Highly conductive homoepitaxial Si-doped Ga₂O₃ films on (010) β-Ga₂O₃ by pulsed laser deposition

The wide band gap, high breakdown strength, and native single crystal substrate availability of β-Ga₂O₃ position the material for use in a wide array of device applications. β-Ga₂O₃ crystals fabricated from the Czochralski,^1–3 floating-zone,⁴ and edge-defined film fed⁵ growth methods allow the deposition of epitaxial doped Ga₂O₃ films vital to achieve carrier concentration control necessary for active channel and contact layers. Shallow doping of β-Ga₂O₃ to achieve carrier concentrations from 10¹⁶ to 10¹⁹ cm⁻³ has been reported using Sn and Ge in molecular beam epitaxy (MBE),^6,7 Si and Sn in metal organic vapor phase epitaxy (MOVPE),^8,9 and Sn in metal organic chemical vapor deposition (MOCVD).¹⁰ Field effect transistors have been realized using channel layers of Sn-doped Ga₂O₃ by MBE,^11–13 Si ion implanted Ga₂O₃ by MBE¹⁴ and Sn-doped Ga₂O₃ by MOVPE.^15,16 In Sn doped β-Ga₂O₃ by MBE, Ahmadi⁷ achieved a mobility of 39 cm² V⁻¹s⁻¹ and concentration of 1 × 10²⁰ cm⁻³, yielding a conductivity of 625 S cm⁻¹. In Si doped β-Ga₂O₃ by MOVPE, Baldini⁹ achieved a mobility of 50 cm² V⁻¹s⁻¹ and concentration of 8 × 10¹⁹ cm⁻³, yielding a conductivity of 641 S cm⁻¹. In addition, low contact resistance through Si ion implantation was achieved with a carrier concentration of 5 × 10¹⁹ cm⁻³ and conductivity of 714 S cm⁻¹.¹⁷ Ohmic contact formation to wide bandgap materials has been a known problem including in β-Ga₂O_3.¹¹ High conductivities are critical to address this limitation through ion implantation or ohmic layer regrowth processes for improved β-Ga₂O₃ wide bandgap device performance. The implications on forming low on-resistance for both power switching and rf applications are widespread.

Pulsed laser deposition (PLD) has also been used to fabricate Sn- and Si-doped Ga₂O₃ films, although primarily on sapphire substrates and with conductivities <10 S cm⁻¹ that are insufficient for application as contact layers. Early reports demonstrated 1 S cm⁻¹ from Sn-doped Ga₂O₃ on silica¹⁸ and an optimized deposition window of 500 °C to 550 °C for heteroepitaxial Sn-doped Ga₂O₃ growth on sapphire.¹⁹ Conductivities of 8.2 S cm⁻¹ at 380 °C and 1.6 S cm⁻¹ at 380 °C growth temperatures were reported using Sn dopant on sapphire, respectively.^20,21 For Si-doped Ga₂O₃ on sapphire, Zhang²² achieved a 9.1 × 10¹⁹ cm⁻³ concentration with a maximum conductivity of 2 S cm⁻¹ and Müller²³ demonstrated conductivity control as a function of oxygen partial pressure during growth. Varying Si content in Ga₂O₃ targets allowed control of carrier density from 10¹⁵ to 10²⁰ cm⁻³ with associated conductivity from 10⁻⁴ to 1 S cm⁻¹ on sapphire.²⁴ Additional reports with other substrates include epitaxial Si-doped γ-Ga₂O₃ on (001) MgAl₂O₄ by PLD with a 1.8 × 10¹⁹ cm⁻³ carrier concentration and conductivity of 4.76 S cm⁻¹.²⁵ 

In this study, Ga₂O₃ films were deposited in a Neocera Pioneer 180 PLD system with a KrF excimer laser (Coherent COMPexPro 110, λ = 248 nm, 10 ns pulse duration). The chamber base pressure was 2.66 × 10⁻⁶Pa with a 5% O₂/95% Ar gas mixture introduced during deposition to a working pressure of 1.33 Pa. Substrates were heated by a backside heater to between 450 °C and 590 °C. The laser operated at a pulse rate of 10 Hz with an energy density of 3 J cm⁻² measured at the target. The target was a 50 mm diameter by 6 mm thick 99.99% pure sintered oxide ceramic disk of Ga₂O₃ with 1 wt. % SiO₂. Films were grown concurrently on Fe-doped semi-insulating (010) β-Ga₂O₃ and (0001) sapphire substrates. Pre-deposition sample preparation entailed solvent cleaning with acetone and isopropyl alcohol followed by an N₂ dry. Prior to deposition, samples were in-situ annealed at 590 °C for 1 h in 13 Pa of O₂. Film thicknesses ranged from 100 nm to 400 nm as measured by contact profilometry.

X-ray diffraction (XRD) analysis was performed using a PANalytical X-Pert diffractometer with a hybrid monochromator (Cu Kα, λ = 1.5406 Å). Atomic force microscopy (AFM) was performed with a Bruker Dimension Icon scanning probe microscope operating in conventional tapping mode in air. Transmission electron microscopy (TEM) was used to evaluate cross-sectional images of the samples with an aberration corrected Titan 80–300 TEM. TEM samples were prepared using a focused ion beam lift-out technique with a Pt cap to preserve the initial surface integrity. Secondary ion mass spectrometry (SIMS) depth profiling was performed by EAG Laboratories. Hall-effect measurements were carried out in an Accent HL-5500-PC system, using a magnetic-field strength of 0.5 T. Sample sizes were 5 mm × 5 mm and Ti/Al/Ni/Au (20/100/50/50 nm) contacts were evaporated on the corners and rapid thermal annealed for 1 min. at 470 °C in N₂.

Figure 1 shows representative 2Θ XRD scans of Si-doped Ga₂O₃ (Ga₂O₃:Si) films fabricated during the same 590 °C deposition on (0001) Al₂O₃ and (010) β-Ga₂O₃ substrates. Films on Al₂O₃ were single phase with reflections at 18.89°, 38.3°, 59.01°, and 82.06° corresponding to the (20$1¯$⁠), (40$2¯$⁠), (60$3¯$⁠), and (80$4¯$⁠) family of planes, respectively. The (20$1¯$⁠) orientation is consistent with other studies of Ga₂O₃ films on sapphire.^20,22–25 In the thickest 400 nm films, several new XRD reflections appear at: 44.43° [60$1¯$], 57.65° [31$3¯$], 30.32° [110] and 59.91° [113]. This effect of reduced film quality with thicker films is nearly analogous to the described deterioration of PLD Ga₂O₃ film quality as the partial pressure of oxygen increases at constant temperature.²³ In our case, under similar circumstances, the increased film thicknesses on sapphire substrates may cause a deviation from a highly oriented crystalline texture and may lead to the formation of poly-oriented crystallites of Ga₂O₃:Si thin films. 2Θ XRD scans of films deposited at lower temperatures showed the same orientation as displayed in Fig. 1(a).

For Ga₂O₃:Si films deposited on β-Ga₂O₃ substrates between 500 °C and 590 °C, only the (020) plane was observed in the θ/2θ XRD scan as shown in a representative plot in Fig. 1(b). The measured 2θ position of the [020] peak at 60.91° for the film agrees with the 2θ [020] peak of substrate and literature data.²⁶ The observed [020] peak with a full-width at half-maximum (FWHM) of ∼0.04° indicates the presence of a pure phase and high crystalline quality of the homoepitaxial layer of β-Ga₂O₃. However, at the lowest deposition temperature of 450 °C, very low intensity satellite peaks were also measured at 57.65° [31$3¯$] and 59.91° [113] as seen in the samples deposited on sapphire.

The distribution of film crystallites with definite orientation with respect to the substrate normal is reflected in the rocking curve (RC) plots in Fig. 2. While the FWHM of the RC of β-Ga₂O₃ substrate was ∼0.0053°, the FWHM values from the RC of Ga₂O₃:Si films on β-Ga₂O₃ substrates varied between 0.0063° and 0.0075°, evidence of high quality alignment of the epitaxial [020] β-axis of the Ga₂O₃:Si film with the (020) substrate normal. In most of the observed rocking curves, differentiation of signals generated from the Ga₂O₃ film and the Ga₂O₃ substrate was not possible. Such overlapping of signals is also evidence of achievement of perfect epitaxy. If a rocking curve plot displays shoulders relative to the substrate peak, as is presented in Fig. 2, this corresponds to Ga₂O₃:Si epitaxial films under compressive or tensile strain in (020) plane, which typically is exposed by the shift in the rocking curve film peak maximum to the left or to the right, respectively, relative to the substrate Bragg reflection. Similar to the bare β-Ga₂O₃ substrate in Fig. 2, Ga₂O₃:Si films deposited at lower temperatures (450 °C to 500 °C) were unstrained and did not exhibit shoulders on rocking curves. Increasing the deposition temperature from 550 °C to 590 °C leads to the appearance of right-side shoulders on the rocking curve seen in Fig. 2, indicating the presence of the (020) in-plane tensile strained films.

Representative AFM images of Ga₂O₃:Si films deposited at 590 °C in Fig. 3 reveal surface morphologies that depend on the substrate. A granular surface structure is evident in films deposited on Al₂O₃ [Fig. 3(a)] with a root mean square (RMS) roughness of 4.7 nm and a grain size range of 4 × 10² nm² to 2 × 10⁴ nm². Films deposited between 500 °C and 590 °C on β-Ga₂O₃ [Fig. 3(b)] exhibit a smoother, uniform morphology with a 0.2 nm RMS roughness, slightly higher than the 0.1 nm RMS roughness measured on a bare substrate. A textured structure with an 8.4 nm RMS roughness was observed on films deposited at 450 °C on β-Ga₂O₃ indicative of a lower quality film as observed by XRD.

Cross sectional TEM images of a Ga₂O₃:Si film deposited at 590 °C shown in Fig. 4 confirm that homoepitaxial deposition was achieved. In Fig. 4(a), the β-Ga₂O₃ substrate is visible in the lower left corner followed by a 218 nm thick Ga₂O₃:Si film. On the surface of the Ga₂O₃:Si film is 20 nm of atomic layer deposited SiO₂ in addition to the TEM sample preparation Pt layer previously discussed. The cross sectional film microstructure differs from the uniform substrate; periodic, high contrast striations in the film perpendicular to the substrate are visible. These features could be attributed to Si segregation in the film, the presence of micro-twins, or in-plane domains, although additional analysis is required for complete identification. A higher magnification image in Fig. 4(b) shows the continuity of (020) substrate lattice planes at the film-substrate interface and through part of the film.

Hall-effect results from Ga₂O₃:Si films deposited on sapphire showed an average conductivity of 1 S cm⁻¹ corresponding to a carrier concentration of 1.81 × 10¹⁹ cm⁻³ and mobility of 0.37 cm² V⁻¹s⁻¹, consistent with conductivities from other PLD studies of Sn-doped Ga₂O₃¹⁸ and Si-doped Ga₂O₃ films.^22–24 The observed crystal structure and granular surface morphology are also consistent with the same reports.

In contrast, significantly higher conductivity was achieved in films deposited on native substrates. In Fig. 5, conductivity and carrier concentration as measured by Hall-effect are plotted as a function of Ga₂O₃:Si film deposition temperature. Plotted data points are averages of multiple samples and error bars depict one standard deviation. A maximum average carrier concentration of 1.74 × 10²⁰ cm⁻³ occurred at a 550 °C deposition temperature and decreased to 2.61 × 10¹⁹ cm⁻³ at 450 °C and 1.26 × 10²⁰ cm⁻³ at 590 °C. Similarly, average Hall-effect mobility (not shown) peaked at 26.5 cm² V⁻¹s⁻¹ for films deposited at 550 °C and decreased to 18.6 cm² V⁻¹s⁻¹ at 450 °C and 19.8 cm² V⁻¹s⁻¹ at 590 °C. A maximum average conductivity of 732 S cm⁻¹ was observed in films deposited at 550 °C with a decrease to 78 S cm⁻¹ at 450 °C and 376 S cm⁻¹ at 590 °C. Mobility and concentration were not found to be dependent on the film thickness within the thickness range studied. Additional Hall-effect measurements were performed on a bare β-Ga₂O₃ substrate and a 250 nm thick un-doped Ga₂O₃ film deposited on a (010) β-Ga₂O₃ substrate. The bare substrate and un-doped film were prepared with in-situ pre-deposition anneals and ohmic metal contacts in the same manner as the Ga₂O₃:Si films. Both samples exhibited GΩ sheet resistance indicating that the conduction mechanism in Ga₂O₃:Si films did not arise from potential substrate alterations due to oxygen annealing or from inherent target impurities transferred to the film. Temperature-dependent Hall-effect measurements on samples grown at 550 °C show classically degenerate characteristics.²⁷ Over the range 10 to 320 K, the carrier concentration is nearly constant, and mobility is also constant at the lowest temperatures, reflecting ionized-Si-impurity scattering, and decreased only about 15% at 320 K, due to phonon scattering. Details will be published separately.

The high conductivity of the Ga₂O₃:Si films deposited on native substrates was attributed to the controlled PLD process of replicating target composition within the deposited film. Although the focus of this study was the influence of deposition temperature and substrate on film properties, many PLD deposition parameters were established to achieve conductive and homoepitaxial films. A 5% oxygen content during deposition was particularly critical as was addressed in previous studies addressing relative oxygen partial pressure.^19,23 Depositions with higher oxygen content resulted in lower mobilities and carrier concentrations. An Ar balance to maintain a 1.33 Pa working pressure mitigated film deposition on the laser window. The difference in Hall-effect results from Ga₂O₃:Si films deposited on β-Ga₂O₃ and Al₂O₃ substrates is attributed to the improved film crystal structure observed on native substrates.

In Fig. 6, SIMS depth profiles of Si and C are plotted from a Ga₂O₃:Si film deposited at 550 °C with the highest measured carrier concentration of 2.16 × 10²⁰ cm⁻³ and a mobility of 27.2 cm² V⁻¹s⁻¹. The Si concentration in the Ga₂O₃:Si film maintains an average of 4.15 × 10²⁰ cm⁻³ throughout the film thickness. Preliminary Hall-mobility fitting yields a donor concentration (N_d) and acceptor concentration (N_a) of about 6.3 × 10²⁰ cm⁻³ and 4.1 × 10²⁰ cm⁻³, respectively. Note that N_d is reasonably consistent with the SIMS Si concentration of 4.15 × 10²⁰ atoms cm⁻³ while the origin of N_a is unknown at this time but could possibly be related to Ga vacancies. Disregarding measured surface contamination, the C concentration averaged 4 × 10¹⁸ cm⁻³ in the 550 °C Ga₂O₃:Si film. Although the C levels are low compared with the donor and acceptor concentrations, the influence of C on the film transport properties is not fully understood.

In conclusion, highly conductive, homoepitaxial Ga₂O₃:Si films were fabricated by PLD from a Ga₂O₃—1 wt. % SiO₂ target on (010) β-Ga₂O₃ substrates. Films deposited at 550 °C had a maximum average conductivity of 732 S cm⁻¹ corresponding to an activated Si carrier concentration of 1.74 × 10²⁰ cm⁻³ and mobility of 26.5 cm² V⁻¹s⁻¹. A high activation efficiency was obtained in the as-deposited condition. The influence of the substrate on deposited film quality was evident as Ga₂O₃:Si films deposited between 500 °C and 590 °C on (010) β-Ga₂O₃ substrates exhibited a single crystal structure with rocking curve FWHM values on the same order as the substrate and a 0.2 nm surface roughness. In comparison, Ga₂O₃:Si films deposited on Al₂O₃ substrates exhibited conductivities < 1 S cm⁻¹ with a (20$1¯$⁠) orientation and RMS roughness of 4.7 nm. The high doping level results presented directly address the critical necessity of n⁺ β-Ga₂O₃ device layers through an ohmic layer regrowth process that has extensive value for improved wide bandgap device performance. In addition to ohmic contacts, this process opens the possibility of β-Ga₂O₃ epitaxial layer schemes for devices including, but not limited to, heterostructures by pulsed laser deposition.

AFRL authors were supported by Air Force Office of Scientific Research under AFOSR LRIR No. 15RYCOR163 (Program Officer Dr. Ali Sayir). We acknowledge Dr. Arnold Kiefer for helpful discussions on XRD data.