Barrier inhomogeneity of Ni/AlN Schottky Barrier Diode

An aluminum nitride (AlN) Schottky barrier diode (SBD) was fabricated on an AlN single-crystal grown by physical vapor transport (PVT). The Ni/Au-AlN SBD features a low ideality factor n of 3.3, an effective Schottky barrier height (SBH) of 1.05 eV, and an on/off ratio of ~106 under forward biases at room temperature. This work presents the lowest ideality factor of AlN SBDs that have been reported. As temperature increases, the effective SBH extracted from the current-voltage characteristics becomes larger while the ideality factor n becomes smaller. The temperature dependences of SBH and n were explained using an inhomogeneous Schottky junction model. A mean SBH of 2.105 eV was obtained by analyzing the inhomogeneity of the Ni-AlN Schottky junction. This work reveals the potential of AlN for advanced SBDs with large barrier height. An equation with parameters having explicit physical meanings in thermionic emission theory to describe the current-voltage characteristics of inhomogeneous SBDs was proposed.

Aluminium nitride (AlN), with a bandgap energy of ~6.0 eV, a thermal conductivity up to 3.2 Wcm -1 K -1 at room temperature and a high critical electric field up to 12 MV/cm, possesses a great potential in power electronics and deep UV optoelectronics. [1][2][3] Though reported almost a century earlier 4 than its counterpart gallium nitride (GaN) 5 which has been the main force of power electronics 6,7 and optoelectronics 8 , AlN based electronics are still in their infant. On one hand, the growth of high-quality AlN crystals or epilayers is still a big challenge. [9][10][11][12] On the other hand, the much larger bandgap results in the higher activation energy of n/p dopants 2 and a wider accommodation energy range for mid-gap defects or traps 4,13 . Schottky barrier diodes (SBDs) on AlN single crystal grown by physical vapor transport (PVT), 14 on AlN layers homoepitaxially grown by hydride vapor phase epitaxy (HVPE), 15 and on AlN epilayers grown on sapphire by metal organic chemical vapor deposition (MOCVD) 16 have been reported. An AlN metalsemiconductor field-effect transistor (MESFET) on the MOCVD-grown epilayer was also demonstrated recently. 17 These SBDs and MESFETs, though with their performances far from the predicted material limit, indicate the feasibility to develop AlN electronic devices.
The ideality factor n and the Schottky barrier height (SBH) are the two critical parameters to evaluate the junction quality and the potential for high-voltage blocking capability, respectively, of a SBD. [18][19][20] The ideality factor n, which is a unity in the ideal case, reveals the junction quality of a unipolar rectifying diode. Compared with GaN and SiC Schottky junctions, of which the ideality factor n near a unity has been achieved, 21-25 the n of AlN Schottky junctions is still much larger. Irokawa et al. 14 and Kinoshita et al. 15 reported ideality factors of ~11.7 and ~8 for a Pt/AlN and a Ni/AlN Schottky junctions at room temperature, respectively. By inserting a 2-nm GaN layer, Fu et al. 16 reported an ideality factor of 5.5 for a Pt/GaN/AlN Schottky junction. Due to the ultrawide bandgap and a large value of the charge neutral level (CNL) of AlN, 26,27 larger SBHs have always been expected for AlN SBDs. By conducting the X-ray photoelectron spectroscopy (XPS) measurements, Reddy et al. 28 reported SBHs of 1.6 to 2.3 eV (2.2 to 2.4 eV) for different Schottky metals on m-plane (c-plane) AlN. However, the SBHs extracted from the current-voltage (J-V) characteristics of fabricated AlN SBDs were much smaller, ~ 1.0 eV. [14][15][16] In this letter, we reported a Ni/AlN SBD fabricated on a non-polar AlN single-crystal grown by PVT, with a low ideality factor n of 3.3, an effective SBH of 1.05 eV, and an on/off ratio ~10 6 under forward bias at room temperature. As temperature increases from 240 to 400 K, the ideality factor n becomes smaller from 5.8 to 2.6, and the effective SBH extracted from J-V curves becomes larger from 0.8 eV to 1.3 eV. These temperature dependences indicate an inhomogeneous Schottky junction at the Ni-AlN interface. 19,[29][30][31][32] By analyzing the barrier inhomogeneities, a mean SBH of 2.1 eV was obtained for the Ni/AlN SBD. Capacitance-voltage (C-V) measurement was also conducted to evaluate the effective dopant density and the SBH at a flat band condition. The AlN single crystal was grown by PVT at ~2200 o C in the N2 atmosphere on a tungsten substrate through spontaneous crystallization. The temperature gradient was optimized to minimize surface reconstruction or decomposition during the cooling down process. An AlN single crystal with the non-polar surface was used for material characterizations and device fabrication. Optics spectrometer, consists of a strong bandedge emission with peak of ~6.0 eV, a UV band and a weak red band. The strong bandedge emission also confirms that the PL emission is from a nonpolar surface. 2,33 The peak-height-ratio (R), between the bandedge emission and the UV emission, is approximately 3.2. This large R value indicates the high quality of the AlN crystal. 34,35 The surface morphology of the AlN crystal was examined by atomic force microscopy (AFM). The root-mean-square (RMS) roughness of a 3×3 μm 2 area, as shown in Fig. 1(b), is about 3 nm. Figure 1(c) shows the X-ray photoelectron spectroscopy (XPS) of the AlN single crystal.
Oxygen is the only impurity in the AlN crystal grown by PVT on a tungsten substrate. This is consistent with the peak positions of the UV band and the red band, which are commonly identified to be the emissions from (VAl-ON) complexes. 34,36 The weak Ar-related peaks result from the insitu surface cleaning by Ar plasma in the XPS chamber.  , where (1) Here, J is the current density, A * is the Richardson constant of the semiconductor, T is temperature, n is the empirical ideality factor, k is the Boltzmann constant, q is the element charge, and ∅ ,(0, ) − is the zero bias SBH with image-force lowering. The subscript (0,T) is due to the fact that the SBH here is calculated from J0 which is the intercept at zero bias of the extrapolation of the linear segment of (lnJ)-V curve at temperature T. Taking the value of A* ~57.6 A/cm 2 K 2 for AlN, 15 the ideality factor n and SBH of the Ni/AlN SBD at 300 K were extracted to be 3. Schottky junction achieved in this work. It is worth to be pointed out that the current-voltage characteristics between the two ohmic contacts are sublinear. Higher annealing temperature or contact metal with lower work functions is required to further decrease the contact resistance.
Nevertheless, as shown in the inset of Fig. 2(b), the resistance of the non-ideal ohmic contact is several orders smaller than that of the Schottky junction contact, and its effect on the extraction of the junction parameters from the linear segment of (lnJ)-V curve is negligible.  Assuming the local Schottky barriers at the Ni-AlN interface follow a Gaussian distribution (∅ ) with a standard deviation of σ around a mean value of ∅ mean , 19,29 , and .
Here the barrier potential fluctuation or distribution is assumed to be independent of temperature T but may be affected by the applied biases due to the existence of image-force lowering. 18 The current density from metal to semiconductor in thermionic emission theory can be written as, , Integrating of Eq. (3) directly results in that, , Therefore, the junction inhomogeneity of which the potential fluctuation or distribution is temperature independent leads to the temperature dependence of the effective SBH extracted from the J-V curve of a SBD, mathematically. For an ideal homogeneous Schottky interface, the SBH becomes lower slightly at higher temperatures due to the temperature dependence of the semiconductor's bandgap. 18 For an inhomogeneous Schottky interface, however, the effective SBH described by Eq. (4) increases at higher temperatures. The underlying physical mechanism of this temperature dependence can be revealed by simply dividing the inhomogeneous interface into low SBH region and high SBH region. The ratio of the junction current through high SBH region over that through the low SBH region in thermionic emission theory is ), which increases at higher temperatures, consequently leading to the increase of the effective SBH.
With a SBH which is temperature and bias dependent, the J-V behavior of a SBD in thermionic emission theory can be written as , Equating Eqs. (1) and (5), the ideality factor n in the empirical equation can be obtained as Here 1 is a coefficient which is temperature dependent.
According to Eq. (4), the bias induced change of SBH can be written as, , where , Here, 2 and 3 are temperature independent coefficients, representing the voltage dependences of the mean value and the standard deviation of the inhomogeneous Schottky junciton, respectively.
Combining Eq. (6b) and (7), , Then, the physical meaning of the ideality factor n in the widely used empirical Eq. (1) is revealed as that it represents the voltage deformation of the barrier distribution at the inhomogeneous Schottky junction. The as-grown AlN crystal is n-type. 14,15 Assuming a constant electron mobility in the temperature range studied, the conductance or inverse of resistance (1/R) can be described as, 2,15 where ED is the activation energy of the donor level. The bulk resistances of the as-grown AlN crystal was estimated as the differential resistance from the current-voltage characteristics between the two ohmic contacts. An activation energy ED of 371 meV can be extracted from the slope of ln(1/R) ̶ 1/T relation, as shown in Fig. 5. This value may be underestimated due to the inclusion of The SBH can also be estimated by conducting the capacitance-voltage (C-V) measurement. 18 As the capacitance is mainly determined by the bulk properties of the semiconductor, the inhomogeneity of the Schottky interface plays little role in the estimation of SBH from the C-V curve. Therfore, the value of ∅ − is expected to be close to that of the ∅ ,(0,0) mean .
To verify the validity of the inhomogeneity analysis of the AlN Schottky junction, we measured the C-V curves of the Ni/AlN SBD. Figure 6(a) shows the C-V curve measured at 1 kHz at room temperature. The donor concentration ND can be estimated from the 1/C 2 -V plot shown in Fig. 6(b) through the equation, 18 , Here, ε0 the is the vacuum permitivity, and εr is the relative permitivity of AlN. The net donor Here, NC is the effective density of states in the conduction band. Taking the effective electron mass in AlN as 0.48m0, where m0 is the electron mass, NC was calculated to be 8.3 × 10 18 cm -3 .
The electron density ne can also be written as , Here, EF is the energy difference between the Fermi level and the conduction band minimum.
The extraction procedure of the parameters in Eq. (14) is the same as that in the empirical Eq.