. THE INFRARED RESPONSE TECHNIQUE

NBS efforts concerning two of the major germanium problems, driftability and carrier trapping, resulted in more meaningful measurements

and procedures for characterizing material in terms of driftability and

Ge(Li) detectors in terms of trapping. However, there was, as yet, no method that could be used to rapidly determine the suitability of a germanium crystal for detector use, particularly with regard to carrier

trapping. Neither was there much information about the nature of the

factors that affected germanium quality.

To proceed further, an infrared response (IRR) technique was evaluated

for its utility in qualifying germanium for detector use. In this

technique, the diode impurity photovoltage is measured as a function of

energy of incident Infrared radiation. Features in the resulting IRR

spectra can be interpreted in terms of impurity and defect levels lying

within the forbidden energy gap. Because of several improvements of the

technique made during this evaluation, it was possible to observe because

of increased sensitivity a number of discrete energy levels lying within

the forbidden gap of germanium which had passed unobserved in previous

studies. These levels were correlated with the type of defects and

vacancies introduced by radiation damage into germanium as measured using

such techniques as photoconductivity and Hall 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 in detector fabrication such as low lithium drift

mobility. Three of the four spectriom types representative of poor crystal

quality could be duplicated by suitably treating specimens of a good

quality crystal (introducing neutron damage or lithium precipitation)

so as to degrade them. The material and detector characteristics of

crystals within each spectrum type were found to be similar. Of the 55

crystals studied that yielded usable IRR spectra, about 56 percent exhibited

the two spectrum types that were found to be indicative of carrier trapping.

It was possible to assign the proper position of energy levels detected

6
by IRR in the upper or lower half of the energy gap. It was thus possible,

in some instances, to determine the nature of the defects responsible for

the observed energy level from results reported in the literature. The

results of this study are detailed in the Appendix (NBS Special Publication

400-13).

4. SIGNIFICANCE AND IMPACT

The objectives of the germanium study were: 1) the development of a method for the rapid specification of germanium quality for nuclear

radiation detector use, and 2) the determination of factors affecting

germanium quality. The method developed at NBS for measuring lithiumion drift mobility [7] and the infrared response technique fulfill the

first objective. The mobility is obtained in about three hours of drifting; less by a factor of about 50 than the time required for drifting a 5-mm-thick detector for evaluating crystal quality; the IRR technique can

be applied to a planar diode after another 24 hours or so of drifting.

In terms of the second objective, the results obtained from IRR spectrum

types showing carrier trapping and subsequent linking to known energy levels seem to confirm the role of oxygen in the creation of trapping centers in

germanium. In addition, the results of the work disclosed the inadequacy

of numerous methods which had been employed in this field and led to the

adoption of new standards which would promote more effective and efficient

evaluation of crystals and detectors.

5. ACKNOWLEDGEMENTS

A program of the scope of this one could not have been carried out

successfully without the help and cooperation of a large number of individuals and laboratories, particularly with regard to the collection of

germanium crystals. Grateful acknowledgement is given the assistance,

criticism, and helpful discussion provided by: G. A. Armantrout (Lawrence

Livermore Laboratories), I. L. Fowler and T. A. McMath (Chalk River

Nuclear Laboratories), R. N. Hall and R. D. Baertsch (General Electric,

Schenectady), W. Hansen (Lawrence Berkeley Laboratories), H. Kraner

(Brookhaven National Laboratory), H. Mann (Argonne National Laboratory),

G. L. Miller (Bell Laboratories), J. Palms (Emory University), R. J. Starks

(Eagle-Picher) , R. Trammel and S. Wagner (ORTEC) , R. D. Westbrook (Oak

Ridge National Laboratory), W. E. Wheeler (Nucleonic Products Company, Inc.),

and V. S. Vavilov (Lebedev Institute, U.S.S.R.). At NBS, M. Cosman,

W. J. Croll, H. E. Dyson, W. J. Keery, J. Krawczyk, Y. M. Liu, R. L. Mattis,

W. R. Thurber, and J. A. Coleman contributed to the technical effort;

W. M. Bullis provided consultation on material characterization and

valuable critism of the work; L. R. Williams prepared the figures; and,

E. Y. Trager prepared this manuscript.

7
6. REFERENCES

1. Fox, R. J., Lithium Drift Rates and Oxygen Contamination in Germanium, IEEE Trans.

Nuol. Soi. NS-13 , No. 3, 367-369 (1966).

2. Armantrout, G. A., Correlation Between Lithium Drift Mobility and Minority-Carrier

Drift Mobility in Germanium, IEEE Trans. Nuol. Soi. NS-13 , No. 3, 370-372 (1966).

3. Bullis, W. M., and Coleman, J. A., Characterization of Germanium and Silicon for Nuclear

Radiation Detectors, Nucleonics in Aerospace^ P. Polishuk, ed. (New York, Plenum Press,

1968) pp. 166-175.

4. Sher, A. H., Lithium-Ion Drift Mobility in Germanium, J. Appl. Phys. 40, 2600-2607

(1969).

5. Fuller, C. S., and Severiens, J. C, Mobility of Impurity Ions in Germanium and Silicon,

Phys. Rev. 96, 21-24 (1954).

6. Pell, E. M. , Study of Li-0 Interaction in Si by Ion Drift, J. Appl. Phys. 32^, 1048-1051,

(1961).

7. Tentative Method of Measuring Lithium-Ion Drift Mobility in Germanium Single Crystals,

F334-71T, 1973 ASTM Book of Standards, Part 8.

8. Sher, A. H.,and Coleman, J. A., Lithium Driftability in Detector-Grade Germanium,

IEEE Trans. Nucl. Sci. NS-17 , No. 3, 125-129 (1970).

9. Thurber, W. R. , Determination of Oxygen Concentration in Silicon and Germanium by

Infrared Absorption, NBS Tech. Note 529 (May 1970).

10. Sher, A. H. , Croll, W. R. , and Thurber, W. R. , Determination of Oxygen in Germanium

Below 20 Parts per Billion by Measurements of Lithium Mobility and Precipitation,

Anal. Chem. 43, 1831-1834 (1971).

11. Methods of Measurement for Semiconductor Material, Process Control, and Devices,

W. M. Bullis, ed., NBS Tech. Note 488, pp. 14-15 (July 1969).

12. Sher, A. H., and Thurber, W. R., Minority Carrier and Lithium-Ion Drift Mobilities and

Oxygen Concentration in p-Type Germanium, J. Appl. Phys. 42^, 4508-4509 (1971).

13. Methods of Measurement for Semiconductor Materials, Process Control and Devices,

W. M. Bullis, ed., NBS Tech. Notes 472, pp. 18-19 (December 1968) and 520, p. 24

(March 1970)

14. Sher, A. H., and Keery , W. J., Variation of the Effective Fano Factor in a CTe(Li)

Detector, IEEE Trans. Nucl. Soi. NS-17 , No. 1, 39-43 (1970).

15. Pehl., R. H., and Goulding, F. S., Recent Observations on the Fano Factor in Germanium,

Nucl. Instr. Meth. 81, 329-331 (1970).

16. Sher, A. H., Carrier Trapping in Ge(Li) Detectors, IEEE Trans. Nucl. Sci. NS-18 , No. 1, 175-183 (1971).

17. Sher, A. H., Nomographs for Use in the Fabrication and Testing of Ge(Li) Detectors,

NBS Tech. Note 537 (August 1970).

18. IEEE and American National Standard, Test Procedures for Germanium Gamma-Ray Detectors,

IEEE Std. 325-1971/ANSI N42. 8-1972.

USCOMM-NBS-DC 8
NDS-:I^A fREV. 7-73)

J.S. DEPT. OF COMM.

J 1. p u ii 1. IC A TH IN OK KI.PORT NO.

BIBLIOGRAPHIC DATA I , , SHEET , NBSIR 74-626

2. Gov't Accession

No.

3, Re'jipient's -Acces.sicn No.

4. THLt AND SUbTITI.H

Semiconductor Nuclear Radiation Detector Studies - A Final Report

5. Fublicaticn Date September 1974

6. Performing Organization Code

7. AUTKOR(S)

A. H. Sher

3. Performing Organ. Report No. NBSIR 74-626

9. PERFOR.MING ORGANIZATION NAMH AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete AJdrcss (Street, City, State, ZIP)

U. S. Atomic Energy Commission

Division of Biomedical and Environmental Research

Washington, D.C. 20545

13. Type of Report Si Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

NBS-SP-400-13 is the appendix to this report.

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

In response to a problem that arose with regard to the availability

of germanium for lithium-drifted germanium detectors [Ge(Li) detectors],

a comprehensive program was undertaken aimed toward the development of

a method for the rapid specification of germanium quality for nuclear

radiation detector use, and the determination of factors affecting germanium

quality. Measurements on a large number of germanium crystals, most of which

had been rejected for detector use, and intercomparison of these measurements

and the methods employed^led to significant developments in the measurement

of lithium mobility and drif tability , carrier trapping, and semiconductor

defect and impurity determination via an improved infrared response (lElR)

technique. The present improvement of the infrared response technique resulted

in the observation of a number of discrete energy levels lying within the

forbidden gap of germanium unobserved in previous studies. It was possible

to assign the proper position of energy levels detected by IRR in the upper

or lower half of the energy gap. It was thus possible, in some instances,

to determine the nature of the defects responsible for the observed energy

level from results reported in the literature. The goal of developing a method for the rapid specification of germanium quality was achieved.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

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

silicon.

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