Ab initiostudy of the strain dependent thermodynamics of Bi doping in GaAs

Abstract

The thermodynamics of Bi incorporation into bulk and epitaxial GaAs was studied using density functional theory (DFT) and anharmonic elasticity calculations. The equilibrium concentration of Bi was determined as a function of epitaxial strain state, temperature, and growth conditions. For a bulk, unstrained system, Bi in GaAs under typical growth conditions (Ga-rich and Bi-metal-rich at 400° C) has a dilute heat of solution of 572 meV/Bi and a solubility of , representing approximately a hundred times increase in solubility over the unstrained bulk case. Despite these potentially large increases in solubility, the equilibrium solubility is still very low compared to values that have been achieved experimentally through non-equilibrium growth. These values of solubility are also sensitive to choice of the Bi reference state. If the primary route for phase separation is the formation of GaBi within the same structure, rather than Bi metal, GaBi would serve as the source/sink for Bi. If GaBi is used as the Bi reference state, the epitaxial formation energy on a bulk unstrained GaAs substrate is reduced dramatically to 144 meV/Bi, yielding a Bi solubility of 0.083 x = in GaAs 1-x Bi x . These calculations suggest that Bi solubility could be greatly enhanced if Bi metal formation is inhibited and the system is forced to remain constrained to the GaAs 1-x Bi x structure. Although GaBi is not a naturally stable compound, it could potentially be stabilized through a combination of kinetic limitations and alloying.

Raman scattering studies of strain effects in (100) and (311)B GaAs1−xBix epitaxial layers

“…[1][2][3][4][5][6][7] This is because growers are currently confronted with some physical and technical issues that have thus far restricted the amount of Bi able to be introduced into the GaAs matrix. 8 The largest Bi incorporation experimentally reported has been x ¼ 0.22 for GaAs 1Àx Bi x films grown on bulk (100) GaAs substrate. 9 Recently, high-index (311)B-oriented substrates were shown to allow for a sizably larger Bi incorporation at stoichiometric conditions to that of (100) GaAs.…”

Understanding and reducing deleterious defects in the metastable alloy GaAsBi

“…7 In the usual growth temperature range of GaAs, 500-600°C, the Bi incorporation is nearly zero, which has been confirmed by both experiments 8 and theoretical claculations. 9 To enhance the Bi incorporation, growth temperatures as low as 300-400°C are widely used to decelerate the growth dynamics that lead to low Bi incorporation. However, the low growth temperature also induces numerous defects, which result in short carrier lifetimes and low photoluminescence intensity.…”

First-principles studies on molecular beam epitaxy growth ofGaAs1−xBix

“…This result is somewhat surprising, since intuition and previous thermodynamic study [13] suggest that the large Bi atoms would be stabilized in GaAs by tension. However, even though the thermodynamic equilibrium solubility is strongly enhanced by biaxial tension [13], the result here indicates that the kinetic barriers do not behave in such a simple manner. The remaining two strain conditions, compression along both directions and tension along [01 1] combined with compression along…”

“…Previous ab initio calculations predicted that the equilibrium solubility of Bi atom in GaAs is less than 0.01% under Ga-and Bi-rich conditions at 400 ºC [13], primarily due to a 24.4% larger covalent radius of Bi than As [14]. A much greater amount of Bi have been incorporated in growth far from equilibrium, such as metalorganic vaporphase epitaxy [3, 5-8, 10, 15-18] and molecular beam epitaxy (MBE) [4,9,[19][20][21][22][23][24][25][26][27].…”