Optical and structural properties of Cu-doped β-Ga2O3 films

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  • Materials Science and Engineering B 176 (2011) 846849

    Contents lists available at ScienceDirect

    Materials Science and Engineering B

    journa l homepage: www.e lsev ier .co

    Optical -

    Yijun Zh hanSchool of Physi

    a r t i c l

    Article history:Received 7 JanReceived in reAccepted 10 A

    Key words:Cu-doped -GOptical properOptical band gThermal anneaRF magnetron

    lmsixtucturtran

    s werlms wualitved a

    475n

    1. Introdu

    A need f(TCO) lmsers of phaselectrodes fparent conductive oxides such as ITO and ZnO are opaque in thedeep-UV region (

  • Y. Zhang et al. / Materials Science and Engineering B 176 (2011) 846849 847

    Fig. 1. XRD patterns of un-doped and Cu-doped -Ga2O3 lms deposited on Sisubstrate and post-annealed in N2 ambient at 800 C for 1h.

    N2 ambient at 800 C for 1h. Polycrystalline -Ga2O3 lms areobtained afthe (111) pother phasestituted byafter thermthe diffractlarger degrmeans thatCu doping,the Ga and

    Fig. 2 shCu-doped for 1h. Thegrain size olms is hincrystallizatiin accordan

    3.2. Optical

    Fig. 3 shlms befor100nm. Asdoped lmssubstrates)more thanthe transmannealing,deterioratedoptant Cu

    Fig. 4 shGa2O3 lmAfter annea

    Fig. 3. Transmittance spectra of the as-deposited and post-annealed-Ga2O3 lms.

    ptica

    sampsharp absorption edge in the deep ultraviolet region. Therear-edge absorption in the post-annealed Cu-doped -Ga2O3hich indicates that the Cu atoms are activated and serve asve acceptors in the post-annealed lm. Thus, the origin of thedge absorption is related to theCuacceptor states in thebandCu-doped -Ga2O3 lm. The near-edge absorption repre-

    ransitions fromsingly ionizedCuacceptors to shallowdonorse conduction band bottom or from singly ionized Cu accep-deep level donors such as oxygen vacancy (Vo) or intrinsic. The absence of near-edge absorption in the as-depositeded-Ga2O3 lmpresumably suggests that theCu impuritiesost not activated in this case. Therefore, the deterioration

    transmittance is mainly caused by the near-edge absorptionpost-annealed Cu-doped -Ga2O3 lm.optical band gaps of-Ga2O3 lms are estimated by extrap-the linear portion of the square of absorption coefcientter post-annealing as shown in Fig. 1. The intensity ofeak decreases after Cu-doping. Diffraction peaks froms are not observed. It indicates that the Ga ions are sub-the Cu ions without changing the monoclinic structureal annealing at 800 C. The (111) peak (2 =35.59) ofion pattern of Cu-doped sample is slightly shifted toee (2 =35.62) as compared to the un-doped one. Itsmall variation in the lattice parameters occurs afterwhich results from the ion radius difference betweenCu in -Ga2O3 lms.ows the surface morphology of un-doped -Ga2O3 and-Ga2O3 lms post-annealed in N2 ambient at 800 Cgrain size of un-doped -Ga2O3 lm is bigger than thef Cu-doped -Ga2O3 lm. The grain growth of -Ga2O3dered by Cu-doping. An obvious deterioration of theon quality is observed after Cu-doping. The results arece with the XRD results.

    properties

    ows the optical transmission spectra of the -Ga2O3e and after annealing. The sample thickness is aboutshown in Fig. 3, the transmittance spectra of the un-have a high transmittance more than 83% (includingin the visible region and an excellent transmittance70% in the UV region (285400nm). Before annealing,ittance slightly decreases after doping. After post-the transmittance of the Cu-doped sample obviouslys in the whole optical region. This indicates that theis activated by post-annealing.ows that the average absorption of the Cu-doped -s is higher than the un-doped one before annealing.ling, the absorption was greatly decreased for the un-

    Fig. 4. Olms.

    dopedhave ais a nelm, weffectinear-egap ofsents tand thtors todefectsCu-dopare almof thein the

    TheolatingFig. 2. SEM images of un-doped (a) and Cu-doped (b) -Ga2O3 lms post-l absorption spectra of the as-deposited and post-annealed -Ga2O3

    le and increased for the Cu-doped sample. All the lmsannealed in N2 ambient at 800 C for 1h.

  • 848 Y. Zhang et al. / Materials Science and Engineering B 176 (2011) 846849

    Fig. 5. (hv)2 versus photon energy plots of the -Ga2O3 lms as-deposited (a) andpost-annealed in N2 ambient at 800 C for 1h (b).

    against photon energy using the equation.

    (h)2 = B(hv Eg) (1)Here B is a constant. Fig. 5(a) shows the (hv)2 versus pho-

    ton energy plots of the as-deposited -Ga2O3 lms. The un-doped-Ga2O3 lm has an optical band gap of 4.83 eV. This value is inagreement with the reported values 4.9 eV for un-doped -Ga2O3[16]. The optical band gap of the-Ga2O3 lms decreases to 4.73 eVafter Cu-doping. The decrement of the band gap is 0.1 eV, which isprobably calms.

    Fig. 5(b)Ga2O3 lmdramatic choptical bandever, the op4.88 eV. Thindicates thgreatly impbidden banobvious shrdoped-Galargely activacceptor en

    3.3. Photolu

    Fig. 6 shannealed

    Fig. 6. Room tfor 1h in N2 am

    un-doped-Ga2O3 lmshowsabroadUVBlueemission, very sim-ilar to that in Cu-doped -Ga2O3 lm. However, the Cu-dopingenhances the PL emission intensity. This emission band can bedivided into three Gaussian bands as shown in Fig. 6(a). Besidesthe commocerned. In tare equally406nm andtred at 498nCu-dopingpeak centre

    The emibination of[17]. The brated by thethe electronblue lumineband gapofground statsion woulddonor and aturedvia a texciton whthe donor avacancy pa

    e lumincrstallrigining paccen thet abo

    clus

    ycrysystalintri-Ged ost-atly dtionfor tand

    ue emd at 4

    wled

    s woof Chused by impurity energy level of Cu in the -Ga2O3

    shows (hv)2 versus photon energy plots of the -s post-annealed in N2 atmosphere at 800 C for 1h. Aange occurs after the post annealing treatment. Thegap of the Cu-doped-Ga2O3 shrinks to 4.65 eV. How-tical band gap of the un-doped -Ga2O3 increases to

    e appearance of the (111) peak in the XRD patternsat the crystal quality of the un-doped -Ga2O3 lms isrovedafter post-annealing. So thedefect state in the for-d is reduced and the optical band gap is expanded. Theinkage of the optical band gap of the post-annealed Cu-2O3 lm indicates that the impurityCuatomshavebeenated as acceptors in the-Ga2O3 lmand introduce theergy level on the top of the valence band.

    minescence spectra

    ows the room temperature PL spectra of the post--Ga2O3 lms excited at 280nm. As illustrated, the

    the blutrationthe cryin the oannealdefectlevels ipeak a

    4. Con

    PolThe crsize ofdopedCu-dopAfter pnicanabsorpgreatlycence band blcentre

    Ackno

    Thidationemperature PL spectra of the -Ga2O3 lms post-annealed at 800 Cbient excited at 280nm.

    of Shandonof ShandonProgram (J1

    References

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    209213n UV and blue emission, the green emission can be dis-he un-doped sample, both the UV and blue emissionsexcited. Three major emissions are centred at 376nm,425nm respectively, while one green emission is cen-m. Thesepeakpositions undergo a slight change for the

    sample. It is worth noticing that there is an additionald at 475nm for the Cu-doping sample.ssion peak in UV region is associated with the recom-the self-trapped excitons, which is an intrinsic processoad green band emission centred at 498nm is gener-radial recombination of a photo generated hole within ionized VO, VGa or VGa:VO and CuGa. Moreover, the

    scence is excitedwith photon energy slightly below the-Ga2O3,which suggests that the acceptor defectswithe close to the valence band are involved. The blue emis-originate from the recombination of an electron on ahole on an acceptor. An electron in a donor band is cap-unnel transferbyaholeonanacceptor to forma trappedich can emit the blue photon. These defects includend the acceptor, the acceptor contains galliumoxygenir (VO,VGa) and copper substituting gallium (CuGa

    ). So

    inescence is greatly enhanced when the CuGa concen-eases after Cu-doping. A lot of doped defects occur inization process and a large quantity of Cu atoms residesal Ga site of the Cu-doped-Ga2O3 lms in the thermalrocess. These impurity defects generally serve as deepptors in semiconductors and would induce new energyband gap [18]. As a result, a new emission band with a

    ut 475nm appears.

    ions

    talline -Ga2O3 lms are obtained by post-annealing.lization quality deteriorates after Cu-doping. The grainnsic -Ga2O3 lm is bigger than the grain size of Cu-a2O3 lm. The average absorption of the as-deposited-Ga2O3 lms is higher than that of the un-doped one.nnealing, the transmittance and optical band gap sig-ecrease for the Cu-doped -Ga2O3 lms. The averagedecreases for theun-doped-Ga2O3 lmsand increasesheCu-doped-Ga2O3 lms. The characteristic lumines-s appear in UV, blue and green spectral regions. The UVissions are enhanced and a new blue emission peak

    75nm appears for the Cu-doped -Ga2O3 lms.

    gements

    rk is supported by the National Natural Science Foun-ina (Grant No. 10974077), Natural Science Foundation

    g Province, China (Grant No. 2009ZRB01702), A Projectg Province Higher Educational Science and Technology0LA08).

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    Optical and structural properties of Cu-doped -Ga2O3 films1 Introduction2 Experimental3 Results and discussion3.1 Crystal structure and surface morphology3.2 Optical properties3.3 Photoluminescence spectra

    4 ConclusionsAcknowledgementsReferences

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