Metal contacts on bulk ZnO crystal treated with remote oxygen plasma

Abstract

To study the quality of thin metal/ZnO Schottky contacts ͑SCs͒, temperature-dependent current-voltage ͑I-V͒, capacitance-voltage, deep level transient spectroscopy, and photoluminescence measurements were performed using bulk, vapor-phase ZnO, treated by remote oxygen plasma ͑ROP͒. Au/ ZnO and Pd/ ZnO contacts on both O and Zn faces are compared as a function of the ROP processing sequence and duration. We find that ͑i͒ as the duration of ROP treatment increases from 2 to 4 h, Au/ ZnO contacts on the Zn face, deposited before ROP treatment, become rectifying, while those on the O face remain Ohmic; ͑ii͒ with long-term ROP treatments prior to metallization, both Au/ ZnO and Pd/ ZnO show high-quality SCs; however, their I-V characteristics can be significantly degraded by electric field and high temperatures; ͑iii͒ ROP treatment can cause more H removal on the Zn face than on the O face, resulting in a decrease in the near-surface carrier concentration for the Zn face only; ͑iv͒ in addition to the dominant bulk-trap E3, surface traps, E6/E7 and E8, and Es, can be observed in Au/ ZnO and Pd/ ZnO SCs, respectively, on the Zn face, with shorter ROP treatment; and ͑v͒ with long-term ROP treatment, E3 ͑or L2͒ significantly increases and shifts in Au/ ZnO SCs on the Zn face.

Defects at oxygen plasma cleaned ZnO polar surfaces

“…The peak temperature shift of the E3 trap corresponding to an increased binding energy may be related to the electric field-related Poole-Frenkel effect. 24 The carrier concentration in the near-surface region on the Zn-face decreases with increasing ROP duration. This decrease in carrier concentration can lead to a lower electricfield in the depletion region, resulting in a shift of E3 to higher temperatures.…”

“…23 Our photoluminescence ͑PL͒ results support this scenario by showing a decreased H donor bound exciton peak ͑I 4 = 3.3628 eV͒ only on ROP cleaned Zn face. 24 Considering the near-surface region, ROP generates V Zn on the Zn face while only depressing V O -related defects on the O face. Our most recent study on ion-implanted ZnO combining DRCLS and positron annihilation spectroscopy ͑PAS͒ demonstrate that 2.1 emission is related to V Zn or clusters.…”

Polarity-related asymetry at ZnO surfaces and metal interfaces

“…29 Consistent with hydrogen removal, recent 4.2 K PL shows that ROP reduces the I 4 line at 3.368 eV assigned to hydrogen donor bound exciton on the Zn-face but much less on the O-face for the same samples. 30 First principles calculations provide further evidence that it is much easier for hydrogen to diffuse out of the Zn face. 31 However, there is no asymmetry for oxygen diffusion within ZnO.…”

“…12 The detailed DLTS results will be reported elsewhere. 30 The surface conductive layer at Pd/ ZnO interfaces is due to hydrogen diffusion. It is easy for hydrogen to penetrate the thin Pd film and accumulate at interfaces.…”

“…25,26 Temperature-dependent I-V characteristics for Au/ ZnO and Pd/ ZnO SBDs will be reported elsewhere. 30 We have carried out C −2 -V measurements at 1 MHz, 100 kHz, and 10 kHz. Figure 6 shows the room temperature C −2 -V characteristics at 1 MHz for the same Au-and PdSBDs.…”