The host III–V compound material which a small quantity of bismuth (Bi) atoms is incorporated into has attracted much attention due to its unique properties of the strong enhancement of spin-orbit splitting and large band-gap reduction.^[1–5] Bismuth is the largest and heaviest group V element with its isoelectronic energy level that resides in the valence bands of most III–V materials,^[6] therefore, substituting As and/or P with a small amount of Bi into InP, GaAs, or InGaAs will result in anomalously narrow bandgaps^[7,8] due to valence band anticrossing (VBAC).^[9–12] Engineering the band structure for potential optoelectronic and electronic device applications is possible because of these interesting properties.^[13–16] Also, InGaAs alloy, which is widely used as a fundamental material in photonic devices, is expected to be turned into a promising temperature-insensitive semiconductor In_xGa_1−xAs_1−yBi_y by Bi incorporation.

In 2005, the first InGaAsBi quaternary alloy was grown on InGaAs buffer layer on an InP substrate by Gan et al. using molecular beam epitaxy (MBE).^[12] Good crystalline quality was achieved with 2.5% Bi, which was confirmed by x-ray diffraction (XRD). Two years later, the same research group investigated temperature dependence of Bi behavior during MBE growth.^[11] They achieved a maximum of 6% Bi concentration at a growth temperature as low as 260 °C. The growth at a high temperature of 450 °C, normally for InGaAs epitaxy growth, revealed no detectable Bi incorporation but the decrease of surface roughness from 1.067 nm to 0.328 nm.^[17] In 2017, Zhou et al. demonstrated that up to 7.5% of Bi incorporation was achieved in InGaAsBi,^[18] which was the highest Bi content until now. For the applications of InGaAsBi in electronic and optoelectronic devices, a short-wave infrared detector with quaternary InGaAsBi as an absorption layer and the bismuth content of about 3.2% has red-shifted the 50% cut-off wavelength from about to at room temperature.^[19]

In this work we systematically study the effects of growth condition, including Bi flux and pressure of AsH₃ on Bi incorporation, surface morphology and optical properties of InGaAsBi grown by the gas source molecular beam epitaxy(GSMBE) method.

All InGaAsBi samples were grown on (100) semi-insulating InP substrates by a V90 GSMBE system. Elements In, Ga, Bi, and As₂ coming from arsine (AsH₃) at 1000 °C were used as source materials. Elements In, Ga, and Bi fluxes during growth were controlled by adjusting the respective effusion cell temperatures. Prior to growth, by slowly ramping up the manipulator temperature, the substrate surface was deoxidized under P₂ pressure until the reflection high-energy electron diffraction (RHEED) pattern showed a clear and abrupt transformation to a (2 × 4) surface reconstruction. This surface reconstruction was adopted as a means for calibrating the substrate temperature. After that, InGaAsBi epilayers with a thickness of about were grown at a temperature as low as 300 °C measured by thermocouple for efficient Bi incorporation. Bi cell temperatures of 0, 510, 520, 530, and 540 °C were adopted. To study the influence of the As flux on the incorporation of Bi into InGaAsBi, the pressure of AsH₃ was changed from 270 Torr to 360 Torr (1 Torr = 1.33322 × 10² Pa) with the Bi cell temperature unchanged. For simplicity, the InGaAsBi samples with varying Bi flux were renamed as a1, a2, a3, a4, and a5 as shown in Table 1. The InGaAsBi samples with varying pressures of AsH₃ were renamed as b1, b2, b3, b4, and b5 as shown in Table 2.

Table 1.

Table 1.

Table 1. Material growth parameters and measured properties of the grown InGaAsBi at different Bi effusion cell temperatures. . Sample No. Ts/°C Bi/°C AsH3/Torr XRD FWHM/(°) Bi content/% Surface roughness/nm a1 300 360 0.013 0 a2 300 510 360 0.025 0.70 0.232 a3 300 520 360 0.159 0.98 0.247 a4 300 530 360 0.18 0.99 0.319 a5 300 540 360 0.175 1.13 0.792

Table 1. Material growth parameters and measured properties of the grown InGaAsBi at different Bi effusion cell temperatures. .

Table 2.

Table 2.

Table 2. Material growth parameters and measured properties of grown InGaAsBi at different pressures of AsH3. . Sample No. Ts/°C Bi/°C AsH3/Torr Surface roughness/nm b1 300 530 240 3.210 b2 300 530 270 0.311 b3 300 530 300 0.196 b4 300 530 330 0.280 b5 300 530 360 0.361

Table 2. Material growth parameters and measured properties of grown InGaAsBi at different pressures of AsH3. .

After growth, the Bi compositions were determined by Rutherford backscattering spectroscopy (RBS) with 2.275-MeV ⁴He²⁺ ions. The structures of all samples were characterized by ω–2θ scans along the (004) direction, performed in a Philips X’pert high resolution x-ray diffractometer (HRXRD) equipped with a Ge (220) four-bounce monochromator using Cu–Kα₁ (λ =0.15406 nm) radiation. Their surface morphologies and roughness values were measured by atomic force microscopy (AFM). Photoluminescence (PL) and absorption measurements were conducted in a Fourier transform infrared (FTIR) spectroscopy by using a liquid nitrogen-cooled InSb detector and a CaF₂ beam splitter. A diode-pumped solid-state (DPSS) laser (λ =532 nm) was used as an excitation source for PL measurement.

The RBS is a powerful method of determining the concentration of heavy atoms in a crystal lattice composed of light elements. A typical result of sample a2 is shown in Fig. 1. The peak/step-like signal of the element Bi is observed at channel number around 480. This signal is strong evidence for Bi incorporation. From the simulations, the Bi concentration is calculated to be 0.7% for this sample. The high resolution x-ray diffraction (HRXRD) ω–2θ scans of samples a1–a5 are shown in Fig. 2. The narrow diffraction peaks correspond to InP (004) diffractions, and the relatively broad peaks correspond to InGaAsBi epilayers. It is clear that the mismatch among diffraction peaks of InGaAsBi and InP increases with the Bi content going up, which indicates that the lattice constant increases with the addition of Bi. As shown in Table 1, increasing Bi content brings about a larger full width at half maximum (FWHM) due to the increase of Bi-related dislocations in the alloy of sample.^[20] From the peak separation between InP and InGaAs_1−xBi_x in the XRD spectrum, each lattice constant can be estimated. For convenience, the variations in In composition and lattice deformation have been ignored.^[21] Based on Vegard’s law, the lattice constant (C) of the quaternary alloy In_0.53Ga_0.47As_1−xBi_x can be expressed as follows:

Fig. 1.

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	Fig. 1. (color online) RBS spectrum from the InGaAsBi film with a Bi concentration of 0.7%. The black, violet, red, blue, magenta, olive, and navy lines represent the randomly and simulated spectra of the InGaAsBi films with the individual P, Ga, As, In, and Bi, respectively.

Fig. 2.

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	Fig. 2. (color online) HRXRD (004) ω–2θ scans for InGaAsBi epilayers on InP with Bi content of 0% (a1), 0.7% (a2), 0.98% (a3), 0.99% (a4), and 1.13% (a5).

Lattice constants are assumed to be 6.639, 6.234, 6.058, and 5.653 Å for InBi, GaBi, InAs, and GaAs in zinc blende structures, respectively.^[22] Therefore, the amounts of Bi in the InGaAsBi alloys estimated from the lattice constant are around 0%, 0.70%, 0.98%, 0.99%, and 1.13%. The Ga has an effect to reduce the lattice constant after being incorporated into InP and InAs lattice.^[23] It is expected that the lattice constant of InGaAsBi can be matched to that of InP by appropriately controlling the Ga and Bi content.

The surface morphologies are characterized by AFM scan using tapping mode for samples a2–a5. The root-mean-square (RMS) roughness versus the Bi content of the InGaAsBi epilayer is shown in Table 1. Figure 3 shows AFM images of the samples. All the samples contain some holes. The formation of these holes is likely to be due to thermal evaporation of oxides from the InP substrates. While the Bi concentration increases to 1.13% in sample a5, big Bi related metallic dots can be seen in the fourth image of Fig. 3, which is in the center of white rectangle. We suggest that Bi surface migration plays an important role. The rough starting surface of InP substrate after oxide desorption restricts the Bi surface diffusion, resulting in a local increase of Bi concentration. So a smooth surface facilitates Bi surface diffusion and thus uniform Bi incorporation.^[24]

Fig. 3.

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	Fig. 3. (color online) Surface morphologies of the InGaAsBi samples a2 (a), a3 (b), a4 (c), and a5 (d).

Figure 4(a) shows the PL spectra of the samples a1–a5 at room temperature. The InGaAs reference sample (sample a1) shows a weak PL signal at 0.74 eV which is close to the bandgap of In_0.53Ga_0.47As^[25] and originates from the band-to-band (BB) transition. After a small amount of Bi is incorporated (sample a2), a strong and broad PL spectrum appears between 0.78 eV and 0.66 eV, peaked at about 0.715 eV. As expected, the PL peak energy decreases with increasing Bi content, implying that the band gap of InGaAs_1−xBi_x alloydecreases with increasing Bi content. The peak energies are 0.714, 0.691, 0.688, and 0.682 eV for the samples with 0.70% (a2), 0.98% (a3), 0.99% (a4), and 1.13% (a5) Bi, respectively. A bandgap reduction coefficient of 48 meV/Bi% is deduced and calculated from the experimental data. The PL intensity decreases dramatically when the Bi content increases from 0.7% to 0.98%, which implies that non-recombination centers in In_0.53Ga_0.47As_1−xBi_x increase because of the deterioration of crystalline quality. The FWHMs of the PL spectra of the samples a2–a5 are nearly 0.060, 0.064, 0.065, 0.075 eV, respectively, which confirms that the deterioration in crystalline perfection of InGaAsBi is due to the increasing Bi content. Figure 4(b) shows the variation of the FWHM of the PL spectra of samples a2–a5 with Bi content for samples a2–a5. The trend implies that the deterioration process in crystalline perfection of InGaAsBi with higher Bi content becomes quicker than with relatively low Bi content. It is because at higher Bi effusion cell temperature, more Bi atoms will be accumulated on surface, forming large metallic Bi droplets on the growth front surface, which hinders the InGaAsBi from uniformly growing.

Fig. 4.

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	Fig. 4. (color online) Room-temperature PL spectra for the samples with different Bi content. (b) Variations of the FWHM of the PL spectra of samples a2–a5 with their Bi content.

The InGaAsBi samples, with pressures of AsH₃ varied but other conditions fixed, are renamed as b1, b2, b3, b4, and b5 as shown in Table 2. The HRXRD ω–2θ scans of samples b1–b5 are shown in Fig. 5. Through the simulations and comparison of the HRXRD ω–2θ scans of samples b1–b5 with the HRXRD ω–2θ scan of sample a2, the amounts of Bi in samples b1–b5 are estimated from the lattice constant at about 1.05%, 0.48%, 0.21%, 0.10%, and 0%. It is obivious that the lattice constant increases with the pressure of AsH₃ decreasing. As is well known, both As and Bi are group-V atoms which tend to bond with group-III atoms. So an excess As supply will outcompete Bi since III–As bonding is stronger than III-Bi bonding. Hence, the decreasing of AsH₃ pressure will spontaneously enhance the possibility of III–Bi bonding and thus increase the Bi content. Then, the diffraction peak in Fig. 5 shifts toward the small-angle side with the pressure of AsH₃ decreasing.

Fig. 5.

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	Fig. 5. (color online) HRXRD (004) ω–2θ scans for InGaAsBi epilayers (b1–b5) on InP at different AsH3 pressures.

Figure 6(a)–6(e) show AFM images of b1–b5 samples. These images reveal that as the pressure of AsH₃ decreases from 360 Torr to 300 Torr, the root mean square (RMS) roughness values of samples decrease from 0.361 nm to 0.196 nm. After the pressure of AsH₃ reaches 300 Torr, the RMS roughness value begins to increase, which peaks at 3.210 nm when the pressure of AsH₃ is 240 Torr. Figure 6(f) shows the variation of the RMS roughness values of samples with the pressure of AsH₃. This trend and the relationship between RMS roughness value and pressure of AsH₃ can be qualitatively comprehended like this: both As and Bi are group-V atoms which tend to bond with group-III atoms. Hence, the decreasing of AsH₃ pressure will increase the Bi content. So when the AsH₃ pressure decreases from 360 Torr to 300 Torr, this trend will facilitate the incorporation of Bi atoms so that fewer excess Bi atoms will stay or accumulate on the surfaces of the samples. Then, the RMS roughness values of samples decrease. However, the continuous decreasing of AsH₃ pressure will not result in a monotonic increase of Bi content, but leads to forming droplets because excess group-III atoms on the growing InGaAsBi surface cannot be evaporated. So when the AsH₃ pressure continously decreases from 300 Torr to 270 Torr, the RMS roughness values of samples increase from 0.196 nm to 0.311 nm because the excess group-III atoms on the growing InGaAsBi surface cannot be evaporated at such a low growth temperature. The RMS roughness value of sample peaks at 3.210 nm when the pressure of AsH₃ is 240 Torr.

Fig. 6.

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	Fig. 6. (color online) Room temperature PL spectra for the samples b2–b5 at different AsH3 pressures.

The PL spectra at room temperature of b2–b5 samples at different AsH₃ pressures are shown in Fig. 7. As the AsH₃ pressure decreases, the PL peak is red-shifted, which confirms that the lower AsH₃ pressure enhances the incoporation of Bi atoms. The PL intensities show their small differences at high AsH₃ pressures ranging from 360 Torr to 300 Torr but decrease dramatically at about 270 Torr AsH₃ pressure and we cannot observe the PL peak of sample b1, which implies the deterioration of the crystalline quality because the excess group-III atoms on the growing InGaAsBi surface cannot be evaporated from the surface.

Fig. 7.

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	Fig. 7. (color online) (a)–(e) Surface morphologies of InGaAsBi samples b1, b2, b3, b4, b5. (f) Variations of the RMS roughness values of the InGaAsBi samples b1, b2, b3, b4, b5 with AsH3 pressure.

In this work, we investigate the effects of Bi flux and AsH₃ pressure during growth on Bi incorporation, surface morphology and optical properties of InGaAsBi grown by GSMBE on InP substrate. It is found that using high Bi flux and low AsH₃ pressure during growth can strengthen the effect on the incorporation of Bi. The Bi content increases linearly with Bi flux until it reaches nearly a saturation value. The maximum incorporated Bi content is found to be 1.13%. A relatively low Bi flux and a moderate AsH₃ pressure during growth can improve the surface morphology. We obtain the photoluminescence spectra of InGaAsBi at room temperature, which show strong and broad light emission at near infrared wavelength. The low AsH₃ pressure will remarkably damage the optical properties of InGaAsBi grown by gas source molecular beam epitaxy.