- Email: [email protected]
II–VI compounds & related optoelectronic materials
Microelectronics Journal, 24 (1993) 8
II-Vl Compounds & Related Optoelectronic Materials Kevin Prior, Dept. of Physics, Heriot-Watt University, Edinburgh, EH14 4AS Scotland. Tel: [44] (0)31 449 5111. Fax: [44] (0)31 451 3136.
Blue lasers,LEDs, SEEDs, contactsand the merits of MBE vs MOVPE growth for II- VI devices - Kevin Prior of Heriot- Watt University, Edinburgh, Scotland, reportsfrom the exciting 6th International Conference on II-VI Compounds and Related Optoelectronic Materials which was held on September 13-17th, 1993, in Newport, Rhode Island, USA. t
This conference was different from the others in the same series which I have attended. There was a different, more upbeat atmosphere, a different emphasis, too. And let's not forget the different logo. Now, previous II-VI semiconductor conferences have adopted modest, unassuming logos showing bulk crystals (polycrystalline lump, Berlin 1989) or attractive abstract designs (Tamano, 1991). This year the logo was a multicoloured schematic laser, with a bright blue beam issuing from one facet. Very loud, very aggressive and very bullish, but completely in tune with the dominant theme of the meeting, which was the latest developments o f the blue and blue/green laser diodes produced over the past two years. It sometimes is hard to remember that at the previous conference there were only two (late news) papers from the first two groups to have produced lasers. At this conference the key papers in the sessions devoted to lasers and LEDs, of which there were four, were all on the "second generation" of II-VI lasers, and the
Figure 1. Buried-ridge laser diode - second generation lasers incorporating the quaternary alloy ZnMgSSe, which can be lattice matched to GaAs substrates. (Courtesy of Dr Jun Qiu, 3M Photonics Research, 3M Corporate Research Labs).
original structures were referred to as the "classic" 3M design on more than one occasion - despite being only 2 years old! N o w there were plenty of papers devoted to the other core areas of interest to the II-VI communi~, such as semi-magnetics and HgCdTe but as these
0026-2692/93/$6.00 © 1993, Elsevier Science Publishers Ltd.
707
areas do not attract the funding that lasers do there was a slight feeling that they were perhaps not subjects to hold the centre stage. Also, given the change in emphasis towards devices, it does make it more interesting to specialists in other areas. In this review, I have concentrated on the lasers, LEDs and other devices, and a number of other areas which I feel may be of interest.
Blue-green lasers There were four invited talks on laser production from 3M, Sony, Philips and Matsushita research groups, with each giving their own personal slant on the way the subject is developing.Jun Qiu from the 3M Corporation opened the conference by talking about their latest lasers. In common with the other invited presentations, these are second generation lasers incorporating the quaternary alloy ZnMgSSe, which can be lattice matched to GaAs substrates. The lasers are all of the type GaAs(n+) / ZnMgSSe(n) / ZnSSe(n) / ZnCdSe / ZnSSe(p) / ZnMgSSe(p), where ZnCdSe is the quantum well and the ZnSSe is the barrier material, with the ZnMgSSe providing optical confinement. With 6% S in the ZnSSe and 10°,4 Mg and 12% S in the ZnMgSSe, the structure is totally lattice matched to the substrate, except for the quantum well, which is under compressive strain. Similar lasers have been reported previously by the Sony and Philips research groups, the latter having obtained C W lasing at R o o m Temperature. The 3M variation on this theme was to etch a ridge waveguide structure into the top layer and re-grow some polycrystalline ZnSe over the ridge. With this extra confinement the lasers can be made to work C W at R o o m Temperature for a second or so with a 50% duty cycle, or for 1 hour with a 0.1% duty cycle. Typical operating currents for a 2 micron stripe of length 180 micron are Ith = 2.5 mA and efficiency = 25%. See Figures 1 and 2. Both 3M and Sony believe that there is a trade-off between the operating wavelength, and amount of carrier and optical c o n f i n e m e n t . 3 M have approached this problem by keeping the operating wavelength of their devices about 515 nm, which keeps Jth b e l o w 500 A c m -2. Decreasing the
708
wavelength below this value causes the threshold current density to rise dramatically. Ishibashi from Sony explained how they have approached this problem by trying to increase the magnesium content in their alloy, and thus increasing the optical and carrier confinement. The unfortunate side effect of this is that the p-type doping decreases sharply. They have had lasers which work up to 70°C with Ith of 50 A and quoted an output power of 330 m W for this laser, which is not quite up to the 500 m W quoted recently by Philips, but their lasers are probably lasting slightly longer - several seconds C W at room temperature. The Sony group have also improved the contacts.
Contacts One problem which has faced anyone trying to turn a laser structure into a laser is the problem of contacting to p-type ZnSe. There is no metal available with a large enough work function, and the + material can't be doped high enough to create a p contact layer. Turning the laser structure upside down and growing on a p-type GaAs substrate doesn't work either - there is a large valence band discontinuity
Figure 2. Far field pattern of 3M laser. (Courtesy of Dr Jun Qiu, 3M Photonics Research, 3M Corporate Research Labs).
Microelectronics Journal, Vol. 24, No. 8
between the two materials. The Purdue University group came up with a novel method which was demonstrated briefly last year and in more detail at this meeting byJung Han. This consists of forming a graded contact region composed of ZnSe/ZnTe muhilayers. The ZnSe fraction is gradually reduced to zero, leaving a ZnTe layer which can be doped p+. The variant proposed by the Sony group takes into account the band bending at the surface of the structure and uses a series of ZnTe wells with thin ZnSe barriers. By changing the well widths at the same time as the bands bend near the surface, it is possible to keep the hole states of all the quantum wells lined up. Holes can then tunnel through the barriers from well to well. A third invited talk on lasers from Petruzzello of Philips showed their approach to producing a very similar laser to the previous two groups. It's possible that the Philips lasers aren't lasting quite as long as the Sony and 3M lasers, but things are changing rapidly here• They do have low current thresholds (320 A cm -2 with coated facets) and very high output -2 -4 powers of 500 m W cm when pulsed at a 10 duty cycle. This talk, however, concentrated on some of the structural analysis which has been performed at Philips. Using the quaternary material grown on to GaAs substrates, with ZnSe guiding layers, the only non-lattice matched layer is the ZnCdSe well. The laser as a whole has <4 x 106 cm -2 defects as seen by TEM, but the quaternary material can contain stacking faults. Using non-lattice matched ZnSe instead of ZnSSe introduced mismatch dislocation networks at the 107 cm -2 level at the interface nearest to the substrate. This talk finished with a word of caution, as even with a completely lattice matched system it is still possible to grow a laser structure • . 9 . . . . which contains 10 stacking faults orlganaung at the GaAs surface. As is always the case in II-VI heteroepitaxial growth, substrate preparation is crucial. Overall, however, Philips believe that the weakest part of their present laser structure is the contacts, and that they are seeing contact degradation first.
Metal Electrode HgSe Zn'l-eSe p-type ZnSSe ZnSSeTe QW (a)
n-type ZnSSe GaAs Buffer Layer GaAs Substrate Au-Ge Electrode
Metal Electrode HgSe ZnTeSe p-type ZnSSe ZnCdSSe MQW (b)
n-type ZnSSe GaAs Buffer Layer GaAs Substrate Au-Ge Electrode
Figure 3. Schematic diagram of ZnSSeTe-based green LED structure. (from "Quaternary ll-Vl alloys for blue and green light emitting diode applications" by DB Eason et al., Dept of Physics, N. Carolina State University, and NA EI-Masry, Dept. of Materials Science and Engineering, N. Carolina State University).
IIl-V comparison The attentions of the various groups presenting papers on LEDs and lasers are now focused on the problems of increasing these lifetimes to usable levels• The dominant defects seem to be the same as in the case of III-V lasers - the dark line defect. Salokatve from the Brown/Purdue collaboration showed a fascinating video o f the degradation of a laser. By observing the spontaneous emission through a transparent top contact on the laser under normal operating conditions, it is possible to watch the dark hne defects propagate over the sample from one side of the stripe to the other• On one particularly memorable sequence, it was possible to see a train of
709
small dark spots produced at a defect centre rapidly moving from one side of the laser to the other. The degradation mechanisms were watched with interest by invited speakers from the III-V community. It was an excellent idea to devote one session to III-V lasers and the problems which have been overcome in this area. The message from these speakers was very reassuring: basically, we have seen all these degradation mechanisms before, and there is nothing here that you are seeing which we have not already overcome. This was a very positive note to end the section of the conference devoted to lasers.
LEDs The drive to a commercially useful LED is also well advanced, with the best results in this area presented in two papers by Eason and Schetzina from North Carolina State University. These LEDs are of the form p-ZnSSe / active layer / n-ZnSSe, where the active layer is a quantum well of ZnSSeTe in the green and ZnCdSSe in the blue LEDs. The structures that are produced are 500 micron square mesas which typically run with drive currents of about 20 mA. See Figures 3 and 4. These authors have their own recipe for producing a good ohmic contact which includes a 3 nm ZnSeTe layer and a contact made of the semi-metal HgSe.
Eason presented some interesting comparisons between these LEDs and comparable commercially available ones, which are given in the table below. The authors claim that these are the most efficient green and blue LEDs ever produced. Schetzina was able to go one better in his talk and produced some LEDs and a power supply to - literally - dazzle us. One great advantage of working in the visible is that you can see the difference with your own eyes between a silicon carbide and a II-VI based LED, and believe me, there is no comparison. The green LED is also an aesthetic advance over its III-V counterpart, as it operates in the range 504-520 in the deep green, while commercially available diodes are a sickly yellow-green. When these II-VI LEDs become commercially available the improvement in the quality of large area displays will be enormous. The lifetime of these LEDs has improved greatly recently and is a strong function of the dislocation density. A hundredfold reduction in these defects has increased the LED lifetime from 2-5 min to 200-300 hours and the authors expect lifetimes 10 000 hours to be achievable, although perhaps only when high quality ZnSe substrates become available.
Materials problems All the lasers produced to date have been made with traditional solid source MBE systems. So far,
Table 1. Green L E D s Power, [.tW o) External efficiency Wavelength
ZnSSeTe 0.53% 506 n m
GaP 0.1% 555 n m
Blue I.~Ds Power, l.tW (1) External efficency Wavelength
ZnCdSe 370 0.57% 482-490 n m
SiC 12-18 0.02-0.03% 470 n m
Notes: 1. Measured at 20 mA drive current.
710
Microelectronics Journal, VoL 24, No. 8
1
U~
T
[
506 nm
¢-
Output Power = 238 l.tW
Efficiency = 0.62 %
L
Color Purity = 62 %
>!.Z LL! i--
T = 300 K
~
(C350)
400
500
600
--__. 1 0 m A 3.8 V
700
WAVELENGTH (nm) Figure 4. Light output characteristics at 300K for ZnSSeTe-based green LED. (from "Quaternary lI-VI alloys for blue and green light emitting diode applications" by DB Eason et aL, Dept of Physics, N. Carolina State University, and NA EI-Masry, Dept. of Materials Science and Engineering, N. Car61ina State University).
MOVPE has not produced a laser, although both Fujita from Kyoto University using photo-assisted growth and Yanashima from the Tokyo Institute of Technology, showed that they have come close with spectral narrowing of the electroluminescence. The main problems with MOVPE are that the source m a t e r i a l s are u s u a l l y i m p u r e , w i t h g r e a t batch-to-batch variation, and that p-type doping is still very difficult. True, the p-type levels available in MBE are not spectacular, with 2 x 10 TM cm -3 being the highest available, but this is an order of magnitude higher than any material reported at this conference or elsewhere grown by MOVPE.
In a late news paper a group from Lawrence Berkeley Labs, University of California at Berkeley and Philips Labs, showed that they may have found the reason for the low doping levels. Using infrared absorption on a highly nitrogen doped but electrically inactive MOVPE grown sample, they detected the presence of N - H complexes within the sample. They suggest that these are electrically neutral. If this is the case, then it is probable that improved electrical activity means new nitrogen precursors with an inert carrier gas.
711
Other devices There were very few optoelectronic devices shown, with the exception of our own self electro-optic devices (SEEDs) operating in the blue-green region. This device, consisting o f 30 Z n C d S e / Z n S e quantum wells placed in a ZnSe p-n junction, operates at the heavy hole exciton wavelength of 488 nm. Coupling two SEED devices together to form a symmetric SEED (S-SEED) device enabled a contrast ratio of 1.5:1 to be obtained. The devices are large and slow compared to the fully integrated state of the art III-V devices, but with a shorter free carrier lifetime there is no reason why a properly designed and fabricated device should not operate faster than a III-V SEED.
Photogrowth effects With a large bandgap and weak chemical bonds, it's amazing what a photon can do. To anyone familiar with electrochemical CV profiling, the fact that semiconductors can be e t c h e d u n d e r photoirradiation will come as no surprise.With II-VI semiconductors, photoirradiation can also have some interesting effects when used during growth. In MOVPE, it has been shown to increase the growth
Figure 5. RT PL showing the variation in intensity due to doping stripes produced on a ZnSe sample by Heriot-Watt.
712
rate substantially, and Fujita from Kyoto University presented interesting results showing that the photogenerated holes arriving at the surface are able to cleave the methyl-zinc bonds in the surface layer. In MBE other effects can be observed. At low substrate temperatures, such as 150°C, ZnSe typically grows polycrystalline. Under photoirradiation, the surface mobility is enhanced and the material is single crystal. Ichikawa, from the Kyoto Institute o f Technology, showed that at 250°C the photoeffect can be long lived, lasting 5-10 ms after the light has been removed. Another interesting photoeffect is the removal of the n-type dopant iodine from the ZnSe surface. Mullins from Heriot-Watt showed that by projecting a pattern of fine lines on to the semiconductor surface, the incorporation of iodine could be ch~mged between the light and dark areas by x30. See Figure 5. The freest features which have been written to date are only 40 microns across, but with very little modification to the optics it should be possible to grow stripes of dopant to form contacts to lasers between 5 and 10 microns wide. I hope that this has given something of the flavour of the meeting. It was certainly one of the most exciting that I have attended, and the feeling of optimism amongst the participants for the future of II-VI semiconductors was very strong. The advances in the II-VI field have been rapid, and a whole new range o f devices will soon be available. Keep watching this space! O h yes, and one final g r a t u i t o u s plug for Heriot-Watt. We will be hosting the next II-VI conference here in Edinburgh in August in two years time. I hope to see perhaps one or two of you there.
II-Vl Compounds & Related Optoelectronic Materials Kevin Prior, Dept. of Physics, Heriot-Watt University, Edinburgh, EH14 4AS Scotland. Tel: [44] (0)31 449 5111. Fax: [44] (0)31 451 3136.
Blue lasers,LEDs, SEEDs, contactsand the merits of MBE vs MOVPE growth for II- VI devices - Kevin Prior of Heriot- Watt University, Edinburgh, Scotland, reportsfrom the exciting 6th International Conference on II-VI Compounds and Related Optoelectronic Materials which was held on September 13-17th, 1993, in Newport, Rhode Island, USA. t
This conference was different from the others in the same series which I have attended. There was a different, more upbeat atmosphere, a different emphasis, too. And let's not forget the different logo. Now, previous II-VI semiconductor conferences have adopted modest, unassuming logos showing bulk crystals (polycrystalline lump, Berlin 1989) or attractive abstract designs (Tamano, 1991). This year the logo was a multicoloured schematic laser, with a bright blue beam issuing from one facet. Very loud, very aggressive and very bullish, but completely in tune with the dominant theme of the meeting, which was the latest developments o f the blue and blue/green laser diodes produced over the past two years. It sometimes is hard to remember that at the previous conference there were only two (late news) papers from the first two groups to have produced lasers. At this conference the key papers in the sessions devoted to lasers and LEDs, of which there were four, were all on the "second generation" of II-VI lasers, and the
Figure 1. Buried-ridge laser diode - second generation lasers incorporating the quaternary alloy ZnMgSSe, which can be lattice matched to GaAs substrates. (Courtesy of Dr Jun Qiu, 3M Photonics Research, 3M Corporate Research Labs).
original structures were referred to as the "classic" 3M design on more than one occasion - despite being only 2 years old! N o w there were plenty of papers devoted to the other core areas of interest to the II-VI communi~, such as semi-magnetics and HgCdTe but as these
0026-2692/93/$6.00 © 1993, Elsevier Science Publishers Ltd.
707
areas do not attract the funding that lasers do there was a slight feeling that they were perhaps not subjects to hold the centre stage. Also, given the change in emphasis towards devices, it does make it more interesting to specialists in other areas. In this review, I have concentrated on the lasers, LEDs and other devices, and a number of other areas which I feel may be of interest.
Blue-green lasers There were four invited talks on laser production from 3M, Sony, Philips and Matsushita research groups, with each giving their own personal slant on the way the subject is developing.Jun Qiu from the 3M Corporation opened the conference by talking about their latest lasers. In common with the other invited presentations, these are second generation lasers incorporating the quaternary alloy ZnMgSSe, which can be lattice matched to GaAs substrates. The lasers are all of the type GaAs(n+) / ZnMgSSe(n) / ZnSSe(n) / ZnCdSe / ZnSSe(p) / ZnMgSSe(p), where ZnCdSe is the quantum well and the ZnSSe is the barrier material, with the ZnMgSSe providing optical confinement. With 6% S in the ZnSSe and 10°,4 Mg and 12% S in the ZnMgSSe, the structure is totally lattice matched to the substrate, except for the quantum well, which is under compressive strain. Similar lasers have been reported previously by the Sony and Philips research groups, the latter having obtained C W lasing at R o o m Temperature. The 3M variation on this theme was to etch a ridge waveguide structure into the top layer and re-grow some polycrystalline ZnSe over the ridge. With this extra confinement the lasers can be made to work C W at R o o m Temperature for a second or so with a 50% duty cycle, or for 1 hour with a 0.1% duty cycle. Typical operating currents for a 2 micron stripe of length 180 micron are Ith = 2.5 mA and efficiency = 25%. See Figures 1 and 2. Both 3M and Sony believe that there is a trade-off between the operating wavelength, and amount of carrier and optical c o n f i n e m e n t . 3 M have approached this problem by keeping the operating wavelength of their devices about 515 nm, which keeps Jth b e l o w 500 A c m -2. Decreasing the
708
wavelength below this value causes the threshold current density to rise dramatically. Ishibashi from Sony explained how they have approached this problem by trying to increase the magnesium content in their alloy, and thus increasing the optical and carrier confinement. The unfortunate side effect of this is that the p-type doping decreases sharply. They have had lasers which work up to 70°C with Ith of 50 A and quoted an output power of 330 m W for this laser, which is not quite up to the 500 m W quoted recently by Philips, but their lasers are probably lasting slightly longer - several seconds C W at room temperature. The Sony group have also improved the contacts.
Contacts One problem which has faced anyone trying to turn a laser structure into a laser is the problem of contacting to p-type ZnSe. There is no metal available with a large enough work function, and the + material can't be doped high enough to create a p contact layer. Turning the laser structure upside down and growing on a p-type GaAs substrate doesn't work either - there is a large valence band discontinuity
Figure 2. Far field pattern of 3M laser. (Courtesy of Dr Jun Qiu, 3M Photonics Research, 3M Corporate Research Labs).
Microelectronics Journal, Vol. 24, No. 8
between the two materials. The Purdue University group came up with a novel method which was demonstrated briefly last year and in more detail at this meeting byJung Han. This consists of forming a graded contact region composed of ZnSe/ZnTe muhilayers. The ZnSe fraction is gradually reduced to zero, leaving a ZnTe layer which can be doped p+. The variant proposed by the Sony group takes into account the band bending at the surface of the structure and uses a series of ZnTe wells with thin ZnSe barriers. By changing the well widths at the same time as the bands bend near the surface, it is possible to keep the hole states of all the quantum wells lined up. Holes can then tunnel through the barriers from well to well. A third invited talk on lasers from Petruzzello of Philips showed their approach to producing a very similar laser to the previous two groups. It's possible that the Philips lasers aren't lasting quite as long as the Sony and 3M lasers, but things are changing rapidly here• They do have low current thresholds (320 A cm -2 with coated facets) and very high output -2 -4 powers of 500 m W cm when pulsed at a 10 duty cycle. This talk, however, concentrated on some of the structural analysis which has been performed at Philips. Using the quaternary material grown on to GaAs substrates, with ZnSe guiding layers, the only non-lattice matched layer is the ZnCdSe well. The laser as a whole has <4 x 106 cm -2 defects as seen by TEM, but the quaternary material can contain stacking faults. Using non-lattice matched ZnSe instead of ZnSSe introduced mismatch dislocation networks at the 107 cm -2 level at the interface nearest to the substrate. This talk finished with a word of caution, as even with a completely lattice matched system it is still possible to grow a laser structure • . 9 . . . . which contains 10 stacking faults orlganaung at the GaAs surface. As is always the case in II-VI heteroepitaxial growth, substrate preparation is crucial. Overall, however, Philips believe that the weakest part of their present laser structure is the contacts, and that they are seeing contact degradation first.
Metal Electrode HgSe Zn'l-eSe p-type ZnSSe ZnSSeTe QW (a)
n-type ZnSSe GaAs Buffer Layer GaAs Substrate Au-Ge Electrode
Metal Electrode HgSe ZnTeSe p-type ZnSSe ZnCdSSe MQW (b)
n-type ZnSSe GaAs Buffer Layer GaAs Substrate Au-Ge Electrode
Figure 3. Schematic diagram of ZnSSeTe-based green LED structure. (from "Quaternary ll-Vl alloys for blue and green light emitting diode applications" by DB Eason et al., Dept of Physics, N. Carolina State University, and NA EI-Masry, Dept. of Materials Science and Engineering, N. Carolina State University).
IIl-V comparison The attentions of the various groups presenting papers on LEDs and lasers are now focused on the problems of increasing these lifetimes to usable levels• The dominant defects seem to be the same as in the case of III-V lasers - the dark line defect. Salokatve from the Brown/Purdue collaboration showed a fascinating video o f the degradation of a laser. By observing the spontaneous emission through a transparent top contact on the laser under normal operating conditions, it is possible to watch the dark hne defects propagate over the sample from one side of the stripe to the other• On one particularly memorable sequence, it was possible to see a train of
709
small dark spots produced at a defect centre rapidly moving from one side of the laser to the other. The degradation mechanisms were watched with interest by invited speakers from the III-V community. It was an excellent idea to devote one session to III-V lasers and the problems which have been overcome in this area. The message from these speakers was very reassuring: basically, we have seen all these degradation mechanisms before, and there is nothing here that you are seeing which we have not already overcome. This was a very positive note to end the section of the conference devoted to lasers.
LEDs The drive to a commercially useful LED is also well advanced, with the best results in this area presented in two papers by Eason and Schetzina from North Carolina State University. These LEDs are of the form p-ZnSSe / active layer / n-ZnSSe, where the active layer is a quantum well of ZnSSeTe in the green and ZnCdSSe in the blue LEDs. The structures that are produced are 500 micron square mesas which typically run with drive currents of about 20 mA. See Figures 3 and 4. These authors have their own recipe for producing a good ohmic contact which includes a 3 nm ZnSeTe layer and a contact made of the semi-metal HgSe.
Eason presented some interesting comparisons between these LEDs and comparable commercially available ones, which are given in the table below. The authors claim that these are the most efficient green and blue LEDs ever produced. Schetzina was able to go one better in his talk and produced some LEDs and a power supply to - literally - dazzle us. One great advantage of working in the visible is that you can see the difference with your own eyes between a silicon carbide and a II-VI based LED, and believe me, there is no comparison. The green LED is also an aesthetic advance over its III-V counterpart, as it operates in the range 504-520 in the deep green, while commercially available diodes are a sickly yellow-green. When these II-VI LEDs become commercially available the improvement in the quality of large area displays will be enormous. The lifetime of these LEDs has improved greatly recently and is a strong function of the dislocation density. A hundredfold reduction in these defects has increased the LED lifetime from 2-5 min to 200-300 hours and the authors expect lifetimes 10 000 hours to be achievable, although perhaps only when high quality ZnSe substrates become available.
Materials problems All the lasers produced to date have been made with traditional solid source MBE systems. So far,
Table 1. Green L E D s Power, [.tW o) External efficiency Wavelength
ZnSSeTe 0.53% 506 n m
GaP 0.1% 555 n m
Blue I.~Ds Power, l.tW (1) External efficency Wavelength
ZnCdSe 370 0.57% 482-490 n m
SiC 12-18 0.02-0.03% 470 n m
Notes: 1. Measured at 20 mA drive current.
710
Microelectronics Journal, VoL 24, No. 8
1
U~
T
[
506 nm
¢-
Output Power = 238 l.tW
Efficiency = 0.62 %
L
Color Purity = 62 %
>!.Z LL! i--
T = 300 K
~
(C350)
400
500
600
--__. 1 0 m A 3.8 V
700
WAVELENGTH (nm) Figure 4. Light output characteristics at 300K for ZnSSeTe-based green LED. (from "Quaternary lI-VI alloys for blue and green light emitting diode applications" by DB Eason et aL, Dept of Physics, N. Carolina State University, and NA EI-Masry, Dept. of Materials Science and Engineering, N. Car61ina State University).
MOVPE has not produced a laser, although both Fujita from Kyoto University using photo-assisted growth and Yanashima from the Tokyo Institute of Technology, showed that they have come close with spectral narrowing of the electroluminescence. The main problems with MOVPE are that the source m a t e r i a l s are u s u a l l y i m p u r e , w i t h g r e a t batch-to-batch variation, and that p-type doping is still very difficult. True, the p-type levels available in MBE are not spectacular, with 2 x 10 TM cm -3 being the highest available, but this is an order of magnitude higher than any material reported at this conference or elsewhere grown by MOVPE.
In a late news paper a group from Lawrence Berkeley Labs, University of California at Berkeley and Philips Labs, showed that they may have found the reason for the low doping levels. Using infrared absorption on a highly nitrogen doped but electrically inactive MOVPE grown sample, they detected the presence of N - H complexes within the sample. They suggest that these are electrically neutral. If this is the case, then it is probable that improved electrical activity means new nitrogen precursors with an inert carrier gas.
711
Other devices There were very few optoelectronic devices shown, with the exception of our own self electro-optic devices (SEEDs) operating in the blue-green region. This device, consisting o f 30 Z n C d S e / Z n S e quantum wells placed in a ZnSe p-n junction, operates at the heavy hole exciton wavelength of 488 nm. Coupling two SEED devices together to form a symmetric SEED (S-SEED) device enabled a contrast ratio of 1.5:1 to be obtained. The devices are large and slow compared to the fully integrated state of the art III-V devices, but with a shorter free carrier lifetime there is no reason why a properly designed and fabricated device should not operate faster than a III-V SEED.
Photogrowth effects With a large bandgap and weak chemical bonds, it's amazing what a photon can do. To anyone familiar with electrochemical CV profiling, the fact that semiconductors can be e t c h e d u n d e r photoirradiation will come as no surprise.With II-VI semiconductors, photoirradiation can also have some interesting effects when used during growth. In MOVPE, it has been shown to increase the growth
Figure 5. RT PL showing the variation in intensity due to doping stripes produced on a ZnSe sample by Heriot-Watt.
712
rate substantially, and Fujita from Kyoto University presented interesting results showing that the photogenerated holes arriving at the surface are able to cleave the methyl-zinc bonds in the surface layer. In MBE other effects can be observed. At low substrate temperatures, such as 150°C, ZnSe typically grows polycrystalline. Under photoirradiation, the surface mobility is enhanced and the material is single crystal. Ichikawa, from the Kyoto Institute o f Technology, showed that at 250°C the photoeffect can be long lived, lasting 5-10 ms after the light has been removed. Another interesting photoeffect is the removal of the n-type dopant iodine from the ZnSe surface. Mullins from Heriot-Watt showed that by projecting a pattern of fine lines on to the semiconductor surface, the incorporation of iodine could be ch~mged between the light and dark areas by x30. See Figure 5. The freest features which have been written to date are only 40 microns across, but with very little modification to the optics it should be possible to grow stripes of dopant to form contacts to lasers between 5 and 10 microns wide. I hope that this has given something of the flavour of the meeting. It was certainly one of the most exciting that I have attended, and the feeling of optimism amongst the participants for the future of II-VI semiconductors was very strong. The advances in the II-VI field have been rapid, and a whole new range o f devices will soon be available. Keep watching this space! O h yes, and one final g r a t u i t o u s plug for Heriot-Watt. We will be hosting the next II-VI conference here in Edinburgh in August in two years time. I hope to see perhaps one or two of you there.