Vapor Phase Epitaxy Of GaAs Research Articles

Features of the Vapor-Phase Epitaxy of GaAs on Nonplanar Substrates

The features of the surface shape of GaAs epitaxial layers grown on grooves several micrometers wide with vertical walls and an aspect ratio close to unity are investigated. The grooves are formed on the surface of a GaAs wafer in a plasma-chemical etching installation and overgrown by organometallic vapor-phase epitaxy under reduced pressure in a reactor.

Controlled formation of GaAs pn junctions during hydride vapor phase epitaxy of GaAs

Interface formation in HVPE GaAs was investigated through the growth of multilayer test structures with alternately doped and undoped layers and subsequently, pn diode devices. Two growth procedures were used in device formation: continuous growth of all layers, and a growth interruption with simultaneous equilibration of new gas flows for subsequent layers. These junctions were probed using SIMS to determine the doping profiles and impurity incorporation near the interfaces and throughout the bulk of the layers. The junction I–V characteristics were measured with and without illumination to correlate the junction properties with the measured photovoltaic performance. It was discovered that the use of a growth interruption leads to doping transitions up to 6x narrower than samples grown without interruption. The growth interruption leads to an interfacial Si spike that is not observed in the uninterrupted samples during growth of GaAs doped with silane. This spike does not appear to degrade either the material quality or pn junction quality, and pn diodes grown with interruption have exhibited enhanced device efficiencies under solar simulation compared with devices grown without interruption, reaching efficiencies of up to 9.2% without the use of antireflective coatings.

Metalorganic Vapor Phase Epitaxy of GaAs with AlP Surface Passivation Layer for Improved Metal Oxide Semiconductor Characteristics

Growth of an AlP epitaxial layer on the top of GaAs was proposed as a novel in situ passivation method. The AlP layer was almost converted to AlOx upon air exposure, forming a part of a gate dielectric. Removal of AlAs at the GaAs/AlP interface was mandatory for avoiding As-oxide formation upon air exposure, which necessitated complete As removal from the surface when switching growth from GaAs to AlP. H2S treatment allowed us to obtain low-As-content c(8×2) surface reconstruction for the first time with metalorganic vapor phase epitaxy (MOVPE). AlP on the c(8×2) GaAs surface has made it possible to obtain a smooth morphology and complete suppression of arsenic oxide. PL intensity increased by a factor of three with the AlP growth for 10 s at 500 °C, corresponding to a thickness of 0.5 nm. Accumulation capacitance obtained from capacitance–voltage (C–V) curves was the largest with that growth a condition of AlP, suggesting reduction in interface states.

Surface Reaction Kinetics in Metalorganic Vapor Phase Epitaxy of GaAs through Analyses of Growth Rate Profile in Wide-Gap Selective-Area Growth

Wide gap selective area growth was proposed as a probe for obtaining information of surface kinetics in the metalorganic vapor phase epitaxy (MOVPE) of GaAs. GaAs was grown over patterned substrates using a SiO2 mask. The characterized growth-rate enhancement profiles for several total pressures were successfully fitted with two-dimensional simulation, taking only gas phase diffusion into account, except for the profiles in the vicinity of the mask edge, which seemed to reflect surface diffusion. The only parameter governing the profile was D/ks, the ratio of the gas-phase diffusion coefficient (D) to the surface reaction rate constant (ks) of a film precursor. The value of D can be estimated using the Chapman–Enskog kinetic theory, the accuracy of which was validated by observing a reactor-scale growth-rate profile under the mass-transfer-limited regime, and thus we obtained the value of ks. The sticking probability of a precursor, which was converted from ks, ranged between 0.06 and 0.33 for the temperature range of 823 to 973 K. The values are smaller than unity which was often assumed in growth simulations. The value of ks was measured as a function of growth temperature. Its activation energy changed at around 873 K: 8 kJ/mol for higher temperature and 121 kJ/mol for lower temperature. This information is important not only in the design of a selective-area growth process but also for constructing a more realistic reaction model of MOVPE.

Study of the Surface Processes in Vapor-Phase Epitaxial GaAs: Asymmetric Trapping of Atoms at the Step

Experimental proofs of asymmetric trapping of atoms at the growth step in vapor-phase epitaxy of gallium arsenide in the GaAs–AsCl3–H2 system are given. The data obtained confirm the important role of the surface diffusion mass transfer in the growth of epitaxial GaAs layers on vicinals in the neighborhood of (111)A. The effective diffusion length is estimated.

Development of low-pressure vapour-phase epitaxial GaAs for medical imaging

Abstract A summary is given of progress accomplished with the development of low-pressure vapour-phase epitaxial GaAs as a material for X-ray detectors. As the III–V concentration ratio is altered from Ga-rich to As-rich, the material is shown to improve from p-type, to n-type with compensation via deep levels, to n-type with a doping density of 1.7×1014 atoms cm−3. The measured barrier height is 0.8 V, as expected for the Ti contact used. Overdepletion was obtained before breakdown, enabling a layer thickness of 41 μ m to be deduced for the final sample. For the later samples, charge collection for 60 keV Am-241 gammas was bias independent at a value of 100±8%. Spectra were also obtained from Sr-90 electrons. The most probable value of the charge collected as a function of the bias reached a plateau and from this value a depletion width of 40 μ m was found for the final sample, equal to the epitaxial layer thickness. Results from detailed alpha and low-energy proton spectroscopy are shown for diodes fabricated from this material. A charge collection efficiency of 100% was obtained when the diode could be depleted sufficiently. The concept of a charge collection depth was introduced, since a significant amount of charge was collected without bias. The minimum depth of such a region was shown to be 10.8 μ m at 0 V reverse bias, far greater than the 1.1 μ m predicted for the depletion depth. Charge coupling between the guard ring and the pad was observed and successfully modelled.

Characterization of interface deep levels in As vapor grown EPI-GaAs

In this work vapor phase epitaxial GaAs structures are investigated by the method of transient acoustoelectric spectroscopy. At the interface between a substrate and epilayer we identified four types of defects (EL3, EL5, EL6, EL17). We have measured the cross section for electron capturing, relaxation times of trapped charge and energetic levels in forbidden zone for said defects.

Characterisation of low-pressure VPE GaAs diodes before and after 24 GeV/ c proton irradiation

GaAs Schottky diode particle detectors have been fabricated upon low-pressure vapour-phase epitaxial GaAs. The devices were characterised with both electrical and charge collection techniques. The height of the TiGaAs barrier used was determined via two electrical methods to be (0.81±0.005) and (0.85±0.01) eV. The current density was greater than that expected for an ideal Schottky barrier and the excess current was attributed to generation current in the bulk of the material. A space charge density of (2.8±0.2)×10 14 cm −3 was determined from capacitance voltage characterisation. The charge collection efficiency was determined from front alpha illumination and 60 keV gamma irradiation to be greater than 95% at a reverse bias of 50 V. The diodes were characterised after an exposure to a radiation fluence of 1.25×10 14 24 GeV/ c protons cm −2. The reverse current measured at 20°C increased from 90 to 1500nA at an applied reverse bias of 200 V due to the radiation induced creation of extra generation centres. The capacitance measurements showed a dependence upon the test signal frequency which is a characteristic of deep levels. The capacitance measured at 5 V reverse bias with a test frequency of 100 Hz fell with radiation from 300 to 40pF due to the removal of measurable free carriers. The charge collection of the device determined from front alpha illumination also fell to (32±5)% at a reverse bias of 200 V.

Compensation of shallow impurities in oxygen-doped metalorganic vapor phase epitaxy grown GaAs

Intentional oxygen incorporation, using diethyl aluminum ethoxide [(C2H5)2AlOC2H5] during metalorganic vapor phase epitaxy GaAs was found to compensate C and Zn shallow acceptors as well as Si and Se shallow donors, due to the oxygen-related multiple deep levels within the GaAs band gap. Deep level transient spectroscopy (DLTS) was used to characterize the energy levels associated with these oxygen deep centers. The total measured trap concentration from DLTS can account for the observed compensation in n-type GaAs:O. The total trap concentration in p-type GaAs:O, however, was found to be lower than the observed compensation by a factor of ∼100. These oxygen deep centers exhibit multiple electronic states which have been associated with the local number of Al nearest neighbors and the microscopic structure of the defect. The concentration and nature of these deep levels were not influenced by the chemical identity of the shallow dopants.

Step-free surface grown on GaAs (111)B substrate by selective area metalorganic vapor phase epitaxy

Selective metalorganic vapor phase epitaxy of GaAs within a finite area was investigated on (111)B substrate. The surface stoichiometry is determined based on surface photo-absorption measurement. High-temperature growth at 800 °C on stable GaAs (111)B and a cooling procedure with the (2×2)-like surface to improve flatness result in an intentionally step-free surface with device dimensions as wide as 8 μm on a selectively grown GaAs mesa.

Antimony doped GaAs: Role of the isoelectronic dopant in defect evolution

Metalorganic vapor phase epitaxial GaAs:Sb layers and Schottky barrier structures based on them have been investigated regarding defect evolution versus doping level. Three types of defects are considered such as surface morphology imperfections, structural defects in the epilayers, and interface defects at the Schottky barriers. A correlation between the defect behavior in the layers and the Schottky structures has been observed. It has been shown that an optimal Sb doping region exists where the defect density decreases compared with the undoped and highly Sb doped samples. The dominating defects in the Schottky barrier interface are dislocation-related states which can be effectively influenced by the isoelectronic doping.

Sites for arsine adsorption on GaAs(001)

Arsine adsorption on c(2 × 8) and (1 × 6) GaAs(001) at 303–573 K has been studied by internal-reflection infrared spectroscopy. We have discovered that arsine adsorbs onto two sites: second-layer Ga atoms and Ga dimers. On the c(2 × 8), arsine dissociatively adsorbs on second-layer Ga atoms, forming arsenic monohydrides and transferring two H atoms each to nearby As dimers. On the (1 × 6), arsine dissociatively adsorbs on Ga dimers, and also transfers H atoms to As sites. The saturation coverage of arsine at 303 K on the (1 × 6) is twice that on the c(2 × 8). Also, more As-H infrared bands are observed, indicating that several AsH x s are formed. Dosing the surfaces with arsine at successively higher temperatures from 303 to 473 K leads to the loss of adsorbed AsH x species. At 573 K, no change in the c(2 × 8) occurs upon extended exposure to arsine. However, on the (1 × 6), the Ga dime are replaced by As dimers during arsine dosing, and at 573 K, 1800 L of AsH 3 is sufficient to convert the (1 × 6) into the c(2 × 8) reconstruction. We conclude that during vapor-phase epitaxy of GaAs with trimethylgallium and arsine, AsH 3 decomposes on second-layer Ga atoms and Ga dimers. However, on the latter sites the arsenic is more likely to incorporate into the crystal, instead of desorbing as As 2 or As 4 .

Growth oscillations with monolayer periodicity monitored by ellipsometry during metalorganic vapor phase epitaxy of GaAs(001)

In this letter we report on the observation of growth oscillations with monolayer periodicity by ellipsometry. An oscillation amplitude of δ〈ε1〉=0.05 was measured using an optimized spectroscopic in situ ellipsometer whose wavelength was tuned to the 2.65 eV resonance energy of the arsenic dimers covering the GaAs (001) growth surface. The monolayer periodicity was verified by parallel monitoring of the growth with reflectance anisotropy spectroscopy (RAS).

Te doping with dimethylditelluride during organometallic vapor phase epitaxy of GaAs

Dimethylditelluride has been used as a dopant source for GaAs epilayers grown by atmospheric pressure oganometallic vapor phase epitaxy. The highest electron concentration obtained, n = 2.7 × 10 19 cm −3, appears to be the highest electron concentration ever obtained in VPE GaAs consistent with mirror-like morphology. The electron and Te concentrations are virtually the same, as determined by SIMS, CV, and Hall effect. Electron concentrations are routinely obtained above 1 × 10 19 cm −3 over a range of growth temperatures and low frsol|V/III ratios of 5–10. The relationship between electron concentration and dopant mole fraction, group V mole fraction, growth temperature and growth rate have been studied. A model of Fermi-level pinning at the surface is consistent with the observed Te incorporation. Excellent morphology combined with high electron concentration requires the use of high growth rates, r g ≥ 6 μm/h. We have extended the measured mobility vs electron concentration curve to n = 3.3 × 10 19 cm −3. Our results also indicate that the highest Te concentrations obtained are in excess of the solubility limit.

Behavior and mechanism of step bunching during metalorganic vapor phase epitaxy of GaAs

The behavior of step bunching during GaAs growth for metalorganic vapor phase epitaxy is investigated by varying growth temperatures and substrate misorientation angles. It is found that monolayer step-flow growth changes to step-bunching growth when the misorientation angle, that is, the substrate surface step density, increases to a certain value, showing the existence of a critical step density. This critical value decreases with increasing growth temperature. The bunched terrace width does not depend on the misorientation angle, but it increases with growth temperature. We propose that the transition to step bunching from step-flow growth occurs when the concentration of Ga adatoms at the step edges is less than the equilibrium concentration. Various bunching characteristics are explained using this model.

Ultrafast photodetector materials based on oxygen-doped metalorganic vapor phase epitaxy GaAs

Controlled incorporation of deep level states into III/V materials tailor their electrical and optical characteristics for ultrafast optoelectronics. We investigate the photoinjected carrier dynamics in GaAs grown by metalorganic vapor phase epitaxy (MOVPE) with (C2H5)2AlOC2H5 as an oxygen precursor providing the deep level state control. Photoconductor switching was used to determine the free-carrier trapping times, and femtosecond photoreflectance at 2 eV was used to measure the trapped charge recombination time as a function of oxygen concentration. We also use electro-optic sampling to demonstrate a photodetector with 650-fs measured response time and to show the material applicability to ultrafast optoelectronics.

Effect of Fermi Level Pinning at the Surface During OMVPE Growth

AbstractDuring organometallic vapor phase epitaxy of GaAs, the Fermi energy is found to be pinned ≈200 meV below the intrinsic Fermi energy on a (lOO)-oriented surface. This was determined by making the first comparison of Zn concentration, Nzn. obtained from growth with that obtained at equilibrium, all under the same nominal ambient conditions. Donor solubility has been found to be virtually unchanged during growth of n- or p-type GaAs, and this is strong evidence that the Fermi level remains pinned at essentially the same energy during growth of n- or p-type GaAs. Consistent with Fermi level pinning, we find (1) the NZn is supersaturated in the epilayer for the ambient growth conditions used, (2) a large excess concentration of positively charged Ga interstitials can be grown into n-type GaAs and which diffuses into nearby p-type layers, causing (3) the Zn diffusivity, DZn ∼10−13 cm2/s, out of buried npn layers to be essentially independent of the ambient conditions. Annealing of a heavily Zn-doped layer at the surface can lead to 10−16<DZn<10−13 cm2/s at T=800°C simply by varying the Zn partial pressure over its commonly used range. Use of In-doped spikes in n- and p-type GaAs suggest that interstitial point defects have a strong effect on the group III diffusion.

Vapour phase epitaxial grown GaAs films with a very low deep level concentration

Isovalent indium doping was employed in order to decrease the deep level concentration in n-type GaAs:S films grown by the vapour phase epitaxy in a chloride system. The EL2 electron trap was found to be the dominant deep level in the films with a low donor concentration. Using isovalent In doping, the concentration of this deep level was reduced from about 1 × 1014 cm−3, which was typical of the GaAs:S films, to a value less than the deep level transient spectroscopy (DLTS) sensitivity threshold (< 2.5 × 1012 cm−3). In the case of heavily sulphur doped films, a strong decrease in the deep level concentration was detected by photoluminescence when isovalent indium doping was used. The suppression of the deep-level-related lines was accompanied by an enhancement of the band-to-band radiative recombination. All the effects were found to appear in a narrow region close to an indium content of 1 × 1020 cm−3.

Multiatomic step formation mechanism of metalorganic vapor phase epitaxial grown GaAs vicinal surfaces and its application to quantum well wires

The multiatomic steps formed on GaAs vicinal surfaces by metalorganic vapor phase epitaxy (MOVPE) are studied by atomic force microscopy (AFM). An AFM image of an epitaxially grown GaAs surface showed coherent multiatomic steps with extremely straight edges over a wide area. The average height and spacing of the multiatomic steps are 1.2–8 and 30–110 nm, respectively. These terrace widths change with the growth conditions. Narrower terrace widths are obtained at higher growth rates, and under higher AsH3 partial pressures and higher impurity doping conditions. The results suggest that the migration distance of a Ga atom on the terrace and the sticking coefficient at the step sites depend on these growth conditions. Using multiatomic steps, GaAsAlGaAs quantum well wires (QWWs) were grown on a GaAs vicinal surface. Cross-sectional transmission electron microscopy and photoluminescence show the successful fabrication of QWWs.

Dependence of carbon incorporation on crystallographic orientation during metalorganic vapor phase epitaxy of GaAs and AlGaAs

We studied the dependence of carbon (C) incorporation on the crystallographic orientation during metalorganic vapor phase epitaxy (MOVPE) of GaAs and AlGaAs. We performed C doping to GaAs grown with trimethylgallium (TMGa) and AsH 3 and Al 0.35Ga 0.65As grown with trimethylaluminum (TMAl), TMGa or triethylgallium (TEGa), and AsH 3 on the surface lying between (100) and (111) A/B. C and hole concentrations nearly coincided over the entire orientation range, showing that C is preferentially incorporated at As sites, irrespective of orientation. We found that the dependence of C incorporation on the orientation is more complex than previously thought. The dependence of carrier concentration on the AsH 3 partial pressure reflected the degree of C incorporation very well. Using simultaneous doping with C acceptors and Si donors, we fabricated lateral p-n junctions on patterned substrates composed of the (100) principal surface and (411)A/B sidewalls.