n-Type Doping of Vapor–Liquid–Solid Grown GaAs Nanowires
© Gutsche et al. 2010
Received: 8 September 2010
Accepted: 17 September 2010
Published: 7 October 2010
In this letter, n-type doping of GaAs nanowires grown by metal–organic vapor phase epitaxy in the vapor–liquid–solid growth mode on (111)B GaAs substrates is reported. A low growth temperature of 400°C is adjusted in order to exclude shell growth. The impact of doping precursors on the morphology of GaAs nanowires was investigated. Tetraethyl tin as doping precursor enables heavily n-type doped GaAs nanowires in a relatively small process window while no doping effect could be found for ditertiarybutylsilane. Electrical measurements carried out on single nanowires reveal an axially non-uniform doping profile. Within a number of wires from the same run, the donor concentrations ND of GaAs nanowires are found to vary from 7 × 1017 cm-3 to 2 × 1018 cm-3. The n-type conductivity is proven by the transfer characteristics of fabricated nanowire metal–insulator-semiconductor field-effect transistor devices.
Novel, quasi one-dimensional structures, like III-V semiconductor nanowires, may act as key elements in future nanoscaled optoelectronic devices [1–3]. They offer intriguing electrical and optoelectronic properties and the ability to combine material systems that are impossible in conventional semiconductor layer growth due to lattice mismatch issues . The large surface to volume ratio, which is already utilized in nanowire sensor applications [5, 6], allows to improve light extraction and light collections when compared to planar devices making especially nanowires ideal candidates for light emitters and photo voltaics [7–9]. However, the future of any semiconductor nanowire technology will inherently rely on their doping capability. Only this way, the control of carrier type and density representing the unique advantage of semiconductors will be available . Unfortunately, the specific parameters for nanowire growth do often not favor the incorporation of doping atoms. Moreover, both n- and p-type doping within the same semiconductor has to be provided for most optoelectronic applications.
There are only a very few publications describing initial doping results of III-V compound semiconductor nanowires with a high charge carrier density. Most of them focus on the material systems InAs  and InN , which is not astounding since at the surface of these semiconductors, the surface Fermi level is pinned  in the conduction band. This effect makes n-type conductivity easy to the expense of difficulties for p-type doping. In other semiconductors like GaAs, the Fermi level at the surface is pinned approximately in the center of the band gap resulting in a substantial surface depletion that may lead to non-conducting nanowires even at elevated doping levels. On the other hand, both a controlled p- and n-type doping might be available. Doping of GaAs nanowires grown by molecular beam epitaxy (MBE) has been demonstrated in different means. LaPierre et al. used Be and Te as p- and n-type dopant precursors , while Fontcuberta i Morral et al. pointed out that Si may act as both by just changing the operating temperature during growth [14, 15]. The incorporation of Si and Be into GaAs nanowires was investigated in a further study . Nevertheless, the growth and dopant mechanisms of GaAs nanowires grown by MBE differ to some extend from chemical vapor deposition (CVD) methods, since the growth temperatures of the first-mentioned are usually much higher (500°C < Tg < 650°C). Till now, just in case of InP nanowires, both a successful n- and p-type doping, respectively, have been obtained in the core of untapered III-V nanowires synthesized via metal–organic vapor phase epitaxial (MOVPE) growth. Here, hydrogen sulfide (H2S)/tetraethyl tin (TESn) and diethyl zinc (DEZn)/dimethyl zinc (DMZn) were used as dopant sources [7, 17] in the vapor–liquid solid (VLS) growth mode. p-doping of VLS-grown GaAs nanowires was demonstrated supplying DEZn during MOVPE growth , but a study on n-type doping is pending.
In this letter, n-type doping of GaAs nanowires grown by VLS using two different precursor materials, ditertiarybutylsilane (DitBuSi) and tetraethyl tin (TESn), is reported. Structural and morphological changes possibly induced by dopant incorporation were analyzed. Ohmic contacts to single n-GaAs nanowires and their electrical measurements are described. The n-type conductivity is proven by measuring the transfer characteristics of fabricated GaAs nanowire field-effect transistors. By adopting a transport model , the carrier concentrations of GaAs:Sn wires are estimated in the presence of surface depletion.
GaAs nanowires were grown on GaAs (111)B substrates by metal–organic vapor phase (MOVPE) epitaxy in an AIX200 RF system with fully non-gaseous source configuration . Monodisperse as well as polydisperse Au nanoparticles were deposited as growth seeds prior to growth. Monodisperse nanoparticles with a diameter of 150 nm were taken from a colloidal solution. Polydisperse metal seeds for VLS growth of the nanowires were formed by evaporation and subsequent annealing of a thin Au layer of nominally 2.5 nm thickness. The anneal step was carried out at 600°C for 5 min under group-V overpressure and resulted in nanoparticles with diameters from 30 nm to some 100 nm. Nanowires were grown at a total pressure of 50 mbar, using Trimethylgallium (TMGa) and Tertiarybutylarsine (TBAs) as precursors with a constant V/III ratio of 2.5. The total gas flow of 3.4 l/min was provided by N2 as carrier gas, while H2 was used for the bubblers. After the growth start, initiated at 450°C for 3 min, the final growth temperature was adjusted to 400°C, to exclude almost completely additional VS growth on the nanowire side facets . n-doping effect was investigated by an additional TESn (0.02 ≤ IV/III ≤ 0.16) or DitBuSi (IV/III ≤ 0.52) supply.
Morphological characterization of the nanowires was performed via scanning electron microscopy (LEO 1530). Electrical results were obtained with standard DC-measurements setup. Therefore, the as-grown structures were transferred to special pre-patterned carriers and finally contacted by electron beam lithography (E-Beam) or optical lithography, respectively. The carrier consists of a semi-insulating GaAs substrate that was covered with 300-nm-thick silicon nitride (SiNx) for improved isolation. The ohmic contacts were formed by evaporation of Ge (5 nm)/Ni (10 nm)/Ge (25 nm)/Au (400 nm), which is known to be a typical contact system for n-GaAs . To improve the contact properties, a rapid thermal annealing was carried out for 30 s or 300 s at 320°C. In addition, metal–insulator-semiconductor field-effect transistor (MISFET) devices were fabricated with about 30 nm SiNx gate dielectric and Ti/Au gate metal  to verify the type of conductivity.
Results and Discussion
If TESn at IV/III = 0.08 is used as dopant precursor, the current of 2 μA at 1 V applied bias is about six orders of magnitude higher than for the nid sample, giving evidence of the doping effect. The corresponding I–V characteristic is not perfectly ohmic, which indicates a small remaining contact barrier, while no blocking region is observable. The realization of ohmic contacts on n-GaAs is known to be challenging specially at low annealing temperatures due to the already mentioned Fermi level pinning and high density of surface states . This well-known classical problem becomes much more serious in nanowire devices due to the increase in surface to volume ratio, which in turn complicates the ohmic contact fabrication even on relatively high-doped n-GaAs nanowires. However, annealing at higher temperatures than 320°C leads to an increased out-diffusion of Ga into the Au contact layer. This effect is also reported for bulk material , but gets crucial in the nanoscale since it destroys the nanowire and has to be avoided. Regarding the following analysis of the doping concentration, it should be noted that the nanowire resistances are extracted for voltages ≥ 1 V, where the remaining contact barrier is just a small series resistance. Therefore, the later given ND values might be slightly underestimated, but in the same order of magnitude. Further, we assume that in case of the nid- and Si-doped nanowires, the I–V behavior is dominated by the high wire resistance and hence completely ohmic in the investigated regime.
Here, we used a value of μ0 = 8,000 cm2/Vs. It should be pointed out that this is a simplification since the Hilsum formula is employed for bulk material and the carrier mobility μ0 is also set to that of bulk GaAs. Therefore, scattering via surface states and stacking faults are not considered. In literature, carrier mobility measured via the transconductance of the nanowire device, which utilizes simplifications to the same degree, reveals lower mobility than known bulk values. If e.g. μ0 is reduced to 4,000 cm2/Vs, the doping concentration for a nanowire with rNW = 100 nm and RNW(1 μm) = 2 kΩ changes to 2 × 1018 cm-3, which also suggests that our NDs might be underestimated (1 × 1018 cm-3 for μ0 = 8,000 cm2/Vs).
With these experiments, we have found the relatively small process window (0.04 ≤ IV/III ≤ 0.08) for the successful n-type doping of VLS-grown GaAs nanowires with high charge carrier densities using TESn.
Finally, this experiment proves the n-type doping effect using TESn, which is to our knowledge the first successfully n-doped GaAs nanowire grown by VLS in an MOVPE apparatus. An additive proof was given by measuring low and room temperature electroluminescence of axial pn-junctions in single GaAs nanowires. More details about this topic will be given in a subsequent study.
The successful n-type doping during the VLS growth of GaAs nanowires is reported using tetraethyltin as doping precursor. DitBuSi shows no doping effect, which is attributed its amphoteric behavior and to the low nanowire growth temperature resulting in a low cracking efficiency. In contrast to p-type doping, using diethyl zinc, no influence on the crystal structure was observable, despite relatively high dopant supplies. From the experimental resistance data, we were able to estimate a donor concentration ND varying from 7 × 1017 cm-3 to 2 × 1018 cm-3. The data spreading is attributed mainly to an axially non-uniform doping profile. Transfer characteristic of multi-channel MISFETs, fabricated from these nanowires, proved that the doping of the nanowire is n-type, though the gate control is reduced due to Fermi level pinning and interface states.
The described route for the n-type doping of GaAs nanowires is of general interest for all compound semiconductor nanowires and for future nanoscaled devices. It points out fundamental aspects regarding the doping capability using different precursors within MOVPE and should provide the basics to synthesize GaAs nanowire pn-junctions, which may act as key element in nanowire optoelectronics.
The authors gratefully acknowledge financial support of the Sonderforschungsbereich SFB 445 "Nanoparticles from the gas-phase".
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