Ines Pietzonka

Recent results of blue and green InGaN laser diodes for laser projection

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and the wall plug efficiency of blue and green lasers. We report on improvements of the performance of true blue riedge waveguide InGaN lasers at 452nm with cw-output power up to 800mW in overstress and mono mode operation up to 500mW in a temperatures range of 20°C to 80°C. We succeeded in high and almost temperature independent wall plug efficiencies >20% at stable output power levels from 200 to 500mW in cw-operation. Due to several improvements of our blue laser diodes we now estimate life times is in the order of 40khrs for 80mW output power in cw-operation at 40°C. Additional overstress degradation tests at power levels up to 200mW show a strong dependency of lifetime with output power. Furthermore, we present pioneering results on true green InGaN laser diodes on c-plane GaN-substrates. The technological challenge is to achieve In-rich InGaN-quantum wells with sufficiently high material quality for lasing. We investigated the competing recombination processes below laser threshold like nonradiative defect recombination by electro-optical measurements, such confirming that low defect densities are essential for stimulated emission. A model for alloy fluctuations in In-rich InGaN-MQWs based on spectral and time resolved photoluminescence measurements yields potential fluctuations in the order of E0=57meV for our blue laser diodes. To get a closer insight into the physics of direct green InGaN-Laser we investigated the inhomogeneous broadening of experimentally measured gain curves via Hakki-Paoli-measurements in comparison to calculated gain spectra based on microscopic theory showing the importance of strong LO-phonon coupling in this material system. Investigations of current dependent gain measurements and calculations yield a factor of 2 higher inhomogeneous broadening for our green lasers than for our blue laser diodes on c-plane GaN. Based on the improvements of the material quality and design we demonstrate true green InGaN-Laser in cw-operation at 522nm with more than 80mW output power on c-plane GaN. The combination of low laser threshold ~60-80mA, high slope efficiency ~0.65W/A and low operating voltage 6.9-6.4V of our green monomode RWG-Laser results in a high wall plug efficiency of 5-6% in a temperature range of 20-60°C.

Temperature-Dependent Internal Quantum Efficiency of Blue High-Brightness Light-Emitting Diodes

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

Determination of recombination coefficients in InGaN quantum-well light-emitting diodes by small-signal time-resolved photoluminescence

We suggest a novel technique for the evaluation of the recombination coefficients corresponding to Shockley–Read–Hall, radiative, and Auger recombination that occur in InGaN/GaN-based light-emitting diodes (LEDs). This technique combines the measurement of the LED efficiency as a function of LED drive current with a small-signal time-resolved photoluminescence measurement of the differential carrier life time (DLT). Using the relationships between the efficiency and DLT following from the empirical ABC-model, one can evaluate all three recombination coefficients. The suggested technique is applied to a number of single- and multiple-quantum well LEDs to gain a deeper insight into the mechanisms ultimately limiting their efficiency.

Vertical-External-Cavity Surface-Emitting Laser With Monolithically Integrated Pump Lasers

The monolithic integration of pump lasers and optically pumped vertical-external-cavity surface-emitting lasers is demonstrated. An innovative contacting scheme for the pump lasers offers high design flexibility and scalability. First devices at 1000 nm generate output powers of 2.5 W in pulsed and 0.65 W in continuous-wave operation.

Influence of the domain size on the band gap of ordered (GaIn)P

The domain size and the band-gap energy of ordered epitaxial (GaIn)P layers have been determined by means of transmission electron microscopy and photoluminescence measurements, respectively. With decreasing domain size the symmetry of the ordered layers is reduced from the trigonal space group R3m (CuPtB type) to the monoclinic Pm space group. Pseudopotential band-structure calculations reveal an increase of the band-gap energy of ordered (GaIn)P with diminishing domain size. This would affect the determination of the degree of order by methods, which rely on the band-gap energy of perfectly ordered (GaIn)P. By correlating the theoretical with the experimental band-gap energies we demonstrate that the order parameter η of layers with small domains is higher if the dependence of the band-gap energy on the domain size is considered. Hence, for the determination of the degree of order not only the band-gap energy, but also the domain size have to be taken into account.

Recent progress of AlGaInP thin-film light-emitting diodes

The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized. Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected. Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.

Scalability of buried microreflector light-emitting diodes for high-current applications

The combination of wafer soldering using metal layers and the introduction of buried micro-reflector structures has proven to be a promising approach to fabricate high brightness, substrate-less LEDs in the AlGaInP material system. In addition to the enhanced light output, the scalability of this approach has been predicted as a major advantage. In contrast to other approaches, larger area LEDs can be fabricated without altering the epitaxial structure and thickness of layers simply by offering a larger area for light generation. First samples of amber (λ = 615 nm) buried micro-reflector LEDs with side-length up to 1000 μm have been realized. Devices mounted in packages with improved heat sinks are capable of low voltage CW operation with currents as high as 600 mA (Vfw≤ 2,8 V) without significant thermal flattening of the light-current characteristics. The maximum luminous flux achieved at these oeprating conditions is 46 lumen. Already these first experiments demonstrate the potential of the concept of buried micro-reflector LEDs not only for high-brightness but also for high-current operation. The results are among the best values of high-flux LEDs in this wavelength range.

Progress in ultra-compact green frequency doubled optically pumped surface emitting lasers

Compact, stable and efficient green lasers are of great interest for many applications like mobile video projection, sensing, distance measurement and instrumentation. Those applications require medium values of output power in the 50mW range, good wall-plug efficiency above 5 % and stable operation over a wide temperature range. In this paper we present latest results from experimental investigations on ultra-compact green intracavity frequency doubled optically pumped semiconductor InGaAs disk lasers. The green laser setup has been limited to a few micro optical and semiconductor components built on a silicon backplane and fits within an envelope of less than 0.4 cc. An optical frequency looking scheme in order to fix the fundamental wavelength over varying operating conditions like changing output power and ambient temperature has been applied. The cavity has been optimized for fast modulation response and high efficiency using quasi-phase matching non-linear material. Recent data from cw and high-frequency characterization is presented.

Recent advances in VECSELs for laser projection applications

Laser projectors integrated in portable devices offer a new platform for media display but put strong demands on the laser sources in terms of efficiency, modulation band width, operating temperature range and device cost. Osram Opto Semiconductors has developed and produces synthetic green lasers for projection applications on which the latest results are reported. Based on vertical external cavity surface emitting laser (VECSEL) technology and second harmonic generation an output power of >75mW has been achieved. The maximum output power is to a large extent limited by the high thermal resistance of the monolithic VECSEL chip used. To overcome the thermal limitations a new thinfilm VECSEL chip design is proposed where the epitaxial layers are transferred to a silicon carrier and processed on wafer level, thus significantly lowering the thermal resistance and improving the maximum output power.

Transport and capture properties of Auger-generated high-energy carriers in (AlInGa)N quantum well structures

Recent photoluminescence experiments presented by M. Binder et al. [Appl. Phys. Lett. 103, 071108 (2013)] demonstrated the visualization of high-energy carriers generated by Auger recombination in (AlInGa)N multi quantum wells. Two fundamental limitations were deduced which reduce the detection efficiency of Auger processes contributing to the reduction in internal quantum efficiency: the transfer probability of these hot electrons and holes in a detection well and the asymmetry in type of Auger recombination. We investigate the transport and capture properties of these high-energy carriers regarding polarization fields, the transfer distance to the generating well, and the number of detection wells. All three factors are shown to have a noticeable impact on the detection of these hot particles. Furthermore, the investigations support the finding that electron-electron-hole exceeds electron-hole-hole Auger recombination if the densities of both carrier types are similar. Overall, the results add to the evidence that Auger processes play an important role in the reduction of efficiency in (AlInGa)N based LEDs.