Low-field transport properties and scattering mechanisms of degenerate n-GaN by sputtering from a liquid Ga target

In this work, degenerate n-type GaN thin films prepared by co-sputtering from a liquid Ga-target were demonstrated and their low-field scattering mechanisms are described. Extremely high donor concentrations above 3 × 1020 cm−3 at low process temperatures (<800 °C) with specific resistivities below 0.5 mΩcm were achieved. The degenerate nature of the sputtered films was verified via temperature-dependent Hall measurements (300–550 K) revealing negligible change in electron mobility and donor concentration. Scattering at ionized impurities was determined to be the major limiting factor with a minor contribution of polar optical-phonon scattering at high temperatures.

][3] To meet the increased data rate requirements, higher frequencies with improved efficiency and bandwidth are targeted. 3,4)However, downscaling of the gate length to address higher frequencies requires a significant reduction of parasitic resistances in the devices. 3,5)The access resistance of highly-scaled highelectron-mobility transistors (HEMTs) or multi-channel devices 6) suffers from the inherent metal-semiconductor barrier for high Al-content barriers. 4,7)A current transport mechanism completely determined by tunneling is desirable to achieve the lowest possible voltage drop at the metalsemiconductor interface.Removing the AlGaN-barrier and/or rendering the sub-contact area n-type is the only possibility to change the electron transport across the barrier to the 2dimension electron gas from a thermionic to field-emission type at higher Al-contents.However, achieving high carrier density (n > 1 × 10 20 cm −3 ) for a field-emission-based current transport is difficult due to the decreasing crystal quality.In addition, low-temperature processes on a large wafer scale are beneficial to protect the AlGaN/GaN interface in a HEMT.
In this work, the transport properties of heavily Si-doped GaN thin films deposited by co-sputtering from Si and from a liquid Ga target and on 4-inch sapphire substrates are investigated.Extremely high carrier concentrations (n > 3 × 10 20 cm −3 ) are demonstrated.Specific resistivities below 0.5 and 3.5 mΩcm are achieved at growth temperatures of 800 °C and 590 °C, respectively.Carrier mobilities were found to be limited by scattering at ionized impurities at high Si doping levels.The donor-acceptor compensation ratio was found to be nearly constant below θ ≈ 0.2 even beyond n > 1 × 10 20 cm −3 different from the reported data by metalorganic CVD (MOCVD) and MBE.Temperature-dependent Hall measurements revealed negligible change in carrier density (n) and mobility with increasing temperature indicating Motttransition from a semiconducting to a metallic character.Carrier densities and specific resistivity at low growth temperatures (⩽800 °C) are found well beyond reported data of MOCVD-grown thin films.The results demonstrate the feasibility of sputtered GaN from a liquid Ga target as an alternative process with high n and easy upscaling beyond 4inch suitable for future process integration into future radiofrequency or optoelectronic (e.g., tunnel junctions) GaNbased devices.
The recent focus of advanced contact processes is generally driven by CMOS-compatible, Au-free, and/or low-temperature budget ohmic contacts, [8][9][10][11][12] or n-GaN regrowth for highlyscaled AlGaN 5) or novel Al(Sc)N-based HEMTs [13][14][15] with high Al-content.The fabrication of highly n-type doped GaN films was demonstrated via Si implantation, 13,16,17) MBE, [18][19][20][21] MOCVD [22][23][24][25][26] (Si or Ge), pulsed laser deposition (PLD) 27) and reactive, pulsed sputtering (PVD) from a solid Ga target.) MBEregrown layers are commonly applied due to the low growth temperature but face issues in terms of upscaling, throughput, and homogeneity.20) MOCVD regrowth was demonstrated at low growth temperatures 23) with reduced growth rate, but is limited in terms of achievable n > 5 × 10 19 cm −2 at 550 °C and does not offer a non-selective growth mode 24,25) to avoid growth inhomogeneities. 25,32)Typical growth with high growth rates is conducted at higher temperatures (>950 °C).24,25) Reactive sputtering was demonstrated with high n and high mobility 29,30) on 300-600 nm thick films.However, the used Ga-targets need to be heavily cooled to remain solid during sputtering. 33) In adition, the cooling system gets more complex for larger wafer diameters. Alternively, ceramic GaN targets can be used but are difficult to handle due to high impurity concentrations. The us of a liquid Ga target avoids the need for a complex cooling system and can be easily filled up while no sputter craters occur during growth. In dition, the liquid target can be easily upscaled to larger wafer diameters and lower parasitic doping occurs when compared to ceramic GaN targets.
Three samples (A-C) were initially grown by MOCVD on sapphire with an Fe-doped buffer topped with a 200 nm unintentionally-doped GaN layer to compensate for Fe segregation. 34)The sheet resistance of samples A, B, and C was measured after MOCVD buffer growth via a contactless eddy current revealing an R S > 100 kΩ/sq.GaN films were deposited by co-sputtering of a Si bar and a liquid Ga target in an Evatec Clusterline ® 200II with a modified process module.A detailed description of the basic sputtering setup/ process can be found elsewhere. 35)150 nm Si-doped GaN (± 6%) was sputtered on top of the MOCVD-GaN with nominal growth temperatures of 590, 700, and 800 °C.Sample D was prepared by directly sputtering GaN:Si on sapphire.Si contamination of the target could be present but not quantifiable so far.FWHM were derived from fitting two pseudo-Voigt functions assuming that the peak with lower intensity is related to the sputtered, Si-doped GaN.The FWHM of the MOCVD-grown buffer was found to be FWHM = 0.064°.Fitting the sample with the lowest growth temperature was not possible due to the low side-peak intensity.
Atomic force microscopy was used to determine the rms to reveal the smoothest surface morphology with the lowest growth temperature.A summary of the structural properties is given in Table I.
The theoretical metal-semiconductor transport properties (thermionic emission -TE, thermionic field emission -TFE; field emission-FE) are dependent on the characteristic energy E 00 , which in turn is dependent on the n in the subcontact area as following: 36) With q, h, ε r (9.5), m tun * and m (m tun */m = 0.22) are the elementary charge, Planck constant, relative dielectric constant, effective tunneling mass, and electron mass, respectively.We use the simple differentiation: 36) FE: E 00 ⩽ 0.5kT; TFE: 0.5kT ⩽ E 00 ⩽ 5kT and E 00 ⩾ 5kT.Thus, to achieve E 00 ⩾ 5kT a donor concentration of n > 1 × 10 20 cm −3 is required for GaN.Fundamental low-field transport properties are characterized by Hall measurements at RT. 200 μm Tibased contacts pads were evaporated via shadow masks and mechanically isolated (1 × 1 mm 2 isolation, field = 0.52 T).The use of co-sputtering a Si bar comes along with the possibility of achieving a gradient in Si concentration over the same wafer depending on the distance of the wafer area to the Si bar.Thus, several carrier concentrations and corresponding carrier mobilities can be measured on the same wafer.Carrier densities of n = 6.7 × 10 19 to 3.7 × 10 20 cm −3 with carrier mobilities of μ = 21 to 42 cm 2 Vs -1 were found over all samples.No Si segregation was observed in the films via transmission-electron microscopy.Improved compensation ratios were found with increasing heater temperature as given in Fig. 1.The achieved carrier densities are well beyond the state-of-the-art reported for MOCVD and MBE at lower growth temperatures [Fig.1(c)].The extremely high carrier concentrations would be well suited to address source starvation issues causing linearity distortion in highly-scaled GaN-HEMTs. 37)n general, several scattering mechanisms could be assumed for the co-sputtered GaN:Si even though impurity scattering is most likely dominating at high donor concentrations. 38)Scattering at ionized impurities in dependence of the compensation ratio θ = N D /N A can be expressed by: 39) And N I and m* F are the ionized impurity concentration and effective mass at the Fermi energy, which is given by: and: Where α (0.64) and m * e (0.22m e ) are the non-parabolic conduction band coefficient [α = 1(m* e /m e )] 2 , and the electron effective mass, respectively.High carrier compensation ratios θ = N D /N A were found for MOCVD and MBEgrown samples beyond n > 1 × 10 20 cm −3 as shown in Fig. 1(b).As a consequence, a strong mobility decrease was reported limiting the achievable n and specific resistivity.Compensation ratios of the co-sputtered thin films in this work were found to be rather constant up to n = 3.7 × 10 20 cm −3 at higher growth temperatures.A significant increase in compensation ratio is observed at lower growth temperatures (θ = 0.8), which could be attributed to a decrease in crystal quality where e.g., an increasing amount of point defects 40) or an increase in impurity incorporation (e.g., carbon).The assumption is consistent with the increase in FWHM.Specific resistivity was derived and compared to reported data from the literature 18,19,[21][22][23][24]29,30,41,42) (Fig. 1). Lowest speciic resistivities in this work were obtained for highest growth temperature (800 °C) with ρ < 0.5 mΩcm.An increase in resistivity is observed by lowering the growth temperature as a result of the increasing compensation ratio.The lowest resistivity of sample A (590°) was found to be 3.5 mΩcm.To verify Mott transition temperature-dependent Hall measurements were carried out for T = 300-575 K.No significant change in electron concentration or mobility was observed indicating the degenerate nature of the Si-doped GaN.Temperature-dependent polar optical-phonon scattering was modeled via: 43) eNm where Ћω0 is the optical-phonon energy (100 meV).Scattering at dislocations is neglected since their impact on transverse mobility above n > 1 × 10 20 cm −3 for dislocation densities below N DISL < 1 × 10 11 cm −2 is not relevant. 44)islocation densities of MOCVD-grown GaN on sapphire are generally found to be much lower and the interface of sputtered GaN on MOCVD-GaN is not expected to generate new dislocations.Scattering at grain boundaries could be assumed for sputtered GaN, however, the associated potential barriers would lead to a thermal activation of μ or n.In addition, at high doping levels, most of the grain boundaryrelated trap states are filled and the potential barriers would decrease in height and width. 45)Temperature-dependent fitting of μ was achieved using Eqs.( 2) and ( 7) by Matthiessen's rule given in Fig. 2.Only a minor contribution of μ POP was found while μ II clearly dominates the overall low-field scattering in the samples.
In conclusion, heavily doped GaN prepared by co-sputtering from a liquid Ga target was demonstrated.Extremely high donor concentrations above 3 × 10 20 cm −3 at low process temperatures (<800 °C) with specific resistivities below 0.5 mΩcm were achieved.Mott-transition was verified via temperature-dependent Hall measurements revealing neither a change in mobility nor carrier concentration in the range of 300 to 550 K. Impurity scattering was determined to be the major low-field mobility limiting factor with a minor contribution of polar optical-phonon scattering at elevated

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd temperatures.Scattering at dislocations or grain boundaries was ruled out to impact the total mobility.The results demonstrate the huge potential of sputtering as an alternative route for low temperature, high throughput, and easy upscaling of regrown n-type GaN.
This work was supported by the Fraunhofer Internal Programs under Grant PREPARE 40-02419 SCALING.
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Fig. 1 .
Fig. 1.(a) n versus μ reported in the literature for various growth methods: MOCVD (quarters with a cross); MBE (diamonds); HVPE (pentagons) reactive sputtering (grey circles); Si implantation (half-filled hexagons).Samples in this work are colored: sample A (yellow); B (blue); C (red); and D (green).Solid and dotted lines represent fitting from Schwierz et al. based on Caughey-Thomas approximation; 38) (b) n versus μ in the range of E 00 > 5kT.(c) Peak process temperature versus measured n.Reported range of alloying temperatures of ohmic contacts to AlGaN/GaN are added; (d) n versus specific resistivity.

Fig. 2 .
Fig. 2. Temperature-dependent electron mobility of the heavily doped GaN.Scattering by ionized impurities μ II (black)b and polar optical-phonons μ POP (green) was modeled to fit the experimental data (red).

Table I .
Structural properties of co-sputtered n-GaN: t n-GaN -thickness of Si-doped GaN-layer, T H-nom.Heater temperature, GR-growth rate, rms-root mean square, FWHM -full-width half maximum of the 00.2.