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InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy
Infrared Physics & Technology 55 (2012) 205–209
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Optical investigation of InAs/InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy S.G. Li a,⇑, Q. Gong b, C.F. Cao b, X.Z. Wang a, L. Yue b, Q.B. Liu b, H.L. Wang c, Y. Wang c a
Department of Electronic Communication and Technology, Shenzhen Institute of Information Technology, 2188 Longxiang Road, Shenzhen 518172, People’s Republic of China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, 865 Changning Road, Shanghai 200050, People’s Republic of China c College of Physics and Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 27 August 2011 Available online 1 February 2012 Keywords: PL spectrum InAs/InP quantum dot GSMBE Growth temperature
a b s t r a c t We report on the optical characteristics of InAs quantum dots based on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other methods. The photoluminescence was carefully investigated by adjusting the thickness of InAs layers and the growth temperature. A wide range of emitting peaks is obtained with the increase in the thickness of InAs layers. In addition, we find that the morphology and shape of quantum dots also greatly depend on InAs layers. The images of atomic force microscopy show that the quantum dots like forming into quantum dashes elongated along the [0 1 1] direction when the thickness of InAs layers increased. A critical thickness of formation quantum dots or quantum dash is obtained. At the same time, we observe that the growth temperature also has a great impact on the emission wavelength peaks. High qualities of InAs/InP(1 0 0) quantum dots providing their emission wavelength in 1.55 lm are obtained, and good performances of quantum dots lasers are fabricated. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing interest in employing low dimensional quantum dots (QDs) for optoelectronic devices, since QDs exhibit unique electronic and optical properties compared with conventional quantum-well (QW) structure [1–3]. Up to now, most of studies have been carried out on the system of InAs/GaAs quantum dots due to a larger lattice mismatch between the InAs and GaAs. A high density of InAs/GaAs QDs and good performance of QDs laser have been obtained [4–7]. However, for the system of InAs/InP quantum dots, providing their wavelength in the optical communication region of 1.55 lm is still in its infancy. For the InAs/InP system of QDs, the properties, that is, the density, size distribution, the morphology and shape of QD, greatly depend on the crystal orientation of InP substrate ((3 1 1)B InP, InP(1 0 0)). On a high-index (3 1 1)B InP substrate, it offers a high density of nucleation points for dot formation, which can strongly reduce the indium surface migration effects and leads to the formation of more symmetric QDs, and then a high gain profile and high density of 1011 cm2 InAs QDs can be easily obtained [8,9]. Unfortunately, high-index substrates have great disadvantages in device processing, like facet cleaving and anisotropic etching. Therefore, growth on InP(1 0 0) substrates is performed to address ⇑ Corresponding author. E-mail address: [email protected] (S.G. Li). 1350-4495/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2012.01.004
better the manufacturability issue [10]. However, the growth process of InAs QDs on InP(1 0 0) substrate is more complex than that of InAs QDs on (3 1 1)B InP substrate, which greatly depends on the growing procedures and equipments. In particular, for high quality of InAs/InP(1 0 0) QDs, three methods have been reported to obtain the desired emission wavelength: (1) two-step growth of the InAs QDs on InP substrate [11], (2) applying ultrathin GaAs or GaP interlayer between InAs QD layer and the buffer layer [12,13] and (3) formation of QDs by (In,Ga)As layers with very low Ga composition instead of pure InAs layer [14]. Most studies were carried out on metal–organic vapor-phase (MOVPE) [14] and chemical-beam epitaxy [12,13]. Recently, Lelarge et al. [15], reported that buried ridge QDs laser was directly grown by gas source molecular beam epitaxy (GSMBE) with two steps of growing process. However, the high temperature MOVPE regrowth of p-doped InP cladding and InGaAs contact layers has a great impact on the size of QDs, which leads the emitting peaks shift to short wavelength. So far, there are a few reports about the InAs/InP(1 0 0) QD grown by GSMBE; further studies are required. In this article, we investigate the properties of InAs QDs on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other particular procedure mentioned above. The shape of the QDs and the emission wavelength are found to depend greatly on the thickness of InAs layers and the growth temperature. With the increase in InAs layers, the QDs on InP(1 0 0) substrate like forming into one-dimension quantum dash
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elongated along the [0 1 1] direction, which decreases the size uniformity and density of quantum state. In addition, we study the growth temperature dependent emission wavelength of QDs under a fixed thickness of InAs layers. When the growth temperature increased, we observe that the emission wavelength shifts to longer wavelength. At the same time, a high density of 1010 cm2 InAs QDs being suitable for active region of QD laser is obtained. Ridge waveguide quantum dots laser with 3.0MLs InAs layers grown at temperature of 485 °C is processed. The laser can operate up to 70 °C under continuous-wave mode. 2. Experiment The samples of InAs QDs were grown on nominally (1 0 0) exact oriented n-type InP(1 0 0) substrates by gas source molecular beam epitaxy using gallium and indium as sources of III element. On the other hand, the V element sources are obtained by introducing AsH3 and/or PH3 through high temperature where the gases are thermally decomposed at 1000 °C. The InAs QDs were directly grown on a lattice matched quaternary 200 nm InGaAsP optical confinement layer (kg = 1.18 lm) without any capping layers. The bottom cladding layers are 600 nm n-typed InP buffer on InP(1 0 0) substrate. The QDs layer was formed with InAs growth rate of 0.1ML/s, while the AsH3 pressure in the gas line as set at 630 Torr and the growth chamber pressure was measured as 1.5 105 Torr during the InAs QD growth. In order to investigate the size of quantum dots against emission wavelength, the thickness of InAs layer was chosen as 2.0MLs, 3.0MLs, 3.5MLs and 4.0MLs, respectively. The growth temperature is 485 °C. At the same time, another two samples with 3.0MLs InAs layers were grown at the temperature of 455 and 515 °C, respectively. The surface morphologies of InAs QDs were measured by an atomic force microscope. The photoluminescence (PL) spectra were collected by a Fourier transform infrared spectrometer. Two laser structures consisting of five-stacked InAs QD layers embedded in InGaAsP waveguide were processed with InAs layers of 3.0MLs and 3.5MLs, respectively. The growth temperature is the same as 485 °C. 3. Results and discussions In InAs/InP(1 0 0) quantum dots, the emission wavelength is greatly dependent on the thickness and growth procedures of InAs layer. In addition, the growth equipment also plays an important role in the formation of QDs. For example, when the InAs QDs are deposited on metal–organic vapor-phase and chemical-beam epitaxy, a short time growth interruption is needed under As flux. To find available growth conditions for InAs/InP QDs by GSMBE, we deposit the samples without assisting any other methods but adjusting the thickness of InAs layers and growth temperature. The room temperature PL spectra of QDs with different thickness of InAs layers are shown in Fig. 1. With the increase in InAs layers, the PL peaks continuously shift to longer wavelength and reach to 1648 nm when the thickness of InAs layer rises up to 4.0MLs. However, for InAs QD layers of 2.0MLs, four peaks are observed in the PL spectrum, as shown in the inset of Fig. 1. The central wavelength peaks are around at the 1180 nm, 1422 nm, 1783 nm and 1970 nm, which are marked by peak 1, peak 2, peak 3 and peak 4, respectively. Peak 1 is from the optical confinement’s layer of InGaAsP, and peak 2 comes from the wetting layer, while the longer wavelength of peak 4 and peak 3 originates from the ground-state (GS) and excited-state (ES) of larger quantum islands. The energy separation between the GS and ES is only 65 meV, which is close to the value of 55 meV reported on InP(3 1 1)B substrate [16]. In contrast, the InAs/GaAs QDs have GS–ES splitting
Fig. 1. PL spectra at room temperature of InAs QDs on the lattice matched InGaAsP with the thickness of InAs layers of 2.0MLs, 3.0MLs, 3.5MLs and 4.0MLs. The inset is magnifying the PL spectra of 2.0MLs InAs layers.
more than 90 meV [17]. At the same time, the intensity of PL is the lowest, indicating that the density of nonradiative center is smaller than that of the other three samples. For InAs QDs on InP(1 0 0), the thickness of wetting layer is little larger than that of InAs/GaAs quantum dots because of a small lattice mismatch between the InAs and InP (3%). On the other hand, thickness of InAs layers would be formed on the surface due to As/P exchanges. Typically, the critical thickness of InAs layer is 1–2MLs on the InP substrate [18]. So, the thickness of 2.0MLs is little above the critical thickness of InAs QDs on InP(1 0 0), only a few larger islands are formed, which lead to the longer wavelength, while for InAs thickness of 3.0MLs, the emission wavelength just happens to be the interesting optical communication window of 1.55 lm. When the thickness of InAs layers is above 3.0MLs, the emission wavelength shifts to longer wavelength and reaches to 1648 nm as InAs layers rise up to 4.0MLs. Moreover, we find that the PL linewidth is related to the thickness of InAs layers. The PL linewidth reduces from 108 meV to 92 meV when the thickness of InAs layers increases from 3.0MLs to 3.5MLs. Compared with a high-index (3 1 1)B InP substrate, the InP(1 0 0) substrate shows a relative lower density of nucleation points for island formation, which largely increases surface migration effects. The morphology and shape of QDs largely depend on the thickness of InAs layers. The morphology of the InAs surface QDs grown on the InGaAsP barrier at the temperature of 485 °C is measured by atomic force microscope (AFM), as shown in Fig. 2. The InAs layers with thickness of 3.0MLs, 3.5MLs and 4.0MLs were measured. For InAs QDs of 3.0MLs, the QDs show a uniform size distribution with a mean dot height and mean base diameter of 2.9 nm and 76 nm, respectively, as shown in Fig. 2(a). A high density of QDs in the 1010 cm2 range is obtained. The typical dimensions of the QDs for thicknesses of 3.5MLs and 4.0MLs are the mean dot height of 3.1 nm, 2.8 nm and base diameter of 100 nm and 160 nm, respectively. With the increase in thickness of InAs layers, the QDs like forming into quantum dash elongated along the [0 1 1] direction due to the cation of indium migration in the surface of InGaAsP along [1 0 0] direction. The elongated quantum dash performs like the one-dimension quantum wire, which reduces the quantum confinement for carriers in this direction and then decreases the density of quantum state and quantum efficiency. As can be seen from Fig. 2(b), with the thickness of InAs layer rising up to 3.5MLs, only the larger size of QDs is elongated along (1 0 0) direction and forms into quantum dash. However, when the thickness of InAs rises up to 4.0MLs, one-dimension quantum dash/or wire is formed. By further increasing the thickness of InAs layers, all the QDs are formed into quantum dash, which have been reported by other groups [19]. In the GSMBE growth system, the thickness
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of emission wavelength, we fabricate three samples with a fixed thickness of InAs layers of 3.0MLs. The growth temperatures are at 455 °C, 485 °C and 515 °C, respectively. The PL spectra of the three samples are shown in Fig. 3. For the growth temperature of 455 °C and 485 °C, the wavelength peaks change a little with a central wavelength of 1529 nm and 1550 nm. However, the emission peaks greatly shift to a longer wavelength and reach 1624 nm when the temperature rises up to 515 °C, as shown in the inset of Fig. 3. For a QDs laser, the emitting wavelength is related to the size of QDs. The larger size of QDs will provide a longer emission wavelength. When the growth temperature raises, the heat speeds up the cation of indium migration on the surface, which induces a larger indium migration length. In other words, the larger indium migration length leads to less nucleation formation into QDs; thus, a larger size of QDs is formed, and the longer emitting wavelength is observed. At the same time, the PL intensity of QD decreases a little as growth temperature increased, as shown in Fig. 3. The decrease of PL intensity is from a decreasing density of QDs due to a less nucleation formation into QDs at high growth temperature. In addition, when growth temperature increased, the As/P exchanges induce an already overlaying InAs layers, which leads to less InAs formation into QDs, and thus further decrease the density of QDs, which causes a lower intensity of PL spectrum. The linewidths of PL spectrum are also carefully studied, and a linewidth of 110 meV, 108 meV and 112 meV is obtained for growth temperature of 455 °C, 485 °C and 515 °C, respectively. The almost same linewidth demonstrates that the size uniformities of QDs are not related to the growth temperature under a fixed thickness of InAs layers. From the points of emission wavelength of 1.55 lm, the suitable growth temperature of QDs is 485 °C. To further investigate the growth conditions dependence of laser performances of QD laser, two laser structures consisting of five-stacked InAs QD layers embedded in 200 nm InGaAsP waveguide are preformed with InAs layers of 3.0MLs and 3.5MLs, respectively. The growth temperature is 485 °C. Each InAs layer is separated by a 40 nm InGaAsP barriers. The bottom and top cladding layers are the 600 nm n-InP buffer and 1.5 lm p-InP followed by a 200 nm p-InGaAs contact layer. Ridge waveguide QD lasers were fabricated with stripe width of 6 lm and different cavity length. The chips with cleaved facet without coating were bonded on a heat sink whose temperature can be adjusted, and all devices tests were carried out under continuous-wave (CW) mode. The lasing spectrum of a laser with InAs layers of 3.0MLs is shown in Fig. 4(a). For a QDs laser with stripe width of 6 lm and cavity length of 0.7 mm, multimode lasing starts at 140 mA with a central wavelength of 1556 nm at the temperature of 20 °C. At 20 °C, the
Fig. 2. AFM images of InAs QDs on the lattice matched InGaAsP with different thickness of InAs layers. The InAs layer thickness is (a) 3.0MLs, (b) 3.5MLs and (c) 4.0MLs. The black-to-white height is 10 nm.
of 3.5MLs may be the critical thickness of formation QDs or quantum dash. At the same time, when the thickness of InAs layers increased, the size uniformity decreases. For a QDs laser diode, the performance largely depends on the size uniformity and the density of dots. So, the InAs layers of 3.0MLs are considered to be suitable thickness for good performance of QDs laser diode, which is shown in Ref. [20]. The growth temperature also has a great impact on the size of quantum dots. To investigate the growth temperature dependence
Fig. 3. PL spectra at room temperature of InAs QDs grown at the temperature of 455 °C, 485 °C and 515 °C under a fixed the thickness of InAs layers of 3.0MLs. Inset is peak wavelength vs. growth temperature.
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depend on the thickness of InAs layer and the growth temperature. For quantum dots provided their wavelength in the optical communication region of 1.55 lm, suitable growth conditions are obtained, which are the thickness of InAs layers of 3.0MLs and growth temperature of 485 °C. At the same time, a critical thickness of formation QDs or quantum dash is obtained. When the thickness of InAs layers rises up to 4.0MLs, InAs QDs like forming into quantum dashes elongated along the [0 1 1] direction, which decreases the density of quantum state and quantum efficiency. In addition, the emission wavelength is largely dependent on the growth temperature. When the growth temperature increased, the wavelength shifts to low energy level due to the larger indium migration length on the surface and As/P exchanges. Acknowledgments This work is financially supported by the Foundation of Shenzhen institute of information technology (Grant No. YB201006) and the National Natural Foundation of China (Grant Nos. 10990103, 61021064 and 60976015). References
Fig. 4. The lasing spectrum and output power of 3.0MLs InAs QDs laser under continuous-wave mode. (a) The lasing spectrum of a laser with stripe width of 6 lm and cavity length of 0.7 mm at 20 °C and (b) the output power of a QDs laser with stripe width of 6 lm and cavity length of 1.5 mm in the operation temperature range of 20–70 °C.
laser can even lase under CW mode when the cavity length ascleaved facet shortens to 0.45 mm. The output power of a laser with stripe width of 6 lm and cavity length of 1.5 mm is performed in Fig. 4(b). The maximum operation temperature is 70 °C under CW mode. At 20 °C, the QDs laser has a maximum output power above 38 mW from one facet, with slope efficiency of 136 mW/A just above the threshold. When the heat sink temperature is raised up to 70 °C, the maximum output power drops gradually to about 4 mW. At the same time, the slope efficiency drops to 55 mW/A due to the thermal effect diffusion of the carriers, while for a same size of laser with InAs layers of 3.5MLs, the maximum temperature is only 40 °C under CW mode (not shown here). The maximum output power from one facet is only 9.4 mW with a lower slope efficiency of 43 mW/A. When the operation temperature increases up to 40 °C, the maximum output power deteriorates to only 1.2 mW. At the same time, when cavity length shortens to 1 mm, the laser can only lase at pulse mode at 20 °C. In fact, the deteriorated performances of laser with 3.5MLs InAs layers are owning to their elongated QDs and nonuniform size distribution, which decreases the quantum efficiency and operation temperature. In conclusion, the deposition thickness of 3.0MLs InAs layers on the InP(1 0 0) is available to obtain good performance of QDs laser. At the same time, the laser just provides their wavelength in the fields of 1.55 lm. 4. Summary High density of InAs QDs is fabricated directly by the gas source molecular beam epitaxy without assisting any other methods. The emission wavelength peaks and shapes of quantum dot greatly
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Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Optical investigation of InAs/InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy S.G. Li a,⇑, Q. Gong b, C.F. Cao b, X.Z. Wang a, L. Yue b, Q.B. Liu b, H.L. Wang c, Y. Wang c a
Department of Electronic Communication and Technology, Shenzhen Institute of Information Technology, 2188 Longxiang Road, Shenzhen 518172, People’s Republic of China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, 865 Changning Road, Shanghai 200050, People’s Republic of China c College of Physics and Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China b
a r t i c l e
i n f o
Article history: Received 27 August 2011 Available online 1 February 2012 Keywords: PL spectrum InAs/InP quantum dot GSMBE Growth temperature
a b s t r a c t We report on the optical characteristics of InAs quantum dots based on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other methods. The photoluminescence was carefully investigated by adjusting the thickness of InAs layers and the growth temperature. A wide range of emitting peaks is obtained with the increase in the thickness of InAs layers. In addition, we find that the morphology and shape of quantum dots also greatly depend on InAs layers. The images of atomic force microscopy show that the quantum dots like forming into quantum dashes elongated along the [0 1 1] direction when the thickness of InAs layers increased. A critical thickness of formation quantum dots or quantum dash is obtained. At the same time, we observe that the growth temperature also has a great impact on the emission wavelength peaks. High qualities of InAs/InP(1 0 0) quantum dots providing their emission wavelength in 1.55 lm are obtained, and good performances of quantum dots lasers are fabricated. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction There is an increasing interest in employing low dimensional quantum dots (QDs) for optoelectronic devices, since QDs exhibit unique electronic and optical properties compared with conventional quantum-well (QW) structure [1–3]. Up to now, most of studies have been carried out on the system of InAs/GaAs quantum dots due to a larger lattice mismatch between the InAs and GaAs. A high density of InAs/GaAs QDs and good performance of QDs laser have been obtained [4–7]. However, for the system of InAs/InP quantum dots, providing their wavelength in the optical communication region of 1.55 lm is still in its infancy. For the InAs/InP system of QDs, the properties, that is, the density, size distribution, the morphology and shape of QD, greatly depend on the crystal orientation of InP substrate ((3 1 1)B InP, InP(1 0 0)). On a high-index (3 1 1)B InP substrate, it offers a high density of nucleation points for dot formation, which can strongly reduce the indium surface migration effects and leads to the formation of more symmetric QDs, and then a high gain profile and high density of 1011 cm2 InAs QDs can be easily obtained [8,9]. Unfortunately, high-index substrates have great disadvantages in device processing, like facet cleaving and anisotropic etching. Therefore, growth on InP(1 0 0) substrates is performed to address ⇑ Corresponding author. E-mail address: [email protected] (S.G. Li). 1350-4495/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2012.01.004
better the manufacturability issue [10]. However, the growth process of InAs QDs on InP(1 0 0) substrate is more complex than that of InAs QDs on (3 1 1)B InP substrate, which greatly depends on the growing procedures and equipments. In particular, for high quality of InAs/InP(1 0 0) QDs, three methods have been reported to obtain the desired emission wavelength: (1) two-step growth of the InAs QDs on InP substrate [11], (2) applying ultrathin GaAs or GaP interlayer between InAs QD layer and the buffer layer [12,13] and (3) formation of QDs by (In,Ga)As layers with very low Ga composition instead of pure InAs layer [14]. Most studies were carried out on metal–organic vapor-phase (MOVPE) [14] and chemical-beam epitaxy [12,13]. Recently, Lelarge et al. [15], reported that buried ridge QDs laser was directly grown by gas source molecular beam epitaxy (GSMBE) with two steps of growing process. However, the high temperature MOVPE regrowth of p-doped InP cladding and InGaAs contact layers has a great impact on the size of QDs, which leads the emitting peaks shift to short wavelength. So far, there are a few reports about the InAs/InP(1 0 0) QD grown by GSMBE; further studies are required. In this article, we investigate the properties of InAs QDs on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other particular procedure mentioned above. The shape of the QDs and the emission wavelength are found to depend greatly on the thickness of InAs layers and the growth temperature. With the increase in InAs layers, the QDs on InP(1 0 0) substrate like forming into one-dimension quantum dash
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elongated along the [0 1 1] direction, which decreases the size uniformity and density of quantum state. In addition, we study the growth temperature dependent emission wavelength of QDs under a fixed thickness of InAs layers. When the growth temperature increased, we observe that the emission wavelength shifts to longer wavelength. At the same time, a high density of 1010 cm2 InAs QDs being suitable for active region of QD laser is obtained. Ridge waveguide quantum dots laser with 3.0MLs InAs layers grown at temperature of 485 °C is processed. The laser can operate up to 70 °C under continuous-wave mode. 2. Experiment The samples of InAs QDs were grown on nominally (1 0 0) exact oriented n-type InP(1 0 0) substrates by gas source molecular beam epitaxy using gallium and indium as sources of III element. On the other hand, the V element sources are obtained by introducing AsH3 and/or PH3 through high temperature where the gases are thermally decomposed at 1000 °C. The InAs QDs were directly grown on a lattice matched quaternary 200 nm InGaAsP optical confinement layer (kg = 1.18 lm) without any capping layers. The bottom cladding layers are 600 nm n-typed InP buffer on InP(1 0 0) substrate. The QDs layer was formed with InAs growth rate of 0.1ML/s, while the AsH3 pressure in the gas line as set at 630 Torr and the growth chamber pressure was measured as 1.5 105 Torr during the InAs QD growth. In order to investigate the size of quantum dots against emission wavelength, the thickness of InAs layer was chosen as 2.0MLs, 3.0MLs, 3.5MLs and 4.0MLs, respectively. The growth temperature is 485 °C. At the same time, another two samples with 3.0MLs InAs layers were grown at the temperature of 455 and 515 °C, respectively. The surface morphologies of InAs QDs were measured by an atomic force microscope. The photoluminescence (PL) spectra were collected by a Fourier transform infrared spectrometer. Two laser structures consisting of five-stacked InAs QD layers embedded in InGaAsP waveguide were processed with InAs layers of 3.0MLs and 3.5MLs, respectively. The growth temperature is the same as 485 °C. 3. Results and discussions In InAs/InP(1 0 0) quantum dots, the emission wavelength is greatly dependent on the thickness and growth procedures of InAs layer. In addition, the growth equipment also plays an important role in the formation of QDs. For example, when the InAs QDs are deposited on metal–organic vapor-phase and chemical-beam epitaxy, a short time growth interruption is needed under As flux. To find available growth conditions for InAs/InP QDs by GSMBE, we deposit the samples without assisting any other methods but adjusting the thickness of InAs layers and growth temperature. The room temperature PL spectra of QDs with different thickness of InAs layers are shown in Fig. 1. With the increase in InAs layers, the PL peaks continuously shift to longer wavelength and reach to 1648 nm when the thickness of InAs layer rises up to 4.0MLs. However, for InAs QD layers of 2.0MLs, four peaks are observed in the PL spectrum, as shown in the inset of Fig. 1. The central wavelength peaks are around at the 1180 nm, 1422 nm, 1783 nm and 1970 nm, which are marked by peak 1, peak 2, peak 3 and peak 4, respectively. Peak 1 is from the optical confinement’s layer of InGaAsP, and peak 2 comes from the wetting layer, while the longer wavelength of peak 4 and peak 3 originates from the ground-state (GS) and excited-state (ES) of larger quantum islands. The energy separation between the GS and ES is only 65 meV, which is close to the value of 55 meV reported on InP(3 1 1)B substrate [16]. In contrast, the InAs/GaAs QDs have GS–ES splitting
Fig. 1. PL spectra at room temperature of InAs QDs on the lattice matched InGaAsP with the thickness of InAs layers of 2.0MLs, 3.0MLs, 3.5MLs and 4.0MLs. The inset is magnifying the PL spectra of 2.0MLs InAs layers.
more than 90 meV [17]. At the same time, the intensity of PL is the lowest, indicating that the density of nonradiative center is smaller than that of the other three samples. For InAs QDs on InP(1 0 0), the thickness of wetting layer is little larger than that of InAs/GaAs quantum dots because of a small lattice mismatch between the InAs and InP (3%). On the other hand, thickness of InAs layers would be formed on the surface due to As/P exchanges. Typically, the critical thickness of InAs layer is 1–2MLs on the InP substrate [18]. So, the thickness of 2.0MLs is little above the critical thickness of InAs QDs on InP(1 0 0), only a few larger islands are formed, which lead to the longer wavelength, while for InAs thickness of 3.0MLs, the emission wavelength just happens to be the interesting optical communication window of 1.55 lm. When the thickness of InAs layers is above 3.0MLs, the emission wavelength shifts to longer wavelength and reaches to 1648 nm as InAs layers rise up to 4.0MLs. Moreover, we find that the PL linewidth is related to the thickness of InAs layers. The PL linewidth reduces from 108 meV to 92 meV when the thickness of InAs layers increases from 3.0MLs to 3.5MLs. Compared with a high-index (3 1 1)B InP substrate, the InP(1 0 0) substrate shows a relative lower density of nucleation points for island formation, which largely increases surface migration effects. The morphology and shape of QDs largely depend on the thickness of InAs layers. The morphology of the InAs surface QDs grown on the InGaAsP barrier at the temperature of 485 °C is measured by atomic force microscope (AFM), as shown in Fig. 2. The InAs layers with thickness of 3.0MLs, 3.5MLs and 4.0MLs were measured. For InAs QDs of 3.0MLs, the QDs show a uniform size distribution with a mean dot height and mean base diameter of 2.9 nm and 76 nm, respectively, as shown in Fig. 2(a). A high density of QDs in the 1010 cm2 range is obtained. The typical dimensions of the QDs for thicknesses of 3.5MLs and 4.0MLs are the mean dot height of 3.1 nm, 2.8 nm and base diameter of 100 nm and 160 nm, respectively. With the increase in thickness of InAs layers, the QDs like forming into quantum dash elongated along the [0 1 1] direction due to the cation of indium migration in the surface of InGaAsP along [1 0 0] direction. The elongated quantum dash performs like the one-dimension quantum wire, which reduces the quantum confinement for carriers in this direction and then decreases the density of quantum state and quantum efficiency. As can be seen from Fig. 2(b), with the thickness of InAs layer rising up to 3.5MLs, only the larger size of QDs is elongated along (1 0 0) direction and forms into quantum dash. However, when the thickness of InAs rises up to 4.0MLs, one-dimension quantum dash/or wire is formed. By further increasing the thickness of InAs layers, all the QDs are formed into quantum dash, which have been reported by other groups [19]. In the GSMBE growth system, the thickness
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of emission wavelength, we fabricate three samples with a fixed thickness of InAs layers of 3.0MLs. The growth temperatures are at 455 °C, 485 °C and 515 °C, respectively. The PL spectra of the three samples are shown in Fig. 3. For the growth temperature of 455 °C and 485 °C, the wavelength peaks change a little with a central wavelength of 1529 nm and 1550 nm. However, the emission peaks greatly shift to a longer wavelength and reach 1624 nm when the temperature rises up to 515 °C, as shown in the inset of Fig. 3. For a QDs laser, the emitting wavelength is related to the size of QDs. The larger size of QDs will provide a longer emission wavelength. When the growth temperature raises, the heat speeds up the cation of indium migration on the surface, which induces a larger indium migration length. In other words, the larger indium migration length leads to less nucleation formation into QDs; thus, a larger size of QDs is formed, and the longer emitting wavelength is observed. At the same time, the PL intensity of QD decreases a little as growth temperature increased, as shown in Fig. 3. The decrease of PL intensity is from a decreasing density of QDs due to a less nucleation formation into QDs at high growth temperature. In addition, when growth temperature increased, the As/P exchanges induce an already overlaying InAs layers, which leads to less InAs formation into QDs, and thus further decrease the density of QDs, which causes a lower intensity of PL spectrum. The linewidths of PL spectrum are also carefully studied, and a linewidth of 110 meV, 108 meV and 112 meV is obtained for growth temperature of 455 °C, 485 °C and 515 °C, respectively. The almost same linewidth demonstrates that the size uniformities of QDs are not related to the growth temperature under a fixed thickness of InAs layers. From the points of emission wavelength of 1.55 lm, the suitable growth temperature of QDs is 485 °C. To further investigate the growth conditions dependence of laser performances of QD laser, two laser structures consisting of five-stacked InAs QD layers embedded in 200 nm InGaAsP waveguide are preformed with InAs layers of 3.0MLs and 3.5MLs, respectively. The growth temperature is 485 °C. Each InAs layer is separated by a 40 nm InGaAsP barriers. The bottom and top cladding layers are the 600 nm n-InP buffer and 1.5 lm p-InP followed by a 200 nm p-InGaAs contact layer. Ridge waveguide QD lasers were fabricated with stripe width of 6 lm and different cavity length. The chips with cleaved facet without coating were bonded on a heat sink whose temperature can be adjusted, and all devices tests were carried out under continuous-wave (CW) mode. The lasing spectrum of a laser with InAs layers of 3.0MLs is shown in Fig. 4(a). For a QDs laser with stripe width of 6 lm and cavity length of 0.7 mm, multimode lasing starts at 140 mA with a central wavelength of 1556 nm at the temperature of 20 °C. At 20 °C, the
Fig. 2. AFM images of InAs QDs on the lattice matched InGaAsP with different thickness of InAs layers. The InAs layer thickness is (a) 3.0MLs, (b) 3.5MLs and (c) 4.0MLs. The black-to-white height is 10 nm.
of 3.5MLs may be the critical thickness of formation QDs or quantum dash. At the same time, when the thickness of InAs layers increased, the size uniformity decreases. For a QDs laser diode, the performance largely depends on the size uniformity and the density of dots. So, the InAs layers of 3.0MLs are considered to be suitable thickness for good performance of QDs laser diode, which is shown in Ref. [20]. The growth temperature also has a great impact on the size of quantum dots. To investigate the growth temperature dependence
Fig. 3. PL spectra at room temperature of InAs QDs grown at the temperature of 455 °C, 485 °C and 515 °C under a fixed the thickness of InAs layers of 3.0MLs. Inset is peak wavelength vs. growth temperature.
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depend on the thickness of InAs layer and the growth temperature. For quantum dots provided their wavelength in the optical communication region of 1.55 lm, suitable growth conditions are obtained, which are the thickness of InAs layers of 3.0MLs and growth temperature of 485 °C. At the same time, a critical thickness of formation QDs or quantum dash is obtained. When the thickness of InAs layers rises up to 4.0MLs, InAs QDs like forming into quantum dashes elongated along the [0 1 1] direction, which decreases the density of quantum state and quantum efficiency. In addition, the emission wavelength is largely dependent on the growth temperature. When the growth temperature increased, the wavelength shifts to low energy level due to the larger indium migration length on the surface and As/P exchanges. Acknowledgments This work is financially supported by the Foundation of Shenzhen institute of information technology (Grant No. YB201006) and the National Natural Foundation of China (Grant Nos. 10990103, 61021064 and 60976015). References
Fig. 4. The lasing spectrum and output power of 3.0MLs InAs QDs laser under continuous-wave mode. (a) The lasing spectrum of a laser with stripe width of 6 lm and cavity length of 0.7 mm at 20 °C and (b) the output power of a QDs laser with stripe width of 6 lm and cavity length of 1.5 mm in the operation temperature range of 20–70 °C.
laser can even lase under CW mode when the cavity length ascleaved facet shortens to 0.45 mm. The output power of a laser with stripe width of 6 lm and cavity length of 1.5 mm is performed in Fig. 4(b). The maximum operation temperature is 70 °C under CW mode. At 20 °C, the QDs laser has a maximum output power above 38 mW from one facet, with slope efficiency of 136 mW/A just above the threshold. When the heat sink temperature is raised up to 70 °C, the maximum output power drops gradually to about 4 mW. At the same time, the slope efficiency drops to 55 mW/A due to the thermal effect diffusion of the carriers, while for a same size of laser with InAs layers of 3.5MLs, the maximum temperature is only 40 °C under CW mode (not shown here). The maximum output power from one facet is only 9.4 mW with a lower slope efficiency of 43 mW/A. When the operation temperature increases up to 40 °C, the maximum output power deteriorates to only 1.2 mW. At the same time, when cavity length shortens to 1 mm, the laser can only lase at pulse mode at 20 °C. In fact, the deteriorated performances of laser with 3.5MLs InAs layers are owning to their elongated QDs and nonuniform size distribution, which decreases the quantum efficiency and operation temperature. In conclusion, the deposition thickness of 3.0MLs InAs layers on the InP(1 0 0) is available to obtain good performance of QDs laser. At the same time, the laser just provides their wavelength in the fields of 1.55 lm. 4. Summary High density of InAs QDs is fabricated directly by the gas source molecular beam epitaxy without assisting any other methods. The emission wavelength peaks and shapes of quantum dot greatly
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