Optical and vibrational properties of doped zinc selenide epitaxial layers

Abstract

In this review article, we present photoluminescence and/or Raman scattering properties of ZnSe epitaxial layers which were doped with either nitrogen, oxygen, indium, or lithium. In particular, we present photoluminescence data due to recombination at discrete donor-acceptor pairs in ZnSe epitaxial layers implanted with nitrogen with a dose of 10 13 cm -2. The experimental pair data is explained with the help of a theoretical computation of a type-II spectrum involving In donors substituting for Zn and N acceptors for Se. We also report two-hole transitions involving up to 6S 3 2 states of the nitrogen acceptor. The experimental values of the energy positions of these excited states are in good agreement with those obtained using an effective-mass calculation. A sharp line superimposed on the broad donor-acceptor pair band, whose peak position has a constant separation from the excitation energy, is also observed. This separation is ≈0.9 meV larger than the 1S-2S energy spacing for the nitrogen acceptor. We show that this line could be either due to resonant inelastic scattering of the exciting photons by the acceptor impurities, or due to selective excitation of the discrete donor-acceptor pairs. We also present photoluminescence and excitation data to show that oxygen substituting isoelectronically in ZnSe gives rise to a pair of transitions A + (2.7895 eV) and B (2.7877 eV) as a result of the exchange interaction between the trapped electron-hole pair. The former is attributed to total angular momentum J= 1, Г 4 representation and is electric-dipole allowed while the latter is assigned to J=2 belonging to Г 3+Г 5 representation and is electric-dipole forbidden. Based on this model, we explain several experimental observations including varying PL intensity of the B line from sample to sample, relatively rapid disappearance of the B line as function of increasing temperature and strong longitudinal-optical (LO) couplings of A + and B lines. Raman spectroscopy was used to investigate the coupling mechanisms between LO-phonons and electronic excitations in n-type ZnSe layers. The layers, grown by molecular-beam epitaxy, were intentionally doped below the Mott criterion for the insulator-metal transition. The nature of the electron-phonon interaction is determined by the degree of electron localization, which was effectively changed by temperature and donor concentration. The LO-phonons couple to plasmons when electrons are thermally excited into the conduction band or at donor sites. In both cases, unbound phonons are observed. From the renormalized phonon frequencies at high temperature, values of free electron concentration as a function of temperature were established. They are in excellent agreement with Hall-effect determinations. At low temperatures, the phonon Raman profiles are asymmetric and show Fano-type line shapes. The electronic continuum responsible for the phonon self-energies at low temperatures was identified as Raman scattering by bound electrons. In addition, Raman spectroscopy was used to establish the presence of a free hole gas and its coupling to the longitudinal optical phonons in Li-doped ZnSe epilayers. The phonon spectra shift to higher frequencies and broaden with increasing acceptor concentration and temperature, as was the case for indium-doped layers, in accordance with the expectation for coupled phonon-plasmon modes. Values for the concentration and mobility of the holes were obtained from an analysis of the spectral lineshapes. They agree with those determined by other methods. A linear relationships was found between the spectral broadening and the hole concentration. In addition, electronic Raman scattering (ERS) from holes bound to Li acceptors was also studied in ZnSe epilayers and correlated with the net acceptor concentration determined by capacitance versus voltage measurements. The ERS spectra reveal several transitions between the ground 1s and the excited states of the Li acceptors as well as transitions to a continuum of delocalized valence band states. Values of excitation energies for the bound hydrogenic states and the ionization energy of the acceptors were measured. The strength of the ERS signal normalized to the phonon scattering depends linearly on net acceptor concentration.