A. S. Jordan

An evaluation of the thermal and elastic constants affecting GaAs crystal growth

The thermal and elastic constants essential to a realistic modeling of GaAs crystal growth have been critically evaluated. High temperature values are recommended for the thermal expansion coefficient, elastic stiffness, thermal stress modulus, critical resolved shear stress, thermal conductivity and diffusivity of GaAs. Furthermore, the radiative and convective heat transfer coefficients, h, have been determined from Newtons Law of Cooling in combination with the Stefan-Boltzmann relation and a hydrodynamic approximation, respectively. The evaluation of these coefficients has been facilitated by the recent calculation of the total emittance of GaAs and the availability of the thermal and transport properties of the ambient fluids. The coefficients hconv for B2O3 (1), He (g), and A (g) and hrad for a series of doping levels are presented in a convenient graphical form. Due to its low viscosity at elevated temperatures, hconv for anhydrous B2O3 (1) is ∼0.2 cm-1 which is more than an order of magnitude larger that that for any one of the gaseous ambients; hrad increases with ambient temperature and doping level, ranging from ∼0.15 to 0.4 cm-1 at 1300 K. This indicates that radiation and free convection via B2O3 (1) are competitive heat dissipating processes. The effect of these parameters is illustrated by means of the quasi-steady state heat transfer/thermal stress model for slip-induced dislocation generation in GaAs. A novel plotting technique maps the thermal stress corresponding to the {111}, slip system in excess of the critical resolved shear stress for a {100} wafer, a quantity defined to be proportional to the dislocation density. It is shown that the central area (∼75%) of a 2 cm diameter boule with 1016 cm-3 n-type impurities grown in N2 at an ambient temperature 20 K below the melting point would contain no dislocations. A lower ambient temperature, B2O3 (1) encapsulation and/or an increase in doping level leads to a progressive elemination of dislocation-free areas with a concomitant rise in the dislocation density.

Carbon doping of III-V compounds grown by mombe

Recent advances in heterostructure bipolar transistor (HBT) technology have created a need for p-type doping at levels ≥1020 cm-3. We have achieved p-type doping levels as high as 5×1020 cm-3 using C, which is introduced through the use of trimethylgallium (TMG) during metalorganic molecular beam epitaxy (MOMBE) growth of GaAs. By utilizing the atomic planar doping method, we have also been able to grow C-doped spikes with hole concentrations as high as 7×1019 cm-3, with a full width at half maximum of ∼50 A at 300 K. This level is among the highest reported for planar doping. By switching out the TMG, and switching in the triethylgallium (TEG) to continue to growth of C-free GaAs, we have grown sandwich-type structures with C levels of 1020 cm-3, which fall off within 210 A to C levels of <1017 cm-3. High temperature annealing of such structures reveals a C diffusion coefficient of <10-16 cm2 s-1 at 950°C, in agreement with other reports. The electrical properties of layers annealed at high temperatures appear to be influenced by the presence of strain arising from the high C concentration. X-ray diffraction patterns of 3 μm layers doped in excess of 1020 cm-3 show a lattice constant which corresponds roughly to that calculated by assuming a Vegard law mixture of GaAs and 0.7% GaC. Preliminary results of C-doping of InGaAs will also be discussed. Finally, the usefulness of carbon doping has been demonstrated in ohmic contact formation, Schottky barrier height enhancement in MESFETs and as the base layer in HBTs.

The theoretical and experimental fundamentals of decreasing dislocations in melt grown GaAs and InP

Abstract The availability of high quality substrate material is a prerequisite to future advances in the emerging high-speed electronic and photonic device/IC technologies. Consequently, significant progress has been made recently in reducing the dislocation densities in the bulk growth of GaAs and InP. We shall review the evidence implicating thermal-stress induced slip as the predominant mode of dislocation generation in these compounds. Then, the quasi-steady state heat transfer/thermal stress theory for defect generation in LEC growth together with its key predictions are outlined. Particular emphasis is placed on the degree of impurity hardening (reflected by an increase in the critical resolved shear stress) and ambient temperature gradient reduction as they relate to suppressing dislocation formation. We conclude by discussing the currently used experimental approaches to grow virtually defect-free single crystals, explain their shortcomings, and examine the outlook for future improvements.

Estimated thermal diffusivity, Prandtl number and Grashof number of molten GaAs, InP, and GaSb

We have determined the thermal diffusivity, Prandtl number and Grashof number of molten GaAs, InP, and GaSb, all necessary parameters in coupled fluid flow and heat transfer modeling of crystal growth. Among the primary quantities there is reliable experimental data for the density as a function of temperature for each liquid which then permits the derivation of the volume expansion coefficient. Since the experimental data for the viscosity of the compounds are a complicated though monotomic function of temperature, we have performed a least-square analysis (log viscosity versus 1/T) including only results relevant in the vicinity of the melting points (Tf). Lacking measurements for the thermal conductivity, KL, of molten GaAs and InP, we introduce a novel scaling technique. An exhaustive survey of the literature on the thermal conductivity of semiconductors and subsequent analysis has led to the discovery that the ratio KL/KS for the liquid and solid phases at Tf is nearly constant for Si, Ge, InSb, and GaSb, ranging between 2.13 and 2.93. Thus we could approximate KL for GaAs and InP from KS at Tf by employing an average multiplying factor of 2.5. Combination of the assembled properties yields for the thermal diffusivity (cm2/s), Prandtl number, and Grashof number of GaAs the values 0.072, 0.068, and 2.9 × 107, respectively. Likewise, the recommended values for InP are 0.103, 0.015, and 6.2 × 108, and for GaSb ar 0.087, 0.044, and 2.4 × 107. Finally, the implications of these estimates with regard to fluid flow and heat transfer simulations of Czochralski growth are discussed.

Some thermal and mechanical properties of InP essential to crystal growth modeling

Abstract The key physical constants required in a realistic modeling of the Czochralski growth of InP have been critically assessed. In particular, we evaluated the (1) thermal expansion coefficient (α) and density from the fractional change in length, (2) thermal conductivity ( K ) and diffusivity (κ), (3) elastic stiffness constants, and (4) heat capacity over a wide temperature range for this compound. Semi-empirical formulae are provided from which one can obtain reliable property values up to the melting point of InP ( T f ). At T f -200 K (the average ambient temperature of the B 2 O 3 encapsulant), we recommend α=6.8×10 -6 K -1 , K =0.11 W/cm·K, and κ=0.0624 cm 2 /s. These parameters in conjunction with the recently determined critical resolved shear stress of undoped and heavily doped InP at elevated temperatures indicate a lesser tendency for thermal stress induced dislocation generation during the LEC pulling of InP when compared with GaAs. As an illustrative example, we computed the dislocation distribution in a 111} InP wafer. It is shown that an undoped 4.5 cm diameter crystal prepared by the LEC technique contains a high density of dislocations (especially at the edge) and exhibits a sixfold “Star of David” pattern. On the other hand, material incorporating 1.3×10 19 cm -3 Ge with the same diameter should be dislocation-free apart of a 0.1 cm rim along its circumference.

Effect of source chemistry and growth parameters on AlGaAs grown by metalorganic molecular beam epitaxy

Abstract We have investigated the effect of V/III ratio and substrate temperature on the growth rate, Al composition, crystallinity, and impurity concentration of AlGaAs grown by metalorganic beam epitaxy (MOMBE). The effect of these growth parameters on the deposition rates of both GaAs and AlAs has also been determined. By comparing films grown from various combinations of triethylgallium (TEGa), trimethylgallium (TMGa), triethylaluminum (TEAl), and trimethylamine alane (TMAA1), we have been able to further identity the surface reactions which are most important in determining film composition and quality.

Redistribution of Zn in GaAs-AlGaAs heterojunction bipolar transistor structures

The redistribution of Zn in the base region of GaAs‐AlGaAs heterojunction bipolar transistor structures during growth by organometallic vapor phase epitaxy has been examined with respect to the presence of Si doping in the emitter‐contact, emitter, and collector/subcollector layers, and as a function of Zn doping concentration and Si counterdoping level in the p+ base. For a growth temperature of 675 °C the Zn shows no significant redistribution up to concentrations of 3×1019 cm−3 without Si doping. The addition of Si to the adjacent AlGaAs emitter and collector/subcollector layers causes substantial diffusion of Zn from the base, while Si doping of the GaAs emitter contact results in even greater Zn redistribution. Under these conditions, the Zn concentration in the base attains a maximum value of ∼7×1018 cm−3. Silicon counterdoping in the base region retards the Zn diffusion, while strain introduced by an InGaAs cap layer has no effect on the Zn redistribution.

Growth and characterization of low defect GaAs by vertical gradient freeze

Abstract As the diameter of GaAs substrates has increased, so has the difficulty and cost of growing GaAs by the liquid encapsulated Czochralski (LEC) method. One alternative crystal growth method with offers promise is growth by vertical gradient freeze (VGF). This technique avoids the problems associated with diameter control during low gradient LEC growth. In this paper we discuss the growth of 2 inch diameter undoped and In-alloyed GaAs by VGF. Through the use of low gradients, undoped material with a dislocation density of (2–6) x 103 cm-2 has been obtained. With the addition of 0.6 at% InAs to the melt, this density is further reduced to 0–1000 cm-2. In contrast to standard LEC, GaAs grown by VGF shows low concentrations of the EL2 deep level and of the C shallow acceptor level. The material is semi-insulating ρ ⩾ 107 ω cm, and is thus suitable for device applications. From temperature dependent Hall measurements, the semi-insulating (SI) behavior is believed to be caused by Fermi level pinning at a defect with an energy level of ∼ 0.5 eV.

Residual oxygen levels in AlGaAs/GaAs quantum‐well laser structures: Effects of Si and Be doping and substrate misorientation

Oxygen forms nonradiative recombination centers in GaAs and AlGaAs, and is a common contaminant in AlGaAs, irrespective of the growth technique. We find that O tends to accumulate near the GaAs active region of an AlGaAs/GaAs quantum‐well laser prepared by molecular beam epitaxy. Moreover, the Be‐doped Al0.6Ga0.4As cladding layer has a higher O content than its Si‐doped counterpart. We present evidence that Si‐doping suppresses, and Be doping favors incorporation of O in AlGaAs. In undoped and Si‐doped AlGaAs, the incorporation of O is further reduced by tilting the (100) GaAs substrates towards 〈111〉A. We propose that Be forms stable Be‐O complexes in AlGaAs, and thus, there is virtually no desorption of incorporated O. But in Si‐doped AlGaAs, O content is reduced due to reaction between group III suboxides and Si, resulting in the formation and desorption of volatile SiO (g). The study suggests that Be doping should be avoided in the p‐side of the GRIN region of a laser structure.

The role of crystal diameter and impurity hardening on the threshold for dislocation formation in LEC GaAs

Abstract We have calculated explicitly the dependence of the dislocation density on crystal radius and ambient temperature gradient in GaAs grown by the LEC technique. In addition, the effects of “impurity hardening” via ln-alloying were investigated by varying systematically the critical resolved shear stress threshold according to estimated bounds given by Ehrenreich and Hirth. We show that for standard LEC growth conditions, it is not possible to grow dislocation-free crystals for diameters beyond ∼ 1 cm. Furthermore, in extremely low gradients impurity hardening is not essential to produce low defect density crystals of diameters in the range 4–8 cm. However, to achieve stable growth of such crystals a compromise between the effects of a moderate thermal gradient and the addition of a small amount of hardening agent is recommended. The predictions are in good agreement with LEC growth experiments and are also applicable to Bridgman methods.