High-speed devices using III-V compound materials have become one of the international research hotspots^［1-2］. The narrow band-gap materials of InAs，InSb and InAsSb in III-V compounds not only have high electron mobility and electron saturation drift velocity，but also can form diverse quantum well band structures with AlSb，GaSb and other related ternary broadband gap materials^［3-7］. These excellent characteristics allow the electrical devices to have the advantages of ultra-high speed and low power consumption. There has been some literature ^［8-10］ on using InAs materials as channel layers to prepare high electron mobility transistor（HEMT）. To pursue higher working speed and lower power consumption，the highest electron mobility in all III–V binary compounds of InSb has generated considerable interest for the fabrication of HEMT^［11-12］. However，the growth of high-quality InSb is challenging due to the large lattice mismatch between InSb and GaAs substrates. To minimize the problem of lattice strain and improve mobility，InAsSb ternary alloy is anticipated to substitute of InSb and InAs. Meanwhile，the straddle energy band structure of Al_0.2In_0.8Sb/InAs_0.4Sb_0.6 heterostructure is used to replace the interleaved energy band structure of InAs/AlSb heterostructure，which can effectively reduce the lattice mismatch and the gate leakage current caused by holes. Therefore，InAsSb material is expected to become a strong competitor as a channel material in the next generation of HEMT^［13-14］.

Since the InAsSb material contains two V elements of As and Sb，the composition of group V elements cannot be accurately calculated by the ratio of growth rates. More influencing factors need to be considered when growing InAsSb materials because the adhesion of As and Sb elements varies under different growth conditions. Therefore，it is necessary to study the composition control of InAsSb materials grown by molecular beam epitaxy. Based on the theoretical calculation results reported in the current literature^［15-17］，the electron mobility of InAsSb as a channel material can reach more than 30000 cm²/V·s. However，the current experimental results are far lower than the simulation results due to challenges in the growth process such as interface mismatch and dislocation scattering^［18-20］. In comparison，fewer reports on experimental results on the transport characteristics of the InAs_xSb_{1-xwith different Sb content are available currently.}

This paper discusses the influence of Sb/In ratio on the transport properties and crystal quality of the 200 nm InAs_xSb_1-x thin film. The Sb content of InAs_xSb_1-x thin film in all samples was verified by HRXRD via the symmetrical 004 reflections and asymmetrical 115 reflections. In addition，the influence of Sb/In ratio and As/In ratio on the transport properties and crystal quality of Al_0.2In_0.8Sb/ InAs_xSb_1-x quantum well heterostructures has also been investigated. By optimizing the Sb/In ratio and As/In ratio，Al_0.2In_0.8Sb/ InAs_xSb_1-x quantum well heterostructures with good surface morphology and high electron mobility were obtained. All samples were confirmed by atomic force microscopy（AFM），high-resolution X-ray diffraction（HRXRD），reciprocal space map（RSM） and Hall measurement.

All samples were grown on GaAs substrate by Gen-II solid-source MBE system. After deoxidation of GaAs substrate at 690 ℃ for 5 minutes，a 100 nm GaAs was grown at 650 ℃ and a 100 nm GaSb was grown at 540 ℃ to ensure that the substrate surface was flat. For one structure，a 1.5 μm Al_0.2In_0.8Sb metamorphic buffer layer was used to study the transport properties of the 200 nm InAs_xSb_1-x layer，as shown in Fig. 1. The 200 nm InAs_xSb_1-x thin films of the first group of samples A1，A2 and A3 were grown at different the Sb/In ratios of 5，6 and7，while the As/In ratio is kept at about 3.

For another structure，the Al_0.2In_0.8Sb/ InAs_xSb_1-x quantum well heterostructures were grown for preparing high electron mobility transistors，as shown in Fig. 2. For the Al_0.2In_0.8Sb/ InAs_xSb_1-x quantum well heterostructures，the 1.5 μm Al_0.2In_0.8Sb that acted as a lower barrier layer and buffer was directly deposited on the GaSb buffer layer. Then a different thicknesses InAs_xSb_1-x channel，a 20 nm Al_0.2In_0.8Sb upper barrier layer and a 5nm InSb cap layer were deposited on the top of the Al_0.2In_0.8Sb layer.

The first group of samples B1，B2，and B3 with different Sb/In ratios（5，6，7） were grown to investigate the effects of different Sb components on the crystal quality and electron mobility of Al_0.2In_0.8Sb/ InAs_xSb_1-x quantum well heterostructures，while the As/In ratio was also kept at about 3. Meanwhile，the channel thickness of this group of samples was 15 nm.The second group of samples C1，C2，and C3 with different As/In ratios（1，2，3） were grown to investigate the effects of different As components on the crystal quality and electron mobility of Al_0.2In_0.8Sb/InAs_xSb_{1-xquantum well heterostructures，while the Sb/In ratio was kept at about 6. Meanwhile，the channel thickness of this group of samples was increased from 15nm to 30nm. The third group of samples D1，D2，D3，D4 and D5 corresponds to InAs0.4Sb0.6 channel layer thicknesses of 15，20，25，30 and 35 nm to investigate the effect of different channel thicknesses on the crystal quality and electron mobility of Al0.2In0.8Sb/ InAsxSb1-x quantum well heterostructures. The Sb/In ratio was kept at about 6 and the As/In ratio was kept at about 3.}

Fig. 3 displays 2×2 μm² AFM images of samples A1，A2 and A3 with an RMS roughness of 0.287 nm，0.28 nm and 0.6 nm，respectively. Atomic steps can be clearly seen in samples A2 and A3，indicating that the surfaces of the samples are very flat. However，some bright spots can be seen in sample A3. These were due to the excess Sb beam while the Sb/In ratio was 7，resulting in residual Sb elements on the surface. The 10×10 μm² AFM images of samples A1，A2 and A3 can also be found that sample A2 has the smallest RMS value of 0.7 nm，which is much smaller than a recent literature report（1.9 nm）^［21］.

The Sb content of the InAs_xSb_1-x layer in all samples was verified by HRXRD via the symmetrical 004 reflections and asymmetrical 115 reflections，as shown in Fig. 4（a） and 4（b）. The calculation results are shown in Table 1. The Sb composition of sample A1 was calculated to be 0.68 and the lattice constant of InAs_0.32Sb_0.68 was found to be 6.3464 Å. The Sb composition of sample A2 was 0.6 and the lattice constant of InAs_0.4Sb_0.6 was 6.3068 Å. The Sb composition of sample A3 was 0.84 and the lattice constant of InAs₁₆Sb_0.84 was 6.4099 Å. The diffraction peaks of GaAs，GaSb，InAs_xSb_1-x and Al_0.2In_0.8Sb can be clearly observed for samples A1 and A2. In sample A3，only the diffraction peaks of GaAs，GaSb and Al_0.2In_0.8Sb can be observed. This is because the InAs₁₆Sb_0.84 lattice constant of sample A3 is close to the lattice constant of Al_0.2In_0.8Sb（6.4106 Å），causing their diffraction peaks to overlap.

The crystalline quality of the epitaxial layers was further assessed by XRD RSM measurements. Fig. 5（a），（b） and（c） show the logarithmic XRD RSM for the symmetrical（004） for sample A1，sample A2 and sample A3，respectively. Apart from the GaAs substrate peak denoted by S，three epitaxial peaks were also identified from Fig. 5（a），（b） and（c） denoted by L1，L2 and L3，respectively. L1 represents the epitaxial peak of GaSb，L2 represents the epitaxial peak of InAs_xSb_1-x and L3 represents the epitaxial peak of Al_0.2In_0.8Sb. Corresponding to the analysis in Fig. 4，it can be seen that the epitaxial peak of InAs_0.16Sb_0.84 in sample A3 is indeed close to the epitaxial peak of Al_0.2In_0.8Sb.

The influence of the V/III ratio on the electrical properties of InAs_xSb_1-x thin films was examined by determining the Hall properties. The electron mobility μ refers to the average speed of electron units under the electric field intensity. The value of μ can be obtained from the following formula：

Where them，m* represents the electron effective massive，τ represents the mean free time of electrons and q represents electron charge. InAs_xSb_1-x is a compound of InSb and InAs materials，so its crystal structure is relatively stable. The room temperature electron effective mass of InAs_xSb_1-x is 0.023-0.039（1-x）+0.03（1-x）² m₀. Therefore，InAsSb with a 60% Sb component has the lowest electron effective mass among III-V compound semiconductors，resulting in the highest electron mobility^［22-23］. As shown in Fig. 6，Hall measurements were performed on 1 cm × 1 cm sample pieces at 300 K to obtain the electron mobility，results of 24 540 cm²/V·s were obtained for sample A1，28 560 cm²/V·s for sample A2 and 25 850 cm²/V·s for sample A3. It can be seen from the above results that the highest mobility is indeed obtained when the Sb component is 0.6. The results of electron mobility in this paper are much better than previously reported，as shown in Table 2. For comparison with the literature，the density of 2DEG concentration was converted into a volume density value of 1.01^{×1018 cm-3.}

Although InAsSb has excellent transmission properties，the lack of matching high-quality semi-insulating substrates limits its development. Therefore，an Al_0.2In_0.8Sb strain buffer layer was used to release the stress caused by the lattice mismatch between InAsSb and GaAs substrates. The AFM images of Al_0.2In_0.8Sb/InAs_xSb_1-x quantum well heterostructures grown under different Sb/In ratios are shown in Fig. 7. It showed a 10×10 μm² AFM images of sample B1，B2 and B3 with an RMS roughness of 2.794 nm，1.725 nm and 3.359 nm，respectively. It can be seen that when the Sb/In ratio was 6，the surface of the sample was the smoothest and with its RMS roughness at the lowest of the batch of samples.

The（004） HRXRD scanning curves of samples with different Sb components are shown in Fig. 8. The diffraction intensity of the InAs_xSb_1-x channel layer is very weak because its thickness is too thin. From Fig. 8，it can be observed that the Bragg angles of Al_0.2In_0.8Sb in all samples are the same，indicating that the Al composition is the same. Additionally，it was assumed that the contribution of FWHM mainly comes from lattice distortion caused by dislocations，and the dislocation density in Al_0.2In_0.8Sb thin film samples can be calculated based on FWHM. The FWHM of the Al_0.2In_0.8Sb buffer layer in samples B1，B2，and B3 are 1109 arcsec，997 arcsec，and 1033 arcsec，respectively，indicating that the quality of the three samples is equivalent.

As shown in Fig. 9，the electron mobilities of samples B1，B2 and B3 at 300 K are 17500 cm²/V·s，18500 cm²/V·s and 17700 cm²/V·s，respectively. According to the calculation results in Table 1，the channel materials of samples B1，B2，and B3 are InAs_0.32Sb_0.68，InAs_0.4Sb_0.6 and InAs_0.16Sb_0.84，respectively. Fig. 9 shows that the Al_0.2In_0.8Sb/ InAs_xSb_1-x quantum well heterostructures obtain the maximum electron mobility when the Sb component is 0.6. This result is consistent with the Hall test results in Table 1. This is because the effective mass of electrons reaches a minimum value when the Sb component is 60%. The 2DEG concentrations in the channel are 9.44×10¹¹ cm^-2，1×10¹² cm^-2 and 7.89×10¹¹ cm^-2 respectively，with little change.

It can be observed from the comparison between Fig. 6 and Fig. 9 that the electron mobility of sample A2 in InAs_xSb_{1-xwith 15 nm thicknesswas significantly lower. Therefore，three samples with different As/In ratios were grown for study after changing the channel thickness in the quantum well from 15 nm to 30 nm. Fig. 10 displays the images of a 10 μm×10 μm surface scan of samples C1，C2 and C3 with an RMS roughness of 1.757 nm，1.785 nm and 0.68 nm，respectively. It shows that sample C3 has a smoother surface than other samples.}

The（004） HRXRD scanning curves of samples with different As components are shown in Fig. 11. In Fig. 11，there are only three peaks corresponding to the GaAs substrate，GaSb buffer layer，and Al_0.2In_0.8Sb strain buffer layer. Because the thickness of the channel layer InAs_xSb_1-x was too thin to be observed. The peaks of the Al_0.2In_0.8Sb strain buffer layer in all samples are clearly visible. The FWHM of samples C1，C2 and C3 are 903 arcsec，936 arcsec，and 986 arcsec，respectively. The similar FWHM and Bragg peak positions indicate that the crystalline quality of all samples is similar.

Electron mobility is an important electrical parameter that can be used to evaluate whether Al_0.2In_0.8Sb/InAs_xSb_1-x quantum well heterostructures grown by MBE can be used to prepare high mobility transistors. As shown in Fig. 12，the electron mobility of samples C1，C2，and C3 at 300 K is 10 100 cm²/V·s，22 020 cm²/V·s and 28 300 cm²/V·s，respectively. Because In atoms will occupy a portion of As and Sb atomic positions as well as interstitial positions in the lattice at the lower V/III flux ratio，this can easily cause In atom clusters. This situation will cause a decrease in the electron mobility as seen for samples C1 and C2.

Based on the previous optimization results，it was found that the thickness of the InAs_xSb_1-x layer has a significant impact on electron mobility. Therefore，the influence of channel thickness on electron mobility and 2DEG concentration was studied while fixing the Sb/In ratio at 6 and As/In ratio at 3. Fig. 13 shows the electron mobility and 2DEG concentration dependence on channel thickness at 300 K. It is evident that the electron mobility of samples increases quickly from 18 500 cm²/Vs to 28 300 cm²/Vs with the increase of the channel width from 15 nm to 30 nm. When the InAs_0.4Sb_0.6 channel width is 30 nm，the mobility reaches the maximum of 28 300 cm²/Vs. When the channel width is larger than 30 nm，the electron mobility decreases slowly from 28 300 cm²/Vs to 27 400 cm²/Vs. Interface roughness scattering is the main factor limiting the mobility in InAs_0.4Sb_0.6 channels thinner than 30 nm，while dislocation scattering is the main factor limiting the mobility in InAs_0.4Sb_0.6 channels above 30 nm. Therefore，the electron mobility is no longer increasing with channel layer thickness after 30 nm. After our literature search，the highest electron mobility reported for the InAsSb quantum well heterostructures is currently 28 000 cm²/Vs^［27］. They used a digital alloy method to grow InAs_0.125Sb_0.875 material as the channel layer. However，this method of growing InAs_0.125Sb_0.875 channel layers using digital alloys introduces more interfaces. Moreover，interface roughness scattering will have a significant impact on electron mobility. Therefore，the results obtained by the growth method used in this article have obvious advantages. From Fig. 13，it can be seen that the 2DEG in the quantum well is 6.03×10¹¹ cm^-2~1.01×10¹² cm^-2. The overall trend change is not significant.

In summary，the influence of the V/III ratio on the transport properties and crystal quality of the 200 nm InAs_xSb_1-x thin film and Al_0.2In_0.8Sb/InAs_xSb_1-x quantum well heterostructures has been investigated. The calculation results indicated that the Sb component is 0.6 in the InAs_xSb_1-x thin film when grown under the conditions of Sb/in ratio of 6 and As/in ratio of 3. Meanwhile，the highest electron mobility of InAs_xSb_1-x thin film measured at room temperature was 28 560 cm²/V·s. In addition，the highest electron mobility of the Al_0.2In_0.8Sb/InAs_0.4Sb_0.6 quantum well heterostructures was obtained at 28 300 cm²/V·s for a sample with a channel thickness of 30 nm grown under the conditions where Sb/in ratio was 6 and As/in ratio was 3. This investigation reports the high-quality film and high electron mobility obtained for Al_0.2In_0.8Sb/InAs_0.4Sb_0.6 heterostructures lattice-matched to GaAs and opens the exploration of their uses in high electron mobility transistors.