Schottky barrier diodes on :Sn heteroepitaxial thin films grown by pulsed laser deposition on m-plane sapphire substrates are reported. Sets of co-planar diodes were fabricated with different metals and different deposition methods. The current rectification and effective Schottky barrier height of oxidized contacts realized by reactive sputtering significantly exceed the values of non-oxidized contacts realized by thermal evaporation or sputtering in an inert argon atmosphere. The best values obtained are rectification of about eight orders of magnitude ( V) and 1.3 eV effective barrier height. The current-voltage characteristics of selected non-oxidized and oxidized platinum diodes have been studied as a function of measurement temperature. The temperature dependence of the effective barrier height and the ideality factor of the diodes were fitted taking into account the lateral potential fluctuations of the barrier potential. The determined mean barrier heights and standard deviations are in the range of 1.76–2.53 and 0.2–0.33 eV, respectively, and are classified with respect to the literature and fulfill a well-established empirical correlation (Lajn’s rule) for a variety of Schottky barrier diodes on different semiconducting materials.
I. INTRODUCTION
is emerging as a viable alternative to GaN or SiC for the fabrication of high-power electronic devices.1–4 It may crystallize in various polymorphs,5 among which the monoclinic -gallia structure is thermodynamically most stable. Moreover, the substrate and wafering technology is available for , so most of the scientific and technological studies are related to this polymorph. In addition, the orthorhombic modification, which is the only polymorph with spontaneous polarization,6 and rhombohedral , being isostructural to commonly used sapphire substrates, are also promising for high-power devices. For both, first Schottky barrier diodes (SBDs)7–13 and -heterodiodes have been demonstrated.13,14 So far, SBDs on have been realized on layers grown by MIST-CVD7–9,11,12,15 (with the exception of a UVC photo detector on grown by halide vapor phase epitaxy10). A rectification ratio of a lateral MESFET device structure of was achieved for reactively sputtered SBD on Sn-doped on c-plane sapphire with Ti/Au ohmic contacts.7 A first vertical SBD was fabricated by Oda et al. by electron beam evaporation (EB-Ev) of Pt Schottky contacts and removal of the c-plane sapphire substrate used for epitaxy. Subsequent EB-Ev of Ti/Au ohmic contacts resulted in low on-resistance of .8,9 The properties of various Schottky contact metals have been investigated by Shiojima et al. as a function of annealing temperature.11 The contacts were deposited by EB-Ev on a MIST-CVD-grown layer stack on c-plane sapphire. The stack consisted of a bottom and -top layer allowing a quasi-vertical diode layout (ohmic contact metals were not specified). As-deposited Ti, Pt, Fe SBDs have an effective barrier height (ideality factor) of , , and eV (1.04, 1.38, and 1.32), respectively, as determined from current-voltage ( ) measurements considering thermionic emission as the transport mechanism. For annealing temperatures of 400 C and higher, interfacial reactions were observed for all contact metals resulting in higher leakage current, lower effective barrier height, and higher ideality factor. Temperature-dependent current-voltage ( ) measurements were conducted for Ti SBDs on a similar - layer stack (with Ti ohmic contacts to the bottom layer) on m-plane sapphire15 for 298 423 K. The forward current is well described by the thermionic emission (TE) theory the reverse current follows the thermionic field emission (TFE) theory. The effective barrier height and the ideality factor were independent of temperature with values of eV and 1.03, respectively. Despite these partially encouraging results, further research on (i) different Schottky contact metals for SBD performance optimization, (ii) the influence of the Schottky contact metal deposition process, and (iii) the temperature-dependent electrical properties of SBDs on are needed to accelerate the exploitation of this polymorph in applications.
The electrical properties of SBDs on :Sn grown by pulsed laser deposition (PLD) on m-plane sapphire substrates are discussed in this study. The use of m-plane sapphire substrates was shown to be advantageous for the growth and stabilization of the metastable corundum phase of .16 On c-plane sapphire or on c-facets formed during growth on r-plane sapphire, ( ) is obtained by physical vapor deposition. For growth on m-plane sapphire, the c-direction is in-plane (perpendicular to the growth direction), allowing the growth of phase-pure .17,18 The room temperature (RT) current-density-voltage ( ) characteristics of SBDs with different contact metals on :Sn(10.0) are compared and discussed first. Then, for Pt-SBDs, the properties of thermally evaporated contacts are compared to inert and reactively sputtered layers. Reactive sputtering produces oxidized Schottky contacts. The effects of oxidation on the properties of Schottky contacts have been investigated and described in detail for various semiconducting oxides and also for different contact metals (e.g., Ag, Au, Ir, Rh, Pd, and Pt) and show (i) significant lower leakage current, (ii) increased rectification, and (iii) improved thermal and temporal stability as demonstrated for ZnO,19,20 ,21,22 zinc-tin-oxide,23,24 and .25,26 In general, near-surface oxygen vacancies, which are donor-like defects in such materials, are saturated due to the transfer of oxygen from the contacts to the semiconductor (reduction of oxidized metal electrode) and/or the surface downward band bending and electron accumulation layers are removed during reactive sputtering or oxygen plasma treatment.19,22,23 Furthermore, the deposition conditions influence the rectification and the homogeneity of the Schottky barrier.20,25 For SBDs on heteroepitaxial thin films the highest rectification and highest barrier height are obtained for oxidized contacts realized by reactive sputtering while most homogeneous Schottky barriers are obtained for deposition methods with low kinetic energy of the deposited species such as evaporation and to some extent long-throw reactive sputtering.25 Finally, the temperature dependence of the effective barrier height and the ideality factor are evaluated by the model of Splith et al.27,28 and Werner and Güttler,29 respectively, and the derived parameters are classified in relation to the literature, and it is shown that Lajn’s rule30 also applies to SBDs on the group-III sesquioxides. Lajn’s rule is an empirically derived correlation between the homogeneous barrier height and the corresponding standard deviation of laterally inhomogeneous Schottky barrier potentials.
II. EXPERIMENTAL METHODS
:Sn was grown by PLD on an buffer layer on a 2-in.-diameter m-plane sapphire substrate. The thickness of the active and the buffer layer are 700 and 100 nm, respectively. The deposition parameters were optimized by Vogt et al.18 The wafer was cut into 5 5 mm sized pieces for maximum comparability of results. Co-planar ohmic and Schottky contacts were patterned by photolithography and lift-off. Prior to the deposition of the Schottky contact metals the ohmic Ti/Al/Au (30/30/30 nm) layer stack, realized by thermal evaporation, was rapidly thermally annealed for 2 min at 400 C in air using a laser. Schottky contact metals were thermally evaporated (Au, Cu, Ni, Pt), sputtered in inert (Nb, Pt), or reactive (Pt) ambient, respectively. Furthermore, sets of Pt contacts were prepared as a function of oxygen and argon volume flow ratio /Ar ranging from 0%/100% to 100%/0%. Current-voltage characteristics were acquired using an Agilent Semiconductor Parameter Analyzer 4155C and 4156C, respectively, inside a SUESS WaferProber PA200 for RT measurements or inside a Lake Shore Cryotronics Cryogenic Probestation for IVT measurements.
III. RESULTS AND DISCUSSION
A. Variation of Schottky contact metal
B. Variation of deposition conditions for Pt-SBDs
The influence of deposition conditions on RT Schottky diode -characteristics is shown in Fig. 2 for typical Pt-SBDs realized by evaporation, inert sputtering, and reactive sputtering. The reverse current is the highest for the evaporated and inert sputtered contact in agreement with other semiconducting oxides.19–26 Ohmic behavior is observed for Pt contacts sputtered in an inert Ar ambient. The reverse current decreases by orders of magnitude for oxygen-containing contacts deposited by reactive sputtering. We note that the forward current density of all types of sputtered Pt-contacts is below the evaporated layer, indicating an increase in the series resistance induced by the sputtering process. The logarithm of the current rectification ratio
IV. TEMPERATURE DEPENDENCE OF DIODE PROPERTIES
Method . | (eV) . | σ0 (eV) . | ρ2 . | ρ3 (eV) . |
---|---|---|---|---|
Ev | 1.985 ± 0.1 | 0.273 ± 0.015 | 0.22 ± 0.01 | −0.02 ± 0.005 |
25/75 O2/Ar | 2.57 ± 0.034 | 0.284 ± 0.0035 | 0.042 ± 0.015 | −0.037 ± 0.000 84 |
50/50 O2/Ar | 2.46 ± 0.017 | 0.25 ± 0.002 | 0.19 ± 0.013 | −0.019 ± 0.000 5 |
75/25 O2/Ar | 1.76 ± 0.039 | 0.20 ± 0.005 | 0.27 ± 0.0075 | −0.021 ± 0.000 4 |
100/0 O2/Ar | 2.53 ± 0.55 | 0.33 ± 0.075 | 0.48 ± 0.013 | −0.012 ± 0.000 6 |
Method . | (eV) . | σ0 (eV) . | ρ2 . | ρ3 (eV) . |
---|---|---|---|---|
Ev | 1.985 ± 0.1 | 0.273 ± 0.015 | 0.22 ± 0.01 | −0.02 ± 0.005 |
25/75 O2/Ar | 2.57 ± 0.034 | 0.284 ± 0.0035 | 0.042 ± 0.015 | −0.037 ± 0.000 84 |
50/50 O2/Ar | 2.46 ± 0.017 | 0.25 ± 0.002 | 0.19 ± 0.013 | −0.019 ± 0.000 5 |
75/25 O2/Ar | 1.76 ± 0.039 | 0.20 ± 0.005 | 0.27 ± 0.0075 | −0.021 ± 0.000 4 |
100/0 O2/Ar | 2.53 ± 0.55 | 0.33 ± 0.075 | 0.48 ± 0.013 | −0.012 ± 0.000 6 |
In Fig. 5, is plotted against the reciprocal temperature for the determination of the voltage coefficients by linear fitting. The fits and the derived parameters are depicted in the figure (dashed lines) and are summarized in Table I, respectively. Non-oxidized Cu contacts to heteroepitaxial have voltage coefficients and eV that are very similar to those of non-oxidized Pt/ , although the RT ideality factor and the standard deviation of the latter are significantly higher. Similar values, together with the different values of and , are attributed to the significantly larger mean barrier height eV of the non-oxidized Pt/ compared to the non-oxidized Cu/ diode with eV, which means that the forward bias of the Pt/ diode must be 660 mV higher than that of the Cu/ diode to achieve flatband conditions (for this bias, the barrier potential is by definition homogeneous). Among the oxidized platinum contacts, the lowest bias-dependent variation of the mean barrier height is found for an ambient during sputtering of 25%/75% /Ar. Combined with the highest mean barrier value and high current rectification at room temperature, these preparation conditions are the most promising for achieving the highest diode performance.
The derived barrier parameters are plotted in Fig. 6 along with data compiled from the literature.17,29,30,35–57 The published data cover various semiconductor materials, including both covalently and more ionically bonded single crystals and thin films, as well as amorphous semiconductors such as zinc-tin oxide. Also included are various diode arrangements and a variety of contact deposition methods. The figure clearly demonstrates that the correlation between the standard deviation and the mean barrier height , originally established by Lajn et al.,30 also holds for the group-III sesquioxides. The figure includes data of the discussed here and of diodes to 34,57 and bixbyite .54 The line in the figure corresponds to Lajn’s empirical rule with Lajn’s factor .30
V. SUMMARY
In conclusion, we have compared the properties of sets of Au, Cu, Nb, Ni, and Pt Schottky contacts with heteroepitaxial PLD :Sn on m-plane sapphire. The lateral Schottky diodes were realized by thermal evaporation, inert, and reactive sputtering, respectively. In agreement with literature results, highest RT current rectification (about eight orders of magnitude) and highest RT effective barrier heights (about 1.3 eV) are obtained for oxidized platinum Schottky contacts. Selected non-oxidized and oxidized platinum Schottky barrier diodes were investigated by current-voltage measurements as a function of temperature. The variation of the effective barrier height and the ideality factor is consistent with thermionic emission across laterally varying potential barriers as introduced by Werner et al.29 and refined by Splith et al.27,28 The mean barrier height and standard deviation of oxidized (non-oxidized) platinum diodes ranges between 1.76 and 2.53 eV (is 1.98 eV) and 0.2 and 0.33 eV (0.27 eV), respectively. The values of the standard deviation and the mean barrier height follow the empirical rule of Lajn et al.:30 with .
ACKNOWLEDGMENTS
This work was partially performed in the framework of GraFOx a Leibniz-Science Campus partially funded by the Leibniz Association.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
S. Köpp: Data curation (lead); Formal analysis (equal); Investigation (lead). C. Petersen: Conceptualization (equal); Methodology (equal); Supervision (lead); Validation (equal); Writing – review & editing (equal). D. Splith: Data curation (equal); Formal analysis (equal); Software (lead); Validation (equal); Writing – review & editing (equal). M. Grundmann: Funding acquisition (equal); Project administration (equal); Writing – review & editing (equal). H. von Wenckstern: Conceptualization (equal); Data curation (equal); Funding acquisition (equal); Project administration (equal); Supervision (equal); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.