1Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices; Xinjiang Technical Institute of Physics and Chemistry of CAS, Urumqi, China
Received date: 04/07/2017; Accepted date: 18/07/2017; Published date: 25/07/2017
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Carbon-doped InGaAsBi films on InP:Fe (100) substrates have been grown by molecular beam epitaxy. It has been found that Bismuth incorporation induces extremely high n-type carbon-doped InGaAsBi films and its electron concentration increases linearly up to 1021 cm3 (highest reported to date for n-type III-V semiconductor) with increased CBr4 supply pressure, implying InGaAsBi to be a prospective ohmic contact material for InP-based terahertz transistors. It also has been proved by secondary ion mass spectroscopy that the alloy composition of carbon-doped InGaAsBi is altered by the preferential etching effect of CBr4, but the etching effect on the Bi content is negligible.
InGaAsBi, Bismuth, Molecular beam epitaxy, Ohmic contact, III-V Semiconductor
With increased scaling of InP-based high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) to abtain terahertz devices for high-speed communications, achieving low-resistance ohmic contacts with minimal lateral diffusion and shallow penetration depths are very crucial [1-7]. Specific contact resistance (ρC) is an important parameter characterizing the ohmic contact resistance. For heavily doped n-type semiconductor, the specific contact resistance depends greatly on the doping level and Schottky barrier height, which is described as 
where h is the Planck’s constant, εS is the semiconductor dielectric constant, is the Schottky barrier height in the metalsemiconductor interface, Nd is the electron concentration and m* is the effective mass of the tunneling electron. Usually, a narrow band-gap InGaAs layer has been generally used as a contact layer to obtain low contact resistance in InP-based HEMTs and HBTs [8-12].
Now, dilute InGaAsBi, which is lattice-matched to InP, are relatively new and promising ohmic contact materials for InP-based terahertz transistors because of the expected narrower bandgap [13,14] and higher doping soluility limit  due to valence band anticrossing (VBAC) [16-19] and bismuth being a large and heavy atom, compared with InGaAs. Previously, the majority of the work was focused on characterizing the MBE growth and material properties of undoped InGaAsBi [20-26]. However, few work were focused on characterizing the electrical properties of doped (n-type) InGaAsBi. Pernell Dongmo et al were the first to report room temperature electronic and thermoelectric properties of Si-doped In0.52Ga0.48BiyAs1-y with varying Bi concentrations and the carrier concentration increases to 6.4 × 1019 cm-3 (compared to Si:InGaAs) at the Bi concentration of 1.6% . In order to fully make use of its potentials for optoelectronics and high-speed electronics device applications, it is important to investigate the doping property of InGaAsBi alloys carefully.
In this paper, we firstly report the effects of Bismuth (Bi) incorporation and CBr4 supply pressure on the electrical properties and element content of carbon-doped InGaAsBi films grown at a relative low temperature (LT) by gas source MBE. It is found that Bismuth incorporation induces extremely high n-type carbon-doped InGaAsBi films and its electron concentration increases linearly up to 1021 cm-3 with increased CBr4 supply pressure, which is the highest electron concentration reported to date for n-type semiconductor. It also has been proved by secondary ion mass spectroscopy that the alloy composition of n-type carbon-doped InGaAsBi is altered by the preferential etching effect of CBr4, but the etching effect of CBr4 on the Bi content is negligible.
All carbon-doped InGaAsBi films were grown on (100) semi-insulating InP substrate by a V90 GSMBE system. Elemental In, Ga, Bi and As cracked from AsH3 at 1000°C were used. Carbon tetrabromide (CBr4) served as the doping sources and the carbon beam intensity was adjusted by CBr4 pressure. The substrate was pre-degassed at about 300°C in the preparation chamber for 1 hour to evaporate most of the volatile species, followed by heating to 480°C for 2 min to desorb the native oxides in the P2 flux inside the growth chamber and then gradually raised to 515°C for 5 min to force the appearance of the (4×2)-(100) In-stable reconstruction. The substrate temperature was measured with an infrared radiation thermometer, and In situ Reflection High-Energy Electron Diffraction (RHEED) was used to monitor the reconstruction of the substrate surface. Before the growth of carbon-doped InGaAsBi samples, a 50 nm undoped In0.53Ga0.47As buffer layer was grown at 450°C. Then, carbon-doped InGaAsBi samples were grown at Tsub=275°C. The growth rates for In1-yGayAs1-xBix (y ~ 0.5) epilayer and In0.53Ga0.47As buffer layer were approximately 1 μm/h. The mobility and carrier concentrations were measured by room temperature Hall effect. We used square samples for Hall effect experiment with indium dots as ohmic contacts located on the sample periphery. Structural properties of the grown samples were characterized by high resolution X-ray diffraction (HRXRD) and secondary ion mass spectroscopy (SIMS).
Effect of Bi incorporation
To explore the effect of Bi incorporation on the electrical properties of carbon-doped InGaAsBi films, a set of samples with thickness of about 500 nm were grown at 275°C by gas source MBE and their growth parameters are listed in Table 1, in which samples c and d were grown under the CBr4 supply pressure of 0.12 Torr and samples a and b without the CBr4 supply pressure as references. Details of the growth procedure can be found in Experiment. As listed in Table 1, the conduction type of samples a, and b is n-type and both of electron concentrations are below 1.0 × 1018 cm-3. Compared with samples a and b, it is found that Bi incorporation in InGaAs causes no degradation of the electron mobility and induces p-type carriers (<7.1 × 1017 cm-3) that compensate the background n-type carriers resulting in mobility enhancement  When samples c and d were grown under the CBr4 supply pressure of 0.12 Torr, the conduction type of sample d with Bi incorporation is still n-type and its electron concentration increases to 3.0 × 1019 cm-3, but the sample c was of high resistivity, being no longer measurable with the Hall measurement setup used, which is due to the strong carrier self-compensation of carbon in InGaAs as carbon can occupy both donor and acceptor sites. G. M. Schott reported that LT(Tsub=270°C) GaAs:C revealed the same electrical activation of carbon of about 50% at a high doping level p=5 × 1019 cm-3 as observed in high-temperature GaAs:C . Schoenfeld  reported that the electron concentration of InAs:C is increased to 5 × 1019 cm-3 with dopant CBr4 flow and decreased substrate temperature. Similar behavior has been reported for InGaAs alloys, where the growth conduction-type transition of carbon-doped InGaAs was observed, depending on growth conditions [29,30]. Compared with samples c and d, it implies that Bi incorporation is benefit to induce carbon occupying donor sites under above growth conditions, which leads to extremely high n-type carbon-doped InGaAsBi films.
Table 1. Growth parameters and electrical properties parameters measured from room temperature Hall effect of the four InGaAsBi samples.
|Sample No.||Material||Tln (ºC)||TGa (ºC)||TBi (ºC)||AsH3 (Torr)||CBr4 (Torr)||Conduction type||Concentration (cm-3)||Mobility (cm2/vs)|
("×"indicated that hall cannot measured)
Effect of CBr4 supply pressure
To investigate the effect of CBr4 supply pressure on the crystal quality and electrical properties of carbon-doped InGaAsBi films, a set of samples with thickness of about 500 nm under the AsH3 supply pressure of 350 Torr were grown at 275°C under different CBr4 supply pressure by gas source MBE. Details of the growth procedure can be found in Experiment. Figure 1 shows the X-ray diffraction (XRD) (004) ω-2θ rocking curves for the carbon-doped InGaAsBi samples grown under different CBr4 supply pressure, which are characterized by a Philips X’pert MRD high resolution XRD equipped with a four-crystal Ge (220) monochromator using Cu Kα1 radiation. The sharp tall peak is from the InP substrate indicated by the vertical dashed arrows and the broad bump at the left of the substrate peak in Figure 1 comes from the In0.53Ga0.47As buffer layer indicated by the vertical dotted arrows. The carbon-doped InGaAsBi epitaixal peak, marked by the horizontal solid arrows, shifts from the lower angle to the higher angle with an increase of CBr4 supply pressure, which indicates the increasing incorporation of carbon in InGaAsBi. When CBr4 supply pressure is higher than 0.12 Torr, the crystal quality starts to degrade, probably due to the parasitic etching effect of CBr4 [28,31].
Figure 1: XRD (004) ω-2θ rocking curves from the InGaAsBi samples grown at 275°C under different CBr4 supply pressure. The supply pressure of AsH3 is 350 Torr. The horizontal solid arrows, the vertical dashed arrows and the vertical dotted arrows indicate the peaks of InGaAsBi, InP substrate and In0.53Ga0.47As buffer, respectively.
Figure 2 shows the dependence of carrier concentration and mobility on CBr4 supply pressure for n-type InGaAsBi grown at 275°C in this work and p-type InGaAs grown at 560°C in previous work . As shown in Figure 2, the net electron concentration of carbon-doped InGaAsBi increases almost linearly with the increase of CBr4 supply pressure and a maximum concentration of 1 × 1021 cm-3 under the CBr4 supply pressure of 0.18 Torr is achieved, which is the highest electron concentration reported to date for n-type III-V semiconductor [32-34]. According to this trend, it can reach beyond the level of 1021 cm-3 if the CBr4 supply pressure increases. However, the net hole concentration of carbon-doped InGaAs reaches a saturation of 1 × 1020 cm-3 when the CBr4 supply pressure is above 0.15 Torr, which means carbon incorporation in InGaAs has arrived its solid solubility nearly under this growth condition. Compared with p-type InGaAs, it is found that the solid solubility of carbon incorporation in InGaAsBi is significantly increased beyond 1 × 1021 cm-3, which is induced by Bi incoporation. Similar behavior has been observed in Si-doped InGaAsBi . This can be understood by considering the fact that bismuth is known to be a large and heavy atom, which can act as a surfactant to help improve the overall film quality, and it may also create a large density of step edges, allowing easier carbon adsorption . Since Bi incorporation is benefit to induce carbon occupying donor sites under above growth conditions, more of the adsorbed C atoms are activated as donors, leading to the increase of the effective electron concentration with the increase of CBr4 supply pressure.
Etching effect of CBr4
To invested the influence of CBr4 supply pressure on the element content of In, Ga, Bi and As in carbon-doped InGaAsBi, sample A consisting of 250 nm C:InGaAsBi, 250 nm InGaAsBi and 250 nm In0.53Ga0.47As buffer was grown by gas source MBE and its growth structures and parameters are listed in Table 2. Figure 3 shows the ω-2θ x-ray diffraction (XRD) scans using Cu Kα1 radiation for sample A along the (004) direction. The sharp peak at 0 arcsec represents the substrate peak and the peak of the In0.53Ga0.47As buffer layer overlaps with the substrate peak. The peak at a bigger angle corresponds to the carbon-doped InGaAsBi film peak, which proves lattice constant decrease brought by the incorporation of small carbon atoms, while the peak at a smaller angle corresponds to the InGaAsBi film peak. From the XRD curves, clear Pendellösung fringes are observed, indicating our sample has very smooth epitaxial interfaces and excellent crystalline quality.
Table 2. Growth structures and parameters of sample A.
|Layer No.||Material layer||Thickness (nm)||Ts (°C)||Tin (°C)||Tga (°C)||AsH3 (Torr)||Tbi (°C)||CBr4 (Torr)|
Figure 4 shows the SIMS depth profiles of this sample. The thickness of each layer is about 250 nm in accordance with the design result. From the distributions of the element content and carbon concentration, we found that the observed In content decreases for the carbon-doped InGaAsBi layer after the opening of the carbon-source while Ga increases, keeping the sum of the two roughly constant. This phenomenon has been observed in InGaAs  and InGaAsP . When CBr4 is introduced, CBr4 is likely to be decomposed to Br and C compounds. They will react with Ga, In and As and suppress the rate of effective incorporation of these elements to solid GaAs and InAs. As the etching rate of InAs is faster than that of GaAs, it will make In content decrease, while keeping the sum of the two roughly constant. However, the bismuth content is almost kept at a constant of 4.0% regardless of the carbon-source on or off, which suggest that Bi incorporation is independence of the CBr4 supply pressure. Since bismuth is known to be a large and heavy atom. With a proper Bi flux, the growth front could be covered with Bi surfactants of at least a few monolayer thickness. When CBr4 is introduced, the carbon-doped InGaAsBi surface is covered by a complete atomic Bi layer, by which the growth surface and Br compounds is separated as shown in Figure 5. Accordingly, the atoms such as In, Ga, As and Bi on the top of the complete atomic Bi layer will be etched by Br. Since the incorporation of elemental Ga and In come from the surface of complete atomic Bi layer, the rate of effective incorporation of elemental In and Ga to solid GaAs and InAs is suppressed. But the bismuth incorporation into InGaAs comes from the complete atomic Bi layer which was almost not affected by the etching of Br, therefore, the etching effect on the Bi incorporation is negligible. Thus, the alloy composition of n-type carbondoped InGaAsBi is altered by the preferential etching effect of CBr4, but the etching effect of CBr4 on the Bi content is negligible.
In summary, we have investigated the effects of Bismuth incorporation and CBr4 supply pressure on the crystal quality, electrical properties and element content of carbon-doped InGaAsBi films grown at a relative low temperature by gas source MBE. It is found that Bismuth incorporation induces extremely high n-type carbon-doped InGaAsBi films and its electron concentration increases linearly up to 1021 cm-3 with increased CBr4 supply pressure, which is the highest electron concentration reported to date for n-type semiconductor. It also has been proved by secondary ion mass spectroscopy that the alloy composition of n-type carbon-doped InGaAsBi is altered by the preferential etching effect of CBr4, but the etching effect of CBr4 on the Bi content is negligible. InGaAsBi grown by a more mature InP technology is very promising as a contact material alternative to InP-based compounds for Terahertz transistors.
The authors wish to acknowledge the financial support of National Natural Science Foundation of China (Grant No. 61434006), National Basic Research Program of China (Grant No. 2014CB643902), and the West Light Foundation of the Chinese Academy of Sciences (Grant No. 2016-QNXZ-B-10).