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APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 25 17 DECEMBER 2001n-type conductivity by controllable extrinsic doping. Unfor-tunately, however, ZnO is naturally only n-type due to thepresence of native defects making acceptor doping of ZnOdifficult.

Yamamoto et al. proposed, however, a acceptor dopingmethod, donoracceptor co-doping based upon first-principle calculations.4 They reported two main conclusions.First, nitrogen ~N! incorporation on oxygen ~O! sites in ZnOincreases the Madelung energy, causing localization of the Nlevels. Second, codoping with aluminum ~Al!, gallium ~Ga!,or indium ~In! enhances the incorporation of N and the for-mation of 2NGa complex in ZnO decreases the Madelungenergy, making p-type doping of ZnO possible.

Based on these predictions, Joseph et al. recently re-ported that p-type ZnO films with low resistivities can befabricated by pulsed laser deposition using Ga and Ncodoping.5 However, they used glass substrates, and conse-quently their ZnO films were polycrystalline with low carriermobilities. To make ZnO-based semiconductor optoelec-tronic devices, epitaxial p-type ZnO with good crystallinityis needed.

We have previously reported on the growth of high qual-

using RS-MBE, the results reported here indicated that theGa and N co-doping is fraught with serious problems.

Samples were grown by RS-MBE. The machine con-figuration is described in Ref. 9. Source materials were el-emental Zn ~7N!, elemental Ga ~7N!, oxygen gas (6N, O2),and nitrogen gas (6N, N2). Metal sources were supplied viaconventional Knudsen cells, and gas sources were separatelysupplied via rf RS cells. Both the O2 and N2 gas flow rateswere 0.3 sccm with a rf power 300 W. The operating condi-tions of the RSs were fixed in all experiments.

We used both sides polished (1120) plane sapphire assubstrates. Both of the substrate treatment methods beforegrowth and the growth conditions used in this letter havebeen reported in Ref. 9. As discussed in the same reference,a high Tsub ~600 C! is effective in reducing the residual car-rier concentration of undoped ZnO films, and therefore dop-ing. Experiments were performed using Tsub5600 C.

A two-layer structure was fabricated for all films in orderto facilitate currentvoltage (I V) measurements. An un-doped ~n-type! layer 0.50.6 mm thick was first grown, andthen the sample was removed from the vacuum chamber anda Inconel shadow mask was affixed. Subsequently, thesample was moved back to the growth chamber and thedoped layer was grown. I V measurements were then madeInteractions between gallium and nitroby radical-source molecular-beam epi

K. Nakaharaa) and H. TakasuOptical Device Research and Development Division, ROHKyoto 615-8585, Japan

P. Fons, A. Yamada, K. Iwata, K. Matsubara, R.Optoelectronics Division, Electrotechnical Laboratory, 1-1

~Received 22 January 2001; accepted for publication

It has been recently predicted that the co-doping of agallium, indium! in a 2:1 ratio will dope ZnO p-typmaking the nitrogen acceptor energy level more shnitrogen co-doped ZnO films by radical-source monitrogen radicals supplied via rf radical source cells. Ddonor acceptor pair-like photoluminescence emissiongrown on an undoped ZnO buffer layer. However, Halwas n-type. Formation of a non-ZnO phase in the saspectroscopy and x-ray diffraction measurements. Zsharply by two orders of magnitude in going from thZnO. X-ray diffraction measurements indicated theInstitute of Physics. @DOI: 10.1063/1.1424066#

ZnO has a 3.37 eV room-temperature direct band gapand has also attracted attention as a useful material for UVoptoelectronic applications. The large excitonic binding en-ergy ~60 meV! of ZnO also raises the interesting possibilityof utilizing excitonic effects in room-temperature devices. Infact, room-temperature optically pumped UV emission fromZnO films has been reported.13

In order to realize ZnO-based semiconductor devices, ita!Electronic mail: Ken.Nakahara@dsn.rohm.co.jp

4130003-6951/2001/79(25)/4139/3/$18.00Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tgen dopants in ZnO films grownxy

Corporation Limited, 21 Mizosaki-cho Saiin,

unger, and S. NikiUmezono, Tsukuba, Ibaraki 305-8568, Japan

6 September 2001!

acceptor ~nitrogen! and a donor ~aluminum,due to a reduction in the Madelung energyllow. We have been growing gallium andcular-beam epitaxy by use of oxygen andode-like currentvoltage characteristics andre observed for a Ga and N doped ZnO filmeasurements revealed that the conductivity

ple was confirmed by secondary ion massand Zn1O secondary ion intensities fell

undoped ZnO layer to the highly co-dopedrmation of ZnGa2O4. 2001 American

ity undoped ZnO films with high carrier mobilities and lowresidual electron concentrations grown on sapphire sub-strates by radical-source molecular-beam epitaxy~RS-MBE!.69 There has been no detailed reports on the be-havior and mechanisms of N or Ga1N doping into singlecrystalline ZnO films, and thus, we report here on the behav-ior and the complications associated with Ga and N co-dopedZnO films grown by RS-MBE. Although the initial motiva-tion for the current work was to grow epitaxial p-type ZnOusing In contacts between the undoped layer and the doped

9 2001 American Institute of Physicso AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

layer. Beam fluxes were measured by a nude ion gauge ro-tated into the sample position. The Zn flux was fixed in therange of 1 231026 Torr, and the Ga flux was varied in therange of 231029 231027 Torr.

Absolute N and Ga concentrations were determined bysecondary ion mass spectroscopy ~SIMS! measurements. AllSIMS data were calibrated by simultaneous measurements ofGa or N ion implanted undoped ZnO layers.

Experiments in which films were doped with only Nwere carried out before Ga and N co-doping. Figure 1 showsa SIMS measurement of a N doped ZnO film grown at Tsub5600 C. The peak at the interface between the N doped andundoped ZnO films is due to a SIMS matrix effect caused byair exposure after growth of the undoped buffer layer asmentioned, and thus irrelevant to the discussion here. The Nconcentration in the ostensibly N doped and undoped layersis unchanged, and the signal levels indicate that the N con-centrations are below the SIMS sensitivity limits(

layer. As the Zn and Zn1O ion intensities would not beexpected to fluctuate if the matrix remained ZnO, this im-plies the presence of a non-ZnO phase in the highly co-doped ZnO film.

The presence of the additional non-ZnO phase was alsoconfirmed by XRD measurements. An v-2u scan of the co-doped ZnO film exhibited peaks from other than ZnO andsapphire. The peak positions from the phase could be in-dexed as the spinel ZnGa2O4 phase,10 suggesting that thenon-ZnO phase is ZnGa2O4. As ZnGa2O4 is a semiconductorwith a ;5 eV band gap,10 it is doubtful that the presence of

4141Appl. Phys. Lett., Vol. 79, No. 25, 17 December 2001 Nakahara et al.peak position blueshifted with increasing excitation power~Fig. 4!. The I V and PL results suggests the presence ofacceptor levels. However, these results alone are insufficientto unambiguously determine the conductivity type of thesample.

In order to determine the conductivity type of the co-doped sample, Hall measurements were carried out. Thesemeasurements showed that the sample was n type with acarrier concentration of 1.9831019 cm23 and a mobility of17.8 cm2 V21 s21. However, SIMS measurements and x-raydiffraction ~XRD! measurements indicated the complicatingfactors, specifically the presence of an additional phase.

SIMS measurements ~Fig. 5! showed that Zn and Zn1Oion intensities sharply decreased by two orders of magnitudatthe interface between the co-doped layer and the undoped e

FIG. 4. PL measurements for a highly co-doped sample. ~a! Excitationpower density (;8.831022 W cm22) and ~b! excitation power density~;88 W cm22!.

FIG. 5. SIMS depth profile of ~a! Ga concentration and Zn secondary ionintensity and ~b! nitrogen concentration and Zn1O secondary ion intensity.Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tthis phase could explain either the PL or diode-like I Vbehavior observed. Hence, the presence of the additionalphase casts some doubt on the validity of the Hall effectresults and makes it impossible to unambiguously determinethe conductivity type of the co-doped ZnO film.

In conclusion, N doped and Ga1N co-doped ZnO filmswere grown by RS-MBE. While N did not incorporate inZnO films grown at 600 C, the presence of Ga enhanced Nincorporation, consistent with the co-doping theoretical pre-dictions. The as-grown films display some aspects reminis-cent of p-type conductivity, namely diode-like I V behaviorand DAP-like PL emissions, however, from SIMS and XRDmeasurements, the formation of a non-ZnO phase was con-firmed, which is identified as ZnGa2O4. The presence of theadditional phase makes unambiguous determination of car-rier type difficult and will undoubtably place severe restric-tions on the practical utilization of the co-doping technique.

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6 P. Fons, K. Iwata, S. Niki, A. Yamada, and K. Matsubara, J. Cryst. Growth201, 627 ~1999!.

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10 T. Omata, N. Ueda, K. Ueda, and H. Kawazoe, Appl. Phys. Lett. 64,1077 ~1994!.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp