In this work, we report on defect-free homogeneous behavior of Ni Schottky contacts patterned on surface treated n-GaN by photolithography with unity ideality factor, high temperature stability, and low reverse leakage. The barrier height (0.7 eV) and ideality factor (1.02) are found to be independent of temperature, indicating a highly homogeneous contact. The contacts are found to be stable with no significant change in ideality factor or leakage current up to an annealing temperature of 600 °C. Temperature dependence of the reverse leakage current shows no evidence for the existence of surface defects that would provide leakage paths, and the behavior was modeled by ATLAS simulations with an ideal homogeneous barrier of 0.7 eV. Consequently, the forward and reverse bias characteristics are explained by a common set of parameters. The surface treatment after the development and prior to metallization included an acid-based surface treatment. X-ray photoelectron spectroscopy (XPS) studies indicate that the hydroxide-based development process during photolithography changes the nitride surface composition by introducing excess C that degrades the ideality factor and introduces barrier inhomogeneity and high leakage currents. XPS studies further demonstrate that the restoration of a stable, Ga-rich surface, similar to as-grown surface, occurs due to the acid-based surface treatment, which is responsible for the observed unity ideality factor, homogeneous barrier, low leakage current, and high temperature stability.

GaN is of high interest for high-power electronics based on vertical structures due to its high breakdown field and electron mobility. Recent studies indicate a dielectric strength in the range of 3.3–3.75 MV cm−1, resulting in a Baliga's figure of merit several times larger than that of SiC.1–3 However, high breakdown voltages (>4 kV) are currently confined to p-n junctions indicating surface instabilities.1 Further, Schottky diodes fabricated on n-GaN have showed a significant variation in performance with ideality factors varying from <1.2 (Refs. 4–6) to ∼1.8 (Refs. 7 and 8) and leakage currents varying over several orders of magnitude with contacts exhibiting low ideality factors also exhibiting relatively low leakage currents.4–8 In this work, we studied the effect of surface composition and surface treatments on the barrier height, ideality factor, and temperature stability and report defect-free behavior in Ni/GaN Schottky diodes fabricated by photolithography that also exhibit high temperature stability.

Hetero-epitaxial films of GaN (0.7 μm) were grown on c-plane sapphire via metalorganic chemical vapor deposition (MOCVD). Details on III-nitride growth are provided elsewhere.9–11 Si was employed as n-type dopant. The carrier concentration was measured to be ∼2 × 1016 cm−3 with mobility of ∼800 cm2/Vs. Surface chemical composition was characterized by X-ray photoelectron spectroscopy (XPS) utilizing a dual anode X-ray source with Al (hν = 1486.6 eV) and Mg (hν = 1253.6 eV) anodes and a base pressure of ∼10−10 Torr. A concentric hemispherical analyzer was used to detect the photoelectrons. Elemental analysis was performed using both Al anode (to study Ga and C) and Mg anode (to study N and O). Mg anode was used since Ga Auger peaks overlap with the photo-electron peaks of N and O while using Al anode. Atomic sensitivity factors of the corresponding elements were utilized to obtain quantitative information.12 Electrical measurements were performed using a Keithley 4200 semiconductor characterization system. The circular Schottky contacts (diameter of ∼100 μm) with concentric large area Ohmic contacts were fabricated via photolithography. 3% Tetra-methyl ammonium hydroxide (TMAH) was employed as developer. An intermediate acid-based surface treatment after development (lithography) and before metallization (Schottky contacts) was used to achieve near ideal contacts. For comparison, Schottky contacts were also fabricated using shadow mask on as-deposited surface and using photolithography without the surface treatment. The final lift-off step involved ultra-sonication in organic solvents followed by oxygen plasma ashing.

The current-voltage characteristic of the Ni/GaN Schottky diode (diameter = 100 μm) fabricated by photolithography without surface treatment is shown in Figure 1. The ideality factor is ∼1.4 with a “shoulder” at low voltages (<0.2 V). The log(I)-log(V) plot (Figure 1(b)) reveals the shoulder to be resistive (slope ∼1), indicating a high-resistance parallel leakage path. In contrast, the Ni Schottky contacts fabricated by a shadow mask show near unity ideality factor with no low voltage shoulders. The low voltage shoulder observed in contacts patterned by photolithography may be removed via annealing.7 Indeed, annealing the contacts at ∼400 °C improves the ideality factor to ∼1.1 while eliminating the parallel leakage path as shown in Figure 2(a). Further, the reverse characteristic exhibits lower leakage for the annealed contacts (<400 °C) as shown in Figure 2(b). However, the reverse leakage is still several orders of magnitude larger than those exhibited by Schottky contacts fabricated using a shadow mask on a pristine surface. Temperature stability is poor as further annealing at 500 °C results in complete loss of its rectifying nature (Figure 2(b)). In comparison, contacts made using the shadow mask show no loss of rectification even at an annealing temperature of 600 °C.

FIG. 1.

I-V characterization ((a) log(I)-V and (b) log(I)-log(V)) of Ni Schottky diodes on n-GaN without the surface treatment exhibiting high ideality factor and a “shoulder” at low voltages, which is found to be a large parallel resistance.

FIG. 1.

I-V characterization ((a) log(I)-V and (b) log(I)-log(V)) of Ni Schottky diodes on n-GaN without the surface treatment exhibiting high ideality factor and a “shoulder” at low voltages, which is found to be a large parallel resistance.

Close modal
FIG. 2.

(a) I-V characteristics of annealed contacts, metallized without surface treatment, showing improved ideality factor and barrier height. Note the absence of additional leakage at low voltage. (b) Reverse leakage characteristics of Ni/n-GaN Schottky diodes processed by photolithography annealed at various temperatures.

FIG. 2.

(a) I-V characteristics of annealed contacts, metallized without surface treatment, showing improved ideality factor and barrier height. Note the absence of additional leakage at low voltage. (b) Reverse leakage characteristics of Ni/n-GaN Schottky diodes processed by photolithography annealed at various temperatures.

Close modal

The high ideality factor, high leakage current, and poor temperature stability on as-deposited contacts (by photolithography) may be understood to be consequences of surface chemical reactions with the basic developer (3% TMAH). XPS studies of the surface before and after development indicate a chemical change at the surface from a Ga-rich composition (Ga/N∼1.3), typically observed on as-deposited surfaces13 with low C contamination, to a more stoichiometric composition (Ga/N∼1) with significant presence of C exhibiting a second, higher binding energy peak likely corresponding to C-N bonding, as shown in Figures 3(a) and 3(b). The resulting surface/interfacial phases may introduce the observed inhomogeneity and parallel leakage paths across the barrier. The reactions of GaN with the developer may also introduce defects at the surface that promote additional defect-assisted tunneling paths across the barrier. The Poole-Frenkel emission through such traps may also be responsible for the observed parallel low voltage resistive leakage.14,15 Such tunneling paths, in addition to surface phases, introduce barrier inhomogeneities and temperature instabilities and also collectively increase the ideality factor and reverse leakage current. Note that all surfaces show about ∼5% of oxygen.

FIG. 3.

(a) XPS core level analysis of C 1s showing C compounds on the developed surface partly comprising of C-N bonds and (b) XPS quantitative chemical analysis showing surface chemical composition of GaN at various stages of contact processing. O is ∼5% at all stages. Higher Ga/N ratio is associated with Schottky contacts with unity ideality factor and low leakage.

FIG. 3.

(a) XPS core level analysis of C 1s showing C compounds on the developed surface partly comprising of C-N bonds and (b) XPS quantitative chemical analysis showing surface chemical composition of GaN at various stages of contact processing. O is ∼5% at all stages. Higher Ga/N ratio is associated with Schottky contacts with unity ideality factor and low leakage.

Close modal

The removal of the surface phases and restoration of the developed surface to as-deposited composition is possible via acid based treatment as shown in Figure 3(b). In order to determine whether the hydroxide-based developer is responsible for contact degradation and whether surface restoration produces better contacts, Ni/GaN Schottky diodes were fabricated via photolithography with the surface treatment step after development and before metallization. The resulting I-V-T and C-V-T characteristics are shown in Figure 4. The low voltage parallel leakage path is not observed. This implies any surface defect states or phases responsible for parallel leakage introduced during the development are passivated or removed by the acid treatment. The ideality factors and barrier heights were extracted from I-V-T and C-V-T characteristics using the equations

ln(IT2)=ln(AA**)qϕkT+qVηkT,
(1)
ln(I0T2)=ln(AA**)qϕkT,and
(2)
1C2=2εqND(VVbi+kT/q),
(3)

where the symbols have their usual meanings and the built-in potential and barrier height in Equation (3) are related by φ = Vbi + (EC − EF). The ln(I/T2)/V slope in Equation (1), i.e., q/ηkT, exhibits a temperature independent ideality factor with an average value of 1.02 ± 0.01.

FIG. 4.

(a) I-V-T and (b) C-V-T characterization of Ni/GaN contacts with post development acid surface treatment.

FIG. 4.

(a) I-V-T and (b) C-V-T characterization of Ni/GaN contacts with post development acid surface treatment.

Close modal

The temperature dependence of the reverse saturation current (from y-intercepts in Figure 4(a) and represented by Equation (2)) was used to determine the barrier height and modified Richardson's constant as shown in Figure 5. They are φ = 0.66 eV (and flat band barrier height, φFB = 0.69 eV) and A** ∼ 0.4 A cm−2 K−2. The measured barrier height is in agreement with the Fermi level pinning energy at the Ga-rich surface measured by XPS.13,16 The linear dependence of log(I0/T2) with 1/T indicates no measurable temperature dependence of the barrier height. The barrier height extracted using Equation (3) from C-V-T characterization (Figure 4(b)) also exhibits temperature independence and exhibits a slightly higher value of 0.83 eV ± 0.01 eV. The observed temperature independent barrier height and ideality factor indicate highly reduced barrier inhomogeneity.17 Note that the C-V measurements were performed with Schottky contacts of diameter ∼500 μm. The observed C-V barrier is ∼20% larger than the barrier height measured by I-V-T characterization. Inclusion of effects of image force lowering reduces the observed difference from 20% to ∼15% since I-V measured barrier increases from 0.66 eV to 0.69 eV (with C-V measured barrier at 0.83 eV). The 15% difference may not be due to typical causes including interfacial layers and barrier inhomogeneities since they increase the ideality factor beyond the observed 1.01–1.02 (the slight increase in ideality from 1 to 1.01–1.02 may be explained by image force effects) and introduce a temperature dependence that is not experimentally observed.18 Further, the barrier height is highly homogeneous. It is possible that the additional capacitances that produce the observed difference in barrier heights may be due to interface states/dipoles observed in earlier studies.13 

FIG. 5.

The plot of log(I0/T2) vs. 1/T revealing a temperature independent barrier height.

FIG. 5.

The plot of log(I0/T2) vs. 1/T revealing a temperature independent barrier height.

Close modal

In addition to the near-ideal forward characteristics, the reverse leakage improves by orders of magnitude and is similar to the contacts made by shadow masks on as-deposited surface, as shown in Figure 6. Temperature dependence of reverse leakage current (Figure 7) shows no evidence for existence of surface defects or inhomogeneities that could provide leakage paths as confirmed by ATLAS Simulations with a homogeneous diode with a flat band barrier height of 0.7 eV and Richardson constant of 0.35 A cm−2 K−2 with image force barrier lowering model, calculated with known carrier concentration (2 × 1016 cm−3) and mobility (∼800 cm2/Vs). The Richardson constant obtained from the reverse bias (where the current scales linearly with it) is expected to be more accurate than the value obtained from the forward bias (where the dependence is logarithmic). It is significant that the forward and reverse bias characteristics were modeled by a common set of parameters without the need of a second diode (typically due to interface defects) to explain the reverse bias behavior.

FIG. 6.

Reverse leakage characteristics of Ni/GaN Schottky diodes processed with acid based surface treatment before photolithography relative to those without surface treatment and those with clean contact deposition via shadow mask.

FIG. 6.

Reverse leakage characteristics of Ni/GaN Schottky diodes processed with acid based surface treatment before photolithography relative to those without surface treatment and those with clean contact deposition via shadow mask.

Close modal
FIG. 7.

The temperature dependence of the measured reverse leakage current (solid lines) in comparison to ATLAS simulated reverse leakage current (dashed lines) with homogeneous diode (flat band barrier height of 0.7 eV and A** = 0.35 A cm−2 K−2) and the image force barrier lowering model enabled.

FIG. 7.

The temperature dependence of the measured reverse leakage current (solid lines) in comparison to ATLAS simulated reverse leakage current (dashed lines) with homogeneous diode (flat band barrier height of 0.7 eV and A** = 0.35 A cm−2 K−2) and the image force barrier lowering model enabled.

Close modal

Further, the restoration of the surface also ensures high temperature stability where the annealing of Schottky contacts to 600 °C produces a slight decrease in the leakage current, as shown in Figure 8. Corresponding forward characteristics exhibits a slight increase of the ideality factor to 1.04 from 1.01, indicating a minor introduction of inhomogeneities, which are likely pinched off in reverse bias operation. Note that contacts made without surface treatment becomes Ohmic already when annealed to 500 °C. Hence, the acid-based surface treatment after development removes any additional surface phases or defects introduced during development and results in a defect-free behavior.

FIG. 8.

The effect of contact annealing on the reverse leakage current (at 300 K) in diodes with and without surface treatment.

FIG. 8.

The effect of contact annealing on the reverse leakage current (at 300 K) in diodes with and without surface treatment.

Close modal

In conclusion, we have achieved defect-free behavior in the Ni/GaN Schottky contacts patterned on surface treated n-GaN by photolithography with a unity ideality factor and high temperature stability (up to 600 °C). The barrier height and ideality factor are found to be independent of temperature, indicating highly homogeneous contact. XPS studies indicate that the large ideality factors and leakage currents in Schottky contacts fabricated via photolithography reported in literature were due to a shift in the surface composition away from as-grown gallium rich surface or introduction of carbon-based surface phases during development. The removal/passivation of interface defects and phases by the acid treatment is confirmed by XPS. The Ni Schottky diodes on treated surface are found to exhibit defect-free behavior with unity ideality factors, homogeneous barriers, low leakage currents, and high temperature stability.

Partial financial support from NSF (DMR-1312582, ECCS-1508854, ECCS-1610992, and DMR-1508191), ARO (W911NF-15-2-0068 and W911NF-16-C-0101). This work was performed in part at the NCSU Nanofabrication Facility (NNF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant No. ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). ATLAS simulations were performed using SILVACO.

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