Unstrained GaN Research Articles

Giant electric fields in unstrained GaN single quantum wells

We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.

Strong piezoelectric effects in unstrained GaN quantum wells

Optical properties of GaN/AlGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary AlGaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.

GaN Homoepitaxy for Device Applications

Epitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec.Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pn- and InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

GaN Homoepitaxy for Device Applications

AbstractEpitaxial growth on GaN single bulk crystals sets new standards in GaN material quality. The outstanding properties provide new insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) being not accessible by heteroepitaxial growth on sapphire or SiC. With MOVPE and MBE we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. Those layers revealed an exceptional optical quality as determined by a reduction of the photoluminescence linewidth from 5 to 0.1 meV and a reduced XRD rocking curve width from 400 to 20 arcsec.Only recently, progress in surface preparation allowed morphologies of the layers suitable for device applications. We report on InGaN/GaN MQW structures as well as the first GaN pn- and InGaN/GaN double heterostructure LEDs on GaN single bulk crystals. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping. Time resolved electroluminescence measurements proof that band/band recombination is the dominant emission mechanism for the InGaN/GaN LEDs.

Strong Piezoelectric Effects in Unstrained GaN Quantum Wells

AbstractOptical properties of GaN/AIGaN single quantum well (SQW) and multi quantum well (MQW) structures grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire have been characterised by low temperature photoluminescence. Photoluminescence (PL) peaks corresponding to emission from very narrow (10 Å wide) SQWs are blue shifted with respect to the bulk GaN emissions but reveal strong red shift for wider SQWs (40 and 60 Å wide). Structural properties of SQW and MQW structures have been characterised by X-ray reciprocal lattice mapping in order to determine the strain conditions and composition of ternary A1GaN alloys. The results clearly demonstrate that in all structures under investigation the ternary (barrier) alloys are fully strained to underlying MOCVD-grown GaN/sapphire substrates. Despite of the fact that GaN quantum wells (QW) are unstrained, photoluminescence experiments clearly reveal the presence of extremely strong electric fields in the QW regions, which can be attributed to the interplay of the piezoelectric-type polarisation in the well and barrier layers due to Fermi level alignment.

Photoluminescence properties of GaN grown on compliant silicon-on-insulator substrates

A compliant substrate approach has been employed to release lattice-mismatch caused strain in GaN epilayers through stress absorption in the substrate. GaN layers have been grown on silicon-on-insulator (SOI) substrates by low-pressure metalorganic chemical vapor deposition. Photoluminescence measurements at 4 K show the spectrum of grown GaN being dominated by UV emission around 3.47 eV related to neutral-donor bound excitons. The much weaker yellow luminescence shows a broad spectrum around 2.16 eV. Peak position of the UV emission changes both with measurement temperature and strain. At room temperature, the UV peak is red shifted by 64 meV corresponding well to the band-gap temperature dependence. Strain-induced blue shift of the peak, compared to unstrained GaN, is much less than for growth on sapphire, indicating strain relief in the GaN by growth on SOI. Further reduction of the blue shift is consistent with increased electron mobility.

Optical and magneto-optical characterization of heteroepitaxial gallium nitride

We describe some recent work on the characterization of strain effects on excitons and band structure in heteroepitaxial GaN, as well as magnetoluminescence studies of excitons and donors in this material. Epilayer strains were measured directly from wafer curvature, which avoids the need to know the lattice constants of unstrained GaN in advance. The change in strain between room temperature (where it is usually measured) down to low temperature, where detailed optical spectroscopy is usually performed, was estimated and found to be significant. Low temperature photoluminescence and reflectance were used to determine the exciton energies as a function of measured strain, yielding some of the deformation potentials and band structure parameters of the material. Magnetospectroscopy and reflectance were used to identify excited states of the excitons, yielding an exciton binding energy around 26.4 meV. Residual donors and their binding energies were studied from two-electron transitions, which have been identified for the first time by their Zeeman splittings in high magnetic fields. Resonant electronic Raman scattering was also used to study excited states of shallow donors, and yielded improved resolution compared to photoluminescence.

Small valence-band offsets at GaN/InGaN heterojunctions

The band discontinuities between GaN and InN, as well as InGaN alloys, are key parameters for the design of nitride-based light emitters. Values reported to date are subject to large uncertainties due to strain effects at this highly mismatched interface. We have investigated the band lineups using first-principles calculations with explicit inclusion of strains and atomic relaxations at the interface. We find that the “natural” valence-band offset between unstrained InN and GaN is 0.3 eV. Prescriptions are given, including the band shifts, due to strains at a pseudomorphic interface.

The excitonic bandgap of GaN: Dependence on substrate

The large variation in the bandgap and exciton energies reported in the literature for GaN is discussed. We compare our photoluminescence data for unstrained bulk GaN with thin GaN layers grown on sapphire and SiC, respectively. It is demonstrated that the built-in strain has a strong effect on the spectral positions of the intrinsic excitons, and consequently also on the bandgap. GaN layers on sapphire have an increased bandgap, while growth on SiC leads to a lower bandgap, compared to unstrained bulk GaN. The temperature dependence of the free excitons has been studied for all three cases, and remarkably strong differences are reported.