Molecular Beam Epitaxial GaAs Layers Research Articles

Excess arsenic and point defects in GaAS grown by molecular beam epitaxy at low temperatures

Defect formation in low temperature molecular beam epitaxy GaAs layers (LT-GaAs) is discussed. The influence of growth conditions (temperature, III/V flux ratio) on internal structure, type, and concentration of electrically and optically active point defects is discussed. The effect of annealing on the formation of arsenic precipitates (clusters) is considered.

Investigation of defects created by growth of InAs quantum dots in GaAs

Defects created by self-organized InAs quantum dot growth in a GaAs matrix were investigated. Growth of the quantum dots was found to enhance strongly the tendency towards inhomogeneous growth. The inhomogeneity seems to stem from point defect clusters generated in the vicinity of a molecular beam epitaxy (MBE)/atomic layer MBE (ALMBE) interface. The point defects identified are characteristic of MBE and ALMBE GaAs layers. The InAs quantum dot states and interface states are not clearly detected since they are located in a depleted layer surrounding the point defects clusters.

On the properties of the Be-doped low temperature molecular beam epitaxy GaAs layers

High resolution X-ray diffraction technique was used to evaluate sets of samples which consisted of an molecular beam epitaxy (MBE) grown low temperature (LT) GaAs layer on semi-insulating GaAs substrate. The GaAs layers were Be doped with three different concentrations and subsequently annealed at temperatures ranging from 500 to 800 °C. X-ray results were compared with resistivity measurements showing similarities in their annealing temperatures dependencies. X-ray double crystal topography was also employed to evaluate the overall quality of the samples and macro-defect content. The topographies show no misfit dislocations in all annealed samples.

Magnetoresistance in low-temperature grown molecular-beam epitaxial GaAs

Conductivity, Hall effect as well as “physical” and “geometrical” magnetoresistances were measured at 290–440 K in molecular-beam epitaxial GaAs layers grown at 200–400 °C. The experimental data were analyzed taking into account the combined band and hopping conductance regime. Positive hopping magnetoresistance parameters (Δρ/ρ0B2)h≈10−4 T−2 and hopping Hall mobilities lower than 1×10−4 m2 V−1 s−1 were determined in the as-grown layers. A transverse-to-longitudinal hopping magnetoresistance ratio of about 2, consistent with hopping transport theories, was obtained. In the annealed layer grown at 200 °C (J200a) the band mobility determined from the geometrical magnetoresitance (GMR) mobility was found to be significantly higher than the band Hall mobility. It is related to a mixed band conductivity regime with the hole concentration p exceeding the electron one n. The difference between GMR and Hall mobilities decreases with increasing growth temperature as far as a typical single-carrier band conductivity regime (n>p) is present in the layer grown at 400 °C. In contradiction to the layers grown at higher temperatures, the J200a layer showed the opposite (positive) sign of the hopping Hall coefficient as well as the largest hopping magnetoresistance parameter (≈3×10−2 T−2).

Model for the linear electro-optic reflectance-difference spectrum of GaAs(001) aroundE1andE1+Δ1

We developed a model to describe the linear electro-optic (LEO) component of the reflectance-difference (RD) spectrum of zinc-blende semiconductors for energies around the ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ critical points. The model is based on a piezoelectric effect due to the semiconductor surface electric field and predicts a LEO line shape given by the superposition of two terms, a first one proportional to the energy derivative of the reflectance spectrum and a second one associated with the undifferentiated reflectance spectrum. We compared the predictions of the model to experimental LEO line shapes for molecular-beam epitaxy GaAs(001) layers doped with $1.4\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{3}$ Si donors and found an excellent agreement. The results reported in this paper help develop the LEO RD spectroscopy as a characterization tool for zinc-blende semiconductors.

Hall mobility analysis in low-temperature-grown molecular-beam epitaxial GaAs

Temperature dependent conductivity and Hall effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 200–420 °C and separated from the substrate. An analysis of experimental data with and without considering the hopping Hall mobility was made. An extremely low room temperature Hall mobility of 0.14 cm2 V−1 s−1 was measured in the 250 °C layer, which could be interpreted as the hopping Hall mobility. The room temperature band Hall mobility (μHb) increases from 500 to 6000 cm2 V−1 s−1 and the power (n) of the temperature dependence of μHb (∼T−n) increases from 0.5 to 1.2 with increasing growth temperature from 300 to 420 °C.

On Hall scattering factors for holes in GaAs

Hall scattering factors for electrons and holes in molecular beam epitaxial GaAs layers have been determined by comparing carrier concentrations measured by the Hall effect with those measured by the electrochemical capacitance–voltage technique. The conclusion is that both the electron and hole scattering factors are near unity for n ranging from 2×1016 to 7×1017 cm−3, and p ranging from 5×1016 to 4×1019 cm−3. This conclusion is consistent with the present theory for electrons, but not with that for holes.

Characterization of Annealed Low-Temperature GaAs Layers Grown by Molecular Beam Epitaxy with n-i-n Structure

Deep levels and resistivity in annealed low-temperature molecular beam epitaxial (LT-MBE) GaAs layers were characterized using n- i-n structure. We found that annealing at 620°C resulted in the highest attainable resistivity of LT GaAs layers. However, deep-level transient spectroscopy (DLTS) and photocapacitance quenching measurements showed no signal of the midgap (EL2) level. Only a “shallower” EL3-like level was observed in the layer of high resistivity. The fact that no midgap level was detected in the highest-resistivity LT GaAs layer means that the “compensation" mechanism does not play an important role in controlling the semi-insulating properties of LT GaAs layers. On the other hand, a midgap level appeared upon annealing at higher than 620°C, although the resistivity of LT GaAs layers decreased more than five orders of magnitude. The photoquenching measurements suggest that this level is identical to the EL2 level. These results support the “buried Schottky barrier" model.

Semi-insulating GaAs layers grown by molecular-beam epitaxy

Temperature dependent conductivity and Hall-effect measurements were carried out on molecular-beam epitaxial GaAs layers grown at 420 °C and separated from the substrate. The layers exhibit semi-insulating properties with a room temperature resistivity of (6–7)×106 Ω cm. The electron Hall mobility is proportional to temperature as ∼T−1.1 and the room temperature value is 5900 cm2 V−1 s−1. From the (nHT−3/2) vs T−1 plot, a donor activation energy of 0.68 eV, different than in bulk semi-insulating GaAs, has been evaluated. A similar activation energy is observed in as-grown and annealed GaAs layers grown at 200–350 °C. This indicates that the high resistivity of GaAs grown at low temperatures might be explained by deep donor defects rather than As precipitates.

Electrical properties of molecular beam epitaxial GaAs layers grown at low temperature

The electrical properties of low-temperature-grown (LTG) molecular beam epitaxial GaAs layers were investigated in the temperature range 300–440 K. The resistivity, Hall mobility and Hall concentration of LTG GaAs were evaluated by applying precise conductivity and Hall effect measurements. The sheet conductivity and Hall effect were measured on double-layer structures (i.e. LTG layer and substrate) and on a semi-insulating substrate. The properties of as-grown (250°C) and annealed (310–590°C) LTG GaAs are presented and discussed. It is shown that reasonable resistivity data can be obtained without separating the LTG layer from its substrate. On LTG GaAs annealed at 350°C, the resistivity and Hall effect data exhibit a temperature-dependent character different from all other samples. This could be due to the compensation mechanism between deep-level states and shallow impurities.

Molecular beam epitaxial GaAs grown at low temperatures

Molecular beam epitaxy (MBE) GaAs layers grown at substrate temperatures of 200–400 °C, well under the normal growth temperature of about 600 °C, exhibit unique electrical and optical properties and can be profitably used as buffer layers for metal-semiconductor field effect transistors (FETs), gate-insulator layers for metal-insulator-semiconductor FETs, and channel layers in fast photoconductive switches. Other III–V materials, such as InP, GaP, AlGaAs, InAlAs, InGaAs, and InGaP, have also been grown at low temperatures (LTs) and are, in some cases, useful in device applications. The most outstanding features of LT MBE GaAs are very high point-defect densities, e.g. about 10 20 As Ga centers cm −3 in 200 °C material, and high concentrations (about 10 17 cm −3) of large (about 30 Å diameter) As precipitates after annealing such material for 10 min at 600 °C. However, there is still much speculation and controversy regarding the relative roles of the point defects and precipitates in determining the optical and electrical characteristics of LT GaAs. Topics discussed here include growth, structural properties, point defects, annealing properties, As precipitates, and devices.

Donor and acceptor concentrations in molecular beam epitaxial GaAs grown at 300 and 400 °C

The first Hall-effect measurements on molecular beam epitaxial GaAs layers grown at the low temperatures of 300 and 400 °C are reported. Two independent methods were used to determine donor ND and acceptor NA concentrations and activation energy ED0, with the following combined results: ND≂3±1×1018, NA≂1.5±1×1017 cm−3, and ED0=0.645±0.009 eV for the 300 °C layer; ND≂2±1×1017, NA≂7±3×1016 cm−3, and ED0=0.648±0.003 eV for the 400 °C layer. Thus, the deep donor is not the expected EL2, which has ED0=0.75±0.01 eV.

AsGa antisite defects in LT GaAs as studied by magnetic resonance and magneto-optical techniques

GaAs epitaxial layers grown by molecular beam epitaxy (MBE) at low temperature (LT) have been characterized by electron spin resonance (ESR) infrared absorption, magnetic circular dichroism (MCD) and optically detected magnetic resonance (ODMR) via MCD. The data prove that AsGa antisite defects contribute to the below-gap infrared absorption of LT MBE GaAs layers. Furthermore it is found that ESR-spectra of LT GaAs epitaxial layers are very similar to those observed in fast neutron irradiated bulk GaAs crystals.

Micro-Raman study on GaAs layers directly grown on (100) Si by molecular beam epitaxy

Micro-Raman spectroscopy is applied to evaluate change of the crystal quality in molecular beam epitaxial GaAs layers on Si after rapid thermal annealing (RTA). The forbidden transverse optical phonon is observed in the GaAs layers especially near the interface. In the as-grown state, the Raman frequency of the longitudinal optical phonon shifts toward higher frequency near the interface. This blue shift indicates the existence of the compressive stress due to the lattice mismatch between GaAs and Si. On the other hand, after the RTA, the Raman peak shifts toward lower frequency. This red shift indicates that the tensile stress exists near the interface because of the difference in thermal expansion. The stress change indicates the relaxation of the lattice mismatch stress near the interface by formation of dislocations during the RTA.

Defect models in electron-irradiated n-type GaAs

1 MeV electron irradiation has been performed in degenerate, n-type (n≂2×1017 cm−3), molecular beam epitaxial GaAs layers, and Hall effect measurements have been carried out during the irradiation in order to get accurate defect production data. The results have been fitted with statistical models, and are most consistent with the usual E1 (EC−0.045 eV) and E2 (EC−0.15 eV) levels being the (−/0) and (0/+) transitions of the As vacancy, respectively. Also, an acceptor well below EC−0.15 eV is produced at a much higher rate than that of E1 and E2.

Incorporation of silicon and aluminum in low temperature molecular beam epitaxial GaAs

The localized vibrational modes (LVMs) of silicon donor (SiGa) and aluminum isovalent (AlGa) impurities in molecular beam epitaxial GaAs layers grown at various temperatures are studied using the infrared absorption technique. It is found that the total integrated absorption of these impurities LVMs is decreased as the growth temperature decreases. This finding suggests a nonsubstitutional incorporation of Si and Al in GaAs layers grown at 200 °C. On the other hand, a subtitutional incorporation is obtained in GaAs layers grown at temperatures higher than 350 °C. A recovery of the SiGa LVMs in GaAs layers grown at 200 °C has not been achieved by thermal annealing.

Ohmic contact formation on GaAs layers with low-temperature molecular-beam epitaxial caps

Molecular-beam epitaxial GaAs layers, grown at 200 or 400 degrees C, show great promise as passivating or insulating films on top of standard n-type MESFET layers (n approximately=1.6*10/sup 17/ cm/sup -3/) grown at normal temperatures (580-600 degrees C). It is shown that ohmic contacts, with specific contact resistances of 10/sup -6/ Omega -cm/sup 2/, can easily be fabricated without removing the cap layers. Preliminary results on capped p-type MESFET layers (p=1.5*10/sup 17/ cm/sup -3/) suggest that a 200 degrees C cap may degrade the contact resistance by a factor of 3-10. >

Electrical characterization of low temperature GaAs layers, and observation of the extremely large carrier concentrations in undoped material

We present here the results of a study on the effect of substrate temperature Ts on the electrical and physical characteristics of low temperature molecular-beam epitaxy GaAs layers. Based on the x-ray results, three temperature ranges have been defined for Ts: 1) high range (Ts≥ 460 °C), 2) intermediate range (260 ≤ Ts ≤ 450 °C), and 3) low range (the amorphous range) (Ts ≤ 250 °C). Hall measurements have been performed on the as-grown samples and on samples annealed at 610 and 850 °C. Si implantation into these layers has also been investigated. From an electrical stand point, the most striking difference between the low range and the intermediate range is the fact that after annealing at 850 °C, the undoped layers grown below or at 250 °C have a low resistivity (net electron concentrations as high as 1.5×1018 cm−3 and mobilities as high as 920 cm2 V−1 s−1), while after anneal the undoped layers grown in the intermediate range have extremely high resistivity (about 8×105 Ω cm).

The role of As in molecular-beam epitaxy GaAs layers grown at low temperature

Annealed GaAs layers grown at low temperatures (180–300 °C) by molecular-beam epitaxy (MBE) were studied by transmission electron microscopy (TEM). These layers were used as buffers for a field-effect transistor (FET) device structure and effectively eliminated sidegating effects. All these layers were found to contain As precipitates. Precipitate size and separation between them differed from sample to sample. The smallest precipitates were coherent ‘‘pseudocubic.’’ Larger precipitates had hexagonal structure. The distance between precipitates was estimated to be in the range of 10–40 nm. These results are consistent with the buried Schottky model by Warren et al., although other explanations cannot be excluded at present.

Electrical Characterization of Low Temperature GaAs Layers, and Observation of the Extremely Large Carrier Concentrations in Undoped Material

ABSTRACTWe present here the results of a study on the effect of substrate temperature, Ts, on the electrical and physical characteristics of low temperature (LT) molecular beam epitaxy GaAs layers. Hall measurements have been performed on the asgrown samples and on samples annealed at 610 °C and 850 °C. Si implantation into these layers has also been investigated.