Some aspects of gunn instabilities in long n-GaAs samples

Solid State Communications,

Vol. 9, pp. 201—203, 1971.

Pergamon Press.

Printed in Great Britain

SOME ASPECTS OF GUNN INSTABILITIES IN LONG n-GaAs SAMPLES S.M. Wu,* H.C. Law, K.C. Kao and E. Bridges Department of Electrical Engineering, University of Manitoba, Winnipeg, Canada (Received 23 November 1970 by R. Barrie)

We report the observation of coherent current oscillations in a long n-GaAs sample of 10 mm in length. The electronic properties of the sample are found to have undergone drastic change with storage time, and this ageing effect is attributed to the diffusion of impurities from the electrodes into n-GaAs.

THE GUNN effect samples used for this investigation were oxygen-doped n-GaAs slices cut perpendicular to the <111> direction and had a size of 12mm x 15mm x 0.45 mm with the ohmic contacts (Sn covered with Au and heat-treated at 450°C for 4 mm) alloyed to the end faces. The effective length of the samples was 10 mm which is longer than any samples so far reported for • • this type of investigation. To avoid heating the sample, rectangular pulses of 1 ~ sec duration • with a repetition frequency of 25 pulses/sec were used. In this communication we report some new results as follows:

for triggered operation. His prediction is in close agreement with the observed value of 0.85 kV/cm. The repetitive nature of the oscillation under such a condition is not yet fully understood. It is possible that the presence of inhomogeneities in the sample is responsible for this phenomenon.

(1) When a pulse with a sharp and high initial overshoot was applied to the sample, spiky and non-coherent current oscillations were observed at room temperature at a field as low as 0.85 kV/cm. This field is much lower than the threshold field for Gunn instabilities normally observed in n-GaAs (from 1.28 to 3.70 kV/cm for sample lengths from 5 to 0.02mm1). The resistivity and carrier concentration of the sample used were respectively 23Q cm and 5.8 x 10 ‘~ cm~, and the NL (the carrier concentration times the sample length) product was therefore 5.8 x 10 cm2. According to Heinle’s diagram2 the threshold field for this value of NL product is below 1 kV/cm

the form of incoherent spikes typical for long samples. 1,3 But for the sample of 23 a-cm resistivity the oscillations appeared to be coherent and sinusoidal as shown in Fig. 1. A similar coherent oscillation hehaviour in GaAs samples of 5mm length with NL products 2 has been reported by of the order of 10~~ cm Guetin.4 There are approximately 24.5 oscillations in 0.7 ~i sec corresponding to a period of 2.9 x 10_2 ~ sec which is of the same order as the transit time for the domain to travel across the sample. When the temperature of the sample was lowered to 77°K,no appreciable effect on both the threshold field and oscillation frequency was observed but the magnitude of the current was reduced as expected.

(2) V~hena clean pulse without initial overshoot was applied to the sample the threshold field for the onset of current oscillations was 2.2 kV/cm which is within the range of threshold fields— reported in the literature. For the sample of 1.,2 ~ cm resistivity the oscillations were in

_____________

*

Now with the Research and Development Laboratories, Northern Electric Co. Ltd., Ottawa, Canada. 201

202

GUNN INSTABILITIES IN LONG n-GaAs SAMPLES

—

—

—

—

—

—

—

=

=

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

— —

—

—

—

—

—

—

=

~

——

—

—

—

—

—

—

(0)

—

—

—

attributed to the presence of deep trapping centers in the energy gap. Slow response to light illumination with times of the order of a few seconds has also been observed in photo6 conductors with traps.

—

—

(5) The electronic behaviour of the sample (23 ~l-cm) was found to have undergone drastic. change with time. After the sample had been stored in a dry desicator for about six months, the resistance of the sample changed from the original 230 Q to about 150 Il, and the threshold field for the onset of Gunn instabilities increased from the original 2.2 kV/cm to about 3.0 kV/cm. Moreover, the Gunn oscillations occured only transiently, even at an applied field as high as 5.0 kV/cm. These oscillations died down in

—

—

—

—

—

Vol. 9, No. 3

—

(b)

FIG. 1. Gunn oscillation at room temperature. Time scale: O.O4 /isec/di vision. Length of n-GaAs sample: 10mm. Resistivity of n-GaAs sample: 23 cl-cm.

(3) A transverse magnetic field of 19 kG applied to the Gunn effect sample (23 a-cm) did not change either the threshold field or the oscillation frequency, but it did diminish the amplitude of oscillations. This phenomenon may be attributed to the decrease of the domain field 5

as predicted theoretically be Law and Kao. (4) When the sample (23 cl-cm) was illuminated by a sodium light with an emission spectrum lying mainly in the energy range of 2—3 eV, the current level and the amplitude of oscillation were increased significantly as expected, but no change in threshold field and oscillation frequency was observed. It is interesting to note that at a bias field slightly above the threshold value at the temperature of —10°C, the time required for the current to reach steady state immediately after illumination was of the order of a few seconds. The slow response to illumination is

about 30 sec and the rate of such decay increased rapidly if the sample surface had been cleaned several times with acetone or methyl alcohol. Sustained oscillations were observed, however, when the sample surfaces were blown over with a jet of dry air or dry nitrogen. Furthermore, during the process of cleaning the sample surfaces with acetone or methyl alcohol or of blowing over the surfaces with a jet of dry air or dry nitrogen, the resistance of the sample was increased to about 200 ft After such a cleaning process was ceased, the resistance dropped back to 150 ~ within a few minutes. This ageing phenomenon may be attributed to a surface effect, the mechanism of which is not yet understood• One possible cause is the migration of tin or gold atoms (since a gold—tin alloy was used for ohmic contact electrodes) into n-GaAs, and particularly into the region close to the surfaces because the ohmic contacts were alloyed to the end faces. These impurity atoms may form surface states, the emptying and occupation of which may depend greatly on the surrounding medium and thus effect the overall electronic properties of the sample. Ageing effects on bulk GaAs devices observed by Al-r,loufti et al. ~are quite different from what we observed since the length of their samples was about 20 ~i. their observed effect would thus be principally due to the degradation of bulk properties. Acknowledgements & We thank the National Research Council of Canada for research support under Grant A—3339.

Vol. 9, No.3

GUNN INSTABILITIES IN LONG n-GaAs SAMPLES

REFERENCES 1.

GUNN J.B., IBM J. Res. Devel. 8, 141 (1964).

2.

HEINLE W., Solid-St. Electron., 11, 583 (1968).

3.

FOYT A.G. Jr., Technical Report 385, MIT Lincoln Laboratory, (1968).

4.

GUETIN P., IEEE Trans. Electron Devices, ED-14, 552 (1967).

5.

LAW H.C. and KAO K.C.,Solid-St. Electron.,

6.

ROSE A., Concept in Photoconductivity and Allied Problems, Wiley, New York (1963).

7.

AL-MOUFTI M.N., JASKOLSKI S.V. and ISHII T.K., Proc. IEEE, 56, 236 (1968).

13, 1223 (1970).

Nous décrirons l’observation d’oscillations d’un courant coherent dans un échantillon de GaAs — type n ayant 10 mm de longueur. On remarque que les propriétés electroniques de l’échantillon subissent des changenients tres importants avec le temps et cet effet est attribué ~ la diffusion d’impuretés des electrodes dans le GaAstype n.

203