Selected Topics in Quantum Electronics, IEEE Journal on - IEEE Xplore

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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 4, JULY/AUGUST 2002

Low-Energy Ion-Implantation-Induced Quantum-Well Intermixing Vincent Aimez, Member, IEEE, Jacques Beauvais, Member, IEEE, J. Beerens, Denis Morris, H. S. Lim, Member, IEEE, and Boon-Siew Ooi, Member, IEEE

Abstract—In this paper, we present the attractive characteristics of low-energy ion-implantation-induced quantum-well intermixing of InP-based heterostructures. We demonstrate that this method can fulfill a list of requirements related to the fabrication of complex optoelectronic devices with a spatial control of the bandgap profile. First, we have fabricated high-quality discrete blueshifted laser diodes to verify the capability of low-energy ion implantation for the controlled modification of bandgap profiles in the absence of thermal shift. Based on this result, intracavity electroabsorption modulators monolithically integrated with laser devices were fabricated, for the first time, using this postgrowth technique. We have also fabricated monolithic six-channel multiple-wavelength laser diode chips using a novel one-step ion implantation masking process. Finally, we also present the results obtained with very low-energy (below 20 keV) ion implantation for the development of one-dimensional and zero-dimensional quantum confined structures. Index Terms—Intracavity modulators, low-energy ion implantation, photonic integrated circuits, quantum-well intermixing, semiconductor laser diode.

I. INTRODUCTION

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HE TREMENDOUS increase in optical telecommunications, together with the development of complex dense wavelength-division-multiplexing (WDM) systems and all optical approaches, calls for the use of multiple functions devices [1], [2]. The availability of photonic integrated circuits (PICs) is a key element to meet these requirements. New fabrication techniques must be developed that will allow mass production of advanced, low-cost, reliable, fully packaged, and robust PICs. Indeed, photonic device integration provides several advantages in terms of improved performance, reliability, and cost reduction [3]. The development of active PICs on III–V heterostructures for optical telecommunication systems typically requires multiple wavelength laser sources, low-absorption passive waveguides, and semiabsorbing regions for modulation or variable optical attenuators as illustrated in Fig. 1. Selective area epitaxy [4] as well as etching and regrowth [5] techniques are being used for the fabrication of advanced PICs. Manuscript received April 2, 2002; revised May 15, 2002. V. Aimez, J. Beauvais, and J. Beerens are with the Centre de Recherche sur les Propriétés Électroniques de Matériaux Avancés and Département de Génie Électrique et Génie Informatique, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada. D. Morris is with the Centre de Recherche sur les Propriétés Électroniques de Matériaux Avancés and Département de Physique, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada. H. S. Lim is with the Photonics Research Group, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798. B. S. Ooi is with Phosistor Technologies Inc., Pleasanton, CA 94588 USA. Digital Object Identifier 10.1109/JSTQE.2002.800846.

However, both these techniques involve repeated use of expensive epitaxial growth systems whose throughput is limited, thus reducing the prospects of low-cost volume production of PICs. Quantum-well intermixing (QWI) [3], [6], [7] techniques offer the possibility to control heterostructure bandgap profiles at the postgrowth level. This can be a decisive advantage in terms of manufacturing costs. The work presented here relies on the use of a low-energy Varian DF-3000 ion implanter, widely available in the microelectronics industry, to generate point defects close to the surface of laser heterostructures, i.e., well above the active region, through ion-implantation-induced damage. Following the implantation, a rapid thermal annealing step is carried out to induce diffusion of the point defects through the active region, thus promoting QWI while partially healing the damage within the shallow implanted layer. QWI results in an increase of the bandgap, i.e., blueshifting of the associated emission wavelength. A specific advantage of this technique is linked to InP–InGaAs–InGaAsP heterostructures, where the amount of defects generated is strongly correlated to the amount of blueshifting [8], thus allowing controlled modification of the bandgap energy. Spatial selectivity of this process is obtained using implantation masks with appropriate shape and thickness. With this low-damage process, it is possible to obtain a large blueshift in implanted areas while avoiding thermal shift in masked (unimplanted) regions of the sample [9]. Some critical restrictions imposed on PIC fabrication can be described as follows [3]. 1) The control of the bandgap profiles must make it possible to obtain large modifications of the emission wavelength, i.e., 100-nm blueshifts and up. 2) The spatial control of the bandgap profiles will have to be selective and precise. In the case of ion-implantationinduced intermixing, this precision is on the order of 2 m [10]. 3) There should be zero or insignificant degradation of the intermixed material optical and electrical quality through the fabrication process. This is an especially important issue with QWI processes since they are based on an impurity generation mechanism. 4) The lifetime of the intermixed devices should not be altered by the fabrication process. 5) Unimplanted regions of integrated devices should not be affected by the processing. For ion-implantation-induced QWI, the implanted ions can be blocked with patterned dielectric masking layers. The thermal stability of the material is a very important issue for all QWI techniques; the

1077-260X/02$17.00 © 2002 IEEE

AIMEZ et al.: LOW-ENERGY ION-IMPLANTATION-INDUCED QWI

Fig. 1.

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Schematic of a typical photonic integrated component with required blueshifted bandgap control values.

unimplanted material characteristics have to remain comparable to as-grown characteristics. In this paper, we have fabricated and characterized high-quality discrete blueshifted laser diodes (LDs) to verify the integration capabilities offered by low-energy ion-implantation-induced QWI. No thermal shift occurred during the fabrication of these devices. Quantum confined Stark effect (QCSE) electroabsorption (EA) modulators fabricated on multiple-quantum-well (MQW) heterostructures were shown to allow efficient high-speed intensity modulation [11]. Monolithic integration of EA modulators with LDs on InP–InGaAs–InGaAsP heterostructures has already been reported with techniques such as butt coupling multiepitaxy regrowth [5], selective area epitaxy [4], and dielectric capping QWI [12]. The results presented here are the first demonstration of integrated laser-modulator devices operating at 1.55 m fabricated using QWI in the absence of thermal shift. These results serve as a proof of concept to demonstrate that low-energy ion implantation QWI is an excellent candidate as a method suitable for the industrial production of high-performance PICs on InP–InGaAs–InGaAsP heterostructures. A novel multiple thickness masking technique has also been developed for this QWI process allowing single multiple bandgap generation in a single implantation step. The fabrication of six-channel monolithic LDs with wavelength span over 70 nm is also demonstrated. The latter approach is offering the possibility to fabricate coarse WDM components. Finally, an alternative high-resolution ion-implantation-induced QWI process under development, compatible with DFB-first growth heterostructures, is presented. In this approach, ions are implanted at very low energy within a sacrificial layer close to the active region in order to minimize defect diffusion lengths. Some essential characteristics, such as material quality preservation and large blueshifts, are demonstrated, thus, possibly allowing the fabrication of one-dimensional (1-D) and zero-dimensional (0-D) quantum confined structures. II. QUANTUM WELL INTERMIXING The InGaAs–InGaAsP laser heterostructure, with low etch pit density (below 1000 cm ) and, therefore, a high thermal stability [13], used for the active devices discussed here, was grown by metal–organic chemical vapor deposition (MOCVD) on a (100)-oriented n -type, S-doped InP substrate. The active region consists of five 55-Å InGaAs wells with 120-Å InGaAsP barriers, bounded by a stepped graded index (GRIN)

waveguide core with InGaAsP confining layers (from the QWs outward) with thicknesses of 500 Å ( m) and m), respectively. The structure, which 800 Å ( was lattice-matched to InP, was completed with a 1.4- m InP upper cladding layer doped with Zn at a concentration of 6 10 cm and a 200-nm InGaAs contact layer Zn-doped 2 10 cm . The lower cladding layer was Si-doped at a concentration of 2 10 cm . This structure has a nominal emission wavelength of 1600 nm at room temperature. The implantation of the heterostructures was carried out with 360 keV ions of electrically neutral species, i.e., either phosphorus or arsenic. The samples were tilted at a 7 angle during implantation in order to reduce channeling effects. 10 ions cm and 1 10 ions cm Doses of 1.5 were selected. The sample temperature was elevated to 200 C throughout implantation in order to avoid the formation of defect aggregates which have higher diffusion activation energy [9]. TRIM [14] simulations (see Fig. 2) were performed to verify the depth of penetration of the implanted ions, as well as the distribution of induced vacancies. Knowing that the active region and GRIN layers are situated at 1550 nm from the surface, we can see that even in the case of phosphorus implantation, there is almost no implantation damage within the first 900 nm above the active region and GRIN layers. Following ion implantation of the devices, rapid thermal annealing was carried out at 650 C using a silicon substrate as a heating element [15] for 120 s, except for the multiple wavelength laser devices which were annealed at 590 C in a lamp heating rapid thermal annealing (RTA) for 120 s. These annealing conditions do not exceed the thermal stability limit of the material, which is important in order to avoid thermally induced QWI. III. DISCRETE BLUESHIFTED LASER DIODES Discrete blueshifted LDs were fabricated in order to verify low-energy ion-implantation-induced QWI capability for PIC fabrication. Two different implantation doses were used in order to get, first, small bandgap shifts in the suitable range for multiple laser sources, and second, large bandgap blueshifts appropriate for low-loss waveguides. In each case, reference material was annealed together with the implanted samples to verify that no thermal shift was induced by the process. The LDs were tested under pulsed current conditions at room temperature. 10 arsenic The first samples were implanted with 1.5 ions cm . The – curves from the processed samples are shown in Fig. 3. We can see that the curves from implanted

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(a)

(b) Fig. 2. (a) TRIM simulation of As and P ion implantation impurity range distributions. (b) TRIM simulation of As and P ion-implantation-generated vacancies distribution. Incident ion energy of 360 keV was assumed in the calculation.

Fig. 3. Light-intensity curves from oxide stripe lasers fabricated on material implanted with 1.5 10 arsenic ions1cm and annealed at 650 C for 120 s and from annealed-only devices. The threshold current density of the devices is 1.18 kA1cm .

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annealed and annealed-only devices are almost identical indicating that the threshold current density of the intermixed devices have not been altered by the QWI process. The slope of these curves is almost the same in both cases, which gives an

Fig. 4. Spectra from oxide stripe lasers fabricated on material implanted with 1.5 2 10 arsenic ions1cm and annealed at 650 C for 120 s and from annealed-only devices. The implanted samples emission wavelength is blueshifted by 26 nm.

indication that the external quantum efficiency of the discrete LDs has not been affected by the processing. The threshold current density ( th) of the implanted and/or annealed LDs is 1.18 kA cm , the same as that of reference lasers fabricated on as-grown material. These results are very encouraging since they show that there is no degradation of the device characteristics after going through this QWI process. This implies that the ion implanted damage has been almost fully healed during the rapid thermal annealing step. The spectra of the diodes are shown in Fig. 4. From this figure, we can see that the emission wavelength of the annealed-only samples is centered at 1548 nm, i.e., no thermal shift occurred. Indeed, the reference material is designed to operate at 1550 nm, the 2-nm difference is explained by the multimode operation of these oxide stripe lasers. The implanted and annealed LD emission wavelength has been blueshifted by 26 nm. Taking into account the broadband efficiency of MQW heterostructures (5–10 nm), this 26-nm bandgap tuning range allows full coverage of the ITU’s C-band grid. Noel et al. have reported aging tests on 30-nm blueshifted LDs fabricated using similar ion implantation energy with predicted lifetimes in excess of 25 years [16]. In the second set of devices, the implantation dose of arsenic 10 arsenic ions cm . The – ions was increased to 1 curves from these samples, including as-grown, are shown in Fig. 5. We can see that the implanted sample threshold current has increased when compared with the as-grown and annealed-only samples, while the slope efficiencies from all – curves remain comparable. The threshold current density of the as-grown and annealed-only LDs is 1.18 kA cm . In the case of the implanted samples, the threshold current density has increased to 1.35 kA cm . The increase in th is limited to 15%, indicating that high material quality has been well preserved through the QWI process. This increase can be explained in part by the loss of two-dimensional (2-D) confinement after QWI of the active region of the devices [7] in addition to implantation damage not fully treated during annealing. A slight reduction of the ion implantation dose and an increase of the ion implantation temperature would presumably limit further the increase in th. Phosphorus ions could also be used instead of arsenic; this would lower the maximum concentration of defects within


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Fig. 7. Top view of integrated device with wire bonds. Section A (1 mm long) corresponds to the laser section, with an 40-m-wide longitudinal oxide stripe contact window. Section B is a 100-m-long low-loss waveguide that provides electrical isolation between Sections A and C. Section C (600 m long) is the modulator section, covered by a broad area contact. Fig. 5. Light-intensity curves from oxide stripe lasers fabricated on material implanted with 1.5 10 arsenic ions1cm and annealed at 650 C for 120 s, from annealed-only, and from as-grown devices. The threshold current density of the unimplanted devices is 1.18 kA1cm , the implanted annealed LDs threshold current density is 1.35 kA1cm .

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Fig. 6. Spectra from oxide stripe lasers fabricated on material implanted with 1 2 10 arsenic ions1cm and annealed at 650 C for 120 s, from annealed-only and from as-grown devices.

the material [see Fig. 2(b)]. The spectra from these diodes are shown in Fig. 6. From this figure, we can see that the emission wavelength from the as-grown and annealed-only lasers are essentially identical. This is once again a demonstration that no thermal shift occurred during processing. The implanted LD emission wavelength has been blueshifted by 100 nm down to 1447 nm. The material quality of these active devices and the emission wavelength blueshifting obtained after QWI indicates that high-quality low-loss passive waveguides could be fabricated using these processing conditions. IV. MONOLITHIC LASER-MODULATOR DEVICES The integrated devices fabricated include three main regions, as illustrated in Fig. 7. Region A consists of the (unshifted) laser section, where a 40- m-wide oxide stripe contact window was defined. Region B is a 100- m-long blueshifted low-loss waveguide, which was used for electrical isolation between the laser and the modulation sections. Region C corresponds to the modulator itself, which was blueshifted as Region B and also equipped with a top contact for biasing. The integration of an efficient EA modulator with a laser requires the absorption edge of the modulator section to be selectively blueshifted [12] in such a

Fig. 8. Electroluminescence versus wavelength from (i) integrated laser modulator device, with the laser biased above threshold (40 m oxide window stripe contact), (ii) integrated laser modulator device, with the laser biased below threshold (40 m oxide window stripe contact), and (iii) a single, standalone modulator (broad area contact).

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way that the optical loss at 1.55 m, in the absence of electrical pumping, is strongly reduced in that region [17], [18]. The ion implantation and annealing conditions were similar to those used for discrete devices except for the dose which was set to 3 10 ions cm , and the species selected, phosphorus in this case, in order to limit the maximum defect concentration within the upper cladding layer, thus keeping high optical quality of the material. Details of the devices fabrication are given elsewhere [19]. The samples were tested under pulsed-current conditions at room temperature. The spectra obtained from standalone modulators with bias applied on the broad area contact, and from integrated devices (detailed below), with bias voltage applied to the laser contact only, are illustrated in Fig. 8. The integrated devices lasing wavelength is at 1544 nm, i.e., at the center of the spectra for the same device below threshold, while the emission wavelength blueshift obtained for the modulator section of the devices with respect to the unimplanted gain section is close to 75 nm. Integrated devices with 1000- m-long current injection region and 600- m-long modulator section exhibited a threshold current density th 1.08 kA cm . This low value of th, comparable to values obtained on standalone oxide stripe lasers made out of the as-grown heterostructure when taking into account the

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Fig. 9. Relative light intensity output at 1.55 m from integrated devices as a function of bias voltage applied to modulator top contact. The slope of this curve corresponds to 1.95 dB/V/100 m.

difference in cavity length for as-grown devices detailed in Section III, indicates that the material quality is not significantly affected by the processing. It also shows that in the absence of bias, the blueshifted modulator section acts as a low-loss waveguide. In order to assess the modulation depth of the EA section, the laser injection current of the integrated device was fixed below threshold and electroluminescence spectra were obtained for various bias voltages of the modulator section. We note here that at a bias voltage of 0.75 V, a leakage current of less than 0.5% of the lasing threshold was measured in the modulator section. The intensity output from the device as a function of applied bias voltage on the modulator contact was acquired at 1550 nm (Fig. 9). These results show that for a bias voltage swing between 0.25 V and 0.75 V, the ON–OFF attenuation ratio below lasing threshold is close to 12 dB. The attenuation capacity per unit length extrapolated from this graph is close to 1.95 dB/V/100 m. This value is of the same order as previously published results on electroabsorption modulators fabricated by QWI [12], [20]. This electroabsorption coefficient could be greatly improved by the use of specially designed heterostructures, comprising a larger number of QWs. We have carried out preliminary QWI tests on 30 QW structures and photoluminescence results show that the low-energy ion-implantation-induced QWI process gives excellent results as well on these materials. Furthermore, the compatibility of our fabrication method with DFB first growth heterostructures [21] shows that PIC incorporating DFB gratings could also be fabricated. Also, the fabrication of ridge waveguide lasers on DFB substrates prepared with this QWI method would allow the assessment of devices in a high data rate configuration. V. MULTIPLE BANDGAP GENERATION PROCESS During ion implantation, the ions may be blocked in selected regions using a patterned silicon dioxide layer on the surface of the material [3], [16], [19], as in the case of laser sections within laser-modulator devices described previously. By implanting through a thin enough layer of masking material, a reduced number of ions can reach the heterostructure surface; the effective average dose and energy of the ions going through the

Fig. 10. SiO implantation mask thickness and monolithic lasers emission wavelength versus photomask transmittivity indexed from level 1, darker region, to level 10, clearer region. From this figure, it is possible to detect seven clearly different SiO thicknesses.

masking layer will then depend on its thickness. Using this approach, it is therefore possible to obtain bandgap profiles with a wide range of values using a single implantation step [3], [16], [19], [22], due to the close relationship between the amount of point defects and the resulting blueshift associated with InPbased heterostructures [8]. From the fabrication point of view, only one ion implantation step is necessary and all the required photolithographic steps can be performed successively. However, the requirements on the mask preparation involves repetitive photolithography and etching steps. In order to circumvent this tedious multiple step fabrication process required for complex structures, we have developed a novel approach based on gray mask photolithography and dry etching [23]. Following the definition of the graded thickness SiO mask, ion implantation of arsenic or phosphorus ions is carried out in a similar way as for discrete devices. The implantation mask is then completely removed, and rapid thermal annealing is carried out to induce the multiple bandgap profiles according to the mask thickness. According to TRIM [14] calculations, it is estimated that a 450-nm-thick SiO layer is required to completely block 360-keV arsenic ions. Alignment marks were first etched into the top contact layer of the structure, including 20- m trenches between individual lasers in order to simplify further processing. A 450-nm-thick SiO layer was then deposited by PECVD onto the samples, followed by a 690-nm-thick photoresist layer. Using specifically designed photomasks with varying opacity (ten levels), a multiple thickness photoresist pattern was defined onto the samples after UV exposition and resist development. This multiple thickness pattern was transferred into the underlying SiO layer through reactive ion etch (RIE) dry etching using a CF –O mixture with 1 : 0.8 selectivity between photoresist and SiO . Details of this three-dimensional (3-D) SiO pattern definition can be found elsewhere [23]. The variable thickness of the SiO mask is illustrated in Fig. 10. In this experiment, 360-keV 1 10 arsenic ions cm were implanted into the structure through the varying thickness SiO implantation mask using similar ion implantation conditions as for discrete devices. Samples were then annealed for 120 s at 590 C using a conventional lamp heating RTA. The SiO mask was etched away after annealing using a buffered oxide etch aqueous solution. Monolithic PICs holding ten lasers each were


Fig. 11. devices.

Multiple wavelength emission spectra from the monolithic laser

then fabricated from this material. Oxide stripe lasers were then fabricated with a 50- m-wide top electrical contact, and the cavity length was set to 500 m by cleaving. The top electrical contact was lifted off over the 20- m etched contact layer corresponding to alignment marks to ensure electrical isolation of individual devices. After monolithic fabrication of the multiple wavelength laser (MWL) chips, individual lasers were cleaved from the rows for light–current – and spectroscopy measurements. The – characteristics were measured and the lasing spectra recorded using a spectrum analyzer. The samples were tested under pulsed current conditions at room temperature. The lasers were pumped individually at just above threshold current to assess the lasing wavelength. The MWL emission wavelength has been reported into Fig. 10 for comparison with the SiO ion implantation mask thickness. From this figure, it can be noticed that six groups of lasing wavelengths (namely 1.555, 1.544, 1.535, 1.518, 1.503, and 1.484 m), separated by at least 9 nm from each other were obtained. The multimode lasing spectra devices from within each group are illustrated in Fig. 11. The – characteristics of the multiple wavelength lasers were measured and the threshold currents and the slope efficiencies were then analyzed. Fig. 12 shows the – curves for samples with the least intermixed (Laser 1: 400 nm SiO mask) and the most intermixed (Laser 10: without SiO mask) lasers. The threshold current density increase for these devices is close to 25%, slightly higher than for discrete devices implanted with similar conditions; this higher th can be explained by the lower annealing temperature in the case of MWL which might not have been sufficient for higher defect density removal. However, the slope efficiency shows very little change, which indicates that the material quality remains high after intermixing using this technique. The relationship between ion-implantation mask thickness and emission wavelength blueshift is plotted in Fig. 13. This graph further confirms the close relationship between the point defect concentrations generated within the samples and the degree of intermixing, thus showing the possibility to control QWI across a wafer through SiO mask thickness. Further optimization of the gray mask parameters will allow better control of the actual SiO ion-implantation mask thickness after pattern transfer. We believe that this fabrication ap-

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Fig. 12. L–I characteristic curves for Channel 1 (region with full oxide and no blueshift) and Channel 10 (region with no oxide and most-blueshifted) from the multiple wavelength lasers.

Fig. 13. MWL emission wavelength blueshift versus SiO ion-implantation mask thickness.

proach could be used for the production of low-cost monolithic coarse wavelength-division-multiplexing (CWDM) sources. VI. HIGH SPATIAL RESOLUTION PROCESS It has been demonstrated that quantum wire and quantum dot semiconductor heterostructures exhibit highly attractive characteristics thanks to their specific density of states [24]. Devices fabricated on such structures are expected to have extremely low threshold current densities, very low emission wavelength drift with temperature, and high spectral purity. However, fabrication of 1-D and 0-D confinement laser devices for room temperature operation is a complex task, as the required dimensions are typically smaller than 50 nm [24], [25], [26]. Several methods have been studied for the realization of 1-D or 0-D structures for photonic device fabrication. A lot of interesting results have been obtained using self assembled growth techniques using InAs–GaAs heterostructures [26], which do not require nanolithography. However, self assembled techniques offer only a partial control of quantum structure spatial localization and size. Up to now, results obtained with these techniques did not reach theoretical expectations [27], and most of the material systems used so far do not allow operation around 1.55 m. Therefore, new approaches are being intensively developed. Results obtained with QWI of laser heterostructures show that large blueshifts can be obtained while keeping high ma-

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terial quality. With a process having an adequate spatial resolution and capable of increasing the total bandgap by values close to 100 meV, it would be possible to create 1-D and 0-D quantum confined structures with predetermined size and distribution. To achieve this purpose, the dimension of the quantum confined structures following the QWI process has to be much lower than 2 m, ideally below 50 nm, for room temperature operation [25], [28]. Recent advances in nanolithography allow the definition of high-aspect ratio structures that could serve as ion-implantation masks [29], [30]. In order to develop a high-resolution quantum well intermixing process (HRQWI), there is a list of requirements that have to be fulfilled. 1) The bandgap increase has to be sufficiently high to obtain quantum confinement. 2) The high-quality optical characteristics of the material has to be conserved through processing. 3) The morphologic surface quality of the material following HRQWI has to remain very high in order to allow the eventual regrowth of upper cladding layers (in the case of lasers for instance). 4) The spatial resolution of the process has to be on the order of 100 nm. The results presented here fulfill the first three requirements while allowing further work to be carried out in order to assess the fourth requirement. For the early development of the HRQWI process, we have used a specific single quantum well heterostructure. The lattice-matched structure was grown by MOCVD on a (100) semi-insulating InP substrate in order to allow transport measurements to be carried out. The active region held a 5.5-nm single quantum well bounded by (from m), a 10-nm the QW upwards) a 40-nm InGaAsP ( InGaAs n-doped contact layer, and a 70-nm sacrificial InP layer. The lower cladding layers (from the QW downward) m), a 50-nm consisted of a 12-nm InGaAsP ( m), 80-nm InGaAsP ( m), InGaAsP ( and a 1- m InP buffer layer. The thickness of the top InP layer (70 nm) has been set on the basis of TRIM simulations, and is sufficiently thick to absorb all direct implantation damage for 18-keV phosphorus ions. Thus, in a very low energy configuration, it is possible to etch out this most severely damaged layer, which then acts as a sacrificial layer following the implantation-annealing process. Given the large amount of blueshift required for quantum confinement, phosphorus ions were implanted at a dose of 5 10 ions cm , with energies from 18 keV up to 180 keV. TRIM simulations were carried out to study the range and maximum depth of phosphorus implanted ions. The results obtained are depicted on a schematic of the heterostructure, shown in Fig. 14. From this figure, it can be confirmed that at 18 keV all ion-implantation damage is restricted within the sacrificial layer, while maximum defect concentrations are centered within the quantum well for 120 keV, and below the QW for 180 keV. Following ion implantation of test samples with energies of 18, 120, and 180 keV, rapid thermal annealing was carried out at 650 C for 120 s. In order to study the material characteristics and, more specifically, carrier lifetime following processing, time-resolved photoluminescence spectroscopy

Fig. 14. Schematic of range and maximum depth of phosphorus ions implanted at various energy into the single quantum well heterostructure according to TRIM simulations.

Fig. 15. Carrier lifetimes extrapolated from time-resolved photoluminescence spectroscopy measurements for reference and implanted samples.

(TRPL) was carried out on implanted samples. A Ti : sapphire laser was used as the excitation source, while an up-conversion detection technique with subpicosecond resolution and a GaAs photomultiplier tube were used for signal detection. The PL data was obtained at a sample temperature of 77 K with an excitation beam power of 20 mW. From Fig. 15, it is clear that the carrier lifetime for samples implanted at 120 and 180 keV has been reduced by almost one order of magnitude, indicating the presence of non radiative recombination centers due to the processing. However, blueshifted samples implanted at 18 keV do not show any significant degradation of carrier lifetime, suggesting that the number of nonradiative recombination centers is kept small. The photoluminescence analysis, shown in Fig. 16, indicates that the samples implanted at 18 keV have been blueshifted by 95 meV with respect to annealed-only samples, a value that would enable the additional quantum confinement needed in 0-D and 1-D devices. With an acceleration energy of 18 keV, the implanted ion implantation damage generates point defects within a small volume, thus minimizing straggling effects close to the quantum well, in order to limit the diffusion length as


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confirming the absence of detectable defects at shallow depth. The combination of low-energy implantation annealing with the use of a sacrificial layer, therefore, appears to provide the appropriate conditions for the fabrication of quantum confined structures. VII. CONCLUSION

Fig. 16. Photoluminescence spectra from reference, annealed-only, and 18-keV phosphorus ion implanted and annealed samples.

Fig. 17. Raman spectra from reference, 18-keV phosphorus ion implanted and annealed samples following sacrificial layer wet etch.

required for HRQWI. The ion straggling associated with 18 keV phosphorus ions implanted within this heterostructure is about 13 nm according to TRIM calculations [14]. Taking into account lateral diffusion during rapid thermal annealing [10] through this structure, with the QW located 120 nm below the surface, we would expect the spatial resolution under these conditions to be close to 150 nm. Under these processing conditions, the defect density generated within the sacrificial layer exceeds the amorphization threshold. The selective wet-etch removal of the sacrificial layer (using H PO –HCl) following rapid thermal annealing is used to get rid of this damaged layer as well as to recover a good surface morphology for further regrowth steps to take place in good conditions [21]. The surface quality of the blueshifted samples following the sacrificial layer wet etch was verified using Raman spectroscopy. The Raman spectra, shown in Fig. 17, were obtained at room temperature in the backscattering configuration with a 612-nm HeNe laser excitation source. Under these conditions, the Raman signal is generated within a depth of 50 nm into the material, i.e., directly within the potentially damaged area. From this graph, it can be noticed that spectra from reference and blueshifted devices are essentially identical, thus, further

We have shown that low-energy ion-implantation-induced QWI is an attractive method for PIC fabrication. The characteristics ( th and slope efficiencies) of LDs moderately blueshifted (i.e., by about 30 nm) are identical to that of devices from as-grown material, thereby demonstrating that the implantation process is not detrimental to the device quality. Using high implantation doses, the emission wavelength of QWI processed samples has been blueshifted by 100 nm while devices that were only annealed (without ion implantation) did not suffer any thermally induced shift. For the first time using this postgrowth QWI process, successful monolithic integration of a laser device and a QCSE modulator was achieved, with an attenuation depth close to 12 dB/V at 1550 nm. A novel one-step multiple bandgap QWI approach, based on gray mask photolithography, has also been demonstrated. With this technique, six-channel monolithic multiple wavelength lasers with a wavelength span over 70 nm have been fabricated. We believe that low-energy ion-implantation-induced QWI is a potential, industrially transferable option for the realization of active monolithic coarse WDM components. Finally, new developments on a HRQWI process, using very low-energy ion implantation has been reported. This approach fulfills essential requirements, such as high blueshifts, carrier lifetime conservation and capability for regrowth of cladding layer, for the realization of controlled quantum confined structures. Further work is currently under way for the optimization of the HRQWI spatial resolution, and the fabrication of devices that include such structures. ACKNOWLEDGMENT The single quantum well heterostructure was obtained through a collaboration with J. H. Marsh and A. C. Bryce of the University of Glasgow, as part of the CERION project. The authors would like to thank J. Corbin for technical assistance regarding the evaporation of metal contacts and P. Lafrance for technical assistance with the ion implantation. They also thank S. Jandl for providing access to the Raman spectroscopy set up. Finally, the authors are grateful to Dr. Y. L. Lam and Dr. Y. C. Chan for providing the fabrication and measurement facility for the development of MWL. REFERENCES [1] A. Girard, “DWDM spurs all-optical networks,” Eval. Eng., pp. 30–36, Oct. 1999. [2] E. Modiano, “Traffic grooming in WDM networks,” IEEE Commun. Mag., vol. 39, pp. 124–129, 2001. [3] S. Charbonneau, E. S. Koteles, P. J. Poole, J. J. He, G. C. Aers, H. Haysom, M. Buchanan, Y. Feng, A. Delage, F Yang, M. Davies, R. D. Goldberg, P. G. Piva, and I. V. Mitchell, “Photonic integrated circuits fabricated using ion implantation,” IEEE J. Select. Topics Quantum Electron., vol. 4, pp. 772–793, July–Aug. 1998.

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Vincent Aimez (S’98–M’00) received the B.Sc. degree in applied physics with microelectronics and computing from the University of Kingston, Kingston, U.K., in 1996, and the M.Sc. and Ph.D. degrees in electrical engineering from the Université de Sherbrooke, Quebec, Canada, in 1998 and 2000, respectively. His work involved quantum-well intermixing using low energy ion implantation for optoelectronic device integration. Since fall 2000, he has been Assistant Professor in the Electrical Engineering Department at the Université de Sherbrooke, Quebec, Canada. His research interests include novel micro/nanofabrication techniques applied to hybrid photonic integrated circuits.

Jacques Beauvais (M’00) received the B.Sc. and M.Sc. degrees in physics from the University of Ottawa, Canada, in 1985 and 1987, respectively, and the Ph.D. degree in physics from the Université Laval, Quebec, Canada, in 1990. His work at Laval involved a study of the effects of ion implantation on tellurium thin films for optical information storage. He was a Postdoctoral Fellow at the University of Glasgow, Glasgow, Scotland, for two years, working on the fabrication of quantum wires and diode laser fabrication and characterization. He joined the Department of Electrical and Computer Engineering at the Université de Sherbrooke, Quebec, Canada, in 1993, where he is now Full Professor. His current research interests are concerned with optoelectronic device fabrication using quantum-well intermixing, nanolithography techniques for electronics and photonics applications, as well as development of new lithographic techniques for sub-100-nm device fabrication.

J. Beerens received the B.Sc. degree in engineering physics from the École Polytechnique de Montréal, Montréal, Canada, in 1979, and the M.Sc. and Ph.D. degrees in physics from the Université de Sherbrooke, Québec, Canada, in 1981 and 1986, respectively. During his graduate studies, he spent several years in Toulouse and Grenoble, France, working on the transport properties of III–V heterostructures under hydrostatic pressure and high magnetic fields. After a postdoctoral stay at the CNET in Bagneux, France, on MBE, he joined the Université de Sherbrooke in 1988, where he is now a Research Scientist in the Electrical Engineering Department. His current interests include nanofabrication as well as transport and optical properties of semiconductor nanostructures.


Denis Morris received the Ph.D. degree from the Université de Montréal, Montréal, Canada, in 1990. He received a postdoctoral fellowship for a two-year project done at the CNET France Telecom, where he worked on hot carrier relaxation in semiconductor quantum wells. He has been a Professor at the Université de Sherbrooke, Quebec, Canada, since 1993. His research activities deal with the physics of ultrafast phenomena occurring in solids, particularly in semiconductor nanostructures.

H. S. Lim (S’97–M’00) received the B.Eng. and M.Eng. degrees from the Department of Electrical and Electronics Engineering, Microelectronics Division, Nanyang Technological University, Singapore, in 1998 and 1999, respectively. His Master’s dissertation was on the fabrication of InP–InGaAsP-based photonics integrated circuits with the combination of both grey-mask and implantation-induced quantum-well-intermixing process techniques. He is currently with DenseLight Semiconductors Pte. Ltd., Singapore, working on process and device development for InP-based photonic devices.

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Boon-Siew Ooi (M’95) was born in Penang, Malaysia, in 1967. He received the B.Eng. and Ph.D. degrees (First Class Honors) from the Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow, Scotland, in 1992 and 1994, respectively. His Ph.D. dissertation was on the fabrication of GaAs–AlGaAs photonic integrated circuits (PICs) using quantum-well intermixing. He was subsequently a Postdoctoral Research Assistant at the same university working on III–V based PICs before joining Nanyang Technological University (NTU), Singapore, as Assistant Professor in July 1996. In NTU, he supervised a group of eight researchers studying integration processes for InP-based photonic devices and plasma etching of III–V semiconductors. In May 2000, he joined Phosistor Technologies Inc., Pleasanton, CA. He is responsible for the development of process technology for InP-based photonic devices at Phosistor. His research interests include optoelectronic integration using quantum-well intermixing and nanofabrication of III–V semiconductors. Dr. Ooi is a member of the Institute of Physics (U.K.) and is a Chartered Physicist (U.K.).

Selected Topics in Quantum Electronics, IEEE Journal on - IEEE Xplore

Selected Topics in Quantum Electronics, IEEE Journal on - IEEE Xplore

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