A novel contactless method for characterization of semiconductors: Surface electron beam induced voltage in scanning electron microscopy

We present a novel contactless and nondestructive method called the surface electron beam induced voltage (SEBIV) method for characterizing semiconductor materials and devices. The SEBIV method is based on the detection of the surface potential induced by electron beams of scanning electron microscopy (SEM). The core part of the SEBIV detection set-up is a circular metal detector placed above the sample surface. The capacitance between the circular detector and whole surface of the sample is estimated to be about 0.64 pf It is large enough for the detection of the induced surface potential. The irradiation mode of electron beam (e-beam) influences the signal generation. When the e-beam irradiates on the surface of semiconductors continuously, a differential signal is obtained. The real distribution of surface potentials can be obtained when a pulsed e-beam with a fixed frequency is used for irradiation and a lock-in amplifier is employed for detection. The polarity of induced potential depends on the structure of potential barriers and surface states of samples. The contrast of SEBIV images in SEM changes with irradiation time and e-beam intensity.

Capacitance-voltage characteristic as a trace of the exciton evolvement from spatially direct to indirect in quantum wells

We have investigated the photo-excited capacitance-voltage (C-V) characteristics as well as the photoluminescence spectra under different biases of a wide quantum well (QW) embedded in an n(+)-i-n(+) double-barrier structure. The pronounced peak feature at zero bias in the C-V spectrum observed upon illumination is regarded as a kind of quantum capacitance related to the quantum confined Stark effect, originating from the spatial separation of the photo-generated electron and hole gas in the QW. This fact is further demonstrated through the comparison between the C-V curve with the PL intensity versus applied voltage relationship under the same excitation. The results may provide us with a more direct and sensitive means in the detection of the separation and accumulation of both types of free carriers-electrons and holes-in low-dimensional semiconductor structures, especially in a new type of optical memory cell.

Quality improvement of GaInNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The optimum growth condition of GaInNAs/GaAs quantum wells (QWs) by plasma-assisted molecular beam epitaxy was investigated. High-resolution X-ray diffraction and photoluminescence (PL) measurements showed that ion damage drastically degraded the quality of GaNAs and GaInNAs QWs and that ion removal magnets can effectively remove the excess ion damage. Remarkable improvement of PL intensity and obvious appearance of pendellosung fringes were observed by removing the N ions produced in the plasma cell. When the growth rate increased from 0.73 to 1.2 ML/s, the optimum growth temperature was raised from 460 degreesC to 480 degreesC and PL peak intensity increased two times. Although the N composition decreased with increasing growth rate, degradation of optical properties of GaInNAs QWs was observed when the growth rate was over 0.92 ML/s. Due to low-temperature growth of GaInNAs QWs, a distinctive reflection high-energy electron diffraction pattern was observed only when the GaAs barrier was grown under lower As-4 pressure. The samples with GaAs barriers grown under lower As-4 pressure (V/III ratio about 24) exhibited seven times increase in PL peak intensity compared with those grown under higher As-4 pressure (V/III ratio about 50). (C) 2001 Elsevier Science B,V. All rights reserved.

The improvement characteristics of homoepitaxial GaAs grown by atomic hydrogen-assisted molecular beam epitaxy

The electrical activity of defects in GaAs grown on GaAs substrates doped with Si and Be by both conventional molecular beam epitaxy (MBE) and atomic hydrogen-assisted MBE (H-MBE) were characterized by deep level transient spectroscopy. The trap densities are significantly reduced in the homoepitaxial GaAs grown by H-MBE compared to that grown by MBE. The reduction of trap densities is attributed to in situ passivation of these defects by atomic H during the growth. The improvement characteristics of GaAs materials will be significance for fabrication of semiconductor devices.

Improvement of thin silicon on sapphire (SOS) film materials and device performances by solid phase epitaxy

The increased emphasis on sub-micron CMOS/SOS devices has placed a demand for high quality thin silicon on sapphire (SOS) films with thickness of the order 100-200 nm. It is demonstrated that the crystalline quality of as-grown thin SOS films by the CVD method can be greatly improved by solid phase epitaxy (SPE) process: implantation of self-silicon ions and subsequent thermal annealing. Subsequent regrowth of this amorphous layer leads to a greater improvement in silicon layer crystallinity and channel carrier mobility, evidenced, respectively, by double crystal X-ray diffraction and electrical measurements. We concluded that the thin SPE SOS films are suitable for application to high-performance CMOS circuitry. (C) 2000 Elsevier Science S.A. All rights reserved.

Dynamic dc voltage band observed within each current branch in the transition from static to dynamic electric-field domain formation in a doped GaAs/AlAs superlattice

A dynamic dc voltage band was found emerging from each sawtooth-like branch of the current-voltage characteristics of a doped GaAs/AlAs superlattice in the transition process from static to dynamic electric-field domain formation caused by increasing the sample temperature. As the temperature increases, these dynamic dc voltage bands expand within each sawtooth-like branch, squeeze out the static regions, and join up together to turn the whole plateau into dynamic electric-field domain formation. These results are well explained by a general analysis of stability of the sequential tunneling current in superlattices. (C) 1999 American Institute of Physics. [S0003-6951(99)04443-5].

Improvement of photoluminescence of strained SiGe/Si layers on patterned Si substrate

A strained SiGe/Si superlattice structure has been grown on a patterned Si substrate and its photoluminescence has been studied. The patterned substrate is composed of pyramid-like structures. It is found that there are Ge-rich SiGe quantum wires (QWR) at the crossings of adjacent planes that form the pyramid-like structure. Photoluminescence of strained the SiGe layer grown on a planar substrate and a patterned substrate was compared. The total intensity of photoluminescence from the patterned substrate was 5.2 times larger than that from the planar substrates. The result is discussed and it is believed that this increase in photoluminescence is related to the observed QWRs. (C) 1999 Elsevier Science Ltd. All rights reserved.

Improvement of the photoluminescence from gallium nitride layers grown by MBE with an additional incident indium flux

The effect of using an indium flux during the MBE growth of GaN layers was investigated. The properties of these layers were studied using electron probe microanalysis, secondary ion mass spectroscopy, photoluminescence and cathodoluminescence. The optical properties of the GaN layers are shown to improve as compared with undoped GaN layers grown under nominally the same conditions but without an additional indium flux.

Conduction-band offset in a pseudomorphic GaAs/In0.2Ga0.8As quantum well determined by capacitance-voltage profiling and deep-level transient spectroscopy techniques

The conduction-band offset Delta E-C has been determined for a molecular beam epitaxy grown GaAs/In0.2Ga0.8As single quantum-well structure, by measuring the capacitance-voltage (C - V) profiling, taking into account a correction for the interface charge density, and the capacitance transient resulting from thermal emission of carriers from the quantum well, respectively. We found that Delta E-C = 0.227 eV, corresponding to about 89% Delta E-g, from the C - V profiling; and Delta E-C = 0.229eV, corresponding to about 89.9% Delta E-g, from the deep-level transient spectroscopy (DLTS) technique. The results suggest that the conduction-band discontinuity Delta E-C obtained from the C-V profiling is in good agreement with that obtained from the DLTS technique. (C) 1998 American Institute of Physics.

Improvement of stoichiometry in semi-insulating gallium arsenide grown under microgravity

A semi-insulating (SI) GaAs single crystal was recently grown in a retrievable satellite. The average etch pit density (EPD) of dislocations in the crystal revealed by molten KOH is 2.0 x 10(4) cm(-2), and the highest EPD is 3.1 x 10(4) cm(-2) This result indicates a quite good homogenity of the EPD which is much better than the ground-grown crystals. A similar better homogenity of the stoichiometry i.e., the [As]/([As] + [Ga]) ratio has been found in the space-grown SI-GaAs single crystal studied nondestructively using a new mapping method based upon X-ray Bond diffraction. The average stoichiometry in the space-grown crystal is 0.50007 with mean-square deviation of 6x10(-6), while the average stoichiometry in ground-grown SI-GaAs crystal is more than 0.50010. (C) 1998 Elsevier Science B.V. All rights reserved.

Stability improvement of selective oxidation during the fabrication of InGaAs/GaAs vertical cavity surface emitting laser

The effects of the carrier gas flow and water temperature on the oxidation rate for different reaction temperatures were investigated. The optimum conditions for stable oxidation were obtained. Two mechanisms of the oxidation process are revealed. One is the flow-controlling process, which is unstable. The other is the temperature-controlling process, which is stable. The stable region decreases for higher reaction temperatures. The simulation results for the stable oxidation region are also given. With optimum oxidation conditions, the stability and precision of the oxidation can be dramatically improved.

Improvement of Ge self-organized quantum dots by use of Sb surfactant

A Sb-mediated growth technique is developed to deposit Ge quantum dots (QDs) of small size, high density, and foe of dislocations. These QDs were grown at low growth temperature by molecular beam epitaxy. The photoluminescence and absorption properties of these Ge QDs suggest an indirect-to-direct conversion, which is in good agreement with a theoretical calculation. (C) 1998 American Institute of Physics. [S0003-6951(98)00420-3].

A voltage-adjustable three-phase rectifier with constant power flow

An three phase adjustable output voltage rectifier with constant power flow based on waveform gap patching principle is resented. By patching the gapes in the phase currents in parallel way as well as the ripple of the output voltage in series way, it implements the constant power flow from the three-phase line to the DC output without using any line frequency (and its harmonics) energy storage components. Principally, by treating only 22.4% power of the needed power output, this rectifier can supply constant power flow with adjustable output voltages without bring about any harmonic interferences to the power utility and achieve unite power factor.

Improvement of GaN-based light emitting diodes performance grown on sapphire substrates patterned by wet etching - art. no. 684107

An effective approach to enhance the light output power of InGaN/GaN light emitting diodes (LED) was proposed using pyramidal patterned sapphire substrates (PSS). The sapphire substrates were patterned by a selective chemical wet etching technique. GaN-based LEDs were fabricated on patterned sapphire substrates through metal organic chemical deposition (MOCVD). The LEDs fabricated on patterned sapphire substrates exhibit excellent device performance compared to the conventional LEDs fabricated on planar sapphire substrates in the case of the same growth and device fabricating conditions. The light output power of the LEDs fabricated on patterned sapphire substrates was about 37% higher than that of LEDs on planar sapphire substrates at an injection current of 20 mA. The significant enhancement is attributable to the improvement of the quality of GaN-based epilayers and improvement of the light extraction efficiency by patterned sapphire substrates.

The improvement of thick oxidized porous silicon layer growth process - art. no. 60290S

Oxidizing thick porous silicon layer into silicon dioxide is a timesaving and low-cost process for producing thick silicon dioxide layer used in silicon-based optical waveguide devices. The solution of H2O2 is proposed to post-treat thick porous silicon (PS) films. The prepared PS layer as the cathode is applied about 10 mA/cm(2) current in mixture of ethanol, HF, and H2O2 solutions, in order to improve the stability and the smoothness of the surface. With the low-temperature dry-O-2 pre-oxidizations and high-temperature wet O-2 oxidizations process, a high-quality SiO2 30 mu m thickness layer that fit for the optical waveguide device was prepared. The SEM images show significant improved smoothness on the surface of oxidized PS thick films, the SiO2 film has a stable and uniformity reflex index that measured by the prism coupler, the uniformity of the reflex index in different place of the wafer is about 0.0003.