Temperature dependence of electrical characteristics of Si-implanted AlN layers on sapphire substrates

AlN with a large bandgap energy is one of the most attractive materials for high-temperature applications. However, performance of AlN devices at high temperatures has been limited by technical problems with electrical characterization systems. Here, we show that Schottky-barrier diodes (SBDs) and metal-semiconductor field-effect transistors with Si-implanted AlN channels can operate at 1100 K and 1000 K, respectively. The breakdown voltage and barrier height of the AlN SBD were 610 V and 3.5 eV, respectively. We found that the high barrier height and thermal stability of the Ni contact on AlN greatly contributed to high-temperature operation of the devices.

S emiconductor devices that can operate at high temperatures are useful for automotive, space exploration, and deep-well drilling applications. Here, thermal agitation increases current flows through unintentional regions in semiconductor devices due to the continuous excitation of electrons from the valence band to the conduction band, degrading device performance. Semiconductors with a large bandgap energy (E g ) have low leakage currents at elevated temperatures because of their low intrinsic-carrier concentrations (n i ). For instance, SiC junction field-effect transistors can operate at 800°C because p-n junctions are free from unreliable gate oxides and Schottky-contact degradation. 1) Further high-temperature operation requires semiconductors with larger E g . β-Ga 2 O 3 with E g = 4.6-4.8 eV and diamond with E g = 5.5 eV have been used to make Schottky-barrier diodes (SBDs) with on-off ratios over 10 5 at 500°C. 2,3) However, Ga 2 O 3 and diamond suffer from an inability of p-type doping 4) and high ionization energy (E d ) of donors, 5) respectively, putting limits on their applicable device structures. Moreover, SBDs and metal-semiconductor field-effect transistors (MESFETs) fabricated from Ga 2 O 3 and diamond are subject to contact degradation at high temperature. [6][7][8] By contrast, AlN is more attractive than these materials for high-temperature applications because of its high E g of 6.1 eV and controllable p-and n-type doping.
High-quality AlN layers have been grown on Si, sapphire, and SiC substrates by metal-organic chemical-vapor deposition (MOCVD), [9][10][11][12][13] molecular-beam epitaxy (MBE), [14][15][16] sputtering, 17) and hydride vapor epitaxy (HVPE). 18,19) Si can be used as an n-type dopant of AlN. We reported that MESFETs with a Si-implanted AlN channel can operate at 250°C. 20) Recently, MESFETs with an MOCVD-grown Sidoped AlN channel operated at 500°C. 21) Still, the hightemperature performance of these AlN-channel devices has been limited by technical problems with electrical characterization systems. In this paper, we report the electrical characteristics of Si-implanted AlN layers at temperatures from room temperature up to 900°C.
We used 3 μm thick unintentionally doped AlN layers grown on c-plane sapphire substrates by MOCVD, which were supplied by DOWA Electronics Materials. Si was implanted into the as-grown AlN layers at room temperature at an incident angle of 7°from [0001] by the Ion Technology Center. The Si concentration ([Si]) of 2 × 10 19 cm −3 in the 150 nm deep box profile was formed by implanting the Si ions at ion-beam energies of 90, 40, and 10 keV, where the corresponding dosages were 2.8 × 10 14 , 1.6 × 10 14 , and 6 × 10 13 cm −2 . After pumping to a high vacuum of 5 × 10 −4 Pa in a chamber to suppress the incorporation of impurities from the surface, 22) the Si-implanted AlN layers were annealed without protective caps at 1500°C for 30 min in N 2 ambient at 1 × 10 4 Pa. The annealing temperature means the susceptor temperature monitored using a pyrometer.
SBDs and MESFETs with a Si-implanted AlN channel were fabricated. After ion implantation and subsequent hightemperature annealing, a 50 nm thick Ni metal mask was deposited using electron-beam evaporation. A 300 nm deep mesa termination was fabricated by reactive-ion etching (RIE) with Cl 2 /BCl 3 mixing gas of 25/5 sccm at inductively coupled plasma (ICP)/bias powers of 150/30 W and the chamber pressure of 0.6 Pa for 2 min. 23) After removing the Ni mask by using piranha solution for 1 min, the surface was cleaned using ICP-RIE with BCl 3 gas of 5 sccm at ICP/bias powers of 100/15 W and the chamber pressure of 0.25 Pa for 3 min. For the cathode electrode of the SBDs and source/ drain contacts of the MESFETs, a Ti (20 nm)/Al (100 nm)/Ti (10 nm)/Au (50 nm) metal stack was deposited using electron-beam evaporation. The metal stack was sintered at 950°C for 1 min in N 2 ambient at 1 × 10 4 Pa to reduce the contact resistance. For the anode electrode of the SBDs and a gate contact of the MESFETs, a Ni (20 nm)/Au (50 nm) metal was deposited by electron-beam evaporation. Current density-voltage (J-V ) measurements were performed in a highvacuum chamber at ∼10 −6 Pa and temperatures (T m ) between 300 and 1173 K by using a Keysight B2902B source-measure unit (Sect. 1 in supplementary data).
The sheet resistance (R s ) and specific contact resistivity (ρ c ) of the Si-implanted AlN layer were determined with the transmission-line method using 50 μm × 100 μm rectangular Ti/Al/Ti/Au electrode pads with gap lengths from 4 to 15 μm. The current-voltage (I-V ) characteristics at T m ⩽ 400 K showed nonlinear behavior near 0 V due to the high Schottky-barrier height (f B ), while those at T m ⩾ 500 K showed almost linear behavior due to the thermionic field emission (TFE), in which some of the thermally activated electrons can have a larger energy than the top of the potential barrier. The I-V measurement at T m = 1150 K was nonrepeatable due to the deterioration of the electrodes. The temperature dependences of R s and ρ c for T m = 500-1100 K are shown in Fig. 1(a). R s decreased with increasing T m , reaching 1 × 10 5 Ω/square at T m = 1100 K. R s at T m = 500 K is comparable to the value of the Si-doped AlN layers at T m = 300 K. [24][25][26][27][28][29] ρ c decreased with increasing T m due to the enhanced TFE, reaching 4.0 × 10 −3 Ω cm 2 at T m = 1100 K. ρ c of the Si-implanted AlN layer was higher than those of Si-doped AlN layers with Si-doped graded AlGaN and Si-doped GaN regrown contacts due to the low electrical activation ratio of the Si-implanted AlN layer. [30][31][32] The electron concentration (n e ) and electron mobility (μ e ) of the Si-implanted AlN layer were determined at a chamber pressure of ∼10 −4 Pa by making DC Hall-effect measurements in a 0.6 T field. We used a 3 × 3 μm 2 Van der Pauw pattern. The data at T m ⩽ 400 K and at T m ⩾ 1000 K were not reliable due to the Schottky contact and small μ e , respectively. The temperature dependences of n e and μ e for T m = 500-900 K are shown in Fig. 1(b). As T m increased from 500 to 900 K, μ e slightly decreased from 19 to 3 cm 2 V −1 s −1 due to phonon scattering and n e significantly increased from 2.5 × 10 16 to 1.9 × 10 18 cm −3 due to the enhanced donor ionization, leading to the reduction of R s with increasing T m . The layer resistivity (ρ s ) is expressed as where e is the elementary charge. ρ s of the Siimplanted AlN layer with [Si] = 2 × 10 19 cm −3 at T m = 500 K is 13 Ω cm, which is comparable to those of an HVPE-grown AlN layer ([Si] = 3 × 10 17 cm −3 , μ e = 115 cm 2 V −1 s −1 , n e = 2 × 10 14 cm −3 at 300 K) and MOCVDgrown AlN layer ([Si] = 2 × 10 18 cm −3 , μ e = 156 cm 2 V −1 s −1 , n e = 1 × 10 15 cm −3 at 300 K). 21,33) Still, n e at T m = 900 K is one order of magnitude lower than [Si]. n e decreases as the net donor concentration (N d -N a ) decreases and E d of Si donors increases. Si at an Al substitutional site forms a shallow donor state, d 0 , with E d = 64-86 meV. [34][35][36][37] However, Si in AlN also forms a deep state, DX, through a large lattice relaxation, leading to self-compensation of the Si donor through the capture of a second electron and the reaction  + 38) The conditions close to thermal equilibrium, such as epitaxial growth and hightemperature annealing, favor forming a DX − center with E d = 250-320 meV, [38][39][40][41] while a non-equilibrium process, such as ion implantation, can increase the population of d 0 . 36,42,43) N d , N a , and E d can be estimated from the charge neutrality conditions for a non-degenerate n-type semicon- ) the density of states effective mass for electrons, 44) and h the Planck constant. The experimental results were fitted using N d = 2 × 10 18 cm −3 , N a = 1 × 10 17 cm −3 , and E d = 320 meV, which is in good agreement with the E d of the DX − center of Si. N d was one order of magnitude higher than the O concentration in the channel area, 22) indicating that the DX − center of O is ignorable. 45,46) The donor activation ratio (N d /[Si]) and compensation ratio (N a /N d ) were estimated to be 10% and 5%, respectively. We consider the cause of the small N d /[Si] to be that some Si atoms did not substitute for Al atoms in the AlN layer. N a increased with increasing the number of compensation defects, which are created by the ion bombardment. Implantation at high temperature and a long annealing time would reduce the implantation-induced defects and make more Si atoms act as donors by substituting at Al sites, leading to a high N d /[Si], low N a /N d , and low ρ c . Also, reducing the annealing temperature to 1200°C-1300°C may enhance the formation of d 0 , 36,42,43) leading to a high electrical activation ratio and low R s .
The J-V characteristics of the AlN SBD at T m = 300-1100 K are shown in Fig. 2(a). The circular anode contact had a diameter of 100 μm. A clear rectifying characteristic was observed for all T m . The turn-on voltage (V on ) decreased from 1.47 to 1.16 V, as T m was increased from 500 to 1000 K because E g shrank with increasing T m . 47) The value of V on  agrees with the previously reported values. 33,48) The forward current density increased with increasing T m due to the increase in n e and decrease in ρ c . The specific on-resistances of the AlN SBDs at T m = 300 K and 1100 K were 3.8 × 10 3 and 4.2 × 10 −1 Ω cm 2 , respectively. The maximum current density at T m = 300 K and 1100 K under a bias of +3 V were 3.9 × 10 −4 and 3.6 A cm −2 , respectively. The on-off ratio and reverse leakage current at T m = 600-1000 K were ∼10 5 and <10 −5 A cm −2 respectively. The reverse current density increased with increasing T m . n i is given by with Varshni coefficients of α (=2.59 meV K −1 ) and β (=2030 K), 49) and E(0) (=6.066 eV) the transition energy at 0 K. n i at T m = 1100 K was calculated to be 6 × 10 8 cm −3 , which is much smaller than n e , and thus it contributed to the high on-off ratio of ∼10 4 . AlN SBDs would be able to operate at high temperatures by suppressing the cathode-contact degradation. The breakdown voltage (V B ) at 300 K was 610 V, which is comparable to the value in the other report. 33) The forward J-V characteristics were analyzed with the thermionic emission (TE) model expressed as  Fig. 2(b). The apparent f B,JV at T m = 300 K was 1.31 eV, which is close to V on . n at T m = 300 K was much larger than unity because of the defect-induced current. n decreased with increasing T m , reaching 1.01 at T m = 1000 and 1100 K. This indicates that nearly ideal J-V characteristics were realized without degradation of the Ni/AlN interface or AlN surface.  The Japan Society of Applied Physics by IOP Publishing Ltd the barrier-height inhomogeneity is given by 51) where f B0 represents the mean zero-bias barrier height and σ the zero-bias standard deviation. The experimental results were well fitted using σ = 110 meV and f B0 = 3.5 eV, which is close to the reported f B of Ni/AlN. 52,53) The high f B of Ni/AlN contributed to the excellent rectifying characteristics at T m = 1100 K. At thermal equilibrium, the built-in potential (V bi ) at zero bias can be obtained from ) is the energy difference between the bottom of the conduction band and the Fermi level in AlN. The imageforce-induced lowering of f B is negligibly small (<0.1 eV). V bi at T m = 1000 K was calculated to be 3.4 V, which was larger than V on of the AlN SBD. V on may be dominated by low f B .
A schematic cross-section of the fabricated AlN MESFETs is shown in Fig. 3(a). The AlN MESFETs had a gate length (L g ) of 2 μm, gate width (W g ) of 100 μm, and source-gate spacing (L sg ) of 2 μm. The output characteristics of the AlN MESFET with gate-drain spacing (L gd ) of 6μm at T m = 1000 K are shown in Fig. 3(b). The AlN MESFET had a normally-on operation and almost pinch-off characteristics at the gate-source voltage (V gs ) < −30 V. The onresistance extracted from the linear region of the output curves for V gs = 0 V were 5.2 × 10 4 Ω mm. The drain current (I d ) was effectively modulated by V gs and showed good saturation due to the small n i and stable Ni/AlN interface even at T m = 1000 K. The maximum I d was 2.2 mA mm −1 for V gs = +20 V, which is one order of magnitude smaller than that of an MOCVD-grown AlN channel at T m = 773 K due to the smaller μ e . 21) The off-state I d at V gs = −36 V was ∼0.1 mA mm −1 , which is the leakage through the bottom undoped AlN layer. 21) The off-state leakage current may be reduced by decreasing the concentrations of defects and unintentional impurities in the undoped AlN layer. Neither breakdown events nor other irreversible degradation was observed for the drain-source voltage (V ds ) < +100 V, suggesting stable device operation in the whole temperature range from 300 to 1000 K (Sect. 2 in supplementary data). The threeterminal V B at V gs = −34 V at T m = 1000 K was 176 V, which is smaller than V B at room temperature in the previous report due to increased gate leakage at high temperature. 20) The transfer characteristics for the AlN MESFET with L gd of 5 μm at V ds = +20 V and T m between 300 and 1000 K are shown in Fig. 4(a). The subthreshold swings at T m = 300 K and 1000 K were 0.42 and 15 V/decade, respectively. The off-state I d at T m = 300 K was less than 30 pA mm −1 , which is comparable to the gate current. The I d on/off ratios at T m = 300 K and 1000 K were as large as 10 4 and over 10 2 , respectively. The performance of the AlN MESFET improved with increasing T m from 300 to 500 K due to the enhanced donor ionization. However, the off-state I d increased with increasing T m from 600 to 1000 K due to leakage through the bottom undoped AlN layers. The offstate I d at T m ⩾ 600 K was larger than that of an MOCVDgrown AlN channel due to the smaller f B . 21) A high-quality AlN homoepitaxial channel and p-type back barrier would reduce the off-state I d at higher temperatures and thereby maintain a high I d on/off ratio. The results show that AlN MESFETs have the potential to operate at T m ⩾ 1000 K.
The temperature dependence of maximum transconductance (g m,max ) and threshold voltage (V th ) estimated from the I d -V gs characteristics is shown in Fig. 4(b). g m,max increased with increasing T m because the current of the AlN MESFET was more strongly affected by n e than by μ e or the saturation velocity. g m,max at T m = 300 K and 1000 K was 7.5 × 10 −9 and 2.8 × 10 −5 S mm −1 , respectively. The field-effect mobility (μ FE ) can be calculated using μ FE at T m = 500 K and 900 K was 2.2 × 10 −2 and 8.7 × 10 −1 cm 2 V −1 s −1 , respectively. Although μ FE at T m = 900 K was close to μ e , μ FE at T m = 500 K was much lower than μ e due to the small g m . Meanwhile, V th decreased with increasing T m perhaps due to the defect states at the Ni/AlN interface. 55) V th at T m = 300 K and 1000 K was −3.2 and −34 V, respectively.  57) V th at T m = 1000 K was calculated to be −34 V, which corresponds to the experimental data. We consider that the limited I d on/off ratio and small g m at T m ⩽ 500 K arise from the high source/ drain contact resistances, the high ionization energy of Si donors, and the low electrical activation ratio of the Si-implanted AlN layer. Figure 5(a) benchmarks the trade-off between on-off ratio and the measurement temperature of our AlN SBD and other SBDs in the literature. 2,31,53) The on-off ratios of β-Ga 2 O 3 and diamond SBDs decrease at T m > 600 K, while the on-off ratio of our diode was almost constant at T m ⩽ 1100 K. This indicates that AlN SBDs have superior potential for hightemperature applications. The MOCVD-grown and MBEgrown AlN SBDs have a higher on-off ratio than our diode. High-quality Si-doped AlN homoepitaxial growth and heavily Si-doped AlN contact layers could further improve the performance of AlN SBDs. Figure 5(b) benchmarks the trade-off between on-off ratio and the measurement temperature of our AlN MESFET and other FETs in the literature. 1,8,21,58,59) Our transistor has already showed good high-temperature performance in comparison with GaN, β-Ga 2 O 3 , and diamond FETs. Despite smaller E g , the SiC JFET shows excellent high-temperature performance. We expect that AlN JFETs would operate at a further high temperature.
In summary, we reported the electrical characteristics of Si-implanted AlN layers in the high-temperature range of 300-1150 K. Current-voltage characteristics at temperatures above 500 K showed almost linear behavior, while the sheet resistance was 1.0 × 10 5 Ω/square and contact resistivity was 4.0 × 10 −3 Ω cm 2 at 1100 K. The Ti/Al/Ni/Au contacts at 1150 K were deteriorated. As the temperature increased from 500 to 900 K, the electron mobility decreased from 19 to 3 cm 2 V −1 s −1 , while the electron concentration increased from 2.5 × 10 16 to 1.9 × 10 18 cm −3 . The AlN SBD showed a clear rectifying characteristic at 300-1100 K. The breakdown voltage and barrier height of the AlN SBD were 610 V and 3.5 eV, respectively. The high barrier height and thermal stability of the Ni contact on AlN contributed to the hightemperature operation at 1100 K. The AlN MESFET showed almost pinch-off characteristics at 1000 K and the drain current was modulated by the gate-source voltage. The maximum drain current, on/off ratios, and three-terminal breakdown voltage were 2.2 mA mm −1 , over 10 2 , and 176 V, respectively. We found that AlN-channel SBDs and MESFETs have great potential for high-power applications at temperatures above 1000 K.