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InP single quantum well structure on GaAs substrate with metamorphic buffer grown by molecular beam epitaxy
ARTICLE IN PRESS
Journal of Crystal Growth 261 (2004) 16–21
Characterization of InGaAs/InP single quantum well structure on GaAs substrate with metamorphic buffer grown by molecular beam epitaxy K. Radhakrishnan*, K. Yuan, Wang Hong Blk S1, School of Electrical and Electronic Engineering, Microelectronics Centre, Nanyang Technological University, Singapore 639798, Singapore Received 8 August 2003; accepted 10 September 2003 Communicated by M. Schieber
Abstract InGaAs/InP single quantum well (SQW) structure grown by solid source molecular beam epitaxy has been investigated to assess the quality of various low-temperature (LT) metamorphic buffer schemes developed on GaAs substrate. Atomicforce microscope, X-ray diffraction and photoluminescence measurements have been used to characterize a series of samples. It is found that the buffer layer scheme consisting of LT grown GaAs and InP (20 nm thick each grown at 400 C) and normal temperature grown InP (1.5 mm, 480 C) was effective in improving the crystalline quality. The full-width at half-maximum values obtained from the XRD $ scan around the InP peak and the PL linewidth of the SQW peak were 519 arcsec and 50 meV, respectively, for the optimized structure. The optimized metamorphic buffer scheme was further tested by characterizing the SQW structure as a function of thickness and growth rate of the InP buffer layer structure. Results indicate that the reduction of thickness and growth rate will bring about larger mosaicity and greater surface roughness. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15.H; 78.55.E; 61.72.D Keywords: A1. Metamorphic buffer; A3. Molecular beam epitaxy; A1. X-ray diffraction; A1. Atomic force microscopy; A1. Photoluminescence
1. Introduction InP-based devices have demonstrated superior performance for applications in the fields of optoelectronics and high-speed electronics. While InP long wavelength devices are important for fiber-optic *Corresponding author. Tel.: +67904549; fax: +67933318. E-mail address: [email protected] (K. Radhakrishnan).
communication technology, high electron mobility transistors (HEMT) and heterojunction bipolar transistors (HBT) grown on InP have demonstrated superior performance over GaAs-based transistors. However, high cost, limited size and brittle nature of the InP substrate bring difficulties to low-cost and high-volume production of InP-based devices. Thus, the metamorphic growth of these devices on GaAs substrate is highly desirable.
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.09.014
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
To accommodate the lattice mismatch between top active layers and GaAs substrate, compositionally graded buffer layers are widely employed in the metamorphic devices. The material schemes of the graded buffer studied include InGaAs [1–3], InAlAs [4–6], InGaP [7] and quaternary alloys such as AlGaInAs [8] and AlGaAsSb [9]. The metamorphic HEMT devices using the graded buffer have demonstrated good performance and have been applied in the circuit applications [10,11]. Recently, the metamorphic HBTs using graded buffer layers have also been studied [12,13]. However, the low thermal conductivity of the conventional graded buffer layers, such as InAlAs, InGaP or AlGaAsSb, results in the substantial increase of junction temperature and degradation of device performance [13–15]. In this case, the InP buffer layer is preferred for the metamorphic HBT applications as its thermal conductivity is very high. The large lattice mismatch between InP and GaAs (B3.8%) is a big challenge to grow InP directly on GaAs substrate without any compositionally graded layer. However, there are a few reports on the growth of InP on GaAs by metal organic vapor phase epitaxy (MOVPE) and chloride vapor phase epitaxy [16–18]. According to Hirokawa et al. [16], the buffer layer scheme including a low-temperature (LT) GaAs buffer and a two-step InP growth is found to provide smoother surface morphology and better crystalline quality. In this paper, we report on the growth and characterization of metamorphic InP layers grown on GaAs substrate using solid source molecular beam epitaxy (SSMBE). Samples with different buffer layer schemes have been grown and studied. Atomic force microscope (AFM), Xray diffraction (XRD) and photoluminescence (PL) have been used to characterize the material properties. The results reported here will benefit in the design of InP buffer layer for metamorphic high-speed transistor applications.
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solid-source Riber MBE32 system equipped with valved arsenic and phosphorus cracker cells. Prior to the growth, oxide desorption was carried out under As4 flux at a beam equivalent pressure (BEPAs) of 5 106 Torr. The process of surface oxide desorption involved slowly ramping up the substrate temperature at a rate of 30 C/min until the reflection high-energy electron diffraction (RHEED) pattern showed clear 2 4 and 4 2 surface reconstruction. This surface reconstruction transformation was adopted as a means for calibrating the substrate temperature (Ts ), which was set to be 590 C as measured by an IRCON infrared pyrometer of appropriate wavelength sensitivity and temperature range. The layer structure of the samples as shown in Fig. 1 includes LT GaAs buffer layer, LT InP buffer layer, and a thick InP layer that was grown at an optimized temperature of 480 C. On top of the InP layer, a 4 nm lattice-matched In0.53Ga0.47As and a 50 nm InP cap layer were also grown at 480 C to form a single quantum well (SQW) structure. A series of samples with various combinations of LT buffer layers, different thickness and growth rate of InP layers have been grown and studied. The detailed layer structure of each sample is described in Tables 1 and 2. After the growth, the surface morphology of the samples was characterized by contact mode AFM using Shimadzu SPM-9500 scanning probe microscope. The XRD measurement was carried out using Philips MRD high-resolution X-ray diffractometer. A Ge (2 2 0) channel-cut analyzer crystal was inserted before the detector for the measurement of mosaicity in the layer structure. PL 50 nm InP 4 nm In0.53Ga0.47As Normal InP LT InP LT GaAs GaAs Substrate
2. Experimental procedure The samples were grown on epiready semiinsulating GaAs (1 0 0) substrates using an all-
Fig. 1. Layer structure of the SQW samples grown on GaAs substrate with various buffer schemes as summarized in Tables 1 and 2.
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
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Table 1 The buffer layer scheme, surface roughness, FWHM of the XRD $ scans and FWHM of the PL peaks of the samples Sample no. Buffer layer scheme
Roughness XRD FWHM (nm) (arcsec)
PL FWHM (meV)
1 2
7.81 10.54
603 694
51 94
6.49
519
50
9.16 8.58
526 588
59 56
3 4 5
SQW/normal InP (1.5 mm)/GaAs substrate SQW/normal InP (1.5 mm)/LT InP(20 nm, 300 C)/LT GaAs (20 nm, 400 C)/GaAs substrate SQW/normal InP(1.5 mm)/LT InP (20 nm, 400 C)/LT GaAs (20 nm, 400 C)/GaAs substrate SQW /normal InP (1.5 mm)/LT InP (20 nm, 400 C)/GaAs substrate SQW/normal InP(1.5 mm)/LT GaAs(20 nm, 400 C)/GaAs substrate
Table 2 The surface roughness, FWHM of XRD $ scans and FWMH of PL peaks for the samples Sample no.
Thickness (mm)
Growth rate (mm/h)
Roughness (nm)
XRD FWHM (arcsec)
PL FWHM (meV)
3 6 7 8 9
1.5 1.25 1.0 1.5 1.5
1.0 1.0 1.0 0.5 0.75
6.49 11.07 12.39 13.8 16.49
519 572 642 573 534
50 84 70 57 78
Thickness refers to normal InP layer thickness. Growth rate corresponds to both LT and normal InP layer.
measurements were carried out using a SPEX 750 M system. The PL spectra were detected using a liquid-nitrogen cooled germanium detector with a 600 g/mm grating spectrometer in association with a conventional lock-in technique.
3. Results and discussion During the growth, the sample surface was monitored using the RHEED system. At the beginning of the InP growth, the pattern was spotty and very weak, indicating a three-dimensional growth due to the large lattice mismatch. The RHEED gradually recovered to streaky pattern during the growth of the normal InP layer. Fig. 2 is the typical AFM image of the surface morphology after the growth, which shows smaller corrugations extending in the ½1 1% 0 direction. However, there are no obvious periodic structures in the [1 1 0] direction, which is unlike the crosshatched surface morphology widely observed in the strain relaxed epitaxial layer. It was pointed out by Haupt et al. [8] that the cross-
Fig. 2. Typical AFM image of the surface morphology (30 mm 30 mm).
hatched surface morphology indicated the existence of misfit dislocations with a two-dimensional growth mode. For the samples studied here, the three-dimensional growth mode at the initial stage of InP growth, which is confirmed by the RHEED
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
Intensity( a.u.)
observations, is the reason for the observed morphology. There are also a number of squareshaped surface defects or pits observed in the figure, which results from the high density of threading dislocations in the highly mismatched layer structures. The pits are primarily aligned along the ½1 1% 0 direction similar to the case of lattice-mismatched InGaAs films grown on GaAs substrate [19] where the nucleation of pits was identified with an additional mechanism by which films roughen to relieve the strain efficiently. Fig. 3 is the typical XRD $ 2y scans of the samples analyzed with open detector. The diffraction peak corresponding to InP and top layers is obviously broadened, which is due to the mosaicity in these strain relaxed layer structures. To characterize the mosaicity, XRD $ scans were made around this broadened diffraction peak. In this measurement, a Ge (2 2 0) channel-cut analyzer crystal was inserted before the detector. The full-width at half-maximum values (FWHM) of the XRD curves (figures not shown) and the root mean square (rms) surface roughness values are listed in Table 1. This table also provides the buffer layer scheme adopted for different samples: sample 1 corresponds to the growth of 1.5 mm thick normal InP layer on GaAs directly without any LT buffer layers, sample 2 employs 20 nm GaAs LT buffer grown at 400 C followed by 20 nm InP LT buffer grown at 300 C, sample 3 uses the LT GaAs buffer similar to sample 2 but the LT InP buffer is grown at 400 C, sample 4 involves the growth of 20 nm LT InP at 400 C on GaAs directly without LT GaAs buffer, and 10
9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
-9000
GaAs substrate Top layers
-6000
-3000
0
3000
Relative Angle (arc sec.) Fig. 3. Typical XRD $ 2y scans of the samples in (0 0 4) reflection.
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sample 5 is to grow normal InP layer on LT GaAs buffer layer without LT InP. It is clear from the table that the combination of LT GaAs, LT InP and normal InP (sample 3) offers smoother surface and smaller FWMH of XRD, which corresponds to less mosaicity in the layer structures. Based on the results obtained from all the five samples, it can be confirmed that the buffer scheme adopted for sample 3 offers better material quality compared to others such as the ones without LT GaAs (sample 4), without LT InP (sample 5) or without any LT buffer (sample 1). This observation is consistent with the XRD FWHM study by Hirokawa et al. [16], where the buffer layer scheme involved a LT GaAs buffer and a two-step InP growth by MOVPE. The optimized growth temperature for their InP buffer layer was 550 C compared to the value of 400 C used in this investigation. In our case, hightemperature growth of InP buffer layer was not attempted, as the optimized temperature for InP growth by SSMBE is only 480 C in our laboratory. Moreover, higher-temperature buffer layer growth will require longer growth interruption time to reduce the growth temperature (to avoid possible indium desorption) before initiating InGaAs layer growth in the SQW structure. On the other hand, our results also show that the growth temperature for the LT InP buffer layer should not be too low during the SSMBE growth, otherwise it results in large mosaicity and very rough surface (sample 2) indicating poor material quality. Sample 3 not only provides smaller XRD FWHM, but also shows better PL result. Fig. 4 shows the normalized room-temperature PL spectra of the samples investigated. All the samples exhibit PL peak around 0.82–0.96 eV, which corresponds to E1–HH1 transition in the InP/ InGaAs SQW structure on the top of the buffer layer structures. PL intensity was found to be approximately the same for all the samples studied. However, the small shift in the energy observed between the peaks is probably due to the variation of the well thickness or composition during the individual growth process. From the FWHM value of the PL peak listed in Table 1, it can be seen that sample 3 exhibits smaller PL linewidth, which is consistent with its better
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
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PL Intensity (a.u.)
6000
No.1
5000
No.2
4000
No.3
3000
No.4
2000
No.5
1000
LM
0 0.7
0.8
0.9
1.0
1.1
Photon Energy (eV) Fig. 4. Normalized room-temperature PL spectra of samples 1–5. LM refers to the sample with the same SQW layer structure grown lattice matched to InP.
material quality as revealed by the XRD results and surface roughness measurement. Sample 2, which has the LT InP buffer grown at an LT of 300 C, shows the largest PL linewidth of all the samples investigated in addition to its large mosaicity. The PL spectrum obtained for the sample LM with the same SQW layer structure grown lattice matched to InP is also included in Fig. 4 comparison. A narrow linewidth of about 38 meV was measured for this sample, and the PL intensity was not significantly different from the values obtained for the samples grown on GaAs. All the samples 1–5 have 1.5 mm normal InP layers, and the growth rate of the LT InP buffer and the normal InP layers is set to 1 mm/h. To study the effect of the thickness and the growth rate of InP layers, samples 6–9 have been grown and studied. All these samples have the same buffer layer scheme as sample 3, that is, the combination of LT GaAs (20 nm, 400 C), LT InP (20 nm, 400 C) and a normal InP layer. All the other growth conditions were kept the same, except for the thickness of the normal InP layer and the growth rate of both LT and normal InP layer. Table 2 summarizes the experimental data for the thickness, growth rate, surface roughness, FWHM of XRD $ scans and FWHM of PL peaks of samples 6–9. For comparison, the results obtained from sample 3 are also included in Table 2. From the results obtained for samples 6 and 7, it can be seen that the reduction of the thickness of the normal InP layer results in higher surface roughness. The reduction of thickness makes the
sample surface closer to the misfit dislocation array lying at the bottom interface, thus a rougher surface can be expected because of the greater strain field of the misfit dislocations. Samples 6 and 7 also show larger FWHM of XRD and PL peaks. Samples 8 and 9 were grown at lower growth rate. Compared to the results obtained for sample 3, it can be seen that reduction of growth rate does not benefit as seen from the surface roughness, FWHM of XRD and PL spectra results. The data in Tables 1 and 2 show that sample 3 exhibits the best experimental results compared to all other samples, which suggests that the optimized buffer layer scheme and the growth conditions designed are effective to produce better material quality for this sample.
4. Conclusion The quality of various LT metamorphic buffer schemes developed on GaAs substrate has been evaluated using InGaAs/InP SQW structure grown by SSMBE. The samples were investigated using AFM, XRD and PL measurements. It is confirmed that the buffer layer scheme consisting of LT grown GaAs and InP (20 nm thick each grown at 400 C) and normal temperature grown InP (1.5 mm, 480 C) was effective in improving the crystalline quality. The growth optimized layer structure exhibited the FWHM values obtained from the XRD $ scan around the InP peak and the PL linewidth of the SQW peak of 519 arcsec and 50 meV, respectively. The reduction of the growth temperature for the LT InP buffer growth was found to increase the mosaicity and surface roughness of the samples. Similar effects were observed when the thickness and/or growth rate of the InP buffer layer was reduced. These results will be useful for the buffer layer design of metamorphic high-speed transistor structures.
Acknowledgements This investigation is a part of the project funded by Agency for Science, Technology and Research
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
(A-STAR), Singapore under the grant EMT/990/ 11. The authors wish to thank for their support.
References [1] M.T. Bulsara, C. Leitz, E.A. Fitzgerald, Appl. Phys. Lett. 72 (1998) 1608. [2] B. Lee, J.H. Baek, J.H. Lee, S.W. Choi, S.D. Jung, W.S. Han, E.H. Lee, Appl. Phys. Lett. 68 (1996) 2973. [3] A. Saced!on, F. Gonz!alez-Sanz, E. Calleja, E. Mun˜oz, S.I. ! R. Garc!ıa, M. Louren@o, Molina, F.J. Pacheco, D. Araujo, Z. Yang, P. Kidd, D. Dunstan, Appl. Phys. Lett. 66 (1995) 3334. [4] S. Gozu, K. Tsuboki, M. Hayashi, C. Hong, S. Yamada, J. Crystal Growth 201/202 (1999) 749. [5] J.I. Chyi, J.L. Shieh, J.W. Pan, R.M. Lin, J. Appl. Phys. 79 (1996) 8367. [6] J.A. Olsen, E.L. Hu, S.R. Lee, I.J. Fritz, A.J. Howard, B.E. Hammons, J.Y. Tsao, J. Appl. Phys. 79 (1996) 3578. [7] K. Yuan, K. Radhakrishnan, H.Q. Zheng, G.I. Ng, J. Vac. Sci. Technol. B 19 (2001) 2119. . [8] M. Haupt, K. Kohler, P. Ganser, S. Emminger, S. Muller, . K. Rothemund, Appl. Phys. Lett. 69 (1996) 412. [9] M. Behet, K. van der Zanden, G. Borghs, A. Behres, Appl. Phys. Lett. 73 (1998) 2760.
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[10] D.C. Dumka, H.Q. Tserng, M.Y. Kao, E.A. Beam, P. Saunier, Electron. Dev. Lett. 24 (2003) 135. [11] M.S. Heins, C.F. Campbell, M.Y. Kao, M.E. Muir, J.M. Carroll, IEEE MTT-S International Microwave Symposium Digest, Vol. 2, Seattle, USA, 2002, p. 1061. [12] H.Q. Zheng, K. Radhakrishnan, H. Wang, K. Yuan, S.F. Yoon, G.I. Ng, Appl. Phys. Lett. 77 (2000) 869. [13] Y.M. Kim, M. Dahlstrom, S. Lee, M.J.W. Rodwell, A.C. Gossard, Solid State Electron. 46 (2002) 1541. [14] H.F. Chau, W. Liu, E.A. Beam III, Seventh International Conference on Indium Phosphide and Related Materials, Hokkaido, Japan, 9–13 May 1995, Conference Proceedings, p. 640. [15] K. Kiziloglu, S. Thomas III, F. Williams, B.M. Paine, International Conference on Indium Phosphide and Related Materials, Williamsburg, VA, USA, 14–18 May 2000, Conference Proceedings, p. 294. [16] H. Horikawa, Y. Ogawa, Y. Kawai, M. Sakuta, Appl. Phys. Lett. 53 (1988) 397. [17] T. Sugino, M. Maeda, K. Kawarai, J. Shirafuji, J. Crystal Growth 166 (1996) 628. [18] S. Sawada, S. Matsukawa, T. Iwasaki, Y. Miura, M. Yokogawa, International Conference on Indium Phosphide and Related Materials, Nara, Japan, 14–18 May 2001, Conference Proceedings, p. 303. [19] N. Chokshi, M. Bouville, J. Mirecki Millunchick, J. Crystal Growth 236 (2002) 563.
Journal of Crystal Growth 261 (2004) 16–21
Characterization of InGaAs/InP single quantum well structure on GaAs substrate with metamorphic buffer grown by molecular beam epitaxy K. Radhakrishnan*, K. Yuan, Wang Hong Blk S1, School of Electrical and Electronic Engineering, Microelectronics Centre, Nanyang Technological University, Singapore 639798, Singapore Received 8 August 2003; accepted 10 September 2003 Communicated by M. Schieber
Abstract InGaAs/InP single quantum well (SQW) structure grown by solid source molecular beam epitaxy has been investigated to assess the quality of various low-temperature (LT) metamorphic buffer schemes developed on GaAs substrate. Atomicforce microscope, X-ray diffraction and photoluminescence measurements have been used to characterize a series of samples. It is found that the buffer layer scheme consisting of LT grown GaAs and InP (20 nm thick each grown at 400 C) and normal temperature grown InP (1.5 mm, 480 C) was effective in improving the crystalline quality. The full-width at half-maximum values obtained from the XRD $ scan around the InP peak and the PL linewidth of the SQW peak were 519 arcsec and 50 meV, respectively, for the optimized structure. The optimized metamorphic buffer scheme was further tested by characterizing the SQW structure as a function of thickness and growth rate of the InP buffer layer structure. Results indicate that the reduction of thickness and growth rate will bring about larger mosaicity and greater surface roughness. r 2003 Elsevier B.V. All rights reserved. PACS: 81.15.H; 78.55.E; 61.72.D Keywords: A1. Metamorphic buffer; A3. Molecular beam epitaxy; A1. X-ray diffraction; A1. Atomic force microscopy; A1. Photoluminescence
1. Introduction InP-based devices have demonstrated superior performance for applications in the fields of optoelectronics and high-speed electronics. While InP long wavelength devices are important for fiber-optic *Corresponding author. Tel.: +67904549; fax: +67933318. E-mail address: [email protected] (K. Radhakrishnan).
communication technology, high electron mobility transistors (HEMT) and heterojunction bipolar transistors (HBT) grown on InP have demonstrated superior performance over GaAs-based transistors. However, high cost, limited size and brittle nature of the InP substrate bring difficulties to low-cost and high-volume production of InP-based devices. Thus, the metamorphic growth of these devices on GaAs substrate is highly desirable.
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.09.014
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
To accommodate the lattice mismatch between top active layers and GaAs substrate, compositionally graded buffer layers are widely employed in the metamorphic devices. The material schemes of the graded buffer studied include InGaAs [1–3], InAlAs [4–6], InGaP [7] and quaternary alloys such as AlGaInAs [8] and AlGaAsSb [9]. The metamorphic HEMT devices using the graded buffer have demonstrated good performance and have been applied in the circuit applications [10,11]. Recently, the metamorphic HBTs using graded buffer layers have also been studied [12,13]. However, the low thermal conductivity of the conventional graded buffer layers, such as InAlAs, InGaP or AlGaAsSb, results in the substantial increase of junction temperature and degradation of device performance [13–15]. In this case, the InP buffer layer is preferred for the metamorphic HBT applications as its thermal conductivity is very high. The large lattice mismatch between InP and GaAs (B3.8%) is a big challenge to grow InP directly on GaAs substrate without any compositionally graded layer. However, there are a few reports on the growth of InP on GaAs by metal organic vapor phase epitaxy (MOVPE) and chloride vapor phase epitaxy [16–18]. According to Hirokawa et al. [16], the buffer layer scheme including a low-temperature (LT) GaAs buffer and a two-step InP growth is found to provide smoother surface morphology and better crystalline quality. In this paper, we report on the growth and characterization of metamorphic InP layers grown on GaAs substrate using solid source molecular beam epitaxy (SSMBE). Samples with different buffer layer schemes have been grown and studied. Atomic force microscope (AFM), Xray diffraction (XRD) and photoluminescence (PL) have been used to characterize the material properties. The results reported here will benefit in the design of InP buffer layer for metamorphic high-speed transistor applications.
17
solid-source Riber MBE32 system equipped with valved arsenic and phosphorus cracker cells. Prior to the growth, oxide desorption was carried out under As4 flux at a beam equivalent pressure (BEPAs) of 5 106 Torr. The process of surface oxide desorption involved slowly ramping up the substrate temperature at a rate of 30 C/min until the reflection high-energy electron diffraction (RHEED) pattern showed clear 2 4 and 4 2 surface reconstruction. This surface reconstruction transformation was adopted as a means for calibrating the substrate temperature (Ts ), which was set to be 590 C as measured by an IRCON infrared pyrometer of appropriate wavelength sensitivity and temperature range. The layer structure of the samples as shown in Fig. 1 includes LT GaAs buffer layer, LT InP buffer layer, and a thick InP layer that was grown at an optimized temperature of 480 C. On top of the InP layer, a 4 nm lattice-matched In0.53Ga0.47As and a 50 nm InP cap layer were also grown at 480 C to form a single quantum well (SQW) structure. A series of samples with various combinations of LT buffer layers, different thickness and growth rate of InP layers have been grown and studied. The detailed layer structure of each sample is described in Tables 1 and 2. After the growth, the surface morphology of the samples was characterized by contact mode AFM using Shimadzu SPM-9500 scanning probe microscope. The XRD measurement was carried out using Philips MRD high-resolution X-ray diffractometer. A Ge (2 2 0) channel-cut analyzer crystal was inserted before the detector for the measurement of mosaicity in the layer structure. PL 50 nm InP 4 nm In0.53Ga0.47As Normal InP LT InP LT GaAs GaAs Substrate
2. Experimental procedure The samples were grown on epiready semiinsulating GaAs (1 0 0) substrates using an all-
Fig. 1. Layer structure of the SQW samples grown on GaAs substrate with various buffer schemes as summarized in Tables 1 and 2.
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
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Table 1 The buffer layer scheme, surface roughness, FWHM of the XRD $ scans and FWHM of the PL peaks of the samples Sample no. Buffer layer scheme
Roughness XRD FWHM (nm) (arcsec)
PL FWHM (meV)
1 2
7.81 10.54
603 694
51 94
6.49
519
50
9.16 8.58
526 588
59 56
3 4 5
SQW/normal InP (1.5 mm)/GaAs substrate SQW/normal InP (1.5 mm)/LT InP(20 nm, 300 C)/LT GaAs (20 nm, 400 C)/GaAs substrate SQW/normal InP(1.5 mm)/LT InP (20 nm, 400 C)/LT GaAs (20 nm, 400 C)/GaAs substrate SQW /normal InP (1.5 mm)/LT InP (20 nm, 400 C)/GaAs substrate SQW/normal InP(1.5 mm)/LT GaAs(20 nm, 400 C)/GaAs substrate
Table 2 The surface roughness, FWHM of XRD $ scans and FWMH of PL peaks for the samples Sample no.
Thickness (mm)
Growth rate (mm/h)
Roughness (nm)
XRD FWHM (arcsec)
PL FWHM (meV)
3 6 7 8 9
1.5 1.25 1.0 1.5 1.5
1.0 1.0 1.0 0.5 0.75
6.49 11.07 12.39 13.8 16.49
519 572 642 573 534
50 84 70 57 78
Thickness refers to normal InP layer thickness. Growth rate corresponds to both LT and normal InP layer.
measurements were carried out using a SPEX 750 M system. The PL spectra were detected using a liquid-nitrogen cooled germanium detector with a 600 g/mm grating spectrometer in association with a conventional lock-in technique.
3. Results and discussion During the growth, the sample surface was monitored using the RHEED system. At the beginning of the InP growth, the pattern was spotty and very weak, indicating a three-dimensional growth due to the large lattice mismatch. The RHEED gradually recovered to streaky pattern during the growth of the normal InP layer. Fig. 2 is the typical AFM image of the surface morphology after the growth, which shows smaller corrugations extending in the ½1 1% 0 direction. However, there are no obvious periodic structures in the [1 1 0] direction, which is unlike the crosshatched surface morphology widely observed in the strain relaxed epitaxial layer. It was pointed out by Haupt et al. [8] that the cross-
Fig. 2. Typical AFM image of the surface morphology (30 mm 30 mm).
hatched surface morphology indicated the existence of misfit dislocations with a two-dimensional growth mode. For the samples studied here, the three-dimensional growth mode at the initial stage of InP growth, which is confirmed by the RHEED
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
Intensity( a.u.)
observations, is the reason for the observed morphology. There are also a number of squareshaped surface defects or pits observed in the figure, which results from the high density of threading dislocations in the highly mismatched layer structures. The pits are primarily aligned along the ½1 1% 0 direction similar to the case of lattice-mismatched InGaAs films grown on GaAs substrate [19] where the nucleation of pits was identified with an additional mechanism by which films roughen to relieve the strain efficiently. Fig. 3 is the typical XRD $ 2y scans of the samples analyzed with open detector. The diffraction peak corresponding to InP and top layers is obviously broadened, which is due to the mosaicity in these strain relaxed layer structures. To characterize the mosaicity, XRD $ scans were made around this broadened diffraction peak. In this measurement, a Ge (2 2 0) channel-cut analyzer crystal was inserted before the detector. The full-width at half-maximum values (FWHM) of the XRD curves (figures not shown) and the root mean square (rms) surface roughness values are listed in Table 1. This table also provides the buffer layer scheme adopted for different samples: sample 1 corresponds to the growth of 1.5 mm thick normal InP layer on GaAs directly without any LT buffer layers, sample 2 employs 20 nm GaAs LT buffer grown at 400 C followed by 20 nm InP LT buffer grown at 300 C, sample 3 uses the LT GaAs buffer similar to sample 2 but the LT InP buffer is grown at 400 C, sample 4 involves the growth of 20 nm LT InP at 400 C on GaAs directly without LT GaAs buffer, and 10
9
10
8
10
7
10
6
10
5
10
4
10
3
10
2
-9000
GaAs substrate Top layers
-6000
-3000
0
3000
Relative Angle (arc sec.) Fig. 3. Typical XRD $ 2y scans of the samples in (0 0 4) reflection.
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sample 5 is to grow normal InP layer on LT GaAs buffer layer without LT InP. It is clear from the table that the combination of LT GaAs, LT InP and normal InP (sample 3) offers smoother surface and smaller FWMH of XRD, which corresponds to less mosaicity in the layer structures. Based on the results obtained from all the five samples, it can be confirmed that the buffer scheme adopted for sample 3 offers better material quality compared to others such as the ones without LT GaAs (sample 4), without LT InP (sample 5) or without any LT buffer (sample 1). This observation is consistent with the XRD FWHM study by Hirokawa et al. [16], where the buffer layer scheme involved a LT GaAs buffer and a two-step InP growth by MOVPE. The optimized growth temperature for their InP buffer layer was 550 C compared to the value of 400 C used in this investigation. In our case, hightemperature growth of InP buffer layer was not attempted, as the optimized temperature for InP growth by SSMBE is only 480 C in our laboratory. Moreover, higher-temperature buffer layer growth will require longer growth interruption time to reduce the growth temperature (to avoid possible indium desorption) before initiating InGaAs layer growth in the SQW structure. On the other hand, our results also show that the growth temperature for the LT InP buffer layer should not be too low during the SSMBE growth, otherwise it results in large mosaicity and very rough surface (sample 2) indicating poor material quality. Sample 3 not only provides smaller XRD FWHM, but also shows better PL result. Fig. 4 shows the normalized room-temperature PL spectra of the samples investigated. All the samples exhibit PL peak around 0.82–0.96 eV, which corresponds to E1–HH1 transition in the InP/ InGaAs SQW structure on the top of the buffer layer structures. PL intensity was found to be approximately the same for all the samples studied. However, the small shift in the energy observed between the peaks is probably due to the variation of the well thickness or composition during the individual growth process. From the FWHM value of the PL peak listed in Table 1, it can be seen that sample 3 exhibits smaller PL linewidth, which is consistent with its better
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
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PL Intensity (a.u.)
6000
No.1
5000
No.2
4000
No.3
3000
No.4
2000
No.5
1000
LM
0 0.7
0.8
0.9
1.0
1.1
Photon Energy (eV) Fig. 4. Normalized room-temperature PL spectra of samples 1–5. LM refers to the sample with the same SQW layer structure grown lattice matched to InP.
material quality as revealed by the XRD results and surface roughness measurement. Sample 2, which has the LT InP buffer grown at an LT of 300 C, shows the largest PL linewidth of all the samples investigated in addition to its large mosaicity. The PL spectrum obtained for the sample LM with the same SQW layer structure grown lattice matched to InP is also included in Fig. 4 comparison. A narrow linewidth of about 38 meV was measured for this sample, and the PL intensity was not significantly different from the values obtained for the samples grown on GaAs. All the samples 1–5 have 1.5 mm normal InP layers, and the growth rate of the LT InP buffer and the normal InP layers is set to 1 mm/h. To study the effect of the thickness and the growth rate of InP layers, samples 6–9 have been grown and studied. All these samples have the same buffer layer scheme as sample 3, that is, the combination of LT GaAs (20 nm, 400 C), LT InP (20 nm, 400 C) and a normal InP layer. All the other growth conditions were kept the same, except for the thickness of the normal InP layer and the growth rate of both LT and normal InP layer. Table 2 summarizes the experimental data for the thickness, growth rate, surface roughness, FWHM of XRD $ scans and FWHM of PL peaks of samples 6–9. For comparison, the results obtained from sample 3 are also included in Table 2. From the results obtained for samples 6 and 7, it can be seen that the reduction of the thickness of the normal InP layer results in higher surface roughness. The reduction of thickness makes the
sample surface closer to the misfit dislocation array lying at the bottom interface, thus a rougher surface can be expected because of the greater strain field of the misfit dislocations. Samples 6 and 7 also show larger FWHM of XRD and PL peaks. Samples 8 and 9 were grown at lower growth rate. Compared to the results obtained for sample 3, it can be seen that reduction of growth rate does not benefit as seen from the surface roughness, FWHM of XRD and PL spectra results. The data in Tables 1 and 2 show that sample 3 exhibits the best experimental results compared to all other samples, which suggests that the optimized buffer layer scheme and the growth conditions designed are effective to produce better material quality for this sample.
4. Conclusion The quality of various LT metamorphic buffer schemes developed on GaAs substrate has been evaluated using InGaAs/InP SQW structure grown by SSMBE. The samples were investigated using AFM, XRD and PL measurements. It is confirmed that the buffer layer scheme consisting of LT grown GaAs and InP (20 nm thick each grown at 400 C) and normal temperature grown InP (1.5 mm, 480 C) was effective in improving the crystalline quality. The growth optimized layer structure exhibited the FWHM values obtained from the XRD $ scan around the InP peak and the PL linewidth of the SQW peak of 519 arcsec and 50 meV, respectively. The reduction of the growth temperature for the LT InP buffer growth was found to increase the mosaicity and surface roughness of the samples. Similar effects were observed when the thickness and/or growth rate of the InP buffer layer was reduced. These results will be useful for the buffer layer design of metamorphic high-speed transistor structures.
Acknowledgements This investigation is a part of the project funded by Agency for Science, Technology and Research
ARTICLE IN PRESS K. Radhakrishnan et al. / Journal of Crystal Growth 261 (2004) 16–21
(A-STAR), Singapore under the grant EMT/990/ 11. The authors wish to thank for their support.
References [1] M.T. Bulsara, C. Leitz, E.A. Fitzgerald, Appl. Phys. Lett. 72 (1998) 1608. [2] B. Lee, J.H. Baek, J.H. Lee, S.W. Choi, S.D. Jung, W.S. Han, E.H. Lee, Appl. Phys. Lett. 68 (1996) 2973. [3] A. Saced!on, F. Gonz!alez-Sanz, E. Calleja, E. Mun˜oz, S.I. ! R. Garc!ıa, M. Louren@o, Molina, F.J. Pacheco, D. Araujo, Z. Yang, P. Kidd, D. Dunstan, Appl. Phys. Lett. 66 (1995) 3334. [4] S. Gozu, K. Tsuboki, M. Hayashi, C. Hong, S. Yamada, J. Crystal Growth 201/202 (1999) 749. [5] J.I. Chyi, J.L. Shieh, J.W. Pan, R.M. Lin, J. Appl. Phys. 79 (1996) 8367. [6] J.A. Olsen, E.L. Hu, S.R. Lee, I.J. Fritz, A.J. Howard, B.E. Hammons, J.Y. Tsao, J. Appl. Phys. 79 (1996) 3578. [7] K. Yuan, K. Radhakrishnan, H.Q. Zheng, G.I. Ng, J. Vac. Sci. Technol. B 19 (2001) 2119. . [8] M. Haupt, K. Kohler, P. Ganser, S. Emminger, S. Muller, . K. Rothemund, Appl. Phys. Lett. 69 (1996) 412. [9] M. Behet, K. van der Zanden, G. Borghs, A. Behres, Appl. Phys. Lett. 73 (1998) 2760.
21
[10] D.C. Dumka, H.Q. Tserng, M.Y. Kao, E.A. Beam, P. Saunier, Electron. Dev. Lett. 24 (2003) 135. [11] M.S. Heins, C.F. Campbell, M.Y. Kao, M.E. Muir, J.M. Carroll, IEEE MTT-S International Microwave Symposium Digest, Vol. 2, Seattle, USA, 2002, p. 1061. [12] H.Q. Zheng, K. Radhakrishnan, H. Wang, K. Yuan, S.F. Yoon, G.I. Ng, Appl. Phys. Lett. 77 (2000) 869. [13] Y.M. Kim, M. Dahlstrom, S. Lee, M.J.W. Rodwell, A.C. Gossard, Solid State Electron. 46 (2002) 1541. [14] H.F. Chau, W. Liu, E.A. Beam III, Seventh International Conference on Indium Phosphide and Related Materials, Hokkaido, Japan, 9–13 May 1995, Conference Proceedings, p. 640. [15] K. Kiziloglu, S. Thomas III, F. Williams, B.M. Paine, International Conference on Indium Phosphide and Related Materials, Williamsburg, VA, USA, 14–18 May 2000, Conference Proceedings, p. 294. [16] H. Horikawa, Y. Ogawa, Y. Kawai, M. Sakuta, Appl. Phys. Lett. 53 (1988) 397. [17] T. Sugino, M. Maeda, K. Kawarai, J. Shirafuji, J. Crystal Growth 166 (1996) 628. [18] S. Sawada, S. Matsukawa, T. Iwasaki, Y. Miura, M. Yokogawa, International Conference on Indium Phosphide and Related Materials, Nara, Japan, 14–18 May 2001, Conference Proceedings, p. 303. [19] N. Chokshi, M. Bouville, J. Mirecki Millunchick, J. Crystal Growth 236 (2002) 563.