Laser annealing of capped and uncapped GaAs

Ion-implanted GaAs samples have been annealed using a Q-switched ruby laser. Both Si3N4 capped and uncapped samples decomposed and gallium precipitates were formed at the surface. The gallium could be removed by dissolution in HCl to leave high-quality GaAs.     

Structural characterisation of CdS layers deposited on porous p-type GaAs

The purpose of this paper is to investigate the initial stage of cadmium sulphide (CdS) layer deposited on porous p-type GaAs substrate by vacuum evaporation technique. The deposited CdS layer was investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM imaging shows that the CdS was penetrated deeply in the porous structure down to the bottom and reaching the interface GaAs/porous GaAs. The AFM image demonstrates that the CdS deposited are grains of several nanometres and XRD patterns exhibit that the deposited layer has a hexagonal prominent phase.     

Optical properties of porous nanosized GaAs

The optical properties of porous GaAs layers obtained by electrochemical etching of single-crystal n-and p-GaAs(100) wafers are studied. It is shown that the shape of the nanocrystals, their mean diameter, and their surface states depend on the conductivity type of the initial crystal. A shift of the peaks corresponding to the main optical phonons to lower frequencies in the Raman spectra is observed. Surface-phonon frequencies determined from the Raman spectra coincide with those determined from the reflection spectra in the infrared spectral region. The forms of the spectral dependences of the reflection coefficient in the phonon-resonance region in bulk GaAs differ from those in porous GaAs. This is caused by the appearance of additional oscillators related to spatially confined lattice vibrations in GaAs nanocrystals. Atomic-force microscopy is used to study the surface morphology of porous GaAs samples formed on the n-GaAs substrates, and a nanosized surface profile is observed. Estimations made for the mean diameter of GaAs nanocrystals based on data from the Raman scattering, infrared spectroscopy, photoluminescence, and atomic-force microscopy yield results that are in good agreement with each other.     

Structural and electrical characteristics of lanthanum oxide gate dielectric film on GaAs pHEMT technology

This paper investigates the feasibility of using a lanthanum oxide thin film (La_2O_3) with a high dielectric constant as a gate dielectric on GaAs pHEMTs to reduce gate leakage current and improve the gate to drain breakdown voltage relative to the conventional GaAs pHEMT. An E/D mode pHEMT in a single chip was realized by selecting the appropriate La_2O_3 thickness. The thin La_2O_3 film was characterized: its chemical composition and crystalline structure were determined by X-ray photoelectron spectroscopy and X-ray diffraction, respectively.La_2O_3 exhibited good thermal stability after post-deposition annealing at 200, 400 and 600 ℃ because of its high binding-energy (835.6 eV). Experimental results clearly demonstrated that the La_2O_3 thin film was thermally stable.The DC and RF characteristics of Pt/La_2O_3/Ti/Au gate and conventional Pt/Ti/Au gate pHEMTs were examined.The measurements indicated that the transistor with the Pt/La_2O_3/Ti/Au gate had a higher breakdown voltage and lower gate leakage current. Accordingly, the La_2O_3 thin film is a potential high-k material for use as a gate dielectric to improve electrical performance and the thermal effect in high-power applications.     

氧化鑭薄膜閘極結構與電性並應用在砷化鎵高速場效應電晶體研製

This paper investigates the feasibility of using a lanthanum oxide thin film （La2O3） with a high dielectric constant as a gate dielectric on GaAs pHEMTs to reduce gate leakage current and improve the gate to drain breakdown voltage relative to the conventional GaAs pHEMT. An E/D mode pHEMT in a single chip was realized by selecting the appropriate La2O3 thickness. The thin La2O3 film was characterized： its chemical composition and crystalline structure were determined by X-ray photoelectron spectroscopy and X-ray diffraction, respectively. La2O3 exhibited good thermal stability after post-deposition annealing at 200, 400 and 600℃ because of its high binding-energy （835.6 eV）. Experimental results clearly demonstrated that the La2O3 thin film was thermally stable. The DC and RF characteristics of Pt/La2O3/Ti/Au gate and conventional Pt/Ti/Au gate pHEMTs were examined. The measurements indicated that the transistor with the Pt/La2O3/Ti/Au gate had a higher breakdown voltage and lower gate leakage current. Accordingly, the La2O3 thin film is a potential high-k material for use as a gate dielectric to improve electrical performance and the thermal effect in high-power applications.     

GaAs Schottky diodes with near-ideal characteristics

The I-V characteristics of some GaAs Schottky-barrier IMPATT diodes are found to be nearly ideal at operating temperatures (∼200°C). Fabricated on epitaxial n-type substrate with platinum contacts, the diodes use a truncated cone shape to avoid soft breakdown. The breakdown voltage and its temperature dependence are close to calculated values for one-sided abrupt junctions.     

Device characteristics of amorphous indium-gallium-zinc-oxide channel capped with silicon oxide passivation layers

It was investigated how the amorphous indium-gallium-zinc-oxide (a-IGZO) channel of a back-gate of thin film transistor (TFT) is affected by the deposition of silicon oxide layers on their top surfaces by radio frequency magnetron sputtering. Preliminary investigations showed that the deposition of silicon oxide layer caused damages to the surfaces of pristine silicon wafers resulting in substantial roughening. However, bombardments by the energetic particles involved in the sputtering process seem to have played beneficial roles in that the a-IGZO channel TFTs showed improved performances in respect of the carrier density, field effect mobility, and on-off current ratio. Such improvements are attributed to the modification of the a-IGZO channel to decrease the concentration of oxygen vacancy sites and/or to average the oxygen vacancy sites thereby increasing the carrier concentrations and decreasing the density of trap sites, as revealed in the negative shift of the threshold voltage. On the other hand, such channel modification by the passivation process resulted in the slight increase in the subthreshold swing. It is suggested that the a-IGZO channel TFTs can be passivated by simple sputtering process without etch stop layer since the process rather improved the device performances despite some damages to the passivated surfaces.     

Comparison of doping profiles in capless annealed and dielectrically capped — Annealed ion implanted GaAs

Recent results of a capless method of annealing ion implanted GaAs are reported. The physical mechanism and effectiveness of the process are described and comparison of doping profiles from wafers annealed with a reactively sputtered Si₃N₄ dielectric encapsulation and with the capless process is given. Capless annealing is shown to consistently result in narrower profiles for various dopants and implant energies. The observed differences are shown to be consistent with enhanced diffusion in the dielectric capped samples, and the effective diffusion coefficients, which are of the order of 10^?15 cm²/s for Se, differ by as much as a factor of two.     

Structural, morphological, optical and photoluminescent properties of spray-deposited ZnSe thin film

Zn Se thin films are successfully deposited by spray pyrolysis deposition technique.Deposited thin films are characterized by X-ray diffraction study,and it reveals that spray-deposited Zn Se thin films are polycrystalline with hexagonal crystal structure.Surface morphology is carried out by scanning electron microscopy.It shows cotton-like morphology,and optical properties,such as absorbance,transmittance,reflectance,band gap,refractive index,extinction coefficient are studied.Photoluminescence shows strong emission at 497 nm.Also,spraydeposited Zn Se thin films are hydrophilic in nature,which is shown by contact angle meter.     

Structural and electrical contact properties of LPE grown GaAs doped with indium

We have studied the effects of In doping on the structural and electrical properties of a liquid phase epitaxially (LPE) grown GaAs. The results of surface morphology studies show that macroscopically, a terrace-free area in certain regions can be seen on the surface of a GaAs layer doped with In of 2.4 × 10¹⁹ cm³. The full widths at half-maximum (FWHM) of x-ray double crystal rocking curves show that a GaAs epi-layer of good crystalline quality can be obtained by doping In to a concentration up to 4.3 × 10¹⁹ cm^-3, beyond which a sharp increase in the FWHM is observed. Etch pit density (EPD) study also shows that the dislocation density is reduced by doping the epi-layer with In. At an optimum In concentration, 2.4 × 10¹⁹ cm^-3, the EPD was reduced by a factor of 20 when measured at the surface of a 9μm thick epi-layer. The I-V characteristics of Au-GaAs Schottky diodes show, for the layer with an optimal In concentration, an ideality factor close to 1.04 over more than seven decades of current. For the same layer, the reverse I-V characteristics are close to an ideal Schottky diode and could be fitted by a theoretical curve, combining the thermionic field emission and thermionic emission. For doping levels higher than 6 × 10¹⁹ cm^-3, the epitaxial layer quality deteriorated. We report the results obtained from the Nomarski optical microscope, double crystal x-ray rocking curves, etch pit density, forward and reverse I-V characteristics, and the theoretical current transport models.     

GaAs optoelectronic integrated receiver with high-output fast-response characteristics

For obtaining high-output level and high-speed response in a GaAs-based optoelectronic integrated receiver, a circuit involving a two-stage amplifier was first designed and fabricated. A high sensitivity of 400 V/W has been achieved while preserving a high-speed response of 2-Gbit/s nonreturn to zero (NRZ).     

Quantum confinement model for electric transport phenomena in fresh and stored photoluminescent porous silicon films

A quantum confinement model is proposed to explain the electrical transport properties in fresh and stored porous silicon (PS) films. In the present paper, the model is verified for the temperature dependence of the dark current. The studied samples are formed mainly by a network of nanowires, suggesting the separation of the electron Hamiltonian into a longitudinal and a transversal part. The last one can be well described by a two-dimensional (2D) cylindrical infinite quantum well, whose levels determine the dark current activation energies. The storage oxidation induces modifications both in the number and the values of the activation energies, which are in excellent agreement with our model.     

Low-noise characteristics of pulse-doped GaAs MESFETs with planarself-aligned gates

The authors report on the low-noise characteristics of pulse-doped GaAs MESFETs. The pulse-doped structure consists of an undoped GaAs buffer layer, a highly doped thin GaAs active layer, and an undoped GaAs cap layer grown by organometallic vapor phase epitaxy. Even though the electron mobility of this structure is 1500 cm²/V-s, the noise figures obtained are 0.72 dB at 12 GHz and 1.15 dB at 18 GHz. In addition, the noise figures are insensitive to the drain current. It was found that the noise characteristics improve as the active layer of the pulse-doped MESFET becomes thinner. These mechanisms can explain the cancellation effect between the drain noise current and gate-induced noise current as reported for HEMTs     

Modification of surface characteristics in GaAs with dry processing

We have studied the effects of plasma exposure on GaAs with Raman scattering, Hall mobility, capacitance-voltage (C-V), and Secondary ion mass spectroscopy (SIMS) measurements. The Raman studies show that the dependence of carrier removal on plasma species and substrate temperatures is consistent with the thermal migration of the plasma species. The free-carrier removal due to a plasma exposure is apparently dependent on the impurities present in the material. After a hydrogen plasma exposure, a decrease in carrier concentration and mobility occurs for the S-doped samples, while an increase in mobility and a decrease in carrier concentration occurs for the Si-doped samples. The production of: surface roughness due to various surface treatments was also observed by means of Raman spectroscopy and ellipsometry. The observed narrowing of the longitudinal optical (LO) phonon line and the plasmon-phonon (L⁺) line may be correlated with the degree of annealing. The degradation at the interface between Si-N films and GaAs substrates was observed to depend on the means of film deposition such as plasma-enhanced deposition (PED)and chemical-vapor deposition (CVD).     

Current-voltage characteristics of silicon-doped GaAs nanowhiskers with a protecting AlGaAs coating overgrown with an undoped GaAs layer

A technique for measurement of longitudinal current-voltage characteristics of semiconductor nanowhiskers remaining in contact with the growth surface is suggested. The technique is based on setting up a stable conductive contact between the top of a nanowhisker and the probe of an atomic-force microscope. It is demonstrated that, as the force pressing the probe against the top of the nanowhisker increases, the natural oxide layer covering the top is punctured and a direct contact between the probe and the nanowhisker body is established. In order to prevent nanowhiskers from bending and, ultimately, breaking, they need to be somehow fixed in space. In this study, GaAs nanowhiskers were kept fixed by partially overgrowing them with a GaAs layer. To isolate nanowhiskers from the matrix they were embedded in, they were coated by a nanometer layer of AlGaAs. Doping of GaAs nanowhiskers with silicon was investigated. The shape of the current-voltage characteristics obtained indicates that introduction of silicon leads to p-type conduction in nanowhiskers, in contrast to n-type conduction in bulk GaAs crystals grown by molecular-beam epitaxy. This difference is attributed to the fact that the vapor-liquid-solid process used to obtain nanowhiskers includes a final stage of liquid-phase epitaxy, a characteristic of the latter being p-type conduction obtained in bulk GaAs(Si) crystals.     

Structural and photoluminescence properties of low-temperature GaAs grown on GaAs(100) and GaAs(111)A substrates

Undoped, uniformly Si-doped, and δ-Si-doped GaAs layers grown by molecular-beam epitaxy on (100)- and (111)A-oriented GaAs substrates at a temperature of 230°C are studied. The As₄ pressure is varied. The surface roughness of the sample is established by atomic-force microscopy; the crystal quality, by X-ray diffraction measurements; and the energy levels of different defects, by photoluminescence spectroscopy at a temperature of 79 K. It is shown that the crystal structure is more imperfect in the case of GaAs(111)A substrates. The effect of the As₄ flux during growth on the structure of low-temperature GaAs grown on different types of substrates is shown as well.     

Frequency-dependent electrical characteristics of GaAs MESFETs

Output resistance and transconductance of GaAs MESFETs have been observed to change significantly at very low frequencies. Extensive measurements of these characteristics as a function of device bias are reported. Direct measurements of the dispersive behavior between DC and 100 kHz and over a broad temperature range have been made on ion-implanted monolithic microwave IC (MMIC) devices. Conductance deep level transient spectroscopy (DLTS) and microwave S-parameter measurements have also been made to investigate this behavior. These measurements reveal that surface or channel-substrate interface traps in the material are most likely to be responsible for the observed behavior. A new equivalent-circuit model which accounts for many of the observed characteristics is developed. Unlike previously proposed equivalent circuits, the model does not rely on physically unrealistic circuit element values in order to obtain accurate performance predictions. The bias dependence of circuit element values is computed for one device. Effects not described by the model are also discussed     

Backgate-induced characteristics of ion-implanted GaAs MESFET's

Three important characteristics of GaAs ion-implanted MESFET's associated with the phenomenon of backgating have been identified and measured. These include a negative backgate capacitance, initiation and/or control of low-frequency oscillations, and enhancement of g-r noise, all related to the deep-level electron traps present in the semi-insulating substrate beneath the implanted layer. Low-frequency oscillations have been observed mostly in devices with high gate-leakage current under conditions involving zero to large negative backgate bias. The frequency of oscillations and the backgate negative-capacitance magnitude have been found to decrease and increase, respectively, with the increase of the negative backgate bias voltage. This implies a decrease in the capture/emission cross section of traps at high fields.     

Growth and characteristics of p-type doped GaAs nanowire

The growth of p-type GaAs nanowires (NWs) on GaAs (111) B substrates by metal-organic chemical vapor deposition (MOCVD) has been systematically investigated as a function of diethyl zinc (DEZn) flow.The growth rate of GaAs NWs was slightly improved by Zn-doping and kink is observed under high DEZn flow.In addition,the I-V curves of GaAs NWs has been measured and the p-type dope concentration under the Ⅱ/Ⅲ ratio of 0.013 and 0.038 approximated to 1019-1020 cm-3.     

DC and AC characteristics of delta-doped GaAs FET

Molecular-beam-epitaxy-grown GaAs field-effect transistors (FETs) using a delta-doped channel are discussed. FETs having gate lengths of 1.3 μm showed transconductances as high as 290 mS/mm at a current density of 200 mA/mm. The measured f_T was 14.5 GHz and the extracted f_max was 30 GHz. These results are the best reported in a FET with this material structure having a delta-doped channel