Type GaAs Research Articles

Absorption spectrum of EL2 defect in p-type GaAs

Absorption of p-type GaAs crystals grown by horizontal Bridgman, liquid-encapsulated Czochralski, and liquid-phase electroepitaxy methods were measured at 4 K. The spectra of melt-grown crystals show a wide absorption band extending from 0.6 eV to the fundamental absorption edge. The comparison with the photocapacitance spectrum of EL2 defect and deep-level transient spectroscopy measurements allowed to interpret this absorption as due to transitions between the valence band and the doubly ionized EL2 level. Absorption can be bleached by intense illumination at temperatures below 60 K with the dark spectrum recovering upon heating with an activation energy of 55 meV.

Thermally stable ohmic contacts to n-type GaAs. VII. Addition of Ge or Si to NiInW ohmic contacts

The effects of Si or Ge addition to NiInW ohmic contacts on their electrical behavior were studied, where the samples were prepared by evaporating Ni(Si) or Ni(Ge) pellets with In and W and annealed by a rapid thermal annealing method. An addition of Si affected the contact resistances of NiInW contacts: the resistances decreased with increasing the Si concentrations in the Ni(Si) pellets and the lowest value of ∼0.1 Ω mm was obtained in the contact prepared with the Ni-5 at. % Si pellets after annealing at temperatures around 800 °C. The contact resistances did not deteriorate during isothermal annealing at 400 °C for more than 100 h, far exceeding process requirements for self-aligned GaAs metal-semiconductor field-effect-transistor devices. In addition, the contacts were compatible with TiAlCu interconnects which have been widely used in the current Si process. Furthermore, the addition of Si to the NiInW contacts eliminated an annealing step for activation of implanted dopants and low resistance (∼0.2 Ω mm) contacts were fabricated for the first time by a ‘‘one-step’’ anneal. In contrast, an addition of Ge to the NiInW contacts did not significantly reduce the contact resistances.

The synthesis and properties of low barrier Ag-Ga intermetallic contacts to n-type GaAs

It is demonstrated that uniform silver-gallium alloy films, containing up to 30% gallium, can be controllably and reproducibly fabricated by electron-beam evaporation from a single alloy source. As expected from bulk thermodynamic arguments, these films are metallurgically stable with respect to GaAs up to their melting point. The alloying of silver with gallium results in a substantial reduction in n-type barrier height. Analogous increases in barrier height were observed for contacts to p-type material. Barrier heights as low as 0.59–0.62 eV were measured for annealed ζ-AgGa contacts to n-type material.

Inelastic electron scattering investigation of the Sb/GaAs(110) system.

A high-resolution electron-energy-loss spectroscopy study of the Sb/({ital n}-type GaAs (110)) system grown at room temperature is presented. A very wide range of antimony coverages was exploited (from 0.02 to 200 monolayers). The amorphous-polycrystalline transition accompanied by the semiconductor-metal transition, taking place in the deposited overlayer, was brought into evidence through the analysis of the energy-loss structures related to the electronic and the vibrational excitations of the system in different energy-loss regions. In particular, the intensity change and the energy shift undergone by the substrate's Fuchs-Kliewer optical phonon, and the intensity modification of the dopant-induced free-carrier plasmon, marked a critical value for 15 monolayers coverage. The band-bending change upon antimony chemisorption has also been estimated from the plasmon-energy position, and an origin for the states inducing the Fermi-level pinning has been suggested. The evolution as a function of coverage of an electronic excitation proper of Sb, which shifts from 90 to 125 meV of loss energy, has been explained as being due to a size effect'' due to the varying overlayer thickness. This result has been obtained through the comparison of the experimental loss function with a model one. Moreover, the dielectric function of antimony has been determined in themore » 0.045--0.5-eV energy range. The high-resolution electron-energy-loss technique is also presented as a superb probe for the study of the electronic structure of narrow-band-gap surface and interface systems.« less

Native and irradiation-induced monovacancies in n-type and semi-insulating GaAs.

Defects induced by electron irradiation in semi-insulating and {ital n}-type GaAs crystals have been characterized by positron-lifetime measurements. We conclude that electron irradiation with energies of 1.5--3 MeV produces negative monovacancies and negative ions at low and room temperature. The results also show that the native monovacancy defects in lightly {ital n}-type GaAs change their properties under irradiation. We relate this change to the existence of an ionization level {minus}{r arrow}0 or 0{r arrow}+ of the native monovacancy defects in the upper half of the band gap. We propose that irradiation produces negative Ga{sub As} antisites and negative {ital V}{sub Ga} vacancies. In {ital n}-type GaAs the behavior of the native defects under irradiation is in agreement with their earlier assignment to {ital V}{sub As}.

Interaction of longitudinal-optic phonons with free holes as evidenced in Raman spectra from Be-doped p-type GaAs.

The interaction between small-wave-vector, longitudinal-optic (LO) lattice vibrations and free-hole plasmas in Be-doped p-type GaAs is studied with use of nonresonant, allowed Raman scattering. In contrast to the coupled LO-phonon--plasmon modes, ${\mathrm{\ensuremath{\omega}}}_{+}$ and ${\mathrm{\ensuremath{\omega}}}_{\mathrm{\ensuremath{-}}}$ typically observed in n-type zinc-blende-structure semiconductors, only one mode is observed for hole densities between 1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ and 1.6\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. With increasing hole density, this single mode shifts first to higher energies, then back to lower energies, between that of the LO and transverse-optic (TO) phonons, and finally asymptotes at the TO-phonon energy. The observed Raman spectra are accurately fitted with calculated coupled-mode spectra which take into account wave-vector-dependent intra- and inter-valence-band transitions within the heavy- and light-hole bands. Intra-light-hole and inter-heavy- to light-hole transitions are shown to make very significant contributions to the spectra, although they alone cannot account for the novel density dependence of the coupled-mode energy. A detailed analysis of the coupled-mode dependence on wave vector and phenomenological damping reveals that the observed density dependence can, in principle, occur even in a single-component plasma due to two distinctly different physical mechanisms. In the case of the p-type GaAs studied here, it is shown that the novel density dependence is primarily due to the overdamped nature of intra-heavy-hole transitions.

Low-field magnetotransport in p-type GaAs in the regime of variable-range-hopping conductivity.

It is shown that phonon-assisted and activationless variable-range-hopping conduction in p-type GaAs, including transport in zero magnetic field and low-field magnetotransport, are very similar to findings in n-type GaAs previously reported. However, in p-type samples the low-field magnetoresistance is positive rather than negative. The positiveness is suggested to be a manifestation of spin-orbit scattering effects on quantum interference between nearly oriented paths containing local loops much smaller than the size of the coherence volume. The possibility to alter the nature of the paths contributing to interference by the application of a strong electric field is also discussed.

Injection and drift of a positively charged hydrogen species in p-type GaAs

Transport of the acceptor-passivating hydrogen species in p-type GaAs has been observed in reverse bias annealed Al Schottky diode samples. The motion of the positively charged hydrogen across the depletion region of these diodes is confirmed both by changes in the electrically active acceptor profiles with time, and by direct measurement of the migration using secondary-ion mass spectrometry on deuterated samples. Acceptor passivation is unstable under minority-carrier injection by illumination at 25 °C. Hydrogen injection into p-type GaAs during boiling in water or etching in H2SO4:H2O2:H2O has also been demonstrated.

A non-alloyed ohmic contact formation on n-type GaAs

The ohmic contact characteristics of nickel antimonide in n-type GaAs semiconductor circuits are evaluated. Deposition techniques, structural, and electrical properties were studied. The structural analysis revealed a well-defined, sharp interface of a polycrystalline nickel antimonide phase with various solid-phase interactions at higher annealing temperatures. The sheet resistance of 1000-Å-thick films decreased to 7–10 Ω/⧠ after annealing at 400 °C. The ohmic contact formation was demonstrated using transmission line measurements. Subtractive etching of the contacts was accomplished by a wet etchant system which was found not to attack the GaAs substrate.

Material properties of p-type GaAs at large dopings

We summarize the room-temperature minority-carrier mobility, minority-carrier lifetime, and effective band-gap shrinkage for p-type GaAs at large dopings, as determined from measurements on heterostructure bipolar transistors and published literature. The minority-carrier mobilities are significantly smaller than the majority-carrier mobilities, the lifetime data show a change in dependence on doping at 1×1019 cm−3, and the effective band-gap shrinkage is ≊5% at 1×1019 cm−3. The fits to electrical parameters described here should be of interest in modeling of minority-carrier devices.

The carrier effects on the change of refractive index for n-type GaAs at λ=1.06,1.3, and 1.55 μm

A numerical Kramers–Kronig analysis is used to calculate the refractive-index change Δn caused by the injection/depletion of free carriers in various doped n-type GaAs. The analysis makes use of a carrier-related absorption spectrum, which is established by using quantum theory as well as empirical relations and is confirmed by the experimental absorption data in the literature. We obtain the Δn values for various doping concentrations ND and carrier concentrations N at three wavelengths; λ=1.06, 1.3, and 1.55 μm. The Δn value is positive for low-N region, and increases gradually to its maximum which is around 10−4 for λ=1.06 μm. Thereafter, Δn decreases rapidly to 0 as N increases. The linear relation between Δn and N, as predicted by the Drude theory, only happens when N is beyond a certain value. In this region, the Δn value may attain to −10−2 at N=5×1018 cm−3 for λ=1.55 μm. Because of this significant Δn value and its linear relation with N, the free-carrier induced index-change effect may find the applications in integrated optics and optical probing.

Transistor-based measurements of electron injection currents in p-type GaAs doped 1018–1020 cm−3

Measurements of electron currents injected into p+-GaAs are presented for molecular beam epitaxially grown material doped from 2×1018 to 8×1019 cm−3 with Be. The collector current versus base-emitter voltage characteristics of n-p+-n GaAs homojunction bipolar transistors are analyzed, and the results are interpreted in terms of the quantity (n0Dn), where n0 is the equilibrium minority-carrier concentration and Dn is the minority-carrier diffusion coefficient. The results are consistent with earlier measurements of (n0Dn) made using metalorganic chemical vapor deposited p+-n GaAs solar cells, Zn doped as heavily as 1×1019 cm−3. The large electron injection currents observed are interpreted as evidence for significant effective band-gap shrinkage. These effects must be accounted for in the modeling and design of GaAs-based heterojunction bipolar transistors and solar cells.

Comparison of Au/Ni/Ge, Au/Pd/Ge, and Au/Pt/Ge Ohmic contacts to n-type GaAs

Au/Ge-based Ohmic contacts to n-type GaAs with an addition of a near-noble-metal, Ni, Pd, or Pt, have been studied. It was found that both the electrical characteristics and the material properties depend very much on this added metal. The material properties of the contacts after thermal alloying were studied using transmission electron microscopy and auger electron microscopy. The contact resistances obtained with different alloying conditions and their reliability have also been studied. It was found that Au/Ni/Ge had the roughest surface while Au/Pd/Ge had the highest contact resistance. The surface morphology was attributed to the difference in the sizes of the grains formed after Ohmic contact alloying. Au/Pt/Ge was found to be the best contact in terms of its electrical characteristics as well as its surface morphology.

Band-gap narrowing in highly doped n- and p-type GaAs studied by photoluminescence spectroscopy

Band-gap narrowing of GaAs as a function of doping concentration has been measured using photoluminescence spectroscopy on samples grown by molecular beam epitaxy. Both n- (Si) and p- (Be) doped samples with concentrations varying from 3×1017 to 3×1018 cm−3 have been measured. The experimental results obtained from a line-shape analysis of the spectra taking tailing effects into account are in good agreement with recent theoretical calculations. A simple expression for the band-gap narrowing as a function of concentration for both n-and p-doped GaAs is given.

The effect of annealing treatments on defect structure and diffusion lengths in bulk n-type GaAs

Specimens from n-type GaAs wafers have been annealed at high temperature (900–1050 °C) and then characterized using deep-level transient spectroscopy (DLTS) and electron-beam-induced current techniques. Relatively short anneals result in substantial changes to the electron trap structure in this material. Diffusion lengths are, at best, marginally increased by short (16 min and below) anneals but have been found to be significantly increased by longer anneals of between 40 and 80 min. DLTS measurements of hole traps in both unannealed and annealed n-type material suggest that a hole trap we term HCX may be an important recombination center in this class of material. The results obtained demonstrate the effectiveness of wafer annealing as a technique for creating a (10 μm deep) near-surface zone in n-type bulk material in which defects are suppressed and diffusion lengths improved.

Selective metallization of n-type GaAs formed by projection-patterned excimer laser doping of Si

Resistless microfabrication of the metallization of n-type GaAs formed by projection-patterned doping using a KrF excimer laser is described. Silane (SiH4 ) gas is used as a source material of the n-type dopant of Si. Copper thin films with a linewidth as narrow as 3.4 μm are deposited selectively on the doped region by electroplating using a CuSO4 aqueous solution. Using the selective metallization process, nonalloyed ohmic contacts can be formed with a specific contact resistance of 2.32×10−5 Ω cm2, which is one-thirtieth of that of the conventional alloyed contacts.

Low-resistance nonalloyed ohmic contacts on p-type GaAs using GaSb/GaAs strained-layer superlattices

Employing a GaSb/GaAs strained-layer superlattice and a GaSb cap, specific contact resistivities as low as 3.2×10−7 Ω cm2 have been realized for nonalloyed ohmic contact to p-type GaAs. This contact structure is shown to give low contact resistances irrespective of the contact metals, including AuBe, AuGe/Ni/Au, and Au. Excellent thermal stability for AuBe contacts was obtained when sintered.

Dopant concentration dependences and symmetric Fermi-level movement for metal/n-type and p-type GaAs(110) interfaces formed at 60 K.

The coverage-dependent Fermi-level movement for Ag, Co, and Ti interfaces formed at 60 K on $n$-type and $p$-type GaAs is shown to be symmetric but dependent on the bulk dopant concentration. Photoemission results show that ${E}_{F}$ remains close to the band edges until \ensuremath{\sim}1 monolayer for doping of 1\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ while ${E}_{F}$ movement is induced by far fewer adatoms for doping of 2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ with overshooting for $p$-type GaAs. Remarkable surface chemical and structural insensitivity is reflected by similar band-bending trends for adatoms which exhibit very different reactivities and amounts of substrate disruption. We conclude that ${E}_{F}$ movement is controlled by the coupling between adatom-induced states and those of the substrate at low temperature, with strong dependence on the bulk doping of the semiconductor.

Optical activity of the EL2 metastable state under hydrostatic pressure.

We present experimental results showing that at hydrostatic pressure exceeding approximately 0.3 GPa the $\mathrm{EL}2$ metastable state becomes optically active in both $n$-type and semi-insulating GaAs, and the pressure-induced optical recovery (PIOR) can be easily observed. PIOR is practically full and extremely efficient (with an efficiency comparable with that of the photoquenching process). The proper choice of the energy of the illuminating light allows reversible cycles of photoquenching and subsequent optical recovery of the characteristic intra-$\mathrm{EL}2$ absorption at 5 K. PIOR is a few orders of magnitude more efficient than pure optical recovery processes already reported for atmospheric pressure.

Thermally stable ohmic contacts to n-type GaAs. V. Metal-semiconductor field-effect transistors with NiInW ohmic contacts

Thermal stability of self-aligned refractory metal-semiconductor field-effect transistors (MESFETs) with the universally used AuNiGe ohmic contact metallurgy or the recently developed NiInW ohmic contact metallurgy has been studied. In these devices WSi0.1 films with length down to 1 μm were used as the gate material, and AlCu was used as the overlayer material on the ohmic contacts, where a very thin Ti layer was deposited prior to the AlCu deposition. The performance of the as-fabricated devices with the NiInW ohmic contacts was as good as those with the AuNiGe ohmic contacts. During subsequent annealing at 400 °C, deterioration of the device performance (defined by the decrease in the FET square-law coefficient) was observed after annealing at 400 °C for 2 h in the devices with the AuNiGe ohmic contacts. However, excellent stability was observed in the devices with the NiInW ohmic contacts; no deterioration was observed at 400 °C for 180 h, 450 °C for 18 h, and 500 °C for 2 h. The device deterioration with NiInW contacts, which was observed after annealing for longer times, is believed to be due to an increase in the contact resistances caused by In movement away from the metal/GaAs interfaces. Based on this assumption, an activation energy for In diffusion in GaAs was determined from onset times of the device deterioration to be ∼2.0 eV, which is close to the reported value of 1.9 eV.