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GaAs multiple quantum well and superlattice structures for optical modulators
PII:
Solid-State Electronics Vol. 42, No. 2, pp. 263±267, 1998 Copyright # Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1101/98 $19.00 + 0.00 S0038-1101(97)00223-2
MOCVD GROWTH OF InxGa1 ÿ xAs/GaAs MULTIPLE QUANTUM WELL AND SUPERLATTICE STRUCTURES FOR OPTICAL MODULATORS S. HASENOÈHRL1, M. KUCÏERA1, J. NOVAÂK1, M. BUJDAÂK2, P. ELIAÂSÏ1 and R. KUÂDELA1 Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Republic 2 Slovak Technical University, Bratislava, Slovak Republic
1
(Received 6 January 1997; in revised form 1 May 1997) AbstractÐSemiconductor epitaxial structures were designed for use in an optical modulator in the 980 nm wavelength region. Strained InxGa1 ÿ xAs/GaAs superlattices and multiple quantum wells were chosen for the optically active part of the modulator as they can be tailored for the desired working wavelength. The optically active region was embedded in a PIN photodiode structure. Test InxGa1 ÿ x As/GaAs structures were prepared by low pressure metalorganic chemical vapor deposition (LPMOCVD). The InxGa1 ÿ xAs composition was determined by X-ray diractometry, and it was con®rmed by photoluminescence spectroscopy. Photore¯ectance spectroscopy was employed to determine energy level positions in the quantum wells. The quality of the samples was checked by photoluminescence. Photocurrent spectroscopy was used for the characterization of electro-optical properties of the structures. Their suitability for optical modulation was discussed. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Advance in optical signal processing by using semiconductor devices is a prerequisite for a further development in optical networks. Materials with suitable optical non-linearity at room temperature are necessary for the fabrication of optical switches, modulators, and signal ampli®ers. Quantum-well structures are better suited for optical signal processing than bulk semiconductors. They exhibit stronger non-linearity eects, which are observable at room temperature[1]. Furthermore, the absorption edge of a quantum-well structure can be tuned in a wide wavelength range by changing its composition and geometry. In electrooptical modulators and switches the refractive index and the absorption coecient of the optically active region are altered by an external electric ®eld. Absorption spectra of quantum-well structures change signi®cantly with a constant electric ®eld as the result of the quantum-con®ned Stark eect. Multiple quantum-well structures show a dierent absorption behavior than superlattices when an external electric ®eld is applied to them. The absorption edge of a multiple quantum-well structure is shifted to a lower energy due to the ®eld-induced polarization of bound electron and hole eigenstates. On the contrary, the absorption edge of a superlattice shifts towards a higher energy. This is caused by the ®eld-induced suppression of tunnelling between adjacent quantum wells of the superlattice. Hence transitions between bot263
toms of minibands in the superlattice are suppressed in favor of transitions between corresponding energy levels in the quantum wells[2]. Strained InGaAs/GaAs heterostructures have been investigated for optoelectronic applications in the near-infrared region. It was recently reported that low-pressure MOCVD can be successfully used to produce high-quality InGaAs/GaAs multiple quantum wells[3]. The aim of this work is the preparation of test epitaxial structures based on the InxGa1 ÿ xAs/ GaAs quantum-well system and the veri®cation of their suitability for the optically active region of an optical modulator working near 980 nm. We report on the design, growth, and characterization of multiple quantum-well (MQW) and superlattice (SL) InxGa1 ÿ xAs/GaAs structures embedded in a PIN photodiode. The test structures consist of 10 InxGa1 ÿ xAs±GaAs pairs. It is assumed the ®nalized optically active region will contain more pairs of this quantum-well system. The modulator has been developed for use in a ®bre optical measurement system. It is operated in a front-end mode, i.e. input and output signals to and from the modulator are guided through a single optical ®bre. A Ti:sapphire laser is used as the source of continuous wave light. It is directed through the optical ®bre to the modulator, modulated using a bias circuit, re¯ected, and collected into the same ®bre.
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Fig. 1. In content xIn in InxGa1 ÿ xAs as a function of the ratio between partial vapor pressures of TMIn and TMGa in the reactor. At xIn=0.532, InxGa1 ÿ xAs is lattice matched to InP. The dependence can be ®tted by xIn/ (1 ÿ xIn) = 0.029 + 0.636(pTMIn/pTMGa).
Fig. 2. Correlation between the In content xIn and the gap energy Eg of the well material. The In content xIn (x-coordinate) was determined by the X-ray measurement and Eg (y-coordinate) was obtained from 6 K photoluminescence measurements. The experimental values are compared with the dependence Eg=0.475(1 ÿ xIn)2+0.6337(1 ÿ xIn) + 0.4105 from[5].
2. EXPERIMENTAL
The InxGa1 ÿ xAs/GaAs structures were grown on Si- and Te-doped (100) GaAs substrates in a horizontal low-pressure reactor AIXTRON AIX 200. Pd-diused H2 was used as the carrier gas. The sources were AsH3, TMGa, TMIn, SiH4 and DEZn. The reactor pressure 20 hPa, growth temperature 6508C, and the total gas velocity in the reactor 2.4 msÿ1 were the growth process parameters. The growth rate was 3.2 mm/h for InxGa1 ÿ xAs. The growth rate of GaAs was varied from 2.5 to 2.8 mm/h. The partial V/III vapor pressure ratio in the reactor was 84 for InxGa1 ÿ xAs. It was varied from 108 to 118 for GaAs. Layer thicknesses were determined on thick layers grown under the same conditions as the ®nal structures. The thickness of GaAs layers was determined by weighing. The thickness of InGaAs layers was measured at cleavage by an optical microscope after selective etching in K3[Fe(CN)6]:KOH:H2O (4 g:6 g:50 ml). The In content xIn in the InxGa1 ÿ xAs was determined by X-ray diraction using a conventional double-crystal diractometer. Cu Ka irradiation in the (004) re¯ection was employed. The measurement was carried out on 1 mm-thick layers. They were grown under the same growth conditions as used for the SL and MQW growth. The results are given in Fig. 1. The dependence of xIn/(1 ÿ xIn) versus the ratio between partial vapor pressures of TMIn and TMGa in the reactor is practically linear. Figure 2 shows a correlation between Eg and xIn. Eg was obtained from photoluminescence measurements at 6 K. Photore¯ectance measurements (PR) were performed at room temperature using a 1 mW HeNe laser (632.8 nm) as the pumping source. A 50 mW tungsten halogen lamp supplied light for the probe beam. The beam was ®ltered by a quarter-meter
monochromator and directed onto a sample. A silicon photodiode detected the probe beam after re¯ection. The PR signal was measured by a standard lock-in technique. Photoluminescence measurements (PL) were carried out using a 20 mW HeNe laser (632.8 nm) as the excitation source. The samples were placed in an optical cryostat and cooled to 6 K. The PL signal was collected employing the same experimental arrangement as for the PR measurement. Photocurrent measurements (PC) were done at room temperature and 6 K. The samples were front-side illuminated. The PIN modulator structures, with the QW placed in the intrinsic region, were furnished with two ohmic contacts formed on the p and n regions. An external electric ®eld was applied perpendicular to the structures by linking the samples in series to a constant voltage source and a load resistor. Its resistance was set to a certain value in order to keep linear the dependence of the photocurrent versus light intensity. The cryostat, light source and monochromator were the same as those used for the PL and PR measurements. The signal was measured by the lock-in ampli®er. 3. RESULTS AND DISCUSSIONS
To tailor the modulator structure for the 980 nm target wavelength, theoretical simulations were carried out to select the widths of the InxGa1 ÿ xAs wells and GaAs barriers, as well as to specify the composition of the well material. The transition between the ®rst electron subband and the ®rst heavy hole subband in the well (e1±hh1) was simulated as a function of the barrier and well widths and the InxGa1 ÿ xAs composition. The computation was done for strained InxGa1 ÿ xAs/GaAs SL and MQW pseudomorphic structures with 10 well/
MOCVD growth of InxGa1ÿxAs/GaAs
265
Fig. 4. Schematic cross-section of the epitaxial structures under investigation. The GaAs 100 nm thick buer layer was undoped, with a background doping density n = 1 1015 cm.3. The n GaAs layer was 300 nm thick and Si-doped to n = 5 1016 cmÿ3. The top p+ GaAs layer was 300 nm thick and Zn-doped to p = 5 1017 cmÿ3.
Fig. 3. Transition wavelength calculations as a function of the well and barrier widths for various composition of the well material.
barrier pairs on GaAs substrate. The ¯at-band condition without external or built-in voltage was considered. The exciton binding energy was not taken into account. The material constants of strained InGaAs (Eg, me, mhh, DEc) were taken from[4]. The SchroÈdinger equation was solved using a transfer matrix method. Figure 3 presents the calculated dependence of the e1±hh1 transition wavelength versus the well width with the barrier width as the parameter. The experimental epitaxial structures consist of 10 InxGa1 ÿ xAs±GaAs pairs, which are embedded in the intrinsic region of a GaAs PIN photodiode. It is necessary to prevent the interdiusion of Zn from the p type region of the photodiode and the interdiusion of Si and Te from the substrate into the quantum-well region. Therefore the quantumwell structure (MQW or SL) is sandwiched between two ``intrinsic'' GaAs layers. The buer layer under the QW structure is 100 nm thick and the upper intrinsic layer is 300 nm thick. Figure 4 depicts a schematic cross-section of the typical epitaxial structure. Parameters of the MQW and SL structures are summarized in Table 1. The structures were ®rst characterized by PR. It is a sensitive technique both to critical-point transitions in energy spectra of electrons and holes in
bulk semiconductors and transitions between energy levels in quantum-well structures. Figure 5 shows evident oscillations in the GaAs gap-energy region (at approx. 1.42 eV), as well as the e1±hh1 transition at lower energies. The e1±hh1 energy corresponds to 960±980 nm for various samples. The measurement was performed on various positions across the wafer. The dispersion was 25 nm. Typical low-temperature PL spectra of the studied structures are shown in Fig. 6. Line (a) represents a PL spectrum of as-grown sample MO 139. Line (b) stands for a new spectrum of the sample after 500 nm of GaAs was etched o from the top
Table 1. Parameters of the SL and MQW parts of the structures Structure In content in well Well width [nm] Barrier width [nm] Type
MO 138 x = 0.15 13 3 SL
MO 139 x = 0.18 7 10 MQW
MO 140 x = 0.20 5 3 SL
Fig. 5. PR spectra of experimental samples MO 138, MO 139 and MO 140. The energies corresponding to the GaAs band gap and the e1±hh1 transition are indicated by arrows.
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Fig. 6. PL spectra of the MQW sample MO 139 at 6 K. (a) A spectrum of the whole as-grown structure. (b) The spectrum after etching away approximately 500 nm of the doped GaAs from the top.
GaAs layer. Both (a) and (b) lines show the band originating in the GaAs layers and the peaks at lower energies, where the transition e1±hh1 is supposed to give o radiation. There are dierences between (a) and (b). Compared with the ``GaAs'' band related to line (a), the ``GaAs'' band associated with line (b) is shifted to higher energies. It is narrower and a small excitonic feature appears in it at the high energy region. Luminescence from the highly-doped uppermost GaAs layer prevails in the spectrum represented by line (a). In the case of line (b) this layer is etched o and only part of the lowdoped n-GaAs layer is left. The quantum-well-related part of this spectrum is also changed. Its intensity is much higher after the etching. The spectrum consists of two overlapping peaks at 1.360 eV and 1.376 eV. The peak at 1.376 eV corresponds to the e1±hh1 transition. As regards the 1.360 eV peak, it is most probable that some impurities or defects play a role in this transition. Electro-optical properties of the structures were characterized by PC. Sample MO 139 with the MQW structure exhibits the typical quantum-con-
Fig. 7. Room temperature PC spectra of the MQW sample MO 139 at various biases.
Fig. 8. 6 K PC spectra of the SL sample MO 140 at various biases.
®ned Stark eect[3,6] in the whole temperature range up to room temperature. Figure 7 shows PC spectra of the sample measured at 300 K. They contain the e1±hh1 excitonic feature with a maximum at 963 nm and another excitonic feature around 920 nm. With increasing reverse bias the e1±hh1 absorption edge shifts to longer wavelengths, broadens, and loses its oscillator strength. This red shift is approximately 6.5 nm for bias varied from 0 V to ÿ3 V. At 975 nm the photocurrent for bias ÿ3 V is about 2.5 times higher than the photocurrent at zero bias. Spectra of SL structures are in¯uenced by the eect of Wannier-Stark localization[2]. Apart from direct e1±hh1 inter-subband transitions, the spectra also contain spatially indirect transitions that occur between adjacent wells. Hence electro-optical properties of SL-based samples MO 138 and MO 140 are qualitatively dierent compared with those of the MQW sample MO 139. Figure 8. presents PC spectra of the SL sample MO 140 at 6 K. The superlattice is in the ¯at-band state at a weak positive bias voltage. When the sample is negatively biased, the main excitonic peak e1±hh1 of the spectrum is blue shifted (about 10 nm), compared with the spectrum at the ¯at-band condition. This is a consequence of the Wannier±Stark localization: absorption between minibands is suppressed in favor of absorption between energy levels in individual quantum wells. Spatially indirect transitions appear in the spectra for intermediate bias voltages (0 V to ÿ1 V). The transition at longer wavelengths (around 915 nm) is clearly seen. At the same time the main excitonic peak develops as the localization enhances the probability of optical transitions. A further increase in bias voltage (from ÿ1 V to ÿ4 V) makes the spatially indirect transitions move and weaken, and the main excitonic peak exhibits a small red shift and broadening. Applying an electric ®eld at room temperature alters optical transitions in the SL structures much less than it does in the MQW structure. At a certain bias voltage the e1±hh1 transition is slightly
MOCVD growth of InxGa1ÿxAs/GaAs
enhanced. The electric-®eld-induced localization increases absorption at the wavelength corresponding to this transition. Apart from this observation, the broadening of the absorption edge suppresses any other electric-®eld eects, such as the blue shift or spatially indirect transitions. Electric-®eld eects observed in the spectral characteristics of the SL structures are too weak for practical use in modulator devices. As seen in Fig. 7, the MQW sample MO139 exhibits very promising room temperature electro-optical characteristics. However, in the SL structures thermal broadening results in considerable tunnelling through the barriers at room temperature. The electric-®eld-induced localization is not sucient to suppress the tunnelling satisfactorily. No typical Stark-eect shifts are observed in their spectra. On the contrary, much broader barriers isolate individual wells in the MQW structure. Hence the tunnelling is suppressed and carriers are localized even at room temperature. 4. CONCLUSION
Strained InxGa1 ÿ xAs/GaAs superlattice and multi-quantum-well epitaxial structures were designed, prepared and characterized. The aim is to realize an optimized structure for use in the optically active region of a PIN optical modulator. Energy levels of electrons and holes in the SL and MQW structures were simulated. The results were satisfactorily con®rmed by experiment. The e1±hh1 transition measured on various test samples deviates from the desired wavelength of operation of the
267
modulator. The dispersion ranges from 960 nm to 980 nm. Moreover, certain features in the PL spectra, such as the peak width and the occurrence of a transition originating in some impurity or defect, indicate that the growth process has to be further optimized. Based on the electro-optical characteristics, the MQW structure appears to be the most promising candidate for the optically active region of the modulator operated at 980 nm. The SL-based structures exhibit considerable electro-optical eects only at cryogenic temperatures. A broadening of the superlattice absorption edge at 300 K renders the SL structures ineective for optical signal processing.
REFERENCES
1. Chemla, D. S., Miller, D. A. B. and Smith, P. W., Semiconductors and Semimetals, 1987, Vol. 24, pp. 279±318, Academic Press. 2. Bastard, G., Brum, J. A. and Ferreira, R., Solid State Physics, Advances in Research and Applications, 1991, Vol. 44, pp. 316±330, Academic Press. 3. Di Dio, M., Lomascolo, M., Passaseo, A., Geraldi, C., Giannini, C., Quirini, A., Tapfer, L., Giugno, P. V., De Vittorio, M., Greco, D., Convertino, A. L., Vasanelli, L., Rinaldi, R. and Cingolani, R., J. Appl. Phys., 1996, 80, 482. 4. Kolbas, R. M., Anderson, N. G., Laidig, W. D., Sin, Y., Lo, Y. C., Hsieh, K. Y. and Yang, Y. J., IEEE J. Quantum Electron. 1988, 24, 1605. 5. Madelung, O. and Schulz, M. (Eds.), LandoltBoÈrnstein, New Series Group III, 1992, Vol. 22a, Springer-Verlag Berlin. 6. Woodward, T. K., Cunningham, J. E. and Jan, W. Y., J. Appl. Phys., 1995, 78, 1411.
Solid-State Electronics Vol. 42, No. 2, pp. 263±267, 1998 Copyright # Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1101/98 $19.00 + 0.00 S0038-1101(97)00223-2
MOCVD GROWTH OF InxGa1 ÿ xAs/GaAs MULTIPLE QUANTUM WELL AND SUPERLATTICE STRUCTURES FOR OPTICAL MODULATORS S. HASENOÈHRL1, M. KUCÏERA1, J. NOVAÂK1, M. BUJDAÂK2, P. ELIAÂSÏ1 and R. KUÂDELA1 Institute of Electrical Engineering, Slovak Academy of Sciences, Bratislava, Slovak Republic 2 Slovak Technical University, Bratislava, Slovak Republic
1
(Received 6 January 1997; in revised form 1 May 1997) AbstractÐSemiconductor epitaxial structures were designed for use in an optical modulator in the 980 nm wavelength region. Strained InxGa1 ÿ xAs/GaAs superlattices and multiple quantum wells were chosen for the optically active part of the modulator as they can be tailored for the desired working wavelength. The optically active region was embedded in a PIN photodiode structure. Test InxGa1 ÿ x As/GaAs structures were prepared by low pressure metalorganic chemical vapor deposition (LPMOCVD). The InxGa1 ÿ xAs composition was determined by X-ray diractometry, and it was con®rmed by photoluminescence spectroscopy. Photore¯ectance spectroscopy was employed to determine energy level positions in the quantum wells. The quality of the samples was checked by photoluminescence. Photocurrent spectroscopy was used for the characterization of electro-optical properties of the structures. Their suitability for optical modulation was discussed. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
Advance in optical signal processing by using semiconductor devices is a prerequisite for a further development in optical networks. Materials with suitable optical non-linearity at room temperature are necessary for the fabrication of optical switches, modulators, and signal ampli®ers. Quantum-well structures are better suited for optical signal processing than bulk semiconductors. They exhibit stronger non-linearity eects, which are observable at room temperature[1]. Furthermore, the absorption edge of a quantum-well structure can be tuned in a wide wavelength range by changing its composition and geometry. In electrooptical modulators and switches the refractive index and the absorption coecient of the optically active region are altered by an external electric ®eld. Absorption spectra of quantum-well structures change signi®cantly with a constant electric ®eld as the result of the quantum-con®ned Stark eect. Multiple quantum-well structures show a dierent absorption behavior than superlattices when an external electric ®eld is applied to them. The absorption edge of a multiple quantum-well structure is shifted to a lower energy due to the ®eld-induced polarization of bound electron and hole eigenstates. On the contrary, the absorption edge of a superlattice shifts towards a higher energy. This is caused by the ®eld-induced suppression of tunnelling between adjacent quantum wells of the superlattice. Hence transitions between bot263
toms of minibands in the superlattice are suppressed in favor of transitions between corresponding energy levels in the quantum wells[2]. Strained InGaAs/GaAs heterostructures have been investigated for optoelectronic applications in the near-infrared region. It was recently reported that low-pressure MOCVD can be successfully used to produce high-quality InGaAs/GaAs multiple quantum wells[3]. The aim of this work is the preparation of test epitaxial structures based on the InxGa1 ÿ xAs/ GaAs quantum-well system and the veri®cation of their suitability for the optically active region of an optical modulator working near 980 nm. We report on the design, growth, and characterization of multiple quantum-well (MQW) and superlattice (SL) InxGa1 ÿ xAs/GaAs structures embedded in a PIN photodiode. The test structures consist of 10 InxGa1 ÿ xAs±GaAs pairs. It is assumed the ®nalized optically active region will contain more pairs of this quantum-well system. The modulator has been developed for use in a ®bre optical measurement system. It is operated in a front-end mode, i.e. input and output signals to and from the modulator are guided through a single optical ®bre. A Ti:sapphire laser is used as the source of continuous wave light. It is directed through the optical ®bre to the modulator, modulated using a bias circuit, re¯ected, and collected into the same ®bre.
264
S. HasenoÈhr et al.
Fig. 1. In content xIn in InxGa1 ÿ xAs as a function of the ratio between partial vapor pressures of TMIn and TMGa in the reactor. At xIn=0.532, InxGa1 ÿ xAs is lattice matched to InP. The dependence can be ®tted by xIn/ (1 ÿ xIn) = 0.029 + 0.636(pTMIn/pTMGa).
Fig. 2. Correlation between the In content xIn and the gap energy Eg of the well material. The In content xIn (x-coordinate) was determined by the X-ray measurement and Eg (y-coordinate) was obtained from 6 K photoluminescence measurements. The experimental values are compared with the dependence Eg=0.475(1 ÿ xIn)2+0.6337(1 ÿ xIn) + 0.4105 from[5].
2. EXPERIMENTAL
The InxGa1 ÿ xAs/GaAs structures were grown on Si- and Te-doped (100) GaAs substrates in a horizontal low-pressure reactor AIXTRON AIX 200. Pd-diused H2 was used as the carrier gas. The sources were AsH3, TMGa, TMIn, SiH4 and DEZn. The reactor pressure 20 hPa, growth temperature 6508C, and the total gas velocity in the reactor 2.4 msÿ1 were the growth process parameters. The growth rate was 3.2 mm/h for InxGa1 ÿ xAs. The growth rate of GaAs was varied from 2.5 to 2.8 mm/h. The partial V/III vapor pressure ratio in the reactor was 84 for InxGa1 ÿ xAs. It was varied from 108 to 118 for GaAs. Layer thicknesses were determined on thick layers grown under the same conditions as the ®nal structures. The thickness of GaAs layers was determined by weighing. The thickness of InGaAs layers was measured at cleavage by an optical microscope after selective etching in K3[Fe(CN)6]:KOH:H2O (4 g:6 g:50 ml). The In content xIn in the InxGa1 ÿ xAs was determined by X-ray diraction using a conventional double-crystal diractometer. Cu Ka irradiation in the (004) re¯ection was employed. The measurement was carried out on 1 mm-thick layers. They were grown under the same growth conditions as used for the SL and MQW growth. The results are given in Fig. 1. The dependence of xIn/(1 ÿ xIn) versus the ratio between partial vapor pressures of TMIn and TMGa in the reactor is practically linear. Figure 2 shows a correlation between Eg and xIn. Eg was obtained from photoluminescence measurements at 6 K. Photore¯ectance measurements (PR) were performed at room temperature using a 1 mW HeNe laser (632.8 nm) as the pumping source. A 50 mW tungsten halogen lamp supplied light for the probe beam. The beam was ®ltered by a quarter-meter
monochromator and directed onto a sample. A silicon photodiode detected the probe beam after re¯ection. The PR signal was measured by a standard lock-in technique. Photoluminescence measurements (PL) were carried out using a 20 mW HeNe laser (632.8 nm) as the excitation source. The samples were placed in an optical cryostat and cooled to 6 K. The PL signal was collected employing the same experimental arrangement as for the PR measurement. Photocurrent measurements (PC) were done at room temperature and 6 K. The samples were front-side illuminated. The PIN modulator structures, with the QW placed in the intrinsic region, were furnished with two ohmic contacts formed on the p and n regions. An external electric ®eld was applied perpendicular to the structures by linking the samples in series to a constant voltage source and a load resistor. Its resistance was set to a certain value in order to keep linear the dependence of the photocurrent versus light intensity. The cryostat, light source and monochromator were the same as those used for the PL and PR measurements. The signal was measured by the lock-in ampli®er. 3. RESULTS AND DISCUSSIONS
To tailor the modulator structure for the 980 nm target wavelength, theoretical simulations were carried out to select the widths of the InxGa1 ÿ xAs wells and GaAs barriers, as well as to specify the composition of the well material. The transition between the ®rst electron subband and the ®rst heavy hole subband in the well (e1±hh1) was simulated as a function of the barrier and well widths and the InxGa1 ÿ xAs composition. The computation was done for strained InxGa1 ÿ xAs/GaAs SL and MQW pseudomorphic structures with 10 well/
MOCVD growth of InxGa1ÿxAs/GaAs
265
Fig. 4. Schematic cross-section of the epitaxial structures under investigation. The GaAs 100 nm thick buer layer was undoped, with a background doping density n = 1 1015 cm.3. The n GaAs layer was 300 nm thick and Si-doped to n = 5 1016 cmÿ3. The top p+ GaAs layer was 300 nm thick and Zn-doped to p = 5 1017 cmÿ3.
Fig. 3. Transition wavelength calculations as a function of the well and barrier widths for various composition of the well material.
barrier pairs on GaAs substrate. The ¯at-band condition without external or built-in voltage was considered. The exciton binding energy was not taken into account. The material constants of strained InGaAs (Eg, me, mhh, DEc) were taken from[4]. The SchroÈdinger equation was solved using a transfer matrix method. Figure 3 presents the calculated dependence of the e1±hh1 transition wavelength versus the well width with the barrier width as the parameter. The experimental epitaxial structures consist of 10 InxGa1 ÿ xAs±GaAs pairs, which are embedded in the intrinsic region of a GaAs PIN photodiode. It is necessary to prevent the interdiusion of Zn from the p type region of the photodiode and the interdiusion of Si and Te from the substrate into the quantum-well region. Therefore the quantumwell structure (MQW or SL) is sandwiched between two ``intrinsic'' GaAs layers. The buer layer under the QW structure is 100 nm thick and the upper intrinsic layer is 300 nm thick. Figure 4 depicts a schematic cross-section of the typical epitaxial structure. Parameters of the MQW and SL structures are summarized in Table 1. The structures were ®rst characterized by PR. It is a sensitive technique both to critical-point transitions in energy spectra of electrons and holes in
bulk semiconductors and transitions between energy levels in quantum-well structures. Figure 5 shows evident oscillations in the GaAs gap-energy region (at approx. 1.42 eV), as well as the e1±hh1 transition at lower energies. The e1±hh1 energy corresponds to 960±980 nm for various samples. The measurement was performed on various positions across the wafer. The dispersion was 25 nm. Typical low-temperature PL spectra of the studied structures are shown in Fig. 6. Line (a) represents a PL spectrum of as-grown sample MO 139. Line (b) stands for a new spectrum of the sample after 500 nm of GaAs was etched o from the top
Table 1. Parameters of the SL and MQW parts of the structures Structure In content in well Well width [nm] Barrier width [nm] Type
MO 138 x = 0.15 13 3 SL
MO 139 x = 0.18 7 10 MQW
MO 140 x = 0.20 5 3 SL
Fig. 5. PR spectra of experimental samples MO 138, MO 139 and MO 140. The energies corresponding to the GaAs band gap and the e1±hh1 transition are indicated by arrows.
266
S. HasenoÈhr et al.
Fig. 6. PL spectra of the MQW sample MO 139 at 6 K. (a) A spectrum of the whole as-grown structure. (b) The spectrum after etching away approximately 500 nm of the doped GaAs from the top.
GaAs layer. Both (a) and (b) lines show the band originating in the GaAs layers and the peaks at lower energies, where the transition e1±hh1 is supposed to give o radiation. There are dierences between (a) and (b). Compared with the ``GaAs'' band related to line (a), the ``GaAs'' band associated with line (b) is shifted to higher energies. It is narrower and a small excitonic feature appears in it at the high energy region. Luminescence from the highly-doped uppermost GaAs layer prevails in the spectrum represented by line (a). In the case of line (b) this layer is etched o and only part of the lowdoped n-GaAs layer is left. The quantum-well-related part of this spectrum is also changed. Its intensity is much higher after the etching. The spectrum consists of two overlapping peaks at 1.360 eV and 1.376 eV. The peak at 1.376 eV corresponds to the e1±hh1 transition. As regards the 1.360 eV peak, it is most probable that some impurities or defects play a role in this transition. Electro-optical properties of the structures were characterized by PC. Sample MO 139 with the MQW structure exhibits the typical quantum-con-
Fig. 7. Room temperature PC spectra of the MQW sample MO 139 at various biases.
Fig. 8. 6 K PC spectra of the SL sample MO 140 at various biases.
®ned Stark eect[3,6] in the whole temperature range up to room temperature. Figure 7 shows PC spectra of the sample measured at 300 K. They contain the e1±hh1 excitonic feature with a maximum at 963 nm and another excitonic feature around 920 nm. With increasing reverse bias the e1±hh1 absorption edge shifts to longer wavelengths, broadens, and loses its oscillator strength. This red shift is approximately 6.5 nm for bias varied from 0 V to ÿ3 V. At 975 nm the photocurrent for bias ÿ3 V is about 2.5 times higher than the photocurrent at zero bias. Spectra of SL structures are in¯uenced by the eect of Wannier-Stark localization[2]. Apart from direct e1±hh1 inter-subband transitions, the spectra also contain spatially indirect transitions that occur between adjacent wells. Hence electro-optical properties of SL-based samples MO 138 and MO 140 are qualitatively dierent compared with those of the MQW sample MO 139. Figure 8. presents PC spectra of the SL sample MO 140 at 6 K. The superlattice is in the ¯at-band state at a weak positive bias voltage. When the sample is negatively biased, the main excitonic peak e1±hh1 of the spectrum is blue shifted (about 10 nm), compared with the spectrum at the ¯at-band condition. This is a consequence of the Wannier±Stark localization: absorption between minibands is suppressed in favor of absorption between energy levels in individual quantum wells. Spatially indirect transitions appear in the spectra for intermediate bias voltages (0 V to ÿ1 V). The transition at longer wavelengths (around 915 nm) is clearly seen. At the same time the main excitonic peak develops as the localization enhances the probability of optical transitions. A further increase in bias voltage (from ÿ1 V to ÿ4 V) makes the spatially indirect transitions move and weaken, and the main excitonic peak exhibits a small red shift and broadening. Applying an electric ®eld at room temperature alters optical transitions in the SL structures much less than it does in the MQW structure. At a certain bias voltage the e1±hh1 transition is slightly
MOCVD growth of InxGa1ÿxAs/GaAs
enhanced. The electric-®eld-induced localization increases absorption at the wavelength corresponding to this transition. Apart from this observation, the broadening of the absorption edge suppresses any other electric-®eld eects, such as the blue shift or spatially indirect transitions. Electric-®eld eects observed in the spectral characteristics of the SL structures are too weak for practical use in modulator devices. As seen in Fig. 7, the MQW sample MO139 exhibits very promising room temperature electro-optical characteristics. However, in the SL structures thermal broadening results in considerable tunnelling through the barriers at room temperature. The electric-®eld-induced localization is not sucient to suppress the tunnelling satisfactorily. No typical Stark-eect shifts are observed in their spectra. On the contrary, much broader barriers isolate individual wells in the MQW structure. Hence the tunnelling is suppressed and carriers are localized even at room temperature. 4. CONCLUSION
Strained InxGa1 ÿ xAs/GaAs superlattice and multi-quantum-well epitaxial structures were designed, prepared and characterized. The aim is to realize an optimized structure for use in the optically active region of a PIN optical modulator. Energy levels of electrons and holes in the SL and MQW structures were simulated. The results were satisfactorily con®rmed by experiment. The e1±hh1 transition measured on various test samples deviates from the desired wavelength of operation of the
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modulator. The dispersion ranges from 960 nm to 980 nm. Moreover, certain features in the PL spectra, such as the peak width and the occurrence of a transition originating in some impurity or defect, indicate that the growth process has to be further optimized. Based on the electro-optical characteristics, the MQW structure appears to be the most promising candidate for the optically active region of the modulator operated at 980 nm. The SL-based structures exhibit considerable electro-optical eects only at cryogenic temperatures. A broadening of the superlattice absorption edge at 300 K renders the SL structures ineective for optical signal processing.
REFERENCES
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