CBE growth of high-quality AlGaAs/GaAs heterostructures for HEMT applications

AlGaAs/GaAs heterostructures were grown by chemical beam epitaxy using triethylgallium, triisobutylaluminium and pure arsine in flow control mode with hydrogen as carrier gas. For substrate temperatures of 580°C and V/III ratios of 10, high quality AlGaAs layers are obtained; heterostructures show abrupt and smooth interfaces. Modulation doping with silicon evaporated from a conventional effusion cell gives two-dimensional electron gases with carrier densities up to 1×10¹² cm^-2. Mobilities of 70000 cm²/V·s are obtained at 77 K for carrier densities of 4×10¹¹ cm^-2. The lateral homogeneity of the heterostructures in layer thickness, composition and doping level is excellent. Perfect morphology with defect densities of about 100 cm^-2 is observed. High electron mobility transistors (gate length 0.3 nm) fabricated from quantum well structures show a transconductance of about 380 mS/mm.     

Monitoring real-time CBE growth of GaAs and AlGaAs using dynamic optical reflectivity

Dynamic optical reflectivity (DOR) uses the interference oscillations arising from the multiple reflections, of a normally incident CW laser beam, between the surface of a growing film and the film-substrate interface. The oscillations have a period determined by the refractive index of the film and the laser wavelength. DOR measurements have been made, in real time, during the CBE growth of Al_xGa_1−xAs layers on a GaAs(100) substrate. The results show that the growth rate and the aluminum composition x can be monitored.     

CBE growth of GaAs/GaAs, GaAs/Si and AlGaAs/GaAs using TEG, AsH3 and amine-alane precursors

The growth of high quality AlGaAs by CBE has been limited by the high levels of carbon and oxygen contamination. The use of alane based precursors offers a significant reduction in such contamination. We report for the first time the CBE growth of Al_xGa_1−xAs from triethylgallium, dimethylethylamine-alane and arsine, and compare with. growth from triethylgallium, trimethylamine-alane and arsine. Some preliminary results of work on the CBE growth of GaAs on silicon will also be reported.     

Growth of GaAs/AlGaAs HBTs by MOMBE (CBE)

In this paper, we will discuss how the unique growth chemistry of MOMBE can be used to produce high speed GaAs/AlGaAs heterojunction bipolar transistors (HBTs). The ability to grow heavily doped, well-confined layers with carbon doping from trimethylgallium (TMG) is a significant advantage for this device. However, in addition to high p-type doping, high n-type doping is also required. While elemental Sn can be used to achieve doping levels up to 1.5×10¹⁹ cm^-3, severe segregation limits its use to surface contact layers. With tetraethyltin (TESn), however, segregation does not occur and Sn doping can be used throughout the device. Using these sources along with triethylgallium (TEG), trimethylamine alane (TMAA), and AsH₃, we have fabricated Npn devices with 2 μm×10 μm emitter stripes which show gains of ≥ 20 with either ƒ_t = 55 GHz and ƒ_max = 70 GHz or ƒ_t = 70 GHz and ƒ_max = 50 GHz, depending upon the structure. These are among the best RF values reported for carbon doped HBTs grown by any method, and are the first reported for an all-gas source MOMBE process. In addition, we have fabricated a 70 transistor decision circuit whose performance at 10 Gb/s equals or exceeds that of similar circuits made from other device technologies and growth methods. These are the first integrated circuits reported from MOMBE grown material.     

CBE growth of GaAs/GaAlAs HBTs using the new DEAlH-NMe3 precursor and all-gaseous dopants

This paper describes the first reported use of diethylaluminium hydride-trimethylamine adduct (DEAlH-NMe₃) for the growth of GaAs/GaAlAs power heterojunction bipolar transistors (HBTs) by chemical beam epitaxy (CBE). This precursor possesses a significantly higher vapour pressure than the more conventionally used triethylaluminium (TEA), and leads to much less stringent requirements for bubbler and gas-line heating, and also much-improved GaAs/GaAlAs heterojunction definition when no carrier gas is employed. The use of all-gaseous n- and p-type dopants offers significant technological advantages in CBE, and the current paper also provides the first report of the use of hydrogen sulphide for n-type doping of CBE-grown GaAlAs HBT emitter regions. In conclusion, DC and RF data obtained from the heterojunction bipolar transistors fabricated to date are described. A DC gain of 40 has already been measured and encouraging early data obtained from RF-probed devices are also presented.     

Reaction kinetics for the CBE growth of GaAs from triethylgallium; computer modelling studies incorporating recent surface spectroscopic data

A new model for the decomposition of triethylgallium on GaAs(100), with kinetic parameters derived from the results of surface science experiments, is presented. Deficiencies of early models are corrected by including surface coverage and site blocking effects, and lateral interactions between absorbed DEG species are included. The model successfully predicts variations in the rate of CBE growth of GaAs with substrate temperature, and addresses effects induced by variations in arsenic overpressure. This dependence of growth rate on the arsenic flux is modelled by computing the steady state concentrations of absorbed arsenic as a function of temperature and As₂ and TEG flux. Excess arsenic is shown to inhibit GaAs growth by blocking sites for TEG absorption.     

Anisotropy in InGaAs/GaAs heterostructures grown by low-pressure MOVPE and CBE

InGaAs/GaAs heterostructures grown on (001) substrates by low-pressure MOVPE exhibit a measurable anisotropy in their structural, optical and electrical properties. This anisotropy occurs in structures which have undergone partial or complete strain relaxation and it can be strongly reduced by using slightly misoriented substrates. A comparison with similar structures grown by CBE indicates that this anisotropy is less important. This study suggests that strain relaxation is achieved by a combination of several mechanisms whose relative importance depends on the orientation of the substrate and on growth temperature which varies with the growth technique.     

CBE growth of InGaAsP/InP multiple quantum wells for optical modulator applications

The incorporation of group III and group V in the chemical beam epitaxy of InGaAsP/InP multiple quantum well structures has been studied in the temperature range of 470 to 550°C. Both Ga/In and P/As composition ratios are found to be strongly dependent on the growth temperature. The enhancement of phosphorus incorporation at high temperature is identified for the first time, which has a profound impact on the incorporation of group III, in particular the adsorption/pyrolysis of triethygallium. With accurate growth temperature control, high quality InGaAsP/InP superlattices with a large number of periods can be grown under continuous growth mode. Clear quantum confined Stark effect near 1.5 μm has been observed in a p-i-n test modulator structure.     

Low temperature growth of AlGaAs by MOMBE (CBE) using trimethylamine alane

In order to gain further insight into the surface chemistry of AlGaAs growth by metalorganic molecular beam epitaxy, we have investigated the deposition behavior and material quality of AlGaAs grown at temperatures from 350 to 500°C using trimethylamine alane (TMAA), triethylgallium (TEG) and arsine (AsH₃). Though the Al incorporation rate decreases with decreasing temperature, Ga-alkyl pyrolysis, and hence Ga incorporation rate, declines more rapidly. Thus the Al content increases from X_AlAs = 0.25 at 500°C to X_AlAs = 0.57 at 350°C. Below 450°C, the Ga incorporation rate appears to be determined by the desorption of diethylgallium species, rather than interaction with adsorbed AlH₃. Similarly, carbon incorporation is enhanced by 2 orders of magnitude over this temperature range due to the increasingly inefficient pyrolysis of the Ga-C bond in TEG. Additionally, active hydrogen from the TMAA1, which normally is thought to getter the surface alkyls, is possibly less kinetically active at lower growth temperatures. Contrary to what has been observed in other growth methods, low growth temperatures produced a slight decrease in oxygen concentration. This effect is likely due to reduced interaction between Ga alkoxides (inherent in the TEG) and the atomic hydrogen blocked Al species on the growth surface. This reduction in oxygen content and increase in carbon concentration causes the room temperature PL intensity to actually increase as the temperature is reduced from 500 to 450°C. Surprisingly, the crystalline perfection as measured by ion channeling analysis is quite good, χ_min≤5%, even at growth temperatures as low as 400°C. At 350°C, the AlGaAs layers exhibit severe disorder. This disorder is indicative of insufficient Group III surface mobility, resulting in lattice site defects. The disorder also supports our conclusions of kinetically limited surface mobility of all active surface components.     

Fluorinated silica glass ablated with ArF excimer laser at low fluence

Amorphous SiO₂ (a-SiO₂) was formed by liquid-phase deposition (LPD) at room temperature. As a result of one shot of ArF excimer laser irradiation, LPD-formed a-SiO₂ shows a threshold fluence for ablation of below than 200 mJ/cm², which is much lower than the threshold fluence (∼1 J/cm²) of a-SiO₂ formed by thermal oxidation of silicon. Raman scattering spectroscopy revealed that two sharp lines at 495 cm⁻¹ and 606 cm⁻¹, respectively, labeled D₁ and D₂, had disappeared, and the main band at 430 cm⁻¹ was sharpened in LPD-formed a-SiO₂. It is presumed that the fluorine broke the silica network, relaxing the Si-O-Si bond angle and dramatically reducing the threshold energy for ablation of a-SiO₂.     

Lateral OMVPE growth of GaAs on patterned substrates

GaAs was grown on patterned 1 0 0 on- and off-axis GaAs substrates by organometallic vapor-phase epitaxy (OMVPE). Patterned mesas were observed to change shape because lateral growth rates varied by more than an order of magnitude in different crystallographic directions. For this study, misoriented GaAs (1 0 0) wafers were polished 3° toward the nearest [1 1 0] or [1 1 1] family of directions, and 320 nm high cross-shaped mesas were fabricated. OMVPE growth was performed between 550°C and 650°C for 1 h at a vertical growth rate of approximately 1.3 μm/h. Atomic force microscopy showed that three effects have a powerful influence on lateral growth initiated at mesa sidewalls. First, the symmetry of the dominant surface reconstruction has a major effect on the diffusion of Ga adatoms. Rapid Ga diffusion occurs along the 0 1 1–0−1−1 axis in OMVPE, or the perpendicular 0−1 1–0 1−1 axis in molecular beam epitaxy, and appears to be a result of the different surface reconstructions which exist in the two growth ambients. Second, misorientation of the wafer causes a growth asymmetry as Ga adatoms move preferentially from high-to-low terraces. When terrace steps descend toward a mesa wall, rapid lateral growth away from the wall is always observed. When terrace steps descend away from a mesa wall, little lateral growth occurs and even reduced vertical growth may be observed. When the misorientation and reconstruction symmetries align, the surface acts like an atomic diode and the rapid lateral growth can exceed the vertical growth rate by more than an order of magnitude. Third, on misoriented substrates, step bunching increases with increasing temperature, and this can lead to significant changes in the original shape of a mesa. A growth model is presented which relates the lateral growth rate in different crystallographic directions to the substrate misorientation, the growth temperature, and the partial pressure of As during growth. It is also shown that different surface reconstruction patterns are related to chemical species with continuously varying concentrations rather than thermodynamically distinct phases.     

The kinetics of parasitic growth in GaAs MOVPE

Gallium arsenide (GaAs) deposition was carried out in a horizontal quartz reactor tube with trimethylgallium (TMGa) and arsine (AsH₃) as precursors, using a hydrogen (H₂) carrier gas. Temperatures were in the range 400–500 °C, where surface reactions limit deposition rate. Nucleation time and deposition rate were monitored using laser interferometry, optimum reflectance was gained by aligning a quartz wafer to back reflect the incident beam. The 980 nm infrared laser beam was sufficiently long in wavelength to be able to penetrate the wall deposit. Results showing the effect of temperature and V/III ratio on the nucleation time and deposition rate are presented, where with temperature the nucleation delay was observed to reduce and the growth rate to increase. The nucleation delay is consistent with a thermally activated surface nucleation for the parasitic GaAs. A theoretical growth rate model, based on a restricted set of reaction steps was used to compare with the experimental growth rates. Without any free parameters, the growth rates from theoretical calculation and experiment agreed within a factor of two and showed the same trends with V/III ratio and temperature. The non-linearity of the theoretical growth rates on an Arrhenius plot indicates that there is more than one dominant reaction step over the temperature range investigated. The range of experimental activation energies, calculated from Arrhenius plots, was 17.56–23.59 kJ mol⁻¹. A comparison of these activation energies and minimum deposition temperature with the literature indicates that the wall temperature measurement on an Aixtron reactor is over 100 °C higher than previously reported.     

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.     

CBE growth of low threshold 1.5 μm InGaAs/InGaAsP MQW lasers

This paper reports on low-threshold InGaAs/InGaAsP multiple quantum well (MQW) lasers emitting at a wavelength of 1.52 μm. Separate confinement heterostructure (SCH) lasers were grown using chemical beam epitaxy (CBE) with source material pressure-control systems. A continuous wave threshold current of 12 mA and internal quantum efficiency of 73% (both facets) are observed in uncoated double-channel planar buried heterostructure (DCPBH) lasers. The internal loss is 15 cm^-1. More than 90% of 50 laser chips have a threshold current of 15±3 mA.     

Effect of an applied electric current in epitaxial growth of GaAs layer on patterned GaAs substrate

Selective epitaxial growth of a GaAs layer on SiN_x masked Si-doped semi-insulating (1 0 0) GaAs substrate was performed by current-controlled liquid-phase epitaxy (CCLPE) in the conventional liquid-phase epitaxy. Experiments were carried out with and without the application of electric current. Surface morphology of (1 0 0) facet of the grown layer and the vertical and lateral growth rates were significantly improved under applied electric current. A thick layer of about 330 μm was achieved at relatively low growth time of 6 h with a current density of 20 Acm⁻². The epitaxial growth is realized by both electromigration of the solute and supercooling under a constant rate of furnace cooling. The dislocation density of the grown layer was significantly reduced, compared with that of the substrate (4×10⁴ cm²).     

LPE growth of GaAs by yo-yo solute feeding method

The effect of gravity on both dissolution and growth of GaAs in the Ga---As system has been investigated using a horizontal “sandwich” system consisting of a substrate-solution-substrate configuration. Remarkable differences were observed in growth and dissolution between upper and lower substrates. These phenomena were attributed to the solutal convection driven by a concentration gradient. Based on these facts, a layer with a thickness of about 80 μm was successively grown by the yo-yo solute feeding method with 8 yo-yo repetition times between 700 and 650°C.     

Thermodynamics of GaAs nanowire MBE growth with gold droplets

The thermodynamics of growth conditions of GaAs nanowires using gold droplets is analyzed. Equilibrium conditions for steady-state growth using experimental molecular beam epitaxy (MBE) impinging molecular flows, as previously published, are calculated in the range 793–893 K. These show that: (i) the tie line for Ga liquidus composition in equilibrium with GaAs(s) is in the 0.4–0.6 mole fraction range, close to the GaAu–GaAs pseudo-binary section, (ii) the As content of the droplet is in the 0.2–0.4×10⁻³ mole fraction range and (iii) the growth rate is mainly governed by the contact angle that determines the droplet section. Different cooling conditions are analyzed using the Scheil–Guliver assumptions to compare final phases after solidification, as analyzed by X-ray diffraction (XRD), with our calculations. The agreement is very good and this feature demonstrates that quasi-equilibrium conditions prevail in the growth process of nanowires.     

Initial growth behavior of GaAs on ZnSe in MOVPE

We have used atomic force microscopy to investigate the initial stages of the growth of GaAs on ZnSe by metalorganic vapor phase epitaxy. Underlying ZnSe with an atomically flat surface is achieved by growth at 450°C and post-growth annealing at the same temperature. The growth modes of GaAs on the ZnSe surface strongly depend on growth temperatures. The growth carried out at 450°C is 2-dimensional, while that at 550°C is highly 3-dimensional (3D), where the 3D islands are elongated in the [110] direction. The growth behavior, unlike homoepitaxy, is well interpreted in terms of low sticking coefficient and anisotropic lateral growth rate in the heterovalent heteroepitaxy.     

GaAs LPE growth and its application to FET

Residual impurities and deep levels in the LPE GaAs layers grown by a sliding boat method were studied. Residual impurities were investigated by monitoring oxygen and water vapor contents in the exhaust gas during heat treatment. The results are satisfactorily explained by assuming oxygen as a dominant residual impurity. Electron traps with a density higher than 5 × 10¹² cm^-3 were not observed in the LPE layers, whereas in VPE layers, 0.82 eV electron traps were always observed. LPE double layers (high purity buffer layer and active layer) were fabricated into FET's. GaAs FET's with a 1 μm gate showed no hysteresis loops in the I–V characteristics and had fairly good high-frequency characteristic (f_max = 70 GHz, NF = 2.4 dB at 10.4 GHz).     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.