or insulator samples

Solid-Stare Electronics Vol. 35, No. 2, pp. 228-230, Printed in Great Britain. All rights reserved

1992 Copyright

0038-l 101/92 165.00 + 0.00 0 1992 Perpmon Press plc

A NEW METHOD TO MEASURE FAST DEFECT TRANSIENTS IN SEMICONDUCTOR AND/OR INSULATOR SAMPLES (Received 30 May 1991; in revised form 18 July 1991)

The knowledge of electrically active defects-shallow and deep levels in semiconductor and insulator materials is important for the production of semiconductor devices. The deep-level defects are usually measured by deep-level transient spectroscopy (DLTS) originally proposed by Lang[l]. A drawback of the DLTS method is that the excitation pulses (typically several volts) overload the measuring device and the much smaller deep-level transients can be measured only after some time delay. The measurement of fast deep-level transients is desirable, as the measurable time constant range is extended and it gives the opportunity to measure deep-level transients near room temperature, the characteristic work temperature of the devices. In this Note a procedure is described to measure fast free carrier emission and/or dielectric loss relaxation of the deep-level defects. The method is based on two ideas. The first is to suppress the excitation pulses by compensating the capacitance, the parallel leakage and series resistance of the sample via a branch connected to the sample. That branch is fed by compensating pulses synchronixed and of opposite sign to the excitation pulses. A measuring device is connected to the sample and the compensating branch (Fig. 1). If the compensating pulse amplitude is properly set then the excitation pulses do not load the measuring device input, as the charge transfer caused by the excitation pulse is carried away by the opposite sign charge transfer caused by the compensating pulse. The second idea is that the relaxations of a sample can be determined by measuring the voltage transient with a large input impedance device. The block scheme of the measuring arrangement is shown in Fig. 1. It consists of pulse generators la, lb, pulse generator output units 2a, 2b, sample holder, sample, variable capacitor, current generator 6, current generator output unit 6k, variable resistor 7, voltage transient measuring unit 8 and of pre-amplifier 8b of measuring unit 8. The pulse generators la, lb and the current generator 6 are synchronized by a common timing. The pulses U, are superimposed on the measuring voltage as usual in the DLTS method. The CJ,pulses are of polarity opposite to the U, pulses. The amplitude of the CJ,, U, pulses can be independently regulated. The 6k output unit has a small output capacitance and known value Rk output resistance. The Rk resistance is favourably larger than the equivalent low-frequency differential parallel resistor of the biased sample. The current generator compensates the leakage or the excitation current flowing through the sample. It is not necessary when the sample leakage is negligible, for example when a good-quality capacitor is measured. The sampb temperature can be set and stabilized within 1 K. The sample 4 is any element, in which a poorly-conducting material is placed between two good conductor layers. Thus the sample can be a semiconductor p-n junction, a metal-insulator-semiconductor or a metal-insulator-metal 228

structure. The sample 4 must have a small high-frequency series resistance and low leakage. Strong leakage restricts the measurement to fast transient time constants. In that case the slower relaxations can be determined by capacitance transient measurements suppressing the excitation pulses as described in this Note. The compensating capacitor 5 is a low loss, low-inductivity variable capacitor and can be set approximately equal to the sample capacitance. The resistor 7 compensates the sample series resistance. It is necessary only when the sample series resistance is large compared to the pulse generator output resistance and the measured time constant is near to the series RC time constant of the sample and pulse generator. The measuring unit 8 is a high-sensitivity, high limiting frequency oscilloscope. The pre-amplifier 8b has a large input resistance and small input capacitance. The sample 4, capacitor 5, resistor 7, units Za, 2b, 6k and pre-amplifier 8b are built-in one device with the sample holder 3 to avoid overload of the connection cables. This device is characterized by good electric isolation of the above elements in the measured frequency range, and by small scatter capacitances and inductivities. The detection time constant 7 charactrizes the sample and the setup and is expressed by 7 = C. R, where C is the sum of the sample capacitance, of the compensating capacitance and of the input capacitance of the pre-amplifier 8b, R stands for the parallel combination of the sample equivalent differential parallel resistance., of the current generator output resistance and of the input resistance of the pre-amplifier 8b. 7 gives the longest time constant, which can be measured with optimum sensitivity and signal-to-noise ratio by the method. At lower speed the measurement is equivalent to a current measurement with detection impedance R. Procedure to measure the defect transients caused by electrical pulse excitation Measurement is carried out in the following steps: 1. The measuring voltage is selected and current generator 6 is set to keep the stationary input voltage of the pre-amplifier at a small value. 2. The current generator is regulated similarly at the excitation voltage. 3. The compensating capacitor 5 is set to a known value approximately qual to the sample capacitance and the compensating voltage pulses of pulse generator lb is set to minimize the voltage transient following the excitation. When the capacitance of the measured sample is independent of the measuring frequency, the voltage transients can be completely suppressed. When it is nv the compensating series resistor 7 can also be set to compensate the sample series resistance, however the measurable time constant is anvay limited by the series time constant of the sample and pulse generator.

Notes

229

1

L ___-_----_--

Fig. 1. The block scheme of the measuring arrangement. The elements of the figure are: la, 1b = pulse generators with 2a, 2b = special output tits; 6 =current generator with special output unit 6k; 3 = temperature controller sample holder; 4 = sample; 5 = variable capacitor; 7 = variable resistor; and 8 = voltage transient measuring unit with special pre-amplifier 8b.

4. The transients following the excitation are measured by the oscilloscope. Any deviation from the equivalent circuit represented by the compensating elements will be apparent in the measured voltage transient. The time decay is analyzed and its exponential components are determined. 5. According to the sample t-for example capacitor, MOS structure, p-n junction etc.-the measured transient can be interpreted as a voltage transient generated by the charges emitted from deep levels, charges generated by dielectric loss relaxation in the isolating material or as the frequency-dependent capacitance of the sample. Knowing the geometry of the samples, the usual parameters (defect concentration, emission time constant) can be calculated. Measured at different temperatures the activation energy and capture cross section can be determined by an Arrhenius plot. The method is demonstrated by Figs 2 and 3. In Fig. 2 curve A shows the compensated voltage transient measured on the circuit in the insert. Without the elements additional to the 100 pF capacitor the transient is negligible. The excitation and compensation pulses have a 1OV amplitude, the compensation is made by setting the

Fig. 2. Measurement on the circuit in the insert. Curve A shows the compensated voltage transient on logarithmic time scale. The excitation and compensating pulses are of 1OV amplitude, the compensation is set by the variable capacitor. Curve B shows the spectra calculated using the derivatives to tind the exponential components. The peaks marked Tl, T2, T3 describe the three additional RC branches of the circuit in the insert. SSB 35/2-H

variable capacitor. A special method based on the analysis of the derivatives was developed to find the exponential components. Curve B shows the spectra derived by the above method. The peaks marked Tl, T2, T3 describe the three additional RC branches of the circuit in the insert. In Fig. 3 the spectra of an amorphous Si Schottky barrier are shown. Curve A is measured at room temperature (20”(Z),after 4 V excitation pulse. Curve B is measured at a higher (approx. 50°C) temperature with the same excitation. The dominating peaks are near the measuring time constant of the setup. The spectra of the transient after switching back to 0 V bias is shown by curve C. For deep-level defects the switching back must be a fast process as it is governed by the capture process. Thus curve C proves that the measured transients originate from parasitic parallel RC elements, in this case it probably from precipitates in the amorphous layer. The equivalent circuit of the sample is shown in the insert, where C, is the high-frequency capacitance, R, and C, characterize the precipitates in the material. The typical values of the capacitances C, and C, are about 150 pF, that of the resistor Ra is a few MQ. In summary, by the method described the free carrier emission or dielectric loss relaxation transients of electrically active defects in semiconductors or in insulators have been measured in a wide time constant range. The procedure realizes a new type of DLTS, extending the measured relaxation time constant from the usual ms to the ns range.

-l.O°C

t

(microseconds)

Fig. 3. The spectra of an amorphous Si Schottky barrier. Curve A is measured at room temperature (20’(Z), atter a 4 V excitation pulse. Curve B is measured at higher (approx. 50°C) temperature with the same excitation. Curve C is the transient atIer switching back to OV bias. The equivalent circuit of the sample determined is shown in the insert.

230

Notes

In DLTS applications a further advantage is that the emission from deep-level defects can be distinguished from scattered RC element-related transients. The capacitance of the sample can be determined by this method in a wide frequency range, typically from 1 kHz to 100 MHz; so capacitor circuit elements can be tested by the method. Acknowledgements-The author is grateful to G. SzAraz and T. Stint6 at MIKI Instrumentation, Budapest for their

technical help and to G. Zentai at Central Research Institute of Physics, Budapest for supply of amorphous Si samples. Research Institute for Technical Physics of the Hungarian Acadmy of Sciences P.O. Box 76, 1325 B&pest, Hungary

LAszti WzsA

REFERENCE

1. D. V. Lang, J. appl. Phys. 45, 3023 (1974).