APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 1
7 JULY 2003
Photoluminescence enhancement by inductively coupled argon plasma exposure for quantum-well intermixing H. S. Djie,a) T. Mei, and J. Arokiaraj Photonics Research Group, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
共Received 6 January 2003; accepted 6 May 2003兲 The exposure of InGaAs/InGaAsP quantum-well 共QW兲 structures to argon 共Ar兲 plasma in an inductively coupled system has been studied. An increase in photoluminescence 共PL兲 intensity without PL peak shift was observed for 5-min Ar plasma exposure compared to the as-grown sample. The exposure creates point defects, and upon rapid thermal annealing produces intermixing between barriers and QWs, resulting in the blueshift of QWs. A selective intermixing using a 200-nm-thick of SiO2 layer as an intermixing mask exhibited a differential band-gap blueshift of 86 nm, with a differential linewidth broadening of 0.3 nm between masked and unmasked section. The improvement of PL intensity in combination with selective intermixing process can pave the way for high-quality hybrid photonic and optoelectronic integrated circuits. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1591063兴
The InGaAsP quaternary lattice matched to InP is an attractive and widely used system for optical fiber communications. It can be used to design a variety of components, such as 共quantum-well兲 共QW兲 lasers, optical waveguides, and Stark-effect modulators operating in the 1.55-mwavelength region. For applications in photonic integrated circuits 共PICs兲, different devices have to be integrated on a single substrate, which demand stable device performance after post-growth control of the band gap of the QW structures. Post-growth band-gap engineering through a quantum well intermixing 共QWI兲 technique has generated considerable interest due to its simplicity and effectiveness.1 QWI is a means of adjusting the band gap of a QW structure via the controlled interdiffusion of beneficial defects between the QW and barrier material, usually resulting in a blueshift of the energy gap. QWI techniques, which offer spatial selectivity of band-gap tailoring, include ion-implantationinduced disordering,2 impurity free vacancy disordering,3 focused ion beam,4 and laser-induced disordering.5 The use of plasma-induced QWI is attractive since the low-energy ions 共with hundreds of eV of ion impact energy兲 generated in the plasma chamber generates mobile point defects on the near sample surface. The high density of point defects far from the active QW region results in no direct damage to QW active region, thereby promising high-quality photonic devices for PIC applications. Initial work has been done using H2 plasma, generated by reactive ion etcher 共RIE兲 on GaAs/ AlGaAs QW structures, and a maximum wavelength blueshift of 24 nm was obtained using up to nine cycles of exposure and annealing.6 In this letter, we have investigated the QWI effect of high-density Ar plasma on InGaAs/InGaAsP QW structures generated in the inductively coupled plasma 共ICP兲 machine and band-gap shift after annealing step using photoluminescence 共PL兲 spectroscopy. In this report, we speculate on improvement in optical properties immediately a兲
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after Ar plasma exposure without the annealing step. During the subsequent annealing step, the mobile point defects are diffused through the active QW region only in the unmasked section of QW sample, and subsequently promote intermixing. The lattice-matched InGaAs/InGaAsP QW samples used in the present investigation were grown by metalorganic vapor phase epitaxy on 共100兲 oriented n ⫹ -type S-doped InP substrate with etch-pit density below 1000 cm⫺2 . The laser structure7 consists of five periods of 55-Å In0.53Ga0.47As QWs with 120-Å InGaAsP barriers. The active region is sandwiched by a step-graded index waveguide core consisting of InGaAsP confining layers. The thicknesses of these confining layers are 500 and 800 Å, respectively. The structure was completed by an upper cladding InP layer of 1.4 m with Zn doping of 5⫻1017 cm⫺3 . The contact layers consist of 500-Å InGaAsP 共Zn-doped of 2⫻1018 cm⫺3 ) and 1000-Å InGaAs 共Zn-doped of 2⫻1019 cm⫺3 ), respectively. The samples resulted in a PL peak at 1.51⫾0.02 m at room temperature 共RT兲 and 1.43⫾0.02 m at 77 K. The plasma source generator ICP180 used in this experiment was built by Plasmalab System100. The system uses inductive coil to generate high-density ‘‘remote’’ plasma with no direct contact between the plasma and the substrate. The 13.56-MHz rf and ICP power supply can provide the independent control on ion bombardment energy and ion current density with power up to 500 and 3000 W, respectively. The ICP parameter settings for the experiments were optimized by Taguchi’s method,8 with a 100-sccm Ar flow rate, 80-mTorr chamber pressure, and 480-W rf power. After Ar plasma exposure, the samples were annealed using singlestep annealing in a flowing nitrogen ambient. Two fresh pieces of GaAs proximity caps were used to provide an As over-pressure environment during the annealing process and further to prevent the sample surface from outdiffusion. The annealing conditions were determined from a thermal stability test performed on as-grown samples with and without dielectric cap layer. A control sample was annealed to deter-
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Appl. Phys. Lett., Vol. 83, No. 1, 7 July 2003
FIG. 1. 77-K PL measurements with normalized intensity with respect to the as-grown sample were performed at different Ar exposure times: 共a兲 0 min, 共b兲 annealed only, 0 min, 共c兲 2 min, 共d兲 3 min, 共e兲 5 min.
mine the thermal shift without plasma exposure. PL was then performed to assess the degree of band-gap shift and linewidth broadening. The PL measurements were carried out at 77 K and RT using an Nd:YAG laser 共1.064 m兲 for excitation, a monochromator, and a TE-cooled InGaAs photodetector associated with a SR-830 lock-in amplifier. Figure 1 shows the PL intensity of samples under different Ar exposure times without an annealing step. The small increase in PL intensity and 6-nm shift were observed for the control sample, which had undergone an annealing treatment only. During the annealing stage, the thermal diffusion of grown-in defects within the heterostructure will cause removal of some defects, agglomeration of others giving rise in small increase in PL intensity and a small shift. For samples exposed at various durations to Ar plasma 共2, 3, and 5 min兲, there were no PL shifts and only a small amount of linewidth broadening. This indicates that point defects have been introduced to samples without damaging the QW structures, which are located 1.5 m below the surface. The increase in PL intensities as a function of Ar exposure time may be at present attributed to reduction of nonradiative carrier recombination center, taking place after Ar exposure. The increase in PL intensity was even evident from the samples selected from the periphery of the samples, which had low PL intensity before exposure. Both RT and 77-K PL measurements retrieved from the back side of the exposed samples show a similar trend of the increase in PL intensity as a function of the exposure time up to 5 min. The PL intensities measured from QW samples with InGaAs/ InGaAsP cap layer and without InGaAs/InGaAsP cap layer show the similar level of PL intensity. This evidence suggests that the PL enhancement after Ar exposure is not due to the changes in the cap properties of InGaAs/InGaAsP layers at the QW sample surfaces from the highly energetic ionbombardment-induced damage. Previous studies have shown that the role of Ar-assisted epitaxy of InGaAsP/InP has greater effect on the PL intensities with reduced linewidth broadening. The plasma-assisted epitaxy, however allows the tensile strained layers to approach the quality of the lattice-matched layers by drastically reducing the lateral composition modulation, which gives rise to relatively narrow linewidths and higher PL intensity.9
Djie, Mei, and Arokiaraj
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FIG. 2. PL peak intensity changes and QWI shifts as a function of different exposure times.
Comparing the plasma-assisted epitaxy with the present postgrowth plasma exposure, we believe that the remote plasma in the ICP creates a radiation regime in the vacuum ultraviolet energy band of 4 –30 eV, where most population are located above 9 eV, during Ar plasma exposure.10 This radiation can have a pronounced effect on the thin QW structure leading to annealing out of grown-in defects, thereby enhancing the PL intensity. We argue that the improved PL efficiency is due mainly to enhanced annealing of grown-in defects since the abrupt change in growing composition between the barriers and wells usually result in a high density of point defects.11 Since the emission is from the QWs, the effect of Ar exposure does not damage the QW region, which is very important for any post-growth processing. The point defects created near to the surface region are driven into the QW by rapid thermal annealing the samples at 600 °C for 2 min. Figure 2 shows the change in PL intensity and wavelength shift of the exposed samples after annealing as a function of Ar exposure time. The created point defects are mobile in nature, causing QWI and hence the blueshifts in the PL peak. The optimum wavelength shift of 79.8 nm from RT PL was achieved after a 10-min exposure, with a similar trend for 77 K as well. The band-gap shift achieved is comparable with that achieved in Ar-plasmainduced QWI work using a conventional parallel plate RIE machine in the InGaAs/InGaAsP QW structures.12 The PL intensity increases for up to a 5-min exposure and further decreases as the exposure time increases. The consistency between the PL intensity of Ar-exposed QW samples before annealing 共in Fig. 1兲 and after annealing 共in Fig. 2兲 suggests that the mobile point defects are diffused into the QW region to promote intermixing without damaging the QW structures such that the quality of the QW layers was not degraded. The agglomeration of created point defects after annealing caused the degradation of PL intensity, and hence the saturation of PL shift was observed after a 15-min exposure. A similar trend in relative PL efficiency was also observed for rapid thermal annealing of SiO2 -encapsulated GaInAs/AlInAs heterostructures.13 Initial study of a selective QWI technique by Ar plasma was prepared using QW samples with half of the surface area deposited with a 200-nm-thick SiO2 layer. The dielectric acts as an exposure mask, which will block ions from penetrating
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Appl. Phys. Lett., Vol. 83, No. 1, 7 July 2003
Djie, Mei, and Arokiaraj
In summary, we have performed a RT and 77-K PL study on Ar-plasma-induced QWI to assess the material quality, PL intensity, and selectivity of intermixing. The increase in PL intensity after Ar exposure without annealing treatment could be due to annihilation of grown-in defects in the QW wells. The selectivity study using a patterned SiO2 mask achieved a differential PL shift of 88 nm and differential linewidth broadening of 0.3 nm. Hence, this plasma-induced QWI technique paves way for the active photonic devices to be integrated on monolithic chips, where high crystalline quality after post-processing is demanded.
FIG. 3. Selective intermixing result across a sample partially masked with a 200-nm-thick SiO2 layer. Annealing was carried out at the optimum condition below critical temperature (600 °C for 120 s兲.
the structure during exposure. Figure 3 shows the 77-K PL spectra obtained from the partially masked sample after Ar plasma exposure and subsequent annealing treatment. As can be seen from the spectrum, the same phenomena occur in PL intensity after QWI between the masked and unmasked SiO2 sections. The PL intensity is increased by three folds. The section, which was masked with a layer of SiO2 during plasma exposure, underwent a small amount of band-gap shift 共6 nm兲, which is mainly attributed to thermal shift after annealing. The portion exposed to the plasma had a much larger band-gap shift of 92 nm, thus producing a differential band-gap difference of 86 nm between masked and unmasked region with a narrow linewidth broadening of 0.3 nm. This result strongly points to the high selectivity obtainable in the QW samples using 200-nm SiO2 as a masking layer. The low ion energy Ar-plasma-induced QWI can achieve comparable or relatively higher selective band-gap shift than the implantation-induced QWI using a 30-keV Ar ion14 and 20-keV plasma immersion Ar ion15 in InGaAs/InP QWs.
1
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