Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S18: Focus Session: Wide Band Gap Semiconductors IV |
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Sponsoring Units: DMP Chair: Jennifer Zinck, HRL Room: LACC 406A |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S18.00001: Defect generation and annealing of Al-implanted 6H-SiC Gary Gerardi, Ken Jones, Mark Wood, M.A. Derenge, R.D. Vispute, S.S. Hulllavarad Aluminum implantation and high-temperature annealing of 6H-SiC was carried out to determine the thermal treatment needed to produce the shallow Al acceptor and remove intrinsic defects resulting from the implant. A p-type wafer was box implanted with Al to a depth of 0.4 $\mu $m resulting in a concentration of 6 x10$^{19 }$cm$^{- 3}$ on both sides of a wafer originally doped to 1.3 x 10$^{18}$ cm$^{-3}$. Samples were annealed at temperatures ranging from 1300 to 1600$^{\circ}$C. Implantation resulted in a large concentration of intrinsic defects and charge trapping as evidenced by the loss of EPR signal of the shallow Al acceptor from the substrate. Optical absorption measurements indicate amorphization, which was removed by the 1300 $^{\circ}$C anneal. Intrinsic defects are completely removed after the 1600 $^{\circ}$C anneal. No measurable increase of the Al shallow acceptor was found as a result of the implant and annealing suggesting that the implanted Al does not behave in the same manner as Al incorporated during growth even after the implant has been electrically activated. EPR results suggest that the annealing at different temperatures produces different Al-related defects. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S18.00002: Time-dependent photo-induced electron paramagnetic resonance of Vc+ in semi-insulating 4H SiC: evidence of defect relaxation Haiyan Wang, Mary Zvanut SiC is widely studied because of its superior electronic and physical properties. Many investigations focus on defect levels that act as efficient recombination centers and influence the carrier lifetime. We conducted photo-induced electron paramagnetic resonance (photo-EPR) studies of high purity semi-insulating 4H SiC by illuminating the sample with light of selected energy at 4K. Although several different defects were detected in the samples, the presentation will focus on the defect level of the carbon vacancy, V$_{c}$. Steady state photo-EPR indicates that the intensity of V$_{c}^{+}$ increases at 1.8 eV and reaches a peak at 2.3 eV. Time-dependent photo-EPR results for V$_{c}^{+}$ are consistent with those obtained from steady state measurements. The data suggest that when V$_{c}^{+}$ captures an electron from the valence band becoming V$_{c}^{0}$ the energy required is about 1.8 eV, and when V$_{c} ^{0}$ releases an electron to the conduction band becoming V$_{c}^{+} $ the energy required is 2.3 eV. Initial analysis indicates that lattice relaxation accounts for the energy difference between the plus- to-neutral and neutral-to-plus states. At the talk, we will present the details of these measurements and discuss additional support for the defect relaxation model. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S18.00003: Activation of dopants in SiC: theoretical study Oleg Pankratov, Michel Bockstedte, Alexander Mattausch The electrical activation and solubility of dopants are the limiting factors that hamper the manufacturing of a highly doped SiC. In nitrogen-doped SiC a complete electrical activation was achieved \footnote{M.~Laube {\em et~al\/}, J.~Appl.~Phys.~{\bf 91}, 549 (2002).}\textsuperscript {,} \footnote{D.~Schulz {\em et~al\/}, Mat. Sci. Forum {\bf 338-342}, 87 (2000).} only for impurity concentration below $2-5\times 10^{19}$cm$^{-3}$. Yet for phosphorus a full activation was obtained \footnotemark[1] up to $10^{20}$cm$^{-3}$. We study the different activation behavior of these dopants theoretically by an {\em ab initio\/} DFT approach. We find that phosphorus mainly substitutes for silicon, whereas nitrogen is incorporated exclusively into the carbon sites. In a thermodynamic equilibrium the activation of both donors is not limited by the self-compensation. Phosphorus is found to be fully activated until the onset of precipitation. In contrast, nitrogen preferentially incorporates in the neutral nitrogen-vacancy complexes at concentrations above $2\times10^{19}$cm$^{-3}$. This leads to the nitrogen passivation, in agreement with the experimental findings \textsuperscript {1,2}. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S18.00004: Defect studies of Vanadium doped 4H-SiC using optical admittance spectroscopy Wonwoo Lee, Mary E. Zvanut Semi-insulating SiC is an excellent candidate for a variety of applications, including microwave FET's and other devices for high power and high temperature applications. Vanadium acts as an amphoteric impurity in 4H-SiC with a V$^{3+/4+}$ acceptor level thought to be within 1 eV of the conduction band edge and a V$^{4+/5+}$ donor level known to be 1.6 eV below the conduction band edge. Vanadium is an efficient carrier trap and recombination center. We have studied vanadium doped 4H-SiC with the optical admittance spectroscopy (OAS) at room temperature. After taking into account phonon-assisted optical transitions, the estimated threshold energies can be compared with defect levels measured using thermal techniques. Compared with data reported in the literature, our results show that the defect level E$_{c}$ -1.5 eV is close to the vanadium donor level and the other level E$_{c}$ -0.67 eV is within the range of the value attributed to the vanadium acceptor level. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S18.00005: Electrical and optical properties of Mg doped Al$_{0.7}$Ga$_{0.3}$N Alloys M.L. Nakarmi, N. Nepal, J.Y. Lin, H.X. Jiang Al-rich AlGaN alloys are ideal materials for the development of chip-scale optoelectronic devices such as deep ultraviolet (UV) emitters and detectors operating at wavelengths down to 200 nm. Mg doped Al-rich Al$_{x}$Ga$_{1-x}$N epilayers with Al content as high as 0.7 are required as an electron blocking layers in deep UV LEDs ($\lambda <$ 300 nm). However, little has been reported regarding how to achieve p-type Al$_{x}$Ga$_{1-x}$N epilayers with x $>$ 0.3. Achieving Al-rich AlGaN with high conductivities remains as one of the foremost challenges for the Nitride community. We report on the growth and studies of the electrical and optical properties of Mg doped Al$_{x}$Ga$_{1-x}$N epilayers for x $\sim $ 0.7 grown on AlN/sapphire templates by metal-organic chemical vapor deposition (MOCVD). We found the epilayers to be semi-insulating at room temperature and confirmed p-type conduction at high temperatures ($>$ 700 K) with a resistivity of about 40 $\Omega $cm at 800 K. From the temperature dependent Hall-effect measurement, the Mg acceptor activation energy was estimated to be about 0.4 eV for Al$_{0.7}$Ga$_{0.3}$N alloys. Deep UV photoluminescence (PL) was employed to probe the impurity transitions. We found that the intensity of the 4.2 eV emission line in Al$_{0.7}$Ga$_{0.3}$N is strongly correlated with the resistivity of the materials. Fundamental limit for achieving p-type Al-rich AlGaN alloys and the effects of the Mg doped Al$_{0.7}$Ga$_{0.3}$N electron blocking layer on the deep UV LED performance will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S18.00006: Electrical and optical activation studies of Si-implanted AlxGa1-xN Mee-Yi Ryu, Timothy Zens, Yung Kee Yeo, Robert Hengehold, Todd Steiner Comprehensive and systematic electrical and optical activation studies of Si implanted Al$_{x}$Ga$_{1-x}$N grown on sapphire substrates by molecular beam epitaxial method have been made as a function of ion dose, anneal temperature, and anneal time. Si ions were implanted at 200 keV with doses ranging from 1x10$^{13}$ to 5x10$^{15}$ cm$^{-2}$. The samples were proximity cap annealed from 1100 to 1250$^{\circ}$C for 5 to 25 min with a 500 {\AA} thick AlN cap in a nitrogen environment. Both the sheet carrier concentration and electrical activation efficiency of Si-implanted Al$_{x}$Ga$_{1-x}$N increase continuously with anneal temperature and/or anneal time. The mobility also increase along with the increase in sheet carrier concentrations as the anneal temperature and/or anneal time increases, indicating successive damage recovery with increased anneal temperature and/or anneal time. The luminescence observations are consistent with the results of electrical activation studies. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S18.00007: The Status of p-type ZnO Invited Speaker: In recent years, ZnO has been proposed as an ideal material for new electronic and optoelectronic devices, such as transparent transistors and UV light-emitting diodes (LEDs). However, the LED application will require both n-type and p-type ZnO and the latter is difficult to produce. Potential acceptors include Group V and Group I elements, substituting for O and Zn, respectively. Unfortunately, the Group I elements (e.g., Li) tend to produce semi-insulating material, because they can, in some cases, enter the lattice either as acceptors or donors. On the other hand, the Group V elements, N, P, and As, have all proven to be viable acceptor dopants. Interestingly, theory predicts that P$_{O}$ and As$_{O}$ should be \textit{deep} acceptors, and not highly soluble in ZnO. Thus, it has been proposed that the As acceptor is not As$_{O}$, but rather As$_{Zn}$-2V$_{Zn}$, which should be more soluble and also should have a lower transition energy. Compensating donors must also be minimized, and the most prominent of these are Al$_{Zn}$, interstitial H, and possibly interstitial-Zn complexes. Some ZnO homojunction p-n UV LEDs have already been produced, but more success has been achieved with heterojunction LEDs, using AlGaN as the hole injector. Future prospects will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S18.00008: New insights in the role of native defects in ZnO Anderson Janotti, Chris G. Van de Walle ZnO is a wide-band-gap semiconductor with unique piezoelectric, optical, and electronic properties suitable for use in optoelectronic devices. The availability of bulk single crystals and a large exciton binding energy of 60 meV make ZnO a serious alternative to GaN. However, as-grown ZnO is nearly always $n$-type and the lack of reliable $p$-type doping still hinder its application for light-emitting devices. The unintentional $n$- type conductivity has been attributed to native defects, but the role of individual defects is still controversial. Here we investigate the electronic and structural properties of native defects in ZnO using Density Functional Theory within the Local Density Approximation. We will discuss methods for correcting the band-gap error inherent in density-functional theory. We explore the local atomic relaxations and their direct effect on the electronic structure of each native defect, diffusion barriers, and defect complexes. We will also report results for previously unexplored configurations. Finally, we discuss the influence of native defects on the control of $p$-type doping. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S18.00009: Light-induced metastability in the wide-gap ZnO and CuGaSe$_{2}$ caused by anion vacancies Stephan Lany, Alex Zunger First-principles electronic structure calculations [1] show that anion vacancies in II-VI and chalcopyrite I-III-VI$_{2}$ semiconductors are a class of intrinsic defects that can produce metastable behavior and persistent photoconductivity (PPC), arising from a pronounced coupling between electronic and structural degrees of freedom. In ZnO, V$_{O}^{0}$ has a deep localized donor state in the gap, while V$_{O}^{2+}$ has a shallow level near the CBM. Illumination excites V$_{O}^{0}$ to V$_{O}^{+}$+e and to V$_{O}^{2+}$+2e, and this transition is accompanied by large lattice relaxation. The latter state is metastable and acts as a shallow donor, leading to persistent \textit{electron} photoconductivity ($n$-type PPC), which persists until it is thermally activated into the deep V$_{O}^{0} $. Comparing the behavior of the anion vacancy in the wide-gap chalcopyrite CuGaSe$_{2}$ to that in ZnO, we find an interesting asymmetry: V$_{Se}$ produces persistent \textit{hole} photoconductivity in $p$-CuGaSe$_{2}$, constituting the unusual case where a donor-like defect creates $p$-type PPC. \par [1] Stephan Lany and Alex Zunger, Phys. Rev. Lett. \textbf{93}, 156404 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S18.00010: Resonant State of Substitutional Oxygen in ZnSe W. Walukiewicz, W. Shan, Y.M. Yu, J.W. Ager III, Y. Nabetani We have studied the effect of hydrostatic pressure on low- temperature photoluminescence (PL) spectra of ZnSe doped with oxygen. MBE-grown samples containing up to 2x10$^{19}$/cm$^{3}$ of oxygen have been studied. A broad PL spectral feature associated with the O-states emerges at the pressures around 30- 40 kbar as the fundamental bandgap of ZnSe increases with pressure. It gradually becomes the predominant emission structure and shifts towards higher energy with increasing pressure but at a much slower rate than the exciton emission in ZnSe. By extrapolating the experimental data to the atmospheric pressure, the energy position of the resonant O-states is found to be $\sim$0.20 eV above the conduction-band edge of ZnSe. The location of the resonant state provides a key parameter for modeling the electronic structure of highly mismatched ZnO$_{x} $Se$_{1-x}$ alloy. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S18.00011: Combined Excitation Emission Spectroscopy of Eu ions in GaN V. Dierolf, Z. Fleischman, C. Sandmann, C. Munasinghe , A.J. Steckl Rare earth doped GaN is a promising candidate for light emitters and electrically pumped laser for various applications, such as displays. The rare earth ions are incorporated in this material in various sites, which exhibit different excitation efficiencies and therefore a precise control of these sites is crucial for optimizing potential devices. To this end, we have performed site-selective combined excitation emission spectroscopy studies on Eu-doped GaN layers that have been prepared using the \textit{Interrupted Growth Epitaxy} (IGE) growth technique. In the spectral regime of the single $^{4}$D$_{0}-^{4}$F$_{0}$ excitation transition of Eu$^{3+}$, a large number of excitation peaks ($>$7) can be observed. At least four of them can unambiguously be assigned to different sites. Their relative number depends on growth condition. Selective excitation allows to produce fingerprints of their crystal field splitting in the $^{4}$F$_{1,2}$ states which allow to identify the sites in electro-luminescence and in PL under above-bandgap excitation. In contrasts to this assignment, other strong excitation peaks exist that yield identical emission spectra and must originate therefore from a \textit{single} defect site. The separation of these peaks coincides with GaN Raman frequencies. We therefore suspect that electron-phonon coupled transitions may account for the discrepancy with the single zero-phonon excitation peak that is expected for a single site. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S18.00012: Change in the Structure and Enhancement of Rare-Earth Emission in amorphous GaN and AlN Thin Films S.B. Aldabergenova, H. Mendel, H.P. Strunk We report strong enhancement of Er$^{3+}$, Ce$^{3+}$ , Tb$^{3+}$, Eu$^{3+ }$and Ho$^{3+}$ emission with annealing in mostly amorphous GaN and AlN thin films prepared by DC magnetron co-sputtering in different laboratories. We observe sharp characteristic emission peaks of intra-4f-shell transitions of Er $^{3+}$, Tb$^{3+}$, Eu$^{3+}$ and Ho$^{3+}$ions and a strong but broad peak of 5d-4f emission from Ce$^{3+}$ ions over the temperature range 2-300K. During annealing small crystallites form in the amorphous matrix. The crystallite diameters are between 4 and 7 nm as analyzed by high resolution transmission electron microscopy. We relate strong enhancement of the rare-earth emission with the occurrence of these small crystallites. Different mechanisms of energy transfer from absorbing states in the nanostructured wide band gap GaN and AlN matrix to the rare-earth ions are discussed [Preview Abstract] |
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S18.00013: Can wide-gap chalcopyrite be doped $n$-type Stephan Lany, Yu-Jun Zhao, Clas Persson, Alex Zunger Wide-gap materials are more difficult to dope than their lower- gap counterparts, as evidenced by $n$-doping of Ge $\rightarrow$ Si $\rightarrow$ C or $p$-doping of GaN $\rightarrow$ AlN. Similarly, whereas CuInSe$_{2}$ (E$_{g}$ = 1.1eV) can be $n$- doped via stoichiometry control, its wider-gap counterpart CuGaSe$_{2}$ (Eg = 1.8eV) so far resisted $n$-type doping. Using the defect formation energies calculated from first-principles supercell calculations, we have studied theoretically doping of CuInSe$_{2}$ and CuGaSe$_{2}$ by Cl, Br, I (on Se-site) and Zn, Cd (on metal sites), as a function of chemical thermodynamic boundary conditions. We find that the bottlenecks are proportional to (a) the ease of forming V$_{Cu}$ (an electron killer) and (b) the ease of doping on the wrong site (e.g. Cd-on-In rather than Cd-on Cu). In CIS, halogen doping does not improve over intrinsic doping by InCu, which yields a net donor concentration of ~10$^{18}$cm$^{-3}$ at T = 800 K. A higher net donor doping can be achieved with Cd and Zn doping, but a high compensation ratio is always present. In CuGaSe$_{2}$, both anion-site and cation-site donor doping is intrinsically hampered by overcompensation due to V$_{Cu}$ formation. [Preview Abstract] |
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S18.00014: Localized states due to oxygen in II-VI semiconductors Paul Kent, Clas Persson, Alex Zunger We study the electronic structure of dilute oxygen impurities in II-VIs (ZnTe, ZnSe, ZnS, CdTe, CdSe, CdS) using fully-relaxed large supercell density functional calculations. We analyze the ensuing localized electronic states, finding that oxygen induces ``cluster states'' (CS) near the conduction band edge, similar to nitrogen localized states in the III-Vs. In contrast to nitrogen in the III-Vs however, the CS in II-VI's exhibit a greater degree of delocalization owing to a greater hybridization with the host band structure. The fundamental character, origin, and energetic alignment of the CS is analyzed using the spectral projection method and hydrostatic pressure calculations. We discuss the ramifications of the CS on the band gap bowing, effective masses, absorption and emission spectra of dilute oxygen alloys. [Preview Abstract] |
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