- Email: [email protected]
Effects of well widths and well numbers on InP-based triangular quantum well lasers beyond 2.4 µm
Journal of Crystal Growth 425 (2015) 376–380
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effects of well widths and well numbers on InP-based triangular quantum well lasers beyond 2.4 mm Y. Gu, Y.G. Zhang n, X.Y. Chen, Y.Y. Cao, L. Zhou, S.P. Xi, A.Z. Li, Hsby. Li State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
art ic l e i nf o
a b s t r a c t
Available online 6 March 2015
The effects of well widths and well numbers of InGaAs triangular quantum well lasers in 2.30–2.44 μm range using antimony-free material system on InP substrates are investigated. The triangular quantum well was equivalently formed by using gas source molecular beam epitaxy grown InAs/In0.53Ga0.47As digital alloy and the pseudomorphic growth was confirmed by the X-ray diffraction measurements. Lasing at 2.30 μm above 330 K under continuous wave operation has been achieved for the laser with four 13 nm quantum wells. By increasing the well width to 19 nm, the continuous wave wavelength has been extended to 2.44 μm at 290 K, whereas the epitaxial quality and laser performances are deteriorated. For those lasers with well width up to 19 nm, the moderate reduction of the quantum well numbers can restrict the strain accumulation and improve the laser performances. Continuous wave lasing at 2.38 μm above 300 K has been achieved. & 2015 Elsevier B.V. All rights reserved.
Keywords: A3. Molecular beam epitaxy B1. Arsenates B2. Semiconducting III–V materials B3. Laser diodes
1. Introduction Semiconductor laser diodes emitting in 2–2.5 mm wavelength range are very attractive for free space communications and light detection and ranging (LIDAR) due to the low absorption by atmospheric water molecules in this range [1]. Such devices are also of great interests in atmospheric pollution monitoring and medical diagnostics by tunable diode laser absorption spectroscopy (TDLAS) gas sensing [2]. Furthermore, it is also desired for the characterization and evaluation of photodetectors and focal plane arrays operating in this wavelength range, because wavelength match is very important to determine their actual features [3]. Compressively strained type-I InGaAs quantum well (QW) lasers on InP substrate offer an attractive alternative to traditional GaSb-based structures for semiconductor lasers in this wavelength range. The emission wavelength can be tailored by the indium composition and well width of InGaAs QWs, and the main obstacle is how to control the significant strain and keep good structural quality in the QWs when increasing the indium contents [4,5]. To improve the material quality, several methods have been applied, such as growing the QWs at a relatively low temperature [6], applying a relatively high growth rate [7], and using surfactant [8]. By using 5 nm thick pure InAs in the QWs, lasers extended to 2.33 μm have been demonstrated under continuous wave (CW) n
Corresponding author. Tel.: þ 86 21 62511070; fax: þ 86 21 52419931. E-mail address: [email protected] (Y.G. Zhang).
http://dx.doi.org/10.1016/j.jcrysgro.2015.02.091 0022-0248/& 2015 Elsevier B.V. All rights reserved.
operation at room temperature (RT) [9,10]. On the other hand, the use of triangular QW instead of rectangular one can increase the lasing wavelength while keeping the same strain extent [11,12]. The triangular QW can be grown by digital alloy technology, which is very effective to restrain the formation of three dimensional growth in the strained QW thus improve the laser performances [13,14]. By this approach, the InAs/InGaAs triangular QW lasers on InP substrate with wavelength up to 2.4 mm have been reported recently [15]. To control the strain in the QW, the QW structures, especially the well widths and numbers, play a very crucial role. In this work, InAs/InGaAs triangular QW lasers with different well widths and well numbers were demonstrated and investigated in detail. The well widths were changed to tailor the wavelength, and the well numbers were adjusted to optimize the laser performances.
2. Experiments The epitaxial laser wafers were all grown on n-type (001)oriented InP epi-ready substrates in a VG Semicon V80H gas source molecular beam epitaxy (GSMBE) system. The elemental indium, gallium and aluminum were used as group III sources, while As2 and P2 cracked from Arsine and phosphine were used as group V sources. The InP substrates were at first heated to about 500 1C measured by thermocouple under P2 flux to carry out the surface oxide desorption and then reduced to 440 1C for growth.
Y. Gu et al. / Journal of Crystal Growth 425 (2015) 376–380
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0.4
p-InGaAs contact, 300 nm
well width
p-InP cladding, 1700 nm
EC
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uid-triangular QWs uid-InGaAsP waveguide, 120 nm
Energy (eV)
uid-InGaAsP waveguide, 120 nm 0.0
In 0.53Ga 0.47 As InAs -0.2
n-InP buffer, 1000 nm EV
-0.4
n-InP substrate
10
20 30 Growth thickness (nm)
40
Fig. 1. (a) Schematic structure of the grown triangular QW laser samples. (b) Schematic diagram of the appeared (solid line) and effective (dotted line) band energy of the digital alloy for one QW.
Table 1 The well widths and numbers of samples 1–3. Sample no.
Well width (nm)
Well number
1 2 3
13 19 19
4 4 2
As shown in Fig. 1(a), the growth started from a 1000-nm silicon doped n-type InP buffer, followed by a 120-nm unintentionally doped (uid) lattice-matched InGaAsP (Eg ¼1.1 eV) waveguide layer which was grown at 460 1C. The substrate temperature was then decreased to 420 1C to grow the uid active triangular QWs. In the QW regions, the indium composition was approximately increased from 0.53 to 1 and then decreased to 0.53, which was approximated by using InAs/In0.53Ga0.47As digital alloy technology. Fig. 1 (b) shows the appeared and effective band energy of one QW. The designed period thickness of the digital alloy including one InAs layer and one In0.53Ga0.47As layer was 1 nm. The thicknesses of the InAs layer (d1) and In0.53Ga0.47As layer (1 d1) are obtained from d1 þ0.53 (1 d1)¼α, where α is the expected average indium composition in each short period. These layers are very thin, and the thinnest layer is even thinner than one monolayer, therefore these thin layers are supposed to intermix during the growth. When growing the QW structures, only the Ga shutter was opening and closing, without any growth interruption. The growth rates of InAs and InGaAs were 0.152 and 0.287 nm/s, respectively. Three samples with different well widths and well numbers were grown and the parameters were listed in Table 1. In0.53Ga0.47As layers were grown as barriers and the barrier thickness were 20 nm except for the first and last barrier layers, which were thickened to 100 nm to enhance the confinement. Therefore, the active QW layers were grown keeping the indium and gallium fluxes unchanged. Afterwards, a 120-nm uid InGaAsP upper waveguide, a 1700-nm beryllium doped p-type InP upper cladding layer and a 300-nm p-type In0.53Ga0.47As contact layer were grown at 440 1C. The substrate was undergoing rotation during the growth to get uniform thickness. After growth, the structural properties of the wafers were characterized by X-ray diffraction (XRD) scanning curves using a
Philips X’pert MRD high resolution X-ray diffractometer equipped with a four-crystal Ge (220) monochromator. Then, the ridge waveguide lasers with strip width of 6 mm were fabricated by using standard lithography and wet chemical etching. 300-nm Si3N4 layers were deposited by plasma enhanced chemical vapor deposition for isolation, and 4-mm-wide windows were opened on the top of the ridges. Sputtered Ti/Pt/Au and evaporated Ge/Au/Ni/ Au were formed as top p-type and bottom n-type contacts, respectively. After an alloyed step, the wafers were diced into bars of 800 mm leaving as-cleaved laser facets, mounted on copper heat sinks, wire bonded and installed into an Oxford Optistat DN-V variable temperature liquid nitrogen cryostat. The lasers were CW driven by a Keithley 2420 source meter, and the spectral characteristics were characterized by a Nicolet 860 Fourier transform infrared spectrometer using a liquid-nitrogen cooled InSb detector and a CaF2 beam splitter. For the I–P measurements, a Coherent EMP1000 power meter was used.
3. Results and discussions The XRD (004) ω/2θ scanning curves of the grown laser wafers with various well widths and numbers in the active QW region are shown in Fig. 2, the strongest peak corresponds to InP and the envelope signals in the left side correspond to the active QWs. The XRD curves at different positions on the wafer are nearly the same indicating the pretty good uniformity of the QWs. Distinct satellite peaks are observed in sample 1. For sample 2 where the QW number is still four but the well width increases to 19 nm, the satellite peaks in the measured XRD curve become dimer, indicating the deteriorated QW quality with the rough heterostructure interfaces due to the accumulated strain. For sample 3, the satellite peaks become much more distinct for the QW number of two and keeping the well width of 19 nm, suggesting that the material quality of 19 nm QWs is improved by decreasing the QW number from four to two. In Fig. 2, the simulated curves from lattice dynamics for samples 1–3 are also shown. The simulated curves were all achieved by the simulation from the exact structure as designed. The good agreements of the diffraction peaks between the measured and simulated curves confirm the pseudomorphic
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Fig. 2. Measured (upper curve) and simulated (bottom curve) XRD (004) ω/2θ scan curves of the triangular QW laser samples 1–3.
2.45
1.41 nm/K 2.40
Wavelength ( μm)
growth of all samples. For these whole laser structures, the PL signals from QWs cannot be observed as about 2 μm thick top layers were grown on the QWs. In the PL measurements of QW test samples with different well widths, the PL intensity was observed a bit decreased as the increased QW width [16,17]. As shown in Fig. 3 the temperature dependent CW lasing wavelengths of the lasers have shown various performances, and the typical lasing spectra of samples 1–3 are shown in Fig. 4. The lasing wavelength is 2.30 mm at 300 K for sample 1 with four 13 nm QWs, and redshifts to 2.44 mm at 290 K as the well width increases to 19 nm for sample 2. However, the maximum lasing temperature is decreased from 330 K for sample 1 to 290 K for sample 2 due to the deteriorated material quality. For the 19 nm QW lasers, as the well number decreases from four to two, the maximum lasing temperature is increased to beyond 300 K and the lasing wavelength is around 2.38 mm at 300 K for sample 3. From linear fitting, the temperature dependent wavelength shift values are 1.00 nm/K, 1.41 nm/K and 1.01 nm/K for samples 1, 2 and 3, respectively. It is noted that the lasing wavelengths of sample 2 are longer than those of sample 3 at all temperatures. It is probably because that the QW wave functions are partly coupled as the carrier confinement is relatively weak in this triangular QW structure due to the inclined energy band. Fig. 5 shows the light output characteristics of the three triangular QW lasers under CW operation. The output power is about 34 mW/facet at 350 mA injection current at 200 K and 15 mW/facet at 300 K for sample 1 with 13 nm QWs. The threshold current is about 17 mA at 200 K and increases to 58 mA at 300 K. As the well width increases to 19 nm while keeping four QWs in the active region (sample 2), the power at 200 K is decreased to only 11 mW/facet at 350 mA injection current. The threshold current of sample 2 is significantly increased to 72 mA and 260 mA at 200 K and 290 K, respectively. As the QW number of 19 nm QW laser decreases from four to two, the output power is increased to 16 mW/facet at 350 mA injection current at 200 K
2.35
1.01 nm/K Sample 2
1.00 nm/K
2.30 Sample 3 2.25
Sample 1
2.20 200 220 240 260 280 300 320 340 Temperature (K) Fig. 3. Temperature dependence of the CW lasing wavelengths for the triangular QW laser samples 1–3. The temperature dependent wavelength shift values for the three samples are all indicated.
although there are less QWs in the active region. The maximum output power at 300 K is 1.8 mW/facet. The threshold current of sample 3 is reduced to about 10 mA at 200 K and 96 mA at 300 K. The improved output characteristics are mainly due to the better material quality. The laser threshold current densities at various temperatures and derived characteristic temperatures for the three samples are shown in Fig. 6. For sample 1 with the 13 nm QWs, the threshold current density is about 1.2 kA/cm2 at 300 K (300 A/cm2 per QW). Two different slopes of the threshold current density exist in different temperature ranges. The characteristic temperature T0 is derived to be about 75 K in the temperature range of 200–290 K, and decreases to 36 K in the temperature range of 290–330 K. For sample 2 with the increased well width of 19 nm, the threshold current density is significantly increased. The characteristic temperature T0 is about 81 K and 48 K in the temperature ranges of 200–260 K and 260–
Y. Gu et al. / Journal of Crystal Growth 425 (2015) 376–380
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Fig. 4. CW lasing spectra of the triangular QW laser sample 1 at 300 K, sample 2 at 290 K and sample 3 at 300 K injected by 1.2 times of the threshold current.
Current (mA)
Sample 3 Voltage (V)
2.5
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1.5 1.0
5
280K 0.5 0.0
0
100
300K 200 300 Current (mA)
400
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20
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0
Fig. 5. Output powers at various temperatures and voltages at 300 K as a function of injected currents of samples 1–3.
290 K respectively, both close to those of sample 1. Comparing to those of sample 2, the threshold current density of sample 3 is significantly reduced as the well number decreases from four to two. At 200 K the threshold current density of sample 3 is even smaller than that of
T0=22 K
T0=36 K
Sample 1 Sample 2 Sample 3
200 220 240 260 280 300 320 340 Temperature(K)
Fig. 6. Temperature dependence of threshold current densities and characteristic temperatures (T0) of samples 1–3. Two different T0 values are derived in different temperature ranges for each sample.
sample 1 probably due to the less QWs of sample 3, however the threshold current density of sample 3 increases much more quickly as the temperature increases, reaching 2.0 kA/cm2 at 300 K. The characteristic temperature T0 of sample 3 is 50 K and 22 K for 200–280 K and 280–300 K temperature ranges, smaller than those of samples 1 and 2, which is probably because of the reduced gain as the decreased QW number. For this kind of highly strained triangular QW laser, a tradeoff should be considered between the strain control and sufficient gain. For the structure with thinner QWs, more QW number can be applied to achieve higher output power. In our experience, the performances of the 4 QWs laser structures do not degrade a lot when the well width enlarged from 10 nm to 16 nm [14,15]. As a rough estimation, 4 QWs could be around the maximum QW number for the 16 nm QW laser structure, but more QW number could be applied for thinner QWs whereas less QW number for wider QWs.
4. Conclusion In conclusion, InP-based InGaAs triangular QW lasers with different well widths and numbers have been grown by GSMBE. The triangular QW was equivalently formed by using InAs/In0.53Ga0.47As digital alloy with carefully adjusted growth thickness of each layer. X-ray diffraction measurements have confirmed the pseudomorphic growth. For the laser with four 13 nm QWs, CW lasing at 2.30 μm above 330 K has been achieved. By increasing the QW width to 19 nm, the CW lasing wavelength is extended to 2.44 mm at 290 K although the material qualities and laser performances are deteriorated. For the 19 nm triangular QW lasers, the reduction of the QW number from four to two can improve the laser performances and CW lasing at 2.38 μm above 300 K has been achieved.
Acknowledgments This work was supported by the National Basic Research Program of China under Grant no. 2012CB619202 and the National Natural Science Foundation of China under Grant nos. 61275113, 61204133 and 61405232. References [1] A. Bauer, K. Rößner, T. Lehnhardt, M. Kamp, S. Höfling, L. Worschech, A. Forchel, Semicond. Sci. Technol. 26 (2011) 014032. [2] W. Lei, C. Jagadish, J. Appl. Phys. 104 (2008) 091101.
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[3] Y. Gu, Y.G. Zhang, K. Wang, X. Fang, C. Li, L. Zhou, A.Z. Li, Hsby. Li, J. Cryst. Growth 378 (2013) 65. [4] S. Forouhar, A. Ksebdzov, A. Larsson, H. Temkin, Electron. Lett. 28 (1992) 1431. [5] J.S. Wang, H.H. Lin, L.W. Sung, IEEE J. Quantum Electron. 34 (1998) 1959. [6] S. Jourba, M. Gendry, P. Regreny, G. Hollinger, J. Cryst. Growth 201/202 (1999) 1101. [7] S. Kim, J. Kirch, L. Mawst, J. Cryst. Growth 312 (2010) 1388. [8] T. Sato, M. Mitsuhara, T. Watanabe, Y. Kondo, Appl. Phys. Lett. 87 (2005) 211903. [9] T. Sato, M. Mitsuhara, Y. Kondo, Electron. Lett. 43 (2007) 1143. [10] T. Sato, M. Mitsuhara, N. Nunoya, T. Fujisawa, K. Kasaya, F. Kano, Y. Kondo, IEEE Photonics Technol. Lett. 20 (2008) 1045.
[11] G. Boehm, M. Grau, O. Dier, K. Windhorn, E. Roenneberg, J. Rosskopf, R. Shau, R. Meyer, M. Ortsiefer, M.-C. Amann, J. Cryst. Growth 301–302 (2007) 941. [12] Y. Gu, Y.G. Zhang, S. Liu, Chin. Phys. Lett. 24 (2007) 3237. [13] S. Jourba, M. Gendry, O. Marty, M. Pitaval, G. Hollinger, Appl. Phys. Lett. 75 (1999) 220. [14] Y.Y. Cao, Y.G. Zhang, Y. Gu, X.Y. Chen, L. Zhou, Hsby. Li, IEEE Photonics Technol. Lett. 26 (2014) 571. [15] Y. Gu, Y.G. Zhang, Y.Y. Cao, L. Zhou, X.Y. Chen, Hsby Li, S.P. Xi, Appl. Phys. Express 7 (2014) 032701. [16] Y. Gu, Y.G. Zhang, K. Wang, A.Z. Li, Y.Y. Li, J. Cryst. Growth 311 (2009) 1935. [17] Y.Y. Cao, Y. Gu, Y.G. Zhang, Y.Y. Li, X. Fang, A.Z. Li, L. Zhou, Hsby. Li, J. Infrared Millim. Waves 33 (2014) 213.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effects of well widths and well numbers on InP-based triangular quantum well lasers beyond 2.4 mm Y. Gu, Y.G. Zhang n, X.Y. Chen, Y.Y. Cao, L. Zhou, S.P. Xi, A.Z. Li, Hsby. Li State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
art ic l e i nf o
a b s t r a c t
Available online 6 March 2015
The effects of well widths and well numbers of InGaAs triangular quantum well lasers in 2.30–2.44 μm range using antimony-free material system on InP substrates are investigated. The triangular quantum well was equivalently formed by using gas source molecular beam epitaxy grown InAs/In0.53Ga0.47As digital alloy and the pseudomorphic growth was confirmed by the X-ray diffraction measurements. Lasing at 2.30 μm above 330 K under continuous wave operation has been achieved for the laser with four 13 nm quantum wells. By increasing the well width to 19 nm, the continuous wave wavelength has been extended to 2.44 μm at 290 K, whereas the epitaxial quality and laser performances are deteriorated. For those lasers with well width up to 19 nm, the moderate reduction of the quantum well numbers can restrict the strain accumulation and improve the laser performances. Continuous wave lasing at 2.38 μm above 300 K has been achieved. & 2015 Elsevier B.V. All rights reserved.
Keywords: A3. Molecular beam epitaxy B1. Arsenates B2. Semiconducting III–V materials B3. Laser diodes
1. Introduction Semiconductor laser diodes emitting in 2–2.5 mm wavelength range are very attractive for free space communications and light detection and ranging (LIDAR) due to the low absorption by atmospheric water molecules in this range [1]. Such devices are also of great interests in atmospheric pollution monitoring and medical diagnostics by tunable diode laser absorption spectroscopy (TDLAS) gas sensing [2]. Furthermore, it is also desired for the characterization and evaluation of photodetectors and focal plane arrays operating in this wavelength range, because wavelength match is very important to determine their actual features [3]. Compressively strained type-I InGaAs quantum well (QW) lasers on InP substrate offer an attractive alternative to traditional GaSb-based structures for semiconductor lasers in this wavelength range. The emission wavelength can be tailored by the indium composition and well width of InGaAs QWs, and the main obstacle is how to control the significant strain and keep good structural quality in the QWs when increasing the indium contents [4,5]. To improve the material quality, several methods have been applied, such as growing the QWs at a relatively low temperature [6], applying a relatively high growth rate [7], and using surfactant [8]. By using 5 nm thick pure InAs in the QWs, lasers extended to 2.33 μm have been demonstrated under continuous wave (CW) n
Corresponding author. Tel.: þ 86 21 62511070; fax: þ 86 21 52419931. E-mail address: [email protected] (Y.G. Zhang).
http://dx.doi.org/10.1016/j.jcrysgro.2015.02.091 0022-0248/& 2015 Elsevier B.V. All rights reserved.
operation at room temperature (RT) [9,10]. On the other hand, the use of triangular QW instead of rectangular one can increase the lasing wavelength while keeping the same strain extent [11,12]. The triangular QW can be grown by digital alloy technology, which is very effective to restrain the formation of three dimensional growth in the strained QW thus improve the laser performances [13,14]. By this approach, the InAs/InGaAs triangular QW lasers on InP substrate with wavelength up to 2.4 mm have been reported recently [15]. To control the strain in the QW, the QW structures, especially the well widths and numbers, play a very crucial role. In this work, InAs/InGaAs triangular QW lasers with different well widths and well numbers were demonstrated and investigated in detail. The well widths were changed to tailor the wavelength, and the well numbers were adjusted to optimize the laser performances.
2. Experiments The epitaxial laser wafers were all grown on n-type (001)oriented InP epi-ready substrates in a VG Semicon V80H gas source molecular beam epitaxy (GSMBE) system. The elemental indium, gallium and aluminum were used as group III sources, while As2 and P2 cracked from Arsine and phosphine were used as group V sources. The InP substrates were at first heated to about 500 1C measured by thermocouple under P2 flux to carry out the surface oxide desorption and then reduced to 440 1C for growth.
Y. Gu et al. / Journal of Crystal Growth 425 (2015) 376–380
377
0.4
p-InGaAs contact, 300 nm
well width
p-InP cladding, 1700 nm
EC
0.2
uid-triangular QWs uid-InGaAsP waveguide, 120 nm
Energy (eV)
uid-InGaAsP waveguide, 120 nm 0.0
In 0.53Ga 0.47 As InAs -0.2
n-InP buffer, 1000 nm EV
-0.4
n-InP substrate
10
20 30 Growth thickness (nm)
40
Fig. 1. (a) Schematic structure of the grown triangular QW laser samples. (b) Schematic diagram of the appeared (solid line) and effective (dotted line) band energy of the digital alloy for one QW.
Table 1 The well widths and numbers of samples 1–3. Sample no.
Well width (nm)
Well number
1 2 3
13 19 19
4 4 2
As shown in Fig. 1(a), the growth started from a 1000-nm silicon doped n-type InP buffer, followed by a 120-nm unintentionally doped (uid) lattice-matched InGaAsP (Eg ¼1.1 eV) waveguide layer which was grown at 460 1C. The substrate temperature was then decreased to 420 1C to grow the uid active triangular QWs. In the QW regions, the indium composition was approximately increased from 0.53 to 1 and then decreased to 0.53, which was approximated by using InAs/In0.53Ga0.47As digital alloy technology. Fig. 1 (b) shows the appeared and effective band energy of one QW. The designed period thickness of the digital alloy including one InAs layer and one In0.53Ga0.47As layer was 1 nm. The thicknesses of the InAs layer (d1) and In0.53Ga0.47As layer (1 d1) are obtained from d1 þ0.53 (1 d1)¼α, where α is the expected average indium composition in each short period. These layers are very thin, and the thinnest layer is even thinner than one monolayer, therefore these thin layers are supposed to intermix during the growth. When growing the QW structures, only the Ga shutter was opening and closing, without any growth interruption. The growth rates of InAs and InGaAs were 0.152 and 0.287 nm/s, respectively. Three samples with different well widths and well numbers were grown and the parameters were listed in Table 1. In0.53Ga0.47As layers were grown as barriers and the barrier thickness were 20 nm except for the first and last barrier layers, which were thickened to 100 nm to enhance the confinement. Therefore, the active QW layers were grown keeping the indium and gallium fluxes unchanged. Afterwards, a 120-nm uid InGaAsP upper waveguide, a 1700-nm beryllium doped p-type InP upper cladding layer and a 300-nm p-type In0.53Ga0.47As contact layer were grown at 440 1C. The substrate was undergoing rotation during the growth to get uniform thickness. After growth, the structural properties of the wafers were characterized by X-ray diffraction (XRD) scanning curves using a
Philips X’pert MRD high resolution X-ray diffractometer equipped with a four-crystal Ge (220) monochromator. Then, the ridge waveguide lasers with strip width of 6 mm were fabricated by using standard lithography and wet chemical etching. 300-nm Si3N4 layers were deposited by plasma enhanced chemical vapor deposition for isolation, and 4-mm-wide windows were opened on the top of the ridges. Sputtered Ti/Pt/Au and evaporated Ge/Au/Ni/ Au were formed as top p-type and bottom n-type contacts, respectively. After an alloyed step, the wafers were diced into bars of 800 mm leaving as-cleaved laser facets, mounted on copper heat sinks, wire bonded and installed into an Oxford Optistat DN-V variable temperature liquid nitrogen cryostat. The lasers were CW driven by a Keithley 2420 source meter, and the spectral characteristics were characterized by a Nicolet 860 Fourier transform infrared spectrometer using a liquid-nitrogen cooled InSb detector and a CaF2 beam splitter. For the I–P measurements, a Coherent EMP1000 power meter was used.
3. Results and discussions The XRD (004) ω/2θ scanning curves of the grown laser wafers with various well widths and numbers in the active QW region are shown in Fig. 2, the strongest peak corresponds to InP and the envelope signals in the left side correspond to the active QWs. The XRD curves at different positions on the wafer are nearly the same indicating the pretty good uniformity of the QWs. Distinct satellite peaks are observed in sample 1. For sample 2 where the QW number is still four but the well width increases to 19 nm, the satellite peaks in the measured XRD curve become dimer, indicating the deteriorated QW quality with the rough heterostructure interfaces due to the accumulated strain. For sample 3, the satellite peaks become much more distinct for the QW number of two and keeping the well width of 19 nm, suggesting that the material quality of 19 nm QWs is improved by decreasing the QW number from four to two. In Fig. 2, the simulated curves from lattice dynamics for samples 1–3 are also shown. The simulated curves were all achieved by the simulation from the exact structure as designed. The good agreements of the diffraction peaks between the measured and simulated curves confirm the pseudomorphic
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Fig. 2. Measured (upper curve) and simulated (bottom curve) XRD (004) ω/2θ scan curves of the triangular QW laser samples 1–3.
2.45
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Wavelength ( μm)
growth of all samples. For these whole laser structures, the PL signals from QWs cannot be observed as about 2 μm thick top layers were grown on the QWs. In the PL measurements of QW test samples with different well widths, the PL intensity was observed a bit decreased as the increased QW width [16,17]. As shown in Fig. 3 the temperature dependent CW lasing wavelengths of the lasers have shown various performances, and the typical lasing spectra of samples 1–3 are shown in Fig. 4. The lasing wavelength is 2.30 mm at 300 K for sample 1 with four 13 nm QWs, and redshifts to 2.44 mm at 290 K as the well width increases to 19 nm for sample 2. However, the maximum lasing temperature is decreased from 330 K for sample 1 to 290 K for sample 2 due to the deteriorated material quality. For the 19 nm QW lasers, as the well number decreases from four to two, the maximum lasing temperature is increased to beyond 300 K and the lasing wavelength is around 2.38 mm at 300 K for sample 3. From linear fitting, the temperature dependent wavelength shift values are 1.00 nm/K, 1.41 nm/K and 1.01 nm/K for samples 1, 2 and 3, respectively. It is noted that the lasing wavelengths of sample 2 are longer than those of sample 3 at all temperatures. It is probably because that the QW wave functions are partly coupled as the carrier confinement is relatively weak in this triangular QW structure due to the inclined energy band. Fig. 5 shows the light output characteristics of the three triangular QW lasers under CW operation. The output power is about 34 mW/facet at 350 mA injection current at 200 K and 15 mW/facet at 300 K for sample 1 with 13 nm QWs. The threshold current is about 17 mA at 200 K and increases to 58 mA at 300 K. As the well width increases to 19 nm while keeping four QWs in the active region (sample 2), the power at 200 K is decreased to only 11 mW/facet at 350 mA injection current. The threshold current of sample 2 is significantly increased to 72 mA and 260 mA at 200 K and 290 K, respectively. As the QW number of 19 nm QW laser decreases from four to two, the output power is increased to 16 mW/facet at 350 mA injection current at 200 K
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2.20 200 220 240 260 280 300 320 340 Temperature (K) Fig. 3. Temperature dependence of the CW lasing wavelengths for the triangular QW laser samples 1–3. The temperature dependent wavelength shift values for the three samples are all indicated.
although there are less QWs in the active region. The maximum output power at 300 K is 1.8 mW/facet. The threshold current of sample 3 is reduced to about 10 mA at 200 K and 96 mA at 300 K. The improved output characteristics are mainly due to the better material quality. The laser threshold current densities at various temperatures and derived characteristic temperatures for the three samples are shown in Fig. 6. For sample 1 with the 13 nm QWs, the threshold current density is about 1.2 kA/cm2 at 300 K (300 A/cm2 per QW). Two different slopes of the threshold current density exist in different temperature ranges. The characteristic temperature T0 is derived to be about 75 K in the temperature range of 200–290 K, and decreases to 36 K in the temperature range of 290–330 K. For sample 2 with the increased well width of 19 nm, the threshold current density is significantly increased. The characteristic temperature T0 is about 81 K and 48 K in the temperature ranges of 200–260 K and 260–
Y. Gu et al. / Journal of Crystal Growth 425 (2015) 376–380
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Fig. 4. CW lasing spectra of the triangular QW laser sample 1 at 300 K, sample 2 at 290 K and sample 3 at 300 K injected by 1.2 times of the threshold current.
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Fig. 5. Output powers at various temperatures and voltages at 300 K as a function of injected currents of samples 1–3.
290 K respectively, both close to those of sample 1. Comparing to those of sample 2, the threshold current density of sample 3 is significantly reduced as the well number decreases from four to two. At 200 K the threshold current density of sample 3 is even smaller than that of
T0=22 K
T0=36 K
Sample 1 Sample 2 Sample 3
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Fig. 6. Temperature dependence of threshold current densities and characteristic temperatures (T0) of samples 1–3. Two different T0 values are derived in different temperature ranges for each sample.
sample 1 probably due to the less QWs of sample 3, however the threshold current density of sample 3 increases much more quickly as the temperature increases, reaching 2.0 kA/cm2 at 300 K. The characteristic temperature T0 of sample 3 is 50 K and 22 K for 200–280 K and 280–300 K temperature ranges, smaller than those of samples 1 and 2, which is probably because of the reduced gain as the decreased QW number. For this kind of highly strained triangular QW laser, a tradeoff should be considered between the strain control and sufficient gain. For the structure with thinner QWs, more QW number can be applied to achieve higher output power. In our experience, the performances of the 4 QWs laser structures do not degrade a lot when the well width enlarged from 10 nm to 16 nm [14,15]. As a rough estimation, 4 QWs could be around the maximum QW number for the 16 nm QW laser structure, but more QW number could be applied for thinner QWs whereas less QW number for wider QWs.
4. Conclusion In conclusion, InP-based InGaAs triangular QW lasers with different well widths and numbers have been grown by GSMBE. The triangular QW was equivalently formed by using InAs/In0.53Ga0.47As digital alloy with carefully adjusted growth thickness of each layer. X-ray diffraction measurements have confirmed the pseudomorphic growth. For the laser with four 13 nm QWs, CW lasing at 2.30 μm above 330 K has been achieved. By increasing the QW width to 19 nm, the CW lasing wavelength is extended to 2.44 mm at 290 K although the material qualities and laser performances are deteriorated. For the 19 nm triangular QW lasers, the reduction of the QW number from four to two can improve the laser performances and CW lasing at 2.38 μm above 300 K has been achieved.
Acknowledgments This work was supported by the National Basic Research Program of China under Grant no. 2012CB619202 and the National Natural Science Foundation of China under Grant nos. 61275113, 61204133 and 61405232. References [1] A. Bauer, K. Rößner, T. Lehnhardt, M. Kamp, S. Höfling, L. Worschech, A. Forchel, Semicond. Sci. Technol. 26 (2011) 014032. [2] W. Lei, C. Jagadish, J. Appl. Phys. 104 (2008) 091101.
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