GaAs Capacitors Research Articles

Heteroepitaxy of La2O3 and La2–xYxO3 on GaAs (111)A by Atomic Layer Deposition: Achieving Low Interface Trap Density

GaAs metal-oxide-semiconductor devices historically suffer from Fermi-level pinning, which is mainly due to the high trap density of states at the oxide/GaAs interface. In this work, we present a new way of passivating the interface trap states by growing an epitaxial layer of high-k dielectric oxide, La(2-x)Y(x)O(3), on GaAs(111)A. High-quality epitaxial La(2-x)Y(x)O(3) thin films are achieved by an ex situ atomic layer deposition (ALD) process, and GaAs MOS capacitors made from this epitaxial structure show very good interface quality with small frequency dispersion and low interface trap densities (D(it)). In particular, the La(2)O(3)/GaAs interface, which has a lattice mismatch of only 0.04%, shows very low D(it) in the GaAs bandgap, below 3 × 10(11) cm(-2) eV(-1) near the conduction band edge. The La(2)O(3)/GaAs capacitors also show the lowest frequency dispersion of any dielectric on GaAs. This is the first achievement of such low trap densities for oxides on GaAs.

Sub-nm equivalent oxide thickness on Si-passivated GaAs capacitors with low Dit

A thin amorphous silicon interlayer, inserted between the III-V semiconductor and the gate dielectric is expected to prevent III-V oxidation, as required for high-mobility channel transistors. We demonstrate that the addition of a thin Al2O3 barrier layer between the a-Si and the high-k HfO2, together with optimized post-metallization annealing, is the key to reduce the a-Si consumption and to achieve a highly scaled gate stack with equivalent oxide thickness of ∼0.8 nm. The evolution of the interfaces during growth and the quality of the stack are investigated by in-situ X-ray photoelectron spectroscopy and electrical measurements on metal-oxide-semiconductors capacitors.

Comparison of n-type and p-type GaAs oxide growth and its effects on frequency dispersion characteristics

The electrical characteristics of n- and p-type gallium arsenide (GaAs) capacitors show a striking difference in the “accumulation” capacitance frequency dispersion. This difference has been attributed by some to a variation in the oxide growth, possibly due to photoelectrochemical properties of the two substrates. We show that the oxide growth on n- and p-type GaAs substrates is identical when exposed to identical environmental and chemical conditions while still maintaining the diverse electrical characteristics. The difference in electron and hole trap time constants is suggested as the source of the disparity of the frequency dispersion for n-type versus p-type GaAs devices.

Chemical and physical interface studies of the atomic-layer-deposited Al2O3 on GaAs substrates

In this work, we study the chemical and physical properties of the interface between Al2O3 and GaAs for different surface treatments of GaAs. The interfacial layer between the high-κ layer and GaAs substrate was studied using x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The reduction in native oxide layer was observed upon atomic layer deposition of Al2O3 on nontreated GaAs using trimethyl aluminum precursor. It was also observed that the sulfide treatment effectively mitigates the formation of the interfacial layer as compared to the surface hydroxylation using NH4OH. The electrical characteristics of GaAs capacitors further substantiate the XPS and TEM results.

Fabrication and characterization of metal-oxide-semiconductor GaAs capacitors on Ge∕Si[sub 1−x]Ge[sub x]∕Si substrates with Al[sub 2]O[sub 3] gate dielectric

In this article, we have studied fabrication and characterization of GaAs metal-oxide-semiconductor (MOS) capacitors with Al2O3 gate dielectric. 300nm thick GaAs layers were grown epitaxially on Ge∕Si1−xGex∕Si substrates. Cross-sectional transmission electron microscopy (TEM) confirmed a threading dislocation density of ∼107∕cm2 in the GaAs layer. In addition, it was observed that threading dislocations were mainly confined within the first ∼50nm of the GaAs layer, adjacent to the Ge film. Interfacial self-cleaning attribute of GaAs upon atomic layer deposition of Al2O3 was confirmed by x-ray photoelectron spectroscopy (XPS) analysis. However, the Al2O3∕GaAs interface properties were remarkably improved by GaAs native removal in dilute HF (1%) followed by sulfur treatment in (NH4)2S, substantiated by probing electrical characteristics of the MOS capacitors and cross-sectional TEM analysis. Thermodynamic properties of Al2O3/sulfide-treated GaAs interface was also studied by monitoring the C-V characteristi...

Unpinned metal gate/high-κ GaAs capacitors: Fabrication and characterization

Fabrication of GaAs metal-oxide-semiconductor capacitors (MOSCAPs) with an unpinned interface is reported. The MOSCAP structure consists of a few monolayers of germanium grown in a molecular beam epitaxy (MBE) system in order to terminate an MBE-grown silicon-doped (100) GaAs layer. An ex situ HfO2 high-κ dielectric with an equivalent oxide thickness of 12Å was deposited by using a dc magnetron sputtering system. A midgap interface state density (Dit) of 5×1011eV−1cm−2 was measured using the high-frequency conductance technique. A rapid thermal annealing study was performed in order to examine the integrity of the gate stack at different temperatures. In addition, a forming gas anneal at 400°C appears to significantly reduce the midgap Dit revealed by probing the frequency dispersion behavior of the MOSCAPs.

Passivation of GaAs surfaces and AlGaAs/GaAs heterojunction bipolar transistors using sulfide solutions and SiNx overlayer

Stable passivation of GaAs surfaces and AlGaAs/GaAs heterojunction bipolar transistors has been achieved using sulfide solutions and SiNx overlayers. The SiNx layers are deposited at ∼200 °C using electron cyclotron resonance plasma-enhanced chemical vapor deposition technique. The capacitance–voltage measurements indicate a substantial reduction in the density of electronic defects at the SiNx/S–GaAs interface as a result of annealing in N2 ambient. The base current of a 36×36 μm2 AlGaAs/GaAs heterojunction bipolar transistor is reduced by approximately two orders of magnitude in the low collector current regime by using sulfide treatment, SiNx deposition, and anneal. Both the Al/SiNx/S–GaAs capacitors and transistors are stable for several months with no noticeable degradation in their electrical characteristics.

Si/Ge/S multilayer passivation of GaAs(100) for metal-insulator-semiconductor capacitors

We describe a new multilayer passivation method on GaAs(100), an ex situ method that doesn’t require GaAs epitaxy. Thin 20 Å Si films or thin films consisting of 15 Å Si and 5 Å Ge were grown on (NH4)2S cleaned and S passivated GaAs(100) wafers. High-energy resolution x-ray photoelectron spectroscopy has been used to study the chemical structure of the buried GaAs surfaces. As and Ga 3d core level studies show that the surface is free of gallium and arsenic oxides as well as elementary As. The Ga-S-Ga bridge bond termination is found preserved on the buried GaAs surface. SiNx/Si/Ge/S/GaAs and SiNx/Si/S/GaAs capacitors have been made and analyzed using quasistatic and high frequency capacitance-voltage measurements. Using the high-low method, an interface state density of about 1012 cm−2 eV−1 is obtained. The modulation of the surface potential is the highest reported for a method not requiring GaAs epitaxy.

Incoherent mesoscopic hole tunneling through barrier states in p-type AlxGa1-xAs capacitors.

Hole tunneling from an accumulation layer in single-barrier ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$-type GaAs--undoped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As--${\mathit{p}}^{+}$-type GaAs capacitors results in complex current-voltage (I-V) characteristics. At low bias, in the direct tunneling regime, several reproducible voltage-controlled negative resistance regions can occur. I-V curves for a given sample are reproducible while I-V curves for nominally identical samples vary from sample to sample. I-V curves are exponential in voltage with fluctuations in ln(dJ/dV)\ensuremath{\sim}1. Detailed structure in curves of d(lnJ)/dV versus voltage is temperature dependent for T70 K. At 1.7 K structure in derivative curves is independent of magnetic field. The observed behavior is consistent with the models reviewed by Raikh and Ruzin for incoherent mesoscopic tunneling through states in a randomly nonuniform barrier. The origin of the states in the nominally undoped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier is probably Be diffusing from regions of high doping sample growth.

Admittance measurements of acceptor freezeout and impurity conduction in Be-doped GaAs.

Carrier freezeout and impurity conduction in lightly doped p-type GaAs have been studied in two groups of ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$-type GaAs--undoped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As--${\mathit{p}}^{+}$-type GaAs capacitors. For one group the substrate doping was ${\mathit{N}}_{\mathit{S}}$\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$; for the other ${\mathit{N}}_{\mathit{S}}$\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. From the temperature dependence of the appearance of a Gray-Brown dip in the capacitance-voltage curves at fixed frequency and temperature, the separation of the Be acceptor from the valence band, ${\mathit{E}}_{\mathit{A}}$, is \ensuremath{\sim}20 meV for both groups. The activation energy for ac transport in ${\mathit{p}}^{\mathrm{\ensuremath{-}}}$-type GaAs, ${\mathit{E}}_{\mathit{I}}$, is determined from admittance measurements at fixed bias and variable temperature and frequency. For the lighter-doped samples, ${\mathit{E}}_{\mathit{I}}$=17--23 meV, which is consistent with the value of ${\mathit{E}}_{\mathit{A}}$. For the heavier-doped samples, ${\mathit{E}}_{\mathit{I}}$\ensuremath{\sim}6 meV; ac transport is either through a well-defined impurity band separated by \ensuremath{\sim}6 meV from the energy level of an isolated acceptor, or by activated hopping.

A temperature-independent capacitance in semiconductor-insulator-semiconductor capacitors

Both measurements of capacitance-voltage (C-V) curves of n− GaAs-undoped AlxGa1− xAs- n+ GaAs (AlGaAs) capacitors at different temperatures, and calculations of C-V curves of semiconductor-insulator-semiconductor (SIS) capacitors at different temperatures, show that there is a temperature-invariant capacitance CC and voltage VC at which C-V curves at different temperatures intersect. We show that this is a general property of SIS capacitors having a degenerate gate and nondegenerate substrate of the same doping type, and that qVC, where q is the electron charge, is approximately equal to the Fermi energy of the degenerate GaAs gate. VC provides a good estimate for the voltage required to establish an accumulation layer on n− GaAs at low temperatures, which is determined from magnetotunneling measurements on AlGaAs capacitors.

Observation of bulk Landau levels in transverse magnetotunneling in Al xGa 1-xAs capacitors

Two phenomena are observed for transverse magnetotunneling (B⊥J) from an accumulation layer in n − GaAs-undoped Al xGa 1−xAs− n + GaAs capacitors. One effect is a strong dependence of tunnel currents, J, at high applied voltage and high current densities, on the angle between J and the magnetic field, B. The second effect is the observation of structure in tunnel currents for applied voltages between 0.15 V and 0.6 V which is interpreted to result from tunneling into Landau levels formed in the n + GaAs electrode.

Magnetotunneling from accumulation layers in Al xGa 1− xAs capacitors

Observations of magnetic freeze-out, sequential single-phonon emission, and the effect of Landau levels on tunneling from accumulation layers in n −-GaAs-undoped Al x Ga 1− x As-n + GaAs capacitors are reviewed.

Magnetotunneling in AlxGa1−xAs capacitors

Abstract Observations of magnetic freezeout, sequential-single phonon emission, and the effect of Landau levels on tunneling in n-GaAs-undoped AlxGa1−xAs-n+GaAs capacitors are reviewed.

Electrical characterization of GaAs/AlGaAs semiconductor-insulator-semiconductor capacitors and application to the measurement of the GaAs/AlGaAs band-gap discontinuity

Electrical measurements were made on n+GaAs-(Al,Ga)As-n−GaAs and p+GaAs-(Al,Ga)As-p−GaAs capacitors. Current conduction is due to thermionic emission at low bias voltages and high temperatures. At low temperatures and high fields Fowler-Nordheim tunneling contributes to the conduction process. An inverted n-type capacitor analogous to an inverted modulation doped structure shows electrical characteristics comparable to the ‘‘normal’’ structure grown under optimized conditions. Conduction- and valence-band discontinuities were calculated from measurements of thermionic emission barrier heights as a function of Al mole fraction in the (Al,Ga)As. A conduction-band discontinuity of 65% of the total band-gap discontinuity between GaAs and (Al,Ga)As, independent of Al mole fraction, was deduced. A classical theory for the capacitance voltage characteristics of p+GaAs-(Al,Ga)As-p−GaAs structures is presented.

Negative charge, barrier heights, and the conduction-band discontinuity in AlxGa1−xAs capacitors

We have combined current-voltage (I-V) and capacitance-voltage (C-V) measurements on n− GaAs–Al0.4Ga0.6As–n+ GaAs capacitors with different thicknesses of Al0.4Ga0.6As to determine the conduction-band discontinuity at the Al0.4Ga0.6As–n+ GaAs interface. Undoped AlxGa1−xAs deposited by molecular-beam epitaxy contains negative charge. We calculate the effect of band bending in undoped AlxGa1−xAs due to negative charge on measured barrier heights and on C-V curves. The temperature dependence of I-V curves is analyzed in terms of thermionic emission to derive barrier heights at the n+ GaAs–Al0.4Ga0.6As interface φG. Measured values of φG are proportional to the square of the insulator thickness, w, showing that negative charge is uniformly distributed through undoped Al0.4Ga0.6As. Combining values of Fermi energy and band bending at zero bias with φG at zero Al0.4Ga0.6As thickness, we find that the value of the barrier discontinuity for n+ GaAs–Al0.4Ga0.6As is ∼0.30 V. This corresponds to a ratio of conduction-band discontinuity ΔEC to band-gap difference ΔEG of 0.63±0.03, which is appreciably less than the value 0.85±0.03 that is widely accepted. We find that the dielectric constant of Al0.4Ga0.6As is more temperature dependent than that of GaAs between 77 and 250 K.

Determination of the valence-band discontinuity between GaAs and (Al,Ga)As by the use of p+-GaAs-(Al,Ga)As-p−-GaAs capacitors

The valence-band discontinuity between GaAs and AlxGa1−xAs (0.3≪x≪1.0) was determined from measurements on p+-GaAs-(Al,Ga)As-p−-GaAs capacitors from current-voltage measurements as a function of temperature. It was found that thermionic emission dominated the conduction process in these structures for temperatures above 140 K and for low bias voltages. The barrier height was determined from the slope of ln(J/T2) vs l/T. From this, a valence-band discontinuity of 35% of the total direct band-gap (EgΓ) discontinuity, being independent of the mole fraction x, between GaAs and (Al,Ga)As was calculated. The corresponding value for the conduction-band discontinuity, 65%, is considerably lower than the commonly accepted value of 85%.

Resonant Fowler–Nordheim tunneling in n−GaAs-undoped AlxGa1−xAs-n+GaAs capacitors

Periodic structure is observed in the current-voltage characteristics at 4.2 K of n−GaAs-AlxGa1−xAs-n+GaAs capacitors, grown by molecular beam epitaxy, which have AlxGa1−xAs thicknesses of 30–35 nm. The periodicities can be explained quantitatively by the theory of resonant Fowler–Nordheim tunneling.