High-voltage AlN Schottky barrier diodes on bulk AlN substrates by MOCVD

Aluminum nitride (AlN) is a promising ultra-wide bandgap (UWBG) semiconductor for next-generation power electronics due to its remarkable material attributes, including the largest bandgap of 6.2 eV in the UWBG semiconductor family, high breakdown field of ∼12–15 MV cm⁻¹, and superior thermal conductivity of 340 W m⁻¹·K⁻. ^1–5) Baliga's figure of merit (BFOM) is a metric to evaluate the performance of semiconductor material in power electronics. ⁶⁾ In terms of BFOM, AlN outperforms wide bandgap SiC and GaN by several orders of magnitude. ⁷⁾ In AlGaN/GaN high electron mobility transistors, AlN can be used as a barrier layer to reduce alloy scattering ⁸⁾ and increase the density of two-dimensional electron gas. ⁹⁾ In recent years, AlN Schottky barrier diodes (SBDs) ^10–13) and MESFETs ^7,14) have already shown decent progress with 1 kV breakdown voltages (BVs) and high-temperature stability. Irokawa et al. ¹⁰⁾ demonstrated lateral AlN SBDs on unintentional n-doped bulk AlN substrates with high-temperature stability up to 573 K. However, these devices showed a large ideality factor of 11.7, indicating the electron transport mechanism deviating from the well-known thermionic emission (TE) model. Kinoshita et al. ¹¹⁾ developed vertical AlN SBDs by removing the seed layer used to grow hydride vapor phase epitaxy AlN epilayers. These diodes were able to achieve BV between 550 and 770 V with an improved ideality factor of ∼8. However, due to the lack of conductive AlN substrates, the substrate removal process used to fabricate vertical AlN SBDs could induce device damage. Fu et al. ¹²⁾ reported lateral AlN SBDs with over 1 kV BV, showcasing the potential of AlN-based power electronics. Maeda et al. ¹³⁾ recently showed MBE grown AlN SBDs with an AlGaN current spreading layer, and the devices exhibited good thermal stability and inhomogeneous apparent Schottky barrier. It was found that both forward and reverse current transports were dominated by defects in the AlN epilayers. Okumura et al. ⁷⁾ demonstrated a Si-ion implanted AlN MESFET with a three-terminal BV of 2.4 kV. Hiroki et al. ¹⁴⁾ demonstrated a high-temperature performance AlN MESFETs with AlGaN-graded Ohmic contacts. The key to improving AlN device performance is to reduce defects in the material by reducing the threading dislocation using homoepitaxy. ⁵⁾ Recently, HexaTech Inc. and Crytal IS have commercialized physical vapor transport (PVT) grown bulk AlN substrates with low dislocation densities on the order of 10³–10⁴ cm^–2. ^13,15) This enables the growth of high-quality AlN epilayers by MBE and metalorganic chemical vapor phase deposition (MOCVD), where the latter is the industrial standard tool for mass production. Moreover, there are still open challenges in controllable n-type doping ^16–18) and the formation of good Ohmic contacts. ^14,19,20) At low doping levels, C_N and dislocations are the main compensators in AlN, which makes the free electron concentration independent from Si doping. ¹⁶⁾ In contrast, at higher Si doping levels, V_Al + nSi_Al complexes restrict the free electron concentration, ¹⁸⁾ resulting in a "knee" behavior in Si doping and free electron concentration in AlN films. To address this, one strategy involves elevating the temperature to diminish C_N and dislocations. However, at high temperatures, the formation of V_Al + nSi_Al is more likely, making the films more resistive. Consequently, a trade-off emerges between minimizing C_N and dislocations while reducing V_Al + nSi_Al. Recently, Bagheri et al. ¹⁶⁾ demonstrated a high degree of control in Si doping by point and extended defect management. Furthermore, there have not been appropriate Ohmic contacts established for AlN due to the large Schottky barrier between AlN and most metals for contact fabrication. Nevertheless, the exploration of diverse metal stacks, combined with optimized annealing ¹⁹⁾ and ion implantation techniques, ^7,17) presents a promising future for addressing this issue. Due to these challenges, the demonstration of power devices using AlN is still scarce.

In this work, we demonstrated the first 3 kV AlN SBDs on bulk AlN substrates by MOCVD with excellent high-temperature performance. The devices showed good rectifying behaviors with ON/OFF ratios of 10⁶–10⁸ from 298 to 623 K and good thermal stability. The device Schottky barrier height increased from 0.89 to 1.85 eV, and the ideality factor decreased from 4.29 to 1.95 with increasing temperature due to the inhomogeneous metal/AlN interface. This work can serve as a reference for the development of multi-kV AlN high-voltage high-power devices.

AlN epilayers were grown using MOCVD on $(0001)$ bulk PVT AlN substrates. Trimethylaluminum (TMAl) and ammonia (NH₃) were used as the Al and N sources, respectively, whereas N₂ diluted silane (SiH₄) was used as the n-type dopant Si. ^16,21) The device structure, as illustrated in Fig. 1(a), consisted of a 1 μm thick unintentionally doped (UID) AlN layer as a resistive buffer, a 200 nm highly Si-doped n-AlN layer, and a 2 nm UID GaN capping layer. The GaN capping layer was used to prevent oxidation of the underlying AlN epilayers upon exposure to air, which could degrade device performance. ²²⁾ The Si doping concentration in the n-AlN layer was 1 × 10¹⁹ cm^–3. To study the crystal quality of the MOCVD-grown AlN sample, high-resolution X-ray diffraction (HRXRD) measurements were conducted using the Rigaku SmartLab X-ray diffractometer system. Figures 2(a) and 2(b) depict the $(0002)$ symmetric and $(10\mathop{1}\limits^{\unicode{x00305}}2)$ asymmetric rocking curves (RCs) for the AlN sample, with a full-width half maximum (FWHM) of 17.6 arcsec for $(0002)$ and 19.08 arcsec for $(10\mathop{1}\limits^{\unicode{x00305}}2).$ The dislocation density was estimated to be in the range of 10⁴−10⁵ cm⁻² using the equations in Ref. 23, which is three orders of magnitude lower in defect density than AlN on sapphire. ¹²⁾ Furthermore, the surface morphology of the AlN sample was assessed using Bruker MultiMode 8 atomic force microscope (AFM), revealing a RMS roughness of ∼0.2 nm over a 2 × 2 μm² scanning area [Fig. 2(c)]. These HRXRD and AFM results indicate that the MOCVD-grown AlN epilayers possessed a low dislocation density and a smooth surface. The as-grown sample underwent a cleaning process involving acetone, isopropyl alcohol, and deionized water aided by ultrasonication. Subsequently, it was immersed in a hydrochloric acid (HCl) solution with a 1:2 (HCl:H₂O) ratio. The fabrication of AlN SBDs was performed using conventional optical photolithography and lift-off processes. Ohmic contacts were formed using Ti/Al/Ni/Au (25/100/25/50 nm) metal stacks deposited via electron beam deposition, followed by rapid thermal annealing at 1000 °C in N₂ for 1 min. The circular Ohmic contact had a width of 100 μm [Fig. 1(b)]. For Schottky contacts, Pt/Au (30/120 nm) metal stacks were deposited via electron beam evaporation. The distance between the anode and cathode contacts d, was varied between 50 and 350 μm. No field plate, passivation, or edge termination structures were implemented on the devices. Electrical measurements were performed on a probe station equipped with a Keithly 4200 SCS semiconductor analyzer and a thermal chuck. Reverse I–V characteristics were measured using Keysight B1505A Power Device Analyzer/Curve Tracer, and reverse breakdown measurements were conducted in insulating Fluorinert liquid FC-70 at RT.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Figure 3(a) shows the forward I–V characteristics of the AlN SBDs. The devices showed ON/OFF ratios on the order of 10⁵–10⁶ and turn-on voltage of ∼2.5 V, which are comparable to those of previously reported AlN SBDs. ^10–13) The general diode equation for an SBD can be written as ²⁴⁾

$$\begin{eqnarray}&&J={J}_{s}\left[\exp \left(\frac{{qV}}{{nkT}}\right)-1\right]\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{J}_{s}={A}^{* }{T}^{2}\exp \left(-\frac{{q\varphi }_{b}}{{kT}}\right)\end{eqnarray} \tag{ 2 }$$

where $J$ is the current density, ${J}_{s}$ is the saturation current density, ${A}^{* }$ is the Richardson constant, $T$ is the temperature in Kelvin, $q$ is the electron charge, ${\varphi }_{b}$ is the Schottky barrier height, $n$ is the ideality factor, and $k$ is the Boltzmann constant. The Richardson constant used in the calculation was 57.7 Acm⁻²K⁻² using the effective electron mass of 0.48${m}_{0}$ ¹¹⁾ where ${m}_{0}$ is the free electron mass. Based on Eqs. (1), and (2), similar Schottky barrier height (${\varphi }_{b}$) of ∼0.9 eV was obtained for the devices with contact distances of 50, 200, and 350 μm. However, the ideality factor $(n)$ varied with increasing distance between Ohmic and Schottky contacts. The minimum value of 4.29 was obtained for the devices with d = 50 μm, which is comparable to previously obtained $n$ for AlN devices. ^12,13) However, other devices with larger contact distances exhibited a slightly larger $n$ (6.11 and 7.52 for d = 200 and 350 μm devices, respectively). This indicates that the current transport mechanism is likely to be influenced by surface states and/or resistance of the AlN epilayers due to relatively low carrier concentration. The widely accepted electron affinity of AlN is (χ) 1.9 eV, ²⁵⁾ whereas the work function (${\varphi }_{m}$) of Pt is 5.65 eV. ²⁶⁾ The experimental Schottky barrier height obtained in this work deviates from the Schottky–Mott rule, ${\varphi }_{b}={\varphi }_{m}-\chi ,$ owing to Fermi level pinning caused by surface states at the metal/AlN interface. ²⁷⁾ Figure 3(b) shows the I–V curves of AlN SBDs at different temperatures. The devices showed good temperature stability from 298 up to 623 K, and the device ON/OFF ratio increased from 10⁶ to 10⁸ as more carriers contributed to the current transport at higher temperatures.

Zoom In Zoom Out Reset image size

Figure 4(a) shows the temperature-dependent ${\varphi }_{b}$ and $n$ of the AlN SBD (d = 50 μm). The ${\varphi }_{b}$ increased from 0.89 to 1.85 eV, and $n$ decreased from 4.29 to 1.95 with increasing temperature. Figure 4(b) shows a linear relationship between ${\varphi }_{b}$ and $n$ of the devices. This can be ascribed as an inhomogeneous metal/semiconductor interface with distributed low and high Schottky barrier regions. ^28,29) In the metal/AlN interface, there are low and high Schottky barrier regions with a Gaussian distribution, ³⁰⁾ originating from non-uniformities under the Schottky metal contact. As a result, at lower temperatures, electrons are limited to passing through regions with lower Schottky barriers. As temperatures increase, electrons gain the energy needed to overcome higher Schottky barrier regions, leading to an increase in the effective ${\varphi }_{b}$ with increasing temperature. This behavior has also been commonly observed in previous reports. ^12,13,30) In addition, the devices exhibited behaviors that are more closely aligned with the TE model with increasing temperature, as evidenced by the decreasing $n.$ ¹³⁾ This is mainly attributed to the fact that at elevated temperatures more electrons are excited and improved ohmic contacts compared to the RT.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Furthermore, C–V measurements can be used to extract the carrier concentration of the AlN epilayers using the following equations ²⁴⁾

$$\begin{eqnarray}&&\frac{1}{{C}^{2}}=\frac{2}{q{\varepsilon }_{0}{\varepsilon }_{r}{N}_{D}}\left({V}_{\mathrm{bi}}-V-{kT}/q\right)\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&d\left(\frac{1}{{C}^{2}}\right)/{dV}=-\frac{2}{q{\varepsilon }_{0}{\varepsilon }_{r}{N}_{D}},\end{eqnarray} \tag{ 4 }$$

where ${V}_{\mathrm{bi}}$ is the built-in voltage, ${\varepsilon }_{0}$ is the permittivity of the vacuum, and ${\varepsilon }_{r}$ is the relative permittivity of AlN (${\varepsilon }_{r}$ = 9.2). ³¹⁾ Figure 5(a) shows the C–V and 1/C²–V plots of the device. C–V measurements of the devices were performed at 10 kHz. The 1/C²–V plot had two regions, corresponding to the AlN UID layer and n-doped AlN, respectively. The extracted carrier concentration of the UID layer was 2.3 × 10¹⁶ cm^–3, whereas the n-doped region has a carrier concentration of 5.7 × 10¹⁷ cm^–3, which is much smaller than the Si doping concentration due to dopant compensation ^16,18,32,33) and high Si donor ionization energy in AlN (∼250 meV). ^16,17,34,35) To better understand the device behavior, temperature-dependent C–V measurements were performed. Figure 5(b) shows temperature-dependent carrier concentration extracted from temperature-dependent C–V measurements of the devices as shown in the inset. As the temperature increased, the carrier concentration varied between 5.7 × 10¹⁷ and 1.6 × 10¹⁸ cm^–3 from 298 to 623 K. With increasing temperature, more carriers are excited and contribute to the additional capacitance observed.

Figure 6(a) shows the reverse I–V characteristics of the AlN SBDs with different contact distances up to −3 kV. All the devices exhibited BV of over 3 kV. It should be noted that no destructive breakdown of the devices was observed up to –3 kV (the limit of the current setup). The reverse leakage current increased with increasing contact distance, and the dominant reverse leakage mechanism warrants further investigation. Reverse leakage can be effectively mitigated through the implementation of passivation and edge termination, which will be explored in future work. Figure 6(b) compares the BV and turn-on voltages of reported AlN SBDs. ^10–13) Our work showed record-high BV of over 3 kV with the smallest contact distance and comparable turn-on voltages. The use of AlN bulk substrates in our work resulted in a substantial reduction in defects, paving the way for enhanced device performance compared with our prior work on sapphire substrate. ¹²⁾

Zoom In Zoom Out Reset image size

In summary, lateral 3 kV AlN SBDs were grown and fabricated on bulk AlN substrates by MOCVD. The devices showed excellent rectifying behaviors with ON/OFF ratios of 10⁶–10⁸ from 298 to 623 K and good thermal stability. With increasing temperature, ${\varphi }_{b}$ increased from 0.89 to 1.85 eV, while $n$ decreased from 4.29 to 1.95. These results show the great potential of AlN and serve as an important reference for the future development of multi-kV AlN power electronics.

Acknowledgments