Au Schottky contacts to n-type ZnO

Applied Surface Science 236 (2004) 387–393

Effect of ozone cleaning on Pt/Au and W/Pt/Au Schottky contacts to n-type ZnO K. Ipa, B.P. Gilaa, A.H. Onstinea, E.S. Lambersa, Y.W. Heoa, K.H. Baika, D.P. Nortona, S.J. Peartona,*, S. Kimb, J.R. LaRocheb, F. Renb a

Department of Materials Science and Engineering, University of Florida, PO Box 116400, Gainesville, FL 32611, USA b Department of Chemical Engineering, University of Florida, PO Box 116400, Gainesville, FL 32611, USA Received in revised form 31 March 2004; accepted 11 May 2004 Available online 28 July 2004

Abstract The role of UV ozone cleaning on the characteristics of Pt contacts on n-type (n
1017 cm3) bulk single-crystal zinc oxide (ZnO) is reported. The contacts are Ohmic for samples that were not exposed to ozone prior to Pt deposition, but exhibit excellent rectifying behavior with ozone cleaning. The barrier height of these contacts obtained from current–voltage measurements was 0:70  0:04 eV at 25 8C with an ideality factor of 1.49 and a saturation current density of 6:17  106 A cm2. There is a significant decrease in surface carbon concentration after the ozone cleaning (29.5 at.% down to 5.8 at.%, as determined from Auger electron spectroscopy). The measured barrier height for Pt on ZnO is similar to the value reported for both Au and Ag rectifying contacts on this material. By sharp contrast, sputter-deposited W contacts are Ohmic, independent of the use of ozone cleaning and become rectifying after 700 8C annealing to repair sputter-induced damage. The barrier height is
0.45–0.49 eV, with the ozone cleaning producing values at the high end of this range. # 2004 Elsevier B.V. All rights reserved. Keywords: UV ozone cleaning; n-type ZnO; Barrier height

1. Introduction There is much current interest in the development of zinc oxide (ZnO) for applications in short wavelength optoelectronics and transparent electronics [1–9]. It is wide band gap semiconductor available in bulk substrate form with good carrier mobility, direct bandgap energy of 3.3 eV and large exciton binding energy of 60 meV. To realize UV light emitters/detectors or field effect transistors with acceptable characteristics, high *

Corresponding author. Tel.: þ1-3528461086; fax: þ1-3528461182. E-mail address: [email protected] (S.J. Pearton).

quality contacts are necessary. A variety of metallization schemes for Ohmic contacts have been reported, including Pt-Ga, Ti/Au, non-alloyed Al, Ta/Au, Al/Pt and Ti/Al [10–20]. In terms of rectifying contacts it has been reported that low-reactive metals such as Au, Ag, Pd form Schottky barriers of 0.6–0.8 eV with nZnO [21–30] with the barrier heights not following the difference in the work function value. This is a strong indication that interface defect states play a major role in determining the contact characteristics. There is clearly a need to study the effect of surface cleaning on the electrical properties of contacts on ZnO. The thermal stability of the Schottky diodes on n-ZnO has not been extensively studied but there are indications

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.013

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K. Ip et al. / Applied Surface Science 236 (2004) 387–393

that Au is unstable even at 60 8C [23–26]. Therefore, more thermally stable metallization schemes are also desirable for ZnO. For the Au Schottky diodes on epiready (0 0 0 1)Zn surfaces of n-ZnO (n
1017 cm3) the lowest reverse current values were obtained with simple cleaning of the surface in organic solvents [29]. The Schottky barrier height of Au deduced from C–V measurements was 0.65 eV, while for Ag the value was 0.69 eV. The ideality factor was close to 2, independent of surface preparation [29]. In this paper we report on the effect of UV ozone cleaning on the properties of both Pt and W-based contacts on bulk n-type ZnO. The ozone cleaning produces a conversion of the Pt contacts from Ohmic to rectifying behavior, while the W contacts are Ohmic as-deposited independent of the surface cleaning.

Microscope after post-sputtering anneals up to 700 8C, under a N2 ambient for 1 min.

3. Results and discussion There was a slight improvement in the overall roughness of the ZnO surface after ozone treatment. Fig. 1 shows the morphology of the bulk ZnO before (top) and after (bottom) a 30 min ozone clean. As

2. Experimental The samples were (0 0 0 1) undoped grade I quality bulk ZnO crystals from Cermet. The samples were epiready, one-side-Zn-face-polished by the manufacturer. The room temperature electron concentration and mobility established by van der Pauw measurements were 1017 cm3 and 190 cm2/Vs, respectively. To study the effects of UV ozone treatment (UVOCS UV Ozone Cleaning System Model 70606B) prior to Schottky metal deposition, samples were exposed for 30 min at room temperature and then characterized with atomic force microscopy (AFM) (tapping mode, 1 mm  1 mm) and also Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS/ ESCA) in a Perkin-Elmer PHI 5100 system. The contact study was performed by depositing a full backside Ohmic contact by E-beam evaporation, con˚ ). This was sisting of Ti/Al/Pt/Au (200/800/400/800 A annealed at 200 8C, 1 min, N2 ambient. The subsequent contacts were deposited either with and without 30 min UV ozone treatments. The Pt Schottky contact was patterned by standard lift-off photolithography ˚ ). The after E-beam evaporation of Pt/Au (200/1000 A W contacts were also patterned by standard lift-off photolithography after deposition of W/Pt/Au (720/ ˚ ) by sputtering in a Kurt J. Lesker CMS-18 200/1000 A system. In each case the samples were characterized with an HP 4156 Semiconductor Parameter Analyzer and AES Perkin-Elmer PHI 660 Scanning Auger

Fig. 1. AFM scans of ZnO surfaces over 1 mm  1 mm area, either before (top) or after (bottom) UV ozone cleaning.

K. Ip et al. / Applied Surface Science 236 (2004) 387–393

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Fig. 2. AFM scans from the samples of Fig. 1, showing the RMS roughnesses.

shown in Fig. 2, the RMS roughness and average roughness of the samples before ozone treatment were 0.248 and 0.347 nm, respectively. After 30 min ozone treatment, the RMS roughness was decreased to 0.162 nm, and the average roughness was 0.322 nm. The main effect of the ozone exposure is expected to desorption of surface C contamination through conversion to volatile CO and CO2 products. To examine this in more detail, XPS and AES measurements were carried out. Fig. 3 shows the AES surface surveys before (top) and after (bottom) ozone exposure for 30 min, with a much higher concentration of C on the untreated sample. The binding energies were calibrated by taking the C1s peak at 284.6 eV as a reference. The 500 eV Arþ sputtering was used for depth profiling. Carbon atomic concentrations were calculated using the

standard sensitivity factors and are tabulated in Table 1, as a function of Ar sputter time. Note that the average surface concentration of C was decreased from 29.5 to 5.8 at.% as a result of the ozone treatment. Given the approximate Ar sputter rate of ˚ min1, it is seen that this decrease is predo
60 A minantly a near-surface effect. The O1s spectra was resolved into two components, O1 and O2, for no ozone and 30 min ozone treated bulk ZnO on the surface and after 1 and 2 min of Arþ sputtering. The O1 line is due to O in the form of ZnO, while the O2 line is due to O in the form of O–H and related species. Fig. 4 shows these spectra for the before and after ozone cases. There is a decrease in the O2 component relative to the O1 component. Table 2 summarizes the XPS data as a function of Ar sputter time.

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No Ozone

No Ozone

60000 Intensity (arb. unit)

30000

40000 20000 N(E)

0 O

-20000 -40000

O

C

Zn

Zn

-60000 Zn 1000

25000 20000 15000 10000 5000

800

600

400

200

0 525

0

530 535 Energy (eV)

Energy (eV) 30 min Ozone

30 min Ozone 30000 Intensity (arb. unit)

150000 100000

N(E)

50000 0 C

O

-50000

Zn

Zn -100000

540

Zn 1000

25000 20000 15000 10000 5000

800

600

400

200

0 525

0

Energy (eV)

530 535 Energy (eV)

540

Fig. 3. AES survey spectra of ZnO before (top) and after (bottom) UV ozone cleaning.

Fig. 4. XPS spectra from the region of O-bonded transitions, before (top) and after (bottom) UV ozone cleaning.

It is clear from the chemical data discussed above that the ozone exposure removes C-related contamination from the ZnO surface. This process also had a major influence on the electrical characteristics of the contacts deposited on these surfaces. The current– voltage (I–V) characteristics for the Pt metallization was measured on 50 mm diameter contacts. The sam-

ple without ozone treatment resulted in linear I–V characteristics, as shown in Fig. 5. By sharp contrast, the sample treated with ozone exhibited rectifying behavior (Fig. 6). In the latter case, the barrier height

Table 1 Carbon atomic concentrations determined by AES on the ZnO surface before and after ozone treatment

Treatment

Treatment

C1s

No ozone No sputter 1 min sputter 2 min sputter

29.5 5.3 2.6

30 min ozone No sputter 1 min sputter 2 min sputter

5.8 1.1 0.1

Table 2 Summary of XPS data for O-related species before and after ozone cleaning Peak energy (eV) Intensity O1

O2

O1

O2

Ratio (O1/O2)

No ozone No sputter 1 min sputter 2 min sputter

529.9 529.8 529.8

531.5 530.4 530.3

22365 27531 31326

18518 18751 15837

1.21 1.47 1.98

30 min ozone No sputter 1 min sputter 2 min sputter

530.0 529.8 529.9

531.1 530.4 530.4

17745 18745 21456

17450 15227 12781

1.02 1.23 1.68

Peak assignments (eV): 530.4—ZnO; 531.5—OH; 533.2—water.

K. Ip et al. / Applied Surface Science 236 (2004) 387–393

No ozone

1.0 Current (mA)

Current (mA)

1.0 0.5 0.0 -0.5 -1.0 -0.10

391

As-sputtered 30 min ozone no ozone

0.5 0.0 -0.5 -1.0

-0.010 -0.005 0.000 0.005 0.010 -0.05

0.00

0.05

Bias (V)

0.10

Bias (V)

0.3

derived from the forward characteristics from the relation for the thermionic emission over a barrier 

efb eV  2 JF ¼ A T exp  exp nkT kT

Current (mA)

Fig. 5. I–V characteristic from Pt/Au contacts on ZnO without any ozone cleaning prior to metal deposition.

0.1 0.0 -0.1

30 min ozone

700 ˚C, 1 min anneal 30 min ozone No ozone

0.2

-0.4

Current (mA)

0.020 0.015 0.010

0.1

0.2

0.3

0.4

0.5

-2

0

30 min ozone 0.0 -0.5 Current (mA)

0.2

0.4

Fig. 7. I–V characteristics from W/Pt/Au contacts on ZnO both asdeposited (top) and after annealing at 700 8C (bottom).

fB = 0.696 eV h = 1.49 -6 -2 Js = 6.17 ¥ 10 A-cm

Bias (V)

-1.0 -1.5 -2.0 -2.5 -3.0 -10

0.0 Bias (V)

0.005 0.000 0.0

-0.2

-8

-6

-4

Bias (V) Fig. 6. Forward (top) and reverse (bottom) I–V characteristics from Pt/Au contacts on ozone cleaned ZnO.

where J is the current density, A the Richardson’s constant for n-ZnO, T the absolute temperature, e the electronic charge, fb the barrier height, k the Boltzmann’s constant, n the ideality factor and V the applied voltage. From the data, fb was obtained as 0.70 eV for the as-deposited Pt on the ozone cleaned surface. The ideality factor was
1.5, suggesting transport mechanisms other than thermionic emission, such as recombination. The saturation current density was 6:17  106 A cm2. It is expected that the transition from Ohmic to rectifying behavior as a result of the ozone cleaning is due to the removal of a defective surface layer that contains C contamination and which prevents electrical contact of the Pt with the ZnO surface. The bottom part of Fig. 6 shows the reverse I–V characteristics from the diodes. In presence of imageforce induced barrier lowering, the reverse current can be expressed as IR ¼ AA T 2 exp

he i ðfb  Dfb Þ kT

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Table 3 Summary of electrical characteristics for W-based contacts after 700 8C anneals

Barrier height (eV) Ideality factor Saturation current (A cm2)

No ozone

30 min ozone

0.45 4.5 8.43  102

0.49 3.2 2.11  102

where Dfb is the image force barrier lowering given by (eEm/4pe)0.5, Em the maximum electric field strength at the contact and e the ZnO permittivity. The reverse current shown in Fig. 6 (bottom) is several orders of magnitude higher than expected from this relation and indicates other current transport mechanisms are present. By sharp contrast to the results for Pt contacts on ZnO, the W contacts displayed non-rectifying beha-

4. Summary and conclusions

30 min ozone, as-deposited Intensity (arb. unit)

30000 Zn

25000 20000 C

O W

15000 Au

10000

Pt

5000 0

0

200

400

600

800

vior for as-sputtered, 250 and 500 8C annealed conditions, independent of the use of ozone cleaning. However, as shown in Fig. 7, after subsequent annealing at 700 8C, both uncleaned and ozone-exposed samples showed rectifying characteristics. Table 3 summarizes the electrical properties of these contacts. The barrier heights are significantly lower than for the Pt contacts, with maximum values of
0.49 eV for the ozone-cleaned samples. The need for post-deposition annealing shows that the initial damage from the sputter process itself dominates the electrical characteristics of the as-deposited diodes. In addition, the post-deposition annealing produced some intermixing of the contact metallurgy, as shown in Fig. 8. After 700 8C anneal, the depth profiles show that Zn diffuses out to the Au–Pt interface. The ozone exposure had no effect of the thermal stability of the contacts.

1000

UV-ozone cleaning is found to have beneficial effects in terms of improving the performance of both Pt and W-based Schottky contacts on n-type ZnO. The removal of surface contamination produces a transition from Ohmic to rectifying behavior in Pt contacts and increases the barrier height of sputtered, annealed W-based contacts. These results show the necessity to control the surface cleanliness and are important in advancing ZnO contact technology for applications such as transparent electronics.

Time (s)

Acknowledgements

30 min ozone, 700 ˚C anneal Intensity (arb. unit)

25000 Zn

20000

C

O

15000

W

10000

Au

Pt

5000

References

Zn 0

0

200

This research was sponsored by the Army Research Office under grant no. DAAD19-01-1-0603, the Army Research Laboratory, the National Science Foundation (DMR 0101438), and the Air Force Office of Scientific Research.

400

600

800

1000

Time (s) Fig. 8. AES depth profiles of W/Pt/Au contacts both as-deposited (top) and after 700 8C annealing (bottom).

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