Formation of Three Red-Shift Emissions in Heavily Germanium-Doped P-Type GaAs Grown By MBE

<jats:title>Abstract</jats:title><jats:p>Molecular beam epitaxy (MBE) of Ge-doped GaAs was made, in which As4 to Ga flux ratio :γ and Ge concentration :[Ge] were used as growth parameters. Photoluminescence (PL) spectra at 2K for slightly Ge-doped GaAs revealed that for γ =1 the emission of excitons bound to neutral Ge acceptors (A°,X) was the dominant one. With increasing γ ,(A°,X) was found to be steeply suppressed and at around γ=1.1, (A°,X) was totally quenched. For γ higher than 1.4, the emission of excitons bound at neutral Ge donors (D°,X) was gradually enhanced and for γ =11, (D°,X) became the principal one. Through van der Pauw measurements, samples with [Ge] around 1×10<jats:sup>17</jats:sup>cm<jats:sup>-3</jats:sup> presented type conversion at around γ=1.7. In this series, the sample with γ =1.0 indicated a strong specific emission, [ g-g], which is formed just below (A°,X) and exhibited a strong energy shift towards lower energy sides (red shift) with increasing [Ge]. [g-g] was theoretically attributed to the pairs between excited-state acceptors. Since [g-g] is known to be easily quenched by small amount of donors, the formation of predominant [g-g] for γ =1 assures that very low-compensated p-type GaAs were grown by using this typically am-photeric impurity. We fabricated a series of p-type Ge-doped GaAs by keeping γ =1 in which the net hole concentration, │ N<jats:sub>A</jats:sub>-N<jats:sub>D</jats:sub> │ as high as 1×10<jats:sup>20</jats:sup>cm<jats:sup>-3</jats:sup> was attained. We found four emissions which exhibited significant energy shifts with increasing │ N<jats:sub>A</jats:sub>-N<jats:sub>D</jats:sub> │ . From │ N<jats:sub>A</jats:sub>-N<jats:sub>D</jats:sub> │ ~1×10<jats:sup>16</jats:sup> cm<jats:sup>-3</jats:sup>, [g-g] begins to appear as a dominant emission and at │ N<jats:sub>A</jats:sub>-N<jats:sub>D</jats:sub> │ ~ 1×10<jats:sup>17</jats:sup> cm<jats:sup>-3</jats:sup>, another red shift emission, [g-g]2 begins to be formed parralelly on the higher energy side of [g-g]. It is interesting to note that both [g-g] and [g-g]2 seem to be totally quenched by the further increase of [Ge]. The emission due to band to Ge acceptor,(e,Ge) does not change its central energy until [Ge]= 5×-10<jats:sup>18</jats:sup>cm<jats:sup>-3</jats:sup> and for larger [Ge] it turned into a new broad emission,[g-g]β showing a steep red energy shift. [g-g]α was formed on the higher energy side of (e,Ge) and indicated a systematic blue energy shift with growing [Ge] larger than 1×10<jats:sup>19</jats:sup>cm<jats:sup>-3</jats:sup>. [g-g]α was theoretically explained to be the emission due to the pairs between ground-state acceptors.</jats:p>