Deep-Level Spectroscopy by Transient Capacitance Techniques ...

16

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-33, NO. 1, MARCH 1984

[9] E. Kettel and W. Schneider, "An accurate analog multiplier and ACKNOWLEDGMENT divider," IRE Trans. Electron. Comput., pp. 269-272, June 1961. J. Lowe carried out most of the computer programming, [10] J. A. Rosenthal, "A pulse modulator that can be used as an amably assisted by J. R. Kinard. Their assistance was invaluable plifier, a multiplier, or a divider," IEEE Trans. Instrum. Meas., vol. IM-12, pp. 125-134, Dec. 1963. and is gratefully acknowledged. Some of this work was done M. Tomota, T. Sugiyama, and K. Yamaguchi, "An electronic [11] during the author's study leave in 1981 at the Division of multiplier for accurate power measurements," IEEE Trans. InApplied Physics CSIRO. Many discussions with B. Inglis and strum. Meas., vol. IM-17, no. 4, pp. 245-251, Dec. 1968. M. Gibbes of that division have helped to complete this paper. [12] T. Sugiyama and K. Yamaguchi, "Precision measurement of audio-frequency power using a time-division multiplier," ElecThe assistance of CSIRO over this time is greatly appreciated. trical Engineer. Japan, vol. 89, no. 8, pp. 26-34, 1969. [13] R. Bergeest and P. Seyfried, "Evaluation of the response of time-

REFERENCES [1] G. A. Korn and T. M. Korn, Electronic Analog and Hybrid Computers. New York: McGraw Hill, 1964. [2] E. A. Goldberg, "A high accuracy time-division multiplier," RCA Rev., vol. 13, pp. 265-274, Sept. 1952. [3] S. Sternberg, "An accurate electronic multiplier," RCA Rev., vol. 16, pp. 618-634, Dec. 1955. [4] C. D. Morrill and R. V. Baum, "A stabilized electronic multiplier," Trans. IRE Prof Group on Electronic Computers, vol. 1, pp. 52-59, 1952. , "Stabilized time-division multiplier," Electron., pp. 139[5] 141, Dec. 1952. , "A stabilized electronic multiplier," in Proc. Nat. Electronics [6] Conf., vol. 8, 1952, pp. 710-715. [7] M. L. Lilamand, "A time-division multiplier," IRE Trans. Electron. Comput., pp. 26-34, Mar. 1956. [8] H. Schmid, "A transistorized four-quadrant time division multiplier with an accuracy of 0.1 per cent," IRE Trans. Electron. Comput., vol. EC-7, pp. 41-47, Mar. 1958.

[14] [15]

[16]

[17] [18]

division multipliers to ac and dc input signals," IEEE Trans. Instrum. Meas., vol. IM-24, no. 4, pp. 296-299, Dec. 1975. V. V. Pavlov and N. V. Belikov, "Four quadrant time-pulse multiplying unit using integrated microcircuits," trans. from Pribory i Teknika Eksperimenta, no. 4, pp. 154-156, July-Aug. 1978. M. M. Stabrowski, "Modern numerical analysis of time-division multipliers," IEEE Trans. Instrum. Meas., vol. IM-28, no. 1, pp. 74-78, Mar. 1979. P. Filipski, "Comments on modern numerical analysis of timedivision multipliers," IEEE Trans. Instrum. Meas., vol. IM-29, no. 1, pp. 80-81, Mar. 1980. "Frequency-type transducing errors of active power converters based on TDM," Normalizacja, vol. XLVII, no. 12, pp. 26-31, 1979, in Polish. Yokagawa Electric Works, Ltd, Tokyo, Japan, Watt-Converter

Type 2885/20.

[19] M. Ruegger, "Solid state reference standard meter TVH2 for the measurement of active and reactive energy," Landis and Gyr Rev., vol. 26, pp. 2-10, 1979. [20] Scientific Columbus, Solid State Watt/Watthour Standard Type SC-60, Scientific Columbus, Ohio, July 1977.

Deep-Level Spectroscopy by Transient Capacitance Techniques under Electrical Resonance JOSE IGNACIO IZPURA, JOSE MIGUEL HERRERO, FRANCISCO SANDOVAL, ENRIQUE CALLEJA, ADALBERTO LA CRUZ, AND ELIAS MUNOZ, MEMBER, IEEE

Abstract-Transient capacitance studies for deep-level spectroscopy

are suggested to be performed under electrical resonance conditions. Variable-frequency impedance bridges allow work at resonance, and an incremental gain for capacitance transients is obtained. This new method is presented and a few examples are discussed.

I. INTRODUCTION

THE STUDY of defect centers and impurities in semiconductors by thermal or optical activation of a diode capacitance is a very powerful technique in semiconductor analysis today [1] . Most of the standard deep-level spectrosManuscript received March 11, 1983; revised September 30, 1983. This work has been supported in part by the ComisionAsesora de Investigacion Cientifica y Tecnica and by the U.S.-Spain Coop. Project 793031. The authors are with the Department of Electronics, Telecommunication Engineering School, Polytechnical University of Madrid, Madrid, Spain.

copy systems (DLTS, DDLTS, etc.) perform differential capacitance measurements at 1 MHz by using Boonton model 72 bridges. To extend trap concentration detection sensitivity, a variety of signal-processing techniques have been proposed, and center concentrations five orders of magnitude below the doping level have been determined [2]. The recent commercial availability of variable-frequency automatic-capacitance bridges allows the examination, both theoretically and experimentally, of new techniques for performing transient capacitance measurerements. In this paper we address the just-mentioned objective, and a new method of performing deep-level spectroscopy is proposed. It is shown that transient capacitances determined at a frequency at which device capacitance is at resonance allows either an increase in trap detection sensitivity or the use of variable-frequency automatic-capacitance bridges with sensitivities below those needed for deep-level spectroscopy.

0018-9456/84/0300-0016$01.00 © 1984 IEEE

IZPURA et al.: SPECTROSCOPY BY TRANSIENT CAPACITANCE TECHNIQUES

L~ ~ ~

IbrideII

pacitance, the Q factor is now

Qp ((r) = wrR IKjACI = (rR R2

C

I

17

(a)

= (OrRACQs((Or)2 rR,ACQ,(Wr

3pfGp

C

3

Ki(r)AC

(c)

Fig. 1. (a) Equivalent circuit for the device under test. (b) Equivalent parallel admitance as determined by the impedance bridge. (c) Equivalent circuit for the device under electrical resonance.

II. THEORETICAL ANALYSIS An equivalent circuit for the diode under test is shown in Fig. 1 (a), where CD is the diode capacitance, R and L are the total series resistance and inductance (device plus external), and C is the parasitic capacitance [3] The equivalent parallel conductance and capacitance determined by the bridge of Fig. 1 (b) are +C CP= CD(1 LCD 2) _ R2 CD 2+ (LCD2 1)2 .

-

.(1)

RD

Gp =

R2C

2

2

+(LC

Equation (1) indicates diode capacitance is multiplied by a gain factor of Ks(co)=

R2C

Io2 +(LCD(O2

1)2

(2)

At co = Wr = (LCD)-112, KS = 0. This is the principal advantage of the present method, because it provides the same benefits of true differential measurements, i.e., the use of the lowest bridge range. For transient measurements, small changes in the diode capacitance, due to trap charging and discharging processes, are sensed by the bridge as -C= Ki(a): a CD

At co =

(LCD [R22

'AC

32 R

L12

(5)

This result shows which Q factor should be allowed by the system bridge if a given AC is to be detected.

(b)

R

=

AC

1)2

C + (LCD

R22 -2

CD

1)2]2

w

==- [Qs(rnr2 (4) Ki((Or)= -2 Xr CD So, any ACD produced in the device is seen by the bridge as multiplied by Ki(c). Equation (4) indicates that low series resistances and low-frequency resonances, may produce large Ki(cr) factors and a potential improvement in the trap detection sensitivity. At resonance, the equivalent circuit is seen in Fig. l(c), only the parasitic capacitance and the transient Ki(COr)AC(t) remain. If we do not consider the parasitic ca-

III. MEASUREMENTS AND RESULTS The previous method has been implemented with an HP4 191 A bridge, that allows measurements in the 1 MHz-1 GHz range. With this approach, sensitivity at the central range is 10 fF, going to 100 fF at the low range end, and close to the femtofarad units above 100 MHz. These data are an oversimplification because the bridge resolution is a complex function of frequency, junction capacitance, and the total Q factor. GaAs1 -P, LED's, previously characterized, have been used as test devices [4] for electron trap energies and concentrations. The experimental conditions have been chosen to address extreme or quite unfavorable trap detection problems. First measurements with the HP4191A bridge revealed that at 1 MHz, the 0.1-pF resolution obtained was not adequate. Transient-capacitance experiments could not be carried out, and in going to higher frequencies the effects illustrated in Fig. l(a) were encountered. Standard device jig and cryostat mounting leads were used. As discussed in the following examples, working at resonance was tried next. A first case was studied in GaAso.15PO.85 LED's. An electron trap with thermal emission energy AEe = 0.27 eV was detected [41, showing AC= 50 fF (1 MHz-Boonton) at T = 105 K. This level is found close to carrier freeze-out, causing a large increase in device series resistance. In the HP4191A, at this temperature, R = 61.4 Q and fr = 40.218 MHz. Working at resonance produces Ki((r) = -2.2 and AC(wr) = -0.11 pF. This example reflects the influence of R on the incremental gain, although some gain is still realized. At T = 114 K, R = 44.6 Q,fr = 40.756 MHz, Ki(wr) = -4.06, and AC((r) = -0.21 pF. A situation of large transients is considered next. For a GaAs 0.7PO.3 diode, the dipole equivalent parallel capacitance is shown in Fig. 2(a), and reflects (1). In this device (CD = 105 pF), a level at AEe = 0.87 eV giving AC = 1.2 pF (Boonton system) at T= 355 K, was then studied. In the HP4191A, working at resonance (fr = 25.45 MHz), a transient AC(r) = -684 pF was found. Such a large transient produces an approximation where (3) has to be substitued by an incremental definition, where Ki((or) is a function of AC(cor). The experimental gain is now Ki((O) = -536, as compared to the theoretical incremental gain of -531 (Fig. 2(b)), and smaller than Ki(wOr) = -680 predicted by (3). A third level was studied in the same device. Traces of this trap (AEe - 0.15 eV) produced AC