- Email: [email protected]
Si templates by MOCVD
Accepted Manuscript Evaluation of GaN/AlGaN THz quantum-cascade laser epi-layers grown on AlGaN/Si templates by MOCVD Sachie Fujikawa, Toshiya Ishiguro, Ke Wang, Wataru Terashima, Hiroki Fujishiro, Hideki Hirayama PII: DOI: Reference:
S0022-0248(18)30639-0 https://doi.org/10.1016/j.jcrysgro.2018.12.027 CRYS 24901
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
14 July 2018 14 November 2018 26 December 2018
Please cite this article as: S. Fujikawa, T. Ishiguro, K. Wang, W. Terashima, H. Fujishiro, H. Hirayama, Evaluation of GaN/AlGaN THz quantum-cascade laser epi-layers grown on AlGaN/Si templates by MOCVD, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/j.jcrysgro.2018.12.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of GaN/AlGaN THz quantum-cascade laser epi-layers grown on AlGaN/Si templates by MOCVD Sachie Fujikawa1, 2,*, Toshiya Ishiguro 2, 3, Ke Wang2,*, Wataru Terashima2, Hiroki Fujishiro3, Hideki Hirayama2 1 Department of Electrical and Electronic Engineering, Tokyo Denki University, 5 Senju, Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan 2
3
Quantum Optodevice Laboratory, RIKEN, 2-1 Hirosawa Wako, Saitama 351-0198, Japan Department of Applied Electronics, Tokyo University of Science, 6-3-1niijuku, Katsushika-ku, Tokyo 125-8585, Japan
Corresponding author. E-mail address: [email protected], [email protected] Tel.: +81-3-5284-5417; fax: +81-3-5284-5690 Abstract GaN/AlGaN THz quantum-cascade laser (QCL) structures were grown on AlGaN/Si templates by a metal-organic chemical vapor deposition (MOCVD). We aim to fabricate a double metal waveguide (DMW) based GaN THz QCL through removing the Si substrate by wet etching. The DMW is essential for realizing low cavity loss and a very high optical confinement factor (99%). We grew 100 periods of GaN/Al0.21Ga0.79N two quantum wells (QW) type active region on Al0.08Ga0.92N/Si templates. Samples were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). We found that the full width at half maximum (FWHM) of the XRD satellite peaks became narrower when the growth temperature decreased, and the steepness of the QC layer hetero-interfaces has been improved. Highlights GaN-based THz-QCL active region was grown by MOCVD on Si substrate. A GaN/AlGaN QC active layer consisting of two-QW for one-period was grown. Samples were evaluated by HR-XRD and cross-sectional TEM. FWHM of the XRD satellite peaks became narrower when growth temperature decreased. Keywords: A3. Metalorganic chemical vapor deposition; A3. Superlattices; B1. Nitrides 1. Introduction Terahertz quantum cascade lasers (THz-QCLs) [1,2] are attracting much attention as a compact THz light source which can achieve high output power, narrow line-width and continuous wave (cw) operation. THz-QCLs are expected to be used for variety of applications such as in medical imaging, 1
security screening or wireless communications, etc. The operation frequency of usual GaAs/AlGaAs THz-QCLs has been limited to be between 1.2 and 5.4 THz [3-5]. The highest operating temperature of the THz-QCL ever reported is 199.5 K [6]. Recently, we developed several GaAs/AlGaAs THz-QCLs operating at around 150 K [7-9]. The operation temperature of the lower frequency (<2 THz) QCL is mainly limited by thermal limit line (hν=k BT). Also, the GaAs QCL operation is inhibited in the frequency range above ~5.4 THz, which is attributed to the strong THz absorption due to the electron-longitudinal optical phonon (e-LO) scattering peaked in 8-9 THz range. GaN THz QCLs are expected to expand the operation frequency and to realize higher operation temperature. Nitride semiconductor is a material having potential for realizing wide frequency range of QCL, i.e., 3~20 THz and 1~8 µm, including an unexplored terahertz frequency from 5 to 12 THz. This is because: (1) GaN-based semiconductor has much higher LO phonon energies (ELO > 90 meV) in comparison with that of GaAs (~36 meV), and (2) the forbidden Reststrahlen band is shifted to 21-22 THz, a contrast to 8-9 THz for GaAs. Also, the conduction-band discontinuity of GaN/AlN (1.82 eV) is approximately 3 times larger than that of InAlAs/InGaAs (0.6 eV). Then, the mid-infrared (MIR) range of GaAs or InP-based QCL (3~16 µm) can be shifted to 1~8 µm by GaN-based QCL. Sapphire substrates have been used for the first trial of realizing GaN-based QCLs [10-12]. However, it is a problem that the absorption of THz light by sapphire becomes larger as the temperature rises up to 50K. The refractive index of sapphire is 3.4 at 1 THz, which is larger than 3.22 for GaN. It is still not easy to obtain a sufficient high optical confinement factor and low waveguide loss by using the usual single metal waveguide (SMW) design fabricated on a sapphire substrate, though the refractive index can vary due to doping and the tail of Reststrahlen band, and the bottom n++ plasmonic layer offers confinement to some extent [13]. On the other hand, a double metal waveguide (DMW) has an ideal high (>99%) optical confinement factor and a low waveguide loss required for a QCL lasing [13]. The properties of a DMW are very different from typical semiconductor laser diodes. A detailed analysis of the optical confinement and waveguide loss can be found in Ref 13. However, the removal technologies of sapphire substrates, which can hardly be etched by any chemicals, have been proved to be not easy, involves some complicated processes. In contrast, Si wet etching technologies have already been well established. In this study, we fabricated GaN/AlGaN THz-QCL structures on Si substrates for the purpose of fabricating a DMW having a very high optical confinement factor and low waveguide loss. Recently, we conducted a simulation of optical gain of GaN/AlGaN THz QCLs by using a non-equilibrium Green’s function (NEGF) method, and confirmed that a sufficiently high optical gain can be obtained at room temperature by introducing an appropriate QC design [14]. The QC epi-layers were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). 2. Experiment 2
Figure 1 shows a schematic structure of a GaN/AlGaN QCL structure grown on an n-AlGaN/Si template. We used Si-doped n-Al0.08Ga0.92N template on Si substrate provided by Enkris Semiconductor Co., Ltd. The stress management has been carefully developed, and the bow of 2-inch templates is below or around 10 µm. We grew 100 periods of two-quantum-well (QW) type GaN/Al0.17Ga0.83N QCL structure by a low-pressure metal-organic chemical-vapor deposition (LP-MOCVD). The thickness of the n-AlGaN template on Si was approximately 3 μm. The X-ray diffraction ω-scan rocking curves (XRC) of (002) and (102) plain were 370 and 500 arcsec, respectively. The Si-doping concentration of the template layer was 5×1018 cm-3. And, the edge-type threading dislocation density (TDD) was 8×108 cm-2. The layer sequence of the superlattices (SLs) was designed as GaN/AlGaN/GaN/AlGaN (60 /15 /40 /15 Å); one period was 13 nm and the total thickness of the QC layer was 1.3 μm. Trimethylgallium (TMGa), trimethylaluminum (TMAl), tetraethylsilane (TESi), and ammonia (NH 3 ) were used for the group III, the dopant and the group V sources. The growth pressure was 76 Torr, and the growth temperature was changed from 1170 to 1210 °C. The crystalline phase and quality of the sample were assessed with a HR-XRD, and the cross sectional image of the sample was observed by a TEM. 3. Results and discussion Figure 2 shows the HR-XRD (0002) 2θ-ω scans for a series of samples grown at 1210, 1190 and 1170 °C respectively. We observed clear 0th to -5th satellite peaks originated from QC layers as seen in the HR-XRD of the sample grown at 1190 °C. The typical (0002) 2θ-ω scan of an AlGaN/AlN-GaN-SL/AlN-buffer/Si template is shown in Figure 2(a) for comparison. The corresponding peak identification is indicated. For the three QC samples, these peaks are very similar to each other, suggesting good reproducibility of the AlGaN/Si templates. The highest peaks actually consist of two overlapped peaks, the AlGaN template and the 0th QC peaks. The insert in Figure 2(c) highlights the double peaks and corresponding fitting. The (-1 -1 4) plane reciprocal space mapping (RSM) in HR-XRD of GaN/AlGaN QC structure grown at 1190 °C is shown in Figure 3, which also clearly reveals the overlapped double peaks and the SL structures for both the template and QC structure. On the left side of the main peak in Figure 2(b, c, d), QC satellite peaks are clearly revealed. The FWHM of the satellite peaks originated from the QC structure obviously become narrower as the growth temperature is reduced. The narrower linewidths of the satellite peaks indicate that the steepness of the QC layer hetero-interfaces has been improved. For the sample grown at 1210 °C, the linewidths of the satellite peaks are the widest and their relative intensities are low. This is considered due to the deterioration of the crystalline quality of the GaN/AlGaN QCL structure. The enhanced diffusion of Al atoms in AlGaN into the GaN matrix at higher Tg should be responsible for the broadening of the satellite peaks at higher Tg. The cross-sectional TEM images of the GaN/AlGaN QC structure grown 1190 °C are shown in Figure 4. The zoom-in TEM image 3
demonstrates such a diffusion effect by the rough interfaces between barriers and wells. The TDD of SLs was about 2.2 × 109 cm-2. From the XRD RSM image shown in Figure 3, one can see that the AlN is under slight tensile strain with a relaxation factor of 85 %. However, the AlGaN template is almost fully relaxed with a relaxation factor of 90 % and its Al composition is 7 %, slightly lower than the nominated value. Combined together, we can extract the thickness and Al composition of the QC structures from the fitting to the 2θ-ω scans, as shown by the fit curve in Figure 2(c) for example. In the fitting, the AlN/GaN SL buffer is not included, but the AlN buffer and AlGaN template layer are included for clarity. For the sample grown at 1190 °C, the fit thicknesses of the QC structure is 77/19/51.5/19 Å with 21 % of Al in the AlGaN barrier layers. One period is 16.7 nm. From the cross-sectional TEM image of the SLs, the film thicknesses are 80.3/20/53.5/20 Å, and one period is 17.4 nm. The two methods give ±2% variation, matching to each other quite well. This is approximately 30% thicker than the designed value. While the samples at 1190 and 1210 °C show very close satellite peaks and thus similar thicknesses, the sample at 1170 °C demonstrates obviously increased thicknesses, which are 99/25/66/25 Å, with one period of 21.5 nm, and the Al composition is also fit by 21 %. We must say that determination of both Al composition and stain is not easy due to the overlap of the 0th peak with the Al0.07Ga0.93N peak. The QC structure seems slightly relaxed from the AlGaN template, but a quantitative determination is difficult. Nevertheless, the broadening effect of the QC structure satellite peaks with increasing T g is clearly revealed. And the TEM results confirm the rough interface due to diffusion of the Al in the AlGaN barrier layers into the GaN matrix. From these results, the growth temperature window is too high to achieve sharp barrier/well interfaces, which is a must for QCL operation. Reducing the Tg further is perhaps necessary for this purpose. 4.
Summary and conclusions We have fabricated GaN/AlGaN QCL structures on AlGaN/Si templates by MOCVD. A 100 periods of GaN/Al0.21Ga0.79N QC active layer consisting of two-QW for one-period was grown and evaluated by HR XRD and cross-sectional TEM. The XRD results demonstrate clear satellite peaks from QC structures. The linewidths of the satellite peaks of the QC layer become narrower when the growth temperature is reduced. However, the growth temperature window 1170-1120 °C is still too high and the interface is not sharp enough due to Al diffusion into GaN matrix. The results suggest that the steepness of the QC layer hetero-interfaces can be improved by reducing growth temperature further.
References 4
[1] R. Köhler, A.Tredicucci, F. Beltram, H.Beere, E.Linfield, A.Davies, D.Ritchie, R.Iotti, and F.Rossi, Nature (London) 417 (2002) 156. [2] J. Faist, F. Capasso, D. Sivco, C. Sirtori, A. Hutchinson, and A. Cho, Science vol.264 (1994) 553. [3] C. Walther, M. Fischer, G. Scalari, R. Terazzi, N. Hoyler, and J. Faist, Appl. Phys. Lett. 91 (2007) 131122. [4] M. Wienold, B. Roben, X. Lu, G. Rozas, L. Schrottke, K. Biermann, H. T. Grahn, Appl. Phys. Lett., 107 (2015) 202101. [5] TT. Lin, and Hideki Hirayama, Phys. Stat. Sol. A, 215 (2018) 1700424. [6] S. Fathololoumi, E. Dupont, C.W.I. Chan, Z. R. Wasilewski, S. R. Laframboise, D. Ban, A. Matyas, C. Jirauschek, Q. Hu, and H. C. Liu, OPT. EXPRESS 20 (2012) 3866. [7] TT. Lin, and H. Hirayama, Phys. Status Solidi A, 215 (2018) 1700424. [8] TT. Lin, and H. Hirayama, Phys. Stat. Sol. C 10, 11 (2013) 1430. [9] M. Sasaki, L.Tsung-Tse, and H.Hirayama, Phys. Stat. Sol. C 10, 11(2013) 1448. [10] W. Terashima, H. Hirayama, Applied Physics Express 3 (2010) 125501. [11] H. Hirayama, W. Terashima, T.-T. Lin and M. Sasaki, Proc. SPIE 9382 (2015) 938217. [12] W. Terashima, Hideki Hirayama, Proc. SPIE 9483 (2015) 948304. [13] K. Wang, TT. Lin, L. Wang, W. Terashima and H. Hirayama, Japanese Journal of Applied Physics, 57 (2018) 8. [14] K. Wang, T. Grange, TT. Lin, L. Wang, S. Birner, J. Yun, W. Terashima, H. Hirayama, Appl. Phys. Lett. 113 (2018) 061109.
5
Figure captions Fig. 1.
Schematic diagram of a GaN/AlGaN QCL structure grown on an n-AlGaN/Si template.
Fig. 2. Growth temperature dependence of the GaN/AlGaN QCL structures grown on AlGaN/Si templates evaluated by HR-XRD (0002) plane 2θ-ω scan. (a) The AlGaN/AlN-GaN-SL/AlN-buffer/Si template, (b)
Tg:1210 °C, (c) Tg:1190 °C and fitting, (d) Tg:1170 °C. The insert highlights the overlapped Al0.07Ga0.93N and 0th QCL peaks, and the fitting curve. Fig. 3. (-1 -1 4) plane RSM in HR-XRD of GaN/AlGaN QC structure grown at 1190 °C. Fig. 4. Cross-sectional TEM images of the GaN/AlGaN QC structure grown 1190 °C.
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Fig. 1.
Fujikawa et al.
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Fig. 2.
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(-1-14)
AlN buffer
QCL satellite peaks AlN/GaN SL buffer
Al0.07Ga0.93N
QCL 0th
QCL satellite peaks
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GaN AlGaN GaN AlGaN
GaN
GaN/AlGaN SL on Si 60/15/40/15Å(Design value) Actually, 80.3/20/53.5/20Å 3 0 nm
Fig. 4.
Fujikawa et al.
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S0022-0248(18)30639-0 https://doi.org/10.1016/j.jcrysgro.2018.12.027 CRYS 24901
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
14 July 2018 14 November 2018 26 December 2018
Please cite this article as: S. Fujikawa, T. Ishiguro, K. Wang, W. Terashima, H. Fujishiro, H. Hirayama, Evaluation of GaN/AlGaN THz quantum-cascade laser epi-layers grown on AlGaN/Si templates by MOCVD, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/j.jcrysgro.2018.12.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Evaluation of GaN/AlGaN THz quantum-cascade laser epi-layers grown on AlGaN/Si templates by MOCVD Sachie Fujikawa1, 2,*, Toshiya Ishiguro 2, 3, Ke Wang2,*, Wataru Terashima2, Hiroki Fujishiro3, Hideki Hirayama2 1 Department of Electrical and Electronic Engineering, Tokyo Denki University, 5 Senju, Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan 2
3
Quantum Optodevice Laboratory, RIKEN, 2-1 Hirosawa Wako, Saitama 351-0198, Japan Department of Applied Electronics, Tokyo University of Science, 6-3-1niijuku, Katsushika-ku, Tokyo 125-8585, Japan
Corresponding author. E-mail address: [email protected], [email protected] Tel.: +81-3-5284-5417; fax: +81-3-5284-5690 Abstract GaN/AlGaN THz quantum-cascade laser (QCL) structures were grown on AlGaN/Si templates by a metal-organic chemical vapor deposition (MOCVD). We aim to fabricate a double metal waveguide (DMW) based GaN THz QCL through removing the Si substrate by wet etching. The DMW is essential for realizing low cavity loss and a very high optical confinement factor (99%). We grew 100 periods of GaN/Al0.21Ga0.79N two quantum wells (QW) type active region on Al0.08Ga0.92N/Si templates. Samples were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). We found that the full width at half maximum (FWHM) of the XRD satellite peaks became narrower when the growth temperature decreased, and the steepness of the QC layer hetero-interfaces has been improved. Highlights GaN-based THz-QCL active region was grown by MOCVD on Si substrate. A GaN/AlGaN QC active layer consisting of two-QW for one-period was grown. Samples were evaluated by HR-XRD and cross-sectional TEM. FWHM of the XRD satellite peaks became narrower when growth temperature decreased. Keywords: A3. Metalorganic chemical vapor deposition; A3. Superlattices; B1. Nitrides 1. Introduction Terahertz quantum cascade lasers (THz-QCLs) [1,2] are attracting much attention as a compact THz light source which can achieve high output power, narrow line-width and continuous wave (cw) operation. THz-QCLs are expected to be used for variety of applications such as in medical imaging, 1
security screening or wireless communications, etc. The operation frequency of usual GaAs/AlGaAs THz-QCLs has been limited to be between 1.2 and 5.4 THz [3-5]. The highest operating temperature of the THz-QCL ever reported is 199.5 K [6]. Recently, we developed several GaAs/AlGaAs THz-QCLs operating at around 150 K [7-9]. The operation temperature of the lower frequency (<2 THz) QCL is mainly limited by thermal limit line (hν=k BT). Also, the GaAs QCL operation is inhibited in the frequency range above ~5.4 THz, which is attributed to the strong THz absorption due to the electron-longitudinal optical phonon (e-LO) scattering peaked in 8-9 THz range. GaN THz QCLs are expected to expand the operation frequency and to realize higher operation temperature. Nitride semiconductor is a material having potential for realizing wide frequency range of QCL, i.e., 3~20 THz and 1~8 µm, including an unexplored terahertz frequency from 5 to 12 THz. This is because: (1) GaN-based semiconductor has much higher LO phonon energies (ELO > 90 meV) in comparison with that of GaAs (~36 meV), and (2) the forbidden Reststrahlen band is shifted to 21-22 THz, a contrast to 8-9 THz for GaAs. Also, the conduction-band discontinuity of GaN/AlN (1.82 eV) is approximately 3 times larger than that of InAlAs/InGaAs (0.6 eV). Then, the mid-infrared (MIR) range of GaAs or InP-based QCL (3~16 µm) can be shifted to 1~8 µm by GaN-based QCL. Sapphire substrates have been used for the first trial of realizing GaN-based QCLs [10-12]. However, it is a problem that the absorption of THz light by sapphire becomes larger as the temperature rises up to 50K. The refractive index of sapphire is 3.4 at 1 THz, which is larger than 3.22 for GaN. It is still not easy to obtain a sufficient high optical confinement factor and low waveguide loss by using the usual single metal waveguide (SMW) design fabricated on a sapphire substrate, though the refractive index can vary due to doping and the tail of Reststrahlen band, and the bottom n++ plasmonic layer offers confinement to some extent [13]. On the other hand, a double metal waveguide (DMW) has an ideal high (>99%) optical confinement factor and a low waveguide loss required for a QCL lasing [13]. The properties of a DMW are very different from typical semiconductor laser diodes. A detailed analysis of the optical confinement and waveguide loss can be found in Ref 13. However, the removal technologies of sapphire substrates, which can hardly be etched by any chemicals, have been proved to be not easy, involves some complicated processes. In contrast, Si wet etching technologies have already been well established. In this study, we fabricated GaN/AlGaN THz-QCL structures on Si substrates for the purpose of fabricating a DMW having a very high optical confinement factor and low waveguide loss. Recently, we conducted a simulation of optical gain of GaN/AlGaN THz QCLs by using a non-equilibrium Green’s function (NEGF) method, and confirmed that a sufficiently high optical gain can be obtained at room temperature by introducing an appropriate QC design [14]. The QC epi-layers were evaluated by cross-sectional transmission-electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD). 2. Experiment 2
Figure 1 shows a schematic structure of a GaN/AlGaN QCL structure grown on an n-AlGaN/Si template. We used Si-doped n-Al0.08Ga0.92N template on Si substrate provided by Enkris Semiconductor Co., Ltd. The stress management has been carefully developed, and the bow of 2-inch templates is below or around 10 µm. We grew 100 periods of two-quantum-well (QW) type GaN/Al0.17Ga0.83N QCL structure by a low-pressure metal-organic chemical-vapor deposition (LP-MOCVD). The thickness of the n-AlGaN template on Si was approximately 3 μm. The X-ray diffraction ω-scan rocking curves (XRC) of (002) and (102) plain were 370 and 500 arcsec, respectively. The Si-doping concentration of the template layer was 5×1018 cm-3. And, the edge-type threading dislocation density (TDD) was 8×108 cm-2. The layer sequence of the superlattices (SLs) was designed as GaN/AlGaN/GaN/AlGaN (60 /15 /40 /15 Å); one period was 13 nm and the total thickness of the QC layer was 1.3 μm. Trimethylgallium (TMGa), trimethylaluminum (TMAl), tetraethylsilane (TESi), and ammonia (NH 3 ) were used for the group III, the dopant and the group V sources. The growth pressure was 76 Torr, and the growth temperature was changed from 1170 to 1210 °C. The crystalline phase and quality of the sample were assessed with a HR-XRD, and the cross sectional image of the sample was observed by a TEM. 3. Results and discussion Figure 2 shows the HR-XRD (0002) 2θ-ω scans for a series of samples grown at 1210, 1190 and 1170 °C respectively. We observed clear 0th to -5th satellite peaks originated from QC layers as seen in the HR-XRD of the sample grown at 1190 °C. The typical (0002) 2θ-ω scan of an AlGaN/AlN-GaN-SL/AlN-buffer/Si template is shown in Figure 2(a) for comparison. The corresponding peak identification is indicated. For the three QC samples, these peaks are very similar to each other, suggesting good reproducibility of the AlGaN/Si templates. The highest peaks actually consist of two overlapped peaks, the AlGaN template and the 0th QC peaks. The insert in Figure 2(c) highlights the double peaks and corresponding fitting. The (-1 -1 4) plane reciprocal space mapping (RSM) in HR-XRD of GaN/AlGaN QC structure grown at 1190 °C is shown in Figure 3, which also clearly reveals the overlapped double peaks and the SL structures for both the template and QC structure. On the left side of the main peak in Figure 2(b, c, d), QC satellite peaks are clearly revealed. The FWHM of the satellite peaks originated from the QC structure obviously become narrower as the growth temperature is reduced. The narrower linewidths of the satellite peaks indicate that the steepness of the QC layer hetero-interfaces has been improved. For the sample grown at 1210 °C, the linewidths of the satellite peaks are the widest and their relative intensities are low. This is considered due to the deterioration of the crystalline quality of the GaN/AlGaN QCL structure. The enhanced diffusion of Al atoms in AlGaN into the GaN matrix at higher Tg should be responsible for the broadening of the satellite peaks at higher Tg. The cross-sectional TEM images of the GaN/AlGaN QC structure grown 1190 °C are shown in Figure 4. The zoom-in TEM image 3
demonstrates such a diffusion effect by the rough interfaces between barriers and wells. The TDD of SLs was about 2.2 × 109 cm-2. From the XRD RSM image shown in Figure 3, one can see that the AlN is under slight tensile strain with a relaxation factor of 85 %. However, the AlGaN template is almost fully relaxed with a relaxation factor of 90 % and its Al composition is 7 %, slightly lower than the nominated value. Combined together, we can extract the thickness and Al composition of the QC structures from the fitting to the 2θ-ω scans, as shown by the fit curve in Figure 2(c) for example. In the fitting, the AlN/GaN SL buffer is not included, but the AlN buffer and AlGaN template layer are included for clarity. For the sample grown at 1190 °C, the fit thicknesses of the QC structure is 77/19/51.5/19 Å with 21 % of Al in the AlGaN barrier layers. One period is 16.7 nm. From the cross-sectional TEM image of the SLs, the film thicknesses are 80.3/20/53.5/20 Å, and one period is 17.4 nm. The two methods give ±2% variation, matching to each other quite well. This is approximately 30% thicker than the designed value. While the samples at 1190 and 1210 °C show very close satellite peaks and thus similar thicknesses, the sample at 1170 °C demonstrates obviously increased thicknesses, which are 99/25/66/25 Å, with one period of 21.5 nm, and the Al composition is also fit by 21 %. We must say that determination of both Al composition and stain is not easy due to the overlap of the 0th peak with the Al0.07Ga0.93N peak. The QC structure seems slightly relaxed from the AlGaN template, but a quantitative determination is difficult. Nevertheless, the broadening effect of the QC structure satellite peaks with increasing T g is clearly revealed. And the TEM results confirm the rough interface due to diffusion of the Al in the AlGaN barrier layers into the GaN matrix. From these results, the growth temperature window is too high to achieve sharp barrier/well interfaces, which is a must for QCL operation. Reducing the Tg further is perhaps necessary for this purpose. 4.
Summary and conclusions We have fabricated GaN/AlGaN QCL structures on AlGaN/Si templates by MOCVD. A 100 periods of GaN/Al0.21Ga0.79N QC active layer consisting of two-QW for one-period was grown and evaluated by HR XRD and cross-sectional TEM. The XRD results demonstrate clear satellite peaks from QC structures. The linewidths of the satellite peaks of the QC layer become narrower when the growth temperature is reduced. However, the growth temperature window 1170-1120 °C is still too high and the interface is not sharp enough due to Al diffusion into GaN matrix. The results suggest that the steepness of the QC layer hetero-interfaces can be improved by reducing growth temperature further.
References 4
[1] R. Köhler, A.Tredicucci, F. Beltram, H.Beere, E.Linfield, A.Davies, D.Ritchie, R.Iotti, and F.Rossi, Nature (London) 417 (2002) 156. [2] J. Faist, F. Capasso, D. Sivco, C. Sirtori, A. Hutchinson, and A. Cho, Science vol.264 (1994) 553. [3] C. Walther, M. Fischer, G. Scalari, R. Terazzi, N. Hoyler, and J. Faist, Appl. Phys. Lett. 91 (2007) 131122. [4] M. Wienold, B. Roben, X. Lu, G. Rozas, L. Schrottke, K. Biermann, H. T. Grahn, Appl. Phys. Lett., 107 (2015) 202101. [5] TT. Lin, and Hideki Hirayama, Phys. Stat. Sol. A, 215 (2018) 1700424. [6] S. Fathololoumi, E. Dupont, C.W.I. Chan, Z. R. Wasilewski, S. R. Laframboise, D. Ban, A. Matyas, C. Jirauschek, Q. Hu, and H. C. Liu, OPT. EXPRESS 20 (2012) 3866. [7] TT. Lin, and H. Hirayama, Phys. Status Solidi A, 215 (2018) 1700424. [8] TT. Lin, and H. Hirayama, Phys. Stat. Sol. C 10, 11 (2013) 1430. [9] M. Sasaki, L.Tsung-Tse, and H.Hirayama, Phys. Stat. Sol. C 10, 11(2013) 1448. [10] W. Terashima, H. Hirayama, Applied Physics Express 3 (2010) 125501. [11] H. Hirayama, W. Terashima, T.-T. Lin and M. Sasaki, Proc. SPIE 9382 (2015) 938217. [12] W. Terashima, Hideki Hirayama, Proc. SPIE 9483 (2015) 948304. [13] K. Wang, TT. Lin, L. Wang, W. Terashima and H. Hirayama, Japanese Journal of Applied Physics, 57 (2018) 8. [14] K. Wang, T. Grange, TT. Lin, L. Wang, S. Birner, J. Yun, W. Terashima, H. Hirayama, Appl. Phys. Lett. 113 (2018) 061109.
5
Figure captions Fig. 1.
Schematic diagram of a GaN/AlGaN QCL structure grown on an n-AlGaN/Si template.
Fig. 2. Growth temperature dependence of the GaN/AlGaN QCL structures grown on AlGaN/Si templates evaluated by HR-XRD (0002) plane 2θ-ω scan. (a) The AlGaN/AlN-GaN-SL/AlN-buffer/Si template, (b)
Tg:1210 °C, (c) Tg:1190 °C and fitting, (d) Tg:1170 °C. The insert highlights the overlapped Al0.07Ga0.93N and 0th QCL peaks, and the fitting curve. Fig. 3. (-1 -1 4) plane RSM in HR-XRD of GaN/AlGaN QC structure grown at 1190 °C. Fig. 4. Cross-sectional TEM images of the GaN/AlGaN QC structure grown 1190 °C.
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Fig. 1.
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Fig. 2.
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(-1-14)
AlN buffer
QCL satellite peaks AlN/GaN SL buffer
Al0.07Ga0.93N
QCL 0th
QCL satellite peaks
Fig. 3.
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GaN AlGaN GaN AlGaN
GaN
GaN/AlGaN SL on Si 60/15/40/15Å(Design value) Actually, 80.3/20/53.5/20Å 3 0 nm
Fig. 4.
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