Strain is the N a m e of the Game

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 Phos- phide 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 sub- strates 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 per- mitted 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 occa- sion, claims to have the "best result" on something when better results already exist in the literature. Is this sloth or a reflection on library cut- backs?

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 pre- sented 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 communica- tions and covered the RACE pro- gramme 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

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 IPRM-V ad- dressed both the growth and assess- ment of wafers. At present, there's a trend towards "epi-ready" but as one wafer qua l i f i ca t ion exper 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 chemi- cal cleaning procedures followed by the growth of 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. Re- sults 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 of 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 MQW structures. The etch pit density of VCZ (vapour pressure control led Czochralski) and LEC grown semi-insulating InP single crystals were compared by Yabuhara et al. An order of magnitude improve- ment was observed for the VCZ crystals over the conventional LEC material.

Selective area epitaxy is receiving a lot of attention these days, particu- larly 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 electro- absorpt ion 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 rol l -off frequency of the modulated output was 4-5GHz. A novel extension of selective area epitaxy was presented by Tsang et al. who per formed etching and epitaxy without remov- ing 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

TFR there was some discussion on the practical relevance of CBE to device research or was it just another way to add to the publication moun- tain? I don't think this is true for two reasons: The relatively low growth temperature should more easily per- mit the growth of highly strained layers (ie metastable) in device struc- tures 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 concentra- tions as high as 9 × 1019 cm -3 are achievable.

Strained layer and strain compen- sated devices dominated many of the papers. Zucker discussed the advan- tage 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 compensa- tion 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 activa- tion. Cammassel et al, examined the thermal stability of strain compen- sated and lattice matched structures and found no difference, probably due to relatively small amount of strain in the structures. Interiffusion seemingly taking place before disloca- tion 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 incorporat ing such layers would survive extended life-test. Tak- ing the concept of strain compensa- tion to the limit was the work of

Freundlich et al. They grew a short period superlattice of InP/GaAs/ G a P / G a A s by C B E . I n P is +3 .8%(compres s ion ) and GaP 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 appl icat ion 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 com- positions 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 com- positions, they suggest that the con- duct ion 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 NTT for their work on InAsP 1.3 pm MQW laser diodes. The PL da ta f rom their layer s t ruc tu re showed narrow line widths, suggest- ing 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 ther- mal performance and low thresholds were observed.

The one topic I felt was greatly under represented was visible lasers based on AIGaInP/GaInP. Consider- ing the relative commercial impor- tance 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 con- ference.

All in all, a good conference show- ing steady progress in the develop- ment 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.

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