High Breakdown Strength Schottky Diodes made from

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High Breakdown Strength Schottky Diodes made from Electrodeposited ZnO for Power Electronics Applications Mourad Benlamri, Benjamin Daniel Wiltshire, Yun Zhang, Najia Mahdi, Karthik Shankar, and Douglas W. Barlage ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00053 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Electronic Materials

High Breakdown Strength Schottky Diodes made from Electrodeposited ZnO for Power Electronics Applications Mourad Benlamri,† Benjamin D. Wiltshire,‡ Yun Zhang,† Najia Mahdi,† Karthik Shankar*,† and Douglas W. Barlage† †Department

of Electrical and Computer Engineering, University of Alberta, Edmonton AB,

T6G 1H9, Canada ‡School

of Engineering, University of British Columbia, V1V 1V7, Canada

*Email address of corresponding author: [email protected]

ABSTRACT: The synthesis of ZnO films by optimized electrodeposition led to the achievement of a critical electric field of 800 kV/cm. This value, which is 2 to 3 times higher than in monocrystalline silicon, was derived from a vertical Schottky diode application of columnarstructured ZnO films electrodeposited on platinum. The device exhibited a free carrier concentration of 2.5  1015 cm-3, a rectification ratio of 3  108 and an ideality factor of 1.10, a value uncommonly obtained in solution-processed ZnO. High breakdown strength and high thickness capability make this environment-friendly process a serious option for power electronics and energy-harvesting.

KEYWORDS: zinc oxide, electrodeposition, solution-processed, Schottky diode, power diode, ideality factor, breakdown voltage, critical electric field

ACS Paragon Plus Environment

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ZnO is a material of interest for high temperature and high power electronics due to its direct wide band gap (3.37 eV), high electron saturation velocity, high voltage breakdown strength and good thermal conductivity.1 Several researchers have studied wide band gap compound semiconductors with the aim of forming the optimal active layer for electronic devices to be used under severe electrical and environmental conditions.2-5 Semiconductor alternatives to silicon such as SiC, GaN and diamond have been studied for decades and are in production for power electronic devices.6-7 ZnO is a comparable material to GaN in many aspects (band gap, saturation velocity, thermal conductivity) and can be synthesized at much lower temperatures unlike GaN. In addition, ZnO is amenable to low-temperature wet and dry chemical etching,8-9 and offers the possibility of high-quality growth by the low-cost solution-processing techniques.10 Insights into the suitability of a material for the fabrication of high-performance power devices can be provided by the Baliga’s figure of merit (BFOM):11 BFOM = 𝜀0𝜀𝑠𝜇𝐸3𝑐

(1)

where 0 is the vacuum permittivity, s the dielectric constant,  the electron mobility and EC the critical electric field for breakdown. For ZnO, the critical electric field is a parameter that still continues to increase as material processing improves. In this work, we focus on assessing this critical field parameter for an electrodeposited zinc oxide film which features a columnar crystalline structure containing relatively defect-free crystallites. We recognize that this process has the potential to produce a low-cost, high-performance power diode for use in applications such as a full-wave rectifier in energy harvesting, a building block for extremely low-cost vertical power transistors, and power electronics on flexible substrates. To our knowledge, this is the first investigation of breakdown field strength in electrodeposited ZnO.


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To achieve significantly high blocking voltage capability in ZnO-based devices, the ZnO grown layers and structures should exhibit: 1) High electron mobilities 2) Sizable ZnO layer thicknesses and 3) Moderately low carrier concentrations. All of the above-mentioned material requirements can be potentially met by using the low-cost electrodeposition technique instead of the other known solution-processing methods, as we have already demonstrated in earlier works.12-13 In the case of vertical power transistors, a collector drift region made of a thick and low-doped ZnO layer is where high electric fields occur, thereby enabling the device as a whole to withstand high voltages before breakdown. In the present work, our objective is to optimize electrodeposited ZnO material to yield an enhanced reverse bias breakdown voltage by studying the multiple material growth conditions: precursor concentration, electrochemical potential, solution temperature, etc. We opted to use vertical platinum (Pt)/ZnO Schottky diodes as test vehicles to characterize the material properties. Therefore, by finding the best growth conditions to obtain the highest breakdown voltage in Pt/ZnO Schottky diodes, we can eventually apply those findings to other ZnO-based power devices such as vertical power transistors. The field-effect scanning electron microscopy (FESEM) investigation of the crystalline morphology of the electrodeposited ZnO films on Pt reveals a columnar ZnO film structure with highly crystalline grains. Vertical carrier transport along the columnar vertical crystallites benefits from mobility enhancement resulting from the low density of scattering centers and grain boundaries in the direction normal to the Pt substrate. This, in turn, should lead to a better BFOM. The deposition rates and deposited thicknesses on the Pt substrate depend on the combined growth conditions but are highly favorable when compared to vacuum deposited films. Certainly, the possibility of achieving high thicknesses ultimately allows us to seek higher breakdown voltages.


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One-step and two-step deposition processes were carried out under similar growth conditions, where applicable, to investigate the effect of the metal-semiconductor interface in these two cases on the expected high electric fields under large reverse bias in a Schottky diode configuration, which in turn directly impact the breakdown voltages that a ZnO-based device can withstand. For these two deposition processes, the growth current-time evolution in Figure 1 shows different nucleation stage evolutions toward the growth stage. Comparatively, the two-step deposition process occurs more instantaneously. The device configuration is illustrated in the inset of Figure 3a. High work function platinum is used as the top substrate layer. The Pt-capped substrate preparation is described in Supporting Information. The low-cost cathodic electrodeposition method was employed to grow ZnO films on the evaporated Pt thin films. A simple electrolytic solution consisting of 100 mM zinc nitrate hexahydrate (99%, Sigma-Aldrich) dissolved in deionized water was prepared and stirred for 15 min. The solution pH was initially 5.3  0.2. Prior to electrodeposition, the Pt substrate was ultrasonically rinsed in acetone and methanol and blow-dried. The electrodeposition process was carried out in potentiostatic mode using a standard three-electrode cell configuration in a glass beaker. A standard Ag/AgCl electrode (+200 mV vs. SHE, the standard hydrogen electrode) was used as the reference electrode. A sputtered Pt film on glass and the pretreated Pt substrate were set up as counter electrode and working electrode respectively. A computer-controlled CHI660E electrochemical workstation was used to control the applied potential and monitor the evolutions of current and charge over deposition time. One-step electrodeposition was conducted for 20 min at a fixed cathodic potential EV of −800 mV and a low solution temperature of 70  2 C with moderate stirring. The two-step electrodeposition, on the other hand, was carried out at the same temperature of 70  2 C with moderate stirring but with a sequence of two steps: fixed EV of


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−1120 mV for 10 s to form the seed layer, followed by a fixed EV of -800 mV for 20 min. The respective electrodeposition current-time recordings, shown in Figure 1, reveal a more instantaneous nucleation mechanism for the two-step electrodeposition. The final fabrication steps of the test Schottky diode are described in Supporting Information.

Figure 1. Variations of current (normalized to maximum current) versus time for the one-step and two-step deposition processes respectively.

The major reasons behind our choice of 100 mM zinc nitrate concentration, low deposition potential and low deposition temperature can be summarized as follows: 1) this choice gives us the best ideality factor-breakdown voltage combination, and 2) lower deposition rates are expected to yield dense ZnO films with wider crystallite sizes.14 The deposition potential lies at the low end of the potential range that yields ZnO synthesis, determined through linear sweep voltammetry


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results that are presented in Supporting Information. The choice of these growth conditions through an optimization process is presented in detail in Topic 4 in Supporting Information. Both one-step and two-step electrodeposited ZnO films were subjected to XRD peak profiling. We observed that the peaks profiles of both types of films are highly similar and hence the use of the seed layer did not affect the crystalline texture under the present substrate and growth conditions. Only the two-step electrodeposited films are profiled in Figure 2b. The resulting peaks profile reveals the presence of the three major diffraction peaks (100), (002), and (101) of hexagonal wurtzite ZnO (PDF 01-089-1397). The narrowness of the peaks qualitatively suggests large crystallite sizes and minimal strain.15 The (101) peak stands out as the most dominant. Further film characterizations by the XRD technique are presented in Supporting Information. Absorbance spectra were extracted through spectrophotometry and are presented in Supporting Information. They show high ZnO film quality with a sharp absorption edge. The cross-sectional morphology of a two-step electrodeposited ZnO film was further investigated by FESEM as depicted in Figure 2a. The film is coated with thin layers of Aluminum and Gold. The film thicknesses average around 550 nm roughly and thus agree, within the margins of uncertainties, with the thickness values extracted from the capacitance data in the subsequent text. The FESEM image reveals a dense film and columnar structures with crystallite widths tending toward 1 m. Vertical carrier transport is particularly enhanced along these columnar structures because of the low scattering centers and grain boundary barriers along this direction. A deeper insight into the evolution of the crystallinity of columnar structures with thickness is shown in Supporting Information.


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Figure 2. (a) Cross-sectional FESEM images of cleaved ZnO films on Pt substrate, and (b) the x-ray diffractogram of the same ZnO electrodeposited film.

Electrical conductivity and carrier concentration in ZnO films are strongly correlated to intrinsic defects and contamination impurities.16 Their study gives insight into the films’ crystalline and stoichiometric qualities. In this respect, the performance parameters of the fabricated test Schottky diodes were investigated through current-voltage (I-V) and capacitance-voltage (C-V)


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measurements at room temperature. The charge transport through a Schottky diode can be described by a thermionic emission model featuring an ideality factor η that incorporates all the non-ideality factors.17 Accordingly, the I-V relation is given by

[ (

𝐼 = 𝐼𝑠 exp

𝑒(𝑉 ― 𝑅𝑠𝐼)

𝑘𝐵𝑇

) ] ―1

(2)

IS, the reverse saturation current, is 𝐼𝑠 = 𝐴𝐴 ∗ 𝑇2exp

( ) ― 𝛷𝐵 𝑘𝐵𝑇

(3)

where A is the diode area, T the absolute temperature, V the applied voltage, B the Schottky barrier height, and RS the series resistance. The theoretical value of 32 A cm-2 K-2 is usually used for the Richardson's constant A* in ZnO by assuming an effective mass m* ~ 0.27m0.18 The top ohmic contact of the vertical diode has a circular geometry with a radius of 50 m. The I-V plots allow the extraction of IS, η, B, and RS. In a semi-log scaled I-V plot, equation 1 corresponds to a linear region with linear fit L(V) = (q/ηkT)V + ln(IS) at forward bias voltages V where V > 3kT/q and V >> RSI concomitantly. The slope and intercept of the fit L(V) readily allow the derivation of the ideality factor η and reverse saturation current IS respectively: η = (q/kT)(dL/dV)-1 and IS = exp[L(V=0)]. Solution-processed ZnO exhibits, in general, much lower quality than vapor-deposited ZnO made under vacuum conditions.19-20 It suffers from higher densities of native defects and grain boundaries as well as higher levels of extrinsic contamination. The associated unintentional n-type doping is consistently reported to be higher than 1017 cm-3.21-22 As a consequence, Schottky barrier contacts formed to solution-processed ZnO have, in general, much lower quality with much higher ideality factors (~ 8 to 10) and leakier contacts (on-off ratio < 104).


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Figure 3. Diode Current-Voltage characteristics at room temperature for a circular contact of 50 m radius. The inset in part (a) shows the vertical Schottky diode structure. Plot (a) displays data in semi-log scale and plot (b) shows data in linear scale until breakdown occurs.

In Figure 3, the measured room temperature I-V characteristics of our fabricated Schottky diode, based on the two-step electrodeposited ZnO, are plotted in linear and semi-log scales. The linear-


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scaled plot shows the extent of the breakdown voltage. An excellent rectification ratio of ~ 3  108 is achieved in spite of a non-optimized Al/Au ohmic contact. The reverse leakage current is kept relatively low with a reverse saturation current density of 1.8  10-9 A/cm2 which is favorably comparable to values obtained from Schottky contacts made of Pt on hydrothermally-grown bulk ZnO.23 The low leakage current is a first indicator that intrinsic defects in the ZnO film are either reasonably low or electrically inactive for the most part. It also indicates that unintentional doping, due mainly to these intrinsic defects and extrinsic contamination, is relatively low. An ideality factor η of 1.10 has been extracted. This constantly reproducible value of the ideality factor is outstanding for a solution-processed ZnO film and even for vapor-deposited ZnO films.24-25 It unequivocally attests to the high quality of the electrodeposited film. In addition, a turn-on voltage VON of 0.91 V has been obtained at a forward current density of 1 A/cm2 from Figure 3a.26 These extracted I-V parameters compare favorably with those obtained from Schottky contacts made on sputter-deposited amorphous gallium-indium-zinc-oxide thin films.27 The assumption of low doping can be verified by carrying out C-V measurements on the same test device to determine the free carrier concentration N. At a detection signal frequency of 100 kHz, the measured diode capacitance is given in Figure 4 as a plot of A2/C2 versus V which conveniently allows the determination of N by applying Mott-Schottky theory. The slope of the linear fit is inversely proportional to N:

[ ( )]

2 𝑑 𝐴2 𝑁=― 𝑒𝜀𝑠𝜀0 𝑑𝑉 𝐶2

―1

(4)

From data plotted in Figure 4, we observe that the ZnO region reaches full depletion at a low reverse bias voltage of −0.45 V corresponding to the maximum value of A2/C2. For a circular electrode of 50 m radius and assuming a dielectric constant s = 8.9, we obtain a free carrier


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concentration N ~ 2.5  1015 cm-3. The x-axis intercept of the linear fit gives a junction built-in potential Vbi of 0.58 V, which is lower than VON as expected. The free carrier concentration is lower even than those achieved in the highest quality ZnO crystals grown by advanced vapor-phase techniques.28-29 Only hydrothermally-grown single crystals of bulk ZnO show lower carrier concentration values because of the enhancing effect of lithium compensation.30 Low carrier concentration and large depletion width are prerequisites to achieve high breakdown voltages. The low N also results in a fully depleted ZnO region, under reverse bias voltages < -0.45 V, with a maximum depletion width Wmax = As0/C ~ 520 nm. The latter value is in close agreement with the average thickness of the ZnO film measured by cross-sectional FESEM. The linear I-V plot in Figure 3b reveals a high breakdown voltage VBD of 20.75 V at room temperature. The combined values of the bandgap, carrier concentration and breakdown voltage suggest a breakdown mechanism resulting from the avalanche effect but studies of the temperature effect on breakdown voltage are needed to ascertain the type(s) of breakdown mechanism. Knowing Wmax, we can readily determine the critical electric field at breakdown EC by using the relation EC ≈ 2VBD/Wmax. The obtained EC corresponding to the present doping level is 800 kV/cm and is 2 to 3 times higher than the value usually reported for silicon. No breakdown voltage enhancement technique such as passivation, field plate, ring guard or other edge termination technologies was employed in these diodes. It is noteworthy that breakdown voltages as high as 33 V, for the same deposition time, have been obtained under slightly different growth conditions as presented in Supporting Information.


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Figure 4. C-V results plotted as A2/C2 versus V, where A is the area of the circular top contact of the diode.

We have demonstrated that electrodeposited ZnO can be a good candidate for low-cost electronic power device technologies. In addition to being simple and environment-friendly, ZnO electrodeposition presents distinct advantages to attain appealing breakdown performance. It has the capability to generate high film thicknesses at reasonably short deposition rates and the possibility to achieve high electron mobilities along the structural columns which feature good crystalline order and low grain boundaries density. Test device processing involved the simple fabrication of a Schottky diode with no passivation, no edge effect reduction and no dry etch with only one standard lithography step. The device exhibited high electrical performance with an ideality factor of 1.10, a carrier concentration of 2.5  1015 cm-3 and a critical electric field of 800


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kV cm−1 which attest to the suitable quality of the ZnO film. The critical electric field has potential for future improvements with more optimized material and device processing techniques. Our results show that electrodeposited ZnO is an inexpensive and environment-friendly option for power electronic devices and energy-harvesting systems including full-wave bridge rectifiers and vertical power transistors. Supporting Information Platinum-capped substrate preparation, Schottky diode processing, Linear sweep voltammetry, Optimization of the growth process, Further XRD characterizations of one-step and two-step electrodeposited ZnO films, Absorbance spectra, Evolution of the crystallinity of columnar structures with thickness, Breakdown voltage dependence on electrodeposition times.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT All authors acknowledge NSERC (Discovery and I2I) and CMC Microsystems for funding this research. D.W.B. thanks nanobridge (Alberta Innovates) while K.S. thanks NRC-NINT and Future Energy Systems for financial assistance. Equipment purchased using a Leader Opportunity Fund grant to K.S. from the Canada Foundation for Innovation (CFI) and matched by the Alberta Small Equipment Research Program (SEGP), was used to perform some of the synthesis and


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characterization reported in this work. B.D.W. thanks Alberta Innovates for scholarship support. The authors would also like to acknowledge the University of Alberta nanoFab for allowing the use of their facility. REFERENCES (1) Look, D. C. Recent Advances in ZnO Materials and Devices. Materials Science and Engineering B-Solid State Materials for Advanced Technology 2001, 80 (1-3), 383-387, DOI: 10.1016/s0921-5107(00)00604-8. (2) Chow, T. P.; Tyagi, R. Wide Bandgap Compound Semiconductors for Superior HighVoltage Unipolar Power Devices. IEEE T Electron Dev 1994, 41 (8), 1481-1483, DOI: Doi 10.1109/16.297751. (3) Ramirez, J. I.; Li, Y. Y. V.; Basantani, H.; Leedy, K.; Bayraktaroglu, B.; Jessen, G. H.; Jackson, T. N. Radiation-Hard ZnO Thin Film Transistors. IEEE Transactions on Nuclear Science 2015, 62 (3), 1399-1404, DOI: 10.1109/tns.2015.2417831. (4) Kim, M.; Seo, J. H.; Singisetti, U.; Ma, Z. Q. Recent Advances in Free-Standing Single Crystalline Wide Band-Gap Semiconductors and their Applications: GaN, SiC, ZnO, beta-Ga2O3, and diamond. J Mater Chem C 2017, 5 (33), 8338-8354, DOI: 10.1039/c7tc02221b. (5) Kim, J.; Mastro, M. A.; Tadjer, M. J.; Kim, J. Heterostructure WSe2-Ga2O3 Junction Field-Effect Transistor for Low-Dimensional High-Power Electronics. Acs Applied Materials & Interfaces 2018, 10 (35), 29724-29729, DOI: 10.1021/acsami.8b07030. (6) Kizilyalli, I. C.; Xu, Y. A.; Carlson, E.; Manser, J.; Cunningham, D. W. In Current and

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High Breakdown Strength Schottky Diodes made from

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