Band-to-acceptor transitions in the low-temperature-luminescence spectrum of Li-doped p-type ZnSe grown by molecular-beam epitaxy

Photoluminescence (PL) and magnetospectroscopy (in magnetic fields up to 12 T) have been used to characterize the properties of Li-doped p-type heteroepitaxial ZnSe on GaAs grown by molecular-beam epitaxy (MBE). A conduction-band-to-acceptor (e-A0) peak at 2.706 eV has been observed and identified in low-temperature (1.7 K), low-excitation-level PL measurements, in addition to the more commonly reported donor-to-acceptor (D0-A0) pair recombination peak at about 2.692 eV. The e0-A0 peak appears to occur at low temperature only in p-type-doped material, which may explain why it has not previously been detected in ZnSe below about 25 K. PL measurements as a function of temperature, excitation intensity, and magnetic field have been performed to confirm and study the nature of the peak. The e-A0 peak shifts and broadens linearly with increasing temperature as expected, but does not show strong excitation intensity-dependent shifts or quench at high temperatures as the D0-A0 peak does. The e-A0 peak becomes narrower and shifts linearly to higher energy in applied magnetic fields, reflecting the expected behavior of the lowest-energy Landau level in the conduction band. A light-hole binding energy of 114.1±0.4 meV in strained material is obtained from the intercept of a linear fit to the temperature-dependent e-A0 peak positions, which corresponds to 114.4±0.4 meV in unstrained material. The inapplicability of a linear Haynes's-rule-type of relationship to shallow acceptors in ZnSe is emphasized. An anomalous initial quenching of the e-A0 peak intensity is observed as the temperature is raised, followed by the more normal increase due to the thermal ionization of the donors. This observation is modeled in terms of the temperature dependence of the competing (nonradiative, etc.) recombination rates. From the position of the e-A0 peak as a function of magnetic field, we obtain an electron effective mass me*=0.17, neglecting spin splittings, which were not well resolved. In previous measurements of these samples, the e-A0 peak was tentatively identified as the R-band, which has been associated with transitions between preferentially paired interstitial Li donors and substitutional Li acceptors on Zn sites. Based on the present results, we show that, in fact, no direct evidence exists in the PL spectrum for the presence of interstitial Li donors in this MBE material, which has important implications when attempting to explain the present limitations on p-type-doping levels achievable with Li. The possibility of observing e-A0 peaks at low temperature should be considered in future analyses of PL spectra of p-type-doped ZnSe, to avoid similar errors in interpretation. The occurrence of the e-A0 peak as a function of growth temperature and Li doping concentration is discussed. We describe and model the splitting of the Li acceptor-bound exciton into a doublet in the strained material. Finally, we discuss the observation of an excited-state-donor-to-acceptor peak involving Li acceptors, and the observation of discrete donor-acceptor pair lines involving Li.