AVS2001 Session TF-MoP: Multilayers and Thin Film Characterization Poster Session

Monday, October 29, 2001 5:30 PM in Room 134/135
Monday Afternoon

Time Period MoP Sessions | Topic TF Sessions | Time Periods | Topics | AVS2001 Schedule

TF-MoP-1 The Interfacial Reaction in the W/WNx/poly Si1-xGex with Ge Contents and Annealing Process
S.-K. Kang, J.J. Kim, D.-H. Ko (Yonsei University, Korea); H.B. Kang (Sungkyunkwan University, Korea); T.H. Ahn, I.S. Yeo (Hynix Semiconductor Inc., Korea); T.W. Lee, Y.H. Lee (Ju-Sung Eng., Korea)
As CMOS device dimensions are continuously scaled down to achieve high performance, new gate structures with low resistivity materials are required to enhance the electrical properties. In order to reduce the resistivity of gate electrode materials, metal silicide/poly Si structures have been employed. However, with metal silicide/poly Si structures, it is difficult to obtain low gate resistance in a narrow gate line. As a low resistivity gate electrode, metal/barrier/poly Si or poly Si1-XGeX gate stack is one of the candidates for the gate structures in the high performance CMOS-FET. In addition, poly Si1-XGeX films have been suggested as a promising alternative to the poly-Si gate electrode for CMOS technology due to the low resistivity, variable workfunction, and compatibility with Si processes. In order to investigate the interfacial reactions in the W/WNx/Poly Si1-xGex structure, poly poly Si1-XGeX films with 0%, 20%, and 60% Ge content were deposited using LPCVD (EUREKA 2000, Ju-Sung Co. Ltd.) on the 8in silicon wafers. Following the deposition process of poly Si1-XGeX, WNx and W films were deposited using sputtering method. To study the interfacial reactions in the W/WNx/Poly Si1-XGeX with Ge contents in poly Si1-XGeX and annealing process, the films were annealed at temperatures between 600°C and 900°C in N2 ambient. The interfacial reactions in the W/WNx/poly Si1-XGeX films were observed by AES, XPS, HR-TEM, and EDX. After annealing treatment, N composition decreased in WNx and the interfacial layer was formed between WNx and poly Si capping layer. The interfacial layer was expected to be composed of W, Si, Ge, and N by the analysis of XPS and EDX. In addition, the formation of interfacial layer with the increase of Ge contents in poly Si1-XGeX will be discussed.
TF-MoP-2 The Role of Interfaces in Fe/W and Co/W Multilayers
E. Majkova, A. Anopchenko, Y. Chushkin, M. Jergel, S. Luby, R. Senderak (Institute of Physics SAS, Slovak Republic)
With GMR multilayers (MLs) the attention has been mostly paid to the combinations of transition metals with Ag, Au and Cu. Due to the immiscibility of constituents the multilayer interfaces are chemically sharp, formation of magnetically dead layers at the interfaces is suppressed and electron transport does not suffer from additional scattering phenomena. For applications, the use of refractory metals (W, Mo) as non- magnetic spacer provides an interesting alternative. In this paper the interfaces in e-beam deposited Fe/W and Co/W multilayers (MLs) with 5 and 10 periods and magnetic layers 1 or 2nm thick are studied. For analyses the X-ray diffraction (XRD), X-ray reflectivity (XRR) and diffuse scattering at grazing incidence (GIXDS) completed by simulations of the spectra were used. All MLs showed regular periodic structure with layer thicknesses close to the nominal values. The interfacial roughness was 0.5-0.7nm, slightly higher for W/on- Co or W/on -Fe interfaces. >From the GIDS spectra vertical correlation of the interfaces across the whole ML stack was obtained. The XRD patterns revealed polycrystalline structure of the Fe, Co and W layers with structural coherency between fcc Co(111) and/or bcc Fe(110) and bcc W(110) lattice planes across the ML stack. The coherency is affected by the lateral waviness of the Fe/W or Co/W interfaces what results in relatively wide angular distribution of the column orientation. The data point at the growth induced mixing and roughness at the ML interfaces. There is no evidence of compound formation at the interfaces even at the deposition at elevated temperatures (<=200C). The miscibility of Co/W and Fe/W systems and the possible formation of magnetically dead layers due to the mixing and/or roughness at Fe/W and Co/W interfaces are discussed.
TF-MoP-3 Intermixing Phenomena in Immiscible Ag/Co Bilayers and Co/Ag/Co Trilayers under KrF Laser Annealing
S. Luby, E. Majkova, M. Jergel, R. Senderak (Institute of Physics SAS, Slovak Republic); P. Mengucci, G. Majni (Uni. Ancona, Italy); E. D'Anna, G. Leggieri, A. Luches, M. Martino (Uni. Lecce, Italy)
The discovery of GMR in multilayers of immiscible pairs of metals or alloys is the stimulus for the progress in the field of microdevices. Consequently, the study of the thermal stability of GMR structures becomes topical. In the last years a couple of papers on excimer laser induced diffusion in layered structures appeared. In one of them we have studied intermixing in Co/Ag/Co trilayers under XeCl laser thermal treatment. In this paper we have employed thermally more effective KrF laser and the processed structures were composed of layers only several nm thick. Bilayers and trilayers were e-beam deposited in UHV at room temperature onto oxidized Si substrates. The samples were processed by fluences F=0.1-0.25 Jcm-2 with number of pulses ≤1000 directed to the same irradiation spot.The samples were studied by RBS, XRD, grazing incidence XRD, X-ray reflectivity and sheet resistance measurements. The temperature and depth of melting vs. time in irradiated structures was obtained by numerical computations. Using a broad interval of heat treatments we were able to study the intermixing phenomena at various interfaces. At solid- solid interfaces the grain boundary diffusion of Ag into Co layers dominates the intermixing. The preexponential factor and activation energy of diffusion are D0 ~ 10-10 m2s-1 and E= 0.4 eV. At solid liquid interfaces the sharpening of previously deteriorated profiles (probably by back diffusion in immiscible systems) was observed at F=0.2 Jcm-2. Here, the value E=1.5 eV was typical. The jumping of Ag clusters seems to be an explanation of the observed value. Using liquid-liquid interactions granular Ag-Co films were obtained. The phenomena in laser treatment are complex, however, they provide more degrees of freedom due to the controlled depth of melting and delivery of energy in well-defined quanta.
TF-MoP-4 The Study of Growth of Al on Polyimide Free-standing Films
X.-F. Lin (Charles Evans & Associates); D.A. Grove (Luxel Corporation); T.F. Fister, L.P. Bisaha, G.S. Strossman, L.-C. Wei (Charles Evans & Associates); G. Lefever-Button (Luxel Corporation); J.R. Kingsley, I.D. Ward, R.W. Odom, P.M. Lindley (Charles Evans & Associates)
Al thin film growth on polyimide free-standing films has been studied by several different characterization techniques. We focused our attention on investigating Al growth, nucleation, and the resulting surface structures. Atomic force microscopy (AFM) and energy dispersive x-ray spectroscopy (EDS) were used to monitor surface morphological evolution and variations of stoichiometries of the Al/polyimide complex as a function of the increasing thickness of Al thin films. The chemical bonding states between Al and polyimide complex were obtained by using x-ray photoelectron spectroscopy (XPS); while the local surface electronic density distribution about the Fermi-level was revealed by scanning tunneling spectroscopy (STS) measurements. We found that during the initial interaction between the condensing Al adatoms and the polyimide substrate, the Al adatoms interact with carbonyl groups of the polyimide to form small clusters which conform to the surface morphology of the polyimide film. Further Al growth produces oriented crystallized sub-micron islands that enlarge with additional Al deposition. The STS I-V spectra acquired at different stages of Al growth show variations of local surface electric conductivity which are affected by adsorbate-substrate and adsorbate-adsorbate interactions, as well as external environmental conditions. These results are compared for Al grown on both free-standing and Si substrate-supported polyimide films.
TF-MoP-5 Amorphous Silicon Photodiodes for Image Sensing
M. Ristova (Texas A&M University and University in Skopje, Republic of Macedonia); Y. Kuo, H.H. Lee, S. Lee, J.Y. Tewg (Texas A&M University)
The aim of this research was to develop amorphous silicon (a-Si:H) thin film photodiodes for image sensing using the He-Ne (632 nm) laser source. Three different kinds of thin film diode structures, i.e., metal-(a-S:H)-metal, n+-(a-S:H)-metal, and n+-(a-S:H)-n+, were prepared. The a-Si:H (between 200 and 1000 nm) and n+ (20 nm) layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) at 250°C. The molybdenum metal was deposited with a magnetron-sputtering gun at 13.56 MHz. All a-Si:H layers, which contain about 10 % of hydrogen, were deposited under the same condition. The n+ layer's conductivity was about 2.3E10 (ohm-cm)-1, while the conductivity of the undoped a-Si:H was about 1-2E9 (ohm-cm)n-1. The complete diode was fabricated using photolithography and reactive ion etching (RIE) methods. Factors, such as electrode geometry, contact resistance, a-Si:H thickness, and the thermal treatment, were studied for their influence on the diode performance. The diode's dark and illumination currents, i.e., Idark and Iillumination, were determined from current-voltage (IV) measurement. The following conclusions are summarized from this study: 1) the highest current (in either dark or under illumination condition) can be obtained with the pattern with highest value for the ratio between the illuminated area and the average distance between the gridlines 2) the contact resistance can be the dominating factor in the current path, 3) the thick film has the large light absorption efficiency and the high current ratio, and 4) the annealing step repairs the RIE damaged film and greatly improves the diode performance.
TF-MoP-6 Li3PO4:N/LiCoO2 Coatings for Thin Film Batteries
M.E. Gross, P.M. Martin, D.C. Stewart, J.W. Johnston, C.F. Windisch, G.L. Graff (Pacific Northwest National Laboratory); P.L. Rissmiller, E.L. Dudeck (Mine Safety Appliances Company)
Li3PO4:N/Li1.04CoO2 thin film battery structures were deposited by reactive RF magnetron sputtering. Li3PO4:N (LIPON) thin films up to 1 µm thick were deposited using a 6" diameter Li2.9PO3.5 pressed powder target. Li1.04CoO2 thin films were deposited using a 6" diameter LiCoO2 pressed powder target. LIPON films were deposited in He + N2 mixtures and LiCoO2 films were deposited in Ar + O2 mixtures. Total chamber pressure during deposition ranged between 10 and 20 mTorr and RF power to the sputtering targets ranged from 100 W to 400 W. Because XPS gave ambiguous compositional results, the films were optimized for AC and DC conductivity. Electrical conductivity was extremely sensitive to deposition conditions (deposition rate, sputtering gas pressure, and reactive gas partial pressure). AC conductivity measurements were made at a frequency of 10 kHz, and were correlated to DC conductivity measurements. LIPON films had highest conductivities in the 130 S.cm-1 range and the highest AC conductivity of Li1.04CoO2 films was near 0.24 S.cm-1. The most conductive films were deposited at 20 mTorr pressures and 100 W target power. X-ray diffraction analysis showed that the films were mostly amorphous. Films deposited under these conditions were transparent at visible wavelengths with a refractive index of 1.6. Lower conductivity films were brownish in appearance and less transmissive than films with high conductivity. The rechargeable battery structures consisting of a gold cathode, 0.4 µm LIPON, 0.6 µm Li1.04CoO2, Li metal anode, and Cu substrate are currently under test. The thin film batteries have been cycled successfully. Performance results are correlated with film properties and reported. Future work will involve optimization of battery performance on a large scale and scale up of the deposition process.
TF-MoP-8 Degradation Mechanisms of Low-Temperature Poly-Si Thin-Film Transistors with PECVD TEOS Oxide
H.W. Zan, P.S. Shih, T.C. Wu (National Chiao Tung University, Taiwan); T.C. Chang (National Sun Yat-Sen University, Taiwan); C.Y. Chang, D.Z. Peng (National Chiao Tung University, Taiwan)
For being applied on low temperature poly-Si AMLCD, we fabricate poly-Si TFTs with PECVD TEOS oxide as gate insulator. It is found that the output characteristics of our devices are well enough with mobility as 37 cm2/V.s and quite low leakage current. However, due to the high density of trap states localized within the grain boundaries, the electric fields across the grain boundaries near the drain side are considerably high even under moderate biases. The hot-carrier effects are therefore more pronounced in poly-Si TFTs than in MOSFETs. To investigate the reliability issues of poly-Si TFTs, we carefully apply several kinds of electrical stresses on our devices and study their degradation phenomena. Two different degradation mechanisms can be concluded. For the stress under linear region, stress-induced defects are located in the whole gate insulator and are uniformly distributed from source to drain. These defects are positive fixed oxide charges and the acceptor-type interface states in the upper half of the band gap. It is also noted that the degradation degree depends on the magnitude of gate current and the electric field across the gate oxide. For stress under saturation region, however, additional defects near the drain side are generated. This asymmetric degradation phenomenon is attributed to the presence of avalanche-generated carriers. These defects could be acceptor-type interface states in the upper half of the bandgap and donor-type interface states in the lower half of the bandgap. In addition, stress under saturation region produces severer degradation than that under linear region.
TF-MoP-9 Atomic Force Microscopy Observation of TiO2 Films Deposited by dc Reactive Sputtering
T. Takahashi, H. Nakabayashi, N. Sasai, K. Masugata (Toyama University, Japan)
A bombardment of energetic particles such as secondary electrons and recoiled ions ejected from the target plane is very useful technique for modifying the structure of the film in reactive sputtering. It leads to the interesting effects such as enhancement of reactivity, adatom mobility and atomic peening. Therefore, the state on the surface of the as-deposited film may significantly depend on the bombardment condition. So, in this study, the relationship between the surface morphology and the crystal structure of TiO2 films has been investigated in detail. TiO2 films with thickness of about 2-3 µm were deposited by dc reactive sputtering on glass-slide substrates. The crystal structure and surface roughness of films were measured with a X-ray diffractometry and an atomic force microscopy (AFM), respectively. The A(101), A(200), A(112) and A(220) peaks were observed from the X-ray diffraction patterns, where A shows an anatase of TiO2. With increasing energy and number of bombarding particles, X-ray peak intensities IP of A(101) and A(200) gradually decreased, and IP of A(112) and A(220) gradually increased, respectively. A(220) peak was significantly higher than others. The crystallinity of TiO2 film was highly improved and its crystallite size became larger with an increase of them. The roughness parameter Ra on the surface of TiO2 films also depended on them. Ra increased in the range of 4 to 51 nm with increasing energy and number of bombarding particles. Consequently, it was found that the surface morphology and the crystal orientation of TiO2 films strongly affected to the bombardment of energetic particles to the growing film.
TF-MoP-10 Effects of Annealing Conditions on Doping Efficiency in the Indium Tin Oxide Thin Films Deposited at Low Temperature
J.W. Bae, S.D. Park, D.H. Lee, Y.J. Lee, G.Y. Yeom (Sungkyunkwan University, Korea)
Tin-doped indium oxide (ITO) thin films were deposited on glass substrates at low temperature(below 90°C) by a dual oxygen beam assisted evaporator system and the effect of various annealing conditions such as annealing atmosphere and annealing temperature on tin-doping efficiency affecting electrical, physical, and optical properties of the ITO were investigated. The tin composition of the source was varied by mixing indium oxide and tin oxide(0 wt.% ~ 30wt.%). Two rf oxygen sources mounted in the chamber were used for oxygen ion and radical sources, respectively. Atomic percentages of tin in the deposited ITO thin films were investigated by X-ray photoelectron spectroscopy. Carrier density and Hall mobility changed with the doped tin concentration and annealing conditions were measured by Hall effect measurement. Crystallinity, sheet resistance, and optical transmittance were observed by a X-ray diffractometer, a four point probe, and an UV-spectrometer, respectively. Carrier concentration of the undoped indium oxide (0% tin) thin film deposited at the optimum deposition condition of O/In compositions (by controlling the flux ratios of two rf oxygen sources and the evaporation source) was decreased with annealing but Hall mobility was increased from ~20 cm2/Vs to 105 cm2/Vs. On the other hand, in the case of tin doped indium oxide (> 0% tin) film deposited at optimum deposition conditions, both the carrier concentration and the mobility were increased with the increase of the annealing temperature and, therefore, the resistivity was decreased from 5 X 10-4 ohm-cm to below 2 X 10-4 ohm-cm due to the increase of tin doping efficiency. Tin atoms located at interstitial sites during the deposition at low temperature appear to move to indium substitutional sites by annealing. It implies that tin atoms in the indium tin oxide deposited at low temperature could act as impurity instead of dopant.
TF-MoP-12 Electrical Properties for Si Doped Glass Light Emitter
T. Ichinohe (Tokyo National College of Technology, Japan); S. Nozaki, H. Morisaki (The University of Electro-Communications, Japan); S. Masaki (Tokyo National College of Technology, Japan); K. Kawasaki (TDY Co. Ltd., Japan)
Nanometer-sized Si ultrafine particle films (the Si nano-structured films) have been studied extensively as a candidate of Si-based photo-electronic coupled devices. Some of the authors have discussed the emission mechanism of the visible light emission and others have emphasized the possibility on the application to light emitters, although the emission intensity of the Si nano-structured films still remains to be quite low at present. The correlation between the light emission and the carrier injection mechanism in the Si nano-structured films should be clarified to realize Si based light emitter. We have studied both photoluminescence (PL) and electroluminescence (EL) of the Si doped glass (Si-DG) films fabricated by the ion beam sputter-deposition. The transmission electron microscopic(TEM) study has shown that the presence of nano-crystalline Si particles formed by annealing at 900 °C in nitrogen-gas atmosphere is essential for the visible light emission. PL spectra have been influenced by the excitation light energy, indicating that the emission is via the emission centers distributed in the interface between the crystallites and the surrounding SiO2. EL devices with indium-tin-oxide (ITO) top electrodes have shown the emission peak at 650 nm, which is considerably longer than the PL peaks between 520 and 580 nm. The EL devices have shown the luminescence only when the diode is in forward bias, that is, the Si back contact is positive to the ITO top electrode. The integrated intensity increases almost in proportion to the injection current. When the forward bias voltage exceeded 5 V, EL became to be strong enough to be noticed with the naked eye in dark. We noticed that EL spectra tend to become broad with increasing current. C-V measurement has shown that the reverse bias characteristics are explained by the MOS-like structure with the flat band potential of about -4V.
TF-MoP-14 Characterization of Pb(Zr0.52Ti0.48)O3 Films in the Thickness Range of 0.4-6.0µm Prepared by Pulsed Laser Deposition
M.C. Kim (Yonsei University, Korea); J.W. Choi, S.J. Yoon, H.J. Kim (Korea Institute of Science and Technology); K.Y. Yoon (Yonsei University, Korea)
The films of nominal composition of Pb(Zr0.52Ti0.48)O3 (PZT) in the thickness range of 0.4-6.0µm were fabricated on Pt/Ti/SiO2/Si substrate using a pulsed laser deposition (PLD). The PZT films were deposited at 500°C with single process and post annealed at 650°C in oxygen atmosphere because the deposition rate of PLD was fast. The variations in crystallite orientation, microstructure, and ferroelectric and dielectric properties were determined as a function of film thickness. The preferred orientation (111) was changed to (110) above the thickness of 8000Å. As film thickness increased, gain size increased and cross-sectional microstructure showed columnar structure. Remnant polarizations increased from 20 to 40µC/cm2 approximately, and relative permittivity and coercive field decreased from 1800 to 1000 and 45 to 30KV/cm, respectively. The changes of property would be discussed in terms of the presumed influence of interfacial phenomena.
TF-MoP-16 The Effect of the Process Parameters on the Electrical Properties of Ni-Cr-Al-Mn-Si Alloy Thin Films
B.J. Lee (Inha University, Korea); C.S. Kim (Korea Electronics Technology Institute); G.B. Park (Yuhan College, Korea); D.C. Lee (Inha University, Korea)
We have fabricated thin resistor films using the DC/RF magnetron sputtering of 75wt%Ni-20wt%Cr-3wt%Al-4wt%Mn-1wt%Si alloy target and studied the effect of the process parameters on the electrical properties for low TCR(Temperature Coefficient of Resistance) films. In sputtering process, pressure and substrate temperature, are varied as controllable parameters. The films are annealed to 400°C in air and nitrogen atmosphere. We have investigated the microstructure using TEM, XRD, EPMA and EDS and measured the electrical properties, the sheet resistance and TCR. The oxygen content and TCR of the films decreased as the sputtering pressure decreased. The oxygen content were 8.9, 8.5 and 1.5wt%, TCR were 105, 85 and 54ppm/°C for 25, 15 and 5mTorr of the pressure, respectively. The sheet resistance, TCR and crystallinity of the films increases with increasing the substrate and annealing temperature. The sheet resistance and TCR abruptly increased as annealing temperature increased over 300°C in air atmosphere. >From TEM and XRD, it is found that these results are due to the existence of NiO on film surface formed by annealing. As a results of them, it is suggested that the sheet resistance and TCR of thin films can be controlled by variation of sputter process parameter and annealing of thin film below 300°C in nitrogen.
TF-MoP-17 Electrical Properties of the Novel Semiconductor Alloy CuxCd1-xTe Prepared by rf Sputtering
E. Garnett-Ruiz, G. Torres-Delgado, O. Jiménez-Sandoval, R. Perez-Castanedo, P. Garcia-Jimenez (Cinvestav-IPN, Mexico); B.S. Chao (Energy Conversion Devices); S. Jiménez-Sandoval (Cinvestav-IPN, Mexico)
It has been demonstrated that the incorporation of Cu into Cd sites at concentrations around 0.3 at% produces high quality films when compared to pure CdTe or to CuxCd1-xTe with higher copper concentrations. The improvement on the structural properties produces an important effect on the transport properties of this semiconducting alloy. For instance, we have observed that the resistivity drops 4 to 8 orders of magnitude depending upon copper content. In this work we present the results of a study on the electrical properties of thin films, grown by rf sputtering, of the novel semiconductor alloy CuxCd1-xTe as a function of copper concentration. This study is based on the current-voltage characteristics of a capacitor-like structure metal/semiconductor alloy/metal (suitable for measurement of polycrysdtalline materials) which allows to determine the values for electrical parameters when this structure is measured for different thicknesses of the semiconducting material.
TF-MoP-18 Characterization of Hydrogenated Amorphous Germanium, a-Ge:H, Thin Films Deposited by a Low Pressure Hollow Cathode Plasma-jet Reactive Sputtering System
G. Pribil, R.J. Soukup, N.J. Ianno (University of Nebraska, Lincoln); Z. Hubicka (Academy of Sciences)
Hydrogenated amorphous germanium, a-Ge:H, thin films were deposited by means of a low pressure dc hollow cathode plasma-jet system. A high density plasma was excited in a cylindrical nozzle of polycrystalline Ge working as a hollow cathode. An adjustable magnetic field was used for dc hollow cathode discharge stability and for partial confinement of the reactive plasma at the cathode outlet. The germanium nozzles were reactively sputtered in this high density hollow cathode discharge. Only nontoxic gases, argon and hydrogen, were used as a working gas mixture. Different conditions for the dc hollow cathode system were used for deposition. The a-Ge:H thin films were characterized by FTIR spectroscopy, Tauc band gap measurements, measurements of conductivity in the light and in the dark, and thickness measurements. The growth rate achieved was in the range of from 2 to 6 µm/h. This system has already been used for the deposition of high quality hydrogenated amorphous silicon, a-SiH, thin films using silicon nozzles. These experiments lead to the deposition of hydrogenated amorphous silicon/germanium alloy films, a-SiGe:H, without the use of silane or germane, using co-sputtering of Si and Ge nozzles in a multi-hollow cathode plasma jet system.
TF-MoP-19 Metal-insulator Transition in Ultrathin Copper Film Observed By Impedance Spectroscopy
X. Jin, Y. Zhou, Y.H. Hyun, T.-U. Nahm, C.O. Kim, Y.P. Lee (Hanyang University, Korea)
Ultrathin Cu films with a thickness of 0.3- 9 nm were thermally evaporated on a glass at room temperature in an ultrahigh vacuum chamber. The simultaneous in-situ monitoring of the complex impedance spectrum and the electrical resistance of the growing films was carried out. A metal-insulator transition was observed at a percolation onset thickness of 2.5 nm. The complex impedance of the films could be described by a parallel R-C equivalent circuit when the film thickness is smaller than the percolation onset thickness. With the further growth of film, the complex impedance undergoes a transition to an inductive equivalent circuit. A change in the electrical resistance at the percolation onset thickness follows a scaling law R ~ (d-dc)-t where t is an exponent, and are consistent with the observed transition.
TF-MoP-20 In-situ Buried Multiquantum Well Structures Studied by Photoreflectance and Photoluminescence Spectroscopy
A. Perez-Centeno, M. Lopez-Lopez, M. Melendez-Lira, M. Tamura (CINVESTAV-IPN, Mexico); T. Ishikawa (OTL, Tsukuba, Japan)
In this work we present the optical and structural characteristics of AlGaAs/GaAs multiquantum wells (MQWs) that were in-situ etched into mesa-stripe structures and then buried avoiding air-exposure of the active region at the sidewalls. The samples were fabricated using the following steps: First, by conventional photolithography and etching techniques mesa stripe-arrays of different widths and along different crystallographic directions were patterned on the surface of a 250nm thick GaAs protective layer of MQWs grown on a GaAs(001) wafer. The mesas depth was 200nm, in order to protect the MQWs a 50nm-thick GaAs layer was left unetched. Next the wafer was introduced into an UHV multichamber system with an MBE chamber and an etching chamber connected through UHV tunnels. The initial pattern was transferred down to the MQWs by in-situ Cl2-etching to a depth of ~200nm. Then, the sample was transferred in UHV to the MBE chamber, where an AlGaAs layer was overgrown thus completing the in-situ buried structures. Photoluminescence (PL) studies showed a decrease in the MQWs PL intensity and PL lifetime for mesa stripes of several microns in width, thus showing the effects of carrier trapping and non-radiative recombination at the etched-regrown interfaces on the sidewalls. The PL degradation was stronger for mesas along the [110] direction. In the photoreflectance (PR) spectra the transitions associated to the MQWs were clearly observed for mesas along [1-1 0], however for mesas along [110] we only observed the signal associated to the GaAs band-gap. PR spectra also showed the presence of oscillations above the GaAs band-gap value associated to built-in internal electric fields. The strength of the internal electric fields was in the order of 10 kV/cm, as obtained by the Franz-Keldysh model. We discuss the degraded optical properties for mesas along [110] in terms of the increased difficulties to smoothly overgrow mesa-sidewalls along this direction.
Time Period MoP Sessions | Topic TF Sessions | Time Periods | Topics | AVS2001 Schedule