Electroluminescence from polycrystalline selenium rectifiers

Solid State Communications,

Vol. 8,

pp. 327-331,

1970.

Pergamon P r e s s .

Printed in Great Britain

ELECTROLUMINESCENCE FROM POLYCRYSTALLINE SELENIUM RECTIFIERS* H.P.D. Lanyon and R.E. Richardson .It. Electrical Engineering Department, Worcester Polytechnic Institute, Worcester, Massachusetts 01609

(Received 19 December 1969 by N.B. Hannay)

Electroluminescence has been observed from Ni/Se/AI rectifiers under both forward and reverse bias at 113 ° and 296CK. Measurements of the spectral dependence at 113°K show the emission to occur both by band-to-band recombination and through intermediate centers. In the reverse sense radiation has been observed up to a photon energy of 6 e V consistent with an avalanche mechanism of breakdown. In the forward sense the presence of 4 eV photons demonstrates double injection currents with warm carriers produced by the applied field. The emissive bandgap is estimated as 2.3 eV. The luminescent efficiency is approximately I photon/10 7 carrier transits at room temperature, increasing as the sample is cooled.

SELENIUM is a wide bandgap material which has long been the b a s i s of commercial rectifiers. In this paper we report the first observation of electroluminescence from such diodes. The luminescence is observed both in the forward s e n s e when both types of carrier are injected into the exhaustion region next to the blocking contact and in the reverse s e n s e when additional electron-hole pairs are created by avalanche multiplication. 1

We have measured the spectral distribution of the emitted radiation at ll3CK and have shown the emission to extend from the ultraviolet (6eV) through the visible into the near infrared (1 eV). The current dependence of the intensity shows the higher energy radiation to be caused by band-toband recombination whereas the lower energy radiation results from recombination through intermediate centers. The extremely low level of radiation has limited the resolution available so that only the broad features of the spectral distribution have been defined.

Photoluminescence has previously been observed in single crystal selenium when illuminated by a helium-neon laser.2, s Such luminescence consisted of a number of discrete lines whose intensity diminished rapidly with increasing temperature and which was completely quenched thermally at SOCK. The lum-nescence that we report here is visible when the diodes are operated at room temperature. The intensity is much greater when the sample is immersed in liquid nitrogen.

The samples used in these measurements were composed of polycrystalline selenium layers approximately 7 5 ~ thick on a nickel plated aluminum substrate. An aluminum counter electrode was evaporated which transmitted 5 - 1 0 per cent of the incident light. The emitted radiation was detected with cooled photomultipliers: an EMI 6256B with an $13 response for the visible and ultraviolet spectrum (greater than bandgap energy) and an RCA 7102 with an $1 response for the near

* Supported, in part, by grants from the SeleniumTellurium Development Association.

327

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POLYCRYSTALLINE SELENIUM RECTIFIERS

Vol. 8, No. 5

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infrared spectrum (less than bandgap energy). For the spectral dependence a monochromator was placed between the source and the photomultiplier. The resolution was 23 m/~ in the ultraviolet and visible (200-700 m~) and 39 m/~ in the near infrared (> 700 m~z). The sample temperature was measured by a chromel-alumel thermocouple; water condensation was prevented by circulating dray nitrogen through the sample chamber. The spectral dependence of the light emitted from a sample operating at l13°K is shown in Fig. 1. The spectra are shown for both forward and reverse currents of essentially the same magnitude (100 mA forward; 110 mA reverse). In the reverse sense we observe radiation from 1.0 to 6eV in energy with a maximum at approximately 1.15 eV. There is a sharp break in the l o g - l i n e a r plot at about 2.3 eV. Essentially the same spectral distribution is seen in the forward

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sense except that the higher energy radiation falls off at 4 eV. The ordinate in these curves is the number of p h o t o n s / e V / s e c arriving at the photomultiplier assuming the standard photomultiplier sensitivity curves to hold. No attempt has been made to account for the spectral dependence of the collection efficiency nor for the self absorption of higher energy radiation within the sample. From photoconductivity measurements 4 the bandgap of selenium at this temperature has been estimated as 2.47eV so that higher energy radiation would be strongly absorbed. The absorption coefficient is greater than 10 5cm-1 in this region, s The luminescence spectrum consists of radiation both greater and smaller in energy than the bandgap of selenium. We interpret the radiation having a lower photon energy than 2.3 eV as being due to recombination via intermediate centers in the bandgap of selenium. Such radiation is not

Vol. 8, No. 5

POLYCRYSTALLINE SELENIUM RECTIFIERS

appreciably absorbed so that the peak at l.ISeV represents the true maximum in emission efficiency. It has been shown that the maximum recombination efficiency for a ce1:ter occurs when it lies at the middle of the bandgap 6 so that, in the absence of any discrete level with a large density of states, the peak in recombination efficiency, and hence in electroluminescent output, should correspond to a photon energy equal to half the bandgap. Consequently, the peak in efficiency and the knee in the spectrum both define the bandgap energy as 2.3 eV.

3.S eV is required for impact ionization. The luminescence in the forward sense implies that the current flow in selenium rectifiers is a double injection phenomenon. The presence of high energy radiation (4 eV) is at first sight surprising since it implies the presence of hot carriers under forward bias. However, the zero bias width of the exhaustion region is several hundred m~ and twenty to thirty volts forward bias may be required to maintain a current of 100 mA, most of the bias developing

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FIG. 2. Current dependence of visible electroluminescent output at 296°K under forward and reverse bias.

FIG. 3. Current dependence of visible electroluminescent output at II3°K under forward and reverse bias.

The presence of luminescence in the reverse sense of the characteristic is consistent with recombination of the electrons and holes created by impact ionization. The presence of radiation up to 6eV in energy in the spectrum corresponds to the recombination of hot electrons and holes and implies that an energy of approximately

across the exhaustion region. As the mean free path for phonon creation is about 20m~z v it is reasonable to expect such hot carriers in the junction region. The current dependence of the electroluminescent output has been measured as shown

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CURRENT DENSITY (mA/crr~) FIG. 4. Current dependence of infrared electroluminescent output at l l 3 e K under forward and reverse bias. in Figs. 2 - 4 . Figures 2 and 3 show the dependence for radiation greater than the bandgap (6256B output without a monochromator) for samples at 296°K and 113°K respectively. Some systematic error would result if the spectral output is voltage dependent. It can be seen that the light intensity varies as l'~where n lies between 1.5 and 2.8. The data in Fig. 4 shows that in the near infrared portion of the spectrum of a sample at 113°K the output is e s s e n t i a l l y linear both for forward and reverse bias with e s s e n t i a l l y the same efficiency.

The efficiency of the electroluminescence is extremely low. Typically we estimate that approximately one transit in 10 7 is terminated radiatively at 296°K. The efficiency typically increases tenfold as the sample is cooled to 113°K. Acknowledgements - The selenium substrates used in this work were obtained through the courtesy of the Westinghouse Electric Corporation. The work would have been impossible without grants received from the Selenium-Tellurium Development Association.

REFERENCES 1.

LANYON H.P.D., Bull, Am. phys. Soc. 13, 951 (1968).

2.

QUIESSER H.J. and STUKE J., Solid State Cornmun. 5, 75 (1967).

3.

ZETSCHE H. and FISCHER R., J. Phys. Chem. Solids 30, 1425 (1969).

4.

KOLOMIETS B.T., ROMANOV V.G. and KHODEVICH P.K., Fiz. tverd. Tela 7, 2534 (1965). Engl. trans. Soviet P h y s i c s - Solid State 7, 2042 (1966).

5.

GILLEO M.A., J. Chem. Phys. 19, 1291 (195I).

6.

See, for example, GROVE A.S., P h y s i c s and Technology of Semiconductor Devices p. 134. Wiley, New York (1967).

7.

LANYON H.P.D., (to be published - submitted to Phys. S~atus Solidi).

Vol. 8, No. 5

POLYCRYSTALLINE SELENIUM RECTIFIERS Elektrolumineszenz in Ni/Se/AI- Gleichrichtern unter Vorw~rts- und Sperrspanungen f~ir die Temperaturen!13°K und 296°K wurde beobschtet. Spektralmessungen Fur 113°K lassen auf direkte und indirekte Band~berg~nge schliessen. In der Sperrichtung wurden Strahlungen mit Photonenergien his zu 6eV beobschtet, was auf einen lswinenartigen Zussmmenbruch schliessen l~sst. Photonenergien yon 4 eV wurden unter Vorw~rtsspannungen gemessen als Folge der Doppelinjektion yon warmen Ladungstr~gern im Gebiete hoher elektrischer Feldst~rke. Das der Emission entsprechende verbotene Energieband wird auf 2.3 eV ein.gesch~tzt. Die Lumineszenzausbeute ist ungef~hr 1 Photon pro 10 ~ Uberg~nge flit Zimmertemperatur, mit grSsseren Werten f~r niedrigere Tempersturen.

331