K. Tominaga

Dependence of electrical properties on film thickness in Pb(ZrxTi1−x)O3 thin films produced by metalorganic chemical vapor deposition

Strongly c‐axis oriented Pb(ZrxTi1−x)O3 (PZT) thin films with tetragonal perovskite structure (0.45≤x≤0.52) were epitaxially grown on (100)Pt/(100)MgO substrates using metalorganic chemical vapor deposition. Film thickness could be varied by altering the growth time. The electrical properties of PZT thin films sharply change below a thickness of 0.5 μm: the dielectric constant and remanent polarization decrease while the coercive field increases. These phenomena are explained by a model in which a layer with low dielectric constant exists in series with the normal PZT layer. The origins of this layer are systematically studied and found to be the intrinsic stress produced by the coalescence of crystal grains.

Preparation and electrical properties of MOCVD‐deposited PZT thin films

Strongly [001] oriented lead zirconate‐titanate [Pb(ZrxTi1−x)O3] thin films with tetragonal structure (0<x<0.52) have been successfully grown on (100)Pt/(100)MgO substrates by using metalorganic chemical vapor deposition (MOCVD). The metalorganic precursors were Pb(C2H5)4, Zr(DPM)4 and Ti(i‐OC3H7)4. Scanning electron micrographs showed dense and noncolumnar growth with good surface morphology. The relative dielectric constants at room temperature were 200–350, and appeared to have less dependence on composition x than for PZT bulk ceramics. Typical D‐E hysteresis loops which occur with PZT bulk ceramics were observed. Remanent polarizations were 30–40 μC/cm2. The coercive field decreased from 65 to 40 kV/cm with increases in Zr content. The pyroelectric coefficients without poling treatment were about 3×10−8 C/cm2 K, showing almost the same value as that of poled PZT bulk ceramics.

Electrical Properties for Capacitors of Dynamic Random Access Memory on (Pb, La)(Zr, Ti)O3 Thin Films by Metalorganic Chemical Vapor Deposition

Lanthanum-modified lead zirconate titanate (PLZT) thin films (0.2–0.35 µ m) have been prepared on Pt/ SiO2/Si substrates at 650° C by the metalorganic chemical vapor deposition (MOCVD) method. The electrical properties of the films were examined as a function of lanthanum content in the range of 0–22 at.%. The remanent polarization and coercive field decreased as the La content increased in the range of 0–14 at.%, whereas they were maintained at constant values of 1 µ C/cm2 and 20 kV/cm, respectively, even in the paraelectric region where La content exceeds 15 at.% in ceramics. The effective dielectric constant of 1200 was obtained at La=10 at.%, and the effective charge densities were about 30 fF/µm2 when La=8–22 at.%. It has been suggested that the breakdown of the films does not occur up to about 1× 1015 cycles at an access voltage of 3 V by the unipolar acceleration test.

Quantum-Confined Stark Shift Due to Piezoelectric Effect in InGaAs/GaAs Quantum Wells Grown on (111)A GaAs

In0.2Ga0.8As/GaAs strained quantum wells (SQWs) were grown on GaAs (111)A just-oriented, 1° off and 5° off toward [110]- and [001]-oriented substrates. Dependence of strain relaxation on substrate orientation was studied by photoluminescence (PL) spectroscopy. Samples grown on GaAs (111)A 5° off toward [001]-oriented substrates showed the best optical characteristics and this substrate orientation was chosen for making p-i-n diodes. The PL spectrum shows the influence of a built-in electric field due to the piezoelectric effect. The blueshift of PL peaks with applied bias was demonstrated in a p-i-n structure. The PL peak corresponding to a 10 nm SQW blueshifted as much as 24 meV with only 1.2 V applied reverse bias.

Current self‐oscillations in photoexcited type‐II GaAs‐AlAs superlattices

Self‐oscillations of the photocurrent have been observed in type‐II GaAs‐AlAs superlattices. In addition to the fundamental frequency, several higher harmonics are present. The frequency of the oscillations can be tuned for a fixed carrier density from 15 to 120 MHz by simply changing the applied bias. The frequency distribution within a certain voltage range can be varied by changing the density of photoexcited carriers. For larger carrier densities, higher frequencies are observed in a different voltage range. This system could therefore be used as a high‐frequency oscillator, which can be controlled by two external parameters, the applied voltage and the light intensity.

Influence of Γ-X resonances on Γ1 ground state electron occupation in type-I GaAs/AlAs superlattice

The influence of Γ-X resonances on Γ1 ground state electron occupation in type-I direct-gap GaAs/AlAs superlattices was studied under an applied electric field. Photoluminescence and photocurrent-voltage characteristics showed anomalous behaviors at corresponding Γ-X resonance voltages. The experimental results demonstrate that Γ1-Xn transfer degrades the sweep-out of carriers, while X1-Γ2 transfer promotes it.

Preparation and electrical properties of ferroelectric (Pb,La) (Zr,Ti)O3 thin films by metalorganic chemical vapor deposition

Abstract Lanthanum-modified lead zirconate titanate (PLZT) thin films have been epitaxially grown on MgO (100) and Pt/SiO 2 /Si substrates at 650°C by the metalorganic chemical vapor deposition (MOCVD) method. The properties of the films were examined as a function of lanthanum content. The films were of a single perovskite structure, and highly oriented to the c -axis of the tetragonal phase on a MgO (100) substrate. The films were transformed from tetragonal to nearly cubic as the La content increased. The relative dielectric constants increased, and the remanent polarization and coercive field decreased as the La content increased. The switching time and leakage current densities decreased with increasing La content. The fatigue of the films was relaxed as the La content increased. The fatigue of the PLZT films does not occur below 1.0× 10 13 cycles at a working voltage of 3 V by means of the acceleration test.

Negative peaks in photocurrent spectra of thick barrier GaAs/AlAs multiple quantum wells

We have experimentally studied photocurrent (PC) spectral features of a relatively thick barrier multiple quantum well (MQW) p-i-n diode at 18 and 80 K as a function of axial electric field. It is found that PC spectra do not always reflect the photoabsorption spectral line shape under the low field condition and show negative peaks at the exciton resonance wavelengths. These PC dips are qualitatively explained by considering the distribution of photogenerated carriers within the intrinsic region and competition between carrier transit and recombination times. Furthermore, the dominance of electron-tunneling transport for the low temperature PC mechanism explains enhanced negative peaks in the PC spectra at 80 K when the tunneling assisted drift is reduced.

Avalanche breakdown mechanism originating from Γ–X–Γ transfer in GaAs/AlAs superlattices

An avalanche breakdown mechanism in GaAs/AlAs type-I superlattices is demonstrated. This mechanism shows its power at a bias voltage where both of the following two conditions are met. One is electron transfer from the Γ ground state to the X ground state (Γ1-X1), and the other is the escape of electrons from the X1 state to the second Γ state (X1-Γ2). Under both conditions, because the AlAs barriers become transparent for electron transport due to the Γ1-X1-Γ2 path, the drift speed (i.e., the acceleration of electrons) grows, and then the superlattice shows the phenomenon of avalanche breakdown. From our experimental results for various GaAs/AlAs superlattices, it is thought that such avalanche breakdown frequently occurs when type-I GaAs/AlAs superlattices have thick barrier widths.

Delayed photocurrent affected by Γ‐X resonance in GaAs/AlAs type‐I short‐period superlattices

Delayed photocurrents were observed in GaAs/AlAs type‐I short‐period superlattices by measuring time‐resolved photoresponses under ultrashort optical pulse excitation. According to the envelope function calculations, the X1 state in AlAs barriers resonates with the Γ2 state in the adjacent GaAs wells at a bias voltage where the delayed photocurrents were conspicuous. These results strongly suggest that the dynamic carrier transport process is significantly influenced by X1‐Γ2 resonance effects in the superlattices.