Photoluminescence and Photoluminescence Excitation Spectroscopy of Cu(In,Ga)Se2 Thin Films

The role of intrinsic point defects on radiative recombination in Cu(In,Ga)Se2 thin films was investigated by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Experiments were performed on device-grade polycrystalline layers and single crystal thin films. PL transitions identified by others as indicating a shallow state with an ionization energy of ∼16 meV is proposed to be a transition into band tail states rather than a distinct shallow defect. The presence of deep levels contributing to radiative recombination does not necessarily preclude the material from producing a high efficiency device and may suggest the absence of dominant non-radiative recombination pathways. The band edge width as measured by PLE and the separation of this edge from defect states are suggested to be potentially effective indicators of the quality of a material. Luminescence that appears to be connected with the absence of Na in the growth process persists in high Ga alloy, Na containing materials, suggesting that Na may become ineffective in passivating or eliminating certain defects in high Ga material.

Identification of Carbon-related Bandgap States in GaN Grown by MOCVD

ABSTRACTCarbon doping of GaN is of great interest in part because it has been shown to result in semi-insulating (SI) behavior. However, determination of the bandgap states and hence the exact mechanism responsible for the SI behavior is, to date, an unresolved issue. A key impediment is that the presumed C acceptor levels are likely near the minority carrier (valence) bandedge of otherwise background n-type GaN, and hence their precise detection by usual methods is difficult. In this paper, we exploit the inherent ability of deep level optical spectroscopy (DLOS) to detect states near the minority carrier band edge, as well as potentially deep states associated with C in background n-type GaN. This is accomplished by comparing unintentionally doped (uid) GaN grown by atmospheric pressure (AP) MOCVD, which has residual n-type conductivity, with LP MOCVD GaN that incorporates a large concentration of C for both uid and intentionally Si co-doped conditions. The results show the emergence of a shallow state at Ec - 3.28 eV (Ev + 0.16 eV) for the LP samples with a minimum concentration of 3.6 × 1016 cm-3 that efficiently compensates Si donors for the co-doped sample, and results in semi-insulating behavior for the uid-LP sample. In contrast, this state is not observed for the AP GaN material, which incorporates a factor of ∼10 times less C, and instead only the expected residual Mg acceptor level at Ec - 3.22 eV is observed. Additionally, a state at Ec - 1.35 eV, near the theoretically expected C split-interstitial level in n-type GaN, is observed to increase significantly in concentration with increased C concentration.

Characterization of the shallow state of iodine chloride

A weakly bound Ω = 1 state of ICl, [Formula: see text], which converges to the ground state 1(2P3/2) + Cl(2P3/2) of the separated atoms, has been identified and characterized. Spectroscopic constants of this state are Te = 17 338.0(13), ωe = 32.85(48), ωexe = 1.272(40), 103Be = 38.2(13), 104αe = 8.89(34), 105γe = −8.1(20) cm−1, and re = 4.01(6) Å. The dissociation energy De = 219.6 cm−1 is consistent with the value predicted for a Morse function, [Formula: see text]. Transitions [Formula: see text] are allowed owing to homogeneous coupling between ã and the well-defined A(3π1) state; in fact, at medium-long range (r = 6–6.5 Å, D–Gν = 20–30 cm−1), the diabatic ã and A curves cross at a small angle. Principal features of the crossing are explained if the electronic interaction matrix element is ca. 4 cm−1, corresponding to weak coupling. Heterogeneous perturbations of the A and ã states in the range D–Gν < 200 cm−1 are attributed to coupling with high vibrational levels of the ground state X(1Σ+).