AVS1997 Session TF-WeM: Semiconductor Thin Films

Wednesday, October 22, 1997 8:20 AM in Room B3/4

Wednesday Morning

Start	Invited?	Item
8:20 AM		TF-WeM-1 Microstructure and Surface Morphology Studies of Si_1-xC_x on Si by Atomic Force Microscope L. He, M. Costello (Northern Illinois University); Z. Shi (Hughes STX Corp.) Epitaxial Si_1-xGe_x on Si has been investigated with considerable attention due to its narrower bandgap, and larger optical absorption cut-off which could be suitable for fiber-optic communications. The critical thickness of the strain layer, however, decreased rapidly with the higher Ge composition which limits the device application of Si_1-xGe_x. The incorporation of carbon in substitution sites can lead to the relaxation of strained Si_1-xGe_x alloys due to the small covalent radius of the carbon atom. In this work, Si_1-xC_x was grown on various n and p-type doped Si substrates by rapid thermal process, very low pressure chemical vapor deposition (RTP/VLP-CVD). The growth temperatures were 650 - 700°C, and pressure was 3.5mTorr. An atomic force microscope(AFM) was used to study the surface morphology variation with the growth parameters such as substrate temperature, mass flow ratio and growth rate. It was found that the surface roughness proportional to the growth rate, and reverse proportional to mass flow ratio. Double-crystal x-ray diffraction was conducted to study the crystal structure and to estimate the C concentration. The current-voltage characteristics was studied by Schottky diodes fabricated on the surface of Si_1-xC_x on Si sample. A current-voltage-temperatature (I-V-T) study was carried out to explore the conduction mechanism with material growth conditions.
8:40 AM		TF-WeM-2 The Effect of Adsorbed Atomic Hydrogen on the Growth of the Si/Ge(100) Interface Studied by Positron-Annihilation-Induced Auger Electron Spectroscopy (PAES) J.H. Kim, S.L. Wheeler (University of Texas, Arlington); A. Nangia (Motorola); E. Jung (Samsung Co., South Korea); A.H. Weiss (University of Texas, Arlington) The effect of adsorbed atomic hydrogen on the stability of Si films grown on a Ge(100) substrate was studied using Positron-annihilation-induced Auger Electron Spectroscopy (PAES). Research carried out at The University of Texas at Arlington has demonstrated that PAES is uniquely sensitive to the topmost atomic layer due to the trapping of positrons in an image potential well just outside the surface before annihilation. The concentration of Si and Ge atoms in the surface layer were monitored by measuring PAES intensities of the Si LVV and Ge MVV peaks after deposition of Si on Ge(100) with and without the prior adsorption of H on the substrate. The PAES results indicated that prior exposure to H significantly suppressed the segregation of substrate Ge atoms to the surface during the growth of thin Si overlayers. PAES was also applied to the study of the thermal stability of Si films grown on a Ge(100) substrate with the prior adsorption of 5000 L atomic hydrogen. The Si overlayer was stable until 573 K, but after annealing the sample at 773 K the Ge atoms had moved onto the top surface layer. The corresponding Electron-induced Auger (EAES) spectra show only small variations with temperature. This work was supported by the NSF Grant #DMR9502459 and the Welch Foundation.
9:00 AM	Invited	TF-WeM-3 A Roadmap Runs Through It - Research in Copper Thin Films for Silicon Technology J.M.E. Harper (IBM T.J. Watson Research Center) Interconnections between transistors in microelectronics are currently made from aluminum alloys. The National Technology Roadmap for Semiconductors predicts that the minimum interconnection linewidth will decrease to below one-tenth micrometer by the year 2010. At these dimensions, the resistivity and reliability of aluminum present almost insurmountable challenges to process technologists and circuit designers. In response, the semiconductor industry is preparing to use copper interconnections, but the differences between aluminum and copper are sufficiently great that new process sequences must be developed. Exploring the materials science of copper interconnections has revealed several surprising phenomena associated with copper, which will be described. These include catalytic oxidation in the reaction of Cu with Si, an unexpected phase formation sequence in Cu-Sn alloys, and the evolution of a highly oriented microstructure coupled with abnormal grain growth in Cu-Co alloys. These examples continue to teach us that our ability to predict materials structure and properties in sub-micrometer dimensions is still very limited. It will also be argued that a new approach to evaluating multi-parameter materials choices is needed if materials science is to contribute most effectively to silicon technology development.
9:40 AM		TF-WeM-5 Ultra-high Doping during Si(001) Gas-Source Molecular Beam Epitaxy: Growth kinetics, B-Incorporation, Strain, and Electrical Activation G. Glass, H. Kim, P. Desjardins, M.R. Sardela, Jr., O. Gurdal, N. Taylor, T. Spila, J.E. Greene (University of Illinois, Urbana-Champaign) Si(001) layers doped with B concentrations C_B, as determined by SIMS, to 1.2x10²² cm^-3 were grown on Si(001)2x1 substrates at T_s = 500- 850 °C by GS-MBE using Si₂H₆ and B₂H₆. With increasing C_B ≥ 2x10¹⁹ cm^-3, film growth rates R_Si decreased at Ts ≥ 600 °C, but increased at T_s ≤ 550 °C. Deuterium temperature-programmed desorption measurements as a function of increasing C_B show strong B surface segregation, decreased steady state H coverages θ_H, and lower dangling bond densities. Si:B growth kinetics are well described by a model showing that at low temperatures, where steady state θ_H values are high, increased H desorption rates from B-backbonded Si adatoms dominate leading to an enhancement in R_Si whereas at higher temperatures, R_Si decreases due to a decreased adsorption site density. B was incorporated into substitutional electrically-active sites at concentrations up to 2.5x10²⁰ cm^-3 at T_s ≤ 600 °C. At higher B concentrations there is a large discontinuous decrease in the activated fraction of B coupled with a rapid decrease in hole mobilities. Nevertheless, the total activated B concentration N_B continues to increase, and reaches a saturation value at N_B = 8x10²⁰ cm^-3 at C_B = 1.2x10²² cm^-3. From high-resolution XRD and reciprocal space mapping measurements, all films were found to be fully strained and the lattice constant in the growth direction decreased linearly with C_B up to the limit of full electrical activation, N_B = C_B = 2.5x10²⁰ cm^-3. The combination of the XRD results with high-resolution TEM/XTEM analyses showing no observable B precipitates, even in films with C_B = 1.2x10²² cm^-3, suggests that electrically-inactive B is present in the form of highly dispersed B-containing nanoclusters.
10:00 AM		TF-WeM-6 Growth of Si_1-xGe_x on Si(011)16x2 by Gas Source Molecular Beam Epitaxy N. Taylor, H. Kim, T. Spila, J.R.A. Carlsson, G. Glass, J.E. Greene (University of Illinois, Urbana-Champaign) Strained Si_1-xGe_x/Si(011) heterostructures provide additional degrees of freedom over the more common Si_1-xGe_x/Si(001) system for bandgap engineering. The maximum film/substrate conduction-band offset is predicted to be substantially larger, the optical bandgap decreases more rapidly with increasing x, and optical selection rules allow hole-intersubband transitions to be excited by light parallel to Si_1-xGe_x/Si(011) multiple quantum well layers. Deuterium temperature programmed desorption (TPD) was used to show that the unit cell of the Si(011)16x2 reconstructed surface is composed of 8 dimers and 16 adatom sites and that hydrogen desorption from the dihydride, adatom monohydride, and surface atom monohydride phases occurs at 430, 520, and 550 C, respectively. GS-MBE was then used to grow Si_1-xGe_x(011) layers with x = 0.02-0.35 from Si₂H₆ and Ge₂H₆ at T_S = 400-950 C. Film thicknesses and compositions were measured by Rutherford backscattering spectroscopy. Ge film concentrations were found to be linearly proportional to the Ge₂H₆/Si₂H₆ flux ratio but independent of T_S. Growth kinetics results show that at T_S ≤ 500 C, the deposition rate R_SiGe decreases exponentially with 1/T_S in a surface -reaction-limited growth mode where the rate limiting step is hydrogen desorption from the monohydride phase. At T_S between 500 and 700 C, the steady-state hydrogen coverage is low and R_SiGe is essentially independent of T_S as growth proceeds in an impingement-flux-limited growth mode. The surface reconstruction remains 16x2 at low alloy compositions and is 2x8 at x ≥ 0.14. For all alloy compositions investigated there is a sharp change in the growth kinetics at T_S ≥ 700 C as the surface reconstruction transforms to 1x1 and R_SiGe again varies exponentially with 1/T_S. The 16x2 to 1x1 transition is reversible as samples are heated and cooled through 700 C.
10:20 AM	Invited	TF-WeM-7 Growth Induced InAs Quantum Dot Columns J.S. Harris, G.S. Solomon (Stanford University) Using a growth induced islanding technique, we have constructed vertical columns of InAs QDs in a GaAs matrix, without post growth lithography. The InAs quantum dots are approximately 180 Å in base width and 40 Å high. The quantum dots form spontaneously in a random array in response to the 7% lattice mismatch between InAs and GaAs. After the InAs quantum dots have formed, further GaAs growth quickly smoothes the growth front. When additional InAs is deposited a new quantum dot layer forms such that the new dots nucleate directly above dots from the previous quantum dot layer. If this growth sequence is continued, vertical columns of InAs QDs are formed. Transmission-electron microscopy indicates that the dots remain in the column formation up to the 20 layers of QDs investigated, and no large scale plastic relaxation is observed. Atomic-force microscopy (AFM) indicates the QD size and density is unchanged between the first layer of QDs and the fifth layer, while the dot density is reduced by the tenth dot layer and the dot size is increased. Two effects are observed in these quantum dot columns. First, AFM results indicated a decrease in the island size distribution of the QD ensemble when the column structures are compared to single QD layers, which is accompanied by the appearance of ordering in the dot plane. Thus, these dot columns increase the ordering in this quantum dot system. Second, our 8K photoluminescence measurements show a red shift in the spectral peak position, and a reduction in the spectral linewidth as the number of dots in a column is increased. We attribute these spectral feature changes to electronic coupling within the vertical dot columns. This coupling is adjusted by adjusting the GaAs spacer region between the dots in a column. Room temperature electroluminescence on single dot layers show a spectral peak shift with increased bias that we have attributed to saturation of lower energy states. This peak shift is significantly reduced in the QD column structures.
11:00 AM		TF-WeM-9 Microstructure Evolution of GaSe Thin Film Grown on GaAs(100) by van der Waals Epitaxy Z.R. Dai (University of Washington); L.E. Rumaner (Intel Corporation); F.S. Ohuchi (University of Washington) GaSe belongs to a family of layered III-VI semiconductors, and has highly anisotropic transport and optical properties. Potential application in the opto-electronic field are considered due to its strong non-linear optical properties. A layer of GaSe consists of four, two-dimensional monoatomic sheets in the sequence of Se-Ga-Ga-Se, where atoms within each layer are tightly bound with a mixture of covalent and ionic bonds, and the bonding between layers involve much weaker van der Waals forces. This results in the formation of different polytypes, leading to strong electrical and optical anisotropy of GaSe. In this paper, GaSe films were grown on GaAs(100) by the van der Waals epitaxy technique. Microstructure of the thin film systems was characterized by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Combined with image processing, image simulation and fast Fourier transform (FFT) techniques, we found that the relationship of crystallography between the grown GaSe film and the GaAs substrate was [011]_GaAs \|\| [1120]_GaSe / (100)_GaAs \|\| (0001)_GaSe. The dominant polytype found in the GaSe film is a γ-type, which has a 3R-rhombohedral structure with R3m space group. This crystal structure is non-center symmetrical, which might be responsible for the non-linear optical properties of GaSe. Another interesting phenomenon is that there is a thin crystallione layer at the interface, the crystal structure of which can be clearly distinguished from both GaAs and GaSe, but is associated with Ga₂Se₃. The microstructural evolution of GaSe film growth will be discussed.
11:20 AM		TF-WeM-10 The Effects of CdCl₂ Treatment on the Recrystallization and Electro-Optical Properties of CdTe Thin Films H.R. Moutinho, L.L. Kazmerski (National Renewable Energy Laboratory) The microstructural properties of CdTe films vary with the conditions used in the common CdCl₂ processing for solar cells. Grain growth, for example, is critically related to film deposition technique, as well as post-deposition procedures. We were able to explain the mechanisms involved in the changes after this treatment by analyzing films deposited by physical vapor deposition (PVD) and close-spaced sublimation (CSS) before and after CdCl₂ treatment at 350 and 400øC. The PVD samples treated at 350øC exhibited ongoing recrystallization (with two types of grains observed by atomic force microscopy (AFM)), the existence of two different lattice parameters and a decrease in the degree of preferential orientation (identified in X-ray diffraction (XRD) analysis). After treatment at 400øC, these films had completely recrystallized, indicated by a substantial grain growth, almost complete loss of preferential orientation, and the presence of only one lattice parameter. In contrast, CSS samples did not show major changes in microstructure after the treatment. The reason is that CSS films are grown at higher temperatures than PVD films, and have larger initial grain sizes and less lattice strain energy. This prevents recrystallization at the temperatures and times used in this treatment. However, results from time-resolved photoluminescence (TRPL) and photoluminescence (PL) show that the eletro-optical properties of these two film types are similar. The minority-carrier lifetimes and PL intensities for untreated films are low. Treatment at 400øC increases the lifetime and the luminescence intensity significantly, due to an elimination of deep levels within the semiconductor bandgap. We also present results showing the formation of a Cd(S_xTe_1-x) region at the CdTe/CdS interface, and the evolution of homogeneous and inhomogeneous stress in the films after the treatment. Finally, changes in the PL spectra as a function of processing and substrate type, are presented.
11:40 AM		TF-WeM-11 Effects of Hydrogen on the Material Properties of r.f.-Sputtered ZnTe:Cu Thin-Films T.A. Gessert (National Renewable Energy Laboratory); H. Modrow (Louisiana State University); A.R. Mason, P. Sheldon (National Renewable Energy Laboratory) We have observed that the optical band gap of r.f.-sputter-deposited ZnTe:Cu films containing ~0.5 at.% Cu increases from ~2.1 to ~2.3 eV when small amounts of hydrogen are added to the argon sputtering ambient (~0.1-1.3 vol.% hydrogen). The increased optical transparency of these films could be beneficial if the film is used as a thin-film transparent contact interface to p-CdTe. Unfortunately, we have not achieved low-resistance tunneling contacts between this ZnTe:Cu and metals, apparently because the carrier concentration remains too low (film resistivity ~50 kOhm-cm). Recently, we have expanded the study to include ZnTe films containing higher Cu concentrations (~1-10 at.%). In addition to an increase in optical band gap, these films exhibit a significant reduction in resistivity. Specifically, the resistivity for ZnTe:Cu films containing ~2 at.% Cu is reduced from ~50,000 to ~0.5 ohm-cm when ~1 vol.% hydrogen in added during sputtering. Electron-probe (wavelength-dispersive) microanalysis has been combined with Hall measurements to study the effect of hydrogen on the Cu concentration, hole concentration, and hole mobility. Results indicate that although variations in Cu concentration result from adding hydrogen to the sputtering ambient, the hole concentration increases beyond that expected from increases in Cu concentration alone; this indicates that more of the Cu is becoming electrically active. The hole mobility is also observed to increase significantly when hydrogen is added. To investigate changes in local atomic structure caused by the sputtering ambient, X-ray absorption spectroscopy (XAS) has been performed. XAS indicates that the addition of Cu to the ZnTe:Cu does not lead to changes in the Te LIII absorption edge, suggesting that CuTe phases do not form readily when Cu is added to these films. The XAS analysis further indicates that addition of hydrogen affects the distribution of Cu, with Cu forming clusters in the films deposited in hydrogen.