Μm Thick GaAs Layer Research Articles

Massive Onset Potential Shifting with an Epitaxial GaAs/Si Photocathode for Solar H2 Production

Solar photoelectrochemical (PEC) H2 production has the potential to solve one of the biggest issues confronting humanity nowadays: producing inexpensive, clean and renewable energy. Among various semiconductor materials, GaAs is an excellent candidate for the development of photoelectrodes within PEC cells for unassisted solar water splitting thanks to its appropriate band gap, good transport and optical properties and high quantum yield. However, the main limitations of GaAs photoelectrodes are related to the high substrate cost, their huge overpotentials and the short lifetime under operation. To address these issues, in this work, we report on the photocathode performance of 1 µm-thick GaAs layers grown on a low-cost Si p-doped substrate by MBE (Molecular Beam Epitaxy) and compare them to those of GaAs:p doped wafers . The photocathodes were investigated in 0.2 M H2SO4 (aq) electrolyte under 1 sun (100 mW/cm2) illumination. The onset potential (Vonset) of bare GaAs/Si and bare GaAs:p wafer is quite comparable, at around -0.2 V vs reversible hydrogen electrode (RHE). The significant reduction of surface states density with a sulfur passivation (S-passivation) by immersing samples in (NH4)2S solution is demonstrated but with very limited changes on the Vonset value, for both GaAs:p wafer and GaAs/Si photocathodes. In contrast, the deposition of thin Pt catalyst layers on both photocathodes by electroless deposition technique leads to a large positive shift Vonset. The Vonset of GaAs/Si and GaAs:p wafer photocathodes are increased up to 0.34 and 0.15 V vs RHE respectively. We show that the Vonset of GaAs/Si can be even improved by combining the Pt catalyst and the S-passivation process, reaching 0.4 V vs RHE (Fig. 1), a record value for GaAs-based Schottky-like photocathodes.STEM-EDX analysis was performed to investigate further the difference between Pt/GaAs/Si and Pt/GaAs:p wafer photocathodes. The EDX-line scans show that after the Pt electroless deposition, the Pt layer on GaAs:p wafer is homogeneous and shows an As0-rich Pt surface that can explain the weaker catalytic activity in comparison with GaAs/Si. Indeed, for the latter, Pt is deposited inhomogenously in the form of clusters leaving some bare surface. Here, As0 is expected to be more easily removed from the surface during the electroless deposition, resulting in more efficient catalyst properties of Pt. The morphology differences between the two Pt-coated electrodes are confirmed by XPS measurements. Since the process of electroless deposition strongly depends on the doping concentration of the materials deposited, the difference between GaAs:p wafer and GaAs/Si photocathodes is attributed to variations in their doping concentrations. Furthermore, the inhomogeneity of the Pt coating on GaAs/Si can also benefit to the passivation process by allowing the (NH4)2S solution to reduce more efficiently the surface defects at the oxide/semiconductor interface. Finally, the stability of Pt/GaAs/Si photocathodes was evaluated, and a lifetime larger than 112 h was established. These findings provide guidance for further studies towards the fabrication of scalable, cost-efficient and stable unassisted PEC cells for green hydrogen production. This research is supported by the “France 2030” French National Research Agency NAUTILUS Project (Grant no. ANR-22-PEHY-0013). Figure 1

Ultrafast InAs Quantum Dot Scintillation Detector

High-efficiency, picosecond scintillation detectors are critically needed for many applications, particularly in the fields of medical imaging, high energy physics and nuclear security. Recently, we proposed a design and considerations for a semiconductor scintillator composed of InAs Quantum Dots (QDs) acting as luminescence centers in a GaAs stopping matrix/waveguide, integrated with a photodetector grown epitaxially on top of the waveguide (WG) [1]. This material has appealing potential photonic properties including high light yield (~240,000 photons/MeV) due to the narrow semiconductor bandgap, fast capture of electrons in QDs (2-5ps) and high radiation hardness (>104 Gy). The high refractive index of GaAs (n=3.4) ensures light emitted by the QDs is waveguided within the GaAs matrix, which can then be collected by an integrated InGaAs p-i-n photodiode (PD). A proof-of-concept integrated scintillation detector (Fig. 1) was grown using molecular beam epitaxy on a GaAs substrate. It has a 20 μm thick GaAs layer with embedded sheets of modulation p-type doped InAs QDs. A generic excitation source is shown interacting with the GaAs matrix, resulting in QD luminescence, which is then waveguided towards an InGaAs PD, grown on top of the WG and tuned to the QD luminescence band (~1150 nm). Employing QD structure engineering improves the internal efficiency of the QD luminescence to ~60% at room temperature and reduces overall waveguide attenuation to about 3-4 cm-1. A sample, as shown in Fig. 1, was used to evaluate the timing characteristics of 5.5 MeV alpha particle responses. The source had a radioactivity of 1 µCi, suspended in air approximately 1 cm above the QD waveguide. The scintillation response (Fig. 2) had a decay time of 0.3 ns, corresponding to QD recombination time, and a noise-limited time resolution of 54 ps (Fig. 3). With a 2D waveguide and an integrated PD covering only 16% of the WG width, the collected charge averaged 1.5x105 electrons (Fig. 4), corresponding to a collection efficiency of about 14%. This can be estimated as 34,000 photoelectrons per 1 MeV of incident energy, given about 1 MeV of alpha particle energy loss in air. This data confirms the unique photonic properties of this scintillation detector, which has the potential to be much faster than any currently used inorganic scintillator. Additionally, technologies for separating an epitaxial film from a substrate and bonding multiple waveguides using organic adhesives are tested and evaluated. [1] S. Oktyabrsky, et al. "Integrated Semiconductor Quantum Dot Scintillation Detector: Ultimate Limit for Speed and Light Yield," IEEE TNS, 63, 656 (2016). Figure 1

Threading dislocations in GaAs epitaxial layers on various thickness Ge buffers on 300 mm Si substrates

We have grown GaAs epitaxial layers on Ge buffers, themselves on Si (001) substrates, using an Applied Materials 300mm metal organic chemical vapor deposition tool. We varied the Ge buffer thickness between 0.36 and 1.38µm and studied the properties of a 0.27µm thick GaAs layer on top. We found that increasing the Ge buffer thickness yielded smoother GaAs films with an rms surface roughness as low as 0.5nm obtained on a 5×5µm2 area. The bow of the substrate increased following a linear law with the epitaxial stack thickness up to 240µm for a 1.65µm stack. We have also characterized the threading dislocations present in the GaAs layers using X-ray diffraction and cathodoluminescence. Increasing the Ge buffer thickness resulted in lower threading dislocation densities, enabling us to obtain anti-phase boundary – free GaAs films with a threading dislocation density as low as 3×107cm−2. In addition, atomic force microscopy surface topology measurements showed the presence of pits in the GaAs layers whose density agreed well with other threading dislocation density assessments. It thus seems that threading dislocations can in certain cases induce some growth rate variations, making them visible in as-grown GaAs films. Using thicker Ge buffers results in smoother films with less threading dislocations, with the side effect of increasing the bow on the wafer. If bow is not an issue, this is a practical approach to improve the GaAs (on Ge buffer) on silicon quality.

Picosecond resolving detection method and experiment for ultrafast X-ray by modulation of an optical probe

Diagnostic measurement of single picosecond event in high energy density physics, laser fusion, plasma radiation, and combustion, is of great importance. However, the measuring of the shape of the single X-ray pulse and the synchronization of X-ray and the laser pulse in picosecond resolution is still a great challenge. Restricted by the transit time of electrons, the time-resolution limit of a conventional framing camera based on the microchannel plate is 40 ps. Centered on the full-optical modulation effect of the light-probe, a novel method for X-ray detection of picoseconds temporal resolution based on low temperature GaAs is proposed in this work. The basic physical mechanism of the detector can be explained in both macroscopical and microcosmic ways. In the macroscopical way, the X-ray radiation absorption in the sensor material produces a transient, non-equilibrium electron-hole pair distribution that results in a transient differential change of the local refractive index, which is then sensed by the reflectivity changes of the optical probe beam. In the microcosmic way, X-ray absorption creates photoelectrons and the core level holes are subsequently filled through Auger or fluorescence processes. These excitations ultimately increase conduction and valence band carriers that perturb optical reflectivity.#br#To verify the proposed X-ray detection method, a Fabry-Perot detector is designed, which consists of a 5 μm thick GaAs layer surrounded by a GaAs/AlAs distributed Bragg reflector. The test is carried out on a femtosecond laser facility, where the X-ray source is produced by focusing the 56 fs Ti: Sapphire facility laser, with a central wavelength of 800 nm, onto an aluminum foil. Then the X-ray pulse induces a transient optical reflectivity change in GaAs, which is a powerful tool for establishing the high-speed X-ray detection.#br#The experimental results indicate that this technology can be used to provide X-ray detectors with a temporal resolution of tens of picoseconds. By optimizing the material, the temporal resolution can be enhanced to be less than 1 ps. Through further development, this X-ray detector could provide an insight into previously unmeasurable phenomena in many fields. Future work will focus on developing much faster devices characterizing both the rise and fall time and imaging array technology.

Wavelength-stabilized tilted wave lasers with a narrow vertical beam divergence

We studied laser diodes grown in the tilted wave geometry with cleaved facets. In this approach a cavity with the gain medium is coupled to the second cavity, while the phase matching of the modes of the two cavities results in the wavelength stabilization. The mode separation can be controlled by the tilt angle of the leaky wave emission and the thickness of the coupled cavity. In one case a ∼100 µm thick transparent substrate with a polished and dielectric-coated back surface was used as a coupled waveguide. In the second case, a 10 µm thick GaAs layer followed by an InGaP evanescent reflector was applied. We observed an increase in the lasing mode wavelength spacing and the width of the vertical far-field lobes from ∼0.7° to 5° (full width at half maximum, FWHM) with the reduction of the thickness of the coupled cavity, in agreement with expectations. The FWHM numbers correspond to the diffraction limit for 100 and 10 µm thick coupled waveguides, respectively. A high temperature stability of the lasing wavelengths (0.1 nm K−1) was revealed. The results indicate that a new generation of wavelength-stabilized lasers for applications requiring ultrahigh brightness and wavelength stabilization can be developed.

Effect of rapid thermal annealing on the noise properties of InAs∕GaAs quantum dot structures

Self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on n+-GaAs substrates, capped between 0.4μm thick n-type GaAs layers with electron concentration of 1×1016cm−3. The effect of rapid thermal annealing at 700°C for 60s on the noise properties of the structure has been investigated using Au∕n-GaAs Schottky diodes as test devices. In the reference sample without containing QDs, the noise spectra show a generation-recombination (g-r) noise behavior due to a discrete energy level located about 0.51eV below the conduction band edge. This trap is ascribed to the M4 (or EL3) trap in GaAs MBE layers, related to a chemical impurity-native defect complex. In the structure with embedded QDs, the observed g-r noise spectra are due to a midgap trap level ascribed to the EL2 trap in GaAs, which is related to the InAs QDs dissolution due to the thermal treatment.

Electronic characterization of several 100 μm thick epitaxial GaAs layers

Non intentionally doped thick epitaxial GaAs layers, grown by chemical vapour phase epitaxy using a high growth rate, are characterized by different electrical techniques applied on a junction (forward and reverse Current–Voltage characteristics, Current versus time and temperature, Deep Level Transient Spectroscopy, Capacitance–Voltage measurements) in order to detect the defects they contain. Experimental data are coupled with theoretical modelling to determine the electrical characteristics of the layers: type and concentration of the carriers, energy levels and concentrations of the defects, associated minority carrier lifetime. The results obtained indicate that i) the concentration of the residual impurities is in the order of 1013 to 1014 cm−3, ii) the layers contain two shallow defects (at 0.12 and 0.21 eV below the conduction band Ec) slightly compensated by a deep defect (at Ec-0.5 eV) and iii) the deep defect is in low concentration (1012 cm−3), resulting in a minority carrier lifetime still limited by the radiative recombination process. Therefore, epilayers obtained using high growth rates exhibit electronic properties very similar to the ones obtained using conventional epitaxial techniques. The good and uniform electronic properties of these layers coupled with the low yield of the fluorescence of GaAs is interesting for high resolution X-ray detection.

Optical characterization of ultra‐pure GaAs

The problem of ultra-pure thick GaAs layers has been important for a long time. Growth of such material is an important technological problem and the basis of application in electronic devices. Epitaxial (100–1000) μm thick GaAs layers were grown by vapour phase epitaxy in a chloride system (HVPE) on N+ and semi insulating 2″ substrates horizontally; the growth zone extended over ∼19 cm with a temperature gradient of ∼3 °C/cm. The total impurity concentration N was in the range ≤ 1012–1014 cm–3. These epilayers have been investigated using low temperature photoluminescence (LTPL), C–V, and Hall measurements. However, estimation of the electro-physical parameters of ultra-pure (N ≤ 1012 cm–3) GaAs often causes difficulties. Because of the low concentration of impurities (N ≤ 1012 cm–3) electro-physical measurements are difficult in many cases or even impossible. In those cases where electro-physical measurements proved feasible a direct relationship was established between the variation of the electro-physical parameters and the evolution of the LTPL spectra at 2 K. By extrapolating the obtained data to regions of lower impurity concentration, ND and μe values corresponding to certain values of the parameters of the LTPL spectra could be found. Thus, analysis of the LTPL spectra can yield estimates of ND and μe when direct determination is difficult or impossible. We consider that from a detailed analysis of LTPL spectra at 2 K the concentration of deep levels in ultra-pure GaAs can be determined.

Reduced-stress GaN epitaxial layers grown on Si(1 1 1) by using a porous GaN interlayer converted from GaAs

This paper reports the reduced-stress GaN epitaxial growth on Si (1 1 1) using a porous GaN interlayer which is formed from GaAs layer by a novel nitridation process. Initially a 2 μm thick GaAs layer is grown on a Si(1 1 1) substrate by MBE. Then, a GaN buffer layer of 20 nm thick is grown on the GaAs layer at 550°C in a MOVPE reactor. The GaAs layer capped with the GaN buffer layer is annealed in NH 3 to 1000°C. Through this process, a porous GaN layer is formed beneath the GaN cap layer. An epitaxial GaN layer is grown on the GaN buffer layer at 1000°C in the MOVPE reactor. The epitaxial layer grown on the porous-GaN/Si(1 1 1) structure is found to have no cracks on the surface. In contrast, an epitaxial layer grown on the GaAs layer nitrated without a cap layer many cracks are found in the epilayer and the layer is sometimes peeled off from the substrate. It is found that the surface morphology of the GaN/porous-GaN/Si(1 1 1) sample is markedly improved by employing a 40 nm-thick interlayer grown at 800°C in addition to the above processes. A PL spectrum with a high intensity ratio between the excitonic emission and the deep yellow emission is obtained for the GaN/porous-GaN/Si(1 1 1) sample. E 2 peak position in Raman scattering spectrum also shows a reduced stress for the GaN epilayers grown on the porous-GaN/Si(1 1 1).

Gas source molecular beam epitaxy as a multi-wafer epitaxial production technology

We present the total merits of gas source MBE (GSMBE) in practical operation as a mass production tool by showing the actual application data. GSMBE realized high-quality crystal growth where at 77 K mobilities as high as 1.21×10 5 cm 2/V s have been obtained from a 7 μm-thick GaAs layer with a carrier concentration of 3.9×10 14 cm −3. The uniformity of sheet resistance in n-GaAs and AlAs mole fractions in AlGaAs over a diameter of 27 cm is excellent with a deviation is less than 2% (1.5 and 0.27%, respectively). High throughput production, dealing with four 4″ diameter wafers at a time, has also been realized with low cost for purchase and operation while maintaining high crystal quality.

Characteristics of GaAs solar cells on Ge substrate with a preliminary grown thin layer of AlGaAs

Characteristics of GaAs solar cell on Ge substrate with a new buffer layer structure is reported. The buffer layer structure, which consisted of a preliminarily grown thin layer of A1 x Ga 1− x As and a 1 μm thick GaAs layer, was designed to obtain a high quality GaAs layer on Ge substrate by metalorganic chemical vapor deposition (MOCVD). Performance of a GaAs solar cell fabricated on Ge substrate with the buffer layer structure was compared with that fabricated on Ge substrate with a conventional GaAs buffer layer and also that fabricated on GaAs substrate. A conversion efficiency of 23.18% (AM1.5G) was successfully obtained for the cell fabricated on Ge substrate with the new buffer layer structure, while it was 20.92% for the cell fabricated on Ge substrate with the conventional GaAs buffer layer. Values of V oc and J sc, for the cell fabricated on Ge substrate with the new buffer layer structure were approximately comparable to those of a 25.39% efficiency GaAs solar cell fabricated on GaAs substrate.

Improvement of life time of minority carriers in GaAs epi-layer grown on Ge substrate

A buffer layer structure on Ge substrate was studied for MOCVD growth of a high-quality GaAs layer. The buffer layer structure was designed taking into consideration both lattice constants and thermal expansion coefficients of GaAs and Ge. It consisted of a preliminarily grown thin layer of Al x Ga 1− x As and a GaAs layer. Photoluminescence (PL) decay of a GaAs layer in an Alo 0.2Ga 0.8As-GaAs-Al 0.2Ga 0.8As double-hetero (DH) structure, which was grown on the buffer layer structure, was observed by time-resolved PL method to estimate the quality of epilayers in the DH structure. The PL decay time strongly depended on Al content ( x) of the Al x Ga 1− x As preliminary layer, and the highest value was obtained when the x was 0.25. A PL decay time above 20 ns was successfully obtained for the DH structure grown on the buffer layer structure, which consisted of a 0.05 μm thick Al 0.25Ga 0.75As layer and a 1 μm thick GaAs layer. Although this value was half of that for the DH structure grown on GaAs substrate, it was much longer than the value of 3 ns for the DH structure grown on Ge substrate with a conventional GaAs buffer layer 1 μm thick.

Improved MBE-grown GaAs using a novel, high-capacity Ga effusion cell

A novel PBN crucible has allowed the design of a new high-capacity Ga source incorporating a unique heat shielding cap. The extra heat shielding at the front of the source allows hot lip operation with considerably lower power than for previous designs, thus completely eliminating Ga droplets without an inordinately high heat load. Compared with another high-capacity, low-flux transient cell (an EPI 125DF), the new source operated with significantly less power and produced lower defect densities. Specifically, defect densities of 22–27 defects/cm 2 for ∼ 1 μ m thick GaAs layers have been obtained with the new source, compared with 88–92 defects/cm 2 from the 125DF. In addition, an 18.7 μm thick GaAs layer grown with the new cell had only 277 defects/cm 2 . The new source also produced GaAs with low background doping (< 2 × 10 13 /cm 3 ), low total trap density (< 2.0 × 10 12 /cm 3 ), and good uniformity over a 3 in diameter wafer (center-to-edge decrease in GaAs thickness of ∼ 2.2%). It also exhibited low flux transients (< 2%) and low-long term drift.

GaAsInP heteroepitaxy and GaAsInP MESFET fabrication by MOVPE

Lattice-mismatched heteroepitaxy has attracted considerable attention in recent years. A great interest of these systems is the possibility of integrating devices from different materials on a single substrate. 1.3 and 1.5 μm InGaAs(P)/InP laser diodes are essential for optical communication, whereas InP field effect transistor technology is less developed than that of GaAs MESFET. The performances of laser diodes are much more sensitive to a high density of dislocations, so it would be interesting to grow GaAs MESFET on InP for integration with 1.3 and 1.5 μm lasers. Due to the large difference of the thermal expansion coefficient and lattice parameter between GaAs and InP, it is very difficult to grow GaAs epilayers of high quality on it is very difficult to grow of GaAs epilayer high quality on InP substrates due to the large difference of the thermal expansion coefficient and lattice parameter between GaAs and InP. A new method, metalorganic source modulation epitaxy (MOSME), which improves the crystal quality of GaAs epilayers on InP substrates by MOVPE, has been adopted in our laboratory. The lowest full width at half maximum (FWHM) of the double crystal X-ray (DCX) diffraction spectra reaches as low as 120 arcsec for a 5 μm thick layer. Structural properties (misorientation, lattice parameters and crystal quality) of 1.0-5.0 μm thick GaAs layers grown on InP have been measured by DCX diffraction. On GaAs MESFETs grown on InP, we have measured g m = 100 ms/mm.For these transitions, the current gain cut-off frequency (F t ) is around 12 GHz and the maximum frequency of oscillation (F max ) is higher than 30 GHz.

Characterization of MOCVD-grown GaAs on Si by spectroscopic ellipsometry

This paper describes the evaluation of the crystalline quality, the stress and surface roughness of GaAs layer grown on Si substrate by using spectroscopic ellipsometry (SE). GaAs layers were grown by metalorganic chemical vapor deposition with changing the intermediate layer and growth conditions. The nine parameters were determined by the fitting of the calculated SE curve to the experiment one, assuming that the structure of surface is (GaAs oxide)/(GaAs + void)/(GaAs layer). It is shown that the tensile stress is applied to 3 μm thick GaAs layer and the stress value is about the same for various intermediate layer structure. However, the surface roughness greatly depends on the intermediate layer structure. The surface roughness of GaAs on Si with AlGaP intermediate layer is smallest, which is consistent with the TEM observation.

Optimization of GaAs epitaxy on CaF 2/Si(111) substrates

Strain-free GaAs epitaxial layers on Si can be achieved by introducing a CaF 2 buffer layer. Here, molecular beam epitaxial growth of GaAs on CaF 2/Si(111) substrates has been thoroughly investigated. Three growth regimes have been distinguished. In the low temperature regime, growth was always in a three-dimensional mode, while, in the high temperature regime, two-dimensional nucleation was achieved. However, continued growth in the high temperature regime resulted in the formation of twins after epitaxial initiation. Between those two regimes, there is a narrow temperature window in which high quality GaAs can be grown on the top of CaF 2/Si(111). Under optimal growth conditions in this window, 1 μm thick GaAs layers were grown which exhibited good crystal quality (a minimum ion channeling yield of 3.5% was achieved at the GaAs surface) along with specular surface morphology. Such high quality GaAs on the top of CaF 2/Si(111) substrates was achieved previously only by using electron-beam modification of the CaF 2 surface.

Silicon cross doping and its effect on the Si or Be implantation doping of gallium arsenide grown on (100) silicon by metalorganic chemical vapor deposition

A study of the cross doping of GaAs layers grown by a two-step metalorganic chemical vapor deposition on Si substrates is reported. All as-grown, unintentionally doped layers of GaAs were n-type, and the carrier profiles tracked the Si atomic profiles. Furnace annealing at 850 °C for 30 min in an arsine overpressure, which is used to improve the crystalline quality of the GaAs near the heterointerface, caused additional Si to diffuse into the GaAs layer. Comparison of the Si concentration at the interface with the carrier concentration suggested the presence of compensating acceptors. Resonance Raman scattering by the SiAs local vibrational mode near the interface shows that a fraction of the Si atoms are localized at the As sites. The furnace annealing increased the Si concentration in the 1.7–1.8 μm thick initially grown GaAs layer. This, in turn, influenced the electrical profiles created with Si or Be implantation on a 2.3 μm thick GaAs layer.

Crack-free and low dislocation density GaAs-on-Si grown by 2-reactor MOCVD system

A 2-reactor MOCVD system is effective to reduce the surface defect which is the trigger for the crack formation, that enables one to obtain a 7 μm thick and crack-free GaAs layer on a 3 inch Si substrate. The etch pit density of the GaAs layer on Si substrate is reduced to 5 × 10 5 cm −2 by the combination of thermal cycle annealing and 2 sets of strained-layer superlattice buffer layers. The minority carrier lifetime measured by time resolved photoluminescence is also studied. It is found that PL lifetime mapping is useful as a non-destructive dislocation evaluation method.

Control of initial surface configuration for GaAs-on-Si MBE using a Si buffer layer?

Gallium arsenide layers were grown by molecular beam epitaxy on (100) Si substrates misoriented by 1.7° and 4° toward [011], with a thin Si buffer layer deposited before GaAs growth for some samples. Reflection high energy electron diffraction observations showed that the substrates misoriented by 1.7° had mostly single-layer steps, while those misoriented by 4° had mostly double-layer steps, regardless of the conditions for the Si buffer layer growth. For 3 μm thick GaAs layers, the measured full widths at half-maximum of the GaAs (400) X-ray reflection ranged from 680 to 870 μrad (about 140 to 180 arc sec), with linewidths about 15% lower for the layers grown on the 4° misoriented substrates. Photoluminescence spectra for the GaAs layers were dominated by carbon-related luminescence, possibly caused by contamination during in situ annealing cycles. However, a GaAs/AlAs superlattice grown on a GaAs-on-Si buffer layer exhibited a single luminescence peak with a linewidth of 8 meV.

A comprehensive study and methods of elimination of oval defects in MBE-GaAs

We have studied the mechanisms of formation and microstructure of oval defects in GaAs films grown by molecular-beam epitaxy, and discovered a novel technique for elimination of Ga-cell related oval defects. Plan-view and cross-sectional transmission electron microscopy (TEM) suggest that the nucleation of oval defects involves imperfections on the surface of the growing crystal such as excess Ga, Ga droplets, or contamination. TEM study also reveals massive dislocation tangles in the core region of oval defects. The majority of the large size oval defects with a core have dark centers in spatially resolved photoluminescence images. which suggests that the central part of such defects is optically inactive. Because of “Ga spitting”, the initial density of oval defects in our system was very high, varying between 10 3 and 10 5 cm -2 depending on the GaAs thickness, growth rate, and growth conditions. By evaporating Ga from an Al-treated PBN crucible, Ga spitting and formation of Ga-cell related oval defects were eliminated. MBE-GaAs layers as thick as 20 μm were totally free from the Ga-cell related oval defects. Remaining oval defects were those related to particulates or substrate contamination and their densities were 100 and 500 cm -2 on 5 and 20 μm thick layers of GaAs on GaAs substrates. These particulates related oval defects were, however, hardly seen on up to 4 μm thick GaAs layers grown on Si substrates. Aluminum wets and reacts with the PBN crucible when heated up to 1300°C during evaporation of Al. Condensed gallium near the orifice wets the aluminum treated surface and is not expected to spontaneously flow to the Ga charge in the form of droplets. As a result, Ga-spitting does not occur and related oval defects do not form. With this technique, MBE-GaAs layers on contamination and particular free substrates can be free from all kind of oval defects.