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Electroluminescence in polycrystalline ZnTe
Solid-State
Electronics
Pergamon
Press 1968. Vol. 11, pp. 513-515.
ELECTROLUMINESCENCE
IN
D. I. KENNEDY Bowmar (Received
Canada Ltd.,
3 November
Printed in Great Britain
POLYCRYSTALLINE
ZnTe
and M. J. RUSS Ottawa,
Ontario,
1967; in revised form
Canada
16 January
1968)
Abstract-The properties of electroluminescent devices fabricated from polycrystalline ZnTe are described. Broad emission bands are observed at room temperature under direct current excitation and 77°K under alternating current excitation. The emission is associated with impact ionization in localized high-field regions. Emission efficiencies at room temperature are in the range 1O-6 to lo-” photons/electron. R&urn&-Les propriCtCs des appareils electroluminescenses fabriquCs de polycristalline ZnTe sont dCcrits. De larges bandes d’emission sont observCes sous la tempbrature intCrieure sur I’excitation du courant direct et 77°K sur l’excitation du champ alternatif. L’tmission est associ.5 avec la collision d’ion 1ocnlisCes dans la region des haut champs. L’dfficacitCs d’Cmission 21tempkrature inti-rieure est dans la catCgorie de 10 -6 B 10 -’ photons/electron. Zusammenfassung-Die Eigenschaften der Elektrolumineszenz-Bautcile, hergestellt aus polykristallinen ZnTe sind ausreichend beschrieben. Breite Strahlungsbaender registrieren bei Raumtcmperatur unter Gleichstrom-Xnregung und 77°K \Vechselstrom-Anregung. Die Ausstrahlungen sind verbunden mit der Aufschlag-Ionisierung in lokalisierter Hochfeld-Sphaere. Der optimale \Vert unter Bcruecksichtigung der Raumtemperatur liegt zwischen der Spannc 10mG bis 10m7 Photo-Elektron.
PREVIOUS
single
investigations
crystal
of electroluminescence
in
ZnTe’lV3’ have indicated that the emission may result from high field processes in the vicinity of the contacts. Since long range order and high quality bulk material may not be essential for these processes to be operative and since single crystal ZnTe is difficult to produce it was of some importance to determine whether emission could be obtained from devices fabricated from polycrystalline material. The material was prepared by sealing a quantity of high purity Zn and Te in amounts deviating from stoichiometry by + 1 wt. y/, Te in an evacuated quartz ampoule, and annealing the reacted mixture for 15 hr at 1080°C. The resulting ZnTe, excluding the remaining cxccss elemental Te, was placed in a quartz tube and inserted in a furnace maintained at 950’ C. The tube extended from the furnace into the room temperature ambient and was connected to a running vacuum. Over a period of 2 hr the ‘LnTe vaporized from the 950°C zone and recondensed in an intermediate temperature
zone at about 7OO’C. The condensate was in the form of a polycrystalline boule of grain size 2 x lop3 to 5 x lo- 2 cm. Simple measurements on the material indicated that the effective resistivity was 4 x lo5 R-cm and thus that the material was semi-insulating. Devices were fabricated from 1 mm2 chips of material which were sawn from the boule. The chips were reduced to a thickness of 0.1-0.5 mm by grinding with alumina and were subsequently etched in a CP-4 solution to remove the surface layers. Three techniques were used to make contact to the chips: first, simple In contacts were made to the top and bottom surfaces; second, the top surface was coated with electroless Au and, third, the top surface was subjected to an In vacuum-alloying process. In all cases the bottom contacts of pure In were attached to conventional transistor headers. Since the brightest emission was obtained from the first technique using pure In on both sides of the chip, this contacting process was adopted for the subsequent investigationg. 513
514
D.
I.
KENNEDY
The I-V characteristics of a typical device at 300” and 77°K are shown in Fig. 1. The linear, square-law, and higher power-law dependencies in
FIG. 1. I-V characteristics of a polycrystalline ZnTe device at 300°K and 77’K. A-top contact negative, B-top contact positive.
the room temperature characteristti are similar to those observed in single crystal material. Also the onset of emission usually coincided with the commencement of a steep portion of the I-V characteristic and occurred at voltage levels of approx. 6 V. As the temperature was lowered pronounced reductions were observed in the conductivity of the material as shown in the 77°K I-V characteristic. The displacement of the d.c. I-V curve to lower currents with cooling was accompanied by a reduction in emission intensity and it was not possible to detect emission from the devices at 77°K using an RCA 7625 photomultiplier. By excitation of the devices with alternating
and M.
J.
RUSS
fields at 4 kHz, however, the 77’K emission characteristics of the material were readily measurable-in this case the emission peaks were found to occur at maxima and minima in the applied voltage waveform. At room temperature, emission was obtained from these devices for both directions of current flow. The emission appeared to originate from localized regions near the In contacts and was always associated with the positive contact. On polarity reversal the emission switched to the opposite contact. The variation in the blocking nature of the two contacts was reflected in higher current levels being obtained for one polarity; concomitantly the emission inter+, for a given applied voltage, was normally higher for this current direction (the latter situation occrurcd for both d.c. and a.c. excitation). At 77°K the ZnTe exhibited marked photoconductivity, and in the dark the current was further reduced by a factor of 100. The reduction of current on the extinction of the illumination was exponential with time, a period of approximately 15 min being required for the current to achieve ;I final stabilized value. Exposure of the device to light of greater than band-gap energy resulted in an immediate incr’kase in current to the original level. At room-kmperature or under a.~. cscitation no significanrphotoconductivity could be detected. The emission spectra were investigated using a 250 mm monochromator in conjunction with a photomultiplier having an S-20 cathode response. Typical; @suits obtained from de. excitation at 300°K d a.c. excitation at 77°K arc shown in Fig._&&se results are corrected for the photoThe room temperature multiplier response. emission spectrum shows a broad emission band centered at 6800 A with a half-width of 1900 A. The reduction of emission at wavelengths below 6550” A was correlated with the distributed absorption edge of the polycrystalline material. A similar broad emission spectrum is frequently observed in measurements on single crystal ZnTe devices and is commonly associated with an impactionization excitation process-in these cases the attenuation of the peak at higher energies is altered because of the sharper absorption edge. At 77°K the emission consisted of a major peak at 7550 A, a secondary peak at 5900 A and a minor peak at 6550 A.
ELECTROLUMINESCENCE
a
0
'
8000
IN
-
7000 WAVELENGTH
6000
5000
- i
FIG. 2. Emission spectrum of a polycrystalline ZnTe device at 300°K and 77°K. Excitation-300°K : d.c., 20 V, 2 mA. 77°K : 4 KHz, 175 V, 2 mA.
At room temperature the emission intensity of these devices as a function of current was measured under both d.c. and pulsed excitation and a linear relationship was obtained. This was true for both types of excitation and for both directions of current flow. Using pulse excitation of repetition rate 50 Hz and a duty cycle of 1 : 1000 the measurements were extended to current densities in the range of 30 A/cm ‘. The emission rise-time (from 10 to 90 per cent of full emission) was found to be 2.5 psec. While caution must be exercised in interpreting results from the poIycrystalline material used in this investigation, some general observations are valid. First, there is a strong similarity between the I-V characteristics and the room temperature emission properties of the polycrystalline and single crystal devices. The broad emission spectrum is suggestive of impact-ionization processes. The localization of emission to the contact regions also suggests that high-fields in the vicinity may lead to the impact-excitation. The similarity of I-V characteristics between single crystal and polycrystalline material suggests that the conduction processes are ‘contact-dominated’. Although the room-temperature results on both types of material are similar, the emission from polycrystalline devices diminishes at low temperatures. The extinction of emission at 77°K is
POLYCRYSTALLINE
ZnTe
515
attributed to the reduction of bulk conductivity of the polycrystalline material to a point where the conduction processes become limited by bulk effects (such as the impedance of grain boundaries, for example). Under a.c. excitation it is not necessary for carriers to traverse the bulk of the material and charge transport within the surface and barrier layers can lead to a.c. current flow and, presumably, to concomitant excitation and emission processes. The complex distributed emission spectrum obtained at 77°K is still in keeping with an impact ionization mechanism. The shape and position of the peak at 7550 A indicates the presence of a deep recombination center of unknown origin and the peak at 5900 A appears to result from recombination of hole-electron pairs of near bandgap energy. An emission peak at 6580 A was previously reported (3) to occur in the 77°K spectrum of devices fabricated from ZnTe single crystals incorporating oxygen centers. The possibility exists that the minor peak observed here at 6550 A is attributable to this same center. Because of the abundance of energy states anticipated at grain boundaries in polycrystalline material a more detailed speculation would be of no value. As anticipated these investigations revealed that electroluminescence observed in this form of ZnTe was inferior to that obtained in monocrystalline material. One of the disadvantages stems from the high resistivity of the material and the high voltage levels required to induce electroluminescence. The room temperature external quantum efficiency estimated from the observed emission intensities was of the order of 10-6-10-7 photons/electron. Acknowledgements-This work was carried out as part of a research program jointly supported by Bowmar Canada Limited and the Defence Industrial Research Program operated by the Canadian Defence Research Board. REFERENCES 1. B.L. CROWDER,F,F.MOFIEHEAD~~~P.R.WAGNER, Appl. Phys. Lett. 8, 148 (1966). 2. D. I. &NNEDY and M. J. RUSS, Solid-St. Electron. 10,125 (1967). 3. D. I. KENNEDY and M. J. Russ, /. oppl. Phys. 38, 4387 (1967).
Electronics
Pergamon
Press 1968. Vol. 11, pp. 513-515.
ELECTROLUMINESCENCE
IN
D. I. KENNEDY Bowmar (Received
Canada Ltd.,
3 November
Printed in Great Britain
POLYCRYSTALLINE
ZnTe
and M. J. RUSS Ottawa,
Ontario,
1967; in revised form
Canada
16 January
1968)
Abstract-The properties of electroluminescent devices fabricated from polycrystalline ZnTe are described. Broad emission bands are observed at room temperature under direct current excitation and 77°K under alternating current excitation. The emission is associated with impact ionization in localized high-field regions. Emission efficiencies at room temperature are in the range 1O-6 to lo-” photons/electron. R&urn&-Les propriCtCs des appareils electroluminescenses fabriquCs de polycristalline ZnTe sont dCcrits. De larges bandes d’emission sont observCes sous la tempbrature intCrieure sur I’excitation du courant direct et 77°K sur l’excitation du champ alternatif. L’tmission est associ.5 avec la collision d’ion 1ocnlisCes dans la region des haut champs. L’dfficacitCs d’Cmission 21tempkrature inti-rieure est dans la catCgorie de 10 -6 B 10 -’ photons/electron. Zusammenfassung-Die Eigenschaften der Elektrolumineszenz-Bautcile, hergestellt aus polykristallinen ZnTe sind ausreichend beschrieben. Breite Strahlungsbaender registrieren bei Raumtcmperatur unter Gleichstrom-Xnregung und 77°K \Vechselstrom-Anregung. Die Ausstrahlungen sind verbunden mit der Aufschlag-Ionisierung in lokalisierter Hochfeld-Sphaere. Der optimale \Vert unter Bcruecksichtigung der Raumtemperatur liegt zwischen der Spannc 10mG bis 10m7 Photo-Elektron.
PREVIOUS
single
investigations
crystal
of electroluminescence
in
ZnTe’lV3’ have indicated that the emission may result from high field processes in the vicinity of the contacts. Since long range order and high quality bulk material may not be essential for these processes to be operative and since single crystal ZnTe is difficult to produce it was of some importance to determine whether emission could be obtained from devices fabricated from polycrystalline material. The material was prepared by sealing a quantity of high purity Zn and Te in amounts deviating from stoichiometry by + 1 wt. y/, Te in an evacuated quartz ampoule, and annealing the reacted mixture for 15 hr at 1080°C. The resulting ZnTe, excluding the remaining cxccss elemental Te, was placed in a quartz tube and inserted in a furnace maintained at 950’ C. The tube extended from the furnace into the room temperature ambient and was connected to a running vacuum. Over a period of 2 hr the ‘LnTe vaporized from the 950°C zone and recondensed in an intermediate temperature
zone at about 7OO’C. The condensate was in the form of a polycrystalline boule of grain size 2 x lop3 to 5 x lo- 2 cm. Simple measurements on the material indicated that the effective resistivity was 4 x lo5 R-cm and thus that the material was semi-insulating. Devices were fabricated from 1 mm2 chips of material which were sawn from the boule. The chips were reduced to a thickness of 0.1-0.5 mm by grinding with alumina and were subsequently etched in a CP-4 solution to remove the surface layers. Three techniques were used to make contact to the chips: first, simple In contacts were made to the top and bottom surfaces; second, the top surface was coated with electroless Au and, third, the top surface was subjected to an In vacuum-alloying process. In all cases the bottom contacts of pure In were attached to conventional transistor headers. Since the brightest emission was obtained from the first technique using pure In on both sides of the chip, this contacting process was adopted for the subsequent investigationg. 513
514
D.
I.
KENNEDY
The I-V characteristics of a typical device at 300” and 77°K are shown in Fig. 1. The linear, square-law, and higher power-law dependencies in
FIG. 1. I-V characteristics of a polycrystalline ZnTe device at 300°K and 77’K. A-top contact negative, B-top contact positive.
the room temperature characteristti are similar to those observed in single crystal material. Also the onset of emission usually coincided with the commencement of a steep portion of the I-V characteristic and occurred at voltage levels of approx. 6 V. As the temperature was lowered pronounced reductions were observed in the conductivity of the material as shown in the 77°K I-V characteristic. The displacement of the d.c. I-V curve to lower currents with cooling was accompanied by a reduction in emission intensity and it was not possible to detect emission from the devices at 77°K using an RCA 7625 photomultiplier. By excitation of the devices with alternating
and M.
J.
RUSS
fields at 4 kHz, however, the 77’K emission characteristics of the material were readily measurable-in this case the emission peaks were found to occur at maxima and minima in the applied voltage waveform. At room temperature, emission was obtained from these devices for both directions of current flow. The emission appeared to originate from localized regions near the In contacts and was always associated with the positive contact. On polarity reversal the emission switched to the opposite contact. The variation in the blocking nature of the two contacts was reflected in higher current levels being obtained for one polarity; concomitantly the emission inter+, for a given applied voltage, was normally higher for this current direction (the latter situation occrurcd for both d.c. and a.c. excitation). At 77°K the ZnTe exhibited marked photoconductivity, and in the dark the current was further reduced by a factor of 100. The reduction of current on the extinction of the illumination was exponential with time, a period of approximately 15 min being required for the current to achieve ;I final stabilized value. Exposure of the device to light of greater than band-gap energy resulted in an immediate incr’kase in current to the original level. At room-kmperature or under a.~. cscitation no significanrphotoconductivity could be detected. The emission spectra were investigated using a 250 mm monochromator in conjunction with a photomultiplier having an S-20 cathode response. Typical; @suits obtained from de. excitation at 300°K d a.c. excitation at 77°K arc shown in Fig._&&se results are corrected for the photoThe room temperature multiplier response. emission spectrum shows a broad emission band centered at 6800 A with a half-width of 1900 A. The reduction of emission at wavelengths below 6550” A was correlated with the distributed absorption edge of the polycrystalline material. A similar broad emission spectrum is frequently observed in measurements on single crystal ZnTe devices and is commonly associated with an impactionization excitation process-in these cases the attenuation of the peak at higher energies is altered because of the sharper absorption edge. At 77°K the emission consisted of a major peak at 7550 A, a secondary peak at 5900 A and a minor peak at 6550 A.
ELECTROLUMINESCENCE
a
0
'
8000
IN
-
7000 WAVELENGTH
6000
5000
- i
FIG. 2. Emission spectrum of a polycrystalline ZnTe device at 300°K and 77°K. Excitation-300°K : d.c., 20 V, 2 mA. 77°K : 4 KHz, 175 V, 2 mA.
At room temperature the emission intensity of these devices as a function of current was measured under both d.c. and pulsed excitation and a linear relationship was obtained. This was true for both types of excitation and for both directions of current flow. Using pulse excitation of repetition rate 50 Hz and a duty cycle of 1 : 1000 the measurements were extended to current densities in the range of 30 A/cm ‘. The emission rise-time (from 10 to 90 per cent of full emission) was found to be 2.5 psec. While caution must be exercised in interpreting results from the poIycrystalline material used in this investigation, some general observations are valid. First, there is a strong similarity between the I-V characteristics and the room temperature emission properties of the polycrystalline and single crystal devices. The broad emission spectrum is suggestive of impact-ionization processes. The localization of emission to the contact regions also suggests that high-fields in the vicinity may lead to the impact-excitation. The similarity of I-V characteristics between single crystal and polycrystalline material suggests that the conduction processes are ‘contact-dominated’. Although the room-temperature results on both types of material are similar, the emission from polycrystalline devices diminishes at low temperatures. The extinction of emission at 77°K is
POLYCRYSTALLINE
ZnTe
515
attributed to the reduction of bulk conductivity of the polycrystalline material to a point where the conduction processes become limited by bulk effects (such as the impedance of grain boundaries, for example). Under a.c. excitation it is not necessary for carriers to traverse the bulk of the material and charge transport within the surface and barrier layers can lead to a.c. current flow and, presumably, to concomitant excitation and emission processes. The complex distributed emission spectrum obtained at 77°K is still in keeping with an impact ionization mechanism. The shape and position of the peak at 7550 A indicates the presence of a deep recombination center of unknown origin and the peak at 5900 A appears to result from recombination of hole-electron pairs of near bandgap energy. An emission peak at 6580 A was previously reported (3) to occur in the 77°K spectrum of devices fabricated from ZnTe single crystals incorporating oxygen centers. The possibility exists that the minor peak observed here at 6550 A is attributable to this same center. Because of the abundance of energy states anticipated at grain boundaries in polycrystalline material a more detailed speculation would be of no value. As anticipated these investigations revealed that electroluminescence observed in this form of ZnTe was inferior to that obtained in monocrystalline material. One of the disadvantages stems from the high resistivity of the material and the high voltage levels required to induce electroluminescence. The room temperature external quantum efficiency estimated from the observed emission intensities was of the order of 10-6-10-7 photons/electron. Acknowledgements-This work was carried out as part of a research program jointly supported by Bowmar Canada Limited and the Defence Industrial Research Program operated by the Canadian Defence Research Board. REFERENCES 1. B.L. CROWDER,F,F.MOFIEHEAD~~~P.R.WAGNER, Appl. Phys. Lett. 8, 148 (1966). 2. D. I. &NNEDY and M. J. RUSS, Solid-St. Electron. 10,125 (1967). 3. D. I. KENNEDY and M. J. Russ, /. oppl. Phys. 38, 4387 (1967).