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NBS TECHNICAL NOTE 594-5

Optical Radiation Measurements:

Stability and Temperature

Characteristics of Some Silicon

and Selenium Photodetectors
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences 2 — Applied Radiation 2 — Quantum Electronics * — Electromagnetics s — Time

and Frequency 3 — Laboratory Astrophysics 3 — Cryogenics s

, THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

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organizational units: Office of Standard Reference Data — Office of Technical Information and Publications —

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Optical Radiation Measurements:

Stability and Temperature Characteristics

of Some Silicon and Selenium Photodetectors

Kshitij Mohan, A. Russell Schaefer

and Edward F. Zalewski

Heat Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

r W \

' V*„ <* ^

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued June 1 973
National Bureau of Standards Technical Note 594-5

Nat. Bur. Stand. (U.S.), Tech. Note 594-5, 16 pages (June 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. 03.46:594-5). Price $0.35 domestic postpaid or $0.25 G.P.O. Bookstore.
Preface

This is the fifth issue of a series of Technical Notes entitled

OPTICAL RADIATION MEASUREMENTS. The series will consist primarily of

reports of progress in or details of research conducted in radiometry

and photometry in the Optical Radiation Section of the Heat Division

and will appear about every six weeks.

The level of presentation in OPTICAL RADIATION MEASUREMENTS will

be directed at a general technical audience. The equivalent of an undergraduate degree in engineering or physics plus familiarity with

the basic concepts of radiometry and photometry [e.g., G. Bauer,

Measurement of Optical Radiations (Focal Press, London, New York,

1965)] should be sufficient for understanding the vast majority of

material in this series. Occasionally a more specialized background

will be required. Even in such instances, however, a careful reading

of the assumptions, approximations and final conclusions should permit

the non-specialist to understand the gist of the argument if not the

details.

At times, certain commercial materials and equipment will be

identified in this series in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the material or equipment identified is necessarily

the best available for the purpose.

Any suggestions readers may have to improve the utility of this

series are welcome.

Henry J. Kostkowski, Chief

Optical Radiation Section

National Bureau of Standards

111
TABLE OF CONTENTS

Page

1. Introduction 1

2. Experiments and Results 3

2.1 Stability and Fatigue Effects 3

2.2 Temperature Dependence 7

3. Conclusions 7

4. References 10

Appendix A 11

LIST OF TABLES

1. Percent drift in detector output over approximately

twenty hours 5

2. Detector output drift during ten minutes after shuttering

for twenty minutes 6

3. Temperature dependence of detector output in the interval

20 to 30 °C 6

LIST OF FIGURES

1. Initial short term drift in detector output. Output

normalized to initial reading 8

xv
STABILITY AND TEMPERATURE CHARACTERISTICS

OF SOME SILICON AND SELENIUM PHOTODETECTORS

Kshitij Mohan*j A. Russell Sohaefer and Edward F. Zalewski

This paper describes the comparison of some characteristics of selenium barrier layer photocells and silicon

PIN and PN type photodiodes operated in the photovoltaic

or non biased mode. The work was done to study the suitability of these detectors specifically for goniometric

measurements of flux and possibly for other photometric

(or radiometric) measurements. The characteristics studied

were the stability of detector output over approximately

twenty hours, fatigue or light memory effects over short

periods of time, and the temperature dependence of detector

output.

Key words: Fatigue; light memory; photocells; photodiodes,

photometry; radiometry; selenium; silicon; stability;

temperature dependence.

1. Introduction

Since the advent of physical photometry, barrier layer selenium

photovoltaic cells have been used extensively in photometry at NBS. The

selenium cells used were equipped with photopic filters that approximately corrected the response of the detector to match the CIE defined

spectral luminous efficiency in photopic vision.

Selenium cells have been found [1,2,3] 1 to exhibit certain characteristics, such as fatigue, that are undesirable in goniometric measurements

of total flux. In the last decade solid state technology has made

available several new kinds of photodetectors, some of which seem promising for photometric applications. In particular, silicon photodiodes

appear to be a possible replacement for selenium barrier layer photocells.

We have examined and compared certain characteristics of two

selenium detectors and several different types of silicon detectors to

ascertain their suitability for use in our work. The study is by no means exhaustive or definitive and was initially meant to be an in-house

guide for the selection of a specific detector for a specific purpose.

It is being published because we are unaware of any other comparative

study of these detectors and also because it may give potential photometric users of these devices a feel for the problems involved.

We have tested photodetectors for three characteristics: the

long-term stability over periods of about 20 hours, light memory or fatigue effects over short time periods, and the sensitivity to changes

*National Bureau of Standards research associate from the Electrical

Testing Laboratories (acting on behalf of the Lamp Testing Engineers Conference) . figures in brackets indicate the literature references at the end of

this paper.

1
in temperature. Not all the detectors were tested for all these characteristics. Appendix A is a list according to the type and manufacturer

of the detectors that were tested. In this report we shall refer to each

detector by the numbers assigned in Appendix A.

Among the silicon detectors, we have studied the PN type of photodiodes and the PIN type of both the Schottky and planar diffuse varieties.

Descriptions of these devices are available in the manufacturers'

literature, but for the sake of completeness and as an introduction we

shall include descriptions of some typical devices here.

The PN type of photovoltaic cell consists essentially of a junction

of two types of material, referred to as p-type and n-type respectively.

The p-type is doped with impurities such that it is a "hole" conductor;

i.e. there is a deficiency of electrons in the material. The n-type

material has been doped to provide a surplus of electrons. At the

interface of these two materials the surplus electrons and holes diffuse

across the junction to combine with each other and form a depleted

"barrier" region which is now free of holes and electrons. The p-type

material near the junction, having lost holes, acquires a negative space

charge and correspondingly the n-type material near the junction, having

lost electrons, acquires a positive space charge. Thus an electric field

is maintained across the depletion barrier. This electric field will

then inhibit further diffusion of the electrons and holes so that a state

of equilibrium is reached. Now, when a photon is incident on the junction

it can excite an electron from the valence band to the conductance band.

This will happen only if the incident photon has an amount of energy

greater than the energy difference between the valence and the conductance

bands. This difference is known as the "energy gap" for the material

and is different for different kinds of material. The excitation of the

electron makes it free to move in the material and a corresponding free

"hole" is also created. The process is known as the creation of a hole-electron pair. Ordinarily, if other free holes or electrons are present there would be a recombination of the holes with the electrons

or the pair could recombine itself. But if the electron-hole pair is

created in the depleted region where there are no other free holes and

electrons present then the electron will be accelerated to the positive

space charge in the n-region and the hole to the negative space charge

in the p-region. If there is a load resistor across the ohmic (external)

contacts on the n-region and the p-region, there will be a flow of

electrons from the n-region through the load to the p-region. Since by

convention a current flow is said to be in the opposite direction of the

electron flow, there will be a current flow from the p-region through the

load to the n-region.

If every photon incident on the detector were to produce an electron-hole pair, and if all these electrons and holes were to migrate

as predicted, the detector would be absolutely linear.

The PIN type of silicon photodiode derives its name from the fact

that it consists of a p-type semiconductor and a n-type semiconductor

with a layer of nearly intrinsic (i.e. almost pure, non-doped) region

of silicon in between. This intrinsic layer acts as an extended depletion
layer which can, under appropriate conditions, improve the linearity and

response time of the photodiode. In the photovoltaic mode these PIN

photodiodes operate in a manner similar to the PN photodiodes described

above. In the photoconductive or biased mode, a voltage is externally

applied across the electrodes such that there is an electric field

completely across the photodiode and the intrinsic region is fully

depleted. When a photon is absorbed in the silicon, an electron-hole

pair is created and propelled by the external field to their respective

electrodes, creating a current through an external load as before.

In the Schottky barrier PIN silicon photodiode, the p- and n-regions

are created by depositing a thin gold layer on one side of a pure silicon

wafer and an aluminum (or other metal) layer on the other side. The

gold layer is transparent and forms the front side of the detector. In

the planar diffuse type of silicon photodiodes, a layer of aluminum

(or other metal) is evaporated on one side of a silicon wafer and on the

other side silicon dioxide is formed. Then through an opening in the

oxide, an impurity is diffused into the silicon to form a p-type layer.

The spectral response of the detector depends not only on the energy

gap of the pure silicon but also on the spectral transmittance of the

material on the front surface of the detector.

All the detectors that we studied were operated in the unbiased

or photovoltaic mode rather than the photoconductive mode. The advantages of the photoconductive mode are an improved linearity and a faster

response time. These arise from the fact that the strong electric field

applied externally to the junction fully depletes the intrinsic region

and creates greater carrier acceleration. However, the application of

an external voltage also leads to a leakage or dark current which is

large compared to the signal being measured. This dark current is not

constant in time and, therefore, makes the photoconductive mode of

operation unsuitable for dc measurements. In the photovoltaic mode the

dark current was usually quite small (less than 0.1% of our measurement

signals)

2. Experiments and Results

2.1 Stability and Fatigue Effects

One important requirement for a detector that is used in goniometric

photometry is that it should be stable; that is, it should maintain its

calibration during the course of a set of measurements which could take

up to several hours to complete. Another requirement is that the detector

should achieve a stable output within seconds after a change of illumination.

We have studied the stability of detector output for periods of

approximately twenty hours. We have also investigated the short term

stability in a 10-15 minute interval by noting any fatigue effects or other kinds of instabilities that might result after a change in

illumination level.

The experimental apparatus consisted of a 120 volt, 750 watt

quartz-halogen EHF type lamp that was mounted on one end of a 3 meter
optical bench. The lamp was operated at a constant current of 4.8300

± 0.0001 amperes and the voltage was approximately 75 volts. Mounted at

the other end of the optical bench was a detector holder that could hold

two test detectors simultaneously. A third detector with an independent

current-to-voltage converting amplifier was placed about 50 cm from the

lamp and to one side of the optical axis. This detector was used to

monitor the output of the lamp in each test.

One may question the stability of the lamp and monitor detector

combination. However, previous work by one of the authors (E.F. Zalewski)

that compared several lamps of the above type using detector No. 1 had

shown this lamp and detector combination to be stable to less than 0.1%.

Of the six runs in table 1 in which this detector and the monitor detector

are compared, only one run shows a disagreement of more than 0.2% (run

No. 3). This run, it should be noted, shows a larger change in the lamp

voltage than was observed during any other run. We believe, therefore,

that any detector drift that differs by more than 0.2% from the monitor

detector is actually a change in detector response and not a change in

lamp output.

When examining detectors which did not have built-in photopic

filters, a set of infrared and ultraviolet cut-off filters was used to

block radiation from all but the visible part of the spectrum. The ambient

temperature near the detector holder was monitored by using thermocouples.

Except in the case of detectors with built-in amplifiers, the outputs of

all detectors were read by using either of the two current-to-voltage

converters described in an earlier paper [4] and a digital voltmeter. Automatic data acquisition equipment was used to collect data.

For the long term stability measurements, the outputs of the test

detectors and the monitoring detectors , the voltage and current of the

lamp, and the temperature of the detector holder were noted at intervals

of 20 minutes over approximately 20 hours. As mentioned earlier, all the

test detectors were operated in the photovoltaic mode.

Table 1 summarizes the results of the long term stability measurements for the different detectors. The detectors are briefly distinguished in the following manner: Si-S, silicon Schottky; Si-D, silicon

diffuse; and Se, selenium. The numbers for the range of noise are the

short term fluctuations above and below a straight line approximation of

the detector drift. The numbers for the amount of drift for individual

detectors have not been adjusted for lamp drift as sensed by the monitor

detector. Each row in table 1 represents a single run so that detector

drifts appearing in the same row indicate that those detectors were used

simultaneously to provide comparisons between them. Where a plus or a minus appears before the number for percent drift, it indicates that the

increase or decrease in detector output was approximately monotonic. The

cases where the detector output first drifted up then down or vice versa, are indicated in the table. The temperature of the test detector holder

was monitored in several runs and found to be within ±0.5 °C. Except for

Nos. 2, 4, 5, and 10, the detectors appear to be stable to within 0.02%

per hour—a fact that should meet most photometric requirements for

stability.

All the measurements had been taken after the lamp had been warmed

up for about 20 minutes, during which time the detector was illuminated.

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TABLE 2

DETECTOR OUTPUT DRIFT DURING 10 MINUTES AFTER SHUTTERING FOR 20 MINUTES

DETECTOR

1 (Si-S)

3 (Si-D)

4 (Si-S)

5 (Se)

7 (Si-D)

8 (Si-D)

9 (Si-S)

DRIFT

less than noise

less than noise

less than noise

-0.4%

less than noise

less than noise

less than noise

RANGE OF NOISE

Before After

0.1% 0.1%

0.05 0.05

0.06 0.06

0.06 0.20/2 min

then 0.06

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0.1 0.3

0.1 0.1

TABLE 3

TEMPERATURE DEPENDENCE OF DETECTOR OUTPUT IN THE INTERVAL 20 TO 30 °C

DETECTOR

1 (Si-S)

2 (Si-S)

3 (Si-D)

4 (Si-S)

5 (Se)

7 (Si-D)

9 (Si-S)

11 (Si-D)

BLOCK

TEMPERATURE DEPENDENCE

-0.06% per °C

-0.02

±0.07*

-0.12

+0.06

±0.07*

±0.08*

+0.12

REMARKS

Minimum near 24 °C

Maximum near 24 °C

Maximum near 24 °C

*These temperature dependences went through a minimum or a maximum.

The numbers represent the slope on the steep part of the temperature

dependence curve.
This was done to avoid any short term fatigue effects. These were

studied separately.

Using the same experimental set-up described above, detectors were

checked for any fatigue or light hysteresis effects by first warming up

the lamp and illuminating the test detector and then shuttering the

detector for a period of about 20 minutes. The shutter was then opened

and the detector output and other parameters were read out at 20 second

intervals for a period of 10 minutes. Table 2 summarizes the drift and

noise that were observed after the shutter was opened. The numbers for

range of noise Before and After1 refer to the noise ranges observed before

the detector was shuttered for 20 minutes and after it was again illuminated. No drifts above the noise level were observed except for detector

5 which was a selenium photocell. This detector showed the expected

fatigue effect [2,3]. A comparison plot of the output of this detector

and detector 1 is shown in the figure. Detectors 7 and 8 did not show

any drift, but the noise on both showed a large increase in the 10 minutes

after the shutter was opened. It seems that a longer time period is

required for them to achieve equilibrium.

2.2 Temperature Dependence

The temperature dependence of a set of detectors was studied over

the range of approximately 20 °C to 30 °C. The experimental apparatus

remained the same as in the stability studies except that the detectors were mounted in a metal block which was cooled or heated by two thermoelectric plates. The detectors were shielded from room temperature

changes by a transparent plastic box surrounding the detector holder.

The temperature of the detector was changed by altering the amount

of current passing through the thermoelectric plates. The current polarity

determined whether the detector was being cooled or heated. Temperature

was measured by two thermocouples , one of which was embedded in the metal

block that held the detectors. The metal detector cases were in good

thermal contact with this block. The other thermocouple measured the

temperature of the air surrounding the detector in the plastic box.

Readings of the detector outputs and other monitoring parameters

were taken when both the temperature of the block and the ambient temperature within the box had achieved equilibrium. Table 3 gives the

changes in detector output per °C change of block temperature. Two

detectors, No. 4 which is a Schottky type PIN device and No. 11 which

is a diffuse PIN device, showed temperature dependences slightly greater

than 0.1% per °C. Some of the detectors had a temperature coefficient

which went to zero (minimum or maximum in the response curve) near room temperature. The maximum slope is, therefore indicated in table 3.

3. Conclusions

As mentioned in the introduction, this study was not meant to be a definitive or exhaustive study of all the different kinds of photodetectors

available today, hence we can not draw any general conclusions of the

relative merit of any manufacturer of these detectors. Detectors of the
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same type and manufacturer may at times perform differently as is

evidenced by the differences in the stabilities of detectors 1 and 2;

both were of the same type, were made by the same manufacturer, and had

the same model numbers.

We can, however, draw some conclusions and also get an indication

of where problems may exist. The study also gives us estimates of a part

of the uncertainty that should be taken into account when these detectors

are used in specific applications.

1. In photometric applications (dc measurements) the silicon

detectors of the kinds that we studied should be operated in

the photovoltaic or non-biased mode.

2. Several of the silicon detectors studied proved to be

sufficiently stable for applications in precise photometry:

the instability of output was less than 0.02% per hour.

However, some of the detectors were significantly less

stable. Thus silicon detectors have to be individually

tested and selected for optimum performance.

3. The silicon detectors did not show any identifiable

fatigue or light memory effects, whereas the selenium photocells studied showed the expected fatigue. Some silicon

detectors are very noisy when first illuminated and it

sometimes can take several minutes before the noise decreases

and the cells achieve equilibrium. In goniometric measurement

of flux, effects due to changing levels in illumination are undesirable. One cannot therefore use the selenium detectors

without some sort of arrangement to take account of these

effects. However, there are some silicon detectors that can be used; again, the characterization of the specific detector

is required.

4. All the detectors showed sensitivity to temperature

changes and if they are to be used in photometric work at

the few tenths of a percent level of precision, temperature

control within ±0.5 °C or less is necessary. For precise

photometric work the specific detector chosen should first

be characterized for its temperature sensitivity and then

the required amount of temperature control should be built

into the detector holder.

Detectors 9 and 10 were silicon detectors that had a current-tovoltage converting amplifier built into the detector case. Detector 9

had stability and temperature dependence characteristics that were of

the same order as the other silicon detectors. Detector 10, however,

showed larger drift and noise. Since these detectors are used without

external operational amplifiers, laboratories that do not now have such

amplifiers may consider using them.

For goniometric measurements of flux the detectors should also be

tested for linearity and spectral response. The linearity, however is

more critical than spectral response for our purposes. Some preliminary

linearity tests using the inverse square approximation have shown that
silicon cells operated in the photovoltaic mode are linear within 0.1% over two decades. Future studies of these detectors will include more

detailed linearity and spectral response measurements.

4. References

1. Wilkinson, F. J. , "Heterochromatic Photometry with Selenium Photovoltaic Cells", NSL (Australia) Tech. Paper No. 23, 15 pages, 1968,

2. Preston, J. S. , Rev. d'Opt. 27, 513 (1948).

3. Preston, J. S., J. Sci. Instrum. 27_, 135 (1950).

4. Zalewski, E. F. ; Schaefer, A. R. ; Mohan, K. and McSparron, D. A.;

"Optical Radiation Measurements: Photometric Instrumentation and

Research (1970-71)". NBS Tech. Note 594-2, 44 pages, Sept. 1972.

10
Appendix A

List of Detectors Tested with Assigned Numbers

Assigned No. Detector Type Manufacturer

1 Silicon PIN Schottky, photopically A

corrected

2 Silicon PIN Schottky, photopically A

corrected

3 Silicon PIN diffuse A

4 Silicon PIN Schottky A

5 Selenium barrier-layer, photopically B

corrected

6 Selenium barrier-layer, photopically C

corrected

7 Silicon PIN diffuse D

8 Silicon PIN diffuse D

9 Silicon PIN Schottky with built in A

op amp, photopically corrected

10 Silicon PN diffuse with built in D

op amp, UV optimized

11 Silicon PIN diffuse A

* U. S. GOVERNMENT PRINTING OFFICE: 1973 --511-327/277

11
IFORM NBS-114A (1-71)

U.t*. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-594-52. Gov't Accession No.3. Recipient's Accession No.

4. TITLE AND SUBTITLE

OPTICAL RADIATION MEASUREMENTS:

STABILITY AND TEMPERATURE CHARACTERISTICS

OF SOME SILICON AND SELENIUM PHOTODETECTORS

5. Publication Date June 1973

6. Performing Organization Code

7. AUTHOR(S)

Kshitij Mohan, A. Russell Sohaefer and Edward F. Zalewski

M. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Task/ Work Unit No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

1 1. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as No. 9

13. Type of Report & Period

Covere"d Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

This paper describes the comparison of some characteristics of selenium

barrier layer photocells and silicon PIN and PN type photodiodes operated in the

photovoltaic or non biased mode. The work was done to study the suitability of

these detectors specifically for goniometric measurements of flux and possibly

for other photometric (or radiometric) measurements. The characteristics studied were the stability of detector output over approximately twenty hours, fatigue

or light memory effects over short periods of time, and the temperature dependence

of detector output.

17. KEY WORDS (Alphabetical order, separated by semicolons) Fatigue; light memory; photocells;

photodiodes; photometry; radiometry; selenium; silicon; stability; temperature

dependence

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16

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