Comparator Method for Optical Lifetime Measurements on SemiconductorsH. L. Armstrong Citation: Review of Scientific Instruments 28, 202 (1957); doi: 10.1063/1.1746486 View online: http://dx.doi.org/10.1063/1.1746486 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/28/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Impedance independent optical carrier lifetime measurements in semiconductor lasers Rev. Sci. Instrum. 69, 4247 (1998); 10.1063/1.1149239 Bulk lifetime and surface recombination velocity measurement method in semiconductor wafers J. Appl. Phys. 79, 6954 (1996); 10.1063/1.361459 Lifetime measurements in semiconductors by infrared absorption due to pulsed optical excitation Appl. Phys. Lett. 51, 1445 (1987); 10.1063/1.98652 New contactless method for lifetime measurement in semiconductor wafers Rev. Sci. Instrum. 54, 1386 (1983); 10.1063/1.1137251 Phase Shift Method of Carrier Lifetime Measurements in Semiconductors Rev. Sci. Instrum. 29, 889 (1958); 10.1063/1.1716029

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THE REVIEW OF SCIENTIFIC INSTRUMENTS

Notes

BRIEF contributions in any field of instrumentation or technique within the scope of the Journal can be accorded earlier publication if submitted for this section. Contributions should in general not exceed 500 words.

Comparator Method for Optical Lifetime Measurements on Semiconductors

H. L. ARMSTRONG

National Research Council, Ottawa Z, Canada (Received December 6, 1956)

T HE minority carrier lifetime in semiconductors is often measured by an optical "pulsed light" method. A sample of

the semiconductor is illuminated by short flashes of suitable light. Excess carriers are thus produced to reduce the resistance of the sample. This change of resistance is monitored on an oscilloscope; the time constant with which the resistance returns to its un­disturbed value after the illumination is finished is equal to the lifetime of the minority carriers.l

The pulses of light can be produced either by electronic means, or by mechanical means, such as a rapidly rotating mirror. The latter method has some advantages; the pulsed light does not produce any electrical transients in the vicinity, and since, the actual source of light may be almost anything, almost any spectral characteristics can be used. The chief drawback is the length of the flash produced mechanically: it is quite difficult to get below five or ten microseconds. If the length of the flash is not much shorter than the lifetime to be measured, the interpretation of the oscilloscope display becomes more difficult, since it does not represent a simple exponential decay.

Figure 1 shows a way of obtaining the interpretation by a simple analog with some saving of trouble.

The rotating mirror and slit arrangement produce short flashes of light, which illuminate both the semiconductor sample and a vacuum photocell. A constant current, obtained by a large dropping resistor, passes through the sample. If the intensity of illumination as a function of time is let), the signal, (transient) voltage Ll. V across the sample is given by the equation

d Ll.V d-(Ll.V)+-=a[(t), t T

(1)

with the condition Ll. V =0 when t=O. a is a constant of proportion­ality, and T the carrier lifetime.

The photocell is operated in the region where its current is proportional to the illumination, which, of course, is also [(t). Then the signal voltage Ll. V across the CR load for the photocell is given by

d Ll.V b -(Ll.V)+-=~(t) dt CR C

(2)

with the same initial condition. b is another constant. It is ap­parent that if CR= T and b/C=a, the Ll. V for Eq. (2) will be identi-

(A ROTATING

J<J MI~ROR --gj,L

SAMPLE +SUPPLY

~v

PHOTO

TUBE

- SUPPLY

IJV

FIG. 1. Schematic diagram of the analog lifetime measure­ment. The two .:lV·s are com­pared on the oscilloscope. by dual beams or otherwise I and C and R adjusted until they are identical functions of time.

VOLUME 28, NUMBER 3 MARCH,1957

cal with that for Eq. (1). Thus, if Rand C are adjusted until the two Ll. V's are identical functions of time, then T=CR. The two Ll. V's can be compared with a dual-beam oscilloscope, or by elec­tronic switching or other such techniques.

By the use of circuits more complicated than the simple CR load, more involved modes of decayl could be studied. However a consideration of such circuits is beyond the scope of the present discussion.

I D. T. Stevenson and J. R. Keyes, J. Appl. Phys. 26, 190 (1955).

Low-Cost Microvolt Potentiometer WALTER H. WOOD

National Bureau of Standards, Washington, D. C. (Received December 17, 1956)

CONTRARY to a general belief of long standing, an un­saturated standard cell can be used to supply a small cur­

rent to a circuit continuously, for considerable periods of time, without substantial injury to the cell. The present article describes a low-cost potentiometer, of simple construction, in which an un­saturated cadmium cell is used as the supply battery. The range of the instrument is 1110 p.v, in steps of 1 p.v. It can be used for the measurement of any emf within its range; however, two im­portant uses for it would be the measurement of thermocouple emfs, and the difference between two standard cells connected in opposition.

As shown in the accompanying diagram, the circuit consists of three decades, with steps of 10, 1, and 0.1 ohm, in series with an unsaturated standard cell, having an internal resistance of about 500 ohms, and a resistor, R, of approximately 100000 ohms. The resistance of R is adjusted to give an emf drop of one microvolt across each step of the 0.1 ohm-per-step decade. The resistors

s.c. + _ R r------~~I---~~~.-------~

E

10 .I OHM/STEP . 6

~~ + EMF - GALV

FIG. 1.

which make up the decades and resistor R should be wire-wound and have sufficient stability to give the accuracy desired from the potentiometer. The switches used for the three decades and switch G should be as free from thermal emfs as possible. It is best to have all binding posts and connecting wires of copper in order to de­crease the possibility of stray thermal emfs in the circuit. The possibility of stray thermal emfs in the galvanometer circuit should not be overlooked. Switch E is left open when the instru­ment is not in use.

A potentiometer of this type was constructed at the National Bureau of Standards and has been used occasionally over a period of two years, without replacement of the standard cell. Periodic checks of the standard cell have shown the decrease in its emf with time to be no greater than that expected of similar cells in normal use, i.e., the small intermittent drain of 10 p.a has not

202

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NOTES 203

significantly affected the emf of the cell to date. In a recent test, switch E was left closed during a two-week interval, resulting in a continuous 10-~ drain on the standard cell. Measurements made during this interval showed errors of less than 2 /oIV in the full-scale reading of the potentiometer.

The manganin resistors in the ten-ohm decade were adjusted to within 0.5%, and those in the lower decades to 1% of nominal values. The desired resistance value of R was obtained by using three commercial wire-wound hermetically-sealed resistors with low temperature coefficients. The switches used for the decades have solid silver contacts, and are designed to be thermally astatic. Switch G is a commercial plunger type key in which the metal plunger was replaced by bakelite to minimize heat flow from the operator's finger to the active parts of the switch.

Constant Current Generator H. S. SOMMERS, JR.

RCA Laboratories, Princeton, New Jersey (Received November 15, 1956; and in final form December 27, 1956)

I N the study of current-voltage characteristics of nonohmic materials, a very steep rise of current with voltage is often

encountered. For example, many insulating crystals show a re­versible characteristic which closely resembles breakdown.1 To facilitate measurements of such characteristics, it is customary to limit the current with a large external resistor.

If the power supply furnishes a constant current rather than a constant voltage, the study of these nonlinear characteristics is much easier, since at constant current there is less chance of burning out the crystal being studied. Also, the constant current lines intersect the steep I-V characteristic at a much more favor­able angle than do the constant voltage lines.

A constant current generator designed specifically for use in studying the I-V characteristics of insulating crystals was designed

+300

and tested. It will deliver any preset direct current from 0.01 ~ to 3.0 rna. The voltage, which automatically adjusts itself to the resistance of the crystal, can vary from 0 to 3000 v dc. The out­put current can be modulated by an external signal to give a wave form which reproduces the voltage form of an input signal of frequency between zero and a few kcps. The voltage swing needed to modulate the output current depends on the setting of the fine control; at 10% of full scale, 10 v peak-to-peak will give 100% output modulation.

The supply can also be used as a constant voltage source with equivalent characteristics.

Figure 1 is a circuit diagram of the amplifier. This circuit has the configuration of a conventional shunt feedback amplifier, the load being one of the feedback resistors.

The current through the output, 1o, is2

Io=E./R.; (1)

for high gain the current is essentially that driven by the standard voltage E. through the standard resistor R •.

The improvement factor 'Y, which gives the ratio of the regula­tion with and without feedback, is

'Y=/oIR./(R.+R.), (2)

where /01 is the forward gain of the amplifier, R. the crystal resistance.

The amplifier has an open loop gain of 200 000 at dc, a turnover frequency of 200 cps, and a rate of attenuation of 9 db/octave. Above 5 kcps, the phase shift is controlled by the last stage; hence, additional capacitative loading will improve rather than harm the stability against oscillation.

The positions of the selector switch give full-scale output currents from 1 /oIa to 3 rna, the range being limited at the small end by the grid current of the input stage and at the high end by the current rating of the output stage. The grid current of a selected 6AK5 operated as shown should be between 1 and 2Xl(J9 amp.8 Such a grid current affects the accuracy of the meter

.3 tty

33K 12 5. 6AK5

POWER

••• IIOK

3000

20k

-JOOV.

OB2 ~--"-105V

A· TAANSPAK C.V.· 300

B- TAAHSPAK C.V.- 300

C- PLASTIC CAP co. H.V.50-502

-!OOV

10'" D.C.'

-I05V.

'0 O.C.F.

FIG. 1. Feedback amplifier circuit diagram.

-IDey

P. AMPS Full. SCALE

10'1. leA.

'00 '0 1000

10000

NORMA&.

30M

OUTPUT

"", o-tIOOov. .. IOK-/V

i~UT!R.A~ MODE t EXTEANAL D.C, I"PUT

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