What Doping Is Required for N-type Gaas to Beconsidered â€å“degeneratelyã¢â‚¬â Doped?

• Journal List
• Nanoscale Res Lett
• v.six(i); 2011
• PMC3212212

Nanoscale Res Lett. 2011; half-dozen(one): 65.

n-Blazon Doping of Vapor–Liquid–Solid Grown GaAs Nanowires

Christoph Gutsche

¹Solid State Electronics Department and CeNIDE, University of Duisburg-Essen, Lotharstr. 55, 47048, Duisburg, Federal republic of germany

Andrey Lysov

^aneSolid State Electronics Department and CeNIDE, Academy of Duisburg-Essen, Lotharstr. 55, 47048, Duisburg, Germany

Ingo Regolin

^oneSolid State Electronics Department and CeNIDE, University of Duisburg-Essen, Lotharstr. 55, 47048, Duisburg, Deutschland

Kai Blekker

^oneSolid State Electronics Section and CeNIDE, University of Duisburg-Essen, Lotharstr. 55, 47048, Duisburg, Germany

Werner Prost

¹Solid State Electronics Department and CeNIDE, University of Duisburg-Essen, Lotharstr. 55, 47048, Duisburg, Germany

Franz-Josef Tegude

¹Solid State Electronics Section and CeNIDE, University of Duisburg-Essen, Lotharstr. 55, 47048, Duisburg, Germany

Received 2010 Sep 8; Accepted 2010 Sep 17.

Abstract

In this letter, northward-type doping of GaAs nanowires grown past 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 beat growth. The touch 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 event could exist found for ditertiarybutylsilane. Electric 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 N _D of GaAs nanowires are found to vary from vii × 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.

Keywords: Nanowires, MOVPE, Gallium arsenide, Doping, Silicon, Tin can, Optoelectronics

Introduction

Novel, quasi one-dimensional structures, like III-V semiconductor nanowires, may act every bit key elements in future nanoscaled optoelectronic devices [1-3]. They offer intriguing electric and optoelectronic properties and the ability to combine material systems that are incommunicable in conventional semiconductor layer growth due to lattice mismatch issues [4]. The big surface to volume ratio, which is already utilized in nanowire sensor applications [5,6], allows to meliorate light extraction and low-cal collections when compared to planar devices making particularly nanowires ideal candidates for low-cal emitters and photo voltaics [seven-9]. Yet, the hereafter of whatever semiconductor nanowire technology will inherently rely on their doping adequacy. Merely this way, the control of carrier type and density representing the unique advantage of semiconductors volition exist available [three]. Unfortunately, the specific parameters for nanowire growth exercise often not favor the incorporation of doping atoms. Moreover, both north- and p-blazon doping within the same semiconductor has to be provided for near optoelectronic applications.

There are only a very few publications describing initial doping results of Iii-5 chemical compound semiconductor nanowires with a loftier charge carrier density. Well-nigh of them focus on the cloth systems InAs [10] and InN [xi], which is not astounding since at the surface of these semiconductors, the surface Fermi level is pinned [12] in the conduction band. This effect makes due north-type electrical conductivity like shooting fish in a barrel to the expense of difficulties for p-blazon doping. In other semiconductors similar GaAs, the Fermi level at the surface is pinned approximately in the eye of the band gap resulting in a substantial surface depletion that may lead to not-conducting nanowires even at elevated doping levels. On the other hand, both a controlled p- and due north-blazon doping might exist available. Doping of GaAs nanowires grown by molecular axle epitaxy (MBE) has been demonstrated in different ways. LaPierre et al. used Be and Te every bit p- and northward-type dopant precursors [13], while Fontcuberta i Morral et al. pointed out that Si may act as both by just irresolute the operating temperature during growth [14,xv]. The incorporation of Si and Be into GaAs nanowires was investigated in a further study [xvi]. Yet, the growth and dopant mechanisms of GaAs nanowires grown past MBE differ to some extend from chemical vapor degradation (CVD) methods, since the growth temperatures of the commencement-mentioned are usually much college (500°C < Tg < 650°C). Till at present, but in case of InP nanowires, both a successful n- and p-blazon doping, respectively, have been obtained in the core of untapered Iii-V nanowires synthesized via metallic–organic vapor phase epitaxial (MOVPE) growth. Here, hydrogen sulfide (H₂South)/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 [18], only a study on n-type doping is pending.

In this letter, due north-type doping of GaAs nanowires grown past VLS using two different forerunner materials, ditertiarybutylsilane (DitBuSi) and tetraethyl tin (TESn), is reported. Structural and morphological changes possibly induced past dopant incorporation were analyzed. Ohmic contacts to single n-GaAs nanowires and their electrical measurements are described. The n-blazon electrical conductivity is proven by measuring the transfer characteristics of fabricated GaAs nanowire field-effect transistors. By adopting a transport model [18], the carrier concentrations of GaAs:Sn wires are estimated in the presence of surface depletion.

Experimental

GaAs nanowires were grown on GaAs (111)B substrates past metal–organic vapor phase (MOVPE) epitaxy in an AIX200 RF system with fully not-gaseous source configuration [nineteen]. Monodisperse besides every bit 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 two.5 nm thickness. The amalgamate 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/Three ratio of 2.5. The full gas menstruation of three.4 l/min was provided by N₂ every bit carrier gas, while H₂ was used for the bubblers. After the growth start, initiated at 450°C for three min, the final growth temperature was adjusted to 400°C, to exclude nearly completely additional VS growth on the nanowire side facets [xx]. north-doping effect was investigated past an additional TESn (0.02 ≤ Four/Three ≤ 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 every bit-grown structures were transferred to special pre-patterned carriers and finally contacted past electron beam lithography (E-Axle) or optical lithography, respectively. The carrier consists of a semi-insulating GaAs substrate that was covered with 300-nm-thick silicon nitride (SiN₁₀) for improved isolation. The ohmic contacts were formed by evaporation of Ge (v nm)/Ni (10 nm)/Ge (25 nm)/Au (400 nm), which is known to be a typical contact arrangement for n-GaAs [21]. To better the contact properties, a rapid thermal annealing was carried out for 30 due south or 300 s at 320°C. In addition, metal–insulator-semiconductor field-effect transistor (MISFET) devices were fabricated with nigh xxx nm SiN_x gate dielectric and Ti/Au gate metal [22] to verify the type of conductivity.

Results and Discussion

Growth Results

SEM micrographs of 3 different samples are depicted in Figure 1a–c. The selected growth temperature of 400°C suppresses the conventional layer growth on the side facets [20], leading to a very high aspect ratio up to gr, _VLS/gr, _VS > 1,000. Hence, the doping machinery through side facet deposition, reported in various publications [14,23], tin be excluded. This enables a separate investigation of VLS-grown GaAs nanowires. The wires given in Figure ​ 1a and ​ 1b are grown from colloidal Au seed particles with 150 nm diameter and under supply of TESn (Figure ​ 1a, Four/Iii = 0.08) and DitBuSi (Figure ​ 1b, IV/III = 0.52), respectively. In addition, nanowires grown from polydisperse seed particles nether the same conditions every bit in (a) are shown in Figure ​ 1c. All of the nanowires adopted the crystal orientation of the growth substrate and are ethical in (111)B management. Furthermore, no wire kinking or other structural defects, even at higher TESn supply up to Iv/III = 0.16, were observable (for TEM analysis refer to [24]). In contrast, p-type doping with diethylzinc (DEZn) revealed a strong influence on the crystal structure, even at depression II/3 ratios higher than 0.008, as reported previously [18]. One possible reason may exist that the solubility of Sn and Si in the Au particle is much lower than for Zn at the selected growth parameters. The phase diagrams of Au–Sn [25], Au–Si [26] and Au-Zn [27] substantiate this assumption, since there exists no eutectic point for the binary Au-Zn alloy at 400°C. Hence, more and more than Zn might be solved in the Au particle during the nanowire growth process. With college 2/III ratios, this leads into an increased number of structural defects and wire kinking. For n-type doping, using TESn and DitBuSi, respectively, the solubility of dopants in the seed particle is lower, which accounts for the adept crystal construction despite relatively loftier dopant supplies. Of course, the nanoscale may differ to some extent and adding a third component (Gallium) complicates the chemistry/physics at the droplet. Nevertheless, the reported differences regarding n- and p-type doping go more comprehensible.

SEM micrographs of GaAs nanowires grown on GaAs (111)B substrates: a from colloidal nanoparticles with 150 nm diameter under TESn supply (IV/Three = 0.08), b from colloidal nanoparticles with 150 nm bore under DitBuSi supply (Iv/Three = 0.52), c grown under the same weather condition every bit in a simply from polydisperse seed particles formed by annealing of a 2.5 nm Au layer. The different nanowire density in a and b is only accidental.

Electrical Characterization

Representative I–V characteristics for nanowires grown without dopant supply, with supply of DitBuSi (IV/Three = 0.52) and with supply of TESn (IV/III = 0.08) are displayed in Figure ​ two. The not-intentional doped (nid) GaAs nanowires let pass a current of a few pA at 1 V applied bias, corresponding to a resistance in the GΩ range. Adding DitBuSi to the gas phase during growth has no remarkable event on the electrical conductivity of nanowires, even at relatively loftier Iv/Three ratios. This can easily exist interpreted since Si is an amphoteric impurity in GaAs [28,29]. Beginning, principle calculations claim that this also holds for nanowires [thirty]. In addition, the growth temperature of 400°C might be to low for a sufficient dandy of the DitBuSi precursor [31]. The latter statement can not be the but reason for the non-existing doping result using DitBuSi, since we already carried out doping experiments on GaAs nanowire shells at growth temperatures up to 650°C (e.thou. same temperature as for GaAs layer growth), which also failed.

Top: SEM epitome of a GaAs nanowire from sample c continued to two electrodes for electrical measurements. The contact spacing is ane.3 μm. Bottom: I-5 characteristics of the untapered GaAs nanowires grown at 400°C: a grown without dopant supply, b grown under supply of DitBuSi (IV/3 = 0.52), c grown under supply of TESn (Iv/III = 0.08). The second inset shows the I–V curves of a and b in a more acceptable electric current scale.

If TESn at Iv/III = 0.08 is used as dopant forerunner, the current of 2 μA at i V applied bias is about 6 orders of magnitude college than for the nid sample, giving bear witness of the doping effect. The corresponding I–Five 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 [12]. This well-known classical problem becomes much more serious in nanowire devices due to the increment in surface to volume ratio, which in plough complicates the ohmic contact fabrication even on relatively high-doped northward-GaAs nanowires. Even so, annealing at higher temperatures than 320°C leads to an increased out-diffusion of Ga into the Au contact layer. This result is also reported for bulk material [32], but gets crucial in the nanoscale since it destroys the nanowire and has to be avoided. Regarding the following assay of the doping concentration, information technology should be noted that the nanowire resistances are extracted for voltages ≥ one Five, where the remaining contact bulwark is just a small-scale series resistance. Therefore, the later given N _D values might be slightly underestimated, just in the same gild 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.

In order to determine the carrier concentration of the Sn-doped GaAs nanowires, we adopted the model used for p-GaAs (for detailed informations see [18]) and exchanged the varying parameters. For (100) due north-GaAs, the value for the surface potential φ_S is 0.vi eV [33]. The dependence between carrier concentration and mobility μ is given by the Hilsum formula [34]:

Here, we used a value of μ₀ = 8,000 cmⁱⁱ/Vs. Information technology should be pointed out that this is a simplification since the Hilsum formula is employed for majority material and the carrier mobility μ₀ is as well ready 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 majority values. If eastward.g. μ₀ is reduced to 4,000 cm²/Vs, the doping concentration for a nanowire with r _NW = 100 nm and R _NW(1 μm) = 2 kΩ changes to ii × 10¹⁸ cm^-three, which besides suggests that our N_Ds might be underestimated (i × 10^xviii cm^-iii for μ₀ = 8,000 cm²/Vs).

The electrical conductivity of a number of nanowires with various radii (thirty nm < r₀ < 70 nm) were analyzed in the linear regime. Since the contact resistances were located in the low kOhm range, which is only a few percent of the total device resistance, we neglected information technology during the following analysis. Taking it into account would again just lead to a marginal shift to slightly higher carrier concentrations. In Effigy ​ three, the respective experimental wire resistances for a IV/Iii ratio of 0.08, normalized to a contact spacing of L = ane μm, are depicted. Rhombuses stand for contact annealing for 30 south, rectangles for 300 s, respectively. No dependence on the duration of the annealing step can be observed from this figure. In addition, modeled data for 3 different values of carrier concentration (five × x¹⁷, one × x¹⁸, 2 × 10^xviii cm^-3) are given in dashed lines. The wire resistance decreases with both increasing carrier concentrations and wire radius, respectively. It is evident that the experimental resistance information are spreading between the three modeled lines. We conclude that the doping density N _D varies in the range of 7 × x¹⁷ cm^-3 ≤ N _D ≤ 2 × ten¹⁸ cm^-3. The spreading is attributed to both a limited precision of geometrical wire data and a possible doping inhomogenity, i.east. a realistic precision of ± 5% in the measurement of the wire diameter and the wire length, respectively, may sum up to a variation of up to ± xv% of the evaluated doping density. The experimental spreading of ± 32% is substantially college such that an inhomogenity of doping density, which was already reported for GaAs:Zn [18], is causeless.

Measured wire resistance versus the wire radius for a IV/III ratio of 0.08 for two different annealing cycles. The resistance is normalized to wires with 1-μ length. In addition, modeled data for three dissimilar carrier concentrations (5 × 10¹⁷ cm^-iii, ane × x¹⁸ cm^-3, two × ten^xviii cm^-iii) are given in dashed lines.

In order to investigate whether the doping profile is axially graded, nosotros carried out electrical measurements on dissimilar parts of the nanowires separately (e.g. we fabricated four or v contacts along the length of the NW). These measurements were performed on nanowires grown under various IV/III ratios to analyze the correlation betwixt Iv/III ratio and carrier concentration additionally. In Figure ​ iv, we plotted the carrier concentration against the location on the wire for IV/III ratios from 0.02 up to 0.16. The given data for the previously described TESn supply (IV/Three = 0.08) reveal an axially non-uniform doping profile with N _D values spreading in the same range equally the ones estimated before (7 × ten¹⁷ cm^-3 ≤ N _D ≤ 2 × x¹⁸ cm^-3). We suggest that Sn accumulates within the Au (or Au/Ga, respectively) particle during growth. Hence, the probability of dopant incorporation increases in the same manner. Simplified, we conclude that the Au seed particle acts like a commencement-order time-filibuster element for the dopant atoms. If the Iv/III ratio is decreased (IV/III = 0.04), just the upper function of grown nanowires show heavy doping event (Due north _D ≥ ane × x¹⁷ cm^-3), with graded carrier concentrations in the same range as described earlier (run across Figure ​ 4 black dots). Recently, Wallentin et al. reported on InP/GaAs esaki diodes, indicating a sharp onset of the doping [35]. We therefore conclude that the lower parts of these nanowires (Four/III = 0.04) are doped at relatively low doping levels (Due north _D ≤ 1 × x¹⁷ cm^-three). By farther decreasing the dopant supply to a Iv/III ratio of 0.02, nosotros observed that the nanowires showroom the same electrical backdrop as nid ones over the whole length of almost 20 microns. We assume that the amount of dopant atoms accumulated within the Au seed particle during growth is to depression to induce a remarkable doping effect. To further increase the carrier concentration (IV/Three ratio), we decreased the Ga menses (note that the TESn menses is limited by our mass flow controller configuration), while the As flow was kept constant, leading into an Five/III ratio of 5. Hence, we accomplished a Iv/Iii ratio of 0.16 that is doubled compared to the standard sample. Curiously, the corresponding I–V characteristics of the contacted nanowires revealed that the conductivity as well as the contact backdrop was not enhanced, but got even poorer. The current menses was decreased by orders of magnitude, indicating a carrier concentration lower than 1 × 10¹⁷ cm^-3 (Figure ​ 4 crosses). In add-on, we observed that the growth charge per unit of the nanowires grown at 4/Three = 0.16 is higher than for the ones grown at IV/III = 0.08 though the Ga catamenia is halved (gr_0.sixteen ≈ 425 nm/min, gr_0.08 ≈ 390 nm/min). This effect might be attributed to a higher diffusion length of Ga atoms induced by the inverse growth conditions, and then that the reduced Ga flow is overcompensated. Borgström et al. reported a comparable consequence for doping of InP nanowires using dimethylzinc (DMZn). As the group-Iii species at the growth front is increased, the doping efficiency is reduced and the enhanced growth rates finer dilute the dopant incorporation [17].

Carrier concentration confronting the location on the wire for various 4/Iii ratios from 0.02 up to 0.16. Length zero represents the wire bottom. An axially graded doping profile is visible.

With these experiments, we take found the relatively minor procedure window (0.04 ≤ IV/3 ≤ 0.08) for the successful northward-type doping of VLS-grown GaAs nanowires with high charge carrier densities using TESn.

Using TESn as dopant precursor implies a n-type conductivity of the GaAs nanowires. Nosotros fabricated multi-channel MISFET devices with the field-assisted self-associates (FASA) approach [36], to verify the type of doping. Plotting the drain electric current I_D versus gate-source voltage 5_GS proves the n-channel beliefs as the channel conductance increases with positive gate bias (see Effigy ​ 5c). Transfer characteristics of the samples grown without dopant supply and grown under supply of DitBuSi evidence both p-aqueduct beliefs with currents in the pA range (Figure 5a, b). This can exist interpreted hands, since carbon residuals out of the methyl groups may cause p-type electrical conductivity. Unfortunately, the gate control of GaAs nanowire MISFET is poor as already reported for nid GaAs nanowires [37] likewise as for other materials like GaSb nanowires [38]. This is attributed to a high density of surface states. Effects of such surface/interface states on nanodevices are described and discussed in detail elsewhere [12]. With this measured poor transconductances, we were unable to estimate realistic doping levels.

Transfer characteristics of made multi-channel GaAs nanowire MISFETs with xxx nm SiN_ten gate dielectric. The drain-source voltage is 2 V. a grown without dopant supply, b grown nether supply of DitBuSi (4/III = 0.52), c grown nether supply of TESn (IV/III = 0.08). Typical p-channel behavior is observable for a, b while c proves the north-aqueduct behavior of the TESn-doped sample.

Finally, this experiment proves the n-type doping effect using TESn, which is to our cognition the starting time 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 volition be given in a subsequent study.

Conclusion

The successful due north-blazon doping during the VLS growth of GaAs nanowires is reported using tetraethyltin as doping precursor. DitBuSi shows no doping issue, which is attributed its amphoteric beliefs and to the depression nanowire growth temperature resulting in a depression cracking efficiency. In dissimilarity to p-type doping, using diethyl zinc, no influence on the crystal structure was appreciable, despite relatively high dopant supplies. From the experimental resistance information, we were able to estimate a donor concentration Northward _D varying from 7 × ten¹⁷ cm^-3 to two × 10^xviii cm^-iii. The data spreading is attributed mainly to an axially non-uniform doping contour. Transfer feature of multi-channel MISFETs, made from these nanowires, proved that the doping of the nanowire is north-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 hereafter nanoscaled devices. Information technology points out fundamental aspects regarding the doping adequacy using dissimilar precursors within MOVPE and should provide the basics to synthesize GaAs nanowire pn-junctions, which may act as key element in nanowire optoelectronics.

Acknowledgements

The authors gratefully acknowledge financial support of the Sonderforschungsbereich SFB 445 "Nanoparticles from the gas-phase".

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