Experimental results of the modal gain and recombination currents of a GaAsSb/InGaAs type-II quantum well laser structure emitting at 1.3 mum as a function of current injection and temperature are presented. The radiative efficiency versus injection level at different temperatures is analyzed.

We measure the temperature dependence of the components of threshold current of 1300 nm undoped and p-doped quantum-dot-lasers and show that the temperature dependence of gain is the largest factor in producing the observed negative T0

The authors measure the temperature dependence of the components of threshold current of 1300?nm undoped and p-doped quantum dot lasers and show that the temperature dependence of the injection level necessary to achieve the required gain is the largest factor in producing the observed negative T0 in p-doped quantum dot lasers.

We have experimentally observed the time evolution of the photoluminescence spectra of InGaN/GaN quantum wells with widths 3 and 4 nm in response to pulsed excitation at room temperature. We find that for both well widths the time evolution of the energy- integrated photoluminescence increases initially then decays and the spectrum displays a blue shift of the peak energy which then reverses. Through an iterative simulation of the carrier density, piezoelectric field and radiative recombination rate we calculate the behavior of these quantum well systems and find good agreement with the experimental data. The internal field present in the InGaN/GaN system is screened as carrier density increases, which combined with band filling and coulomb interactions result in a blue shift as the system is pumped and as recombination of the carriers occur a red shift is simulated. Although screening of the internal fields occurs our calculations show that at laser threshold there is still a large internal field present, 1.0 MVcm(-1), which is 75 % of the unscreened value.

Self-assembled In(Ga)As quantum dot (QD) lasers incorporating p-type modulation doping have generated much interest recently due to reports of a temperature insensitive threshold current and increased modulation bandwidth. The mechanism by which p-type doping improves the performance of QD lasers is thought to be similar to that envisaged for quantum well lasers, where increased gain is expected for a given quasi-Fermi level separation due to a shift in both quasi-Fermi levels towards the valence states. However, the benefits may be much more pronounced in quantum dot structures since the population of the smaller number of dot states can be dramatically affected using relatively low doping levels, which may incur less penalty with regard to increased non-radiative recombination and internal optical mode loss.We present results of direct measurements of the modal gain measured as a function of the quasi-Fermi level separation for samples with different degrees of doping, which demonstrate unambiguously the increased gain that can be obtained at a fixed quasi-Fermi level separation. In addition, we have measured the internal optical mode loss and radiative and non-radiative recombination currents for samples containing 0, 15 and 50 dopant atoms per dot and show that, although the internal optical mode loss is similar for all three samples, the non-radiative recombination current increases for samples containing p-doping. We show that our experimental results are consistent with a simple computer simulation of the operation of our structures.

In many studies, the value of the experimentally determined internal piezoelectric field has been reported to be significantly smaller than theoretical values. We believe this is due to an inappropriate approximation for the electric field within the depletion region, which is used in the analysis of experimental data, and we propose an alternative method. Using this alternative, we have measured the strength of the internal field of InGaN p-i-n structures, using reverse bias photocurrent absorption spectroscopy and by fitting the bias dependent peak energy using microscopic theory based on the screened Hartree-Fock approximation. The results agree with those using material constants interpolated from binary values.

We have measured the pulsed light-current characteristics of a series of InGaN/GaN quantum well light-emitting diodes which were annealed post-growth at different temperatures as a function of their operating temperature. The light output at a fixed current density increases with the temperature of measurement, reaches a maximum and then decreases for all the diodes. The measurement temperature at which the maximum light output occurs and the magnitude of the light output depend on the post-growth thermal anneal temperature. The thermal anneal temperature is thought to affect the acceptor concentration in the p-doped cap layer, which also changes the carrier mobility. A simulation, incorporating carrier leakage, is used to reproduce the experimental behavior where the acceptor concentration is changed to represent the effects of the different anneal temperatures.

Pulsed light–current characteristics of InGaN/GaN quantum welllight-emitting diodes have been measured as a function of temperature, with sublinear behavior observed over the whole temperature range, 130–330 K. A distinctive temperature dependence is also noted where the light output, at a fixed current, initially increases with temperature, before reaching a maximum at 250 K and then decreases with subsequent increases in temperature. On the basis of a drift diffusion model, we can explain the sublinear light–current characteristics and the temperature dependence by the influence of the large acceptor ionization energy in Mg-doped GaN together with a triangular density of states function characteristic of localized states. Without the incorporation of localization effects, we are unable to reproduce the temperature dependence whilst maintaining emission at the observed wavelength. This highlights the importance of localization effects on device performance.

In this paper, we describe methods for analysis of edge-emitted amplified spontaneous emission spectra measured as a function of the pumped stripe length. We show that both the modal gain and the unamplified spontaneous emission spectra can be extracted from the data, and we describe a means of calibrating the spontaneous emission in real units, without requiring the carrier populations to be described by Fermi functions. The gain and emission spectra can be determined for transverse electric and transverse magnetic polarizations and by summing, the recombination currents for each polarization the total radiative current can be measured. This enables the overall internal radiative quantum efficiency to be calculated. Once the calibration factor is known the internal stimulated recombination rate at the facet can also be estimated. The experiment can be configured to give a measurement of the passive modal absorption of the gain medium. The internal optical mode loss can be determined from the long-wavelength region of the gain spectrum or the modal absorption spectrum. In summary, we show that measurements of amplified spontaneous emission spectra provide a full characterization of the gain medium.

We have measured piezoelectric fields in p-i-n LED structures using the quantum confined Stark effect and photocurrent absorption. The results agree with calculations of the absorption where material parameters are interpolated from the binaries.

The critical role of localised states in determining the behaviour of blue emitting GaN/InGaN quantum well LEDs is highlighted using detailed analysis of the temperature dependent light-current characteristics and measured absorption and emission spectra.

Measurement of the spontaneous emission and gain spectra provides a complete characterization of a semiconductor gain medium, however, this requires the observation of emission in two directions to avoid amplification of the spontaneous emission spectrum. We show that both the gain spectrum and the true spontaneous emission spectrum can be obtained from amplified spontaneous emission (ASE) spectra measured from the end of a segmented-contact device. The spontaneous emission spectra agree with spectra measured through a top contact window. If the carrier populations are fully inverted at low photon energy, it is possible to convert the ASE-derived spontaneous emission into real units.

The reduction in penetration of the optical mode into the cladding layers in large optical cavity (LOC) laser structures offers the possibility of reducing the cladding-layer thickness. This could be particularly beneficial in GaInP-AlGaInP high-power devices by reducing the thermal impedance and the electrical series resistance. We have designed and characterized 650-nm LOC lasers by modeling the optical loss due to incomplete confinement of the optical mode by the cladding layers and calculating the thermally activated leakage current. This indicated that the cladding thickness could be reduced to 0.5 mum without adversely affecting performance. We investigated devices with 0.3-, 0.5-, and 1-mum-wide cladding layers. The measured optical mode loss of the 0.3 mum-wide cladding device was 36.2 cm(-)1 compared with 12.4 and 11.3 cm(-1) for the 0.5- and 1 mum-wide cladding samples, respectively. The threshold current densities of the 0.5- and 1.0-mum devices were similar over the temperature range investigated (120-320 K), whereas the 0.3-mum devices had significantly higher threshold current density. We show that this can be attributed to the higher optical loss and increased leakage current through the thin cladding layer. The intrinsic gain characteristics were the same in all the devices, irrespective of the cladding-layer thickness. The measured thermal impedance of 2-mm-long devices was reduced from 30.7 to 22.3 K/W by reducing the cladding thickness from I to 0.5 mum. Our results show that this can be achieved without detriment to the threshold characteristics.

The radiative efficiency is critical in applications and it is therefore necessary to have a good understanding of the factors which determine this in GaN based LEDs. To this end we have studied light output versus current characteristics as a function of temperature to separate out the relevant physical processes. We have measured the pulsed light-current characteristics of GaN/(x5)InGaN quantum well (QW) LEDs as a function of temperature using a low duty cycle to avoid self-heating.

In this paper we summarise recent developments in the experimental study of the intrinsic gain and recombination characteristics of GaInP quantum wells. Derivation of gain spectra from spontaneous emission spectra observed through a top-contact window is limited to radiation of TE polarisation and it is necessary to assume the carrier system is in quasi equilibrium to calibrate the data into real units. These difficulties are overcome by deriving the gain and spontaneous emission spectra from the amplified spontaneous emission spectra observed from the end of the structure as a function of the pumped stripe length. The emission spectra can be calibrated by identifying the region where the carrier distributions are fully inverted, without assuming that quasi-equilibrium conditions are established. We have determined the modal gain and spontaneous emission spectra for both TE and TM polarisation for a tensile strained GaInP quantum well structure, and have obtained the TM and TEy peak gains as functions of the total experimentally-determined radiative recombination current.

The spontaneous emission and optical gain spectra from an InGaAs quantum dot laser have been independently measured under the same operating conditions. Using these spectra a combined probability-distribution function describing the electron occupancy in the conduction and valence bands has been experimentally determined. Comparison of this function with theoretical curves based on Fermi-Dirac statistics shows that for temperatures down to 100 K the carrier occupancy statistics are accurately described by thermal distributions. Measurements at 70 K show a breakdown of thermodynamic equilibrium indicated by non-thermal carrier distributions.

Using a segmented-contact method, we have measured the optical mode loss in a series of AlGaInP 650 nm large optical cavity laser diodes with cladding layer thicknesses of 1.0, 0.5 and 0.3 µm. For the thinnest cladding layer the loss is 24 cm-1 greater than the other devices and by comparison with transfer matrix calculations we show that this is due to penetration of the mode into the outer GaAs layers. The results show that the cladding thickness can be reduced to about 0.5 µm without significant increase in loss and threshold current and this could be beneficial in reducing the electrical and thermal resistance of the cladding layer in high power structures.

Summary form only given. The assumption of quasi-equilibrium within the electron and hole energy distributions is at the heart of most semiconductor laser diode gain theories and is the basis for models used to design and optimise devices. One of the consequences of the existence of such distributions is that there is a known detailed balance relation between the spontaneous emission and gain spectra of excited semiconductor systems. This relation is used explicitly in some theories to derive the spontaneous emission spectrum from the material gain/absorption spectrum to calculate the intrinsic radiative current. The recent development of a segmented-contact, single-pass method for measuring optical gain has enabled us to measure the optical modal gain spectrum, the spontaneous emission and the quasi-Fermi level separation simultaneously under the same pumping conditions. We have measured modal gain and spontaneous emission spectra for segmented-contact GaInP/AlGaInP quantum well structures.

We validate a microscopic laser model based on the quantum kinetic equations using experimentally determined threshold current, gain, spontaneous emission and quasi-Fermi level separation data taken on GaInP/AlGaInP lasers. By comparison of further experimental and calculated optical properties we find that there is a significant contribution to the threshold current from non-radiative recombination within the quantum wells.