N-type AlGaAs Research Articles

Theoretical modeling and analyzing structural characteristics of AlGaAs/GaAs negative electron affinity array cathode with optically and electrically injected variable bandgap

In order to obtain high emission current efficiency of the AlGaAs/GaAs NEA array cathode, this array cathode has two ways to form electron emission, i.e. optical injection and electrical injection. The two-dimensional continuity equation of electronic transport in the variable bandgap emission array is solved numerically by using the finite volume method thereby obtaining the emission current and emission current efficiency. Simulation obtains the optimal parameter range for each layer of the AlGaAs/GaAs NEA array cathode under both optical injection and electrical injection. The results show that the optimal angle of incident light for the array cathode under light injection is 10°–30° for selecting an array micro-nano column with a duty cycle of 2/3. Under the condition of light injection, the P-type variable bandgap AlGaAs layer array micro-nano column height ranges from 0.3–0.6 μm. Under the condition of electrical injection, the height of the micro-nano column of the P-type variable bandgap AlGaAs layer array is 0.1–0.3 μm. The optimal thickness range of N-type variable bandgap AlGaAs layer, N-type AlGaAs layer, and P-type AlGaAs layer under light injection are 0.5–2.5 μm, 0.5–1.0 μm and 0.2–0.5 μm, respectively. The optimal thickness range of N-type variable bandgap AlGaAs layer, N-type AlGaAs layer, and P-type AlGaAs layer under electrical injection conditions are 2–3 μm, 0.8–1.2 μm and 0.1–0.3 μm, respectively. The optimal doping concentration of P-type AlGaAs layer and N-type AlGaAs layer under light injection are range from 5×10<sup>18</sup> to 1×10<sup>19</sup> cm<sup>–3</sup> and from 1×10<sup>18</sup> to 5×10<sup>18</sup> cm<sup>–3</sup>, respectively. The optimal doping concentration of the P-type AlGaAs layer and the N-type AlGaAs layer under electrical injection range from 1×10<sup>18</sup> to 5×10<sup>18</sup> cm<sup>–3</sup> and from 5×10<sup>17</sup> to 1×10<sup>18</sup> cm<sup>–3</sup>, respectively. The maximum efficiency of the emission current under the light injection is 35.04%, and the maximum emission current per unit length is 10.3 nA/μm. The maximum efficiency of the emission current under electrical injection is 31.23%, and the maximum emission current per unit length is 105.5 μA/μm. Electric injection cathode does not need expensive and complex drive laser system, and the light injection control mode is simple, so light injection or electrical injection control mode can be chosen as needed. The research of array cathode, which integrates the advantages of many advanced technologies, is of great significance for enriching the cathode array cathode emission theory and expanding its application field.

Optical absorption in a waveguide based on an n-type AlGaAs heterostructure

Free carrier absorption of optical radiation in layers of an AlGaAs/GaAs heterostructure is studied by the method of probe radiation coupling in order to determine the absorption cross section parameter in the AlGaAs material with a high (22%) aluminium concentration. For this purpose, we have fabricated special samples based on AlGaAs/GaAs heterostructures simulating an n-type-doped laser waveguide with carrier concentrations in the range 5 × 1016 − 3 × 1017 cm−3. The doping profile and the composition and thickness of layers are measured and the temperature and spectral dependences of the absorption coefficient are studied. It is shown that an increase in temperature and in the probe wavelength leads to an increase in the absorption in the heterostructure layers.

Si-doped AlGaAs/GaAs(6 3 1)A heterostructures grown by MBE as a function of the As-pressure

The effects of doping with silicon (Si) AlGaAs layers grown by molecular beam epitaxy on GaAs (631)-oriented substrates as a function of the arsenic pressure (PAs) is presented and compared with layers grown on (100) oriented substrates. The surface texture of the AlGaAs (631) films is composed by nanogrooves, whose dimensions depend on PAs. On the contrary, the MBE growth on the (100) plane resulted on rough surfaces, without evidence of formation of terraces. Mobility and carrier density of AlGaAs:Si layers grown on substrates (631) were studied as a function of PAs. The doping type conversion from p-type to n-type as a function of the As pressure is corroborated for high index samples. All the films grown on (100) exhibited silicon n-type doping. These observations were related with the amphotericity of Si, where it acts as a donor impurity occupying Al or Ga-sites or as an acceptor when it takes an As-site, depending on the competition that the Si atoms encounters with As for any of these sites. The acceptor and donor lines close to the AlGaAs transition observed by photoluminescence spectroscopy (PL) were affected by the incorporation of Si. When increasing PAs the energy of the main PL peak is redshifted for n-type AlGaAs layers, but it is shifted back towards high energy once the conduction type conversion takes place. X-ray diffraction patterns revealed high crystalline quality for samples grown at the highest PAs.

Structural and Magnetic Properties of Epitaxial Fe and Ni Thin Films Grown on n-AlGaAs(001) Using Electrodeposition

We report epitaxial growth of body-centered cubic Fe and face-centered cubic Ni thin films on n-type AlGaAs(001) substrates using electrochemical deposition with the cubic axes of Fe oriented parallel to the substrate lattice while the Ni is rotated in-plane 45°. The magnetic properties in either case are dominated by the fourfold crystalline anisotropy, a high remanence, and saturation magnetization values similar to bulk properties. The growth of such ferromagnetic contacts on AlGaAs might find applications in magneto-electronics.

Deep level transient spectroscopy of AlxGa1−xAs/GaAs single-quantum-well lasers

Deep level transient spectroscopy (DLTS) and capacitance–voltage (C–V) measurements have been made on AlGaAs/GaAs single quantum well (QW) laser diode structures. Si-related DX centres in the n-type AlGaAs layer adjacent to the QW have been observed by DLTS with reverse bias voltage and reduced bias pulse excitation. The DLTS peak of the DX centre is wiped out at zero bias by a large injection pulse excitation and a new peak appears which is interpreted as being due to an electron trap inside the QW. Identification of the electron trap has been discussed.

Computer analysis of photon-induced non-equilibrium phenomena at Si and AlGaAs surfaces

A theoretical analysis of the surface recombination, surface band bending and photoluminescence (PL) under strongly absorbed light illumination was performed for n-type Si and AlGaAs surfaces. A very strong dependence of the so-called effective surface recombination velocity ( S eff), surface photovoltage (SPV) and photoluminescence quantum efficiency ( Y PL) on both the excitation intensity and surface Fermi level position was found from rigorous computer calculations. The initial surface band bending was varied from accumulation to inversion by introducing the surface fixed charge ( Q FC) in the Si case, and from weak to strong depletion by increasing the surface state density ( N SS0), in the AlGaAs case. It was revealed that S eff can be significantly reduced (almost four orders of magnitude on Si surface) by shifting the surface Fermi level towards band edges. On the other hand, a deep quenching of PL efficiency was observed for the values of N SS around midgap above 5×10 10 cm −2 eV −1 on AlGaAs surface. In addition, a comparison of simulated Y PL dependencies versus photo-excitation with the experimental data obtained for MBE-grown AlGaAs layers passivated by ultrathin Si interface control layer was performed. This technique of passivation is known to reduce the interface state density down to the range of 10 10 cm −2 eV −1.

Microstructural and electrical investigation of Pd/Ge/Ti/Au ohmic contact to pseudomorphic high electron mobility transistor with undoped cap layer

Microstructural reactions of Pd/Ge/Ti/Au contact to AlGaAs/InGaAs pseudomorphic high electron mobility transistor with an undoped GaAs/AlGaAs cap layer have been investigated using cross-sectional transmission electron microscopy, and the results are used to interpret the electrical properties of the ohmic contact. In the as-deposited state, a quaternary phase of PdxAlGaAs containing excess Ge atoms is formed at the interface of Pd/AlGaAs and some microvoids exist at the Pd layer in the vicinity of the interface. When the ohmic metals deposited on the undoped cap layer were annealed, the lowest contact resistivity of 9.1×10−5 Ω cm2 is obtained at 380 °C. AuGa compound is formed at the PdGe/undoped-AlGaAs interface as a result of the reaction between the ohmic metal and the undoped GaAs cap. This is due to the fast in-diffusion of Au toward the undoped AlGaAs through grain boundaries of the PdGe compound. The layer structure is changed to TiO/AuGa/PdGe/AuGa(TiAs+epi-Ge)/undoped-AlGaAs. The AuGa compound enhances the creation of group III vacancies, and the in-diffused Ge atoms occupy the vacancies. Thus, a number of electrons are produced below the contact, which plays a role in reducing the contact resistivity. For the ohmic metals deposited on n-AlGaAs by removing the undoped cap layer, the annealing temperature at which the contact resistivity has the minimum value of 2.3×10−6 Ω cm2 increases to 460 °C. The Au2Al is additionally observed at the PdGe/n-AlGaAs interface. Consequently, the layer structure is changed to TiO/AuGa/PdGe/AuGa+Au2Al(TiAs+epi-Ge)/n-type AlGaAs. The formation of Au2Al at the PdGe/n-AlGaAs interface creates more group III vacancies. Thus, the contact resistivity is further reduced by the incorporation of the in-diffused Ge into the group III vacancies. The InGaAs channel layer is observed to be intermixed in the annealed sample. This evidences the production of a large number of the group III vacancies via electrons below the contact.

Short wavelength bottom-emitting vertical cavitylasers fabricated using wafer bonding

Selectively oxidised 850 nm vertical cavity surface emitting laser diodes which emit through transparent n-type GaP and AlGaAs substrates are reported. The short wavelength bottom-emitting lasers are fabricated using a low temperature inert gas wafer fusion process. Compared to top-emitting lasers, the bottom-emitting lasers bonded to n-type AlGaAs substrates show comparable electrical characteristics, while lasers bonded to n-type GaP substrates exhibit higher series resistance and voltage.

Edge quantum wire structures with novel doping profiles and their electronic states

We propose and analyze a new type of edge quantum wire structure (EQWR), where an n-type AlGaAs layer is deposited on the edge plane of a GaAs/AlGaAs n–i–p–i structure where acceptors and donors are planar-doped at the center of GaAs and AlGaAs layers, respectively. Since a sharp electric field generated by the sheet doping raises the potential at the center of GaAs layer, electrons supplied by extra donors in the overgrown AlGaAs layer are driven to the two corner ridges of the GaAs layer and confined two-dimensionally by electrostatic fields. Wave functions of one-dimensional electrons in these quantum wires are analyzed and found to be efficiently squeezed. Features of electronic levels are clarified.

Surface band-bending assessment by photocurrent techniques. Application to III - V semiconductors

In epitaxial layers with a large area to thickness ratio, surface and interface space charge regions behave as transverse resistance - capacitance systems modulating the effective volume that takes part in the electrical conductivity. In this work it is shown that in photoconductivity spectroscopy experiments these surface and interface photovoltaic effects can become dominant, therefore giving information about the sample surface and interface band-bending. In photoconductivity spectroscopy, procedures to identify the signatures of such band bending are described. The present technique to assess surface bending is first applied to gated and ungated GaAs samples, to validate the present model. N-type AlGaAs and undoped InGaAs strained buffer layers, with various degrees of strain and surface roughness, are also characterized by this technique.

Effect of p and n doping on neutral impurity and dielectric cap induced quantum well intermixing in GaAs/AlGaAs structures

We report the diffusion and quantum well intermixing (QWI) effects of the neutral impurities fluorine and boron in p- and n-type AlGaAs in the GaAs/AlGaAs system. Boron was found to show significant diffusion in p-doped AlGaAs material but only exhibited a very small amount of diffusion in n-doped material after furnace annealing. Boron was found to retard intermixing in p-AlGaAs. This may be due to the existence of interstitial B, which may reduce the native group III interstitial concentration. The mechanism of boron impurity-induced disordering (IID) in n-AlGaAs was proposed to be attributed to both the diffusion of point defects generated during ion implantation and the Fermi level effect from the deep acceptor . Extremely fast fluorine diffusion rates, which may be correlated to an interstitial diffusion mechanism, have been observed. Results from fluorine IID suggest the possibility of the ionization of fluorine which gives rise to a higher electron concentration during annealing, leading to higher degrees of intermixing in n-AlGaAs. The intermixing rate of impurity free vacancy disordering (IFVD) using an cap was found to be higher in n-type and intrinsic AlGaAs than in p-type AlGaAs, which suggests that the diffusion of the group III vacancy is inhibited in p-type material which has a higher group III interstitial concentration. These results imply that n-i-p structures should be more effective than conventional p-i-n structures for the IFVD process.

Reduction of gap states of ternary III–V semiconductor surfaces by sulfur passivation: Comparative studies of AlGaAs and InGaP

The effects of sulfur passivation on liquid-phase-epitaxy-grown n-type InGaP and AlGaAs surfaces have been studied using x-ray photoelectron spectroscopy. The surfaces were simultaneously prepared through degreasing and the use of an aqueous (NH4)2Sx treatment in air. For InGaP, sulfur atoms initially reacted with both surface In and Ga atoms and reacted negligibly with P atoms. The band bending was reduced by 0.7 eV compared to a sputter-cleaned surface. Presumably, sulfur eliminated P-vacancy-related gap states by occupying P sites and forming In–S and Ga–S bonds. By postheat treatment at 180 °C, S atoms were not removed from the surface and band bending was reduced further by 0.1 eV. For AlGaAs, S atoms initially reacted with Ga and As, but this treatment could not remove the Al oxide previously formed in the air. Postheat treatment at 180 °C simply induced S redistribution from As to Ga and As desorption, which reduced the band bending by 0.3 eV compared to the sputter-cleaned surface—a result similar to that for GaAs.

Contact Resistivity and Adhesion of Ni/AuGe/Ag/Au Ohmic Contact Directly to n-Type AlGaAs

The direct ohmic contact to AlGaAs, Ni/AuGe/Ag/Au=5/120/100/580 nm has been studied. Contact resistivity of 3.8×10-5 Ω cm2 is obtained after thermal treatment for 10 min at 450° C. This is one order lower than that of the conventional Ni-Ge-Au system. The new metallization also indicates strong adhesion on SiNx film, does not peel off even after thermal treatment during all device processes and withstands the wire bonding process. We conclude that the metallization is useful and applicable for AlGaAs-related devices.

Degradation related defects in [formula omitted] visible lasers

The defects resulting in degradation of AlGaAs GaAs visible lasers have been studied in this paper by deep level transient spectroscopy (DLTS) and ultrasonic Abrahams-Buiocchi (AB) etching (USABE) or ultrasonic Doniach-Šunjić (DS) etching (USDSE). The dislocations and deep levels in AlGaAs GaAs degrade the visible lasers seriously. The donor-related deep centers (DX centers), are non-irradiating traps and attached to the dislocations in n-type AlGaAs. While a laser is in light-emitting operation, most of the injected carriers focus on the dislocations in company with DX centers, which results in the degradation of the AlGaAs GaAs lasers. The effects of defects in AlGaAs GaAs on relative lasers can be avoided by growing an InGaAs buffer layer between the AlGaAs cladding layer and GaAs substrate.

Surfactant assisted growth of 850 nm light modulators

We compare growth of multiple quantum well modulators atop a p-type or n-type AlGaAs layer. We find the excitonic absorption edge of the quantum wells broadens 30% less under applied field when grown on a p-down AlGaAs layer compared to the n-down case. Auger spectroscopy shows Be surface concentration builds up to 0.3 ML on the p-type AlGaAs layer whereas Si is smaller than 0.07 ML on an n-type AlGaAs layer. X-ray diffraction reveals quantum well crystallinity and interfacial roughness to be improved when grown on p-down AlGaAs layer. We conclude Be is a surfactant that assists AlGaAs epitaxy.

On the spectroscopy of DX centres by transient techniques at constant capacitance

In systems with no shallow levels, the features of constant-capacitance techniques for mirroring the emission processes from a deep donor are presented. The main objective is to determine the intrinsic sources of non-exponentiality in these experiments. The analysis is focused on the problem of DX centres as controlling the electrical properties of n-type AlGaAs alloys for Al mole fractions (x) in the 0.2<x<0.4 range. Using a Poisson plus a dynamical evolution equations solver, the presence of intrinsic non-exponentiality in constant-capacitance transients under thermal emission conditions, and its absence under optical excitation, is shown. Experimental transients taken under constant-capacitance conditions for Si and Se donors were analysed and fitted by sums of exponential decays. While the idea of DX local environment effects is confirmed, a single photoionization rate was detected. It is concluded that constant-capacitance techniques must be used to characterize systems governed by a deep level.

Pressure Dependence of the Schottky Barrier Heights in Al/AlGaAs Junctions

The influence of hydrostatic pressure up to 8 kbar on the barrier height of epitaxially MBE-grown Al on AlGaAs metal—semiconductor junctions is reported. The pressure change of the Schottky barrier on n-type AlGaAs is the same as that of the energy gap (for both direct and indirect-gap AIGaAs compositions), while for p-type AlGaAs it is negligible. This result is in direct conflict with a class of models of the Schottky barrier formation based on a concept of a semiconductor neutrality leve1 alignment with the metal Fermi level. PACS numbers: 73.30.+y, 73.40.Kpa, 73.20.Ηb

DX Centers in Reduced Dimensionality n-Type AlGaAs Structures

DX Centers in Reduced Dimensionality n-Type AlGaAs Structures

A critical analysis of capacitance transient signals due to DX centres in n-type AlGaAs

In n-type AlGaAs layers, the evolution of high frequency capacitance with time due to electron emission from DX centres has been computer simulated. The electron density profiles during the emission and capture processes are also presented. The effects of switching voltage amplitude and DX level position in the gap have been considered. A critical discussion of the signals obtained, the intrinsic sources of non-exponentiality and the reliability of the DX parameters extracted, are presented. The artifacts due to the DLTS processing of the capacitance transients generated were also considered. The subject of alloy broadening has been addressed, and it has been unambiguously shown that intrinsic broadening may only account for less than half the measured broadening in Al0.3Ga0.7As:Si.

Si-acceptor passivation on n-type (AlGa)As post-growth hydrogenation

We have studied AlGaAs:Si grown by molecular beam epitaxy before and after post-growth hydrogen plasma treatment by low temperature photoluminescence (PL) capacitance-voltage, and Hall effect. The PL strength increases after hydrogen plasma treatment. We observe, for the first time, that in n-type AlGaAs:Si not only the shallow donors but also the Si acceptor can be passivated by a long time (3 h) hydrogen diffusion, at 570 K. The change of the relative PL intensities associated with the Si and C acceptors demonstrates the dependence of the passivation effect on the chemical species.