Source: http://ufdc.ufl.edu/UFE0041602/00001
Timestamp: 2019-04-19 23:08:59+00:00

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Genre: Materials Science and Engineering thesis, Ph.D.
Abstract: SYNTHESIS AND CHARACTERIZATION OF SILVER DOPED ZINC OXIDE THIN FILMS FOR OPTOELECTRONIC DEVICES The synthesis and properties of Ag-doped ZnO thin films were examined. Epitaxial films of 0.6 at.% Ag doped ZnO grown at moderately low temperatures (300 masculine ordinalC to 500 masculine ordinalC) by pulsed laser deposition yielded p-type material as determined by room temperature Hall measurements. Carrier (hole) concentrations ranging on the order mid-1015 cm-3 to mid-1019 cm-3 were realized. Growth at higher temperatures yielded n-type material, suggesting that the Ag substitution yielding an acceptor state is metastable. Photoluminescence measurements showed strong near-band edge emission with little to no mid-gap emission. The stability of the Ag-doped films was examined as well. Persistent photoconductivity was observed. ZnO buffer layers drastically improved the surface morphology of films thicker than 1.0 micron. Photoluminescence studies showed that Ag inclusion resulted in smaller non-radiative relaxation rates over surface states, which lead to UV emission enhancement. Room temperature PL measurements also showed a suppression of ZnO visible luminescence suggesting that Ag does not occupy interstitial sites or an antisite. Low temperature and temperature dependent PL spectroscopy revealed strong and dominant emissions originating from free electron recombination to Ag-related acceptor states around 3.31eV. The Amasculine ordinalX emission at 3.352 eV was also observed at low temperatures. Enhancement of the PL intensity with increasing grain size was observed. The nature of the acceptor related emissions was confirmed. The acceptor energy was estimated to be 124 meV. Weak deep level emission at low temperatures indicated that in the p-type ZnO:Ag native donor and acceptor defects are suppressed suggesting the observed acceptor related PL emissions and hole concentration are from the Ag in ZnO instead of native defects. High temperature ZnO buffers and lattice matched MgCaO buffers helped improve the UV emission of the Ag doped films. The room temperature PL spectrum of Ag-doped ZnO was compared to that of undoped, P-doped, Ga-doped, and Ag-Ga- codoped ZnO. The Ag-doped ZnO films showed superior optical properties. Finally, the fabrication and properties of rectifying Ag-doped ZnO/Ga-doped ZnO thin film junctions were reported. A rectifying behavior was observed in the I-V characteristic, consistent with Ag-doped ZnO being p-type and forming a p-n junction. The turn on voltage of the device was 3.0 V under forward bias. The reverse bias breakdown voltage was approximately 5.5 V. The highest light emission output power measured was 5.2x10-8 mW. At excitation currents of 10 mA, the applied voltage was approximately 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conduction and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag-doped ZnO on bottom, Ga-doped ZnO on top) did not result in rectifying I-V characteristics. The reason for this is unclear but may relate to the differing growth temperatures used for the two layers.
Statement of Responsibility: by Fernando Lugo.
Local: Adviser: Norton, David P.
4 ACKNOWLEDGMENTS I would like to thank to my advisor, Dr David P. Norton, for his guidance patience, and unconditio nal support throughout my undergraduate and graduate school years. I thank every committee member, Dr Stephen J. Pearton, Dr. Fan Ren, and Dr. Franky So for their advice and support on my research. A special thanks to Dr. Brent Gila for countless hours o f labor and advice in setting up photoluminescence equipment, and Dr. Cammy R. Abernathy for trus ting me with her lab equipment I would like to express a more than special thanks to all the group members in Dr. Erie, Dr. Yuanjie Li, Dr. Seemant Rawal. Dr. Li Chia Tien, Dr. Hyun Sik Kim, Dr. Patrick Sadik, Dr. Daniel Leu, Dr. Charlee Cal lender, Ryan Pate Joe Cianfrone, Seon Hoo Kim, and Kyeong Won Kim. It has truly been a pleasure to meet them and share 6 years o f research with every one of them. I am especially grateful to my collaborato Yu Lin Wang. They were kind enough to give some of their time to help me fabricate LED devices. I would also like to thank Ritesh Das, Andrew Gerge r, Galileo Sarasqueta and Sergey Maslov for their unconditional help with device fabrication and characterizations. This project would have been impossible to finish without their help. I must ackn owledge the Board of Eductation the Naval Office of Resear ch and Scientific Development, the National Scienc e Foundation and the College of Engineering at The University of Florida for their financial support and fellowships. My deepest gratitude goes to my roommates and ultimate club teammates. They provided me and still continue to provide their unconditional friendships and support over many years of training and competition They gave me the joy and privilege to compete and represent the University of Florida at the highest stage in ultimate.
5 Finally, I would like to express my deepest appreciation and love to my parents brother and sister for their infinite love and support Simply, there are no words to express my undying admiration.
14 characteristics. The reason for this is unclear but may relat e to the differing growth temperatures used for the two layers.
15 CHAPTER 1 INTRODUCTION In recent years, the market of electronic devices that source, detect, and control light has grown rapidly. Light emitting d iodes (LEDs) and laser advancements have significantly contributed to this rapid growth. Nitride based devices, in particular, have entered the communication, display, traffic signal, and automotive industry. In the near future, white LEDs are expected to develop as a major market replacing incandescent and fluorescent lamps in general lighting applications. The GaN semiconductor system has dominated the solid state lighting field for approximately two decades. The need for short wavelength photonic devices high power, and high frequency electronic devices in addition to the high quality synthesis of GaN has established its dominance. ZnO, however, has gained substantial interest in part because of its advantages over GaN and thus i s considered an alternati ve material. Initially, ZnO was studied for its polycrystalline properties and applications to facial powders, varistors, piezoelectric transducers, and transparent conductive films. Lately, however, large area bulk ZnO growth has been achieved , and e pitaxial thin film growth optimized . Hence, the motivation for renewed focus on ZnO photonics research. ZnO has several advantages over GaN: ZnO has an exciton binding energy of 60 meV, while that of GaN is only 26 meV. This large binding energy is of particular interest because its excitonic emission may be used to obtain lasing action above room temperature . ZnO is available in large area bulk wafers while no bulk wafers are available for GaN. Single crystal growth by seed vapor phase (SVP) is the method used to fabricate commercially available 2 inch wafers . High quality homo epitaxial ZnO growth is possible using these native substrates. Thus, concentrations of dislocations and point defects due to lattice mismatch are relatively low in Zn O films compare to GaN .
17 doping on ZnO thin film prop erties grown by pulsed laser deposition (PLD). In addition, it addresses the fabrication of rec tifying junctions, and p MOS devices.
19 lasing and stimulated emission at temperature up to 550 K, establishing ZnO as an interesting photonic semiconducting oxide . The synthesis of heavily doped n type ZnO is easily accomplished via group III cation doping. However, the control over dopants and defects that may lead to high quality and robust p type still remains a major challenge to the fabrication of practical devices. 2.2.1 Undoped ZnO and Its Native D efects Undoped ZnO normally exhibits n t ype conductivity. The role of native defects such as vacancies (V O and V Zn ), interstitials (Zn i and O i ), and antisites (Zn O and O Zn ) in undoped ZnO is not yet clearly understood. Several studies [15 17] claim that such native defects create shallow donor s tates. D.C. Look et al suggested that Zn i rather than V O are the main cause for n type conductivity, acting like shallow donors, in ZnO . However, more recent theoretical and experimental studies [19 24] argue that Zn i are unstable and diffuse at room temperature, while V O are deep compensating defects not responsible for the n type material. These studies suggest that hydrogen and group III elements impurities are more likely to be responsible for the intrinsic conductivity in ZnO. Theoretical work by Van de Walle  showed that interstitial H is a shallow donor in ZnO. This was confirmed by experimental results  that showed a three orders of magnitude increase in conductivity in ZnO films when grown in H 2 by pulsed laser deposition (PLD). Second ary ion mass spectroscopy (SIMS) analysis revealed Ca H complexes, where Ca donates an electric charge to a neighboring O atom that traps a H atom, allowing it to act as a shallow donor. Another first principle study  demonstrated that H can substitute an O atom an d act as a shallow donor as well.
23 Capacitance Voltage (CV) measurements showed that Zn 0.9 Mg 0.1 O can be made p type using P 2 O 5 as the phosphorus source, but p type ZnO was not achieved . The lack of reproduci bility, carrier type changes and lattice constant relaxation over time has raised doubts about the validity of the reports of p type ZnO. In addition, the apparent p type conductivity may be the result of interface and near surface states  and/or inhom ogeneous samples . Therefore, a better understanding of the physical properties of point defects may be useful. Silver Doping In comparison to the group V elements, studies on group IB dopants, namely Cu or Ag, in ZnO have been rather limited [66 68]. Early reports argued that Ag substitution in ZnO forms a deep acceptor state 0.23 eV below the bottom of the conduction band . However, recent studies suggest this may not to be the case. One study reported an acceptor state binding energy for the Ag 3d 10 states of only 200 meV . Another study of the behavior of Ag in bulk ZnO suggests that Ag acts as an amphoteric dopant, yielding an acceptor state for substitution on the Zn site, and a donor state for interstitial defects . First principles c alculations have examined the dopant energy levels and defect formation energies for group IB elements in ZnO . The calculations estimate the acceptor state ionization energies for substitutional Ag, Cu, and Au to be 0.4, 0.7, 0.5 eV, respectively. Alt hough these represent relatively high ionization energies, the formation energies for these substitutional defects (Cu Zn Ag Zn and Au Zn ) are predicted to be low; energies for interstitial defects are predicted to be high. These calculations suggest that s olubility and self compensation may be less of an issue for group IB elements as compared to the group V dopants.
24 Within the group IB elements, Ag has the lowest predicted transition energy (0.4 eV) , reflecting a weaker p d orbital repulsion as compar ed to Cu or Au. This weak repulsion is rooted in the large size and low atomic d orbital energy of Ag. Interestingly, the O rich conditions that have been suggested for preventing oxygen vacancy (V O ) and/or Zn interstitial (Zn i ) defects are consistent with the required conditions for substituting Ag onto the Zn site. A few groups have experimentally examined the properties of Ag doped ZnO. H. S. Kang et al. have reported the formation of p type ZnO via Ag doping in thin films grown by pulsed laser depositio n . The formation of p type material was limited to deposition temperatures of 200 250C. Studies on Ag implanted ZnO suggest that Ag substitution on the Zn site becomes unstable at temperatures greater than 600C . This is consistent with the esti mated 0.08 mol% bulk solid solubility of Ag in ZnO . 2.3 ZnO Band Gap Engineering and Devices As mentioned before, p type ZnO must be accomplished in order to fabricate practical devices and therefore the vast majority of research has been dedicated to its realization. However, another important step in realizing ZnO based optoelectronic devices is bandgap modulation and indeed it has been demonstrated by Mg [74 79] and Cd [80 86] alloying. 2.3.1 Bandgap Engineering The band gap energy of a tern ary alloy AxZn1 xO (where A = Mg or Cd) is given by the following equation : Eg(x) = (1 x)E ZnO + xE AO b x(1 x) (1) Where b is the bowing parameter and E AO and E ZnO are the band gap energies of compounds AO and ZnO, Respectively.
26 luminescence (EL) was observed under reve rse bias; however, the blue white light under forward bias was clearly seen even with naked eyes in the dark. EL and PL measurements from the ZnO based device are shown in F igure 2 3 Similar results were obtained for ZnO films deposited on GaAs substrate [ 89]. A rsenic diffused from GaAs substrate was used to dope ZnO p type, while ZnO:Al was used as the n type layer. EL measurements revealed an emission peak centered at ~ 2.5 eV and a weaker shoulder at ~ 3.0 eV. Later, Sun et al.  reported EL emissions centered at 3.2 and 2.4 eV under forward biased for films doped with N and Ga as p type and n type dopants, respectively. EL spectrum of the device under a direct forward bias current of 40 mA at room temperature and IV characteristics is shown in Figure 2 4. The UV emission reported was comparable with the visible emission in the EL spectrum, which is a significant step forward in the performance of ZnO homojunction LEDs. ZnO heterostructure devices In the absence of reliable p type ZnO resea rch groups, in an effort to exploit ZnO many advantages, have spent a great deal of attention making heterostructure devices. When Sun et al  used Cu 2 O and ZnO substrate as p and n layers respectively, measurements revealed EL in both forward and rever se biases. Later, Tsurkan et al.  used p type ZnTe on n ZnO substrates varying carrier concentrations for each layer. Although, strong EL emissions were observed for all carrier concentrations under forward bias, the EL spectrum was dominated by differ ent emission bands as the result of carrier diffusion from low to high resistive layers. Other materials such as Si , GaN , AlGaN , SrCu 2 O 2 , NiO , CdTe , and SiC  have been used with n type ZnO to create useful heterostructure devices.
28 room temperatures using n type Si substrate s and 100 nm thick ZnO. High fi e l d effect mobilities of 1.2 cm 2 /Vs and I on / I off ratio o f 1.6x10 6 with drain current greater than 10 5 A was achieved. The charge accumulation transistor curves are shown in Figure 2 8. Masuda et al. [104 ], succeeded at fabricating ZnO TFTs by pulsed laser deposition on glass substrates. A double layer insulato r (SiO 2 + SiN x ) was used to obtain an I on /I off ratio of 10 5 and an optical visible transmittance of more than 80%. The transistor curves for the double insulator TFTs are shown in Figure 2 9. Many other studies [ 105 112 ] have reported successful fabricatio n of ZnO TFTs grown at both low and room temperature in a variety of substrates such as amorphous glasses, plastics or metal foil. More reliable and efficient ZnO TFTs are expected to be fabricated in the near future, making invisible electronics possible.
31 Figure 2 3 The PL spectrum, EL spectrum, and EL image of the ZnO light emitting device Reprinted with perm ission from Figure 4 of H.W. Liang, Q.J. Feng, J.C. Sun, J.Z. Zhao, J.M. Bian, L.Z. Hu, H.Q. Zhang, Y.M. Luo and G.T. Du, Semicond. Sci. Technol. 23, (2008) 025014 Figure 2 4 ZnO p n homojunction a) EL spectrum under forward current injection and b) r oom tem perature I V characteristic Reprinted with permission from Figure 4 of J.C. Sun, H.W. Liang, J.Z. Zhao, J.M. Bian, Q.J. Feng, L.Z. Hu, H.Q. Zhang, X.P. Liang, Y.M. Luo, G.T. Du Chemical Ph ysics Letters 460 (2008) 548.
32 Figure 2 5. EL spectrum o f an n ZnO/p GaN heterostructure Reprinted with permission from Figure 4 of Y. I. Alivov, J. E. Van Nostrand, D. C. Look, M. V. Chukichev, and B. M. Ataev, Appl. Phys. Lett. 83 (2003) 2943. Figure 2 6 EL spectra of n ZnO/ p Al0.12Ga0.88N heterostruct ure LED at 300 K and 500 K Reprinted with permission from Figure 4 of Y. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83 (2003) 4719.
33 Figure 2 7. Room temperatur e spectral photoresponsivity of the n ZnO/ p SiC photodiode illuminated both from the ZnO and 6H SiC (inset) sides for various reverse biases Reprinted with permission from Figure 3 of Y. I. Alivov, and H. Morko, Appl. Phys. Lett. 86 (2005) 241108. Figure 2 8. ( a ) is a set of transistor curves of drain current ( I d ) vs source drain voltage ( V d ) at gate v oltages ( V g ) between 0 and 50 V for a ZnO TFT The corresponding transfer characteristic, I d vs V g at a fixed Vd equal to 20 V, for the same ZnO TFT is shown in ( b ) Reprinted with permission from Figure 3 of P. F. Carcia, R. S. McLean, M. H. Reilly, and G Nunes, Appl. Phys. Lett. 82 (2003) 1117.
35 CHAPTER 3 MATERIALS AND CHARAC TERIZATION TECHNIQUE S The synthesis and characterization of Ag doped ZnO thin film s will be discussed in this chapter. Pulsed laser deposition (PLD) was employed to grow films with thickness ranging from 250 nm to 1.0 m. In order to find optimal doping conditions, oxygen partial pressures (PO 2 ), temperature, and doping levels were varied systematically. To better understand the doping effects on electrical properties Hall Effect measurements were performed, while the optical properties were studied by photoluminescence (PL) measurements. Scanning Electron Microscopy (SEM), powder and high resolution X ray diffraction we re used to investigate the microstructure of the films The surface morphology of the films was chara cterized by Atomic Force Microscopy (AFM). 3.1 Thin Film Synthesis 3.1.1 Pulsed Laser Deposition (PLD) Pulsed laser deposition (PLD) is a common thin film growth technique used in research studies because it allows for growths ranging from 25 C to 1000 C in nearly any desirable background gas. It consists of high power energy pulses that evaporate material from a target surface producing a plasma or plume of atoms, ions, and molecules. The ablated material then condenses on a substrate positioned opposi te to the ablation target, forming a thin film with the same composition as the target. Figure 3 1 shows a schematic of the PLD system (growth chamber and laser) used in this research. The quality of the films strongly depends on the laser wavelength, stru ctural and chemical composition of the ablation target and background gas, chamber pressure, and substrate temperature and distance to ablation target.
39 crystal quality and symmetry was obtained, from omega and phi scans, using t he Philips 3.2.4 Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) is a tool used to characterize the surface topology and morphology of thin films. It consists of a cantilever with a sharp tip at its end t hat is used to scan the sample surface (Figure 3 4) In this research, tapping mode AFM (Digital Instruments Dimension 3100) was employed to map out the topology of the ZnO film surface. In tapping mode, the cantilever oscillates near its resonanc e frequency. The oscillation amplitude decreases or increases due to interaction of forces acting on the cantilever as the AFM tip comes close the sample surface; thus, imaging the force of the oscillating contacts of the tip with the sample surface.
44 defects [ 7 1 ]. First principles calculations have examined the dopant energy levels and defect formation energies for group IB elements in ZnO [66 ]. The calculations estimate the acceptor state ionization energies for substitutional Ag, Cu, and Au to be 0.4, 0.7, 0.5 eV, respectively. Although these represent relatively high ionization energies, the formation energies for these substitutional defects (Cu Zn Ag Zn and Au Zn ) are predicted to be low; energies for interstitial defects are predicted to be high. These calculations suggest that solubility and self compensation may be less of an issue for gr oup IB elements as compared to the group V dopants. Within the group IB elements, Ag has the lowest predicte d transition energy (0.4 eV) [66 ], reflecting a weaker p d orbital repulsion as compared to Cu or Au. This weak repulsion is rooted in the large siz e and low atomic d orbital energy of Ag. Interestingly, the O rich conditions that have been suggested for preventing oxygen vacancy (V O ) and/or Zn interstitial (Zn i ) defects are consistent with the required conditions for substituting Ag onto the Zn site. A few groups have experimentally examined the properties of Ag doped ZnO. H. S. Kang et al. have reported the formation of p type ZnO via Ag doping in thin films grown by pulsed laser deposition [71 ]. The formation of p type material was limited to depo sition temperatures of 200 250C. Studies on Ag implanted ZnO suggest that Ag substitution on the Zn site becomes unstable at tem peratures greater than 600C [72 ]. This is consistent with the estimated 0.08 mol% bulk s olid solubility of Ag in ZnO [73 ]. I n this chapter, the synthesis and properties of Ag doped ZnO films grown by pulsed laser deposition is examined, focusing on the formation of p type material, as well as delineating the stability of the transport properties.
48 type an d density for these highly compensated samples, the Hall measurements were performed at various magnetic field values and over a large magnetic field range. Figure 4 6 shows a plot of V H all d/I as a function of applied magnetic field, where V H all is the Ha ll voltage, d is the film thickness, and I is the measurement current. It shows a plot of VdI 1 as a function of magnetic field for two samples, a Ag doped ZnO film grown at 600 C that is n type (as determined by the full Van der Pauw four point calcula tion), and another film grown at 300 C that is p type. For the sample grown at 600 C, the slope is clearly negative, indicating n type. In Figure 4 6b, there is an obvious positive slope to the data as field is increased, indicating p type behavior. F rom the slope of the curve, the extracted hole carrier density is 5.2x10 16 cm 3 Figure 4 7 shows the results for resistivity and carrier concentration of 0.6 at% SZO films for some of th e growth conditions considered. For most deposition conditions, the films were n type (Table 4 1) For growth temperatures in the range of 300 500C, results showed an initial drop in film resistivity, followed by a rise as growth pressure was increased. P type ZnO was realized for films grown at 400 500 C in oxyg en pre ssure of 10 and 25 mTorr. For p type material, these conditions were optimal. For these films, the hole carrier concentration was in the mid 10 19 cm 3 The mobility for films grown at 400 C and 500 C was 10.7 and 2.9 cm 2 /Vs, respectively. Note that for g rowth at 300 C, P(O 2 )=75 mTorr, and 400 C and 500 C in 10mTorr of oxygen, low carrier concentration p type ZnO:Ag was also realized. At a growth temperature of 600 o C, the carrier concentration and resistivity were independent of growth pressure. Presum ably, the Ag was driven out of the Zn site yielding only n type films. All films grown with Ag content other than 0.6 at% were n type.
50 films sh owed well defined band edge emission around 377 nm This emission line is due to free excit on recombination around 3.27 eV, which is very close to the bandgap (3.25 eV) found by room temperature absorption measurements shown in Figure 4 11. The photoluminescence intensity was highest for films grown at 400 500C. This enhancement may be due to a reduction in surface states, which are deleterious to UV emission. Other studies on the effect a monovalent dopant on the photoluminescence of ZnO showed that Ag enhances the efficiency of exciton recombination, so as to have b etter optical properties . Note that there is little visible emis sion often seen in ZnO films [14 0] due to recombination involving mid gap states suggesting lower concentration of compensating defects in the films. 4.4 Summary The synthesis and proper ties of Ag doped ZnO thin films were examined. Epit axial ZnO films doped with 0.6 at% Ag content grown at moderately low temperatures (300 C to 500 C) by pulsed laser deposition yielded p type material as determined by room temperature Hall measurements Hole concentrations on the order mid 10 1 5 to mid 10 19 cm 3 range were realized. Growth at higher temperatures yielded n type material, suggesting that the Ag was driven out of the substitutional site above 500 C and that Ag substitution yielding an acce ptor state is metastable. Photoluminescence measurements showed strong near band edge emission with little mid gap emission as the result of Ag substitution for Zn (Ag Zn ) and reduction of surface states deleterious to UV photoluminescence emission The st ability of the Ag doped films was examined as well. Presumably hydrogen incorporation caused the films to turn n type after about 120 days. Persistent photoconductivity was also observed. High temperature ZnO buffer layers drastically imp roved the surface morphology of films thicker than 1.0 m.
51 roughness below 10 nm were observed. Finally, i n developing electroluminescent junctions, the realization of robust UV photoluminescence in p type ZnO may prove advantageous. A detailed PL study and results for the rectifying junctions utilizing Ag dop ed ZnO films are reported in the following chapters.
53 Figure 4 1 Pow der XRD pattern for films grown in (a) 25 mTorr for dep osition temperature range of 300 600 C and (b) films grown at 300 C in oxygen pressures ranging from 1 75 mTorr.
56 Figure 4 6 Plot of Vd/I as a function of magnetic field for (a) an n type and (b) a p type Ag doped ZnO film.
60 Fig ure 4 10 Room temperature photoluminescence for Ag doped ZnO films grown at various temperatures in 25 mTorr of oxygen showing (a) large wavelength and (b) narrow wavelength plots.
69 spectrum of Ag doped ZnO was compared to that of undoped, P doped, Ga doped, and Ag Ga codoped ZnO. The Ag doped ZnO films showed better optical propert ies.
81 2.0 V. After each measurement the light intensity decreased and the junction became Ohmic. The instability appears to be related to surface conducti on and perhaps hydrogen incorporation. Finally, deposition of layers in reversed order (Ag doped ZnO on bottom, Ga doped ZnO on top) did not result in rectifying I V characteristics. The reason for this is unclear but may relate to the differing growth tem peratures used for the two layers. Thin film transistor structures were also fabricated. Although no p channel behavior was observed, the measured output and transfer characteristics revealed a mobility of 3.5 cm 2 /Vs and a I on /I off ratio of 10 5 The subthe shold slope was determined to be 4.4 V/decade.
83 Figure 6 1 Plot of (a) V H all d/I as a function of applied magnetic field and (b) room temperature photoluminescence for a Ag doped ZnO was grown at 500 C in an oxygen pressure of 25 mTorr.
86 Figure 6 4 The ZnO:Ga/ZnO:Ag/sapphire junction (a) schematic of structure and (b) junction I V characteristic.
92 conductivi ty is expected to remain p type however a top gate TFT structure would be required.
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