Effect of thermal annealing on structural properties of GeSn thin films grown by molecular beam epitaxy

Effect of thermal annealing on structural properties of GeSn thin films grown by molecular beam epitaxy Z. P. Zhang,1,2,3 Y. X. Song,1,a Y. Y. Li,1 X. Y. Wu,1,3,5 Z. Y. S. Zhu,1,2 Y. Han,1,3 L. Y. Zhang,1 H. Huang,1 and S. M. Wang1,2,4,a 1State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China 2School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China 3University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China 4Department of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg 41296, Sweden 5State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Key Laboratory on Navigation and Location-Based Service and Center of Quantum Information Sensing and Processing, Shanghai Jiao Tong University, Shanghai 200240, China


INTRODUCTION
Group IV materials with engineered band structures have gain increased attentions in recent years. 1,2GeSn alloy is one of the most promising semiconductor materials with a tunable bandgap.Although Ge is an indirect bandgap semiconductor which has a 136 meV gap between the L-valley and the Γ-valley, 3 introducing Sn in Ge forming a GeSn alloy could make transformation of the L-valley and the Γ-valley to convert it into a direct bandgap semiconductor.6][7] Noteworthy milestones like GeSn laser 2 have spurred a fast development in this research field.Besides the tunable bandgap, GeSn alloy is also predicted to hold high electron and hole mobility, which makes it a potential candidate material for both optoelectronic and electronic devices integrated on the Si platform. 8,9he thermal equilibrium solubility of the Ge-Sn binary system is as low as 1%, 10 and surface energy of Sn is smaller than that of Ge. 11 Therefore, Sn segregation is the largest challenge for formation of homogeneous GeSn alloys during the epitaxial growth.GeSn layers with a high Sn concentration have been grown by low temperature molecular beam epitaxy (MBE), 12 chemical vapor deposition [13][14][15][16] or solid phase epitaxy.17 Moreover, some ultra-high Sn concentration GeSn alloy has been grown by MBE at a growth temperature less than 200 • C, for example 25% 18 and 27%.19 The non-equilibrium processes during MBE are effective to avoid Sn segregation. 2022 Therefore, to obtain high quality and direct bandgap GeSn alloy, the dilemma between the Sn segregation at a high growth temperature and the high defect density introduced at a low growth temperature must be overcome.23 Thermal annealing leads to decrease in defect density and improves optical quality of the GeSn alloy.24 At the same time, thermal annealing could also increase the portion of substitutional Sn atoms to reduce the difficulty of realizing direct bandgap luminescence.25 Previous reports have shown that when the annealing temperature of GeSn/Ge alloy is above 400 • C, strain couldn't be maintained along with structure relaxation.24,25 Proper annealing temperature has been found to be 500 • C by measuring the root mean square (RMS) roughness and average of aligned yield (from RBS) 26 or 550 • C by using photoluminescence (PL).23 For a partially relaxed GeSn alloy, the bandgap transition occurs when the Sn concentration is 6∼11%.[1][2][3]27,28 When the GeSn alloy is fully strained on a Ge substrate, the bandgap transformation from the indirect bandgap to the direct bandgap even occurs at 12%. 2 However, growth of high quality and high Sn concentration GeSn alloy has encountered great challenges.18,19 To reduce the difficulty of bandgap transformation, a feasible path is reducing or eliminating the compressive strain by thermal annealing.
In this work, 200 nm thick GeSn thin film with 7.68% Sn concentration and 25% initial strain relaxation grown by MBE has been investigated by rapid thermal annealing at different temperatures between 300-800 • C. Structural properties are analyzed by two-dimensional reciprocal space mapping (2DRSM) in the symmetric (004) and asymmetric (224) planes by high resolution X-ray diffraction (XRD).The lateral correlation length (LCL) and the mosaic spread (MS) are extracted for the epilayer peaks in the asymmetric (224) diffraction.The larger the LCL, the higher the lattice quality, while the trend for MS is opposite.Strain relaxation has also been studied by 2DRSM.The relaxation rate of the annealed GeSn alloy increases gradually when the annealing temperature is up to 600 • C. The most suitable annealing temperature to improve both the GeSn lattice quality and strain relaxation rate is about 500 • C.

METHODS
200 nm GeSn thin film with 7.68% Sn concentration was grown by a DCA MBE system with a base pressure of 5×10 -10 Torr.The Ge source is an electron beam evaporator and the Sn source is an effusion cell.The absolute flux of Ge and Sn was calibrated by both electron impact emission spectroscopy and Q-pod quartz crystal monitor.Background pressure and impurities were recorded by an ultra-sensitive ion gauge and quadrupole mass spectrometer, and surface evolution during growth was monitored by in situ reflection high-energy electron diffraction (RHEED).
Prior to the growth, surface oxide desorption of the Ge (001) substrate was carried out by heating the substrate at 550 • C for 30 min.After that, a 200 nm thick Ge buffer layer was grown to provide an atomically clean and flat surface.Then, the sample holder was cooled to 200 • C and remained stable for about 5 min followed by deposit of a 200 nm thick GeSn thin film.The Ge flux was 0.75 Å/s and the Sn flux was 0.06 Å/s, and the background pressure during epitaxy was (5-6)×10 -9 Torr.The sample was diced into small pieces which were then rapidly annealed for 1 min at 300 • C, 400 • C, 500 • C, 600 • C, 700 • C and 800 • C, respectively, in a N 2 ambient condition.Surface morphology was characterized by atomic force microscopy (AFM).Structural property and Sn concentration were analyzed by 2DRSM using high resolution XRD and scanning electron microscope (SEM).Both the LCL and the MS were investigated for the epi-layer peaks in the asymmetric (224) diffraction.

RESULTS AND DISCUSSION
FIG. 1(a) shows the HRXRD (004) ω-2θ rocking curve of the GeSn sample with 7.68% Sn concentration.The right narrow peak corresponds to the Ge substrate while the left wide peak corresponds With increasing annealing temperature up to 500 • C, both surface morphology and roughness values are not significantly changed (at about 6 nm).However, when the annealing temperature is 600 • C, the surface roughness value is reduced to 2 nm and the density of the pyramid shaped features also decreases to about 1.5 × 10 9 /cm 2 .As shown in FIG.2(f) and (g), when the annealing temperature is   To further analyze crystal quality of the GeSn thin film, the MS and the LCL of the GeSn epilayers are investigated by extracting from the asymmetric (224) mapping.FIG.5(c) and (d) separately shows the MS and the LCL change with annealing temperature.MS may arise either because of a fine mosaic structure giving a variation in the tilt of the reflecting plane for different mosaic grains or it may be a result of "rippled" reflecting planes due to the presence of a uniform dislocation density. 29hen lattice quality is high, the value of the MS is small conversely.FIG.5(b) shows the relaxation rate of GeSn thin film goes up greatly after annealing at 300 • C, and the reason for the increase of relaxation rate is likely from the increase of the dislocation density.Therefore, the lattice quality of the GeSn thin film is also correspondingly reduced, which is in accordance with the great increase of the MS in the annealed GeSn thin film at 300 • C. LCL gives an indication of the lateral uniformity of the layer.If a layer is perfectly smooth and of uniform thickness in all directions parallel to the interface, the breadth of this peak would be related to an average value of the separation of GeSn thin film lateral irregularities. 20,30When lattice quality is high, the value of the LCL is also high.Therefore, FIG.

CONCLUSIONS
GeSn alloy with 7.68% Sn concentration grown by MBE has been rapidly annealed at different temperatures from 300 • C to 800 • C. The surface morphology and roughness of the samples annealed up to 500 • C for 1 min have no obvious change, except for the increase in strain relaxation rate.
When the annealing temperature is above or equal to 600 • C, significant changes occur in surface morphology and roughness, and Sn precipitates at 700 • C. The structural properties are analyzed by reciprocal space mapping in the symmetric (004) and asymmetric (224) planes by high resolution XRD.The LCL and the MS are extracted for the epi-layer peaks in the asymmetric (224) diffraction.The most suitable annealing temperature to improve both the GeSn lattice quality and relaxation rate is about 500 • C.

FIG. 1 .
FIG. 1.(a) HRXRD (004) ω-2θ rocking curve of the GeSn sample with 7.68% Sn concentration.(b) TEM image of the as grown sample.(c) A typical EDX spectrum of the GeSn thin film from the green rectangle area in (b).

FIG. 5 .
FIG. 5. (a) Sn concentration (b) strain relaxation rate (c) MS and (d) LCL extracted from the 2DRSM as a function of annealing temperature.
5(b)-(d) jointly indicate that 500 • C is the most suitable annealing temperature to improve both the GeSn lattice quality and relaxation rate.