Low-temperature Molecular-beam Epitaxy GaAs Research Articles

Variable range hopping conduction in low-temperature molecular beam epitaxy GaAs

Electric transport properties measured by Van der Pauw resistivity experiments of Low-Temperature Molecular Beam Epitaxy (LT-MBE) GaAs samples are used to identify a method to improve the resistivity of GaAs material. We present results on five samples grown at 265, 310, 315, 325, and 345 ºC. The electric measurements were carried out at temperatures ranging from 130 to 300 K. In this temperature range the dominant transport process is identified as variable range hopping. The hopping parameter plotted against the growth temperature is shown to present a maximum. The mechanisms responsible for this behavior are discussed in relation to the compensation ratio.

Reduction of variable range hopping conduction in low-temperature molecular-beam epitaxy GaAs

Studying the transport properties via Hall and resistivity measurements of low-temperature molecular-beam epitaxy (LT-MBE) GaAs samples, the optimal conditions for fabricating high-resistivity material are found. We present results on three LT-MBE GaAs samples grown at 215, 265, and 315 °C. The measurements were carried out at temperatures ranging from 130 to 300 K, and the hopping conduction mechanism in this range is identified as variable range hopping. The sample grown at 315 °C presents the highest hopping parameter; this appears to be due to a reduction in the density of hopping centers. The mechanisms responsible for this are discussed.

DLTS Study on Annealed Low-Temperature GaAs Layers with An n-I(LT)-n Structure Grown by MBE

AbstractDeep levels in the annealed low-temperature molecular beam epitaxial (LT-MBE) GaAs layer were successfully characterized by using the capacitance deep-level transient spectroscopy (C-DLTS) as well as photocapacitance quenching technique in combination with a unique sample structure. In this work, we have fabricated the samples by inserting the LT-GaAs layer into two n-type semi-conductive layers, like a sandwich (n-LT-n structure), grown at normal substrate temperatures. DLTS measurements have revealed that one electron trap dominates the annealed LT-MBE GaAs. The dominant electron trap was very similar to the so-called EL3 level. Moreover, we found the midgap level appeared upon 800-900°C RTA, although no midgap level was detected in the as-grown n-LT-n sample (annealed at 620°C) and confirmed with photoquenching measurements that it is the EL2 level.

In situ ellipsometric study of As capping and low temperature molecular-beam epitaxy GaAs growth and implications for the low temperature critical thickness

Low temperature GaAs (LT–GaAs) has been demonstrated to be a useful high resistivity buffer layer for subsequent growth of a variety of epitaxial device layers. It has proven difficult to reproducibly grow these films due to the low (∼250 °C) substrate temperatures involved and the need for tight III/V flux control. In this work in situ ellipsometry was used to reproducibly set the substrate temperature through a study of the As capping process on a GaAs surface. A hysteresis of ∼200 °C occurred in the temperature at which As was deposited and subsequently removed. Subtle changes occur in the ellipsometric response in this temperature window which suggest minimal variation in the adsorbed As behavior on the GaAs surface. A point occurs in the ellipsometric response which characterizes that temperature at which As deposition begins for a given As overpressure. Using this signature, reproducible LT–GaAs films were grown and followed by ellipsometry. A uniform growth spiral was initially obtained indicating the formation of a homogeneous LT–GaAs layer. The ellipsometric response deviates substantially from the uniform growth spiral as the film thickness exceeds a critical value and may correspond to the formation of an amorphous or polycrystalline structure previously reported [Z. Lilental-Weber, W. Swider, K. M. Yu, J. Kortright, F. W. Smith, and A. R. Calawa, Appl. Phys. Lett. 58, 2153 (1991)], [D. J. Eagelsham, L. N. Pfeiffer, K. W. West, and D. R. Dykaar, Appl. Phys. Lett. 58, 65 (1991)]. Subsequent low angle thin film x-ray diffractometry showed that the formation of polycrystalline LT–GaAs at submicron thicknesses was commensurate with the above signature. Finally variations in the index for LT–GaAs were determined from the growth spirals.

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.

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).

Nonalloyed ohmic contacts on low-temperature molecular beam epitaxial GaAs: Influence of deep donor band

The Ohmic nature of the nonalloyed metal contact on molecular beam epitaxial GaAs grown at 200 °C was studied. The specific contact resistances at room temperature and 120 K were 1.5×10−3 and 7.0×10−1 Ω cm2, respectively. These values are anomalously low considering that the conduction-band electron concentration in this material is less than 1011 cm−3 at room temperature. The experimental results indicate that the carrier transport at the metal/semiconductor interface is dominated by a dense (∼3×1019 cm−3) EL2-like deep donor band, rather than the usual conduction band.<squeeze;1.6p>