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Strain is the name of the game
Strain is the N a m e of the G a m e Mike Davies Following up our first report last issue, Mike Davies of the Institute for Microstructural Sciences, National Research Council of Canada, presents his personal over-view of IPRM-V held at the Maison de la Chimie, Paris, in April. I
PRM-V, the Fifth International Conference on Indium Phosphide and Related Materials was held in Paris this year, a city known for its beautiful architecture and interesting night life. I arrived early on Saturday morning and took the bus to the Arc de Triomphe, where I met my first Parisian who was walking down the street stark naked at about 8 am in the morning. Evidently Friday nights are pretty wild in Paris. This was going to be an interesting conference. The whole gamut of InP-based research was represented from substrates to integrated electronic and optoelectronic circuits. Even though InP is a very dynamic field I got the impression that there was a lot of work presented which had already been published but this provided a good opportunity for someone to get an up to date overview of lnP based research. This is also almost certainly a reflection of the economic times in which we live, in that, for some of us, conference attendance is only permitted if the attendee is presenting a paper. This makes life tough for conference organisers as they have to maintain standards yet get people to come to make the conference viable. I also heard, on more than one occasion, claims to have the "best result" on something when better results already exist in the literature. Is this sloth or a reflection on library cutbacks? The topic which transcended the whole spectrum of activity in InP based research is strain. For InP based devices strain is the name of the game.
Schematic view of a circular-grating surface emitting DBR laser as developed by NRC-Canada.
Of the plenary talks, Greiling presented some of the excellent work going on at Hughes. This shows what can be done if sufficient investment is made. Hadjifotou looked at the role of InP in optical fibre communications and covered the RACE programme in particular. Personally, I'd like to know which is the most important wavelength to be working at, 1.3 ~tm or 1.5 p.m? As most of the fibre in the ground isn't dispersion shifted, are the telecoms companies going to dig it all up and replace it? I think not, so I'm sure device people would really like some input from
Page 21
Vol 6 No 4
systems people when it comes to optical fibre communications, lga clearly explained the applications waiting for surface emitting lasers, but the present dominance of this area by vertical cavity surface emitting lasers is under attack by circular grating DBR devices, a point the speaker missed. Several papers at I P R M - V addressed both the growth and assessment of wafers. At present, there's a trend towards "epi-ready" but as one wafer q u a l i f i c a t i o n e x p e r t says, there's no such thing as epi-ready. It really depends on what qualification
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criteria are applied. Overs et al. in their evaluation of epi-ready wafers emphasised the importance of chemical cleaning procedures followed by the growth o f a controlled oxide which then facilitates homogeneous oxide desorption prior to epitaxial growth. Yamada et ai. looked at the residual stress distribution determined by strain-induced birefringence. Results were similar to those seen by scanning photoluminescence or x-ray topography. In a poster presentation by Watkins et al. the donor bound exciton transition in low temperature PL was used to determine small variations in lattice parameter due to strain in the substrate. Perhaps a critical review o f these substrate assessment techniques would be a good subject for the next IPRM. The various types of crystal quality problems in epitaxial layers were reviewed by Huber and Grattepain along with their possible sources such as substrate quality, parasitic arsenic contamination of InP layers, Si and Fe doping level and interface quality in M Q W structures. The etch pit density o f VCZ (vapour pressure controlled Czochralski) and LEC grown semi-insulating InP single crystals were compared by Yabuhara et al. An order of magnitude improvement was observed for the VCZ crystals over the conventional LEC material. Selective area epitaxy is receiving a lot of attention these days, particularly as it could well be the way to integrate optoelectronic components. An example of this approach was presented by Schilling et al. in a post deadline paper with their results on an integrated DFB laser and electroabsorption modulator. The active devices were grown on SiO2 masked substrates prior to grating formation and then over grown to form a buried heterostructure. The 3dB roll-off frequency of the modulated output was 4-5GHz. A novel extension of selective area epitaxy was presented by Tsang et al. who p e r f o r m e d etching and epitaxy without removing the sample from vacuum. They used the PC13 precursor as an etch gas, SiO2 being used as a mask. Several contributions featured device results from material which was purchased commercially, it's going to be interesting to see if this trend continues. In the previous issue of
T F R there was some discussion on the practical relevance of CBE to device research or was it just another way to add to the publication mountain? I don't think this is true for two reasons: The relatively low growth temperature should more easily permit the growth of highly strained layers (ie metastable) in device structures allowing new device structures. Secondly, lower growth temperatures should permit control of highly doped p-type layers, eg HBT applications where out diffusion of Be from the base is a problem. Tu reviewed the state of play of carbon doping of InGaAs, which is a possible p-type dopant alterative to Be, but with a much lower diffusion coefficient. He noted that p-type doping concentrations as high as 9 × 1019 cm -3 are achievable. Strained layer and strain compensated devices dominated many of the papers. Zucker discussed the advantage of using tensile strained wells to get equal index change for both TE and TM modes in electro-optic MQW switches. Such devices often require several wells and so strain compensation is a technique used to render the stack strain neutral permitting the growth of superlattices as discussed in a poster presentation by Houghton et al. The thermal stability of strained MQW stacks is an important topic as these structures often have to undergo extensive thermal treatments as part of further device processing such as grating overgrowth or implant activation. Cammassel et al, examined the thermal stability of strain compensated and lattice matched structures and found no difference, probably due to relatively small amount of strain in the structures. Interiffusion seemingly taking place before dislocation injection. Similarly, Bennett and del Alamo considered the thermal stability of InGaAs and InA1As on InP. Surprisingly, or not depending on your point of view, they found that it was possible to grow layers beyond the Matthews and Blakeslee limit which were stable up to 700-800C. They ascribe the stability of these pseudomorphic layers to the quality and preparation of the substrate. There is the question of whether any devices incorporating such layers would survive extended life-test. Taking the concept of strain compensation to the limit was the work of
Page 22
Vol 6 No 4
Freundlich et al. They grew a short period superlattice of I n P / G a A s / GaP/GaAs b y C B E . I n P is + 3 . 8 % ( c o m p r e s s i o n ) and G a P 3.6%(tension) lattice mismatch with respect to the GaAs substrate. In a post deadline invited paper, Mircea et al. put strain compensation into practical application in the growth of strained MQW structures. As it is generally preferable to have quaternaries as the well material so that wide wells may be used, it makes the growth of strain compensated structures difficult. They chose compositions for wells and barriers which allowed the growth of the structure at constant y (ie arsenic concentration). They claim the absence of an As/P gradient leads to improved well/ barrier interfaces and, for their compositions, they suggest that the conduction band and valence band offsets are favourable for improved device performance. However, their To values were not untypical for 1.5 pm lasers. The award for best paper was deservedly given to Yamamoto et al. of N T T for their work on InAsP 1.3 pm MQW laser diodes. The PL d a t a f r o m their layer s t r u c t u r e showed narrow line widths, suggesting that the energy levels were deep in the InAsP wells, as expected as InAsP has a large conduction band offset (0.7Eg). Greatly improved the thermal performance and low thresholds were observed. The one topic I felt was greatly under represented was visible lasers based on AIGaInP/GaInP. Considering the relative commercial importance of these devices only one paper, a doping study by Kadoiwa et al., was presented. Indeed, visible lasers are the subject of a forthcoming LEOS Summer Topical Meeting in July at Santa Barbara. Maybe a session dedicated to these devices would be appropriate for the next IPRM conference. All in all, a good conference showing steady progress in the development of InP technology. Let's all hope this technology will soon achieve its full commercial potential. Mike Davies, Institute Jor Microstructural Sciences, National Research Council of Canada, Ontario, Canada. Tel. [1] (613) 998-3476; .[hx.. [1] (613) 941-9794 ./957-8734.
PRM-V, the Fifth International Conference on Indium Phosphide and Related Materials was held in Paris this year, a city known for its beautiful architecture and interesting night life. I arrived early on Saturday morning and took the bus to the Arc de Triomphe, where I met my first Parisian who was walking down the street stark naked at about 8 am in the morning. Evidently Friday nights are pretty wild in Paris. This was going to be an interesting conference. The whole gamut of InP-based research was represented from substrates to integrated electronic and optoelectronic circuits. Even though InP is a very dynamic field I got the impression that there was a lot of work presented which had already been published but this provided a good opportunity for someone to get an up to date overview of lnP based research. This is also almost certainly a reflection of the economic times in which we live, in that, for some of us, conference attendance is only permitted if the attendee is presenting a paper. This makes life tough for conference organisers as they have to maintain standards yet get people to come to make the conference viable. I also heard, on more than one occasion, claims to have the "best result" on something when better results already exist in the literature. Is this sloth or a reflection on library cutbacks? The topic which transcended the whole spectrum of activity in InP based research is strain. For InP based devices strain is the name of the game.
Schematic view of a circular-grating surface emitting DBR laser as developed by NRC-Canada.
Of the plenary talks, Greiling presented some of the excellent work going on at Hughes. This shows what can be done if sufficient investment is made. Hadjifotou looked at the role of InP in optical fibre communications and covered the RACE programme in particular. Personally, I'd like to know which is the most important wavelength to be working at, 1.3 ~tm or 1.5 p.m? As most of the fibre in the ground isn't dispersion shifted, are the telecoms companies going to dig it all up and replace it? I think not, so I'm sure device people would really like some input from
Page 21
Vol 6 No 4
systems people when it comes to optical fibre communications, lga clearly explained the applications waiting for surface emitting lasers, but the present dominance of this area by vertical cavity surface emitting lasers is under attack by circular grating DBR devices, a point the speaker missed. Several papers at I P R M - V addressed both the growth and assessment of wafers. At present, there's a trend towards "epi-ready" but as one wafer q u a l i f i c a t i o n e x p e r t says, there's no such thing as epi-ready. It really depends on what qualification
iiiiiiiiiii!!!i iili~ !!iii!i!!!i~!iiiiiU +~+~iiiL!!!~!ii!:~:iiiiiiiTiii!i iiiiiiiii!iiiii
criteria are applied. Overs et al. in their evaluation of epi-ready wafers emphasised the importance of chemical cleaning procedures followed by the growth o f a controlled oxide which then facilitates homogeneous oxide desorption prior to epitaxial growth. Yamada et ai. looked at the residual stress distribution determined by strain-induced birefringence. Results were similar to those seen by scanning photoluminescence or x-ray topography. In a poster presentation by Watkins et al. the donor bound exciton transition in low temperature PL was used to determine small variations in lattice parameter due to strain in the substrate. Perhaps a critical review o f these substrate assessment techniques would be a good subject for the next IPRM. The various types of crystal quality problems in epitaxial layers were reviewed by Huber and Grattepain along with their possible sources such as substrate quality, parasitic arsenic contamination of InP layers, Si and Fe doping level and interface quality in M Q W structures. The etch pit density o f VCZ (vapour pressure controlled Czochralski) and LEC grown semi-insulating InP single crystals were compared by Yabuhara et al. An order of magnitude improvement was observed for the VCZ crystals over the conventional LEC material. Selective area epitaxy is receiving a lot of attention these days, particularly as it could well be the way to integrate optoelectronic components. An example of this approach was presented by Schilling et al. in a post deadline paper with their results on an integrated DFB laser and electroabsorption modulator. The active devices were grown on SiO2 masked substrates prior to grating formation and then over grown to form a buried heterostructure. The 3dB roll-off frequency of the modulated output was 4-5GHz. A novel extension of selective area epitaxy was presented by Tsang et al. who p e r f o r m e d etching and epitaxy without removing the sample from vacuum. They used the PC13 precursor as an etch gas, SiO2 being used as a mask. Several contributions featured device results from material which was purchased commercially, it's going to be interesting to see if this trend continues. In the previous issue of
T F R there was some discussion on the practical relevance of CBE to device research or was it just another way to add to the publication mountain? I don't think this is true for two reasons: The relatively low growth temperature should more easily permit the growth of highly strained layers (ie metastable) in device structures allowing new device structures. Secondly, lower growth temperatures should permit control of highly doped p-type layers, eg HBT applications where out diffusion of Be from the base is a problem. Tu reviewed the state of play of carbon doping of InGaAs, which is a possible p-type dopant alterative to Be, but with a much lower diffusion coefficient. He noted that p-type doping concentrations as high as 9 × 1019 cm -3 are achievable. Strained layer and strain compensated devices dominated many of the papers. Zucker discussed the advantage of using tensile strained wells to get equal index change for both TE and TM modes in electro-optic MQW switches. Such devices often require several wells and so strain compensation is a technique used to render the stack strain neutral permitting the growth of superlattices as discussed in a poster presentation by Houghton et al. The thermal stability of strained MQW stacks is an important topic as these structures often have to undergo extensive thermal treatments as part of further device processing such as grating overgrowth or implant activation. Cammassel et al, examined the thermal stability of strain compensated and lattice matched structures and found no difference, probably due to relatively small amount of strain in the structures. Interiffusion seemingly taking place before dislocation injection. Similarly, Bennett and del Alamo considered the thermal stability of InGaAs and InA1As on InP. Surprisingly, or not depending on your point of view, they found that it was possible to grow layers beyond the Matthews and Blakeslee limit which were stable up to 700-800C. They ascribe the stability of these pseudomorphic layers to the quality and preparation of the substrate. There is the question of whether any devices incorporating such layers would survive extended life-test. Taking the concept of strain compensation to the limit was the work of
Page 22
Vol 6 No 4
Freundlich et al. They grew a short period superlattice of I n P / G a A s / GaP/GaAs b y C B E . I n P is + 3 . 8 % ( c o m p r e s s i o n ) and G a P 3.6%(tension) lattice mismatch with respect to the GaAs substrate. In a post deadline invited paper, Mircea et al. put strain compensation into practical application in the growth of strained MQW structures. As it is generally preferable to have quaternaries as the well material so that wide wells may be used, it makes the growth of strain compensated structures difficult. They chose compositions for wells and barriers which allowed the growth of the structure at constant y (ie arsenic concentration). They claim the absence of an As/P gradient leads to improved well/ barrier interfaces and, for their compositions, they suggest that the conduction band and valence band offsets are favourable for improved device performance. However, their To values were not untypical for 1.5 pm lasers. The award for best paper was deservedly given to Yamamoto et al. of N T T for their work on InAsP 1.3 pm MQW laser diodes. The PL d a t a f r o m their layer s t r u c t u r e showed narrow line widths, suggesting that the energy levels were deep in the InAsP wells, as expected as InAsP has a large conduction band offset (0.7Eg). Greatly improved the thermal performance and low thresholds were observed. The one topic I felt was greatly under represented was visible lasers based on AIGaInP/GaInP. Considering the relative commercial importance of these devices only one paper, a doping study by Kadoiwa et al., was presented. Indeed, visible lasers are the subject of a forthcoming LEOS Summer Topical Meeting in July at Santa Barbara. Maybe a session dedicated to these devices would be appropriate for the next IPRM conference. All in all, a good conference showing steady progress in the development of InP technology. Let's all hope this technology will soon achieve its full commercial potential. Mike Davies, Institute Jor Microstructural Sciences, National Research Council of Canada, Ontario, Canada. Tel. [1] (613) 998-3476; .[hx.. [1] (613) 941-9794 ./957-8734.