Metalorganic Vapor Phase Epitaxy Conditions Research Articles

InAs/InP Quantum Dots in Etched Pits by Droplet Epitaxy in Metalorganic Vapor Phase Epitaxy

The growth of InAs quantum dots (QDs) on InP(100) via droplet epitaxy in a metalorganic vapor phase epitaxy (MOVPE) reactor is studied. Formation of indium droplets is investigated with varying substrate temperature, and spontaneous formation of nanoholes is observed for the first time under MOVPE conditions. Indium droplets are crystallized into QDs under arsenic flow at different temperatures. For temperatures greater than 500 °C, a local etching takes place in the QD vicinity, showing an unexpected morphology which is found to be strongly dependent on the crystallization conditions. Such QDs are structurally and optically investigated and emission from single QDs in the telecom C‐band is detected via microphotoluminescence at low temperature.

A model for calculating the composition of GaAs x P1–x solid solutions under metalorganic vapor phase epitaxy conditions

A model is proposed for calculating the composition of GaAsxP1–x solid solutions, based on the iterative determination of the equilibrium partial pressures of the pyrolysis products of the starting reagents, followed by calculation of the composition of the solid solution in an adsorption–desorption model. The proposed model ensures good agreement between calculation results and experimental data.

Growing InAs/GaAs quantum dots by droplet epitaxy under MOVPE conditions

Results of studying the formation of InAs quantum dots (QDs) on GaAs(100) substrates by droplet epitaxy using trimethylindium and arsine (AsH3) as precursors are presented. The growth process was carried out at temperatures within 230–400°C in a horizontal reactor for metalorganic vapor phase epitaxy (MOVPE) using high-purity hydrogen as the carrier gas. Data on the influence of process temperature on the QD size and the density of QD array and results of investigation of the low-temperature photoluminescence of obtained samples are presented.

A quantum chemical study on gas phase decomposition pathways of triethylgallane (TEG, Ga(C2H5)3) and tert-butylphosphine (TBP, PH2(t-C4H9)) under MOVPE conditions.

The gas phase decomposition reactions of precursor molecules relevant for metal-organic vapour phase epitaxy (MOVPE) of semiconductor thin films are investigated by computational methods on the density-functional level as well as on the ab initio (MP2, CCSD(T)) level. A comprehensive reaction catalogue of uni- and bimolecular reactions is presented for triethylgallium (TEG) as well as for tert-butylphosphine (TBP) containing thermodynamic data together with transition state energies. From these energies it can be concluded that TEG is decomposed in the gas phase under MOVPE conditions (T = 400-675 °C, p = 0.05 atm) to GaH3via a series of β-hydride elimination reactions. For elevated temperatures, further decomposition to GaH is thermodynamically accessible. In the case of TBP, the original precursor molecule will be most abundant since all reaction channels exhibit either large barriers or unfavorable thermodynamics. Dispersion-corrected density functional calculations (PBE-D3) provide an accurate description of the reactions investigated in comparison to high level CCSD(T) calculations serving as benchmark values.

Formation conditions for InAs/GaAs quantum dot arrays by droplet epitaxy under MOVPE conditions

The first stage of formation of InAs/GaAs quantum-dot heterostructures by droplet epitaxy is investigated. Factors influencing the geometrical size and density of arrays of indium nanodrops deposited by trimethylindium pyrolysis on the GaAs(100) substrate are analyzed, and the possibility of using these factors in the process of metal-organic vapor phase epitaxy (MOVPE) are studied. To refine the temperature dependence of the In evaporation rate, a computational experiment taking into account real MOVPE conditions is conducted. An ultimate change in the composition of In droplets contacting the substrate at a high temperature is estimated, and the thickness and composition of crystallizing In-Ga-As solid solution are calculated. It is shown that the size and density of the droplet array to a great extent depend on the crystallochemical structure of the substrate surface and deposition conditions.

On the possibility of growth quantum dot arrays in InAs/GaAs system by droplet epitaxy under MOVPE conditions

We have experimentally studied the possibility of obtaining InAs quantum dot arrays on GaAs(100) substrates by droplet-island growth under low-temperature (160–360°C) metalorganic vapor phase epitaxy (MOVPE) conditions. It is established that trimethylindium (In source) exhibits decomposition even at the lower boundary of the indicated temperature interval. The height of In drops formed on the substrate surface was 3–12 nm with a density of ∼(0.4−1.4) × 109 cm−2 depending on the H-MOVPE conditions. In order to retain the dimensions of InAs nanocrystals formed at the subsequent stage, the process should be carried out at an increased rate of arsine supply.

Analysis of twin defects in GaAs nanowires and tetrahedra and their correlation to GaAs(1 1 1)B surface reconstructions in selective-area metal organic vapour-phase epitaxy

We analyzed twin defects in GaAs nanowires as thin as 100–400 nm and tetrahedral structures as small as 1.0 μm, which were selectively grown by metal organic vapour-phase epitaxy (MOVPE) within a SiO 2 mask window fabricated on GaAs(1 1 1)B substrates. In particular, we focused on the correlation between the twins and GaAs(1 1 1)B surface reconstructions. We confirmed that the shape of GaAs crystals selectively grown on GaAs(1 1 1)B substrates changed from hexagonal nanowires to truncated tetrahedra when the size of the mask opening was increased from 100 to 1000 nm under the same growth conditions. The shape also changed from tetrahedral to hexagonal with decreasing growth temperature ( T g : 600–800 °C) and with increasing arsine (AsH 3) partial pressure (1.0×10 −4 to 5.0×10 −4 atm). Rotational twins around the 〈1 1 1〉 axis were found in the tetrahedra by transmission electron microscopy (TEM) and scanning electron microscopy observations. In addition, the probability of twins developing in the tetrahedra increased with decreasing mask opening size, with decreasing T g , and with increasing AsH 3 partial pressure. The TEM study also revealed the existence of a high density of rotational twins in the nanowires, and their density increased with decreasing nanowire diameter, suggesting a correlation between the twins and the shape/size of GaAs crystals. These findings were semi-quantitatively compared with a reported phase diagram for GaAs(1 1 1)B surface reconstruction. By analyzing the relationship between twin development and MOVPE conditions, we found that the shape change of GaAs crystals on GaAs(1 1 1)B and the formation of twins coincided well with the transition of GaAs surface reconstruction between the (2×2) and (√19×√19) structures.

Optical monitoring of technological parameters in metal-organic vapor-phase epitaxy

The possibility of applying low-coherent tandem interferometry to optical monitoring of the temperature of a semiconductor substrate and the thickness of a deposited layer in metal-organic vapor-phase epitaxy (MOVPE) is demonstrated for the first time. The absolute accuracy in the temperature measurements of Si, GaAs, and sapphire substrates under MOVPE conditions is limited by the calibration accuracy and is ±1°C. The accuracy in the measurement of the deposited layer thickness is 2 nm. A considerable (10–100°C) deviation of the temperature measured by a thermocouple placed inside a susceptor from the actual substrate temperature is found. A significant temperature gradient along the susceptor depending on the gas flow rate and other factors is revealed. It is shown that, owing to the high heating efficiency of sapphire substrates, there is no need to coat their reverse with absorbing layers upon heating up to 300°C or in the presence of hydrogen pressure of higher than 100 mbar.

Growth monitoring of GaAsSb:C/InP heterostructures with reflectance anisotropy spectroscopy

Using time-resolved reflectance anisotropy spectroscopy (RAS) we studied in situ the dynamics of surface processes of Sb-covered GaAs(1 0 0) and InP(1 0 0) under metalorganic vapour-phase epitaxy conditions. Both Sb removal during a hydride purge and Sb incorporation during GaAs and InP overgrowth were monitored enabling the investigation of group V exchange reactions. Sb surface layers are found to be much more stable during purge with PH 3 as compared to AsH 3. Estimated activation energies are 2.5 eV for the Sb–P exchange processes and 1.1 eV for the Sb–As exchange reactions. These values reflect the low volatility of Sb compared to other group V adsorbates. During InP overgrowth Sb is rapidly buried at temperatures of 510 °C and below while it is rather persistent on the surface at temperatures of 550 °C and above. This transition indicates the existence of a highly mobile quasi-liquid InSb surface layer at temperatures above its melting point (∼525 °C).

Size- and shape-controlled GaAs nano-whiskers grown by MOVPE: a growth study

We have investigated the Au-catalyzed GaAs 〈 1 ̄ 1 ̄ 1 ̄ 〉 B whisker growth under low-pressure metal-organic vapour phase epitaxy conditions. By varying the growth temperature we found a maximum in the whisker growth rate at about 450–475°C. With increasing temperature the growth rate decreases due to competing growth at the ( 1 ̄ 1 ̄ 1 ̄ ) substrate surface and at the {1 1 0} whisker side facets, which leads to significant tapering of the whiskers. For low temperatures, the growth rate R in the ln R=f(1/T) -plot results in an Arrhenius activation energy of about 67–75 kJ/mol, a value which is in agreement with activation energies reported for low-temperature planar growth of GaAs from TMG and AsH 3. The Au acts as a local catalyst and as a collector of reactants, enabling a liquid-phase-epitaxy-like growth with high growth rates at the GaAs ( 1 ̄ 1 ̄ 1 ̄ ) B/(Au,Ga) interface.

Surface quality of InP etched with tertiarybutylchloride in an MOVPE reactor

In situ etching of InP with tertiarybutylchloride under metalorganic vapor phase epitaxy conditions was investigated with respect to etching profiles, surface morphology, and lateral etching uniformity. Etching of mesa structures resulted in positively sloped sidewalls independent of crystal orientation and etching parameters. Depending on specific etching temperature regimes, excellent surface morphology could be achieved with the addition of PH 3, or TBP, or even without the presence of any group(V) species. However, strong generation of etch pits was encountered with materials containing high dislocation densities. Preliminary measurements suggest the lateral etched rate uniformity to be very sensitive to the thermal conditions in the reactor owing to the kinetic nature of the etching process.

Optical response of reconstructed GaP(001) surfaces

GaP(001) surfaces were found to reconstruct with (2\ifmmode\times\else\texttimes\fi{}1) or (2\ifmmode\times\else\texttimes\fi{}4) symmetry under surface conditions corresponding to a high or low V/III surface stoichiometry ratio, respectively. These surface reconstructions, identified by reflection high-energy electron diffraction, have been prepared in a chemical beam epitaxy (CBE) system by varying the sample temperature. Reflectance anisotropy spectra (RAS) of these surfaces have been taken under both CBE and metal-organic vapor phase epitaxy conditions. Three different phases of (2\ifmmode\times\else\texttimes\fi{}4) symmetry have been distinguished according to their characteristic RAS response in agreement with recent theoretical predictions [A.M. Frisch, W.G. Schmidt, J. Bernholc, M. Pristovsek, N. Esser, and W. Richter, Phys. Rev B 60, 2488 (1999)]. For the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction a line-shape analysis of the RAS signatures was performed and their temperature shifts have been compared to the respective shifts of the bulk critical points. These experiments indicate the GaP(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface dielectric anisotropy originating from transitions between bulk states modified by the surface due to band-folding effects and anisotropic shifts of the ${E}_{1}$ and ${E}_{0}^{\ensuremath{'}}$ critical point energies.

Theoretical reflectance anisotropy spectroscopy and scanning tunnelling microscopy study of the gaas(001) (2 × 4) surface

The method of reflectance anisotropy spectroscopy (RAS) has proved extremely useful in monitoring the structure of semiconductor surfaces during growth under metal-organic vapour phase epitaxy conditions. Similarly, scanning tunnelling microscopy (STM) has proved invaluable in providing information in real space, with atomic resolution, to determine surface atomic structure. It is therefore clear that the combination of these two techniques could provide vital evidence in determining the reconstructions of the GaAs(001) (2 × 4) surface. Both techniques, however, suffer from the lack of supporting theoretical calculations. This is necessary for an analysis of the RAS spectra and for an interpretation of the STM images. Using an ab initio pseudopotential method we have calculated the RAS spectra and generated STM images for three different models of the GaAs(001) surface. We are thus able to provide a link between theoretical RAS spectra, STM images and experiment.

Selective area epitaxy of GaInAs using conventional and novel group III precursors

Selective area growth of GaInAs has been performed by low pressure metalorganic vapor phase epitaxy (MOVPE) on SiO 2-masked InP substrates. The growth rate and composition variations induced by the mask pattern have been evaluated by cathodoluminescence for two In precursors, trimethylindium (TMI) and dimethylaminopropyl-dimethyl-indium (DADI) in combination with triethylgallium (TEG) and AsH 3, and compared to model calculations taking gas phase diffusion into account. The resulting characteristic parameter D/ k could be attributed to the chemical stability of the precursors. We found that the use of DADI results in a less pronounced growth rate and In incorporation enhancement at the mask edges than the use of TMI, but DADI still has to be labeled “chemically less stable” than TEG under MOVPE conditions. Gas phase reactions with the hydrides probably have to be taken into account to understand these findings.

In situ time-resolved monitoring of PH3 induced exchange reactions on GaAs under metalorganic vapor phase epitaxy conditions

Exposure of GaAs and InGaAs to PH3 is a standard step in gas switching sequences for metalorganic vapor phase epitaxy (MOVPE) growth of heterostructures in the technologically important GaAsP, InGaP, and InGaAsP material systems. The exchange of group-V atoms was monitored in situ by reflectance anisotropy spectroscopy when GaAs is exposed to PH3. The c(4×4) reconstructed, As-terminated GaAs surface is then replaced by a P-terminated structure. At standard MOVPE growth temperatures and pressures the time constant for this reaction is of the order of 100 ms. The temperature and pressure dependence of the As by P exchange is reported, and the activation energy was determined to be 1.64 eV. It is concluded that PH3 enhances the desorption of As.