H. M. Ng

The role of dislocation scattering in n-type GaN films

The lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy doped n type with Si to the levels of 1015–1020 cm−3 was investigated. The room temperature electron mobility versus carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model of scattering by charged dislocations. The mechanism of this scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. It was found that in the low carrier concentration region (<1017 cm−3), the electron mobility is thermally activated with an activation energy half of that of carrier concentration. This is in agreement with the prediction of the dislocation model.

High reflectivity and broad bandwidth AlN/GaN distributed Bragg reflectors grown by molecular-beam epitaxy

A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwave layers have been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples a...

Broadening of near-band-gap photoluminescence in n-GaN films

This letter addresses the broadening mechanism of the near-band-gap photoluminescence in GaN films doped n type with silicon. The films were produced by plasma assisted molecular beam epitaxy and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He–Cd laser at 77 K. At low carrier concentration ( 1018 cm−3) the full width at half maximum increases monotonically with carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to tailing of the density of states caused by potential fluctuations due to randomly distributed impurities. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [Phys. Rev. 139, A343 (1965)], and the agreement between model and experim...

Distributed Bragg reflectors based on AlN/GaN multilayers

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.

PHOTOCONDUCTIVE DETECTORS BASED ON PARTIALLY ORDERED ALXGA1-XN ALLOYS GROWN BY MOLECULAR BEAM EPITAXY

Photoconductive detectors based on partially ordered AlxGa1−xN alloys with AlN mole fractions up to 45% were fabricated and evaluated. The degree of ordering in these alloys was found to increase with the AlN mole fraction and it has a maximum value at about 50%. The resistivity of the AlxGa1−xN films was found to increase from 10 to 108 Ω cm by increasing the Al content in the films. Correspondingly, the mobility-lifetime (μτ) product, which was determined by measuring the photoconductive gain, was found to decrease from 10−2 to 10−5 cm2/V. These high values of the μτ product at the high AlN mole fraction are attributed to spatial separation and indirect recombination of the photogenerated electron hole pairs, due to band-gap misalignment of the ordered and disordered domains in these films.

MBE growth and doping of III–V nitrides

We report on the growth and doping of GaN by molecular-beam epitaxy on the c-plane of sapphire. We find that the steps of nitridation and low-temperature buffer have a significant effect on the structure, microstructure, defects and opto-electronic properties of the grown GaN films. The electron mobility in Si-doped GaN films was found to be controlled both by the density of ionized impurities and the density of dislocations. This result is consistent with the model which assumes that dislocations introduce acceptor centers. Films were doped p-type with Mg with carrier concentration up to 6 x 10 18 cm -3 , but relatively low hole mobilities (0.3 cm 2 /V s). These low mobilities can be improved by thermal annealing, a result attributed to the removal of static disorder.

Growth and doping of GaN directly on 6H-SiC by MBE

The authors report on methods for the growth of GaN by MBE directly on 6H-SiC substrates. The films were doped p-type by the incorporation of Mg and the samples were characterized by studying their structure and morphology by RHEED, XRD and SEM and their recombination properties by photoluminescence measurements. The undoped films were found to be atomically smooth with 2 x 2 surface reconstruction and have an x-ray rocking curve with a FWHM of 3.5 arcmin. The photoluminescence spectra indicate that recombination is dominated by transition across the gap. The p-type doped films have a rocking curve with FWHM of 6.5 arcmin, and the majority of recombination occurs through D-A transitions at 3.26 eV.

Metal Contacts to n- Al X Ga 1-x N

In this paper we report on the formation of ohmic contacts to n- Al x Ga 1-x N alloys. The films were produced by plasma-assisted MBE and doped n- type with silicon at doping levels between 10 18 to 10 19 cm -3 . Contacts were formed by sequential deposition of 200 A of Ti and 2000 A of Al and the contact resistivities were determined from TLM measurements. For low Al- content (x -4 to 10 -5 cm 2 . The contacts become progressively non-ohmic at Al concentrations greater than 10%. There results are consistent with the Schottky limit being applicable to these alloys and thus the Ti/Al contact forms Schottky barriers with higher barrier height as the conduction band of the alloy moves towards the vacuum level.

GaN photodiodes grown by MBE on HVPE and ELO-HVPE GaN\sapphire substrates

In this paper, we report on the growth by molecular beam epitaxy (MBE), the fabrication and the characterization of GaN diodes on HVPE n+-GaN/sapphire and ELO-HVPE n+-GaN/sapphire substrates. Specifically, such diodes were fabricated in the form of vertical schottky diodes or p-n junctions. In both cases we have seen a dramatic decrease in the leakage current in the reverse direction which is consistent with the reduction of threading dislocations in the active area of the device. The lowest leakage current measured at -5 V bias was approximately 10-8 A/cm2 for p-n junctions grown on ELO-HVPE n+-GaN/sapphire substrates. The spectral response of the vertical schottky diodes were evaluated and compared to similar devices grown wholely by MBE on sapphire substrates. The device grown on HVPE n+-GaN/sapphire substrate shows nearly ideal responsivity below 355 nm but also poorer visible light rejection than the fully grown MBE device. The observed exponential tail in the spectral response of the vertical schottky grown on the HVPE n+-GaN/sapphire substrate is attributed to the absorption and collection in the thick n+ GaN substrate.

Electron Mobility of N-Type GaN Films

The lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy (ECR-MBE) doped n-type with Si to the levels of 10 15 − 10 20 cm −3 was investigated. The room temperature electron mobility vs carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model in which dislocations with an edge component introduce acceptor centers along the dislocation lines which capture electrons from the conduction band in n-type GaN. The mechanism of scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. We found that for the samples with high carrier concentration (> 10 18 cm −3 ), the scattering at low temperatures is dominated by impurities while for samples with low carrier concentration ( 17 cm −3 ), the scattering is dominated by charged dislocations.