740 nm InP/GaInP quantum-dot laser with 190 A cm/sup -2/ room temperature threshold current density

InP quantum-dot lasers grown on GaAs substrates and emitting in the 730-740 nm band with threshold current density as low as 190 A cm/sup -2/ for a 2000 /spl mu/m-long device with uncoated facets are reported.     

Nonradiative Recombination in Multiple Layer In(Ga)As Quantum-Dot Lasers

The segmented contact technique has been used to study the effects on nonradiative recombination of stacking multiple quantum-dot layers. As the number of stacked layers is increased a shift in the balance of dots in the bimodal dot distribution is observed due to a reduction in the number in the smaller dot size subset. This is attributed to an increase in the density of defect islands, as the number of layers is increased, that preferentially take material from the smaller dots, and lead to an increased level of nonradiative recombination per layer at low injection level. A second nonradiative process is apparent at higher injection level, which is related to the population of the small dot size subset. Spontaneous radiative efficiency was improved in a five-layer sample where the large to small dot size energy separation and relative density of the large dot size subset were increased.     

Threshold current of 670-nm AlGaInP strained quantum well lasers

By means of gain-current calculations we have examined the factors which determine the threshold current of compressively strained Ga_x In_1-xP/AlGaInP quantum well lasers for the various well width/composition (x) combinations which give a transition wavelength of 670 nm. In addition to valence band modifications we find that the increasing depth and decreasing width of the well are important in decreasing the current as the strain increases. We reveal the important role of well width fluctuations in devices with high compressive strain     

The role of high growth temperature GaAs spacer layers in 1.3-/spl mu/m In(Ga)As quantum-dot lasers

We investigate the mechanisms by which high growth temperature spacer layers (HGTSLs) reduce the threshold current of 1.3-/spl mu/m emitting multilayer quantum-dot lasers. Measured optical loss and gain spectra are used to characterize samples that are nominally identical except for the HGTSL. We find that the use of the HGTSL leads to the internal optical mode loss being reduced from 15 /spl plusmn/ 2 to 3.5 /spl plusmn/ 2 cm/sup -1/, better defined absorption features, and more absorption at the ground state resulting from reduced inhomogenous broadening and a greater dot density. These characteristics, together with a reduced defect density, lead to greater modal gain at a given current density.     

InP-GaInP quantum-dot lasers emitting between 690-750 nm

We describe the growth, material characterization, and device characterization of InP-GaInP quantum-dot lasers for operation in the wavelength range 690-750 nm. We show that the growth conditions have a major influence on the form of the gain spectrum. Relatively flat gain can be achieved over a spectral width of 90 nm at 300 K using samples containing a bimodal distribution of dot sizes, or narrower gain spectra at shorter wavelength can be achieved by suppressing the bimodal distribution by using (211)B substrates. Optimization of samples grown on substrates with the growth surface of (100) misorientated by 10/spl deg/ toward [111] results in laser operation between 729 and 741 nm and with a room temperature threshold current density as low as 190 A/spl middot/cm/sup -2/ for a 2000-/spl mu/m-long device with uncoated facets.     

Minority carrier effects in GaInP laser diodes

Using a top-contact window, we have observed emission from a direct-gap monitor layer placed at the interface between the p-cladding and contact layers of an AlGaInP laser diode when driven under forward bias, thereby providing direct evidence for minority carrier (electron) leakage in these devices. We have further shown that the leakage is due to both drift and diffusion and, using pulsed optical excitation of a device under bias, we have determined a value of 170±10 cm² V^-1 s^-1 for the mobility of minority carriers in the p-type cladding layer by a time-of-flight experiment. The data was analyzed using a simulation which takes account of the influence of recombination times in the well and monitor layer on the overall time response of the structure. The measured mobility corresponds to electron transport through the X-conduction band. We show that the drift component of the leakage current reduces the differential efficiency and is responsible for the decrease in external differential efficiency with increasing temperature. Because the leakage occurs by a mixture of drift and diffusion, the transit time does not decrease significantly with increasing drive current; however the impact of leakage on the modulation response is predicted to be very small unless the leakage becomes a substantial fraction of the total current     

InGaAs量子点的自发发射及光增益

研究了InGaAs量子点材料自发发射、放大自发发射及光增益特性.实验发现InGaAs量子点材料随着注入电流密度的增加,其自发发射及放大自发发射光谱峰蓝移,表现出明显的能带填充现象.由于量子点材料尺寸及形状等存在一定的分布,在光谱中没有明显的对应量子点激发态的谱峰.由单程增益放大自发发射得到了量子点材料在不同注入电流密度下的光增益谱.结果表明存在由于量子点大小分布造成的量子点态非均匀展宽引起的增益峰蓝移和增益谱展宽现象.     

State filling in InAs quantum-dot laser structures

We have measured the passive modal absorption, modal gain, and spontaneous emission spectra of a quantum-dot system where the inhomogeneous broadening is sufficiently small so that the ground- and excited-state transitions can be spectrally resolved. Absorption by ground- and excited-state transitions is in the ratio 1:1.88 which is close to the ratio of 2 expected for dots with similar dimensions in two directions. The absorption cross section per dot is measured to be 1.1 /spl times/ 10/sup -14/ cm/sup 2/. Optical gain from the ground-state saturates with current at a maximum value of one third of that predicted from the measured absorption if the system is fully inverted. The measured population inversion factor spectrum shows that the carrier distributions cannot be described only by a single global Fermi distribution and that the system is not in overall equilibrium. However, using parameters obtained by fitting the absorption spectrum, we find that for these particular samples the inversion factor spectrum can be described by a possible model where the ground- and excited-state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations. We speculate that photon mediation within the homogeneous linewidth could be one possible process which establishes quasi-equilibrium within each of the ground- and excited-state inhomogeneous distributions.     

The effect of cladding layer thickness on large optical cavity650-nm lasers

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 μm without adversely affecting performance. We investigated devices with 0.3-, 0.5-, and 1-μm-wide cladding layers. The measured optical mode loss of the 0.3-μm-wide cladding device was 36.2 cm^-1 compared with 12.4 and 11.3 cm^-1 for the 0.5- and 1-μm-wide cladding samples, respectively. The threshold current densities of the 0.5- and 1.0-μm devices were similar over the temperature range investigated (120-320 K), whereas the 0.3-μm 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 1 to 0.5 μm. Our results show that this can be achieved without detriment to the threshold characteristics     

Editorial: Selected papers inspired by the Semiconductor and Integrated Optoelectronics (SIOE 2008) Conference

It is a pleasure to introduce this special issue of IET Optoelectronics focused on Semiconductor Optoelectronics. A Special Issue on this theme is published in IET Optoelectronics once a year and is linked to the Semiconductor and Integrated Optoelectronics (SIOE) conference held in Cardiff.