A. Hangleiter

Radiative carrier lifetime, momentum matrix element, and hole effective mass in GaN

By using picosecond time-resolved photoluminescence we have measured the lifetime of excess charge carriers in GaN epitaxial layers grown on sapphire at temperatures up to 300 K. The decay time turns out to be dominated by trapping processes at low excitation levels. The radiative lifetime derived from our data is dominated by free excitons at temperatures below 150 K, but also clearly shows the gradual thermal dissociation of excitons at higher temperatures. From our data, we are able to determine the free exciton binding energy and the free carrier radiative recombination coefficient. By combining these data with optical absorption data, we find the interband momentum matrix element and an estimate for the hole effective mass, which is much larger than previously thought.

The role of piezoelectric fields in GaN-based quantum wells

In this contribution, we focus on the consequences of the piezoelectric field, which is an inherent consequence of the commonly used wurtzite phase of GaN, on the optical properties of strained GaN-based quantum well structures. We demonstrate that both in GaN/AlGaN and in GaInN/GaN single quantum well structures, the piezoelectric field leads to a Stark-shift of the fundamental optical transitions, which can lead to luminescence emission far below the bulk bandgap. Due to the spatial separation of the electron and hole wavefunctions in such structures, the oscillator strength of these transitions may become extremely small, many orders of magnitude lower than in the field-free case. From specially designed structures, we can even determine the sign of the piezoelectric field and relate it to the polarity of the layers. Under high-excitation conditions, as found in a laser diode, the piezoelectric field is almost completely screened by the injected carriers. As a consequence, the stimulated emission is significantly blue-shifted compared to the photoluminescence, which has sometimes been confused with localization effects.

Auger recombination in bulk and quantum well InGaAs

We report the determination of Auger recombination coefficients in bulk and quantum well InGaAs by time‐resolved luminescence measurements. In bulk InGaAs the coefficient is C=3.2×10−28 cm6/s and has the temperature dependence of the valence‐band Auger effect involving the split‐off valence band. In 11 nm quantum well InGaAs we find C=0.9×10−28 cm6/s, independent of temperature. The Auger coefficient decreases slightly with decreasing well width.

Intervalence band absorption in strained and unstrained InGaAs multiple quantum well structures

We report the direct determination of absorption losses in unstrained InGaAs/InGaAsP and InGaAs/InGaAlAs and strained InGaAs/InGaAsP layer multiple quantum well (MQW) laser structures. In the case of the unstrained structures we find a strong dependence of the absorption on carrier density indicating the presence of an intrinsic optical loss mechanism, the intervalence band absorption (IVBA). In the strained layer InGaAs/InGaAsP structures, IVBA is completely switched off. Our results explain the superiority of strained layer InGaAs MQW lasers.

Auger recombination in strained and unstrained InGaAs/InGaAsP multiple quantum‐well lasers

We report the determination of the Auger recombination coefficient in strained and unstrained InGaAs/InGaAsP/InP separate‐confinement multiple quantum‐well laser structures. For a temperature of 300 K and a well width of 100 A, we find an Auger coefficient of C=1.0×10−28 cm6 s−1, independent of strain and only weakly dependent on temperature. These properties of the Auger coefficient indicate the dominance of phonon‐assisted Auger recombination. Our model calculations based on a six‐band kp theory explain the experimentally found dependency on temperature and strain. The consequences on laser performance are discussed.

Optical gain in GaInN/GaN heterostructures

By optical gain spectroscopy we have studied the fundamental laser properties of GaInN/GaN heterostructures grown on sapphire. Utilizing the stripe excitation method we have measured optical gain spectra at room temperature. Due to the low symmetry of the wurtzite structure and the resulting splitting of the uppermost valence bands, we find optical gain only for the TE mode. Our analysis shows that the optical gain is due to direct band‐to‐band transitions in an electron‐hole plasma. For gain amplitudes typically found in lasers, we find carrier densities up to 3×1019 cm−3, which are likely to lead to rather large threshold current densities.

Composition dependence of polarization fields in GaInN/GaN quantum wells

We report an experimental determination of the internal polarization field in GaInN/GaN quantum wells, due to piezoelectric and spontaneous polarization, utilizing the quantum confined Stark effect, with fields as large as 3.1 MV/cm at 22% In. From its dependence on quantum well composition and strain, we find that the total field in GaInN is a linear combination of polarization charges from GaN and InN. The piezoelectric constants d31 for GaN and InN derived from our data are 1.05±0.05 pm/V and 3.7±0.5 pm/V, in fair agreement with theoretical data.

Intrinsic upper limits of the carrier lifetime in silicon

We have measured the carrier lifetime in ultrapure n‐ and p‐type silicon with carrier densities between 1015 cm−3 and 1019 cm−3. At high carrier densities the dominant recombination mechanism is known to be band‐to‐band Auger recombination. At carrier densities below some 5×1018 cm−3 experimentally determined carrier lifetimes in n‐ and p‐type silicon were always found to be smaller than expected for band‐to‐band Auger recombination. Our results are explained by taking into account electron–hole correlations in the Auger process. Since Auger recombination is an unavoidable material property this represents a new intrinsic upper limit of the carrier lifetime in silicon.

Rare earth ions in LPE III-V semiconductors

Abstract InP : Yb LPE layers have been grown by a supercooling process at high growth temperatures up to 800°C. The Yb concentrations in the In growth melts ranged between 0.001 and 0.005 mole fraction. Photoluminescence and Hall measurements gave information about the incorporation behaviour of the rare earth ion Yb 3+ (4f 13 ) and about its influence on the electrical properties of InP. Time resolved photoluminescence measurements at low temperatures indicated a lifetime of about 12 μs of the excited Yb 3+ state. LPE grown InP:Yb p-n electroluminescence diodes were fabricated, which emit the intracentre Yb luminescence at 1 μm wavelength. High purity InP layers could be grown, using Yb in low concentrations to remove residual impurities from the growth melts. Net carrier concentrations N D - N A of 2 × 10 14 cm −3 at 300 K (1.5 × 10 14 cm −3 at 77 K) and carrier mobilities of 5700 cm 2 /V·s at 300 K (90,000 cm 2 /V·s at 77 K) could be achieved.

Excitation and decay mechanisms of the intra‐4f luminescence of Yb3+ in epitaxial InP:Yb layers

The decay and excitation processes of internal transitions of Yb3+(4f13) incorporated in InP were investigated by means of time‐resolved photoluminescence and photoluminescence excitation spectroscopy. From the temperature dependence of the excited state lifetime we find several decay mechanisms, including a bound‐exciton‐like Auger process, energy transfer, and thermal depopulation. Excitation spectroscopy reveals that free carriers are needed during the excitation process of Yb centers.