We report on the time-resolved luminescence of the defect-related violet band from undoped AlN epitaxial layers grown on sapphire and SiC. For both measurements in photoluminescence and in cathodoluminescence a decay of algebraic nature at long times is observed. This is typical for donor-acceptor pair transitions. We compare the behavior of this band to that of the generically yellow luminescence of GaN.

We investigate by photoluminescence (PL) nominally undoped, commercially available Zinc Oxide substrates (from Eagle Picher) grown by seeded chemical vapor transport technique in order to identify residual donors and acceptors. In low temperature PL spectra the dominant emission comes from the decay of bound exciton lines at around 3.36 eV. Zeeman measurements allow the identification of the two strongest lines and some weaker lines in-between as donorrelated. From the associated two-electron satellite lines binding energies of the major donors of 48 meV and 55 meV, respectively, can be deduced.

Piezoelectric AlGaN/GaN FETs on SiC with high carrier mobility have been fabricated yielding IDS=450 mA/mm and gm=200 mS/mm. In the on-state, under UV-illumination, the devices sustain a drain voltage of VDS=49 V, corresponding to a power dissipation of 26.5 W/mm. On turn-on of the device from the pinch-off state, a significant delay in the drain current build-up is observed. This effect depends on the pinch-off time and the pinch-off voltage and can be removed by either a brief UV-illumination or a VDS>25 V applied in the on-state. The drain current transients are characterized by a relaxation time τ, which is in the order of several hundred seconds. From the temperature dependence of τ, an activation energy of about 280 meV and a capture cross section of 4.4·10−18cm2 were determined. The devices show pronounced persistent photoconductivity (PPC) and the drain current ID is sensitive to illumination.

Cathodoluminescence measurements at cryogenic temperatures are reported on boron- and phosphorus-doped CVD-diamond films grown on silicon substrates. Boron and phosphorus concentrations were determined by SIMS measurements; for boron, they reached from unintentional background doping levels up to 3500 ppm. At increasing boron concentrations, the radiative recombination of boron bound excitons (BEto) at 5.22 eV photon energy systematically broadens and shifts down to 4.99 eV whereas the free exciton emission (FEto) disappears for 40 ppm and higher. In the phosphorus-doped films we observe new lines at 5.16 eV and 4.99 eV which we ascribe to TO- and (TO+Or)-phonon assisted transitions of an exciton bound to a shallow impurity other than boron, possibly phosphorus or a phosphorus-related shallow complex.

The photoluminescence (PL) infrared (IR)-band of p-doped porous Si (PS) films is studied by steady-state and time-resolved PL and by photoluminescence excitation (PLE) in detail. In analogy to the S-band in the visible the IR-band shifts to higher energies with reduced average nanocrystal size. The IR- and S-bands are very different in their decay behavior and in their recombination lifetimes. The temperature-dependent PL intensity shows non-exponential decay with lifetime distributions in the nsec-µsec range in contrast to the stretched exponential decay shape of the S-band corresponding to lifetime distributions in the μsec -msec range. The origin of the IR-band is likely related to radiative recombination at deep defects in Si nanocrystals with quantum-upshifted band gaps.

Sil-xGex/Si(100) single quantum well samples were grown by MBE with x varying from 18.5% to 36%, well widths Lz from 1.1 nm to 175 nm, and growth temperatures To in the range from 350°C to 750°C. We studied the photoluminescence (PL) properties in detail using Fourier transform spectroscopy. In most of the samples, the dominant and phononresolved SiGe band edge PL exhibits exclusively free exciton luminescence even at sample temperatures as low as 2 K. From the absence of bound excitons we conclude that the background concentration of shallow impurities is low in our SiGe layers. Variation of the growth temperature of the Si cap layer strongly influences the efficiency of the SiGe band edge PL at higher temperatures, whereas the PL efficiency is less affected at T=4 K. This indicates a reduction of the concentration of non-radiative defects in the cap layer when it is grown at higher temperatures. A broad PL band 150 meV below the SiGe bandgap, supposed to be typical for MBE-grown material, is weakly observed in only a few samples. The strength of the broad PL band does not unambiguously depend on the strain energy density in the SiGe layer as recently suggested.

For the thermal quenching of quantum well (QW) photoluminescence (PL) controversal de-activation energies have been reported. Values range from the total exciton (or electron-hole) binding energy Ex to half of Ex or to the binding energy of the shallower bound particle. We resolve this controversy by performing steady-state measurements both under high and extremely low (Iex≈l μW/mm2) excitation conditions on a series of multiple QW structures of the material systems InxGa1-xAs/GaAs and GaAs/AlyGa1-yAs. For high and low excitation, we find that the final de-activation step is associated with Ex. In an intermediate temperature range, the first decade of PL quenching is governed by a de-activation energy close to the binding energy of the shallower bound particle.

Also performed were temperature dependent photoluminescence excitation (PLE) measurements under high injection in InxGa1-xAs/GaAs double QWs with additional AlyGa1-yAs cladding barriers. Vertical transport between the QWs is observed and the underlying de-activation energies are found to be equal to the total exciton binding energies Ex We discuss our findings in the frame of a simple model for the thermalization of electrons and holes.

Steady state and time-resolved photoluminescence (PL) and Fourier-transform infrared (FTIR) spectroscopy have been performed in situ during etching, on “as prepared” porous Si in air under laser exposure and on chemically oxidized porous Si. We suppose that PLdegradation of “as prepared” porous Si is caused by creating non-radiative defect centers during photooxidation. Chemically oxidized porous Si shows increased PL intensity and longer recombination lifetimes as compared to non-oxidized samples. We conclude, that an oxide layer with low defect density on the inner surface of chemically oxidized porous Si reduces the non-radiative recombination rate.

The 0.5 eV (2.5 μm 4000 cm1) emission band in InP has been studied by optical spectroscopy. By the use of Fourier-transform-infrared photoluminescence we have been able to observe at least a three-fold fine structure in the zero-phonon transitions at ∼ 4300 cm−1 which are studied at different temperatures. Based on the fine structure and the long decay time of 1.1 ms we ascribe the 0.5 eV emission to the 4T1 → 6A1 spin-flip transition of Fe3+. The excitation spectrum of this Fe3+-related emission shows a characteristic fine structure at ∼ 1.13 eV which belongs to a charge-transfer process of the type: Fe3+ + hv (1.13 eV) → [Fe2+, bound hole]. We discuss the excitation mechanism of the 0.5 eV emission by charge-transfer states and compare the results with an emission at 3057 cm1 in GaAs, which we attribute to the same Fe3+ transition (decay time: 1.9 ms).

In the present paper, optical absorption studies on the neutral charge state of the double acceptor zinc in silicon are presented. Measurements were carried out in the mid infrared (MIR) and in the near infrared (NIR) region. The MIR absorption spectra show the excitation series of an effective-masslike hole, from which the Zn° level position is calculated to be at Ev + 319. 1 meV. A splitting of the ground state into 3 sublevels is assigned to hole-hole coupling and crystal-field splitting. Absorption spectra obtained in the NIR are interpreted in terms of an A° X-type bound exciton.

We observe in photoconductivity (PC) measurements EMT-like excited states of the pseudodonor CrB, which are responsible for the capture of free electrons due to their short lifetime. The energetic position of the ground state can be located at Ev + (292 ± 5) meV. A finestructure of the (CrB)° transition consisting of at least 15 components is observed in PC and high-resolution Fourier transform photoluminescence. In acceptor-free silicon a step in photoconductivity at 205 ± 10 meV is ascribed to interstitial Cr, whereas a second step at ~ 425 meV could be due to substitutional Cr.