Proton irradiation was used to create interdiffusion in In₀.₅Ga₀.₅Asquantum dots(QDs), grown by low-pressure metalorganic chemical vapor deposition. After 25-keV proton irradiation, the QD samples were annealed at two temperatures (700 or 750 °C) for 30 s. It was found that much lower annealing temperatures were needed to recover the photoluminescence signals than in the quantum-well case. Large blueshifts (120 meV) and narrowing of the photoluminescence spectra were seen. Various doses (5×10¹³–1×10¹⁵ cm⁻²) and implant temperatures (20–200 °C) were used to study the interdiffusion processes in these samples. In QD samples, much lower doses were required to achieve similar energy shifts than reported in quantum-well samples

Gal, M.

Jagadish, C.

Lever, P.

Reece, P.

Tan, Hark Hoe

Applied Physics Letters

The Australian National University

APPLIED PHYSICS LETTERS VOLUME 82, NUMBER 13 31 MARCH 2003Proton-irradiation-induced intermixing of InGaAs quantum dotsP. Lever,a) H. H. Tan, and C. JagadishDepartment of Electronic Materials Engineering, Research School of Physical Sciences and Engineering,The Australian National University, Canberra, ACT 0200, AustraliaP. Reece and M. GalSchool of Physics University of New South Wales, Sydney, NSW 2052, Australia~Received 4 November 2002; accepted 20 January 2003!Proton irradiation was used to create interdiffusion in In0.5Ga0.5As quantum dots~QDs!, grown bylow-pressure metalorganic chemical vapor deposition. After 25-keV proton irradiation, the QDsamples were annealed at two temperatures~700 or 750 °C! for 30 s. It was found that much lowerannealing temperatures were needed to recover the photoluminescence signals than in thequantum-well case. Large blueshifts~120 meV! and narrowing of the photoluminescence spectrawere seen. Various doses (531013– 131015 cm22) and implant temperatures~20–200 °C! wereused to study the interdiffusion processes in these samples. In QD samples, much lower doses wererequired to achieve similar energy shifts than reported in quantum-well samples. ©2003 AmericanInstitute of Physics.@DOI: 10.1063/1.1561153#umardyschoafir-ritcaetuicsheterucumdm-inonibhgdiiem-ex-s.ofornn ofro-m-andles..th. Aep tomi-r theots.en-5o-fteram-ionalernd, theonen-ct.eak°CThere has been a lot of interest in the study of quantdots~QDs! due to the three-dimensional confinement of criers leading to an atom-like density of states. QD lasers1 andphotodetectors2 with improved characteristics have alreabeen demonstrated. Intermixing techniques allow pogrowth tuning of the emission wavelength. Using these teniques, lasers of different wavelengths can be fabricatedthe same chip for wavelength division multiplexing applictions. The control of the band gap in various regions ochip is critical for the fabrication of photonic integrated ccuits. There are several methods to achieve this. Impufree vacancy disordering uses dielectric caps to create vacies in selected regions, while ion implantation inducintermixing creates vacancies through bombardment. In sies on QD intermixing annealing, with and without dielectrcapping, has been shown to cause a large blueshift andnificant narrowing in the QD photoluminescence~PL!spectra.3,4 In this process, interdiffusion occurs between tGaAs barrier layers and the InGaAs or InAs QDs, leadingthe confining potential of the QDs becoming shallowHowever, there have been studies indicating that if too minterdiffusion takes place, the QDs merge into a quantwell.5Ion-implantation-induced intermixing is a widely usetechnique for modifying the potential profile of a quantuwell and quantum-well lasers.6–9 Ion implantation has several advantages over other techniques used for intermixIon implantation is compatible to planar processing ascan accurately control the concentration and depth distrtion of the defects by varying the ion dose and energy. Twill allow creation of controlled interdiffusion in QDs inselected regions.Although the interdiffusion in QDs by thermal annealinhas been widely studied, there have only been a few stuon ion-implantation-induced interdiffusion. These studused medium-mass ions~Mn and Cr!10,11 and were done ona!Electronic mail: penny_lever@ieee.org2050003-6951/2003/82(13)/2053/3/$20.00Downloaded 14 Apr 2010 to 150.203.243.38. Redistribution subject to AI-t--n-ay-n-dd-ig-o.hg.eu-isessInAs dots grown by molecular beam epitaxy. The QD coposition, growth methods, and implant conditions arepected to play a major role in the interdiffusion of the QDIn this work, proton irradiation was used on a single layerIn0.5Ga0.5As QDs, grown by metalorganic chemical vapdeposition~MOCVD!. Based on previous studies of protoirradiation into quantum wells,7 these light ions create dilutecascades that lead to generation of a large concentratiopoint defects and hence a large degree of interdiffusion. Pton irradiation was carried out with various doses and teperatures. Low-temperature PL spectra were measuredused to characterize the interdiffusion created in the sampIn0.5Ga0.5As QDs were grown by low-pressure MOCVDA buffer layer of GaAs was grown at 650 °C, then the growtemperature was reduced to 550 °C for the dot growthtotal of 6 monolayers~ML ! of material was used to form thdots. After the dot growth, the temperature was ramped u650 °C during the GaAs capping layer growth. A 200-nGaAs cap was grown on top of the dots. Atomic force mcroscopy was used on uncapped samples grown undesame conditions to measure the size and density of the dThe dots are 3–5 nm high and 20–30 nm wide, with a dsity of 3.631010 dots/cm2.After growth, samples were irradiated with H ions at 2keV to doses of 531013 to 131015 cm22 at temperaturesranging from RT to 200 °C. The energy of the ions was chsen to position the damage peak over the QD region. Aimplantation, the samples were annealed to remove the dage caused by implantation and initiate the interdiffusprocess. This was carried out in a rapid thermal anneunder Ar ambient. The samples were proximity capped aannealed at 700 or 750 °C for 30 s. For PL measurementsamples were excited with the 514.5-nm line of an Ar-ilaser and the spectra were collected using a liquid-nitrogcooled Ge detector.Thermal interdiffusion in QD samples is a large effeAn energy shift of 143 meV is seen in the luminescence pof the sample used in this study when it is annealed at 8503 © 2003 American Institute of PhysicsP license or copyright; see http://apl.aip.org/apl/copyright.jspaathsie-uethhishonigtuesnhegt crugeefuotthInfoontouhehThadsisev-h.faceho-oreheredick-hedheislestheta-fu-asthet ofwnthehera-(1lesuchan-tedaAsyling.des,ter-ut is, them-al fer-2054 Appl. Phys. Lett., Vol. 82, No. 13, 31 March 2003 Lever et al.for 30 s. The linewidth was reduced to 59% of the originvalue. With so much interdiffusion occurring due to thermenergy, there are concerns that dissolution of the dots5 cantake place. Strain relaxation could also be a factor. Instudy, the annealing temperatures are kept as low as posto minimize the effect of thermal interdiffusion.Annealing at 700 °C was found to be sufficient to rcover the PL intensity to more than half of the original valfor the lowest dose. Figure 1 shows the PL spectra fromQD sample annealed at 750 °C for 30 s. The energy sfrom the as-grown to the annealed~unimplanted! sample is45 meV. The additional energy shift from implantationfrom 40 to 75 meV, depending on the implantation dose. TPL intensity from the lowest dose implant returns to 76%its original value in the unimplanted sample. The PL intesity of the highest dose sample recovered to 22% of its ornal value. Even annealing at 900 °C, InGaAs/GaAs quanwells implanted with the same dose have PL intensity of lthan one tenth of the original value.12 The energy shifts seein this study lie between the energy shifts seen in two otstudies10,11 when compared against the amount of damacreated in the sample. There are several differences thabe expected in the behavior of the samples previouslyported in literature and the In0.5Ga0.5As QDs grown in thisstudy. The InAs QDs used in the previous studies shoshow a larger effect from interdiffusion as they have a larIn concentration gradient to cause interdiffusion. Howevall these QDs are highly strained, and during the interdifsion process this strain could partially relax. As the InAs dhave more strain than the In0.5Ga0.5As dots, they are morelikely to show the strain relaxation effects.Two different annealing temperatures were used afterimplant, resulting in different amounts of energy shift.Fig. 2, the implantation-induced energy shift is plottedthe different annealing temperatures. The implantatiinduced energy shift is taken to be the difference betweensample that has been annealed but not implanted and thethat has been implanted and annealed. As noted in this figthere is less implantation-induced energy shift at the higannealing temperatures. However, the overall energy swas larger after annealing at the higher temperature.FIG. 1. PL spectra measured at 10 K from proton-implanted and anne~at 750 °C for 30 s! QD samples:~a! as-grown,~b! annealed with no im-plantation, ~c! 531013 H/cm2, ~d! 131014 H/cm2, and ~e! 131015 H/cm2.Downloaded 14 Apr 2010 to 150.203.243.38. Redistribution subject to AIllisbleeiftef-i-msreane-ldrr,-ser-henere,riftelarger thermal interdiffusion at the higher temperature leto smaller differential energy shifts.The linewidth reduction in the luminescence spectraproportional to the amount of blueshift. There could be seral factors involved in the reduction of the PL linewidtThe shape of the potential profile is changed as the interbecomes graded. As-grown dots are known to have an inmogeneous In distribution, and at the areas of high In, minterdiffusion will take place. This will lead to a smootpotential profile and will also homogenize the dots. Anothfactor is that the energy shift from as-grown to interdiffusquantum wells has been shown to be dependent on the thness of the quantum well.13 This is due to the position of theenergy levels within the as-grown well with respect to tbarrier level. It follows that this would also occur in QDs ancould contribute to the narrowing of the PL spectra. Tnarrowing of the linewidth from the implanted samplesslightly less than the linewidth reduction for the sampannealed without implantation. This is expected, asbroadening due to residual defects introduced by implantion offsets the narrowing gained from the added interdifsion.The effect of different implantation temperatures walso studied. The implantations were carried out usingtwo highest doses at 20, 100, and 200 °C. The amounenergy shift seen after annealing at 750 °C for 30 s is shoin Fig. 3. As the implantation temperature increasesamount of implantation induced interdiffusion decreases. Timplantation induced blueshift is 75 meV at room tempeture, but only 19 meV at 200 °C for the highest dose31015 cm22). The luminescence intensities in the sampimplanted at the elevated temperatures are also mbrighter. Both these effects can be attributed to dynamicnealing processes occurring during the implants at elevatemperatures. This has been seen previously in InGaAs/Gquantum wells.12 The interdiffusion process is controlled bthe concentration of point defects present before anneaSince proton implantation creates dilute defect cascathere could be significant annihilation of vacancies and institials. This process occurs even at room temperature, benhanced at the elevated implant temperatures. Henceconcentration of residual point defects after elevated teedFIG. 2. The implantation-induced PL energy shift after annealing at difent annealing temperatures.P license or copyright; see http://apl.aip.org/apl/copyright.jspomthveoeconthketaterD. Ptherede-°Chansere-eryv,.Sci.ne,.ski,, M.rg,pl.ys.J.P.h,aried2055Appl. Phys. Lett., Vol. 82, No. 13, 31 March 2003 Lever et al.perature implantation is less than after implantation at rotemperature. As the degree of interdiffusion is related toconcentration of point defects, less interdiffusion is obseras the temperature is increased. Another process that coccur at elevated temperatures is the clustering of definto large clusters or loops. These would lead to large nradiative recombination centers; if they were present insamples, the intensity of the luminescence would be weaHowever, in our case, we see brighter luminescence atelevated temperatures than in the room temperature cTherefore, the reduction in interdiffusion must be relatedthe dynamic annealing occurring at the elevated temptures.In summary, the interdiffusion processes in InGaAs Qcaused by proton irradiation and annealing were studiedFIG. 3. The implantation-induced PL energy shift after implantation at vous temperatures, for different proton doses. Samples were anneal750 °C for 30 s.Downloaded 14 Apr 2010 to 150.203.243.38. Redistribution subject to AIeduldts-er.hese.oa-sLenergy shifts were found to increase with proton dose fordoses studied. It was found that the PL intensity recovemuch better in In0.5Ga0.5As QD samples than has been rported for InGaAs quantum wells. Annealing even at 750for 30 s has been found to be sufficient to recover more t20% of the original PL intensity, even for the highest doused in this study. Implantation at elevated temperaturesduced the amount of PL energy shift, but enhanced recovof the luminescence was observed.1D. Bimberg, M. Grundmann, F. Heinrichsdorff, N. Ledentsov, V. UstinoA. Zhukov, A. R. Kovsh, M. Maximov, Y. Shernyakov, B. Volovik, ATsatsul’nikov, P. Kop’ev, and Z. I. Alferov, Thin Solid Films367, 235~2000!.2A. Stiff-Roberts, S. Krishna, P. Bhattacharya, and S. Kennerly, J. Vac.Technol. B20, 1185~2002!.3R. Leon, Y. Kim, C. Jagadish, M. Gal, J. Zou, and D. J. H. CockayAppl. Phys. Lett.69, 1888~1996!.4D. Bhattacharyya, A. S. Helmy, A. C. Bryce, E. A. Avrutin, and J. HMarsh, J. Appl. Phys.88, 4619~2000!.5A. Babinski, J. Jasinski, R. Bozek, A. Szepielow, and J. M. BaranowAppl. Phys. 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Proton-irradiation-induced intermixing of InGaAs quantum dots

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