Defect segregation and optical emission in ZnO nano- and microwires

Abstract

The spatial distribution of defect related deep band emission has been studied in zinc oxide (ZnO) nano- and microwires using depth resolved cathodoluminescence spectroscopy (DRCLS) in a hyperspectral imaging (HSI) mode within a UHV scanning electron microscope (SEM). Three sets of wires were examined that had been grown by pulsed laser deposition or vapor transport methods and ranged in diameter from 200 nm-2.7 μm. This data was analyzed by developing a 3D DRCLS simulation and using it to estimate the segregation depth and decay profile of the near surface defects. We observed different dominant defects from each growth process as well as diameter-dependent defect segregation behavior.

Wavelength‐Tunable Electroluminescent Light Sources from Individual Ga‐Doped ZnO Microwires

“…And the emission band obviously redshifted as Ga atoms substitute for Zn sites in ZnO crystalline host lattice, which suggests that the V O ‐related defect modes decrease due to Ga‐doped effects. For Sample‐4, the redshift of the visible emission bands can be obtained as well . As the Ga content increases, the incorporated Ga ions exert a stronger effect on the optical properties of ZnO:Ga MWs in combination with the surface defect modes changing from V O ‐related donor levels to complex V O –Ga Zn related impurity levels, and finally turning into Ga Zn related impurity bands.…”

“…In addition, defect spectroscopy of single ZnO MWs has been indicated that there is a characteristic deep defect levels inside the gap at 0.88 eV from the top of the VB, and a defect level next to the bottom of the CB at 0.11 eV (Figure S5, Supporting Information). The presence of these defect levels attribute to Zn i +2 and V Zn −2 , respectively . Moreover, it has been verified that the oxygen vacancies are the dominant acceptor defects in as‐grown ZnO MWs.…”

“…No defect emission can be observed for undoped ZnO, indicating the good crystallinity and purity, which is consistent with XRD results. Interestingly, a weak visible emission (≈500 nm) can be observed for the four ZnO:Ga MWs samples (Sample‐1–Sample‐4), which may originate from the intrinsic defects, such as zinc interstitials (Zn i ), zinc antisite (Zn O ), and/or oxygen vacancies (V O ) . With increasing Ga‐doping amounts, the weak visible emissions change from green (Sample‐1) to yellow (Sample‐2), orange (Sample‐3), and even red (Sample‐4).…”

“…Furthermore, the temperature‐dependent PL spectra in the visible range for Sample‐2 and Sample‐4 are displayed in Figure b,c, respectively. For Sample‐2, the visible emission peak around 500 nm is attributed to the defect states such as surface defects and oxygen vacancies (V o ) . And the emission band obviously redshifted as Ga atoms substitute for Zn sites in ZnO crystalline host lattice, which suggests that the V O ‐related defect modes decrease due to Ga‐doped effects.…”

“…The EL spot and electrically injected breakdown behavior a can be well explained by Joule heating mechanism. Nevertheless, thermal heating can also be ruled out as a source by the experimental observations, such as increasing the injected current brings redshift of the EL, increasing Ga‐doping concentration in ZnO MWs brings redshift of the EL in the visible bands, together with the enhanced EL under low‐temperature environment . Therefore, the electrically injected light‐emitting mechanism of ZnO:Ga MWs may provide another electrically driven light emission as following: (1) Rearrangement of localized electric field can be formed toward to the center of ZnO:Ga MWs, so accelerated electrons become spatially localized toward the center and inelastic collision with ZnO host lattices; (2) An impact excitation process under electrical transport can generate an abrupt potential drop and a strong electric field due to Joule heating, and the carriers are accelerated to scatter inelastically and produce electron–hole pairs, then the EL emission can occur through the radiative electron–hole recombination; and (3) Different complex defect levels due to the Ga‐impurity content in ZnO:Ga MWs can be used to adjust the EL emission wavelengths.…”