- Email: [email protected]
InGaAsP quantum dots with additional confinement structures
Journal of Crystal Growth 393 (2014) 59–63
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Investigation on the lasing characteristics of InAs/InGaAsP quantum dots with additional confinement structures Byounggu Jo a, Cheul-Ro Lee a, Jin Soo Kim a,n, Won Seok Han b, Jung Ho Song b, Jae-Young Leem c, Sam Kyu Noh d, Jae-Hyun Ryou e, Russell D. Dupuis f a Division of Advanced Materials Engineering, Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 561-756, Republic of Korea b IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Republic of Korea c Department of Nano Systems Engineering, Inje University, Gimhae 621-749, Republic of Korea d Nano Materials Evaluation Center, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea e Department of Mechanical Engineering and Texas Center for Superconductivity at the University of Houston (TcSUH), University of Houston, Houston, TX 77204-4006, USA f Center for Compound Semiconductors and School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA
art ic l e i nf o
a b s t r a c t
Available online 21 November 2013
We report on morphological, optical, and lasing characteristics of InAs quantum dots (QDs) embedded in an In0.69Ga0.31As0.67P0.33 quantum well (having a bandgap energy corresponding to a wavelength of 1.35 μm (1.35Q-InGaAsP)), which formed a dot-in-a-well (DWELL) structure. This DWELL was further sandwiched in In0.85Ga0.15As0.32P0.68 layers (1.15 μm, 1.15Q-InGaAsP). A 2 monolayer-thick GaAs layer was simultaneously introduced right below the InAs QD layer in the DWELL structure (GDWELL). The emission wavelength of the InAs GDWELL was 1490 nm, which was slightly shorter than that of the InAs QDs embedded only in 1.15Q-InGaAsP layers. To evaluate the effects of the GDWELL structure on lasing characteristics, gain-guided broad-area (BA) and index-guided ridge-waveguide (RW) laser diodes (LDs) were fabricated. The BA-LDs with the InAs QDs embedded only in 1.15Q-InGaAsP layers did not show the lasing at room temperature (RT) even in pulsed mode. For the GDWELL structure, however, the lasing emissions from both the BA-LDs and RW-LDs were successfully achieved at RT in continuous-wave mode. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Quantum dots B2. InAs B2. InGaAsP B3. Laser diodes
1. Introduction Unique properties of nano-scale three-dimensionally-confined quantum dots (QDs) have motivated research on many photonic devices such as laser diodes (LDs) and photodetectors. In particular, the LDs with self-assembled InAs (or InGaAs) QDs on GaAs substrates as an active medium have been extensively studied for the applications of optical communication, medical, and industrial systems [1–3]. The QD-based LDs (QD-LDs) can offer several superior device performance characteristics, including low threshold current densities, improved thermal stability, and high material gain compared to those of other lower-dimensionallyconfined quantum structures [4,5]. The LDs employing InP-based InAs QDs are a potential candidate for high-power lasers emitting at wavelengths near 1.55 μm, thanks to their relatively narrower beam divergence than that of the quantum-well (QW)-based LDs [6,7]. In the previous report on the comparative study for QW- and
n
Corresponding author. Tel.: þ 82 63 270 2291; fax: þ82 63 270 2305. E-mail address: [email protected] (J.S. Kim).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.038
QD-LDs of identical structure, the emission characteristics including beam quality of QD-LDs were significantly improved compared to those of QW-LDs [7]. Technical challenges, however, remain for the growth of high-quality self-assembled QDs with low defects, high-density, and high-uniformity, mainly due to complicated formation mechanism of Stranski and Krastanow growth [4,5]. In particular, the formation of InAs QDs on InP substrates is significantly affected not only by growth parameters but also by the strain between the InAs and barrier materials. For the InAs QDs with an InGaAsP matrix, an As/P exchange reaction between QDs and the barrier should be considered during the growth of the QD region. That is, the As/P exchange effect can reduce the structural uniformity of QDs resulting in the degradation in zerodimensionality [8,9]. The QD-LDs on InP substrates employing self-assembled InAs QDs have been less explored compared to those of the GaAs-based epitaxial structures. Previous studies on the InAs QD-LDs on InP substrates have focused on the demonstration of lasing at room temperature (RT). Caroff et al. reported the lasing of InAs QDs at RT by using an InP (1 1 3)B substrate. Lelarge et al. demonstrated the RT operation of buried ridge-stripe LDs in continuous-wave
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B. Jo et al. / Journal of Crystal Growth 393 (2014) 59–63
(CW) mode, employing InAs QDs in an InGaAsP-InP (1 0 0) heterostructure [10,11]. Also, Kim, et al. demonstrated the lasing at temperatures higher than RT and single-mode lasing emission from InAs QDs in an InAlGaAs-InAlAs-InP heterostructure [12]. However, the lasing characteristics of the LDs with the InP-based InAs QDs were generally inferior to those with GaAs-based QDs. This is partially due to the reduced height of the InAs QDs buried in InP-based heterostructures. This morphological change in the self-assembled QDs results in reduced confinement of carriers in the vertical direction of a QD. As a result, the overlap integral between electron and hole wave-functions in a QD is not large enough, leading to low modal gain of the QD-LD structures. Recently, in order to improve capturing, thermalizing, and confinement of carriers, “dot-in-a-well (DWELL)” structures, where QDs are embedded in a QW, have been used in the active region of the GaAs-based InAs QD devices [13–16]. In this paper, we describe on the influences of the DWELL structure with a thin GaAs layer (GDWELL) on the optical and lasing characteristics of InAs QD-LDs fabricated on InP (0 0 1) substrates. In order to investigate the effects of the GDWELL structure, gainguided broad-area (BA) LDs and index-guided ridge-waveguide (RW) LDs were fabricated and characterized. Table 1 Comparison of different structures of the InAs quantum-dot samples used in this study. Samples
GDWELL structure
Nominal thickness of InAs (ML)
CQD GDWELL1 GDWELL2 GDWELL3
No Included Included Included
2.4 2.0 2.4 2.8
2. Experimental details The InAs QD samples (both exposed QDs on bottom cladding and waveguide layers and embedded QDs sandwiched between top and bottom waveguide and cladding layers) and QD-LD structures (multiple-layered QDs in LD structures) used in this study were grown on n-type InP (0 0 1) substrates by metal-organic chemical vapor deposition in a vertical-type reactor with the sources of trimethylindium, trimethyl-gallium, arsine, phosphine, and hydrogen gas. Before the deposition of the QD layer, a two-step 1.15Q-InGaAsP barrier with the growth rate of 3.16 Å/s was deposited at the growth temperature (Tg) of 640 and 520 1C. The thickness of the bottom 1.1Q-InGaAsP layer was 100 nm. And then, In0.69Ga0.31As0.67P0.33 (having a bandgap energy corresponding to a wavelength of 1.35 μm, 1.35Q-InGaAsP) and a GaAs layer with the thickness of 2 monolayers (MLs) were successively grown on the 1.1Q-InGaAsP layer. InAs with different nominal thicknesses of 2.0, 2.4, and 2.8 MLs were deposited on GaAs/In0.69Ga0.31As0.67P0.33 (GDWELL structures) to evaluate the formation characteristics of the QDs. The III/V ratio and the Tg for the QD layer were 0.81 and 520 1C, respectively. The insertion of this GaAs layer was intended for the modulation in strain between the InAs and InGaAsP, and the suppression of the inter-diffusion of As and P between the InAs QDs and the InGaAsP barriers. Detailed growth conditions of InAs QDs with the DWELL and GDWELL structures were described in the previous report [17]. Also, the InAs QDs with a nominal thickness of 2.4 MLs were formed on In0.85Ga0.15As0.32P0.68 (1.15 μm, 1.15Q-InGaAsP) for comparison. The layer structures of the QD samples to investigate surface morphologies and optical properties are summarized and compared in Table 1. In order to investigate the device characteristics of the QD-LDs with the GDWELL structures, we fabricated BA-LDs and RW-LDs.
Fig. 1. Surface morphologies of (a) 2.4 monolayer (ML)-thick InAs QDs on 1.15-μm quaternary (1.15Q)-InGaAsP, and (b) 2.0 ML-thick InAs QDs, (c) 2.4 ML-thick InAs QDs, and (d) 2.8 ML-thick InAs QDs on GaAs/1.35Q-InGaAsP/1.15Q-InGaAsP.
B. Jo et al. / Journal of Crystal Growth 393 (2014) 59–63
The fabrication processes are relatively simple, and thus should not influence LD performance characteristics. Therefore, the dependence of intrinsic QD properties on the lasing characteristics can be evaluated. The width of a metal stripe for the BA-LDs and a ridge width for the RW-LD were 100 and 4 μm, respectively. The LD with conventional InAs QDs without the GDWELL structure (labeled as CQD-LD) was compared as a reference. The CQD-LD structure consists of seven-layer-stacked InAs QDs formed at a nominal thickness of 2.4 MLs and separated by a 10 nm-thick 1.15QInGaAsP spacer, which is further sandwiched by a spin index separate confinement heterostructure (SPIN-SCH) with an 800 nm-thick 1.15QInGaAsP waveguide layer and InP cladding layers. For comparison, the same LD structure with the seven-layer-stacked GDWELL inserted in the same SPIN-SCH was fabricated (GDWELL-LD). The structural and optical properties of the InAs QDs were investigated by atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy, respectively. Lasing characteristics of the QD-LDs were measured using a multi-function optical meter and a spectrum analyzer.
3. Results and discussion Fig. 1 shows surface morphologies of the exposed InAs QD samples measured by AFM. A spatial density and an average height of the InAs QDs with a nominal thickness of 2.4 MLs on a simple 1.15Q-InGaAsP layer (SL) (shown in Fig. 1(a)) are estimated to be 2.6 1010 cm 2 and 43.1 nm, respectively. Densities of the InAs QDs on GaAs/1.35Q-InGaAsP/1.15Q-InGaAsP layers (GWL) are measured to be 0.8 1010 cm 2, 1.9 1010 cm 2, and 2.4 1010 cm 2 for InAs nominal thicknesses of 2.0 MLs (GWL1), 2.4 MLs (GWL2), and 2.8 MLs (GWL3), respectively. The densities of these QDs of GWLs are lower than that of the QDs on the SL. Average lengths of the InAs QDs are 43.7, 46.2, and 48.1 nm for the GWL1, GWL2, and GWL3 samples, respectively. For the GWL2 sample with the same nominal thickness of InAs as the QDs on the SL, the average size is relatively large. The changes in the density and size of the InAs QDs are believed to depend on the presence of indium (In) atoms on the surface during the In and arsenic (As) precursor supply. That is, for the formation of the InAs QDs on the InGaAsP layer, the newly supplied In atoms for the QD layer are affected by the presence of In atoms on the surface of 1.15Q-InGaAsP. However, In atoms for the QD layer on the GaAs layer are not affected by In atoms of the buried 1.35Q-InGaAsP layer during the migration process. As a result, the migration characteristics of the In atoms on GaAs are altered, leading to decreased QD density and increased QD size. Also, the structural variation may occur due to the modulation in
Fig. 2. Photoluminescence (PL) spectra of the CQD and GDWELL samples.
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the strain between the InAs and 1.15Q-InGaAsP layers by the additional GaAs layer [18]. In addition, the diffusion of In atoms from the 1.15Q-InGaAsP layer into the QD layer and the well-known As/P exchange at the interface between the InAs QDs and InGaAsP layers can effectively be suppressed by the GaAs layer [19,20]. Fig. 2 shows the PL spectra for the embedded QD samples measured at RT. The emission wavelength, full-width at halfmaximum (FWHM), and intensity ratio for the QD samples are summarized in Table 2. The emission wavelength and FWHM for the GDWELL2 sample are 1490 nm. Even though the size of the QDs for the GDWELL2 sample is slightly larger than that of the CQD sample, the emission wavelength is blue-shifted by 13 nm from that of the CQD sample. The blue-shift in the peak position for the GDWELL2 sample is attributed to the effect of the highpotential barrier provided by the GaAs layer. In the previous work,
Table 2 Summary on the optical properties of the QD samples. Samples
Emission wavelength (nm)
FWHM (meV)
I/ICQD
CQD GDWELL1 GDWELL2 GDWELL3
1503 1470 1490 1564
92 78 69 68
1 2.6 4.3 3.6
Fig. 3. (a) Light output vs. current (L–I) curves for the BA-LDs with the CQDs and GDWELL structure, and (b) lasing spectra for the BA-LDs with GDWELL with increasing injection current in CW mode.
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the emission wavelength of InAs QDs in a GaAs matrix was significantly blue-shifted by changing the position of a thin AlAs layer below the QD layer [21]. Also, the introduction of gallium (Ga) atoms into QDs from GaAs layer may partially be responsible for the blue-shift in the emission wavelength. The FWHM for the GDWELL samples are narrower than that of the CQD sample with an FWHM of 92 meV, which is interpreted as improved size uniformity of the QDs (as shown in Fig. 1) and the reduced compositional fluctuation in QDs due to the suppressed As/P exchange [19]. The PL intensity for the GDWELL2 sample is more than four times stronger than that of the CQD sample, possibly due to the combined effects of the enhanced carrier-capturing probability by the additional QW structure and the structural modification by the GaAs layer. With increasing nominal thickness of the InAs QDs for the GDWELL samples, the emission wavelength is red-shifted due to the increase in the size of QDs, as shown in the AFM images of Fig. 1. Fig. 3(a) shows the light output power with respect to the injection current curves (L–I curves) for the BA-LDs with the CQDs and GDWELL structures as an active layer, respectively. For the CQDLDs, the lasing emission was not observed at RT even in pulsed mode. This could be attributed to the insufficient gain for the lasing emission. The low gain for the CQD-LDs is largely related to the low spatial confinement of carriers in QDs, especially in the vertical direction. Lasing operation for the GDWELL-LDs with a cavity length of 1 mm was achieved with a threshold current density of 900 A/cm2 in CW mode. The slope efficiency is measured to be 0.14 A/W.
Fig. 4. (a) L–I curves and (b) lasing spectra for the RW-LD with GDWELL structure.
The improvement in the lasing characteristics of the GDWELL-LDs is believed to be due to the increase in the carrier-capturing probability and structural modification, similar to the enhancement in the PL intensity. Fig. 3(b) shows the lasing spectra of the GDWELLLDs under CW condition with increasing injection current above the threshold current. The multi-mode lasing emission was observed with a center wavelength of 1437 nm. Considering the maximum peak wavelength in the PL spectrum (1490 nm), the lasing emission wavelength is relatively short. This shift in the lasing wavelength is mainly related to the gain required for the lasing operation. Fig. 4(a) shows the L–I curve for the RW-LDs with the same cavity length of 1 mm in CW mode. The threshold current density and slope efficiency are measured to be 1.1 kA/cm2 and 0.364 W/A, respectively, which are better than those of the previous reports. In particular, the slope efficiency of the RW-LDs with the GDWELL structure was significantly improved. The slope efficiencies of the QDLDs based on InP in the previous reports were less than 0.1 W/A even in pulsed mode or low temperature [22–25]. The improvement in the lasing characteristics is largely related to the increase in the spatial confinement of carriers and the structural uniformity by the additional QW and GaAs layer. The lasing emission was observed at the wavelength of 1445 nm at an injection current of 50 mA as shown in Fig. 4(b). With increasing injection current, the additional lasing modes were observed due to the increase in the
Fig. 5. (a) Temperature dependence of L–I curves for the RW-LDs with the GDWELL and (b) summary on the threshold current density with temperature. The characteristic temperature is estimated to be 51 K.
B. Jo et al. / Journal of Crystal Growth 393 (2014) 59–63
gain required for the lasing operation depending on the carrier injection. Also, the emission wavelength is slightly red-shifted mostly because of the interaction between carriers. To the best of our knowledge, this is the first observation on the lasing emission from the RW-LDs with the DWELL structures in CW mode. Fig. 5(a) shows the temperature dependence of L–I curves for the RW-LDs with the GDWELL in CW mode. With increasing temperature, the threshold current increases and the output power decreases. As a result, the slope efficiency is gradually decreased. This is mostly due to the enhancement of the nonradiative recombination with increasing temperature. Fig. 5(b) shows the summary on the threshold current density with respect to ambient temperature in CW mode. By fitting the experimental data using the relationship, Jth ¼J0exp(T/T0), where the Jth and J0 are the threshold current density at a given temperature (T) and 0 K, respectively, the characteristic temperatures (T0) of the RW-LD is measured to be 51 K at the temperature ranging from 20 to 65 1C.
4. Conclusion Lasing from InAs QDs with GDWELL structures was successfully achieved both from the BA-LDs and RW-LDs at temperatures higher than RT in CW mode, while the lasing operation was not observed from the LDs with CQDs. The improvement in the lasing characteristics of the LDs by adopting the GDWELL is due to improved carrier capture probability and structural modification. This result suggests that the DWELL and GDWELL structures can provide effective technical solutions for improved QD-LDs fabricated on InP substrates, similar to the GaAs-based InAs QD-LDs.
Acknowledgements This work was financially supported by the grant from the Industrial Source Technology Development Program (1415115378) of the Ministry of Knowledge Economy (MKE) of Korea. This work also partly supported by National Research Foundation of Korea Grant funded by the Korean Government (MEST) (No. 20100019626 and No. 2011-0008161)
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Investigation on the lasing characteristics of InAs/InGaAsP quantum dots with additional confinement structures Byounggu Jo a, Cheul-Ro Lee a, Jin Soo Kim a,n, Won Seok Han b, Jung Ho Song b, Jae-Young Leem c, Sam Kyu Noh d, Jae-Hyun Ryou e, Russell D. Dupuis f a Division of Advanced Materials Engineering, Research Center of Advanced Materials Development, Chonbuk National University, Jeonju 561-756, Republic of Korea b IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Republic of Korea c Department of Nano Systems Engineering, Inje University, Gimhae 621-749, Republic of Korea d Nano Materials Evaluation Center, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea e Department of Mechanical Engineering and Texas Center for Superconductivity at the University of Houston (TcSUH), University of Houston, Houston, TX 77204-4006, USA f Center for Compound Semiconductors and School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA
art ic l e i nf o
a b s t r a c t
Available online 21 November 2013
We report on morphological, optical, and lasing characteristics of InAs quantum dots (QDs) embedded in an In0.69Ga0.31As0.67P0.33 quantum well (having a bandgap energy corresponding to a wavelength of 1.35 μm (1.35Q-InGaAsP)), which formed a dot-in-a-well (DWELL) structure. This DWELL was further sandwiched in In0.85Ga0.15As0.32P0.68 layers (1.15 μm, 1.15Q-InGaAsP). A 2 monolayer-thick GaAs layer was simultaneously introduced right below the InAs QD layer in the DWELL structure (GDWELL). The emission wavelength of the InAs GDWELL was 1490 nm, which was slightly shorter than that of the InAs QDs embedded only in 1.15Q-InGaAsP layers. To evaluate the effects of the GDWELL structure on lasing characteristics, gain-guided broad-area (BA) and index-guided ridge-waveguide (RW) laser diodes (LDs) were fabricated. The BA-LDs with the InAs QDs embedded only in 1.15Q-InGaAsP layers did not show the lasing at room temperature (RT) even in pulsed mode. For the GDWELL structure, however, the lasing emissions from both the BA-LDs and RW-LDs were successfully achieved at RT in continuous-wave mode. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Quantum dots B2. InAs B2. InGaAsP B3. Laser diodes
1. Introduction Unique properties of nano-scale three-dimensionally-confined quantum dots (QDs) have motivated research on many photonic devices such as laser diodes (LDs) and photodetectors. In particular, the LDs with self-assembled InAs (or InGaAs) QDs on GaAs substrates as an active medium have been extensively studied for the applications of optical communication, medical, and industrial systems [1–3]. The QD-based LDs (QD-LDs) can offer several superior device performance characteristics, including low threshold current densities, improved thermal stability, and high material gain compared to those of other lower-dimensionallyconfined quantum structures [4,5]. The LDs employing InP-based InAs QDs are a potential candidate for high-power lasers emitting at wavelengths near 1.55 μm, thanks to their relatively narrower beam divergence than that of the quantum-well (QW)-based LDs [6,7]. In the previous report on the comparative study for QW- and
n
Corresponding author. Tel.: þ 82 63 270 2291; fax: þ82 63 270 2305. E-mail address: [email protected] (J.S. Kim).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.11.038
QD-LDs of identical structure, the emission characteristics including beam quality of QD-LDs were significantly improved compared to those of QW-LDs [7]. Technical challenges, however, remain for the growth of high-quality self-assembled QDs with low defects, high-density, and high-uniformity, mainly due to complicated formation mechanism of Stranski and Krastanow growth [4,5]. In particular, the formation of InAs QDs on InP substrates is significantly affected not only by growth parameters but also by the strain between the InAs and barrier materials. For the InAs QDs with an InGaAsP matrix, an As/P exchange reaction between QDs and the barrier should be considered during the growth of the QD region. That is, the As/P exchange effect can reduce the structural uniformity of QDs resulting in the degradation in zerodimensionality [8,9]. The QD-LDs on InP substrates employing self-assembled InAs QDs have been less explored compared to those of the GaAs-based epitaxial structures. Previous studies on the InAs QD-LDs on InP substrates have focused on the demonstration of lasing at room temperature (RT). Caroff et al. reported the lasing of InAs QDs at RT by using an InP (1 1 3)B substrate. Lelarge et al. demonstrated the RT operation of buried ridge-stripe LDs in continuous-wave
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(CW) mode, employing InAs QDs in an InGaAsP-InP (1 0 0) heterostructure [10,11]. Also, Kim, et al. demonstrated the lasing at temperatures higher than RT and single-mode lasing emission from InAs QDs in an InAlGaAs-InAlAs-InP heterostructure [12]. However, the lasing characteristics of the LDs with the InP-based InAs QDs were generally inferior to those with GaAs-based QDs. This is partially due to the reduced height of the InAs QDs buried in InP-based heterostructures. This morphological change in the self-assembled QDs results in reduced confinement of carriers in the vertical direction of a QD. As a result, the overlap integral between electron and hole wave-functions in a QD is not large enough, leading to low modal gain of the QD-LD structures. Recently, in order to improve capturing, thermalizing, and confinement of carriers, “dot-in-a-well (DWELL)” structures, where QDs are embedded in a QW, have been used in the active region of the GaAs-based InAs QD devices [13–16]. In this paper, we describe on the influences of the DWELL structure with a thin GaAs layer (GDWELL) on the optical and lasing characteristics of InAs QD-LDs fabricated on InP (0 0 1) substrates. In order to investigate the effects of the GDWELL structure, gainguided broad-area (BA) LDs and index-guided ridge-waveguide (RW) LDs were fabricated and characterized. Table 1 Comparison of different structures of the InAs quantum-dot samples used in this study. Samples
GDWELL structure
Nominal thickness of InAs (ML)
CQD GDWELL1 GDWELL2 GDWELL3
No Included Included Included
2.4 2.0 2.4 2.8
2. Experimental details The InAs QD samples (both exposed QDs on bottom cladding and waveguide layers and embedded QDs sandwiched between top and bottom waveguide and cladding layers) and QD-LD structures (multiple-layered QDs in LD structures) used in this study were grown on n-type InP (0 0 1) substrates by metal-organic chemical vapor deposition in a vertical-type reactor with the sources of trimethylindium, trimethyl-gallium, arsine, phosphine, and hydrogen gas. Before the deposition of the QD layer, a two-step 1.15Q-InGaAsP barrier with the growth rate of 3.16 Å/s was deposited at the growth temperature (Tg) of 640 and 520 1C. The thickness of the bottom 1.1Q-InGaAsP layer was 100 nm. And then, In0.69Ga0.31As0.67P0.33 (having a bandgap energy corresponding to a wavelength of 1.35 μm, 1.35Q-InGaAsP) and a GaAs layer with the thickness of 2 monolayers (MLs) were successively grown on the 1.1Q-InGaAsP layer. InAs with different nominal thicknesses of 2.0, 2.4, and 2.8 MLs were deposited on GaAs/In0.69Ga0.31As0.67P0.33 (GDWELL structures) to evaluate the formation characteristics of the QDs. The III/V ratio and the Tg for the QD layer were 0.81 and 520 1C, respectively. The insertion of this GaAs layer was intended for the modulation in strain between the InAs and InGaAsP, and the suppression of the inter-diffusion of As and P between the InAs QDs and the InGaAsP barriers. Detailed growth conditions of InAs QDs with the DWELL and GDWELL structures were described in the previous report [17]. Also, the InAs QDs with a nominal thickness of 2.4 MLs were formed on In0.85Ga0.15As0.32P0.68 (1.15 μm, 1.15Q-InGaAsP) for comparison. The layer structures of the QD samples to investigate surface morphologies and optical properties are summarized and compared in Table 1. In order to investigate the device characteristics of the QD-LDs with the GDWELL structures, we fabricated BA-LDs and RW-LDs.
Fig. 1. Surface morphologies of (a) 2.4 monolayer (ML)-thick InAs QDs on 1.15-μm quaternary (1.15Q)-InGaAsP, and (b) 2.0 ML-thick InAs QDs, (c) 2.4 ML-thick InAs QDs, and (d) 2.8 ML-thick InAs QDs on GaAs/1.35Q-InGaAsP/1.15Q-InGaAsP.
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The fabrication processes are relatively simple, and thus should not influence LD performance characteristics. Therefore, the dependence of intrinsic QD properties on the lasing characteristics can be evaluated. The width of a metal stripe for the BA-LDs and a ridge width for the RW-LD were 100 and 4 μm, respectively. The LD with conventional InAs QDs without the GDWELL structure (labeled as CQD-LD) was compared as a reference. The CQD-LD structure consists of seven-layer-stacked InAs QDs formed at a nominal thickness of 2.4 MLs and separated by a 10 nm-thick 1.15QInGaAsP spacer, which is further sandwiched by a spin index separate confinement heterostructure (SPIN-SCH) with an 800 nm-thick 1.15QInGaAsP waveguide layer and InP cladding layers. For comparison, the same LD structure with the seven-layer-stacked GDWELL inserted in the same SPIN-SCH was fabricated (GDWELL-LD). The structural and optical properties of the InAs QDs were investigated by atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy, respectively. Lasing characteristics of the QD-LDs were measured using a multi-function optical meter and a spectrum analyzer.
3. Results and discussion Fig. 1 shows surface morphologies of the exposed InAs QD samples measured by AFM. A spatial density and an average height of the InAs QDs with a nominal thickness of 2.4 MLs on a simple 1.15Q-InGaAsP layer (SL) (shown in Fig. 1(a)) are estimated to be 2.6 1010 cm 2 and 43.1 nm, respectively. Densities of the InAs QDs on GaAs/1.35Q-InGaAsP/1.15Q-InGaAsP layers (GWL) are measured to be 0.8 1010 cm 2, 1.9 1010 cm 2, and 2.4 1010 cm 2 for InAs nominal thicknesses of 2.0 MLs (GWL1), 2.4 MLs (GWL2), and 2.8 MLs (GWL3), respectively. The densities of these QDs of GWLs are lower than that of the QDs on the SL. Average lengths of the InAs QDs are 43.7, 46.2, and 48.1 nm for the GWL1, GWL2, and GWL3 samples, respectively. For the GWL2 sample with the same nominal thickness of InAs as the QDs on the SL, the average size is relatively large. The changes in the density and size of the InAs QDs are believed to depend on the presence of indium (In) atoms on the surface during the In and arsenic (As) precursor supply. That is, for the formation of the InAs QDs on the InGaAsP layer, the newly supplied In atoms for the QD layer are affected by the presence of In atoms on the surface of 1.15Q-InGaAsP. However, In atoms for the QD layer on the GaAs layer are not affected by In atoms of the buried 1.35Q-InGaAsP layer during the migration process. As a result, the migration characteristics of the In atoms on GaAs are altered, leading to decreased QD density and increased QD size. Also, the structural variation may occur due to the modulation in
Fig. 2. Photoluminescence (PL) spectra of the CQD and GDWELL samples.
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the strain between the InAs and 1.15Q-InGaAsP layers by the additional GaAs layer [18]. In addition, the diffusion of In atoms from the 1.15Q-InGaAsP layer into the QD layer and the well-known As/P exchange at the interface between the InAs QDs and InGaAsP layers can effectively be suppressed by the GaAs layer [19,20]. Fig. 2 shows the PL spectra for the embedded QD samples measured at RT. The emission wavelength, full-width at halfmaximum (FWHM), and intensity ratio for the QD samples are summarized in Table 2. The emission wavelength and FWHM for the GDWELL2 sample are 1490 nm. Even though the size of the QDs for the GDWELL2 sample is slightly larger than that of the CQD sample, the emission wavelength is blue-shifted by 13 nm from that of the CQD sample. The blue-shift in the peak position for the GDWELL2 sample is attributed to the effect of the highpotential barrier provided by the GaAs layer. In the previous work,
Table 2 Summary on the optical properties of the QD samples. Samples
Emission wavelength (nm)
FWHM (meV)
I/ICQD
CQD GDWELL1 GDWELL2 GDWELL3
1503 1470 1490 1564
92 78 69 68
1 2.6 4.3 3.6
Fig. 3. (a) Light output vs. current (L–I) curves for the BA-LDs with the CQDs and GDWELL structure, and (b) lasing spectra for the BA-LDs with GDWELL with increasing injection current in CW mode.
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the emission wavelength of InAs QDs in a GaAs matrix was significantly blue-shifted by changing the position of a thin AlAs layer below the QD layer [21]. Also, the introduction of gallium (Ga) atoms into QDs from GaAs layer may partially be responsible for the blue-shift in the emission wavelength. The FWHM for the GDWELL samples are narrower than that of the CQD sample with an FWHM of 92 meV, which is interpreted as improved size uniformity of the QDs (as shown in Fig. 1) and the reduced compositional fluctuation in QDs due to the suppressed As/P exchange [19]. The PL intensity for the GDWELL2 sample is more than four times stronger than that of the CQD sample, possibly due to the combined effects of the enhanced carrier-capturing probability by the additional QW structure and the structural modification by the GaAs layer. With increasing nominal thickness of the InAs QDs for the GDWELL samples, the emission wavelength is red-shifted due to the increase in the size of QDs, as shown in the AFM images of Fig. 1. Fig. 3(a) shows the light output power with respect to the injection current curves (L–I curves) for the BA-LDs with the CQDs and GDWELL structures as an active layer, respectively. For the CQDLDs, the lasing emission was not observed at RT even in pulsed mode. This could be attributed to the insufficient gain for the lasing emission. The low gain for the CQD-LDs is largely related to the low spatial confinement of carriers in QDs, especially in the vertical direction. Lasing operation for the GDWELL-LDs with a cavity length of 1 mm was achieved with a threshold current density of 900 A/cm2 in CW mode. The slope efficiency is measured to be 0.14 A/W.
Fig. 4. (a) L–I curves and (b) lasing spectra for the RW-LD with GDWELL structure.
The improvement in the lasing characteristics of the GDWELL-LDs is believed to be due to the increase in the carrier-capturing probability and structural modification, similar to the enhancement in the PL intensity. Fig. 3(b) shows the lasing spectra of the GDWELLLDs under CW condition with increasing injection current above the threshold current. The multi-mode lasing emission was observed with a center wavelength of 1437 nm. Considering the maximum peak wavelength in the PL spectrum (1490 nm), the lasing emission wavelength is relatively short. This shift in the lasing wavelength is mainly related to the gain required for the lasing operation. Fig. 4(a) shows the L–I curve for the RW-LDs with the same cavity length of 1 mm in CW mode. The threshold current density and slope efficiency are measured to be 1.1 kA/cm2 and 0.364 W/A, respectively, which are better than those of the previous reports. In particular, the slope efficiency of the RW-LDs with the GDWELL structure was significantly improved. The slope efficiencies of the QDLDs based on InP in the previous reports were less than 0.1 W/A even in pulsed mode or low temperature [22–25]. The improvement in the lasing characteristics is largely related to the increase in the spatial confinement of carriers and the structural uniformity by the additional QW and GaAs layer. The lasing emission was observed at the wavelength of 1445 nm at an injection current of 50 mA as shown in Fig. 4(b). With increasing injection current, the additional lasing modes were observed due to the increase in the
Fig. 5. (a) Temperature dependence of L–I curves for the RW-LDs with the GDWELL and (b) summary on the threshold current density with temperature. The characteristic temperature is estimated to be 51 K.
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gain required for the lasing operation depending on the carrier injection. Also, the emission wavelength is slightly red-shifted mostly because of the interaction between carriers. To the best of our knowledge, this is the first observation on the lasing emission from the RW-LDs with the DWELL structures in CW mode. Fig. 5(a) shows the temperature dependence of L–I curves for the RW-LDs with the GDWELL in CW mode. With increasing temperature, the threshold current increases and the output power decreases. As a result, the slope efficiency is gradually decreased. This is mostly due to the enhancement of the nonradiative recombination with increasing temperature. Fig. 5(b) shows the summary on the threshold current density with respect to ambient temperature in CW mode. By fitting the experimental data using the relationship, Jth ¼J0exp(T/T0), where the Jth and J0 are the threshold current density at a given temperature (T) and 0 K, respectively, the characteristic temperatures (T0) of the RW-LD is measured to be 51 K at the temperature ranging from 20 to 65 1C.
4. Conclusion Lasing from InAs QDs with GDWELL structures was successfully achieved both from the BA-LDs and RW-LDs at temperatures higher than RT in CW mode, while the lasing operation was not observed from the LDs with CQDs. The improvement in the lasing characteristics of the LDs by adopting the GDWELL is due to improved carrier capture probability and structural modification. This result suggests that the DWELL and GDWELL structures can provide effective technical solutions for improved QD-LDs fabricated on InP substrates, similar to the GaAs-based InAs QD-LDs.
Acknowledgements This work was financially supported by the grant from the Industrial Source Technology Development Program (1415115378) of the Ministry of Knowledge Economy (MKE) of Korea. This work also partly supported by National Research Foundation of Korea Grant funded by the Korean Government (MEST) (No. 20100019626 and No. 2011-0008161)
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