Low-temperature GaAs Layer Research Articles

The electrical breakdown properties of GaAs layers grown by molecular beam epitaxy at low temperature

The electrical properties of as-grown low temperature (LT) GaAs grown at 200-300 degrees C have been investigated in the temperature range of 100-400 K. It was found that the resistivity of the LT GaAs layer increased as the growth temperature was increased from 200 degrees C to 300 degrees C. Correspondingly, over the same growth temperature range, the breakdown field decreased from 320 kV cm-1 to 80 kV cm-1, yet was more than one order of magnitude higher than that of semi-insulating GaAs. The breakdown voltage VBD was found to increase as the measurement temperature was decreased, differing from the behaviour of conventional avalanche breakdown. The transport properties of LT GaAs were characterized by hopping conduction at low electric field and low temperature. The particular properties of the as-grown LT GaAs layers suggest a useful application as in insulator in GaAs field effect transistors (FETS). The predominant ohmic behaviour resulting from hopping conduction, produces a uniform field in the material, which prevents the breakdown at the gate edge of the FETS or at the surface. The high breakdown strength suggests enhanced performance in power devices.

Studies on deep levels in GaAs epilayers grown on Si by metal-organic chemical vapour deposition, Part II: 1.13 eV photoluminescence emission

Midgap deep levels have enjoyed a special status in semiconductor technology ever since Shockley and Read [1] and Hall [2] first pointed out their role as most efficient recombination centres on the basis of kinetic considerations. Several midgap luminescence bands trapped at deep levels have been found in GaAs and some of them are not yet well understood. The 0.635 eV photoluminescence (PL) emission in oxygen-doped semi-insulating GaAs was attributed to a complex centre involving the deep oxygen donor on an arsenic site and a gallium vacancy as a nearest neighbour [3, 4]. The 0.64, 0.68, 0.77, and 0.8 eV PL bands in undoped semi-insulating GaAs were related to the electron traps EL2 and EL6, respectively [5-10]. The origin of the 0.8 eV emission in Sidoped GaAs layers was suggested to arise from an ASGa--VAs centre [11]. The emission at about 0.85 eV in low-temperature GaAs was associated with emission from the surface states [12, 13]. However, Yu et al. [14] recently studied the 0.8 eV emission from low-temperature GaAs layers and attributed the 0.8 eV emission to an Asi-VG, centre (EL6). In this work, the 0.78 and 0.84 eV photoluminescence bands present in GaAs epitaxial layers grown on Si by metal-organic chemical vapour deposition were studied at various excitation intensities and temperatures and it was shown that they originate from the recombination of arsenic defects such as As interstitials (Asi), and gallium vacancies (VG,). The temperature dependence of the PL intensities obeys the relation IpL oc [1 + A exp (r/To)] -1, previously reported for amorphous semiconductors. A model for the recombination mechanism is given and the results are explained well by thermally activated tunnelling. GaAs epitaxial layers used in the experiment were grown on 4 ° off toward [110] (100) n-type Si substrates using the two-step method by MOCVD. The Si substrates-were chemically treated in NH4OH, H202, H20 and HC1, H2Oz, H20, and etched in HF for i min. In a U 2 / A s H 3 ambient, they were heated initially at 950 °C for 10 min, and then the temperature was lowered to 450 °C for GaAs

EL2-like defects in low temperature GaAs

Two different techniques, namely optical absorption and X-ray diffraction, were extensively used to study arsenic antisite (As Ga ) defects in low temperature (LT) GaAs. A standard temperature and illumination cycle following the ground-tomestable and reverse transitions was employed in both experiments. Similarities between EL2 and the defects in question were found, leading to the establishment of three types of As Ga -related centres in LT GaAs. The underlying role of the strained lattice in LT GaAs layers was pointed out by the X-ray experiment.

Characterization of crystalline low temperature GaAs layers annealed from an amorphous phase

The results from an in-depth characterization of as-grown and annealed low-temperature GaAs layers deposited at less than 260°C are presented. The layers, amorphous as grown, became crystalline after annealing. The crystallization was documented by several characterization techniques including photoreflectance, Raman spectroscopy, photoluminescence, transmission electron microscopy, and double-crystal x-ray diffraction. The n-type conductivity of the annealed films was exploited for the construction of a diode structure.

A scanning tunneling microscopy study of low-temperature grown GaAs

Scanning tunneling microscopy (STM) has been used to investigate the effect of low-temperature (LT) growth of GaAs by molecular beam epitaxy on the morphology of the resulting surface. We present STM images of a GaAs(001) surface that was grown at ∼300°C and subsequently annealed at 600°C and show that there is a recovery of the (2×4) reconstruction. We also report images of a surface grown on top of a buried LT GaAs layer and show that the LT layer has little effect on the resulting surface morphology. In addition, scanning tunneling spectroscopy spectra are presented which demonstrate that the current-voltage characteristics of annealed and unannealed LT grown GaAs are significantly different.

Structure and carrier lifetime in LT-GaAs

The relationship between the structural quality of low-temperature GaAs layers and the photoexcited carrier lifetime has been studied. Transmission electron microscopy, x-ray rocking curves, time-resolved reflectance methods, and photoconductive-switch-response measurements were used for this study. For a variety of samples grown at temperatures in the vicinity of 200°C, subpicosecond carrier lifetimes were observed both in as-grown layers, as well as in the same layers after post-annealing and formation of As precipitates. These results suggest that the carrier lifetime, which was found to be shorter in the as-grown layers than in the annealed ones, might be related to the density of AsGa antisite defects present in the layers. The annealed layers which contained structural defects before annealing appeared to exhibit the longest carrier lifetime due to gettering of As on these defects (and formation of relatively large As precipitates) and depletion of extra As (AsGa) defects from the layer. It was found as well that the responsivity of detectors fabricated on these layers depended strongly on the structural quality of the layers, with the greatest response obtained not for the layers with the fewest defects, but for the layers with 107–108/cm2 of pyramidal defects.

Application of convergent-beam illumination methods to the study of lattice distortion across the interface

A high accuracy measurement of the lattice parameter can be obtained by x-ray diffraction. However, these methods have low spatial resolution and are limited by sample thickness. Therefore, for heterolayers much better information about local distortions near the interface can be obtained by electron microscopy using convergent-beam illumination. Large-angle convergent-beam electron diffraction (LACBED) patterns and convergent-beam imaging (CBIM) were applied to study of the lattice distortion across the interface of the GaAs epilayer grown on [001] GaAs substrate and the GaAs layer grown at 200°C (called the low-temperature (LT) layer). These LT GaAs layers are grown by molecular beam epitaxy from As oversaturation and are known to be As rich. Particle-induced x-ray studies reveal up to 1.5 % extra As. This excess As leads to the expansion of the lattice parameter up to 0.15% when measured by x-ray diffraction along the (004) reflection. From this study, it was expected that a cubic expansion of the lattice parameter would be equal in all three directions. However, TEM studies do not reveal the formation of dislocations at the interface, which would be expected taking into account the difference in the lattice parameter across the interface . Convergence beam illumination methods were applied to study in detail the strain, lattice parameter variation, and crystallographic distortion across the interface, since these methods have spatial resolution several orders of magnitude higher than x-ray diffraction. A CBED pattern taken in the substrate and in the layer with an incidence close to the [530] direction (exact orientation [0.869 0.495 0]) reveals the lattice parameter change, since the position of the cross of the 1113 and 1113 lines changes when an electron beam is placed in the substrate and the layer, respectively (Fig. la,b).

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).

Optically detected electron paramagnetic resonance of arsenic antisites in low-temperature GaAs layers

Arsenic antisites in GaAs layers grown by molecular-beam epitaxy at low substrate temperatures (∼200 °C) were observed using electron paramagnetic resonance (EPR), magnetic circular dichroism in absorption (MCDA), and MCDA-detected EPR. This observation confirms that there is a MCDA band directly associated with arsenic antisites in the GaAs layers.

Transmission electron microscopy study on GaAs grown by molecular beam epitaxy at low substrate temperature

Report of the microstructure of the low temperature-GaAs layers before and after annealing, examined by high resolution electron microscopy. The layers grown at a substrate temperature of about 200°C contained twin blocks consisting of (111) twin lamellae, and the excess arsenic seems to be distributed at the twin boundaries. After annealing the sample at 600°C for 10 min the excess arsenic redistributed so that the twin blocks disappeared and some arsenic precipitates were formed

Microstructure of annealed low-temperature-grown GaAs layers

The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180° to 210° C. In the monocrystalline LT-GaAs layers small “pseudocubic” As precipitates (2–3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (>4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.

X-ray analysis of GaAs layers on GaAs(001) and GaAs(111)B surfaces grown at low temperatures by molecular beam epitaxy

We report on the simultaneous, molecular beam epitaxy growth of GaAs on GaAs(001) and GaAs(111)B substrates at low temperatures. The crystallinity of the low-temperature GaAs layers was assessed using a double-crystal x-ray diffractometer and a wide-angle diffractometer with a rotating specimen stage. Layers were grown at 200 and 250 °C to a thickness of 3 μm on both (001) and (111) orientated substrates and an additional 3 μm layer was grown on GaAs(111)B at 300 °C. Double-crystal diffractometry confirmed the presence of a single crystalline layer, with a growth-temperature-dependent excess As concentration, on the (001) substrates. On the (111) substrates, only a polycrystalline layer was observed. A possible explanation for these observations based on growth surface roughening is presented.

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.

Structural properties of As-rich GaAs grown by molecular beam epitaxy at low temperatures

GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x-ray diffraction, and electron paramagnetic resonance (EPR) techniques. High-resolution TEM cross-sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x-ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE-grown GaAs. These results are related to off-stoichiometric, strongly As-rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low-temperature MBE-grown GaAs layers.