N-type GaAs Epitaxial Layers Research Articles

A novel dual-band terahertz metamaterial modulator

This paper presents a novel terahertz (THz) dual-band metamaterial modulator on a gallium arsenide (GaAs) substrate with an n-type GaAs epitaxial layer. By adopting split strips of two different lengths in one cell of metamaterial structure, the metamaterial can resonate at 0.58 THz and 1.0 THz. The electric-controlled depletion region of Schottky contacts is numerically analyzed to explain the mechanism of modulator and estimate the modulation depths. The measurements show that the modulation depths at the resonant frequencies (0.61 THz and 1.03 THz) can reach 50% and 85% under an inverse bias of 17 V in good agreement with simulations. The surface plasmons are utilized to explain the difference of modulation depths at two bands, and the tri-band modulator is also discussed for the scalability of the metamaterial structure. The presented dual-band modulator can help to design THz modulators with multi-band modulations and high modulation depths.

Enhancement of terahertz electromagnetic wave emission from an undoped GaAs∕n-type GaAs epitaxial layer structure

We have investigated the emission of the terahertz electromagnetic wave from an undoped GaAs (200nm)∕n-type GaAs (3μm) epitaxial layer structure (i-GaAs∕n-GaAs structure), where the doping concentration of the n-GaAs layer is 3×1018cm−3. It is found that the first-burst amplitude of terahertz wave of the i-GaAs∕n-GaAs sample is remarkably larger than that of a n-GaAs crystal, which means that the i-GaAs layer enhances the terahertz emission intensity. The first-burst amplitude of the i-GaAs∕n-GaAs sample, by tuning the pump-beam energy to the higher energy side, exceeds that of an i-InAs crystal that is known as one of the most intense terahertz emitters. We, therefore, conclude that the i-GaAs∕n-GaAs structure is useful to obtain intense terahertz emission.

Chemical characterization by XPS of Cu/Ge ohmic contacts to n-GaAs

Chemical composition of Cu/Ge layers deposited on a 1 μm thick n-type GaAs epitaxial layer (doped with Te to a concentration of 5 × 10 18 cm −3) and its interface were examined ex situ by XPS combined with Ar + sputtering. These measurements indicate a diffusion of Cu and Ge from the Cu/Ge layer towards GaAs and, also, an out-diffusion of Ga and As from the GaAs layer to the metallic films. The Auger parameter corrected Auger spectra and XPS spectra show only Cu and Ge metals in the in the Cu/Ge layer and in the interface.

Osmium impurity-related deep levels in n-type GaAs

The 5d transition-metal impurity, osmium, has been incorporated during the growth of n-type GaAs epitaxial layers using low-pressure metal-organic chemical-vapor deposition to characterize defect states associated with this heavy and, therefore, thermally stable dopant impurity. Deep-level transient spectroscopy has been employed to investigate the electrical characteristics of Os-related deep levels. Two prominent deep levels have been identified with Os at Ec−0.28eV (Os1) and Ec−0.41eV (Os2) in the upper half-band-gap of GaAs, while no Os-related level has been clearly detected in the lower half-band-gap. The detailed characteristics determined for the two levels include thermal emission rate signatures, leading to the above cited thermal activation energies, electron-capture cross sections, and their temperature dependence, measured by direct pulse-filling technique and deep-level concentrations. Further, both levels are found to exhibit a significant dependence of thermal emission rates on the junction electric field. While Os1 does not allow accurate quantitative investigations of this field dependence due to its relatively low concentration, detailed quantitative data on the field dependence of the level Os2 are reported. Analysis of these data in light of the available theoretical models allows us to conclude that Os2 is probably a substitutional donor-type defect in GaAs.

Photovoltaic effects on Franz–Keldysh oscillations in photoreflectance spectra: Application to determination of surface Fermi level and surface recombination velocity in undoped GaAs∕n-type GaAs epitaxial layer structures

We demonstrate that the surface Fermi level and surface recombination velocity in undoped GaAs∕n-type GaAs (i-GaAs∕n-GaAs) epitaxial layer structures can be simultaneously estimated from Franz–Keldysh oscillations (FKOs) in photoreflectance spectra, taking account of the photovoltaic effects. Initially, we performed computational studies on the surface electric fields in i-GaAs∕n-GaAs structures under the illumination of a probe beam. The surface electric-field strength is sensitive to the surface Fermi level and surface recombination velocity. We have found that these parameters can be evaluated from the dependence of the surface electric-field strength on the probe-beam power density. Next, we estimated experimentally the surface Fermi level and surface recombination velocity in an as-grown i-GaAs∕n-GaAs structure by analyzing the photovoltaic effect on the FKOs. The period of the FKOs increases with a decrease in the probe-beam power density. The surface Fermi level and surface recombination velocity are estimated from the probe-beam power dependence of the surface electric-field strength that is obtained from the analysis of the FKOs. We have also applied the analysis of the photovoltaic effect to the assessment of the GaAs surfaces exposed to the nitridation and the catalytic chemical vapor deposition of SiNx. In addition, we have derived a line-shape function of the FKOs from i-GaAs∕n-GaAs structures, which is applicable even to the FKOs influenced by a probe-beam interference phenomenon in a layered structure.

Osmium related deep levels in n-type GaAs

Deep levels due to 5d transition metal, osmium (Os), in n-type GaAs epitaxial layers grown by metal-organic vapour-phase epitaxy are investigated using deep level transient spectroscopy (DLTS). Two clear and prominent peaks are observed in majority carrier (electron) emission spectra in GaAs:Os samples in addition to a weaker, broad feature at higher temperatures, which are absent in the Os-free, reference samples. The well-resolved, prominent peaks are attributed to deep levels associated with Os impurity having activation energies Ec−0.28eV and Ec−0.41eV at a junction electric field of ∼1.4×105V/cm. Both of these peaks are found to be field dependent. Detailed data on emission rate signatures, apparent electron capture cross-sections and defect concentrations are reported for these levels. No Os-related deep level peaks could be clearly delineated in minority-carrier injection DLTS, at this stage, although some evidence is found for a low-energy deep level band.

A Comparison of Low-Energy As Ion Implantation and Impurity-Free Disordering Induced Defects in N-Type GaAs Epitaxial Layers

We have compared the electrical properties of n-type GaAs layers disordered by either 40 keV As ion implantation or an impurity-free process employing an SiO2 capping layer. Current–voltage and capacitance–voltage measurements on Au Schottky barrier diodes fabricated on the processed layers showed that the impurity-free method retained the much better electrical quality of the GaAs epitaxial layers. Different sets of defects were observed in the implanted samples and impurity-free disordered samples, which meant that the charge transfer across the Schottky barriers was different in the two cases. Our results further reveal that the concentrations and diffusion lengths of defects created by ion implantation were much larger. The impurity-free method retains the better electrical quality of the semiconductor material.

Comparisons using optoelectronic modulation spectroscopy of n-type GaAs epitaxial layers formed on buffer layers prepared at normal and low temperatures

Optoelectronic modulation spectroscopy (OEMS) has been used to examine defect-related states in GaAs metal-semiconductor, field-effect transistor (MESFET) structures prepared by molecular-beam epitaxy (MBE) on buffer layers formed at normal and low temperatures. The technique was used to simultaneously observe the spectra of defect states near to the interface with the buffer layer and in the active layer. There were few responses that were common to the structures. The most prominent was an electron trap with a depth of 0.92 eV. This was present throughout both structures and may be from a native defect in GaAs. With few exceptions the states seen in a particular structure were present in the active layer as well as near to the interface with the buffer layer suggesting that, in each case, the defects originated from the active/buffer layer interface or from the buffer layer. The most significant difference was that states in the low-temperature (LT)-based structure generally exhibited replicating responses with specific energy periodicities. A model describes this in terms of optical-absorption transitions to the local vibrational states of defects. Five different replica series were observed. The vibrational characteristics of the defects found in the LT material suggest that they are different in character and extent to those defects found in material formed on a normally prepared buffer layer. Their Frank-Condon energies ranged from 9–332 meV.

Photoacoustic measurement of transport properties in doped GaAs epitaxial layers

The photoacoustic technique under heat transmission configuration is used to determine the effect of doping on both the thermal and transport properties of p- and n-type GaAs epitaxial layers grown on GaAs substrate by the molecular beam epitaxial method. Analysis of the data is made on the basis of the theoretical model of Rosencwaig and Gersho. Thermal and transport properties of the epitaxial layers are found by fitting the phase of the experimentally obtained photoacoustic signal with that of the theoretical model. It is observed that both the thermal and transport properties, i.e. thermal diffusivity, diffusion coefficient, surface recombination velocity and nonradiative recombination time, depend on the type of doping in the epitaxial layer. The results clearly show that the photoacoustic technique using heat transmission configuration is an excellent tool to study the thermal and transport properties of epitaxial layers under different doping conditions.

Microstructure and properties of aluminum contacts formed on GaAs(100) by low pressure chemical vapor deposition with dimethylethylamine alane source

We report on a low pressure chemical vapor deposition of metallic thin aluminum films on GaAs (001) with a dimethylethylamine alane (DMEAA) source and H2 as a carrier gas. The deposition temperatures varied in the range 130–360°C. Integrated volumes for Al (111), (100), (110)R, and (110) grains were estimated by the x-ray diffraction technique and the growth temperature values preferred for every type of grains were observed. The experimentally observed dominance of Al(110)R over Al(110), irrespective of the substrate miscut direction, supports the GaAs(100) inner anisotropy effect on the Al grain orientation. Electrical resistivity was 5 ·cm for the best Al films. The Schottky barrier heights were near a 0.7 eV level and the ideality factor n=1.1. Nonalloyed ohmic contacts were fabricated on an n-type GaAs epitaxial layer with an additional set of Si-layers near the Al/GaAs interface. Specific contact resistance, c=7 cm2, was measured. Best contacts were obtained at a deposition temperature lower than 250°C.

Electronic and isochronal annealing properties of electron traps in rapid thermally annealed SiO2-capped n-type GaAs epitaxial layers

We have recently shown [P. N. K. Deenapanray et al., Appl. Phys. Lett. 77, 626 (2000)] that four electron traps S1(Ec−0.23 eV), S2(Ec−0.46 eV), S3(Ec−0.72 eV), and S4(Ec−0.74 eV) are introduced in rapid thermally-annealed (RTA) SiO2-capped n-type GaAs epitaxial layers. In the present study, we have used deep level transient spectroscopy to investigate the electronic and annealing properties of these deep levels. The electron emission kinetics of S1 is enhanced by an electric field, and the activation energy of S1 decreases linearly from ∼233 to ∼199 meV when the field is increased from 7.5×104 to 13.4×104 V cm−1. The intensities of S1, S2, and S4 show Arrhenius-like dependencies on the RTA temperature, which relate to the outdiffusion of Ga atoms into the SiO2 layer. The intensity of S2(VGa–SiGa) also increases exponentially with the square of the annealing time for RTA at 800 °C. Isochronal annealing experiments show that S1 and S2 are thermally stable below 500 and 400 °C, respectively. S4, which is a member of the EL2 family, is stable up to 600 °C. Secondary defects are introduced during isochronal annealing above 400 °C, and some of these defects are thermally stable at 600 °C.

Electrical and optical properties of a polymer semiconductor interface

We have fabricated a stable electrical interface between a range of n-type, p-type and undoped GaAs epitaxial layers and a film of conductive p-type polyaniline. The IV characteristics of this device show rectifying behaviour consistent with a potential barrier formed at the interface. We have used simple thermionic emission theory for a Schottky type barrier to model the results. This has shown no significant change in the barrier height as the fermi level moves through the semiconductor bandgap by changing from n-type to p-type substrates. We have measured the temperature dependence of the barrier height. Using n-type GaAs we can observe electroluminescence centred at 680 nm with a FWHM of 160 nm at a current density of 5 mA cm −2. Our data suggests that the interface characteristics are controlled by surface states on the GaAs layer.

Photoreflectance spectra from a surface and an interface of n-type GaAs epitaxial layers and their modulation frequency dependence

We have performed photoreflectance (PR) measurements for n-type GaAs epitaxial layers grown on n + and S.I. substrates and have observed that the PR spectrum is a superposition of two signals which originate from the surface and from the interface between a grown layer and a substrate. From the modulation beam power dependence, we reveal that the photo-carriers, which cause the PR signal from the interface, are generated at the vicinity of the interface. We have also demonstrated that the PR signal from the surface can be separated from the other contributions at a high modulation frequency. Through measuring the modulation frequency dependence, we have confirmed that this results from the difference of the lifetimes of photo-carriers at the surface and the interface.

Intermittent breakdown of current-oscillation tori in n-type GaAs epitaxial layers.

Self-sustained current oscillations in high-purity n-type GaAs epitaxial layers were investigated for intermittent behavior in the control parameter plane constituted by the constant bias voltage and an external magnetic field. Intermittent windows were observed in a sequence of Hopf bifurcations. Quasiperiodic and frequency-locked oscillations were found to follow an intermittency type-II or type-III route to chaos in the Pomeau-Manneville classification scheme. Additionally, intermittent bursts due to noise were observed at voltages well below the threshold of bifurcation.

Quantum well character of semiconductor homostructures underlying the occurrence of a negative differential mobility

The present work is seeking to trace a quantum well character in the functionality of the active epitaxial layer of semiconductor homostructures as underlying the occurrence of a negative differential mobility in the persistent photoconductive response of illuminated devices. The proposed scheme is argued to adequately interpret the characteristics of a wide variety of differently prepared n-type GaAs epitaxial layer samples.

Regular and chaotic current oscillations in n-type GaAs in transverse and longitudinal magnetic fields.

Self-sustained current oscillations due to impurity breakdown in n-type GaAs epitaxial layers are reported for various magnetic-field strengths and orientations of the field with respect to the direction of current. In all cases relaxation oscillations at the onset of breakdown and formation of a current filament in the postbreakdown regime are observed. A magnetic field normal to the epitaxial layer destabilizes the filament and causes multifrequency oscillations and chaotic fluctuations. On the other hand, if the magnetic field lies in the plane of the layer the filamentary current flow is stable up to field strengths of 2 T.

Photoconductive response of GaAs epitaxial layers

In the present work the photoconductive response of low resistivity, n-type GaAs epitaxial layers is studied by experimentally monitoring the dependence of the photoconductive gain (PG) optoelectronic parameter upon incident photon flux and temperature. The characterized samples fall into three major categories: ion implanted (II) GaAs epilayers formed within undoped, semi-insulating GaAs substrates; GaAs epitaxial layers grown by liquid phase epitaxy (LPE) on Cr-doped, semi-insulating GaAs substrates; and ungated GaAs MESFETs.

Improvement of interface electronic properties of GaF 3/GaAs MIS structures

The interface electronic properties of GaF 3/GaAs structures are investigated for three different deposition procedures, the GaF 3 films have been deposited on n-type GaAs epitaxial layers without breaking the vacuum, or after being exposed to air, or after being treated in (NH 4) 2S 1 solution. It has been found from low-temperature C-V measurements in MIS diodes that a real modulation of the Fermi level in the forbidden band gap of GaAs occurs only when the GaAs epitaxial layer is treated in the sulfur solution and annealed at an optimum temperature prior to deposition of GaF 3. The unpinning mechanism is also discussed in conjunction with the simplified model for sulfur-treated GaAs surfaces.

Blue-green injection laser diodes in (Zn,Cd)Se/ZnSe quantum wells

Laser diode action in the blue-green has been observed from (Zn,Cd)Se quantum wells within ZnSe/Zn(S,Se) p-n heterojunctions up to 250 K. Operation is reported for two different configurations for which the GaAs substrate serves either as the n- or p-type injecting contact. In pulsed operation, output powers exceeding 0.6 W have been measured in devices prepared on both n-type and p-type GaAs epitaxial buffer layers and substrates.

Midgap electron traps in n-type GaAs epitaxial layers grown by the close-spaced vapor transport technique

The deep-level transient spectroscopy technique was applied to the study of deep electron traps existing in n-type GaAs epitaxial layers that were prepared by the close-spaced vapor transport technique using three kinds of sources (semi-insulator-undoped, Zn-doped and Si-doped GaAs). Two midgap electron traps labelled ELCS1 and EL2 were observed in all layers regardless of the kind of source used. In addition, the effect of the electric field on the emission rate of ELCS1 is discussed and its identification to ETX2 and EL12 is suggested.