Volker Härle

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.

Deep‐level defects and n‐type‐carrier concentration in nitrogen implanted GaN

We analyzed the intrinsic defects and the n‐type‐carrier concentration generated by nitrogen ion implantation in n‐type GaN by deep‐level‐transient spectroscopy and by capacitance–voltage measurements, respectively. The samples were grown on sapphire by metalorganic vapor‐phase epitaxy. Nitrogen implantation with different ion doses and postimplantation rapid‐thermal annealing (RTA) were investigated. We observed a growing n‐type‐carrier concentration and increasing defect concentration with increasing nitrogen ion implantation doses. After RTA the concentration of free carriers and deep levels as found in the as‐grown state are restored. We also address contrarily seeming results from measurements of sheet resistance after N implantation published recently.

Mechanisms of recombination in GaN photodetectors

Steady‐state and transient responses of a nonintentionally doped GaN photodetector are investigated. The kinetics of the photoresponse demonstrate the existence of deep levels in the gap, acting as recombination centers with an acceptor character. The photoresponse displays two competing processes: a bimolecular recombination, dominating at high optical power range, and a monomolecular recombination involving long response times. The observed persistent photoconductivity and the huge photoconductive gain are due to the small electron capture cross section and a much faster hole capture rate.

Facet degradation of GaN heterostructure laser diodes

We investigated the degradation of cleaved facets of (Al,In)GaN laser diodes in different atmospheres. We found that operation in water-free atmospheres with sufficient oxygen shows a slow degradation. Operation in atmospheres with water vapor causes a fast degradation and an oxidation on the facet. This deposition is a permanent damage to the laser diode. If the laser diode is operated in pure nitrogen, we find a thick deposition on the facet, which shows high absorption. This deposition can be removed by either high optical output powers or by operation in atmospheres with sufficient oxygen. We also explain the influence of these coatings to the degradation behavior and see these coatings as the reason for unstable kinks in the L–I characteristics during operation.

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.

Microscopic analysis of optical gain in InGaN∕GaN quantum wells

A microscopic theory is used to analyze optical gain in InGaN∕GaN quantum wells (QW). Experimental data are obtained from Hakki–Paoli measurements on edge-emitting lasers for different carrier densities. The simulations are based on the solution of the quantum kinetic Maxwell–Bloch equations, including many-body effects and a self-consistent treatment of piezoelectric fields. The results confirm the validity of a QW gain description for this material system with a substantial inhomogeneous broadening due to structural variation. They also give an estimate of the nonradiative recombination rate.

Optical gain, carrier-induced phase shift, and linewidth enhancement factor in InGaN quantum well lasers

Adapting the Hakki Paoli method to group III nitrides, we measure gain, differential gain, carrier-induced change of refractive index, carrier-induced phase shift, and the antiguiding factor. Our measurements also cover the low-carrier-density regime, in which spontaneous and piezoelectric fields and Coulomb interaction are only partially screened. This regime is most interesting as a comparison with existing theoretical simulations, including many-body effects.

Substrate Modes of (Al,In)GaN Semiconductor Laser Diodes on SiC and GaN Substrates

In semiconductor laser diodes layers with high refractive index can act as parasitic waveguides and cause severe losses to the optical mode propagating in the longitudinal direction. For (Al,In)GaN laser diodes, the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers, hence the name substrate modes. A set of four different experiments shows the effect of substrate modes in the near-field (the most direct evidence of substrate modes), as side lobes in far-field, oscillations of the optical gain spectra, and as dependency of threshold current on n-cladding thickness. We derive several basic properties of the substrate modes by simple estimates. For a quantitative analysis we employ a 2-D finite element electromagnetic simulation tool. We simulate periodic variations in the cavity gain spectrum that explain the measurements in terms of absolute value and oscillation amplitude. We show that it is necessary to include the refractive index dispersion in order to get the correct period of the gain oscillations. Furthermore, we use the simulations to optimize the laser diode design with respect to substrate mode losses within the constraints given, e.g., by growth conditions