P. Ludewig

Electrical injection Ga(AsBi)/(AlGa)As single quantum well laser

The Ga(AsBi) material system opens opportunities in the field of high efficiency infrared laser diodes. We report on the growth, structural investigations, and lasing properties of dilute bismide Ga(AsBi)/(AlGa)As single quantum well lasers with 2.2% Bi grown by metal organic vapor phase epitaxy on GaAs (001) substrates. Electrically injected laser operation at room temperature is achieved with a threshold current density of 1.56 kA/cm2 at an emission wavelength of ∼947 nm. These results from broad area devices show great promise for developing efficient IR laser diodes based on this emerging materials system.

Physical properties and optimization of GaBiAs/(Al)GaAs based near-infrared laser diodes grown by MOVPE with up to 4.4% Bi

This paper reports on progress in the development of GaAsBi/(Al)GaAs based lasers grown using metal-organic vapour phase epitaxy and focuses on the underlying processes governing their efficiency and temperature dependence. Room temperature lasing has been achieved in devices with 2.2% Bi and lasing in devices with 4.4% Bi was observed up to 180 K. We show that the device performance can be improved by optimizing both electrical and optical confinement in the laser structures. Analysis of the temperature dependence of the threshold current together with pure spontaneous emission and high hydrostatic pressure measurements indicate that device performance is currently dominated by non-radiative recombination through defects (>80% of the threshold current at room temperature in 2.2% Bi samples) and that to further improve the device performance and move towards longer wavelengths for optical telecommunications (1.3–1.5 µm) further effort is required to improve and optimize material quality.

Optical gain in GaAsBi/GaAs quantum well diode lasers

Optical gain, absorption and spontaneous emission spectra for GaAs_0.978Bi_0.022/GaAs laser diodes are measured experimentally and compared with theory. Internal optical losses of 10-15 cm^-1 and peak modal gain of 24 cm^-1 are measured at threshold. The results of calculations showed excellent agreement with the experiment, key for future laser design.

Determination of type-I band offsets in GaBixAs1–x quantum wells using polarisation-resolved photovoltage spectroscopy and 12-band k.p calculations

Using photovoltage (PV) spectroscopy we analyse the electronic structure of a series of GaBixAs/(Al)GaAs dilute bismide quantum well (QW) laser structures. The use of polarisation-resolved PV measurements allows us to separately identify transitions involving bound light- and heavy-hole states in the QWs, as well as bound-to-continuum transitions from the QWs to the barriers. Analysis of these transitions enables us to probe the GaBixAs/(Al)GaAs conduction and valence band offsets, thereby quantifying the band offsets. Using a 12-band Hamiltonian, we extract the band offsets in the QWs explicitly by constraining the Bi-related parameters of the model against the experimentally measured transition energies. The PV measurements and calculations we present provide the first explicit confirmation of a type-I band offset at the GaBixAs/GaAs heterointerface near x = 2%. This result, combined with the theory we present for calculating the band offsets at GaBixAs/(Al)GaAs heterointerfaces, can be used to determine the band offsets at arbitrary Bi composition x.

Properties of hybrid MOVPE/MBE grown GaAsBi/GaAs based near-infrared emitting quantum well lasers

A combined growth approach involving both molecular-beam epitaxy and metal-organic vapor phase epitaxy has been developed to fabricate GaAsBi/GaAs-based quantum well (QW) laser structures with a Bi composition up to 8%. Lasing operation has been demonstrated at room temperature at 1.06 μm in laser diodes containing 3QWs that in turn contain approximately 6% Bi. A 5QW device demonstrated lasing at 1.09 μm at 80 K. Using temperature- and pressure-dependent measurements of stimulated emission as well as pure spontaneous emission measurements, we show that the threshold current of the devices is limited by non-radiative defect-related recombination and an inhomogeneous carrier distribution. This is suspected to be due to inhomogeneity of the QW width as well as non-uniform Bi composition in the active region.

Interface morphology and composition of Ga(NAsP) quantum well structures for monolithically integrated LASERs on silicon substrates

Highly efficient light sources are the remaining item required for the realization of optoelectronically integrated circuits on exactly oriented Si(0 0 1). Here, we present—using transmission electron microscopy—an investigation on the structure and stability of Ga(NAsP), which is a direct bandgap semiconductor. It is shown that Ga(NAsP) can be grown on Si(0 0 1) substrates at a wide range of growth temperatures. No sign of defect formation and phase separation is observed even for the highest growth temperatures used. The interfaces of the quaternary alloys with the GaP barriers roughen significantly with increasing growth temperature. On the contrary, the material deposited at high temperatures is more homogeneous than the one deposited at low temperatures. This is highly surprising as dilute nitride III/V alloys are commonly thought to be metastable. This is resolved by density functional theory calculations, which show that Ga(NAsP) becomes significantly more stable when grown on substrates which have a smaller lattice constant than the equilibrium lattice constant of the alloy. This stability together with the strong room-temperature photoluminescence shown by all samples, make the Ga(NAsP) material system highly promising for laser applications on Si substrates.

Thermal quenching of photoluminescence in Ga(AsBi)

We report on a comparative experimental and theoretical study of the thermal quenching of the photoluminescence (PL) intensity in Ga(AsBi)/GaAs heterostructures. An anomalous plateau in the PL thermal quenching is observed at intermediate temperatures under relatively low excitation intensities. Theoretical analysis based on a well-approved approach shows that this peculiar behavior points at a non-monotonous density of states (DOS) in the disorder-induced band tails with at least two-energy-scales. While in previous studies carried out at relatively high excitation intensities a single-energy-scale was sufficient to fit the thermal quenching of the PL in Ga(AsBi), our study at lower excitation intensities proves that two-energy-scales of disorder contribute to the thermal quenching of the PL. Possible energy shapes of the DOS, which can fit experimental data, are revealed.

GaAs1−xBix/GaNyAs1−y type-II quantum wells: novel strain-balanced heterostructures for GaAs-based near- and mid-infrared photonics

The potential to extend the emission wavelength of photonic devices further into the near- and mid-infrared via pseudomorphic growth on conventional GaAs substrates is appealing for a number of communications and sensing applications. We present a new class of GaAs-based quantum well (QW) heterostructure that exploits the unusual impact of Bi and N on the GaAs band structure to produce type-II QWs having long emission wavelengths with little or no net strain relative to GaAs, while also providing control over important laser loss processes. We theoretically and experimentally demonstrate the potential of GaAs1−xBix/GaNyAs1−y type-II QWs on GaAs and show that this approach offers optical emission and absorption at wavelengths up to ~3 µm utilising strain-balanced structures, a first for GaAs-based QWs. Experimental measurements on a prototype GaAs0.967Bi0.033/GaN0.062As0.938 structure, grown via metal-organic vapour phase epitaxy, indicate good structural quality and exhibit both photoluminescence and absorption at room temperature. The measured photoluminescence peak wavelength of 1.72 μm is in good agreement with theoretical calculations and is one of the longest emission wavelengths achieved on GaAs to date using a pseudomorphically grown heterostructure. These results demonstrate the significant potential of this new class of III-V heterostructure for long-wavelength applications.

Compositional dependence of the band gap in Ga(NAsP) quantum well heterostructures

We present experimental and theoretical studies of the composition dependence of the direct band gap energy in Ga(NAsP)/GaP quantum well heterostructures grown on either (001) GaP- or Si-substrates. The theoretical description takes into account the band anti-crossing model for the conduction band as well as the modification of the valence subband structure due to the strain resulting from the pseudomorphic epitaxial growth on the respective substrate. The composition dependence of the direct band gap of Ga(NAsP) is obtained for a wide range of nitrogen and phosphorus contents relevant for laser applications on Si-substrate.

MOVPE growth mechanisms of dilute bismide III/V alloys

This paper summarizes the present understanding of the growth of Ga(AsBi) on GaAs substrates using metal organic vapor phase epitaxy (MOVPE). A growth model including Bi segregation is developed and the influence of several growth parameters, such as the applied growth temperature, the growth rate and the partial pressures of the precursors, are investigated in detail. Also, effects, beyond pure source decomposition, of the low growth temperature needed for the deposition of the highly metastable material system are summarized. Optimizing the growth conditions enables the deposition of Ga(AsBi) layers with more than 7% Bi that show strong room temperature photoluminescence without the necessity of annealing. Bi acts as a surfactant during the growth that reduces the defect density and unintentional carbon doping of the crystals. Besides using the established Bi precursor trimethylbismuth (TMBi), the growth of Ga(AsBi) with alternative Bi precursors tritertiarybutylbismuth (TTBBi) and triisopropylbismuth (TIPBi) is discussed. Furthermore, first results on Ga(AsBi) containing an electrically pumped single quantum well laser grown with MOVPE are presented. These devices might enable high efficiency infrared laser devices in future.