Remote hydrogen plasma doping of single crystal ZnO
Yuri M. Strzhemechny, Howard L. Mosbacker, David C. Look, Donald C. Reynolds, Cole W. Litton, Nelson Y.
小猪回家
Garces, Nancy C. Giles, Larry E. Halliburton, Shigeru Niki, and Leonard J. Brillson
Citation: Applied Physics Letters 84, 2545 (2004); doi: 10.1063/1.1695440
View online: dx.doi/10.1063/1.1695440
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Remote hydrogen plasma doping of single crystal ZnO
Yuri M.Strzhemechny a)
Center for Materials Rearch,The Ohio State University,Columbus,Ohio 43210
Howard L.Mosbacker
Department of Physics,The Ohio State University,Columbus,Ohio 43210
David C.Look
Semiconductor Rearch Center,Wright State University,Dayton,Ohio 45435
and Air Force Rearch Laboratory,AFRL/MLPS,Wright-Patterson Air Force Ba,Ohio 45435
Donald C.Reynolds and Cole W.Litton
Air Force Rearch Laboratory,AFRL/MLPS,Wright-Patterson Air Force Ba,Ohio 45435
Nelson Y .Garces,Nancy C.Giles,and Larry E.Halliburton
Department of Physics,West Virginia University,Morgantown,West Virginia 26506
Shigeru Niki
AIST,1-1-1Umezono,Tsukuba,Ibaraki 305-8568,Japan
Leonard J.Brillson
Department of Electrical Engineering,Department of Physics and Center for Materials Rearch,The Ohio State University,Columbus,Ohio 43210
͑Received 5January 2004;accepted 9February 2004͒
We demonstrate that remote plasma hydrogenation can increa electron concentrations in ZnO single crystals by more than an order of magnitude.We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I 4for a variety of ZnO single crystals–bulk air annealed,Li doped,and epitaxially grown on sapphire.Hydrogen increas I 4intensity in conducting samples annealed at 500and 600°C and partially restores emission in the I 4range for Li-diffud ZnO.Hydrogenation increas carrier concentration significantly for the mi-insulating Li doped and epitaxial thin film samples.The results indicate a strong link between the incorporation of hydrogen,incread donor-bound exciton PL emission,and incread n -type conductivity.©2004American Institute of Physics.͓DOI:10.1063/1.1695440͔
ZnO has attracted considerable attention from the mi-conductor community in recent years as a promising material for important new applications,yet many fundamental ques-tions on the nature of its conductivity remain open.Thus,high-quality p -type ZnO with reproducible properties re-mains a contested goal.Similarly,theory 1–3suggests hydro-gen is a shallow donor impurity rather than a compensating center,a prediction supported by a number of spectroscopic studies.4–14Some electrical measurements 12–18are also con-sistent with H donor character,although the polycrystalline samples,direct plasma exposure,or the ion implantation in-volved may introduce additional complications.This letter prents strong transport and spectroscopic evidence in favor of the hydrogen shallow donor hypothesis.
In our studies,we employ a remote hydrogen plasma treatment of the surface of ZnO.The main advantage of this approach,as oppod to the direct plasma treatment tech-niques,is a parate control and measurement of the tem-perature and pressure at the free surface of the specimen.Therefore,chemically driven changes occur without major thermal variations.
黑洞与时间弯曲Recently,we described the effects of remote hydrogen plasma on the optoelectronic properties of high-quality
single-crystalline ZnO.19,20We obrved,among other ef-fects,that H plasma induces a relative intensity increa in the luminescent features often associated with shallow do-nors.Thus,in the low temperature ͑LT ͒photoluminescence ͑PL ͒spectra,the intensity of the peak commonly designated as I 4͑photon energy ϳ3.363eV at 4K ͒appeared to grow after exposure to the H plasma.Several authors have argued in favor of attributing the I 4to a neutral donor bound exciton ͑BEx ͒.4,21–25It ems very likely that the I 4-related donors are the shallow donors responsible for the predominantly n -type conductivity in otherwi undoped material.The na-ture of the shallow donors is the primary focus of this letter.
Here,we demonstrate that H-plasma processing intro-duces changes in both the electrical and optical properties of ZnO that are consistent with the introduction of new shallow donors.Specifically,we show that H-plasma-induced in-creas in BEx luminescence in ZnO correlate with increas in free carrier concentrations from the Hall effect measure-ments.
The first t of samples included three single crystal ZnO specimens grown by a chemical vapor transport ͑CVT ͒21at Eagle-Picher Technologies.They were cut and polished nor-mal to the crystallographic c axis and later annealed in air for 30min at 500,600,and 700°C.Results from a similar t of annealed samples appeared previously 23,25and showed that,
a ͒
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subquent to annealing,the BEx region in the LT PL trans-formed,with the emission intensity shifting towards lower-energy BEx peaks.In particular,the I 4intensity decread significantly as annealing temperature incread.The prent study reproduced the BEx results.
The three annealed specimens were expod at room temperature to the remote hydrogen plasma produced by an inductive coupling with a rf generator.The remote plasma was created from hydrogen gas with a pressure of 13mTorr,flow rate 3sccm,employing rf power of 40W and the samples at room temperature.A more detailed explanation of the experimental tup appears elwhere.26Figure 1shows spectra of the BEx regions for the three samples before and after.In two of the three samples after hydrogenation ͑A ͒and ͑B ͒,the I 4emission line remarkably regains most of the intensity lost due to thermal treatments.Only a modest in-crea in I 4as a shoulder is evident for the specimen an-nealed at 700°C ͑C ͒following H plasma.
Temperature-dependent Hall measurements were per-formed on the three air-annealed bulk samples before and after hydrogenation.For experimental details,e Ref.21.No conspicuous change was recorded for either concentra-tion or mobility of the free charge carriers.An exception was the 700°C specimen—it showed low-temperature anomalies that a subquent H-plasma treatment removed.Given the small—tens of nanometers—penetration depth of the H-plasma ions in the ZnO ͓estimated from our condary-ion mass spectroscopy ͑SIMS ͒data,e also Ref.27͔compared to the thickness of the bulk wafer,the abnce of bulk effects is not surprising.In order to detect such Hall variations,one requires either a reduced initial bulk carrier concentration or a reduced thickness
of the conductive layer comparable to the diffusion depth of hydrogen species.We report here the results of both approaches using,respectively,bulk CVT-grown ZnO rendered mi-insulating via Li diffusion and using an epitaxial thin film of ZnO on sapphire.Figure 2prents LT PL results for the CVT Eagle-Picher sample that was Li-diffud and then hydrogenated.
The Li diffusion was carried out by placing the sample on top of a 2-cm-high open quartz tube,placing LiOH powder in the bottom of the tube,and then heating the whole asm-bly in a furnace at 800°C.The as-received ZnO exhibits strong excitonic features,including a pronounced I 4peak.Li doping removes this I 4line.Subquent hydrogenation in-creas and broadens the BEx features,creating a conspicu-ous shoulder where the I 4is expected.Prior to hydrogena-tion,no Hall measurements were possible,as expected for the mi-insulating Li-doped ZnO crystal.Hydrogenation re-stores measureable conductivity and Hall coefficient,as shown in Fig.3.Here,only sheet carrier concentration n can be plotted,since the electrical thickness is not known.However,the rather weak temperature dependence of n suggests that the donor concentration is well above 1017cm Ϫ3,the value found before Li diffusion.This obr-vation is consistent with a new donor being added by
the
FIG.1.PL results for the air-annealed CVT samples in the near band edge region,T ϭ10K,incident power ϭ0.2W/cm 2.The BEx peaks shown for the as-received CVT material are commonly obrved.͑A ͒After anneal in air for 30min at 500°C,the relative intensity of the I 4line (E ϭ3.363eV)is reduced.1h of remote H-plasma treatment partially restores the intensity of I 4.͑B ͒After anneal in air for 30min at 600°C,the I 4line is not discernible.2h of hydrogenation bring back the I 4.͑C ͒After anneal in air for 30min at 700°C,the I 4line is not discernible,3h of hydrogenation restore a small shoulder around 3.363eV
.
FIG.2.PL results for the Li-doped CVT sample in the near band edge region,T ϭ10K,incident power ϭ0.2W/cm 2.After Li-diffusion treatment,the I 4line is suppresd.Subquent H-plasma treatment broadens the bound-exciton emission and restores a shoulder at ϳ3.364eV
.
FIG.3.Sheet carrier concentration for the Li-doped CVT sample after hy-drogenation.Hall parameters were unobrvable before H-plasma treatment since Li diffusion makes ZnO mi-insulating.
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hydrogenation,rather than an alternative explanation in which the Li would simply deplete near the surface and ‘‘re-expo’’the existing donors.Indeed,SIMS measurements of the average surface Li concentration show no such depletion.Furthermore,the mobility is lower than that before Li diffu-sion,which is consistent with a larger donor concentration as well as the additional acceptors introduced by the Li.
Finally,we hydrogenated a thin film of ZnO grown on sapphire by molecular beam epitaxy.28Figure 4compares its free temperature-dependent Hall carrier concentration plots before and after remote H-plasma treatment.Here,the sig-nificant ri in free electron concentration indicates an order of magnitude increa in the number of donors created by hydrogenation.Electron mobility ͑not shown ͒a
lso increas after hydrogenation in most of the temperature range.Unfor-tunately,the excitonic PL emission ͑not shown ͒is veral meV wide and does not reveal very sharp features such as I 4either before or after H-plasma treatment.Hydrogen,how-ever,induced obrvable changes in other parts of the PL spectrum,such as a passivation of the ‘‘green’’band,a higher intensity of the violet/near UV emission,and a suppression of a free exciton recombination,consistent with our previous reports.19,20
In summary,we have shown that the remote hydrogen plasma treatment of ZnO is an effective tool to control its transport and optoelectronic properties.The strong depen-dence of the Hall parameters and the I 4BEx luminescence on the prence of hydrogen,described above,indicates a convincing connection between the shallow donor and a hydrogen-related impurity.
This work is supported by the Office of Naval Rearch
͑C.Wood ͒,the National Science Foundation ͑V .Hess ͒,the Air Force Office of Scientific Rearch ͑G.Witt ͒,the Depart-ment of Energy ͑J.Zhu,optical studies ͒,and the Air Force Rearch Laboratory ͑J.Brown ͒.The authors would like to thank G.Lucovsky for uful discussions and plasma hardware provided,and T.A.Cooper,W.Rice,and J.E.Hoelscher for technical assistance.
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FIG.4.Hall results for the epitaxial film on sapphire.Remote hydrogen plasma treatment increas free electron concentration by at least an order of magnitude in the entire range of temperatures.
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