H. P. Gislason

Properties of Zn‐doped VPE‐grown GaN. I. Luminescence data in relation to doping conditions

Experimental results on optical and electrical properties of VPE‐grown GaN doped with Zn under various conditions of growth are presented. The incorporation of Zn into GaN was found to be critically dependent on growth conditions. Four different Zn‐related acceptorlike centers A–D were found to occur, with broad radiative emissions peaking at 2.87 (A), 2.6 (B), 2.2 (C), and 1.8 eV (D). Their broad shape was found to be due to a moderate lattice relaxation upon optical transitions. A linear model for phonon coupling was found adequate, with a principal mode resonant with the optical band [h/ω, varying between 74 (A level) and about 84 meV (C level)] and an additional lower‐energy mode, yielding Franck‐Condon shifts for these centers ΔFC,A =0.25±0.03, ΔFC,B=0.25±0.04, ΔFC,C =0.28±0.04 eV, and ΔFC,D=0.28±0.05 eV. Data for electrical compensation related to the occurrence of these different emissions indicate that the A level could be the ZnGa substitutional acceptor, while the more efficiently compensating B...

Acceptor associates and bound excitons in GaAs:Cu

The effect of several parameters in the diffusion conditions for copper into GaAs on the 1.36‐eV photoluminescence (PL) band and the C0 (1.5026 eV) and F0 (1.4832 eV) bound excitons (BE) has been investigated using photoluminescence techniques. Also, the different behavior of the corresponding centers under heat treatment and cool down after diffusion is reported. The results are discussed in relation to the vast literature on the GaAs:Cu, which is confusing on the relation between the 1.36‐eV PL band, C0 and F0 bound excitons and the well‐known 0.156‐ and 0.45‐eV acceptors. Here it is concluded that there exists no simple relation between the C0 BE and the 1.36‐eV PL band nor is the F0 BE related to any other Cu‐related PL band in our samples. Models for the identities of these centers are suggested on the basis of the new experimental data presented in this paper.

Properties of Zn‐doped VPE‐grown GaN. II. Optical cross sections

Experimental data are presented for optical cross sections σ0n(hν) related to the four deep acceptorlike Zn‐related radiative levels A–D in GaN. Very accurate spectral data were obtained by the photoluminescence excitation (PLE) technique, at temperatures varying from 4.2 up to 293 K. From a detailed comparison of the spectral behavior of the emission and absorption spectra, the size of electronic broadening effects could be determined in the overlap region. The binding energies for Zn‐related acceptor levels were obtained as EA=0.34±0.04, EB=0.65±0.08, EC=1.02±0.05, and ED=1.42±0.08 eV at low temperature. Optical transmission data give estimates of absolute values for optical cross sections at saturation, varying from σ0nA(sat) ?1×10−16 to σnD(sat) ?5×10−18 cm2. Temperature broadening effects in optical cross sections are well explained by a linear model for phonon coupling in the optical transitions, employing the same phonon energies and coupling strengths as were evaluated from emission spectra.

Electrical characterization of Mg-related energy levels and the compensation mechanism in GaN:Mg

We investigated GaN:Mg samples grown by metal-organic chemical vapor deposition using various electrical measurement techniques. Annealing of highly resistive as-grown samples for different duration of time gives gradual activation of acceptors to concentrations up to 1×1017 cm−3. Conductance measurements of the annealed samples show the presence of two hole traps H1 and H2 with activation energies 130 and 170 meV from the valence band, respectively. The concentration of the H2 trap is always found to be low in our samples. The H1 trap is the shallowest one in our samples. Its concentration is directly proportional to the electrically active acceptor concentration in the samples, increasing with annealing. Hence, we attribute it to a Mg-related acceptor. This assignment is confirmed by dark current measurements. Two electron traps at 280 and 580 meV from the conduction band are observed in optical deep-level transient measurements. They have too weak concentrations to influence the free carrier concentrat...

Properties of GaN tunneling MIS light‐emitting diodes

MIS structures on GaN consisting of Au‐NaI‐GaN or Au‐Al2O3‐GaN with insulator thickness <100 A have been fabricated with the aim of producing light‐emitting diodes with emission in the uv and blue spectral region at low bias. I‐V characteristics are consistent with a tunneling carrier transport mechanism, and light emission typically occurs for voltages 2–3 V. The spectral behavior of electroluminescence (EL) is in good agreement with photoluminescence (PL) data in the uv and blue spectral region. Suggestions for further work to improve radiant power output from such devices are presented.

Evidence for complex acceptors related to Cu and Li in GaAs

New photoluminescence (PL) bands are reported in GaAs doped with Cu and Li as well as with Li only. A study of a PL band at 1.41 eV appearing after both subsequent and simultaneous diffusions of Cu and Li into a range of different GaAs samples under varying doping conditions shows an unequivocal correlation between this band and the presence of both Cu and Li in the samples. Also, an observed decrease of the 1.36‐eV Cu PL band as the 1.41‐eV Cu‐Li band increases suggests a common component of the corresponding two centers, presumably CuGa. A model is proposed, assigning the 1.41‐eV PL band to recombination involving a 0.11‐eV CuGa‐Lii acceptor level. It is suggested that a PL peak at 1.34 eV appearing after Li doping of bulk material involves transitions to the doubly ionized LiGa acceptor. Other new PL bands appear after Li diffusion as well. The absence of bound exciton (BE) states associated with these acceptors is also discussed.

Complex defects in ZnTe created by Cu diffusion

Abstract An extensive study of complex defects induced by Cu diffusion in ZnTe is reported. About thirty-five different bound excitons are observed in optical spectra at low temperatures in the range 2.0–2.4 eV, all related to different neutral (“isoelectronic”) or acceptor-like Cu-related complexes. The electronic structure of these bound excitons is studied in detail with a combination of optical emission and absorption spectra, together with Zeeman data and FTIR transmission. In most cases the electron is the loosely bound particle in the bound exciton for both acceptor complexes (“pseudo-donor model”) and neutral complexes (HTL model). The hole is more localized, presumably due to the hole-attractive CuZn pseudopotential. For acceptor-like complexes usually only one electronic bound exciton line is seen, while other electronic states are lifted above the bandgap energy by the action of the local stress field. Similarly neutral complexes usually have a single bound hole state in the gap, derived from a CuZn central cell. Further, novel electronic satellite spectra in photoluminescence and resonant Raman scattering, seem to be common for Cu-related complex defects at high doping levels. Another interesting effect is a strongly enhanced phonon coupling observed in the absorption spectrum of these complex bound excitons, compared to the coupling strength in the emission case. The possible identities of these Cu-related complex defects are discussed, considering their observed symmetries and the available information on chemical impurities present in the starting material. Several of these complex defects are thermally unstable at room temperature, presumably typical for the cases where an interstitial Cui is part of the complex.

Influence of initial surface reconstruction on nitridation of Al2O3 (0001) using low pressure ammonia

The purpose of this study is to investigate the effect of initial surface reconstruction on the nitridation process of Al2O3 (0001). This was done by exposing differently reconstructed sapphire sub ...

Effect of the interface on the electrical properties of ZnSe/GaAs heterojunctions grown by molecular beam epitaxy

Using current–voltage and ac conductance measurements, nitrogen-doped ZnSe grown by molecular beam epitaxy on p-GaAs substrates heterostructures was studied. The reverse current–voltage characteristics of the heterojunction show a soft saturation and a hysteresis. A slow current increase takes place following the application of a constant reverse bias until a steady-state value is reached. This behavior is explained in a model involving slow interface states at the heterointerface that result in a voltage-induced barrier lowering. The observation of a broad peak in the ac conductance vs temperature spectra confirms the model. The presence of such states may seriously affect the performance of ZnSe/GaAs-based devices.

Lithium–gold‐related defect complexes in n‐type crystalline silicon

Using deep level transient spectroscopy combined with secondary‐ion‐mass spectroscopy and capacitance–voltage profiling, it is demonstrated that lithium diffusion into gold‐doped n‐type silicon at temperatures between 200 and 300 °C results in the formation of two lithium–gold‐related complexes. One of the Au–Li complexes appears to be electrically passive and is observed indirectly as gold acceptor passivation. Virtually all passivated gold acceptors are reactivated after 30 min annealing at 400 °C of samples with comparable Au and Li concentrations in the 1014 atoms/cm3 range. The process can be reversed again by additional heat treatment at lower temperatures. The passivation–reactivation cycle can be repeated as long as there is enough Li present in the crystal. This reaction can be described by a mass‐action law between negatively charged gold atoms and positively charged lithium (Au−+Li+) with a free binding energy of approximately 0.87 eV. The other Au–Li complex has a deep level (labeled L1) withi...