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InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy
Solid-State Electronics 46 (2002) 877–883
InGaAs/InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy K. Radhakrishnan *, K. Yuan, H.Q. Zheng Blk. S1, School of Electrical and Electronic Engineering, Nanyang Technological University, Microelectronics Centre, Nanyang Avenue, Singapore 639798, Singapore Received 7 April 2000; received in revised form 21 September 2001
Abstract Optical and structural properties of InGaAs/InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell have been investigated. X-ray diffraction measurements were carried out to investigate the strain relaxation behavior of the metamorphic buffers grown at different temperatures, and the degree of relaxation was found to be 96%. Photoluminescence (PL) measurements show comparable results with the PL response observed for a lattice matched InGaAs/InP quantum well grown on InP substrate. The growth temperature of the metamorphic buffer layer ranging from 430 to 480 °C was found not to influence the strain relaxation ratio and the PL results. The results presented suggest that linearly graded InGaP metamorphic buffer layers grown by SSMBE can be used to decouple the InGaAs/InP quantum well structure from the GaAs substrate. Ó 2002 Published by Elsevier Science Ltd. Keywords: Metamorphic; Solid source molecular beam epitaxy; Quantum well; Graded buffer layer; X-ray diffraction; Photoluminescence
1. Introduction In recent years InP-based heterostructures have demonstrated superior performance for applications in optoelectronic and high-speed devices. While InP-based long wavelength devices are important for fiber-optic communication technology, InP-based high-electron mobility transistors (HEMT) have shown superior performance over GaAs-based transistors. However, the manufacture of InP-based devices is difficult because of the limited size, high cost and brittle nature of the InP substrate. In order to overcome these difficulties several investigations have been carried out in recent times to grow InP-based heterostructures on GaAs substrate.
*
Corresponding author. Tel.: +65-790-4549; fax: +65-7933318. E-mail address: [email protected] (K. Radhakrishnan).
The challenge in this case is to accommodate the large lattice mismatch (4%) between InP and GaAs. In this direction, many researchers have used compositionally graded metamorphic buffer layers, such as InGaAs [1–3], and InAlAs [4–7] between the active layers and the GaAs substrate. Quaternary alloys such as AlGaInAs [8] and AlGaAsSb [9] have also been attempted. Another possible candidate for the metamorphic graded buffer layer scheme is InGaP. Since InGaP has a larger bandgap compared to InP, a fully transparent buffer layer can be achieved, which is advantageous to long wavelength optoelectronic devices. Furthermore, the InGaP/InP system does not have composition control problem at the top of the buffer layer so that the transition between the graded buffer (Inx Ga1x P, x ! 1) and the InP layer can be very smooth. This is unlike in the case of AlInAs or InGaAs buffer layers where the switching of As to P at the interface is known to provide a thin ternary or quaternary
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interfacial layer. However, so far only a few research works has been reported on the growth and characterization of the InGaP graded buffer layers [10,11]. Moreover, these InGaP buffer layers and heterostructure materials were grown by gas-source molecular beam epitaxy (GSMBE) [10] or metalorganic vapor phase epitaxy (MOVPE) [11]. To our knowledge, there has been no report on the characterization of InGaP graded buffer layers grown by solid source molecular beam eitaxy (SSMBE). In this paper, we report on the growth and characterization of InGaAs/InP single quantum wells grown on GaAs (1 0 0) substrate with linearly graded metamorphic InGaP buffer layer by SSMBE using a valved phosphorus cracker cell. This new evaporation method for phosphorus overcomes the conventional problems associated with the handling of hydrides and metalorganics employed in the growth processes involving gas sources. The use of a value in the cracker cell also enables precise delivery of the required phosphorus flux for growth of epitaxial layers. Our previous studies have shown the successful growth of InP [12] and In0:48 Ga0:52 P [13] epitaxial layers using this technique. High-resolution X-ray diffraction (XRD) and photoluminescence (PL) measurements have been performed to characterize the InGaP metamorphic graded buffer layers grown at different temperatures and the quantum well structures.
the growth, the following As/P switching sequence was used at each interface of the arsenic/phosphorus heterostructure: after the growth of an As or a P containing layer the group-V flux was maintained for 10 s to smoothen the grown surface before closing all the sources for 5 s to evacuate the residual group-V atoms; then, the new group-V source was switched on for 10 s to stabilize the flux before starting the growth of the subsequent layer. Three samples, A, B and C, were grown with various buffer layer growth temperatures 480, 430 and 380 °C, respectively while keeping all other growth conditions constant. During the growth of active layers, the substrate temperature was maintained at 480 °C. The exact layer structure of the samples studied is shown in Fig. 1. The sample surface morphology was observed using a scanning electron microscope (SEM). A JEOL JEM2010 transmission electron microscope (TEM) operating at 200 kV was used to observe the cross-section of the samples. XRD measurements were carried out using Philips MRD high-resolution X-ray diffractometer using CuKa1 wavelength. Both the rocking curve (-–2h scan) and the reciprocal space mapping measurements were performed. PL measurements were carried out using a SPEX 750M system. The samples were mounted in a closed cycle He cryostat and excited at near normal incidence using a 514 nm Arþ laser at an excitation power density of 5 W/cm2 . The PL spectra were detected using a liquid nitrogen cooled germanium detector with
2. Experimental procedure Samples were grown on epiready semiinsulating GaAs (1 0 0) substrates using an all-solid-source Riber MBE32 system equipped with a valved phosphorus cracker cell and a valved arsenic cracker cell. 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 ðTs Þ, which was set to be 590 °C measured by an IRCON infrared pyrometer of appropriate wavelength sensitivity and temperature range. Before the growth of InGaP graded buffer layer, a GaAs layer was grown on the substrate. To 1000 A achieve a linearly varying composition profile (from x ¼ 0:48 to 1) in Inx Ga1x P grade layer the gallium cell temperature was decreased gradually while maintaining the same temperature for the indium cell. At the top of the 1.5 lm graded layer a single quantum well was re latticealized with three layers grown as: 1 lm InP, 40 A InP cap layer. During matched In0:53 Ga0:47 As and 500 A
Fig. 1. Layer structure of InGaAs/InP single quantum well on GaAs substrate with linearly graded metamorphic InGaP buffer layer grown by SSMBE.
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a 1200 gr/mm grating spectrometer in association with a conventional lock-in technique.
3. Results and discussion During the growth, the sample surface was monitored with a RHEED system. Streaky patterns of the reconstructed surface were observed through the graded InGaP buffer layers and the top active layers for sample A and B that was grown at 480 and 430 °C, respectively. This indicated that the growth of the highly mismatched InGaP graded layers and the top layers on GaAs substrate was two-dimensional. However, in the case of sample C that had the buffer layer grown at 380 °C showed spotty RHEED patterns during the entire growth except for the first 1000 A GaAs layer. This indicates that when the growth temperature is too low, the graded layer cannot maintain a two-dimensional growth. Sample C showed rough surface under SEM while sample A and B exhibited mirror-like surface. Fig. 2a shows the image of the surface morphology of sample A when observed under SEM. It can be seen that parallel corrugations extend along [1 1 0] direction and perpendicular corrugations extend along the [1 1 0] direction. This kind of cross-hatched surface has been widely observed in strain-relaxed epitaxial layers grown on lattice-mismatched substrate when the critical thickness is exceeded. Similar observation has been reported by Haupt et al. [8] for InAlAs/InGaAs heterostructures with AlGaInAs graded buffer layer grown on GaAs substrate. It was pointed out that the cross-hatched surface morphology indicated the existence of misfit dislocations with a two-dimensional growth mode. Fig. 2b shows the typical cross-sectional TEM image of the samples with graded buffer. It can be seen that there are a large number of misfit dislocations confined at lower part of the graded buffer, which is the origin of the crosshatched surface morphology. No obvious threading dislocations are observed in the top region. High-resolution XRD measurements were carried out to study the structural properties of the grown layers and understand the strain relaxation in the buffer and active layers. Rocking curves were obtained by performing -–2h scan, where - was the angle between the incident beam and the sample surface, and 2h was the angle between the incident beam and the diffracted beam. Fig. 3a shows the (0 0 4) XRD rocking curve of the sample A detected by a normal open detector. A four-crystal Bartels monochromator with Ge(2 2 0) crystal module was used in the primary beam side. A simulated curve for the fully relaxed InGaAs/InP quantum well structure on GaAs substrate is presented in Fig. 3b. In the experimental curve, the highest peak corresponds to GaAs substrate, while the broad diffraction peak obtained corresponds to the InP layer and
Fig. 2. (a) SEM image of sample A showing the cross-hatching and (b) cross-sectional TEM image showing the misfit dislocations.
Fig. 3. XRD curves in (0 0 4) reflection: (a) rocking curve (-–2h scan) with normal open detector and (b) simulated curve of fully relaxed InP/InGaAs/InP single quantum well structure grown on GaAs substrate.
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the quantum well structure. The plateau between the GaAs substrate and the top layers corresponds to the InGaP graded buffer layer. Since this graded layer changes its composition gradually along the thickness, the corresponding XRD feature is a plateau instead of a peak. It is also noticed that the broadened peak for the top layers is not centered at 480000 , where the simulated InP peak is located. Similar phenomenon has been reported in (0 0 4) reflection of mosaic GaAs substrate, which is attributed to the mosaicity in the sample [14]. In general, broadening of the diffraction peak can be caused by several reasons. Non-uniform spacing of the lattice planes due to the composition variation or the strain in the layer may cause peak broadening. The variation in the orientation of the lattice planes due to defects (mosaic structures) or wafer curvature can also broaden. If the wafer curvature is the cause for the broadening, the substrate peak should also be broadened, which is not the case in Fig. 3. On the other hand, it is known that a tilt between the epitaxial layers and the substrate always occurs for the samples that have large strain relaxation, and hence the angular separation between the diffraction peaks may vary. However, for the samples studied here, normal XRD rocking curve is insufficient to distinguish the strain and mosaic structures. In order to investigate the reasons for the peak broadening in the rocking curves a (0 0 4) reciprocal space mapping measurement was performed. For the reciprocal space mapping measurements, a three-reflection Ge(2 2 0) channel-cut analyzer crystal was employed for the diffracted beam. The reciprocal space mapping is illustrated in Fig. 4. Isointensity lines are plotted in a logarithmic scale in arbitrary units. The relatively weak intensity region between the sharp GaAs substrate and top layers shows the reflection of the graded buffer layers. From the change in Qy direction, it can be seen that the lattice parameter of this graded layer is varying from the value lattice matched to GaAs to InP. The reciprocal space mapping also clearly shows that the diffraction peak corresponding to top layers is obviously broadened in Qx direction. The broadening provides evidence for the mosaicity in the layers that is due to bulk-like defects. A gradual tilt between the epitaxial layers and the substrate is also apparent since the diffraction peak of the epitaxial layers does not exactly lie on the line between the substrate peak and the reciprocal space origin. When the composition is uniform in the layer, the epitaxial strain due to the lattice misfit is accommodated by the misfit dislocations confined at the interface to the underlying layer. On the other hand, layers with compositionally graded profiles have misfit dislocations distributed along the film thickness. For the sample studied here, the network of misfit dislocations beneath the active layers causes the mosaic structure, which is the reason for the broadening of the diffraction peak.
Fig. 4. Reciprocal space mapping of sample A in (0 0 4) reflection. Qx coordinates are linked to the plattice parameter ffiffiffi parallel to the layer plane ajj by Qx ¼ 2k=ð 2ajj Þ, Qy coordinates are linked to the lattice parameter perpendicular to the layer plane a? by Qy ¼ 2k=a? . Here, k is the X-ray wavelength for CuKa1 .
XRD measurements were performed using both the (0 0 4) and (2 2 4) reflection to determine the degree of strain relaxation. A Ge(2 2 0) channel-cut analyzer crystal was inserted before the detector for rocking curve measurements in order to distinguish the strain effect from the mosaic structure [15,16]. To eliminate the effect of tilt between the epitaxial layers and the substrate two sets of (0 0 4) and (2 2 4) scans were performed at two orientations by rotating the sample by 180° around the [0 0 1] axis. The average angular separation between the top layer peak and the GaAs substrate peak for each set of scans was calculated. Then, the Bragg angle corresponding to the top layer can be determined by adding the angular separation to the standard Bragg angle of GaAs substrate. In the XRD measurement of a tetragonally distorted lattice with the lattice parameter a? and ajj , where a? and ajj are the lattice parameters perpendicular and parallel to the layer plane, respectively, the following equations are given as [17]: 2 sin hB cos u ¼
lk a?
ð1Þ
2 sin hB sin u ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k 2 k ajj
ð2Þ
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where hB is Bragg angle, u is the angle between the diffraction place and the sample surface, and k is the wavelength of X-ray. The strain relaxation ratio R can be defined as [18]: R¼
ajj as a0 as
ð3Þ
where a0 is the lattice parameter of the unconstrained epitaxial layer and as is the lattice parameter of the substrate. The strain relaxation ratio for sample A and B was calculated using Eqs. (1)–(3), and it was found that both the samples exhibited the same relaxation ratio R ¼ 96%. This indicates that the epitaxial layers are nearly fully relaxed. The PL spectra measured at room temperature and 4.5 K for samples A and B are shown in Fig. 5, along with a PL spectrum obtained for an identical single quantum well structure grown on InP substrate for comparison. At 4.5 K, it can be seen that all the samples exhibit PL peak around 1.0 eV, which corresponds to E1-HH1 transition in the quantum well. The small shift
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in energy between the peaks is probably due to the variation of well thickness or composition during the individual growth process. It is noticed that the relative intensity and the value of full width at half maximum (FWHM) measured for sample A and sample B are almost the same as listed in Table 1. However, the FWHM value determined for the sample grown on the InP substrate (sample D) is smaller, and this lattice-matched sample also shows higher PL intensity. At room temperature, the PL peaks for all the samples are broadened, and the peak position shifts to lower energy. To compare the temperature-dependent PL response of the metamorphic sample with the lattice-matched sample, the PL spectra were recorded at various temperatures ranging from 4.5 K to room temperature. Fig. 6 shows the PL spectra of the sample A and D measured at various temperatures. It can seen from Fig. 6a that the metamorphic samples PL peak position shifts to lower energy when the temperature is increased gradually to room temperature. The peak also becomes broadened and the intensity decreases. At room temperature, the PL peak becomes weak but is still visible. The temperature dependence on the PL spectrum of the metamorphic sample is consistent with that of the lattice-matched sample, as shown in Fig. 6b. It is noticed that the linewidth of the PL peaks in Fig. 5 is relatively broad compared to the PL features of the same quantum well structure grown on InP substrate. The possible reasons are the fluctuation of the quantum well thickness and the influence of threading dislocations. It is known that the effect of strain on the semiconductor epitaxial layers produces a variety of changes to the surface morphology. The strain fields from the dislocations in the relaxed layers modulate the film
Table 1 Growth conditions, strain relaxation ratio and PL results
Fig. 5. The PL spectra of sample A, sample B and sample D (a lattice-matched sample with the same QW structure grown on InP substrate): (a) Room temperature and (b) 4.5 K.
Sample
A
B
C
Da
Growth temperature of buffer layer ð°CÞ RHEED pattern Strain relaxation ratio (%)
480
430
380
500
Streaky 96
Streaky 96
Spotty –
Streaky –
1783
1576
–
2494
64
64
–
38
7301
7005
–
11206
38
38
–
13
Room temperature PL Relative intensity (a.u.) FWHM (meV): 4.5K PL Relative intensity (a.u.) FWHM (meV) a
Lattice matched sample with the quantum well structure grown on InP substrate.
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mode but relax to accommodate the large lattice mismatch between the InGaAs/InP quantum well structure and the GaAs substrate.
4. Conclusion Linearly graded InGaP metamorphic buffers have been used to accommodate the large lattice mismatch between InGaAs/InP single quantum well structure and the GaAs substrate. The layer growth was carried out by all-SSMBE employing a valved phosphorus cracker cell. XRD measurement was used to study the stress relaxation of the metamorphic buffer layers grown at different temperatures. The degree of relaxation obtained was 96%, which indicates that the epitaxial layers are nearly fully relaxed. PL measurements show consistent results with the PL response observed for the lattice matched InGaAs/InP quantum wells grown on InP substrate. The growth temperature of the metamorphic buffer layer ranging from 430 to 480 °C did not influence the strain relaxation ratio and the PL results. The results presented here suggest that linearly graded InGaP metamorphic buffer layers grown by SSMBE can be used to decouple the InGaAs/InP quantum well structures from the GaAs substrate.
Fig. 6. The PL spectra of (a) sample A and (b) sample D (a lattice-matched sample with the same QW structure grown on InP substrate) measured at various temperatures.
thickness producing cross-hatched patterns on the surface and increase the interface roughness at both sides of the quantum wells. This influences the uniformity of the quantum well thickness and increases the linewidth of the PL peak. On the other hand, when a dislocation loop grows during the relaxation process, it develops into two threading dislocation arms connected by one or two misfit arms [19]. Thus, the threading dislocation density in the metamorphic sample is expected to be higher than that in the lattice-matched sample. The threading dislocations generated in the epitaxial layers may also degrade the optical characteristics of the quantum wells. The growth conditions, the strain relaxation ratio and the PL results the samples studied are listed in Table 1. Our previous work has shown that the growth temperature of In0:48 Ga0:52 P should be kept below 520 °C in order to have high optical and structural quality [20]. Moreover, the results presented in Table 1 show that the growth temperature of buffer layers ranging from 430 to 480 °C does not influence the strain relaxation ratio and the PL results. In this temperature range, the graded buffer layers still maintain the two-dimensional growth
References [1] Bulsara MT, Leitz C, Fitzgerald EA. Appl Phys Lett 1998;72:1608. [2] Lee B, Baek JH, Lee JH, Choi SW, Jung SD, Han WS, Lee EH. Appl Phys Lett 1996;68:2973. [3] Saced on A, Gonzalez-Sanz F, Calleja E, Mu~ noz E, Molina SI, Pacheco FJ, Ara ujo D, Garcıa R, Lourencßo M, Yang Z, Kidd P, Dunstan D. Appl Phys Lett 1995;66:3334. [4] Gozu S, Tsuboki K, Hayashi M, Hong C, Yamada S. J Cryst Growth 1999;201/202:749. [5] Chyi JI, Shieh JL, Pan JW, Lin RM. J Appl Phys 1996;79:8367. [6] Olsen JA, Hu EL, Lee SR, Fritz IJ, Howard AJ, Hammons BE, Tsao JY. J Appl Phys 1996;79:3578. [7] Cordier Y, Bollaert S, Zaknoune M, diPersio J, Ferre D. Proceedings of the 10th International Conference on InP and Related Materials, Tsukuba, Japan, 11–15 May 1998. p. 221. [8] Haupt M, K€ ohler K, Ganser P, Emminger S, M€ uller S, Rothemund K. Appl Phys Lett 1996;69:412. [9] Behet M, van der Zanden K, Borghs G, Behres A. Appl Phys Lett 1998;73:2760. [10] Chin TP, Hou HQ, Tu CW, Chang JCP, Otsuka N. Appl Phys Lett 1994;64:2001. [11] Kim AK, McCullough WS, Fitzgerald EA. J Vac Sci Technol B 1999;17:1485. [12] Radhakrishnan K, Zheng HQ, Zhang PH, Yoon SF, Ng GI. J Cryst Growth 1999;204:275. [13] Yoon SF, Mah KW, Zheng HQ. J Appl Phys 1999;85:7374.
K. Radhakrishnan et al. / Solid-State Electronics 46 (2002) 877–883 [14] Krost A, Bauer G, Woitok J. High resolution X-ray diffraction. In: Bauer G, Richter W, editors. Optical characterization of epitaxial semiconductor layers. New York: Springer; 1996. p. 300. [15] Fewster PF. Semicond Sci Technol 1993;8:1915. [16] Hartmann JM, Gallas B, Zhang J, Harris JJ, Joyce BA. J Appl Phys 1999;86:845.
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[17] Halliwell MAC, Lyons MH, Davey ST, Hockly M, Tuppen CG, Gibbings CJ. Semicond Sci Technol 1989;4:10. [18] Li JH, Mai ZH, Cui SF. J Appl Phys 1994;76: 810. [19] Schwarz KW. J Appl Phys 1999;85:120. [20] Yoon SF, Mah KW, Zheng HQ, Zhang PH. J Cryst Growth 1998;191:613.
InGaAs/InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy K. Radhakrishnan *, K. Yuan, H.Q. Zheng Blk. S1, School of Electrical and Electronic Engineering, Nanyang Technological University, Microelectronics Centre, Nanyang Avenue, Singapore 639798, Singapore Received 7 April 2000; received in revised form 21 September 2001
Abstract Optical and structural properties of InGaAs/InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell have been investigated. X-ray diffraction measurements were carried out to investigate the strain relaxation behavior of the metamorphic buffers grown at different temperatures, and the degree of relaxation was found to be 96%. Photoluminescence (PL) measurements show comparable results with the PL response observed for a lattice matched InGaAs/InP quantum well grown on InP substrate. The growth temperature of the metamorphic buffer layer ranging from 430 to 480 °C was found not to influence the strain relaxation ratio and the PL results. The results presented suggest that linearly graded InGaP metamorphic buffer layers grown by SSMBE can be used to decouple the InGaAs/InP quantum well structure from the GaAs substrate. Ó 2002 Published by Elsevier Science Ltd. Keywords: Metamorphic; Solid source molecular beam epitaxy; Quantum well; Graded buffer layer; X-ray diffraction; Photoluminescence
1. Introduction In recent years InP-based heterostructures have demonstrated superior performance for applications in optoelectronic and high-speed devices. While InP-based long wavelength devices are important for fiber-optic communication technology, InP-based high-electron mobility transistors (HEMT) have shown superior performance over GaAs-based transistors. However, the manufacture of InP-based devices is difficult because of the limited size, high cost and brittle nature of the InP substrate. In order to overcome these difficulties several investigations have been carried out in recent times to grow InP-based heterostructures on GaAs substrate.
*
Corresponding author. Tel.: +65-790-4549; fax: +65-7933318. E-mail address: [email protected] (K. Radhakrishnan).
The challenge in this case is to accommodate the large lattice mismatch (4%) between InP and GaAs. In this direction, many researchers have used compositionally graded metamorphic buffer layers, such as InGaAs [1–3], and InAlAs [4–7] between the active layers and the GaAs substrate. Quaternary alloys such as AlGaInAs [8] and AlGaAsSb [9] have also been attempted. Another possible candidate for the metamorphic graded buffer layer scheme is InGaP. Since InGaP has a larger bandgap compared to InP, a fully transparent buffer layer can be achieved, which is advantageous to long wavelength optoelectronic devices. Furthermore, the InGaP/InP system does not have composition control problem at the top of the buffer layer so that the transition between the graded buffer (Inx Ga1x P, x ! 1) and the InP layer can be very smooth. This is unlike in the case of AlInAs or InGaAs buffer layers where the switching of As to P at the interface is known to provide a thin ternary or quaternary
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interfacial layer. However, so far only a few research works has been reported on the growth and characterization of the InGaP graded buffer layers [10,11]. Moreover, these InGaP buffer layers and heterostructure materials were grown by gas-source molecular beam epitaxy (GSMBE) [10] or metalorganic vapor phase epitaxy (MOVPE) [11]. To our knowledge, there has been no report on the characterization of InGaP graded buffer layers grown by solid source molecular beam eitaxy (SSMBE). In this paper, we report on the growth and characterization of InGaAs/InP single quantum wells grown on GaAs (1 0 0) substrate with linearly graded metamorphic InGaP buffer layer by SSMBE using a valved phosphorus cracker cell. This new evaporation method for phosphorus overcomes the conventional problems associated with the handling of hydrides and metalorganics employed in the growth processes involving gas sources. The use of a value in the cracker cell also enables precise delivery of the required phosphorus flux for growth of epitaxial layers. Our previous studies have shown the successful growth of InP [12] and In0:48 Ga0:52 P [13] epitaxial layers using this technique. High-resolution X-ray diffraction (XRD) and photoluminescence (PL) measurements have been performed to characterize the InGaP metamorphic graded buffer layers grown at different temperatures and the quantum well structures.
the growth, the following As/P switching sequence was used at each interface of the arsenic/phosphorus heterostructure: after the growth of an As or a P containing layer the group-V flux was maintained for 10 s to smoothen the grown surface before closing all the sources for 5 s to evacuate the residual group-V atoms; then, the new group-V source was switched on for 10 s to stabilize the flux before starting the growth of the subsequent layer. Three samples, A, B and C, were grown with various buffer layer growth temperatures 480, 430 and 380 °C, respectively while keeping all other growth conditions constant. During the growth of active layers, the substrate temperature was maintained at 480 °C. The exact layer structure of the samples studied is shown in Fig. 1. The sample surface morphology was observed using a scanning electron microscope (SEM). A JEOL JEM2010 transmission electron microscope (TEM) operating at 200 kV was used to observe the cross-section of the samples. XRD measurements were carried out using Philips MRD high-resolution X-ray diffractometer using CuKa1 wavelength. Both the rocking curve (-–2h scan) and the reciprocal space mapping measurements were performed. PL measurements were carried out using a SPEX 750M system. The samples were mounted in a closed cycle He cryostat and excited at near normal incidence using a 514 nm Arþ laser at an excitation power density of 5 W/cm2 . The PL spectra were detected using a liquid nitrogen cooled germanium detector with
2. Experimental procedure Samples were grown on epiready semiinsulating GaAs (1 0 0) substrates using an all-solid-source Riber MBE32 system equipped with a valved phosphorus cracker cell and a valved arsenic cracker cell. 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 ðTs Þ, which was set to be 590 °C measured by an IRCON infrared pyrometer of appropriate wavelength sensitivity and temperature range. Before the growth of InGaP graded buffer layer, a GaAs layer was grown on the substrate. To 1000 A achieve a linearly varying composition profile (from x ¼ 0:48 to 1) in Inx Ga1x P grade layer the gallium cell temperature was decreased gradually while maintaining the same temperature for the indium cell. At the top of the 1.5 lm graded layer a single quantum well was re latticealized with three layers grown as: 1 lm InP, 40 A InP cap layer. During matched In0:53 Ga0:47 As and 500 A
Fig. 1. Layer structure of InGaAs/InP single quantum well on GaAs substrate with linearly graded metamorphic InGaP buffer layer grown by SSMBE.
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a 1200 gr/mm grating spectrometer in association with a conventional lock-in technique.
3. Results and discussion During the growth, the sample surface was monitored with a RHEED system. Streaky patterns of the reconstructed surface were observed through the graded InGaP buffer layers and the top active layers for sample A and B that was grown at 480 and 430 °C, respectively. This indicated that the growth of the highly mismatched InGaP graded layers and the top layers on GaAs substrate was two-dimensional. However, in the case of sample C that had the buffer layer grown at 380 °C showed spotty RHEED patterns during the entire growth except for the first 1000 A GaAs layer. This indicates that when the growth temperature is too low, the graded layer cannot maintain a two-dimensional growth. Sample C showed rough surface under SEM while sample A and B exhibited mirror-like surface. Fig. 2a shows the image of the surface morphology of sample A when observed under SEM. It can be seen that parallel corrugations extend along [1 1 0] direction and perpendicular corrugations extend along the [1 1 0] direction. This kind of cross-hatched surface has been widely observed in strain-relaxed epitaxial layers grown on lattice-mismatched substrate when the critical thickness is exceeded. Similar observation has been reported by Haupt et al. [8] for InAlAs/InGaAs heterostructures with AlGaInAs graded buffer layer grown on GaAs substrate. It was pointed out that the cross-hatched surface morphology indicated the existence of misfit dislocations with a two-dimensional growth mode. Fig. 2b shows the typical cross-sectional TEM image of the samples with graded buffer. It can be seen that there are a large number of misfit dislocations confined at lower part of the graded buffer, which is the origin of the crosshatched surface morphology. No obvious threading dislocations are observed in the top region. High-resolution XRD measurements were carried out to study the structural properties of the grown layers and understand the strain relaxation in the buffer and active layers. Rocking curves were obtained by performing -–2h scan, where - was the angle between the incident beam and the sample surface, and 2h was the angle between the incident beam and the diffracted beam. Fig. 3a shows the (0 0 4) XRD rocking curve of the sample A detected by a normal open detector. A four-crystal Bartels monochromator with Ge(2 2 0) crystal module was used in the primary beam side. A simulated curve for the fully relaxed InGaAs/InP quantum well structure on GaAs substrate is presented in Fig. 3b. In the experimental curve, the highest peak corresponds to GaAs substrate, while the broad diffraction peak obtained corresponds to the InP layer and
Fig. 2. (a) SEM image of sample A showing the cross-hatching and (b) cross-sectional TEM image showing the misfit dislocations.
Fig. 3. XRD curves in (0 0 4) reflection: (a) rocking curve (-–2h scan) with normal open detector and (b) simulated curve of fully relaxed InP/InGaAs/InP single quantum well structure grown on GaAs substrate.
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the quantum well structure. The plateau between the GaAs substrate and the top layers corresponds to the InGaP graded buffer layer. Since this graded layer changes its composition gradually along the thickness, the corresponding XRD feature is a plateau instead of a peak. It is also noticed that the broadened peak for the top layers is not centered at 480000 , where the simulated InP peak is located. Similar phenomenon has been reported in (0 0 4) reflection of mosaic GaAs substrate, which is attributed to the mosaicity in the sample [14]. In general, broadening of the diffraction peak can be caused by several reasons. Non-uniform spacing of the lattice planes due to the composition variation or the strain in the layer may cause peak broadening. The variation in the orientation of the lattice planes due to defects (mosaic structures) or wafer curvature can also broaden. If the wafer curvature is the cause for the broadening, the substrate peak should also be broadened, which is not the case in Fig. 3. On the other hand, it is known that a tilt between the epitaxial layers and the substrate always occurs for the samples that have large strain relaxation, and hence the angular separation between the diffraction peaks may vary. However, for the samples studied here, normal XRD rocking curve is insufficient to distinguish the strain and mosaic structures. In order to investigate the reasons for the peak broadening in the rocking curves a (0 0 4) reciprocal space mapping measurement was performed. For the reciprocal space mapping measurements, a three-reflection Ge(2 2 0) channel-cut analyzer crystal was employed for the diffracted beam. The reciprocal space mapping is illustrated in Fig. 4. Isointensity lines are plotted in a logarithmic scale in arbitrary units. The relatively weak intensity region between the sharp GaAs substrate and top layers shows the reflection of the graded buffer layers. From the change in Qy direction, it can be seen that the lattice parameter of this graded layer is varying from the value lattice matched to GaAs to InP. The reciprocal space mapping also clearly shows that the diffraction peak corresponding to top layers is obviously broadened in Qx direction. The broadening provides evidence for the mosaicity in the layers that is due to bulk-like defects. A gradual tilt between the epitaxial layers and the substrate is also apparent since the diffraction peak of the epitaxial layers does not exactly lie on the line between the substrate peak and the reciprocal space origin. When the composition is uniform in the layer, the epitaxial strain due to the lattice misfit is accommodated by the misfit dislocations confined at the interface to the underlying layer. On the other hand, layers with compositionally graded profiles have misfit dislocations distributed along the film thickness. For the sample studied here, the network of misfit dislocations beneath the active layers causes the mosaic structure, which is the reason for the broadening of the diffraction peak.
Fig. 4. Reciprocal space mapping of sample A in (0 0 4) reflection. Qx coordinates are linked to the plattice parameter ffiffiffi parallel to the layer plane ajj by Qx ¼ 2k=ð 2ajj Þ, Qy coordinates are linked to the lattice parameter perpendicular to the layer plane a? by Qy ¼ 2k=a? . Here, k is the X-ray wavelength for CuKa1 .
XRD measurements were performed using both the (0 0 4) and (2 2 4) reflection to determine the degree of strain relaxation. A Ge(2 2 0) channel-cut analyzer crystal was inserted before the detector for rocking curve measurements in order to distinguish the strain effect from the mosaic structure [15,16]. To eliminate the effect of tilt between the epitaxial layers and the substrate two sets of (0 0 4) and (2 2 4) scans were performed at two orientations by rotating the sample by 180° around the [0 0 1] axis. The average angular separation between the top layer peak and the GaAs substrate peak for each set of scans was calculated. Then, the Bragg angle corresponding to the top layer can be determined by adding the angular separation to the standard Bragg angle of GaAs substrate. In the XRD measurement of a tetragonally distorted lattice with the lattice parameter a? and ajj , where a? and ajj are the lattice parameters perpendicular and parallel to the layer plane, respectively, the following equations are given as [17]: 2 sin hB cos u ¼
lk a?
ð1Þ
2 sin hB sin u ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k 2 k ajj
ð2Þ
K. Radhakrishnan et al. / Solid-State Electronics 46 (2002) 877–883
where hB is Bragg angle, u is the angle between the diffraction place and the sample surface, and k is the wavelength of X-ray. The strain relaxation ratio R can be defined as [18]: R¼
ajj as a0 as
ð3Þ
where a0 is the lattice parameter of the unconstrained epitaxial layer and as is the lattice parameter of the substrate. The strain relaxation ratio for sample A and B was calculated using Eqs. (1)–(3), and it was found that both the samples exhibited the same relaxation ratio R ¼ 96%. This indicates that the epitaxial layers are nearly fully relaxed. The PL spectra measured at room temperature and 4.5 K for samples A and B are shown in Fig. 5, along with a PL spectrum obtained for an identical single quantum well structure grown on InP substrate for comparison. At 4.5 K, it can be seen that all the samples exhibit PL peak around 1.0 eV, which corresponds to E1-HH1 transition in the quantum well. The small shift
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in energy between the peaks is probably due to the variation of well thickness or composition during the individual growth process. It is noticed that the relative intensity and the value of full width at half maximum (FWHM) measured for sample A and sample B are almost the same as listed in Table 1. However, the FWHM value determined for the sample grown on the InP substrate (sample D) is smaller, and this lattice-matched sample also shows higher PL intensity. At room temperature, the PL peaks for all the samples are broadened, and the peak position shifts to lower energy. To compare the temperature-dependent PL response of the metamorphic sample with the lattice-matched sample, the PL spectra were recorded at various temperatures ranging from 4.5 K to room temperature. Fig. 6 shows the PL spectra of the sample A and D measured at various temperatures. It can seen from Fig. 6a that the metamorphic samples PL peak position shifts to lower energy when the temperature is increased gradually to room temperature. The peak also becomes broadened and the intensity decreases. At room temperature, the PL peak becomes weak but is still visible. The temperature dependence on the PL spectrum of the metamorphic sample is consistent with that of the lattice-matched sample, as shown in Fig. 6b. It is noticed that the linewidth of the PL peaks in Fig. 5 is relatively broad compared to the PL features of the same quantum well structure grown on InP substrate. The possible reasons are the fluctuation of the quantum well thickness and the influence of threading dislocations. It is known that the effect of strain on the semiconductor epitaxial layers produces a variety of changes to the surface morphology. The strain fields from the dislocations in the relaxed layers modulate the film
Table 1 Growth conditions, strain relaxation ratio and PL results
Fig. 5. The PL spectra of sample A, sample B and sample D (a lattice-matched sample with the same QW structure grown on InP substrate): (a) Room temperature and (b) 4.5 K.
Sample
A
B
C
Da
Growth temperature of buffer layer ð°CÞ RHEED pattern Strain relaxation ratio (%)
480
430
380
500
Streaky 96
Streaky 96
Spotty –
Streaky –
1783
1576
–
2494
64
64
–
38
7301
7005
–
11206
38
38
–
13
Room temperature PL Relative intensity (a.u.) FWHM (meV): 4.5K PL Relative intensity (a.u.) FWHM (meV) a
Lattice matched sample with the quantum well structure grown on InP substrate.
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mode but relax to accommodate the large lattice mismatch between the InGaAs/InP quantum well structure and the GaAs substrate.
4. Conclusion Linearly graded InGaP metamorphic buffers have been used to accommodate the large lattice mismatch between InGaAs/InP single quantum well structure and the GaAs substrate. The layer growth was carried out by all-SSMBE employing a valved phosphorus cracker cell. XRD measurement was used to study the stress relaxation of the metamorphic buffer layers grown at different temperatures. The degree of relaxation obtained was 96%, which indicates that the epitaxial layers are nearly fully relaxed. PL measurements show consistent results with the PL response observed for the lattice matched InGaAs/InP quantum wells grown on InP substrate. The growth temperature of the metamorphic buffer layer ranging from 430 to 480 °C did not influence the strain relaxation ratio and the PL results. The results presented here suggest that linearly graded InGaP metamorphic buffer layers grown by SSMBE can be used to decouple the InGaAs/InP quantum well structures from the GaAs substrate.
Fig. 6. The PL spectra of (a) sample A and (b) sample D (a lattice-matched sample with the same QW structure grown on InP substrate) measured at various temperatures.
thickness producing cross-hatched patterns on the surface and increase the interface roughness at both sides of the quantum wells. This influences the uniformity of the quantum well thickness and increases the linewidth of the PL peak. On the other hand, when a dislocation loop grows during the relaxation process, it develops into two threading dislocation arms connected by one or two misfit arms [19]. Thus, the threading dislocation density in the metamorphic sample is expected to be higher than that in the lattice-matched sample. The threading dislocations generated in the epitaxial layers may also degrade the optical characteristics of the quantum wells. The growth conditions, the strain relaxation ratio and the PL results the samples studied are listed in Table 1. Our previous work has shown that the growth temperature of In0:48 Ga0:52 P should be kept below 520 °C in order to have high optical and structural quality [20]. Moreover, the results presented in Table 1 show that the growth temperature of buffer layers ranging from 430 to 480 °C does not influence the strain relaxation ratio and the PL results. In this temperature range, the graded buffer layers still maintain the two-dimensional growth
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