ISSN 1063-7826, Semiconductors, 2017, Vol. 51, No. 5, pp. 663–666. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.Ya. Aleshkin, N.V. Baidus, A.A. Dubinov, Z.F. Krasilnik, S.M. Nekorkin, A.V. Novikov, A.V. Rykov, D.V. Yurasov, A.N. Yablonskiy, 2017, published in Fizika i Tekhnika Poluprovodnikov, 2017, Vol. 51, No. 5, pp. 695–698.
PHYSICS OF SEMICONDUCTOR DEVICES
On the Stimulated Emission of InGaAs/GaAs/AlGaAs Laser Structures Grown by MOCVD on Exact and Inclined Ge/Si(001) Substrates V. Ya. Aleshkina, b, N. V. Baidusc, A. A. Dubinova, b*, Z. F. Krasilnika, b, S. M. Nekorkina, c, A. V. Novikova, b, A. V. Rykovc, D. V. Yurasova, b, and A. N. Yablonskiya a
Institute for Physics of Microstructures, Russian Academy of Sciences, ul. Ul’yanova 46, Nizhny Novgorod, 603950 Russia b Lobachevsky State University of Nizhny Novgorod, pr. Gagarina 23, Nizhny Novgorod, 603950 Russia c Physical-Technical Research Institute, Lobachevsky State University of Nizhny Novgorod, pr. Gagarina 23/3, Nizhny Novgorod, 603950 Russia * e-mail:
[email protected] Submitted November 16, 2016; accepted for publication November 21, 2016
Abstract—GaAs/AlGaAs laser structures with InGaAs quantum wells are grown by metal-organic chemical vapor deposition (MOCVD) on exact Si(001) substrates and substrates inclined by 4° to the [011] axis with a relaxed Ge buffer layer, emitting in the transparency region of bulk silicon (the wavelength is longer than 1100 nm at room temperature). The threshold power densities of stimulated emission, observed for the structures grown on exact and inclined substrates are 45 and 37 kW/cm2, respectively. DOI: 10.1134/S1063782617050037
The transition from copper wires to optical interconnections in high-performance processors can be implemented using hybrid III–V heterolasers compatible with the complementary metal–oxide semiconductor (CMOS) technology on silicon [1]. Significant progress has been achieved in the development of lasers on Si substrates using GaAs/AlGaAs heterostructures with InGaAs quantum wells (QWs) [2–4] and InAs quantum dots (QDs) [5, 6]. Almost in all cases, III–V laser structures were grown on silicon substrates inclined by several degrees from the [001] axis to avoid the formation of antiphase defects at the interface of polar and nonpolar materials [7]. At the same time, the existing fabrication technologies of silicon processors are developed for exactly oriented Si(001) substrates, with a deviation not exceeding 0.5° [8]. Recently, progress has been achieved in the development of a GaAs/AlGaAs laser with three InGaAs QWs, grown by metal-organic vapor-phase epitaxy (MOVPE) on an exact Si(001) substrate with a Ge buffer layer [9]. The laser emits at a wavelength of 992 nm at room temperature. However, the use of III–V lasers in silicon optical interconnections requires that the laser emission wavelength (λ) be in the transmission band of bulk silicon (λ > 1100 nm at room temperature). We note that it is possible to reach this wavelength region in a laser structure with double InGaAs/GaAsSb/GaAs quantum wells grown on an
inclined Si(001) substrate using a relaxed Ge buffer layer, but only at liquid-nitrogen temperature [10]. Therefore, in the present study, GaAs/AlGaAs laser structures with single InGaAs QWs with a higher indium content in comparison with the structure studied in [9] to increase the emission wavelength were developed and studied. The laser structures were grown by the MOVPE method on Si(001) substrates using a relaxed Ge buffer layer. To compare the radiative characteristics under optical pumping, the laser structures were grown on both an exactly oriented (exact) Si(001) substrate and on a substrate inclined by 4° from the [001] axis to the [011] axis. The exact Ge/Si(001) virtual substrate and that inclined by 4° to the [011] axis were grown in a Riber SIVA-21 ultrahigh-vacuum molecular-beam epitaxy (MBE) system by the technique of so-called “twostage” growth [11–13]. The growth temperature was measured using a calibrated thermocouple [14] and an IMPACIS 12 specialized infrared pyrometer. Si and Ge were deposited using electron-beam evaporation. The surface state during epitaxy was monitored by the high-energy electron diffraction (HEED) method. The surface morphology of the grown samples was studied by atomic-force microscopy (AFM) using an NTEGRA Prima microscope. The crystalline quality of the samples was studied using X-ray diffraction analysis and the selective etching of defects [15].
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Parameters of laser structures Layer number
Layer composition
Thickness, nm
1 2 3 4 5
AlAs GaAs AlAs GaAs GaAs → Al0.3Ga0.7As gradient
10 50 10 500 100
6
Al0.3Ga0.7As
7 8
GaAs InxGa1 − xAs
9 10
GaAs Al0.3Ga0.7As
1000 250 10 350 20
11
Al0.3Ga0.7As → GaAs gradient
12
GaAs
10 10
The essence of the two-stage method is as follows. In the first stage, a relatively thin (~50 nm) Ge layer is deposited at a lower temperature (275°C) to exclude elastic-strain relaxation via the formation of threedimensional islands. In this case, relaxation occurs via the formation of a large number of misfit dislocations, and a relatively smooth (~1 nm) layer with a high density of threading dislocations (>1010 cm–2) is formed. In the second stage, the growth temperature is increased to 600°C and the major part of the Ge layer (~1 μm in this study) is grown at this temperature, which promotes crystalline quality. To lower the density of threading dislocations after structure formation, cyclic annealing was used: {850°C/2 min, 550°C/2 min} × 5 repetitions. According to AFM measurements, it is possible to retain a low surface roughness after an annealing cycle due to a rather short exposure time at high temperature (the rootmean-square RMS roughness is 1130 nm, respectively. We note that the emission wavelength is in the transmission band of bulk silicon in both cases; therefore, these sources are applicable to channeling and lowloss radiation propagation in silicon waveguides. Figure 2 shows the dependences of the total intensity of stimulated emission of three laser structures on SEMICONDUCTORS
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ON THE STIMULATED EMISSION
Intensity, arb. units
0.20
0.15
3
1
2
0.10
0.05 0 950
1000
1050 1100 1150 Wavelength λ, nm
1200
Integral intensity, arb. units
Fig. 1. Stimulated emission spectra of laser structures: (1) the structure used in [9] (the In fraction is x ≈ 0.17), (2) the structure on the inclined Ge/Si substrate (x ≈ 0.33), and (3) the structure on the exact Ge/Si substrate (x ≈ 0.35).
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structure on the Ge/Si substrate, but without such a buffer layer [3]. Although the lasing threshold for structure 2 is lower than for structure 3, the emission intensity of structure 3 is higher than that of structure 2 beyond the lasing threshold, which suggests that the quantum efficiency of the structure on the exact substrate is higher. The larger slope of the dependence of the total intensity of stimulated emission on the pump-power density for structure 1 in Fig. 2 is probably associated with the larger number of QWs (3 QWs) in this structure in comparison with the other structures (1 QW). Thus, it was shown that the use of a buffer layer consisting of alternating AlAs and GaAs layers leads to a significant decrease in the number of defects in III–V laser structures grown on both exact Ge/Si(001) substrates and those inclined by 4° to the [011] axis. This is confirmed by a twofold decrease in the excitation threshold power density. The formation of antiphase defects on the exact substrate has an insignificant effect on radiative properties. The results obtained suggest the possibility of fabricating lowthreshold hybrid lasers emitting in the transmission band of bulk silicon on exactly oriented Ge/Si(001) substrates.
2 1.0
ACKNOWLEDGMENTS This study was supported by the Russian Science Foundation, project no. 14-12-00644.
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25 50 75 100 125 Pump power density, kW/cm2
150
Fig. 2. Dependences of the total intensity of stimulated emission of laser structures: (1) the structure studied in [9], (2) the structure on the inclined Ge/Si substrate, and (3) the structure on the exact Ge/Si substrate.
the power density of optical excitation. The measured dependences were used to determine the threshold power density of stimulated emission: ~37 kW/cm2 and ~45 kW/cm2 for structures on inclined and exact substrates, respectively. We note that the room-temperature lasing threshold for the laser diode made of structure 1 was ~5.5 kA/cm2 [9]; hence, similar threshold current densities during electrical pumping are also expected for laser diodes fabricated from structures 2 and 3. Furthermore, we can see that the use of a buffer layer consisting of alternating AlAs and GaAs layers significantly decreases the number of defects and results in a twofold decrease in the threshold power density in comparison with the III–V laser SEMICONDUCTORS
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Translated by A. Kazantsev
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