Infrared optical properties of GaAs after N+ ion implantation

By analyzing the infrared reflection spectra of ion implanted GaAs, the optical dispersion parameters have been determined as function of depth from the implanted surface. Measured spectra were fitted to the calculated reflectance from two homogeneous implant layers on the unaffected bulk; the dispersion parameters in both layers were adjusted with different sets of layer thicknesses sampling the implanted region. Fluences ranging from 3x10¹³ to 2x10¹⁷ N⁺ ions/cm² were implanted at 1, 2, and 3 MeV. The resulting profiles of the frequency, strength and damping of the transverse optical lattice oscillator and of the high-frequency dielectric constant show maximum change at depth d_o determined by the ion energy. These maximum changes are comparable to the literature values for amorphous GaAs. Annealing from 300 to 600°C leads to gradual recovery, except near d_o. Sectioning experiments showed amorphous electron diffraction patterns in buried layers of about 1 μm thickness, encompassing d_o.     

MOCVD-grown AlGaAs/GaAs HBTs with epitaxially embedded p+ layers in extrinsic base

AlGaAs/GaAs HBTs having a 300 ? base were grown by two-stage MOCVD. In the second stage, p+ layers were epitaxially embedded in the extrinsic base to make the base contact. The current through the parasitic pn interface between the emitter and the embedded p+ layers was negligible. The technique employed here makes undamaged narrow-base HBTs possible.     

Switching conduction in ion-irradiated layers in GaAs

Experiments have been performed to investigate whether switching phenomena are observable in ion-irradiated GaAs. It is found that the V/I characteristics of H2+-and N+-irradiated layers show both negative resistance and switching from a high resistance to a low resistance state. Ion irradiation may thus be a useful method of fabricating switching devices.     

Electron transport in implant isolation GaAs layers

We report results of ac electrical characterisation of diode structures with Hg electrodes and an implant isolation layer formed on a n⁺GaAs substrate. The C–f and G–f characteristics at varying bias voltage between 0 and 10 V at room temperature were examined. The frequency of 50 mV measuring signal was changed from 100 Hz to 10 MHz. The implant isolation layers were performed by ion implantation of oxygen with 100 keV followed by 250 keV process on well conducting commercial GaAs substrate. Ion doses ranging from 10¹² to 5×10¹³ cm⁻² were used. The high frequency C–V characteristics showed that all tested implant isolation layers were fully depleted beginning from zero bias voltage. The G–f characteristics showed that ac conductance increases as a function of frequency and approximately follows ω^s dependence with s=0.6–0.9. These results are consistently interpreted as the result of the transport of injected carriers through the implant isolation layer via a hopping mechanism involving defects. Only in the samples with low level of defect density (low ion dose of 10¹² cm⁻²) for low frequency from 100 Hz to 5 kHz the high level conductance does not depend on frequency. This can be interpreted as the result of the transport of injected carriers through the extended states in conduction band.     

Incoherent annealing of implanted layers in GaAs

Incoherent light from filament lamps focused by elliptical mirrors has been used to activate implanted layers in GaAs. 4 × 10¹⁴Si⁺cm^-2and 2 × 10¹⁴Zn⁺cm^-2implants were annealed with Si3N4deposited by CVD at 400°C providing a surface protective layer. By taking advantage of the focusing properties of elliptical mirrors, most of the emitted light could be concentrated onto the GaAs to give annealing times × 1 sec. Differential Hall measurements show peak carrier concentrations of 6.5 × 10¹⁸cm^-3and 50% activation for the n+ layers. The Zn implants were completely activated and doped to ∼ 2 × 10¹⁹cm^-3. These results, together with the short annealing times, suggest the present approach to be an attractive alternative to both laser and conventional thermal annealing.     

Photoluminescence of Si-doped GaAs epitaxial layers

The effect of arsenic pressure on the amphoteric behavior of Si during the growth of the Si-doped (100)-, (111)Ga-, and (111)As-oriented GaAs layers is studied by photoluminescence spectroscopy. The edge luminescence band is examined, and the concentration and the degree of compensation as functions of the arsenic pressure are determined. Nonstoichiometry defects in GaAs layers grown with a deficit and an excess of arsenic are studied. It is shown that the defects formed in the (111)Ga- and (111)As-oriented layers are different in nature.     

Photoelastic optical directional couplers in epitaxial GaAs layers

Photoelastic channel optical waveguides showing strong lateral confinement at ?=1.55 ?m have been fabricated by etching a narrow slot through a Au Schottky contact deposited on an n/n+ GaAs layer. Such structures allow electro-optic directional coupler switches of short coupling length (~2 mm) to be easily realised.     

Photoelastic channel optical waveguides in epitaxial GaAs layers

Photoelastic channel optical waveguides have been fabricated in GaAs epitaxial layers. These waveguides exhibit strong lateral confinement in a single region of their cross-section. They compare favourably with previously demonstrated integrated optical waveguide structures.     

Observation of bias-dependent capture-emission processes in MBE-grown GaAs layers

The capture-emission process in a semiconductor sample was greatly affected by a varying electric field in DLTS analysis. The sample was an n-type GaAs layer grown on a 〈100〉-oriented seimi-insulating substrate by molecular beam epitaxy. An E_c − 0.5 eV electron trap level was found under lower bias voltage (−6 V) while both E_c − 0.40 eV and E_c − 0.50 eV traps were observed under higher bias voltage (−7.0 to −8.0 V). Emission rates have been measured that increased with increasing bias voltage from 0 V to −5 V and then decreased when biases were higher than −5 V. In the capacitance transient, when it revealed a delay, two levels, namely at −0.4 and −0.5 eV, appeared in the DLTS signals. However, the EL2 trap (E_c − 0.78 eV), found on another sample, revealed monotonically increasing emission with increasing bias.An explanation of these phenomena was proposed in light of the interaction of these two levels due to the electric field effect.     

Characterisation of Se implanted layers for GaAs FETs

Results on doping profile, uniformity of doping and mobility for Se implanted LEC grown semi-insulating GaAs substrates are presented as a function of Se ion energy and annealing temperature. SiO₂, RF sputtered Si₃N₄ and Ga₂O₃+Al are used as encapsulating layers. Surface effects (stress and damage) on doping efficiency of implanted Se ions are also included.     

Design of enhanced Schottky-barrier AlGaAs/GaAs MODFET's using highly doped p+surface layers

The design and performance of enhanced Schottky-barrier height modulation-doped AlGaAs/GaAs field-effect transistors (ESMODFET's) is discussed. Results are presented showing that the addition of a thin highly doped p⁺layer under the gate can increase the forward biased gate turn-on voltage from 0.8 V (conventional MODFET) to as high as 1.6 V. A mathematical model is presented that predicts the thickness and doping of the heterostructure layers required to obtain a given threshold voltage and effective Schottky-barrier height. It is predicted that this enhanced Schottky barrier will allow increased gate-voltage swings and thus significantly improve the noise margin of enhancement-mode MODFET circuits.     

Electric current controlled liquid phase epitaxy of GaAs on N+ and semi-insulating substrates

Electric current controlled liquid phase epitaxy (LPE) of GaAs has been performed on both n⁺ and semi-insulating substrates. Growth is induced by current flow across the substrate-melt interface. The furnace temperature is held constant during growth so that direct electrical control of the growth process is achieved. The dependence of the growth rate on both the electric current density across the substrate-melt interface and the ambient furnace temperature was determined. Current densities from 5 to 20 A/cm² were employed and furnace temperatures ranging from 680 to 800°C were used. Sustained steady state growth rates as small as 0.022μm/min and as large as 1.4μm/min were obtained. For a given furnace temperature and current density, the measured growth rates on semi-insulating substrates range from 48% to 77% of the rates obtained on n⁺ n substrates. The surface morphology of the epitaxial layers is observed to depend on the electric current density employed during growth. Electric current controlled doping modulation was studied in epitaxial layers grown from unintentionally doped melts. The degree of doping modulation achieved is approximately proportional to the change in applied current density. Approximately a 40% increase in the net electron concentration is obtained by changing the current density from 10 to 30 A/cm² during growth. Preliminary experiments with tin doped epitaxial layers indicate that similar changes in the amount of tin incorporation can be achieved.     

Role of substrate in electrical properties of GaAs implanted layers

The variation of implantation efficiency in different semi-insulating GaAs materials is shown to be correlated with he purity of the starting substrate, i.e. its Cr concentration and more especially its value of ND?NA (shallow levels). This last value appears to be a key parameter in the electrical properties of implanted layers.     

Microwave MESFET's fabricated in GaAs layers grown on SOS Substrates

Device-quality GaAs layers have been grown on silicon-on-sapphire (SOS) substrates by molecular beam epitaxy (MBE). Microwave MESFET's (gate length of ~0.8 µm) with transconductance of 140 mS/ mm,f_{max} = 20GHz, andf_{T} = 8.3GHz have been fabricated in these layers.     

LSA operation of GaAs layers in large-scale tunable microwave circuits

It is shown that the attainment of LSA oscillations in epitaxial layers of GaAs does not rely on operation of the layers in small primary resonant circuits. Restrictions on circuit dimensions have thus been relaxed and LSA oscillations obtained in large scale microwave cavities. Layers of thickness 9-12.5 microns have been operated at frequencies from 26.5-40 GHz, the oscillations being tuned over this band by a conventional short-circuit plunger. The frequency of LSA oscillation is shown to be determined entirely by the natural circuit frequency. The tuning characteristics of the oscillations in various waveguide circuits are described and some general circuit features emerge which are of importance for a tunable LSA source. In particular it is noted that in some circuits a localized and therefore fixed frequency resonance occurs. It is also noted that LSA oscillations cannot occur if circuit Q is insufficiently high. The work has been carried out on a pulse basis to avoid thermal effects, and most of the experiments described have been carried out using unencapsulated devices. The maximum efficiency observed in these experiments was 4 percent.     

Characterization of GaAs layers grown on polycrystalline GaAs by LPE and current controlled LPE

It is the purpose of this paper to investigate the suitability and effectiveness of growth of thin GaAs layers on polycrystalline GaAs substrates by liquid phase epitaxy (LPE) and current controlled LPE (CCLPE). During each growth run LPE and CCLPE were used to grow thin GaAs layers on two large-grain polycrystalline GaAs substrates cut from the same wafer and simultaneously placed in the same growth system. The grain boundary was exposed by cleaving the samples perpendicular to the grain boundary. Notnarski contrast, SEM, C-V and Hall measurements were performed in order to determine the surface morphology, discontinuity of epilayer at the grain boundary, epilayer thickness unform-ity, resistivity (in directions parallel and perpendicular to the grain boundary), and dopant concentration. The CCLPE system was carefully designed so that growth would take place only by electrotransport in the absence of convection or Peltier cooling. The results indicate that CCLPE yields layers with improved surface morphology and thickness uniformity as compared to those grown by LPE. In some samples the epilayer was discontinuous at certain grain boundaries. Results are presented on CCLPE growth rate dependence upon grain orientation, current density, and continuity of the epilayer at the grain boundary.     

High-performance AlGaAs/GaAs HBT's utilizing proton-implanted buried layers and highly doped base layers

Fabrication of AlGaAs/GaAs heterojunction bipolar transistors (HBT's) using a proton-implanted external collector layer and a highly doped base layer is presented. Influence of the proton implantation on base-collector junction characteristics is systematically investigated. At the optimized implantation condition, a buried semi-insulating layer beneath the external base is formed without deteriorating the junction current-voltage characteristics. In a fabricated HBT with 2 µm × 10 µm emitter size, a cutoff frequency fTof 50 GHz and a maximum oscillation frequency fmaxof 70 GHz have been achieved.     

Performance of power FET's fabricated on MBE-Grown GaAs layers

Power GaAs FET's of various sizes have been fabricated using MBE material containing a 1 µm-thick semi-insulating buffer layer. These devices, when operated between 6 and 12 GHz, exhibited state-of-the-art microwave performance. For example, the 3 mm devices gave an output power of 1.5 W with 10.9 dB associated gain at 6.4 GHz, a power-added efficiency of 42.6% with 1.4 W output power and 6.4 dB associated gain at 8 GHz. The results confirm the capability of MBE for producing high quality material with a sharp active layer/buffer interface.