RHEED studies of MOMBE growth using TMGa or TEGa with As2

MOMBE and CBE growth has until recently been based on largely empirical studies of the epitaxial process. We have used reflection high energy electron diffraction (RHEED), previously applied to the study of MBE, to study the growth GaAs using TMGa and As₂. In this work we have extended our previous studies to include a detailed study of the effect of As₂ flux on growth rate and to compare data on singular and vicinal plane surfaces cut off orientation in two orthogonal {110} directions. Clear evidence for site blocking mechanisms is observed together with an indication that the concentration of elemental Ga present on the surface during growth is negligible even under conditions where the arrival rate of TMGa exceeds that As₂. We have compared this behaviour with that observed using TEGa and As₂ under identical conditions. Using TEGa a conversion from a (2×4) to (4×2) reconstructed surface is observed under As deficient conditions indicating the presence of elemental Ga on the surface. This is accompanied by an abrupt change in growth rate similar to that secn in MBE.     

Heavily carbon-doped p-type InGaAs by MOMBE

Carbon-doped In_xGa_1−xAs layers (x=0−0.96) were grown by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG), solid arsenic (As₄) and solid indium (In) as sources of Ga, As and In, respectively. The carrier concentration is strongly affected by growth temperature and indium beam flux. Heavy p-type doping is obtained for smaller In compositions. The hole concentration decreases with the indium composition from 0 to 0.8, and then the conductivity type changes from p to n at x=0.8. Hole concentrations of 1.0×10¹⁹ and 1.2×10¹⁸ cm^-3 are obtained for x=0.3 and 0.54, respectively. These values are significantly higher than those reported on carbon-doped In_xGa_1−xAs by MBE. Preliminary results on carbon-doped GaAs/In_xGa_1−xAs strained layer superlattices are also discussed.     

Surface diffusion lenght during MOMBE and CBE growth measured by μ-RHEED

The growth rates of layers grown on a mesa-etched (001) GaAs surface were measured by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED) from the period of the RHEED intensity oscillation in real time. The diffusion lenght of the surface adatoms of column III elements was determined from the gradient of the variation of the growth rates in the cases of MBE, MOMBE using trimethylgallium (TMGa) and CBE using TMGa or triethylgallium (TEGa) and arsine (AsH₃). The obtained values of the diffusion lengths were of the order of a micrometer in every case of the source-material combination. In the case of metalorganic materials as Ga source, it was found that the diffusion length was larger than that of Ga atom from metal Ga source. Since the substrate temperature of the present experiment is high enough to decompose TMGa and TEGa on the surface, Ga adatoms are considered to be responsible to the surface diffusion. Therefore, it is considered that the derivatives of the metalorganic molecules such as methyl radicals affect the diffusion of Ga adatoms.     

The incorporation behavior of arsenic and antimony in GaAsSb/GaAs grown by solid source molecular beam epitaxy

GaAsSb ternary epitaxial layers were grown on GaAs (0 0 1) substrate in various Sb₄/As₂ flux ratios by solid source molecular beam epitaxy. The alloy compositions of GaAs_1−ySb_y were inferred using high-resolution X-ray symmetric (0 0 4) and asymmetric (2 2 4) glance exit diffraction. The non-equilibrium thermodynamic model is used to explain the different incorporation behavior between the Sb₄ and As₂ under the assumption that one incident Sb₄ molecule produces one active Sb₂ molecule. It is inferred that the activation energy of Sb₄ dissociation is about 0.46 eV. The calculated results for the incorporation efficiency of group V are in good agreement with the experimental data.     

Molecular beam epitaxy of GaSb/AlxGa1 − xSb quantum well structures

Molecular beam epitaxy (MBE) is used to grow GaSb/Al_xGa_{1 − x}Sb quantum well (QW) structures on GaSb(001) substrates using both Sb₂ and Sb₄ molecules. While the optoelectronic properties of thick GaSb epitaxial layers are significantly affected by the type of molecule used, no influence is noted on the QW photoluminescence properties. It is shown that MBE allows a very precise and reproducible control of the QW structure parameters such as QW widths for which monolayer precision is obtained. Through the variation of the QW associated PL energy as a function of the growth temperature, the occurrence of a surface segregation-like phenomenon is evidenced. However, this effect is rather weak so that a good estimation of the valence band offset through the PL energy variation with the QW width can be made. Moreover, the QW width for which the Γ-L crossover occurs is very precisely determined.     

Characteristics of GaSb growth using various gallium and antimony precursors

We report the epitaxial growth of GaSb using trimethylgallium (TMGa) or triethylgallium (TEGa) with trimethylantimony (TMSb), triethylantimony (TESb), or trisdimethylaminoantimony (TDMASb) in a low-pressure vertical rotating-disk reactor. Growth is kinetically limited for TMGa in the temperature range 560 to 640°C, and is mass-transport limited for TEGa in the range 525 to 640°C. A minimum V/III ratio is necessary to obtain stoichiometric GaSb, and is dependent on the pyrolysis temperature of Ga and Sb precursors. Featureless morphology is achieved for layers grown with TMGa or TEGa and TMSb, while surface defects are observed for layers grown with TEGa and TESb or TDMASb. These observations are consistent with Fourier transform infrared measurements, which indicate interactions between TEGa and TESb or TDMASb. All nominally undoped layers are p-type, with overall superior properties being obtained for layers grown with TEGa and TMSb. However, growth conditions that yield layers with the best electrical properties do not necessarily correspond to the same conditions for highest optical quality.     

p-Type GaAs doped by diiodomethane (CI2H2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI₂H₂) as a carbon source. In the low 10¹⁹ cm⁻³ range, almost all carbon atoms are electrically activated and at 9×10¹⁹ cm⁻³, 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.     

Low temperature GaAs grown by gas source molecular beam epitaxy

Low temperature growth of GaAs by gas source molecular beam epitaxy (GSMBE) is investigated. Reflection high energy electron diffraction is used to monitor the low temperature buffer (LTB) growth and anneal conditions. Growth at low temperatures with dimeric arsenic is more sensitive to the V/III flux ratios and substrate temperatures than with As₄ used in solid source MBE. Temperature dependent conductivity and deep level transient spectroscopy measurements are presented to observe trap outdiffusion from the LTB into subsequently grown FET channels. Low temperature photoluminescence spectra show degradation of quantum well properties when LTBs are grown with increasing V/III flux ratios.     

Simultaneous reflection high-energy electron diffraction oscillations and mass spectroscopy investigations during molecular beam epitaxy growth of (001) GaAs - smooth surfaces or stoichiometric films?

Film composition and surface morphology of molecular beam epitaxy (MBE)-grown GaAs(001) surfaces were investigated in situ as a function of flux ratio J_Ga/J_As4. The flux of As₄ molecules desorbing from the sample surface was measured with a quadrupole mass spectrometer (QMS) simultaneously with the observation of reflection high energy electron diffraction (RHEED) intensity oscillations. The incorporation ratio, given by the number of incorporated Ga atoms per As atom, was calculated independent of both the QMS data and the decreasing growth rate for growth conditions with As deficiency, as obtained from the RHEED oscillation frequency. Stoichiometric growth was found up to a flux ratio J_Ga/J_As4 ≈ 0.9. At flux ratios of 1.1 to 1.2, a minimum of the damping of the RHEED oscillation amplitude indicates a very smooth growth front profile, but Ga excess appears to be incorporated mainly in As sites without disturbing the crystal lattice. This assumption was confirmed by photoluminescence (PL) spectroscopy of films grown at a flux ratio J_Ga/J_As4 of 1.2. An additional PL peak was observed, which indicates the incorporation of Ga atoms on As sites.     

Heavily carbon-doped GaAs grown on various oriented GaAs substrates by MOMBE

Heavily carbon-doped GaAs epitaxial layers have been grown simultaneously on (100), (111)A, (111)B, (411)A, (411)B and (711)A semi-insulating (SI) GaAs substrates by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG) and elemental As (As₄). The hole concentration and surface flatness strongly depend on the substrate orientation. The highest carbon incorporation was observed for the layers grown on a (411)A substrate with a hole concentration of 1.0 × 10²¹ cm^{− 3} and a lattice mismatch of Δd/d = −0.48%. Atomic force microscope (AFM) images reveal that the epilayers grown on (411)A substrates exhibit extremely flat surfaces, although these layers contain the highest carbon concentration.     

Theoretical consideration of the growth kinetics for GaAs and GaSb

An extended MOMBE growth kinetics model is proposed, based on the Robertson model, to explain both the GaAs growth rate variation and modulated beam mass spectroscopy data reported by Martin and Whitehouse. In this model, we assume that (1) MEGa molecules react with ethyl-radicals to form DEGa, (2) excessive group-V molecules on the surface suppress the decomposition of DEGa and enhance the desorption of DEGa, (3) reaction of DEGa with ethyl-radicals to form TEGa is negligible, and (4) effective surface coverage of excessive group-V atoms during growth is determined by the double layer adsorption model including desorption parameters for group-V molecules. The first assumption (1) is found to be a dominant process to explain the behaviour of DEGa desorption at high temperatures. This model can reproduce the dependences of both growth rate and desorbing rate of Ga alkyls on substrate temperature during GaAs MOMBE growth. The use of Sb instead of As produces a significant change in the growth rate variation with substrate temperature and group-V flux for the growth of GaSb, in spite of the use of the same TEGa flow rate. This can be rationalized by the difference in the desorption parameters for Sb and As.     

Growth of 1.55 μm DH laserstructures using TBAs and TBP in MOMBE

In a comparative study we have chosen TBAs and TBP as well as AsH₃ and PH₃ for the growth of InP/GaInAs(P) heterostructures for laser applications in a production metalorganic molecular beam epitaxy (MOMBE) system. The n-type doping was performed with Si from an effusion cell, whereas for the p-type doping Be and DEZn were utilized. InP layers using TBP under optimized cracking conditions exhibit excellent surface morphology with good electrical properties in the low 10¹⁵ cm⁻³ range of carrier concentrations. The MOMBE growth mechanism is not disturbed by the hydride replacement compound. This allows for a convenient replacement without losing calibration data from the hydride process. Broad-area DH laserstructures with GaInAsP (λ = 1.55 μm) active regions were grown with AsH₃/PH₃ and TBAs/TBP. Comparable threshold current densities in the range of 1.6-2.3 kA/cm² are achieved for the lasers, grown with both sets of precursors combined with DEZn source doping. These results are in good agreement with the standard set by the hydride MOVPE process.     

Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Heavily carbon-doped p-type In_xGa_1−xAs (0≤x<0.49) was successfully grown by gas-source molecular beam epitaxy using diiodomethane (CH₂I₂), triethylindium (TEIn), triethylgallium (TEGa) and AsH₃. Hole concentrations as high as 2.1×10²⁰ cm⁻³ were achieved in GaAs at an electrical activation efficiency of 100%. For In_xGa_1−xAs, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x, increased from 0.41 to 0.49. Hole concentrations of 5.1×10¹⁸ and 1.5×10¹⁹ cm⁻³ for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As₄ pressure), the hole concentration increased to 6.2×10¹⁸ cm⁻³ for x = 0.49, probably due to the activation of hydrogen-passivated carbon accepters.     

Hillock formation observed in MOMBE of InGaAs grown on a patterned GaAs substrate

We observed hillock formation during metalorganic molecular beam epitaxy (MOMBE) of InGaAs on a mesa-grooved (100) GaAs substrate. Hillocks were formed under specific growth conditions and comprised mostly InAs. The distribution of hillocks formed in InGaAs MOMBE using trimethylindium (TMIn) and metal Ga depended strongly on the widths of mesa-grooves; the density decreased with decreasing width and hillocks were hardly observed on the ridges. The hillock density also varied, depending on the off-angle of the substrate from the (100) plane. This indicates that the observed anomalous distribution of InGaAs hillocks was caused by both the formation of facets and a vicinal tilted surface near the edge of mesa-grooves, due to the growth of a GaAs buffer layer on a patterned substrate.     

A novel technique to reduce the concentration of carbon in LEC gallium arsenide

An in-situ method has been developed to reduce the concentration of carbon in the liquid encapsulated growth of gallium arsenide. Sublimated As₂O₃ was bubbled through a GaAs melt to remove carbon via the production of CO₂. When a melt of known high starting carbon concentration ( 30 × 10¹⁴ atoms/cm³) was bubbled with As₂O₃, the subsequent crystal contained 4 × 10 ¹⁴ carbon atoms/cm³. This low carbon crystal was subsequently carbon doped and regrown, demonstrating the ability to return As₂O₃ treated GaAs to desired carbon concentrations and electrical properties.     

Structural and electrical properties of GaSb, AlGaSb and their heterostructures grown on GaAs by metalorganic chemical vapor deposition

A systematic study of structural and electrical properties of GaSb and AlGaSb grown on GaAs by metalorganic chemical vapor deposition is reported. In general, the results obtained from surface morphologies, X-ray linewidths and Hall properties are consistent with each other and indicate that the optimal growth conditions for GaSb are at 525°C around V/III = 1. A highest hole mobility of 652 cm²/V · s at RT (3208 cm²/V · s at 77 K) and a lowest concentration of 2.8 × 10¹⁶ cm⁻³ (1.2 × 10¹⁵ cm⁻³ at 77 K) were obtained for GaSb grown under this optimal condition. Compared to the GaSb growth, a smaller V/III ratio is needed for the AlGaSb growth to protect the surface morphology. When Al was incorporated into GaSb growth, mobility decreased and carrier concentration increased sharply. The AlGaSb grown at 600°C had a background concentration about one order of magnitude lower than the AlGaSb grown at 680°C. Room-temperature current-voltage characteristics of GaSb/Al_xGa_{1 − x}Sb/GaSb show a rectifying feature when Al composition x is higher than 0.3, suggesting a valence-band discontinuity at the AlGaSb/GaSb interface. A leakage current much higher than the value predicted by the thermionic emission theory is observed at 77 K, presumably due to a large number of dislocations generated by the huge lattice mismatch between GaSb and GaAs.     

Optimal crystal growth conditions of thin films of Bi2Te3 semiconductors

Crystal growth conditions of Bi₂Te₃ narrow bandgap semiconductors have been studied using molecular beam epitaxy method. It was applied to the growth of Bi₂Te₃ on Bridgman single-crystal substrate Sb₂Te₃. Substrate ingots were taken from the natural cleavage along the (0001) plane. The deposited conditions have been studied as a function of substrate temperature (T_s) and flux ratio (F_R=F(Te)/F(Bi)). The quality of deposited layers was controlled by X-ray diffraction, scanning electron microscope (SEM), secondary ion mass spectroscopy (SIMS) depth profiling and energy-dispersive X-ray (EDX) microanalyser. The sticking coefficients K_s(Te) and K_s(Bi) of the elements that compose Bi₂Te₃ were determined. It was found that the stoichiometry of deposited layers depended on substrate temperature and flux ratio. It was observed that all deposited layers were single-crystal in the orientation of their substrates with a small shift due to the stress in layer.     

Confinement of δ Be at the one monolayer level in GaAs

We investigate growth dependences to the planar confinement of Be near the 1 monolayer, ML, level when δ doped in GaAs. We examine concentration dependences on the rates of segregation and diffusion, as well as the effect of As₂, As₂ and AsH₃ overpressure on Be confinement during growth. Evidence from both chemical profiling and surface electron diffraction point to a Be surface dimerization process that drives dopant segregation.     

The influence of the growth rate and V/III ratio on the crystal quality of InGaAs/GaAs QW structures grown by MBE and MOCVD methods

The influence of the growth rate and V/III ratio on the crystal quality of In_0.2GaAs/GaAs quantum well structures was examined. The investigated heterostructures were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD). Reflection high energy electron diffraction (RHEED), photoluminescence measurements (PL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) were applied for evaluation of the interfaces smoothness and the overall layer quality. Comprehensive characterisation of InGaAs/GaAs structures allowed us to establish optimal values of analysed technological parameters. Moreover, the comparison between the results obtained for samples grown by two different epitaxial techniques allowed us to find, which of the analysed growth parameters has the strongest influence on the quality of MBE and MOCVD grown structures. In contrast with the growth temperature and the interruption time, which in different manner impact on the crystal quality of QWs obtained by different method, the growth rate and the V/III ratio play similar role in both epitaxial techniques.     

GaSb薄膜生长的RHEED研究

采用分子束外延技术,在GaAs衬底上生长GaSb薄膜时,利用反射式高能电子衍射仪(RHEED)对衬底表面清洁状况、外延层厚度等进行在线监控.通过RHEED讨论低温缓冲层对GaSb薄膜表面结构和生长机制的作用,可以估算衬底温度,并能计算出薄膜的生长速率.实验测量GaSb的生长周期为1.96s,每秒沉积0.51单分子层.低温缓冲层提高了在GaAs衬底上外延GaSb薄膜的生长质量.